2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 9
Presentation Time: 8:00 AM-12:00 PM


MEMETI, Vali, Department of Earth and Planetary Sciences, Washington University in St. Louis, 1 Brookings Dr, St. Louis, MO 63130-4862 and PATERSON, Scott, Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, memeti@usc.edu

Microgranitoid enclaves are common in granitoids and have various shapes, sizes and compositions. They are generally thought to form by mingling and disaggregation of more mafic magmas in dikes, sheets or blobs intruding into chambers. Less well understood is whether there are other mechanisms involved in their formation, and how enclaves are both widely dispersed and locally accumulated in magma chambers. In the 95-85 Ma Tuolumne batholith, Sierra Nevada, California, enclaves show large regions of dispersion and local areas of accumulation. Rare examples of enclave formation are also preserved. We examined enclaves in the geologically simpler, ~40 km2 Kuna Crest lobe that formed and crystallized within ~1 m.y. and is characterized by a normally zoned pattern of compositionally and texturally differing granodiorites separated by gradational contacts. This lobe contains a moderate amount of distributed enclaves and local enclave accumulations. Locally, it also preserved dm to m scale mafic sheets and zones of enclave formation. We see a number of processes that form enclaves and affect their distribution: (1) mingling of mafic sheets or blobs with more felsic magmas, sometimes resulting in elongate shapes controlled by the shape of the mafic intrusion, although, a few dioritic bodies froze without disintegration; (2) examples where deformation such as magmatic boudinage, folding, and shearing plays an important role in the formation and break-up of enclaves; (3) examples of older KC phases getting mingled with younger – thus forming enclaves that are cognate inclusions; (4) wide dispersion of enclaves by presumably broad flow of magma and accumulations in local flow channels; (5) cases where enclave accumulations formed earlier and are now in blocks getting recycled into younger magmas. These enclave-rich blocks often have fairly rectangular shapes, sharp, intrusive margins, sometimes even cutting through a single enclave, and have a plutonic matrix enclosing enclaves in the block that is distinctly different than the plutonic matrix around the block. Thus even in the shorter-lived lobes of the Tuolumne batholith, we find a number of processes that form enclaves and influence their shapes and distribution. Both, widespread enclave dispersion and local accumulation require regional and localized magma flow.