西准噶尔托里县都伦河东岩体锆石U-Pb年龄及地质意义

2015-05-05 01:01段丰浩李永军王冉纪征宝程万里郭鑫
西北地质 2015年3期
关键词:包体准噶尔闪长岩

段丰浩,李永军,2,王冉,2,纪征宝,程万里,郭鑫

(1.长安大学地球科学与资源学院,陕西 西安 710054;2.西部矿产资源与地质工程教育部重点实验室,陕西 西安 710054;

西准噶尔托里县都伦河东岩体锆石U-Pb年龄及地质意义

段丰浩1,李永军1,2,王冉1,2,纪征宝3,程万里1,郭鑫1

(1.长安大学地球科学与资源学院,陕西 西安 710054;2.西部矿产资源与地质工程教育部重点实验室,陕西 西安 710054;

3.新疆维吾尔自治区地质矿产勘查开发局第七地质大队,新疆 乌苏 833000)

西准噶尔地体晚古生代岩浆活动强烈,其中以达尔布特构造-岩浆带最为显著。近年来,对该带大岩基研究较多,而对该带小岩体的研究主要集中在包古图一带成矿斑岩上,对其余小岩体研究较为局限。位于西准噶尔托里县的都伦河东岩体内外接触带有较强的孔雀石化、硅化、碳酸盐化、绿泥石化、黄铁矿化等蚀变与矿化,岩体内部发育镁铁质微粒包体。本次于寄主石英闪长岩中获得(312.6±4.2)Ma(n=21,MSWD=0.15,95%置信度)的LA-ICP-MS锆石U-Pb年龄,其与包古图小岩体(Au-Cu矿化)和夏尔莆小岩体(Cu-Mo矿化)的成岩时代基本一致,各岩体形成的构造环境大致相同。该可靠年龄为研究本区与小岩体有关,与岩浆混合作用有关的Au-Cu-Mo成矿作用时代提供了年代学约束。

岩浆混合花岗岩;锆石U-Pb年龄;成矿作用时代;都伦河东岩体;西准噶尔

西准噶尔地区产出大量的中酸性侵入岩,其中以达尔布特构造-岩浆带最为显著。除少数岩体研究程度较高以外,大部分岩体研究程度均很低。根据产状可将这些侵入体分为以庙尔沟、阿克巴斯陶、红山、克拉玛依岩体等为代表的以巨大岩基形式产出的花岗岩体和以包古图Ⅱ号、V号岩体等为代表的以小岩株或岩脉形式产出的小岩体(图1)。近年来,对该带出露的大岩基研究较多(伍建机等,2001;韩宝福等,2006;高山林等,2006;庞振甲等,2010;贺敬博等,2011;冯乾文等,2012;高睿等,2013;胡洋等,2015),而对出露的小岩体(岩脉)研究较为局限,且主要集中在包古图一带的含矿斑岩上(张连昌等,2006;宋会侠等,2007;唐功建等,2009;刘玉琳等,2009;Shen et al., 2009,2012;申萍等,2010;魏少妮等,2011,2015;尹继元等,2013),对同一构造-岩浆带上的其他小岩体研究较少。

都伦河东岩体位于西准噶尔托里县扎依尔山阔依塔斯地区,前人对该岩体研究工作只停留在野外地质调查上,尚缺乏高精度的同位素年代学资料,很大程度上制约了区域岩浆活动和构造演化的研究。本次研究主要对都伦河东岩体中石英闪长岩进行锆石U-Pb年代学工作,确定该岩体的精确侵位时代,为探讨达尔布特构造-岩浆带铜、金金属成矿提供年代学证据。

1 岩体地质特征

都伦河东岩体位于达尔布特断裂西南端,呈不规则椭圆状,长轴近南北向,长约5.5km,东西约3.8km,出露面积约18km2,侵入于石炭纪地层中(图1a),与围岩多为港湾状外倾接触。围岩局部有混染交代现象,岩体外接触带往往呈较深色调,是由接触变质导致的角岩化等蚀变引起。围岩蚀变较发育,岩石呈碎裂状,内外接触带有较强的孔雀石化、硅化、碳酸盐化、绿泥石化、黄铁矿化等蚀变与矿化。笔者等在野外地质工作中,首次发现都伦河东岩体为岩浆混合花岗岩,这是继夏尔莆岩浆混合花岗岩(张兵等,2009;李永军等,2013)之后,西准地区再次发现的发育镁铁质微粒包体的岩浆混合花岗岩体。寄主岩石主要为一套中粒石英闪长岩、中粗粒花岗闪长岩、中粗粒黑云母花岗闪长岩组合。镁铁质微粒包体岩性主要为闪长(玢)岩,细粒-微粒结构,粒径大都小于1mm,多为0.5~0.8mm,部分包体粒径可达到1.5~2mm,但粒度总体来说要比其寄主岩石细,两者相差近一个数量级。包体大小不一,相差悬殊,形态主要有椭圆状、浑圆状,部分呈透镜状以及撕裂状,部分包体呈现出“拖尾”等流线型形态。此外,野外露头尺度上显示包体与寄主岩石强烈成分交换的现象极为普遍,包体中还可见寄主岩石斜长石捕掳晶(图2)。镁铁质包体在寄主岩石中均有分布但极不均匀,其中花岗闪长岩中镁铁质包体数量最少,出露面积仅占整体的0.1%左右,分布零星;黑云母花岗闪长岩中包体数量也极少,出露面积约占整体的0.3%;石英闪长岩中包体最为发育,数量最多,出露面积约占整体的15%。包体越密集分布的岩石,其寄主岩石基性程度越高,这与经典的岩浆混合花岗岩体中包体分布规律基本相同(朱金初等,2006;李永军等,2013)。

