Comparative isotopic and chemical geochronometry of monazite, with implications for U-Th-Pb dating by electron microprobe: An example from metamorphic rocks of the eastern Wyoming Craton (U.S.A.)

Polygenetic monazite grains in diverse Precambrian crystalline rocks from the Black Hills, South Dakota, have been analyzed in situ by ion and electron microprobe methods (SHRIMP and EMP), to evaluate the accuracy and precision of EMP ages determined using a new analytical protocol that incorporates improved background acquisition and interference corrections. Parallel evaluations were conducted by comparing EMP chemical and SHRIMP isotopic ages at regional-, rock-, and grainscales. The monazite data set includes 354 EMP chemical analyses from 26 grains in six metamorphic rocks, which resolve into 54 age-composition domains, and 31 SHRIMP isotopic ages from 13 grains in one of the rocks, with six grains microanalyzed in common by the two methods. The data set also includes monazite-bearing garnets in two of the rocks, whose isotopic compositions were analyzed using Pb stepwise-leaching (PbSL) methods. Both the EMP and SHRIMP data sets reveal a continuum of apparent monazite ages spanning a ~1790–1680 Ma timeframe, with a relatively high probability of ages at ~1755 and ~1715 Ma that correspond spatially to core and rim domains. PbSL ages of ~1742 and ~1734 Ma obtained from monazite-bearing garnet in two rocks are intermediate compared to the corresponding EMP ages, and are thereby interpreted as mixed ages. EMP data for two grains in the structurally deepest of the six rocks record ~1785 and ~1755 Ma ages in the cores and (higher-Y and lower-Th) rims, respectively, and these results are duplicated by SHRIMP ages in these and/or other grains from the same rock. Overall, the EMP, SHRIMP, and PbSL ages are internally consistent at the various scales of observation, which serves to validate EMP chemical dating as an accurate and precise method of discerning monazite age populations in polymetamorphic terrains. The EMP data set is interpreted geologically as reß ecting multiple episodes of monazite growth that are provisionally related to known metamorphic events in the Black Hills. Taking the most precise EMP data at face value, it is possible to resolve the timing of the two older events at ≤1784 ± 4 Ma (or ≤1786 ± 6 Ma) and 1756 ± 3 Ma (or 1753 ± 4 Ma), with 95% conÞ dence. These events are considered to be related to sequential episodes of N-directed thrusting and ~E-W compression associated with Paleoproterozoic crustal assembly in the mid-continent. A younger metamorphism, related to granite intrusion known to have occurred at 1715 ± 3 Ma, is dated independently at 1717 ± 2 Ma from the EMP monazite ages. DAHL ET AL.: MONAZITE GEOCHRONOLOGY OF THE BLACK HILLS 620 ration). Within the last decade, in situ monazite geochronometry has matured into a routine and viable approach for placing ages on thermotectonic events observable as microtextures in thin section. The earliest in situ studies employed U-Th-Pb isotopic dating by ion-microprobe methods (e.g., DeWolf et al. 1993; Vry et al. 1996; Zhu et al. 1997; Ireland and Gibson 1998; Catlos et al. 2002; etc.), and this technique has evolved into a widely accepted method of choice. Other recent studies have advanced U-Th-Pb chemical dating by electron-microprobe methods (EMP) as an alternative in situ technique (e.g., Suzuki et al. 1994: Montel et al. 1996, 2000; Braun et al. 1998; Crowley and Ghent 1999; Vavra and Schaltegger 1999; Williams et al. 1999; Zhu and OʼNions 1999a; Terry et al. 2000; Cocherie et al. 1998; Cocherie and Albarède 2001; Shaw et al. 2001; Williams and Jercinovic 2002; Pyle et al. 2003; and references therein). What emerges from these and other studies is that chemical dating of monazite by EMP methods offers potentially better spatial resolution and lower cost, on a “per-spot” basis, than isotopic dating by secondary ion mass spectrometry (SIMS; e.g., SHRIMP, Cameca IMS 1270, etc.). On the other hand, patterns of apparent Pb loss and/or mixed ages in monazite are readily