西藏山南地区花岗质岩石成因及其对地壳结构变化的记录
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摘要
西藏拉萨地块南部发育大规模东西带状展布的花岗质岩石,记录了新特提斯洋壳俯冲晚期及随后印度-欧亚大陆碰撞、后碰撞过程的重要信息,受到了学者的广泛关注.对拉萨地块南部山南地区采集的6件花岗质岩石样品进行了LA-ICPMS锆石U-Pb年代学、Hf同位素及全岩主微量元素和Sr-Nd同位素地球化学分析,获得了～90 Ma、65 Ma及23 Ma三阶段的锆石年龄,显示区域内发育了三期岩浆活动事件.三个时代的岩石样品均为亚碱性系列,具类似埃达克质岩特征,富集高场强元素并亏损大离子亲石元素,稀土元素分布图呈右倾型,具有弧岩浆的地球化学特征.本文所有样品的锆石εHf(t)均为正值(+5.6～+14.6),暗示它们可能来源于新生下地壳物质的部分熔融.结合前人已有数据,采用花岗质岩石的La/Yb比值定量还原了山南地区100 Ma以来的地壳厚度演化历史.从晚中生代开始,区域内地壳厚度由厚减薄,到新生代早期达到最薄,此后逐渐增厚.这与中新生代以来新特提斯洋俯冲至印度-欧亚大陆碰撞-后碰撞过程引起地壳结构变化较好地契合.
In the southern part of the Lhasa block in southern Tibet, large-scale zonal distribution of granitic rocks is developed,where the important information on the late subduction of the Neo-Tethys oceanic crust and subsequent collisions and post-collision processes in the Indian-Eurasia continent is recorded. In this paper, LA-ICP-MS zircon U-Pb geochronology, Hf isotope and total trace elements and Sr-Nd isotope geochemical analysis of six granitic rock samples collected in the southern Shannan area of the Lhasa block were obtained. The three-stage zircon ages of ～90 Ma, 65 Ma and 23 Ma show that three phases of magmatic activity occurred in the area. The rock samples of the three eras are all subalkalic series, with similar adakite characteristics, enriched with high field strength elements and depleted with large ion lithophile elements. The distribution of rare earth elements is right-dip,geochemical with the feature of arc magma. The zircon εHf(t) of all samples in this paper are positive(+5.6-+14.6), suggesting that they may be derived from the partial melting of the new lower crust. Based on the previous data, the La/Yb ratio of granitic rocks is used to quantitatively show the evolution of crustal thickness in the Shannan area for 100 Ma. From the late Mesozoic, the thickness of the crust in the area was thinned from thick to thin in the Early Cenozoic, and gradually thickened thereafter. This is in line with the changes in the crustal structure caused by the subduction of the Neo-Tethys and the Indian-Eurasia collision-postcollision process since Mesozoic to Cenozoic.
In the southern part of the Lhasa block in southern Tibet, large-scale zonal distribution of granitic rocks is developed,where the important information on the late subduction of the Neo-Tethys oceanic crust and subsequent collisions and post-collision processes in the Indian-Eurasia continent is recorded. In this paper, LA-ICP-MS zircon U-Pb geochronology, Hf isotope and total trace elements and Sr-Nd isotope geochemical analysis of six granitic rock samples collected in the southern Shannan area of the Lhasa block were obtained. The three-stage zircon ages of ～90 Ma, 65 Ma and 23 Ma show that three phases of magmatic activity occurred in the area. The rock samples of the three eras are all subalkalic series, with similar adakite characteristics, enriched with high field strength elements and depleted with large ion lithophile elements. The distribution of rare earth elements is right-dip,geochemical with the feature of arc magma. The zircon εHf(t) of all samples in this paper are positive(+5.6-+14.6), suggesting that they may be derived from the partial melting of the new lower crust. Based on the previous data, the La/Yb ratio of granitic rocks is used to quantitatively show the evolution of crustal thickness in the Shannan area for 100 Ma. From the late Mesozoic, the thickness of the crust in the area was thinned from thick to thin in the Early Cenozoic, and gradually thickened thereafter. This is in line with the changes in the crustal structure caused by the subduction of the Neo-Tethys and the Indian-Eurasia collision-postcollision process since Mesozoic to Cenozoic.
