Date of Award
2019
Document Type
Open Access Master's Thesis
Degree Name
Master of Science in Geology (MS)
Administrative Home Department
Department of Geological and Mining Engineering and Sciences
Advisor 1
Chad Deering
Committee Member 1
Theodore Bornhorst
Committee Member 2
Olivier Bachmann
Abstract
The Southern Complex is part of the Archean Superior Province in the Upper Peninsula of Michigan and includes a batholithic sized body of Archean high-K megacrystic granitoid rocks informally called the Bell Creek granite. U-Pb zircon ages of the granitoid from previous studies suggest an emplacement age of ~2.6 Ga (Tinkham, 1997). Based on those ages the Bell Creek granite formed around the Archean-Proterozoic transition. This transition is a crucial time period in Earth’s history for crustal growth because of the onset of subduction and increased sedimentary environments at the end of the Archean (Taylor and McLennan, 1995). In this study we tested two models hypothesized for the origin of granitoids. The first model involves juvenile magma from the mantle that undergoes differentiation and the remaining silica-rich melt separates, ascends and cools to form a granitoid; may include assimilation of country rock. The second model involves the partial melting of preexisting lower crustal basement lithologies; may include assimilation of country rock. To test the two models a combination of U-Pb and O-Hf isotopes was employed, in addition to petrological observations. The results of the U-Pb zircon dating suggests a 2.5-2.6 Ga emplacement age for the Bell Creek granite. Inherited zircons range in age from 2.7-4.2 Ga, suggesting that supracrustal material of various ages contributed to the overall petrogenesis of the Bell Creek granite. The δ18O (VSMOW) values of the zircons ranges from +5.0 to +13.0‰, representing a range from mantle (+5.5‰, Valley, 2005), to crustal values (>8.0‰, Valley, 2005). The εHf(t) values range from -20 to -5, and yield a Hf model age of approximately 3.3 Ga, signifying isotopic ingrowth. The high-K nature of the granites suggests that a pelitic material was assimilated during petrogenesis; the range in δ18O values of the zircons with magmatic U-Pb ages is evidence that supports this hypothesis. Hoffman (1987) suggested that the Bell Creek was emplaced in a suite of metasedimentary and metavolcanics rocks, this could be the source of the assimilated pelitic material. Remnants of pelitic material are present in the high-K granites as biotite-chlorite-garnet dominated clots. Hoffman (1987) concluded that the Bell Creek granite is an S-type granite and a product of partial melting of sediments. However, magmatic zircons with mantle-like δ18O values supported by bulk-rock major and trace elements suggest the Bell Creek granite is an I-type. The δ18O isotopic results supports partial melting and assimilation of sedimentary material which is consistent with Hoffman’s (1987) previous conclusion of assimilation. Comparison of the emplacement age of the Bell Creek (2.55 Ga) and the Hf model age (3.3 Ga) supports a hypothesis that the Bell Creek granite is a product of partial melting of a possible amphibolite source rock that was produced 1 Ga prior to emplacement of the granite.
Recommended Citation
Petryk, Brandi, "THE ORIGIN OF AN ARCHEAN BATHOLITH IN MICHIGAN’S UPPER PENINSULA", Open Access Master's Thesis, Michigan Technological University, 2019.
Digital Appendices