DEPTH-DEPENDENT GEOCHEMICAL CHANGES IN LATE CRETACEOUS GRANITIC ROCKS, TRANSVERSE RANGES, SOUTHERN CALIFORNIA

Hypotheses about depth-dependent changes in plutonism are difficult to test because of the lack of 3D control. Collectively, the Transverse Ranges in southern California constitute a cross-section through the Mesozoic magmatic arc, offering an opportunity to test such depth-dependent hypotheses. Major element geochemistry from a suite of Late Cretaceous (85 Ma – 74Ma) plutonic rocks ranging from lower to upper crustal intrusion depths we collected to evaluate the geochemical links between plutons as a function of emplacement depth. Lower crustal rocks from the Central Transverse Ranges include tonalite and diorite from the Waterman Mountain Batholith and tonalite and granite from the Josephine Mountain intrusion. Tonalite compositions from the lower crust are characterized by 48-68% silica, 4.38-7.82 Fe₂O₃ and 0.60-0.92 wt% TiO₂, whereas granite compositions are characterized by 68-74% silica, 0.30-2.03% Fe₂O₃, and 0.06-.42 wt% TiO₂. (Barth et al., 1995; Buehrer, 2003). The volumetrically dominant components of the mid-crustal exposures in the Eastern Transverse Ranges are biotite-hornblende tonalite, granodiorite, and muscovite-garnet granite sheets that are interlayered at meter to decimeter scale. Geochemically, the middle crust is compositionally more felsic than lower crustal exposures but more mafic than upper crustal exposures, ranging from metaluminous to peraluminous with 56-76% silica, 0.4- 6.0 % Fe₂O₃, and 0.07- 1.3 %wt TiO₂. Upper crustal plutons are characterized by metaluminous hornblende biotite granodiorite to peraluminous biotite granite, with 64-76 % silica, 0.43-2.75 % Fe₂O₃, and 0.07-0.46% TiO₂ (Palmer et al., 2006). This data shows that 1.) mid-crustal compositions overlap well with lower crustal compositions and 2.) mid-crustal plutonism is more heterogeneous than upper crustal plutonism, recording both more primitive and more evolved compositions. Furthermore, we contend that upper crustal plutonism could originate from the mixing of ascending hydrous magmas that stall in the mid-crustal sheeted complex. Continued work will explore trace element compositions and geochemical mixing models to support the preliminary depth-dependent geochemical links.