Petrogenesis of Middle Triassic A₂‐Type Granitic Pluton in Shigongmiao Area, Hainan Island: Insights on Early Mesozoic Tectonic Evolution of South China Block

The ongoing controversy regarding the Early Mesozoic tectonic regimes of Hainan Island (influenced by Paleo‐Tethyan or Proto‐Pacific dynamics) hinders a proper understanding of the geological evolution of the South China Block. A 2 ‐type granites, typically associated with extensional environments, play a crucial role in addressing this issue as they form either in post‐collisional or subduction‐related extensional settings. In this study, we conducted a comprehensive analysis using whole‐rock geochemistry, Sr‐Nd‐Pb isotopes, zircon U‐Pb ages, and Hf isotopes on a Triassic granite from the Shigongmiao district on the east coast of Hainan Island. The new LA‐ICP‐MS U‐Pb age reveals that the studied granites were emplaced in the Middle Triassic (U‐Pb ages of ∼242 Ma), contemporaneous with the A‐type granitic rocks distributed along the southeast coast of South China, which have ages ranging from ca. 230 to 250 Ma. Geochemically, these samples exhibit high SiO 2 (76.4–77.2 wt%), total alkalis (Na 2 O + K 2 O = 7.5–7.9 wt%), Ga/Al (> 2.6), Rb/Nb (10.3–11.9), and Y/Nb (1.8–2.5), collectively supporting an A 2 ‐type affinity for the Shigongmiao pluton. All samples consistently display enriched ε Nd ( t ) and ε Hf ( t ) values, along with old two‐stage crustal Nd‐Hf isotopic model ages (1.70–2.10 Ga) and high zircon Ti thermal temperatures (796°C–851°C). Geochemical and isotopic characteristics suggest that the Shigongmiao granites formed by partial melting of intermediate‐felsic calc‐alkaline granitoids under high‐temperature, low‐pressure conditions. Integrating regional geological data, we conclude that the Shigongmiao A 2 ‐type granite originated in an extensional setting associated with Proto‐Pacific plate subduction. This finding supports the existence of pre‐existing continental arc magmatism along the southeastern margin of the South China Block during the Early Mesozoic era.