VOLUMINOUS STOPING IN THE MITCHELL PEAK GRANODIORITE, SIERRA NEVADA, CALIFORNIA

The importance of stoping during pluton emplacement remains controversial. This mechanism has recently fallen out of vogue, in favor of other more complex mechanisms, largely due to the apparent lack of preserved stoped blocks. The preservation of stoped blocks in plutons is in fact fortunate, since the rate at which blocks sink is much greater than the rate at which magmas crystallize, and only blocks formed during final crystallization should be trapped within a chamber (Paterson and Okaya, 1999). In reality, there are many examples of plutons that show evidence for stoping, which is not surprising, since stoping should be favored when large thermal gradients at magma-host rock boundaries are present, leading to the thermal- mechanical erosion of host rock (Pignotta et al., 2000). The Mitchell Peak granodiorite, Sierra Nevada, preserves nearly 50% stoped blocks in its youngest intrusive phase, indicating that in this pluton, stoping is an important process. It thus is an excellent locale to further study the effect of stoping on the evolution of magmatic systems. Preliminary work in the Mitchell Peak area has shown a complex pattern of stoping at pluton margins, with abundant evidence for mechanical disaggregation of the host granodiorite, with preserved blocks ranging in size from one hundred meters in diameter down to xenocrystic feldspars. Many stoped blocks have sharp contacts with the surrounding pluton. Others have more diffuse, partially melted contacts with abundant xenocrysts, providing good evidence for mechanisms of contamination. In addition, a variety of magmatic features are concentrated along margins such as mafic enclave swarms, voluminous aplite blocks, and orbicular granodiorite, suggesting that these are areas of repeated injection of hotter mafic magmas and phases with high volatile content. We speculate that this plays an important role in the mechanical and chemical breakdown of host rock margins and of stoped blocks. Thus, stoping may play an important role in the thermal, mechanical, and chemical contamination of magmas in arcs and deserves further attention.