VOLCANIC CLASTS WITHIN ORDOVICIAN STRATA IN THE MARATHON FOLD-THRUST BELT, WEST TEXAS, REVEAL EVIDENCE FOR NEOPROTEROZOIC (~706 MA) ALKALINE INTRAPLATE MAGMATISM ALONG THE SOUTHERN LAURENTIAN MARGIN

The Marathon fold-thrust belt (MFTB) in West Texas provides a critical window into the western part of the Ouachita orogenic belt, which is buried over much of its length. Upper Cambrian to Devonian strata in the MFTB were deposited in off-shelf settings along the Laurentian continental margin after the southern Iapetus Ocean began to open, and were subsequently thrust over the shelf for as much as 200 km from the SE during late Paleozoic collisional orogenesis. Ordovician strata within the MFTB include numerous sediment-gravity flow deposits that transported carbonate debris from the shelf into deeper water settings. Sparse volcanic clasts up to 70 cm in length are intermixed with the abundant carbonate debris in some deposits and contain a previously untapped record of pre-Ordovician magmatism along the southern margin of Laurentia.

We have analyzed 36 volcanic clasts from the Lower, Middle and Upper Ordovician Marathon, Fort Peña and Woods Hollow Formations in the NW part of the MFTB for major and trace elements; the majority of samples are from the Fort Peña Formation. The clasts are unmetamorphosed but have undergone low-T alteration and range in composition from basalt through trachyandesite to trachyte. Trace element patterns indicate within-plate settings for the volcanism, with derivation of the magmas from ocean-island-basalt-type sources, coupled with limited interaction with continental crust. Three Fort Peña trachytic clasts have yielded U-Pb zircon crystallization ages of ~706 Ma, dating at least one of the volcanic terranes that supplied the basaltic to trachytic clasts contained in the Ordovician strata. These data provide the first robust evidence for Neoproterozoic intraplate alkaline volcanism along this part of the Laurentian margin. At least locally, this part of the margin appears to have undergone a similar tectonomagmatic evolution to that recorded in the Appalachians to the east, where Iapetus opening in the Cambrian was preceded by intraplate magmatism extending back to ~765 Ma. However, unlike the situation in the Appalachians, Neoproterozoic volcanic rocks along the southern Laurentian margin must have been exposed intermittently to erosion well into the Ordovician, possibly in block uplifts in an extensional tectonic regime.