图1 都伦河东岩体地质简图Fig.1 The geological map of the Dulunhe granite

(a).包体中斜长石捕掳晶; (b).包体与寄主岩石强烈成分交换; (c).石英闪长岩; (d).花岗闪长岩 Pl.斜长石; Qz.石英; Am.角闪石; Bi.黑云母图2 都伦河东岩体岩浆混合特征及镜下显微照片Fig.2 The Macroscopic characteristics of magma mixing and Microscopic photos of the Dulunhe granite

2 样品与分析方法

用于LA-ICP-MS锆石U-Pb测年的样品采自都伦河东岩体近中心部位寄主石英闪长岩中(样品编号DLHRZ1-1)。采样点坐标:北纬45°14.422',东经83°14.165′(图1a)。

锆石分选在廊坊诚信地质服务有限公司完成。首先使用重液浮选和电磁分离方法挑选锆石,在双目镜下挑选出晶形和透明度较好的代表性锆石以环氧树脂固定制成样品靶,抛光至暴露出锆石中心面,用反射光、透射光照相。然后用体积百分比为3%的HNO3清洗样品并镀碳,进行阴极发光(CL)照相。照相之后用3%的HNO3清洗靶标的表面。

锆石的激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)原位U-Pb定年在西北大学大陆动力学国家重点实验室完成。实验采用的ICP-MS为美国Agilent 公司生产的Agilent 7500 a,激光剥蚀系统为德国M-icroLas公司生产的GeoLas 200 M,激光剥蚀束斑直径约为30μm,激光剥蚀样品的深度为20~40 μm。锆石年龄采用标准锆石91 500作为外部标准物质。笔者择29Si作为内标,采用Glitter(ver4.0,Macquarie University)对锆石的同位素比值及元素含量进行计算,最终的年龄计算及谐和图是采用Isoplot(ver3.0)完成的,详细的实验原理和流程及仪器参数参见(柳小明等,2002;YUAN et al.,2008)。

3 锆石特征及分析结果

本次处理所获得的的石英闪长岩锆石大多为无色透明或浅黄色,晶形发育较好,多呈长柱状、正方双锥状自形晶体,长约100~250μm,宽约80~160μm,长宽比3∶1~1∶1;阴极发光图像(图3)显示大部分锆石均具有清晰的内部结构和典型的岩浆震荡环带以及明暗相间的条带结构,属于岩浆结晶锆石(吴元保等,2004),可以反映岩浆冷却结晶及岩体侵位的时代。

图3 都伦河东岩体石英闪长岩中典型锆石阴极发光(CL)图像Fig.3 CL images of zircon gains from the quartiz diorite of the Dulunhe granite

锆石U-Pb测试分析结果列于表1。21个样品点的207Pb/206Pb值非常接近(0.051 95~0.053 70),表明该批锆石为同期岩浆成因锆石。

21个样品的Th含量为21.97×10-6~448.42×10-6,U含量为45.06×10-6~456.92×10-6,Th/U值较高(0.34~0.98),并呈现出较好的正相关,指示锆石属典型岩浆成因(吴元保等,2004)。21个测点年龄值较集中,206Pb/238U年龄为(312.6±4.2)Ma(n=21,MSWD=0.15,95%置信度,图4),时代为晚石炭世早期。

4 分析与讨论

西准噶尔地区晚古生代岩浆活动强烈,时代主要集中在石炭纪—早二叠世(韩宝福等,2006;苏玉平等,2006;Tang et al., 2010;尹继元等,2011;Geng et al., 2009)。从成岩时代上看,以碱性花岗岩类为主的大岩基时代主要集中在308~296Ma(韩宝福等,2006;苏玉平等,2006;Geng et al., 2009),而对于以石英闪长岩和花岗闪长岩等为主的中酸性小岩体而言,前人研究主要局限在包古图一带的含矿斑岩上。本次研究获得的都伦河东岩体成岩时代,与该岩体侵入于早石炭世希贝库拉斯组这一地质事实相吻合。该结果也与近年来报道的包古图一带小岩体的成岩时代(310~320Ma)(宋会侠等,2007;刘玉琳等,2009;唐功建等,2009;申萍等,2010;魏少妮等,2011,2015;Shen et al., 2012;张照伟等,2014)基本一致,证实这些小岩体是近同期形成的,略早于区域大岩基的成岩时代,且两者均位于达尔布特深大断裂两侧,成因相近。因此,推断其与包古图一带的小岩体相似,是俯冲作用的产物。

