西藏冈底斯南缘中侏罗世辉长闪长岩锆石U-Pb定年和Lu-Hf同位素组成及其构造意义
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摘要
为解释冈底斯南缘新发现的中基性岩体构造环境及物质来源等信息,以西藏自治区贡嘎县昌果乡辉长闪长岩为研究对象,通过开展锆石U-Pb定年、Hf同位素组成、全岩地球化学分析,得到岩体锆石年龄为(165±1. 1) Ma。样品富集大离子亲石元素,亏损高场强元素,具有富铝(Al2O3=18. 84%～20. 70%)、富钾的特征,在构造判别图解中样品多落在挤压岛弧区域,表明其形成构造环境应为俯冲岛弧环境。样品具较为平缓的稀土配分曲线,岩浆分异程度较低,无明显Eu异常,(La/Yb)N平均值为2. 30(<4. 3),与幔源岩浆的稀土配分型式相似。锆石Hf同位素显示εHf(t)值为很高的正值(12. 97～18. 74),二阶段模式年龄集中在150～218 Ma,略大于其侵位年龄,表明幔源岩浆在侵位过程中受到地壳物质的混染。结合区域构造演化史认为该岩体形成于新特提斯洋向冈底斯俯冲过程中的成熟岛弧期,从而也进一步证实了南冈底斯在中侏罗世处于俯冲岛弧构造环境。
In order to elucidate the tectonic setting and magma source of the gabbro-diorite in the southern margin of the Gangdese magmatic belt,we conducted zircon U-Pb age and Lu-Hf isotope analyses,and whole-rock major and trace elements analyses. The zircon U-Pb dating yielded( 165 ± 1. 1) Ma for the gabbro-diorite. Geochemically,the gabbro-diorite is enriched in LILEs,Al2 O3 and K2 O,but depleted in HFSEs. The sample data points are distributed in the island arc field in tectonic discrimination diagrams,and have clear island arc geochemical affinities. The chondrite-normalized REE patterns are flat with no Eu anomalies,showing low degree of magma differentiation. The mean( La/Yb)Nvalue is 2. 30(< 4. 3),and the rocks have similar REE patternsto typical mantle-derived magmas. The gabbro-diorite samples have highly depleted zircon Hf isotopic compositions,with εHf( t) values of 12. 97-18. 74 and two-stage Hf model ages of 150-218 Ma,indicating that the mantle-derived magma was influenced by crustal assimilation. Integrated whole-rock geochemical and zircon isotopic analyses suggest that the gabbro-diorite was formed in a mature island arc formed by the Neo-Tethyan subduction,which further indicates that the Gangdese magmatic arc was in a subduction setting during the Middle Jurassic.
In order to elucidate the tectonic setting and magma source of the gabbro-diorite in the southern margin of the Gangdese magmatic belt,we conducted zircon U-Pb age and Lu-Hf isotope analyses,and whole-rock major and trace elements analyses. The zircon U-Pb dating yielded( 165 ± 1. 1) Ma for the gabbro-diorite. Geochemically,the gabbro-diorite is enriched in LILEs,Al2 O3 and K2 O,but depleted in HFSEs. The sample data points are distributed in the island arc field in tectonic discrimination diagrams,and have clear island arc geochemical affinities. The chondrite-normalized REE patterns are flat with no Eu anomalies,showing low degree of magma differentiation. The mean( La/Yb)Nvalue is 2. 30(< 4. 3),and the rocks have similar REE patternsto typical mantle-derived magmas. The gabbro-diorite samples have highly depleted zircon Hf isotopic compositions,with εHf( t) values of 12. 97-18. 74 and two-stage Hf model ages of 150-218 Ma,indicating that the mantle-derived magma was influenced by crustal assimilation. Integrated whole-rock geochemical and zircon isotopic analyses suggest that the gabbro-diorite was formed in a mature island arc formed by the Neo-Tethyan subduction,which further indicates that the Gangdese magmatic arc was in a subduction setting during the Middle Jurassic.
引文
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[22]PLANK T,LANGMUIR C H.The chemical composition of subducting sediment and its consequences for the crust and mantle[J].Chemical Geology,1998,145(3/4):325-394.
[23]AYERS J.Trace element modeling of aqueous fluid-peridotite interaction in the mantle wedge of subduction zones[J].Contributions to Mineralogy and Petrology,1998,132(4):390-404.
[24]吴福元,李献华,郑永飞,等.Lu-Hf同位素体系及其岩石学应用[J].岩石学报,2007,23(2):185-220.
[25]BUSBY C.Continental growth at convergent margins facing large ocean basins:a case study from Mesozoic convergent-margin basins of Baja California,Mexico[J].Tectonophysics,2004,392(1/4):241-277.
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[27]耿全如,潘桂棠,金振民,等.西藏冈底斯带叶巴组火山岩地球化学及成因[J].地球科学,2005,30(6):747-760.