引文
Belousova,E.A.,Griffin,W.L.,O′Reilly,S.Y.,et al.,2002.Igneous Zircon:Trace Element Composition as an Indicator of Source Rock Type.Contributions to Mineralogy and Petrology,143(5):602-622.
Chapman,J.B.,Ducea,M.N.,DeCelles,P.G.,et al.,2015.Tracking Changes in Crustal Thickness during Orogenic Evolution with Sr/Y:An Example from the North American Cordillera.Geology,43(10):919-922.
Chen,J.L.,Xu,J.F.,Zhao,W.X.,et al.,2011.Geochemical Variations in Miocene Adakitic Rocks from the Western and Eastern Lhasa Terrane:Implications for Lower Crustal Flow beneath the Southern Tibetan Plateau.Lithos,125(3-4):928-939.https://doi.org/10.1016/j.lithos.2011.05.006
Chiaradia,M.,2015.Crustal Thickness Control on Sr/Y Signatures of Recent Arc Magmas:An Earth Scale Perspective.Scientific Reports,5:8115.https://doi.org/10.1038/srep08115
Chung,S.L.,Chu,M.F.,Ji,J.Q.,et al.,2009.The Nature and Timing of Crustal Thickening in Southern Tibet:Geochemical and Zircon Hf Isotopic Constraints from Postcollisional Adakites.Tectonophysics,477(1-2):36-48.https://doi.org/10.1016/j.tecto.2009.08.008
Dai,Z.W.,Li,G.M.,Ding,J.,et al.,2018.Late Cretaceous Adakite in Nuri Area,Tibet:Products of Ridge Subduction.Earth Science,43(8):2727-2741(in Chinese with English abstract).
Defant,M.J.,Drummond,M.S.,1990.Derivation of Some Modern Arc Magmas by Melting of Young Subducted Lithosphere.Nature,347:662-665.
Hou,Z.Q.,Duan,L.F.,Lu,Y.J.,et al.,2015.Lithospheric Architecture of the Lhasa Terrane and Its Control on Ore Deposits in the Himalayan-Tibetan Orogen.Economic Geology,110(6):1541-1575.https://doi.org/10.2113/econgeo.110.6.1541
Hu,F.Y.,Ducea,M.N.,Liu,S.W.,et al.,2017.Quantifying Crustal Thickness in Continental Collisional Belts:Global Perspective and a Geologic Application.Scientific Reports,7:7058.
Huang,F.,Xu,J.F.,Chen,J.L.,et al.,2016.Two Cenozoic Tectonic Events of N-S and E-W Extension in the Lhasa Terrane:Evidence from Geology and Geochronology.Lithos,245:118-132.https://doi.org/10.1016/j.lithos.2015.08.014
Huang,F.,Xu,J.F.,Zeng,Y.C.,et al.,2017.Slab Breakoff of the Neo-Tethys Ocean in the Lhasa Terrane Inferred from Contemporaneous Melting of the Mantle and Crust.Geochemistry,Geophysics,Geosystems,18(11):4074-4095.https://doi.org/10.1002/2017gc007039
Ji,W.Q.,Wu,F.Y.,Chung,S.L.,et al.,2009a.Zircon U-Pb Geochronology and Hf Isotopic Constraints on Petrogenesis of the Gangdese Batholith,Southern Tibet.Chemical Geology,262(3-4):229-245.
Ji,W.Q.,Wu,F.Y.,Liu,C.Z.,et al.,2009b.Geochronology and Petrogenesis of Granitic Rocks in Gangdese Batholith,Southern Tibet.Science in China(Series D),52(9):1240-1261.
Jiang,Z.Q.,Wang,Q.,Wyman,D.,et al.,2011.Origin of~30Ma Chongmuda Adakitic Intrusive Rocks in the Southern Gangdese Region,Southern Tibet:Partial Melting of the Northward Subducted Indian Continent Crust?Geochimica,40(2):126-146(in Chinese with English abstract).
Jiang,Z.Q.,Wang,Q.,Wyman,D.A.,et al.,2014.Transition from Oceanic to Continental Lithosphere Subduction in Southern Tibet:Evidence from the Late Cretaceous-Early Oligocene(~91-30 Ma)Intrusive Rocks in the ChanangZedong Area,Southern Gangdese.Lithos,196-197:213-231.
