Virtual Field Trip—Group 1
Features of the Porphyritic Rocks
of the Sierra Blanca Igneous Complex
We are a research and service division of:
Flow-banded microdiorite porphyry dike—Sierra Blanca microdiorite porphyry dike, seven-meters thick.
Shift in flow banded dike—Location: UTM zone 13S, 0431780E/ 3694745N (1927 NAD). Dike with slight offset. Notice the dike has been shifted southwest due to a left oblique normal fault.
Vertical flow foliations—Vertical flow foliations, (subtle, thin white streaks) in a moderately phenocryst rich (large gray crystals) andesite porphyry dike. Flow bands parallel the chilled margins of this nearly vertical NNW trending dike. Approximately 1 meter thick.
Flow bands and phenocrysts—Color bands in the center of the flow banded dike showing moderately abundant phenocrysts (large white crystals).
Microdiorite porphyry dike with elbow—Location: UTM zone 13S, 0431825E/3694740N (1927 NAD). NNW-trending, seven meters thick, microdiorite porphyry dike. Box indicates horizontal elbow where dike steps to the left.
Columnar features of microdiorite dike—Polygonal columnar joints indicate the direction of cooling (nearly horizontal).
Andesite porphyry dikelet—Andesite porphyry dikelet (small dike) 3 meters E of microdiorite dike. Proximity and similarity of texture to andesitic margin of the microdiorite suggests the two are physically continuous at depth. Dikes are younger molten material that pushed into fractures in rock. In this case, older material is much more coarse.
"Cooled" margin between dike and flow—Rapidly cooled (chilled) margin between the two types of porphyritic rock. The different crystal size indicates they cooled differently. They are probably from two different magma sources.
Porphyritic rock (lava, see Group 2)
Igneous rock composed of large crystals in a fine-grained matrix. Large phenocrysts are plagioclase feldspar.