Clément Gariépy

Heat flow and structure of the lithosphere in the Eastern Canadian Shield

Twenty-two new determinations of heat flow and radiogenic heat production in the Superior and Grenville provinces of the Canadian Shield are presented. The new data and previously published measurements strongly constrain the thermal structure of the eastern Canadian Shield. In the Abitibi greenstone belt, heat flow gradually increases from 29 mW m−2 near the Grenville Front to 44 mW m−2 east of the Kapuskasing uplift. This heat flow variation is interpreted in terms of crustal thickening and increased thickness of a tonalitic layer with average heat production of about 1.1 μW m−3. This interpretation, based on estimated heat production of major rock types in the region, is consistent with crustal models derived from recent seismic reflection and refraction studies. It also leads to an estimate of about 12 mW m−2 for the mantle heat flow beneath the area. The average heat flow in the Grenville Province, 41 ± 10 mW m−2, is the same as that of the Superior Province. This similarity and the lack of significant variation of heat flow across the Grenville Front indicate that the crust on both sides of the front has similar heat production and thus composition. In the western part of the Grenville Province, heat flow reaches high values in the vicinity of the boundary between the Allochtonous Polycyclic and Monocyclic belts where enriched granitic plutons are found. In the crystalline terranes in the central part of the Grenville Province, heat flow and heat production are related to each other. The parameters of the linear heat flow-heat production relationship (Qr = 30 ± 2 in mW m−2 and D = 7.1 ± 1.7 km) are close to those of the much younger Appalachian Province, implying that the higher Appalachian heat flow is due solely to higher heat production in the upper crust. The data provide no evidence for variation of mantle heat flow between the Superior, Grenville, and Appalachian provinces, whose tectonic ages range between 2700 and 400 Ma. The small value of the mantle heat flow, about 12 mW m−2, implies that the depth to the 450°C isotherm, which controls the effective elastic thickness of the lithosphere, is very sensitive to crustal heat production.

The tectonic evolution of the Abitibi greenstone belt of Canada

Based on structural, geochemical, sedimentological and geochronological studies, we have formulated a model for the evolution of the late Archaean Abitibi greenstone belt of the Superior Province of Canada. The southern volcanic zone (SVZ) of the belt is dominated by komatiitic to tholeiitic volcanic plateaux and large, bimodal, mafic-felsic volcanic centres. These volcanic rocks were erupted between approximately 2710 Ma and 2700 Ma in a series of rift basins formed as a result of wrench-fault tectonics. The SVZ superimposes an older volcanic terrane which is characterized in the northern volcanic zone (NVZ) of the Abitibi belt and is approximately 2720 Ma or older. The NVZ comprises basaltic to andesitic and dacitic subaqueous massive volcanics which are cored by comagmatic sill complexes and layered mafic-anorthositic plutonic complexes. These volcanics are overlain by felsic pyroclastic rocks that were comagmatic with the emplacement of tonalitic plutons at 2717 ±2 Ma. The tectonic model envisages the SVZ to have formed in a series of rift basins which dissected an earlier formed volcanic arc (the NVZ). Analogous rift environments have been postulated for the Hokuroko basin of Japan, the Taupo volcanic zone of New Zealand and the Sumatra and Nicaragua arcs. The difference between rift related ‘submergent’ volcanism in the SVZ and ‘emergent’ volcanism in the NVZ resulted in the contrasting metallogenic styles, the former being characterized by syngenetic massive sulphide deposits, whilst the latter was dominated by epigenetic ‘porphyry-type’ Cu(Au) deposits.

U-series measurements in tyrrhenian deposits from mallorca — Further evidence for two last-interglacial high sea levels in the Balearic Islands

The Campo de Tiro type-section for the Tyrrhenian ecostratigraphic beds of the Balearic Islands shows four indurated littoral conglomerates and beach-rocks, unconformably superimposed. The lower two units (1 and 2) are separated by a thin layer of reddish continental silts; both contain a typical Eutyrrhenian fauna, whereas a Neotyrrhenian fauna characterizes the overlying units 3 and 4. ThU measurements by thermal ionisation mass spectrometry on mollusk shells from these deposits yielded ages of ∼135 ka (unit 1), ∼117 ka (units 2 and 3), and a scatter of ages around ∼100 ka (unit 4). The stratigraphic relationships and ThU data indicate (i) that uranium was uptaken by mollusks shells (but Arca sp.) each time during a relatively short early diagenetic interval before cementation ensured a ‘fair’ closure of the radioactive system, (ii) two high sea stands characterized the Last Interglacial (Isotopic Substage 5e) of the Balearic Islands area, (iii) the duration of this episode was ∼17 ka, and (iv) the change in faunal assemblages and disconformity observed between units 2 and 3 are due to fluctuations in sea level and surface water conditions which occurred during the second high sea level episode of the Last Interglacial.

