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王建,谢亘,施光海,王妍. 2018. 北祁连川刺沟A型花岗岩的年代学及其意义. 岩石学报, 34(6): 1657-1668
北祁连川刺沟A型花岗岩的年代学及其意义
作者单位E-mail
王建 中国地质大学地质过程与矿产资源国家重点实验室, 北京 100083  
谢亘 中国地质大学地质过程与矿产资源国家重点实验室, 北京 100083
武警黄金部队第十一支队, 拉萨 850000 
 
施光海 中国地质大学地质过程与矿产资源国家重点实验室, 北京 100083 shigh@cugb.edu.cn 
王妍 中国地质大学地质过程与矿产资源国家重点实验室, 北京 100083  
基金项目:本文受地质过程与矿产资源国家重点实验室和武装警察部队黄金指挥部研究项目(201404020604)联合资助.
摘要:
      关于北祁连南蛇绿岩带蛇绿岩,前人多认为是形成于早古生代洋中脊的大洋岩石圈残片。本次对川刺沟A型花岗岩和其周围超基性岩的研究,有助于较全面认识该区时代序列与构造格架。川刺沟A型花岗岩的组成矿物为碱性长石(~45%)、石英(~30%)、斜长石(~15%)以及少量的霓辉石(~5%)等,发育条纹和显微文象等结构。岩石具有高SiO2(72.58%~73.96%)和高Na2O+K2O(10.12%~10.39%),A/NK<1,10000Ga/Al值为2.59~3.49。微量元素型式显示该花岗岩富集Rb、Ba、Th、K,强烈亏损Sr、P、Ti、Nb、Ta,稀土元素型式呈明显负铕异常(δEu=0.56~0.60)的海鸥形。岩相学和地球化学特点均符合A型花岗岩特征,进一步细分可将其归属于A2型,形成于后造山环境。其周围的超基性岩稀土型式呈"U"型,富集Th、Zr、Hf,推测其经历过俯冲熔/流体交代。SHRIMP锆石U-Pb定年给出一组该A型花岗岩的谐和年龄501±5Ma,可以代表其侵位年龄。该年龄早于北祁连造山带陆-陆碰撞时间,晚于大陆裂谷发生时间。由于A2型花岗岩后造山环境有两亚类:陆-陆碰撞后造山和活动大陆边缘拉张环境,本次研究结果更加倾向后者,即,该A型花岗岩为在中-晚寒武世俯冲作用影响下,活动大陆边缘弧前发生拉张,软流圈部分熔融产生幔源岩浆底侵形成。进一步推测该地区超基性岩可能与俯冲有关,为非经典定义的蛇绿岩。
英文摘要:
      Some geologists believe that South Ophiolite Belt in North Qilian is a fragment of oceanic lithosphere formed in ocean ridge in Early Paleozoic. Characteristics of an A-type granite and a peridotite in the belt will help to understand the time sequence and structural framework of this area. This A-type granite is mainly composed of alkali feldspar (~45%), quartz (~30%), plagioclase (~15%), and aegirine augite (~5%) and other accessory minerals as well, with perthitic and micrographic textures. The granite is high in SiO2 (72.58%~73.96%), Na2O+K2O (10.12%~10.39%) and Ga/Al (10000Ga/Al=2.59~3.49). Its A/NK ratios are below 1.0, which means the granite is peralkaline. Normalized rare earth element and trace element patterns show that this pluton has seagull-shaped REE patterns with obvious negative Eu anomaly (δEu=0.56~0.60), enriched in Rb, Ba, Th, K and depleted in Sr, P, Ti, Nb, Ta. Petrological and geochemical characteristics suggest that this granite is an A-type one, which is subdivided into A2-type, i.e., formed in post-orogenic environment. U-shaped REE patterns and the enrichment of Th, Zr, Hf of the Chuancigou peridotite illustrate a source of fossil mantle wedge modified by melts or fluids released from subduction slab. SHRIMP U-Pb analysis on zircon of the granite yields a weighted mean age of 501±5Ma, interpreted as the emplacement age, which is earlier than the event of continent-continent collision and later than continent rifting of the North Qilian orogenic belt. As there are two sub-types for A2 granite:post-collision and active continental margin, for the Chuancigou A2 granite, the forming environment should be active continental margin. In the Middle-Late Cambrian fore-arc of active continental margin of North Qilian, it is inferred that asthenosphere melted due to the extension caused by subduction and the magma underplated to form the Chuancigou A-type granite, and the formation of the ultramafic rock in the studied area is estimated to be related to subduction zone and it is not a classically defined ophiolite.
关键词:北祁连  早古生代  A型花岗岩  SHRIMP锆石测年  活动大陆边缘  蛇绿岩
投稿时间:2017-05-05  修订日期:2018-02-01
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