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1.
Testing the fidelity of thermometers at ultrahigh temperatures   总被引：1，自引：0，他引：1  
A highly residual granulite facies rock (sample RG07‐21) from Lunnyj Island in the Rauer Group, East Antarctica, presents an opportunity to compare different approaches to constraining peak temperature in high‐grade metamorphic rocks. Sample RG07‐21 is a coarse‐grained pelitic migmatite composed of abundant garnet and orthopyroxene along with quartz, biotite, cordierite, and plagioclase with accessory rutile, ilmenite, zircon, and monazite. The inferred sequence of mineral growth is consistent with a clockwise pressure–temperature (P–T) evolution when compared with a forward model (P–T pseudosection) for the whole‐rock chemical composition. Peak metamorphic conditions are estimated at 9 ± 0.5 kbar and 910 ± 50°C based on conventional Al‐in‐orthopyroxene thermobarometry, Zr‐in‐rutile thermometry, and calculated compositional isopleths. U–Pb ages from zircon rims and neocrystallized monazite grains yield ages of c. 514 Ma, suggesting that crystallization of both minerals occurred towards the end of the youngest pervasive metamorphic episode in the region known as the Prydz Tectonic Event. The rare earth element compositions of zircon and garnet are consistent with equilibrium growth of these minerals in the presence of melt. When comparing the thermometry methods used in this study, it is apparent that the Al‐in‐orthopyroxene thermobarometer provides the most reliable estimate of peak conditions. There is a strong textural correlation between the temperatures obtained using the Zr‐in‐rutile thermometer––maximum temperatures are recorded by a single rutile grain included within orthopyroxene, whereas other grains included in garnet, orthopyroxene, quartz, and biotite yield a range of temperatures down to 820°C. Ti‐in‐zircon thermometry returns significantly lower temperature estimates of 678–841°C. Estimates at the upper end of this range are consistent with growth of zircon from crystallizing melt at temperatures close to the elevated (H2O undersaturated) solidus. Those estimates, significantly lower than the calculated temperature of this residual solidus, may reflect isolation of rutile from the effective equilibration volume leading to an activity of TiO2 that is lower than the assumed value of unity.  相似文献   

2.
Dokukina  K. A.  Konilov  A. N.  Van  K. V.  Mints  M. V. 《Doklady Earth Sciences》2017,477(1):1353-1357

In the Salma eclogite of the Belomorian eclogite province, a dumortierite–phengite–corundum–bearing quartz–feldspar rock has been studied: its primary HP mineral paragenesis included garnet, phengite, and quartz. The phengite–quartz rocks were formed during dehydration and/or melting of boroncontaining rocks when they were dipped in the Meso- Neoarchaean subduction zone to a depth of not less than 70 km. As a result of the subsequent superimposed high-temperature metamorphic events under PT conditions of high-pressure granulite facies, the phengite in quartz underwent incongruent dehydration melting with formation of complex polymineral pseudomorphs, consisting of feldspars, biotite, newly formed muscovite, kyanite, corundum, and dumortierite. New estimates of the metamorphic temperature (850–900°C according to the melting reactions of phengite and the dumortierite field of stability; about 1000°C by the reintegrated composition of feldspar–mesoperthite) that affected the HP parageneses of Salma eclogitized rocks are at least 50–100°C (or even more) higher than them estimated earlier.

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3.
Oxygen isotope ratios of quartz inclusions (QI) within garnet from granulite and amphibolite facies gneisses in the Adirondack Mountains, NY were analysed and used to determine metamorphic temperatures. Primary QI for eight of 12 samples have δ18O values significantly lower than matrix quartz (MQ). The primary QI retain δ18O values representative of thermal conditions during garnet crystallization, whereas the δ18O values of MQ were raised by diffusive exchange with other matrix minerals (e.g. mica and feldspar) during cooling. The δ18O differences between QI and MQ show that garnet (a mineral with slow diffusion of oxygen) can armour QI from isotopic exchange with surrounding matrix, even during slow cooling. These differences between δ18O in MQ and QI can further be used to test cooling rates by Fast Grain Boundary diffusion modelling. Criteria for identifying QI that preserve primary compositions and are suitable for thermometry were developed based on comparative tests. Relations between δ18O and inclusion size, distance of inclusion to host–garnet rim, core–rim zonation of individual inclusions, and presence or absence of petrological features (healed cracks in QI, inclusions in contact with garnet cracks lined by secondary minerals, and secondary minerals along the inclusion grain boundary) were investigated. In this study, 61% of QI preserve primary δ18O and 39% were associated with features that were linked to reset δ18O values. If δ18O in garnet is homogeneous and inclusions are removed, laser‐fluorination δ18O values of bulk garnet are more precise, more accurate, and best for thermometry. Intragrain δ18O(Grt) profiles measured in situ by ion microprobe show no δ18O zonation. Almandine–rich garnet (Alm60–75) from each sample was measured by laser‐fluorination mass‐spectrometry (LF‐MS) for δ18O and compared with ion microprobe measurements of δ18O in QI for thermometry. The Δ18O(Qz–Grt) values for Adirondack samples range from 2.66 to 3.24‰, corresponding to temperatures of 640–740 °C (A[Qz–Alm] = 2.71). Out of 12 samples that were used for thermometry, nine are consistent with previous estimates of peak temperature (625–800 °C) based on petrological and carbon–isotope thermometry for regional granulite and upper amphibolite facies metamorphism. The three samples that disagree with independent thermometry for peak metamorphism are from the anorthosite–mangerite–charnockite–granite suite in the central Adirondacks and yield temperatures of 640–665 °C, ~100 °C lower than previous estimates. These low temperatures could be interpreted as thermal conditions during late (post‐peak) crystallization of garnet on the retrograde path.  相似文献   

