Open-System, Sub-Volcanic Magmatic Evolution: Constraints on the Petrogenesis of the Mount Brome Alkaline Complex, Canada |
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| Authors: | CHEN JIANGFENG; HENDERSON C M B; FOLAND K A |
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| Institution: | 1 Department of Geological Sciences, Ohio State University Columbus, Ohio 43210, USA
2 Department of Geology, University of Manchester Manchester M13 9 PL, UK |
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| Abstract: | The Mount Brome alkaline complex of southern Quebec (Canada)comprises gabbroic to silica-oversaturated and -undersaturatedfelsic rocks which have variable initial 87Sr/86Sr and 143Nd/144Ndratios: 0.70345–0.70431 and 0.51270–0.51258, respectively,in mafic lithologies (gabbro, nepheline diorite, and biotitemonzodiorite); 0.70353–0.70403 and 0.51270–0.51263in silica-undersaturated felsic units (pulaskite and nephelinesyenite); and 0.7051–0.7115 and 0.51262–0.51259in silica-oversaturated nordmarkite. Negatively correlated 87Sr/86Srvs. 143Nd/144Nd trends for the various rock types appear toconverge at the composition 87Sr/86Sr = 0.70345 and 143Nd/144Nd= 0.51270 ( Nd = 4.3). This suggests that all rock types sharea common parental magma composition, either through derivationfrom a single batch of liquid, or (more probably) through twoepisodes of melting of the same depleted mantle source region.Delta 18O ranges from + 5.5 to + 8% and is broadly correlatedwith radiogenic isotopes and bulk composition. Isotopic, and major and trace element compositions suggest thatfractional crystallization (FC) of the parental magma at deeplevels produced evolved magmas, apparently without significantcrustal contamination FC without assimilation (A)]. Duringascent and emplacement into the upper crust, these magmas thenevolved by simultaneous crystal fractionation and country-rockassimilation (AFC). Within both basic and felsic rocks thereis a clear relationship between silica saturation and degreeof contamination. Indeed, the oversaturated nordmarkites owetheir origin to assimilation of siliceous country rocks by acritically undersaturated magma. More generally, it seems likelythat this type of process is the normal mode of origin for coexistingquartz and nepheline syenites in many sub-volcanic alkalineigneous complexes. Additionally, such complexes would be likelyto develop by punctuated FC and AFC processes throughout theirpetrogenetic history. |
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