北秦岭西段三叠纪超镁铁岩-正长岩体的确定及其地质意义  

The determination of Triassic ultramafic-syenite intrusive body and its geological significance,western North Qinling

作　　者：宫相宽[1] 陈丹玲[1,2] 朱小辉[1,2] 董增产[1,2] 郭彩莲[1,3] 

机构地区：[1]大陆动力学国家重点实验室,西北大学地质学系,西安710069 [2]中国地质调查局西安地质调查中心,西安710054 [3]西北有色地质研究院,西安710054

出　　处：《岩石学报》2016年第1期177-192,共16页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41090374;41472053;41430209;41072051)资助

摘　　要：陕西凤县九子沟地区的奥陶纪草滩沟群中出露一套由透辉石岩组成的超镁铁质岩体,依据岩性和区域对比,被认为是形成于古生代而且可能是大洋岩石圈残片,但缺乏详细研究。野外地质考察发现,该透辉石岩在露头上与一套黑云母透辉石岩和(含透辉石)正长岩关系密切,而且以黑云母透辉石岩为主体。本文在详细岩石学研究的基础上,对其中的透辉石岩、黑云母透辉石岩以及(含透辉石)正长岩进行了透辉石矿物成分、锆石U-Pb年代学和Hf同位素研究。结果表明,三种岩石中的透辉石皆为岩浆型透辉石,而且透辉石成分在透辉石岩和(含透辉石)正长岩中完全相同,而在黑云母透辉石岩中出现向普通辉石转变的趋势。LA-ICP-MS锆石定年结果,得到透辉石岩、黑云母透辉石岩和(含透辉石)正长岩的形成年龄分别为235.2±1.6Ma、232.8±4.3Ma和230.7±2.1Ma,表明九子淘超镁铁岩-正长岩形成于中生代。透辉石岩和黑云母透辉石岩锆石具有相似的Hf同位素组成,初始^(176)Hf/^(177)Hf分别变化于0.282405~0.282589和0.282397~0.282607,ε_(Hf)(t)值介于-8.26^-1.37和-8.41^-0.84之间,显示富集岩石圈地幔特征;(含透辉石)正长岩锆石^(176)Hf/^(177)Hf介于0.282577~0.282825之间,ε_(Hf)(t)值为-2.06^+5.83,是富集岩石圈地幔低程度部分熔融的产物,与超镁铁岩来自不同的岩浆源区。综合区域地质已有研究,九子沟中生代超镁铁岩-正长岩体是在华北、扬子两大板块碰撞过程中构造体制由挤压向伸展转换阶段,由两个富集岩石圈地幔不同程度部分熔融的、独立演化岩浆先后侵位形成。The Jiuzigou ultramafic rock located in Jiuzigou area, Feng County, Shaanxi Province. It intruded in the Ordovician Caotangou Group and used to be regarded as a Paleozoic pluton or ultramafic part of Erlangping ophilite so far, but lack of detailed study. Field geological investigation found that the diopsidite has a close relationship with a suite of biotite diopsidite and diopsidebearing syenite and all three together construct an uhramafic-syenite complex. Electron probe analyses reveal that diopsides in the three rocks show magmatic origin in their composition and those in diopsidite and syenite have almost the same composition, however those in biotite diopsidite show the changing trend from diopside in the grain cores and augite in the rims. Zircons separated from the three rocks show typical magmatic origin with wide oscillatory zoning structure, high content of Th, U and HREE, high Th/U ratios and HREE enrichment REE patterns. LA-ICP-MS zircon U-Pb dating yield the crystallization age of 235.2 ±1.6Ma, 232. 8 ± 4. 3Ma and 230. 7 ± 2. 1Ma for diopsidite, biotite diopsidite and diopside-bearing syenite, respectively, suggesting that the Jiuzigou uhramafite-syenite complex formed during Mesozoic instead of Paleozoic. Zircon Lu-Hf isotope analyses show that diopsidite and biotite diopsidite have homogeneous and very similar Hf isotopic composition, their initial 176 Hf/177 Hf ratios are between 0. 282405 -0. 282589 and 0. 282397 -0. 282607, and 6Hf（t） are between -8.25 - - 1.36 and -8.4 - -0. 8, respectively, indicating an enriched mantle origin. The initial 176 Hf/177 Hf ratios of zircons from diopside-bearing syenite vary from 0. 282577 to 0. 282825 and εHf （t） from -2. 06 to ＋ 5.83, obviously differ from the first two rocks, suggesting that the syenite derived from a different magma source, resemble to that of low grade partial melting of enriched lithospheric mantle. Combined these characteristics with the previous studies of Mesozoic granitoid rocks in the western Qinling mountains, the Jiuz

关 键 词：三叠纪 超镁铁岩-正长岩 锆石U-PB定年 HF同位素 北秦岭 

分 类 号：P588.124[天文地球—岩石学] P588.125[天文地球—地质学]