4.
New data on the metamorphic petrology and zircon geochronology of high‐grade rocks in the central Mozambique Belt (MB) of Tanzania show that this part of the orogen consists of Archean and Palaeoproterozoic material that was structurally reworked during the Pan‐African event. The metamorphic rocks are characterized by a clockwise P–T path, followed by strong decompression, and the time of peak granulite facies metamorphism is similar to other granulite terranes in Tanzania. The predominant rock types are mafic to intermediate granulites, migmatites, granitoid orthogneisses and kyanite/sillimanite‐bearing metapelites. The meta‐granitoid rocks are of calc‐alkaline composition, range in age from late Archean to Neoproterozoic, and their protoliths were probably derived from magmatic arcs during collisional processes. Mafic to intermediate granulites consist of the mineral assemblage garnet–clinopyroxene–plagioclase–quartz–biotite–amphibole ± K‐feldspar ± orthopyroxene ± oxides. Metapelites are composed of garnet‐biotite‐plagioclase ± K‐feldspar ± kyanite/sillimanite ± oxides. Estimated values for peak granulite facies metamorphism are 12–13 kbar and 750–800 °C. Pressures of 5–8 kbar and temperatures of 550–700 °C characterize subsequent retrogression to amphibolite facies conditions. Evidence for a clockwise P–T path is provided by late growth of sillimanite after kyanite in metapelites. Zircon ages indicate that most of the central part of the MB in Tanzania consists of reworked ancient crust as shown by Archean (c. 2970–2500 Ma) and Palaeoproterozoic (c. 2124–1837 Ma) protolith ages. Metamorphic zircon from metapelites and granitoid orthogneisses yielded ages of c. 640 Ma which are considered to date peak regional granulite facies metamorphism during the Pan‐African orogenic event. However, the available zircon ages for the entire MB in East Africa and Madagascar also document that peak metamorphic conditions were reached at different times in different places. Large parts of the MB in central Tanzania consist of Archean and Palaeoproterozoic material that was reworked during the Pan‐African event and that may have been part of the Tanzania Craton and Usagaran domain farther to the west.  相似文献   

5.
New results on the pressure–temperature–time evolution, deduced from conventional geothermobarometry and in situ U‐Th‐total Pb dating of monazite, are presented for the Bemarivo Belt in northern Madagascar. The belt is subdivided into a northern part consisting of low‐grade metamorphic epicontinental series and a southern part made up of granulite facies metapelites. The prograde metamorphic stage of the latter unit is preserved by kyanite inclusions in garnet, which is in agreement with results of the garnet (core)‐alumosilicate‐quartz‐plagioclase (inclusions in garnet; GASP) equilibrium. The peak metamorphic stage is characterized by ultrahigh temperatures of ～900–950 °C and pressures of ～9 kbar, deduced from GASP equilibria and feldspar thermometry. In proximity to charnockite bodies, garnet‐sillimanite‐bearing metapelites contain aluminous orthopyroxene (max. 8.0 wt% Al2O3) pointing to even higher temperatures of ～970 °C. Peak metamorphism is followed by near‐isothermal decompression to pressures of 5–7 kbar and subsequent near‐isobaric cooling, which is demonstrated by the extensive late‐stage formation of cordierite around garnet. Internal textures and differences in chemistry of metapelitic monazite point to a polyphasic growth history. Monazite with magmatically zoned cores is rarely preserved, and gives an age of c. 737 ± 19 Ma, interpreted as the maximum age of sedimentation. Two metamorphic stages are dated: M1 monazite cores range from 563 ± 28 Ma to 532 ± 23 Ma, representing the collisional event, and M2 monazite rims (521 ± 25 Ma to 513 ± 14 Ma), interpreted as grown during peak metamorphic temperatures. These are among the youngest ages reported for high‐grade metamorphism in Madagascar, and are supposed to reflect the Pan‐African attachment of the Bemarivo Belt to the Gondwana supercontinent during its final amalgamation stage. In the course of this, the southern Bemarivo Belt was buried to a depth of >25 km. Approximately 25–30 Myr later, the rocks underwent heating, interpreted to be due to magmatic underplating, and uplift. Presumably, the northern part of the belt was also affected by this tectonism, but buried to a lower depth, and therefore metamorphosed to lower grades.  相似文献   

