Exotic mineral products from the Chupa gneisses of the Belomorides

Abstract

Exotic mineral products (EМP) in the Chupa paragneisses of the Belomorian rock complex occur as an association of submicroscopic “spherules”, “pipes”, “chips” and their combined varieties. EМPs were derived by multi-stage allochemical stress-metamorphism of host gneisses at high pressure in temperature regimes typical of amphibolite (in the Late Archean) and epidote-amphibolite (in the Early Proterozoic) facies. One distinctive feature of EMPs is the presence of carbon in all of their mineral phases. The metal-like substance of “chips” and the cores of “spherules” and “pipes” is comparable in chemical composition and the Ме/С index (Ме = Fe + impurity elements) with known iron carbides, and is similar in composition to terrestrial minerals, such as chalypite, yarlongite, cohenite, and haxonite. The average bulk carbon content of core-free EMPs and core margin substance (at.%) is 7 for Archean and 7-11 for Proterozoic rocks. The average carbon content of iozite, a mineral phase prevalent in “spherules” and “pipes”, is 11 at.% for Archean and 14 at.% for Proterozoic units. The carbon content of the mineral fragments of host rocks (quartz, plagioclase, garnet, and kyanite), which form inclusions in EMPs, is 7-20 to 46 at. %. The distinctive characteristics of the EMPs, publications on the gas composition and soluble carbonaceous endogenic material of gas-liquid inclusions in the rock-forming minerals of Chupa gneisses, and the presence of graphite therein provide evidence for the significant, yet poorly understood contribution of carbon and its chemical compounds as fluid constituents to the highgrade metamorphism of the Belomorides.