Symplectite Assemblages Research Articles

Petrological implications of element redistribution during metamorphism: insights from meta-granite of the South Delhi Fold Belt, Rajasthan, India

AbstractMeta-granites of the South Delhi Fold Belt, northwestern India, contain spectacular reaction textures formed during the metamorphic replacement of primary minerals. Textural relationships imply that amphibole was replaced sequentially in two stages. Epidote + titanite + quartz symplectite formed syn-tectonically on amphibole grain boundaries/fractures, followed by post-deformational growth of euhedral garnet overprinting amphibole grains. Besides occurring as symplectite grown during deformation, titanite in this rock also developed as a post-tectonic corona around magnetite. Parent magnetite contains exsolutions of ilmenite and/or ultrafine lamellae of Ti-rich oxide (Ti-oxd). Textures involving coronal titanite suggest their formation through a magnetite + ilmenite(/Ti-oxd) + plagioclase → titanite reaction. Compositional attributes and the calculation of the gain versus loss of components during the reaction suggest that the Mn2+for garnet (XSpss= 0.23–0.29) that grew replacing amphibole was supplied by ilmenite (Mn2+is 0.118–0.128 apfu) as it disintegrated to form coronal titanite. The redistribution of components between the metamorphic reaction sites connects the texturally unrelated domains and suggests that these zones were in chemical equilibrium during metamorphism. We estimated theP–Tconditions of metamorphism for these post-tectonic assemblages as ∼650–700 °C from pseudosection modelling and conventional thermometry. Zircon data from this study suggest that the granitic rock crystallized at 988.8 ± 8.8 Ma. We propose that the metamorphic phases replaced the primary minerals during the mid Neoproterozoic tectonic activity reported from this terrane. The syn-tectonic symplectitic assemblage formed as the temperature increased during prograde metamorphism, and the post-tectonic minerals developed at peak conditions following the cessation of deformation.

Paleoproterozoic oceanic subduction in the North China Craton: Insights from the metamorphic P–T–t paths of the Chicheng Mélange in the Hongqiyingzi Complex

How and when modern-style plate tectonics started in the Earth’s history is one of the most important scientific issues. The Chicheng mélange in the north margin of North China Craton (NCC) presents a new insight into the Paleoproterozoic cold oceanic subduction and the beginning of modern-style plate tectonic regime. The mélange comprises blocks of retrograde eclogites, amphibolites and peridotites in a matrix of metasediments. The retrograde eclogites mainly contain garnet, amphibole and symplectites consisting of fine-grained plagioclase, clinopyroxene and amphibole. Three metamorphic stages were derived, involving the prograde, peak, and post-peak decompression stages. The P–T conditions for the prograde and peak stages range from ~17 kbar/660 °C to >22/~750 °C kbar based on mineral composition isopleths in P–T pseudosections, involving core–to–mantle increasing pyrope and decreasing grossular in garnet and Ti content in amphibole. The post-peak decompression is proposed to be isothermal under melt (fluid)-absent conditions, recorded a pressure of 8–11 kbar on the basis of symplectite assemblages. For the amphibolites pre-peak heating, peak temperature, and post-peak cooling metamorphic stages were derived. They experienced anataxis during this overprinting metamorphism when heated to above the solidus. The peak temperature conditions are constrained to be 5–7 kbar/780–830 °C on the basis of the contours of the maximum Ti in amphibole and anorthite of plagioclase in P–T pseudosections. Zircon U–Pb dating for the selected samples and a summary of published age data reveal that the eclogite facies metamorphism may have occurred at 1.85–1.80 Ga, and the eclogitic rocks exhumed to a lower crust level during or after subduction to be overprinted by multi-phase thermal events at ~1.66 Ga and 460–420 Ma with undefined conditions, and at 360–270 Ma under amphibolite facies. The eclogite facies metamorphism from the Chicheng mélange shows a geothermal gradient of ~9 °C/km, pointing to a cold oceanic subduction at late Paleoproterozoic that occurred along the northern margin of the NCC. The overprinting metamorphism with anataxis recognized from the amphibolites represents a low-pressure type with a geothermal gradient of ~35 °C/km, which was interpreted to occur at 360–270 Ma governed by extension.

