Growth, Behavior, and Textural Sector Zoning of Biotite Porphyroblasts During Regional Metamorphism and the Implications For

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Growth, behavior, and textural sector zoning of biotite porphyroblasts during regional metamorphism and the implications for interpretation of inclusion trails: Insights from the Pequop Mountains and Wood Hills, Nevada, USA

Phyllis A. Camilleri Geosciences, Austin Peay State University, Box 4418, Clarksville, Tennessee 37044, USA

ABSTRACT terns. Therefore, caution must be used when continuously dilating pressure shadows where inferring strain histories on the basis of inclu- the matrix separates from the {001} faces (e.g., Metapelites in the Pequop Mountains and sion trails. Furthermore, although textural Lister et al., 1986; Barker, 2002). This type of Wood Hills, Nevada, contain biotite porphy- sector zoning has been reported in a variety growth produces zones of relatively inclusion- roblasts that are part of a Barrovian meta- of other porphyroblast species, where it is free, clear biotite at the ends of porphyroblasts morphic sequence that formed in response thought to develop in a state of hydrostatic and in intragrain extension fractures. to tectonic burial. Inclusion trails and pat- stress in pretectonic or intertectonic porphy- This study focuses on the growth of biotite in terns in these biotite porphyroblasts provide roblasts, zoning in biotite is signifi cant in that metapelite in the Pequop Mountains and Wood a remarkable record of their growth and it is strain induced and hence an indicator of Hills, Nevada (Fig. 1), and (1) provides an addi- behavior in this environment. Accompanied syntectonic growth. tional example of crack-fi ll porphyroblastesis, by a strong component of coaxial strain, the which has not been widely recognized; and porphyroblasts underwent a constructive INTRODUCTION (2) demonstrates that this growth mechanism is phase that involved growth characterized by probably an important process even in the fi rst textural sector zoning followed by a destruc- This paper documents the growth mecha- stage of growth. Furthermore, this paper shows tive phase involving fracturing, rotation, and nisms and behavior of biotite porphyroblasts that the growth of biotite involves a natural pro- minor residual growth. Textural sector zon- during progressive Barrovian metamorphism gression through a constructive phase in which ing is the result of uninhibited syntectonic and focuses on how strain controls the three- diverse inclusion trail patterns develop, and a growth in all directions. Growth along por- dimensional development, preservation, and subsequent destructive phase that obscures these phyroblast margins that parallel foliation modifi cation of passive inclusions and their pat- patterns. The destructive phase is similar to the involved incorporation of inclusions, whereas terns. This study builds on work in Australia, the second growth stage recognized by others, but growth along margins perpendicular to folia- Pyrenees, Japan, the Scottish Highlands, and the early constructive growth phase is different tion involved syntaxial precipitation of biotite Korea by Vernon and Flood (1979), Lister et al. because it involves the crack-fi ll growth mecha- in dilating strain shadows, which generally (1986), Miyake (1993), Barker (2002), and Kim nism and results in textural sector zoning of pas- precluded development of inclusions. This and Cho (2008), respectively. These authors sive inclusions (e.g., Fig. 2F). This paper also growth mechanism partially accommodated collectively showed that the growth of biotite shows how the zoning patterns can be used as strain and produced porphyroblasts with in upper greenschist to lower amphibolite facies an indicator of the style of strain accompany- a characteristic hourglass-shaped included is a two-stage process. The fi rst stage involves ing growth and why caution must be used when core bounded by zones of relatively unin- growth by matrix replacement and incorporation making strain assessments of biotite, and other cluded biotite. Cessation of growth of bio- of passive inclusions that represent an excess Barrovian index minerals, on the basis of inclu- tite triggered onset of the destructive phase reactant of the porphyroblast-forming reaction sion trails. and ultimately resulted in the transference or matrix material not involved in the reaction. of some strain to the porphyroblasts and the The second stage involves a syntectonic process BACKGROUND ON INCLUSION TRAIL fi lling of strain shadows with mostly quartz known as crack-fi ll porphyroblastesis (Barker, PATTERNS AND TEXTURAL SECTOR instead of biotite. Residual growth of biotite 2002). The crack-fi ll growth mechanism is ZONING in the destructive phase was largely restricted facilitated by fracturing along {001} in porphy- to strain shadows and extension fractures. roblasts where {001} is at a high angle to folia- Porphyroblasts that contain evenly distrib- Progression through the constructive and tion. Growth in these porphyroblasts occurs by uted trails of passive inclusions are commonly destructive phases results in production of precipitation of biotite in {001}-parallel exten- designated as syntectonic, intertectonic, or inclusion trails with a diversity of dip angles, sion fractures within the porphyroblasts, as well post-tectonic (e.g., Figs. 2A–2C). For example, dip directions, and trail geometries and pat- as by syntaxial precipitation in episodically or porphyroblasts that contain trails that curve into

Geosphere; June 2009; v. 5; no. 3; p. 215–251; doi: 10.1130/GES00184.1; 22 ﬁ gures; 1 supplemental ﬁ le.

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foliation are commonly interpreted as syntec- complicated patterns in garnet (e.g., Andersen, textural sector zoning is an indicator of growth tonic, and porphyroblasts with straight inclu- 1984; Burton, 1986; Rice and Mitchell, 1991; in a hydrostatic state, and hence the porphy- sion trails can be interpreted as intertectonic if Rice, 1993; Rice et al., 2006; see Figs. 2D, 2E roblasts are either pretectonic or intertectonic. foliation wraps around the grain margin, or as for examples). Textural zoning occurs when Although textural sector zoning in biotite has post-tectonic if foliation is not deﬂ ected around there is preferential incorporation of inclu- not been studied heretofore, this paper will show the porphyroblast (Figs. 2A–2C; Passchier sions along particular crystallographic faces that it is a syntectonic phenomenon. and Trouw, 2005). However, some porphyro- (growth sectors) or boundaries between faces blasts contain inclusions that are not distributed during growth (e.g., Harker, 1932; Rast, 1965; GEOLOGIC SETTING evenly throughout the grain, but rather occur Spry, 1969; Rice and Mitchell, 1993; see also in a geometric pattern that inherently reﬂ ects Barker, 1998; Vernon, 2004, for overviews). The Pequop Mountains, Wood Hills, and adja- the crystal’s structure (cf. Figs. 2A–2C and Rice and Mitchell (1991) observed that there is cent Ruby Mountains–East Humboldt Range, Figs. 2D–2F). These porphyroblasts exhibit tex- a common co-occurrence of cleavage or graph- Nevada, expose parts of a metamorphosed tural sector zoning and are regarded as pretec- ite domes along the margins of texturally zoned thrust footwall that formed in the hinterland of tonic or intertectonic (Rice and Mitchell, 1991; porphyroblasts. Because the domes represent the Mesozoic Sevier fold-and-thrust belt; these Rice, 1993). Well-known examples of zoning material displaced by the porphyroblast dur- metamorphic rocks were subsequently exhumed include the characteristic cross or hourglass ing growth in a hydrostatic state of stress (e.g., by normal faulting during Mesozoic and Ceno- patterns observed in some cut orientations of Ferguson et al., 1981), Rice and Mitchell (1991) zoic extension (Fig. 1; Camilleri and Cham- chiastolite and chloritoid, as well as the more and Rice (2001) suggested that the presence of berlain, 1997). The exhumed thrust footwall

115˚ 1(a-g) 41˚ 2 114˚ 30´ Pequop Wells 3(a-b) Mountains N

> 9 kb 8(a-b) 7 4 Garnet- and 9 tremolite-in 5.5-6.4 kb 10 isograd WH1(a-c) 11(a-b) WH2 Biotite-in 12(a-c) 5(a-b) isograd Wood Hills 13(a-b) 6 sillimanite-in 14

East Humboldt Range Independence 15(a-b) thrust 16(a-c) 40˚ 45´ 114˚ 30´ 2 km Ruby Mountains 10 km 114˚ 37´ 30˝ 40˚37´ 30˝

Unmetamorphosed strata that are structurally above or lie depositionally on metamorphosed strata Nevada Low-angle normal fault Map area Unmetamorphosed United States of America High-angle normal fault chlorite zone Greenschist facies Sillimanite-in Isograd biotite zone Thrust fault Axial trace of anticline Basin and Range Amphibolite facies* garnet, staurolite, kyanite thrust footwall Extensional Province and sillimanite zones Axial trace of * = garnet zone in the overturned anticline Pequop Mountains Sample location

Figure 1. Simpliﬁ ed geologic map depicting distribution of metamorphic facies and sample locations in the exhumed metamorphosed thrust footwall. The enlarged map of the Pequop Mountains shows sample locations. Many sample locations represent a suite of samples of varying lithology and fabric elements denoted by letters following the numbered locations. For example, location #1 has 7 samples (i.e., a–g). Not all samples are referenced in this paper. Photomicrographs of unreferenced samples are presented in Supple- mental File 1 (see footnote 1). Maps are modiﬁ ed from Camilleri and Chamberlain (1997) and Coats (1987) and show metamorphic pressures in kilobars, based on thermobarometric data in the East Humboldt Range from McGrew et al. (2000) and in the Wood Hills from Hodges et al. (1992).

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constitutes a Barrovian metamorphic terrain that the metamorphic rocks as well as a macroscopic Camilleri and Chamberlain, 1997). In both

ranges from a sillimanite zone to the west and to megascopic scale pinch-and-swell structure ranges the S1 foliation is locally folded, and in

progressively decreases in metamorphic grade that accommodated as much as ~30% attenu- a few places, weakly crenulated (S2) by the sub- and pressure to the east (Fig. 1). The footwall ation of stratigraphic units (Camilleri, 1998). sidiary structures in the hanging wall and foot- contains Paleozoic carbonate and clastic strata Crystallographic preferred orientations of quartz wall of the Independence thrust (see Appendix 1 that underwent metamorphism synchronous c-axes in quartzite indicate that coaxial ﬂ atten- in Supplemental File 11 for more information). with extensional collapse in response to tectonic ing accompanied attenuation (Camilleri, 1998). This study focuses on biotite porphyroblasts burial. Footwall collapse was accommodated by The footwall rocks in the adjacent Wood Hills in biotite-zone phyllite and schist primarily dominantly coaxial strain that resulted in devel- differ from the Pequop Mountains in that units from Cambrian Dunderberg Shale that is not

opment of a regional prograde metamorphic are more attenuated and they are largely in the overprinted by the S2 crenulation in the Pequop

foliation (S1) and as much as 50% attenuation of kyanite zone. Following peak metamorphism, Mountains. In addition, to more fully assess the stratigraphic units (Camilleri, 1998). rocks in both ranges underwent a second defor- The exposed footwall in the Pequop Moun- mation that involved the development of the out- 1Supplemental File 1 is a PDF ﬁ le containing tains consists of a continuous transition of of-sequence, small-displacement Independence 20 appendices. If you are viewing the PDF of this unmetamorphosed to garnet zone rocks that thrust exposed in the Pequop Mountains as well paper or reading it ofﬂ ine, please visit http://dx.doi .org/10.1130/GES00184.S1 or the full-text article at contain the S1 foliation. A west-northwest– as associated folds and small-scale thrusts in the http://geosphere.gsapubs.org to view Supplemental trending mineral lineation is present in many of hanging wall and footwall of the thrust (Fig. 1; File 1.

Texturally unzoned porphyroblasts

A Intertectonic B Syntectonic Syntectonic C Post-tectonic

Texturally zoned porphyroblasts (metamorphic index minerals) D Staurolite or Chiastolite E Garnet F Biotite (andalusite) (this study) Intertectonic Intertectonic Syntectonic

Pretectonic Pretectonic

Figure 2. Sketches illustrating patterns of trails of passive inclusions in texturally zoned and unzoned porphyroblasts. Unzoned porphyroblasts have evenly distributed inclusion trails, whereas inclusion trails in zoned crystals are restricted to certain growth sectors or boundaries between growth sectors. (A–C) Examples of texturally unzoned intertectonic, syntectonic, and post-tectonic porphyroblasts highlighting the relationship of inclusion trails in porphyroblasts (yellow) to the external foliation. Traces of foliation and the inclusion trails in this and all other ﬁ gures are indicated by black lines. (D, E) Examples of textural sector zoning as typically developed in staurolite, chiastolite, and garnet. Zoning in these minerals can be pretectonic or intertectonic, whereby inclusion trails in zoned intertectonic porphyroblasts reﬂ ect foliation traces, and inclusions in pretectonic porphyroblasts are randomly oriented and do not represent a foliation. (F) Example of textural sector zoning observed in biotite in this study. Sketches in A–C are after Passchier and Trouw (2005). Sketches in D and E represent generalizations based, in part, on line drawings and photomicrographs of texturally zoned minerals shown in Harker (1932), Rice and Mitchell (1991), and Rice (1993). Note that these sketches display only part of the spectrum of sector zoning patterns for a given mineral.

