THE MYSTERIOUS MACHINATIONS OF MUSCOVITE AND MONAZITE DURING METAMORPHISM OR HOW THE CVS (CONNECTICUT VALLEY SYNCLINORIUM) SURVIVED PMS (POST-METAMORPHIC-STRETCHING)

Monazites ages from 21 pelitic schists combined with existing 40Ar/39Ar ages of muscovite in many of the same samples places fundamental constraints on the tectonic assembly of central New England. The samples are from two areas, nine are from the Chester Dome and vicinity in VT and twelve are from MA. In MA, most are along a transect west across the CVS into the Rowe-Hawley zone (RHZ) near the CT border. The ion microprobes at UCLA and WHOI were used to obtain 69 Th-Pb spot ages on monazites from VT and 48 spot ages from MA.

In MA, SIMS ages from monazite in both the Siluro-Devonian rocks of the CVS and the Cambro-Ordovician rocks of the RHZ are nearly the same. The ages range from 354±3 (1 s.e.) to 403±6 Ma in the RHZ and from 363±3 to 389±5 Ma in the CVS. The youngest monazite inclusions in garnet, staurolite and in kyanite are all within error of 372 ±5 Ma. Muscovite from most of these samples were previously dated by 40Ar/39Ar and yielded ages that range from 332±3 to 356±5 Ma (Henderson et al., 1996).

The MA ages are consistent with the four spot ages from two CVS samples from VT that range from 342±3 to 373±3 similarly EMP ages on two CVS samples average 356±36 (sd) and 357±28. Five of the VT samples are from the Eo-Cambrian Cavendish Formation. SIMS ages range from 287±3 to 443±4. The youngest monazite inclusions are 369±4 in garnet, 355±3 in staurolite and 361±4 in kyanite. 40Ar/39Ar fusion ages from muscovite at the Gassetts road cut range from 326±2 to 360 ±2Ma and average 348±2 (Hames and Cheney, in prep)

The monazite ages from these rocks are consistent with early metamorphism in pre-Silurian rocks of the domes at 410-430 Ma followed by the main event at 350-390. The similar inclusion ages for garnet in VT & MA indicate that the peak pressures in these rocks of 8-10kbar were attained by ca. 370 Ma. Staurolite and kyanite formed later possibly during exhumation at 350-360 Ma. Muscovite ages differ from younger monazite ages by ca. 10 Ma. The resulting cooling rate of ca. 20 degrees C/ m.y. was accommodated by differential uplift and shearing of the CVS.