c. 1.85 Ga HP granulite-facies metamorphism in the Dunhuang block of the Tarim Craton, NW China: evidence from U–Pb zircon dating of mafic granulites

Mafic granulites of the Dunhuang block, eastern Tarim Craton, record high-pressure granulite-facies metamorphism followed by a medium- to low-pressure granulite-facies metamorphic overprint, suggesting a clockwise P–T path and implying an environment of collisional orogenesis. Zircon U–Pb ages of two samples are 1834 ± 12 Ma and 1842 ± 5 Ma. Mineral inclusions in zircon indicate that these ages record the high-pressure granulite-facies event, suggesting that the Tarim Craton may be related to the c. 1.85 Ga assembly of the Columbia supercontinent. Similar c. 1.85 Ga metamorphism in both the Dunhuang block and the Alxa block of the North China Craton supports their correlation across the Altyn Tagh fault. Supplementary material: Analytical procedures, mineral abbreviations, mineral chemistry and U–Pb dating data are available at www.geolsoc.org.uk/SUP18541.

M afic granulites of the Dunhuang block, eastern Tarim Craton, record high-pressure granulitefacies metamorphism followed by a medium-to low-pressure granulite-facies metamorphic overprint, suggesting a clockwise P-T path and implying an environment of collisional orogenesis. Zircon U-Pb ages of two samples are 1834 ± 12 Ma and 1842 ± 5 Ma. Mineral inclusions in zircon indicate that these ages record the high-pressure granulite-facies event, suggesting that the Tarim Craton may be related to the c. 1.85 Ga assembly of the Columbia supercontinent. Similar c. 1.85 Ga metamorphism in both the Dunhuang block and the Alxa block of the North China Craton supports their correlation across the Altyn Tagh fault.
Supplementary material: Analytical procedures, mineral abbreviations, mineral chemistry and U-Pb dating data are available at www.geolsoc.org.uk/SUP18541. Metamorphism at conditions of (ultra)high-pressure, high-pressure (HP) granulite facies and ultrahigh temperature is sparsely preserved but widely distributed in Phanerozoic orogenic belts and Precambrian cratons (e.g. Santosh & Kusky 2010;Santosh et al. 2012;and references therein). This metamorphism preserves important records of the tectonic history of orogenic belts and the formation of cratons. High-pressure granulite is typically inferred to record subduction-to-collision orogenesis, and studies of highpressure granulite in the Precambrian cratons can provide crucial constraints on the evolution of early continental assembly and crust evolution.
China consists mainly of three Precambrian cratons (the North China Craton, the South China Craton and the Tarim Craton) that amalgamated during Phanerozoic orogenic processes (e.g. Zhao et al. 2005;Zhai & Santosh 2011). Palaeoproterozoic highpressure granulite has been recognized in the North China Craton and has attracted global attention in recent years, particularly in relation to Palaeoproterozoic subduction-collision and the assembly of the Palaeoproterozoic supercontinent Columbia. However, compared with the North China and South China Cratons, the Pl coexisting with Cpx has An = 22-28 mol%, whereas Pl coexisting with Opx has a higher An content (An = 50-57 mol%).
Later metamorphic reactions led to the growth of Opx. Using Grt-Opx-Pl-Qtz thermobarometry yields 0.6-0.7 GPa and 700-770 °C (Newton & Perkins 1982;Harley 1984), indicating a significant drop in pressure for similar temperatures. Combining petrographic observations with P-t estimates from two mafic granulite samples suggests a clockwise loop with inferred prograde amphibolitefacies conditions, peak conditions in the HP granulite facies and nearly isothermal decompression through medium-to low-pressure granulite facies to amphibolite facies (Fig. 3).
Analytical procedures and U-Pb results. The zircons were mounted in epoxy resin and were polished to expose the cores of the grains for cathodoluminescence (CL) and U-Pb analyses. Prior to analysis, the mineral inclusions in zircons were identified by laser Raman spectrophotometry. The U-Pb analyses were performed at the Institute of Mineral Resources, Chinese Academy of Geological Sciences using the multicollector laser ablation inductively coupled plasma mass spectrometry facility. Detailed analytical procedures have been given by Hou et al. (2009) andLiu et al. (2010).
All zircons from the two mafic granulite samples show similar characteristics. These zircons are clear, and form spherical, oval or irregular crystals (Fig. 3). In the CL images, most of the zircon grains show low to medium luminescence with fir-tree sector zoning or are unzoned. These morphology and CL features are characteristic of metamorphic zircon grains. Minor mineral inclusions of Grt, Cpx, Pl and Qtz in zircons indicate that these zircons grew under high-pressure granulite-facies conditions (Fig. 3). A narrow CL-bright rim was also observed for most zircon grains, probably implying a later metamorphic overprint or fluid modification.
