Post-Orogenic Granites in Pingwu Region, Northwest ... - Springer Link

Journal of Earth Science, Vol. 20, No. 2, p. 250–273, April 2009 Printed in China DOI: 10.1007/s12583-009-0024-1

ISSN 1674-487X

Post-Orogenic Granites in Pingwu Region, Northwest Sichuan: Evidence for North China Block and Yangtze Block Collision during Triassic Pei Xianzhi* (裴先治) State Key Laboratory of Continental Dynamics, Northwest University, Xi’an 710069, China; Key Laboratory of Western China’s Mineral Resources and Geological Engineering of Ministry of Education, Chang’an University, Xi’an 710054, China Li Zuochen (李佐臣), Ding Saping (丁仨平), Feng Jianyun (冯建赟), Li Ruibao (李瑞保), Sun Yu (孙雨), Zhang Yafeng (张亚峰), Liu Zhanqing (刘战庆) Key Laboratory of Western China’s Mineral Resources and Geological Engineering of the Ministry of Education, College of Earth Science and Land Resources, Chang’an University, Xi’an 710054, China ABSTRACT: The Nanyili (南一里), Laohegou (老河沟), and Shaiziyan (筛子岩) granitic intrusions are located in the southern margin of the Bikou (碧口) block in Pingwu (平武) area, Northwest Sichuan (四川). The petrography and geochemical characteristics of the granitic intrusions as well as their source and tectonic settings are reported and discussed in this article. The Laohegou and Shaiziyan granites are with high SiO2 (69.89 wt.%–73.05 wt.%) and Al2O3 contents, and A/CNK=1.04–1.12. They are typical strongly peraluminous granites, with supersaturation in Al and Si. The abundance of ∑REE varies in the range of (33.13–89.12)×10-6. The rocks show an LREE enrichment pattern and obvious Eu negative anomaly. The trace element geochemistry is characterized evidently by a negative anomaly of Ta, Nb, Ti, etc. and a positive anomaly of Rb, Ba, Sr, etc.. Zircons of the Nanyili granite have higher Th/U ratios, and their CL images have internal oscillatory zoning, suggesting that the zircons of the samples are igneous in origin. The LA ICP-MS zircon U-Pb isotopic concordia diagram yields an age of 223.1±2.6 Ma (MSWD=1.4), which indicates that the granodiorite intrusions formed in the early Late Triassic. The Nanyili, Laohegou, and Shaiziyan granites have the characteristics of post-collisional granites and are regarded as post-orogenic granites. Thus, the granite intrusions are interpreted as syn-collisional granites that resulted from the crustal thickening caused by the collisions between the North China plate and the Yangtze plate during This study was supported by the National Natural Science

the Indosinian. The granitic intrusions formed in

Foundation of China (Nos. 40572121 and 40234041), and

a transitional environment from syn- (compres-

MOST Special Fund from the State Key Laboratory of Conti-

sional environment) to post-collision (extensional

nental Dynamics, Northwest University.

environment).

*Corresponding author: [email protected]

KEY WORDS: strongly peraluminous granite, LA ICP-MS dating, geochemistry, tectonic set-

Manuscript received November 6, 2008. Manuscript accepted February 7, 2009.

ting, post-collision, Bikou block.

