Search

The spatial distribution of isolated ultra-high-temperature (UHT) granulites in Sri Lanka corresponds to the exposure level of the deepest granulites of the Highland Complex (HC). Their spatial distribution, and that of ultramafic bodies in the HC, defines a relatively thin (several km) lowermost crustal layer that has been folded into a N-plunging asymmetric fold (F3 in our scheme), Z-shaped looking N. We postulate that the UHT granulites represent suitable lithologies within this lowermost crustal layer above an ultrahot palaeo-Moho. Prior to D3, this layer had already been imbricated and intensely folded with the HT granulites and with ultramafic units after peak metamorphism, leading to a thick package of HT granulites with isolated UHT assemblages. This concept has some important implications for granulite terrains in general: (i) UHT granulites elsewhere may also represent a thin zone above an ultrahot palaeo-Moho and overlain by HT granulite terrains, and their spatial relationships may have been complicated by ductile deformation during early cooling; (ii) many other HT granulite terrains may be underlain by a similar zone of UHT granulites, and, if so, new discoveries of UHT terrains may be expected.

We report here rare evidence for the early prograde P-T evolution of garnet-sillimanite-graphite gneiss (khondalite) from the central Highland Complex, Sri Lanka. Four types of garnet porphyroblasts (Grt(1), Grt(2), Grt(3) and Grt(4)) are observed in the rock with specific types of inclusion features. Only Grt(3) shows evidence for non-coaxial strain. Combining the information shows a sequence of main inclusion phases, from old to young: oriented quartz inclusions at core, staurolite and prismatic sillimanite at mantle, kyanite and kyanite pseudomorph, and biotite at rim in Grt(1); fibrolitic sillimanite pseudomorphing kyanite +/- corundum, kyanite, and spinel + sillimanite after garnet + corundum in Grt(2); biotite, sillimanite, quartz +/- spinel in Grt3; and ilmenite, rulite, quartz and sillimanite in Grt(4). The pre-melting, original rock composition was calculated through stepwise re-integration of melt into the residual, XRF based composition, allowing the early prograde metamorphic evolution to be deduced from petrographical observations and pseudosections. The earliest recognizable stage occurred in the sillimanite field at around 575 degrees C at 4.5 kbar. Subsequent collision associated with Gondwana amalgamation led to crustal thickening along a P-T trajectory with an average dP/dT of similar to 30 bar/degrees C in the kyanite field, up to similar to 660 degrees C at 6.5 kbar, before crossing the wet-solidus at around 675 degrees C at 7.5 kbar. The highest pressure occurred at P > 10 kbar and T around 780 degrees C before prograde decompression associated with further heating. At 825 degrees C and 10.5 kbar, the rock re-entered into the sillimanite field. The temperature peaked at 900 degrees C at ca. 9-9.5 kbar. Subsequent near-isobaric cooling led to the growth of Grt(4) and rutile at T similar to 880 degrees C. Local pyrophyllite rims around sillimanite suggest a late stage of rehydration at T < 400 degrees C, which probably occurred after uplift to upper crustal levels. U-Pb dating of zircons by LA-ICPMS of the khondalite yielded two concordant Pb-206/U-238 age groups with mean values of 542 +/- 2 Ma (MSWD = 0.24, Th/U = 0.01-0.03) and 514 +/- 3 Ma ( MSWD = 0.50, Th/U = 0.01-0.05) interpreted as peak metamorphism of the khondalite and subsequent melt crystallization during cooling. (C) 2017, China University of Geosciences ( Beijing) and Peking University. Production and hosting by Elsevier B.V.

Here we report the occurrence of garnet porphyroblasts that have overgrown alternating silica-saturated and silica deficient microdomains via different mineral reactions. The samples were collected from ultrahigh-temperature (UHT) metapelites in the Highland Complex, Sri Lanka. In some of the metapelites, garnet crystals have cores formed via a dehydration reaction, which had taken place at silica-saturated microdomains and mantle to rim areas formed via a dehydration reaction at silica-deficient microdomains. In contrast, some other garnets in the same rock cores had formed via a dehydration reaction which occurred at silica-deficient microdomains while mantle to rim areas formed via a dehydration reaction at silica-saturated microdomains. Based on the textural observations, we conclude that the studied garnets have grown across different effective bulk compositional microdomains during the prograde evolution. These microdomains could represent heterogeneous compositional layers (paleobedding/laminations) in the precursor sediments or differentiated crenulation cleavages that existed during prograde metamorphism. UHT metamorphism associated with strong ductile deformation, metamorphic differentiation and crystallization of locally produced melt may have obliterated the evidence for such microdomains in the matrix. The lack of significant compositional zoning in garnet probably due to self-diffusion during UHT metamorphism had left mineral inclusions as the sole evidence for earlier microdomains with contrasting chemistry., Geoscience Frontiers, 8 (5)