THE DUFEK COMPLEX: GROWTH BY PERIODIC REPLENISHMENT IN ONE OF THE WORLD’S LARGEST LAYERED MAFIC INTRUSIONS

The 182-183 Ma Dufek Layered Mafic Intrusion (LMI) located in Antarctica’s Pensacola Mountains, is one of the largest known layered intrusions on earth but due to its location it remains understudied relative to other layered intrusions. It is exposed in 3500m of non-overlapping stratigraphy: the lower and more mafic Dufek Massif section and the higher and less mafic Forrestal Range section. The lowermost ~500m of the Dufek Massif was sampled by ~900 closely spaced oriented minicores. This traverse crosses the contact between two major units: the Walker Anorthosite and the overlying Aughenbaugh Gabbro. Layering within units is complex and occurs on many scales, with the Walker Anorthosite being composed of well layered anorthositic and gabbronoritic cumulates, and the Aughenbaugh Gabbro composed of layered gabbronoritic cumulates with a few laterally extensive pyroxenites. Overall, mineral compositions become more evolved up-section, with plagioclase becoming more sodic and pyroxenes becoming more Fe-rich. Over the 500m section, Mg# ranges from 74-54 for inverted pigeonite, 81-61 for diopside, and plagioclase An content varies from An₈₃-An₆₁; this degree of chemical evolution occurs over a shorter stratigraphic interval than in other large LMIs, suggesting the early Dufek magma chamber was thin compared to the thickness it would eventually attain. Sr isotope compositions become less radiogenic up-section, suggesting a decreasing degree of crustal contamination as the LMI evolved. The studied section also contains two subtle chemical “reversals” (abrupt increases in the Mg#) in pyroxenes at the contact between the Walker Anorthosite and Aughenbaugh Gabbro and at the base of the Neuberg Pyroxenite, which may indicate replenishment by primitive magma. There are also intervals where both plagioclase and pyroxenes become anomalously Fe-rich. They are usually accompanied by lithological changes, an increase in pyroxene incompatible element content, country rock xenoliths, and particularly at the base of the Neuberg anomalously radiogenic Sr; these are interpreted to be the result of crustal contamination during magma influxes. Together this evidence suggests that the Dufek LMI likely grew by multiple replenishment events.