Post-Orogenic Granites in Pingwu Region, Northwestern Sichuan

INTRODUCTION The Northwest Sichuan and its adjacent regions are at the junction of the Qinling orogenic belt, the Songpan-Ganze orogenic belt, and the Yangtze block, which are also the eastern margin of the Tibetan plateau. It is a crucial area to approach continental tectonics and dynamics in China as well as a natural laboratory in which to study the three-dimensional structure between the crust and the mantle, the assembling of multiple blocks, and the growth of the continent. The region had experienced complex tectonic movements and accretion histories in geological periods. Previous studies suggest that the Qinling-Dabie orogenic belt formed during the Late Triassic collision between the Yangtze block and the North China block (Zhang G W et al., 2004a, 2003, 2001). Although the Songpan-Ganze orogenic belt had been deformed strongly in the Cenozoic (Xu et al., 1992; Dewey et al., 1988), the predominant deformation occurred in the Late Triassic or Indosinian (Yin and Harrison, 2000; Burchfiel et al., 1995; Hsu et al., 1995). Recent studies revealed that the Indosinian granitic plutons of about 223–205 Ma were discovered south of the Mianlue suture or are sandwiched between the Mianlue suture and the Shangdan suture as well as the north of the Shangdan suture. They are distributed in a wide area and show vague spatial relationships with the surrounding orogenic belts. What are the relationships among the syn-orogenic granites in terms of geochemical characteristic, petrogenesis, and tectonic environment as they are exposed in different blocks or orogenic belts? Therefore, there is a vital geological significance in studying the geochemistry of these granites in detail so that their tectonic settings, the assembling of the block relations, and the continental dynamics can be discussed. The Bikou block is situated in the northwest edge of the Yangtze block and is connected with the West Qinling orogenic belt, the Longmenshan orogenic belt, and the Songpan-Ganze orogenic belt through the Mianlue belt, the Qingchuan-Yangpingguan fault, and the Huya fault, respectively. It pinches out eastward in structure and shows an extended triangular-shaped terrane (Zhang G W et al., 1996, 1995). The strata of the Bikou block consist of the Bikou Group and overlying sedimentary rocks from the Sinian and Paleozoic.

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Plenty of work on the Bikou block has been done previously. However, they were mainly limited to volcanic rocks of the Bikou Group; multifarious origin theories of the volcanic rocks have been proposed, such as the viewpoint of island arc setting (Yan et al., 2004; Lu et al., 1997; Zhang J R, 1990; Xia et al., 1989), the view of intra-arc rift valley background (Dong et al., 1998), the perspective of a continental rift environment (Xia et al., 2007, 1996a, b; Xu et al., 2001; Kuang et al., 1999; Ding et al., 1998), the concept of mid-ocean ridges and/or oceanic intra-plate background (Lai et al., 2007; Liu et al., 1993; Tao et al., 1993; Zhang E P et al., 1993; Pei, 1989), and the assumption of paleo-landmass in the Proterozoic (Zhang G W et al., 2001). However, little is known about the characteristics of it and activities within the deep-seated crust of the Bikou block, making it difficult to some extent to discuss the tectonic evolution history of the Bikou block in depth. As granitoid study is an effective device to uncover the deep-seated crustal composition and geodynamic process (Kemp and Hawkesworth, 2003), some researchers have recently studied the granites of the Bikou block in detail and arrived at two different recognitions. Qin et al. (2005) argued that the forming of the rocks has something to do with the detachment of the lithosphere and the underplating of mantle-origin magma in the Qinling orogenic belt after its main orogenesis, while Zhang H F et al. (2007) held the idea that the magma originated from partial melting of the lower crustal basaltic rocks induced by the detachment and thickening of the lithosphere in the Indosinian. Therefore, it is very necessary to carry out a further study on the magma origin and tectonic settings of granites in the Bikou block. This can be brought to light the identities of the deep-seated crust in the Bikou block. For these reasons, we pick out Nanyili, Shaiziyan, and Laohegou intrusions in the south margin of the Bikou block (Fig. 1) as objects of study. After detailed petrology, geochemistry, and LA ICP-MS zircon U-Pb geochronology studies, discussions are given about the origin and tectonic settings of granites in the area, the geological relationships between the rocks and synchronous rocks in adjacent areas, and the inheritance involved in the rocks and the orogenic collision.

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Pei Xianzhi, Li Zuochen, Ding Saping, Feng Jianyun, Li Ruibao, Sun Yu, Zhang Yafeng and Liu Zhanqing

Figure 1. Geological sketch map showing the Bikou block and contiguous areas in Northwest Sichuan. Q. Quaternary; K. Cretaceous; J. Jurassic; T3x. Xujiahe Formation of Triassic; T1–2. Middle–Upper Triassic; P2. Upper Permian; P1. Lower Permian; C–P. Carboniferous–Permian; D–C. Devonian–Carboniferous; D. Devonian; Smx. Maoxian Group of Silurian; S. Silurian; ∈–O. Cambrian–Ordovician; Z–Nh. Sinian– Nanhuaian; Pt3bk. Bikou Group of Meso- and Neoproterozoic; Pt3. Neoproterozoic; γ. granite; F. fault. GEOLOGICAL SETTING AND PETROLOGIC CHARACTERISTICS Nanyili intrusions, which outcrop in the southwest margin of the Bikou block in the Nanyili area, north of Pingwu County, exist as the biggest rock body in the Bikou block and take on intrusive contacted relationships both with metamorphic basaltic lava, pyroclastic volcanic tuff, etc. of the Bikou Group in the north and with metamorphic fine-grained sandstone, silty slate, etc. of the Devonian in the south. Both the Senalu-Tongqian fault and the Zhuolong fault are cut across by the rocks, and a little xenolith can be found in the rim of the rockbodies. The rock bodies display in a nearly triangular shape, reaching approximately 140 km2 in area. They are single and relatively uniform in lithology, like biotite granodiorites, which take on a gray-white color, a medium–fine grained granitic texture, and a blocky structure without deformation and metamorphism. Their essential minerals are described in micro-scale as follows: quartzes that are xenomorphic-granular in texture and are 30%– 35% in volume percentage; plagioclases that

