MODERN INTRA-OCEANIC ARCS AND THEIR BACK-ARCS AS ANALOGS FOR THE EASTERN THRUST SLICES OF THE ROWE-HAWLEY ZONE IN WESTERN CONNECTICUT

Several modern intra-oceanic arcs are characterized by back-arc basins that can be hundreds of kilometers wide, formed by rifting of an active arc. Magmatism along the volcanic arc can be sustained for >10 m.y. before rifting; rifting and back-arc spreading can mature in <5 m.y. Back-arc spreading typically occurs along separate segments of variably propagating rifts whose spreading rate and volcanic activity can be simultaneously fast/enhanced close to the volcanic arc and slow/diminished further away into the basin, respectively. Along rifts close to the active arc, boninites can erupt. During early stages of rifting and in rifts close to the volcanic arc, basalts can have arc-affinity (LREE-depleted, LILE enriched, low Ti). As the back-arc opens and spreading centers separate from the volcanic front, MORB chemistry becomes dominant. In western Connecticut, eastern thrust slices of the Rowe-Hawley Zone are exposed in a series of domes, separated from Bronson Hill domes by the Mesozoic Hartford Basin. In the Collinsville and Bristol domes are calc-alkaline tonalitic and trondhjemitic orthogneisses and the amphibolite-rich Collinsville Fm, correlative with the Hawley Fm in western Massachusetts. There are two varieties of orthogneiss (based on twenty analyses): Group 1 has low Ti (0.3-0.5 wt%), Nb (2-8 ppm), and Y (12-26 ppm); ORG-normalized multi-element profiles that overlap with arc granites with only slightly depleted HFSE > 0.3x ORG; and weakly concave up REE patterns at 15-35x chondrite similar to the REE patterms of arc-like meta-basalts of the Collinsville Fm which overlies the orthogneisses. Group 2 has very low Ti (0.2-0.3 wt%), Nb (<5 ppm), and Y (5-10 ppm); depleted ORG-normalized multi-element profiles with HFSE 0.1 < x < 0.5 ORG and negative Ta; and U-shaped chondrite-normalized REE patterns with HREE <10x chondrite and positive Eu anomalies. Both groups have (Nb vs. Y) and (Ta vs. Yb) signatures of VAG. Collinsville Fm amphibolites are mostly MORB but arc-like basalts and boninites are also present, in proportions similar to a back-arc. Direct age constraints are lacking, but these arc and back-arc rocks could have accreted to the Laurentian margin before 450 Ma. Their present position, however, reflects their Acadian-age thrust juxtaposition with already-metamorphosed rocks of the Waterbury dome.