ASTHENOSPHERIC UPWELLING AND SLAB ROLL-BACK OF THE SUBDUCTING AVALON PLATE: GEOCHEMICAL EVIDENCE FROM THE LEBANON GABBRO, EAST-CENTRAL CONNECTICUT

The geochemical affinities of the Lebanon Gabbro in southeastern Connecticut suggest a transition between volcanic arc and mid-ocean ridge tectonic setting. It intrudes as a sill into the Late Silurian Hebron Formation, and thus must be Early Devonian or younger. Subsequent deformation has folded the body into a pincer-shaped outcrop pattern. The Lebanon Gabbro is an orthogneiss that now contains primarily hornblende and plagioclase with biotite present locally. The gabbro is variably but weakly foliated with no mafic magmatic minerals surviving amphibolite facies metamorphism. Plagioclase may be an exception; it locally contains the strong albite twinning that may be inherited from the igneous protolith. Normative calculations indicate an olivine tholeiite protolith, with minor websterite cumulates.

To infer tectonic setting of the tholeiitic protolith, preliminary geochemical data are plotted on discrimination diagrams utilizing Zr, Ti, Sr, Y, Nb, Rb, FeO, MgO, and SiO₂. Data plotted on SiO₂ –FeO/MgO (Miyashiro, 1974), Zr-Y-Nb, Zr-Y-Ti, and Sr- Ti-Zr (Pearce and Cann, 1973) diagrams define a calc-alkaline trend, and lie in the volcanic arc field. On Ti-V (Shervais, 1981) and T-Zr (Pearce and Cann, 1973) diagrams the data lie in the back-arc basin/mid-ocean ridge basalt (MORB) field, while on a Zr/Y-Zr (Pearce and Norry, 1979) diagram the data lie across the MORB and within plate fields. The consistency of the interpretations made from the Sr-Ti-Zr diagram with diagrams utilizing only the high field strength elements suggests that metamorphism may have been nearly isochemical, and least for Sr.

The tectonic setting indicated by the data in the various discrimination diagrams suggests that the original basaltic liquid was derived from an environment intermediate between a volcanic arc and a mid-ocean ridge. Such a setting supports a tectonic model in which an Early Devonian volcanic arc overlying the subducting Avalon plate preceded slab roll-back driving asthenospheric upwelling in connection with crustal thinning, and intrusion of the Lebanon magma.