A ZONED PLUTON FORMED IN THE TRANSVERSE RANGES, CALIFORNIA BY NESTED DIAPIRS OF FRACTIONATED CRETACEOUS MAGMAS

In the San Bernardino Mountains, Transverse Ranges of southern California, three nested magma pulses consisting of diorite, biotite granite, and muscovite granite intrude the Precambrian to Paleozoic Wood Canyon Formation and the Paleozoic Carrara Formation of Cactus Flat. U-Pb ages of 81.9 ± 2.2 Ma for the diorite, 83.6 ± 2.2 for the biotite granite, and 77.5 ± 4.8 Ma for the muscovite granite indicate inward younging and an ~3-5 m.y history of emplacement. Geochemical data from all three units show LREE enrichment, increasing negative Eu anomalies, and Rb/Sr, K/Rb, A/CNK, and Zr/SiO₂ concentrations increasing with SiO₂. This is consistent with formation of all three pulses by fractionation from a single source but may also reflect variable source melting and/or contamination.

Contact metamorphic effects from emplacement of these three pulses overprint regional metamorphism. Intrusion of the diorite produced an aureole of upper pyroxene hornfels to lower sanidinite facies metamorphism with maximum temperatures between 735-775ºC. The final episode of contact metamorphism occurred as the granitic intrusions caused static, retrograde metamorphism in the albite-epidote-hornfels facies at temperatures of about 350-450˚C. 

Country rock -pluton and internal contacts between pulses are sharp and steeply dipping and only an extremely narrow structural aureole is formed in the host rock. Steep magmatic lineation is common in the units. Metamorphism of host rock indicates a large body of magma was emplaced at temperatures consistent with a diorite, yet nearly all of the diorite has been removed during emplacement of younger pulses with very little evidence to suggest the mechanism by which this occurred.   Structural evidence precludes lateral expansion or movement on faults as space-making mechanisms for these units and instead requires upward or downward transport of host rock and the older pulses. Evidence for limited stoping exists and may have masked other emplacement mechanisms.