Petrogenesis and tectonic implications of Late Jurassic shoshonitic lamprophyre dikes from the Liaodong Peninsula, NE China

Abstract

This paper presents detailed mineral chemical, element geochemical and Sr–Nd–Hf isotopic data for the Late Jurassic (155 ± 4 Ma) lamprophyre dikes in the Liaodong Peninsula, NE China. The lamprophyres are shoshonitic and geochemically fall into three groups: Group I has relatively high SiO2 (52.5–57.0 wt.%), low MgO (5.5–8.3 wt.%) and compatible trace element (e.g. Cr = 128–470 ppm) contents, high initial 87Sr/86Sr ratios (0.7093–0.7117), and low eNd (T) values (−9.6 to −12.1); Group II has relatively low SiO2 (44.8–50.0 wt.%), high MgO (10.8–14.2 wt.%) and compatible trace element (e.g. Cr = 456–1,041 ppm) contents, low initial 87Sr/86Sr ratios (0.7073–0.7087), and high eNd (T) values (−1.4 to −2.9); Group III is transitional between the two in all elemental and isotopic compositions. Interpretation of the elemental and isotopic data suggests that the lamprophyric melts were derived by partial melting of subcontinental lithospheric mantle (SCLM) at a depth of 60–80 km (group I), decompression melting of upwelling asthenosphere at 60–100 km (group II), and mixing between the SCLM-derived and asthenosphere-derived melts (group III). It is assumed that the local SCLM was detached at a depth of 60–80 km by the 155 Ma ago. A continental arc-rifting related to the Palaeo-Pacific plate subduction is favored as a geodynamic force for such a cratonic lithosphere detachment.