Petrogenesis and geodynamics of the Dashenshan I-type granite and its implications for the Triassic tectonic evolution of the South China Block

Abstract

There is significant debate concerning the tectonic characteristics and evolutionary understanding of the South China Block (SCB) during the Early Mesozoic. One of the key points of contention is the tectonic–magmatic activity during the Triassic and its dynamic mechanisms. However, research on the detailed chronology and tectonic settings of granite plutons in key regions remains insufficient, limiting the understanding of the tectonic–magmatic dynamic mechanisms in the interior of SCB during the Triassic. In this contribution, we present whole-rock major and trace elemental data, Sr–Nd isotope data, LA-ICP-MS zircon U–Pb age dating, and Lu–Hf isotope data for granites of Dashenshan pluton in the Xiangzhong, northwest part of SCB. The results indicate that the Dashenshan granite has an emplacement age of 208.4–212.5 Ma, characterized by high SiO₂, Na₂O, and K₂O contents and low MgO and CaO. The Dashenshan granite is enriched in light rare-earth elements with a significant negative Eu anomaly (average δEu = 0.42). It is also enriched in Rb, K, and Th and shows pronounced depletion in Nb, Ta, and Ti, classifying it as peraluminous calc-alkaline granite, specifically of the I-type. The zircon ε_Hf(t) values range from − 8.39 to − 4.4, with an average of − 5.82, and the Sr–Nd isotopes are relatively enriched [ε_Nd(t) = − 9.31 to − 6.8]. Combining these geochemical characteristics, it is revealed that the Dashenshan granite was derived from the partial melting of middle to upper crustal metamorphic basement materials under medium- to low-temperature conditions, with possible minor contributions from mantle-derived materials. Furthermore, it underwent fractional crystallization, including plagioclase differentiation. By integrating the geochemical features and spatial distribution of Triassic granites in SCB, this study suggests that the regional tectonic evolution of SCB during the Triassic was primarily controlled by the collision of the SCB with the Indochina Block and the North China Block. In Xiangzhong, the tectonic setting transitioned from syn-collisional compression to post-collisional extension during the Late Triassic. The Dashenshan pluton formed in a post-collisional extensional setting, resulting from the decompression melting of middle-to-upper crustal rocks. The upwelling of the asthenosphere and upward heat transfer likely played a significant role in the formation of the Dashenshan granitic magma.