Monteregian Hills

Petrology and Geochemistry of Mount Yamaska, Quebec, Canada: a Mafic Representative of the Monteregian Hills Igneous Province

Eby, G. N.

The Monteregian Hills igneous province is located in southwestern Quebec, Canada, and consists of a number of small plutons and dikes encompassing a wide range of alkaline rock types. Mount Yamaska is a predominantly ultramafic to mafic pluton in which the major lithologies are pyroxenite and gabbro. The pyroxenites and gabbros are nepheline-normative, but do not contain modal nepheline, and show a wide range of textures and chemistry typical of crystal accumulation. These cumulate rocks were subsequently intruded by essexites and nepheline syenites, which comprise a strongly silica-undersaturated magmatic trend. A partial outer annulus of monzonitic rocks apparently represents melted and remobilized country rock.

Initial Sr ratios vary from 0.7032 to 0.7051 and support the contention that the mafic magmas were derived from a depleted mantle, and that the magmas subsequently interacted with the rocks of the crustal section. Two distinct mafic magmas have been identified, one of alkali picrite composition which originated in a garnet lherzolite mantle and the other of basanite composition which originated by limited partial melting of a spinel lherzolite mantle . The alkali picrite magma gave rise to the pyroxenites and gabbros while the basanite magma was the precursor for the essexites and nepheline syenites.

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The Monteregian Hills and White Mountain Alkaline Igneous Provinces, Eastern North America

Eby, G. N.

The Monteregian Hills and White Mountain provinces consist of stocks, plugs, ring-dyke complexes and several large granite bodies emplaced into Precambrian gneisses, flat-lying Cambro-Ordovician sediments and the deformed Lower Palaeozoic section of the Appalachian fold belt. Felsic rocks dominate in the Appalachian fold belt, while elsewhere mafic and ultramafic rocks are significant components of the plutons. Igneous activity extended from 240 to 90 Ma ago with two major periods of magmatism, correlated with events in the opening of the N Atlantic Ocean, occurring between 200 - 165 Ma and 140 - 110 Ma.

Five major rock series have been identified: (1) undersaturated CO₂-rich rocks, carbonatite and alnöite; (2) moderately to strongly undersaturated diorites - nepheline syenites; (3) slightly undersaturated to slightly oversaturated pyroxenites - gabbros - diorites - syenites; (4) alkali syenite - quartz syenite - granite; (5) metaluminous biotite granite. Series (1), (2) and (3) magmas were drawn from an isotopically depleted mantle which was enriched in incompatible elements shortly before or synchronous with melting. These magmas were produced by variable degrees of melting of garnet or spinel lherzolite. Series (4) and (5) magmas represent partial melts of a heterogeneous crustal section consisting of both meta-sedimentary and meta-igneous rocks of either Grenville (Precambrian) or Lower Palaeozoic age.

The Petrology of the Mont Saint Hilaire Pluton, Southern Quebec: an Alkaline Gabbro - Peralkaline Syenite Association

Currie, K. L, Eby, G. N., and Gittins, J.

The Mont Saint Hilaire complex consists of an older (133 Ma) suite of layered cumulates of titanaugite, kaersutite, plagioclase and titaniferous magnetite, and two younger (122 Ma) suites, one a thick ring dyke of nepheline-olivine diorite to monzonite, and the other a pipe or funnel-like mass of peralkaline nepheline-sodalite syenite and porphyry associated with a variety of breccias. Trace element data suggest derivation of the older suite from a garnet-bearing source. Only minor amounts of possible liquid compositions are preserved in this suite. The nepheline and olivine-bearing suite followed a course of fractionation from gabbroic to monzonitic compositions involving fractionation of pyroxene, magnetite, apatite and plagioclase. Field and trace element data suggest mixing of the evolved liquid with a saline brine at crustal depths produced the strongly nepheline-normative peralkaline magma. Rich in Na and Cl, the brine was poor in other major and trace elements, and had a high initial Sr ratio. The localization and extended time of emplacement of the complex appear to be due to upward migration of a thermal anomaly from the base of a lithospheric plate.

