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

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...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 T-DM ranging from 250Ma to 600Ma.SCI(E)中文核心期刊要目总览(PKU)中国科技核心期刊(ISTIC)中国科学引文数据库(CSCD)0051324-13402