V31C-2153
Rheology of Mafic Granulite at High Pressure and Temperature: Implications for Crust- Mantle Interactions
The continental lower crust, dominated by mafic granulite in composition, is where many important chemical exchanges and physical interactions occur between the upper mantle and the crust. The rheology of the lower crust may shape the Earth's surface, affects the tectonic evolution of continental lithosphere and plays an important role in many geodynamic processes, such as delamination and underplating. However, controversies have been developed recently on the rheological properties of continental lower crust. In order to address directly rheology of the continental lower crust, we have performed systematic experiments to quantify the deformation of a two-pyroxene mafic granulite (57% Plag + 24% Cpx + 14% Opx + 5% opaque minerals + trace amount of K-feldspar) under high pressure and temperature. The experimental starting materials are hot-pressed well-mixed fine-grained (32-53μm) powders of granulite. Deformation experiments were carried out in a modified Griggs-type apparatus at confining pressures of 1.1-1.2 GPa, temperatures of 1100 K to 1250 K and strain rates of 3.7 × 10-4 s-1 to 9.0 × 10- 6 s-1 with an oxygen fugacity buffered at Ni/NiO. The mechanical data are fit by a power law, yielding an activation energy Q = 247 ± 32 kJ/mol, a stress exponent n = 3.3 ± 0.2, and a preexponential factor LogA = -2.1 ± 0.5, suggesting a plastic deformation in the dislocation creep regime. Our results suggest that mafic granulite has a strength significantly stronger than that of quartzite, significantly weaker than those of eclogite and peridoitite, comparable to those of diorite, anorthosite and diabase. A soft lower crust is estimated from the strength profile of continental lithosphere, supporting strongly the traditional "jelly sandwich" strength model. Our results are also consistent with the hypothesis of the lower crust acting as a low-viscosity zone to promote the lateral crustal flow underneath the Tibetan plateau. Our results suggest that continental lower crust is likely to decouple from the upper mantle if they are subjected to lateral uniform stress deformation. Delamination may happen between granulite and eclogitic lower crust in deep levels of subducted continental crusts.
V31C-2154
Viscosity Profile of the Cratonic Lithosphere of Eastern China and its Implications for Craton Reactivation
It is now well known that the cratonic lithosphere of eastern China has experienced strong reactivation since the Mesozoic, including a dramatic reduction of lithosphere thickness from ~200 km to 60-80 km and a remarkable alteration of lithospheric mantle chemistry from depleted to fertilized. Many controversial hypotheses have been proposed, mainly based on geochemical evidence, to explain the craton reactivation, including chemical and thermal erosion, delamination, lithosphere extension and subducting slab dehydration. However, craton reactivation is a sophisticated process involving both chemial and physical interactions between the crust and the mantle. In order to test the existing hypotheses, here we provide evidence extrapolated directly from laboratory rheologcial studies of quartzite, diorite, granulite, eclogite and peridotite. We compared the viscosity profile of the cratonic lithosphere of eastern China before and after the reactivation. Our results reveal that the precambrian lower crust has a viscosity much lower than the upper crust and the unaltered dry upper mantle. Thickening and detachment of lower crust from upper mantle are expected if the cratonic lithosphere is subject to deformation. Weakening of the upper mantle by both water and partial melts are prerequisite for the eclogitilized lower crust to sink down through the otherwise strong lithospheric upper mantle. We hypothesize that the craton reactivation could be initiated by subduction around edges of the craton which caused lower crust deformation and thickening, followed by alteration and weakening of the upper mantle due to dehydration of the subducting slabs. Delamination of eclogtic lower crust and underplating happen last to reconstruct the physical and chemical balance between the crust and the mantle.
V31C-2155
Composition and origin of Archean lower crust, Northern Tanzania
Granulite-facies xenoliths from tuff cones erupted on the margin of the Tanzanian craton and within the adjacent Mozambique belt in northern Tanzania offer an opportunity to assess the role of lower crustal processes in the tectonic evolution of these two terranes. Both terranes are Archean, but record very different histories, starting in the Proterozoic and continuing today. Whereas the craton experienced little metamorphism or igneous activity following its stabilization around 2.8 Ga, Archean rocks of the Mozambique belt in the study area experienced at least one episode of high-grade metamorphism during the East African orogeny (ca. 640 Ma). Today, the East African rift exists at the contact between the Mozambique belt and the craton, implying a fundamental lithospheric weakness at this boundary. Granulite xenoliths come from Labait, on the craton margin, and Lashaine and Naibor Soito in the metamorphic belt. Most xenoliths are mafic and all are igneous in origin. Cratonic xenoliths (pl- opx±cpx±gt±hbl) are primarily anhydrous two-pyroxene granulites that likely originated as crystallized high-Ni, Archean basaltic melts. Xenoliths from the Mozambique belt are dominated by mafic granulites (pl-cpx-gt±opx) at Lashaine and banded, mafic to intermediate granulites at Naibor Soito. Positive Sr and Eu anomalies imply that the Lashaine granulites originated as plagioclase cumulates. The wide range in SiO2 (47-65 wt%) and correlation of Ni-MgO in the Naibor Soito xenoliths suggests they may have originated as igneous rocks that subsequently underwent partial melting to form the mafic (pl- opx±cpx±gt±hbl±bt) and felsic bands (pl-qtz-opx±kfs). U-Pb zircon ages for xenoliths from both terranes are Archean, as are most TDM ages, though younger TDM ages are seen in some Lashaine samples that were contaminated by rift magma. High pressures (up to 2.7GPa) are recorded by the Mozambique belt xenoliths, suggesting equilibration in thickened crust during the East African orogeny, but no igneous activity related to metamorphism has been detected and our samples suggest that the Tanzanian lower crust has persisted without significant chemical modification since the Archean. Proterozoic magmatism is also absent from the upper crust in this section of the Mozambique belt, raising the question of the heat source during metamorphism.
V31C-2156
The stability of Cl-CO3-scapolite relative to plagioclase + CaCO3 + CaSO4 in the presence of NaCl brines as a function of P-T-XNaCl
Cl-CO3-scapolite [(Na,Ca)4[Al3 (Al,Si) 3 Si3 O24](Cl, CO3 , SO4 )] occurs as a common partial to total alteration of plagioclase in deep-crustal xenoliths, skarns, marbles, gabbros, metabasites, calc-silicate gneisses, as well as in quartzofeldspathic granulite-facies rocks in general (Moecher and Essene, 1990, J Petrol 31, 997). Alteration of plagioclase to Cl-CO3-scapolite is presumed due to metasomatism by CO2-NaCl-H2O fluids (Satish-Kumar and Santosh, 1998, Geol Mag 135, 27). Previous experimental work on CO3-scapolite has focused on reversing the equilibrium 3 CaAl2 Si2 O8 + CaCO3 = Ca4 Al6 Si6 O24 CO3 in either pure CO2 (Goldschmidt and Newton, 1977, Am Mineral 62, 1063) or in CO2-H2O (Huckenholz and Seiberl, 1989 Abs IGC 28, 2.79). These experiments have determined that the anorthite- calcite-scapolite equilibrium is nearly pressure-invariant in P-T space (200 to 1500 MPa) occurring at approximately 790 to 820 °C (Huckenholz and Seiberl, 1989). In this study, a series of experiments, involving the equilibrium 3 Plagioclase(An60) + 0.5 CaCO3 + 0.5 CaSO4 = [(Na,Ca)4[Al3 (Al,Si)3 Si3 O24](Cl, CO3, SO4 )] plus an NaCl brine (10/90, 20/80, 30/70, and 50/50 molar NaCl/H2O) have been done at 500, 1000, and 1500 MPa and 600 to 900 °C. Natural plagioclase and scapolite, along with synthetic calcite and anhydrite, were lightly ground together in equi-molar amounts in ethanol. The mineral mix (10 mg) + NaCl brine (5 mg), or pure H2O (1.5 mg), were loaded into 3 mm diameter/1.3 mm long Pt capsules which were arc-welded shut, folded, and placed horizontally in a CaF2 setup (with graphite oven), such that the thermocouple tip touched the Pt capsule, or placed in a hydrothermal autoclave (600 and 700 °C; 500 MPa) with an internal thermocouple. A series of duplicate experiments for the same mineral mix, at the same P-T conditions, were done utilizing pure H2O as the flux. The H2O-only experiments duplicated the P-T reversals of Huckenholz and Seiberl (1989). In contrast, the NaCl/H2 O experiments indicate that the stability field of Cl-CO3-scapolite (SiO2 =52.1, Al2O3=24.9, CaO=11.9, Na2O=7.60, Cl=1.86, CO3=2.00, SO4=0.45), relative to plagioclase, greatly expands in the presence of NaCl brines at NaCl concentrations above 10 percent NaCl such that Cl-CO2-scapolite is stable over 600 to 900 °C and 500 to 1500 MPa. This result further strengthens the proposition that NaCl brines, coupled with CO2-bearing fluids, can be and probably are involved during high-grade scapolitization of plagioclase-bearing rocks in the mid to lower crust and upper mantle.
