Petrogenesis of the Laoshan Suite in the Jiaodong Peninsula (Eastern China): An Oxidized Low Ba–Sr A1-Type Granite

Abstract

The Jiaodong Peninsula is closely related to Mesozoic granites in terms of spatial and temporal aspects. However, the specific association between the genesis of gold mineralization and these granites remains unclear. It is also ambiguous why Laoshan-type granites, which are similar to Mesozoic granites, are not gold mineralized. In this study, we analyzed the Laoshan granites and compiled Mesozoic magmatic rock data (Linglong, Guojialing, and Weideshan suites) of the Jiaodong Peninsula. We performed whole-rock major and trace elements, LA–ICP–MS zircon U–Pb geochronology and geochemistry analyses. Our zircon U–Pb data denote that the Laoshan granite was emplaced during the 118 ± 1 Ma. The Laoshan granite is characterized by high SiO2 content (76.03–80.28 wt.%), high TFe2O3/MgO (11.1–27.1) and Ga/Al (3.0–3.5) ratios, high zircon saturation temperature (809–850 °C), and negative Eu (Eu/Eu* = 0.05–0.08) anomalies, showing A-type granite characteristics. Furthermore, the Laoshan granite is identified as an A1-type granite with low Ba (33.1–42.0 ppm) and Sr (14.1–21.0 ppm) contents. It was formed in an extensional tectonic environment induced by the subducting slab roll-back of the Paleo-Pacific Plate, mainly from the partial melting of lower crustal materials, mixed with a small amount of mantle components. Zircon trace elements indicate that the Laoshan suite had relatively high oxygen fugacity and temperature (ΔFMQ = + 2.43 to + 4.22, T-Ti in zircon (mean) = 796 °C) compared to the pre-mineralization and contemporaneous mineralization magma. We propose that oxidized adakitic magma (Weideshan suite) may contribute to the enrichment and mineralization of gold. Although Laoshan-type granite also has a high oxygen fugacity, its location is distant from the ore-controlling faults, which ultimately hinders the formation of gold mineralization. From the Jurassic to the Cretaceous, the tectonic and geochemical properties of magmatic rocks in the Jiaodong Peninsula have changed, and gold deposits are formed in such a transitional process (compression to extension, reduction to oxidation).