Reconstructing the kinematics of magmatic flow during pluton emplacement in the Coastal Batholith, central Chile
Abstract
Unlike solid-state deformation fabrics, the kinematic significance of structures produced during hypersolidus deformation remains equivocal. Nevertheless, integrating field-based observations and quantitative fabric analyses of magmatic structures can yield important information on how arcs are constructed, and how magma migrates through the crust. The Coastal Range of central Chile (33.5° S) records the emplacement of a Permian to early Mesozoic batholith situated along the western margin of Gondwana. This batholith is hosted by locally migmatitic orthogneiss, amphibolite, and paragneiss of the Late Paleozoic Valparaíso Metamorphic Complex; and preserves structures that record magmatic flow during the formation of at least six igneous units. In order to evaluate and reconstruct the kinematic history of magmatic flow during emplacement, we identified key features in mineral fabrics and microgranitoid enclaves that formed prior to full crystallization of the magma. The oldest unit is a coarse-grained tonalite with abundant microgranitoid enclaves that create a compact network with a porphyritic matrix, and display a high degree of enclave impingement. This unit is intruded by a second tonalite body that is distinguished by relatively low enclave volumes, such that isolated enclaves are suspended within a coarse-grained matrix. Enclaves in both phases display features that indicate compositional heterogeneity, variable viscosity contrasts, and rheological changes. Orientations of feldspar phenocrysts, as well as enclaves, indicate relatively uninhibited object rotation within planes that define feldspar and hornblende mineral foliations, with little to no rotation of long axes out of these planes. Although the matrix and enclave networks are significantly different in terms of scale, viscosity, and geometry; both rotate in a similar manner. Three-dimensional strain analyses applied to enclaves within both phases were used to evaluate fabric development, and indicate a significant increase in the magnitude of shape fabric ellipsoids, with a shift from slightly flattened to highly elongate geometries as new magma was introduced. Therefore, high degrees of enclave interaction appear to inhibit fabric development. Where enclaves are highly concentrated, magmatic flow is characterized by the migration of matrix material through an enclave-supported network where phenocrysts are able to spin about an axis parallel to the transport direction, but can also rotate within the foliation plane. This enclave network records both sinistral and dextral rotations perpendicular to the bulk flow direction, and define mesoscopic flattened helicoidal flow cells that formed during emplacement. Enclave geometries may indicate matrix transport in an upward direction through the enclave network, implying magma migration from deep to shallow levels through the crust. This model may be an analog for how magma can migrate through plutonic complexes within arc systems globally.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.T53A2490W
- Keywords:
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- 8011 STRUCTURAL GEOLOGY / Kinematics of crustal and mantle deformation;
- 8035 STRUCTURAL GEOLOGY / Pluton emplacement;
- 8145 TECTONOPHYSICS / Physics of magma and magma bodies;
- 8178 TECTONOPHYSICS / Tectonics and magmatism