A priorievidence for zircon antecryst entrainment in megacrystic Proterozoic granites

Research Article| March 01, 2016 A priori evidence for zircon antecryst entrainment in megacrystic Proterozoic granites A. Heinonen; A. Heinonen * 1Department of Geosciences and Geography, Division of Geology and Geochemistry (DiGG), PO Box 64, Gustaf Hällströmin katu 2a, University of Helsinki, 00014 Helsinki, Finland *E-mail: aku.heinonen@helsinki.fi Search for other works by this author on: GSW Google Scholar I. Mänttäri; I. Mänttäri 2Research Laboratory, Geological Survey of Finland, PO Box 96, 02151 Espoo, Finland Search for other works by this author on: GSW Google Scholar O.T. Rämö; O.T. Rämö 1Department of Geosciences and Geography, Division of Geology and Geochemistry (DiGG), PO Box 64, Gustaf Hällströmin katu 2a, University of Helsinki, 00014 Helsinki, Finland Search for other works by this author on: GSW Google Scholar T. Andersen; T. Andersen 3Department of Geosciences, University of Oslo, PO Box 1047, Blindern, N-0316 Oslo, Norway Search for other works by this author on: GSW Google Scholar K. Larjamo K. Larjamo 1Department of Geosciences and Geography, Division of Geology and Geochemistry (DiGG), PO Box 64, Gustaf Hällströmin katu 2a, University of Helsinki, 00014 Helsinki, Finland Search for other works by this author on: GSW Google Scholar Geology (2016) 44 (3): 227–230. https://doi.org/10.1130/G37696.1 Article history received: 14 Aug 2015 rev-recd: 18 Jan 2016 accepted: 21 Jan 2016 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation A. Heinonen, I. Mänttäri, O.T. Rämö, T. Andersen, K. Larjamo; A priori evidence for zircon antecryst entrainment in megacrystic Proterozoic granites. Geology 2016;; 44 (3): 227–230. doi: https://doi.org/10.1130/G37696.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Entrainment of antecrystic zircon in early saturated main silicate phases has been recognized as a complicating factor in high-precision U-Pb geochronology of Phanerozoic granitic systems, but has not been demonstrated for Precambrian rocks. We report U-Pb ages (secondary ion mass spectrometry) and trace element (laser ablation–inductively coupled plasma–mass spectrometry) compositions of zircon from three main ferroan granite types of the late Paleoproterozoic Wiborg rapakivi granite batholith of southeastern Finland and vicinity. Zircon was extracted from the interior parts of megacrystic (diameters as large as 15 cm) rapakivi-textured alkali feldspar ovoids using a microdrilling technique. Compared to zircon in ovoid groundmass, zircon in the ovoids has higher incompatible element values (e.g., uranium and rare earth elements) and, for two of the three samples examined, slightly older (by ∼1 m.y.) average U-Pb ages. These observations suggest that the ovoid material of the rapakivi granites entrained zircon from earlier magmas that were compositionally different than the final magmatic host of the ovoids. Groundmass zircon implies similar average U-Pb ages for all the studied samples, ca. 1628 Ma; this is regarded as the final crystallization age of the granites. These observations show that it is viable to measure statistically valid age differences between antecrysts and groundmass of Proterozoic granites. Overall, our sampling technique provides enhanced textural control of antecrystic zircon in a wide range of sample materials. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.