New Occurrence of Sapphirine–spinel–bearing Granulite from NW of Chilka Lake, Eastern Ghats Belt, Odisha

Abstract A new locality of sapphirine-spinel-bearing granulites from the Kaithapalli area which lies NW of Chilka Lake, Odisha is reported. The area tectonically forms a northern part of Eastern Ghats belt. It occurs as small enclaves within the khondalite and pelitic granulite. The mineral assemblage includes spinel-sapphirine-garnet-cordierite-orthopyroxene-sillimanite-biotite-k-feldspar-plagioclase-quartz. Development of reaction textures and symplectites are common in the sapphirine-spinel granulites which have been used to describe reaction history. The relative XMg values among various minerals are as follows: cordierite > biotite > sapphirine > orthopyroxene > garnet > spinel. The P-T evolution of these sapphirine-spinel granulites constrained through the pseudosection modelling in the NCKFMASH model system using Perple_X software indicate conditions of ultra-high temperature (UHT) metamorphism. The P-T estimates computed by isopleths define a retrograde trajectory with decompression of c.2.5 kbar from P-Tmax of c.10.5 kbar at c.950 °C. The sequence of reactions as deduced from the symplectite assemblages, together with pseudosection modelling, from the Kaithapalli area, offer greater opportunities for providing a better picture of petrological evolution of northern part of the Eastern Ghats Belt (EGB).

Metamorphic Evolution and Zircon U-Pb Ages of the Nanshankou Mafic High Pressure Granulites from the Jiaobei Terrane, North China Craton

Petrological analysis and LA-ICP-MS zircon U-Pb dating were conducted on high-pressure mafic granulites, which occured as xenolith within TTG gneisses, from the Nanshankou Village of the Jiaobei terrane, Shandong Peninsula in the north-eastern part of the North China Craton (NCC). The mafic HP granulite is composed of garnet, clinopyroxene, orthopyroxene, amphibole and symplectitic clinopyroxene, orthopyroxene, plagioclase, ilmente and magnetite which were formed after the decomposition of porphyroblastic garnet and clinopyroxene. Four stages of metamorphic mineral assemblages for the mafic HP granulites were constrained by detail petrological and mineralogical investigations. The early prograde assemblage is represented by the mineral inclusions within garnet and clinopyroxene porphyroblasts (Opx1+Pl1+Qtz1), recording the metamorphic conditions at ~754–757 °C, 0.63–0.71 GPa; peak metamorphic conditions were determined at ~874–891 °C, 1.32–1.35 GPa with the mineral assemblage of Grt2+Cpx2+Amp2+Pl2+Qtz2. Retrograde minerals derived from symplectitic assemblage Opx3+Cpx3+Amp3+Pl3+Qtz3+Ilm3±Mag3 were formed at 693–796 °C, 0.60–0.84 GPa. A final greenschist to sub-greenschist facies event was recorded by the exsolution of actinolite and albite within a retrograded clinopyroxene, as well as the occurrence of prehnite, chlorite and calcite minerals. Accordingly, a clockwise P-T path was concluded on the basis of the different stages of mineral asseblage. Cathodoluminescence imaging, trace element and U-Pb dating of zircons from the mafic HP granulites recorded similar charactistics for three episodes of Paleo–Meso Proterozoic metamorphic events. These are the metamorphic events preserved in mafic and pelitic granulites in the Jiao-Liao-Ji belt (JLJB) with 207Pb/206Pb ages of 2.0‒1.9 Ga for peak metamorphism and of 1.86‒1.84 Ga for decomposing process, followed by a retrograde amphibolite facies metamorphic event related to the post-orogenic extension at the age of 1.76‒1.74 Ga, resulting the exhumation of the granulite to the upper crust level.

