Pluton Emplacement Along an Active Ductile Thrust Zone, Piute Mountains, Southeastern California: Interaction Between Deformationai and Solidification Processes

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Pluton emplacement along an active ductile thrust zone, Piute Mountains, southeastern California: Interaction between deformationai and solidification processes

K. E. KARLSTROM University of New Mexico, Albuquerque, New Mexico 87131 C. F. MILLER | Vanderbilt University, Nashville, Tennessee 37235 J. A. KINGSBURY J. L. WOODEN U.S. Geological Survey, Menlo Park, California 94025

ABSTRACT id-state deformationai fabrics and brittle syn- space with regional, as opposed to only em- thetic thrusts that offset the pluton margin placement-related, deformation. Plutons that are emplaced synchronously record sharp temperature decline during and Two end-member models for granitoid as- with regional deformation offer the opportu- after crystallization. cent and emplacement are shown in Figure 1. nity to understand the partitioning of defor- Because plutons crystallize rapidly, they can Although ascent and emplacement may often mation and metamorphism in time and space be used to estimate strain rates during orogeny. involve a combination of mechanisms, exam- during orogeny. The 85 Ma East Piute pluton Thermal modeling suggests that the transition ination of end members highlights some of the of the eastern Mojave Desert is an unusually from rheological liquid to solid in the East Piute controversies surrounding the interpretation dear example of a syntectonic pluton; it was pluton occurred during an interval of about 104 of the relative timing of pluton emplacement emplaced during thrusting in the southern yr. As final increments of ductile deformation and regional deformation. Diapiric ascent is Cordillera. Combined petrologic and struc- coincided with final crystallization of evolved often considered to be linked to models of tural studies lead to empirical models for the compositional units, total strain recorded by forceful emplacement, where buoyancy- dynamic interaction of magmatic and defor- fabric in the pluton apparently accumulated driven ascent of the pluton induces ductile mationai processes during granitoid emplace- during this interval. Shear strains estimated deformation of the wall rock. Thus, rock fab- ment and final crystallization. from the S-C fabric in the granodiorite unit and rics in and around plutons are attributed to Trace- and m^jor-element and isotopic data rotation of pegmatite-filled vein arrays are on rise and emplacement of magma (Holder, suggest that closed-system differentiation of a the order of 1. Thus, strain rates in the shear 1979; Bateman, 1985; Ramsay, 1989). Defor- -12 single magma batch was responsible for com- zone were on the order of 10 /sec during mation models of pluton emplacement (Fig. positional variation (granodiorite to leucocratic crystallization of the pluton. This corresponds 1) also rely on buoyant forces, but magma granite). Correlation between degree of chem- to displacements of tens of centimeters per ascends opportunistically in conduits that ical evolution of compositional units and in- year. These high strain rates suggest punctu- may follow shear zones and other crustal dis- trusive sequence suggests that discrete melt ated, melt-enhanced movement episodes dur- continuities. In these models, emplacement is segregation events punctuated fractional crys- ing contractional orogeny. often viewed as predominantly passive in the tallization. Modeling of trace-element data sug- sense that space for pluton emplacement is gests segregation of monzogranite melts after created by crustal movements during regional 50%-70% crystallization of the magma and INTRODUCTION deformation, and rock fabrics are interpreted segregation of leucocratic granite after about to reflect regional strains. 80% crystallization. Granitoid magmatism is spatially and tem- Numerous workers have documented syn- Deformation promoted segregation of melt porally related to orogeny, yet causal rela- tectonic pluton emplacement during regional fractions at discrete times during crystalliza- tionships between magmatic and deforma- strike-slip and transpressive deformations tion. During final ascent, and after the magma tionai processes remain controversial. It is (Bran and Pons, 1981; Davies, 1982; Hutton, had crystallized sufficiently (50%-70%) to at- becoming increasingly clear that a precise 1982; Guineberteau and others, 1987; Castro, tain shear strength, monzogranite liquid was knowledge of the relative and absolute timing 1987) or during extensional and transtensional incompletely segregated along the active thrust of pluton emplacement can provide insight deformations (Hutton, 1988a, 1988b). This zone at the southern margin of the pluton. At into orogenic processes, rates, and history study and many others suggest that syntec- essentially the present crustal level, and after (for example, see Pitcher, 1979, 1987; Pater- tonic emplacement during thrusting and about 80% crystallization, the remaining leu- son and Tobisch, 1988). Using plutons to con- crustal shortening is also important in oro- cocratic granite melts were more effectively struct orogenic history, however, requires genic belts (see also Bran and Pons, 1981; segregated and migrated into thrust-related ex- careful multidisciplinary studies and a rather Schmidt and others, 1988; Karlstrom, 1989; tensional openings, including tension gashes. precise definition for syntectonic emplace- Tobisch and Paterson, 1990). Undeformed pegmatite and aplite dike arrays ment (see Paterson, 1989; Karlstrom, 1989). This paper documents the history and tim- indicate that final crystallization coincided with As used here, syntectonic plutons are those ing of emplacement of a small, well-exposed, final increments of shear-zone movement. Sol- whose crystallization coincides in time and in Late Cretaceous pluton in the eastern Mojave

Geological Society of America Bulletin, v. 105, p. 213-230, 12 figs., 4 tables, February 1993.

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MODELS FOR GRANITOID ASCENT AND EMPLACEMENT Hoisch and others, 1988; Reynolds and others, 1986; Fletcher and Karlstrom, 1990; Fos- ter and others, 1989,1992). This belt appears to be a southward, ductile extension of the I DIAPIR MODEL Mesozoic foreland fold and thrust belt ASCENT:driven by bouyant forces (Howard and others, 1980). It is located along the eastern margin of the Cretaceous magmatic arc, and it is parallel to a belt of Tertiary EMPLACEMENT: forceful core complexes to the east (Fig. 2). "ballooning" of early xlized skin caused by continued Figure 3 shows a generalized geologic map bouyant rise of tail of the Piute and Old Woman Mountains areas. The Proterozoic basement consists of a complex of Proterozoic gneiss, amphibolite, schist, and minor quartzite, injected by Prot- erozoic granitoids (Miller and others, 1982; II DEFORMATION MODEL Wooden and Miller, 1990). Metamorphosed Paleozoic strata unconformably overlie the Proterozoic rocks (Stone and others, 1983). ASCENT:through shear zones; Both gneisses and Paleozoic metasedimen- driven by bouyancy tary rocks are ductilely deformed by shear zones and related recumbent folds (Miller and EMPLACEMENT:room created during others, 1982; Fletcher and Karlstrom, 1990). regional deformation by strain Cretaceous metamorphism was variable in incompatibilities, crustal antisotropy, the Piute Mountains. Much of the Piute and deformation partitioning Mountains remained at temperatures less than 450 °C, as shown by preserved Prot- erozoic (1.6-1.3 Ga) 40Ar/39Ar dates on hornblende from early Proterozoic (>1.7 Ga) am- phibolites (Foster, 1989; Foster and others, A. CONTRACTION B. EXTENSION 1989, 1992). Some areas adjacent to Creta- ceous plutons experienced temperatures of 500-650 °C (Hoisch and others, 1988; Fletcher, 1989). Pressures in the northern Piute Mountains were about 2.5-3.5 kb, cor- responding to depths of about 9-13 km (Fos- ter and others, 1992). Both metamorphism C. STRIKE SLIP and plutonism were synchronous with, and in most areas outlasted, deformation (Rothstein and others, 1990; Nicholson and Karlstrom, 1990; compare with Carl and others, 1991).

