Evaluating chemical equilibrium in metamorphic rocks using major element and Sm–Nd isotopic age zoning in garnet, Townshend Dam, Vermont, USA Matthew P. Gatewood a, ⁎, Besim Dragovic b,1 , Harold H. Stowell a , Ethan F. Baxter b , David M. Hirsch c , Rose Bloom c a Department of Geological Sciences, University of Alabama, Box 870338, Tuscaloosa, AL 35487-0338, USA b Department of Earth and Environment, Boston University, 685 Commonwealth Ave., Boston, MA 02215, USA c Geology Department, Western Washington University, 516 High St., Bellingham, WA 98225-9080, USA abstract article info Article history: Received 15 March 2014 Received in revised form 21 January 2015 Accepted 18 February 2015 Available online 26 February 2015 Editor: L. Reisberg Keywords: Metamorphism Chemical equilibrium Garnet Sm–Nd geochronology Porphyroblast growth rates Townshend Dam, VT A quantitative assessment of metamorphic chemical equilibrium derived from correlation of spessartine content and garnet Sm–Nd ages suggests that major element matrix equilibrium was maintained (to a first order) throughout a ca. 40 cm-wide rock sample during garnet growth; however cm-scale Sm–Nd isotopic heterogene- ity limits the Sm–Nd age precision required to evaluate more subtle age differences within individual garnet crystals. Central wafers from 1–3 cm diameter garnet grains within a 1.21 × 10 4 cm 3 block of pelitic schist were used to document concentric growth zoning of major elements, with decreasing Mn and Ca and increasing Fe and Mg from cores to rims. Garnets also preserve growth zoning patterns for HREE and MREE and show evidence for resorption and partial recrystallization of the outermost rims. Similar garnet core compositions and identical garnet rim compositions for large like sized porphyroblasts throughout the sample suggest that garnet growth occurred at near equilibrium P–T–X conditions for major elements over the sample volume. Comparison of 28 rock Sm–Nd isotope values from the sample indicates substantial cm-scale heterogeneity, which precludes meaningful use of local garnet rock isotope pairs for isochron age calculation. Therefore, Sm–Nd isotopic compositions of thirty-eight concentric core to rim garnet segments from ten large (1–3 cm) garnets and two small (1–4 mm) bulk garnets, with narrow ranges of Mn content, are paired with sixteen matrix/whole-rock Sm–Nd isotopic compositions collected over the rock volume to define a range of isochron ages from 383.1 ± 6.8 Ma to 324.5 ± 3.3 Ma. Four of the garnets have anomalously young rims that likely result from post-growth alteration. Chlorite, quartz, and xenotime haloes around garnet suggest that anomalously young garnet rim ages reflect post-growth resorption/recrystallization effects. Excluding these young rims yields a range of ages from 383.1 ± 6.8 (oldest core) to 374.9 ± 1.8 Ma (youngest rim). Sm–Nd age precisions N 1.5 m.y. (and high MSWD) result primarily from isotopic heterogeneity in the finely layered metasedimentary rock matrix. However, garnet cores with high Mn (n = 7), mantles with intermediate Mn (n = 14), and rims with low Mn (n = 8; including the 2 smaller bulk garnet analyses), define three distinct multi-grain isochrons of 380.3 ± 2.0 Ma (n = 23, MSWD = 14), 377.3 ± 1.4 Ma (n = 30,MSWD = 18), and 376.5 ± 1.0 Ma (n = 24, MSWD = 18), respectively, yielding an average garnet growth duration of 3.8 ± 2.2 m.y. These three composite Mn-age zones define a Mn vs. age relationship that reflects depletion of Mn in the rock matrix as it is sequestered by growing garnet. Correlation of garnet major element compositions throughout the sample suggests that major element matrix equilibrium was generally maintained (to a first order) throughout the ca. 4 m.y. duration of garnet growth. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Metamorphic rocks are generally considered as equilibrium pressure– temperature–composition-mineralogical (P–T–X-M) systems. Although this equilibrium view of metamorphism is widely used for interpreting P–T histories of rocks, it is disequilibrium that drives metamorphic reac- tions and produces mineral compositional zoning and metamorphic rock textures (e.g. Carlson, 2002). Therefore, to fully understand the Chemical Geology 401 (2015) 151–168 ⁎ Corresponding author. Tel.: +1 251 709 8634. E-mail addresses: mpgatewood@crimson.ua.edu (M.P. Gatewood), dragovic@vt.edu (B. Dragovic), hstowell@geo.ua.edu (H.H. Stowell), efb@bu.edu (E.F. Baxter), dave@davehirsch.com (D.M. Hirsch), rose.vail.bloom@gmail.com (R. Bloom). 1 Present address: Department of Geological Sciences, Virginia Polytechnic Institute and State University, 1405 Perry St., Blacksburg, VA 24061, USA. http://dx.doi.org/10.1016/j.chemgeo.2015.02.017 0009-2541/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo