Mineral composition magmatic rocks

Barker (1978) advocated plotting mineral compositions on an AFM diagram in addition to rock compositions to assess mineralogical control of magmatic processes. However, this approach can only be semiquantitadve since the lever rule.(see below), which works well in bivariate plots, cannot be applied because of the disparate proportions of the compositions before projection. Thus AFM diagrams cannot be used in petrogenetic studies to extract quantitative information about... 

The mineral composition (mode) of the Wyatt Formation in the Scott-Glacier area was determined by Minshew (1967) and is listed in Appendix 6.7.3.1 followed by chemical analyses in Appendix 6.7.3.2. The rocks of the Wyatt Formation in the La Gorce Mountains were altered either by the original magmatic fluid or during later metamorphism. In extreme cases, plagioclase was completely altered to sericite or saussurite (Stump et al. 1986). Saussurite is a mineral aggregate composed of albite and zoisite or epidote with variable amounts of calcite, sericite, and prehnite (Jackson 1997). 

Table 1.1 shows the mean mineral composition of magmatic rocks. 

Direct sampling of mantle rocks and minerals is limited to tectonic slices emplaced at the surface (see Chapter 2.04), smaller xenoliths transported upwards by magmatic processes (see Chapter 2.05), and still smaller inclusions in such far-traveled namral sample chambers as diamonds (see Chapter 2.05). Because of such limited direct access to mantle materials, knowledge of mantle structure, composition, and processes must be augmented by geophysical remote sensing. What can various seismological observations tell us about the major-element composition of the upper mantle How can they constrain possible differences in chemical composition between the upper... 

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