DID WEST TEXAS CLOSE, THEN OPEN, THEN CLOSE AGAIN? A POSSIBLE EXPLANATION FOR THE SOUTHERN GRANITE-RHYOLITE TERRANE

The SE margin of Laurentia was convergent for most of the Mesoproterozoic, but hosted basins unrelated to subduction at 1.4-1.3 Ga. The basins, represented by the Carrizo Mountain Group (CMG) of west Texas and (at least part of) the east continent rift basin (ECRB), reflect localized extension that may explain the coeval southern granite-rhyolite terrane (SGRT). The CMG, which includes the only exposed sedimentary section of this age in the US, is predominantly metarhyolite, cross-bedded quartzite and meta-arkose, as well as schist and marble. Metarhyolites locally retain pyroclastic textures, have within-plate chemistry and protolith ages that range from 1380 +/- 20 to 1327 +/- 28 Ma, and have been correlated with the SGRT. The CMG, nearly devoid of conglomerate, does not resemble deposits of a forearc or backarc basin so much as distal rift or passive-margin sediments. CMG deposition was framed in time by NW-dipping subduction events: 1.5-1.4 Ga emplacement of juvenile crust the entire length of the granite-rhyolite terrane, and a 1.29-1.23 Ga arc event in Texas, which included back-arc extension in west Texas and the intrusion of mafic sills in the CMG at 1286 +/- 3 Ma. Because active margins do not easily convert to passive margins (if anything, they close while an outboard passive margin is accreted), and because there is no (short wavelength, high amplitude) geophysical signature of ophiolite in the subsurface, it is unlikely that an ocean basin opened at 1.4-1.3 Ga. Instead, I suggest that the basin-opening extension was analogous to late Cenozoic extension of the N. American cordillera. Thus, features of the 1.4-1.3 Ga extension included localized rifting, reworking of the 1.5-1.4 Ga arc and Paleoproterozoic crust that produced felsic magmatism (including explosive volcanism, similar to the ignimbrite flare-up in the western US), and possibly even core-complex formation. Similarly, the cause of such extension would be the migration of a triple junction, possibly with subduction of a spreading center and sinking of an extinct plate. This hypothesis explains the SGRT as a zone of extended, reworked crust produced in an event that, further east, produced rifting in the form of the ECRB. This extension event, however, was a subduction hiatus, and the SE Laurentian coast was an active margin throughout the Mesoproterozoic.