T21A-0500 0800h
Petrology of UHP calcite marble from the Kokchetav Massif
In the Kumdy-kol area, Kokchetav Massif, northern Kazakhstan, three types of UHP marbles have been described: diamond-bearing dolomite marble, Ti-clinohumite-bearing dolomitic marble (Ogasawara et al., 2000) and titanite-bearing calcite marble (Ogasawara et al., 2002). UHP calcite marble is distinguished from other types of UHP marbles by pure calcite (after aragonite) as a dominant carbonate phase. This calcite marble has unique evidence of UHP metamorphism; titanite with coesite exsolution and its precursor compositions indicated that the peak P-T conditions was $>$ 6 GPa and 980-1250 C (Ogasawara et al., 2000; 2002). This rock shows typical granoblastic texture consisting of calcite, diopside, K-feldspar, titanite and symplectite (diopside + zoisite) after garnet. The peak assemblage was aragonite + diopside + K-feldspar + garnet + titanite. Based on the phase relations in the system CaO-MgO-TiO2-SiO2-CO2-H2O, aragonite + diopside + rutile tie-triangle is stable under UHP conditions and divides the compositional space into dolomite-bearing or dolomite-free tetrahedrons (Kikuchi et al., 2003). The presence of titanite in calcite marble means that P-T condition was located at the right-hand side of the reaction rutile + aragonite + coesite = titanite + CO2. Previously described titanite-bearing calcite marble is diamond-free (A-type) and is characterized by titanite with coesite exsolution (Ogasawara et al., 2002). Recently, we found a small amount of diamond in calcite marble (B-type) that is characterized by microdiamond in diopside, and by the lack of K-feldspar and low amount of titanite. No diamond occurs in titanite. Rutile, aragonite and calcite inclusions in titanite were found in titanite of B-type calcite marble. These three inclusion phases in titanite that were confirmed by laser Raman spectroscopy are the evidence for titanite formation reaction described above. This titanite forming reaction occurs at extremely low XCO2 conditions as 0.02. In B-type calcite marble, microdiamond occurs locally and its amount is low; only 61 grains were found in two thin sections. Distributions of titanite, K-feldspar and diamond are heterogeneous and seem to form layers. Diamond occurs in the domain where amounts of titanite and K-feldspar are relatively low. Low amount of microdiamond may be related with extremely low-XCO2 condition under UHP metamorphism. References Ishida et al. (2003: Journal of Metamorphic Geology, Vol. 21, p. 515-522. Kikuchi et al. (2003): EOS Transactions AGU, Vol. 84, F1532. Ogasawara et al. (2000): The Island Arc, Vol. 9, p 400-416. Ogasawara et al. (2002): American Mineralogist, Vol. 87, p. 454-461.
T21A-0501 0800h
Hydroxyl in diopside in diamond-free UHP dolomitic marble from the Kokchetav Massif
The amount of hydroxyl incorporated in diopside in diamond-free UHP marble from the Kokchetav, northern Kazakhstan was examined with micro-Fourier transform IR spectroscopy. The examined diamond-free UHP marble has the assemblage under UHP conditions dolomite + aragonite + diopside + garnet + Ti-clinohumite (+ forsterite) and is categorized as diamond-free dolomitic marble previously described (Ogasawara et al., 2000). This marble was subjected to the physical condition of $>$ 6 GPa and $>$ 1000 C, and extremely low XCO2 condition ($<$ 0.01). Diamond could be unstable in such extremely low XCO2 condition (Ogasawara et al., 2000; 2004). Diopside in dolomitic marble contains no lamella and has no evidence of retrograde alteration. Diopsides in other types of UHP marbles from the same region have K2O-bearing lamellae such as K-feldspar and phengite. Two stage exsolution was confirmed in diopside in calcite marble (K-feldspar at first stage and phengite at second one), implying that OH could survive at lower P than K2O component (Ogasawara et al., 2002). Therefore, diopside in dolomitic marble has been expected to contain significant amount of hydroxyl. Doubly polished thin section of sample no. Y676 with thickness of 150 micrometers was prepared for FTIR analysis. The polished section was kept in acetone and put in a desiccator. Thirty diopside grains in the same thin sections were analyzed. All diopsides exhibit only one major hydroxyl absorption band at 3645 cm-1. Johnson et al. (2002) reported similar absorption band in diopside from marble xenolith from the Cascade Slide. Intensity of hydroxyl bands of the present samples varied because of the random orientation of diopside grains. Amphibole (probably tremolite) band appeared at 3685 cm-1 in almost all diopsides. No amphibole was recognized in diopside under microscope; the intensity of amphibole bands was heterogeneous even in the same diopside grain. This may be caused by submicroscopic amphibole lamellae as described by Skogby (1990). In order to estimate the amount of hydroxyl in diopside, we chose areas in 30 diopside grains where amphibole band did not appear or was weak. Non-polarized spectra of 30 analyzed spots were averaged. Applying the procedures described by Katayama & Nakashima (2003), we obtained that diopside contained 1020 ppm H2O by weight. The source of OH in submicroscopic amphibole could be derived from precursor diopside, not a fluid infiltration origin during retrogression. Microscopic observations and isotopic studies on diamond-free dolomitic marble have shown no evidence for retrograde fluid effect (Ogasawara et al., 2000; Ohta et al., 2003). Therefore, the precursor diopside might contain much higher hydroxyl than 1000 ppm. Diopside in diamond-free dolomitic marble from the Kokchetav could contain at least 1000 ppm OH under UHP conditions. Large part of hydroxyl survived in diopside during the journey to the surface. These results are new evidence for UHP metamorphism in diamond-free carbonate rocks.
