V41C-0711
Monazite and Zircon Record Proterozoic and Paleozoic Metamorphism of Paragneiss, North Qaidam UHP Terrane, NW China
Amphibolite-facies felsic gneisses near Dulan, at the southeast end of the North Qaidam terrane, enclose minor (<10 vol%) eclogite and peridotite which record ultra-high pressure (UHP) metamorphism. Field relations, and coesite inclusions in zircons from paragneiss suggest that felsic, mafic, and ultramafic rocks all experienced UHP metamorphism and a common amphibolite-facies retrogression. SHRIMP-RG U-Pb geochronology and trace element analyses of four paragneiss samples constrains the ages of inheritance and metamorphism. Inherited zircon cores contain high Th/U, and REE concentrations consistent with magmatic zircon of detrital origin. Ages are variably discordant, dominantly 1.4-2.5 Ga, and suggest that the sedimentary protoliths were derived from cratonal continental crust. For three samples, zircon rims and whole grains are medium to dark in CL , with high U and low Th/U (0.04- 0.004). Negative Eu-anomalies are absent, and HREE are depleted relative to detrital cores, suggesting growth in the absence of plagioclase and in the presence of garnet in the eclogite-facies. Monazite is weakly zoned in BSE images. Weighted mean ages are: 430 ± 4 Ma (Zrn) and 438 ± 4 Ma (Mnz); 427 ± 5 Ma (Zrn) and 419 ± 3 Ma (Mnz); and 421 ± 3 Ma (Zrn; no Mnz from this sample). Zircon rims in the fourth sample contain higher Th/U (~0.1), negative Eu-anomalies, and are enriched in HREE, suggesting growth in the presence of plagioclase and in the absence of garnet. With the exception of two Paleozoic analyses, zircon rims yield a weighted mean age of 927 ± 6 Ma. Monazite from this sample yields a weighted mean age of 433 ± 3 Ma, consistent with the two Paleozoic zircon analyses. The Proterozoic zircon age is identical to ages of metagranites from this area, suggesting metamorphism of sediments accompanied granite intrusion, as has recently been reported from the Xitieshan and Lüliangshan localities 200-300 km to the northwest. The Paleozoic zircon and monazite ages overlap the 449-422 Ma range of zircon ages from eclogite enclosed in the paragneiss, consistent with in situ metamorphism of both rock types during ~25 m.y. duration of eclogite-facies conditions.
V41C-0712
Coupled Sm-Nd and Lu-Hf geochronology constraints prograde garnet growth in ultrahigh- pressure eclogites from the Dabie orogen
Ultrahigh-pressure eclogites from the Dabie orogen formed over a spread rang in temperature (~{}600 oC to >700 oC) have been investigated with combined Sm-Nd and Lu-Hf geochronology. Three eclogites, sampled from Zhujiachong, Huanghzhen and Shima, yield Sm-Nd ages of 222.5 ± 4.4 Ma, 217.8 ± 4.2 Ma and 224.2 ± 5.6 Ma and corresponding Lu-Hf ages of 240.0 ± 5.0 Ma, 224.4 ± 1.9 Ma and 230.8 ± 5.0, respectively. Well-preserved major- and trace element growth zoning in garnets in the Zhujiachong eclogite suggest the Lu-Hf age mostly reflects early garnet growth and garnet growth is estimated to have occurred over a time interval of ~{}22 Myr. Whereas, despite of the preserved element growth zoning in garnets in the Huangzhen eclogite, combined texture study reveal that the obtained Lu-Hf age was skewed towards the last garnet growth episode rather than an average estimate of the overall growth span. The narrow time interval of less than 6 Myr defined by the difference between Sm-Nd and Lu-Hf ages indicates a short growth period response to the final garnet growth episode and thus a rapid cooling stage. In contrast, the rather flat element zoning in garnets in Shima eclogite suggests the obtained Sm-Nd and Lu-Hf ages are cooling ages. Furthermore, Hf isotope disequilibrium was observed in several meters scale. The new Lu-Hf ages point an initiation of prograde metamorphism prior to ~{}240 Ma and the exact peak metamorphism timing is subjected to specific samples are largely range between ~{}220 to ~{}230 Ma.
V41C-0713
Microfabric Characteristics and Rheological Importance of UHP Jadeite-Quartzite from Shuanghe, Dabie Mountains
Jadeite-quartzite is one of the important UHP rock types occurring in the Dabie Mountains ultrahigh-pressure metamorphic belt. It is significant to study the structural evolution of jadeite-quartzite in order to understand more completely the formation and evolution of UHP orogenic belts. Moreover, to understand the jadeite-quartzite's deformation mechanisms and geodynamical parameters (stress, strain, strain rate) the study of the microstructure and LPO (lattice preferred orientation) of deformed jadeite and quartz is essential; until now this kind of study has not been performed extensively. Petrofabric data and preliminary TEM work on six representative jadeite-quartzite samples from Shuanghe (Dabie Mountains) are presented. Petrofabric data obtained by EBSD (Electron Backscattered Diffraction) show that the jadeite LPOs patterns are similar to those of omphacite and vary between L- and S-type, which correlate with prolate and oblate grain shape fabrics; quartz LPOs are monoclinic; garnet has no crystallographic preferred orientation. TEM study shows that dominant slip systems of jadeite in one sample are (100)[001], (110)[001] and (-110)1/2[110], while in another sample no dislocation is observed. Abundant dislocations in quartz are operated by the dominant slip system (0001)[11-20] which is basal glide and represents regional shearing during the exhumation process. This suggests that dislocation creep is the dominant deformation mechanism in jadeite. Further study is needed to determine whether other factors such as diffusion creep, grain boundary migration, cation ordering and strain symmetry play an important role in deformation mechanism of jadeite in the jadeite- quartzites of Shuanghe, Dabie Mountains.
