Campaign-style titanite U-Pb dating by ICP - Earth Science ...

K.J. Spencer et al. / Chemical Geology 341 (2013) 84–10185on quantifying how the rates of phase transformations depend onintensive parameters (e.g., stress and temperature) and materialproperties (Rubie and Thompson, 1985; Hacker and Kirby, 1993).Geodynamic models that include phase transformation are forcedto make general assumptions about rates (e.g., Sung and Burns,1976; Behn et al., 2011) or to extrapolate experimental data (e.g.,Mosenfelder et al., 2001)—which, in spite of our best efforts, remainuncertain because of an inability to quantify nucleationrates with sufficient precision (e.g., Rubie et al., 1990; Hacker etal., 2005). The common observations of incomplete phase transformationsin rocks (e.g., Mørk, 1985; Austrheim, 1987; Koons et al.,1987; Wayte et al., 1989; Hacker, 1996; Engvik et al., 2000;Krabbendam et al., 2000; Lenze and Stöckhert, 2007; Peterman etal., 2009) and the presence of slow waveguides in subductingslabs (e.g., Hori et al., 1985; Abers et al., 1999) aretworatherclear indicators that mineral transformation rates in Earth neednot follow equilibrium. The factors that influence transformationrate are similar to those listed above for rheology. This studyshows that titanite U–Pb dates can be used as a sensitive monitorof phase transformations in continental crust at high P and T. Theresults expand our understanding of phase transformations in general,and re-emphasize that models based on chemical equilibriumare only endmember models.1.3. Titanite closure to PbEstimating the whole-grain closure temperature of titanite commencedwith the concerted effort to date titanite by thermal-ionizationmass spectrometry (TIMS) in the late 1980s (Fig. 1). Tucker et al. (1987,1990, 2004) and Tucker and Krogh (1988) measured 56 multigraintitanite samples and used the data to define a single U–Pb discordia betweenca. 1657 Ma and 395 Ma. They explained this discordia as a resultof diffusive Pb loss during a single, short-lived thermal event at 395 Ma,based on titanite textures and because the lower intercept of the discordiais well defined. Other natural titanite datasets (Gromet, 1991; Mezger etal., 1993; Scott and St-Onge, 1995; Corfu, 1996; Pidgeon et al., 1996;Verts et al., 1996), and an experimental study by Cherniak (1993) are inFig. 1. Titanite closure to Pb diffusion estimated from previous TIMS studies (boxes)and calculated from experiments (lines) of Cherniak (1993). Colors identify 10-folddifferences in cooling rates. Lines calculated from Cherniak (1993); have experimentaluncertainties of roughly −45/+90 °C. (For interpretation of the references to color inthis figure legend, the reader is referred to the web version of this article.)general agreement that cooling at rates of 10–100 K/Myr from temperaturesof 700–800 °C should be sufficient to cause measurable Pb lossin titanite as coarse as 2–10 mm, and to cause total Pb loss from1 mm radius grains heated to 750 °C (Fig. 1). As demonstrated below,our conclusion is that titanite is much more retentive, and can remainclosed to Pb for tens of millions of years at length scales of 1 mm andtemperatures as high as 750 °C.2. Study area: the Western Gneiss RegionThe Western Gneiss Region (WGR) of Norway contains one ofEarth's giant ultrahigh-pressure terranes (Fig. 2). Like most wellexposedand extensively investigated UHP terranes, the WGR isdominated by quartzofeldspathic gneiss; eclogite and peridotite compriseonly a few volume percent of inclusions within the gneiss. Assuch, the WGR is typical of the bulk of the continental crust. Thequartzofeldspathic gneiss and minor mafic rocks formed chiefly bymagmatism at ~1.7–1.6 Ga, 1.2 Ga, and ~970–950 Ma (Corfu andAndersen, 2002; Walsh et al., 2007; Krogh et al., 2011). The eclogitesformed from mafic protoliths during subsequent inferred northwestwardsubduction of the Baltica continental margin (Krogh, 1977;Andersen et al., 1991; Hacker and Gans, 2005). They crop out overan area of ~30,000 km 2 (Hacker et al., 2010) and range in peak metamorphicconditions from ~650 °C and 1.8 GPa near the foreland to~850 °C and 3.6 GPa (Figs. 2 and 3) in the core of the orogen (seegeothermometry reported below and summaries in Cuthbert et al.,2000; Ravna and Terry, 2004; Hacker, 2006). The eclogitesrecrystallized during the late, Scandian, phase of the Caledonian orogeny,between ~425 and ~400 Ma based on U–Pb zircon, Lu–Hf garnet,Sm–Nd garnet, and Rb–Sr white mica dates (see summary in Glodnyet al., 2008; Kylander-Clark et al., 2009; Krogh et al., 2011).The host gneiss consists almost exclusively of (garnet-) amphibolitefaciesminerals with local (garnet-bearing) granulite. U–Pb titanite andSm–Nd garnet geochronology has revealed that these minerals in thehost gneiss are i) Proterozoic in the eastern half of the WGR toward theforeland, and thus predate the UHP metamorphism; and ii)400–380 Ma in the western, core of the orogen, and formed during thepost-UHP exhumation (Tucker et al., 1987, 1990; Johnston et al., 2007;Walsh et al., 2007; Kylander-Clark et al., 2009; Peterman et al., 2009).The peak temperatures of this exhumation-related metamorphism were650–800 °C, and the exhumation was nearly isothermal (Fig. 3) (Terryet al., 2000; Labrousse et al., 2004; Walsh and Hacker, 2004; Root et al.,2005); new Zr-in-titanite data presented below add measurably to thisdataset. 40 Ar/ 39 Ar muscovite dates are 400 Ma in the eastern WGR and385–375 Ma in the core of the orogen (Fig. 2) (Hacker et al., 2005;Walsh et al., 2007; Young et al., 2011), and presumed herein to recordcooling below ~400–500 °C (Harrison et al., 2009). This relativelyextensive geochronologic dataset shows that the (U)HP event enduredfor 25 Myr and that the post-UHP amphibolite-facies eventlasted 10–15 Myr; the total time at temperatures of 650–800 °Cwasthus25–40 Myr, depending on location (Fig. 3).Although there is general agreement that the host gneissexperienced the same (U)HP conditions as the included eclogite blocks,direct evidence of that is limited to one non-eclogite gneiss withdiamond (Dobrzhinetskaya et al., 1995) and a few locations wherethe quartzofeldspathic gneiss has quartz pseudomorphs after coesitein garnet or clinozoisite (Wain, 1997; Walsh and Hacker, 2004); indirectevidence of local (U)HP recrystallization of the host gneiss includesgarnet with Sm–Nd dates similar to those of (U)HP eclogite(Peterman et al., 2009). Quartzofeldspathic gneiss with possible (U)HP matrix minerals—such as kyanite, phengite, zoisite, sodic clinopyroxene,or garnet—is restricted to rare domains, generally interlayeredwith eclogite or in strain shadows around eclogite blocks (Wain,1997; Cuthbert et al., 2000). The near-total absence of (U)HP mineralsin the quartzofeldspathic gneiss requires that either the gneiss did nottransform to (U)HP minerals or that all such phases disappeared during