Mid-Late Triassic-age alkali-basalt dikes were emplaced along the coast of New England between 240-200 Ma. Known as the Coastal New England (CNE) magmatic province, this dike swarm is the immediate magmatic predecessor to the formation of the Central Atlantic Magmatic Province large igneous province at 201 Ma and the breakup of Pangea. The intent of this study is to determine the melt source and mechanisms for melting which produced the Triassic coastal dikes. To achieve this goal, major and trace element compositions were analyzed for 53 CNE dikes from Maine and Nova Scotia. Radiogenic Nd-Sr-Pb-Hf ratios, representing some of the first 176Hf/177Hf data for CNE, are reported for 12 of the dikes. Taken together, the compositional data implicate melting of a deep mantle source that is relatively enriched in incompatible elements, such as a mantle-plume similar to those hypothesized as the source of melting in modern ocean-island basalts (i.e. Hawaii). Dike compositions are inconsistent with melts generated at typical spreading-center ridges (i.e. MORB). Modeling suggests that CNE melts ascended through thick continental crust, consistent with the incipient stages of rifting of Pangea, as evidenced by a heterogeneous mix of melting and crystallization depths, between 0-70km, with no clear geographic pattern. Radiogenic isotope data are relatively consistent and represent a mixture between HIMU, EMI and DMM mantle reservoirs, implying component consisting of relict subducted oceanic crust (or other similarly evolved material). CNE magmatism may have contributed to the breakup of Pangea by destabilizing the lower crust in the limited local area where it erupted, but its true relationship with the breakup of Pangea and later CAMP event requires more study. / Master of Science / Approximately 200-250 million years ago, hundreds of sheets of lava, called dikes, erupted along what is today the coast of New England. As these volcanic dikes rose up from the Earth’s mantle, they traveled along cracks and weak areas of the Earth’s crust. Today, these dikes are found along the New England coast as far south as Rhode Island and as far north as Nova Scotia, Canada. Based on the similarity of their geochemistry and petrology, as well as their geologic age and geography of their eruption, geologists group these dikes and similar volcanics together as a single, related magmatic event. This magmatic event produced the Coastal New England (CNE) magmatic province. 250 million years ago, the coast of New England was actually an interior part of the supercontinent known as Pangea. Around 250 m.yr. ago, Pangea slowly began rifting apart, which is when CNE volcanism began. By 200 m.yr. ago, Pangea had broken up, and CNE volcanism had ended. Further complicating the story, a large-igneous province (LIP) also erupted 200 m.yr. ago. Known as the Central Atlantic Magmatic Province (CAMP), this volcanism consisted of enormous volumes of lava that flooded over the entire east coast of the United States. The intent of this study is to determine what geological conditions led to the CNE volcanism. By learning which part of the Earth melted and why, CNE volcanism’s role in the breakup of Pangea, and the much larger CAMP eruptions that coincided with it, will become clearer. For instance, did the geologic events that resulted in CNE volcanism contribute to the breakup of Pangea, or did the breakup of Pangea cause CNE volcanism followed by CAMP volcanism? To achieve this goal, the geochemical compositions of 53 CNE dikes from Maine and Nova Scotia were analyzed. Radiogenic Nd-Sr-Pb-Hf ratios for a subset of the dikes (12) were also analyzed. This study presents some of the first radiogenic hafnium data for rocks from CNE. The data indicate that the melting which produced the CNE dikes began in the deep mantle, similar to the melting of mantle plumes beneath modern ocean-islands such as Hawaii. In contrast, shallow mantle melting, like the melting at mid-ocean ridges where oceanic crust is produced, is not consistent with the geochemical evidence presented for CNE in this study. Modeling suggests that CNE magmas rose through thick continental crust, which caused them to begin forming crystals at relatively high depths. Radiogenic isotope data suggests that part of the mantle that melted was old, recycled oceanic crust or similar mantle material. CNE magmatism may have contributed to the breakup of Pangea by destabilizing the lower crust in the limited local area where it erupted, but its true relationship with the breakup of Pangea and later CAMP event requires more study.

Indiana University-Purdue University Indianapolis (IUPUI) / The Antler and Sonoma orogenies occurred along the southwest-trending passive Pacific margin of North America during the Paleozoic concluding with the accretion of the McCloud Arc. A southeast-trending sinistral transform fault truncated the continental margin in the Permian, becoming a locus for initiation of an east-dipping subduction zone creating the Sierran magmatic arc. Constrained in age between two early Triassic tuff layers, the volcanic clasts in the breccia of Frog Lakes represent one of the earliest records of mafic magmatism in the eastern Sierra Nevada. Tholeiitic rock clasts found in the breccia of Frog Lakes in the Saddlebag Lake pendant in the east central Sierra Nevada range in composition from 48% to 63% SiO2. Boninites produced by early volcanism of subduction initiation by spontaneous nucleation at the Izu-Bonin-Mariana arc are more depleted in trace element concentrations than the clasts while andesites from the northern volcanic zone of the Andes produced on crust 50 km thick have similar levels of enrichment and provide a better geochemical modern analogue. Textural analysis of the breccia of Frog Lakes suggest a subaqueous environment of deposition from a mature magmatic arc built on continental crust > 50 km thick during the Triassic. The monzodiorites of Saddlebag and Odell Lakes are temporal intrusive equivalents of the breccia of Frog Lakes and zircon geochemistry indicates a magmatic arc petrogenesis.