Midcrustal emplacement of the Sausfjellet pluton, central : Ductile fl ow, stoping, and in situ assimilation

Gregory Dumond† Aaron S. Yoshinobu‡ Calvin G. Barnes Department of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053, USA

ABSTRACT side-up kinematic indicators. In addition to such studies have been carried out on shallow to the abundance of dioritic, calc-silicate, and middle crustal plutons; fewer focused on con- Midcrustal (25–30 km) emplacement of quartzo-feldspathic gneiss xenoliths and struction of magma chambers in the middle to the dioritic Sausfjellet pluton into rocks of geochemical evidence for assimilation in the deep crust (Paterson and Miller, 1998). the Nappe Complex, central Nor- western/annular zone, regional discordance During magma ascent and emplacement, host way, occurred in two stages. Stage 1 consists of the pluton–host-rock contacts indicates rocks may not respond in either a strictly brittle of two-pyroxene hornblende gabbro and that stoping was also an important emplace- (e.g., diking) or ductile (e.g., diapirism) manner diorite. Stage 2 is asymmetrically zoned, with ment process at midcrustal depths. Follow- (Rubin, 1993; Miller and Paterson, 1999). This a modally layered central zone of diorite and ing magma emplacement, foundering of the is particularly true at middle to deep crustal lev- anorthosite and a western/annular zone of central portion of the chamber combined els, where diverse emplacement-related features quartz-bearing monzodioritic rocks. Igneous with possible ca. 445 Ma regional contrac- suggest the operation of both brittle and ductile layering was locally attenuated, folded, and tion produced the map-scale synform defi ned (i.e., viscoelastic) processes during magma boudinaged in the hyper-solidus state. The by magmatic foliations and igneous layers. chamber construction (Rubin, 1993). magmatic foliation trajectory pattern in the This study demonstrates that stoping and Many structurally oriented emplacement pluton defi nes a shallowly southwest-plung- assimilation may occur simultaneously with models do not take into account the compo- ing synform that crosscuts compositional host-rock ductile fl ow during magma cham- sitional evolution of the magma, which com- zones. Mineral lineations plunge shallowly ber evolution at midcrustal levels and offers monly evolves as an open system. For example, to moderately to the southwest. The pluton an explanation of why xenolith preservation while assimilation and hybridization are well intruded a major lithologic boundary within may be compositionally dependent. documented in arc magmas, it is less clear what the nappe; migmatitic pelitic gneisses are the physical processes initiate assimilation during main host rocks to the western part of the Keywords: pluton emplacement, stoping, the evolution of a magma. Magmatic stoping pluton, whereas the eastern and central parts layered intrusions, Caledonides, (Daly, 1903) is one mechanism that can incor- are hosted predominantly by metacarbonate Batholith. porate crustal material into a magma chamber rocks. Calc-silicate, marble, quartzo-feld- and initiate assimilation (e.g., Clarke et al., spathic, and dioritic xenoliths (up to 200 m INTRODUCTION 1998). However, such processes are generally in length) are present throughout the pluton; not considered in magma emplacement models. they are most common in Stage 1 and the Since A.F. Buddington’s 1959 seminal Since the effi cacy of stoping is contentiously central zone of Stage 2. Metapelitic xeno- examination of North American pluton–host- debated (compare Marsh, 1982; Clarke et al., liths are conspicuously absent. Ductile fl ow rock systems, geologists have recognized the 1998; Yoshinobu et al., 2003; with Clemens and during emplacement produced an ~1-km- fi rst-order depth dependence of various magma Mawer, 1992; Petford, 1996; Tikoff et al., 1999), wide structural aureole in which host-rock emplacement mechanisms. Many workers have evidence for stoping and for its consequences structures were defl ected into subparallelism documented the diversity of physical processes on magmatic evolution will provide interesting with the steeply inward-dipping margin of that attend magma emplacement with respect to constraints on the compositional evolution of the pluton. Tight antiforms developed along depth and temperature (e.g., Buddington, 1959; magmas and the mechanisms of emplacement. the northeastern, southeastern, and south- Pitcher, 1979; Marsh, 1982; Paterson et al., Here we report on the emplacement and western margins. Amphibolite-grade shear 1996; McNulty et al., 2000), yet many published structural evolution of the Sausfjellet pluton, a zones in the host rocks preserve pluton- magma emplacement models attempt to identify laterally zoned dioritic body in the Norwegian a single prevailing mechanism for the construc- Caledonides (Fig. 1A and 1B). The pluton tion of plutons (e.g., Holder, 1979; Hutton, 1988; was emplaced at middle crustal depths by a Tikoff and Teyssier, 1992; Morgan et al., 1998; combination of stoping and ductile fl ow of its †Present address: Department of Geosciences, and others). In general, such models involve host rocks. Geologic and geochemical evidence University of Massachusetts, 233 Morrill Science Center, Amherst, Massachusetts 01003-9297, USA. either brittle or ductile deformation in the form presented in Barnes et al. (2004) indicates ‡Corresponding author e-mail: aaron.yoshinobu@ of faulting/shearing (e.g., Tikoff and Teyssier, that lateral zoning developed, at least in part, ttu.edu. 1992) or ductile fl ow (e.g., Holder, 1979). Most because of differences in compositions of the

GSA Bulletin; March/April 2005; v. 117; no. 3/4; p. 383–395; doi: 10.1130/B25464.1; 9 fi gures; 1 table.

For permission to copy, contact [email protected] © 2005 Geological Society of America 383 DUMOND et al.

host rocks, such that the parts of the pluton 14 E B) hosted by metacarbonates show little chemical AP evidence of host-rock contamination, whereas parts hosted by metapelitic migmatites show chemical and isotopic evidence for stoping and assimilation. In this report we fi rst present the y geologic setting of the pluton and document Finland Sweden HP emplacement-related deformation of the host Norwa rocks. This is followed by a description of the AD structural features of the pluton and then by a model for magma emplacement. A) Vega SP

