SILURO-DEVONIAN GEOLOGY IN NORTHERN NH – STRATIGRAPHY, U-PB GEOCHRONOLOGY, GEOCHEMISTRY AND REGIONAL IMPLICATIONS

Recent and ongoing mapping of six 7.5’ quadrangles in northernmost NH (NHGS-USGS StateMap project) on the eastern flank of the Connecticut Valley–Gaspé Trough focuses on the following challenges: ages (limited fossils); stratigraphic relationships and sediment sources; deformation and metamorphic history; and tectonic environment and evolution.

Bedrock is comprised of pre-Acadian, Siluro-Devonian fine-grained metasedimentary (slate, graded graywacke, siltstone, fine-grained sandstone, carbonate-rich siltstone, grit and volcaniclastic), extrusive (pillow basalt, rhyolite, ignimbrite, tuff) and intrusive (diabase, gabbro, biotite granite, tonalite) rocks of the Waits River, Gile Mt (GM), Ironbound Mt (IBM) including the Halls Stream Grits (HSG), formations west of the Monroe Fault and the Frontenac (FF) formation east of the Monroe Fault. Sedimentary stratigraphy is addressed by Summa et al. (this meeting).

Twelve maximum depositional ages (MDA) from detrital zircons and seven zircon crystallization ages (CZ) range from ~388-434 Ma and ~403 – 432 Ma respectively. A HSG sample (debrite?) with a ~ unimodal zircon population yielded an MDA age of ~ 391 Ma and a nearby GM pillow basalt had a CZ age of ~ 403 Ma. Two samples from HSG along strike (1, 5 miles away) are in analysis (LA-ICP-MS) to test these ages. One sample from a graded graywacke (IBM) is also in analysis.

Two major Acadian events (D1 and D2) are expressed as folds (F1, F2) and cleavages (S1, S2) and as high-angle reverse faults. Locally, evidence suggests a D3 event. The orientation and style of D1 and D2 folds and cleavages are remarkably similar throughout the area, including on opposite sides of the Monroe Fault (NH) and correlative Bella Fault (Qc). The bedrock was metamorphosed to greenschist (chlorite rising to biotite zones) near Late Devonian Acadian intrusive rocks.

Geochemical data from Siluro-Devonian igneous rocks are used with published geochemical data from NH, ME, VT, and Qc along strike to evaluate tectonic environments and genetic relationship of igneous units. In addition to the interpretation that the basalts are with-in plate tholeiites, locally other interpretations are possible (e.g., MORB and/or plate margin basalts). Geochemical differences exist between Clinton River and Frontenac basalts, and along strike within units.