松潘—甘孜造山带南缘二叠系变质玄武岩的成因与构造意义
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摘要
松潘—甘孜造山带南缘的二叠系之中发育一套顺层产出的变玄武岩,岩石具有明显的变余枕状构造及气孔构造。本文对其进行了主微量元素分析,并探讨其岩石成因与成岩构造背景。变玄武岩具有低的SiO_2质量分数(47.56%~37.98%)与Nb/Zr值,其MgO(平均为18.95%)、Mg#值(平均为74.1)和Cr(平均为1 712×10~(-6))含量较高,表明原岩岩浆很可能起源于亏损地幔,主要由石榴石橄榄岩发生20%~25%的部分熔融形成。岩石MgO与Cr含量呈现良好的正相关,具有极低的La/Sm(2.86~1.02)、(Th/Yb)PM(2.42~0.56)、(Th/Ta)PM(1.21~0.43)和(La/Nb)PM(1.55~0.27)比值,说明岩浆演化过程中发生了含铬矿物的分离结晶,且没有遭受地壳物质的混染。变玄武岩稀土、微量配分型式及构造背景判别显示,岩石的化学成分与E-MORB及N-MORB相当。野外考察显示,与变玄武岩共生的岩石主要为二叠系大理岩,暗示变玄武岩原岩很可能形成于成熟的弧后盆地环境。结合二叠系还发育超基性岩的事实,本文认为该套变玄武岩可能是古特提斯洋的洋壳残余。另外成分对比显示,松潘—甘孜造山带南缘的二叠系变玄武岩并非峨眉山玄武岩浆活动的产物。
A set of bedded metabasalts occurred in the Permian strata,in the southern margin of the SongpanGarze orogenic belt.These rocks have apparent pillow and vesicular structures.In this paper,we performed geochemical analyses on these rocks and studied their petrogenesis and diagenetic tectonic settings.The metabasalts exhibit low SiO_2 contents(47.56%-37.98%)and Nb/Zr ratios and relative high abundance of MgO(average 18.95%),Mg(Mg~#average value 74.1)and Cr(average 1712×10~(-6)).These geochemical features indicate that the magma of their protolith was most likely derived from the depleted mantle and formed mainly by the partial melting(20%-25%)of garnet peridotite.Their MgO and Cr contents show a positive correlation with the extremely low La/Sm(2.86-1.02),(Th/Yb)PM(2.42-0.56),(Th/Ta)PM(1.21-0.43)and(La/Nb)PM(1.55-0.27)ratios,suggesting fractional crystallization of the Cr-bearing minerals and no contamination with crustal materials during the magma evolution processes.REE,trace element patterns and tectonic discriminations show that the chemical compositions of metabasalts are similar to those of E-MORB and N-MORB.In the field,the metabasalts are commonly sandwiched in the Permian marble,indicating a mature back-arc basin setting.Combined with the fact that some ultrabasic rocks were also developed in the Permian strata,we propose that the metabasalts are most likely a remnant of the Paleo-Tethys oceanic crust.When compared with the chemical compositions of Emeishan basalts,it is clear that the Permian metabasalts in the southern margin of the Songpan-Garze orogenic belt were not the products of the Late Permian Emeishan basaltic magma activities.
A set of bedded metabasalts occurred in the Permian strata,in the southern margin of the SongpanGarze orogenic belt.These rocks have apparent pillow and vesicular structures.In this paper,we performed geochemical analyses on these rocks and studied their petrogenesis and diagenetic tectonic settings.The metabasalts exhibit low SiO_2 contents(47.56%-37.98%)and Nb/Zr ratios and relative high abundance of MgO(average 18.95%),Mg(Mg~#average value 74.1)and Cr(average 1712×10~(-6)).These geochemical features indicate that the magma of their protolith was most likely derived from the depleted mantle and formed mainly by the partial melting(20%-25%)of garnet peridotite.Their MgO and Cr contents show a positive correlation with the extremely low La/Sm(2.86-1.02),(Th/Yb)PM(2.42-0.56),(Th/Ta)PM(1.21-0.43)and(La/Nb)PM(1.55-0.27)ratios,suggesting fractional crystallization of the Cr-bearing minerals and no contamination with crustal materials during the magma evolution processes.REE,trace element patterns and tectonic discriminations show that the chemical compositions of metabasalts are similar to those of E-MORB and N-MORB.In the field,the metabasalts are commonly sandwiched in the Permian marble,indicating a mature back-arc basin setting.Combined with the fact that some ultrabasic rocks were also developed in the Permian strata,we propose that the metabasalts are most likely a remnant of the Paleo-Tethys oceanic crust.When compared with the chemical compositions of Emeishan basalts,it is clear that the Permian metabasalts in the southern margin of the Songpan-Garze orogenic belt were not the products of the Late Permian Emeishan basaltic magma activities.
