Abstract

A Precambrian complex of granulite, gneiss, and migmatite, intruded by numerous plutons of granitic rocks correlated with the Silver Plume granite, is exposed in a long narrow belt along the crest and upper slopes of the Tenmile Range, Colorado. The metamorphic rocks are predominantly felsic; bands, lenses, and irregular bodies of mafic rocks rich in biotite, hornblende, and locally in sillimanite and garnet, are interlayered with the felsic rocks. The major lithologic variations in the metamorphic rock complex are believed to be due chiefly to variations in the original sedimentary rocks, which probably were interbedded sandstone, shale, and limestone.

The metamorphic rocks and the Silver Plume granite reveal the age relations of quartz and the feldspars, and these relations afford considerable information on the origin and progressive transformation of the rocks. Quartz is the earliest mineral in the metamorphic rocks and is probably a relict mineral of a sandstone. It has been partially replaced by feldspar. It occurs chiefly in irregular clusters, some of which show sutured grains, enclosed in a ramifying network of feldspar. Irregular small apophyses, barbs, and prongs of feldspar penetrate the quartz clusters along grain boundaries and healed fractures in the quartz. In some of the least feldspathized quartzose metamorphic rocks the feldspar is clearly interstitial to the quartz. Quartz also occurs in feldspar as small spherical inclusions. The relations of the quartz to the feldspars show clearly that a quartzose host rock was replaced by feldspar along quartz grain boundaries, pre-existing healed fractures, and margins of shadowy areas in strained quartz grains.

The textural relations of the other principal minerals in the metamorphic rocks show that plagioclase formed earlier than the microcline and that the micas were the last of the principal minerals to form.

Identical paragenetic relations are found in the Silver Plume granite, and the writer concludes that the Silver Plume granite was derived by partial fusion of quartzose metamorphic rocks.

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