GEOLOGIC SETTING Ursfjord Fig. 2

The Sausfjellet pluton intrudes the Helgeland Bronnoysund Nappe Complex, which is the uppermost nappe complex in the Caledonides of north-central Fig. 1B KP Norway (Fig. 1A and 1B). In the Velfjord region, Kollstraumen the Helgeland Nappe Complex has been divided Detachment Tosenfjord into four nappes; from structurally lowest, they Zone Leka 0 10 km are the Soren-Torghatten, lower, middle, and Kvaloy 65 N Upper Nappe upper nappes (Fig. 1B; Thorsnes and Løseth, Middle Nappe 1991; Yoshinobu et al., 2002). The Soren- Lower Nappe Torghatten and middle nappes consist of basal Sauren-Torghatten Nappe ophiolitic rocks that are unconformably overlain AP - Andalshatten Pluton HP - Hillstadfjellet Pluton by medium-grade metasedimentary sequences. thrust fault The lower and upper nappes consist of migma- 25 km AD - Akset-Drevli Pluton SP - Sausfjellet Pluton titic paragneiss, marble, calc-silicate rocks, and normal detachment fault KP - Krakfjellet Pluton sparse amphibolite. The nappes are separated by east-dipping shear zones. Displacement on these Bindal Batholith shear zones was originally in a west-vergent Metasupracrustal rocks of the Helgeland.. .. Nappe Complex reverse sense; however, the shear zones between Uppermost Metasupracrustal rocks of the Rodingsfjallet Nappe Complex Allochthon the Sauren-Torghatten and the lower nappes, and Greenstone, gabbro, ultramafic rocks between the lower and the middle nappes show Leka ophiolite complex down-to-the-east, normal displacement reactiva- Koli Nappe Complex - Upper Allochthon tion (Yoshinobu et al., 2002). The timing and Central Norway basement window - Northern Western Gneiss Region nature of the regional deformation during the interval when the Sausfjellet pluton was con- structed is summarized in Table 1. Figure 1. (A) Location map modifi ed after Nordgulen and Sundvoll (1992) illustrating the During an arc collisional event, migmatiza- geologic setting of the Helgeland Nappe Complex, Scandinavian Caledonides. (B) Inset depicts tion of pelitic and quartzo-feldspathic rocks of major shear zones in the Helgeland Nappe Complex and the localities discussed in the text. the lower nappe accompanied upper amphibo- lite facies regional metamorphism and emplace- ment of a number of peraluminous plutons. The regional metamorphic event occurred from in-hornblende barometry on samples with the into steeply dipping orientations subparallel to 477 to 468 Ma (Barnes et al., 2002; Yoshinobu appropriate mineral assemblage (Barnes and the intrusive contacts (Fig. 3) and on the west et al., 2002) based on U-Pb (zircon) ages of Prestvik, 2000). by disruption of layering in regional migmatite synmetamorphic granites. Between 448 and to form diatexite (disrupted migmatite; e.g., 430 Ma (Nordgulen et al., 1993; Yoshinobu et GEOLOGY OF THE SAUSFJELLET Barnes et al., 2002). The aureole’s southern al. 2002), the compositionally diverse Bindal PLUTON AND AUREOLE extent is inferred from the defl ection of folia- Batholith was emplaced. In the Velfjord region, tion trajectories and the change in dip direction the batholith consists of the predominantly Host Rocks of lithologic units (Fig. 3). As described below, dioritic Velfjord plutons (Fig. 1B; Akset-Drevli, discontinuously developed shear zones in the 447.8 ± 2.3 Ma; Hillstadfjellet, 447.0 ± 3.2 Ma; The western side of the pluton is hosted pri- inner aureole of the pluton are generally defi ned and Sausfjellet, 445 ± 11 Ma; Yoshinobu et al., marily by migmatitic pelitic gneiss with minor by pluton-side-up kinematics. 2002) and the 447 ± 7 Ma Andalshatten pluton marble and amphibolite; the eastern half is The western contact with the migmatitic (Nordgulen et al., 1993). Emplacement pres- hosted by marble, calc-silicate rocks, and minor gneiss is locally defi ned by an ~100–200-m- sures for these four plutons were in the 600–800 migmatitic pelitic gneiss (Fig. 2). The contact wide zone of interfi ngering, contact-parallel MPa range (20–30 km) based on garnet-alumi- aureole is ~1 km wide and is defi ned by the dikes of diorite, diatexite, and two-mica granite nosilicate-quartz-plagioclase and aluminum- defl ection of foliations and lithologic contacts (Fig. 2; Barnes et al., 2002). As this contact is

