The extent of inter-mineral isotope equilibrium: a systematic bulk U-Pb and Pb step leaching (PbSL) isotope study of individual minerals from the Tertiary granite of Jerissos (northern Greece)
European Journal of Mineralogy Volume 8 Number 5 (1996), p. 1175 - 1190
published: Oct 30, 1996
manuscript accepted: Apr 18, 1996
manuscript received: Oct 9, 1995
ArtNo. ESP147050805020, Price: 29.00 €
Abstract Pb stepwise leaching (PbSL) has been successfully used to date rock-forming silicates. In the present study this technique was applied to magmatic minerals (titanite, epidote, allanite, biotite, apatite and zircon) from the Tertiary granite of Jerissos (northern Greece) with the aim to constrain whether or not isotope equilibrium among different phases in a young, undeformed pluton has been reached and to which extent equilibrium is preserved. The minerals responded positively towards the preferential release of radiogenic Pb during acid leaching and an enhanced data spread was obtained in both common Pb-Pb diagrams. A Pb-Pb isochron of 51 ± 16 Ma for titanite, representing an undisturbed date for the emplacement of the Jerissos granite, is in accordance with a conventional U-Pb date of 53.6 ± 6.2 Ma obtained from fractions of uranothorite. Furthermore it supports the assumption - which is based on geochemical data - that the emplacement of the 50 Ma old Sithonia plutonic complex to the south could be genetically related to the intrusion of Jerissos granite. Conventional U-Pb dating of accessory phases (zircon, uranothorite, brannerite) is complicated because they are disturbed by variable postigneous mobility of U, Th, or Pb and by inherited, inhomogeneous, older Pb components pointing to assimilation of material from a composite country rock terrane of at least Jurassic age, most probably of rocks from the Serbomacedonian Massif (SMM) in northeastern Greece. Besides revealing the presence of microscopic solid inclusions, PbSL revealed that isotope equilibrium is retained between plagioclase, titanite, epidote, allanite, and apatite, and that secondary biotite and K-feldspar were formed during a late magmatic alteration stage from a Pb isotopically contaminated K-rich fluid.