150 / 2015-06-10 10:23:33

Disintegration and Age of “ Tuolai Complex-Group ”in the West Segment of Central Qilian Block and Its Implication

7041,7040,7039,7038,7037

Disintegration and Age of “ Tuolai Complex-Group ”in the West Segment of Central Qilian Block and Its Implication
LIU Changfeng , YANG Xin, SONG Zhijie, XIA Yang, WU Chen, ZHOU Zhiguang, LIU Wencan1
1 China University of Geoscience(Beijing),100083; E-mail:nose010@sohu.com
Abstract: Various views on the age and attribute of Qilian Block have been presented. However, we found the supposed Paleoproterozoic Tuolai Complex-Group actually comprises complex geologic bodies with diverse rock types, different metamorphic degrees, formation ages and nature, through regional geological investigation at 1:50000 scale in areas such as Shangrimuceer in Qinghai. Based on large scale geologic mapping, outcrops measuring and zircon dating, the Tuolai Complex-Group is divided into four parts: metamorphic supracrustal rocks, Neoproterozoic acid intrusive rocks, early-middle Ordovician acid intrusive rocks and middle Ordovician basic intrusive rocks. Zircon U-Pb LA-MC-ICP-MS dating was conducted on samples of quartzite, two-mica quartz schist with sillimanite and garnet and medium-coarse grained biotite monzonitic granite gneiss to constrain the depositional time of Tuolai Complex-Group. The youngest age assembly from two-mica quartz schist is ca.1178Ma, implying the TuoLai Complex-Group is younger than ca.1178Ma. The medium-coarse grained biotite monzonitic granite gneiss probably intruded into Tuolai Complex-Group at 896.7 ±3.6Ma（MSWD= 1.7，n=31）. Therefore, the age of Tuolai Complex-Group can be constrained at 896.7 ~1178Ma, belonging to late Mesoproterozoic and early Neoproterozoic. The detrital zircon age distribution characteristic (peaked at ca.1780Ma) is comparable to the Mesoproterozoic and Neoproterozoic detrital zircons from northern and eastern North China Craton, suggesting the Qilian Block has a closer affinity with North China Craton.

