Zircon extraction from rock

Size-fractions of zircon extracted from a pluton of unfoliated quartz diorite which intrudes the intensely folded metasedimentary rocks of the Anthill and Cocks formations in the Skelton Glacier-Cocks Glacier area at 78°39 and 161°00 E, were dated by Rowell et al. (1993) by the U-Pb method. The data define a chord that intersects concordia at a point that corresponds to a well-constrained date of 551 4 Ma. Zircon from a second pluton, composed of granite and located only 1 km from the quartz-diorite pluton, yielded an identical date of 551 4 Ma (Encarnacion and Grunow 1996). This date not only establishes the age of the two plutons, but also sets a lower limit to the time of deposition and deformation of the metasedimentary rocks that were intruded by these plutons. 

In the Taupo volcanic zone of New Zealand, the 26.5 ka Oruanui eruption was studied by Charlier and Zellmer (2000). Three fractions of zircons (sub 63 pm 63-125 pm 125-250 pm) were extracted from the rhyolitic pumice, which together with the whole rock respectively define three ages from 5.5 to 12.3 ka before eruption (Fig. 12b). Microscopic observation of the zircons showed that they are composed of a core surrounded by euhedral rims, and the preferred explanation of the authors is that zircons represent mixtures in variable proportions of old crystal cores crystallized 27 ka before eruption and crystal rims crystallized just before eruption. 

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