科研之友
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摘要
A mineral chemistry and whole-rock major element, platinum group element (PGE), and Re-Os isotope dataset is presented for a large suite of mantle xenoliths from 13 kimberlites that erupted through the Proterozoic mobile belts surrounding the Archean Kaapvaal craton of South Africa. The peridotites have compositions that are unusually infertile compared with post-Archean lithosphere outside southern Africa (e.g. mean olivine Mg-number = 91 center dot 4; mean whole-rock Al(2)O(3) = 1 center dot 3 wt %) but are similar to xenoliths from the Gibeon kimberlites of southern Namibia, which also erupted through Proterozoic lithosphere. Rhenium depletion model ages (T(RD)) determined from 58 Os isotope compositions of peridotites span a range from 2 center dot 6 to 0 center dot 6 Ga, with an average of 1 center dot 67 Ga. Latest Archean T(RD) model ages were determined for three samples from different localities, but whether these indicate the off-craton presence of reworked Archean lithosphere or are simply artifacts of heterogeneity in the convecting mantle is unclear. Low and relatively restricted Al(2)O(3) contents combined with variable (187)Os/(188)Os(i) in the peridotites are most consistent with a two-stage melt extraction history for southern African off-craton lithosphere, with initial formation in the earliest Proterozoic by widely varying (but, on average, moderate) degrees of melting, followed by a second melt extraction episode associated with the Namaquan orogeny at 1 center dot 3-1 center dot 0 Ga. Extensive metasomatism in the lithosphere beneath East Griqualand, SE of the craton, resulted in extreme clinopyroxene enrichment and addition of ilmenite along with disturbance of PGE abundances in many samples. This is probably due to percolation of melts similar to those parental to the Cr-poor megacryst suite, and related pyroxenites, which are abundant in East Griqualand kimberlites. In other regions, there is evidence for less extensive clinopyroxene addition and cryptic metasomatism. Southern African off-craton mantle xenoliths record evidence of a Mesozoic heating episode probably brought about by mantle upwelling linked to continental break-up and/or Karoo flood basalt magmatism. Prior to this, the thermal state, and hence the thickness, of southern African off-craton and cratonic lithosphere were probably roughly similar. The mantle upwelling responsible for lithospheric heating also appears to have caused moderate (approximate to 30 km) thermal erosion of the off-craton portions of the southern African lithosphere.
- 出版日期2010-9
