豫西龙王幢岩体地球化学特征及其演化探讨——来自锆石U-Pb定年以及岩石地球化学的约束

Study on the geochemical characteristics and evolution of Longwangzhuang rock body,west Henan province——derived from constraints of zircon U-Pb dating and rock geochemistry

龙王幢岩体位于华北克拉通南缘,具明显的高硅、富碱特点,岩石为准铝质至弱过铝质碱性岩。微量元素分配曲线呈右倾型,富集大离子亲石元素(Rb、Th),亏损高场强元素Nb、Ta,显示明显的Nb、Ta、Sr、P及Ti负异常。稀土元素配分模式具有右倾、轻稀土富集及重稀土亏损的特征,显示了较强的负Eu异常,早期岩浆中斜长石的晶出使得晚期岩浆结晶中钾长石更富集,是伸展机制下晚期富碱岩浆活动产物。岩体锆石LA-ICP-MS年龄为(1613.5±4.3)Ma,属中元古代碱性侵入岩。岩体的主微量元素及含铁指数表明岩体属于碱性-碱钙性铁质A型花岗岩。岩体构造环境元素分析表明为板内A型花岗岩。龙王幢岩体形成于板内构造环境,是华北克拉通南缘1.8～1.6 Ga期间大规模裂解事件中最晚期的一次碱性岩浆活动的产物。

Longwangzhuang rock body is located in the south edge of north China craton, with obvious characteristics of high silicon and rich alkali,belongs to quasi-aluminous to weakly peraluminous alkaline rock. The distribution curve of trace elements is right-sided, with the enrichment of large ionic lithophile elements (Rb, Th) and the loss of high field strength elements (Nb and Ta), showing obvious negative anomalies of Nb, Ta, Sr, P and Ti. The distribution pattern of rare earth elements has the characteristics of right-leaning, light rare earth enrichment and heavy rare earth loss, showing strong negative Eu anomaly. The crystallization of plagioclase in the early magmatic activity makes the potassium feldspar in the late magmatic crystallization more enriched, which is the typical characteristic of the late alkali-rich magmatic activity under the extension mechanism. The age of zircon LA-ICP-MS is 1613.5±4.3Ma, belonging to the middle proterozoic alkaline intrusive rocks body. The main trace elements and ferric index indicate that the rock body belongs to the alkali - alkali calcium type A-type granite. The analysis of rock structure environmental elements shows that is A-type granite in plate. In a word, the Longwangzhuang rock body was formed in the plate tectonics environment, which is the product of alkaline magmatic activity in the late stage of large-scale pyrolysis events in the southern margin of north China craton from 1.8 to 1.6 Ga.