新疆南天山红柳沟辉长岩地球化学特征及地质意义

Geochemical Characteristics and Geological Significance of Gabbro from the South Tianshan Hongliugou in Xinjiang

红柳沟辉长岩位于南天山北缘东段哈尔克-萨阿尔明晚古生代沟弧带东部,是南天山北缘-中天山南缘早古生代弧岩浆带内出露为数不多的基岩侵入岩体。辉长岩主要由斜长石(±48%)、普通辉石(±42%)、黑云母(±5%)、角闪石(±5%)组成。岩石化学成分显示辉长岩Na_(2)O含量为2.49%~3.08%,平均为2.83%;K_(2)O含量为1.6%~2.4%,平均为2.02%,里特曼指数为1.78~2.09,均小于3.3;分异指数为50.3~57.06,主、微量图解显示其为拉斑-钙碱性系列;辉长岩稀土元素球粒陨石标准化配分模式与E-MORB相似,轻稀土富集,重稀土亏损,未见明显的δEu异常,微量元素具有Rb、Ba、Th、U等大离子亲石元素富集和Ta、Nb、Ti等高场强元素亏损;锆石原位εHf(t)值为-1.27~8.75,反映辉长岩来源于亏损地幔,并上升的过程中受到地壳明显混染,基性岩的(Na_(2)O+K_(2)O)/TiO2判别图解显示红柳沟辉长岩源区碳酸盐化橄榄岩部分熔融。锆石U-Pb测年显示辉长岩形成年龄为为(410.4±2.3)Ma(MSWD=0.51,n=25)。本次研究补充了南天山构造带辉长岩年代学、地球化学证据,结合区域地质资料,笔者提出在早泥盆世,南天山洋存在洋陆俯冲作用,并伴随有基性岩侵入。

The Hongliugou gabbro is located in the eastern part of the Halke Saarming Late Paleozoic arc belt on the northern edge of the South Tianshan Mountains.It is one of the few intrusive rock masses exposed in the Early Paleozoic arc magmatic belt on the northern edge of the South Tianshan Mountains and the southern edge of the Central Tianshan Mountains.Gabbro is mainly composed of plagioclase(±48%),common pyroxene(±42%),biotite(±5%),and hornblende(±5%).The chemical composition of the rock shows that the Na_(2)O content of gabbro ranges from 2.49%to 3.08%,with an average of 2.83%;The K_(2)O content ranges from 1.6%to 2.4%,with an average of 2.02%.The Rittman index is 1.78 to 2.09,both of which are less than 3.3%;The differentiation index ranges from 50.3 to 57.06,and the main trace diagram shows that it belongs to the plaque calcium alkaline series;The standardized distribution pattern of rare earth elements in gabbro chondrite meteorites is similar to that of E-MORB,with enrichment of light rare earth elements and depletion of heavy rare earth elements,and no significant difference observedδEu anomaly,trace elements with enrichment of large ion lithophilic elements such as Rb,Ba,Th,U,and depletion of high field strength elements such as Ta,Nb,Ti;Zircon in-situ theεHf(t)value ranges from 1.27 to 8.75,indicating that the gabbro originated from the depleted mantle and was significantly contaminated by the crust during its ascent.The w(Na_(2)O+K_(2)O)/w(TiO2)discrimination diagram of the basic rocks shows partial melting of carbonated peridotite in the Hongliugou gabbro source area.The zircon U-Pb dating shows that the age of gabbro formation is(410.4±2.3)Ma(MSWD=0.51,n=25).This study supplemented the dating and geochemical evidence of gabbro in the South Tianshan tectonic belt,and combined with regional geological data,we proposed that in the Early Devonian,there was oceanic continental subduction in the South Tianshan Ocean,accompanied by the intrusion of basic rocks.