塔里木盆地西缘上寒武统下丘里塔格群热液白云岩改造时限:来自古地磁的约束

Timing of hydrothermal alteration on the Lower Qiulitag Group dolomites of the Upper Cambrian,western margin of Tarim Basin:Palaeomagnetic constraint

成岩流体活动时间的确定一直都是学界难以解决的问题。上寒武统下丘里塔格群广泛发育有热液白云石,但热液改造的时间缺乏可靠约束。基于热液改造形成的广泛重磁化对热液改造时间进行约束,在巴楚永安坝和柯坪水泥厂下丘里塔格群中共采集43个热液白云石脉定向古柱状样品,并制样测试了64件古地磁样品,再对其中的29件样品进行了岩石磁学分析。大多数样品磁性在580℃时也没有被完全清洗掉,一部分在高外部磁场上才得到饱和等温剩磁,指示高矫顽力、高阻挡温度载磁矿物,推测为赤铁矿;另一部分样品在低外部磁场上迅速达到饱和,指示低矫顽力、高阻挡温度载磁矿物,推测为磁铁矿。许多样品的退磁数据较离散,但少数样品退磁曲线具有典型的双分量特征。柯坪水泥厂2号采样点和永安坝2号采样点的高温特征剩磁分量对应的古地磁极位置能与前人在同地区测的早二叠世与晚二叠世古磁极位置良好对应。考虑到所测样品均为热液白云石脉,这部分高温特征剩磁分量显然是热液活动引起化学重磁化的结果,说明热液白云岩改造与塔里木板块二叠纪强烈的岩浆活动有关系。

Timing of fluid activity during diagenetic process keeps to be a difficult problem,notably in carbonate successions.The Lower Qiulitag Group dolomites of the Upper Cambrian in the Tarim Basin experienced extensive hydrothermal alteration through fracturing,crystallization and saddle dolomite precipitation;however,the timing of hydrothermal alte ration was poorly constrained.To solve this problem,the extensive remagnetization data are used to constrain the timing of hydrothermal alteration on the Upper Qiulitag Group dolomites.We drilled 43 oriented plugs from saddle dolomite veins of the Lower Qiulitag Group in Keping and Yong’anba in the western flank of Tarim Basin,from which 64 samples were prepared for paleomagnetic analysis and 29 samples for rock maganetic studies.The magnetism of most samples cannot be washed out at a temperature over 580℃,and only some samples can yield saturation isothermal remanent magnetism in a high magnetic field,which resides in the high-coervicity,high-unblocking temperature magnetic carrier minerals,i.e.,hematite.However,the other samples can rapidly reach saturation isothermal remanent magnetism in a low magnetic field,which is likely yielded by the low-coervicity,high-temperature magnetic carrier such as magnetite.Although many samples yield discrete demagnetization data,minor samples do yield bimodal fractions on the demagnetization curve.The paleomagnetic polar positions calculated from the characteristic high-temperature,remanent magnetism fractions from the two sampling sites(Keping and Yong’anba)agree well with those of the Early Permian and Late Permian in the study area,respectively.Considering that the measured samples are mainly obtained from hydrothermal dolomite veins,the high-temperature characteristic remanent magnetism fractions apparently resulted from chemical remagnetization induced by hydrothermal activity,which was temporally linked to the intense Permian large igneous province(LIP)activity in the Tarim block initiated in the Early Permian.