摘要
新疆北部中新生代岩石圈深部过程一直备受关注但研究较少。在详细的野外地质和室内岩相学的研究基础上,对西准噶尔克拉玛依侏罗纪玄武岩开展了全岩主元素、微量元素和Sr-Nd同位素以及锆石U-Pb年代学和Hf同位素分析。这些玄武岩与下伏下石炭统太勒古拉组地层呈角度不整合接触关系,具有明显柱状节理、块状构造、斑状结构,斑晶主要由富Ca斜长石、普通辉石和橄榄石组成,部分含有菱铁矿,基质为间粒间隐结构。玄武岩属于碱性玄武岩系列(σ=3.87~4.68),强烈富集大离子亲石元素(如Rb、Ba、K等)和轻稀土元素,没有Eu的负异常(δEu=1.02~1.06),具有相对较低的正εNd(t)值(+2.95^+3.02)和较高的(87Sr/86Sr)i值(0.7048~0.7049),暗示其具有与OIB型源区相似但较为亏损的源区特征。这些玄武岩可能来源于80 km以下深处的石榴石二辉橄榄岩的低程度部分熔融作用。玄武岩中获得锆石的U-Pb谐和年龄为(357.3±5.1)Ma,这一年龄与围岩太勒古拉组的时代相当,指示这些锆石主要来自早期岩浆作用。与晚古生代岛弧及后碰撞基性火山岩不同,侏罗纪玄武岩形成于相对稳定的板内构造环境,其地幔源区亏损程度不及晚古生代火山岩源区。这样的源区可能与俯冲下沉洋壳和拆层岩石圈发生变质重熔形成富集熔体并不断交代亏损地幔有关。
Mesozoic-Cenozoic deep lithospheric process in the northern Xinjiang has been much concerned but not studied in detail. In this paper, systematic studies on Karamay Jurassic basalts in the Western Junggar were carried out, including field geology, petrography, whole-rock major and trace elements, Sr-Nd isotope, zircon U-Pb geochronology and Hf isotope. These basalts unconformably overlie rocks of the Early Carboniferous Tailegula Formation, and show obvious columnar jointing, massive structure and porphyritic texture. The phenocryst minerals are mainly composed of Ca-rich plagioclase, augite, olivine, and siderite for some samples and the groundmass displays intergranular and intersertal texture. They belong to alkaline series （a=3.87-4.68）, and are strongly enriched in large ion lithophile elements （such as Rb, Ba, K） and light rare earth elements, with no Eu anomaly （Eu=l.02-1.06）. They also have a relatively low positive ENd（t） values （＋2.95-＋3.02） and high （87Sr/86Sr）i values （0.7048-0.7049）, demonstrating their mantle source is similar to that of OIB-type basalts. These basalts may be produced by low degree of partial melting in the garnet lherzolite stability field at depths of more than 80 km.Zircon from the Karmay basalts yielded a U-Pb concordant age of （357.3＋5.1） Ma, similar to the age of their country rocks from the Tailegula Formation, indicating that the zircons were inherited from early magmatism. Quite different from the late Palaeozoic island-arc and post-collisional mafic volcanic rocks, the Jurassic basalts were derived from relatively less depleted mantle source in a stable intraplate tectonic environment. This mantle source was related to metasomatism of enriched melt, which were produced by metamorphic remelting of the subducting slab and sinking lithosphere.
出处
《地球化学》
CAS
CSCD
2013年第1期29-45,共17页
Geochimica
基金
国家自然科学基金(40803007,41072047)
中央高校基本科研业务费专项资金资助项目(CUG090101)
关键词
玄武岩
中新生代
OIB型源区
板内环境
西准噶尔
basalt
Mesozoic-Cenozoic
OIB-type mantle source
intraplate setting
western Junggar
