Combined U–Pb and Hf isotopic analysis of detrital and igneous zircon provides information about variations in crustal addition versus reworking through time. However, whereas detrital zircon data from only a few samples can provide information about extended periods of orogenic activity, this approach lacks the geological context that comes with studying igneous zircon and may not record mid- and deep-crustal sources or sources isolated from sites of deposition. Conversely, igneous zircon is likely to miss sources that were removed by later tectonic activity, but which may be preserved in the detrital record. The Central Gneiss Belt of the southwest Grenville Province in Ontario has a relatively well-understood orogenic architecture and records active-margin growth and reworking between ca. 1900 and 1200 Ma. We present U–Pb and Hf isotope data of zircon from 30 orthogneiss samples ranging in age from ca. 1750 to 1250 Ma, covering all tectonostratigraphic levels of the Central Gneiss Belt. As expected, the detrital and igneous datasets display many of the same overall features, such as crustal growth along the southwest Laurentian active margin; however, there are also some distinct differences. In particular, sedimentation appears to have taken place near the active margin, with little or no input from distal inboard sources, thus missing this significant component of reworked crust. This study highlights the need for a combination of detrital and igneous samples to provide the most complete picture of long-lived accretionary orogenic systems.
Abstract We report structural and metamorphic data from a c. 25‐km transect across the eastern Grenville Front Tectonic Zone (GFTZ) to the Britt domain at the northern end of Georgian Bay near Key Harbour, Ontario. Constrasting Grenvillian structural and fabric elements characterize the eastern GFTZ, northern Britt domain and a narrow Transition Zone between them. Moderately to steeply dipping foliations with strong down‐dip lineations in all three divisions appear to be associated with NW‐directed thrusting. In the Transition Zone and northern Britt domain, early S = L fabrics with steep lineations are overprinted by younger structures (S > L) with shallow, SE‐SSE‐plunging lineations in which sparse, dominantly (but not exclusively) normal‐sense kinematic indicators are recognized. Pressure and temperature estimates from Grenvillian metamorphic assemblages in metadi‐abase indicate that conditions of P ± 12 kbar and T c 800° C were achieved before or during the thrust‐related deformation, with P‐T‐t paths that indicate near‐isothermal decompression to P c. 4 kbar and T c. 700° C. Correlation of fabric elements with points on the P‐T‐t paths suggests that exhumation occurred during two stages, the first associated with thrusting (≥1035 Ma) and the second with extension and thrusting (pre‐1003 Ma). The GFTZ contains steeply to moderately dipping, thrust‐related fabrics and lacks shallow, extensional structures; the latest episode of thrusting in the GFTZ is inferred to have taken place at 990‐980 Ma. The data are interpreted in terms of a tectonic model involving two stages of propagation of the Grenville orogen towards its foreland (≥1035 Ma and ≥980 Ma), with an intervening period of extension, although the tectonic regime probably remained compressional on the scale of the orogen.
Research Article| March 01, 1994 1.45 Ga granulites in the southwestern Grenville province: Geologic setting, P-T conditions, and U-Pb geochronology J.W.F. Ketchum; J.W.F. Ketchum 1Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada Search for other works by this author on: GSW Google Scholar R. A. Jamieson; R. A. Jamieson 1Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada Search for other works by this author on: GSW Google Scholar L. M. Heaman; L. M. Heaman 2Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario M5S 2C6, Canada Search for other works by this author on: GSW Google Scholar N. G. Culshaw; N. G. Culshaw 1Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada Search for other works by this author on: GSW Google Scholar T. E. Krogh T. E. Krogh 2Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario M5S 2C6, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information J.W.F. Ketchum 1Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada R. A. Jamieson 1Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada L. M. Heaman 2Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario M5S 2C6, Canada N. G. Culshaw 1Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada T. E. Krogh 2Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario M5S 2C6, Canada Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1994) 22 (3): 215–218. https://doi.org/10.1130/0091-7613(1994)022<0215:GGITSG>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation J.W.F. Ketchum, R. A. Jamieson, L. M. Heaman, N. G. Culshaw, T. E. Krogh; 1.45 Ga granulites in the southwestern Grenville province: Geologic setting, P-T conditions, and U-Pb geochronology. Geology 1994;; 22 (3): 215–218. doi: https://doi.org/10.1130/0091-7613(1994)022<0215:GGITSG>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract In the southwestern Grenville province, the parautochthonous Britt domain includes a variety of pre-Grenvillian metamorphic and plutonic rocks that were reworked at upper amphibolite facies during the Grenvillian orogeny. Near Pointe-au-Baril, Ontario, a crustal block containing pre-Grenvillian granulite facies mineral assemblages and pre-Grenvillian to early Grenvillian tectonic fabrics has been identified. The block is bounded on the northwest and southeast by extensional shear zones that may have isolated it from regional late Gren- villian deformation. Multiequilibria pressure-temperature (P-T) calculations for orthopyroxene-bearing mafic rocks suggest conditions of 625-700 °C and 7.2-8.4 kbar for the pre-Grenvillian metamorphism. The granulite facies assemblages were locally overprinted during higher pressure Grenvillian metamorphism, which peaked at 720-775 °C and 10.8-11.5 kbar. U-Pb zircon data from migmatitic, mafic supracrustal gneiss indicate metamorphism and leucosome development at ca. 1450-1430 Ma, in agreement with other pre-Grenvillian metamorphic ages for the Central gneiss belt and Grenville Front tectonic zone. An expanding data base on pre-Grenvillian events in the southwestern Grenville province indicates that high-grade metamorphism at ca. 1450-1430 Ma affected a large region of crust and was coeval with widespread felsic to intermediate plutonism. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
