内蒙古呼和浩特变质核杂岩韧性拆离带40Ar-39Ar定年及其构造含义

晚中生代,内蒙古大青山依次经历晚侏罗世盘羊山逆冲推覆、早白垩世呼和浩特变质核杂岩伸展、早白垩世大青山逆冲推覆断层及早白垩世以来高角度正断层复杂构造演化。其中,呼和浩特变质核杂岩韧性剪切带的冷却时间和抬升机制的制约尚不明确。本文在野外考察和显微构造分析基础上,采用逐步加热40Ar-39Ar定年法对韧性剪切带内不同单矿物的冷却年龄进行了测定。角闪石、白云母、黑云母和钾长石单矿物40Ar-39Ar冷却年龄处于120~116Ma之间。结合已有年龄数据及单矿物封闭温度,构建了韧性剪切带的冷却曲线。结果表明,韧性剪切带在122~115Ma期间存在一个明显的快速冷却过程。这一阶段快速冷却是与变质核杂岩拆离断层相关核部杂岩拆离折返作为大青山逆冲推覆断层上盘抬升的结果。

40Ar-39Ar dating of the detachment fault of the Hohhot metamorphic core complex, Inner Mongolia, China

Daqing Shan in central Inner Mongolia had experienced a complex structural evolution during the Late Mesozoic. The Panyang Shan Thrust (PST), the Hohhot metamorphic core complex (Hohhot MCC), the Daqing Shan thrust-nappe system (DST) and high angle normal faults are the major structures formed during this stage. However, the temporal and special relationships among these events are still not clear. Based on field observations, we give a chronological sequence on them. Then we measured the ⁴⁰Ar-³⁹Ar ages of different minerals from the detachment fault zone of the Hohhot MCC and explain the tectonic implications of these ages based on the newly constructed structural evolution of the Daqing Shan area. The south-directed PST overlay pre-Cambrian rocks on Late Paleozoic-to-Mesozoic rocks. The PST was active at roughly the same time as the deposition of the Daqingshan Formation during the time from Late Jurassic to Early Cretaceous. The Hohhot MCC consists of metamorphic core complex, hanging wall unites and detachment faults. The metamorphic core complex is composed of pre-Mesozoic metamorphic rocks and Late Mesozoic granitoids. The hanging wall is made of pre-Cambrian low grade metamorphic rocks and syn-extensional sediments. There are three detachment faults outcropped representing different evolutionary phases of the detachment fault. The detachment faults from south to north are Hohhot fault zone, Xiaojing fault zone and Deshengying fault zone, which are the master detachment fault, early and deep detachment fault, and late and shallow fault, respectively. Lineation and fabrics of all these ductile shear zones indicate a common top-to-southeast shear in the detachment faults zone. Zircon ages of granitoids which intruded into the detachment fault zones indicate that the deformation of ductile shear zone had ended before ~131Ma, much earlier than the ⁴⁰Ar-³⁹Ar ages of the detachment fault zones. The DST is made up of a series of top-to-the-north or to-the-northwest reverse faults. These faults moved Archean crystalline rocks, Proterozoic gneissic granite, and Proterozoic low-grade metamorphic rocks on top of the autochthonous system composed of Paleozoic-to-Mesozoic rocks and the younger rocks. The DST cut the Hohhot fault zone and the mylonite was a part the hanging wall of DST. Both newly formed syn-deformational phyllonite in the fault zone and hornblende from a weakly deformed granodiorite dike that intruded into the thrust fault have ⁴⁰Ar-³⁹Ar ages of ~120Ma, representing the time when these fault were active. Then, the Daqing Shan area entered into a new south-north directed extensional stage represented by high angle normal faults. These brittle normal faults strike east-west and cut all the above structures. Combined with previous published papers, we estimate these normal faults were mostly active between 100Ma and 90Ma. Though Hohhot MCC is one of the most intensely studied MCCs in China, the cooling history and uplifting mechanism of its ductile shear zone is unclear. Here we adopts step heating ⁴⁰Ar-³⁹Ar method to date the cooling ages of different minerals from the ductile shear zone of Hohhot MCC, and 4 samples of hornblende, muscovite, biotite and K-feldspar give cooling ages ranging from 116Ma to 120Ma. A cooling curve is constructed according to the cooling ages and the related closure temperatures of the various minerals dated by ⁴⁰Ar-³⁹Ar method from this paper and Davis and Darby (2010), zircon U-Pb ages of the granitoids intruded into the detachment fault zone and apatite fission track ages of the fault zone. The cooling curve shows that the ductile shear zone experienced a rapid uplifting since 122Ma to 115Ma. Since the deformation of detachment fault zone had ended before ~131Ma, the ⁴⁰Ar-³⁹Ar ages do not represent the rapid lifting stage of the ductile shear zone which was caused by the detachment fault zones themselves. However, this period is roughly in consistent with the dating data from the DST fault zone. Combined with the field observations that the DST cut the ductile shear zones of Hohhot MCC, we propose the rapid cooling of the detachment fault zone from 122Ma to 115Ma is the result of the DST, in which the Hohhot MCC played as a part of its hanging wall.