384 Geological Society of America Bulletin, March/April 2005 DUCTILE FLOW, STOPING, ASSIMILATION: SAUSFJELLET PLUTON

TABLE 1. SUMMARY OF REGIONAL STRUCTURAL/METAMORPHIC EVOLUTION OF ROCKS WITHIN THE schists and adjacent to the southwestern margin LOWER NAPPE OF THE HELGELAND NAPPE COMPLEX, CENTRAL NORWAY. of the pluton (Fig. 5C). Together these obser- Structural event Age Reference vations indicate pluton-side-up displacement (Ma) along the structures near the southern contact. West-directed contraction and associated folding, migmatization, 477–466 Yoshinobu et al. (2002); Along the southwestern margin a deformed and granite petrogenesis associated with arc collision Barnes et al. (2002) monzodioritic dike that intruded marble and West-directed contraction associated with “Taconic”-like arc- 468–448 Yoshinobu et al. (2002) calcareous schists is subparallel to the steeply continent collision NE-dipping contact of the pluton. The dike Emplacement of Velfjord plutons, development of the Sausvatnet ca. 447–445 Barnes and Prestvik (2000, is axial-planar to a shallowly SE-plunging shear zone within the aureole of the Sausfjellt pluton, and local 2002); Dumond (2002); z-fold that displays pluton-side-up sense of north-south contraction associated with exhumation of lower Yoshinobu et al. (2002) nappe along the Skogmoen detachment fault displacement. In summary, lithologic contacts and folia- tions are conformable with the inward dip of the intrusive contact (Fig. 2). Shallow to moderately approached, fold axes in the migmatitic gneiss gneiss and by graphite and mullions in the lay- plunging lineations and asymmetric folds in the change orientation from moderate SE–plunging ered calc-silicate rocks and marble plunge shal- gneiss and calcareous schists in the contact to shallowly SSE plunging. Sillimanite linea- lowly to moderately to the southeast (Fig. 4A). aureole are consistent with a component of tions plunge moderately to the SE (Fig. 2). Synkinematic sillimanite porphyroblasts along lateral expansion of the magma chamber into With approach to the northern contact, the northeastern side of Sausvatnet display the host rocks. However, at the map scale the calc-silicate layers in marble are ptygmatically top-to-the-northwest reverse displacement (i.e., pluton is discordant; host-rock markers cannot folded and locally boudinaged. Moderately to pluton-side-up) (Fig. 5A). Crudely asymmetric be restored to their pre-emplacement confi gu- steeply plunging mullions occur at the contact garnet porphyroblasts with tails of sillimanite ration by “removing” the pluton from the map between the marble and the diorite. Apophyses show a similar sense of displacement in mig- plane (Fig. 2; see also Discussion). The pluton of diorite inject coarsely recrystallized marble matitic gneiss adjacent to the pluton (Fig. 5B). cuts across an extensive body of migmatitic and locally contain xenoliths of the marble. Sil- Calc-silicate layers in the marble are deformed pelitic gneiss along the southwestern contact, limanite lineations in migmatitic gneiss near the into sigma-clasts and asymmetric boudins that even though foliation trajectories and fold axial contact plunge shallowly to moderately to the also show dextral top-to-the-northwest reverse surfaces are brought into subparallelism with southeast and northwest, whereas in the plu- displacement on surfaces viewed perpendicular the contact (e.g., Fig. 4). ton, lath-shaped hornblende lineations plunge to the foliation and subparallel to the lineation. steeply to the southeast (Fig. 2). Along the western part of the shear zone, linea- Sausfjellet Pluton Along the northeastern and southeastern mar- tions in the migmatitic gneiss locally plunge gins of the pluton, the migmatitic gneiss pinches shallowly to the southwest (Fig. 2). Kinemat- The Sausfjellet pluton is asymmetrically and swells along strike and becomes laterally ics of this part of the shear zone inferred from zoned and was emplaced in distinct stages. The discontinuous (Figs. 2 and 3). In these areas dynamically recrystallized asymmetric K-feld- fi rst stage underlies the southeastern part of the and along the southwestern margin, shallowly spar porphyroclasts yield dextral down-to-the- pluton. It consists of biotite-bearing two-pyrox- to moderately plunging tight antiformal folds southwest normal-sense displacement. ene (clinopyroxene > orthopyroxene) horn- defi ned by metamorphic layering in the marble As the eastern contact of the intrusion is blende gabbro and diorite (Fig. 6). Stage 1 can and gneiss are present (Figs. 3 and 4). These approached, calc-silicate layers are ptygmati- be distinguished in the fi eld by its rusty orange structures form discontinuous shear zones cally folded, sheared, and boudinaged. Leuco- weathering and the presence of dark brown within the inner aureole of the pluton. somes in migmatitic gneiss locally intrude along hornblende oikocrysts up to 3 cm in diameter. Along the northern side of the pluton, an the contact between the marble and the gneiss. A poorly exposed, sharp intrusive contact sepa- ~1-km-wide shear zone strikes west-northwest, Calc-silicate xenoliths (0.5–5 m in length) and rates rocks of stage 1 from those of stage 2. subparallel to the steeply southwest-dipping mafi c enclaves in the pluton have long axes sub- A small mass of gabbroic rock crops out intrusive contact (Figs. 3 and 4A). The shear parallel to magmatic foliation and the host-rock southwest of the Sausfjellet pluton. The two zone is bounded on its northern side by the foliation, both of which dip steeply to the south- bodies are separated by a septum of marble and Akset-Drevli pluton, whose margin dips mod- west (Fig. 2). Lineations defi ned by prismatic pelitic migmatite (Fig. 2). This small gabbroic erately northward (Fig. 4A). This shear zone hornblende are subhorizontal. body, herein named the Nonsdalen pluton, con- is termed the “Sausvatnet shear zone” after the Along the southern margin, lineations in sists of coarse- to medium-grained hornblende lake that parallels its length (Fig. 3). Adjacent the migmatitic gneiss, calcareous schists, and pyroxene gabbro and diorite. The unit is textur- to the northeastern contact of the Sausfjellet quartzo-feldspathic gneiss plunge shallowly ally similar to stage 1, but poor exposure and pluton, leucosomes in migmatitic pelitic gneiss to moderately to the northeast and southwest extensive deuteric alteration limited our collec- within the Sausvatnet shear zone are parallel (Fig. 2). A map-scale, sinistral defl ection of tion and description to a few samples. to foliation and also occur in shear bands that foliation trajectories was mapped southwest of Stage 2 is asymmetrically zoned from east to cut across foliation. At a distance greater than Strauman (Fig. 3). This is consistent with sinis- west (Fig. 6) and is subdivided into central and ~1.2 km from the pluton, leucosomes are no tral top-to-the-southwest normal displacement western/annular zones on the basis of internal longer apparent in the gneiss (near the bend in observed in sigma-type garnet porphyroblasts structure, mineral assemblages, and geochemi- cross-section A–A′; Fig. 4A). in the migmatitic gneiss along the southeastern cal characteristics, as described below. The cen- Foliations in the shear zone generally dip margin, and sinistral, asymmetric folds (Fig. 4C) tral zone is predominantly biotite two-pyroxene steeply to the southwest (Fig. 4A). Lineations and top-to-the-southwest reverse displacement diorite (Fig. 6). Orthopyroxene is more abun- defi ned by sillimanite in migmatitic pelitic defi ned by C′-shear bands in folded calcareous dant than clinopyroxene, and hornblende is an

Geological Society of America Bulletin, March/April 2005 385 DUMOND et al.

53 65 60 66 54 51 47 40 447.8 ± 2.3 Ma AÕAÕ 0261 58 50 59 78 35 38 48 Akset-Drevli Pluton 60 61 26 Mpg 30 66 71 55 42 65 43 km 80 19 64 74 70 50 62 60 84 73 40 82 Mpg 63 58 47 67 42 68 38 60 61 59 Mpg 67 57 Mpg 9 18 33 57 87 75 60 N 44 50 50 84 73 68 58 40 60 41 65 64 47 40 20 35 45 50 59 74 36 82 74 55 50 40 65 35 Sausvatnet Mpg 40 50 65 30 43 65 58 25 42 52 laterally extensive zone 44 of anorthosite layering 59 50 68 24 70 48 75 53 45 71 20 40 50 A 66 62 47 43 76 62 72 64 63 B 34 78 71 40 35 73 60 Mpg 84 57 60 32 52 78 63 49 73 50 76 64 55 57 80 48 51 50 45 79 53 62 76 64 73 65 16 55 64 84 34 62 57 68 71 80 75 77 49 80 70 72 30 83 58 69 56 74 44 58 49 73 74 72 N 65 20’ 39 35 61 76 62 55 12 35 59 71 79 53 35 5 65 50 59 49 56 64 45 51 37 55 51 71 80 38 48 63 55 61 54 60 66 55 46 44 32 63 64 52 42 60 73 12 65 51 57 56 Fuglvatnet 57 38 40 6 15 32 77 Mpg 28 45 59 66 68 51 69 66 65 29 35 50 50 41 65 41 60 36 50 61 58 54 445 ± 11 Ma 24 39 58 58 Sausfjellet 57 40 57 61 Pluton Qfg 55 ΄ 48 50 63 C 73 80 31 65 68 83 40 40 30 Nonsdalen 60 44 75 65 42 Mpg 56 45 55 70 53 6 71 56 35 35 60 58 45 70 63 55 70 B΄35 84 35 20 41 65 55 62 56 47 57 20 50 11 37 22 68 33 44 67 41 74 50 Mpg 52 67 49 43 66 73 67 77 Strauman 72 53 70 48 73 74 56 45 46 33 66 794638 46 46 44 81 32 25 Qfg 82 49 66 75 76 41 85 64 24 24 38 51 68 63 55 72 89 35 75 66 62 55 61 77 70 86 77 62 41 61 29 67 44 72 58 Mpg 75 15 37 37 70 9 49 55 55 C 51 63 76 87 46 35 80 43 43 69 59 76 21 60 56 9 56 46 80 50 56 45 70 40 88 75 36 62 29 15Vassbygda 15 50 55 45 80 64 60 38 52 72 66 44 9 75 17 57 49 19 55 Mpg 38 74 65 15 28 37 20 29 84 73 6 54 48 Qfg 61 51 65 56 31 49 61 31 80 50 Qfg 35 39 79 30 55 67 60 34 44 72 61 71 67 84 15 12 35’ E 61 33 63 Explanation Water/unmapped contact, Internal igneous dashed where approx. compositional boundary Middle Nappe undivided Skogmoen ductile detachment fault, block on Diorite and gabbro/xenoliths down-thrown side 2-mica granite Lower Marble and calc-silicates Nappe shear zone with sense of displacement

