T24A-01 16:00h
Subduction and Exhumation Processes Associated with UHP-HP Metamorphism in pre-, syn- and post-collisional Settings: Examples from Oman, Himalaya and Hindu Kush
High-pressure and Ultrahigh pressure eclogites occur along continental plate margins associated with ophiolite obduction (Oman), arc-continent collision (Kaghan and Tso Morari, Ladakh Himalaya) and along continent-continent collision zones (western Alps, Western gneiss region Norway, Dabie Shan-Sulu massif, China, Kokchetav massif, Kazakhstan). Both the Oman and western Himalayan eclogites are metamorphosed Permian basalts that were originally part of the thinned passive margin of the Arabian and Indian plates. The thinned margin was stretched and sheared during rapid subduction to ca 20 kbar (during ophiolite emplacement) and 27 kbar (during Kohistan arc obduction) respectively. Old oceanic crust originally attached to the thinned continental crust was eclogitised, and provided the dense anchor to pull the slab down. When this anchor broke off, buoyancy forces rapidly forced the thin UHP-HP slab back to the surface by return channel flow. Fabrics within and above the HP rocks are extensional and related to rapid exhumation of footwall eclogites back up the same subduction zone. U-Pb dating of zircon, allanite, titanite and rutile from Oman and Pakistan shows that exhumation rates were approximately 1 cm/yr. The Hindu Kush seismic zone along the NW frontier of the Himalaya provides a modern-day analogue for deep subduction of thinned continental crust in a continental collision setting.
T24A-02 16:20h
Early, steep subduction of India beneath Asia required by timing of UHP metamorphism
Ultrahigh-pressure (UHP) rocks in the NW Himalaya are the youngest on earth, and allow testing of critical questions of UHP formation and exhumation and the India-Asia collision. The Tso Morari Complex (TMC) is a UHP subduction zone complex in eastern Ladakh in the western Himalaya, south of the Indus-Tsangpo suture zone. New U-Pb SHRIMP data from zircon shows the TMC has a Proterozoic protolith with a Pan-African history, and defines three stages in the Eocene history of the TMC, hence constraining the timing of collision, subduction, and exhumation in the western Himalaya. Our precise new single zircon U-Pb ages indicate that UHP eclogite-facies metamorphism occurred at 53.3$\pm$0.7 Ma followed by a HP eclogite-facies event at 50.0$\pm$0.6 Ma, and amphibolite-facies metamorphism at 47.5$\pm$0.5 Ma. Retrograde zircon growth may result from dehydration reaction-related fluids evolved during exhumation; the intermittent presence of these fluids would allow zircon crystallization during exhumation that would document the changing P-T conditions. Today India subducts beneath Tibet at an angle of only c. $10\deg$; at this low angle, rocks must travel over 500 km along the subduction thrust to reach the depth of c. 100 km required for UHP metamorphism. Initial collision of India with Asia, based on stratigraphic and paleomagnetic data, is widely cited as being at 55$\pm$1 Ma. Here we show that Indian rocks reached UHP depths at 53.3$\pm$0.7 Ma. Given the fast India-Asia convergence rate of 69 km/m.y., our result implies an earlier onset of collision (c. 57 Ma) and that UHP rocks must have traveled a short, hence steep path into the mantle. We infer that early continental subduction was at a steep angle, probably vertical, comparable to modern continental subduction in the Hindu Kush.
