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A, Map showing relationship of the Black Hills, South Dakota, to major cratonic blocks and the Pans-Hudson orogen (THO; after Hoffman, 1990). Inset shows location of the map within North America. The dates show model mantle-extraction ages, based mostly on Sm-Nd isotopic data, for Precambrian rocks within each tectonic province. B, Interpretation of the COCORP transect indicated in part A by Baird et al. (1996), showing the Wyoming province, Dakota block, and wedge of continental arc rocks. The depths shown are below the present surface. BH, Black Hills.
Source publication
Intrusion and crystallization of the Harney Peak Granite and associated plutons and pegmatites were the culminating events of the Trans-Hudson orogeny as expressed in the Black Hills. The granite was emplaced as thousands of sills and dikes at 1715 Ma, following an approximately 45 m.y. period of regional metamorphism and deformation of now exposed...
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... of the southern Ti-ans- Hudson orogen is now buried under Phanerozoic sedimentary rocks. However, it is exposed in the Black Hills as the result of the Laramide orogeny, and these rocks provide a window on Prot- erozoic processes that occurred in the region ( Redden et al., 1990; Fig. 1, Thble 1). In Canada, the Trans-Hudson terrane contains relatively juvenile lithologies (such as accreted island-arc ter- ranes) that generally yield dates > 1800 Ma for the orogeny (Lewry and Collerson, 1990). In contrast, metamorphic rocks in the south- ern Black Hills are dominantly quartzites and quartz-muscovite- biotite ...
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... orogen just south of the U.S.-Canadian border, interpreted with the aid of dates from basement cores, suggests that there may have been a small Archean crustal block (the Dakota block) between the two cratons ( Baird et al., 1996). The Dakota block, rather than the Superior craton, collided di- rectly with the Wyoming craton (Baird et al., 1996; Fig. 1). The dominantly metasedimentary se- quences and leucogranites in the Black Hills prob- ably are within what Baird et al. (1996) identified as a wedge of continental arc plutonic rocks between the Wyoming province and the Dakota block. Redden et al. (1990) obtained a concordant 1715 f 3 Ma U-Pb age on a monazite taken from a thin ...
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... the source of the 10wS'~o suite. Krogstad and Walker (1996) ob- tained a range of &Na (1715 Ma) of -6.4 to -9.9 for ten samples of the 1 0 ~ 4 ~ ~ 0 suite and -2.0 to -7.7 for ten samples of the high-6180 suite (Fig. 7). One sample of the high- 6l80 suite has an unusually low average value of -12.6, whereas the higher values for that suite over- Geology, v. 34, no. 2, p . 165-181, 13 figs., 2 tables, Novembm, 1999 Q ~ ~ I I I I I I I I I I ~ 6 8 7 0 7 2 7 4 7 6 7 8 8 0 sio, (wt. ...
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... able because there is no evidence that the magma ascended as a crystal mush. The types of source rocks and melt-producing reactions are indicated by minor and trace elements in the HPG. For ex- ample, biotite-containing granites have on average higher concentrations of TiO, and lower concen- trations of B than the tourmaline-containing gran- ites (Fig. 10). The contrasting B/TiO, ratios are thought to indicate that the high-6l80 granites were generated by dehydration-melting of metapelites that involved only the breakdown of muscovite, the major site of B in the metapelites at melting condi- tions. Generation of most of the 10w-6~~0 granites, in contrast, also involved the breakdown of ...
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... and Thompson, 1988), which lies at tem- peratures higher than the muscovite dehydration- melting reaction 2 (Patiiio-Douce and Harris, 1998) below ~ 1 0 kbar. However, trace elements that are controlled by major minerals during melting, in- cluding Rb, Cs, Sr, Ba, Sc, Cr, Zn, and Th, have simi- lar concentration ranges in both granite suites (Fig. 11) Fignre 8. Oxygen-isotope isotherm plot for a granite sample. Regression of observed fractionations in terms of experimentally determined fractionation coefficients gives a temperature. Error on the regression indicates deviation from isotopic ...
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... orogen as interpreted from seismic studies; (2) available dates for re- gional metamorphism, deformation, and HPG em- placement; (3) petrogenetic and emplacement conditions of the granite; and (4) character of its source rocks. We use numerical modeling to re- construct crustal pressure-temperature-time paths that led to genesis of the granite (Fig. 12). Salient constraints on the model include the following ...
