Petrology and tectonic significance of the early Mesozoic granulite xenoliths from the eastern Inner Mongolia, China
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Granulite xenoliths are found in the early Mesozoic diorite intrusions from Chifeng and Ningcheng areas, eastern Inner Mongolia. The granulites are granoblastic and weakly gneissic with mineral assemblage of hypersthene, diopside, plagioclase and minor biotite, amphibole and ilmenite. Some samples contain the intergrowth composed of labradorite and vermicular hypersthene, and some coarse-grained plagioclases of andesine and labradorite composition occasionally develop bytownite rims with vermicular hypersthene, indicating a possible presence of garnet. Presence of blastogabbroic texture and hypersthene with diopside exsolution lamellae in some samples suggests that the protolith of the granulite is norite or gabbro. On the basis of metamorphic research and thermobaric calculation, the evolution of the granulite xenoliths is summarized into the following stages: (1) Isobaric cooling of underplated noritic or gabbroic magma in the lower crust led to the formation of probable garnet-bearing medium-high pressure granulite. (2) These higher pressure granulites were adiabatically uplifted to upper crust by dioritic magma and transformed to low pressure two-pyroxene granulite during an isothermal decompression. (3) The two-pyroxene granulite underwent retrograde metamorphism of different degrees during an isobaric cooling process as a result of crystallization and cooling of the dioritic magma. The pyroxenite-dominated cumulates and the medium-high pressure granulites may have rejuvenated the lower crust during the early Mesozoic.Keywords:
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Abstract High-pressure mafic granulites (retrograded eclogites?) were discovered as minor lenses enclosed in garnet-kyanite-cordierite gneiss from the Badu Complex of the East Cathaysia terrane in South China. These rocks consist mainly of garnet, clinopyroxene, hornblende, quartz, and rutile/ilmenite with or without omphacite pseudomorphs that are indicated by clinopyroxene + sodic plagioclase symplectic intergrowths. Mineral textures and reaction relationships suggest three metamorphic stages: (1) an eclogite-facies stage (M1) characterized by the mineral assemblage of garnet + clinopyroxene (omphacite) + hornblende + rutile + quartz; (2) a high-pressure granulite-facies (M2) stage mainly represented by garnet + clinopyroxene + plagioclase + hornblende + rutile + quartz in the matrix; and (3) an amphibolite retrograde stage (M3) defined by hornblende + plagioclase + ilmenite + quartz symplectites surrounding garnet porphyroblasts. Conventional geothermometers and geobarometers in combination with phase equilibria modeling constrain metamorphic P–T conditions of 15.8–18.2 kbar/625–690 °C (M1), 11.8–14.5 kbar/788–806 °C (M2), and 5.4–6.4 kbar/613–668 °C (M3), respectively. Two-staged decompression processes are defined after the peak pressure, which suggests a two-staged exhumation of these deeply buried rocks. Secondary ion mass spectrometry (SIMS) zircon U-Pb dating and trace element analysis show that the high-pressure metamorphism occurred at 240–244 Ma. Complete early Mesozoic orogenic processes characterized by initial subduction and/or crustal thickening and subsequent exhumation followed by rapid uplift are reconstructed for this part of the East Cathaysia terrane, South China.
