Sub-Greenschist to Lower Amphibolite Facies Metamorphism of Basalts: Examples from Flin Flon, Manitoba and Rossland, British Columbia
Abstract
This study documents the metamorphic evolution of two sequences of metabasaltic rocks: one within the Flin Flon Greenstone Belt, Manitoba/Saskatchewan, and the other in the Rossland area, British Columbia.
The Flin Flon area comprises a varied igneous stratigraphy that has been affected by seafloor alteration, contact metamorphism and pervasive regional metamorphism. The regional metamorphism is split into six metamorphic zones spanning the prehnite-pumpellyite to lower amphibolite facies, separated by five major isograds. The pressure and temperature conditions for the prehnite-pumpellyite facies are ~200-280°C, 1.4-3.2kbar, whilst conditions of ~430-470°C and 3.3-4.3kbar are estimated for the greenschist-amphibolite transition. A domain of high temperature, hydrothermal alteration occurs within the Flin Flon block in the lower parts of the hanging wall stratigraphy. This is interpreted to be the product of prolonged syn-volcanic hydrothermal activity within the underlying hydrothermal system that generated the Flin Flon volcanogenic massive sulphide deposits.
Metamorphism of the Rossland Volcanic sequence has resulted in the development of five metamorphic zones spanning the sub-greenschist to amphibolite facies separated by four metamorphic isograds. The pattern of metamorphism indicates that greenschist to upper amphibolite facies ‘regional contact’ metamorphism, associated with the emplacement of plutons, was superimposed on regional sub-greenschist burial metamorphism.
Documentation of coexisting minerals from Flin Flon and Rossland suggests the existence of miscibility gaps involving amphibole, plagioclase and epidote group minerals. Actinolite and hornblende are interpreted to crystallise initially across a miscibility gap within the hornblende-actinolite zone. However, increasing compositional separation between actinolite and hornblende above the oligoclase-in isograd is interpreted as a kinetic effect, with hornblende being stable whilst actinolite persists upgrade as a metastable relict.
The greenschist-amphibolite transition zone is characterized by several devolatilisation reactions. This fluid release occurs predominantly within the hornblende-actinolite-oligoclase zone, characterized by very low XCO2 compositions (<0.05). Fluid release with higher XCO2 values (>0.2) may occur in rocks with greater amounts of carbonate. Petrological and modelling observations suggest that the Flin Flon sequence would predominantly produce fluid compositions with low XCO2 (<0.1) and thus is unlikely to be a source for fluids associated with orogenic gold mineralization that are typified by higher XCO2 compositions (0.2-0.3).
Description
Keywords
Geochemistry, Geology, Mineralogy
Citation
Starr, P. (2017). Sub-Greenschist to Lower Amphibolite Facies Metamorphism of Basalts: Examples from Flin Flon, Manitoba and Rossland, British Columbia (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/27926