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

Abstract

The Mesozoic intermediate-silicic intrusive rocks in the Tongling area, Anhui Province, eastern China, include a high-K, calcalkaline series and a shoshonitic series . Rocks of the calc- alkaline series comprise more than 90% of the total and consist chiefl y of gabbro-diorite , granodiorite, quartz monzodiorite, 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, mafi c quartz monzodiorite, and microdiorite. The shoshonitic series consists of pyrox ene monzodiorite, monzonite, and quartz monzonite, which are commonly asso ciated 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 micro probe (SHRIMP) U-Pb age data suggest that the granodiorites, quartz monzo diorites, 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 refl ects mix