ISOTOPIC COMPOSITION OF EPIPHYTIC LICHENS AS A TRACER OF THE SOURCES OF ATMOSPHERIC LEAD EMISSIONS IN SOUTHERN QUEBEC, CANADA

Abstract Lead isotopic data are reported for epiphytic lichens, vegetation samples, and lacustrine sediments collected in the boreal forest of Quebec between 47° and 55°N, and along the St. Lawrence Valley between 45° to 48°N. Lichens located up to 500 km north of Noranda (48°N) record a significant input of anthropogenic Pb emitted to the atmosphere from smelting activities. This input is not apparent beyond 53°N where only the isotopic signal typical of Canadian aerosols is recorded. Lichens along the St. Lawrence Valley show evidences for a dominant input from U.S. sources. The lead isotopic composition of lichens allow quantitative monitoring of the sources of atmospheric Pb. However, their slow metabolism and their unknown age detract from recording the Pb signal on short and precise timescales. Spruce needles have isotopic compositions undistinguishable from that of lichens; this reflects integration of the atmospheric Pb signal over a comparable time span, a result confirmed by the lead isotopic record in lacustrine sediments. Vegetation samples such as spruce bark, spruce wood, and deciduous tree leaves are more radiogenic than lichens from the same site. This may reflect mixing of radiogenic Pb metabolized from soil solutions through the root system with atmospheric Pb.

Pb-Sr-He isotope and trace element geochemistry of the Cape Verde Archipelago

Abstract New lead, strontium and helium isotopic data, together with trace element concentrations, have been determined for basalts from the Cape Verde archipelago (Central Atlantic). Isotopic and chemical variations are observed at the scale of the archipelago and lead to the definition of two distinct groupings, in keeping with earlier studies. The Northern Islands (Santo Antao, Sao Vicente, Sao Nicolau and Sal) present Pb isotopic compositions below the Northern Hemisphere Reference Line (NHRL) (cf. Hart, 1984) , unradiogenic Sr and relatively primitive 4He/3He ratios. In contrast, the Southern Islands (Fogo and Santiago) display Pb isotopes above the NHRL, moderately radiogenic Sr and MORB-like helium signatures. We propose that the dichotomy between the Northern and Southern Islands results from the presence of three isotopically distinct components in the source of the Cape Verde basalts: (1) recycled ∼1.6-Ga oceanic crust (high 206Pb/204Pb, low 87Sr/86Sr and high 4He/3He); (2) lower mantle material (high 3He); and (3) subcontinental lithosphere (low 206Pb/204Pb, high 87Sr/86Sr and moderately radiogenic 4He/3He ratios). The signature of the Northern Islands reflects mixing between recycled oceanic crust and lower mantle, to which small proportions of entrained depleted material from the local upper mantle are added. Basalts from the Southern Islands, however, require the addition of an enriched component thought to be subcontinental lithospheric material instead of depleted mantle. The subcontinental lithosphere may stem from delamination and subsequent incorporation into the Cape Verde plume, or may be remnant from delamination just before the opening of the Central Atlantic. Basalts from Sao Nicolau reflect the interaction with an additional component, which is identified as oceanic crustal material.

Probing Archean lithosphere using the Lu^Hf isotope systematics of peridotite xenoliths from Somerset Island kimberlites, Canada