Figure 1. End-member models for granitoid ascent and emplacement. Emplacement of magma PETROLOGY OF THE EAST during contractional deformation is facilitated by local extensional regimes; for example, tension PIUTE PLUTON gashes, saddle reefs (phaccoliths), pressure shadows, and accumulation at thrust ramps (Schmidt and others, 1990). The East Piute pluton is exposed over an area of 14 km2, with nearly 100% outcrop along pluton margins (Fig. 4). It intrudes Prot- Desert. This pluton is syntectonic relative to strain rate can be done for discrete plutons of erozoic gneisses and is bounded on the south ductile mid-crustal thrusting. Combined relatively simple geometry if observed strains by the Fenner shear-zone system, which can structural and petrologie studies indicate that accumulated between the time the magma be traced continuously into the southwest- episodic melt segregation during crystalliza- passed through its critical melt fraction and verging thrust zone that bounds the Paleozoic tion can be tied to the deformational se- final crystallization. outcrops farther to the west (Fletcher and quence. We are thus able to document the Karlstrom, 1990). interaction of magmatic and deformational GEOLOGY OF THE PIUTE MOUNTAINS The East Piute pluton is composed of two processes. Further, and perhaps of general granitoid units that differ strikingly in their significance for orogenic studies, thermal The Piute Mountains are located within a appearance, one highly felsic and leucocratic constraints on the duration of pluton crystal- belt of rocks that experienced middle-crustal and the other considerably darker (Fig. 4). lization, coupled with estimates of strain mag- (9-13 km) ductile deformation, granitoid The more abundant mafic unit, here called the nitude, provide an estimate of orogenic strain magmatism, and regional metamorphism in main unit (units 1 and 2 of Fig. 4), ranges from rate. This type of empirical calculation of the Late Cretaceous (Miller and others, 1982; granodiorite to monzogranite in composition

214 Geological Society of America Bulletin, February 1993

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of massive granite are also present within this protrusion. Smaller, less regular shaped bod- ies of massive granite, pegmatite, and aplite extend into the country rock at the northeast- ern and southwestern margins of the pluton. A discontinuous border zone of fine-grained monzogranite is present along the southern and southeastern margins. Aplite and pegmatite dikes are common near the margins of the pluton and in the adjacent country rocks, especially where other felsic unit rocks are present. The remaining 90% of the exposed area of the pluton is main unit rock, which fills much of the western angular protrusion and almost all of a bulbous protrusion on the south side of the pluton. The main unit rock of the western protrusion (unit 2 of Fig. 4) is relatively felsic and in part muscovite-bearing. There is a subtle increase in biotite and decrease in muscovite from the protrusion east into the main part of the pluton. The sequence of emplacement as indicated by cross-cutting relationships was (1) main unit, (2) fine-grained monzogranite and massive granite, and (3) aplite and pegmatite dikes. The fine-grained monzogranite and massive granite are not in contact with each other, but we infer, on the basis of their geochemical characteristics and intensity of deformation, that monzogranite preceded granite.

AGE OF THE EAST PIUTE PLUTON Figure 2. Regional tectonic setting of the Old Woman Mountains. This area is south of the southern limit of the Sevier thrust belt and near the eastern edge of the Cretaceous magmatic arc. Foster (1989) dated minerals from the East Tertiary extensional strain is lower here than in adjacent regions to the east and west. Piute pluton by the ^Ar/^Ar method, deter- mining ages of 71 Ma for muscovite and biotite, and about 65 Mafor K-spar. These ages are interpreted to reflect regional cooling and (Sparkes, 1981). The felsic unit includes fine- tite, sparse sphene (most or all secondary), provide only a minimum age for the pluton. grained foliated monzogranite (unit 3 in Fig. and allanite. The felsic unit contains sparse We have analyzed three zircon fractions 4); massive, medium- to coarse-grained leu- apatite, monazite, and zircon. Both units con- from the main unit and one monazite fraction cocratic granite that is primarily alkali-feld- tain epidote. Plagioclase, which is strongly from the leucocratic granite phase of the west- spar granite because plagioclase is generally zoned, and accessory minerals began crys- ern protrusion (Table 1; Fig. 5). The zircon albitic (unit 4 in Fig. 4); and aplite-pegmatite tallization early, followed by K-feldspar fractions are highly discordant, but they de- dikes of alkali-feldspar granite (unit 5 in Fig. and quartz in rocks of both units. In aplite fine a line with an upper intercept of 1629.2 ± 4). The most obvious difference between the dike rocks of the felsic unit, all major 6.1 Maandalower intercept of84.8 ± 2.7Ma. two granitoid units lies in the modal abun- mineral phases appear to have crystallized We interpret the lower intercept to be the dance of biotite; leucocratic units have little simultaneously. crystallization age, and the upper intercept to biotite, commonly zero, reaching 2% in fine- The felsic unit as a whole (units 3,4, and 5 be the average age of zircons inherited from grained monzogranite, whereas the main unit of Fig. 4) makes up about 10% of the exposed Proterozoic basement (Foster and others, typically has 5%— 10%. The felsic unit is rich area of the pluton. Of this, approximately 1989; Wooden and Miller, 1990; Miller and in primary muscovite (—5%), and garnet is an two-thirds is massive granite, one-third is others, 1990a). The monazite fraction shows almost ubiquitous accessory; the main unit fine-grained monzogranite, and less than 1% less discordance than the zircons and lies very has zero to sparse muscovite, rarely exceed- is aplite and pegmatite. Most of the massive close to the line defined by the zircons. The ing 1%, and garnet is absent. granite is concentrated at the western margin, U-Pb data thus support the interpretation that Accessory minerals in the main unit in- at the outer edge of an angular protrusion of the main and leucocratic units are part of a clude magnetite, zircon (± monazite), apa- the pluton into the country rock. Large dikes single magma crystallization event.

