“L s t. u. :.l_3 .Ev. -. 4 . (I 59 6 114‘ -‘_._ / '-."/?« - 1/ 6/ U/ /-’__’ /C-—" fl..-- ABSTRACT PETROGRAPHY AND PETROLOGY OF THE MIDDLEFIBLD GRANITE by Russell Gould Clark, Jr. The Middlefield Granite is a small elliptical body, roughly three miles long and one and a half miles wide, enclosed in a series of eugeosynclinal metasediments and located in the Chester quadrangle in west central Massa- chusetts. The purpose of the study was to map and describe the Middlefield Granite and to determine through field and petrographic studies as much as possible about the history, origin, mode of formation of the rock, and its relation (time and space) to the surrounding country rocks. A geologic map of the granite and surrounding rocks, based on the author's field investigations, is presented. As a result of the petrographic examination of gran- ite thin sections, the minerals present are described, with estimated modes tabulated: plagioclase, averaging An (38.7%); perthitic microcline (24.0%); quartz (24.1%); 12’ biotite (6.0%); muscovite (4.0%); epidote (2.5%); and other accessories (0.8%). Russell Gould Clark, Jr. The technique used for selectively staining potas- sium feldspar and plagioclase in thin section is described in the Appendix. Macroscopically, the granite has a gneissic and por- phyritic texture. Microscopically, the rock appears to have a hypidiomophicfgranular texture with a cataclastic .gneissic texture superimposed on it. Two foliations were observed: 1) a dominant folia- tion of subparallel microcline phenocrysts, elongated packets of biotite, and individual biotite flakes, which parallels the regional schistosity; and 2) a weak folia- tion of biotite flakes, which was observed only in a few places and may be primary. Five stages of folding, previously described by Hatch gt, El- (1967), which affected the area,are used to date the age of the granite. Three major conclusions are reached: 1) the granite was probably emplaced in the liquid state; 2) it was pro- bably emplaced during the early stages of the Acadian Oro- ’geny; and 3) subsequent deformation imposed gneissic and cataclastic textures upon the granite. PETROGRAPHY AND PETROLOGY OF THE MIDDLEFIBLD GRANITE BY Russell Gould Clark, Jr. A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Geology 1968 G 5/ ACKNOWLEDGMENTS The writer wishes to recognize and sincerely thank the following for their aid in the preparation of this paper: The United States Geological Survey for its finan- cial support during the field investigation and for supplying the thin sections used in this study, and to Lincoln R. Page and Norman L. Hatch, Jr. of the United States Geological Survey for their helpful suggestions and reading of the manuscript. The Department of Geology, Michigan State University, for the use of its petrographic facilities. Dr. Harold B. Stonehouse and Dr. Robert Ehrlich for their critical reading of the manuscript and many sugges- tions during this study. Dr. James W. Trow for his guidance, suggestions, and interest throughout the preparation of this paper. And, especially my wife, Jane, who endured long separ- ations, encouraged the author in every way, and typed the final draft of this paper. ii TABLE OF CONTENTS ACKNOWLEDGMENTS LIST OF TABLES LIST OF FIGURES INTRODUCTION Purpose of Study Geographic Setting Method of Study . Previous Work . REGIONAL SETTING Regional Structure Metamorphic Units THE MIDDLEFIBLD GRANITE . General Statement Areas of Outcrop Contacts with Older Rocks Foliations Petrography of the Middlefield Granite. SUMMARY‘. Origin of the Middlefield Granite Crystallization History . . . Subsequent Metamorphic Effects Conclusions . . . . . . BIBLIOGRAPHY, . iii Page ii . vi H 0301 ox ->FJHHd 11 ll 15 17 23 35 35 39 40 42 iv APPENDIX . Procedure for Uncovering Thin Sections in Prep- aration for Staining . . Selective Staining of Potassium Feldspar and Plagioclase in Thin Sections Revised Staining Technique Procedure for Covering Stained Thin Sections Page 46 46 48 51 LIST OF TABLES Table Page 1. Estimated modes across a granite sill at Glendale Falls . . . . . . . . . . . . 12 2. Estimated modes of granite samples . . . . . 14 Figure 10. 11. LIST OF FIGURES Map of Massachusetts, showing location of Chester quadrangle Geologic map of the northwestern part of the Chester quadrangle Sketch showing location of samples listed in Table 1 Detailed sketch showing alternating granite sills and schist inclusions at Glendale Falls Negative print of a thin section showing sharp nature of the contact between a granite sill and slab-like inclusion of schist Sketch showing granite cutting through a schist inclusion . . . . . . Sketch showing a small schist inclusion which has been partially assimilated by the granite . . . . . . . . . . . Photomicrograph of a Carlsbad twinned, perthitic microcline phenocryst Photomicrograph of a group of small micro- cline grains . Negative print of a granite thin section . Photomicrograph showing slightly zoned inclusion of plagioclase, with myrme- kite border, in microcline vi Page 13 16 20 21 22 24 25 26 28 Figure 12. 13. 14. 15. 16. vii Photomicrograph showing part of the myrmekite border on the microcline phenocryst shown in Figure 10 Negative print of a granite thin section . Photomicrograph showing elongate packet of biotite, muscovite, epidote and other accessories . Photomicrograph showing euhedral sphene and epidote replacing biotite Sketch showing 8- shaped schist inclusion which has been partly assimilated by the granite . . . . . . . . . Page 29 30 32 33 37 INTRODUCTION Purpose of Study The purpose of this study was to map and describe the Middlefield Granite and to determine through field and petrographic studies as much as possible about the history, origin, mode of formation of the rock, and its relation (time and space) to the surrounding country rocks. Geographic Setting The Middlefield Granite is an irregular but roughly elliptical body located in the northwest ninth of the Chester quadrangle in west central Massachusetts. The body underlies about three square miles of the Town of Middlefield. (see Figures 1, 2) Method of Study Field work was done during the summer of 1967, while the author was working under the direction of Norman L. Hatch of the United States Geological Survey, Branch of Regional Geology in New England, in a c00perative map- ping project with the Massachusetts Department of Public Works. Traverses were run along stream valleys and ridge _.mmohm vowwnmv oawthwwsc Houmonu mo :oaumooa maflsonm .mpuomsaowmmmz we no: -- .H ohnmwm 3235.8 o co. on non A z \ .f. .