西准噶尔达尔布特蛇绿混杂岩中碱性玄武岩的成因：晚泥盆世地幔柱的产物？

Petrogenesis of alkaline basalt from the Darbut ophiolitic mélange in West Junggar: The product of a Late Devonian mantle plume?

西准噶尔是巨型中亚造山带的重要组成部分。该地区分布有多条蛇绿混杂岩带,其中达尔布特蛇绿混杂岩带被认为是西准噶尔规模最大的蛇绿混杂岩带,主要是以方辉橄榄岩为主的超镁铁质岩、镁铁质橄长岩、辉长(绿)岩,枕状熔岩、块状玄武岩及硅质岩。对达尔布特蛇绿混杂岩带中的块状玄武岩进行详细的岩石学、岩相学、同位素年代学及地球化学分析。结果表明,块状玄武岩的LA-ICP-MS锆石U-Pb(加权平均206Pb/238U)年龄为(375±2)Ma,时代属于晚泥盆世。岩石SiO2质量分数为45.24%～49.86%,具有高镁、钛和碱的特征(w(MgO)=2.45%～6.85%,w(TiO2)=1.68%～3.74%,w(Na2O+K2O)=2.37%～5.34%),属于碱性玄武岩系列。碱性玄武岩具有典型洋岛玄武岩的稀土元素和微量元素模式,且相对富集轻稀土元素和大离子亲石元素Rb、Ba、Th等。高场强元素Zr、Hf、Nb、Ta没有明显负异常,显示其来源于富集地幔,主要组成为尖晶石和石榴石二辉橄榄岩,并发生了5%～10%的部分熔融。结合研究区与碱性玄武岩伴生的沉积岩特征、前人资料及对比邻区同期火山岩,可知达尔布特蛇绿混杂岩碱性玄武岩形成于大洋板内与地幔柱有关的洋岛环境,即碱性玄武岩是晚泥盆世地幔柱活动的产物,暗示中亚造山带在晚泥盆世就有地幔柱活动。假若这里提出的晚泥盆世地幔柱模式是正确的,将意味着地幔柱活动在显生宙中亚造山带地壳增生过程中起到重要作用。

As a part of Central Asian Orogenic Belt （CAOB）, the West Junggar is located at the southern mar- gin of the CAOB. Several ophiolitic m^langes were formed and preserved in the West Junggar. The Darbut ophiolitic mfilange （DOM） in the West Junggar is the largest and one of the best-preserved Paleozoic ophiolitic mlanges in the CAOB. Harzburgite, lherzolite, dunite, chromitite, troctolite, gabbro, diabase, pillow lava, massive basalt, and chert occurred as blocks within the serpentinite m^lange. Based on petrology, petrogra- phy, isotopic geochronology and geochemical analysis for the alkaline basalt in the DOM, the alkaline basalt yielded a weighted mean 2~6Pb/23aU age of （375 4-2） Ma by Laser Ablation Inductively Coupled Plasma Mass Spectrometry （LA-ICP-MS）, suggesting a Late Devonian emplacement. Furthermore, the alkaline basalt have typical geochemical characteristics to those of OIB, with moderate SiO2 （45. 24 %- 49. 86 %）, high MgO （2.45 -6.85%）, TiO2 （3. 17%-43.77%） and Na20＋K20 （2.37%-5.34%）, characterized by alkaline basalt. The rare earth element （REE） and trace element distribution patterns are similar to those of oceanic island basalt （OIB）, with light REE （LREE） and large ion lithophile element （LILE, Rb, Ba, Th） enrich- ment, no obvious high field strength element （HFSE, Zr, Hf, Nb, Ta） negative anomaly, indicating the ori gin from enriched mantle that contains 5%-10% spinel and garnet lherzolite partial melting. These observa tions, in combination with sedimentary and volcanic rocks, and previous work in this region, indicate that the alkaline basalt from the DOM might be correlated with Late Devonian mantle plume-related magmatism of ocean island setting within the ocean floor. That is to say, the alkaline basalt from the DOM is the product of a Late Devonian mantle plume, showing that the Late Devonian mantle plume-related magmatism must exist in the CAOB. If the plume model as proposed here is correct, it would suggest that mantle plume activity might have significantly contributed to the crustal growth in the CAOB.