The Implications of Porphyroblast Fabric Obliquity on the Timing of Deformation and Metamorphism at Coos Canyon, Maine

Transcription

1 The Implications of Porphyroblast Fabric Obliquity on the Timing of Deformation and Metamorphism at Coos Canyon, Maine Lauren Steely Advisor: Dr. Michael Brown GEOL 394

2 PORPHYROBLAST FABRIC OBLIQUITY AT COOS CANYON, MAINE 1 Abstract A new suite of samples from Coos Canyon, Maine, provides evidence for late syntectonic metamorphism of the Devonian Perry Mountain Formation. Porphyroblast growth in western Maine metapelites has traditionally been considered to be post-tectonic. This theory was tested at one locality via microstructural analysis of a suite of mica schists from the Coos Canyon outcrop near Byron, Maine. Porphyroblast matrix fabric obliquities were the principal tool used to determine the relative timing of metamorphism and deformation. Because porphyroblast inclusion fabrics are interpreted to record relict foliation, it has been suggested that the angle that they form with the modern matrix fabric is qualitatively related to the amount of matrix strain accommodated during or after porphyroblast growth. Solar & Brown (1999) have studied such fabric obliquities in rocks over a km-scale study area between the Mooselookmeguntic and Lexington plutons and have proposed that the obliquities developed due to matrix flattening (pure shear) as regional folds tightened. According to their model, fabric obliquities in western Maine are indicative of Acadian deformation outlasting porphyroblast growth. The present investigation suggests that this model is partially applicable to folding at the outcrop scale, based on fabric analysis of a highresolution suite of samples from a continuous strip of outcrop. 45 oriented samples were collected from a traverse spanning 132 meters across the strike of the Perry Mountain Formation at Coos Canyon. Fabric obliquities in staurolite and garnet porphyroblasts were measured in lineationparallel thin sections from each of the 45 samples. The resulting data set shows statistically significant variation in obliquity magnitude between staurolite and garnet populations, and between porphyroblasts in mica- vs. quartz-dominated mineralogical domains. The data suggest that both garnet and staurolite were late-syntectonic, but staurolite growth commenced after garnet growth and likely outlasted the tail end of regional deformation. Porphyroblasts in different mineralogical domains may have had different angular velocities with respect to the matrix foliation plane due to differences in the competency of the domains. Finally, all populations of porphyroblasts show the same sense of obliquity regardless of location on the outcrop, contrary to what is predicted by Solar and Brown s fold tightening model of fabric obliquity formation.

3 PORPHYROBLAST FABRIC OBLIQUITY AT COOS CANYON, MAINE 2 Table of Contents I. INTRODUCTION REGIONAL GEOLOGY OF WESTERN MAINE LITHOLOGY AND STRUCTURE OF COOS CANYON TIMING OF PLUTONISM AND METAMORPHISM KINEMATICS OF FABRIC OBLIQUITY FORMATION II. OBJECTIVES OF RESEARCH III. METHOD OF ANALYSIS FIELD WORK SAMPLE PREPARATION DATA COLLECTION ANALYSIS OF ERROR IV. DATA AND OBSERVATIONS FIELD DATA MICROSTRUCTURAL OBSERVATIONS V. GEOCHEMICAL DATA VI. DISCUSSION OF RESULTS FABRIC OBLIQUITY DATA STRAIN SHADOWS LACK OF VARIATION ACROSS THE OUTCROP THERMOCHRONOLOGY VII. DIRECTIONS FOR FUTURE WORK THERMOCHRONOLOGICAL WORK ON INCLUDED MONZONITE TO CONSTRAIN THE P-T-T PATH SERIAL SECTIONING TO DETERMINE THE GARNET ROTATION AXES IN THREE DIMENSIONS VIII. CONCLUSIONS REGIONAL DEFORMATION OUTLASTED METAMORPHISM AT COOS CANYON PORPHYROBLAST GROWTH MAY HAVE BEEN INTERKINEMATIC STAUROLITE GROWTH COMMENCED AFTER THE INITIATION OF GARNET GROWTH NO APPARENT CORRELATION BETWEEN FOLDING AT THE OUTCROP SCALE AND OBSERVED FABRIC OBLIQUITIES 28 IX. ACKNOWLEDGEMENTS X. REFERENCES XI. APPENDIX I: PORPHYROBLAST DATA XII. APPENDIX II: TABULATED FIELD DATA XIII. APPENDIX III: BASELINE S E MEASUREMENTS XIV. APPENDIX IV: SI ERROR ANALYSIS... 37

4 PORPHYROBLAST FABRIC OBLIQUITY AT COOS CANYON, MAINE 3 List of Figures and Tables Figure 1 Geological map of New England showing the juxtaposition of terranes that accreted to North America during the Appalachian orogenies (adapted from Brown and Solar, 1998). The study area lies within the Central Maine Belt (CMB), a reverse-dextral shear zone that was activated by the early Devonian collision of the Avalon terrane (ACT) with North America Figure 2 Geological map of the area surrounding Coos Canyon, western Maine (adapted from Brown and Solar, 1998). Metasedimentary rocks of the Rangeley Stratigraphic Sequence (Moench, 1971), including the Perry Mountain Formation, are shown in green and purple. Note that plutons are highly discordant to the stratigraphy. Coos Canyon sits in a narrow belt of country rock between the ~366 Ma Mooselookmeguntic pluton and a zone of anatectic migmatites Figure 3 Aerial photo of Coos Canyon, an outcrop along the Swift River near Byron, ME that is easily accessible from State Route 17. The red lines indicate the approximate extent of the sample transect, which is normal to the strike of the foliation and compositional layering. Station #1 is located at the far east end, directly under the centerline of the bridge, and station numbers increase to the west. The transect was offset at the west end to include a section of rocks farther upstream Figure 4 A view to the west down Coos Canyon, showing the Devonian Perry Mountain formation. The rocks are garnet-staurolite-mica schists that have weathered into slabs parallel to the subvertical compositional layering Figure 5 Interbedded pelitic (dark) and psammitic (light) layers in outcrop. Note the visibly abundant light-brown staurolite crystals, which occur exclusively in the more aluminous pelitic layers. Scale in inches Figure 6 An along-strike view of the south wall of Coos Canyon. Evidence for high strain visible in this photo includes: tight folds of the layering visible on the left; a boudinaged pegmatite dike; and parallelism of all structures, including compositional layering, foliation, dikes, and fold axial planes Figure 7 Soft-sediment deformation structures have been preserved in the psammitic layers. No such structures are observed in the pelitic layers. Scale in inches Figure 8 Outcrop view of a foliation-parallel surface, showing the well-developed biotite lineation which plunges steeply to the northeast, parallel to the regional mineral elongation lineation. Note that staurolite long axes are oblique to the lineation. Scale in inches Figure 9 One of the many tight, cm- to m-scale folds of the compositional layering that occur in outcrop. This is Fold C, between field stations 32 and Figure 10 Three end-member models for the development of folds in layered materials (adapted from Williams and Jiang, 1999). Pure shear works by magnifying pre-existing undulations in the layers. Flexural flow involves slip between layers and flow within layers; thus, the layers are subject to a component of non-coaxial deformation as well. Slip folding utilizes slip planes at a high angle to the layering, and is the only mechanism shown that does not result in layer-parallel shortening. Actual geologic folds may form via a combination of these mechanisms, depending on the stress regime, the relative rheologies and competencies of the individual layers, and the geologic environment Figure 11 A small pegmatite dyke intruded across the metapelitic layers at Coos Canyon shows evidence for 1) deformation by simple shear, 2) a dextral sense of shear, and 3) strain

