Petrology of ultramafic lamprophyres from the Beaver Lake area of Eastern Antarctica and their relation to the breakup of Gondwanaland

Abstract

Mesozoic melilite-bearing ultrama®c lamprophyres are developed as sill, dyke and plug-like intrusive bodies in the East Antarctic Beaver Lake area. They consist of varying amounts of olivine, melilite, phlogopite, nepheline, titanomagnetite and perovskite as major phases, accompanied by minor amounts of apatite, carbonate, spinel, glass and, rarely, monticellite. The rocks are mineralogically and geochemically broadly similar to olivine melilitites, differing in higher CO 2 and modal phlogopite and carbonate contents. The ultrama®c lamprophyres are MgO-rich (13.4±20.5 wt%) and SiO 2 -poor (32.8±37.2 wt%), indicative of a near-primary nature. Major and trace element features are consistent with minor fractionation of olivine and Cr-spinel from melts originating at depths of 130±140 km.Primary melts originated by melting of upper mantle peridotite which had been veined by phlogopite carbonate clinopyroxene-bearing assemblages less than 200 Ma before eruption. The presence of the veins and their time of formation is required to explain high incompatible trace element contents and growth of 87 Sr= 86 Sr, leaving 143 Nd= 144 Nd unaffected. The major element, compatible trace element, and most radiogenic isotope characteristics are derived from melting of the wall-rock peridotite. The depth of about 130 km is indicated by the presence of phlogopite rather than amphibole in the veins, by control of the REE pattern by residual garnet, by the high MgO content of the rocks, and by the expected intersection of the rift-¯ank geotherm with the solidus at this depth. The higher CO 2 contents than are characteristic for olivine melilitites favoured the crystallization of melilite at crustal pressures, and suppressed the crystallization of clinopyroxene. The Beaver Lake ultrama®c lamprophyres are a distal effect of the breakup of Gondwanaland, too distal to show a geochemical signature of the Kerguelen plume. Upward and outward movement of the asthenosphere-lithosphere boundary beneath the Lambert-Amery rift led ®rst to the production of phlogopite-and carbonaterich veins, and later to the generation of the ultrama®c lamprophyres themselves.

Continuous cratonic crust between the Congo and Tanzania blocks in western Uganda

“…The kamafugites and carbonatites of the Toro-Ankole volcanic field are broadly similar in composition to ultramafic lamprophyres that are characteristic igneous features of rift activity in cratonic regions (Foley et al 2002;Tappe et al 2006;Foley 2008). The association of carbonate-rich melt production and continental rifts can be attributed to oxidation of the mantle (Foley et al 2002;Tappe et al 2006Tappe et al , 2007, whereby the carbon was concentrated by the earlier deposition of graphite or diamond in more reducing conditions characteristic of cratonic mantle lithosphere (Foley 2008). Experimental investigations confirm that melts similar to these magmas are produced in oxidized conditions at depths of 120-180 km ).…”

Between carbonatite and lamproite—Diamondiferous Torngat ultramafic lamprophyres formed by carbonate-fluxed melting of cratonic MARID-type metasomes

“…Recently, Foley et al (2002) and Tappe et al (2006) have invoked a vein-and-wall rock melting mechanism for the genesis of UML magmas similar to the model developed for the generation of lamproite melts by Foley (1992a). The veins reside in the lower reaches of the peridotite dominated cratonic SCLM and represent frozen convective mantle-derived low-degree melts produced under various oxygen fugacity and volatile conditions (cf.…”

The discovery of kimberlites in Antarctica extends the vast Gondwanan Cretaceous province

“…1b). Interestingly, the UML occur in the northern part of the region (mainly around Beaver Lake) [9][10][11][12] , whereas the current kimberlites and the carbonatite-kimberlite reported by Belyatsky et al 15 occur in the southern part. This may reflect progressive stages of development of the Lambert rift, with the UML representing a greater extent of rifting in the north, more erosion of the base of the lithosphere leading to melting at shallower subcontinental lithospheric levels, relative to the kimberlites in the south.…”

“…The association of xenocrystic and phenocrystic olivine and phlogopite macrocrysts set in a matrix containing perovskite, spinel, apatite and carbonate is a distinctive feature of kimberlites 4,7 . Notably, clinopyroxene does not occur as phenocrysts nor in the groundmass, precluding classification of these samples as ultramafic lamprophyres (UML) 8 , examples of which have been reported from other localities in the nPCM region [9][10][11][12] .…”