南天山库米什蛇绿混杂岩带中硅质岩的元素地球化学特征及其形成环境

Geochemical characteristics and sedimentary environments of cherts from kumishi ophiolitic mélange in southern Tianshan.

南天山库米什铜花山地区蛇绿混杂岩带不同岩块中的硅质岩高Al2O3和Al2O3／TiO2，Zr、Nb、Hf、Ta和Th与Al2O3等陆源元素具较好正相关性，北美页岩标准化稀土模式呈现无Ce负异常（Ce／Ce＊=1．05～1.33）的典型大陆边缘硅质岩平坦稀土谱型，它们的（La／Ce）SN=0．74～0．98，Ti/V：5．10～122．18，V／Y=0．39～6．03，均与大陆边缘硅质岩值相当，表明这些硅质岩形成在明显受陆源物质影响与大陆边缘密切相关的环境。但不同岩块硅质岩的物源区仍存在一定差异。其中，与泥岩成互层产出的红色硅质岩的Th/Sc=0.56—4．35，具Eu负异常（Eu／Eu＊=0．56～0．71）明显的较陡右倾球粒陨石标准化稀土谱型（LaN／YbN=3．92～7．43），与分异岩浆弧源区特征类似，可能形成在相对近陆缘的弧前盆地，其源区物质主要来自分异岩浆弧。而成夹层产出于基性熔岩中的绿色硅质岩呈现为Eu负异常较弱（Eu／Eu＊=0．72～0．93）右倾较缓的稀土谱型（Lan／Ybn=4．15～6．69），其Th／Sc=0．57～0．87，反映有洋内弧物质加入的影响，因此它们可能形成于相对远离大陆、除有未分异基性程度较高的洋内弧物质输入外，仍受到陆缘物质影响的有限洋盆环境。根据硅质岩晚古生代初的放射虫化石推断，该地区硅质岩在晚古生代初期形成，南天山构造带已进入板块汇聚俯冲消减洋盆萎缩的发展演化阶段。

The cherts, developed in various blocks of the ophiolitic mrlange of the Tonghuashan area to the south of Kumishi in southern Tianshan, have high Al2O3 and Al2O3/TiO2 values, and a good positive correlation between Al2O3 and trace elements including Zr, Nb, Hf, Ta and Th. Moreover, they are typical NASC-normalized fiat REE patterns without visible Ce negative anomalies, similar to those of cherts in continental margin settings. They have Ce/Ce＊ values of 1.05 - 1.33, （La/Ce） sN of 0.74 - 0.98 ,Ti/V of 5.10 - 122.18 and V/Y of 0.39 - 6.03, respectively, consistent with the values of cherts formed in continental margin settings, suggesting that they were formed in sedimentary environments closely related to continental margin influenced by the input of terrigenous component. However, their sources are more or less different. Of those, the red cherts, interbedded in mudstone, have Th/Sc ratio ranging between 0.56 -4.35, and chondrite-normalized REE patterns characterized by negative Eu anomalies （ Eu/Eu＊ = 0.56 - 0.71 ） and LREE enrichment （ LaN/YbN = 3.92 - 7. 43 ）, implying a setting of forearc basin with terrigenous dements dominantly derived from differentiated magmatic arc. In contrast, the green charts in basic lavas show slightly right-dipping chondritenormalized REE patterns （ La/YbN = 4, 15 - 6.69） with small Eu anomalies （ Eu/Eu ＊ = 0.72 - 0.93 ）. Their Th/Sc values are from 0.57 to 0.87, displaying some input of materials from oceanic arc. Therefore, they were formed in limited ocean basin relatively apart from continent and mainly affected by input of terrigenous and oceanic arc components. Consequently, based on the occurrence of early later Paleozoic radiolarian in cherts, it could be concluded that a tectonic regime of gradually closed-ocean basin and slab-subduction in active continental margin started in southern Tianshan during early later Paleozoic.