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RELATIONSHIP TO OTHER LATE Si02 ranging from 68 to 77 wt% (Table 3). The cons are small and acicular in the felsic unit, CRETACEOUS PLUTONS weakly peraluminous main unit rocks aver- in contrast to the larger and more equant zir- age about 0.6 wt% normative corundum, con population (with strong inheritance as The East Piute pluton is regionally unique whereas the strongly peraluminous felsic unit noted above) in the main unit. in age and composition. The only plutons with rocks average 1.1 wt% corundum. Elemental Assuming that the felsic rocks represent similar reported ages in the eastern Mojave compositional trends are grossly smooth and pure melt compositions and that the mean Desert and adjacent western Arizona are lo- continuous, but there is a clear compositional composition of the pluton represents the ap- cated in the Whipple Mountains (ca. 89 Ma; gap between the main and felsic units (for proximate initial melt composition, Ba and Sr Wright and others, 1987). All other Creta- example, between 72 and 74 wt% Si02; 1 and contents suggest that 50% to 70% of the initial ceous granitoids in the Piute and adjacent Old 2 wt% CaO; Fig. 6A). All major and trace magma had crystallized at the time of segre- Woman Mountains are 72 to 73 Ma according elements are depleted or enriched, as would gation of the fine-grained monzogranite, and to zircon and monazite U-Pb and hornblende be predicted for closed-system crystal-liquid that the massive granite and dikes represent ""Ar/^Ar dating (Miller and others, 1990a; differentiation of a granitoid magma crys- segregation after —80% crystallization (Fig. Foster and others, 1989). The 73 Ma com- tallizing the observed mineral assemblage 7). If present exposure is an indicator of the posite Old Woman-Piute batholith has a com- (Fig. 6). mass of melt segregated in each event, a great positional gap between dominantly metalu- The correlation between the intrusion se- majority of the melt must have remained minous granodiorite plutons and more felsic, quence and degree of chemical evolution sug- trapped in the residue during the initial strongly peraluminous plutons (Miller and gests that the East Piute pluton formed from (monzogranite) separation; the second (gran- others, 1990a). The main unit of the East Piute a single magma, emplaced during a single ite) segregation event must have been much pluton spans this gap and is broadly consis- event, with compositional differences reflect- more effective. tent with most compositional trends of the ing progressive segregation of differentiated batholith, but it is distinctly poorer in Rb, Ba, liquids. This interpretation is supported by DEFORMATIONAL HISTORY OF THE and light rare-earth elements (REE) and far the fact that the pluton is a single coherent EAST PIUTE PLUTON AREA poorer in heavy REE (compare Mittlefehldt body; all intrusive rocks are confined within and Miller, 1983; Miller and others, 1990a). a well-defined perimeter or occur as dikes just In order to interpret the relative timing of Furthermore, it is less radiogenic in Sr and outside this perimeter and the distribution of pluton emplacement and regional deforma- especially in Pb (Table 2). Initial ^Sr/^Sr for later units is consistent with control by me- tion, it is necessary to understand the kine- main unit samples and monzogranite of the chanical properties of the evolving pluton and matic framework of regional deformation East Piute pluton (at 85 Ma) are 0.709-0.710, its host rocks (see below). We interpret the events. Figure 4 shows that the East Piute compared with 0.7095-0.7190 for the Old felsic unit to represent melt segregated very pluton occupies the hanging wall of the north- Woman-Piute batholith. East Piute leu- effectively (facilitated by deformation) from to northeast-dipping Fenner shear-zone sys- cocratic granites have initial ratios of —0.717, the evolving magma. The magma was under- tem that extends generally east-west across possibly reflecting local contamination of going continuous fractional crystallization 206 204 the mountains. This shear zone is spatially these Sr-poor samples. Feldspar Pb/ Pb (Michael, 1984), but segregation events were associated with isoclinal folding, attenuation, is tightly constrained between 16.69 and 16.75 discrete. and transposition of bedding in Paleozoic for all units (Table 2), compared with 17.3- Continuous (or nearly continuous) chemi- rocks, ductile reorientation and reactivation 18.8 forthe Old Woman-Piute batholith (Mill- cal fractionation is required by the extreme of gneissic layering in Proterozoic rocks, and er and others, 1990a). The felsic units of the fractionation of trace elements. For example, mylonitization of the East Piute pluton (Fig. East Piute pluton, although broadly similar to maximum solid/liquid partition coefficients 3). Structural data (Fig. 8) suggest that Prot- evolved peraluminous granites of the batho- for Sr and Ba for the crystallizing assemblage erozoic fabrics were variably folded and re- lith, are also distinct (for example, in their of the East Piute pluton are less than 3-4, yet activated by Mesozoic deformation such that lower Rb and Ba contents). These character- both elements diminish by more than a factor present orientations are due to Mesozoic de- istics suggest a different source (different rock of 30 from least evolved to most evolved formation even though original gneissic lay- types and/or different depth of melt genera- rocks (Fig. 7). Such depletion can be ac- ering formed in the Proterozoic. Stretching tion) for the East Piute pluton than for the Old counted for only by effective fractional crys- lineations in the granite and in the country Woman-Piute batholith. tallization, and not by equilibrium batch seg- rock plunge at moderate to shallow angles regation. Batch segregation, including restite and variable trends, but chiefly northeast PETROCHEMISTRY AND unmixing (Chappell and others, 1987), re- and southwest (Fig. 8). Kinematic indicators DIFFERENTIATION HISTORY quires that melt and crystal-rich residue be in show that the shear zone in its present orien- or close to equilibrium at the time of separa- tation has southwest-vergent thrust displacement in northwest-striking segments The rocks of the East Piute pluton show tion, a condition not met by the rocks of the and left-lateral/thrust oblique-slip in east- considerable variation in composition, with East Piute pluton. west-striking segments (see below). If 20°- We assume that the felsic rocks represent < 60° west-dipping Teritary strata (Fig. 4) are almost pure liquids, because they are near restored to horizontal, however, the stretch- minimum melt composition in both Qz-Ab- ing lineation is rotated to a more nearly dip- Figure 3. Generalized geologic map of the Or-An and mafic components (see Winkler, parallel orientation (Fig. 8A). Thus, although Old Woman Mountains area, showing Creta- 1976; Miller and others, 1985), they are almost in its present orientation slip is predominantly ceous granitoids and mqjor shear zones. Rect- free of biotite and moderately calcic plagio- left-lateral strike-slip, the Fenner shear-zone angle represents the East Piute pluton area and clase cores, and zircon is very sparse and system is interpreted to have initially been is also the area of Figure 4. almost entirely magmatic in appearance. Zir-

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Figure 4. Geologic map of part of the Piute Mountains showing the East Piute pluton and the east-west-striking Fenner shear-zone system.

TABLE 1. Pb AND U CONCENTRATIONS AND ISOTOPIC COMPOSITIONS FOR ZIRCON AND MONAZITE FROM THE EAST PIUTE PLUTON

Fraction ppm U ppm Pb ^Pb/^Pb 20Spb/206pb ^Pb/^U Error Age "'Pb/^U Error Age Error Age (%) ^Pb/^Pb (Ma) (%) (Ma) (%) (Ma)

Zircon from JW87-82 M-63 626 35.04 6461 0.09613 0.05465 0.15 343 0.67395 0.23 523 0.08944 0.16 1413 Nm -63 531 37.33 9791 0.08621 0.06917 0.16 431 0.88057 0.26 641 0.09233 0.19 1474 Nm 102-163 448 40.33 10156 0.10429 0.08717 0.16 539 1.1349 0.19 770 0.09442 0.11 1517 Monazite from JW89-116 -63 777 139 99 8.6433 0.01741 2.1 111 0.13468 33.5 128 0.05611 31.4 457

Abbreviations for zircon fractions: M = magnetic, Nm- 63 = non-magnetic, 63 microns. U and Pb concentrations are for total amounts. Pb and U blanks are 0.3 ng and 0.01 ng or less, respectively. Measured Pb isotopic ratios corrected for fractionation of 0.11% pmu. Pb isotopic composition of feldspar from JW87-82 used for common Pb correction.

an east-west-striking thrust system (Figs. morphic assemblages (biotite, quartz, mus- lineations (compare Figs. 8B and 8C). These 8A, 9). covite) in pelitic rocks in the western part of folds are interpreted to have developed syn- The schematic cross section shown in Fig- the shear zone (away from later higher tem- chronously with emplacement of the 72 Ma ure 9 shows that Proterozoic rocks are struc- perature metamorphism related to the 72 Ma Lazy Daisy pluton (Fletcher and Karlstrom, turally above overturned and highly attenu- Lazy Daisy pluton), and preserved Protero- 1990), and they cause a dispersion of early ated Paleozoic metasedimentary rocks. The zoic 40Ar/39Ar ages all suggest that tempera- stretching lineations (Fig. 8C). If of large am- footwall of the shear zone contains a large- tures during thrust movement in the western plitude, the presence of these folds would in- scale, overturned, thrust-related syncline part of the zone were below —450 °C validate the construction of the eastern part of whose axial surface is subparallel to the shear (Fletcher and Karlstrom, 1990; Foster, 1989; the area shown in Figure 9. In the area of the zone (Figs. 4 and 9). The syncline is defined Foster and others, 1992). In the eastern Fen- folded Paleozoic rocks, however, amplitudes by folded Paleozoic strata, and, to the east, by ner shear zone, adjacent to the East Piute are on the order of tens to several hundred folded Proterozoic gneissic layering and com- pluton, Proterozoic gneisses and supracrustal meters. positional interlayering of Proterozoic units. rocks record higher temperature defor- This layering is less regular in the hanging wall mation. The mylonite zone is wider than else- SYN-THRUSTING EMPLACEMENT OF of the shear zone, which is dominated by where, suggesting more distributed move- THE EAST PIUTE PLUTON more massive Fenner Gneiss. Paleozoic units ment. Mylonite textures indicate that are not exposed in the hanging wall so that the recovery and recrystallization facilitated Interpretation of the relative timing of plu- magnitude of displacement on the shear zone crystal plastic deformation of both feldspar ton crystallization and regional deformation is is not well constrained, although Figure 9 sug- and quartz (see below). These features indi- a complex problem because of the changing gests a minimum displacement of several cate that temperatures of deformation were rheology of magma as it crystallizes, and be- kilometers. probably greater than 500 °C near the pluton cause of incomplete understanding of the The western part of the shear zone, away (Simpson, 1985; Tullis, 1983). The increased range of possible fabrics that can form in the from the East Piute pluton, is characterized ductility of the area adjacent to the pluton transition from magma to crystalline solid by a network of anastomosing zones of my- suggests that emplacement of the East Piute (Paterson and others, 1989). Experimental lonitic Proterozoic gneiss and by slices of pluton was synchronous with mylonitization. work (Arzi, 1978; van der Molen and Pater- overturned and folded Paleozoic rocks. Nu- Shear zones and country-rock foliation are son, 1979), and materials science research

merous relatively narrow zones of intense folded by upright (F2) folds that plunge shal- (MUler and others, 1990b, Wickham, 1987) grain size reduction, generally brittle behav- lowly to the northeast and southwest, sub- suggest that there is a critical melt fraction, in ior of feldspars, apparently low-grade meta- parallel to the dominant earlier (L,) stretching the range 509^-70% crystals, which marks a sharp drop in effective viscosity of a magma and a transition from suspension-like behavior to more complex solid-state deformation. 0.14 - Because of the relatively short time necessary for a crystallizing magma to pass through its 0.12 critical melt fraction relative to the time span •Q of regional deformation, a major goal of stud- Q. 0.10 CO ies of syntectonic plutons should be to iden- m tify the phase or phases of deformation during CM 0.08 Figure 5. U-Pb compo- !d sitions of zircons and mon- which a given magmatic unit passed through Q. its critical melt fraction and became entirely CD 0.06 azite from the East Piute O CM pluton. The best-fit discor- crystallized. 0.04 dia line is based on zircons The following section reviews evidence only. that the East Piute pluton was syntectonic 0.02 relative to progressive deformation in the 0.00 - Fenner shear zone. We document that the main unit passed through its critical melt fraction during thrusting. Leucocratic units, 207pb/235pb which represent fractionated melts that seg-