T21A-0502 0800h
Laser Raman study on microdiamond in UHP marbles from the Kokchetav Massif
Laser Raman analyses were conducted on the microdiamonds in dolomite marble, calcite marble and garnet-biotite gneiss from Kumdy-kol, Kokchetav Massif, northern Kazakhstan. Raman spectra were obtained by a Jobin Yvon LabRam300 Raman Micro-Spectrometer equipped with confocal optics and air cooled CCD detector at Waseda University. The laser beam was 514.4 nm Ar laser at 10 mW and was focused at 1 micrometer diameter. Acquisition time was 5s and accumulation number was 5 times. Peak positions and full width at half maximum (FWHM) of a Raman band were determined by the Gaussian/Lorentzian fitting of spectra. The number of analyzed spots are 98 spots of 27 grains of S-type in dolomite marble (core: 42 spots, rim: 46 spots), 27 spots of 9 grains in calcite marble, and 15 spots of 13 grains in garnet-biotite gneiss. The peak positions of Raman band were at about 1332 cm-1, and significant differences of the peak positions did not detected. Significant differences in FWHM of Raman band were confirmed in each occurrence as follows: 1.S-type core in dolomite marble: 4.26 to 5.73 cm-1 (average: 4.69 cm-1) 2.S-type rim in dolomite marble: 3.59 to 5.01 cm-1(average: 4.17 cm-1) 3.Calcite marble: 2.99 to 3.64 cm-1 (average: 3.28 cm-1) 4.Garnet-biotite gneiss: 4.69 to 5.61 cm-1 (average: 5.17 cm-1) On S-type microdiamond, the rim has smaller FWHM than the core, and this is consistent with those by Ishida et al. (2003). Microdiamond in calcite marble has the smallest FWHM among all occurrences although other features are similar to R-type and the core of S-type. Microdiamond in garnet-biotite gneiss has relatively large FWHM. The cause of such differences in FWHM is still unclear; however, we may use FWHM of Raman band to characterize microdiamond in UHP metamorphic rocks. We also conducted mappings of Raman bands and related features covering area of several tens square micrometers for diamond-bearing composite inclusions in garnet. This technique is highly effective to understand FWHM distribution in diamond aggregates, phase identification and distribution in micrometer scale in composite inclusions. References Ishida et al. (2003): Journal of Metamorphic Geology, Vol. 21, p. 515-522.