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growth and behavioral characteristics of biotite subparallel to foliation but normal to lineation through the spectrum of metamorphic grades, the (e.g., porphyroblasts M and O, respectively, Figure 3. Sketches and photomicrographs same rocks were also observed in the adjacent in Figs. 3A–3B). Furthermore, crystals with illustrating how the general shapes and chlorite zone and in the kyanite zone in the Wood c-axes that are normal to foliation appear to be dimensions of biotite crystals in the Pequop Hills. Figure 1 shows the locations of the sam- thinnest parallel to the c axis (e.g., porphyrob- Mountains differ from ideal and how they ples. These metapelite samples are lithologically last N in Figs. 3A, 3B). Porphyroblasts in rock relate to fabric elements (foliation and lin- diverse and have varying proportions of graphite, that has no appreciable lineation have more eation). All photomicrographs are in plane- quartz, and carbonate minerals. In addition, the equant shapes (cf. Figs. 3C, 3D). polarized light. (A) Sketch illustrating the metapelites are structurally diverse and range Although the c-axes of porphyroblasts are ideal shape and relative dimensions of biotite from rocks that contain little tectonic fabric to dominantly at a low angle to foliation, they have versus the approximate end-member shapes S to S > L tectonites with biotite porphyroblasts a diversity of orientations relative to lineation, if (N, M, and O) of biotite porphyroblasts in defi ning the lineation (Figs. 3C–3D). present. Consequently, in any foliation-normal the Pequop Mountains. The shapes shown thin section, porphyroblast cross sections have are typical of those found in schist where CRYSTALLOGRAPHIC ORIENTATION, a diversity of shapes. These include elliptical, biotite forms a lineation (an S-L tectonite). GRAIN SHAPES, AND SIZES OF circular, rectangular, parallelogrammatic, and An N-type grain has the c axis normal to BIOTITE parabolic shapes (Figs. 4–8). foliation, {001} faces approximately parallel to foliation, and is not texturally zoned. Characterization of the orientation, size, and NUCLEATION OF BIOTITE The c axis of an M-type grain is approxi- shapes of biotite porphyroblasts in three dimen- PORPHYROBLASTS mately parallel to foliation and lineation sions is important to understanding growth pro- with {001} at a high angle to foliation. An cesses and textural zoning patterns. This was To understand the predominance of biotite O-type grain has the c axis approximately accomplished by examination of thin sections porphyroblasts with {001} at a high angle to parallel to foliation but perpendicular to lin- cut (1) parallel to foliation, and (2) perpendicu- foliation, which is unusual for phyllosilicates, eation with {001} at a high angle to foliation. lar to foliation in both lineation-parallel and lin- possible porphyroblast nucleation sites were Both M- and O-type grains are typically tex- eation-normal sections (all photomicrographs in observed in phyllite in the proximity of the turally zoned. (B) Foliation-parallel photo- this paper are shown in plane-polarized light). biotite-in isograd (Fig. 1). Chlorite-zone phyl- micrograph and line drawing of sample 1a These observations indicate that most biotite lite near this isograd is characterized by folia- showing two-dimensional examples of the porphyroblasts in the Pequop Mountains are tion defi ned by white mica, chlorite, deformed N, M, and O shapes illustrated in A. These subhedral with undefi ned {010} and {110} detrital quartz, and carbonate (ankerite?) porphyroblasts have graphite inclusions faces and well to moderately defi ned {001} grains, and sparse extension fractures (veins) (dark areas) and relatively unincluded clear faces (Fig. 3A). Biotite composes a few percent that are at a low angle or parallel to foliation zones (light areas). The cores of the M and to as much as ~35% of the volume of the rock and are fi lled with chlorite, quartz, and plagio- O grains are texturally zoned; areas 1 and 2 and crystals are as large as ~1.5 mm in their lon- clase [see Fig. 9A and inset photomicrograph represent the zoned core, and 1 = included gest dimension. Biotite is present as both por- in Fig. 9B; see also additional discussion biotite and 2 = unincluded, clear biotite. The phyroblastic and foliation-defi ning matrix crys- about the origin of these veins in Appendix 2 areas marked by 3 represent a poikiloblastic tals with matrix grains typically having {001} at (see footnote 1)]. The same phyllitic unit just rim. Lineation in this rock is approximately a low angle or parallel to foliation. The porphy- across the isograd in the biotite zone contains parallel to crystal O, which has the trace of roblastic crystals have a preferred orientation in small biotite grains that may refl ect the ini- {001} parallel to lineation, the least width, the sense that their crystallographic c-axes are tial stage of growth of biotite porphyroblasts and thinnest clear zones. In comparison, most commonly oriented at a low angle, or par- (Figs. 9C–9F). Figures 9C–9F show an exam- crystal M, the {001} trace of which is nor- allel, to foliation; i.e., the {001} cleavage tends ple of a biotite-bearing phyllite with unfoliated mal to lineation, has the greatest width and to be at a high angle to foliation. Porphyroblasts laminations (bedding) of quartz, carbonate, largest clear zones. Crystal M has partially with {001} parallel or subparallel to foliation and mica that are separated by foliated layers engulfed another unlabeled N-type crystal are generally sparse, but are more abundant in defi ned by fl attened detrital quartz grains, and (dark area in top left corner of crystal M). rocks where biotite composes a large part of the chlorite and white mica with {001} at a low Arrows point to locations of extension frac- volume of the rock. angle to parallel to foliation (Fig. 9C). Both tures fi lled with biotite and quartz. (C) Pho- An ideal biotite crystal is monoclinic, thin foliated and unfoliated parts in this sample have tomicrograph of sample 1a showing biotite in the direction of the crystallographic c axis, biotite, but the biotite tends to be larger in the defi ning a well-developed lineation. In this and wider in the directions of the a- and b-axes undeformed layers. Biotite in the undeformed sample, porphyroblasts that are texturally (Fig. 3A); however, biotite porphyroblasts layers has no apparent preferred orientation, zoned contain clear zones that broadly par- in the Pequop Mountains differ from ideal in whereas biotite in the foliated parts tends to allel the trace of {001} in individual porphy- many respects. Their relative crystallographic be oriented with {001} either at a low angle roblasts. (D) Photomicrograph of sample dimensions appear to be related to strain or parallel to foliation, or at any angle when 16a showing a very subtle lineation defi ned because crystals are generally elongated paral- in the strain shadow of a quartz grain (Figs. 9 by the elongation of biotite. These por- lel to lineation regardless of crystallographic D–9F). Consequently, biotite with {001} at a phyroblasts are texturally zoned and have orientation (Fig. 3). This has an effect on the high angle to foliation may have nucleated in quartz inclusions, but these features are not dimensions of biotite in that porphyroblasts no-strain to low-strain areas. Additional but apparent at this scale and resolution. (Fig- having c-axes that are approximately paral- sparse sites of nucleation include strain shad- ure 6B shows an example of these features lel to foliation and lineation tend to be wider ows on diagenetic(?) pyrite and in veins as a at higher magnifi cation.) Double-headed parallel to the c axis than those with c-axes replacement of chlorite (Fig. 9B). arrows indicate lineation direction.

218 Geosphere, June 2009

A shapes in 3D Ideal biotite crystal N M O {110} a {001} c-axis

shapes in 2D c foliation parallel cut trace of {001} lineation cleavage

b {010} trace of foliation

foliation-normal, lineation-parallel cut

2 3 O 1 M N 2 2 1 2 lineation 3 Foliation parallel view 1 mm 1 = included area; 2 = conical clear zones; 3 = dendritic or poikiloblastic overgrowth rim; = approximate trace of c-axis C D

5 mm 5 mm Foliation parallel view of an S-L tectonite Foliation parallel view of an S>>L tectonite

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c axis

c axis cc c c

{001} cleavage

inclusions trace of {001} cleavage trace of foliation c = conical clear zones

B perpendicular to foliation parallel to foliation A1 section A2 section A3 section B1 sections or

B2 sections c axis or

C perpendicular to foliation

B1 sections B2 sections or or

B1 B2 C1 section C2 section C1 D C2 D section

Trace of sections on a foliation- c axis parallel B section

Figure 4. Diagrams illustrating the character of textural sector zoning in biotite porphyroblasts and the diversity of crystal shapes versus inclusion patterns for various cut orientations. (A) Sketch showing an example of a texturally zoned porphyroblast with a crystallographic c axis that is approximately parallel to foliation. The length of the porphyroblast is exaggerated for illustrative purposes. The sketch shows a biotite crystal with conical clear zones that terminate at the {001} crystal faces. Although the clear zones are shown as separated cones, in many porphyroblasts they nearly connect at a point (see Fig. 6B). (B) Diagram showing crystal shapes versus inclusion patterns in sections cut perpendicular to

foliation and the c axis (A1, A2, and A3 sections), and parallel to foliation and the c axis (B1 and B2 sections). The B sections are commonly shaped like a rectangle or parallelogram, depending on where the section is cut relative to the a and b crystallographic axes. (C) Diagram showing crystal shapes versus inclusion patterns in sections that are perpendicular to foliation and parallel or at an angle to the c axis. This diagram shows the same porphyroblast as B, but it has been

rotated 90° for illustrative purposes. B1 and B2 sections in this diagram are similar to those described in B.

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CONSTRUCTIVE PHASE: patterns within, the biotite porphyroblasts. texturally sector-zoned core that in some cases DEVELOPMENT OF TEXTURAL In contrast, the destructive phase involves is surrounded by a poikiloblastic or dendritic SECTOR ZONING AND SUBSEQUENT destruction or modiﬁ cation of constructive overgrowth. The inclusion trails are generally MATRIX OVERGROWTH phase inclusion patterns by fracturing, rota- straight to gently curved, and the inclusions tion, and residual growth around fractured are mostly quartz and graphite, although in The constructive phase involves growth and rotated porphyroblasts. The constructive some rocks a few large porphyroblasts have processes that are responsible for the bulk phase of a biotite porphyroblast is typically overgrown smaller biotite grains as whole or of growth of, and development of inclusion demarcated by passive inclusions that deﬁ ne a partial inclusions.

A B

A1 section A2 section

lineation parallel lineation parallel

1 mm 0.5 mm

C D

A2 section A3 section

lineation normal lineation parallel

1 mm 0.5 mm

Figure 5. Photomicrographs of A sections, which are cuts that are perpendicular or approximately normal to the c axis. All sections

are cut perpendicular to foliation. (A) Example of an A1 section (sample 1b). A part of a B2 section is shown in the lower right corner, where a quartz-ﬁ lled extension fracture separates a clear zone on the left from an included graphitic core on the right. (B) Example of

a lineation-parallel A2 section (sample 16a). Dark bubble in the lower right corner of the porphyroblast is an air bubble. (C) Example

of a lineation-normal A2 section (center; sample 16a). Dark bubble to the right of this porphyroblast is an air bubble. (D) Example of

an A3 section (sample 14). Inclusions in B–D are quartz.

