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

Abstract

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 fl ow of fi rst felsic melts followed by intermediate melts led to growth of magma fi ngers, which in turn led to increased porosity and loss of host-rock cohesion; and (3) connection of mag matic fi ngers resulted in the formation of dike-like channels in which fl ow 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 fl ow 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).

The Late Mesoproterozoic Sirdal Magmatic Belt, SW Norway: Relationships between magmatism and metamorphism and implications for Sveconorwegian orogenesis

“…The significance of these zones is unclear, but we speculate that they may represent contact zones between different intrusions/plutons, i.e., are similar to 'screens' of wall-rock described in more recent Cordilleran batholiths such as Sierra Nevada (e.g. Bateman, 1983;Bartley et al, 2006;Paterson et al, 2012). Individual screens can be hundreds of meters long and several meters wide and typically dip shallowly (30-20 • ) east-or westwards.…”

Erupted frothy xenoliths may explain lack of country-rock fragments in plutons

“…ref. 9). Our models simulate the effect of a mild density decrease on the dynamics of xenolith sinking during melting.…”

“…In addition, processes such as xenolith fragmentation and expansion due to thermal stresses and volatile dissolution (cf. refs 9, 17, 35 and 41) would aid and accelerate xenolith melting. Although our models do not consider these processes and only take a mild decrease in density into account, large effects for the xenolith can be anticipated, i.e.…”

“…thermal stresses (e.g. refs 1, 9 and 10). Hence, magmatic stoping should occur preferentially in the upper crust where temperature differences between the country rock and magma are largest5.…”

Structure, Construction, and Emplacement of the Late Cretaceous Sonora Pass Intrusive Suite

“…Stoping operates as a material transfer process during the emplacement of many plutons (Hutton, 1982;Paterson and Fowler, 1993;McNulty et al, 1996;Paterson et al, 1996;Clarke et al, 1998;Yoshinobu et al, 2003;Paterson et al, 2012). It is likely restricted, however, in the volume of material transferred (Glazner and Bartley, 2006).…”