THE ROLE OF STOPING IN THE EVOLUTION OF MAGMATIC SYSTEMS: FIELD AND MODELING CONSTRAINTS FROM PLUTONS IN THE SIERRA NEVADA BATHOLITH, CALIFORNIA

The role of stoping during pluton emplacement and crustal evolution 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. We have examined many plutons in the Cordillera (from Alaska to Argentina), the Appalachians and the French Variscans that show evidence for stoping. In reality, many more plutons show evidence for stoping, which should be favored when large thermal gradients exist at magma-host rock boundaries. Preservation of stoped blocks however, is unusual, 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. Furthermore, field and petrographic evidence along with thermal-mechanical models suggest that mechanical disintegration of blocks is much more rapid than chemical breakdown (melting). 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. Work in the area has shown a complex pattern of stoping at pluton margins, preserved stoped blocks throughout the pluton, and excellent evidence for mechanical disintegration of blocks. Preserved blocks range in size from hundreds of meters in diameter down to xenocrystic feldspars. Many have sharp contacts with the surrounding pluton, while some have more diffuse, partially melted contacts with preserved xenocrysts. This field evidence supports both thermal and chemical contamination of plutons via mechanical disintegration of blocks. Also, a variety of magmatic features are concentrated along margins such as mafic enclave swarms, schlieren layers, voluminous aplite blocks, and orbicular granodiorite, indicating that these are areas of repeated injection of hotter mafic magmas and other magmatic phases. Thermal-mechanical models, using the Mitchell Peak area as a case study, indicate that thermal-mechanical breakdown of blocks is an extremely efficient way to reduce block size and contaminate magmas compared to chemical contamination. Stoping may be a rapid and effective means to thermally, mechanically, and chemically contaminate magmas in arcs and deserves further consideration.