The petrology and tectonics of the mesoproterozoic margin of southern Laurentia
Barnes, Melanie A. W.
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During the evolution of Laurentia, a Mesoproterozoic granite-rhyolite belt extended from Finland through Canada to the southwestern United States. This graniterhyolite province forms much of tiie west Texas and eastern New Mexico basement, where it is known locally as the Panhandle tertane and underiies the Debaca tertane (a metasedimentary and metavolcanic unit intruded by gabbro). The first study is a regional geological investigation of the Panhandle tertane using subsurface samples. The second study presents data from 41 closely spaced wells which penetrated several hundred meters into basement, providing the first opportunity to develop a 3-D view of the Panhandle terrane. Geochemical and isotopic data are presented for the Panhandle terrane, Debaca terrane, and deformed 'crystalline terrane' which consists of felsic intrusive rocks of unknown affinity. Characterization of the Panhandle terrane felsic rocks revealed three main groups: (1) early quartz monzonite and granite (-1.38 Ga), (2) a granite-rhyolite association (-1.36 Ga), and (3) late quartz syenite (-1.34 Ga). In addition, there are mafic rocks, the majority of which are thick sills intruding the granite-rhyolite pile. These sills have an alkaline, OIB-like signature, and Nd model ages imply they are contemporaneous with the felsic rocks. A neodymium model age for a Debaca terrane gabbro (1.26 Ga) is distinctly younger than a Panhandle terrane gabbro, as are Nd model ages (1.35 to 1.47 Ga) for the crystalline terrane granites, indicating that both tertanes are distinct from the granite-rhyolite province. In addition, a meta-arkose of the Debaca tertane containing detrital zircons (1.69 and 1.32 Ga) uncomfortably overlies a Panhandle terrane quartz syenite (1.33 Ga), suggesting derivation of the Debaca sediments from older terranes. The most significant petrologic features for understanding the tectonic regime are (1) the bimodal nature of the suite, (2) the alkaline character of both the mafic and felsic samples, and (3) evidence from Nd isotopes that the felsic magmatism is crustal. These features indicate that there was not subduction during this time period and instead agree with previous models where decompression melting of the mantle results in large volumes of mafic magmas which trigger extensive crustal anatexis.