U–Pb geochronology of zircon and polygenetic titanite from the Glastonbury Complex, Connecticut, USA: an integrated SEM, EMPA, TIMS, and SHRIMP study John N. Aleinikoff a, * , Robert P. Wintsch b , C. Mark Fanning c , Michael J. Dorais b,1 a U.S. Geological Survey, Denver Federal Center, P.O. 25046, MS 963, Denver, CO 80225, USA b Department of Geological Sciences, Indiana University, Bloomington, IN 47405, USA c Research School of Earth Sciences, Australian National University, P.O. Box 4, Canberra ACT 2601, Australia Received 24 January 2001; accepted 30 April 2002 Abstract U – Pb ages for zircon and titanite from a granodioritic gneiss in the Glastonbury Complex, Connecticut, have been determined using both isotope dilution thermal ionization mass spectrometry (TIMS) and the sensitive high resolution ion microprobe (SHRIMP). Zircons occur in three morphologic populations: (1) equant to stubby, multifaceted, colorless, (2) prismatic, dark brown, with numerous cracks, and (3) elongate, prismatic, light tan to colorless. Cathodoluminescence (CL) imaging of the three populations shows simple concentric oscillatory zoning. The zircon TIMS age [weighted average of 207 Pb/ 206 Pb ages from Group 3 grains — 450.5 F 1.6 Ma (MSWD = 1.11)] and SHRIMP age [composite of 206 Pb/ 238 U age data from all three groups — 448.2 F 2.7 Ma (MSWD = 1.3)], are interpreted to suggest a relatively simple crystallization history. Titanite from the granodioritic gneiss occurs as both brown and colorless varieties. Scanning electron microscope backscatter (BSE) images of brown grains show multiple cross-cutting oscillatory zones of variable brightness and dark overgrowths. Colorless grains are unzoned or contain subtle wispy or very faint oscillatory zoning. Electron microprobe analysis (EMPA) clearly distinguishes the two populations. Brown grains contain relatively high concentrations of Fe 2 O 3 , Ce 2 O 3 (up to f 1.5 wt.%), Nb 2 O 5 , and Zr. Cerium concentration is positively correlated with total REE + Y concentration, which together can exceed 3.5 wt.%. Oscillatory zoning in brown titanite is correlated with variations in REE concentrations. In contrast, colorless titanite (both as discrete grains and overgrowths on brown titanite) contains lower concentrations of Y, REE, Fe 2 O 3 , and Zr, but somewhat higher Al 2 O 3 and Nb 2 O 5 . Uranium concentrations and Th/U discriminate between brown grains (typically 200 – 400 ppm U; all analyses but one have Th/U between about 0.8 and 2) and colorless grains (10 – 60 ppm U; Th/U of 0 – 0.17). In contrast to the zircon U – Pb age results, SHRIMP U – Pb data from titanite indicate multiple growth episodes. In brown grains, oscillatory zoned cores formed at 443 F 6 Ma, whereas white (in BSE) cross-cutting zones are 425 F 9 Ma. Colorless grains and overgrowths on brown grains yield an age of 265 F 8 Ma (using the Total Pb method) or 265 F 5 Ma (using the weighted average of the 206 Pb/ 238 U ages). However, EMPA chemical data identify zoning that suggests that this colorless titanite may preserve three growth events. Oscillatory zoned portions of brown titanite grains are igneous in origin; white cross-cutting zones probably formed during a previously unrecognized event that caused partial dissolution of earlier titanite and reprecipitation of a slightly younger generation of brown titanite. Colorless titanite replaced and grew over the magmatic titanite during the Permian Alleghanian orogeny. These 0009-2541/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII:S0009-2541(02)00076-1 * Corresponding author. Tel.: +1-303-236-7884; fax: +1-303-236-4930. E-mail address: jaleinikoff@usgs.gov (J.N. Aleinikoff). 1 Present address: Department of Geology, Brigham Young University, Provo, UT 84602, USA. www.elsevier.com/locate/chemgeo Chemical Geology 188 (2002) 125 – 147