LASER-ABLATION U Th Pb IN SITU DATING OF ZIRCON AND ALLANITE: AN EXAMPLE FROM THE OCTOBER HARBOUR GRANITE, CENTRAL COASTAL LABRADOR, CANADA

U–Th–Pb geochronology based on data acquired by laser-ablation microprobe – inductively coupled plasma – mass spectrometry (LAM–ICP–MS) has been applied to in situ dating of allanite and zircon in a single section of the October Harbour granite, Labrador. This is the first report of Th–Pb dating of allanite by the LAM–ICP–MS method, confirmed by zircon ages from the same section. Crystals as small as 20 20 m have been analyzed in a standard polished section 100 m thick. Zircon crystals were examined using back-scattered electron images to identify features such as inherited cores, overgrowths and oscil-latory growth. In this study, 45 LAM–ICP–MS analyses were carried out on 35 crystals of zircon. The U–Th–Pb zircon data suggest that the age of the granite is 1657 ± 10 Ma (MSWD = 0.77, weighted mean 207Pb/206Pb ages from 25 concordant points). The data also indicate 1800–1850 Ma inheritance and patterns of multiple loss of lead in many grains. The allanite occurs as large euhedral crystals; they show elevated U and Th contents. Twenty-six analyses of two such crystals of allanite gave sixteen concordant points, obtained mainly from the low-Th core of a single large crystal, and a weighted mean 208Pb/232Th age of 1635 ± 17 Ma (MSWD = 2.1). Although complex Pb-loss is also recorded in portions of the allanite crystals analyzed, no evidence of inheritance was recorded. The age of the October Harbour granite is constrained to be ca. 1640–1660 Ma; it thus is significantly younger than the previously defined conventional multi-grain U–Pb zircon age of 1719 ± 3 Ma attributed to the postulated parental intrusive complex, the Strawberry intrusive suite. The Strawberry suite thus may not be a single ca. 1720 Ma suite of