Petrogenesis of the ultrapotassic Lincoln Syenite, Maine: Late Silurian-Early Devonian melting of a source region modified by subduction driven metasomatism

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ID: 303226
2007
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Abstract
The Late Silurian-Early Devonian (418 ± 1 Ma) Lincoln syenite, 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. Detailed field and petrographic study reveals that most of the exposed syenites have been variably affected by post-magmatic recrystallization. However, a small proportion of the syenites in south-central Maine preserve original igneous minerals and textures and allow for an unobstructed study of the igneous petrogenesis. The syenites are 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 SiO~2~ content (55--59 wt. %), yet are ultrapotassic (6--7 wt. % K~2~O) 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 chondritic abundances). The syenites show limited radiogenic isotopic variability with relatively high initial ^87^Sr/^86^Sr values (0.707 to 0.710), relatively low initial ε~Nd~ values (−4.8 to −6.7), and initial Pb isotopic compositions characteristic of igneous rocks in central Maine (for example, ^207^Pb/^204^Pb = 15.577 to 15.639). 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. Crystallization of this magma was influenced by the presence of significant amounts of water that suppressed plagioclase feldspar crystallization, facilitated the crystallization of Mg-rich biotite, and ultimately resulted in the nucleation and growth of the alkali feldspar megacrysts. 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. 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.
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Authors David P. West, Paul B. Tomascak, R. A. Coish, Martin G. Yates, Michael J. Reilly
Journal american journal of science
Year 2007
DOI
10.2475/01.2007.08
URL
https://ajsonline.org/article/60894.pdf
Keywords Keywords not found

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