9 Petrology, Igneous / Metamorphic / Experimental (Posters)

Sunday, 23 March 2014
8:00 AM - 12:00 PM
Freedom Hall A

PALEOCENE GRANITIC PLUTONS AND SHALLOW INTRUSIONS IN A TERRANE SUTURE ZONE IN SOUTH-CENTRAL ALASKA: ZIRCON U-PB AGE, GARNET SEM-EDS, AND WHOLE-ROCK GEOCHEMICAL CONSTRAINTS ON MAGMA ORIGINS AND ECONOMIC POTENTIAL

Paper number: 9-6
Program: Technical Sessions
Time: 8:00 AM – 12:00 PM

Authors

Abstract

Paleocene granitic plutons and shallow intrusions in the suture zone of the Wrangellia composite terrane record an episode of collisional magmatism that punctuated ongoing arc activity of southern Alaska. We report 29 LA-ICPMS zircon U-Pb ages for these plutons which range from 63.1 to 56.1 Ma. Most of the granitic samples range from calc-alkalic to alkali-calcic, are peraluminous, ferroan, and range between A-type and S-type composition. They have moderate La/Yb ratios (<70), show enrichment in large-ion-lithophile and fluid-mobile elements (Ba, K, Pb, U), and a negative Nb-Ta trend on primitive-mantle-normalized plots. A subset of early Paleocene granitic samples have high La/Yb and Sr/Y ratios (up to 184 and 267, respectively), similar to adakites. The plutons are cross-cut by garnet-bearing leucocratic granitic dikes and 56-57 Ma rhyo-dacite and basaltic dikes. The garnet-bearing granitic dikes have high SiO2 (73-76 wt. %), are peraluminous, alkalic, and contain tourmaline, biotite, and minor muscovite. The garnets are euhedral with Mn-rich cores (SEM-EDS analyses), which together with the high silica content and field relationships of their host dikes is consistent with their forming as low temperature magmatic phases during the last stage of Paleocene magmatism. A migmatitic gneiss intermingled with some of the granitic plutons also contains garnet. These garnets are euhedral with overgrowths; their Mn content decreases outward from their cores but increases again in the overgrowth rims. This is consistent with magmatic overgrowths on metamorphic cores.

The Paleocene granitic magmas were formed under relatively dry ‘intra-plate’ conditions not typical of arc processes. Their magmas attained arc-like characteristics (fluid-mobile element enrichment) by contamination from crustal rocks. The adakite-like chemistry of some plutons and the garnet-bearing dikes indicate that some magmas were formed by melting of aluminous sedimentary rocks (Kahiltna assemblage flysch) in the collisional suture zone. Comparison of granite chemistry with U.S. Geological Survey stream sediment data within the suture zone shows a correlation of REE, Be, and Sn. Locally high concentrations of Ag are present along fault zones in the suture zone, but overall the Paleocene plutons do not have strong economic potential.