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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. 相似文献
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The Monteregian Hills consist of a series of alkaline intrusionsand associated dikes and sills emplaced along a linear west-easttrend extending from 35 km west of Montreal, Quebec, to 190km east of Montreal. The igneous activity occurred between 117Ma and 141 Ma and the age data show a distinct bimodality, withstrongly undersaturated magmas emplaced at c. 118 Ma and slightlyundersaturated magmas emplaced at c. 136 Ma. The eastern intrusionslargely consist of gabbros, diorites, and a variety of felsicrocks as compared to the more mafic character of the westernintrusions which largely consist of pyroxenites, gabbros, anddiorites. The eastern intrusions were emplaced through a thicksequence of folded and faulted geosynclinal sediments whichseems to have played a role in determining their felsic character. Mounts Brome and Shefford are located approximately 80 km eastof Montreal. The dominant rock type at Mount Shefford is a dioritewhich 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 bodydivided into a slightly quartz-undersaturated unit (pulaskite)and a slightly oversaturated unit (nordmarkite). The two unitsare chemically similar, but the nordmarkite has elevated initialSr ratios suggesting that crustal contamination is responsiblefor its oversaturation. Both complexes have been intruded bynepheline-bearing diorites, and at Mount Brome, foyaites, tinguaites,and laurdalites. Mount Megantic is located approximately 190km east of Montreal and consists of an outer normarkite annulus,separated by a gabbro-dioritic body from an inner granite plug. Five different magma types have been identified in the easternMonteregian Hills. Type 1, precursor to the gabbros and syenitesat Mounts Shefford and Brome, is an alkali picrite generatedby limited partial melting of a garnet lherzolite, with subsequentevolution controlled by the removal of olivine, pyroxene, andplagioclase. Type 2, precursor to the gabbros and nordmarkitesat Mount Megantic, is apparently produced by moderate partialmelting of a spinel lherzolite, and this magma subsequentlyevolved to a quartz-saturated residuum. Type 3, which producedthe rocks of the strongly undersaturated series, is equivalentto the basanite magmas generated by small degrees of meltingof a spinel lherzolite, which formed similar rocks in the westernMonteregian Hills. The high concentration of incompatible elementsand high-charge-density cations in these magmas indicates thatthe mantle source regions were enriched in these elements. Type4, Mount Shefford nordmarkite, and Type 5, Mount Megantic granite,magmas apparently originated in the crust by partial meltingof, respectively, amphibolite or granulite facies metadioriteand metagraywacke sources. 相似文献
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