Clay Minerals ( 1998) 33, 373-393 Diagenesis-very of clastic sedimentary low-grade metamorphism Cambrian and Ordovician rocks in the Iberian Range (Spain) B. BAULUZ, C. FERNANDEZ-NIETO AND J. M. GONZALEZ LOPEZ Area de Cristalografia y Mineralogia. Dpto. Ciencias de la Tierra, Universidad de Zaragoza, Zaragoza, Spain (Received 21 January 1997; revised I December 1997) ABSTRACT: Cambrian and Ordovician pelites and greywakes from the Iberian Range were studied using XRD, EMPA and AEM/TEM techniques, lllite and chlorite are the main phases in Cambrian <2 gm fractions. Illites show crystallinities (IC) from 0.28 to 0.23~ the 2M1 is almost the only polytype; they do not have expandable layers, and they are heterogeneous in composition. Chlorites show two compositional types, both containing variable proportions of smectite-like layers. Clinochtores growing over bedding surfaces are also present. An evolutionary trend from anchizone to near the anchizone-epizone limit has been proposed for Cambrian samples. In the Ordovician <2 pm fractions, illites are associated with minor chlorites and kaolinites. The ICs range from 0.68-0.28~ the 2M1 polytype dominates over 1M, and it is associated with illite- smectite mixed-layering (R3 ordering). An evolutionary trend from diagenesis to low grade anchizone has been inferred. Chemical analyses of coarser detrital micas indicate muscovite and phengite compositions. The compositional heterogeneity exhibited by authigenic and detrital phases suggests that chemical equilibrium was not attained. In general, detrital sediments which later evolve to greywackes and pelites, are composed of a complex mixture of heterogeneous coarser grains and finer fractions containing phyllosilicates such as micas, chlorite, smectite, kaolinite and various mixed- layered minerals when initially deposited. Later, during diagenesis and low-grade metamorphism they evolve toward a simpler mineral assemblage in which the main phyllosilicates are micas and chlorites. During the last 30 years, many detailed studies on clay mineral evolution through diagenesis and to the onset of metamorphism have been performed (e.g. Frey, 1987, and references therein). The crystallochemical parameters of phyllosilicates that have mainly been used as monitors of diagenetic and incipient metamorphic changes are 'crystallinity', b0 and polytypism of illites, and 'crystallinity' and polytypism of chlorite. Illite and chlorite 'crystal- linity' and polytypism have been related to temperature (Frey, 1987), although other factors such as lithology, stress, and rock and mineral chemistry may play a role in these parameters (Kubler, 1968; Frey, 1987). In the realm of diagenesis/metamorphism, the b0 parameter in white K-micas appears to be controlled mainly by pressure and mineral assemblages (Frey, 1987; Franceschelli et al., 1986). Therefore, in general, knowledge of the phyllosilicate assemblage and their crystallochemical parameters allows an assessment of the general chemical environment as well as the physical conditions of diagenesis-incipient metamorphism to which the rocks were subjected. However, since mineralogical assemblages and crystallochemical parameters are classically deter- mined from X-ray diffraction (XRD) data, only mean values of the samples are obtained. Thus, these ~-~)1998 Thc Mincralogical Socicty