Test of a Spreading-Ridge Subduction Model for the Origin of Granitoid

INTERNATIONAL GEOLOGY REVIEW, 2016 http://dx.doi.org/10.1080/00206814.2016.1185651

Test of a spreading-ridge subduction model for the origin of granitoid plutons and metavolcanic complexes in the Palaeoproterozoic Pinal forearc/subduction complex of Southern Arizona, USA Arend Meijer Department of Geosciences, University of Arizona, Tucson, AZ, USA

ABSTRACT ARTICLE HISTORY The Pinal terrane of southern Arizona has been characterized as a Palaeoproterozoic forearc/subduction Received 21 January 2016 complex (Meijer 2014). It contains granitoid plutons and metavolcanic rocks with arc affinities that have Accepted 30 April 2016 a spatial distribution reminiscent of plutons and volcanic rocks that intruded into and erupted onto KEYWORDS – the early Cenozoic forearc/subduction complex of south-central Alaska (i.e. Chugach Prince William Spreading-ridge subduction; terrane). The magmatic rocks that intruded into and erupted onto the Alaskan forearc/subduction forearc magmatism; complex are believed to have been produced largely as a result of a spreading-ridge subduction event Palaeoproterozoic; Pinal (SRSE) during the early Cenozoic (Bradley et al. 2003). A characteristic of these rocks is that they were terrane emplaced in a relatively short interval of time (10–30 million years) between the arc and associated trench. In the Pinal forearc/subduction complex of southern Arizona, there are numerous plutons and several metavolcanic complexes with arc affinities scattered over a distance of approximately 300 km perpendicular to the NE–SW Palaeoproterozoic orogenic trend. If these magmatic rocks are the result of an SRSE, they should have similar emplacement ages. New zircon age data for 16 samples from metavolcanic units and granitoid plutons in the Pinal forearc/subduction complex collected over a distance of 250 km perpendicular to the orogenic trend show a narrow range of emplacement ages that average 1650 ± 20 million years. The restricted age range and the spatial distribution of the plutons and metavolcanic rocks within this forearc/subduction complex are consistent with them having originated as a result of a Palaeoproterozoic SRSE.

1. Introduction operated continuously since then, there could be as many as 357 SRSEs in the pre-Mesozoic geologic record. If plate Although the possibility of spreading-ridge subduction tectonics started around 1.0 Ga, as others believe (e.g. Stern events (SRSEs) has long been recognized (e.g. Pitman and 2008), there could be at least 114 SRSEs. According to Hayes 1968;GrowandAtwater1970;UyedaandMiyashiro Abbot and Menke (1990), there was a greater length of 1974; DeLong and Fox 1977), very few of these events have ridge systems and a greater number of plates in the Downloaded by [Arend Meijer] at 10:26 01 June 2016 been documented in geologic records (Brown 1998; Sisson Archaean. This would imply that there might have been et al. 2003). The term spreading-ridge subduction event is an even greater number of SRSEs. used loosely here to refer to any spreading ridge–trench To date, only four SRSEs have been reasonably well interaction while acknowledging that there is a large documented (inferred?) in pre-Mesozoic rocks: one in the range of possible interactions involved in these features late Palaeozoic Taconic arc terrane of northern Maine, USA (e.g. Burkett and Billen 2009). There have been at least 11 (Schoonmaker and Kidd 2006), one in the mid-Palaeozoic relatively well-documented SRSEs in the last 80 million Central Asian Orogenic Belt of China (Tang et al. 2010), one years (Table 1). These 11 SRSEs suggest there is, on average, in the Neoproterozoic Damara belt of Namibia (Meneghini at least one SRSE at least every 7 million years or so, et al. 2014), and one in the Palaeoproterozoic Pinal terrane although this is probably a conservative estimate. For of southern Arizona, USA (Meijer 2014; this study). Other example, in the case of the southern Antarctic Peninsula, SRSEs have been postulated in pre-Phanerozoic rocks, but it has been suggested there might have been two SRSEs in critical details remain unresolved (e.g. Kusky 1990;Brown the last 80 million years (McCarron and Larter 1998). If, as 1998;Sissonet al. 2003; Manikyamba et al. 2007;Zhang some believe (e.g. Brown 2007; Condie and Kroner 2008), et al. 2015). The reasons for the lack of recognition of plate tectonics started at approximately 2.7 Ga and has SRSEs in the geologic record are likely complex and may

CONTACT Arend Meijer [email protected] Department of Geological Sciences, University of Arizona, 1040 E. 4th Street, Tucson, AZ 85721, USA Supplemental data for this article can be accessed here. © 2016 Informa UK Limited, trading as Taylor & Francis Group 2 A. MEIJER

