Rocky Mountain (56th Annual) and Cordilleran (100th Annual) Joint Meeting (May 3–5, 2004)

Paper No. 7
Presentation Time: 10:40 AM


HOWARD, K.A., U.S. Geol Survey, 345 Middlefield Road, MS 975, Menlo Park, CA 94025, BARNES, C.G., Geosciences, Texas Tech Univ, Lubbock, TX 79409-1053, SNOKE, A.W., Dept. Geology and Geophysics, Univ of Wyoming, Laramie, WY 82071, WRIGHT, J.E., Department of Geology, Univ of Georgia, Athens, GA 30602 and LEE, S-Y., Geosciences, Texas Tech Univ, Lubbock, TX 79409,

East of the main plutonic arc in the Sierra Nevada, Late Cretaceous back-arc magmatism was mostly granite emplaced at mid-crustal or greater depths. From the Ruby Mountains of northeast Nevada we describe the intrusive style of two-mica leucogranites dated by monazite U-Pb methods as Late Cretaceous (two-mica granites were also intruded here in the Jurassic, Eocene, and Oligocene). The Cretaceous granites comprise large amounts of ~84-Ma sillimanite-bearing, pegmatitic leucogranite gneiss, and small amounts of ~93-Ma equigranular granite gneiss, both derived from melting of crustal rocks at deeper levels. Pegmatitic textures and estimated near-solidus temperatures indicate intrusion at or near H2O saturation. The granites intimately intrude a framework of sillimanite-grade metamorphosed Neoproterozoic and lower Paleozoic strata of quartzite and carbonate, such that the rock volume is roughly half framework and half Cretaceous granite. Despite the pervasive Cretaceous intrusion, the host-rock stratigraphic framework exhibits undisrupted fold nappes, and the fabric in metasedimentary relicts of all sizes is regionally consistent. Marble hosts subhorizontal sheets and sills of granite ranging in thickness from a few centimeters to >100 meters, and irregular bodies and swarms or networks of crosscutting dikes. Metaquartzite hosts granite sheets and sills; sprays of dikes occur adjacent to marble contacts. Cretaceous intrusion of pegmatitic granite spanned and likely accompanied the main pre-extensional folding and metamorphic fabric development, as suggested by (1) granite sills folded with the hosts, (2) crosscutting irregular masses and dikes connected to these sills, (3) a higher proportion of granite in the lower limb of the Lamoille Canyon fold nappe than in the upper limb, and (4) nappe-related fabric in the pegmatitic granite gneiss. The proportion of pegmatitic granite gneiss relative to host rocks increases structurally and topographically downward from less than 20% at high levels to nearly 100% in the deepest canyons as well as southward where deep levels project up plunge. The intrusion of abundant small granite sheets and other bodies into a structurally coherent host-rock ghost stratigraphy support a model of batholith construction by incremental coalescence of small intrusions.