Thursday, January 15, 2015 AGS Luncheon
"Late Cretaceous Through Oligocene Magmatic and Tectonic Evolution of the Western Alaska Range"
Noon Luncheon 11:30-1:00 pm
New U-Pb zircon ages together with geochemistry and radiogenic isotopes for Late Cretaceous to Oligocene igneous rocks in the western Alaska Range constrain the spatial distribution, petrogenesis, and tectonic setting of magmatism through time. These igneous rocks were emplaced across multiple basement domains that include Neoproterozoic to Jurassic carbonate and siliciclastic strata of the Farewell terrane, Mesozoic plutonic and volcanic rocks of the Peninsular terrane, and Cretaceous turbiditic strata of the Kahiltna basin. They also host multiple types of mineralization including intrusion-related Au, porphyry Cu-Mo-Au, polymetallic veins and skarns, and peralkaline intrusion-related REE. The oldest intrusive suite is ca. 104 to 78 Ma; the youngest plutons of this relatively localized suite were intruded during folding of the Kahiltna succession. This deformation is interpreted to represent closure of the retroarc Kahiltna basin and a transition to transpression-dominated tectonics along the southern Alaska margin. More widespread magmatism ca. 75– 55 Ma occurred in two general pulses, each with contrasting geochemistry. The first pulse was dominantly magnesian ca. 75–67 Ma, and the second pulse was dominantly ferroan ca. 63–55 Ma. The latter pulse is associated with widespread andesitic to rhyolitic volcanic rocks and was coeval with dextral transpressional deformation. Emplacement of widespread and voluminous intermediate to mafic dikes ca. 59–51 Ma overlaps the final phase of the second pulse. Magmatism waned during inferred late Paleocene to early Eocene ridge subduction along the southern Alaska margin but then resumed ca. 45 Ma, around the same time as the initiation of the Aleutian/Meshik arc. In the western Alaska Range, this dominantly subalkaline Eocene magmatism included emplacement of the elongate N–S Merrill Pass pluton and large volumes of associated ca. 44–37 Ma andesitic flows, tuffs, and lahar deposits. Finally, an Oligocene magmatic pulse involved emplacement of a compositionally variable suite of subduction-related magmas ranging from gabbro to peralkaline granite ca. 31–25 Ma, followed by waning magmatism that coincided with initiation of Yakutat slab subduction.
Jamey Jones is a Research Geologist with the U.S. Geological Survey Alaska Science Center. Originally from northern Louisiana, Jones received a B.S. in Geology from the University of the South in Tennessee, an M.S. from the University of Wyoming working on a metamorphic core complex in northeastern Nevada, and a Ph.D. from the University of Texas at Austin working on the Proterozoic tectonic evolution of western North America. His research interests and expertise include regional tectonics, structural geology, hard-rock petrology and geochronology. He held tenure-track positions for three years each at University of Minnesota Morris and University of Arkansas at Little Rock prior to joining the USGS Alaska Science Center, where he has been working since 2011. His work at the USGS is primarily funded by the Mineral Resources Program, and is focused on framework geology, tectonic evolution, and mineral resources of Alaska and the northern North American Cordillera.