T21A-0503 0800h
Occurrence of microdiamond in UHP calcite marble from the Kokchetav Massif
Small amount of microdiamond was discovered in UHP calcite marble from Kumdy-kol, the Kokchetav Massif northern Kazakhstan. UHP calcite marble is characterized by 1) pure calcite (after aragonite) is only one carbonate phase, 2) presence of titanite, K-feldspar and diopside with phengite and K-feldspar lamella, 3) coesite exsolution from supersilicic titanite. This marble contains UHP evidence as coesite of exsolution origin in titanite and lamella texture in diopside. Minimum P condition was constrained as 6 GPa by precursor composition of titanite. So far, we have described this calcite marble as diamond-free UHP carbonate rock (Ogasawara et al., 2002); however, our recent observation found microdiamond in some domains (B-type) of this rock. Microdiamond occurs only in diopside and its amount is very low compared with diamond-bearing dolomite marble. We found 61 microdiamond grains in two thin sections (c.f. in dolomite marble, highest concentration domain contains over 4000 grains in one thin section). Microdiamond grains are heterogeneously distributed in some layers. The domain (A-type) containing relatively large amount of titanite does not contain diamond. The microdiamond in calcite marble shows 3 to 20 micrometers in diameter, rounded or cubic form, pale yellowish color and translucent character. Laser Raman spectroscopy indicated that FWHM of a Raman band at about 1332 cm-1 ranges from 2.99 to 3.64 cm-1 (average: 3.28 cm-1) and this average value is the smallest among core of S-type, rim of S-type, R-type and diamond in gneiss. Based on the morphology and other features except for FWHM of Raman band, the microdiamond in calcite marble is similar to R-type in dolomite marble (Ishida et al., 2003). No S-type diamond (_gStar_h-shaped form with core and rim) occurs in this calcite marble, and this indicates that the second stage growth of microdiamond probably from multicomponent aqueous fluid had not occurred in calcite marble. Low amount of microdiamond occurs only in diopside, no diamond occurs in titanite (stable at extremely low-XCO2 conditions), and lack of the second stage growth of microdiamond in calcite marble; all these features may be related with extremely low-XCO2 condition under UHP metamorphism. One of the possible explanations for this diamond occurrence is that microdiamonds in calcite marble are relic crystal formed before the second stage growth (the rim stage of S-type microdiamond in dolomite marble). Therefore, it is very important to confirm whether these microdiamond in B-type calcite marble had been growing or dissolving under such extremely low-XCO2 conditions. References Ishida et al. (2003): Journal of Metamorphic Geology, Vol. 21, p. 515-522. Ogasawara et al. (2000): The Island Arc, Vol. 9, p. 400-416. Ogasawara et al. (2002): American Mineralogist, Vol. 87, p. 454-461.
T21A-0504 0800h
UHP Ilmenite Exsolution from Iron-bearing Rutile in Eclogite from the Altyn Tagh, NW China
Rutile transforms to TiO$_{2}$ (II) with the $\alpha$-PbO$_{2}$ structure with increasing pressure. The latter has been found recently in UHP metamorphic rocks as well as in shocked gneisses, suggesting a peak metamorphic pressure in excess of 7-8 GPa. TiO$_{2}$ (II) has been determined experimentally to have a much higher solubility of Fe$_{2}$O$_{3}$ than does rutile. Unlike rutile, that commonly incorporates thousands of ppm hydrogen in its structure, TiO$_{2}$ (II) has essentially no solubility of hydrogen. We report here preliminary study of ilmenite exsolution from rutile in eclogite from the Altyn Tagh, China. The Altyn Tagh has been recognized recently as a new UHP metamorphic terrane of China. Rutile is the most abundant accessory mineral in eclogite. Exsolved ilmenite platelets, 0.5-5$\mu$m thick and 10-100$\mu$m long, and parallel to (010) and (100) of the host rutile, are abundant. The area proportions of the exsolved ilmenite vary from 7.6 to 22.2% with an average of 14%. The reconstructed precursor rutile has a Ti content of 0.93 and a Fe content of 0.07 per formula unit, corresponding to an average Fe$_{2}$O$_{3}$ content of 5.3 wt %. This Fe$_{2}$O$_{3}$ content is significantly higher than those reported from other natural eclogitic rutiles and from experimental rutiles at high pressure. In addition, infrared spectra of these rutiles show that they have a single OH absorption band around 3290 cm$^{-1}$. The average hydroxyl contents in rutile calculated by using three different published coefficients for rutile are 0.028 wt% (Johnson et al., 1973), 0.128 wt% (Hammer and Beran, 1991) and 0.011 wt% (Maldener et al., 2001) respectively. The high calculated Fe$_{2}$O$_{3}$content and low present hydrogen content suggest the earlier presence of TiO$_{2}$ (II) at a pressure greater than 6-7 GPa that, upon transformation to rutile during exhumation, exsolved the observed large amounts of ilmenite. The very large amount of ilmenite observed here suggests that the Fe$_{2}$O$_{3}$ content within the rutile precursor could potentially be used as a barometer for very high pressure metamorphism.
