Andrew Hynes

Carbonatization and mobility of Ti, Y, and Zr in Ascot Formation metabasalts, SE Quebec

The middle Ordovician Ascot Formation of southeastern Quebec consists of greenschist facies metamorphosed silicic to mafic pyroclastic rocks and lava flows and associated metasediments. Chemical analyses of lavas reveal a preponderance of metarhyolites and metabasalts, together with some porphyritic rocks with intermediate SiO2 contents. The metabasalts exhibit wide ranges in concentrations of TiO2 (0.25–2.0 wt.°), Y (9–46 ppm), and Zr (5–135 ppm). The extent of the ranges, and unusual interelement ratios, suggest that the concentrations of these normally immobile elements have been affected by secondary processes.There is a strong correlation between trace-element concentrations and the degree of carbonatization of the metabasalts. Low carbonate rocks are severely depleted in Ti, Y, and Zr whereas high carbonate rocks are depleted in Y and Zr and enriched in Ti. The differing movement of Ti can be explained in terms of variable chemical potential due to the various carbonatization reactions affecting titaniferous phases. Overall mobility of these generally “immobile/rd elements is attributed to high CO2 levels in the fluid phase during metamorphism.Extrapolation of the two alteration trends to a common origin enables one to infer primary concentrations of the trace-elements. Primary inter-element ratios arrived at in this way are compatible with an island-arc origin for the Ascot Formation although TiO2 concentrations are a little high (1.5 wt/%).

Plume-modified orogeny: An example from the western United States

Plate tectonic theory implies that orogeny at convergent margins results from several processes, including subduction of oceanic crust, subduction of aseismic ridges, accretion of terranes, and continental collision. Each of these processes involves the consumption of an oceanic tract, and each has its own characteristic style of tectonothermal activity. An additional, potentially important, orogenic process has been largely overlooked. Oceanic regions contain chains of volcanic islands formed at hotspots, which are generally considered to reflect the sites of rising plumes. In a hotspot reference frame, where active continental margins advance and override a plume, the plume9s buoyant swell may profoundly change the tectonothermal expression of subduction and hence orogenic processes at the continental margin. The Late Cretaceous–Tertiary evolution of the western United States may be an example of this process, which has been overlooked in the development of orogenic paradigms.

Seismic images of eclogites, crustal-scale extension, and Moho relief in the eastern Grenville province, Quebec

Seismic images from a 250 km Lithoprobe reflection profile in the interior of the eastern Grenville province provide important new constraints on the crustal architecture of this part of the orogen. Prominent upper-crustal reflections can be correlated with exposures of high-pressure metamorphic rocks in the Manicouagan shear belt, providing the first direct evidence for eclogite reflectivity in the Grenville province. The eclogites are cut by major late Grenvillian normal faults that penetrate the deep crust and preserve evidence of extensional collapse of the overthickened orogen. North-to-south crustal thinning, indicated by a change in Moho reflection time from 16 to 13 s, correlates well with regional Bouguer gravity trends and is accompanied by a dramatic increase in the reflectivity of the lower crust. These features underscore the significance of recently recognized along-strike variations in tectonic style within the Grenville province and point to the internal complexity that characterizes the root zones of collisional orogens.

A transect of the early Proterozoic Cape Smith foldbelt, New Quebec

Abstract A transect of the Cape Smith foldbelt near Asbestos Hill consists of eight blocks separated by north-dipping, high-angle reverse faults. The most southerly block (Block 1) contains a succession of quartz-rich, continent-derived clastic sediments resting unconformably on Archean gneisses. Mafic volcanic rocks occur in the next block to the north (Block 2) and become progressively more important in the two subsequent blocks (Blocks 2a and 3), along with fining of the clastic sediments. These blocks are interpreted as lateral equivalents, representing the collapsed southern edge of a basin. A similar succession is preserved in the northernmost two blocks (Blocks 6 and 7), and may be part of the northern edge of the basin. The mafic rocks of Blocks 2, 2a, 6 and 7 are Ti-rich tholeiites with all the chemical features of contemporary continental tholeiites, and are associated with minor amounts of rhyolite. They are thought to reflect an episode of continental rifting. Thick high-Mg (komatiitic) layered sills are present in many of the blocks, and the two central blocks (Blocks 4 and 5) consist almost exclusively of komatiitic basalt, cogenetic low-Ti tholeiitic basalt and associated intrusives. The basalts occur in orderly sequences changing progressively upwards from komatiitic to tholeiitic, and have built up a succession over 4 km thick. The more northerly block (Block 5) is on average much less magnesian than the one further south (Block 4). The tholeiitic rocks from these two blocks are similar to modern MORB in both major and trace-element characteristics. The less magnesian block may represent early Proterozoic oceanic crust, with the more magnesian block formed at an earlier stage in the spreading before a steady state had been achieved. Although the foldbelt very probably represents a continental rift zone, and quite possibly one in which true oceanic crust was generated, the general absence of sediment in the “oceanic” blocks, the absence of subduction-related volcanics and the similarities of the two margins suggest that it does not reflect a major suture zone. It was probably an ocean-basin of limited width, destroyed by north-dipping subduction.