6.
Abstract Finite difference models of Fe-Mg diffusion in garnet undergoing cooling from metamorphic peak conditions are used to infer the significance of temperatures calculated using garnet-biotite Fe-Mg exchange thermometry. For rocks cooled from high grades where the garnet was initially homogeneous, the calculated temperature (Tcalc) using garnet core and matrix biotite depends on the size of the garnet, the ratio of garnet to biotite in the rock (Vgarnet/Vbiotite) and the cooling rate. For garnets with radii of 1 mm and Vgarnet/Vbiotite<1, Tcalc is 633, 700 and 777°C for cooling rates of 1, 10 and 100°C/Ma. For Vgarnet/Vbiotite= 1 and 4 and a cooling rate of 10° C/Ma, Tcalc is approximately 660 and 610° C, respectively. Smaller and larger garnets have lower and higher Tcalc, respectively. These results suggest that peak metamorphic temperatures may be reliably attained from rocks crystallized at conditions below Tcalc of the garnet core, provided that Vgarnet/Vbiotite is sufficiently small (<0.1) and that the composition of the biotite at the metamorphic peak has not been altered during cooling. Numerical experiments on amphibolite facies garnets with nominal peak temperatures of 550–600° C generate a ‘well’in Fe/(Fe + Mg) near the rim during cooling. Maximum calculated temperatures for the assemblage garnet + chlorite + biotite + muscovite + plagioclase + quartz using the Fe/(Fe + Mg) at the bottom of the ‘well’with matrix biotite range from 23–43° C to 5–12° C below the peak metamorphic temperature for cooling rates of 1 and 100° C/Ma, respectively. Maximum calculated temperatures for the assemblage garnet + staurolite + biotite + muscovite + plagioclase + quartz are approximately 70° C below the peak metamorphic temperature and are not strongly dependent on cooling rate. The results of this study indicate that it may be very difficult to calculate peak metamorphic temperatures using garnet-biotite Fe-Mg exchange thermometry on amphibolite facies rocks (Tmax > 550° C) because the rim composition of the garnet, which is required to calculate the peak temperature, is that most easily destroyed by diffusion.  相似文献   

7.
High‐pressure kyanite‐bearing felsic granulites in the Bashiwake area of the south Altyn Tagh (SAT) subduction–collision complex enclose mafic granulites and garnet peridotite‐hosted sapphirine‐bearing metabasites. The predominant felsic granulites are garnet + quartz + ternary feldspar (now perthite) rocks containing kyanite, plagioclase, biotite, rutile, spinel, corundum, and minor zircon and apatite. The quartz‐bearing mafic granulites contain a peak pressure assemblage of garnet + clinopyroxene + ternary feldspar (now mesoperthite) + quartz + rutile. The sapphirine‐bearing metabasites occur as mafic layers in garnet peridotite. Petrographical data suggest a peak assemblage of garnet + clinopyroxene + kyanite + rutile. Early kyanite is inferred from a symplectite of sapphirine + corundum + plagioclase ± spinel, interpreted to have formed during decompression. Garnet peridotite contains an assemblage of garnet + olivine + orthopyroxene + clinopyroxene. Thermobarometry indicates that all rock types experienced peak P–T conditions of 18.5–27.3 kbar and 870–1050 °C. A medium–high pressure granulite facies overprint (780–820 °C, 9.5–12 kbar) is defined by the formation of secondary clinopyroxene ± orthopyroxene + plagioclase at the expense of garnet and early clinopyroxene in the mafic granulites, as well as by growth of spinel and plagioclase at the expense of garnet and kyanite in the felsic granulite. SHRIMP II zircon U‐Pb geochronology yields ages of 493 ± 7 Ma (mean of 11) from the felsic granulite, 497 ± 11 Ma (mean of 11) from sapphirine‐bearing metabasite and 501 ± 16 Ma (mean of 10) from garnet peridotite. Rounded zircon morphology, cathodoluminescence (CL) sector zoning, and inclusions of peak metamorphic minerals indicate these ages reflect HP/HT metamorphism. Similar ages determined for eclogites from the western segment of the SAT suggest that the same continental subduction/collision event may be responsible for HP metamorphism in both areas.  相似文献   

8.
The granulites of the Fraser Range are assumed to have formed in a carbon-rich fluid, and are generally devoid of hornblende, and lack obvious hydrous retrograde features. In these granulites, pyroxene, garnet, plagioclase and quartz are the minerals most likely to retain the oxygen isotope ratios fixed at an early stage of initial granulite metamorphism. Temperature estimates using these minerals commonly suggest that oxygen isotopic exchange ceased in the range 600 to 680°C. The peak metamorphic temperature was probably ～ 850°C as based on the stability fields of the coexisting minerals and some cation temperatures from coexisting pyroxenes in these rocks. Ilmenite may be slightly out of isotopic equilibrium with the other minerals. Thus, grains of quartz, feldspar, pyroxene and ilmenite have suffered considerable oxygen isotopic exchange during the retrogressive phase of the metamorphism, in spite of the fact that very little water was present in these granulites. The observed deviation from the peak metamorphic temperatures can be explained by essentially closed system solid-state diffusion (on at least a scale of centimetres) during slow cooling of the rocks from ~850 to 650°C, followed by more rapid cooling down to ～ 300°C. Such an explanation is not at variance with the radiometric data available for rocks from the area, which suggest that the latter phase could have involved uplift rates of ?0.5 mm/yr for a period of about 40 Ma. Wholerock δ18O values on non-quartzose mafic granulites, about 7.2%., fall within the range of basalts affected by seafloor weathering.  相似文献   

