北山南带新场岩体地球化学特征及构造意义

Geochemical characteristics of Xinchang pluton in southern Beishan and its tectonic significance

新场岩体位于北山造山带南带中段地区,岩性以花岗闪长岩和二长花岗岩为主。主要针对新场岩体的地球化学特征进行了研究。新场花岗岩为准铝质-弱过铝质的钙碱性和高钾钙碱性系列,大离子亲石元素K、Rb、Ba和高场强元素Th呈富集状态,亏损Hf、Zr、Sm和Y等元素。岩石轻重稀土分馏明显,总体呈现较弱的负Eu异常。新场花岗岩属于未分异的I型花岗岩,岩体中的花岗闪长岩和二长花岗岩应是同源岩浆不同演化阶段的产物。受岩石圈地幔岩浆作用影响,敦煌地块北缘下地壳岩石发生了部分熔融,岩浆在上升的过程中混合了下地壳物质,因此新场花岗岩应属壳幔混合成因的花岗岩。综合本次研究所得数据和前人研究成果,认为新场岩体应为俯冲-碰撞构造背景条件下的岩浆产物。

Xinchang pluton is located in the middle part of the southern belt of Beishan orogenic belt,and mainly granodiorite and monzonitic granite in lithology.This study mainly focused on the detailed geochemistry of Xinchang pluton.Xinchang granite falls to the quasi aluminous-weakly peraluminous calc-alkaline and high-k calc-alkaline series,enriched in The LILE K,Rb,Ba and HFSE Th,and depleted in Hf,Zr,Sm and Y.The granite is obvious in LREE and HREE fractionation and with weak negative Eu anomaly.Xinchang granite belongs to undifferentiated I-type granite.The granodiorite and monzogranite in Xinchang pluton should be the products of different evolution stages of homologous magma.Under the influence of lithospheric mantle magmatism,the lower crust rocks in the northern margin of Dunhuang block were partially melted,and the magma mixed the lower crust materials in the process of rising.Therefore,Xinchang granite should be the mixed crust mantle origin granite.Based on the data obtained in this study and previous research results,it was considered that Xinchang pluton should be a magmatic product under the background of subduction-collision structure.