15 Migmatitic pelitic gneiss (Mpg) and 55 quartzo-feldspathic gneiss (Qfg) 35 foliation mineral lineation magmatic layering

Figure 2. Geologic map of the Sausfjellet region, Velfjord, Norway. The map includes previously published data from Kollung and Myrland (1970), Skaarup et al. (1974), Thorsnes (1985), and Barnes et al. (1992). Unpublished data collected between 1987 and 1988 in the northwest quadrant of the map by M. Schöenfeld and colleagues from Technische Universität Clausthal, Germany, have also been included. Lines of cross sections (A–A′, B–B′, and C–C′) in Figure 4 are indicated.

386 Geological Society of America Bulletin, March/April 2005 DUCTILE FLOW, STOPING, ASSIMILATION: SAUSFJELLET PLUTON

Mpg 447.8 ± 2.3 Ma 02 Akset-Drevli Pluton km Mpg 8080 Mpg N Mpg

Sausvatnet

6060 Mpg 65 20’ N 65 20’ Mpg

445 ± 11 Ma Sausfjellet ? Nonsdalen Pluton Mpg Qfg 6767 Mpg 8585 Strauman 7272 4646 Qfg 6666

Mpg

Vassbygda

Mpg 3838 Qfg Qfg

Qfg 12 35’ E Explanation Ductile detachment fault, antiform 6767 dip and dip direction of block on down-thrown side lithologic unit synform Shear zones direction of hanging foliation trajectory wall movement inferred extent of with dip direction foliation triple point structural aureole trend of mineral lineation Internal compositional contact

Figure 3. Foliation trajectory map based on data presented in Figure 2; lithologic legend same as in Figure 2.

Geological Society of America Bulletin, March/April 2005 387 DUMOND et al.

A) Akset-Drevli pluton N SW Sausvatnet NE = pole to host rock foliation shear zone bend 400 = mineral lineation = fold axis diorite 200 d migmatitic and quartzo- 0 feldspathic gneiss meters u marble, calc-silicates, and calcareous schists AA'0 0.5 km xenoliths

N N = pole to magmatic foliation/layering = magmatic mineral lineation, n = 200 n = 20, average = 32°, S39°W

B) boundary between NW stage 1 and 2 SE bend 300 meters 0

0 0.5 km Zone of laterally extensive and B variably deformed anorthositic layering B'

bend N C) = pole to host rock foliation SW NE = mineral lineation = fold axis 400 200 meters 0

C 0 0.5 km C'

Figure 4. Interpretive cross sections of the Sausfjellet pluton and aureole with lower hemisphere equal-area projection stereonet data. (A) Dextral top-to-the-northwest reverse Sausvatnet shear zone and projection of moderately north-dipping contact of Akset-Drevli pluton. (B) Magmatic foliation/layering and lineations within the pluton. (C) Sinistral shear zone along the southern and southwest margin of the pluton displaying pluton-up kinematics. Position of cross sections through pluton indicated in Figure 2.

accessory phase. Centimeter- to decimeter-thick lithology. This transition is defi ned on the basis some samples (Barnes et al., 2004). Thus, the anorthosite and pyroxene-rich diorite layers of petrographic features, the absence of layer- western/annular zone is distinguished by its (<1 to >300 m long) crop out in the northern ing, a decrease in the intensity of foliation, and greater modal abundances of quartz, biotite, part of the central zone (Fig. 7A–7C). Contacts geochemical compositions (Barnes et al., 2004) K-feldspar, and hornblende, paucity of modal between anorthosite layers and the more abun- and occurs over widths of tens to perhaps 100 m. layering, and generally weaker foliation. dant two-pyroxene diorite layers are sharp and We refer to these petrographically diverse rocks locally lobate. Some diorite layers are modally that underlie the western part of the pluton, as Foliation and Microstructures graded, with pyroxene-rich and less commonly well as a nearly continuous annular ring around magnetite-rich basal zones that grade upward stage 1 and the central zone of stage 2, as the Foliation is defi ned by pyroxene and laths of into plagioclase-rich rock (Fig. 7B). Layering western/annular zone. The annular zone var- subhedral to euhedral plagioclase with strong is discontinuous along strike, may be folded ies in width from 100 m to 1 km and contains shape-preferred and lattice-preferred orienta- (e.g., Fig. 7B), and commonly displays pinch numerous dikelike bodies of variable grain size tions (Fig. 7D and 7E). Plagioclase contains and swell structures and tapered ends (Fig. 7C). and rock type, whereas the western zone is gen- deformation twins and undulose extinction but Synmagmatic normal-sense shear bands that erally massive (Fig. 6). little evidence for pervasive dislocation creep. displace layers down to the southwest are Rock types in the western/annular zone are Microfractures in pyroxene are generally subper- locally common (Fig. 7D). Schlieren are pres- diorite, quartz diorite, monzodiorite, and quartz pendicular to foliation. Weakly to well-developed ent throughout the central zone and consist of monzodiorite. As in the central zone, the propor- mineral lineations defi ned by plagioclase laths diffuse segregations of mafi c minerals. tion of low-Ca pyroxene is greater than augite. and elongate prismatic pyroxene plunge shal- Dioritic rocks of the central zone grade west- However, hydrous mafi c minerals are more lowly to moderately to the southwest (Figs. 3 and ward and outward to a zone of more variable abundant and inverted pigeonite is present in 4B). We interpret the foliation and lineation to be

