The K-Ca radioactive parent-daughter system provides a tool for tracing the origins of igneous rocks. It is complementary to other isotopic systems because as stoichiometric constituents of major minerals, the concentrations of K and Ca, and the K/Ca ratio in rocks, are simply related to mineralogy. In this paper we report the first high-precision calcium isotopic analyses of continental granitic rocks, island arc rocks, and mid-ocean ridge basalts. These data show that mid-ocean ridge basalts have the low 40Ca /42Ca ratios expected for the Earth's mantle, but that island arc rocks have slightly higher 40Ca /42Ca ratios indicative of crustal calcium in their magma sources. Many granitic rocks have high initial 40Ca /42Ca ratios, and in conjunction with independent evidence for the age of the crustal sources, these ratios provide constraints on the K/Ca ratios, and in turn on the silica contents and residual mineralogy, of the deep crustal magma sources.