Age Relations, Chemistry, and Petrogenesis of Mafic Alkaline Dikes from the Monteregian Hills and Younger White Mountain Igneous Provinces

Eby, G. N.

The mafic alkaline dikes of the Monteregian Hills and younger White Mountain igneous provinces can be divided into three groups: (1) K₂O-rich alnöites; (2) moderately to strongly undersaturated monchiquites, camptonites, and basanites; and (3) slightly undersaturated to critically saturated camptonites and alkali olivine basalts. The dikes were emplaced between 139 and 107 Ma, with the bulk of the activity occurring in three discrete intervals: 139 - 129, 121 - 117, and 110 - 107 Ma. The first two intervals correspond to the times of emplacement of the main Monteregian intrusions. There is no apparent geographic pattern to the ages.

Chemical evolution of the group 2 and group 3 magmas was largely controlled by the removal of olivine, clinopyroxene, and Fe-Ti-rich oxides. The group 2 dikes are generally enriched in REE and have higher La/Yb ratios (18 - 28) than the group 3 dikes (La/Yb = 9 - 23). For the majority of the samples Zr/Hf ratios (30 - 43) and Rb/Ba x 10² ratios (4.8 - 11.6) fall in the range of primary basalts, but some samples have higher ratios, indicating crustal contamination.

Trace-element models indicate that group 2 and group 3 magmas originated by variable degrees of melting of a metasomatized spinel lherzolite whereas the group 1 magmas most likely originated in a carbonated garnet lherzolite mantle. The thermal energy for the melting may have been provided by a mantle plume.

Sr and Pb Isotopes, U and Th Chemistry of the Alkaline Monteregian and White Mountain Igneous Provinces, Eastern North America

Eby, G. N.

The Monteregian Hills and younger White Mountain alkaline intrusions were emplaced into the Cambro-Ordovician sediments of the St. Lawrence Lowlands and the folded and thrusted Lower Paleozoic sequence of the Appalachian orogen. Age relations indicate that there is a fine-scale structure to the igneous activity, with slightly undersaturated to critically saturated rocks emplaced between 141 and 128 Ma and strongly undersaturated rocks emplaced between 121 and 117 Ma.

Sr and Pb isotopic data for the mantle-derived alkali picrite, alkali olivine basalt and basanite magmas, indicate derivation from a depleted mantle similar to that which produces present-day oceanic island basalts. For the most isotopically primitive samples, decay corrected ⁸⁷Sr/⁸⁶Sr = 0.7030 - 0.7037, ²⁰⁶Pb/²⁰⁴Pb = 19.05 - 19.72, ²⁰⁷Pb/²⁰⁴Pb = 15.56 - 16.65, and ²⁰⁸Pb/²⁰⁴Pb = 38.64 - 39.26. On Pb-Sr isotope correlation diagrams the data define trends similar to those for MOR basalts, implying mantle heterogeneity which requires the presence of a component enriched in radiogenic Pb relative to Sr. The interaction of these isotopically primitive magmas with the crust can be defined in terms of a three component system: depleted mantle - Grenville age crust - Lower Paleozoic age crust. The granitic magmas were apparently derived from the Lower Paleozoic crust of the Appalachian orogen.

For the mantle-derived magmas, Th/U ratios vary from 2.5 (estimated ratio for MORB source) to 5.1, with the mean value near that of the bulk earth. The variations in Th/U suggest mantle heterogeneity on a local scale, and the high Th/U of some samples suggests that the mantle was enriched in incompatible elements shortly before melting. The magmas derived by partial melting of the crust have Th/U of 3.3 to 8.7, and the higher ratios are associated with rocks crystallized from magmas that originated by melting of Lower Paleozoic sediments.