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... of both Archean and Proterozoic crustal sources in the same region of the crust. B, Metasedimentary rocks in the Black Hills have Proterozoic model mantle-extraction ages, suggesting derivation from young crustal rocks such as island arcs. The metasedimentary rocks were thrust up over the Wyoming craton as sug- gested by the COCORP cross section (Fig. 1) IXgum 9. Histograms of apparent oxygen-isotope equili- bration temperatures in the HPG based on the isotherm method (Fig. 8 ) . Note that for many samples tempera- tures are nearly 8OO0C, implying that this is the mini- mum temperature of melt formation in the source ...
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... the loga- rithmic scale for B. High boron concentrations imply breakdown of muscovite during melting, and high TiO, concentrations imply break- down of biotite. Geology, v. 34, no. 2, 165-181, 13figs., 2 tables, November, 1999Hudson orogen is N45 km. How- ever, during initial stages of re- gional M, metamorphism, these rocks were at a depth of 20-25 km, indicating that the total thickness of the crust may have reached approximately 70 km during the collision. ...
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... is N45 km. How- ever, during initial stages of re- gional M, metamorphism, these rocks were at a depth of 20-25 km, indicating that the total thickness of the crust may have reached approximately 70 km during the collision. The essential component in our model is shear-heating along a shear zone in thickened crust that was undergoing unroofing (Fig. 13). Shear-heating is per- haps the most feasible way to pro- duce sufficiently high temperatures for dehydration- melting reactions in relatively shallow parts of the thickened Trans-Hudson orogen. Meta- Osedimentary rocks with short crustal histories most probably were located in such parts (Nabelek et al., in review). An- other way to ...
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... simulated evolving geotherms and P-T-t paths of the crust (Fig. 12) using numerical techniques outlined by Liu and Furlong (1993). The model pa- rameters selected are listed in Thble 2. The total initial thickness of the model lithosphere is 125 km with total crust thickness of 70 km, although location of the Moho plays no role in the model thermal structure. Thermal boundary conditions are fixed at ...
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... exponential decay of radioactive heat produc- tion (A) with depth (A = AoeZlD), where Z is depth and D is taken to be 10 km. Crustal thickening during orogeny is approximated by thrusting a 35 km crustal sheet onto the reference lithosphere, although in reality crustal thick- ening is often accommodated by a system of imbricate listric faults (Fig. 13). It is tempting to suggest that the north-northwest-striking faults in the Black Hills are subvertical expressions of a listric thrust system, although the ...
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... Hills Figure 13. A, Schematic drawing, based on COCORP cross section (Fig. l), showing presumed source region of HPG within a ductile shear zone at the interface located where Proterozoic metasedimentary rocks were thrust over the Archean Wyoming basement. ...
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... folding of the metasedimentary rocks occurred prior to melting. B, Alternative schematic drawing (analogous to part A) that may be more consistent with evidence for Archean basement bounding the Black Hills to the east (DeWitt et al., 1986). ing of movement on the fault sys- tems is not well understood. The initial geotherm i n the lower plate (Fig. 12A) is at steady state, characterized by parameters listed in Thble 2. In contrast, the initial temperature of the bottom 20 km of the upper plate is arbi- trarily set at 250" C, so that initial heating of relatively cold, thrusted, sedimentary sequences can be approximated. It is noted, however, that within 10 million years after ...
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... assumed that thrusting of sedimentary sequences over the basement continued for 60 mil- lion years. The duration is given by the approximate time it takes for rocks at N7 kbar to reach the garnet isograd (15 m.y.; Fig. 12B) plus the time difference between the oldest dates for regional meta- morphism and intrusion of the HPG (45 m.y.). The chosen rate of thrusting is 3 cm y', which is appropriate for rates of crustal convergence. The resulting inte- grated period of thrusting permits stacking of the thrusted material to thicken the upper plate along ...
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... calculated evolving geotherms are shown in Fig. 12A. They show the enhancement of temperatures to nearly 800" C near the shear zone while thrust- ing continued. They also show that it is possible to develop a n inverted sequence of isograds be- low the thrust fault, as is observed below the Main Central Thrust in the Himalayas (Hubbard, 1989). However, the temperature anomaly due to ...
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... P-T-t paths for four ini- tial depths, with 10 m.y. intervals marked, are shown in Fig. 12B. The figure also includes the stau- rolite-in and garnet-in (for garnet with Mn/(Mn + Fe + Mg) = 0.4; Friberg et al., 1996;Helms and Labotka, 1991) isograds (Spear and Cheney, 1989), stability fields for the alumino- silicates, and relevant dehydration-melting reac- tions. The most important feature reproduced by the combined ...
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... proposed model (Figs. 12 and 13) explains the isotopically distinct suites of leucogranites ob- served in the Black Hills. Source rocks in the lower part of the shear zone, within the Archean crust of the Wyoming province, would have reached the mus- covite + biotite dehydration melting reaction. The reaction would have led to relatively high TiO, con- centrations, as ...
