Stop #6 — Narragansett Pier Granites

Introduction:

Felsic magmatism from 270-280 Ma occurred across the Avolonian basement resulting in extensive pegmatitic/granitic dikes and several larger plutonic bodies (Zartman and Hermes, 1987; Wintsch and Aleinikoff, 1987; Aleinikoff and others, 1979; Getty and Gromet, 1991). One of these, the Narragansett Pier granite (275 Ma; Zartman and Hermes, 1987), intrudes polydeformed Pennsylvanian-age metasedimentary rocks. The granite is largely undeformed, but is interpreted to have intruded, at least partially during sinistral shearing (Reck and Mosher, 1989).

U-Pb geochronology on monazite and zircon from the Narragansett Pier granite yields a radiometric age of 275 +/- 2 Ma (Kocis, 1981; Zartmann and Hermes, 1987); an Ar40/Ar39 biotite cooling age of 238 Ma was reported by Dallmeyer (1982). Moreover, zircon isotopic systematics show a pronounced radiogenic Pb inheritance implying a Late Archean source region. No other pre-Permian rocks in southeastern New England show such an inheritance, leading Zartman and Hermes (1987) to propose that the Archean crustal component was derived from underplating of Archean source rock in the late Paleozoic during collision of Gondwana with Avalonia. Roof pendants within the granite contain Annularia stellata plant fossils, which are considered to be of Stephanian A or younger in age, further constraining the age of the granite (Brown and others, 1978). The granite truncates a crenulated foliation on the roof pendants, indicating that the regional metamorphism and deformations (D1 and D2) occurred in the interval between cessation of deposition (Stephanian A or younger; ~290 Ma) and granite emplacement (275 Ma). Taken together, these dates place tight constraints on the timing of the contractional deformation associated with Alleghanian orogeny in southeastern New England.

Recently, Whitehead and Gromet (1998) have argued that some alleged Precambrian granites in eastern Connecticut (i.e., Potter Hill granite) are also Permian-aged Alleghanian granitoids. Moreover, S-type granite in the New Bedford region may represent an eastward extension of the Narragansett Pier Granite (Murray et al, 1990). If so, the extent of Alleghanian plutonism may be more extensive than previously thought.

Description:

The Narragansett Pier Plutonic Suite forms an elongate, east-trending outcrop pattern from southeastern Connecticut across southern Rhode Island to the eastern boundary of the Narragansett basin. Most of the pluton consists of pink-colored, medium- to course-grained, equigranular to porphyritic, two-feldspar granite (the pink facies on geologic map). Common accessory minerals include biotite, muscovite, magnetite, ilmenite, apatite, garnet, monazite, sphene, zircon, allanite and pyrite. The rocks typically are slightly peraluminous, containing 1-3% corundum in their norms (Hermes et al, 1981); high Ba contents ranging from 1,000-3,000 ppm Ba which distinguish the rocks from older plutonic rocks in Rhode Island. Local hornblende-bearing varieties occur at several localities in eastern Connecticut (Goldsmith, 1985). Cross-cutting aplites and pegmatites are abundant. In western Rhode Island, some aplites are 10-30 meters thick and occur as gently south-dipping, east-trending dikes (formerly called Westerly Granite). Mineralogy of these aplites is similar to coarser-grained granite varieties, and they are considered to be a late-stage, but comagmatic facies of the plutonic suite.

(source: Burks, Mosher and Murray)

Figure 1: Narragansett Pier Granite with crosscutting fine-grained aplites and coarser-grained pegmatites (center)