Preservation of evidence for prograde metamorphism in ultrahigh-temperature, high-pressure kyanite-bearing granulites, South Harris, Scotland

Mineralogical and mineral chemical evidence for prograde metamorphism is rarely preserved in rocks that have reached ultrahigh-temperature (UHT) conditions (>900 °C) because high diffusion and reaction rates erase evidence for earlier assemblages. The UHT, high-pressure (HP) metasedimentary rocks of the Leverburgh belt of South Harris, Scotland, are unusual in that evidence for the prograde history is preserved, despite having reached temperatures of ∼955 °C or more. Two lithologies from the belt are investigated here and quantitatively modelled in the system NaO-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O: a garnet-kyanite-K-feldspar-quartz gneiss (X _Mg = 37, A/AFM = 0.41), and anorthopyroxene-garnet-kyanite-K-feldspar quartzite (XMg = 89 A/AFM = 0.68). The garnet-kyanite gneiss contains garnet porphyroblasts that grew on the prograde path, and captured inclusion assemblages of biotite, sillimanite, plagioclase and quartz (<790 °C, <9.5 kbar). These porphyroblasts preserve spectacular calcium zonation features with an early growth pattern overgrown by high-Ca rims formed during high-P metamorphism in the kyanite stability field. In contrast, Fe-Mg zonation in the same garnet porphyroblasts reflects retrograde re-equilibration, as a result of the relatively faster diffusivity of these ions. Peak P-T are constrained by the occurrence of coexisting orthopyroxene and aluminosilicate in the quartzite. Orthopyroxene porphyroblasts [y(opx) = 0.17-0.22] contain sillimanite inclusions, indicative of maximum conditions of 955 ± 45 °C at 10.0 ± 1.5 kbar. Subsequently, orthopyroxene, kyanite, K-feldspar and quartz developed in equilibrated textures, constraining the maximum pressure conditions to 12.5 ± 0.8 kbar at 905 ± 25 °C. P-T-X modelling reveals that the mineral assemblage orthopyroxene-kyanite-quartz is compositionally restricted to rocks of X _Mg > 84, consistent with its very rare occurrence in nature. The preservation of unusual high P-T mineral assemblages and chemical disequilibrium features in these UHT HP rocks is attributed to a rapid tectonometamorphic cycle involving arc subduction and terminating in exhumation.