Geochemistry and petrology of amphibolites, granites, and metasedimentary rocks, Monashee Mountains, Southeastern Canadian Cordillera

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Graphitic metapelites from Howard Ridge, British Columbia, underwent metamorphism at 610-625 ± 30 °C and 6.7 ± 1.0 kbar. The equilibrium, paragonite-quartz-albite-kyanite-H20 gives significantly different estimates of XH20 using different paragonite solution models. These estimates range from 0.80 to 0.35. Based on consistent estimates of fluid composition between C-0-H and mineral equilibria and published fluid inclusion data, it is proposed that a H20-rich fluid phase evolved during the metamorphism of graphitic metapelites at Howard Ridge. Amphibolites of the Late Proterozoic Horsethief Creek Group consist of a mineralogically diverse suite of flows, transposed dykes, and sills represented by metamorphosed alkaline and tholeiitic basalts. Alkaline basalts are distinguished from the tholeiites by their light rare earth element (LREE) enrichment and low Y/Nb. Tholeiites comprise three distinct groups that were derived by partial melting of a heterogeneous mantle source. The basalts have undergone variable degrees of crystal fractionation and, perhaps, crustal contamination. Partially recrystal­lized ultramafic rocks occur as boudins of Cr-spinel-bearing harzburgite and/or lherzolite, enriched in REE, high field strength elements, Cao, Ti02, and Al203 relative to depleted mantle. The ultramafic rocks represent either crustally contaminated primary mantle melts or olivine, pyroxene, and Cr-spinel cumulates enriched by a basaltic melt. In the northern Adams River of the Monashee Mountains, British Columbia, the axial trace of the Scrip Nappe, a large-scale southwest­verging F1 anticline, can be traced upward through all stratigraphic units of the Late Proterozoic Horsethief Creek Group. This nappe has been coaxially refolded by a northeast-verging, isoclinal F2 synform with an amplitude of about 6 km. Metamorphic isograds cut across strati­graphic units and F2 structures and thus, metamorphism post-dates F2. The muscovite + quartz-out isograd defines a metamorphic culmination expressed as a series of isogradic "synforms" and "antiforms" in the northern Adams River area. Syn- and post-kinematic (i.e., kinematic with respect to F2) per­aluminous granites with U-Pb ages of 105-110 and 63 Ha, respectively, occur as dykes, sills, and sheets up to 50 m thick within and adjacent to the metamorphic culmina- tion in the northern Adams River area. Geo­barometric estimates suggest that both generations of granites equili­ 87 brated at 6-8 kbar (22-30 km). High initial Sr/86 Sr ratios and evidence of Precambrian Pb inheritance indicates that both syn- and post-kinematic granites were derived from a crustal source. Geochemical modelling demonstrates that the syn- kinematic granites were derived by a low degree of partial melting of Late Proterozoic Horsethief Creek Group metapelites leaving a REE enriched upper amphibolite facies residue. Post-kinematic granites were derived from a geochemically and isotopically similar metapelitic source. The suite of post-kinematic granites underwent a small amount (up to 0.1%) of monazite crystal fractionation.
Bibliography: p. 143-149.
Sevigny, J. H. (1988). Geochemistry and petrology of amphibolites, granites, and metasedimentary rocks, Monashee Mountains, Southeastern Canadian Cordillera (Unpublished doctoral thesis). University of Calgary, Calgary, AB. doi:10.11575/PRISM/17983