Stoped blocks in plutons: paleo-plumb bobs, viscometers, or chronometers?

Abstract

Stoped blocks potentially provide information about paleohorizontal during emplacement of plutons, and about the timing of, and viscosities and kinematics during magmatic fabric formation. In the Mount Stuart batholith, Washington, we have examined magmatic fabrics patterns around stoped blocks in tonalite located near the pluton roof and completed 1:1 scale, three-dimensional mapping of fabric patterns around stoped blocks in diorite located ∼1000 m below the roof. In both settings, no structural record of magma ascent, early emplacement, or settling of the blocks is preserved. Instead, fabric formation largely post-dates the settling and trapping of stoped blocks. Two-dimensional finite difference thermal modeling and calculations of sinking rates support short (<2 ×10 3 years) residence times in the magma chamber for these blocks. Relatively symmetric foliation deflections around blocks indicate largely coaxial flattening during fabric formation and that magma viscosity must be lower than the viscosity of the weakest block (metapsammite) but high enough to trap the most dense block (dunite). Block settling calculations indicate that effective magma viscosities were from 10 14 to 10 15 Pa s and/or that yield strengths were large, which suggests that preserved fabrics formed in crystal-rich magmas near the tonalite and diorite solidi. Thus in the Mount Stuart batholith, stoped blocks fail to record information about paleohorizontal, but do indicate that the magmatic fabrics formed after chamber construction, during coaxial strain, and in crystal-rich magma near its solidus.