Diversity among peridotites in the Western Gneiss Region, Norwegian Caledonides

Garnet-bearing peridotites in the Western Gneiss Complex (WGC) have been classed into two groups based upon bulk composition – the Mg-Cr type (generally accepted to be derived from subcontinental mantle) and the Fe-Ti type (metamorphosed ultramafic parts of layered intrusions into continental crust). The distribution of both types broadly coincides with the occurrence of UHP (coesite-stable) eclogites. In Sunnfjord, in the southern WGC, where only quartz-stable eclogites are known, major peridotite bodies of the Fe-Ti type are characterized by the assemblage ol + opx + chl + Ca-amp and lack garnet or any evidence that it has previously existed. While this paragenesis may be co-facial with the predominant amphibolite-facies mineral assemblages in the host gneisses, it is also consistent with estimated P-T conditions for the eclogites in this area at ~600^oC and 2.3GPa, thus chlorite peridotites may have equilibrated at pressures under which eclogites were stabilized. Mg-Cr garnet-bearing peridotites in the WGC have often been grouped together and their features combined to determine a common evolution, but in fact two distinct groups appear to exist – a southern group including Almklovdalen, Aldalen and Tafjord, and a northern group including Fjortoft, Otroy and possibly Gurskoy. The northern group is dominated by harzburgites and less common dunites, with garnet-bearing varieties widely distributed throughout. Independent P-T estimates here are consistent with regional diamond stability. As in the southern group, the peridotites are porphyroclastic, but the neoblastic matrix has stable garnet. Porphyroclastic assemblages include relics of majoritic garnets, apparently formed first, during the mid-Proterozoic, then later during the Scandian continental subduction event, the latter associated with localized metasomatic microdiamond formation associated with development of strongly radiogenic Sr-isotopic compositions, presumably due to contamination by fluids or melts derived from adjacent, subducted continental crust. In contrast, southern group peridotites are predominantly dunites with only localized development of garnetiferous varieties, these resulting from localized Proterozoic refertilisation of pre-existing, severely depleted dunites. No strong evidence for a majoritic history has yet been published, and porphyroclast assemblages give a range of Proterozoic ages. Neoblastic matrix assemblages appear to lack stable garnet, consisting of ol + opx + cpx + Ca-amp +/- sp, and their age is unknown. Also, chlorite is abundant and forms a prominent foliation, in contrast to the northern group where it appears to be only locally developed along narrow late shear zones and fractures. No microdiamonds have yet been discovered, and Sr isotopic compositions are much less radiogenic than in the northern group. P-T conditions in the crust associated with the southern belt peridotites are ~650^oC and 2.5GPa (Almklovdalen), and as in Sunnfjord this may be consistent with equilibration conditions for chlorite peridotites.