PETROGENESIS AND REGIONAL TECTONIC SIGNIFICANCE OF LATE DEVONIAN MAFIC INTRUSIONS IN THE MEGUMA ZONE, NOVA-SCOTIA

Late Devonian (377-368 Ma, Ar-40/Ar-39; 376 Ma, U-Pb) mafic intrusions in the Meguma Zone crop out as dykes, plugs, and synplutonic bodies o f gabbro, diorite, or lamprophyre. All of the intrusions have similar lithologic characteristics and hydrous ferromagnesian mineral assembla ges, and they appear to represent a genetically related series of mafi c bodies with similar petrogenetic histories in the crust of the Megum a Zone. The intrusions show wide chemical variation of SiO2 (45.7-65.7 wt. %), Al2O3 (8.9-26.5 wt.%); MgO (2.8-26.5 wt.%), CaO (1.2-11.2 wt. %), and K2O (0.1-4.4 wt.%), and they have calc-alkaline, high-g calc-a lkaline, and shoshonitic characteristics. Large-ion lithophile element s (LILE) are present at variable but high concentrations (e.g., Ba = 6 2-1920 ppm, Sr = 176-2567 ppm) relative to most high field strength el ement (HFSE) abundances (e.g., Y = 10-37 ppm, Zr = 8-421 ppm), and lig ht rare-earth elements (LREE) have much higher concentrations than hea vy rare-earth elements (HREE) (La/Lu = 24-330). Initial Sr isotopic ra tios (0.7044-0.7079) and epsilon(Nd) values (-4.36 to 3.69) are highly variable. Scatter on major oxide variation diagrams probably results from the fractionation of all the major modal phases in the intrusions (olivine, augite, hornblende, and (or) plagioclase), and the cumulate characteristics of some bodies support this suggestion. Nevertheless, parallel patterns for the intrusions on mid-ocean ridge basalt (MORE) normalized spider diagrams support the notion of similar mafic parent melts, and Sr-Nd isotopic data identify contamination by continental crust in only one of the intrusions. The most primitive picrite contai ns approximately basaltic HFSE in conjunction with HREE at 5-11(CN), p erhaps suggesting that the magmas emanated from depleted peridotite or pyroxenite, but high alkalies, LILE (< 60(MN)): and LREE (10-100(CN)) , and elevated initial Sr ratios in all of the intrusions, also requir e the existence of an enriched source component. Troughs in the spider diagrams at Ta, Nb, and Ti, and Sr-Nd isotopic values comparable with modern island-are basalts, suggest that fluids derived from subducted ocean lithosphere metasomatized the mantle. Tectono-magmatic discrimi nators imply a continental margin are environment rather than an islan d are, and the intrusions record either Early Devonian subduction of I apetus Ocean lithosphere beneath the Avalon terrane, Middle Devonian s ubduction of Theic Ocean lithosphere beneath the Meguma terrane, or an inherited subduction signature formed during a much older event.