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西藏北冈底斯巴木错安山岩的年代学、地球化学及岩石成因 预览 被引量:32

Geochronology,geochemistry and petrogenesis of the Bamco andesites from the northern Gangdese,Tibet
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摘要 青藏高原南部的冈底斯带中北部地区广泛分布着中生代火山岩及相关侵入岩。由于缺少可靠的年代学和高质量的地球化学数据,使得其成因及地球动力学背景长期以来存在较大争议。为探讨这一问题,本文报道了出露于北冈底斯巴木错东岸安山岩的锆石U-Pb定年、地球化学和Sr-Nd-Hf同位素数据。巴木错安山岩锆石U-Pb年龄为122.1±0.9Ma。岩石为斑状结构,斑晶为斜长石、角闪石和少量辉石、黑云母。岩石SiO2为58.65%~60.16%,全碱(Na2O+K2O)含量为(5.25%~5.52%,平均5.38%),K2O含量为2.67%~2.81%之间,K2O/Na2O均】1,TiO2含量较低(0.61%~0.72%),MgO含量为2.78%~3.10%,Mg#值为45~48。岩石相对富集Rb、Th、U、Pb,亏损Ba、Nb、Ta和Ti等元素,显示出弧特征。样品具有较冈底斯成熟大陆地壳(如宁中早侏罗世强过铝质花岗岩)低的(87Sr/86Sr)t(0.7102~0.7103)和高的εNd(t)(-9.3~-9.8)值,Nd同位素单阶段模式年龄为1.61~1.76Ga;锆石εHf(t)值为-4.6~-0.3,地幔模式年龄tDM为795~965Ma。综合岩石学和地球化学研究表明,巴木错安山岩为高钾钙碱性系列,产于大陆岛弧向同碰撞转换的构造环境。该安山岩可能是班公湖-怒江洋向南俯冲于冈底斯带之下的洋壳在重力拖拉下发生板片回转,导致软流圈物质上升提供热量,从而诱发受到俯冲流体改造的古老岩石圈地幔部分熔融并与壳源酸性熔体混合而成的产物。 The Mesozoic volcanic rocks and associated intrusive rocks are widely distributed in the northern and central Gangdese Belt of southern Tibetan Plateau.The petrogenesis and geodynamic setting of these rocks remain hotly debated so far due to the limitation of reliable geochronological and good quality geochemical data.To explore these issues,we report the petrological,zircon LA-ICPMS U-Pb dating,whole-rcok geochemical,and zircon Hf isotopic data for andesites from eastern Bamco,northern Gangdese.One Bamco andesite sample was dated to be 122.1±0.9Ma.The samples are porphyric,with dominant plagioclase,hornblende phenocrysts,together with minor pyroxene,biotite phenocrysts.They have SiO2 of 58.65%~60.16%,total alkalis(K2O+Na2O)of 5.25%~5.52%,K2O of 2.67%~2.81%,TiO2 of 0.61%~0.72%,MgO of 2.78%~3.10%,with Mg numbers of 45~48,and K2O/Na2O>1.The Bamco andesites are enriched in Rb,Th,U,K and Pb,and depleted in Ba,Nb,Ta,P and Ti,indicative of arc-related rocks.The andesite samples have higher εNd(t)values(-9.3~-9.8),higher zircon εHf(t)values(-4.6~-0.3),and lower initial 87Sr/86Sr ratios(0.7102~0.7103)relative to melts derived from mature continental crust in the Gangdese(e.g.,Ningzhong Early Jurassic strongly peraluminous granite).The ancient Nd model ages(1.61~1.76Ga)and zircon Hf mantle model ages(795~965Ma)suggest an origin of ancient lithosphere mantle for their generation.Petrological and geochemical data reported here indicate that the Bamco andesites are high-K calc-alkaline,and were emplaced at a transitional setting from continental arc to syn-collisional setting.The Bamco andesites may be the products of magma mixing between ancient lithosphere mantle-derived melts and crust-derived felsic melts resulting from rising asthenosphere material following slab rollback.Such a slab can be correlated with the Bangong-Nujiang Ocean seafloor that has been subducted southward towards beneath the Gangdese Belt.
作者 陈越 朱弟成 赵志丹 张亮亮 刘敏 于枫 管琪 莫宣学 CHEN Yue1,ZHU DiCheng1,ZHAO ZhiDan1,ZHANG LiangLiang1,LIU Min1,YU Feng1,GUAN Qi1,2 and MO XuanXue1 1. State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences,School of Earth Science and Mineral Resources,China University of Geosciences,Beijing 100083,China2. College of Resources,Shijiazhuang University of Economics,Shijiazhuang 050031,China
出处 《岩石学报》 SCIE CAS CSCD 北大核心 2010年第7期2193-2206,共14页 Acta Petrologica Sinica
基金 教育部新世纪优秀人才项目(NCET-10-0711) 国家自然科学重点基金(40830317) 国家“973”项目(2009CB427002) 111计划(B07011) 中国地质调查局综合研究项目(1212010610104)联合资助
关键词 锆石U-PB年代学 地球化学 岩石成因 巴木错安山岩 西藏冈底斯 Zircon U-Pb geochronology Geochemistry Petrogenesis Bamco andesites Northern Gangdese Tibet
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