Two Cenozoic prominent features are spatio-temporally associated in central Mongolia, i.e., the continental basalts and regional uplift, but their genesis and relationship remain unclear. This study presents major- and trace-element compositions for olivine phenocrysts and xenocrysts, as well as data of bulk-rock geochemistry and Sr-Nd-Hf isotopes for the host basalts. The studied basalts mostly have trachybasalt compositions with high total alkali (Na2O + K2O = 5.1–8.2 wt%) contents and all display OIB-like trace element patterns (e.g., spikes of Ba, Nb, and Ta and troughs of Th and U) and EM1-like Sr-Nd-Hf isotopic compositions. Compared to the partial melting products of mantle peridotite, these basaltic samples have higher FeO/MnO, Zn/Mn, and Zn/Fe ratios. Meanwhile, phenocrystic olivines are characterized by lower Ca, Mn, Mn/Zn, and Mn/Fe but higher Ni than their counterparts in the peridotitic melts, indicating a pyroxenite-rich mantle source. The above geochemical data suggest that the source of the studied basalts was mainly made up of secondary pyroxenite produced by the reaction of recycled oceanic crust with its ambient mantle peridotite. The calculated magma oxygen fugacities (ΔFMQ-0.26 to +0.42) and mantle melting temperatures (1343–1430 °C) do not support a genetic link with the stagnant Pacific slab or with a deep mantle plume. Instead, the far-field effect of India-Eurasia convergence possibly tapped the upper asthenospheric mantle, subsequent melting of which gave rise to the dispersive Cenozoic basalts. On the other hand, the xenocrystic olivines exhibit zoned textures with high-Fo (up to 92) cores and low-Fo (down to 76) rims, reflecting the melt-rock interaction. Preservation of zoned olivine xenocrysts indicates rapid magma ascent and widespread melt-rock reaction in the mantle lithosphere, which may modify the rheology and accelerate the mechanical erosion of mantle lithosphere. Consequently, mass deficit in the lithosphere could have caused isostatic uplift of central Mongolia in the Cenozoic.

中文翻译：

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蒙古中部新生代玄武岩中的异相和异相橄榄石的成分对比：对源岩性和区域隆升的限制

蒙古中部地区有两个新生代突出的时空特征，即大陆玄武岩和区域隆升，但它们的成因和关系仍不清楚。这项研究介绍了橄榄石隐晶和异晶晶的主要和微量元素组成，以及主体玄武岩的块状岩石地球化学和Sr-Nd-Hf同位素数据。所研究的玄武岩大多具有高总碱含量（Na2O + K2O = 5.1–8.2 wt％）的钙钛矿成分，并且都显示出类似OIB的痕量元素模式（例如，Ba，Nb和Ta的尖峰以及Th和U的凹坑）和类似EM1的Sr-Nd-Hf同位素组成。与地幔橄榄岩的部分熔融产物相比，这些玄武岩样品具有较高的FeO / MnO，Zn / Mn和Zn / Fe比。同时，单晶橄榄石的特征是较低的Ca，Mn，Mn / Zn，橄榄岩熔体中的锰，铁，锰比铁高，但镍含量高，说明富含辉石的地幔源。上述地球化学数据表明，所研究的玄武岩的来源主要是次生辉石矿，次生辉石矿是由再生海洋地壳与其周围的地幔橄榄岩反应生成的。计算得出的岩浆氧逸度（ΔFMQ-0.26至+0.42）和地幔融化温度（1343–1430°C）不支持与停滞的太平洋板块或地幔柱深的遗传联系。取而代之的是，印度-欧亚大陆融合的远场效应可能利用了软流圈上地幔，随后地幔融化产生了新生代玄武岩。另一方面，异晶橄榄石显示出带高Fo（最多92个）芯和低Fo（最多76个）边缘的分区纹理，反映熔岩相互作用。区域橄榄石异晶的保存表明地幔岩石圈中岩浆上升较快，熔岩反应广泛，这可能会改变流变学并加速地幔岩石圈的机械侵蚀。因此，岩石圈的质量赤字可能导致新生代中部蒙古的等静压上升。