Neoarchean UHT Metamorphism and Paleoproterozoic UHT Reworking at Uweinat in the East Sahara Ghost Craton, SW Egypt: Evidence from Petrology and Texturally Controlledin situMonazite Dating

The Uweinat-Kamil basement complex, in the central part of the East Sahara Ghost Craton (ESGC) in NE Africa, is the only inlier in the craton that contains rocks with Archean formation ages, and is hence a key to understanding the ancient crustal evolution of the otherwise enigmatic and poorly known ESGC. The craton is thought to have been decratonized during the Neoproterozoic after thickening as a result of Pan-African collisional events along its margins. Textural and compositional relationships preserved in the metapelitic granulites from the Uweinat-Kamil inlier suggest a two-stage metamorphic evolution of the rocks. Stage I saw the growth of sapphirine + quartz + garnet 1 at similar to 10 kbar and similar to 1050 degrees C, from an initial assemblage containing kyanite, sillimanite +/- biotite, which are now preserved only as inclusions in porphyroblastic garnet 1. This stage was followed by near-isobaric cooling stabilizing the assemblage garnet 1 + sillimanite 1 +/- orthopyroxene 1 +/- sapphirine coexisting with melt. Stage II saw the breakdown of this assemblage forming a variety of symplectite assemblages (orthopyroxene 2 + cordierite or orthopyroxene 2 + cordierite + sapphirine +/- sillimanite 2 or cordierite + spinel) through near-isothermal decompression from similar to 9 kbar to similar to 6 kbar at ultrahigh temperatures of 900-1000 degrees C. This was followed by near-isobaric cooling of the rocks to temperatures of 700 degrees C at similar to 5.5-6 kbar, as evidenced by the growth of garnet 2 and the formation of late-stage biotite, owing to back-reaction of melt with residual garnet 1 and symplectite minerals. The second stage of the evolution is also observed in the associated metabasic granulites. Complete to partial replacement of garnet 1 porphyroblasts and clinopyroxene by orthopyroxene + plagioclase + hornblende +/- spinel symplectites represents a stage of near-isothermal decompression, whereas the growth of garnet 2 around the symplectitic minerals represents a stage of isobaric cooling. Texturally controlled in situ Th-U-total Pb monazite dating of the metapelitic granulites reveals the polymetamorphic nature of the rocks. Stage I occurred at similar to 2.6 Ga, as indicated by monazite inclusions within garnet 1 porphyroblasts (coexisting with sapphirine + quartz), and represents a previously unknown Neoarchean ultrahigh-temperature metamorphism. Stage II occurred 700 Myr later at similar to 1.9 Ga, as indicated by monazite grains in the symplectites and matrix, and represents a previously uncharacterized Paleoproterozoic ultrahigh-temperature isothermal decompression event. No evidence of any metamorphism during the Neoproterozoic has been found. In this context, it is possible that the proposed decratonization occurred during a Paleoproterozoic decompression event instead of in the Neoproterozoic. The Paleoproterozoic evolution of the Uweinat-Kamil inlier is very similar to that described from other Paleoproterozoic orogenies across the world, which are considered to have resulted from collisions during the formation of the supercontinent Columbia. Future attempts at Columbia reconstructions should take into account the Paleoproterozoic evolution of the Uweinat-Kamil inlier in NE Africa.

Metamorphic evolution and Zircon ages of Garnet–orthoamphibole rocks in southern Hengshan, North China Craton: Insights into the regional Paleoproterozoic P–T–t history

