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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Rhyodacites of Kulshan caldera, North Cascades of Washington: Postcaldera lavas that span the Jaramillo
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Rhyodacites of Kulshan caldera, North Cascades of Washington: Postcaldera lavas that span the Jaramillo

机译：华盛顿北部小瀑布库尔山破火山口的假丝酵母：横跨Jaramillo的Postcaldera熔岩

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摘要

Kulshan caldera (4.5x8 km), at the northeast foot of Mount Baker, is filled with rhyodacite ignimbrite (1.15 Ma) and postcaldera lavas and is only the third Quaternary caldera identified in the Cascade arc. A gravity traverse across the caldera yields a steep-sided, symmetrical, complete Bouguer anomaly of ―16 mGal centered over the caldera. Density considerations suggest that the caldera fill, which is incised to an observed thickness of 1 km, may be about 1.5 km thick and is flat-floored, overlying a cylindrical piston of subsided metamorphic rocks. Outflow sheets have been stripped by advances of the Cordilleran Ice Sheet, but the climactic fallout (Lake Tapps tephra) is as thick as 30 cm some 200 km south of the caldera. Ten precaldera units, which range in ~(40)Ar/~(39)Ar age from 1.29 to 1.15 Ma, are dikes and erosional scraps that probably never amounted to a large edifice. A dozen postcaldera rhyodacite lavas and dikes range in age from 1.15 to 0.99 Ma; rhyodacites have subsequently been absent, the silicic reservoir having finally crystallized. At least 60 early Pleistocene intermediate dikes next intruded the caldera fill, helping energize an acid-sulfate hydrothermal system and constituting the main surviving record of an early postcaldera andesite-dacite pile presumed to have been large. Most of the pre- and postcaldera rhyodacites were dated by ~(40)Ar/~(39)Ar or K-Ar methods, and 13 were drilled for remanent magnetic directions. In agreement with the radiometric ages, the paleomagnetic data indicate that eruptions took place before, during, and after the Jaramillo Normal Polarity Subchron, and that one rhyodacite with transitional polarity may represent the termination of the Jaramillo. Most of the biotite-hornblende-orthopyroxene-plagioclase rhyodacite lavas, dikes, and tuffs are in the range 68-73% SiO_2, but there were large compositional fluctuations during the 300-kyr duration of the rhyodacite episode. The rhyodacitic magma reservoir was wider (11 km) than the caldera that collapsed into it (8 km).
机译：位于贝克山东北脚的库尔山破火山口（4.5x8公里）充满了流纹辉石火成岩（1.15毫安）和破火山口熔岩，仅是喀斯喀特弧中发现的第三纪第四纪破火山口。横跨火山口的重力运动会产生一个陡峭的，对称的，完整的布格异常，其中心位于火山口上方，约为16 mGal。出于密度方面的考虑，火山口填充物被切成观察到的1 km的厚度，厚度可能约为1.5 km，并且是平底的，覆盖了下沉变质岩的圆柱形活塞。山脉冰盖的发展已经清除了流出的表层，但在火山口以南约200公里处，高空沉降物（Tephra湖）却高达30厘米。十个破火山口单位，在〜（40）Ar /〜（39）Ar年龄范围在1.29至1.15 Ma之间，是堤防和侵蚀性废料，可能从未构成大建筑物。十二种火山口流纹岩熔岩和堤坝的年龄范围为1.15至0.99 Ma；随后不存在流纹岩，硅质储层最终结晶。接下来至少有60个更新世早期的中间堤防侵入了火山口填充物，有助于向酸性硫酸盐热液系统供能，并构成了早期火山灰后安山岩-ac石堆的主要存活记录。破火山口前后的大多数流苏都通过〜（40）Ar /〜（39）Ar或K-Ar方法确定日期，并为剩余的磁方向钻了13个。与辐射年龄一致，古磁数据表明火山喷发发生在Jaramillo正极性次同步子之前，之中和之后，并且具有过渡极性的流纹岩可能代表了Jaramillo的终止。黑云母-角闪石-邻苯二酚-斜长石流纹岩熔岩，堤坝和凝灰岩的SiO_2范围在68-73％之间，但在流纹岩发作的300年期间，成分波动较大。流纹岩浆岩储层比塌陷的火山口（8公里）更宽（11公里）。

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