Determining Relative Magma and Host Rock Xenolith Rheology During Magmatic Fabric Formation in Plutons: Examples from the Middle and Upper Crust

Determining relative magma and host rock xenolith rheology during magmatic fabric formation in plutons: Examples from the middle and upper crust

Aaron S. Yoshinobu* Jeannette M. Wolak*† Department of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053, USA Scott R. Paterson* Geoffrey S. Pignotta*§ Department of Earth Sciences, University of Southern California, Los Angeles, California 90089-0740, USA Heather S. Anderson* Department of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053, USA

ABSTRACT tilely at presumably fast strain rates. Axial- to arrest the xenoliths in their fi nal position planar magmatic foliations within folded and allow deformation. Estimated effective Field observations, structural analysis, and granodioritic dikes within xenoliths are par- viscosities considering magma yield strength analytical calculations are utilized to evaluate allel to magmatic foliations throughout the and measured density variables (melt and the strength of intermediate magmas during Jackass Lakes pluton and metamorphic foli- solid) are ~1013 Pa s. crystallization in a regional strain fi eld. Two ations within the host rocks, indicating that plutons are examined, the subvolcanic 98 Ma the xenolith deformation occurred within the INTRODUCTION old Jackass Lakes pluton, central Sierra regional 98 Ma old strain fi eld that affected Nevada, California, and the voluminous mid- the pluton. The strength of magmas as they crystallize dle crustal 442 Ma old Andalshatten pluton, The behavior of these xenoliths suggests remains poorly constrained in natural environ- central Norway. The Andalshatten example that late in the crystallization history, mag- ments. However, the rheological transitions that contains millimeter- to kilometer-scale xeno- mas in both middle crustal and subvolcanic occur during crystallization (and melting) must liths that display evidence for synmagmatic settings behaved as a high-strength crystal- play a pivotal role in the mechanical evolution deformation, including fold reactivation melt mush capable of transmitting deviatoric of the lithosphere, the segregation and migra- and boudinage, after being isolated in the stresses, which drove both elastic and plastic tion of magma, and the eventual solidifi cation magma. Fabrics within the pluton adjacent deformation in the enclosed xenoliths. Simul- and thus emplacement of igneous bodies. In this to the xenoliths are usually magmatic, with taneously, intercrystalline melt, and in some article we explore how magma-xenolith rela- only local, discontinuous zones of crystal- cases magma, was drained from the host tions in plutons may provide information on the plastic deformation <1 m from the xenolith intrusions into the xenoliths. Rheological viscosity of partially molten systems and the contact. Examination of particularly well modeling based on geochemical data yields strength of magmas as they crystallize. Many exposed mafi c metavolcanic xenoliths in the an effective viscosity of a crystal-free melt of experimental and theoretical investigations have Jackass Lakes pluton indicates that all were ~104 Pa s and increased to ~107 Pa s as cool- demonstrated the complexity and diffi culty in strained prior to incorporation and then ing proceeded to 758 °C and crystal content quantifying the rheology of magmas because of separated from the remaining host rock by approached 40% for the Jackass Lakes plu- their multiphase nature across a range of tem- brittle cracking. Once isolated from the host ton. Such viscosities are too low to impart or peratures, pressures, ambient deviatoric stresses, rocks, some of these xenoliths were intruded transmit deformation into the xenoliths. The and fl uid compositions and/or concentrations by veins fed by the in situ draining of melt and preservation of xenoliths in both plutons is (e.g., Arzi, 1978; Van der Molen and Paterson, magma from the surrounding crystal mush compatible with higher crystallinities and/ 1979; McBirney, 1993; Lejeune and Richet, zone. The xenoliths continued to deform duc- or magma yield strengths as an explanation 1995; Rutter and Neumann, 1995; Barboza

*Emails: Yoshinobu: [email protected]; Wolak: [email protected]; Paterson: [email protected]; Pignotta: [email protected]; Anderson: [email protected]. †Present address: Department of Earth Sciences, Montana State University, Bozeman, Montana 59717, USA. §Present address: Department of Geology, University of Wisconsin-Eau Claire, Eau Claire, Wisconsin 54702, USA.

Geosphere; June 2009; v. 5; no. 3; p. 270–285; doi: 10.1130/GES00191.1; 12 ﬁ gures.

