天山及邻区石炭纪—早二叠世裂谷火山岩岩石成因  

Petrogenesis of Caboniferous-Early Permian Rift-Related Volcanic Rocks in the Tianshan and its Neighboring Areas,Northwestern China

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

机构地区：[1]西安地质矿产研究所,陕西西安710054

出　　处：《西北地质》2008年第4期1-68,共68页Northwestern Geology

基　　金：中国地质调查局综合研究项目(编号:1212010611804);国家自然科学基金项目(编号:40472044)

摘　　要：中国西北部天山石炭纪—早二叠世裂谷火山作用代表了一个新近被认可的大火成岩省,其分布范围至少有170万km2。该火山岩系主要由玄武质熔岩组成,其次有中性和酸性熔岩及火山碎屑岩。它们是地幔柱活动的产物,该地幔柱的组分为:εNd(t)≈+5,87Sr/86Sr(t)≈0.704和La/Nb≈0.9。根据岩石地球化学数据,石炭纪—早二叠世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熔岩又可进一步划分为LT1、LT2、LT3和LT4等4个亚类。LT1、LT2(天山中段和甘肃北山)、LT4(天山西段、新疆北山和准噶尔)和LT3、HT(塔里木)熔岩的化学演化系受控于橄榄石(ol)+单斜辉石(cpx)结晶分离作用;而天山东段的的LT4熔岩的化学变异则是经受了辉长岩质结晶分离作用。元素和同位素数据表明,天山及邻区石炭纪早二叠世裂谷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LT3和LT4熔岩的Sr-Nd同位素变化特点与其地幔柱源熔体上升喷发所通过的岩石圈的性质有关。古老(前寒武纪)岩石圈的卷入,导致天山西段的石炭纪LT4熔岩和柯坪裂谷的早二叠世LT3熔岩具有低-负εNd(t)值(-2.91～+6.1)和中等—高87Sr/86Sr(t)值(0.7036～0.7081);相反,天山东段和准噶尔的石炭纪LT4熔岩是以高εNd(t)值(+4.2～+9.7)和低87Sr/86Sr(t)值(0.7035～0.7044)为特征,这乃是与其遭受了含有早古生代—泥盆纪弧-盆系火山岩的上地壳的混染有关,或者是与其岩石圈地幔源区遭受前石炭纪消减富集有关。天山及邻区石炭纪—早二叠世裂谷基性熔岩中观察到的地球化学变异与AFC作用一致。天山及邻区石炭纪—早二叠世裂谷火山岩显示时间上和空间上的岩石地球化学变化。石炭纪时,未遭受混染的石炭纪LT1熔岩和受到轻微混染的石炭纪LT2熔岩喷发于天山中段裂谷,而遭受强烈混染的石炭纪LT4熔岩则喷发于天山中段裂谷The Tianshan Carboniferous-Early Permian rift-related volcanism in northwestern China represents a newly-recognized large igneous province extending over at least 1. 7 X 106 kmz. The volcanic successions comprise thick piles of basaltic lavas and subordinate intermediate and silicic lavas and pyroclastics, and are interpreted to result from a mantle plume with component of εNd （t）≈＋5, 87Sr/S6Sr （t） ≈0. 704 and La/Nb ≈ 0. 9. On the basis of petrogeochemical data, the Carboniferous-Early Permian basic lavas can be classified into high-Ti/Y （HT, Ti/Y〉500） and low-Ti/Y （LT, Ti/Y 〈 500） magma types. The LT lavas can be further divided into four subtypes： LT1, LT2, LT3 and LT4. The chemical evolution of the LT1, LT2 （in central Tianshan and Baishan of Gansu）, LT4 （in western Tianshan, Jungar and Baishan of Xinjiang） and LT3, HT （Tarim） lavas is controlled by an olivine （ol） ＋ clinopyroxene （cpx） fractionation, but gabbroic fractionation accounts for the chemical variation of the LT4 lavas from eastern Tianshan. Elemental and isotopic data suggest that the chemical variation of the Carboniferous-Early Permian rift-related basic lavas in the Tianshan and their neighboring areas can not be explained by crystallization from a common parental magma. The Sr-Nd isotopic variation of the crustally contaminated LT3 and LT4 lavas is related to the nature of lithosphere through which the plume-derived melts have erupted. The involvement of an older （Precambrian） lithosphere led the Carboniferous LT4 lavas in western Tianshan and the Early Permian LT3 lavas in Keping rift to have lower to negative εNd （t） （-2. 91 to＋6.1） and middle to high 87Sr/86Sr （t） （0. 7036-0. 7081）, whereas the Carboniferous LT4 lavas from eastern Tianshan and Jungar are characterized by high εNd （t） （.1.1.4.2 to ＋9.7） and low 87Sr/S6 Sr （t） （0. 7035-0. 7044）, that are related to the contamination of upper crust containing early Paleozoic and Devonian arc-basin vol

关 键 词：天山及邻区石炭纪-早二叠世裂谷火山岩 岩石成因 地球化学 地幔柱 地壳混染 大火成岩省 中国西北 

分 类 号：P581[天文地球—岩石学]