Plutonic rocks of Jurassic age in the Alaska-Aleutian Range Batholith; chemical variations and polarity

The plutonic rocks represent the roots of a magmatic arc. Tonalite and quartz diorite are the dominant rock types. Trend-surface analysis was used to examine the geographic variation of major oxides. If the chemical trends reflect the approximate geometry of a paleosubduction zone, the polarity of the Jurassic magmatic arc is to the northwest. 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.--Modified journal abstract.