THEMED ISSUE: EarthScope IDOR project (Deformation and Magmatic Modification of a Steep Continental Margin, Western Idaho–Eastern Oregon) Construction and preservation of batholiths in the northern U.S. Cordillera R.M. Gaschnig1*, J.D. Vervoort2, B. Tikoff 3, and R.S. Lewis4 1SCHOOL OF EARTH AND ATMOSPHERIC SCIENCES, GEORGIA INSTITUTE OF TECHNOLOGY, 311 FERST DRIVE, ATLANTA, GEORGIA 30332, USA 2SCHOOL OF THE ENVIRONMENT, WASHINGTON STATE UNIVERSITY, PO BOX 642812, PULLMAN, WASHINGTON 99164, USA 3DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF WISCONSIN–MADISON, 1215 W DAYTON STREET, MADISON, WISCONSIN 53703, USA 4IDAHO GEOLOGICAL SURVEY, 875 PERIMETER DRIVE, MS 3014, MOSCOW, IDAHO 83844, USA ABSTRACT There is a nearly continuous record of magmatism through the Late Cretaceous–early Paleogene in Idaho and adjacent areas of Oregon and Montana, including the various phases of the Idaho batholith. We suggest that much of this magmatic record, however, has been obscured by subsequent tectonic deformation, erosion, and magmatic disruption and cannibalization, the latter of which can be tracked by zircon inheritance. Specifically, a mid-Cretaceous magmatic arc was significantly deformed by the western Idaho shear zone and intruded by the 83–67 Ma Atlanta peraluminous suite of the Idaho batholith. The northern part of the Atlanta peraluminous suite was, in turn, intruded by the 65–55 Ma Bitterroot lobe of the Idaho batholith. Consequently, the present age distribution of magmatism is strongly biased toward the youngest phases of plutonism; much of the older phases were destroyed by tectonic, magmatic, and erosional processes. The destruction of granitic batholiths may characterize Cordilleran-style orogens worldwide, which can lead to significant underestimates of magmatic fluxes. LITHOSPHERE; v. 9; no. 2; p. 315–324; GSA Data Repository Item 2016265 | Published online 19 August 2016 doi: 10.1130/L497.1 INTRODUCTION magmatism, but it is important that subduction- In this contribution we argue that magma- related magmatism continued until at least ca. tism in Idaho was relatively continuous from Mesozoic–Paleogene granitic coastal batho- 50 Ma (Gehrels et al., 2009). ca. 110 to 43 Ma and that the Idaho batholith liths characterize the North American Cordillera. The transition between northern and south- underwent the same Early to Late Cretaceous They are thought to be a hallmark of Andean- ern magmatic patterns occurs in Idaho, and, flare-up as the other coastal batholiths. The style subduction of oceanic lithosphere under although it has similarities to both, it also has record of this flare-up has been lost to a com- the western edge of North America (Dickinson, important differences. The Idaho batholith is bination of tectonic shortening, magmatic dis- 2004). There are, however, fundamental along- a massive ~23,500 km2 magmatic center that ruption and cannibalization, and erosion. The strike differences in the timing and longevity of was entirely emplaced in Precambrian continen- former has been discussed (Giorgis et al., 2005), magmatism along the North American Cordil- tal crust, unlike the other large coastal batholiths. in which transpressional deformation associated lera. The southern Cordillera is typified by the The precise chronology of the Idaho batholith, with the western Idaho shear zone significantly Sierra Nevada batholith in California and Pen- which is geographically divided into a northern shortened an existing magmatic arc. Evidence insular Ranges batholith further south. These Bitterroot and a southern Atlanta lobe (Fig. 1), for the loss of other major elements of the ear- Cretaceous, subduction-related magmatic arcs was unclear for many years because of the lier plutonic record through a combination of were emplaced within the transition between large amount of Precambrian zircon inheri- magmatic disruption and cannibalization and continental and oceanic crust, and they ceased tance. When finally dated, it was discovered erosion is the focus of this manuscript. The activity by ca. 85 Ma (Bateman, 1992). In con- that the timing of Idaho batholith magmatism tendency for specific plutons to contain mixed trast, the magmatic record of the northern North is distinct from that of other coastal Cordil- crystal populations, including crystals from ear- American Cordillera in Canada is complicated leran batholiths. In terms of exposed granites, lier magmatic pulses, is well established (e.g., by episodes of terrane accretion and probable there was apparently little granitic magmatism Miller et al., 2007; Claiborne et al., 2010). The translation, with the active magmatic arc occur- in Idaho during the key interval of 110–85 Ma, record of crystal recycling has been particularly ring on the outboard oceanic Insular terrane which is a time of voluminous magmatism in well documented in recent years and the detailed (Monger et al., 1982; Butler et al., 2001). In all other coastal batholiths. Rather, most of geochronological and geochemical studies that this northern section of the North American Cor- the exposed portions