Elsevier

Lithos

Volume 105, Issues 3–4, October 2008, Pages 301-318
Lithos

High-pressure metamorphic rocks from Tongbaishan, central China: U–Pb and 40Ar/39Ar age constraints on the provenance of protoliths and timing of metamorphism

https://doi.org/10.1016/j.lithos.2008.04.009Get rights and content

Abstract

The Tongbaishan area is situated at the junction between the Paleozoic North Qinling orogen and the Mesozoic South Qinling–Dabie–Sulu orogen. The area contains an important geological section for understanding the assembly of the Sino-Korean and Yangtze cratons. Two eclogite zones were recently recognized in this area. The peak metamorphic conditions are estimated to be 530–610 °C and 17–20 kbar for the northern eclogite zone and 460–560 °C and 13–19 kbar for the southern eclogite zone. SHRIMP U–Pb analyses on zircons of eclogite, metagabbro and orthogneiss indicate that their protoliths were formed during the Neoproterozoic, and experienced high-pressure (HP) recrystallization during the Permian–Triassic period. Therefore, the Tongbaishan area belongs to the western extension of the Dabieshan HP/UHP terrane. In addition, U–Pb ages of detrital zircons from quartzite, paragneiss and schist show three age populations of 2.49 Ga, 1.93 Ga and 1.85–1.82 Ga, suggesting that extensive Paleoproterozoic orogeny or tectonothermal events took place in the northern margin of the Yangtze craton. The SHRIMP dating and complementary 40Ar/39Ar dating of muscovite from quartzofeldspathic gneiss and quartzite reveal that the crustal rocks were subducted to great depths and underwent HP eclogite facies metamorphism at ca. 255 Ma, slightly earlier than the timing of deep subduction of the UHP slice. The HP rocks were later exhumed to mid- to upper-crustal levels and suffered from retrograde recrystallization at ca. 238 Ma. The dating also discloses a still later phase of deformation and recrystallization at ca. 215 Ma. The available age data throughout the Tongbai–Dabie–Sulu orogen reflect diachronous exhumation from ca. 238 Ma for the HP slice to ca. 213 Ma for the UHP slice. Such a phenomenon could be explained by successive subduction and exhumation of the broken slices of the subducting Yangtze slab.

Introduction

The Paleozoic–Mesozoic Qinling–Dabie–Sulu orogen marks the suture zone between the Sino-Korean and Yangtze cratons. The most important feature of the orogen is the preservation of high-pressure and ultrahigh-pressure (HP/UHP) metamorphic rocks of different ages. Regionally, Cambrian HP/UHP rocks occur in North Qinling (Qinling Group) (Hu et al., 1995, Hu et al., 1996, Yang et al., 2003a), whereas Triassic HP/UHP rocks are mainly distributed in the Dabieshan and Sulu areas (Hacker et al., 1996, Liou et al., 1996). Furthermore, some HP eclogites from the northwestern corner of western Dabieshan (i.e. Hong'an block) appear to have formed during the Silurian or Carboniferous time (Jian et al., 1997, Jian et al., 2000, Xu et al., 2000, Li et al., 2001, Sun et al., 2002). Since HP/UHP metamorphic rocks record the history of continental subduction, collision and subsequent exhumation, the multiple HP/UHP events registered in the Qinling–Dabie–Sulu orogen have well constrained the geodynamic evolution during the assembly of the Sino-Korean and Yangtze cratons.

