DEFORMATIONAL HISTORY OF THE NOVILLO GNEISS, CIUDAD VICTORIA, MEXICO

The Novillo Gneiss constitutes one of four outcrops of Grenville-aged basement in eastern Mexico, much of which is considered to be underlain by a large, coherent orogenic segment of Grenville age termed Oaxaquia. Whether this orogenic belt is of Laurentian or Gondwanan affinity, however, is uncertain since its paleogeography is unknown and the tectonic history of the Novillo Gneiss is too poorly constrained to permit its correlation with the other basement outcrops. The gneiss comprises garnet-rich rocks that range in composition from gabbro-anorthosite to granite and occur in association with amphibolite, metaquartzite, quartz and feldspar-rich gneisses, talc-silicate rock, and marble. Metamorphism to granulite facies is thought to have occurred in the interval 0.9-1.1Ga, based on K-Ar and Sm-Nd dates of 860-930 Ma, a Rb-Sr date of 1140±80 Ma, and a U-Pb zircon date of 1018±3 Ma. Two sets of igneous intrusions post-date granulite facies metamorphism: (1) A sheared plagiogranite borders the gneiss along its contact with the Paleozoic Granjeno Schist generating non-sheared dikes that intrude only the gneiss. The granite is thought to be Precambrian based on an imprecise Rb-Sr age of 774±256 Ma. (2) Multiple sets of variably metamorphosed and deformed, NE-trending mafic dikes cut the NW-trending gneissic foliation but do not cut the granite. The dikes are of unknown age but may include feeders for mafic volcanics in the Granjeno Schist. The gneiss has been interpreted to record arc-continent collision above an E-dipping subduction zone. It is unconformably overlain by Silurian-Permian sediments. At least three phases of Precambrian deformation are expressed in the gneisses: (1) The gneissic banding (S₀)is folded by tight to isoclinal folds (F₁) of unknown kinematics and possess an axial planar foliation (S₁).(2) Tight to isoclinal NW- to SE-plunging sheath folds (F₂)deform S₀ and S₁ and are associated with an ESE-plunging mineral stretching lineation (L₂)that indicate a top to the WNW ductile shear. (3) Large scale, open to tight, gently ESE-plunging folds (F₃) deform all previous structures. deformation of L₂ indicates a dextral shear component during D₄. A set of mafic dykes crosscut D2 while a second cuts all three phases. This deformational sequence invites comparison with grenville basement outcrops further south.