西藏纳木错早白垩世流纹岩锆石U-Pb年龄、地球化学及其构造意义
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摘要
西藏纳木错流纹岩构造上处于北冈底斯南缘,地层角度不整合于中—晚侏罗世拉贡塘组之上,归属于卧荣沟组。其中流纹岩的LA-ICP-MS锆石U-Pb年龄为120～112 Ma,时代为早白垩世中期。岩石具高Si O_2、富Al_2O_3和K_2O,贫Ti O_2、MgO、Ca O特征,A/CNK=0.94～1.91(平均1.16),属于高钾钙碱性-钾玄岩系列过铝质岩石。(La/Yb)_N为1.49～11.08,δEu=0.12～0.55,球粒陨石标准化稀土元素分布模式显示为略右倾V字形。大离子亲石元素及高场强元素Rb、K、Th、Ta、Ce、Zr、Hf相对富集,Nb相对亏损,Ba、Sr、P、Ti强烈亏损,成岩岩浆源自地壳,可以与多尼组酸性火山岩对比。流纹岩具同碰撞型火山特征,不具传统岛弧环境特征,研究认为在早白垩世中期(120 Ma左右),北冈底斯发生了一次重大的构造事件,即班公湖—怒江新特提斯洋俯冲已经结束,冈底斯带与羌塘地块陆-陆碰撞开始,碰撞过程中下地壳增厚并部分发生熔融,纳木错流纹岩是碰撞后部分熔融的产物。
The Namtso rhyolite is located in the southern margin of the North Gangdese Belt.It belongs to the Woronggou Formation and overlies the Middle-Late Jurassic Lagongtang Formation along an angular unconformity.The rhyolite is LA-ICP-MS zircon U-Pb dated to be Early Cretaceous(120-112 Ma).The rocks are characterized by high SiO_2,Al_2O_3and K_2O,and low Ti O_2,MgO and CaO,A/CNK=0.94-1.91(average 1.16).The rhyolite is high-K calc-alkaline and peraluminous.The chondrite-normalized patterns show a gently right-inclining((La/Yb)_N=1.49 to 11.08)"V"shape and weakly negative Eu anomalies(δEu=0.12 to 0.55).The elements of Rb,K,Th,Ta,Ce,Zr and Hf are enriched,while Nb and Ba,Sr,P and Ti are strongly depleted,suggesting a crustal inheritance.The geochemistry can be compared with the Duoni Group felsic volcanic rocks.The Namtso rhyolite does not have the characteristics of typical island arc rhyolites,but mimics those of collisional felsic volcanic rocks.We considered that during the middle Early Cretaceous(ca.120 Ma),a major tectonic event may have occurred in the North Gangdese Belt,i.e.,subduction of Neo-Tethys Ocean in the Bangong Lake and Nujiang River region had ended,and the continent-continent collision in the Gangdese and Qiangtang region had begun.During the collision,the lower crust may have thickened and partially melted,forming the Namtso rhyolite.
The Namtso rhyolite is located in the southern margin of the North Gangdese Belt.It belongs to the Woronggou Formation and overlies the Middle-Late Jurassic Lagongtang Formation along an angular unconformity.The rhyolite is LA-ICP-MS zircon U-Pb dated to be Early Cretaceous(120-112 Ma).The rocks are characterized by high SiO_2,Al_2O_3and K_2O,and low Ti O_2,MgO and CaO,A/CNK=0.94-1.91(average 1.16).The rhyolite is high-K calc-alkaline and peraluminous.The chondrite-normalized patterns show a gently right-inclining((La/Yb)_N=1.49 to 11.08)"V"shape and weakly negative Eu anomalies(δEu=0.12 to 0.55).The elements of Rb,K,Th,Ta,Ce,Zr and Hf are enriched,while Nb and Ba,Sr,P and Ti are strongly depleted,suggesting a crustal inheritance.The geochemistry can be compared with the Duoni Group felsic volcanic rocks.The Namtso rhyolite does not have the characteristics of typical island arc rhyolites,but mimics those of collisional felsic volcanic rocks.We considered that during the middle Early Cretaceous(ca.120 Ma),a major tectonic event may have occurred in the North Gangdese Belt,i.e.,subduction of Neo-Tethys Ocean in the Bangong Lake and Nujiang River region had ended,and the continent-continent collision in the Gangdese and Qiangtang region had begun.During the collision,the lower crust may have thickened and partially melted,forming the Namtso rhyolite.