The Tongbaishan area lies at the junction between the Paleozoic North Qinling orogen and the Mesozoic South Qinling–Dabie–Sulu orogen (Fig. 1). HP eclogites and their retrograde products have recently been recognized in several localities in this area (Wei et al., 1999, Suo et al., 2001, Liu et al., 2005). These rocks clearly holds key to the understanding of the evolution of the Qinling–Dabie–Sulu orogen, and therefore, geochronological data are most critical. Up to the present, however, the ages of these HP rocks remain unconstrained. In this paper, we report SHRIMP U–Pb ages on zircon and 40Ar/39Ar ages on muscovite from metabasites (eclogite and metagabbro) and their country rocks (gneiss, quartzite and schist) from the Tongbaishan area in order to (1) delineate the formation time and provenance of protoliths of the HP rocks, and (2) to date the event of HP metamorphism. The geodynamic evolution between the Sino-Korean and Yangtze cratons will be further discussed in conjunction with the available age data from the Dabie–Sulu HP/UHP metamorphic terrane.

Section snippets

Regional geology and HP metamorphism

The Tongbaishan area is separated from Qinling in the west by the Nanyang basin, and from western Dabieshan in the east by the Dawu fault (Fig. 1). The area comprises three Paleozoic and five Mesozoic metamorphic units. These metamorphic units are separated by large-scale shear zones as a result of extensional detachment at a late stage of exhumation (Zhong et al., 1999, Zhong et al., 2001, Suo et al., 2001). A major shear zone along the southwest boundary of the Tongbai complex (see below) has

Sample description

In order to precisely define ages of protolith and HP metamorphism, two fresh eclogites (samples HYH01-3 and SHZ08-1), a retrograde eclogite (sample GM02-1), a metagabbro (sample DFS03-1), a quartzofeldspathic orthogneiss (sample HYH01-11), a quartzofeldspathic paragneiss (sample SHZ03-5), a pelitic schist (sample TB02-1) and a quartzite (sample DFS06-1) from the northern and southern eclogite zones were chosen for SHRIMP U–Pb zircon dating. Furthermore, samples HYH01-11, SHZ03-5, DFS06-1 and

Sample HYH01-3: eclogite from the northern eclogite zone

Zircons from eclogite sample HYH01-3 are short prismatic and irregular in shape, with 50–150 μm in length. CL images reveal that most zircon grains are oscillatory-zoned, but the oscillatory bands have commonly been thickened and somewhat blurred (Fig. 2a,b). This feature reflects a magmatic origin of the zircons, which have undergone a post-magmatic solid-state recrystallization (Hoskin and Black, 2000). In addition, two zircon grains show grey homogeneous or cloudy-zoned pattern (Fig. 2c).

Age and provenance of protoliths

For the Qinling–Dabie–Sulu orogen, the ages of inherited zircons can be used to determine the provenance of the Sino-Korean or Yangtze craton and therefore the location of the suture (Hacker et al., 1998, Hacker et al., 2006). As generally accepted, the distinctive ca. 750 Ma bimodal magmatism are diagnostic of the Yangtze craton (Hacker et al., 1998, Hacker et al., 2000) and its occurrence was related to the rifting of the supercontinent Rodinia (Li et al., 2003). Four Neoproterozoic protolith

Conclusions

  • (1)

    U–Pb zircon analyses of eclogite, metagabbro and orthogneiss from the Tongbaishan area reveal an extensive bimodal magmatism in the period of 768–662 Ma, whereas detrital zircons from metasedimentary rocks reveal three major tectonothermal events of 2.49 Ga, 1.93 Ga and 1.85–1.82 Ga. The new age data suggest that the northern margin of the Yangtze craton involves a Paleoproterozoic orogenesis and a Neoproterozoic rifting.

  • (2)

    40Ar/39Ar dating of muscovite from quartzofeldspathic gneiss and quartzite

Acknowledgements

We would like to thank Ping Jian, Wei Zhang and Biao Song for assistance in the SHRIMP U–Pb zircon analyses and Wen Chen and Yan Zhang in the 40Ar/39Ar muscovite analyses. Critical reviews by two anonymous reviewers and editorial comments by I. Buick substantially improved the manuscript. The research of the senior author was supported by the National Natural Science Foundation of China (No. 40672047), Geological Investigation Project of China Geological Survey (1212010711812) and Free Research

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