Rb-Sr and K-Ar geochronometry of Mesozoic granitic rocks and their Sr isotopic composition, Oregon, Washington, and Idaho
Rb-Sr and K-Ar geochronometry of Mesozoic granitic rocks and their Sr isotopic composition, Oregon, Washington, and Idaho
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俄勒冈州、华盛顿州和爱达荷州中生代花岗岩的 Rb-Sr 和 K-Ar 测年法及其 Sr 同位素组成
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发表时间：
1977
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影响因子：
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通讯作者：
Peter O. Hales
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作者：
R. Armstrong;W. H. Taubeneck;Peter O. Hales
中生代造山运动和岩浆活动开始于美国西北部二叠纪地层沉积后不久，但还没有岩石从二叠纪-三叠纪造山期。中三叠世至晚侏罗世沉积序列包括主要的不整合面和几个火成岩活动事件的证据。岩浆活动的早期高潮发生在晚三叠世和早侏罗世时期（200 - 217百万年）前）在远西爱达荷州优地槽部分。晚侏罗世的一次广泛而强烈的高潮是俄勒冈州优地槽最后的主要造山事件。秃头山（147至158百万年）老），瓦洛瓦（可能143至160百万年。古老但受白垩纪变质作用影响）、深溪（至少1.37亿年）老），以及其他许多位于俄勒冈州的布鲁和克拉马斯山区以及爱达荷州西部的岩体都是在侏罗纪结束前不久侵位的。爱达荷州岩基的大部分是在白垩纪岩浆活动达到顶峰时侵位的--南部（亚特兰大）岩瓣约在7500万年至1亿年之间。大约70到80百万年前和北方（Bitterroot）叶瓣前大部分岩基受到始新世岩浆活动的影响，导致较老岩石的同位素年龄普遍重置为5000万年。5500万至7000万年之间。在此之前，美国西北部的火成岩活动暂时停止。Sr同位素初始比值在爱达荷州西部的边界上突然变化，从西边的0.7040或更小，到东边的0.7060或更大。这一变化标志着前寒武纪地壳与中生代优地槽的分界。所观察到的转变的地质背景和它的时间独立性表明，这是由于污染和同化过程涉及岩浆从地幔和封闭的地壳岩石。放射性Sr的岩浆污染使得Sr全岩等时线技术在测定爱达荷州中部和东部的爱达荷州岩基和其他侵入岩的年代时变得无用，这些地区位于前寒武纪基底之下。
Mesozoic orogeny and magmatism began in the northwestern United States soon after deposition of Permian strata, but no rocks have yet been dated from the Permian-Triassic orogenic period. Middle Triassic to Late Jurassic sediment sequences include major unconformities and evidence of several episodes of igneous activity. An early culmination of magmatism occurred in Late Triassic and Early Jurassic time (200–217 m.y. ago) in eugeosynclinal parts of far western Idaho. A widespread and intense culmination in Late Jurassic time was the final major orogenic event in the Oregon eugeosyncline. The Bald Mountain (147 to 158 m.y. old), Wallowa (probably 143 to 160 m.y. old but affected by Cretaceous metamorphism), Deep Creek (at least 137 m.y. old), and many other plutons in the Blue and Klamath Mountain regions in Oregon and in western Idaho were emplaced shortly before the end of Jurassic. The bulk of the Idaho batholith was emplaced during a Cretaceous culmination of igneous activity — the southern (Atlanta) lobe about 75 to 100 m.y. ago and the northern (Bitterroot) lobe about 70 to 80 m.y. ago. Much of the batholith was affected by Eocene magmatism which resulted in widespread resetting of isotopic dates for older rocks to values of 50 m.y. or less. Between 55 and 70 m.y. ago, there was a lull in igneous activity in the northwestern United States. Sr isotope initial ratios change abruptly across a boundary in western Idaho from ∼0.7040 or less, to the west, to ∼0.7060 or greater, to the east. This change marks the boundary between Precambrian crust and Phanerozoic eugeosyncline. The geologic setting of the observed transition and its time independence suggest that it is due to contamination and assimilation processes involving magmas from the mantle and enclosing crustal rocks. Contamination of magmas with radiogenic Sr renders the Sr whole-rock isochron technique useless in dating the Idaho batholith and other intrusive rocks in central and eastern Idaho, areas underlain by Precambrian basement.