Magmatic Tubes, Pipes, Troughs, Diapirs, And

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Magmatic tubes, pipes, troughs, diapirs, and plumes: Late-stage convective instabilities resulting in compositional diversity and permeable networks in crystal-rich magmas of the Tuolumne batholith, Sierra Nevada, California

Scott R. Paterson Department of Earth Sciences, University of Southern California, Zumberge Hall of Science, 3651 Trousdale Parkway, Los Angeles, California 90089-0740, USA

ABSTRACT the case of the Tuolumne batholith involved al., 2004; Gray et al., 2008). This view has been crystals with ages ranging over ~10 m.y. A questioned by authors describing a number of A complex array of widespread, but likely solution is that crystals in subvolca- processes that occurred in largely constructed domainally developed, structures is pre- nic chambers become armored (rimmed) chambers, such as local ponding of mafi c served in the Tuolumne batholith, includ- by other crystals or exist in crystal clusters magmas, convection, mixing, and fraction- ing stationary and migrating tubes, pipes, that, in spite of changing environmental ation, which occurred in crystal mush zones troughs, diapers, and plume heads. These conditions, prevent rapid chemical commu- that approached and eventually exceeded 50% structures, all formed by local magma fl ow nication with the surrounding melts. These crystals (e.g., Wiebe, 1996; Wiebe and Collins, through crystal-mush host magmas, are structures also challenge many aspects of 1998; Marsh, 1996, 2006; Miller and Miller, often associated with the formation of schlie- the incremental chamber growth model 2002; Hersum et al., 2005; Walker et al., 2007), ren rich in accessory and mafi c minerals, and resulting in sheeted bodies championed by and is being challenged further by recent stud- are associated with fi lter pressing and accu- Glazner, Bartley, Coleman, and colleagues ies concluding that processes leading to compo- mulations of crystals with diverse magma for the Tuolumne batholith. The thousands sitional diversity occur in magmas with >50% histories and ages. Together they represent a of preserved internal structures provide crystals (e.g., Bachmann and Bergantz, 2004, network in which channelized fl ow occurred clear evidence against late annealing and 2008; Žák and Klomínský, 2007). For example, in an existing chamber of crystal-rich mag- removal of internal contacts, and are diffi - Bergantz (2000) used numerical modeling to mas, resulting in local compositional and cult to reconcile with either vertical sheeted examine rheological controls of internal magma structural diversity. or subhorizontal laccolith models; however, boundaries and concluded that if an intrusive These structures also are useful structural they are permissive of early pulsing leading unit along an internal margin is not fairly crys- tools for evaluating the internal evolution of to one or more large magma chambers. tal rich and thus stiff, its boundary is not stable magma chambers. For example, the consis- and would collapse, making it less likely that tently steep tube and pipe axes indicate that INTRODUCTION margins between crystal poor magmas are pre- neither the pluton nor features in the plu- served in chambers. A related issue comes from ton were tilted during growth, thus exclud- It has been suggested that as magmas a dramatic proposal that many of these internal ing models in which subhorizontal layers approach their solidus and become crystal contacts, including internal contacts between tilted to form the existing steep contacts. rich, their viscosities rise to such a degree that magma pulses, are entirely removed due to Although the overall direction of young- it becomes essentially impossible for them to late, thermally driven annealing (Glazner et al., ing established by geochronologic studies is convect, fractionate, and/or erupt and thus to 2008a, 2008b) a suggestion that was challenged toward the batholith center, local younging form the compositional and structural diversity by Vernon and Paterson (2008a, 2008b). Thus, directions determined from troughs cutoffs often preserved in chambers (Barriere, 1976; the timing of formation of magmatic structures indicate that outward growth occurred in Brandeis and Marsh, 1989; McBirney, 1993; and associated crystal percent will dramatically many zones. The highly variable movement Vigneresse et al., 1996; Scaillet et al., 2000; infl uence our interpretation of chamber con- directions of local diapirs and plumes require Dingwell, 2006). This has led some to conclude struction and evolution and inferences about interactions between buoyancy forces and that most preserved internal compositional and when magmas can mix, mingle, and fractionate other gradients. structural variations in plutons refl ect either in chambers and their subsequent behavior dur- The existence and characteristics of these the juxtaposition of many pulses derived from ing volcanic eruptions. structures have several other implications. the melting of a heterogeneous lower crust or These issues also are critical for evaluating Interpretations derived about crystal resi- mantle, or the processes that operated early another recent conclusion that both volcanic dence times in chambers and about crystal during ascent and chamber construction rather and plutonic rocks are mixtures of crystals mixing during eruptions need to be treated than processes operating in an already con- with distinct histories. For example, Davidson with caution, since mixed crystal popula- structed magma chamber (e.g., McNulty et al., et al. (2001, 2005, 2007) and others (Broxton tions existed well prior to eruptions, and in 1996; Coleman et al., 2004, 2005; Glazner et et al., 1989; Christensen et al., 1995; Claiborne

Geosphere; December 2009; v. 5; no. 6; p. 496–527; doi: 10.1130/GES00214.1; 18 ﬁ gures; 1 table.

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et al., 2006; Cooper and Reid, 2003; Costa et the Tuolumne batholith, central Sierra Nevada, batholith, based on Al-in-hornblende barometry al., 2003; Barbey et al., 2008; Ramos and Reid, California. Their characteristics and relative (Ague and Brimhall, 1988; Webber et al., 2001; 2005; Wallace and Bergantz, 2005; Walker et timing indicate that their formation and pres- Gray, 2003; Anderson et al., 2007), indicate a al., 2007) have demonstrated, using isotopic ervation required development in crystal-rich depth of 6–10 km, consistent with widespread fi ngerprinting in single minerals, that crystal (e.g., >50%) magmas, and thus indicate that andalusite and local sillimanite in the surround- exchange between different melts is a com- late, local movement of magmas resulted in ing host rocks (Rose, 1957; Memeti et al., mon phenomenon and that the resulting crystal crystal accumulations (of minerals with diverse 2005a; Anderson et al., 2007). populations are often accumulated from two histories), fractionation, and formation of these The Tuolumne batholith (Fig. 1) consists of or more sources. Recent high precision U/Pb structures (Weinberg et al., 2001; Paterson et al., four nested, progressively more evolved inward, thermal ionization mass spectrometry zircon 2005; Žák and Klomínský, 2007; Vernon and intrusive units: (1) the outer Kuna Crest unit dating of multiple single grains supports this Paterson, 2008a; Ruprecht et al., 2008; Bach- to the east and its equivalents along the west- conclusion through the recognition that zir- man and Bergantz, 2008). ern and southern margins (tonalites of Glen con populations in a single sample are a mix An examination of these structures also indi- Aulin and Glacier Point, granodiorite of Gray- of xenocrysts, antecrysts, and autocrysts (e.g., cates that they are useful tools for addressing ling Lake) and inner phases, including (2) the Brown and Fletcher, 1999; Charlier et al., the internal evolution of magma chambers (e.g., Half Dome granodiorite, (3) the K-feldspar 2005; Bindeman et al., 2006; Claiborne et al., Fernandez and Gasquet, 1994; Wiebe and Col- megacrystic Cathedral Peak granodiorite, and 2006; Matzel et al., 2006b; Miller et al., 2007; lins, 1998; Pignotta et al., 2006). For example, (3) a central phase, the Johnson granite porphyry Walker et al., 2007). The resultant volcanic or tubes and pipes, but not plumes and diapirs, (Bateman, 1992; Bateman and Chappell, 1979). plutonic rock is thus a mechanical mixture of may preserve information about paleohorizon- The Kuna Crest unit is mostly fi ne- to medium- crystals, for which (1) the bulk rock geochem- tal and local fl ow directions. Troughs provide grained, dark colored, equigranular tonalite, istry may say little about equilibrium processes information about local younging (or growth) quartz diorite, and biotite-hornblende grano- of melting and crystallization; (2) the contrib- directions as well as directions of magma fl ow. diorite, typically with a strong magmatic fabric uting sources (mantle, crust, subsequent con- Diapirs record information about local gradients and abundant mafi c enclaves. The Half Dome tamination) may be obscured; and (3) the age and displacement fi elds. All of these magmatic granodiorite, consisting of an outer equigranular may be misleading or at best leaves untapped a structures record information about processes and inner porphyritic phase, is generally much great deal of useful information. by which crystals and melt are sorted, some- coarser grained than the Kuna Crest unit and is Another implication of the above observa- times separated, and other times mixed, and thus characterized by the presence of large (to ~2 cm tions is that some volcanic eruptions are derived about processes resulting in compositional and in length) prismatic euhedral hornblendes and from magma chambers constructed from structural diversity formed in middle and upper conspicuous sphene. The porphyritic variety numerous pulses of magma that accumulated crustal magma chambers. contains large (to ~6 cm in length) K-feldspar over time and are integrated just prior to or even phenocrysts. The Cathedral Peak granodiorite, during eruption. This requires fairly late mixing OVERVIEW OF TUOLUMNE which forms the most voluminous part of the of diverse crystal populations in magma cham- BATHOLITH currently exposed Tuolumne batholith, consists bers. Alternatively, could mixing of diverse of biotite granodiorite containing abundant large crystal populations occur during ascent and The Tuolumne batholith is an ~1100 km2 K-feldspar phenocrysts (on average 4–5 cm, but magma chamber construction and then this Late Cretaceous composite batholith exposed locally to ~12 cm in length) and large quartz disequilibrium assemblage be preserved until in the central Sierra Nevada, California (Fig. 1). crystals in a medium-grained matrix. The cen- eruptions occur? For example, in the Tuolumne It is emplaced into Cretaceous granitoids (e.g., tral phase, the Johnson granite porphyry, is batholith, California, mixing of distinct crystal 102 Ma El Capitan granite) and amphibolite- fi ne-grained equigranular biotite granite, locally populations is nicely supported by recent U/Pb grade metasedimentary rocks (Kings Sequence) containing sparse antecrystic K-feldspar phe- zircon data (of Matzel et al., 2005, 2006b, 2007; to the west and older plutonic and greenschist- nocrysts and other fragments from the Cathe- Miller et al., 2007) in which widespread zir- grade metavolcanic rocks of Triassic to Creta- dral Peak granodiorite. Work by Memeti et al. con antecrysts with concordant ages identical ceous age to the east (Huber et al., 1989; Sch- (2005b, 2007) indicates that all these main units to older units in the batholith are found in the weickert and Lahren, 1993, 2006). Contacts of show internal compositional variations, par- younger phases, an observation increasingly the batholith with host rocks are steeply dipping ticularly in lobes extending out from the main established in other magmatic systems (Gardner and generally discordant to the host rock struc- batholith, in which the main units fractionate to et al., 2002; Bacon and Lowenstern, 2005; Clai- tures, although local (100 m) domains occur form central leucogranite lenses (Fig. 1). borne et al., 2006; Bachmann et al., 2007). How in which older structures are defl ected and In contrast to the model of in situ fraction- and when did this mixing of zircons occur? margin-parallel foliations and steeply plung- ation of single parent melt proposed by Bateman Answers to these questions depend on to ing lineations occur. Local relief establishes and Chappell (1979), geochronological stud- what degree late processes in middle and upper that these steep contacts extend at least 2 km in ies (Kistler and Fleck, 1994a, 1994b; Coleman crustal crystal-rich magma chambers can sort height. Gravity data (Oliver, 1977; Oliver et al., and Glazner, 1997; Matzel et al., 2005, 2006b, magma into compositionally different com- 1987) indicate that many of these vertical con- 2006c, 2007; Miller et al., 2007; Memeti et al., ponents, form magmatic structures that do not tacts may extend downward at least to depths 2007) indicate that the batholith was constructed refl ect source or ascent processes, and mix 6–12 km below the present erosion level for over a 10 m.y. duration between 95 and 85 Ma. diverse crystal populations. Here I describe an most units, although a recent gravity study con- Based on Sr and Nd isotopic analyses, Kistler impressive array of compositionally and textur- cluded that the central Johnson granite porphyry et al. (1986) concluded that the inner and outer ally defi ned magmatic structures such as station- may only extend down a few kilometers (Titus intrusive units evolved separately and were ary and migrating tubes, pipes, troughs, small- et al., 2005). Estimates of emplacement depth of largely derived from mantle (outer units) and scale within-chamber diapers, and plumes in the currently exposed surface of the Tuolumne crustal (inner units) derived magmas. Coleman

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119°45' 119°30' 119°15' 119°10' 38°13 ' 38°13' Legend Plutonic Rocks CP lobe leucogranite (lg) JP lg CP CP ? eHD/pHD KC

HD lg lobe pHD eHD ? KC

38° 38° California Nevada Sawmill Fig. 6a Canyon

SF Sierra Nevada TB batholith Glen Aulin

Tuolumne Pacific Meadows Ocean LA JP 0 100 200 km eHD KC pHD Potter Point lg CP KC lobe eHD pHD KC 37°45 37°45

HD lobe 0 15 10km Scale 1 : 125 000

119°45' 119°30' 119°15' 119°10'

Figure 1. Geologic map of Tuolumne batholith and its host rocks (after Huber et al., 1989). Includes new mapping (by Paterson and colleagues) of much of the batholith at a scale of 1:24,000. Note the ﬁ ve main units in the central batholith and presence of additional internal zoning in the four lobes extending out from the units in the central batholith (after mapping by Memeti and colleagues). The structures discussed in this paper occur in all units and in lobes. Index map shows location of Tuolumne batholith (TB) in California. Box indicates location of Figure 6A. KC—Kuna Crest unit; CP—Cathedral Peak granodiorite; JP—Johnson granite porphyry; HD—Half Dome granodiorite (p is porphyritic, e is equigranular); SF—San Francisco; LA—Los Angeles.

