Weathering Products of a Dismantled Variscan Basement. Minero-Chemical Proxies to Insight on Cretaceous Palaeogeography and Late Neogene Palaeoclimate of Sardinia (Italy)

Abstract

This study compares, for the first time, the mineralogy and geochemistry of two residual-clay deposits in NW Sardinia (Nurra district) that formed at different times in tropical and subtropical climates. Both deposits represent palaeosols with deep-weathered residual profiles and overlie Mesozoic carbonate rocks that were deposited on the south European palaeomargin. The older alterite is Cenomanian-Turonian in age and grades upward into a horizon of karstic bauxite, whereas the younger unit occurs within alluvial deposits of Late Neogene age. The Cretaceous palaeosol represents the precursor of the overlying bauxite and formed from unknown sedimentary parent rocks. In contrast, the Messinian weathering products formed by alluvium that was sourced from the Variscan metamorphic basement. Chemical Index of Alteration values, REE fractionation index values, and the results of R-mode factorial analysis suggest a common initial weathering path and a common precursor for the deposits. However, the latter stages of weathering of the Cretaceous palaeosols resulted in lateritic alteration and bauxite production, whereas weathering of the Late Neogene palaeosols produced 2:1 clay minerals under less extreme conditions. Comparison of these residual products constrains the parental material and weathering trends and allows insight into the relationship between palaeoclimate and regional/local palaeogeography of southern Europe during Upper Cretaceous and during Messinian.

Geochemistry as a Clue for Paleoweathering and Provenance of Southern Apennines Shales (Italy): A Review

“…Proxies based on low-mobility elements, affected by minor fractionation during intense weathering and recording chemical differentiation, are used to identify the parent rock(s) of shales [2,3,7,8,10,11,14,15]. Among them, the Eu anomaly is retained as the more conservative provenance proxy [3,10,11,14].…”

“…However, weathering conditions affect the leaching of elements during the path from the protolith(s) to the sediment(s). Low-field-strength elements have a high affinity for aqueous phases, although some of them can be further partially incorporated into secondary clay minerals (see [10,11], and references therein). Mobile elements can be used to evaluate the degree of chemical weathering [12][13][14][15][16] and to characterize the paleoclimate and paleoprecipitation [17][18][19][20][21][22][23].…”

Geochemistry and Geometrical Features of the Upper Cretaceous Vitulano Para-Autochthonous Karst Bauxites (Campania Region, Southern Italy): Constraints on Genesis and Deposition

“…In order to redefine the processes and paleo-conditions that led to the formation of bauxites, geochemical element discrimination and heavy mineral tracing methods have been widely used. These models investigate the distribution of weathering-resistant elements in bauxites, such as Ti, Zr, Nb, Ta, and Rare Earth Elements (hereafter REEs) as a tool to unroof parental affinity by comparing the abundances of immobile elements in both bauxite deposits and potential source rocks [10][11][12][13][14][15][16][17][18]. Great interest has been focused on the Mediterranean Bauxite Province [1,19] since this bauxite province is closely related to the tectonic evolution and paleoclimatic conditions of the Tethyan realm [20,21].…”

“…The Cenozoic bauxites are classified as Salento-type deposits, formed by the erosion and re-deposition of pre-existing Cretaceous bauxite rocks [22]. Previous studies on Italian bauxite deposits focused on ore deposition processes [19], mineralogy [23], elemental geochemistry [8,11,17,22,24,25], and zircon age for paleogeographic restoration [5,18,26]. These studies provided constraints for the provenance of the southern Italy bauxite deposits, the main mineralogical composition, the fractionation of REEs and other critical raw materials, and paleoenvironmental-paleoclimatic conditions.…”

“…Minerals 2023, 13, x FOR PEER REVIEW 2 of 16 elements in bauxites, such as Ti, Zr, Nb, Ta, and Rare Earth Elements (hereafter REEs) as a tool to unroof parental affinity by comparing the abundances of immobile elements in both bauxite deposits and potential source rocks [10][11][12][13][14][15][16][17][18]. Great interest has been focused on the Mediterranean Bauxite Province [1,19] since this bauxite province is closely related to the tectonic evolution and paleoclimatic conditions of the Tethyan realm [20,21].…”