Plutonic rocks of Jurassic age exposed on the eastern, or Pacific, side of the Alaska–Aleutian Range batholith represent the roots of a magmatic arc generally considered to have been generated in response to northwest-directed subduction. These rocks form a compositionally continuous calc-alkaline suite that ranges from hornblende gabbro through quartz monzonite. Tonalite and quartz diorite are the dominant rock types.
Trend-surface analysis was used to examine the geographic variation of major oxides and a few simple oxide ratios for 102 samples from widely separated localities. Statistical tests indicate that most of the trends, although weak, are real. The direction of slope of the trends is approximately normal to the Jurassic magmatic arc K2O and SiO2 increase toward the east-southeast; the other oxides increase toward the west-northwest. The K2O trend accounts for about 19% of the variance in the data and is significant at > 99.9% level of confidence.
If the chemical trends reflect the approximate geometry of a paleosubduction zone, the polarity of the Jurassic magmatic arc is to the northwest. That is, the paleosubduction zone was on the northwest side of the arc, and subduction was directed toward the southeast. The paleosubduction zone is on the opposite side of the arc from the position that has generally been assumed, indicating that the Jurassic plutonic rocks were not generated in response to classical Andean-type convergent plate margins. The magmatic arc may have formed in an intra-ocean environment, and subsequently has been rafted northward and accreted to this part of the northern Pacific rim during the late Mesozoic. Middle and Upper Jurassic clastic sediments underlying Cook Inlet to the southeast, and derived from the magmatic arc, are classified as backarc deposits, rather than as an arc-trench gap sequence.