Evolution of the Platoro caldera complex and related volcanic rocks, southeastern San Juan Mountains, Colorado

Abstract

Volcanic rocks in the San Juan Mountains constitute the largest erosional remnant of a once nearly continuous volcanic field that extended over much of the southern Rocky Mountains and adjacent areas in Oligocene and later time. Recent regional studies have shown that the gross petrologic evolution of the San Juan remnant of this field was relatively simple, beginning with intermediate-composition lavas and breccias, followed closely in time by more silicic ash-flow tuff, and ending with a bimodal association of basalt and alkali rhyolite.In the southeastern San Juan Mountains, voluminous early lavas and breccias of the Conejos Formation-mainly alkali andesite, rhyodacite, and mafic quartz latite-were erupted from numerous scattered central volcanoes onto an eroded tectonically stable terrane. These rocks were erupted mostly during the interval 35-30 m.y. (million years) ago, and they volumetrically represent about two-thirds of the volcanism.About 30 m.y. ago, major volcanic activity changed to explosive ashflow eruptions of quartz latite and low-silica rhyolite that persisted until about 26 m.y. ago. Source areas for the ash flows are marked by large calderas in the central San Juan Mountains. Among the earliest and most southeastern of these is the Platoro caldera complex, a composite collapse structure about 20 kilometers in diameter that formed as a result of ash-flow eruption of the 29-to 30-m.y.-old Treasure Mountain Tuff. Three major ash-flow sheets of the Treasure Mountain Tuff-the La Jara Canyon, Ojito Creek, and Ra Jadero Members in ascending order-are approximately coextensive and cover about 5,000 square kilometers in the southeastern San Juan Mountains. Major collapse of the Platoro caldera occurred during eruption of phenocrystrich quartz latitic ash flows that form the La Jara Canyon Member, and late ash flows of this member accumulated within the collapsing caldera to a thickness of more than 800 meters. The core of the collapsed block was resurgently uplifted to form an unbroken homoclinally tilted block, in contrast to the fractured domical uplifts that characterize other known resurgent calderas; and the marginal moat was filled by as much as 1 kilometer of lavas and interbedded volcaniclastic sedimentary rocks of the lower member of the Summitville Andesite (name reinstated in this report).Renewed ash-flow eruption of the quartz latitic Ojito Creek and Ra Jadero Members resulted in further collapse to form the Summitville caldera within the northwest part of the Platoro caldera. No resurgence is associated with this late caldera, but it was also filled to overflowing by a thick accumulation of upper member lavas of the Summitville Andesite. 2PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO caldera comple~, as well as similar ash-flow assemblages elsewhere in the San Juan field, contrast with other continental-interior ash-flow associations which are predominantly rhyolitic in composition. Close spatial and temporal association of the quartz latitic ash-flow tuffs and the voluminous andes...

Mount Belknap and Red Hills calderas and associated rocks, Marysvale volcanic field, west-central Utah

“…Whereas the general model of ash-flow eruption and caldera development (Ratte and Steven, 1964;Smith and Bailey, 1968;Lipman, 1975;Steven and Lipman, 1976;Bailey and others, 1976) was applied to studies of both areas, the results were quite different. Minor differences in timing and conditions of eruption led to strongly contrasting volcanic accumulations and structures; depth and configuration of the source magma chambers may have had a major controlling influence.…”

Mineral resource potential of National Forest RARE II and wilderness areas in Colorado

“…Adjacent to, and partly within the southwestern part of the study area, is the Jasper district, where there are several small mines and prospect pits in gold, silver, lead, and zinc-bearing veins, though it is not known what production occurred there . All of these altered and mineralized regions occur at intersections between the northwesttrending Pass Creek fault zone and ring fracture zones of the Platoro and Summitville calderas (Lipman, 1975). A widespread zone of sporadic alteration and pyritization not associated with this structural setting occurs with the Cat Creek stock in the southern part of the study area.…”

“…A widespread zone of sporadic alteration and pyritization not associated with this structural setting occurs with the Cat Creek stock in the southern part of the study area. There are numerous prospect pits and adits here, but production is not known (Lipman, 1975). There are no other areas of mineralization or alteration within the study area.…”

Contact zones and hydrothermal systems as analogues to repository conditions

“…The Alamosa River stock, of monzonitic composition, was emplaced at 29.1 ± 1.2 MYBp into predominantly andesitic volcanics forming and filling caldera structures (Lipman, 1975). The La jara is predominantly a densely welded and devitrified ash flow tuff primarily of phenocryst rich quartz latite, ejected by an explosive event at 29.8 ±1.2 MYBP.…”

“…Based on analyses of oxygen isotope ratios in 260 minerals and whole rock samples from the Alamosa River stock region, Williams (1980) suggested that the heat from the stock and other small intrusions, which range in age from 28 to 22.8 MY (Lipman, 1975) , Initiated extensive hydrothermal circulation. A roughly concentric aureole of S 18g depletion was noted around the south and east margins of the stock, with depletion enhanced along two major faults, attesting to the importance of fracture permeabil ity in the circulation of this system.…”