闽东南基性岩脉成因及动力学背景研究：Sr-Nd同位素、元素地球化学

Genesis and dynamic setting of mafic dikes in southeastern Fujian evidence from Sr-Nd isotopic and major and trace element geochemistry

闽东南的海边斜闪煌斑岩脉和赤湖辉长岩脉分别形成于晚白垩世和古新世,为高钾钙碱性和钙碱性系列岩石,具有高Al、Na_2O>K_2O的特征;岩浆演化过程中可能经历了以橄榄石、单斜辉石为主的结晶分异作用。海边和赤湖基性岩脉具有相对富集大离子亲石元素和轻稀土元素,亏损高强场元素特征,不相容元素蛛网图显示出大陆边缘弧的地球化学特征,以Nb、Ta、Ti负异常为特征;海边和赤湖明显富集轻稀土元素,(La/Yb)_N分别为5.0～10.9和11.2～12.0。具有高Sr同位素初始值和低ε_(Nd)(t)值,海边的(^(87)Sr/^(86)Sr)_i:0.70577～0.707574,ε_(Nd)(t):-8.1～-1.8,赤湖的(^(87)Sr/^(86)Sr)_i:0.70547～0.70552,ε_(Nd)(t):-0.2～0.6。根据Sr、Nd同位素、微量元素研究及野外地质观察,本区基性岩脉的岩浆在上升侵位过程中未发生地壳物质混染。通过Sr、Nd同位素和微量元素等研究,认为地幔源区的演化与古太平洋板块俯冲密切相关,俯冲流体交代地幔楔、消减洋壳携带的海洋与陆源沉积物参与地幔源区的混合,形成本区基性岩脉的富集地幔源区,大离子亲石元素和轻稀土元素特别富集是俯冲流体与沉积物共同参与源区演化的结果。海边和赤湖基性岩脉形成的构造背景属于活动大陆边缘弧,构造性质应为活动陆缘拉张带(或裂谷带)。晚白垩世和古新世,闽东南发生了地壳拉张事件,与中国东南部晚白垩世以来的地壳拉张期次是一致。

In southeastern Fujian, the Haibian spessartite dike and Chihu gabbro dike emplaced at late Cretaceous and Paleocene, respectively, are high - K calcalkali and calcalkali rocks. They are characterized by high Al content and Na2O 〉 K2O, and possibly formed by fractional crystallization of olivine and clinopyroxene. The mafic dikes show enrichment in IdLE （ such as K, Rb, Ba, Sr, Th） and LREE, depletion in HFSE （such as Nb, Ta, and Ti）. In primary - mantle - normalized spidergram, they have the feature of continental marginal arc and negative anomalies of Ta, Nb and Ti. Chondrite - normalized REE patterns for the mafic dikes are LREE enrichment （La/Ybn = 5.0 -10.9 and 11.2 -12.0 for Haibian and Chihu, respectively）. They are characterized by high initial Sr isotopes （ Haibian ： 0. 70577 - 0. 707574 ; Chihu ： 0.70547-0.70552） and low εnd （t） （ Haibian： - 8.1 ～ - 1.8 ; Chihu ： - 0.2 - 0.6）. According the Sr - Nd isotopic and trace element characteristics and field observation, the mafic dikes are not affected by crustal material during emplacement. Their mantle source may be related with subduction of the Pacific plate, and mixed by normal mantle wedge and oceanic and terrigenous deposits. This results in enrichment of LILE and LREE. The Haibian and Chihu dikes formed extensional continental margin or rifting environment. From late Cretaceous and Paleocene, the crustal extension may happen at southeastern Fujian, which is consistent with the tectonic setting of southeastern China.