北祁连西段熬油沟组玄武岩地球化学特征及构造意义

Geochemical Characteristics and Tectonic Significance of Aoyougou Group Basalts in Western North Qilian

北祁连造山带是一典型的板块缝合带,其中镶嵌着众多大小不一的由前震旦系变质岩系组成微陆壳残块,为解析北祁连前造山构造过程提供了重要的研究载体。北祁连西段卡瓦—祁青一带分布有较为完整的中元古界长城系朱龙关群熬油沟组火山岩—碎屑岩—碳酸盐岩建造。岩石地球化学研究表明,熬油沟组玄武岩主量元素含量变化较为稳定,具高K_2O(4.27%~6.07%),TFe_2O_3(10.49%~13.01%),Ti O_2(1.96%~2.90%)以及中等的Mg O(5.37%~6.71%,Mg~#48~51),Ca O(2.57%~5.51%),为钾玄碱性玄武岩。岩石Cr和Ni含量随着Mg~#降低而减小、Ca O与Ca O/Al_2O_3呈正相关,明显富集轻稀土元素(LREE)和大离子亲石元素(LILE),轻微正Eu异常,高含量的高场强元素(Nb,Ta,Ti),以及元素对Gd/Yb(2.5~3.0),Zr/Y(8.6~10.2),Ta/Yb(0.96~1.23),Ti/Yb(5 074~6 021),Zr/Yb(81~97),Ce/Nb(1.87),Zr/Nb(6.41)值均揭示了熬油沟组玄武岩具有洋岛玄武岩(OIB)地球化学特征。结合区域地质背景研究表明,熬油沟组碱性玄武岩源于类似于OIB的深部富集地幔,在岩浆作用过程中发生过橄榄石和单斜辉石的分离结晶作用以及一定程度的大陆地壳物质的混染作用,形成于大陆裂谷背景,并认为其是对中元古代Columbia超大陆裂解的响应,进一步为中元古代华北克拉通裂解与Columbia超大陆裂解事件的关系提供了重要的岩石学证据。

The North Qilian orogenic belt is a typical plate suture zone, which is studded with numerous differ- ent sizes of fragments of micro-continental crust consisting of pre-Sinian metamorphic rocks and the North Qilian orogenic belt provides important study carrier tor studying the pre-orogenic tectonic process of the North Qilian. The relatively complete volcanic-c｜astic-carbonate rocks formation of Zhulongguan Group, Changchengian System, Me- soproterozoic are distributed in the Kawa-Qiqing area, Western North Qilian orogenic belt. Geochemical characteris- tics of the Aoyougou Group basalts in the North Qilian indicated that the contents of major element were relatively stable, high K20 contents（4.27%-6.07%）, TFe203（10.49%-13.01%） ,TiO2（ 1.96%-2.90%）and middle MgO （ 5.37 % - 6.71%, Mg# 48 - 51 ）, CaO （ 2.57% - 5.51% ） content. Thus, the Aoyougou Group basalts belong to potas- sium alkaline basalt. The contents of Cr and Ni of the basalts decreased with the decreasing of Mg#, and there was a positive correlation between CaO and CaO/Al2O3, Furthermore, the Eu anomaly of samples was slightly negative, high contents of high field strength elements （HFSE, Nb, Ta, Ti）, and the ratios of Gd/Yb （2.5 - 3.0）, Zr/Y （ 8.6- 10.2）, Ta/Yb（0.96 - 1.23）, Ti/Yb（ 5 074-6 021 ）, Zr/Yb （ 81 -97）, Ce/Nb（ 1.87）, Zr/Nb（ 6.41 ） indi- cated that the Aoyougou Group basahs had obvious OIB-type magmatic affinity. In combination with the regional geological background, the study results showed that Aoyougou Group alkaline basalt generated from deep enrichment mantle similar to the source of OIB, with fractional crystallization of olivine and monoclinic pyroxene and a certain degree of contamination of continental crust in the process of magmatism, formed in the background of continental rift, might response to the Proterozoic Columbia supercontinent ri^ing, which further provides important petrological evidences for the relationship between North China Craton rifting events and Columbia supercontinent rifting during Proterozoic.