新疆阿尔泰山南缘玛音鄂博断裂南侧变质基性岩的发现及其地质意义

Metabasic Rock on the South Side of Mayin'ebo Fault in the South Margin of Altay Mountains, Xinjiang, and Its Geological Implications

位于额尔齐斯-玛音鄂博大断裂带南侧的阿热勒托别变质基性岩产于下石炭统姜巴斯套组一套细碎屑沉积岩中.岩石组成以变质玄武岩为主,它们具有相对高的TiO2、MgO和低SiO2、K2O的特点;在稀土元素组成上,显示轻稀土弱富集的配分模式,无明显Eu异常;微量元素组成显示大离子亲石元素富集,并具有明显的正Th异常和弱的Nb负异常、高的Nb含量(>2×10-6)以及HFSE基本上和MORB相当的特点;其Zr/Nb比值落在MORB范围之内,而Ti/V比值略比MORB高,表明其为MORB源的亏损地幔在相对较低熔融程度下熔融的产物.其εNd(t)值为+7.40～+8.35,略低于MORB,但高于洋岛以及大陆板内玄武岩.因此该变质基性岩总体上兼有火山弧和洋中脊玄武岩特征,因而其可能形成于弧后环境,代表了弧后盆地扩张早期的产物.

The Areletuobie metabasic rocks occur in the Lower Carboniferous Jiangbasitao Formation, which consists of sedimentary fine clastic sequences on the south side of the Ertix-Mayin＇ebo deep-large fault that marks the boundary between the Altay and Junggar orogenic belts. The metabasic rocks display relatively high TiO2 （1.15%- 2. 20%） and MgO contents （7. 03%- 9.54 %）, and relatively low SiO2 （45.72%- 48. 31% ） and K20 contents （0.13 %-0. 41 % ）. They exhibit slight LREE enrichment on the chondrite-normalized REE patterns without Eu anomalies. Their MORB-normalized trace element patterns are characterized by large ion lithophile element （LILE） enrichment with significantly positive Th anomalies, slightly negative Nb anomalies and high Nb concentrations （2. 6 × 10^-6--8. 7 × 10^-6）, whereas their high field strength element （HFSE） contents resemble those of MORB. The Zr/Nb ratios range from 18. 9 to 32.7, similar to those of MORB. In contrast, their Ti/V ratios are higher than those of MORB. All of these features suggest that they were generated by relatively low degrees of melting of MORB-like depleted mantle source. Their εNd （t） values range from 7. 40 to 8. 35, lower than those of MORB, but higher than those rocks derived from oceanic island and intracontinental basalts. Consequently, the metabasic rocks have the somewhat transitional features of the volcanic rocks between a volcanic arc and an oceanic island. It is therefore inferred that they were formed from a back-arc basin, and represent the products in the early stage of back-arc basin spreading.