Diamond and coesite discovered in Saxony-type granulite: Solution to the Variscan garnet peridotite enigma

Abstract

The pressures required for diamond and coesite formation far exceed conditions reached by even the deepest present-day orogenic crustal roots. Therefore the occurrence of metamorphosed continental crust containing these minerals requires processes other than crustal thickening to have operated in the past. Here we report the first in situ finding of diamond and coesite, characterized by micro-Raman spectroscopy, in high-pressure granulites otherwise indistinguishable from granulites found associated with garnet peridotite throughout the European Variscides. Our discovery confirms the provenance of Europe's first reliable diamond, the “Bohemian diamond,” found in A.D. 1870, and also represents the first robust evidence for ultrahigh-pressure conditions in a major Variscan crustal rock type. A process of deep continental subduction is required to explain the metamorphic pressures and the granulite–garnet peridotite association, and thus tectonometamorphic models for these rocks involving a deep orogenic crustal root need to be significantly modified.

Orogenic, Ophiolitic, and Abyssal Peridotites

“…Examples of HT metamorphism are observed in the country rocks of the Beni Bousera (northern Morocco) and Ronda (southern Spain) massifs (Loomis, 1972), as well as in the limestones hosting the Pyrenean lherzolites (Goldberg and Leyreloup, 1990). However, several orogenic peridotites are associated with HP to UHP terranes, notably in the Central Alps (Alpe Arami, Cima di Gagnone, and Ulten peridotites), China (Dabie-Sulu, Qinling, and North Qaidam-Altyn-Tagh UHP metamorphic belts), the Western Gneiss Region (WGR) in Scandinavian Caledonides, and the Bohemian Massif in Central Europe Ernst, 1981;Ernst and Liou, 2008;Kotkova et al, 2011;Liou et al, 2002aLiou et al, ,b, 2009Liu et al, 2010;Reverdatto and Selyatitskii, 2005;Schmaedicke et al, 2010;Terry et al, 2000;van Roermund, 2009b;Zhang et al, 2005).…”

“…These include HP and UHP ultramafic bodies dominated by garnet peridotites. These rocks have attracted much attention recently as a potential source of information on relatively deep mantle processes such as accretion of lithospheric roots, transport, reaction and storage of fluids/melts in suprasubduction mantle wedges, and subduction/exhumation mechanisms Brueckner and Medaris, 2000;Brueckner et al, 2004;Ernst and Liou, 2008;Green et al, 1997;Green et al, 2010;Katayama et al, 2003Katayama et al, , 2005Kotkova et al, 2011;Liou et al, 2002aLiou et al, ,b, 2007Liou et al, , 2009Liu et al, 2007Liu et al, , 2009MacKenzie et al, 2005;Malaspina et al, 2009Malaspina et al, , 2010Paquin et al, 2004;Sapienza et al, 2009;Scambelluri et al, 2006Scambelluri et al, , 2008Scambelluri et al, , 2010Song et al, 2004;van Roermund et al, 2001;Yoshida et al, 2004;Zhang and Liou, 1998;Zhang et al, 2005;Zhang et al, 2003Zhang et al, , 2004aZheng et al, 2003Zheng et al, , 2011. Numerous studies were published, mostly in the last decade, providing data on the petrology, geochronology, and geochemistry of HP/UHP garnet peridotites and related rocks from four principal regions: Liati and Gebauer, 2009;Medaris et al, 1995Medaris et al, , 2005Medaris et al, , 2006Massonne and Bautsch, 2002;Masso...…”

Role of strain localization and melt flow on exhumation of deeply subducted continental crust

“…1A) and large-scale relamination of the subducted and buoyant lower plate felsic crust under the mafic lower crust of the upper plate. The subducted felsic crust likely plunged to upper mantle depths, as indicated by diamondand coesite-bearing felsic gneisses and granulites (Kotková et al, 2011;Haifler and Kotková, 2016;Nasdala and Massonne, 2000;Stöckhert et al, 2001) associated with peridotites and eclogites (Kotková and Janák, 2015). These rocks extruded together to the shallower crustal levels of the orogeny along the former suture interface.…”

Evidence for partial melting of eclogite from the Moldanubian Zone of the Bohemian Massif, Czech Republic

“…Geothermo-barometric calculations suggest that several eclogite and garnet peridotite masses in the Gföhl unit experienced UHP conditions (e.g., Nakamura et al, 2004;Medaris et al, 2006;Naemura et al, 2009). On the other hand, evidence for UHP metamorphism of felsic granulite and gneiss in the Bohemian Massif is rare, but a few findings of microdiamond and coesite from such felsic rocks have been recently reported (e.g., Kobayashi et al, 2008;Kotková et al, 2011;Perraki and Faryad, 2014), indicating that some garnet peridotite masses and surrounding felsic rocks may have shared the same P-T history, although peak P-T conditions of granulite in the Gföhl unit have been estimated to be approximately 1.6-2.0 GPa, 900-1100°C (e.g., Carswell and O'Brien, 1993).…”