V41C-0714
On the Microstructure of Omphacite in Eclogite from Dabie Mountains, China
Observations of omphacite in eclogite from Shuanghe massif of Dabie Mountains, Eastern China by transmission electron microscope show banded subgrains of omphacite with C2/c space group and high dislocation densities exist in the matrix of omphacite with P2/n space group and low dislocation density. Barrosites and albites are distributed along with the banded omphacite subgrains defined by dislocation walls. Based on the P-T path of eclogite and jadeite-quartzite from Shuanghe, we studied the relationship of barrosites, albites, and omphacites. Our results suggest that the banded C2/c omphacite subgrains were formed by dynamic recrystallization under high pressure and temperature during the UHP metamorphic peak stage and/or early exhumation stage. The barrosite and albite should form within the C2/c omphacite before it transforms to P2/n structure at T > 700 °C during exhumation. The omphacite preserves its C2/c structure because of the loss of Na component to the albite. This evolution processes suggest that the P-T path of retrogression of eclogite and jadeite-quartzite from Shuanghe should move toward the high temperature side comparing to previous studies. Barrosites formed under high-pressure before the C2/c to P2/n transformation in omphacite indicates that nominally anhydrous omphacite can preserve considerable amounts of water at great depth. Barrosites and albites distributed along with the high dislocation zones of omphacite suggest that dislocation tubes may act as channels and reservoirs that transport active components including the hydroxyls.
V41C-0715
Trace elemental records of short-lived heating during exhumation of the CCSD eclogites
Trace element compositions of garnet, omphacite and apatite in ultrahigh-pressure eclogites from the main hole of the Chinese Continental Scientific Drilling (CCSD) project were in situ analyzed by the LA-ICP-MS method. Although both garnet and omphacite have homogeneous major element compositions, their trace elements show zonations from core to rim in rare earth elements. In particular, middle rare earth elements in the garnet, heavy rare earth elements in the apatite and all rare earth elements in the omphacite increase from core to rim, respectively. Based on dependence of partition coefficients on temperature and pressure in these minerals, we suggest that the trace elemental zonations in these minerals may record a short-lived heating event during exhumation.
V41C-0716
Structural Geometry of an Exhumed UHP Terrane in the Sulu Orogen, China: Implications for Lower Crustal Deformation and Continental Collisional Processes
Yangkou Bay in the eastern Sulu Orogen, China, preserves significant exposures of UHP eclogites, representing pieces of continental and oceanic crust caught between the colliding North and South China continents in the Paleozoic-Mesozoic, subducted to depths > 200 km, then exhumed along thin shear zones. Narrow coesite- eclogite shear zones are folded and progressively overprinted by amphibolite and greenschist facies shear zones that become wider with lower grade. Detailed mapping shows eclogite facies fabrics (S0 consisting of garnet+omphacite+phengite+rutile+quartz) preserved in relatively weakly deformed zones, where statically recrystallized garnet and omphacite are aligned with a weak shape preferred orientation. Phengite formed during an early eclogite-facies retrogression stage has a different orientation than S0, meaning S0 formed before the phengite grew. A thin layer of fine-grained hornblende and albite coronas grew on the omphacite. Elongated garnet, omphacite, phengite and rutile define S1 and L1. Garnet shows dynamic recrystallization and omphacite shows plastic deformation, representing eclogite facies deformation during early stages of exhumation. Mylonitic to ultramylonitic shear zones separate 5-10-meter-thick nappes of ultramafic-mafic UHP rocks and banded granitic gneiss. Ultramafic-mafic UHP rocks form a magmatic series ranging from dunite-gabbro- gabbroic eclogite- eclogite- retrogressed eclogite, bounded by ultramylonite, with interlayered, strongly foliated and lineated granitic and eclogitic mylonites. The protolith of these rocks was a mafic/ultramafic sequence, probably ophiolitic. In gabbroic to granitic mylonite most minerals except quartz and augite are sheared into thin bands. Quartz and augite are preserved as deformed porphyroblasts in a ductile matrix and are mantled by small grains of omphacite and garnet, which are present in the deformed tails of augite. Rutile is preferentially crystallized in pressure shadows. K-feldspar is dynamically recrystallized. UHP-HP minerals in the mantles, tails, and pressure shadows of the porphyroclasts confirm UHP deformation. Small lineated tonalite plugs intruded after F2 and before F3 between the banded granitoid-gabbro gneiss and ultramylonite. S3 in amphibolite is defined by oriented mica and amphibole, while in lighter colored granitic gneiss, it is mainly defined by stretched quartz and feldspar. This foliation is widely developed in the country rocks parallel to compositional layering S0. Dark mafic layers and amphibolites are stretched into lenses and ultimately, into thin layers parallel to S0. F3 folds are defined by compositional layers and the regional F2 foliation that are both folded and deformed into isoclinal folds. Greenshist facies deformation and metamorphism overprints the granitoid gneiss. F3 is refolded by F4 and shows more angular fold styles whose fabrics developed together with greenschist facies retrogression. In hinges, biotite-chlorite crenulation lineations surround F3 fold axes, displaying radial patterns. In some cases, greenschist facies metamorphism and deformation are overprinted on both interlayered granitoid gneiss and retrogressed amphibolite facies eclogite layers. Isoclinal recumbent F3 folds are refolded by open folds. F4 folds with axial surfaces striking NW and dipping moderately SW exhibit a new greenshist facies foliation (S4) that displaces the S3 amphibolite facies foliation forming a crenulation foliation.
