MESOZOIC MAGMATISM IN MONTANA Kaleb C. Scarberry,1 Petr V. Yakovlev,1 and Theresa M. Schwartz2 1Montana Bureau of Mines and Geology, Butte, Montana 2U.S. Geological Survey, Denver, Colorado ABSTRACT From crystalline batholiths with footprints larger than 4,500 km2 to beds of micron-sized ash particles, a record of Mesozoic magmatism is found throughout Montana. Mesozoic igneous rocks are an important natu- ral resource in the State because of their association with precious metal ores and industrial mineral deposits. Mesozoic magmatism in Montana is a tale of volcanic arc eruptions, pluton emplacement, crustal magma dif- ferentiation, melting, and assimilation. Explosive caldera-forming eruptions, magmatic hydrothermal and meta- morphic mineralization, tectonic uplift, fl uvial erosion, and redeposition of igneous rocks and mineral resources are all chronicled by Mesozoic igneous rocks. This article off ers a summary of the types, locations, extents, and ages of Mesozoic igneous rocks in Montana. There follows a discussion of the generation and emplacement of magmas and intrusive rocks, their geometries, and the style and timing of magmatism. The article ends with a summary of what remains unknown and off ers suggestions for future research. INTRODUCTION Montana prospectors have long recognized the tinent. In Montana, arc-related deposits include ashes association among lode gold systems, stream placers, derived from the Jurassic–Early Cretaceous arc that and crystalline igneous rocks. Gold strikes at Grass- was located along the continental margin (the Sierra hopper Creek (1862), Alder Gulch (1863), and Last Nevada Batholith) as well as ashes derived from the Chance Gulch (1864) drew swarms of prospectors to Late Cretaceous arc located in the continental interior southwestern Montana (summary in Albright, 2004) (the Idaho and Boulder Batholiths; Christiansen and and sparked the charge toward scientifi c inquiries into others, 1994). the nature of Mesozoic igneous systems and their min- Mesozoic igneous rocks dominantly formed in eral deposits. Lindgren (1886) fi rst described granite Montana between about 120 Ma and 66 Ma (fi g. 3) at Mullan Pass, about 10 mi (16 km) northwest of and include batholiths, plutons, volcanic fi elds, epi- Helena (fi g. 1), and laid the foundation for subsequent clastic volcanic sedimentary aprons, and pyroclastic research. ash beds (bentonite). Magmatism continued into the North American Cordilleran arc magmatism was Cenozoic (fi g. 3) as the Cordilleran arc collapsed and underway by the Jurassic in southern California, pri- regional stresses transitioned to an extensional regime or to breakup of the supercontinent Pangea (Coney, (Mosolf and others, this volume). Most workers agree 1972; Dickinson and others, 1988; Miller and Snoke, that (1) batholiths and plutons formed in an evolving 2009; Sauer and others, 2017), and reached Montana thrust wedge during Late Cretaceous contractional de- during the Late Cretaceous. Pyroclastic ash-fall depos- formation (Tilling and others, 1968; Hamilton, 1988; its from silicic eruptions settled in the Western Interior Lageson and others, 2001); (2) batholiths, plutons, of the United States (fi g. 2), in shallow marine and and related volcanic rocks are a record of Cordilleran continental settings and inland of the Cordilleran arc arc magmatism (Rutland