PII S0016-7037(98)00303-2 Palaeoproterozoic thermal events recorded in the 4.0 Ga Acasta gneiss, Canada: Evidence from SHRIMP U-Pb dating of apatite and zircon YUJI SANO,* ,1 KENTARO TERADA, 1 HIROSHI HIDAKA, 1 KAZUMI YOKOYAMA, 2 and ALLEN P. NUTMAN 1 1 Department of Earth and Planetary Sciences, Hiroshima University, Kagamiyama 1-3, Higashi-Hiroshima, 739, Japan 2 Department of Geology, National Science Museum, Hyakunincho 3-23-1, Shinjuku, Tokyo 169, Japan (Received September 24, 1998; accepted in revised form November 3, 1998.) Abstract—U-Pb isotopes of apatites and zircons extracted from a sample of the Acasta gneisses that occurs along the western margin of the Slave craton in Northwest Territories of Canada have been measured using a SHRIMP II ion microprobe. Eleven apatite analyses (30 m spot) give 238 U/ 204 Pb- 206 Pb/ 204 Pb and 204 Pb/ 206 Pb- 207 Pb/ 206 Pb isochron ages of 1905  86 Ma (2 ) and 1936  28 Ma (2 ), respectively. Analyses (20 m spot) on the partly recrystallised central portions of fourteen zircons mostly define a mixing array on a 238 U/ 206 Pb*- 207 Pb*/ 206 Pb* concordia plot with concordia intercepts at 4014  25 Ma (2 ) and 1967  93 Ma (2 ). The former agrees with 3.96 – 4.02 Ga protolith ages obtained on the oldest components of the Acasta gneisses by other workers. The lower concordia intercept for the zircon data is consistent with the U-Pb age of apatites and may be related to a 500°C thermal event, perhaps early in the Palaeoproterozoic Wopmay orogeny. Copyright © 1999 Elsevier Science Ltd 1. INTRODUCTION The early Archaean (3.6 – 4.0 Ga) Acasta gneisses are exposed in the foreland and metamorphic internal zone of the Wopmay orogen. They occur in Palaeoproterozoic structural basement culminations, which are contiguous with the westernmost part of the Archaean Slave province, Northwest Territories of Can- ada. Some components of the Acasta gneisses have been es- tablished as the oldest known intact terrestrial rocks (Bowring et al., 1989; Stern et al., 1997). This observation was based on SHRIMP (Sensitive High Resolution Ion MicroProbe) U-Pb analyses of zircons. The geochronology indicates that the Acasta gneisses are a complex group of rocks, with protolith ages of ca. 3.7 to 4.0 Ga, and a marked thermal event at 3.6 Ga, shown by dating of zircon overgrowths (e.g., Bowring et al., 1989). Apatite, which is common in the Acasta gneisses, incorpo- rates both U and Pb when it crystallises, and has an effective closure temperature of 500 – 600°C for the U-Pb system (Cher- niak et al., 1991; Krogstad and Walker, 1994). Pristine zircon, on the other hand, has an effective closure temperature of 1000°C for U-Pb (Lee et al., 1997), and thus can be a robust tool for obtaining protolith ages of components in gneisses with protracted, severe, thermal histories (e.g., Bowring et al., 1989; Schiøtte et al., 1989; Friend and Nutman, 1992; Nutman et al., 1992, 1996). However, if the zircon lattice becomes damaged by radioactive decay, radiogenic Pb can be easily lost at 1000°C, giving younger apparent U/Pb ages. Furthermore, areas of damaged zircon can be recrystallised (e.g., Greenland metatonalite zircon shown in Fig. 1 and Pidgeon, 1992), with the expulsion of previously accumulated radiogenic Pb. Thus, loss of radiogenic Pb from “damaged” and “recrystallised” zircon occurs at temperatures below 1000°C, in the realm of temperatures realised during greenschist– granulite facies meta- morphism. It is important that this open system behaviour for Pb located in “damaged” and “recrystallised” domains in zircon be distinguished from the much more sluggish solid-state dif- fusion through a pristine zircon lattice. In this study we have obtained U-Pb data on apatites and variably damaged zircon from a sample of Acasta gneiss, using the SHRIMP II ion microprobe recently installed at Hiroshima University. The purpose of these measurements is to under- stand the thermal history of the sample. Of particular interest is the time when temperatures in the range 500 – 600°C (the effective closure temperature in apatite) were last experienced. 2. SAMPLE AND ANALYTICAL METHODS The Acasta gneiss sample is a leucocratic quartzofeldspathic gneiss consisting mainly of plagioclase, alkali-feldspar, quartz, and biotite, collected from the Slave Province of the Northwest Territories of Canada, where Archaean ortho- and paragneisses with formation ages of 2.9  4.0 Ga are exposed (Isachsen and Bowring, 1994). Zircon and apatite were separated from the rock sample using standard crushing and heavy-liquid techniques. Zircon grains were mounted in epoxy with several grains of two standard zircons, “SL13” and “QGNG.” SL13 is the well-known Sri Lanka megacryst with the age of 572 Ma extensively used by the Australian National University SHRIMP group as a U/Pb and abundance calibration standard (Roddick and van Bree- men, 1994; Claoue ´-Long et al., 1995; Williams, 1997) and QGNG is a new multicrystal zircon standard from Quartz-Gabbro-Norite-Gneiss (QGNG) from Cape Donnington, Eyre Peninsula, South Australia whose TIMS U/Pb age is 1850  2 Ma (2 ) (C.M. Fanning, personal communication, 1997). Apatite grains were also mounted in an epoxy disc with several grains of standard apatite, “PRAP,” derived from an alkaline rock of Prairie Lake circular complex in the Canadian Shield (Bell et al., 1987) dated at 1156  45 Ma (2 ) derived from pooled concordant 207 Pb/ 206 Pb ages (Sano et al., 1999). Zircons and apatites were polished until they were exposed through their mid-sections to provide a flat surface for sputtering of secondary ions. After the surface was finished using 0.25 m diamond paste, the standard and unknown zircons and apatites were imaged using cathode-luminescence and electron probe microanalyser (EPMA) in order to locate inclusion-free homogeneous regions suitable for analysis. Then following cleaning to minimize surface contaminant Pb, they were gold-coated to prevent charging of the sample surface by the primary ion beam. The samples were evacuated in the sample lock overnight in order to *Author to whom correspondence should be addressed (ysano@ue.ipc. hiroshima-u.ac.jp). Pergamon Geochimica et Cosmochimica Acta, Vol. 63, No. 6, pp. 899 –905, 1999 Copyright © 1999 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/99 $20.00 + .00 899