THE GEOLOGY AND PETROLOGY OF ANORTHOSITIC ROCKS IN THE DULUTH COMPLEX, SNOWBANK LAKE QUADRANGLE, NORTHEASTERN MINNESOTA. (VOLUMES I AND II) (PLAGIOCLASE, PETROGENESIS, PETROCHEMISTRY, TROCTOLITE)

Field, petrographic and petrochemical studies of the Middle Proterozoic (Keweenawan) Duluth Complex in the Snowbank Lake 7.5" quadrangle have identified six major anorthositic rock types (Pl > 82%) and their petrogenetic relationship to one another and to younger troctolitic rocks. Rock types are distinguished by their modal compositions (mostly Pl + Ol + Cpx + Feox), olivine habit, and plagioclase grain size. Stratiform and intrusive contact relationships indicate that anorthositic rocks formed in at least three major intrusive episodes.;Intrusive contact relationships, plagioclase zoning varieties, and noncotectic cumulus plagioclase and olivine modes imply that anorthositic rocks crystallized from tholeiitic basaltic magmas laden with up to 50% intratelluric labradoritic plagioclase in subvolcanic chambers (5-10 km). Troctolitic magmas intruded with few or no intratelluric crystals.;The apparent lack of cryptic variations and the overall fractionated composition of anorthositic rocks implies that most of the compositional diversity of anorthositic parent magmas was inhereted from fractional crystallization in deeper chambers. Although parent magmas were saturated with olivine at low pressures, evolved magmas were probably undersatured at depth, devoid of nucleii upon intrusion, and crystallized poikilitic low-Fo (55-38) olivine. Less fractionated magmas were saturated with olivine at depth and crystallized granular high-Fo (66-49) olivine. Augite and iron oxides were consistently undersaturated at low pressures and crystallized interstitially to plagioclase and olivine. The nearly constant average composition of plagioclase (An 55-65) relative to olivine implies that plagioclase fractionated less efficiently than mafic minerals due perhaps to its suspension in deep crustal chambers (40-15 Km). Such a mechanism may also have acted to segregate plagioclase into expelled magmas.;Petrochemical modelling of the Duluth Complex and Keweenawan volcanic rocks indicate that all Keweenawan magmas could have been derived by fractional crystallization of combinations of Pl + Ol + Cpx + Sp from a high-Al olivine tholeiitic basalt at lower crustal pressures (10-7 kb). The composition of such a magma was similar to primitive MORB in major and compatible trace elements but was variably enriched in incompatible minor and trace elements. The modelling also supports the suggestion that mafic minerals fractionated more efficiently than plagioclase.