新疆南天山东段库兹山基性超基性岩特征和成因探讨

Characteristics and genesis of ultrabasic and basic rocks in the Kuzi Mountain on the eastern margin of Southern Tianshan,Xinjiang

新疆南天山东段库兹山基性超基性岩带,前人缺乏研究,但该带的岩石成因和形成地质背景对认识南天山的构造格架具有重要意义。该带中的主要岩石类型有单辉橄榄岩,辉绿岩和辉长岩等。单辉橄榄岩的橄榄石为富镁型:Fo为93～96;副矿物铬尖晶石属于高铬低镁型,Cr#(Cr/(Cr+Al)×100)值78～99,Mg#(Mg/(Mg+Fe)×100)值1.8～41.4,属于造山带中的阿拉斯加型超基性岩类型。辉绿岩中辉石为普通辉石,Mg#(Mg/(Mg+Fe)×100)显示一个连续的从高到低的演化趋势,反映了岩浆的分异作用;长石为拉长石和中长石。辉长岩中辉石为普通辉石;斜长石已经蚀变成钠长石,Ab在95以上。辉绿岩和辉长岩K2O含量均很高,平均值分别为0.75和0.85,且LILE相对HFSE明显富集,认为它们可能与单辉橄榄岩同属造山带中的侵入体,形成在南天山的造山阶段。基性岩的同位素年代学指示该岩浆事件由石炭纪延续到二叠纪(331～286 Ma)。

Researches on the ultrabasic and basic rock belt in the Kuzi Mountain on the eastern margin of eastern Southern Tianshan in Xinjiang have been very insufficient;however,the genesis and the formation of the geological setting of this rock belt is of important significance for the tectonic framework of Southern Tianshan.This belt consists of wehrlite,diabase and gabbro.The olivines in wehrlite are forsterite,whose Fo is about 93-96;the accessory mineral chrome spinel has Cr#=Cr/（Cr＋Al）×100 values of 75.6-98.8 and Mg#=Mg/（Mg＋Fe）×100 values of 1.8-41.4,suggesting a high chromium,low magnesium and Alaskan-type complexes in the orogenic belt.The pyroxenes in diabase are augites,whose Mg#=Mg/（Mg＋Fe）×100 values show a continuous trend of evolution in decreasing order,reflecting differentiation of magma;The feldspars are composed of labradorite and andesine.The pyroxenes in gabbro are augites,the Ab values of feldspars are above 95,indicating albite.The values of K2O and TiO2 are high,and LILE content is obviously higher than HFSE content.It is considered that they are intrusions of the same orogenic belt as the wehrlite.Isotopic chronology suggests that the magmatic event might have continued from Carboniferous to Permian（331-286 Ma）.