UHT sapphirine granulite metamorphism at 1.93–1.92 Ga caused by gabbronorite intrusions: Implications for tectonic evolution of the northern margin of the North China Craton
Graphical abstract
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Highlights
► A new Pt1 UHT granulite from Dongpo, the North China Craton, is reported. ► UHT metamorphism was caused by ∼1.93 Ga mantle-derived magma injection. ► Sapphirine granulite shows a decompressional P–T path following magma heating.
Introduction
Silica-undersaturated sapphirine-bearing granulites, an important and distinctive type of ultrahigh-temperature (UHT) metamorphic rocks, have been reported from a number of high-grade terrains worldwide including: Antarctica, India, Sri Lanka, Australia, South Africa, Algeria, Canada, Brazil, Italy, Norway, Scotland, Japan, and Peru (see Kelsey et al., 2005, Kelsey, 2008 and references therein). Such rocks are mineralogically and texturally complex, and document abundant metamorphic reactions and derivative processes that provide important insights into understanding the tectonothermal evolution of UHT metamorphism (Harley, 1989, Harley, 1992, Harley, 1998a, Harley, 1998b, Harley, 1998c, Kelsey et al., 2005, Kelsey, 2008). Although UHT is now generally accepted as a part of regional crustal orogenesis, there is still in controversy on the relations of UHT rocks with other rocks, their tectonic settings, and the geodynamic models that have been proposed, i.e., back-arc basins, ridge subduction, self heating of the orogen, accretionary or collisional orogens (e.g. Brown, 2006, Brown, 2007a, Brown, 2007b, Brown, 2009, Kelsey, 2008, Santosh and Kusky, 2010). Therefore it is timely and necessary to document in detail further examples, as in the North China Craton, where new relations suggest a specific metamorphic history and tectonic setting.
In the North China Craton sapphirine granulites have been described from two localities: the Daqingshan area (Jin, 1989, Liu et al., 1993, Liu et al., 2000, Guo et al., 2006, Wan et al., 2009), and the Jining area (Santosh et al., 2006, Santosh et al., 2007a, Santosh et al., 2007b, Liu et al., 2010). Both locations are situated in the EW-trending, Palaeoproterozoic, ∼1.95 Ga Khondalite belt, which separates the Yinshan block to the north from the Ordos block to the south (Zhao et al., 2003a, Zhao et al., 2005, Zhao et al., 2010, Yin et al., 2009, Yin et al., 2011, Zhou et al., 2010). In the Jining area Santosh et al., 2006, Santosh et al., 2007a, Santosh et al., 2007b, Santosh et al., 2008, Santosh et al., 2009a, Santosh et al., 2009b, Santosh et al., 2010 demonstrated that the UHT metamorphism occurred at about ∼1.92 Ga, with an anticlockwise P–T path. Although the timing of the UHT metamorphism is broadly coincident with that of the collisional event that led to formation of the Khondalite belt, the UHT anticlockwise P–T evolution is inconsistent with published clockwise P–T paths documented by non-UHT rocks in the Khondalite belt (Jin et al., 1991, Liu et al., 1993, Liu et al., 1997, Lu et al., 1992, Zhao et al., 1999, Wang et al., 2011). Thus, the significance of the UHT metamorphism of the sapphirine granulites in the Khondalite belt remains ambiguous. Compared with the Jining area, the sapphirine granulites in the Daqingshan area are more silica-undersaturated and preserve mineral assemblages and reaction textures that formed during the peak and exhumation stages.
In this study, we present detailed textural and compositional data on mineral assemblages, symplectites and coronas from silica-undersaturated sapphirine granulites from the Daqingshan area. Using the THERMOCALC program (Holland and Powell, 1998), we estimate the P–T conditions of the UHT metamorphism and define the ambient P–T trajectory. In combination with the field relationships of the sapphirine granulites and their adjacent rocks, our results place new constraints on the origin of the UHT metamorphism and on the tectonic evolution of the Khondalite belt in the Western Block of the North China Craton.
Section snippets
Geological setting
It is widely accepted that the North China Craton formed by collisional amalgamation of the Eastern and the Western Blocks along the central Trans-North China Orogen (TNCO) at ∼1.85 Ga (Fig. 1; Zhao et al., 1998, Zhao et al., 1999, Zhao et al., 2000, Zhao et al., 2001a, Zhao et al., 2002a, Zhao, 2001, Wilde et al., 2002, Guo et al., 2005, Kröner et al., 2005, Kröner et al., 2006, Zhang et al., 2007, Zhang et al., 2009, Lu et al., 2008, Wang et al., 2010a, Li et al., 2010, Liu et al., 2011),
The Daqingshan Sapphirine granulites
The silica-undersaturated sapphirine-bearing granulites in the Daqingshan area are located in the easternmost part of the Daqingshan–Ulashan complex, which is separated from the Archaean Wuchuan complex to the north by the Jiuguan-Xiashihao Fault (Fig. 2, Fig. 3). Being part of the Archaean Yinshan block, the Wuchuan complex consists of late Archaean TTG gneisses and minor mafic granulites; the latter contain garnet + quartz symplectitic coronas surrounding plagioclase and pyroxene grains (the
Petrography and metamorphic textures
Of three types of rocks in the sapphirine granulite layer, only the spinel–garnet–sillimanite–biotite–plagioclase–sapphirine gneisses and the spinel–garnet granulites contain tiny sapphirine grains sparsely disseminated within garnet porphyroblasts, whereas the sapphirine granulite contains numerous grains of sapphirine up to 30% in some samples. Other major phases in the sapphirine granulites are garnet (30–50%), spinel (5–15%), sillimanite (5–15%), biotite (10–20%) and plagioclase (10–20%),
Mineral chemistry
All mineral analyses were made with an electron microprobe (Cameca SX50) housed at the Institute of Geology and Geophysics, Chinese Academy of Sciences in Beijing. Operating conditions were 15 kV and 15 nA with a point beam. Count times were 20 s on peaks, and 10 s on each background. Natural and synthetic phases were used as standards. The data were processed with an online ZAF-type correction. Representative mineral analyses of garnet, sapphirine and biotite are listed in Table 1, Table 2, Table 3
Development of the matrix assemblage (M1)
The major phase change from the mineral inclusion assemblage (M0) to the matrix assemblage (M1) is characterized by the appearance of sapphirine. Using the SiO2–Al2O3–(FeO + MgO) diagrams (Fig. 13) for the inclusion and matrix mineral assemblages, the growth of sapphirine mainly consumed garnet, spinel and sillimanite; no quartz was involved. Accordingly, the major sapphirine-producing reaction can be illustrated as follows:
Tectonic environment
The tectonic environment(s) responsible for UHT metamorphism is an interesting and topical tectono-metamorphic problem. However, in spite of numerous case studies and innovative models there is still little unanimity, because a variety of environments seem to be capable of providing the required UHT conditions. Possible settings include: 1. A back-arc basin of an active accretionary-extensional margin or orogen (Harley, 1989, Harley, 2008, Collins, 2002, Brown, 2006, Brown, 2007a, Brown, 2007b,
Acknowledgements
We thank L.Q. Zhang, T.S. Li, F. Liu, L. Chen, Z.W. Lan, and X.D. Ma for their assistance in the field work, and Q. Mao and Y.G. Ma for their technical support with the microprobe analyses. J. Wheeler and S.G. Song are thanked for constructive reviews. We also thank S. Harley, E. Grew and M. Santosh for their helpful comments on the early version of this manuscript. Most of all, J.H. Guo is very grateful to G.C. Zhao for thorough and constructive discussions and valuable assistance on this
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