Time span of plutonism, fabric development, and cooling in a Neoproterozoic magmatic arc segment: UPb age constraints from syn-tectonic plutons, Sark, Channel Islands, UK
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New U-Pb zircon and titanite dates from syn-tectonic plutons on the British Channel Island of Sark constrain the time span of plutonism, fabric development, and cooling in this part of the Neoproterozoic Cadomian magmatic arc. The Tintageu leucogneiss is a mylonitic unit that was dated previously at 615.6-2.3+4.2 Ma. The Port du Moulin quartz diorite, which intruded the Tintageu unit, contains a high-strain solid-state deformation fabric that is less intense than, but parallel to, fabrics in the leucogneiss and yields a U-Pb zircon date of 613.5-1.5+2.3 Ma. The Little Sark quartz diorite also displays solid-state deformation fabrics in addition to relict magmatic textures, and yields a U-Pb zircon date of 611.4-1.3+2.1 Ma. The North Sark granodiorite is largely penetratively undeformed, exhibits mainly magmatic fabrics and textures and has a U-Pb zircon date of 608.7-1.0+1.1 Ma. Two fractions of titanite from each intrusion are essentially concordant and are identical within error, with mean dates of 606.5 0.4 Ma (Port du Moulin quartz diorite), 606.2 0.6 Ma (Little Sark quartz diorite), 606.4 0.6 Ma (North Sark granodiorite). The new U-Pb data, in combination with previous U-Pb and 40Ar/39Ar data and previous field studies, confirm the syn-tectonic nature of the Sark plutons and quantify the time span (ca. 7 m.y.) required for intrusion and sufficient crystallization of each body to record incremental strain during waning deformation. Titanite U-Pb and hornblende 40Ar/39Ar dates mark final cooling about 2 m.y. after intrusion of the last pluton.
