Magma–magma interaction in the mantle recorded by megacrysts from Cenozoic basalts in eastern China

Recently, besides magma–rock and rock–rock reaction, magma–magma interaction at mantle depth has been proposed as an alternative mechanism to produce diverse compositions of mantle. Clinopyroxene and garnet megacrysts can be formed at this condition since this process is suggested to trigger the high-pressure crystallization of these minerals. Studying on this type of megacrysts provides us important information on the genesis of intraplate basalts and the chemical heterogeneity of mantle, which has not been reported before. Here we present major, trace elements and Sr isotopes of clinopyroxene and garnet megacrysts hosted by Cenozoic basalts from Penglai, Shandong province of eastern China. The megacrysts are suggested to be formed by crystallization from magma because of their moderate Mg^# (74.0–79.9 for clinopyroxene and 58.8–65.0 for garnet) and good correlations between Mg^# and other elements (e.g. CaO, TiO₂, Nd and Lu). The potential crystallized temperature and pressure are estimated to be ~1156°C at 2.6–3.2 GPa, which should occur at the top of asthenosphere or lithosphere–asthenosphere boundary based on the lithospheric thickness in this area (~60–70 km). Since the megacrysts show variable Sr isotopes, and their primary magmas show negative correlation between ⁸⁷Sr/⁸⁶Sr and Hf/Sm ratios, as well as positive correlation between Ba/Th and Nb/U for clinopyroxenes, it indicates a mixing origin. Cenozoic basalts from Shandong show a mixing trend, and high-pressure fractionation of clinopyroxene and garnet is suggested to occur during the mixing process because some basalts show significantly higher Sm/Yb and lower Ca/Al ratios than others, which again supports our interpretations. When compared to megacrysts and host basalts from other locations of eastern China, similar geochemical variations and a deviation trend relative to the mixing trend are also observed. It indicates that magma–magma interaction can be a common process for formation of intraplate basalts and basalt-borne megacrysts.

近年来，除了岩浆-岩石反应与岩石-岩石反应之外，地幔深度处的岩浆-岩浆相互作用已被提出作为形成多样地幔成分的另一机制。在此条件下可形成单斜辉石（clinopyroxene）与石榴石（garnet）巨晶（megacrysts），因为该过程被认为会触发这些矿物的高压结晶。对这类巨晶的研究能够为我们提供板内玄武岩成因以及地幔化学不均一性的重要信息，此前尚无相关报道。本文报道了中国东部山东省蓬莱新生代玄武岩中赋存的单斜辉石与石榴石巨晶的主量元素、微量元素及锶（Sr）同位素数据。这些巨晶的镁指数（Mg#）适中（单斜辉石为74.0–79.9，石榴石为58.8–65.0），且Mg#与其他元素（如CaO、TiO₂、Nd及Lu）之间呈现良好的相关性，表明它们是由岩浆结晶形成的。经估算，其潜在结晶温度与压力约为1156℃，压力介于2.6–3.2 GPa，结合该区域岩石圈厚度（约60–70 km），该结晶深度应位于软流圈（asthenosphere）顶部或岩石圈-软流圈边界处。由于巨晶的Sr同位素组成存在差异，且其原始岩浆的⁸⁷Sr/⁸⁶Sr比值与Hf/Sm比值呈负相关，单斜辉石的Ba/Th比值与Nb/U比值呈正相关，这表明其成因属于混合起源。山东新生代玄武岩呈现出混合趋势，且部分玄武岩的Sm/Yb比值显著更高、Ca/Al比值显著更低，表明在混合过程中发生了单斜辉石与石榴石的高压分异作用，进一步支持了我们的解释。将本次研究的巨晶与寄主玄武岩与中国东部其他区域的同类样品对比后发现，二者存在相似的地球化学变化特征，且均呈现出相对于混合趋势的偏离趋势。这表明岩浆-岩浆相互作用可能是板内玄武岩及其寄主玄武岩巨晶形成的普遍过程。