今天是2018年10月18日

中天山南缘乌瓦门早志留世安第斯型安山岩的发现及意义

DOI:10.3799/dqkx.2018.725

作者:牛晓露,刘飞,冯光英,杨经绥

联系地址:地幔研究中心,国土资源部深地动力学重点实验室,中国地质科学院地质研究所,北京 100037

摘要:安山岩是俯冲相关构造环境的特征岩石;对安山岩的研究,可以获得俯冲作用发生时代和俯冲过程的相关信息.报道了中天山地块南缘乌瓦门地区早志留世安山岩的年代学和地球化学特征,探讨了其岩石成因和构造属性,为南天山洋及中天山陆块南缘构造演化提供制约.研究表明,乌瓦门安山岩具有安山结构,组成矿物为普通辉石、钙质角闪石、斜长石和钾钠长石,为高钾钙碱性粗面安山岩;全岩SiO2=56.23%~59.28%,K2O=2.70%~3.37%,Na2O=3.32%~4.11%.其锆石U-Pb年龄为430 Ma,形成于早志留世晚期.微量元素组成上,富集轻稀土、亏损重稀土,(La/Yb)N=18.5~20.9;Eu负异常不明显,δEu= 0.82~0.88;富集Ba、Th、U,亏损Nb、Ta、Ti;初始87Sr/86Sr比值为0.706 2~0.707 5, ε Nd( t )=+2.96~+3.01;这些数据揭示乌瓦门安山岩为安第斯型陆弧岩石,起源于被俯冲带相关流体交代的地幔楔,并在上升过程中受到了古老地壳的改造.早志留世晚期,南天山洋向中天山陆块下俯冲,构成成熟的洋-陆俯冲体系;中天山陆块南缘为活动大陆边缘,发育典型的安第斯型陆弧岩石组合.

关键字:安山岩;锆石U-Pb定年;地球化学;安第斯型陆弧;南天山洋;中天山.

Discovery and Significance of Early Silurian Andesites in Wuwamen Area, Southern Margin of Central Tianshan Block

Author:Niu Xiaolu, Liu Fei, Feng Guangying, Yang Jingsui

Author Address:Center for Advanced Research on the Mantle (CARMA), Key Laboratory of Deep-Earth Dynamics, Ministry of Land and Resources,Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China

Abstract:Andesite is a typical rock type in subduction-related environment, and can reveal important information on the subduction processes. In this paper, it is presented of zircon U-Pb geochronology, whole-rock major and trace element and Sr-Nd isotope data for the Wuwamen andesites in the southern margin of Central Tianshan block, to provide constraints on the evolution processes of the South Tianshan ocean and Central Tianshan block. The andesites are high-K, calc-alkaline trachyandesite composed of augite, calcic amphibole, plagioclase and K-Na feldspar, with SiO2=56.23%-59.28%, K2O=2.70%-3.37%, and Na2O=3.32%-4.11%. They are characterized by fractionated rare earth element patterns with (La/Yb) N=18.5-20.9, and show negligible Eu anomalies with δEu=0.82-0.88; they are highly enriched in Rb, Ba and Sr, and depleted in Nb, Ta and Ti. Their initial 87Sr/86Sr ratios are in the range of 0.706 2-0.707 5 and ε Nd( t ) in the range of 2.96-3.01. The data suggest that the Wuwamen andesites are typical Andean-type continental arcs, which originated from the partial melting of the mantle wedge that had been previously metasomatized by subduction-related fluids; the parental magma has been modified by the old crustal rocks during the magmas ascent. The origin and tectonic affinity of the Wuwamen andesites indicate that the South Tianshan ocean has subducted northward beneath the southern margin of the Central Tianshan block at the end of Early Silurian, forming a mature oceanic-continental subduction system; the southern margin of the Central Tianshan block has been a typical active continental margin characterized by development of Andean-type continental arc rocks.

Keyword:andesite; zircon U-Pb dating; geochemistry; Andean-type continental margin; South Tianshan ocean; Central Tianshan block.

 

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作者简介:牛晓露(1983-),女,副研究员,博士,主要从事岩石学和岩石地球化学研究.ORCID: 0000-0003-0862-0675.E-mail: niuxiaoludx@126.com

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