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Transition from Staurolite to Sillimanite Zone, Rangeley Quadrangle, Maine

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Transition from Staurolite to Sillimanite Zone, Rangeley Quadrangle, Maine CHARLES V. GUIDOTTI Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 Transition from Staurolite to Sillimanite Zone, Rangeley Quadrangle, Maine ABSTRACT GENERAL GEOLOGICAL SETTING Ordovician and Silurian to Devonian pelitic schist, conglomerate, quartzite, calc-silicate Study of pelitic schists in the Rangeley Figure 1 shows the location of the area granulite, and biotite schist. Post-tectonic, area, Maine, by means of petrographic, and a generalized geologic map of the shallow-dipping, adamellite sheets intrude x-ray, and electron-microprobe techniques southwestern third of the Rangeley quad- the metamorphosed strata. As illustrated in enables definition of the isogradic reaction rangle based upon Moench (1966, 1969, Figure 1, the isograds have a clear spacial relating the staurolite and lower sillimanite 1970a, 1970b, 1971). The rocks in this area relation to the distribution of the adamel- zones. The reaction is a discontinuous one consist of tightly folded, northeast-trending lites; but in a few cases, the adamellite and can be shown on an AFM projection as the tie line change from staurolite + chlorite to sillimanite 4- biotite. This topology change, in conjunction with the min- eralogical data provides the equation: Staur + Mg-Chte + Na-Musc + (Gam?) Sill + Bio + K-richer Muse + Ab + Qtz + H20. This reaction should result in a sharp isograd in the field but in fact is found to be spread out over a zone which is called the transition zone. It is proposed that this zene results from buffering of fH20 by means of the equation above. Buffering of fH.,o by continuous reactions also appears to be taking place in the lower sillimanite zone. Metamorphic conditions for the isogradic reaction are believed to be near 4 kb and P fluid was probably equal to Pt„tai but somewhat greater than PH2o- INTRODUCTION This study considers the prograde transi- tion from the upper part of the staurolite (u. st.) zone to the lower sillimanite (1. s.) zone — that is, where sillimanite + staurol te coexist. This transition can be approxi- mated by the tie-line change, staurolite + chlorite to sillimanite + biotite. Theoreti- cally, this change should eliminate chlorite from staurolite-bearing rocks. However, minor chlorite persists in the assemblage sillimanite + staurolite + biotite + garret for a short distance upgrade of the first appearance of sillimanite. This overlap between u. st. and 1. s. zone assemblages is called the "transition zone" (tr. zone). This study is part of a larger study of the ADAMELLITE ISOGRAD,HACHURES ON metamorphism of northwestern Maine on HIGH-GRADE SIDE rocks ranging from biotite zone to D- K-feldspar + sillimanite zone. Reports on UMBAGOG GRANODIORITE GEOLOGIC CONTACT the higher grades include Guidotti (1963, 1970a) and Evans and Guidotti (1966). SILURIAN & DEVONIAN S-D GEOLOGIC CONTACT OF Green (1963) studied rocks of similar grade STRATIFIED ROCKS OaS STRATIFIED ROCKS to the west of this area. Other petrologic 0 ORDOVICIAN studies in nearby areas include the unpub- STRATIFIED ROCKS lished work of Milton (1961), Pankiwskyj Figure 1. Geologic map of portions of the Oquossoc (Guidotti, unpub.), Old Speck Mountain (modified from (1964), Drake (1968), and Warner (1967). Milton, 1961), Rangeley (Moench, 1971), and Rumford (Moench, unpub.) quadrangles. Geological Society of America Bulletin, v. 85, p. 475—190,13 figs., March 1974 475 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/3/475/3433654/i0016-7606-85-3-475.pdf by guest on 30 September 2021 476 C. V. GUIDOTTI bodies truncate isograds. The map pattern zone. Several prominent textural and fabric 4. Ragged megacrysts of andalusite are of the isograds and their relation to features are related to the polymetamorphic :airly common (see Fig. 2A) in all three topography indicates shallow-dipping iso- history of the are;. (Guidotti, 1970b). These :;ones; in virtually all cases, they are largely gradic surfaces — an observation which include: eplaced by coarse muscovite. seems to be related to the attitudes of the 1. A well-developed slip cleavage which 5. Staurolite in the u. st. zone commonly adamellite sheets. predates most o: the metamorphism. The xcurs as swarms of oriented 4-mm euhedra Stratified rocks in this area have been micas forming this cleavage are now issociated with aggregates of quartz, metamorphosed at least twice in Silurian or polygonized (Zwart, 1962). nuscovite, and chlorite, the shape of which Devonian time. The first metamorphism was 2. In many 1. s. zone specimens, sub- ndicates the extent of earlier, larger (to high enough grade to produce the assem- hedral to euhsdral garnet is made 2-cm) staurolite crystals. Quartz, muscovite, blage andalusite + staurolite + biotite pseudomorphous by coarse biotite plates ind chlorite in the aggregates are well throughout the area considered in this study. (commonly with intergrown sillimanite) in a :rystallized and somewhat coarser than The second metamorphism which is spa- manner which retains the original these same minerals elsewhere in the cially related to the adamellite upgraded dodecahedral shapes. specimens. some of these rocks to 1. s. zone but 3. Coarse muscovite plates commonly 6. Andalusite and staurolite (paragraphs downgraded others to the u. st. zone. Details rim staurolite in the tr. and 1. s. zones. 