Building Arc Crust: Plutonic to Volcanic Connections in an Extensional Oceanic Arc, the Southern Alisitos Arc, Baja California

Abstract

The ∼50 km long Rosario segment of the Cretaceous Alisitos oceanic arc terrane provides undeformed three-dimensional exposures of the upper 7 km of an oceanic extensional arc, where crustal generation processes are recorded in both the volcanic and underlying plutonic rocks. These exceptional exposures allow for the study of the physical and chemical links between the rock units and help constrain the differentiation processes active during the growth and evolution of arc crust. This study focuses on the southern third of the Rosario segment, previously referred to as the southern volcano-bounded basin, and its plutonic underpinnings. Upper crustal rocks in the Rosario segment consist of a 3–5 km thick volcanic–volcaniclastic section with hypabyssal intrusions. Plutons intrude these units at various levels along-strike, but at each intrusive contact the transition is complete over a distance of <150 m, where stoped volcanic blocks are present. There is striking compositional overlap in whole-rock and mineral chemistry between the plutonic and volcanic units, suggesting a comagmatic source. Whole-rock geochemistry shows coherent trends in major and trace elements in mafic to intermediate compositions, but less coherent trends above 63 wt % SiO2. Units are predominantly low-K with flat rare earth element patterns, and show large ion lithophile element enrichment and high field strength element depletion. Initial Nd and Pb isotope ratios overlap for all units and imply no cratonic continental involvement. This agrees with low Sr/Y ratios of all rock types, indicative of thin, immature oceanic arc crust. Modeling results show that closed-system fractional crystallization drove crustal differentiation from mafic to intermediate compositions, but open-system processes likely occurred to produce some of the felsic compositions. Differentiation occurred in a two-step fractionation process. Step 1, from basaltic andesite to andesite, fractionated an anhydrous gabbroic cumulate (∼40% crystallization). Step 2, from andesite to rhyolite, fractionated a hydrous amphibole cumulate (∼65% crystallization, total), which is similar to what fluid dynamical models suggest for production of rhyolite (between 50–70% crystallization). Our results can be used as a reference model for differentiation processes relating to the growth of the middle and upper crust within active extensional arc systems. The Rosario segment plutonic rocks may be analogous to the low-velocity zone (Vp = 6·0–6·5 km s–1) imaged within the extensional Izu–Bonin arc. The chemistry of the plutonic and volcanic rocks is most similar to those of volcanic rocks in the Izu–Bonin active rift.

Geochemical and Stratigraphic Analysis of the Chisana Formation, Wrangellia Terrane, Eastern Alaska: Insights Into Early Cretaceous Magmatism and Tectonics Along the Northern Cordilleran Margin

“…Other trace elements also show subduction influence; for example, a plot of La versus Nb (Figure 8a) shows that Chisana samples are characterized by La enrichment and Nb depletion, consistent with melting of subduction‐affected mantle (Pearce & Peate, 1995). Chisana lavas studied here are also geochemically similar (e.g., light rare‐earth element [LREE]‐enriched, La/Yb = 2.6 to 12.1) to igneous rocks sourced primarily in the rear arc segment (also LREE‐enriched; La/Yb > 2.2) of the Izu‐Bonin arc (Figure 8b), an archetypal example of an extensional, intra‐oceanic magmatic arc (Ducea et al, 2015; Morris et al, 2019; Stern, 2010).…”

Across‐Arc Diversity in Rhyolites From an Intra‐oceanic Arc: Evidence From IODP Site U1437, Izu‐Bonin Rear Arc, and Surrounding Area

“…The fractionation sequence used was based on observed minerals within Site U1437 tephras, as well as observed minerals in the Cretaceous Alisitos Arc (Baja California/Mexico) (Morris et al, ). The Alisitos Arc is an accreted midcrustal to upper‐crustal section from an extensional intra‐oceanic arc that has been cited as an analog to the Izu‐Bonin arc (Busby et al, ; Medynski et al, ; Morris et al, ).…”

“…The fractionation sequence used was based on observed minerals within Site U1437 tephras, as well as observed minerals in the Cretaceous Alisitos Arc (Baja California/Mexico) (Morris et al, ). The Alisitos Arc is an accreted midcrustal to upper‐crustal section from an extensional intra‐oceanic arc that has been cited as an analog to the Izu‐Bonin arc (Busby et al, ; Medynski et al, ; Morris et al, ). Mineral compositions are from mineral populations observed within the Site U1437 tephra suite as well as one amphibole composition from the Jurassic Talkeetna Arc (Johnsen, ) which we used to represent a felsic amphibole (Mg# 49) typical of hydrous fractionation in a middle‐to‐upper crustal arc pluton.…”

“…This has been recently highlighted in both modern arcs (e.g., Barker et al, 2012;Bryant et al, 2003;Machida & Ishii, 2003;Wade et al, 2005) and ancient accreted analogs (cf. Greene et al, 2006;Jagoutz, 2010;Morris et al, 2019).…”

“…An important result from this major and trace element modeling is the fractionation of amphibole. Amphibole is an important (although sometimes cryptic) crystallizing phase in arc magmatism and in generating a continental crust-like geochemical signature (e.g., Ridolfi et al, 2010), but there are different scenarios for how, where, and when amphibole crystallization occurs: early in the differentiation process (Davidson et al, 2007;Greene et al, 2006;Jagoutz, 2010) or as a late-stage crystallizing phase (Morris et al, 2019), in lower crustal hot zones (Annen et al, 2005;Davidson et al, 2007), or in midcrustal magma chambers (Barker et al, 2012;Morris et al, 2019).…”