新疆西准噶尔达尔布特蛇绿岩套柳树沟镁铁质杂岩SHRIMP锆石U-Pb年龄及地球化学特征
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摘要
对新疆西准噶尔达尔布特蛇绿岩套柳树沟镁铁质杂岩中橄榄辉长岩、蛇纹石化橄榄辉长岩、辉长岩和蚀变辉长岩地球化学分析表明:辉长岩和蚀变辉长岩具有钙碱性和拉斑玄武岩的双重特征,橄榄辉长岩和蛇纹石化橄榄辉长岩属镁铁质堆积岩,为蛇绿岩组成单元;稀土总量较高,具微弱正Eu异常,稀土元素配分模式为略左倾平坦型,与SSZ型镁铁质堆晶岩稀土配分模式相同;微量元素蛛网图上,富集大离子亲石元素Cs、K、Th、U,相对亏损高场强元素Nb、Ta,可能代表俯冲板片的流体交代上覆地幔楔使地幔岩石发生部分熔融。地球化学构造环境判别柳树沟镁铁质岩石岩浆源区为亏损型地幔向富集型地幔过渡的适度富集型地幔,其形成的最佳模型是在成熟岛弧基础上裂谷化形成的一个不成熟的类似边缘海性质弧后盆地,不具成熟大洋或盆地那样的洋壳-上地幔结构,在盆地扩张初期岩浆具岛弧特征,随着盆地被进一步打开,镁铁质岩石具N-MORB特征。对辉长岩采用SHRIMP锆石U-Pb同位素年龄测试,获得特柳树沟镁铁质岩石的结晶时间为314.9±1.7 Ma。
Geochemical characteristics of olive gabbro, serpentinizated olive gabbro, gabbro and altered gabbro in the mafic intrusive complex located in Liushugou-Darbut,western Junggar, Xinjiang are reported in this paper.The gabbro and altered gabbro have a dual characteristic of Ca-Alkali and tholeiite. Olive gabbro and serpentinizated olive gabbro are mafic accumulated rocks which are the units of ophiolite. The amount of rare element is high which is faint-abnormal in positive-Eu, and distributed mode of rare element is left-leaning and smooth which is the same as SSZ-type mafic cumulate. When normalized to primitive mantle, they are rich in large-ion lithophile element Cs, K, Th and U, and relatively deficient in high field strongth element Nb, Ta, which probably indicate that originate from mantle wedge metasomatized by the liquid of subducted panel make pyrolite partly melt. The geochemical tectonic environment distinguish that mafic rocks in Liushugou Channel is moderately enriched mantle which transits from depleted mantle into enriched mantle. The best model of its formation is that: a mature island arc rifting formed on the basis of an immature nature similar to the edge of the sea-arc basin, without upper mantle structure of mature oceanic or oceanic basins, and then the early expansion of the magma basin with arc characteristics, as the basin is further opened, mafic rocks with N-MORB features. SHRIMP Zircon U-Pb dating of gabbro in Liushugou-Darbut mafic complex is yielded ages of 314.9±1.7 Ma.
Geochemical characteristics of olive gabbro, serpentinizated olive gabbro, gabbro and altered gabbro in the mafic intrusive complex located in Liushugou-Darbut,western Junggar, Xinjiang are reported in this paper.The gabbro and altered gabbro have a dual characteristic of Ca-Alkali and tholeiite. Olive gabbro and serpentinizated olive gabbro are mafic accumulated rocks which are the units of ophiolite. The amount of rare element is high which is faint-abnormal in positive-Eu, and distributed mode of rare element is left-leaning and smooth which is the same as SSZ-type mafic cumulate. When normalized to primitive mantle, they are rich in large-ion lithophile element Cs, K, Th and U, and relatively deficient in high field strongth element Nb, Ta, which probably indicate that originate from mantle wedge metasomatized by the liquid of subducted panel make pyrolite partly melt. The geochemical tectonic environment distinguish that mafic rocks in Liushugou Channel is moderately enriched mantle which transits from depleted mantle into enriched mantle. The best model of its formation is that: a mature island arc rifting formed on the basis of an immature nature similar to the edge of the sea-arc basin, without upper mantle structure of mature oceanic or oceanic basins, and then the early expansion of the magma basin with arc characteristics, as the basin is further opened, mafic rocks with N-MORB features. SHRIMP Zircon U-Pb dating of gabbro in Liushugou-Darbut mafic complex is yielded ages of 314.9±1.7 Ma.
引文
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[2]Zhu Y F,Zhang L F,Gu L B,Guo X and Zhou J.The zircon SHRIMP chronology and trace element geochemistry of the Carboniferous volcanic rocks in western Tianshan Mountains[J].ChineseScienceBulletin,2005,50:2201-2212.
[3]Zhu Y F and Xu X.The discovery of Early Ordovician ophiolite melange in Taerbahatai Mts,Xinjiang,NW China[J].ActaPetrologicaSinica,2006,22(12):2833-2842.
[4]肖序常,汤耀庆,李锦轶,等.新疆北部及其邻区大地构造[M].北京:地质出版社,1992.
