American Mineralogist, Volume 75, pages 1092-1 105, 1990 Compositionalvariations in pyroxenesand amphibolesof the Belknap Mountain complerqNew Hampshire: Evidencefor the origin of silica-saturatedalkaline rocks Mrcneer, J. Doruls Department of Geology, Indiana University, Bloomington, Indiana 47405, U.S.A. Arsrucr Compositions of amphibole and pyroxene in the various units of the Belknap Mountain complex of central New Hampshire indicate a diversity of parental magmas.Amphibole and pyroxene in camptonites and Moat Volcanic trachyandesite,and amphibole in the Vent Agglomeratedike, indicate crystallization from silica-undersaturatedmagnas similar to those of the Monteregian Hill intrusions. The Belknap Syenite, Cobble Hill Syenite, and Conway Granite are silica-saturated;amphibole and pyroxene from these rocks are typical of the White Mountain Magma Series. Pyroxenein the camptonites,Gilford Gabbro, Vent Agglomeratedike, and the Endicott Diorite has similar Mg/(Mg + Fe,o,)values, but displays different t41Al,Ti, Na, and Ca contents and zoning. These differencesimply that even the units that may have had a common parent evolved independently from each other and are probably not related by shallow level processes. Amphibole samples from the Conway Granite have higher M/(Mg + Fe,o,)values and are richer in Si and poorer in Al, Ti, and Na + K than those from the Vent Agglomerate dike. Amphibole from the Belknap Syenite forms a compositional continuum between amphibole in the Vent Agglomerate dike and Conway Granite. The nature of Belknap Syenite amphibole evolution, coupled with the presenceof nepheline inclusions in oligo- clase and interstitial qvartz, suggeststhat the Belknap Syenite may have originated through mixing of a silica-poor magma similar to the Vent Agglomeratedike with a magma similar to the Conway Granite. Underplating of the crust by strongly silica-undersaturatedmagmas similar to those in the Monteregian Hills may have partially melted lower crustal rocks to form the Conway Granite. Mixing of thesetwo main magma types can account for the origin of the Belknap Syenite and may serve as a model for many of th€ alkaline, silica-saturatedrocks of the White Mountain Magma Series. ftlITnonucrroN Eby, 1985; Woolley and Jones, 1987; Bailey, 1987).In An aspect of alkaline magmatism of considerablein- this scenario, associatedmafic rocks represent magmas terest is the origin of the Daly gap and the relationship that provide a heat source for melting (Loiselle and Hart, between mafic, silica-undersaturatedmagmas and felsic, 1978;McHone and Butler, 1984;B6dard et al., 1987).(3) silica-saturatedmagmas. Bimodal associationsof gabbro- Crustal contamination or mixing with anatectic melts im- diorite and syenite-graniteoccur at many ring-dike com- poses silica saturation on initially silica-undersaturated plexesthroughout the world (Martin and Piwinsk'r, 1972; parental magmas (Barker et al., 1975; Randall and Fo- Bonin, 1986),and there are three main schoolsof thought land, 1986; Baker, 1987;Neumann et al., 1988;Foland that explain these relationships: (l) Felsic magmas asso- et al., 1988a,1988b). ciated with such complexesare residual liquids resulting This paper presentsmineralogical data that constrain from extensivefractionation of mafic parents (Bonin and the above-mentionedpossibilities for the origin of silica- Giret, 1984;Foland et al., 1985, 1988a).Silica-undersat- saturated alkaline rocks of the Belknap Mountain com- urated magmas with compositions close to the olivine- plex of the White Mountain Magma Series. plagioclase-diopside thermal divide can fractionate to sil- ica saturation, provided pressure and volatile content Gnor,ocrcll sETTTNG:Tnn Wnrrn MouNrnN increasethe stability of silica-poor minerals (Giret et al., Macru Spnrns AND THE Morqrnn-r,crll.l pnovrNcn 1980; Bonin and Giret, 1984, 1985a).(2) Felsic,anoro- Hu.l genic magmas are derived from anatexis of granulitic or The Monteregian Hill and the White Mountain prov- other crustal rocks (Loiselle and Wones, 19791,Collins et inces of Quebec and northern New England contain nu- al.,1982:'Creasy and Eby, 1983;Eby and Creasy,1983; merous plutons that were emplacedbetween 240 and 90 0003-o04x/90/09l 0-l 092$02.00 1092 DORAIS: PYROXENES AND AMPHIBOLES OF THE BELKNAP MOUNTAINS 1093 Ma (Foland eIal., l97L;' Foland and Faul, 1977;Eby and, 730 720 70o Creasy,1983; Eby, 1985, 1987).The MonteregianHill intrusions define a west, northwesterly lineament in which the majority of plutons are composedof silica-undersatu- rated rocks (Fig. l).Quartz-bearing rocks are abundant only in the easternregions of the province. The Triassic- Cretaceousintrusions of the White Mountain province are called the White Mountain Magma Series(WMMS), and trend in a north-northwesterly