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TABLE 2. WHOLE-ROCK Pb AND Sr ISOTOPIC DATA FOR THE EAST PIUTE PLUTON they passed into the shortening field during progressive deformation. Rotations are gen- Sample no. ^Pb/^Pb ^Pb/^Pb ^Pb/^Pb "Sr/^Sr "Rb/^r Initial Sr erally less than 45° (Fig. 8E), however, and

Main unit therefore shear strains were generally less E-17 16.754 15.453 37.248 0.71020 0.479 0.7096 than one. Because these dikes of the felsic E-30 16.686 15.444 37.313 0.71097 0.667 0.7102 P-34 16.720 15.437 37.251 0.71063 1.021 0.7094 unit represent the end of the fractional crys- Monzogranite EP-90 16.772 15.437 37.262 0.71260 2.380 0.7097 tallization of the magma, they indicate that Granites last increments of movement on the shear E-23 16.718 15.440 37.305 0.72644 7.618 0.7172 E-26 16.748 15.435 37.272 0.73306 12.20 0.7183 zone coincided in the southern margin of the E-60 16.720 15.447 37.227 0.77700 50.80 0.7156 Feldspar 16.710 15.451 37.211 — — — pluton with final crystallization of highly JW87-82 evolved phases of the East Piute pluton.

Note: ''Sr/^Sr normalized to 8.3752. NBS-987 gave an average of 0.71026 ± 2 during this analysis period. Reported Pb isotopic In outcrops of the Fenner shear zone at the compositions are corrected for fractionation by 0.11% per mass unit based on comparisons to NBS-981 and -982. southwestern margin of the pluton, irregular pegmatite pods occupy small thrust planes that offset mylonitic layering in a synthetic regated from the differentiating magma, par- weak solid-state deformation of quartz is in- sense. In Figure 10B, the pegmatite obviously ticipated in different stages of progressive dicated by undulose extinction and subgrain postdated emplacement and mylonitization of deformation in the shear zone and also crys- development, but quartz pods are equant monzogranite and was apparently synchro- tallized during thrusting. We start with a dis- (Fig. 11 A). These crosscutting dikes are nous with thrusting. Thus, highly evolved cussion of the syntectonic character of peg- therefore interpreted to be only very weakly units were emplaced in small shear zones as matites and aplites, the last melt increments, deformed in the solid state. Dikes in the east- well as in tension gashes late during defor- and work back to progressively older segre- west-striking section of the shear zone at the mation in the shear zone. gation and crystallization events. southeast margin of the pluton are tabular, many with their poles oriented —45° from the Syntectonic Leucogranite: Microcracking of Latest Syntectonic Pegmatites and Aplites stretching lineation (Fig. 8E). These are in- a Grain Network terpreted as fillings of tension gashes in the Vein and dike arrays of aplite and pegma- shear zone because the dikes are perpendic- Coarse-grained, muscovite-rich, leuco- tite are present in a number of areas imme- ular to the incremental extension direction of cratic granite (unit 2 in Fig. 4) in the western diately adjacent to the East Piute pluton. The the shear zone (Fig. 8F). Dikes have appar- part of the pluton occupies an angular pro- dikes are variably deformed and cut mylonitic ently been variably rotated from their inferred trusion. The south side of the protrusion is main unit rocks, mylonitic felsic monzogran- original orientation perpendicular to the in- bounded by a shear zone in the country rock ite, and mylonitic country rock (Fig. 10A). cremental extension direction (poles to dikes that is subparallel to, and probably kinemat- They are generally unfoliated and have grain are displaced within the XZ plane toward the ically related to, the Fenner shear zone. This size zoning parallel to dike margins. Very stretching lineation), and some were folded as zone does not pass into the granitoids of

TABLE 3. COMPOSITIONS OF EAST PIUTE PLUTON GRANITOIDS

Main unit Monzogranites Granites

E-6 E-10 E-17 E-30 E-40 E-51 P-34 E-33 EP-3 EP-90 E-8 E-23 E-26 E-29 E-59 E-60

Si02 70.0 68.4 68.8 70.1 68.9 71.8 71.0 74.8 73.2 74.7 74.90 77.0 74.3 75.1 76.6 73.7 Ti02 0.4 0.38 0.39 0.36 0.42 0.25 0.24 0.11 0.07 0.08 0.09 0.06 0.08 0.08 0.09 0.07 AI2O3 15.6 15.6 15.9 15.6 15.6 15.3 15.0 14.20 13.80 13.60 14.7 13.0 14.6 14.7 13.0 14.8 Fe203 2.48 2.47 2.53 2.20 3.36 1.66 2.22 1.11 0.93 0.97 0.47 0.43 0.44 0.44 0.38 0.44 MnO 0.04 0.06 0.05 0.04 0.04 0.03 0.06 0.06 0.08 0.03 0.10 0.02 0.07 0.13 0.02 0.17 MgO 0.72 0.60 0.76 0.66 0.87 0.47 0.48 0.20 0.14 0.17 0.13 0.11 0.15 0.13 0.12 0.11 CaO 2.90 2.67 2.80 2.58 2.80 1.95 2.07 0.90 0.80 ' 1.11 0.65 0.88 0.82 0.64 0.62 0.76 Na20 4.08 4.02 4.24 3.99 4.05 4.13 4.21 3.85 4.19 3.54 4.54 4.00 40.7 4.52 3.53 4.12 k2o 3.58 3.27 3.37 3.75 2.98 4.11 3.56 4.18 4.65 5.35 4.10 4.64 4.59 4.14 4.68 5.10 P20, 0.16 0.16 0.16 0.15 0.20 0.10 0.11 0.04 0.06 0.03 0.04 0.04 0.03 0.05 0.03 0.05 LOI 0.39 0.69 1.00 .85 1.00 0.62 1.08 0.77 0.47 0.47 0.93 0.47 0.77 0.77 1.00 0.70 100.35 98.32 100.0 100.28 100.22 100.42 100.03 100.22 98.39 100.05 100.65 100.65 99.92 100.70 100.07 100.02

Rb 87 nd 78 107 81 105 114 133 134 102 184 129 156 207 139 158 Sr 464 nd 471 464 478 443 323 124 25 124 14 49 37 15 14 9 Ba 1000 nd 1100 1100 940 1300 810 280 120 230 80 230 90 80 130 40 Zr 140 nd 195 141 161 119 105 50 31 70 14 58 28 10 17 21 La 30.9 nd nd 34.2 35.2 31.2 nd 13.6 nd 13.0 7.5 17 nd 8.7 nd 19 Ce 58.2 nd nd 63 58 54 nd 22 nd 27 18.6 41 nd 19 nd 46 Nd 26 nd nd 26 30 20 nd 10 nd 15 11.3 nd nd 10 nd nd Sm 5.34 nd nd 4.12 4.39 3.45 nd 3.77 nd 3.88 7.09 8.3 nd 4.38 nd 11.5 Eu 1.28 nd nd 1.40 1.56 1.22 nd 0.58 nd 0.71 0.13 0.6 nd 0.31 nd 0.2 Tb 0.58 nd nd 0.5 0.4 0.3 nd 0.8 nd 0.9 1.09 1.4 nd 1.1 nd 1.4 Dy nd nd nd 1.8 1.8 2.1 nd 4.9 nd 5.3 nd nd nd 6.1 nd nd Yb 0.41 nd nd 0.59 0.57 0.60 nd 2.01 nd 2.21 2.26 3.70 nd 2.55 nd 5.50 Lu 0.091 nd nd 0.08 0.11 0.07 nd 0.27 nd 0.32 0.30 .30 nd 0.35 nd 0.60

T: Total Fe as Fe203. LOI: loss on ignition. nd: no data. Major elements and Ba by XRF as NAS, Inc.; Rb, Sr, and Zr by XRF at Vanderbilt University; all REE by INAA (E-30, E^O, E-51, E-33, EP- 90, E-23, E-60: NAS, Inc.; E-6, E-8: by A. F. Glazner at UCLA).

