郯庐断裂带中部基性岩墙成因：K-Ar年代学、地球化学和Sr-Nd同位素证据

K-Ar Geochronological,Geochemical,and Sr-Nd Isotopic Constraints on the Origin of Mafic Dykes in the Central Tancheng-Lujiang Fault,Shandong Province,China

为查明郯庐断裂带中部基性岩墙的形成机制,对其开展了岩石学、K-Ar年代学、元素地球化学和Sr-Nd同位素研究。研究表明,研究区基性岩墙属于碱性粗玄岩系列,且为晚中生代白垩纪(122.2±2.8~128.3±2.4Ma)岩浆活动的产物。基性岩墙主量素组成范围较窄,富集Rb、Ba、U、Pb和Eu、亏损Nb、Ta和Ti,具有较高的(^(87)Sr/^(86)Sr)i初始比值(0.7068~0.7104)和负的εNd(t)值(-15.2^-16.2)。矿物学和地球化学研究显示,基性岩墙为石榴石二辉橄榄岩地幔低程度(<5.0%)部分熔融作用的产物,形成过程经历了橄榄石、单斜辉石和角闪石的分离结晶作用,同时受到一定程度太古-元古代物质混染作用的影响。富集源区形成机制与榴辉岩化下地壳拆沉作用及强烈的交代作用有密切的联系。

Based on the petrologic,K-Ar geochronological,geochemical,and Sr-Nd isotopic data of the mafic dykes from the central Tancheng-Lujiang Fault,as well the controvercy on the origin of the igneous rocks,this study is focused on the origin of these rocks. The study dykes belong to alkaline shoshonite series,were formed in Cretaceous between 122. 2 ±2. 8Ma and 128. 3 ± 2. 4Ma. In addition,these mafic dykes are characterized by narrow variation range of major elements,enrichment of Rb,Ba,U,Pb,and Eu,depletion of Nb,Ta and Ti,relatively high initial ratios of ^87Sr/^86Sr（ 0. 7068- 0. 7104）,and negative Nd isotopic values（- 15. 2 -- 16. 2）. The mineralogical and geochemical evidences indicate that the mafic dykes were formed by the magma derived from low degree（ 〈5. 0%） partial melting of the garnet lherzolite mantle,through the process of fractional crystallization of olivine,clinopyroxene,and hornblende and the crustal contamination. Moreover,the enriched mantle sources of the study dykes are related to the delamination of eclogitic lower crust and strong metasomatism.