direction in northern New England (Fig. l; Billings, 1956). Intrusions of the WMMS were emplaced into the deformed Paleozoic rocks of the Appalachian orogen, whereasthose of the Monte- regian Hills were emplaced into the Grenville province and the flat-lying Carnbro-Ordovician sediments of the St. Lawrence lowland. Intrusions of the WMMS were therefore emplaced into thicker Paleozoic sections than those in the Monteregian Hill province (Billings, 1956; Dooland et al., 1982;Eby, 1987).They consistprimarily of silica-saturatedrocks. Several WMMS complexes consist of bimodal suites dominated by gabbro-diorite and syenite-granite:Ascut- ney, Pliny, Belknap, Ossipee,Merrymeeting Lake, Paw- (Billings, tuckaway 19561,Czamanske et al., 1977; Loi- Fig. l. Generalizedmap showingthe MonteregianHills and selle,1978; Carr, 1980;Randall, 1985; Folandetal., 1985; White Mountain provinces and the location of the Belknap Eby, 1987). Geochemical studies of mafic rocks at these Mountaincomplex (after B6dard, 1988). The stippledline rep- complexes indicate that WMMS parental magmas were resentsthe boundary line between the two provincesas proposed slightly SiOr-undersaturatedto SiOr-saturated mantle- by B6dard(1988). derived magmas (Loiselle, 1978; Can, 1980; Foland et al., 1985, 1988a),yet the relationshipbetween mafic and felsicrocks of the WMMS remainstenuous. Because many of the WMMS rocks are at least of partly cumulate origin Tnn Bnr,rxAP MoUNTATN coMPLEx (Size,1972;Loiselle, 1978; Foland et al.; 1985),it is dif- The Belknap Mountain complex is dated at 168 + 2 ficult to determine and isolate the effectsof assimilation, Ma by the RblSr isochron method (Loiselle, 1978). The mixing, and fractionation on liquid evolution and to elu- complex is exposedover 70 km2 along the south shore of cidate the relationship between mafic and felsic rocks. Lake Winnipesaukeein central New Hampshire. Modell Basedupon Sr isotopic evidence,Loiselle (1978) dem- (1936) originally mapped the complex and provided the onstrated that several units of the Belknap Mountain first detailed petrologic descriptions ofthe various units. complex of the WMMS had experiencedlittle or no upper Subsequentremapping by Bothner and Gaudette (1971) crustal contamination, although he could not rule out and Bothner and Loiselle (1987) forms the basisofFigure contamination from lower crustal rocks with low Sr iso- 2. The units defined by theseworkers, listed from young- topic ratios. Theseunits rangefrom gabbro to granite and est to oldest, are Trap Syenite Breccia, Rowes Vent Ag- all have initial 875r/865rratios of 0.70395. The Belknap glomerate, Conway Granite, Albany Quartz Syenite, Lake Mountain complex was chosenfor this study becauseup- Quartz Syenite, Sawyer Quartz Syenite, Belknap Syenite per crustal contamination could be eliminated as a hy- and Cobble Hill Syenite,Gilmanton Monzodiorite, Ames pothesis.Because coarse-grained WMMS rocks common- Monzonite, Endicott Diorite, (Gilford Gabbro), and (Moat ly do not represent liquid compositions (Size, 1972; Volcanics). Loiselle, 1978; Foland et al., 1985),mafic silicateswere The relative ages of the Gilford Gabbro and the Moat studied with the aim of deciphering the nature of liquid Volcanics are uncertain. Based on whole-rock, major- and evolution. The compositions of amphiboles and pyrox- trace-element characteristics, Gaudette and Bothner enesin alkaline rocks vary widely as a function of magma (1969) and Bothner and Gaudette (1971) suggestedthat composition, temperature, pressure,fv2o, und fo, (Cza- the Moat Volcanics are the extrusive equivalent of the manskeand Wones, 1973;Thompson, 1974;Neumann, Belknap Syenite. Mafic and felsic dikes are present 1976;Larsen,1976; Wass, 1979;Parsons, 1979, l98l; throughout the complex but generally are of insufficient Giret et al., 1980; Stephensonand Upton, 1982; Bonin extent to determine their age relationships with units oth- and Giret, 1984, 1985b),and their compositionscan be er than the unit into which they intrude. Many of these used to evaluatepotential relationshipsamong rock units dikes are synplutonic, indicating coexistenceof multiple (Czamanskeetal., 1977; Brooks and Gill, 1982; Lalonde magma types. and Martin, 1983;B6dard, 1988;B6dard et al., 1988). Loiselle (1978) demonstratedthat the Gilford Gabbro, lo94 DORAIS: PYROXENES AND AMPHIBOLES OF THE BELKNAP MOUNTAINS 7f20' The gabbro is medium- to coarse-grainedand consists of labradorite, two amphiboles, clinopyroxene, magne- tite, and ilmentite. Brown, kaersutitic amphibole forms poikilitic knots up to 3 cm in diameter and enclosespy- roxene, plagioclase,and oxides. Green amphibole com- monly fringes brown amphibole and pyroxene. Endicott Diorite The Endicott Diorite is intensely brecciated by the Conway Granite.