A 45 km wide, shear-zone-bounded segment of the northwestern Makkovik Province, Labrador, is underlain by Archean gneisses derived from the adjacent Nain craton. This lithotectonic block (Kaipokok domain) was reworked at high metamorphic grade, overthrust by supracrustal sequences (Lower Aillik and Moran Lake groups), and intruded by granitoid plutons during the Paleoproterozoic. Initial amphibolite-facies reworking of the Kaipokok domain at 1896 ± 6 Ma is indicated by U–Pb ages of metamorphic zircon from a foliated Kikkertavak metadiabase dyke. This is one of the oldest Paleoproterozoic tectonic events dated thus far in northeast Laurentia and may be linked with ca. 1890 Ma plutonism documented elsewhere in the Kaipokok domain. Intrusion of granitoid plutons at [Formula: see text], 1877 ± 5, and [Formula: see text] in the Kaipokok Bay area postdates early thick- and thin-skinned thrusting (possibly east to northeast directed) that involved Lower Aillik Group strata. U–Pb titanite ages of 1866–1847 Ma in part record a metamorphic event that followed this plutonic–tectonic activity. These early events are temporally and kinematically difficult to reconcile with accretion of juvenile Makkovikian terranes in the southeast and may instead be related to early stages of the ca. 1.91–1.72 Ga Torngat orogeny along the western margin of the Nain craton. In contrast, high-grade metamorphism, dextral shearing, and northwestward thrusting between 1841 and 1784 Ma, including crystallization of an Iggiuk granitic vein at 1811 ± 8 Ma, are in accord with accretion of Makkovikian terranes in a dextral transpressional regime (Makkovikian orogeny sensu stricto). Coeval sinistral transpression in the Torngat orogen suggests that both otogenic belts accommodated relative northward tectonic escape of the Nain craton during this interval.
The Lac Ghyvelde-Lac Long area exposes mainly granulite-grade paragneiss, gabbro, anorthosite, and charnockite. The Wilson Lake block, the northern two-thirds of the area, grades from sillimanite-cordierite assemblages in the north to sillimanite-orthopyroxene in the south. Sapphirine-bearing assemblages occur in both parts, and sapphirine-quartz assemblages occur in the southern part. Metamorphism and plutonism of Paleoproterozoic or Archean paragneisses of the Wilson Lake block occurred about 1650 Ma during the Labradorian orogeny. They have been variably, but slightly, overprinted by Grenvillian (1200-1000 Ma) orogeny. Variably metamorphosed gabbro, anorthosite, and monzonite of the Atikonak River massif intrude the Wilson Lake block in the central part of the area. The massif consists of anorthosite suite rocks, probably of Labradorian age, intruded by tholeiitic rocks about 1120 Ma. The deformed and metamorphosed Lac Long igneous suite, characterized by orthopyroxene-garnet-two feldspar assemblages, underlies the southern part of the area. This suite was emplaced at 1225 Ma and metamorphosed at 1008 Ma under amphibolite to granulite grade conditions. The Lac Long lineament, a broad zone of cataclasis, crosses the area from east to west. At present it separates granulites of Labradorian age from those of Grenville age. It appears to be a major and persistent crustal break. The Lac Ghyvelde-Lac Long area exhibits deep crustal rocks brought to shallower depth and imbricated northward during Grenvillian orogeny. In this region the orogeny affected crust already thickened and metamorphosed during the previous Labradorian event.
New UPb zircon, titanite, and monazite ages reported here, along with existing age data, demonstrate that granitoid bodies in the northwestern segment of the Paleoproterozoic Makkovik Province, Labrador, are of three distinct ages. The redefined Island Harbour Bay plutonic suite consists of varied dioritic to granitic units that were syntectonically emplaced into Archean crust of the Kaipokok domain between ca. 1895 Ma and 1870 ± 2 Ma. This plutonism occurred during the early stages of the Makkovikian Orogeny in an obliquely convergent, Andean continental margin setting. The Hares Islands and Drunken Harbour granites form smaller, discrete plutons that were emplaced in or adjacent to the Island Harbour Bay plutonic suite at 1805 ± 5 Ma and 1791 ± 2 Ma, respectively, during dextral strike-slip deformation that accompanied accretion of an outboard juvenile terrane. Magmatic activity during this period was preferentially sited along active structural zones, but also occurred outside of these zones. The undeformed Blacklers Bight A-type granite was emplaced in the Kaipokok domain at 1716 ± 1 Ma, late in the development of the orogen. A-type granites of this age form a significant component of the southeastern Makkovik Province, and their generation is linked to mafic underplating and heterogeneous regional extension. The new age data support the conclusion of earlier workers that crustal growth via synorogenic and postorogenic plutonism was episodic, and allow, along with field and geochemical data, inferences to be made regarding the tectonic setting of individual plutonic events.
As mountains develop, the lower parts of the thickened continental crust may flow, analogous to a very slow moving fluid. My primary project uses field studies and lab analyses to discover how the architecture, and thus the strength, of the lower crust changes during such flow. The work is based in a part of the Canadian Shield, the Grenville Province, that exposes the deep roots of an ancient mountain belt. A second project uses geochronological and geochemical studies to illuminate the history of the rocks were before they were incorporated into the mountain belt.
