柯克亚剖面新生代晚期沉积物中磷灰石裂变径迹年龄及意义

APATITE FISSION-TRACK AGES OF LATE CENOZIOC SEDIMENTS FROM KOKYAR SECTION IN WESTERN KUNLUN FORELAND BASIN AND THEIR SIGNIFICENCE

西昆仑山前柯克亚剖面厚度约7 000 m,通过对该剖面4个样品的磷灰石裂变径迹特征进行的详细分析,按地层厚度计算,样品埋深应为3 000~5 000 m,按古地温梯度计算埋深温度至少为60℃,进入磷灰石裂变径迹的退火带温度.分析结果表明样品并未发生典型的退火过程,退火程度较弱.径迹长度的双峰态分布显示,样品曾经历部分退火带,形成短径迹,后期经抬升后,古地温低于60℃,长径迹形成.剖面上部厚层砾岩中样品磷灰石裂变径迹年龄的分散性反映碎屑来源较复杂,埋藏受热时间较短,代表碎屑是经快速堆积的产物,与沉积记录反映的磨拉石成因与形成时代相一致.

An apatite fission track investigation was carried out from the Cenozoic (N) sandstones along Kokyar section in the western Kunlun foreland basin. Apatite fission track ages vary from 16 Ma to 214 Ma. The ages of all samples are older than the depositional age of the sandstones. This means that all sandstone samples were heated under 110 ℃ during burial, and a partial annealing of the fission tracks occurred in apatite. Mean track lengths range from 10um to 12 um. All the track length distributions suggest the presence of two groups of track lengths. Such a distribution is indicative of a sample that has spent some time within the apatite partial annealing zone(between 60 ℃ and 110 ℃) to produce the shorter tracks, then cooled to below 60 ℃ to allow the accumulation of the longer tracks. Apatite FT data show that post-depositional heating of the sediments has not been to the levels sufficient to cause partial track annealing, therefore all measured AFT ages record original cooling of the source rock, it is indicative of clasts source of the sediments. This is in good agreement with the sedimentary analysis. Dispersed ages also reflect the mobility of tectonic movement in source regions, indicating their significance in understanding tectonic evolution.