Geochronology and geochemistry of igneous rocks from the Kongling terrane: Implications for Mesoarchean to Paleoproterozoic crustal evolution of the Yangtze Block

摘要

The Kongling terrane is a high grade metamorphic complex, exposed in the northern part of the Yangtze Block. It mainly consists of TTG gneisses, amphibolites, migmatites and metasedimentary rocks. The amphibolites occur as enclaves or layers in the TTG gneisses, and both the amphibolites and the TTG gneisses are intruded by granitic and diabasic dykes. U-Pb ages of zircons from the amphibolite, migmatite, granite and diabase are determined by LA-ICP-MS. Igneous zircons from an amphibolite have an upper intercept age of 2945 13 Ma (MSWD = 1.2) while metamorphic zircons yield an upper intercept age of 2870 +/- 13 Ma (MSWD = 0.9). Igneous zircons from a migmatite and a granite have weighted average Pb-207/Pb-206 age and upper intercept age of 1987 +/- 5 Ma (MSWD = 1.7) and 1985 +/- 6 Ma (MSWD = 1.3), respectively. Igneous zircons from a diabase dyke yield an upper intercept age of 1783 +/- 23 Ma (MSWD = 0.86). The protoliths of the amphibolites are tholeiitic basalts and exhibit slightly depleted to variably enriched LREE patterns with moderate enrichment of Rb and Ba and-depletion in Nb and Zr, similar to oceanic island arc volcanics. They have high Cr, Co, Ni, MgO and FeOT content with negative epsilon(Nd)(t) values, indicating that the tholeiitic basalts resulted from high degree batch melting of enriched mantle wedge. The granites are strongly peraluminous with A/CNK > 1.1 and they have relatively low HREE and Y and negative epsilon(Nd)(t) values. Detailed elemental and isotopic data suggest that they formed by partial melting of recycled products of the TTG gneisses, amphibolites and older crustal material in a continental collisional setting. The diabases are characterized by relatively high Al2O3, FeOT and TiO2 contents and low Zr/Ba ratios. Their high initial Sr-87/Sr-86 ratios and negative epsilon(Nd)(t) values indicate that they were derived from enriched lithospheric mantle in extensional setting. Therefore, the Kongling terrane occurred as an oceanic island arc at ca. 2.95 Ga, and experienced earliest metamorphism at ca. 2.87 Ga which probably contributed to the formation of the high grade metamorphic terrane. This terrane experienced a strong collisional event at similar to 2.0 Ga which may have led to cratonization over all of the Yangtze Block, and also experienced lithospheric extension during 1.85-1.78 Ga. The global similar to 2.0 Ga Paleoproterozoic collisional events were related to the assembly of supercontinent Columbia. (C) 2014 Elsevier B.V. All rights reserved.