16-45 Bimodal Plutonism During the Devonian Acadian Phase of the Appalachian Orogen, Maine (USA)

Session: From Thin Section to Outcrop: Exploration of Undergraduate Research (Posters)

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Abstract:

The Appalachian orogen, which formed along the Iapetan margin of Laurentia during the Paleozoic, is considered an archetype of accretionary orogenesis. The orogen records multiple phases of microcontinent accretion, including the Devonian Acadian episode, which is well preserved in the Central Maine Belt (CMB) around Rangeley, Maine, USA.

 The CMB consists of a series of early Paleozoic sedimentary deposits that were regionally metamorphosed under medium- to high-pressure–temperature conditions and intruded by magmas during the Acadian orogeny. These magmas form kilometer-scale plutons composed predominantly of either granitic or gabbroic rocks, with some locations also containing ultramafic lithologies. Previous studies in this region have focused primarily on the timing of felsic magmatism and its relationship to spatially associated regional metamorphism. However, the timing and tectonic framework for mafic and ultramafic magmatism, and their implications for tectonic processes in the CMB during the Acadian orogeny, remain poorly constrained.

         In this study, we present field observations and major, minor, and trace element geochemical data for 23 plutonic samples collected during field campaigns conducted in the summers of 2024 and 2025. Field relationships between the felsic and mafic–ultramafic plutons suggest structural continuity along strike of the CMB and coeval emplacement. Geochemical analyses show an obvious distinction in magma composition between the mafic–ultramafic and felsic samples, with the mafic-ultramafic rocks plotting within the tholeiitic series and the felsic rocks plotting within the calc-alkaline series. This compositional gap suggests that the mafic–ultramafic plutons may be associated with decompression melting of the mantle, whereas the felsic plutons may have formed via flux melting in a subduction zone. Preliminary data further indicates a mid-oceanic ridge basalt-like composition for the mafic–ultramafic plutons and an arc-like signature for the felsic plutons, potentially reflecting magmatism in distinct tectonic regimes.

Further study focusing on mineral chemistry and U-Pb zircon geochronology will be necessary to better constrain the relative timing of the felsic and mafic magmatism and to confirm the tectonic regime in which these bimodal magmas were emplaced. This study contributes to a broader understanding of heat and mass exchange between the crust and mantle during orogenesis and the geodynamics of accretionary orogenic systems.

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