Characterization of multiple fluid-granite interaction processes in the episyenites of Avila-Béjar, Central Iberian Massif, Spain

Abstract

Episyenites are small granite-hosted pipe-like masses characterized by hydrothermal alteration and the major loss of silica compared with the host granites. Such bodies are fairly common in the Hercynian granites of western Europe. This study deals with late Hercynian amphibole-bearing and biotite granodiorites/monzogranites from the Central Iberian Massif using petrological, geochemical (major oxides, trace elements and REE), and isotopic (O, H) data to constrain the paragenesis of alteration, the apparent changes in bulk composition, and the sources and conditions of fluids involved in fluid-rock interactions. Episyenitization leads to reduction of quartz content, albitized plagioclases and chloritized ferromagnesian phases, and the process results in increasingly ordered alkali feldspars. There are noticeable variations in whole-rock Na 20, K 2O, MgO, CaO, Rb and Sr contents with SiO 2, in some cases behaving differently depending on whether the episyenite is hosted in amphibole granite or biotite granite. The LREE, Y, Zr and Nb have also been modified in those syenites associated with amphibole-bearing granites, this being attributed to destabilization of accessory minerals and recrystallization of new stable mineral assemblages. The process is complex and two generations of chlorite (early high-Fe and late low-Fe) are observed. Whole rocks and mineral separates were studied for O and H isotopes. The data rule out meteoric fluids as the cause of syenitization (and the later resiliciftcation of some) and suggest that at least two different fluids were involved in generating the syenites. The first is interpreted to be a high-temperature (300–450°C) late-magmatic deuteric fluid, as recorded by feldspar-O isotope data. Later, a different fluid withδ 18O≈ −1.4‰ and δD ≈ −13‰, isotopically indistinguishable from seawater on a deglaciated Earth, produced the chlorite, secondary quartz and minor epidote. Such a fluid was only available for a short period, since feldspars did not re-equilibrate. The fault systems occupied by the episyenites provided suitable pathways for fluids of both shallow and deep origins over a long term during the Carboniferous.