Kang,Z.Q.,Xu,J.F.,Wilde,S.A.,et al.,2014.Geochronology and Geochemistry of the Sangri Group Volcanic Rocks,Southern Lhasa Terrane:Implications for the Early Subduction History of the Neo-Tethys and Gangdese Magmatic Arc.Lithos,200-201:157-168.https://doi.org/10.1016/j.lithos.2014.04.019
Li,X.W.,Mo,X.X.,Scheltens,M.,et al.,2016.Mineral Chemistry and Crystallization Conditions of the Late Cretaceous Mamba Pluton from the Eastern Gangdese,Southern Tibetan Plateau.Journal of Earth Science,27(4):545-570.https://doi.org/10.1007/s12583-016-0713-5
Ma,L.,Wang,Q.,Wyman,D.A.,et al.,2013.Late Cretaceous Crustal Growth in the Gangdese Area,Southern Tibet:Petrological and Sr-Nd-Hf-O Isotopic Evidence from Zhengga Diorite-Gabbro.Chemical Geology,349-350:54-70
Ma,L.,Wang,Q.,Wyman,D.A.,et al.,2015.Late Cretaceous Back-Arc Extension and Arc System Evolution in the Gangdese Area,Southern Tibet:Geochronological,Petrological,and Sr-Nd-Hf-O Isotopic Evidence from Dagze Diabases.Journal of Geophysical Research:Solid Earth,120(9):6159-6181.
Mantle,G.W.,Collins,W.J.,2008.Quantifying Crustal Thickness Variations in Evolving Orogens:Correlation between Arc Basalt Composition and Moho Depth.Geology,36(1):87.https://doi.org/10.1130/g24095a.1
McDonough,W.F.,Sun,S.S.,1995.The Composition of the Earth.Chemical Geology,120(3-4):223-253.https://doi.org/10.1016/0009-2541(94)00140-4
Mo,X.X.,Hou,Z.Q.,Niu,Y.L.,et al.,2007.Mantle Contributions to Crustal Thickening during Continental Collision:Evidence from Cenozoic Igneous Rocks in Southern Tibet.Lithos,96(1-2):225-242.https://doi.org/10.1016/j.lithos.2006.10.005
Owens,T.J.,Zandt,G.,1997.Implications of Crustal Property Variations for Models of Tibetan Plateau Evolution.Nature,387:37-43.
Profeta,L.,Ducea,M.N.,Chapman,J.B.,et al.,2016.Quantifying Crustal Thickness over Time in Magmatic Arcs.Scientific Reports,5:17786.https://doi.org/10.1038/srep17786
Rapp,R.P.,Watson,E.B.,1995.Dehydration Melting of Metabasalt at 8-32 kbar:Implications for Continental Growth and Crust-Mantle Recycling.Journal of Petrology,36(4):891-931.
Wang,B.D.,Wang,L.Q.,Chung,S.L.,et al.,2016.Evolution of the Bangong-Nujiang Tethyan Ocean:Insights from the Geochronology and Geochemistry of Mafic Rocks within Ophiolites.Lithos,245:18-33.https://doi.org/10.1016/j.lithos.2015.07.016
Wang,Q.,McDermott,F.,Xu,J.F.,et al.,2005.Cenozoic K-Rich Adakitic Volcanic Rocks in the Hohxil Area,Northern Tibet:Lower-Crustal Melting in an Intracontinental Setting.Geology,33(6):465.https://doi.org/10.1130/g21522.1
Wen,D.,Liu,D.,Chung,S.,et al.,2008.Zircon SHRIMP U-Pb Ages of the Gangdese Batholith and Implications for Neotethyan Subduction in Southern Tibet.Chemical Geology,252(3-4):191-201.https://doi.org/10.1016/j.chemgeo.2008.03.003
Xu,B.,Griffin,W.L.,Xiong,Q.,et al.,2017.Ultrapotassic Rocks and Xenoliths from South Tibet:Contrasting Styles of Interaction between Lithospheric Mantle and Asthenosphere during Continental Collision.Geology,45(1):51-54.https://doi.org/10.1130/g38466.1
Yin,A.,Harrison,T.M.,2000.Geologic Evolution of the Himalayan-Tibetan Orogen.Annual Review of Earth and Planetary Sciences,28(1):211-280.