Table 1. Spreading-ridge subduction events (late Cretaceous to recent). Location Age References Southern Chile Recent Forsythe et al.(1986); Breitsprecher and Thorkelson (2009) Solomon Islands Recent Luyendyk et al.(1973); Chadwick et al.(2009) Western North America Mid-Cenozoic–recent Dickinson and Snyder (1979); Johnson and O’Neil (1984);Wilson et al.(2005) Panama Plio-Pleistocene Johnston and Thorkelson (1997) South Scotia Ridge Mid-Miocene Barker et al.(1984) Japan (Shikoku Islands) Mid-Cenozoic Hibbard and Karig (1990); Kimura et al.(2005) Alaska (south-central) Early Cenozoic Marshak and Karig (1977); Sisson et al.(2003); Bradley et al.(2003) British Columbia Early Cenozoic Breitsprecher et al.(2003); Madsen et al.(2006) Antarctic Peninsula Early-Late Cenozoic Barker (1982); Breitsprecher and Thorkelson (2009) Northeastern Japan Late Cretacious–early Cenozoic Uyeda and Miyashiro (1974); Brown (1998); Raimbourg et al.(2014) Sunda–Java Late Cretaceous–early Cenozoic Whittaker et al.(2007)

include (1) the lack of accessible oceanic lithosphere (i.e. largely by the melting of sediments in the accretionary magnetic anomalies) in pre-Mesozoic terranes, (2) limita- prism. This melting concept was subsequently rein- tions to our current understanding of the effects of SRSEs forced with some modification by trace element and on forearc regions (see below), (3) possibly the low prob- isotopic data presented by Hudson et al.(1979), Barker abilities of preservation of forearc/subduction complexes, et al.(1992), and Ayuso et al.(2009). Marshak and Karig (4) a wide (i.e. non-unique) range of petrologic types (1977) speculated that this melting process was a result observed among magmatic rocks associated with SRSEs of heat added to the accretionary prism by the subduc- (e.g. Hole et al. 1991; Cole and Basu 1995;Guivelet al. tion of the Kula Ridge beneath the south-central Alaska 1999; Pallares et al. 2007), and (5) overprinting of the margin. DeLong et al.(1979) presented a thermal model effects of SRSEs by post-ridge subduction tectonism (e.g. for this process and discussed the implications of the terrane collisions). model for forearc magma genesis and the metamorph- DeLong and Fox (1977) proposed a model for the ism of forearc accretionary prisms. They suggested geological consequences of SRSEs, which included: (1) bimodal magmas could be produced in the forearc uplift and subsequent subsidence of the forearc region region by the melting of subducted lithosphere and and arc complex, (2) a lull in magmatism in the volcanic by the melting of sediments in the forearc prism, arc during ridge subduction, and (3) moderate T/P meta- respectively. Since 1979, forearc silicic plutons have morphism of the forearc region. This model was based been described in association with the Cenozoic to largely on data from the central Aleutian arc and from the recent SRSEs in southern Chile (Kaeding et al. 1990; southern Chile SRSE. In the central Aleutian example, it Guivel et al. 1999; Anma et al. 2009), the southern was believed the Kula Ridge had been subducted Antarctic Peninsula (McCarron and Millar 1997), south- beneath the Aleutian arc during the early Cenozoic. western Japan (Ōba 1977; Kimura et al. 2005), and However, data obtained on marine magnetic anomalies British Columbia (Madsen et al. 2006). The distribution in the oceanic plate directly south of the Aleutian arc led of plutons in these forearcs is quite variable. In south-

Downloaded by [Arend Meijer] at 10:26 01 June 2016 Lonsdale (1988) to the conclusion that the Kula Ridge central Alaska, the plutons are located at various died well before it reached the central Aleutian Trench. distances from the trench and they form a diachronous Thus, subduction of the Kula Ridge might not have been age progression younging to the east (Bradley et al. the cause of the geological effects noted by DeLong et al. 2003). This progression is considered to be due to the (1978) in the central Aleutian arc. Nonetheless, the model progressive subduction of the Kula spreading-ridge at by DeLong and Fox (1977) remains consistent with an oblique angle to the margin from the west to the observations in other Cenozoic to recent SRSEs, including east (in modern-day coordinates). However, this is only southern Chile (Breitsprecher and Thorkelson 2009), the one of a large number of possible combinations of southwestern Antarctic Peninsula (Hole and Larter 1993; subduction geometries and forearc types. For most of Maldonado et al. 1994), and southwestern Japan (Kimura the SRSEs listed above, there are insufficient data avail- et al. 2005). able to deduce ridge subduction geometry from pluton Two important geologic characteristics of SRSEs not distributions. Dickinson and Snyder (1979) proposed included in the original DeLong and Fox (1977) model that a slab window formed along the coast of the are magmatism in forearc regions and the formation of western California and northwestern Mexico as a result slab windows in the subducted oceanic lithosphere of the Neogene SRSE along the coast of California (Dickinson and Snyder 1979). Hudson et al.(1977) although they did not associate slab windows with proposed that silicic plutons in the forearc region of forearc magmatism. However, slab window and/or south-central Alaska (Chugach Terrane) were produced slab-tear magmatism now play(s) a prominent part in INTERNATIONAL GEOLOGY REVIEW 3