5 PORPHYROBLAST FABRIC OBLIQUITY AT COOS CANYON, MAINE 4 partitioning between the compositional layering. The thick psammitic layer in the center of the photo experienced relatively little strain compared with the thinner interbedded pelitic and psammitic layers on the top and bottom, where the limbs of the pegmatite S-fold have been rotated into parallelism with the layering Figure 12 A view of the north rim of Coos Canyon showing 10 of the 45 sampling sites, presented to show the sampling resolution and cross-strike geometry of the traverse Figure 13 Diagram illustrating the two mutually-perpendicular thin sections that were cut from each sample. Both sections are perpendicular to the foliation plane. The biotite lineation is visible on the foliation-parallel surface, taken from a photograph of an actual sample surface Figure 14 Equal-area stereonet showing structural data collected at 45 stations along the 132- meter traverse of Coos Canyon. Lineation data is contoured using the 1% area method; contour interval = 9. The data show a consistent NNE-trending subvertical foliation, and a biotite lineation plunging steeply to the NE. The data is tightly clustered and shows little variation across the length of the outcrop Figure 15 Boundary between a Q-domain (left) and a P-domain (right) in lineation-parallel view, showing the difference in distribution of biotite and staurolite between the two. Staurolite is largely restricted to the more aluminous P-domains. Note the abundance of assymmetric biotite fish in the P domain. (PPL, long dimension is 15 mm) Figure 16 Close-up of Figure 14. Typical view of a Q domain in thin section, showing the biotite lineation and garnet porphyroblasts. As seen here, garnets throughout a single thin section generally exhibit fabric obliquities of similar magnitude and always in the clockwise sense. (Lineation-parallel view, lineation plunges toward the top, PPL, long dimension is 5 mm). 18 Figure 17 Garnet and staurolite porphyroblasts from a P-domain in lineation-perpendicular view. In this view, both the matrix and inclusion fabrics are poorly defined. (XPL, long dimension is 4 mm) Figure 18 Garnets in a Q domain showing weakly sigmoidal inclusion trails and discontinuous fabric obliquities up to 26 o in the core. The matrix foliation is deflected around the garnets, though it is cut by a thin inclusion-free overgrowth on the larger garnet. (PPL view from section 00-6B, lineation-parallel view. Diameter of large garnet is ~2 mm.) Figure 19 Microprobe scans of the euhedral garnet from Figure 18 (left) and a euhedral garnet from thin section (right). Scans were made using the University of Maryland s JEOL electron probe microanalyser in WDS mode. Clockwise from top-left: scans of Ca, Mg, Mn, and Fe. Note the thin Ca-depleted rims on both garnets, corresponding to the inclusion-free overgrowths seen in thin section Figure 20 Rose plots of fabric obliquity magnitudes (S i -S e ) from all lineation-parallel thin sections, divided into populations according to porphyroblast type and mineralogical domain. North in all plots represents the orientation of the matrix foliation, S e, when viewed down the plunge of the biotite lineation. Data is binned into sections 2 degrees wide. The radial scale in percentage units varies. Arrows indicate mean values for each population Figure 21 Pie charts showing the proportion of each type of porphyroblast with clockwise obliquities (S i - S e > 0), anticlockwise obliquities (S i - S e < 0), and no measurable obliquities (S i - S e = 0). Data is from all lineation-parallel thin sections Figure 22 A profile across Coos Canyon from east to west, showing the mean fabric obliquity for each lineation-parallel thin section plotted versus the cross-strike distance of the associated

6 PORPHYROBLAST FABRIC OBLIQUITY AT COOS CANYON, MAINE 5 sample from the starting point of the traverse. The locations of the three fold noses identified in the field are shown in green

7 PORPHYROBLAST FABRIC OBLIQUITY AT COOS CANYON, MAINE 6 I. Introduction Western Maine is a well-studied example of a high-t, low-p metamorphic belt. The traditional view of metamorphism in this region is that it was driven by post-tectonic episodes of plutonism (De Yoreo et. al., 1989). However, the timing of metamorphism, deformation, and plutonism in western Maine has become a source of controversy over the last 15 years. This paper describes an investigation into one aspect of this controversy by means of microstructural analysis of a suite of rocks collected from a single locality in western Maine. 1. REGIONAL GEOLOGY OF WESTERN MAINE Western Maine is characterized by kilometer-scale plutons of granodiorite, leucogranite, and gabbro that are broadly elliptical in map view, surrounded by metasedimentary sequences that were metamorphosed and deformed during the Taconic and Acadian orogenic phases (Figure 1). The dominant structure in Western Maine is the Central Maine Belt (CMB), a northeaststriking, kilometer-scale shear zone that was last active during the Acadian orogeny. This orogeny is thought to have reached its peak in Maine between 400 and 405 Ma, in the Early Devonian (Smith and Barreiro, 1990; Johnson, T.E., et al., 2003). The CMB formed in response to the transpressive collision of the Avalon terrane with eastern North America, producing reversedextral kinematics along the shear zone; the resulting deformation is recorded at all scales in the country rocks. De Yoreo et al. and others have shown that the paleo-depth exposed by the present erosion surface increases along strike to the southwest. Metamorphic grade increases in the same direction, from greenschist to upper amphibolite facies rocks, with migmatites occurring in discreet domains (Solar and Brown, 1999). The specific area of concern for this study is a region roughly bounded by the Mooselookmeguntic, Reddington, and Lexington plutons, near the towns of Rangeley, Weld, and Rumsford (Figure 2). Within this region, Solar and Brown identified alternating km-scale belts of metasedimentary host rocks that are characterized by differences in grain-shape fabric and structural style (Solar and Brown, 1999). Two distinct structural zones were identified: belts of tight, sub-parallel folds characterized by S>L fabrics (oblate strain ellipsoid); and belts of more Figure 1 Geological map of New England showing the juxtaposition of terranes that accreted to North America during the Appalachian orogenies (adapted from Brown and Solar, 1998). The study area lies within the Central Maine Belt (CMB), a reverse-dextral shear zone that was activated by the early Devonian collision of the Avalon terrane (ACT) with North America.

