OPEN-SYSTEM, SUB-VOLCANIC MAGMATIC EVOLUTION - CONSTRAINTS ON THE PETROGENESIS OF THE MOUNT BROME ALKALINE COMPLEX, CANADA

The Mount Brome alkaline complex of southern Quebec (Canada) comprises gabbroic to silica-oversaturated and -undersaturated felsic rocks which have variable initial Sr-87/Sr-86 and Nd-143/Nd-144 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 Sr-87/Sr-86 vs. Nd-143/Nd-144 trends for the various rock types appear to converge at the composition Sr-87/Sr-86 = 0.70345 and Nd-143/Nd-144 = 0.51270 (epsilon(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 O-18 ranges from + 5.5 to + 8parts per thousand 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 subvolcanic alkaline igneous complexes. Additionally, such complexes would be likely to develop by punctuated FC and AFC processes throughout their petrogenetic history.