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et al. (2004) and Glazner et al. (2004), using a in sections perpendicular to tube axes, display meters and thus were probably signifi cantly new geochronologic data set, concluded that the numerous, enclosed (if not removed by subse- longer before disruption. entire batholith was constructed of thousands quent magmatic erosion), elliptical schlieren Crosscutting relationships between of dikes or sills, and that these dikes did not bounded layers (Figs. 2 and 3; Table 1). At least schlieren-bounded layers, comparable to trough interact signifi cantly with neighboring pulses. some and occasionally all layers in the tubes cutoffs in sedimentary rocks (Figs. 2–5) and In their model, numerous internal contacts have compositions and/or textures distinct from mineral grading in schlieren, provide informa- formed, and little to no fractionation, mingling, the surrounding host magma (Figs. 3–5). Two tion about younging directions in both station- convection, internal margin collapse, stoping, general types of tubes occur in the Tuolumne ary and migrating tubes and thus information and related processes occurred because a large batholith: stationary tubes in which the tube axes about the temporal evolution of tubes. In migrat- magma chamber never existed in this batho- do not migrate with time (Figs. 2 and 3) (see ing tubes, rare reversals in migration direction lith. Coleman et al. (2008) and Glazner et al. also Weinberg et al., 2001), and migrating tubes are sometimes preserved, but in general tube (2008a, 2008b) recently tempered this view and in which the tube axis moves and develops path centers migrate in a single direction, both in a concluded that a large magma chamber formed lengths of meters to many tens of meters (Fig. 4) single tube and in adjacent tubes (Fig. 4). It is by stacked laccoliths may have existed (Cole- (see also ladder dikes of Reid et al., 1993). Par- not yet clear whether these migration directions man et al., 2008), but that widespread annealing ticularly in the migrating tubes the displacement follow any chamber-wide pattern(s). In station- removed evidence of earlier internal contacts. of tube centers result in crescent-shaped patterns ary tubes these crosscutting relationships indi- of alternating light and dark schlieren along the cate that tube diameters decrease with time and OVERVIEW OF MAGMATIC path lengths (Fig. 4; Burgess, 2006; Burgess and young toward tube centers (Fig. 3). STRUCTURES PRESERVED IN THE Miller, 2008). The compositions and textures in tubes vary TUOLUMNE BATHOLITH Both stationary and migrating tubes are tremendously, from fairly simple cases of mafi c found in all four units in the Tuolumne batho- schlieren grading into more intermediate to An impressive variety of internal magmatic lith, although they are rare in the Johnson gran- felsic compositions that are macroscopically structures is preserved in the Tuolumne batholith. ite porphyry and in similar leucocratic granitic similar to the host magma (Figs. 2A, 2B), to Here I focus on fi ve types of locally developed lenses recently discovered in other Tuolumne very complex cases where each layer displays but widely distributed structures (Fig. 2; Table 1): batholith units by Economos et al. (2005) and compositions and textures different from both (1) stationary and migrating tubes (Figs. 3–6) Memeti et al. (2005b, 2007). Initial estimates nearby layers and from the host composition (magma tubes have been called snail structures suggest that there are thousands of tubes in the (Fig. 2C, 2D). In the fi rst case, which is par- or ladder dikes; e.g., Reid et al., 1993; Wein- Tuolumne batholith with a roughly equal num- ticularly common in the Cathedral Peak unit, berg et al., 2001); (2) pipes (Figs. 7 and 8; see ber of stationary and migrating tubes. Tube the lighter layers are composed of plagioclase also Wiebe, 1996); (3) troughs (Figs. 9 and 10); densities measured to date vary from 0 to hun- with lesser amounts of quartz, K-feldspar, bio- (4) diapirs (Figs. 11 and 12); and (5) plumes dreds per square kilometer, indicating that they tite, and rare hornblende. The mafi c layers con- (Fig. 13). Table 1 and Figure 2 contrast the main are spatially clustered. This clustering does not tain dominantly biotite, and abundant accessory differences between these structures. My research appear to be related to internal contacts. minerals with small amounts of quartz, plagio- group’s recent 1:24,000 mapping of ~60% of the Maximum tube diameters vary from a few clase, K-feldspar, and hornblende. More com- batholith indicates that all 4 major units preserve centimeters (Fig. 3C) to >40 m. Larger tubes plex compositional variations are common in examples of these structures, that their forma- may exist but may be diffi cult to recognize tubes in the Kuna Crest and Half Dome units. tion was times transgressive from after 95 Ma because schlieren boundaries of large tubes Schlieren compositions and chemistries in tubes to ca. 85 Ma, and that they are most common in would appear as planar schlieren zones unless are presented in a later section. the Half Dome and Cathedral Peak granodiorites, they are followed along strike. The crosscutting In many cases the composition and textures less common in the Kuna Crest granodiorite, and relationships between schlieren-bounded lay- of the youngest part of the tubes are similar to rare in the Johnson granite porphyry and other ers (Figs. 3–5) and mineral grading in schlieren those of the host magma (Figs. 3–5). In exam- leucogranite lenses. Our mapping also indicates indicate that both stationary and migrating tubes ples where tube layers feather into host mag- that thousands of tubes, plumes, and troughs decrease their tube radii through time, often mas, the felsic layers between schlieren can exist, whereas pipes and diapirs are much less resulting in fairly small (centimeters to decime- be traced continuously into the host magmas common: the latter two structures are more com- ters) fi nal tube diameters. In stationary tubes the (Figs. 3–5) (see also Burgess, 2006; Burgess monly located in areas of well-developed mag- decrease in diameters can be either symmetri- and Miller, 2008). K-feldspar megacrysts occur matic sheeting (Žák and Paterson, 2005; Paterson cal or slightly asymmetrical, but the tubes are in some layers in tubes in the porphyritic Half et al., 2008). Troughs are common along internal always nested (Fig. 3). When exposed in three Dome granodiorite and Cathedral Peak grano- contacts between the main units but also occur in dimensions, most tubes do not change their gen- diorite units. In rare cases more mafi c dioritic zones between these contacts. eral characteristics over the short vertical dis- magmas, enclaves, or xenoliths occur in tubes I also examine herein the possibility that there tances exposed (Fig. 5). (Figs. 4E, 4F): in just a few examples the last may be gradations and/or genetic connections Almost all tube axes plunge >70° (Figs. 5 pulse of magma broke through the slightly older between some of these structures (Figs. 14–18); and 6). Since most tube axes are steep, the outer tube rings (Fig. 3D). however, I fi rst describe compositional and struc- maximum vertical dimensions of tubes are Magmatic mineral foliation(s) and less com- tural characteristics of end-member examples. not well constrained: local relief indicates monly lineation(s) are well defi ned in tubes minimum vertical dimensions of at least sev- (Fig. 5). Two types of foliations are preserved Stationary and Migrating Tubes eral meters. In rare cases tubes get reintruded in tubes: an early foliation subparallel to tube by host magmas (Fig. 17), which can lead to margins, best preserved in the more mafi c layers Magma tubes are defi ned as cylindrical or disruption and rotation of tube segments. Bro- (Type 1 of Žák et al., 2007), and a younger folia- tube-shaped structures in three dimensions that ken and rotated tube segments range to tens of tion that ignores layer orientations in tubes and

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o is parallel to regional foliations in the batholith Pipes l l i t a t a (Type 3 or 4 of Žák et al., 2007). This latter folia- n e o n

z l i

tion is best seen in the more felsic layers in tubes Pipes are enclosed, sometimes funnel-shaped s r i c i o

x t

h (Figs. 5 and 17D). Subvertical, layer-parallel sec- to more commonly cylindrical-shaped bod- a a

l y h l m b

g tions appropriate to examine mineral lineations ies, geometrically similar to tubes, but distinct e g a i r u a r a o a r in tubes are rare. In the few examples located, from tubes in that they have no repeated internal r M

t v

o l t o

e lineations are best displayed by the alignment of layering and thus have a single, dominant com- t l

l p e a e r r large, euhedral hornblende crystals in the maﬁ c position distinct from and more felsic than the e a a teep to rarely horizontal t P R S base of schlieren (Fig. 5). Lineations both paral- surrounding magma (Fig. 7). Pipes are less com- lel (steeply plunging) and perpendicular (shal- mon than tubes and troughs in the Tuolumne lowly plunging) to tube axes occur. batholith, but have been found in all four units:

e s Stationary tube r r e e

i Diapir l y h a

l Closed c Migrating tube S d schlieren Inner ring younger e t

a perpendicular than outer ring

ral weak layers Rare to variable

e Simple e e

n to tube axis r r v u a a e schlieren front r T S R R Graded schlieren

s l a r e n i m

t s o

h E

Graded A f e of host Multiple, nested rings Steep to rarely horizonta

t o n schlieren a Pipe o n n i t o i o i i t s t

a Plume head c o l a p u r f m

m r

o t u o s c / C c o d

Simple

a n h

t e r a f

l schlieren front s

o o b o

m a h i e e

o t t r r o a a f a t

v n n B t

r c o o y i i a l t t l n i i h c c t F g s a a m i i r r S D F F C

l Trough b a i r

a e v

Horizontal Face b y l p p a i h e e r g e e i t a t H S V S TABLE 1. COMPARISON OF MAGMATIC STRUCTURES DISCUSSED IN TEXT

cumulates Truncated layers perpendicular to trough axis

Vertical Face s s s

l D r r e e e n d d Open schlieren on both ends n n n i i a l l of the trough axis y h y C Spherical heads, broad tails C C Figure 2. Main structures discussed in this paper. (A) Stationary tubes (Figs. 3 and 5A). (B) Migrating tubes (Figs. 4 and 5). (C) Pipes (Figs. 7 and 8). (D) Troughs (Figs. 9 and 10).

s (E) Diapirs (Figs. 11 and 12). (F) Plume heads (Fig. 13). The distinguishing features are e b

u listed in Table 1. Graded schlieren layers shown where developed in structures. Troughs in T

y D include typical relationships between trough cutoffs, mineral fabrics, magma ﬂ ow direc- r

s a

h n tions (double-ended arrow, as absolute fl ow direction is uncertain), and crystal accumula- g o s i t u e tions seen in the fi eld. Some drawings are infl uenced by diagrams in Weinberg et al. (2001), a o p t i r S Type of StructureType Migrating Tubes cylinders Multiple intersecting Inferred Shape Steep Orientation Distinct from and/or fractionate of host Multiple, truncated rings S P Diapirs Spherical heads, narrow tails Highly variable Hi Plumes T but with modifi cations.

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A B

Figure 4. Photos of steeply plunging, migrating tubes in the Tuolumne batholith. Arrows show direction of migration. Ruler in photos A, B, E, and F is 15 cm. (A) Overview of migrating (southwest to northeast in photo) tube in the Cathedral Peak (CP) granodiorite; ruler near northeast end of tube for scale. (B) Close-up of one part of migrating tube shown in A. Note that the composition of felsic layers in tube are identical to host magma. (C) A row of several separate migrating tubes in the CP granodiorite. The fi rst schlieren ring in one tube always truncates the last rings in the slightly older tube. All migrated in the same direction (left to right in photo). Some layers in tubes have K-feldspar megacrysts, others do not. Photo ~3 meter across. (D) Fairly mafi c layers bounding migrating tube in the Half Dome granodiorite. Tube walls interdigitate with host magma and some rings in tube have fi eld characteristics identical to those in the host granodiorite. Brunton compass for scale. (E) Migrating tube in transition zone between Half Dome and Kuna Crest unit granodiorites near Potter Point, Lyell Canyon. Tube is reintruded by host magma and decreases its diameter as it youngs (parallel to arrow) toward top of photo. Ruler is in region of reintrusion. (F) Close-up of one part of tube in E showing heterogeneous compositions and truncation of tube walls indicating younging (migration of) tube to right in photo (parallel to arrow).

