Lewis D. Ashwal

Heat production in an Archean crustal profile and implications for heat flow and mobilization of heat-producing elements

We have measured concentrations of heat producing elements (Th, U, and K) in 58 samples representative of the main lithologies in a 100 km transect of the Superior Province of the Canadian Shield, from the Michipicoten (Wawa) greenstone belt, near Wawa, Ontario, through a domal gneiss terrane of amphibolite grade, to the granulite belt of the Kapuskasing Structural Zone, near Foleyet. This transect has been interpreted as an oblique cross section through some 25 km of crust, uplifted along a major thrust fault, and thus provides an opportunity to examine in detail a continuous profile into deep continental crust of Archean age. Mean heat production values for these terranes, based on aereal distribution of major rock types and calculated from their Th, U, and K concentrations are: Michipicoten greenstone belt = 0.72 μW m−3; Wawa domal gneiss terrane (amphibolite grade) = 1.37 μW m−3; Kapuskasing granulites = 0.44 μW m−3. Among the silicic plutonic rocks (tonalites, granites, and their derivative gneisses), the relatively large variation in heat production correlates with modal abundances of accessory minerals including allanite, sphene, zircon, and apatite. We interpret these variations as primary (pre-metamorphic). The relatively high weighted mean heat production of the domal gneiss terrane can be accounted for by the larger proportion there of late-stage Th-, U-, and K-rich granitoid plutons. These may have been derived from the underlying Kapuskasing granulite terrane, leaving it slightly depleted in heat producing elements. Transport of Th, U, and K, therefore, could have taken place in silicate melts rather than in aqueous or carbonic metamorphic fluids. This conclusion is supported by the lack of a statistically significant difference in heat production between tonalites, tonalite gneisses and mafic rocks of amphibolite versus granulite grade. The pre-metamorphic radioactivity profile for this crustal section is likely to have been uniformly low, with a mean heat production value less than 1 μW m−3. This result is distinctly different from measured profiles in more silicic terranes, which show decreasing heat production with depth. This implies fundamental differences in crustal radioactivity distributions between granitic and more mafic terranes, and may be an important factor in selective reactivation of lithologically different terranes, possibly resulting in preferential stabilization of basic terranes in the geological record. Our results indicate that a previously determined apparently linear heat flow-heat production relationship for the Kapuskasing area does not relate to the distribution of heat production with depth. Low, but significant heat production, 0.4–0.5 μW m−3, continues to lower crustal depths with no correlation to the depth parameter from the linear relationship. This low heat production may be a minimum average granulite heat production and suggests that, in general, heat flow through the Moho is 8–10 mW m−2 lower than the reduced heat flow calculated from the heat flow-heat production regression.

African lithospheric structure, volcanism, and topography

In Africa volumetrically minor, mid-plate volcanic rocks of Cenozoic age are concentrated in areas affected by Pan-African (500 ± 150Ma) crustal reactivation, and are virtually absent from cratonic ares. We interpret this as indicating that Pan-African areas are underlain by fertile lithospheric mantle and cratons by depleted lithospheric mantle, and propose a model to explain both the distribution of volcanism and the two kinds of African mantle lithosphere: (1) Cratons were formed by the assembly of collided island arcs, and for this reason are underlain by depleted, sub-oceanic-type mantle lithosphere. (2) Comparable depleted mantle lithosphere has been delaminated from beneath areas thickened during Tibetan-style continental collision (especially in Pan-African times) and replaced by fertile material which now forms the lithospheric mantle below the Pan-African reactivated crust. (3) When the African plate came to rest with respect to sub-lithospheric mantle circulation patterns at about 30 Ma, heating from below extracted magmas from the fertile but not from the depleted mantle lithosphere. (4) As a result of interaction with an underlying convective pattern for the last 30 Ma, Pan-African reactivated areas display both Neogene elevations and volcanism but cratonic areas display only elevations.