V31C-2157
Partial Melting Of Metapelitic Rocks And Melt Extraction In The Lowest Continental Crust
This study reports Ca-rich garnet-bearing metapelitic rocks, which preserve high pressure granulite facies mineral assemblages and mineral compositions: biotite + garnet (almandine-grossular-rich) + K-feldspar + plagioclase (An-rich) + quartz + ilmenite, in the low-P/high-T Higo metamorphic rocks. Peak P-T condition of the metapelitic rocks is estimated to be 12 kbar and 800 ° C by using Grt-Bt thermometer and Grt-Pl-Bt-Qtz barometer. The peak P-T condition almost corresponds to the depth of the lowest crust. Whole-rock major elements analyses show that the Ca-rich garnet-bearing metapelitic rocks are more melt- depleted (e.g. enriched in Fe2O3 + MgO + TiO2 and depleted in Na2O + K2O and SiO2) than metapelitic gneisses, migmatites, such as metatexite and diatexite, and leucogranites in the Higo metamorphic rocks, and that the metapelitic rocks are compositionally complemented by the leucogranites, making a mixing line with average compositions of metapelitic gneisses and leucogranites, while there is no difference among average compositions of metatexites, diatexites and meapelitic gneisses. Mineral equilibria modeling in the NCKFMASH system of an average composition of Higo metapelitic gneisses shows that effective melt loss formed the Ca-rich garnet-bearing metapelitic rocks with the granulite facies assemblage. The melt compositon calculated at the peak P-T condition is consistent with that estimated in terms of mass balance by using isocon method. The modeling also shows that influx of H2O was required for high proportion of melt needed to form the diatexite, and that the metatexite did not involve little added H2O. Source region for the melt, which is considered to be a principal cause for the low-P/high-T Higo metamorphism, might be metapelitic rock in the lowest crust. Most of metapelitic rocks in the mid- to lower crust might be partially melted but no melt was effectively extracted from the metapelitic rocks. Melt production was promoted by influx of H2O, which might be derived from the solidified melt ascending from the lowest crust, and, as a consequence, diatexite was formed in the mid- to lower crust.
V31C-2158
Silicic Volcanics in the South Mountain Region: A Volcanic Center with the Breakup of Rodinia
Commonly, large igneous provinces are associated with the break-up of continents. The waning stages of large igneous provinces prior to continental rifting and breakup are characterized by volumetrically less significant felsic volcanics. The origin of these felsic volcanics remains a source of significant debate – do they represent additions of new crust derived from the mantle, or the melting and recycling of continental crust? We examine the South Mountain felsic volcanicism in Pennsylvania, part of a sequence of metabasalts and metarhyolites of the Catoctin Formation. These Late Neoproterozoic magmas are associated with the breakup of Rodinia and the formation of the Iapetus Ocean. The South Mountain felsic volcanics are predominantly rhyolitic and can be subdivided into high Al2O3 (>11.8 wt.%) and low Al2O3 (<11.8 wt.%) groups. Each group forms distinctive trends against differentiation indices. Cl Chondrite normalized spider diagrams show that both groups are parallel to each other, the low-Al group having higher REE concentrations. The low-Al group forms REE trends that decrease at a greater magnitude than the high-Al group with increasing SiO2, excluding Eu. A Eu anomaly is present in both groups but is more pronounced in the low-Al group. Models of REE variations in felsic liquids [Brophy, 2008] indicate that amphibolite melting yields magmas with decreasing La and Yb with increasing SiO2. In both groups, La and Yb indicate amphibolite partial melting trends. Decreasing Dy/Yb with increasing SiO2 is an indicator of a hornblende rich source for the South Mountain rhyolites. To explain these geochemical trends, we propose that the South Mountain rhyolites were produced by the partial melting of a plagioclase-bearing amphibolite source. The difference in magnitude of the Eu anomaly between the two groups may be produced by variable concentrations of plagioclase in the source amphibolite. Comparison of the South Mountain rhyolites with other large igneous provinces may provide insight into the petrogenic processes generating felsic volcanism during continental break-up. Brophy, James G (2008), A study of rare earth element (REE)—SiO2 variations in felsic liquids enerated by basalt fractionation and amphibolites melting: a potential test for discriminating between the two different processes, Contrib Mineral Petrol, 156, 337-337.
V31C-2159
Zircon U-Pb SHRIMP Ages From Eastern Ghats Belt, India and Their Implication on the Indo–Antarctic Correlation
We analyze zircon using U-Pb SHRIMP method to constrain the timing of important thermal events in the Eastern Ghats Belt (EGB) which shows a comparable geological history with the Precambrian rocks of east Antarctica. Zircon grains show contrasting morphological and chemical characters depending on the rock types and locality. Zircon from UHT aluminous granulites from the central part show detrital (c. 1820 Ma) and igneous cores (c. 1760 Ma) occasionally surrounded by oscillatory-zoned mantle (c. 1650 Ma). Thick metamorphic rim (U-rich) over detrital core is and simple concentric zoned zircon grains show well-grouped concordant age of c. 950 Ma. This event is correlated with the Rayner structural event (980-930 Ma) strengthening the fact that the Eastern Ghats-Rayner Complex evolved as a composite orogen during Meso- Neoproterozoic time. The c. 1650 Ma event possibly suggests the timing of UHT metamorphism and anatexis. Absence of Pan-African ages (550-500 Ma) in all of these samples suggests thermal input was not strong enough to cause zircon growth during this event. Zircon grains from leptynite from the southern part shows well-grouped concordant age of c. 1760 Ma with partial resetting at c. 1690 Ma due to UHT metamorphism. The c. 950 Ma event was unseen by rocks of this area. Rocks of the northern part bear distinct geochronological signatures. Zircon grains in the HT/UHT aluminous granulites are sector zoned, cloudy homogeneous metamorphic grains with nearly concordant age of c. 780 Ma. Few metamorphic grains show c. 520 Ma age which possibly is responsible for partial modification and recrystallization of the c. 780 Ma grains. Our data suggest that the northern part of Eastern Ghats is exotic in origin and was possibly amalgamated to the rest of the Eastern Ghats during the Neoproterozoic-Phanerozoic time. The immediate counterpart of this segment is not properly known, but Prydz Bay area of east Antarctica shows a comparable evolutionary history. The tectonic boundary separating the Rayner Complex and Prydz Bay region can be extended to the Nagavalli-Vamsdhara Shear Zone that separates the northern and central parts of EGMB.
V31C-2160
Mineral Chemistries And K-Ar Ages Of Hundar Igneous Complex In Ladakh Himalaya, Northern India
The Shyok Suture Zone is situated between Ladakh and Karakoram batholiths in northwestern Trans- Himalaya and consists of mafic - ultramafic, sedimentary and metamorphic rocks. The sedimentary rocks are considered to have been deposited on the continental shelf, which probably has matured continental hinterland because they comprise terrigenous clastic rocks, including quartzose sandstones and conglomerate with granitic clasts, and yield shallow marine molluscs and land plants. We discovered Hundar igneous complex that intruded into the sedimentary rock sequence in the Hundar river section and carried out their petrographic, mineral chemistry and K-Ar analyses to compare with the calc- alkaline Ladakh batholith that formed as a paleovolcanic arc at a convergent margin of the Tethyan oceanic plate and Eurasian continental plate. The Hundar igneous complexes consist of granite, diorite and dykes of andesite and leucogranite. They have intruded into the sedimentary sequences in the Shyok Suture Zone and caused thermal metamorphism on the sedimentary rock to form the contact aureole. The granite and diorite occur as a complex magmatic sequence depicting mechanical mixing of granite and diorite magmas. We collected the granite, diorite, leucogranite, hornfels from the Hundar river section and coarse-grained granite, granodiorite from central part of Ladakh batholith in Khardung La and Karakoram batholith in Panamik for K-Ar dating of biotite and muscovite. Electron Probe Micro analyses of the constituent minerals reveal that the Ladakh batholiths, Hundar igneous rocks and Karakoram granites are calc-alkaline and formed in a subduction related tectonic setting. The leucogranites occur as dykes and small bodies intruding into the main Karakoram batholith and represent S- type granite related to post-collision tectonic setting in Karakoram block. Samples of the Ladakh batholiths give ages from 49.2±1.1Ma to 51.2±1.1Ma, suggesting the last major magma pulse in Ladakh batholith. The Hundar complex yields ages from 60.8±1.4Ma to 65.8±1.4Ma. The age differences between the Ladakh batholith and the Hundar complexes are significant, suggesting that the igneous activity of the Hundar complexes took place before the emplacement of the main Ladakh batholith. Thus, the subduction related igneous activity in Ladakh-Shyok area took place at least for 16My, from 50Ma to 66Ma, before the collision event between the Indian and Asian plates. The Karakoram granite samples give ages from 95.7±2.1Ma to 96.7±2.1Ma, suggesting that one of the phases of the Karakoram batholith was emplaced in the 21 My before the collision (75Ma) between Ladakh Arc and south Asian margin. The leucogranite samples of the Tangtse gorge yield ages of 9.18±0.21Ma to 9.45±0.21Ma, representing one of the post-collision phases that may be related to an activation of the Karakoram fault in Karakoram block.
