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

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.

Supplemental Material: Low-d18O A-type granites in SW China: Evidence for the interaction between the subducted Paleotethyan slab and the Emeishan mantle plume

Analytical methods, supplementary figures and tables.

Supplemental Material: Low-d18O A-type granites in SW China: Evidence for the interaction between the subducted Paleotethyan slab and the Emeishan mantle plume

Analytical methods, supplementary figures and tables.

Late Permian basalts in the northwestern Sichuan Basin, SW China: Implications for the geodynamics and thermal effect of the Emeishan mantle plume

<p>The Middle-Late Permian Emeishan large igneous province (ELIP), located in the western margin of Yangtze craton, SW China, is regarded as the result of the impingement of a mantle plume onto the lithosphere. However, little is known about the petrogenesis of Late Permian basalts in Sichuan Basin, which were previously considered to be located outside the ELIP. Here we report new petrographic, major elements, trace elements and isotopic data (Sr-Nd-Pb) for Late Permian basalts in the boreholes from the northwestern Sichuan Basin. These basaltic rocks are characterized by low SiO<sub>2</sub> contents (47.17-49.40 wt.%), high TiO<sub>2</sub> contents (3.38-4.11 wt.%) and Ti/Y ratio (539-639), moderate total alkalis contents (Na<sub>2</sub>O+K<sub>2</sub>O, 3.36-6.01 wt.%) and Mg<sup>#</sup> values (40.93-46.04), which geochemically resemble the Emeishan high-Ti basalts. These rocks are enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREE), and have (La/Yb)<sub>N</sub> ranging from 9.95 to 11.78, showing that typical oceanic island basalt (OIB)-like normalized patterns. The fractionation of MREE to HREE suggests that the basalts were generated by low degree of partial melting within the garnet stability field. Low initial <sup>87</sup>Sr/<sup>86</sup>Sr ratios (0.70572-0.70676; t=260 Ma), Pb isotopic ratios [<sup>206</sup>Pb/<sup>204</sup>Pb<sub>(t)</sub> (18.062-18.637), <sup>207</sup>Pb/<sup>204</sup>Pb<sub>(t)</sub> (15.574-15.641), <sup>208</sup>Pb/<sup>204</sup>Pb<sub>(t)</sub> (38.33-38.98)], and slightly high εNd(t) values (-0.03 to +1.34) indicate that the magma formed from a deep mantle source that may possibly be a mantle plume and have negligibly been affected by crustal contamination. This inference is further supported by high Nb/U ratios (20.56-25.70), low Th/Nb (0.17-0.19) and Th/Ta ratios (2.77-3.14), and no visible Nb and Ta anomalie. In addition, thermal history reconstruction using paleogeothermal indicators in the study area shows that the Lower Paleozoic to Middle Permian formations experienced an intensive thermal event and abnormal high heat flow value reached 118.0 mW/m<sup>2</sup> at the Late Permian, which may be due to the mantle plume magma upwelling. The geochemical and geothermal characteristics all demonstrate that these basalts were probably generated in response to the Emeishan mantle plume. Thus, we conclude that the ELIP may have larger areal extent and has been played an important role on the thermal evolution of source rocks in the Sichuan basin.</p>