Cryptomelane: A tool to determine the age and the physical–chemical regime of a Plio-Pleistocene weathering zone in a granitic terrain (Hagendorf, SE Germany)

Cryptomelane, a K–Mn-bearing oxide, is widespread in the lowermost part of the weathering/supergene alteration zone of the Hagendorf Pegmatite, SE Germany. This supergene Mn mineral has been used in the present study for K/Ar dating of weathering and paleohydraulic processes as young as Plio-Pleistocene. Different cryptomelane age data found in the same weathering and paleohydraulic system resulted from continuous hydraulic processes in the weathering system rather than from a stepwise uplift that shaped the landscape in the granitic area of the NE Bavarian Basement. An almost pure cryptomelane mineralization is indicative of the basal zone of supergene alteration, because a complete separation of Mn from Fe can favorably be achieved only in the infiltration zone under slightly reducing conditions or in the percolation zone under slightly oxidizing conditions. Cryptomelane is precipitated under slightly oxidizing conditions from alkaline solutions in the percolation zone and cast as a marker for fluctuating (paleo)water levels. Manganese oxides, uranyl phosphates and silicates were found at different depths in the weathering zone of this granitic terrain. They were dated by different dating methods (K/Ar and U/Pb) and yielded the same age of formation. Consequently, the 4 Ma signal of both groups of supergene minerals has geomorphological and paleohydraulic connotation and is related to a unique geomorphological process, the Pliocene peneplanation. Gradual hydraulic adjustments gave rise to the younger Pleistocene age found at shallow depth in the weathering zone.