ridge subduction models (e.g. Thorkelson and Taylor southwesternLaurentianmargintotestthehypothesis 1989; Pallares et al. 2007). In fact, Madsen et al.(2006) that they were produced by an SRSE. The degree of used the distribution of forearc magmatism to infer exhumation of the Pinal terrane is particularly favourable the location of slab windows beneath British Columbia for this purpose. during early Cenozoic time. DeLong and Fox (1977) noted that ridge subduction would likely increase the 2. Geological setting metamorphic grade of rocks in the forearc region and Brown (1998) has discussed the potential impact of The Palaeoproterozoic Pinal terrane consists of exten- SRSEs on metamorphism in forearc regions. sive sequences of schist, metagraywacke, and phyllite, The Palaeoproterozoic Pinal terrane of southern Arizona considered meta-turbidites (Condie and DeMalas 1985; has been characterized as a relatively well-preserved Copeland and Condie 1986; Swift and Force 2001). The forearc/subduction complex with a wide accretionary schists and metagraywackes are locally interbedded prism (Meijer 2014). Other models that have been proposed with lesser quartzite, minor meta-conglomerate, and for this terrane or parts of it (Condie and DeMalas 1985; rare limestone. The terrane also contains tectonic Condie et al. 1985;Condie1986, 1992;Copelandand mélange (Swift 1987; Swift and Force 2001), pods/slivers Condie 1986;Anderson1989;Keep1996)arelessconsistent of oceanic metavolcanic rocks overlain by metachert, with currently available data (see discussion in Meijer 2014 minor ultramafics, and local accumulations of metavol- and discussion below). The Pinal forearc/subduction canic rocks with arc-like trace element concentrations complex contains numerous granitoid plutons and several (Meijer 2014). Granitoid plutons were intruded over volcanic complexes that appear to have resulted from an most of the width of the Pinal forearc (Figure 1). These SRSE. This article presents locations and new zircon ages of plutons are mostly I-type granodiorites, tonalities, and Palaeoproterozoic magmatic rocks emplaced across the diorites. The metamorphic grade of rocks in the Pinal Downloaded by [Arend Meijer] at 10:26 01 June 2016

Figure 1. Map showing the locations of Palaeoproterozoic rocks in central and southern Arizona and locations of samples analysed in this study. Palaeoproterozoic arc volcanic-plutonic belts are outlined to the north of the Pinal terrane. Sampling locations for new zircon age dating samples (solid stars) include: Ajo (Aj), Bisbee (Mu), Casa Grande Mountains (Cg), Empire Mountains (Em), James Wash (Jw), Javelina Mountain (Ja), Johnny Lyon Hills (Jl), Nogales (No), Ray (Ra), Santa Rita Mountains (Sr), Shaw Butte (Sb), Thunderbird Park (Th), Tortolita Mountains (Tm), Union Hills (Uh), and Zapata Wash (Zw). Other locations referenced in the text are Buckeye Hills (BH), Dos Cabezas Mountains (DC), Four Peaks (FP), Hess Canyon (HC), Peñaleno Mountains (PM), and Webb Mountain (WM). Sampling locations for ‘older data’ (shown as open stars) are approximate. 4 A. MEIJER

terrane ranges from greenschist facies to amphibolite For a few samples, fewer zircons were available and, thus, facies, although there are no clear regional patterns in fewer were analysed. The zircons were mostly euhedral, metamorphic gradients (Meijer 2014). High P/T rocks although the gneissic samples contained small fractions (blueschist facies) are unknown within the terrane. of rounded grains and/or grains with rounded cores. The Pinal terrane is located directly south of the Scanning electron microscopy (SEM) images indicate the Palaeoproterozoic Yavapai and Mazatzal arc volcanic grains were generally zoned, but not many grains showed belts in central Arizona. Each of these volcanic belts evidence of rounded cores. Some grains contained small include extrusive and intrusive arc rocks (Figure 1) that inclusions and/or fractures/cavities. The age data reported range in age from approximately 1.75 to 1.68 Ga in Table 3 are mostly for the outer portions of the zircon (Karlstrom and Bowring 1991). Various interpretations grains (not outermost rims or cores) although some core of the genetic relationships between these volcanic analyses are included in the results. Analytical errors were belts have been presented (e.g. Karlstrom et al. 1978; calculated based on counting statistics and systematic Silver et al. 1986; Anderson 1989; Conway and Silver errors associated with the LA-ICP-MS technique (see 1989). However, resolution of this issue is not critical Gehrels and Pecha 2014 for details). Two error estimates to an analysis of the origin of magmatic rocks in the are provided for each sample. The smaller number Pinal terrane. primarily reflects the measurement error, whereas the larger number reflects the measurement error in addition to systematic contributions to the total error estimate, 3. Sampling locations, sample descriptions, including uncertainties in decay constants, common lead and analytical procedures corrections, and ages for standards (Ludwig 2008). Sample locations are shown in Figure 1 and coordinates and sample descriptions are provided in Table 2. All the 4. Results samples were taken from units previously identified on quadrangle maps by the US Geological Survey or the The new zircon dating results are presented in Table 3. Arizona Geological Survey (Table 2). Most samples are of Detailed analytical results are provided in Supplementary plutonic rock with only two (meta)volcanic samples. Table 1: U–Pb zircon ages. The data for samples from the The procedures for the extraction of zircons followed arc volcanic belts (SB-1Z, TH-1Z, UH-4Z, UH-5Z) suggest standard techniques (i.e. crushing, milling, shaker table these analyses sampled nearly normal distributions with water flow, magnetic separation, and heavy liquid (e.g. on a plot of ‘best ages’; see Supplementary Table 1: separations). Techniques for the processing and mounting U–Pb zircon ages) of co-magmatic zircon ages. The of zircon grains and laser ICP-MS analysis closely followed Gaussian statistics used in Isoplot (Ludwig 2008)are those reported by Gehrels and Pecha (2014). For most probably appropriate for these samples. Samples from samples, 50 zircons were mounted in epoxy at the base of the Pinal forearc terrane generally show more complex a 2.5 cm cylinder (puck) and 35 of these zircons were distributions and it is likely these distributions reflect analysed (see Supplementary data for analytical results). zircons with disparate histories. As shown in Downloaded by [Arend Meijer] at 10:26 01 June 2016

