Genesis of Cenozoic low‐Ca alkaline basalts in the Nanjing basaltic field, eastern China: The case for mantle xenolith‐magma interaction

Abstract

[1] Although peridotite xenoliths are common in alkaline basalts, it is still unclear whether the chemical compositions of their host rocks have been affected by these mantle fragments and, if so, what processes are involved in this alteration of the host basalts. Here, we document a kind of xenolith-rich alkaline basalts from the Nanjing basaltic field, eastern China. These basalts contain lower concentrations of CaO (4.1-7.8 wt %) and Sc (3.3-17.8 ppm) and have lower Ca/Al (0.3-0.6) and higher Na/Ti ratios (2.8-11.2) than other Cenozoic basalts in this area. These xenolith-rich basalts show good correlations between elemental ratios (e.g., Lu/Hf and Ca/Al) and e Hf values, which are indicative of mixing of two distinct components during the genesis of the magmas that formed these basalts: a high-e Hf end-member (with low Lu/Hf and Ca/Al ratios) and the primitive melt-related low-e Hf end-member. In addition, peridotite xenoliths hosted in these basalts have distinct core-mantle textures, with the margins having higher modal olivine abundances (70%) than the xenolith cores (52%). Within the xenolith margins, some orthopyroxenes are enclosed in the olivines, and all clinopyroxenes are sponge textured. These sponge-textured clinopyroxenes have higher CaO and Sc concentrations, higher Ca/Al ratios, and lower Na/Ti ratios than clinopyroxenes within the cores of the xenoliths, suggesting that the xenoliths underwent low-pressure melting within the host magma. This indicates that xenolith-rich magmas within the study area were contaminated during ascent by melts derived from mantle xenoliths within the magmas, transforming the magmas into the low-Ca alkaline basalts.

Light Mg Isotopic Composition in the Mantle Beyond the Big Mantle Wedge Beneath eastern Asia

“…Data for Cenozoic basalts in Eastern China are shown for comparison (NE China: Basu et al, ; Chu et al, ; Kuritani et al, ; Liu, Chen, et al, ; Rasskazov et al, ; Sun, Teng, et al, ; Sun et al, ; Wang et al, ; Yan & Zhao, ; Yan et al, ; Zhang et al, ; Zhang et al, ; Zhao et al, ; Zou et al, ; Zou et al, . North China: Basu et al, ; Chen et al, ; Choi et al, ; Guo et al, ; Han et al, ; Ho et al, ; Li, Ma, et al, ; Wang et al, ; Xu et al, ; Yu et al, ; Zeng et al, ; Zhang et al, ; Zhang et al, ; Zou et al, . South China: Li, Xu, et al, ; Yu et al, ; Ho et al, ; Huang et al, ; Sun et al, ; Zeng et al, ; Zou et al, ), including data for the Chaihe basalts (big gray dots; Ho et al, ; Li, Ma, et al, ; Zhao & Fan, ; Xue et al, ).…”

The role of melt‐rock interaction in the formation of Quaternary high‐MgO potassic basalt from the Greater Khingan Range, northeast China

“…Experimental studies have confirmed that reaction between a siliceous melt and peridotite produces silica‐undersaturated alkali melts owing to the formation of orthopyroxene and/or garnet at the expense of olivine [ Mallik and Dasgupta , , , ], where the degree of alkalinity and silica undersaturation of the resulting melt increase with increasing system CO 2 content [ Mallik and Dasgupta , ]. Despite abundant evidence that melt‐rock interaction occurs in the shallow mantle and contributes to the compositional diversity of intraplate basalt [ Xu et al ., ; Søager and Holm , ; Zeng et al ., ; Liu et al ., ], it is still not well understood how these reactions and thus the erupted basalt compositions spatially and temporally evolve, which we do in this contribution.…”

“…The most important controls on the compositional diversity are inferred to be heterogeneous mantle sources with variable amounts of recycled crustal components and/or variable degrees of mantle partial melting [ Hofmann and White , ; McKenzie and O'Nions , ; Weaver , ; Hofmann , , ; Sobolev et al ., , ; Willbold and Stracke , , ; Walter et al ., , ; Stracke , ]. In addition to source heterogeneity, interaction between mantle‐derived melt and surrounding rocks during ascent may also contribute to the compositional diversity of the erupted magma [ Wagner and Grove , ; Xu et al ., ; Tang et al ., ; Lambart et al ., ; Mallik and Dasgupta , , , ; Søager et al ., ; Zeng et al ., ; Mallik et al ., ; Liu et al ., ]. Interaction between asthenosphere‐derived alkali‐rich melt and refractory lithospheric mantle has, for example, been proposed to explain the high SiO 2 and Cr and low Al 2 O 3 and CaO content, and the “kinked” REE patterns of Hawaiian [ Wagner and Grove , ] and Datong [ Xu et al ., ] tholeiitic basalt, reflecting the crystallization of olivine and the consumption of orthopyroxene in melt‐rock reactions.…”

An Investigation of the Reliability of HF Acid Mixtures in the Bomb Digestion of Silicate Rocks for the Determination of Trace Elements by ICP‐MS

“…(), s Zeng et al . (), t Webb et al . (), u Vasilieva (), v Petrov (), w Kuznetsova and Chumakova ().…”