Mafic Rocks of the Bowers Terrane and Along the Wilson-Bowers Terrane Boundary: Implications for a Geodynamic Model of the Ross Orogeny in Northern Victoria Land, Antarctica

A key area for the construction of a geodynamic model for the Ross Orogeny in northern Victoria Land is the boundary zone between the Wilson and Bowers terranes, along with the Bowers terrane itself. These areas are characterized by the occurrence of an abundance of mafic volcanic and plutonic rocks (locally with variable metamorphic overprint) that is unparalleled in the remainder of the Wilson Terrane. The age of these mafic igneous rocks is not always unequivocally related to the Ross Orogeny. In the Lanterman Range, the Wilson Terrane close to the boundary to the Bowers Terrane is characterized by the occurrence of mafic and ultramafic rocks with a metamorphic grade ranging from amphibolite to greenschist facies, locally including lenses of well preserved medium-temperature eclogites and their variously retrogressed products (Gateway Hills Metamorphic Complex). All these mafic rocks are included within an amphibolite-grade metasedimentary sequence consisting of dominant gneisses and minor quartzites. The age of the mafic protoliths is still poorly constrained. The geochemical affinity is variable from E-MORB throughout T-MORB to orogenic calc-alkaline. A mafic-ultramafic cumulate intrusive complex (Tiger Gabbro) crops out at the SE termination of the Bowers terrane; the age of intrusion is close to 500 Ma. The geochemical affinity is orogenic calc-alkaline. As the Tiger Gabbro is in fault contact with the adjoining rocks of the Bowers Terrane, no constraints exist regarding the original country rock. The Bowers terrane includes formations of mafic volcanic rocks (Glasgow Volcanics) and metamorphosed volcaniclastic sediments (Molar Formation). The age of deposition could be middle-late Cambrian. Geochemical data indicate affinities including N-MORB, E-MORB, T-MORB, arc tholeiite, calc-alkaline and OIB products. Scattered paleontological evidence suggests that volcanism, erosion, and sedimentation of the Bowers mafic rocks occurred within middle-late Cambrian time. If this is the case, the geochemical affinities of the Bowers mafic rocks precludes that this elongated belt simply represented the remnants of a disrupted island arc. Rather, the variety of igneous rocks seems to be more typical of a marginal back arc basin in an extensional setting, associated with a primitive island-arc. The striking similarity of the variety of geochemical signature between Bowers mafic rocks and the protoliths of eclogites and amphibolites suggests that scattered mafic rocks, included within continental material at the border between the Wilson terrane and the arc-back arc (Bowers terrane), could have been subducted/underthrusted at depths in excess of 90 km.