西藏神公地区典中组火成岩锆石U-Pb年龄及地球化学特征
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摘要
新特提斯洋俯冲消减、印度-亚洲板块碰撞导致西藏冈底斯构造带分布大量火成岩,其形成年龄尚存在一定争议,形成时代及构造意义对揭示板块运动信息具有重要意义。对神公地区典中组火成岩样品分别进行了锆石U-Pb同位素测年和元素地球化学分析,结果表明,典中组底顶部火成岩锆石U-Pb年龄值分别为65.37±0.58Ma和57.42±0.20Ma;该套火成岩主要为钙碱性系列岩石,稀土元素配分曲线表现为轻稀土元素富集、重稀土元素亏损的右倾形态,且负Eu异常明显,K_2O/Na_2O值平均为1.57,微量元素Rb/Sr值平均为1.78,Nb/Ta值平均为13.28,Rb、Th、U、Pb等大离子亲石元素富集,Nb、Ta、Ti亏损。由此认为,典中组火成岩是在俯冲碰撞背景下,来自地幔和地壳的岩浆以不同比例混合形成的,可能指示了新特提斯洋关闭引起的洋壳俯冲作用。
The subduction of the Neo-Tethyan Ocean and the collision of the India-Asia plate resulted in the distribution of large amounts of igneous rocks in the Gangdise tectonic belt in Tibet, whose age remains in dispute, and the exact age and tectonic significance are important for revealing the information of plate movement. In this study, the U-Pb isotopic age of zircon samples from the Dianzhong Formation in Shongong area was determined, and the geochemical analysis of major elements, trace elements and REE was carried out. The result indicates that the age of igneous rocks at the bottom and top of the Dianzhong Formation is65.37±0.58 Ma and 57.42±0.20 Ma, respectively. The distribution curve of REE shows the rightward form of LREE concentration and HREE loss, with obvious negative Eu anomalies. The average ratio of K_2 O/Na_2 O, Rb/Sr and Nb/T is 1.57, 1.78 and 13.28,respectively. The Dianzhong Formation igneous rocks in Shongong area are enriched in Rb, Th, U and Pb and depleted in Nb, Ta and Ti. The authors hold that the igneous rocks of the Dianzhong Formation were formed by magmatic mixing of the mantle and the crust in different proportions in the background of subduction collision, which may indicate the subduction of oceanic crust caused by the closure of the Neo-Tethyan Ocean.
The subduction of the Neo-Tethyan Ocean and the collision of the India-Asia plate resulted in the distribution of large amounts of igneous rocks in the Gangdise tectonic belt in Tibet, whose age remains in dispute, and the exact age and tectonic significance are important for revealing the information of plate movement. In this study, the U-Pb isotopic age of zircon samples from the Dianzhong Formation in Shongong area was determined, and the geochemical analysis of major elements, trace elements and REE was carried out. The result indicates that the age of igneous rocks at the bottom and top of the Dianzhong Formation is65.37±0.58 Ma and 57.42±0.20 Ma, respectively. The distribution curve of REE shows the rightward form of LREE concentration and HREE loss, with obvious negative Eu anomalies. The average ratio of K_2 O/Na_2 O, Rb/Sr and Nb/T is 1.57, 1.78 and 13.28,respectively. The Dianzhong Formation igneous rocks in Shongong area are enriched in Rb, Th, U and Pb and depleted in Nb, Ta and Ti. The authors hold that the igneous rocks of the Dianzhong Formation were formed by magmatic mixing of the mantle and the crust in different proportions in the background of subduction collision, which may indicate the subduction of oceanic crust caused by the closure of the Neo-Tethyan Ocean.
引文
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[12]付文春,康志强,潘会彬.西藏冈底斯带西段狮泉河地区林子宗群火山岩地球化学特征、锆石U-Pb年龄及地质意义[J].地质通报, 2014, 33(6):850-859.
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[14]杨辉,向树元,王欣,等.西藏马乡地区典中组年龄厘定及其构造背景[J].地质科技情报, 2013, 32(4):89-96.
[15]韩吟文,马振东.地球化学[M].北京:地质出版社, 2003:181-202.
[16]Sun S S, Mcdonough W F. Chemical and isotopic systematics of oceanic basalt:Implications for mantale composition and processes[C]//Saunders A D, Norry M J. Magmatism in the Ocean Basins.London:Geological Society of London special Publications, 1989,42:313-345.