‘ . zI\) . .dmillv .HI..«H x. . 1‘. ”a . _ oo‘ ' a7'so‘ 42° 22' 30‘ MIIDLEFIELD GRANITE ULTRAMAFIC GOSHEN FM. HAWLEY FM. 00/ STRIKE AND DIP OF GRANITE FOLIATION _ N OF COUNTRY ROCK SCHISTOSITY MORETOWN FM, ZIIS‘ OUTCROP WITH SAMPLE STATION NUMBER ROWE SCHIST Isosx HILLTOP AND ELEVATION _......_ m. councr I000 0 I000 3000 6000 FEET L. . 1 .¥1 . L 4... HOOSAC FM. @ D CI Figure 2. -- Geologic map of the northwestern part of the Chester quadrangle; showing outcrops, sample sta- tions and locations referred to in the text. crests, with a traverse spacing, in so far as possible, of no more than 1000 feet. Location of supplementary traverses was determined by the amount of outcrop seen on the ridges and streams and by the complexity of the _geology. Field data were plotted on a 1:24,000 t0po- graphic sheet. Detailed studies on a scale of 1:120 were made at Glendale Falls. Laboratory work and writing were done during the period September, 1967, to May, 1968, at Michigan State University under the supervision of Dr. James W. Trow. Thin sections were observed through a Leitz petrographic microscope. Estimated modes were made from point counts using a Chayes stage. Feldspar identification was facil- itated by using the technique for selectively staining plagioclase and potash feldspar described in the Appendix. Previous Work The author was unable to locate any detailed descrip- tions of the Middlefield Granite in any previous litera- ture. The earliest mention of the granite is by Emmons (1824) and also Dewey (1824). The most recent mention of the Middlefield Granite in the literature is by B. K. Emerson in a monograph on old Hampshire County (1898). Since it is the most recent account of the granite and it is brief, Emerson's des- cription of the "Middlefield Porphyritic Granite" is here quoted in its entirety: The great dike of granite in Middlefield, about 6 miles long, is widely separated from all other outcrOps, and is unlike all other masses of granite in the region. It is purely a biotitngranite, small pOrphyritic in its central portions. The feldspars are about three-fourths of an inch long, rarely show Carlsbad twinning, and are microcline with- out albite bands. A few rounded spots, ap- parently of albite, break the continuity of the cleavage surface. These feldspar crys- tals are at times bounded by a layer of secondary muscovite plates, and this is the only appearance of muscovite in the granite. The biotite is aggregated in groups of rather dull-black plates, with epidote,_garnet and rarely white apatite needles accompany- ing it. The yellowish-white background is a somewhat friable mixture of much granular orthoclase and a little bluish qUartz, which is characterized by the presence of small, elongate cavities. At the border the por- phyritic feldspars and the biotite aggregates disappear, and the friable ground with small distinct spots of biotite and the small cav- ities remain unchanged. Regional stratigraphic and structural studies are recently completed or currently underway by the United States Geological Survey in many of the neighboring quadrangles. REGIONAL SETTING The Chester quadrangle is geologically located in the Northern Appalachian Mountains on the east limb of the Green Mountain-Berkshire Anticlinorium. The anti- clinorium trends north-south and has Precambrian crystal- line gneisses at its core. The west limb of the anti- clinorium is composed of a sequence of miogeosynclinal Paleozoic quartzites and carbonates. These rocks have most recently been studied in north western Massachusetts by Herz (1958, 1961) and Norton (1967). The Middlefield Granite is on the east limb, which is composed of a sequence of eugeosynclinal Paleozoic schists and gneisses. These rocks have been studied most recently in north western Massachusetts by Hatch and Hartshorn (in press), Osberg, Hatch and Norton (unpublished map), and Hatch (in press). Regional Structure Description of the structural features found in the Chester quadrangle (and in the Worthington and Plainfield quadrangles just to the north) is summarized in Hatch, 33. al., (1967). The description by Hatch is quoted in part here: (1) Deformation in pre-Goshen time produced north-west trending tight or iso- clinal folds. These folds are assumed originally to have had near-horizonal or horizonal axes. (2) The next deformation, which affected all the rocks, produced isoclin- al folds with north-trending, vertical or steeply east- dipping axial surfaces. Axes are generally steeply plunging in pre- Goshen rocks and gently plunging or horizonal in Goshen rocks. The prominent regional schistosity is parallel to the aXial surfaces of these folds. (3) Schistosity and the axial surfaces of the folds produced in stage-2 deforma- tion are locally warped into open folds a few inches to a few feet in amplitude. Axial surfaces of these stage-3 folds strike northeast and dip moderately to steeply to the northwest; axes plunge mod- erately to the north. . . . (4) The doming of the gneiss around Hallockville Pond (north 0f Chester Quad- rangle) produced minor folds with an axial plane slip cleavage in the rocks in and immediately around the dome. . . . (5) Folds with north-trending subver- tical axial planes and associated slip cleav- age and gently plunging axes are present along the western edge of the area. . . . This stage of folding is interpreted as being synchronous with the formation of the preSent Berkshire anticlinorium. According to Hatch, stage 1 is probably Taconic (Ordovician) in age, whereas stages 2, 3, 4, and 5 are probably all related to the Acadian Orogeny. Hatch also believes that the regional metamorphism is Acadian, as is true in most of the rest of New England. (personal communication, April, 1968). Metamorphic Units Five formations of metamorphic rocks older than the Middlefield Granite occur in the Chester quadrangle. (see Figure 2) All of these were sedimentary units orig— inally except for the smaller amphibolites, some of which were probably intrusive dikes and sills. Two of these formations are in contact with the Middlefield Granite: l) the Cambrian and Ordovician Rowe Schist; and 2) the Ordovician