The protoliths of central Himalayan eclogites

Eclogites represent the highest pressure conditions yet observed from rocks thrust to the surface in the central Himalaya. A detailed investigation of the protolith nature of these eclogites is needed to better understand pre-Himalayan geological history. Retrogressed eclogites were collected from Thongmön (Dingri County) and Riwu (Dinggye County), central Himalaya, China. We investigated the bulk rock major and trace elements, Sr-Nd isotopes, zircon geochronology, and Hf-O isotopes. These retrogressed eclogites experienced five stages of metamorphic evolution from prograde amphibolite-facies to peak eclogite-facies, and high pressure granulites-facies, granulites-facies then final amphibolite-facies overprinting during exhumation. Geochemically, they are subalkaline basalts with high FeO contents and a tholeiitic affinity; trace elements show similarities with enriched mid-ocean ridge basalts. Bulk rocks have a wide range of εNd(t) values from −0.24 to +7.08, and an unusually wide range of initial 87Sr/86Sr ratios of 0.705961−0.821182. Zircon relict magmatic cores from both Thongmön and Riwu eclogites yield a consistent protolith age of ca. 1850 Ma, with enriched heavy rare earth element patterns and significant negative Eu anomalies. These relict cores have oxygen isotopes signatures of δ18O = 5.8−8.1‰, εHf(t) values of −4.85 to +9.59, and two-stage model ages (TDM2) of 1.91−2.81 Ga. Metamorphic overgrowth zircons yield much younger ages of ca. 14 Ma. Integration of all of the above data suggests that the protolith of these central Himalayan retrogressed eclogites might be Proterozoic continental flood basalts of the North Indian Plate, generated under a post-collisional extension setting during the assembly of the Columbia Supercontinent. Occurrence of both Neoproterozoic−early Paleozoic rocks and ca. 1.85 Ga rocks in the regional crystalline rocks may reflect either unrecognized sub-horizontal Main Central Thrust exposure(s) or exhumation of a deeply cut part of the Greater Himalayan Crystalline complex. In combination with previous reports of Late Cretaceous, Neoproterozoic, and similar but younger Paleoproterozoic protolith, it is clear that the central Himalayan eclogites originate from multiple sources of protolith.

Rapid eruption of the Emeishan continental flood basalts: New paleomagnetic and geochronologic constraints

The duration of the eruption of the Emeishan large igneous province is hotly debated. We conducted a magnetostratigraphic and geochronological study of the core area of the large igneous province in the Binchuan area of Yunnan Province, southwestern China, in order to constrain the duration of the eruption. The results of detailed thermal demagnetization experiments revealed two remanent magnetic components from the volcanic rocks of 11 composite sections. A low-temperature component separated below 300 °C is interpreted as a recent viscous remanence. Additionally, reliable characteristic remanent magnetizations were revealed above 400 °C, with unblocking temperatures up to 580−680 °C, which passed the fold test and record three magnetozones. Zircons from the felsic ignimbrites exposed in the final stage of the mafic volcanism are dated to 258.2 ± 0.7 Ma (n = 15; mean square of weighted deviates = 1.3) by sensitive high-resolution ion microprobe. Stratigraphic and magnetostratigraphic correlations of the Emeishan basalts in the Binchuan sections indicate that the eruption of the mafic rocks of the Emeishan large igneous province can be clearly divided into early (reverse polarity subzone), middle (normal polarity subzone), and late (reverse polarity subzone) stages, with a total duration of less than 1.7 m.y. (260.8−259.1 Ma). However, by combining this chronology with previously reported conodont biostratigraphic results from locations around the Emeishan large igneous province, and comparing the dominant normal-reverse polarity sequence in the Emeishan large igneous province with the geomagnetic polarity time scale, we obtain a much shorter duration of the main eruptive stage of <0.8 m.y. (260.4−259.6 Ma). About three quarters of the basalts of the Emeishan large igneous province record have a normal polarity and erupted within 0.4 m.y., while the other quarter, mainly distributed in the central zone, shows a reverse polarity and much shorter duration. Given the short duration of the eruption, gas volatiles would have been released into the atmosphere at high rates, which might provide a causal link between the rapid eruption and the end-Guadalupian mass extinction. Before the mantle plume eruption, localized eruptions probably occurred. After eruption of the mafic Emeishan flood basalts, an acid volcanic eruption occurred in the early Wuchiapingian, which was sporadically distributed in the Emeishan large igneous province.

Supplemental Material: Rapid eruption of the Emeishan continental flood basalts: New paleomagnetic and geochronologic constraints

Table S1: The characteristic remanent magnetization for the samples of Emeishan basalts from the Binchuan area; Table S2: SHRIMP zircon U-Pb analytical data for sample DY65-7 of this study.

Supplemental Material: Rapid eruption of the Emeishan continental flood basalts: New paleomagnetic and geochronologic constraints

Table S1: The characteristic remanent magnetization for the samples of Emeishan basalts from the Binchuan area; Table S2: SHRIMP zircon U-Pb analytical data for sample DY65-7 of this study.

