EMERGING DATA ON RODINIA RIFTING FROM MARATHON OROGENIC BELT, SOUTHERN LAURENTIA
These new data for samples from the Marathon Fm constitute the westernmost evidence for two magmatic pulses in southern Laurentia as Rodinia broke apart – one Neoproterozoic (750 to 700 Ma) and one Eocambrian (580 to 520 Ma) – as have been documented along the Appalachian margin. Our major and trace element data for 36 MFTB volcanic clasts, including the three dated to the earlier pulse (~706 Ma), are characteristic of rift-related intraplate magmatism. Trace element patterns indicate derivation from ocean-island-basalt-type sources and limited interaction with continental crust. Admixed sediment rendered the tuff unsuitable for whole-rock geochemical analysis.
Along the southern margin, potential rift-related volcanic rocks lie buried beneath Ouachita thrust sheets and younger strata. Prior to analyses of the Marathon boulders, Devils River Uplift (subsurface ~ 75 km ESE of Marathon belt) provided the sole evidence for co-located Neoproterozoic and Eocambrian magmatism in the region. Dated well cores (Rb/Sr, whole rock) included metarhyolites at 711 ± 23 Ma, whereas metavolcanic rocks within metasediments yielded ages between 524 ± 31 and 529 ± 31 Ma. Forthcoming geochemical analyses should reveal if either or both magmas were products of intraplate extension during the opening of Iapetus.