MONAZITE MICROPROBE AGES FROM A STRUCTURALLY COMPLEX OUTCROP, CENTRAL MASSACHUSETTS: EXPLORATION OF “ACADIAN” TECTONOTHERMAL EVENTS

Four years ago, regional assessment of isotope dilution U-Pb ages on zircon in metamorphosed igneous rocks and on metamorphic monazite indicated that peak granulite-facies metamorphism in central Massachusetts was during Late Devonian – Early Mississippian (366-359 Ma), with development of late E-W trending high-T mylonite fabrics. We now assess that idea using detailed microprobe analyses of Th, U, Pb, Y and age in monazite in sill-grt-crd-kfs gneiss and mylonite from the highest-grade part of the zone. The rock is in a normal pyrrhotite gneiss west of Late Silurian Smalls Falls equivalent (Spsq on State Map), interpreted on State Map as Late Ordovician Partridge (~449-445 Ma), but since as Lower Silurian Rangeley (~440-428 Ma). Detailed analyses are in large (~100 micrometer) monazites in gneiss and small (~20 micrometer) monazites in cross-cutting mylonite. The gneiss assemblage qtz-plg-kfs-sill-grt-crd-bt-ilm-gr-po yields peak metamorphic T of 700 C at 5.8 kbar. The very fine-grained mylonite has incipient recrystallization, with coarsened bt and skeletal neoblasts of grt, plus qtz, plg and sil, and yields T estimate of 550 C at 7-8 kbar. Monazite element maps show complex zoning in Y, Th and U, with abrupt chemical breaks correlated with age breaks. Statistical analysis shows three age groups in each rock. Means (1 sigma) and % in each age group [ ] for gneiss are: 469 Ma (10) [33%], 439 (11) [50%] and 392 (12) [17%]; for mylonite: 463 Ma (11) [12%], 432 (8) [28%] and 388 (13) [60%]. Because analysis points are analogous to point or line counts (rigorous 3 micron spacing), percentages are proportional to volumes of monazite of each age. Metamorphism much prior to 400 Ma has not been recognized in this terrane, and the putative Late Ordovician or Early Silurian protoliths would preclude ages greater than 450 Ma. Areal distribution of the oldest ages in the gneiss monazites allows them to be interpreted as relict detrital cores, but the 439-432 Ma age dominates the volume and is areally distributed so as to make detrital interpretation more difficult. The “true” Acadian age of ~390 for overgrowths on gneiss monazites and most recrystallized monazite in the mylonite suggests that as the age of mylonization. A few ~375-350 ages in gneiss and particularly in mylonite indicate the possibility of minor Neo-Acadian monazite recrystallization.