V31C-2161
New Geological, Petrological, and Geochemical Investigations of Alishar Volcanic rocks, NW of Saveh, Iran
Two slivers of Eocene volcanic rocks, separated by Pliocene-Quaternary conglomerate, are exposed, northwest of the city of Saveh, along the Tertiary Uromieh-Dokhtar magmatic zone. The area north of Alishar, between 49o, 53'E and 50o,00'E longitudes and 35o, 17' N and 35o, 20' N latitudes, is located in the northern sliver. The oldest rocks north of Alishar have a Middle Eocene age, and are exposed in the core of an anticline which is faulted in its southern limb. These rocks are crystal and lithic tuff, and covered by rhyolitic ignimbrite. They are disconformably overlain by conglomerate, sandstone, marl, lenses of nummulite- bearing limestone, and green tuff. Fossils in the limestone give a Middle Eocene age. Bimodal, Late Eocene volcaniclastic and volcanic rocks cover these rocks. Evidence for acidic and intermediate volcanic activities is apparent at two levels. The early products of the explosive Late Eocene volcanic/volcaniclastic activity in this area were dacitic, and laid down on wet, green Middle Eocene tuff. These rocks are characterized by their vitroclastic texture and abundance of volcanic glass shard mixed with oxidized basic glass. The first level of basic-intermediate lavas is andesitic, with coarse clinopyroxene phenocrysts at the base, and plagioclase phenocrysts at the top, suggesting crystal separation at the scale of this unit. Extensive ignimbrites, including ignimbrite-breccia, ignimbrite-tuff, and ignimbrite-lava pairs cover this unit. The ignimbrites are rhyolitic, trachy-dacitic, dacitic, and trachy-andesitic. Mafic inclusions inside felsic groundmass are evidence for magma mingling in the ignimbrites and their compositional variation from rhyolite to trachy-andesite. The ignimbrites are cut by basaltic dikes and by pipes which probably were conduits for the movement of hydrothermal fluids. Epiclastic breccia and overlying level-2 andesitic lava cover the ignimbrite. These rocks are cut by masses of post-Eocene quartz diorite. Studies of the major, trace, and rare earth elements, show a wide range of variation in the felsic rocks and progressive differentiation from mafic to felsic rocks, and suggest a bimodal volcanism in the area north of Alishar. Primitive mantle- and chondrite-normalized spider diagrams show a moderate enrichment of HREE (with La/Sm=3.97-6.86 and La/Yb=6.11-12.11) and negative Nb and Ti anomalies, suggesting backarc basin volcanism. A comparison of the REE in the Alishar rocks with those in the upper crust, in addition to revealing similarities, indicates the enrichment in the LILE. It seems that during Middle and Late Eocene, intermediate- basic magma in the upper crustal levels, in addition to undergoing differentiation, mingled and mixed with the rhyolitic magma which formed from the melting of the rocks around the magma chamber.
V31C-2162
Magmatism in the Tsagaandelger, Eastern Mongolian volcanic belt: Petrological, geochemical and isotopic constraints on Mesozoic geodynamic setting
The vast territory of Mongolia lies in the heart of the Central Asian Orogenic Belt, one of largest provinces of the Phanerozoic continental growth on Earth (Jahn et al., 2004). We present new petrographic, geochemical and Sr–Nd isotopic analyses on Mesozoic igneous rocks emplaced in Central Mongolia. The Mesozoic igneous suites, those exposed in the Tsagaandelger area, pass upwards from alkaline series trachytic rocks and overlain by tuffaceous sediments. Those are intruded by calc alkaline leucocratic granites and covered by Late Mesozoic calc alkaline bimodal volcanic rocks consisting of basalts and rhyolite. Alkaline series volcanic sequences were erupted in Early-Middle Triassic (241 Ma) and characterized by LILE, LREE enrichment and significant Nb-Ta depletion. Rocks have weakly enriched initial 87Sr86Sr ratios of 0.705 to 0.706 and positive εNd(t) values (0.7 to 4). The crystallization age of intrusive rocks is 231 Ma. The majority of samples is slightly peraluminous and can be classified as granite, including monzogranite, granodiorite and aplite. Granites are characterized by near- zero εNd(t) values (0.7 to 2) and tetrad effect in their REE distribution patterns. Further Cretaceous volcanic sequences have lower contents of LILE and higher contents of HFS and REE, comparing with Triassic volcanic sequences. The Cretaceous volcanic rocks have the initial 87Sr86Sr ratios between 0.705 and 0.719 and near-zero εNd(t) values (-0.7 to 1.6). Trace element geochemistry indicates that Mesozoic volcanic rocks from the studied area are arc related. The Triassic volcanic and plutonic rocks could be emplaced in active continental margin settings. Post collisional extensional regime could be started with Early Cretaceous volcanism. The mass balance calculation suggests that the all Mesozoic volcanic and plutonic rocks were derived from sources composed of more than 80% juvenile mantle-derived component. Our data confirm the earlier observations of similar isotopic characteristics, in other Paleozoic to Mesozoic granitic plutons in the Central Mongolia as well as Central Asia (Kovalenko et al. 1996, Jahn et al. 2004). Thus, our new data provide evidence for a significant production of juvenile crust, and hence growth of the continental crust, in the Phanerozoic.
V31C-2163
Geochemical Constraints For The Genesis Of A-type Granite From Southeastern Korea
Mineralogical, geochemical characteristics and Sr-Nd isotopic data are presented for the Namsan A-type granite and Gyeongju I-type granitic rocks in Gyeongju area, Gyeongsan Basin, Korea. The Namsan A-type granite is alkali feldpar granite and consists of quartz, perthite, sodic amphibole (riebeckitic arfvedsonite), annite, and fluorite. Quartz and perthite occurs as early crystallized phase, and others as interstitial phase. The Gyeongju I-type granitic rocks are divided into four plutons based on their petrographical features; The Maseoksan biotite granite(MBG), Tohamsan biotite granite(TBG), Gyeongju hornblende biotite granodiorite(GHBGd), and Gyeongju biotite porphyritic granite(GBPG). Aplitic dykes intrude these granite rocks. Petrochemically, the Namsan alkali feldspar granite is distinctly different from other I-type granites, and is characterized by higher Fe2O3, Fe2O3T, Na2O, Rb, Nb, Y, Ce, Ga, Zr contents and Ga/Al2O3 ratio, and lower TiO2, Al2O3, MgO, CaO, P2O5, Ba and Sr contents. The rocks show flat chondrite-normalized REE patterns with strong Eu negative anomaly. The MBG and TBG, the GHBGd, and the GBPG show the geochemistry of general calc-alkaline I-type granitic rocks, and they have almost constant Ga/Al2O3 ratio and LREE-enriched pattern. Although the aplitic dykes of the Maseoksan and Tohamsan biotite granites are extensively fractionated, compared with the Namsan A-type granite, the former has significantly lower large highly charged cations such as Nb, Y, Ce, Ga, Zr. This geochemical feature suggests that the Namsan A-type granite can not be derived from MBG and TBG magma by fractional crystallization process. Geochemical data indicate that the GBPG was derived from the GHBGd magma by fractionation of amphibole, biotite, plagioclase, sphene, and magnetite. On discrimination diagrams, the Namsan A-type granite is plotted in the field of anorogenic, within plate granites, whereas I-type granitic rocks in the field of subduction-related, volcanic arc granites. The Namsan A-type granite shows a Rb-Sr whole rock isochron age of 49.0 Ma¡¾2.0 Ma with an initial Sr isotope ratio (Sri ) of 0.7065¡¾16 (2¥ò). Initial 143Nd/144Nd ratios range from 0.512709 to 0.512734 (eNd= +2.9~+3.1). The MBG and TBG show same Rb-Sr age of 51.4¡¾6.5 Ma with Sri of 0.7042¡¾8 (2¥ò), and 51.4 Ma¡¾4.1 Ma with Sri of 0.7040¡¾5 (2¥ò), respectively. These isotopic data as well as mineralogy and geochemical data indicate that the two granites were derived from a same magma. All samples from the both granites yield a well defined Rb-Sr isochron age of 51.5¡¾3.5 Ma with Sri of 0.7041¡¾4 (2¥ò). Initial 143Nd/144Nd ratios of the two granites range from 0.512664 to 0.512766 (eNd= +1.80~+3.78). During the GHBGd have emplaced, the magma may have been contaminated with sedimentary rocks, resulting in having the scattered isotope data. The GBPG shows a Rb-Sr whole rock isochron age of 57.6¡¾5.6 Ma with Sri of 0.7048¡¾4 (2¥ò). Initial 143Nd/144Nd ratio of the GHBGd and GBPG show lower values ranging from 0.512565 to 0.512603 (eNd= +0.02+0.77) than those of other granites. These isotope data may suggest the GHBGd was derived from a different source from the MBG and TBG. The high fluorine content of the Namsan A-type granite would have contributed to enrich Nb, Ga, Na, Ce, Zr and REEs in the magma. The fluorine-enriched A-type magma may have been produced by low degree of partial melting of the source at high temperature under extensional environments, which were provoked by the resetting of the subduction mode of the Pacific plate from normal to oblique during early Tertiary.