Moretown Formation. The Hoosac Formation which is the oldest unit in the area, consists largely of light- to darkegrey quartz- albite-biotite-muscovitejgarnet schist. The Rowe Schist (Hatch at 31., 1966) includes three distinctive lithologic types: black;_green; and amphi- bolite. The Black Schist member of the Rowe Schist is pri- marily dark grey carbonaceous quartz-feldspar-muscovite- biotite schist. It commonly weathers rusty brown or a yellow sulfidic color. The rock is moderately well- bedded in relatively more schistose and granulose beds. Quartz pods up to 4 centimeters wide and 8 centimeters long are present locally. The carbonaceous material occurs in thin layers parallel to schistosity and is a distinctive feature of this member. The Green Schist member of the Rowe Schist is a light green fineegrained muscovite-quartz-chlorite-feld- sparfgarnet-magnetite schist. Fresh surfaces are light ‘grey green to silvery green. It weathers grengreen to brown. The rock varies somewhat in prOportions of micas and quartz, but is_generally highly schistose. Stringers and blebs of quartz as much as 5 centimeters long are common. Bedding is generally not evident. The amphibolite member of the Rowe Schist is a fine- to mediumfgrained hornblende-plagioclase schist. Fresh surfaces are_greenish black with lighter plagioclase lay- ers in some places. It weathers to a dark greenfgrey with a characteristic lineation. Local banding (1-2 millimeters) results from the alternation of hornblende- rich and plagioclase-rich layers. The Moretown Formation is predominantly grey fine- to mediumfgrained quartz-plagioclase-muscovite-biotite ‘granulite* and schist. The rock in many places approaches quartzite in composition. The layers or partings of mica- rich rock between 1-3 millimeter layers of quartz-feldspar- rich rock impart to the rock a characteristic thin-laminated * The term granulite is used here purely in the textural sense to indicate a rock composed predominantly of even- sized interlocking granular minerals. No implication as to the grade of metamorphism is intended by this usage. (Hatch g£_al., in preparation) ' 10 appearance commonly known as "pinstripe" structure. Many garnets are one half inch in diameter. The Rowe- Moretown boundary is gradational by interbedding of Rowe and Moretown lithologies. The Rowe-Moretown boundary is mapped at the base of the first granulite bed more than about 6 inches thick. Many small finefgrained amphibo- lites, similar in composition to the Amphibolite member of the Rowe Schist, occur near the top of the Moretown FOrmation. Some of these are cross-cutting and thus in- trusive. Stratigraphically above the Moretown Formation is the Hawley Formation of Ordovician age and the overlying Goshen Formation of Silurian and Devonian age. Generally, the mica-quartz rich Green Schist member of the Rowe Schist is in contact with the granite on the west, and the quartz-plagioclase rich Moretown Formation is in contact with the granite on the east. THE MIDDLEFIELD GRANITE General Statement The Middlefield Granite is a roughly elliptical body about three miles long and one and a half miles wide. It is a light- to mediumjgrey, or buff, medium grained gneissic granite composed predominantly of microcline (commonly perthitic), plagioclase, and quartz, with lesser amounts of biotite, muscovite, epi- dote, sphene, allanite, and opaques. Microcline com- monly forms Carlsbad twinned phenocrysts as much as 3 centimeters long that may constitute as much as 36 per- cent of the rock. The parallelism of individual flakes of biotite and small lenses of biotite, epidote, and sphene give the rock a prominent foliation that parallels the regional schistosity of the country rock. The modal composition of the rock (see Tables 1, 2) is that of a granodiorite according to Johannsen (Vol. II, p. 320.), but is that of a granite according to Moorehouse (p. 256). The term nganite" is herein retain- ed to conform with Emerson's (1898) description of the body. Microsc0pically, the texture is porphyritic and hypidiomorphicegranular, but in most places a cataclastic ll 12 .00:0 moamsmm 000:00m.00m 00 .monEmm mo CO0pmooH 000 m QHDM0m mom 0 om000>0 was .opfinmhm 600 z I m moamsmm .000500 00 o oHQEmm 0.000 0.000 0.00 0.000 0.00 0.000 0.000 0.00 0.00 00000 0.0 0.0 0.0 000000 0.0 4.0 0.0 0000000< 4.0 0.0 0.0 0.0 0.0 0.0 00 00 000000 0.0 0.0 0.0 0.0 4.0 0.0 0.0 0.0 0.0 000000< 0.0 0.0 0.0 0.0 0.4 0.0 0.0 0.0 0.0 0000000 0.0 0.4 0.0 0.0 0.00 0.0 0.0 0.00 0.0 000>o0002 5.5 4.40 0.0 0.4 0.4 4.0 5.4 0.00 0.50 0000000 4.00 0.0 0.0 0.0 5.00 0.00 4.00 0.00 0.00 000000 0.04 0.50 0.04 0.00 0.00 0.54 0.50 5.00 0.04 00000000000 00.00 00.00 00.40 00.00 05.00 00.40 00.00 00.0 00.0 0000000002 00000>< z 2 0 0 0 0 m 0 «mHme OHNHZHQHO Hm HHHw muwfimhm d mmOHUN mmwofi UmumEHu—mm I: .H OHQ'NH/ 13 .fiommm nOHkum mo :0000000 now v.005m0m 0600 .0 00309 :0 copm00 monEmm mo :O0pmooH mn030aM 0000: @0000 0.00:0:0 map £00m nopoxm .: .m 60=m0m 200.0. .0: _ 0.470; l4 Table 2. -- Estimated modes of granite samples (1) (2) (3) (4) (5) Average Microcline 24.0% 29.9% 24.6% 16.2% 25.3% 24.0% Plagioclase 34.7 38.5 32.8 43.8 43.6 38.7 Quartz 26.3 23.8 33.4 23.8 13.4 24.1 Biotite 7.1 4 5 3 9 6.7 7 7 6 0 Muscovite 3.0 l 2 2 7 6.2 6 9 4 0 Epidote 3.9 2 2 1 7 2.7 1 9 2 5 Apatite 0 3 0 2 0.4 0 3 0.2 Sphene 0.3 0 l 0.6 0 3 0.4 0.3 Opaque 0.3 tr 0.1 0.1 Allanite 0.5 0.2 0.1 0.2 Total 100.1 100.5 100.1 100.1 99.8 100.1 1. Granite Sample Number 2161* 2. Granite Sample Number 2124 3. Granite Sample Number 2133 4. Granite Sample Number 2115 5. Average of Granite Sill from Table 1. * See Figure 2 for location of samples. 