| Text Description: |
Leuco albite quartz monzonite from an irregular aplitic body at the margin of the 3600 Mountain roof pendant, 0.7 mile southwest from the summit of the 3,600-foot peak for which the pendant is named. The texture is allotriomorphic-granular, locally subgraphic, locally pseudoperthitic. The specimen contains 94-98 percent felsic minerals: 20-24 percent quartz, 25-34 percent orthoclase slightly altered to kaolin, and 40-49 percent plagioclase (only one-sixth of which is twinned) slightly altered to sericite. The plagioclase composition, by extinction angle, is An7, [mean of 12 crystals, standard deviation is 3 percent An, average amount of zoning per individual plagioclase crystal is 0 percent An per crystal]. There are 2-6 percent mafic minerals of which 2-6 percent is biotite (?) completely altered to magnetite (0.5-1.5 percent), penninite (2-3 percent), epidote ( <0.1 percent), and sphene (0.1-1 percent), and <0.3 percent is allanite associated with minor epidote. The discrepancy in the amounts of normative and modal quartz may be due to unrecognized fine-grained quartz in the groundmass, or perhaps some of the quartz was mistakenly identified with the abundant untwinned plagioclase. |
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