THE GEOCHEMISTRY OF INTERMEDIATE VOLCANIC ROCKS OF CRATACEOUS LAIYANG GROUP IN HAIYANG AREA, SULU OROGENIC BELT AND ITS IMPLICATION FOR THE SLAB-MANTLE INTERACTION IN THE CONTINENTAL SUBDUCTION ZONE

Massive volcanic rocks developed in the eastern North China during the cretaceous Qingshan period. Unlike the Qingshan group’s volcanic rocks, the volcanic rocks of Laiyang group developed earlier but distributed limitedly. They exposure to the surface well in Haiyang area, Shandong, China, which is located in the north zone of Su-lu orogen. Their major and trace element and Sr-Nd isotope compositions, combined with zircon U-Pb ages, provide an insight into the nature of their mantle source and the slab-mantle interaction in the continental subduction zone. Their lithologies include basaltic trachyandesite, trachyandesite and less amount of rhyolite, characterized by intermediate-felsic(SiO₂=50.06-76.96%), high Mg(Mg#=46.5-74.6), high-K(K₂O/Na₂O>0.4), low-Ti(TiO₂<1.5%), peraluminous calc-alkaline to alkaline series. They are enriched in LILEs and LREEs, and depleted in HFSEs(Nb,Ta, Ti), with high initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7067-0.7087 and highly negative Ɛ_Nd(t) values of -11.9 to -16.2. Zircon U-Pb dating yields 120.1±0.7 Ma for magma emplacement. Additionally, residual zircon cores yield Triassic and Proterozoic U-Pb ages. An integrated interpretation of their geochemistry is that they were derived from partial melting of metasomatized zone in the orogenic lithospheric mantle.

More interestingly, the suprachondritic Nb/Ta ratio(19.3-22.9) of the Haiyang Crataceous high Mg# intermediate rocks suggests that the intermediate magma formation should be include two stages. Firstly, during continental collision in the Triassic, the felsic melts with high Nb/Ta ratio were originated from the low-degree melting of the deeply subducted continental crust in the presence of both rutile and a hydrous fluid, the slab-derived melt carried the high Nb/Ta value into the overlying SCLM wedge to generate the metasomatities. Secondly, during the postcollisional extension, the metasomatities were melted to generate the high-Mg# intermediate magma. In this process, the trace element characteristics provide the evidences that the lavas were formed at relative shallower levels or preferentially melted garnet-free metasomaitic body and there were the disseminated amphibole, but not residual rutile and mica in the source.