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Textural Sector Zoning and post-zoning dendritic or poikiloblastic the clear zones are broadly cone shaped, con- overgrowth by 3. In addition, photomicro- cave outward, and terminate at {001} crystal Textural sector zoning appears only in por- graphs of texturally zoned porphyroblasts faces (Fig. 4). The axes of cones are roughly phyroblasts whose inclusion trails are at a high from samples not shown in this paper are pre- parallel to the c axis. The shape of the hour- angle to {001}, and hence it developed in crys- sented in Supplemental File 1 (see footnote 1), glass, however, varies depending on proximity tals that grew with {001} oriented at a high which includes those that exhibit more subtle of the c axis to the section cut. For example, angle to foliation. Figure 4 shows a schematic manifestations of zoning and those with zon- a section cut nearly parallel to, and through, illustration of the three-dimensional geom- ing patterns that vary from ideal. the c axis yields an hourglass shape with a thin

etry of, and two-dimensional cuts through, an The two-dimensional zoning patterns viewed or no neck (B1 sections in Figs. 4C, 6B, and ideal textural sector-zoned porphyroblast with in any thin section are diverse and reﬂ ect cut 7C), whereas a section cut closer to the margin {001} normal to foliation, and the photomi- effects (Fig. 4). Porphyroblasts that are cut par- of the porphyroblast will yield the same pat- crographs in Figures 5–8 show examples of allel or subparallel to the c axis contain a cen- tern but have a much thicker neck and corre-

these cuts. Many of the photomicrographs of tral hourglass-shaped distribution of passive spondingly smaller clear zones (B2 sections in

zoned porphyroblasts are accompanied by line inclusions (B1 and B2 sections in Figs. 4, 6, and Figs. 4C and 7A, 7B, and 7D). drawings where the included area is always 7). The included hourglass shape is bounded Porphyroblasts that are cut highly oblique to

indicated by a number 1, the areas that lack by zones of clear biotite that contain a low den- the c axis differ from the B1 and B2 sections in signiﬁ cant inclusions (i.e., clear zones) by 2, sity of, or no, inclusions. In three dimensions, that they are either broadly elliptical or parabolic

A 3

221 3

B2 section

1 mm

2 1 3

1 2

3 B1 section

Figure 6. Photomicrographs and line drawings of foliation-parallel B sections. In this and other ﬁ gures the areas marked 1 and 2 represent the zoned core, where 1 = included biotite and 2 = unincluded, clear biotite. The areas marked by 3 represent a post-zoning

overgrowth rim. (A) B2 section rimmed by a dendritic overgrowth (zone 3) from sample 1c. This porphyroblast has an hourglass

shape with a thick neck and graphite inclusions. (B) B1 section rimmed by a poikiloblastic overgrowth (zone 3) from sample 16a. This porphyroblast has an hourglass shape with quartz inclusions and a negligible neck. The scale for B is the same as in A.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/5/3/215/3338228/i1553-040X-5-3-215.pdf by guest on 01 October 2021 Growth, behavior, and textural sector zoning of biotite porphyroblasts roxi- 4 lineation parallel lineation parallel lineation parallel rk circle adjacent to lower left adjacent to lower rk circle 2 ned by graphite in A and B by A ned by graphite in 1 1 1mm of biotite growth. The arrow points to a biotite- The arrow of biotite growth. 2 1mm 22 3 3 1& 3 4 1 -section 2 section section 2 2 C B B section (sample 1c) with a negligible zone 3. Inclusion trails in zones 1–4 in the porphyroblasts section (sample 1c) with a negligible zone 3. Inclusion trails in zones 1–4 the porphyroblasts 2 section (sample 1a) that has a large overgrowth rim (zone 3) and locally abuts a biotite grain (colored orange) with {001} app rim (zone 3) and locally abuts a biotite grain (colored section (sample 1a) that has a large overgrowth 2 ). Photomicrographs and line drawings of foliation-normal B sections. Included hourglass shapes are deﬁ and line drawings of foliation-normal B sections. Included hourglass shapes are ). Photomicrographs section is an air bubble in the thin section. (B) Example of a B section is an air 2 lled extension fracture that also formed in the destructive phase. Development of zone 4 and the fracture postdates zone 3. Da that also formed in the destructive phase. Development of zone 4 and fracture lled extension fracture continued on next page A B in A and B are broadly convergent (cf. Figs. 11F, 11G), as is the foliation surrounding the porphyroblasts. the porphyroblasts. as is the foliation surrounding 11G), convergent (cf. Figs. 11F, broadly and B are A in Figure 7 ( Figure quartz in C and D. (A) Example of a B mately parallel to foliation. Zone 4 represents biotite and quartz precipitated in strain shadows during the destructive phase biotite and quartz precipitated mately parallel to foliation. Zone 4 represents and quartz-ﬁ corner of the B corner

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/5/3/215/3338228/i1553-040X-5-3-215.pdf by guest on 01 October 2021 Camilleri lineation normal lineation parallel lineation parallel section contains an extension 1 2 f center (shown by arrows); this may be a (shown by arrows); f center 1 1 1 ble. The scales in C and D are the same. The scales in C and D are ble. 2 convergence. The B 1mm 3 22 3 2 1 + 3 Section 2 section section A 2 1 B B sections from sample 16a. The inclusion trails in the bulk of zoned parts sample 16a. sections from 2 and B 1 ). (C, D) Examples of lineation-parallel and lineation-normal B continued and a crude line of {001}-parallel “inclusions” to the right o of the porphyroblast lled with biotite and quartz in the center C D crystals are straight, and trails in the poikiloblastic rim (zone 3) and minor parts of the adjacent zoned area exhibit subtle parts of the adjacent zoned area straight, and trails in the poikiloblastic rim (zone 3) minor crystals are fracture ﬁ fracture relict fracture. The white area with a dark circle beneath the porphyroblast in C is a void in the thin section with an air bub in C is a void the thin section with an air beneath the porphyroblast with a dark circle The white area fracture. relict Figure 7 ( Figure

224 Geosphere, June 2009

in shape and have different inclusion patterns. Origin of Textural Sector Zoning the clear zones, which is similar to that observed Elliptical sections that do not cut the {001} faces The origin of the conical clear zones in bio- in the Pequop Mountains. display an off-centered, internal clear zone if cut tite porphyroblasts is probably, in part, similar The textural sector-zoned porphyroblasts approximately through one of the conical clear to the ﬂ uid-induced crack-seal growth mecha- from the Pequop Mountains differ from those zones, or have no apparent clear zones if the nism described by Lister et al. (1986) for biotite in the Pyrenees primarily in the shapes of the cut is through the center of the hourglass shape porphyroblasts in the Pyrenees. They described included versus clear areas, and more spe-

(C1 and C2 sections in Figs. 4C, 8A, and 8B). A porphyroblasts with {001} at a high angle to foli- ciﬁ cally with regard to the hourglass-shaped parabolic section cuts one of the {001} faces and ation that initially grew over foliation incorporat- included core in the Pequop Mountains (cf. contains one arcuate clear zone that terminates ing inclusions followed by growth in a direction Figs. 10E and 10L). The typical inclusion pat- at the (001) face (D section in Figs. 4C and 8C). parallel to foliation in dilating strain shadows. terns observed in the Pequop Mountains can be The appearance of sections cut approximately Growth in the strain shadows occurs when pore- produced if growth (1) occurs in all directions perpendicular to the c axis varies depending on ﬂ uid pressure exceeds the tensile strength of the away from the core, rather than being inhibited where the section is cut. If the cut is through one matrix-{001} interface, and biotite is precipitated in directions perpendicular to foliation, and of the clear zones, then inclusions are distributed in the void created by separation of matrix from (2) is accompanied by protracted shortening in an elliptical pattern along the grain boundary the porphyroblast {001} faces. This process pro- approximately perpendicular to foliation and

with a central clear zone (A2 section in Figs. 4B, duces an included core bounded by paired zones extension parallel to foliation. Figures 10F– 5B, and 5C). If the cut is through the center, then of clear biotite with little or no inclusions at the 10K show the progressive two-dimensional the porphyroblast will appear replete with inclu- ends of the porphyroblast (e.g., Figs. 10A–10D). development of an hourglass-shaped distribu-

sions (A3 section in Figs. 4B and 5D). In con- Lister et al. (1986) also indicated that overall tion of straight inclusion trails due to growth trast, if the section is cut near the end of the por- growth rate kept pace with matrix separation, in all directions. Growth occurs on the {001} phyroblast, then it should be mostly clear with but that occasionally, in places, either strain faces where matrix separates from the por- sparse inclusions along the margin of the grain decreased or growth rate increased, resulting in phyroblast in symmetric dilating strain shad-

(A1 section in Figs. 4B and 5A). matrix overgrowth yielding sparse inclusions in ows at the same time as growth in a direction

A lineation parallel B lineation parallel

C1 section C2 section

1 mm

C lineation normal

Figure 8. Photomicrographs of C and D sections from sample 16a. View in all sections is perpendicular to foliation. All porphyroblasts have quartz inclusions and minor matrix overgrowth following textural sector growth. Scale for all photomicrographs is the same as in A.

D section {001}

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/5/3/215/3338228/i1553-040X-5-3-215.pdf by guest on 01 October 2021 Growth, behavior, and textural sector zoning of biotite porphyroblasts otite zone. (A) Photomicrograph otite zone. (A) Photomicrograph ite zone. The inset photomicrograph shows an The inset photomicrograph ite zone. 0.25 mm variably oriented brown biotite grains that are biotite grains that are variably oriented brown f foliated layers in C with biotite grains indicated ame. the plane of thin section (e.g., see biotite grains ch layers and lighter unfoliated quartz-, mica-, and ch layers and lighter ces of {001} cleavage visible). (B) Photomicrograph ces of {001} cleavage visible). (B) Photomicrograph phyroblast. The biotite is clear where it has replaced it has replaced where The biotite is clear phyroblast. lled extension fracture. The arrow in the large photomi- The arrow lled extension fracture. F ). Foliation-normal photomicrographs showing the character of chlorite zone phyllite and nucleation sites for biotite in the bi of chlorite zone phyllite and nucleation sites for showing the character ). Foliation-normal photomicrographs continued E Figure 9 ( Figure quartz (clear) and chlorite–white mica stacks (gray with tra of graphitic chlorite zone phyllite (sample 5b). Oblate grains are some vein chlorite that formed in the chlor has replaced a biotite porphyroblast showing biotite zone schist (sample 16a) where the s are The scales in E and F has {001} aligned with foliation. The biotite grain in F indicated by downward-pointing arrow). example of such a vein in chlorite zone phyllite (sample 5a), which is (gray)-, plagioclase-, and quartz (clear)-ﬁ in C showing of unfoliated layer bedding. (D) Photomicrograph layers represent carbonate-rich layers (sample 4). Light colored o (E, F) Photomicrographs round. quartz grains. Grains with {001} at a high angle to foliation are than clear generally smaller in the strain shadows of quartz grains (clear) and out reside Many of the biotite grains in E probably by yellow arrows. crograph points to a depletion halo in the matrix (lighter colored matrix), which is also present on the top of biotite por matrix), which is also present colored points to a depletion halo in the matrix (lighter crograph foliated mica-ri of phyllite with darker, vein chlorite and has passive quartz inclusions outside the vein. (C) Photomicrograph

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/5/3/215/3338228/i1553-040X-5-3-215.pdf by guest on 01 October 2021 Camilleri A Porphyroblast core B Growth into strain shadow C Continued growth

porphyroblast {001} {001}

matrix trace of foliation

Porphyroblast with D 3D shapes of clear zones rectangular clear zones E

CZIC CZ CZ IC CZ IC = included core to a direction parallel to foliation to a direction parallel CZ = clear zone included core when growth is restricted included core when growth

Development of rectangular clear zones and of rectangular clear zones Development {001} normal {001} at a high angle, but not to foliation perpendicular to, foliation

F Porphyroblast core G Growth increment H Growth increment {001} {001}

trace of foliation

Porphyroblast with con- Growth increment Growth increment K I J cave outward clear zones

L 3D shapes of clear zones

CZ CZ Development of textural sector zoning with conical clear zones and an with conical clear zones sector zoning of textural Development hourglass-shaped included core when growth direction is not restricted hourglass-shaped included core when growth {001} normal to foliation {001} at a high angle, but not perpendicular to, foliation

Figure 10. Growth models for the development of rectangular- and hourglass-shaped included cores in B section porphyroblasts. The diagrams represent cuts that are perpendicular to foliation and the {001} faces of biotite. The trace of the {001} cleavage is not shown. (A–E) Growth model illustrating the development of a rectangular-shaped included core and clear zones. The sketch in A represents an included biotite before development of the clear zones and B and C show growth increments involving precipitation of biotite into strain shadows, which does not leave inclusions. The arrows in B denote shortening and extension directions during growth. The fi nal two- and three-dimensional shapes of the clear zones and included core are shown in D and E. (F–L) Growth model showing the development of an hourglass-shaped included core and conical clear zones (CZ). The sketch in F represents an included biotite before development of the clear zones and G–J show growth increments. Growth rate in a direction perpendicular to foliation is assumed to be the same as parallel to foliation. Arrows in G indicate shortening and extension directions that prevail in growth increments in H–J, which collectively refl ect coaxial strain. The fi nal two- and three-dimensional shapes of the clear zones are shown in K and L. See text for explanation.

