北疆阿尔泰南缘泥盆系浅变质碎屑沉积岩地球化学特征及其形成环境  

Geochemical characteristics and sedimentary environments of Devonian low metamorphic clastic sedimentary rocks in the southern margin of the Chinese Altai,North Xinjiang.

作　　者：龙晓平 袁超[1] 孙敏[2] 肖文交[3] 林寿发[4] 王毓婧[1] 蔡克大[2] 

机构地区：[1]中国科学院广州地球化学研究所,广州510640 [2]香港大学地球科学系,中国香港薄扶林道 [3]中国科学院地质与地球物理研究所岩石圈构造演化重点实验室,北京100029 [4]滑铁卢大学地球科学系,加拿大滑铁卢ONN2L 3G1

出　　处：《岩石学报》2008年第4期718-732,共15页Acta Petrologica Sinica

基　　金：国家973项目(2007CB411308);自然科学基金(40772130,40572043);香港RGC项目(HKU7040/04P);香港大学研究项目(200711159058);中国科学院知识创新工程领域前沿项目(GIGCX-07-03)资助成果。

摘　　要：对于北疆阿尔泰地区泥盆纪所处的大地构造环境，目前仍旧存在不同观点。前人基于阿尔泰南缘泥盆纪火山岩地球化学研究，分别提出了活动大陆边缘和被动大陆边缘裂谷等不同构造观点。阿尔泰造山带南缘的泥盆纪浅变质碎屑沉积岩地球化学研究表明，该套浅变质碎屑沉积岩原岩主要为泥质或砂质沉积岩。尽管不同岩性样品主量元素含量不同，但其化学蚀变指数（CIA）小于75，成分变异指数（ICV）接近或小于1．0，斜长石蚀变指数（PIA）平均70，说明其源区物质比较新鲜，成熟度相对较低，化学风化作用较弱。同样，不同岩性样品微量元素含量差别较大，但表生过程中不活泼的微量元素比值却比较一致，轻稀土（LREE）中度富集（LaN／YbN=2．88-9．90），重稀土（HREE）比较平坦，并伴有明显的Eu负异常（Eu／Eu^＊=0．45-0．89）。绝大多数样品具有高的La／Sc（1～3）、La／Y（0．5～1）和Ti／Zr（10～35），以及较低的Sc／Cr（0．1～0．3）比值，类似于大陆岛弧相关环境碎屑沉积物。在La—Th—Sc和Th—Sc—Zr／10构造环境判别图解中，除一千枚岩样品外，其他所有样品均落入大陆岛弧区。以上地球化学特征明显不同于大洋岛弧和被动陆缘沉积物，说明该套浅变质碎屑沉积岩可能沉积于活动大陆边缘的大陆岛弧相关环境，为认识阿尔泰造山带泥盆纪岛弧增生构造演化过程提供了一个重要证据。Devonian is a key period for the tectonic evolution of the Chinese Altai and several competitive models, e, g. active continental margins and passive continental margins, have been suggested for this region. These models were mainly based on studies of the Devonian volcanic rocks in the southern margin of the Chinese Altai, and geochemical research for the sedimentary rocks is still rare. In this paper, systematic geochemical study for the Devonian low-grade metamorphosed clastic rocks was conducted to constrain the tectonic background of this orogen. The major element data show that protoliths for the metaclastic rocks are dominated by argillaceous and arenaceous sedimentary rocks. Although differing in major element contents among rocks of various types, their low Chemical Index of Alteration （CIA 〈 75 ）, Plagioclase Index of Alteration （mean PIA - 70） and Index of Compositional Variability （ ICV ≤ 1.0） indicate relatively low maturity and weak chemical weathering for their source. The samples are moderately enriched in light REE （ LaS/YbN = 2.88 - 9.90） and show flat heavy REE with strong negative Eu anomalies （ Eu/Eu ^＊ = 0.45 - 0.89）. Most samples have high La/Sc （ 1 - 3 ） , La/Y （0.5 - 1 ） , Ti/Zr （ 10 - 35 ） and low Sc/Cr （0.1 - 0, 3 ） ratios, which are similar to clastic rocks deposited in continental arc settings. In both La-Th-Sc and Th-Sc-Zr/10 discriminate diagrams, almost all of the clastic rocks plot in the continental arc field, These geochemical characteristics are different from those of sedimentary rocks deposited in oceanic island arcs or passive continental margins and suggest a sedimentary environment related to continental island arcs for the Devonian clatic rocks in the southern margin of the Chinese Altai,

关 键 词：地球化学 碎屑沉积岩 阿尔泰造山带 北疆 构造环境 

分 类 号：P588.21[天文地球—岩石学] P595[天文地球—地质学]