Tectonic Relationships Between E-Type Cratonic and Ultra-High-Pressure (UHP) Diamond: Implications for Craton Formation and Stabilization

Abstract

Many eclogite xenoliths from kimberlites show geochemical and isotopic signatures compatible with an origin by subduction of oceanic crustal rocks, and it has been proposed earlier that progressively metamorphosed coesite- and diamond-bearing eclogitic assemblages and coesite inclusions in E-type diamonds may be viewed as an expression of Archean and Proterozoic UHP metamorphic events. Yet the observation that kimberlite-born cratonic eclogitic diamonds and diamonds in UHP metamorphic terranes lie on opposite ends of the geological age spectrum, is still commonly used to infer that UHP metamorphism is a Neoproterozoic or younger process that was not possible in a much hotter Archean Earth. The present paper re-evaluates the UHP model for the Archean eclogitic upper mantle sample by discussing (1) examples for igneous exhumation of subducted Phanerozoic and Proterozoic UHP eclogites and diamonds; (2) seismic images showing that tectonic accretion of oceanic lithosphere was a major factor during assembly of Paleoproterozoic and Archean cratons, and (3) recently discovered Archean metabasic eclogites representing the oldest surface record for subduction zone metamorphism. It is concluded that crustal UHP melanges and the diamondiferous eclogite upper mantle sample are complementary end products of subduction zone metamorphism. Lower density crustal melanges are tectonically exhumed soon after continental or microcontinental collision. The mantle eclogites and their E-type cratonic diamonds are parts of the higher density oceanic slab subducted prior to collision and accreted to the roots of microcontinental nuclei. They had long mantle residence times and could be exhumed only when picked up by younger igneous transport media (e.g. kimberlites). As shown by ~2.9 Ga ages of the oldest known eclogitic diamonds and xenoliths with subduction signatures, deep subduction along UHP-gradients began latest in the Mesoarchean, producing lithospheric roots sufficiently thick and cool to extend into the diamond stability field. The range of Proterozoic E-type diamond ages on several diamondiferous cratons, even within individual kimberlites, suggests that craton roots were repeatedly modified during the Proterozoic.