8 PORPHYROBLAST FABRIC OBLIQUITY AT COOS CANYON, MAINE 7 open, variably-oriented folds characterized by L>S fabrics (prolate strain ellipsoid) (Solar and Brown, 2001). Brown and Solar interpreted these zones to be the result of crustal-scale deformation partitioning during synchronous regional metamorphism, transpressive regional deformation, and plutonism (Brown and Solar, 2001). At outcrop scale, metasedimentary rocks from both structural regimes exhibit both a subvertical foliation that strikes subparallel to the strike of the CMB, and a moderatelyto steeply-plunging mineral elongation lineation that also trends subparallel to the CMB (Solar and Brown, 1999). The foliation is better developed and more consistent in orientation in the S>L zones. The main feature of the S>L zones is a parallelism of structures at all scales: stratigraphic contacts and fold hingelines at map-scale; compositional layering and fold hingelines at outcrop scale; and the penetrative foliation and mineral elongation lineation apparent in thin section. Asymmetric shear structures also exist at all scales, and these always indicate a dextral sense of displacement, consistent with the reverse-dextral motion along the CMB. 2. LITHOLOGY AND STRUCTURE OF COOS CANYON Figure 2 Geological map of the area surrounding Coos Canyon, western Maine (adapted from Brown and Solar, 1998). Metasedimentary rocks of the Rangeley Stratigraphic Sequence (Moench, 1971), including the Perry Mountain Formation, are shown in green and purple. Note that plutons are highly discordant to the stratigraphy. Coos Canyon sits in a narrow belt of country rock between the ~366 Ma Mooselookmeguntic pluton and a zone of anatectic migmatites. For this study, samples were collected from Coos Canyon, a dramatic outcrop of the Devonian Perry Mountain Formation located in one of the tight (S>L) fold belts described by Solar and Brown, 2001 (Figure 3, Figure 4). As with most of the outcrops in this area, Coos Canyon is exposed along the course cut by the Swift River. The Perry Mountain Formation is part of the Rangeley Stratigraphic Sequence, a group of metasedimentary country rocks first defined by Moench (1971). Perry Mountain is an upperamphibolite grade garnet-staurolite-mica schist consisting of centimeter-scale interbedded pelitic and psammitic layers (Figure 5) that are interpreted to be the relict bedding (ibid.). The layers are subvertical and strike approximately parallel to the strike of the CMB in the region, suggesting that they have been rotated into alignment with the shear zone and have experienced qualitatively high strain. Remarkably, sedimentary features such as graded bedding, soft-sediment deformation

9 PORPHYROBLAST FABRIC OBLIQUITY AT COOS CANYON, MAINE 8 structures, and even large tabular cross-beds are still preserved in the psammitic layers and together suggest a shallow marine protolith, possibly a turbidite deposit. These structures are not observed in the pelitic layers, suggestion that strain may have partitioned preferentially into these weaker layers, obliterating any relict structures (Figure 6). Extensive petrologic mapping in this area has shown that Coos Canyon lies in a narrow transition zone in which metamorphic grade increases from garnet to upper-sillimanite grade as one moves south, over a distance of five kilometers (Guidotti, 1974; Brown and Solar, 1998). Coos Canyon is located in the staurolite zone, and staurolite porphyroblasts are visibly abundant in the pelitic layers of the outcrop. Garnet and muscovite pseudomorphs after andalucite also occur in these layers. The phase assemblage corresponds to P-T conditions of about 3.5 kbar (12.3 km) and 500 o C (De Yoreo et al., 1989). South of Coos Canyon, metamorphic grade increases abruptly to lower- and uppersillimanite zones, and migmatites occur just two kilometers to the southeast. N The meso- and microscopic structures observed at Coos Canyon mimic larger structures at the regional scale, revealing a similarity of structures at all scales (Figure 7). The rocks exhibit a well-developed subsolidus foliation defined by biotite and muscovite which is parallel to the compositional layering and to the general strike of the CMB in turn. A well-developed biotite lineation plunges steeply (Figure 8), parallel to the regional mineral elongation lineation (RMEL). Tight, centimeter- to meter-scale folds of the layering and foliation are visible in outcrop (Figure 9). The hingelines of these folds are subparallel to the RMEL. Staurolite long axes lie within the plane of foliation and show only a weak statistical orientation parallel to the RMEL (Solar and Brown, 1999). 3. TIMING OF PLUTONISM AND METAMORPHISM Figure 3 Aerial photo of Coos Canyon, an outcrop along the Swift River near Byron, ME that is easily accessible from State Route 17. The red lines indicate the approximate extent of the sample transect, which is normal to the strike of the foliation and compositional layering. Station #1 is located at the far east end, directly under the centerline of the bridge, and station numbers increase to the west. The transect was offset at the west end to include a section of rocks farther upstream. Plutonism in western Maine was episodic and was associated with crustal thickening from the accretion of the Bronson Hill and Avalon terranes to Laurentia during the Taconic and Acadian orogenies, respectively (Bradley, 1983). Three discreet episodes of pluton emplacement are recognized at 450 Ma, Ma, and 330 Ma (De Yoreo et. al., 1989).

