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

八个大体积（100-300 km 3）的灰流凝灰岩从 位于圣胡安火山中部的一系列火山口 在不到200万年的时间里， 晚渐新世（26- 28 M.年）。 这些火山灰流 夹层火山岩和浅成深成岩代表了 一个大型岩浆岩发展的一系列阶段 系统，也许是一个岩基，在这个地区的地壳。 两 对灰流凝灰岩进行了详细的研究， 单位正在进行中。 我们可以估计 温度、压力和深度，以及水、氧气， 硫和卤素的逸度在发展中的岩浆使用 斑晶矿物分析。 数值模拟还 被执行。似乎有三种主要的岩浆类型 在这个地区发现的。 火山灰流凝灰岩是作为一种 这是历史上从未观察到的巨大火山爆发的结果。 次 仔细研究这些大量的火山产物 将能够与可比规模的深成系统进行比较。 安山质岩浆显然与 地球化学和热演化 灰流这项工作将继续研究这些岩石单元， 应该有助于了解火山爆发 现象、地热能、某些矿床等等 一般来说，在大陆地壳中运作的过程。 * //

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. m***//