| Text Description: |
Granite, the matrix of a "plum pudding" rock, from the shore bench one-half mile southeast of the tip of Kof Point. The texture is hypidiomorphic, micrographic; the original rock contains many inclusions but this analysis is of the matrix only. Felsic minerals compose 75-80 percent of the rock: 5-7 percent quartz occurring in myrmekitic to micrographic intergrowths with much alkalic feldspar, 45-55 percent alkalic feldspar partly altered to kaolin (5-7 percent), and 15-25 percent plagioclase partly altered to sericite ( 2-4 percent), kaolin (2-5 percent), albite (lumped with alkalic feldspar) and epidote (1-2 percent). The plagioclase composition, by extinction angle, is An41 [mean of 10 crystals, standard deviation is 16 percent An, average amount of zoning per individual plagioclase crystal is 13 percent An per crystal], by index oils, is An37. Mafic minerals compose 20-25 percent: 12-15 percent clinopyroxene mostly replaced by actinolitic hornblende (10-13 percent), sphene (< 1 percent), and magnetite (< 1 percent), 5-7 percent biotite completely replaced in most areas by epidote (< 1 percent), penninite ( 3-4 percent), titanite (0.5-1.5 percent) and magnetite-ilmenite (0.5-1.5 percent), 2-4 percent magnetite-ilmenite, < 1 percent apatite, < 0.1 percent epidote, sphene, penninite, and a fibrous amphibole that fills tiny holes in the groundmass. The discrepancy in the plagioclase composition between the mode and the norm is probably due to the presence of secondary albite in the rock. Also, the alkalic feldspar probably contains large quantities of both orthoclase and albite molecules |
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