Centro de Oportunidades para la Mujer Proyectista: Arq Sharon Davis Design / Bruce Engel

Timing and geometry of Rodinian rifting continues to be debated, with different models placing various continental blocks along the western Laurentian margin (Li et al., 2008 and references therein). It is not clear what continent(s) have rifted away from the western margin of Laurentia, but possibilities include: Antactica, Australia, west Africa, and various portions of China (e.g., Moores, 1991; Hoffman, 1991, Dalziel, 1997; Sears and Price, 2003; Li et al., 2008; Evans, 2009; and Fu et al., 2015). The western

Laurentian margin has been appreciably modified by younger deformation and accretion, resulting in significant uncertainties regarding plate reconstruction. In addition to

improving the resolution of timing and number of magmatic events in the region, these dates may provide piercing points potentially useful for plate reconstruction.

CA-IDTIMS data presented here, along with ~726 Ma (Alexander, 2007) and ~676-692 Ma (Link and Christie-Blick, 2011; Link et al., 2014) show a prominent, at least 70 Ma period of magmatism across Idaho from ~720-649 Ma. In addition to the direct magmatic dates, age components from Cryogenian detrital zircon patterns indicate even more extensive volcanism. The combination of 224 CA-IDTIMS detrital analyses from the Pocatello Formation produce a unique record that further supports protracted rifting between ~720 and 649 Ma. Collectively, these dates are indicative of a cyclic, waxing-waning pattern of magmatism in at least four broad phases: an initial pulse at ~720 Ma, with renewed activity 1) between ~705 Ma and 693 Ma; 2) from ~686 to 681 Ma; 3) episode at ~675, 667, and 661 Ma; and 4) alkaline plutonism followed in the

period between ~654 to 649 Ma (Figure 13). It is unclear how this magmatism directly relates to the process of rifting or the identification of what conjugate margin may have been rifted away.

Details of regional rift processes have been complicated by younger geologic events, thus reconstruction of events is difficult. However, the cycles of magmatism are consistent with episodic, incomplete rift event models. In a symmetric-style of rifting, initially strong lithosphere may be weakened by dike intrusion during extension/thinning and subsequently strengthened as dikes solidified, thereby episodically slowing/reviving magmatism during extension. The lack of distinctly tilted strata in Utah, Nevada, and southern Idaho is interpreted as consistent with more symmetric-rifting (Yonkee et al., 2014). Alternatively, cyclic magmatic patterns may also be interpreted to be a product of progressive thinning of a lower-plate and underplating/intrusion forming arches in an upper-plate during an asymmetric-style of rifting (Lister, Etheridge, and Symonds, 1991). Sedimentary basins are created along low-angle detachments as the lower-plate margin thins and subsides across a broad shelf (Figure 14). Underplating the complementary upper-plate limits thinning and effectively forms a topographic high that separates local depositional basins and provides a source for juvenile detritus. This asymmetry may explain seemingly missing detrital sources as the original provenances may have drifted away on conjugate margins.

For example, the detrital record of southeastern Idaho does not possess significant ~665-649 Ma grains clearly correlative with Idaho magmatism. Assuming the magmatic bodies were part of an upper-plate arch and sufficiently exposed, they may have been cut off from the southeast and, instead, shed detritus more toward the west (i.e., the Pocatello

Formation was not in communication with igneous bodies of central Idaho). However, diamictite-bearing strata of southeastern Australia does possess ~662-653 Ma dates (Calver et al., 2013) that may correlate as a former lower-plate conjugate depositional basin from central Idaho (southeastern Australia was in communication with central Idaho). Similarly, southeastern Idaho may have represented a lower plate conjugate to Antarctic. Neoproterozoic strata in east Antarctica contain abundant Paleo- through Neoproterozoic detrital dates – including one with a prominent ~675 Ma peak – and associated ~668 Ma volcanic rocks (e.g., Goodge et al., 2002; Goodge et al., 2004) that potentially correlate with dates found in the Pocatello Formation and the diamictite of Daugherty Gulch.

5. Conclusions

The 693 and 667 Ma dates for the Hogback Rhyolite and diamictite of Daugherty Gulch place robust age constraints on diamictite-bearing strata in south-central Idaho. These dates support other findings that regional glacigenic strata are younger than other well dated Sturtian diamictite deposits, e.g., the ~716 Ma basal Upper Mount Harper Group in Canada (Macdonald et al., 2010), and the ~711 Ma basal Gubrah Formation in Oman (Bowring et al., 2007). Despite somewhat equivocal lithological correlations across a substantial unconformity, the 693 and 667 Ma dates are indicative that the

Sturtian Placer Creek – Daugherty Gulch diamictite was deposited over a period spanning at least 25 My.

CA-IDTIMS results revise previous volcanic and plutonic SHRIMP dates and uncertainties (Lund et al., 2003, 2010) from central Idaho: Hogback Rhyolite from 684 ±4 Ma to 693.03 ±0.73 Ma; tuff of Daugherty Gulch from 664 ± 6 Ma to 667.76 ±0.22

Ma; Acorn Butte suite from 665 ±6 Ma (with younger overgrowths at 638 ±13 Ma) to 652.43 ±0.17 Ma; Rush Creek Point suite from 651 ±5 Ma to 650.82 ±0.15 Ma; and Ramey Ridge Suite from 651 ±5 Ma to 649.26 ±0.16 Ma. Further, the discrepancy in calculated dates between in situ and IDTIMS analytic techniques highlight the need for CA-IDTIMS to accurately date deposits of this time and place.

