Abstract
The Modoc Lava-Bed quadrangle includes an area in which is seen the transition from the Cascade Mountain province to the Great Basin plateau province, with mountains of volcanic accumulation (dissected and undissected) characteristic of the former, and fault-block mountains and high plateaus, typical of the latter.
The oldest volcanic rocks, the Cedarville andesite of Miocene age, are a series of pyroclastic formations, with a few interbedded flows, chiefly andesitic. After warping and the development of fault blocks, renewed volcanic activity built several volcanic cones from massive lava flows, chiefly andesitic, called the Massive Lava Group These eruptions are correlated with the major Cascade activity of Pliocene age.
Lake beds (Lacustrine group) and widespread flows of olivine basalt (Warner basalt) were deposited in the grabens In late Pleistocene time, the lavas of the Platy Andesite Group were erupted from a number of vents located on a fracture zone which encircled the top of one of the Pliocene volcanoes New fault-grabens were formed and further lacustrine deposition took place. In post-Glacial time the Modoc basalts were erupted from a number of parasitic vents on the north and south flanks of the Medicine Lake Highland, the last flow is probably less than 500 years old. During this basaltic activity, several small eruptions of dacite and rhyo-lite took place on the top of the Medicine Lake Highland; the youngest rhyolite is probably less than 300 years old.
Many of the basalts of the area are abnormally rich in olivine and calcic plagi-oclase, thus resembling the Porphyritic Central Magma Type of Mull; but they were, in contrast, completely liquid at the time of extrusion.
In attempting to explain the difference in composition between several pairs of associated lavas by fractional crystallization, it was found that the residual liquid after partial crystallization of the less siliceous magma was richer in iron than the more siliceous rock of the pair.
In a partly glassy basalt the pyroxene and iron oxide had not begun to crystallize though over half of the plagioclase had formed crystals. The texture of the basalts suggests that pigeonite is a late-forming mineral in less siliceous basalts which crystallize with ophitic or sub-ophitic texture; and hypersthene and augite are early-forming minerals in more siliceous basalts and andesites which crystallize with intergranular texture.
The abnormally high lime content of the plagioclase phenocrysts of the Lake basalt may be explained with no less difficulty by assuming that the composition of the liquid surrounding the phenocrysts was changed, than by assuming movement of the crystals from one liquid to another.
Strange globular bodies, found in the vesicular phase of both dacite and rhyolite flows, may represent a liquid fraction, high in volatile constituents, and with the silicate composition approaching Vogt's anchi-eutectic granite, which separated as an immiscible liquid from the dryer Iava of the flows.