V41C-0717
Multi-stage Development From Ultrahigh-pressure Environment to Low-pressure Magmatic Processes: New Insight From Podiform Chromitites in the Luobusa Ophiolite, Southern Tibet
Unusual silicate lamellae were discovered within chromites of podiform chromitites in the Luobusa ophiolite, southern Tibet, especially from massive- and nodular-type chromites. Using analytical transmission electron microscopy (ATEM), we found coesite, clinopyroxene and MgSiO3 phase as exsolution lamellae from the host chromites. There is no evidence that the Luobusa ophiolite itself formed at great depth, and the presence of coesite lamellae in a chromite directly indicate that podiform chromitites originate from deep mantle environment. On the other hand, magmatic structures are still observed in the podiform chromitite ore-body, such as banding chromites and the sharp contact between host peridotite and dunite envelopes. The characteristic features of the disseminated- and banded-type chromites, such as their interstitial distribution, euhedral to subhedral morphology and absence of exsolution lamellae, suggest they formed under low-pressure magmatic conditions. According to petrographic investigations, nodular-type chromites with numerous lamellae seem to be changing into disseminated-type chromite with no lamellae. Therefore, we conclude that the podiform chromitites at Luobusa retain evidence of their multi-stage development from ultrahigh-pressure environment to low-pressure magmatic processes under a ridge. On the basis of our results, we propose that a significant component of the podiform chromitite at Luobusa originate from the ultra-deep environment, and that chromitites were transported with mantle upwelling from a deep mantle to a shallow level under a mid-ocean ridge. Subsequently, the mantle peridotite with its podiform chromitite underwent partial melting, and the chromites without exsolution lamellae were largely recrystallized or newly precipitated by a shallow-level magmatic process under the mid-ocean ridge. As chromite is a highly refractory mineral, the petrological ultrahigh-pressure evidence can be preserved in spite of its long evolved history. Moreover, our new approach of nano-scale measurement with ATEM for refractory chromites may become a new tool to trace ultra-high pressure signature not only for ophiolitic massifs but also for ultra-high pressure massifs or mantle xenoliths.
V41C-0718
Sm--Nd Garnet Geochronology Demonstrates Wholesale Transformation of Continental Crust During UHP Subduction---Western Gneiss Region, Norway
Determining the extent to which ultrahigh-pressure (UHP) terranes transform to high-pressure (HP) minerals during subduction is central to understanding the processes attending the subduction of continental margins. The Western Gneiss Region (WGR) of Norway is one of two giant UHP terranes on Earth, and as such constitutes an important natural laboratory for investigating these processes. The distribution of UHP and HP eclogites shows that a 60,000 km2 area was subducted, but these eclogites are sparse (1--2 vol%) blocks in a sea of chiefly quartzofeldspathic, amphibolite-facies orthogneiss. Aside from the eclogites, the only widespread indicator of (U)HP metamorphism in this enclosing orthogneiss is garnet. This study uses high- precision Sm–-Nd geochronology of these garnets to evaluate the degree of transformation of the WGR to high- pressure minerals during subduction and back to low-pressure minerals during exhumation. We compare these garnet ages to the 420–-400 Ma Sm-–Nd ages reported for WGR eclogites (e.g., Kylander-Clark et al., 2007). Eight garnet samples dated using the garnet–-whole-rock isochron technique yielded the following results: 1) 921.7 ± 1.3 Ma for a sample from the easternmost section of the WGR; 2) 418.1 ± 1.7 Ma, 417.3 ± 1.2 Ma and 403.9 ± 0.8 Ma for three UHP samples; 3) 410.3 ± 2.5, 406.9 ± 1.5 Ma and 398.5 ± 0.8 Ma for three HP samples; 4) 587.3 ± 4.3 Ma for a fourth HP sample. These ages are interpreted to represent 1) the Proterozoic granulite-facies metamorphism of ca. 950 Ma; 2 & 3) (U)HP prograde metamorphism of the continental crust; and 4) a mixed age derived from the granulite-facies and (U)HP metamorphic events. The Proterozoic garnet age implies that the easternmost WGR did not transform to eclogite-facies minerals. In contrast, the good agreement between the 420–-400 Ma eclogite ages (e.g., Kylander-Clark et al., 2007) and the 418–-398 Ma gneiss ages reported here, indicates that the bulk of the WGR continental crust underwent wholesale transformation at (U)HP conditions. That the garnets are the only HP silicate mineral (other than quartz) surviving within a quartzofeldspathic gneiss composed of otherwise amphibolite-facies minerals indicates wholesale retrogression during exhumation.
V41C-0719
Peculiar Carbonate-Rich HP/UHP Rocks from the Tromso Nappe – Metacarbonatite or Remobilized Marble?
The common occurrence of marble and calc silicate gneiss interlayered with garnet mica schist and quartz- feldspathic hosting UHP mafic and ultramafic bodies within the uppermost allochthonous unit of the Scandinavian Caledonides of northern Norway is well known (Krogh et al. 1990; Ravna et al., 2006; Ravna & Roux, 2006), and the UHP (730 oC/3.36 GPa) event is dated to 452 +/- 1.7 Ma (U-Pb zircon; Corfu et al., 2003). Recently, a massive medium to coarse grained carbonate-rich body intimately associated with carbonate-bearing phlogopite-rich garnet pyroxenite and relatively pure banded marble has been discovered. The rock has an almost isotropic fabric with aggregates of silicate minerals evenly distributed in a matrix of calcite, giving it an igneous appearance. Garnet, clinopyroxene and biotite are easily recognized in hand specimens. Locally megacrysts (up to 4x6 cm) of green clinopyroxene occur. The contact toward the adjacent pyroxenite exposed in situ and in large blasted blocks shows an intricate and chaotic relationship. The massive carbonate rock and the marble show distinctly different primary mineralogy. The primary minerals of the massive carbonate rock is Sr-bearing Fe-Mg-calcite (Ca90Fe3Mg7), with lamellae and recrystallized grains of Fe-dolomite (Ca52Fe9Mg39), garnet (Grs26Sp1Alm44Py24And5), sodic clinopyroxene (Jd + Ac)20-25, Ti- (3.6 wt % TiO2) and Ba- (0.85 wt % BaO) rich biotite, F-rich apatite and rutile. Secondary minerals are hornblende after clinopyroxene, LREE-rich (up to 15 wt % LREE-ox) allanite growing on biotite and titanite (XAl = 0.09) after rutile. Garnet contains inclusions of apatite and calcite surrounded by radial cracks. The associated marbles contain calcite (Ca96Fe1Mg3), Ca-rich garnet (Grs56Sp1Alm30Py13), clinopyroxene (Jd + Ac)20-25, zoisite, K-feldspar and titanite (XAl = 0.20), with secondary diopside and plagioclase after Na-rich clinopyroxene. Major and trace element concentrations also display a marked difference between the massive carbonatite-like rock and marble. The former have high contents of TiO2 (1.7-4.0 wt %) and P2O5 (0.1-2.1 wt %) as compared to 0.2-0.3 wt% and <0.1 wt %, respectively, in the marble. REE-patterns of the carbonatite-like rocks show strong LREE enrichment (up to 1000 times chondrite vs. ca. 100 for marble) and (La/Yb)N of 31-77 vs. 12-17 for marble. The absence of a distinct Eu-anomaly in the massive carbonate rock also distinguishes it from the marble. Based on the distinct differences in bulk chemical composition between the massive carbonate rock and marble, they certainly are of different origin. The massive carbonate rock has REE-patterns similar to true carbonatites, whereas the patterns for the marble have typical limestone patterns. Also the high content of Ti, P and REE and the virtual absence of a europium anomaly in the massive carbonate rock bear similarities with carbonatites. We thus suggest that the massive carbonate rock is a meta-carbonatite, and the marble is of metasedimentary origin. This indicates that the eclogite-bearing Tromsø Nappe may represent a rifted continental margin subducted to UHP conditions during Ordovician time.