15 _gneissic texture has been superimposed on this to vary- ing degrees. Only a few of the outcrops located by this present author were entirely lacking gneissosity. Areas of Outcrop The actual outcrOps located by the author during field work are shown on Figure 2. Outcrops are scarce in the center and along the northeastern and southwestern edges of the granite body. The largest single outcrOp is at Glendale Falls, a Massachusetts State Reservation, near the eastern edge of the body. Here there is almost continuous exposure of alternating bands of granite and Moretown Schist for about 1200 feet along Glendale Brook (see Figure 4). The shape of the body was roughly determined from the locations of the outcrops of granite and country rock. Although the granite contacts with the country rock were not located precisely, the author feels that with the exception of the northeastern portion of the body, the contacts as shown on the map (see Figure 2) are fairly accurate. Falls) 0 IO 20 FEET I 1 I I >"F MIDDLEFIELD MORETOWN APLITE GRANITE SCHIST DIKE Figure 4. -- Detailed sketch showing alternating granite sills and schist inclusions at Glendale Falls (drawn from author's > v 7 7 < V v v 7 V "1 A 4 (a plane table mapping at Glendale 17 Contacts with Older R0cks The contact of the main granite body with the sur- rounding rocks is very poorly exposed. The exact con- tact was never observed. The contact at 1) the base of Glendale Falls was determined to within 10 feet, 2) the northwest side of hill 1577 was determined to within 125 feet, and 3) the north fork of Smith Brook was deter- mined to within 120 feet. (see Figure 2) The contact at each of these three places is thought by the author to be sharp, since at no place was there any sign of tex- turally or compositionally gradational contact between the granite and the country rock. The contact at the southern end of the body is not located with any precision by the outcrOps but is based on the attitude of the schistosity in the adjacent coun- try rock (see Figure 2). Assuming that the schistosity here smoothly wraps around the granite (as is apparently the case) the southwestern contact of the granite trun- cates the Moretown Formation-Rowe Schist boundary. The nature of the contact at the extreme northern end of the granite body is uncertain due to lack of out- crops. From the available field evidence, however, two possibilities can be suggested: 1) one sill of granite was located in the brook north of the granite (see Figure 18 2); if more sills like this exist, the granite may have an interfingering contact with the country rock; or 2) if the obscure foliation of biotite flakes found on the northern sides of hills 1509 and 1577 is primary (see page 19), it may indicate a smooth curved contact simi- lar to one inferred at the southern end of the granite. The author favors the second possibility, since this would give the granite body similar contacts at both ends, and such a smooth contact would also be parallel to the schistosity of the two Moretown outcrops found in the brook just northeast of hill 1509. On the geologic map (Figure 2) this contact is shown as a group of jag- ged sills with a generally curved northern outline. This is an attempt by the author to show that this may be either a smooth or interfingering contact. Many pieces of country rock are included or partly included in the granite, particularly along the eastern margin of the body. Only two such large inclusions, which may be roof pendants, were noted in the western half of the granite. These inclusions are best seen at Glen- dale Falls, where they range in width from a few feet to several tens of feet (see Figure 4). Most are exposed along their length for less than a hundred feet but may extend for thousands of feet. The contacts of the granite 19 and the inclusions parallel the schistosity in the coun- try rock and are sharp (see Figure 5). Foliation observed in the granite is parallel both to the contacts and the schistosity in the wall rocks, except locally where the .granite cuts across the schistosity of the inclusions (see Figure 6). One gradational contact between the granite and a small inclusion is shown in Figure 7. ' Foliations Almost every outcrOp seen by the author was folia- ted. Most outcrops had only one foliation, but some had two. In the field the two foliations were distinguished as 1) a relatively weak foliation consisting of a sub- parallel alignment of individual biotite flakes, and 2) a relatively stronger foliation consisting of a subpar- allel alignment of elongate packets of biotite, individ- ual flakes of biotite, microcline phenocrysts, or any combination of these three. Microsc0pica11y, the elon- _gate packets of biotite were seen to also generally con- tain epidote, sphene, and muscovite. Where only one foliation was observed at an outcrop, it was invariably the second type, and was parallel or subparallel to the regional schistosity in the country rock. Where both types of foliation were present in an outcrOp, the second type was parallel or subparallel to Figure 5. -- Negative print of a thin section showing sharp nature of the contact between a granite sill (top) and slab—like inclusion of schist at Glendale Falls. (X4) 21 ‘1 A < > J .0 e f .e d .0 'MI Ie'7 A L 7 x v n] ‘I L 9V1 LGfO ion. lus 1nc te cutting through a schist Figure 6. -- Sketch from the author's field notes ingvgrani show I0”. 0,- H.” 0‘s CF" - -. .‘r .. I. ' L’- '\ l A l I ' . » o '7 i l ' ' * i.‘ c J g .‘ ‘fi‘ .At'” 0“... V t , O , \ ‘ U Q . ; \ -0 '0' ' . , t ‘. . ‘II V [- 'l‘ . In “an." 40‘"... a- o“- . J. ’1 II. . h I I 5 "\ I I ._ 0 .4 R I. . v' J‘m 22 A L. >partial|y A assimilated schist < > ............ OOOOOOOOOOOOOOOOOO 000000000000000000 IIIIIIIIIIIIIIIIIII OOOOOOOOOOOOOOOOOO IIIIIIIIIIIIIIIIII IIIIIIIIIIIIIIIIII ooooooooooooooooooo IIIIIIIIIIIIIIIIIII OOOOOOOOOOOOOOOOOOO oooooooooooooooooooooooooooooooooooo oooooooooooooooooooooooooooooooooo ooooooooooooooooooooooooooooooooooooooo ooooooooooooooooooooooooooooooooooooo O 47V A>AP¢Q7PA Ia b .ALq ‘ ‘I\( Middlefield Granite E Figure 7. -- Sketch from the author's field notes showing a small schist inclusion which has been partially assimilated by the granite. 23 the regional schistosity and the weaker first type of foliation was at an angle to this. The location and orientation of these foliations are noted on the accompanying geologic map. 