T24A-03 16:35h
Geology of the North Qaidam HP-UHP Terrane, Western China
The Early Paleozoic North Qaidam ultrahigh-pressure (UHP) metamorphic belt is a recently discovered UHP terrane, located along the southwestern boundary of the Qilian Shan orogenic belt, Western China. It preserves Precambrian to Mesozoic rocks with continuous exposure over tens of kilometers. Pre-Mesozoic rocks can be divided into two units: epidote-amphibolite grade late Proterozoic rocks (Dakendaben gneiss) which include heterogeneous biotite gneiss, amphibolite, and quartzo-feldspathic gneiss containing eclogite and minor garnet peridotite. The second major unit is a suite of early Cambrian (545.5$\pm$6.0 Ma) epidote-amphibolite facies ophiolitic rocks: vari-textured gabbro, plagiogranite, mafic dikes, vesicular basalt, and metasediments (marble, chert and flysch). The contact between the ophiolite and UHP gneiss is folded and strongly transposed by later ductile folding, making it approximately parallel to the pervasive regional foliation (N10-45$\deg$W), with top to N and top-NE sense kinematic indicators which become subhorizonital when the effects of Cenozoic thrusting are removed. Serpentinized ultramafic bodies commonly occur close to this contact. Middle Paleozoic granites (510-400 Ma) intrude the belt placing a lower bound on the cessation of the ductile phase of deformation. Regional muscovite cooling ages of 460-360 Ma indicate that the metamorphic complex reached the middle crust in the Ordovician and unmetamorphosed near-source Devonian and Carboniferous sediments lie unconformably on the ophiolite indicating that the metamorphic complex was at the surface by 360 Ma. Early Cenozoic imbricate thrust faults placed UHP gneiss over unlithified Tertiary sediments of the Qaidam Basin and generated broad regional folding and repetition of UHP units. Preliminary data indicate regional variations in age and peak metamorphic pressure within the eclogite-bearing gneisses. U/Pb dating of metamorphic zircon from individual eclogite blocks yields statistically different ages of UHP metamorphism, ranging from 425$\pm$7 (MSWD = 0.26) to 523$\pm$8 Ma (MSWD = 3.9). Preliminary regional thermobarometry also indicates spatial variation in peak pressure ranging from 25 to 20 kbar. No regional variation in peak T has been identified. A random spatial distribution of peak pressure and metamorphic zircon age implies mixing of blocks within the eclogite-bearing gneiss during the early stages of exhumation. Whereas, a systematic spatial variation in pressure and time imply a coherent unit which preservation of geometric relationships developed during UHP metamorphism. Preliminary data in the North Qaidam support the former and predicts regionally variable pressure-temperature-time (P-T-t) paths.
T24A-04 16:50h
Age of eclogite facies metamorphism of metabasic boudins within schist and gneiss of the Monte Rosa nappe, Val di Gressoney, Western Alps, Italy
High precision 238U-206Pb geochronology of rutile from quartz-carbonate-rutile tension veins in eclogite boudins indicate that eclogite-facies metamorphism may have lasted at least from 52 to 46 Ma. The eclogite boudins occur within quartzo-feldspathic schist and gneiss of the Monte Rosa nappe near the Indren glacier, Val di Gressoney, Western Alps, Italy. The association of a few eclogite boudins with serpentenite and gabbro suggests that some of the boudins may be tectonic slices of the Zermatt-Saas ophiolite, which lies structurally above the Monte Rosa nappe. The sampled tension veins, which typically consist of quartz, carbonate, and rutile, are usually oriented at high angles to the long axis of the boudins and are likely synchronous with boudinage of more coherent tracts of metabasite. The sampled tension veins cross-cut eclogite facies minerals but there is little or no associated retrogression of the eclogite-facies minerals adjacent to the veins, suggesting that vein formation occurred under eclogite-facies conditions. The age data suggests that rutile growth was episodic, perhaps associated with fluid pulses, and likely represents a possible age range of eclogite facies conditions recorded in these boudins. The eclogite boudins are locally isoclinally folded, possibly under lower eclogite to greenschist facies conditions based on the mineral assemblages in the fold hinges. This indicates that the eclogites, if they are exotic to the Monte Rosa nappe, were juxtaposed with schist and gneiss at least prior to this folding event. Regardless of the exotic or in-situ origin of the eclogites, if the boudinage of eclogites occurred within the Monte Rosa nappe, the age of high pressure metamorphism in the Monte Rosa may also be between 46 and 52 Ma. The geodynamic implications are that the Monte Rosa nappe experienced high pressure conditions synchronously with the Zermatt-Saas ophiolite and that they were in contact during exhumation (e.g. Reddy et al., 1999), raising the possibility that buoyancy-driven processes in part aided in the exhumation of the Zermatt-Saas ophiolite complex.
