Ca-rich garnets and associated symplectites in mafic peraluminous granulites from the Gföhl Nappe System, Austria

Abstract

Abstract. Mafic peraluminous granulites associated with the mantle-derived peridotites of the Dunkelsteiner Wald provide evidence of the tectono-metamorphic evolution of the Gföhl Nappe System, Austria. They contain the primary assemblage garnet + Al-rich clinopyroxene + kyanite. Large Ca- and Mg-rich garnets are embedded in a granoblastic matrix of Al-rich clinopyroxene, Ca-rich plagioclase and minor hornblende. They were partially replaced by different generations of symplectites: (a) corundum + sapphirine + spinel + plagioclase formed around kyanite inclusions, (b) orthopyroxene + spinel + plagioclase ± hornblende formed at their rims and (c) clinopyroxene + orthopyroxene + spinel + plagioclase ± hornblende formed within cracks. Large garnets show complex compositional structure comprising several repeatedly occurring garnet types, which are characterized by specific compositions. The areal extent and the cross-cutting relations observed in element distribution maps allowed for the derivation of the relative timing of the formation of the different garnet types. The compositional features of the garnets indicate post-formational modification by intra-crystalline diffusion and metasomatic agents. The garnet composition isopleths in equilibrium assemblage diagrams are in line with compositions modification as indicated by the element distribution maps. They confirm the deviation of composition from equilibrium for all garnet types. Furthermore, at least the youngest garnet types show evidence of metasomatic (Fe + Mg) loss affecting their Ca content. Pressure–temperature (P–T) estimates are based on equilibrium assemblage diagrams that reproduce satisfactorily the observed mineral assemblages and measured mineral compositions. Criteria for checking the existence of preserved equilibrium compositions are suggested. The results call into question the invariability of the assumption that the Ca content and/or zoning in garnet preserves primary P–T information from garnet growth in every case. Recrystallization and compositional readjustment of the reactive garnet volume during symplectite formation led to the development of pronounced, secondary diffusion-induced zoning profiles overprinting the different garnet types and post-dating the complex garnet compositional structure. The primary assemblage is stable between 760 and 880 °C and pressures > 11 kbar. The bulk composition of the crack symplectites is almost isochemical to the oldest, broken-down garnet type. These symplectites were formed above 730 °C and pressures between 5 and 7.5 kbar. The rocks studied underwent more or less isothermal decompression from pressures above 11 to ∼ 6 kbar at temperatures of about 800 °C. Crack and rim symplectites were formed after decompression during the early stage of approximately isobaric cooling under conditions of low differential stress. Due to limited availability of fluids promoting symplectite formation, the timescale of symplectite formation calculated from secondary diffusion profiles associated with crack symplectites is shown to be geologically very short (< 0.5 ka).

First report of ultra-high pressure metamorphism in the Paleozoic Dunhuang orogenic belt (NW China): Constrains from <i>P</i>-<i>T</i> paths of garnet clinopyroxenite and SIMS U-Pb dating of titanite

“…5A-C, H). Similar decomposition textures in mafic granulites can be found elsewhere and are repeatedly demonstrated to be formed under severe decompression during tectonic exhumation (e.g., Wang et al, 2016Wang et al, , 2017aPetrakakis et al, 2018;Zhang et al, 2020). Other retrograde assemblages include finegrained ilmenite (Ilm3) ± plagioclase (Pl3) ± hornblende (Hbl3) ± titanite (Ttn3) lamellae exsolved from within the clinopyroxene (Cpx2) (Fig.…”

Paleoproterozoic UHT metamorphism with isobaric cooling (IBC) followed by decompression–heating in the Khondalite Belt (North China Craton): New evidence from two sapphirine formation processes

“…The microstructures of sapphirine‐bearing symplectites have generally been interpreted as reflecting near‐ITD in an order of 0.1–0.3 GPa or heating with or without decompression in other granulite terranes (Brandt et al, ; Cadéron et al, ; Harley, ; Harley et al, ; Jiao, Guo, Wang, & Peng, ; Kawasaki & Motoyoshi, ; Kriegsman & Schumacher, ; Waters, ). In addition, Spr + Pl intergrowths, associated with Spl + Pl and Crn + Pl intergrowths after Ky have been commonly interpreted as reflecting rapid extensive decompression (from 1.5–3.5 GPa to 1.1 GPa or below) with or without slight heating over a wide T range of 700–1,000ºC, such as in Ky eclogites (Baldwin, Powell, Williams, & Goncalves, ; Liati & Seidel, ; Tumiati et al, ), and in HP mafic/felsic granulites (Baldwin et al, ; Dong, Wei, & Zhang, ; Johansson & Möller, ; Petrakakis, Schuster‐Bourgin, Habler, & Abart, ; Rötzler & Romer, ; Zhang, Mattinson, Meng, & Wan, ). Our interpretation for the second type of Spr is consistent with the ITD path inference.…”

Compositional trends in Ba-, Ti-, and Cl-rich micas from metasomatized mantle rocks of the Gföhl Unit, Bohemian Massif, Austria

“…Multiple genetic models have been proposed for garnet pyroxenites and eclogites occurring within garnet peridotites of the Gföhl and Saxothuringian granulite massifs, including: 1) HT-HP crystallization from silicate melts containing components of the oceanic crust (Medaris et al, 1995;Medaris et al, 2013); 2) high pressure cumulates of hot melts derived from the asthenosphere (Becker 1997;Schmädicke et al 2010); and 3) metasomatism of peridotites by melts derived from a phengite-bearing protolith already present in the peridotites or more probably within the deeply subducted continental crust (Borghini et al 2018(Borghini et al , 2020. Furthermore, mafic and intermediate granulites sometimes occur in association with pyroxenite bodies at the contact with felsic granulites (Cooke, 2000;Petrakakis et al, 2018;Racek et al, 2008).…”

“…The decompression conditions for felsic, intermediate, and mafic granulites have been estimated to be 5-13 kbar and 650-910 °C (Cooke, 2000;Racek et al, 2008;Štípská et al, 2004;Petrakakis et al 2018), for peridotites with garnet pyroxenites 12-13 kbar and 840-860 °C were estimated (Schmädicke et al 2010), and for eclogites 18-20 kbar and 835-935 °C were obtained (Medaris et al, 2006).…”