Lifting the 'Metamorphic veil': Tom Krogh's Contributions to Understanding the Geology and Tectonics of the Grenville Orogen
In a session focusing mainly on Tom Krogh's contributions as an experimentalist and as a geochronologist, it is important to remember that Tom had an equally keen interest in geology and tectonics, particularly when it came to understanding the Grenville Orogen. Tom's nearly 50-year fascination with the Grenville Orogen began as an undergraduate at Queen's University, long before he became a geochronologist, and continued through his stints at MIT, the Carnegie Institution, the Royal Ontario Museum and the University of Toronto. Regardless of the decade, Tom was always applying cutting-edge methods, many of which he developed, to solving Grenville problems, be it using Rb-Sr methods to determine the age of rocks in the French River area or to applying high-precision U-Pb zircon methods on eclogite pods in Muskoka, Ontario. In fact, many of his contributions to dating techniques, especially those applicable to high-grade terrains, resulted from the honing of his skills in the Grenville crucible. A list of Tom's contributions to our understanding of the Grenville Orogen, by no means comprehensive, includes the following: 1) By dating zircon in pegmatite dikes along the entire 2000 km length of the Grenville Front, he found that they all had Grenvillian metamorphic ages of 988 to 995 Ma, dating the last major orogenic event in the orogen. Furthermore, at most sites, he was able to precisely date the time of earlier high-grade metamorphic events. 2) In conjunction with the pegmatite study, in Labrador, he demonstrated that partially reset titanite and monazite for samples up to 225 km apart recorded metamorphic events at 1651±5 Ma and 995±12 Ma. This work showcased the effectiveness of using other uranium-bearing accessory minerals to unravel complex metamorphic histories in high-grade terrains. 3) He pioneered the use of zircon in amphibolites to date metamorphism in high-grade terrains, which he subsequently undertook in both the Grenville and the Kapuskasing structure. 4) He recognized, through the use of Rb-Sr and U-Pb dating in Labrador that the style and degree of Grenvillian metamorphic overprinting differed in terms of intensity and pervasiveness throughout the orogen. 5) He pioneered the use of melt pods in pressure shadows in boudin necks and similar structural features to date the timing of metamorphism. This led to the recognition in the Muskoka area that metamorphism was not an instantaneous event but a prolonged one, consistent with predictions from subsequent finite-element modeling of the orogen. An indirect, but nonetheless important contribution was his long-term interest in the rocks of the Laurentian Margin in Ontario, which have received far less study than their counterparts in the Composite Arc or Frontenac-Adirondack belts. Furthermore, Tom was instrumental in training the next generation of geochronologists currently working in the Grenville Orogen in Canada, and imparting his enthusiasm and approach to tackling Grenville problems. An important lesson to be learned from Toms work in the Grenville is that many of his most significant contributions result from a long-term approach and commitment to a particular problem and region that may never have occurred through short-term and/or localized studies.
Deciphering Multiple Orogenic Events in the Grenville Province in Eastern Labrador Using U-Pb ID-TIMS Geochronology at Key Outcrops
The Grenville Province in eastern Labrador experienced four major orogenic events. These occurred at 1800 - 1770 Ma (unnamed pre-Labradorian event), 1710 - 1600 Ma (Labradorian), 1520 - 1460 Ma (Pinwarian) and 1085 - 985 Ma (Grenvillian). Using carefully selected key outcrops where field relationships are unequivocally preserved, it has proved possible to unravel this complex history and deliver it with confidence. The presentation will demonstrate how such key sites are used to build a sequential orogenic history. The key- outcrop approach involves collecting several samples from a single well-exposed site, recognizing that having an independent temporal control (such as cross-cutting intrusive or deformational relationships) is critical to the successful interpretation of complex U-Pb isotopic data. Also contingent to the success of this field-based method are high-precision ages and a tenacity to persist analytically to 'get it right'. Despite having used this approach in eastern Labrador since 1984, only 25 sites can be designated as first-rank key sites, as critical outcrops are rare and a considerable investment in geochronological investigation (both time and money) is required. The key sites have been augmented, however, by other outcrops where only single-sample dating was carried out. The selection of these second-rank sites was also determined by the field relationships displayed, and their potential contribution to understanding the orogenic history - beyond simply determining the age of the rock present. Essential in deciphering multiple orogeneses is a requirement that not all events through time occurred with comparable intensity everywhere. This means that the absence of one or more orogenic events at a site enables the timing of the remaining events to be resolved. The tectonic significance that can be attached to the presence/absence of an orogenic event will be addressed.