228 Geosphere, June 2009

perpendicular to foliation along the porphy- a high angle to foliation was favorable. Once allows the zoned crystals to become large, i.e., roblast margins that parallel foliation. Only these biotite crystals grow larger than the sur- porphyroblastic, unlike the smaller unzoned growth in directions perpendicular to folia- rounding grains, the separation of matrix from biotite crystals with {001} parallel to foliation tion involves matrix replacement and hence {001} faces becomes permissible and textural that have a slow growth direction perpendicu- incorporation of inclusions, which ultimately sector growth can begin. For example, a biotite lar to foliation. yields the hourglass-shaped included core. In crystal with {001} perpendicular to foliation three dimensions this process would produce may nucleate in the strain shadow of a detrital Control of Strain and Growth Rates on paired clear zones that have a broadly conical quartz grain, but then grow out of the shadow Hourglass and Porphyroblast Shapes geometry (Fig. 10L). Restricted growth of bio- and into the deforming matrix, at which point Although the growth model in Figure 10 tite as shown in Figures 10A–10D could reﬂ ect the biotite will generate its own strain shad- shows development of a porphyroblast with an a relatively high strain rate inhibiting growth ows and textural sector growth will ensue. hourglass-shaped included core under condi- perpendicular to foliation, or perhaps the redis- This scenario requires the growth of biotite out tions where growth rate is the same in directions tribution of biotite by solution on grain mar- of a strain shadow in a direction perpendicu- parallel and perpendicular to foliation, many gins parallel to foliation and reprecipitation in lar to foliation and parallel to {001}, which porphyroblasts show evidence that growth dilating strain shadows. is the natural fast-growth direction of biotite, rate varied in different directions, resulting The textural sector zoning patterns in the and hence it probably facilitates this process. in shapes that deviate from that shown in the Pequop Mountains probably developed on Furthermore, the alignment of the biotite fast- model. The spectrum of crystal and hourglass

small unzoned cores that represent grains growth direction perpendicular to foliation, shapes in B1 and B2 sections that can result from that initially grew in strain-free or low-strain coupled with growth parallel to foliation facili- different relative growth rates in directions per- areas where growth of crystals with {001} at tated by matrix separation along {001} faces, pendicular and parallel to foliation are shown

ABC Growth rate in a direction Growth rate in a direction Growth rate in a direction parallel to foliation* is parallel to foliation* is parallel to foliation* is

section

2 {001}

B approximately the same as slower than perpendicular faster than perpendicular

B perpendicular to foliation to foliation to foliation

c-axis c-axis Figure 11. Diagrams illustrating variations in hourglass

{001}

{001} section shapes and inclusion trail 1

B patterns in B sections. The (cut through growth * c-axis is parallel to foliation crystallographic c axis is hor- center of porphyroblast) izontal in all porphyroblasts and is parallel to foliation (not shown). (A–D) Sketches showing how hourglass shape varies as function of growth section

2 rate in directions parallel and porphyroblast) B

(cut near margin of perpendicular to foliation. (E–G) Sketches highlighting D Growth rate perpendicular to Non-rotational inclusion trail geometries in sector zoned variations in inclusion trail foliation diminishes during parts of porphyroblasts ( B2 sections) geometries in unrotated, tex- the latter stages of growth turally zoned porphyroblasts. E F G Although inclusions in clear zones are sparse and generally faint, they are shown as overt in diagrams E–G for illustrative purposes only. B1 section Variant

Straight Convergent Straight to Convergent

B2 section Growth increment* Included area Clear zone

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in Figures 11A–11D. In the Pequop Moun- different matrix rheologies with rocks that Strain Signiﬁ cance of Biotite Porphyroblasts tains, the end-member crystal and hourglass contain a signiﬁ cant graphite component hav- with and without Textural Sector Zoning shapes in Figures 11A and 11C tend to form ing greater ductility (and possibly strain rate), In comparison to other texturally zoned when the c axis is aligned parallel to lineation, resulting in convergence of foliation in strain mineral species that require a hydrostatic state whereas the shapes shown in Figure 11B form shadows during textural sector growth. In of stress and no distortional strain to produce when the c axis is aligned normal to this direc- addition, some porphyroblasts exhibit straight textural sector zoning (e.g., Rice and Mitch- tion (cf. the M and O porphyroblasts in Fig. 3B to weakly convergent trails in the center with ell, 1991; Rice et al., 2006), biotite is unique

with the shapes of B2 sections in Figs. 11A and more strongly convergent trails and graph- in that it requires strain to develop. Therefore, 11B and the porphyroblasts in Figs. 7C and 7D ite concentration on the margins, which may zoning in biotite indicates syntectonic growth,

with the shapes of BI sections in Figs. 11C and refl ect an overall decrease in growth rate rela- which contrasts with the pretectonic or inter- 11B, respectively). These observations imply tive to strain rate (Fig. 11G). tectonic origin of other zoned minerals. The that growth rate in the direction of the c axis In thin sections that have porphyroblasts presence of zoning can also be used to imply is (1) generally enhanced when it is aligned with straight to convergent trails, it is not the style of strain during growth. As shown in parallel to the direction of maximum extension uncommon to fi nd some with rotated cores the growth model in Figure 10 (F–L), the typi- (lineation), resulting in development of a crys- that refl ect dextral and sinistral senses of rota- cal zoning patterns observed in biotite can form tal that is generally long parallel to the c axis, tion during the latter stages of textural sector if growth is accompanied by progressive short- and (2) slower when the c axis is aligned paral- growth (e.g., Figs. 13A–13D). This is appar- ening perpendicular to foliation and extension lel to the minimum extension direction, result- ent in both lineation-parallel and lineation- parallel to foliation, which implies that each ing in crystals that are shorter parallel to the normal thin sections. Inclusion trails in the increment of strain accumulates coaxially. The c axis. The dependence of crystal and hourglass rotated core generally form an acute angle to model therefore illustrates the development of shapes on the orientation of the c axis relative {001}, with the sense of rotation commonly zoning under progressive end-member coaxial to lineation suggests at least a partial control predictable by observing the restored (unro- strain, i.e., no rotation of fi nite stretching axes. of strain on growth rates. In addition, in some tated) position of {001} relative to foliation, However, textural sector growth is probably per- porphyroblasts the extremities of clear zone– which was probably the approximate long missible with a small component of noncoaxial hourglass boundaries are arcuate (cf. Fig. 11D dimension of the crystal prior to rotation. For strain, but prolonged textural sector growth and and Figs. 12A–12B) rather than straight, as example, when the cores are restored to their full development of the patterns in Figures 10 depicted in Figures 11A–11C, which indicates pre-rotation positions, those with a dextral and 11 are probably not favored in a regime of a general slowing of growth rate in a direc- sense of rotation had {001} canted to the right general shear with a large component of nonco- tion perpendicular to foliation during the latter and those with a sinistral sense were canted axial strain. This is because a strong noncoaxial stages of growth (additional data on variations to the left (Figs. 13C, 13D). The observed component would likely result in biotite crystals of hourglass shapes are in Appendix 3 in Sup- opposing senses of rotation are consistent with {001} at a high angle to foliation being plemental File 1; see footnote 1). with strain that involves a signifi cant compo- rotated toward foliation and hence away from nent of shortening perpendicular to foliation. the optimum angle for tensile separation of the Behavior of Biotite and the Matrix during Such opposing senses of rotation can develop matrix from the {001} faces, resulting in ces- Textural Sector Growth and the Effect on the when the long axes of some porphyroblasts sation of precipitation of clear biotite on these Development of Inclusion Trails are inclined in opposite directions and are not faces. Consequently, prolonged textural sector Development of the ideal zoning patterns parallel to the instantaneous stretching axes, growth is likely only operative in regimes where shown in Figures 10K and 11A–11C does one of which is presumably parallel or at a low the coaxial component of strain is signifi cant not involve rotation of the porphyroblast and angle to foliation (e.g., Ghosh and Ramberg, and persistent. Therefore, textural sector zoning results in production of straight inclusion trails. 1976). It is also plausible that porphyroblasts patterns in biotite (e.g., Figs. 10K and 11) are Although this is apparent in the zoned parts of may rotate, regardless of orientation, due to inferred to require a strong coaxial component many porphyroblasts (e.g., Figs. 7C–7D), it is strain interference with neighboring porphy- of strain during growth, and hence the presence also common to fi nd a diversity of curved inclu- roblasts. Furthermore, rotation also has the of such patterns may be used as an indicator of sion trail patterns, which yields information on effect of inducing cessation of textural sector this type of strain. Furthermore, the presence of the behavior of biotite and the matrix during growth as the {001} faces rotate toward folia- zoning in porphyroblasts that have c-axes nearly growth. Some curved inclusion trails are gen- tion and away from the optimum angle for ten- parallel to foliation but normal to lineation sig- tly convergent toward the center of the porphy- sile separation of the matrix from the {001} nifi es extension perpendicular to lineation, and roblast and some are parallel to foliation on the faces. When this happens, growth continues in can be used to suggest that fl attening strain margin, but have been rotated in the center. Fur- all directions with incorporation of inclusions accompanied growth. In the Pequop Mountains,

thermore, some curved inclusion trails appear (e.g., see rotated B2 section with clear zones porphyroblasts of this orientation are zoned, to represent growth over foliation defl ected by surrounded by included biotite in Fig. 13D). indicating fl attening strain. This is consistent a neighboring porphyroblast. Some curved inclusion trails may be unrelated with patterns of crystallographic preferred ori- Porphyroblasts with straight to convergent to rotation and are probably a result of growth entation of quartz c-axes in quartzite in the adja- inclusion trails exhibit no apparent rotation of a porphyroblast over foliation that has been cent garnet zone that are indicative of coaxial during textural sector growth (Figs. 11E– defl ected around a neighboring porphyroblast fl attening (Camilleri, 1998). 11G). Straight trails are typical of porphyro- (e.g., Figs. 13E, 13F). All of the foregoing Porphyroblasts without textural sector zon- blasts with quartz inclusions, whereas broadly types of inclusion trails serve to illustrate that ing occur in two settings: (1) in rocks where convergent trails are more characteristic of trail shapes refl ect matrix rheology, fl ow per- porphyroblasts with {001} at a low angle or those with graphite trails (e.g., Figs. 7C and turbations, and growth over matrix defl ected parallel to foliation are unzoned and porphy- 7D, and 12, respectively). This may refl ect by a neighboring porphyroblast. roblasts with {001} at a high angle to foliation

230 Geosphere, June 2009

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/5/3/215/3338228/i1553-040X-5-3-215.pdf by guest on 01 October 2021 Camilleri playing a 4 4 2 A and B is negligible limited to a A 3 1 general slowing of growth rate in a direction rate in a direction general slowing of growth 1 mm se microstructure present in C includes biotite present se microstructure overlying the porphyroblast in A is an extension A in overlying the porphyroblast m is thinner along grain margins parallel to folia- m is thinner 2 sections are orange in the line drawings; {001} these sections are 2 4 section with a well-developed overgrowth rim from sample 1a. Convergence of rim from section with a well-developed overgrowth 2 sections. View in all photomicrographs is perpendicular to foliation and parallel is perpendicular in all photomicrographs View sections. 2 sections that lack overgrowth rims from sample 1g. In these porphyroblasts growth in a direction in a direction growth sample 1g. In these porphyroblasts rims from sections that lack overgrowth 2 ned primarily by graphite and are broadly convergent (see Figs. 11F, 11G) with the foliation surrounding the porphyroblast dis the porphyroblast with the foliation surrounding 11G) convergent (see Figs. 11F, broadly ned primarily by graphite and are ). Photomicrographs and line drawings of hourglass zoning patterns in B ). Photomicrographs continued lled mostly with quartz, carbonate minerals, and sparse biotite. The destructive phase microstructure in the porphyroblasts in in the porphyroblasts The destructive phase microstructure lled mostly with quartz, carbonate minerals, and sparse biotite. section 2 B C similar convergence. The extremities of clear zone–hourglass boundaries are broadly arcuate in all porphyroblasts, indicating a in all porphyroblasts, arcuate broadly zone–hourglass boundaries are of clear The extremities convergence. similar that impinge on the B Biotite porphyroblasts stages of growth. during the latter to foliation (see Fig. 11D) perpendicular to lineation. Inclusion trails are deﬁ to lineation. Inclusion trails are parallel to foliation. (A, B) Examples of B is at a low angle or porphyroblasts fracture ﬁ fracture perpendicular to foliation was locally inhibited due to impingement with neighboring biotite porphyroblasts (orange). The vein (orange). to foliation was locally inhibited due impingement with neighboring biotite porphyroblasts perpendicular right sides. (C) Example of a B in strain shadows on their amount of quartz precipitated minor Figure 12 ( Figure and quartz precipitated in extension fractures (indicated by arrows) and in strain shadows (zone 4). (indicated by arrows) in extension fractures and quartz precipitated inclusion trails in this porphyroblast persists from the zoned interior through the overgrowth rim. Note that the overgrowth ri rim. Note that the overgrowth the overgrowth through the zoned interior persists from inclusion trails in this porphyroblast The destructive pha to foliation during overgrowth. perpendicular in a direction rate was slower tion, implying that the growth

