HIGH-MgO MAGMA COMPOSITIONS AS INDICATORS OF PLUME HEAD DECOMPRESSION IN THE 1.1 GA MIDCONTINENT RIFT SYSTEM

High-MgO basalts and picrites form some of the oldest lavas and intrusions of the 1.1 Ga Midcontinent Rift System (MRS). These high-MgO compositions fall into distinct groups based on mineralogic, geochemical, and isotopic characteristics. In western Lake Superior (including the Nipigon area and Baraga Basin), a few of the oldest (first) flows (the Lower Siemens Creek composition, LSC) have high MgO (about 15 wt %), high Fe₂O₃ (11-14 wt %), high Sm/Yb ratios (4.3-7.4), and contain clinopyroxene phenocrysts. A second (slightly younger) picritic composition occurs throughout the Lake Superior basin and is typified by the Upper Siemens Creek composition (USC), having high MgO (11.2-22.9 wt %) with lower Sm/Yb ratios (2.6-3.2) and olivine phenocrysts. A third high-MgO composition is typified by the Lower Kallander Creek (LKC) composition which resembles the LSC composition, but is generally more strongly fractionated (MgO=4.5-12.7 wt %; Sm/Yb= 4.6-6.4) and contains plagioclase phenocrysts.

Incompatible trace elements patterns in the LSC strongly resemble those of ocean island basalts, with no Nb-Ta anomaly and e _Nd (1100 Ma) of ~ 0. The steep REE patterns suggest garnet was a residual phase and that melting took place at depths of >75 km. In contrast, the high-MgO composition of USC is characterized by a prominent negative Nb-Ta anomaly, less-steep REE patterns, and an e _Nd (1100 Ma) of -4; these compositional characteristics suggest the presence of another component (likely continental lithospheric mantle, CLM) and that melting occurred at a shallower depth than that indicated by the older picrites. The third composition, although more strongly fractionated, has overall trace element patterns that mimic those of the LSC, implying a large ocean island component; however, an e _Nd (1100 Ma) of -1 suggests a small but distinctive role for a second component (likely CLM). Thus, the succession of MRS high-MgO magmas records the decompression history of a probable plume head as it rose through the mantle, initially melting at depths of at least 75 km (LSC), and then, as it became ponded at the base of the continental lithosphere, incorporated more (USC) or lesser amounts of CLM (LKC).