摘要
拉萨地块南缘日多地区叶巴组火山岩以中酸性熔岩及火山碎屑岩占绝对优势为特征。以墨竹工卡县以东日多地区叶巴组火山岩代表性岩石组合为对象进行了地球化学、LA-ICP-MS锆石U-Pb年龄及锆石Lu-Hf同位素研究。研究结果表明,叶巴组火山岩具有轻稀土元素富集,富集大离子亲石元素Rb、Th、K,亏损高场强元素Nb、Ta、Ti(P、Hf)的地球化学特征。其中,基性火山岩具低钾、低钛和富钠、富铝的特征,Nb、Zr含量和Th/Y、Th/Yb、Ta/Yb值较高,而La/Nb值较低,呈现出大陆地壳组分增加的趋势。中酸性火山岩属中钾-高钾钙碱性系列,具有低钛、低镁和高铝的特征,微量元素含量及比值与大陆岛弧安山岩接近。叶巴组火山岩总体地球化学特征与陆缘弧火山岩相似。LA-ICP-MS锆石U-Pb测年获得英安岩和流纹质晶屑凝灰岩~(206)Pb/~(238)U年龄加权平均值分别为176.9±2.3Ma和162.2±3.3Ma,表明研究区叶巴组酸性火山岩形成于中侏罗世。锆石Hf同位素测试结果显示,εHf(t)值为2.43~11.42,二阶段模式年龄(t_(DM)~C)为482~1065Ma,暗示叶巴组酸性火山岩源区除新生地壳物质的部分熔融外,还明显受到古老结晶基底的影响。结合前人研究成果,认为叶巴组形成于早中侏罗世雅鲁藏布江洋北向俯冲于拉萨地块南缘之下的陆缘弧环境。
The volcanic rocks of Yeba Formation in Riduo area on the southern margin of Lhasa massif are characterized by the absolute superiority of intermediate acid lava and pyroclastic rock. In this paper, the authors selected geochemistry, LA-ICP-MS zircon U-Pb geochronology and zircon Lu-Hf isotopes of representative rock combination of Yeba Formation in Riduo area of eastern Mozhugongka County as the study objects. Geochemical analysis shows that the volcanic rocks of the Yeba Formation are characterized by LREE enriched type pattern, enrichment of LILES such as Rb, Th and K, and depletion of HFSE such as Nb,Ta and Ti(P, Hf).The basic volcanic rocks are characterized by low potassium and titanium as well as rich sodium and aluminum. The values of Nb, Zr and ratios of Th/Y, Th/Yb and Ta/Yb are relatively high, but the ratio of La/Nb is relatively low, showing a trend of increasing continental crustal components. The intermediate acid volcanic rocks belong to the middle-high potassium calc alkaline series. They are characterized by low titanium and magnesium as well as high aluminum. The values and ratios of trace elements are close to those of andesite from the continental island arc. The overall geochemical characteristics of the volcanic rocks of the Yeba Formation are similar to features of the continental margin arc volcanic rocks. LA-ICP-MS Zircon U-Pb dating of dacite and rhyolitic crystal tuff yielded ~(206) Pb/~(238) U weighted average ages of 176.9±2.3 Ma and 162.2±3.3 Ma, showing that acidic volcanic rocks of Yeba Formation in the study area were formed in Middle Jurassic. Zircon Hf isotope analytical results show that the values ofεHf(t) range from 2.43 to 11.42,and the values of t_(DM)~C range from 482 Ma to 1065 Ma, suggesting that the source area of the acid volcanic rocks of Yeba Formation was affected not only by the partial melting of the new crustal material but also by the ancient crystallization basement. Combined with previous research results, the authors hold that the volcanic rocks of Yeba Formation was formed in a tectonic setting of continental arc,which was the subduction of the Yarlung Zangbo Ocean to the southern margin of Lassa massif during Early-Middle Jurassic period.
引文
[1]潘桂棠,莫宣学,侯增谦,等.冈底斯造山带的时空结构及演化[J].岩石学报,2006,(3):3-15.
[2]许志琴,杨经绥,李海兵,等.印度-亚洲碰撞大地构造[J].地质学报,2011,85(1):1-33.
[3]朱弟成,潘桂棠,王立全,等.西藏冈底斯带侏罗纪岩浆作用的时空分布及构造环境[J].地质通报,2008,27(4):458-466.
[4]董颜辉,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.
[5]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—中国地质大学学报,2005,30(6):747-760.
[6]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.
[7]Zhu D C,Pan G T,Chung S L,et al.SHRIMP zircon age and geochemical constraints on the origin of Lower Jurassic volcanic rocks from the Yeba Formation,southern Gangdese,South Tibet[J].International Geology Review,2008,50(5):442-471.
[8]曾忠诚,刘德民,泽仁扎西,等.西藏冈底斯东段叶巴组火山岩地球化学特征及其地质构造意义[J].吉林大学学报(地球科学版),2009,39(3):435-445.
