Neoarchean greenstone volcanism and continental growth, Dharwar craton, southern India: Constraints from SIMS U-Pb zircon geochronology and Nd isotopes
Résumé
We present SIMSU-Pbzircon ages andNdisotope data for the felsic volcanic rocks from seven Neoarchean greenstone belts of the Eastern Dharwar craton (EDC) and from the Chitradurga greenstone belt in the Western Dharwar craton (WDC). Zircon ages show bimodal age distribution of felsic volcanism. The ca. 2.70-2.65 Ga felsic volcanic event is contemporaneous with 2.7 Ga mafic greenstone volcanism and emplacement of juvenile tonalitic to granodioritic crust, while 2.58-2.54 Ga felsic volcanics are coeval and spatially (and probably genetically in some cases) linked to the major episode calc-alkaline magmatic accretion in the EDC. The Chitradurga and Veligallu greenstone belts host felsic volcanics of the first generation, the latter showing inheritance at ca. 2.95 Ga. Four of the five greenstone belts hosting the second generation of felsic volcanics (Chitradurga, Kolar, Kadiri, Hutti) show crustal inheritance at ca. 2.6, 2.7, 2.9, 3.0, 3.1 and 3.3 Ga. åNdt indicate derivation of the felsic volcanics from juvenile sources or short-lived crustal sources with minor influence of older crust. The new and existing data are consistent with two-stage growth of the Dharwar craton in the Neoarchean. First-stage accretion led to the growth of a 2.7-2.6 Ga juvenile crustal province of mafic volcanics and felsic plutons along the eastern margin of the WDC. Second-stage accretion (2.58-2.52 Ga) led to the emplacement of TTG and calc-alkaline plutons and felsic volcanics throughout the Eastern Dharwar craton. An active margin context could apply for the two-stage accretion scenario considering a west-dipping subducting slab beneath the craton in the framework of long-lived ultra-hot accretionary orogen. But the great width of influence of magmatic accretion and/or reworking, particularly during the second stage, would suggest large-scale mantle flow reorganization that would have generated large plume(s).