Abstract A knowledge of the Hf isotopic composition of the subcontinental lithosphere beneath Archean cratons is essential to constrain the Hf isotope budget of the Earth’s mantle. Hf isotopic measurements were obtained by MC-ICP-MS for a suite of refractory peridotite xenoliths and constituent garnets from the Nikos kimberlite (100 Ma) on Somerset Island in order to constrain the isotopic composition and age of the lithosphere beneath the northern Canadian craton. The low-temperature Nikos peridotites ( 1100°C; 0.004–0.03, 0.28265–0.28333, respectively). These differences in Hf isotope signatures suggest that shallow and deep subcontinental lithosphere beneath Somerset Island represent isotopically distinct domains and do not share a common petrogenetic history. The Lu–Hf isotope systematics of the shallow low-temperature peridotites define a positively sloped line that plot along a 2.8 Ga reference isochron. A number of these peridotites are characterized by highly radiogenic Hf isotopic compositions suggestive of long-term radiogenic ingrowth (billions of years). These findings are consistent with an interpretation that the shallow Somerset lithosphere (to depths of ∼150 km) stabilized in the Archean. The majority of the high-temperature peridotites plot closer to the composition of the host kimberlite. Although the observed isotopic variation may be attributed in part to kimberlite-related Hf addition, it is possible that these deep-seated xenoliths represent younger mantle. The superchondritic 176Lu/177Hf ratios observed for a number of the shallow low-temperature peridotites indicate strong fractionation of Lu and Hf, suggesting mantle root formation in the garnet stability field (depths >80 km). The Hf isotope compositions for the Somerset low-temperature peridotites indicate that part of the mantle root beneath the North American craton is characterized by a more radiogenic Hf isotope signature than that estimated for a typical ‘depleted’ mantle.

Pb and Sr isotopic compositions of snowpack from québec, canada: inferences on the sources and deposition budgets of atmospheric heavy metals

Elemental concentrations of Al, Ba, Cd, Cu, Mg, Mn, Pb, Rb, Sr, and Zn, as well as Pb and Sr isotopic compositions were determined in samples of snowpack obtained along two main transects from the province of Quebec (Canada); one north-south (between 47°N and 55°N; 1994) and the other within the St. Lawrence Valley (1997). Median enrichment factors (relative to upper crustal abundances) for Cd, Cu, Mn, Pb and Zn for all samples range from ≈300 to ≈42,000 and are indicative of an anthropogenic origin. Pb isotope ratios for snow samples retrieved in 1994 are highly variable (206Pb/207Pb = 1.148 to 1.193) and are characterized by the most radiogenic Sr isotope values (87Sr/86Sr ≥ 0.710). In contrast, the Pb and Sr isotope results for 1997 snow samples collected along the St. Lawrence Valley (below latitude 47°N), yield the most radiogenic Pb isotope ratios (206Pb/207Pb = 1.180 to 1.190) and 87Sr/86Sr ratios between 0.708 and 0.710. The former indicate that the atmospheric pollution in this region of Quebec is dominated by a mixture of anthropogenic emissions from U.S. (206Pb/207Pb ≈ 1.20) and Canadian (206Pb/207Pb ≈ 1.15) sources. Pb isotope ratios (206Pb/207Pb = 1.160 to 1.180) for 1997 samples collected north of latitude 47°N indicate input of an additional anthropogenic component, possibly that of Eurasian pollution being transported over the high Arctic during the winter season. A comparison of the Pb isotope results between lichens and snow samples from identical sample locations indicate that these either overlap (along St. Lawrence Valley), or are significantly different (north–south transect). The latter discrepancy may be attributed to either: (1) different time scales for the integration of the atmospheric signal (months for snow vs. years for lichens); (2) recording of the atmospheric signal at substantially different altitudes; or (3) the presence of an important, local point source of atmospheric pollution. Annual depositional budgets have been estimated for Pb, Cd, Zn, Cu, and Mn, and average values (g km−2 yr−1) are 1500, 130, 196,000, 1900, and 6400, respectively. Compared to previous depositional fluxes (1993–1994) estimated from adjacent regions in North America, those reported here are slightly lower with the exception of Mn and Zn. The nondecrease in depositional fluxes of Mn may be attributed to combustion of Mn-bearing fossil fuels by automotive vehicles. The exact cause for the elevated annual depositional values for Zn, however, remains enigmatic.

