A geochemical study was undertaken on basaltic lava flows intercalated with Oligocene to Miocene strata in the Sabzevar region, northern Iran, to examine their petrogenesis in a regional tectonic framework. The lavas are either aphyric or have phenocrysts and micro-phenocrysts of olivine and, to a lesser extent, clinopyroxene. Geochemically, the lavas are silica-undersaturated alkali basalts characterized by relatively high Mg# (~ 57–66) and Na 2 O/K 2 O (~ 2.0–6.7). They have distinctive trace element patterns characterized by strong rare-earth element fractionation, negative Nb–Ta and Zr-Hf anomalies and a positive Sr anomaly. Significant contamination by crustal materials either in the magma source or during ascent is ruled out on the basis of trace element compositions and Sr–Nd isotopic compositions ( 87 Sr/ 86 Sr = 0.7037–0.7048 and 143 Nd/ 144 Nd = 0.5128–0.5130), both of which differ markedly from continental crustal rocks. Phenocryst assemblage, analysis of multiple saturation points in lherzolite systems, and covariations of La/Yb with MgO of the studied lavas are generally consistent with an origin involving high-pressure fractionation of peridotite-derived melts. Primary magma compositions calculated by reversed fractionation of clinopyroxene and olivine for the relatively primitive samples (> 9 wt.% MgO), however, do not plot on the lherzolite multiple saturation points. Also, high-pressure fractionation predicts increasing trends of silica undersaturation and alkalinitiy with differentiation, and such trends are not indicated by the geochemical data. We suggest that the mixed trends shown by the data might be related to melt generation from both peridotite and silica-deficient pyroxenite sources, superimposed by variable degrees of high-pressure fractionation. The role of pyroxenite in magma genesis is indicated not only by the positive Sr anomaly shown by the trace element patterns, but also first-row transition element systematics of the studied lavas. The silica-deficient pyroxenites contributing to melt generation might have been transformed from mafic–ultramafic cumulates in subducted, lower oceanic crust, or might have formed in the lower crust or mantle lithosphere under continents during earlier magmatic episodes.

中文翻译：

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伊朗北部 Sabzevar 地区大陆碱性玄武岩的地球化学：辉石岩在岩浆成因中的作用

对伊朗北部 Sabzevar 地区的渐新世至中新世地层中的玄武岩熔岩流进行了地球化学研究，以检查它们在区域构造框架中的岩石成因。熔岩要么是无晶的，要么有橄榄石的斑晶和微斑晶，在较小程度上，还有单斜辉石。在地球化学上，熔岩是二氧化硅不饱和的碱性玄武岩，其特征是相对较高的 Mg# (~ 57-66) 和 Na 2 O/K 2 O (~ 2.0-6.7)。它们具有独特的微量元素模式，其特征是强烈的稀土元素分馏、负的 Nb-Ta 和 Zr-Hf 异常以及正的 Sr 异常。根据微量元素组成和 Sr-Nd 同位素组成 ( 87 Sr/ 86 Sr = 0.7037-0. 7048 和 143 Nd/ 144 Nd = 0.5128–0.5130），两者都与大陆地壳岩石有显着差异。斑晶组合、对锂辉石系统中多个饱和点的分析，以及所研究熔岩的 La/Yb 与 MgO 的协变通常与涉及橄榄岩衍生熔体的高压分馏的成因一致。然而，对于相对原始的样品（> 9 wt.% MgO），通过单斜辉石和橄榄石的反向分馏计算的初级岩浆成分没有绘制在锂辉石多个饱和点上。此外，高压分馏预测二氧化硅不饱和度和碱度随着分化的增加趋势，地球化学数据并未表明这种趋势。我们认为，数据显示的混合趋势可能与橄榄岩和缺乏二氧化硅的辉石岩来源的熔体生成有关，并叠加了不同程度的高压分馏。辉石岩在岩浆成因中的作用不仅通过微量元素模式显示的 Sr 正异常表明，而且还表明所研究熔岩的第一行过渡元素系统学。导致熔体生成的缺硅辉石岩可能是从俯冲的下洋地壳中的镁铁质-超镁铁质堆积物转化而来的，或者可能是在早期岩浆活动期间在大陆下的下地壳或地幔岩石圈中形成的。辉石岩在岩浆成因中的作用不仅通过微量元素模式显示的 Sr 正异常表明，而且还表明所研究熔岩的第一行过渡元素系统学。导致熔体生成的缺硅辉石岩可能是从俯冲的下洋地壳中的镁铁质-超镁铁质堆积物转化而来的，或者可能是在早期岩浆活动期间在大陆下的下地壳或地幔岩石圈中形成的。辉石岩在岩浆成因中的作用不仅通过微量元素模式显示的 Sr 正异常表明，而且还表明所研究熔岩的第一行过渡元素系统学。导致熔体生成的缺硅辉石岩可能是从俯冲的下洋地壳中的镁铁质-超镁铁质堆积物转化而来的，或者可能是在早期岩浆活动期间在大陆下的下地壳或地幔岩石圈中形成的。