THE LAYERED SERIES AT DULUTH: EVIDENCE FOR A PLUTONIC-VOLCANIC CONNECTION

The Duluth Complex is a multiple intrusive mafic complex that comprises the largest exposed plutonic component of the 1.1 Ga Midcontinent Rift. It was emplaced beneath a comagmatic volcanic edifice termed the North Shore Volcanic Group (NSVG). The Duluth Complex at Duluth is composed of two major lithologic series: the Anorthositic Series (DAS), a ~1-km thick cap of structurally complex, plagioclase-rich gabbros, and the Layered Series at Duluth (DLS), a 3-4.5 km thick, well differentiated, stratiform sequence of troctolitic to gabbroic cumulates. Based on cumulate lithology, the DLS can be subdivided into five major lithostratigraphic zones: basal contact zone, troctolite zone, cyclic zone, gabbro zone and upper contact zone. The DAS had long been interpreted to be significantly older than the DLS based on abundant DAS inclusions in the DLS and on the occurrence of a fine-grained mafic rock at the sharp upper contact between the DLS and the DAS, referred to as the DLS “chill”. However, high precision U-Pb ages from the DAS and DLS show that these two rock series are essentially identical in age at 1099±0.5 Ma, which precludes the DLS “chill” being a thermal quench of DLS parental magma against the DAS.

An alternative explanation is that quenching of DLS magma was caused by decompression of a volatile-saturated magma accompanying volcanic venting from the subvolcanic DLS chamber. Evidence in support of this interpretation include: 1) evolved composition of the DLS “chill”, 2) presence of biotite phenocrysts in the “chill”, and 3) extensive hydrothermal alteration of overlying DAS rocks. We show further evidence that decompression attending periodic venting of hydrous magma can also explain the cumulate regressions and microgabbro occurrences that characterize the medial cyclic zone of the DLS.

A venting origin for the DLS "chill" is further supported by the discovery of matching compositions found in several mafic lavas occurring 3-5 km above the top of the DLS. The most interesting match occurs among a sequence of 8 lavas that define a progressively differentiated series. If these flows formed from venting of differentiated DLS magmas, it would imply that the top of the DLS was emplaced at a depth of ~4-5km. This depth is in the 2-4 km range estimated for magma reservoirs feeding mafic volcanic centers like Kilauea in Hawaii.