Geochemistry and geohronology of Hualicun mafic rocks in North Daba Mountain and tectonic control

Hualicun mafic rocks are exposed in Langao County, Shaanxi Province. They are dominated by diabase-porphyrite, gabbro-diabase, diabase, and gabbro. The main minerals are clionopyroxene and plagioclase, and the accessory minerals are Fe-Ti oxides. Major elements show high TiO₂ (2.26%~7.30%), high TFe₂O₃ (11.17%~17.0%), low MgO (2.96%~5.52%), and high alkaline content(K₂O+Na₂O=3.07%~6.76%), thus belonging to alkaline gabbro series. Mg^# (36~42) of Hualicun mafic rocks suggest that they possibly originated from an extremely evolved magma. Primitive mantle-normalized multi-element patterns show enrichment of Ba and Pb for large ion lithophile elements, and depletion of K. Chondrite-normalized REE patterns display LREE enrichment (La_N/Yb_N=9.34~13.05), and normalized patterns for trace elements and REE similar to features of typical OIB. Detailed LA-ICPMS zircon U-Pb dating yielded emplacement ages of 436.9±2.4Ma, corresponding to the age of regional mafic rock and indicating that regional magma occurred in early Silurian. The enriched mantle characteristics for Hualicun mafic rocks suggest that mafic rocks in the North Daba Mountain were derived from a mixture of EMⅠ, EMⅡ and a small amount of HIMU components. Furthermore, the mantle geodynamic analysis of Hualicun mafic rocks that occurred in the Silurian or earlier period indicates an extension related to subduction and uplift of magma.