TESTING THE NATURE OF THE BASEMENT BELOW THE EDIACARAN BOSTON BASIN, EASTERN MASSACHUSETTS: GEOCHRONOLOGY OF XENOLITHS IN A TRIASSIC CAMPTONITE DIKE, AND OF POSSIBLY RELATED ROCKS

U-Pb LA-ICPMS zircon, monazite and rutile ages were acquired from xenoliths in a Triassic camptonite dike from the Porter Square subway station in Cambridge, MA, to test the nature of the basement rocks below the Ediacaran Boston Basin and perhaps below the Avalon terrane. Data were compared with those from two potentially equivalent rocks to the south: a staurolite schist from Dartmouth, MA, and the Pennsylvanian Sachuest arkose of the Narragansett Basin, RI.

Two metasedimentary gneiss xenoliths yielded ~541-228 Ma zircon, with a main peak at ~246 Ma and a shoulder at ~270 Ma. Monazite yielded ~360-230 Ma ages with a ~277 Ma peak. Rutile from these two samples and a third xenolith of sulfide-bearing granulite yielded ~230 Ma ages. The camptonite dike has a published 246 ± 4 Ma ⁴⁰Ar/³⁹Ar kaersutite phenocryst age. All zircon grains are rounded with aspect ratios <1:1.5, with concentric, patchy or sector zoning, and weakly zoned overgrowths. The youngest ~246 Ma zircon population suggests Pb loss and/or new growth during dike crystallization. The ~277-270 Ma ages reflect formation of Pangea. Any older grains may be detrital and/or metamorphic, and given the spread of ages, they likely lost some Pb. A detrital zircon age spectrum is difficult to interpret. The rutile ages record cooling or the onset of the breakup of Pangea.

The staurolite schist yielded only 21 analyses from 16 grains. Zircon yielded one Archean and three Proterozoic grains, and ~595 Ma and ~246 Ma age populations. The Proterozoic ages resemble the Avalonian signature. Apatite yielded a 272±39 Ma age, consistent with monazite ages in the area and in the metasedimentary gneiss xenoliths of Porter Square. The Sachuest arkose is from one of the lowest parts of the southern Narragansett Basin. Zircon yielded ~691-252 Ma ages. The youngest detrital zircon age population is ~542 Ma, and younger grains probably lost Pb or grew during the Alleghanian orogeny.

The metasedimentary gneiss xenoliths of Porter Square may be equivalents of the Narragansett Basin, but not of the staurolite schist. The fact that they reached upper amphibolite or granulite facies conditions, and came from below the Boston Basin, remains enigmatic, and their nature and origins warrant further investigation.