CHARACTERIZATION OF MULTIPLE FLUID-GRANITE INTERACTION PROCESSES IN THE EPISYENITES OF AVILA-BEJAR, CENTRAL IBERIAN MASSIF, SPAIN

Episyenites are small granite-hosted pipe-like masses characterized by hydrothermal alteration and the major loss of silica compared with th e host granites. Such bodies are fairly common in the Hercynian granit es of western Europe. This study deals with late Hercynian amphibole-b earing and biotite granodiorites/monzogranites from the Central Iberia n Massif using petrological, geochemical (major oxides, trace elements and REE), and isotopic (O, H) data to constrain the paragenesis of al teration, the apparent changes in bulk composition, and the sources an d conditions of fluids involved in fluid-rock interactions. Episyeniti zation leads to reduction of quartz content, albitized plagioclases an d chloritized ferromagnesian phases, and the process results in increa singly ordered alkali feldspars. There are noticeable variations in wh ole-rock Na2O, K2O, MgO, CaO, Rb and Sr contents with SiO2, in some ca ses 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 miner als and recrystallization of new stable mineral assemblages. The proce ss is complex and two generations of chlorite (early high-Fe and late low-Fe) are observed. Whole rocks and mineral separates were studied f or O and H isotopes. The data rule out meteoric fluids as the cause of syenitization (and the later resilicification of some) and suggest th at at least two different fluids were involved in generating the syeni tes. The first is interpreted to be a high-temperature (300-450 degree s 0) late-magmatic deuteric fluid, as recorded by feldspar-O isotope d ata. Later, a different fluid with delta(18)O approximate to -1.4%0 an d delta D approximate to -13%0, isotopically indistinguishable from se awater 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 b y the episyenites provided suitable pathways for fluids of both shallo w and deep origins over a long term during the Carboniferous. (C) 1997 Elsevier Science B.V.