Facies dependence of the mineralogy and geochemistry of altered volcanic ash beds: An example from Permian-Triassic transition strata in southwestern China

Abstract

Factors controlling variations in the clay mineralogy and geochemical composition of volcanic ash beds deposited in different types of depositional environments remain incompletely understood. The well-correlated, altered volcanic ash beds (tuffs or K-bentonites) of Permian-Triassic boundary (PTB) successions in South China provide an opportunity to better understand processes of alteration and diagenesis of volcanic materials in terrestrial, paralic, and marine sedimentary facies. The types of authigenic clay minerals produced vary among facies: mainly mixed-layer illite/smectite (I/S) clays in deep-marine, shallow-marine, and lacustrine facies, and kaolinite and mixed-layer I/S or kaolinite/smectite (K/S) clays in paludal and mixed marine-terrestrial facies. Furthermore, the stacking sequences of I/S clays in the ash beds are notably facies-dependent: mainly R3 with minor R1 in deep-marine facies, mainly R1 in shallow-marine facies, and mainly R3 in lacustrine and paludal facies. Ordering of I/S clays seems to have been controlled by porewater K+ availability, solution pH, and energy levels of the depositional environment. High-pH conditions favored the formation of feldspar and inhibited the illitization of smectite, and loss of K+ during early alteration of ash was related to environmental energy levels. However, the stacking sequences of I/S clays in terrestrial facies were dominantly controlled by burial conditions. The MgO and K2O contents of altered volcanic ash were related to sediment clay-mineral composition and thus also facies-dependent. Marine-facies ash beds contain relatively more MgO (i.e., MgO/K2O > 0.30; MgO/Al2O3 > 0.056) than terrestrial-facies ash beds (MgO/K2O < 0.30; MgO/Al2O3 < 0.056). MgO was generally retained during alteration of ash beds, and its concentration was largely a function of parent magma type and depositional and diagenetic conditions. K2O was almost completely leached out of volcanic glass, leading to formation of I/S clays by reaction with smectite.Rare earth elements (REE) generally exhibit little variation within a single section but strongly facies-dependent distributions across South China. The REE distributions of deep- and shallow-marine ash beds are similar, characterized by enrichment of light REEs, slight depletion of heavy REEs, and negative Eu anomalies. The REE distributions of terrestrial ash beds are notably different, with weak negative Eu anomalies in lacustrine facies and little to no Eu anomaly in paludal facies. REE distributions can be influenced by adsorption-desorption processes on clay minerals and by dissolution-precipitation reactions involving accessory minerals. Thus, the use of REE patterns of volcanic ashes as fingerprints of source-magma chemistry must take into consideration their sedimentary and diagenetic histories.Trace elements such as Nb, Cu, Zn, Co, Cr, Ta, Mo, V, La, Th, and U exhibit different concentration patterns between ash samples of different facies and even between samples from a single section. Devitrification of volcanic glass to clay minerals can lead to redistribution of TiO2, which shows a close association with the clay fraction of altered ash beds, especially in terrestrial facies. TiO2/Al2O3 ratios of felsic ashes reflect the effects of physical reworking and chemical alteration. It is proposed that TiO2/Al2O3 values <0.055 are indicative of primary ash composition, 0.055–0.140 of moderate secondary overprints, and > 0.140 of strong secondary overprints. The use of Zr/TiO2 vs Nb/Y diagrams for source rock discrimination must take into account such reworking and alteration processes. In terrestrial and shallow-marine facies in which ash-bed reworking is common, the ratios of immobile elements of the clay fraction can still indicate magma source within the range of sensitivity afforded by empirical discrimination diagrams.