232 Geosphere, June 2009

2 fracture fill fracture section 2 A 1 xtension fracture ﬁ xtension fracture section porphyroblasts with inclusion trails that section porphyroblasts 2 1 phs in B–D. (B) Enlarged view of the porphyroblast phs in B–D. (B) Enlarged view of the porphyroblast 1 mm the general trace of inclusion trails in, and foliation 1 mm 2 2 c 1 2 sections 2 B , and D section porphyroblasts with a diversity of inclusion trail geometries. These porphy- with a diversity of inclusion trail geometries. , and D section porphyroblasts 3 , A 2 ). Photomicrographs and line drawings of B ). Photomicrographs continued on next pages A B roblasts have sparse quartz and graphite inclusion trails and are from the same thin section (sample 1b). (A) View of three B of three View the same thin section (sample 1b). (A) from have sparse quartz and graphite inclusion trails are roblasts Figure 13 ( Figure show dextral (d), sinistral (c), and no apparent rotation (b) of inclusion trails. Solid black lines on the drawing illustrate rotation show dextral (d), sinistral (c), and no apparent shown in the photomicrogra that are letters b, c, and d indicate the porphyroblasts The circled outside of, the porphyroblasts. has convergent inclusion trails. Dashed purple lines denote boundaries of an e This porphyroblast rotation. showing no apparent

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/5/3/215/3338228/i1553-040X-5-3-215.pdf by guest on 01 October 2021 Camilleri sts that partially impinge 2 2 1 ts a larger degree of rotation than the grain of rotation degree ts a larger 1 1 mm 1 mm same lines are shown in the lower left corners of shown in the lower same lines are rotated porphyroblasts in the line drawings C porphyroblasts rotated 2 2 IT IT {001} {001} unrotated unrotated ects rotation of the {001} face away from the optimal angle for tensile separation of the matrix, the optimal angle for of the {001} face away from ects rotation Z Y ). (C) Enlarged view of the porphyroblast showing sinistral rotation. X, Y, and Z indicate positions of neighboring porphyrobla Y, X, showing sinistral rotation. ). (C) Enlarged view of the porphyroblast X continued C D Figure 13 ( Figure exhibi This porphyroblast showing dextral rotation. (D) Enlarged view of the porphyroblast porphyroblast. on the large, rotated in C and also has a poikiloblastic rim along the margins of grain, which reﬂ lines on the the matrix along {001} faces. Solid red over and hence growth growth in cessation of textural sector resulting These of the porphyroblast. the orientations of traces inclusion trails and {001} cleavage in center and D represent the line drawings in their pre-rotation orientations, where IT is trace of inclusion trails. IT orientations, where pre-rotation the line drawings in their

234 Geosphere, June 2009

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/5/3/215/3338228/i1553-040X-5-3-215.pdf by guest on 01 October 2021 Growth, behavior, and textural sector zoning of biotite porphyroblasts 1 D section 2 away from the host. away from n trails that probably represent growth growth represent n trails that probably 1 section 3 1 mm A 1 mm 2 1 section. Black arrows in all line drawings indicate positions section. Black arrows 3 D section section. From a two-dimensional perspective this can be visualized as the D section. From 3 area of photo in F and skeletal D sections. (F) Enlarged view of the section on left. Because this porphy- 3 ). (E) Photomicrograph and line drawing of an A and line drawing of an ). (E) Photomicrograph continued E F Figure 13 ( Figure rotation has undergone minor a thin fragment of the porphyroblast on the corners of grains, where of small extension fractures roblast is skeletal, it represents an edge cut and therefore the last vestige of growth. This porphyroblast has curved inclusio This porphyroblast the last vestige of growth. an edge cut and therefore is skeletal, it represents roblast A the by, the left side of strain cap developed on, and partially overgrown over section growing into the plane of the thin section and inheriting the matrix fabric perturbed by the A into the plane of thin section and inheriting matrix fabric perturbed by section growing

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are zoned, and (2) in rocks where porphyro- show negligible, examples of overgrowth from thin sections. In order of decreasing abundance, blasts appear to lack zoning regardless of crys- the same area). they are (1) extension fractures along {001}, tallographic orientation. In the fi rst setting, the The overgrowth rim could be interpreted (2) shear fractures along {001}, (3) extension apparent lack of zoning in porphyroblasts with as indicating a change to either growth rate fractures oblique to {001}, and (4) shear frac- {001} parallel to foliation may be a function of exceeding strain rate, hence precluding matrix tures oblique to {001} (Figs. 16 and 17). alignment of the biotite fast growth direction separation at the {001} faces, or simply growth with the extension direction, thereby enhancing under static conditions. However, observation of Extension Fractures the probability of growth rate outpacing retreat inclusion trails in the overgrowth rims suggests Extension fractures that developed parallel of the matrix in strain shadows. This would that some strain accompanied overgrowth. This to {001} constitute nearly all of the fractures generally preclude development of a void in is suggested by the persistence of the conver- in biotite porphyroblasts and they tend to have which to precipitate clear biotite along the gence of inclusion trails from the zoned parts of a predictable spacing and geometry. To under- foliation-normal, non-{001} faces, and hence the crystals through the overgrowth rim coupled stand the distribution of these fractures, it is growth would proceed by matrix replacement. with a geometrically similar convergence of In samples where all biotite appears unzoned, foliation surrounding the porphyroblasts (e.g., porphyroblasts either do not have ample inclu- Figs. 7A and 12C). This implies that the gen- sions to indicate zoning, or have inclusions but eral strain regime that produced textural sector no overt evidence of zoning (e.g., Figs. 14A, growth continued through overgrowth. Figure 14. Photomicrographs of biotite por- 14B). There are several possible reasons for the phyroblasts that lack evidence of textural lack of zoning. First, the rock may not be of DESTRUCTIVE PHASE: RESIDUAL sector zoning or have a subtle manifesta- appropriate composition to develop inclusions, GROWTH AND STRAIN tion of zoning. (A) Foliation-parallel view e.g., insuffi cient graphite or quartz. Second, the of numerous unzoned B sections (sample relative rates of strain versus growth may not The inception of the destructive phase is gen- 7). (B) Foliation-normal, lineation-parallel be appropriate for textural sector growth, i.e., erally marked by (1) the cessation of porphyro- view of unzoned B sections (indicated by there may be low or negligible strain resulting blastic growth in all directions and (2) the begin- arrows) in sample 7. The porphyroblasts in apparent growth under static conditions (e.g., ning of intragrain strain coupled with restricted have straight inclusion trails and probably Figs. 14A, 14B). On a local to regional scale, residual growth of biotite in strain shadows and grew over foliation in a low-strain or no- the absence of textural sector zoning, although extension fractures. These processes occurred strain environment. These porphyroblasts sparse, in part refl ects the partitioning of strain. as the growth of biotite diminished and the fi ll exhibit variable amounts of dextral rota- Figures 14C–14E show an example of this, of dilating strain shadows transitioned from tion. (C) Foliation-normal photomicrograph where schist with a graphite-poor layer contain- precipitation of biotite during textural sector of a rock that has a graphite-rich layer with ing crudely zoned porphyroblasts is overlain growth to predominantly quartz. Overall, the unzoned porphyroblasts at the top and an by a graphite-rich layer with unzoned, highly destructive phase appears to have taken place underlying graphite-poor layer where some included porphyroblasts that did not undergo during a broad continuation of the strain regime biotite porphyroblasts exhibit crude zoning the textural sector growth process. that characterized textural sector growth, but (sample 13B). The dashed yellow line shows it involved straining more than growing of the the boundary between the two layers. The Matrix Overgrowth porphyroblasts. The destructive phase involved predominantly quartz-fi lled strain shadows fracturing, rotation, dissolution, and minor kink- and extension fractures evident in both lay- For many rocks the last vestige of biotite’s ing and subgrain development on the corners of ers formed during the biotite post-zoning constructive phase is marked by cessation of grains. These processes collectively obscured or destructive phase. (D) Example of unzoned textural sector growth and overgrowth of matrix modifi ed the textural sector zoning and inclu- B section (indicated by arrow) from the in all directions. The overgrowth appears to sion trail patterns developed during the con- unzoned layer from sample 13b. On the left, refl ect a strain regime similar to that that pro- structive phase. this porphyroblast has a strain shadow fi lled duced the zoning, but with growth rate exceed- with quartz and a minor amount of biotite.

ing strain rate. In B section porphyroblasts the Fracturing of Biotite (E) Examples of crudely zoned B2 sections overgrowth appears as a poikiloblastic, or less (indicated by arrows) from the zoned layer commonly, dendritic rim framing the included Following development of poikiloblastic or in sample 13b. These porphyroblasts were hourglass shape and clear zones (Figs. 6A, dendritic rims, fracturing in biotite appears to rotated following textural sector growth. 15A, and 15C). However, thin sections that mark a fundamental transition from growth to The top porphyroblast shows a dextral contain porphyroblasts with overgrowth rims strain of porphyroblasts and a shift from strain sense of rotation, and the bottom shows a also generally contain a few B sections that are focused in the matrix that was accommodat- sinistral sense of rotation. Dashed black entirely dendritic and lack any evidence of tex- ing textural sector growth (i.e., as in Fig. 10) lines indicate the positions of the hourglass tural sector zoning (e.g., Fig. 15B). These por- to transference of some strain to the porphy- shape—clear zone boundaries. The bottom phyroblasts probably represent edge cuts that roblast. Fractures are mostly present in porphy- porphyroblast has a poorly deﬁ ned clear only penetrate the dendritic rim (cf. Figs. 15A, roblasts with {001} at a high angle to foliation. zone on the right. The right clear zone in the 15B). The degree of overgrowth is usually The fractures do not penetrate the matrix, sug- top porphyroblast is slightly misoriented consistent on a thin-section scale, but can vary gesting a rheologic contrast between a ductile with respect to the host and is deformed. from negligible to extreme in the same outcrop matrix and brittle porphyroblast at the time Note that although the photomicrographs in area, and therefore the driving forces for over- of fracture. Fractures can be divided into four C, D, and E are from sample 13B, the por- growth are probably localized (Figs. 7A and categories based on apparent slip or separation phyroblasts in D and E are not present in 12C show extreme, and Figs. 12A and 12B viewed in lineation-parallel, foliation-normal the photomicrograph in C.

236 Geosphere, June 2009

A B

1mm 1mm

C D A graphite-rich layer graphite-rich

0.25 mm graphite-poor layer graphite-poor

1mm E

0.25 mm

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2 Foliation-normal, Foliation-normal, lineation parallel 3 2 section in a thin cut per- 2 1 n on the appearance of porphyroblasts in n on the appearance of porphyroblasts scales in A and C are the same. and C are A scales in section 0.5 mm 2 2 B 4 section, and an unzoned B 2 2

B section 1 & 3 & 1 of a foliation-parallel B ned by graphite inclusions. (A) Photomicrograph section 1 C Foliation-normal, Foliation-normal, lineation parallel section, a zoned B 1 B section shown B section shown in photo B 2 in photo C B 1 mm section 2 B section shown 1 C in photo C Foliation parallel Foliation A C pendicular to foliation. (B, C) Examples of what these cross sections would look like in the foliation-normal thin section. The sections would look like in the foliation-normal thin section. to foliation. (B, C) Examples of what these cross pendicular Figure 15. Photomicrographs and line drawings of zoned porphyroblasts that highlight the effects of cut orientation and locatio and line drawings of zoned porphyroblasts 15. Photomicrographs Figure a thin section cut perpendicular to foliation and parallel lineation (sample 1a). Zoning is deﬁ a thin section cut perpendicular section with a dendritic rim and lines showing cross sections that would yield a zoned C section with a dendritic rim and lines showing cross

238 Geosphere, June 2009

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/5/3/215/3338228/i1553-040X-5-3-215.pdf by guest on 01 October 2021 Camilleri lled primarily with quartz and lled extension fracture that is oriented lled extension fracture quartz-ﬁ lusion trails and a {001}-parallel shear fracture fracture lusion trails and a {001}-parallel shear F is the same as in E. F of the line drawings in their pre-rotation orientations pre-rotation of the line drawings in their B2 section F lled with biotite. Solid white lines on the rotated porphyroblasts in A and B represent the and B represent A in porphyroblasts lled with biotite. Solid white lines on the rotated denote the trace of {001} The yellow lines in E and F ned by sparse quartz and graphite inclusions. 1 mm ). Photomicrographs of fractured biotite porphyroblasts. (A–D) Examples of zoned porphyroblasts with extension fractures ﬁ with extension fractures (A–D) Examples of zoned porphyroblasts biotite porphyroblasts. of fractured ). Photomicrographs

continued {001} D section E (sample 14). Arrows show shear sense. Dark bubble in the center is an air bubble in the thin section. (F) Porphyroblast with a bubble in the thin section. (F) Porphyroblast is an air sense. Dark bubble in the center show shear Arrows (sample 14). (IT—trace of inclusion trails). The scale in B–D is the same as in A. (E) Zoned D section porphyroblast with rotated quartz inc with rotated A. (E) Zoned D section porphyroblast The scale in B–D is the same as (IT—trace of inclusion trails). Figure 16 ( Figure mostly ﬁ that are denote positions of fractures sample 16a). Black arrows biotite (from left corners shown in the lower These same lines are orientations of the traces quartz inclusion trails and {001} cleavage. oblique to {001} (sample 1b). This porphyroblast exhibits subtle zoning deﬁ This porphyroblast oblique to {001} (sample 1b). The scale in zones and the included core. cleavage and the dashed black lines highlight subtle boundaries between clear

