Cordillera Zealandia: A Mesozoic arc flare-up on the palaeo-Pacific Gondwana Margin

Two geochemically and temporally distinct components of the Mesozoic Zealandia Cordilleran arc indicate a shift from low to high Sr/Y whole rock ratios at c. 130 Ma. Recent mapping and a reappraisal of published Sr-Nd data combined with new in-situ zircon Hf isotope analyses supports a genetic relationship between the two arc components. A reappraisal of geophysical, geochemical and P-T estimates demonstrates a doubling in thickness of the arc to at least 80 km at c. 130 Ma. Contemporaneously, magmatic addition rates shifted from ~14 km3/my per km of arc to a flare-up involving ~100 km3/my per km of arc. Excursions in Sr-Nd-Hf isotopic ratios of flare-up rocks highlight the importance of crust-dominated sources. This pattern mimics Cordilleran arcs of the Americas and highlights the importance of processes occurring in the upper continental plates of subduction systems that are incompletely reconciled with secular models for continental crustal growth.

. This study includes Cretaceous ARC samples that have not been formally assigned to a suite however are considered a correlative of the Darren Suite based on age and geochemistry 25,53,54 . A further sample defined as Darren Suite is also included. Modified from Turnbull et al. 20

Crustal Profile
Crustal profiles of the magmatic arc before and after c. 130 Ma (Fig. 3) were reconstructed by incorporating published seismic velocity and gravity geophysical data, Ce/Y ratios for mafic rocks 40 , Sr/Y ratios for intermediate rocks [41][42][43] and maximum pressures recorded throughout the arc. Sr/Y data were filtered by MgO (1-6 wt%) and SiO 2 (55-70 wt%) content, and then Sr/Y outliers were removed via the modified Thompson tau statistical method, similar to the methods described by Chapman et al. 41 21 , gabbroic rocks of the Darran Suite have maximum Ce/Y ratios of 2.5, equating to a crustal thickness in excess of 35 km 40 . The Darran Suite whole rock data also has median Sr/Y ratios of 28, equating to a crustal thickness of ~40 km 41,42 . Seismic velocity data delineates the Moho at approximately 60 km depth beneath the whole of Fiordland 45 . However, it is unclear to what degree subsequent arc magmatism and the recent transpressional plate tectonic setting 46 and subduction initiation modified the pre-c. 130 Ma crustal profile. Geochemical data are consistent with arc material older than c. 130 Ma having been emplaced in an arc approximately 40 km thick (Fig. 3).

Flare-up (Post-c. 130 Ma: Separation Point Suite and Western Fiordland Orthogneiss).
The maximum-recorded emplacement depth of the Western Fiordland Orthogneiss is 18 kbar 47 , indicating a crustal thickness in excess of 60 km at c. 130 Ma, confirming early inferences of crustal thickness up to 70 km 48 . Utilizing the whole rock geochemical data of Allibone et al. 22 , gabbroic rocks from these plutons have maximum Ce/Y ratios >5, extending the crustal thickness estimates for Fiordland of Mantle and Collins 40 to greater than 60 km. Median Sr/Y ratios of 68 for the Western Fiordland Orthogneiss whole rock data set of Allibone et al. 22 estimate a thickness in excess of ~70 km 41,42 . The Separation Point Suite's median Sr/Y ratios of 108 fall outside the correlations established by Profeta et al. 42 and Chapman et al. 41 and may reflect higher degrees of differentiation 43 . Seismic velocity data extends the current surface geology to at least 10 km depth 45 . Higher velocity material, inferred by Eberhart-Phillips and Reyners 45 to be dense residues/cumulates left behind in the source region of the Western Fiordland Orthogneiss, extends to more than 40 km depth where it is juxtaposed against the newly subducting Australian plate. These observations suggest that the crust may have been more than 80 km thick after c. 130 Ma (Fig. 3). Ducea 13 and Ducea and Barton 6 posit the residue/melt ratio in a batholith as 1/1 to 3/1. This implies that the 30 km of residues/cumulates imaged by Eberhart-Phillips and Reyners 45 beneath the Western Fiordland Orthogneiss is smaller than expected for the predicted 80 km thick arc. The missing residue/cumulates may have either delaminated after flare-up or been tectonically removed during the recent transpressional plate tectonic setting and subduction initiation. The preservation of residue/cumulate materials [49][50][51] is unusual as they commonly founder due to having densities greater than the mantle. The root of residue/cumulates may have been preserved by a buoyant subducting slab, and partially removed by recent subduction similar to the Laramides 13, 16 .  The Darran Suite magmatic flux rate is 14 km 3 /my per km of arc. As the Separation Point Suite and Western Orthogneiss were coeval, their combined magmatic flux rate is greater than 100 km 3 /my per km of arc and possibly up to 150 km 3 /my per km of arc depending upon the thickness of the arc at the time. Individual plutons within these suites were assigned ages based on available geochronology or field relationships to create a histogram of age versus exposed area (Fig. 4).
Zircon grains from five rock samples of Darran Suite and their corellatives 25, 53, 54 (Fig. 1), were previously dated via SHRIMP by Hollis et al. 25 and selected for Lu-Hf analyses via MC-LA-ICP-MS at Macquarie University, Sydney, Australia. See Supplementary Information for raw data and Milan et al. 28 , for methods and operating conditions for Lu-Hf data collection.
The initial Hf isotopic ratio (Hf i ) of the Darran Suite samples is consistent with recycling of a common ancient source with a c. 500 Ma average model age for all samples (this study, Scott et al. 29 ) during a 40 my period of low magmatic flux. The Hf i of the Separation Point Suite 24 , and Western Fiordland Orthogneiss 28 shows an excursion to less radiogenic values over time, during a brief pulse of high-flux magmatism. Sr i and εNd data (Fig. 3 inset) from previous studies 27,32,33 show an identical excursion towards an evolved component in the source region.  in Sr-Nd-Hf isotopic ratios and the modification of pre-existing arc crust through melt-rock interaction 54,57,58 . Flare-ups and isotopic excursions have been explained in the Americas via the underthrusting of a melt-fertile, lower crustal foreland into the base of the arc 5, 6 . In Fiordland, zircon inheritance age spectra in the Western Fiordland Orthogneiss are dominated by c. 2480, 770 and 555 Ma peaks which are consistent with the underthrusting or burial, and partial melting of an amalgam of foreland Gondwana margin crust 28 . The excursion to more evolved isotopic signatures, coupled with the high-flux event (Figs 3 and 4) attests to the importance of this crustal contribution to the magmatic flux.

