Collaborative Research: The Origin and Magmatic Development of the Central San Juan Volcanic Field

在晚近新世（26-28M）不到200万年的时间里，圣胡安火山场中部的一系列巢状破火山口喷发出了8个大体积（100-300 km3）的火山灰流凝灰岩。年)。这些火山灰流和穿插的火山和浅深成岩代表了该地区地壳中一个大型硅质岩浆体系（可能是一个岩基）发展的一系列阶段。已经对两个灰流凝灰岩进行了详细的研究，其他单元的工作正在进行中。通过对斑晶矿物的分析，可以估计岩浆发育过程中的温度、压力和深度，以及水、氧、硫和卤素的逸出度。并进行了数值模拟。在这个地区发现了三种主要的岩浆类型。火山灰流凝灰岩是历史上从未见过的大规模喷发的结果。对这些大体积火山产物的仔细研究将使其与同等规模的深成体系进行比较。安山岩岩浆显然在地球化学和热上都与硅灰流的发展密切相关。这项工作将继续研究这些岩石单元，并将有助于了解爆炸性火山现象、地热能、某些矿床以及更一般地说，在大陆地壳中起作用的过程。

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.