and Bergantz, 1998; Renner et al., 2000; Rosen- ination of xenoliths in both plutons indicates includes diorite, quartz diorite, biotite ± horn- berg, 2001; Petford, 2003). Our approach is to that they were all pervasively deformed during blende granodiorite, granite, leucogranite, and utilize fi eld relations, structural analysis, and magmatic fabric formation. While this ductile additional hybrid phases (McNulty et al., 1996; geochemistry to place constraints on the rheol- deformation was ongoing, the xenoliths were Coyne et al., 2004). Generally these various ogy of magmas during xenolith incorporation brittlely cracked, sometimes injected by melts phases form north-northwest–trending, steeply and deformation in shallow crustal and middle and magmas from the surrounding magma, and dipping sheet-like bodies ranging in width from crustal environments. Our results may bear in some cases these veins were boudinaged and/ meters to kilometers with variable contacts on the effi cacy of magma emplacement pro- or folded at relatively fast strain rates. ranging from gradational to locally sharp. How- cesses such as diking and stoping, as well as The above observations are interpreted to ever, the central and western portions of the plu- provide constraints on magma viscosity at high indicate that late in the crystallization history ton contain distinct compositional units several crystal fractions. of both plutons, the magma behaved as a high- kilometers in width with quite variable shapes, In this paper we use the term xenolith to strength crystal-melt mush capable of trans- although generally elongate in the north- describe any body of rock that is foreign to and mitting deviatoric stresses, which drove both west direction. Mafi c microgranitoid enclave entirely surrounded by the host igneous rock elastic and plastic deformation in the xenoliths, swarms of dioritic composition as wide as 50 m in contact with the xenolith. This may include while intercrystalline melt drained from the host occur in the more felsic units and are often stoped blocks from the roof, walls, or fl oor of the magma into the xenoliths. Furthermore, com- northwest trending. Textures vary considerably, magma chamber, cognate xenoliths (autoliths) parison of structures in the host pluton and in from aphanitic to coarse grained, and from equi- derived from earlier crystallized portions of the the xenoliths indicates that the strength of the granular to porphyritic. magma system, and screens (kilometric-scale crystal-melt mush during magmatic fabric for- Numerous metavolcanic and metasedimen- xenoliths, sometimes referred to as pendants), mation was equal to or greater than that of the tary xenoliths occur throughout the Jackass or raft trains of xenoliths (e.g., Pitcher, 1970). metavolcanic xenoliths in the Jackass Lakes Lakes pluton and vary from kilometer-scale We differentiate xenoliths from mafi c magmatic pluton. The magmatic foliations and lineations screens to millimeter-scale xenoliths (Figs. 1 enclaves or microgranitoid enclaves (e.g., Didier in both plutons are interpreted to refl ect regional and 2). The largest screens resemble lithologies and Barbarin, 1991). Xenocrysts are defi ned as strain. Thus, both plutons preserve evidence for exposed in the Minarets caldera sequence to mineral fragments that are encapsulated in the the orientation of the regional strain fi eld during the east (Fig. 1) and include both metavolcanic host igneous rock and have no direct chemical magma emplacement. and metasedimentary rocks. Some pendants are relationship with the host magma. The study of also intruded by a porphyritic leucogranite with such xenoliths may provide information about XENOLITHS IN THE JACKASS LAKES miarolitic cavities called the Post Peak phase, magma compositional changes due to assimila- PLUTON, CENTRAL SIERRA NEVADA, which probably is a slightly earlier, subvolcanic tion (e.g., Barnes et al., 2004), the paleohori- CALIFORNIA leucogranitic phase of the Jackass Lakes pluton zontal at the time of fi nal chamber construction, (Peck, 1980). Individual xenoliths and/or raft timing of formation of structures in plutons, The Jackass Lakes pluton (Fig. 1) in map view trains of xenoliths are particularly common near kinematics of magma fl ow, and the rheology is an ~13 × 17 km rectangular body that intruded the large pendants, but occur throughout the plu- of magmas at the time of xenolith capture (e.g., slightly older metavolcanic and plutonic pen- ton with variable sizes (meters to several hun- Paterson and Miller, 1998). Xenoliths also may dants and screens (Peck, 1980; McNulty et dred meters) and rock types (metavolcanic, leu- preserve examples of the complex interactions al., 1996). McNulty et al. (1996) analyzed two cogranitic, and metasedimentary; Wolak, 2004). that occur along chamber margins, including the samples of the granodiorite for U-Pb zircon geo- Magmatic foliations in the Jackass Lakes plu- interplay of processes such as host rock melting chronology, and reported a concordant age of ton strike north to northwest, dip steeply, and are (McLeod et al., 1998), thermal cracking (Clarke 98.5 ± 0.3 Ma old from a population of zircons subparallel to elongate host-rock screens within et al., 1998), high-temperature creep (Langdon, from the northeast portion of the pluton and a the pluton (Figs. 1, and 2). Magmatic linea- 1985), the initiation of dikes (Rubin, 1993a, discordant age of 97.1 ± 0.7 Ma old from the tions typically plunge steeply to shallowly north 1993b), and magma fractionation and crystalli- southwest portion. The pluton is truncated to within the foliation plane. Magmatic lineations zation along boundaries (Mahood and Cornejo, the south by the ca. 90 Ma old Mount Givens in the western quarter of the pluton are shal- 1992; McBirney et al., 1987; McBirney, 1993). pluton (McNulty et al., 2000) and to the north lowly north plunging (Krueger, 2005). McNulty Two pluton–host rock systems are examined by the 95 ± 2 Ma old Red Devil Lake pluton et al. (1996) interpreted the magmatic fabric in that preserve abundant well-exposed xenolith- (Tobisch et al., 1995). Aluminum-in-hornblende the Jackass Lakes pluton to refl ect kinematics magma interactions. The 98 Ma old Jackass geobarometric calculations yield anomalously of magma fl ow during chamber construction. Lakes pluton, Sierra Nevada, is a subvolcanic high (~400 MPa) pressures of crystallization Wiebe (1999, 2000) suggested that Jackass Lakes pluton that intruded its own volcanic ejecta (i.e., 13–15 km depths; Ague and Brimhall, pluton formed by episodic input of mafi c and fel- and minor metasedimentary rocks and contains 1988). Geologic reconstructions of the overly- sic sheets that accumulated on a subhorizontal numerous metavolcanic and metasedimentary ing ca. 98–101 Ma old Minarets caldera (Fiske (10°–30°) chamber fl oor and were subsequently xenoliths (see Peck, 1980; McNulty et al., 1996). and Tobisch, 1978, 1994), the presence of miaro- tilted to their present steep dips. Magmatic fab- The Jackass Lakes pluton was emplaced during litic cavities, and the observation that the pluton rics in this model refl ect compaction in cumulate regional transpressive deformation (Krueger, intruded its own volcanic ejecta and subvolcanic piles below layers and local fl ow during convec- 2005). The 442 Ma old Andalshatten pluton, plutonic rocks point to a subvolcanic emplace- tion above mafi c sheets. However, magmatic central Norway, was constructed in the middle ment depth for the pluton (Peck, 1980; Stern et foliations cut across compositional boundaries crust during regional contractional deforma- al., 1981; Fiske and Tobisch, 1978; 1994; Coyne and are subparallel to regional structural trends tion and preserves numerous xenoliths at scales et al., 2004; Wolak, 2004; Krueger, 2005). in the xenoliths and host rocks (see follow- ranging from millimeters to kilometers (Nord- The composition of the Jackass Lakes pluton ing; Peck, 1980; McNulty et al., 1996). There- gulen et al., 1992; Anderson et al., 2007). Exam- can be variable down to the meter scale, and fore, in contrast to the previous interpretations

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noted above (e.g., Wiebe, 1999, 2000), we Descriptions of Xenoliths (Figs. 1 and 2). The xenolith is 45 × 25 m with suggest that this fabric refl ects regional strain a long dimension that strikes ~N25°W and is imprinted on the still hypersolidus magma Black Beauty Xenolith exposed on a glacially polished subhorizontal 97–98 Ma ago (see also Krueger, 2005). The metavolcanic xenolith described here, surface. The xenolith is metamorphosed and In the following sections, different xenolith informally named the Black Beauty xenolith, is contains a well-developed foliation oriented suites are characterized in terms of the mag- andesitic in composition and is located ~100 m ~N35°W/83°NE defi ned by aligned phenocrysts matic structures surrounding the xenoliths and from the margin of the nearest exposure of of plagioclase and hornblende and fi ne-grained internal structures within the xenoliths. the pluton roof called the Post Peak pendant biotite and quartz (Fig. 3A). Metamorphic foliations in the xenolith are subparallel to foliations in the nearby Post Peak pendant. Around the xenolith magmatic foliation (N5°W/84°NE) Sierra and lineation (70° at S30°E) occur in the host Nevada granodiorite and are well defi ned by the align- batholith ment of magmatic biotite ± hornblende ± feld- K spar and fl attened mafi c microgranitoid enclaves ca. 95-85 Ma plutons 85 (Fig. 2D). Little to no defl ection of this folia- Field tion occurs as the xenolith is approached at the area two ends of the xenolith; i.e., in regions where Khd 60 the foliation and xenolith margins are at high angles (Fig. 2). Local defl ections at the meter 80 scale occur along other portions of the xenolith margins. Both fi eld and microstructural obser- Fig. 2 70 vations (Figs. 3B–3D) indicate that this foliation is entirely magmatic, using criteria outlined in Post K K Peak Paterson et al. (1998). 50 Pendant K MCs Xenolith Margins 85 At the meter to centimeter scale the mar- K 85 gins of the Black Beauty xenolith are defi ned by straight to undulating segments connected by lobate to angular corners where margins 80 abruptly change directions (Fig. 4). Undulating K and lobate margins tend to have amplitudes and 85 85 wavelengths of centimeter to decimeter dimen- 75 sions. Straight margin segments in both the 85 larger xenolith and smaller pieces usually have 80 K K cutoff angles between metamorphic foliations in the xenolith and the xenolith contact of 10° K 75 or more, indicating that xenolith internal anisot-