引文
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[26]汪云峰,张招崇,王丽娟,等.峨眉山大火成岩省虎跳峡和金安二叠纪玄武岩的地球化学特征及其对源区的约束[J].岩石学报,2013,29(12):4387-4403.
[27]代堰锫,张连昌,王长乐,等.辽宁本溪歪头山条带状铁矿的成因类型、形成时代及构造背景[J].岩石学报,2012,28(11):3574-3594.
[28]POLAT A,HOFMANN A W,ROSING M T.Boninite-like volcanic rocks in the 3.7-3.8Ga Isua greenstone belt,West Greenland:geochemical evidence for intra-oceanic subduction zone processes in the early Earth[J].Chemical Geology,2002,184:231-254.
[29]XU Y G,CHUNG S L,JAHN B M,et al.Petrologic and geochemical constraints on the petrogenesis of Permian-Triassic Emeishan flood basalts in southwestern China[J].Lithos,2001,58:145-168.
[30]DUPUY C,DOSTAL J.Trace element geochemistry of some continental tholeiites[J].Earth and Planetary Science Letters,1984,67(1):61-69.
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[34]GILL R.Igneous rocks and processes:apractical guide[M].Chichester:Wiley-Blackwell,2010:1-472.
[35]LIU S,HU R Z,GAO S,et al.Zircon U-Pb geochronology and major,trace elemental and Sr-Nd-Pb isotopic geochemistry of mafic dykes in western Shandong province,east China:constraints on their petrogenesis and geodynamic significance[J].Chemical Geology,2008,255(3):329-345.
[36]DEPAOLO D J.Trace element and isotopic effects of combined wallrock assimilation and fractional crystallization[J].Earth and Planetary Science Letters,1981,53(2):189-202.
[37]LASSITER J C,DEPAOLO D J.Plume/lithosphere interaction in the generation of continental and oceanic flood basalts:chemical and isotopic constraints[M]∥MAHONEY J,MILLARD C.Large igneous provinces:continental,oceanic,and planetary flood volcanism.Washington:American Geophysical Union,1997:335-355.
[38]QI L,ZHOU M F.Platinum-group elemental and Sr-Nd-Os isotopic geochemistry of Permian Emeishan flood basalts in Guizhou Province,SW China[J].Chemical Geology,2008,248(1/2):83-103.
[39]PENG Z X,MAHONEY J,HOOPER P,et al.A role for lower continental crust in flood basalt genesis?Isotopic and incompatible element study of the lower six formations of the western Deccan Traps[J].Geochimica et Cosmochimica Acta,1994,58:267-288.
[40]SHERVAIS J W.Ti-V plots and the petrogenesis of modern and ophiolitic lavas[J].Earth and Planetary Science Letters,1982,59(1):101-118.
[41]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(1):11-30.
[42]PEARCE J A,PEATE D W.Tectonic implications of the composition of volcanic arc magmas[J].Annual Review of Earth and Planetary Sciences,1995,23:1073-1109.
[43]FRETZDORFF S,LIVERMORE R A,DEVEY C W,et al.Petrogenesis of the back-arc East Scotia Ridge,South Atlantic Ocean[J].Journal of Petrology,2002,43:1435-1467.
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[2]ZHANG H F,PARRISH R,ZHANG L,et al.A-type granite and adakitic magmatism association in Songpan-Garze fold belt,eastern Tibetan Plateau:implication for lithospheric delamination[J].Lithos,2007,97:323-335.
