陈蔡早古生代俯冲增生杂岩对华夏与扬子地块拼合过程的指示意义

Chencai Early Paleozoic Subduction-Accretionary and Their Restrictions on Collage Between Cathaysia and Yangtze Block

华夏地块与扬子地块的拼合时限与方式长期存在争议。本文对出露于浙江诸暨一带的原"陈蔡岩群"进行了详细的露头尺度解剖。野外地质调查表明,原"陈蔡岩群"主要由不同性质的外来岩块与基质组成。其中:代表外来岩块的大理岩及斜长角闪岩的变质年龄分别为(424.7±2.9)和(420.6±1.8)Ma,成岩年龄分别为(479.2±9.5)^(424.7±2.9)Ma和(507.7±7.8)^(420.6±1.8)Ma,斜长角闪岩原岩为具OIB(洋岛玄武岩)特征的碱性玄武岩,大理岩的原岩为海相碳酸盐岩,二者共同构成了洋岛海山组合。代表原地岩块的变长石石英砂岩主要物源区为3 620~1 530 Ma形成于活动大陆边缘和大陆岛弧环境下的古老地壳物质;与之构造混杂接触的斜长角闪岩变质年龄为(438.0±2.5)Ma,其原岩分别为形成于消减带岛弧环境的岛弧拉斑玄武岩、形成于俯冲环境下的富Nb玄武岩和洋岛海山环境下的具OIB特征的碱性玄武岩类。代表基质的含榴黑云斜长片麻岩LA-MC-ICP-MS锆石U-Pb年龄测试结果表明,其变质年龄为(441.0±3.0)Ma,碎屑206Pb/238 U年龄多数为840~780 Ma,反映其物源主要来自于新元古代,且最年轻的沉积年龄限定在598 Ma,说明片麻岩原岩可能为早古生代沉积地层。陈蔡地区该套岩石组合的发现表明,原"陈蔡岩群"的构造属性应为早古生代俯冲增生杂岩。结合测区及龙游地区新发现的加里东期麻粒岩和退变榴闪岩,提出扬子与华夏两大地块碰撞于445~420 Ma。

The timing and model of the collage between Cathaysia block and Yangtze block have been debated for a long time. In this study, a detailed profile anatomy from ＂Chencai Group＂ was carried out on outcrop scale in Zhejiang. The field geological survey showed that the ＂Chencai Group＂ mainly consists of exotic rocks and matrix. The metamorphic age of the marble and amphibolite from the exotic rocks are （424.7±2.9） and （420.6±1.8） Ma respectively, and their diagenesis ages fall between （479.2±9.5）-（424.7±2.9） Ma and （507.7±7.8）-（420.6±1.8） Ma respectively. The protoliths of amphibolites show the OIB characteristics similar to those of alkaline basalts, and the marble protoliths belong to marine carbonate rocks that together make up the combination of ocean island seamount. The provenance of autochthonous feldspar quartz sandstone was the ancient crust in a continental margin and continental arc environment aged between 3 620-1 530 Ma. The protoliths of amphibolites were island arc tholeiite formed in a subduction zone, and Nb-rich basalts formed in a subduction environment,and alkaline basalts with OIB features in an ocean island seamount environment. The LA-MC-ICP-MS zircon U-Pb data indicated that the metamorphic age of the matrix garnet-bearing biotite-plagioclase gneiss is （441.0±3.0） Ma, with detrital age of 840-780 Ma. These data demonstrated that the source of the gneisses was mainly formed in the Neoproterozoic, with the youngest depositional age of 598 Ma, an Early Paleozoic sedimentary strata. The recognition of the rock units in the Chencai region showed that the ＂Chencai Group＂ belonged to the early Paleozoic subduction accretionary complex. In combination with the Caledonian granulite and garnet amphibolite in the Longyou area, the collision between the Cathaysia and Yangtze block occurred during 445-420 Ma.