Date of Award
Master of Science (MS)
The middle to late Tertiary Black Face-Ames plutons are irregularly-shaped, intrusive bodies located in the San Miguel Mountains of southwestern Colorado. The Ames pluton is generally a composite sill; the Black Face pluton is an asymmetric laccol1th. The two plutons are probably joined at depth, and together they form a crudely annular outcrop pattern, open toward the west. The principal rock type within the plutons is granodiorite porphyry, although part of the Ames pluton consists of porphyritic rhyodacite.
Two major zones of xenoliths of Precambrian rock occur within the Black Face-Ames plutons; scattered xenoliths are found at widely separated locations. One zone, located near the summit of Black Face, contains xenoliths of shale, quartzite, and granitic rocks, ranging in diameter from 2 cm to 5.4 m. The other zone is exposed near Ames, where xenoliths of amphibole-pyroxene rocks, mafic schists, and occasional granites occur, ranging in diameter from 2 cm to J.6 m.
Measurements near Ames indicate that the xenoliths of high specific gravity have tended to sink to the lower parts of the granodiorite porphyry sill, whereas the lighter granitic rocks have generally remained in the middle portions. In contrast, the xenoliths on Black Face, which are generally of lower specific gravity than the host granodior1te porphyry, have apparently floated to the top of the magma. chamber.
Xenolith orientation measurements near Ames suggest imbricate structure, which may indicate the direction of magmatic flow.
Microscopic study of contact zones between xenoliths and host rocks indicates that granitic xenoliths have reacted with their hosts to a greater degree than have xenoliths of other rock types, In the contact zone, pyroxene in both maficxenoliths and in the granodiorite porphyry is usually converted to amphibole and amphibole is generally altered to chlorite. Concentration of mafic minerals at the contact is common,
The presence of a small mafic-rich intrusive cutting the granodiorite porphyry near the summit of Black Face, as well as the zone of concentrated xenoliths in this area, suggest that a zone of weakness existed, through which first the xenolith-bearing granodiorite porphyry and later the mafic-rich plug intruded,
The zone of xenoliths and Ophir stock-Ames pluton con tact phenomena near Ames is located near the intersection of two major structures: the northeast-trending monoclinal flexure associated with the San Juan domal uplift during Laramide time, and a west-trending fracture radiating from the middle to late Tertiary Silverton caldera. The concentration of xenoliths in this area·1s believed to be a prod uct of this apparent conduit from the Precambrian basement.
Moore, Richard B., "Petrography of xenolith zones in the Black Face-Ames plutons, western San Juan Mountains, Colorado" (1970). Theses and Dissertations. 203.