TY - JOUR
T1 - The oxygen-isotopic composition of granulites and retrogressed granulites from the Limpopo Belt as a monitor of fluid-rock interaction
AU - Hoernes, S.
AU - van Reenen, D. D.
PY - 1992/3
Y1 - 1992/3
N2 - From the Southern Marginal Zone of the Limpopo Belt three rock systems: cordierite-bearing metapelites, mafic rocks, and ultramafic rocks are studied by whole-rock oxygen isotope analysis. In order to decide wether the observed variation in the O-isotopic composition of the metapelites is due to granulite-facies fluid-rock interactions, or due to earlier processes, a discrimination index (pelite index: Pi) is proposed. In combination with the whole-rock oxygen-isotopic composition this index enables one to put some constraints on the origin of the precursor rocks. In the case of the Limpopo metapelites it is concluded, that the cordierite-anthophyllite-bearing metapelites are only in part true metasediments: a metasomatic alteration of mafic precursors in an ocean-crust environment could be responsible for both, low Pi indices and high (with respect to unaltered mantle material) δ18O values. Fluid-rock interactions in this environment are also thought to be responsible for the observed chemical and O-isotopic variation of mafic and ultramafic rocks. The data do not support significant large-scale CO2 infiltration during granulite-facies metamorphism. The retrograde, regionally extended anthophyllite formation in the Southern Marginal Zone of the Limpopo Belt is certainly triggered by some fluid infiltration. Model calculations show that the amount of fluids necessary for this readjustment, is too small to have an imprint on the whole-rock oxygen-isotope composition.
AB - From the Southern Marginal Zone of the Limpopo Belt three rock systems: cordierite-bearing metapelites, mafic rocks, and ultramafic rocks are studied by whole-rock oxygen isotope analysis. In order to decide wether the observed variation in the O-isotopic composition of the metapelites is due to granulite-facies fluid-rock interactions, or due to earlier processes, a discrimination index (pelite index: Pi) is proposed. In combination with the whole-rock oxygen-isotopic composition this index enables one to put some constraints on the origin of the precursor rocks. In the case of the Limpopo metapelites it is concluded, that the cordierite-anthophyllite-bearing metapelites are only in part true metasediments: a metasomatic alteration of mafic precursors in an ocean-crust environment could be responsible for both, low Pi indices and high (with respect to unaltered mantle material) δ18O values. Fluid-rock interactions in this environment are also thought to be responsible for the observed chemical and O-isotopic variation of mafic and ultramafic rocks. The data do not support significant large-scale CO2 infiltration during granulite-facies metamorphism. The retrograde, regionally extended anthophyllite formation in the Southern Marginal Zone of the Limpopo Belt is certainly triggered by some fluid infiltration. Model calculations show that the amount of fluids necessary for this readjustment, is too small to have an imprint on the whole-rock oxygen-isotope composition.
UR - http://www.scopus.com/inward/record.url?scp=0026482890&partnerID=8YFLogxK
U2 - 10.1016/0301-9268(92)90034-L
DO - 10.1016/0301-9268(92)90034-L
M3 - Article
AN - SCOPUS:0026482890
SN - 0301-9268
VL - 55
SP - 353
EP - 364
JO - Precambrian Research
JF - Precambrian Research
IS - 1-4
ER -