The once designated Zhucun Formation in Xiaoxiong basin is investigated to be geochronologically earlier (128Ma) not only than the overlying upper volcanic series (110 similar to 88Ma), but also than the underlying Jiuliping Formation (122 similar to 120Ma), but coeval to the Xishantou Formation (130 similar to 128Ma). Zircon Hf isotopic compositions of the studied Zhucun Formation rhyolitic tuff show enriched characteristics with Paleoproterozoic crustal model ages (epsilon(Hf)(t) =-16.4 similar to-13. 6, t(DM2) = 2. 01 similar to 2. 09Ga), also similar to the Lower Moshishan Group, such as the Xishantou Formation. Note these and the pyroclastic-rhyolitic volcanic rock assemblages and other regional geological features, the foundation basis of the Zhucun Formation is considered to be insufficient, and the Formation should be disintegrated. On the other hand, the enriched zircon Hf isotopic compositions of the studied Zhucun Formation rhyolitic tuff show that it was derived mainly from partial melting of the Paleoproterozoic crustal basement, further indicating that the Lower Moshishan Group volcanic rocks inland were derived with depleted-mantle derived material contribution, which was barely noticed in previous studies. The increased depleted-mantle derived material contribution in the volcanic petrogenesis and the pyroclastic-rhyolitic-bimodal volcanic assemblages both imply increased lithospheric extension, plausibly caused by rollback of the subducted paleo-Pacific slab.

Late Permian mafic flows and dikes are prominent features in and around the Western Guangxi region in southern China. Based on petrographic, geochemical and Sr-Nd isotopic data, the western Guangxi mafic rocks are geochemically akin to the Emeishan large igneous province (ELIP) high-Ti basalts, except that they possess extremely elevated Ti/Y ratios (750-2000). The Dy/Yb and Ti/Y vs. Dy/Dy* covariations of the mafic rocks indicate a garnet-controlled magmatic differentiation of a mafic melt at relatively great depth. The limited epsilon(Nd)(t) range from +0.41 to +1.81 also suggests minimal crustal contamination of such a melt. Geochemical modeling using TiO2/Yb vs. Nb/Yb and Zr/Y vs. Nb/Y projections indicate that the parental melts of the western Guangxi mafic rocks formed at a low degree (<5%) of partial melting at or over 3.5 GPa, consistent with a deep mantle plume source under a thick continental lithosphere. Thus, the Guangxi extremely high Ti/Y mafic rocks most likely represent a part of outer zone of the ELIP plume magmatism. Results of this study reinforce the previously proposed temporal and spatial distribution of the ELIP. (C) 2016 Elsevier Ltd. All rights reserved.

The Napo area in the western Guangxi province, South China, is located in the southwestern margin of the South China Block, and to the north of the North Vietnam Block. The Upper Permian mafic rocks well crop out in the west-southwest of Napo County, and are composed of layered, sub-volcanic diabases and orbiculite. Geochemically, the Napo mafic rocks can be subdivided into two groups: high-Ti group (TiO2>2.8%, Ti/Y>500) and low-Ti group. The high-Ti group is mainly composed of alkaline basalts, and the low-Ti group of tholeiite. The high-Ti rocks have lower SiO2, MgO, higher FeOt, P2O5 than the low-Ti ones. The high-Ti group shows an enrichment of LILE, HFSE, and significant fractionation between LREE and HREE, akin to ocean island basalts (OIB), suggestive of an affinity with the Emeishan high-Ti basalts. In contrast, the low-Ti rocks display relatively higher SiO2, MgO, lower FeOt, P2O5, enriched LILE, depleted Nb, Ta, and relatively flat REE patterns, indicative of island-arc geochemical signatures. The trace element ratios and the tectonic discrimination diagrams indicate that the Napo high-Ti mafic rocks were most likely derived from the enriched OIB mantle source, while the low-Ti group showed the transitional features from OIB- to island arc-like signatures. Integrating the geochemistry and regional geological background, we suggest that the Napo high-Ti mafic rocks were probably related to the Emeishan mantle plume magmatism; however, the low-Ti mafic rocks were possibly produced by interaction between the subducting Paleotethyan slab and the Emeishan mantle plume.

The mid-ocean ridge volcanic glass is of mantle partial melting carrying lots of information on formation of oceanic crust and its magmatism. The major element geochemical characteristics of mid-ocean ridge glass from Pacific mid-ocean ridge system is studied,and the element distribution characters of magma is discussed in this paper. Results indicate that the major element composition of glass displays normal evolution,with Si O_2 increasing,the Na_2 O increasing from 2% to 6%,K_2 O increasing from 1% to 5%,but Ca O decreasing from10%- 13% to 1%,Fe O decreasing from 9%- 13% to 3%,and Al_2O_3 decreasing from 18% to 13% when Si O_2 increasing. The distribution pattern of major elements suggests the heterogeneity of magma,which was likely caused by mantle heterogeneity. Moreover,the glass from same mid-ocean ridge system also shows the composition heterogeneity,which is probably due to the locality geodynamic difference( es) inducing from major element heterogeneous distribution.

Permian- Trassic mafic rocks,well cropped out in Youjiang Basin,have a clear spatial distribution of high-Ti and low-Ti mafic rocks. The high-Ti mafic rock mainly distributes in the central and southwest of Youjiang Basin,while the low-Ti mafic rock almost distributes at Funing-Napo-Jingxi-Pinxiang area,NW trending and controlled by the Indosinian fracture structure. The geochemical results indicate that the high-Ti mafic rock is alkaline basalts,enriched in LREE and depleted in HREE with( La / Yb)N= 6. 49- 8. 83,similar with OIB. However the low-Ti mafic rock has a wider transitional signatures ranging from subalkaline basalt to andesite,displays a weak fractionation between LREE and HREE with( La / Yb)N= 1. 1- 4. 3,but a strong depletion of Nb and Ta. Trace elements ratios suggest that the high-Ti mafic rock is likely derived from the Emeishan mantle plume,but the low-Ti mafic rock experienced partial melting of E-MORB-and N-MORB-like magma.The spatial overlap of low-Ti and high-Ti mafic rocks,as well as the medium-acid rocks,formed by subduction orogenic and Emeishan mantle plume in Southwest Guangxi and North Vietnam,suggest that the low-Ti mafic rock is likely formed by a mixed compound in Emeishan mantle plume or asthenosphere and continental crust,and this process may be associated with the plate motion of Indo-Chinese epoch.