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... require source rocks with mostly Proterozoic T, , ages of source rocks. It is noted, however, that the two dis- tinct melting reactions for the two granite suites in the model are possible in part by the arbitrary lo- cation of the shear zone at 35 km, which happens to be at the approximate pressure of the intersec- tion of the melting reactions (Figs. 3 and ...
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... schematic cross section in Fig. 13A is based in part on the COCORP study of Baird et al. (1996), which indicates a fairly wide wedge of Proterozoic rocks near the U.S.-Canada border. However, in the Black Hills geophysical evidence indicates that the foliated Archean Little Elk Creek Granite, exposed along the eastern margin of the Precambrian ter- rane, is quite a ...
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... of the Dakota block. Thus, the wedge of Proterozoic metasedimentary rocks may be much narrower here than at the latitude of the COCORP cross section. Therefore, an alternative cross sec- tion consistent with the proposed shear-heating model for generation of the HPG and perhaps more in accord with the available geologic evidence is shown in Fig. ...
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Citations
... In the exposed sTHO in Canada, metamorphism has been interpreted to be associated with the terminal collision of the Superior and Churchill Provinces (Corrigan et al., 2007(Corrigan et al., , 2009). Similar models have been proposed for the Black Hills (Dahl et al., 2005;Kilian et al., 2016), although the pre-1715 Ma metamorphic record is somewhat obscured by the thermal overprint associated with emplacement of the 1715 ± 3 Ma Harney Peak Granite (Hrncir et al., 2017;Nabelek et al., 1999;Redden et al., 1990). The difference in timescales ( Figure S14 in Supporting Information S1) and P-T paths, as well as the generally earlier timing of peak metamorphism in sample 21IWH16 suggest different drivers of peak metamorphism: collision in the sTHO of Canada and the Black Hills versus back-arc processes in sample 21IWH16. ...
Plain Language Summary
The Trans‐Hudson orogeny was a Himalayan‐scale collisional mountain building event approximately 1.8 billion years ago that formed the core of North America. The southern portion of the Trans‐Hudson orogen (THO) is poorly understood because it is almost entirely buried by younger rocks. It is unknown whether the southern THO is underlain by Archean or Proterozoic crust. The timing, conditions, and setting of high‐grade metamorphism in this region are also poorly known. We carried out petrologic, geochemical, and geochronologic analyses on a metasedimentary rock sample recovered from drill core. Detrital zircon and tracer isotopic data show derivation from both Paleoproterozoic and Archean sources. This suggests that Paleoproterozoic magmatic arc(s) developed on or near Archean crust. The rock sample experienced extreme temperatures (≥900°C, ≥1 GPa) and partial melting between approximately 1870 and 1850 million years ago perhaps in a back‐arc tectonic setting. This marks the first recognition of extreme, ultra‐high temperature metamorphism in the THO. Later retrograde metamorphism occurred in the Mesoproterozoic and Phanerozoic. These constraints provide new understanding of the enigmatic southern THO and highlight the ability of a combined petrologic, geochemical, and geochronologic study to extract a comprehensive record of crustal evolution from a single sample.
... The former (e.g. Uchi, Winnipeg River, Wabigoon, Wawa-Abitibi subprovinces) are considered Dawson, 1966;Bourne & Danis, 1987;Feng & Kerrich, 1991Feng et al., 1993;Mulja et al., 1995aMulja et al., , 1995bDucharme et al., 1997. 2 Allison Lake batholith Boundary of English Southwick & Sims, 1979;Percival et al., 1985;Day & Weiblen, 1986;Sawyer & Barnes, 1988;Percival & Williams, 1989;Southwick, 1991 14 Bayley et al., 1973;Stuckless et al., 1985;Stuckless, 1989;Hausel (continued) Green et al., 1968;Kretz, 1968Kretz, , 1982Kretz, , 1985Green & Baadsgaard, 1971;Drury, 1979;Meintzer et al., 1984;Meintzer, 1987;Davis & Bleeker, 1999;Palmer et al., 2016;Palmer, 2018 20 Davis, 1992;Davis & Hegner, 1992;King et al., 1992;van Breemen et al., 1992a;Davis et al., 1994 Yilgarn Craton Sinceni pluton associated Blamart et al., 1993;Maphalala & Krö ner, 1993;Trumbull, 1993; Meyer et al., Bostock et al., 1987Bostock et al., , 1991Bostock & Loveridge, 1988;Thé riault, 1992;van Breemen et al., 1992b; (continued) Redden et al., 1982Redden et al., , 1990Duke et al., 1992;Nabelek et al., 1992aNabelek et al., , 1992bNabelek et al., , 1999Nabelek et al., , 2001 Mesoproterozoic SW USA micro-continental fragments or arc-related plutonic-volcanic rocks, which evolved independently between 3Á6 and 2Á7 Ga. The latter (e.g. the English River, Quetico, and Pontiac subprovinces) are dominated by variably metamorphosed greywackes and volcanogenic turbidites, but also contain pelagic and chemical sediments such as carbonates, chert, and mudstones, and fragments of greenstone lithologies (Percival & Williams, 1989;Breaks & Moore, 1992;Davis, 2002). ...
