Christopher P. Gulino1, Gary S. Solar1, Structural, textural and petrographic variations in rocks [email protected] and Paul B. Tomascak2 on Bruce Hill, eastern Sebago Migmatite Domain, 1 Laboratory for Orogenic Studies, Department of Earth Sciences SUNY College at Buffalo, Buffalo, NY 14222 2 Buffalo State southern Maine. Department of Earth Sciences, SUNY Oswego, Oswego, NY 12326 State University of New York Department of Earth Sciences REGIONAL GEOLOGICAL MAP OF THE SE SEBAGO PLUTON, E SEBAGO MIGMATITE DOMAIN AND ADJACENT NORUMBEGA SHEAR ZONE SYSTEM, S ME Introduction 70°30’ W 70°15’ W 44°00’ N 70°00’ W Results: Rock Types If magma flow through deforming rocks is recorded by structures in migmatites and associated granites, 25 26 ROCK TYPES Ms-Bt granite, Bt granite, schleiric granite, pegmatite A n y Rocks at Bruce Hill are of five units that are all typical of the SMD in general, but a study of such rocks requires multiple scales of observation. As part of a larger multi-disciplinary study of d stromatic migmatite, diatexite B r with associated schlieric C o g n r s i granite, foliated granite, 21 t o c 20 e 19 e granite, pegmatite o S o particularly of the eastern SMD. Stromatic migmatite is the dominant rock unit on k rocks of the northern Appalachian migmatite-granite belt, this study is one of several ongoing concurrent g 68 m e g 14 y Hbl-bearing granite E r d i r n e 86 syenite (Rattlesnake Mtn. R B M R 67 i igneous complex) v i A v the NW and SE flanks of the hill, and is uniformly plastically deformed, including e 06 35 and related projects that together form a database of fine-scale documentation of the Sebago migmatite e gabbro, metagabbro r 69 74 84 r 55 Standish Form. (CMB) G 40 06 Windham Form. (CMB) O 38 62 00 52 boudinaged leucosomes and granite layers with S-C fabrics. The other units are local domain (SMD) and related rocks in S Maine and New Hamphire. For this work, we document variations Hutchins Corner Form. (CMB) 38 Avalon Composite Terrane M 02 86 44 08 I 46 73 of mineral content, structures and fabric at the m-mm scale in rocks of Bruce Hill, Cumberland Ctr, ME, G 56 semipelitic schist, and granites of three varieties: (1) S-C augen gneiss that composes MAP SYMBOLS 11 basalt dyike M 2 78 50 the SW flank of the hill, (2) pegmatite (locally with solid-state fabrics) that strike and dip of foliation in A 00 located in the E part of the SMD that mantles the 400 km Permian Sebago granite pluton on its W, N and 61 04 metasedimentary rock, 33 T 32 stromatic migmatite 24 80 I 58 56 trend and plunge of mineral dominates the hill top, and (3) unfoliated to weakly foliated 2-mica granite. Where 33 T 64 E. We expect this documentation to be useful in determining the interrelation of the processes of 00 06 00 lineation in 34 06 metasedimentary rock, E 86 86 79 migmatite, granite gneiss 37 86 fabrics exist, they are consistently 048-055-striking and shallowly to moderately SE- D 62 metamorphism, melting, melt segregation, magma flow, magma ascent and emplacement, and plastic 33 trend and plunge of fold SEBAGO PLUTON 06 hinge line O 77 Brunswick strike and dip of solid-state 04 31 dipping. Pegmatite grain size increases with elevation, and is Grt+Tm-bearing where deformation. 33 foliation in granite gneiss sensu stricto M strike and dip of S-C fabric in 24 The Study Area and Southern Maine 33 pegmatitic granitic gneiss A 49 foliated. (See Detail Map at right.) strike and dip of foliation in I 22 metagabbro Sebago N Freeport 30 32 12 04 32 LOCATION: NE Appalachains, USA The exposure is in the central part of the eastern SMD (see Regional Sebago Lake 47 03 00 Lake 21 75 stromatic migmatite structural boundaries of the 21 54 47 C transition zone between 20 Central Maine belt the NSZS (SE) and the 45 CMB A Map at right) that was previously shown as part of the Sebago rocks of the Sebago area N 09 15 A 32 Migmatite, stromatic BHB Bronson Hill belt batholith (on the state geological map). New mapping and western boundary of the D Location of Study 00 domain SE of the 48 47 sensu 54 Sebago pluton A 79 05 stricto R 2000 (layered) from pelitic