IGNEOUS PETROLOGY OF THE METAMORPHIC SOLE AT BELMINA RIDGE, THETFORD MINES OPHIOLITE COMPLEX (TMOC), QUEBEC, CANADA

Belmina Ridge, western mantle section of TMOC in Quebec Appalachians, is associated to the obduction of an ophiolitic complex located around 100 kilometers south-west of Quebec City. By studying the 800 m thick amphibolitic sole underlying this part of the complex, the goal is to determine the protolith of this metamorphic sequence and the geodynamic environment in which the ancient oceanic basin has evolved during closure of Iapetus proto-ocean. Since summer 2013, twenty-two thin sections analyzed by microprobe have proven presence of mineral zonations in amphibole and garnet interpreted as results of intern diffusion during prograde evolution. Some minerals, as chlorite and hydrogrossular, are interpreted as retrograde minerals formed during exhumation of the sole. Other minerals are mostly prograde and show a strong metamorphosed texture formed during early stage of subduction before exhumation. Some thin sections show a small partial melt texture around some grains. Thirty-three geochemistry analyses based on major, minor and trace elements show geochemical attributes with island arc tholeiite. Based on discrimination diagrams, some amphibolites show a similar REE patterns to E-MORB involving possibly OIB enrichment. Such enrichments of LREE values are also easily recognized in most REE patterns. LREE range from 6x rock/C1 to more than 100x rock/C1 with a (La/Sm)_Nc1 ratio around 0,8 to more than 3,5 characteristic of T-MORB to in excess of E-MORB values. Extended-spider diagrams normalized to primitive mantle show that most of the samples have little Nb-Ta positive anomalies that could be characteristic of OIB contamination and strong Zr-Hf negative anomalies that could correspond to specific mineral composition of the source. Preliminary geodynamic model, could involve thickened OIB-modified arc crust generating typical OIB highs such as seamounts or aseismic ridge that could perturbate plate subduction and control the exhumation of the sole.