Zhang,Z.M.,Zhao,G.C.,Santosh,M.,et al.,2010.Late Cretaceous Charnockite with Adakitic Affinities from the Gangdese Batholith,Southeastern Tibet:Evidence for Neo-Tethyan Mid-Ocean Ridge Subduction?Gondwana Research,17(4):615-631.
Zhu,D.C.,Wang,Q.,Cawood,P.A.,et al.,2017.Raising the Gangdese Mountains in Southern Tibet.Journal of Geophysical Research:Solid Earth,122(1):214-223.
Zhu,D.C.,Zhao,Z.D.,Niu,Y.L.,et al.,2011.The Lhasa Terrane:Record of a Microcontinent and Its Histories of Drift and Growth.Earth and Planetary Science Letters,301(1-2):241-255.https://doi.org/10.1016/j.epsl.2010.11.005
Zhu,D.C.,Zhao,Z.D.,Niu,Y.L.,et al.,2013.The Origin and Pre-Cenozoic Evolution of the Tibetan Plateau.Gondwana Research,23(4):1429-1454.https://doi.org/10.1016/j.gr.2012.02.002
Zou,J.Q.,Yu,H.X.,Wang,B.D.,et al.,2018.Petrogenesis and Geological Implications of Early Jurassic Granodiorites in Renqinze Area,Central Part of Southern Lhasa Subterrane.Earth Science,43(8):2795-2810(in Chinese with English abstract).
代作文,李光明,丁俊,等,2018.西藏努日晚白垩世埃达克岩:洋脊俯冲的产物.地球科学,43(8):2727-2741.
姜子琦,王强,Wyman,D.A.,等,2011.西藏冈底斯南缘冲木达约30 Ma埃达克质侵入岩的成因:向北俯冲的印度陆壳的熔融?地球化学,40(2):126-146.
邹洁琼,余红霞,王保弟,等,2018.南拉萨地块中部早侏罗世仁钦则花岗闪长岩成因及其地质意义.地球科学,43(8):2795-2810.
Chapman,J.B.,Ducea,M.N.,DeCelles,P.G.,et al.,2015.Tracking Changes in Crustal Thickness during Orogenic Evolution with Sr/Y:An Example from the North American Cordillera.Geology,43(10):919-922.
Chen,J.L.,Xu,J.F.,Zhao,W.X.,et al.,2011.Geochemical Variations in Miocene Adakitic Rocks from the Western and Eastern Lhasa Terrane:Implications for Lower Crustal Flow beneath the Southern Tibetan Plateau.Lithos,125(3-4):928-939.https://doi.org/10.1016/j.lithos.2011.05.006
Chiaradia,M.,2015.Crustal Thickness Control on Sr/Y Signatures of Recent Arc Magmas:An Earth Scale Perspective.Scientific Reports,5:8115.https://doi.org/10.1038/srep08115
Chung,S.L.,Chu,M.F.,Ji,J.Q.,et al.,2009.The Nature and Timing of Crustal Thickening in Southern Tibet:Geochemical and Zircon Hf Isotopic Constraints from Postcollisional Adakites.Tectonophysics,477(1-2):36-48.https://doi.org/10.1016/j.tecto.2009.08.008
Dai,Z.W.,Li,G.M.,Ding,J.,et al.,2018.Late Cretaceous Adakite in Nuri Area,Tibet:Products of Ridge Subduction.Earth Science,43(8):2727-2741(in Chinese with English abstract).
Defant,M.J.,Drummond,M.S.,1990.Derivation of Some Modern Arc Magmas by Melting of Young Subducted Lithosphere.Nature,347:662-665.
Hou,Z.Q.,Duan,L.F.,Lu,Y.J.,et al.,2015.Lithospheric Architecture of the Lhasa Terrane and Its Control on Ore Deposits in the Himalayan-Tibetan Orogen.Economic Geology,110(6):1541-1575.https://doi.org/10.2113/econgeo.110.6.1541
Hu,F.Y.,Ducea,M.N.,Liu,S.W.,et al.,2017.Quantifying Crustal Thickness in Continental Collisional Belts:Global Perspective and a Geologic Application.Scientific Reports,7:7058.
