MAGMA SOURCE CHARACTERISTICS OF CAMBRIAN VOLCANIC ROCKS IN THE ELLSWORTH TERRANE, PENOBSCOT BAY AREA, MAINE

The Ellsworth terrane in the Penobscot Bay area of Maine consists of Cambrian bimodal volcanic rocks of the Castine Volcanics (~502 Ma), Ellsworth Schist (~509 Ma), and North Haven Greenstone. The volcanic rocks are divided into four geochemical types based on data for immobile trace elements: two types of tholeiitic basalt (Tb-1a, Tb-1b) and two types of rhyolite (Rhy-1, Rhy-2). Here we report Nd isotope data that provide further constraints on the source characteristics of these bimodal volcanics.

Tb-1a basalts, present in all units of the Ellsworth terrane, have compositions transitional between N-MORB and E-MORB, including slightly depleted LREE and primitive mantle-normalized trace element patterns (PMNP) with no Th or Ta anomalies. They have large positive ε_Nd500 values of +7.9 to +8.6 (mean +8.2; n = 6) indicating a long-term depleted mantle source and no crustal contamination. Tb-1b basalts, present only in the Castine Volcanics, are compositionally similar to E-MORB, including enriched LREE and PMNP with no Th or Ta anomalies. They have lower positive ε_Nd500 values of +5.6 and +7.0 (mean +6.3; n = 2) and T_DM model ages of 452 and 718 Ma. Rhy-1 rhyolites, present mainly in the Castine Volcanics, have tholeiitic compositions with immobile trace element ratios overlapping those of the Tb-1b basalts. Their positive ε_Nd500 values (mean +6.5; n = 3) also overlap those of the Tb-1b basalts, supporting a genetic relationship. Rhy-2 rhyolites, present mainly in the older Ellsworth Schist, are calc-alkaline with enriched LREE and PMNP with positive Th and negative Ta and Ti anomalies. Relatively low ε_Nd500 values near zero (-0.9 to +0.5) and T_DM model ages ranging from 1067 to 1130 Ma indicate an evolved crustal component in the source area of the Rhy-2 rhyolites.

The geochemical and Nd isotope data support a non-arc setting for the Ellsworth terrane. Further, the compositional similarity of the Ellsworth basalts to those found in rift basins in the present Gulf of California suggests that the Ellsworth evolved in a similar tectonic regime. The older Rhy-2 rhyolites may reflect crustal contamination/ melting during initial crustal rifting. Such a rift setting is compatible with the Baja rift model proposed for the Neoproterozoic-Cambrian evolution of peri-Gondwanan terranes (Keppie et al., 2003, Tectonophysics 365: 195-219).