U-Pb dating of major and accessory minerals formed during metamorphism and deformation of metapelites

Abstract

U-Pb isotope analyses on major and accessory minerals from pelitic schists in of southwestern Connecticut are used to date metamorphism and deformation. Most of the major minerals have 206Pb/ 204Pb ratios less than 100. However, U-Pb mineral isochron and mineral-whole rock isochrons, constructed using aliquots of the same mineral, yield concordant ages with uncertainties as low as 5 Ma. Staurolite, kyanite, and garnet are older than metamorphic monazites and zircons. These data are summarized below: 5 mineral 206Pb/ 204Pb System The Straits Schist Trap Falls Fm. monazite, zircon — 207Pb- 206Pb 379–384 Ma 380 ± 2 Ma staurolite 48–623 238U- 206Pb 394–400 Ma 398 ± 5 Ma garnet 22–46 238U- 206Pb 383 Ma 414 ± 10 Ma 235U- 207Pb 418 Ma — 207Pb- 206Pb 623 Ma — kyanite 21–27 238U- 206Pb 399 Ma — chlorite, anatase, ilmenite 20–27 238U- 206Pb 350–370 Ma — apatite, quartz, Kspar 18–19 — — Transposed pegmatitic intrusions in The Straits Schist yield monazites with U-Pb ages analytically indistinguishable from those of metamorphic monazites and zircons in the schists. Growth of monazite and zircon in the schists may be related to high- T fluid influx associated with pegmatite intrusion. Age discordance in the garnets from The Straits Schist is most consistent with continuous loss of intermediate daughter products of the 238U decay chain from about 420 Ma to 380 Ma. Garnets from the adjacent Trap Falls Formation do not display this behavior.