FINGERPRINTING NEO-ACADIAN THERMAL AND TECTONIC ACTIVITY, SW NEW HAMPSHIRE, VIA REE PHOSPHATE COMPOSITIONAL VARIATION

Pyle and Spear (2005) showed that the high-grade metamorphism near Gilsum, NH was Neo-Acadian, culminating in anatexis quenched by west-directed overthrusting of the Chesham Pond nappe on to the Fall Mountain nappe ca. 355 Ma. Detailed study of phosphate accessory phase paragenesis in the Gilsum samples provided crucial thermal and chronological constraints. This approach was extended to regional scale in a study of the Monadnock, NH 15' quadrangle. EPMA monazite ages and monazite-xenotime thermometry (MXT) delineate distinct thermal events within the quadrangle. Monazite growth in the region of the Brennan Hill Thrust Fault (BHT, western portion of the quadrangle) occurred primarily between 380-360 Ma, with samples adjacent to the 354 Ma Fitzwilliam Granite recording monazite growth domains at 349±18 Ma (2 s.e.), and younger monazite domains ca. 300 Ma in two samples. MXT for samples astride the BHT yields heating rates of 9°C/m.y. (lower plate), and ~3°C/m.y. (upper plate), prior to 360 Ma, suggesting movement on the BHT occurred after 360 Ma. Samples in the middle and western portions of the quadrangle, astride the Chesham Pond fault, and adjacent to ~410 Ma plutons, return significantly older monazite ages, with compositionally-delimited monazite domains yielding average ages between 420 and 380 Ma (± 15-25 Ma), with an upper-plate cooling rate of 7°C/m.y. from 420-408 Ma, calculated from MXT. For the quadrangle overall, accessory phase thermometry suggests two thermal maxima; one at 415-420 Ma, and the other ca. 360 Ma. Restricted to the upper plate of the BHT is a inter-formational monazite compositional signature characterized by cores with widely dispersed Sm/Nd ratios plus depletion in La/Ce and enrichment in La/Th ratios relative to all other characterized monazite domains. This La- and Th- depleted domain yields EPMA ages of 480-430 Ma, older than the known plutonism in the area. SIMS monazite ages for the Monadnock quadrangle samples are forthcoming.