The Sr and Pb isotopic data support the conclusion of Bell et al. (1982) that the subcontinental mantle under eastern Canada underwent a Precambrian depletion event. This depleted mantle apparently extends under the White Mountain province and is isotopically similar to the mantle that gives rise to oceanic island basalts. In contrast, Pb isotopic ratios for the New England Seamount chain (Tara and Hart, 1983), which apparently represents the oceanic extension of this magmatic activity, are significantly more radiogenic. It is possible that a mantle plume provided the heat energy, and perhaps metasomatic fluids, to trigger melting in the subcontinental mantle. whereas in the case of the oceanic extension the plume directly contributed to the observed magmatism, as reflected in the more radiogenic Pb ratios.

Rock isotopic and chemical data (Excel spreadsheet)

Monteregian Hills II. Petrography, Major and Trace Element Geochemistry, and Strontium Isotopic Chemistry of the Eastern Intrusions: Mounts Shefford, Brome, and Megantic

Eby, G. N.

The Monteregian Hills consist of a series of alkaline intrusions and associated dikes and sills emplaced along a linear west-east trend extending from 35 km west of Montreal, Quebec, to 190 km east of Montreal. The igneous activity occurred between 117 Ma and 141 Ma and the age data show a distinct bimodality, with strongly undersaturated magmas emplaced at c. 118 Ma and slightly undersaturated magmas emplaced at c. 136 Ma. The eastern intrusions largely consist of gabbros, diorites, and a variety of felsic rocks as compared to the more mafic character of the western intrusions which largely consist of pyroxenites, gabbros, and diorites. The eastern intrusions were emplaced through a thick sequence of folded and faulted geosynclinal sediments which seems to have played a role in determining their felsic character.

Mounts Brome and Shefford are located approximately 80 km east of Montreal. The dominant rock type at Mount Shefford is a diorite which has been intruded by arcuate bodies of pulaskite and nordmarkite. Mount Brome consists of an outer crescent-shaped gabbro body, composed of several cyclical units, and a large syenite body divided into a slightly quartz-undersaturated unit (pulaskite) and a slightly oversaturated unit (nordmarkite). The two units are chemically similar, but the nordmarkite has elevated initial Sr ratios suggesting that crustal contamination is responsible for its oversaturation. Both complexes have been intruded by nepheline-bearing diorites, and at Mount Brome, foyaites, tinguaites, and laurdalites. Mount Megantic is located approximately 190 km east of Montreal and consists of an outer nordmarkite annulus, separated by a gabbro-diorite body from an inner granite plug.

Five different magma types have been identified in the eastern Monteregian Hills. Type 1, precursor to the gabbros and syenites at Mounts Shefford and Brome, is an alkali picrite generated by limited partial melting of a garnet lherzolite, with subsequent evolution controlled by the removal of olivine, pyroxene, and plagioclase. Type 2, precursor to the gabbros and nordmarkites at Mount Megantic, is apparently produced by moderate partial melting of a spinel lherzolite, and this magma subsequently evolved to a quartz-saturated residuum. Type 3, which produced the rocks of the strongly undersaturated series, is equivalent to the basanite magmas generated by small degrees of melting of a spinel lherzolite, which formed similar rocks in the western Monteregian Hills. The high concentration of incompatible elements and high-charge-density cations in these magmas indicates that the mantle source regions were enriched in these elements. Type 4, Mount Shefford nordmarkite, and Type 5, Mount Megantic granite, magmas apparently originated in the crust by partial melting of, respectively, amphibolite or granulite facies metadiorite and metagraywacke sources.

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Geochronology of the Monteregian Hills Alkaline Igneous Province, Quebec

Eby, G. N.

Three distinct groups of alkaline rocks can be recognized, on the basis of silica content, in the Monteregian Hills, Quebec: strongly undersaturated, slightly undersaturated to critically saturated, and oversaturated. The first two groups were derived from basanite and alkali picrite magmas, respectively, and yield distinct ages of intrusion clustered about 118 Ma and 136 Ma. The age data are not readily explained by the translation of the North American plate across a fixed mantle hotspot, but because the alkali picrite magmas were derived from greater mantle depths than the basanite magmas, the data do suggest the upward migration of the zone of melting. This change in depth of melting may be a response to the upward transport of a metasomatic fluid, perhaps with a coupled rise in mantle isotherms.

