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There is increasing evidence indicating that melts derived from subducted oceanic crust and sediments may have played a key role in building continental crust. This mechanism predicts that juvenile arc crust should have oxygen isotope characteristics ranging from mantle-like to supracrustal, but consistent mantle-like radiogenic (Nd-Hf) isotopic signatures. Here we present in-situ zircon U-Pb dating, Hf-O isotope analyses, and whole rock major-trace element and Nd isotope analyses of a granitoid from NW India. In-situ secondary ion mass spectrometry (SIMS) zircon U-Pb dating yields a weighted mean 207Pb/206Pb age of 873±6 Ma for the granitoid. It displays mantle-like zircon εHf(εHf(873 Ma)=+9.3 to +10.9) and whole-rock Nd (εNd(873 Ma)=+3.5) values but supracrustal δ18O values, the latter mostly varying between 9‰ and 10‰. The calculated whole-rock δ18O value of 11.3‰±0.6‰ matches well with those of hydrothermally-altered pillow lavas and sheeted dykes from ophiolites. The major and trace element composition of the granitoid is similar to petrological experimental melts derived from a mixture of MORB+sediments. Thus, the granitoid most likely represents the product of partial melting of the uppermost oceanic crust (MORB+sediments). We propose that the decoupling between Hf-Nd and O isotopes as observed in this granitoid can be used as a powerful tool for the identification of slab melting contributing to juvenile continental crustal growth. Such isotopic decoupling can also account for high δ18O values observed in ancient juvenile continental crust, such as Archean tonalite-trondhjemite-granodiorite suites. [ABSTRACT FROM AUTHOR]

Abstract: The Sausar Mobile Belt at the southern margin of the Central Indian Tectonic Zone (CITZ) is a key unit in constraining the timing of continent–continent collision between the North and South Indian cratonic blocks. It consists of three domains – northern, central and southern – that record the history of this collision. Rocks from the northern and central domains were metamorphosed under medium pressure–medium temperature conditions and define a clockwise P–T trajectory. Monazite spot ages in garnet–cordierite migmatite from the western extremity of the northern domain yield a weighted mean age of 1043±18Ma (MSWD=0.26) for monazite cores and 955±11Ma (MSWD=0.43) for monazite rims. These ages are correlated with peak granulite facies metamorphism and post-peak decompression, respectively. Garnet–orthoamphibole gneiss and garnet–cordierite migmatite from the central and eastern sectors of the northern domain, respectively, yield weighted mean ages (for monazite) of 974±9Ma (MSWD=0.53) and 989±9Ma (MSWD=1.3). These ages mark the timing of retrograde metamorphism in the northern domain. Charnockite in the northern domain, which was emplaced during post-peak decompression, records a SHRIMP U–Pb zircon concordia age of 938±3Ma (MSWD=1.9). Lu–Hf isotopic analyses of these zircons show a restricted range of εHf(T) values (from −2.9 to −5.8 with a mean of −4.3) and have TDMc model ages from 1995 to 2163Ma (with a mean of 2064Ma), implying derivation of charnockitic magma from a Paleoproterozoic crustal source. In contrast, garnet–staurolite–kyanite schist and garnet–biotite–muscovite–quartz schist from the central domain of the Sausar Mobile Belt yield weighted mean monazite ages of 1062±13Ma (MSWD=1.2) and 993±19Ma (MSWD=0.16), respectively, which are broadly correlated with the peak and retrograde stages of metamorphism from this domain. Geochronological data thus tightly bracket the collisional orogeny in the CITZ at between 1.06Ga and 0.94Ga. When combined with recent data from collisional belts further east of the CITZ, namely the Chhotanagpur Gneissic Complex and Shillong Plateau Gneissic Complex, a uniformity of Mesoproterozoic to Early Neoproterozoic events is evident, implying final amalgamation of the North and South Indian blocks at this time along a ~1500km long orogenic belt. This means that two or more separate blocks were in existence at the time of incorporation within Rodinia, not a single block as suggested in most current models. [Copyright &y& Elsevier]