Sr, Nd, and Pb isotopes in Proterozoic intrusives astride the Grenville Front in Labrador - Implications for crustal contamination and basement mapping

Abstract We report Sr, Nd and Pb isotopic compositions of mid-Proterozoic anorthosites and related rocks (1.45-1.65 Ga) and of younger olivine diabase dikes (1.4 Ga) from two complexes on either side of the Grenville Front in Labrador. Anorthositic or diabasic samples from the Mealy Mountains (Grenville Province) and Harp Lake (Nain-Churchill Provinces) complexes have very similar major, minor and trace element compositions, but distinctly different isotopic signatures. All Mealy Mountains samples have ISr = 0.7025−0.7033, eNd = +0.6 to +5.6 and Pb isotopic compositions consistent with derivation from a mantle source depleted with respect to Nd/Sm and Rb/Sr. Pb isotopic compositions for the Mealy Mountains samples are slightly more radiogenic than model mantle compositions. All Harp Lake samples have ISr = 0.7032−0.7066, eNd = −0.3 to −4.4 and variable, but generally unradiogenic 207Pb/204Pb and 206Pb/204Pb compared to model mantle, suggesting mixing between a mantle-derived component and a U-depleted crustal contaminant. Crustal contaminants are probably a variety of Archean high-grade quartzofeldspathic gneisses with low U/Pb ratios and include a component that must be isotopically similar to the early Archean (>3.6 Ga) Uivak gneisses of Labrador or the Amitsoq gneisses of west Greenland. This would imply that the ancient gneiss complex of coastal Labrador and Greenland is larger than indicated by present surface exposure and may extend in the subsurface as far west as the Labrador Trough. If Harp Lake and Mealy Mountains samples were subjected to the same degree of contamination, as suggested by their chemical similarities, then the Mealy contaminants must be much younger, probably early or middle Proterozoic in age. The Labrador segment of the Grenville Front, therefore, appears to coincide with the southern margin of the Archean North Atlantic craton and may represent a pre mid-Proterozoic suture.

Sr and Nd isotope geochronology, geologic history, and origin of the Adirondack Anorthosite

Abstract We have analyzed samples from the Adirondack Marcy massif for Rb-Sr and Sm-Nd isotopes in an attempt to determine directly the primary crystallization age of a Proterozoic massif-type anorthosite rock suite. The oldest age obtained (1288 ± 36Ma) is from a 4 point Sm-Nd isochron defined by igneous-textured whole-rock and mineral separate data from a local layered sequence gradational from oxiderich pyroxenite to leuconorite. This age is older than Silvers (1969) 1113 Ma zircon age of associated charnockites, but is within the window of permissible anorthosite ages based on previous geochronology and field relationships. As such, 1288 Ma may represent the time of crystallization of the massif. For the most part, however, both Sm-Nd and Rb-Sr isotopic systems did not survive granulite facies metamorphism. Internal isochrons based on whole rocks and minerals yield ages between 995 and 919 Ma. These isotopic data suggest that the granulite fades metamorphism experienced by the massif was a prograde event that occurred a minimum of 100 Ma and as much as 350 Ma after crystallization of the massif. The relatively large range in Rb abundance, and in calculated initial 87 Sr 86 Sr (0.7039–0.7050) and 143 Nd 144 Nd ratios among anorthosite suite rocks, particularly those at or near the contacts of the Marcy massif is explicable by variable contamination with “crustal” materials and/or fluids, derived from surrounding acidic metaplutonic rocks, paragneisses, and marbles. Despite uncertainies caused by crustal contamination and metamorphic resetting of primary ages, Marcy samples have epsilon Nd values between +0.44 and +5.08, implying a source for the massif with long-term depletion in light rare earth elements. A probable source material would be depleted mantle.

Origin of Archean anorthosites - Evidence from the Bad Vermilion Lake anorthosite complex, Ontario

The Bad Vermilion Lake anorthosite complex (2,700 m.y.) is exposed over an area of about 100 km2 near Rainy Lake, Ontario. As is typical of other Archean anorthosites, it is composed of coarse (1–30 cm across), equidimensional, euhedral to subhedral, calcic (An80) plagioclase, in a finer grained mafic matrix. The amount of mafic matrix in individual samples ranges from none to about 70% by volume. The complex has been variably metamorphosed to greenschist facies. Zoisite, chlorite, and hornblende are abundant, but primary plagioclase is preserved in many places.The anorthosite complex is associated with gabbro and with mafic to felsic metavolcanic rocks, and is cut by tonalite plutons and by mafic dikes. Some gabbros contain local concentrations of Fe-Ti oxides and/or apatite, but no chromite. The mafic groundmass of the anorthositic rocks is similar in major and trace element chemistry, including rare earth elements, to the associated basaltic metavolcanics, suggesting that the anorthositic complex may have accumulated from a subvolcanic magma chamber which fed mafic lavas to the surface during its crystallization. Mafic flows and dikes chemically similar to the mafic metavolcanics contain plagioclase megacrysts akin to those of the anorthositic rocks, and thus may represent a link between the anorthosite complex and associated mafic lavas. Elongate pretectonic tonalite intrusions were comagmatic with the felsic metavolcanics, but not with the anorthosites or metabasalts. These silicic rocks may represent low-pressure partial melts of the mafic rocks.There is no direct or indirect evidence for significant volumes of ultramafic material at the present exposure level of the complex. An estimate of the bulk composition of all rocks presumed to be comagmatic with the anorthosites, including gabbros and mafic metavolcanics, is an aluminous basalt with about 20 wt.% Al2O3. This composition has REE abundances unlike those of typical Archean high-Al basalts and probably does not represent that of a primary or evolved melt. The possibility must be considered, therefore, that a substantial fraction of material comagmatic with the anorthosites has been separated from the complex, either by magmatic or tectonic processes.