'Petrography g: the Middlefield Granite Two types of microcline are present: 1) large grains, the white phenocrysts seen in hand specimen, are typically perthitic (vein and patch types of perthite), commonly exhibit the characteristic gridiron twinning, and gen- erally.contain inclusions of plagioclase (other inclu- sions of biotite, sphene, and epidote may also be present); these large grains generally have a subhedral rectangular shape with irregular borders; and 2) small approximately equidimensional, well-twinned grains, containing few or no inclusions, commonly in aggregates, composed solely of microcline, elongated parallel to the secondary folia— tion of the granite; these aggregates are commonly seen near, or in contact with, the larger first type of micro- cline. (see Figures 8, 9, 10) Plagioclase occurs in three forms as: l) inclusions in microcline phenocrysts; 2) grains in, but not part of, the groundmass; and 3) myrmekitic intergrowths of quartz and plagioclase. Plagioclase in the first two forms is subhedral with corroded borders,generally albite-twinned, 24 Figure 8. -— Photomicrograph of a Carlsbad twin- ned, perthitic microcline phenocryst; an epidote veinlet fills the twin plane and cuts through a zoned plagio- clase inclusion (upper right). (X80) (crossed nicols). f 25 Figure 9. -- Photomicrograph of a group of small microcline grains (right center) adjacent to, and pre- sumably fractured from,the larger microcline phenocryst. (X80) (crossed nicols). 26 Figure 10. -- Negative print of a granite thin sec- tion: 'unstained (top) and stained (bottom). In stained half: Quartz is black; plagioclase is dark-grey; micro- cline and muscovite are light-grey; and biotite is white. A corroded phenocryst of micr6cline with a myrmekitic border is located at the upper left in the stained half. (X4) 27 and commonly contains muscovite (and/or paragonite?), epidote, and, in some places, quartz inclusions. The third (myrmekitic) form of plagioclase is commonly found 1) as partial rims on microcline phenocrysts, and 2) in the cataclastic groundmass. The composition of the plagioclase in the first two forms was found, using the Michel-Levy method, to be approximately An12° (see Figures 8, 10, ll, 12) Quartz occurs in 1) elongate lenses, the sides of which are generally smooth and sharp against adjacent minerals and the groundmass; the long directions of these lenses parallel the regional secondary foliation of the (granite, and the quartz within the lenses is fractured, with individual_grains showing undulating extinction; 2) myrmekitic intergrowths with plagioclase, 3) inclusions in plagioclase and microcline, and 4) small distinct grains in the cataclastic_groundmass. The first type of quartz described is distinctive and the most abundant type. (see Figures 10, 13) In the first type, bubble trains are com- mon and generally cross boundaries between individual frac- tured grains within the lenses; and in some places epidote, biotite, or sphene occur in fractures within these lenses. Biotite occurs as 1) individual flakes with ragged ends in the groundmass, and 2) inclusions in plagioclase and microcline, but its most common occurence is as 3) 28 Figure 11. -- Photomicrograph showing slightly zoned inclusion of plagioclase, with myrmekite border,in microcline. Plagioclase twinning continues through zon- ing. (X330) (crossed nicols). 29 Figure 12. -- Photomicrograph showing part of the myrmekite border on the microcline phenocryst shown in Figure 10. (X80) (crossed nicols). 30 Figure 13. -- Negative print of a granite thin sec- tion: 'unstained (bottom) and stained (top). Quartz is black; plagioclase is dark-grey; microcline and muscovite are lightEgrey; and biotite is white. (X4) 31 elongate packets of flakes, associated with epidote, sphene, and muscovite and other accessories. This third form (elongate packets) commonly appears vein-like and wraps around larger grains of feldspar. The subparallel- ism of these elongate packets forms the predominant foliation of the granite which parallels the regional schistosity. The biotite in all three forms is gener- ally brown, although the pleochroism tends toward olive- green when the flakes are viewed normal to the cleavage. "Bird's—eye" extinction is commonly strong. Biotite may alter to muscovite or chlorite. (see Figures 10,13,14,15) White mica occurs as 1) flakes associated with the elongate packets of biotite mentioned above, 2) ragged flakes in the groundmass, 3) small flakes included in plagioclase, often oriented along plagioclase cleavage planes, and 4) symplectic intergrowths with plagioclase. The first three types are muscovite. The fourth type of white mica, which may be paragonite, has a 2V of about 20°. "Bird's-eye" extinction is common in the first two forms of muscovite. (see Figures 10, l3, l4) Epidote occurs as l) anhedral to subhedral grains associated with biotite in the elongate packets mentioned above and included in plagioclase and in some microcline, and 2) green cryptocrystalline veinlets filling fractures through other_grains, spaces between other grains, and 32 Figure 14. —- Photomicrograph showing elongate packet of biotite, muscovite, epidote, and other accessories. (X80) (uncrossed nicols). 33 Figure 15. -- Photomicrograph showing euhedral sphene (left), and epidote replacing biotite (right). (X330) (uncrossed nicols). 34 some twin planes in both plagioclase and microcline. In some places this second (veinlet) form of epidote appears to be iron—stained. (see Figures 8, 14, 15) Sphene generally occurs as dirty-brown anhedral to euhedral grains associated with biotite and epidote in the elongate packets (described above), but it may occur as euhedral inclusions in microcline or quartz, or anhedral grains in the_groundmass. (see Figure 15) Apatite occurs as anhedral to subhedral grains in the cataclastic groundmass. Subhedral to anhedral, red-brown to orange allan- ite is found intimately associated with epidote, commonly in the elongate packets of biotite, epidote, and sphene. Normally, allanite shows little extinction, but rather transmits polarized light in all orientations and has a blotchy appearance; other crystals are zoned and show more extinction, or are botryoidal. The few opaques observed were 1) magnetite included in muscovite, and 2) ilmenite included in sphene. SUMMARY A Origin pf the Middlefield'Granite Several field observations point toward the conclu- sion that the Middlefield Granite was forcefully injec- ted into its present position as a small elliptical stock. 