V41C-0720
Si-Al-Cl-Mg-Ca Aqueous Fluids in Dora-Maira Pyrope: new Contributions for an old Question
Recent discovery of multiphase solid inclusions (MSI) in peak minerals from ultra-high pressure (UHP) terranes opened up new prospects for understanding the fluid-rock interaction during deep subduction in both crust and mantle. The first report on MSI in UHP rocks was from Dora-Maira (DM; Case Parigi; western Alps) whiteschists, more than ten years ago (Philippot et al, 1995, CMP, 121, 29-44). Nevertheless, the nature of such a fluid, and its role on the origin of the unusual composition of these rocks is still matter of debate. We report data on inclusions in DM UHP pyropes and HP prograde kyanite, part of them from a new sampling site (SSW Case Parigi). Primary MSI (30 micron) are present only in small UHP pyropes (1 - 6 cm) and often show post-entrapment decrepitation. Each MSI contains Mg-chlorite, Na-phlogopite, minor Cl-rich apatite, talc, pyrite, magnesite, Ca-rich chlorides +/- liquid water. Maps of total water concentrations collected in MSI-rich pyropes by infrared synchrotron radiation show gradients that suggest considerable H diffusion from inclusions into the host garnet (Frezzotti et al, 2007, abstract ECROFI XIX). In prograde kyanite, rare fluid inclusions are high salinity brines, containing different salts. Present data indicate that at HP conditions brines were present in the rocks and that at UHP peak aqueous fluids were enriched in Si, Al, Mg, Na, Ca, but still containing significant amounts of Cl, P, S, C. DM whiteschists are commonly considered metasomatic rocks from a granitic protolith. Our data on MSI in UHP pyrope and on rare brines in prograde HP kyanite strongly support metasomatism by external high-Ca-Mg fluids, probably evolved during serpentinite dehydration as proposed by Sharp and Barnes (2004, EPSL, 226, 243-254). Present data support the model of Compagnoni and Hirajima (2001, Lithos, 57, 219-236), who proposed that metasomatic fluids were introduced into the system during prograde metamorphism, channelled along shear zones cutting across the precursor granitoid/orthogneiss.
V41C-0721
Development of Three-dimensional Stress Imaging around Mineral Inclusions in Diamond
\hspace*{5mm}Most natural diamonds trap the mantle minerals and/or fluids as inclusions during its formation in the Earth's interior. Residual stress of diamond can be measured using micro-Raman spectroscopy, as Raman peak of diamond shifts to higher wavenumber when pressure is applied. Two-dimensional Raman mapping of diamond samples have disclosed the stress distribution around mineral inclusions. Due to the anisotropic elastic properties of minerals, however, three-dimensional Raman mapping is necessary for detailed and precise estimations of stress distributions. In the present study, we aimed at three-dimensional visualization of stress distribution using micro Raman spectroscopy. By measuring residual stresses around at least two mineral species with different bulk modulei and thermal expansivitivties, pressure and temperature when the inclusions were trapped in the diamond can be estimated. \hspace*{5mm}Raman spectra of diamond were measured using a confocal Raman microscope equipped with an automatic X-Y-Z stage. High spectroscopic resolution is required for measuring subtle difference (c.a. 0.1 cm-1) in residual stresses in diamond. Furthermore, high stability of spectrometer is also required because it takes long time to obtain three-dimensional mapping. Our previous studies revealed that peak position of Raman spectra contained the periodic oscillation of the peak positions with amplitude of 0.15 cm-1 synchronized the temperature regulation. In the present study, neon atomic lines for the wavenumber standard were collected into the spectrometer simultaneously with the Raman spectrum of samples. \hspace*{5mm}The studied sample was from Internationalnaya pipe (Russia) and contains olivine and chromites as inclusions. The sample was polished to optical grade and a volume containing a couple of inclusions were scanned three dimensionally. By digitizing the Raman shift of diamond, stress distribution around the inclusions can be three-dimensionally visualized and the difference in the maximum residual pressures surrounding mineral inclusions between olivine and chromite were revealed, respectively. Using the obtained maximum residual pressures, the compressibilities and thermal expansivities of diamond and inclusions, source temperature and pressure were estimated. The obtained P-T value was in the stability field of diamond.