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30 cm Vertical

orizontal H ruler against vertical surface for scale. Subhorizontal surface immediately below ruler with closed schlieren layers. (D) Hornb with closed schlieren scale. Subhorizontal surface immediately below ruler against vertical surface for ruler layers, but many deﬁ aligned parallel to schlieren few hornblendes are A tube in HD granodiorite. scale. Note vertical mineral lineation deﬁ for experiments (Ildefonse et al., 1997); 15 cm ruler leucocratic layer Also note that more layers in the subhorizontal surfaces B–D. aligned parallel to schlieren random or more host granodiorites. diorite. (B) Half Dome (HD) granodiorite; 15 cm ruler against vertical surface for scale. Subhorizontal surfaces at top of phot against vertical surface for diorite. (B) Half Dome (HD) granodiorite; 15 cm ruler Figure 5. Three-dimensional view of tubes showing subvertical schlieren-bounded walls and steeply plunging tube axes (parallel view of tubes showing subvertical schlieren-bounded Three-dimensional 5. Figure

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they are most common in the porphyritic Half Dome and Cathedral Peak granodiorites. Cylindrical pipe diameters vary from a few centimeters to >10 m and maintain these diameters along their axes (Figs. 7 and 8). Pipes with diameters >~10 cm have axes consistently CP steeply plunging in Tuolumne batholith, and pHD thus their maximum vertical dimensions are eHD KC unknown. Where good three-dimensional (3D) exposures of pipes occur, the vertical dimensions or long axes of the pipes are always Average = 89/266 2km greater than their subhorizontal diameters and are at least a few meters in length (Fig. 7). Pipes N=68 with diameters <10 cm have slightly more vari- B able axis orientations but in general still plunge A steeply (Figs. 7 and 8). Less common funnel- shaped pipes have decreasing diameters with depth (Fig. 7F), although they share all other characteristics with the cylindrical pipes discussed above. Pipes in the porphyritic Half Dome and Cathedral Peak granodiorites tend to be dominated by K-feldspar megacrysts, sometimes making up >80% of pipe minerals (see also Pole to GC= 65/156 Average= 89/246 Burgess and Miller, 2008), but still with at least minor amounts of all minerals seen in the surrounding host magma (Figs. 7 and 8). N = 585 K-feldspar megacrysts in the pipes show ﬁ eld and thin section characteristics identical to those C D in the host magma around the pipes (Fig. 16B) (Vernon and Paterson, 2008a, 2008b; Burgess al Area Equal area Equal area and Miller, 2008). These larger pipes may also include small microgranitoid enclaves and host rock xenoliths (Fig. 7). K-feldspar megacryst abundances in host magmas are neither depleted nor enriched as pipes are approached (Figs. 7 and 8). One sample has been analyzed from a pipe in the northern part of the Cathedral Peak

granodiorite. This pipe has 70% SiO2; is enriched (relative to nearby Cathedral Peak

and Half Dome granodiorite analyses) in K2O N= 7 (~6 wt% versus <4 wt% in regular Cathedral N= 24 N= 51 Peak and Half Dome granodiorite analyses), F Ba (~2450 ppm versus ~1000 ppm in normal E Cathedral Peak granodiorite analyses) and Sr (>600 ppm); remains unchanged in Al O ; and Figure 6. (A) Map of one corridor through the central part of the Tuolumne batholith 2 3 is depleted in Na O, TiO , MgO, CaO, P O , showing locations of domains with a few to >70 tubes or pipes. Note that tubes occur in all 2 2 2 5 compositional units. KC—Kuna Crest unit; CP—Cathedral Peak granodiorite; HD—Half Rb (~150 ppm), Zr, Y, and La. It has a nor- Dome granodiorite (p is porphyritic, e is equigranular). (B) Equal-area plot of 68 tube mal distribution of rare earth elements (light, axes: average shown in red and is vertically plunging. (C) Equal-area plot of 585 poles to LREE enriched compared to heavy, HREE) schlieren in troughs. Note wide scatter although weak maxima of NW-SE–striking schlie- and no Eu anomaly. ren. (D) Equal-area plot of 45 trough axes from one domain in central part of Tuolumne Some pipes preserve a single, weakly devel- batholith. (E) Measurements of two different magmatic foliations in a small domain in the oped schliere-like layer, rich in biotite, along CP near Tuolumne Meadows. The west-northwest fabrics tend to be stronger and more the outer pipe margin (Figs. 7C, 7D). A close common. (F) Rose diagram of the long axes (in subhorizontal surface) of schlieren ellipses examination of these biotite-rich layers shows (plume heads) and tubes. The blue and green great circles represent the average orienta- that they are deﬂ ected around phenocrysts in tions of the two magmatic foliations shown in E. the pipes, indicating that some compaction, presumably during ﬁ lter pressing, occurred during layer formation (Figs. 16A, 16B). Compaction

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A B

Figure 7. Photos of pipes. (A) Vertical surface through granodiorite pipes in quartz diorite sheets in Kuna Crest unit; 15 cm ruler for scale. (B) Subhorizontal surface through the pipes in A; 15 cm ruler for scale. Note that all pipes are roughly subvertical even in cases where the maﬁ c layering is also steeply dipping (in contrast to Wiebe and Collins, 1998). (C) Horizontal surface through a number of small leucocratic granite pipes in Johnson granite porphyry; 6.5 cm lip balm for scale. (D) Vertical surface through cylindrical pipe K-feldspar megacryst– rich pipes (typically consisting of 60%–80% megacrysts by volume); 25 cm hammer for scale. A minor amount of biotite accumulation occurs along the pipe walls. (E) Closeup of one margin of a K-feldspar megacryst–rich pipes (typically consisting of 60%–80% megacrysts by volume) in the Cathedral Peak granodiorite. Thin schlieren along margin; 6.5 cm lip balm for scale. (F) Vertical surface through funnel- shaped pipe. Top of pipe is ~1 m across.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/5/6/496/3338206/i1553-040X-5-6-496.pdf by guest on 26 September 2021 Paterson D B cult to see in photo due rite; 15 cm ruler for scale. (B) Vertical Vertical scale. (B) for rite; 15 cm ruler e te t for scale. (D) Horizontal surface through scale. (D) Horizontal surface through for a la eldspar megacryst–rich pipe in sheeted zone eldspar l to 15 cm ruler is difﬁ l to 15 cm ruler u mu m u cumulate cu sm s f Kf K Kfs megacryst C A cumulate Kfs megacryst cumulate Kfs megacryst Figure 8. Photos of large K-feldspar (Kfs) megacryst rich pipes. (A) Horizontal surface through pipe in Half Dome (HD) granodio (Kfs) megacryst rich pipes. (A) Horizontal surface through 8. Photos of large K-feldspar Figure K-f bands at the edge of ~20-m-diameter scale. (C) Schlieren for in HD granodiorite; 15 cm ruler A same pipe in surface through locally truncated by the pipe margin; 25 cm hammer are (Žák and Paterson, 2005). Note that schlieren in the Sawmill Canyon area magmatic foliation paralle Weak metavolcanic and plutonic xenoliths enclosed in pipe. of same pipe in C. Small host rock center to large grain size.

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of crystals in the pipes, and thus melt removal, orientations of trough axes are highly variable Magmatic mineral fabrics in troughs are is also indicated by the indentation of one phe- in their plunges, which range from 0° to 90°. A similar to those in tubes in that foliations vary nocryst by another (e.g., Fig. 16B) leading to stereonet plot of unequivocal trough axes in one from approximately margin parallel in mafi c melt assisted dissolution of zoning (Park and domain in the main chamber shows some scat- layers (type 1 fabrics) to regional orientations Means, 1996). ter but with many with steep plunges (Fig. 6D). (types 3 or 4) in the more felsic interiors. Sub- Magmatic mineral fabrics in pipes tend to However, these trough axis orientations vary hedral to euhedral hornblende and biotite are be weaker than those seen in the host magmas tremendously from one domain to the next, such sometimes imbricated relative to schlieren or in tubes and troughs. Type 1 margin-parallel as documented (in Paterson et al., 2008) in a bases (Solgadi and Sawyer, 2007; Burgess and fabrics are less common and regional type 3 domain near the eastern margin of the batholith. Miller, 2008). Only a few examples of mag- or 4 fabrics more typically overprint the pipes In another domain in the Cathedral Peak grano- matic lineations have been seen in troughs: (see fabric nomenclature in Žák et al., 2007). In diorite, Burgess and Miller (2008) noted that in these vary from those defi ned by large euhe- rare cases subhorizontal fabrics, formed at high general the schlieren strike northwest, have vari- dral hornblende crystals in basal schlieren lay- angles to pipe margins and to regional fabrics, able dips, and are oblique to the major internal ers, which are always oriented down the cen- suggest that late vertical compaction may have contacts with the Half Dome granodiorite and tral axis of the trough (Fig. 10), to a steeply aligned crystals within the pipes. Appropriate Johnson granite porphyry units. Troughs orien- plunging lineation in the more felsic parts of surfaces to observe mineral lineations along tations are further complicated since they also some troughs. As noted here, K-feldspar mega- pipe margins have not yet been found. are occasionally reintruded, broken apart, and crysts, when present in troughs, sometimes A few of the largest pipes have migrating sometimes rotated by the host magma (Fig. 17). show excellent alignment in schlieren but tubes preserved along the pipe margins (Fig. 8). Spectacular truncations or cutoffs of one much weaker alignment in more leucocratic The presence of these migrating tubes raises trough by another, geometrically identical to layers (Figs. 9 and 10). In the few cases where the question of whether formation of the pipes trough truncations seen in sedimentary rocks, layer-parallel surfaces in schlieren are visible sometimes caused complex fl ow along their are common in the Tuolumne batholith; trunca- in troughs with megacrysts, these megacrysts margins, resulting in tubes. tion angles vary from ~50° to as low as 5°–10° did not form obvious lineations in the foliation (Fig. 9). These truncations provide trough plane (Fig. 10C). Troughs younging or growth directions, measured in this study as the pole to the tangent plane of Within-Chamber Diapirs Troughs are open and thus asymmetrical, schlieren at the point where the older layer is schlieren-bounded channels with curvatures truncated (Fig. 9), that usually preserve a single A number of small-scale diapirs (within- commonly less than tubes (Fig. 9). As trough direction at any one fi eld location. It is surpris- chamber diapirs of Weinberg et al., 2001) occur curvatures decrease, they grade into and are ing that these trough younging directions typi- in the Tuolumne batholith. These small-scale commonly associated with layered schlieren cally trend either parallel to nearby margins or diapirs are herein defi ned as moderately to non- zones (Žák and Paterson, 2005; Burgess and toward older units (outward) rather than inward layered, irregularly shaped batches of magma, Miller, 2008). Trough widths vary from a few toward younger magmatic units as defi ned from intruding host magma of a different composi- centimeters to hundreds of meters, but many are geochronologic studies (Bateman and Chap- tion (Figs. 11 and 12). Small-scale diapirs are in the 1 m to 20 m range. Trough amplitudes are pell, 1979; Matzel et al., 2006b. 2006c). These less common than tubes and troughs in the typically no more than 1 or 2 m, but can some- schlieren-bounded troughs were recognized by Tuolumne batholith, but have been found in all times reach the 10 m scale. Where 3D exposures Bateman (1992), who described them as form- units except the Johnson granite porphyry. All of troughs were found, trough long axes were ing an outward-facing schlieren arch (hinted diapirs found to date occur in compositionally always much greater than trough heights and at in the weak maxima of schlieren in Fig. 6). layered domains and/or near internal contacts, widths (Fig. 9). Locally schlieren in troughs Potential causes of these unexpected trough although caution is suggested since it may be are offset by magmatic faults, which in turn are younging directions were discussed in Paterson particularly diffi cult to recognize diapirs in non- truncated by younger troughs (Fig. 9A). et al. (2008) and Žák et al. (2009). layered regions if they are compositionally simi- Troughs in the Tuolumne batholith are also Trough compositions are much less vari- lar to their host (e.g., see plume discussion). Dia- spatially clustered in that they commonly occur able than in tubes, and except for the schlieren pir shapes are highly irregular, but often include in zones of multiple troughs separated by zones typically appear similar to host compositions. narrow tails and bulbous or mushroom-shaped where few if any are preserved. They are com- Schlieren in troughs show the same features as heads where the diapirs are largely separated mon along the main internal contacts and local those seen in tubes and are discussed in detail in from their origin (mature diapirs) and are more sheeted zones in the Tuolumne batholith, but a later section. K-feldspar megacrysts occur in cylindrical if still connected to layers (immature also are found well away from any obvious troughs in porphyritic Half Dome and Cathedral diapirs). The size of within-chamber diapirs rec- internal contact. Peak granodiorite units (Fig. 9) and show sev- ognized to date in the Tuolumne batholith does Schlieren and schlieren-bounded trough ori- eral interesting features. They do not follow the not exceed several meters. entations in the Tuolumne batholith are more usual mafi c to felsic zoning and reverse grain Diapir compositions are highly variable rela- complex than tubes and more diffi cult to evalu- size grading seen in schlieren. They also some- tive to both the immediate host rock and to other ate because clearly defi ned troughs grade into times show defl ection of schlieren around them, diapirs. Diapirs in the Tuolumne batholith are fairly planar schlieren. Orientations of 585 suggesting that signifi cant compaction of the sometimes slightly more mafi c or more felsic schlieren (which include both troughs and planar schlieren occurred around these resistant crys- than host magma, and sometimes rich in mafi c schlieren) show a wide scatter, although there is tals (Fig. 17). Their alignment is variable from minerals, K-feldspar megacrysts, and enclaves a weak maximum, indicating that a number of one schliere to the next, and sometimes is differ- (Figs. 11 and 12). Internal layering can be pres- schlieren have northwest-southeast strikes and ent than the alignment of mafi c minerals in the ent or absent but is typically less prominent than fairly steep dips (Fig. 6C). Unlike tubes, the same layer (Fig. 10). in tubes and troughs.

