NEW AGES FROM THE CRYSTALLINE HAYMOND BOULDERS, MARATHON BASIN, TEXAS

During the middle Pennsylvanian a thick pile of deepwater flysch beds were deposited in the Marathon depositional basin. Conspicuous in this sequence are the boulder beds in the Haymond Formation deposited as submarine flows. The diverse boulders include a small percentage of crystalline cobbles. The origin of these crystalline boulders remains unknown but is clearly from the unroofing of an outboard continental block to the southeast during the closing of the Marathon depositional basin.

The petrography of the crystalline cobbles differs from known basement rocks in southern Laurentia. The cobbles are mostly finer grained silicic igneous and metaigneous rocks, those most likely to travel well. Rhyolite, metarhyolite and two mica granitic rocks are most common. A limited number of Rb/Sr and K/Ar determinations were published from these cobbles in the 1960s. The mica ages indicated an earlier Devonian metamorphism. The whole rock Rb/Sr results yielded an age in the same range with a relatively high initial ratio, suggesting a sedimentary protolith. Three new K/Ar ages from muscovites from an expanded range of rock types gave ages between 381 and 383 Ma, within the range of previous earlier Devonian determinations. In this study we have analyzed individual zircons separated from two cobbles, a granodiorite gneiss and an unmetamorphosed rhyolite, by laser ablation icp-ms. The U-Pb zircon data are complex due to significant discordance in the granodioritic gneiss and inheritance in the unmetamorphosed rhyolite. The data from the gneiss yielded formed a poorly constrained chord with intercept ages of 1436 +/- 160 and 330 +/- 72 Ma. We interpret the upper intercept to be the age of the protolith while the lower intercept may record a time of metamorphism. The weighted mean of the most concordant zircons from the rhyolite cobble have a 206Pb/238U age of 371 +/- 12 Ma, which we consider to be the best estimate of the age of the rhyolite. The rhyolite contains a significant component of inherited zircon with mid- and early Proterozoic ages. The dated cobbled reflect a source area having a Proterozoic history that was strongly overprinted by metamorphism during the Devonian. No basement exposures having these two age components have are known from the region.