Major and rare earth element mineral chemistry of low-grade assemblages inform dynamics of hydrothermal ocean-floor metamorphism in the Dinaridic Neotethys

Abstract

This contribution provides insights into ocean-floor hydrothermal metamorphism of the fast-evolving Dinaridic Neotethys. Mineralogical, geochemical and Sr isotope data collected from altered ophiolites and non-ophiolite basalts/andesites and tuffs of the active continental margin are consistent with hydrothermal alteration trajectories that reflect the host-rock composition. This suggests that hydrothermal fluxes were restricted within a simple closed seawater-fed system. Based on the initial isotopic ratios of Sr, two fluid–rock interaction trends are established: (a) low-to-medium degrees of metasomatism in pre-Middle Jurassic anorogenic ophiolites that progressively abated, and (b) increased intensities of metasomatism in post-Middle Jurassic orogenic ophiolites. This agrees with chlorite thermometry and Ca-Al-(Fe)-silicate phase chemistry. The metamorphic assemblages belong to the zeolite, prehnite-pumpellyite, prehnite-actinolite and greenschist facies. The facies is reliant on the temperature of hydrothermal systems and their fluid chemistry. Rare earth element (REE) phase geochemistry shows (a) variable fluid–rock ratios in chlorite and pumpellyite dependent on fluid temperatures, (b) prominent Eu and Ce anomalies that reflect the fluid oxidation state, (c) light REE/heavy REE mobilization attributed to prevalent ligand complexation, and (d) multi-phase fluid percolation across reaction zones of heterogeneous permeability. This study proposes initiation of simple hydrothermal system(s) at or near a spreading centre(s) in the infancy of the Dinaridic Neotethys. Such a system became more complex during Middle Jurassic and Early Cretaceous time with reactive hydrothermal fluids passing the recharge area and reaching the hot reaction zone. An abrupt obliteration of the established high-temperature regime ensued, following the final closure of the Neotethys.

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“…Petrographic, XRD and geochemical analysis revealed significant sea-floor hydrothermal alterations in the rocks, confirmed by high loss on ignition and selective mobilisation of LILE (Table 3; POLAT et al, 2002;POLAT & HOFMANN, 2003). Here reported post-magmatic parageneses (Table 2) are in line with those of similar hydrothermally altered mafic rocks from the area (SLOVENEC et al, 2012;ŠEGVIĆ et al, 2023). The formation of the investigated pyroclastic rocks is associated with alkali suites (Nb/ > 1 and high Ti/V = 62-91; SHERVAIS, 2023) formed in divergent within-plate settings (Fig.…”

“…Finally, chlorite-smectite (C-S) intermediates seem to be the major alteration products of the vitreous component in both studied lithotypes. While chlorite-rich C-S is clearly of hydrothermal origin (ŠEGVIĆ et al, 2023), relative intensities of the 002 reflection at ~7.1 Å and the 001 reflection at 14.1-14.3 Å of such C-S (WEAVER, 1956), are taken as evidence of chlorite vermiculitization which took place during oxidative dissolution (GU et al, 2020;KRZESIŃSKA et al, 2021), following the emplacement of the analyzed rocks. Further weathering likely led to C-S dissolution and emergence of illite and illitesmectite interlayers as documented in sample PB-1 (Table 2).…”