Re-Os dating of the Raobazhai ultramafic massif in North Dabie

Abstract

The ultramafic massif at Raobazhai in North Dabie is located in the suture zone between the Yangtze craton and North China craton. The Re-Os isotope compositions of the massif are used to decipher the origin and tectonics of the ultramafic rocks involved in continental subduction and exhumation. Fifteen samples were collected from five drill holes along the main SE-NW axis of the Raobazhai massif. Major and trace element compositions of the samples show linear correlations between MgO, Yb and Al 2 O 3 . This suggests that the massif experienced partial melting with variable degrees and is from fertile to deplete in basaltic compositions. Nine selected samples were analyzed for Re-Os isotope compositions. Re contents range from 0.004 to 0.376 ng/g, Os contents from 0.695 to 3.761 ng/g, 187 Re/ 188 Os ratios from 0.022 to 2.564 and 187 Os/ 188 Os ratios from 0.1165 to 0.1306. These indicate that the massif is a piece of continental lithospheric mantle with variable depletion. Using the positive correlations between 187 Os/ 188 Os and Yb and Al 2 O 3 respectively, a proxy isochron age of 1.8±0.1 Ga is obtained for the Raobazhai ultramafic massif, which is interpreted to represent a fragment of the ancient subcontinental lithospheric mantle. During Triassic subduction of the Yangtze craton beneath the North China craton, part of the wedge of subcontinental lithospheric mantle was intruded into the subduction belt, and then exhumed to crustal level together with the subducted crustal plate after ultrahigh pressure metamorphism at mantle depths. This ancient lithospheric mantle is now exposed as an orogenic peridotite massif.