摘要
新疆西准噶尔玛依勒地区的拉巴岩体侵入于下石炭统包古图组中,由花岗闪长岩、石英闪长岩、英云闪长岩、花岗岩组成,以花岗岩为主。其中花岗岩和花岗闪长岩属钙碱性、高钾钙碱性系列,为偏铝质至过铝质岩石,并具有轻稀土-大离子亲石元素富集、Nb-Ta-Ti等高场强元素亏损的地球化学特征。该岩体两个代表样品的锆石La-ICP-MS法206Pb/238U加权平均年龄分别为287±5Ma和295.1±2.3Ma,表明岩体的结晶年龄为早二叠世,与西准噶尔地区后碰撞花岗岩的时代(年龄峰值310～295Ma)相近。综合岩体的地质、地球化学特征和区域地质特征,认为拉巴花岗岩体为准噶尔地区后碰撞阶段伸展期的岩浆活动产物。
Plenty of granitic rocks are exposed in the western Junggar region which has been regarded as one of the most typical areas in the world where the post-collisional vertical continental growth has been recorded.Previous studies were concentrated on the representative intrusions exposed in the eastern part of this region,such as Miaoergou granite pluton,Akebasitao granite pluton,Kelamayi granite pluton,Hongshan granite pluton,Hatu granite pluton and Baogutu adakitic pluton.However,rather insufficient studies were devoted to the granite pluton exposed in Tangbale-Mayile area in the western part of this region.In this paper,the study was concentrated on the geology,geochemistry and U-Pb chronology of the Laba intrusion in the Mayileshan Mountain in order to provide some new data for the discussion on the post-collisional magmatism along the southwestern margin of the western Junggar accretionary orogenic belt.The Laba pluton intruded into the Lower Carboniferous Baogutu Formation.This intrusion occupies about 23.95 km2 in the plan view,with the long axis extending about 8 km in NS direction and an EW width of about 5 km.The pluton is composed of granodiorites,quartz-diorites,tonalites and granites,predominantly of granites.Among them,the granodiorite and the granite belong to calc-alkaline to high-K calc-alkaline series and metaluminous to peraluminous rocks.The σ value of granitic rocks varies from 1.09 to 1.91,averagely 1.48;and the A/CNK value varies from 0.87 to 1.34,averagely 1.07,indicating that these rocks are metaluminous to peraluminous.The A/CNK values of two samples are 1.25 and 1.34 respectively,suggesting that they are peraluminous.The chondrite-normalized patterns of the samples assume LREE-enriched and HREE flat with a （La/Yb）N value varying from 1.04 to 1.39,1.19 on average.Relative to the primitive mantle,the trace element spider diagram of the samples indicates that these rocks are enriched in large ion lithophile elements （K,Rb and Ba） and depleted in high strength field elements （Nb,Ta,Ti and P）.The Laser-ICP-MS mean 206Pb/238U ages of 287 ± 5 Ma and 295.1 ± 2.3 Ma were obtained for two representative samples from this pluton,indicating that the age of this pluton is Early Permian.This age is similar to the age of post-collisional granites in the western Junggar region with a peak of 310～295 Ma.On the basis of an integration of geological and geochemical characteristics of this pluton and regional geology,the Laba pluton is held to be a magmatic product of the post-collisional extension in the western Junggar region.The pluton has similar trace element geochemical characteristics to island-arc granites,implying that its magma source had been metasomatized by previous subduction fluids.The closure of paleo-oceans in western Junggar and the adjacent northern Tianshan region happened earlier than 316 Ma.In other words,these regions were under a post-collisional extension episode during the period of Late Carboniferous to Early Permian.Both the oceanic ridge subduction and the subducted oceanic crust melting models cannot be employed to interpret the formation of the Laba granitic pluton.
出处
《岩石矿物学杂志》
CAS
CSCD
北大核心
2010年第6期663-674,共12页
Acta Petrologica et Mineralogica
基金
新疆维吾尔自治区1:5万区域地质矿产调查项目
