David B. Stewart

The Ellsworth terrane, coastal Maine: Geochronology, geochemistry, and Nd-Pb isotopic composition—Implications for the rifting of Ganderia

The Ellsworth terrane is one of a number of fault-bounded blocks that occur along the eastern margin of Ganderia, the westernmost of the peri-Gondwanan domains in the northern Appalachians that were accreted to Laurentia in the Paleozoic. Geologic relations, detrital zircon ages, and basalt geochemistry suggest that the Ellsworth terrane is part of Ganderia and not an exotic terrane. In the Penobscot Bay area of coastal Maine, the Ellsworth terrane is dominantly composed of bimodal basalt-rhyolite volcanic sequences of the Ellsworth Schist and unconformably overlying Castine Volcanics. We use new U-Pb zircon geochronology, geochemistry, and Nd and Pb isotopes for these volcanic sequences to constrain the petrogenetic history and paleotectonic setting of the Ellsworth terrane and its relationship with Ganderia. U-Pb zircon geochronology for rhyolites indicates that both the Ellsworth Schist (508.6 ± 0.8 Ma) and overlying Castine Volcanics (503.5 ± 2.5 Ma) are Middle Cambrian in age. Two tholeiitic basalt types are recognized. Type Tb-1 basalt, present as pillowed and massive lava fl ows and as sills in both units, has depleted La and Ce ([La/Nd] N = 0.53‐0.87) values, fl at heavy rare earth element (REE) values, and no positive Th or negative Ta anomalies on primitive mantle‐ normalized diagrams. In contrast, type Tb-2 basalt, present only in the Castine Volcanics, has slightly enriched LREE ([La/Yb] N = 1.42‐ 2.92) values and no Th or Ta anomalies. Both basalt types have strongly positive e Nd (500) values (Tb-1 = +7.9‐+8.6; Tb-2 = +5.6‐+7.0) and relatively enriched Pb isotopic compositions ( 206

Isotopic and Paleontologic Evidence for Correlating Three Volcanic Sequences in the Maine Coastal Volcanic Belt

The correlation of the Castine Volcanics, the Thorofare Andesite–Vinalhaven Rhyolite sequence, and the Cranberry Island Series of Shaler (1889), suggested by their similar lithologies, appearance, and structural histories, is supported by the results of Rb-Sr whole-rock isotopic analyses and by the faunal assemblages from old and new fossil localities in the Castine Volcanics, Ames Knob Formation, and Thorofare Andesite. The volcanic rocks are partly Late Silurian, but mostly Early Devonian in age and yield an average radiogenic age of 390 ± 5 m.y. The use of calcite-bearing volcanic samples for whole-rock Rb-Sr dating degrades the method by greatly increasing the uncertainty of the isochron and initial Sr 87 /Sr 86 . Lower to Middle Devonian granitic plutons have initial ratios of Sr 87 and Sr 86 similar to those in the volcanic formations. The Castine Volcanics and the Lower Devonian granite of Sedgwick may be comagmatic, but the time interval between the extrusion of the Vinalhaven Rhyolite and its intrusion by the Middle Devonian granite of Vinalhaven Island is too long to support the comagmatic hypothesis.

Crystallography of Some Lunar Plagioclases

Crystals of calcic bytownite from type B rocks have space group I1 with c ≈ 14 angstroms. Bytownite crystals from type A rocks are more sodic and have space group C1, c ≈ 7 angstroms. Cell parameters of eight bulk feldspar separates from crystalline rocks indicate that the range of angle gamma is about 23 times the standard error of measurement, and its value might be useful for estimation of composition. Cell parameters of seven ilmenites are close to those of pure FeTiO3.