V31C-2164
The Late-Eocene Adakitic Granite In The Yardoi Gneiss Dome, Southern Tibet
The Yardoi gneiss dome is located to the easternmost of the North Himalayan Gneiss Dome (NHGD), southern Tibet. It consists of metapelite, garnet graphite schist, garnet amphibolite, Cenozoic two-mica granite and leucogranite. The Dala two-mica leucogranite intruded into the Yardoi gneiss dome, and is located at the southeastern of the Yardoi gneiss dome. It consists of quartz + alkali-feldspar + plagioclase + biotite + muscovite. SHRIMP zircon U/Pb dating indicates that it formed at ~ 44.0 Ma. This pluton has (1) high Al2O3 (16.0~ 17.0 wt% ), Na2O/K2O ratios (>1.20) and A/CNK(>1.05); (2) highly enriched in LREE (La/Yb)N = 39.5~ 45.5) and depleted in HREE (Gd/ Yb)N = 5.5~6.4), slightly negative or no Eu anomalies; and (3) high Sr concentrations (355~ 416 ppm) and Sr/Y ratios (>59.0). High Sr/Y and La/Yb ratios, and Sr contents, but low Y and HREE concentrations in the Dala leucogranite suggests it is of an adakitic composition. Sr and Nd isotope analyses show that they have relatively radiogenic Sr (87Sr/86Sr (t) = 0.71754~ 0.71785) and unradiogenic Nd (ƒÕNd(t) = -9.38~ -12.6) isotope compositions. Such a Sr-Nd systematics is similar to that of another suite of younger leucogranites formed at~ 35 Ma, within the same gneiss dome (Zeng et al., 2008; Gao et al. 2008). Our data suggests that the Dala adakitic granite was derived from partial melting of mafic lower crustal materials in the overthickened crust of the Himalayan orogenic belt.
V31C-2165
Fluxed-melting of shallow and hot high-grade metamorphic rocks in the Namcha Barwa Massif: A new mechanism for the generation of adakitic melts
Extremely rapid exhumation has been a major factor leading to rapid advection of relatively hot crustal rocks into shallow crustal levels in the Namcha Barwa massif. In such a hot tectonic regime, it is conceivable that hot crustal material can undergo partial melting assisted by excessive water. Previous studies indeed have documented extremely young Na-rich granitic rocks (<5.0 Ma) in the Namcha Barwa massif (Burg et al., 1998; Booth et al., 2000). To constrain the nature and mechanism for the production of these rocks, we have carried out major and trace element, Sr and Nd radiogenic isotope geochemistry studies on a suite of migmatites and their wall-rocks. The analytical data show that (1) a majority of young leucosomes are of adakitic composition with elevated Na2O/K2O, Sr and Sr/Y (>63.5 and up to 237.1), consistent with garnet either as a non-reactant phase or as a residue phase; (2) these leucosomes have similar Sr isotope compositions to their host rocks, but some of them have Nd isotope compositions significantly deviated from their host by ~4-5 epsilon units. Since high-grade metamorphic rocks in the Namcha Barwa were already shallower than 10 km in the past 5 million years, these data is best explained by water- fluxed melting of granitic components in the source rocks. This interpretation is also consistent with at water- present and relatively lower temperature conditions, both apatite and garnet behaves as non-reactant phases during a partial melting event, which could lead to depletion of garnet-compatible elements as well as negative shift in Nd isotope compositions in the amphibolite- or metapelite-derived melts. Our data emphasize that at rapidly exhumed and relatively hot tectonic regimes (e.g. the Nanga Parbat and Namcha Barwa areas), fluxed-melting of high-graded metamorphic rocks at relatively shallower crustal levels could be another important mechanism to produced adakitic magma and associated porphyritic Cu-Au deposits.
V31C-2166
Sources and petrogenesis of the Gangdese Batholith: Evidence for episodic crustal growth in southern Tibet
The Gangdese batholith is renowned for its prolonged magmatism that spanned the India-Asia collision, but still poorly constrained in its processes and genesis. To delineate the source characteristics, and investigate the tectonomagmatic evolution of the continental margin in southern Tibet, we here integrate new geochemical data with the relevant geochronological results of the Gangdese plutonic complex. The entire batholith is profoundly characterized by mantle source components with ε Nd(T) from +5.3 to -0.5, and 87Sr/86Sri from 0.7032 to 0.7069. Among the 4-stage plutonism we observe, the Cretaceous (103-85 Ma) Gangdese arc magmatism exhibits the most primitive compositions such as the overall most depleted Sr-Nd isotopic signatures. The subsequent ca. 80 Ma intrusives are marked by the relatively uniform and silicic characteristics, particularly the adakitic geochemical features as the ending phase of the Cretaceous arc magmatism. Reactivating after an abrupt increase of the plate convergence rate and decrease in obliquity of convergence, the arc magmatism proceeded at 65-60 Ma and refined into more enriched components in the terminal stage before the closure of Tethys. Shortly after the waning interval related to the continental collision until ca. 55 Ma, the Eocene (53-45 Ma) magmas flared up with the largest isotopic and compositional diversity that contrast them by fractionation and mixing petrogenesis to the previous arc magmas, marking their post-collisional characters. The observation that the Mg numbers of the Gangdese magmas remain roughly constant at 40-50 with increasing SiO2, together with the evidence of lower crust melting, suggest that basaltic underplating or recharging near the Moho interface is the main drive for the generation and refinement of the continental arcs. Thus the melting regime of the juvenile crust can be utilized for determination of relative crustal thickness. Our data show that, after the accretionary orogeny at 80 Ma, the Tibetan continental crust was not significantly thickened until the Oligocene. Collectively, the crustal growth in southern Tibet is deciphered as a juvenile arc developing toward a relatively evolved continental arc from the Cretaceous to Paleocene, and then being rejuvenated by asthenospheric sources during the Eocene magmatic flare-up, a series of magmatic addition associated with the slab geodynamics that is inferred to be preceding from common to shallow subduction, and then rolling back until breakoff of the Neo-Tethyan slab.
V31C-2167
U-Pb and Lu-Hf Isotope Systematics of Zircons From the Mesoproterozoic Lower Crust of Central-Southern Mexico
Lower crustal rocks, including AMCG suites, are widely exposed in the Oaxacan Complex of S Mexico and in the Novillo, Huiznopala, and Guichicovi gneisse of eastern and SE Mexico, also referred to as 'Oaxaquia' [1-4 and refs. therein]. To characterize the different basement exposures, we combined U-Pb dating by LA-MC- ICPMS and single-grain ID-TIMS with single-grain Lu-Hf analysis by solution MC-ICPMS. In the Oaxacan complex, zircon grains from a ferrodiorite dike associated with anorthosites yield homogeneous CL images but have >100 Myr age differences from core to rim. Zircon grains from the host migmatites are complex, with zoned cores, high-U mantles, and low-U metamorphic overgrowths. Laser ablation age mapping revealed, however, that most of the zircon crystallized at ~1.2 Ga. This was followed by a single, long- lasting event at ~1 Ga. Zircon from Guichicovi, Novillo, and Huiznopala gneisses records the same two major geologic events. Huiznopala zircon also contains another inherited component. Zircon grains from a 1 Ga enderbite and ferrodiorite-, and the 1.2 Ga migmatite of the Oaxacan complex yield a narrow range of two-stage T(DM) Hf model ages (1.5-1.6 Ga), suggesting that the 1 Ga magmatic suite is reworked from the same crustal protolith that formed the ca. 1.2 Ga migmatites. Hafnium isotope ratios of zircons from Guichicovi and Novillo gneisses are similar to those of the Oaxaca complex, suggesting a common evolution in a geologic system that was isolated from cratonic crust. Lower εHf was only found in grains with >1.3 Ga inherited cores and from a charnockite (992 ± 5 Ma, concordant; εHfi = +0.3). The latter yields a two-stage T(DM) of 1.7-1.8 Ga, indicating a distinct, older component in the lower crust of southern Mexico. [1] Ortega-Gutiérrez et al. 1995 Geology 23: 1127-1130 [2] Solari et al 2003 Tectonophysics: 257-282 [3] Cameron et al. 2004 GSA Mem. 197: 755-769; [4] Lawlor et al. 1999 Precamb. Res. 94: 73-79.