A U-Pb Discordia Line for Carbonate of the Mooidraai Formation, Transvaal Supergroup: Paleoproterozoic Formation and Neoproterozoic Fluid Infiltration
Dating of supracrustal successions that lack interleaved zircon-bearing felsic volcanic horizons is generally a challenging task. Alternative approaches include dating detrital zircon populations, which yield maximum ages for deposition, or whole-rock isotopic techniques (Rb-Sr, Sm-Nd, Pb-Pb). The latter are susceptible to post- depositional disturbances, which decrease the age, or to mixing effects, which often have the reverse result by increasing the age. These difficulties are of particular importance for the studies of the Paleoproterozoic chronostratigraphy of the Transvaal Supergroup, which covers the Kaapvaal Craton and is know to have been deposited over a period of some 500 m.y. starting at about 2600 Ma. This period included a major global glaciation and the transition from anoxic to oxygenated atmospheric conditions. One point of contention is the correlation between strata of the Griqualand West Basin in the west with those of the Kanye and Transvaal basins farther north and east, specifically the correlation, or not, of the lower part of the Postmasburg Group with similar units in the Pretoria Group. The Postmasgroup includes the glacial deposits of the Makganyene Formation, overlain by Ongeluk andesitic volcanic strata, BIF and Mn deposits of the Hotazel Formation, and the Mooidraai carbonate. The latter had provided a Pb-Pb age of 2394 +/- 26 Ma based on the carbonate fraction separated from diagenetic quartz (Bau et al. 1999). This age is older than the Pb-Pb age of about 2220 Ma reported for Ongeluk lavas (Cornell et al. 1996), and older than apparent correlative strata in the Kanye and Transvaal basins. In our study we have addressed this discrepancy by investigating the U-Pb relationships in Mooidraai limestone from a drill core. The rock is heterogeneous on a mm-scale with alternations of homogeneous grey carbonate, green streaked carbonate and thin back coatings, presumably Fe-oxides. The identity of these coatings is under investigation. The U-Pb analyses show that they are strongly enriched in U (1.5 ppm) with respect to the 'clean' carbonate (0.08 ppm), the common Pb concentration also increasing, but only by a factor of 3 to 4 (0.1 vs. 0.35 ppm). The least radiogenic grey carbonates plot on the isochron of Bau et al. (1999). From this we can thus obtain reasonable values for the initial common Pb correction of the U-Pb data. The U-Pb data are variously discordant, the enriched black coatings showing up to 80% normal discordance. A few analyses are reversely discordant. Most of the data define a discordia line with an upper intercept age of 2381+/- 44 Ma and a lower intercept age of 590 +/- 63 Ma. Two data points deviate from the line probably indicating some more recent U mobility. These results are interpreted as indicating metamorphic infiltration of the carbonate by U-bearing fluids during the Panafrican events. Such an overprint has been recorded before in the Ongeluk lavas (Dorland 2004) where it resulted in Neoproterozoic zircon resetting and new-growth. The upper intercept age, and the consistency of the least radiogenic isotopic data with those of Bau et al. (1999), confirms their 2396 Ma age and supports the regional stratigraphic model proposed by Moore et al. (2001).