引文
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[36]郑有业,次琼,吴松,等.西藏班公湖—怒江成矿带荣嘎斑岩型钼矿床的发现及意义[J].地球科学,2017,42(9):1441-1453.
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(1)中国地质科学院地质力学研究所. 1∶25万当雄幅区域地质调查报告. 2003.
(2)四川省地质矿产勘查开发局区域地质调查队. 1∶5万尼玛幅、孔玛幅、弄清幅、果业多幅区域地质调查报告. 2015.
(3)四川省地质矿产勘查开发局区域地质调查队. 1∶5万文布当桑幅、物玛错幅、阿里淌幅、绒玛幅幅区域地质调查报告. 2016.
[2]王立全,潘桂棠,丁俊,等.青藏高原及邻区地质图及说明书(1∶500000)[M].北京:地质出版社,2013:56-57.
[3]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.
[4]LUDWING K R. Isoplot/Ex Version 3. 00:A Geochronological Toolkit for Microsoft Excel[M]. Berkeley:Berkeley Geochronology Center,2003:1-70.
[5]GUNNARSSON B,MARSH B D,TAYLOR JR H P. Generation of Icelandic rhyolites:Silicic lavas from the Torfajo Kull central volcano[J]. Journal of Volcanology and Geothermal Research,1998,83:1-45.
[6]THY P,BEARD J S,LOFGREN G E. Experimental constraints on the origin of Icelandic rhyolites[J]. The Journal of Geology,1990,98:417-421.
[7]丁烁,黄慧,牛耀龄,等.东昆仑高Nb-Ta流纹岩的年代学、地球化学及成因[J].岩石学报,2011,27(12):3603-3614.
[8]莫宣学,赵志丹,邓晋福,等.印度—亚洲大陆主碰撞过程的火山作用响应[J].地学前缘,2003,10(3):135-148.
[9]BARBARIN B. A review of the relationships between granitoid types,their origins and their geodynamic environments[J].Lithos,1999,46(3):605-626.
[10]PEARCE J A,HARRIS N B W,TINDLE A G. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks[J]. Journal of Petrology,1984,25(4):956-983.
[11]MASUDA A,NAKAMURA N,TANAKA T. Fine structures of mutually normalized rare-earth patterns of chondrites[J].Geochimica et Cosmochimica Acta,1973,37(2):239-248.
[12]SUN S S,MCDONOUGH W F. Chemical and isotopic systematics of oceanic basalts:implications for mantle composition and processes[M]//SAUNDERS A D,NORRY M J. Magmatism in Ocean Basins. London:Geological Society of London Specical Publication,1989,42:313-345.
[13]CONDIE K C. Mantle Plume and Their Record in Earth History[M]. London:Cambridge University Press,2001.
[14]ROLLINSON H R. Using Geochemical Data:Evaluation,Presentation,Interpretation[M]. New York:Longman Group U K Ltd,1993:352.
[15]迟清华,鄢明才.应用地球化学元素丰度数据手册[M].北京:地质出版社,2007:103-104.
[16]肖庆辉,邓晋福,马大铨,等.花岗岩研究思维与方法[M].北京:地质出版社,2002:12-20.
[17]WEDEPOHL K H. The composition of the continental crust[J]. Geochimica et Cosmochimica Acta,1995, 59:1217-1232.
[18]程银行,李影,刘永顺,等.松辽盆地西缘早白垩世伸展事件:流纹岩锆石U-Pb年龄、地球化学研究[J].地质学报,2016,90(12):3492-3507.
[19]GREEN T H. Significance of Nb/Ta as an indicator of geochemical processes in the crust-mantle system[J]. Chemical Geology,1995,120(3):347-359.
[20]BARTH M G,MCDONOUGH W F,RUDNICK R L. Tracking the budget of Nb and Ta in the continental crust[J]. Chemical Geology,2000,165(3):197-213.