V31C-2168
Reassessing Melt Depletion Indices of Peridotites: Implications to the Origin of Subcontinental Lithospheric Mantle
The correct evaluation of the maximum degree of depletion experienced by peridotites and the physical conditions at which their melting occurred is critical to constraining the origin of sub-continental lithospheric mantle (SCLM). For example, whole-rock (WR) MgO-Al2O3 and MgO-Yb trends observed in various peridotites suites can be used to retrieve both the minimum degree of depletion and the maximum pressure of melting, thereby placing constraints on whether SCLM peridotites were formed by basal melting of the lithosphere, melting in a plume head, or represent shallowly-formed mantle that was transported to greater depths by accretion and thickening. The advantage of this approach is that most known post-melting processes do not further enrich residual peridotites in Mg, or deplete them in Al or Yb, and therefore the minimum degree of melting experienced by SCLM can be reasonably estimated. In the case of mantle xenoliths collected in various continental environments, due to small sample size or alteration, WR compositions are not always easy to analyze or estimate, and thus, as an alternative, often trace element concentrations in fresh clinopyroxenes are assumed to be representative of melting degree experienced by their host peridotites. However, the extent of depletion estimated from clinopyroxene compositions is heavily model-dependent and our analysis reveals that melting degrees assessed by this approach systematically underestimate the more robust results based on peridotite WR chemistry. This problem is most critical in the case of harzburgites, for which current formulations based on clinopyroxene trace element concentrations indicate depletion degrees that are up to 8-10 melt percent lower than those estimated from WR chemistry. In the case of polybaric fractional melting, such differences translate to a 25-30 km underestimate of melting column length, a fundamental constraint of the depth of melting and hence the origin of SCLM. The main reason for this discrepancy is that clinopyroxene trace element compositions in harzburgites are not representative of the WR due to subsolidus redistribution of these elements into other minerals. In order to correct for this discrepancy, we derive an expression that matches heavy REE concentrations in clinopyroxenes equilibrated in subsolidus peridotites to the WR chemistry based melting degrees. Using this expression, we revisit a series of clinopyroxene REE data reported in worldwide SCLM xenoliths, estimate the corresponding minimum melt depletions, and approximate their corresponding melting pressures. We find that harzburgites formed by more than 20 percent melt depletion in spinel facies conditions are common in xenolith suites from all peri-cratonic areas. Our observations corroborate the view that SCLM mainly formed by low-pressure melt extraction and that modern continents mainly grew by lateral accretion of oceanic and/or arc lithosphere.
V31C-2169
Two Distinct Sets of Magma Sources in Cretaceous Rocks From Magnet Cove, Prairie Creek, and Other Igneous Centers of the Arkansas Alkaline Province, USA
Two distinct sets of magma sources from the Arkansas alkaline province (~106-89 Ma) are revealed by Sr-Nd-Pb isotopic compositions of olivine lamproites vs. other alkalic rock types, including carbonatite, ijolite, lamprophyres, tephrite, malignite, jacupirangite, phonolite, trachyte, and latite. Isotopic compositions of diamond-bearing olivine lamproites from Prairie Creek and Dare Mine Knob point to Proterozoic lithosphere as an important source, and previous Re-Os isotopic data indicate derivation from subcontinental mantle lithosphere. Both sources were probably involved in lamproite generation. Magnet Cove carbonatites and other alkalic magmas were likely derived from an asthenospheric source. Lamproite samples are isotopically quite different from other rock types in Sr-Nd-Pb isotopic space. Although three lamproite samples from Prairie Creek have a large range of SiO2 contents (40-60 wt %), initial values of εNd (-10 to -13), 206Pb/204Pb (16.61-16.81), 207Pb/204Pb (15.34-15.36), and 208Pb/204Pb (36.57-36.76) are low and similar. Only 87Sr/86Sr(i) displays a wide range in the Prairie Creek lamproites (0.70627-0.70829). A fourth lamproite from Dare Mine Knob has the most negative εNd(i) of -19. Lamproite isotope values show a significant crustal component and isotopically overlap subalkalic rhyolites from the Black Hills (SD), which assimilated Proterozoic crust. Six samples of carbonatite, ijolite, and jacupirangite from Magnet Cove and Potash Sulphur Springs exhibit the most depleted Sr-Nd isotopic signatures of all samples. For these rock types, 87Sr/86Sr(i) is 0.70352 - 0.70396, and εNd(i) is +3.8 - +4.3. Eight other rock types have a narrow range of εNd(i) (+1.9 - +3.7), but a wide range of 87Sr/86Sr(i) (0.70424 - 0.70629). These 14 samples comprise a fairly tight cluster of Pb isotopic values: 206Pb/204Pb (18.22-19.23), 207Pb/204Pb (15.54-15.62), and 208Pb/204Pb (38.38-38.94), suggesting very little crustal assimilation. They are most similar to EM-2 (sub-group of OIB). Published ages of crustal amphibolite xenoliths from the Prairie Creek lamproite are Proterozoic (~1.32- 1.47 Ga), in keeping with isotopic evidence for crustal assimilation, including Tdm = 1.3-1.7 Ga. Published ages of lamproite (~106 Ma) indicate that these magmas intruded first, whereas carbonatites and other alkalic magmas were later (~102 to ~89 Ma). Asthenospheric upwelling first melted lithospheric mantle and crust, producing lamproitic magmas; asthenospheric magmas followed as swelling of the lithosphere ensued.
V31C-2170
Geochemical and Sr-Nd Isotopic Compositions of Cenozoic Granitoids in Western Anatolia (Turkey): Spatial and Temporal Evolution of Continental Magmatism and Extension in the Aegean Province
Western Anatolia (Turkey) is part of the Aegean extensional province, which is situated in an active collision zone between the African and Eurasian Plates. The Cenozoic magmatism in western Anatolia started after the collision of the Sakarya continent (SC) and Anatolide-Tauride continental blocks (ATP) in the late Paleocene. The collisional front is today marked by the Izmir-Ankara suture zone (IASZ), which includes late Cretaceous Tethyan ophiolites, melanges, and HP/LT blueschist assemblages. Comparison of the isotope geochemistry and the petrogenesis of different plutons on both sides of the IASZ provide important geochronological and geochemical constraints on the nature of the late Cenozoic magmatism associated with crustal extension in the Aegean province. Irrespective of the lithological make-up of the collided blocks, the Oligo-Miocene granitoids that were emplaced into the SB and ATP show similar major and trace element and Sr-Nd isotopic compositions, indicating common melt sources and evolutionary trends. These granitoids are mainly metaluminous, medium to high-K calc-alkaline rocks with their silica contents ranging from 57.0 wt.% to 72.2 wt.%. They display enrichment in LILE and strong negative anomalies in Nb, Ta, P, Ti, and Zr, and they have initial 87Sr/86Sr values of 0.705186 - 0.711437 and 143Nd/ 143Nd values of 0.512615-0.512266. These isotopic signatures and trace element characteristics are considered to reflect the composition of the magmas derived from a metasomatized lithospheric mantle beneath NW Anatolia and from the overlying mafic lower crust. This inferred melt source readily explains the I-type granitoid nature of most Cenozoic plutons in western Anatolia regardless of their temporal and spatial position. The heat and the basaltic material to induce this partial melting were provided by aesthenospheric upwelling caused by partial lithospheric delamination and/or convective thinning. Slab rollback of the Hellenic subduction zone may have triggered the inferred lithospheric delamination and/or convective thinning. The widespread early to middle Cenozoic magmatism caused thermal weakening and played a significant role for the initiation of lithospheric- scale extension, and crustal exhumation and thinning in the broader Aegean extensional province.
