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白垩纪时期东南沿海出露了大量的流纹质火山碎屑岩，这些岩石物源复杂，含有大量异源碎屑，限制了学界通过全岩化学组成探讨火山岩的成因与演化，异源碎屑物质来源的研究一直较薄弱。此外，这些火山碎屑岩也是我国重要的热液型铀矿床赋矿围岩类型，对其物源开展分析有助于推动铀元素来源的研究。本文对浙西北大桥坞地区的火山碎屑岩开展研究，野外观察和岩相学鉴定发现火山碎屑岩中裹挟的异源碎屑包括正变质岩、副变质岩、花岗岩岩屑等。对火山岩中的异源碎屑开展挑纯工作并进行主量、微量元素分析。对比分析结果发现，部分主量元素和微量元素含量受到异源碎屑的影响，表现出一定程度的变化，特别是Ce、Gd、Tb等，例如挑纯后的样品Gd含量集中分布在9.4×10-6～13.6×10-6，高于挑纯前4.9×10-6～11.1×10-6；挑纯后样品Tb含量变化于1.3×10-6～2.2×10-6，也高于挑纯前0.8×10-6～1.8×10-6，表明该地区的火山碎屑岩受到一定程度异源碎屑物质的混染，改变了初始火山岩的物质组成。通过元素的协变图解，发现异源碎屑物质的来源可能主要同浙西北陈蔡群副变质岩有关，而花岗岩、石英岩混染作用不明显。此外，前人对火山碎屑岩中锆石的研究发现了陈蔡群同期的720～850 Ma的基底捕获锆石，也印证了陈蔡群副变质岩的混入。因此，浙西北大桥坞火山碎屑岩在喷发的过程中，穿过基底变质岩地层时炸碎并裹挟了一部分碎屑物质，其中主要以陈蔡群副变质岩为主。
Abstract：Large volumes of rhyolitic pyroclastic rocks were exposed along the southeastern coastal region during the Cretaceous， these rocks show complex provenance and contain abundant exotic clasts， which lowers the potentials of whole-rock geochemical compositions to explore the genesis and evolution of these volcanic rocks. Researches on the sources of these exotic detrital materials have rarely been carried out previously. In addition， these pyroclastic rocks are important host rocks for hydrothermal uranium deposits in China. Therefore， provenance analysis of these rocks is of great significance for advancing studies on the sources of uranium. Here we present a case study of the pyroclastic rocks in the Daqiaowu area. Field investigations and petrographic studies suggest that xenolithic debris entrained within the pyroclastic rocks includes fragments of ortho- and parametamorphic rocks， as well as granite clasts. After removing the xenolithic debris， major and trace element analyses were carried out on both debris-removed and untreated samples. The analytical results indicate， to some extent， variations in several major and trace elements between the two groups， such as Ce， Gd， and Tb. For example， Gd contents in the debris-removed samples range from 9.4×10-6 to 13.6×10-6， which is higher than those of the untreated samples (4.9×10-6–11.1×10-6). A similar pattern is observed for Tb， with concentrations of 1.3×10-6–2.2×10-6 in the debris-removed samples compared to 0.8×10-6–1.8×10-6 in the untreated samples. This suggests that pyroclastic rocks were indeed contaminated by xenolithic debris， to some extent， changing original geochemistry for volcanic rocks. Element co-variation diagrams reflect a source of the Chencai Group parametamorphic rock， rather than granite and quartzite， for xenolithic debris. Furthermore， previous zircon dating of pyroclastic rocks revealed some xenocryst zircons of 720～850 Ma simultaneous with the Chencai Group， this also demonstrate a contribution of basement parametamorphic rock debris. Thus， during the eruption of pyroclastic rocks in the Daqiaowu area， some basement metamorphic rocks were broken to debris as explosion and entrapped in the magma， xenolithic debris was dominately sourced from the Chencai Group parametamorphic rocks.
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基本信息:
中图分类号:P588.211
引用信息:
[1]张智星,顾陆宁,郭芳菊,等.浙西北大桥坞地区黄尖组火山碎屑岩异源碎屑特征及物源分析[J].南通大学学报(自然科学版)().
基金信息:
国家自然科学基金青年科学基金项目（41703042）
2026-06-05
2026-06-05
2026-06-05