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Presence and geodynamic significance of Cambro-Ordovician series of SE Karakoram (N Pakistan)

Abstract : New geological, geochemical and geochronological data from the Southern Karakoram (NE Pakistan) indicate the presence of several unexpectedly old and well preserved units along the Asian margin: (1) a Precambrian basement, displaying a minimum amphibole Ar-Ar age of 651 Ma; (2) a thick Cambro-Ordovician platform-type sedimentary unit overlying the Precambrian basement. These series are dated by graptolite and crinoid faunas, and are confirmed by concordant 87Sr/ 86Sr and 13C "ages" of the marbles; (3) a dismembered ophiolitic series formed by slices of metagabbros and metabasalts separated by ultramafic lenses (the Masherbrum Greenstone Complex). The occurrence of such Cambro-Ordovician series overlying a Precambrian basement in south-eastern Karakoram similar to the south-western Karakoram shows that the Karakoram constitutes a continuous tectonic block. The petrology and geochemistry of the Masherbrum Greenstone Complex (mineral chemistry, major and trace element and Sr-Nd isotopic data) are indicative of a supra-subductive environment. The presence of LREE-enriched calc-alkaline rocks 〚(La/Yb) N = 4.4-5.6; (Nb/La) N = 0.2-0.3; ɛNd 565 = 5.1-7.1〛 and LREE-depleted tholeiitic rocks 〚(La/Yb) N = 0.5-1.3; (Nb/La) N = 0.6-0.9; ɛNd 565 = 5.6-7.8〛 are consistent with arc and back-arc settings, respectively. A high-Mg andesitic dolerite and an OIB-type metabasalt, with lower ɛNd ratios (ɛNd 565 = 0.5 and 4.5) are in accordance with source heterogeneity beneath the arc. The Masherbrum Greenstone Complex, along with other Cambro-Ordovician central-eastern volcanic series give evidence of a tectonic situation governed by micro-plate convergent-divergent systems with occurrence of arc - back-arc settings during the Lower Palaeozoic, comparable to that of the current SW Pacific area.
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Yann Rolland, Christian Picard, Arnaud Pêcher, Elisabeth Carrio, Simon M. F. Sheppard, et al.. Presence and geodynamic significance of Cambro-Ordovician series of SE Karakoram (N Pakistan). Geodinamica Acta, 2002, 15, pp.1-21. ⟨10.1016/S0985-3111(01)01075-0⟩. ⟨insu-03607079⟩



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