... The Trans-Hudson Orogeny (THO) was a major collisional event closing the Manikewan Ocean and suturing the Superior Province with the Wyoming and Rae-Hearne cratons at c. 2Á0-1Á8 Ga, and marks the largest Paleoproterozoic orogenic belt in the world (Corrigan et al., 2009). Two well-documented occurrences of SPGs occur in the THO, including those in the Wollaston domain of Saskatchewan (Annesley et al., 2005;McKechnie et al., 2012aMcKechnie et al., , 2012b and the Harney Peak Granite and satellite intrusions in the Black Hills of South Dakota (Redden et al., 1982(Redden et al., , 1990Duke et al., 1992;Nabelek et al., 1992aNabelek et al., , 1992bNabelek et al., , 1999Nabelek et al., , 2001Shearer et al., 1992). The Wollaston domain comprises metasedimentary rocks deposited unconformably on the margins of the Hearne craton during rifting and formation of the Manikewan Ocean in the early Proterozoic (Madore & Annesley, 1996). ...
... The Harney Peak granite (1715 6 3 Ma; Redden et al., 1990) marks the culmination of the Trans-Hudson Orogeny during collision of the Wyoming Province and the western boundary of either the Superior Craton or the Dakota block (DeWitt et al., 1986;Nabelek et al., 1999). This collision resulted in the thrusting of Proterozoic metasedimentary rocks to the west over the Wyoming Craton and low-pressure (0Á3-0Á4 GPa), hightemperature regional metamorphism of late Archean and early Proterozoic (deposited 2100-1880 Ma) metasedimentary rocks exposed around and within the granite (Redden et al., 1982(Redden et al., , 1990DeWitt et al., 1986;Helms & Labotka, 1991;Nabelek & Bartlett, 1998). ...
Strongly peraluminous granites (SPGs) form through the partial melting of metasedimentary rocks and therefore represent archives of the influence of assimilation of sedimentary rocks on the petrology and chemistry of igneous rocks. With the aim of understanding how variations in sedimentary rock characteristics across the Archean–Proterozoic transition might have influenced the igneous rock record, we compiled and compared whole-rock chemistry, mineral chemistry, and isotope data from Archean and Paleo- to Mesoproterozoic SPGs. This time period was chosen as the Archean–Proterozoic transition broadly coincides with the stabilization of continents, the rise of subaerial weathering, and the Great Oxidation Event (GOE), all of which left an imprint on the sedimentary rock record. Our compilation of SPGs is founded on a detailed literature review of the regional geology, geochronology, and inferred origins of the SPGs, which suggest derivation from metasedimentary source material. Although Archean and Proterozoic SPGs are similar in terms of mineralogy or major-element composition owing to their compositions as near-minimum melts in the peraluminous haplogranite system, we discuss several features of their mineral and whole-rock chemistry. First, we review a previous analysis of Archean and Proterozoic SPGs biotite and whole-rock compositions indicating that Archean SPGs, on average, are more reduced than Proterozoic SPGs. This observation suggests that Proterozoic SPGs were derived from metasedimentary sources that on average had more oxidized bulk redox states relative to their Archean counterparts, which could reflect an increase in atmospheric O2 levels and more oxidized sedimentary source rocks after the GOE. Second, based on an analysis of Al2O3/TiO2 whole-rock ratios and zircon saturation temperatures, we conclude that Archean and Proterozoic SPGs formed through partial melting of metasedimentary rocks over a similar range of melting temperatures, with both ‘high-’ and ‘low-’temperature SPGs being observed across all ages. This observation suggests that the thermo-tectonic processes resulting in the heating and melting of metasedimentary rocks (e.g. crustal thickening or underplating of mafic magmas) occurred during generation of both the Archean and Proterozoic SPGs. Third, bulk-rock CaO/Na2O, Rb/Sr, and Rb/Ba ratios indicate that Archean and Proterozoic SPGs were derived from partial melting of both clay-rich (i.e. pelites) and clay-poor (i.e. greywackes) source regions that are locality specific, but not defined by age. This observation, although based on a relatively limited dataset, indicates that the source regions of Archean and Proterozoic SPGs were similar in terms of sediment maturity (i.e. clay component). Last, existing oxygen isotope data for quartz, zircon, and whole-rocks from Proterozoic SPGs show higher values than those of Archean SPGs, suggesting that bulk sedimentary 18O/16O ratios increased across the Archean–Proterozoic boundary. The existing geochemical datasets for Archean and Proterozoic SPGs, however, are limited in size and further work on these rocks is required. Future work must include detailed field studies, petrology, geochronology, and constraints on sedimentary source ages to fully interpret the chemistry of this uniquely useful suite of granites.