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2 3 1

A B

C D

E F

Figure 9. Schlieren-bounded troughs in the Tuolumne batholith largely made of granodiorite magma or their fractionated and/or accumulated components. (A) Moderately plunging trough sets younging to left in photo (see numbers) in the Sawmill Canyon area discussed in Paterson et al. (2008). Note magmatically folded troughs near 1 and magmatic fault in troughs below 2. All schlieren show grading. K-feldspar megacrysts occur in all three sets, showing best alignment at the base of schlieren and less so in felsic layers. Map case at base is ~20 cm long. (B) Gently plunging troughs also in Sawmill Canyon area that young upward. The late leucocratic dikes and some layers rich in K-feldspar megacrysts (lower right in photo) are intruded from the nearby Cathedral Peak granodiorite (CP) unit. Blue hammer is ~20 cm long. (C, D) Steeply plunging troughs exposed in subhorizontal surfaces in CP. Note truncations of schlieren layers at the base of younger troughs. These truncations indicate that erosion of older troughs occurred during magma ﬂ ow in younger troughs followed by renewed deposition in the younger trough. Cutoffs also indicate direction of younging measured in this study as pole to tangent of plane parallel to younger schlieren at truncation of older (symbols in C). Troughs in CP younging to left in photos; ~25 cm hammer for scale. (E, F) Subhorizontally trending troughs exposed in three dimensions in the Sawmill Canyon area. Some late magmatic folding and local diapiric movement locally deform schlieren; 25 cm hammer for scale in E.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/5/6/496/3338206/i1553-040X-5-6-496.pdf by guest on 26 September 2021 Magmatic structures formed in crystal-rich mushes D ndes at base of schlieren in Half Dome grano- ndes at base of schlieren A, they do not form a lineation in the trough; A, they do not form a lineation in the trough; near Teneya Lake. Note increase in K-feldspar in K-feldspar Lake. Note increase Teneya near these layers are not as well aligned; 15 cm ruler not as well aligned; 15 cm ruler these layers are est lineation; 10 cm pen for scale. (C) Surface of est lineation; 10 cm pen for AB C base of schlieren in trough showing that although K-feldspar megacrysts may form a foliation parallel to schlieren, as seen in megacrysts may form a foliation parallel to schlieren, showing that although K-feldspar in trough base of schlieren diorite. Note how a few hornblendes are oriented ~90° to main lineation. Flow in trough is interpreted to be parallel east-w is interpreted oriented ~90° to main lineation. Flow in trough diorite. Note how a few hornblendes are layers. Small hornblendes in Alignment of euhedral hornblende phenocrysts at the base schlieren scale. (D) 10 cm pencil for scale. for megacrysts away from base of troughs. Swiss army knife for scale. When closed, the knife is ~6 cm. (B) Aligned euhedral hornble When closed, the knife is ~6 cm. (B) scale. Swiss army knife for base of troughs. megacrysts away from Figure 10. Photos of magmatic mineral fabrics in troughs. (A) Troughs with grading in schlieren in Cathedral Peak granodiorite with grading in schlieren Troughs (A) 10. Photos of magmatic mineral fabrics in troughs. Figure

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/5/6/496/3338206/i1553-040X-5-6-496.pdf by guest on 26 September 2021 Paterson z B it granodiorite in Sawmill Canyon push- spar megacryst–rich diapir in Cathedral Peak megacryst–rich diapir spar par megacryst–bearing HD granodiorite, Saw- par A CD c minerals (mainly biotite and accessories) at the top of diapir; 25 cm hammer for scale. (C) Laterally moving diapir of quart scale. (C) Laterally moving diapir for c minerals (mainly biotite and accessories) at the top of diapir; 25 cm hammer layers in magmatic troughs. ecting schlieren Figure 11. Photos of local diapirs with arrows showing inferred movement direction. (A) Mushroom-shaped diapir of Kuna Crest un of Kuna Crest diapir (A) Mushroom-shaped movement direction. showing inferred Photos of local diapirs with arrows 11. Figure scale. (B) K-feld for batholith; 10 cm ruler Tuolumne the eastern margin of laterally away from ing metavolcanic host rocks granodiorite unit. Note accumulation of maﬁ diorite in transitional phase of the Half Dome (HD) granodiorite; 15 cm ruler for scale. (D) Obliquely moving diapir of K-felds scale. (D) Obliquely moving diapir for diorite in transitional phase of the Half Dome (HD) granodiorite; 15 cm ruler mill Canyon area, moving into and deﬂ mill Canyon area,

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/5/6/496/3338206/i1553-040X-5-6-496.pdf by guest on 26 September 2021 Magmatic structures formed in crystal-rich mushes B D na Crest unit magma near the Mam- unit magma near na Crest lations surrounded by Half Dome (HD) lations surrounded nt in exposed surface. (A) Hornblende- granite; 15 cm ruler for scale. for granite; 15 cm ruler ect the movement of slurries of CP magma into HD ect the movement of slurries CP CP HD A C granodiorite; 25 cm hammer for scale. (C) Microgranitoid enclave and hornblende phenocryst–rich magma intruding transitional Ku scale. (C) Microgranitoid for granodiorite; 25 cm hammer Figure 12. Photos of downward-moving diapirs, informally called “drips“ in this study. Arrows show inferred direction of moveme direction show inferred Arrows 12. Photos of downward-moving diapirs, informally called “drips“ in this study. Figure megacryst accumu (B) K-feldspar shown is ~3 m across. Lake granite; area Turner rich Half Dome granodiorite drip moving through to reﬂ the contact (marked with black line) Cathedral Peak (CP) granodiorite. Interpreted granodiorite near Lake Turner scale. (D) Hornblende phenocryst- and enclave-bearing HD granodiorite intruding for moth sheeted zone; 15 cm ruler

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Several features provide information about was it established that their longest axes were bounded troughs (Fig. 11D). Examples in the the movement directions of these diapirs: (1) the steeply plunging approximately parallel to the Sawmill Canyon area (Fig. 1) were discussed in presence of diapir tails and heads (Figs. 11 and steep mineral lineation seen throughout the Paterson et al. (2008). 12); (2) the intrusion into and resulting defl ec- Tuolumne batholith. It is thus assumed that in Tubes, pipes, plume heads, and diapirs all tion of older features (Figs. 11A, 11D); (3) the the approximately horizontal surface the ellipses involve the buoyant movement of lower viscosity accumulation of mafi c minerals (mostly biotite) refl ect intermediate and short axes: intermediate magmas through a higher viscosity host magma. to form weak schliere on margins of the diapir axes ranging from 17 cm to 6.3 m and ellipse When combined with the potential gradations margin head (Fig. 11B) and not along the tail, ratios range from 1.0 to 2.9 (average = 1.7). The between these structures (presented in previ- which probably refl ects fi lter pressing of melt intermediate tube axes show a wide range in ori- ous sections), these observations raise the issue and accumulation of residual minerals during entations with a potentially weak correlation to of whether there may be genetic links between diapir movement (Weinberg et al., 2001). Rarely two magmatic fabrics measured in this domain them. For example, tubes, pipes, plume heads, do diapirs in the Tuolumne batholith record (Fig. 6). However, there is certainly no strong and diapirs show gradations in characteristics. simple vertical movement: examples of dia- alignment of intermediate tube axes. pirs moving up, horizontally (Figs. 16A, 16C, A number of other shapes probably related Nature of Schlieren 16D), obliquely (Fig. 16B), and down relative to plume heads also occur in the Tuolumne to present-day horizontal (Figs. 11 and 12) are batholith. Elongate shapes with small protru- All fi ve of the magmatic structures discussed all preserved (Žák and Paterson, 2005). Diapirs sions (Fig. 13C), mushroom- or jellyfi sh-shaped in this paper may be associated with schlieren moving downward relative to present-day hori- bodies that grade into the surrounding matrix either within or along their boundaries: thus the zontal (Fig. 12) are herein referred to as drips (Figs. 13D, 13E), and diapir-like heads attached nature of the schlieren is important in under- (Bergantz and Ni, 1999). to dike-like stems or tails (Fig. 13F) all occur. standing the development of the structures. Two Weak schlieren layers bound the heads of these distinct types of schlieren occur: those consist- Plume Heads bodies but tend to disappear at the bottoms ing of well-developed, repeated, graded lay- (Fig. 13D) or tails (Fig. 13F). Slices through the ers in tubes and troughs (Figs. 2–10), and the Weinberg et al. (2001) described elliptical, heads of all of these structures appear identi- much fainter biotite-rich layers discontinuously schlieren-bounded, magmatic ellipsoids or ther- cal to plume heads, as would the compositions. formed along margins of some pipes, plume mal plume heads in the Tavares pluton, Brazil. However, the 3D shapes are more complex and heads, and diapirs (Figs. 3–14). The former, Plume heads are particularly common in the merge into those described above for diapirs. herein called graded schlieren, have received a Cathedral Peak unit and rarely occur in leuco- They are distinct from these diapirs in that their fair amount of attention in both the Tuolumne granites in the Tuolumne batholith such as the compositions more closely match host magma batholith (e.g., Barbarin et al., 1989; Bateman, Johnson granite (Fig. 13), although some of compositions and that there is no evidence that 1992; Reid et al., 1993; Solgadi and Sawyer, the characteristics of those in the batholith dif- they formed in compositionally layered regions. 2007; Burgess and Miller, 2008) and in many fer from those described in the Tavares pluton. other plutons (Lykhovich, 1964; Barriere, 1981; Elliptical, schlieren-bounded regions with fairly Combinations of Structures Trent, 1981; Hanson and Nash, 1996; Antipin et homogeneous internal compositions, often simi- al., 1997; Ventslovaite, 1998; Wiebe and Col- lar to the host rock compositions, defi ne plume It is not uncommon in the Tuolumne batho- lins, 1998; Murray et al., 1993; Kagashima, heads in the Tuolumne batholith. The most com- lith to fi nd combinations of the above structures 1999; Stallings and Hogan, 1999; Preston et mon compositional difference is the reduction at a single locality and fi eld evidence that these al., 2000; Gibson et al., 2001; Weinberg et al., or lack of K-feldspar megacrysts in the ellipses, structures formed at the same time. Figure 14 2001; Clarke, 2003; Getsinger, 2004). The latter resulting in a slightly more mafi c composition. displays some typical examples. K-feldspar schlieren, herein called weak schlieren, have not Only rarely is more than one schliere ellipse megacryst–rich, funnel-shaped pipes sometimes been rigorously studied in the Tuolumne batho- preserved in a single plume head (e.g., Fig. 13). have their tails merging into schlieren troughs lith, but appear identical to schlieren in the Tava- Thus these features are distinct from tubes in the (Fig. 14A), suggesting that the K-feldspar-rich res pluton discussed by Weinberg et al. (2001). lack of multiple rings, the faintness of the schlie- magmas were separating from these mafi c- In the Tuolumne batholith, graded schlieren ren layers, and the similarity in compositions dominated layers in the troughs. Sometimes are centimeters to tens of centimeters in width with the host magmas. Weinberg et al. (2001) these pipes feed into troughs with no K-feldspar and typically have sharp bottoms and diffuse noted that the 3D shapes of plume heads in the megacrysts, even though adjacent portions of the tops. Reverse size grading of minerals is the Tavares pluton were typically ellipsoidal. This troughs do have some megacrysts (Fig. 14A). norm, but with some exceptions, such as exam- is sometimes true in the Tuolumne batholith, but However, this is not a general rule. K-feldspar ples where K-feldspar megacrysts and large in other cases, dimensions in one direction were accumulations identical to those found in some euhedral hornblende phenocrysts occur at the signifi cantly greater than the other two, result- pipes are also found in troughs (Fig. 14B). base of schlieren. As the sharp bases of schlie- ing in elongate bodies geometrically closer to K-feldspar megacryst–rich clusters, sometimes ren are approached, a modal increase in biotite those described as pipes or tubes in this paper. forming pipes, are sometimes associated with and hornblende and an unusually high increase One cluster of 51 plume heads was exam- either migrating or stationary schlieren tubes in accessory minerals, including zircon, sphene, ined in the Cathedral Peak granodiorite near (Figs. 8, 14C, and 14D) (see also Burgess and apatite, rare allanite, and oxides, occurs (see Tuolumne Meadows (Figs. 6E, 6F, and 13). Miller, 2008). The close association of these also Reid et al., 1993; Burgess and Miller, 2008; These all have fairly weak schlieren rings and K-feldspar-rich and K-feldspar-poor structures Paterson et al., 2008). This dramatic increase macroscopic characteristics (composition and suggests a potential genetic link, a possibility I in accessory minerals is a common feature in texture) that closely match the host magma explore in a later section. schlieren worldwide (Lyakhovich, 1964; Han- characteristics. They were best exposed in Diapirs are common in layered zones, which son and Nash, 1996; Antipin et al., 1997; Vent- subhorizontal surfaces and only in a few cases sometimes consist of numerous schlieren- slovaite, 1998; Murray et al., 1993; Kagashima,

