Monteregian Hills I. Petrography, Major and Trace Element Geochemistry, and Strontium Isotopic Chemistry of The Western Intrusions: Mounts Royal, St. Bruno, and Johnson |
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| Authors: | EBY G NELSON |
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| Institution: | Department of Earth Sciences, University of Lowell Lowell, MA 01854, U.S.A. |
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| Abstract: | The Monteregian Hills petrographic province of southwesternQuebec, Canada, consists of a series of alkaline intrusionsemplaced along faults associated with the St. Lawrence graben.The intrusions are crudely cylindrical in shape, show verticalcontacts, and apparently extend to great depths. Where observed,igneous foliation is generally steeply dipping. The western intrusions consist of two petrographically distinctgroups. One group is composed of slightly undersaturated tocritically saturated pyroxenites and gabbros, largely of cumulateorigin, and associated slightly quartz-saturated syenites. Thesecond group is composed of strongly to moderately undersaturateddiorites, monzonites, and syenites which contain significantamounts of feldspathoidal minerals. The Oka carbonatite complexbelongs to the latter group. Available age data indicate that these two petrographic groupsrepresent separate periods of igneous activity. The slightlyundersaturated to critically saturated series has a mean ageof 136 Ma, while the strongly to moderately undersaturated serieshas a mean age of 118 Ma. Mounts Royal and St. Bruno are largely composed of gabbros andpyroxenites which belong to the slightly undersaturated to criticallysaturated series. These units consist of variable amounts ofcumulus pyroxene and olivine and intercumulus minerals. Someof the finer-grained gabbros approximate liquid compositions.Major and trace element rock and mineral chemistry demonstratethat the evolution of these magmas was largely controlled bypyroxene and olivine fractionation, with plagioclase appearingon the liquidus late in the crystallization history. The quartz-bearingsyenites at Mt. St. Bruno represent a late stage differentiatewhich was contaminated by siliceous crustal material. The strongly to moderately undersaturated series is representedby the essexites and pulaskites at Mount Johnson and the nepheline-bearingdiorites and feldspathoidal monzonites and syenites at MountRoyal. The petrogenetic relationships between these rocks arecomplex and apparently involve a number of processes includingliquid immiscibility, contamination, and alkali transport. Low initial Sr isotope ratios (0.7032 to 0.7035) for both ofthese rock series indicate a mantle origin. Calculated initialmelts are alkali picrites for the slightly undersaturated tocritically saturated series and basanites for the strongly tomoderately undersaturated series. The alkali picrites can beproduced by an 8 per cent melt of a light rare-earth enrichedgarnet lherzolite source. The basanites require a much morelimited degree of melting (1–2 per cent) of a spinel lherzolitesource. In the case of the basanites, CO2 may have played animportant role in determining the nepheline-normative characterof the magmas. |
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