THERMAL PULSE ASSOCIATED WITH CRETACEOUS ALKALINE MAGMATISM IN THE NORTHWESTERN APPALACHIANS: EVIDENCE FROM LOWER CRUSTAL XENOLITHS

Lower crustal xenoliths in Cretaceous lamprophyre dikes preserve a record of lithospheric thermal evolution associated with alkaline magmatism in northern New York, Vermont, and southern Quebec. The dikes occur in the Taconic supra-crustal fold and thrust belt and the xenoliths contain higher metamorphic grade assemblages than are found in neighboring outcrops.

Several xenoliths from Cumberland Head, near Plattsburg, New York contain the assemblage garnet + cpx + plagioclase (~An₃₀) + rutile + illmenite. Quartz is absent. The mineral cores are unzoned, reflecting extensive diffusive annealing. Garnet-cpx thermometry (Grt: Alm₄₃Prp₄₂Grs₁₄Sps₁; Cpx: Hed₂₄; Ravna, 2000) and quartz-absent barometry (Mukhopadhyay et al., 1992) from one sample give 850°C and 0.9 GPa for these cores. The garnet cores are surrounded by reaction coronas too extensive to be the result of the one day to several day transport in the host dike melt required by Stokes settling velocities.

Garnet and kyanite-bearing xenoliths from Ayer's Cliff, Quebec (700-800°C, 0.8-1.0 GPa equilibration conditions; Trzcienski & Marchildon, 1989) also contain sillimanite needles partially replacing kyanite. This suggests that these rocks were subjected to a relatively short-lived thermal pulse prior to being entrained in dike magma to shallow crustal levels. These xenoliths also contain monazite. Monazites located along grain boundaries have Pb contents near the detection limit of the electron microprobe and give Cretaceous (upper limit) U-Th-Pb ages. Given the sluggish nature of lead diffusion, we interpret the low lead values to be the result of diffusion and/or recrystallization prior to diking. Monazite inclusions within other minerals may retain lead recording growth during earlier metamorphic events.

The petrography, thermobarometry and monazite chronology suggest that these are mid- to lower-crustal assemblages whose protoliths may be derived from Grenvillian basement. These samples underwent extensive diffusive annealing before an interval of heating that may have occurred as part of the lithospheric thinning and trace-element enrichment precursor to the mantle melting event that produced the lamprophyre melts.