T24A-05 INVITED 17:05h
Microstructures Developed During Natural and Experimental Decompression of Peridotite From Pressures of 10-15 GPa
Evidence is now robust that continental rocks and sediments can be subducted to P $>$ 6 GPa during continental collision and returned to the surface. Moreover, mantle rocks exhumed with this subducted material carry evidence of P $>$ 9 GPa and perhaps much more. We present a short review of natural examples and discuss them in the context of decompression experiments conducted on garnet lherzolite over the range 14 $\rightarrow$ 5 GPa. Experiments at 14 GPa dissolved all enstatite (En) and about 85% diopside (Di) into garnet, yielding run products of 40% Ol + 55% Grt + 5% Di. Re-annealing this product at 13 or 12 GPa resulted in exsolution of Di as blebs at garnet grain boundaries and oriented platelets of Ol chemistry within grt. Specimens first equilibrated at 8 GPa dissolved abundant En but little Di. When re-annealed at 5 GPa, En exsolved as blebs at garnet boundaries -- very similar to interstitial blebs of enstatite along grt grain boundaries in UHP ($>$200 km) Norwegian grt-harzburgite. In the latter rocks, abundant En and rare Di exsolution lamellae are also found in the cores of large garnets. Our experiments do not show such lamellae, supporting the arguments of van Roermund and Drury (1998) that they are produced only in the cores of large grains and that the interstitial pyroxenes found in their specimens are also exsolution products. Ol has not been reported with exsolution morphology in natural UHP products, nor did we observe it in our experiments at P = 5 GPa. On the other hand, our observation that Ol may be exsolved during decompression of majoritic garnet during decompression at higher P is consistent with expansion of the garnet field at the expense of wadsleyite at P $>$ 13 GPa reported by Ringwood (1991). Di, En, and/or Ol do occur along grain boundaries within larger polycrystalline garnets and within embayments at the margins of smaller amoeboid garnets in subduction zone garnet peridotites. Such garnets also may contain rounded, non-oriented, inclusions of each of these minerals, or all three together, consistent with the results of majoritic garnet decompression presented above. Our results suggest that some Ol in this microstructure may have exsolved during decompression of majoritic garnet.
T24A-06 17:25h
Fate Of Continental Crust Subducted to the Mantle Transition Zone: Experimental Investigations
Although some debate about the depth to which continental crust may be subducted continues among petrologists, geophysical and experimental modeling suggests that continental crust constituents may be carried to great depth into the Earth's upper mantle. Ultra-high pressure metamorphic rocks of granitoid bulk chemistry were only recently recognized due to the discovery of coesite, diamond, garnet and titanite containing six-fold coordinated Si and TiO$_{2}$ with the $\alpha$-PbO$_{2}$ structure. Despite considerable scientific conservatism in accepting inferred subduction depths for continental materials, a wealth of new finds over the last decade have confirmed the subduction depths inferred from earlier discoveries of coesite and diamond in those terranes. These observations have pushed the accepted `limit' of continental metamorphism ever deeper and have given rise to new questions about the exhumation of UHP granitoid rocks. We have designed a new experimental program to explore the following questions: What fraction of the continental crust may survive transportation from shallow depth to great depth within the mantle? What is the proportion of solid versus melt? What high-pressure phases are stable in certain lithologies at different depths? We report here our preliminary results on experimental studies of mineralogical assemblies of granitoid rocks subjected to recrystallization at P=10 to 12 GPa and T=1173 to 1474K in a Walker-style multianvil apparatus. The run products consist of K-(Na)-hollandite, stishovite, jadeite-rich clinopyroxene and poikilitic intergrowths of majoritic garnet (Si=3.11 c.p.f.u.) with phengite. Tiny crystals of an Al-silicate phase were also observed. The stoichiometry of the Al-silicate phase greatly differs from all currently known high-pressure Al-silicates such as kyanite, topaz-OH, but it is close to the hydrous aluminosilicate phase, AlSiO$_{3}$(OH), where ratio of Al to Si is 1:1. Although we have not yet verified the structure of this mineral due to its small crystal size (0.5-1 $\mu$m), we assume that this is a phase similar to that observed by Ono (1998) in his 15 GPa `wet' experiments on a similar bulk chemistry. Presence of `garnet-phengite' poikilitic domains, an unusual texture for such high-pressure experiments, may reflect local partial melting initiated by an excess of H$_{2}$O-fluid migrating at the grain boundaries due to the breakdown of biotite. Our results show that `continental crust' mineralogical systems need to be explored in detail by experimental determination of phase relations, phase equilibrium, reproduction of mineralogical diversity, calculations of the density of newly synthesized aggregates and evaluation of the proportion between melt and solids.