9.
ABSTRACT The high-grade rocks (metapelite, quartzite, metagabbro) of the Hisøy-Torungen area represent the south-westernmost exposures of granulites in the Proterozoic Bamble sector, south Norway. The area is isoclinally folded and a metamorphic P–T–t path through four successive stages (M1-M4) is recognized. Petrological evidence for a prograde metamorphic event (M1) is obtained from relict staurolite + chlorite + albite, staurolite + hercynite + ilmenite, cordierite + sillimanite, fine-grained felsic material + quartz and hercynite + biotite ± sillimanite within metapelitic garnet. The phase relations are consistent with a pressure of 3.6 ± 0.5 kbar and temperatures up to 750–850°C. M1 is connected to the thermal effect of the gabbroic intrusions prior to the main (M2) Sveconorwegian granulite facies metamorphism. The main M2 granulite facies mineral assemblages (quartz+ plagioclase + K-feldspar + garnet + biotite ± sillimanite) are best preserved in the several-metre-wide Al-rich metapelites, which represent conditions of 5.9–9.1 kbar and 790–884°C. These P–T conditions are consistent with a temperature increase of 80–100°C relative to the adjacent amphibolite facies terranes. No accompanying pressure variations are recorded. Up to 1-mm-wide fine-grained felsic veinlets appear in several units and represent remnants of a former melt formed by the reaction: Bt + Sil + Qtz→Grt + lq. This dehydration reaction, together with the absence of large-scale migmatites in the area, suggests a very reduced water activity in the rocks and XH2O = 0.25 in the C–O–H fluid system was calculated for a metapelitic unit. A low but variable water activity can best explain the presence or absence of fine-grained felsic material representing a former melt in the different granulitic metapelites. The strongly peraluminous composition of the felsic veinlets is due to the reaction: Grt +former melt ± Sil→Crd + Bt ± Qtz + H2O, which has given poorly crystalline cordierite aggregates intergrown with well-crystalline biotite. The cordierite- and biotite-producing reaction constrains a steep first-stage retrograde (relative to M2) uplift path. Decimetre- to metre-wide, strongly banded metapelites (quartz + plagioclase + biotite + garnet ± sillimanite) inter-layered with quartzites are retrograded to (M3) amphibolite facies assemblages. A P–T estimate of 1.7–5.6 kbar, 516–581°C is obtained from geothermobarometry based on rim-rim analyses of garnet–biotite–plagioclase–sillimanite–quartz assemblages, and can be related to the isoclinal folding of the rocks. M4 greenschist facies conditions are most extensively developed in millimetre-wide chlorite-rich, calcite-bearing veins cutting the foliation.  相似文献   

10.
The East Hebei terrane of North China Craton is characterized by the dome-and-keel structure, a common feature in most Archean cratons, where supracrustal rocks of granulite facies commonly occur as enclaves or rafts in tonalite–trondhjemite–granodiorite (TTG) gneisses. The metamorphic P–T paths of the granulites are significant for addressing the Archean tectonic regimes. Two types of granulite facies paragneiss with pelitic and greywacke compositions from the western margin of Qian'an gneiss dome are documented for their petrography, mineral chemistry, phase equilibria modelling using thermocalc, and zircon dating. Anticlockwise P–T paths involving the pre-peak pressure increase to the ultra-high temperature peak conditions and post-peak cooling and decompression processes were recognized. The pre-peak pressure increase process was constrained for a pelitic granulite mainly based on the spinel and cordierite inclusions in garnet and rutile corona around ilmenite, where the transition from spinel to garnet is modelled at 6–7 kbar at a fixed T = 1,000°C. For greywacke granulite, the pre-peak pressure increase evolution can be ascertained from the textural relation that orthopyroxene is surrounded by garnet, and the outwards increasing grossular (from 0.03 to 0.05) in the core of the atoll-like garnet (Grt-A), to occur from ~7 kbar at ~1,000°C. The peak P–T conditions for pelitic granulite are roughly limited to 7–11 kbar/890–1,050°C on the basis of the stability of the inferred peak assemblage involving garnet, perthite, sillimanite, rutile/ilmenite, and quartz. The peak P–T condition for greywacke granulite can be well constrained as 9–10 kbar/>1,000°C on the basis of the maximum grossular content (XGrs = 0.045–0.050) in the core of subhedral garnet (Grt-B) and the mantle of Grt-A together with an average re-integrated anorthite content (XAn = 0.07) in K-feldspar. The peak temperature condition is consistent with the ternary feldspar thermometer results mostly of 950–1,020°C for antiperthite and perthite in greywacke granulite, and in accordance with the development of oriented needle-like exsolution of Ti±Fe oxides in garnet from pelitic granulite. The post-peak cooling and decompression process was consistent with the decreasing XGrs in the mantle of Grt-A and core of Grt-B in greywacke sample, and the final-stage cooling conditions can be well constrained from the stability of final assemblages marked by the later growth of biotite, as 8–9 kbar/820–880°C for pelitic granulite and 6–9 kbar/840–890°C for greywacke granulite. Zircon dating yields provenance ages from 3.34 to 2.57 Ga and metamorphic ages of c. 2.50 Ga for the two types of granulite. The metamorphic ages overlap the final pulse of the Neoarchean magmatic activity of TTGs that ranges from c. 2.56 to c. 2.48 Ga with a peak at c. 2.52 Ga. Combining the development of dome-and-keel structures, the penecontemporaneity between the metamorphism of supracrustal rocks and TTG magmatic activity, and also the unique anticlockwise P–T paths, we prefer a vertical sagduction regime to interpret the tectonic evolution of the East Hebei terrane, which may be also significant for other Archean cratons.  相似文献   

