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 x 20 mum have been analyzed in a standard polished section 100 mum thick. Zircon crystals were examined using back-scattered electron images to identify features such as inherited cores, overgrowths and oscillatory 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 Pb-207/Pb-106 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 Pb-208/Th-232 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 rocks, but a more complex composite unit represented by rocks of widely differing ages.