V41C-0722
First Discovery of Eclogite in West Papua (Wandamen Peninsula)
Exceptional boulders of fresh eclogites and large garnets (up to 10 cm) were discovered in schist in the Wandamen peninsula (West Papua) The eclogites are located in a context of very active oblique convergence between the Pacific plate and the Australian plate. The Wandamen peninsula is located in the Cenderawasih bay (West Papua). An EW metamorphic gradient is observed from the western unmetamorphosed Lengguru sedimentary prism to the Wandamen peninsula. The metamorphic Wandamen peninsula may therefore represent the inner part of the Lengguru belt, and may also be in the continuation of the inner part of the Central range of Papua New Guinea farther east. The eclogite occurs as centimetric lenses within garnet bearing metasedimentary rocks. Sediments look like the Cenozoic sediments of the internal zone of the Lengguru FTB. The eclogites are medium to coarse grained. The eclogitic paragenesis consists of clinopyroxenes, garnets, amphiboles, rutiles, quartz and accessory minerals like apatites. Clinopyroxenes have omphacitic composition, they are poecilitic, and very fresh as rare textures of retrogression (symplectite) are observed. Garnets are micrometric to centimetric. They present atoll shapes, with a core often filled by slightly green amphibole, or by quartz. They contain minute inclusions of omphacite, amphibole, apatite, quartz. Many rutiles exsolutions are observed within the garnets, suggesting high metamorphic conditions. Inclusions of quartz in the garnet or in the omphacite are surrounded by many fractures suggesting the previous occurrence of coesite. The sodic calcic greenish amphiboles and rutiles are also important minerals of these eclogites. Migmatites and resulting leucogranite cross cut the eclogite indicating a later HT event. The leucogranite is rich in tourmaline and muscovite that suggest their anatectic origin (melting of the black micashists metamorphosed under migmatitic conditions). Miocene pebbles were found within an unmetamorphosed conglomerate (no metamorphic or magmatic pebbles were in the conglomerate) that overlies the eastern flank of the Wandamen massif suggesting that exhumation of eclogite was active after Miocene.
V41C-0723
The timing of partial melting and UHP metamorphism in the Kumdy-Kol region (Kokchetav Massif, Northern Kazakhstan)
The Kokchetav Massif of northern Kazakhstan is the best-known metamorphic diamond locality among numerous ultrahigh-pressure (UHP) terranes in the world. At the Kumdy-Kol deposit, diamondiferous rocks are interbedded with granitic gneisses, and biotite gneisses; some have been migmatized. Some granite gneisses and migmatites were formed by partial melting of diamondiferous rocks. To verify such suggestion, sensitive high resolution ion microprobe (SHRIMP) U-Pb dating of zoned zircons from migmatites at the Kumdy-Kol region was performed to constrain the age of partial melting of the Kokchetav UHP metamorphic rocks. Most age data from core and rim domains of zircon separates are concordant. The apparent 206/Pb238U ages for core (524 ± 6 Ma) and rim domains of zircons (522 ± 7 Ma) are nearly identical within analytical error. All SHRIMP analyses of zircons from three samples fall in the range 507 - 538 Ma with the weighted mean age for all zircon domains at 523 ± 4 Ma (MSWD = 4.6). Taking the mean ages of UHP metamorphism (530 - 337 Ma) and late amphibolite-facies overprinting (517 ± 5 and 515 ± 5 Ma) we conclude that exhumation from diamond-grade depths (> 150 km) to mid-crustal depths (~ 30 km) must have been completed within about 15 - 20 Ma. Our data show that migmatization of UHP pelites occurred slightly later than the peak metamorphism (~ 530-537 Ma), and the decompression, partial melting took place during exhumation of diamondiferous rocks from mantle depths to amphibolite-facies conditions at mid-crustal levels.
V41C-0724
Boron-cycling by subducted lithosphere; insights from boron-isotope compositions of the Kokchetav tourmalines
For understanding the Earth's chemical evolution, the role of subducting plates has long been focused on. Stable isotopes can provide critical evidence to identify materials experienced geological processes near the surface. In this study, we examined B-isotope compositions of tourmalines from the Kokchetav diamondiferous UHP metamorphic belt, particularly a recently discovered high-K tourmaline from Kumdy-kol[1]. The high-K tourmaline occurs in Qtz-Kfs layers, alternating with Grt-Cpx-Bt-Qtz rocks. It has microdiamond- bearing and K-rich (K2O=~2.38 wt.%) cores, which yield heavy B-isotope ratios (δ11B=+3.2~+7.7, analyzed by SIMS). Our results suggest that the high-K tourmaline would be crystallized under high-pressure within the diamond stability from fluids or melts with the surficial B-isotope signature, which is clearly different from that of ordinary tourmalines (δ11B=-16.6~-2.3) experienced the isotope fractionation through subduction-related dehydration reactions. Tourmalines with heavy B-isotope ratios have been described from marine evaporites and carbonates[2]. The presence of silicate-carbonate melt inclusions with microdiamonds in metacarbonate rocks from Kumdy- kol[3] implies that the heavy B-isotope in the high-K tourmaline might have derived from the melt preserved in the metacarbonate rocks. However, recrystallized carbonates can yield light B-isotope ratios (δ11B=~-5[4]), and it is doubtful that carbonates could have retained the heavy B-isotope signature during subduction to the depths. The alternative source of the heavy B-isotope is serpentinite in hydrated lithospheric mantle, because serpentinized peridotites are enriched in heavy B-isotope (δ11B=+5.4~+25[5]). Serpentinization of subducting lithospheric mantle, with enrichment of heavy B-isotope, can be realized by transform faulting near oceanic ridges and normal faulting at trench-outer rise regions, followed by penetration of seawater into the lithospheric mantle prior to subduction[6]. As high- pressure stability limits of serpentines[7] are comparable to the peak metamorphic pressure of the Kokchetav diamond-grade rocks, the fluids derived from serpentine-breakdown in the lithospheric mantle would ascend to accelerate the partial melting of overlying crustal rocks. The serpentine-breakdown is the first, major dehydration reaction in subducting lithospheric mantle, the fluids from serpentine-breakdown should have retained the heavy B-isotope ratios, differing from subducting crustal rocks that have already experienced the isotope fractionation through dehydration reactions. Consequently, we conclude that the heavy B-isotope signature would be inherited from serpentinites in subducted lithospheric mantle, hydrated prior to subduction. The subducted lithospheric mantle is also an essential reservoir for the geochemical recycling of surficial materials, as implied by recent studies with radiogenic and stable isotopes in oceanic island basalts. [1]Shimizu & Ogasawara (2005) Mitt Öterr Miner Ges 150:141 [2]Palmer & Slack (1989) 103:434-451 [3]Korsakov et al.(2004) Terra Nova 16:146-151; Korsakov & Hermann (2006) EPSL 241:104-118 [4]Spivack & You (1997) EPSL 152:113-122 [5]Spivack & Edmond (1987) GCA 51:1033-1043; Benton et al.(2001) EPSL 187:273-282 [6]Peacock (2001) Geology 29:299-302 [7]Ulmer & Trommsdroff (1995) Science 268:858-861; Wunder & Schreyer (1997) Lithos 41:213-227 http://www.misasa.okayama- u.ac.jp/eng/
V41C-0725
The coexistence of jadeite-omphacite-diopside/hedenbergite in an albite-omphacite rock from the Kurosegawa zone, southwest Japan.
