NEAR INFRARED SPECTROSCOPY OF WHITE MICA AND IMPLICATIONS FOR METAMORPHISM IN THE BLACK HILLS, SOUTH DAKOTA

Visible/near infrared spectroscopy of the Al-OH absorption feature of white mica offers a field-based technique to quickly and economically map regional trends in metamorphic intensity. Previous studies in the Lower Proterozoic metamorphic rocks of Black Hills, South Dakota, suggested a strong relationship between Al-OH wavelength and metamorphic grade in the biotite through sillimanite zones. However, those studies included a limited range in rock types, so that the effect of bulk rock composition remains unclear. Furthermore in previous studies, Al-OH wavelength spatial variations suggested the possibility of unmapped anomalies in metamorphic intensity.

Field spectra of 1953 rock samples were acquired in greenschist and lower amphibolite facies metapelitic and metapsammic rocks in the Black Hills. The wavelength of the Al-OH band shifts from 2218 nm in the biotite zone to 2195 nm in the staurolite zone in response to substitutions in the octahedral layer of white mica. The principal substitution is the aluminoceladonite exchange [(Mg,Fe)²⁺_Oct + Si⁴⁺_Tet = Al³⁺_Oct + Al³⁺_Tet] which drives white mica to more Al-rich composition as metamorphic temperature increases. When mapped out, variation of Al-OH values follow regional metamorphic patterns in the Black Hills, demonstrating the potential to detect and map metamorphic intensity in similar terranes.

In addition to field spectra, 45 polished thin sections were prepared from representative samples for further petrologic analysis through optical microscopy and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy. Samples cover the geographic distribution of the study area and span the complete range of Al-OH wavelength values. White mica with wavelengths spanning 2217 to 2196 nm showed a linear increase in Al_Oct from ~1.62 to ~1.92 cations per 11 O and a linear decrease in (Fe+Mg)_Oct from ~0.42 to ~0.09 cations per 11 O. Samples with anomalously lower wavelength values than regional trends are strongly correlated with increasing interlayer Na content in the white micas, up to ~0.22 Na/(K+Na). However, the wavelength for these samples remain consistent with white mica Al_Oct content variation.