甘肃柳园地区晚三叠世辉绿岩脉年代学和地球化学研究及其对北山造山带早中生代构造演化的指示  

Early Mesozoic tectonic evolution of Beishan Orogenic Belt:Constraints from chronology and geochemistry of the Late Triassic diabase dyke in Liuyuan area,Gansu Province

作　　者：孙海瑞[1,2] 吕志成 于晓飞[1,2] 李永胜 杜泽忠[1,2] 吕鑫 杜轶伦[1,2] 公凡影 SUN HaiRui;LüZhiCheng;YU XiaoFei;LI YongSheng;DU ZeZhong;Lü Xin;DU YiLun;GONG FanYing(Development Research Center of China Geological Survey,Beijing 100037,China;Mineral Exploration Technical Guidance Center of Ministry of Natural Resources,Beijing 100083,China)

机构地区：[1]中国地质调查局发展研究中心,北京100037 [2]自然资源部矿产勘查技术指导中心,北京100083

出　　处：《岩石学报》2020年第6期1755-1768,共14页Acta Petrologica Sinica

基　　金：国家重点研发计划课题(2018YFC0603704、2018YFC0603806);中国地质调查局项目(DD20160050);中央高校基本科研业务费专项资金项目(310827171122)联合资助。

摘　　要：位于中亚造山带中段南缘的北山地区,不仅是我国北方重要的三叠纪多金属成矿带,也是诠释中亚造山带南缘早中生代构造演化的关键区域。甘肃柳园地区位于北山南带,区内缺失三叠纪沉积记录,花岗质复式岩体和中酸性岩脉十分发育,基性岩脉也有分布,从而为探究北山早中生代构造演化提供了重要条件。基于此,本文对甘肃柳园地区晚三叠世辉绿岩脉开展了系统的锆石U-Pb-Hf同位素和全岩主量、微量元素分析。LA-ICPMS锆石U-Pb分析显示,辉绿岩形成于227.5±1.4Ma,年代为晚三叠世。全岩地球化学分析显示,辉绿岩具有高Mg#(65~70)、较高Ni含量(110×10^-6~157×10^-6)、轻重稀土分馏不明显((La/Y)N为1.42~1.94)、无或弱Eu正异常、强烈亏损Nb、Ta和Zr、Hf弱富集的特征。较高的(Th/Nb)N(>1)和较低的Nb/La(<1)比值,稳定的La/Nb和Th/Nb比值说明其岩浆侵位过程中壳源物质的混染不强;而较高的Th/Yb比值,较低的Th/U和La/Nb比值说明源区存在一定俯冲物质的参与;此外,较低的Nb/Yb、Th/U,明显的Nb和Ta负异常,平坦的HREE配分形式排除了来自OIB类似岩浆源区的可能;而较低的Nb/Yb比值及较高的εHf(t)值(平均值为1.08)说明镁铁质岩浆主要来自亏损的软流圈地幔。因此,该辉绿岩更可能源于受俯冲作用改造的亏损软流圈地幔的部分熔融,此幔源熔体在上升过程中与大陆岩石圈发生相互作用,导致少量壳源物质的混染。结合区内二叠纪基性-超基性岩脉的地球化学特征,本文认为辉绿岩岩浆源区受到先前俯冲物质的混染,辉绿岩形成于晚三叠世软流圈上隆的岩石圈伸展构造背景。在系统分析前人关于柳园地区早二叠至晚三叠世中酸性和基性-超基性侵入岩锆石Hf同位素数据后,本文发现:自~280Ma至~220Ma,北山南部中-酸性侵入岩锆石εHf(t)值具有先降后升的变化趋势,而同期基性-超基性岩锆石εHf(t)值主要呈逐渐降低Beishan Orogenic Belt(BOB),located in the southern section of Central Asia Orogenic Belt(CAOB),is an important region to discuss the accretionary evolution of the CAOB,and it is also one of the most important Triassic metallogenic belts in the northern China.Regionally,the Triassic sediment is absent,while,many granitoid plutons and dikes exposed with minor mafic dikes during the Late Paleozoic to Early Mesozoic.Therefore,it is an ideal area to probe the evolution of BOB.U-Pb zircon dating yields magma crystallization age of the Late Triassic(weighted 206Pb/238U ages of 227.5±1.4Ma)for a diabase dyke.Geochemically,the diabase has relatively high Mg#(65~70)and Ni(110×10^-6~157×10^-6)with weakly fractionated REE patterns((La/Yb)N=1.42~1.94),flat HREE patterns,no or weakly positive Eu anomaly,pronouncedly negative Nb and Ta anomalies and enrichment of Zr and Hf.Isotopically,the diabase dikes display positiveεHf(t)values of-2.21^+4.02 with an average of+1.08.Their relatively high ratios of(Th/Nb)N and Th/Yb,and low ratios of Nb/La and Th/U,and constant La/Nb and Th/Nb ratio suggest that slab-derived components were probably involved in the magmatic source with minor crustal contamination.In addition,the diabase show positiveεHf(t)values,low ratios of Nb/Yb and Th/U,and depletion of Nb and Ta with flat HREE pattern,which indicates that it is mainly derived from a depleted asthenosphere mantle,but not an Ocean Island Vasalts(OIB)-like source.Accordingly,it is more likely that the mafic magma originated from partial melting of a depleted asthenosphere mantle overprinted by slab-derived components,with minor crustal contamination during the ascending through continental lithosphere.Integrated the geochemistry of the Permian mafic dykes in this region,it is considered that the Late Triassic diabase of this study was formed in a lithospheric extensional tectonic setting which leads to melt uplift of depleted asthenosphere with overprinting of slab-derived components and experienced interaction with crustal material.By

关 键 词：辉绿岩脉 三叠纪 构造背景 北山造山带 中亚造山带 

分 类 号：P588.124[天文地球—岩石学] P597.3[天文地球—地质学]