[28]ZHU D C,ZHAO Z D,NIU Y,et al.The origin and pre-Cenozoic evolution of the Tibetan Plateau[J].Gondwana Research,2013,23(4):1429-1454.
[29]MURPHY M A,YIN A N,HARRISON T M,et al.Did the Indo-Asian collision alone create the Tibetan plateau?[J].Geology,2000,8:719-722.
[30]TAPPONNIER P,XU Z Q,ROGER F,et al.Oblique stepwise rise and growth of the Tibet Plateau[J].Science,2001,23:1671-1677.
[31]朱杰,杜远生,刘早学,等.西藏雅鲁藏布江缝合带中段中生代放射虫硅质岩成因及其大地构造意义[J].中国科学D辑:地球科学,2005,35(12):1131-1139.
[32]ZHU D C,PAN G T,WANG L Q,et al.Spatial-temporal distribution and tectonic setting of Jurassic magmatism in the Gangdise belt,Tibet,China[J].Geological Bulletin of China,2008,27(4):458-468.
[33]ZHU D C,PAN G T,WANG L Q,et al.Tempo-spatial variations of Mesozoic magmatic rocks in the Gangdise belt,Tibet,China,with a discussion of geodynamic setting-related issues[J].Geological Bulletin of China,2008,27(9):1535-1550.
[34]张宏飞,徐旺春,郭建秋,等.冈底斯南缘变形花岗岩锆石U-Pb年龄和Hf同位素组成:新特提斯洋早侏罗世俯冲作用的证据[J].岩石学报,2007,23(6):1347-1353.
[35]裴树文.拉萨地块火山岩系内早-中侏罗世双壳类动物群及其古生物地理[J].现代地质,1999,13(3):291-296.
[36]CHU M F,CHUNG S L,SONG B,et al.Zircon U-Pb and Hf isotope constraints on the Mesozoic tectonics and crustal evolution of southern Tibet[J].Geology,2006,34(9):745-748.
[37]ZHU D C,PAN G T,CHUNG S L.SHRIMP zircon age and geochemical constraints on the origin of Early Jurassic volcanic rocks from the Yeba Formation,southern Gangdese in south Tibet[J].International Geology Review,2008,50(5):442-471.
[2]RENNA M R,TRIBUZIO R,TIEPOLO M.Origin and timing of the post-Variscan gabbro-granite complex of Porto(Western Corsica)[J].Contributions to Mineralogy and Petrology,2007,154(5):493-517.
[3]ZHANG S H,ZHAO Y,KRNER A,et al.Early Permian plutons from the northern North China Block:constraints on continental arc evolution and convergent margin magmatism related to the Central Asian Orogenic Belt[J].International Journal of Earth Sciences,2012,98(6):1441-1467.
[4]邱检生,王睿强,赵姣龙,等.冈底斯中段早侏罗世辉长岩-花岗岩杂岩体成因及其对新特提斯构造演化的启示:以日喀则东嘎岩体为例[J].岩石学报,2015,31(12):3569-3580.
[5]朱弟成,潘桂棠,王立全,等.西藏冈底斯带中生代岩浆岩的时空分布和相关问题的讨论[J].地质通报,2008,27(9):1535-1536.
[6]朱弟成,潘桂棠,莫宣学,等.冈底斯中北部晚侏罗世-早白垩世地球动力学环境:火山岩约束[J].岩石学报,2006,22(3):534-546.
[7]董彦辉,许继峰,曾庆高,等.存在比桑日群弧火山岩更早的新特提斯洋俯冲记录么?[J].岩石学报,2006,22(3):661-668.
[8]黄丰,许继峰,陈建林,等.早侏罗世叶巴组与桑日群火山岩:特提斯洋俯冲过程中的陆缘弧与洋内弧?[J].岩石学报,2015,31(7):2089-2100.
[9]ZHU D C,ZHAO Z D,NIU Y L,et al.The Lhasa Terrane:Record of a microcontinent and its histories of drift and growth[J].Earth and Planetary Science Letters,2011,301(1/2):241-255.
[10]YIN A,HARRISON T M.Geologic evolution of the HimalayanTibetan Orogen[J].Annual Review of Earth&Planetary Sciences,2000,28:211-280.
[11]潘桂棠,王立全,朱弟成.青藏高原区域地质调查中几个重大科学问题的思考[J].地质通报,2004,23(1):12-19.
[12]LIU Y S,GAO S,HU Z C,et al.Continental and oceanic crust recycling-induced melt-peridotite interactions in the Trans-North China Orogen:U-Pb dating,Hf isotopes and trace elements in zircons from mantle xenoliths[J].Journal of Petrology,2010,51:537-571.
[13]侯可军.LA-MC-ICP-MS锆石Hf同位素的分析方法及地质应用[J].岩石学报,2007,23(10):2595-2604
[14]WU F Y,YANG Y H,XIE L W.Hf isotopic compositions of the standard zircons and baddeleyites used in U-Pb geochronology[J].Chemical Geology,2006,234:105-126.