Garnet–orthoamphibole rock (GOR) from southern Hengshan of the North China Craton (NCC) shows a prominent porphyroblastic texture involving garnet, orthoamphibole, kyanite, sillimanite, chlorite, rutile and quartz, with or without talc, hornblende and staurolite. Garnet develops coronae or symplectite assemblages of cordierite, anthophyllite, ilmenite and quartz. The P–T and P–H2O evolution for three representative samples were determined by using pseudosections calculated with THERMOCALC. Generalized prograde paths are inferred from subtle garnet growth zoning, chlorite compositions and inclusion assemblages in garnet. The peak mineral assemblages are predicted to be characteristic of presence of garnet, gedrite, chlorite, kyanite and rutile under P–T conditions estimated to be 9–12kbar and 650–670°C. The post-peak history is characterized by isothermal decompression, developing coronae and symplectite assemblages under a mix of fluid-present and fluid-absent conditions. An idealized P–XMg [=Mg/(Mg+Fe) in mole] pseudosection drawn at T=655°C and a P–M(H2O) pseudosection at T=630°C suggest that the garnet-free cordierite–orthoamphibole rock (COR) can form from decompression of GOR assemblages when the rock is high-in XMg value or the decompression temperature is low. Zircon U–Pb dating yields a metamorphic age of 1964±25Ma (∼1.95Ga) and a protolith age of 2533±14Ma, where the former was interpreted to represent the prograde- or peak stages of metamorphism. This paper validates an argument that the amalgamation of the NCC via a crust thickening event occurred at ∼1.95Ga, followed by an isothermal decompression and a prolonged nearly isobaric cooling process from 1.90 to 1.78Ga.

Reaction textures and metamorphic evolution of sapphirine–spinel-bearing and associated granulites from Diguva Sonaba, Eastern Ghats Mobile Belt, India

AbstractThe Diguva Sonaba area (Vishakhapatnam district, Andhra Pradesh, South India) represents part of the granulite-facies terrain of the Eastern Ghats Mobile Belt. The Precambrian metamorphic rocks of the area predominantly consist of mafic granulite (±garnet), khondalite, leptynite (±garnet, biotite), charnockite, enderbite, calc-granulite, migmatic gneisses and sapphirine–spinel-bearing granulite. The latter rock type occurs as lenticular bodies in khondalite, leptynite and calc-granulite. Textural relations, such as corroded inclusions of biotite within garnet and orthopyroxene, resorbed hornblende within pyroxenes, and coarse-grained laths of sillimanite, presumably pseudomorphs after kyanite, provide evidence of either an earlier episode of upper-amphibolite-facies metamorphism or they represent relics of the prograde path that led to granulite-facies metamorphism. In the sapphirine–spinel-bearing granulite, osumilite was stable in addition to sapphirine, spinel and quartz during the thermal peak of granulite-facies metamorphism but the assemblage was later replaced by Crd–Opx–Qtz–Kfs-symplectite and a variety of reaction coronas during retrograde overprint. Variable amounts of biotite or biotite+quartz symplectite replaced orthopyroxene, cordierite and Opx–Crd–Kfs–Qtz-symplectite at an even later retrograde stage. Peak metamorphic conditions of c. 1000°C and c. 12 kbar were computed by isopleths of XMg in garnet and XAl in orthopyroxene. The sequence of reactions as deduced from the corona and symplectite assemblages, together with petrogenetic grid and pseudosection modelling, records a clockwise P–T evolution. The P–T path is characteristically T-convex suggesting an isothermal decompression path and reflects rapid uplift followed by cooling of a tectonically thickened crust.

XMapTools: A MATLAB©-based program for electron microprobe X-ray image processing and geothermobarometry

XMapTools is a MATLAB©-based graphical user interface program for electron microprobe X-ray image processing, which can be used to estimate the pressure–temperature conditions of crystallization of minerals in metamorphic rocks. This program (available online at http://www.xmaptools.com) provides a method to standardize raw electron microprobe data and includes functions to calculate the oxide weight percent compositions for various minerals. A set of external functions is provided to calculate structural formulae from the standardized analyses as well as to estimate pressure–temperature conditions of crystallization, using empirical and semi-empirical thermobarometers from the literature. Two graphical user interface modules, Chem2D and Triplot3D, are used to plot mineral compositions into binary and ternary diagrams. As an example, the software is used to study a high-pressure Himalayan eclogite sample from the Stak massif in Pakistan. The high-pressure paragenesis consisting of omphacite and garnet has been retrogressed to a symplectitic assemblage of amphibole, plagioclase and clinopyroxene. Mineral compositions corresponding to ~165,000 analyses yield estimates for the eclogitic pressure–temperature retrograde path from 25kbar to 9kbar. Corresponding pressure–temperature maps were plotted and used to interpret the link between the equilibrium conditions of crystallization and the symplectitic microstructures. This example illustrates the usefulness of XMapTools for studying variations of the chemical composition of minerals and for retrieving information on metamorphic conditions on a microscale, towards computation of continuous pressure–temperature-and relative time path in zoned metamorphic minerals not affected by post-crystallization diffusion.