... The Trans-Hudson orogen formed during the assembly of the Hearne, Wyoming, and Superior cratons into the core of Laurentia following the closure of the Manikewan Ocean (Hoffman, 1988;Lewry & Stauffer, 1990;Weller & St Onge, 2017). Although primarily located in Canada, the Trans-Hudson orogen extends into the northern USA, where the final suturing is marked by the Harney Peak granite ($1715 Ma) and other satellite intrusions in the Black Hills of South Dakota (Redden et al., 1990;Nabelek et al., 1999). We examined previously published mineral and whole-rock data for the Harney Peak granite for nine samples (Nabelek et al., 1992a), and we re-analyzed four of those samples (provided by P. Nabelek) for major element mineral chemistry. ...
We constrain the oxygen fugacity (fO2) of strongly peraluminous granites (SPGs; i.e., granites (sensu lato) generated through the partial melting of sediments) across the Archean-Proterozoic boundary, which coincides roughly with the Great Oxygenation Event (GOE), in order to understand whether secular changes in atmospheric O2 levels may be imprinted on the fO2 of igneous rocks. SPGs were chosen to maximize the potential effects of sediments in their sources on the fO2 of the magmas. We studied 28 Archean (2685-2547 Ma) and 31 Meso- to Paleoproterozoic (1885-1420 Ma) geographically distributed samples from North America, spanning two cratons (Superior and Wyoming) and both orogenic and anorogenic Proterozoic provinces (Trans-Hudson Orogen, Wopmay Orogen, and SW United States). We present an analysis of both new and previously published whole-rock major and trace element data and mineral major element chemistry from the samples. All the studied samples are peraluminous high-silica plutonic rocks (all contain >67 wt % SiO2, and 92% are true granites with >69 wt % SiO2), and biotite+muscovite±garnet±tourmaline±sillimanite are the primary aluminous minerals in all samples. Whole-rock major element and trace element abundances of all samples are consistent with derivation by partial melting of aluminous sediments. In order to constrain the fO2 of crystallization of the SPGs, we developed an alphaMELTS-based method that takes advantage of the sensitivity of biotite FeT/(FeT+Mg) ratios to fO2. This method is able to reproduce experimental and empirical data where biotite compositions and whole-rock compositions, pressures and temperatures of crystallization and fO2are known.
For the SPGs in this study, alphaMELTS modeling indicates that 68% of Proterozoic samples crystallized at an fO2 between NNO -1 to NNO +1.1, whereas the remaining Proterozoic samples (32%) and most of the Archean samples (75%) crystallized at ≤NNO -2. The simplest explanation of these results is that the Proterozoic SPGs were derived from metasedimentary source rocks that on average had more oxidized bulk redox states relative to their Archean counterparts. The bulk redox state of the metasedimentary source rocks of SPGs of all ages is defined by the relative abundances of oxidized (e.g., Fe³⁺ and S⁶⁺) and reduced (e.g., organic matter) material. The crystallization of both Archean and Proterozoic samples at fO2 values of ≤NNO -2 is consistent with them having their fO2 buffered by graphite (formed from organic carbon) in their source regions. However, the dominantly low fO2 (≤NNO -2) values of the Archean SPGs plausibly reflects the presence of organic material and relatively reduced metasedimentary rocks in their source region prior to the GOE. In contrast, the elevated fO2 values of the majority of the Proterozoic SPGs may reflect enhanced sulfate contents or increased Fe³⁺/FeT in sediments after the GOE which, in terms of the bulk redox state of their metasedimentary source region, would have offset the reducing nature of organic matter present there.
