Abstract
The initial Sr isotopic compositions of acid rocks from world-wide occurrences are plotted against their whole rock isochron ages.
They fall within a limited region slightly different from that of assumed source material basaltic magmas. The former has steep inclination than the latter. These two regions intersect at about 3×109 years ago each other. The Rb/Sr value calculated from the inclination of the region of acid rocks is in a range from 0.06 to 0.07, which corresponds to that of andesitic basalt.
These may suggest following fact. Namely, the most acid rocks in geologic ages are derived by partial fusion of certain material similar to the chemical composition of andesitic basalt that was already formed by partial fusion of more primitive material probably of peridotitic composition at about 3×100 years ago.
This hypothesis seems to be consistent with the geologic evidences that the activities of acid magmas have abruptly increased since about 3×109 years before.