ca. 107-99 Ma plutons ca. ropy did not control fracture formation. Fig. 7 K Exceptions to the development of planar K 60 N margins occur at the centimeter scale in loca- Jr tions where the overall xenolith margins strike at Sing high angles to the magmatic foliation, i.e., at the Peak ends of elongate north-northwest–trending xeno- Pendant 50 liths. In the Black Beauty example, the margins 70 K 01 2 become concave with respect to the granodiorite, 37°30′ km with centimeter scale V-shaped cusps of host rock at the magma-xenolith interface pointing outward ca. 90 Ma Mount Givens pluton 119°30′ toward the host granodiorite and subparallel to the magmatic foliation (Figs. 4A, 4B). Between 98 Ma Jackass Lakes pluton Magmatic foliation inclined; trace these cusps, lobate margins of the granodiorite are concave toward the xenolith. Such cuspate- Jr Jurassic (Jr) metasedimentary and K Metamorphic foliation in- lobate patterns may form between two materi- Cretaceous (K) metavolcanic rocks of clined; vertical the Minarets Caldera sequence (MCs). als with different effective viscosities, with the material forming the cusps having the lower Figure 1. Simplifi ed geologic map of the ca. 98 Ma old Jackass Lakes granodiorite viscosity (e.g., Fletcher, 1982; Smith, 2000). pluton, modifi ed after Peck (1980), McNulty et al. (1996), and Wolack (2004). Only the The occurrence of these margins only on the kilometer-scale xenoliths and screens are depicted on this map. For larger-scale maps sides of the xenolith at high angles to the grano- of xenolith fi elds contained within the pluton, see Wolak (2004) and Krueger (2005). diorite magmatic foliation and subparallelism

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/5/3/270/3338380/i1553-040X-5-3-270.pdf by guest on 30 September 2021 Relative magma and host rock xenolith rheology during fabric formation 0 80 73 70 lineation 69 foliation; Magmatic Lithologic Contact Km 88 Metavolcanics Jackass Lakes pluton Kj

Kj 65 0000 Km 1 Metamorphic foliation; lineation Fig. 2 C 85 70 70 84 Kj 85 75 Km 82 Magmatic foliation; N=20 Magmatic lineation; N=12 Metamorphic foliation; N=14 Metamorphic lineation; N=1 83 83 82 -119.315° Stereonet Key 75 54 73 81 80 Km Perspective of Perspective 2A & C Figs.

76 10400 88 ca. 95 Ma old; McNulty et al., 1996); Km— Kj 82 90 83 pendant shown in Figure 1. (B) Geologic map of pendant shown in Figure 55 axis. (D) Lower-hemisphere stereonet displaying stereonet axis. (D) Lower-hemisphere 85 80 79 88 72 Km 86 80 89 86 85 83 87 65 85 N Kj 82 meters 84 Contours in feet 64 67 80 84 0 100

77 10400 66 66 68 Km 37.646° D B Kr View looking south View Km Xenolith long axis, N30°W Xenolith long axis, Km Kr N35°W/83°SW View looking south View Metamorphic foliation, Metamorphic foliation, Kj Magmatic foliation, N5°W/84°SW foliation, Magmatic Fig. 2 C C Kj Kj A the xenolith and surrounding rocks. (C) Outcrop photograph displaying variation in orientations of foliations and xenolith long (C) Outcrop rocks. the xenolith and surrounding Figure 2. (A) Location of Black Beauty xenolith with respect to host rocks. Kj—Jackass Lakes pluton; Kr—Red Devil Lake pluton ( to host rocks. 2. (A) Location of Black Beauty xenolith with respect Figure of the Post Peak the metavolcanic rocks contact separates the Jackass Lakes pluton from The red metavolcanic rocks. Cretaceous 45 m long. and C is approximately A the xenolith. Xenolith in in and around orientations of structures

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between the cusps and magmatic foliation indistinguishable from the surrounding ground- is largely a function of the vein orientation rela- suggest that these margins and the magmatic mass (Figs. 4B–4D). Vein tips taper to <1 mm tive to the xenolith foliation. Those striking at foliation formed at the same time during east- in width and typically die out in preserved frac- low angles to the foliation (set 1) display boudi- northeast–west-southwest contraction. tures that extend for short distances in front nage (Fig. 5B), whereas those striking at high of the vein tip. It is not possible to determine angles are folded (sets 2 and 3; Fig. 5C). Field Dikes and Veins in Xenoliths whether these fractures were preexisting or observations (Figs. 3–5) and microstructural Granodioritic dikes and quartzofeldspathic formed during vein formation. However, frac- observations (Fig. 3D) indicate that the veins veins of various sizes intrude virtually all of the tures with no vein material exist in the xenolith. underwent little to no subsolidus crystal-plastic xenoliths observed within the Jackass Lakes In no case are dikes or veins observed to be trun- deformation (beyond minor dislocation glide in pluton. The Black Beauty xenolith displays cated at the xenolith-granodiorite contact, indi- quartz and feldspar). structures associated with these dikes and veins cating that dikes and veins must have formed One granodioritic dike is unusually thick and that bear on magma-xenolith rheology. Millime- once the xenolith was stationary and could not shows several features not seen in other veins ter- to centimeter-thick veins and dikes intrude move within the magma. within the xenolith. (Figs. 5A and 6). This dike the xenolith and taper to thin tips (Figs. 4B–4D). Veins range from strongly deformed to late, is no longer planar and shows both fold pat- In some cases these veins can be traced continu- crosscutting, relatively undeformed examples terns and lobate margins (Figs. 6A, 6B). It is ously across the xenolith-granodiorite contact and are divided into three sets (Figs. 5 and 6). by far the thickest dike in the xenolith and has and into the granodiorite, where they become For the deformed veins, the type of deformation walls that are subperpendicular to the xenolith

A 1cm B

0.5 mm

C D

4mm

Figure 3. (A) Photomicrograph of metamorphic foliation in the metavolcanic xenolith shown in Figures 2A–2C; also shown are granitic veins. (B) Magmatic foliation, deﬁ ned by aligned igneous minerals and enclaves in the Jackass Lakes pluton. This outcrop is a short distance from xenolith in Figure 2. (C) Folded and locally sheared schlieren layers and granodiorite several hundred meters south of examined xenolith in Figure 2. (D) Photomicrograph of magmatic foliation in the Jackass Lakes granodiorite; white line in upper right depicts average orientation of foliation in rock. The main foliation shown in D is axial planer to folds in C. Ruler in top center of C is 6 cm long.

