Metamorphism and partial melting at UHP conditions revealed by microdiamonds and melt inclusions in metapelitic gneiss from Heia, Arctic Caledonides, Norway

Abstract Primary multiphase inclusions trapped in host minerals such as garnet may provide important information about the nature of fluids and melts and their crystallization products. The occurence of melt and fluid inclusions in the same cluster suggests primary fluid-melt immiscibility during partial melting. Here we report the coexistence of diamond-bearing fluid inclusions with melt inclusions in metasedimentary UHP rocks of the Nordmannvik Nappe at Heia, in the Arctic Caledonides of Norway. Multiphase fluid inclusions (Type I) and primary melt inclusions (Type II) have been identified in garnet and studied in detail. Microscopic observations, Raman spectroscopy, FIB-SEM, and EDS analysis show that microdiamonds occur in situ, in multiphase fluid inclusions (Type I) distributed in clusters in the garnet mantle. The Raman spectra suggest partial transformation of diamond to disordered sp2-bonded carbon structure. Along with diamond, Type I inclusions contain (i) rutile and apatite as trapped solid phases, (ii) carbonates (magnesite-siderite) and Al-phyllosilicates (white mica, phlogopite, pyrophyllite) as daughter or step-daughter minerals, and (iii) CO2 as a residual fluid phase. Former melt inclusions (Type II) occur in the same microstructural position in the host garnet. They contain muscovite, paragonite, phlogopite, K-feldspar, plagioclase, albite and quartz as solid phases crystallized from a melt, and kyanite as accidentaly trapped mineral. The occurrence of melt inclusions in the inner part of garnets thus suggests that garnet was growing in the presence of melt. Garnet is nearly homogeneous with respect to major elements Mg, Fe, Ca and Mn, expressed by pyrope (0.18-0.22 XPrp), grossular (0.09-0.12 XGrs), almandine (0.67-0.70 XAlm) and spessartine (0.01-0.03 XSps) except local Ca enrichment in the rim. Trace elements show decreasing HREE and Y patterns from the core to the mantle suggesting garnet growth according to the Rayleigh fractionation model. Phosporus shows an elevated content in the core and mantle. A positive correlation between P and Na indicates a coupled NaPM2+-1Si-1 substitution in garnet. Minimum P-T conditions of 3.7-3.8 GPa and 840-870°C for the peak metamorphic stage were estimated from garnet composition (XPrp = 0.22), zirconium-in rutile thermometry of rutile inclusions in garnet (747-977 ppm of Zr) and diamond/graphite stability boundary. Partial melting on a prograde P-T path was controlled by the decomposition of phengite in the presence of C-O-H fluid, producing peritectic garnet ± kyanite along with melt, in the diamond stability field. The coexistence of diamond-bearing multiphase fluid inclusions with melt inclusions suggests primary fluid-melt immiscibility at UHP conditions. During exhumation, the rock underwent decompression and second partial melting, leading to enrichment in Ca, Y, Cr and Sc of garnet rims. Microdiamonds found in metasedimentary crustal rocks at Heia provide new evidence of UHP metamorphism in the Nordmannvik Nappe of the Arctic Caledonides. The results favor a correlation with the pre-Scandian subduction and arc-continent collision events of the Caledonian Orogeny.