Abstract

We present new geochemical, Sr, Nd, and Pb isotope, and ⁴⁰Ar/³⁹Ar data from Pleistocene basalts of the Western Snake River Plain (WSRP), Idaho, USA to explore their petrogenesis and to investigate the nature of the lithosphere at the western boundary of the North American craton. The basalts are divided into three groups based on their geochemical and isotopic characteristics. Prior to ∼1 Ma, volcanoes in the WSRP erupted iron-rich tholeiites (FeB1), but subsequent volcanism was dominated by concurrent eruptions of mildly alkaline, alumina-rich lavas (AlB) and iron-rich tholeiites (FeB2) with isotopic signatures similar to the AlB lavas. New ⁴⁰Ar/³⁹Ar dates of AlB and FeB2 basalts range from 0·920 ± 0·049 to 0·287 ± 0·014 Ma. MELTS models of FeB1 differentiation trends indicate that the range of compositions in this suite can be produced by 10–15 % crystallization of olivine and plagioclase at low pressure using the least evolved FeB1 composition as a parental magma; isotopic ratios can be produced via combined assimilation of a Miocene rhyolite and fractional crystallization. Additional modeling suggests that parental magmas at AlB centers were produced by 3–12 % equilibrium melting of a garnet–spinel-enriched mantle source, slightly different from that proposed for the youngest mildly alkaline lavas of the eastern and central Snake River Plain. Our new geochemical, isotopic, and geochronological data for the FeB2 basalts suggests that they are related to AlB-type magmas via a combination of fractional crystallization and assimilation of evolved mafic crust. MELTS models suggest that crystallization of an AlB parental melt at a depth of 6–8 km (2·5 kbar) could produce residual liquids having many of the major oxide characteristics of FeB2 ferrobasalts. Sr–Nd–Pb isotopic signatures of these three suites indicate a dominant contribution from an enriched plume source. FeB1 lavas are probably products of mixing between melts of an enriched plume mantle source (represented by Imnaha and Steens Basalts of the Columbia River Basalt Group) and isotopically heterogeneous sub-continental lithospheric mantle (SCLM) that has been isolated from the convecting mantle since the Archean. Isotopic ratios of FeB2 and AlB lavas capture mixing between enriched plume mantle and a more isotopically homogeneous ancient SCLM domain characteristic of the eastern and central Snake River Plain, with a coupled decrease in lithospheric contribution and degree of partial melting through time to the present. Mixtures of enriched asthenospheric reservoirs with lithospheric mantle have been proposed for neighboring volcanic fields to the east along the strike of the Yellowstone–SRP hotspot track, and to the west owing to differences in the mantle underlying the boundary of the North American craton and accreted terranes. Our petrogenetic model for the Pleistocene WSRP basalts suggests that there is also a lateral, across-strike gradient in the geometry and interaction of enriched plume mantle and ancient lithosphere. We reiterate suggestions that the WSRP is a lithosphere-scale conduit connecting initial plume-head impingement in east–central Oregon with the subsequent Yellowstone–SRP hotspot plume-tail track.

中文翻译：

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爱达荷州西蛇河平原更新世玄武岩的成岩作用

摘要

我们提供了新的地球化学，Sr，Nd和Pb同位素以及来自美国爱达荷州西蛇河平原（WSRP）的更新世玄武岩的⁴⁰ Ar / ³⁹ Ar数据，以探讨它们的岩石成因并研究该地区岩石圈的性质。北美克拉通的西边界。根据玄武岩的地球化学和同位素特征将其分为三类。在大约1 Ma之前，WSRP中的火山喷发了富铁的冲铁质（FeB1），但随后的火山活动主要由中等同位素，富含氧化铝的熔岩（AlB）和具有类似同位素特征的富铁的冲铁质（FeB2）共同爆发所主导。到AlB熔岩。新⁴⁰ Ar / ³⁹AlB和FeB2玄武岩的Ar日期范围为0·920±0·049至0·287±0·014 Ma。FeB1分化趋势的MELTS模型表明，该套件中的成分范围可以通过使用演化最少的FeB1成分作为母岩浆在低压下通过橄榄石和斜长石的10-15％结晶来产生。同位素比率可以通过中新世流纹岩和部分结晶的联合同化来产生。其他模型表明，AlB中心的母岩浆是由富含石榴石-尖晶石的地幔源的3-12％平衡融化产生的，与东部和中部Snake River Plain的最年轻的轻度碱性熔岩所提议的略有不同。我们新的地球化学同位素 FeB2玄武岩的地质年代学数据表明，它们通过分步结晶和同化的镁铁质地壳的同化作用与AlB型岩浆有关。MELTS模型表明，AlB母体熔体在6–8 km（2·5 kbar）的深度结晶可能会产生残留液体，这些残留液体具有FeB2铁玄武岩的许多主要氧化物特征。这三个套件的Sr–Nd–Pb同位素特征表明，来自丰富羽流来源的主要贡献。自从太古代。FeB2和AlB熔岩的同位素比值捕获了富烟羽幔​​与东部和中部Snake河平原的同位素更均一的古代SCLM域之间的混合，岩石圈的贡献和随时间变化的部分融化程度到现在为止。已经提出了沿黄石-SRP热点轨迹走向向东的邻近火山场，以及由于北美克拉通边界和增生的地层边界之下的地幔的差异，建议将富集的软流圈储层与岩石圈地幔混合，形成东部的邻近火山场。 。我们对更新世WSRP玄武岩的岩石成因模型表明，在富集羽幔和古代岩石圈的几何形状和相互作用中，还存在横向，横向走向的走向梯度。