title: Origin and geochemical evolution of mafic magmas from Mount Baker in the northern Cascade arc, Washington: probes into the mantle and crustal processes
Creators: name: Moore, Nicole E.
Publisher: name: Western Washington University
Year: 2010

description: <p>Five mafic lava flows located on the southern flank of Mount Baker are among the most primitive in the volcanic field. A comprehensive whole rock and mineral chemistry dataset (including major, trace, REE and isotopic abundances as well as petrography and mineral chemistry) reveals the diversity between these mafic lavas, which 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, demonstrated by a lack of variation in the most highly incompatible elements, insignificant addition of crustal material, lack of an obvious crustal radiogenic source, and minimal chemical differences between lavas that are the result of mixing and their mafic endmembers. In evaluating mantle sources, four endmember lava types are represented. These include modified low-K tholeiitic basalt (LKOT-like), typical calc-alkaline (CA) lavas, high- Mg basaltic andesite (HMBA), and a second but distinct basaltic andesite. The first type, the basalt of Park Butte (49.3-50.3 wt.% SiO2, Mg# 64-65), is classified as low-K, and 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. A second lava type is represented by the basalts of Lake Shannon (50.7- 52.6 wt.% SiO2, Mg# 58-62) and Sulphur Creek (51.2-54.6 wt.% SiO2, 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 in abundances of REE, indicating variation in degree of partial melting or fractionation. The third lava type is represented by the basaltic andesite of Tarn Plateau (51.8-54.0 wt.% SiO2, Mg# 68-70), which has characteristics similar to high-Mg andesite worldwide. The second basaltic andesite unit, Cathedral Crag (52.2- 52.6 wt.% SiO2, Mg# 55-58), can be distinguished from the HMBA endmember because it has much lower MgO, Mg#, Ni and Cr, and is the least primitive of all the lavas in this study. The strongly depleted HREE nature of both basaltic andesite units suggests fractionation from a high-Mg basaltic parent (to a greater extent in the less primitive Cathedral Crag lavas) derived from a source with residual garnet is responsible for their generation. Assessment of the relative enrichment of a slab component in the lavas reveals that the calc-alkaline units are least enriched, the HMBA and basaltic andesite are intermediate, and the LKOT-like unit is most enriched in slab fluids. Petrogenetic indicators point to sediment as the primary slab fluid contributor, with a small role for altered oceanic crust fluids in the LKOT-like unit. Modeled fluid compositions support this assertion. Melt modeling indicates the LKOT-like and calc-alkaline lavas were generated by partial melting of a depleted spinel harzburgite, while the results for modeling HMBA and basaltic andesite units suggest a depleted garnet harzburgite source. Differences in primitiveness (i.e. MgO, Ni and Cr content) among the units are the result of differences in depth and degree of fractionation in the crust. A summary model is presented where 1) varying degrees of a primarily sediment-derived slab component infiltrate the mantle wedge; 2) slab flux assists in melting two distinct mantle sources; 3) melting occurs at various fractions, and the resulting melts stall at different levels in the crust; and 4) differentiation at a range of pressures to different degrees produces mafic magmas with diverse chemistry.</p>