[9]陈炜,马昌前,边秋娟,等.西藏得明顶地区叶巴组火山岩地球化学特征和同位素U-Pb年龄证据[J].地质科技情报,2009,28(3):31-40.
[10]魏友卿.藏南叶巴组火山岩的年代学、地球化学及岩石成因[D].中国地质大学(北京)硕士学位论文,2014.
[11]魏友卿.西藏拉萨地体南缘中生代火山岩与碎屑沉积岩的年代学、地球化学及构造意义[D].中国地质大学(北京)博士学位论文,2017.
[12]熊秋伟,陈建林,许继峰,等.拉萨地块南部得明顶地区叶巴组火山岩LA-ICP-MS锆石U-Pb年龄、地球化学特征及其成因[J].地质通报,2015,34(9):1645-1655.
[13]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):2089-2098.
[14]董昕,张泽明.拉萨地体南部早侏罗世岩浆岩的成因和构造意义[J].岩石学报,2013,29(6):1933-1948.
[15]阴家润,苟金.拉萨地块叶巴组内中侏罗世双壳类动物群及其古地理意义[J].中国区域地质,1998,17(2):132-135.
[16]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,2004,22(7):12-19.
[17]Zhu D C,Zhao Z D,Niu Y,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):241-255.
[18]朱弟成,赵志丹,牛耀龄,等.拉萨地体的起源和古生代构造演化[J].高校地质学报,2012,18(1):1-15.
[19]路远发.Geo Kit:一个用VBA构建的地球化学工具软件包[J].地球化学,2004,33(5):459-464.
[20]Sun S S and Mc Donough W F.Chemical and isotopic systematic of oceanic basalts:Implications for mantle composition and processes[C]//Saunders A D,Norry M J.Magmatism in the Ocean Basins.Geological Society,London,Special Publicatin,1989,42(1):313-345.
[21]Yuan H L,Gao S,Dai M N,et al.Simultaneous determinations of U-Pb age,Hf isotopes and trace element compositions of zircon by excimer laser ablation quadrupole and multiple collector ICPMS[J].Chemical Geology,2008,247:100-117.
[22]Le Bas M J,Le Maitre R W,Streckeisen A,et al.A chemical classification of volcanic rocks based on the total alkali-silica diagram[J].Journal of Petrology,1986,27:745-750.
[23]Irvine T N,Baragar W R A.A guide to the chemical classification of the common volcanic rocks[J].Canadian Journal of Earth Sciences,1971,8:523-548.
[24]Wilson M.Igneous Petrogenesis[M].London:Unwin Hyman,1989:1-466.
[25]Atherton M,Petford N.Generation of sodium-rich magmas from newly underplated basaltic crust[J].Nature,1993,362(6416):144-146.
[26]Peccerillo A,Talor A R.Geochemistry of Eocene calcalkaline volcanic rocks from the Kastamonu area,northern Turkey[J].Contribution to Mineralogy and Petrology,1976,58:63-81.
[27]Wood D A,Joron J L,Treuil M.A re-appraisal of the use of trace elements to classify and discriminate between magma series erupted in different tectonic setting[J].Earth Planet.Sci.Lett.,1979,45:326-336.
[28]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.
[29]Vervoort J D,Patchett P J,Gehrels G E,et al.Constraints on early earth differentiation from hafnium and neodymium isotopes[J].Nature,1996,379:624-627.
[30]Ringwood A E.Composition and Petrology of the Earth’s Mantle[M].New York:Mc Graw-Hill,1975:1-618.
[31]Watson E B,Harrison T M.Zircon saturation revisited:temperature and composition effects in a variety of crustal magma types[J].Earth and Planetary Science Letters,1983,64(2):295-304.
[32]Jiang Y H,Jiang S Y,Ling H F,et al.Low-degree melting of a metasomatized lithospheric mantle for the origin of Cenozoic Yulong monzogranite-porphyry,east Tibet:geochemical and SrNd-Pb-Hf isotopic constraints[J].Earth and Planetary Science Letters,2006,241(3):617-633.
[33]Bea F,Arzamastsev A,Montero P,et al.Aonmalous alkalin rocks of Soustov,Kola:evidence of mantle derived matasomatic fluids affecting crustal materials[J].Contrib Mineral Petrol,2001,140:554-566.
[34]周金城,蒋少涌,王孝磊,等.华南中侏罗世玄武岩的岩石地球化学研究——以福建藩坑玄武岩为例[J].中国科学(D辑),2005,35(1):927-936.
[35]徐夕生,邱检生.火成岩岩石学[M].北京:科学出版社,2010:204-205.
[36]张立雪,王青,朱弟成,等.拉萨地体锆石Hf同位素填图:对地壳性质和成矿潜力的约束[J].岩石学报,2013,29(11):3681-3688.
[37]夏代祥,刘世坤.西藏自治区岩石地层[M].武汉:中国地质大学出版社,1997:72-83.
[38]胡道化,吴珍汉,江万,等.西藏念青唐古拉岩群SHRIMP锆石U-Pb年龄和Nd同位素研究[J].中国科学(D辑),2005,35(1):29-37.
