GEOLOGY, MINERALOGY, AND GEOCHEMISTRY OF THE SERPENTINITE OF DEER ISLE AND THE PERIDOTITE OF LITTLE DEER ISLE, PENOBSCOT BAY, MAINE: IMPLICATIONS FOR MID-CAMBRIAN TECTONICS OF THE APPALACHIAN OROGEN

In coastal Maine, USA, the Ellsworth terrane hosts two unique serpentinite bodies. The Deer Isle (DI) body, faulted against the Ellsworth Schist, has a protolith of harzburgite, with minor dunite. Little Deer Isle (LDI) has a protolith of clinopyroxene-bearing dunite and an unknown relationship with the nearby Castine Volcanics. We employ bulk-rock and phase major- and trace-element compositions to address the petrogenesis of these rocks and implications for paleotectonic setting.

Deer Isle samples comprise relict olivine (Fo_92.3-81.3), spinels that range from Mg#_73.1-13.8 and Cr#_92.2-1.3, and orthopyroxene (Mg#_91.1, En_90.9Wo_0.13) and clinopyroxene (Mg#_92.7, En_48.6Wo_47.6). In Little Deer Isle samples, we observed neither relict olivine nor orthopyroxene, spinels range from Mg#_61.3-8.7 and Cr#_80.0-28.4, and 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). Bulk-rock compositions display two distinct groups: a high-MgO (>37 wt%) group (i.e., DI) and a low-MgO group (<32 wt%: LDI). The high-MgO group generally displays low TiO₂, Al₂O₃, and FeO_t. Both localities display flat-to-enriched REE profiles, exemplified by La/Lu values from 6-40, and positive Eu-anomalies.

The REE patterns and other trace-element ratios (Th/Yb, La/Nd) are consistent with an enriched-MORB (EMORB) source region, which has been previously modeled as a proto-oceanic extensional rift between Ganderia and Gondwana (Schulz et al. 2008). Yet, up to 20% anhydrous batch melting of an EMORB source (e.g., Mertz et al., 2001) in the spinel peridotite facies does not reproduce the depleted REE patterns, and fractional melting models do not replicate light-REE enrichment. Additional evidence new to our study includes the presence of muscovite in DI samples, LDI titaniferous clinopyroxene, spinels that share compositions with oceanic core complexes, enstatite “dykes” bordered by “diffused” chromite, and the positive Eu-anomaly observed in some DI serpentinites. In total, our observations and models are consistent with an extensional setting in the Penobscot back-arc or a transtensional setting, such as the Cayman Trough in the Caribbean.