中天山奎先达坂一带晚志留世侵入岩地球化学特征及构造意义

Geochemical Characteristics and Tectonic Significance of Late Silurian Intrusive Rocks in Kuixiandaban,Central Tianshan

中天山奎先达坂一带大量发育花岗质侵入岩,其中,晚志留世侵入岩位于研究区中部东侧,对其开展岩石学、岩石化学、地球化学及LA-ICP-MS锆石U-Pb测年研究,以探讨岩体成因类型、侵位时代及形成的构造背景,旨在对南天山洋盆俯冲时限,乃至中亚造山带构造演化提供新的科学依据。结果表明,奎先达坂一带晚志留世侵入岩为过铝质、高钾钙碱性“S”型花岗岩。岩体富硅(67.22%~74.88%)、富碱(7.65%~8.66%)、富钾(K_(2)O/Na_(2)O=1.16~1.87),里特曼指数σ均小于3.3;铝饱和指数A/CNK为1.01~1.06;CIPW标准矿物中,均见有刚玉分子;具远高地壳均值的Rb/Sr比值;稀土元素富集轻稀土,发育中等铕负异常,稀土分布模式曲线为右倾“海鸥”型曲线;微量元素与原始地幔相比,明显富集Rb,Th,La,Nd,Hf,亏损Ba,Sr,P,Ti。钾长花岗岩锆石206Pb/238U同位素加权平均年龄为(422.9±3.3)Ma,为晚志留世。结合前人研究成果和区域地质特征,认为奎先达坂一带晚志留世侵入岩为中天山北缘洋盆闭合后的陆陆碰撞和南天山洋的北向俯冲消减共同作用形成的,为造山晚期环境的陆缘火山弧花岗岩,表明南天山洋盆于晚志留世仍在向中天山地块之下俯冲。

A large number of granitic intrusive rocks are developed in the Kuixiandaban area of the Central Tianshan Mountains,among which the Late Silurian intrusive rocks are located on the east side of the central part of the study area.Carry out petrology,petrochemistry,geochemistry,LA-ICP-MS zircon U-Pb dating studies on its Late Silurian intrusive rock,and explore its genetic type,emplacement age and the tectonic background of its formation.It provides a new scientific basis for the subduction time limit of the Southern Ocean Basin and even the tectonic evolution of the Central Asian Orogenic Belt.The results show that the Late Silurian intrusions in the Kuixiandaban area are peraluminous,highpotassium calc-alkaline S-type granites:Rich in silicon(67.22%~74.88%),rich in alkali(7.65%~8.66%),rich in potassium(K_(2)O/Na_(2)O=1.16~1.87),the whole rock Ritman indexσis less than 3.3;aluminum saturation index A/CNK is between 1.01 and 1.06;corundum molecules are found in CIPW standard minerals;and the ratio of Rb/Sr is far higher than the average value of the crust;Rare earth elements are enriched in light rare earths,with moderate negative europium anomalies,and the rare earth distribution model curve is a right-dipping"seagull"curve;Compared with the original mantle,trace elements are obviously enriched in Rb,Th,La,Nd,Hf,and depleted in Ba,Sr,P,and Ti.The zircon 206Pb/238U isotope weighted average age of the potash feldspar granite is 422.9±3.3 Ma,which is the Late Silurian.Combining previous research results and regional geological characteristics,This paper believes that the Late Silurian intrusive rocks in the Kuixiandaban area are continental marginal volcanic arc granites formed in the late orogenic environment after the closure of the northern margin of the Central Tianshan Ocean Basin and the northward subduction and subduction of the South Tianshan Ocean,This indicates that the Southern Tianshan Ocean Basin was still subducting under the Zhongtianshan block in the Late Silurian.