Petrogenesis of high-K, calc-alkaline and shoshonitic intrusive rocks in the Tongling area, Anhui Province (eastern China), and their tectonic implications

摘要

The Mesozoic intermediate-silicic intrusive rocks in the Tongling area, Anhui Province, eastern China, include a high-K, calc-alkaline series and a shoshonitic series. Rocks of the calc- alkaline series comprise more than 90% of the total and consist chiefly of gabbro-diorite, granodiorite, quartz monzo-diorite, and porphyritic quartz monzodiorite. These rocks are associated with important skarn-type copper-iron deposits. They contain three types of enclaves: mica-rich varieties that appear to be residues of partially melted pelitic rock, mafic quartz monzodiorite, and microdiorite. The shoshonitic series consists of pyroxene monzodiorite, jnonzonite, and quartz monzonite, which are commonly associated with skarn-type gold deposits. Enclaves in these rocks are typically pyroxene-rich or amphibole-rich varieties or amphibole gabbros. Zircon sensitive high-resolution ion microprobe (SHRIMP) U-Pb age data suggest that the granodiorites, quartz monzodiorites, and gabbro-diorites of the calc-alkaline series were generated at ca. 146-142, 143, and 140 Ma, respectively. The shoshonitic rocks range in age from 143 to 136 Ma. Although there is some overlap in reported ages of the two series, contact relations indicate that the shoshonitic rocks postdate the calc-alkaline varieties. On the basis of the geochemistry of the two series and the character of their enclaves, the shoshonitic series is thought to have formed primarily by differentiation of a mantle-derived, weakly contaminated, alkali basalt magma, whereas the high-K, calc-alkaline series reflects mix- ing of differentiated mantle and crustal melts, followed by assimilation-fraction crystallization (AFC) processes. The magmatic activity may have been related to reactivation of the Tongling-Deijiahui structural zone in response to rapid, highly oblique subduction of the paleo-Pacific plate beneath South China.