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

K. De Jong et al., Excess argon incorporation in phengite of the Mulhacen Complex: submicroscopic illitization and fluid ingress during late Miocene extension in the Betic Zone, south-eastern Spain, CHEM GEOL, 178(1-4), 2001, pp. 159-195
Citations number
Categorie Soggetti
Earth Sciences
Journal title
ISSN journal
0009-2541 → ACNP
Year of publication
159 - 195
SICI code
Ar-40/Ar-39 induction furnace and laser step-heating of well-crystallised p ost-tectonic phengitic mica single grains from gneisses of the Mulhacen Com plex with an early Alpine tectonic fabric has resulted in: (1) highly scatt ered integrated ages, (2) an abnormally high atmospheric contamination and (3) often anomalously old apparent ages during early Ar-39 release that is associated with a high Ar-36(AIR) and Ar-37(Ca) contamination. This low-tem perature excess argon (Ar-40(XS)) component is probably released from carbo nate formed during slight alteration of the mica. More than 50% of the samp les yielded plateau ages ranging from 15.8 +/- 0.4 to 90.1 +/- 1.0 Ma. Samp les 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 hal f that differ by 12%. The age variation on these different scales is explai ned by heterogeneous Ar-40(XS) incorporation during a period with a high tr ansient partial argon pressure in the metamorphic fluid, resulting from a l ate stage reheating event. The very swift cooling of 50-100 degreesC/Ma dur ing exhumation of the Mulhacen Complex concomitant with late Miocene extens ion may have prevented the equilibration of different Ar-40(XS) levels in t he mica. HRTEM images of the oldest and youngest phengite specimens show that at lea st 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 mum thick crys tallites of three illitic micas types, which are chemically and structurall y 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 dep leted in K. The oldest sample is derived from a coarse-grained augen gneiss with extensively developed hydraulic cracks, which are lacking in the youn gest sample, a fine-grained mylonitic gneiss. Fluid-rock interaction and co nsequently sub-microscopic illitization were therefore more intense in the coarser-grained rocks. Growth of the illitic micas in equilibrium with a hi gh partial Ar-40 pressure could account for 40Ar,, incorporation in K-vacan cies and other lattice imperfections; Variation in illitisation and associa ted textural dissimilarities between the oldest and youngest mica permit th e different levels of Ar-40(XS), incorporation that account for the observe d age discordance. The finding of Ar-40(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 l attices of phengite and the illitic micas between 800 degreesC and 1000 deg reesC. (C) 2001 Elsevier Science B.V. All rights reserved.