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Hornblende
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The Bohemian Massif (BM), the easternmost part of Variscan chain in Europe, contains both autochthonous and allochthonous terrains. Thin skinned units (nappes) characterize parts of Moldanubian, and Saxothuringian and cover the deep structure of BM. The occurrences of granulites in Moldanubian (Gfohl unit) and the crustal (granulite) xenoliths in alkaline basalts of North Bohemian Cenozoic province indicate the presence of two principally different lower crust sections in and under the nappes. The southern part of the Bohemian Massif (exposed at the surface) is characterized by the presence of high-P high-T “ga–ky granulites” (accompanied by garnet peridotites) whereas the northern part, containing pyroxene granulites (charnockites) with high-T, low-P paths, is accompanied by upper mantle four-phase lherzolites. Both granulites are compositionally close to granites in respect to Ab–Or–Oz relations and differ in respect to trace element compositions. Garnet–kyanite granulites have trace-element abundances with granite features: high LILE contents, LREE-enriched patterns, negative Eu anomaly (Fig. 1), but low Th and U and varied Zr abundances. The contents of siderophile and transitional elements are slightly higher than corresponding contents in granites. Positive correlation of Zr with Th and U, and negative correlation of Zr with LILE elements, e.g., Rb suggests that melting or removal of zircon played a major role in generating the granulite trace element signature. North Bohemian charnockites that are LILE poor with Eu positive anomalies resemble “shield granulites” (Fig. 2). Southern Bohemian granulite bodies contain blocks and xenoliths of ultramafic rocks not related to metamorphic structures of granulites. Xenoliths occur in both ga–ky (not retrogressed) and ga–bi (retrogressed) rocks. Ultramafic rocks were incorporated into granulites in the solid state and their mineral equilibria indicate origin in the lithospheric as well as asthenospheric mantle and consequent fast exhumation. The interval of time of protolith crystallization of granulites (i.e., 370–340 Ma) is narrow and indicates a “granulite event” in the early Variscan. The event is contemporaneous with the emplacement of “igneous rocks” that share granulite facies features (e.g., opx–cpx parageneses), have strong geochemical mantle signatures and are part of major batholiths (durbachite types) in Moldanubian terrane (Tabor, Jihlava, Weinsberg, Rastenberg). Differences in metamorphic grade of granulites and surrounding gneisses, composition, and presence of ultramafic xenoliths suggest that ga– ky granulites are pristine dehydration melts that may have formed at collisional environment at the expense of crustal material of lower plate. These melts have crystallized with ga and ky as a near liquidus phases or as a product of dehydration incongruent melting. On the rise from the lower plate the melts captured ultramafic rocks and reaction of ga → opx + plg has taken place. The intrusions of ga–ky granulites stopped in the middle crust (amphibolite facies). Structure of granulite bodies and metamorphic rock textures relate to the retrogressive changes, postdate ga–ky and px-bearing parageneses and represent post-emplacement features. The granulites (charnockites) of North Bohemia are distinctly different and may represent fragments of Baltica or may be remnants of such microcontinents as Armorica, Perunica, or Avalonia.
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A region of metamorphosed supracrustal rocks (pelite, quartzite, marble, and graywacke) and coeval intrusive igneous rocks crop out in a 250 km long orogenic belt in northern Madagascar. The NW-SE trending belt is situated between a juvenile Neoproterozoic magmatic arc terrane (to the north) and an Archean craton, strongly reworked in early Neoproterozoic times (~800-670 Ma), to the south. Pelitic schist and granulite exposed along a ~70 km long transect from Andapa to Sambava contain assemblages ranging from sillimanite-garnet-biotite-orthoclase-cordierite to sillimanite-garnet- biotite-orthoclase and sillimanite-garnet-biotite-muscovite. These assemblages crop out over much of the area in which migmatites and hornblende + augite ± hypersthene ± biotite + perthite granites are common. Partial melting, biotite dehydration reactions, and granite emplacement are interpreted to have been nearly synchronous on the basis of field, structural, and petrographic observations.
