Late Mesozoic high-K calc-alkaline magmatism in Southeast China: the Tongling example

摘要

We report new zircon U-Pb ages, Hf isotopic and geochemical results for the Tongling granitic plutons of Southeast China. SHRIMP U-Pb ages for the Miaojia quartz monzodiorite porphyrite,the Tianebaodan and Tongguanshan quartz monzodiorites, the Xinqiaotou granodiorite porphyry, and the Shatanjiao and Nanhongchong granodiorite are 143 +/- 2, 141 +/- 1 and 142 +/- 1, 147 +/- 1, and 145 +/- 1 and 139 +/- 1Ma, respectively. Combined with previous geochronological data, our results indicate that the porphyritic rocks are older than rocks of the same type lacking porphyritic texture. Geochemically, these high-K calc-alkaline intrusive rocks are characterized by arc-like trace element distribution patterns, with significant enrichment in LILE and LREE but depletion in HFSE. Lu-Hf isotopic compositions of zircons from the high-K calc-alkaline (HKCA) rocks have epsilon(Hf)(t) values of magmatic 139-147Ma zircons from -8.1 to -25.6, with two-stage model ages (t(DM2)) of 1.71-2.67Ga, whereas epsilon(Hf)(t) values of inherited 582-844Ma zircons range from 5.4 to -9.5, with t(DM2) of 1.39-2.22Ma, younger than t(DM2) values of igneous zircon, indicating that newly formed mantle material was added to the continental crust of the Yangtze Block. Moreover, epsilon(Hf)(t) values of inherited zircon cores older than 1000Ma are from -7.8 to -26, similar to magmatic zircons, and the t(DM2) values are all greater than 3.0Ga (3.16-3.75Ga), reflecting partial melting of ancient sialic material. We conclude that the plutonic melts were derived from both the enriched mantle and the ancient crust. The HKCA Tongling intrusions coincide temporally with the J(3)-K-1 magmatic event that was widespread in Southeast China. This igneous activity may have accompanied sinistral slip along the Tan-Lu fault due to oblique subduction of the Palaeo-Pacific plate.