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