Pan-African Granulite Facies Metamorphism in the Grove Mountains East Antarctica
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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.Keywords:
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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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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.
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Research Article| November 01, 1985 A Proterozoic granulite-facies terrane on Roan Mountain, western Blue Ridge belt, North Carolina–Tennessee GERALD L. GULLEY, JR. GERALD L. GULLEY, JR. 1Shell Western E & Pine., P.O. Box 60775, New Orleans, Louisiana 70160 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1985) 96 (11): 1428–1439. https://doi.org/10.1130/0016-7606(1985)96<1428:APGTOR>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation GERALD L. GULLEY; A Proterozoic granulite-facies terrane on Roan Mountain, western Blue Ridge belt, North Carolina–Tennessee. GSA Bulletin 1985;; 96 (11): 1428–1439. doi: https://doi.org/10.1130/0016-7606(1985)96<1428:APGTOR>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Polymetamorphosed gneisses of Proterozoic age crop out on Roan Mountain, North Carolina–Tennessee, near the western edge of the Blue Ridge belt. These gneisses display Grenville-age, granulite-facies assemblages, including two-pyroxene metabasites and gar-net-sillimanite-orthoclase metapelites. All gneisses were intruded by diabase dikes of the late Precambrian Bakersville Gabbro. Paleozoic amphibolite-facies metamorphism locally formed garnet and amphibole in mafic rocks and kyanite in metapelitic rocks.Two pyroxene geothermometry and mineral stability curves define temperature-pressure ranges of 750–847 °C and 6.5–8.0 kbar for the granulite-facies metamorphism. This metamorphism probably occurred within the lower crust at depths of 23–28 km. Pressure-temperature conditions for the major Paleozoic metamorphism are not well constrained but are estimated to be 700–740 °C and 7.5–9.0 kbar. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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Abstract Proterozoic mafic dykes from the southwestern Vestfold Block experienced heterogeneous granulite facies metamorphism, characterized by spotted or fractured garnet‐bearing aggregates in garnet‐absent groundmass. The garnet‐absent groundmass typically preserves an ophitic texture composed of lathy plagioclase, intergranular clinopyroxene and Fe–Ti oxides. Garnet‐bearing domains consist mainly of a metamorphic assemblage of garnet, clinopyroxene, orthopyroxene, hornblende, biotite, plagioclase, K‐feldspar, quartz and Fe–Ti oxides. Chemical compositions and textural relationships suggest that these metamorphic minerals reached local equilibrium in the centre of the garnet‐bearing domains. Pseudosection calculations in the model system NCFMASHTO (Na 2 O–CaO–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O–TiO 2 –Fe 2 O 3 ) yield P – T estimates of 820–870 °C and 8.4–9.7 kbar. Ion microprobe U–Pb zircon dating reveals that the NW ‐ and N‐trending mafic dykes were emplaced at 1764 ± 25 and 1232 ± 12 Ma, respectively, whereas their metamorphic ages cluster between 957 ± 7 and 938 ± 9 Ma. The identification of granulite facies mineral inclusions in metamorphic zircon domains is also consistent with early Neoproterozoic metamorphism. Therefore, the southwestern margin of the Vestfold Block is inferred to have been buried to depths of ~30–35 km beneath the Rayner orogen during the late stage of the late Mesoproterozoic/early Neoproterozoic collision between the Indian craton and east Antarctica (i.e. the Lambert Terrane or the Ruker craton including the Lambert Terrane). The lack of penetrative deformation and intensive fluid–rock interaction in the rigid Vestfold Block prevented the nucleation and growth of garnet and resulted in the heterogeneous granulite facies metamorphism of the mafic dykes.
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