Geochemistry, zircon U-Pb age and Hf isotope compositions of Paleoproterozoic aluminous A-type granites from the Kongling terrain, Yangtze Block: Constraints on petrogenesis and geologic implications

An integrated study of petrology, zircon U-Pb age, and geochemistry was reported for a Paleoproterozoic granitic pluton from the Kongling terrain in the northern part of the Yangtze Block. The Quanqitang pluton is composed mainly of alkali feldspar, quartz, biotite and plagioclase. Zircon U-Pb dating yields a Pb-207/Pb-206 age of ca.1.85 Ga, which represents its intrusion age. The Quanqitang pluton has high SiO2 (72.6-74.4 wt.%), K2O (5.09-5.56 wt.%) and Na2O (3.02-3.36 wt.%), and low Fe2O3 (2.56-3.19 wt.%) and MgO (0.25-0.37 wt%). It is metaluminous (A/CNK = 0.93-0.98) and belongs to the high-K calc-alkaline series. On the chondrite-normalized REE diagram, all the samples invariably show a relatively enrichment in light rare earth elements (LREEs) with high (La/Yb)(N) ratios (10.6-21.7) and strong negative Eu anomalies. Their primitive-mantle normalized spidergrams display negative Ba, Nb, Ta, Sr, P, Eu and Ti anomalies. The calculated zirconium saturation temperatures range from 802 to 861 degrees C with an average of 845 degrees C. Zircons from the granites have epsilon(Hf)(t) values ranging from -17.6 to -20.9, corresponding to two-stage Hf modal ages of 3.6 to 3.8 Ga. Based on the geochemical features, it is inferred that the granites were generated by the dehydration melting of Archean tonalitic gneisses with a plagioclase-rich residual assemblage. The granites chemically belong to the A(2)-type granites, and have an intrusion age younger than the possible ca. 2.0 Ga collisional event in the Yangtze Block. This suggests that they formed in a post-collisional tectonic setting, and thus a tectonic regime switch from collision to extension might have occurred in the Yangtze Block before ca. 1.85 Ga. The occurrence of the A(2)-type granite indicates that the Kongling microcontinental block became a stabilized craton at ca. 1.85 Ga. (C) 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.