10 PORPHYROBLAST FABRIC OBLIQUITY AT COOS CANYON, MAINE 9 Figure 4 A view to the west down Coos Canyon, showing the Devonian Perry Mountain formation. The rocks are garnet-staurolite-mica schists that have weathered into slabs parallel to the subvertical compositional layering. Figure 5 Interbedded pelitic (dark) and psammitic (light) layers in outcrop. Note the visibly abundant light-brown staurolite crystals, which occur exclusively in the more aluminous pelitic layers. Scale in inches. Figure 6 An along-strike view of the south wall of Coos Canyon. Evidence for high strain visible in this photo includes: tight folds of the layering visible on the left; a boudinaged pegmatite dike; and parallelism of all structures, including compositional layering, foliation, dikes, and fold axial planes. Figure 7 Soft-sediment deformation structures have been preserved in the psammitic layers. No such structures are observed in the pelitic layers. Scale in inches. Metamorphism in this region appears to be a direct result of these plutonic episodes. A plutondriven model of metamorphism is supported by the distribution of metamorphic isograds in the region, which show a broad correlation with pluton geometry (Solar and Brown, 1999, Figure 4) but can extend several kilometers beyond pluton margins, suggesting that the intrusions may have been more expansive above or below the present level of erosion (Johnson, et. al., 2011). This observation led Holdaway to describe the western Maine region as an example of regional con-

11 PORPHYROBLAST FABRIC OBLIQUITY AT COOS CANYON, MAINE 10 tact metamorphism (Holdaway, 1982). In the case of Coos Canyon, at least two phases of metamorphism are recognized. A regional phase associated with the tail end of Acadian folding and metamorphism has been dated to Ma, corresponding with an intense period of plutonism in western Maine (Smith and Barreiro, 1990). A later phase is associated with the emplacement of the nearby Mooselookmeguntic Pluton, which U-Pb monzonite dating indicates occurred in two main stages between Ma (Solar, et. al, 1998). Work done in the 1980s suggested that plutonism and metamorphism were post-tectonic with respect to the deformation recorded by the CMB. De Yoreo et al. (1988) used 40 Ar/ 39 Ar blocking temperatures in biotite and muscovite from plutonic rocks to constrain the temperature versus time path of plutons in western Maine. From these data, they determined that the plutons were emplaced in much cooler rocks, because the plutons appear to have cooled rapidly upon emplacement. Thus, they concluded that plutonism and its resultant metamorphism occurred some time after the main episode of deformation, following significant exhumation and cooling of the country rocks. Lending further support to this hypothesis is the fact that in map view, the plutons cut across structural domains (Figure 2). More recently, Solar and Brown have contested the post-tectonic hypothesis, asserting that metamorphism, deformation, and plutonism occurred contemporaneously via a feedback relationship between crustal anatexis, transpressional shear, and melt transport (Brown and Solar, 1998; Solar and Brown, 1999). Much of their evidence comes from microstructural studies of metasedimentary country rocks collected from the area between the Mooselookmeguntic and Lexington plutons. One of their principal pieces of evidence was the observation that inclusions in garnet and staurolite porphyroblasts form sub-planar fabrics (S i ) that are discordant with the dominant matrix foliation (S e ). These fabric obliquities are commonly interpreted to record deformation in the matrix during, or between periods of, porphyroblast growth ( syntectonic or inter-tectonic fabrics in the sense of Passchier et. al., 1992). Solar and Brown argued that the fabric obliquities Figure 8 Outcrop view of a foliation-parallel surface, showing the well-developed biotite lineation which plunges steeply to the northeast, parallel to the regional mineral elongation lineation. Note that staurolite long axes are oblique to the lineation. Scale in inches. Figure 9 One of the many tight, cm- to m-scale folds of the compositional layering that occur in outcrop. This is Fold C, between field stations 32 and 33.

12 PORPHYROBLAST FABRIC OBLIQUITY AT COOS CANYON, MAINE 11 formed during non-coaxial tightening of folds, in which the matrix foliation on the limbs of the folds rotated passively as porphyroblasts continued to grow (Figure 10d). From this they concluded that porphyroblast growth, and therefore, metamorphism, was syntectonic. Guidotti challenged this hypothesis, arguing that the fabric obliquities could be explained by helicitic overprinting of a pre-existing crenulation cleavage (Guidotti, 2000; see Bell, 1985 and Bell and Forde, 1995 for theoretical discussion of this process). In this model, garnet and staurolite growth was therefore static and post-tectonic. Guidotti identified at least two episodes of post-tectonic metamorphism, both of which he associated with the Ma episode of plutonism (the M2 and M3, of De Yoreo et. al., 1988). 4. KINEMATICS OF FABRIC OBLIQUITY FORMATION Porphyroblast matrix fabric obliquities are observed in metamorphic terranes worldwide and have been variously interpreted, sometimes controversially (Bell, 1985; Passchier et. al., 1992; Bell and Forde, 1995; Solar and Brown, 1999; Guidotti, 2000; Williams and Jiang, 1999). To model the formation of fabric obliquities within folded rocks, the relative rotation of fold limbs, porphyroblasts, and matrix foliation during folding must all be considered. Therefore, it is necessary to study the kinematics of folding in order to determine what properties of the rock govern these rotation rates. A major complication is that the rocks in which porphyroblasts grow are rarely homogenous. The Perry Mountain Formation, for example, consists of alternating bands of relatively competent psammitic layers and relatively weak pelitic layers. During folding, strain partitions preferentially into the weaker layers. Fold kinematics in heterogeneous layered materials can be described by a combination of four end-member models: flexural flow/flexural slip, pure shear (flattening), slip folding, and longitudinal strain (Figure 10; see Ramsay, 1967 for an overview of the various models). Each model predicts different relative rates of rotation of layering, matrix foliation, and porphyroblasts, and therefore different observed porphyroblast-matrix fabric obliquities. Figure 10 Three end-member models for the development of folds in layered materials (adapted from Williams and Jiang, 1999). Pure shear works by magnifying pre-existing undulations in the layers. Flexural flow involves slip between layers and flow within layers; thus, the layers are subject to a component of non-coaxial deformation as well. Slip folding utilizes slip planes at a high angle to the layering, and is the only mechanism shown that does not result in layer-parallel shortening. Actual geologic folds may form via a combination of these mechanisms, depending on the stress regime, the relative rheologies and competencies of the individual layers, and the geologic environment.