Coexisting pseudobrookite, ilmenite, and titanomagnetite in hornblende andesite of the Coleman Pinnacle flow, Mount Baker, Washington: Evidence for a highly oxidized arc magma

Pseudobrookite microphenocrysts occur in cognate inclusions in the ~305 ka Coleman Pinnacle hornblende andesite flow from the Mount Baker volcanic field, Washington. Pseudobrookites are associated with hornblende phenocrysts and glomerophyric clusters of orthopyroxene, clinopyroxene, plagioclase, ilmenite, titanomagnetite, apatite, and zircon in a matrix of fresh rhyolitic glass. Grains of pseudobrookite are rimmed by or intergrown with ilmenite. These textures are analogous to those observed between armalcolite and ilmenite in high-Ti lunar basalts. In a unique occurrence, pseudobrookite, and titanomagnetite form a symplectitic intergrowth surrounding a core of ilmenite. Mass balance calculations show that the pseudobrookite + titanomagnetite assemblage is not an isochemical decomposition of ilmenite. In the TiO 2 -FeO-Fe 2 O 3 system (Mg-free), pseudobrookite and titanomagnetite solid solutions do not coexist. However, all three Fe-Ti oxides in the symplectitic assemblage contain significant amounts of Mg. In the TiO 2 -MgO-FeO-FeO 1.5 system at high oxygen fugacities, the Mg-rich pseudobrookite + titanomagnetite assemblage is stable relative to the conjugate pair of Mg-bearing ilmenite solid solutions. At lower f O 2 , Fe 2+ increases, Mg/(Mg+Fe 2+ ) (Mg no.) decreases and the conjugate ilmenite pair becomes the stable assemblage at Mg no. T = 900–1000 °C and f O 2 = NNO + 1.5 to + 1.75, one of the highest redox states on record for arc magmas. The calculated f O 2 range is consistent with the composition of the ilmenite in equilibrium with pseudobrookite ± rutile and with Fe 3+ -rich cores in hornblende phenocrysts.

Chromite symplectites in Mg-suite troctolite 76535 as evidence for infiltration metasomatism of a lunar layered intrusion

