FORMATION AND TRANSFER OF STOPED BLOCKS INTO MAGMA CHAMBERS: THE HIGH TEMPERATURE INTERPLAY BETWEEN CRACKING, FOCUSED POROUS FLOW, AND UNIDIRECTIONAL GROWTH OF SHEETED COMPLEXES

Magmatic stoping, the formation, transfer into, and movement through magma chambers of host rock xenoliths, is widespread in the Sierra Nevada batholith, California. However, the common view that blocks form by thermal shattering and rapid collapse into chambers may not be the most common process of block formation and transfer. In detailed studies around the Tuolumne batholith, we see evidence for the following history of block formation in older, fairly isotropic plutonic units: (1) low stress sites develop leading to planar zones of increased porosity; (2) focused porous flow of first siliceous, volatile-rich? melts, then by more mafic melts along the low stress sites, presumably leading to increased porosity and loss of host rock cohesion; and (3) channel flow and formation of dike-like bodies resulting in loss of all host rock material in these planar zones. Once formed blocks are initially displaced by repeated magma injections along these zones resulting in sheeted complexes, often with magma erosion and redeposition in, and unidirectional growth directions of the sheeted complexes. Free block rotation occurs when sufficient non-layered magma surrounds the host block, although we find examples where segments of former sheeted zones remain attached to rotated blocks.

In more anisotropic metamorphic host rocks, cracking parallel and at high angles to the anisotropy and intrusion of magma sheets dominate over focused porous flow during initial block formation. But the other processes of block formation, initial displacement, and eventual rotation are identical. Typically the principal direction of host rock dilation is towards the magma chamber. But the driving force(s) for this dilation, the development of low stress sites, and the cracking remain uncertain but almost certainly reflect the interplay between regional stress, magma buoyancy stresses, thermal gradients, and host rock properties and not just thermal heating.