Usually when we think of anthracite coal in the United States, we think of the Late Paleozoic anthracite fields of eastern Pennsylvania. However, the Narragansett basin in Rhode Island/Massachusetts contains similar age (Pennsylvanian period, 323-299 million years ago) anthracite- to meta-anthracite rank coals. These coal-bearing fluvial sediments were deposited after the Devonian Acadian orogeny whose imprint dominates the metamorphic terrane of New England. Columbus Day weekend, I attended the 106th New England Intercollegiate Geological Conference (NEIGC), an annual regional field conference headquartered this year at Wellesley College, Massachusetts, and went on two trips to the coastal portion of the Narragansett basin.
Anthracite coal is defined as fixed carbon 92-98%, vitrinite reflectance 2.5-6%; meta-anthracite has vitrinite reflectance >6%. It is a higher rank, with more carbon and less hydrogen and oxygen than bituminous coal. Metamorphically, anthracite rank is correlated with the anchizone, prehnite-pumpellyite grade or subgreenschist metamorphism. The metamorphic grade in the Narragansett basin increases from anthracite-coal-rank in the north to sillimanite-grade in the southwest, and this metamorphic gradient can be seen across the basin primarily within one unit, the Rhode Island Formation. This year's NEIGC Rhode Island field trips were in the southern garnet-to-staurolite metamorphic zones where carbon occurs as mineral graphite, but there have been earlier field trips to the lower-grade RI anthracite fields.
Following dramatic increases in oil prices in the early 1970's, old US domestic fossil fuel resources were re-evaluated, including the RI anthracites. These had been exploited earlier in the 20th century, but due to high ash (mineral content including quartz veining), discontinuous seams, and incipient graphitization, the RI anthracites and meta-anthracites never were as profitable as the PA anthracites. Their utilization, besides as a fuel, included lightweight aggregate and foundry graphite. Intense deformation and contact metamorphism from granitic plutons are among processes related to the Late Paleozoic Alleghanian orogeny that compromised the economic value of the RI anthracites. The Narragansett basin was perched on peri-Gondwanan terranes outboard of the craton whereas the PA anthracites were inboard on the craton and, therefore, more "sheltered", so deformation in RI during the Late Paleozoic collision of North American and Africa was, analogously, more like a head-on crash rather than the Labrador-Retriever-skidding-on-the-front-hall-rug folding/thrusting of the PA Valley and Ridge province.