南拉萨地块高分异S型花岗岩锆石U-Pb年龄、地球化学特征及地质意义
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摘要
拉萨地块中-南部广泛分布着三叠—早侏罗世花岗岩,拉龙松多花岗岩为其中一个岩体,通过对该岩体样品的U-Pb年代学和地球化学测试分析发现,拉龙松多花岗岩中的岩浆锆石U-Pb加权平均年龄为178.3 Ma±2.3 Ma,MSWD=1.4,表明拉龙松多花岗岩的形成时代为早侏罗世。岩石地球化学方面,拉龙松多花岗岩具富w(SiO_2)77.62%～78.94%,w(K_2O)3.34%～4.35%,w(Na_2O)1.86%～2.28%,贫w(CaO)0.05%～0.48%,w(Ti_2O)0.09%～0.10%,富w(Al_2O_3)11.78%～12.80%的特征;属于强过铝质高钾钙碱性系列-钙碱系列(A/CNK=1.43～1.61)。拉龙松多花岗岩轻稀土元素(LREE)含量介于82.01×10~(-6)～91.97×10~(-6)之间,重稀土元素(HREE)含量介于12.67×10~(-6)～15.94×10~(-6)之间,LREE/HREE介于5.49～6.58之间,具明显轻稀土相对富集,重稀土相对亏损特征,Eu负异常明显(δEu=0.25～0.34),球粒陨石标准化分布模式呈典型的右倾V型。微量元素显示,拉龙松多花岗岩样品中的大离子亲石元素(Rb,Th,U等)相对富集,高场强元素(Nb,Ta,Sr,P,Ti等)相对亏损,同时相对Rb而言亏损Ba。上述岩石地球化学特征显示出其具有S型花岗岩的特征;在相关地球化学图解中样品位于S型花岗岩区域。综合分析,认为拉龙松多S型花岗岩应该形成于洋壳俯冲向高角度转换,俯冲带之上出现的伸展环境下。
Triassic to early Jurassic granite is widely distributed in the middle-south Lhasa terrane.U-Pb dating geochronology and geochemical analysis are carried out to study the formation age and geochemical characteristics of the Lalongsong granite in the area.Magmatic zircon U-Pb age dating for Lalongsong granite yields a weighted mean age of 178.3 Ma±2.3 Ma(MSWD=1.4),an early Jurassic magmatic product.Geochemical analysis reveals that Lalongsong granite is rich in w(SiO_2) 77.62%～78.94%,w(K_2O) 3.34%～4.35%,w(Na_2O)1.86%～2.28%,w(Al_2O_3)11.78%～12.80%; poor in w(CaO)0.05%～0.48%,w(Ti_2O)0.09%～0.10% and belongs to strong aluminous high potassium calc-alkaline series-calcium base series(A/CNK=1.43～1.61).Light rare earth element(LREE) content of the rock is in the range of 82.01×10~(-6)～91.97×10~(-6),and heavy rare earth element(HREE) content is between 12.67×10~(-6)～15.94×10~(-6),with LREE/HREE ranging from 5.49 to 6.58,showing obvious enrichment of light rare earths,relative loss of heavy rare earths and obvious Eu negative anomaly(δEu=0.25～0.34).The normal distribution pattern of chondrite of the Lalongsong granite is typical right-inclined V-type.Trace element analysis shows that the large ion lithophile elements(Rb,Th,U,etc.) in the Lalongsong granite are relatively enriched,and the high field strength elements(Nb,Ta,Sr,P,Ti,etc.) are relatively depleted,while Ba is depleted relative to Rb.The geochemical characteristics and relevant geochemical diagrams of the Lalongsong granite display characteristics of S-type granites.Based on the comprehensive analysis,it is considered that the Lalongsong S-type granite is formed in the extension environment on the upper slab of the subduction zone.
