Late Permian High-Ti Basalt in Western Guangxi, SW China and Its Link With the Emeishan Large Igneous Province: Geochronological and Geochemical Perspectives

High-Ti (Ti/Y) flood basalts are widely distributed in the Late Permian Emeishan large igneous province (LIP), SW China, and their spatial distribution and genetic mechanism are important to reveal the role of plume-lithosphere interactions in the LIP origin. Western Guangxi is located on the eastern edge of Emeishan LIP. To explore the genesis of the high-Ti basalt in western Guangxi and any genetic link with the Emeishan LIP, we performed whole-rock geochemical and Sr-Nd isotope and zircon U-Pb-Hf isotope analyses on the Longlin basalts from western Guangxi. The results indicate that the Longlin basalt from Tongdeng area has relatively high SiO2 but low MgO and TFe2O3 contents. The rocks have zircon εHf(t) = −0.42 to 6.41, whole-rock (87Sr/86Sr)i = 0.707167–0.707345, and εNd(t) = −2.5 to −2.14. In contrast, the Longlin basalt from Zhoudong area has relatively low SiO2 but high MgO and TFe2O3 contents. The rocks have whole-rock (87Sr/86Sr)i = 0.706181–0.706191 and εNd(t) = −0.57 to 0.69. Four Longlin basalt samples display LREE enrichments and HREE depletions, and with indistinct δEu and δCe anomalies. LA-ICP-MS zircon U-Pb dating on three Longlin basalt samples (from different localities) yielded consistent weighted average age of 257.9 ± 2.6 Ma (MSWD = 0.55), 259.5 ± 0.75 Ma (MSWD = 3.0), and 256.7 ± 2.0 Ma (MSWD = 0.68), indicating a Late Permian emplacement. Considering the similar age and geochemical features between the Longlin basalt and Emeishan flood basalts, we interpret that the former is spatially, and temporally associated with the Emeishan LIP. Geochemical features show that the high-Ti basalts in western Guangxi resemble Deccan-type continental flood basalts (CFBs), which were derived by decompression melting of the mantle plume. Combined with previous geochemical studies, we suggest that the difference in Ti content and Ti/Y ratio in CFBs are related to the depth and melting degree of mantle source, in which high-Ti features may have been linked to low degree of partial melting in the deep mantle.

Low-Ti gabbroic pluton in Dali, SW China: new evidence for back-arc lithospheric melting inducing the early-stage magmatism of the Emeishan large igneous province

The latest studies proved contribution of the Emeishan mantle plume (the widely-regarded origin of the Emeishan LIP in the western Yangtze Plate. LIP: large igneous province) to the Palaeo-Tethys subduction. However, whether the Palaeo-Tethys subduction oppositely affected the formation of the Emeishan LIP remains poorly understood. Here, we report geochronological, petrological, geochemical and isotopic studies of a gabbroic intrusion in this LIP, located in Jiangwei, the Dali area. The gabbro has a weighted mean SHRIMP U-Pb age of ∼262 Ma. Key geochemical features include Nb, Ta and Ti depletion; Th, U and Sr enrichment, low light/heavy rare earth element ratios and ∼0.707 87Sr/86Sr(t) and ∼-0.21 εNd(t) values. We conducted pMELTS thermodynamic modeling and batch melting calculations to evaluate the origin and evolution of the gabbro, based on real components of low-Ti picrites and xenolith of the Yangtze lithosphere. The results support 3% melting of a hydrated spinel peridotite source from the Yangtze lithosphere can produce magma equivalent to the gabbro components. Integrating this conclusion with tectonic background of the western Yangtze Plate and volcano-stratigraphic record of the Emeishan LIP, we infer the early-stage magmatism of the Emeishan LIP was triggered by Paleo-Tethys back-arc extension with fluid modification from subductional slab.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5433267

World-Class Fe-Ti-V Oxide Deposits Formed in Feeder Conduits by Removing Cotectic Silicates

