大同第四纪玄武岩成因:主微量元素及Sr-Nd-Pb-Hf同位素研究

大同火山区位于大兴安岭-太行山重力梯度带西侧，所发育的第四纪玄武岩岩石地球化学特征为探索该区火山岩成因提供了重要约束，同时也为华北克拉通西部岩石圈地幔与软流圈的相互作用提供重要依据。根据火山地貌和岩性的不同，沿着北东向陈庄-许堡断裂可将大同玄武岩大致分为东、西两区，西区火山多呈锥状，以碱性玄武岩为主；东区则以溢流拉斑玄武岩为主，锥体少。镜下岩相学研究观察到大量橄榄石和单斜辉石斑晶，结合Ni、Cr两种元素随着MgO含量降低而减小，这两种矿物应是分离结晶作用下的主要产物。这些玄武岩的SiO₂和（K₂O+Na₂O）含量分别为45.02%~53.3%和3.60%~6.53%，相对富集轻稀土元素（（La/Yb）_N=5.8~31.6），并显示富集LILE（Rb、Ba、Sr正异常）以及HFSE（Nb、Ta、Zr正异常）的洋岛玄武岩（OIB）特征。根据La/Yb-Sm/Yb图解模拟计算得出大同玄武岩均是石榴石相二辉橄榄岩低程度部分熔融的结果，其中碱性玄武岩部分熔融程度约为1.5%~3%，拉斑玄武岩约为4%~8%。所研究的玄武岩有较低⁸⁷Sr/⁸⁶Sr（0.703302~0.705102）、较高¹⁴³Nd/¹⁴⁴Nd（0.512561~0.512963）和¹⁷⁶Hf/¹⁷⁷Hf（0.282922~0.283072）比值。在¹⁴³Nd/¹⁴⁴Nd-⁸⁷Sr/⁸⁶Sr图解上大同玄武岩落在OIB范围内；²⁰⁷Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb和¹⁴³Nd/¹⁴⁴Nd-²⁰⁶Pb/²⁰⁴Pb图解表明它们来自PREMA和EMⅠ端元的二元混合。大同碱性玄武岩和拉斑玄武岩的地球化学特征不同可以用两点原因来解释：（1）主量与微量元素特征的差异是两种玄武岩部分熔融程度不一样形成的；（2）同位素特征表明两种玄武岩都来自软流圈亏损端元的部分熔融并存在少量岩石圈富集端元物质的加入，其差异则是加入比例不同造成的。

Petrogenesis and characteristics of the mantle source for the Quaternary Datong basalt: Research on the major, trace elements and Sr-Nd-Pb-Hf isotopes

The Quaternary Datong basalt is located to the west of the Daxing'anling-Taihangshan Gravity Lineament (DTGL). The research of geochemical characteristics of the basalt will place important constraint on the origin as well as the interaction between lithosphere mantle and asthenosphere in the western North China Craton. According to the different landform and lithology, Datong basalt can be roughly divided into east and west regions by the NE-oriented Chenzhuang-Xubao fault. The western region is characterized by numbers of volcanic cones with dominant alkaline basalt, whereas the eastern region is characterized by overflowing tholeiitic basalt with few cones. Many olive and clinopyroxene phenocrysts are observed under the microscope. As the Ni and Cr abundance decrease with that of MgO, we conclude that these phenocrysts should be formed by crystallization. The SiO2 contents of these rocks vary from 45.02% to 53.3% and (K2O+Na2O) from 3.60% to 6.53%. The trace element normalized curves show relatively rich light rare earth elements ((La/Yb)N=5.8~31.6), and the primitive mantle-normalized spider patterns show both positive Rb, Ba, and Sr anomalies (LILE enrichment) and Nb, Ta, and Zr anomalies (HFSE enrichment), similar to those of Ocean Island-like basalt (OIB). According to the simulation calculations of La/Yb-Sm/Yb, it is suggested that the Datong basalts derive from low degree partial melting of garnet-bearing lherzolite, with the melting degree of the alkaline basalt about 1.5%~3.0% and the tholeiitic basalt about 4%~8%. The Datong basalts have relatively lower 87Sr/86Sr (0.703302~0.705102) ratios and higher 143Nd/144Nd (0.512561~0.512963) and 176Hf/177Hf (0.282922~0.283072) ratios, falling into the OIB range. The 207Pb/204Pb-206Pb/204Pb and 143Nd/144Nd-206Pb/204Pb diagrams both show that the Datong basalts are a binary mixture of PREMA and EMⅠ. The differences of the geochemical characteristics can be explained by the two reasons:(1) The different main and trace element features are formed by various degrees of partial melting; (2) The isotope characteristics indicate that the Datong basalts are mainly produced by the partial melting of the depleted asthenosphere, with the varied addition of the few amount of enriched lithospheric components.