(U-TH)/HE THERMOCHRONOLOGICAL ANALYSIS OF APATITE, ZIRCON AND HEMATITE TO MODEL THE EXHUMATION HISTORY OF PRECAMBRIAN GRANITE NEAR HUNTERS POINT, DEFIANCE UPLIFT, ARIZONA

Abstract

With permission of the Navajo Nation*, we studied the structural-tectonic development of the Hunters Point monocline in northeastern Arizona near Window Rock, Arizona. Studying the monocline allows for a closer look at the effects burial and erosion have had on the Eastern margin of the Colorado Plateau since 1.7-Ga. The known orogenic history of the Defiance Uplift includes the Ancestral Rockies deformation, Laramide deformation, and overall Colorado Plateau uplift. The Hunters Point monocline is located within the Defiance Uplift and is defined by folded Permian strata, which abruptly bend from horizontal to vertical. The folded strata consist of De Chelly Sandstone (Permian) overlying Supai Formation (Pennsylvanian-Permian), which in turn rests nonconformably on 1.7-Ga Precambrian granite. The Precambrian granite is exposed in a quarry located ~1 km south of Hunters Point. Granite samples collected from the quarry provide an opportunity to understand the timing and rates of exhumation of the Precambrian basement by using (U-Th)/He-dating methods on apatite, zircon, and hematite grains. The apatite and zircon He systems have closure temperatures that make them sensitive to exhumation through depths of roughly 2-3 km and 6-9 km, respectively. We processed three samples of apatite and zircon for (U-Th)/He dating. Apatite (U-Th)/He analyses show a central age of 50.6 +/- 7.0 (1 stdev) Ma, with no significant differences between samples, over a range of eU from 10-175 ppm. Whereas, the zircon samples showed a strong inverse age-eU correlation ranging from 882-593 Ma over an eU range of 142-235 ppm. Apatite fission track (AFT) data records ages of 52.7 +/- 5.54 Ma (1 stdev) and 45.5 +/- 4.56 (1 stdev) from samples HP-1 and HP-2, respectively. Specularite ages (samples HP-4 and 5) are centered around 946 +/- 14.6 (1 stdev) Ma. Using this data, we model the thermal history of the top of the Precambrian basement through time-temperature space. Our model suggests that the granite was held at temperatures around 170 °C from soon after formation at 1.7-Ga until around 1.0-Ga, when the rock was cooled by exhumation. By 540 Ma, the granite would be at the surface, with likely only minor burial until 300 Ma, to account for the unconformity between the Precambrian basement and Permian Supai Formation. Our model suggests burial by sedimentation resulted in atemperature of the granite at 140 °C by roughly 55 Ma, followed by rapid exhumation soon after to the present exposed surface. If the geothermal gradient in this region were 20-30 °C/km, the thickness of overlying rock during the Precambrian would be around 8.5-5.7 km. A similar geothermal gradient would predict burial by 7.0-4.6 km of sedimentary rock just prior to exhumation around 55 Ma. According to observed thicknesses for the Paleozoic/Mesozoic strata in the surrounding area, a thickness of 3.3-3.6 km would likely be the thickness above the Precambrian granite. Therefore, we conclude that in this region, either the geotherm was around 38-40 °C or more sediment would have to have been present on top of the Precambrian graniteprior to Eocene exhumation.