High-pressure mafic granulites of the South Muya Block (Central Asian Orogenic Belt)
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Abstract The metamorphic conditions of the Natal Metamorphic Province (NMP) have been the focus of previous studies to assist with Rodinia reconstructions but there are limited constraints on the age of metamorphism. We use a combination of modern techniques to provide new constraints on the conditions and timing of metamorphism in the two southernmost terranes: the Mzumbe and Margate. Metamorphism reached granulite facies, 780–834°C at 3.9–7.8 kbar in the Mzumbe Terrane and 850–892°C at 5.7–6.1 kbar in the Margate Terrane. The new pressure and temperature constraints are supportive of isobaric cooling in the Margate Terrane as previously proposed. Peak metamorphism of the two terranes is shown to have occurred c. 40 myr apart, which contrasts strongly with previous assumptions of coeval metamorphism. While the age of peak metamorphism of the Margate Terrane (1032.7 ± 4.7 Ma) coincides with the tectonism and magmatism associated with the emplacement of the Oribi Gorge Suite ( c. 1050–1030 Ma), the age of metamorphism of the Mzumbe Terrane (987.4 ± 8.1 Ma) occurs c. 30–40 myr after tectonism is previously thought to have finished. We propose that models of advective cooling during transcurrent shearing can explain the metamorphic conditions and timing of the NMP.
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Early Precambrian rock units in the Urals are present in several polymetamorphic complexes, which are exposed in the Urals in the form of small (<1500 km2) tectonic blocks. Their ages are Archaean (as old as 3.5 Ga) and Palaeoproterozoic. During the formation of these complexes in the early Precambrian, two stages of ultra-high-temperature (granulite) metamorphism occurred. The maximum age of the early Neoarchaean stage of metamorphism is 2.79 Ga. Evidence of this metamorphic event includes the dating of the Taratash gneiss-granulite complex of the South Urals. Gneiss-migmatite complexes, which dominate the lower Precambrian section of the Urals, were formed in the Palaeoproterozoic during the sequential appearance of granulite facies metamorphism followed by amphibolite facies metamorphism and accompanying granitization. The maximum age of the Palaeoproterozoic stage of granulite metamorphism in the Alexandrov gneiss-migmatite complex, the most well-studied complex in the South Urals, is 2.08 Ga.
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The Larsemann Hills are located in the central part of the Prydz tectonic belt in East Antarctica. The study of granulite metamorphism of this area is essential to understanding the tectonic nature of the Prydz belt. The petrographic study of the garnetbearing mafic granulite boulder collected from the region demonstrates that pre-peak prograde mineral assemblage( M1) consists of hb+ opx ± cpx + pl + bi + ilm ± q ± mt,and peak metamorphic assemblage( M2) comprises g + opx + cpx + hb + ilm ± mt ± q,whereas the symplectite opx + pl + bi + ilm ± mt represents a superimposed assemblage( M3) associated with a decompression event. Mineral chemistry shows that garnets and orthopyroxenes in the mafic granulite have weak compositional zonings. Through the use of THERMOCALC program,the thermodynamic modeling in the NCFMASHTO system has been undertaken for the mafic granulite.Combined with conventional thermobarometers and the average P-T estimates,P-T conditions of the different metamorphic stages are estimated as 650 ~ 750℃ /5. 5 ~ 6. 5kb( M1),850 ~ 950℃ /8 ~ 8. 5kb( M2),and 800 ~ 900℃ /5. 5 ~ 6. 5kb( M3),respectively.Its metamorphism evolution of the mafic granulite is a typical post-peak isothermal decompression( ITD) clockwise P-T path. Compared with the mafic granulites in this region,we argue that this sample may originate from basement in Larsemann Hills. Combined with available chronological data in the region,it is suggested that the peak metamorphism likely corresponds to the Greenville high-grade tectonism,whereas post-peak retrograde metamorphism may be related to the Pan-African high-grade tectonism,implying that the PanAfrican Prydz belt could be an intracontinental mobile belt.
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The types of the high-pressure(HP) mafic granulites from high-grade metamorphic basement of the Jiaobei terrane are mainly composed of Grt mafic granulites,Grt-Hy granulites and Grt amphibolites.The HP mafic granulites are mainly occurred in TTG gneisses or granitic gneisses as enclaves or deformed dikes in the Jiaobei terrane.Mobile elements such as the large ion lithophile elements(K,Na,Sr,Rb) of the HP mafic granulites changed in significant levels,high field strength elements(Th,Nb,Zr,Ti) and rare earth elements basically unchanged and keep stable during the metamorphic process.The HP mafic granulites from the Jiaobei terrane belong to tholeiitic-basaltic rock series,with the concentrations of SiO2 at 44.04%~53.54%,and Mg# at 35~60.The composition of the rare earth elements of the HP mafic granulites is characterized by the flat(ΣREE=21.13×10-6~78.49×10-6,(La/Yb)CN=1.03~2.86) and right-inclined(92.74×10-6~133.5×10-6,(La/Yb)CN=2.93~4.56) on the chondrite-normalized REE patterns with no Eu anomalies(Eu/Eu*=0.93~1.04).The variation of Cr and Ni concentrations is large,however Cr and Ni have strong correlation with MgO.All most samples of the HP mafic granulites share obvious negative anomaly in Nb,Zr and Ti of the Phanerozoic island arc basalts,with the positive eNd(t) of(+2.70~+4.77).The composition of the major and trace elements of the HP mafic granulites in the Jiaobei terrane is characterized by the island arc tholeiitic basaltic rocks,and the protoliths may be gabbro instrusions or diabase dykes,and the corresponding basic volcanic rocks in back-carc extension tectonic setting.
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The various models for the nature and origin of fluids in granulite facies metamorphism were summarized. Field and petrologic evidence exists for both fluid-absent and fluid-present deep crustal metamorphism. The South Indian granulite province is often cited as a fluid-rich example. The fluids must have been low in H2O and thus high in CO2. Deep crustal and subcrustal sources of CO2 are as yet unproven possibilities. There is much recent discussion of the possible ways in which deep crustal melts and fluids could have interacted in granulite metamorphism. Possible explanations for the characteristically low activity of H2O associated with granulite terranes were discussed. Granulites of the Adirondacks, New York, show evidence for vapor-absent conditions, and thus appear different from those of South India, for which CO2 streaming was proposed. Several features, such as the presence of high-density CO2 fluid inclusions, that may be misleading as evidence for CO2-saturated conditions during metamorphism, were discussed.
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