Abstract

Approximately 130 samples of plutonic rocks were collected from two sections across the Coast Plutonic Complex of British Columbia, between lat 53° and 55°N. Major-element oxide (SiO2, TiO2, Al2O3, ΣFe2O3, MgO, CaO, Na2O, K2O, P2O5) and trace-element (Cr, Mn, Ni, Cu, Zn, Rb, Sr, Y, Zr, Ba) contents of the samples were determined by X-ray fluorescence spectroscopy.

Chemical and mineralogical variation within the plutons of the complex suggest that they may have formed by differentiation of magmas comprising a mixture of crystals and liquid. The data also suggest that these magmas formed from a source material of tonalitic or quartz dioritic bulk composition, probably the Central Gneiss Complex, by equilibrium fusion.

Statistical analysis of the raw chemical data reveals systematic increases in K2O, Rb, and Ba contents and decreases in CaO, Na2O, Mn, and Sr contents from west to east across the complex. The patterns of variation are not identical in the northern and southern sections, and analysis of the variance between the two populations confirms that chemical differences exist between them. Statistical analysis of the data normalized to 60% SiO2 shows that there are systematic decreases in MgO and Na2O and an increase in P2O5 contents from west to east across the pluton.

Differences in chemical composition and in the patterns of spatial chemical variation between the northern and southern sections across the complex and between raw and normalized data are interpreted as resulting largely from differential uplift and erosion of the plutons. Variations in composition of the source material and/or in the physical conditions of fusion are also significant.

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