The Neoproterozoic Mozambique Belt in the Gol Mountains (N Tanzania): Structural Analysis of Amphibolite and Granulite‐Facies Nappe Tectonites From a Crustal Orogenic Channel
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Abstract A structural analysis of the Gol Mountains of North Tanzania is presented in which we identify and characterize a sheet‐like deformation‐localization zone of regional scale. Syn‐metamorphic rock‐deformation within the localization zone gave rise to widespread tectonite development under high‐T in lower‐mid crustal realms. Regional foliations and mineral/stretching lineations, and associated non‐coaxial mineral petrofabrics can be correlated with ductile flow shear planes. At the macro‐scale these structures are genetically connected with tangential movement zones involving large a‐type and sheath folds, thrusts and ductile shear zones, and hydrothermal quartz megaveins plastically deformed. The dimension of the area affected by this structural style is c. 30 km across the Mozambique Belt and >50 km parallel to it. The associated tectonic displacements likely attained several tens of km in the context of an overall W‐directed tectonic vergence transversal to the Mozambique Belt orogenic trend. It is interpreted as a segment of a mid‐lower crustal orogenic channel active during late Ediacaran amalgamation of the Congo‐Tanzania Archean Craton (and its discontinuous fringe of Paleoproterozoic and Mesoproterozoic orogens) with the amalgamated arcs and terranes of the Arabian‐Nubian Shield.Keywords:
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Abstract This work presents the results of a fluid inclusion study of an amphibolite‐granulite facies transition in West Uusimaa, S.W. Finland. Early fluid‐inclusions in the granulite facies area are characteristically carbonic (CO 2 ), in contrast to predominantly aqueous early inclusions in the amphibolite facies area. These early inclusions can be related to peak metamorphic conditions (750‐820°C and 3‐5 kbar for peak granulite facies metamorphism). Relatively young CO 2 inclusions with low densities (<0.8g/cm 3 ) indicate that the first part of the cooling history of the rocks was characterized by a near isothermal uplift. N 2 ‐CH 4 inclusions, with compositions ranging between pure CH 4 and pure N 2 (Raman spectral analysis), were found in the whole area. They are probably syn‐ or even pre‐early inclusions. Only nearly critical homogenizing inclusions have been found (low density). Pressure estimates, based on densities of early fluid inclusions, show that the rapid transition of amphibolite towards granulite facies metamorphism is virtually isobaric. Granulite facies metamorphism in West Uusimaa is a thermal event, probably induced by the influx of hot, CO 2 ‐bearing fluids.
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Shear zones in the boundary between Eastern Ghats Province (EGP) and the cratons of Singhbhum in the north and Bastar in the west provide an excellent opportunity to study the tectonics of shear zone development and its timing in relation to the evolutionary history of the granulite suites. Detailed structural, microfabric and quartz C-axis patterns revealed a high temperature shear zone, at the western boundary between EGP and Bastar Craton (BC) around Paikmal. Petrological studies in this shear zone indicated decompression coeval with stretching in the sheared granulites. Geochronological constraints provided here indicate rapid exhumation of deep seated granulites in this boundary shear zone; the timing also is late in relation to the long-lived thermal (granulite formation) event in the EGP. Additionally, our geochronological data demonstrated the ~1600 Ma event in the Eastern Ghats Belt (EGB) involving sedimentation, magmatism, metamorphism and crustal anatexis, as a significant world event.
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A Thesis Submitted to the Faculty of Science
University of the Witwatersrand, Johannesburg
for the Degree of Doctor of Philosophy.
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The Southern Granulite Terrane (SGT) of southern India being a regional granulite-facies terrane with exposed mid- to lower-crustal rocks has been the attention of several studies focusing on amalgamation of Gondwana supercontinent. It comprises of a collage of several crustal blocks bisected by crustal scale shears [1]. Among these, the Madurai Granulite Block (MGB) forms the central and largest block in SGT, bounded by Palghat-Cauvery Shear Zone (PCSZ) to the north and Achankovil shear zone (AKSZ) in the south. Within the MGB, a V-shaped shear zone extending towards SW direction from Karur to Kambam, then taking a sharp NW turn at Painavu Shear Zone (KKPTSZ) in the central region of the MGB. Previous studies, however, contradict on the nature and evolution of the KKPTSZ [2,3]. The lithological makeup north of the shear zone is more comparable to the counterparts of Dharwar Craton, while the rocks south of the KKPTSZ are more akin to those of the Eastern Ghats. A recent tectonic model suggests the extension of Karur–Kambam lineament up to the AKSZ, and demarcated it as Kambam ultrahigh-temperature (UHT) belt [2] This has been interpreted to mark a fundamental collisional crustal boundary between eastern and western MGBs. Though, the newly suggested eastern and western crustal block model has greatly aided in understanding the evolution of the HP-UHT belt in north-central MGB, it suffered with inadequate data in identifying basement characteristics and age variations in southern part of the MGB. The present study attempts to synthesize multifarious geological information across the terrain integrated with new petrological, geochemical data for a comprehensive understanding of tectonic and metamorphic processes and thereby crustal evolution in the central Madurai block.  The petrological and geochemical characteristics of the granulite-facies rocks suggest igneous origin of the protolith by partial melting of the source region. They are enriched in Na2O over K2O, thus the K2O/Na2O ratio is less than one suggesting it is Tonalitic charnockite [4]. The K/Rb values of the charnockite vary between 81 and 400 with an average of about 245. Ba/Rb ratios in the charnockites are high, between 3.95 and 27.58 (average 12.23) indicating that they are not derived directly from a mantle melt, rather suggesting the role of internal differentiation of a pre-existing TTG-type crust through intra-crustal melting [5]. The result gives similarity to arc granitoid, while from the major and trace element data it is inferred that the formation is during a collisional event. With limited isotope geochronology data and field evidence, the argument of KKPTSZ as a possible terrain boundary is withered. Therefore, more convincing field-based data, integrated with petrological, geochronological, and phase equilibria models are required from this belt for a comprehensive understanding of the crustal evolution in Madurai Block.[1] Braun & Kriegsman (2003) Spec. Publ., Geol. Soc., London, 206:169–202.[2] Brandt et al (2014) Precambrian Research, 246: 91–122.[3] Plavsa et al (2014) Geol. Soc. of America Bulletin, 126: 791–811.[4] Ravindra Kumar & Sreejith (2016) Lithos, 262: 334–354.[5] Elis Hoffmann et al (2014) Earth & Planetary Sciences Letters, 388: 374-386. 
