ORIGINAL PAPER J. Dostal Æ A. K. Chatterjee Æ D. J. Kontak Chemical and isotopic (Pb, Sr) zonation in a peraluminous granite pluton: role of fluid fractionation Received: 22 January 2003 / Accepted: 30 October 2003 / Published online: 20 January 2004 Ó Springer-Verlag 2004 Abstract The Davis Lake pluton (DLP, 800 km 2 ) of southwestern Nova Scotia, Canada, part of the large peraluminous South Mountain batholith of ca. 380 Ma (U/Pb zircon, Ar/Ar mica), consists of granite and subordinate topaz–muscovite leucogranite that hosts greisen tin-base metal mineralization. A new Pb–Pb isochron age for leucogranite from the most evolved part of the DLP indicates a crystallization age of 378±3.6 Ma, coincident with other radiometric ages of the DLP (Rb–Sr, Re–Os, Pb–Pb). The intrusion displays a compositional zonation defined by lead and strontium isotopic ratios, as well as some major elements (e.g., Si, F), incompatible trace elements (e.g., Li, Rb, Ta, U, Sn), and elemental ratios (e.g., K/Rb and Nb/Ta). The grei- sens and the leucogranites that host them are charac- terized by extreme radiogenic compositions for Pb and Sr, and their chemical-isotopic trends are extensions of the trends displayed by the less evolved granites. The covariations of the isotopic ratios with several major and trace elements and elemental ratios as well as the Pb–Pb and Rb–Sr isochrones indicate that all phases of the intrusion originated from a homogeneous parental magma. The granitoid magma underwent extensive fractional crystallization of feldspars, minor biotite and accessory minerals (monazite, apatite and zircon) in a compositionally zoned magma chamber that was sub- sequently accompanied by fluid fractionation, during which time the internally derived fluorine-rich fluids modified the Rb/Sr, U/Pb and Th/Pb ratios, leading to distinct variations of 87 Sr/ 86 Sr, 206 Pb/ 204 Pb, 238 U/ 204 Pb and 232 Th/ 204 Pb isotopic ratios. These data therefore document the evolution of a granitic magma through magmatic (i.e., crystal fractionation), orthomagmatic (i.e., crystal-fluid fractionation) and hydrothermal (i.e., fluid fractionation) stages that culminated in the for- mation of a tin-base metal deposit. The Pb isotope data also constrain the source region for the DLP as being Avalonian basement that, by inference, must underlie much of the Meguma Terrane. Introduction The isotopic abundances of lead in geological systems are variable and reflect the system’s age, source reservoir and geologic history. In addition to age determinations, radiogenic Pb isotopes have been extensively used as tracers for a wide variety of geological processes, including petrogenetic modeling (e.g., Zindler and Hart 1986; Dickin 1997; Faure 2001). Since physicochemical processes do not fractionate Pb isotopes from one an- other, they preserve a time-integrated record of the U/Pb and Th/U ratios of a reservoir. These elemental ratios can, however, be fractionated during various geological processes as a consequence of the contrasting geo- chemical behavior of these three elements. One such process is fluid fractionation, which is known to lead to a distinct enrichment of some incompatible trace ele- ments (e.g., Tatsumi et al. 1986; Stolz et al. 1996) and could, therefore, potentially modify Pb isotope system- atics. A fractionated, peraluminous, F-rich granitic intru- sion (Davis Lake pluton, DLP) in southwestern Nova Scotia, Canada, provides an ideal opportunity to examine the effects of fluid fractionation on trace ele- ment composition and integrate this with Pb isotopic systematics. Our previous studies of this pluton (Dostal Editorial responsibility: T.L. Grove J. Dostal (&) Æ A. K. Chatterjee Department of Geology, Saint Mary’s University, Halifax, Nova Scotia, B3H 3C3, Canada E-mail: jdostal@smu.ca Tel.: +902-420-5747 Fax: +902-420-5261 D. J. Kontak Nova Scotia Department of Natural Resources, P.O. Box 698, Halifax, Nova Scotia, B3J 2T9, Canada Contrib Mineral Petrol (2004) 147: 74–90 DOI 10.1007/s00410-003-0547-x