TOURMALINE AS AN INDICATOR OF THE PROVENANCE OF DETRITAL SEDIMENTS: LINKAGE BETWEEN THE CALC-ALKALINE ARC AND FOREARC BASIN OF THE ROSS OROGEN, TRANSANTARCTIC MOUNTAINS, ANTARCTICA

Tourmaline is a common accessory mineral, crystallizing over wide ranges in temperature, pressure, and bulk rock compositions, and it retains a chemical signature of the rock in which it formed. Thus, tourmaline is useful as a provenance indicator, an aspect of sedimentary geology explored by Bob Dott. Analyses of tourmaline by Laser-Induced Breakdown Spectroscopy (LIBS) provide rich chemical information about the concentrations of most elements. As such, LIBS spectra are excellent for comparing the compositions of detrital tourmaline grains to their potential source rocks.

Detrital tourmalines from syn- to late-orogenic forearc basin clastic sedimentary rocks from the modern central Transantarctic Mountains, deposited during the Cambrian-Ordovician Ross Orogeny, were analyzed using LIBS to compare tourmaline sources in order to evaluate whether sediment sources changed during orogenesis. Using a series of Partial Least Squares Regression (PLSR) models of complete tourmaline spectra, tourmalines can be categorized into the six most common lithologies in which tourmaline crystallizes: pegmatites (LCT and NYF), pelitic metamorphic rocks, calcareous metamorphic rocks, silicic igneous rocks, and hydrothermal deposits. This multivariate approach takes advantage of the complexity of tourmaline composition and the large amount of data revealed in LIBS spectra.

Spectra of syn-orogenic detrital tourmalines were sorted into the six lithologic groups; spectra in each group were subdivided into eleven clusters using cluster analysis. LCT pegmatitic tourmaline grains comprise 4 clusters; NYF pegmatite, 3; pelitic metamorphic rocks, 2; and hydrothermal deposits, 2. These clusters represent individual tourmaline sources. Only 5 grains were identified as derived from calcareous metamorphic rocks, and 4 from silicic igneous rocks. PLSR models constructed for each cluster were used to process spectra of late-orogenic detrital tourmalines for comparison. All 11 syn-orogenic tourmaline sources are represented in late-orogenic detrital tourmalines, but only four persist in the youngest late-orogenic sample. These data confirm the Ross arc source indicated by detrital zircon and muscovite ages, and suggest that the sources of tourmaline during Ross orogenesis remained relatively constant through time.