Important role of hornblende fractionation in generating the adakitic magmas in Tongling, Eastern China: evidence from amphibole megacryst and cumulate xenoliths and host gabbros

摘要

Tongling, in eastern China, is an area well-known for intra-plate adakites. Here, we present the mineral chemistry and zircon U-Pb ages for amphibole cumulate xenoliths, the mineral chemistry of amphibole megacrysts, and the whole-rock chemistry, zircon U-Pb age and Sr-Nd isotopic compositions of host gabbros from Tongling. Zircon U-Pb dating yields a crystallization age of 120.6 +/- 1.2Ma (MSWD=4.2) for the host gabbros, which are characteristically depleted in high field strength elements (Nb, Ta, and Ti) and enriched in large ion lithophile elements (Ba and Sr), with epsilon Nd (t) of -3.00 to -4.52 and initial Sr-87/Sr-86 ratios of 0.7068-0.7072, suggesting an enriched mantle source. Parental melts, as estimated from average amphibole megacryst and cumulate compositions, have Mg# values of 26-33, are enriched in Ba, Th, U, and Nd, and depleted in Nb, Ta, Zr, Hf, and Ti, similar to 136Ma mafic magmas in Tongling. Zircon U-Pb dating yields a crystallization age of 135.4 +/- 1.0Ma (MSWD=1.6) for the amphibole cumulates. It is concluded that the Tongling adakitic rocks were formed by polybaric crystallization involving early high-pressure intracrustal fractional crystallization of cumulates comprising hornblende and clinopyroxene, and late low-pressure fractional crystallization of hornblende and plagioclase phenocrysts. The flat subduction of Pacific plate and its subsequent foundering during the Cretaceous may have triggered the generation of extensive adakitic magmas and lithospheric thinning in the Lower Yangtze Region.