Occurrence, Mineral Chemistry, and Metamorphism of Precambrian Carbonate Rocks in a Portion of the Grenville Province

Marble occurs abundantly in a 31,000 km² segment of the southernGrenville Province of the Canadian Precambrian Shield, whereit is associated with quartzite, biotite-garnet gneiss, andamphibolite to form the Grenville Group. An 1800 km² area onthe western margin of this segment, north of the Ottawa river,displays a great variety of carbonate rocks, which may be dividedinto two groups: (I) major marble, with calcite, dolomite, graphite, phlogopite,Ca amphibole, Ca pyroxene, forsterite, humite group minerals, (II) minor marble, with pink calcite, phlogopite, Ca amphibole,Ca pyroxene, K feldspar, scapolite, sphene. Rocks of the first group are associated with plagioclase gneissand amphibolite, and are metamorphosed limestone, little affectedby metasomatism; rocks of the second group, which are less common,are associated with potassium feldspar gneiss and heterogeneousgranitic and syenitic rocks, and are inferred to be metasomaticrocks. Numerous mineral reactions have taken place in the carbonaterocks during metamorphism. The calcite-dolomite reaction, whichgoverns the Mg content of calcite, indicates a metamorphic temperatureof about 650 °C. Forsterite was possibly produced from low-Alamphibole, and forsterite + spinel from high-Al amphibole. Thecrystallization of some silicate minerals in the minor marbleunits, and the enrichment in the contained calcite in Fe andSr are attributed to metasomatic reactions. Metamorphic ion-exchangereactions involving carbonates produced the following distributioncoefficients: Sr in calcite/Sr in dolomite = 2.5 Mn in calcite/Mn in dolomite = 0.89 Fe in calcite/Fe in dolomite = 0.29 from which inferences may be drawn concerning the distributionof these elements between the Ca and Mg sites within dolomiteduring metamorphic crystallization. Ion-exchange reactions involvingsilicates produced the following distribution of Mn: humite group Ca pyroxene.Ca amphibole phlogopite where the numbers are distribution coefficients. An equilibriumdistribution of Fe between silicates and calcite in the minormarble was evidently not attained during metasomatic crystallization.Numerous retrograde reactions have taken place, including thealteration of pyroxene to amphibole, forsterite to serpentine,and the exsolution of dolomite from calcite. Forsterite in marble, and orthopyroxene in the associated gneissesand amphibolites crystallized sporadically in the Laurentianhighlands, but not in the lowlands of the Ottawa rift valley,where peak metamorphic temperatures may have been slightly lower.In the highlands, reactions to produce forsterite and orthopyroxenewere initiated in response to a local increase in temperature,local peculiarities in the chemical composition of amphibole,which produced these minerals, or a local decrease in the activityof CO₂ and H₂O in the grain-boundary phase.