Orogenic ultramafic rocks of UHP (diamond facies) origin

Abstract

Abstract Our physical understanding of tectonic processes is predominantly influenced by evidence from crustal rocks. There are few constraints on the coupling between crust and mantle and the degree to which mantle is physically involved in orogenic events. Occurrences of alluvial diamonds in mountain belts have caused episodic speculation that some tectonically emplaced ultramafic rocks may originate from within the diamond stability field. Since the mid–1980s substantive evidence has come to light to support this view. This chapter reviews the evidence for the tectonic emplacement of large fragments of mantle (up to 300 km 2 ) from the diamond stability field into the crust. Orogenic peridotite massifs from the Betico-Rifean tectonic belt and ophiolitic peridotite bodies from the Indian–Tibetan suture zone are discussed in detail. Constraints are placed on the origin of these ultrahigh pressure (UHP) rocks, their P-T histories are constrained by geochemical and mineralogical data and their implications for orogenesis are discussed. Pyroxenites within the Beni Bousera and Ronda orogenic peridotite bodies contain multicrystalline aggregates of graphite as octahedra and other forms of cubic symmetry that are interpreted as graphitized diamonds. Stable and radiogenic isotope data for the Beni Bousera pyroxenites indicate that some of them originated as high pressure (HP) cumulates from melts of subducted oceanic crust. Anomalously light carbon isotope values (δ13C = –16 to –27.6%0 0 ) for the graphite suggest crystallisation of diamond from subducted kerogenous carbon. Small, submillimeter diamonds have been recovered from two ophiolitic peridotite bodies in Tibet.