T24A-07 17:40h
Experimental Constraints on Pelite Melting in Subduction Zones: a New Approach Using HP Metapelites
During subduction, sea-floor pelites melt and enrich the mantle wedge with many of the elements that give arc magmas their unique chemistry. This melt allows us to constrain subduction thermal models; e.g., the plate must be hotter than the experimentally determined pelite solidus. Previous melting experiments used marine red clays, but we know from high pressure and ultra-high pressure terrains that prograde metamorphism changes the composition of sediments before they reach sub-arc depths. A prime example of this alteration comes from the G?setjorn unit in the Norwegian Caledonides. Here, 400-410 million years ago, ocean sediments were subducted beyond 50 km and heated to 600$\deg$C. These minimally retrograded metapelites have a higher aluminum to silica ratio than any modern sea-floor sediment and they are depleted in potassium, sodium, magnesium, and calcium. Thus, the G?setjorn metapelite is a more appropriate starting material than marine clay for deep melting experiments. We performed high pressure melting experiments on a sample from the G?setjorn unit and we found a 20 kb fluid-saturated solidus between 625$\deg$C and 650$\deg$C. This is the lowest pelite solidus yet obtained at these depths.
T24A-08 17:55h
In Search of the Mixed Derivative \partial$^{2}$M/\partial${P}\partial${T} (M = G, K): Joint Analysis of Ultrasonic Data for Polycrystalline Pyrope From gas- and Solid-Medium Apparatus
Robust extrapolation of laboratory-derived elasticity data to the conditions of the deep mantle requires knowledge/assumptions about \partial$^{2}$M/\partial${P}\partial${T}- so far the subject of very few experimental studies. Accordingly, we have begun to explore the circumstances under which constraints on these mixed derivatives might be extracted from joint analysis of data obtained by high-temperature ultrasonic interferometry on fine-grained polycrystals in gas- and solid-medium high-pressure apparatus. In a first case study, elastic wave velocities have been newly measured on dense isotropic polycrystalline Mg$_{3}$Al$_{2}$Si$_{3}$O$_{12}$ to 1000 K and 300 MPa in an internally heated gas-medium high-pressure apparatus for comparison with the results of a previous study in multi-anvil apparatus {\it (Gwanmesia et al., submitted)}. For this study dense fine-grained polycrystals were prepared by hot pressing within a 2000-ton uniaxial split-sphere apparatus at 10 GPa and 1275 K for 1 hour, from a homogeneous glass starting material. The physical properties of the recovered specimens have been characterized with density measurements, x-ray diffraction and scanning electron microscopy (SEM). The measured elastic compressional and shear wave velocities determined are in excellent agreement with the temperature-dependent Hashin-Shrikman averages calculated from single-crystal elasticity data. Progress in the analysis of the combined gas- and solid-medium M (P, T) dataset for (\partial${M}/\partial${P})$_{T}, (\partial${M}/\partial${T})$_{P} ${and} \partial$^{2}$M/\partial${P}\partial${T} will be reported.