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