Isotopic and trace element constraints on the genesis of the Faeroe lava pile

Abstract Basaltic lavas from the Faeroe Islands form three stratigraphic series which define two geochemical groups. Both the lower and middle series are LREE enriched ((La/Yb) e.f. : 2–3) and are characterized by convex LREE profiles; in contrast, the upper series comprises both depleted ((La/Yb) e.f. : 0.45–0.6) and enriched lavas. This twofold geochemical division is also evident from the incompatible trace elements such as Zr, Nb, Hf and Ta and the compatible trace elements Cr, Ni, Sr and Y. Nd-Sr-Pb isotopic measurements show that the basalts are contaminated by crustal materials, implying the presence of Precambrian sialic basement underneath the Faeroes block, a conclusion supported by geophysical data [35,36]. The uncontaminated end-members, for the LREE-depleted basalts ( 87 Sr/ 86 Sr) 0 ∼ 0.7026 and eNd 0 + 10 and for the LREE-enriched basalts ( 87 Sr/ 86 Sr) 0 ∼ 0.7034 and eNd 0 + 9, require two different mantle source regions thus posing serious problems for petrogenetic models such as dynamic partial melting which have been proposed for the Faeroes. We interpret the LREE-depleted basalts as partial melts of the oceanic asthenosphere whilst the LREE-enriched basalts may result either from the partial melting of deep mantle blobs or of the subcontinental lithosphere during upwelling of the asthenosphere.

Heat flow, gravity and structure of the Abitibi belt, Superior Province, Canada: implications for mantle heat flow

The results of new heat flow measurements are presented for eleven sites located in the Abitibi greenstone belt of the Archean Superior Province. These new and previously published heat flow data are used with gravity and seismic data to constrain the crustal structure and composition of the Abitibi subprovince, and to determine the mantle heat flow beneath the Canadian Shield. In order to analyze the long-wavelength variations in heat flow and Bouguer gravity anomaly, the crust was divided into three layers with distinctive composition and physical properties. Average density, heat production and thickness for each layer were varied within a range compatible with measurements and yielded crustal models consistent with gravity and heat flow data. The dominant contribution to crustal heat production is that of a tonalitic type layer which has the lowest density. The models require mantle heat flow to range between 10 and 14 mW m -2. The westward increase in heat flow requires the tonalitic layer to thicken from the Grenville Front toward the Kapuskasing uplift at the expense of the greenstone supracrustals. Long-wavelength gravity data are more affected by changes in crustal thickness than by composition, and the low Bouguer anomaly near the eastern Grenville Front is caused by 5-6 km of crustal thickening. In greenstone terranes, the long-wavelength variations in heat flow are more useful for determining average crustal composition and the thickness of supracrustals than the gravity data.

Late Archean Mantle Composition and Crustal Growth in the Western Superior Province of Canada: Neodymium and Lead Isotopic Evidence from the Wawa, Quetico, and Wabigoon Subprovinces

Abstract Neodymium and lead isotopes are presented for late Archean rocks from the Wawa, Quetico, and Wabigoon subprovinces (Sp) in the Western Superior Province (Ontario, Canada). The isotopic compositions were determined on greenstone volcanic sequences, pre-tectonic (2.73-2.69 Ga) trondhjemite-tonalite-granodiorite (TTG) suites, metasedimentary rocks, and post-tectonic (2.69-2.67 Ga) dioritic to granitic plutons in order to characterize the mantle and crustal reservoirs involved in the evolution of the southern part of the Western Superior Province. Although derived from a depleted mantle, almost all the greenstone volcanism and pre-tectonic TTG suites record contamination by crustal material as old as 3.2 Ga. Neodymium and lead isotopic compositions of the pre-tectonic TTG can be modeled as products of the melting of amphibolitic crust (e.g., subducted basalts) contaminated by 1-10% of subducted sediments. Moreover, lead isotopes of TTG suites provide evidence that the isotopic compositions of these subducted sediments varied as a result of the addition of new ca. 2.7 Ga juvenile material, in agreement with neodymium isotopic differences recorded by sedimentary units of the Quetico Sp (50:50 mix of young:old crusts) and Wawa Sp (75:25 young:old). Neodymium and lead isotopes of the post-tectonic REE-enriched Archean sanukitoid suites define an isotopic trend which is distinct from that of the TTG suites and which is interpreted to reflect the importance of crustal assimilation in the evolution of these suites. Furthemore, such plutons found in the Wawa Sp have more juvenile neodymium and lead isotopic compositions and higher Nd contents than those intruded into the Quetico and Wabigoon Sp which have more crustal isotopic signatures and lower Nd concentrations. This geographical difference could be due either to differences in Nd contents of initial Archean sanukitoid magmas, lesser assimilation in the Wawa Sp or, more probably, because the Wawa crust is on average more juvenile than the Quetico and Wabigoon crusts in agreement with the fact that the geographical difference mirrors that recorded by metasedimentary rocks deposited in Quetico and Wawa Sp.