Geochemical and zircon U–Pb geochronological study of the Yangshan A-type granite: Insights into the geological evolution in south Anhui, eastern Jiangnan Orogen

Abstract

The Early Cretaceous Yangshan granite is an A-type granitic intrusion that was emplaced along the eastern Jiangnan Orogen in southern Anhui Province, South China. The Yangshan intrusion mainly consists of syenite porphyry (127.0±0.6Ma) and alkali-feldspar granite porphyry (126.0±1.0Ma). As a part of Qingyang–Jiuhuashan complex intrusion, the Yangshan A-type granites have lower MgO, CaO, Co, Sr, and higher Rb, Nb, Th and HREE contents, with enrichment in large-ion lithophile elements (LILE) and light rare earth elements (LREE), and slightly negative Eu anomalies. However, the syenite porphyry and the alkali-feldspar granite porphyry differ in terms of zircon εHf(t) values: small variations in the syenite porphyry from −5.5 to −3.7, corresponding to Hf model ages (tDMC) between 1.42Ga and 1.53Ga, and large variations in the alkali-feldspar granite porphyry from −6.4 to +4.4, yielding tDMC of 0.90–1.59Ga. While both rocks also have similar εNd(t) values ranging from −7.02 to −5.47, corresponding Nd model ages (TDMC) are 1.37–1.49Ga, falling within the Hf model ages. We take these features to indicate that the Qingyang I-type granites, which are isotopically similar, and Yangshan A-type granites were originated from partial melting of Mesoproterozoic–Neoproterozoic crust, with minor juvenile crust input for the alkali-feldspar granite porphyry, followed by fractional crystallization. In combination with previous studies, we propose that the Cretaceous A-type granitic rocks formed between 135Ma and 122Ma, implying an important Mesozoic extensional event in eastern Jiangnan Orogen, which facilitated underplating of mantle-derived magma and crustal heating. This may have occurred in a back-arc extension in response to the drift of subduction direction of the Paleo-Pacific plate, which started as early as ~135Ma ago.