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

Abstract

Plutons that are emplaced synchronously with regional deformation offer the opportunity to understand the partitioning of deformation and metamorphism in time and space during orogeny. The 85 Ma East Piute pluton of the eastern Mojave Desert is an unusually clear example of a syntectonic pluton; it was emplaced during thrusting in the southern Cordillera. Combined petrologic and structutal studies lead to empirical models for the dynamic interaction of magmatic and deformational processes during granitoid emplacement and final crystallization. Trace- and major-element and isotopic data suggest that closed-system differentiation of a single magma batch was responsible for compositional variation (granodiorite to leucocratic granite). Correlation between degree of chemical evolution of compositional units and intrusive sequence suggests that discrete melt segregation events punctuated fractional crystallization. Modeling of trace-element data suggests segregation of monzogranite melts after 50%-70% crystallization of the magma and segregation of leucocratic granite after about 80% crystallization. Deformation promoted segregation of melt fractions at discrete times during crystallization. During final ascent, and after the magma had crystallized sufficiently (50%-70%) to attain shear strength, monzogranite liquid was incompletely segregated along the active thrust zone at the southern margin of the pluton. At essentially the present crustal level, and after about 80% crystallization, the remaining leucocratic granite melts were more effectively segregated and migrated into thrust-related extensional openings, including tension gashes. Undeformed pegmatite and aplite dike arrays indicate that final crystallization coincided with final increments of shear-zone movement. Solid-state deformational fabrics and brittle synthetic thrusts that offset the pluton margin record sharp temperature decline during and after crystallization. Because plutons crystallize rapidly, they can be used to estimate strain rates during orogeny. Thermal modeling suggests that the transition from rheological liquid to solid in the East Piute pluton occurred during an interval of about 10 4 yr. As final increments of ductile deformation coincided with final crystallization of evolved compositional units, total strain recorded by fabric in the pluton apparently accumulated during this interval. Shear strains estimated from the S-C fabric in the granodiorite unit and rotation of pegmatite-filled vein arrays are on the order of 1. Thus, strain rates in the shear zone were on the order of 10 -12 /sec during crystallization of the pluton. This corresponds to displacements of tens of centimeters per year. These high strain rates suggest punctuated, melt-enhanced movement episodes during contractional orogeny.