The petrogenesis of carbonatite,melnoite and kimberlite from the Eastern Goldfields Province,Yilgarn Craton

摘要

We have obtained major and trace element data for a suite of rocks emplaced over an area of approx45,000 km~2 in the Eastern Goldfields Province (EGP),Yilgarn Craton,that are petrographically and mineralogically described as kimberlites,melnoites and carbonatites.Kimberlites dominate the rock types found in the west whereas carbonatites and melnoites are common in the east.Compatible element data from the carbonatites and melnoites tend to lie along trends that imply silicate-carbonate fractionation.The kimberlites exhibit a much greater amount of compatible element scatter as a result of the variable contribution from continental lithospheric mantle (CLM).When compared to southern African kimberlites,the EGP kimberlites have consistently lower MgO and Os concentrations at comparable Ni concentrations.The opposite is true for Ti-Ni variation where the EGP kimberlites have higher Ti than the southern African kimberlites at comparable Ni concentrations.These data are interpreted to suggest that the CLM beneath the province was either melt metasomatised some time prior to kimberlite emplacement,or that the EGP CLM is less refractory (melt depleted) than the Kaaapvaal CLM.In contrast,the incompatible element ratios and initial eNd values are constant throughout the entire rock suite.Carbonate C and O isotope data show a broad positive correlation,consistent with magmatic-hydrothermal trends found in many carbonatite complexes.These incompatible element and isotope data link all of the rocks within the province to the same mantle source that was similar to modern-day mantle plume sources.Re-Os data for the various samples,including oxide minerals from all of the petrographic types,xenocryst-poor kimberlites and melnoites yield a precise Re-Os isochron of 2025+-10 Ma and an initial gammaOs of 5.3+-3.1 (MSWD=5.8).These data support the conclusion based on incompatible element,stable isotope and Sm-Nd isotope data that the rocks are comagmatic.Initial Os isotopic compositions and Re/Os ratios for the xenocryst-rich kimberlites are also correlated.However,the correlation does not pass through the calculated initial gammaOs and Re/Os of the isochron.The Re-Os data show that the incompatible element-enriched melt exhibited very little control on the Re-Os variations of the xenocryst-rich kimberlites.Correlations between deep mantle seismic velocities,petrology and whole-rock compatible element geochemistry suggest that the rheology and history of the EGP CLM played a significant role in determining the petrographic characteristics of t he magma that were utimately emplaced into the EGP crust.