T21A-0505 0800h
A Early Paleozoic HP/UHP metamorphic belt truncated by the Altyn Tagh fault in NW China-GPetrological and geochronological constaints
The Altyn Tagh and North Qaidam Mountains at the northern margin of Qinghai-Tibet plateau are separated by the Altyn Tagh sinistral strike-slip fault, which is one of the largest strike-slip fault systems in the world and was considered as the key element in the escape tectonics model for Euraisa-India continent-continent collision. In recent years, the eclogites, HP granulite and garnet peridotites were recognized in the Altyn Tagh and the North Qaidam Monutains (NQD), respectively. They occur as lens or boundins within quratzofeldspathic gneisses or pelitic gneisses characterized by amphibolite-facies parageneses. Along NQD, eclogites occur in the the Yuka, Xitieshan and Dulan area, respectively. Garnet peridotite lenses occur within quartzofeldspathic gneisses in the Luliangshan between Xitieshan and Yuka. Some UHP metamorphic evidences, such as quartz pseudomorphs after coesite from eclogites, coesite inclusions in zircons from the host paragneisses as well as unusual exsolution textures in ompacites, have been recognized. Zircon U-Pb and Sm-Nd isochron ages from the eclogites gave ages of 450-500Ma for eclogite-facies metamorphism. These eclogites and associated rocks constitute an early Paleozoic HP/UHP metamorphic belt which extends from the Dulan in the east to the Altyn Tagh fault to the west along the NQD. To the northwest of the Altyn Tagh fault, eclogites and HP granulite-garnet peridotite associations were discovered respectively in the Jianggelesayi and Bashiwake area. Polycrystalline quartz inclusions within eclogite garnets, P-T estimates (P = 2.8-3.0 GPa, T = 780-820 ,aC) and unusual exsolution textures suggest UHP conditions of eclogite formation. Petrological data gave peak metamorphic conditions of P = 1.9-2.7 GPa, T = 870-1050 ,aC for HP granulite-garnet peridotite associations. Zircon U-Pb and Sm-Nd isochron analyses from the eclogites, HP granulites and garnet peridotites indicate HP/UHP ages of around 500 Ma The similarity of age, occurrence, and P-T conditions of the NQD and Altyn Tagh HP/UHP rocks suggests that the Altyn Tagh is the northwestward extension of the NQD metamorphic belt, formed by early Paleozoic subduction and collision of the Qaidam and Qilian blocks, and subsequently offset by left-lateral slip on the Altyn Tagh fault. This situation might be similar to the Dabie-Sulu HP-UHP metamorphic zone in eastern China, which was split into Dabie and Sulu regions by the sinistral Tanlu strike-slip fault.
T21A-0506 0800h
Geochronology and Mineral Inclusions of Eclogite, Ortho-, and Paragneiss Zircon, North Qaidam UHP Terrane, Northwest China
Amphibolite-facies felsic gneisses of the North Qaidam Mountains enclose minor ($<$10 vol%) eclogite, peridotite and pyroxenite ($\pm$garnet) which record ultra-high pressure (UHP) metamorphism. Field relations, and the presence of coesite inclusions in zircons from paragneiss suggest felsic, mafic, and ultramafic rocks all experienced UHP metamorphism and a common amphibolite-facies retrogression. Cathodoluminescence (CL) and SHRIMP-RG U/Pb and REE analyses of zircons from two granitic orthogneisses reveal oscillatory zoning, moderate to high Th/U, U, and REE concentrations, and large negative Eu anomalies, consistent with magmatic crystallization at ca.~936 Ma (all ages are $^{238}$U/$^{206}$Pb, $^{207}$Pb corrected for common Pb, except as noted). Inclusions identified by EMP include biotite, muscovite, plagioclase, and SiO$_2$. Zircon cores from two paragneisses contain high Th/U, moderate to high U and REE concentrations, prominent Eu anomalies, and yield discordant Early Proterozoic $^{207}$Pb/$^{206}$Pb ages, and are probably of detrital origin, but zircon rims contain garnet, rutile, phengite, and SiO$_2$ inclusions, very low Th/U, high U, low REE abundances, and Eu anomalies are small or absent, suggesting eclogite-facies growth. Weighted mean ages of the rims are 418$\pm$3 Ma and 411$\pm$3 Ma. Two fresh eclogites contain zircon with inclusion-rich CL dark cores, and rounded, inclusion-poor, CL grey overgrowths. Analyses of cores (partial overlap on rims) yield moderate to high Th/U and U, and ages up to 475 Ma, which places a minimum age on eclogite protolith crystallization; albite and Th + REE-rich epidote inclusions suggest greenschist- or epidote-amphibolite-facies growth. The rims contain garnet, omphacite, rutile, and phengite, yield low Th/U, moderate to high U, and weighted mean ages of 449$\pm$3 Ma and 440$\pm$4 Ma, reflecting eclogite-facies growth. Two retrogressed eclogites contain inclusion-poor zircon with CL bright, mottled cores ($\pm$faint oscillatory zoning; garnet, omphacite, and rutile inclusions) surrounded by irregular, dark rims characterized by low Th/U, and low to very low U, which yield weighted mean ages of 421$\pm$5 Ma and 415$\pm$12 Ma. Amphibole and plagioclase inclusions in the rims suggest the measured ages reflect retrogression. The spread in ages (449--411 Ma) is probably too large to be explained by a single metamorphic event, and suggests that mafic enclaves record polymetamorphic/tectonic histories prior to their incorporation in the surrounding gneisses. Juxtaposition of the different rock types probably occurred during exhumation from UHP conditions.
