扬子陆块西缘寒武系砂岩的物源分析：对古地理位置重建和构造背景的指示

扬子陆块西缘寒武系主要为一套碎屑岩- 碳酸盐岩的岩石组合，前人研究多认为形成于相对稳定的克拉通盆地。但同时期出现的大陆岩浆作用显然与前期认定的克拉通盆地性质不符，需要借助扬子西缘的物质来源探讨构造背景。基于野外露头等资料，本文通过对扬子陆块西缘会泽和会东附近寒武系3件砂岩样品进行重矿物分析、电气石电子探针和碎屑锆石U- Pb测年分析，确定扬子西缘寒武纪沉积物的源区；并结合沉积序列等综合探讨扬子陆块西缘寒武纪的构造背景。沉积序列表明，扬子西缘寒武系沧浪铺组、西王庙组和二道水组主要由砂岩和白云岩等组成，沉积环境为滨岸—潮坪。细—粗砂岩碎屑颗粒为次棱角状—次圆状，分选较差；碎屑组分主要为石英，岩屑几乎全部为燧石，长石含量较少。测试分析结果表明：重矿物分析指示扬子西缘寒武系砂岩重矿物主要由锆石、赤—褐铁矿、电气石、钛铁矿、金红石、磷灰石等组成，重矿物组合指示岩浆岩为其主要母岩；电气石电子探针分析结果表明，物源主要来自于贫锂花岗岩和变砂岩、变泥岩；碎屑锆石测年分析表明物源区母岩主要为983~540 Ma岩浆岩。碎屑锆石年龄对比等综合分析表明，寒武系沉积物部分源自康滇古陆983~708 Ma的岩浆岩和变沉积岩，部分源自冈瓦纳大陆东非造山带663~540 Ma的岩石，物源区岩石经历再旋回产物作用。扬子西缘寒武系的沉积序列、碎屑锆石年龄谱图和碎屑组成等特征综合分析表明，扬子陆块西缘寒武系形成于前陆盆地。

Provenance analysis of Cambrian sandstones in the western margin of Yangtze block: Implication for paleogeography reconstruction and tectonic setting

During the Cambrian period, a set of clastic and carbonate rock associations was mainly deposited in the western margin of the Yangtze block. These sedimentary deposits have commonly been interpreted as representing a vast and stable craton sourced from older rocks in the Oldland. However, the widely contemporaneous magmatism recorded within the sediments is obviously inconsistent with the craton basin identified earlier. Therefore, the tectonic background needs to be determined based on the material sources of the western margin of the Yangtze block. Field outcrops and other data are used to determine the source areas of the Cambrian sediments in the western margin of the Yangtze block through heavy mineral analysis, tourmaline electron probe microanalysis, and detrital zircon U- Pb dating of three sandstone samples from Huize and Huidong counties. We then discuss the tectonic background of the Cambrian period in combination with a comprehensive analysis of sedimentary successions. These successions show that the Canglangpu, Xiwangmiao, and Erdaoshui formations are mainly composed of sandstone and dolostone, which were deposited in shore and tidal flat environments. The sandstones are fine- to coarse- grained, with subangular to subrounded and poorly sorted fabric. Quartz is the dominant constituent, while lithic fragments consist mainly of cherts, and feldspar grains are very rare. The detrital heavy minerals are mainly zircon, hematite- limonite, tourmaline, ilmenite, rutile, and apatite, suggesting a magmatic rock source. The results of tourmaline electron probe microanalysis further confirmed that the Cambrian sediments are mainly derived from granitoids, their associated pegmatites and aplites, metapelites, and metapsammites. Detrital zircon dating provided valuable insights into the age of the parent rocks in the provenance area, indicating a distribution ranging from 983 Ma to 540 Ma. Comparisons of the geochronological data with detrital zircon ages from different blocks reveal that the sediments were derived from 663~540 Ma rocks of the East African orogeny, as well as the underlying old sediments (dated 983~708 Ma) in the Kangdian Oldland. Contrary to previous studies suggesting a simple craton setting for the Cambrian period, our analysis of detrital zircon spectra indicates an active continental margin setting during this time. The sedimentary successions, detrital zircon spectra, and detrital grain framework collectively suggest that the Cambrian sediments were formed in a collisional setting, especially a foreland basin.