Mineral chemistry, geochemistry and geological significance of Early Cretaceous mafic rock in Longyou area, Gan-Hang belt

There have been reports of diamonds found in the mafic rocks of Longyou, Zhejiang Province. However, the tectonic setting and host rock characteristics still lack detailed studies. In this paper, the authors carried out a detailed study of petrology, major and trace elements, Sr-Nd isotopic compositions and mineral chemistry for the Longyou mafic rocks in an attempt to unravel the petrogenesis, magma evolution and tectonic setting during Late Mesozoic period. Longyou mafic rocks can be classified as olivine gabbro. Isotope characteristics indicate that the olivine gabbro was derived from a deep mantle source. The major and trace elements show that they mainly underwent fractional crystallization of olivine and pyroxene, without any obvious crustal contamination. Olivine grains show a core-rim structure with the core being Mg-rich and Fe-depleted and Fo values being 90.1~91.8, which indicates that the original magma was an asthenospheric mantle. In contrast, the rim of the olivine grains is Fe-rich and Mg-depleted with Fo values being 77.4~85.3. The remarked difference in Fo values for the core and rim may indicate that the original olivine was replaced by mantle melt/fluid. Most pyroxene xenocrystals belong to diopside, and a small amount of aegirite augite is developed at the edge of some diopside xenocrystals. The diopside xenocrystal generally has a core-mantle-rim structure, exhibiting a decrease in SiO₂ and increase in TiO₂, Al₂O₃ and temperature of magmatic emplacement from the core to rim. The authors hold that this corerim trend may indicate that the pyroxenes were metasomatized by later higher temperature mantle melt/fluid. It is argued that the Longyou olivine gabbro was formed during the roll-back of the Paleo-Pacific plate along the deep faults of the Gan-Hang tectonic belt. The asthenospheric mantle melts moved upwards along these deep faults and the magma responsible for the Longyou olivine gabbro experienced the fractional crystallization between olivine and pyroxene with very low degree of or without crustal contamination. Furthermore, it is also suggested that the Longyou olivine gabbro experienced a metasomatism of higher temperature mantle melt/fluids.