Mid-Neoproterozoic ridge subduction and magmatic evolution in the northeastern margin of the Indochina block: Evidence from geochronology and geochemistry of calc-alkaline plutons

摘要

The mid-Neoproterozoic medium-to high-K calc-alkaline magmatic rocks in the northeastern margin of the Indochina block, SW China, provide important insights into the relationship of the Indochina block with the Gondwana supercontinent. Here we report zircon LA-ICP-MS U-Pb data from the early and late stage plutons which yield weighted mean Pb-206/U-238 ages of 765 Ma and 732-739 Ma suggesting mid-Neoproterozoic emplacement. The zircon epsilon(Hf)(t) values show a range of -3.2 to +2.4 (average +0.1 +/- 0.9) with T-DM(C) of 1510 to 1870 Ma for the early plutons, and -5.4 to +5.1 (average +2.1 to -3.9) with T-DM(C) of 1366 to 1985 Ma for late plutons. Both groups show similar geochemical characteristics including high Mg#, enrichment of LILE and LREE, slight negative Eu anomalies, and strongly negative Nb, Ta and Ti anomalies, with all the samples falling within the continental/island arc field in tectonic discrimination diagrams. These features suggest that the early and late stage magmas were produced by the mixing of mantle-derived magma and crust-derived magma in different proportion within an active continental margin, in subduction-related continental-arc tectonic setting. The linear zoning and roughly parallel distribution of the two generations of intrusions with a hiatus of 20 Ma might suggest an episode of ridge subduction with asthenosphere upwelling through the slab window that generated the second phase of plutons. (C) 2016 Elsevier B.V. All rights reserved.