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A B

C D

E F

Figure 13. (A, C, D, E) Photos of magma ellipsoids, or “thermal plume heads“ as defi ned by Weinberg et al. (2001), in Cathedral Peak granodiorite. (B) Johnson granite porphyry. Note that compositions in plume centers are similar to host magmas, although mafi c minerals are locally enriched and K-feldspar megacrysts depleted or absent. Schlieren rings are not nearly as well developed as in tubes. In D–F, mafi cs and accessories are enriched at head of plume (inferred movement direction) and eventually disappear on sides or tail. In F, plume head is attached to a dike-like stem that extends to left of photo for tens of meters. Scales in photos: A—10 cm pencil; B—15 cm ruler; C–E—6 cm lip balm tube; F—19 cm book.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/5/6/496/3338206/i1553-040X-5-6-496.pdf by guest on 26 September 2021 Paterson B D cm book for scale. cm book for ard from the trough. Note that K-feldspar Note that K-feldspar the trough. ard from feldspar megacrysts in CP unit cored by accu- unit cored megacrysts in CP feldspar Arrow shows direction of tube migration based shows direction Arrow bounded troughs with aligned K-feldspar mega- with aligned K-feldspar bounded troughs C A c tube in the center of a K-feldspar megacryst–rich cluster, possibly a pipe in CP unit. Note the megacrysts aligned possibly a pipe in CP megacryst–rich cluster, of a K-feldspar c tube in the center Figure 14. Photos of spatially associated combinations of previously discussed structures in Tuolumne batholith. (A) Schlieren- Tuolumne in discussed structures 14. Photos of spatially associated combinations previously Figure unit. pipe in CP scale. (C) Migrating tube immediately adjacent to large K-feldspar-rich mulation of K-feldspars; boot toe for crysts in Cathedral Peak granodiorite (CP) unit next to a funnel-shaped K-feldspar–rich pipe (just left of arrow) extending upw crysts in Cathedral Peak granodiorite (CP) unit next to a funnel-shaped K-feldspar–rich pipe (just left of arrow) K- with rare Troughs in photo is ~3–4 m wide. (B) Area layers below this funnel. schlieren the trough absent for megacrysts are scale. (D) Maﬁ layers; 6.5 cm lip balm for on crosscutting (60%–80%); 20 magma (10%–25%) and the cluster much less common than in host CP they are in tube; however, parallel to schlieren

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1999; Stallings and Hogan, 1999; Preston et al., during the formation of schlieren or in the devel- (Fig. 16). For example, the bending of schlieren 2000; Gibson et al., 2001; Getsinger, 2004). opment of compositional diversity in general in around large crystals (Fig. 16A), indentation of Figure 15 compares geochemical analyses the Tuolumne batholith is debated (Bateman one crystal by another (Fig. 16B), subhorizon- of graded schlieren in the Tuolumne batholith and Chappell, 1979; Reid et al., 1993; Glazner tal foliations at high angles to steep tube walls (black symbols) to analyses of the main units in et al., 2004; Burgess and Miller, 2008). How- (Fig. 8B), and veins of felsic melt rising off the batholith (colored symbols). Whether these ever, fi eld evidence of compaction and removal particular domains, that form “dish and pillar- geochemical results are in part due to fraction- of melt associated with schlieren and other like” structures (Figs. 16C, 16D), provide clear ation by crystal compaction and fi lter pressing structures is present in the Tuolumne batholith evidence of compaction and fi lter pressing. Our

10 12 14 16

8 Figure 15 (continued on following page). Summary of selected schlieren geochem- 45 50 55 60 65 70 75 istry in comparison to host geochem- SiO2 istry. (A) Al vs. Si. (B) Sr-Ba-Rb plot. Legend: KC—Kuna Crest unit; CP— Legend Cathedral Peak granodiorite; JP—Johnson granite porphyry; HD—Half Dome grano- B Sr diorite (p is porphyritic, e is equigranular). See Janousek et al. (2006) for plotting techniques and software.

Ba Rb

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810

CaO

MgO

0246

45 50 55 60 65 70 75 80 45 50 55 60 65 70 75 80 45 50 55 60 65 70 75 80

SiO2 SiO2 SiO2

3.0 3.5

60.81.0

TiO

.5 1.0 1.5 2.0 2

234

0.0 0.2 0.4 0

0.0 0 45 50 55 60 65 70 75 80 45 50 55 60 65 70 75 80 45 50 55 60 65 70 75 80

SiO2 SiO2 SiO2

Legend D

0 1000 1500

0 20 60 100 140

050

45 55 65 75 45 55 65 75

SiO2 SiO2

Figure 15 (continued). (C) Harker diagrams

of major elements. (D) Trace vs. SiO2 diagrams. In each ﬁ gure, colored circles show

distribution of data from (~300 analyses of) La

Ce host units in main Tuolumne batholith chamber. Black squares show schlieren analyses

0 100 150 200 and white square shows single analyses from a K-feldspar megacryst–rich pipe in the

0 100 200 300 400 Cathedral Peak unit. Legend: KC—Kuna 05 Crest unit; CP—Cathedral Peak granodio- 45 55 65 75 45 55 65 75 rite; JP—Johnson granite porphyry; HD— Half Dome granodiorite (p is porphyritic, SiO2 SiO2 e is equigranular). See Janousek et al. (2006) for plotting techniques and software.

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geochemical analyses (Fig. 15) clearly support or erosion and recycling of material from older evidence of subsolidus alteration (for summa- observations noted by Reid et al. (1993), that the pulses must be invoked (Reid et al., 1993). This ries see Moore and Sisson, 20008; Vernon and chemistry of the schlieren moves dramatically conclusion is supported by least squares model- Paterson, 2008a, 2008b). (2) The concentration away from host magma compositions, do not ing in which all calculations required little or no of magmatic minerals in schlieren requires that follow the trends seen in the main units, and are preferential accumulation of feldspars, whereas a large percentage of the crystals were already compatible with mixing plus crystal-liquid frac- the accumulation of amphibole + biotite + mag- present during ﬂ ow and crystal sorting. (3) Some tionation of certain minerals (see discussion). netite > titanite > apatite + zircon is important layers, particularly in tubes and troughs, grade

Initial results for any given SiO2 value support (Paterson et al., 2008). into the host magmas (Figs. 3–5 and 9). (4) A

depletion in Al2O3, Na2O3, and enrichment in Burgess and Miller (2008) examined graded number of the structures were reintruded by host

Rb, MgO, CaO, K2O, TiO2, P2O5, Zr, Y, La, and schlieren within the Cathedral Peak granodio- magmas, during which time the structures were Ce (Fig. 15). rite and concluded that they represent locations sometimes broken into segments and/or rotated Solgadi and Sawyer (2007) noted that whole- in the batholith where crystals have been seg- (Fig. 17). (5) Most if not all of these structures rock geochemical analyses of graded schlieren regated in some fashion from melt, and thus are partially or completely overprinted by late in the Sawmill Canyon area (Fig. 1) show trends are cumulates. They noted that the schlieren magmatic fabrics (Fig. 17D) (Žák et al., 2007; compatible with the redistribution of mafi c and in some cases have extraordinarily high REE Paterson et al., 2008). accessory minerals, but have different geo- contents and much higher Zr contents (from These observations indicate that the above chemical trends than seen in nearby nonlayered accumulated zircon) than average Cathedral structures formed by fl ow of crystal-bearing granodiorite. They also noted that microprobe Peak granodiorite (e.g., Reid et al., 1993; Miller magmas through hosts of crystal-rich magma analyses of hornblendes from these schlieren et al., 2007). For example, analyses from two mushes. The structures also formed before defi ne three distinct populations that composi- schlieren are characterized by overall REE formation of fi nal magmatic fabrics and thus tionally match those seen in the three main units enrichment reaching 800–1000× chondrite for above, but probably fairly near, the solidus of of the Tuolumne batholith (Kuna Crest unit, LREEs in the most mafi c schlieren. One sample the magmas in the structures. Field observations Half Dome granodiorite, Cathedral Peak grano- contained >1500 ppm Zr, >60 ppm Nb and Y, also indicate that the tubes, troughs, and pipes diorite). Furthermore, no systematic decrease >400 ppm Ce, and >100 ppm Th, and is cor- formed at the same time at any one locality. This in Mg number occurred from bottom to top of respondingly depleted in feldspar compatible conclusion implies that a great deal of localized schlieren, which Solgadi and Sawyer (2007) elements and quartz. Schlieren they analyzed fl ow, resulting in differentiation and formation concluded argued against simple fractionation also display very slight negative Eu anomalies. of compositional and structural complexity, can of a single magma to form these layers: they Burgess and Miller (2008) noted that anorthite occur in crystal-rich mushes (>50% crystals)

concluded that mechanical erosion, mixing of content in maﬁ c schlieren increased to An48 but still above the solidus.