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stretching direction. The vein is coarser grained, lobate protrusions, indicating that the xenolith display similar geometries as the Black Beauty and muscovite and biotite locally form comb was ductile and ﬂ owing during magmatic defor- xenolith: discordant hypersolidus foliations in layering where crystals nucleated on and grew mation of the vein. the granodioritic host occur at the ends of the perpendicular to the margin (Fig. 6C), indicat- xenolith, whereas concordant fabrics occur ing that any deformation in this vein occurred Snake Xenolith along the sides (Fig. 7A). Large-scale mapping in the magmatic state. This dike has curving Mapping throughout the Jackass Lakes pluton (1:300) reveals a meter-wide granodioritic dike margins that, over several wavelengths, form reveals similar xenolith-magma relationships. In that can be traced continually from the south- cuspate-lobate patterns similar to those seen another example, a large (50 m × 55 m) andes- ern to northern edge of the xenolith and into along the main xenolith margin (cf. Figs. 4 and itic xenolith crops out along the eastern margin the host granodiorite (Figs. 7A, 7B). This dike 6). Here also the andesitic xenolith forms cusps of the Sing Peak pendant (Figs. 1 and 7). The is folded about axial planes that are subparal- perpendicular to the xenolith-vein margin, sug- xenolith contains a northwest-trending, steeply lel to the metamorphic foliation in the xenolith gesting that it had a lower effective viscosity east dipping metamorphic foliation that is sub- and magmatic foliation within the granodioritic during folding of the vein. Locally foliation in parallel to a well-developed magmatic foliation host (Figs. 7C–7F). Schlieren layering within the xenolith is deﬂ ected around fold hinges or within the host granodiorite. Contact relations the dike is folded. The dike contains an axial

A B

C D

Figure 4. Field photos of xenolith margins. (A) Sharp margin between xenolith (dark color) and granodiorite (light color) showing ~60° jog in margin. (B) Cuspate-lobate pattern at ends of xenolith. Black and white arrows in A and B point toward concave granodiorite-xenolith contacts where cusps of the meta-andesite point into the granodiorite. Three veins of late melt extend into the xenolith, are continuous across the xenolith margin, and have no subsolidus deformation. The largest vein (blue arrow) extends a short distance into the granodiorite before merging into the granodiorite matrix. (C) Close-up of ~90° step along xenolith margin with both straight and slightly curved margins. More felsic magma (lighter-colored material) accumulated along margin and a thin vein intruded into xenolith at inner step (arrow). (D) Vein of late melt extending into xenolith. Note that vein is continuous across xenolith margin. Orange ruler in all images is 15 cm long.

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Fig. 6A

Set 3

Fig. 6B Set 2

B C

D Extension = 32%

Shortening = -49%

Figure 5. Photos of granodiorite and granite veins in meta-andesitic xenolith. (A) Best-exposed portion of xenolith and contact with granodiorite showing three vein sets. Set 1 is subparallel to metamorphic foliation in the xenolith; set 2 is subperpendicular to the foliation; set 3 is at high angles to foliation and typically deﬁ ned by thicker dikes. Single veins can abruptly change orientation from one set to the next. Crosscutting relationships are complex, but typically suggest that set 1 formed before set 2, followed by set 3. (B) Boudinaged vein set 1; note rectangular shapes and lack of change in thickness of most boudins. (C) Close-up of folded vein (set 2); 5 cm of ruler for scale. (D) Folded and boudinaged veins showing estimates of extension and shortening in subhorizontal plane, i.e., at high angles to mineral lineation. Orange scale in A and B is 15 cm long.

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planar, magmatic foliation defi ned by subhedral morphic temperatures were estimated by Barnes eral to tens of meters in length exposed near the plagioclase laths and amphibole crystals that is and Prestvik (2000) to be >500 °C at the time western contact of the pluton (Fig. 9). In con- subparallel to the metamorphic foliation within of emplacement. Along its western margin, the trast to the Jackass Lakes pluton, xenoliths and the xenolith and magmatic foliation within the pluton intrudes amphibolite-grade interbedded screens within the Andalshatten pluton show host granodiorite. In addition, centimeter-scale marbles, calc-silicate, and metaarkosic rocks more evidence for ductile modifi cation of con- elongate xenoliths of andesitic composition that contain bedding and other structures that tacts and internal strain. occur within the dike and share the axial planar are broadly concordant with the contact at the A spectacular example of synmagmatic orientation (Fig. 7D). map scale (Fig. 8). Prior to emplacement of the deformation of a screen occurs along shore- Andalshatten pluton, these rocks were deformed line exposures near the southwest margin of XENOLITHS IN THE ANDALSHATTEN during ca. 480–470 Ma ago regional contraction the pluton (Fig. 9). At this locality, termed the PLUTON (Yoshinobu et al., 2002; Barnes et al., 2007). “taffy” screen, an ~100-m-long, elongate screen The Andalshatten pluton is rich in xenoliths of of folded calc-silicate rock was intruded by The Andalshatten pluton, central Norway, is all lithologies observed in the host rocks and at north-trending sheets of coarse-grained grano- a large (~630 km2) elongate intrusive complex centimeter to kilometer scales. diorite and shows evidence for boudinage in the with exposed dimensions of 18 × 35 km (Fig. 8). Regional mapping (Nordgulen et al., 1992; granodioritic magma. The contacts of the screen The pluton consists of a large K- feldspar mega- Anderson et al., 2007) has demonstrated that dip steeply to shallowly east and broadly defi ne crystic granodiorite unit with lesser tonalite, magmatic fabrics in the Andalshatten pluton an S shape in map view (Fig. 9B). The screen diorite, and leucogranite. Nordgulen et al. cut across compositional boundaries at the map contains sets of upright, tight to isoclinal folds (1993) published a U-Pb zircon age of 447 scale. We interpret this relationship to indicate defi ned by transposed layers in calc-silicate ± 8 Ma. More recent chemical abrasion–thermal that the magmatic fabrics formed late in the and metaarkosic rocks. A spaced axial planar ionization mass spectrometry (CA-TIMS) dat- crystallization history of the Andalshatten plu- cleavage is well developed in the folds. Both ing on two samples from the granodiorite phase ton. Ongoing structural work along the north the axial planar cleavage and the limbs of the yields ages of 442.66 ± 0.18 Ma and 442.86 and eastern margin of the pluton indicate a per- folds mimic the S shape of the screen. At the ± 0.20 Ma old (Anderson et al., 2007). The plu- vasive crystal-plastic overprint that is interpreted neck of the boudin, point P in Figure 9A, the ton was emplaced at pressures of ~670 MPa, to refl ect ca. 442 Ma ago regional deformation. interlimb angles of the folds approach 0° and the based on Al-in-hornblende barometry using the Therefore, we view the magmatic fabrics within axial planar cleavage becomes penetrative with Anderson and Smith (1995) calibration (Yoshi- the Andalshatten pluton as the hypersolidus lithons that decrease in width to <2 cm. nobu et al., 2002). This pressure is consistent record of this deformation. with the presence of magmatic epidote + horn- Xenolith Margins blende and with pressure estimates for regional Descriptions of Xenoliths The contacts of the taffy screen are variable metamorphism using the GASP (garnet- in terms of the relative concordance and shape aluminosilicate-plagioclase-quartz) barometer The following descriptions are from a suite of with respect to internal structure within the (Barnes and Prestvik, 2000). Regional meta- xenoliths and screens ranging in size from sev- screen. At the meter scale, undulating, lobate