are euhedral columnar in texture and are 32%± in volume percentage; K-feldspars that are irregular and tabular shaped and are 25%± in volume percentage; and muscovites that are sub-euhedral flake in structure and are 3%± in volume percentage. Biotites occupy most of the dark minerals overwhelmingly with a volume of 8%± in the whole rock; the dominant accessory minerals are sphenes and apatites, while zircons, orthites, clinozoisites, magnetite, etc. are the secondary accessory minerals. Laohegou intrusions are located in the Laohegou area of Northeast Pingwu County and emplaced into the epimetamorphic sandstone and shale of the Wugongkou Formation of Lower Sinian with apparent contacted metamorphism in their rims. The rocks are oval shaped in appearance and distribute over a large range nearly 11 km2 in area. They are single and relatively uniform in lithology, and are granites with a light gray-gray color, a medium-macrograined granitic texture, and a coarse-grained inner structure, up to 0.8 cm in diameter for a single granule. Nevertheless, the rocks have a fine-grained texture on the outside rang-


ing for 0.2–0.5 cm in diameter and have a massive structure without obvious deformation and metamorphism. Their major minerals consist of quartze, which are white, of irregular allotriomorphic-granular texture, and 25%–30% in volume percentage; and plagioclases, which are gray-white, euhedral columnar in texture, and 60%–65% in volume percentage. The dark minerals are mainly biotites, which are dark tan in color, sub-euhedral flake in structure, and 3%–5% in volume percentage. Their dominant accessory minerals are sphenes and apatites; zircons, orthites, clinozoisites, magnetites, etc. are the secondary accessory minerals. Shaiziyan intrusions are exposed in the south Mupi Village of Pingwu County and intrude into the light gray-light gray-green biotite-sericite-quartzalbite schist and gray-green chlorite-epidote-albite schist of the Bikou Group with obvious chloritization and epidotization in country rocks. The rocks show a nearly oval shape on the map and cover a small area approximately 3.2 km2. They are single and relatively uniform in lithology like biotite-granites, which appear as gray-dark gray in color, porphyroid granitic in texture, and massive in structure, with little deformation and metamorphism. The essential minerals of the rocks are quartzes, which are white, of irregular allotriomorphic-granular texture, and 30%–35% in volume percentage; and plagioclases, which are gray-white in color, euhedral columnar in texture, and 45%–50% in volume percentage. The dark minerals of the rocks are mainly biotites that are dark tan in color, euhedral flake in structure, and 10%–15% in volume percentage. The phenocrysts consist of gray-white plagioclases, which have a high degree of euhedral extent and arrange in (0.5–1) cm×(0.8–2) cm size. Their dominant accessory minerals are sphenes and apatites; while zircons, orthites, clinozoisites, magnetites, etc. are the secondary accessory minerals, the same as those of the two other rocks. ANALYTICAL METHODS LA ICP-MS Testing Method One sample (D202) was sampled in Nanyili intrusions for isotopic dating, and its geographic coordinates are 32°43'12.3" N, 104°21'54.6" E. Rock samples were crushed to 80–100 meshes using conventional methods and were separated by