... A possible analog for this kind of infl ation may be the splaying of ghost stratigraphy defi ned by enclave trains in the Main Donegal Granite pluton into wider zones than in the host rocks, as mapped (but not then so interpreted) by Pitcher and Read (1959). The intrusion complex represented by the Harney Peak Granite and associated pegmatites in South Dakota is another analog (Nabelek et al., 1999;Redden and DeWitt, 2008). Infl ation of the host-rock structure in the Ruby Mountains by injection of primarily subhorizontal sheets may have been accompanied by downward displacement of the lower part of the framework as partial melt was transferred from deeper crustal levels (cf. ...
Gneissic pegmatitic leucogranite forms a dominant component (>600 km3) of the midcrustal infrastructure of the Ruby Mountains-East Humboldt Range core complex (Nevada, USA), and was assembled and modified episodically into a batholithic volume by myriad small intrusions from ca. 92 to 29 Ma. This injection complex consists of deformed sheets and other bodies emplaced syntectonically into a stratigraphic framework of marble, calc-silicate rocks, quartzite, schist, and other granitoids. Bodies of pegmatitic granite coalesce around host-rock remnants, which preserve relict or ghost stratigraphy, thrusts, and fold nappes. Intrusion inflated but did not disrupt the host-rock structure. The pegmatitic granite increases proportionally downward from structurally high positions to the bottoms of 1-km-deep canyons where it constitutes 95%-100% of the rock. Zircon and monazite dated by U-Pb (sensitive high-resolution ion microprobe, SHRIMP) for this rock type cluster diffusely at ages near 92, 82(?), 69, 38, and 29 Ma, and indicate successive or rejuvenated igneous crystallization multiple times over long periods of the Late Cretaceous and the Paleogene. Initial partial melting of unexposed pelites may have generated granite forerunners, which were remobilized several times in partial melting events. Sources for the pegmatitic granite differed isotopically from sources of similar-aged interleaved equigranular granites. Dominant Late Cretaceous and fewer Paleogene ages recorded from some pegmatitic granite samples, and Paleogene-only ages from the two structurally deepest samples, together with varying zircon trace element contents, suggest several disparate ages of final emplacement or remobilization of various small bodies. Folded sills that merge with dikes that cut the same folds suggest that there may have been in situ partial remobilization. The pegmatitic granite intrusions represent prolonged and recurrent generation, assembly, and partial melting modification of a batholithic volume even while the regional tectonic environment varied dramatically from contractile thickening to extension and mafic underplating.
... Several studies have documented the orogenic processes and aspects of metamorphism in the Black Hills (e.g., Duke et al. 1990b;Redden et al. 1990;Terry and Friberg 1990;Helms and Labotka 1991;Nabelek et al. 1992Nabelek et al. , 1999Nabelek et al. , 2001Friberg et al. 1996;Holm et al. 1997;Dahl et al. 1999Dahl et al. , 2005aDahl et al. , 2005b. In * E-mail: nabelekp@missouri.edu this paper, we synthesize the previous work with new mineral compositional data, textures, geochronological data, and phaseequilibria calculations, including pseudosections, to refi ne previous views of the pressure, temperature, and fl uid conditions that existed during syncollisional regional and subsequent contact metamorphism by the late-orogenic Harney Peak Granite. ...
Late Archean and Early Proterozoic continental margin pelites, graywackes, and quartzites in today's Black Hills were regionally metamorphosed during the collision of the Archean Wyoming and Superior provinces beginning at similar to 1755 Ma. During east-west regional compression, metamorphism reached incipient garnet-biotite grade conditions at similar to 400 degrees C. Unzoned garnet (Sps(44)Alm(44)Pyr(3)Grs(9)) characterizes the mineral assemblage in graphitic metapelites. The low-temperature onset of the garnet-biotite assemblage is attributed to high Mn concentrations and low a(H2O) due to the presence of CH4 and CO2. During late stages of the collision, the rocks were intruded by the Harney Peak leucogranite (HPG). Vigorous fluid flow, evidenced by abundant quartz veins, metasomatism, and consumption of graphite from metapelites, occurred in the granite aureole. The lowest-grade aureole assemblage includes chlorite that overgrows the early regional foliation and new, clear, almandine rims on garnet. Higher-grade facies include staurolite, then sillimanite, and finally second-sillimanite. The prograde, mineral assemblages are consistent with calculated pseudosection assemblages for the average metapelite. Beginning with the sillimanite zone, almandine garnet is mostly inclusion-free. Textures suggest that this new garnet grew after dissolution of the old garnet, for which the only remaining evidence are remnant inclusions or coarse quartz-biotite clots. The new garnet grew in response to more elevated temperatures. Garnet compositions projected onto the pseudosection indicate nearly isobaric heating between 3.5-4.5 kbar during prograde metamorphism. Occasional replacements of staurolite by muscovite and biotite are attributed to infiltration of the rocks by K-bearing magmatic fluids. Andalusite occurs as euhedral crystals in quartz veins or as late poikiloblasts along foliations planes, where it appears to have grown while Si-rich fluids passed through the rocks. Typically, it replaces earlier muscovite, suggesting that the fluids had low a(H2O). The andalusite indicates decompression of the still hot, buoyant, fault-bounded block that included the HPG magma, relative to surrounding blocks in the terrane. This decompression contributed to the high-T/low-P conditions that existed in this portion of the thickened orogen and suggests that the various fault-bounded blocks in the Black Hills may have had different P-T histories.
