DOUGLAS RUMBLE III Department of Geology, University of California, Los Angeles, California 90024 Andalusite, Kyanite, and Sillimanite from the Mount Moosilauke Region, New Hampshire ABSTRACT of andalusite is wedge-shaped and terminates with increasing pressure. A number of prob- Andalusite, kyanite, and sillimanite occur lems important to the interpretation of the in regionally metamorphosed rocks of the Al2SiOo distribution, such as metamorphic Mount Moosilauke region, New Hampshire. chronology and postmetamorphic tectonics, Andalusite and sillimanite are found together were raised in their paper; however, in a frequently in the same thin sections. Kyanite report of such scope these questions could not appears once in the same thin section with be answered in detail. Furthermore, there sillimanite but never with andalusite. were then no systematic data available on the Andalusite and sillimanite mica schists chemical compositions of the aluminum were subjected to multiple episodes of recrys- silicates; therefore, the trace-element contents tallization that resulted in the reaction se- of the polymorphs could not be used to test quence andalusite —» sillimanite —» andalusite. the hypothesis of attainment of chemical Kyanite crystallized in an adjacent area during equilibrium. the same time period. Kyanite from quartzite The purpose of this paper is to present the containing hematite is enriched in Fe203 but the results of a detailed study of the distribu- andalusite. and sillimanite from rutile and tion, metamorphic chronology, and chemical ilmenite—or graphite-bearing mica schists— compositions of the aluminum-silicate minerals are relatively impoverished in Fe2Os. The occurring in the Mount Moosilauke region, Al2Si05 minerals crystallized during post- New Hampshire. The Mount Moosilauke Lower Devonian, pre-Triassic regional meta- region is one of the New England localities morphism under conditions approaching chem- where the trace of the "fossil isobaric surface" ical equilibrium. (Thompson and Norton, 1968, p. 324) cross- cuts middle-grade metamorphic rocks. In such INTRODUCTION an area, occurrences of all three polymorphs Thompson and Norton (1968) published a are to be expected (see Billings, 1937, p. 482, synthesis of Paleozoic regional metamorphism 493); therefore, the most favorable conditions of New England in which they distinguish be- are afforded for testing the hypothesis of tween the andalusite-sillimanite terrane of Thompson and Norton. northeastern New England and the kyanite- sillimanite terrane of southwestern New England. A line was drawn across the map of METHOD OF STUDY New England "such that all occurrences of andalusite believed to have formed during The initial phase of the work was facilitated regional metamorphism lie to the east or by the comprehensive petrographic collections northeast of it" (Thompson and Norton, 1968, of the Department of Geological Sciences, p. 324). Assuming the mapped Al2Si03 Harvard University. One thousand thin sec- distribution to be due to local equilibration in tions of rock samples from the Moosilauke, a rock volume subjected to continuous pres- Littleton, Franconia, and Woodsville quad- sure and temperature gradients, they inferred rangles (all collected by M. P. Billings), the the line so drawn to be the approximate trace Mount Cube quadrangle (collected by J. B. of the isobaric surface representing the pres- Hadley), and the Plymouth quadrangle sure of the invariant point in the system (collected by C. B. Moke) were studied with Al2SiOs. This is so because the stability field the petrographic microscope. Two-thousand Geological Society of America Bulletin, v. 84, p. 2423-2430, 1 fig., July 1973 2423 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/7/2423/3433439/i0016-7606-84-7-2423.pdf by guest on 25 September 2021 2424 D. RUMBLE III rock samples and 800 thin sections were col- folded and refolded into north-northeast- lected and studied by me. trending anticlinoria and synclinoria (Rumble, Two important sampling biases affect the 1971b; Thompson and others, 1968). mapped distribution of Al2Si05 polymorphs The kyanite-bearing rocks located on Black (Fig. 1). In order to avoid the problem of Mountain and to the southwest were meta- interpreting relic mineral assemblages of pre- morphosed during the penetrative deforma- Silurian age, sampling efforts were concentrated tion that accompanied recumbent folding and on rocks of Silurian and Devonian age. All the developement of the anticlinoria (Rumble, major outcrop areas of Silurian or Devonian 1971b). The growth of porphyroblastic min- mica schist and quartzite, whose bulk com- erals in the andalusite and sillimanite mica positions favored the crystallization of alumi- schists