冀北晚古生代（325± Ma）变质事件：起因、过程与意义

The zircon SHRIMP U-Pb isotopic dating for the overgrowth rims of zircon by proposer from the different metamorphic rock samples such as the garnet -biotite plagiogneiss, retrograded eclogite and rodingite within the Hongqiyingzi Group in North Hebei area defines the weighted mean ages ranging from 320.8 Ma-327.4 Ma(with the mean age of 325 ± Ma), which may be interpreted as recrystallization ages of the metamorphic recrystallized zircons(Th/U<0.1) and possibly characterize the age of Late Paleozioc(325 ± Ma) metamorphic event.On the basis of these ages, the traditionary viewpoint about the Hongqiyingzi Group, which it is previously described as the Early Precambrian metamorphic block (amphibolite facies volcano -sedimentary rocks),need to be rectified immediately.But then the problem is how to understand the genesis, evolution and geological implications of the Late Paleo -zioc metamorphic event wihtin North Hebei area,and the genetic relationship between .the metamorphic event and the subduction of Paleo-Asian Oceanic crust beneath the North Chian Craton during the Late Paleozioc.It is possible that we obtain the new understanding about the tectonic evolution of the central part of the northern margin of the North China Craton (North Hebei area) after the above mentioned problems may be solved. Therefore this requisitiuon, based on the detailed field investigation of the regional distribution,deformation and metamorphism about the different metamorphic rock samples related to the Late Paleozoic metamorphic event from the Hongqiyingzi Group, will research their rock combination, protoliths, evolutional history and tectonic setting, and characterize the genesis,metamorphic characteris -tics and tectonic implications of the Late Paleozoic metamorphic event. We may finally provide the new scientific insight for farther understanding of tectonic setting and metamorphic evolution of the northern margin of the North China Craton(North Hebei area), and the relationship between the Paleo-Asian Ocean and the North China Craton(North Hebei area).

申请者在冀北地区不同类型变质岩中获得的新生变质成因锆石SHRIMP U-Pb年龄数据为320.8 Ma～327.4 Ma，显示冀北存在有晚古生代（325±Ma）变质事件的可能，从而颠覆了"冀北属于早前寒武纪变质地块"这一传统观点。那么，冀北晚古生代变质事件的性质和起因是什么？其与古亚洲洋洋壳的向南俯冲消减又有何成因联系？这些问题的明确有可能改变我们对华北克拉通北缘中段（冀北）大地构造演化的认识。因此，本申请项目试图在深入调查冀北不同类型变质岩的空间分布特征以及变形与变质历史的基础上，查明不同类型变质地质体（变质地层或岩石）的岩石类型、原岩性质及其大地构造属性，理清晚古生代变质事件所卷入的变质地质体的类型及其空间展布方式，明确晚古生代变质事件的性质、起因与过程，最终可为华北克拉通北缘中段（冀北）的构造属性和演化历史，以及古亚洲洋与华北克拉通北缘中段（冀北）相互关系的系统研究提供新的科学依据。

赤城红旗营子群（石榴黑云斜长片麻岩、退变榴辉岩）存在有晚古生代（325±Ma）变质事件的年龄记录。在此基础上，为了获得冀北其它地区可能存在的同时期变质事件的年龄数据，先后对崇礼、尚义和康保红旗营子群（包括赤城）以及平泉五道河群的相关岩石进行了锆石SHRIMP（SIMS、LA-ICPMS）U-Pb以及黑云母或角闪石40Ar-39Ar年龄测定。.冀北红旗营子群没有获得新太古代变质作用的年龄数据，故不宜再将其作为新太古代变质地层来加以对待。其中，康保红旗营子群应为古元古代（1875Ma~1903Ma）沉积变质岩系；赤城、崇礼和尚义等地红旗营子群则为晚古生代沉积变质岩系或火山-沉积变质岩系，但它们发生区域变质作用的时代是有差异的，分别为325±Ma、280±Ma和260±Ma。.冀北晚古生代经历了多期次的构造（岩浆、变质）热事件。其中，康保主要表现为构造-岩浆（热）事件，相应的酸性岩浆活动始于285±Ma，终于255±Ma，时间间隔4~5Ma左右；赤城、崇礼和尚义等地则主要表现为构造-变质（热）事件，相应的变质作用始于280±Ma，终于250±Ma。冀北晚古生代构造（岩浆、变质）热事件应该与古亚洲洋的俯冲消减作用有关。.平泉太古宙五道河群主体应为变质深成侵入岩（花岗片麻岩），其岩浆结晶年龄为2481±Ma。花岗片麻岩中存在有数量众多、大小不等、形态各异的基性麻粒岩岩块，其原岩形成时代为1833±Ma。.冀北的崇礼、尚义、康保及平泉等地没有晚古生代（325± Ma）变质事件的年龄记录，赤城红旗营子群中的晚古生代（325± Ma）变质事件，应该是一种“异地”变质作用，不是冀北区域角闪岩相变质作用的反映。如果考虑到冀北在325± Ma之后所经历的多期次构造（岩浆、变质）热事件，至少能够说明冀北在325± Ma前还没有受到古亚洲洋构造演化的影响。