388 Geological Society of America Bulletin, March/April 2005 DUCTILE FLOW, STOPING, ASSIMILATION: SAUSFJELLET PLUTON

NWNW SESE form that plunges shallowly to the southwest Some quartzo-feldspathic and calc-silicate (Figs. 3 and 7B). Within tens of meters of the xenoliths are fractured along and across foliation host-rock contact, magmatic foliations are com- planes. These fractures are typically fi lled with monly subparallel to the contact. The foliation veins of fi ne-grained diorite (Fig. 8B). At some trajectory pattern cuts across the compositional localities in the central zone, xenoliths were frag- zoning in the pluton. There is not a strong mented in situ by apophyses of the diorite. increase in magmatic fabric intensity approach- Structural relationships between magmatic ing the host-rock contact. fabrics in the host pluton, xenolith geometry, Asymmetric fabrics, tiled plagioclase laths, and internal structures within the xenoliths are and magmatic C-S fabrics are visible in thin complex. The long axes of the xenoliths are typ- A 2 mm sections cut perpendicular to the foliation and ically aligned subparallel to the strike of mag- parallel to the mineral lineation (Fig. 7D). matic foliation and modal layering (Fig. 8C), NWNW silsil SESE Shear sense indicators typically record top-to- and metamorphic foliations within calc-silicate, the-southwest normal displacement, although marble, and quartzo-feldspathic xenoliths are shear sense inferred from tiled plagioclase laths predominantly subparallel to magmatic folia- from a few localities in the eastern half of the tion in the host pluton. In contrast, the magmatic central zone show top-to-the-northeast reverse foliation of Akset-Drevli–type diorite xenoliths, displacement (see Fig. 3). if present, is more commonly discordant to silsil silsil Near the margins of the intrusion (i.e., in the magmatic foliation in the host pluton. Layering annular zone), samples are commonly inequi- is common below and above the “stratigraphic” granular with interlobate grain boundaries. location of the xenoliths. In some localities, lay- B 0.5 mm Plagioclase displays deformation twins, undu- ering developed structurally above the blocks lose extinction, kink bands, and well-devel- is tapered and boudinaged, indicating that the WSWWSW ENEENE oped shape- and lattice-preferred orientation layers formed during or soon after xenolith tr (Fig. 7E). Quartz displays undulose extinction entrainment and may have been “draped” on bt and subgrain development (Fig. 7E). Some top of the xenolith (Fig. 8D). In other cases the samples have polygonal arrangements of pla- magmatic foliation is defl ected and truncated gioclase and quartz with well-developed triple by the xenoliths (Fig. 8D). Beyond a few cen- junctions, no apparent shape-preferred orienta- timeters to meters from the xenoliths, magmatic C' tion in plagioclase, and weakly developed lat- foliations within the Sausfjellet pluton maintain tice-preferred orientation in both plagioclase their regional NNE trend (Fig. 8E). These C 0.5 mm and quartz. A few localized, discrete anastamos- observations indicate that the crystal mush had ing plastic shear zones occur along the southern suffi cient yield strength to trap the blocks yet Figure 5. Kinematics in shear zones devel- and northern margins of the pluton. still deform at hypersolidus conditions. oped in the contact aureole of the Sausfjel- let pluton. Photomicrographs are in plane Xenoliths DISCUSSION light and are perpendicular to foliation and parallel to lineation. (A) Photomicrograph of Xenoliths range in outcrop area from <1 m2 We summarize the following observations/ synkinematic sillimanite porphyroblast from to >400 m2; they consist of marble, calc-silicate interpretations that bear on the discussion of the migmatitic pelitic gneiss, Sausvatnet shear rocks, quartzo-feldspathic gneiss, and diorite. emplacement and evolution of the Sausfjellet zone, northern portion of aureole. (B) Asym- The dioritic xenoliths are characterized by a pluton in the middle crust. (1) Lithologic con- metric garnet porphyroblast with inclusions trachytoid texture defi ned by cm-scale, elon- tacts and structures were defl ected into subparal- of fi brolite and tails of sillimanite (sil), Saus- gate, tabular laths of plagioclase. This fabric is lelism with the steeply inward-dipping contact vatnet shear zone, northeastern portion of strikingly similar to the trachytoid texture of the of the pluton and shear zones developed within aureole. (C) C′ shear bands (extensional Akset-Drevli pluton (Barnes et al., 1992). No ~1 km of the contact. (2) Tight antiforms formed crenulation cleaveage) in calcareous tremo- xenoliths of the pelitic migmatite were observed. along the margins of the pluton; shear zones in lite (tr) + biotite (bt) schist in the limb of a Marble xenoliths generally crop out in the host rocks close to the contact preserve pluton- fold along the southwest contact of the plu- annular zone along the northern and eastern side-up kinematics. (3) Dioritic, calc-silicate, and ton. Kinematics inferred from outcrop fold parts of the pluton, in some places less than quartzo-feldspathic gneiss xenoliths are well pre- asymmetry, shear bands, and C′ shear bands 1 m from the pluton–host-rock contact (see served within the pluton, whereas pelitic migma- are consistent with pluton-side-up sense of Fig. 8A). In contrast, calc-silicate xenoliths are titic xenoliths are conspicuously absent. (4) The displacement (top to WSW; see Fig. 4C). present throughout the pluton: they decrease in pluton is asymmetrically zoned, petrographically abundance from stage 1 to the central zone and and geochemically, with the most evolved rocks then the western/annular zone of stage 2. They in the western and annular zone (Barnes et al., of hypersolidus origin on the basis of the well- typically consist of diopside ± plagioclase ± 2004). The zoning is compatible with host rock developed shape- and lattice-preferred orienta- quartz ± epidote. Such xenoliths are much richer and xenolith lithology. (5) A southwest-plung- tion in plagioclase and on the lack of pervasive in diopside than are the calc-silicate host rocks. ing synform defi ned by magmatic foliations and intercrystalline plastic deformation. Akset-Drevli–type dioritic xenoliths are most igneous layers cuts across compositional bound- In the central zone of stage 2, the foliations abundant in stage 1, but are also present in the aries, zonation of the magma, and igneous layers. are parallel to igneous layers and defi ne a syn- central zone of stage 2 (Fig. 8B–8E). Foliation also postdates “capture” of some of

Geological Society of America Bulletin, March/April 2005 389 DUMOND et al.

