Tracking fluid action in Little Sark Pluton (Channel Islands, UK): a UV laserprobe Ar/Ar study of excess 40Ar

Schwenzer, Susanne P.; Schwanethal, James; Tindle, Andrew G.; Kelley, Simon Peter and Sherlock, Sarah C. (2011). Tracking fluid action in Little Sark Pluton (Channel Islands, UK): a UV laserprobe Ar/Ar study of excess 40Ar. In: AGU Fall Meeting, 5-9 Dec 2011, San Francisco, CA, USA.

Abstract

Little Sark pluton (Channel Islands, UK) is a weakly to moderately deformed pluton of quartz diorite composition. Zircon from Little Sark has been dated as 611.4(+2.1/-1.3) Ma [Miller et al. 1991, Tectonophysics, 312], whereas titanite has an age of 606.2±0.6 Ma. Hornblende yields an Ar-Ar age of 606.4±3.4 Ma [Dallmeyer et al., 1991, J. Geol. Soc. London 148]. The rocks show evidence for post-magmatic hydrous alteration including chloritisation of biotite, and sericite formation in feldspar. We investigated the alteration in order to understand its influence on the Ar-Ar system. Detailed petrographic investigations of two samples indicate different deformation levels – S1, (less deformed), and S2 (more deformed). The feldspar is andesine to oligoclase in composition, but contains K-rich areas with patchy and sometimes cleavage guided sericitization. Mafic minerals include hornblende and biotite (the latter partially chloritized). Ar-Ar investigations were carried out using high spatial resolution UV laser ablation gas release in combination with a Nu instruments Noblesse gas mass spectrometer using spots of 80 to 140 µm diameter.
Mafic minerals in S1 have highly variable 38Ar/39Ar and 37Ar/39Ar ratios and unusually high atmospheric contents for such old samples, possibly reflecting the presence of fluid inclusions associated with the sericite, particularly in the feldspar. The apparent ages in their entirety range from 429 to 668 Ma. Hornblende yields homogeneous apparent ages, whereas the younger and older ages are measured in both micas and feldspars, whose apparent ages range from 245–670 Ma. The modal age for feldspar is around 400–450 Ma, but this may not have geological meaning. Sample S2 shows many of the same features as S1. Mafic minerals span a slightly smaller age range and feldspars display apparent ages of ~245–645 Ma with a mode around 480 Ma.
The alteration of both samples appears to have been accompanied by the introduction of 40Ar-rich fluids. At least one fluid appears to have been a chlorine-rich brine with excess 40Ar, the presence of excess argon is confirmed by very high apparent ages in quartz. A second end member was a chlorine-poor fluid with higher 36Ar contents and associated with lower apparent ages. The combination of observed mineralogical changes and spatially resolved Ar-Ar data indicate the pathways of the fluids, and demonstrates the power of high spatial resolution Ar-Ar dating to unravel a succession of geologic events.