甘肃马庄山地区花岗岩类的元素地球化学、U-Pb年代学及Nd-Hf同位素特征

The element geochemistry,zircon U-Pb chronology and Nd-Hf isotope of the granitoids in Mazhuangshan area,Gansu Province

北山造山带位于中亚造山带南缘,连接了东天山和索伦缝合带,对认识中亚造山带的构造演化具有十分重要的意义。本文选择北山造山带西段马庄山地区的花岗闪长岩、钾长花岗岩和闪长岩进行岩石学、地球化学、Nd-Hf同位素及LAICP-MS锆石U-Pb年代学的研究。结果表明,英云闪长岩、花岗闪长岩、钾长花岗岩具有高硅(64.54%~78.48%)、高钾(1.53%~4.95%)、低磷(0.01%~0.21%)的特征,显示出高钾钙碱性-钙碱性岩石的特点。岩石整体相对富集Rb、K、Th和U大离子亲石元素,亏损P、Ta、Nb和Ta高场强元素,与弧火成岩的微量元素特征一致。花岗闪长岩和英云闪长岩的SiO2与P2O5呈负相关关系,Rb与Y、Th均呈现正相关关系,说明其属于Ⅰ型花岗岩。钾长花岗岩和花岗闪长岩的LA-ICP-MS锆石U-Pb年龄分别为317.7±1.0Ma和320.2±0.8Ma,表明岩浆侵位时代为晚石炭世。它们的初始εNd(t)值介于-5.31~-4.24之间,锆石εHf(2)介于-3.6~-0.2之间,Hf的tDM2为1318~1537Ma,暗示岩石来自于下地壳物质的部分熔融,原始岩浆在上侵过程中有部分地幔物质的加入。综合地质、地球化学和年代学特征,认为北山西段,在晚石炭世,马庄山地区处于与俯冲相关的弧环境。

The Beishan orogenic belt is located in the south margin of the Central Asia Orogenic Belt CCAOBD,connecting the eastern Tianshan and the Solonker suture belt,which would provide important clues for to understand the tectonic evolution of CAOB.The Mazhuangshan area lies in the western part of Beishan orogenic belt,and a lots of granitoids exposed there.In this manuscript,we present the new petrology,geochemistry,Nd-Hf isotopes and LA-ICP-MS zircon U-Pb geochronology results of granodiorite,K-feldspar granite and diorite in the Mazhuangshan area.K-feldspar granite,tonalite and granodiorite are characterized by high SiO2 C64.54%~78.48% D,high K2 O Cl.53%~4.95% D and low P2 O5 C0.01%~0.21% D contents,showing the characteristics of high potassium,calc alkaline and calc alkaline rocks.These granotoids are relatively enriched in Rb,K,Th,U elements and depleted in P,Ta,Nb and Ti elements,which is similar to the features of the rocks formed in arc setting.Granodiorite and tonalite samples show negative correlation between P205 and Si02 elements,positive correlation between Y and Rb,and Th and Rb elements,which are consistent with the typical Ⅰ-type granite trends.The zircons from K-feldspar granite and granodiorite yield U-Pb concordant ages of 317.7±1.OMa and 320.2±0.8 Ma respectively,indicating that they intruded at Late Carboniferous.The εNd dCtD values of the granotoids vary between-5.31 and 4.24,the zircon εHf CtD values are-3.6~-0.2 and the corresponding tDM2 model ages are 1318~1537 Ma.These features illustrate that the magmas were derived from the parial melting of the lower crustal materials that involved some mantle component.Comprehensive geological,geochemical and chronological studies demonstrate that the Mazhuangshan area was in an arc environment related to subduction during the Late Carboniferous.