[39]Dong X,Zhang Z,Santosh M.Zircon U-Pb Chronology of the Nyingtri Group,Southern Lhasa Terrane,Tibetan Plateau:Implications for Grenvillian and Pan-African Provenance and MesozoicCenozoic Metamorphism[J].Journal of Geology,2010,118(6):677-690.
[40]Zhu D C,Zhao Z D,Niu Y L,et al.The origin and Pre-Cenozoic evolution of the Tibetan Plateau[J].Gondwana Research,2013,23(4):1429-1454.
[41]胡培远,翟庆国,唐跃,等.青藏高原拉萨地块新元古代(~925Ma)变质辉长岩的确立及其地质意义[J].科学通报,2016,(19):2176-2186.
[42]付燕刚.西藏拉萨地块南部新特提斯洋俯冲成岩成矿作用[D].中国地质科学院博士学位论文,2017.
[43]纪伟强,吴福元,锺孙霖,等.西藏南部冈底斯岩基花岗岩时代与岩石成因[J].中国科学(D辑),2009,(7):849-871.
[44]Ji W Q,Wu F Y,Chung S L,et al.Zircon U-Pb geochronology and Hf isotopic constraints on petrogenesis of the Gangdese batholith,southern Tibet[J].Chemical Geology,2009,262(3):229-245.
[45]莫宣学.岩浆作用与青藏高原演化[J].高校地质学报,2011,17(3):351-367.
[46]Wood D A.The application of a Th-Hf-Ta diagram to problems of tectonomagmatic classification and to establishing the nature of crustal contamination of basaltic lavas of the British Tertiary volcanic province[J].Earth and Planetary Science Letters,1980,50:11-30.
[47]Pearce J A.The role of sulrcontinental lithosphere in magma genesis at destructive plate margins[C]//Hawkesworth C J,Norry M J.Continental Basalts and Mantle.Xenoliths Nantwich Shiva Publications,1983:230-249.
[48]路凤香,桑隆康,邬金华,等.岩石学[M].北京:地质出版社,2001:324-327.
[49]胡享生,莫宣学,范例.西藏江达古沟-弧-盆系的火山岩岩石学与地质学标志[C]//地质矿产部青藏高原地质文集编委会编,青藏高原地质文集(20).北京:地质出版社,1990:1-15.
[50]Bailey J C.Geochemical criteria for a refined tectonic discrimination of orognnic andesites[J].Chemical Geology,1981,32:139-154.
[51]Pearce J A,Harris N B W,Tindle A G.Trace element discrimination diagrams for the tectonic interpretation of granitic rocks[J].Journal of Petrology,1984,25:956-983.
[52]王玉净,杨群,松冈笃,等.藏南泽当雅鲁藏布缝合带中的三叠纪放射虫[J].微体古生物学报2002,19(3):215-227.
[53]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),2005,35(12):1131-1139.
[54]孟元库.藏南冈底斯中段南缘构造演化[D].中国地质科学院博士学位论文,2016.
[55]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
[56]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.
[57]曲晓波,辛洪波,徐文艺.三个锆石U-Pb SHRIMP年龄对雄村特大型铜金矿容矿火成岩时代的重新厘定[J].矿床地质,2007,26:512-518.
[58]谭陈诚.日喀则东嘎乡冈底斯岩体的形成年代及成因[D].中国地质大学(北京)硕士学位论文,2012.
[59]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.
[60]曾云川.拉萨地块晚三叠世—晚侏罗纪岩浆-构造演化[D].中国科学院大学博士学位论文,2017.
[61]宋绍玮,刘泽,朱弟成,等.西藏打加错晚三叠世安山质岩浆作用的锆石U-Pb年代学和Hf同位素[J].岩石学报,2014,30(10):3100-3112.
[62]Guynn J H,Kapp P,Pullen A,et al.Tibetan basement rocks near Amdo reveal“missing”Mesozoic tectonism along the Bangong suture,central Tibet[J].Geology,2006,34(6):505-508.
[63]杨经绥,许志琴,耿全如,等.中国境内可能存在一条新的高压/超高压(?)变质带——青藏高原拉萨地体中发现榴辉岩带[J].地质学报,2006,80(12):1787-1792.
[64]李化启,许志琴,杨经绥,等.拉萨地体内松多榴辉岩的同碰撞折返:来自构造变形和40Ar-39Ar年代学的证据[J].地学前缘,2011,18(3):66-78.
(1)张显廷,周光弟,杨延兴.1∶20万下巴淌(沃卡)幅区域地质调查报告.青海地矿局区调队,1994.
(2)李定洪,刘鸿飞,徐开锋,等.1∶20万拉萨幅区域地质调查报告.西藏地矿局区域地质调查大队,1990.
(3)周斌,韩奎,潘亮,等.西藏日多地区H46E015009、H46E015010、H46E015011幅1∶5万区域地质调查2017年度总结报告.陕西省地质调查中心,2018.