Triassic to early Jurassic granite is widely distributed in the middle-south Lhasa terrane.U-Pb dating geochronology and geochemical analysis are carried out to study the formation age and geochemical characteristics of the Lalongsong granite in the area.Magmatic zircon U-Pb age dating for Lalongsong granite yields a weighted mean age of 178.3 Ma±2.3 Ma(MSWD=1.4),an early Jurassic magmatic product.Geochemical analysis reveals that Lalongsong granite is rich in w(SiO_2) 77.62%～78.94%,w(K_2O) 3.34%～4.35%,w(Na_2O)1.86%～2.28%,w(Al_2O_3)11.78%～12.80%; poor in w(CaO)0.05%～0.48%,w(Ti_2O)0.09%～0.10% and belongs to strong aluminous high potassium calc-alkaline series-calcium base series(A/CNK=1.43～1.61).Light rare earth element(LREE) content of the rock is in the range of 82.01×10~(-6)～91.97×10~(-6),and heavy rare earth element(HREE) content is between 12.67×10~(-6)～15.94×10~(-6),with LREE/HREE ranging from 5.49 to 6.58,showing obvious enrichment of light rare earths,relative loss of heavy rare earths and obvious Eu negative anomaly(δEu=0.25～0.34).The normal distribution pattern of chondrite of the Lalongsong granite is typical right-inclined V-type.Trace element analysis shows that the large ion lithophile elements(Rb,Th,U,etc.) in the Lalongsong granite are relatively enriched,and the high field strength elements(Nb,Ta,Sr,P,Ti,etc.) are relatively depleted,while Ba is depleted relative to Rb.The geochemical characteristics and relevant geochemical diagrams of the Lalongsong granite display characteristics of S-type granites.Based on the comprehensive analysis,it is considered that the Lalongsong S-type granite is formed in the extension environment on the upper slab of the subduction zone.
引文
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[26] 王德滋,刘昌实,沈渭洲,等.桐庐I型和相山S型两类碎斑熔岩对比[J].岩石学报,1993,9(1):44-54.
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[29] Harris N B W,Inger S.Trace element modelling of pelite-derived granites[J].Contributions to Mineralogy and Petrology,1992,110(1):46-56.
[30] 李献华,周汉文,刘颖,等.粤西阳春中生代钾玄质侵入岩及其构造意义:Ⅰ.岩石学和同位素地质年代学[J].地球化学,2000,29(6):513-520.
[31] Sylvester P J.Post-collisional strongly peraluminous granites[J].Lithos,1998,45(1-4):29-44.
[32] Mclennan B.The continental crust:its composition and evolution[M].Blackwell scientific publication,1985.
[33] Jung S,Pfander J A.Source composition and melting temperatures of orogenic granitoids:constraints from CaO/Na2O,Al2O3/TiO2 and accessory mineral saturation thermometry[J].European Journal of Mineralogy,2007,19(6):859-870.
[34] Skjerlie K P,Johnston A D.Vapor-absent melting at 10 kbar of a biotite- and amphibole-bearing tonalitic gneiss:Implications for the generation of A-type granites[J].Geology,1992,20(3):263-266.
[35] Ma X,Xu Z,Chen X, et al.The origin and tectonic significance of the volcanic rocks of the Yeba Formation in the Gangdese magmatic belt,South Tibet[J].Journal of Earth Science,2017,28(2):265-282.
[36] Collins W J,Richards S W.Geodynamic significance of S-type granites in circum-Pacific orogens[J].Geology,2008,36(7):559-562.
[2] Yin A,Harrison T M.Geologic evolution of the Himalayan-Tibetan orogen[J].Annual Review of Earth & Planetary Sciences,2003,28(28):211-280.
[3] Huang F,Xu J F,Chen J L, et al.Two Cenozoic tectonic events of N-S and E-W extension in the Lhasa Terrane:Evidence from geology and geochronology[J].Lithos,2016,245:118-132.
[4] 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 & Planetary Science Letters,2011,301(1-2):241-255.
[5] 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.
[6] 莫宣学,董国臣,赵志丹,等.西藏冈底斯带花岗岩的时空分布特征及地壳生长演化信息[J].高校地质学报,2005,11(3):281-290.
[7] Di-ChengZhu,Gui-TangPan,Sun-LinChung, 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):30.
[8] Yang J,Xu Z,Li Z, et al.Discovery of an eclogite belt in the Lhasa block,Tibet:A new border for Paleo-Tethys?[J].Journal of Asian Earth Sciences,2009,34(1):0-89.
[9] Huaqi Li,Zhiqin Xu,Jingsui Yang, et al.Records of Indosinian orogenesis in Lhasa terrane,Tibet[J].Journal of China University of Geosciences,2009,20(2):348-363.
[10] 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):1 429-1 454.
[11] 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):1 645-1 655.
[12] Liu Y S,Hu Z C,Zong K Q, et al.Reappraisement and refinement of zircon U-Pb isotope and trace element analyses by LA-ICP-MS[J].Chinese Science Bulletin,2010,55(15):1 535-1 546.
