THE ORIGIN AND EVOLUTION OF A CONTINENTAL VOLCANO -- INDEPENDENCE, MONTANA (CONTINENTAL LITHOSPHERE, BEARTOOTH MOUNTAINS, ABSAROKA VOLCANICS).

摘要

Continental volcanoes provide windows to the lithosphere which may preserve evidence of the processes of continent evolution. Continental rocks have proven complex to study and difficult of interpretation. This study concentrated on one volcano--Independence in the Absaroka Mountains, Montana--and attempted to use its igneous record to elucidate the genesis of the volcanic rocks and the composition of the lithosphere in the Beartooth region.;The magma parental to most of the extrusive rocks was a high-magnesium andesite; the parent of the Independence stock was a basaltic andesite. Both these magmas evolved by precipitation of plagioclase-rich, olivine-free, magnetite-bearing assemblages of potassic compositions--probably at moderate P and low P(,H(,2)O). Crystallization at lower pressures or with higher water contents would have produced "calc-alkaline" or "island-arc tholeiitic" rocks.;Crustal contamination-fractional crystallization produced liquid lines of descent with only moderate potash enrichment. Contamination of mafic magmas can occur locally by melting wall rocks around conduits, with small volumes of crust sampled. Minor crustal rock types can thus be important contaminants and can produce unexpectedly large deviations in the isotopic compositions of contaminated rocks.;The mantle-derived rocks have low (('143)Nd/('144)Nd)(,i) and moderate (('87)Sr/('86)Sr)(,i), which are considered to reflect the composition of the sub-Beartooth mantle. The mantle is enriched in LREE, alkaline earths, U, Th, Pb, P. The Pb-isotopic compositions of the high-magnesium andesites suggest that enrichment occurred 3.8 By ago. Those of the basaltic andesites require a 1.9 By source.;Volcanic activity commenced at Independence 92 My ago with intrusion of intermediate-acid sills. This was followed by faulting along the Cooke City Fault Zone and development of the volcano. Volcanism at the cone terminated 84 My B.P. with intrusion of the Independence stock. Following erosion and deposition of volcaniclastic sediments, the area was buried by flood flows in the Eocene.;The Pb-isotopic compositions of the anatectites define a 3.3 By isochron--interpreted as the age of regional homogenization of the deep crust. Some of this crust melted at 2.7 By to give the granites of the upper crust.