crystals, and redeposition played a dominant (versus the highest An content of An39 in nearby role in schlieren formation. units) and had more variable core compositions Rheological Characteristics of Magmas Paterson et al. (2008) also examined graded that those from other Cathedral Peak samples. during Formation schlieren in tubes and troughs and layered sequences in the Sawmill Canyon area. Major DISCUSSION Weinberg et al. (2001) argued that marginal and trace element analyses for schlieren, nor- schlieren and extraction structures, and conse- malized by the typical composition of the Timing of Formation of Structures quently ladder dikes, snail structures, diapirs, nearby Half Dome granodiorite (the likely and plume heads, are ideally developed when

source), show that with decreasing SiO2 a I noted that because all 4 major units preserve the magma consists of a permeable crystal more than threefold enrichment in Ti, Fe, and examples of tubes, troughs, pipes, and diapers, mush. They specifi cally noted that the presence P occurs, with less pronounced increases in Mn and because these units have well-established of similar structures in K-feldspar megacryst- and Mg (Fig. 15). The contents of Al, Na, Ca, crystallization ages ranging from 95 to 85 Ma, bearing granites of the Sierra Nevada suggests and K are largely unchanged. Among trace ele- the formation of these structures must be time that these structures may best be developed ments, the most pronounced changes are enrich- transgressive over ~10 m.y. This is because it and preserved in high-K, calc-alkaline granite- ments in Zr and Hf (>4× ), followed by LREE is highly unlikely that any part of the batho- granodiorite with heterogeneously distributed and middle (M) REE (~3.5×), Nb and Ta (close lith formed and stayed above its solidus for K-feldspar megacrysts (in general 15%–25% to 3×), Th, U, and HREE (slightly above 2×). longer than a few million years, a conclusion modal megacrysts). One implication of my The contents of K, Sr, Na, K, Al, and Rb remain supported by thermal modeling of and cooling study, besides establishing that similar struc- almost constant. We (Paterson et al., 2008) con- ages from the Tuolumne batholith (Mundil et tures formed in magmas without K-feldspar cluded that such a variation is compatible with al., 2004; Matzel et al., 2006b, 2006c; Pater- megacrysts, is that they formed in higher crys- progressive accumulation of mainly mafi c and son et al., 2007b). Direct evidence of the time- tallinity magmas than suggested by Weinberg et accessory phases, most notably amphibole (Fe, transgressive nature of these structures is found al. (2001), that is, in magmas with fairly high Mg, Ti), titanite (Ti, Nb, Ta, U, Th, LREE), along the main internal contacts, where tubes effective viscosities (Bergantz and Ni, 1999, magnetite (Fe, Ti, Mn), apatite (P, MREE), and and troughs in older units are intruded and trun- Bergantz, 2000; Walker et al., 2007). The fol- zircon (Zr, Hf, and HREE) with a lesser role for cated by the younger intrusive phases (Fig. 17). lowing observations in the Tuolumne batho- feldspar(s), a conclusion consistent with what is Additional observations establish the devel- lith address specifi cally the above hypothesis. seen in the fi eld and in thin sections of schlieren. opment at any one location of these structures (1) When structures are reintruded, broken Our data confi rmed the notion that the origin of relative to the magma solidus and other struc- apart, and/or rotated by host magmas, the struc- schlieren cannot be due to a simple accumula- tures. (1) All crystals in these structures are tures act as rigid objects and do not completely tion of crystals during fractional crystallization. dominated by magmatic characteristics with collapse (mingle) or deform internally (Fig. 17). Instead, magma mixing and/or mingling and/ only very localized and minor microstructural (2) When layers in structures were cut by

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magmatic faults, the movement on these faults Weinberg et al. (2001) suggested that in the tube walls form by combined fl ow sorting and rigidly displaced but did not internally disrupt Tavares pluton tubes represent cylindrical chan- fi lter pressing. these layers (e.g., Fig. 9). The same is true when nels of magma fl ow through crystal mushes, This model explains well the tubes in the structures at internal margins were intruded and and that the main fl ow direction is parallel to the Tuolumne batholith with the following refi ne- truncated by younger pulses, which required tube walls (Fig. 18). They interpreted slightly ments. In the Tuolumne batholith, the composi- partial removal of the structure without inter- (snail structures) to fully migrating tubes (ladder tions and textures of magma that fl owed through nally deforming the remainder (Fig. 17). dikes) as similar structures that result from the the tubes can be quite variable, implying that (3) Many troughs formed during magmatic ero- superposition of sequential cylindrical magma different magma sources were tapped. Also both sion of older layers prior to renewed deposition pathways in which each new, curved schliere stationary and most migrating tubes decrease in to form new layers: this erosion removed parts represents the walls of a former cylindrical diameter with time, an observation interpreted of the older layer without otherwise deform- magma path. Magma fl ow through tubes, poten- to refl ect decreasing fl ow velocities and thus ing the remaining layering (Fig. 9). (4) All of tially either up or down, is driven by thermal or shrinking tube diameters. Mineral fabrics in these structures form during intrusion of one compositional buoyancy (e.g., Griffi ths, 1986; tubes supports vertical motion parallel to tube magma pulse into another along relatively sharp Martin et al., 1987; Weinberg et al., 2001; Bach- margins, sometimes followed by vertical com- contacts across which no obvious mingling and mann and Bergantz, 2008). Schlieren along the paction in tubes (as fl ow ceased?), and fi nally only rare mixing has occurred (Figs. 3–17). The formation and preservation of these sharp contacts suggests that at least the host rock magmas Stationary tube Migrating tube must have been fairly strong due to high crystal percents (Saar et al., 2001; Bergantz, 2000). Tube flow Oldest Diapir (5) Compaction of magmas and indentation of Vertical magma migration Simple schileren crystals (Fig. 16) during transmission of devia- flow through front at toric stresses through magmas also requires that cylinder shaped, Youngest diapir head stationary the magmas are fairly crystal rich. Diapir magma magma body All of these observations support the inter- different from Graded host magma pretation that the magmatic host rocks were schlieren Graded fairly strong and thus crystal rich, at least at schlieren C Kfs Pipe the time scales and rates at which the struc- “log jam” tures formed, although these host rocks must C Kfs Pipe have retained melt since they (1) continued to “log jam” Vertical magma flow through E contribute magma to the developing structures, A cylinder shaped migrating tube (2) fl owed magmatically during reintrusion of in magma body some structures, and (3) strained in a magmatic state during the hypersolidus formation of mag- Plume head matic fabrics (Žák et al., 2007). Magmas in the Host magma developing structures presumably began with motion? fewer crystals than the host magmas, but still B had a signifi cant percentage of crystals, since Flowing Porous Mush Plume head material all crystal phases were sorted during formation Magma and host material Trough of the structures and they had to have enough Flow F are similar strength to preserve steeply dipping layering, Shear sorting resist subsequent erosion, undergo compaction Horizontal Face and fi lter pressing, and act as rigid objects dur- ~5 cm ing reintrusion and rotation. Magma channel flow trough axis shear sorting Processes by Which Structures Formed

The formation of all of these structures involved multiphase magmatic fl ow during initially high and then decreasing temperatures and increasing crystal percents. Unfortunately, multiphase fl ow, particularly during changing Vertical Face D conditions resulting in crystal growth, is not well understood (e.g., Carrigan et al., 1992; Figure 18. Features discussed in this paper, possible relationships between different types Bergantz, 2000; Bergantz, and Breidenthal, of structures, and the relationships between mineral fabrics and accumulations. (A, B) 2001; Petford, 2003; Burgisser et al., 2005; Tubes. (C) Pipes; Kfs—K-feldspar. (D) Troughs. (E) Diapirs. (F) Plume heads. Some draw- Dingwell, 2006). In spite of this limitation, the ings are based on diagrams in Weinberg et al. (2001), but with a number of additions. existing fi eld observations and geochemistry Trough illustrations show typical relationships between trough cutoffs, mineral fabrics, place fairly large constraints on the develop- magma fl ow directions, and crystal accumulations seen in the fi eld. Arrows show implied ment of these structures. magma fl ow directions in each structure.

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regional strain resulting in the development of berg et al. (2001) noted that coarse K-feldspar tially entrained, in which case the plume-head overprinting magmatic fabrics. The locally con- aggregates occur on the upstream side of dike buoyancy may remain constant even though sistent direction of tube migrations raises the necks, as inferred from crosscutting relation- its volume increases and the plume cools. This question of whether source regions of rising ships in the dike. They suggested that fl ow neck- contrasts with compositionally driven diapirs, magma are migrating in consistent directions or ing led to a megacryst “logjam,” which contin- which do not easily entrain surrounding magma if lateral fl ow of host magma past a fairly steady ued to grow by the fi ltering out of megacrysts because of the slower chemical diffusion as magma source may displace preexisting tubes (a upstream (see also Clarke and Clarke, 1998). If compared to thermal diffusion (Weinberg et hotspot analogy). Observations in the Tuolumne these “logjams” occurred in local channels such al., 2001). These observations explain well the batholith indicate that the host magma to tubes as tubes, troughs, and diapers, they would result characteristics of Tuolumne batholith magma was stiff enough over the time scales of tube in K-feldspar–dominated pipe-shaped regions ellipsoids with compositions similar to those formation to resist mingling and collapse of the (Figs. 7, 8, and 18), as seen in the Tuolumne of host magmas and bounded by weak schlie- tube, but weak enough that it could move into batholith. Caution for this interpretation is rec- ren. However, their variable shapes, particularly and through tubes, intrude through tube walls, ommended because K-feldspar accumulations those grading into sheet-like bodies (Fig. 13), or reintrude and break apart tubes. in the Tuolumne batholith have highly variable suggests that these magmas may fi rst move by Troughs in the Tuolumne batholith are consid- geometries, many not being associated with a more channelized fl ow process and then form ered to be analogous to sedimentary fl ow chan- tubes and troughs, implying that there may be plume heads as their fl ow rates decrease. nels in which the channels have a bedload and multiple ways to form them (Vernon and Pat- For example, Olsen and Weeraratne (2008) formed in a porous media (Fig. 18) (e.g., Wah- erson, 2008a). However, one common theme in presented an intriguing set of analogue experi- raftig, 1979; Solgadi and Sawyer, 2007; Dufek all these accumulations is evidence of compac- ments of metal-silicate plumes and/or diapirs and Bergantz, 2007). These channels in the tion of the megacrysts and removal of interstitial and “emulsion diapers,” an interesting analogue Tuolumne batholith vary from fairly localized melt (Fig. 16), suggesting that fi lter pressing is for crystal-mush systems. Even though their with similar width/depth ratios to broad gently an important process during their formation. study focused on a different environment (sink- curved surfaces (large width/depth ratios) prob- The above processes all involved faster chan- ing of metal through mantle to form the Earth’s ably associated with broad sheet fl ows along an nelized fl ow of magma through host magma core), and different materials (liquid gallium internal crystal mush boundary. Magma fl ow that is stationary or moving slowly relative to sinking through corn syrup with no thermal is probably parallel to trough axes, as inferred the channel fl ow. Thus the channel margins may effects), their experiments produced a number by aligned hornblendes in basal schlieren. be eroded but not otherwise displaced by the of features remarkably similar to those seen in Host magmas, including earlier troughs, were channelized fl ow. In contrast, diapirs and plume the Tuolumne batholith. Olsen and Weeraratne’s stiff enough to allow local erosion followed by heads are interpreted to be magma batches that (2008) experiments produced (1) diapir and redeposition and associated fi lter pressing (see moved through host magma that was displaced plume heads, often attached to pipe-like paths Nature of Schlieren discussion) and local lag around the diapir and potentially incorporated that had signifi cant life spans in their models deposits to form (Fig. 14). The similar material into plume heads. The thin tails, broad heads, and sometimes continued to focus channelized in the troughs and in the host magmas suggests and compositions distinct from host magmas fl ow of material (cf. their Figs. 1 and 2 to the local sources: the highly variable orientation of of most Tuolumne batholith diapirs indicate tubes and pipes in the Tuolumne batholith); trough axes opens the door to variable processes that they were detached (typically from unob- (2) half diapirs (cf. their Fig. 6 and Fig. 14 driving magma fl ow in troughs, such as crystal served sources) batches of magma moving herein); (3) a means of mixing of host mate- mush avalanches, gravity-driven channelized through but not signifi cantly incorporating host rial into the diapirs and pipe-like structures; and fl ows, convection driven fl ow along irregular magma (Marsh, 1982; Clemens, 1998; Miller (4) cases where layer instabilities led to the mush zone margins, and downward return fl ow and Paterson, 1999; Olsen and Weeraratne, above structures (their Figs. 4 and 5). Further- during ascent of new magmas. The most intrigu- 2008). They are nicely explained as Rayleigh- more, they presented examples where the chan- ing aspect of Tuolumne batholith troughs is the Taylor instabilities (Berner et al., 1972; Marsh, nelized movement of plumes and emulsion common outward younging of channels. One 1982; Ronnlund, 1987; Whitehead and Hel- diapirs (their Figs. 7 and 8) resulted in ther- possible explanation is that stresses caused by frich, 1991; Weinberg and Podladchikov, 1994; mochemical plumes forming along the borders local processes (Paterson et al., 2008), regional Dietl and Koyi, 2002; Olsen and Weeraratne, of the features produced by their experiments, tectonic processes (Paterson et al., 1998), and 2008) and since they move through hot, crys- plumes similar to those seen along the borders by the cooling of magma chambers (Žák et al., tal mushes can potentially ascend long dis- of pipes in the Tuolumne batholith. 2009) combine to produce outward-migrating tances relative to their sizes (e.g., Marsh, 1982; The most unusual aspect of diapirs and low stress sites favorable for local redistribution Weinberg and Podladchikov, 1994). A few have plume heads in the Tuolumne batholith is their of late melts in chambers. less well defi ned heads and broader dike-like highly varied movement directions, indicating Two mechanisms for the formation of pipes stems still attached to source layers, and thus that gravity and/or buoyancy is not the domi- have been proposed in the literature. Wiebe fi t the well-known characteristics of immature nant driving mechanism. Buoyancy must have (1996; also see Wiebe and Collins, 1998) sug- Rayleigh-Taylor diapirs (Berner et al., 1972; operated, but was dominated by other poorly gested that immediately after the juxtaposition Dixon, 1975; Whitehead and Helfrich, 1991; constrained gradients in the host magma such of magmas with different compositions, the less Olsen and Weeraratne, 2008). as gradients in effective viscosities, differential dense magma rises up through the more dense Griffi ths (1986) showed that thermal plume stresses, or host magma fl ow (Žák and Pater- magma, largely as a Raleigh-Taylor instability. heads grow during ascent by entraining their son, 2005). This would result in pipes with variable com- surroundings through buoyancy (heat) diffu- Many of the above structures are associated positions and grain sizes relative to the host sion away from the plume head, which heats up with bounding and/or internal schlieren, which magmas, comparable to some of the pipes seen a surrounding boundary later (Weinberg et al., in this paper were grouped into “graded” and in the Tuolumne batholith. Alternatively, Wein- 2001). The entire hot boundary layer is poten- “weak” schlieren. A number of processes have