A B C

Figure 6. Field photos of largest dike within the xenolith (shown in Fig. 5A). (A) Folds, cuspate-lobate margins, deﬂ ection of host foliation (near ruler), and fragments of host in vein. (B) Close-up of cuspate-lobate margin occurring at two scales (white arrow). Red line in A and B is trace of metamorphic foliation in meta-andesite. (C) Large igneous muscovite forming comb layering along lobate margin. No subsolidus deformation observed. Orange ruler is 15 cm long.

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C Figure 7. (A) Geologic map of Fig. D large metavolcanic xenolith Kj (Km—Cretaceous metavolcanic) within the Stanford Lakes xenolith ﬁ eld, located in the Km southwestern Jackass Lakes pluton (Kj; see Fig. 1). Univer- sal Transverse Mercator projec- tion, NAD 1972 Zone 11. Arrow in A denotes area of folded dike GPS 4158860 N show in B–D. (B) Photo of xeno- A D lith. (C) Drawing of B. Note the granodiorite dike that intrudes 87 Kj Km the xenolith and is folded 82 65 about an axial plane parallel 85 to magmatic fabrics in the Kj.

GPS 292100 E 81 74 68 (D) Hypersolidus foliations in Km the dike and metamorphic folia- 82 89 86 85 tions in the enclosing xenoliths are subparallel parallel to the Location 71 of 84 87 pencil. (E) Lower- hemisphere Figs. 7B-D stereonet displaying poles to 79 76 84 magmatic foliations. (F) Lower- 74 84 hemisphere stereonet displaying 87 83 85 86 Km poles to metamorphic foliations.

Kj North 81 76 GPS 292170 E 0 40 m Kj GPS 4158780 N Magmatic foliation Metamorphic foliation 70 60 E F

Magmatic foliations, n = 154 Metamorphic foliations, n = 196 mean plane N14W/ 83NE mean plane N14W/ 81NE

278 Geosphere, June 2009

contacts with amplitudes and wavelengths of to cutoff angles between strata and faults in brit- of synmagmatic deformation of metasedimen- as much as tens of centimeters can be followed tle fault systems (e.g., Boyer and Elliot, 1982). tary xenoliths. In Figure 10 a granodioritic dike that display a transition from obliquely dis- At the lobe apex, bedding, although attenuated, intrudes the eastern margin of the screen at a cordant to concordant (Fig. 9). The maximum is usually concordant to the contact. very shallow angle with respect to layering. length of straight and planar contacts is ~1 m. The dike continues for several meters obliquely Where lobate contacts with granodiorite bow Dikes and Sills in Xenoliths into the screen where it tapers to a pointed end into the screen, the layering is attenuated at the Dikes and sills of granodiorite and leuco- in the eastern limb of a tight syncline. An en apex of the lobe (Fig. 9C). Locally, the layering granite intrude most of the larger xenoliths and echelon vein of granodiorite is observed 20 cm is truncated along the inﬂ ection points of lobes virtually all of the screens. The taffy screen west of this dike that cuts the axial planar cleav- (Fig. 9C) forming a bedding cutoff, analogous contains two examples that bear on the nature age in the core of the syncline (Fig. 10A). The

NORWAY Map area UpperUpper nnappeappe Atlantic Ocean

442 Ma Andalshatten Location of pluton Figs. 9, 10

Atlantic Figure 8. Simpliﬁ ed geologic Velfjord map of the Andalshatten plu- Ocean ton and Helgeland Nappe 12°00' Complex. The Andalshatten 65°30' pluton intrudes rocks of all the major nappes within the Helgeland Nappe Complex. Note that screens and xenoliths Sauren-TorgattenSauren-Torgatten nnappeappe within the Andalshatten con- N tain lithologies that are appar- MiddleMiddle nnappeappe ently observed along strike in LowerLower nnappeappe the host rocks. 10 km

ca. 442-448 Ma ‘Velfjord’ diorite suite

442 Ma Andalshatten pluton Thrust fault; inferred Mafic/ultramafic rock Sauren-Torghatten nappe; calc-silicate rocks/pelitic schist & intrusive rocks Detachment fault; inferred Upper nappe marble Upper nappe quartzo-feldspathic gneiss/migmatite & intrusive rocks Middle nappe

Lower nappe; calc-silicate rocks & intrusive rocks

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/5/3/270/3338380/i1553-040X-5-3-270.pdf by guest on 30 September 2021 Yoshinobu et al. 20 cm 4 m layers (bedding) x and y display progressive layers (bedding) x and y display progressive y x ail of contact between xenolith and granodiorite. row shows perspective of view in Figure 10. Black arrows 10. Black arrows shows perspective of view in Figure row C 65 + P Fig. 9C. Fig. 63 7278000 Anticline/antiform Syncline/synform 60 Calc-silicate, meta-arkose Calc-silicate, Quarternary Granodiorite 80 80