Table 2. Rock type, sampling locations, and geologic map references. Location Geologic Map Sample # Rock Type Symbola Latitude (N)b Longitude (W)b Referencec A-6Z Qtz diorite gneiss Aj 32.359759 −112.931891 Gilluly (1946) AC-22Z Meta-volc breccia Zw 32.807029 −110.579559 Krieger (1968a) AC-23Z Granodiorite Zw 32.838325 −110.579559 Krieger (1968a) B-5Z Quartz diorite Mu 31.523197 −110.004614 Gilluly (1956) CM-19Z Banded gneiss Cg 32.798523 −111.684716 Bergquist and Blacet (1978) CM-20Z Granite Cg 32.803081 −111.685520 EM-8Z Granodiorite Em 31.959915 −110.601383 Spencer et al.(2001) JLH-40Z Granodiorite Jl 32.195806 −110.222823 Cooper and Silver (1964) JM-33Z Diorite Ja 32.777412 −112.381593 Gray et al.(1988) LM-1Z Granodiorite Jw 32.755883 −110.726485 Krieger (1968b) N-13Z Diorite gneiss No 31.415861 −110.702192 Simons (1974) RA-2Z Diorite Ra 33.213858 −111.064766 Creasey et al.(1984) SR-25Z Granitic gneiss Sr 31.631445 −110.965524 Drewes (1976) TMN-20Z Granodiorite Tm 32.635986 −111.093155 Spencer et al.(2002) SB-1Z Granodiorite Sb 33.599826 −112.100130 Johnson et al.(2003) TH-1Z Diorite Th 33.688483 −112.100130 Leighty and Huckelberry (1998) UH-4Z Metarhyolite Uh 33.743814 −112.097927 Holloway and Leighty (1998) UH-5Z Diorite Uh 33.721904 −112.065955 aSee Figure 1. bIn decimal degrees. cIn reference list. INTERNATIONAL GEOLOGY REVIEW 5

Table 3. Zircon U–Pb age results for the analysed samples. Latitude Longitude nb Best Agec Sample # Locationa (degrees) (degrees) (Ma) (2σ) Errord A-6Z Ajo Mtns 32.359759 −112.931891 25/33 1666/1678 7/15 AC-22Z Zapata Wash 32.807029 −110.579559 31 1654 8/15 AC-23Z Zapata Wash 32.838325 −110.579559 29 1654 7/13 B-5Z Mule Mtns 31.523197 −110.004614 33 1653 7/15 CM-19Z Casa Grande Mtns 32.798523 −111.684716 9 1665 24/21 CM-20Z Casa Grande Mtns 32.803081 −111.685520 24/29 1651/1655 8/17 EM-8Z Empire Mts 31.959915 −110.601383 20/30 1646/1663 7/15 JLH-40Z Johnny Lyon Hills 32.195806 −110.222823 32 1649 9/16 JM-33Z Javelina Mtn 32.777412 −112.381593 30 1654 6/15 LM-1Z James Wash 32.755883 −110.726485 24 1652 8/16 N-13Z Nogales 31.415861 −110.702192 26 1649 9/18 RA-2Z Ray 33.213858 −111.064766 25 1656 9/17 SR-25Z Santa Rita Mtns 31.631445 −110.965524 19 1658 12/19 TMN-20Z Tortolita Mtns 32.635986 −111.093155 33 1651 7/15 SB-1Z Shaw Butte 33.599826 −112.100130 32 1733 6/15 TH-1Z Thunderbird Park 33.688483 −112.100130 35 1749 6/15 UH-4Z Union Hills 33.743814 −112.097927 35 1735 5/15 UH-5Z Union Hills 33.721904 −112.065955 35 1719 6/15 aSee Figure 1. bNumber of zircons included in age estimate. Where two numbers appear, the first number is associated with the lower age estimate and the second number is associated with the higher age estimate. cBased on comparison of 238U/206Pb, 235U/207Pb, and 207Pb/206Pb ages. All listed ages are 207Pb/206Pb ages. Where two ages appear, the first age represents the ‘best age’ of the truncated distribution (see text) and the second age is the Isoplot age of the ‘corrected’ distribution (Ludwig 2008). d The first number is a measurement (i.e. counting) error that reflects the ‘internal’ precision for samples on a given loading block. The second number is an estimate of the (‘external’) error associated with the Isoplot age. The latter includes the measurement error as well as contributions from systematic errors including uncertainties associated with the ages of standards, decay constants, and the common lead correction.

