EMERGING DATA ON RODINIA RIFTING FROM MARATHON OROGENIC BELT, SOUTHERN LAURENTIA

Deformed as Pangea was born, the Marathon fold-thrust belt of West Texas provides new insight into the last days of Rodinia as well. Sedimentary gravity flow deposits of the L-M Ordovician Marathon, Ft. Peña and Woods Hollow Formations entrain olistoliths of unmetamorphosed volcanic rocks. A tuff – the first pyroclastic boulder recovered – and a bentonite bed from the Marathon Fm, have yielded zircon populations dated at 750 to 700 Ma (U/Pb, LA-ICPMS), consistent with the average crystallization age of ~706 Ma (U/Pb, SHRIMP, zircon) determined for three trachytic/trachyandesitic clasts from the Ft. Peña Fm. In addition, the tuff and bentonite contained 580- to 520-Ma grains – a suite not represented in the lavas. Grenvillian metamorphic basement rocks of the region furnished the 1300- to 1100-Ma grains common to all. Age of the bentonite bed (L. Ord.) is established biostratigraphically, as the zircons are detrital.

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.