The Sierra Crest Magmatic Event: Rapid Formation of Juvenile Crust during the Late Cretaceous in California

Abstract

The Late Cretaceous was a period of extremely voluminous magmatism and rapid crustal growth in the western United States. From approximately 98 to 86 Ma, greater than 4000 km2 of exposed granodioritic to granitic crust, including the largest composite intrusive suites in the Sierra Nevada batholith, were emplaced in eastern California. Plutons intruded during this period include the highest peaks in the Sierra; we informally refer to this as the Sierra Crest magmatic event. Field, petrologic, geochemical, and geochronologic data indicate that, although they comprise an insignificant volume of exposed rocks (less than 100 km2), mafic magmas were intruded contemporaneously with each episode of intermediate and high-silica magmatism in the event. This observation attests to the fundamental importance of high-alumina basaltic magmas during crustal-growth episodes in continental arcs. Geochemical data for suites of coeval plutonic rocks of the Sierra Crest magmatic event, ranging in composition from basalt to high-silica rhyolite, demonstrate that recycling of pre-existing crust locally played a minor role in the growth of new crust. Thus, major chemical and isotopic characteristics of Sierra Crest plutons, such as variable isotopic compositions, were inherited from the mantle source of the high-alumina basalts and are not necessarily the result of interaction with the overlying crust. Consequently, we interpret isotopic boundaries in the western United States, such as the 87Sr/86Sr = 0.706 isopleth, to be largely features of the continental lithospheric mantle. Furthermore, isotopic data demonstrate that enrichment of the lithospheric mantle in the western United States probably occurred in the Precambrian during assembly of the North American craton. Geophysical and xenolith investigations by other workers support the hypothesis presented here that Cretaceous magmatism in the Sierra Nevada may have locally restructured most, if not all, of the crustal column. The timing of Sierra Crest magmatism correlates with voluminous magmatism elsewhere in the Cordilleran arc. We speculate that this intense episode of magmatism may have played a role in the global marine geochemical excursions and extinctions at the Cenomanian-Turonian boundary.