Two miscibility gaps in the system jadeite - aeigirine - diopside/hedenbergite, one between jadeite and omphacite and the other between omphacite and hedenbergite were examined based on the coexistence of jadeite, omphacite and hedenbergite in an albite-omphacite rock which occurs within serpentinites of the Kurosegawa Zone in the middle Kyusyu, southwest Japan. Identification of simultaneous equilibrium of three pyroxenes is based on the following observation. A.Hedenbergite occurs as a core of omphacite or as a rim of omphacite in contact with K-feldspar vein. B.Omphacite is euhedral and overgrows hedenbergite or taramite. C.Jadeite occurs as euhedral crystals, overgrowing omphacite. D.Taramite is in contact with jadeite or overgrown by omphacite. Mineral chemistry of these minerals indicates miscibility gaps between jadeite and omphacite and between omphacite and hedenbergite in the albite-omphacite rock at the same metamorphic condition. The miscibility gap between omphacite and hedenbergite is similar to that of the earlier studies (Carpenter, 1980; Enami and Tokonami, 1983 etc) and that between omphacite and hedenbergite is narrower than that of Tsujimori(1997). The miscibility gaps in the ternary system are asymmetric (Holland, 1990) rather than symmetric (Carpenter, 1980). The albite-omphacite rock lacks mineral assemblages indicative of P-T condition. Therefore, we use a lawsonite blueschist and an epidote blueschist to estimate the metamorphic condition, which occurs in the same serpentinite melange zone. The lawsonite blueschist indicates schistose textures consisting of Na amphibole, lawsonite, pumpellyite, chlorite and augite, with small amounts of omphacite, quartz and albite. Omphacite occurs as a rim of augite. This occurrence is similar to that in the albite-omphacite rock. The epidote blueschist occurs as a thin horizon in the schistose lawsonite blueschist. This rock includes aegirine, Na amphibole, chlorite, epidote, muscovite, albite and quartz. The metamorphic condition of lawsonite blueschist was inferred from P-T pseudosection. The pseudosection indicates that the metamorphic pressure of the lawsonite blueschist is more than 5kbar and the temperature is not more than 400 Celsius. The metamorphic condition of the epidote blueschist was inferred from average P-T method. These results indicate that the metamorphic condition of lawsonite blueschist and epidote blueschist is 7.3kbar and 383 Celsius.
V41C-0726
Interlayered High-P Granulites and Eclogites, Fiordland, New Zealand
As lithospheric plates are subducted and thickened, rocks are metamorphosed under high- P and UHP conditions to produce granulite and eclogite facies metamorphic rocks. Serendipitous circumstances may facilitate chemical equilibrium at such conditions, but it is rarely entirely achieved. Granulites and eclogites can preserve, in their distinctive mineral assemblages and textures, a record of the pressure, temperature and deformation conditions experienced during subduction, crustal thickening, and subsequent exhumation. Granulite facies rocks reflect the highest temperature conditions, whereas eclogite facies rocks are most commonly associated with subduction and reflect highest-pressure conditions associated for orogenic metamorphism. Eclogite facies assemblages may also reflect upper-mantle conditions. Rocks that record assemblages from both the granuilte and eclogite facies, evidence of the garnulite-eclogite facies transition, are extremely rare. These types of rocks are of interest as they represent the deepest parts of Earth's crust affected by orogenesis and encapsulate processes related to nascent crust and lower crust - upper mantle interaction. Fiordland, New Zealand, offers a unique cross-section through the lower crust root of a Cretaceous magmatic arc. We have recently identified a suite of unusual rocks that contain interlayered eclogite and granulite facies assemblages. On the basis of published literature, rocks with assemblages similar to those occur in relatively few localities - the Bohemian massif (Czech Republic) and the Western Gneiss Region (Norway). Peak metamorphic conditions accompanied by the formation of penetrative gneissic textures displaying interstitial partitions of mafic to felsic bearing assemblages - garnet-omphacite-rutile (mafic) and antiperthite-plagioclase-quartz-rutile-kyanite (felsic). Preliminary thermobarometric results for these uncommonly well preserved high- P high- T orthopyroxene-bearing rocks indicate P~17-19kbar, and T~850-920°C. The metamorphic P- T-t path of these peculiar rocks will be constrained through forward modeling utilizing XRF bulk rock data in conjunction with phase equilibria employing the program THERMOCALC (Powell, 1998). Calculated pseudosections to constrain peak and retrograde conditions reflected by changes in mineral composition and reaction textures; they have proven to be a powerful tool as both the stability of mineral assemblages and mineral compositions can be considered simultaneously. THERMOCALC also offers the potential to study other important physiochemical issues such as the development local-scale chemical potential gradients and volume change during the formation of these unique outcrops.
