TIMING AND NATURE OF MAGMATIC FABRICS FROM STRUCTURAL RELATIONS AROUND STOPED BLOCKS

Structural relations around several large (> 100 m) stoped blocks in t he granodiorite of Castle Creek, Sierra Nevada, California, suggest th at magmatic fabrics formed as or after the space-making phase of pluto n emplacement had ended. The stoped granite blocks are approximately 3 60 m below the exposed pluton roof. Magmatic foliations in granodiorit e surrounding the blocks commonly follow km-scale orientation trends t o within 1 m of block contacts, where they become deflected parallel t o the block boundaries. Locally, undeflected foliations occur within a few centimeters of block contacts. Mafic enclave fabric ellipsoid int ensities (0.2 < Es < 0.7) and shapes (mostly plane to oblate) show lit tle relation to distance from stoped block contacts. Mineral and encla ve linear fabrics vary widely in orientation. In short, magmatic fabri cs in the granodiorite of Castle Creek show virtually no strain record for sinking of these presumably late, stoped blocks. Instead, these f abrics reflect shortening of the surrounding granodiorite magma associ ated with the formation of km-scale fabric trends. We draw the followi ng conclusions concerning magmatic fabrics in the granodiorite of Cast le Creek: (1) magmatic fabrics formed during and/or after the latest s tages of pluton emplacement, and thus, contain little or no informatio n about space-making processes; (2) magmatic fabrics had poor strain m emory; and (3) during magmatic fabric formation, pluton strains were p artially decoupled from host-rock strains. Published data from other p lutons indicate that these relationships may be common, particularly i n the brittle upper crust. It follows from these conclusions that even in the best case, magmatic fabrics contain only qualitative informati on about pluton emplacement mechanisms, regional tectonics, and intern al magma chamber processes. Interpretations of magmatic fabrics must b e corroborated by evaluating causes, kinematics, and magnitudes of syn -emplacement host-rock deformation. In the worst case, the causes of m agmatic fabrics cannot be uniquely determined. (C) 1997 Elsevier Scien ce Ltd.