Oligocene High-MgO Alkali Basalts in Central Tibet: Implications for Magma–Mush Mixing and Mantle Processes.

This article focuses on the petrogenesis and mantle source interpretations of high-MgO (>9 wt %) basaltic rocks from the Lugu area in the southern Qiangtang terrane (SQT) of central Tibet. It distinguishes two types of alkali basalts erupted around 29 million years ago: porphyritic alkali basalts (PABs), identified as near-primary magmas derived from partial melting of enriched garnet peridotite in the presence of H₂O and CO₂ at depths of 95–105 km; and intersertal alkali basalts (INABs), which, despite high MgO contents, represent mixtures of evolved residual melts and cumulate crystals formed through fractional crystallization and crystal–melt mixing in magma reservoirs. The study highlights that assuming all high-MgO basalts as primary magmas can lead to erroneous conclusions about mantle source characteristics, emphasizing the necessity of detailed mineralogical analyses before interpreting mantle processes. These findings also suggest a heterogeneous lithospheric mantle beneath the Qiangtang terrane during the Oligocene, with implications for understanding regional geodynamic evolution.