小秦岭东吉口印支期辉石正长岩年代学及其地质意义  

Geochronology of the Indosinian Dongjikou Pyroxene Syenite from Xiaoqinling Area and Its Tectonic Implications

作　　者：李春麟[1] 余心起[1] 刘俊来[1] 王北颖 陈帅奇 代堰锫[4] 

机构地区：[1]中国地质大学地球科学与资源学院,北京100083 [2]陕西省地质调查院,西安710065 [3]湖南省地质调查院,长沙410116 [4]中国科学院地质与地球物理研究所,北京100029

出　　处：《吉林大学学报（地球科学版）》2012年第6期1806-1816,共11页Journal of Jilin University：Earth Science Edition

基　　金：国家自然科学基金项目(90814006)

摘　　要：东吉口辉石正长岩是小秦岭地区此前唯一获得加里东期年龄的岩体,呈不规则卵圆状侵入于古元古代太华群高级变质岩中,发育弱片麻理。详细的岩相学观察发现,岩体内部可以分为两种具不同结构特征的岩性,从野外接触关系来看:细粒辉石正长岩形成较早,发育烘烤边;中粒含斑辉石正长岩略晚,发育冷凝边。二者的矿物均由斜长石、钾长石、辉石、角闪石、石英等组成,斑晶主要为辉石和斜长石,辉石和角闪石均发生退变质作用。细粒辉石正长岩LA-ICP-MS锆石U-Pb测年结果为(214.8±3.1)Ma(MSWD=0.48),中粒含斑辉石正长岩为(212.9±3.1)Ma(MSWD=5.6),均形成于晚三叠世印支期,而不是此前认为的加里东期。这一组年龄表明,小秦岭地区加里东期岩浆活动可能极其微弱,或者不存在该期岩浆活动。总结前人资料,发现东吉口辉石正长岩的形成时代与小秦岭印支期成矿事件同时,印支期岩浆活动与洋陆俯冲转为陆陆碰撞作用有关,强烈的壳幔相互作用导致了深部物质的上涌,岩浆活动为金的富集提供了热驱动力。The Dongjikou pyroxene sysnite,appearing as irregular ovoid,intruding into the Palaeoproterozoic high-grade metamorphic rocks of Taihua group,and developing gneissic schistosity in Huayangchuan region,Xiaoqinling,is the only known Caledonian rock body.Detailed petrography and field work studies on Dongjikou pyroxene sysnite indicate that inner parts of the rock body,according to the different structural characteristics,can be divided into the earlier-formed fine-grained pyroxene syenite,developing optalic boeder and a little later-formed medium-grained porphyritic pyroxene syenite,developing chilled border.The mineral association in both types of rocks is composed of plagioclase,potassium feldspar,pyroxene,amphibole,quartz,etc.The phenocrysts are mainly composed of pyroxene and plagioclase.Pyroxene and amphibole have experienced the retrogressive metamorphism.LA-ICP-MS zircon U-Pb ages of the fine-grained pyroxene syenite and medium-grained porphyritic pyroxene syenite are（214.8±3.1） Ma（MSWD=0.48）and（212.9±3.1） Ma（MSWD=5.6） respectively,which represents that the formation age of the rock is Late Triassic instead of the Caledonian period.The isotopic dating results show that the Caledonian magmatism event was extremely weak or inexistent in Xiaoqinling area.According to isotopic dating in this paper and a systematical summary of the previous research on the geological characteristics,it can be concluded that the age of the Dongjikou pyroxene sysnite is very close to that of the Indosinian magmatic and metallogenic event in Xiaoqinling area.It＇s concerned that the Indosinian magmatism is related to the transition regime from oceanic slab subduction to intercontinental collision.Moreover,intensive crust-mantle interactions caused upwelling of the deep material and triggered the gold enrichment.

关 键 词：辉石正长岩 LA-ICP-MS锆石U-PB测年 晚三叠世 加里东期 小秦岭 金矿 

分 类 号：P548[天文地球—构造地质学] P597.3[天文地球—地质学]