Excess argon incorporation in phengite of the Mulhacén Complex: submicroscopic illitization and fluid ingress during late Miocene extension in the Betic Zone, south-eastern Spain

Abstract

40Ar/ 39Ar induction furnace and laser step-heating of well-crystallised post-tectonic phengitic mica single grains from gneisses of the Mulhacén Complex with an early Alpine tectonic fabric has resulted in: (1) highly scattered integrated ages, (2) an abnormally high atmospheric contamination and (3) often anomalously old apparent ages during early 39Ar release that is associated with a high 36Ar AIR and 37Ar Ca contamination. This low-temperature excess argon ( 40Ar XS) component is probably released from carbonate formed during slight alteration of the mica. More than 50% of the samples yielded plateau ages ranging from 15.8±0.4 to 90.1±1.0 Ma. Samples taken only a few metres apart may differ in age by as much as 50 Ma; a grain that was split over the basal plane yielded plateau ages for each half that differ by 12%. The age variation on these different scales is explained by heterogeneous 40Ar XS incorporation during a period with a high transient partial argon pressure in the metamorphic fluid, resulting from a late stage reheating event. The very swift cooling of 50–100°C/Ma during exhumation of the Mulhacén Complex concomitant with late Miocene extension may have prevented the equilibration of different 40Ar XS levels in the mica. HRTEM images of the oldest and youngest phengite specimens show that at least 20% of the lattice is affected by submicroscopic illitisation, which is concentrated in several micrometer wide zones and veins that cross-cut the basal cleavage. These are made up of aggregates of 0.07–0.30 μm thick crystallites of three illitic micas types, which are chemically and structurally progressively closer to pure illite and occur in different textures. The oldest specimen is affected most severely as the veins contain newly formed (pseudo)illite that does not inherit its crystallographic orientation and chemistry from the host mica, in contrast to the youngest sample. HRTEM–AEM analyses revealed that phengite and the different illitic micas may be depleted in K. The oldest sample is derived from a coarse-grained augen gneiss with extensively developed hydraulic cracks, which are lacking in the youngest sample, a fine-grained mylonitic gneiss. Fluid–rock interaction and consequently sub-microscopic illitization were therefore more intense in the coarser-grained rocks. Growth of the illitic micas in equilibrium with a high partial 40Ar pressure could account for 40Ar XS incorporation in K-vacancies and other lattice imperfections. Variation in illitisation and associated textural dissimilarities between the oldest and youngest mica permit the different levels of 40Ar XS incorporation that account for the observed age discordance. The finding of 40Ar XS plateau ages, despite the degassing of intimately intergrown micaceous minerals, is interpreted by gas release invoked by in-vacuo chemical and structural changes that led to a joint collapse of the lattices of phengite and the illitic micas between 800°C and 1000°C.