[21]BEA F,ARZAMASTSEV A,MONTERO P,et al. Anomalous alkaline rocks of Soustov, Kola:evidence of mantle-derived metasomatic fluids affecting crustal materials[J]. Contribution to Mineralogy and Petrology,2001,140:554-566.
[22]常承法,郑锡澜.中国西藏南部珠穆朗玛峰地区地质构造特征以及其青藏高原东西向诸山系形成的探讨[J].中国科学(D辑),1973(2):190-201.
[23]潘桂棠,郑海祥,徐耀荣.初论班公湖—怒江结合带[M]//地质矿产部.青藏高原地质文集(12).北京:地质出版社,1983:229-242.
[24]潘桂棠,陈智梁,李兴振,等.东特提斯地质构造形成演化[M].北京:地质出版社,1997:1-218.
[25]肖序常,李廷栋.青藏高原的构造演化与隆升机制[M].广州:广东科技出版社,2000:239-268.
[26]潘桂棠,王立全,朱弟成.青藏高原区域地质调查中几个重大科学问题的思考[J].地质通报,2004,23(1):12-19.
[27]YIN A,HARRISON T M. Geologic evolution of the HimalayanTibetan orogen[J]. Annual Review of Earth and Planetary Sciences,2000,28:211-280.
[28]邱瑞照,周肃,邓晋福,等.西藏班公湖—怒江西段舍马拉沟蛇绿岩中辉长岩年龄测定:兼论班公湖—怒江蛇绿岩带形成时代[J].中国地质,2004,31(3):262-268.
[29]李波,胡道功,林广奇,等.西藏甲玛矿区侵入岩成岩成矿年龄[J].现代地质,2016,30(6):1234-1243.
[30]潘桂棠,莫宣学,侯增谦,等.冈底斯造山带的时空结构及演化[J].岩石学报,2006,22(3):521-523.
[31]康志强,许继峰,王保弟,等.拉萨地块北部白垩纪多尼组火山岩的地球化学:形成的构造环境[J].地球科学,2009,34(1):89-104.
[32]FAN J J,LI C,WANG M,et al. Reconstructing in space and time the closure of the middle and western segments of the Bangong-Nujiang Tethyan Ocean in the Tibetan Plateau[J]. International Journal of Earth Science,2018,107:231-249.
[33]LI G M,QIN K Z,LI J X,et al. Cretaceous magmatism and metallogeny in the Bangong-Nujiang metallogenic belt,central Tibet:Evidence from petrogeochemistry, zircon U-Pb ages, and Hf-O isotopic compositions[J]. Gondwana Research,2017,41:110-127.
[34]丁帅,唐菊兴,郑文宝,等.西藏拿若斑岩型铜(金)矿含矿岩体年代学、地球化学及地质意义[J].地球科学,2017,42(1):1-23.
[35]张志,宋俊龙,唐菊兴,等.西藏嘎拉勒铜金矿床的成岩成矿时代与岩石成因:锆石U-Pb年龄、Hf同位素组成及辉钼矿Re-Os定年[J].地球科学,2017,42(6):862-880.
[36]郑有业,次琼,吴松,等.西藏班公湖—怒江成矿带荣嘎斑岩型钼矿床的发现及意义[J].地球科学,2017,42(9):1441-1453.
[37]LI Y,HE J,WANG C S,et al. Late Cretaceous K-rich magmatism in central Tibet:Evidence for early elevation of the Tibetan plateau?[J]. Lithos,2013,160/161:1-13.
[38]LI Y L,HE H Y,WANG C S,et al. Early Cretaceous(ca.100 Ma)magmatism in the southern Qiangtang subterrane,central Tibet:Product of slab breakoff?[J]. International Journal of Earth Science,2017,106:1289-1310.
[39]WANG Q,ZHU D C,LIU A L,et al. Survival of the Lhasa Terrane during its collision with Asia due to crust-mantle coupling revealed by ca. 114 Ma intrusive rocks in western Tibet[J].Lithos,2018,304/305/306/307:200-210.
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