CONTRASTING ORIGINS OF MAFIC ROCKS IN THE ROWE-HAWLEY BELT OF VERMONT: EVIDENCE FROM GEOCHEMISTRY

Metamorphosed mafic rocks, mainly Cambrian and Ordovician in age, crop out in newly mapped parts of the Rowe-Hawley Belt in north-central Vermont. Meta-mafic rocks within the Prospect Rock slice, the Hyde Park slice, the Worcester Complex, and the Belvidere Mountain Complex of the Rowe Belt are fine to medium-grained, foliated to massive, greenschist and amphibolite facies rocks; blueschist-eclogite facies rocks are found in the Tillotson Peak Complex. Most samples have geochemical characteristics similar to very late-stage rift, or embryonic oceanic, tholeiitic basalts: moderate Ti contents, flat to slightly LREE-enriched REE patterns, slightly elevated Nb/Y ratios relative to mid-ocean ridge basalts (MORB), and no Nb anomalies. A small subset of samples has chemical features like alkali rift basalts: high Ti contents, enriched LREE patterns, high Nb/Y, high Zr/Y, and no Nb anomalies. Both types are interpreted as having formed during the last stages of continental rifting that led to formation of the Iapetus Ocean in the Early Cambrian.

Metamorphosed mafic rocks to the east in the Wrightsville slice of the Moretown Fm (Hawley Belt) occur as dikes that cut the dominant foliation in metasediments and as sills or flows interlayered with metasediments. Chemically, they are tholeiitic basalts with moderate to high Ti contents, flat to slightly-enriched LREE patterns, and prominent negative Nb anomalies. These characteristics, along with some chemical similarities to MORB, suggest they formed in a suprasubduction zone extensional environment.

Whereas most tectonic slices in the Rowe Belt were metamorphosed at medium to medium-high pressure facies series conditions, slices to the east in the Hawley Belt were metamorphosed at low pressure facies series conditions (Laird et al., 1984, 1993). The distinct differences in magmatism and contrasting metamorphic pressures suggest a major suture separates the Rowe and Hawley Belts.