1) The attitude of the wall rocks, as shown on Figure 2, bulge outward on both the east and west sides of the body. Just east of Collins Hill, at the southern end of the granite, the foliation in the country rock appears to wrap around the end of the body; the strike changes from N.43°W. on the southwest side to N.46°E. on the southeast side. 2) The granite appears to cut across the Moretown Formation-Rowe Schist boundary on the southwest side of the body and possibly at the north end. 3) The contacts between granite and the large slab-like inclusions of Moretown are sharp. The three contacts between the main granite body and the enclosing schists described on page 17 gave every indication of being sharp; no evidence of large scale gradational con- tacts were seen. 4) Alternating bands of granite and schist at Glen- dale Falls are probably sills of igneous rock between 35 36 roof pendants of the overlying country rock. 5) A few partially assimilated xenoliths of schist were observed. (see Figures 6, 7, l6) 6) A weak biotite foliation, which may be pri- mary, is visible, where it does not parallel the regional schistosity, but has the stronger SeCOndary foliation superimposed upon it. On hill 1509 primary (?) folia- tion dips to the north on the top of the hill and dips to the south on the south side of the hill, indicating that hills 1509 and 1577 are probably cupolas of an originally igneous body projecting into the overlying country rock. 7 7) At Glendale Falls, within one of the sills of ‘granite, a partially assimilated xenolith of schist about 15 feet long and 6 inches wide is present. The xenolith is elongated perpendicular to the bands of schist on either side of the granite sill and is straight except that the ends bend to the north on the west end and to the south on the east end, so that the xenolith forms an elongate S (see Figure 16). This may indicate that the flowing granite magma rotated the xenolith with a right lateral strike-slip movement. Several lines of evidence pointing to a magmatic origin for the granite have been found by studying the thin sections: 37 1’44hf‘7 '7' 0'8"“ L. ‘7 Figure 16. -— Sketch from the author's field notes showing S- shaped schist inclusion which has been partly assimilated by the granite. (see Figure 4 for location of station 2533). ' 38 1) Several plagioclase grains were zoned with the highest anorthite content in the center; the zoning was fuzzy, and in a few places twinning was SUperimposed on the zoning. (see Figures 8, 11) 2) Several plagioclase grains have inclusions of muscovite and quartz in zones, indicatingthat the crys- tallization of the plagioclase may have been interrupted temporarily. 3) Plagioclase inclusions in microcline grains have been rotated. All of the above field and microscopic observations are believed to point toward a magmatic origin and force- ful intrusion of the granite body. The origin of the magma itself cannot be determined from the available evidence. Crystallization History In view of the complex regional tectonics it is not surprising that very little can be inferred about the crystallization history of the Middlefield Granite. The body most likely crystallized from a granitic magma, or from a quartz monzonitic magma if the anor- thite content of the plagioclase was much higher than it is now (i.e., abouthnlz). 39 Since at least some of the plagioclase present has included biotite flakes and both plagioclase and biotite are found in the groundmass, plagioclase and biotite crystallization probably overlapped. Microcline has included plagioclase, so it probably crystallized later than plagioclase. Most, if not all, of the white mica appears to be secondary after biotite and plagioclase. Epidote also appears to be secondary. Most of the perthite appears to be of the exsolu- tion vein type and probably formed as the body cooled. Myrmekite probably formed as the body cooled,apparently resulting from the reaction between plagioclase and microcline. Subsequent Metamorphic Effects The regional schistosity of the country rocks appears to have been imparted during Hatch's second (early Aca- dian) stage of folding (N. L. Hatch, personal communica- tion, 1968) (see page 7). Since the granite body warps this schistosity outward on both the east and west sides of the body, its emplacement could not have been before the folding. But since the granite also contains a folia- tion parallel to this regional schistosity, it too must have been affected by this stage of folding. The granite 40 is, therefore, presumed to have been emplaced syntecton- ically with the second stage of folding. After the body cooled another tectonic event occurred which caused most of the myrmekite to be broken off from the microcline phenocrysts. This event cannot be defin— itely correlated with any subsequent stage of folding, but must have occurred before stage 5. This can be rea- soned as follows: the regional metamorphism of the area (which produced staurolite in other areas of the quad- rangle) occurred after Hatch's stage 3, but before his stage 5 (N. L. Hatch, personal communication, 1968); both forms of epidote (vein and grain) present in the rock appear to have formed by decalcification of the pla- ,gioclase which probably took place during the regional metamorphism. Since veins of epidote cut through both ‘grains and groundmass alike, it followed any cataclasis of the rock. Conclusions Three reasonably certain conclusions may be reached from the field and petrographic descriptions assembled in this paper: 1) the Middlefield Granite was originally emplaced in a liquid or partially liquid state; 2) the body was emplaced during the early stages of the Acadian 41 Orogeny; and 3) subsequent to its emplacement, the body underwent further deformation, which imposed gneissic and cataclastic textures upon the granite. This defor- mation most likely also occurred during the Acadian Orogeny. The conclusions reached in this paper concerning the crystallization history and the precise order and number of metamorphic events which affected the granite are at best only tentative, but are believed by the author to be the best approximations which can be cur- rently made from the available evidence. BIBLIOGRAPHYfiOF CITED AND RELATED REFERENCES Bailey, E. H. and Stevens, R. H., 1960, "Selective Staining of K-Feldspar and Plagioclase on Rock Slabs and Thin Sections,“ Am. Min., Vol. 45, p. 1020. _— '—__ Billings, M. P., 1956, The Geology of New Hampshire: Part II. Concord, New HampEHire: New Hamp- shire Planning Commission. Bowen, N. L., 1928, The Evolution 9: Igneous Rocks. Princeton: Pr1nceton University Press. Buddington, A. F., 1948, "The Origin of Granitic Rocks ' of the Northwest Adirondacks," Geol. Soc. Am. Memoir, Vol. 28, pp. 21-43. Deer, W. A., 1935, "The Cairnsmore of Carsphairn Igneous Complex," Q. Jour. Geol. Soc. LOndon, Vol. 91. , Howie, R. A. and Zussman, J., 1962-1963, Rock FOrming Minerals (5 Vols.) London: Longmans, Green and Co. Dewey, C., 1824, "Geology of Berkshire County, etc.,” Am. Jour. Sci., Vol. 8. Emerson, B. K., 1898, "Geology of Old Hampshire County," U. S. Geological Survey Monographs, Vol. 29. Emmons, E., 1824, "Notice of Localities," Am. Jour. Sci., Vol. 8. Emmons, R. C. (ed.), 1953, "Selected Petrogenic Relation- ships of Plagioclase," Geol. Soc. Am. Memoir, Vol. 52. "———- Engel and Engel, 1960, "Progressive Metamorphism and ' Granitization of the Major Paragneiss, North- west Adirondack Mountains, New York," Bull. Geol. Soc. Am., Vol. 71. 