Barium in granitoid rocks of the Basin and Range Province of Nevada, Utah, California, and Arizona

Abstract

The relation between CaO and barium is shown for 230 random samples of granitoid rocks collected from 115 plutons in the Basin and Range Province of Nevada, Utah, Arizona, and California. The lowest barium contents are found in the most felsic rocks, the highest in those rocks with a CaO content of 1.5-2.5 percent. Intermediate barium contents are found in the more mafic rocks, those with a CaO content of greater than 2.5 percent. It is concluded that the barium concentrations described can be explained on the basis of the fact that only biotites and potassium feldspars among the major rock-forming minerals have been found to concentrate barium relative to the matrix. Introduction In a magmatic environment, most of the trace elements are taken up by the crystallizing silicates, substituting for the major elements largely on the basis of ionic size. The substitution of barium for potassium is a good example of this principle (Krauskopf, 1967, p. 587-589). Lee and Van Loenen (1971) describe an equivalent of a large part (63-76 percent Si02 ) of the classic differentiation sequence that resulted mainly from assimilation of chemically distinct host rocks. In determining the distribution of barium in this pluton, Lee and Doering (1974) were surprised to find a well-defined trend toward lower barium values in the more felsic rocks. In other words, though barium substitutes for potassium in this pluton, one tends to decrease as the other increases. These writers concluded that the distribution of barium in this hybrid granitoid rock was controlled partly by the barium contents of the various sedimentary rock types assimilated. The present paper presents additional data that suggest that the distribution of barium described by Lee and Doering (1974) is not unusual for granitoid rocks in general. Analytical data The hybrid granitoid rocks studied by Lee and Doering (1974) crop out in the Snake Creek-Williams Canyon area of the southern Snake Range, Nevada. The distribution of barium in those rocks is shown in figure 1, where barium is plotted against CaO (for a better point spread) and the equivalent ranges of K20 and SiOa are indicated. Barium rises from a minimum in the most felsic rocks to a maximum in those rocks containing 2.0-2.5 percent CaO (3.8-3.0 percent K20), and a secondary trend branches from the main trend where the rock contains 2.0-2.5 caO. During the past several years one of us (Lee) has assembled a suite of granitoid rocks from the Basin and Range Province of Nevada, Utah, Arizona, and California. Two random samples were collected from each of two randomly selected plutons within each 1° x 1° area. The final suite includes 230 samples collected from 115 plutons. The distribution of the samples is shown in figure 2, where only odd-numbered samples have been plotted; at the scale of figure 2, the even-numbered samples would plot at the same locations as their odd-numbered counterparts. The 230 samples were analyzed for barium by means of X-ray fluorescence and for calcium by means of flame atomic absorption, The relation between CaO and barium contents in the 230 samples of granitoid rocks from the Basin and Range Province is shown in figure 3. The scatter of points is much greater in figure 3 than in figure 1, which is not surprising in 1