Monteregian Hills I. Petrography, Major and Trace Element Geochemistry, and Strontium Isotopic Chemistry of the Western Intrusions: Mounts Royal, St. Bruno, and Johnson

Eby, G. N.

The Monteregian Hills petrographic province of southwestern Quebec, Canada, consists of a series of alkaline intrusions emplaced along faults associated with the St. Lawrence graben. The intrusions are crudely cylindrical in shape, show vertical contacts, and apparently extend to great depths. Where observed, igneous foliation is generally steeply dipping.

The western intrusions consist of two petrographically distinct groups. One group is composed of slightly undersaturated to critically saturated pyroxenites and gabbros, largely of cumulate origin, and associated slightly quartz-saturated syenites. The second group is composed of strongly to moderately undersaturated diorites, monzonites, and syenites which contain significant amounts of feldspathoidal minerals. The Oka carbonatite complex belongs to the latter group.

Available age data indicate that these two petrographic groups represent separate periods of igneous activity. The slightly undersaturated to critically saturated series has a mean age of 136 Ma, while the strongly to moderately undersaturated series has a mean age of 118 Ma.

Mounts Royal and St. Bruno are largely composed of gabbros and pyroxenites which belong to the slightly undersaturated to critically saturated series. These units consist of variable amounts of cumulus pyroxene and olivine and intercumulus minerals. Some of the finer-grained gabbros approximate liquid compositions. Major and trace element rock and mineral chemistry demonstrate that the evolution of these magmas was largely controlled by pyroxene and olivine fractionation, with plagioclase appearing on the liquidus late in the crystallization history. The quartz-bearing syenites at Mt. St. Bruno represent a late stage differentiate which was contaminated by siliceous crustal material.

The strongly to moderately undersaturated series is represented by the essexites and pulaskites at Mount Johnson and the nepheline-bearing diorites and feldspathoidal monzonites and syenites at Mount Royal. The petrogenetic relationships between these rocks are complex and apparently involve a number of processes including liquid immiscibility, contamination, and alkali transport.

Low initial Sr isotope ratios (0.7032 to 0.7035) for both of these rock series indicate a mantle origin. Calculated initial melts are alkali picrites for the slightly undersaturated to critically saturated series and basanites for the strongly to moderately undersaturated series. The alkali picrites can be produced by an 8 per cent melt of a light rare-earth enriched garnet lherzolite source. The basanites require a much more limited degree of melting (1 - 2 per cent) of a spinel lherzolite source. In the case of the basanites, CO₂ may have played an important role in determining the nepheline-normative character of the magmas.

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Minor and Trace Element Partitioning Between Immiscible Ocelli-Matrix Pairs from Lamprophyre Dikes and Sills, Monteregian Hills Petrographic Province, Quebec

Eby, G. N.

Many lamprophyre dike and sill rocks in the Monteregian Hills petrographic province of southwestern Quebec contain felsic segregations (ocelli) which have been interpreted as globules of immiscible liquid (Philpotts 1976). Ocelli and matrix material were separated from a number of these rocks and analyzed for major and trace elements. The major element data, when plotted on a Greig diagram, outline a field of possible silicate-liquid immiscibility at higher alumina + alkali content than that previously mapped in iron-rich experimental systems. The trace element data support a liquid immiscibility hypothesis for the formation of these ocelli since high-charge-density cations are preferentially concentrated in the matrix (mafic) material, a result which is consistent with theoretical and experimental studies.

The distribution of minor and trace elements between ocelli and matrix indicates that several factors control the partitioning of these elements between immiscible felsic and mafic liquids. These factor include the difference in relative polymerization (as measured by the Si:O ratio) of the two liquids, with an increase in this difference favoring partitioning of the high-charge-density cations into the mafic liquid; the concentration of P₂O₅ in the mafic liquid which favors the partitioning of high-charge-density cations into this liquid; the presence of a CO₂ vapor(?) phase which favors the partitioning of high-charge-density cations into the CO₂ enriched phase; and the presence of solid phases at the onset of immiscibility. These observations indicate that the chemical compositions of two possibly immiscible melts should be known if minor and trace element data are to be used as evidence of silicate-liquid immiscibility.