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flotation and electromagnetism techniques; then the nice-looking crystal-shaped and transparent zircons were picked out under a binocular microscope by handpicking. Zircons were pasted on two-sided glue and fixed with colorless transparent epoxy resin until fully solidified, and the surface was polished to expose the interior of the zircons. The cathodoluminescence (CL) microphotography images were accomplished with a Cameca electron probe X-ray microanalyser at the Institute of Geology and Geophysics, Chinese Academy of Sciences. The analysis voltage was 15 kV and the current was 19 nA. The in situ U-Pb isotopic age analysis of zircons was carried out following the standard test procedure on the LA ICP-MS instrument at the State Key Laboratory of Continental Dynamics, Northwest University. The analysis instruments were an Elan 6100DRC Type Quadrupole Perch Mass Spectrograph and Geolas200M excimer laser ablation system (193 nm, Geolas200M, Lambda Physics). The facula beam’s diameter of laser ablation was 30 μm, and the depth of laser ablation samples was 20–30 μm. The calculation of zircon ages adopted the international standard zircon 91500 as external standard; the element content adopted the artificial synthetic silicate glass NIST SRM610 of the American National Standard Substance Bureau as external standard. 29Si was used to adjust as the internal standard element. The isotopic ratio and element content data were handled with GLITTER (ver 4.0, Macquarie University) software, the general plumbum adjustment was conducted by Andersen software, and the age calculation and concordia diagram drafting were completed with ISOPLOT (2.49 edition) (Ludwig, 2001, 1991). The detailed experiment principle, technological process, and instrument parameters refer to the technical literature (Yuan et al., 2003). Analysis Methods Thirteen samples were selected for the analysis of major elements and trace elements, respectively. The major and trace elements were determined by the State Key Laboratory of Lithosphere Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, after the sample was ground to 200 meshes. The major elements were tested using the method of

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Pei Xianzhi, Li Zuochen, Ding Saping, Feng Jianyun, Li Ruibao, Sun Yu, Zhang Yafeng and Liu Zhanqing

X-ray fluorescence spectrometry (XRF-1500). To get the content of the oxide, a sheet glass made of 0.6 g samples and 6 g lithium tetraborate was tested on the Shimadzu XRF-1500, the precision of which was better than 2%–3%. While the trace elements and rare earth elements (REE) were tested by ICP-MS (ElementⅡ), the samples were prepared using the methods of acid-solubility, which, when the element content is greater than 10×10-6, has an analytic precision better than 5%; otherwise, precision is better than 10%, according to the national standard on the GSR-1 and GSR-2. The testing process of chemical analysis refers to the methods described by Chen Z H et al. (2004). RESULTS U-Pb Dating of Zircons Zircons selected from the sample (D202) of Nanyili intrusions are euhedral, pale-yellow-colorless

transparent, and have a clear internal structure, as the CL images of zircons illustrated in Fig. 2. The images reveal that the zircons have typical magma growth oscillatory zonal texture, rhythm texture, and core-rim texture, and are the products of magma crystallization (Siebel et al., 2005; Wu and Zheng, 2004; Zhang C L et al., 2004; Belousova et al., 2002). The zircons can be divided into three groups in terms of their texture. The first group has three zircons (D202-01, D202-09, D202-19, Fig. 2a) showing long-pyramid shapes, a high length/width ratio of about 2 : 1, and granularities of about 80–200 μm in general. There are five zircons in the second group (D202-06, D202-13, D202-16, D202-21, D202-23, Fig. 2b), which appear to be short-pyramid shaped or nearly oval-shaped with a length/width ratio of about 1 : 1–2 : 1, showing strong luminescence in CL images (except for D202-13), and feature granularities of about 150–250 μm in general.

Figure 2. CL images and ages of single zircon U-Pb of Nanyili intrusions. (a) CL images showing zircons from the Neoproterozoic; (b) CL images showing zircons from the Middle–Late Ordovician; (c) CL images showing zircons from the Middle–Late Triassic.


What’s more, residual cores of inherited zircons can be found in some zircons, which appear white and oval-shaped in the CL images. The other 17 zircons are classified in the third group (Fig. 2c), the granularities of which are about 150–350 μm in general and the crystals of which exhibit long-pyramid and hemi-pyramid shapes with a length/width ratio of about 1 : 1–3 : 1. There are residual cores in inherited zircons among the samples of D202-12, D202-18, and D202-20, which display strong luminescence in CL images. Many studies show that different genetic zircons have different contents of Th and U, and ratios of Th/U. Magmatic zircons have a higher content of Th and U, and a higher ratio of Th/U (>0.4 in general), while metamorphic zircons have a lower Th and U content, and Th/U ratio (