GEOCHEMISTRY OF THE START COMPLEX GREENSCHISTS - RHENOHERCYNIAN MORB

The meta-igneous greenschists of the Start Complex, South Devon, are c omposed of a mineralogically uniform, but texturally variable, actinol ite-epidote-albite assemblage with retrogressed variants containing ch lorite, muscovite, sphene, carbonate and oxidized opaque minerals. Geo chemically they represent a suite of relatively primitive tholeiites, exhibiting mild differentiation, depleted incompatible element abundan ces, and variable light rare-earth-element-depletion patterns comparab le to modem basalts from normal spreading ridge segments (N-MORB). As the Start greenschists exhibit a number of chemical similarities to th e nearby Upper Palaeozoic Lizard ophiolite, and MORB-type clasts withi n the Rhenohercynian Zone generally, they may also represent local Var iscan ocean crust, which floored small oceanic basins that separated t he Old Red Sandstone continent from the Armorican microplate to the so uth. The Start Complex could thus represent a previously unrecognized oceanic component to the Variscan orogenic belt (Rhenohercynian Zone) of Northwest Europe.