240 Geosphere, June 2009

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/5/3/215/3338228/i1553-040X-5-3-215.pdf by guest on 01 October 2021 Growth, behavior, and textural sector zoning of biotite porphyroblasts section 3 lled by growth section 2 2 B 1 3 1 2 2 section denote the boundaries of an extension 2 1 F section 1 1 biotite in the fracture. To the left of the center fracture is a fracture the left of center To in the fracture. 4 pendicular to inclusion trails. pendicular C F 1mm 1 section 1 2 2 3 allanite A 3 lled extension fractures. The scale is the same in both photomicro- lled extension fractures. 1 section with a wide extension fracture in the center and fracture zones and fracture in the center section with a wide extension fracture 2 lled with quartz (clear) and sheaths of biotite (shown as yellow) that are lled with quartz (clear) and sheaths of biotite (shown as yellow) that are on the host and bottom half was fi ll is a syntaxial overgrowth section sections below is a strain cap. Narrow, inclined white zone in center of the A of the inclined white zone in center sections below is a strain cap. Narrow, 2 2 B and B 1 The top half of the fi ll zone. sections with graphite inclusions and biotite-fi 2 section above and the C 3 sections with contiguous grain boundaries (from sample 1g). Purple lines in the B sections with contiguous grain boundaries (from 1 , and C 2 , B 3 B A shows the fi lled with biotite and the double-headed arrow section into the fracture. Dark zone between the A Dark zone between the section into the fracture. 1 of the C is a zone of alteration, possibly along a non-{001}-parallel extension fracture. (B) Photomicrograph of a B (B) Photomicrograph is a zone of alteration, possibly along non-{001}-parallel extension fracture. fracture fi fracture Figure 17. Photomicrographs and line drawings of zoned B 17. Photomicrographs Figure A of graphs. (A) Photomicrograph zone of several small fractures indicated by purple lines. These fractures are delimited by zones of clear biotite oriented per delimited by zones of clear are These fractures indicated by purple lines. zone of several small fractures on either side (locations indicated by arrows; from sample 1a). The extension fracture in the center is fi in the center The extension fracture sample 1a). from side (locations indicated by arrows; on either also present are denoted by F, to the host. Several included fragments of host, which are slightly misoriented with respect

Geosphere, June 2009 241

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important to view B sections so that most or all and number of fractures observed in many bio- sense of shear or rotation. These samples prob- of the porphyroblast can be seen. From these tite porphyroblasts. This is supported by the ably record a small component of noncoaxial observations it appears that porphyroblasts observations that the fractures formed at a time strain signifi cant enough to produce a preferred typically contain one to three fractures, which when there was a rheologic contrast between sense of rotation (a discussion of the regional tend to divide porphyroblasts into segments the matrix and porphyroblasts, and that most tectonic implications of such a noncoaxial com- of approximately equal width, with one com- fractures are equally spaced with at least one ponent is in Appendix 5 in Supplemental File 1; monly being in the center (Figs. 16A–16D). at or near the center, albeit the order of fractur- see footnote 1). Precipitation of biotite, quartz, and sparse ing can’t be ascertained (additional information The observed multidirectional rotation of opaque and epidote minerals by intergranu- about porphyroblasts with off-centered frac- biotite porphyroblasts toward foliation in the lar fl uids followed fracturing (Figs. 16 and tures is in Appendix 4 in Supplemental File 1; Pequop Mountains is similar to that reported 17). Growth of biotite in fractures was either see footnote 1). Moreover, stress transfer from by Miyake (1993) in Japan. Miyake (1993) syntaxial or nonsyntaxial. Syntaxial growth the matrix and resulting fracture was prob- described unzoned biotite porphyroblasts with is easily recognized by uniform extinction ably triggered by attainment of a critical size straight inclusion trails exhibiting multidirec- of the porphyroblast and the fi ll. In fractures coupled with cessation of growth, which may tional senses and amounts of rotation, with that are mostly fi lled with syntaxial biotite, facilitate stress transfer when the porphyroblast some exhibiting no apparent rotation. Rota- quartz appears as elongated blebs parallel to ceases to replace matrix (i.e., grow) in a direction sense and rate were interpreted by Miyake {001} and can resemble inclusions (Figs. 7C tion perpendicular to foliation. (1993) to be controlled by their shape and the and 16C; additional data are in Appendix 4; orientation of {001}. Variable rotation was see footnote 1). Nonsyntaxial growth is more Shear Fractures attributed to strain with a pure shear (coaxial) common and is indicated by various sheaths Porphyroblasts with shear fractures are fl attening component perpendicular to folia- of biotite that are generally only slightly mis- sparse but are present in some thin sections tion and a minor simple shear (noncoaxial) oriented with respect to the host. Nonsyntaxial with abundant extension fractures. These frac- component subparallel to foliation. growth may have resulted from crystal defects tures cannot be explained by the stress-transfer altering growth patterns or syntaxial growth mechanism; however, most shear fractures Dissolution and Quartz Strain Shadows on fragments of the host that sustained minor are parallel to {001}, occur in porphyroblasts rotation during fracturing (e.g., Fig. 17B). with rotated inclusion trails (Fig. 16E), and Dissolution of biotite porphyroblasts and Some porphyroblasts appear to have a com- may have formed when {001} was rotated sparse development of quartz strain shadows bination of syntaxial and nonsyntaxial growth toward foliation to an optimum angle for shear accompanied and outlasted fracturing and rota- in fractures, but in one extension fracture fracture according to distribution of principle tion. Dissolution of biotite and quartz in the observed, half of the fracture was fi lled by syn- stresses for the whole rock (i.e., they formed matrix along porphyroblast margins parallel to taxial growth on the host and the other half by when {001} was rotated to a moderate angle foliation resulted in mild to moderate develop- σ growth of a neighboring porphyroblast into the from 1). Alternatively, some may be extension ment of strain caps and hourglass shapes with void (Fig. 17A). Post-zoning growth of biotite fractures with negligible separation that were missing components (e.g., Fig. 18C), and it in extension fractures results in the production rotated and reactivated as shear fractures. facilitated development of quartz strain shad- of a clear zone that can be distinguished from ows accompanied by minor syntaxial growth those produced during textural sector growth, Rotation of Whole and Fractured Segments of biotite (e.g., Fig. 7A). The quartz-fi lled specifi cally because they are rectangular rather of Porphyroblasts strain shadows have a diversity of shapes (e.g., than conical in shape and they transect zoning Figs. 13C, 14D, 14E, 18A–18D, 19A–19D, patterns (e.g., Fig. 17). In any given foliation-normal thin section it is and 20) and can range from mildly asymmetric Several observations suggest that the exten- evident that some whole and fractured segments to symmetric in the same thin section, but the sion fractures developed by the fi ber-loading or of porphyroblasts rotated toward foliation fol- most unusual and informative types of strain stress-transfer mechanism (Lloyd et al., 1982) lowing textural sector growth (Figs. 18 and 19). shadows are those that have an unusual bicus- as a consequence of stresses imparted to the This rotation, coupled in some cases with minor pate geometry that is present on one or both rigid porphyroblasts by the ductiley fl owing growth (Fig. 18F), yielded B sections that have {001} faces (Fig. 20). The formation of bicus- matrix. It has been shown that the development inclined hourglass shapes with inclusion trails pate strain shadows appears to refl ect coaxial of extension fractures in fi bers in composite that have a diversity of apparent dip angles and strain resulting in divergent separation of the materials occurs when stress is transferred from dip directions (Fig. 19). Rotation of porphyrob- matrix away from the corners of the porphyrob- the fl owing matrix to the fi ber. In these cases lasts in opposing directions is generally evident last (Fig. 20). Many bicuspate shadows appear tensile stress is greatest at the mid-point of the in both thin sections cut parallel and perpendic- to have initiated following fracturing of cusped fi ber (or mineral), and when tensile strength is ular to lineation (e.g., Figs. 16A–16B, 18, and corners of porphyroblasts or a {001} boundary exceeded, a fracture develops in the center (e.g., 19). Direction of rotation for most porphyrob- between a thin overgrowth rim and the zoned Boullier, 1980; Watts and Williams, 1980; White lasts appears to be controlled by the orientation part of the crystal. Development of the shadows et al., 1980; Lloyd et al., 1982; Ji and Zhao, of {001} prior to rotation whereby those with ensued when the fractured fragments separated 1993). Fractured segments will then continue a dextral sense of rotation had {001} canted to and rotated away from the host (Fig. 20). Bicus- to fracture at their midpoints until the length of the right and those with a sinistral sense were pate strain shadows are generally only present a segment is below some critical length where canted to the left (e.g., Figs. 16A, 16B, 18F, and on porphyroblasts whose inclusion trails exhibit stress cannot exceed the tensile strength of the 18G). Such multidirectional rotation is likely little or no apparent rotation and are absent on material (e.g., Lloyd et al., 1982). The stress- a product of strain that is dominantly coaxial. rotated porphyroblasts in the same thin section, transfer mechanism appears to be a plausible However, in some samples the majority of attesting to the heterogeneity of strain on the explanation of the consistency of the geometry porphyroblasts indicate a top-toward-the-west thin-section scale.

242 Geosphere, June 2009

NW SE

{001} {001} inclusion trails parallel foliation: IT inclusion trails with approximately IT B2 section no apparent rotation B2 section 17 degrees apparent rotation unrotated unrotated C D

{001} {001} inclusion trails with approximately IT minority porphyroblast with IT B2 section 30 degrees apparent rotation B2 section opposing sense of rotation unrotated unrotated E

removed by dissolution

B2 section D section removed by unrotated dissolution Foliation and lineation normal section

1 mm Figure 18 (continued on next page). Photomicrographs of zoned, unfractured porphyroblasts that sustained rotation and developed strain shadows during the destructive phase. (A–E) Photomicrographs of porphyroblasts that sustained no apparent rotation (A) to as much as 30°of apparent rotation (B–E) from the same thin section (sample 15a). Zoning patterns range from well defi ned in B and C to subtle in A, D, and E. White lines on the porphyroblasts represent the orientations of the traces of inclusion trails and {001} cleavage. These same lines are shown in their pre-rotation orientation in the lower right corner of the photomicrographs (IT—trace of inclusion trails). The porphyroblasts in A–D are from a foliation-normal, lineation-parallel thin section, and E is from a foliation-normal and lineation-normal thin section. The majority of porphyroblasts in the lineation-parallel section display a sinistral sense of rotation (e.g., B and C), although some have a dextral sense (e.g., D; note also that the strain shadows in D formed after rotation). Evidence of opposing senses of rotation in the lineation normal section in E indicates rotation in diverse directions. Porphyroblasts in B–E contain inclined, rotated hourglass shapes with evidence of dissolution on porphyroblast margins that were rotated into the shortening fi eld. A few inclusions (circled red in the line drawing) along the margin of the porphyroblast in C are at a slight angle to the rest and suggest minor growth during rotation. In addition, most strain shadows are fi lled with quartz, with the exception of the porphyroblast in A, where nonsyntaxial biotite fi lls the lower part of the strain shadow on the right.

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DOES TEXTURAL SECTOR ZONING, persisted to higher grades of metamorphism, tains, biotite porphyroblasts in the Wood Hills OR MICROSTRUCTURE OF THE correlative kyanite zone schist from the adja- are, in places, slightly larger (as much ~2 mm DESTRUCTIVE PHASE, PERSIST AT cent Wood Hills (Fig. 1) was examined. Many in the longest dimensions), matrix grains are HIGHER METAMORPHIC GRADES? of the schists in the Wood Hills contain biotite coarser, chlorite is absent as a prograde phase, as the sole porphyroblast species, but some and they lack characteristic low-grade pressure To assess whether processes operative in the also contain staurolite, kyanite, and garnet solution microstructure such as strain caps. biotite zone and their resultant microstructure (Fig. 21). In comparison to the Pequop Moun- Despite higher metamorphic grade and growth

{001} IT B2 section 1 mm unrotated

{001} IT B2 section unrotated

Figure 18 (continued). (F, G) Examples of porphyroblasts with opposing senses of rotation from a foliation-normal, lineation- parallel thin section from sample 16c. Zoning in porphyroblasts in this sample is subtle because quartz inclusions are sparse. This sample contains fractured and unfractured porphyroblasts that have no apparent preferred sense of rotation (Fig. 19 shows examples of rotated, fractured porphyroblasts from this same thin section). The porphyroblasts in F and G show sinistral and dextral senses of rotation, respectively. The enlarged photomicrograph in F shows growth prongs of biotite (and partial inclusions) extending into the foliation, indicating that growth occurred during and/or following rotation. The scales in A–D are as in E, and the scale for G is the same as in F.

