Geochronology, Geochemistry, and Sr-Nd-Hf Isotopes of the Mengshan Pluton in Jiangxi, South China: Implications for Indosinian Crustal and Tectonic Evolution

The Mengshan pluton provides an opportunity for studying the tectonic setting of the Indosinian orogeny and crustal evolution within the South China Block (SCB). Zircon laser ablation ICP-MS U-Pb dating shows that the Mengshan pluton is mainly composed of medium- to coarse-grained porphyritic biotite granite, fine-grained porphyritic biotite granite, and medium- to fine-grained monzogranite from the middle-late Indosinian period. They have the characteristics of high-silica I-type granites: high SiO2 content, low Zr+Ce+Nb+Y and 10,000Ga/Al values, low A/CNK (molar ratio Al2O3/(CaO+Na2O+K2O)) ratios, and relatively high whole-rock zircon saturation temperatures. During magmatic evolution, the crystallization differentiation process occurred, with potassium feldspar and plagioclase as the main minerals and apatite, allanite, and monazite as accessory minerals. The Mengshan pluton originated from a pelitic-rich source, with high Rb/Sr and Rb/Ba ratios and variable CaO/Na2O and Al2O3/TiO2 ratios. The characteristics of low initial 87Sr/86Sr values and ancient two-stage model ages of Nd isotopes but relatively depleted epsilon Hf(t) values indicate that the genesis of the Mengshan pluton is related to the underplating of basic magma into the Mesoproterozoic crust. Combining these characterisrtics with different tectonic-magmatic activities in the southeastern coast and the southwestern inland, we believe that the Indosinian orogeny is controlled by multiple tectonic domains: the Paleo-Pacific subduction zone, the collision belts between the Indosinian Block and the SCB, and the collision belts between the SCB and the North China Block. The Indosinian Lu-Hf isotope data of igneous rocks in the SCB show that the epsilon Hf(t) values gradually increased over time, which reveals that crustal evolution is dominated by crustal thickening due to tectonic compression (major) and crustal growth in regional deep faults (minor).