[3]胡健民,孟庆任,石玉若,等.松潘—甘孜地体内花岗岩锆石SHRIMP U-Pb定年及其构造意义[J].岩石学报,2005,21(3):867-880.
[4]ZHANG H F,ZHANG L,HARRIS N,et al.U-Pb zircon ages,geochemical and isotopic compositions of granitoids in Songpan-GarzêFold Belt,eastern Tibet Plateau:constraints on petrogenesis,nature of basement and tectonic evolution[J].Contributions to Mineralogy and Petrology,2006,152:75-88.
[5]时章亮,张宏飞,蔡宏明.松潘造山带马尔康强过铝质花岗岩的成因及其构造意义[J].地球科学:中国地质大学报,2009,34:569-584.
[6]蔡宏明,张宏飞,徐旺春,等.松潘带印支期岩石圈拆沉作用新证据:来自火山岩岩石成因的研究[J].中国科学:地球科学,2010(11):1518-1532.
[7]YAN D P,ZHOU M F,SONG H L,et al.Structural style and tectonic significance of the Jianglang dome in the eastern margin of the Tibetan Plateau,China[J].Journal of Structural Geology,2003,25(5):765-779.
[8]傅昭仁,宋鸿林,颜丹平.扬子地台西缘江浪变质核杂岩结构及对成矿的控制[J].地质学报,1997,71(2):113-122.
[9]颜丹平,宋鸿林,傅昭仁,等.扬子地台西缘变质核杂岩带[M].北京:地质出版社,1997:1-94.
[10]HUANG M H,MAAS R,BUICK I S,et al.Crustal response to continental collisions between the Tibet,Indian,South China and North China Blocks:geochronological constraints from the Songpan-Garze orogenic belt,Western China[J].Journal of Metamorphic Geology,2003,21:223-240.
[11]游振东,程素华,赖兴运.四川丹巴穹状变质地体[J].地学前缘,2006,13(4):148-159.
[12]HUANG M H,BUICK I S,HOU L W.Tectonometamorphic evolution of the eastern Tibet Plateau:evidence from the central Songpan-Garze orogenic belt,Western China[J].Journal of Petrology,2003,44:255-278.
[13]颜丹平,周美夫,宋鸿林,等.华南在Rodinia古陆中位置的讨论:扬子地块西缘变质-岩浆杂岩证据及其与Seychelles地块的对比[J].地学前缘,2002,9(4):249-256.
[14]ROGER F,JOLIVET M,MALAVIEILLE J.The tectonic evolution of the Songpan-Garzê(North Tibet)and adjacent areas from Proterozoic to present:a synthesis[J].Journal of Asian Earth Sciences,2010,39(4):254-269.
[15]NIE S,YIN A,ROWLEY D B,et al.Exhumation of the Dabie Shan ultra-high-pressure rocks and accumulation of the Songpan-Ganzi flysch sequence,central China[J].Geology,1994,22:999-1002.
[16]苏本勋,陈岳龙,刘飞,等.松潘—甘孜地块三叠系砂岩的地球化学特征及其意义[J].岩石学报,2006,22(4):961-970.
[17]代堰锫,张惠华,朱玉娣,等.扬子陆块西缘江浪穹隆及“里伍式”富铜矿床研究进展与问题[J].地球科学与环境学报,2016,38(1):66-78.
[18]四川省地质矿产局.四川省区域地质志[M].北京:地质出版社,1990:1-680.
[19]周家云,谭洪旗,龚大兴,等.川西江浪穹隆核部新火山花岗岩LA-ICP-MS锆石U-Pb定年和Hf同位素研究[J].矿物岩石,2013,33(4):42-52.
[20]周家云,谭洪旗,龚大兴,等.乌拉溪铝质A型花岗岩:松潘—甘孜造山带早燕山期热隆伸展的岩石记录[J].地质论评,2014,60(2):348-362.
[21]TAYLOR S R,MCLENNAN S M.The continental crust:its composition and evolution[M].Oxford:Blackwell,1985:1-328.
[22]SUN S S,MCDONOUGH 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.