244 Geosphere, June 2009

of other porphyroblast species, some relict tex- strong relicts of zoning probably because reac- + muscovite + staurolite = biotite + garnet + tural sector zoning patterns and poikiloblastic tions associated with these minerals involved kyanite; Camilleri and Chamberlain, 1997). rims of the constructive phase as well as relicts consumption of old zoned biotite and growth Consequently, if searching for evidence of rel- of extension fractures and evidence of rotation of new biotite. For example, biotite porphyro- ict textural sector zoning in high-grade rocks, from the destructive phase are present in the blasts in these rocks tend to have an included it is best to look in schist that has biotite as the Wood Hills. Figures 21A–21E show examples center surrounded by a clear rim that obscures sole porphyroblast species. of relicts of these features present in a schist what may be relict zoning patterns, which con- In summary, because biotite porphyroblasts sample that contains sparse staurolite and trasts with overtly zoned relicts with poikilo- in the Wood Hills contain relicts of the same kyanite. This sample is similar to the Pequop blastic rims in samples that lack or have minor features as those in the Pequop Mountains, and Mountains sample shown in Figure 19 in that amounts of kyanite, staurolite, and garnet (cf. overall are not much larger, it is reasonable to inclusion trails are variable in apparent dip Figs. 21A–21E with 21H). The growth of the assume that most nearly reached their peak size, direction and angle. Samples with abundant clear rim may be a product of the kyanite- and underwent modiﬁ cation in the destructive kyanite, staurolite, and garnet (Fig. 21G) lack producing reaction in this rock (e.g., quartz phase, in the biotite zone. Therefore, it appears

A B2 section B

D section

B2 section 1 mm 1 mm C

B2 sections

1mm

Figure 19. Photomicrographs of fractured porphyroblasts in sample 16c from a foliation-normal, lineation-parallel thin section. Por- phyroblasts in this sample show no apparent preferred sense of rotation. These zoned porphyroblasts have quartz inclusions and exten-

sion fractures ﬁ lled mostly with quartz. Matrix foliation in all photomicrographs is horizontal. (A) Fractured B2 section with a dextral

sense of rotation. Inclusion trails in the D section underlying the B2 section exhibit no apparent rotation. (B) Fractured B2 section with divergent senses of rotation of fractured segments. (C) Photomicrographs of porphyroblasts with dextral and sinistral senses of rotation or no apparent rotation. Yellow lines denote the orientation of the trace of inclusion trails where the horizontal lines are on top of porphyroblasts with no apparent rotation. Dashed black lines highlight zoning patterns. Unfractured, but rotated, porphyroblasts with sinistral and dextral senses of rotation from this sample are shown in Figure 16F and 16G.

Geosphere, June 2009 245

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that textural sector growth was only operative in the constructive and destructive phases as sector growth. When this happens some strain the biotite zone. depicted in Figure 22. The constructive phase is transferred to the porphyroblasts, resulting involves strain that is accommodated by both in rotation and tensile fracturing (via the stress BIOTITE PORPHYOBLAST GROWTH the textural sector growth mechanism and the transfer mechanism). AND IMPLICATIONS FOR THE shear and reactions in the matrix that facilitate Progression through the constructive and INTERPRETATION OF INCLUSION it. This phase ceases as porphyroblasts mature destructive phases inevitably results in porphy- TRAILS in size and growth rate diminishes as nutrients roblasts with a diversity of inclusion trail patterns become depleted, and may be marked in some that record variable strain and growth histories The growth processes observed in the Pequop areas by dendritic overgrowth as crystals extend at all scales. The variability observed on a small Mountains may characterize low-grade Bar- growth prongs to reach nutrients. The transi- scale in a thin section is a function of where the rovian metamorphism of pelite in parts of tion to the destructive phase is triggered when porphyroblast is cut relative to its growth center orogenic belts undergoing burial, heating, and porphyroblasts cease to replace matrix per- (e.g., Fig. 15) and the heterogeneity and parti- collapse. In essence, in the biotite zone, biotite pendicular to foliation, coupled with markedly tioning of strain. Partitioning and heterogeneity should undergo a natural progression through diminished accommodation of strain by textural results in: (1) development of opposing senses

F-Section B2 section 1 mm

B2 section

Figure 20. Photomicrographs of zoned B2 sections displaying bicuspate, quartz-ﬁ lled strain shadows (from sample 16a). (A) Strain shadows on the top right, lower left, and possibly top left, developed by rotation of fractured slivers of biotite along the porphyroblast margins. The strain shadow on the lower right corner lacks a rotated sliver of biotite. (B) Strain shadows on the left and top right developed by rotation of fractured slivers of biotite. This porphyroblast has overgrown a smaller biotite crystal in the lower right corner. The yellow arrows adjacent to the strain shadows in both photomicrographs denote (1) sense of shear in the matrix relative to the porphyroblast, and (2) the sense of rotation of fractured slivers of biotite that were severed from the host but are present in the strain shadows. Black arrows indicate positions of {001}-parallel extension fractures. The scale in A is the same as in B.

246 Geosphere, June 2009

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/5/3/215/3338228/i1553-040X-5-3-215.pdf by guest on 01 October 2021 Growth, behavior, and textural sector zoning of biotite porphyroblasts lineation- section with a 2 1 mm hyroblast in D has exsolved(?) acicular in D has exsolved(?) acicular hyroblast C section shown in C has relict extension section shown in C has relict 2 tite porphyroblasts in the same thin sec- tite porphyroblasts es of biotite growth. This sample resembles the This sample resembles es of biotite growth. C shown in D). ctions. The porphyroblast in A is a B A in The porphyroblast ctions. of {001}-parallel inclusions. The porphyroblasts in D The porphyroblasts of {001}-parallel inclusions. divided into two crystallographically misoriented parts es a relict clear zone; and dashed white lines denote the clear es a relict ture marked by false {001}-parallel inclusions (indicated ture C KY section 2 section with a sinistral sense of rotation. The B section with a sinistral sense of rotation. D section B 2 B D C C C KY ). Photomicrographs of porphyroblasts in kyanite zone schist from Wood Hills. All photomicrographs represent foliation-normal, represent All photomicrographs Hills. Wood in kyanite zone schist from of porphyroblasts ). Photomicrographs C section section 2 2 B B sections, respectively, with poikiloblastic rims and inclusion trails that exhibit no apparent rotation. In addition, the porp rotation. with poikiloblastic rims and inclusion trails that exhibit no apparent sections, respectively, A C 3 continued on next page parallel thin sections, and the foliation is horizontal. Dashed black lines indicate traces of relict inclusion trails; C denot parallel thin sections, and the foliation is horizontal. Dashed black lines indicate traces of relict of zoned bio GT—garnet. (A–E) Photomicrographs KY—kyanite; ST—staurolite; zones and the included core. boundaries between clear of the constructive and destructive phas have quartz inclusions and display relicts These porphyroblasts WH1a. sample tion from biotite zone samples shown in Figures 18 and 19, where inclusion trails display a diversity of apparent dip angles and dire inclusion trails display a diversity of apparent 18 and 19, where biotite zone samples shown in Figures minerals, most of which are oriented at an angle to the trails of inclusions. The photomicrographs in A–E have the same scale ( in The photomicrographs oriented at an angle to the trails of inclusions. minerals, most of which are fractures indicated by arrows and rotated fractured segments that display a sinistral sense of rotation. This porphyroblast is This porphyroblast segments that display a sinistral sense of rotation. fractured and rotated indicated by arrows fractures side the morphology of biotite on either judging from had a component of dextral shear, probably This fracture by left arrow). by the extension fracture on the right. Biotite to the left of this fracture has uniform extinction and a relict, annealed frac has uniform extinction and a relict, on the right. Biotite to left of this fracture by the extension fracture A D and and E are poikiloblastic rim and a minor amount of dextral rotation. The porphyroblast in B is a The porphyroblast amount of dextral rotation. poikiloblastic rim and a minor Figure 21 ( Figure

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/5/3/215/3338228/i1553-040X-5-3-215.pdf by guest on 01 October 2021 Camilleri ST C = biotite + garnet + kyanite, where kyanite = biotite + garnet kyanite, where C d GT. These porphyroblasts appear to be relict to be relict appear These porphyroblasts d GT. 1 mm 1 mm C section section 2 2 A B of schist with abundant biotite, lled with biotite and quartz. (G, H) Photomicrographs F H section in E that has a poikiloblastic rim). See text for explanation. section in E that has a poikiloblastic rim). See text for 3 ST GT section with sparse quartz and graphite inclusions from sample WH1c, which does not have ST, KY, and GT. This porphyro- and GT. KY, WH1c, which does not have ST, sample section with sparse quartz and graphite inclusions from ST 2 1 mm KY ). (F) Photomicrograph of a B ). (F) Photomicrograph section 3 continued A G E sections rimmed by relatively unincluded biotite (this is in contrast with the A unincluded biotite (this is in contrast with the sections rimmed by relatively 2 Figure 21 ( Figure A blast has uniform extinction and a relict, annealed extension fracture in the center that is ﬁ in the center annealed extension fracture blast has uniform extinction and a relict, quartz + muscovite staurolite WH1b and shows evidence of the reaction sample in G is from The photomicrograph and GT. KY, ST, an KY, schist that has abundant ST, WH2, which is from sample from in H are The biotite porphyroblasts staurolite. has replaced

248 Geosphere, June 2009

A Microstructure of the Constructive Phase Nucleation and Textural sector growth Late stage matrix initial growth (e) overgrowth* (a) (c) (b) (d)

(l) { 0 0 1 } 1 0 0 {

{ 0 1 } { 0 1 } { 0 0 1 } 1 0 0 { { 0 0 1 } 1 0 0 { { 0 0 1 } 1 0 0 { (j)

(h) { 0 1 } { 0 1 } (f) (g) (i) { 0 1 } (k) { 0 0 1 } *not present in { 0 0 1 } all samples { 0 0 1 } { 0 0 1 } { 0 0 1 } { 0 0 1 } { 0 0 1 } { 0 0 1 }

B Microstructure of the Destructive Phase Included biotite (m) Clear biotite* (n) (o) (p) Quartz Pyrite

Trace of foliation (r) (s) (t) (q) Trace of inclusion trails

*sparse inclusions may (u) (v) (w) (x) develop in biotite’s clear zones in the constructive phase.

~1 mm

Figure 22. Synoptic diagram showing the characteristic microstructure of biotite constructive and destructive phases. Development of this microstructure accompanies progressive shortening approximately perpendicular to foliation and extension parallel to foliation. (A) Sketches showing the microstructure of different stages of the constructive phase. Nucleation and initial growth of biotite occur in a diversity of crystallographic orientations but biotite with {001} at a high angle to foliation tends to form in low-strain or no-strain areas, such as strain shadows of other mineral grains. Growth of biotite with {001} at a high angle to foliation will produce textural sector zoning (porphyroblasts a–j), but growth of crystals with {001} parallel or at a low angle to foliation will not produce zoning (e.g., porphyroblast k). Zoned porphyroblasts can develop straight (a, b, d, h, i, and j) to convergent (e) inclusion trails, but also may develop curved trails later in their growth history if the porphyroblast rotates (c, g, and f). If signifi cant rotation occurs, matrix separation from {001} will cease and growth will continue in all directions by matrix replacement (f). Post-zoning dendritic or poikiloblastic overgrowth (l) prior to the destructive phase may occur locally. (B) Sketches showing microstructure of the destructive phase. Typical microstructure includes: (1) {001}-parallel extension fractures fi lled with quartz (m, n, o, p, s, and u), biotite (r), or a combination where quartz can appear as false {001}-parallel inclusions (p, x); (2) rotated whole (t, v, w) and fractured porphyroblasts (m, s) that may have minor curvature of inclusion trails along the grain margin if some growth of biotite followed rotation (e.g., right side of w); (3) single (t) or bicuspate (r) strain shadows fi lled mostly with quartz; (4) strain caps and evidence of dissolution of biotite along porphyroblast margins that parallel foliation (t, v, w); and (5) kinking, subgrain development, and fracturing of cusped corners of porphyroblasts (u). Note that although porphyroblast cross sections that cut both {001} faces are shown as parallelogrammatic or rectangular in shape in this diagram (e.g., a and e), it is also possible to have trapezoidal shapes if non-{001} crystal faces are well developed.