Nashoba terrane 51 o geochemical data (on both the Maine Map below and the Regional y 35 65 a 79 69 19 10 l 39 30 28 R 87 49 iv 05 00 e 05 40 17 00 r 23 46° 34 2127 52 protolith. The migmatite has Avalon C 00 51 ACT Map, right) show that this area is composed of mostly metapelitic 81 10 48 40 Yarmouth M 12 61 71 Composite 10 58 Goose Island S 3631 21 B 60 77 Z 80 Bustins biotite foliation and selvedge terrane S stromatic migmatite and diatexite, with subordinate centimeter- to Island R ) Hays & Solar, A 40 29 S 46 2006 00 E Z melanosomes, and sub- 32 ADA meter-scale bodies of granite with compositions from medium-grained 00 00 74 CAN 00 H 53 15 Mosher H S S i 77 22 Island g 04 77 20 S T a 22 h 86 11 Maine c 25 l 18 S Z C o a N n R 43 concordant cm-scale d 43 ( S i A 2-mica granite to pegmatite, and fabrics from unfoliated to augen v 29 L e 15 a A r N 72 k 47 4816 Sebago e 18 Casco Bay B 36 16 29 19 B 69 60 29 M 48 42 G 32 44 leucosomes that have H M 32 85 d E gneiss and schlieric granite. This in in distinct contrast with the n 49 rocks a E l 46 s 17 I B C n d d h n a n j l a T s l e 36 s l I 44° I tto t i s L B a 17 in o 52 s b 05 38 u le 25 00 o a 18 C S h W varied solid-state fabrics. Sebago pluton (Regional Map) composed consistently of fine- to 28 43 50 M 19 81 VT 43°45’ N 88 80 44 41 Y 22 81 e 25 37 U 33 83 r 67 25 S 13 10 19 i d Migmatite outcrops have R 07 n 19 medium-grained 2-mica granite of uniform composition and la h P Is w 87 e E u r g a e 00 e p b O e e s h 50 40 C 00 area of u 20 78 t N 44 35 a m 29 re N G N m 85 sub-concordant granitic geochemistry (Tomascak et al., 1996a, 1996b). The SMD granites p 41 s O NY a regional map c o 05 t Z H R 26 44 75 iv 36 gneiss layers (cm-scale) have fabrics locally, and are cut by centimeter-scale granite dikes. On e 32 34 r 36 33 26 09 19 N 37 36 34 er N granitic augen gneiss Boston Saco Riv and cm- to m-scale cross- MA the NW, the SMD is apparently part of the NE-SW-trending Devonian d n 34 la Is 42° f f li 30 C 22 20 50 32 29 29 Atlantic cutting unfoliated granites. central Maine-New Hampshire migmatite belt (see Maine Map 28 RI 84 Atlantic Ocean 80 Atlantic Ocean CT Long Island d n a Ocean l Is l Great Diamond e below). Thus the age of the SMD is uncertain, but ongoing work will w Island e 100 km J 5 km 72° 70° address this unknown. The eastern SMD is bounded on the SE by the Portland CMBSZS NSZS Devonian crustal-scale NE-SW-striking dextral Norumbega shear zone transition zone Central Maine belt Norumbega shear NE and S of domain SE of STEREOGRAM SYMBOLS shear zone system zone system system (e.g., West & Hubbard, 1997). Structure of the eastern SMB is the Sebago N the Sebago N N N solid-state fabrics all structures N Granitic augen gneiss has transition NSZS pole to foliation pluton pluton metasedimentary rock, consistent with that zone (see Regional Map), so this area may be key zone quartzo-feldspathic gneiss a strong S-C fabric and and amphibolite stromatic migmatite and diatexite to the Sebago pluton history. granite gneiss and biotite foliation. pegmatite gneiss gabbroic gneiss lineation metasedimentary rock, 68°W quartzo-feldspathic gneiss SIMPLIFIED GEOLOGIC MAP: Maine and New Hampshire 69°W and amphibolite stromatic migmatite and diatexite 50 granite gneiss and 113 36 pegmatite gneiss 35 29 29 1 fabrics in igneous rocks 12 1 T 13 9 6 pole to foliation 9 PLUTONS STRUCTURES 1 17 L 8 17 1 13 4 schlieric granite 13 47°N E 4 12 5 11 3 9 67 Ms-Bt granite and Bt granite B 1 37 2 2 29 21 12 19 Pegmatitic granitic gneiss folds of compositional layers pegmatitic gneiss Norumbega fault total: 24 Mesozoic Y total: 48 total: 178 NF E total: 115 fold hinge line total: 286 sensu stricto LL has variable solid-state Carboniferous 70°W VA fabric defined by quartz and Turtle Head fault T Devonian THF U C feldspar ribbons. Grain size SPF Senebec Pond fault TI Ordovician C Methods E varies with elevation where terrane boundary N N C This work involved both field work and supporting hand-specimen and thin-section petrography of rocks collected at Bruce Hill coarser grains are found at O e fault C n tr outcrops.