Early Cretaceous near‐trench plutonism in southern Alaska: Atonalite‐Trondhjemite Intrusive Complex injected during ductile thrusting along the Border Ranges Fault System

Abstract

A group of leucotonalite to trondhjemite plutons in the western Chugach Mountains, Alaska, record a period of Early Cretaceous near‐trench plutonism along the paleo‐Alaskan/Aleutian subduction zone. The plutons were emplaced along the tectonic join between an older (Jurassic) arc massif and a younger (Cretaceous) melange terrane that was accreted by subduction beneath the crystalline massif. The plutonic rocks were injected during a major thrusting event that placed upper mantle(?) ultramafic masses atop the Cretaceous subduction assemblages. Deformation during this thrusting event apparently progressed from ductile to brittle conditions, and plutonic rocks were injected during both phases. Evidence for plutonism synchronous with ductile deformation includes boudinage or folding of leucotonalite dikes as well as a characteristic weak fabric within larger plutons. The latter is commonly associated with secondary mineral growth as well as deformation‐induced microstructures that suggest deformation under temperatures that decreased from lower amphibolite to subgreenschist facies conditions, conditions which are equivalent to the structural/metamorphic progression in adjacent schists. Paradoxically, however, the plutons locally crosscut low‐grade melange fabrics in the footwall as well as brittle fault networks within structurally higher parts of the crystalline hanging wall. The paradoxical field relationships are partially due to shuffling by early Tertiary(?) dextral strike‐slip faulting but are also indicative of plutonic emplacement over a period of time. The absolute age span of plutonism, however, need not have been long considering the syntectonic emplacement history. In particular, the deformation locally may have been melt‐dominated such that after initial injection of melts, the deformation was concentrated in the igneous masses. Lithologically, the plutons follow a low‐K series from diorite to muscovite‐garnet trondhjemite, a trend that is unusual in Phanerozoic terranes and is typically associated with Archean terranes. Moreover, the compositional trends are unlike the calc‐alkaline trend of near‐trench plutons studied by Hill et al. (1981). The tectonic mechanism for the near‐trench plutonic event is elusive, but shallow melting of amphibolite or metagreywacke along a young subduction zone provides the most reasonable explanations for the observations. However, an alternative model relating magmatic activity to a ridge‐trench encounter is also allowable. Nonetheless, in either model the genesis of the igneous rocks within a high‐temperature ductile thrust zone is atypical of modern subduction zones. Thus structural setting and lithologic association have many similarities to Archean systems, and the rocks may represent a young analog that ultimately may provide key petrogenetic constraints for the Archean system.