Despite the very low chromium concentrations in its cumulus olivine (∼140ppm), lunar troctolite 76535 contains large amounts of Cr sporadically, but highly concentrated, in symplectite assemblages consisting of Mg–Al-chromite and two pyroxenes. Previously proposed symplectite formation mechanisms include crystallization of trapped interstitial melt, diffusion of Cr from cumulus olivine, and/or remobilization of cumulus chromite grains. These mechanisms would imply that the highly Cr-depleted nature of Mg-suite parental magmas and their source materials inferred from cumulus olivine may be illusory. We have conducted a detailed petrologic and textural study of symplectites, as well as chromite veins, intercumulus assemblages, olivine-hosted melt inclusions and clinopyroxene-troilite veins in 76535 with the goals of constraining the origin of the symplectites, and the degree of Cr-depletion in Mg-suite magmas relative to other lunar basalts. Orthopyroxene and clinopyroxene in melt inclusions are depleted in Cr relative to their symplectite counterparts, averaging 900 and 1200ppm vs. 7400 and 8100ppm Cr2O3, respectively. Olivine in contact with symplectite assemblages may exhibit a diffusion profile of Cr going into olivine, whereas olivine boundaries away from symplectites show no diffusion profile. There is also a distinct lack of primary chromite as inclusions in cumulus phases and melt inclusions. Multiple textural observations, melt inclusion chemistry, and modeling of chromite–olivine equilibrium rule out previously proposed symplectite formation mechanisms, and strongly suggest that chromite was not a primary crystallization product of the 76535 parental magma. Accordingly, the post-cumulus addition of Cr and Fe is required to produce the symplectites. After considering multiple models, the addition of Cr and Fe to 76535 via infiltration metasomatism by an exogenous chromite-saturated melt is the model most consistent with multiple textural and geochemical observations. Failure of models that call upon Cr diffusion out of olivine grains imply that the observed Cr-depleted nature of olivine observed in many Mg-suite lithologies is a primary feature of the Cr-depleted nature of the Mg-suite parental magmas and their source materials. This substantial depletion of Cr in the magma relative to mare basalt magmas still requires a satisfactory explanation in order to be consistent with Mg-suite petrogenetic models and currently accepted bulk-Moon compositions. Additionally, if the intimate interaction of migrating melts with early lunar crustal lithologies was a widespread phenomenon after LMO solidification, it provides another mechanism by which to reset or delay closure of radiogenic isotopic systems and explain the Mg-suite-ferroan anorthosite age overlap.

Petrology and Geochronology of Granulites from the McKaskle Hills, Eastern Amery Ice Shelf, Antarctica, and Implications for the Evolution of the Prydz Belt

A combined petrological and geochronological study was carried out on mafic granulites and associated felsic gneisses from the McKaskle Hills, eastern Amery Ice Shelf, East Antarctica. Garnet-bearing mafic granulites exhibit reaction textures and exsolution textures that indicate two-stage metamorphic evolution. Thermobarometric estimates from matrix and symplectite assemblages yield peak and retrograde P^T conditions of 9 0^9 5 kbar and 880^9508C and 6 6^7 2 kbar and 700^7508C, respectively. Similar but slightly scattered peak P^Testimates of 7 9^10 1kbar and 820^9808C are obtained from the core compositions of minerals from felsic paraand orthogneisses. Evidence for the prograde history is provided by muscovite inclusions in garnet from a paragneiss. Sensitive highresolution ion microprobe U^Pb zircon dating reveals an evolutionary history for the granulites, including a mafic and felsic igneous intrusion at 1174^1019Ma, sedimentation after 932^916Ma, and a high-grade metamorphism at 533^529Ma. In contrast, Sm^Nd mineral^whole-rock dating mainly yields a single age population at 500Ma.This suggests that the McKaskle Hills form part of the Prydz Belt, and that the relatively high peak P^Tconditions and a decompression-dominated P^T path for the rocks resulted from a single Cambrian metamorphic cycle, rather than two distinct metamorphic events as formerly inferred for the granulites from Prydz Bay.The age data also indicate that the Precambrian history of the McKaskle Hills is not only distinct from that of the early Neoproterozoic terrane in the northern Prince Charles Mountains, but also different from that of other parts of the Prydz Belt. The existence of multiple basement terranes, together with considerable crustal thickening followed by tectonic uplift and unroofing indicated by the clockwise P^T^t evolution, suggests that the Prydz Belt may represent a collisional orogen that resulted in the assembly of Gondwana during the Cambrian period.

