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现代地质 ›› 2021, Vol. 35 ›› Issue (06): 1651-1676.DOI: 10.19657/j.geoscience.1000-8527.2021.119

• 秦岭金——多金属成矿作用 • 上一篇    下一篇

西秦岭碌础坝石英闪长岩-花岗闪长岩的地球化学、矿物学研究及其地质意义

任廷仙1,2(), 李小伟1,2(), 王可1,2, 葛涵云1,2, 关瑞1,2   

  1. 1.中国地质大学(北京) 地质过程与矿产资源国家重点实验室,北京 100083
    2.中国地质大学(北京) 地质资源勘查实验教学中心 成因矿物学研究中心,北京 100083
  • 收稿日期:2021-06-02 修回日期:2021-09-02 出版日期:2021-12-10 发布日期:2022-02-14
  • 通讯作者: 李小伟
  • 作者简介:李小伟,男,副教授,博士生导师,1985年出生,矿物学、岩石学、矿床学专业,主要从事成因矿物学与岩石学研究。Email: xwli@cugb.edu.cn
    任廷仙,女,硕士研究生,1996年出生,矿物学、岩石学、矿床学专业,主要从事岩浆作用与深部过程研究。Email: 1139269624@qq.com
  • 基金资助:
    国家自然科学基金项目(41730426);国家自然科学基金项目(41872066)

Geochemistry, Mineralogy, and Geological Significance of the Luchuba Quartz Diorite-Granodiorite in the West Qinling Orogen

REN Tingxian1,2(), LI Xiaowei1,2(), WANG Ke1,2, GE Hanyun1,2, GUAN Rui1,2   

  1. 1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Beijing), Beijing 100083, China
    2. Research Center of Genetic Mineralogy, Resources Exploration Experiment and Training Center, China University of Geosciences(Beijing),Beijing 100083, China
  • Received:2021-06-02 Revised:2021-09-02 Online:2021-12-10 Published:2022-02-14
  • Contact: LI Xiaowei

摘要:

不同成因类型的花岗岩组合,反映出不同的物源组成或迥异的岩浆演化过程。为了进一步探讨西秦岭造山带中生代花岗质侵入岩的成因、矿物结晶条件和地球动力学背景,选择西秦岭东部碌础坝岩体内的石英闪长岩和花岗闪长岩为研究对象,对其开展详细的野外地质调查以及系统的岩相学、矿物学和岩石地球化学研究。研究结果表明:碌础坝石英闪长岩和花岗闪长岩的SiO2含量范围为59.65%~67.36%,A/CNK为0.82~1.04,K2O/Na2O=1.11~1.74,Mg#值为47~53,显示出准铝质-弱过铝质特征,属于高钾钙碱性岩石,其中花岗闪长岩为I型花岗岩;岩体具有富集Rb、Th、U、K和Pb等元素,亏损Nb、Ta、P和Ti等元素的特征,具有中等Eu负异常(δEu=0.50~0.77),轻重稀土分馏明显((La/Yb)N=9.43~30.37)。碌础坝花岗质岩石中斜长石的An值介于18~53之间,以中长石为主,部分斜长石显示振荡环带;角闪石为镁角闪石,部分角闪石具有环带结构,且核部Mg/(Mg+Fe2+)值较高(0.88~0.91)并具有富钙特征(CaO含量为17.80%~22.67%),但Al2O3含量较低,指示角闪石核部与边部的形成环境具有明显差异;黑云母Mg/(Mg+Fe2+)值为0.44~0.57,为镁质黑云母。碌础坝花岗质岩石中全岩和各类矿物的温度计算结果显示,全岩锆饱和温度为736~795 ℃,角闪石结晶温度为704~824 ℃,黑云母结晶温度为700~746 ℃。三种方法计算的结晶温度相近,表明岩体形成于中温环境。碌础坝岩体角闪石全铝压力计结果为1.0~3.5 kbar(1 bar=100 kPa),平均形成深度为6.8 km;黑云母结晶压力为0.9~1.4 kbar,平均形成深度为4.1 km。角闪石湿度及氧逸度计显示其相对氧逸度为ΔNNO=0.1~1.3,含水量为3.9%~6.3%。结合前人资料,认为西秦岭碌础坝岩体由角闪岩为主的变基性岩部分熔融形成,幔源组分的参与导致其具有高Mg#值、高Cr和Ni等元素含量的特征。碌础坝岩体形成于洋-陆俯冲向陆-陆碰撞转换的阶段。

关键词: 西秦岭, 碌础坝岩体, 矿物学特征, 岩石地球化学特征

Abstract:

Different felsic rock assemblages can reflect magma source heterogeneity or different magmatic evolution processes. To determine the petrogenesis, crystallization conditions and geodynamic background of Mesozoic granitoids in the West Qinling Orogen, this study analyzed the Luchuba quartz diorite-granodiorite in the eastern West Qinling Orogen, through detailed field geological survey and petrographic, mineralogical, and geochemical analyses. We found that granite samples from the Luchuba pluton have 59.65%-67.36% SiO2, A/CNK=0.82-1.04, K2O/Na2O=1.11-1.74, and Mg#=47-53. The granites are meta-aluminous-weak peraluminous and high-K calcium-alkaline, among which granodiorite is I-type. The Luchuba plutonic rocks are enriched in Rb, Th, U, K, and Pb, and depleted in Nb, Ta, P, and Ti. The samples show moderately negative Eu anomalies (δEu=0.50-0.77) and steep LREE/HREE patterns ((La /Yb)N=9.43-30.37). The plagioclase An contents of the Luchuba pluton are 18 to 53, with some plagioclase grains showing oscillatory zoning. Amphibole is classified as Mg-amphibole, with some showing compositional zoning. The Mg/(Mg+Fe2+) value (0.88-0.91) and Ca content (17.80%-22.67 %) of the core are high, but the Al2O3 content is low, suggesting different crystallization conditions. The biotite Mg/(Mg+Fe2+) value (0.44-0.57) falls into the magnesia biotite range. The crystallization conditions of the Luchuba granitic rocks are featured by 736-795 ℃ zircon saturation temperature, 704-824 ℃ hornblende crystallization temperature, and 700-746 ℃ biotite crystallization temperature. The three thermometric results indicate a medium-temperature environment. The amphibole all-aluminum pressure gauge of the Luchuba pluton is 1.0-3.5 kbar (eqv. 6.8 km depth), the biotite all-aluminum pressure is 0.9-1.4 kbar (eqv. 4.1 km depth). The results of amphibole water content and oxygen fugacity meter show that the relative oxygen fugacity (ΔNNO) is 0.1-1.3, and the water content is 3.9%-6.3%. Accordingly, we argue that the Luchuba pluton was formed by partial melting of meta-mafic rocks dominated by amphibolite, along with the mantle-derived input as evidenced by the high Mg# and Cr-Ni contents. Integrating with published geological, geochronological, and geochemical data from the Triassic West Qinling Orogen, we conclude that the Luchuba pluton was formed under the tectonic transition from oceanic subduction to continent-continent collision.

Key words: West Qinling Orogen, Luchuba pluton, mineralogical characteristic, petro-geochemical characteristic

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