AN EARLY TRIASSIC DUCTILE SHEAR ZONE NEAR CLAREMONT, NH: EVIDENCE FROM 40AR/39AR DATING OF WHITE MICA

Combined microstructural, crystal chemical, and ⁴⁰Ar/³⁹Ar step-heating data indicate that the Devonian Littleton Formation near Claremont, NH cooled through white mica (WM) closure in the late Carboniferous to early Permian and was partially recrystallized in the early Triassic during retrograde ductile deformation in the Bald Mountain shear zone (BMsz). Field and petrographic observations show that the BMsz is defined by an ESE dipping WM-chlorite fabric with top-down shear sense indicators (local S₂), and that S₂ overprints and retrogrades a pre-staurolite bedding-parallel foliation (S₁). The shear zone coincides with the location of the previously mapped staurolite isograd. Electron microprobe analyses of the matrix WM record variations in the paragonite component (Pg) of 7 to 33 mol %. Element mapping shows that the Pg-rich mica is commonly found in the cores of zoned grains, and as grains at a high angle to, and truncated by S₂ micas. These observations demonstrate preservation of prograde S₁ and retrograde S₂white micas.

⁴⁰Ar/³⁹Ar step heating experiments on WM concentrates from 10 samples show a systematic decrease in total gas age from 274 to 258 Ma as the BMsz is approached, and steps in individual spectra range in age by ~20 m.y. Powder XRD experiments show peak splitting of WM 00l reflections (owing to the variation in composition noted above) and indicate a clear correlation between decreasing age and decreasing mode of the Pg-rich WM in the mineral separate. The age and XRD results demonstrate the preservation of not only the geometry and crystal chemistry, but also the argon isotopic composition of two populations of WM. These data support the petrologic conclusion that S₂ WM crystallized under greenschist facies conditions below its closure temperature and that the ⁴⁰Ar/³⁹Ar system records both cooling of S₁ and growth of S₂WM.

Using the single-step age range from the entire suite of samples yields estimates for the maximum age of cooling and minimum age of growth of 282 and 253 Ma, respectively. These ages are further constrained by an amphibole cooling age of 313 ± 3 Ma, and by syn- to post-S₂ pseudomorphic white mica after staurolite that yields ⁴⁰Ar/³⁹Ar ages of 242-247 Ma (n =4). Thus, the samples cooled through WM closure between 285 and 313 Ma and were recrystallized during ductile shearing between 242 and 253 Ma.