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.

Rapid changes from arid to humid conditions during the onset of the Paraná-Etendeka Igneous Provinces: Can volcanic gas emissions from Continental Flood Basalts affect the precipitation regime?

Despite the intriguing correlation between Continental Flood Basalts (CFB) provinces and environmental crises, little is known about how the local/regional sedimentary systems and environment respond to flood basalt volcanism. Active sedimentary systems, and their interaction with volcanism, provides an important rock record to understand palaeoenvironments in volcanic settings. The Paraná-Etendeka Igneous Province is a well-known example of a CFB emplaced on a dry desert environment, but evidence has also shown the existence of humid conditions during the volcanic episode. This work describes and interprets non-volcanic sedimentary and volcaniclastic rocks interbedded with Paraná-Etendeka Igneous Province lavas in southernmost Brazil to better understand palaeoenvironmental process and changes during the onset of volcanism. Non-volcanic sedimentary rocks record the existence of ephemeral sheet-like flows and ponds/lakes while volcaniclastic rocks documents hydromagmatic activity, supporting a change to more humid conditions. Stratigraphic constrains indicate that this change started with the onset of volcanism and affected the whole province. We suggest that SO2 degassing from Paraná-Etendeka province may have caused a net global surface cooling resulting in precipitation redistribution and a local increase in rainfall. This hypothesis may help explaining the cooling and increased humidity observed elsewhere to be closely related with the Paraná-Etendeka emplacement.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5505710

GEODYNAMICS

Purpose. The main purpose of this paper is to investigate the peculiarities of the spatial distribution of thickness and paleorelief of continental flood basalts of Luchychi stratum of Ratne suite of the Ediacaran of the Ratne–Kamin-Kashyrskyi Area in Western Volyn using maps of the thickness and relief of the paleosurface. The study also focuses on correlation between the specified parameters of the stratum and the spatial change of concentrations of native mineralization and its localization relative to the roof (sole) of the stratum. Method. The research applies a number of methods, including field geological surveys; petrography of basalts, structural features (degree of crystallization of mesostasis, structural position of minerals and, in particular, copper, ratio of globular formations and minerals that surround them, relationship between globules), macro- and microtextural features of rocks; geochemical research: determination of the content of the main chemical components of the rocks by the method of gross chemical analysis and copper content; geological and morphostructural studies: construction of a map of paleorelief and thickness of continental flood basalt of the Luchichiv stratum (according to the section of wells). Results. The constructed maps of the thickness and paleo surface of the Luchychi stratum show the spatial variability of the basalt thickness and the reflection of the effective tectonic situation on its paleo surface, which can be "read" by relief elements. It is established that high copper content in basalts is spatially confined to areas of maximum thickness and shifted vertically to the near-roof and, partially, plantar parts of the basalt thickness, and the degree of their crystallization increases in the direction of the inner parts of bodies. This situation with the spatial arrangement of native mineralization indicates the existence within these areas of local isolated thermostated systems. They evolved in their internal parts in conditions close to the intrusive ones (relatively slow decrease in melt temperature, crystallization of rock differences almost devoid of volcanic glass – dolerite-basalts with the transition to dolerites in the central part). Such physicochemical conditions caused long-term migration of gaseous, gaseous-liquid and liquid fluids, providing concomitant extraction, concentration, transfer and deposition not only of native copper, but also a number of petrogenic oxides (alkalis, iron, partially calcium, silicium). The latter subsequently formed a number of low-temperature minerals, the most common of which are zeolites, calcite, and iron compounds. In areas with small and minimum thickness of basalts, the above facts are observed in a reduced form, and in some places are virtually absent. The research has established the complete absence of signs of hydrothermal copper ore mineralization. The latter indicates the lack of evidence about the formation of native mineralization of the hydrothermal type. We provide the geological and petrogeochemical facts that give grounds to consider the scenario of the process of formation of native copper mineralization in relation to the fluid-liquidation hypothesis, earlier developed and proposed by the authors. Scientific novelty. For the first time, the study made it possible to construct maps of the thickness and relief of the paleo surface of the basalts of the Luchychi strata of the Ratne–Kamin-Kashyrskyi area of Western Volyn. Based on the actual material it is shown that the vertical distribution and localization of native mineralization within basaltic bodies are naturally related to their thickness. The value of native copper is directly related to body thickness, reaching a maximum in areas with maximum thickness and background values at the minimum thickness. Practical significance. For the first time, geological, petrographic, geological and morphostructural data were compared with the copper content in basalts of the Luchichi stratum. The proposed approach can be used by geological production organizations in conducting exploration work to prepare promising areas for exploration of the North-Hirnyky and Rafalivka ore fields (ore nodes) within the Ratne Horst anticline. The obtained original material can be distributed as a new method of native mineralization search in other areas of the continental flood basalts distribution.