Silicic dikes in the SMDS range tremendously in texture from glassy aphyric to porphyritic to medium-grained hypabyssal plutonic. Magmas with phenocryst contents more than about 50-60 vol% will have a rheology controlled by deformation rather than magma flow; phenocryst-rich magmas therefore have viscosities too high to be intruded as dikes (Wada, 1994; Kerr & Lister, 1995). This indicates that granitic textures in silicic dikes in the SMDS must reflect in-situ crystallization of magma after dike emplacement. Flow-banding in some rhyolite dikes in the Rancho San Marcos region strongly resemble eutaxitic textures in welded tuffs suggesting that dike emplacment here was shallow enough to allow for violent vesciculation of magma within the dike conduit at present levels of exposure. Local evidence of rheomorphic deformation in flow-banded rhyolite dikes may represent late stage flow features as magma transport comes to a halt in the conduit. Rubin (1995) calculates that dikes can begin to vesciculate at 4 km depth in a 10 m dike supporting these tentative interpretations. Mafic dikes in general have much more uniform aphyric to sparsely porphyritic textures. Chill margins on basaltic-andesite dikes in the San Marcos region are common.

Sample ON99-11. Note the spherulites, readiating intergrowths of quartz or tridymite with orthoclase, sanidine, or sodic plagioclase, and probably formed as the magma was still moving through the dike (Williams et al., 1954).

Sample ON99-02. Note the oscillatory zoned plagioclase crystal, somewhat sericited and equant, indicating episodic growth in a possible magma-mixing environment (Williams et al., 1954). Frames C and D highlight a plagioclase phenocryst with multiple twinning. Although the sample is from a dike, the petrography shows a decidedly extrusive character.