The occurrence of leucocratic rocks in ophiolites, herein collectively named oceanic plagiogranites, indicates that in some instances, differentiation has taken place during the development of oceanic crust. These plagiogranites consist primarily of quartz and plagioclase with only minor amounts of ferromagnesian minerals. High silica, moderate alumina, low iron‐magnesium, and extremely low potassium characterize oceanic plagiogranites and distinguish them from continental granophyres developed from differentiated mafic rocks. The 87Sr/86Sr ratios of the mafic and silicic rocks of the ophiolite suite of Troodos are similar. This similarity is consistent with the idea of low‐pressure differentiation of subalkaline basaltic magma to form K poor leucocratic rocks. Slow‐spreading centers may favor the formation of the oceanic plagiogranites because crystal fractionation is more likely to occur in such situations.

Initial ^Sr/^Sr was determined for samples of Mesozoic granitic rocks in the vicinity of the Garlock fault zone in California. These data along with similar data from the Sierra Nevada and along the San Andreas fault system permit a reconstruction of basement rocks offset by the Cenozoic lateral faulting along both the San Andreas and Garlock fault systems. The location of the line of initial "Sr/^Sr = 0.7060 can be related to the edge of the Precambrian continental crust in the western United States. Our model explains the present configuration of the edge of Precambrian continental crust as the result of two stages of rifting that occurred about 1,250 to 800 m.y. ago, during Belt sedimentation, and about 600 to 350 m.y. ago, prior to and during the development of the Cordilleran geosyncline and to left-lateral translation along a locus of disturbance identified in the central Mojave Desert. The variations in Rb, Sr, and initial "'Sr/^Sr of the Mesozoic granitic rocks are interpreted as due to variations in composition and age of the source materials of the granitic rocks. The variations of Rb, Sr, and initial "'Sr/^Sr in Mesozoic granitic rocks, the sedimentation history during the late Precambrian and Paleozoic, and the geographic position of loci of Mesozoic magmatism in the western United States are related to the development of the continental margin and different types of lithosphere during rifting.

No abstract

Analyses of particle size distributions indicate that clay minerals and other diagenetic and metamorphic minerals commonly undergo recrystallization by Ostwald ripening. The shapes of their particle size distributions can yield the rate law for this process. One consequence of Ostwald ripening is that a record of the recrystallization process is preserved in the various particle sizes. Therefore, one can determine the detailed geologic history of clays and other recrystallized minerals by separating, from a single sample, the various particle sizes for independent chemical, structural, and isotopic analyses.