Huang,F.,Xu,J.F.,Chen,J.L.,et al.,2016.Two Cenozoic Tectonic Events of N-S and E-W Extension in the Lhasa Terrane:Evidence from Geology and Geochronology.Lithos,245:118-132.https://doi.org/10.1016/j.lithos.2015.08.014
Huang,F.,Xu,J.F.,Zeng,Y.C.,et al.,2017.Slab Breakoff of the Neo-Tethys Ocean in the Lhasa Terrane Inferred from Contemporaneous Melting of the Mantle and Crust.Geochemistry,Geophysics,Geosystems,18(11):4074-4095.https://doi.org/10.1002/2017gc007039
Ji,W.Q.,Wu,F.Y.,Chung,S.L.,et al.,2009a.Zircon U-Pb Geochronology and Hf Isotopic Constraints on Petrogenesis of the Gangdese Batholith,Southern Tibet.Chemical Geology,262(3-4):229-245.
Ji,W.Q.,Wu,F.Y.,Liu,C.Z.,et al.,2009b.Geochronology and Petrogenesis of Granitic Rocks in Gangdese Batholith,Southern Tibet.Science in China(Series D),52(9):1240-1261.
Jiang,Z.Q.,Wang,Q.,Wyman,D.,et al.,2011.Origin of~30Ma Chongmuda Adakitic Intrusive Rocks in the Southern Gangdese Region,Southern Tibet:Partial Melting of the Northward Subducted Indian Continent Crust?Geochimica,40(2):126-146(in Chinese with English abstract).
Jiang,Z.Q.,Wang,Q.,Wyman,D.A.,et al.,2014.Transition from Oceanic to Continental Lithosphere Subduction in Southern Tibet:Evidence from the Late Cretaceous-Early Oligocene(~91-30 Ma)Intrusive Rocks in the ChanangZedong Area,Southern Gangdese.Lithos,196-197:213-231.
Kang,Z.Q.,Xu,J.F.,Wilde,S.A.,et al.,2014.Geochronology and Geochemistry of the Sangri Group Volcanic Rocks,Southern Lhasa Terrane:Implications for the Early Subduction History of the Neo-Tethys and Gangdese Magmatic Arc.Lithos,200-201:157-168.https://doi.org/10.1016/j.lithos.2014.04.019
Li,X.W.,Mo,X.X.,Scheltens,M.,et al.,2016.Mineral Chemistry and Crystallization Conditions of the Late Cretaceous Mamba Pluton from the Eastern Gangdese,Southern Tibetan Plateau.Journal of Earth Science,27(4):545-570.https://doi.org/10.1007/s12583-016-0713-5
Ma,L.,Wang,Q.,Wyman,D.A.,et al.,2013.Late Cretaceous Crustal Growth in the Gangdese Area,Southern Tibet:Petrological and Sr-Nd-Hf-O Isotopic Evidence from Zhengga Diorite-Gabbro.Chemical Geology,349-350:54-70
Ma,L.,Wang,Q.,Wyman,D.A.,et al.,2015.Late Cretaceous Back-Arc Extension and Arc System Evolution in the Gangdese Area,Southern Tibet:Geochronological,Petrological,and Sr-Nd-Hf-O Isotopic Evidence from Dagze Diabases.Journal of Geophysical Research:Solid Earth,120(9):6159-6181.
Mantle,G.W.,Collins,W.J.,2008.Quantifying Crustal Thickness Variations in Evolving Orogens:Correlation between Arc Basalt Composition and Moho Depth.Geology,36(1):87.https://doi.org/10.1130/g24095a.1
McDonough,W.F.,Sun,S.S.,1995.The Composition of the Earth.Chemical Geology,120(3-4):223-253.https://doi.org/10.1016/0009-2541(94)00140-4
Mo,X.X.,Hou,Z.Q.,Niu,Y.L.,et al.,2007.Mantle Contributions to Crustal Thickening during Continental Collision:Evidence from Cenozoic Igneous Rocks in Southern Tibet.Lithos,96(1-2):225-242.https://doi.org/10.1016/j.lithos.2006.10.005
Owens,T.J.,Zandt,G.,1997.Implications of Crustal Property Variations for Models of Tibetan Plateau Evolution.Nature,387:37-43.
