大别—苏鲁造山带变质岩原岩组合与闽浙沿海晚中生代岩浆岩组合的对比:对扬子板块北东缘新元古构造属性的启示

Comparison of Protolith Assemblages of Metamorphic Rocks in the Dabie-Sulu Orogen and the Late Mesozoic Magmatic Rock Associations in the Coastal Region of Zhejiang and Fujian Provinces:Implications for the Neoproterozoic Tectonic Setting of Northeastern Y

大别—苏鲁造山带在构造上位于扬子板块北东缘,该造山带主体由花岗质片麻岩所占据,其间尚分布有少量呈似层状、透镜状产出的榴辉岩,二者原岩的时代均为新元古代,原岩的岩性分别为酸性和基性火成岩。本文在对这些原岩岩石组合及其地球化学特征详细分析的基础上,发现它们与闽浙沿海晚中生代的岩浆岩组合十分相似,主要表现在:(1)二地岩浆岩类型均以酸性岩占绝对优势,基性岩所占比例很小(仅约5%),它们均构成以酸性岩占主体的双峰式岩石组合;(2)二地的酸性岩在成因类型上均主要为I型花岗岩类,但部分岩石表现出A型花岗岩特征,其中少数岩体发育碱性铁镁矿物,属典型的过碱性A型花岗岩,它们均与I型花岗岩构成I-A型复合岩体,且二地的A型花岗岩均可进一步归为后造山背景下产出的PA型或A2型花岗岩类;(3)二地基性岩的地球化学组成明显相似,在微量元素蛛网图上均表现出显著的Nb,Ta负异常,在相关的构造环境判别图解上,均主要投影于火山弧或大陆玄武岩区域。鉴于对闽浙沿海晚中生代岩浆作用的构造背景已有较成熟的认识,即普遍认为它们形成于因古太平洋板块俯冲诱发的弧后伸展引张环境,因此,大别—苏鲁造山带变质岩原岩组合与闽浙沿海晚中生代岩浆岩组合所表现出的高度相似性,无疑指示扬子板块北东缘在新元古时期应处于活动大陆边缘的弧后伸展引张背景。

The Dabie-Sulu orogenic belt is tectonically situated in the northeastern margin of the Yangtze Block.This belt is mostly underlain by granitic gneisses（over 80%）,and minor eclogites occur as pods or layers within them.The protoliths of the granitic gneisses and eclogites correspond to acidic and basic igneous rocks,respectively,and both of them are of Neoproterozoic ages.In this paper,the protolith assemblages and geochemical characteristics of these metamorphic rocks are systematically compared with those of the Late Mesozoic igneous rock associations in the coastal area of Zhejiang and Fujian Provinces.The results have shown that they share quite similar features which can be summarized as follows：（1）Igneous rock types in both regions are dominated by acidic members with the basites constituting only about 5%,and there is a bimodal rock association with the acidic rocks as the predominant components.（2）The dominant acidic rocks in both regions can genetically be categorized into I-type granitoids,but a few of them show features of A-type granites.There are several plutons containing alkaline mafic minerals and can be ascribed to peralkaline A-type granites.These A-type granites,spatially coexisted with the I-type granites and commonly occurring as I-and A-type composite plutons,can be further grouped into PA or A2 subtype which generally suggested to have been formed in a postorogenic extensional setting.（3）The geochemical characteristics of basites in both regions are also quite similar.Both of them show Nb and Ta depletions on the primitive-mantle normalized spidergrams,and are mostly plotted in the fields of volcanic-arc or continental basalts on the related tectonic discrimination diagrams.Considering that there is generally a consensus concerning the tectonic setting for the Late Mesozoic magmatism in the coastal area of Zhejiang and Fujian Provinces, i.e.,generated in a back-arc extensional setting induced by the subduction of Paleo-Pacific plate,therefore,the highly similar features between the protolith assemblages of metamorphic rocks in the Dabie-Sulu orogen and the Late Mesozoic igneous rock associations in the coastal area of Zhejiang and Fujian Provinces,undoubtedly indicate that the northeastern margin of Yangtze Block during the Neoproterozoic might have been under a back-arc extensional setting along an active continental margin.