discerned in analysis of SIMS isotopic data sets, wherein U-Pb concordia relationships also pinpoint corresponding isotopic ages that are anomalously young. In contrast, with EMP chemical dating of monazite there is no comparable means of assessing these possibilities. Given the rapid rise of EMP chemical dating of monazite as a complement to in situ SIMS or ion-probe isotopic methods, it is essential to establish its accuracy and precision on monazite of known U-Pb concordance relatively early in the history of this new technique. Other studies of monazite geochronology have certainly touched upon this issue (e.g., Williams et al. 1999; Terry et al. 2000; Kohn and Malloy 2004), having demonstrated general agreement between chemical and isotopic ages as parts of larger studies. This paper builds upon these and related studies by presenting chemical and isotopic ages (and uncertainties) obtained for selected monazites, and then comparing these data both spatially and graphically at various scales of observation (i.e., region, rock, and grain). In particular, the accuracy and precision of the EMP monazite ages are determined by comparing them to isotopic ages determined mostly by SHRIMP methods and to a lesser extent by Pb stepwise leaching techniques (PbSL; Frei and Kamber 1995). In a follow-up study (Dahl et al., in preparation), the ages, microtextures, and compositional zonation of these monazite generations will be integrated and interpreted in an effort to constrain Precambrian thermotectonic evolution in Black Hills exposures of the western mid-continent of the U.S. The present study is based on 34 grains of Paleoproterozoic monazite analyzed both isotopically and chemically in six polymetamorphosed rocks from the Black Hills, South Dakota (Fig. 1). The rock studied in greatest detail is a metapelite (sample PR-1) for which in situ SHRIMP and EMP analyses of monazite permit direct interand intra-grain comparisons of Pb/Pb and U-Th-Pb spot ages, respectively, and their uncertainties. For two other metapelites (samples ST-112 and SC-9a), EMP-derived ages of monazite occurring as matrix grains and as inclusions in garnet are compared to bulk Pb/Pb ages obtained by Pb stepwise leaching (PbSL) of monazite-bearing garnet. In addition, EMP ages obtained for monazite in these and the remaining three rocks (granitic gneisses BM-20 and LEG-8; metapelite EG-56) are assessed by comparing them to published isotopic ages of monazite and known ages of other minerals in the Black Hills. The value of a spot-by-spot approach to age comparison is inherently limited by the 2σ precision of individual SIMS-EMP analyses, coupled with the fact that different micro-volumes of monazite are inevitably sampled and excited by the ion and electron beams, respectively, during in situ microanalysis. To circumvent these limitations in pursuit of meaningful age comparisons, multiple EMP spot analyses of each compositional domain within single grains are averaged to characterize potential age domains, and each such cluster of analyses is screened statistically to verify that it represents a single age population. This approach greatly enhances the age precision of the chemical dating method. Analogously, each isotopic spot age actually FIGURE 1. Generalized geologic map of the crystalline core of the Laramide Black Hills uplift, showing locations of Archean granitoids and Proterozoic metasedimentary rocks sampled for this study. LEG = Little Elk granite; BFG = Biotite-Feldspar gneiss; BMG = granite at Bear Mountain. Left inset (after Redden et al. 1990) shows regional distribution of metamorphic isograds. Right inset is a generalized map showing location of the Black Hills relative to the Archean cratons [Wyoming (WC) and Superior (SC)] and Paleoproterozoic orogens [Black Hills (BHO; Dahl and Frei 1998), Trans-Hudson (THO, Sims et al. 1991), and Central Plains (CPO; Sims and Peterman 1986)]. CPO lies within Yavapai (YAV) arc terrane. Map modiÞ ed after DeWitt et al. (1989), Redden et al. (1990), and Dahl et al. (1999). DAHL ET AL.: MONAZITE GEOCHRONOLOGY OF THE BLACK