REE Variations Across the Peninsular Ranges Batholith: Implications for Batholithic Petrogenesis and Crustal Growth in Magmatic Arcs

Abstract

Rare earth element (REE) patterns of plutonic rocks across the Cretaceous Peninsular Ranges batholith vary systematically west to east, transverse to its long axis and structural trends and generally parallel to asymmetries in petrologic, geochronologic and isotopic properties. The batholith can be divided into three distinct parallel longitudinal regions, each defined by distinct REE pattern types. An abrupt transition occurs between rocks with slightly fractionated REE patterns in the western (coastal) region and rocks with middle to heavy REE fractionated and depleted patterns in the central region. Further to the east a second transition to strongly light REE enriched rocks occurs. The slopes of the REE patterns within each of these regions are largely independent of rock type. The first REE transition is closely coupled to regional discontinuities in other parameters: elimination of negative Eu anomalies, an increase in Sr content, and a marked restriction in petrologic diversity. This transition occurs over a range of initial 87 Sr/ 86 Sr ratios and Ci 18 0 values, but approximately correlates to a major shift in the emplacement style of the batholith from a stationary arc to a rapidly eastward-migrating (cratonward) arc.The sense of the regionally consistent REE trends cannot be explained by crystallization, assimilation, combined crystallization-assimilation, or mixing processes. The consequences of assimilation and high-level differentiation are not observed generally, despite the sensitivity of the REE to these processes. Geochemical and petrological features argue that the partial melting of mafic source rocks in which plagioclase-rich (gabbroic) residual assemblages abruptly gave way laterally and downward to garnet-bearing (eclogitic) residual assemblages produced all the changes associated with the first REE transition. The change in residual assemblages from gabbroic to eclogitic was superimposed on source regions already zoned in light REE abundances, 87 Sr/ 86 Sr and 18 0. Temperature and pressure constraints on the source regions place them in a subcrustallocation. The calcic nature of the batholith and the dominance of tonalite and low-K 2 0 granodiorite in all its regions argue that the source materials are broadly basaltic in composition. Experimental studies are consistent with the generation of the abundant tonalitic magmas by the partial melting of basalt under both low and high pressure conditions. Arc basalts such as those commonly erupted in modern island arcs and continental margins are inferred to have provided much of the source material and the heat. Additional high-18 0 components are needed in the more easterly source regions. These materials must be distributed so as to contribute equally to the range of magmas that occur in a given local region, and must preserve the calcic nature of batholithic sources. Altered basalts of ancient oceanic crust and possibly their associated metasediments, previously incorporated into the lithosphere beneath the continental margin during earlier ...

The Mg isotopic systematics of granitoids in continental arcs and implications for the role of chemical weathering in crust formation

“…The PRB represents part of the north-south trending Cordilleran batholiths emplaced through the western margin of North America during Cretaceous subduction of the Farallon plate. The eastern PRB represents the part of the continental arc that was emplaced through the Paleozoic margin of North America and hence represents mixtures of juvenile magmas and melts of older metamorphic basement (27) (4,20,26). Despite the involvement of older, metamorphic basement, all of the granitic rocks studied here belong to the I-type granitic series (11).…”

“…We selected 11 representative samples in this region. The geology of the PRB has been described in detail (20)(21)(22)(23)(24)(25)(26). The PRB represents part of the north-south trending Cordilleran batholiths emplaced through the western margin of North America during Cretaceous subduction of the Farallon plate.…”

Arc and Slab-Failure Magmatism in Cordilleran Batholiths II – The Cretaceous Peninsular Ranges Batholith of Southern and Baja California

“…The rare earth element (REE) concentrations of the two suites are different as originally noted by Gromet and Silver (1987), who suggested that rocks of the Santa Ana arc were derived from partial melting of plagioclase-rich sources whereas La Posta rocks were derived from garnet-bearing assemblages. They also pointed out that generation of La Posta rocks involved a higher δ 18 O source.…”

Late Early Cretaceous adakitic granitoids and associated magnesian and potassium‐rich mafic enclaves and dikes in the Tunchang–Fengmu area, Hainan Province (South China): Partial melting of lower crust and mantle, and magma hybridization

“…10a-b), suggesting that their source contained residual amphibole in addition to garnet (Gromet and Silver, 1987). Therefore, the Tunchang adakitic granodiorites were most plausibly produced by partial melting of basaltic lower crust in the garnet stability field whereas the Niubiling and Gaotongling adakitic granites were likely derived by partial melting of basaltic lower crust in the stability field of garnet + amphibole.…”