Geochronology and geochemistry of late Carboniferous volcanic rocks from northern Inner Mongolia, North China: Petrogenesis and tectonic implications

摘要

Zircon U-Pb ages, geochemical and Sr-Nd isotopic data are presented for the late Carboniferous Baoligaomiao Formation (BG Fm.) and Delewula Formation (DW Fm.) volcanic rocks, widely distributed in northern Inner Mongolia, in the northern part of the Xing'an-Mongolia Orogenic Belt (XMOB). The BG Fm. rocks mainly consist of basaltic andesites and andesites while the DW Fm. rocks include dacites, trachytes, rhyolites, pyroclastic rocks and minor andesites. New LA-ICPMS zircon U-Pb analyses constrain their eruption to late Carboniferous (317-322 Ma and 300-310 Ma, respectively). The BG Fm. volcanic rocks are characterized by enriched large ion lithophile elements (LILE) and depleted high field strength elements (HFSE), with initial Sr-87/Sr-86 ratios of 0.70854-0.70869 and negative epsilon Nd(t) (-2.1 to -2.4) values. They have low La/Ba (0.03-0.05), high La/Nb (2.05-3.70) ratios and variable Ba/Th (59.5-211) ratios. Such features suggest that they are derived from melting of heterogeneous sources including a metasomatized mantle wedge and Precambrian crustal material. The DW Fm. volcanic rocks are more depleted in HFSE with significant Nb, Ta, P, Ti anomalies. They have high initial Sr-87/Sr-86 ratios (0.72037-0.72234) and strong negative epsilon Nd(t) (-11 to -11.6) values which indicate those igneous rocks were mainly derived from reworking of the Paleoproterozoic crust. The late Carboniferous volcanic rocks have geochemical characteristics similar to those of the continental arc rocks which indicate the northward subduction of the Paleo Asian Ocean may have continued to the late Carboniferous. The Volcanic association of this study together with the early Permian post-collisional magmatic rocks suggests that a tectonic transition from subduction-related continental margin arc volcanism to post-collisional magmatism occurred in the northern XMOB between the late Carboniferous and the early Permian. (C) 2015 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.