FACIES RELATIONSHIPS WITHIN THE AMMONOOSUC VOLCANICS AND PARTRIDGE FORMATION OF LYME, WEST-CENTRAL NEW HAMPSHIRE

Recent mapping at 1:24,000 in the town of Lyme reveals a complex facies pattern within Ordovician rocks of the Orfordville anticline as well as contrasting relationships to the underlying Oliverian rocks in the Mascoma dome to the east. In the Ammonoosuc (formerly Post Pond) Volcanics of the anticline, white aphyric to porphyritic felsite horizons are interlayered at several stratigraphic levels with massive to foliated amphibole-epidote-feldspar gneiss, locally with pillows, and subordinate biotite gneiss. Volcanic breccias are present in rocks of all mineralogy, and are most abundant toward the mafic-felsic contacts, for example toward the south side of Post Hill, where a small body of weakly foliated hornblende-feldspar gneiss suggests an intrusive plug. Metadiabase sills and dikes are abundant throughout. Black, sulfidic phyllites and schists are present within the volcanic sequence, are nearly absent below the Devonian Littleton Formation on the west limb, and are thickest on the east limb and to the north (Partridge Formation). Contacts between mafic and felsic metavolcanics are truncated by the Partridge contact, but felsic horizons persist into the Partridge, which is also cut by both felsic and mafic dikes. Metamorphic grade in the schists ranges west to east from staurolite to garnet and back to staurolite.

The Orfordville and Mascoma structures differ in age. Both limbs of the anticline dip west, with associated overturned minor folds and foliation subparallel to primary layering; these folds are deformed by the dome and its related upright minor folds with steep AP cleavage. The Partridge is absent on the dome, where Clough Quartzite unconformably truncates contacts between coarse biotite gneiss and hornblende gneiss in the Ammonoosuc. These contacts are conformable to the underlying Holts Ledge Gneiss and Mascoma granite gneiss, but are cut by the Smarts Mountain granodiorite gneiss, which contains biotite gneiss xenoliths. Thus rocks in the two bodies seem to bear different relationships to the overlying volcanics. Published radiometric ages for the Mascoma pluton are bracketed by ages elsewhere for the Ammonoosuc. No radiometric ages are available for the Smarts Mountain pluton.