American Mineralogist, Volume 66, pages 70-86, l98l Petrologyof contact metamorphosedargillite from the Rove Formation, Gunflint Trail. Minnesota THeonone C. Lesorre. J. J. PAprKE.D. T. VeNnaaN' Department of Earth and Space Sciences State University of New York at Stony Brook Stony Brook, New York I 1794 exo G. B. Monnv M innesot a Geolo gic al Survey St. Paul, Minnesota 55108 Abstract The contactmetamorphic efects of the Duluth Complex on the argillaceousRove Forma- tion, northeasternMinnesota, are confined within a few hundred metersof the contact. The argillite has recrystallizedfrom the low-grade assemblagechlorite + muscovite+ quartz to assemblagescharacterized by cordierite + biotite + microcline + muscovite+ quartz. Cor- dierite coexistswith biotite over a wide range in Mgl(Mg + Fe) from 0.90 to lessthan 0.50. Andalusiteis restrictedto iron-rich rocks, although it occursin one unusual magnesium-rich assemblage.Locally, corundum + microcliae and cordierite * hyperstheneassemblages oc- cur at the contact with the Duluth Complex. The pressure,P"oria, of metamorphismis esti- mated to have been 1500bars. The temperaturenear the contact ranged from -500o to 600oC,based on the partitioning of Na and K betweencoexisting muscovite and microcline. The equilibrium constant for the continuous reaction muscovite + phlogopite * quartz : cordierite * microcline + HrO is calibrated and used to determine "/n,o attending meta- morphism.Results indicate that P",o rangesfrom P"",,oto jP"",,o.Calculation of the composi- tion of COH fluid for a graphite-bearingsample iadicates that the remainder of the fluid is mostly COr. The Rove Formation doesnot show the extensivemineralogic eflectsof contact metamorphismthat the iron-formation shows,and the differencesreflect the large thermal and compositionalstability field for coexistingcordierite + biotite in pelitic rocks and the rel- atively small stability fields of the iron-rich minerals in the iron-formation. Introduction to changesin metamorphic grade, and rocks as far The -l Gy Duluth Complex in northeasternMin- away as 10 km (along strike) show effectsof contact nesotaintruded an -2 Gy sequenceof sedimentary metamorphism.The zonesof metamorphismrecog- rocks that includes banded iron-formation (Gun- nized by French (1968) and Floran and Papike flint-Biwabik) and argillite and greywacke (Rove- (1978) are low-grade zonescharacterized by green- Virginia Formations).The contact metamorphic ef- alite or minnesotaite,intermediate-grade zones de- fects of the Duluth Complex on the iron-formation fined by grunerite-and fayalite-bearingassemblages, have been extensivelystudied by French (1968), and a high-grade zone characteitzed by ferrohyper- Bonnichsen(1968, 1969),and Morey et al. (1972) stheneor inverted pigeonite. (Biwabik Iron Formation), and by Simmons et al. The Rove Formation consistspredominantly of (1974)and Floran and Papike(1975, 1978) (Gunflint carbonaceousargillite and quartz-rich greywacke, Iron Formation). Theseiron-rich rocks are sensitive and the petrologyof theserocks has been determined in order to comparethe responseof the more K- and Al-rich rocks to that of the iron-formation under rPresent address: Los Alamos Scientific Laboratorv. Los low-pressuremetamorphic conditions. In this r€port, Alamos,New Mexico87545. the mineral assemblagesand mineral compositionsin 0003-o0/.x / 8 | / 0 I 02-0070$02.00 70 LABOTKA ET AL.: ROVE FORMATION, MINNESOTA 7l the Rove Formation are presentedso that the distri- The Rove Formation consistsof a lower argillite bution of elementsamong coexisting minerals in low- unit, a middle transition unit, and an upper thin-bed- pressure,pelitic schistscan be characterized.We use ded greywackeunit. The lower argillite unit is ap- the mineral reactionsthat are inferred to have oc- proximately 150m thick and consistsof thin-bedded, curred to estirnatethe conditions of metamorphism. fissile, fine-grained greywacke, silty argillite, and graphitic argillite. Irregular limestonelenses and cal- Geologicsetting careousconcretions also occur in the lower part of The Rove Formation is part of a sequenceof Pro- the unit. Bedsof greywackeare abundantin the tran- terozoic (-2Gl sedimentaryrocks assignedto the sition unit, and greywackedominates the upper unit. Animikie Group. The group unconformably overlies The complete thickness of the two upper units is 2.7 Gy Archean rocks and consistsof two units, the about 900 m, but the upper parts are generally trun- Gunflint lron-Formation and the overlying Rove cated by the Duluth Complex in the region around Formation. The stratigraphy and sedimentary pe- Gunflint Lake. trography of the Rove Formation have been de- The Duluth Complex,a compositemafic intrusion, scribedby Morey (1969),and the generalgeology of