MAGMATIC FABRICS IN PLUTONS: WHAT DO THEY REALLY REPRESENT?

Our studies of plutons indicate that magmatic fabrics: (1) reflect a late increment of strain of crystal-rich mushes; (2) record strain caused by magma flow during emplacement, internal convection, or regional, tectonic strain superposed on a hypersolidus chamber; (3) may have more than one orientation in a pluton and reflect a composite fabric that need not have orthorhombic symmetry; thus indicating caution when using fabric acquisition techniques (e.g., image analyses, AMS), (4) often crosscut internal contacts and thus cannot record magma ascent or chamber shapes, and (5) may preserve a record of the changing strain fields and thus inferred stress fields in ancient arcs. We have documented four distinct magmatic fabrics in the Tuolumne Batholith, central Sierra Nevada, California. One of these fabrics in the Tuolumne Batholith formed as a result of strain caused by highly localized magma flow, whereas the other three fabrics are chamber wide and record strain during batholith construction superimposed by increments of tectonic strain. Thus magmatic fabrics formed by different processes are preserved in this batholith and provide an extensive strain history. In contrast to previous studies that interpret magmatic fabrics in plutons as simple structures formed by a single process, our studies in the TB emphasize that magmatic fabrics: (i) may form complex polyphase structures, recording multiple strain increments caused by different processes; (ii) may form in relatively static (i.e., not flowing) rheologically complex crystal mushes after construction of a magma chamber and thus provide no information about magma flow during emplacement; (iii) may track tectonic paleostrain and thus inferred stress in arcs through time; (iv) may form composite fabrics when different processes result in a single fabric thus making it difficult to interpret which processes formed the magmatic fabric. We also have shown that some portion of melt must have been present over large portions of the Tuolumne Batholith during fabric formation implying that a large magma chamber did exist in the upper crust for a significant period of time (> 1 m.y.).