Paper No. 21
Presentation Time: 1:30 PM-5:30 PM
LEAD ISOTOPIC SIGNATURE OF CONTINENTAL LITHOSPHERES IN EASTERN NORTH AMERICA, AND ITS ROLE IN THE EVOLUTION OF THE UPPER MANTLE
During the assembly and breakup of Super-continents, the records of complex physical and chemical interactions between continental and mantle reservoirs are often preserved in the geologic record. Along the eastern margin of North America, two episodes of supercontinent assembly (Rodinia and Pangea) and their subsequent dispersals are well documented. We utilize new and published lead isotope data (over 600 analyses) from both crustal and mantle reservoirs to address (1) isotopic provenance of continental lithospheres, (2) isotopic signature of mafic magmatism (LIPS) associated with breakouts, and (3) relationship between continental lithosphere and isotopic segmentation of the mid-Atlantic Ridge. The data from present day continental lithospheres (from the coast to the Arbuckle Mountains of Oklahoma) define four distinct isotopic provinces that become progressively more depleted in 207Pb/204Pb, relative to 206Pb/204Pb.Voluminous mafic magmas associated with the 1.0 Ga (Mid-continent rift), 570 Ma (Catoctin Fm) and Mesozoic (Central Appalachian Magmatic Province, CAMP) are used to characterize the lithospheric mantle. Mafic magmas of the Mid-continent rift show evidence of both plume and lithosphere derived melts. In contrast, mafic magmas related to breakout of the two Super-continents, e.g. Catoctin (Rodinia) and CAMP (Pangea) are correlated with mostly Laurentian (Adirondack Lead Isotope Province) and Amazonian (Blue Ridge Lead Isotope Province) continental lithospheres respectively. It is most likely that these lithospheres were assembled during Rodinian event. Although tectonic transport of the Catoctin formation rocks during the Taconic (Ordovician) orogeny is not well known, but by removing Alleghanian displacement, the two regions of magmatism essentially overlap, and suggest the tectonic stacking of multiple continental lithospheres at depth. We also utilize these complex isotopic datasets from land based observations to evaluate their impact on the isotopic systematics of the proto-Atlantic as mapped along the MAR and fracture zones. Our interpretation of the isotopic segmentation along the MAR favors heterogeneity in the upper mantle caused by delamination and interaction of the lower crustal regions of the continental lithosphere with a depleted upper mantle.