Global variations in the chemical compositions of basalt and andesite
Schutz, Donald F
Adams, John A. S.
Master of Arts
The nonparametric rank-sum test is suggested as an improved nethod for comparing frequency distributions of individual components of chemical analyses which compose different statistical populations. The dalle index calculated on tbs basis of increasing SiO2, Al2O3, Na2O, and K2O and decreasing CaO, MgO, FeC, and TiO2 is used as a summary method for comparing the relative stage of differentiation of different areas. Fourteen areas (which include 744 basalt analyses) are arranged in order of decreasing average sialic rank. Five groups are found and a correlation between average sialic rank and tectonic character of the groups is observed* The two groups of highest sialic rank are composed of the four areas which surround the Pacific Basin. The intermediate group is the largest* being composed of four areas of the Afro-Surasian land case and the South Atlantic Ocean. The next group includes the Korth Atlantic and Indian Oceans and is followed by the most ferric group of all, which includes both areas of the Pacific Basin and the Greenland-Iceland Ami. The hierarchy of these areas arranged by sialic character is considered to be consistent with the hypothesis that the composition of basic extrusives is a function of the tectonic and/or orogenic history of the area involved and the thickness of the crust through which the lava must pass. The andesites of seven areas (224 analyses) are compared and are found to be a relatively homogeneous rook type, iio clear connection between basalts and andesites is apparent In the present data.