Isotopic and geochemical evidence for a recent transition in mantle chemistry beneath the western Canadian Cordillera
Isotopic and geochemical evidence for a recent transition in mantle chemistry beneath the western Canadian Cordillera
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加拿大西部科迪勒拉山脉下地幔化学近期转变的同位素和地球化学证据
DOI：
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发表时间：
2010
期刊：
影响因子：
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通讯作者：
G. Gehrels
中科院分区：
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作者：
C. Manthei;M. Ducea;J. Girardi;P. Patchett;G. Gehrels
[1]新的岩石学、地球化学和同位素数据来自一套从加拿大西部科迪勒拉地区采集的镁铁质岩脉和熔岩流样本。样品的年龄范围从始新世到第四纪，并记录了发生在10 Ma后的地幔化学的重大转变。始新世至晚中新世侵位于海岸山体岩基内的玄武岩含有丰富的角闪石，富含大离子亲石元素(LILE)(Ba，Rb，K)，具有高场强元素(Nb，Ta)负异常，可能来自于该区弧岩浆作用停止后稳定的岩石圈地幔(87Sr/86Sr=0.70353~0.70486；ɛND=+2.5至+5.7.)。相比之下，第四纪熔岩流具有较低的LILE浓度和正的Nb-Ta异常，可能是由软流圈上涌(87Sr/86Sr=0.70266-0.70386；ɛNd=+7.4to+8.8.)或至少一个具有不同化学和同位素特征的地幔源区产生的。对加拿大西部同为始新世至第四纪的镁铁质岩石进行的区域对比表明，在10 Ma以后，地幔化学的转变是普遍和普遍的，并不局限于本研究区。这种转变发生在不列颠哥伦比亚省中部科迪勒弧岩浆作用停止后40 Ma，这表明岩浆弧下的地幔化学可能在大洋岩石圈最终俯冲后数千万年左右发生大规模转变，在这种情况下是大陆伸展减薄岩石圈的结果。
[1] New petrologic, geochemical, and isotopic data are reported from a suite of mafic dike and lava flow samples collected from sites within the western Canadian Cordillera. Samples range in age from Eocene to Quaternary and document a significant transition in mantle chemistry that occurred sometime after 10 Ma. Eocene to late Miocene basalts emplaced as dikes within the Coast Mountains Batholith contain abundant hornblende, are enriched in large ion lithophile elements (LILE) (Ba, Rb, K), have negative high field strength element (HFSE) (Nb, Ta) anomalies, and were likely derived from lithospheric mantle (87Sr/86Sr = 0.70353–0.70486; ɛNd = +2.5 to +5.7) that was stabilized after the cessation of arc magmatism in the area. By contrast, Quaternary lava flows have lower LILE concentrations, positive Nb-Ta anomalies, and were likely generated by upwelling asthenosphere (87Sr/86Sr = 0.70266–0.70386; ɛNd = +7.4 to +8.8) or at least a mantle source with different chemical and isotopic characteristics. A regional comparison of mafic rocks from western Canada that are also Eocene to Quaternary in age indicates that the transition in mantle chemistry after 10 Ma was pervasive and widespread and was not limited to the present study area. This transition occurred ∼40 Ma after the cessation of Cordilleran arc magmatism in central British Columbia, suggesting that large-scale transitions in mantle chemistry beneath magmatic arcs may occur on the order of tens of millions of years after the final subduction of oceanic lithosphere, in this case as a result of lithospheric thinning by continental extension.