Spatial migration and compositional changes of Miocene‐Quaternary magmatism in the Western Grand Canyon
Spatial migration and compositional changes of Miocene‐Quaternary magmatism in the Western Grand Canyon
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西部大峡谷中新世-第四纪岩浆作用空间迁移及成分变化
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发表时间：
1995
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影响因子：
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通讯作者：
B. Blackerby
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作者：
K. Wenrich;G. Billingsley;B. Blackerby
岩石圈伸展似乎是负责侵蚀和构造演化的科罗拉多高原西缘。新生代晚期（20 Ma至10，000年）玄武岩流、火山渣锥、颈、岩脉和岩床暴露在西部大峡谷地区，提供了一个独特的机会，将年龄限制在景观演变。新的K-Ar年龄表明，这些玄武质岩石变得越来越年轻的东北部从华拉派和Shivwits高原到Uinkaret高原，约100公里的距离。岩浆活动似乎已经迁移的速度约为0.45厘米/年，直到13.5和9.2马之间的某个时候，当速率变为约1.15厘米/年。最近的区域地质测绘表明，伸展断层，无处不在的下降到西部，伴随着这种岩浆作用;飓风断层抵消熔岩年轻0.29马。沉积岩中年轻的断层崖，以及沿着这些断层沿着的历史地震活动，表明该地区仍在发生断层作用。根据玄武岩的年龄、化学成分和地理分布，可将其分为三个相对不同的组：（1）20-11 Ma，（2）11- 4.5Ma，（3）≤ 4.5Ma。较老的玄武岩（>11 Ma）在岩石学和成分上与较年轻的玄武岩不同;它们的橄榄石和普通辉石更富Mg，具有较低的TiO 2和较高的Ni、Cr和不相容元素含量。第1组玄武岩具有更多的放射成因Sr和更少的放射成因Nd同位素组成比年轻的第2和第3组玄武岩。总体而言，现今的Sr同位素组成介于0.70344至0.70505之间，143 Nd/144 Nd介于0.51291至0.51227之间（eNd = +5.4至-7.1），206 Pb/204 Pb介于17.951至19.091之间。第1组玄武岩可能来自于经历地壳混染的软流圈地幔熔体。这些玄武岩似乎已经产生了最初从更原始的熔体比组2和3，这可能也来自软流圈幔源岩浆，但只经历了轻微的地壳污染。第2组玄武岩似乎是第1组和第3组之间的过渡。13-9 Ma和4.6-4.3 Ma的时间间隔将西部大峡谷的三个化学性质不同的玄武岩群分开。这些时期与美国西部主要的中新世和上新世板块重组密切对应，大约在12.5-10 Ma和3.9-3.4 Ma。板块运动的这些变化反映在西部大峡谷岩浆迁移速率的变化上。飓风断层和托罗韦普断层的近期运动似乎与<4.5 Ma的火山活动直接相关，可能是近期板块重组的结果。
Lithospheric extension appears to be responsible for erosional and tectonic evolution of the western margin of the Colorado Plateau. Late Cenozoic (20 Ma to 10,000 years) basaltic flows, scoria cones, necks, dikes, and sills exposed in the Western Grand Canyon region offer a unique opportunity to place age constraints on the landscape evolution. New K-Ar ages show that these basaltic rocks become progressively younger to the northeast from the Hualapai and Shivwits Plateaus onto the Uinkaret Plateau, a distance of about 100 km. Magmatism appears to have migrated at a rate of about 0.45 cm/yr until sometime between 13.5 and 9.2 Ma, when the rate changed to about 1.15 cm/yr. Recent regional geologic mapping indicates that extensional faulting, with ubiquitous downdrop to the west, accompanied this magmatism; the Hurricane Fault offsets lavas as young as 0.29 Ma. Young fault scarps in sedimentary rocks, and historic seismic activity along these faults, indicate that the region is still undergoing faulting. The basalts form three relatively distinct groups based on their age, chemistry, and geographic distribution: (1) 20–11 Ma, (2) 11–4.5 Ma, and (3) ≤4.5 Ma. The older basalts (>11 Ma) are petrographically and compositionally distinct from younger basalts; their olivine and augite are more Mg rich with lower TiO2 and higher Ni, Cr, and incompatible element contents. Group 1 basalts have more radiogenic Sr and less radiogenic Nd isotopic compositions than younger group 2 and 3 basalts. Overall, present-day Sr isotopic compositions range from 0.70344 to 0.70505, 143Nd/144Nd from 0.51291 to 0.51227 (eNd = +5.4 to −7.1), and 206Pb/204Pb from 17.951 to 19.091. Group 1 basalts were probably derived from asthenospheric mantle melts that underwent crustal contamination. These basalts appear to have been generated originally from more primitive melts than groups 2 and 3, which were probably also derived from asthenospheric mantle-derived magmas but underwent only minor crustal contaminations. Group 2 basalts appear to be transitional between groups 1 and 3. The intervals 13–9 Ma and 4.6–4.3 Ma temporally separate the three chemically distinct groups of basalts in the Western Grand Canyon. These periods correspond closely with major Miocene and Pliocene plate reorganizations in the western United States at about 12.5–10 Ma and 3.9–3.4 Ma. These changes in plate motion were reflected in changes in the rate of magmatism migration in the Western Grand Canyon. Recent movements on the Hurricane and Toroweap Faults appear directly related to the <4.5 Ma volcanism and are probably a result of recent plate reorganization.