220 Geological Society of America Bulletin, February 1993

78 o o • Main unit 76 + Monzogranite 3 - 0 p + O Granite

74 - CO CM o o CM 2 - 05 72

- • Main unit + 1 - + + 70 + Monzogranite o Granite GŒD 1 • 68 1 1 1 1 ' 0 1 2 3 1 A CaO B CaO 2 3

1.0 1 . 1 300 1 • Main unit • Mai1n unit + Monzogranite 0.8 - + Monzogranite O Granite 200 O o Granite O 6 o °' -Q o> CD 0C ° -b 100 - + •• • S 0.4 - * + 0.2 Ô06

0.0 o 4 1 2 CaO D CaO

300 O Granite + Monzogranite m Main unit 200 J • N • • 100 • ?„ +

1 2 E CaO

F La Ce Nd Sm Eu Tb Dy Yb Lu

Figure 6. Elemental variations of East Piute pluton rocks. (A through E): Ca versus selected major-element oxides and trace elements. (F) Chondrite-normalized, rare-earth-element patterns.

the protrusion or into the main phase. This The angular shape of the protrusion indi- granite dikes in the main unit and dikes be- suggests that movement on this shear zone, cates that the leucocratic granite was proba- tween large screens of country rock, are con- although geometrically related to the devel- bly emplaced along fractures and shear sistent with the interpretation that the leu- opment of the protrusion, predated final crys- zones. The location of the protrusion, and the cocratic granite was emplaced in extension tallization of the pluton. extension direction indicated by leucocratic cracks perpendicular to the incremental ex-

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10 i • main unit : + monzogranite . O granite 100 4 F., 8

Sr, ppm Yb, ppm •rW r\ • initial r • main unit • D=2.5(Ba), 3(Sr) O granite a D=3(Ba), 3.5(Sr) + monzogranite . g F=0.2 h D=3.5(Ba), 4(Sr)

10 100 1000 10000 Ba, ppm B

Figure 7. (A) Modeling of Sr versus Ba fractionation in East Piute pluton rocks. Initial magma composition taken as average pluton composition (aerially weighted mean of each unit multiplied by its outcrop fraction). Bulk solid/melt partition coefficients for Sr and Ba must be subequal and probably are between 2.5 and 3.5; values would have been 3-4 and 2.5-3.5, respectively, based upon published mineral/melt partition coefficients (for example, Arth, 1976; Henderson, 1982; Mittlefehldt and Miller, 1983) and on observed modal abundances of micas (5%-10%), plagioclase (30%-50%), and K-feldspar (20%-30%) (Sparkes, 1981). Calculated melt compositions with remaining melt fractions of 50% and 20% for a range of Dvl's are shown. (B) Yb versus Ba. Strong negative correlation is consistent with fractional crystallization, in this case with Yb behaving incompatibly (D^1 << 1). Yb is highly incompatible with all major crystalline phases, and accessory minerals with a strong affinity for it are sufficiently sparse that overall incompatible behavior is predicted.

tension direction associated with thrusting because there was an interlocking network of recrystallization and recovery by grain- (Fig. 9). Presumably magma accumulation grains through which melt was free to mi- boundary migration, but they retain equant kept pace with extensional strain. Trace-ele- grate. Slight rotation of leucocratic granite shapes, suggesting relatively little solid-state ment modeling suggests that the magma was dikes into a splay of the shear zone at the strain. We interpret this fabric as a magmatic about 80% crystallized at the time of segre- southern edge of the protrusion indicates that foliation with a solid-state overprint because gation of leucocratic granite, and the volume very minor movements along this shear zone strain recorded in the quartz aggregate grains of granite in the protrusion is large (5% of the (gamma « 1) outlasted their emplacement is not large enough to explain the alignment of total area of the pluton) relative to the inferred and crystallization. feldspar and mica grains (Paterson and oth- amount of melt remaining after 80% crystal- Leucocratic granite in the protrusion con- ers, 1989). Microcracking of feldspars and lization. Thus, stresses related to the devel- tains a foliation that dips shallowly east and is garnets and shear offsets of muscovite are opment of this protrusion during thrusting defined by aligned muscovite and plagioclase common (Fig. 11C). Although deformation is were apparently efficient in segregating melt grains (Fig. 1 IB). Quartz pods show dynamic brittle, cracks are sealed with polygonal ag-

• Figure 8. Equal-area projections of structural data. Dots are poles to foliation; triangles are lineations; squares are poles to pegmatite dikes; open symbols are measurements from the East Piute pluton, filled symbols are from country rock. A. Rotation of Tertiary strata (which crop out ~1 km west of the East Piute pluton) to horizontal restores the Fenner shear zone (as exemplified by subarea E) to a more nearly dip-slip orientation (thrust). B. Total poles to foliation for the East Piute pluton and adjacent country rocks show that gneissic layering in Proterozoic rocks and foliation

in the East Piute pluton are subparallel and have been broadly folded by shallowly northeast-plunging F2 folds. Proterozoic layering in the area therefore has been reoriented by Mesozoic deformation both during thrusting (St) and later F2 folding. C. Stretching lineations are dispersed but mainly plunge shallowly to the southwest and northeast; F2 fold axis is subparallel to earlier stretching lineations. D. Northern margin shows F2 folding of St layering in pluton and country rock. E. Southeastern margin: kinematic framework for the Fenner shear zone is inferred from this subarea, where F2 folding is minimal. Poles to St mylonitic foliation define the average shear plane; stretching lineations define the movement direction. The inferred pole to unrotated pegmatite dikes is 45° from the stretching lineation and parallel to the incremental stretching direction. Poles to measured pegmatite dikes show variable rotation toward the finite stretching lineation but less than 45°. The kinematic Y direction (90° from the stretching lineation) is the pole to the great circle (dashed line) defined by variably rotated dikes and foliations. F. Southeastern margin: schematic drawing showing geometry of sinistral drag fold of pluton margin and mylonitic monzogranite, pegmatite-filled tensiongashes , and brittle synthetic thrusts; all record progressive simple shear. G. Southeastern margin (including subarea E): poles to foliation define a sinistral drag fold of the pluton margin and Fenner shear zone. Mylonitic foliation is folded along with the contact, but stretching lineations are not strongly reoriented,

suggesting that folding took place during progressive shearing. Fx fold axis is subparallel to the inferred Y kinematic direction (subarea E), compatible with progressive deformation. H. Southwestern margin shows well-developed girdle of St layering defining F2 fold axis.