13 PORPHYROBLAST FABRIC OBLIQUITY AT COOS CANYON, MAINE 12 Jiang (2001) considered folding kinematics in multi-layer folds during layer-parallel shortening. He assumed that folding is accommodated by a combination of pure shear and flexural flow, and that the relative importance of each mechanism varies as the fold tightens. From continuum-mechanical relationships, he related the finite angular rotation of porphyroblasts to the dip of the fold limb, the competancy history of the layer containing the porphyroblast, the relative proportion of flexural flow and pure shear, and the fold curvature distribution (chevron vs. circular folds). He theorized that for rigid, spherical porphyroblasts in a relatively incompetent layer (for example, garnets in pelitic layers at Coos Canyon), inclusion fabrics from porphyroblasts in the limbs of a fold should define a subdued fold with a similar geometry as the host fold. To an observer, it would appear as though the porphyroblasts had rotated toward the host fold s axial plane. Yet Solar and Brown predict the opposite: inclusion fabrics which describe a mirror image of the host fold, in Figure 11 A small pegmatite dyke intruded across the metapelitic layers at Coos Canyon shows evidence for 1) deformation by simple shear, 2) a dextral sense of shear, and 3) strain partitioning between the compositional layering. The thick psammitic layer in the center of the photo experienced relatively little strain compared with the thinner interbedded pelitic and psammitic layers on the top and bottom, where the limbs of the pegmatite S-fold have been rotated into parallelism with the layering. which the porphyroblasts appear to have rotated away from the host fold s axial plane (Solar and Brown, 1999, Figure 12c). Why the discrepancy between these two models? Jiang s model assumes that folding occurs in response to layer parallel shortening, and that only the pure shear and flexural flow components operate. However, given that folding at Coos Canyon occurred within the transpressional CMB shear zone, it is possible that much of the folding could have been accommodated by slip-fold kinematics, in which some component of folding was not due to layer-parallel shortening (D. Jiang, personal communication). Indeed, the many asymmetric mesostructures (Figure 11) and microstructures (biotite fish and asymmetric strain shadows) in evidence at Coos Canyon indicate that these rocks experienced significant dextral shear that may have activated slip folding along foliation planes. Nonetheless, both models make predictions that can be tested at Coos Canyon: 1. Both models predict the existence of porphyroblast-matrix fabric obliquities, but only if deformation outlasted porphyroblast growth. 2. Both models predict that the magnitude of the observed fabric obliquities will be greater for porphyroblasts that started growing earlier. 3. Both models predict that the sense of the observed obliquities (clockwise or anticlockwise) will be reversed in porphyroblasts on opposite limbs of the host fold.

14 PORPHYROBLAST FABRIC OBLIQUITY AT COOS CANYON, MAINE 13 II. Objectives of Research The most basic purpose of this study was to investigate whether deformation outlasted porphyroblast growth at Coos Canyon. Based on my observations in the field during two summers working in western Maine, as well as the emerging research from Solar and Brown, I hypothesized at the outset that deformation did in fact outlast porphyroblast growth. This hypothesis was tested by observation, in thin section, of microstructures that are diagnostic of syntectonic porphyroblast growth: strain shadows, foliation that is wrapped around porphyroblasts, and porphyroblast-matrix fabric obliquities (Prediction #1 above). Particular emphasis was placed on the porphyroblast-matrix relationships because they can be quantified and thus allow for a rigorous statistical investigation of deformation history and timing. A secondary goal of this research was to test the model of Solar and Brown (1999), which hypothesized that the porphyroblast fabric obliquities seen in western Maine are the result of noncoaxial tightening of regional folds. I tested this model in two ways. Their model predicts that the magnitude of observed fabric obliquities will be greater in porphyroblasts that began growing earlier (Prediction #2 above). Because garnet precedes staurolite in the prograde P-T-t metamorphic path represented by the rocks in Coos Canyon, I hypothesized that the mean obliquity of the population of garnet porphyroblasts should be greater than the mean obliquity of the population of staurolite porphyroblasts, and they should form statistically distinct populations. The model also predicts that the sense of the observed obliquities will be reversed in porphyroblasts on opposite limbs of host folds (Prediction #3 above). The only way to conclusively identify folds at outcrop scale is to look for the fold noses, three of which I located in the course of my traverse across the north wall of the canyon (an experienced sedimentologist might be able to identify folds by locating reversals in the orientation of graded bedding, but I was not confident in my ability to do this). When I located these fold noses I took samples from opposite limbs. There are undoubtedly many other folds included in the traverse whose noses are not visible. If the Solar and Brown prediction is correct, it should be possible to identify these folds by reversals in the sense of fabric obliquity, when measured in oriented thin sections. Solar and Brown studied microstructures in rocks over a kilometer-scale study area. This investigation has attempted to determine whether their model holds for folding at the outcrop scale, using a higher-resolution suite of samples from a single continuous strip of outcrop. Finally, I hoped to be able to say something about the kinematic mechanism(s) by which folding occurred at Coos Canyon, based on the theoretical work of Jiang. III. Method of Analysis The above problems were addressed by examining microstructures in a suite of rocks from Coos Canyon near Byron, Maine. Coos Canyon was chosen for several reasons: 1. It is ideally located within both a staurolite zone (where there are abundant porphyroblasts for study) and one of Brown and Solar s high strain belts, yet just outside of the deformation aureole of the Mooselookmeguntic pluton (in Zone 1 of Johnson, et al., 2006), so that any deformation observed can be attributed to solely to regional deformation and not the emplacement of the pluton;

15 PORPHYROBLAST FABRIC OBLIQUITY AT COOS CANYON, MAINE Its rocks are lithologically representative of metapelites of the Rangeley Stratigraphic Sequence; 3. Its rocks exhibit strong compositional layering, which makes it possible to test various theories of fold kinematics and strain partitioning; 4. It contains numerous structures that indicate qualitatively high strain, including numerous outcrop-scale folds and a well-developed foliation and lineation that align with the regional structures, making it a good locality for comparing the effects of folding at a variety of scales; 5. The easy access and extent of exposure along the canyon walls lends itself to a traverse across the strike of the rocks, making it possible to collect a continuous suite of samples with high spatial resolution (Figure 12). This study can be divided into three stages: collection of samples in the field, preparation of samples for thin sectioning, and measurement of fabric obliquities in thin section. 1. FIELD WORK All field work was completed during the period of August 21-25, I collected a suite of forty-five oriented samples from a cross-strike traverse spanning 132 meters across the north rim of Coos Canyon, starting at the bridge and working upstream (Figure 3, Figure 12). The mean sampling resolution was 3.0 meters, and the individual sampling sites were largely determined by where I could extract a suitably large sample with visibly abundant staurolites. I chose samples that had at least one surface parallel to the foliation plane; this made it simpler to orient the samples and was easily accomplished because the rocks have weathered preferentially into slabs that are parallel to the foliation. Figure 12 A view of the north rim of Coos Canyon showing 10 of the 45 sampling sites, presented to show the sampling resolution and cross-strike geometry of the traverse. At each site, I measured the strike and dip of the in situ foliation-parallel sample surface using a Brunton compass, and I marked strike lines and dip lines on each sample for orientation purposes. I also measured the pitch of the biotite elongation lineation within the foliation plane. To account for random error, I took each strike, dip, and pitch measurement three times in different places along each surface and averaged the values. I then extracted the samples using an airless jackhammer, and measured the cross-strike distance from the previous site with a 100-foot tape. I made a note of where visible fold hinges occur on the surface of the outcrop between sampling stations; I identified three such folds (A, B, C). The field data are presented in Appendix II.