Supplementary Table 1: U–Pb zircon ages, there is that have ages similar to those observed in the ‘fat tails’ of evidence of xenocrystic zircons or zircon cores (i.e. zircons these samples. These zircons have ages that suggest they with ages that are outliers on the high side), zircons were derived mostly from the Yavapai and Mazatzal arc that experienced some Pb loss, metamorphic zircons (i.e. volcanic belts to the north (Karlstrom and Bowring 1991). high U concentrations, high U/Th ratios), and contaminant In two samples (JLH-40Z and SR-25Z), the majority of the zircons (i.e. younger outliers probably as a result of zircons analysed had experienced significant Pb loss and a contamination either in the field by elutriation of younger ‘best age’ was calculated for each of these samples based zircons into cracks and pores in older rocks or during on the upper concordia intercept. The complexity of the processing). Once these xenocrystic, metamorphic, ‘age’ distributions found in samples from the Pinal terrane Pb-loss, and contaminant zircons were removed from is not unexpected for plutons in forearc settings. Forearc the distributions, the remaining ‘corrected’ distributions plutons associated with younger SRSEs have been shown of more or less concordant zircons (see Supplementary to be derived from complex mixtures of mantle-derived Table 1: U–Pb zircon ages) were used to calculate a mean and sediment-derived components (e.g. Barker et al. 1992; Downloaded by [Arend Meijer] at 10:26 01 June 2016 ‘best age’ for each sample using Isoplot (Table 3). In Guivel et al. 1999;Ayusoet al. 2009). three samples (A-6Z, EM-8Z, and CM-20Z), the ‘corrected’ distributions remaining after the removal of xenocrystic, 5. Discussion metamorphic, Pb-loss, and/or contaminant zircons were clearly non-Gaussian (e.g. Figure 2(a)) although the zircons As noted above, the distribution of magmatism in a were mostly concordant on a concordia plot. In effect, the forearc/subduction complex is an important aspect of distributions for these samples have high-side ‘fat tails’ SRSEs (Brown 1998; Madsen et al. 2006). In general, this attached to relatively normal distributions of younger magmatism is distributed in the forearc over a range of ages. In Table 3, two ages are given for each of these distances from the trench. In Figure 3, the zircon ages samples. The older age reflects all the zircons in the of rock samples from the Pinal forearc/subduction ‘corrected’ distribution and the younger age was complex and the arc volcanic belts directly to the calculated based only on that portion of the ‘corrected’ north are plotted as a function of the distance of a distribution that approximated a normal distribution given plutonic or volcanic unit north-northwest of the (see Figure 2(a)). The ‘fat tails’ are believed to reflect town of Bisbee near the border with Mexico (Figure 1). xenocrystic zircons derived mainly from metasediments Distances to individual sampling localities were mea- in the Pinal terrane. Eisele and Isachsen (2001), Doe et al. sured towards the northwest perpendicular to a base- (2012), and Doe (2014) reported zircon age data for line through Bisbee with an azimuth of N72°E. This detrital zircons in schists and quartzites of the Pinal terrane baseline is approximately parallel to the dominant 6 A. MEIJER

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Figure 2. Plots of results for zircon grain analyses for two samples. In plot 2A, the error bars associated with each analysis reflect only the measurement error (2σ). They are plotted as a function of increasing age (‘best age’) simply to show the trend in the ages. The distribution of ages in plot 2A (sample A-6Z) is non-Gaussian with a ‘fat tail’ of older ages. The grains that make up the ‘fat tail’ are believed to have been inherited from either the source rock or the crustal materials through which the magma travelled on the way to its emplacement depth. Plot 2B shows an example (sample JLH-45Z) of the degree of U–Pb age discordance observed in some samples. INTERNATIONAL GEOLOGY REVIEW 7

450

400

350

300

250

200

New Data-plutonic 150 New Data-metavolc x New "corrected" Distance NW of Bisbee (km) 100 New "truncated" Old Data-plutonic 50 Old Data-metavolc

0 1600 1650 1700 1750 1800 Age (m.y.)