T21A-0507 0800h
Clinoenstatite Exsolution in Diopside of Dabieshan, China - Evidence of Origin $>$ 250km
Exsolution lamella of clinoenstatite (Clen) in diopside (Di) were found for the first time by Bozhilov et al. (1999) in the Alpe Arami peridotite, Switzerland. According to its particular micro-characteristics, they deduced that high-pressure clinoenstatite (HPclen) was the originally exsolving phase, which can exist stably only at P-T conditions of more than 250 km in the upper mantle. Here we report similar exsolution lamellae of Clen in Di from garnet peridotite of the Bixiling massif of the Dabie mountains, China. Transmission electron microscopy (TEM) shows oriented exsolution lamellae, 0.3-1$\mu$m long by about 0.1$\mu$m wide. X-ray energy-dispersive spectrometry shows that the host mineral is Di, whereas the composition of the lamella is close to that of enstatite. Selected-area electron diffraction shows that the host mineral is Di with C2/c space group and the lamellae are low clinoenstatite (Lclen) with P2$_{1}$/c space-group. The topotaxy between Lclen and Di is as follows: [100]$_{clen}$$\|$[100]$_{Di}$, [010]$_{clen}$$\|$[010]$_{Di}$ and [001]$_{clen}$$\|$[001]$_{Di}$ approximately, with a deviation of 1-2$\deg$. Lclen exsolution lamellae are oriented parallel to (401) planes of Di. Observation of high-resolution lattice image and diffraction contrast images showed that there exist nanometer-size antiphase domains in the Lclen lamellae, which proves that the Lclen lamellae transformed from a C2/c precursor after exsolution. These micro-characteristics are identical with those reported by Bozhilov et al. (1999). There are two C2/c pyroxenes of approximately enstatite composition, high-temperature clinoenstatite (HTclen) and high-pressure clinoenstatite (HPclen). The Dabie/SuLu UHPM belt has an extremely cold exhumation path, making HPclen the only reasonable choice between these two possible C2/c precursors. We propose extension of the existing exhumation P-T path for the Bixiling massif developed by others to depths in excess of 250 km to reach the stability field of HPclen. In another words, the Bixiling garnet peridotite traveled to the surface from a depth greater than 250 km, placing a new constraint for the peak metamorphic depth of Dabie-Sulu UHP metamorphism belt.
T21A-0508 0800h
Accretionary Complex Origin of the Mafic-ultramafic Bodies of the Sanbagawa Belt, Central Shikoku, Japan
In the high-grade Cretaceous Sanbagawa high pressure (HP) metamorphic belt our new 1:5000 scale mapping of the eclogitic mafic-ultramafic bodies and their surrounding epidote-amphibolite-facies schists, has revealed a duplex structure formed by the subduction of the Izanagi-Pacific oceanic plate. Lithologies of the two largest mafic-ultramafic bodies in the Sanbagawa belt, the Iratsu eclogite and the Higashi-Akaishi peridotite, strike WNW-ESE and dip N; the upper boundary with the surrounding schist is a normal fault, whereas the lower boundary is a thrust. The Iratsu body is subdivided into at least two tectonic units; the unit boundary is sub-parallel to a lithological boundary. The protoliths of the upper unit are gabbro, basalt, minor quartz rock and pelite, and those of the lower unit are pyroxenite, gabbro, basalt, chert and marble, in ascending order. The lower unit is characterized by layers of alternating eclogitic metagabbro and pyroxenite. The layers are extensive at the bottom of the Iratsu eclogite, and transient towards the Higashi-Akaishi body. Eclogite-facies metapsammite is intercalated between the Iratsu and Higashi-Akaishi bodies. Our mapping has revealed the following; (1) a duplex structure of the mafic-ultramafic bodies indicating their accretionary complex origin. (2) Reconstructed oceanic plate stratigraphy in ascending order of peridotite, gabbro, basalt, limestone, minor chert and pelite suggests that different parts of the protolith were derived from a mid-oceanic topographic high, an oceanic island or plateau, and an overlying trench turbidite. (3) The convergent motion of the oceanic plate changed from NW to NE during the accretion of the large oceanic island or plateau.