16 PORPHYROBLAST FABRIC OBLIQUITY AT COOS CANYON, MAINE SAMPLE PREPARATION Preliminary lab work, including preparation of the samples for thin sectioning, was completed during September and October, 2000 at the thin section lab in the Department of Geology at the University of Maryland College Park. Two mutually-perpendicular slabs were cut from each sample, both of which were perpendicular to the foliation plane (Figure 13). The two slabs were oriented parallel and perpendicular to the biotite elongation lineation, respectively. I chose the directions in which to cut by visually estimating the orientation of the biotite lineation, which is well-defined on the foliation-parallel surfaces of most samples. Because the slabs were cut from oriented samples, I was able to orient the thin sections so that the observed sense of fabric obliquity would be consistent among different sections. Many authors have addressed the importance of considering porphyroblast-matrix geometries in three dimensions. Having two perpendicular sections from each sample allowed me to better study the three-dimensional geometry of the porphyroblast inclusions. Also, because most fold hinges at Coos Canyon are sub-parallel to the biotite elongation lineation, the observed fabric obliquities ought to be greatest in lineation-perpendicular section if they have developed due to fold tightening. Thus, the two views allowed conclusions to be drawn regarding the formation of fabric obliquities. 3. DATA COLLECTION The thin sections were analyzed under plane- and cross-polarized light with a petrographic microscope. For each thin section, I systematically measured at least 15 randomly-chosen garnet and staurolite porphyroblasts, recording the following characteristics: length of the longest dimension crystal habit the mineralogical domain (P or Q) in which it occurs the presence or absence of strain shadows the magnitude of any fabric obliquity between the matrix and included fabrics The thin section data is presented in Appendix I. All fabric obliquity measurements were made using the angular graduations on the microscope stage to a precision of 1 degree. White mica grains were used as an indicator of matrix foliation (S e ) orientation. Because the matrix foliation varies locally, I measured S e at nine grid locations spaced 5 mm across each section and calculated a mean value for the entire section (Appendix III). Numerous measurements of many thin sections show that the foliation rarely deviates from the nine- Figure 13 Diagram illustrating the two mutually-perpendicular thin sections that were cut from each sample. Both sections are perpendicular to the foliation plane. The biotite lineation is visible on the foliationparallel surface, taken from a photograph of an actual sample surface.

17 PORPHYROBLAST FABRIC OBLIQUITY AT COOS CANYON, MAINE 16 point mean by more than a few degrees in a given section, except where it locally wraps around porphyroblasts and biotite grains. The orientation of the included fabric (S i ) in each porphyroblast in the thin section was then measured with respect to each section s mean S e. The included fabric was typically defined by quartz and opaque inclusions in garnet (these were nearly always subparallel), and by quartz and white mica inclusions in staurolite. Porphyroblasts with no obvious inclusion patterns were not measured. To avoid measuring each porphyroblast more than once, I worked across each thin section in a grid pattern, measuring every porphyroblast within the field of view and then advancing the microscope stage to the next field of view. 4. ANALYSIS OF ERROR The measurement of S i is subjective because inclusion fabrics are not always well defined, if present at all. Neither are they always linear; many staurolites show sigmoidal or curvilinear inclusion trails. In order to achieve the best possible data set, I measured only those crystals exhibiting a clear, unambiguous fabric. In the case of sigmoidal or curvilinear trails, I measured the maximum observed obliquity, usually found at the core of the crystal. Even with these precautions, random errors are expected. To determine the random error inherent in these measurements, I measured three garnets and three staurolites five times under both low and high power objectives to estimate the repeatability of my measurements and to determine the standard deviation of subsequent measurements (Appendix IV). Staurolite measurements had a higher standard deviation (1.6 degrees) compared to garnets (0.6 degrees) due to higher variability in the orientation of S i. Because S e also varies locally within each thin section, and because accurate measurements of its orientation are needed in order to make meaningful comparisons between populations of porphyroblasts, all measured porphyroblast fabric obliquities for each thin section are referenced to a mean value of S e for the entire thin section, determined by measurement of 9 grid-spaced points spaced 5 mm apart (Appendix III). The average standard deviation of S e in lineationperpendicular thin sections was 2.3 degrees, for lineation-parallel thin sections 1.5 degrees. By summing the standard deviations for S i and S e, the error in the obliquity measurements may be calculated (Table 1). Table 1: Calculated error for individual fabric obliquity measurements Garnet Staurolite ± 2.1 o ± 3.9 o

18 PORPHYROBLAST FABRIC OBLIQUITY AT COOS CANYON, MAINE 17 IV. Data and Observations 1. FIELD DATA The foliation and lineation data from the field traverse are plotted in Figure 14. They reveal that the structures at Coos Canyon parallel the larger regional structures of the Central Main Belt. The matrix foliation trends north-northeast, parallel to the stratigraphic contacts of the Perry Mountain Formation seen in map view (shown in Figure 2), with a sub-vertical dip that also parallels the steep dip of the stratigraphy in cross-section (see Figure 4 of Johnson, et. al., 2011). The biotite lineation lies at an average pitch of 56 o within the foliation plane, plunging to the northeast. This lineation is sub-parallel to the regional mineral elongation lineation and may represent the direction of displacement along the CMB. The small spread of the data across 132 meters of outcrop supports Solar and Brown s hypothesis that the area around Coos Canyon experienced a qualitatively high amount of strain that rotated all structures into close parallelism. In particular, the tight clustering of the lineation data gives confidence that fabric obliquity measurements can be compared meaningfully across the thin sections, which were cut parallel and perpendicular to this lineation. N Foliation (n = 50) Pole to foliation Biotite elongation lineation (n = 51) Figure 14 Equal-area stereonet showing structural data collected at 45 stations along the 132-meter traverse of Coos Canyon. Lineation data is contoured using the 1% area method; contour interval = 9. The data show a consistent NNE-trending subvertical foliation, and a biotite lineation plunging steeply to the NE. The data is tightly clustered and shows little variation across the length of the outcrop.

