老挝沙耶武里B.Talan地区玄武岩锆石U-Pb定年及岩石地球化学特征
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摘要
老挝沙耶武里B.Talan地区主要出露枕状、杏仁状玄武岩、中基性(含安山质)熔结火山角砾岩(含集块岩)。采用LA-(MC)-ICP-MS锆石U-Pb同位素定年技术,获得了该玄武岩的结晶年龄为260.8±4.7 Ma(MSWD=0.005 5),其形成时代为中二叠世晚期。该区玄武岩具低SiO_2(45.82%~47.77%)和富钠贫钾(Na_2O/K_2O>1)特征;富集大离子亲石元素(Rb、Ba、U、K和Th),明显亏损Nb、Ta、P、Ti等高场强元素;稀土总量ΣREE为(41.94~63.97)×10~(-6),δEu=0.96~1.08,具弱的正、负铕异常,总体上表现出LREE富集的右倾型配分模式;Nb/Ta值为3.89~11.56(平均7.25)。岩石地球化学证据表明,B.Talan地区玄武岩的形成构造背景为岛弧环境;岩浆源区来自于岛弧下的俯冲板片熔融体,其成因与交代的地幔楔相关。
The basalt of B. Talan region in Laos is chiefly composed of pillow,amygdaloidal basalt and intermediate-basic weldedly volcanic breccia which consists of agglomerates. LA-( MC)-ICP-MS zircon U-Pb analyses suggest that the crystallization age of the terrace is 260. 8 ± 4. 7 Ma( MSWD = 0. 005 5),implying it is the product of magmatic activity in Middle Permian. The basalts exhibit low and variable SiO_2( 45. 82%-47. 77%),high Na_2O/K_2O( Na_2O/K_2O > 1) ratios with large ion lithophile elements( Rb,Ba,U,K and Th),depletion of high-field strength elements( Nb,Ta and Ti) and relatively low ΣREE(( 41. 94-63. 97) × 10~(-6)) as well as a stable variation with δEu of 0. 96-1. 08,showing a right type distribution patterns of LREE enrichment. In addition,Nb/Ta value 3. 89-11. 56 is an average of 7. 25. Based on the integrating of geological and geochemical evidences,the tectonic setting of the basalt in B. Talan region,Xaignabouli should be the island arc environment. The magma source is derived from the subduction plate melt below the island arc,and its origin is related to the metasomatic mantle wedge.
The basalt of B. Talan region in Laos is chiefly composed of pillow,amygdaloidal basalt and intermediate-basic weldedly volcanic breccia which consists of agglomerates. LA-( MC)-ICP-MS zircon U-Pb analyses suggest that the crystallization age of the terrace is 260. 8 ± 4. 7 Ma( MSWD = 0. 005 5),implying it is the product of magmatic activity in Middle Permian. The basalts exhibit low and variable SiO_2( 45. 82%-47. 77%),high Na_2O/K_2O( Na_2O/K_2O > 1) ratios with large ion lithophile elements( Rb,Ba,U,K and Th),depletion of high-field strength elements( Nb,Ta and Ti) and relatively low ΣREE(( 41. 94-63. 97) × 10~(-6)) as well as a stable variation with δEu of 0. 96-1. 08,showing a right type distribution patterns of LREE enrichment. In addition,Nb/Ta value 3. 89-11. 56 is an average of 7. 25. Based on the integrating of geological and geochemical evidences,the tectonic setting of the basalt in B. Talan region,Xaignabouli should be the island arc environment. The magma source is derived from the subduction plate melt below the island arc,and its origin is related to the metasomatic mantle wedge.
引文
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[26]李兴振,江新胜,孙志明,等.西南三江地区碰撞造山过程[M].北京:地质出版社,2002.
[27]Phajuy B,Panjasawatwong Y,Osataporn P.Preliminary geochemical study of volcanic rocks in the Pang Mayao area,Phrao,Chiang Mai,northern Thailand:tectonic setting of formation[J].Journal of Asian Earth Sciences,2005,24:765-776.
[2]钱鑫.老挝琅勃拉邦及黎府构造带古特提斯构造演化[D].武汉:中国地质大学,2016.
[3]杨文强.泰国和老挝难河-程逸及黎府缝合带构造演化[D].武汉:中国地质大学,2010.
[4]Bunopas S.Paleogeographic history of western Thailand and adjacent parts of South-East Asia:a plate tectonics interpretation[D].New Zealand:Victoria University of Wellington,1981.