U-Pb baddeleyite geochronology of the Bushveld Igneous Complex: Evidence for rapid cooling
U-Pb baddeleyite geochronology is a direct upshot of single zircon geochronology pioneered by Tom Krogh, following more or less the same analytical protocol. It expands the application of U-Pb geochronology to mafic rocks which are typically devoid of zircon. A prominent example is the Bushveld Igneous Complex (BIC), the world's largest layered mafic intrusion and most important primary source of chromium, vanadium, and platinum-group elements The BIC has been extensively studied, though its emplacement age and cooling history have not been well-established. This information is important in constraining the origin of its ore deposits, as well as evaluating the hypothesis that the BIC is the intrusive component of a continental flood basalt province. The dominantly mafic units of the Bushveld's Rustenberg Layered Suite (RLS) have been shown to have cooled rapidly from ca 500-200 ° C between ca. 2060 Ma and 2040 Ma based on 40Ar/39Ar analysis of hornblende, biotite and plagioclase using U-Pb normalized constants (Nomade et al., 2004; Renne et al., 2008). RLS rocks are typically devoid of zircon, making high-closure-temperature ages difficult to establish, though Scoates and Friedman (2008) reported zircon and rutile U/Pb ages (2054.4 ± 1.3 Ma and 2055.0 ± 7.9 Ma, respectively) from the Merensky Reef, further supporting rapid emplacement and cooling. Virtually all high precision work to date has been on the western limb of the BIC, and our ongoing study is intended to broaden the spatial scope of thermochronologic data. To this end, baddeleyite has been extracted from several gabbros of the RLS, and U-Pb analyses using ID-TIMS have been carried out. A gabbro from the eastern limb of the BIC yields results in agreement within error of those of Scoates and Friedman (2008) and with our U/Pb (zircon) age (2057 ± 3 Ma) for the younger Nebo granite in the western limb, and is close to the U-Pb normalized 40Ar/39Ar ages of biotite from the same gabbro, and of hornblende and biotite from other units of the RLS. This age implies rapid emplacement across more than 300 km of lateral extent of the BIC, as well as rapid cooling after emplacement, consistent with estimates for continental flood basalts. Nomade et al. (2004) J. Geol. Soc. Lond. 161: 411-420; Renne et al. (2008) Geochim. Cosmochim. Acta 72: A787; Scoates and Friedman (2008) Econ. Geol. 103: 465-471
Major Advances From U-Pb Geochronology in Determining the Thermotectonic Evolution of the Western Margin of the Trans-Hudson Orogen, Eastern sub-Athabasca Basement, Saskatchewan: Input From Tom Krogh and his Co-workers
U-Pb zircon, monazite, titanite, and xenotime dating of 58 Archean/Paleoproterozoic samples from the sub- Athabasca basement of northern Saskatchewan was carried out at the Jack Satterly Laboratory, Royal Ontario Museum (ROM, Toronto, Ontario) from 1990 to 2005. Here we will present/review the most significant U-Pb geochronology results from the basement rocks for regional stratigraphic correlation and thermotectonic evolution of the western margin of the Trans-Hudson Orogen. A majority of the samples come from drill core in the vicinity of major unconformity-type uranium deposits and showings of the eastern Athabasca Basin; however, a good number are from outcrops peripheral to the southeastern margin of the basin. The samples included: (i) tonalitic-trondhjemitic gneisses of Archean age; (ii) migmatitic tonalitic gneisses; and (iii) granitic- granodioritic gneisses all of Archean age; (iv) monzogabbros; (v) grey granites; (vi) calc- alkaline porphyritic granites; (vii) peraluminous leucogranites-pegmatites; and (viii) skarns, all of presumed Paleoproterozoic age. Despite a variably complex history of polyphase ductile deformation and high-grade metamorphism, all but two samples gave reliable ages. The Archean orthogneisses form clusters at 2785, 2740-2700, 2685, and 2630-2585 Ma. An Archean protolith age was always apparent, except for one sample. An enigmatic Paleoproterozoic age of ca. 1925 Ma was obtained for one of the granitic pegmatite samples, possibly due to inheritence. The remainder of the granitoid rocks yielded Hudsonian magmatic (mainly U-Pb zircon) ages and metamorphic (mainly monazite and titanite) ages of 1850-1750 Ma. One of the grey granites yielded inherited zircon ages of 2150-1900 Ma, similar to the protolith ages of the lower Wollaston Group obtained by other researchers. In summary, this large data set expanded the previously limited geochronological database for this part of the Trans-Hudson Orogen. The results here have many similarities to those obtained and interpreted by other co- workers of Tom Krogh from the eastern side of the Trans-Hudson Orogen. In conjunction with reaction textures, phase equilibria, and petrogenetic grids, high-quality U-Pb geochronology allowed construction of a detailed Hudsonian clockwise P-T-t path, in this part of the western Trans-Hudson foreland margin.
