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. 213 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 KARLSTROM AND OTHERS MODELS FOR GRANITOID ASCENT AND EMPLACEMENT Hoisch and others, 1988; Reynolds and oth- ers, 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 mag- matic 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 ar- eas. 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 horn- blende 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