青藏高原古新世-始新世早期(65～40Ma)火山岩——同碰撞火山作用的产物  

Paleocene-Early Eocene (65-40 Ma) Volcanic Rocks in Tibetan Plateau:the Products of Syn-Collisional Volcanism

作　　者：夏林圻[1] 马中平[1] 李向民[1] 夏祖春[1] 徐学义[1] 

机构地区：[1]中国地质调查局西安地质矿产研究所,陕西西安710054

出　　处：《西北地质》2009年第3期1-25,共25页Northwestern Geology

基　　金：中国地质调查局综合研究项目(1212010611804)

摘　　要：印度-亚洲大陆的碰撞开始于65Ma左右,大约在45/40Ma完成,之后转入碰撞后阶段至今。碰撞过程(～65～40Ma)中,已消减的新特提斯大洋板片回转,不仅导致会聚速率提高,还诱使青藏岩石圈之下的对流软流圈上涌,并发生减压熔融,产生碰撞期(或同碰撞)火山作用。西藏中部和南部的古新世-始新世早期(～65～40Ma)火山岩即是此碰撞期(或同碰撞)火山作用的产物。该碰撞期(或同碰撞)火山岩系并非是单一的长英质中酸性火山岩,其成分变化很宽,从玄武质到流纹质均有发育。它们源于成分为εNd(t)≈+3、87Sr/86Sr(t)≈0.705和La/Nb≈0.8的软流圈源。根据岩石地球化学数据,古新世-始新世早期基性熔岩可以划分为高Ti/Y(HT,Ti/Y≥500)和低Ti/Y(LT,Ti/Y<500)两个岩浆类型。LT熔岩又可以进一步划分为LT1和LT2等两个亚类。HT和LT1熔岩为未遭受地壳混染的基性熔岩,以具有高Nb/La值(0.88～1.53)和原始地幔标准化分配曲线上缺乏Nb、Ta和Ti负异常为特征;而LT2熔岩却为受到了强烈地壳混染的基性熔岩,其Nb/La值很低(0.20～0.49),Nb、Ta和Ti明显亏损。西藏中部拉嘎拉玄武岩和邦达错碱性玄武岩的化学演化受控于橄榄石(ol)+单斜辉石(cpx)结晶分离作用;而西藏南部林子宗火山岩系的化学变异则是经受了辉长质结晶分离作用。元素和同位素数据表明,青藏高原古新世-始新世早期基性熔岩并不是单一母岩浆结晶分离的产物。遭受地壳混染的LT2熔岩的Sr-Nd同位素变化特点与其软流圈源熔体上升过程中所卷入的不同岩石圈组分有关。下地壳组分的卷入导致典中组、帕那组和拉嘎拉玄武岩的LT2熔岩具有低-负εNd(t)值(+1.3～-3.9)和较低87Sr/86Sr(t)值(0.7046～-0.7065);而达孜基性火山岩和年波组的LT2熔岩则是以具较高87Sr/86Sr(t)值(0.7051～0.7084)和变化幅度较大的εNd(t)值(+5.4～-4.0)为特征,这应该是与受到上地壳混染有关;还有It has been accepted that the India-Asia continental collision started from about 65 Ma and completed at -45-40 Ma, followed by continued post-collisional convergence to the present. During the collision between India and Asia （-65-40 Ma）, the flatly subducted Neo-Tethyan oceanic slab might have undergone roll-back to steeper angles. This roll-back would not only account for the increase in the convergence rate but also induce the upwelling and decompression melting of the convective asthenosphere under Tibetan lithosphere, producing the collisional （or syn-collisional） volcanism. The Paleocene-Early Eocene （-65-40 Ma） volcanic rocks in central and southern Tibet are the products of such collisional （or syn-collisional） volcanism. These collisional （or syn-collisional） volcanic successions have a large compositional range from basaltic to rhyolitic, rather than simplex felsic and are interpreted to be the result from an asthenosphere source with component of εNd （t） ≈-＋3,87Sr/86Sr （t） ≈0.705 and La/Nb≈0.8. On the basis of petrogcochemieal data, the Paleocene-Early Eocene basic lavas can be classified into high-Ti/Y （HT, Ti/Y 500） magma type and low-Ti/Y （LT, Ti/Y 〈 500） magma type that can be further divided into two subtypes： LT1 and LT2. The HT and LT1 lavas are uncontaminated basic lavas characterized by relative high Nb/La ratios （0.88-1.53） and absence of negative anomalies of Nb, Ta, and Ti on mantle-normalized plots; whereas the LT2 lavas are crustally contaminated basic lavas with low Nb/La ratios （0. 20-0.49） and obvious depletions in Nb, Ta, and Ti. The chemical evolution of the Lagala basalts and Bangdaco alkaline basalts （in central Tibet） is controlled by an olivine （ol） ＋ elinopyroxene （cpx） fractionation, but gabbroie fractionation accounts for the chemical variation of the Linzizong volcanic succession from southern Tibet. Elemental and isotopic data suggest that the chemical variation of Tibetan plateau Paleocene-Early Eoeene

关 键 词：古新世-始新世早期火山岩 印度-亚洲大陆碰撞 岩石圈-软流圈相互作用 青藏高原 

分 类 号：P588.14[天文地球—岩石学]