Reply: Mount Johnson, Quebec - An Example of Silicate-Liquid Immiscibility?

Eby, G. N.

Mount Johnson, Quebec - An Example of Silicate-Liquid Immiscibility?

Eby, G. N.

Mount Johnson, a member of the Monteregian Hills petrographic province in Quebec, is a cylindrical, pluglike intrusion in which the silicic peripheral unit (pulaskite) appears to have been emplaced prior to the more mafic transition and core units (essexite). Liquid immiscibility has been invoked to explain this emplacement sequence. The partitioning of trace elements between ocelli-matrix pairs (which have been shown to be the result of liquid immiscibility) from two Monteregian sills is compared to the partitioning of the same elements between peripheral-unit rocks and transition-unit rocks at Mount Johnson. In general, the low-field strength elements are relatively enriched in the silicic material and the high-field strength elements are relatively enriched in the mafic material. The distribution of the trace elements is quite similar for the sills and Mount Johnson. It is concluded that two magmas were involved in the formation of Mount Johnson. The first magma split into two immiscible liquids to form the peripheral-unit and transition-unit rocks, and the second magma was subsequently intruded up the center of the conduit to form the core-unit rocks.

Abundance and Distribution of the Rare-earth Elements and Yttrium in the Rocks and Minerals of the Oka Carbonatite Complex, Quebec

Eby, G. N.

Rare-earth (REE) and yttrium abundances were determined, by an ion-exchange - X-ray fluorescence procedure, for whole-rock (14) and mineral (87) samples from the Oka carbonatite complex. Whole-rock and mineral data indicate a trend of total REE + Y enrichment, and relative enrichment in light REE, in the order: ultrafenites < ijolites < okaites. The sövites may show wide variations in total REE + Y concentrations, but relative REE abundance patterns will be similar. The greatest REE and Y concentrations occur in apatite, niocalite, perovskite and pyrochlore. Many of the minerals show europium anomalies (both positive and negative), and these are believed to be the result of closed system competition between the various minerals for divalent Eu. The partition coefficients for mineral pairs are quite variable, indicating that the Oka rocks were emplaced through a wide-range of physicochemical and/or nonequilibrium conditions. A reasonable model for the origin of the complex involves a limited partial melting of mantle material, emplacement of the melt in a magma chamber, crystallization of mafic minerals resulting in a residual liquid which produced ijolite and subsequently okaite, and crystallization of the carbonatites from a volatile-rich, possible immiscible, phase.

Scandium Geochemistry of the Oka Carbonatite Complex, Oka, Quebec

Eby, G. N.

Scandium abundances were determined by X-ray fluorescence for rocks (14 samples) and minerals (84 samples) from the Oka carbonatite complex, Oka, Quebec. Whole-rock data indicate a general trend of scandium enrichment in the order: urtites < ijolites < okaites < sövites < alnöites. The ore minerals (niocalite, perovskite, and pyrochlore) and apatite are the principal carriers of scandium; ferromagnesian minerals contain less scandium and show the order of scandium uptake: biotite < pyroxene < garnet. The scandium content of the calcites is quite variable. Monticellite, melilite, nepheline, albite, the zeolites, and the feldspathoids contain negligible amounts of scandium. Scandium and REE concentrations vary sympathetically for the ore minerals, calcites, and apatites but not for any of the other minerals. Partitioning coefficients for various coexisting phases indicate that, in general, equilibrium was not attained by the Oka rocks. The silicate rocks of the complex crystallized from a differentiating alkali peridotite magma, and the sövites formed from a volatile-rich phase. The selective partitioning of scandium, possibly in the form of complex ions, into the volatile-rich phase may account for the concentration of scandium in the sövites.