42 43 Fowler-Lunn, K. and Kingsley, L., 1937, "Geology of the Cardigan Quadrangle, New Hampshire,"‘ Bull. Geol. Soc. Am., Vol. 48, Pt. 2, pp. 1363-1386. Fyfe, W. S., Turner, F. J. and Verhoogen, J., 1958, "Metamorphic Reactions and Metamorphic Facies,” "Geol. Soc. Am. Memoir, Vol. 73. Gilluly, J. (ed.), 1948, "The Origin of Granite," Geol. Soc. Am. Memoir, Vol. 28. Goldsmith, J. R., 1952, "Diffusion in Plagioclase Feld- spars," Jour. Geol., Vol. 60, pp. 288-291. Hatch, N. L. an E. , Jr., 1967, "Redefinition of the Hawley d Goshen Schists in Western Massachusetts," §.g.§. Bull. 1254+D. , (in press), Geologic Map of the Worthington Quadrangle, Massachusetts: U.S. Geol. Survey Geol. Quad. Map GQ. , Chidester, A. H., Osberg, P. H., and Norton, S. A., 1966, Redefinition of the Rowe Schist in northwestern Massachusetts, in Cohee, G. V., and West, W. 8., Changes in stratigraphic nomencla- ture by the U. S. Geological Survey, 1965. U. S. Geol. Survey Bull. 1244-A, pp. A33-A35. , and Hartshorn, J. H., (in press), Geologic Map of the Heath quadrangle, Massachusetts-Vermont: U. S. Geol. Survey Geol. Quad. Map GQ. , Osberg, P. H. and Norton, S. A., 1967, "Stra- tigraphy and Structure of the East Limb of the Berkshire Anticlinorium,” New England Inter- collegiate Geology Conference Guidebook. , Norton, 8. A., and Clark, R. G., Jr., (in prep- aration), Geologic Map of the Chester Quadrangle, Massachusetts: U. S. Geol. Survey Geol. Quad. Map. Heinrich, E. W., 1965, Micro$copic Identification 9f Minerals. New York: McGraw-HillTBooleo. 44 Herz, N., 1958, Bedrock geology of the Cheshire quad- rangle, Massachusetts: U. S. Geol. Survey Geol. Quad. Map GQ-108. , 1961, Bedrock geology of the North Adams quad- rangle, Massachusetts-Vermont: U. S. Geol. Sur- vey Geol. Quad. Map GQ-l39. Johannsen, A., 1938, Descriptive'Petrology 9f the Igne- ous Rocks. Chicago: University of Ch1cago Press. Laves, F., 1952, "Phase Relations of the Alkali Feldspars," Jour. Geol., Vol. 60, pp. 436-450, 549-574. Leedal, G. P., 1952, "The Cluanie Igneous Intrusion, Inver- ness-shire and Ross-shire," Q. Jour. Geol. Soc. London, Vol. 108, pp. 35-63. Moorehouse, W. W., 1959, The Study of Rocks in Thin Sec- tion. New York: Harper and—Roe, PuBTisHers. Norton, S. A., 1967, Geology of the Windsor quadrangle, Massachusetts: U. S. Geol. Survey, Open-file report, 210 p. Osberg, P. H., Hatch, N. L., Jr., and Norton, S. A., Geologic map of the Plainfield quadrangle, Massa- chusetts: unpublished map. Osterwald., 1955, "Petrology of Pre-Cambrian Granites in Northern Bighorn Mountains, Wyoming," Jour. Geol., Vol. 63. Quinn, A., 1944, "Magmatic Contrasts in the Winnipesau- kee Region, New Hampshire," Bull. Geol. Soc. Am., Vol. 55, pp. 473-496. , 1937, "Petrology of the Alkaline Rocks at Red Hill, New Hampshire," Bull. Geol. Soc. Am., Vol. 48, Pt. 1, pp. 373-402. , and Stewart, G. W., 1941, "Igneous Rocks of the Merrymeeting Lake area of New Hampshire," Am. Min., Vol. 26, pp. 633-645. 45 Ramberg, H., 1952, The Origin 9f Metamorphic and Metaso- matic Rocks. Chicago: University of Chicago Press. ' Raguin, E., 1965, Geology gf Granite. E. H. Kranck and P. R. Eakins (trans.) London: John Wiley and Sons, Ltd. ' Robertson, 1959, "Perthite formed by reorganization of albite from plagioclase during potash feldspar metasomatism,” Am. Min., Vol. 44. Schermerhorn, 1956, "The Granites of Transcose (Portu- gal): A Study of Microclinization," Am. Jour. 'Sci., Vol. 254. Seifert, K. E., 1964, "The Genesis of Plagioclase Twin- ning in the Nonewaug Granite," Am. Min., Vol. 49, pp. 297-320. Shelley, D., 1964, "On Myrmekite," Am. Min., Vol. 49, pp. 41-52. , 1966, "The Significance of Gran0phyric and Myrmekitic Textures in the Lundy Granites," Min. Mag., Vol. 35, No. 273, pp. 678-692. Simpson, D. R., 1962, "Graphic Granite from the Ramona Pegmatite District, California," Am. Min., Vol. 47, pp. 1123-1138. Turner, F. J. and Verhoogen, J., 1960, Igneous and Metamorphic Petrology. New York: McGraw-Hill Book Co. APPENDIX Procedure for UncoVering Thin Sections im Preparation for Staining In most cases, one will want to uncover only half of a thin section, so that part of the slide may be re- tained for comparison purposes, or for the study of tex- tures or structures which might possibly be obscurred by the stain. Apparatus: ice box (or an inhospitable climate) diamond pencil straight edge acetone lint-free cloth or paper razor blade masking tape Procedure: 1. Freeze the thin section, by placing it outdoors if it is cold enough, or in an ice box. Fifteen minutes should be sufficient, but longer freezing, even for days, apparently will not hinder the preparation of the sec- tion. 2. Using a diamond pencil and a straight edge, make a ,groove across the cover_glass on the frozen thin section. 3. Insert the edge of a razor blade (at a low angle) under the edge of the cover glass which is most nearly parallel to the groove which has been made. 46 47 4. Pry up the edge of the cover_glass using a qmigk flipping motion. The portion of the cover glass to be removed will (hOpefully) fly off in one piece. 5. If part of the cover glass does not come free, re- peat the action with the razor blade on another free edge of the cover_glass until all of the glass is com- pletely removed. 6. When the desired portion of the cover glass has been completely removed, the rock chip will still be covered by the mounting medium. This may be removed by washing the slide with acetone, and a slight amount of rubbing with a lint-free cloth or paper. Now the rock chip it- self is exposed. 7. Assuming that only part of the cover glass has been removed, it is necessary to cover the remaining portion with masking tape (or something similar) to prevent the remaining portion of cover glass from being frosted by the hydrofluoric acid. 8. It may also be convenient to form a handle for the thin section by placing a piece of folded masking tape on the back of the thin section. 