Petrologic Study Of The “Chill” Zone Of The Layered Series At Duluth: Testing A Possible Plutonic – Volcanic Correlation Within The Midcontinent Rift

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Petrologic Study Of The “Chill” Zone Of The Layered Series At Duluth: Testing A Possible Plutonic – Volcanic Correlation Within The Midcontinent Rift

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The Duluth Complex is a multiple intrusive mafic complex that represents the largest exposed plutonic component of the 1.1 Ga Midcontinent rift. Results from extensive field mapping and petrologic studies (Miller and Green, 2008a, 2008b; Green and Miller, 2008) of the mafic cumulates comprising the type locality of the Duluth Complex at Duluth have confirmed that it is composed of two fundamentally distinct rock series, the Anorthositic Series (DAS) and the Layered Series at Duluth (DLS). The DAS had long been interpreted to be significantly older than the DLS based on the abundance of DAS inclusions in the DLS and, especially, on the occurrence of a fine-grained mafic rock that occurs at the sharp upper contact of the DLS with the overlying DAS, referred to as the DLS “chill”. However, high precision U-Pb ages from DAS and DLS samples (Paces and Miller, 1993) have shown that these two rock series are essentially identical in age within uncertainty (±0.5 Ma) at 1099 Ma relative to the 30 m.y. window of MCR magmatism. Because the similar ages of the DLS and DAS preclude the DLS “chill” being a thermal quench of DLS parental magma against the DAS, Miller and Ripley (1997) and Miller (2011) have suggested that quenching of DLS magma was caused by the decompression of a volatile-saturated magma accompanying volcanic venting from the subvolcanic DLS chamber. Several features lend evidence in support of a decompression quenching of hydrous magma interpretation, including: 1) the evolved composition of the DLS “chill”, 2) the presence of biotite phenocrysts in the “chill”, and 3) the extensive hydrothermal alteration of overlying DAS rocks. It has been further suggested that periodic venting of hydrous magma may have played an important role in the formation iii of the cyclic zone in the medial part of the DLS, particularly the occurrence of microgabbro cumulates in the upper parts of phase-layered macrocycles. This study seeks to test the decompression quenching model proposed by Miller and Ripley (1996) and evaluate if the DLS “chill” composition is in equilibrium with the cyclic zone cumulates and whether possible volcanic products are represented in the North Shore Volcanic Group (NSVG) overlying the Duluth Complex. To accomplish this, the lithological, petrographic and geochemical attributes of the DLS “chill”, microgabbros and flows from the NSVG were evaluated. The chemostratigraphy of the overlying NSVG, documented in a previous study by Brannon (1984), provided the means to compare the DLS “chill” composition with eruptive products of the layered series. A suite of lavas was identified whose composition and position above the DLS (4.5-5 km) are consistent with being the volcanic products of volcanic venting from the DLS magma chamber. To evaluate the comagmatic relationship between the DLS “chill” and cyclic zone microgabbros, the “chill” composition was applied to the MELTS based modeling program, PELE (Boudreau, 1999) to evaluate the equilibrium mineral phases of the “chill.” PELE was also used to determine oxidation state, water contents and simulate the proposed pressure fluctuations (devolatilization and venting) occurring with the DLS magma chamber. This study concludes the DLS chill formed by venting and decompressional quenching of hydrous roof zone magma. In addition, phase equilibrium modeling shows that cyclical pressure fluctuations caused by crystallization-driven volatile build-up and venting can also explain the macrocyclic cumulus phase layering and microgabbro intervals observed in the cyclic zone.



University of Minnesota M.S. thesis. October 2015. Major: Geology. Advisor: Jim Miller. 1 computer file (PDF); x, 151 pages.

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Sauer, Sarah. (2015). Petrologic Study Of The “Chill” Zone Of The Layered Series At Duluth: Testing A Possible Plutonic – Volcanic Correlation Within The Midcontinent Rift. Retrieved from the University Digital Conservancy,

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