Geochronology and geochemistry of Mashan trachydacite, JiMo District, Shandong Province and their geological implications

Intermediate-acid volcanic rocks with columnar joints expose in the Mashan area, Jimo, Shandong Province. The geochronology and tectonic setting of the volcanic rocks had been rarely studied. The petrographic characteristics and major element results show that the volcanic rocks at Mashan are trachydacite. Zircon LA-ICP-MS U-Pb dating of the trachydacite shows that the volcanic rocks erupted at 113.2 ± 1.3 Ma, that is, in the late Early Cretaceous. In-situ Lu-Hf analysis suggests that the zircon εHf (t) values range from −19.5 to −0.4 and two-stage model ages of zircons range from 1.97 Ga to 3.26 Ga, indicating that the magma was sourced from the melting of ancient crust. The Mashan trachyandesite is characterized by high content of SiO₂ and Al₂O₃, low content of MgO, enriched Na₂O and K₂O. Moreover, all samples are featured by lower total contents of rare earth elements, enriched light rare earth elements, deficit heavy rare earth elements, slightly negative Eu anomaly, high Sr content, low Y and Yb contents. The major and trace elements of the trachydacite in Mashan are quite similar to adakite, and the adakitic magma could have been formed by the melting of the lower old crust after the regional crust thickening in Eastern Norch China Block during the Yanshanian.