Profeta,L.,Ducea,M.N.,Chapman,J.B.,et al.,2016.Quantifying Crustal Thickness over Time in Magmatic Arcs.Scientific Reports,5:17786.https://doi.org/10.1038/srep17786
Rapp,R.P.,Watson,E.B.,1995.Dehydration Melting of Metabasalt at 8-32 kbar:Implications for Continental Growth and Crust-Mantle Recycling.Journal of Petrology,36(4):891-931.
Wang,B.D.,Wang,L.Q.,Chung,S.L.,et al.,2016.Evolution of the Bangong-Nujiang Tethyan Ocean:Insights from the Geochronology and Geochemistry of Mafic Rocks within Ophiolites.Lithos,245:18-33.https://doi.org/10.1016/j.lithos.2015.07.016
Wang,Q.,McDermott,F.,Xu,J.F.,et al.,2005.Cenozoic K-Rich Adakitic Volcanic Rocks in the Hohxil Area,Northern Tibet:Lower-Crustal Melting in an Intracontinental Setting.Geology,33(6):465.https://doi.org/10.1130/g21522.1
Wen,D.,Liu,D.,Chung,S.,et al.,2008.Zircon SHRIMP U-Pb Ages of the Gangdese Batholith and Implications for Neotethyan Subduction in Southern Tibet.Chemical Geology,252(3-4):191-201.https://doi.org/10.1016/j.chemgeo.2008.03.003
Xu,B.,Griffin,W.L.,Xiong,Q.,et al.,2017.Ultrapotassic Rocks and Xenoliths from South Tibet:Contrasting Styles of Interaction between Lithospheric Mantle and Asthenosphere during Continental Collision.Geology,45(1):51-54.https://doi.org/10.1130/g38466.1
Yin,A.,Harrison,T.M.,2000.Geologic Evolution of the Himalayan-Tibetan Orogen.Annual Review of Earth and Planetary Sciences,28(1):211-280.
Zhang,Z.M.,Zhao,G.C.,Santosh,M.,et al.,2010.Late Cretaceous Charnockite with Adakitic Affinities from the Gangdese Batholith,Southeastern Tibet:Evidence for Neo-Tethyan Mid-Ocean Ridge Subduction?Gondwana Research,17(4):615-631.
Zhu,D.C.,Wang,Q.,Cawood,P.A.,et al.,2017.Raising the Gangdese Mountains in Southern Tibet.Journal of Geophysical Research:Solid Earth,122(1):214-223.
Zhu,D.C.,Zhao,Z.D.,Niu,Y.L.,et al.,2011.The Lhasa Terrane:Record of a Microcontinent and Its Histories of Drift and Growth.Earth and Planetary Science Letters,301(1-2):241-255.https://doi.org/10.1016/j.epsl.2010.11.005
Zhu,D.C.,Zhao,Z.D.,Niu,Y.L.,et al.,2013.The Origin and Pre-Cenozoic Evolution of the Tibetan Plateau.Gondwana Research,23(4):1429-1454.https://doi.org/10.1016/j.gr.2012.02.002
Zou,J.Q.,Yu,H.X.,Wang,B.D.,et al.,2018.Petrogenesis and Geological Implications of Early Jurassic Granodiorites in Renqinze Area,Central Part of Southern Lhasa Subterrane.Earth Science,43(8):2795-2810(in Chinese with English abstract).
代作文,李光明,丁俊,等,2018.西藏努日晚白垩世埃达克岩:洋脊俯冲的产物.地球科学,43(8):2727-2741.
姜子琦,王强,Wyman,D.A.,等,2011.西藏冈底斯南缘冲木达约30 Ma埃达克质侵入岩的成因:向北俯冲的印度陆壳的熔融?地球化学,40(2):126-146.
邹洁琼,余红霞,王保弟,等,2018.南拉萨地块中部早侏罗世仁钦则花岗闪长岩成因及其地质意义.地球科学,43(8):2795-2810.
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