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

摘要

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 ~(207)Pb/ ~(206)Pb age of ca.1.85Ga, which represents its intrusion age. The Quanqitang pluton has high SiO _2 (72.6-74.4wt.%), K _2O (5.09-5.56wt.%) and Na _2O (3.02-3.36wt.%), and low Fe _2O _3 (2.56-3.19wt.%) and MgO (0.25-0.37wt.%). 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°C with an average of 845°C. Zircons from the granites have ε _(Hf)(t) values ranging from -17.6 to -20.9, corresponding to two-stage Hf modal ages of 3.6 to 3.8Ga. 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. 2.0Ga 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. 1.85Ga. The occurrence of the A _2-type granite indicates that the Kongling microcontinental block became a stabilized craton at ca. 1.85Ga.