TECTONIC IMPLICATIONS OF ELECTRON MICROPROBE MONAZITE AGES OF 525-535, 500-510, AND ≈475 MA FROM THE WATERBURY DOME, SOUTHWESTERN NEW ENGLAND

The age of high-grade metamorphism recorded by rocks of Rodgers’ (1985) Gneiss Dome Belt (GDB) in western Connecticut is central to understanding their tectonic evolution and testing Iapetian tectonic models.  Using the SX-50 probe at UMass with improved analytical protocols, we have further substantiated pre-Middle Ordovician electron microprobe ages of monazite from two samples of ky + kfs + bt + grt + ms + qtz + pl + rt migmatitic schist and one sample of ky + bt + grt + ms + qtz + pl + rt migmatite gneiss from the core of the Waterbury dome (WD), the southernmost and largest of the GDB domes.  Cover rocks of the dome are floored by a décollement of Silurian age and are composed of thrust sheets of early Ordovician arc-related rocks: the Arenig or older Taine Mountain Formation (forearc sediments) and the Collinsville Formation (metabasites of boninitic, arc-like, and MORB composition).  The final shaping of the dome structure occurred during F3 crenulate folding of migmatitic foliation at »380 Ma at 4.5-6 kilobars and 550-590ºC, after cooling and decompression from the peak of metamorphism at 7.0-9.5 kilobars and 680-780ºC.

All the grains we analyzed are small, less than 75 microns across, without distinct core and rim age domains.  A portion of our data is as follows:

sample	grain	# pts.	age	std er x 2	grain description
gsg.92.9	m2 m4 m7	13 18 16	535 535 507	6 7 4	rounded, xenoblastic, patchy Y rounded, xenoblastic, patchy Y matrix grain in contact w/ prismatic sil, bt, qtz
rs.92.3	m2 m6 m4	9 18 6	525 498 464	4 3 9	high Y core of xenoblastic grain in matrix bt unzoned, elongated , subidioblastic in matrix bt low Y zone in rounded, xenoblastic in matrix bt
gg.92.2	m5 m7 m3	17 8 10	508 486 470	7 7 7	rounded, subidioblastic, patchy Y, leucosome elongated, xenoblastic, patchy Y, leucoosome rounded, xenoblastic, matrix w/ ms, qtz

The texture of m7 in sample gsg.92.9 strongly implies that the age of metamorphism in the WD is at least as old as »505 Ma.  If the older ages of rounded and xenoblastic monazite are interpreted as relic detrital grains, their ages indicate Ganderian margin-related source rocks.  Interpreted in this way, growth of monazite at »475 Ma records metamorphism further related to accretion events along the Gander margin, rather than GDB collision with the Laurentian margin.