FORM AND STRUCTURE OF SILLS NEAR PANDO, COLORADO
Four varieties of early Tertiary quartz monzonite and quartz latite porphyries: the Elk Mountain, Lincoln, and Eagle River porphyries, and one here named the Pando porphyry, form numerous sills in the Pennsylvanian and Permian(?) clastic rocks of the southern part of the Gore Range, Colorado. The major sills are 50–400 feet thick and 1–10 miles across. The sills fall into two groups on the basis of form and structure. Sills of the Pando and Elk Mountain porphyries, the more widespread and uniform although they show local irregularities in form and are locally discordant, are in general tabular in cross section and irregular in ground plan. Near the contacts, the porphyry of the sills shows folds, intrusive-stage faults, platy parting produced by differential movement between thin laminar plates of magma during intrusion, and linear mineral orientation at right angles to the orientation in the inner part of the sill and to the direction of magma flow. Sills of the Lincoln and Eagle River porphyries are less extensive and more irregular in shape. They do not show intrusive-stage folds and faults except locally and poorly, and their linear mineral orientation is normally uniform from contact to contact and parallel to the direction of magma flow. The structural features of the sills of Pando and Elk Mountain porphyry suggest high magmatic viscosity; absence of these structures in sills of Lincoln and Eagle River porphyries suggests that these were more fluid magmas, a conclusion that is supported by the difference in contact metamorphism produced by sills of the two groups. Contrary to what should be expected, the most uniform and widespread sills thus seem to have been formed by viscous magmas, and the most irregular sills, some of which grade into laccolithic bodies, by relatively fluid magma. Emplacement of the viscous Pando and Elk Mountain magmas in uniform and widespread sills may have been aided by the lifting action of laterally compressed competent beds. The Lincoln and Eagle River sill magmas apparently were capable of making room for themselves by lifting the load; in doing so they compacted the host rocks, which being lenticular yielded unevenly, causing smallscale knobs, domes, swellings and constrictions to form in the sills. Large-scale or laccolithic doming occurred near the supply vent, probably as a result of rapid introduction of magma.