Moore, N. and DeBari, S., 2008, Origin and geochemical evolution of mafic magmas from the Cascade arc, Mount Baker, Washington: probes into mantle processes: Geochimica et Cosmochimica Acta, Volume 72, Issue 12, p. A647

Origin and geochemical evolution of mafic magmas from the Cascade arc, Mount Baker, Washington: probes into mantle processes

Mt. Baker is an active stratovolcano in the Garibaldi Belt of the Cascade arc in northern Washington. The composition of this volcanic field is largely andesitic, with basalt comprising only ~1% of the total eruptive components. Heretofore, knowledge of the geochemistry of the mafic flows on Mt. Baker has been restricted to major elements of a very limited number of samples. Whole rock chemistry reveals the diversity between these mafic lavas, which must be derived from distinct sources. Five mafic lava flows have been sampled and analyzed for major, trace and REE chemistry: the basalts of Park Butte (49.3-50.3 wt.% SiO2, 7.9-8.4 wt.% MgO), Lake Shannon (50.7-52.6 wt.% SiO2, 5.3-6.4 wt.% MgO), and Sulphur Creek (51.2-54.6 wt.% SiO2, 5.0-5.5 wt.% MgO), and the basaltic andesites of Tarn Plateau (51.8-53.9 wt.% SiO2, 7.0-7.9 wt.% MgO), and Cathedral Crag (52.1-52.9 wt.% SiO2, 3.8-8.3 wt.% MgO). The Tarn Plateau basaltic andesite and Park Butte basalt flows are the most primitive, with Mg# (100[Mg/(Mg + FeT)]) ranging from 60 to 67, Ni content from 59 to 67 ppm, and Cr content from 176 to 267 ppm. Mg# is not correlated with SiO2 content, as the Tarn Plateau basaltic andesite has the highest Mg# of all the lavas. The Park Butte flow is classified as low-K tholeiitic basalt (compositionally similar to LKOT found elsewhere in the Cascade arc), while the remainder of the flows are classified as medium-K calc-alkaline basalts and basaltic andesites. The REE patterns differ from flow to flow, with varying slopes. Park Butte (LKOT) has the flattest REE pattern and the lowest abundances of REE of all the lavas, with (La/Sm)N at ~1.7 and (LREE)N values from 25-35. Tarn Plateau has a relatively steep REE pattern, with (La/Sm)N at ~2.2 and (LREE)N values from 40-55. However, the most differentiated lava (Cathedral Crag) has the steepest REE pattern, with (La/Sm)N at ~2.3 and (LREE)N values from 45-70. Ba, Sr and Th cannot be correlated between the most primitive flows (Tarn Plateau and Park Butte), demonstrating that these lavas are not genetically related. We describe multiple mantle sources that generate mafic magmas of such varied compositions.