GENERATION OF GRANITIC DIKES: CAN TEXTURE, MINERALOGY, AND GEOCHEMISTRY BE USED AS GUIDES TO DETERMINE THE MECHANISMS OF DIKING?

Textural variation, mineralogy, and relationships of dikes in high-level granitic bodies provide valuable information about late-stage crystallization in plutonic systems. We mapped a dike complex in the Kuna Crest granodiorite (KC) of the Tuolumne Intrusive Suite (TIS), Sierra Nevada Batholith, California; the field area contains a pegmatite plus five related dikes, which range from 6.7-59.5 m in apparent length, <0.5-12.5 cm in width, strike approximately East-West and dip to the South. The pegmatite dike (1 m thick) strikes at a high angle relative to the other dikes and dips 18°-25°S discontinuously exposing both the top and bottom contacts of the pegmatite with KC. Hornblende barometry on five samples from the TIS indicates pressures ranging from 240-290 MPa. Dike 1 originates in the pegmatite in a miarolitic pod (~0.5 m2) of coarse euhedral quartz, plagioclase, and K-feldspar. Dike 1 also exhibits allotriomorphic and graphic textures (suggesting high undercooling) closest to the country rock and farthest from its contact with the miarolitic pod, consistent with the interpretation that the dike originated at a site of volatile accumulation in the pegmatite and propagated toward cold country rock. Dike 2 also originates in a miarolitic pod within the pegmatite; however, it propagates away from the KC/country rock contact, and lacks aplite or graphic texture. It is miarolitic and develops tourmaline ~10 m from the pegmatite. Unlike the others, a fifth dike has poorly defined boundaries, higher biotite concentration, coarse texture, and phenocrysts of quartz ranging from 3-8 mm. It is cut by Dike 4 and is sub-parallel to foliation in the KC (30°-45°E), suggesting this dike is the earliest generation of diking; this is consistent with its granitic texture, but also suggests that generation of quartz phenocrysts represents some degree of undercooling.

We suggest that progressive changes in texture and mineralogy, as well as geochemistry, can be used to ascertain flow directions for magma in dikes, as well as aid in the location of dike entrance regions. Further, studies of this dike complex and others in the TIS show clearly that accumulation of volatiles can give rise to diking, and suggest progressive undercooling yields textures that range from quartz phenocrystic, to pegmatitic, to graphic, to aplitic.