V41C-0727
Phase Equilibria and Thermobarometry of Lawsonite and Pumpellyite-Bearing Metabasalts From Crete
High-pressure, blueschist facies rocks associated with the subduction and subsequent exhumation of sedimentary rocks along the Hellenic subduction zone now outcrop over a large portion of the Greek island of Crete. They host the rare pelitic index mineral, carpholite, and has one of the few occurrences of metamorphic aragonite in marbles. Though largely composed of pelites, many small pods of metabasalts contain index minerals such as lawsonite and glaucophane. The assemblage lawsonite + pumpellyite has been found, providing an excellent opportunity to unravel the pressures and temperatures to which these rocks were subjected during subduction. Blueschists from the central part of Crete are typically glaucophane-bearing, with plentiful epidote, sphene and chlorite. Millimeter scale domains within some of the rocks contain the assemblage, quartz + lawsonite + albite + chlorite + epidote ± pumpellyite ± glaucophane. The presence of lawsonite and epidote roughly locates the rocks between the lawsonite and epidote blueschist facies as defined by Evans (1990). Chemically the lawsonite is fairly pure, with only small amounts of Fe (less than 0.4 wt %). Although some pumpellyite has almost no iron, when in equilibrium with the lawsonite it typically contains subequal amounts of Fe and Mg, (Fe/(Fe+Mg) is on average ~0.45). Chlorite is also roughly halfway between clinochlore and chamosite. The glaucophane is crossitic in composition. Clinozoisite has around 4 weight percent Fe2O3 (0.25 atoms Fe per formula unit). The albite and quartz are pure. The reaction albite + pumpellyite + lawsonite = glaucophane + clinozoisite + quartz + fluid has a somewhat steep slope (74 bar/°C) and can be used to constrain temperatures. Another reaction between lawsonite and pumpellyite is glaucophane + lawsonite = pumpellyite + chlorite + albite + quartz + fluid. The slope of this reaction is (24 bar/°C) and so the intersection of the two provides a reasonable estimate of the P-T conditions of this assemblage. A more accurate activity calculation will allow this estimate to be further refined. Further work on the mineral compositions and assemblages in metabasalts change from west to east across the island investigates the proposed westward increase in metamorphic grade in blueschist outcrops. Other reactions which can be used from this assemblage, and indicate roughly similar conditions of formation include tremolite + clinozoisite + clinochlore + fluid = pumpellyite + quartz or lawsonite + glaucophane = clinozoisite + quartz + albite + clinochlore + H2O and other limits in the absence of calcic-amphibole, or sphene. Initial calculations in the presence of lawsonite and pumpellyite suggest these rocks formed around 350°C at pressures near 7 kbar. Lawsonite + pumpellyite breaks down when the activity of H2O is greater than 0.95 at 6 kbar, suggesting a relatively high water activity. This greatly simplifies P-T calculations. These temperatures are in good agreement with those reported for carpholite-bearing metapelites described from the same unit by Theye & Seidel (1983). Their reported assemblage included quartz + phengite ± sudoite ± carpholite ± chloritoid. Our current estimate is slightly lower in pressure than is required to form metamorphic aragonite (~9 kbar at 350°C), which we have also found in this unit, although not in the metabasalts. The westward increase in grade has been reported as largely a temperature difference, so pressure gradients may explain the infrequency of aragonite occurrences at Crete.
V41C-0728
21 Ma Eclogite From the Main Central Thrust Sheet, Eastern Nepal Himalaya
Though uncommon throughout the Himalaya, eclogites have been documented in the Kaghan Valley of Pakistan, the Tso Morari dome in India, the Kharta region of Tibet, and the Makalu-Everest region of the Arun River valley in eastern Nepal. The Kaghan and Tso Morari UHP eclogites have been dated at ~50 Ma, and are commonly viewed as reflecting aborted subduction of the leading edge of the Indian plate during the initial stages of Indo- Asian collision. Here we show that the Arun eclogites are significantly younger, only ~21 Ma, so reflect either different origins, or substantial time lags in tectonics along strike. The Arun eclogites are stratigraphically continuous with the surrounding Greater Himalayan Sequence (GHS) felsic gneisses, and have been interpreted as metamorphosed basaltic sills. P-T conditions have been estimated at >14 kbar at 670-710 °C. The GHS in this region overlies Lesser Himalayan rocks along the Main Central Thrust (MCT), which can be traced for over 2000 km along strike. Lu-Hf dates from garnet separates in one relict eclogite indicate an age of 20.7±0.4 Ma (MSWD = 2.2). Five garnet amphibolites from nearby were also dated via Lu-Hf, and their ages range from 14-20 Ma (13.9±2.5, 14.1±0.3, 14.5±2.8, 15.1±0.6, and 19.8±1.1 Ma). The ~21 Ma age obtained from the eclogite postdates eclogite ages from the western Himalaya (Kaghan and Tso Morari) by ~30 Myr, and has important implications for tectonic models of Himalayan orogenesis. One possible model is that (aborted) subduction, slab breakoff, and ascent of India's leading edge occurred diachronously: ~50 Ma in the western Himalaya, ~20 Ma in eastern Nepal, and presumably even younger in the eastern Himalaya. Alternatively, because the Arun eclogites did not reach ultra-high pressure conditions seen by western eclogites (only ≥45, not ≥90 km depth), they may simply reflect deepening or longer transport of the MCT in the Arun area. Regardless, a ~21 Ma age for these eclogites combined with several 14-15 Ma ages for nearby garnet amphibolites implies young initial movement on the MCT at Arun, as late as 14 Ma.
