NEOPROTEROZOIC INTRAPLATE MAGMATISM ALONG THE SOUTHERN LAURENTIAN MARGIN: EVIDENCE FROM VOLCANIC CLASTS IN ORDOVICIAN STRATA IN THE MARATHON UPLIFT, WEST TEXAS

The Neoproterozoic history of the southern Laurentian margin preceding breakup of the Rodinia supercontinent and Cambrian opening of the southern Iapetus Ocean is to a large extent unclear, because much of the critical evidence is buried beneath thrust sheets of the Ouachita orogenic belt and extensive younger cover. A previously untapped record of intraplate magmatism along the southern Laurentian margin occurs in the Marathon uplift in the western part of the Ouachita belt. Shelf-derived Ordovician sediment-gravity flow deposits exposed in the uplift accumulated in relatively deep water along the continental margin and contain sparse volcanic clasts up to 70 cm long intermixed with abundant carbonate debris. These strata were thrust over the Laurentian shelf for as much as 200 km from the SE during late Paleozoic collisional orogenesis, and the volcanic clasts contained within them provide a window into one or more buried igneous terrains along this part of the Laurentian margin.

We have obtained geochemical data for 36 volcanic clasts in the Lower to Upper Ordovician Marathon, Fort Peña and Woods Hollow Formations in the NW part of the Marathon uplift. The clasts are unmetamorphosed, precluding derivation from Grenville basement underlying this part of the Laurentian margin, and primarily range in composition from alkaline basalt through trachyandesite to trachyte. Trace element contents indicate the magmas were derived from ocean-island-basalt-type sources in intraplate settings and underwent limited interaction with continental crust. Three Fort Peña volcanic clasts have yielded U-Pb zircon crystallization ages of ~706 Ma, which dates at least one of the volcanic terrains that supplied the clasts. These data indicate that at least parts of the southern Laurentian margin experienced a comparable history to that recorded in the Appalachians, where opening of Iapetus was preceded by protracted rift-related intraplate magmatism going back to ~765 Ma. By analogy with the Appalachian record, it is likely that Neoproterozoic magmatism along the southern Laurentian margin was also directly related to rifting, and additional examples of Neoproterozoic intraplate igneous rocks probably remain to be discovered in the subsurface elsewhere along the southern margin.