| 西准噶尔晚古生代中基性岩墙群岩石学成因及地质意义 |
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| 引用本文： | 贺新星,肖龙,王国灿,高睿,杨钢,鄢圣武.西准噶尔晚古生代中基性岩墙群岩石学成因及地质意义[J].地球科学,2015,40(5):777-796. |
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| 作者姓名： | 贺新星 肖龙 王国灿 高睿 杨钢 鄢圣武 |
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| 作者单位： | 1.江苏省地质调查研究院，江苏南京 210018 |
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| 基金项目： | 新疆1∶25万铁厂沟镇幅，克拉玛依市幅(L45C003001)区调修测项目，西准噶尔克拉玛依后山地区三维地质调查试点项目 |
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| 摘 要： | 中基性岩墙群的形成及产出对研究区域大地构造背景和岩浆演化过程具有重要的地质意义.对西准噶尔地区的夏尔蒲中基性岩墙群和小西湖中基性岩墙群中样品(共18件)进行了岩石学、锆石U-Pb年代学、岩石地球化学和同位素地球化学等方面的研究.结果显示，夏尔蒲和小西湖岩墙群岩石类型以闪长玢岩为主，含少量辉绿岩.LA-ICP-MS锆石U-Pb年代学表明夏尔蒲岩墙群的侵位年龄为308.6±5.5 Ma.岩石均具有高Mg#(＞40)、MgO(＞3%)、Al2O3(＞16%)；全岩A/CNK值多在0.9左右，A/NK＞2，属准铝质岩石；岩石整体属钙碱性玄武岩/安山岩系列.岩石具有较低的稀土总量(多在40×10-6~60×10-6)，具轻稀土富集、重稀土亏损及Eu正异常等特征((La/Yb)N为3.03~11.32，δEu=1.00~1.20)；明显富集大离子亲石元素K、Rb、Ba、Sr，亏损高场强元素Nb、Ta、Ti、Th，呈现了俯冲消减带岩石的地球化学特征.同时，岩石具有较高的Sr(均大于500 ×10-6)、较低的Y(大多小于10×10-6)和Yb(多在1×10-6左右)含量，较高的Sr/Y比值(36~95)，大多数样品具有富镁埃达克质岩石的组成特征.岩石具有亏损的Sr-Nd同位素组成((87Sr/86Sr)i=0.703 58~0.703 80，εNd(t)=5.76~6.34).元素及同位素地球化学资料表明岩浆源区中既有亏损地幔组分的参与，又有俯冲消减作用的印迹.结合区域地质特征及前人研究成果，结果表明晚石炭世时西准噶尔地区已进入后碰撞阶段.由于俯冲残留大洋板片部分熔融，产生的熔体在与亏损地幔一定程度相互作用后，经单斜辉石的分离结晶而形成了夏尔蒲和小西湖岩墙群中富镁埃达克质岩石；而来源于亏损地幔的岩浆同样经单斜辉石的分离结晶后，形成了夏尔蒲岩墙群中的辉绿岩和小西湖岩墙群中的角闪闪长玢岩.大规模中基性岩墙群的产出则进一步表明晚石炭世时西准噶尔地区处于后碰撞的伸展拉张构造背景之下.
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| 关 键 词： | 西准噶尔 中基性岩墙群 残余俯冲大洋板片 亏损地幔 岩石学 地球化学 |
| 收稿时间： | 2014-10-15 |
Petrogenesis and Geological Implications of Late Paleozoic Intermediate-Basic Dyke Swarms in Western Junggar |
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| He Xinxing,Xiao Long,Wang Guocan,Gao Rui,Yang Gang,Yan Shengwu.Petrogenesis and Geological Implications of Late Paleozoic Intermediate-Basic Dyke Swarms in Western Junggar[J].Earth Science-Journal of China University of Geosciences,2015,40(5):777-796. |
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| Authors: | He Xinxing Xiao Long Wang Guocan Gao Rui Yang Gang Yan Shengwu |
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| Abstract: | Understanding of the formation and occurrence of intermediate-basic dyke swarms can facilitate future studies on the magmatic evolution and tectonic settings. The Xiaerpu and Xiaoxihu intermediate-basic dyke swarms in West Junggar are principally composed of dioritic porphyrite, with minor diabase. LA-ICP-MS zircon U-Pb dating results indicate that the Xiaerpu dyke swarm is emplaced with age of 308.6±5.5 Ma. It is concluded that these dykes are the products of magmatism in Late Carboniferous based on the geological relationship between the dykes and host granitic rocks, and previous chronological studies. The dykes are characterized by high Mg#(> 40), high MgO(> 3%) and Al2O3(> 16%), belonging to calc-alkaline rocks and metaluminous rocks (with A/CNK approximately at 0.9 and A/NK > 2). The rocks have lower total REE (most are concentrated in 40×10-6-60 ×10-6), with enriched LREE and slightly positive Eu anomalies ((La/Yb)N=3.03-11.32, δEu=1.00-1.20). All samples are highly depleted in (Nb, Ta, Ti and Th), and enriched in (Sr, Ba, K and Rb). Meanwhile, the high contents of Sr (> 500×10-6), low contents of Y and Yb, high values of Sr/Y ratios (36.98-95.74), indicate the rocks (except sample XEP01, XEP02 and XXH06) are analogous to Mg-enriched adakitic rocks. Dykes have depleted Sr-Nd isotopic composition ((87Sr/86Sr)i=0.703 58-0.703 80, εNd(t)=5.76-6.34). Geochemical and isotopic compositions indicate that depleted-mantle components and materials related to subduction have prominent significance in the origin source of these dykes. Combined with the district geological evolution history and previous studies, we propose that West Junggar was under a post-collisional regime in Late Carboniferous. Due to the melting of remnant subducted oceanic slab, the slab melts interacted with the mantle materials to varying extent during ascent, and then accompanied fractional crystallization of clinopyroxene that generated the Mg-enriched adakitic rocks in Xiaerpu and Xiaoxihu intermediate-basic dyke swarms; and fractional crystallization of clinopyroxene of depleted-mantle materials metasomatised by slab melts can account for the formation of the diabase in Xiaerpu dyke swarm and the amphibolic diorite in Xiaoxihu dyke swarm. The emplacement of large-scale intermediate-basic dyke swarms further show that West Junggar was under a post-collisional extensional environment in Late Carboniferous. |
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| Keywords: | western Junggar intermediate-basic dyke swarm remnant subducted oceanic slab depleted-mantle petrology geochemistry |
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