48 Selective Stainimgrgf Potassium Feldspar and Plagioclase im Thin Sections* Reagents and Apparatus: Hydrofluoric acid, concentrated, 52% HF, Caution: HF can cause painful burns. Barium chloride solution, 5%. Sodium cobaltinitrite solution, saturated. Rhodizonate reagent. Dissolve 0.05 grams of rho- dizonate acid potassium salt in 20 milliliters of distilled water. Make fresh in a small dropping bottle, as the reagent solution is unstable. Etching vessels. Plastic vessels of about 3/4 inches in depth and various diameters slightly less than those of the specimens placed in them for etching. Plastic cover vessels, large enough to cover the etching vessels and specimens placed on them for etching. Ribbed vessel to hold the cobaltinitrite solution. Plastic or paraffin etching vessel to fit thin sections. Procedure: 1. Etch the rock surface by leaving it face down for only ten seconds over hydrofluoric acid at room tempera- ture. Note: Rinsing the slide after etching causes the stains to be uneven. 2. Immerse the slide in the saturated sodium cobalti- nitrite solution for 15 seconds. The potassium feldspar is evenly stained light yellow. 3. Rinse the slide briefly in tap water to remove all of the cobaltinitrite. *From: E. H. Bailey and R. H. Stevens, 1960, "Selective Staining of K-Feldspar and Plagioclase on Rock Slabs and Thin Sections," Am. Min., Vol. 45, p. 1020. 49 4. Dip the slide quickly in and out of the barium chlor- ide solution. 5. Rinse the slide briefly with tap water and then with distilled water. 6. Cover the rock surface with the rhodizonate reagent from the dr0pping bottle. When the plagioclase feldspar has become pink, rinse the slide in tap water. 7. Allow the slide to dry and cover it in the usual way. In the stained thin sections under the microscope, the potassium feldspar can be seen to be stained a pale yellow and the plagioclase pink.** The mineral borders may be outlined by tiny spots of amber red, which seem to be a reaction product of rhodizonate with residual barium chloride left in the cracks between the mineral grains. This defect is not sufficient to interfere with study of the thin section. It was thought that longer washing after the barium chloride treatment would eliminate these amber spots, but after three minutes in tap water the potassium feldspar was also stained red by the rhodizonate. _A v ** Pure albite does not become stained by the treatment with barium ion and rhodizonate, but albite with calcium corresponding to only 3 percent of anorthite was stained red. Apparently sodium in the etch residue from albite is not readily replaced by barium; however, it may be replaced by potassium. After immersing etched albite in a solution of potassium chloride, it may be stained yellow with co- baltinitrite, showing that potassium ion has substituted for sodium in the etch residue. By this method plagioclase, except perhaps for nearly pure anorthite, as well as the potassium feldspar, can be stained yellow with the cobal- tinitrite. 50 Revised Staining Technique The technique described on the previous two pages is the one described by Bailey and Stevens (1960). Through a trial and error method the author found that better staining was achieved by using the following method, which is basically like that of Bailey and Ste- vens, but has a few minor differences. Reagents and Apparatus: It was found that diluting the rhodizonate reagent solution by four times (i.e., use 0.05 grams of the salt in eighty milliliters of distilled water) gives better control over the quality of the stain, since it slows down the staining process. Procedure: 1. Etch the rock surface by leaving it face down for 15-20 seconds at one-quarter inch above the hydrofluoric acid at room temperature. 2. Immerse the slide in the saturated sodium cobaltini- trite solution for 1 minute. 3. Rinse the slide briefly in distilled water to remove all of the excess cobaltinitrite. 4. Dip the slide quickly in and out of the barium chlor- ide (1 second of complete submersion is sufficient). 5. Rinse the slide briefly with distilled water. 51 6. Cover the rock surface with the rhodizonate reagent from the drOpping bottle. 7. Then, immediately dip the slide in distilled water to stop the staining. The plagioclase will be slightly pink. Repeat steps 6 and 7 until the desired shade of pink is obtained. (If the slide is not immediately rinsed after applying the rhodizonic acid for the first time, the staining will be uneven and perhaps too strong. When reapplying the rhodizonic acid, wait just a few seconds before washing the slide.) 8. Allow the slide to dry and cover it in the usual way. The major differences between this technique and that of Bailey and Stevens are l) the immersion times for various reagents; 2) the use of distilled water, wherever rinsing is called for; and 3) the repeated appli- cation of rhodizonic acid followed by immediate rinsing. Procedure for Covering Stained Thin Sections Apparatus: Permount (or some similar mounting medium) glass rod 'cover glasses pencil, with eraser end lamp with 60-watt bulb Procedure: 1. After staining section, let it dry completely. Be careful not to touch the stained portion of the slide, 52 2. In a well-ventilated area, dip the glass rod into the mounting medium and let one dr0p fall on the uncovered stained area of the slide. (Note: one dr0p will cover approximately a 22 millimeter by 22 millimeter area.) 3. Take a cover glass and hold it at a 45° angle to the thin section with one end of the cover glass resting on the section. Let the other end of the cover glass fall slowly, spreading the drop of mounting medium beneath it, so that it now completely covers the rock chip. 4. With the eraser end of a pencil, press lightly on the cover glass, so that the mounting medium spreads evenly over the rock chip. 5. Place the covered thin section under a gentle heat source for about 12-15 hours. (A lamp with a 60-watt bulb placed 12 inches away should be sufficient.) Note: over- heating may cause bubbles to form in the mounting medium. 6. After the mounting medium has hardened, excess medium on the slide may be scraped off, and the slide may be cleaned with an apprOpriate solvent. "ImmmS