GENETIC LINKS BETWEEN LITHIUM PEGMATITES AND THE ACADIAN ALTIPLANO AND THE ROLE OF PALEOCLIMATE

New England’s lithium pegmatites are concentrated within the reconstructed footprint of a former orogenic plateau, the Acadian altiplano (Hillenbrand & Williams, 2021). The altiplano, which is marked by high-grade metamorphic rocks now at the surface, persisted from 380 to 330 Ma, stretched from CT to ME, and had a maximum crustal thickness of ~70 km. On its inboard (W & NW) and outboard (E & SE) flanks, it was bounded by orogen-parallel sinistral and dextral shear zones, respectively. These facilitated northeastward extrusion of altiplano crust. The NE boundary of the altiplano, in western ME, is different. It coincides with a steep northeasterly decrease in metamorphic grade and with a transverse zone of late, NW-trending folds that are nearly perpendicular to the orogenic grain. The implied shortening is predicted by the extrusion model.

New England’s Li-pegmatites mostly crop out along the inboard, outboard and transverse edges of the altiplano’s reconstructed footprint, but are sparse in the middle. Within those three belts, pegmatites crop out in somewhat linear districts. We recognize 18 districts, with the largest, the Grafton district in NH, stretching 25 km along strike. The pegmatites were emplaced as sills, dikes, plugs, and irregular bodies. Fault control is likely for those districts, like Grafton, with strongly aligned pegmatites. U-Pb zircon and cassiterite ages require five Mississippian to Permian episodes of Li-pegmatite emplacement. The oldest two (ca. 360 and 330 Ma) coincided with the tenure of the altiplano; the youngest three (ca. 270, 260, and 250 Ma) were emplaced after thinning of the altiplano but still within its footprint. Pegmatites of the transverse belt in Maine include representatives of all 5 age groups.

During the altiplano’s tenure from 380 to 330 Ma, this part of Laurentia drifted northward through the southern dry belt, from ~40° to ~15 °S. Modern analogues suggest that precipitation and erosion did not keep up with tectonically-driven mountain building, creating an orogenic plateau with an endorheic basin system dotted by salars, with dense, Li- and B-rich brines in the subsurface. The hypothesis that these brines were somehow involved in Li-pegmatite genesis—perhaps by coseismically delivering fluxing elements (e.g. Li, B) to anatectic depths—remains to be tested.