天水地区李子园高压麻粒岩的发现：西秦岭晚泥盆纪造山的证据  

Discovery of Liziyuan high-pressure granulite in Tianshui region: Evidence for Late Devonian orogeny in West Qinling Orogen

作　　者：杨立明 宋述光[1] 苏犁 李艳广 董金龙 李清云 YANG LiMing;SONG ShuGuang;SU Li;LI YanGuang;DONG JinLong;LI QingYun(MOE Key Laboratory of Orogenic Belts and Crustal Evolution,School of Earth and Space Sciences,Peking University,Beijing 100871,China;State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences,Beijing 100083,China;MLR Key Laboratory for the Study of Focused Magmatism and Giant ore Deposits,Xi＇ an Center of Geological Survey,CGS,Xi＇ an 710054,China)

机构地区：[1]北京大学地球与空间科学学院,造山带与地壳演化教育部重点实验室,北京100871 [2]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 [3]中国地质调查局西安地质调查中心,国土资源部岩浆作用成矿与找矿重点实验室,西安710054

出　　处：《岩石学报》2018年第8期2425-2440,共16页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41572040、41372060)资助 中国地质大学(北京)科学研究院激光剥蚀等离子体质谱实验室张红雨老师及实验室的同学们在实验过程中给予了帮助;中国地质调查局西安地质调查中心国土资源部岩浆作用成矿与找矿重点实验室靳梦琪老师在锆石Hf同位素分析中给予了指导;段站站和彭卫刚在岩石学和矿物学方面给予了指导和帮助;审稿人提出了中肯的意见和建议;在此一并表示感谢.

摘　　要：西秦岭造山带北缘的天水李子园地区发现一套高压石榴石单辉麻粒岩,在变质的早古生代岛弧火山-沉积岩系中呈透镜状岩块产出。其峰期矿物组合为石榴石+单斜辉石+角闪石+斜长石+钛铁矿/榍石。利用地质温压计得到的温压条件为T=757~792℃、P=1.3~1.5GPa,达到中压相系的高压麻粒岩相变质条件。LA-ICPMS锆石U-Pb定年显示,锆石变质增生边的谐和年龄为384±1.6Ma,表明高压麻粒岩相变质作用的时代为中-晚泥盆世。结合区域地质资料,这一期变质作用要晚于北秦岭造山带中普遍记录的中低压麻粒岩-角闪岩相变质作用,可能与商丹洋闭合之后碰撞造山阶段的地壳加厚过程有关。岩浆锆石核部的谐和年龄为796±2.2Ma,代表石榴石单辉麻粒岩的原岩形成年代。其锆石核部的Hf同位素组成变化较大,对应的εHf(t)值为-7.3^+13.2,显示出不同源区岩浆混合的特征或者陆壳混染。高压麻粒岩全岩地球化学特征同样显示其经历壳幔岩浆混合作用。结合原岩的形成时代、区域上与裂解相关的岩浆作用和构造背景,我们认为原岩可能是造山带垮塌伸展阶段的壳-幔岩浆混合作用的产物,可能与新元古代Rodinia超大陆裂解事件有关。High-pressure（ HP） Grt-Cpx granulites were discovered in the Liziyuan,Tianshui region,the northern margin of the West Qinling Orogen. It outcrops as tectonic lenticles in the Early Paleozoic metamorphic island arc volcanic-sedimentary series. The peak mineral assemblage of the granulite is composed of garnet,clinopyroxene,amphibole,plagioclase,and ilmenite/titanite. The GrtCpx granulites have experienced HP granulite facies metamorphism with peak P-T conditions of T = 757 ~ 792℃ and P = 1. 3 ~ 1. 5 GPa by using traditional geothermobarometer. Zircon overgrowth rims yielded a concordia age of 384 ± 1. 6 Ma by LA-ICP-MS zircon U-Pb analysis,suggesting that the HP granulite metamorphism occurred in Middle-Late Devonian. Combined with the regional geological research,this metamorphism age is later than that of the medium-low pressure granulite-amphibolite facies metamorphism in the North Qinling Orogen. We believe that it is related to the crustal thickening process during the collision orogeny after closure of the Shangdan Ocean. The magmatic zircon cores gave a concordia age of 796 ± 2. 2 Ma,interpreted to represent the formation age of protolith of the HP granulite. Hf isotopic compositions of the zircon cores exhibit distinct variation with εHf（t） values ranging from-7. 3 to ＋ 13. 2,showing the magma mixing characteristics of different sources and/or crustal contamination. The whole rock compositions of the HP granulites also show mixture of mantle and crustal magmas. Combined with the formation age,regional rifting-related rock assemblage and tectonic setting,we suggest that the protolith may be the product of crust-mantle magma mixing in response to the collapse of the orogenic belt in the post-collision stage,which is related to the breakup of Rodinia.

关 键 词：西秦岭造山带 麻粒岩 商丹洋 Rodinia裂解 

分 类 号：P588.347[天文地球—岩石学] P597.3[天文地球—地质学]