Serra de Tramuntana of Mallorca is a mountain range built of a stack of thrust sheets composed mostly of Mesozoic platform carbonates, and it formed in the Oligocene and Miocene during the Alpine orogeny. Volcanic rocks, intruding the Triassic sediments, and known mostly from the bottom of the lowest thrust sheet, offer an opportunity for dating the post-sedimentary thermal history of this mountain range and for evaluating the maximum palaeotemperatures by studying the mineralogy and K–Ar dating of authigenic illite. Such a study was conducted on 16 samples from two outcrops, employing X-ray diffraction (XRD), optical microscopy, electron probe microanalysis and K–Ar dating of separated clay fractions. Illite was found in 10 samples, but only one sample was identified as pure volcanic rock, not contaminated by older detrital material. This sample yielded a K–Ar age of 133–140 Ma, which is within the experimental error for three grain-size fractions. This was confirmed by extrapolating the ages of a contaminated sample, and it is interpreted as representing the age of the maximum palaeotemperatures. These palaeotemperatures were estimated using several illite characteristics, including the Kübler Index applied to shales as below but close to the diagenesis/anchimetamorphism boundary (180–200°C). The dated pre-tectonic early Cretaceous thermal event is interpreted as recording the extremely high geothermal gradient at the end of the Mesozoic extensional phase. The maximum palaeotemperatures during the Oligocene–Miocene tectonic burial of Mallorca were not high enough to reset the Mesozoic K–Ar age of illite, thus being lower than ∼250°C, and, based on the preserved Cretaceous illite XRD characteristics, lower than 180–200°C.

X-ray powder diffraction (XRD) studies of the clay fraction of Upper Cretaceous mudstones from a shallow (2.5 km) drill hole in the Niger delta indicate a high geothermal gradient (about 100°C/km) during diagenesis. The mineralogy of the clays is similar to that observed elsewhere and consists of interstratified illite/smectite (I/S), kaolinite, and chlorite. Detrital mica and K-feldspar are also present throughout the section. The composition-depth relationship of the I/S is different from that observed in deeply buried Gulf Coast shales. The Niger delta rocks show a linear change in composition of the I/S as a function of depth in the drill hole from 60 to 10% smectite layers. The I/S ordering is R=0 in shallower samples and progresses to R=1 in deeper samples. The I/S in the deepest samples has R=3 ordering. No R=1 I/S showing a first-order 27-Å XRD reflection was found. The Niger delta sequence differs from the Gulf Coast sequence b. (1) a lack of R=1 I/S showing a 27-Å XRD reflection (which are common in Gulf Coast samples and contain 20% smectite layers), an. (2) the existence of a simple, continuous linear relation between the composition of the R=1 and R=0 I/S and depth.

A comparison of composition-temperature curves for I/S formed under different diagenetic regimes shows different types of I/S ordering in which the presence of R=1 I/S showing a 27-Å reflection and R=3 I/S types changes the composition-depth (and thus, composition-temperature) relations. These changes suggest a difference in the energy necessary to form the various ordering types. Also, geothermal gradient during burial appears to be responsible for different composition-temperature gradients found for the same type of I/S ordering.