One form of albitization consists of the replacement (albitization) of Ca-bearing plagioclase by secondary Na-plagioclase (i.e. albite) and is typically accompanied by alteration of biotite to chlorite and the staining of the rock by iron oxide pigments. In crystalline rocks, albitization is rather widespread but is traditionally considered result from deep and high-temperature metasomatic alteration linked to magma cooling during exhumation. Studies of albitization have principally been undertaken by petrologists who are mostly interested in the origin and emplacement of granites and naturally focus on aspects of their petrography and the details of their parageneses in this context. They interpret albitization to be a ‘window’ to post magmatic processes.
But albitization can also be a consequence of surficial weathering. The example we studied in Spain (Fig. 1) is widespread and spatially and temporally associated with a Triassic palaeosurface. Its occurrence forces a completely different point of view about this form of alteration. A major gap separates the two points of view about albitization and our findings have exposed a ‘cultural barrier’ that has to be breached in order to advance knowledge about this process, particularly where it is found in crystalline rocks. Key foci in the scientific exchange ought to include the geometry of the alteration in the field, petrographic observations detailing stages in the alteration, and the dating of these stages.
The outcrop scale
The albitization we have studied in granites is characterised by reddening due to impregnation of the feldspars by iron oxides (Fig. 2). It results in: (1) a pervasive coloration distributed rather homogeneously within the rock volume, (2) fracture-bound red to pinkish granite facies or (3) spotting when the alteration is weak, resulting in pinkish spots distributed within non-reddened rock.
In terms of structure, a vertical profile of albitized granite, with a typical thickness around 150-200 m, occurs below the Triassic unconformity in the Guilleries and Roc de Frausa Massifs in NE Spain (Fig. 3). The most intense alteration occurs at the top, immediately below the unconformity, and progressively decreases in intensity with depth. From the top to the bottom, the alteration facies change from red to pink and ultimately spotted. Unaltered granites are found below (Fig. 3).
The microscope scale
Microscope observations of the albitized granites show a suite of reactions in the major minerals including plagioclase albitization, K-feldspar microclinization, biotite chloritization and quartz recrystallization. Albitization affects the primary plagioclase, where the Ca-bearing cores of the crystals are pseudomorphosed by secondary albite along twin planes and micro-fractures (Fig. 4a, b). Within this secondary albite, part of the calcium released during plagioclase replacement forms small patches of secondary calcite. Primary K-feldspar crystals are partially pseudomorphosed by secondary microcline (Fig. 4a). Biotite and is replaced by secondary chlorite (which contains large numbers of apatite inclusions compared with primary biotite; Fig. 4c), secondary titanite and anatase. Clusters of rare-earth carbonate minerals, probably synchisite-Ce, can be seen inside the secondary porosity of albitized plagioclase and microclinized orthoclase in backscattered electron-microscope images (Fig. 4d). Secondary quartz partially replaces primary quartz in the upper part of the profile (Fig. 4e). Primary and secondary monazite occur at the reaction front between albitized and unaltered granite (Fig. 4f).
Dating of albitized profiles has been conducted at different sites through Europe using a variety of dating methods, including: (1) K-Ar dating of albite and K-feldspar (Bonhomme et al., 1980; Schmitt et al., 1984); (2) palaeomagnetism of Fe-oxides (Ricordel et al., 2007; Parcerisa et al., 2010; Thiry et al., 2011; Yao et al. 2011, Franke et al., 2011, Yao, 2013) and, most recently, (3) K-Ar microprobe dating of albite and K-feldspar and electron microprobe U-Th-Pb dating of monazite (Fabrega et al., 2019). All have established that the albitization and associated alteration occurred during the Late Permian to Early Triassic, around 250 Ma, while the Variscan granites were exposed at the landsurface.
Thus, the red-stained albitization relates specifically to the Triassic palaeosurface and was preserved beneath it by a Mesozoic and Cenozoic sediment cover. The alteration profiles are palaeoweathering features relating to surficial weathering and were nor much eroded. The geopetal organization (decreasing intensity of albitization with depth ending in alteration confined to fractures) is characteristic of weathering and alteration under the influence of geomorphology, climate, and groundwater.
Palaeoenvironmental setting of the alteration
The morphology of the alteration profile and its proximity to the Permo-Triassic palaeosurface suggests that the responsible fluids were Na-rich groundwaters (Fig. 5a). The depth of the alteration profile, the long-lasting tectonic stability in the region, and the continental scale abundance of saline environments in the Permo-Triassic period were probably key factors in the development of this near-surface weathering event across Europe. The albitization profiles remained preserved below the Triassic sedimentary cover or, in some cases (Fig. 5b), directly at the surface. In contrast to the non-albitized granites usually decompose to sandy material when exposed to surface conditions, the albitized granite has an increased resistance to weathering and erosion.
Extent of the Late Permian to Early Triassic palaeoweathering
Thus far, this form of palaeoweathering has been observed in Morocco, the Catalan Coastal Ranges, Eastern Pyrenees, Galicia and La Mancha in Spain; Corsica, the Maures, French Central Massif (Fig. 6), Morvan, the Alps, the Vosges and Brittany in France; in Germany; Great-Britain and Ireland; Sudetes in Poland; and in the Oslo Fiord in Norway.
We have demonstrated that the albitized facies are of surficial origin and bound to the weathering (geochemical) environment linked to the Triassic palaeosurface. This dramatically changes ideas about the geomorphic evolution of the basement areas over a large part of western Europe and is a major contribution to the geodynamic modeling of the region.Indeed, reconstruction of palaeosurfaces is a unique tool in unravelling the evolution of ancient continents. Extending age data from the better-known sedimentary basin deposits to adjacent crystalline basement landscapes (particularly of palaeoweathering features) provides the possibility to set the correct framework for geodynamical models (Fig. 6).
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Dr Carles Fàbrega (Departament d’Enginyeria Minera, Industrial i TIC, Escola
Politècnica Superior d’Enginyeria de Manresa, Spain)