Contrasting Episodes of Regional Granulite-Facies Metamorphism in Enclaves and Host Gneisses from the Aravalli–Delhi Mobile Belt, NW 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.

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Early Archaean continental crust in the Eastern Ghats granulite belt, India: isotopic evidence from a charnockite suite

Geological Magazine ◽

10.1017/s0016756801005702 ◽

2001 ◽

Vol 138 (5) ◽

pp. 609-618 ◽

Cited By ~ 33

Author(s):

S. BHATTACHARYA ◽

RAJIB KAR ◽

S. MISRA ◽

W. TEIXEIRA

Keyword(s):

Continental Crust ◽

Granulite Facies ◽

Crustal Thickening ◽

Eastern Ghats ◽

Mobile Belt ◽

Granulite Facies Metamorphism ◽

Granulite Belt ◽

Facies Metamorphism ◽

Geochemical Studies ◽

Lower Crustal

The Eastern Ghats granulite belt of India has traditionally been described as a Proterozoic mobile belt, with probable Archaean protoliths. However, recent findings suggest that synkinematic development of granulites took place in a compressional tectonic regime and that granulite facies metamorphism resulted from crustal thickening. The field, petrological and geochemical studies of a charnockite massif of tonalitic to trondhjemitic composition, and associated rocks, document granulite facies metamorphism and dehydration partial melting of basic rocks at lower crustal depths, with garnet granulite residues exposed as cognate xenoliths within the charnockite massif. The melting and generation of the charnockite suite under granulite facies conditions have been dated c. 3.0 Ga by Sm–Nd and Rb–Sr whole rock systematics and Pb–Pb zircon dating. Sm–Nd model dates between 3.4 and 3.5 Ga and negative epsilon values provide evidence of early Archaean continental crust in this high-grade terrain.

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Zircons from a Pegmatite Cutting Eclogite (Gridino, Belomorian Mobile Belt): U-Pb-O and Trace Element Constraints on Eclogite Metamorphism and Fluid Activity

Geosciences ◽

10.3390/geosciences10050197 ◽

2020 ◽

Vol 10 (5) ◽

pp. 197 ◽

Cited By ~ 1

Author(s):

Sergey G. Skublov ◽

Aleksey V. Berezin ◽

Xian-Hua Li ◽

Qiu-Li Li ◽

Laysan I. Salimgaraeva ◽

...

Keyword(s):

Trace Element ◽

Granulite Facies ◽

Mobile Belt ◽

Belomorian Mobile Belt ◽

Pegmatite Vein ◽

Granulite Facies Metamorphism ◽

Zircon Population ◽

Element Abundances ◽

Vein Formation ◽

Facies Metamorphism

This report presents new data on U-Pb geochronology, oxygen isotopes, and trace element composition of zircon from a pegmatite vein crosscutting an eclogite boudin on Stolbikha Island, Gridino area, Belomorian mobile belt (BMB). The zircon grains occur as two distinct populations. The predominant population is pegmatitic and shows dark cathodoluminescence (CL); about a third of this population contains inherited cores. The second zircon population is typical of granulite and exhibits a well-defined sectorial (mosaic) zoning in CL. Both the inherited cores and sectorial in CL zircons appear to have been captured from metabasites as xenocrysts during the pegmatite vein formation. A U-Pb age of 1890 ± 2 Ma for the main zircon population is interpreted as the age of the pegmatite injection. This value is close to the age threshold for the BMB eclogites (~1.9 Ga) and unambiguously defines the upper age limit for the eclogite metamorphism. The pegmatite formation is thus related to partial melting events that occurred during the retrograde amphibolite-facies metamorphism shortly after the eclogitization. A U-Pb date of 2743 ± 10 Ma obtained for the sectorial in CL zircons is considered as the age of the granulite-facies metamorphism established previously within the BMB. The values of δ18O in the zircon populations overlap in a broad range, i.e., δ18O in the pegmatitic zircons varies from 6.1‰ to 8.3‰, inherited cores show a generally higher δ18O of 6.7–8.8‰, and in the captured granulitic zircons δ18O is 6.2–7.9‰. As a result of fluid attack during the final stage of the pegmatite vein formation, the composition of the pegmatitic zircons in terms of non-formula elements (REE, Y, Ca, Sr, Ti) has become anomalous, with the content of these elements having been increased by more than tenfold in the alteration zones. Our data provide new constraints on the timing of eclogite metamorphism within the BMB and show that the late-stage pegmatite-related fluids exerted a very pronounced influence on trace element abundances in zircon, yet had no significant impact on the isotopic composition of oxygen.

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High-pressure granulite facies metamorphism of Archean Bastar craton at the interface of Eastern Ghats Belt: Implications for cratonic subduction/underthrusting

10.5194/egusphere-egu21-6976 ◽

2021 ◽

Author(s):

Padmaja Jayalekshmi ◽

Tapabrato Sarkar ◽

Somnath Dasgupta ◽

Rajneesh Bhutani

Keyword(s):

High Pressure ◽

Shear Zone ◽

Granulite Facies ◽

Eastern Ghats ◽

Mobile Belt ◽

High Grade ◽

Granulite Facies Metamorphism ◽

Bastar Craton ◽

High Pressure Granulite ◽

Facies Metamorphism

<p>The Bastar Craton at the interface of Eastern Ghats Belt (EGB) contains a m&#233;lange of rocks from both the Archean cratonic domain and the adjacent Proterozoic mobile belt domain marking a broad shear zone, known as the Terrane Boundary Shear Zone (TBSZ). The TBSZ preserves a very rare occurrence of high-grade metamorphosed Archean cratonic rocks, whose ancestry has been constrained by Nd model ages. This study presents the petrological and geochemical characterization of mafic granulites and orthopyroxene bearing granitoids from the shear zone and its implications on the tectonic evolution of the craton &#8211; mobile belt boundary. Detailed petrographic, geothermobarometric and P-T pseudosection studies indicate that the Bastar cratonic rocks underwent high-pressure granulite facies metamorphism along a clockwise P-T path, reaching ~900&#176;C and 9-10 kbar. The originally amphibolite facies rocks, metamorphosed through dehydration-melting of hornblende (mafic rocks) and biotite (felsic rocks), to attain the peak P-T conditions. We suggest that this high-grade metamorphism was due to the subduction/underthrusting of the Bastar Craton beneath the EGB, supported by the available seismic data, which resulted from far-field stress related to the Kuunga orogeny in an intraplate setting.</p>

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