ABSOLUTE TIMING OF MONAZITE FORMING REACTIONS IN THE CHESHAM POND NAPPE, SW NEW HAMPSHIRE, USA: IMPLICATIONS FOR TECTONIC ASSEMBLAGE OF CENTRAL NEW ENGLAND THRUST SHEETS

Pyle and Spear (2003) investigated a series of low-pressure migmatites from the Chesham Pond nappe, SW New Hampshire, USA. Four distinct generations of monazite growth were identified, and three (zones 2, 3 and 4) were correlated with both temperature and major-phase mineral assemblage; zone 1 cores were interpreted to be detrital relics or vestiges of an earlier Acadian metamorphism. The four monazite zones have been dated by in-situ isotopic and chemical methods, and yield the following domain ages (average ± 1 standard error of the mean): 396±4 Ma (zone 1); 381±4 ma (zone 2); 370±3 (zone 3); 353±6 Ma (zone 4). Heating and cooling rates derived from monazite thermochronometry and Ar-Ar closure temperatures are 3-12°C/My for heating from ~500 to 750°C, and 5-7°C/My cooling from 750 to 350°C. Temperature-time paths calculated from monazite thermochronometry indicate that plutonism at ca. 400 Ma (Kinsman quartz monzonite) was the likely heat source for the formation of zone 1 monazite, but long-lived (380-350 Ma) low-P, high-T metamorphism recorded by monazite zones 2, 3, and 4 was regional in scale, and may have been caused by lithospheric mantle delamination in a eastward-dipping subduction zone, followed by compensatory upwelling of asthenospheric mantle that heated a wide area of the Merrimack basin (SW New Hampshire, central Massachusetts, central Connecticut) to temperatures in excess of 725° C. Monazite ages in the Chesham Pond nappe and adjacent structural units to the west (Fall Mountain nappe, Skitchewaug nappe) constrain the occurrence of nappe overthrusting to between 370 and 350 Ma (Chesham Pond over Fall Mountain) and ~350 Ma (Fall Mountain plus Chesham Pond over Skitchewaug).