Contrasting response of monazite and zircon to a high- T thermal overprint

Contact metamorphism in the aureole of the 1322 Ma Makhavinekh Lake Pluton, northern Labrador, affected monazite and zircon in the adjacent 1850 Ma metapelitic gneisses. Transformation of regional garnet and sillimanite to lower-pressure symplectitic intergrowths of cordierite, orthopyroxene, and spinel was accompanied by resorption of inherited monazite inclusions in garnet coupled with the appearance of coronitic high-Y monazite rims. In situ ion-microprobe dating is used to show that high-Y rims formed during contact metamorphism. Liberation of Y and HREE from garnet also gave rise to new xenotime growth. The coronitic nature of monazite overgrowths reflects the diffusion-controlled nature of net-transfer reactions whereas its higher Y composition reflects equilibration with xenotime at peak T (> 800 °C) conditions in the inner aureole. Very thin overgrowths on inherited zircon were also encountered, but only where zircon is surrounded by the symplectitic assemblage, reflecting liberation of Zr from garnet. Although these overgrowths are too thin to date using conventional ion-microprobe techniques, well-developed triple junctions between zircon and orthopyroxene suggests that they grew in textural equilibrium with the contact metamorphic assemblage. In contrast to monazite, inherited zircon remained intact during contact metamorphism, exhibiting no change in morphology (other than the growth of thin rims) or internal zoning throughout the aureole. However, inherited sector-zoned zircons of anatectic origin display evidence for intracrystalline Pb redistribution in the inner aureole. In these samples, ion-microprobe analyses encountered heterogeneous Pb signals and a dispersion of 207Pb / 206Pb dates away from the well constrained 1850 Ma age of regional metamorphism. Whereas analyses from the outer aureole faithfully record the age of regional metamorphism, those from the inner aureole are normally and reversely discordant and distributed along a line collinear with a 1850 to 1322 Ma discordia. This disturbance is correlated with proximity to the pluton implying that Pb was mobile in the zircon lattice during contact metamorphism. Most grains are characterized by apparent Pb loss from low-U domains and apparent Pb gain in higher-U domains. These data are interpreted to reflect recovery of strained crystalline domains leading to expulsion of Pb* that was able to efficiently diffuse into higher-U domains that were partly amorphous prior to rapid reheating in the inner aureole.

Lunar Mg suite: Thermobarometry and petrogenesis of parental magmas

Using compilations of internally consistent thermodynamic data on minerals and advances in quantifying the energetics of crystalline solid solutions, we have recalculated the pressures and temperatures of equilibration of troctolite sample 76535 and a number of spinel cataclasites. The five phase assemblage (olivine + orthopyroxene + clinopyroxene + Cr‐spinel + plagioclase), which occurs in symplectite assemblages in 76535, gives equilibration temperatures from 800°–900°C and pressures from 200 to 250 MPa, indicating a deep (40–50 km) crustal source. Spinel cataclasite assemblages give minimum pressures of ∼100 MPa. Mg suite magmas were apparently intruded throughout the lunar crust early in lunar history. The textures and equilibration temperatures in many Mg suite samples are consistent with subsolidus recrystallization and the Mg‐rich minerals of some refractory troctolites and dunites may be residues of a crustal melting process or products of subsolidus elemental redistribution. The Sm‐Nd age of 4.26±0.06 Ga of troctolite 76535 most likely represents the time of subsolidus recrystallization and/or partial melting. In an attempt to identify the primary source material for the Mg suite magmas, a variety of models were tested using the MELTS and MAGFOX codes. A good fit between mineral compositions and crystallization sequences is obtained by ∼20% batch melting at 0.5 GPa of an average whole Moon composition with an Mg # of 0.85. Melts formed by batch melting at higher pressures and melts formed by polybaric partial fusion over the pressure range from 2.0 to 0.3 GPa were either too poor in Mg and/or crystallized clinopyroxene before orthopyroxene. Mixing with 10–20% urKREEP, which is required to satisfy the REE constraints of the Mg suite, reduces the Mg # of the parental magmas and requires a whole Moon Mg # closer to that of the terrestrial mantle.