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of shear due to rotation of porphyroblasts during other microstructures that represent the sum of phase involving fracturing, rotation, and resid- textural sector growth (e.g., Figs. 13A–13D), the infl uence of the constructive and destruc- ual growth. Relicts of constructive and destruc- (2) perturbations of matrix around growing and tive phases of biotite growth in the biotite zone tive phase microstructures become obscured at impinging porphyroblasts during textural sector rather than conditions in the garnet zone. Garnet higher metamorphic grades, but are still recog- growth that leads to overgrowth of the perturbed that inherited biotite inclusion trails was docu- nizable in kyanite zone rocks. The implications matrix (e.g., Figs. 13E, 13F), and (3) production mented in the Scottish Highlands by Barker of this study, and important points about the pro- of variable senses and amounts of rotation of (2002) and in the Imjingang belt in Korea by gression from the constructive to the destructive fractured and unfractured porphyroblasts during Kim and Cho (2008). In summary, the forego- phases, are the following: the destructive phase (e.g., Figs. 18, 16A, and ing observations indicate that caution should be (1) Following nucleation of biotite, the growth 16B). Variability in strain history on a moun- used when interpreting inclusion trails in biotite of crystals with {001} at a high angle to folia- tain-range scale is probably also related to strain in general and specifi cally in higher grade index tion will become enhanced when grains attain partitioning and is refl ected in the development porphyroblasts that may have replaced other a size large enough to create their own strain of textural sector zoning in biotite in most rocks, porphyroblast species. shadows and can sustain a growth rate high but not all, during the constructive phase and in enough to overgrow matrix in a direction per- the variability in the style and intensity of the COMPARISON WITH OTHER pendicular to foliation. At this point, the con- destructive phase (e.g., compare the lack versus METAMORPHIC TERRAINS structive growth phase ensues and results in abundance of destructive phase microstructure textural sector zoning of passive inclusions, in Figs. 15 and 21F, respectively). The growth of biotite porphyroblasts in the which ultimately yields an hourglass-shaped Pequop Mountains–Wood Hills area refl ects a distribution of inclusions in cuts viewed par- Implications for the Interpretation of two-stage process similar to that described in allel or subparallel to the porphyroblast c Inclusion Trails other, generally higher grade, regional meta- axis. Zoning requires uninhibited growth in morphic terrains in the Pyrenees, Japan, Scot- all directions and a delicate balance between Understanding the origin of inclusion trails tish Highlands, and Korea (Lister et al., 1986; growth and strain rate such that the growth of in Barrovian index minerals (e.g., biotite, gar- Miyake, 1993; Barker, 2002; Kim and Cho, biotite in the direction of the c axis broadly net, and staurolite) is important because the 2008, respectively), but differs with regard to keeps pace with retreat of the matrix from the trails are used to assess strain during progres- the fi rst stage. The fi rst growth stage in the other {001} faces in dilating strain shadows. sive metamorphism and to aid geothermobaro- terrains is characterized as matrix replacement, (2) The shapes of included hourglasses and metric studies of these minerals (e.g., Johnson, whereas in the Pequop Mountains it is charac- porphyroblasts are variable and, in part, are 1999). Because progression through the biotite terized by textural sector zoning that involves controlled by strain as a function of whether constructive and destructive phases results in both matrix replacement and the crack-fi ll the c axis was aligned with the maximum or production of inclusion trails with a heteroge- growth mechanism, although the crack-fi ll minimum extension directions (i.e., parallel neous array of apparent dip angles, dip direc- mechanism in this stage is restricted to separa- or perpendicular to lineation, respectively). tions, and trail patterns (Fig. 22), caution must tion of the matrix from {001} faces that are at a (3) Inclusion trail geometry developed during be used when inferring strain histories from high angle to foliation (i.e., no intragrain exten- textural sector growth can be variable on a inclusion trails in biotite. Similarly, although it sion fracturing). Nonetheless, the fi rst stages in thin-section scale. Inclusion trails may be is common practice to designate porphyroblasts all these terrains, whether they involve textural straight or convergent refl ecting nonrotation as intertectonic, syntectonic, or post-tectonic on sector zoning, can be considered the construc- or curved refl ecting multidirectional rotation the basis of the nature of inclusion trails relative tive phase, and the second phase, which simi- of grains that grew with {001} inclined at a to the external foliation (e.g., Fig. 2A; Passchier larly involves the crack-fi ll growth mechanism high angle to, but canted toward, foliation. and Trouw, 2005, p. 196), such designations in all terrains, is the destructive phase. In addi- Inclusion trails also may refl ect growth over should be made circumspectly because of the tion, textural sector zoning in biotite may be foliation wrapped around or perturbed by diversity of inclusion patterns. For example, the an underrecognized process for two reasons. neighboring growing porphyroblasts. inclusion trail pattern developed during the con- First, the textural sector growth process would (4) The transition to the destructive phase occurs structive phase in the porphyroblasts shown in not be apparent if a pelitic rock lacks graphite when porphyroblasts mature in size and cease Figures 5D and 8A could be interpreted as inter- or excess quartz, because these are needed to to grow in a direction perpendicular to folia- tectonic, that in Figure 12C syntectonic, and that produce inclusions and hence textural zon- tion. At this point, the textural sector growth in Figure 15B post-tectonic. These porphyro- ing. Second, because of the progression to the mechanism no longer partially accommo- blasts are probably all syntectonic, but the micro- destructive phase within the biotite zone, zon- dates strain and some strain is transferred to structure refl ects variations in rheology, strain ing patterns become naturally obscured and are the porphyroblasts, resulting in fracturing, and growth rate, and cut effects. Furthermore, further obfuscated during progression through rotation, and obscuring of zoning and inclu- growing evidence suggests that some porphy- higher grades of metamorphism, which can sion trail patterns developed in the construc- roblast species that chemically replace others make zoning diffi cult to recognize. tive phase. The destructive phase is also char- may inherit their inclusion trails (e.g., Rubenach acterized by minor growth of biotite in strain and Bell, 1988; Barker, 2002; Passchier and CONCLUSIONS shadows and extension fractures, and by the Trouw, 2005; Kim and Cho, 2008), which could fi lling of strain shadows with mostly quartz lead to erroneous interpretations, especially if If the growth of biotite in the biotite zone is instead of biotite. replacement of biotite is involved. This may be accompanied by a strong component of coax- (5) The partitioning of strain and the construc- an issue for garnet because the garnet-forming ial strain, biotite porphyroblasts will undergo tive and destructive phases ultimately pro- reaction may involve replacement of biotite, and a constructive growth phase characterized by duce biotite porphyroblasts with inclusion hence garnet may inherit inclusion trails and textural sector zoning followed by a destructive trails that have a diversity of apparent dip

250 Geosphere, June 2009

Camilleri, P.A., 1998, Prograde metamorphism, strain evolu- Rast, N., 1965, Nucleation and growth of metamorphic angles and dip directions. Thus, caution must tion, and collapse of footwalls of thick thrust sheets: minerals, in Pitcher, W.S., and Flinn, G.S., eds., be used when inferring strain histories or A case study from the Sevier hinterland, U.S.A: Controls of metamorphism: Edinburgh, Oliver and designating porphyroblasts as syntectonic, Journal of Structural Geology, v. 20, p. 1023–1042, Boyd, p. 73–102. doi: 10.1016/S0191-8141(98)00032-7. Rice, A.H.N., 1993, Textural and twin sector-zoning and dis- intertectonic, or posttectonic on the basis of Camilleri, P.A., and Chamberlain, K.R., 1997, Mesozoic tecton- placement of graphite in chiastolite and pyralspite inclusions. Furthermore, caution should be ics and metamorphism in the Pequop Mountains and and grandite garnets in the Variscides of south- Wood Hills region, northeast Nevada: Implications for west England: Ussher Society Proceedings, v. 9, used when interpreting inclusion trails in the architecture and evolution of the Sevier orogen: Geo- p. 124–131. other mineral species that may chemically logical Society of America Bulletin, v. 109, p. 74–94, Rice, A.H.N., 2001, Displacement textures (cleavage domes) replace zoned biotite because the minerals doi: 10.1130/0016-7606(1997)109<0074:MTAMIT> and sector-zoning as hydrostatic stress indicators in 2.3.CO;2. contact and regional metamorphic terranes: Stras- may inherit the diverse array of inclusion Coats, R.R., 1987, Geology of Elko County, Nevada: Nevada bourg, France, European Union of Geosciences XI trails of biotite. Bureau of Mines and Geology Bulletin 101, 112 p. Abstracts, p. 666. (6) Development of textural sector zoning in Ferguson, C.C., Harvey, P.K., and Lloyd, G.E., 1981, On the Rice, A.H.N., and Mitchell, J.I., 1991, Porphyroblast tex- mechanical interaction between a growing porphy- tural sector-zoning and matrix displacement: biotite during regional metamorphism dif- roblast and its surrounding matrix: Contributions to Mineralogical Magazine, v. 55, p. 379–396, doi: fers from textural zoning reported in other Mineralogy and Petrology, v. 75, p. 339–352, doi: 10.1180/minmag.1991.055.380.08. 10.1007/BF00374718. Rice, A.H.N., Habler, G., Carrupt, E., Cotza, G., Wiesmayr, mineral species because it is strain induced, Ghosh, S.K., and Ramberg, H., 1976, Reorientation of G., Schuster, R., Sölva, H., Thöni, M., and Koller, i.e., it is syntectonic rather than pretectonic inclusions by combination of pure shear and F., 2006, Textural sector-zoning in garnet: Theo- or intertectonic. Recognition of relict tex- simple shear: Tectonophysics, v. 34, p. 1–70, doi: retical patterns and natural examples from Alpine 10.1016/0040-1951(76)90176-1. metamorphic rocks: Australian Journal of Earth tural sector zoning in biotite porphyroblasts Harker, A., 1932, Metamorphism, a study of the transfor- Sciences, v. 99, p. 70–89. may be used to signify growth during either mation of rock masses: London, Methuen and Co. Rubenach, M.J., and Bell, T.H., 1988, Microstructural coaxial strain or general shear with a strong Ltd., 360 p. controls and the role of graphite in matrix/porphy- Hodges, K.V., Snoke, A.W., and Hurlow, H.A., 1992, Ther- roblast exchange during synkinematic andalusite coaxial component. Furthermore, the pres- mal evolution of a portion of the Sevier hinterland: growth in a granitoid aureole: Journal of Meta- ence of zoning in porphyroblasts that have The northern East-Humboldt Range-Ruby Moun- morphic Geology, v. 6, p. 651–666, doi: 10.1111/ tains and Wood Hills, northeastern Nevada: Tecton- j.1525-1314.1988.tb00446.x. c-axes perpendicular to lineation can be used ics, v. 11, p. 154–164, doi: 10.1029/91TC01879. Spry, A., 1969, Metamorphic textures: Oxford, Permagon to infer fl attening strain. Ji, S., and Zhao, P., 1993, Location of tensile fracture Press Ltd, 352 p. within rigid-brittle inclusions in ductiley fl ow- Vernon, R.H., 2004, A practical guide to rock microstructure: ACKNOWLEDGMENTS ing matrix: Tectonophysics, v. 220, p. 23–31, doi: Cambridge, Cambridge University Press, 594 p. 10.1016/0040-1951(93)90221-5. Vernon, R.H., and Flood, R.H., 1979, Microstructural evi- Johnson, S.E., 1999, Porphyroblast microstructures: A dence of time-relationships between metamorphism This study was completed while I was on Faculty review of current and future trends: American Min- and deformation in the metasedimentary sequence Development Leave at Austin Peay State University. I eralogist, v. 84, p. 1711–1726. of the northern Hill End Trough, New South Wales, thank Geosphere reviewers Michael Wells and Colin Kim, Y., and Cho, M., 2008, Two-stage growth of porphy- Australia: Tectonophysics, v. 58, p. 127–137, doi: Shaw and Associate Editor Michael Williams for roblastic biotite and garnet in Barrovian metapelites 10.1016/0040-1951(79)90325-1. providing comments and suggestions that helped to of the Imjingang belt, central Korea: Journal of Watts, M.J., and Williams, G.D., 1980, Strain history of greatly improve this manuscript. 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