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been proposed for the origin of schlieren in gran- cal modeling suggests is compatible with pro- cations for volcanic processes. The complex itoids: (1) partial assimilation of mafi c enclaves, gressive accumulation of amphibole + biotite crystal cargoes seen in erupted units already (2) crystal settling, (3) shearing out of inhomo- + magnetite > titanite > apatite + zircon with existed in the Tuolumne batholith and need not geneities, (4) steep physicochemical gradients little role for feldspar(s) (Paterson et al., 2008; have formed during eruption. at fl ow margins (e.g., Barriere, 1981) leading Žák et al., 2009). These data also indicate that Ruprecht et al. (2008) presented an exciting to preferential crystallization of ferromagnesian magma mixing and/or mingling and/or erosion study of crystal mixing processes, particularly minerals and suppression of crystallization of and recycling of material from older pulses must mixing during magma overturn driven by gradi- felsic minerals (Naney et al., 1980), and (5) shear be invoked either before or during schlieren for- ents in gas bubbles, but also applicable to ther- sorting against an effectively rigid wall (Bhat- mation (see also Reid et al., 1993). mal or compositionally driven overturn: they tacharji and Smith, 1964; Komar, 1972; Barri- The weak schlieren in the Tuolumne batholith experimentally produced structures during mix- ere, 1981). In the Tuolumne batholith, schlie- are not yet well studied. Their common setting ing and overturn (their Fig. 1) that look identi- ren in these structures all share the following: at the outer boundaries of diapirs, plume heads, cal to the diapirs, plume heads, pipes, and tubes (1) they formed along boundaries of differential and pipes, and their greatest intensity along described in the Tuolumne batholith. Ruprecht fl ow; (2) these boundaries varied from verti- boundaries in the direction of inferred move- et al. (2008) concluded that even during a sin- cal to horizontal during schlieren formation; ment of these structures (Fig. 13), supports gle overturn, crystals that were originally hun- (3) size grading (of selective mineral popula- their formation by a fi lter-pressing mechanism dreds of meters apart can be juxtaposed at the tions) occurs with distance from schlieren bases in which mafi c minerals are preferentially col- centimeter scale, and after multiple overturns, in some schlieren, but not all; (4) dramatic com- lected in the host magmas and melts driven off crystals from diverse parts of a chamber can be positional grading occurs with distance from (e.g., Weinberg et al., 2001). assembled. They further concluded that in spite schlieren base and particularly formed by the of magma overturn, some stratifi cation in cham- accumulation of mafi c (biotite and hornblende) Implications bers is typically preserved, which may explain and accessory (sphene, zircon, apatite) miner- the variable magma types seen in the Tuolumne als into the basal zones; (5) mineral alignment In the Introduction, I raised three related batholith tubes. that is subparallel to slightly oblique at bases to issues. (1) Could local convection occur in Another implication of the existence of highly oblique at schlieren tops; (6) evidence crystal-rich magmas? (2) How and when did diverse crystal populations in the Tuolumne for mineral compaction and removal by fi lter mixing of crystal populations occur? (3) To batholith concerns the inferred residence times pressing; (7) formation in crystal-mush sys- what degree do local magmatic structures pro- of crystals in magmas. Studies of diverse crystal tems, in which all mineral phases had begun to vide information about chamber construction populations in volcanic deposits use crystalliza- crystallize; and (8) involved already premixed versus processes that occurred within already tion and/or resorption rates during disequilib- crystal populations, some of which came from constructed chambers? rium to establish potential residence times of other parts of the magma chamber (e.g., zircon The structures described herein clearly indi- crystals in subvolcanic chambers. However, in antecrysts of Matzel et al., 2007; hornblendes of cate that movement of crystal-bearing magmas the Tuolumne batholith there is good evidence Solgadi and Sawyer, 2007). Thus the proposed may continue well below magma liquidi and that complex mineral populations existed that processes for schlieren formation must involve fairly close to magma solidi at least in the wet were not in chemical or textural equilibrium and both physical sorting and/or alignment during granodiorites examined. Magma movement had mineral ages varying by millions of years fl ow and compositional sorting during crystal- continued even after these structures were suffi - (e.g., Matzel et al., 2005, 2006b, 2006c; Miller lization and crystal-liquid fractionation and thus ciently crystallized to act as rigid objects, as indi- et al., 2007). A likely solution is that some crys- are only compatible with a combination of steep cated by their breaking apart and rigid rotations. tals were armored (rimmed) by others or existed physicochemical gradients at fl ow margins and The details of how mixed crystal popula- in crystal clusters that prevented rapid chemical shear sorting against an effectively rigid wall, tions occurred in the Tuolumne batholith remain communication with new melts or crystals with mentioned above. uncertain. However, the sorting and accumula- which they were in disequilibrium. Thus caution Weinberg et al. (2001) postulated that schlie- tion of crystals during formation of the struc- is urged regarding existing conclusions about ren with size sorting, such as the graded schlie- tures involved magmas that already had diverse the length of crystal residence times in subvo- ren, may result from the combined effects of mineral populations, based on geochronologic lcanic chambers. shear fl ow and loss of interstitial melt (by fi l- (Matzel et al., 2005, 2006b, 2007; Miller et al., This study (and Žák and Paterson, 2005; ter pressing) to the permeable magmatic walls 2007), geochemical (Reid et al., 1993; Burgess Paterson et al., 2008) indicates that a complex (Fig. 18). They noted that this requires a nega- and Miller, 2008), and fi eld and microstructural array of compositionally and textural defi ned tive pressure gradient toward the walls, result- studies (Solgadi and Sawyer, 2007; Paterson structures formed in an already amalgamated ing from unbalanced pressures on a wider scale et al., 2008); thus a great deal of crystal mix- magma chamber. Given the large number and (beyond the local fl ow observed), but could not ing occurred before and/or during the formation simultaneous formation of these structures it is rule out other gradients driving melt migration of these structures. Since this mixing involved intriguing to speculate that together they form into the structures. They also argued that shear crystals from all main magmatic units in the a network of increased permeability in a crys- sorting and melt loss are most effective when Tuolumne batholith, mixing must have occurred tal-rich, lower permeability, host magma, and the walls enclosing the fl ow are a crystal-liquid as localized fl ows passed through other units thus aided in the redistribution of magma and mush behaving as a permeable, viscoplastic either in a vertically or laterally extensive cham- in crystal mixing. Whether the Tuolumne batho- material that not only favors the formation of ber. Such mixing is diffi cult to imagine in a lith chamber “boiled” in the sense of forming schlieren, but favors melt extraction from mush system formed of small pulses that did not dra- volatile-rich channels in the chamber (Ruprecht pores. This model is generally compatible with matically interact with one another, such as sug- et al., 2008), whether blocks of crystal mushes Tuolumne batholith graded schlieren forma- gested by Coleman et al. (2004) and Glazner et or crystal avalanches moved through the cham- tion, which our initial least squares geochemi- al. (2004). The timing of mixing also has impli- ber, or whether crystal-bearing currents fl owed

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along crystal-mush margins need further study. bulk compositions, and chemistries (see also that has yet examined tube migration patterns at However, all of these processes imply operation Moore and Sisson, 2007, 2008; Vernon and the batholith scale. Whether such patterns are in a large, mushy, evolving chamber. Paterson, 2008a, 2008b). Most of these inter- random or ordered will be of great interest in I also indicate here how important it is to rec- nal contacts are formed during local fl ow in evaluating chamber evolution models. ognize and look beyond the structures described an existing chamber, not during chamber con- Troughs, mineral grading, and intrusive rela- herein when attempting to locate features that struction or during thermal maturation and tionships give local growth and/or younging might give us clues about how the chambers annealing. Many of these local contacts, such as directions, but at three different scales: min- originally formed. This is because structures defi ned by schlieren, have widely varying ori- eral grading provides information at the scale such as tubes, troughs, and even local sheeted entations (Fig. 6) that are not compatible with of a single pulse of magma, trough cutoffs at zones (Paterson et al., 2008; Žák et al., 2009) either a vertical sheeted dike model or a subho- the scale of one or more pulse(es) fl owing past can form in an existing magma chamber and rizontal laccolith model, unless these models led another, and intrusive relationship at scales thus not record the chamber construction pro- to large magma chambers in which internal con- ranging to large intrusive pulses. These young- cess. The same has been argued for preserved tacts were destroyed by continued movement ing directions need not match the overall young- magmatic fabrics that in many cases may pro- of magma prior to formation of the structures ing of the chamber, but instead provide valuable vide information about regional strain during discussed in this study. information about local processes in an evolving chamber evolution, or strain caused by local All the structures described in this paper also chamber (e.g., Žák and Paterson, 2005; Pater- fl ow gradients in the chamber, but not about provide wonderful examples that continued son et al., 2008). In Paterson et al. (2008) we initial chamber growth (Paterson et al., 1998; movement of magma at the emplacement site led gave one example where an evaluation of trough Žák et al., 2007). to both crystal mixing and formation of compo- younging was useful in determining that crystal A fi nal important implication of these struc- sitional and structural diversity. The magnitude mush domains in the Tuolumne batholith were tures is that they challenge aspects of the incre- of these processes is diffi cult to evaluate at the being disrupted and recycled in the chamber. mental chamber growth model championed by scale of the entire batholith, but they did occur. The movement directions of both diapirs Coleman et al. (2004, 2008), and Glazner et al. These observations do not exclude the possi- and plumes are surprisingly variable in the (2002, 2008a, 2008b) for the construction of bility of incremental growth of the Tuolumne Tuolumne batholith and rarely vertical. Thus the Tuolumne batholith. These authors have batholith, although the location (deep in the these features must be refl ecting fl ow or rheo- suggested: (1) that the batholith “began its crust, during ascent, during chamber growth) of logical gradients and not just gravity, and thus life as a large [vertical] dike swarm“ (Glazner the amalgamation of pulses and the number and provide information about these gradients. et al., 2002, p. 269), an interpretation they size of pulses remain poorly constrained. How- The presence or absence of these structures have recently changed to the interpretation ever, these observations imply that by whatever in plutons may provide information about the that it “was assembled as downward-stacking means, the batholith grew fairly large magma nature of former magma chambers. For exam- laccoliths” (Coleman et al., 2008, p. 23); chamber(s) in which magma pulses moved ple, Weinberg et al.’s (2001) suggestion that (2) that “Early increments cool below the through existing crystal mushes and disperse- these structures may best be developed when solidus quickly” (Coleman et al., 2008, p. 23); ment and/or mixing of crystals occurred. the magma composition is appropriate to form “magma is 50% crystallized (and thus no lon- a porous crystal mush is an idea supported by ger mobile)” (Glazner et al., 2004, p. 6), “in Use of Structures as Tools for the presence of these structures in Sierran plu- situ crystal fractionation and/or magma mix- Understanding Magma Chamber Evolution tons compositionally similar to the Tavares ing cannot account for the zonation” (Glazner pluton. Other hypotheses worth considering et al., 2004, p. 7), and that “the geochemical These structures provide a great deal of infor- are whether some magma systems crystallized variation of the suite refl ects regular changes mation about the local evolution of magma so quickly that there was not suffi cient time in the composition of magmas generated at the chambers. For example, methods of determining to form the structures (do they require longer- source” (Coleman et al., 2004, p. 435–436), all paleovertical in chambers have long been sought lived chambers), whether widespread magma of which imply that the compositional diversity in paleomagnetic studies and have been used to movement may occur after formation of these in the Tuolumne batholith could not form at support chamber growth models (e.g., Wiebe structures and thus destroy them, whether chan- the emplacement site; and (3) that subsolidus and Collins, 1998). In the Tuolumne batholith nelized fl ow and development of permeable net- internal annealing and/or recrystallization led (Frei, 1986), the tubes and pipes provide reli- works are favored at certain crustal levels, and to removal of contacts and thus “the general able indicators of paleovertical when not broken whether subtle compositional differences make rarity of chilled margins in granites, and the apart during reintrusion, but troughs, diapers, it diffi cult to detect these structures in other plu- cryptic character of their internal contacts, to and plume heads do not. Statistical averages of tons. Certainly the plume heads are often easy late-stage textural modifi cation that obscures the plunge of tubes and pipes in the Tuolumne to miss in the Tuolumne batholith, and large much of the record of pluton assembly” batholith are consistently 90° ± 2°, suggesting troughs may appear as planar schlieren zones (Glazner et al., 2008a, p. 10337). that the batholith was not tectonically tilted and and thus be downplayed. The presence of chamber-wide, knife internal parts of the chamber were not magmati- sharp, and gradational internal contacts in the cally rotated during growth (Frei, 1986). CONCLUSIONS Tuolumne batholith (Žák and Paterson, 2005; The consistent direction of tube migration at Žák et al., 2007), plus the thousands of inter- single outcrops is also intriguing and may pro- 1. A complex array of widespread, but nal magmatic contacts associated with the vide information either about the direction of domainally developed, structures is preserved structures, indicates that internal contacts are host magma fl ow in the chamber (if a hotspot in the Tuolumne batholith and includes station- widespread in the Tuolumne batholith, are not analogy is correct) or about some as yet unknown ary and migrating tubes, troughs, pipes, diapers, cryptic nor annealed, and often occur between process that caused tube source regions to unidi- and plume heads. These structures all formed magma pulses with clearly defi ned textures, rectionally migrate. I am not aware of any study by local magma fl ow through crystal-mush

