北大山镁铁质—超镁铁质岩体地球化学特征及形成构造环境

GEOCHEMICAL FEATURES AND TECTONIC SETTING OF BEIDASHAN MAFIC-ULTRAMAFIC ROCKS

阿拉善地块南缘是镁铁质—超镁铁质岩体的重要分布区,自北向南可分为北大山岩带、龙首山岩带和北海子岩带。本文主要以北大山岩带为研究对象,通过对野芨里岩体、青疙瘩岩体及鄂博沟岩体的岩石学、岩石地球化学、微量元素、稀土元素等研究,探讨了北大山镁铁质—超镁铁质岩体的成因类型和形成构造环境。3个岩体主量元素特征显示Mg#=76～87,橄榄岩类w（Al2O3）=3.2%～5.2%,但辉长岩、辉绿岩类明显高于该值,说明北大山镁铁质—超镁铁质岩熔融程度整体不高,该区岩体属于镁质超基性岩,并且具有富铁趋势;稀土元素特征显示稀土总量（ΣREE）在5.68×10-6～11.5×10-6之间,野芨里岩体LREE稍富集,而青疙瘩及鄂博沟岩体LREE明显富集;微量元素特征显示北大山镁铁质—超镁铁质岩体的成岩构造环境为板内拉张环境,岩浆在侵位过程中受到上地壳的混染作用,微量元素原始地幔标准化蛛网图显示Ni明显亏损,有利于镍成矿。

Mafic-ultramafic rocks are widely distributed in the southern margin of the Alashan Block. Three rock zones have been recognized from north to south： Beidashan, Longshoushan, and Beihaizi. Detailed petrological and geochemical studies have been conducted on the Yejili, Qinggeda, and Ebogou plutons in the Beidashan mafic-ultramafic rock zone, with an aim to clarify the genesis and tectonic setting of these rocks.Rocks from these plutons yield Mg#values of 76 to 87, and Al2O3 contents of 3.2% to 5.2%, which are both lower than the typical gabbros and diabases, indicating low melting degree of these rocks. Major element analyses suggest that they belong to magnesian ultrabasic rocks, and have Fe-rich affinity. The total REE contents（ΣREE）of these rocks range from 5.68 ×10-6~11.5 ×10-6. The Yejili pluton is weakly enriched in LREEs, while the Qinggeda and Ebogou plutons show pronounced LREE enrichments. Trace elements characteristics suggest that they were formed in an intraplate extensional environment. Upper crust materials were involved during the emplacement of the parental magma. Primitive Mantle（PM） normalized trace element diagrams exhibit obvious depletion of Ni, indicating that Ni was probably extracted from these plutons, which may be in favour of Ni mineralization.