Anorthositic Rocks of the Duluth Complex: Examples of Rocks Formed from Plagioclase Crystal Mush

Anorthositic rocks compose 35–40% of the Middle Proterozoic(Keweenawan; 1?1 Ga) Duluth Complex—a large, compositemafic body in northeastern Minnesota that was intruded beneatha comagmatic volcanic edifice during the formation of the Midcontinentrift system. Anorthositic rocks, of which six general lithologictypes occur in one area of the complex, are common in an earlyseries of intrusions. They are characterized on a local scale(meters to kilometers) by nonstratiform distribution of rocktypes, variably oriented plagioclase lamination, and compositeintrusive relationships. Variably zoned, subhedral plagioclaseof nearly constant average An (60) makes up 82–98% ofthe anorthositic rocks. Other phases include granular to poikiliticolivine (Fo_66–38), poikilitic clinopyrox-ene (En'_73–37),subpoikilitic Fe-Ti oxides, and various late-stage and secondaryminerals. Whole-rock compositions of anorthositic rocks are modelled bymass balance to consist of three components: cumulus plagioclase(70–95 wt.%), minor cumulus olivine (0–5%), anda gabbroic postcumulus assemblage (5–27%) representinga trapped liquid. The postcumulus assemblage has textural andcompositional characteristics which are consistent with crystallizationfrom basaltic magma ranging from moderately evolved olivinetholeiite to highly evolved tholeiite (mg=60–25). Sympatheticvariations of mg in plagioclase and in mafic minerals suggestthat cumulus plagioclase, though constant in An, was in approximateequilibrium with the variety of basaltic magma compositionswhich produced the postcumulus assemblages. Standard models of mafic cumulate formation by fractional crystallizationof basaltic magmas in Duluth Complex chambers, although ableto explain the petrogenesis of younger stratiform troctoliticto gabbroic intrusions, are inadequate to account for the field,petrographic, and geochemical characteristics of the anorthositicrocks. Rather, we suggest an origin by multiple intrusions ofplagioclase crystal mushes—basaltic magmas charged withas much as 60% intratelluric plagioclase. The high concentrationsof cumulus plagioclase (70–95%) estimated to compose theanorthositic rocks may reflect expulsion of some of the transportingmagma during emplacement or early postcumulus crystallizationof only plagioclase from evolved hyperfeldspathic magma. Althoughthe evolved compositions of anorthositic rocks require significantfractionation of mafic minerals, geophysical evidence indicatesthat ultramafic rocks are, as exposure implies, rare in theDuluth Complex and implies that plagioclase crystal mushes werederived from deeper staging chambers. This is consistent withinterpretations of olivine habit and plagioclase zoning. Moreover,plagioclase could have been segregated from coprecipitatingmafic phases in such lower crustal chambers because of the buoyancyof plagioclase in basaltic magmas at high pressure. The geochemicaleffects of plagioclase suspension in basaltic magmas are consistentwith observed compositions of cumulus plagioclase in the anorthositicrocks and with the geochemical characteristics of many comagmaticbasalts. The petrogenesis of the anorthositic rocks and theoverall evolution of Keweenawan magmas can be related to thedynamics of intracontinental rift formation.