Petrology of the regional sillimanite zone, west-central New Hampshire, U.S.A., with implications for the development of inverted isograds

Other| April 01, 1995 Petrology of the regional sillimanite zone, west-central New Hampshire, U.S.A., with implications for the development of inverted isograds Frank S. Spear; Frank S. Spear Rensselaer Polytechnic Institute, Department of Earth and Environmental Sciences, Troy, NY, United States Search for other works by this author on: GSW Google Scholar Matthew J. Kohn; Matthew J. Kohn Search for other works by this author on: GSW Google Scholar Stephan Paetzold Stephan Paetzold Search for other works by this author on: GSW Google Scholar American Mineralogist (1995) 80 (3-4): 361–376. https://doi.org/10.2138/am-1995-3-418 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Frank S. Spear, Matthew J. Kohn, Stephan Paetzold; Petrology of the regional sillimanite zone, west-central New Hampshire, U.S.A., with implications for the development of inverted isograds. American Mineralogist 1995;; 80 (3-4): 361–376. doi: https://doi.org/10.2138/am-1995-3-418 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract Sillimanite from the regional sillimanite zone in west-central New Hampshire is fibrol-itic and overprints F2 folds (nappe stage) of earlier mica foliation. Regional sillimanite zone samples show no evidence for earlier staurolite parageneses, despite the fact that staurolite is abundant at lower grade, because sillimanite was produced directly from garnet + chlorite by the prograde (heating) reaction garnet + chlorite + muscovite + quartz = sillimanite + biotite + H2O. The pressure at which this reaction occurs is sensitive to the MnO and CaO contents of garnet, and phase-equilibrium arguments reveal that at the regional pressures of west-central New Hampshire (2–4 kbar), staurolite parageneses are only possible in rocks with low MnO + CaO.The inferred P-T path is counterclockwise with nearly isobaric initial heating at 2 kbar, followed by loading (± heating) to a peak metamorphic temperature of 600 ± 25 °C at 4 kbar, followed by nearly isobaric cooling. Garnets were compositionally homogenized near the metamorphic peak, but subsequent cooling was rapid (> 100 °C/m.y.). These data argue against the folded isotherm model of Chamberlain (1986). Instead, the present distribution of metamorphic grades is interpreted to be the result of regional stacking of high-grade thrust sheets on lower grade rocks, followed by depression of high-grade rocks to lower structural levels. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not currently have access to this article.