西准噶尔玛依勒地区枕状玄武岩年代学、地球化学及岩石成因

通过对造山带内洋岛玄武岩的时代及地球化学性质研究,不仅可以进行古海山/大洋高原的识别,而且还可以进行古洋盆演化及古构造格局恢复。笔者等最新在西准噶尔玛依勒山北侧识别出一套枕状玄武岩,其与火山碎屑岩、硅质岩共生。枕状玄武岩斜长石微晶普遍发育中空骸晶结构,是在水下熔岩急剧萃冷条件下迅速结晶的产物。通过LA-ICP-MS锆石U-Pb测年,获得枕状玄武岩206Pb/238U加权平均年龄为437.2±2.2Ma,该年龄的获得填补了志留纪碱性玄武岩的空白。岩石地球化学分析结果显示,玛依勒枕状玄武岩为碱性玄武岩系列,岩石具有中等Si O2(44.89%~47.81%),高Ti O2(3.28%~4.12%)及P2O5(0.50%~0.70%),低Mg O(3.49%~6.79%),轻、重稀土元素分异较为明显((La/Yb)N=5.5~7.3),无明显Eu异常(Eu/Eu*=0.96~1.06),相对富集Rb、Th、U,亏损Ba、K、Sr,没有明显Nb、Ta负异常,这些地球化学特征与洋岛玄武岩(OIB)极其相似。微量元素含量及反映源区性质的比值表明,枕状玄武岩来源于富集地幔源区,主要组成为尖晶石和石榴石二辉橄榄岩,并发生了5%±的部分熔融,其形成于大洋板内与地幔柱有关的海山/大洋岛屿环境。结合前人研究,认为西准噶尔乃至古亚洲洋在中古生代洋内俯冲的同时,大洋板内可能存在地幔柱活动。

Geochronology, geochemistry and petrogenesis of pillow basalts from Mayile region in West Junggar

Through studying on age and geochemical properties of oceanic island basalts(OIB) within orogenic belts, we not only identify paleo-seamount/oceanic plateau, but also rebuild paleo-ocean basin evolution and tectonic framework. Recently, the authors distinguished a suit of pillow basalts associated with volcaniclastic rocks and cherts at north Mayile area in West Junggar. The microcrystalline plagioclase in pillow basalts generally developed hollow skeletal structure, which is product that the lava rapidly crystallized underwater in dramatic cold conditions. U-Pb analyses of zircon grains from basalt by laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS) yielded ²⁰⁶Pb/²³⁸U weighted mean age of 437.2±2.2Ma. The age gets to fill the gap of the Silurian alkaline basalts in this region. Geochemically, the Mayile pillow basalts bear the signature of typical OIB, and are characterized by alkaline affinity with moderate concentrations of SiO₂(44.89%~47.81%), high TiO₂(3.28%~4.12%) and P₂O₅(0.50%~0.70%), low MgO(3.49%~6.79%), clear differentiation of LREE and HREE((La/Yb)_N=5.5~7.3), no obvious Eu anomalies(Eu/Eu^*=0.96~1.06), Rb, Th and U enrichment and Ba, K, Sr depletion, and also no obvious Nb and Ta negative anomalies. The content and ratios containing source characteristics of trace elements suggest that the pillow basalts derived from enriched mantle source, with 5%±degree partial melting of spinel and garnet lherzolite. The pillow basalts formed in plume-related seamount/oceanic island settings within oceanic plate. These observations, in combination with previous work, we believe that not only the intra-oceanic subduction system, but also mantle plume activity in the West Junggar Ocean evolution as well as Paleo-Asian Ocean at Middle Paleozoic.