N 55 Hypersolidus foliation Crystal-plastic foliation Bedding 20 m 68 67 84 “Taffy Screen” “Taffy 55 + 7277920 76 75 0378645 A B Contact, dashed where inferred Bedding/structure lines form Figure 9. (A) North-looking perspective of the taffy screen (see text). Note the attenuation of the screen at point P. White ar at point P. (see text). Note the attenuation of screen 9. (A) North-looking perspective of the taffy screen Figure denote contacts that are impinged by the granodiorite. (B) Geologic map of the taffy screen and other nearby xenoliths. (C) Det and other impinged by the granodiorite. (B) Geologic map of taffy screen denote contacts that are denote cutoffs between layering and xenolith-granodiorite contact. Marker black arrows denotes concordance, whereas White arrow attenuation before they are truncated at the contact. they are attenuation before

280 Geosphere, June 2009

dike is boudinaged in the screen along a ﬂ at- dioritic and/or leucogranitic sill that intruded (e.g., Fig. 7), there is no preserved axial pla- tening plane that is subparallel with the axial parallel to layering within the calc-silicate nar magmatic foliation within the sill. How- planar cleavage contained within the screen rock. Bedding is folded about the same axes ever, within the sill, panels of calc-silicate (Fig. 10A). Cutoffs between the dike and layer- shown in Figure 9. The sill, therefore, has a rock 2–3 cm wide and as much as 1 m long ing are consistently <15°. folded geometry but contains no evidence of are entrained and in some cases tightly to iso- In contrast to the boudinaged dike, Fig- strain associated with folding. In contrast to clinally folded about chaotically oriented fold ures 10B and 10C display a composite grano- the examples from the Jackass Lakes pluton axes (Fig. 10C).

A B 2 m Area shown in Fig. 10C

scale changes with perspective

10 cm

Figure 10. Detail view of internal strain and diking in the taffy screen in Figure 9. (A) Oblique view to south of the east margin of the taffy screen displaying the tightly folded syncline, transposed bedding, and boudinaged granodioritic dikes that emanated from the host magma (bottom and left side of the image). Layering- contact cutoffs are <10°. Crystal-plastic fabrics within the granodiorite are local- ized to within a few centimeters to meters of the screen contact. Note how scale changes with perspective. (B) Folded composite granodioritic-leucogranitic sill that intruded previously folded calc-silicate rocks. (C) Lower limb of folded granodioritic dike in B displaying panels of calc-silicate rock that are folded about randomly oriented axes (black arrow). Coin is ~15 mm in diameter.

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DISCUSSION perpendicular to the steeply plunging lineation Figure 11 depicts a schematic characterization in this region and therefore refl ect the YZ plane of xenolith-magma deformation as observed in Xenolith Formation, Incorporation, and of the strain ellipsoid. Because the thickness of both the Jackass Lakes and Andalshatten exam- Strain the vein boudins does not vary along veins, and ples. Deformation within the xenolith is kine- because they show no signs of necking or other matically compatible with regional deformation Both the subvolcanic Jackass Lakes pluton and internal strain, we treat the boudins as solid rect- associated with fabric development in the host the middle crustal Andalshatten pluton are lit- angles formed by brittle cracking and then rigidly magma. At the point of fabric formation in the tered with pieces of host rock ranging in size from translated during extension. Therefore, we can system, we envision that the rheology of the meter-scale to kilometer-scale xenoliths. In some use their original and deformed lengths to calcu- xenolith and magma are similar. Given the appar- cases it is possible to establish that xenoliths have late extension along the vein in the foliation plane ent fast rate at which dikes in the Jackass Lakes been detached from screens or the host rocks and (Fig. 5D; Ferguson, 1981). We have also treated pluton may have cooled, the xenoliths likely moved through the magma prior to being trapped the folded veins as ptygmatic folds and used their continued to deform ductilely at presumably fast at their present location. In other cases, xenoliths deformed and undeformed lengths to calculate the strain rates so as to maintain the magmatic folia- appear to have been translated away from the host amount of shortening perpendicular to the foliation within the dikes. Albertz et al. (2005), on by an invading dike, but do not appear to have tion plane (Fig. 5D). Our measurements indicate the basis of thermal modeling and fi eld obser- been considerably rotated. Whether most xeno- that the most deformed veins record ~31% exten- vations in shallow crustal settings, suggested liths or screens have remained in situ cannot be sion and 49% shortening in the horizontal surface. that strain rates may approach 10−2 s–1 in order constrained because magma emplacement must We have no quantitative information about strain to preserve magmatic fabrics within the axial move either the host rock contact or the screen (or in the vertical direction in this xenolith. These val- planes of folded granodioritic dikes. Regard- both) during intrusion. ues for strain are interpreted to refl ect the mini- less of the absolute rate of deformation, axial In both plutons, xenoliths and screens have mum strain that affected the xenoliths and magma planar magmatic fabrics in the folded grano- variably discordant margins, or cutoffs, defi ned during folding and boudinage of the granodioritic diorite dikes that are parallel to metamorphic by planar fractures that typically cut across any veins and dikes prior to fi nal cracking and intru- fabrics in the xenoliths imply relatively fast structure in the xenolith (e.g., Figs. 2, 4, 7, 8, sion of the late crosscutting dikes. strain rates in the pluton–host rock system. and 10). Such bedding and/or layering contact cutoff angles provide information about initiation of xenolith incorporation as well as subse- Extension direction quent xenolith deformation within the magma. These planar contacts were deformed to various extents after the xenolith was incorporated into the magma, resulting in tightened cutoff angles, folding, and boudinage (e.g., Fig. 9). In the fol- Lobate contacts with host magma lowing sections we examine how xenoliths may be used to evaluate magma-xenolith rheology and strain fi elds in plutons. The folding and boudinage of veins (Fig. 5), thickening and formation of cuspate-lobate margins of some veins (e.g., Fig. 4), and the defl ection of foliations in the xenoliths around vein margins all indicate that the xenoliths underwent additional ductile strain during vein formation. Maximum The late crosscutting undeformed veins establish shortening that this ductile deformation ceased before fi nal direction crystallization of the Jackass Lakes pluton. The presence of a single foliation in all of the xenoliths examined and the orientation of this foliation parallel to the direction of vein extension and perpendicular to vein shortening (indicated by vein boudinage and folding, respectively) imply that this preexisting foliation was reactivated during Magmatic the in situ ductile deformation of the xenoliths foliation and the formation of the magmatic foliation in the Jackass Lakes pluton. This conclusion is supported by the axial planar, hypersolidus foliation observed in folded dikes (e.g., Fig. 7). In the Black Beauty xenolith we have estimated the magnitude of strain associated with this synmagmatic pulse of ductile deformation Figure 11. Schematic illustration depicting kinematic compatibility in subhori- by using the folded and boudinaged veins. These zontal planes between internal and external structures in an idealized xenolith- veins occur on subhorizontal faces that are sub- magma system.

