The Wulian Metamorphic Core Complex: a Newly Discovered Metamorphic Core Complex Along the Sulu Orogenic Belt, Eastern China
Total Page：16
File Type：pdf, Size：1020Kb
Journal of Earth Science, Vol. 24, No. 3, p. 297–313, June 2013 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-013-0330-5 The Wulian Metamorphic Core Complex: A Newly Discovered Metamorphic Core Complex along the Sulu Orogenic Belt, Eastern China Jinlong Ni (倪金龙) Shandong Provincial Key Laboratory of Depositional Mineralization & Sedimentary Minerals, College of Geological Sciences & Engineering, Shandong University of Science and Technology, Qingdao 266590, China; State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China Junlai Liu* (刘俊来) State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China Xiaoling Tang (唐小玲) College of Chemistry & Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China Haibo Yang (杨海波), Zengming Xia (夏增明) State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China Quanjun Guo (郭全军) College of Geological Sciences & Engineering, Shandong University of Science and Technology, Qingdao 266590, China ABSTRACT: Combined with field studies, microscopic observations, and EBSD fabric analysis, we defined a possible Early Cretaceous metamorphic core complex (MCC) in the Wulian (五莲) area along the Sulu (苏鲁) orogenic belt in eastern China. The MCC is of typical Cordilleran type with five elements: (1) a master detachment fault and sheared rocks beneath it, a lower plate of crystalline rocks with (2) middle crust metamorphic rocks, (3) This study was supported by the National Natural Science Foun- syn-kinematic plutons, (4) an upper plate of dation of China (Nos. 90814006, 91214301), the Natural Science weakly deformed Proterozoic metamorphic Foundation of Shandong Province (No. ZR2009EQ002), the rocks, and (5) Cretaceous volcanic-sedimentary Foundation of the Shandong Provincial Key Laboratory of De- rocks in the supradetachment basin. Some positional Mineralization & Sedimentary Minerals (No. postkinematic incursions cut across the master DMSM201005), and the National Key Basic Research Devel- detachment fault zone and two plates. In the opment Program (973 Program) of China (No. 2012CB723104). upper plate, Zhucheng (诸城) Basin basement *Corresponding author: [email protected] consists of the Proterozoic Fenzishan (粉子山) © China University of Geosciences and Springer-Verlag Berlin Group, Jinning period granite (762–834 Ma). Heidelberg 2013 The supradetachment basin above the Proterozoic rocks is filled with the Early Manuscript received July 12, 2012. Cretaceous Laiyang (莱阳) (~135–125 Ma) and Manuscript accepted September 28, 2012. Qingshan (青山) groups (120–105 Ma), as well 298 Jinlong Ni, Junlai Liu, Xiaoling Tang, Haibo Yang, Zengming Xia and Quanjun Guo as the Late Cretaceous Wangshi (王氏) Group (85–65 Ma). The detachment fault zone is developed at the base and margin of the superposed basin. Pseudotachylite and micro breccia layers located at the top of the detachment fault. Stretching lineation and foliation are well developed in the ductile shear belt in the detachment faults. The stretching lineation indicates a transport direction of nearly east to west on the whole, while the foliations trend WNW, WSW, and SE. Protomylonite, mylonite, and ultramylonite are universally developed in the faults, transitioning to mylonitic gneiss, and finally to gneiss downward. Microstructure and quartz preferred orientation show that the mylonites formed at high greenschist facies to low greenschist facies as a whole. The footwall metamorphic rock series of the Wulian MCC are chiefly UHP (ultrahigh pressure) metamorphic rocks. Syntectonic rocks developed simultaneously with the Wulian MCC detachment and extension. Geological research has demonstrated that the MCC is associated with small-scale intrusive rocks developing in the vicinity of the detachment faults, for instance, dike. Geochronology results indicate that the denudation of the Wulian MCC occurred at about 135–122 Ma. Its development and exhumation was irrelevant to the Sulu UHP metamorphism zone rapid exhumation during Triassic Period but resulted from the crustal extension of North China Craton and adjacent area. KEY WORDS: metamorphic core complex, Late Mesozoic, North China Craton, crustal extension, Sulu orogenic belt. INTRODUCTION alization, the exhumation of the MCCs resulted from The North China Craton has undergone large- the Late Mesozoic lithospheric evolution of North scale lithospheric thinning since the Late Mesozoic (Xu China. and Qin, 2009; Kusky et al., 2007; Liu et al., 2005; As the southeastern margin of the North China Deng et al., 2004; Wu et al., 2003; Xu, 2001; Menzies Craton, the Jiaodong Peninsula also experienced et