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.
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