Collaborative Research: The Origin and Development of Silicic Magmas-Central San Juan Region, Colorado

在晚渐新世(26-28m)不到200万年的时间里，圣胡安火山场中部的一系列嵌套破火山口喷发了8个大体积(100-300 km~3)的火山灰流凝灰岩。年)。这些火山灰流和夹层的火山岩和浅层深成岩代表了该地区地壳中大型硅质岩浆系统(可能是岩基)发展的一系列阶段。已经对两个火山灰流凝灰岩进行了详细研究，其他装置的工作正在进行中。通过对斑晶矿物的分析，已经有可能估计出正在形成的岩浆中的温度、压力和深度，以及水、氧、硫和卤素的逸度。还进行了数值模拟。在该地区似乎发现了三种主要的岩浆类型。火山灰流凝灰岩是由史无前例的大规模喷发产生的。仔细研究这些大体积的火山产物将使我们能够与可压缩的深成系统进行比较。安山质岩浆显然与硅质火山灰流的发展在地球化学和热学上都有密切的联系。这项工作将继续研究这些岩石单位，并将有助于了解火山爆发现象、地热能、某些矿床以及更广泛的陆壳活动过程。M*//

Eight large volume (100-300 km3) ash-flow tuffs wee erupted from a series of nested calderas in the central San Juan Volcanic Field during a period of less than 2 million years during the late Oligocene (26-28M. years). These ash flows and the intercalated volcanic and shallow plutonic rocks represent a series of stages in the development of a large silicic magmatic system, perhaps a batholith, in the crust of this region. Two ash-flow tuffs have been studied in detail and work on the other units is in progress. It has been possible to estimate temperature, pressure, and depth, as well as water, oxygen, sulfur, and halogen fugacities in the developing magmas using analyses of the phenocryst minerals. Numerical modeling has also been carried out. There appear to be three major magma types found in this region. The ash-flow tuffs were produced as a result of giant eruptions of a magnitude not observed in historic times. A careful study of these large volume volcanic products will enable comparison with plutonic systems of comperable size. The andesitic magmas apparently are closely connected both geochemically and thermally with the development of the silicic ash flows This work will continue the study of these rock units, and should contribute to the knowledge of explosive volcanic phenomena, geothermal energy, certain ore deposits and, more generally, processes operating in the continental crust. m***//