Figure 3. Age versus distance diagram for metavolcanic and granitoid rocks from the Pinal terrane and the arc terranes to the north. The distances from Bisbee to the northwest are calculated along lines through each sample collection point that are perpendicular to a baseline thatrunsthroughBisbeewithanazimuthofN72°E(paralleltothemainorogenic trend). A single error bar representing the average total error (2σ) associated with the ‘new’ analyses is shown. The errors associated with ‘old’ ages (all by TIMS) are generally smaller than the errors associated with the ‘new’ ages. Ages based on ‘truncated’ distributions (see text) are connected with brackets to the non-truncated ‘corrected’ ages. Data sources include: DeWitt et al.(2008), Doe (2014), Eisele and Isachsen (2001), Isachsen et al.(1999), Karlstrom and Bowring (1991), Labrenz and Karlstrom (1991), Mako et al.(2015), Powicki (1996), and Spencer et al.(2003).

et al

Downloaded by [Arend Meijer] at 10:26 01 June 2016 orogenic trend (Whitmeyer and Karlstrom 2007). The Shaw Butte in the Phoenix Mountains (Johnson . distances from Bisbee are considered an approximation 2003), and a metavolcanic and a diorite sample from of the distance of the magmatism from a trench the Union Hills (Holloway and Leighty 1998). In addition, believed to have existed south of Bisbee during a zircon date of 1.736 ± 0.001 Ga was reported by Eisele Palaeoproterozoic time (Anderson 1989; Meijer 2014). and Isachsen (2001) for an arc granitoid sample from Figure 3 shows that igneous and meta-igneous Webb Mountain (Figure 1). When combined (Figure 3), samples, collected from the arc volcanic belts approxi- the ‘old’ and ‘new’ arc data indicate 1.75–1.68 Ga arc mately 270–410 km north-northwest of Bisbee, range in rocks extend over much of the Prescott–Payson– age from approximately 1.75 to 1.675 Ga. Thus, Phoenix area and as far south as Webb Mountain Palaeoproterozoic arc magmatism in this region lasted (Figure 1). Overall, these observations offer little support approximately 75 million years. The ‘old’ arc data in to the existence of distinct age provinces within the Figure 3 include most of the zircon ages published for Palaeoproterozoic arc volcanic belts of central Arizona samples from these terranes over the last 50 years (see as first noted by Silver et al.(1986). Moreover, the lack of figure caption for references). Several new zircon dates data points between 1.68 and 1.60 Ma in the arc region for arc rocks are shown in Figure 3 (also see Tables 2 and (270–410 km in Figure 3) indicates magmatism was not 3) and these represent samples from the metropolitan active in the arc volcanic belts during this time. Phoenix area including a diorite from Thunderbird Park The zircon ages for igneous and meta-igneous (Leighty and Huckelberry 1998), a granodiorite from samples from the Pinal terrane show a very different 8 A. MEIJER