11.
Mineralogical and mineral chemical evidence for prograde metamorphism is rarely preserved in rocks that have reached ultrahigh‐temperature (UHT) conditions (>900 °C) because high diffusion and reaction rates erase evidence for earlier assemblages. The UHT, high‐pressure (HP) metasedimentary rocks of the Leverburgh belt of South Harris, Scotland, are unusual in that evidence for the prograde history is preserved, despite having reached temperatures of ～955 °C or more. Two lithologies from the belt are investigated here and quantitatively modelled in the system NaO–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O: a garnet‐kyanite‐K‐feldspar‐quartz gneiss (XMg = 37, A/AFM = 0.41), and an orthopyroxene‐garnet‐kyanite‐K‐feldspar quartzite (XMg = 89 A/AFM = 0.68). The garnet‐kyanite gneiss contains garnet porphyroblasts that grew on the prograde path, and captured inclusion assemblages of biotite, sillimanite, plagioclase and quartz (<790 °C, <9.5 kbar). These porphyroblasts preserve spectacular calcium zonation features with an early growth pattern overgrown by high‐Ca rims formed during high‐P metamorphism in the kyanite stability field. In contrast, Fe‐Mg zonation in the same garnet porphyroblasts reflects retrograde re‐equilibration, as a result of the relatively faster diffusivity of these ions. Peak P–T are constrained by the occurrence of coexisting orthopyroxene and aluminosilicate in the quartzite. Orthopyroxene porphyroblasts [y(opx) = 0.17–0.22] contain sillimanite inclusions, indicative of maximum conditions of 955 ± 45 °C at 10.0 ± 1.5 kbar. Subsequently, orthopyroxene, kyanite, K‐feldspar and quartz developed in equilibrated textures, constraining the maximum pressure conditions to 12.5 ± 0.8 kbar at 905 ± 25 °C. P–T–X modelling reveals that the mineral assemblage orthopyroxene‐kyanite‐quartz is compositionally restricted to rocks of XMg > 84, consistent with its very rare occurrence in nature. The preservation of unusual high P–T mineral assemblages and chemical disequilibrium features in these UHT HP rocks is attributed to a rapid tectonometamorphic cycle involving arc subduction and terminating in exhumation.  相似文献   

12.
The metamorphic evolution of rocks cropping out near Stoer, within the Assynt terrane of the central region of the mainland Lewisian complex of NW Scotland, is investigated using phase equilibria modelling in the NCKFMASHTO and MnNCKFMASHTO model systems. The focus is on the Cnoc an t’Sidhean suite, garnet‐bearing biotite‐rich rocks (brown gneiss) with rare layers of white mica gneiss, which have been interpreted as sedimentary in origin. The results show that these rocks are polymetamorphic and experienced granulite facies peak metamorphism (Badcallian) followed by retrograde fluid‐driven metamorphism (Inverian) under amphibolite facies conditions. The brown gneisses are inferred to have contained an essentially anhydrous granulite facies peak metamorphic assemblage of garnet, quartz, plagioclase and ilmenite (±rutile, K‐feldspar and pyroxene) with biotite, hornblende, muscovite, chlorite and/or epidote as hydrous retrograde minerals. P–T constraints imposed by phase equilibria modelling imply conditions of 13–16 kbar at >900 °C for the Badcallian granulite facies metamorphic peak, consistent with the field evidence for partial melting in most lithologies. The white mica gneiss comprises a muscovite‐dominated matrix containing porphyroblasts of staurolite, corundum, kyanite and rare garnet. Previous studies have suggested that staurolite, corundum, kyanite and muscovite all grew at the granulite facies peak, with partial melting and melt loss producing a highly aluminous residue. However, at the inferred peak P–T conditions, staurolite and muscovite are not predicted to be stable, suggesting they are retrograde phases that grew during amphibolite facies retrograde metamorphism. The large proportion of mica suggests extensive H2O‐rich fluid‐influx, consistent with the retrograde growth of hornblende, biotite, epidote and chlorite in the brown gneisses. P–T conditions of 5.0–6.5 kbar at 520–550 °C are derived for the Inverian event. In situ dating of zircon from samples of the white mica gneiss yield apparent ages that are difficult to interpret. However, the data are permissive of granulite facies (Badcallian) metamorphism having occurred at c. 2.7–2.8 Ga with subsequent fluid driven (Inverian) retrogression at c. 2.5–2.6 Ga, consistent with previous interpretations.  相似文献   

