Chapter 3: Mid-crustal emplacement ofMesozoic plutons, San Gabriel Mountains, California, and implicationsfor the geologic history of the San Gabriel terrane

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Abstract
Mesozoic plutonism in the San Gabriel Mountains has temporal and geochemical similarities to plutonism in the eastern, inboard part of the Sierran–Mojave Desert Mesozoic arc, including the presence of Late Triassic quartz-poor, alkali-enriched plu-tons, Middle Jurassic(?) melanodiorites and porphyritic monzodiorites, a pre–Late Cretaceous bimodal dike swarm, and a Late Cretaceous calc-alkalic suite ranging from metaluminous quartz diorite to peraluminous garnet two-mica granite. Mineralogical evidence demonstrates that these plutonic rocks were emplaced at significantly deeper, mid-crustal levels in comparison to their counterparts on the North American craton. Hornblende barometry in metaluminous rocks yields emplacement depths of 19 to 28 km (5 to 7.5 kbar). The presence of late magmatic epidote in hornblende biotite granodiorite and quartz diorite is consistent with these estimates. Epidote is richer in pistacite component, and thus is indicative of slightly lower pressures of crystallization at higher f O2, in comparison to recent studies of magmatic epidote-bearing plutons. In Late Cretaceous peraluminous rocks, celadonitic muscovite yields estimated emplacement depths of 22 to 33 km (6 to 9 kbar). Calculations indicate that the presence of calcic magmatic garnet in the Late Triassic Mount Lowe intrusion is in accord with these pressure estimates. Mesozoic plutonic rocks in the San Gabriel Mountains are part of an extensive basement terrane (San Gabriel terrane) in southern California, adjacent Arizona, and northern Sonora. The presence of mid-crustal plutonic rocks in the San Gabriel Mountains is geometrically consistent with juxtaposition on the Vincent thrust fault above Pelona Schist, which was metamorphosed at high pressure in a Late Cretaceous–Paleocene subduction event. However, this mid-crustal Mesozoic history appears to characterize only the southern part of the San Gabriel terrane. A northern subterrane of the San Gabriel terrane, containing Proterozoic and Triassic basement rocks correlative with those in the San Gabriel Mountains, had an upper crustal post–Early Jurassic history in close proximity to the North American craton. The mode and timing of juxtaposition of the San Gabriel terrane with autochthonous North America is unclear, but the similarity in histories of Mesozoic arc construction suggests close ties with Mesozoic North America. Much of the distinctive character of the San Gabriel terrane may be a consequence of exposure of rocks formed in the middle crust of an evolving continental margin magmatic arc.

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