Nd and Sr isotopic study of crustal and mantle inclusions from the Sierra Nevada and implications for batholith petrogenesis
Sm-Nd and Rb-Sr systematics have been examined for eight upper-mantle and lower-crustal xenoliths, the xenolith-bearing trachyandesite, and the granodiorite intruded by the trachyandesite in the Sierra Nevada batholith. Isotopic heterogeneity is shown by the xenoliths with 143 Nd/ 144 Nd ranging from 0.51169 to 0.51306, and 87 Sr/ 86 Sr ranging from 0.7031 to 0.7333. These ranges are similar to those of the plutonic rocks of the batholith. This similarity suggests that the xenoliths, believed to be representative of mantle and crustal reservoirs, may approximate the diverse source regions from which the varied granitic rocks of the batholith were derived and that the Sr-Nd isotopic correlation seen in granitic rocks may not be due to simple mixing of two end-member components. The subgranitic crust in this part of the Sierra Nevada apparently consists of metamorphosed sedimentary and igneous rocks which are highly variable isotopically. The mantle beneath the Sierra Nevada is also isotopically heterogeneous both vertically and laterally.