Sedimentary Features and Tectonic Environments of Silurian VolcanicRelated Carbonates in the North Daba Mountains
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摘要: 生物碎屑灰岩、生物礁是造山带内最为常见的岩石类型之一，它们可以形成于多种构造环境。研究这些岩石组合的结构组成及生物赋存状态可为古地理恢复及造山带演化提供依据。分布于北大巴山地区与富TiO2碱性火山岩紧密相关的碳酸盐岩组合长期以来被认为是被动陆缘台地相组合。该套碳酸盐岩组合主要由生物礁、生物碎屑灰岩、砂屑灰岩、泥质灰岩、角砾灰岩共同构成。砂屑灰岩及生物灰岩中常伴随有薄层凝灰岩夹层；同时这些碳酸盐岩中富含不同比例的火山碎屑成分，发育粒序层理、平行层理、波纹斜层理和滑塌构造。生物碎屑灰岩通常与凝灰质砂岩、泥岩构成韵律层，火山质碎屑在类岩石中主要表现为粒径0.5~3 mm的棱角状—次棱角状玄武岩和凝灰岩碎屑，具有近源沉积特征；生物礁中通常出现1~2.5 cm棱角状—次棱角状玄武岩碎屑，且在生物礁之间的砂岩夹层中含有丰富的0.5~1 mm的次圆状玄武岩碎屑；砂屑灰岩中含有棱角状—次棱角状玄武岩和辉石两类碎屑，其中辉石碎屑粒径通常为1~2 mm，同时该类岩石中还含有丰富的黄铁矿，这些黄铁矿通常因其粒径变化而发育粒序结构特征。角砾状灰岩可分别由砂屑灰岩、生物礁及生物碎屑灰岩构成，也可由三者共同构成，玄武岩碎屑仅出现于角砾状生物礁灰岩中。这些碳酸盐岩中的生物化石具有曾经历过明显的搬运改造特征，其中生物碎屑灰岩和砂屑灰岩中的化石碎屑以次圆状为主，生物礁中的生物化石平行于砂岩夹层分布且发生不同程度的压扁和挤压变形，岩石中普遍发育滑塌沉积构造。这些特征共同表明，该套碳酸盐岩与下伏的碱性玄武岩形成密切相关，二者共同构成了与现代大洋中典型洋岛/海山相一致的结构特征，且这些碳酸盐岩多沿着下伏玄武岩的周边沉积，具有深水—斜坡环境的沉积组合，同时因其中所包含的生物化石经历了一定距离搬运作用而发生再沉积，进一步表明这些生物发育时代可能要略微早于该套火山—沉积组合的形成时代。Abstract: Bioclastic and biohermal limestones, which are widely exposed in the orogenic belt, should be formed in different tectonic environments. Identifying their composition and fossil occurrence state is of fundamental importance for palaeogeographical reconstructions and orogenic evolution. Carbonate rocks, which are closely related to alkaline basalts with high TiO2 abundance, have been regarded as platform deposits of passive margin in the North Daba Mountains. They are composed of biohermal limestone, bioclastic limestone, calcarenite, and marlstone with abundant bioclastic and volcanic fragments. Thinner pyroclastic interlayers generally occur in the calcarenite and bioclastic limestone assemblages. Graded, parallel, wavy beddings and slump structure are very common in the outcrops. Bioclastic limestone, tuffaceous sandstone and mudstone generally show rhythmic features. Volcanic fragments within the bioclastic limestone are mainly composed of 0.5 to 3 mm angularsubangular basalt and tuff, indicating proximal sedimentary deposits. Volcanic fragments of the biohermal limestone are dominated by 1~2.5 cm angularsubangular basalts and 0.51 mm subrounded basalt fragments are also abundant in the sandstone beds which are interbeded within the biochermal limestone beds. Two types of volcanic fragments occur in the calcarenite: basalt and pyroxene. They are angular and subangular, and pyroxene fragments range from 1 to 2 mm in size. Abundant pyrite grains in the calcarenite show normal graded structures because of their size variation. Breccia limestone is composed of calcarenite, biohermal and bioclastic limestones, and basalt fragments only exist in the breccia biohermal limestone. Fossil clasts/fragments in the calcarenite and bioclastic limestone are mainly subrounded, suggesting that these fossil fragments had ever been transported for a long distance. Additionally, the growth orientation and distribution of fossils in the biochermal limestone are generally parallel to the sandstone interbeds, and slump structures are very common in this lithology. These characters coevally suggest that these carbonate assemblages are sincerely associated with the underlying basalts with alkaline geochemical affinity spatially; they have the similar framework and constitute characters of modern oceanic island/sea mountain. The carbonate generally developed around the underlying basalts and deposited in the deepwater and slope environments. Moreover, fossils within these carbonates should be derived from other place and were modified during transport, indicating the ages of these fossils should be earlier to the age of carbonate rocks.
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