... The HPG is a peraluminous leucogranite (average aluminum saturation index ϭ 1.2) that was generated 1715 Ma ago at the culmination of the collision of the Superior and Wyoming cratons during the Trans-Hudson orogeny . The granite, pegmatites, and surrounding metamorphic rocks were the subject of several structural, geochemical, chronological, and petrologic studies aimed at understanding the petrogenesis of the HPG granite and the Proterozoic thermotectonic history of the southern Black Hills (Duke et al., 1990;Norton and Redden, 1990;Nabelek et al., 1992aNabelek et al., , 1992bNabelek et al., , 1999Krogstad et al., 1993;Holm et al., 1997;Dahl et al., 1999). ...
... The HPG was generated by partial melting of protoliths similar to the exposed metasedimetary rocks, on the basis of its peraluminous, low-Ca character, as well as radiogenic, stable isotope, and trace element compositions (Nabelek and Bartlett, 1998;Nabelek et al., 1999). By the time of granite emplacement, the metamorphic rocks were between 3.5 and 4 kbar (Helms and Labotka, 1991) and had cooled to Ͻ500°C (Holm et al., 1997). ...
... Nabelek et al., 1992a). The calculated P-T-t path is from Nabelek et al. (1999Nabelek et al. ( , 2001. The marks on the P-T-t path are 10-Ma intervals. ...
A microthermometric study of inclusions in granites and pegmatites in the Proterozoic Harney Peak Granite system identified four types of inclusions. Type 1 inclusions are mixtures of CO2 and H2O and have low salinities, on average 3.5 wt.% NaCleq; type 2 inclusions are aqueous solutions of variable salinities, from 0 to 40% wt.% NaCleq; type 3 inclusions are carbonic, dominated by CO2, with no detectable water; and type 4 inclusions consist of 20 to 100% solids, with the remaining volume occupied by a CO2-H2O fluid. Many inclusions have a secondary character; however, a primary character can be unambiguously established in several occurrences of the type 1 inclusions. These inclusions were trapped above the solidus and represent the exsolved magmatic fluid. The secondary populations of types 1, 2, and 3 probably formed as a result of reequilibration and unmixing of the type 1 fluid that progressively changed composition and density with decreasing temperature and pressure and was finally trapped along healed microfractures under subsolidus conditions. Type 4 inclusions are primary and are interpreted to be trapped, fluid-bearing, complex silicate melts that subsequently solidified or underwent other posttrapping changes.
... A comprehensive summary of the current knowledge of the conditions of metamorphism in the southern Black Hills is given by Nabelek et al. (1999) and Dahl et al. (1999). Therefore, only a brief overview is given here, with the addition of some new information for the lower-grade rocks in the northern part of the Proterozoic terrane. ...
The abundance and distribution of B and Li in metasedimentary rocks and in the Harney Peak Granite (HPG) in the Black Hills, South Dakota, U.S.A., elucidate the behavior of these two elements during (1) regional metamorphism that began at ~1760 Ma; (2) subsequent contact metamorphism associated with emplacement of the HPG and associated pegmatites at ~1715 Ma; and (3) anatexis leading to production of the granite. There is no clear evidence for changes in B and Li concentrations with progressive regional metamorphism of the metapelites from chlorite-biotite grade up to staurolite grade. Rocks outside the pegmatite aureole that surround the HPG have average B contents of ~80 ppm. There is no correlation of B with other elements in the pelitic schists, which indicates that none of the major phases is the dominant host for B. Boron content is mainly con-trolled by small amounts of randomly distributed tourmaline that was identified in alpha-track maps of thin sections. In the aureole of the granite, B concentrations are depleted in many samples. The depletion is attributed to consumption of tourmaline during interaction of the rocks with alkalic fluids. The average Li contents in metapelites far away from the HPG are also ~80 ppm. Near the HPG, Li concentrations reach roughly 190 ppm, indicating significant metasomatism by fluids that emanated from the granite and pegmatites. The concentrations of B and Li in the low-grade metapelites are sufficient to explain their abundances in the HPG, if the granite formed by partial melting of the metapelites and if there was total breakdown of tourmaline. Stabilization of tourmaline in leucogranites is not necessarily related to sources enriched in B, but may be related to its competition for Fe and Mg with biotite.