Arc Migration Over Time
Exhumed examples of long-lived Cordilleran arcs reveal they are constructed from a series of vertically aligned, trench-parallel igneous belts of varying age 7 . This wide footprint is produced by the migration of the active magmatic front inboard or outboard relative to a fixed position on the upper plate 7 . Inboard migrations are well documented in the American Cordilleras 7, 59, 60 , but outboard migrations in arcs also occur 7 .
For the Mesozoic Cordilleran Zealandia arc, the inboard migration of the active magmatic front (westward) during the flare up (Fig. 1) has been attributed to flat slab subduction 36 . The location of pre c. 130 Ma Darran Suite has led to competing tectonic interpretations. The most recent model (Scott et al. 29 ) calling for an allochthonous history of the Darran Suite and excision of a back-arc basin is based upon: (i) contrasting isotopic signatures either side of c. 130 Ma; (ii) the restriction of Gondwana-derived metasedimentary rocks to the west; (iii) a proposed crustal suture; and (iv) the presence of A-type granites. Here we argue against each of these points in turn and reaffirm an autochthonous relationship between the Darran Suite and western parts of the Mesozoic arc.
The apparent contrast in isotopic signatures are resolved in this study by new data defining a smooth excursion towards an evolved character, reflecting the underthrusting or burial of a Gondwanan lower crustal component, as required by the scale and geochemistry of the flare-up event. Though exposures of metasedimentary units in the proposed 'allochthonous terrane' lack Gondwana-derived detrital zircon, the very restricted nature of these units suggests that they represent small intra-arc basins formed during oblique subduction. In addition, significant Gondwana inheritance has been observed in Mesozoic plutons of the proposed 'allochthonous terrane' (e.g. Pomona Island and Clark Hut granites 21,61 ). The proposed crustal suture is unusually narrow (300 m wide), and lacks ophiolitic material that might support the argument for basin closure. The same structure has been interpreted (along strike) to reflect transpression within a Cordilleran setting 34 . Furthermore, intrusive relationships between the Western Fiordland Orthogneiss and parts of the Darran Suite are preserved in northern Fiordland 48,62 .
Though Scott et al. 29 ascribe A-type granites in the Darran Suite to a rift setting, these are restricted to just ~42 km 2 and a number of tectonic settings have been proposed for A-type granites elsewhere [63][64][65][66] . Furthermore, these rocks have similar Sr and Nd isotope ratios 32 to the entire Darran Suite and intruded at a time of thickened crust inconsistent with a rift setting.

Generation of Crust in Arcs
The flux from mantle to crust is basaltic (e.g. Davidson and Arculus 67 ) in contrast to the andesitic composition of average continental crust 1,68,69 . The Fiordland flare-up event represented by the Separation Point Suite and Western Fiordland Orthogneiss equates to two thirds of the total volume of newly created arc crust throughout the Mesozoic in 'Cordillera Zealandia' and highlights the significance of short-lived high-flux episodes in Cordilleran arcs. Long-lived Cordilleran arcs of the Americas record repetitive flare-up contributions as high as 85-90% of the total volume of the arc 5-7, 12, 70-72 . These high-flux episodes require a significant component of the arc magma to be derived from melt-fertile lower crust. These observations diminish the contribution of mantle-derived melts in Cordilleran arcs and in addition to the foundering of dense residues/cumulates from the roots of arcs [13][14][15][16] , contribute to resolving why the average continental crust is andesitic in composition.