Document Type

Article

Publication Date

5-22-2019

Keywords

Extensional oceanic arc, Plutonic-volcanic connection, Closed-system fractional crystallization, Arc magmatism, Izu-Bonin analog

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.

Publication Title

Journal of Petrology

Volume

60

Issue

6

First Page

1195

Last Page

1228

DOI

10.1093/petrology/egz029

Required Publisher's Statement

Copyright: The Author(s) 2019. Published by Oxford University Press

Link to published version:

https://academic.oup.com/petrology/article/60/6/1195/5494693

Subjects - Topical (LCSH)

Geology, Stratigraphic--Cretaceous; Island arcs--California, Southern; Continental crust--California, Southern; Sedimentary basins--California, Southern

Geographic Coverage

Baja California (Mexico : Peninsula)

egz029_supplementary_data.zip (5133 kB)
Supplementary data - zip file

Genre/Form

articles

Type

Text

Rights

Copying of this document in whole or in part is allowable only for scholarly purposes. It is understood, however, that any copying or publication of this document for commercial purposes, or for financial gain, shall not be allowed without the author’s written permission.

Rights Statement

http://rightsstatements.org/vocab/InC/1.0/

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

Language

English

Format

application/pdf

Included in

Geology Commons

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