19 PORPHYROBLAST FABRIC OBLIQUITY AT COOS CANYON, MAINE MICROSTRUCTURAL OBSERVATIONS The phase assemblage of all sections is Qtz + Bt + white mica + St + Gt + And + opaque phases. Guidotti (2000) noted that the opaque phases are primarily platy ilmenite, which are included in garnets and staurolites and often provide a good indication of fabric obliquity, in addition to quartz inclusions. The opaque inclusion fabrics are subparallel to the quartz inclusion fabrics. In thin section, the rock fabric is domainal, with alternating mm- to cmscale quartz-dominated ( Q ) and micadominated ( P ) layers that are parallel to the foliation. These domains probably represent fine interbedding of sand and clayey silt, respectively, in the protolith. The thicker layers correspond to the compositional layering observed at outcrop scale. The domainal fabric appears to control the distribution of staurolite, which is abundant in P-domains but rarely found in Q-domains. Staurolite crystals are even seen to truncate abruptly at domain boundaries (Figure 15). This is consistent with the higher Al content of staurolite and the greater availability of Al 2 O 3 in the micaceous P-domains. Garnet is abundant in both domains. The matrix foliation is well-defined in lineation-parallel view, where it is defined by a strong grain preferred orientation of white mica, quartz, and biotite. The foliation is typically planar but may exhibit a weakly anastamosing pattern near porphyroblasts, especially in P domains. In the Q-domains, polycrystalline quartz ribbons are common. Biotite grains often show a dextral asymmetry, forming shapes known as biotite fish. Porphyroblasts in lineation-parallel view generally exhibit well-defined inclusion trails defined by quartz and opaque minerals. In garnet the inclusion trails are Figure 15 Boundary between a Q-domain (left) and a P-domain (right) in lineation-parallel view, showing the difference in distribution of biotite and staurolite between the two. Staurolite is largely restricted to the more aluminous P-domains. Note the abundance of assymmetric biotite fish in the P domain. (PPL, long dimension is 15 mm) Figure 16 Close-up of Figure 15. Typical view of a Q domain in thin section, showing the biotite lineation and garnet porphyroblasts. As seen here, garnets throughout a single thin section generally exhibit fabric obliquities of similar magnitude and always in the clockwise sense. (Lineation-parallel view, lineation plunges toward the top, PPL, long dimension is 5 mm)

20 PORPHYROBLAST FABRIC OBLIQUITY AT COOS CANYON, MAINE 19 linear or weakly sigmoidal (Figure 18). In staurolites, they may be linear (Figure 15) or curvilinear. In garnets, they tend to be discontinuous with the surrounding matrix foliation, while in staurolites they tend to be continuous. Garnets as a population, regardless of domain, show larger fabric obliquities than staurolites. Within each thin section domain, the angle formed by the inclusion trail and the matrix foliation (the fabric obliquity) is similar from garnet to garnet (Figure 16). Many garnets and staurolites show inclusion-free rims that overprint the matrix foliation (Figure 18). In lineation-perpendicular view, the matrix foliation is less well defined and, in some thin sections, difficult to measure without resorting to statistical methods (Figure 17). In other thin sections it is present as a weak preferred orientation of biotite. Quarts grains appear more equant and quartz ribbons are less apparent. Many porphyroblasts in lineationperpendicular view do not exhibit obvious inclusion trails; the distribution of quartz and opaque inclusions appears pseudo-random (Figure 17). Because of the difficulty found in accurately measuring both S e and S i in lineation-perpendicular thin sections, only the lineation-parallel sections were used for fabric obliquity measurements. Many porphyroblasts, exhibit strain shadows, which I define as lenticular, foliation-bounded zones adjacent to a porphyroblast that are filled by polycrystalline quartz having a grain size larger than that of the surrounding matrix. Even where strain shadows are not present, the matrix foliation may be locally deflected by a porphyroblast (Figure 18). Figure 17 Garnet and staurolite porphyroblasts from a P-domain in lineation-perpendicular view. In this view, both the matrix and inclusion fabrics are poorly defined. (XPL, long dimension is 4 mm). Figure 18 Garnets in a Q domain showing weakly sigmoidal inclusion trails and discontinuous fabric obliquities up to 26 o in the core. The matrix foliation is deflected around the garnets, though it is cut by a thin inclusion-free overgrowth on the larger garnet. (PPL view from section 00-6B, lineationparallel view. Diameter of large garnet is ~2 mm.)

21 PORPHYROBLAST FABRIC OBLIQUITY AT COOS CANYON, MAINE 20 V. Geochemical Data To investigate the thermal history of the porphyroblasts, garnets from one thin section were analyzed with a JEOL electron probe microanalyser operating in wavelength dispersive spectrometer (WDS) mode. Two euhedral garnets were selected for scans of Fe, Mg, Ca, and Mn, which are shown in Figure 19. Three observations can be made about the microprobe data. First, the scans for all elements except calcium reveal continuous zoning without any sharp discontinuities, indicating that nearly all growth occurred over a single period. Second, the scans for iron and magnesium show prograde (that is, up-temperature) zoning, suggesting that temperature was rising as the garnets grew. Third, both garnets exhibit thin calcium-depleted rims about µm in width. These could represent either late-stage overgrowths or retrograde reaction rims. In the case of one garnet, the rim appears to correspond with an inclusion-free overgrowth that cuts across the matrix foliation (compare Figure 18 and Figure 19). The most likely explanation is that both the Ca depletion zone and the helicitic overgrowth are associated with the intrusion of the nearby Mooselookmeguntic Pluton ~30 million years after the main phase of regional deformation and metamorphism ceased. The question arises as to why scans of the other elements do not show similar depleted rims. It is possible that these depletion zones were obliterated via diffusive equilibration at high T, while the calcium zones were preserved because of the calcium cation s larger size and hence its lower rate of diffusion. Ca Mg Ca Mg Mn Fe Mn Fe Figure 19 Microprobe scans of the euhedral garnet from Figure 18 (left) and a euhedral garnet from thin section (right). Scans were made using the University of Maryland s JEOL electron probe microanalyser in WDS mode. Clockwise from top-left: scans of Ca, Mg, Mn, and Fe. Note the thin Ca-depleted rims on both garnets, corresponding to the inclusion-free overgrowths seen in thin section.