V31C-2171
Bimodal magmatism during the Diego Hernández Formation, Tenerife, Canary Islands: genesis and eruption-triggering of phonolitic magmas during ongoing mafic volcanism
The Diego Hernández Formation (DHF) represents the explosive eruption of nearly 70 cubic km of phonolite over approximately 200 k.y. from the Las Cañadas caldera on Tenerife. Four chemostratigraphic units are distinguished on the basis of trace element contents: DHF bs (represented by the 370 ka Fortaleza and 347 ka Roque Members), DHF I (319 ka Aldea, 309 ka Fasnia, and 268 ka Poris Members), DHF II (Arafo and 223 ka Caleta Members), and DHF III (Cruz Sequence and the 196 ka Abrigo Member); all named units involve plinian and/or ignimbrite components that devastated a significant fraction of the island [1]. These chemostratigraphic units demarcate two dominant compositional trends distinct in incompatible element contents, and in Nb/Ta and REE ratios. DHF bs and DHF III plot along a high-Nb trend, and DHF I and DHF II plot along a low-Nb trend, a feature consistent with divergent fractionation histories involving titanite. Mafic magma was an important component of the DHF magmatic system and flanking mafic volcanism was ongoing during DHF time. Major phonolitic eruptions are conformably bounded by basanitic lavas and scoria deposits. Mafic magmatic components are identifiable in many of the phonolitic pyroclastic deposits as mafic, mingled and banded pumices, or as quenched mafic enclaves. Mafic components in the Abrigo, Caleta, and Poris Members are nearly geochemically identical to the underlying scoria or lava, suggesting that flanking mafic volcanism may in some cases be associated with subcaldera intrusive events that remobilize phonolitic magma to trigger major explosive eruptions. We envisage that the DHF represents a time when the intrusion of mantle-derived mafic magma in the lower crust supplied heat sufficient for the generation of intermediate tephriphonolite and phonotephrite magmas via melting of gabbroic/basaltic crust. Some of these intermediate magmas evolved to phonolite by crystal fractionation, a scenario consistent with DHF III-type phonolites. The intrusion of basaltic dikes and sills in the shallow crust lead to the genesis of DHF I phonolite magmas via whole-scale melting of preexisting altered syenitic rocks. Some of this mantle-derived mafic magma did reach the surface having undergone crystal fractionation, and in many cases there is geochemical, or more rarely petrographic, evidence that phonolitic magma contaminated aliquots of basaltic magma. [1] Edgar et al., 2007. JVGR 118, 99-130.
V31C-2172
Clinopyroxene Dissolution and Xenolith Digestion in Basalt
Mg- and Ca-rich clinopyroxene dissolution in a basaltic melt was investigated at 1509-1790 K and 0.47-1.90 GPa using a piston-cylinder apparatus. Clinopyroxene saturation was treated as being determined by the product of MgO and CaO concentrations in the melt. Based on the experimental results, CMgO×CCaO at saturation is modeled as a function of T and P. Chen and Zhang (2008) modeled olivine saturation as being determined by the MgO concentration in the melt, and the saturation MgO concentration depends on T and P. The olivine and clinopyroxene saturation curves defined by the above two models cross in an MgO vs CaO plot, dividing the composition regime into four regions. The saturation status of olivine and clinopyroxene in a given melt at given T and P can be inferred from the position of the melt in the MgO-CaO composition regime. For example, if a melt composition falls to the right of the olivine saturation curve but to the left of the clinopyroxene saturation curve, clinopyroxene would dissolve but olivine would precipitate. The model is tested against xenolith dissolution experiments by Morgan and Liang (2005), in which olivine and clinopyroxene dissolved preferentially over orthopyroxene in a basaltic andesite, while clinopyroxene dissolved preferentially over olivine and orthopyroxene in an alkali basalt. The model prediction is consistent with the experimental results within model uncertainty. When the reaction textures of mantle xenoliths and the composition of the hosting basalt is known, the thermal history can be inferred by comparing the melt composition with the saturation curves in the MgO-CaO composition regime. For example, mantle xenolith in Cenozoic basalt from Northeast China show different reaction textures with the hosting magma (Chen et al. 2007). At Jingbohu, clinopyroxene display significant extent of dissolution, while olivine show smaller extent of dissolution or even possible overgrowth. Comparing the basalt composition at Jingbohu and the saturation curves at different T and small P in the MgO-CaO composition regime suggests that the eruption temperature of the basalt at Jingbohu is above 1473 K. At Kuandian, clinopyroxene display small extent of dissolution, while olivine show less dissolution or possible overgrowth. The eruption temperature of the basalt at Kuandian is inferred to be above 1450 K. References: Morgan Z. and Liang Y. (2005) CMP 150, 369-385. Chen Y., Zhang Y., Graham D., Su S. and Deng J. (2007) Lithos 96, 108-126. Chen Y. and Zhang Y. (2008) GCA. doi:10.1016/j.gca.2008.07.014.
V31C-2173
Late-orogenic mantle garnet pyroxenites evidence mantle refertilization during exhumation of orogenic belt
The petrological and geochemical study of garnet bearing pyroxenites from four localities (FMC, Morocco, Jordan, Cameroon) demonstrates that these rocks are cumulates crystallised in the lithospheric mantle domain. Metamorphic reactions, exsolutions and trace elements WR analysis demonstrate that their crystallisation pressure ranges between 1 and 2GPa (30 to 60km). The elaboration of the PTt paths for the studied samples attests of important movements in the respective lithospheres. Replaced in the geodynamical contexts, the samples are interpreted to represent the crystallisation of melts formed during exhumation of orogenic domains. Radiogenic isotopes (Sr-Nd) show that in a very same region, the samples are isotopicaly heterogeneous but are similar to the respective regional lithosphere. Initial isotopic ratios lead to propose that the FMC samples have crystallised at the end of the Hercynian orogen and that the samples from the other localities (Morocco, Jordan and Cameroon) have crystallised at the end of the Pan-African orogen. After recalculation at the crystallisation time, the isotopic compositions are in good agreement with the respective regional lithosphere ones and so samples of this study could represent the product of the melting of these lithospheres. The analyses of oxygen stable isotopes allow to precise the model; they show that twelve of the samples come from the melting of a lherzolitic mantle and that the four others come from the melting of a heterogeneous mantle formed of lherzolites and eclogites. The presence of some hydrous minerals such as amphiboles and micas and the trace elements WR analyses show that some of the samples were affected by a late metasomatic event. Results of our study show that thermal relaxation following orogenic events lead to the crystallisation of pyroxenites in the lithosphere. The presence of lage amounts of mantle pyroxenites in old orogenic regions confers physical and chemical particularities to these domains. Among others, global solidus temperature of the whole lithospheric domain will be lowered; it follows that old orogenic regions such as FMC, Morocco, Jordan and Cameroon represent refertilized lithospheric zones in which magmatic activity will be facilitated.
V31C-2174
Mafic Inputs to the Augustine Magma System Over the Past 2,200 Years
Andesitic dome material erupted over the past 2,200 years from Augustine Volcano (Alaska) record phenocrystic, compositional, and thermal changes indicative of repeated mafic replenishments into a silicic host. Interestingly, whereas deposits from pre-historic eruptions 2,200 to 125 years ago additionally contain evidence for mafic replenishment in the form of quenched mafic enclaves, deposits emplaced during historic record ~125 years ago to the present are virtually absent of them. Textural evidence such as spherical morphologies, crenulate margins, widespread vesicles, and acicular groundmass indicate that the mafic enclaves formed as a result of rapid cooling of hot mafic magma when intruded into a cooler silicic magma host. Field measurements indicate a decline in the abundance and diameter of mafic enclaves in dome material through time. Enclaves comprise up to 15 volume % of dome material contained in 2,200 year old debris avalanche deposits located on the Southeast Point, where enclave diameters range from <1 to 36 cm. In contrast, enclaves account for only 3 to 5 volume % of dome material contained in debris avalanche deposits emplaced 1,700-125 years ago, where enclave diameters range from <1 to 12 cm. Finally, enclaves are virtually absent from dome material contained in pyroclastic flow and debris avalanche deposits emplaced during historic eruptions of 1883, 1935, 1964, 1976, 1986, and 2006, where enclaves account for less than 1 volume % of the dome material or were not observed at all and range in diameter from <1 to 5 cm. Mafic enclaves consist predominantly of plagioclase (>60 vol%), with lesser amounts of clinopyroxene, orthopyroxene, and olivine. Hornblende is a common phase in some mafic enclaves, such as those found in dome material contained in 2,200-1,100 year ago debris avalanche deposits, but is absent in dome material contained in other debris avalanche deposits, such as the 350 year old deposit of Grouse Point. In contrast, the host andesite phenocryst assemblage appears to change very little over time, where plagioclase and orthopyroxene account for the most common phases followed by clinopyroxene, pargasite, Fe-Ti oxides, and trace amounts of olivine and quartz. This project is designed to better understand how magma mixing processes once occurred in the Augustine magma system and to evaluate how this process has changed over time through a petrological, geochemical, and textural investigation of the mafic input at Augustine Island as enclave formation has almost entirely disappeared favoring a more homogeneous mixing style between the compositional end members.