[5]李锦轶.新疆东部古元古代晚期和古生代构造格架及其演变[J].地质论评,2004,50(3):304-322.
[6]李锦轶,何国琦,徐新,李华芹,孙桂华,杨天南,高立明,朱志新.新疆北部及邻区地壳构造格架及其形成过程的初步探讨[J].地质学报,2006,80(1):148-168.
[7]肖文交,韩春明,袁超,陈汉林,孙敏,林寿发,厉子龙,毛启贵,张继恩,孙枢,李继亮.新疆北部石炭纪-二叠纪独特的构造-成矿作用:对古亚洲洋构造域南部大地构造演化的制约[J].岩石学报,2006,(22)5:1062-1076.
[8]朱永峰,徐新.新疆塔尔巴哈台发现早奥陶世蛇绿岩[J].岩石学报,2006,22(12):2833-2842.
[9]师占义.新疆达尔布特二辉橄榄岩团块的成因类型及其地质特征[J].中国科学院西安地质矿产研究所所刊,1982,(4):21-25.
[10]朱宝清,王来生,王连晓.西准噶尔西南地区古生代蛇绿岩[J].中国地质科学院西安地质矿产研究所所刊,1987,(17):3-64.
[11]新疆维吾尔自治区地质矿产局,新疆维吾尔自治区区域地质志.北京:地质出版社,1993.
[12]张驰,黄萱.新疆西准噶尔蛇绿岩形成时代和环境讨论[J].地质论评,1992,38:509-524.
[13]徐新,何国琦,李华芹,丁天府,刘兴义,梅绍武.克拉玛依蛇绿混杂岩带的基本特征和锆石SHRIMP年龄信息[J].中国地质,2006,33(3):470-475.
[14]辜平阳,李永军,张兵,佟丽莉,王军年.西准达尔布特蛇绿岩中辉长岩LA-ICP-MS锆石U-Pb测年[J].岩石学报,2009,25(6):1364-1372.
[15]Pearce J A,Lippard S J,Roberts S.Characteristics and tectonic significance of supra-subducfion zone ophiolites[A].//In:Kokelaar B P,Howells M E eds.Marginal Basin Geology.Geological Society of London Special Publication,1984,16:77-94.
[16]Pearce J A.Supra-subduction zone ophiolitcs:The search for modem analogues[A].//Ophiolite concept and the evolution ofgeological thought(eds.Dilek Y anf Newcomb S)[C].Geological Society ofAmefican Special Paper,2003,373:269-293.
[17]Robertson A H E.Overview of the genesis and emplacement ofMesozoic ophiolites in the Eastern Mediterranean Tethyan region[J].Lithos,2002,65:l-67.
[18]何国琦,李茂松,刘德权,等.中国新疆古生代地壳演化及成矿[M].乌鲁木齐:新疆人民出版社,1994:130-147.
[19]霍有光.西准噶尔达拉布特蛇绿岩稀土元素岩石化学特征及地质意义[J].西安地质矿产研究所所刊,1985,(10):53-66.
[20]姜勇,李少贞,郑启之,李松龄,冯新昌,董富荣.西准噶尔达拉布特超镁铁岩岩石化学特征及形成环境[J].新疆地质,2003,21(2):260-261.
[21]刘希军,许继峰,王树庆,侯青叶,白正华,雷敏.新疆西准噶尔达拉布特蛇绿岩E-MORB型镁铁质岩的地球化学、年代学及其地质意义[J].岩石学报,2009,25(6):1373-1389.
[22]Coleman R G.Ophiolites[M].Beijing:Geological Publishing,1982.
[23]王希斌,鲍佩声,戎合.中国蛇绿岩中变质橄榄岩的稀土元素地球化学[J].岩石学报,1995,11(增刊):24-41.
[24]Sun and McDonough W F.Chemical and isotopic systematics of oceanic basalts:Implications for mantle composition and processes[J].Geological Society of London,Special Publication,1989,42:331-345.
[25]Boynton W V,Cosmochemistry of the rare earth elements:meteorite studies[A].//In:Henderson,P.(ed.)Rare Earth ElementGeochemistry.Amsterdam:Elsevier,1984:63-114.
[26]Compston W,Williams I S,Kirschvink J L,Zhang Z C.Zircon U-Pb ages ofearly Cambrian time-scale[J].Journal of GeologySociety,London,1992,149:171-184.
[27]Williams I S,Claesson S.Isotope evidence for the Precambrian province and Caledonian metamorphism of high grade paragneiss from the Seve Nappes,Scandinavian Caledonides,Ⅱ.Ion microporpe zircon U-Th-Pb[J].ContributionstoMineralogyand Petrology,1987,97:205-217.
[28]Ludwig K R.Using Isoplot/EX,version 2,a Geolocronolgical Toolkit for Microsoft Excel[M].Berkeley Geochronolobical CenterSpecialPublication,1999:47.
[29]Ludwig K R.Squid 1.02:A User Manual[M].Berkeley GeochronologicalCenterSpecialPublication,2001.
[30]张旗,周国庆.中国蛇绿岩[M].北京:科学出版社,2001.