PETROGRAPHIC AND AERIAL MAPPING OF THE LITTLE DEER ISLE PERIDOTITE, EASTERN PENOBSCOT BAY, MAINE: EVIDENCE FOR HYDROUS MAGMATISM WITHIN THE LOWER CRUST

We employ aerial and petrographic mapping to investigate the Little Deer Isle (LDI) peridotite. Based on petrography and whole-rock and phase chemistry, these rocks are classified as wehrlites, which are typically characterized by high olivine and clinopyroxene content and commonly occur in ophiolitic complexes (e.g., Oman). The presence of this ultramafic body allows us to test different models for emplacement of the Ellsworth terrane during the evolution of the Iapetus Ocean.

Element mapping displays negligible orthopyroxene and no relict olivine. However, detailed petrographic observations reveal cusps, lobes, and facets between altered cumulate olivine and interstitial clinopyroxene, which we relate to porous melt flow, as also evidenced by vermicular spinel. Two clinopyroxene populations are present: low-TiO₂ (Mg#_91.8, En_46.9Wo_48.9) and high-TiO₂ (Mg#_85.6, En_45.1Wo_47.3). Whole-rock REE compositions display enriched concentrations up to 6x chondrite, and a negative MREE to HREE profile.

Evidence bearing on the tectonic setting of these rocks includes: (1) wehrlites are not typically observed in fast- or intermediate-spreading systems (Koepke, 2021), which conflicts with previous studies that suggest an extensional rift; (2) orthopyroxenite dykes, as observed through aerial mapping, are interpreted as a reaction between olivine and an aqueous silicic fluid (Kelemen et al. 1992; 1997); and (3) whole-rock geochemistry; e.g., a Nb-anomaly (< 0.74x chondrite), and the lack of a Eu anomaly, explained by the presence of water that suppresses plagioclase crystallization

In total, our data do not support the origin of the Ellsworth terrane within a proto-oceanic extensional rift. Instead, as proposed by Waldron et al. (2014), a Caribbean-type subduction zone gathered a mosaic of terranes (e.g., the Penobscot arc) as it migrated westward by trench rollback, into the Iapetus Ocean. The process that formed the Little Deer Isle wehrlite likely involved extension above the arc, which allowed for melt-rock interactions that impregnated the precursor rock (harzburgite?), with the melt sourced from hydrous magmas, expressed by LREE enrichment (3-6x chondrite).