pattern from that shown by samples from the arc trailing plate is found in the lack of magmatism in the volcanic belts. As is evident in Figure 3, the forearc arc after approximately 1680 Ma and the general lack of samples form a vertical distribution that shows a magmatism along the southwestern margin of Laurentia more limited age range (~30 million years) than the over a span of approximately 150 million years after arc samples and this distribution extends over much of ~1600 Ma (e.g. Doe et al . 2012;GehrelsandPecha2014). the width of the forearc. This type of age/distance The convergence rate between the trailing plate and the distribution involving granitoid plutons is atypical of margin might have been sufficiently small to allow the the forearcs in younger arc systems except those that trailing plate to become ‘welded’ to the margin, resulting have experienced an SRSE (e.g. Marshak and Karig 1977; in a passive margin. Similar effects have been observed in Bradley et al. 2003). Plutonic and metavolcanic forearc some modern margins that have experienced SRSEs (e.g. samples, where both were analysed from a given area, Maldonado et al. 1994;Jabaloyet al. 2003). The leading show similar ages (Figure 3). Eisele and Isachsen (2001) plate continued to subduct, leaving a slab window in its reported zircon ages for a number of igneous and meta- wake beneath the forearc region. As the leading edge of igneous samples from the Pinal terrane (among the this window moved along the subduction zone with the younger ‘old’ data in Figure 3). The range of ages trailing edge of the leading plate, it produced magmatism (~30 million years) observed for the bulk of the ‘old’ in the forearc region as asthenosphyric melts rose and ‘new’ forearc samples is within the range of ages upwards and interacted with metasediments in the previously associated with SRSE magmatism (e.g. forearc prism. This magmatism was of a relatively short Dickinson 1997; Bradley et al. 2003). It is to be noted duration because the asthenosphere beneath the forearc that the forearc samples do not show a clear increase or was cooled by the materials in the forearc prism as the decrease in ages along the orogenic trend (i.e. NE–SW). leading edge of the slab window moved towards the arc. For example, the diorite sample from Javelina Mountain The gradual cooling may have resulted in the production in the western portion of the terrane (Figure 1) has an of a range of magma volumes and compositions and may age and associated error similar to that for the quartz have added a layer of mafic materials to the base of the diorite from the Bisbee area in the eastern portion of crust in the forearc region. It is possible that the younger the terrane (Table 3). In this sense, the Pinal forearc magmatism in the eastern-most Pinal terrane reflects magmatic rocks differ from the magmatic rocks that subduction of another ridge crest in this region (e.g. invol- intruded into the forearc in south-central Alaska. The ving an oceanic microplate) 20–40 million years after the latter form a diachronous distribution of intrusion ages subduction of the first ridge crest at ca. 1650 Ma. from the west to the east (Bradley et al. 2003). Data obtained on detrital zircons from metasedimentary Two samples from the easternmost portions of the rocks collected from exposures in central and southern Pinal terrane in Arizona appear to be somewhat Arizona (Doe 2014) provide insight into the nature of younger than most of the other Pinal forearc igneous thesouthwesternLaurentianmarginaftertheSRSEat and meta-igneous samples (Figure 3). These samples, approximately 1.65 Ga. The youngest zircons found in analysed by Eisele and Isachsen (2001), include a ‘quartz most of the Palaeoproterozoic quartzites are approximately Downloaded by [Arend Meijer] at 10:26 01 June 2016 porphyry’ from the western Dos Cabezas Mountains 1.65 billion years old and this age is considered to be the (1634 ± 2 Ma) and a gneissic alkali granite from the maximum depositional age for the quartzites (Doe et al. Peñaleno Mountains (1615 ± 5 Ma). Eisele and Isachsen 2012). Very few ca. 1.65 Ga igneous or meta-igneous rocks (2001) also zircon-dated a metatuff (1647 ± 2 Ma) and a have been reported from the Mazatzal and Yavapai arc felsic xenolith in a metatuff (1641 ± 15 Ma) from the volcanic belts although they are common in the Pinal western Dos Cabezas Mountains. In addition, Erickson terrane (Figure 3;Silver1967). This suggests ca. 1.65 Ga and Bowring (1990) zircon-dated two metavolcanic sam- igneous rocks in the Pinal terrane were exposed sub-aerially ples from the western Dos Cabezas Mountains and (i.e. the forearc was uplifted) and intensively weathered obtained ages of 1670 Ma for both samples. The latter (see Medaris et al. 2003;Joneset al. 2009)toproducethe four ages are more or less in line with the zircon ages quartzites not long after the SRSE. obtained on other magmatic rocks in the Pinal terrane Doe et al.(2012) and Doe (2014) also found some (Figure 3). The significance of the two younger ages is 1.60–1.49 Ga zircons (estimated maximum depositional not clear. They may reflect a slightly younger phase of age of 1.488 Ga) in metasedimentary rocks of the Yankee magmatism in the eastern-most part of the Pinal terrane. Joe and Blackjack formations in the northeastern part of Figure 4 shows a conceptual model for the origin of the the Pinal terrane (Hess Canyon; Figure 1). According to Arizona portion of the southern Laurentian margin during Doe et al.(2012), igneous or meta-igneous rocks with the time of the SRSE. In the model, the trailing plate is not zircon ages in this range have not been reported from subducted. Evidence for the lack of subduction of the southern Laurentia, suggesting that these zircons did not INTERNATIONAL GEOLOGY REVIEW 9

Trailing Plate ? ?

400 km Aj + Trench + + No Pinal + + Sr Mu + Bisbee Tucson ? + Em + + + + + Ja Jlh Tm Dc + + + Cg + SOLM + + Wm + Zw Jw + Newly Subcreted Mafic Crust + + and Mantle ++ Bu + + Pe + + + Phoenix + + + + + + + + + Jm ++ Ra + + +Th Pm Sb + Uh + + + Soc > 1.7 Ga

Soc Crust SCLM Older Mazatzal Arc Accreted Slab Materials + + + Yavapai Window SCLM SCLM S Leading Plate W E (Asthenosphere) N

Figure 4. Conceptual model for a portion of the southern margin of Laurentia during the SRSE (modified after Thorkelson 1996). The subsurface cross-section was modified from Plafker et al.(1994). In the vertical section, ‘soc’ represents subducted oceanic crust. Active forearc volcanism is represented by small cones with ‘plumes’. This volcanism could have been subaerial and/or submarine. Forearc plutonism is shown at the surface as irregular patches with plus symbols. Patches without labels represent plutons dated by Eisele and Isachsen (2001). Labels are defined in the caption for Figure 1. The spreading ridge segments in the outline above the

Downloaded by [Arend Meijer] at 10:26 01 June 2016 ‘Pinal Trench’ are drawn to represent the tectonic setting prior to the SRSE. SCLM denotes Subcontinental Lithospheric Mantle. SOLM denoted Sub-Oceanic Lithospheric Mantle. Note that the Mazatzal arc was inactive during the SRSE.