Transition Zone 12 35’ E Stage 2 - Central Zone - biotite two- or three- - biotite two-pyroxene pyroxene (opx>cpx> (opx>cpx) diorite and inverted pigeonite) diorite, anorthosite monzodiorite, and quartz - high MgO (> 5 wt.%) diorite - δ18O = 6.1 to 6.9 An to An Stage 2 - Central Zone An to An 63 33 61 44

Stage 2 - Western and Annular Zone - biotite hornblende two- or three-pyroxene diorite, quartz diorite, monzodiorite, N 65 20’ and quartz monzodiorite - low MgO (< 5.0 wt.%) - δ18O = 6.7 to 8.7 An to An 58 27 Stage 1

Stage 1 - biotite hornblende two- Stage 2 - Western and pyroxene (cpx>opx) gabbro Annular Zone and diorite - high MgO (4.9 - 13.6 wt.%)

- high Fetotal (4.8 - 12.6 wt.%) - δ18O = 5.9 to 6.5 An to An N Explanation 83 48 xenoliths synform 0 1 foliation trajectory kilometers with dip direction

Figure 6. Map of lithologic, geochemical, and isotopic zoning in the Sausfjellet pluton. The contact between the central zone and western/ annular zone is gradational. Note that foliation trajectories and synform axis cut across inferred compositional boundaries. Geochemical data from Barnes et al. (2004) and Dumond (2002).

the xenoliths because magmatic foliations wrap range of host-rock xenoliths within the pluton, pulses of magma (Fig. 6). The central zone of around most xenoliths. but the absence of pelitic xenoliths. Third, con- stage 2 behaved essentially as a closed system in tact metamorphism of pelitic host rocks resulted which accumulation of variable proportions of Petrologic Evolution in the Context of in remelting in the aureole, which produced plagioclase, clinopyroxene, and orthopyroxene Stoping and Emplacement diatexites (migmatites lacking continuous inter- resulted in the observed compositional trend. nal structures) adjacent to the pluton (Barnes et Barnes et al. (2004) suggested this process of in Three aspects of emplacement are notewor- al., 2002). Contact migmatization occurred by situ crystal accumulation also resulted in layer thy in the context of the petrological evolution biotite-dehydration melting in the temperature development and upward/inward aggradation of the Sausfjellet pluton (Barnes et al., 2004). range of 800–850 ºC (Barnes and Prestvik, of a cumulate pile of crystals. In the western/ First, the pluton was emplaced across a major 2000). Based on mass balance calculations, annular zone, however, the crude correlation of lithologic boundary in the lower nappe that Barnes et al. (2002) estimated that a minimum δ18O values with differentiation, the increase separates predominantly calcareous rocks in the of 20%–25% melt was produced in the host in abundance of hydrous mafi c phases, and an east from pelitic migmatitic gneiss in the west. pelitic migmatites during contact melting. increase in incompatible trace elements beyond Second, magmatic stoping of the host rocks The intrusive contact between stage 1 and the that predicted by closed-system fractionation occurred, which produced regionally discordant central zone of stage 2 indicates that the Sausfjel- suggests open-system behavior, specifi cally 18 contacts and resulted in the inclusion of a wide let pluton is the product of at least two discrete contamination by O-, K2O-, and Zr-rich

390 Geological Society of America Bulletin, March/April 2005 DUCTILE FLOW, STOPING, ASSIMILATION: SAUSFJELLET PLUTON

(2) the presence of xenoliths along the margins of the intrusion, in stage 1, and in the central zone of stage 2 (Fig. 8); and (3) geochemical evidence for assimilation of pelitic migmatitic gneiss in the western/annular zone of stage 2 (Fig. 6; Dumond et al., 2002; Barnes et al., 2004). Although fracturing of a partially molten medium like the migmatitic gneiss in order to facilitate stoping at relatively high ambient tem- peratures requires rapidly applied strain (Rubin, 1993), dikes and apophyses from the pluton that intrude host rocks in the aureole suggest the A)A) presence of excess magma pressure, which may have contributed to this process. WNWWNW Incorporation of pelitic migmatite via stoping N85°E is consistent with the observed compositional heterogeneity in the western/annular zone and 65°SE with the regional geology. The Sausfjellet plu- ton intruded across a northwest- to northeast- 19° striking regional contact between the pelitic migmatitic gneiss on the west and the marbles, calc-silicates, and quartzo-feldspathic rocks on the east (Fig. 2). This contact extends for ~45 km from the southern side of Velfjord to south of Tosenfjord (Gustavson, 1981). The gra- dational boundary between the closed-system B) C) ESEESE part of stage 2 (the central zone) and the open- system part (western/annular zone) is essentially D) E)E) coincident with this regional contact. Stoping NENE SWSW of marble, diorite, calc-silicates, and quartzo- feldspathic rocks into stage 1 and the central zone of stage 2 had little chemical effect on the magmas. In contrast, stoping of pelitic migma- tite (with melt present) into the western/annular zone resulted in local, variable assimilation. S Such variations probably refl ect piecemeal stop- ing rather than uniform distribution of stoped 5 mm C 0.5 mm blocks. The absence of such blocks is thought to be due to complete assimilation: The near- Figure 7. Field photos (A–C) and photomicrographs (D–E) of structures in the pluton. (A) liquidus temperatures of the probable parental View looking NNE at thin, well-developed anorthosite and diorite layers (notebook at left is magma (ca. 1240 °C; Barnes and others, 2004) 17 cm wide). (B) NNW view down-plunge of magmatic fold (head of hammer is 17 cm long). are high enough to accommodate dissolution (C) Anorthosite boudins viewed NE along strike with accompanying sketch displaying of hot pelitic rocks (Patiño Douce and John- dextral offset of boudins. (D) Dextral, down-to-SW magmatic C-S fabric in two-pyroxene ston, 1991; Skjerlie et al., 1993). Furthermore, diorite from central zone defi ned by tabular, elongate laths of plagioclase, view to the SE. because the stoped blocks were partly molten, (E) Magmatic foliation in quartz diorite from northern margin of pluton in the annular zone their porosity greatly increased the surface area defi ned by the shape- and lattice-preferred orientation of plagioclase. Interstitial quartz in contact with the host, and they disintegrated contain subgrains (white arrow) and plagioclase contains some deformation twins (black quickly in the host diorite. Petrologic modeling arrow). Thin sections cut parallel to lineation, perpendicular to foliation. presented in Barnes et al. (2004) is consistent with as much as 20% of the mass underlaying the western/annular zone to be derived from material (Barnes et al., 2004). Their modeling magma, their interstitial melts were extracted assimilated pelitic material. is consistent with enrichment of hydrous phases and mingled with the host magma, enriching it 18 18 and the increase in δ O due to a combination in O, H2O, and incompatible elements, specifi - Host-Rock Ductile Deformation of crystal-liquid separation (i.e., fractionation) cally the alkalis (Barnes et al., 2004). and assimilation of the migmatitic gneiss Stoping must have occurred throughout the Ductile attenuation and transposition of the (Fig. 2). We suggest that assimilation resulted emplacement history of the Sausfjellet pluton. host rocks caused by outward expansion of the from stoping of blocks of already partly molten This is supported by (1) the discordant contact magma chamber would be possible at the high migmatitic gneiss into the stage 2 magma. As between the migmatitic pelitic gneiss and the temperatures achieved during contact melting the blocks were incorporated into the stage 2 pluton along the southwestern margin (Fig. 2); of the migmatites (800–850° C; Barnes and

Geological Society of America Bulletin, March/April 2005 391 DUMOND et al.