New U-Pb Geochronological Data From the Aillik Domain, Makkovik Province, Labrador: Implications for the Formation of the Aillik Group and Evolution of the Region
The Makkovik Province of eastern Labrador is part of a Paleoproterozoic accretionary orogenic belt that formed during the ca. 1.9-1.7 Ga Makkovikian orogeny. The Aillik domain represents one of three domains that characterize the Makkovik Province. This domain largely comprises the Aillik Group, a package of Paleoproterozoic volcano-sedimentary rocks, and abundant Paleoproterozoic intrusive suites that intrude the Aillik Group and were variably deformed during the Makkovikian orogeny. The Aillik Group comprises polydeformed, upper-greenschist to lower-amphibolite facies, bi-modal volcanic rocks and sedimentary units which were metamorphosed and deformed during the Makkovikian orogeny. Recent detailed 1:10,000 scale bedrock mapping in conjunction with U-Pb geochronology from two sections of the Aillik Group within the Makkovik Bay area illustrate the complex lithological, structural and deformational history of the Aillik Group. Two areas that are the focus of this study: the Middle Head area and the Pomiadluk Point area. Middle Head is composed of two northeasterly trending sections of Aillik Group which are separated by the intrusion of a foliated syenogranite and a non foliated quartz monzonite to monzogranite. The metavolcanic rocks at Middle Head are dominated by rhyolite, felsic tuff, tuffaceous sandstone and basalt with lesser mafic tuff, while the metasedimentary rocks are dominantly siltstone-sandstone and lesser volcaniclastic breccia. In contrast, Pomiadluk Point is composed dominantly of crystal felsic tuff and conglomerate, with lesser preserved rhyolite and basalt. The Aillik Group at Pomiadluk Point is intruded by the ca. 1657 Ma October Harbour granite, a non- foliated monzogranite. New zircon U-Pb SHRIMP geochronology from four felsic tuff samples from the Aillik Group and one intrusive Paleoproterozoic syenogranite provide additional constraints on the timing of the formation and tectonic evolution of the area. A felsic tuff from Middle Head area yielded an age of 1850 +/-7 Ma; whereas, two crystal felsic tuff samples from the eastern and western exposures of Pomiadluk Point yielded ages of 1855 +/-7 Ma and 1860 +/-7 Ma. An additional age from a thin (10 m) unit of felsic tuff to lapillistone at Pomiadluk Point yielded a mixed zircon population with an interpreted age of formation of 1863 +/-7 Ma, and inherited zircons that were from two populations at 1892 +/-8 Ma and 1911 +/-11 Ma. These new Aillik Group ages indicate that lithologically diverse sections of the Aillik Group were deposited contemporaneously, and that felsic volcanism continued to as late as ca. 1850 Ma. In addition, no inherited Archean zircons were documented in any of the samples, supporting interpretations of the Aillik Group being deposited on a largely juvenile substrate. The foliated Paleoproterozoic syenogranite from Middle Head, yielded an age of 1804 +/-4 Ma; its similar petrology and this new age date support the interpretation that it is part of the Kennedy Mountain Intrusive Suite and intruded the Aillik Group during the final compressional stages of the Makkovikian orogeny. Detailed field mapping, in combination with precise U-Pb geochronological data, have further constrained the timing of formation and tectonic evolution of the Aillik Group and Aillik domain.