Journal of Petrology | Volume 35 | Number 4 | Pages 1127-1153 | 1994
© Oxford University Press 1994
research-article |
Open-System, Sub-Volcanic Magmatic Evolution: Constraints on the Petrogenesis of the Mount Brome Alkaline Complex, Canada
1 Department of Geological Sciences, Ohio State University Columbus, Ohio 43210, USA
2 Department of Geology, University of Manchester Manchester M13 9 PL, UK
Received February 16, 1993; Revised typescript accepted October 6, 1993
ABSTRACT
The Mount Brome alkaline complex of southern Quebec (Canada) comprises gabbroic to silica-oversaturated and -undersaturated felsic rocks which have variable initial 87Sr/86Sr and 143Nd/144Nd ratios: 0.70345–0.70431 and 0.51270–0.51258, respectively, in mafic lithologies (gabbro, nepheline diorite, and biotite monzodiorite); 0.70353–0.70403 and 0.51270–0.51263 in silica-undersaturated felsic units (pulaskite and nepheline syenite); and 0.7051–0.7115 and 0.51262–0.51259 in silica-oversaturated nordmarkite. Negatively correlated 87Sr/86Sr vs. 143Nd/144Nd trends for the various rock types appear to converge at the composition 87Sr/86Sr = 0.70345 and 143Nd/144Nd = 0.51270 (
Nd = 4.3). This suggests that all rock types share a common parental magma composition, either through derivation from a single batch of liquid, or (more probably) through two episodes of melting of the same depleted mantle source region. Delta 18O ranges from + 5.5 to + 8% and is broadly correlated with radiogenic isotopes and bulk composition.
Isotopic, and major and trace element compositions suggest that fractional crystallization (FC) of the parental magma at deep levels produced evolved magmas, apparently without significant crustal contamination [FC without assimilation (A)]. During ascent and emplacement into the upper crust, these magmas then evolved by simultaneous crystal fractionation and country-rock assimilation (AFC). Within both basic and felsic rocks there is a clear relationship between silica saturation and degree of contamination. Indeed, the oversaturated nordmarkites owe their origin to assimilation of siliceous country rocks by a critically undersaturated magma. More generally, it seems likely that this type of process is the normal mode of origin for coexisting quartz and nepheline syenites in many sub-volcanic alkaline igneous complexes. Additionally, such complexes would be likely to develop by punctuated FC and AFC processes throughout their petrogenetic history.
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