Residual clay deposits on basement rocks: The impact of climate and the geological setting on supergene argillitization in the Bohemian Massif (Central Europe) and across the globe

详细信息    查看全文

• 作者：Harald G. Dill ^{haralddill@web.de}
• 关键词：Phyllosilicates ; Chemical weathering ; Basement ; Geodynamic setting ; Climate
• 刊名：Earth-Science Reviews
• 出版年：2017
• 出版时间：February 2017
• 年：2017
• 卷：165
• 期：Complete
• 页码：1-58
• 全文大小：13731 K
• 卷排序：165

文摘

Supergene argillitization on basement blocks shows a great variety of deposits with regard to their mineralogical composition and morphology, particularly those which developed during the Late Cretaceous and the succeeding Cenozoic era, driven by vertical displacements along highland boundary faults. The geodynamic setting and the morpho-climatic zonation across the globe from the pole to the equator have a strong impact on the evolution of these argillitizations and are made accountable for the zonal argillitization. The great variety of parent rocks, closely related to the geodynamic setting cannot be cast aside as the azonal argillitization is concerned. In conclusion, this intimate interrelationship of azonal and zonal processes leads to a tripartite subdivision of argillitization on a global scale into (1) super-mature, (2) mature, and (3) pre-mature stages of supergene argillitization. The basement block of the Bohemian Massif provides a good coverage as to the supergene argillitization and is representative of the mature stage with on one side relics of the super- and on the other side indications of the pre-mature stages. Therefore the evolution of supergene argillitization can well be documented with all its facets there and discussed in comparison to geodynamic settings from elsewhere on the globe being representative of the preceding super-mature and succeeding pre-mature stages.Almost all types of 7, 10 and 14 Ångstrom (Å) phyllosilicates can be identified in the fine-grained residual rocks resulting from zonal and azonal processes of chemical weathering of the Bohemian Massif, which is one of the crustal sections most intensively studied for this sort of near-ambient argillitization and shared in by four Central European countries, Austria, the Czech Republic, Germany and Poland. In this sedimentary review, the uplifted basement block is taken as a reference site to establish the various morphological types of argillitization found across the globe (type A: weathering crust and dissemination, type B: continuous and discontinuous blankets of massive regolith, type C: continuous blanket of zoned argillitization, type D: vein-like and gossan-type argillitization, type E: stratified traps incised in to the basement (perennial lake fed by alluvial-fluvial drainage systems (type E I) and maar lake (type E II)) filled mainly with argillaceous sediments.Moreover, this crustal section in Central Europe offers a good coverage to determine the physical-chemical regime of a wide range of phyllosilicate associations (kaolinite-muscovite-illite-(smectite), nontronite-saponite-smectite, chlorite-vermiculite-illite/muscovite-(biotite), serpentine-talc-chlorite-smectite, halloysite-(kaolinite)) and their accessory minerals (iron sulfides, phosphates and carbonates, iron and manganese oxides-hydrates, aluminum-phosphate-sulfate minerals (APS minerals) and aluminum hydrates, titanium- and titanium-iron minerals, silica, heavy minerals). This is due to the outstanding geodynamic position held by the Bohemian Massif, as the core zone of Mesoeurope. The orogen under consideration took morphological shape by the Late Paleozoic and forms the intermediate and reference part in the tripartite subdivision scheme. Its Precambrian predecessors and Late Mesozoic and Cenozoic followers with regard to the geodynamic settings and morphoclimatic zones across the globe were reviewed as to their basement-related argillitizations in this study.The supermature stage is today well exposed in the lowlands and peneplains of the deeply eroded Precambrian cratons. Large argillaceous deposits of type B (mainly continuous blankets of massive argillitization), locally reworked into type E argillitization are common, e.g., in Africa and South America. The chemical and mineralogical variability of the phyllosilicate assemblages is low to moderate, dominated mainly by kaolin, which often occurs together with bauxite and laterite in mixed-type argillitization-duricrusts deposits, limiting the value of this crustal section as a prime target of supergene argillaceous rocks. Duricrusts are typical of the ultimate stage of near-ambient element mobilization and depletion during supergene alteration.The mature stage is represented by the highly eroded mountain belts of Paleozoic and Early Mesozoic age (e.g. Hercynian/Variscan and Cimmerian basement blocks) which the Bohemian Massif belongs to. Argillitizations in these crustal sections provide the greatest variability in terms of the morphological types as well as the chemical and mineralogical composition that is why it is an ideal objective for reference. On account of the advanced level of dissection the size of the deposits of argillitization