西天山大哈拉军山组火山岩SHRIMP锆石U-Pb年龄及其构造意义

SHRIMP zircon U-Pb geochronological and geochemical data of the Early Carboniferous Dahalajunshan Formation: Evidence for the subduction processes in Western Tianshan

新疆早石炭世大哈拉军山组火山岩由西向东表现出由早到晚的演化规律。伊宁县博尔博松河孟玛热勒林场附近大哈拉军山组底部的英安质晶屑岩屑凝灰岩的SHRIMP锆石U-Pb年龄为359±2Ma,表明西天山西段该组底界时代为晚泥盆世末期—早石炭世之间。岩石地球化学数据统计表明,大哈拉军山组火山岩主要受岩浆结晶分异作用控制,是泥盆纪末期—石炭纪时期洋壳消减、壳幔混熔的产物。其岩石组成、时空分布的变化与天山洋由西向东的俯冲―碰撞过程相对应,不需要对其进行解体。

From east of Xinyuan County through Duku Highway to Yuximolegai area in the middle of the west Tianshan Mountains, the regional unconformity of Carboniferous Akeshake Group upon Lower Carboniferous Dahalajunshan Formation can be easily founded. This paper reports SHRIMP zircon U-Pb age of 359±2Ma from Dahalajunshan dacitic tuff near the Boerbosong River in Yining County, which constrains the age of the lower boundary of the west part of Dahalajunshan Formation. In combination with previously published results, the authors hold that Dahalajunshan Formation was formed at about 40Ma, with the characteristic that the early deposition appeared in the west and the later deposition occurred in the east. Taking into account all available chronological data published in the past ten years, petrological features, and major and trace elements data of these Carboniferous volcanic rocks, the authors consider that the basic lavas were most plausibly formed by mixing between enriched mantle sources and ocean floor sediments （melt） and modified by subduction zone fluids. Changes in volcanic rock composition, spatial distribution and temporal evolution of Dahalajunshan Formation correspond to features of the Tianshan oceanic subduction-collision process （from west to east）, and therefore its disintegration is not necessary.