T21A-0509 0800h
Ancient Protolith Ages and Scandian Metamorphism Across the Western Gneiss Region, Norway
Recent work shows the ultrahigh-pressure (UHP) eclogite-facies event in the Western Gneiss Region (WGR) occurred $\sim$415-400 Ma. However, other (U)HP eclogite-facies events in the Scandinavian Caledonides make it necessary to determine eclogite ages across the WGR before constructing tectonic models. Also, insufficient data exist to constrain the ages of two amphibolite-facies events (at $650-750\deg$C, $\sim$11 kbar and at 6-7 kbar) characteristic to the WGR, which provide important pins on the exhumation process. To these ends, we dated zircon from eclogites, monazite and muscovite from pelites in a 220 x 100 km E-W band across the WGR. Zircon and monazite were dated by secondary ion mass spectrometry and laser-ablation inductively coupled plasma mass spectrometry to extract igneous core and metamorphic rim ages. Most eclogites record Gothian ($\sim$1.7-1.5 Ga) or Sveconorwegian ($\sim$1.2-0.9 Ga) igneous protolith ages similar to those of the Baltica orthogneiss. The easternmost eclogite records a Svecofennian ($\sim$1.8-2.0 Ga) upper intercept age; zircon and monazite from the Trollstigen area record upper intercept and concordia ages $>$ 2 Ga, not previously known in the WGR. While Upper Allochthon eclogites and pelitic gneisses retain the Early to Middle Ordovician and Early Silurian ages of their Iapetan protoliths, the age of the last metamorphism in all the eclogites is Scandian. Basement eclogites were least affected by this Scandian (re)crystallization, and the westernmost eclogites were most affected. Precambrian monazite ages occur only within basement gneisses; Upper Allochthon gneisses yield peak monazite ages of $\sim$424 Ma and $\sim$374 Ma. The most robust geochronological signature of the Scandian metamorphism is a steady westward decrease in muscovite $^{40}$Ar/$^{39}Ar ages from $\sim$394 Ma to $\sim$384 Ma. The entire WGR was involved in the Scandian phase of the Caledonian Orogeny, including the allochthons, which were emplaced onto the Baltica basement before subduction to UHP. Allochthon monazite record this nappe emplacement as well as a late episode of fluid intrusion. The predominance of Precambrian ages in basement rocks may reflect a paucity of fluids. The muscovite age gradient is compatible with near-coeval closure at $\sim$$400\deg$C across the WGR, followed by eastward tilting. If the UHP metamorphism occurred at $\sim$415-400 Ma, complete exhumation to upper crustal levels in the eastern part of the study area may have taken $<$ 5-10 Ma, with the rocks cooling at rates $>$ $50-100\deg$C/Ma. Such rapid cooling indicates tectonic exhumation, likely aided by significant erosion.
