伊宁地块构造单元划分——来自火山岩浆作用的证据

Division of tectonic units in Yining Block:Evidence from volcano-magmatism

伊宁地块不是一个之前一贯认为的"均匀地块"或"均一地块",而是以乌孙山-塔勒得近东西向区域性大断裂为界的南、北两大次级构造带("弧-盆"体系)叠加拼贴增生而成,火山岩浆作用为这一新的构造单元划分与建立提供了佐证。南构造带由喀拉峻岛弧带和其北的阿腾套弧后盆地构成,主要发育于晚泥盆世-早石炭世早期,火山岩同位素年龄峰值为355~350Ma,17个年龄平均值为351Ma;北构造带由北而南可再细分出清水河-苏布台弧后盆地→阿吾拉勒叠加岛弧带→特克斯-新源弧前盆地,主要发育于早石炭世中-晚期,同位素年龄峰值集中于345~329Ma,18个火山岩年龄平均值为340Ma。这两个"弧-盆"体系以大哈拉军山组钙碱性火山岩为主体,共生早石炭世海相阿克沙克组弧前及弧后沉积岩组合。大哈拉军山组火山岩主体以岛弧火山岩为主,见有富Nb玄武岩、高镁安山岩等,共生埃达克岩和高分异Ⅰ型花岗岩等小岩体;在弧后还见有碱性火山岩、碱性球泡流纹岩,共生双峰式火山岩。不仅大哈拉军山组火山岩在各构造相中显著有别,而且共生的阿克沙克组在各构造相中差异极为显著。两大次级构造带具有独立的基底建造史,差异显著的盆地沉积史,独特的火山岩浆史和构造演化史。早晚石炭世之间的鄯善运动使南、北两个次级构造带叠加拼贴,构成统一的伊宁地块,晚石炭世进入统一的陆内构造发展演化阶段,发育以伊什基里克组碱性双峰式火山岩为代表的裂谷火山岩浆建造。

The Yining Block is not a "uniformity terrane"or a "uniform terrane"that is distinctly different from the previous persistent view,but was formed by the superposition and collage of the southern and northern secondary structural belts( two "arcbasin"systems) bounded by the Wusunshan-Talde regional large-scale fault. This papper set up a new division and re-establishment about its tectonic units by using the widespread volcano-magmatism in this block. The southern structural belt is composed of the Kalajun island arc belt to the south and the Atengtao back-arc basin to the north,which mainly developed from the Late Devonian to early stage of Early Carboniferous. The peak isotopic age of volcanic rocks in this belt ranges from 355 Ma to 350 Ma,with an average age of 351 Ma( n = 17). While the northern structural belt can be subdivided into,namely from north to south,the Qingshuihe-Subutai back-arc basin,the Awulale superimposed island arc belt and the Tekesi-Xinyuan fore-arc basin,which was mainly developed in the middle-late stage of Early Carboniferous. The peak isotopic age in this belt concentrates in 345 ~ 329 Ma with an average age of 340 Ma( n = 18). The two "arc-basin"systems are dominated by calc-alkaline volcanic rocks of the Dahalajunshan Formation,and they coexist with the fore-arc and back-arc basin sedimentary associations in the Early Carboniferous Akeshake Formation. The Dahalajunshan Formation is dominated by island arc volcanic rocks,including Nb-enriched basalts,high-magnesium andesites,symbiotic adakites and highly differentiated I-type granites,and some alkaline volcanic rocks,alkaline bimodal rhyolites and symbiotic bimodal volcanic rocks that predominantly cropped out in the back-arc basin. Studies show that not only the volcanic rocks of the Dahalajunshan Formation show significant differences in each tectonic facies,but also the symbiotic sedimentary rocks of the Akeshake Formation are obviously different in each tectonic facies. The above two structural belts have experienced independent histories of the basement formation,the basin sedimentation,and the volcanic magmatic and tectonic evolution. The Shanshan Movement indicates that the southern and northern structural belts were superimposed and collaged together at the end of Early Carboniferous,forming the unified Yining Block. During the Late Carboniferous,the block entered into a unified intracontinental tectonic development and evolution stage,and during this period of time,a series of rift-related volcanic constructions as represented by the alkaline bimodal volcanic rocks of the Late Carboniferous Yishijilike Formation were formed.