西天山托云盆地及周边中新生代岩浆活动的岩石学、地球化学与年代学研究  

Cretaceous-Paleogene alkaline magmatism in Tuyon basin,southwest Tianshan mountains:geochronology,petrology and geochemistry.

作　　者：季建清[1] 韩宝福[1] 朱美妃[2] 储著银[3] 刘玉琳[1] 

机构地区：[1]北京大学造山带与地壳演化教育部重点实验室,北京100871 [2]台湾大学地质科学系 [3]中国科学院地质与地球物理研究所,北京100029

出　　处：《岩石学报》2006年第5期1324-1340,共17页Acta Petrologica Sinica

基　　金：'国家重点基础研究发展计划(973)'之'中国西部中亚型造山与成矿'二级专题'后碰撞地壳垂向生长过程(编号:2001CB409802)'之研究成果;国家自然科学基金项目(49872078)的资助.

摘　　要：天山西南部白垩纪-老第三纪发育的托云盆地及其周边出露的岩浆岩是一套完整的碱性岩浆岩系列,包含了苦橄质玄武岩、玄武岩、碧玄岩、碱玄岩(橄榄玄武岩、黑云母辉长二长岩、辉长辉绿岩、辉石橄榄岩)和响岩等多种岩石类型。野外工作显示有火山喷出岩和侵入岩两种不同的产状。年代学结果指示岩浆岩形成于120～50Ma间,为晚白垩世-老第三纪盆地形成演化阶段岩浆活动的产物。分离结晶作用是岩浆演化和岩浆系列形成最主要的因素,托云岩浆岩大致经历了结晶分异过程的两个阶段:早期苦橄质岩浆中橄榄石、尖晶石的结晶分离,表现为MgO和微量元素Cr含量随SiO_2含量增加大幅度的降低;晚期主要是单斜辉石、斜长石和钛铁矿等矿物的结晶分异,以CaO、FeO、TiO_2等随SiO_2含量增加大幅度的降低为特点。苦橄质岩石的出现指示了地幔较高温熔融事件的存在,进而为托云盆地地幔柱的存在提供了有力的证据。无论如何,碱性岩浆的活动表明托云盆地形成的大地构造背景是大陆主动裂谷环境,对应的深部背景为区域性的地幔柱构造。首次发现的响岩是结晶分异作用的最终产物。响岩较极端地指示了岩浆结晶分离过程对岩浆演化的巨大影响。托云岩浆岩的同位素特征指示其源区是一个接近于PREMA地幔,但微量元素特征显示其受地壳流体交代改造的特点。岩浆岩的Nd同位素T_(DM)集中在250～600Ma之间,反映了一个古生代时期形成的新生岩石圈地幔,与新疆北部地区的晚古生代新生岩石圈地幔的事实相符。There is a series of Cretaceous-Paleogene alkaline extrusives and intrusive units in Tuyon basin in southwestern Tienshan mountains, which include picrobasalt, basalt, basanite, tephrite, phonotephrite, and phonolite emplaced primarily in Cretaceous- Paleogene rocks. The rocks yield K-Ar ages between 120Ma and 50Ma. The characteristics of major and trace elements indicate that fractional crystallization play important influences on the magma evolutionary process. Two fractional crystallization processes result in the magma evolution from picrobasalt to phonolite. First, from picrobasalt to basalt, olivine, ilmenite and spinel are major fractionating minerals from the primary magma; the second, clinopyroxene and plagioclase as the major fractionating minerals from basalt, basanite, tephrite to phonotephrite. As the primary product of mantle melting, the picrobasalt indicates that a mantle plume or upwelling even occurred beneath the Tuyon basin. The phonolite is the last product of crystallization differentiation indicating that alkaline igneous rocks, even alkaline granite and A-type granitic rocks, may be contributed by crystallization differentiation from picrobasalt. Major and trace element analysis combined with Sr, Nd and Pb isotopic data suggest that these rocks were derived from apparently ocean island basalts （OIB）-like mantle melts, which have characteristic close to asthenospheric PREMA, mixed with an enriched component in the continental mantle lithosphere or emplaced with some crustal contamination, though the REE partern and trace element spide diagrams obviously display that the mantle reservoir have been modified by metasomatic fluids. However, it is likely to conclude that the mantle reservoir is a paleozoic fresh lithospheric mantle based on Nd isotopic TDM ranging from 250Ma to 600Ma.

关 键 词：碱性岩浆岩 地幔柱 岩浆系列 晚白垩-老第三纪 托云 

分 类 号：P588.145[天文地球—岩石学] P597.2[天文地球—地质学]