T21A-0510 0800h
Exhumation of Norwegian Ultrahigh-Pressure Rocks: Microstructural Evolution of the Nordfjord-Sogn Detachment Zone, Hornelen Region, Norway
The Nordfjord-Sogn Detachment Zone (NSDZ) of Western Norway juxtaposes the eclogite bearing Western Gneiss Complex with the low-grade Hornelen Basin, and is widely cited as one of the primary extensional structures responsible for the exhumation of the Norwegian ultrahigh-pressure (UHP) rocks. In the Hornelen region, the NSDZ is exposed as a 2-6 km thick section of intensely deformed allochthonous rocks that were isoclinally folded and buried to depths of 50-60km at ~435-465 Ma during early contraction. They then remained at or near the base of the crust until after the UHP event at 410-405 Ma, passing through the muscovite closure temperatures at ~400 Ma during orogen-wide extension. Major questions regarding the kinematics of this extension, however, remain problematic: does the contact between the Western Gneiss Complex and the deformed allochthonous rocks represent a crustal scale extensional fault, and what is the role of coaxial versus non-coaxial deformation during extension and exhumation of high and ultrahigh-pressure rocks? Here, as a first step toward answering these questions, quartz lattice preferred orientations and microstructural analysis provide insight into the development of the NSDZ and the evolution of extensional deformation. Quartz $<$c$>$ and $<$a$>$ axis orientations on quartzites were measured using electron back scatter diffraction and yielded strong lattice preferred orientations from all structural levels within the allochthons and throughout the Hornelen region. Samples from middle to high structural levels give lattice preferred orientations that suggest greenschist to lower-amphibolite facies deformation. While lower temperature samples generally yield asymmetric patterns that indicate non-coaxial, top-W shearing, higher temperature samples predominantly yield symmetric patterns suggesting coaxial shearing. Lattice preferred orientations from the lowermost structural levels, and closest to the contact with the Western Gneiss Complex, exclusively indicate coaxial, amphibolite facies shearing. Extension crenulation cleavage and S-C fabrics implying non-coaxial, top-W sense of shear were observed in the field at all structural levels. However, petrographic observations reveal the growth of chlorite in association with these fabrics suggesting that they represent a lower temperature, later stage of extension, and that they overprint earlier higher temperature quartz fabrics. The lack of high temperature non-coaxial fabrics near the base of the allochthons indicates that the contact between the allochthons and the Western Gneiss Complex does not represent a major extensional fault that was active throughout the thickness of the crust. Furthermore, this new data set suggests that coaxial shear played an important role in the earlier, high temperature stages of extension, while non-coaxial shear became more important in later, low temperature stages of extension.
T21A-0511 0800h
Trace- and major-element zoning in garnets from eclogites of the Zermatt-Saas ophiolite, western Alps: LA-ICP-MS data and implications for Lu-Hf geochronology
Trace element contents, inclusion assemblages, and element zoning patterns found in garnets from several eclogite-facies metabasalts sampled across the Zermatt-Saas ophiolite complex, western Alps, record large segments of the prograde P-T-t path to HP/UHP conditions. Ca, Fe, Mn, and Mg contents in garnet from core to rim are indicative of prograde growth zoning. Laser-ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) on garnets cut to expose the core using 3-D x-ray tomography indicates strong HREE (Er, Yb, Lu) zoning, where the highest concentrations occur in the garnet core. The core-to-rim zoning of HREE is very similar to that of Mn determined by both electron microprobe and LA-ICP-MS along the same transect. The HREE and Mn patterns tend to be bell-shaped and have a moderate spike in concentrations toward the rim that is followed by an increase in Gd concentrations. These complex zoning patterns suggest that a reaction or multiple reactions took place that liberated HREE and possibly MREE as well during growth of garnet. Hf concentrations, estimated from Zr concentrations measured by LA-ICP-MS, are typically constant across garnet traverses. The Lu and Hf garnet zoning patterns have important implications for Lu-Hf geochronology because they indicate that the Lu-Hf ratios are highest in the core of the garnet. The high Lu-Hf ratio in garnet cores results in a measured Lu-Hf age that is strongly skewed toward the early prograde growth, if bulk garnet separates are used for geochronology for units such as the Zermatt-Saas ophiolite that were not metamorphosed to temperatures above the Lu-Hf blocking temperature. Sm-Nd and Lu-Hf geochronology using bulk garnet separates suggests that the duration of prograde metamorphism of the Zermatt-Saas ophiolite in the Lago di Cignana area, Italy, is between 50-38 Ma (Amato et al., 1999; Lapen et al., 2003). An eclogite sample collected from the Pfulwe area, Switzerland, yielded a bulk garnet-cpx-whole rock Lu-Hf age of 54 3 Ma (2-sigma). Based on our Lu and Hf zoning data, we interpret the age from the Pfulwe area as representing a stage in its P-T-t history that precedes attainment of peak metamorphic conditions. The range in ages between Cignana and Pfulwe may record the different partitioning behavior of Lu, Hf, Sm, and Nd in specific samples or the diachronous subduction of the ophiolite sheet.
T21A-0512 0800h
Oxidised Eclogites and Garnet-Blueschists from Oman: PT Path Modelling in the NCFMASHO System.