of the Idaho batholith show inheritance in zircon populations have pro- dillera, there is a large mid-Cretaceous pulse of are younger than 80 Ma (e.g., Foster and Fan- vided important insights into the how individual ning, 1997; Unruh et al., 2008; Gaschnig et volcanic and plutonic systems are constructed al., 2010). Why does the Idaho segment appear (Reid et al., 1997; Miller et al., 2007; Bryan et *Now at the Department of Environmental, Earth, and Atmospheric Sciences, University of Massachu- so different from the other parts of the North al., 2008; Stelten and Cooper, 2012; Zimmerer setts Lowell, Lowell, Massachusetts 01854. American Cordillera? and McIntosh, 2012; Barboni et al., 2013). We LITHOSPHERE© 2016 Geological | Volume Society 9 of| AmericaNumber 2| |For www.gsapubs.org permission to copy, contact [email protected] 315 Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/2/315/1001597/315.pdf by guest on 28 September 2021 GASCHNIG ET AL. 117ºW 114ºW the Bitterroot lobe. The 53–43 Ma Challis intru- 47ºN + + sive province is a belt of compositionally diverse plutons and dikes widely distributed throughout, and to the east of, the Idaho batholith, with con- temporaneous volcanics further east. Orofino Bitterroot lobe Lewiston METHODS WA Geochronology ID Grangeville OR Elk City New U-Pb ages for samples from the cen- 0.704/0.706 isopleth Challis intrusions Salmon River suture zone (53-43 Ma) tral portion of the Idaho batholith are presented here. Sample locations are shown in Figure 1 MT Bitterroot peraluminous suite (66-53 Ma) and results are plotted in Figure 2. Example Riggins ID Late metaluminous suite cathodoluminescence images of zircons are (76-68 Ma) shown in Figure 3. Samples were analyzed by Atlanta peraluminous suite laser ablation–inductively coupled plasma–mass (83-67 Ma) spectrometry (LA-ICP-MS) at Washington State + McCall + Border zone suite University, using a New Wave UV Nd:YAG 213 (92-85 Ma) nm laser coupled to a Thermo-Finnigan Ele- Early metaluminous suite Atlanta lobe ment2 single collector high-resolution ICP- (98-85 Ma) MS. Methods are identical to those described Suture zone suite (110-100 Ma) in Gaschnig et al. (2010). Weighted mean 206 238 New Zircon dates Pb/ U ages, mean square of weighted devi- Published samples ates, and the probabilities of fit are calculated Stanley with Cretaceous inheritance based on internal measurement errors only. The error from the weighted mean calculation was then combined quadratically with the standard deviation derived from the reproducibility of the zircon standard. The given error following the Hailey weighted mean in text is this total uncertainty at Boise the 2σ level. Tabulated results are reported in the Data Repository1 along with sample locations. Compilation of Geochronological Results 43ºN 0 100km + N + from Idaho In addition to the new U-Pb results pre- Figure 1. Simplified geologic map (after Gaschnig et al., 2010) showing major phases of the batholith sented here, we also compile the large number and locations of new zircon samples (black dots), along with previously published zircons samples of instances of earlier Cretaceous inheritance (red dots; from Gaschnig et al., 2010) that showed Cretaceous inheritance. in younger samples of the Idaho batholith, as reported in the literature (Gaschnig et al., 2010, 2013; Braudy et al., 2016). These are cases argue that the same process can occur on the (WISZ), ranging from ca. 110 to 100 Ma. The where zircon populations in the latest Creta- scale of an entire batholith and its importance border zone suite is a belt of mostly ca. 92 Ma ceous and Paleocene peraluminous units of the in the evolution of batholiths is often underes- tonalite plutons immediately east of the WISZ. batholith contain components that are 5–20 m.y. timated. We use information gathered from the The ca. 98–87 Ma early metaluminous suite is older than the dominant zircon rim age. These zircon U-Pb record to provide new and impor- composed of hornblende granodiorite and tonal- instances are listed in Table 1 and their spatial tant insights into the construction and destruc- ite and occurs primarily on the east side of, and distributions are shown in Figures 1, 4B, and 4C. tion of granitic batholiths in Idaho and place this as roof pendants within, the Atlanta lobe. The information within the context of the broader Atlanta peraluminous suite is compositionally Magmatic Flux and Areal Addition Rate Cordilleran magmatic arc. restricted biotite granodiorite and muscovite- Throughout this paper we use the magmatic bearing granite and makes up the bulk of the Knowledge of the age distributions and areas suite classification as described in Gaschnig et Atlanta lobe. The 76–68 Ma late metalumi- of different suites of the Idaho batholith allows al. (2010) for identifying different units of the nous suite is a belt of quartz diorite, tonalite, for the calculation of the apparent areal rate of Idaho batholith (see Fig. 1 for the geographic and hornblende granodiorite stocks forming a magmatic addition at different times during distribution).