al., 1993). Petrological, geochemical, and geophysi- lithospheric extension and thinning, thereby forming a cal studies led to the construction of different tectonic series of extensional structures, such as the Queshan models that elucidate the fundamental processes in- and Linglong (Charles et al., 2011) MCCs in the volved in the tectonic evolution and thinning of the northern Shandong Peninsula. Although considerable cratonic lithosphere (Xu and Qin, 2009; Mao et al., research has been done on the formation and evolution 2007; Deng et al., 2004; Wu et al., 2003). The most of high-pressure (HP) and ultrahigh-pressure (UHP) frequently cited ones are the delamination (Wu et al., rocks on the southern part of the peninsula, little in- 2002), lithosphere derooting (Deng et al., 2004), mantle formation on Late Mesozoic extension in the area has plume (Zheng et al., 2010; Zhi et al., 2001), coupled been provided. In this study, we report a newly MCC chemical thermo-mechanical (Xu and Qin, 2009), and located at the northern part of the Sulu orogenic belt mantle-cell convection (Ren et al., 2002) models. near Wulian City. The discovery of this MCC and de- As a significant tectonic style of continental tailed analysis of its evolution in relation to the de- lithospheric extension and crust thinning, metamor- velopment Sulu orogenic belt can provide direct evi- phic core complexes (MCCs) provide exceptionally dence for the regional lithosphere thinning process in important information on lithospheric evolution the Late Mesozoic. (Lister and Davis, 1989). The Late Mesozoic litho- spheric evolution of North China, for example, is REGIONAL GEOLOGY characterized by the formation of MCCs in the crustal The Jiaodong Peninsula principally comprises level along with regional lithospheric extension and three petrological tectonic units: the Jiaobei meta- thinning. These geological structures include the morphic block, Sulu UHP metamorphic block, and Liaonan (Liu et al., 2006, 2005), Yiwulüshan (Waziyu) Mesozoic Jiaolai Basin, which is superimposed on the (Darby et al., 2004), and Xiaoqinling MCCs (Zhang et two blocks (Yang et al., 2002; Faure et al., 2001; al., 2000). Similar to regional magmatism and miner- Hacker et al., 1998). The Jiaobei metamorphic block The Wulian Metamorphic Core Complex: A Newly Discovered Metamorphic Core Complex 299 is possibly a segment of the Qinling microplate, com- Jingshan Group (Ar3–Pt1), Fenzishan Group (Pt1), and posed of banded ferromagnesian-felsic gneiss of am- Lower Triassic UHP metamorphic rocks. The phibolite facies, and locally granulite facies (Hacker et Fenzishan Groupis also called the Wulian Group in al., 2006). The block is superimposed by the medium this area. Upper rocks are distributed limitedly, with metamorphic to nonmetamorphosed Archean, Pro- the UHP rocks making up the majority of the compo- terozoic, and Early Paleozoic Jiaodong, Fenzishan sition (Yang et al., 2005). (Jingshan), and Penglai (Wulian) Groups (Wu et al., The Jingshan and Fenzishan Group are distrib- 2004). The lowermost part of the Sulu UHP metamor- uted in an NW- or NEE-oriented zone or in a phic block consists of UHP metamorphic rocks. The lens-shaped area. The formations are intruded by Pro- upper section of the UHP metamorphic rocks consists terozoic plutonic bodies and enclosed within the rocks mainly of the metamorphic rocks of the Jingshan Group as relic segments. Most of the rocks experienced in- (Fenzishan Group), with Neoproterozoic granites tense ductile deformation, forming mylonites or (Faure et al., 2001). gneisses (Li et al., 2004). During the Mesozoic, the north and south blocks The Lower Triassic UHP block primarily com- of the Jiaodong Peninsula evolved through multiple prises two sets of rocks, namely, layered stages of magma intrusion and volcanic eruption, meta-sedimentary rocks, and deformed and metamor- producing huge amounts of granitic and mafic rocks. phic Neoproterozoic granites. The layered The volcanic activity was centered in Jiaolai Basin meta-sedimentary rocks are distributed chiefly near (Yang et al., 2005; Zhang Y Q et al., 2005). Zhucheng in the northern part of the study area. Gar- Tan-Lu fault is a very striking structure in the net porphyroclasts, up to 1 mm in diameter, are very Jiaodong Peninsula. Some parts of the main fault zone common. Large eclogite lenses are typically present in form the western boundary of the Mesozoic Jiaolai Basin. the metamorphic rocks. The deformed and metamor- A series of branch faults including the Wulian- phic Proterozoic granites are distributed principally in Qingdao-Yantai fault usually considered as the northern Wulian and Huangdun, with the appearance of mas- boundary of the Sulu orogenic belt (Zhou et al., 2008).