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Ignimbrite Calderas, Regional Dike Swarms, and the Transition from Arc to Rift in the Southern Rocky Mountains

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Ignimbrite Calderas, Regional Dike Swarms, and the Transition from Arc to Rift in the Southern Rocky Mountains Research Paper GEOSPHERE Magmato-tectonic links: Ignimbrite calderas, regional dike swarms, and the transition from arc to rift in the Southern Rocky Mountains 1, 2, GEOSPHERE, v. 15, no. 6 Peter W. Lipman * and Matthew J. Zimmerer * 1U.S. Geological Survey, Menlo Park, California 94025, USA 2New Mexico Bureau of Geology and Mineral Resources, Socorro, New Mexico 87801, USA https://doi.org/10.1130/GES02068.1 18 figures; 3 tables; 1 set of supplemental files ABSTRACT active for at least an additional 9 m.y. Platoro magmatism began to decline at ca. 26 Ma, concurrent with initial basaltic volcanism and regional extension CORRESPONDENCE: [email protected] Radial and linear dike swarms in the eroded roots of volcanoes and along along the Rio Grande rift, but no basalt is known to have erupted proximal rift zones are sensitive structural indicators of conduit and eruption geometry to Platoro caldera prior to ca. 20 Ma, just as silicic activity terminated at this CITATION: Lipman, P.W., and Zimmerer, M.J., 2019, Magmato-tectonic links: Ignimbrite calderas, regional that can record regional paleostress orientations. Compositionally diverse magmatic locus. The large numbers and lengths of the radial andesitic-dacitic dike swarms, and the transition from arc to rift in the dikes and larger intrusions that radiate westward from the polycyclic Platoro dikes, in comparison to the absence of similar features at other calderas of the Southern Rocky Mountains: Geosphere, v. 15, no. 6, caldera complex in the Southern Rocky Mountain volcanic field (southwest- San Juan volcanic locus, may reflect location of the Platoro system peripheral p. 1893–1926, https://doi.org/10.1130/GES02068.1. ern United States) merge in structural trend, composition, and age with the to the main upper-crustal San Juan batholith recorded by gravity data, as enormous but little-studied Dulce swarm of trachybasaltic dikes that con- well as its proximity to the axis of early rifting. Spatial, temporal, and genetic Science Editor: Shanaka de Silva Associate Editor: Valerio Acocella tinue southwest and south for ~125 km along the eastern margin of the links between Platoro radial dikes and the linear Dulce swarm suggest that Colorado Plateau from southern Colorado into northern New Mexico. Some they represent an interconnected regional-scale magmatic suite related to Received 23 September 2018 Dulce dikes, though only 1–2 m thick, are traceable for 20 km. More than 200 prolonged assembly and solidification of an arc-related subcaldera batholith Revision received 16 April 2019 dikes of the Platoro-Dulce swarm are depicted on regional maps, but only a concurrently with a transition to regional extension. Emplacement of such Accepted 18 July 2019 few compositions and ages have been published previously, and relations to widespread dikes during the late evolution of a subcaldera batholith could Platoro caldera have not been evaluated. Despite complications from deuteric generate earthquakes and trigger dispersed small eruptions. Such events Published online 30 September 2019 alteration, bulk compositions of Platoro-Dulce dikes (105 new X-ray fluores- would constitute little-appreciated magmato-tectonic hazards near dormant cence and inductively coupled plasma mass spectrometry analyses) become calderas such as Valles, Long Valley, or Yellowstone (western USA). more mafic and alkalic with distance from the caldera. Fifty-eight (58) new 40Ar/39Ar ages provide insight into the timing of dike emplacement in relation to evolution of Platoro caldera (source of six regional ignimbrites between ■ INTRODUCTION 30.3 and 28.8 Ma). The majority of Dulce dikes were emplaced during a brief period (26.5–25.0 Ma) of postcaldera magmatism. Some northeast-trending Radial and linear dike swarms in the eroded roots of central volcanoes and dikes yield ages as old as 27.5 Ma, and the northernmost north-trending dikes along rift zones have long been recognized as structures that document the have younger ages (20.1–18.6 Ma). In contrast to high-K lamprophyres farther geometry of conduits for eruptions and provide records of paleostress geom- west on the Colorado Plateau, the Dulce dikes are trachybasalts that contain etry (Nakamura, 1977; Aldrich et al., 1986; Acocella, 2014). In comparison, dike only anhydrous phenocrysts (clinopyroxene, olivine). Dikes radial to Platoro swarms appear to be relatively uncommon at large ignimbrite calderas (Smith caldera range from pyroxene- and hornblende-bearing andesite to sanidine and Bailey, 1968; Cole et al., 2005), perhaps because growth of batholithic-scale dacite, mostly more silicic than trachybasalts of the Dulce swarm. Some distal