222 Geological Society of America Bulletin, February 1993

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-POLE TO AVERAGE PBE-TERTIARY BEDDING K Si *POLE TO AVERAGE SHEAR PLANE FROM. NET E

A. Tertiary Rotation B. Poles to Foliation Summary C. Lineations Summary

H.Southwestern Margin G. Southeastern Margin F. Pegmatites Filling Tension Gashes, SE Margin

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û & is LAZY DAISY FENNER GNEISS PLUTON 1.68 Go rsw

Figure 9. Schematic southwest-northeast cross section across the East Piute pluton and Fenner shear zone system. Rocks now exposed were at depths of about 10 km in the Late Cretaceous based on 2- to 3-kb pressures from metamorphic porphyroblasts in the lower Paleozoic rocks (Hoisch and others, 1988). The western part of the cross section was drawn as a schematic cross section (following Fletcher and Karlstrom, 1990). The eastern two-thirds of the cross section was constructed by projecting map data up-dip into the X-Z plane of the shear zone, a plane striking east-west, dipping 50 degrees south, then rotating this projection to restore 30-degree west-dips of Tertiary strata (inset A). This treatment assumes that the present map surface in the eastern part of the area shown in Figure 4 approximates an oblique cross section of the Fenner shear-zone system, which is consistent with the relatively constant east-west trend and north dip of the shear zones and the shallow plunges of the dominant stretching lineation (Fig. 8C). Inset B shows that restoration of the southeastern margin of the East Piute pluton suggests about 20% shortening (1 km) by folding and thrusting after crystallization of the monzogranite.

gregates of quartz, with rarer feldspar and metamorphism in this area. Deformation of tallized main-phase granite (southeast part of muscovite (Fig. 11C). Quartz and feldspar the grain network of the leucocratic granite area shown in Fig. 4). As discussed above, grains within cracks are commonly in optical within the protrusion may be explained by the trace-element modeling suggests that monzo- continuity with grains outside the crack. relatively minor (—20 degrees) rotation of the granite crystallized from melt that segregated These features have also been observed in protrusion during progressive shear, as indi- from the magma after it was 50%-70% crys- other syndeformational granites (Gapais and cated by the rotated dikes discussed above. tallized and was passing through its critical Barbarin, 1986; Tobisch and Paterson, 1990) melt fraction. At this point, the pluton was and are interpreted here as evidence for mi- Syntectonic Monzogranite: Mylonitic Solid- probably just beginning to develop a network crocracking of a grain network in the pres- State Fabrics of interlocking grains which would facilitate ence of melt. The alternative explanation, segregation of melt. Restriction of felsic that crack-filling minerals are metamorphic in Monzogranite of the leucocratic unit crops monzogranite to areas adjacent to the main origin (Tobisch and Paterson, 1990) seems out along the southern margin of the pluton in shear zone suggests that melts migrated less likely because of the similarity between intrusions parallel to foliation in the adjacent toward the active shear zone (Sibson and oth- crack-filling grains and matrix grains in the shear zone, and in dikes oriented parallel to ers, 1975; Beach, 1980). This melt segregation granite and the generally low grade of regional the axial plane of drag folds of earlier crys- event, however, was not especially efficient,

224 Geological Society of America Bulletin, February 1993

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PLUTON EMPLACEMENT, ACTIVE DUCTILE THRUST ZONE, CALIFORNIA

as volumes of monzogranite are much smaller than the inferred volumes of melt of this composition that should have been available after 509^-70% crystallization. Fabrics in the monzogranite record intense mylonitization. Quartz ribbons contain su- tured , but generally straight, grain boundaries that are at a high angle to the ribbon boundary (type 4 ribbons of Boullier and Bouchez, 1978). Feldspar was ductilely deformed by dislocation creep processes as shown by core and mantle textures and strong lattice preferred orientation (Fig. 11D). Evidence for feldspar ductility, and that grain boundary migration facilitated recovery in quartz (Urai and others, 1986), both suggest that solid- state deformation took place at temperatures of perhaps 550-650 °C (Simpson, 1985; Simpson and DePaor, 1991). This deformation apparently required heat from the crystallizing magma, as regional metamorphic temperatures away from the pluton were below 450 °C. Mylonitic monzogranite is crosscut by undeformed pegmatites (Fig. 10), indicating that solid-state deformation of the monzogranite predated final crystallization of the pluton.

Main Phase: Homogeneous Magmatic Fabric Overprinted by S-C Solid-State Fabric

Fabric in the main phase of the pluton generally parallels, but is weaker than, that in the adjacent country rock. The variation in intensity of the fabric reflects location relative to the adjacent shear zones, with the southern portion of the pluton most intensely deformed. Fabric is rather homogeneous in the center of the pluton and is defined by the shape-preferred alignment of biotite, feldspar (especially plagioclase), and quartz pods. This foliation may initially have been magmatic. Solid-state dynamic recrystallization, however, is ubiquitous in all units. Quartz pods are lenticular granoblastic aggregates with straight grain boundaries, indicating appreciable recovery by grain-boundary migra- Figure 10. Mesoscopic features documenting syntectonic emplacement of pegmatites. tion. Fine-grained granoblastic aggregates of A. Tabular, weakly deformed pegmatite (thin white dikes) crosscut mylonitic monzogranite feldspar in the margins and tails around relic (thicker white band) and country rock along the southeastern side of the pluton. Orientations of igneous grains indicate feldspar ductility in- these dikes (Figs. 8E and 8F) suggest emplacement into tension gashes formed during thrusting. volving grain-boundary migration (Urai and View is toward the north; outcrop face is approximately perpendicular to foliation and parallel others, 1986). S-C fabric is increasingly well to lineation. developed toward the shear zone at the south- B. Melt pod of pegmatite (black) accumulated along a small thrust in mylonitic monzogranite ern margin (Fig. 1 IE). The change from ho- (cross-hatched pattern) and Fenner Gneiss (lines) in Fenner shear zone along the southwestern mogeneous foliation to S-C fabric in the main margin of East Piute pluton. Note that the geometry of this mesoscopic thrust is similar to the phase is consistent with syntectonic emplace- macroscopic geometry (Fig. 9), with an overturned syncline in the footwall of the thrusts. View ment, and it suggests a change from homo- is toward the north; outcrop face is approximately perpendicular to foliation and parallel to geneous to heterogeneous flow in response to lineation. higher strain rates and/or more rapid cooling

Geological Society of America Bulletin, February 1993 225

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/2/213/3381748/i0016-7606-105-2-213.pdf by guest on 26 September 2021 Figure 11. Sketches of photomicrographs of deformationa] fabrics. A. Undeformed aplite and pegmatite form tabular dikes that crosscut mylonitic country rock, main phase units, and monzogranite. Garnet (G), black; muscovite (M), fine lines; plagioclase (P), coarse lines; K-spar (K), dots; quartz (Q), clear, with dashed patterns indicating undulose extinction in quartz. Grain- size layering is parallel to dike margins. Dikes show negligible solid-state deformation, as shown by equant quartz pods. View is 4 mm across. B. Leucocratic granite of the protrusion shows subhorizontal magmatic fabric defined by shape-preferred alignment of muscovite and plagioclase (patterns and letters as in A). Mica fish suggest thrust displacement. Quartz pods are recrystallized but still subequant, indicating low solid-state strain. View is 4 mm across. C. Leucocratic granite of the protrusion shows cracking of plagioclase (stripes) and K-spar (thin line pattern), with cracks healed by quartz-feldspar aggregates (clear) that are interpreted to represent residual melt. View is 1 mm across. D. Monzogranite shows isoclinally folded mylonitic fabric that is characterized by feldspar ductility. Dynamically recrystallized feldspar pods (dots, F) have new strain-free grains in a core and mantle structure with good lattice preferred orientation, suggesting recovery by rotation recrystallization. Quartz ribbons (Q) are isoclinally folded and show type 4 grain boundaries (Boullier and Bouchez, 1978). These features suggest solid-state deformation at temperatures above 500 °C. Dashed form lines represent the mylonitic matrix consisting of fine-grained quartz, feldspar, and micas. View is of an entire thin section, 4 cm across. E. S-C fabric is developed in the main phase near the Fenner shear zone. C-planes are defined by zones of intense grain size reduction. S-C angle ranges from 20 to 40 degrees. Clear blebs are ductilely deformed feldspar pheno- crysts; lines represent matrix of quartz, feldspar, and biotite. Rock slab is 13 cm across. F. Delta-type feldspar porphyroclast (F) (Passchier and Simpson, 1986) in the country rock within the Fenner shear zone adjacent to the southeastern margin of the pluton shows sinistral shear and dynamic recrystallization of feldspars and suggests that shear-strain rate was high relative to recrystallization rate. View is 4 mm across. G. Shear band fabric in the Fenner Gneiss shows sinistral shear and feldspar ductility within the Fenner shear zone adjacent to the southeastern margin of the pluton. View is 14 nun across.