图4 (a)都伦河东岩体石英闪长岩锆石U-Pb谐和图和(b)年龄直方图Fig.4 (a) Concordia plots of zircon U-Pb dating results and (b)age histogramof the quartiz diorite of the Dulunhe granite

都伦河东岩体位于西准噶尔铜、钼、金成矿带(何国琦等,2006;朱永峰等,2007,2014),铜、金矿床大多赋存于晚泥盆世—石炭纪岩浆岩中(王强等,2006;朱永峰等,2007;申萍等,2008)。包古图中酸性小岩体和脉岩的成岩时代为310~320Ma(宋会侠等,2007;刘玉琳等,2009;唐功建等,2009;申萍等,2010;魏少妮等,2011,2015),包古图斑岩型铜矿中辉钼矿Re-Os同位素年龄为(310±3.6)Ma(宋会侠等,2007)和(316.0±6.4)Ma(刘玉琳等,2009);夏尔莆岩体成岩时代为297.6~300.8Ma(李永军等,2013;Wang et al.,2013),其铜-钼矿Re-Os同位素年龄为(294.6±4.6)Ma(Wang et al.,

2013),其成岩与成矿年龄仅相差几个百万年,成因关系密切。和夏尔莆岩体一样,都伦河东岩体在成岩过程中显著的壳幔岩浆混合作用与铜、钼、金矿化关系密切(李永军等,2012;王玉往等,2012;邢丽萍等,2014)。此外,都伦河东岩体内外接触带较强的孔雀石化、硅化、碳酸盐化、绿泥石化、黄铁矿化等蚀变与矿化,以及与镁铁质微粒包体密切共生关系等,为成矿作用研究提供了新信息。因此探讨都伦河东岩体的精确锆石U-Pb年龄可为本区铜、钼、金矿化时代提供年代学约束。

5 结论

都伦河东岩体LA-ICP-MS锆石U-Pb年龄为(312.6±4.2)Ma,形成于晚石炭世早期,与包古图一带小岩体成岩时代基本一致。综合本区区域岩浆活动、构造演化等成果,笔者等认为都伦河东岩体可能形成于对成矿极为有利的俯冲相关环境,与西准噶尔达尔布特构造-岩浆带铜、金等成矿作用关系密切,显示较好的成矿潜力,值得进一步研究。

致谢:论文撰写过程中得到了长安大学杨高学副教授的帮助和建议,审稿专家为笔者提出了宝贵的意见和建议,在此对他们的帮助表示真诚的感谢!

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Zircon U-Pb Age and Geological Significance of Dulunhe Granite in Tuoli,Western Junggar

DUAN Fenghao1, LI Yongjun1,2, WANG Ran1,2, JI Zhengbao3, CHENG Wanli1, GUO Xin1

(1. Earth Science and Resources College, Chang’an University, Xi’an 710054, Shaanxi, China;2. Key Laboratory of Western China’s Mineral Resources and Geological Engineering,Ministry of Education, Xi’an 710054, Shaanxi, China; 3. No.7 Geological Survey Party,Xinjiang Bureau of Geology and Mineral Resource Exploration, Wusu 833000, Xinjiang, China)

There are large numbers of intrusive granites exposed along the Daerbute tectonic-magmatic belt in the center of western Junggar. Many researches have been conducted on the batholites instead of small granite intrusions, which mainly distributed in porphyry mineralization belt of the Baogutu region. Wallrock alterations such as malachitization, silicification, carbonatization, chloritization and pyritization are well developed in Dulunhe granite, which is located in Kuoyitasi area of Zayier Mountain, and abundant mafic microgranular enclaves can be found in these host rocks, however, accurate geochronology data is still insufficient. High-precision LA-ICP-MS zircon U-Pb dating reveals that the quartz diorite in the Dulunhe granite is formed at (312.6±4.2)Ma(n=21, MSWD=0.15, 95% confidence degrees) in early late Carboniferous. Combined with previous researches, the authors suggest that the age of Dulunhe granite is consistent with small granites like Baogutu granite (Au-Cu mineralization) and Xiaerpu granite (Cu-Mo mineralization); both of them are probably formed in the subduction-related setting. This reliable age provides chronology constraints for the study of regional mineralization age that related to small granite and magma mixing of Au-Cu-Mo deposit.

magma mixed granite; zircon U-Pb dating; mineralization age; Dulunhe granite; west Junggar

2015-03-14;

2015-04-29

新疆两权中央留成专项资金项目(Y14-5-LQ06)、国家自然科学基金(41303027)、中央高校基本科研业务费专项基金(2014G1271058)

段丰浩 (1991-),男,在读硕士研究生,构造地质学专业。E-mail:chddfh@163.com

P588.12

A

1009-6248(2015)03-0140-09

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