22 PORPHYROBLAST FABRIC OBLIQUITY AT COOS CANYON, MAINE 21 VI. Discussion of Results In the discussion that follows, I will analyze three types of data: magnitude of fabric obliquity, sense of fabric obliquity, and strain shadows (Table 2). Whenever fabric obliquity magnitudes are reported, positive values indicate a clockwise sense of apparent rotation of the porphyroblast with respect to the matrix foliation; negative values indicate an anticlockwise sense of apparent rotation. Because the thin sections were made from oriented samples, and the up side and lineation plunge directions were carefully marked on each section, it is possible to compare the sense of obliquity between all thin sections. 1. FABRIC OBLIQUITY DATA Nearly all porphyroblast inclusion fabrics in a single thin section show the same sense of obliquity with respect to the matrix (for example, Figure 16). In every thin section examined, the sense of obliquity is predominantly clockwise when viewed down the plunge of the biotite lineation. It is helpful to consider the obliquity data along two dimensions: type of porphyroblast (garnet versus staurolite), and mineralogical/structural domain (P versus Q). Analyzing the data this way yields four statistically distinct populations; Table 2 and Figure 20 summarize this information. Garnets versus Staurolites In general, garnets show much larger obliquities than staurolites (+17.4 o versus +3.1 o ) regardless of the domain in which they are found. This effect is especially pronounced in Q domains, where the mean obliquity for garnets is o versus +5.4 o for staurolites. In fact, garnets in Q domains show the largest average obliquity of the four populations. The smaller obliquities observed in the staurolites may indicate that staurolite growth commenced after garnet growth and therefore staurolite experienced less matrix deformation, as is expected from the prograde reaction sequence for pelites. 95% of staurolite crystals show S i surfaces that are within only a few degrees of S e, suggesting that their growth was very late-syntectonic, near the end of Acadian regional deformation. P Domains versus Q Domains Porphyroblasts in Q domains show somewhat larger obliquities than porphyroblasts in P domains, regardless of the type of porphyroblast. This effect is large for garnets (+25.7 o versus +7.4 o ) and small for staurolites (+5.4 o versus +2.7 o ). This effect suggests that differences in the competency of the two domains produced different angular velocities of the porphyroblasts with respect to the matrix foliation plane. 2. STRAIN SHADOWS Quartz strain shadows are present around 43% of all measured garnets and 8% of all measured staurolites. Both garnets (48%) and staurolites (10%) in P domains were more likely than those in Q domains (39% and 0%, respectively) to exhibit strain shadows.

23 PORPHYROBLAST FABRIC OBLIQUITY AT COOS CANYON, MAINE LACK OF VARIATION ACROSS THE OUTCROP The most intriguing question raised by the obliquity data is why the overwhelming majority of porphyroblasts 93% of garnets and 77% of staurolites exhibit a clockwise sense of rotation relative to the matrix (Figure 21). If the fabric obliquities had developed via fold tightening, as predicted by the models of both Solar and Brown (1999) and Jiang (2001), one would expect to observe porphyroblasts with the opposite sense of obliquity on conjugate fold limbs, and thus a more balanced distribution of clockwise and anticlockwise obliquities. To test whether there is any association between the obliquity data and outcrop-scale folds, the mean obliquity magnitudes for each lineation-parallel thin section have been plotted against the cross-strike distances of the associated samples along the 132-meter traverse (Figure 22). This profile reveals that the mean sense of obliquity is clockwise across the entire traverse, showing no reversals, even on opposite sides of the three fold noses identified in the field. Moreover, the average magnitude varies by 15 o (for garnets) and 7 o (for staurolites) but shows no meaningful trend across the outcrop. Two explanations might explain the discrepancy between the observed data and the predictions of the two folding models: 1. Preferential sampling of asymmetric fold limbs. Many of the tight, meter-scale folds in Coos Canyon are asymmetric, with one limb many times longer than the other. My observations in the field indicate that the sense of asymmetry is always the same across the length of the outcrop. It is therefore possible that all of the samples were taken from the long limbs of meter-scale folds. 2. Simple shear was more important than pure shear. It is possible that the observed fabric obliquities were formed by simple shear rather than the pure shear associated with fold tightening. THERMOCHRONOLOGY The microprobe data confirms that the dominant phase of porphyroblast growth was prograde, in which case it is likely that the growth of garnet predates the growth of staurolite. The sequential growth of garnet and staurolite is common in rocks recording high-t, low-p prograde metamorphism (Spears, 1994). Petrologic studies indicate that in aluminous pelitic rocks like those of the Perry Mountain precursor, biotite and chloritoid are among the first metamorphic phases to form, followed by garnet via the reaction: Cld + Bt + H 2 O Gt + Chl At pressures similar to those recorded by the Coos Canyon rocks (~3.5 kbar), this reaction will occur at about 515 o C (Spears, 1994, Figure 10-5). As the temperature rises above 550 o C, garnet converts to staurolite via a dewatering reaction: Gt + Chl St + Bt + H 2 O Geochronological studies of the metamorphic country rocks in western Maine show that an episode of regional metamorphism occurred at ± 2 Ma. The obliquity data suggest that this metamorphism was coincident with regional deformation rather than post-tectonic. The helicitic

24 PORPHYROBLAST FABRIC OBLIQUITY AT COOS CANYON, MAINE 23 overgrowths seen on many porphyroblasts probably represent contact metamorphism associated with the emplacement of the Mooselookmeguntic pluton at ± 2 Ma.

25 PORPHYROBLAST FABRIC OBLIQUITY AT COOS CANYON, MAINE 24 Table 2: Summary of porphyroblast-matrix data for all lineation-parallel thin sections Garnet in P Domains Garnet in Q Domains All Garnets Staurolite in P Domains Staurolite in Q Domains All Staurolites n Mean Obliquity +7.4 o o o +2.7 o +5.4 o +3.1 o Error ±2.1 o ±2.1 o ±2.1 o ±3.9 o ±3.9 o ±3.9 o StDev Obliquity 95% conf. interval Strain shadows present 6.0 o 10.3 o 12.5 o 3.1 o 4.2 o 3.4 o 5 o 19 o 5 o 46 o 8 o 42 o 4 o 9 o 3 o 14 o 4 o 10 o 48% 39% 43% 10% 0% 8% A Garnets P domains n=349 S e C Staurolites P domains n=228 S e B Garnets Q domains n=417 S e 5% 5% S e D Staurolites Q domains n=36 2% 5% Figure 20 Rose plots of fabric obliquity magnitudes (S i -S e ) from all lineation-parallel thin sections, divided into populations according to porphyroblast type and mineralogical domain. North in all plots represents the orientation of the matrix foliation, S e, when viewed down the plunge of the biotite lineation. Data is binned into sections 2 degrees wide. The radial scale in percentage units varies. Arrows indicate mean values for each population.

26 PORPHYROBLAST FABRIC OBLIQUITY AT COOS CANYON, MAINE 25 Figure 21 Pie charts showing the proportion of each type of porphyroblast with clockwise obliquities (S i - S e > 0), anticlockwise obliquities (S i - S e < 0), and no measurable obliquities (S i - S e = 0). Data is from all lineation-parallel thin sections. Figure 22 A profile across Coos Canyon from east to west, showing the mean fabric obliquity for each lineation-parallel thin section plotted versus the cross-strike distance of the associated sample from the starting point of the traverse. The locations of the three fold noses identified in the field are shown in green.