New 40Ar/39Ar age determinations and paleomagnetic results bearing on the tectonic and magmatic history of the northern Madison Range and Madison Valley region, southwestern Montana, U.S.A.
New 40Ar/39Ar age determinations and paleomagnetic results bearing on the tectonic and magmatic history of the northern Madison Range and Madison Valley region, southwestern Montana, U.S.A.
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新的 40Ar/39Ar 年龄测定和古地磁结果与美国蒙大拿州西南部麦迪逊山脉北部和麦迪逊谷地区的构造和岩浆历史有关。
DOI：
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发表时间：
2007
期刊：
影响因子：
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通讯作者：
S. Harlan
中科院分区：
文献类型：
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作者：
K. Kellogg;S. Harlan
对侵入蒙大拿州西南部麦迪逊山脉北部白垩纪沉积岩的英安斑岩岩床和岩脉进行的详细的\(^{40}Ar/^{39}Ar\)定年和古地磁分析表明，拉腊米造山运动的缩短作用在约6900万年前基本完成。负的古地磁褶皱检验表明，拉腊米褶皱作用在英安岩床和岩脉于约6900万年前冷却之前就已发生。拉腊米变形始于约7900万年前利文斯顿组岩石沉积的同时。这些结果与该地区先前的观测结果一致，即显示落基山脉北部拉腊米变形的起始时间向东逐渐变年轻。对麦迪逊山谷北部以及蒙大拿州诺里斯周围的其他火成岩进行的\(^{40}Ar/^{39}Ar\)定年更好地确定了该地区拉腊米运动之后的构造岩浆事件，包括始新世 - 渐新世火山活动以及盆岭区地壳伸展。红山附近的三个流纹岩侵入体的年龄在\(48.71\pm0.18\)百万年到\(49.42\pm0.18\)百万年之间，与弗吉尼亚城火山区（蒙大拿州西南部火山区的一部分）底部硅质流的年龄相似，这表明红山侵入体可能是一些早期喷出岩的源区。弗吉尼亚城火山区的岩浆作用随着时间推移总体上变得更偏镁铁质，诺里斯附近约3000万年前的玄武岩流被认为是该火山区晚期的一个外围成员。麦迪逊山谷半地堑东侧的一层凝灰岩得出了早中中新世的年龄（\(16.2\pm0.19\)百万年），这表明加速的地壳伸展以及相关的快速盆地沉积可能始于早中新世，比先前的估计略早。
Detailed 40 Ar/ 39 Ar dating and paleomagnetic analysis of dacite porphyry sills and dikes that intrude Cretaceous sedimentary rocks in the northern Madison Range in southwestern Montana show that Laramide shortening was essentially complete by ∼69 Ma. A negative paleomagnetic fold test indicates that Laramide folding occurred before cooling of the dacite sills and dikes at ∼69 Ma. Laramide deformation began synchronous with deposition of the Livingston Formation rocks at ∼79 Ma. These results are consistent with previous observations in the region that show the onset of Laramide deformation in the northern Rocky Mountains becoming progressively younger toward the east. 40 Ar/ 39 Ar dating of additional igneous rocks in the northern Madison Valley and around Norris, Montana better define post-Laramide tectonomagmatic events in the region, including Eocene–Oligocene volcanism and Basin and Range crustal extension. Dates from three rhyolitic intrusions near Red Mountain are between 48.71 ± 0.18 Ma and 49.42 ± 0.18 Ma, similar to the dates from basal silicic flows of the Virginia City volcanic field (part of the southwest Montana volcanic province), suggesting that the Red Mountain intrusions may have been the sources for some of the early extrusive rocks. Magmatism in the Virginia City volcanic field became generally more mafic with time, and a ∼30-Ma basalt flow near Norris is considered a late, outlying member of the volcanic field. A tuff along the east side of the Madison Valley half graben yielded a early middle Miocene date (16.2 ± 0.19 Ma), suggesting that accelerated crustal extension and associated rapid basin sedimentation probably began in the early Miocene, slightly earlier than previous estimates.