Mafic magmas from Mount Baker in the northern Cascade arc, Washington: probes into mantle and crustal processes

• Published in: Contributions to mineralogy and petrology 2012-03, Vol.163 (3), p.521-546
• Main Authors: Moore, Nicole E., DeBari, Susan M.
• Format: Article
• Language: English
• Subjects: Basalt; Creeks; Earth and Environmental Science; Earth Sciences; Fluctuations; Fractionation; Geochemistry; Geology; Glaciers; Igneous rocks; Lava; Lava flows; Magma; Melting; Mineral Resources; Mineralogy; Original Paper; Petrology; Rocks; Trace elements; Volcanic fields
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-011-0686-4

Five mafic lava flows located on the southern flank of Mount Baker are among the most primitive in the volcanic field. A comprehensive dataset of whole rock and mineral chemistry reveals the diversity of these mafic lavas that come from distinct sources and have been variably affected by ascent through the crust. Disequilibrium textures present in all of the lavas indicate that crustal processes have affected the magmas. Despite this evidence, mantle source characteristics have been retained and three primitive endmember lava types are represented. These include (1) modified low-K tholeiitic basalt (LKOT-like), (2) typical calc-alkaline (CA) lavas, and (3) high-Mg basaltic andesite and andesite (HMBA and HMA). The Type 1 endmember, the basalt of Park Butte (49.3–50.3 wt% SiO 2 , Mg# 64–65), has major element chemistry similar to LKOT found elsewhere in the Cascades. Park Butte also has the lowest overall abundances of trace elements (with the exception of the HREE), indicating it is either derived from the most depleted mantle source or has undergone the largest degree of partial melting. The Type 2 endmember is represented by the basalts of Lake Shannon (50.7–52.6 wt% SiO 2 , Mg# 58–62) and Sulphur Creek (51.2–54.6 wt% SiO 2 , Mg# 56–57). These two lavas are comparable to calc-alkaline rocks found in arcs worldwide and have similar trace element patterns; however, they differ from each other in abundances of REE, indicating variation in degree of partial melting or fractionation. The Type 3 endmember is represented by the HMBA of Tarn Plateau (51.8–54.0 wt% SiO 2 , Mg# 68–70) and the HMA of Glacier Creek (58.3–58.7 wt% SiO 2 , Mg# 63–64). The strongly depleted HREE nature of these Type 3 units and their decreasing Mg# with increasing SiO 2 suggests fractionation from a high-Mg basaltic parent derived from a source with residual garnet. Another basaltic andesite unit, Cathedral Crag (52.2–52.6 wt% SiO 2 , Mg# 55–58), is an Mg-poor differentiate of the Type 3 endmember. The calc-alkaline lavas are least enriched in a subduction component (lowest H 2 O, Sr/P N , and Ba/Nb), the LKOT-like lavas are intermediate (moderate Sr/P N and Ba/Nb), and the HMBA are most enriched (highest H 2 O, Sr/P N and Ba/Nb). The generation of the LKOT-like and calc-alkaline lavas can be successfully modeled by partial melting of a spinel lherzolite with variability in composition of slab flux and/or mantle source depletion. The HMBA lavas can be successfully modeled by partial me