Extremely thick Fe-Ti-V oxide layers are hosted in mafic-ultramafic intrusions of the Emeishan large igneous province (LIP) in the Pan-Xi district of southwest China, accounting for a quarter of the world’s Ti and V resource. It is unclear why these small intrusions contain such huge ore reserves that form world-class Fe-Ti-V oxide deposits. We find that the Hongge intrusion contains 35% Fe-Ti-V oxides, which is twice the typical content in mafic-ultramafic intrusions worldwide and the experimentally determined cotectic proportion in natural ferrobasaltic magma systems. The V content is almost constant in titanomagnetite across the entire Hongge intrusion in the Emeishan LIP, indicating a small (10–20%) proportion of cotectic Fe-Ti-V oxide during fractional crystallization. The bulk composition of the intrusion indicates an open magma system at the time of its formation. Clinopyroxene phenocrysts from overlying basalts contain Fe-Ti-V oxide inclusions, indicating that the phenocrysts crystallized at depth from magma saturated in Fe-Ti-V oxide and were then transported to the surface. We suggest that these intrusions were feeder conduits to the overlying basalts, where the silicates were cotectic with Fe-Ti-V oxides which were then extracted from the underlying intrusion as phenocrysts. Such a fundamental process is key to increase the proportion of oxide minerals in the residual assemblage, thereby upgrading the barren oxide-bearing rocks to world-class Fe-Ti-V oxide deposits in the small intrusions of the Emeishan LIP. A similar process might have occurred in LIPs elsewhere, meaning that intrusions formed as conduit-like open systems to the basalts in LIPs are good exploration targets for giant high-grade Fe-Ti-V oxide deposits.

Geochemical, biostratigraphic, and high-resolution geochronological constraints on the waning stage of Emeishan Large Igneous Province

The initiation and peak magmatic periods of the Emeishan Large Igneous Province (LIP) are well constrained by both biostratigraphic and radioisotopic dating methods; however, the age of cessation of volcanism is poorly constrained and continues to be debated. Marine carbonates interbedded with volcanic ashes across the Guadalupian–Lopingian boundary (GLB) are widespread in south China, and these ashes provide an opportunity to study its timing, origin, and potential relationship with the Emeishan LIP. Here we present biostratigraphic constraints, mineralogical and geochemical characteristics, and high-resolution geochronology of ash layers from the Maoershan and Chaotian sections. Stratigraphic correlation, especially conodont biostratigraphy, confines these ashes to the early Wuchiapingian. Those altered ashes are geochemically akin to alkali tonsteins from the coal seams of the lower Xuanwei/Lungtan Formation in southwest China. The ashes postdating the GLB yield a coherent cluster of zircon U-Pb ages with weighted mean 206Pb/238U ages of 258.82 ± 0.61 Ma to 257.39 ± 0.68 Ma, in agreement with the ages of intrusive rocks (259.6 ± 0.5 Ma to 257.6 ± 0.5 Ma) in the central Emeishan LIP. Moreover, the ɛHf(t) values of zircons from the ashes vary from +2.5 to +10.6, a range consistent with that of the Emeishan LIP. The results collectively suggest that the early Wuchiapingian volcanic ashes are a product of extrusive alkaline magmatism and most likely mark the waning stage of the Emeishan volcanism, which may have continued until ca. 257.4 Ma in the early Wuchiapingian.

Mantle roots of the Emeishan plume: an evaluation based on teleseismic P-wave tomography

The voluminous magmatism associated with large igneous provinces (LIPs) is commonly correlated to upwelling plumes from the core–mantle boundary (CMB). Here we analyse seismic tomographic data from the Emeishan LIP in southwestern China. Our results reveal vestiges of delaminated crustal and/or lithospheric mantle, with an upwelling in the upper mantle beneath the Emeishan LIP rather than a plume rooted in the CMB. We suggest that the magmatism and the Emeishan LIP formation might be connected with the melting of delaminated lower crustal and/or lithospheric components which resulted in plume-like upwelling from the upper mantle or from the mantle transition zone.

Mantle roots of the Emeishan plume: an evaluation based on telesismic P-wave tomography

The voluminous magmatism associated with Large Igneous Provinces (LIP) is commonly correlated to upwelling plumes from the Mantle Transition Zone or the Core-Mantle Boundary (CMB). Here we analyse seismic tomographic data from the Emeishan LIP in southwestern China. Our results reveal vestiges of delaminated crustal and (or) lithospheric material in the cental part of the study area, and upwelling mantle in the southern part. Our results do not provide any conclusive evidence for upwelling mantle plume rooted in the CMB beneath the Emeishan LIP. We therefore suggest that the magmatism and the Emeishan LIP formation might be connected with the melting of delaminated lower crustal and (or) lithospheric components and associated plume-like upwelling from the mantle transition zone.