西藏八宿吉利地区新发现寒武纪变质花岗岩锆石U-Pb年龄、地球化学特征及其地质意义
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摘要
八宿吉利地区寒武纪变质花岗岩位于曲扎湖-提卡一带,主要由变质二长花岗岩和变质花岗闪长岩组成。这一新发现对于认识和恢复原特提斯构造历史演化具有重要意义。锆石CL图像显示变质花岗岩锆石为岩浆成因。锆石LA-ICP-MS测年得出片理化变质二长花岗岩年龄为503.7±4.7Ma、变质花岗闪长岩年龄为494.7±3.4Ma,表明该岩体形成时代属于寒武纪。通过岩石地球化学分析,变质二长花岗岩SiO_2含量介于69.87%～79.89%之间;变质花岗闪长岩SiO_2含量介于66.63%～70.15%之间。前者Al_2O_3含量变化于12.36%～14.82%,Na_2O含量为2.54%～7.16%,K_2O含量为0.15%～5.95%,K_2O/Na_2O=0.02～2.34;后者Al_2O_3含量变化于14.66%～15.41%,Na_2O含量为3.60%～5.63%,K_2O含量为0.77%～2.78%,K_2O/Na_2O=0.14～0.77,属于钙碱性-碱性过铝质花岗岩。在侵入岩构造环境Rb-(Y+Nb)判别图解、Rb-(Yb+Ta)判别图解中,样品均落入"火山弧花岗岩"区域中,表明其形成于大陆边缘火山弧环境。结合锆石测年结果及区域地质背景分析,认为吉利地区变质花岗岩形成于冈瓦纳大陆裂离卡穷微陆块阶段,同时表明原特提斯洋形成最早时限可追溯至寒武纪。
The Cambrian metamorphic granites in the Jili area, Baxoi, Xizang consist dominantly of metamorphic monzogranites and metamorphic granodiorites. The new findings of the metamorphic granites in this study will be of significance in the understanding and reconstruction of the tectonic evolution of the proto-Tethys. The cathodoluminescence images of the zircons indicate the magmatic origin of the metamorphic granites in this area. The LA-ICP-MS zircon U-Pb dating data gives the ages of 503.7±4.7 Ma for the schistositized metamorphic monzogranites, and 494.7±3.4 Ma for the metamorphic granodiorites, implying that the metamorphic granites were initiated during the Cambrian. Geochemically, the metamorphic monzogranites have the SiO_2 contents of 69.87% to 79.89%, Al_2O_3 contents of 12.36% to 14.82%, Na_2O contents of 2.54% to 7.16%, K_2O contents of 0.15% to 5.95%, and K_2O/Na_2O ratios of 0.02 to 2.34, whereas the metamorphic granodiorites have the SiO_2 contents of 66.63% to 70.15%, Al_2O_3 contents of 14.66% to 15.41%, Na_2O contents of 3.60% to 5.63%, K_2O contents of 0.77% to 2.78%, and K_2O/Na_2O ratios of 0.14 to 0.77, indicating that the above-mentioned metamorphic granites belong to the calc-alkaline to alkaline peraluminous granites. In the Rb vs.(Y+Nb) and Rb vs.(Yb+Ta) discrimination diagrams for the tectonic interpretation, all the granites samples are projected into the volcanic arc granite field, suggesting the continental-margin volcanic arc environments. It follows that the metamorphic granites in the Jili area were initiated in the stages of the Kaqiong micro-continent during the breakup of the Gondwana land, and the timing of the proto-Tethys may be traced back to the Cambrian.
The Cambrian metamorphic granites in the Jili area, Baxoi, Xizang consist dominantly of metamorphic monzogranites and metamorphic granodiorites. The new findings of the metamorphic granites in this study will be of significance in the understanding and reconstruction of the tectonic evolution of the proto-Tethys. The cathodoluminescence images of the zircons indicate the magmatic origin of the metamorphic granites in this area. The LA-ICP-MS zircon U-Pb dating data gives the ages of 503.7±4.7 Ma for the schistositized metamorphic monzogranites, and 494.7±3.4 Ma for the metamorphic granodiorites, implying that the metamorphic granites were initiated during the Cambrian. Geochemically, the metamorphic monzogranites have the SiO_2 contents of 69.87% to 79.89%, Al_2O_3 contents of 12.36% to 14.82%, Na_2O contents of 2.54% to 7.16%, K_2O contents of 0.15% to 5.95%, and K_2O/Na_2O ratios of 0.02 to 2.34, whereas the metamorphic granodiorites have the SiO_2 contents of 66.63% to 70.15%, Al_2O_3 contents of 14.66% to 15.41%, Na_2O contents of 3.60% to 5.63%, K_2O contents of 0.77% to 2.78%, and K_2O/Na_2O ratios of 0.14 to 0.77, indicating that the above-mentioned metamorphic granites belong to the calc-alkaline to alkaline peraluminous granites. In the Rb vs.(Y+Nb) and Rb vs.(Yb+Ta) discrimination diagrams for the tectonic interpretation, all the granites samples are projected into the volcanic arc granite field, suggesting the continental-margin volcanic arc environments. It follows that the metamorphic granites in the Jili area were initiated in the stages of the Kaqiong micro-continent during the breakup of the Gondwana land, and the timing of the proto-Tethys may be traced back to the Cambrian.
引文
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[15] Maniar P D and Piccoli P M.Tectonic discrimination of granitoids [J].Geological Society of America Bulletin,1989,101(5):635-643.
