Paper No. 3
Presentation Time: 8:35 AM
MAGMATIC HISTORY OF THE TORDRILLO MOUNTAINS AND WESTERN ALASKA RANGE AND TECTONIC IMPLICATIONS
New mapping and geochronology in the Tordrillo Mountains and the western Alaska Range provides new insights on the tectonics and magmatism of the region. We report new geochronology obtained from 2002 to 2013: 34 40
Ar ages, 8 U-Pb TIMS ages, 6 LA-ICPMS, and 17 U-Pb SHRIMP ages. The Tordrillo Mountains consist of a large, domed granitic batholith intruding Lower Cretaceous to Paleocene sedimentary and volcanic rocks. The main granitic batholith yields ages between 122 and 55 Ma. One of the older plutons dated is near Whiskey Lakes and yielded an age of 90 Ma (Ar, hb), similar in age to the Pebble deposit. The peak of granitic magmatism was between 70 and 55 Ma, with the most abundant ages around 57 Ma. Next was intrusion of an intermediate to mafic dike swarm with ages from 58 to 51 Ma (Ar, wr). The dikes commonly trend N-S, dip west, and comprise up to 30% of outcrop. There was a pause in magmatism following intrusion of the dikes. A tuff in the West Foreland Fm near the Capps Glacier yielded an age at 43 Ma (U/Pb, zr) and an andesite west of Dinglishna Hill at ~32 Ma (U/Pb, zr). West of the Tordrillos, in the Lime Hills quad, there are ages of 41 to 34 Ma (Ar, wr) from volcanic rocks and ages between 46 and 25 Ma (Ar, wr) from scattered, often smaller, intrusions. Subsequent to 25 Ma, there was a magmatic hiatus until Mount Spurr Volcano began erupting at 1.8±0.5 Ma (Ar, wr).
From a tectonic perspective, granites with older ages (between 122 and ~80 Ma) have stronger solid-state fabrics. Also, the ocean (Kahiltna) basin between the Peninsular (to the south) and Farewell (to the north) terranes likely closed between 100 and 80 Ma, and thus the early stage of Tordrillo magmatism overlaps this time frame. The greatest volume of intrusions, around 57 Ma, is post collisional, but slightly pre-dates the time of inferred ridge subduction along the southern Alaska margin. The timing and presence of the late-stage mafic dike swarm is consistent with the igneous record of slab window magmatism elsewhere along the margin in SE Alaska, British Columbia, and the Cascades. The feeble re-establishment of arc magmatism post ridge-subduction is intriguing. The slab window event may have caused fundamental changes in the supply and (or) transport of magmas to upper crustal levels. The cessation of magmatism at 25 Ma is consistent with the initiation of Yakutat slab subduction.