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

Abstract

Ever since the late 1960s when Warren Hamilton proposed that the great Cordilleran batholiths of the western Americas are the roots of volcanic arcs like the Andes and were generated by longstanding eastward subduction, most geologists have followed suit, despite the evergrowing recognition that many Cordilleran batholiths are complex, composite bodies that developed with intervals of intense shortening and exhumation between and during periods of magmatism. The Peninsular Ranges batholith of Southern and Baja California provides a superb place to unravel the complexities because there is a lot of data and because it is longitudinally composed of two parts: an older western portion of weakly to moderately deformed, low-grade volcanic and epizonal plutonic rocks ranging in age from ~128–100 Ma; and a more easterly sector of deformed amphibolite grade rocks cut by compositionally zoned, mesozonal plutonic complexes of the La Posta suite, emplaced from 99–86 Ma. While plutons of the La Posta suite are generally considered to be the product of continued eastward subduction, they are enigmatic, because they and their wall rocks were rapidly exhumed from as deep as 23 km and eroded during, and just after, their emplacement, unlike plutons in magmatic arcs, which are generally emplaced in zones of subsidence. Here we resolve the enigma with a model where westward-dipping subduction led to arc magmatism of the western sector, the Santiago Peak–Alisitos composite arc, during the period ~128–100 Ma. Arc magmatism shut down when the arc collided with a west-facing Early Cretaceous passive margin at about 100 Ma. During the collision the buoyancy contrast between the continental crust of the eastern block and its attached oceanic lithosphere led to failure of the subducting slab. The break-off allowed subjacent asthenosphere to upwell, adiabatically melt, and rise into the upper plate to create the large zoned tonalite–granodiorite–granite complexes of the La Posta suite. While compositionally similar to arc plutons in many respects, the examples from the Southern California and Baja segments of the batholith have geochemistry that indicates they were derived from partial melting of asthenosphere at deeper levels in the mantle than typical arc magmas, and within the garnet stability field. This is consistent with asthenosphere upwelling through the torn lower-plate slab. We identify kindred rocks with similar geological relations in other Cordilleran batholiths of the Americas, such as the Sierra Nevada, which lead us to suggest that slab failure magmatism is common, both spatially and temporally.SOMMAIREDepuis la fin des années 1960, Warren Hamilton a proposé que les grands batholites de la Cordillère de l'ouest des Amériques sont les racines d’arcs volcaniques andéens issus de la subduction vers l'est de longue durée, et depuis la plupart des géologues ont emboîté le pas, bien qu’un nombre croissant d’indications montrent que de nombreux batholites de la Cordillère sont des entités composites complexes qui se sont développés lors d’intervalles intenses de contraction et d’exhumation, durant et entre les périodes de magmatisme. Le batholite Peninsular Ranges du Sud de la Californie et de Baja California est un excellent endroit permettant de démêler les choses parce qu'il y a beaucoup de données et parce qu'il est composé longitudinalement de deux parties: une partie occidentale plus ancienne, faiblement à modérément déformée, de roches volcaniques de faible métamorphisme et de roches plutoniques épizonales âgées d’environ 128 Ma à 100 Ma; et, d’un segment plus à l'est de roches amphiboliques déformées recoupées par des roches de composition zonée des complexes mésozonaux plutoniques de la suite de la Posta, mises en place entre 99 Ma et 86 Ma. Bien que les plutons de la suite La Posta sont généralement considérés comme le produit d’une subduction soutenue vers l’est, ils posent problème, parce qu'avec leurs roches encaissantes, ils ont été rapidement exhumés de profondeurs aussi grandes que 23 km, et érodées durant et juste après leur mise en place, contrairement aux plutons des arcs magmatiques, qui sont généralement mis en place dans les zones de subsidence. Dans le présent article, nous proposons une solution à ce problème, avec un modèle de subduction vers l'ouest qui conduit à un magmatisme d'arc du secteur ouest, l'arc composite de Santiago Peak-Alisitos, durant la période d’environ 128 Ma à 100 Ma. Le magmatisme d’arc s’est arrêté lorsque l'arc est entré en collision avec une marge passive à pendage ouest du début du Crétacé, il y a environ 100 Ma. Lors de la collision, le contraste de flottabilité entre la croûte continentale du bloc de est et la lithosphère océanique qui y est rattachée a conduit à l'avortement de la plaque plongeante. La cassure a entrainé la remontée de l’asthénosphère sous-jacente, sa fusion adiabatique, et sa remontée dans la plaque supérieure pour former les grands complexes zonés de tonalite-granodiorite-granite de La Posta. Bien que de composition similaire aux plutons d'arc à bien des égards, les exemples des segments de batholites de Californie du Sud et de Baja ont une géochimie qui indique qu'ils proviennent de la fusion partielle de l’asthénosphère à des niveaux plus profonds dans le manteau que les magmas d'arc typiques, à l’intérieur du domaine de stabilité du grenat. Ce qui correspond à une remontée d’asthénosphère à travers une dalle de plaque inférieure cassée. Nous connaissons des roches semblables avec les relations géologiques similaires dans d'autres batholites de la Cordillère des Amériques, tel celles de la Sierra Nevada, ce qui nous amène à penser que le magmatisme de cassure de plaque est commun, tant spatialement et temporellement.