13.
Petrology of high-pressure granulites from the eastern Himalayan syntaxis   总被引：36，自引：0，他引：36  
The eastern Himalayan syntaxis, situated at the eastern terminus of the Himalayas, is the least-known segment of the Himalayas. Recent research in this area has revealed that the syntaxis consists of the Gangdise, the Yarlung Zangbo, and the Himalayan units, each of which is bounded by faults. The Himalayan unit, the northernmost exposed part of the Indian plate, mainly contains amphibolite facies rocks, marked by the assemblages staurolite+kyanite+plagioclase+biotite+muscovite±sillimanite and garnet+amphibole+plagioclase, in the south; to the north, low- to medium-pressure granulite grade pelitic gneisses and marbles are present and are characterized by the assemblages garnet+sillimanite+K-feldspar+plagioclase or antiperthite+biotite+quartz±spinel±cordierite±orthopyroxene in gneisses, and anorthite+diopside±wollastonite and plagioclase+diopside+quartz+phlogopite+calcite in marbles. Within this unit, the Namula thrust system is a series of moderately north-dipping structures that displaced the granulite facies rocks southwards over the amphibolite facies rocks. High-pressure granulites occur as relics within these granulite facies rocks and contain garnet–kyanite granulite and garnet clinopyroxenite. The peak assemblage of the garnet–kyanite granulite includes garnet (core part)+kyanite+ternary feldspar+quartz+rutile. Sillimanite+garnet (rim part)+K-feldspar+ oligoclase+ilmenite+biotite and spinel+albite+biotite or spinel+cordierite±orthopyroxene, which are coronas around sillimanite and garnet, are retrograde products of this peak assemblage. Another peak assemblage includes very-high-Ca garnet (CaO 32–34 wt%, Alm10±Grs>80) and diopside (CaO 22–24 wt%), scapolite, meionite, quartz, and accessory Al-bearing titanite (Al2O3 4–4.5 wt%). The diopside has kink bands. Partial or complete breakdown of Ca-rich garnet during post-peak metamorphism produced pseudomorphs and coronas consisting of fine-grained symplectic intergrowths of hedenbergite and anorthite. Thermobarometric estimates in combination with reaction textures, mineral compositions, and recent experimental studies indicate that these peak assemblages were formed at P=c. 1.7–1.8 GPa, T =c. 890 °C, and the retrograde assemblages experienced near-isothermal decompression to P=0.5±0.1 GPa, T =850±50 °C. The whole-rock compositions indicate that marble and pelite are plausible candidates for the protoliths. These facts suggest the following (1) sedimentary rocks were transported to upper-mantle depths and equilibrated at those conditions to form these high-pressure granulites, which were then emplaced into the crust quickly. During the rapid exhumation of these rocks, the earlier high-pressure assemblages were overprinted by the later low- to medium-pressure assemblages, that is, the high-pressure granulite belt formed in the syntaxis. (2) The Namula thrust system is an important tectonic boundary in the syntaxis, or even in the Higher Himalaya more generally.  相似文献   

14.
Abstract ‘Peak’metamorphic carbon isotope fractionations between calcite and graphite (ΔCal–Gr) in marbles and calc-silicates from the Cucamonga granulite terrane (San Gabriel Mountains, California) range from 3.48 to 2.90%. The data are used to test three previously published calibrations of the calcite–graphite carbon isotope thermometer. An empirical calibration of the calcite–graphite carbon isotope thermometer gives temperatures of 700–750°C; a theoretical–experimental calibration of the system gives temperatures of 760°–870°C; an experimental calibration gives temperatures of 870–1300°C. Temperatures calculated using the empirical calibration are in agreement with those calculated from garnet-based cation exchange thermometry when uncertainty is considered. Temperatures calculated using the theoretical–experimental calibration overlap the upper range of cation exchange thermometry temperatures and range to 50°C higher. The experimental calibration yields temperatures from 50 to 480°C higher than those from cation exchange thermometry. Moreover, temperatures from the experimental calibration are also inconsistent with mineral and melt equilibria in the granulite phase assemblage. Despite the better agreement between cation exchange thermometry and the empirical calibration of the calcite–graphite system, temperatures calculated using the theoretical–experimental calibration may be real peak metamorphic temperatures. If retrograde diffusion partially reset garnet-based cation exchange thermometers by c. 50°C, then the cation exchange temperatures are consistent with those from the theoretical–empirical calibration. Thermometric evidence from biotite dehydration melting equilibria is consistent with either the empirical calibration if melting was fluid-present, or the theoretical–experimental calibration if melting was fluid-absent.  相似文献   