A total of 68 spots were analysed on 68 grains for sample AQ10-4-4.1, and 63 spots were analysed on 63 grains for AQ10-4-2.3. Most of these analyses produced discordant ages, resulting from a marked loss of radiogenic Pb. For sample AQ10-4-2.3, a few analyses are concordant with 207 Pb/ 206 Pb ages ranging from 1805 ± 19 Ma to 1859 ± 29 Ma and give a weighted mean age of 1822 ± 14 Ma (MSWD = 1.8). The upper intercept age of the discordia line defined by all analyses is at 1834 ± 12 Ma (MSWD = 2.8) (Fig. 4a). We interpret the 1834 ± 12 Ma age as the time of granulite-facies metamorphism. For sample AQ10-4-4.1, all analyses are discordant and yield a concordia upper intercept age of 1842 ± 5 Ma and a lower intercept age of 442 ± 11 Ma (MSWD = 1.15, Fig. 4b).
Discussion and conclusions. The ages from the two samples are identical within uncertainty, and on the basis of the mineral inclusions in zircon we suggest that 1.83-1.84 Ga represents the age of the high-pressure granulite-facies metamorphism determined from thermobarometry. The relatively high P/t of peak metamorphism and the clockwise P-t path suggest that these HP rocks are the products of collisional orogenesis. This paper presents the first evidence of Palaeoproterozoic HP granulite in the Tarim Craton. However, HP granulites with ages of c. 1.85 Ga have been commonly reported in the Central Zone of the North China Craton (also named the Trans-North China Orogen) and are considered to be key evidence of collisional orogeny between the western continental block and the eastern continental block, resulting in final accretion of the North China Craton (Zhao et al. 2005, and references therein). Moreover, c. 1.85 Ga high-pressure granulitefacies metamorphism in the North China Craton is not restricted to the 'Trans-North China Orogen' (e.g. Kusky et al. 2007;Kusky 2011), and several workers have suggested that the distribution of 1.85 Ga metamorphic rocks in the North China Craton is more complex than originally proposed by Zhao et al. (2005) (e.g. Wan et al. 2006. This necessitates a re-evaluation of the Palaeoproterozoic tectonic models of the North China Craton. For the Tarim Craton, recently reported 1.85-1.80 Ga metamorphism in the Kuluketage was interpreted as a production of Palaeoproterozoic orogeny, although its metamorphic conditions and P-t path have not been constrained . Therefore, similar to the North China Craton, a c. 1.85 Ma tectonothermal event probably also represents a major orogenic event in the Tarim Craton and is broadly coeval with the late Palaeoproterozoic global orogenic event recorded from many other continental fragments, including Laurentia, Baltica, Amazonia and India, which has been correlated with the time of assembly of the Palaeoproterozoic supercontinent Columbia (Rogers & Santosh 2002Zhao et al. 2002Zhao et al. , 2004Zhao et al. , 2010Santosh et al. 2009;Condie & Aster 2010;Santosh 2010;Meert 2012). Recent models suggest that the assembly of Columbia at c. 1.90-1.85 Ga coincided with major geological events that affected the entire globe (Zhang et al. 2011, and references therein). These observations suggest that the Tarim Craton is part of the Palaeoproterozoic Columbia supercontinent.
Our new data support a hypothesis that the Dunhuang block is the western extension of the Alxa block as result of the sinistral displacement of the Altyn Tagh fault (xu et al. 1999; yang et al. 2001) (see Fig. 1). The Alxa block is the westernmost part of the North China Craton, and has been traditionally considered to be Archaean. However, recent studies indicate that the eastern Alxa block was subjected to high-grade metamorphic events c. 1.89 Ga and c. 1.79 Ga (Dan et al. 2012). We speculate that the c. 1.89 Ga metamorphic event may be related to continental collision, whereas the c. 1.79 Ga event is probably a result of terrane uplift and/or regional extension. Also, a U-Pb zircon age of 1856 ± 6 Ma from an amphibolite sample at Longshoushan (near Jinchang city) in the western Alxa block (see Fig. 1; Gong et al. 2011) was interpreted as the age of high-grade metamorphism, although the metamorphic conditions have not been constrained. The age similarity of highgrade metamorphic events in the Tarim Craton and North China Craton suggests that they experienced a similar Palaeoproterozoic collisional orogeny c. 1.90-1.80 Ga.