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Canyon magma body, Colorado: Contributions to host magmas, and are associated with the for- chambers. An evaluation of these structures in Mineralogy and Petrology, v. 149, p. 338–349, doi: mation of mafi c and accessory mineral–rich the Tuolumne batholith established a number 10.1007/ s00410–005–0653-z. schlieren, evidence of fi lter pressing, and the of interesting results that future models must Bachmann, O., Charlier, B.L.A., Lowenstern, J.B., and Pry- tulak, J., 2007, Zircon crystallization and recycling in accumulations of crystals with diverse magma incorporate. For example, the consistently steep the magma chamber of the rhyolitic Kos Plateau Tuff histories. Together they may form a permeable tube and pipe axes indicate that neither the plu- (Aegean Arc): Geology, v. 35, p. 73–76, doi: 10.1130/ G23151A.1. network in which channelized fl ow occurred in ton nor features in the pluton were tilted, thus Bacon, C.R., and Lowenstern, J.B., 2005, Late Pleistocene the magma chamber. excluding a model for the Tuolumne batholith granodiorite source for recycled zircon and phe- 2. These structures provide examples of local in which subhorizontal layers tilted to form pre- nocrysts in rhyodacite lava at Crater Lake, Oregon: Earth and Planetary Science Letters, v. 233, p. 277– convection in crystal-rich magmas resulting in served steep contacts. The tube and pipe orien- 293, doi: 10.1016/j.epsl.2005.02.012. local compositional and structural diversity. tations and widespread orientations of schlieren Barbarin, B., Dodge, F.C.W., Kistler, R.W., and Bateman, Thus the notion that magmas must have ≤50% in troughs and diapir movement directions are P.C., 1989, Mafi c inclusions, aggregates, and dikes in granitoid rocks, central Sierra Nevada Batholith, Cali- crystals to convect and/or fractionate at the particularly challenging to explain in the dike fornia; analytic data: U.S. Geological Survey Bulletin chamber site and the ability of magmas to form and laccolith models proposed by others. Local 1899 Report B, 27 p. Barbey, P., Gasquet, D., Pin, C., and Bourgeix, A.L., 2008, compositional diversity at these crustal levels younging directions determined from troughs Igneous banding, schlieren and mafi c enclaves in need to be revised. indicate that outward growth occurred in a num- calc-alkaline granites: The Budduso pluton (Sar- 3. These structures formed in an existing ber of zones, although the overall direction of dinia): Lithos, v. 104, p. 147–163, doi: 10.1016/j .lithos.2007.12.004. magma chamber and involved crystals derived younging is toward the batholith center. The Barrière, M., 1976, Flowage differentiation: limitation of the from other parts of the chamber, indicating that it variable movement directions of diapirs and “Bagnold effect” to the narrow intrusions: Contribu- is necessary to fi nd older features of the chamber plumes require interactions between buoyancy tions to Mineralogy and Petrology, v. 55, p. 139–145, doi: 10.1007/BF00372223. to unravel chamber construction models. This is forces and other gradients. Since all of these Barrière, M., 1981, On curved laminae, graded layers, con- also true for preserved, chamber-wide magmatic structures formed in a time-transgressive fash- vection currents and dynamic crystal sorting in the Ploumanac’h (Brittany) subalkaline granite: Contri- fabrics, since these overprint the structures. ion over the duration of batholith growth, but at butions to Mineralogy and Petrology, v. 77, p. 214– 4. Interpretations derived from volcanic stud- essentially the same time at any one locality, it 224, doi: 10.1007/BF00373537. ies about magma residence times of crystals and will be an exciting challenge to integrate these Bateman, P.C., 1992, Plutonism in the central part of the Sierra Nevada Batholith, California: U.S. Geological crystal mixing during eruptions need to treated processes in future models. Survey Professional Paper 1483, 186 p. with caution, since there is good evidence that Bateman, P.C., and Chappell, B.W., 1979, Crystallization, mixed crystal populations existed prior to erup- ACKNOWLEDGMENTS fractionation, and solidifi cation of the Tuolumne intrusive series, Yosemite National Park, Califor- tions, and in the case of the Tuolumne batholith nia: Geological Society of America Bulletin, v. I thank Calvin Miller and Jim Moore for their very involved crystals with ages ranging over 10 m.y. 90, p. 465–482, doi: 10.1130/00 16–7606 (1979) constructive reviews that signifi cantly improved the 90<465:CFASOT>2.0.CO;2. A likely solution is that crystals in subvolcanic manuscript, Robert Miller and Jonathan Miller for Bergantz, G.W., 2000, On the dynamics of magma mixing chambers become armored (rimmed) by other discussions about the Tuolumne batholith, George by reintrusion: Implications for pluton assembly pro- crystals or exist in crystal clusters that, in spite Bergantz and Roberto Weinberg for discussions cesses: Journal of Structural Geology, v. 22, p. 1297– 1309, doi: 10.1016/S0191-8141(00)00053-5. of changing environmental conditions, prevent about magmatic structures, and Stephen Holloway and Randy Keller for editorial assistance and won- Bergantz, G.W., and Breidenthal, R.E., 2001, Non-stationary rapid chemical communication with the melts derful patience. I also thank the Yosemite National entrainment and tunneling eruptions: A dynamic link with which they are in disequilibrium. between eruption processes and magma mixing: Geo- Park Rangers for their constant support and interest physical Research Letters, v. 28, p. 3075–3078, doi: 5. The presence of these structures and result- in our work. I gratefully acknowledge support from 10.1029/2001GL013304. ing implications challenge aspects of the incre- National Science Foundation grants EAR-0537892 Bergantz, G.W., and Ni, J., 1999, A numerical study of and EAR-0073943. sedimentation by dripping instabilities in viscous fl u- mental chamber growth model championed by ids: International Journal of Multiphase Flow, v. 25, Glazner et al. (2002, 2003, 2004) and Coleman REFERENCES CITED p. 307–320, doi: 10.1016/S0301-9322(98)00050-0. et al. 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Vigneresse, J.L., Barbey. P., and Cuney, M., 1996 Rheo- Weinberg, R.F., Sial, A.N., and Pessoa, R.R., 2001, Magma ica Bulletin, v. 117, p. 1242–1255, doi: 10.1130/ logical transitions during partial melting and crystal- fl ow within the Tavares pluton, northeastern Brazil: B25558.1. lization with application to felsic magma segrega- Compositional and thermal convection: Geologi- Žák, J., Paterson, S.R., and Memeti, V., 2007, Four mag- tion and transfer: Journal of Petrology, v. 37, no. 6, cal Society of America Bulletin, v. 113, p. 508–520, matic fabrics in the Tuolumne batholith, central Sierra pp. 1579–1600. doi: 10.1130/0016-7606(2001)113<0508:MFWTTP> Nevada, California (USA): Implications for interpret- Wahrhaftig, C., 1979, Signifi cance of asymmetric schlieren 2.0.CO;2. ing fabric patterns in plutons and evolution of magma for crystallization of granites in the Sierra Nevada Whitehead, J.A., and Helfrich, K.R., 1991, Instability of fl ow chambers in the upper crust: Geological Society of batholith, California: Geological Society of America with temperature-dependent viscosity: A model of America Bulletin, v. 119, p. 184–201, doi: 10.1130/ Abstracts with Programs, v. 11, p. 133. magma dynamics: Journal of Geophysical Research, B25773. Walker, B.A., Jr., Miller, C.F., Claiborne, L.E., Wooden, v. 96, p. 4145–4155, doi: 10.1029/90JB02342. Žák, J., Paterson, S.R., Kabele, P., and Janoušek, V., 2009, J.L., and Miller, J.S., 2007, Geology and geochro- Wiebe, R.A., 1996, Mafi c-silicic layered intrusions: The role The Mammoth Peak sheeted complex, Tuolumne nology of the Spirit Mountain batholith, southern of basaltic injections on magmatic processes and the batholith, Sierra Nevada, California: A record of ini- Nevada: Implications for timescales and physical evolution of silicic magma chambers: Royal Society tial growth or late thermal contraction in a magma processes of batholith construction: Journal of Volca- of Edinburgh Transactions, Earth Sciences, v. 87, chamber?: Contributions to Mineralogy and Petrol- nology and Geothermal Research, v. 167, p. 239–262, p. 233–242. ogy, v. 158, p. 447–470. doi: 10.1016/j.jvolgeores.2006.12.008. Wiebe, R.A., and Collins, W.J., 1998, Depositional features Wallace, G.S., and Bergantz, G.W., 2005, Reconciling het- and stratigraphic sections in granitic plutons: Impli- erogeneity in crystal zoning data: An application of cations for the emplacement and crystallization of shared characteristic diagrams at Chaos Crags, Lassen granitic magma: Journal of Structural Geology, v. 20, Volcanic Center, California: Contributions to Miner- p. 1273–1289, doi: 10.1016/S0191-8141(98)00059-5. alogy and Petrology, v. 149, p. 98–112, doi: 10.1007/ Žák, J., and Klomínsky, J., 2007, Magmatic structures in the s00410-004-0639-2. Krkonoše-Jizera Plutonic Complex, Bohemian Mas- Webber, C.E., Candela, P.A., Piccoli, P.M., and Simon, A.C., sif: Evidence for localized multiphase fl ow and small- 2001, Generation of granitic dikes: Can microstruc- scale thermal-mechanical instabilities in a granitic ture, mineralogy, and geochemistry be used as guides magma chamber: Journal of Volcanology and Geo- to determine the mechanisms of diking?: Geological thermal Research, v. 164, p. 254–267, doi: 10.1016/j Society of America Abstracts with Programs, v. 33, .jvolgeores.2007.05.006. no. 6, p. 138. Žák, J., and Paterson, S.R., 2005, Characteristics of inter- Weinberg, R.F., and Podladchikov, Y., 1994, Diapiric ascent nal contacts in the Tuolumne Batholith, central Sierra of magmas through power-law crust and mantle: Jour- Nevada, California (USA): Implications for episodic MANUSCRIPT RECEIVED 10 OCTOBER 2008 nal of Geophysical Research, v. 99, p. 9543–9560, emplacement and physical processes in a continental REVISED MANUSCRIPT RECEIVED 23 JULY 2009 doi: 10.1029/93JB03461. arc magma chamber: Geological Society of Amer- MANUSCRIPT ACCEPTED 14 AUGUST 2009

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