[5]Bunopas S,Vella P.Opening of the Gulf of Thailand-rifting of continental Southeast Asia,and Late Cenozoic tectonics[J].Journal of the Geological Society of Thailand,1983,6:1-12.
[6]Metcalfe I.Origin and assembly of south-east Asia continental terranes[J].Geological Society London Special Publications,1988,37(1):101-118.
[7]Bunopas S,Vella P,Fontaine H,et al.Growth of Asia in the Late Triassic continent-continent collision of Shan-Thai and Indochina against South China[J].Gondwana Research,2001,4(4):584-585.
[8]高建华,范文玉,王宏,等.老挝沙耶武里玄武岩的地球化学特征、成因及其地质意义[J].地质学报,2015,89(9):1696-1702.
[9]林方成,施美凤,李兴振,等.三江-湄公河成矿带地质背景和成矿规律对比研究专题报告成果[R].成都:成都地质矿产研究所,2010.
[10]唐专红,文件生,李玉坤,等.中老合作项目(老挝1∶20万沙耶武里幅区域地质填图项目2014年度报告)[R].桂林:广西区域地质调查研究院,2014.
[11]Ludwig K R.Users manual for Isoplot/Ex:a geochronological toolkit for Microsoft Excel[K].Berkeley Geochronology Center Spec.Publ.,2000.
[12]Le Maitre R W.A Classification of igneous rocks and glossary of terms:recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks[M].Blackwell,1989.
[13]Peccerillo A,Taylor S R.Geochemistry of eocene calc-alkaline volcanic rocks from the Kastamonu area,Northern Turkey[J].Contributions to Mineralogy and Petrology,1976,58:63-81.
[14]Middlemost E A K.Magmas and magmatic rocks:an introduction to igneous petrology[M].London:Longman,1985.
[15]Ewart A,Collerson K D,Regelous M,et al.Geochemical evolution within the Tonga-Kermadec-Lau arc-back-arc systems:therole of varying mantle wedge composition in space and time[J].Journal of Petrology,1998,39(3):331-368.
[16]Sun S-s,Mc Donough W F.Chemical and isotopic systematics of oceanic basalts:implications for mantle composition and processes[J].Geological Society London Special Publications,1989,42:313-345.
[17]Jochum K P,Hofmann A W.Constraints on earth evolution from antimony in mantle-derived rocks[J].Chemical Geology,1997,139:39-49.
[18]Taylor S R,Mclennan S M.The continental crust,its composition and evolution:an examination of the geochemical record preserved in sedimentary rocks[M].Oxford Melbourne:Blackwell Scientific Publications,1985.
[19]Hergt J M,Peate D W,Hawkesworth C J.The petrogenesis of Mesozoic Gondwana low-Ti flood basalts[J].Earth and Planetary Science Letters,1991,105:134-148.
[20]Saunders A D,Norry M J,Tarney J.Origin of MORB and chemically-depleted mantle reservoirs:trace element constraints[J].Journal of Petrology,1988,Special Lithosphere Issue:415-445.
[21]Pearce J A,Cann J R.Tectonic setting of basic volcanic rocks determined using trace element analyses[J].Earth and Planetary Science Letters,1973,19:290-300.
[22]Wood D A.The application of a Th-Hf-Nb diagram to problems of tectomagmatic classification and to establishing the nature of crustal contamination of the British Tertiary Volcanic Provinic[J].Earth and Planetary Science Letters,1980,50:11-30.
[23]Pearce J A,Norry M J.Petrogenetic implications of Ti,Zr,Y,and Nb variations in volcanic rocks[J].Contributions to Mineralogy and Petrology,1979,69:33-47.
[24]Condie K C.High field strength element ratios in Archean basalts:a window to evolving sources of mantle plumes?[J].Lithos,2005,79:491-504.
[25]莫宣学,沈上越,朱勤文,等.三江中南段火山岩-蛇绿岩与成矿[M].北京:地质出版社,1998.
[26]李兴振,江新胜,孙志明,等.西南三江地区碰撞造山过程[M].北京:地质出版社,2002.
[27]Phajuy B,Panjasawatwong Y,Osataporn P.Preliminary geochemical study of volcanic rocks in the Pang Mayao area,Phrao,Chiang Mai,northern Thailand:tectonic setting of formation[J].Journal of Asian Earth Sciences,2005,24:765-776.