226 Geological Society of America Bulletin, February 1993 226

SC nw SE

-O / WEAKLY FRACTIONATED > A — (BIOTITE POOR) MAIN UNIT \ \\\ (- . ' ~~ MAGMA (UNIT 2) HOMOGENEOUS MAGMA V\\ \ ^ ~ • MIGRATES TOWARDS (UNIT 1 OF FIGURE 4) » I - I X __ "CAP" \ ^ EMPLACED ALONG I — I 50-70% / ACTIVE THRUST ' ^ ^ I CRYSTALLIZED \ , \ ' I /•' ' / _(MONZOGRANITE (UNIT 3) LIQUID MIGRATES TO SHEAR ZONE

'.J.-'LEUCOCRATIC GRANITE (UNIT 4) Jïwë'-'-l FILLS DILATANT -CAP" ZONE X •-'-"" • DILATANT ZONES REFLECT

INCREMENTAL STRAIN _ \ - - ' ^ - ' - X • \

-"Ml / - 80=90% S/C FABRIC DEVELOPS CRYSTALLIZED I \\ W IN MAIN PHASE - - \ , \ - / - / - - / SHEAR ZONE FOLDED INTO LATE PEGMATITE AND _ APLITE DIKES (UNIT 5) ^ I - I INCIPIENT NAPPE FILL TENSION CRACKS

N - I - I

LATE BRITTLE THRUST Figure 12. Inferred sequence of emplacement of the syn-thrusting East Piute phiton; see text for discussion.

near the shear zone (Gapais and Barbarin, DISCUSSION OF EMPLACEMENT OF tion related to shear-zone geometry, for ex- 1986; Gapais, 1989). THE EAST PIUTE PLUTON ample, along a ramp in a thrust (Fig. 12). As The spacing of C-planes in the main unit is true for many plutons, however, present varies from centimeters to tens of centime- Figure 12 is an interpretation of the se- pluton geometry and rock fabrics are related ters. Shear strains in the East Piute pluton quence of emplacement and melt segregation to final emplacement of the pluton, and it is were apparently lower than in the adjacent of the East Piute pluton. We infer the follow- difficult to determine geometry of magma as- shear zones with gamma values of less than 1 ing sequence. (1) A homogeneous magma cent. We think that it is unlikely that pluton suggested by mean angles between the was emplaced along an active thrust zone ca. ascent caused the thrust faulting, because the C-planes and S-planes measured in thin sec- 85 Ma. We suggest that the magmareached its pluton is small and the system of top to the tions ranging from 25 to 40 degrees (Fig. 1 IE). present level in the crust essentially as a single southeast shear zones is of regional extent Higher apparent shear strains in adjacent batch because we see no evidence within the (Fletcher and Karlstrom, 1990; Kelly, 1991). country rock (Figs. 1 IF and 11G) than in the main unit for discrete intrusions and because In addition, there is evidence for earlier main unit of the pluton suggest that apprecia- compositional variations between the main (Jurassic or earlier Cretaceous) movement ble shear-zone movement may have predated and leucocratic units can be explained in across this and other shear zones in the granite emplacement and/or that the melt terms of fractional crystallization of a magma area (Foster, 1989). Space for the emplace- fraction was high enough during early move- whose composition was similar to that of the ment of the main magma was apparently cre- ment that shear strength was minimal and main unit. Magma may have ascended as a ated by crustal movements (passive em- deformation was not recorded. diapir or as numerous pods, with accumula- placement), as we see no wall-rock strains

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TABLE 4. CONSTRAINTS ON COOLING OF THE EAST PIUTE PLUTON tributed (for example, shear strains in the monzogranite are appreciably higher), and to- I. ASSUMPTIONS A. Country rock: infinite reservoir of country rock at 300 °C and 12 km; thermal difliisivity = 1 mm2 s_1; geotherm = 25 °C/km. tal shear strain may not be closely approxi- B. Magma temperatures: based on zircon saturation calculations (Watson and Hanison, 1983). mated by S-C angle if movement is concen- 1. Initial melt - 745 ± 15 °C: mean Zr content of pluton = 140 ppm; assuming 60%~90% of Zr was in melt, with the remaining, 10%-40% in inherited zircons; stated error reflects range of inherited zircons. M value = 1.5. trated in discrete high strain zones, or if 2. Monzogranite segregation - 680 ± 15 °C: mean Zr in monzogranite = 50 ppm, and assuming 10%~40% of Zr in inherited zircons', M - 1.35. appreciable flattening accompanied shearing. 3. Granite and pegmatite segregation = 635 ± 15 °C: mean Zr in granites, aplites, and pegmatites = 25 ppm, and assuming Nevertheless, our strain-rate calculations are 10%-40% of Zr in inherited zircons; M = 1.3. 4. Water-saturated solidus of muscovite granite at 3-4 kb = 645 ± 15 °C: (Huang and Wyllie, 1981). relatively insensitive to shear strain and more C. Heat transport: cooling by simple conduction in all models. Pluton-wide convection would be severely inhibited in the late stages of crystallization that are of greatest interest here, but advection of heat, as late melts migrated to the margin, may have sensitive to duration of strain accumulation. been important. Together, convection and advection may have increased the cooling rate appreciably, and so time estimates Shear strains as high as 10, which are clearly given below are maximum cooling times. unreasonable for observed fabrics in the main II. MODELS FOR DURATION OF CRYSTALLIZATION A. 3- to 4-km-thick sheet: (using equations of Turcotte and Schubert, 1982), with latent heat of crystallization = 3 Jg unit, would yield strain rates near 10~'°/sec; 1. Total time to solidification = 90 to 160 x I03 yr: (length of time until most of pluton is <640 °C). overall strains of 0.1, which fail to account for 2. Time from monzogranite segregation to solidification = 40 to 80 x 103 yr. B. 3- to 4-km-thick sheet: (using the program "1 DT"; Haugerud, 1989); latent heat of crystallization indirectly considered by the S-C angle and rotated tension veins, increasing stated magma T to 1000 °C initially and to 800 °C for magma at time of monzogranite segregation; this is _12 approximately equivalent to including 3 Jg-1 latent heat, and assuming a constant rate of crystallization with declining T. would yield strain rates close to 10 /sec. 1. Total time to solidification = 75 to 130 x 10> yr. 2. Time from monzogranite segregation to solidification = 30 to 50 x 103 yr. Minimum displacement across the shear C. 2- to 3-km radius sphere: (using equations from Ingersoll and others, 1948; and T. D. Hoisch, unpub. data); latent heat zone adjacent to the pluton can be estimated indirectly considered, as above, 1. Total time to solidification = 20 to 50 x 103 yr. from inferred shear strains. The zone of more 3 2. Time from monzogranite segregation to solidification = 15 to 25 x 10 yr. highly strained main unit rocks is on the order of 1 km wide, with shear strains of about 1, that we attribute to diapiric ascent and ESTIMATE OF RATES suggesting displacements of 1 km due to de- emplacement. OF CRYSTALLIZATION velopment of S-C fabric. In addition, the fold (2) During the final stages of ascent to the AND DEFORMATION of the southern contact of the plutort and mi- present crustal level, and after 50%-70% nor brittle offsets of the contact can be recrystallization of the pluton, monzogranite Thermal modeling suggests that the pluton stored (Fig. 9, inset B) and yield about an- melt migrated toward the shear zone. This took between 104 and 105 yr to crystallize. other 1 km of displacement across the shear probably occurred at about the time that the Duration of crystallization of the last 30%- zone. Thus, total displacement after the pluton had developed a weak interlocking 50% of melt (the melt that remained at the magma attained shear strength in the south- network of grains and was solid enough to time of the first segregation event) was closer ern margin of the pluton was perhaps on the sustain appreciable shear stress. to 104 yr (Table 4). The granitic liquids that order of 2 km. This estimate suggests a min- (3) After the pluton reached the present separated during the final segregation event imum displacement rate during this interval of level, and after 80% crystallization, granite were only slightly above their solidi and prob- tens of centimeters per year. Total displace- liquid was segregated from the magma and ably crystallized within only a few thousand ment across the Fenner shear-zone system is began to accumulate in the protrusion which years. Thus, final stages of progressive shear- probably considerably greater, as suggested was progressively opening in response to zone movement (stages 2-4 of Fig. 12) are by thrust displacement of Proterozoic rocks shear-zone movements. Segregation of the precisely constrained by the crystallization over Paleozoic rocks and attenuation of the granite melt was effective because of the pres- age of 85 Ma. This is the general case for Paleozoic section to a few percent of its orig- ence of a strong grain network, and melt mi- granitoids, that duration of pluton crystalli- inal thickness (Fig. 9), reinforcing the inter- gration apparently kept pace with wall-rock zation is very short relative to duration of pretation that the pluton was emplaced late in displacement. The nearly crystallized main orogenic deformation. Thus by linking crys- the thrusting deformation and that the pluton unit of the pluton developed an S-C fabric, tallization sequence to deformation sequence did not cause the thrusting. and the shear zone and mylantic monzogran- it is possible to help to constrain orogenic The proposed orogenic strain rate of 10~11 ite sills near the southeast margin of the plu- deformation rates. to 10~12/sec is high relative to published val- ton became progressively folded into incipi- According to our interpretation, essentially ues of 10" 13/sec to 10" 15/sec (summarized by ent nappes (Figs. 8F, 8G). These Z-shaped all of the deformation recorded in rocks of the Pfiffner and Ramsay, 1982). It may be that drag folds are interpreted to have developed East Piute pluton occurred between the two accumulation of finite strain in an orogenic synchronously with the S-C fabric in the main segregation events. Table 4 suggests that the belt may yield average strain rates of 10 14, unit, because stretching lineations maintain a total strain recorded by fabrics in the pluton but values such as 10"11 represent deforma- fairly consistent orientation across ninety-de- must have accumulated over a period no tion rates during episodic local orogenic strain gree bends in mylonitic foliation around the longer than 10,000-80,000 yr, with shorter events. Likewise, displacements of tens of F, fold (Fig. 8G). time periods favored if cooling was facilitated centimeters per year are several times greater (4) When the pluton was nearly completely by advective heat transport. Main unit rocks than plate-motion estimates, but these data crystallized, late dikes of pegmatite and aplite within about 1 km of the southern margin are likewise averages of relative plate move- were emplaced in tension gashes and along record shear strains on the order of one or ment and need not apply to local strain rates shear planes in the shear zone. Brittle syn- less, as do most of the latest syntectonic peg- over short time intervals. thetic thrusts and development of solid-state matite dikes (tension gashes) in the southeast- One explanation for higher strain rates deformational fabrics document continued ern margin of the pluton. Thus strain rates might involve punctuated strain accumula- n 12 progressive deformation at lower tempera- must have been close to 10~ to 10~ /sec. tions, facilitated by the presence of melt and ture during the latest stages of magma crys- Estimating total shear strain is problematic; local elevated temperatures caused by pluton tallization and cooling. shear strain is obviously heterogeneously dis- emplacement. Hollister and Crawford (1986)