Finally, the discontinuous nature of the pelitic gneiss, particularly where it is concordant to the contact of the pluton, can be attributed to mb large-scale attenuation and boudinage. Outward expansion of the pluton and dextral oblique-slip to strike-slip shearing along the northern contact di of the pluton can explain such deformation on the northern side of the pluton. Similar ductile fl ow would explain the translation of the mig- A) B) matitic pelitic gneiss on the southern margin of the intrusion (northwest of Strauman in Fig. 2). anorthosite layer Prior to emplacement, this gneiss was probably N19E part of the large mass of pelitic gneiss southwest S 60 of the pluton. A maximum amount of fi nite longitudinal AD AD strain produced by lateral expansion can be esti- mated from the map pattern of the southwestern S part of the intrusion (Fig. 2). If one assumes that the northwest-striking contact between the pelitic gneiss (southwestern contact) and adja- C) D) cent metamorphic rocks to the northeast was originally planar, then the amount of fi nite lon- S gitudinal strain produced by lateral expansion of AD the pluton into the pelitic migmatitic gneiss at the southwest portion of the intrusion was ~0.49 (as measured in map view). This estimate assumes plane strain and does not take into account the amount of volume lost due to stoping. S Emplacement as a Steep-Walled Intrusion E) AD F) or a Subhorizontal Tabular Body?

Figure 8. Field photographs of stoped blocks in the Sausfjellet pluton. (A) Apophysis of Sausfjel- Numerous authors have suggested that let diorite (di) at northern margin with included marble xenolith (mb). (B) Large xenolith of subelliptical plutons can be characterized in Akset-Drevli block surrounded by diorite of the Sausfjellet pluton. White dashed line denotes three dimensions as tabular or funnel-shaped contact between xenolith and host; black dash represents magmatic foliation in the Sausfjellet intrusions with fl oors that steepen toward one pluton. (C) 1.5-m-long, deformed, lozenge-shaped Akset-Drevli–type xenolith (AD); accompa- or more conduits (Vigneresse, 1995; Cruden, nying fi eld sketch illustrates the subparallel alignment of plagioclase laths in the xenolith with 1998; Wiebe and Collins, 1998). Floors that the foliation and deformed anorthosite layer; Sausfjellet pluton (S). (D) Close-up view of Akset- steepen toward a feeder are characteristic of Drevli–type xenolith (AD) that defl ects and truncates magmatic foliations and igneous layers in a number of layered igneous intrusions (e.g., the Sausfjellet pluton (S). (E) Akset-Drevli–type diorite xenolith (AD) impinging on the aggraded Muskox: Irvine, 1980; Skaergaard: McBirney, fl oor of layered rocks in the Sausfjellet (S) pluton (hammer is 0.7 m long). (F) Fractured quartzo- 1996). Models for the development of subho- feldspathic xenolith intruded by fi ne-grained diorite (pencil is 13 cm long). rizontal tabular plutons typically involve: (1) horizontal fracture propagation and/or magma accumulation at some anisotropic level in the crust (e.g., Hogan and Gilbert, 1995), (2) lateral Prestvik, 2000; Barnes et al., 2002). Evidence forms in the northeastern, southeastern, and growth or ballooning, and (3) fl oor subsidence for outward expansion of the magma chamber southwestern parts of the aureole. The amount accommodated by either normal-sense move- during emplacement includes the defl ection of of strain that the folds accommodated during ment on basal shear zones or sinking of the lithologic contacts and foliation trajectories in magma emplacement is dependent on the tim- pluton as a deeper source reservoir was drained the surrounding host rocks, particularly near ing of folding. If the folds were already present (Brown and Solar, 1998; Cruden, 1998; Wiebe the southwestern and southeastern contacts of and were tightened during emplacement, their and Collins, 1998). the intrusion (Figs. 3 and 4). Translation and presence signifi es less expansion than if they Assuming relatively straightforward three- ductile fl ow of the host rocks around the mar- formed during emplacement. Because the folds dimensional shapes for the pluton, subsidence gins of the pluton would explain the presence along the northeastern and southeastern margin of the fl oor or sinking of the Sausfjellet pluton of shallow to moderately plunging lineations of the pluton (Fig. 3) are oblique to the preex- would be expected to produce pluton-side-down and asymmetric folds in the gneiss and calcar- isting northwest-striking structural grain, their shear zones in the aureole of the intrusion. How- eous schists in the aureole. present geometry indicates signifi cant transpo- ever, shearing in the aureole of the pluton is pre- Outward expansion of the pluton is also sition of the host-rock folds by emplacement- dominantly pluton side up and/or strike-slip. In implied by the presence of tight plunging anti- related strain (Dumond, 2002). fact, fl oor subsidence is not a necessity for sub-

392 Geological Society of America Bulletin, March/April 2005 DUCTILE FLOW, STOPING, ASSIMILATION: SAUSFJELLET PLUTON

A) Pre-emplacement regional structure Akset-Drevli pluton 0 2 km

Figure 9. (A–D) Step-wise model Akset-Drevli pluton 0 with schematic cross sections for construction of the Sausfjellet

km pluton pluton. Pre-emplacement struc- 2 ture from Dumond (2002). See text for explanation. (Continued on following page). B) Emplacement of Stage 1.