Keywords: Central Qilian Block, Tuolai Complex-Group, Detrital Zircon, Mesoproterozoic, LA-MC-ICP-MS Zircon U-Pb dating
1. Introduction
Tuolai Complex-Group (TCG) can be compared with Paleoproterzoic rocks, such as Beidahe group, Huangyuan group and so on, which compose the crystalline basement of Qilian Block. The formation time of the crystalline base of Qilian Block remains controversial due to a lack of precise isotopic dating data. Regarding the tectonic affinity of TCG, views of Paleoproterzoic (Zhongtiao Cycle) (RGQP, 1991) or Neoproterzoic (Jinning Cycle) (Smith et al., 2000; Guo et al., 1999; 2000) crystalline base were presented respectively. Some scholars considered it had affinity to the North China Craton (NCC), while someone pointed out that it showed affinity to Yangtze Plate (Wan et al.; 2006; Zhang et al., 2006；Xu et al., 2007；Dong et al., 2007；Lu et al.,2009).
2. Geological setting and the disintegration of TCG
The study area is located at northeastern Tibetan plateau, the northern slope of Tuolainanshan, and tectonically, belongs to the middle segment of Qilian orogenic belt of Qinling-Qilian-Kunlun Central Orogenic System (QQKCOS). An assemblage of migmatite, gneiss, schist interbedded by quartzite and marble was defined as Paleoproterozoic TCG, but it lacks reliable dating result. Based on large scale geologic mapping, outcrops measuring and zircon dating, Tuolai Complex-Group is divided into four parts: metamorphic supracrustal rocks, Neoproterozoic acid intrusion, early-middle Ordovician acid intrusion and middle Ordovician basic intrusion.
3. Geological features of TCG and Geochronology
3.1 geological features and sample description
As depicted in Fig.1b, TCG extends in a NW ward, intruded by Neoproterozoic monzonitic granite gneiss in the east and the contact line is modified by consequent tectonic movement and shows hallmarks of ductile shear. TCG shows fault contact with metamorphic volcanic rocks of Early Ordovician Yingou formation and Mesoproterozoic Nanbaishuihe formation along the Hongyahu and Yangyuchi, but it thrusts onto Triassic sandstone to its south. The gneiss assemblage of TCG comprises banding biotite plagiogneiss (two mica garnet bearing plagiogneiss distribute locally), biotite plagioclase granulites, marble and quartz, extending stably with consistent attitudes. The schist assemblage comprises two-mica quartz schist, quartz schist, feldspar quartz schist and quartzite, with palimpsest bedding and palimpsest graded bedding. The two assemblages contact with each other by fault, and both the top and the bottom of TCG can’t be traced.
3.2 Chronology
Sample (SRM001) is sillimanite and garnet bearing two-mica quartz schist, and was collected at Dayanglonggou in Shangrimuce’er(E98°33′24″，N38°37′48″). 80 Zircons yielded 207Pb/206Pb ages ranging from 1178Ma to 2902Ma. Six zircons belong to Neoproterozoic. 49 zircons belong to Paleoproterozoic, ranging from 1612-2307Ma, and the rest 24 zircons possess ages ranging from 1178-1685Ma. The pronounced age peak at 1776Ma has the best repeatability. Besides, there are the other three age peaks, 1429Ma, 2394 Ma and 2794Ma, respectively.
Sample(D4181) of quartzite was collected at south of Xiarenci in Shangrice’er (E98°38′57″,N38°375′12″). Most spots of 31 zircons plotted on or near the Concordia line. Ages of these zircons rang from 1149Ma to 2651Ma, among which one neoproterozoic age, 19 Paleopreoterozic ages and 11 Mesoproterozic ages are included. The sharpest age peak is at 1782Ma, besides two small peaks occur at 1255Ma and 2540Ma.
Sample D4187 (E98°40′45″, N38°39′21″), monzonitic granite gneiss, was collected from Xiarencigoukou. 206U-238Pb ages for 31 zircons vary between 879-914Ma, yielding a weighted mean 206U-238Pb age of 896.7 ±3.6Ma (MSWD= 1.7, n=31), interpreted to be the crystallization age of the monzonitic granite gneiss.
4. Discussion
4.1 Formation time of TCG
It is generally accepted that the youngest detrital zircon age indicates the maximum depositional age of sedimentary rocks. But great caution is required when dealing with the youngest ages. Only the one with good repeatability can be applied to constrain the formation time of a stratum. Detrital zircons of quartz from TCG yield the youngest age of 1149Ma, but it is unique. The youngest zircon from mica-quartz schist aged 1178Ma with great repeatability suggests the maximum depositional time is younger than 1178Ma. Besides, gneissic monzogranite emplaced into TCG yielded age of 896.7±3.6Ma(MSWD= 1.7，n=31), indicating TCG deposited before 896.7±3.6Ma. Hence, the formation time of TCG ranges from 1100Ma to 950Ma, probably at ca.1000Ma, the late Mesoproterozoic.
4.2 Tectonic affinity
Formation time of the crystalline base of Qilian block remains controversial. New evidences on the formation time of Paleopeoterozoic crystalline base have been present. Consequently, based on new zircon ages of detrital rocks and gneissic granitite reported in this paper, we suggest that a great portion of assumed Paleoproterozoic strata probably formed from Mesoproterozoic to late Neoproterozic. Although the age spectra we presented above are different from regional data reported previously, there still are some similarities between them. Firstly, Archaean age population is included in most detrital samples. Secondly, the age peak is dominated by ages from late Paleoproterozoic to Mesoproterozoic. Thirdly, most zircon ages cluster around two peaks, 1700-1800Ma and 1200-1450Ma, respectively. Fourthly, the youngest detrital zircon with excellent repeatability belongs to Mesoproterozoic in age. All characteristics imply the provenance component of the meta-sediments mentioned above is similar, and probably have affinity to the same ancient block in Mesoproterozoic.
Published data support the view that Qilian broke away from NCC before Caledonian. Some scholars thought Qilian block had tight affinity to Yangtze plate due to the Mesoproterozoic and Neoproterozoic detrital zircons in rocks from the base of Qilian(Zhang et al., 2006; Xu et al., 2007; Dong et al., 2007; Lu et al., 2009). Current research results agued for the viewpoint that magmatic events occurred in NCC from Mesoproterozoic to Neoproterozoic. Granite-porphyry vein unconformably overlain by pebbly sandstone of Changchengian Changzhou formation has a U-Pb age of ca.1670Ma (Li et al., 2011). Bentonite from Tieling formation has a zircon U-Pb of 1440Ma (Su et al., 2010). Bentonite from Xiamaling formation has a U-Pb ae of 1380Ma (Gao et al., 2008). Zircon and baddeleyite U-Pb age for basic rock intruded into Xiamaling formation is 1320±Ma (Li et al., 2009). Basic veins, sills (Peng et al., 2011) and acid volcanic rocks, aged from 890Ma to 925Ma, expose in surrounding area of NCC. Zhai et al (2014) sorted magmatic events into four phases and pointed out multistage rifting events occurred in NCC from Paleoproterozic to Neoproterozoic. So, it is doubtful to determine the tectonic affinity of Qilian block by just considering whether it contains Mesoproterozoic and Neoproterozoic detrital zircons or not.
5. Conclusion
(1) Based on large scale geologic mapping, outcrops measuring and zircon dating, the Tuolai Complex-Group is divided into four parts: metamorphic supracrustal rocks, Neoproterozoic acid intrusion, early-middle Ordovician acid intrusion and middle Ordovician basic intrusion.
(2) LA-MC-ICP-MS dating yielded age of 1178Ma for the youngest detrital zircon of redefined TCG，896.7Ma for the gneissic monzogranite emplaced into TCG, which constrains the formation time of TCG to 1178~896.7Ma, belonging to late Mesoproterozoic-Neoproterozoic.
(3) Samples from TCG exhibit major age peak at 1780Ma, which can be compared with Mesoproterzoic-Neoproterzoic detrital zircon age-spectra from northern margin and east part of NCC. Taking magmatic activities of adjacent blocks into account, Qilian block shows intimate affinity to NCC.

Acknowledgement: This study was supported by Metallogenic Belt Geological and Mineral Survey and Evaluation Program in Qai dam and Its Adjacent Areas issued by China Geological Survey (1212011121188). We are grateful to other team members for their filed work.

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