Geochemistry and Age of Magmatic Rocks on Rat Island in the Aleutian Forearc at the Junction of the Aleutian Arc with the Bowers Ridge

Abstract:

Rat (also called Hawadax) Island in the western Aleutian forearc is located at the southern terminus of the Bowers Ridge where the Aleutian and Bowers Ridges meet at ~180° W. Relative to nearby Aleutian Islands, Rat Island is northwest of Amchitka, southeast of Kiska and south of Little Sitkin Volcano. Given the location of Rat Island, the question arises as to whether the geochemistry of the volcanic rocks reflects the Aleutian Arc or Bowers Ridge and what insight can be gained into processes forming the Aleutian and Bowers Ridges. In their reconnaissance mapping of Rat Island, the USGS (Lewis et al., 1960) separated the volcano-sedimentary units on Rat Island into the older Rat and younger Gunners Cove Formations, which were considered to be Oligocene-Miocene in age based on fossil evidence. A new 12.91±0.06 Ma plagioclase Ar/Ar age for a Rat Formation andesite porphyry shows the Rat Island rocks are significantly younger than the ~26 Ma andesitic and ~ 22 Ma seamount rocks dated by Wanke et al. (2012) on the Bowers Ridge. The new age places the Rat Formation andesites in the latest middle Miocene magmatic episode recognized along the Aleutian Arc from Medny Island in the west to Unalaska Island in the east.

Chemically magmatism on Rat Island progressed from the calc-alkaline porphyritic hornblende andesites in the Rat Formation to the tholeiitic basalts in the Gunners Cove Formation, in contrast to much of the Aleutians where the typical sequence is reversed. The Rat Formation andesites have REE patterns like Aleutian volcanic rocks to the east, classic subduction zone depletions in Nb and Ta, slight enrichments in Th and larger enrichments in Ba, U, K, Pb and Sr as do most pre-Pleistocene Aleutian magmatic rocks to the east. Rat andesites (57-65% SiO₂) differ from Bowers Ridge andesites (55-59% SiO₂) in having less adakitic-like Sm/Yb (~ 2 vs. ~3-4) and La/Ta (47-80 vs. 100-160) ratios. εNd values (+7 to +8.9) on Rat Island are similar to those on Adak to the east and lower than those on Attu to the west and on the Bowers Ridge; ⁸⁷Sr/⁸⁶Sr ratios (0.70315-0.70335) are similar to those on Adak and the Bowers Ridge. Overall, the geochemistry of Rat Island magmatic rocks resembles central and eastern Aleutian pre-Pleistocene magmatic rocks and is distinct from that of most pre-Pleistocene rocks in the western Aleutians.