NW SE

Stage 1

horizontal tabular construction of the Sausfjellet Stage 2 magma was emplaced adjacent to shearing within the chamber may refl ect com- magma chamber if vertical displacement of the or above stage 1 (Fig. 9C) and involved stop- paction processes associated with expulsion host rocks by stoping and lateral displacement ing of partially molten pelitic migmatitic host and emplacement of the western annular zone. by ductile fl ow are suffi cient to make space for rocks along the western margin and projected Magmatic foliations probably formed through- the pluton. roof. Continued growth of stage 2 resulted in out the history of chamber formation; however, defl ection of the migmatitic pelitic gneiss due to based on crosscutting relationships between the A Model for Construction and Deformation lateral expansion of the chamber (Fig. 9C). foliations, xenoliths, and compositional boundar- of the Sausfjellet Pluton We propose that foundering of the central ies, we suggest that the fi nal geometry of mag- zone of stage 2 due to tilting of the crystal-mush matic foliations may be the result of combined Figure 9 illustrates a possible geologic recon- pile and contemporaneous regional deformation foundering of the magma chamber and regional struction of the study area prior to and during (e.g., Table 1) resulted in coeval injection of the northwest-southeast–directed contraction during emplacement of the Sausfjellet pluton. The fi g- western/annular zone magma (see schematic fi nal solidifi cation of the pluton (e.g., Benn et ure emphasizes the preexisting NNW-striking cross sections in Fig. 9C). During and after al., 2001). We note, however, structures associ- structural grain of the area (Fig. 9A; Dumond, emplacement of the western/annular zone, ated with a ca. 445 Ma contractional event have 2002; Yoshinobu et al., 2002). translation of the crystal-mush pile toward the only been identifi ed so far in the Sausfjellet plu- Emplacement of stage 1 magma was accom- southwest continued, which resulted in defor- ton–host-rock system. modated by stoping of the overlying Akset- mation of the igneous layering as indicated by Drevli pluton, marble, calc-silicate, and quartzo- the predominant southwest-plunging lineation CONCLUSIONS feldspathic rocks (Fig. 9B). Akset-Drevli–type and top-to-the-southwest normal sense of shear xenoliths occur within stage 1 and the moderately in the pluton (Fig. 3). The Sausfjellet pluton (445 ± 11 Ma) was north-dipping southern contact of the Akset- Because the axis of the synformal magmatic emplaced at a pressure of ~700 MPa (25–30 km) Drevli pluton projects over the Sausfjellet pluton foliation trajectory pattern and magmatic folia- into marbles, calc-silicate, diorite, quartzo- (Fig. 4A). Stoping was accompanied by lateral tions cross lithologic and geochemical bound- feldspathic gneiss, and pelitic migmatite in at and downward ductile fl ow of the host rocks as aries, the fi nal fabric pattern must have been least two stages, one gabbroic and one dioritic. indicated by the pluton-up, host-rocks–down produced after the magma chamber was already Emplacement was accommodated by simultane- kinematic relationships. Lateral and vertical constructed. We suggest that igneous layer- ous stoping and ductile fl ow of the host rocks translation of material around the growing ing and structures associated with southwest- that are divided into predominantly carbonate magma chamber by ductile fl ow, boudinage, and directed foundering (e.g., lineations, magmatic on the east and pelitic/migmatitic on the west. displacement on marginal shear zones, coupled shear bands) formed during the duration of Pluton-bounding folds and shear zones defi ne an with downward transfer of xenoliths through the chamber construction and represent processes ~1-km-wide structural aureole. The presence of feeder conduit would account for the volume loss operative during construction and petrologic carbonate, calc-silicate, dioritic, and in all stages that is required at deeper levels. evolution of the chamber. The southwest-directed of the intrusion implies that stoping occurred

Geological Society of America Bulletin, March/April 2005 393 DUMOND et al.

C) Emplacement of Stage 2. NW SE Stage 2 SW NE

w Stage 2 modal es c e layering te n r tr Stage 1 z n a o /a l z n n o e n n u e la r Stage 1

0 2 km

NW SE

Stage 2 SE

boudinage of gneiss D) Present-day geologic map

Middle Nappe Ophiolitic/supracrustal rocks Lower Nappe 02 km Sausfjellet pluton, Stage 1 Sausfjellet pluton, Stage 2 Nonsdalen pluton Akset-Drevli pluton Pelitic migmatite Marble Quartzo-feldspathic gneiss Porphyritic granite

Figure 9 (continued). throughout the emplacement history of the plu- petrological/geochemical study of Barnes et al. an anonymous reviewer for their comments and sug- ton. The lack of pelitic/migmatitic xenoliths is (2004), this study reveals the complex interplay gestions on the submitted manuscript. Funding to sup- port this research was provided by National Science consistent with assimilation of partially molten of physical emplacement-related processes and Foundation grant EAR-9814280 (Barnes), NSF grant pelitic gneiss. The aggrading, layered fl oor of the magmatic evolution of a magma chamber EAR-0106557 (Yoshinobu), the Texas Tech Univer- cumulus minerals provided a rheologically and suggests that stoping and xenolith assimila- sity Research Enhancement Fund (Yoshinobu), and strong medium that trapped the xenoliths as they tion may be important processes during magma a Texas Tech University Mr. and Mrs. Carl H. Gelin were incorporated into the chamber. evolution in the middle and lower crust. Graduate Student Loan to G. Dumond. The late-stage, shallowly southwest-plung- REFERENCES CITED ing, synformal foliation trajectory pattern and ACKNOWLEDGMENTS mineral lineations in the pluton refl ect, in part, Barnes, C.G., and Prestvik, T., 2000, Conditions of pluton We thank Melanie Barnes, the Norwegian Geo- emplacement and anatexis in the Caledonian Bindal foundering and deformation of a crystal cumu- logical Survey, Tore Prestvik, and Petra and Håkon Batholith, north-central Norway: Norwegian Journal of late pile toward the southwest after construction Aune for outstanding logistical support. Our ideas Geology, v. 80, p. 259–274. of the magma chamber. Regional ca. 445 Ma regarding the research presented here have been Barnes, C.G., Prestvik, T., Nordgulen, Ø., and Barnes, M.A., 1992, Geology of three dioritic plutons in Velfjord, contraction may also have contributed to fold- greatly infl uenced by a number of thought-provoking conversations with Keegan Schmidt, Øystein Nord- : Norges Geologiske Undersøkelse Bulletin, ing of the magma chamber, however structures v. 423, p. 41–54. gulen, Tore Prestvik, Roar Sandøy, Scott Paterson, Barnes, C.G., Yoshinobu, A.S., Prestvik, T., Nordgulen, Ø., of this age are confi ned to the pluton–host- Geoff Pignotta, and Christopher L. Andronicos. We Karlsson, H.R., and Sundvoll, B., 2002, Mafi c magma rock systems. Combined with the companion thank Associate Editor Rick Law, Keith Benn, and intraplating: Anatexis and hybridization in arc crust,

394 Geological Society of America Bulletin, March/April 2005 DUCTILE FLOW, STOPING, ASSIMILATION: SAUSFJELLET PLUTON

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