Emplacement age of the Songshugou ultramafic massif in the Qinling orogenic belt, and geologic implications

The Songshugou ultramafic massif is located to the north of the Shang-Dan fault, the Palaeozoic suture between the North and South China blocks. It is the largest Apline-type ultramafic body in the Qinling orogenic belt of central China, consisting mainly of dunite with a small amount of harzburgite and minor pyroxenite. We present new LA-ICP-MS U-Pb dating and trace element results for zircon from two garnet amphibolite samples in the contact metamorphic zone surrounding the massif. One was sampled 1m from the massif, the other 5m away. The studied zircon grains are small, anhedral, and display typical metamorphic characteristics of low Th/U values (0.1). The U and Th concentrations of zircon range from several hundred ppm to less than 10ppm. Cathodoluminescence images show two apparent generations of zircon, with lighter cores and darker rims. Core and rim ages however, are identical within error. These two samples yield identical concordant ages of 5067 and 5107 Ma, suggesting that the Songshugou ultramafic massif was emplaced at 510 Ma. Low HREE concentrations and the absence of Eu anomalies in most analysed zircons suggest that the studied grains most likely formed during garnet amphibolite metamorphism induced by emplacement of the ultramafic massif. To better understand the cooling history of the massif, 40Ar/39Ar ages of amphibole from three garnet amphibolite specimens in the contact metamorphic zone and one amphobolite sample about 20m away from the massif were determined. The 40Ar/39Ar ages increase from 37215 Ma (JSM-01) near the massif to disturbed, unreliable 'plateau' ages of 4748 Ma (JSM-03) and 781146 Ma (JSM-04) with increasing distance from the ultramafic massif, showing limited heating during exhumation of the massif, followed by slow cooling. Therefore, the Songshugou ultramafic massif does not reflect the Jining orogeny at 1 Ga. Instead, it was emplaced into the Proterozoic, Qinling Group during the Palaeozoic, probably due to the subduction along the Shang-Dan fault.