15.
The Motuo area is located in the east of the Eastern Himalayan Syntaxis. There outcrops a sequence of high-grade metamorphic rocks, such as metapelites. Petrology and mineralogy data suggest that these rocks have experienced three stages of metamorphism. The prograde metamorphic mineral assemblages(M1) are mineral inclusions(biotite + plagioclase + quartz ± sillimanite ± Fe-Ti oxides) preserved in garnet porphyroblasts, and the peak metamorphic assemblages(M2) are represented by garnet with the lowest XSps values and the lowest XFe# ratios and the matrix minerals(plagioclase + quartz ± Kfeldspar + biotite + muscovite + kyanite ± sillimanite), whereas the retrograde assemblages(M3) are composed of biotite + plagioclase + quartz symplectites rimming the garnet porphyroblasts. Thermobarometric computation shows that the metamorphic conditions are 562–714°C at 7.3–7.4 kbar for the M1 stage, 661–800°C at 9.4–11.6 kbar for the M2 stage, and 579–713°C at 5.5–6.6 kbar for the M3 stage. These rocks are deciphered to have undergone metamorphism characterized by clockwise P-T paths involving nearly isothermal decompression(ITD) segments, which is inferred to be related to the collision of the India and Eurasia plates.  相似文献   

16.
The assumption of oxygen isotope and major element equilibrium during prograde metamorphism was tested using staurolite‐grade pelitic schists that have undergone sequential porphyroblast growth and multiple episodes of recrystallization of matrix minerals and foliation development. Textural relationships are used to infer a metamorphic history that involves garnet growth followed by staurolite growth, with each porphyroblast growth event followed by at least one period of recrystallization of matrix minerals. Conventional geothermobarometry using Qtz–Grt–Pl–Ms–Bt ± St equilibria yields peak P–T conditions of c. 625 °C at 9–11 kbar, consistent with KMnFMASH petrogenetic grid predictions for stability of the assemblage Grt + St + Bt. Qtz–Grt oxygen isotope fractionations yield apparent temperatures of c. 590 °C and Qtz–St fractionations yield an apparent temperature of c. 595 °C. Diffusional modelling indicates that quartz isotopic compositions were reset by c. 30 °C via retrograde isotopic diffusional exchange with micas. The isotopic temperatures appear to be in excellent agreement with one another, and suggest oxygen isotope equilibrium was attained between garnet and staurolite at c. 625 °C. However, the agreement of Qtz–Grt and Qtz–Str isotopic temperatures is not consistent with petrographic observations (garnet grew before staurolite) and petrogenetic grid constraints that predict that garnet grows over a temperature interval of c. 525–550 °C. Given that: (i) oxygen diffusion rates in staurolite and garnet are slow enough to render an individual porphyroblast effectively closed to exchange after it forms; and (ii) matrix minerals are able to exchange isotopes via recrystallization during each period of deformation; garnet and staurolite could not have simultaneously achieved oxygen isotope equilibrium with each other or with minerals in the recrystallized matrix. Thus, the Qtz–Grt fractionations, which yield apparent temperatures that are in apparent agreement with peak metamorphic temperature and apparent temperatures for Qtz–St fractionations, cannot be fractionations resulting from equilibrium isotopic exchange. Instead, they are apparent fractionations between porphyroblasts formed at different temperature and times in the prograde P–T–D path, and quartz that recrystallized and exchanged with micas and plagioclase during several phases of deformation.  相似文献   

17.
Oxygen isotope analyses of quartz-Al2SiO5 pairs have been made for samples from the Mica Creek area, British Columbia. We have analysed quartz–kyanite nodules and quartz–kyanite and quartz–sillimanite in multiphase pelitic rocks from the staurolite–kyanite, kyanite, and sillimanite zones. Apparent temperatures calculated from oxygen isotopic fractionation range from 555 °C (staurolite–kyanite zone) to 695 °C (sillimanite zone). Temperatures from the quartz–kyanite nodules range from 630 to 675 °C. Some of the nodules show isotopic disequilibrium. Most of the results confirm predictions that bimineralic rocks will yield an estimate of peak metamorphic temperatures, when the less abundant mineral (an aluminium silicate) is the slower oxygen diffuser. Using cooling rates of 10–100 °C Ma?1 for the multiphase rocks, measured crystal sizes and modes, the Fast Grain Boundary diffusion model with ‘wet’ diffusion data (PH2O?1.0 kbar) yields predicted apparent temperatures which are generally lower than the measured apparent temperatures. The agreement is improved if slower diffusion coefficients are used. This suggests that f (H2O) during cooling was lower than that of the hydrothermal experiments and thus that there was little interaction with aqueous fluids of internal or external origin to modify the isotopic compositions. The measured apparent isotopic temperatures and apparent garnet–biotite Fe–Mg exchange temperatures show very poor agreement for the sillimanite zone samples, with the garnet–biotite Fe–Mg exchange temperatures generally higher than the oxygen isotope temperatures. Compared with the other calibrations that we tested the measured apparent temperatures using the Sharp calibration show the best agreeement with recently published P–T grids, although some variability in agreement is expected due to variable f (H2O) during cooling.  相似文献   

