摘要
岔路口地区超基性熔岩主要由玻基纯橄岩和玻基辉橄岩组成,呈夹层状断续分布于玄武岩、玄武质火山岩及硅质岩中,总体上构成了深海相火山-沉积建造。玻基纯橄岩和玻基辉橄岩中的橄榄石斑晶多呈自形-半自形晶,11件样品25个测点获得的SiO 2含量为38.68%~40.58%,MgO含量主要为44.43%~46.72%,Ni值主要为1100×10-6~2381×10-6,计算得出的Fo值为85.60~87.71。在岩石化学成分上,SiO 2含量为38.51%~42.86%,MgO含量为26.81%~31.28%,TFeO含量为12.17%~14.52%,TiO 2含量为1.00%~1.45%,Na 2O+K 2O含量为0.17%~0.67%,在岩石化学分类上属麦美奇岩。因玻基辉橄岩中存在较多辉石斑晶而与传统麦美奇岩存在明显区别,因此暂将研究区超基性熔岩称为“麦美奇质岩石”。选用橄榄石最高Fo值和其寄主岩石TFeO含量估算出上述麦美奇质岩石原生岩浆中的MgO含量≥18.18%,据此限定地幔熔融温度和压力下限分别为1580℃和3.9 GPa。结合岔路口地区麦美奇质岩石具有洋岛玄武岩微量元素特征以及前人研究成果,认为该区麦美奇质岩石形成于晚二叠世-晚三叠世的伸展环境,其地球动力学背景与地幔柱有关。
The ultramafic lavas in the Chalukou area is mainly composed of vitric dunites and vitric pyroxene peridotites,which are interlayered in basalts,basaltic volcanic rocks and siliceous rocks,and generally constitute a deep marine volcanic-sediment formation.The olivine phenocrysts in the vitric dunites and vitric pyroxene peridotites are mostly euhedral to subhedral.The 25 analysis spots of 11 samples have SiO 2 contents of 38.68%~40.58%,MgO contents of 44.43%~46.72%,Ni concentrations of 1100×10-6~2381×10-6,and the calculated Fo values of 85.60~87.71,respectively.In terms of whole rock geochemical compositions,SiO 2 content is 38.51%~42.86%,MgO content is 26.81%~31.28%,TFeO content is 12.17%~14.52%,TiO 2 content is 1.00%~1.45%,and Na 2O+K 2O content is 0.17%~0.67%.They belong to meimechite by petrochemical classification.In consideration of the presence of pyroxene phenocrysts in the vitric pyroxene peridotites,it is obviously different from the traditional definition of meimechite.Therefore,the authors refer to the newly discovered ultramafic lavas in the study area as“meimechitic rocks”.Using the highest Fo value of olivine and the TFeO content of its host rock,the MgO content in the parent magma of the meimechitic rocks is estimated to be≥18.18%.Based on this evidence,the mantle melting temperature and pressure are limited to be higher than 1580℃and 3.9 GPa,respectively.The meimechitic rocks in the Chalukou area show OIB-like trace element characteristics.This,along with the results of previous studies,suggests that the meimechitic rocks in the area were formed in the Late Permian to Late Triassic extensional setting related to the mantle plume.
作者
杨万志
田江涛
郭秀玮
YANG Wan-zhi;TIAN Jiang-tao;GUO Xiu-wei(Xinjiang Institute of Geological Survey,Urumqi 830000,China;School of Mechanics and Engineering Science,Shanghai University,Shanghai 200444,China)
出处
《世界地质》
CAS
2021年第3期511-524,共14页
World Geology
基金
新疆地质矿产开发局“新疆西昆仑岔路口地区铜镍钨锡银找矿方向研究与靶区优选”项目(XGMB202040)。