V31C-2175
Experimental Constraints on Basalt Differentiation in the Deep Crust
A model for the generation of intermediate and silicic igneous rocks was developed based on existing experimental data and numerical modelling [1]. This model proposed that hydrous basalts are emplaced into the lower crust as a succession of sills generating a deep crustal hot zone. As soon as these sills are emplaced they undergo chemical differentiation as they cool and equilibrate with the surrounding rocks. It was shown experimentally [2], and by field observations [3], that arc basalts have H2O contents in the range of 2-6 wt%. This means that any melts generated from this basalt will be even more H2O-rich due to partial crystalisation of the basalt sill and partial melting of crustal host rocks. H2O - rich melts have a low viscosity and density and will very easily detach from their source and ascend rapidly. Subsequent degassing and crystallisation lead to increased viscosities at shallow depths and this may lead to the formation of a volcano-feeding magma chamber. For this model knowledge of the variation of melt fraction with temperature, pressure and H2O content in arc basalts is important. Unfortunately, few experimental studies have been completed on the chemical differentiation of magmas in the deep crust with various water contents and covering the full temperature range from solidus to liquidus. These data are also difficult to apply to the model due to inconsistencies in the oxygen fugacities at which the experiments were performed. To fill this gap in the experimental data and thus gain better understanding of the chemical evolution of such magmas, a suite of experiments were run at f 'O2 close to NNO (the nickel-nickel-oxide buffer) .The experiments were performed in end-loaded piston cylinder apparatus on a primitive mantle-derived hydrous basalt from the Lesser Antilles at 1-1.3 GPa and temperatures from 1000- 1200°C. Two series of experiments were performed with initial H2O contents of 2.4 and 4.8wt%, respectively. In order to investigate the behaviour of trace elements during crystallisation of hydrous basalts in the lower crust, the starting materials were doped at ppm levels with a wide range of trace elements and the run products analysed by ion-microprobe. [1] Annen et al., 2006, J. Petrol., 47, 505-539; [2] Sisson & Grove, 1993, Contrib. Mineral. Petrol., 113, 143-166; [3] Murphy et al., 2000, J.Petrol, 41, 21-42.
V31C-2176
The relative contribution of partial melting and crustal assimilation for upper continental crust formation: a lower crustal perspective from the Kohistan arc.
A long-standing debate circles around the relative importance of assimilation vs high-pressure fractional crystallization in the lower crust to form the upper continental crust. The lack of well exposed deep levels of magmatic arc hampers process in the discussion. A notable exception is the Kohistan arc in NW Pakistan, where the whole crustal sequence of a paleo intra oceanic arc from upper mantle to unmetamorphosed sediments is exposed. These exposures allow studying deep crustal processes. We will present field, geochronological and petrological evidence from the Kohistan arc, which elucidate the relative importance of partial melting and fractional crystallization: The Kohistan arc is dominated by the voluminous Chilas Complex one of the largest mafic intrusion complex exposed, which intruded during intra arc extension in the deeper level of the arc. Accordingly, intrusion of the Chilas Complex could have provided a favorable environment for granitoid formation by partial melting. U-Pb zircon intrusion ages from I-type granitoids sampled along a crustal transect in the vicinity of the Chilas complex are however, generally younger than the Chilas Complex. The new results indicate, in conjunction with literature data, that granitoid formation in the Kohistan arc was a continuous rather than punctuated process and is not related to the emplacement of voluminous mafic units. Contrary field relations, petrographic observations and major and trace element compositions of the lower crustal Jijal complex, suggest that hydrous, high-pressure fractional crystallization was a dominant crust- forming process in the Kohistan lower crust, even if there is minor evidence for dehydration melting. A fractionation model is presented that incorporates cumulate and partial melt compositions to explain the Kohistan granitoids. Depending on the parental magma composition, assimilation of few percent of melts derived by low degree of amphibole dehydration melting is a crucial process to explain the silica-rich continental upper crust composition. Additionally, the model shows that assimilation in the lower crust tends to reduce initial compositional differences of parental magma compositions. Therefore, we propose that assimilation in a lower crustal "hot zone" acts as an "equalizer" of magma compositions.
V31C-2177
Formation and Cross-Cumulus Migration of Silica-Rich Liquids in the Skaergaard Intrusion, East Greenland
In a recent publication on the Skaergaard intrusion evidence for the formation of silica-rich melts by silicate- silicate liquid immiscibility was proposed (Jakobsen et al., Geology 33, 2005). Coexisting iron-rich and silica- rich microscopic melt inclusions were trapped in apatite during crystallisation of the Skaergaard melts. Given this evidence for liquid immiscibility it is possible to explain the formation of macroscopic accumulations of silica rich entities throughout the magmatic stratigraphy. Previously, the formation and emplacement of these granophyric entities were challenging to explain. Examples include decimetre to metre size granophyric /melano-granophyric aggregates in either gabbroic pegmatite, in chimney shaped columns intersecting the layering or in isolated pods. Particularly, the presence of numerous granophyric pods a few metres above large gabbroic pegmatite were enigmatic. Moving the granophyric melts from the pegmatite where they formed (Larsen and Brooks, Journal of Petrology 35, 1994) and several metres across the magmatic stratigraphy would require unconsolidated cumulates i.e. a crystal mush. Geothermobarometric estimates from fluid inclusions, amphibole and feldspars show that the silica-rich aggregations solidified between 900 and 660 C at P from 1.8 to 2.9 kb. However, to be true products of liquid immiscibility they should form at T > 1050 C. With an average of 960 ppm Zr, the silica-rich aggregates are extremely Zr rich. Zr saturation thermometry imply minimum T's of 1070 (c. 2000 ppm Zr) to 900 C (c. 700 ppm Zr). Ti in Zr thermometry is progressing and may further constrain the T of formation. Although large uncertainties apply, a T of 1070 C or higher, would agree with a formation by liquid immiscibility. Assuming T > 1070 C the cumulus stratigraphy was unconsolidated with > 30 vol% intercumulus melts in the lower part of the magmachamber. With a density of 2.4-2.6 g/cm3, the silica-rich melts were much lighter than the ambient mush (c. 3.2 g/cm3) and may have migrated diapirically and/or along syn-magmatic semi-ductile fault systems (as observed in Lower Zone c and the Middle Zone). With an onset of silica-rich melt migration at T > 1070 C in the Lower Zone, large proportions of the magma chamber was molten and, at least theoretically, it was possible to reintroduce the silica-rich melts in the convecting magma. 19 chimney shaped structures of granophyric and melano-granophyric rocks in MZ with diameters of 2-5 metres, imply that the transfer of silica-rich melts was not only a trivial matter but substantially may have interacted with the cumulates they transgressed or the ambient convecting melt if they migrated this far.
V31C-2178
Petrogenesis of Mafic Volcanic Rocks from the Pribilof Islands, Alaska, by Melting of Metasomatically Enriched Depleted Lithosphere, Crystallization Differentiation, and Magma Mixing
The Pribilof Islands, Alaska, are located in the Bering Sea in a continental intraplate setting. In this study we examine the petrology and geochemistry of mafic volcanic rocks from St. Paul (0.54 to 0.003 Ma) and St. George (2.9 to 1.4 Ma) Islands, the two largest Pribilof Islands. Together these islands offer an opportunity to simultaneously investigate an active and extinct Bering Sea basaltic volcanic field in a setting where features such as lithospheric thickness and composition, distance from the Aleutian arc front, and other tectonic factors are virtually constant. Rocks from St. George can be divided into three groups. Group 1 contains high MgO, low SiO2 rocks that are primarily basanites. Group 2 contains high MgO, high SiO2 rocks that predominantly alkali basalts. Group 3 contains intermediate to low MgO rocks that include alkali basalts and trachybasalts with high modal plagioclase contents. Major and trace element compositions indicate that Groups 1 and 2 formed by partial melting (2-4%) of amphibole-bearing, garnet peridotite. Group 1 rocks were produced from the most hydrous parts of the mantle, as they show the strongest signature of amphibole in their source. Rocks from St. Paul inlcude alkali basalts and basanites with MgO contents from 4.2 to 14.4 wt% at relatively constant SiO2 contents (43.1 to 47.3 wt%). The most primitive St. Paul rocks are interpreted as mixtures between magmas with compositions similar to Groups 1 and 2 from St. George Island. These magmas subsequently fractionated olivine, clinopyroxene, and spinel to form more evolved, plagioclase-rich rocks. Plagioclase-rich Group 3 rocks from St. George can be modeled as mixtures between an evolved St. Paul end-member and a fractionated Group 2 end-member from St. George. Mantle potential temperatures estimated for primitive basanites and alkali basalts average 1370°C and are similar to those calculated for mid-ocean ridge basalts (MORB). Similarly, 87Sr/86Sr and 143Nd/144Nd values for all rocks are MORB-like, ranging from 0.70270 to 0.70304 and 0.51276 to 0.51311, respectively. 208Pb/204Pb vs 206Pb/204Pb values are also similar to MORB, although they deviate slightly towards HIMU (high time-integrated 238U/204Pb). Despite these similarities to MORB, other major and trace element characteristics are similar to ocean island basalts, such as enriched alkali and incompatible trace element contents. These characteristics are interpreted to indicate that the underlying mantle experienced an ancient melt-removal event and then was affected by metasomatism that elevated incompatible trace element contents, but was too young to produce significant time-integrated changes in radiogenic isotopic ratios. Evidence suggests, therefore, that the Pribilof Islands did not form in either a plume or a mid-ocean ridge tectonic setting. Rather, magmas erupted on the islands were produced by melting of depleted, metasomatically enriched peridotite at relatively low temperatures and were able to ascend to the surface through extensional or transtensional faults that acted as conduits.