3, 4, and 5 above) commonly overprint the of this polymetamorphism are treated in Guidotti (1970b). Evidence presented there and in this paper demonstrates that the ASSEMBLAGE DESIGNA- second metamorphism attained approxi- rd TIONS BASED ON THIS rh. DIAGRAM. Mu 8 PI 8 mate equilibrium and was responsible for a ,v'm Ilm PRESENT IN ALL the mineral assemblages which are the Bl° SPECIMENS. concern of this study. Hence, the earlier Po metamorphism need not be considered in OPTICAL 8 X-RAY WORK detail here. ONLY PARTIAL ANALYSES OF SOME MINERALS PETROGRAPHY AND MINERAL MAJOR MINERALS ANALYZED ASSEMBLAGES OF EXAMPLE: Si + St+Bio»Ga+Po PELITIC ROCKS OR I CONTAINS RELIC Both the Ordovician and Silurian to ANDALUSITE Devonian units contain pelitic rocks of 62-NUMERAL GIVES SAMPLE similar chemical composition. All have NUMBER similar textures, fabrics, and mineral ISOGRAD, HACHURES ON HIGH- \GRADE SIDE assemblages' in the various metamorphic zones. Figure 2A gives specimen number, map location, nature of analytical work done on the sample, and assemblage information for the individual specimens in the various grades. The specific assemblage information is keyed to the Thompson projections to be used later (see Fig. 8 below). Detailed assemblage and analytical data are given in Tables 1, 2, and 3.2 The appearance of sillimanite and dis- appearance of chlorite delineate the upper limit of the u. st. zone and lower limit of the 1. s. zone, respectively. The tr. zone is marked by the presence of both phases. Figure 2B presents the evidence on the gradational nature of the tr. zone. In all three zones, rocks are well foliated with a silvery-white luster due to abundant muscovite. Grain size ranges from fine to medium, the latter predominating in the 1. s. 1 Mineral abbreviations employed in this paper include: Mu = muscovite, Chi = chlorite, Bio = biotite, St = staurolite, Si = sillimanite, Gn = garnet, II = ilmenite, Crd = cordierite, Qz -- quartz, PI = plagioclase, Opq = opaques, Po = pyrrhotite, Grp = graphite, Andai = andalusite. 2 Tables 1, 2, 3, 5, 6, 8B, 10B, 11D, HE, 11F, 11G, UH, 12B, 12C, 12D, 12E, and 12F may be obtained by writing to Microfiche Publications, Div. of Microfiche Systems Corp., 305 East 46th Street, New York, New York 10017. Request document #02271 and enclose $7.25 for photocopies or $1.50 for microfiche. Make Figure 2A. Location of specimens and isograds in the southwestern portion of the Rangeley quadrangle. A ' checks payable to Microfiche Publications. adamellite; all other rocks are metasediments. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/3/475/3433654/i0016-7606-85-3-475.pdf by guest on 30 September 2021 TRANSITION FROM STAUROLITE TO SILLIMANITE ZONE, MAINE 477 slip cleavage (1 above). Hence, the minerals for elements constituting more than about and they were iterated using the analyzed replacing the andalusite and staurolite are two percent of the sample, and the relative Fe/Mg ratio. not cut by the slip cleavage. In the tr. zone amounts of elements analyzed together have The University of Wisconsin analyses on and especially the 1. s. zone, the coarse a similar error even for lower concentrations mineral separates were determined under muscovite replacing staurolite and (or) (Albee, 1972). similar conditions relative to working curves andalusite commonly is intergrown with For the partial analyses, Fe-Ti-Mg were based on mineral standards, and no sillimanite. analyzed on biotite and muscovite, Mn- corrections were made. Two points on five 7. As developed in Guidotti (1970b), the Fe-Mg on garnet and chlorite, and Zn-Fe- or more grains were averaged for each various pseudomorphs described above are Mg on staurolite; fewer points and grains specimen. Analysis of 16 individual samples believed to have formed during the final from a single area were analyzed. The of biotite and chlorite analyzed at both the metamorphism which is the concern of this intensity data were reduced with a computer California Institute of Technology and the study. program utilizing general formulas which University of Wisconsin gave an average were derived from the complete analyses, deviation of Mg/Fe of 0.0165, about 1.5 ANALYTICAL TECHNIQUES Detailed petrographic study of thin sections was carried out on all specimens. Refractive indices and x-ray spacings were obtained by procedures described in Guidotti (1970a). In addition to these data, representative samples were chosen fo:: chemical analyses (Fig. 2A). At the California Institute of Technology, complete electron-microprobe analyse:; were made of the coexisting minerals in 12 prime3 specimens (5, u. st. zone; 4, tr. zone; 3, 1. s. zone) and supplemented by partial analyses of 9 other samples (5, u.
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