PALEOPROTEROZOIC TECTONICS OF THE EASTERN MOJAVE DESERT REGION: INVESTIGATION OF CRYSTALLINE BASEMENT COMPLEXES IN THE IVANPAH AND CLARK MOUNTAINS, CALIFORNIA

The Ivanpah and Clark Mountains of the eastern Mojave Desert region of California preserve Paleoproterozoic tectonothermal evidence of the ca. 1.8-1.7 Ga amalgamation of the Mojave crustal province onto the margin of Laurentia. The Ivanpah Mountains contain a sequence of lithologically heterogeneous megacrystic granite intercalated with juvenile supracrustal rocks metamorphosed to metapelitic gneisses and migmatites and local, foliated (S1) amphibolite pods. The Clark Mountains contain similar units, with an abundance of megacrystic granite near the top of the sequence, and the entire basement complex is nonconformably overlain by dolomite of a Cambrian passive margin sequence. Consistent with other portions of the Mojave province, this basement complex was pervasively deformed under upper amphibolite-facies conditions: the migmatite contains cm-scale garnet and microscopic sillimanite, and thermobarometry from the nearby McCullough Mountains establish minimum metamorphic conditions of 700 °C and 4 kbar. In both the Ivanpah and Clark Mountains, a single deformation fabric (D1) is characterized by N-S striking, shallowly west-dipping to subhorizontal foliation (S1) axial planar to east-vergent sheath folds (F1) of migmatitic leucosomes. Stretching mineral lineations (L1) on S1 surfaces in all lithologies throughout the entire area plunge an average of 30º toward the west, and combined with asymmetric feldspar porphyroclasts indicate a top to the east sense of shear. Wooden and Miller (1990) reported U-Pb zircon ages of 1.76-1.74 Ga from plutonic units of the Ivanpahs that are interpreted to represent initial crystallization of the megacrystic granite; Barley et al. (2004) presented similar total-Pb monazite ages from granitic plutons in the Nopah Range to the north in addition to 1.74 Ga metamorphic ages from the crystalline complex. The deformation and metamorphism reported here likely corresponds to widespread Yavapai orogenesis recorded in central Mojavia, which involved transpression-related crustal thickening. These observations add to a growing body of evidence that a geon 17 Yavapai signature is recorded well inboard of the Laurentide margin and crustal growth can be regarded as a continuum of convergence between individual terranes.