282 Geosphere, June 2009

Rheology of the Magma–Host Rock– To estimate magma viscosities over a range of fore, we suggest that the Jackass Lakes magma Xenolith System conditions in the Jackass Lakes pluton, we ana- must have been crystal rich (>>50% crystals) at lyzed fi ve samples of granodiorite from the Jack- the time of xenolith capture and thus at tempera- Several observations suggest that the grano- ass Lakes pluton for major and trace elements tures approaching the solidus. This conclusion dioritic magma in both the subvolcanic Jack- (Wolak, 2004). These data were then used as agrees with: (1) the conclusions of Scaillet et al. ass Lakes chamber and the middle crustal input in the programs MELTS (Ghiorso and Sack, (1997) and Bachmann and Bergantz (2004), who Andalshatten chamber was as strong as or stron- 1995) and PELE (Boudreau, 1999) to model suggested that the greatest increase in viscosity, ger than the andesitic and calc-silicate xenoliths, effective magma viscosities. Although originally due to crystal content, occurs at temperatures respectively, while the xenoliths were internally developed for simulating melt evolution in basal- nearing the solidus; and (2) previous workers who deforming. The ptygmatic folding and boudi- tic magmas, these programs may also be used have estimated magma viscosities ranging from nage of veins (e.g., Figs. 5 and 7) and xenoliths to determine silicic melt parameters at low crys- 1014 to 1016 Pa s in crystal-rich mushes during fab- (e.g., Fig. 9), the presence of cuspate-lobate tal percentages (<40% crystals). Unfortunately, ric formation in granodioritic to gabbroic compo- margins (Fig. 4), and the thin V-shaped cusps the programs cannot account for hydrous mafi c sitions (Nicolas and Ildefonse, 1996; Paterson and all suggest that the magma effective viscosity phases, and are thus less reliable as crystallization Miller, 1998; Yoshinobu and Hirth, 2002). approached and in some cases exceeded the proceeds. However, these results provide mini- Settling rate estimates for a Newtonian liq- xenolith viscosity in both plutons. One conse- mum estimates of the viscosity of the crystal-melt uid, as described above, are well established for quence of these observations is that if the xeno- mush and therefore provide a minimum estimate a wide range of conditions (McBirney, 1993, liths are internally shortening and elongating of the strength of the magma. and others) but are probably not realistic for during fabric formation within the magma, then The shallow emplacement depth, tempera- a magma. Sparks et al. (1977) suggested that the magma must have been capable of transmit- ture (~900–800 °C), and compositions are well magmas may behave as granular materials with ting deviatoric stresses to the xenoliths. This constrained for the pluton (Fiske and Tobisch, some yield strength, i.e., viscoplastic or Bing- implies that the magmas in both cases were non- 1994; McNulty et al., 1996; Coyne et al., 2004; ham behavior. One effect of an increase in yield Newtonian, crystal-rich mushes with signifi cant Wolak, 2004; Krueger, 2005). Although less well strength would be to capture xenoliths at lower strength and that the strength of the xenoliths constrained in our study, volatile contents are magma viscosities. Figure 12 is reproduced

must be equal to or less than the magma. We probably between 3 and 8 wt% H2O (Scaillet et from Sparks et al. (1977) and plots the maxi- also note that the magmatic fabrics in the Jack- al., 1997; Bachmann and Bergantz, 2003). For mum diameter of spheres at excess densities 3 ass Lakes pluton are not strongly deﬂ ected granodioritic melts with 3 wt% H2O, a liquidus of of 300, 500, and 700 kg/m with respect to the around xenoliths, again suggesting that xeno- ~983 °C was calculated. The effective viscosity of enclosing magmas as functions of yield strength. liths were not stronger than the magma. a crystal-free melt of this composition was ~104 Given excess densities of 200–400 kg/m3 Structural relations in the Jackass Lakes Pa s and increased to ~107 Pa s as cooling pro- for metavolcanic xenoliths (diameter 1 m) in pluton–host rock system can be used to con- ceeded to 758 °C and crystal content approached the Jackass Lakes pluton, we estimate that yield

strain relative magma viscosities at the time 40%. Increasing the H2O content to 6 wt% low- strengths needed to trap blocks range from ~250 of fabric formation. For example, the degree ered the effective viscosities to 103–104 Pa s and to 500 N/m2. Like Paterson and Miller (1998), of defl ection of magmatic foliations along the yielded a liquidus temperature of 872 °C. This is we note that our yield strength estimates are margins of xenoliths may provide information compatible with Scaillet et al. (1997) and other much higher than those measured by Sparks et about the viscosity ratio between the crystalliz- studies, which predict that higher volatile contents al. (1977), and therefore require a crystal-rich ing magma and xenoliths (Paterson and Miller, will result in lower magma viscosities. In addi- magma (>50% crystals). Therefore, we con- 1998). Undefl ected magmatic foliations and tion, our values agree with Coyne et al. (2004), tend that higher crystal fractions are required in subparallel metamorphic foliations in xenoliths who suggested an estimated solidus of ~670 °C. order to explain the observations from the Jack- in the Jackass Lakes pluton suggest a very low Modeled densities of the magma described ass lakes pluton. We note that such Bingham viscosity ratio. That is, where xenoliths and their above ranged from 2230–2410 kg/m3, much lower behavior would reduce melt viscosities needed internal foliations are parallel to the magmatic than our average measured density for metavol- to trap blocks by one to two orders of magnitude fabric, and where granodiorite dikes in xenoliths canic xenoliths of 2640 kg/m3 (Wolak, 2004). (Sparks et al., 1977; Paterson and Miller, 1998). are folded about axial planes that are parallel to These data indicate that host rock xenoliths in the We summarize the following steps regarding the magmatic fabrics, then the viscosity of the Jackass Lakes pluton were negatively buoyant changing magma viscosities, incorporation of crystallizing magma was similar to that of host and should have sunk steadily through the crystal- host rock xenoliths, and formation of magmatic rock xenoliths. Assuming that individual xeno- poor magma, assuming xenolith incorporation at fabrics in the granodiorite of Jackass Lakes. liths were entirely surrounded by granodioritic crystallinities <40%. Assuming an average block (1) Xenoliths removed from the margins of the magma, the magma viscosity must have been radius of 1 m, the densities given above, granodio- growing magma chamber, whether via stoping high enough to trap xenoliths (i.e., higher than ritic magma viscosities of 104–107 Pa s, and using or diking, during the fi nal stages of emplace- the most dense block), yet still had suffi cient Stokes’ settling laws for static, Newtonian melts, ment must have been incorporated into a crystal- melt present to allow a magmatic foliation to settling rates of 4.0 × 10−3 to 1.5 × 10−7 m/s (~5 m rich magmatic mush with viscosities ranging form during regional tectonism and after incor- a–1) are predicted for our metavolcanic xenoliths. in excess of 1013 Pa s, or possibly 1–2 orders of poration of some of the xenoliths. To achieve slower settling rates (e.g., <10 cm/ka) magnitude lower if the magma had some yield Granodioritic melt viscosities during ascent, requires signifi cantly higher viscosities in order to strength. (2) The granodioritic mush retained emplacement, and fabric formation are depen- trap xenoliths and allow a magmatic foliation to enough melt to allow reorientation of frame- dent on a number of factors, including compo- form after incorporation of the xenolith into the work grains after xenolith capture, thereby eras- sition, pressure, temperature, volatile content, magma. For example, a magma viscosity on the ing most evidence for xenolith incorporation. and crystallinity (e.g., McBirney, 1993; Scail- order of 1013 Pa s due to increased crystallinity (3) The viscosities and densities of the magma– let et al., 1997; Bachmann and Bergantz, 2004). yields a settling velocity of ~2.13 cm/ka. There- host rock–xenolith system were very similar at the

