Layered Mantle Structure Beneath the Western Guyana Shield, Venezuela: Evidence From Diamonds and Xenocrysts in Guaniamo Kimberlites

The Neo-Proterozoic (712 Ma) kimberlites from Guaniamo, Venezuela contain abundant low-Ca (harzburgitic) Cr-pyrope garnet xenocrysts, as well as lherzolitic Cr-pyropes, but few diamonds belonging to the peridotite-suite. Trace element compositions suggest three groupings of garnet. Most Cr-pyropes, both lherzolitic and harzburgitic, are highly depleted in magmaphile elements (e.g., Y < 5 ppm, Zr < 31 ppm) and have sinusoidal REE patterns. A second group (primarily lherzolites) has relatively undepleted characteristics (e.g., 9 - 14 ppm Y, Zr < 22 ppm), and typical LREE-depleted REE patterns. A few garnets show enrichment typical of peridotites metasomitised by relatively low-temperature mantle fluids (e.g., 7 - 9 ppm Y, 30 - 90 ppm Zr). Ni-in-garnet thermometry allows depths of origin to be estimated for the garnet xenocrysts by projecting temperatures onto the conductive geothermal gradient (40 Mw/m2 heat flow equivalent) obtained from compositions of Guaniamo Cr-diopside xenocrysts using the Nimis and Taylor method. Within the limited depth range of lithosphere sampled by the kimberlites (100-150 km) compositions vary considerably, but regularly, defining a strongly layered mantle section. Major and trace element compositions suggest the following lithologic sequence: highly-depleted lherzolite from 100 to 115 km, mixed ultra-depleted harzburgite and lherzolite from 115 to 120 km, relatively undepleted lherzolite from 120 to 135 km, mixed depleted harzburgite and relatively-undepleted lherzolite from 135 to 150 km. Based on comparison with well-documented mantle peridotites and xenocrysts from elsewhere, we conclude that the Meso-Proterozoic Cuchivero Province of the Guyana Shield (host to the Guaniamo kimberlites) is underlain by depleted and ultra-depleted shallow Archean mantle which was under-plated, and uplifted, by Proterozoic subduction, perhaps more than once. These Proterozoic subduction events introduced less-depleted oceanic lithosphere beneath the Archean section, which remained there and was the source of the abundant Guaniamo eclogite-suite diamonds that have ocean-floor carbon and oxygen isotope signatures. Although diamond-indicative low-Ca Cr-pyropegarnets are abundant, they were derived primarily from the shallow depleted layer within the field of graphite stability, and the rare peridotite-suite diamonds were either metastably preserved at these shallow depths, or were derived from the small amount of depleted lithosphere sampled by these kimberlites that remained within the diamond stability field (the mixture of Archean and Proterozoic mantle in the depth range 135-150 km).