228 Geological Society of America Bulletin, February 1993

have proposed that the presence of melt en- zone, and cessation of deformation after final Foster, D. A., Harrison, M. T„ and Miller, C. F., 1989, Ion probe U-Pb and Âr/Âr geochronology of the Old Woman-Piute hances deformation because melts are appre- crystallization may be explained in terms of batholith, California: Age, inheritance, uplift history, and implications for K-feldspar age spectra: Journal of Geology, ciably weaker than surrounding rocks and rapid cooling of the crust. v. 97, p. 232-243. should flow in response to small differential 5. Thermal modeling indicates that the in- Foster, D. A., Miller, C. F., Harrison, M. T., and Hoisch, T. D., 1992, Timing and character of metamorphism and tectonism stresses. Thermally activated plastic defor- terval between attainment of appreciable in the Old Woman Mountains area, California: Evidence from Âr/Âr thermochronology and thermobarometry: Geolog- mation mechanisms also facilitate deforma- shear strength and final crystallization was ical Society of America Bulletin, v. 104, p. 176-191. tion. Punctuated strain accumulation would brief, on the order of 104 yr. Gapais, D., 1989, Shear structures within deformed granites: Me- chanical and thermal indicators: Geology,v. 17,p. 1144-1147. be compatible with a view of orogenic defor- 6. Fabrics in the pluton developed during Gapais, D., and Barbarin, B., 1986, Quartz fabric transition in a cooling syntectonic granite (Hermitage Massif, France): Tec- mation involving episodic periods of move- crystallization, after the pluton was solid tonophysics, v. 125, p. 357-370. ment and quiescence, and dramatic partition- Guineberteau, B., Bouchez, J.-L., and Vigneresse, J.-L., 1987, The enough to sustain shear stress. Thus, ob- Mortagne granite pluton (France) emplaced by pull apart ing in time and space of deformation events served shear strains of ~1 accumulated in along a shear zone: Structural and gravimetric arguments and 4 regional implication: Geological Society of America Bulletin, (BeU, 1981). about 10 yr, yielding strain rates of 10" " to v. 99, p. 866-879. ,2 Haugerud.R. A., 1989, On numerical modelling of one-dimensional A somewhat similar view of deformation 10~ /sec. geothermal histories: Computers and Geosciences, v. 15, was proposed by Tobisch and Paterson p. 825-836. Henderson, P., 1982, Inorganic chemistry: New York, Pergamon (1990), but they suggested that emplacement ACKNOWLEDGMENTS Press, 353 p. Hoisch, T. D., Miller, C. F., Hiezler, M. T., Harrison, T. M„ and of granite was "intradeformational" in the Stoddard, E. F., 1988, Late Cretaceous regional metamor- sense that emplacement took place during an We acknowledge the work of A. K. phism in southeastern California, in Ernst, W. G., ed., Meta- morphism and crustal evolution of the western United States, orogenic lull. Evidence from the East Piute Sparkes, who was the first to describe the Rubey Volume VII: Englewood Cliffs, New Jersey, Prentice- pluton suggests that this pluton was emplaced Hall, p. 538-571. East Piute pluton. The paper benefitted from Holder, M. T., 1979, An emplacement mechanism for post-tectonic during thrusting and that pluton emplacement discussions with other participants of the Col- granites and its implications for their geochemical features, in Atherton, P. P., and Tamey, J., eds., Origin of granite batho- facilitated high strain rates during an orogenic laborative Old Woman-Piute Investigative liths; geochemical evidence: Orpington, England, Shiva, pulse. 148 p. Effort (COWPIE). In particular, we thank Hollister, L. S., and Crawford, M. L., 1986, Melt-enhanced defor- Eric Bender, John Fletcher, David Foster, mation: A major tectonic process: Geology, v. 14, p. 558-561. Howard, K. A., Miller, C. F., and Stone, P., 1980, Mesozoic thrust- CONCLUSIONS Keith Howard, and Carol Simpson. The pa- ing in the eastern Mojave Desert, California: Geological So- ciety of America Abstracts with Programs, v. 12, p. 112. per was improved following reviews by Ken Huang, W. L., and Wyllie, P. J., 1981, Phase relations of s-type McCaffrey and Scott Paterson. Funding for granite with H20 to 35 kbar: Muscovite granite from Hamey 1. Southwest-directed thrusting in the Peak, South Dakota: Journal of Geophysical Research, v. 86, Piute Mountains was active at 85 Ma, and the research was provided by National Sci- p. 10515-10521. Hutton, D.H. W., 1982, A tectonic model for the emplacement of the ended after emplacement of leucocratic ence Foundation Grants EAR -8609153 and main Donegal granite, NW Ireland: Geological Society of phases of the East Piute pluton. Movement -8904675 (Karlstrom) and -8609801 and London Journal, v. 139, p. 615-631. Hutton, D.H. W., 1988a, Granite emplacement mechanisms and tec- on the Fenner shear zone of several to tens of -8904320 (Miller). tonic controls: Inferences from deformation studies: Royal Society of Edinburgh Transactions: Earth sciences, v. 79, kilometers predated granite emplacement and p. 245-255. 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