Sillimanite
Migmatite
Hornblende
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Garnet-bearing basic granulites crop out near Gushan Town, the eastern end of the Khondalite Belt and just next to the Trans-North China Orogen. They occur as enclaves within sillimanite garnet gneisses and hypersthene monzonitic gneisses. Four metamorphic stages have been recognized on the basis of petrographic features and mineral compositions. The conventional thermobarometry and the calculated P-T pseudosections are used to determine the P-T conditions of different metamorphic stages. The early pre-peak metamorphic stage (M1) is represented by garnets' cores and their mineral inclusions, including clinopyroxene+plagioclase+quartz+ilmenite±rutile. The sodic cores of matrix plagioclases reveal that the highest metamorphic pressure could be over 11kbar. The peak metamorphic stage (M2) is documented by garnet+clinopyroxene+orthopyroxene+plagioclase+quartz+ilmenite, which formed at 850~900℃, 9~10kbar. Symplectites and coronas of clinopyroxene+orthopyroxene+amphibole+plagioclase±ilmenite surrounding garnet porphyroblasts imply a near-isothermal decompression retrograde stage (M3). The P-T conditions of pyroxene+plagioclase symplectites and coronas were yielded at 760~820℃, 5~8kbar. The occurrences of lower amphibolite-facies amphiboles suggest a late cooling stage (M4). The metamorphic temperature was lower than 690℃. These data define a clockwise P-T path with near-isothermal decompressional section for garnet-bearing basic granulites.Metamorphic zircons from the garnet-bearing basic granulites yield two groups of U-Pb ages, 1945±25Ma and 1828±36Ma, which are consistent with main metamorphic episodes recorded in the Khondalite Belt and the Trans-North China Orogen, respectively. The combination of metamorphic and geochronological data indicates that garnet-bearing basic granulites have possibly experienced two independent metamorphic events, suggesting that garnet-bearing basic granulites underwent initial crust thickening followed by the exhumation to some extent at ~1.95Ga caused by the collision between the Yinshan and Ordos blocks to form the Western Block, and that subsequently they were exhumed to the middle crust at ~1.85Ga caused by the collision between the Eastern and Western blocks of the North China Craton.
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The Grove Mountains, East Antarctica, consist of granulite-facies high-grade metamorphic rocks and some granitoids. Among them, the metamorphic rocks are dominated by pale and dark orthopyroxene-bearing felsic gneiss, with minor mafic granulite, metasedimentary rock and occasionally scapolite-bearing calc-silicate rock. All metamorphic rocks exhibit an equilibrium texture, but exsolution lamellae of orthopyroxene (pigeonite) occur in all clinopyroxenes in mafic granulites. A peak metamorphic temperature of c. 850℃ was obtained from the reintegrated compositions of exsolved clinopyroxene, and a pressure of 0. 61 -0. 67 GPa from garnet-orthopyroxene-plagioclase-quartz geobarometer. The preservation of igneous augite megacrysts in mafic granulites suggests a single episode of Pan-African granulite-facies metamorphism developed in the Grove Mountains. The rocks later underwent a slow cooling process, which is attributed to the magmatic underplating of the lower crust.
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ABSTRACT The northern Dabie terrane consists of a variety of metamorphic rocks with minor mafic‐ultramafic blocks, and abundant Jurassic‐Cretaceous granitic plutons. The metamorphic rocks include orthogneisses, amphibolite, migmatitic gneiss with minor granulite and metasediments; no eclogite or other high‐pressure metamorphic rocks have been found. Granulites of various compositions occur either as lenses, blocks or layers within clinopyroxene‐bearing amphibolite or gneiss. The palaeosomes of most migmatitic gneisses contain clinopyroxene; melanosomes and leucosomes are intimately intermingled, tightly folded and may have formed in situ. The granulites formed at about 800–830 °C and 10–14 kbar and display near‐isothermal decompression P–T paths that may have resulted from crust thickened by collision. Plagioclase‐amphibole coronae around garnets and matrix PI + Hbl assemblages from mafic and ultramafic granulites formed at about 750–800 °C. Partial replacement of clinopyroxene by amphibole in gneiss marks amphibolite facies retrograde metamorphism. Amphibolite facies orthogneisses and interlayered amphibolites formed at 680–750 °C and c. 6 kbar. Formation of oligoclase + orthoclase antiperthite after plagioclase took place in migmatitic gneisses at T ≤ 490°C in response to a final stage of retrograde recrystallization. These P–T estimates indicate that the northern Dabie metamorphic granulite‐amphibolite facies terrane formed in a metamorphic field gradient of 20–35 °C km ‐1 at intermediate to low pressures, and may represent the Sino‐Korean hangingwall during Triassic subduction for formation of the ultrahigh‐ and high‐P units to the south. Post‐collisional intrusion of a mafic‐ultramafic cumulate complex occurred due to breakoff of the subducting slab.