[23]MA L,JIANG S Y,HOFMANN A W,et al.Lithospheric and asthenospheric sources of lamprophyres in the Jiaodong peninsula:a consequence of rapid lithospheric thinning beneath the North China craton[J].Geochimica et Cosmochimica Acta,2014,124(1):250-271.
[24]WINCHESTER J A,FLOYD P A.Geochemical discrimination of different magma series and their differentiation products using immobile elements[J].Chemical Geology,1977,20:325-343.
[25]XIAO L,XU Y G,MEI H J,et al.Distinct mantle sources of low-Ti and high-Ti basalts from the western Emeishan large igneous province,SW China:implications for plumelithosphere interaction[J].Earth and Planetary Science Let-ters,2004,228(3/4):525-546.
[26]汪云峰,张招崇,王丽娟,等.峨眉山大火成岩省虎跳峡和金安二叠纪玄武岩的地球化学特征及其对源区的约束[J].岩石学报,2013,29(12):4387-4403.
[27]代堰锫,张连昌,王长乐,等.辽宁本溪歪头山条带状铁矿的成因类型、形成时代及构造背景[J].岩石学报,2012,28(11):3574-3594.
[28]POLAT A,HOFMANN A W,ROSING M T.Boninite-like volcanic rocks in the 3.7-3.8Ga Isua greenstone belt,West Greenland:geochemical evidence for intra-oceanic subduction zone processes in the early Earth[J].Chemical Geology,2002,184:231-254.
[29]XU Y G,CHUNG S L,JAHN B M,et al.Petrologic and geochemical constraints on the petrogenesis of Permian-Triassic Emeishan flood basalts in southwestern China[J].Lithos,2001,58:145-168.
[30]DUPUY C,DOSTAL J.Trace element geochemistry of some continental tholeiites[J].Earth and Planetary Science Letters,1984,67(1):61-69.
[31]GAO S,ZHANG B R,JIN Z M,et al.How mafic is the lower continental crust?[J].Earth and Planetary Science Letters,1998,161(1):101-117.
[32]ROEX A P L,DICK H J B,ERLANK A J,et al.Geochemistry,mineralogy and petrogenesis of lavas erupted along the Southwest Indian Ridge between the Bouvet Triple Junction and 11degrees east[J].Journal of Petrology,1983,24(3):267-318.
[33]XU Y G,MA J L,FREY F A,et al.Role of lithosphere-asthenosphere interaction in the genesis of Quaternary alkali and tholeiitic basalts from Datong,western North China Craton[J].Chemical Geology,2005,224(4):247-271.
[34]GILL R.Igneous rocks and processes:apractical guide[M].Chichester:Wiley-Blackwell,2010:1-472.
[35]LIU S,HU R Z,GAO S,et al.Zircon U-Pb geochronology and major,trace elemental and Sr-Nd-Pb isotopic geochemistry of mafic dykes in western Shandong province,east China:constraints on their petrogenesis and geodynamic significance[J].Chemical Geology,2008,255(3):329-345.
[36]DEPAOLO D J.Trace element and isotopic effects of combined wallrock assimilation and fractional crystallization[J].Earth and Planetary Science Letters,1981,53(2):189-202.
[37]LASSITER J C,DEPAOLO D J.Plume/lithosphere interaction in the generation of continental and oceanic flood basalts:chemical and isotopic constraints[M]∥MAHONEY J,MILLARD C.Large igneous provinces:continental,oceanic,and planetary flood volcanism.Washington:American Geophysical Union,1997:335-355.
[38]QI L,ZHOU M F.Platinum-group elemental and Sr-Nd-Os isotopic geochemistry of Permian Emeishan flood basalts in Guizhou Province,SW China[J].Chemical Geology,2008,248(1/2):83-103.
[39]PENG Z X,MAHONEY J,HOOPER P,et al.A role for lower continental crust in flood basalt genesis?Isotopic and incompatible element study of the lower six formations of the western Deccan Traps[J].Geochimica et Cosmochimica Acta,1994,58:267-288.
[40]SHERVAIS J W.Ti-V plots and the petrogenesis of modern and ophiolitic lavas[J].Earth and Planetary Science Letters,1982,59(1):101-118.
[41]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(1):11-30.
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