Ultra‐high‐temperature metamorphism and multistage decompressional evolution of sapphirine granulites from the Palni Hill Ranges, southern India

Sapphirine granulites from a new locality in the Palni Hill Ranges, southern India, occur in a small enclave of migmatitic, highly magnesian metapelites (mg=85–72) within massive enderbitic orthogneiss. They show a variety of multiphase reaction textures that partially overprint a coarse‐grained high‐pressure assemblage of Bt+Opx+Ky+Grt+Pl+Qtz. The sequence of reactions as deduced from the corona and symplectite assemblages, together with petrogenetic grid considerations, records a clockwise P–T evolution with four distinct stages. (1) Equilibration of the initial high‐P assemblage in deep overthickened crust (12 kbar/800–900 °C) was followed by a stage of near‐isobaric heating, presumably as a consequence of input of extra heat provided by the voluminous enderbitic intrusives. During heating, kyanite was converted to sillimanite, and biotite was involved in a series of vapour‐phase‐absent melting reactions, which resulted in the ultra‐high‐temperature assemblage Opx+Crd+Kfs+Spr±Sil, Grt, Qtz, Bt, coexisting with melt (equilibration at c. 950–1000° C/11–10 kbar). (2) Subsequently, as a result of decompression of the order of 4 kbar at ultra‐high temperature, a sequence of symplectite assemblages (Opx+Sil+Spr/Spr+Crd→Opx+Spr+Crd→Opx+Crd→Opx+Crd+Spl/Crd+Spl) developed at the expense of garnet, orthopyroxene and sillimanite. This stage of near‐isothermal decompression implies rapid ascent of the granulites into mid‐crustal levels, possibly due to extensional collapse and erosion of the overthickened crust. (3) Development of late biotite through back‐reaction of melt with residual garnet indicates a stage of near‐isobaric cooling to c. 875 °C at 7–8 kbar, i.e. relaxation of the rapidly ascended crust to the stable geotherm. (4) A second period of near‐isothermal exhumation up to c. 6–5 kbar/850 °C is indicated by the partial breakdown of late biotite through volatile phase‐absent melting reactions. Available isotope data suggest that the early part of the evolutionary history (stages 1–3) is presumably coeval with the early Proterozoic metamorphism in the extended granulite terrane of the Nilgiri, Biligirirangan and Shevaroy Hills to the north, while the exhumation of the granulites from mid‐crustal levels (stage 4) occurred only during the Pan‐African thermotectonic event, which led to the accretion of the Kerala Khondalite Belt to the south.

Petrology of ultrahigh‐pressure rocks from the southern Su‐Lu region, eastern China

Abstract In the Su‐Lu ultrahigh‐P terrane, eastern China, many coesite‐bearing eclogite pods and layers within biotite gneiss occur together with interlayered metasediments now represented by garnet‐quartz‐jadeite rock and kyanite quartzite. In addition to garnet + omphacite + rutile + coesite, other peak‐stage minerals in some eclogites include kyanite, phengite, epidote, zoisite, talc, nyböite and high‐Al titanite. The garnet‐quartz‐jadeite rock and kyanite quartzite contain jadeite + quartz + garnet + rutile ± zoisite ± apatite and quartz + kyanite + garnet + epidote + phengite + rutile ± omphacite assemblages, respectively. Coesite and quartz pseudomorphs after coesite occur as inclusions in garnet, omphacite, jadeite, kyanite and epidote from both eclogites and metasediments. Study of major elements indicates that the protolith of the garnet‐quartz jadeite rock and the kyanite quartzite was supracrustal sediments. Most eclogites have basaltic composition; some have experienced variable ‘crustal’contamination or metasomatism, and others may have had a basaltic tuff or pyroclastic rock protolith.The Su‐Lu ultrahigh‐P rocks have been subjected to multi‐stage recrystallization and exhibit a clockwise P‐T path. Inclusion assemblages within garnet record a pre‐eclogite epidote amphibolite facies metamorphic event. Ultrahigh‐P peak metamorphism took place at 700–890° C and P>28 kbar at c. 210–230 Ma. The symplectitic assemblage plagioclase + hornblende ± epidote ± biotite + titanite implies amphibolite facies retrogressive metamorphism during exhumation at c. 180–200 Ma. Metasedimentary and metamafic lithologies have similar P‐T paths. Several lines of evidence indicate that the supracrustal rocks were subducted to mantle depths and experienced in‐situ ultrahigh‐P metamorphism during the Triassic collision between the Sino‐Korean and Yangtze cratons.