Feldspar Lead Isotopic Composition of Granitoids from the Eastern Qinling Orogenic Belt and Their Tectonic Significance

Abstract

AbstractThis paper reports 48 feldspar lead isotope analyses from 27 granitic intrusions, which formed from the Late Proterozic to Mesozoic within the Eastern Qinling oregenic belt. It is found that the granitic rocks of South Qinling are characterized by a strong block–effect and depletion in U–Pb and Th–Pb, showing that these rocks came from the same lead isotope tectono–geochemical province, while those of North Qinling are characterized by higher U–Pb and Th–Pb for Late Proterozoic to Early Paleozoic ones and lower U–Pb and Th–Pb for Late—Palaeozoic and younger ones in their feldspar lead isotopic composition. In the North Qinling block, lead isotopic signatures reflect that the source of granitic magma had changed since the Late Palaeozoic. Comparison of feldspar lead isotopic composition between South Qinling and North Qinling shows that there is marked difference in lead isotopic composition for pre–Palaeozoic granitoids, indicating that the South Qinling and the North Qinling blocks belong to different tectonic units, but the similarities in lead isotopic composition are quite clear, which indicates that the South Qinling block had been welded with the North Qinling block and that the magma sources of both blocks were identical. The analysis provides direct evidence for underplating of the continental crust of South Qinling beneath the North Qinling block in the continent–continent interaction stage of the Eastern Qinling oregenic belt.