PETROGENESIS OF THE LINCOLN SYENITE, MAINE: EVIDENCE FOR LATE SILURIAN-EARLY DEVONIAN MELTING OF A SOURCE REGION MODIFIED BY SUBDUCTION DRIVEN METASOMATISM
The Late Silurian-Early Devonian (418 ± 1 Ma) Lincoln syenite (a.k.a. Lincoln sill or Lincoln shonkinite), exposed discontinuously over a distance of 75 kilometers in south-central and mid-coastal Maine, represents one of the most unusual igneous rocks in the Appalachian orogen. Rocks unaffected by post-magmatic recrystallization are highly porphyritic and characterized by large dark colored alkali feldspar megacrysts set in a finer grained matrix dominated by alkali feldspar + clinopyroxene + orthopyroxene + biotite + oxides. The rocks are intermediate in terms of SiO2 content (55-59 wt. %), yet are ultrapotassic (6-7 wt. % K2O) and relatively primitive in terms of their MgO (Mg# > 67), Cr (> 400 ppm), and Ni (> 150 ppm) concentrations. Additionally, the rocks exhibit large ion lithophile element enrichment relative to the high field strength elements along with elevated concentrations of light rare earth elements (200-300 x chondrite). The syenites show limited radiogenic isotopic variability with relatively high initial 87Sr/86Sr values (0.707 to 0.710), relatively low initial eNd values (-4.8 to -6.7), and initial Pb isotopic compositions characteristic of igneous rocks in central Maine (e.g., 207Pb/204Pb = 15.577 to 15.639). Correlation with the geochemically similar Parks Pond pluton in eastern Maine extends the distribution of Late Silurian-Early Devonian ultrapotassic magmatism in Maine to an orogen-parallel distance of over 150 km. The geochemical characteristics of these rocks are most consistent with a model involving the partial melting of a mantle wedge previously metasomatized by subduction derived fluids.
Comparisons with rocks of similar composition in known tectonic environments suggest the Lincoln syenite (and by association the Parks Pond pluton) formed as a result of lithospheric extension superimposed on a sub-continental lithospheric mantle source region previously modified by subduction-driven metasomatism. The timing and structural position of this ultrapotassic magmatism is consistent with a geodynamic model involving slab-break off or the convective removal of lithospheric mantle (delamination) in association with Acadian convergence. Both processes facilitate a brief period of crustal extension and decompression melting of source rocks previously modified by subduction related processes.