HILLS 621 represents the combined result of Þ ve or more “burns” with the ion probe that involves depth-proÞ ling and sampling new material each time. Averaging EMP and SHRIMP analyses to infer a single age in each case permits a more rigorous comparison of chemical vs. isotopic ages than is possible using relatively imprecise individual spot analyses alone. Geological setting and previous geochronology Situated on the eastern ß ank of the Archean Wyoming craton, the Black Hills uplift exposes a crystalline core consisting of Neoarchean gneissic granite nonconformably overlain by thick packages of Paleoproterozoic sedimentary and igneous rocks (Fig. 1; Redden et al. 1990). The Neoarchean basement, exposed only in two erosional windows, comprises, for the most part, the 2595 Ma granite at Bear Mountain (BMG; McCombs et al. 2004), the 2560 Ma Little Elk granite (LEG), and the ≥2560 Ma biotite feldspar gneiss (BFG), which is intruded by the LEG (Gosselin et al. 1988). Two Paleoproterozoic rift successions of mainly sedimentary rocks dominate the crystalline core (Redden et al. 1990). The older succession (~2550–2480 Ma; Dahl et al. 2003) is conÞ ned to the LEG-BFG area (Fig. 1) and consists of sandstones, conglomerates, carbonates, and iron formations. The younger and areally predominant succession (~2015–1885 Ma) is a thick package of greywackes, pelites, sandstones, iron formations, carbonates, and alkalic tuffs locally interlayered with basalt or intruded by gabbro (Redden et al. 1990; Bekker et al. 2003). The Neoarchean basement and Paleoproterozoic cover sequences were intruded by the ~1720–1700 Ma Harney Peak leucogranite (HPG) and associated pegmatites (Riley 1970; Nabelek et al. 1992; Krogstad and Walker 1994); an early HPG sill is precisely dated at 1715 ± 3 Ma (U-Pb, monazite; Redden et al. 1990). The Black Hills crystalline core (Fig. 1) preserves a complex record of Paleoproterozoic thermotectonism and magmatism, and published ages of the principal events are summarized in Table 1 (see also Redden et al. 1990; Holm et al. 1997; Dahl et al. 1999). The main structural elements in the Black Hills include ENE-trending fold nappes/thrusts (F1) that were refolded into NNW-trending upright folds (F2) with a steep, locally penetrative, axial-planar foliation, S2 (Redden et al. 1990). The regional fold interference pattern (F1/F2) is most evident on the map scale just north and west of the Harney Peak dome (Fig. 1). The nappe/ thrusts originated from a N-vergent thin-skinned(?) thrusting event (D1) that deformed bedding (S0) without significant metamorphism in most of the Black Hills (Redden et al. 1990). Dahl et al. (1999) interpreted this event as having resulted from N-directed accretion of the Yavapai island-arc terrane (Fig. 1, lower right inset) possibly beginning as early as ~1780–1790 Ma. The F2 folds and S2 foliation were imposed during a subsequent episode of collisional crustal thickening (D2) and medium-P regional metamorphism interpreted to be associated with ~E-W collision of the Wyoming-Superior cratons (DeWitt et al. 1986, 1989; Terry and Friberg 1990). This major event in the so-called Black Hills orogen (BHO, Fig. 1) is provisionally dated between ~1770 and ~1730 Ma (Dahl and Frei 1998; Dahl et al. 1999), and was superseded by an early phase of unrooÞ ng (Holm et al. 1997) that culminated in localized, ~1720–1680 Ma doming (D3) associated with post-collisional HPG intrusion (Ratté and Zartman 1970; Ratté 1986; Redden et al. 1990; Duke et al. 1990). PostD3 events in the Black Hills included development throughout the crystalline core of a weak NE-trending fabric (S4) during a deformational event (D4) of uncertain age and origin (Redden et al. 1990) followed by an episode of renewed unrooÞ ng beginning at ~1500 Ma (Holm et al. 1997). Ultimately, current exposures of the Precambrian crystalline core were exhumed to the surface by no later than ~520 Ma (nonconformable deposition of upper Cambrian sandstone) and were re-exposed during a period of Laramide uplift and associated magmatism dated at ~60 Ma (Lisenbee 1978).