Direct dating of these volcanic bodies, along with four new igneous samples, across Idaho extends Cryogenian magmatism from ~697 to 649 Ma and, by proxy, up to ~720 Ma. The upper ages, correlative with the ~719 to 716 Ma dates associated with the Mount Harper Volcanic suite and Franklin large igneous province of northwestern Canada (Macdonald et al., 2010), potentially link coeval early Cryogenian magmatism in the northwestern and central Laurentian margin. Collectively, dates determined in this study show that magmatism also non-systematically migrated across the region. When combined with other dated bodies across Canada, magmatism appears to have occurred in at least 18 different episodes, spanning at least 70 Ma across western Laurentia.

High-precision igneous ages determined in this study do no match provenance patterns predicted from CA-IDTIMS detrital compilation of Cryogenian deposits from the Pocatello Formation in southeastern Idaho (Isakson, 2017c). These data do not directly support the hypothesis that the deposits can be traced to central Idaho.

Alternatively, grains were possibly sourced from undiscovered bodies to the east, south, or, more likely, from rifted fragments. While data presented here are not diagnostic as provenance for local deposits, these high-precision dates may provide piercing points that are potentially correlative with other rifted continental blocks and provide a means to identify former conjugate pieces of the Laurentian margin.

Figure 4.1: Map of Neoproterozoic through Cambrian rock in Idaho (from Reed et al., 2012b) and general sample location.

Figure 4.2: CA-IDTIMS results from volcanic clast from cobble conglomerate of the Pocatello Formation, southeast Idaho (sample SMM08-1SG). A - Ranked 206_Pb/238_{U weighted mean plot of single zircons from SMM08-1SG. Solid bars} represent data used in age calculation. Horizontal orange bar represents calculated mean age and uncertainty. B - Corresponding SMM08-1SG Wetherill Concordia plots for CA-IDTIMS analysis.

Figure 4.3: Rhyolitic, pyroclastic sample Scout 3: cathodoluminescence image of representative zircons and, A - Ranked 206_Pb/238_{U weighted mean plot using only} <10% discordant LA-ICPMS analysis. Solid bars represent data used in age calculation. Horizontal grey bar represents calculated mean age and uncertainty. B - Ranked 206_Pb/238_{U weighted mean plot for CA-IDTIMS analysis of single zircons from} Scout 3. Blue bars represent analysis using 180°_{C chemical abrasion and red bars} using 200°_{C chemical abrasion. Solid bars represent data used in age calculation.} Horizontal orange bar represents calculated mean age and uncertainty. C - Scout 3 Wetherill Concordia plots for CA-IDTIMS analysis without chemical abrasion (black), 180°_{C chemical abrasion (blue), and 200}°_{C chemical abrasion (red).}

Figure 4.4: Rhyolitic, pyroclastic sample Scout 3: cathodoluminescence image of representative zircons and, A - Ranked 206_Pb/238_{U weighted mean plot using only} <10% discordant LA-ICPMS analysis. Solid bars represent data used in age calculation. Horizontal grey bar represents calculated mean age and uncertainty. B – Ranked 206_Pb/238_{U weighted mean plot for CA-IDTIMS analysis of single zircons from} Scout 2. Blue bars represent analysis using 180°_{C chemical abrasion and red bars} using 200°_{C chemical abrasion. Solid bars represent data used in age calculation.} Horizontal orange bar represents calculated mean age and uncertainty. C - Scout 2 Wetherill Concordia plots for CA-IDTIMS analysis without chemical abrasion (black), 180°_{C chemical abrasion (blue), and 200}°_{C chemical abrasion (red).}

Figure 4.5: Hogback Rhyolite Member sample 15VI003: cathodoluminescence image of representative zircons and, A - Ranked 206_Pb/238_{U weighted mean plot using} only <10% discordant LA-ICPMS analysis. Solid bars represent data used in age calculation. Horizontal grey bar represents calculated mean age and uncertainty. B - Ranked 207_Pb/206_{Pb weighted mean plot for CA-IDTIMS analysis of single and split} grain zircons from 15VI003. Solid bars represent data used in age calculation. Horizontal orange bar represents calculated mean age and uncertainty. CA represent grains only Chemically Abraded and AA are grains that were both Air Abraded and chemically abraded. C - 15VI003 Wetherill Concordia plot and intercept age for CA- IDTIMS analysis with chemical abrasion (red) and with both air and chemical abrasion (green).

Figure 4.6: Lithic tuff of Daugherty Gulch sample DG-1: cathodoluminescence image of representative zircons and, A - Ranked 206_Pb/238_{U weighted mean plot using} only <10% discordant LA-ICPMS analysis. Solid bars represent data used in age calculation. Horizontal grey bar represents calculated mean age and uncertainty. B - Ranked 206_Pb/238_{U weighted mean plot for CA-IDTIMS analysis of single zircons from} DG-1. Solid bars represent data used in age calculation. Horizontal orange bar represents calculated mean age and uncertainty. C - DG-1 Wetherill Concordia plot for CA-IDTIMS analysis. Solid error ellipses used in age calculation, open ellipse not used.