... Redden et al. (1990) obtained a discordant U-Pb zircon age of 1728 Ma and a concordant 1715 Ma monazite age for the HPG. The metamorphism, deformation, and granite generation are related to the Trans-Hudson orogeny (Nabelek et al. 1999 ...
Proterozoic migmatites in the Black Hills, South Dakota, were examined to determine if partitioning of trace elements hosted
by rock-forming minerals between melanosomes and leucosomes preserves residue-melt equilibrium that was presumably established
during partial melting of pelitic lithologies. Granitic leucosomes in the Black Hills, as is often the case elsewhere, have
positive Eu anomalies, are highly enriched in Sr and Ba, and are depleted in Rb and Cs relative to granites derived by partial
melting of metapelites. Distributions of these trace elements between melanosomes and leucosomes cannot be simulated using
mineral-melt distribution coefficients. A metamorphic reaction-progress method is used to demonstrate that the distributions
approach mineral-mineral equilibrium rather than mineral-melt equilibrium. Application of published diffusion data for the
relevant elements in feldspars shows that solid-state equilibrium is unlikely to have been established during subsolidus cooling.
Instead, it is suggested that partial melts maintained chemical equilibrium with melanosomes during crystallization that lead
to the migmatites.
Using target-matching techniques combining 40Ar/39Ar crystal-mapping with elemental mapping and high-resolution electron microscopy, this study investigates the 40Ar behavior in very-slowly cooled muscovite from the Harney Peak Granite (HPG, South Dakota, USA). Detailed age mapping along (001) in single crystals from different localities of the HPG documents age gradients in excess of ∼ 300-400 m.y., with conspicuous internal 40Ar/39Ar zoning. This suggests (001) layer-parallel 40Ar transport driven by diffusion, consistent with previous 40Ar/39Ar crystal-mapping studies. The age distribution pattern is complex, however, and defines a mosaic of sub-grain domains with more retentive core zones, broadly ∼ 250-300 μm across, separated by zones of high diffusivity varying in shape and extent. The maximum ages preserved in the core domains are independent of their size but vary linearly with the bulk areal extent of the peripheral (or surrounding) high-diffusivity zones. Spatial 40Ar/39Ar relationships inside each grain point to a mechanism of multipath continuum-diffusion interaction between subdomains across the whole crystal, rather than via discrete non-interracting domains such as in K-feldspars. A close spatial correlation exists between younger ages, Na-depleted (K-enriched) zones, and density of microstructural defects. These defects, identified as lenticular voids and basal partings (< 100 nm-long), are developed in response to inward K ↔ Na interdiffusion during late-magmatic stages, in the absence of deformation. Coupled variations in density of microstructural defects and Na-K interchange are inferred to control the bulk diffusion-domain structure of HPG muscovite. Quantitative diffusion modeling of coupled compositional-defect-isotopic variations indicates that 40Ar diffusivity may be enhanced by up to six orders of magnitude in defect-controlled high-diffusivity zones relative to less defective (pristine) domains. On the other hand, empirical diffusivity estimates required to preserve the core ages are commensurate with diffusion estimates independently derived from recent atomistic simulations.
Training geoscience students in the methods of modern structural field analysis occurs in some degree during conventional coursework, on field trip excursions, in the process of undertaking undergraduate or graduate research projects, and while participating in extended programs usually referred to as “field camp.” Until the mid- to later 1980s, field course curricula were required for most university or college geoscience degrees in the USA. This educational fixture has declined over the last few decades, becoming lost or optional from many geology degree programs. Most recently, industry, government, and academia have recognized this decrease in emphasis as a mistake. The basic understanding of geological field methods is essential in many contexts beyond structural analysis. Advanced courses or those that include advanced problems are also of great benefit to students who will study structural geology as application in economic geology, local and regional tectonics, petrology, geochemical and geophysical studies, etc. The Black Hills of South Dakota in the USA is a region long exploited for instruction in field methods, because of its centrality, relatively accessible from much of the nation. It is also a small area rich in many classic, wonderful examples of structural features in diverse tectonic relationship. Certain localities within the Precambrian core of this uplift have provided great case study exercises to extend structural analysis beyond the basics. Five areas described for advanced instruction offer “hard rock” associations of metamorphic and some intrusive complexities.