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time of fabric formation, so that fabrics in each 2. The amount of strain recorded by the 1. The subvolcanic Jackass Lakes pluton, cen- record regional tectonism ca. 98 Ma ago, some deformed veins in the xenoliths is certainly suf- tral Sierra Nevada, California, and the middle time after xenolith removal and incorporation. fi cient enough to form the relatively weak fabric crustal Andalshatten pluton, Norway, contain in the Jackass Lakes pluton, implying that the numerous centimeter- to kilometer-scale xeno- Implications for Fabric Formation in the preserved Jackass Lakes pluton fabric may have liths that display evidence for postincorporation, Jackass Lakes Pluton formed within a short duration and during rapid synmagmatic deformation. strain rates. 2. Late in the cooling history, the crystal-melt One important conclusion is that the pre- 3. The overprinting of internal intrusive mush of both plutons behaved as high-strength served magmatic fabric (i.e., foliation and lin- contacts by the fabric and local discordance materials capable of transmitting deviatoric eation) in the Jackass Lakes pluton (and the between the north-south–striking Jackass Lakes stresses to the xenoliths, which drove both elastic Andalshatten pluton) formed at the same time pluton fabric and chamber margin indicate and crystal plastic deformation in the xenoliths. as the xenoliths were internally deforming. We that the Jackass Lakes pluton fabric formed 3. During synmagmatic deformation melts base this conclusion on the following observa- after chamber construction, and thus refl ects drained into the xenoliths from the surrounding tions: (1) the subparallelism between the mag- regional strain of an existing chamber and not crystal-rich magma. matic foliation and/or lineation and metamor- emplacement-related deformation. If the Jack- 4. Estimates of magma viscosity at the time of phic foliations and/or lineation contained in ass Lakes pluton is a subcaldera chamber, then xenolith capture are in the range of 1013 Pa s for the xenoliths; (2) subparallelism between these this implies that following caldera collapse (rep- the Jackass Lakes pluton, suggesting a crystal- structures and axial planes defi ned by magmatic resented by the numerous pendants and xeno- rich mush in which realignment of grains to foliations within folded granodioritic dikes and liths in the chamber), regional tectonic stresses form a magmatic foliation was still possible. veins; (3) lack of signifi cant defl ection of the dominated over any stresses related to the evolu- We conclude that magma-xenolith systems magmatic foliation around the xenoliths; and tion of the magmatic-volcanic system. may share similar viscosities during near solidus (4) synchronous pluton fabric formation and crystallization. In addition, evidence from the vein and/or dike deformation. Given the above, CONCLUSIONS Jackass Lakes pluton supports the notion that the general characteristics of the xenoliths and deformation of the magma-xenolith system may Jackass Lakes pluton veins (Figs. 6–9), and the Field observations, structural analysis, and refl ect the regional strain fi eld ca. 97–98 Ma map pattern of the Jackass Lakes pluton fabric modeled crystallization histories based on major ago. This study documents the potential of (Fig. 1), we summarize the following. element data may be used to place constraints utilizing detailed fi eld observations of xeno- 1. The preserved Jackass Lakes pluton fabric on the effective viscosity of the magmas in the lith and igneous structures in plutons to better formed under a deviatoric stress very late in the presence of xenoliths during fabric formation. understand magma rheologies and paleostrain crystallization history of the chamber. We summarize our conclusions as follows. fi elds in arcs. An interesting future study would include examination of fabric geometry in different plutons with different ages in the same arc 100 system to attempt to track the changing strain

3 ﬁ eld through time.

90 3 3 ACKNOWLEDGMENTS 80 This research was supported by National Science 300 kg/m 700 kg/m 70 500 kg/m Foundation grants EAR-0106557 and EAR-0439750 (Yoshinobu) and EAR-0073943 and EDMAP (Edu- 60 cational Component of the National Cooperative Geological Mapping Program) grant 02HQAG0089 50 (Paterson). Yoshinobu also acknowledges support from the Department of Geosciences, Texas Tech University, and the Norwegian Geological Survey. Diameter (cm) 40 Pignotta and Anderson acknowledge support from the Geological Society of America and Sigma Xi. We 30 thank Cal Barnes and Øystein Nordgulen for continued discussions about the interactions of magmas and 20 host rocks, and Bob Miller and Brendan McNulty for their thoughtful reviews. Stereonets were made 10 with Rick Allmendinger’s software. We especially thank Øystein Nordgulen for guiding us through the diverse collection of igneous rocks that occur within 100 200 300 400 500 600 700 800 900 1000 the Bindal batholith, Norway, and for his fantastic regional mapping. Yield Strength (N/m2) REFERENCES CITED Figure 12. The maximum diameter of spheres that can be supported in a Bingham liquid, plotted as a function of yield strength Ague, J.J., and Brimhall, G.H., 1988, Magmatic arc symme- try and distribution of anomalous plutonic belts in 3 for spheres with excess densities of 300, 500, and 700 kg/m (taken the batholiths of California: Effects of assimilation, from Sparks et al., 1977). Metavolcanic xenoliths in the Jackass crustal thickness, and depth of crystallization: Geo- logical Society of America Bulletin, v. 100, p. 912– Lakes pluton generally have excess densities ranging from 200 to 927, doi: 10.1130/0016-7606(1988)100<0912:MAA 400 kg/m3 (shaded area). ADO>2.3.CO;2.

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