V31C-2179
A Textural Record of Silicate Liquid Immiscibility in the Skaergaard Intrusion, East Greenland.
The extent of silicate-liquid immiscibility in differentiated basaltic systems is widely debated despite its great potential importance in controlling the liquid line of descent. While the onset of liquid immiscibility in the bulk magma is likely to occur late in the fractionation history in basaltic systems, the interstitial liquid trapped in the developing crystal mush may reach the miscibility gap earlier in the solidification history. We present previously unreported symplectite textures from the Skaergaard Intrusion. The replacement of cumulus crystal rims by reactive symplectites of olivine or orthopyroxene and plagioclase, together with growth of vermicular ortho- and clinopyroxenes, An-rich plagioclase, Fe-Ti oxides and apatite is common in lower and mid-levels of the Layered Series and very common in the Triple Group and mineralized horizons. In contrast, the Upper Zone of the Layered Series and the Marginal Border Series contain co-existing, non-reacting granophyric and ilmenite-rich symplectites filling interstitial pockets between cumulus grains. We suggest that reactive mafic symplectites grew during chemical disequilibrium caused by the separation of conjugate immiscible interstitial liquids and selective loss of the Si-rich component from the crystal mush. We anticipate that Upper Border Series contains reactive granophyric segregations due to the preferential loss of the dense Fe-rich conjugate liquid. Non-reactive ilmenite-rich intergrowths and associated granophyres formed by in-situ crystallisation of late-stage immiscible interstitial liquids. Reactive mafic symplectite formation and, by inference, the best developed interstitial liquid phase separation, coincides with the mineralized horizons of the Triple Group suggesting a genetic link between the two.
V31C-2180
Strontium Isotope Evidence for Numerous Small-Volume Inputs of Hybrid Magma During Growth of the Cuillin Layered Gabbro Complex, Isle of Skye, Scotland
The Cuillin mafic-ultramafic complex on the Isle of Skye is the exposed remnant of a large shallow crustal magma system of Paleocene age. It was fed by basaltic magmas that ascended through a thick Archean gneissic crust via a complicated magmatic plumbing system. Zone II of the Outer Layered Eucrite Series of the complex includes several hundred meters of rhythmically layered gabbro section consisting of alternating layers of medium-grained laminated gabbro and coarse-grained massive leucogabbro. The layers of massive leucogabbro formed by accumulation of Ca-rich plagioclase phenocrysts (An 85-91) during episodic replenishment of the chamber by small batches of plagioclase-phyric magma. The initial 87Sr/86Sr ratios of massive layers are about 0.0002 lower than those of adjacent laminated layers, indicating that the plagioclase-phyric magmas underwent less crustal contamination than did other magmas in this part of the complex. Both types of layers are isotopically variable, implying that all of the layers are hybrids of at least two isotopically distinct magmas. In general, however, 87Sr/86Sr increases upward through the section, indicating that the proportion of material derived from the low-87Sr/86Sr end-member decreased through time. Much of the hybridization probably occurred in external magma reservoirs prior to emplacement in the currently exposed chamber, although some mixing may have occurred within the chamber itself. Earlier studies have shown that, in the context of the entire Cuillin layered mafic-ultramafic complex, the Outer Layered Eucrite Series represents an irregular transition from earlier influxes of isotopically heterogeneous and highly contaminated magmas to later influxes of isotopically homogenous and less contaminated magmas. The section described here appears to record the intermediate stages of this transition, with the massive layers representing numerous minor pulses of the less contaminated magmas that eventually came to dominate the late-stage growth of the complex. Temporary waning of these pulses produced episodic reversals in the larger trend toward lower degrees of contamination, with one such reversal being recorded by the upward increase in 87Sr/86Sr described in this study.
V31C-2181
Chemical Evolution of Intercumulus Liquid, as Seen in Plagioclase Overgrowth Rims from the Skaergaard Intrusion
Closed-system solidification of a crystal mush will produce a cumulate rock with strongly zoned intercumulus minerals, as a result of continual thermal and compositional change in the residual liquid. The physical properties of the residual liquid (viscosity, density) also change, which may allow compositional convection or melt migration through the cumulate pile, depending on the porosity and permeability of the mush. However, for fully solidified cumulates, 'fossil' changes in liquid composition or porosity are difficult to identify. This study investigates the changing concentration of Ti in plagioclase overgrowths from the lower parts of the Skaergaard Intrusion. Ti concentrations in plagioclase reflect the changing Ti concentration of the residual liquid during solidification. The plagioclase overgrowths record Ti increasing in the liquid until local (intercumulus) saturation of Fe-Ti oxides, when Ti contents start to fall. Ti continues to fall in the residual liquid until the most evolved plagioclase compositions (An30), which form when only approximately 4 % porosity remains. Ti in clinopyroxene oikocrysts also falls rimward, but zoning in faster diffusing species (Fe, Mg) is not observed. These results are discussed in the context of the changing porosity and permeability during solidification.
V31C-2182
Relationship between Famatinian Arc Magmatism and Recent Mafic Volcanism in Northwest Argentina: Implications for Lithospheric Composition and Evolution Beneath the Puna Plateau
The tectonic and magmatic evolution of the Puna Plateau (NW Argentina) has generated much debate over the past two decades. This study focuses on the young (< 7 Ma), mafic magmatism that led to the creation of monogenetic and simple polygenetic volcanoes throughout the plateau. These volcanics provide a means to evaluate the recent petro-tectonic development of the plateau and, in combination with Ordovician intrusive rocks, determine the isotopic composition and long term evolution of the sub-continental lithospheric mantle (SCLM) beneath the Andean back-arc domain. Here we present new whole rock major and trace element data and isotopic values for volcanic samples collected from the Antofagasta and Pasto Ventura basins in the southern Puna Plateau. Major element chemistry shows most of our samples are basalt, trachybasalt, basaltic andesite and basaltic trachyandesites, some with < 50.0 wt% SiO2 and > 8.0 wt% MgO, which is indicative of a strong mantle component. The more primitive lavas likely have a sub-crustal origin and experienced minimal interaction with overlying crust during transport to the surface. Two of our samples with low wt% MgO, a silicic andesite and a dacite, indicate an extensive crustal component and possibly a lower crust origin for evolved magmas. All samples have light trace element enrichment compared to NMORB and elevated abundances of LIL and LRE elements compared to HFS and HRE elements, indicating the magmas originated from a metasomatized source region. The samples also have variable (low and high) Nb, Ta and Ti negative anomalies, which are interpreted to be a signature of the source region. Our samples do not have a lithospheric delamination (~OIB) trace element signature as proposed by previous workers in support of a delamination model. Additionally, the samples have isotopic values (e.g. 87Sr/86Sr >0.7055 and εNd <0) that are not comparable to depleted asthenosphere. It is impossible for asthenospheric magma to obtain these isotopic values through crustal assimilation or AFC processes while maintaining a basalt major element composition and high Ni and Cr concentrations. Therefore, we propose the mafic magmas are sourced from a SCLM that, in accord with the LIL and LRE element concentrations, has been metasomatized during dehydration and possibly melting of a subducting oceanic plate. The young volcanics have isotopic values nearly identical to those of Early Ordovician Famatinian gabbros and norites. We suggest the most primitive Puna volcanic and Famatinian samples originated from the same SCLM source region. This implies at least a thin portion of the SCLM has remained intact beneath NW Argentina for the last ~485 million years. Resultantly, the SCLM was likely thinned to its present thickness sometime between the Early Ordovician and the Late Miocene. Thinning may have occurred by long term mantle wedge processes. Steady shortening and thickening of the continental crust and gradual removal of the SCLM by convection is envisioned here. The occurrence of discrete, intermittent delamination events is not favored because removal and then regeneration of the SCLM would not have allowed for preservation of the Famatinian isotopic signature.