GEOCHRONOLOGY AND STRAIN ANALYSIS OF THE JURASSIC PLUTONIC COMPLEX ON THE SOUTHERN FLANK OF THE NORTHERN SNAKE RANGE, NEVADA

The Jurassic Silver Creek and Old Man’s Canyon plutonic complex (Miller et al, 1999 –NBMG MFS Map#21) is exposed in the footwall of the Northern Snake Range Decollement along the southern flank of the range. The Old Man’s Canyon pluton generally grades westward from hornblende diorite to two mica granite over 10 km. Three new LA-ICPMS U-Pb zircon ages indicate an emplacement age of this entire compositional spectrum at 159-160 Ma. The Silver Creek 2 mica granite is compositionally more homogeneous and clearly cuts the Old Man’s Canyon plutonic complex. Two new LA-ICP-MS U-Pb zircon ages suggest a more protracted but distinctly younger emplacement age of 154.21±0.76 to 151.6±0.48 Ma. The plutonic complex and adjacent meta-sedimentary wall rocks are variably deformed by a mylonitic ESE trending mineral stretching lineation and weakly developed subhorizontal foliation. Grain shape fabrics in the plutons and stretched pebbles in metasedimentary pendants consistently indicate a strongly constrictional strain with top-to-SE sense of shear. Microstructural analysis of >20 samples of plutonic rock and mylonitized quartzite pendants within the pluton suggests that strain is extremely heterogeneous; some samples show little or no evidence of deformation while others have strongly developed mylonitic fabrics and crystallographic preferred orientations. EBSD analyses reveal a variety of CPO patterns for quartzite samples including Type II crossed girdle and single Y-axis maxima, suggesting activity of basal <a> slip, rhomb <a> slip, and prism <a> slip. Ubiquitous synkinematic growth of biotite, dynamic recrystallization of k-spar, grain boundary migration, and subgrain rotation recrystallization all suggest that deformation occurred at 450-550^oC (Regime I or II). The mylonitic fabric is interpreted to be late Eocene or early Oligocene and distinctly older than the rapid Miocene slip event on the NSRD based on (a) new 36 Ma (U-Pb zircon) age on a granodiorite dike that is at least partially involved in the mylonitization and (b) a new U-Pb zircon age of 23.3±0.09 Ma age on an undeformed rhyolite dike in the Silver Creek drainage that cuts all of the mylonitic fabric.