Late Mesozoic volcanism in the northern Huaiyang tectono-magmatic belt, central China: partial melts from a lithospheric mantle with subducted continental crust relicts beneath the Dabie orogen?

摘要

Geochemistry of two groups of late Mesozoic volcanic rocks from the north Huaiyang belt (NHB) of the northern Dabie orogen provide evidence for their derivation from an enriched lithospheric mantle with subducted continental crustal relicts. The older group (149-137 Ma) consists of trachyandesites and trachydacites, showing light rare earth element (LREE) and large ion lithophile element (LILE) enrichment and strong high field strength element (HFSE, e.g., Nb, Ta, P and Ti) depletion, and highly enriched Sr-Nd isotopic signatures (Sr-87/Sr-86(i)=0.7082-0.7098 and epsilon(Nd)(t)=-24.4 to -19.1). Despite variations in SiO2, MgO and isotopic ratios, they exhibit slight change in TiO2, P2O5, Nb and LREE, suggesting that source differences rather than magmatic processes could be a major controlling factor for their genesis. The younger group (132-116 Ma) is mainly composed of basaltic trachyandesites and trachyandesites, having more significant LREE and LILE enrichment and HFSE depletion but less enriched Sr-Nd isotopic compositions (Sr-87/Sr-16(i)=0.7080-0.7084 and epsilon(Nd)(t) =-19.2 to -16.2) than the older group. The time-integrated elemental and isotopic variations from groups 1 to 2 show that less continental crustal materials were involved in producing the younger group of basaltic magmas because of easy exhaustion of the crustal rocks trapped in the lithospheric mantle during generation of the older intermediate-felsic melts. The petrogeneses of the two volcanic suites in the NHB imply that, though some of the subducted continental slices were rapidly exhumed to the crust shortly after collision, a significant volume of the subducted continental crust might be trapped in the lithospheric mantle. (C) 2004 Elsevier B.V. All rights reserved.