[17]Rollision H R, Yang X M, Yang X Y, et al. Using geochemical date:evaluation, presentation, interpretation[M]. Hefei:University of Science and Technology Press, 2000.
[18]贾建称,温长顺,王根厚,等.冈底斯地区林子宗群火山岩岩石地球化学特征及地球动力学意义[J].中国地质, 2005,(3):396-404.
[19]Taylor S R, Mclennan S M. The continental crust:Its composition and evolution[M]. New York:Oxford University Press, 1985:1-328.
[20]Condie K C. Mantle Plume and Their Recod in Earth History[M].London:Cambridge University Press, 2001.
[2]莫宣学,赵志丹,邓晋福,等.西藏林周盆地林子宗火山岩及其反演的印度-欧亚大陆碰撞过程[J].地学前缘, 2003,(3):135-148.
[3]刘安琳,朱弟成,王青,等.藏南米拉山地区林子宗火山岩LAICP-MS锆石U-Pb年龄和起源[J].地质通报, 2015, 34(5):826-833.
[4]周肃,莫宣学,董国臣,等.西藏林周盆地林子宗火山岩40Ar/39Ar年代格架[J].科学通报, 2004,(20):2095-2103.
[5]聂国永,钟康惠,苟金,等.西藏自治区堆龙德庆县马区典中组底砾岩特征及其地质构造意义[J].四川有色金属, 2012,(1):37-41.
[6]胡新伟,马润则,陶晓风,等.西藏措勤地区典中组火山岩地球化学特征及构造背景[J].成都理工大学学报(自然科学版), 2007,(1):15-22.
[7]梁银平,朱杰,次邛,等.青藏高原冈底斯带中部朱诺地区林子宗群火山岩锆石U-Pb年龄和地球化学特征[J].地球科学——中国地质大学学报, 2010, 35(2):211-223.
[8]莫宣学.青藏高原新生代碰撞-后碰撞火成岩[M].北京:地质出版社, 2009:1-50.
[9]Liu Yongsheng, Gao Shan, Hu Zhaochun, et al. Continental and oceanic crust recycling-induced melt-peridotite interactions in the Trans-North China Orogen:U-Pb dating, Hf isotopes and trace elements in zircons of mantle xenoliths[J]. Journal of Petrology,2010, 51(1/2):537-571.
[10]吴元保,郑永飞.锆石成因矿物学研究及其对U-Pb年龄解释的制约[J].科学通报, 2004,(16):1589-1604.
[11]Le Maitre R W, Bateman P, Dudek A, et al. 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]. London:Blaskwell Scientific Publications, 1989.
[12]付文春,康志强,潘会彬.西藏冈底斯带西段狮泉河地区林子宗群火山岩地球化学特征、锆石U-Pb年龄及地质意义[J].地质通报, 2014, 33(6):850-859.
[13]李再会,郑来林,李军敏,等.冈底斯中段林子宗火山岩岩石地球化学特征[J].矿物岩石地球化学通报, 2008(01):20-27.
[14]杨辉,向树元,王欣,等.西藏马乡地区典中组年龄厘定及其构造背景[J].地质科技情报, 2013, 32(4):89-96.
[15]韩吟文,马振东.地球化学[M].北京:地质出版社, 2003:181-202.
[16]Sun S S, Mcdonough W F. Chemical and isotopic systematics of oceanic basalt:Implications for mantale composition and processes[C]//Saunders A D, Norry M J. Magmatism in the Ocean Basins.London:Geological Society of London special Publications, 1989,42:313-345.
[17]Rollision H R, Yang X M, Yang X Y, et al. Using geochemical date:evaluation, presentation, interpretation[M]. Hefei:University of Science and Technology Press, 2000.
[18]贾建称,温长顺,王根厚,等.冈底斯地区林子宗群火山岩岩石地球化学特征及地球动力学意义[J].中国地质, 2005,(3):396-404.
[19]Taylor S R, Mclennan S M. The continental crust:Its composition and evolution[M]. New York:Oxford University Press, 1985:1-328.
[20]Condie K C. Mantle Plume and Their Recod in Earth History[M].London:Cambridge University Press, 2001.