Late Permian A-type granites in Ma'andi in the Jinping area, southwestern China: Petrogenesis and implications for plume–slab interaction

The Jinshajiang–Ailaoshan suture zone in southwestern China preserves the magmatic record of the late Permian Paleo-Tethyan oceanic subduction system and the Emeishan mantle plume. However, little is known about whether direct plume–subduction interaction occurred in southwestern China during the late Permian. Here we report a study of late Permian (ca. 254 Ma) quartz monzonites and granites in the locality of Ma'andi in the Jinping area in the southern part of the Ailaoshan suture zone. The quartz monzonites and granites have medium–high SiO2 (62.1–74.9 wt%), high Na2O + K2O (7.8–9.7 wt%), and low TiO2 (0.24–0.84 wt%) and CaO (0.18–4.1 wt%) contents. The studied rocks are enriched in Rb, Th, U, and light rare earth elements and depleted in Ba, Sr, Eu, and Ti. They exhibit A-type granite mineral assemblages and geochemical characteristics, including the occurrence of anhedral amphibole and biotite interstitial to quartz and feldspar, high 10,000 × Ga/Al (1.8–5.1, mean = 2.9) and FeOT/MgO (3.8–23.7) ratios, and high Zr + Nb + Ce + Y contents (345–1024 ppm, mean = 613 ppm). The quartz monzonites and granites have slightly enriched to depleted whole-rock Nd compositions (εNd(t) = −0.9 to +1.3; TDM = 1.2 to 0.8 Ga) and depleted zircon Hf isotopic compositions (εHf(t) = +3.6 to +9.2; TDM = 0.8 to 0.5 Ga) that are similar to those of Emeishan OIB-type high-Ti basalts. The studied rocks also show (206Pb/204Pb)i ratios of 17.206–18.304, (207Pb/204Pb)i ratios of 15.515–15.571, and (208Pb/204Pb)i ratios of 36.404–38.294, respectively, which can best be explained by a mixture of dominant enriched-mantle and subordinate depleted-mantle materials. Combining regional geological data with geochemical characteristics, we suggest that the primary mafic magmas of the quartz monzonites and granites were formed mainly by partial melting of a hybridized source consisting of dominant Emeishan-fossil-plume material (85%–90%) and subordinate (10%–15%) recycled Paleo-Tethyan oceanic crust. Subsequently, these mafic magmas underwent sequential fractional crystallization, forming the A-type quartz monzonites and granites. Plume–slab interaction has been only rarely identified in pre-Cenozoic systems, which casts doubt on the importance of plume–slab interaction through the geological record. However, our study provides direct magmatic evidence for pre-Cenozoic plume–slab interaction.