| Text Description: |
Gabbro from the sea cliff 3 miles north-northeast of the southern tip of Cape Aiak. The texture is porphyritic and hypidiomorphic, and the mafic minerals are locally aggregated. Felsic minerals range in abundance from 50 to 70 percent, of which about 1 percent is orthoclase and the rest is plagioclase replaced by 10-15 percent albite and minor sericite. The unaltered plagioclase composition, by extinction angle, is An78 [mean of 26 crystals, standard deviation is 11 percent An, average amount of zoning per individual plagioclase crystal is 3 percent An per crystal] ; by index oils, is An68. Mafic minerals make up 30-50 percent, of which 25-30 percent is stubby euhedral phenocrysts of clinopyroxene, 2-3 percent clinopyroxene or uralite needles (length-to-width ratio is 20:1) grown on the margins of the large pyroxenes rather than altered from them, 0-1 percent biotite(?) now completely altered to antigorite, 8-10 percent magnetite, <1 percent is apatite, and <1 percent serpentine. Discrepancy in plagioclase compositions (above) is probably due to two causes. (1) Low temperature curves were used; using the same angles, high temperature curves give average plagioclase as An67. (2) Alteration to albite lowers normative plagioclase. Presence of 7.10 percent olivine and 0.85 percent nepheline in the norm is unexplained, but other specimens from the Cape Aiak sill contain abundant zeolites through out the groundmass, and if the analyzed sample contained a pocket of these the total rock might be undersaturated in silica. |
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