found east and northeast of Black num silicates, were sampled thoroughly. Out- Mountain continued after penetrative de- crop areas of amphibolite, quartz-feldspar formation had ceased. gneiss, and calc-silicate rock received cursory Outcrop belts of Bethlehem Gneiss occur- attention only. ring on either side of the Bronson Hill anti- Minor-element contents of aluminum-silicate clinorium are erosional remnants of a large minerals were measured using an ARL-EMX nappelike body whose root zone lies east of microprobe located in the Department cf the anticlinorium and whose leading edge lies Geology, UCLA. Wavelength scans carried ou: to the west (Billings, 19156, Fig. 13, p. 126; at 15 kv and 0.04 pa. sample current revealed Thompson and others, IS68, PI. 15-lb). The Fe as the only detectable impurity (the Kinsman Quartz Monzorite may also occupy detectability limit for transition metals is a similar structural position (Thompson and estimated to be 0.1 weight percent). Quantita- others, 1968, PI. 15-lb). The origin of these tive analyses of Fe content were made at 15 rocks has been interpreted in a variety of kv and 0.03 jua specimen current with Elba ways (Thompson and others, 1968, p. 208; hematite (provided by A. L. Albee) as standard. Page, 1968, p. 377-378; Billings, 1937, p. 537). Individual grains were analyzed using 100-sec Most workers would agree that their emplace- counting times such that the number of mea- ment did not significantly change the thermal surements on the Fe Ka x-ray peak equalled regime imposed during regional metamorphism. the number of measurements of background The French Pond Granite is similar in x-radiation. Counting statistics were calculated mineralogical and chemical composition to using the data reduction scheme of Boyd the Bethlehem and Kinsman rocks but it was (Boyd and others, 1969) and Hadidiacos intruded after regional metamorphism. It (Hadidiacos and others, 1971); matrix cor- superimposed a contact aureole containing rections were calculated by the Bence-Albee fibrolitic sillimanite ori its staurolite- and method (Bence and Albee, 1968). Calculated garnet-bearing country rocks (White and detectability limit of Fe in the quantitative Billings, 1951, p. 692). The White Mountain analyses is 0.01 weight percent. plutonic-volcanic series, is Jurassic in age (Lyons and Faul, 1968, p. 312) and was em- GEOLOGICAL HISTORY placed subsequent to folding and regional The pre-Silurian metamorphosed sedimen- metamorphism of the .Silurian and Devonian tary and igneous rocks exposed in the Bronson rocks. Contact metamorphic effects, including Hill anticlinorium (Fig. 1) are probably the formation of anddusite, have been ob- Ordovician in age (Thompson and others, served adjacent to the stock exposed in the 1968, p. 206); they may have experienced northeast corner of the: Plymouth quadrangle metamorphism and deformation before deposi- (Moke, 1946, quoted by Billings, 1956, p. tion of Silurian sediments but evidence for ':he 142). In the discussion that follows, only those pre-Silurian orogeny has been almost com- aluminum silicates formed during regional pletely erased by post-Lower Devonian, pre- metamorphism will be considered. Triassic metamorphism and deformation (Nay- The Ammonoosuc fault developed after lor, 1969; Rumble, 1969). Metamorphosed regional metamorphism of the Silurian- and sedimentary and igneous rocks of Silu ian Devonian-age rocks. Neither geology nor and Devonian age overlie unconformably the mineral occurrences are shown on Figure 1 for pre-Silurian rocks (Billings, 1937). The entire the area lying west cf the fault (see Albee, stratigraphic succession has been recumbently 1968, Fig. 25-1). The fault is the locus of Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/7/2423/3433439/i0016-7606-84-7-2423.pdf by guest on 25 September 2021 EXPLANATION j White Mountoin plutonic-volcanic series, I Jurassic, post-regionol metomorphism French Pond Granite, Devonian, post- regional metamorphism. 1 Bethlehem Gneiss and Kinsman Quartz Monzon- ] ite, Devonian, syn-regional metamorphism • Silurion and Devonion metamorphic rocks I Ordovician metamorphic rocks Locotion of Figure I ond >5' quadrangles Stourotite isograd defined by first oppeorance of Is WoodsviHe, 2 = Moos'louke, 3= Fronconia, X staurohte, tick morks on high grade side Rocks on 4s Mt. Cube, 5 s Rumney,6s Plymouth x low grade side of isogrod are m garnet zone Figure 1. Aluminum-silicate distribution of the Mount Moosilauke region, New Hampshire. Geology A Andolusite occurrence and mineral distribution after Billings (1937, 1956), f Fibrohtic silhmonite occurrence Hadley (1942), Moke (1946), Page (in Fowler-Billings and Page, 1942), Rumble (1969, 1971b), Williams and K Kyamte occurrence Billings