Catena ( IF 6.2 ) Pub Date : 2019-12-10 , DOI: 10.1016/j.catena.2019.104372 Deborah Biondino , Luigi Borrelli , Salvatore Critelli , Francesco Muto , Carmine Apollaro , Sabrina Coniglio , Vincenzo Tripodi , Francesco Perri
The paper focuses on the weathering processes affecting gneissic rocks of the western Sila Grande Massif (Calabria, Italy) through the development of an interdisciplinary research based on field studies and investigations, minero-petrographical analyses and geochemical modeling. Both physical and chemical weathering affect gneissic rocks of the study area, where the combination of tectonic and past climatic conditions played an important key role in the development of complex and deep weathering profiles. Field surveys and cut slope analyses highlight articulated and complex geometric relationships between various classes of weathering (i.e., out-of-sequence weathering horizons giving rise to a partial, or even complete inversion of the “normal” weathering profile). The weathering profile has turned out particularly intense, as classes IV, V and VI are widespread. Among the class VI, the colluvial soils are prominent in comparison whit the residual soils. Borehole logs, that confirm the intensity and complexity of weathering profiles in deep, allow to estimate the presence of weathered rocks to be at least 70 m in thicknesses.
The main mineralogical modifications linked to weathering processes concern the partial transformation of biotite and the partial destruction of feldspars (mainly plagioclases), associated with the neoformation of secondary mineralogical phases (clay minerals and Fe-oxides). Neoformed clay minerals (such as sericite, illite and mixed layer phases) and ferruginous products replaced feldspars and biotite during the most advanced weathering stage. Results of XRD analyses and geochemical modelling provide a good indication on the secondary mineral assemblage due to the increase of the weathering intensity (form class III to class VI) that reflects the different contributions of chemical elements provided by dissolution of silicate minerals into the surrounding groundwater system. The chemical composition of the studied rock samples and the oxides variations suggest a removal of some alkali (Na and K) and alkaline earth (Ca and Mg) into solution as a consequence of weathering reactions. The chemical analysis and the weathering indices (CIA, PIA, CIW and CIW’) show a marked alteration process, with ferromagnesian minerals and feldspars probably dissolved and leached into the surrounding groundwater system. The CO2-controlled dissolution of plagioclase appears to be the most important reaction during chemical weathering. The progressive dissolution results then dominated by biotite, followed by a minor amounts K-feldspar, chlorite and garnet; whereas the sillimanite shows a neglectable amounts. The secondary solid phases observed during the geochemical modeling (illite, followed by vermiculite, ferrihydrite and saponite) are similar to those found in this natural system. The proposed approach could be used to characterize weathered crystalline rocks and related weathering profiles in similar geological setting, and the obtained results represent a key point for the evaluation of the control exerted by weathering on landscape evolution under current environmental settings in terms of sediment generation, soil erosion rates, and mass movements, and for the mechanical characterization of weathering profiles for engineering geological purposes.
中文翻译:
研究影响片麻岩的风化过程的多学科方法(意大利南部卡拉布里亚)
本文通过基于实地研究和调查,矿物岩石学分析和地球化学模型的跨学科研究,着重研究了影响西席拉格兰德地块(意大利卡拉布里亚)的片麻岩岩石的风化过程。物理和化学风化都会影响研究区的片麻岩,在这里构造和过去气候条件的结合在复杂和深层风化剖面的发展中起着重要的关键作用。现场勘测和坡度分析强调了不同类型的风化之间的明确的和复杂的几何关系(即,不连续的风化层位导致“正常”风化剖面的部分甚至完全反转)。第四类的耐候性特别强,V和VI很普遍。在第六类中,与残留土壤相比,河床土壤是突出的。钻孔测井证实了深层的风化剖面的强度和复杂性,可以估算风化岩石的厚度至少为70 m。
与风化过程有关的主要矿物学修饰涉及黑云母的部分转变和长石(主要是斜长石)的部分破坏,以及次生矿物学相(粘土矿物和铁氧化物)的新形成。在最高级的风化阶段,新形成的粘土矿物(如绢云母,伊利石和混合层相)和铁质产品代替了长石和黑云母。XRD分析和地球化学建模的结果由于风化强度的增加(从III类到VI类)的增加而对次生矿物组合提供了很好的指示,这反映了硅酸盐矿物溶解到周围地下水中所提供的化学元素的不同贡献。系统。研究的岩石样品的化学成分和氧化物的变化表明,由于风化反应,溶液中的一些碱(Na和K)和碱土(Ca和Mg)被去除了。化学分析和风化指数(CIA,PIA,CIW和CIW')显示出明显的变化过程,铁磁矿物和长石可能溶解并浸出到周围的地下水系统中。在化学风化期间,CO2控制的斜长石溶解似乎是最重要的反应。逐步溶解的结果以黑云母为主,其次是少量的钾长石,绿泥石和石榴石。而硅线石的量可忽略不计。在地球化学模拟过程中观察到的次级固相(伊利石,然后是ver石,亚铁酸盐和皂石)类似于在这种天然系统中发现的那些。所提出的方法可用于表征相似地质环境中的风化结晶岩和相关的风化剖面,并且获得的结果代表了评估风化对当前环境下在沉积物产生方面对景观演化的控制作用的关键点,土壤侵蚀速率和质量运动,以及用于工程地质目的的风化剖面的机械表征。