Coesite and diamond inclusions, exsolution microstructures and chemical patterns in ultrahigh pressure garnet from Ceuta (Northern Rif, Spain)

Abstract

Garnet from diamondiferous granulites of Ceuta (Betic-Rif cordillera, Spain and Morocco) contains a variety of inclusion types. To better understand the evolution of these rocks during the ultrahigh pressure event, two samples (1 and 2) were selected for the detailed study of garnet. Primary inclusions of apatite, quartz, coesite, rutile and retrograded pyroxene, and exsolution microstructures of rutile characterize garnet from sample 1, whereas exsolution microstructures of quartz, coesite, apatite and rutile, and inclusions formed from a melt characterize garnet from sample 2, indicating that peak metamorphic conditions were recorded by sample 2. In contrast, the chemical patterns of garnet suggest an inverse situation. Garnet from sample 1 has high Ca-and low Mn contents and high X Mg , characteristic of growth at high pressure and temperature whereas garnet from sample 2 shows high Mn and low Ca contents and low X Mg , characteristic of garnet formed at lower temperature and pressure. The contrasting compositions are interpreted as reflecting differences in the position of the metamorphic path followed by both samples relative to the solidus: Garnets from sample 1 are interpreted as formed below the solidus whereas garnets from sample 2 are interpreted as formed in the presence of a melt, which caused notable enrichment of garnet in Mn and depletion in Ca relative to garnet from sample 1. Due to extensive low-pressure Hercynian melting that caused generalized migmatization and melt mobilization, whole-rock composition of the samples notably changed, thus preventing the accurate estimation of the physical conditions characterizing the older ultrahigh pressure event. Estimations based on experimental determinations of the phosphorous solubility in garnet suggest that peak pressure conditions were on the order of 6-7 GPa, which put the origin of the studied crustal rocks at depths greater than 200 km.

On the forbidden and the optimum crystallographic variant of rutile in garnet

“…In other words, if the oriented rutile needles commonly observed as inclusions in many UHP garnets of both crustal and mantle origin were in fact the solid-state exsolution product by the disproportionation of a Ti-bearing garnet from UHP conditions upon decreasing P-T (e.g. Alifirova et al, 2015;Mposkos & Kostopoulos, 2001;Proyer et al, 2013;Ruiz-Cruz & Sanz de Galdeano, 2013;Song et al, 2004;van Roermund et al, 2000;Ye et al, 2000;Zhang & Liou, 1999), they would follow COR-2 with the minimum topotactic mismatch to the garnet host. Considering that a garnet matrix could host up to 24 variants of h103i rt //h111i grt COR-2 rutile, there seems to be no room for other rutile variants with higher energy barriers to form unless by processes other than solid-state exsolution.…”

Diamond in metasedimentary crustal rocks from Pohorje, Eastern Alps: a window to deep continental subduction

“…In addition to UHP, there are also other factors to control the formation of diamond, like carbon speciation and redox state. Moissanite, a natural silicon carbide, reported for the first time from the Canyon Diablo meteorite (Moissan, 1905), was subsequently found in a wide spectrum of terrestrial rocks, including diamond-bearing kimberlites (Leung, 1990), volcanic rocks (Di Pierro et al, 2003), ophiolites (Trumbull et al, 2009) and felsic granulites (Ruiz-Cruz & Sanz de Galdeano, 2013;Perraki & Faryad, 2014). The occurrence of moissanite is puzzling in terms of redox and equilibrium conditions in crustal rocks.…”

Metamorphic diamonds in a garnet megacryst from the Edough Massif (northeastern Algeria). Recognition and geodynamic consequences

“…The close association of garnet peridotites with UHP granulites, such as in the Rif and Betic Cordillera (Ruiz-Cruz et al, 2011 and references therein) suggests that the peridotites were interdigitated within subducted continental crust rather than being tectonically emplaced at shallow crustal levels. In addition, the recent discovery of microdiamonds and coesite in staurolite-andalusite schists from the Betic Cordillera, far from mantle peridotites, suggests that micrometer-sized diamond is the only preserved record of prograde UHP conditions having affected sediments in a subduction context (Ruiz-Cruz & Sanz de Galdeano, 2013a). In that case, the critical UHP mineral assemblages were totally erased (except coesite and diamond) by later lower pressure recrystallization.…”