V41C-0729
Eclogite Facies Relicts and Decompression Assemblages; Evidence for the Exhumation of a Large Coherent Metabasite Block From > 40 km Depth; Central Metamorphic Terrane, Eastern Klamath Mountains, Northern California
Recent exhumation models for eclogite terranes have focused on the exhumation of sialic rocks. Exhumed high pressure terranes are typically > 85% − 90% sialic material with only minor amounts of mafic and ultramafic rock. Most known metabasitic eclogites are blocks in mélange rather than large coherent bodies. The Central Metamorphic terrane (CMt) is a large (~300 km3) coherent, fault-bounded package of metabasites thought to represent a remnant of a downing plate subducted in an intra-oceanic convergent margin. Thermochronology indicates that the CMt was metamorphosed and later accreted to the base of the Trinity ophiolite along the Trinity fault during Early Permian extension (Hbl and Musc 40Ar/39Ar ages of 275 Ma - 294 Ma). Previous work suggested that the peak metamorphic temperatures and pressures were ~650°C and 0.4 to 0.8 GPa (Peacock and Norris, 1989) which is consistent with the amphibolite facies mineral assemblage. Trace element data confirm the NMORB-like composition of CMt metabasite protoliths. Newly discovered relict textures, however, suggest that CMt amphibolites record much deeper subduction burial with subsequent decompression exhumation. A decompression sequence consisting of rutile cores within ilmenite crystals mantled by titanite is observed in CMt amphibolite samples. Zr-in-rutile thermometry (Watson et al., 2006) combined with experimental data for rutile stability in metabasites (Ernst and Lui, 1998) suggests that relict rutile crystals preserve early P-T conditions of ~600°C and > 1.3 GPa consistent with eclogite facies metamorphism. Transition from eclogite facies is further supported by ilmenite-plagioclase-amphibole symplectites suggesting replacement of garnet (Bhowmik and Roy, 2003) during decompression. Amphibole compositions vary significantly and reflect lower grade (low Na, Al, Ti actinolite) overprint of earlier amphibolite facies compositions (high Na, Al, Ti magnesio- hornblende). Application of the Al-Ti hornblende thermobarometer (Ernst and Lui, 1998) yields upper P-T estimates of ~600°C and > 1.5 GPa, consistent with hornblende eclogite facies. The dominant mineral assemblages and metamorphic fabrics indicate dynamic recrystallization of metabasites during declining P-T conditions through amphibolite - epidote amphibolite facies (ilmenite - titanite transition). Exhumation via extension along the Trinity fault is suggested by the coplanar relationship between metabasite decompression-related deformation fabrics and the Trinity fault. Present models call on the higher buoyancy of the sialic eclogites as the mechanism for exhumation and to explain the volumetric difference between the mafic and felsic eclogites exhumed. The exhumation of the CMt from depths > 40 km requires new models for the exhumation of subducted oceanic crust from eclogite facies conditions. We speculate that the CMt was exhumed from eclogite facies conditions by tectonic unroofing along the Trinity fault. However, the mechanism is still unclear and requires further investigation.
V41C-0730
Two modes of occurrence of eclogites from the Atbashy Range, Southern Tien-Shan, Kyrgyzstan
The eclogite-bearing complexe of the Choloktor suite occurs in the Atbashy Range, Southern Tien-Shan, Kyrgyzstan, and consists of pelitic schists, garnet-amphibolites and eclogites. They have experienced HP and UHP metamorphism with evidence of quartz pseudomorphs after coesite (Tagiri et al., 1995; Puelles and Takasu, 1996; Bakirov et al., 1998). There are non-metamorphosed conglomerates overlying the Choloktor suite and they consist mostly of subrounded cobbles and boulders including eclogites. The eclogites from the Choloktor suite have experienced a prograde metamorphism to the eclogite facies through the greenschist and blueschist facies. The peak metamorphic conditions of the eclogites are estimated as 530- 660°C (Ellis & Green, 1979) and 500-620°C (Powell, 1985) at 15 kbar. Eclogites as the clasts from the conglomerate bed have experienced two events of HP metamorphism. The first HP metamorphic event is defined by inclusions in zoned garnets, which suggest a prograde metamorphism from the greenschist fecies to the eclogite facies. The estimated peak eclogitic conditions are 490-590°C (Ellis & Green, 1979), and 460-560°C (Powell, 1985) at 14 kbar. The second HP metamorphic event is defined by chemically zoned amphiboles in the matrix which show a prograde metamorphism from the greenschist facies through the blueschist facies to the epidote-amphibolite facies. We will discuss the metamorphic relationship between two modes of eclogites, and the study contributes for better understanding of subduction zone metamorphism and exhumation processes of the Atbashy Range, Southern Tien-Shan, Kyrgyzstan. Keywords: Atabshy Range; Southern Tien-Shan; Choloktor suite; conglomerate bed
V41C-0731
Metamorphic decarbonation in the Neoproterozoic ultrahigh-temperature metamorphism and its environmental implication
Metamorphic decarbonation reactions and volcanic degassing leads to significant influx of CO2, a major greenhouse gas, into the ocean-atmosphere system from the solid Earth. The anhydrous mineral assemblages that characterize the Proterozoic granulite facies rocks, including charnockites and ultrahigh-temperature (UHT) rocks, require that water activity was buffered to low levels during their formation. One of the popular models invokes the influx of CO2-rich fluids from the tectosphere mantle to generate dry mineral assemblages. Here we present quantitative estimates on CO2 derived through degassing during UHT metamorphism in the Neoproterozoic through the mineralogical and geological analyses. In an attempt to investigate the link between CO2 liberation from the carbonated tectosphere, UHT metamorphism and major earth processes, we address some of the important issues such as: 1) how the tectosphere had become carbonated; 2) how and when the tectosphere degassed; and 3) what is the difference between Proterozoic orogens and those of the present day. The fate of the Earth as a habitable planet was dictated by a reversal of the fundamental process of formation of oceans by the selective removal of CO2 into mantle in the Hadean time, carbonation of the Archean mantle wedge, and subsequent decarbonation of the carbonated mantle through divergent metamorphism and water infiltration since the late Proterozoic. Our computations show that an extra flux of CO2 was added to the atmosphere through a Himalayan scale UHT metamorphism to the extent of 6E16 to 3E18 mol/my, for a duration of 10 my. A calculation of the impact of the extra CO2 influx to the global mean temperature in the context of carbon cycle and greenhouse effect of CO2 shows that at the peak influx stage, the steady state temperature would be raised by 4 degC from 15 degC and by 13 degC from 4 degC. Our results have important bearing in evaluating the mechanism of melting and the duration of Snowball Earth. Our estimate of the maximum degassing rate during UHT metamorphism suggests that the duration of Snowball Earth in Marinoan was probably shorter, and the recovery from an ice-covered Earth to ocean-covered Earth faster, than in previous estimates.