A neodymium and strontium isotopic study of the Mesozoic calc‐alkaline granitic batholiths of the Sierra Nevada and Peninsular Ranges, California

Abstract

Plutonic igneous rocks of the Sierra Nevada batholith exhibit a range of Nd isotopic composition described by ε Nd = +6.5 to −7.6. Similar rock types from the Peninsular Ranges have ε Nd = +8.0 to −6.4. In both batholiths, ε Nd correlates strongly with initial 87 Sr/86 Sr. Decreasing ε Nd values are accompanied by increasing 87 Sr/86 Sr and increasing δ 18 O; the correlation with δ 18 O being more pronounced for the Peninsular Ranges. The ε Nd values show regular geographic variations, as was found previously for initial 87 Sr/86 Sr. Three metasedimentary country rock samples from the Sierra Nevada region have low ε Nd values (−11 to −16) and Precambrian model Sm‐Nd ages (1.5 to 1.9 Æ). The country rock ε Nd values, and those of primitive oceanic island arcs (ε Nd = +8), bracket the data for the batholith rocks. The Nd, Sr, and O isotopic data can be explained if the batholiths are mixtures of island arc and metasedimentary components, the latter being of both Paleozoic and early Proterozoic age. This model appears to be consistent with existing Pb isotopic data. Consideration of O‐Sr isotopic relations and the variation of 147 Sm/144 Nd with ε Nd suggests that assimilation of crustal rocks by magmas rising from the mantle and undergoing fractional crystallization could have been the major process responsible for the mixing of crustal‐ and mantle‐derived components. The isotopic data, when combined with assumptions about the structure of the crust beneath the batholiths, suggest that about 50% of the crustal material presently within the geographic boundaries of the batholiths and above the Moho represents juvenile crust derived from the mantle in the Mesozoic. The remaining material appears to be mostly derived from 1.8‐Æ crust, yielding an average crust formation age of nearly 1 Æ for this section of the crust. This result, which may apply to large portions of the Cordillera, suggests that the average age of the North American continent may be greater than previously estimated. The concentration of Nd correlates well with ε Nd in the batholith rocks and supports the conclusion that juvenile continental crust is derived from mantle reservoirs that are depleted in incompatible elements. A 1.5‐Æ Sm‐Nd model age for sedimentary rocks of the Mesozoic(?) Calaveras Formation indicates that the Nd in this “oceanic” terrain is dominated by continental detritus and demonstrates the potential of Sm‐Nd isotopic studies for aiding in construction of tectonic models.