Low-δ18O A-type granites in SW China: Evidence for the interaction between the subducted Paleotethyan slab and the Emeishan mantle plume

Abstract

Abstract The mechanisms and processes by which subducted slab interacted with mantle plume remain controversial, as direct observation of such interaction is difficult to impossible. Compositional heterogeneity of large igneous provinces (LIPs) additionally makes plume-slab interaction hard to detect. Oxygen isotopes are sensitive enough to trace the source of magmas. Here we provide evidence for plume-slab interaction mainly based on in situ zircon Hf-O isotope analyses, as well as whole-rock elemental and Sr-Nd-Hf isotope analyses, on the Late Permian and Early Triassic A-type granites on the margin of the Emeishan LIP in SW China. These granites show typical A-type geochemical characters, such as high total alkali (7.93–9.68 wt%) and field strength element (HFSE, e.g., Zr and Nb) contents, and high FeOT/(FeOT+MgO) (0.87–0.98) and Ga/Al (3.67–5.06) values. The Late Permian (ca. 259 Ma) and Early Triassic (ca. 248 Ma) granites show high Nb/Th (>3.0) and low Y/Nb (<1.2) and Yb/Ta (<2.0) ratios similar to the oceanic island basalts and have near-zero εNd(t) (−0.83 to −0.13 and −0.15 to +0.16, respectively) and depleted εHf(t) (+2.71 to +3.39 and +2.62 to +3.55, respectively). In situ zircon O-Hf analyses yielded anomalously low δ18O (0.2–2.0‰ and 3.2–4.8‰, respectively) and positive εHf(t) (1.6–7.0 and 3.9–8.8, respectively), suggesting varying proportions of hydrothermally altered oceanic crust in their source region. Our results imply that significant amounts of altered Paleotethyan oceanic crust have been subducted in the upper mantle beneath the western South China Block. The nearby rising Emeishan mantle plume may have rapidly entrained and incorporated these oceanic crustal materials to the shallow mantle so that their low-δ18O isotope feature was preserved. Subsequent decompression-related partial melting of this hybrid source formed parental rocks of the low-δ18O A-type granites. Our findings also suggest that LIPs could obtain their compositional (especially oxygen isotope) diversity through the interaction between the subducting slab and rising mantle plume.