PETROGENESIS OF THE LATE SILURIAN-EARLY DEVONIAN LINCOLN SHONKINITE, SOUTH-CENTRAL MAINE

Highly distinctive rocks of the Lincoln shonkinite (a.k.a. Lincoln Sill) are exposed discontinuously over a distance of nearly 75 kilometers, from Boothbay Harbor in south-coastal Maine to near Liberty in south-central Maine. The 418 ± 1 Ma (Tucker et al., 2001) shonkinite intrudes rocks of both the Liberty-Orrington and Fredericton lithotectonic belts and has been variably sheared and metamorphosed during Devonian orogenic activity. In south-central Maine, where the width of the intrusive body is greatest, the effects of this subsequent tectonism are minimal and pristine igneous mineralogies and textures are well preserved. This study presents field, petrographic and geochemical data from the Lincoln Shonkinite and interpretations of this data will provide information on the tectonic setting of this region during the early stages of the Acadian orogeny.

Fieldwork in south-central Maine reveals only subtle compositional and textural variability within the undeformed portions of the shonkinite intrusion. The dominant rock type is highly porphyritic, containing megacrysts of alkali feldspar up to 6 cm in length set in a finer-grained matrix dominated by clinopyroxene, orthopyroxene and biotite. Plagioclase and quartz are rare to absent in most samples and olivine has been found at only one locality. Minor accessory minerals include apatite, zircon and Fe-Ti oxide minerals. Fine-grained mafic enclaves can be observed at most localities, although they are not overly abundant. Alkali feldspar megacrysts often show a preferred orientation within a given outcrop, although there are no systematic trends within the intrusive body as a whole.

Whole rock geochemistry from samples unaffected by metamorphism reveals unusual igneous rock compositions. SiO2 contents range from 55-60% and the rocks are noticeably rich in alkalis (K2O=6.0 to 7.5% & Na2O=1.5 to 2.5%) and magnesium (7-9%). Trace element analyses yield relatively high abundances of both compatible and incompatible elements (e.g., Cr=400 to 500 ppm and Ba=1500 to 2000 ppm). Rare earth element plots reveal LREE enrichment (200-300 times chondrites) as compared to the HREEs (10 times chondrites). Future work, including mineral chemistry and isotope geochemistry will further characterize the Lincoln Shonkinite and provide information on the tectonic setting of the region during the time of intrusion.