PETROLOGY AND GEOCHEMISTRY OF THE ALFRED COMPLEX IN SOUTHWESTERN MAINE

The Alfred Complex in southwestern Maine is similar to Cretaceous plutons throughout northern New England and adjacent Quebec. The magmatic origin of these widely distributed plutons is of interest because the region is generally considered to have been tectonically quiescent during the Cretaceous period. This is the first study to address the origin of the Alfred Complex using modern petrologic and geochemical techniques.

The Alfred Complex is composed of three major rock types that form a generalized circular “target” map pattern. From outside to inside the rocks are: biotite-bearing noritic gabbro, monzodiorite, and granodiorite. A porphyritic texture in the dioritic rocks is common, with plagioclase phenocrysts up to 9mm across. Plagioclase phenocrysts in all rock types are compositionally zoned with sodic rims relative to cores. The An content of plagioclase phenocrysts in gabbro ranges from An₂₄ – An₆₂, in monzodiorite from An₁₅ – An₄₃, and in granodiorite from An₁₀ – An₃₆. Average clinopyroxene composition in the gabbro is Wo₄₄En₃₈Fs₁₈, and in monzodiorite is Wo₄₅En₃₁Fs₂₄.

Rocks of the Alfred Complex are classified on an alkali-silica diagram as alkaline gabbro and syenodiorite. K, Na, and Al oxides increase whereas Ca, Fe, Mg, Mn, P, and Ti oxides decrease with increasing SiO₂ content. Bivariate plots of trace elements against SiO₂show strong positive correlations with Hf, Lu, Rb, Th, U, and Zr, and strong negative correlations with Ni, Sr, and V. Samples are enriched (10-400x chondrite) in rare earth elements (REEs), with relative light rare earth element enrichment. Granodiorite and monzodiorite are generally enriched in REEs relative to gabbro. Interestingly, gabbro is enriched in Sr, P, and Ti compared to other rock types

Results suggest that all rocks of the Alfred Complex are comagmatic and that variation in rock type is a result of mineral fractionation processes. The complex could have formed as three distinct pulses of magma from a continually fractionating magma chamber, with each successive pulse intruding earlier crystallized bodies. Further, data is consistent with initial melt generation by partial melting of an enriched mantle source in a within-plate setting.