[15]KELEMEN P B,JOHNSON K T M,KINZLER R J,et al.Highfield-strength element depletions in arc basalts due to mantle-magma interaction[J].Nature,1990,345:521-524.
[16]WANG Y,ZHANG A,FAN W,et al.Origin of paleosubductionmodified mantle for Silurian gabbro in the Cathaysia Block:Geochronological and geochemical evidence[J].Lithos,2013,160/161(1):37-54.
[17]MIDDLEMOST E A K.Naming materials in the magma/igneous rock system[J].Annual Review of Earth&Planetary Sciences,1994,37(3/4):215-224.
[18]PECCERILLO A,TAYLOR S R.Geochemistry of eocene calc-alkaline volcanic rocks from the Kastamonu area,Northern Turkey[J].Contributions to Mineralogy and Petrology,1976,58(1):63-81.
[19]BOYNTON W V.Cosmochemistry of the rare earth elements:meteorite studies[J].Developments in Geochemistry,1984,2(2):63-114.
[20]MCDONOUGH W F,SUN S S.The composition of the Earth[J].Chemical Geology,1995,120(3/4):223-253.
[21]GRIFFIN W L,PEARSON N J,BELOUSOVA E.The Hf isotope composition of cratonic mantle:LAM-MC-ICP-MS analysis of zircon megacrysts in kimberlites[J].Geochimica et Cosmochimica Acta,2000,64(1):133-147.
[22]PLANK T,LANGMUIR C H.The chemical composition of subducting sediment and its consequences for the crust and mantle[J].Chemical Geology,1998,145(3/4):325-394.
[23]AYERS J.Trace element modeling of aqueous fluid-peridotite interaction in the mantle wedge of subduction zones[J].Contributions to Mineralogy and Petrology,1998,132(4):390-404.
[24]吴福元,李献华,郑永飞,等.Lu-Hf同位素体系及其岩石学应用[J].岩石学报,2007,23(2):185-220.
[25]BUSBY C.Continental growth at convergent margins facing large ocean basins:a case study from Mesozoic convergent-margin basins of Baja California,Mexico[J].Tectonophysics,2004,392(1/4):241-277.
[26]耿全如,潘桂棠,王立全,等.西藏冈底斯带叶巴组火山岩同位素地质年代[J].沉积与特提斯地质,2006,26(1):1-7.
[27]耿全如,潘桂棠,金振民,等.西藏冈底斯带叶巴组火山岩地球化学及成因[J].地球科学,2005,30(6):747-760.
[28]ZHU D C,ZHAO Z D,NIU Y,et al.The origin and pre-Cenozoic evolution of the Tibetan Plateau[J].Gondwana Research,2013,23(4):1429-1454.
[29]MURPHY M A,YIN A N,HARRISON T M,et al.Did the Indo-Asian collision alone create the Tibetan plateau?[J].Geology,2000,8:719-722.
[30]TAPPONNIER P,XU Z Q,ROGER F,et al.Oblique stepwise rise and growth of the Tibet Plateau[J].Science,2001,23:1671-1677.
[31]朱杰,杜远生,刘早学,等.西藏雅鲁藏布江缝合带中段中生代放射虫硅质岩成因及其大地构造意义[J].中国科学D辑:地球科学,2005,35(12):1131-1139.
[32]ZHU D C,PAN G T,WANG L Q,et al.Spatial-temporal distribution and tectonic setting of Jurassic magmatism in the Gangdise belt,Tibet,China[J].Geological Bulletin of China,2008,27(4):458-468.
[33]ZHU D C,PAN G T,WANG L Q,et al.Tempo-spatial variations of Mesozoic magmatic rocks in the Gangdise belt,Tibet,China,with a discussion of geodynamic setting-related issues[J].Geological Bulletin of China,2008,27(9):1535-1550.
[34]张宏飞,徐旺春,郭建秋,等.冈底斯南缘变形花岗岩锆石U-Pb年龄和Hf同位素组成:新特提斯洋早侏罗世俯冲作用的证据[J].岩石学报,2007,23(6):1347-1353.
[35]裴树文.拉萨地块火山岩系内早-中侏罗世双壳类动物群及其古生物地理[J].现代地质,1999,13(3):291-296.
[36]CHU M F,CHUNG S L,SONG B,et al.Zircon U-Pb and Hf isotope constraints on the Mesozoic tectonics and crustal evolution of southern Tibet[J].Geology,2006,34(9):745-748.
[37]ZHU D C,PAN G T,CHUNG S L.SHRIMP zircon age and geochemical constraints on the origin of Early Jurassic volcanic rocks from the Yeba Formation,southern Gangdese in south Tibet[J].International Geology Review,2008,50(5):442-471.