Origin of Late Permian syenite and gabbro from the Panxi rift, SW China: The fractionation process of mafic magma in the inner zone of the Emeishan mantle plume
Abstract Contemporaneous alkaline and gabbroic intrusions are commonly observed around high-Ti basalt in the inner zone of Emeishan large igneous province (ELIP), the distribution and origin of these igneous rocks are significant to understand the process associated with a head of Emeishan mantle plume activity. In this study, we present detailed petrology, chronology, major and trace element geochemistry, whole rock Sr-Nd isotopes and zircon Hf isotopes for quartz syenite and hornblende gabbro from Miyi area, middle-southern Panxi rift. The Hengshan quartz syenite closely lies to the west of the Hepingcun hornblende gabbro pluton, and both units have identical U-Pb ages (260 Ma). The quartz syenite has high SiO2 (66.74 to 70.63 wt%), Na2O (4.94 to 5.64 wt%) and K2O (4.95 to 5.35 wt%) contents, with significantly negative Eu and Sr anomalies, while the hornblende gabbro displays lower SiO2 (47.80 to 48.85 wt%) and higher MgO (8.09 to 9.06 wt%), TFe2O3 (10.57 to 11.16 wt%) contents with slightly positive Eu and Sr anomalies. The highest eHf(t) values from quartz syenites (QS-1) is up to +10, which is similar to the values of hornblende gabbro (+6.6 to +9.4), indicating the both magmas were predominantly from a mantle-derived component. The negative eNd(t) values (−3.9 to −5.2) and variations in overall eHf(t) values (−5.4 to +1.3) for quartz syenite indicate the rocks had undergone certain extent of contamination by crustal material, combined with the reciprocal normalized trace element patterns, suggesting a common source of both two units. The syenitic and gabbroic units exhibit well-developed correlation of SiO2 with various major oxides, indicating the fractional crystallization plays a major role in magma evolution. We use the MELTS modeling to evaluate the detail fractional crystallization process and the results show that magma composition similar to the quartz syenite can be produced by fractional crystallization of ~4% olivine, ~11% clinopyroxene, ~9% orthopyroxene, ~40% plagioclase and ~8% oxide from a high-Ti basalt magma at ~900 °C. We propose the syenite-gabbro association in Panxi rift coincides with final activity of the Emeishan mantle plume, and the lower part formed the hornblende gabbroic unit in a closed system, the upper part of the intrusion formed the syenitic unit and mixed with the crust material.