Abstract

The Ellsworth terrane is one of a number of fault-bounded blocks that occur along the eastern margin of Ganderia, the western-most 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 flows and as sills in both units, has depleted La and Ce ([La/Nd]N = 0.53–0.87) values, flat 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 ϵNd (500) values (Tb-1 = +7.9–+8.6; Tb-2 = +5.6–+7.0) and relatively enriched Pb isotopic compositions (206Pb/204Pb = 18.037–19.784; 207Pb/204Pb = 15.531–15.660; 208Pb/204Pb = 37.810–38.817). The basalts have compositions transitional between recent normal and enriched mid-ocean-ridge basalt, and they were probably derived by partial melting of compositionally heterogeneous asthenospheric mantle.

Two types of rhyolite also are present. Type R-1 rhyolite, which mostly occurs as tuffs interlayered with basalt in the Ellsworth Schist, is calc-alkaline and characterized by relatively low REE, Zr, and Hf contents, enriched LREE ([La/Yb]N ~3–6), positive Th and negative Ta anomalies, ϵ Nd (500) values near zero (+0.5 to −0.9), and relatively unradiogenic Pb isotope values (206Pb/204Pb = 18.845; 207Pb/204Pb = 15.625; 208Pb/204Pb = 38.626). The data suggest that R-1 rhyolite magma was likely derived by mixing of basalt with melts from a relatively depleted crustal source. Type R-2 rhyolite, which mostly occurs as lava flows and domes in the Castine volcanics, is tholeiitic and characterized by enriched REE with flat patterns ([La/Yb]N= 1–2.5), moderate negative Eu anomalies (Eu/Eu* = 0.3–0.5), enriched Th, small negative Ta anomalies, and ϵ Nd (500) (+5.8–+7.5) and Pb isotope (206Pb/204Pb = 19.175–19.619; 207Pb/204Pb = 15.605–15.649; 208Pb/204Pb = 38.834–38.851) values that overlap those of the tholeiitic basalts. The data suggest that R-2 rhyolite magma was derived by the partial melting of hydrothermally altered basalt with the addition of a small amount of an enriched component, probably R-1 rhyolite.

The geologic, geochemical, and isotopic characteristics of the bimodal volcanic sequences strongly suggest that the Ellsworth terrane did not evolve as an extensional back-arc basin behind an active arc, but rather it evolved as a proto-oceanic rift petrogenetically similar to Cenozoic rifts like the Gulf of California–Salton Trough and Red Sea–Gulf of Aden rift systems. Such a setting is supported by the presence of serpentinized mantle and zinc-copper–rich massive sulfide deposits in the Ellsworth terrane. We conclude that the Ellsworth terrane developed as a Middle Cambrian oceanic rift that led to the separation of Ganderia from northern Gondwana and the development of the Rheic Ocean. The synchronous development of the Ellsworth terrane with the ensialic Penobscot arc system, which formed on the ocean-facing margin of Ganderia, suggests that the Ellsworth terrane (rift) probably developed inboard from the margin, perhaps in response to far-field plate forces (slab pull or rollback?) in a manner similar to that of the recent Red Sea or Tasman Sea.

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