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[18] 王楠,吴才来,秦海鹏,等.川西义敦岛弧稻城花岗岩体和海子山花岗岩体锆石U-Pb年代学、Hf同位素特征及地质意义[J].地质学报,2016,90(11):3227-3245.
[19] Sun S S and McDonough W F.Chemical and isotopic systematics of oceanic basalts:Implication for mantle composition and processes [J].1989:313-345.
[20] Rapp R P,Watson E B.Dehydration melting of metabasalt at 8-32 kbar:implications for continental growth and crust-mantle recycling [J].Journal of Petrology ,1995,891-931.
[21] Rapp R P,Watson E B,Miller C F.Partial melting of amphibolite /eclogite and the origin of Archean trondhjemites and tonalities [J].Precambrian Research,1991,51(1-4):1-25.
[22] Rollinson H R.Using geochemical data:Evaluation,Presentation,Interpretation [J].Computers & Geosciences,1995,21(3):439.
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[24] Dungan M A,Lindstrom M M,Mcmiland N J,et al .Open system magmatic evolution of the Taos Plateau volcanic field,northern New Mexicol,the petrology and geochemistry of the Servillets basalt [J].Journal of Geophysical Research,1986,91:5999-6028.
[25] Pearce J A.Source and setting of granitic rocks [J].Episodes,1996,19:120-125.
[2] 尹福光,王东兵,王保弟,等.西南三江造山带地层区划[J].沉积与特提斯地质,2017,37(4):1-6.
[3] 谢尧武,彭兴阶,陈德泉,等.八宿县幅1:25万区调区域地质调查报告[R].拉萨:西藏自治区地质调查院,2007.289-299.
[4] 李才,谢尧武,沙绍礼,等.藏东八宿地区泛非期花岗岩锆石SHRIMP U-Pb定年[J].基础地质,2008,27(1):64-67.
[5] 何世平,李荣社,王超,等.青藏高原岗底斯北缘卡穷岩群形成时代的确定[J].地球化学,2012,41(3):217-223.
[6] 任飞,潘桂棠,尹福光,等.西南三江地区洋板块地层特征及构造演化[J].沉积与特提斯地质,2017,37(4):9-15.
[7] 李光明,冯孝良,黄志英,等.西藏冈底斯构造带中段多岛弧-盆系及其演化[J].沉积与特提斯地质,2000,20(4):37-45.
[8] 王铠元.西南三江构造带及扬子西缘前寒武纪基底岩群与构造演化[J].云南地质,1996,(2):138-148.
[9] 黄汲清,陈国铭,陈炳蔚.特提斯-喜马拉雅构造域初步分析[J].地质学报,1984,(1):1-17.
[10] 沙绍礼,谢尧武,陈应明,等.藏东同卡石榴辉石岩和拉萨地体松多蓝片岩[J].中国地质,2009,36(6):1302-1311.
[11] Liu Y S,Gao S,Hu Z C,et al.Continental and oceanic crust recyoling-induced melt-peridotite interactions in the Trans-North China Orogen:U-Pb dating,Hf isotopes and trace elments in zircons of mantle xenoliths [J].Journal of Petrology,2010,51:537-571.
[12] Liu Y S,Hu Z C,Zong,K Q,et al.reappraisement and refinement of zircon U-Pb isotope and trace elment analyses by LA-ICP-MS [J].Chinese Science Bulletin,2010,55(15):1535-1546.
[13] Ludwig K R.ISOPLOT 3.00:A Geochronnlogical Toolkit for Microsoft Excel [M].Berkeley,California:Berkeley Geochronnlogical Center,2003.
[14] Middlemost E A K.Naning materials in the magma/igneous rock system [J].Earth-Science Reviews,1994,37:215-224.
[15] Maniar P D and Piccoli P M.Tectonic discrimination of granitoids [J].Geological Society of America Bulletin,1989,101(5):635-643.
[16] Martin H.Adakitic magmas:modern analogues of Archean granitoids [J].Lithos,1999,46:411～429.
[17] Atherton M P,Petford N.Generation of sodium-rich magmas from newly underplated basaltic crust [J].Nature,1993,362:144～146.
[18] 王楠,吴才来,秦海鹏,等.川西义敦岛弧稻城花岗岩体和海子山花岗岩体锆石U-Pb年代学、Hf同位素特征及地质意义[J].地质学报,2016,90(11):3227-3245.
[19] Sun S S and McDonough W F.Chemical and isotopic systematics of oceanic basalts:Implication for mantle composition and processes [J].1989:313-345.
[20] Rapp R P,Watson E B.Dehydration melting of metabasalt at 8-32 kbar:implications for continental growth and crust-mantle recycling [J].Journal of Petrology ,1995,891-931.
[21] Rapp R P,Watson E B,Miller C F.Partial melting of amphibolite /eclogite and the origin of Archean trondhjemites and tonalities [J].Precambrian Research,1991,51(1-4):1-25.
[22] Rollinson H R.Using geochemical data:Evaluation,Presentation,Interpretation [J].Computers & Geosciences,1995,21(3):439.
[23] Collins W J,Beams S D,White A J R,Chappell B W.Nature and origin of A type granites with particular reference to southeastern Australia [J].Contrib.Mineral.Petrol.,1982,80:189-200.
[24] Dungan M A,Lindstrom M M,Mcmiland N J,et al .Open system magmatic evolution of the Taos Plateau volcanic field,northern New Mexicol,the petrology and geochemistry of the Servillets basalt [J].Journal of Geophysical Research,1986,91:5999-6028.
[25] Pearce J A.Source and setting of granitic rocks [J].Episodes,1996,19:120-125.
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