is reduced and the linear types, e.g., vein-like deposits are more widespread than the blankets of massive argillitization. Duricrusts are encountered in the zone of argillitization, but only as relics at limited amounts, while rivers debouching in the foreland of the uplifted basement blocks, giving rise to world-class kaolin deposits (e.g. Central Europe, Ukraine, and USA).The deeply dissected and most strongly uplifted mountain or modern fold belts of Late Mesozoic to Cenozoic age (e.g. the Andes and the Himalaya) show a great mineralogical and geological variability of argillitization, but frequently with a predecessor stage involving hypogene argillitization. If present at all, due to the strong erosion, zones of supergene argillitization often reflect the vertical extend exceeding the lateral one, with morphological types A, D and E prevailing over type B. Rift valleys or grabens as well as island arcs both create an auspicious framework for supergene argillitization if their rocks are exposed to subaerial weathering. Depending upon the vertical displacement and the lateral extension of the continental grabens, these linear crustal sections may either be more akin of the high-altitude mountain belts or they may closely resemble the mature (e.g. Rhein Graben) and super-mature (e.g. East African Rift) settings being present alongside the graben axis, and mentioned previously. The island arcs have features which resemble as to the outward appearance of the argillitization on their active side the environment known from the modern fold belts and on their passive side the super-mature zone of argillitizations. Supergene argillitization in arc systems are closely related to the subtype of arc system. Compressional subtypes as along the eastern margin of the Pacific Ocean (e.g. South American continent) fail to generate extensive supergene argillitization, whereas on the opposite margin of the Pacific Ocean, extensional ones are prone to supergene argillitization.Alternating wet and dry climatic conditions are not only favorable for the formation of soils (ultisol, oxisol, vertisol) but also very effective during the development of zones of supergene argillitization on basement rocks resulting in argillaceous zones with kaolinite, smectite, halloysite, Ni-bearing serpentine, talc, chlorite and smectite. Abnormally high precipitation as it is the case within the humid tropical zone leads to a complete wash-out of alkaline and earth alkaline elements, thereby hampering widespread built-up of phyllosilicates and sparking formation of Fe-Al-bearing duricrusts. By contrast, abnormally low precipitation common to the arid region is also deleterious to the development of phyllosilicates. It favors the precipitation of evaporites or hampers the formation of a chemical weathering crusts at all. This is true also for the high-altitude areas or regions near the pole in a landscape shaped by periglacial or glacial processes, where mechanical disintegration is abundant.While the climate-induced phyllosilicate assemblages are the proto-types of zonal argillitization, Fe-bearing smectites, (nontronite or Fe-bearing saponite, Fe phosphates, Fe carbonates, Fe-Mn hydrates and Fe-Ti oxides in the argillitization zone are the reference types of azonal argillitization, because of their response to fluctuating ground water levels and taking on the role of an ideal redox meter. They document as its best changing hydraulic conditions in the zone of argillitization, in the course of a continuous or stepwise uplift or the other way round when warped down. These redox meters may form under various climatic regimes irrespective of the geodynamic conditions in pre-mature, mature and supermature stages. The term azonal is also used for those cases where the parent rocks have a strong impact on the composition of the argillitization (e.g. ultrabasic rocks).What might be encountered all across the globe in different sites of “pre-mature”, “mature” and “supermature” stages, with respect to basement-related argillitization, can be studied within the crystalline basement rocks of the Bohemian Massif, in the narrowest of space and confined to two discrete phases, the first one lasting from the Late Carboniferous to the mid Triassic resultant from the uplift of the Variscan crystalline parent rocks and in a second phase of argillitization during the late Cretaceous and Paleogene as a consequence of the Alpine Orogeny that triggered a renewed uplift. The first phase left its imprints mainly in the contemporaneous argillaceous foreland sediments and in the basins subsided into the uplifted basement block, the second one has been preserved atop of the Bohemian Massif dependent upon the position of the argillitization relative to the active and passive margins of the highland.The current review is the logic supplement to the review “Kaolin: soil, rock and ore.From the mineral to the magmatic, sedimentary, and metamorphic environments” where emphasis was placed on the extractive or economic part of clay geology of kaolin (Dill, 2016). In this review on muddy to friable sedimentary rocks the environmental clay geology was given priority.