Fluid inclusions in high-grade gneisses of the Kapuskasing structural zone, Ontario: metamorphic fluids and uplift/erosion path

Fluid inclusions in quartz grains from five samples of high-grade rocks (two paragneisses, an amphibolite, a mafic gneiss and a tonalite dike) from the 2.7 Ga Kapuskasing structural zone (KSZ), Ontario, were examined with petrographic, microthermometric and laser Raman techniques. Three types of fluid inclusions were observed: CO2-rich, H2O-rich and mixed CO2-H2O. CO2-rich fluid inclusions are pseudosecondary or secondary in nature and are generally pure CO2; a few contain varying amounts of CH4·H2O-rich fluid inclusions are secondary in nature, contain variable amounts of dissolved salts, and generally contain daughter crystals. Mixed CO2-H2O fluid inclusions occur where trails of H2O-rich inclusions intersect trails of CO2-rich inclusions. Isochores for high density (p=1.03 g/cm3) pseudosecondary, pure CO2 inclusions intersect the lower pressure portion of the estimated P-T field for high-grade metamorphism, implying that pure CO2 was the peak metamorphic fluid. The variable CH4 content of CO2 inclusions within graphite-bearing samples suggests that CH4 was introduced locally after the formation of the CO2 inclusions; however the origin of the CH4 remains problematic. An aqueous fluid clearly penetrated the gneisses after the peak metamorphism (during uplift/erosion), forming secondary inclusions and contributing to the minor retrogressive hydration observed in these rocks. The presence of the pseudosecondary, high-density CO2 inclusions in quartz crystals in the KSZ rocks constrains the uplift/ erosion path for the KSZ to one of simultaneous decrease in pressure and temperature.

Sm-Nd and Rb-Sr isotope systematics of an Archean anorthosite and related rocks from the Superior Province of the Canadian Shield

Sm−Nd and Rb−Sr isotopic data for the Bad Vermilion Lake anorthosite complex (BVL) in the Rainy Lake area of the Superior Province of northwestern Ontario show that direct ages of Archean anorthosites can be obtained with these isotopic systems despite the effects of low-grade metamorphism. There is sufficient spread in Sm/Nd between plagioclase megacrysts and coexisting mafic groundmass to allow the determination of reasonably precise internal Sm−Nd isochrons. Anorthosite samples from BVL show an unusually large range in LIL concentrations such that there is sufficient spread in Rb/Sr for a whole-rock isochron (2.69±0.10 Ga, ISr=0.70079±8). This variability may have been caused by Rb introduction during hydrothermal alteration and/or low-grade metamorphism. The Sm−Nd isochron for BVL (2747±58 Ma, eNd=+2.0±1.4) includes data for anorthosite, gabbro and metabasalt, and is consistent with the consanguineity of these units in the Rainy Lake area. The age is interpreted as the time of crystallization of the anorthosite complex and related mafic plutonics and volcanics. Visibly altered samples show evidence for disturbance of the Sm−Nd and Rb−Sr isotopic systems. In one altered porphyritic dike plagioclase appears to have exchanged light REE with the relatively REE-rich basaltic matrix. This sample yields an internal Sm−Nd age of 2.16±0.05 Ga, which may correspond to the time of local heating or represent a partial resetting from a still younger event. Initial isotopic ratios of Nd and Sr determined here add to the growing body of data indicating that the Superior Province is underlain by depleted mantle.