18.
The St. Leonhard Granulite Massif, Lower Austria, is one of the small occurrences of high-pressure granulite found in the Gföhl unit at the highest tectono-stratigraphic level of the Moldanubian zone. Although predominantly composed of extremely deformed acidic, garnet+kyanite-bearing rocks, thin conformable layers of intermediate garnet+clinopyroxene-bearing granulites are seen. Pressure-temperature estimates for the peak metamorphic assemblage of garnet+clinopyroxene+ternary feldspar+quartz in these rocks are 15-19 kbar, 950-1050°C. A close coherence between results obtained from a combination of independent geothermobarometers and those derived from an internally consistent thermobarometric method indicate the retention of high-pressure/temperature equilibrium mineral compositions, even though there is a wealth of petrographic evidence for significant post-peak metamorphic decompression. Pressure-temperature estimates for the orthopyroxene-bearing, intermediate-pressure decompression stage, obtained from discrete reaction textures, are 8-12 kbar and 800-900°C. Post-decompressive cooling from 800 to 500°C, at ca. 5-8 kbar, is recorded by the final amphibolite-facies, biotite-bearing assemblage, together with petrological constraints from the enclosing acid granulites.  相似文献   

19.
Garnet–clinopyroxene intermediate granulites occur as thin layers within garnet–kyanite–K–feldspar felsic granulites of the St. Leonhard granulite body in the Bohemian Massif. They consist of several domains. One domain consists of coarser‐grained coexisting ternary feldspar, clinopyroxene, garnet, quartz and accessory rutile and zircon. The garnet has 16–20% grossular, and the clinopyroxene has 9% jadeite and contains orthopyroxene exsolution lamellae. Reintegrated ternary feldspar and the Zr‐in‐rutile thermometer give temperatures higher than 950 °C. Mineral equilibria modelling suggests crystallization at 14 kbar. The occurrence and preservation of this mineral assemblage is consistent with crystallization from hot dry melt. Between these domains is a finer‐grained deformed matrix made up of diopsidic clinopyroxene, orthopyroxene, plagioclase and K‐feldspar, apparently produced by reworking of the coarser‐grained domains. Embedded in this matrix, and pre‐dating the reworking deformation, are garnet porphyroblasts that contain clinopyroxene, feldspar, quartz, rutile and zircon inclusions. In contrast with the garnet in the coarser‐grained domains, the garnet generally has >30% grossular, the included clinopyroxene has 7–27% jadeite and the Zr content of rutile indicates much lower temperatures. Some of these high‐grossular garnet show zoning in Fe/(Fe + Mg), decreasing from 0.7 in the core to 0.6 and then increasing to 0.7 at the rim. These garnet are enigmatic, but with reference to appropriate pseudosections are consistent with localized new mineral growth from 650 to 850 °C and 10 to 17 kbar, or with equilibration at 20 kbar and 770 °C, modified by two‐stage diffusional re‐equilibration of rims, at 10–15 and 8 kbar. The strong pervasive deformation has obscured relationships that might have aided the interpretation of the origin of these porphyroblasts. The evolution of these rocks is consistent with formation by igneous crystallization and subsequent metamorphism to high‐T and high‐P, rather than an origin by ultrahigh‐T metamorphism. Regarding the petrographic complexity, combination of the high grossular garnet with the ternary feldspar to infer ultrahigh‐T metamorphism at high pressure is not justified.  相似文献   

20.
A detailed investigation of the compositional variation in garnet has been undertaken in a garnet–pyroxene‐bearing granulite from the high‐grade Gföhl Unit, Moldanubian Zone, Lower Austria. Textural observations, together with the interpretation of the preserved garnet chemistry, enables the recognition of both prograde core and peak metamorphic garnet mantle growth stages, an extremely rare feature in high‐P–T granulite facies rocks. Initial thermobarometric calculations undertaken across whole garnet zoning profiles show how correct interpretation of a zoning profile is essential if the maximum peak metamorphic P–T conditions are to be recovered. The effect of retrograde decompression‐ and cooling‐driven reactions on inclusion and host garnet compositions has also been assessed. The results indicate that caution should be exercised when utilizing inclusion and adjacent garnet compositions for the thermobarometric evaluation of peak metamorphic equilibration conditions. Peak P–T conditions were determined by the TWEEQU thermobarometric method, utilizing the core compositions of matrix phases combined with the interpreted high‐P–T garnet mantle composition, to give 15.6 kbar and 1090 °C, consistent with previously determined results for Moldanubian granulites. Similar high‐P–T estimates are also provided by a re‐evaluation of previously published results for a granulite sample from the same lithological unit, using a modified interpretation of garnet and plagioclase compositional data. The new estimates presented confirm the previously disputed idea that the Gföhl Unit underwent a high‐pressure granulite facies stage and is therefore distinctly different from the underlying tectonostratigraphic units. It is emphasized that any interpretation of the peak metamorphic conditions in high‐grade rocks must be based on detailed petrographic observations combined with a thorough understanding of the co‐existing equilibrium mineral compositions.  相似文献   

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