Source-inherited compositional diversity in granite batholiths: The geochemical message of Late Paleozoic intrusive magmatism in central Calabria (southern Italy)

“…340-330 Ma, (e.g., Casini et al, 2015;von Raumer et al, 2013), i.e., 30-40 Ma before the onset of intrusive magmatism in Calabria. A time span of only a few Ma is instead indicated between slab failure and related magmatism (e.g., Davies and von Blanckenburg, 1995;Hildebrand and Whalen, 2014;Sui et al, 2013;van de Zedde and Wortel, 2001). According to Hildebrand and Whalen (2014), it is exactly the timing of magmatism during and immediately after collision, associated with nearly simultaneous strong exhumation, that provides the most convincing evidence of a slab failure origin of post-collisional magmas.…”

“…A time span of only a few Ma is instead indicated between slab failure and related magmatism (e.g., Davies and von Blanckenburg, 1995;Hildebrand and Whalen, 2014;Sui et al, 2013;van de Zedde and Wortel, 2001). According to Hildebrand and Whalen (2014), it is exactly the timing of magmatism during and immediately after collision, associated with nearly simultaneous strong exhumation, that provides the most convincing evidence of a slab failure origin of post-collisional magmas. This evidence is lacking in central Calabria, where the magmatic bodies were emplaced long after slab break off and resided near their emplacement depths over further Sun (1995) for primitive mantle and from Sun and McDonough (1989) for chondrite.…”

“…4. tens of millions years. Besides, the Serre granitoids have ambiguous features in terms of possible geochemical parameters (Online Table S1) able to typify slab failure-related granitoid magmas, such as Sr N 400 ppm, Sr/Y N 10, Nb/Y N 0.4, La/Yb N 10, Gd/Yb N 2 (Hildebrand and Whalen, 2014). Actually, the "Cordilleran" signature of post-collisional granitoids may also simply reflect inheritance derived from partial melting of magnesian magma sources (e.g., Frost and Frost, 2008;Kroner and Romer, 2013).…”

Geochronology and lithogeochemistry of granitoid rocks from the central part of the Central plutonic belt, New Brunswick, Canada: implications for Sn-W-Mo exploration

“…10). On this diagram, empirically-derived compositional trends (Whalen and Frost 2013;Hildebrand and Whalen 2014) again illustrate a dominant calc-alkalic character. Trace-element variation diagrams highlight some of the similarities and differences among the various plutonic rock units.…”

“…This is consistent with Nd isotope data from late Silurian-Early Devonian basalts in northern New Brunswick, where the range in ε Nd (T) values and depleted mantle ages also argue against the involvement of juvenile asthenospheric mantle in the genesis of the volcanic rocks (Dostal et al in press). Furthermore, the chemistry of granites from the Central plutonic belt does not match that of slab breakoff-related granitoid suites documented by Hildebrand and Whalen (2014) (Fig. 17).…”