Formation and transfer of stoped blocks into magma chambers: the high temperature interplay between focused porous flow, cracking, channel flow, and host rock anisotropy

Magmatic stoping, i.e., the formation, transfer into, and movement through magma of older plutonic and metamorphic host-rock xenoliths, was widespread in the Mesozoic Sierra Nevada batholith (California, United States). However, the prevailing view that stoped blocks form by rapid thermal shattering and collapse into chambers may not be the dominant process of block formation and displacement into chambers in the Sierra Nevada. In detailed studies in and around the Tuolumne Batholith and Jackass Lakes pluton, we found evidence for the following history of block formation in slightly older, fairly isotropic plutonic host rocks: (1) low stress sites developed, leading to planar zones of increased porosity; (2) focused porous flow of first felsic melts followed by intermediate melts led to growth of magma fingers, which in turn led to increased porosity and loss of host-rock cohesion; and (3) connection of magmatic fingers resulted in the formation of dike-like channels in which flow facilitated removal of all host-rock material in these planar zones. Once formed, blocks
were initially displaced by repeated magma injections along these channels, often resulting in uni directional growth in these zones creating local magmatic sheeted complexes
along block margins. Free block rotation occurred when suffi cient nonlayered magma surrounded the host block; in some cases, segments of former sheeted zones remain
attached to rotated blocks.
In anisotropic metamorphic host rocks, focused porous fl ow may have locally played a role, but the dominant processes during initial
block formation were cracking, parallel and at high angles to anisotropy, and intrusion of magma by channel fl ow. Subsequent initial block displacement and eventual rotation
are identical to those in the nearly iso tropic host rock. The driving forces for the development of low-stress sites, cracking, dilation, and magma flow remain uncertain, but likely
refl ect the interplay between regional stress, magma buoyancy stresses, thermal gradients, and host-rock properties, and not simply rapid heating and thermal expansion cracking.
Thus a number of processes may drive block formation, some of which are rapid (thermal shattering, roof collapse) whereas others occur over longer durations (incremental
magma pulsing and formation of sheeted complexes, regional deformation).