truncatesthe rocksof the Animikie Group with slight northeasternMinnesota is shown in Figure l. angulardiscordance. Consequently, the complexis in Fig. l. Generalgeology of the Gunflint Lake area,northeast Minnesota. Sample localities are indicatedby the lettersKT (Kakakebic Trail), CR (CrossRiver), GT (Gunflint Trail), LL (Loon Lake), GP (Gunflint Palisades),ML (Mayhew Lake), SRL (South Round Lake), RL (RoseLake), and DL (Daniel's Lake). LIIBOTKA ET AL.: ROVE FORMATION. MINNESOTA contact with progressivelyhigher units of the Ani- be usedas phasediagrams if the chemical potentials mikie Group from west to east. Gabbroic sills and of HrO and each additional componentare constant dikes assignedto the Logan Intrusions (Weiblen et for all assemblagesin each projection (Thompson, al., 1972)pervasively invade the Animikie Group in 1957). the vicinity of Gunflint Lake. The Logan Intrusions grade appearto be both older and younger than rocks of Low the Duluth Complex and are part of the sameintru- The rocksfrom the Rove Formation are character- sive complex. ized by detrital texturesexcept near the Duluth Com- plex and adjacentto somesills. The clastic grains in Methods siltstonesand greywackeshave angular to sub- The Rove Formation is not well exposed;outcrops rounded shapes,and the matrix consists of fine- generallyonly occur beneaththe more resistantLo- (<0.25 nm) to very fine-grained (<0.0J mm) mi- gan sills or beneaththe Duluth Complex adjacentto caceousminerals. The micaceousminerals chlorite. the contact. Sampleswere collected at sill contacts muscovite,and biotite have recrystallizedfrom the and in sequencesaway from the sills to determine the presumably original clay matrix. The textures de- regionalthermal environmentand perturbationsim- pend stronglyon the rock type. The graphite-rich ar- posedby the sills. gillite near the baseof the formation contains angu- The rocks were examinedin thin section and the lar quartz and plagioclasegrains, very fine-grained identification of some minerals was aided by X-ray biotite and chlorite, and comparatively large (>0.5 diffractometry.Selected samples were analyzedwith mm) muscovitegrains that have an idioblastic form an ARL-EMXautomated microprobe using simple sili- and appearto be recrystallizeddetrital muscovite.In cate standards.Data were reduced on-line by the many samples,chlorite and muscoviteare finely in- method of Bence and Albee (1968) and the conec- tergrownin the matrix. Albite grains are irregular in tion factors of Albee and Ray (1970).Precision was shapeand contain fine-grainedinclusions of epidote monitoredby analyzingminerals of known composi- ("saussurite").In some rocks, epidote occurs as dis- tion and is -2Vo for major elements and -ll%o for tinct grains. minor elements.Unknown mineralswere analyzedin The observedmineral assemblagesinclude quartz two or three areason a standard thin section, and + muscovite+ chlorite * biotite * plagioclaseand mineral compositionshave rangeswithin the stated quartz * muscovite+ biotite * microcline + plagio- precisionover the area of a thin section. clase.Rocks that were collectedfrom RoseLake and The structural state of potassium-feldsparhas not farther east,near Lake Superior,do not contain bio- been determined,and the use of the term "micro- tite. The diagnosticassemblage quartz * muscovite cline" doesnot imply that K-feldspar has the triclinic + chlorite + biotite + microcline + plagioclaseoc- structure. curs in sample4ls from South Round Lake. Graph- ite is very abundant in rocks from the lower part of Mineral assemblagesin pelitic rocks the formation. near the western end of Gunflint Samplelocations are shown on Figure l, and the Lake. All assemblagescontain ilmenite or sphene mineral assemblagesin analyzed samplesare given and commonly pyrrhotite which is locally altered to in Table l. Rocks that exhibit only incipient recrys- marcasite. The chlorite-free assemblagequartz * tallization and retain detrital textures are termed muscovite+ biotite * cordierite + plagioclaseoccurs "low grade." Rocks that have recrystallizedto fine- in the Rove adjacent to the sill contact at South grained hornfels are termed "medium grade," and Round Lake. thosethat showextensive interaction with the Duluth Figure 2 illustrates the compositionsof minerals Complex are called "high grade." that occur in muscovite + quartz-bearing assem- The assemblagesin quartz-bearing rocks can be blagesfrom low-graderocks of the Rove Formation. representedin the system KrO-FeO-MgO-AlrOr- Chlorite + biotite coexistover a rangein Mgl(Mg + HrO. HrO is assumedto be a boundary-valuecom- Fe) from 0.65 to 0.50, biotite is slightly more iron- ponent, and becauseeither muscovite + quartz or rich than coexistingchlorite, and microcline coexists microcline + quartz