ABSTRACT

The major, trace and REE geochemistry of glauconites (as members of the trioctahedral micas) from the early Cretaceous Kurnub Group of Jordan are discussed. The investigated glauconites, with 7.1 to 9.2% K₂O, are ranked as evolved to highly evolved as defined by Odin and Matter (1981). Al₂O₃ contents show a significant inverse relationship with K₂O and Fe₂O₃. The trace element contents show a broad range of variation in contrast to the major elements and a consistent relationship with major elements is lacking. The studied glauconites show a very wide range in the total amount of REE (ΣREE = 4.5 to 564.5 ppm). Two distinct groups have been differentiated accordingly: I. REE-poor group with ΣREE < 15 ppm and with a weak to moderate LREE/HREE fractionation (1.2 to 8.2); II. REE- rich group with ΣREE up to 565 ppm and with moderately to highly fractionated chondrite-normalized patterns (LREE/HREE ratios 3.5 to 64). The group I glauconites are set in an arenaceous (detrital quartz) dolomitic matrix while group II glauconites are embedded in a mudstone or marlstone or has an argillaceous matrix. The REE abundances are therefore lithologically rather than regionally controlled. When normalized to NASC the investigated glauconites display the distinctive hat-shaped pattern reported for other authigenic minerals of marine origin. The calculated Cc anomaly indicates the formation of glauconites at the boundary between oxic and anoxic environment near the sediment water interface.