THE ORIGIN OF FLUIDS AND THE EFFECTS OF METAMORPHISM ON THE PRIMARY CHEMICAL-COMPOSITIONS OF BARBERTON KOMATIITES - NEW EVIDENCE FROM GEOCHEMICAL (REE) AND ISOTOPIC (ND, O, H, AR-39 AR-40) DATA/

Numerous greenstone relies, all containing the two lowermost formation s of the Onverwacht Group, occur in the Archean trondhjemitic/tonaliti c gneiss terrains south of the Barberton Greenstone Belt. In this stud y, we report detailed petrological, geochemical, and isotopic (Nd, O, H, Ar-40/Ar-39) data obtained on komatiites from the Schapenburg Green stone Remnant (SGR), the largest and best-preserved greenstone relic. The main goals are(l)to date the metamorphism affecting the SGR using the Ar-40/ Ar-39 dating method on amphiboles, (2) to evaluate the effe ct of metamorphism on the preservation of primary isotopic and chemica l signatures, and (3) to estimate the temperature and water/rock ratio s that prevailed during metamorphic recrystallization in order to cons train the composition and origin of the reacting fluid phase. Ar-40/Ar -39 ages of 2.9 Ga obtained on two amphibole separates from the Schape nburg metavolcanics reveal the existence of a metamorphic event younge r than the emplacement age (3.5 Ga). This metamorphic event belongs to a series of discrete periods of thermal activity from 3.4 to 2 Ga, ea ch of which coincides with a major episode of magmatic activity. The u ltrabasic lava flows acquired their delta(18)O values (from +3.2 to +5 parts per thousand) at high temperature (approximate to 450 degrees C ) under high water/rock ratios. The reacting water had initial isotopi c Values typical of metamorphic fluids (delta(18)O = +5 to +7 parts pe r thousand; delta D = -65 to -50 parts per thousand). REE patterns wer e not disturbed by metamorphic recrystallization. Despite the long tim e interval between emplacement and metamorphism (approximate to 600 Ma ), epsilon(Nd)(T) values are uniform throughout the whole magmatic sui te, indicating that the Sm-Nd system was closed on the sample scale du ring metamorphism. The mantle source of these greenstones was depleted in LREE as evidence by epsilon(Nd)(T) approximate to +2.5. Chemical f luxes during metamorphism were calculated for elements unfractionated by olivine removal (e.g. Na, Ca, Ti, Al, and Sr), by normalizing to Nd . They suggest a significant mobility of most major elements in the cu mulate zones of the lava flows. By contrast, spinifex zones appear to have preserved most of their primary chemical signatures during metamo rphic recrystallization. Their CaO/Al2O3 and Al2O3/TiO2 ratios can be used with confidence to determine the PT conditions of melting in the mantle source. A general model of water-rock interactions applied to t he sedimentary and magmatic rocks of the Onverwacht Group is also pres ented. The model involves conditions of metamorphism deduced from this study and available data from the literature. In addition to metavolc anic rocks, most of the oxygen isotope compositions of carbonates and cherts (except the highest values) can be explained by reequilibration with metamorphic fluids under greenschist-amphibolite facies conditio ns.