EXPANDING THE PROVENANCE TOOLKIT BEYOND DETRITAL ZIRCON DATING: A TEST CASE FROM THE MERRIMACK RIVER

U-Pb dating of detrital zircon is an immensely popular tool in provenance studies and provides important information on the chronology of magmatism in the sediment source region, but it provides little information by itself on the composition and tectonic setting of said magmatic rocks or on the metamorphic history of the sediment source region. Far more complete information can be gleaned by interrogating the geochemistry of zircon and measuring the age and geochemistry of other detrital minerals, such as rutile, titanite, and monazite. An example of this multi-proxy approach is presented here in a study of garnet-rich sand from the mouth of the Merrimack River, which drains much of New Hampshire along with parts of eastern Massachusetts. All analyses were conducted in the new Lowell Laser Lab.

Detrital zircon U-Pb age populations match those reported by Bradley et al. (2015) for Merrimack sands, with a main age peak around 400 Ma and lesser peaks at 200 Ma and 124 Ma. The trace element compositions of these zircons can be tied to the magmatic lineage and tectonic setting of their probable source rocks. For example, the Mesozoic zircons, derived from the White Mountain igneous province, have distinctly high Nb/Yb, which is linked to the distinct A-type chemistry of these plutons whereas some of the Devonian grains have distinctly low Th/U, indicating either metamorphic origin or perhaps origin in a highly peraluminous melt.

U-Pb dating and trace element analysis of detrital rutile provide a very different story. Major age peaks are present at ~370, 330, and 250 Ma, and Cr-Nb covariations of the rutile from each age group are distinct, although all are broadly consistent with metapelite source rock. While the first two peaks can be linked to known metamorphic events, the 250 Ma peak appears to significantly postdate most known Alleghanian metamorphism and magmatism in the region. This could record regional cooling of deep seated metasedimentary rocks after the Alleghanian orogeny, but this might be expected to produce diffusional differences in age between rutile cores and rims, which are not observed. Interpretation may ultimately hinge on whether pending analyses of detrital monazite yield similar ages.