新疆西准噶尔红山岩体地质地球化学特征及对下地壳性质的启示

准噶尔盆地基底性质历年来一直存在争议, 而花岗岩地球化学特征对下地壳性质具有指示意义.研究发现, 红山岩体的岩石类型主要为碱长花岗岩, 具有高SiO₂、低Al₂O₃、偏碱性、准铝质、轻重稀土分馏相对明显且富集轻稀土、明显的Eu异常、富集Rb、Th、K等大离子亲石元素及Zr、Hf等高场强元素而强烈亏损Sr、Ba、P、Ti等元素的A型花岗岩特征, 形成于后碰撞的张性环境中.SIMS和LA-ICP-MS测试获得的岩体侵位年龄分别为315.7±2.4Ma(MSWD=0.82)和317.8±3.8Ma(MSWD=2.8), 指示该岩体为晚石炭世早期的产物, 与西准噶尔后碰撞岩浆活动的时限一致.红山碱长花岗岩的结晶温度为680~860℃, Lu-Hf、Sr-Nd同位素特征共同表明西准噶尔的基底是由亏损地幔演化而来的年轻地壳物质组成. 

Geological,Geochemical Characteristics of Hongshan Pluton:Constraint for Lower Crust of West Junggar,Xinjiang

The basement of Junggar basin has been disputed for years, the geochemical characteristics of the granites are indicative of the lower crust, our research shows that, the Hongshan alkali feldspar granites from the West Junggar region, Xingjiang, are characterized by high SiO₂, low Al₂O₃, high total alkali, metalumious, and LREE along with significant negative Eu anomalies, enrichment of large ion lithophile elements (such as Th, Rb and K) and high field-strength elements (such as Zr and Hf), strong depletion of Sr, Ba, P and Ti, showing geochemical features reminiscent of A-type granites. It can further infer that it was formed in post-collisional extensional environment. Zircon U-Pb dating by SIMS and LA-ICP-MS yield ages of 315.7±2.4Ma and 317.8±3.8Ma, suggesting that the pluton was emplaced at early Late Carboniferous, which is contemporary with the timing of the Late Paleozoic post-collisional magmatism in the region. Mineral thermometry implies that the pluton formed at a temperature from 680 to 860℃. On the other hand, zircon Lu-Hf and Sr-Nd isotopic features show a depleted mantle-like signature, suggesting that the magma was sourced from juvenile crust.