SmNd age of the Fisken˦sset Anorthosite Complex, West Greenland

Abstract The Fisken˦sset Anorthosite Complex of southern West Greenland is a sheet-like layered igneous body which has been fragmented by granitoid intrusions, multiply deformed and folded, metamorphosed as high as granulite facies, and in some places retrogressed to lower amphibolite or greenschist facies. In some localities, however, such as at Majorqap qaˆva, some 20 km inland from the coast, these effects are minimal. Here, metamorphic grade reached only amphibolite facies with no apparent retrogression. Igneous textures and structures are spectacularly preserved, detailed stratigraphic sequences have been established, and in some samples primary mineralogy remains. We report a Sm Nd isotopic study on samples from this locality, attempting to constrain the age of crystallization of the Fisken˦sset Complex. This complex represents a useful marker unit with which the age relationships of surrounding units may be compared. A five-point isochron, including data for whole-rock samples of anorthosite, metagabbro, metaperidotite, and separates of calcic plagioclase and mafic matrix from a coarse megacrystic leucogabbro, corresponds to an age of 2.86 ± 0.05 Ga (MSWD = 2.5), with initial eNd of +2.9 ± 0.4. The Sm Nd age is significantly older than previous Pb Pb whole rock ages of the Complex, but is sufficiently close to the 2.8 Ga high-grade metamorphic event which affected the area to cause partial resetting of the Sm Nd system to be considered. However, based on the integrity of the whole-rock Sm Nd isotopic system in similar anorthositic complexes of higher metamorphic grade, and the apparent preservation of magmatic REE patterns in the analysed plagioclase megacryst and coexisting mafic matrix, we conclude that 2.86 ± 0.05 Ga represents the time of crystallization of the Fisken˦sset Complex. This would imply a relatively short time interval, on the order of 70 Ma, during which anorthosite formation, tonalite emplacement, and high-grade metamorphism took place, and this must be accounted for in any tectonic model for Late Archean events in southern West Greenland. In any case, the positive eNd value of the Fisken˦sset Complex requires that it was derived from a source with long-term depletion in light REE. These results add to the increasing body of data indicating the widespread presence of depleted mantle sources during the Archean.

River Valley pluton, Ontario: A late-Archean/early-Proterozoic anorthositic intrusion in the Grenville Province

Abstract The River Valley pluton is a ca. 100 km2 body of anorthositic and gabbroic rocks located about 50 km northeast of Sudbury, Ontario. The pluton is situated entirely within the Grenville Province, but its western margin is a series of imbricate thrust faults associated with the Grenville Front Tectonic Zone. It is dominated by coarse leuconorite and leucogabbro, with lesser anorthosite, gabbro, and rare ultramafics. Igneous textured rocks are abundant and consist of plagioclase (An60–70) charged with Fe-Ti oxide inclusions, low Ca pyroxene (orthopyroxene and/or inverted pigeonite) and augite. The most unfractionated rocks are minor olivine gabbros with Fo70–80. A variety of deformed and recrystallized equivalents of the igneous-textured rocks is also present, and these are composed largely of calcic plagioclase and hornblende. Ten samples, including both igneous and deformed lithologies give a Pb-Pb whole-rock isochron of 2560±155Ma, which is our best estimate of the time of primary crystallization. The River Valley pluton is thus the oldest anorthositic intrusive yet reported from the Grenville Province, but is more calcic and augitic than typical massifs, and lacks their characteristic Fe-Ti oxide ore deposits. The River Valley body may be more akin to similar gabbro-anorthosite bodies situated at the boundary between the Archean Superior Province and Huronian supracrustal belt of the Southern Province west of the Grenville Front. An Sm-Nd isochron from 3 igneous-textured leucogabbros and an augite mineral separate gives 2377 ± 68 Ma, implying slight disturbance of the Sm-Nd whole-rock-mineral system during later metamorphism. The Rb-Sr system has been substantially disturbed, giving an age of 2185 ± 105 Ma, which is similar to internal Pb-Pb isochron ages of 2165 ± 130 Ma and 2100 ± 35 Ma for two igneous-textured rocks. It is uncertain whether these ages correspond to a discrete event at this time or represent a partial resetting of the Rb-Sr and Pb-Pb systems from a younger event such as the Grenvillian orogeny of ca. 1.0 Ga. None of the isotopic systems we investigated, however, gives an age near 1.0 Ga, suggesting that neither the River Valley pluton, nor the immediately surrounding gneisses were strongly affected by metamorphism associated with the Grenvillian orogeny. Initial isotopic ratios for the River Valley pluton correspond to single-stage model parameters of μ = 8.06, ϵNd = 0 to −3, and ISr = 0.7015 to 0.7021. Collectively, these suggest either an enriched mantle source or crustal contamination of a mantle-derived magma. The crustal component involved must have been older and more radiogenic than the majority of rocks exposed at the surface in the nearby Superior Province.