Geodynamic evolution of the Schwarzwald Crystalline Complex – new insights from zircon U-Pb-Hf isotope systematics of (meta)sedimentary rocks and metabasites

目前，对德国南部黑森林结晶杂岩（SCC）的物源、沉积年龄和（变质）沉积岩和变基性岩的Hf同位素组成知之甚少，阻碍了对中欧前-同华力西期地球动力学演化的详细重建，以及与其他基底单元的比较。为了填补这一知识空白，本项目旨在首次对碎屑锆石颗粒进行系统的U-Pb-Hf同位素研究，这些颗粒选自SCC所有主要岩石单元的约20个（Meta）沉积岩样品（灰岩、砂岩、长石砂岩）和15个变质基性岩样品（角闪岩-榴辉岩相）。这包括高级副片麻岩（角闪岩-麻粒岩相），（1）中央黑森林片麻岩杂岩（CSGC）和（2）南部黑森林片麻岩杂岩（SSGC）的不同“推覆体单元”，（3）Badenweiler-Lenzkirch带（BLZ）的奥陶纪至下石炭世的低级变质沉积岩，该带是分离CSGU和SSGU的拟议缝合带，巴登-巴登带（BBZ）的低级至高级杂砂岩和千屈岩，形成SCC的最北单元，传统上被认为是Moldanubian域和Saxothuringian域之间的过渡带。基于代表性的锆石年龄谱（包括至少120个在结构上良好表征的锆石颗粒/域上进行的U-Pb分析），我们将回答以下开放性问题：（1）形成SCC的主要岩石单元（SSGC，CSGC，BLZ和BBZ）的变质沉积岩的最大沉积年龄（MDA）是多少？(2)把MDA和生物地层记录联系起来？(3)SCC中不同单元的锆石年龄谱是否存在显著差异？(4)沉积物的来源是什么？它是否随时间和空间而变化？(5)大陆腹地的壳幔演化是怎样的？(6)锆石碎屑是否与冈瓦纳大陆的形成、冈瓦纳大陆的裂解、“匈奴超大陆”的演化、Rheic洋的闭合和/或完全不同的东西有关？(7)SCC的不同岩石单元是否代表华力西造山运动期间合并的不同岩石？(8)变质基性岩的形成和变质作用的时间是什么？它们是在什么样的大地构造环境中形成的？(8)SCC的不同地质单元如何与整个华力西带的其他基底单元相关联，包括波希米亚地块、Voges、法国中央地块和伊比利亚。

Presently, little is known about the provenance, depositional ages and Hf isotopic composition of (meta)sedimentary rocks and metabasites of the Schwarzwald Crystalline Complex (SCC) in southern Germany, preventing detailed reconstructions of the pre- to syn-Variscan geodynamic evolution in Central Europe, as well as comparison with other basement units. To close this gap of knowledge, this project aims at presenting the first systematic U-Pb-Hf isotope study on detrital zircon grains, selected from about 20 (meta)sedimentary rock samples (greywackes, sandstones, arkoses) and 15 metabasite samples (amphibolite- to eclogite-facies) of all major rock units of the SCC. This includes high-grade paragneisses (amphibolite- to granulite-facies) of different “nappe units” of the (1) Central Schwarzwald Gneiss Complex (CSGC) and (2) Southern Schwarzwald Gneiss Complex (SSGC), (3) low-grade metasedimentary rocks of Ordovician to lower Carboniferous age of the Badenweiler-Lenzkirch Zone (BLZ), a proposed suture zone separating the CSGU from the SSGU, and (4) low- to high-grade greywackes and phyllonites of the Baden-Baden Zone (BBZ), forming the northernmost unit of the SCC, traditionally assumed to host the transition zone between the Moldanubian and Saxothuringian domains. Based on representative zircon age spectra (comprising at least 120 U-Pb analyses carried out on texturally well characterized zircon grains/domains), we will give answers to the follow open questions: (1) What are the maximum depositional ages (MDA) of the metasedimentary rocks forming the major rock units of the SCC (SSGC, CSGC, BLZ, and BBZ), and what was the geotectonic setting during deposition? (2) Correlate the MDA with the biostratigraphic record? (3) Are their significant differences in zircon age spectra between different units of the SCC? (4) What is the sediment provenance, and did it change through space and time? (5) What was the crust-mantle evolution in the hinterland through time? (6) Is the zircon detritus related to Gondwana formation, Gondwana break-up, “Hun superterrane” evolution, Rheic ocean closure and/or something completely different? (7) Do the different rock units of the SCC represent different terranes amalgamated during the Variscan orogeny? (8) What is the timing of metabasite formation and metamorphism, and in which geotectonic environment were these formed? (8) How do the different geological units of the SCC correlate with other basement units throughout the Variscan Belt comprising the Bohemian Massif, Voges, French Massif Central and Iberia.