and others, 1989; Saleeby that developed along the western margin of the con- and others, 1992; Gaschnig and others, 2011); 1 MBMG SpecialPublication122:GeologyofMontana,vol.1:Geologic History 2 116° AK 49° 114° 112° 110° 108° Leading edge of fold- and -thrust belt YK Kalispell BC 48° 55N NORTH Great Falls Lewis and Clark Line Adel Mountain 50N Volcanic Field 47° Missoula Big Belt plutons PACIFIC OCEAN t CANADA l u U.S.A. a Helena F t Sapphire t AMERICAN n l Elkhorn Mtns Volcanic Field 45N e u a m Philipsburg F t WA h t s c Helena Structural Salient n u Bitterroot Lobe a r t e Tectonic Boulder h e T m Crazy MT Idaho Batholith D h d Batholith r Mountains t c a a 46° o b Block t o e m r D o r a L e d Butte Basin t t n i o c OR B na ID A 40N CA Bozeman Billings NV WY Pioneer Tobacco N UT Batholith Root Dillon Batholith Sliderock Mountain CORDILLERA Volcano Volcanic rocks 45° Bannack 35N AZ Plutonic rocks Volcanic Field N Epiclastic volcanic deposit 0 50 mi 310 mi 500 km Bentonitic shale 0 80.4 km Jurassic to Eocene U.S.A. magmatic arc MEXICO 130W 120W 110W Figure 1. Distribution of Mesozoic magmatic arc and related rocks in Montana. Inset panel modifi ed from Sauer and others, 2017. Distribution of the Jurassic to Eocene arc in North America after Burchfi el and others (1992). Western Interior Seaway Scarberry andothers:MesozoicMagmatisminMontana Early Cretaceous (135o130 Ma) Late Cretaceous (87o83 Ma) Early Paleogene (65o62 Ma) Figure 2. Early Cretaceous through early Paleogene paleogeographic maps (Blakey, 2012). 3 MBMG Special Publication 122: Geology of Montana, vol. 1: Geolo gic History 116°W 114°W 112°W 110°W A 48°N Montana alkalic province Idaho ! ! ! Batholith ! ! ! ! ! ! !! ! ! ! !!!!! !!!! ! ! !!! ! X! !! X X X ! ! !! ! !!! X ! ! ! ! Boulder !! X X X!XX!X !!! ! ! X! !! ! !! !! Batholith X X XXX X !! !X XX ! X XXX !! X! X X X ! X! ! ! X ! ! ! ! ! ! X ! ! ! ! ! X X!!! ! ! !!X ! X !! ! !! !!!!! ! !!XXXX !!!!!! !! 46°N ! ! X !!!!X! ! XX ! !!X! !!!! X! ! 46°N 44°N !X!!! ! !! ! XXX ! ! ! ! X !! ! X ! ! ! !! !!!! ! XX !!! X!X! X ! ! !! XXXX Lowland Creek ! ! !! !!!X X !! ! !!!! !!! !!! XX !Volcanic Field !!!!! !! ! X !!! X !!!!!!X! XX X !! ! ! !! X !!! ! XX !!! ! ! ! X ! ! !X ! ! ! ! X ! !!! X!! !!! !!!! ! !! !!! ! ! !! X! X !!! !!! !! !!! ! ! ! XX ! ! !!!!!! ! !! !! ! ! ! !!!! ! X X! ! !! ! ! ! ! ! ! ! !! ! ! X! ! !!! ! ! ! ! ! !! Absaroka XXX ! !! XX ! !! !! ! Volcanic X X !! ! ! X X X ! ! !! X !! !! Field ! X ! ! X!X! !X ! !! X X ! ! ! ! !!!! Challis 44°NX X ! 48°N ! ! Volcanic X !! ! ! X XX ! Field ! !! X XX! ! ! !! ! ! XX ! 0 50 100 km X Zircon U/Pb age X! Eocene crystal age ! Ar/Ar, K/Ar, or fission track age X! Paleocene crystal age X! Miocene crystal age X! Late Cretaceous crystal age 0 30 60 mi X! Oligocene crystal age X! Early Cretaceous crystal age 116°W 114°W 112°W 110°W 1.0 B 0.8 Average of all igneous units (n = 873) Paleogene volcanic fields Average (n = 469) Absaroka Volcanic Field (n = 58) 0.6 Anaconda & Bitterroot core complexes (n = 102) Challis Volcanic Field (n = 150) Dillon Volcanic Field (n = 56) Helena & Lowland Creek Volcanic Fields (n = 44) 0.4 Montana alkalic province (n = 59) Cretaceous intrusive complexes Average (n = 404) 0.2 Adel Mountains Volcanic Field (n = 11) Boulder Batholith & Elkhorn Volcanics (n = 80) Idaho Batholith (n = 202) Cumulative Distribution Pioneer Batholith (n = 107) 0.0 Tobacco Root Batholith (n = 4) Paleogene volcanic fields 48 n = 469 100 Cretaceous intrusive complexes n = 404 75 100 48 All igneous units 48 n = 873 100 75 0 20 40 60 80 100 120 140 Age (Ma) Figure 3. Age constraints on Cretaceous and Paleogene igneous c omplexes in western Montana and adjacent areas. (A) Hillshade m ap showing the spatial distribution of Cretaceous intrusive (pink) and Paleogene volcanic (orange) complexes and the locations of geochro- nologic age constraints from U/Pb, 40Ar/39Ar, K/Ar, and fi ssion track analyses (compilation from Schwartz data from Esri, USGS, and NOAA. (B) Cumulative probability curv and others, 2019b). Basemap es and kernel density estimates (bandwidth = 2; histogram bins Ma) for Cretaceous intrusive and Paleogene volcanic complexes b = 2 ased on compiled U/Pb, 40Ar/39Ar, K/Ar, and fi ssion track ages. 4 Scarberry and others: Mesozoic Magmatism in Montana (3) Paleocene uplift followed by Paleocene–Eocene in this summary because they formed by geologic pro- fl uvial incision (e.g., Houston and Dilles, 2013; cesses identical to their Late Cretaceous precursors. Schwartz and others, 2019a) removed cogenetic vol- Idaho Batholith and Sapphire Block Plutons canic deposits from atop many batholiths and plutons; and (4) collectively, these processes contributed to The Late Cretaceous–Paleogene Idaho Batholith the production and distribution of lode gold systems contains two lobes, the northern Bitterroot lobe and and stream placer deposits throughout southwestern the larger southern Atlanta lobe, which is not exposed Montana (Lyden, 1948; Foster and Childs, 1993; see in Montana. The batholith intruded Proterozoic base- 2 contributions to this special publication, volume 2, ment rocks (Hyndman, 1983). About 200 km of the 2 by Reed and Dilles, 2020, and Gammons and others, Bitterroot lobe, less than 10% of the total 25,000 km 2020). footprint of the Idaho Batholith, extends into Montana (fi g. 1). Both lobes are composed of texturally similar BATHOLITHS AND PLUTONS biotite granodiorite, yet the Bitterroot lobe is conspic- uously younger (66 to 53 Ma) than the Atlanta lobe Mesozoic intrusions in Montana occur primarily (83 to 67 Ma; Foster and Fanning, 1997; Gaschnig and within and adjacent to the Helena structural salient others, 2011). The Atlanta lobe of the batholith con- (fi g. 1) and include large batholiths and smaller plu- tains 98–87 Ma hornblende-bearing roof pendants and tons (fi g. 4). The Idaho, Boulder, and Pioneer Ba- stocks along its southeast margin (Gaschnig and oth- tholiths are the major intrusive centers in Montana. ers, 2011). East of the Bitterroot lobe, several outlier Smaller batholiths, such as the Philipsburg, Mount granitic plutons similar in composition to the Idaho Powell, and Tobacco Root Batholiths, are satellite Batholith are exposed over a 700 km2 arcuate area masses to the major intrusive centers. Paleogene vol- called the Sapphire block (fi g. 1; Hyndman and others, canic fi elds that postdate regional arc-related mag- 1975). matism include the Absaroka, Challis, Dillon, and Lowland Creek volcanic fi elds, and lie adjacent to and Boulder Batholith and Satellite Intrusions partly overprint the Mesozoic intrusive centers (fi g. 3). The Boulder Batholith extends from Butte to Hel- Figure 3 shows the spatial distribution of available ena (fi g.