MID AND LATE CENOZOIC VOLCANISM IN NORTHERN EAST TINTIC MOUNTAINS, WEST-CENTRAL UTAH

A Miocene transition from subduction-related intermediate and silicic volcanism to extensional mafic volcanism is recorded by volcanic rocks in the Allens Ranch quadrangle which lies at the northern end of the East Tintic Mountains west of Utah Lake near the eastern margin of the Great Basin of central Utah. The Paleogene volcanic section is dominated by a suite of high-K calc-alkaline extrusive rocks. The oldest is a rhyolite ignimbrite that we have correlated with the Packard Quartz Latite with a type-locality to the south. It is a variably welded quartz-sanidine-rich, low-silica rhyolite. The Chimney Rock Pass Tuff Member of the Soldiers Pass Formation (34.7 +/- 0.07 Ma; Christiansen et al., 2007) is a pumice-rich, high-silica rhyolite locally overlain by volcaniclastic sediments interlayered with lake and hot-spring deposits. In the tuff, pumice clast sizes and unit thickness increase to the north. The Latite Ridge Latite (~35.3 +/- 1.4 Ma; Morris and Lovering , 1979) is a distinctive alkalic lithic-rich ignimbrite that crops out in the southern part of the area. Chemically, it is a trachyte with high concentrations of TiO2, LREE, and Zr, and low CaO. The Laguna Springs Volcanic Group (32.8 +/- 1.8; Morris and Lovering, 1979) consists of a trachyandesite to trachydacite ignimbrite (with large flattened pumice lapilli), a series of latite lava flows with underlying tuffs, and an overlying coarse volcaniclastic deposit. The volcanic conglomerate contains large blocks of Laguna Springs latite lava and smaller clasts of andesite and shoshonite that are similar to lava flows from Soldiers Pass to the north and Gardison Ridge to the west. The Paleogene volcanic rocks have spikey trace element patterns with large Nb, Ti, and Pb anomalies. The Paleogene intermediate to silicic sequence was followed by the eruption of a mildly alkaline Miocene (19.5 Ma; Christiansen et al. 2007) basaltic lava flow which we correlate with the Mosida Basalt; the basalt may have come from local eruptive centers or it may be erosional remnants of a long flow originating near Soldiers Pass about 15 km north. It has characteristically high Ti, Nb, and other high field strength element concentrations and is similar to other rift-related Miocene and younger basalts of the Great Basin which have relatively smooth trace element patterns lacking Nb anomalies.