Eclogites and garnet-blueschists exposed at the deepest structural levels of the Oman Mountains in north-eastern Saih Hatat, Oman, are evidence for the subduction of the Arabian continental margin. Their peak pressures have been a matter of debate for over a decade, with initial thermobarometric estimates in excess of 20 kbar, based on garnet-clinopyroxene-phengite barometry and the presence of radial cracks around quartz inclusions in garnet, questioned by some workers. The high pressure minerals (glaucophane/crossite, omphacite, epidote) contain significant amounts of ferric iron, postulated to displace the stability fields of the eclogite and blueschist assemblages to less extreme conditions. In this study we have calculated phase diagrams and pseudosections in the model system NCFMASHO, using the program THERMOCALC and the thermodynamic database of Holland and Powell, which incorporates data for Fe$^{3+}$ -bearing end members. We find that the phase compositions and modal abundances for typical bulk compositions are successfully matched at pressures of $\sim$22 kbar for the eclogites and $\sim$18 kbar for the garnet blueschists. This supports the original high P estimates for the eclogites, and indicates that crossitic amphibole and aegirine-rich pyroxene do not necessarily reflect less extreme conditions. The data set and activity models are applicable to other oxidised HP and UHP mineral assemblages.
T21A-0513 0800h
Diamond Synthesis and Carbon Solubility in a Hydrous Granitoid System
An increasing number of UHP metamorphic terranes incorporated in collisional orogenic belts have been identified since the first discovery of diamonds within felsic metamorphic rocks of Kokchetav massif, Kazakhstan, extending up to $\sim$4000 km in length in the Dabie-SuLu-Qinling belt of China. Some have suggested that all orogenic belt diamonds formed from a carbonate or calc-alkaline melt similar to those derived from kimberlitic pipes, or alternatively that they formed from a carbon-enriched silicate melt with a granitoid bulk chemistry composition. Another group has suggested that orogenic belt diamonds crystallized from a COH-rich supercritical fluid. While the diversity of the minor components accompanying the SiO$_{2}$-dominated inclusions from Kokchetav diamonds, as well as the presence of cavities bearing traces of a former fluid, suggest the idea of diamond growth from a COH fluid, a Si-rich melt as a source for diamond formation cannot be ruled out. Although many experiments were performed on diamond synthesis and on the petrology of diamond-bearing rocks, no consensus has been reached as to which of the mentioned growth media is correct to explain orogenic belt diamond formation. We report here the results of an experimental program undertaken to determine the critical point of the Si-Al-K-C-H$_{2}$O system (and thus, to distinguish a melt environment from a fluid one) and to provide an understanding of how diamond is crystallized in hydrous subducted felsic continental crust. Carbon solubility in these systems was qualitatively determined based on the observed diamond growth rates. Experiments were performed at in a Walker-style multianvil apparatus at P=7 GPa and T=1500-1700$\deg$C with SiO$_{2}$ ranging from 90 wt. %, imitating diamondiferous quartzite, to 62 wt. %, imitating a wide range of feldspathic diamondiferous gneisses. An additional parameter, oxygen fugacity, was also varied to test its affects on the solubility of carbon in Si-rich melt/fluid.
T21A-0514 0800h
Eclogite Rheology and Fabrics at High Temperature and High Pressure
Eclogite plays an important role for mantle convection and geodynamics in subduction zones. An improved understanding of processes in the deeper levels of subduction zones and collision belts requires information on eclogite rheology. However, the deformation processes and associated fabrics in eclogite are not well understood. Incompatible views of deformation mechanism have been proposed for both garnet and omphacite. We investigated here the high temperature deformation behavior of eclogite reconstituted from powders of 50% garnet, 40% omphacite, 10% quartz and trace rutile at temperatures of 1000-1600 K, a confining pressure of 2.5-3.5 GPa, and strain rates of 10$^{-4}$/s to 10$^{-5}$/s. We obtained a power-law creep for the high temperature deformation of eclogite with n=3.4$\pm$0.5 and Q=420$\pm$70 KJ/mol. Microscopic fabrics of these experimental eclogites were analyzed with the Electron Back Scattered Diffraction (EBSD) technique. Our results show that: (I) Garnet displays complex pole figures with random distributions of misorientation angles in good agreement with rigid round garnets; (II) Omphacite shows a pronounced S-type crystallographic preferred orientation characterized by [001] axis forming a well-defined girdle near parallel to the foliation and poles of (010) normal to the foliation; suggesting a dislocation creep mechanism. Further investigation into the water effects on eclogite show: (I) Hydroxyl content does not induce a change in the microfabric of omphacite; (II) Grain boundary processes dominate the deformation of garnet under high water fugacity conditions, yielding a random crystallographic preferred orientation similar to that of nondeforming garnet. These results are remarkably similar to observations from deformed eclogites in nature.