originate in southern Laurentia. In any case, the absence environments of deposition, the largely unimodal detrital of zircons of similar ages in other (1.60–1.65 Ga) quart- zircon age spectra of these quartzites (Doe et al. 2012, zites of the Pinal terrane (Doe et al. 2012; Doe 2014) Figure 2) are not consistent with sedimentation in a suggests the latter quartzites were deposited prior to typical passive margin setting, a rift setting, or a colli- ca. 1.6 Ga. The facts that these quartzites (1) are a minor sional setting. However, they are consistent with a fore- component of the Pinal terrane, (2) generally have arc depositional environment (i.e. forearc and slope localized distributions, (3) have depositional ages equiva- basins) along a young compressional margin (Cawood lent in age to the magmatic rocks in the Pinal terrane, (4) et al. 2012). In the Pinal terrane, it appears the forearc contain relatively few zircons with ages representing environment turned into a ‘passive margin’ environment older cratonic terranes to the north (Doe et al. 2012), not long after the SRSE. However, this ‘passive margin’ and (5) are not uncommonly found in proximity to expo- environment was unlike typical passive margins formed sures of Palaeoproterozoic forearc igneous rocks (Doe along rifted continental margins in that it formed along a 2014; this study) suggest they were derived from local compressional margin. This scenario for the post-SRSE sources. Based on a model developed by Cawood et al. evolution of the Pinal terrane is similar to those proposed (2012) relating detrital zircon age spectra to by Maldonado et al.(1994) and Jabaloy et al.(2003) for 10 A. MEIJER

the evolution of the arc-trench system along the central evolution of the Kodiak batholith in southern Alaska: Antarctic Peninsula in the Cenozoic. Tectonophysics, v. 464, p. 137–163. doi:10.1016/j. tecto.2008.09.029 Barker, F., Farmer, G.L., Ayuso, R.A., Plafker, G., and Lull, J.S., 1992, 6. Conclusions The 50 Ma granodiorite of the eastern Gulf of Alaska: Melting in an accretionary prism in the forearc: Journal of Geophysical The various geological consequences of SRSEs enumerated Research, v. 97, p. 6757–6778. doi:10.1029/92JB00257 by DeLong and Fox (1977)andDeLonget al.(1979)are Barker, P., 1982, The Cenozoic subduction history of the Pacific reflected in the rock units within the Pinal terrane. There is margin of the Antarctic Peninsula: Ridge crest-trench interactions: Journal of the Geological Society of London, evidence of shallowing, emergence, and deepening of v. 139, p. 787–801. doi:10.1144/gsjgs.139.6.0787 the forearc region in the presence of conglomerates, Barker, P., Barber, P.L., and King, E.C., 1984, An early Miocene quartzites, and phyllites within the terrane. In addition, ridge crest-trench collision on the south Scotia Ridge near arc magmatism was shut down during the time forearc 36°W: Tectonophysics, v. 102, p. 315–332. doi:10.1016/0040- magmatism was active. Granitoid plutons that intruded 1951(84)90019-2 into the forearc region show a narrow range of crystalliza- Bergquist, J.R., and Blacet, P.M., 1978, Preliminary reconnais- sance bedrock geologic map of part of the Casa Grande tion ages (1650 ± 20 Ma) and are distributed over most of Mountains Quadrangle, Pinal County, Arizona: United States the distance between the arc terrane and the inferred Geological Survey Open-File Report 78-547, scale 1:24,000, trench location. In addition, the metamorphic grade of 1 sheet. most of the exposed Pinal terrane is greenschist facies, Bradley, D.C., Kusky, T.M., Haeussler, P.J., Goldfarb, R., Miller, consistent with an elevated geothermal gradient along M., Dumoulin, J., Nelson, S.W., and Karl, S., 2003, Geologic signature of early Tertiary ridge subduction in Alaska, in this margin at approximately 1.65 Ga. Thus, the combined Sisson, V.B., Roeske, S.M., and Pavlis, T.L., eds., Geology of results are consistent with the idea that the southern mar- a transpressional orogen developed during ridge-trench gin of Laurentia experienced an SRSE in Palaeoproterozoic interaction along the North Pacific margin: Geological time. Society of America Special Paper 371, p. 19–49. Breitsprecher, K., and Thorkelson, D.J., 2009, Neogene kinemati- chistory of Nazca-Antarctic-Phoenix slab windows beneath Acknowledgements Patagonia and the Antarctic Peninsula: Tectonophysics, v. 464, p. 10–20. doi:10.1016/j.tecto.2008.02.013 The author would like to thank Professor George Gehrels of the Breitsprecher, K., Thorkelson, D.J., Groome, W.G., and Dostal, J., University of Arizona for his support of this work and for his review 2003, Geochemical confirmation of the Kula-Farallon slab of an early version of the manuscript. The Arizona Laserchron window beneath the Pacific Northwest in Eocene time: Center (ALC) is supported by NSF grant EAR-1338583. The author Geology, v. 31, p. 351–354. doi:10.1130/0091-7613(2003) would also like to thank Professor David Kimbrough for making 031<0351:GCOTKF>2.0.CO;2 available the rock crushing and mineral separation facilities at San Brown, M., 1998, Ridge-trench interactions and high-T-low-P Diego State University. These facilities are supported by NSF metamorphism with particular reference to the Cretaceous grant EAR-1250434. evolution of the Japanese Islands, in Treloar, P.J., and O’Brien, P.J., eds., What drives metamorphic reactions? The Geological Society of London, Special Publications, v. 138, Downloaded by [Arend Meijer] at 10:26 01 June 2016 Disclosure statement p. 137–169. 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