magma bodies beneath calderas decouples the overlying crust from regional andesite dikes have ages (31.2–30.4 Ma) similar to those of late precaldera stress geometry (Christiansen et al., 1965; Steven and Lipman, 1976). Even lavas; ages of other proximal dikes (29.2–27.5 Ma) are akin to those of caldera- ignimbrite systems that erupted during regional extension, such as Valles, filling lavas and the oldest Dulce dikes. The largest radial dikes are dacites Yellowstone, and Long Valley calderas in the western USA, typically host only that have yet younger sanidine 40Ar/39Ar ages (26.5–26.4 Ma), similar to those sparse fissure-controlled magmatism (the Mono-Inyo chain north of Long of the main Dulce swarm. Valley being an exception). The older andesitic dikes and precaldera lavas record the inception of a In contrast, an enormous system of long-recognized but little-studied long-lived upper-crustal magmatic locus at Platoro. This system peaked in dikes in the southwestern USA, which radiate from the Oligocene Platoro magmatic output during ignimbrite eruptions but remained intermittently caldera in Colorado (Fig. 1) to merge with the Dulce dike swarm that contin- This paper is published under the terms of the ues ~125 km into New Mexico (Fig. 2), provides exceptional opportunities to CC-BY-NC license. *E-mail: [email protected]; [email protected] explore magmato-tectonic–temporal links between dike emplacement and a © 2019 The Authors GEOSPHERE | Volume 15 | Number 6 Lipman and Zimmerer | Magmato-tectonic links Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/15/6/1893/4876434/1893.pdf 1893 by guest on 30 September 2021 Research Paper 108°W 106° 104° 40° N 0 60 mi 0 100 km Denver F r N o n t R a n g S e a GP w South a Park t BP Colorado c x Figure 1. Map of the Southern Rocky Mountain vol- h Springs 39 Mile canic field (SRMVF) and Rio Grande rift in southern volcanic Colorado and northern New Mexico (USA). Large MP area rectangle, location of study area for the Platoro-Dulce MA l dike swarm (Fig. 2); smaller rectangle, Platoro caldera Approx. original limit l West Elk l l R map area (Fig. 3). Also shown are other ignimbrite of volcanic locusrocks Gunnison g calderas of the SRMVF, major erosional remnants e x and inferred original extent of mid- Cenozoic vol- canic cover (Steven, 1975), caldera-related granitic M Bz W intrusions (Tweto, 1979; Lipman, 2007), and later e sedimentary fill in asymmetric grabens of the Rio C t NP S Rosita Grande rift. Rift graben asymmetry and bound- a x M ary-fault geometry reverse from east- dipping in Saguache n 38° SL g t the San Luis Valley segment to west-dipping in r s e . the Sawatch Range–upper Arkansas River valley LGn S San B San Luis Valley segment, segment to the north. Blue-dashed lines, major Juan SC bounding faults of asymmetrical rift grabens. Arrows Cr volcanic Del x indicate the trend of Late Cretaceous–early Cenozoic locus SR Norte Rio Grande rift (Laramide) intrusions of the Colorado Mineral Belt. d Calderas: B—Bachelor; Bz—Bonanza; C—Cochetopa e LGs Fig. 3 Park; Cr—Creede; GP—Grizzly Peak; LGn—La Gar- Approximate original limit ita, north segment; LGs—La Garita, south segment; M—Marshall; MA—Mount Aetna; NP—North Pass; of volcanic rocks Pl Pl—Platoro; S—Silverton; SL—San Luis complex; SR—South River. Other features: BP—Buffalo Peak; C Spanish Line of SC—Summer Coon volcano. CO—Colorado; NM— section, r i Peaks New Mexico. Modified from McIntosh and Chapin Fig. 16 s t Colorado o (2004). Rge—Range; Mts.—Mountains. New Mexico T u s a Questa-Latir s volcanic M M locus t t s s Fig. 2 . Explanation Granitoid intrusion MP Mount Princeton batholith Mid-Cenozoic volcanic areas Trend of Colorado Mineral Belt Sedimentary fill of Rio Grande rift Caldera Late-rift mafic lavas Regional structural attitude GEOSPHERE | Volume 15 | Number 6 Lipman and Zimmerer | Magmato-tectonic links Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/15/6/1893/4876434/1893.pdf 1894 by guest on 30 September 2021 Research Paper Approximate inferred extent of ignimbrites erupted from 26.4 Platoro caldera complex (Treasure Mountain Group) 21.3 26.5 20.9 26.9 26.2 PLATORO 20.4 *27.1 CALDERA 27.4 26.2 *29.0 27.7 * 25.1 * 29.0 AR * Platoro: * JM 25.1 CRETACEOUS * distal 27.5 dikes (PD) ca. 29.2 SEDIMENTARY ROCKS, 31 NE trend, CO-N SAN JUAN BASIN 31.4 (D1) 26.8 29.2 26.5 x Platoro: 19.1 Pagosa Springs 27.5 BB Platoro map (Lipman, 1974) proximal 26.5 dikes (PP) 18.6 27.3 25.3 N trend, CO-N 18.6 25.9 30.2 30.7 (D2A) ~27.5 31.2 VOLCANIC ROCKS, SAN JUAN MOUNTAINS 27.3 24.5 ~21 V- 25.1 NE trend, CO-S Mtn (D2B) 24.6 N trend, CO-S 25.0 (D3) 21.5 20.1 25.5 25.5 37° AM AM Colorado N *15.5 New Mexico 26.4 XRF chem 25.6 25.3 sample sites x Dulce 25.4 Dacite N trend, NM-N Andesite (D4) Trachybasalt 26.0 25.5 Ar/Ar ages Blue, DG (2015); red, no success U-Pb* ages* Blue, DG (2015); black, AG (2018) N trend, NM-mid (D5) Granitoid intrusions N Line of projected chemical plots (Fig.
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