Detrital zircon geochronology of pre‐ and syncollisional strata, Acadian orogen, Maine Appalachians

Abstract

The Central Maine Basin is the largest expanse of deep‐marine, Upper Ordovician to Devonian metasedimentary rocks in the New England Appalachians, and is a key to the tectonics of the Acadian Orogeny. Detrital zircon ages are reported from two groups of strata: (1) the Quimby, Rangeley, Perry Mountain and Smalls Falls Formations, which were derived from inboard, northwesterly sources and are supposedly older; and (2) the Madrid, Carrabassett and Littleton Formations, which were derived from outboard, easterly sources and are supposedly younger. Deep‐water deposition prevailed throughout, with the provenance shift inferred to mark the onset of foredeep deposition and orogeny. The detrital zircon age distribution of a composite of the inboard‐derived units shows maxima at 988 and 429 Ma; a composite from the outboard‐derived units shows maxima at 1324, 1141, 957, 628, and 437 Ma. The inboard‐derived units have a greater proportion of zircons between 450 and 400 Ma. Three samples from the inboard‐derived group have youngest age maxima that are significantly younger than the nominal depositional ages. The outboard‐derived group does not share this problem. These results are consistent with the hypothesised provenance shift, but they signal potential problems with the established stratigraphy, structure, and (or) regional mapping. Shallow‐marine deposits of the Silurian to Devonian Ripogenus Formation, from northwest of the Central Maine Basin, yielded detrital zircons featuring a single age maximum at 441 Ma. These zircons were likely derived from a nearby magmatic arc now concealed by younger strata. Detrital zircons from the Tarratine Formation, part of the Acadian foreland‐basin succession in this strike belt, shows age maxima at 1615, 980 and 429 Ma. These results are consistent with three episodes of zircon recycling beginning with the deposition of inboard‐derived strata of the Central Maine Basin, which were shed from post‐Taconic highlands located to the northwest. Next, southeasterly parts of this succession were deformed in the Acadian orogeny, shedding detritus towards the northwest into what remained of the basin. Finally, by Pragian time, all strata in the Central Maine Basin had been deformed and detritus from this new source accumulated as the Tarratine Formation in a new incarnation of the foreland basin. Silurian‐Devonian strata from the Central Maine Basin have similar detrital zircon age distributions to coeval rocks from the Arctic Alaska and Farewell terranes of Alaska and the Northwestern terrane of Svalbard. We suggest that these strata were derived from different segments of the 6500‐km‐long Appalachian‐Caledonide orogen.

Bedrock Geology of Mt. Washington, Presidential Range, NH

“…For example, Part A of the Rangeley Formation may be mid-Silurian rather than Early Silurian, the Perry Mountain Formation may be Early Devonian rather than mid-Silurian, and the Smalls Falls Formation may be Early Devonian rather than Late Silurian (Bradley and O'Sullivan, 2016). This trend toward younger than expected ages warrants a flurry of new mapping linked with detrital zircon geochronology.…”

“…This was part of a regional detrital zircon age study of the Central Maine Belt in New Hampshire and Maine. The detrital zircon data show a Ganderian signature with peaks at 923, 576, 524 and 494, and 388 Ma, with a lesser peak at 1176 Ma (Bradley and O'Sullivan, 2016).…”

“…It is interesting to note that three formations from the type Rangeley section in Maine sampled for detrital zircon analysis by Bradley and O'Sullivan (2016) gave maximum youngest age peaks much younger than the traditionally accepted stratigraphic ages. For example, Part A of the Rangeley Formation may be mid-Silurian rather than Early Silurian, the Perry Mountain Formation may be Early Devonian rather than mid-Silurian, and the Smalls Falls Formation may be Early Devonian rather than Late Silurian (Bradley and O'Sullivan, 2016).…”

“…Age constraints on the stratigraphy are all by correlation to distant fossiliferous sections. Bradley and O'Sullivan (2016) recently published a detrital zircon age spectrum for a sample of Littleton Formation from a quartzite at the 4,000 ft. elevation level on the Auto Road. This was part of a regional detrital zircon age study of the Central Maine Belt in New Hampshire and Maine.…”

“…Given this, the youngest detrital zircon age peak in the Littleton formation should be older than 408 Ma and that is why Bradley and O'Sullivan (2016) attributed the 388-Ma detrital zircon peak to a metamorphic over-print. However, given that peak's error of +/-11 Ma and that we know from Bradley et al (2000) and Bradley and Tucker (2002) that Littleton sedimentation was syn-collisional and syn-intrusion with the Piscataquis Volcanic arc, it is possible that deposition continued to as young as circa 400 Ma.…”