49. A possible unsuspected significance of isotopic Rb-Sr

Rb-Sr isotopic data that produce "errorchrons " with large ± error values are commonly not published because more accurate dates calculated from isochrons are sought. An assumption is generally made that Sr is the only element that has moved out of the system when an errorchron is produced, but little work has been done to verify this hypothesis. A Rb-Sr errorchron occurs when data are plotted from chemically analyzed samples collected in the Kernville diorite-gabbro, north of the Lake Isabella Reservoir in the southern Sierra Nevada. Detailed structural, mineralogical, chemical, and Rb-Sr isotopic examinations of aplite-pegmatite-migmatite dikes and their transitions to Kernville diorite north of Lake, show how data that produce a Rb-Sr errorchron are generated. The changes in mineralogy and whole-rock major- and trace-element compositions in the modified transition rocks reveal different movements of both Rb and Sr isotopes as well as other trace elements and major oxides. On that basis, other terranes producing Rb-Sr isotopic data that plot as an errorchron should be examined carefully to see if major changes in mineralogy and chemistry are associated with processes that created the errorchron