Petrogeneza węglanów skał ultramaficznych w świetle badań stosunków izotopów trwałych

Abstract

PETROGENESIS OF ULTRAMAFIC-HOSTED CARBONATES IN THE LIGHT OF STABLE ISTOPES RATIO STUDIES Summary Two general forms of carbonates in ultramafic rocks (ophicarbonates) are common, scattered grains and veins. δ13C and δ18O analyses of ophicarbonates from Lower Silesian ophiolite complexes let to discriminate two groups of them which are not related to their form: carbonates of δ13C from -8.9 to -1.2‰ and δ18O from 6.2 to 16.1 ‰ (scattered calcite, vein dolomite, scattered magnesite of Gogołów-Jordanów massif) and carbonates of δ13C from -18.7 to -4.4‰and δ18O from 23.71 do 27.9‰ (vein magnesites and aragonites of Gogołów-Jordanów, Szklary, Braszowice massif, and scattered magnesites of Sobótka massif). The process of precipitation of the ophiolitic Lower Silesian ophicarbonates proceeded probably in four stages: (1) supposed ocean floor metamorphism-precipitation of carbonates from oceanic water supersaturated with endogenous CO2; (2) continental regional metamorphism - carbonates precipitated as co genetic to the main stage of antigorite serpentinization - metamorphic solutions supersaturated with endogenous CO2; (3) serpentinization caused by hydrothermal activity of a granitic intrusion; (4) exogenous - precipitation of aragonite and vein magnesite from meteoric water supersaturated with biogenic CO2 (in part derived from oxidation of biogenic methane). Positive and negative correlation in the ΣCO2 - δ13C and ΣCO2- δ18O, respectively, suggest that dominant part of the scattered ophimagnesite precipitated at the temperature higher than 190°C. The scattered grain-magnesite concentration and the δ13C value slightly increases, and less clearly, the δ18O value decreases, towards the center of the Gogołów-Jordanów massif. Such a distribution of the concentration and δ13C may suggest CO2 autometasomatism - the source of the carbon was the primary ultramafic rocks (fluid inclusion in olivine and pyroxene).