Johnson et al., 2003 equilibrio subsólido y suprasólido a baja presi&Atild

Johnson et al., 2003 equilibrio subsólido y suprasólido a baja presi&Atild

Info iconThis preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
American Mineralogist, Volume 88, pages 624–638, 2003 0003-004X/03/0004–624$05.00 624 INTRODUCTION Accurate characterization of the pressure ( P ) conditions of metamorphism in moderate- to high-temperature ( T ), low- P (andalusite-sillimanite) terranes is problematic. In part this is a consequence of the poorly constrained boundary between the stability fields of andalusite and sillimanite based on experi- mental or extrapolated thermodynamic data (e.g., Pattison et al. 2002). Furthermore, most of the commonly used P -sensi- tive net-transfer reactions involve garnet, which may be absent in metapelitic rocks metamorphosed at low- P conditions, or which may contain an insufficient concentration of the relevant component (e.g., grossular in the GASP barometer) to avoid errors that are large (> or >>1 kbar). In such cases, a powerful method for constraining the P-T evolution of terranes charac- terized by low- P metamorphic field gradients is by compari- son of inferred stable parageneses with equilibrium phase diagrams constructed in P-T- composition (X) space for suit- able bulk compositions (pseudosections) using internally con- * E-mail: timj@geol.umd.edu Low–pressure subsolidus and suprasolidus phase equilibria in the MnNCKFMASH system: Constraints on conditions of regional metamorphism in western Maine, northern Appalachians TIM E. JOHNSON, 1, * MICHAEL BROWN, 1 AND GARY S. SOLAR 2 1 Laboratory for Crustal Petrology, Department of Geology, University of Maryland, College Park, Maryland 20742, U.S.A. 2 Department of Earth Sciences, SUNY College at Buffalo, Buffalo, New York 14222, U.S.A. ABSTRACT The peak of regional metamorphism in western Maine was reached at ca. 404 Ma during the waning stage of Devonian Acadian deformation. Regional metamorphic mineral assemblages in metapelitic rocks range from greenschist to upper amphibolite facies. Subsolidus rocks are charac- terized by the association andalusite + staurolite; at the highest grades, anatectic migmatites are developed. Results of thermodynamic modeling in the MnNCKFMASH system are consistent with field data and imply a metamorphic field gradient that extends from 3.5–4.0 kbar at lower grades (500–520 C) to > 4.5 kbar at suprasolidus temperatures that exceeded 700 C. Regional isotherms that are inferred to have been shallowly inclined at lower grades are closely spaced around synmetamorphic granites and at the migmatite front, consistent with advection-controlled intracrustal redistribution of heat within the regionally extensive thermal high. Peak pressures vary both along and across the strike of the Central Maine belt, which is interpreted to record differential thickening during syntectonic metamorphism. Contact metamorphism associated with the Mooselookmeguntic igneous complex occurred ca. 35 million years after the regional metamorphic peak, and records higher pressure conditions than the regional event. We suggest that the final increment of late-Acadian thickening accounts for the pressure increase, consistent with regional cooling prior to the emplace- ment of the Mooselookmeguntic igneous complex. Pluton emplacement at deeper levels ca. 35 mil-
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 03/19/2012 for the course GEOLOGY 6 taught by Professor Rm during the Spring '12 term at UNAM MX.

Page1 / 15

Johnson et al., 2003 equilibrio subsólido y suprasólido a baja presi&Atild

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online