Abstract We present detailed major-, trace-element and Sr-Nd-Mg isotopic geochemistry for a suite of pyroxenite xenoliths entrained in Oligocene basalts in Yangyuan county, North China Craton, to reveal their origins and to further constrain the evolution of the lithospheric mantle beneath this area. The Yangyuan pyroxenites exhibit a lithologic spectrum ranging from orthopyroxenite, through orthopyroxene-rich websterite and clinopyroxene-rich websterite to clinopyroxenite. Their equilibrium temperatures (874–975 °C), which are within the temperature range of the upper mantle and lower crust, suggest that they came from the crust-mantle transition zone. The Yangyuan pyroxenite xenoliths can be subdivided into three types based on the compositions of their constituent clinopyroxene. Clinopyroxenes in the type I pyroxenites are characterized by high Mg# (88.3–92.0; Mg# = Mg/(Mg + Fe2+)*100) and Cr2O3 contents (0.86–1.63 wt.%). Their resemblance to those from the peridotite xenoliths, together with their light rare earth element (LREE) enrichment, indicates that the type I pyroxenites should be produced by lithological heterogeneity in the mantle, which may have including metasomatism. Clinopyroxenes in the type II pyroxenites have intermediate Mg# (81.0–86.7) and Cr2O3 contents (0.53–0.73 wt.%), which are similar to those of strongly metasomatized mantle-derived Fe-lherzolite xenoliths. Their LREE-enriched compositions, as well as the high Ni contents of the orthopyroxenes and clinopyroxenes, suggest that the type II pyroxenites were produced by melt-peridotite interactions. Clinopyroxenes from type III pyroxenites have the lowest Mg# (65.2–76.5) and Cr2O3 contents (0.05–0.38 wt.%), which are comparable to those formed by fractional crystallization of primary basalts, suggesting that they are the products of fractional crystallization. The high Mg#, Ti/Eu ratios, the low (La/Yb)N and Ca/Al ratios, and Rb and Ba contents of the clinopyroxenes, as well as the depleted Sr-Nd isotopes of most of the Yangyuan pyroxenites, suggest that asthenosphere or juvenile lithospheric mantle-derived silicate melts were probably involved in their formation, whereas the enriched Sr-Nd isotopes of sample YY17-5 indicate that its source may have mixed with melts derived from recycled ancient lower crust. The light and variable Mg isotopic compositions (δ26Mg = −0.23 to −0.46‰) of the Yangyuan pyroxenites suggest contributions from a subducted oceanic slab. The diverse origins of these pyroxenite xenoliths provide evidence that the deep lithosphere beneath Yangyuan has been modified by multiple melt-rock interactions and magma underplating.

中文翻译：

---

华北克拉通阳原辉石岩捕虏体的不同成因：岩浆底侵作用和熔岩相互作用对岩石圈改造的意义

摘要 我们详细介绍了华北克拉通阳原县渐新世玄武岩中夹带的一套辉石岩捕虏体的主要、微量元素和 Sr-Nd-Mg 同位素地球化学，以揭示它们的起源并进一步限制岩石圈地幔的演化。在这个区域之下。阳原辉石岩的岩性光谱范围从斜方辉石，到富含斜方辉石的网辉石和富含单斜辉石的网辉石，再到单斜辉石。它们的平衡温度（874-975°C）在上地壳和下地壳的温度范围内，表明它们来自壳幔过渡带。阳原辉石岩捕虏体根据其组成单斜辉石的成分可分为三类。I 型辉石岩中的单斜辉石具有高 Mg# (88.3–92.0; Mg# = Mg/(Mg + Fe2+)*100) 和 Cr2O3 含量 (0.86–1.63 wt.%) 的特征。它们与橄榄岩捕虏体的相似性以及轻稀土元素 (LREE) 的富集表明 I 型辉石岩应该是由地幔中的岩性非均质性产生的，其中可能包括交代作用。II 型辉石岩中的单斜辉石具有中等的 Mg# (81.0–86.7) 和 Cr2O3 含量 (0.53–0.73 wt.%)，与强交代地幔衍生的 Fe-二锂辉石捕虏体相似。它们富含 LREE 的成分以及斜辉石和单斜辉石的高镍含量表明 II 型辉石岩是由熔体-橄榄岩相互作用产生的。来自 III 型辉石岩的单斜辉石具有最低的 Mg# (65.2–76.5) 和 Cr2O3 含量 (0.05–0.38 wt.%)，可与原生玄武岩分步结晶形成的那些相媲美，表明它们是分步结晶的产物。单斜辉石的高 Mg#、Ti/Eu 比率、低 (La/Yb)N 和 Ca/Al 比率以及 Rb 和 Ba 含量，以及大多数阳原辉石岩的贫化 Sr-Nd 同位素表明软流圈或新生岩石圈地幔衍生的硅酸盐熔体可能参与了它们的形成，而样品 YY17-5 富集的 Sr-Nd 同位素表明其来源可能与来自回收的古代下地壳的熔体混合。轻且可变的 Mg 同位素组成（δ26Mg = -0.23 至 -0。46‰) 的阳原辉石岩表明来自俯冲大洋板块的贡献。这些辉石岩捕虏体的不同来源提供了证据，证明阳原下方的深层岩石圈已被多次熔岩相互作用和岩浆底侵改造。