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Timing and nature of magmatic fabrics from structural relations around stoped blocks

Journal of Structural Geology
Publication Date
DOI: 10.1016/s0191-8141(96)00058-2
  • Earth Science


Abstract Structural relations around several large ( > 100 m) stoped blocks in the granodiorite of Castle Creek, Sierra Nevada, California, suggest that magmatic fabrics formed as or after the space-making phase of pluton emplacement had ended. The stoped granite blocks are approximately 360 m below the exposed pluton roof. Magmatic foliations in granodiorite surrounding the blocks commonly follow km-scale orientation trends to within 1 m of block contacts, where they become deflected parallel to the block boundaries. Locally, undeflected foliations occur within a few centimeters of block contacts. Mafic enclave fabric ellipsoid intensities (0.2 < Es < 0.7) and shapes (mostly plane to oblate) show little relation to distance from stoped block contacts. Mineral and enclave linear fabrics vary widely in orientation. In short, magmatic fabrics in the granodiorite of Castle Creek show virtually no strain record for sinking of these presumably late, stoped blocks. Instead, these fabrics reflect shortening of the surrounding granodiorite magma associated with the formation of km-scale fabric trends. We draw the following conclusions concerning magmatic fabrics in the granodiorite of Castle Creek: (1) magmatic fabrics formed during and/or after the latest stages of pluton emplacement, and thus, contain little or no information about space-making processes; (2) magmatic fabrics had poor strain memory; and (3) during magmatic fabric formation, pluton strains were partially decoupled from host-rock strains. Published data from other plutons indicate that these relationships may be common, particularly in the brittle upper crust. It follows from these conclusions that even in the best case, magmatic fabrics contain only qualitative information about pluton emplacement mechanisms, regional tectonics, and internal magma chamber processes. Interpretations of magmatic fabrics must be corroborated by evaluating causes, kinematics, and magnitudes of syn-emplacement host-rock deformation. In the worst case, the causes of magmatic fabrics cannot be uniquely determined.

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