CHARACTER OF LATE CRETACEOUS MAGMATISM SPANNING THE TRANSITION FROM SUBDUCTION TO LARAMIDE OROGENESIS IN THE WESTERN/CENTRAL MOJAVE REGION

The Mojave region is a critical domain for developing and evaluating crustal models that interpret the passage of Laramide-aged plateaux beneath the western US (Liu et al., 2010). Throughout this transitional period (late Cretaceous), magmatism in the western/central Mojave and eastern Transverse Ranges regions was continuous, spanning intrusion of the Teutonia Batholith at 88 – 82 Ma (Barth et al., 2004), magmatism in the eastern Transverse Ranges from 88 – 72 Ma (Needy et al., 2009; Friesenhahn et al., In review), and culminating in the intrusion of the Cadiz Valley Batholith (~76.5 – 73.5 Ma) in the central Mojave region (Barth et al., 2004; Economos et al., In review).

Igneous-rock chemistry and petrologic characteristics for all these intrusive bodies are consistent with their origin by subduction-related magmatism, but do not uniquely require such a scenario. However, during this late Cretaceous magmatic period evidence is lacking in support of major compositional and petrologic transitions that ostensibly would be expected (for example) during the infiltration of asthenospheric melts--as predicted in a crustal-delamination scenario. Whole-rock La/Yb ratios across the region indicate that the crustal column through which late Cretaceous magmas passed was thick throughout this time period. Moreover, syn-magmatic deformation--particularly of granites observed to intrude the hinges of macro- and outcrop-scale folds—indicates a compressional environment in the mid-crust during the period ~76 – 72 Ma, coincident with the intrusion of the Cadiz Valley Batholith. All this evidence conflicts with the hypothesis that the Shatsky Rise conjugate passed beneath the Mojave region during the late Cretaceous (Liu et al., 2010); instead, available data support continuing subduction-related arc magmatism in the Mojave throughout the late Cretaceous. All available data are consistent with the early Tertiary passage of the Hess Rise conjugate through the Mojave area into Arizona, and shifts of zircon chemistry of igneous rocks between 70 and 40 Ma (e.g. Chapman et al., 2018).

Barth et al. (2004), G.S.A. Bull., v. 116; no. 1/2, p. 142-153.

Chapman et al. (2018), G.S.A. Bull, v. 130; no. 11/12, p.2031-2046.

Liu et al., (2010), Nat. Geosci. V. 3, p. 353-357.

Needy et al. (2009), G.S.A. Special Paper no. 456, p. 187-218.