Origin of early Cretaceous calc-alkaline lamprophyres from the Sulu orogen in eastern China: implications for enrichment processes beneath continental collisional belt

Abstract

K–Ar dating, major- and trace-element and Sr–Nd isotopic analyses were carried out for early Cretaceous (122–127 Ma) lamprophyres from the Sulu orogen in eastern China. The results show strong fractionation in rare-earth elements with (LREE) >100 times chondrite, but HREE <10 times chondrite, indicating the presence of residual garnet in the melting source. These rocks are characterized by significant LILE and LREE enrichment but Nb and Ta depletion with moderate Zr/Hf (39.8–50.8 with regard to 36 for primitive mantle) and Nb/Ta (17.8–23.0, compared with 17.5 for primitive mantle) fractionations, probably as a consequence of carbonate- and rutile-rich melt metasomatism induced by dehydration and/or melting of subducted continental slab at mantle depths. Age-correlated Sr–Nd isotope ratios show moderate ranges of 87Sr/ 86Sr(i) from 0.70787 to 0.70934 and −17.2 to −11.6 of ε Nd( t). The lamprophyres from the Sulu orogen were derived from decompression melting of such a metasomatized lithospheric mantle that was mainly composed of phlogophite garnet peridotites and experienced crystal fractionation of a mineral assemblage of olivine+clinopyroxene±plagioclase en route to the surface. Such geochemical and isotopic signatures are also prevalent in the contemporaneous basaltic lavas in the Dabie–Sulu belt, suggesting predominant enrichment processes by carbonate- and rutile-rich metasomatic assemblage beneath the continental collisional belt.