Dharwar Craton
Indian Shield
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The southernmost Hidaka metamorphic belt consists mainly of cordierite tonalite intrusions and pelitic metamorphic rocks ranging from the greenschist to the granulite facies. Anatectic migmatites are common in the higher amphibolite and granulite facies zones. Compositional changes in major, rare earth elements and some other trace metals are so small that they are undetectable among the pelitic metamorphic rocks of zones A + B + C and D, but they are large enough to be detected in the higher amphibolite (zone D) to the granulite facies rocks (zone E). The enrichment of Fe, Mg, Na, Eu, and Sc, and the depletion of K, P, La, Ce, Nd, Cs and Rb are statistically significant in pelitic granulites, while heavy REEs are very variable. The chemical variation of pelitic granulite was derived from the accumulation of plagioclase + garnet. This suggests that more than 50-60% of the total volume of pelitic granulite was melted to produce a large amount of tonalitic magma, leaving pelitic granulite as a restite. Migmatites of the higher amphibolite facies are anatexites, and their K, P, Cs, Rb and light REE content is the same as that of lower grade metamorphic rocks. Migmatites of the higher amphibolite facies melted incipiently to segregate only a small amount of melt, and could not produce a large magmatic mass such as the cordierite tonalites. Cordierite tonalites are S-type granites, and their major elements, Cs, Rb and light REE concentrations are similar to those of lower grade metamorphic rocks. The chemical variation of cordierite tonalites is explained by the extraction of plagioclase + garnet from a tonalitic magma and the variation of original sedimentary rocks. The small chemical difference between the cordierite tonalites and the lower grade metamorphic rocks suggests that the former was derived from a massive melting of metapelites or that much of the restite is retained. The material migration among higher amphibolite facies rocks, pelitic granulites, migmatites and cordierite tonalites took place through mineral/melt interaction in the lower crust.
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Cordierite
Greenschist
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Indian Shield
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The high‐grade crystalline nappes exposed southeast of the São Francisco craton comprise two distinct units of mainly granulite facies rocks that represent a composite section of Neoproterozoic deep continental crust: the Socorro‐Guaxupé nappe above, derived from an arc terrane, and the Três Pontas‐Varginha nappe below. Metamorphism in the Três Pontas‐Varginha nappe is characterized by the exceptional preservation of kyanite granulites (700–750°C, 15 kbar), and followed by limited retrogression. Maximum temperatures around 900–950°C were reached toward the base of the overlying Socorro‐Guaxupé nappe during the intrusion of charnockitic‐mangeritic magmas. Lower‐pressure metamorphism, accompanied by anatexis, prevailed at shallower crustal levels. Our petrological results document an inverted thermal structure with isobaric heating of the top of the high‐pressure granulite nappe. Both granulite nappes were transported more than 200 km eastward above lower nappes involving reworked basement and passive margin units, both metamorphosed to high‐pressure but lower‐temperature conditions. Significant thinning and cooling of the two granulite nappes may have occurred before their emplacement onto the lower nappes. The proposed geodynamic scenario considers that continental subduction took place westward underneath Neoproterozoic oceanic lithosphere. The two granulite units crystallised at ∼ 45 km depths under distinct paleogeotherms within this subduction zone around 630 Ma. The kyanite granulites were rapidely exhumed through the mechanism of low‐angle “forced” extrusion, whereas syncollisional collapse affected the soft, anatectic middle crust of the overlying arc terrane. The final emplacement of the thinned nappe pile onto the cold São Francisco craton and its platform cover, with at most, anchizonal to greenschist‐facies metamorphism, occurred around 600 Ma.
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