Comparative K–Ar and Rb–Sr age determinations of retrograde processes on rocks from the KTB deep drilling project

Abstract

Conventional K-Ar and Rb-Sr age determinations were carried out on fine mineral fractions of the KTB rocks and of rocks from the surrounding area in order to get an idea of the timing of retrograde processes which the rocks had suffered in the Late Variscan and afterwards. Coarse-grained mica minerals were used as reference minerals. It can be demonstrated that conventional K-Ar dating on fine mineral fractions is a useful tool to date retrograde processes in crystalline rocks. For methodical reasons the Rb-Sr method is less suitable. A complex age pattern was found which could be divided into different age groups representing different periods of either penetrative retrograde overprint or formation and reactivation of distinct cataclastic fault zones. Whereas the rocks of the KTB have undergone a penetrative retrograde overprint during the Permo-Triassic without any visible deformation, the Jurassic brought a first reactivation of cataclastic fault zones. In Cretaceous time an overprint led to a pronounced second reactivation of cataclastic fault zones. This latter process turned out to be the important one for the stacking and a repetition of the drilled profile in the KTB. The data and interpretations obtained on the fine mineral fractions coincide excellently with fission track data on sphene from the drill site, with sedimentological records concerning a larger regional scale and with hydrothermal alterations and ore-forming events in mid-Europe.

Post-Caledonian brittle deformation in the Bergen area, West Norway: results from K–Ar illite fault gouge dating

“…Based on the same diffusion parameters, Duvall et al (2011) estimated the closure temperature of fine-grained muscovite (2-0.05 µm) to be 250-350°C. This is consistent with empirical studies based on present-day borehole temperatures and estimated palaeotemperatures, which consistently indicate closure temperatures >250°C for illite <2 µm (Hunziker et al, 1986;Wemmer & Ahrendt, 1997). Importantly, these closure temperatures of >250°C suggest that illite in shallow crustal faults generally crystallises below the closure temperature of the K-Ar system, providing crystalli sation rather than cooling ages.…”

Exhumation and uplift of the Sierras Pampeanas: preliminary implications from K–Ar fault gouge dating and low-T thermochronology in the Sierra de Comechingones (Argentina)

“…Hunziker et al (1986) reported a closure temperature interval for the mica fractions \2 lm of 260 ± 30°C, whereas Wemmer and Ahrendt (1997) found indication that fine-grained white micas (sericite \2 lm) did not behave as open systems, even at temperatures of 275°C. Therefore, the closure temperature of finegrained mica has to be estimated somewhere between 275 and 350°C (Wemmer and Ahrendt 1997). Furthermore, errors in the acquisition of accurate K-Ar ages can arise from contamination by other K-bearing phases.…”

The tectonic significance of K/Ar illite fine-fraction ages from the San Luis Formation (Eastern Sierras Pampeanas, Argentina)

“…Hunziker et al (1986) reported a closure temperature interval for the mica fractions \2 lm of 260 ± 30°C. Wemmer and Ahrendt (1997) reported Cretaceous K/Ar ages on fine-grained white mica (sericite \2 lm) from the bottom of the German Deep Drilling Bore Hole (KTB). Here, in situ temperatures of 275°C did not reset the Ar system in the \2 lm serecites.…”

“…At high temperature conditions, potential candidates are kyanite, staurolite or garnet that have been shown to be suitable for stepwise lead leaching experiments (Frei and Kamber 1995), while in greenschist facies and lower-grade metamorphic conditions the Ar systematic in K-bearing minerals such as mica becomes of particular interest (e.g. Hunziker et al 1986;Wemmer and Ahrendt 1997;Clauer and Chaudhuri 1998). In pelitic rocks, K-bearing white micas are formed by prograde dynamo-thermal metamorphic reactions that involve chlorite and illite minerals (Frey et al 1980), whereas fluid-assisted retrogression of highgrade psammopelitic assemblages will result in phyllitic rocks.…”

“…The interpretation of the Ar systematic in illite crystals from low-grade shear zones has to be corroborated by X-ray diffraction-based calculation of the Kübler index, in order to distinguish between newly grown metamorphic crystals or detrital material (e.g. Reuter 1987;Reuter and Dallmeyer 1989;Ahrendt et al 1991;Clauer and Chaudhuri 1998;Wemmer and Ahrendt 1997).…”