Highly Refractory Lithospheric Mantle beneath the Tanzanian Craton: Evidence from Lashaine Pre-metasomatic Garnet-bearing Peridotites

Abstract

Here we report the first occurrence of ultra-depleted pyrope garnets with rare earth element (REE) patterns similar to those of hypothetical garnets proposed to have formed in the subcontinental lithospheric mantle prior to those of metasomatic origin. These unique ultra-depleted garnets have not previously been identified in global suites of mantle xenoliths or diamond inclusions. They occur in certain harzburgite members of the xenolith suite from the Lashaine tuff cone, northern Tanzania. The Lashaine ultra-depleted garnets are characterized by low concentrations of CaO (<0·35 wt %), Cr2O3 (2·5 wt %), incompatible trace elements (e.g. Ba, Sr, Ti, Zr and Y), and have chondrite-normalized REE patterns with steep positive middle-to-heavy REE slopes and flat-to-positive, light-to-middle REE slopes [i.e. they are ‘tick’ (√)-shaped]. The ultra-depleted garnets have high Mg# (92·5) and coexist in chemical and textural equilibrium with highly refractory olivine (Fo95·4) and orthopyroxene (Mg# = 96·4), which are all more magnesian than encountered in most global mantle harzburgites and diamond inclusions. The ultra-depleted garnets occur in interconnecting networks around grains of orthopyroxene, which give the rocks a banded appearance. We propose that the ultra-depleted garnets formed by isochemical exsolution from orthopyroxene following a change in geothermal gradient (decrease in temperature and increase in pressure) associated with Archean lithospheric thickening. Metasomatism of refractory mantle is recorded in the Lashaine xenoliths by an almost continuous variation from ultra-depleted to sub-calcic (harzburgitic) to calcic (lherzolitic) garnet compositions. Overall this systematic trend correlates with increasing depth and is accompanied by a change in chondrite-normalized REE patterns, from ‘√’-shaped to sinusoidal to normal (i.e. light REE depleted). We attribute these compositional variations in garnet, together with a general decrease in Mg# in all mineral phases with depth, to a gradual change from low-temperature high-density fluid metasomatism to high-temperature silicate melt-related enrichment. Pressure and temperature estimates suggest that this metasomatism is concentrated at the base of the Tanzanian lithospheric mantle (i.e. between 125 and 160 km depth), which is consistent with the results of previous studies. Harzburgites containing ultra-depleted garnets were entrained from shallower depths (∼125 km) in the lithosphere than most Lashaine lherzolites and we suggest that long-term survival of this refractory mantle reflects its location above the level reached by ascending metasomatic agents. We propose that preservation of widespread highly refractory, low-density and high-viscosity lithospheric mantle at shallow depths beneath the Tanzanian Craton may have ensured its long-term stability and resistance to delamination. The presence of refractory mantle beneath both the core and eastern margin of the Tanzanian Craton may account for the steep gradient in lithospheric thickness relative to other regions of thick stable Archean lithosphere where thinning beneath cratonic margins is more gradual.