[13] Andersen T.Correction of common lead in U-Pb analyses that do not report 204Pb[J].Chemical Geology,2002,192(1):59-79.
[14] Belousova E,Griffin W,O'Reilly S Y, et al.Igneous zircon:trace element composition as an indicator of source rock type[J].Contributions to Mineralogy & Petrology,2002,143(5):602-622.
[15] Maniar P D,Piccoli PM.Tectonic discrimination of granitoids[J].Geol soc am bull,1989,101(5):635-643.
[16] 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.
[17] 李昌年.火成岩微量元素岩石学[M].武汉:中国地质大学出版社,1992,101-113.
[18] 王森,张达,Absai Vatuva,等.福建龙岩大洋-莒舟花岗岩地球化学、年代学、铪同位素特征及其地质意义[J].地球化学,2015,44(05):450-468.
[19] 吴福元,李献华,杨进辉,等.花岗岩成因研究的若干问题[J].岩石学报,2007,23(6):1 217-1 238.
[20] Whalen J B,Currie K L,Chappell B W.A-type granites:Geochemical characteristics,discrimination and pretrogenesis [J].Contrib Mineral Petrol,1987,95(4):407-419.
[21] Lee C T A,Morton D M.High silica granites:Terminal porosity and crystal settling in shallow magma chambers[J].Earth & Planetary Science Letters,2015,409(409):23-31.
[22] Linnen R L,Keppler H.Melt composition control of Zr/Hf fractionation in magmatic processes[J].Geochimica Et Cosmochimica Acta,2002,66(18):3 293-3 301.
[23] Watson E B,Harrison T M.Zircon saturation revisited:temperature and composition effects in variety of crustal magma types[J].Earth & Planetary Science Letters,1983,64(2):295-304.
[24] Eby G N.The A-type granitoids:A review of their occurrence and chemical characteristics and speculations on their petrogenesis[J].Lithos,1990,26(1):115-134.
[25] 李献华,李武显,李正祥.再论南岭燕山早期花岗岩的成因类型与构造意义[J].科学通报,2007,52(9):981-991.
[26] 王德滋,刘昌实,沈渭洲,等.桐庐I型和相山S型两类碎斑熔岩对比[J].岩石学报,1993,9(1):44-54.
[27] Tang J,Lang X,Xie F, et al.Geological characteristics and genesis of the Jurassic No.I porphyry Cu-Au deposit in the Xiongcun district,Gangdese porphyry copper belt,Tibet[J].Ore Geology Reviews,2015,70(4):438-456.
[28] Wang C,Ding L,Zhang L, et al.Petrogenesis of Middle-Late Triassic volcanic rocks from the Gangdese belt,southern Lhasa terrane:Implications for early subduction of Neo-Tethyan oceanic lithosphere[J].Lithos,2016,262:320-333.
[29] Harris N B W,Inger S.Trace element modelling of pelite-derived granites[J].Contributions to Mineralogy and Petrology,1992,110(1):46-56.
[30] 李献华,周汉文,刘颖,等.粤西阳春中生代钾玄质侵入岩及其构造意义:Ⅰ.岩石学和同位素地质年代学[J].地球化学,2000,29(6):513-520.
[31] Sylvester P J.Post-collisional strongly peraluminous granites[J].Lithos,1998,45(1-4):29-44.
[32] Mclennan B.The continental crust:its composition and evolution[M].Blackwell scientific publication,1985.
[33] Jung S,Pfander J A.Source composition and melting temperatures of orogenic granitoids:constraints from CaO/Na2O,Al2O3/TiO2 and accessory mineral saturation thermometry[J].European Journal of Mineralogy,2007,19(6):859-870.
[34] Skjerlie K P,Johnston A D.Vapor-absent melting at 10 kbar of a biotite- and amphibole-bearing tonalitic gneiss:Implications for the generation of A-type granites[J].Geology,1992,20(3):263-266.
[35] Ma X,Xu Z,Chen X, et al.The origin and tectonic significance of the volcanic rocks of the Yeba Formation in the Gangdese magmatic belt,South Tibet[J].Journal of Earth Science,2017,28(2):265-282.
[36] Collins W J,Richards S W.Geodynamic significance of S-type granites in circum-Pacific orogens[J].Geology,2008,36(7):559-562.
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