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The oldest dated rocks from the Acasta gneisses of the western Slave Province, Canada present an igneous age of ~4030 Ma. Following this the detrital zircons from the Jack Hills, Narryer Gneiss Terrane, Yilgarn Craton, Western Australia are identified as 4404 ±8 Ma. These discoveries suggest that crustal formation started as early as the Priscian Eon. Hitherto the Earth has gone through a series of interactions involving the atmosphere, hydrosphere, crust, mantle and core. However, only limited remnants of these early processes remain on the accessible crust due to extensive crustal reworking. The Southern Granulite Terrane (SGT) in the southern part of India represents the most extensive exposure of lower crustal granulite terranes in the world. This study mainly focuses on the characteristics of Archean (~2500 Ma) tectonics and nature of subsequent crustal growth, which led to the formation of Archean Nilgiri Block. Detailed fieldwork in this terrane and subsequent petrographic analysis revealed charnockites, hornblende-biotite gneiss, metagabbro/mafic granulite, websterite, amphibolite, Grt-Ky metasediment, metatuff and banded iron formation as the main rock types in this terrane. Field and petrographic results show a regional trend with garnet-orthopyroxene-biotite-quartz-plagioclase-K- feldspar bearing charnockites in the southern part which gets subsequently enriched in clinopyroxene that forms garnet-absent two pyroxene granulites consisting of orthopyroxene-clinopyroxene-quartz-plagioclase-K-feldspar towards the central part. Further north, metagabbro/mafic granulite is enriched in garnet-clinopyroxene-plagioclase assemblage. Websterite, amphibolite, metasediment, metatuff and banded iron formation are stacked and closely associated within this mafic belt in the north. The metagabbro represents peak P-T conditions of ~850°C and ~14kbar compared to the charnockites, which recorded a peak P-T of ~850°C and 9-10kbar. Petrographic results of oxide minerals show that the southern charnockitic part is abundant in rutile-ilmenite association represent reduced conditions compared to the oxidized magnetite-hematite-ilmenite associations in the mafic rocks. This oxidation trend is followed by pyrrhotite-chalcopyrite enriched southern charnockitic region that transforms to pyrite rich northern mafic belt. Ilmenite¬titanite association with no sulphides characterizes the hornblende-biotite gneiss in the entire Nilgiri Block. The geochemical variations of major, trace and rare earth elements show that the granulite-amphibolite grade felsic rocks evolved in an arc magmatic process leaving behind mafic magma, which later intruded into these rocks, in a subduction related arc magmatic process. The U-Pb LA-ICPMS and SHRIMP dating of charnockite, hornblende-biotite gneiss and met gabbros shows ca. 2550 Ma formation age and ca. 2450 Ma metamorphism in this terrane.
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Hornblende
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Felsic
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The Lišov Granulite Massif differs from neighbouring granulite bodies in the Moldanubian Zone of southern Bohemia (Czech Republic) in including a higher proportion of intermediate–mafic and orthopyroxene-bearing rocks, associated with spinel peridotites but lacking eclogites. In addition to dominantly felsic garnet granulites, other major rock types include quartz dioritic two-pyroxene granulites, tonalitic granulites and charnockites. Minor bodies of high-pressure layered gabbroic garnet granulites and spinel peridotites represent tectonically incorporated foreign elements. The protoliths of the mafic–intermediate granulites (quartz-dioritic and tonalitic) crystallized ∼360–370 Ma ago, as indicated by laser ablation inductively coupled plasma mass spectrometry U–Pb ages of abundant zircons with well-preserved magmatic zoning. Strongly metamorphically recrystallized zircons give ages of 330–340 Ma, similar to those of other Moldanubian granulites. For the overwhelming majority of the Lišov granulites peak metamorphic conditions probably did not exceed 800–900°C at 4–5 kbar; the equilibration temperature of the pyroxene granulites was 670–770°C. This is in sharp contrast to conditions of adjacent contemporaneous Moldanubian granulites, which are characterized by a distinct HP–HT signature. The mafic–intermediate Lišov granulites are thought to have originated during Viséan metamorphic overprinting of metaluminous, medium-K calc-alkaline plutonic rocks that formed the mid-crustal root of a Late Devonian magmatic arc. The protolith resembled contemporaneous calc-alkaline intrusions in the European Variscan Belt.
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Charnockite
Felsic
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