[Display omitted] • Granitoids of Chitradurga belt were emplaced at ∼ 3.2 Ga, ∼3.0 Ga and ∼ 2.6 Ga. • Two metamorphic events are documented at ∼ 3.0 Ga and ∼ 2.56 Ga. • Nd isotopes infer juvenile crustal addition at 3.2 Ga, reworking at 3.0 Ga and 2.6 Ga. • Role of convergent margin tectonics in the evolution of Chitradurga granitoids. The Chitradurga Greenstone Belt is one of the largest belts in the central part of the Western Dharwar Craton (WDC). We present new SHRIMP U-Pb zircon and titanite ages, whole-rock geochemical and Sm-Nd isotopic data for granitoids that developed at both its eastern and western margins, and for those that are intrusive into the belt. Western margin TTG samples yield emplacement ages ofca. 3.21 Ga and are of the high-Al TTG type characterized by weakly negative ɛNd t values. Eastern margin granite samples yield emplacement ages of ∼ 3.0 Ga and 2.56 Ga with negative ɛNd t values. The granites intrusive into the belt are of Neoarchean age (∼2.61 Ga) with more-evolved Nd isotopic signatures consistent with the involvement of older crustal material. Metamorphic events are documented at ca. 3.0 Ga and ca. 2.5 Ga. The ca. 3.0 Ga event is synchronous with the diapiric emplacement of trondhjemites and high-K granitic plutons in the WDC. The peak metamorphic event at ∼ 2.5 Ga is the major tectono-thermal event evident throughout the Dharwar Craton. Geochemical systematics imply the involvement of convergent margin tectonics in the evolution of studied granitoids. The role of horizontal tectonics in the crustal growth of WDC is evident from at least 3.2 Ga, whereas signatures for the reworking of the older crust are more prominent from ∼ 3.0 Ga. [ABSTRACT FROM AUTHOR]

• The Bababudan Group is an intracontinental rift to passive-margin succession. • The Chitradurga Group is divided into unconformity bound Stages I and II. • Stage I is the sedimentary and volcanic record of an extensional back-arc basin. • Stage II is the sedimentary and volcanic record of a transtensional back-arc basin. • Stratigraphic equivalents in the Eastern Dharwar Craton are near-arc basins. The Dharwar Supergroup comprises the unconformity bound Bababudan and Chitradurga groups. The Bababudan Group, which is best preserved in the Western Dharwar Craton, records a divergent margin comprising a basalt-dominant intracontinental rift sequence, a shale-BIF drift sequence, and a sandstone-shale thermal-subsidence sequence. The rift stage evolved from ∼2765 Ma to ∼2720 Ma, whereas the succession was folded, uplifted and eroded during development of a convergent continental margin from ∼2680 Ma. Ocean opening was to the east or southeast. The Chitradurga Group records a two-stage back-arc basin behind an east-facing continental arc. Stage I evolved between ∼2609 Ma and ∼2582 Ma, and Stage II between ∼2582 Ma and ∼2540 Ma. The two stages are separated by a subaerial unconformity. Stage I comprises siliciclastic fluvial, shallow-marine and deep-marine sedimentary rocks and bimodal volcanic rocks. Stage I is best preserved in the Western Dharwar Craton, but there are equivalents in the Eastern Dharwar Craton. Stage II in the Western Dharwar Craton comprises deep-marine turbidites of siliciclastic and volcaniclastic provenance, and basaltic and felsic volcanic rocks. Volcanic Stage II sequences in the Eastern Dharwar Craton define an arc-adjacent position. Tectonically driven cyclic uplift and erosion were responsible for the mismatch between preserved stratigraphic thickness and time, particularly for Stage II sequences. A remnant of a <2540 Ma late-stage basin in the northeast of the Eastern Dharwar Craton defines a syncollisional tectonic setting. The Dharwar Supergroup was deformed in a SW-verging hinterland magmatic fold/thrust belt from ∼2540 Ma. [ABSTRACT FROM AUTHOR]