Garnet exsolution in pyroxene from clinopyroxenites in the Moldanubian zone: constraining the early pre‐convergence history of ultramafic rocks in the Variscan orogen

Abstract

Exsolution lamellae of garnet in clinopyroxene and orthopyroxene porphyroclasts from garnet pyroxenites in the Moldanubian zone were studied to elucidate the pressure-temperature conditions of the exsolution process and to reconstruct the burial and exhumation path of ultramafic rocks in the Variscan orogen. The porphyroclasts occur in a fine-grained matrix with metamorphic fabrics, which consists of clinopyroxene and small amounts of garnet, orthopyroxene and amphibole. The clinopyroxene porphyroclasts contain garnet + orthopyroxene lamellae as well as ilmenite rods that have orientation parallel to (100) planes of the porphyroclasts. Orthopyroxene porphyroclasts host garnet and clinopyroxene lamellae, which show the same lattice preferred orientation. In both cases, lamellar orthopyroxene, clinopyroxene and garnet were partially replaced by secondary amphibole. Composition of exsolution phases and that of host pyroxene were reintegrated according to measured modal proportions and demonstrate that the primary pyroxene was enriched in Al and contained 8-11 mol.% Tschermak components. Conventional thermobarometry and thermodynamic modelling on the reintegrated pyroxene indicate that primary clinopyroxene and orthopyroxene megacrysts crystallized at 1300-1400°C and 2.2-2.5 GPa. Unmixing and exsolution of garnet and a second pyroxene phase occurred in response to cooling and pressure increase before the peak pressure of 4.5-5.0 GPa was reached at 1100°C. This scenario is consistent with a burial of hot upper-mantle ultramafics into a cold subcratonic environment and subsequent exhumation through 900°C and 2.2-3.3 GPa, when the pyroxenites would have partially recrystallized during tectonic incorporation into eclogites and felsic granulites.

Cryptic metasomatic agent measured in situ in Variscan mantle rocks: Melt inclusions in garnet of eclogite, Granulitgebirge, Germany

“…These are mostly eclogites but varieties with low‐Na clinopyroxene also exist. Moreover, also stress‐induced recrystallization of high temperature (HT; 1,300–1,400°C) highly aluminous pyroxene megacrysts can generate, typically at HP and lower temperatures, exsolution‐like lamellar garnet and low‐Al pyroxene growth (Carswell & Jamtveit, ; Faryad, Dolejš, & Machek, ; Massonne & Bautsch, ; Medaris et al, ; Schmädicke et al, ). The megacrysts originally formed from interaction between the mantle and mantle‐derived melts, which produced mono‐mineralic layers of clinopyroxene and/or orthopyroxene and/or garnet (Massonne & Bautsch, ; Schmädicke et al, ).…”

Mineralogical heterogeneity of UH<i>P</i> garnet peridotite in the Moldanubian Zone of the Bohemian Massif (Nové Dvory, Czech Republic)

“…The Gföhl unit, one of the units in the Moldanubian Zone of the Variscan orogenic belt, is a metamorphic belt that mainly consists of felsic granulite along with orthoand paragneisses that enclose numerous small bodies of garnet and spinel peridotites, pyroxenites, and eclogite. Past studies indicate crust-mantle interaction in the Gföhl unit and suggest varying origins of the UHP lithologies on the basis of geochemical and thermobarometrical investi-gations (Medaris et al, 1995;Medaris et al, 2005;Obata et al, 2006;Faryad et al, 2009;Naemura et al, 2009;Svojtka et al, 2016). Trials to reveal the hidden history of high-grade metamorphic rocks from the Gföhl unit have also shown that kyanite-bearing eclogite, which is closely associated with the Nové Dvory garnet peridotite body, was buried to the 150-km depth (Nakamura et al, 2004).…”

On the nature and origin of garnet in highly-refractory Archean lithospheric mantle: constraints from garnet exsolved in Kaapvaal craton orthopyroxenes

“…The scarcity of garnets with exceptionally-low CaO is almost certainly because less than 1% of the mantle sampled by xenoliths and megacrysts is unmetasomatized (Pearson and Wittig, 2013) and the extent to which sub-cratonic lithospheric mantle has been affected by exsolution of garnet from orthopyroxene (enstatite) remains enigmatic; much more widely reported is the exsolution of garnet from clinopyroxene in mantle eclogites and pyroxenites rather than peridotites (Beeson and Jackson, 1970;Faryad et al, 2009;Harte and Gurney, 1975;Jerde et al, 1993;Roach, 2004;Harte, 1988, 1990;Wilkinson, 1976).…”