Crystal fractionation and partial melting in the petrogenesis of a Proterozoic high-MgO volcanic suite, Ungava, Québec

There is little concensus on the relative importance of crystal fractionation and differential partial melting to the chemical diversity observed within most types of volcanic suites. A resolution to this controversy is best sought in suites containing high MgO lavas such as the Chukotat volcanics of the Proterozoic Cape Smith foldbelt, Ungava, Quebec. The succession of this volcanic suite consists of repetitive sequences, each beginning with olivine-phyric basalt (19-12 wt% MgO), grading upwards to pyroxene-phyric basalt (12-8 wt% MgO) and then, in later sequences, to plagioclase-phyric basalt (7-4 wt% MgO). Only the olivine-phyric basalts have compositions capable of equilibrating with the upper mantle and are believed to represent parental magmas for the suite. The pyroxene-phyric and plagioclase-phyric basalts represent magmas derived from these parents by the crystal fractionation of olivine, with minor chromite, clinopyroxene and plagioclase. The order of extrusion in each volcanic sequence is interpreted to reflect a density effect in which successively lighter, more evolved magmas are erupted as hydrostatic pressure wanes. The pyroxene-phyric basalts appear to have evolved at high levels in the active part of the conduit system as the eruption of their parents was in progress. The plagioclase-phyric basalts may represent residual liquids expelled from isolated reservoirs along the crust-mantle interface during the late stages of volcanic activity.A positive correlation between FeO and MgO in the early, most basic olivine-phyric basalts is interpreted to reflect progressive adiabatic partial melting in the upper mantle. Although this complicates the chemistry, it is not a significant factor in the compositional diversification of the volcanic suite. The preservation of such compositional melting effects, however, suggests that the most basic olivine-phyric basalts represent primitive magmas. The trace element characteristics of these magmas, and their derivatives, indicate that the mantle source for the Chukotat volcanics had experienced a previous melting event.

Komatiite-derived tholeiites in the Proterozoic of New Quebec

Abstract Layered sills and flows are conspicuous in the komatiitic volcanics of the Chukotat Group of the Aphebian Cape Smith fold belt in New Quebec. These bodies consist of a lower ultramafic member with an overlying gabbroic complex and are bound by margins of quench-textured, pyroxene-rich melanogabbro. Features such as cyclic layering of pyroxenite and peridotite, successive appearance of euhedral olivine, clinopyroxene, and plagioclase, and polarized compositional variation indicate that the ultramafic member and lower gabbro are crystal cumulates. The uppermost gabbros, however, appear to represent liquids derived by removal of these cumulates. The significance of these bodies is that their initial liquids were at least as basic as pyroxenitic komatiites (14 wt.% MgO) while the residual liquids are Fe-Ti-rich tholeiites. Similarity between the liquid line of descent within these differentiated bodies and the spectrum of volcanic composition of the Chukotat Group as a whole suggests that the komatiites and tholeiites of the latter may constitute a single magmatic suite whose chemical diversity is a function of low-pressure, crystal fractionation.

The onset of interaction between the hydrosphere and oceanic crust, and the origin of the first continental lithosphere

Abstract New continental crust forms above subduction zones through the recycling of hydrated oceanic lithosphere. The most efficient process known for oceanic lithosphere hydration takes place at the submerged mid-ocean ridges where the lithosphere is young and warm, and cools through hydrothermal convection. Such mid-ocean ridge hydrothermal interactions were operative at least as far back as 3.5–3.8 Ga. The apparent absence of preserved continental crust older than 4.0 Ga may reflect the absence of hydrothermal interaction before that time. This model requires that prior to about 4.0 Ga mid-ocean ridges stood above sea level. Our calculations show, however, that on a plate-tectonic early Earth with substantially less continent, realistic higher heat flow, and a volume of sea water similar to that of today’s ocean, Archaean mid-ocean ridges would have remained below sea level. Only with a substantial reduction of surface water would Earth have been able to recycle dry oceanic lithosphere, and thus prevent the present day style of continental crust formation. A 30% reduction of surface water is required to elevate early Earth’s ridges above sea level. This excess water may have been stored in nominally anhydrous minerals of the mantle. Early Earth’s mantle may have released a significant proportion of its initial water only gradually through convective overturn of the oceanic floor. Given realistic ocean-floor creation rates, it would have taken roughly 500 Ma to process the early Earth’s mantle through a MORB generation event if only the upper mantle was involved and considerably longer if whole mantle convection was involved. The inefficiency of water extraction during this process is illustrated by the amount of water apparently present in the source regions for present-day MORB. In this scenario, the Hadean-Archaean transition may mark the time when Earth changed its style of cooling from one dominated by heat exchange directly to the atmosphere to one dominated by heat exchange with the hydrosphere, which still buffers Earth’s heat loss today.

A Comparison of amphiboles from medium- and low-pressure metabasites

A comparison of published metabasite amphibole analyses from medium and low-pressure metamorphic terrains reveals that there is no systematic variation in Na, NaM4, Al or AlVI as a function of pressure. This may be due to blurring of the differences by variation in oxidation state, or by analytical differences between laboratories. It is not due to variable Mg/Fe in whole rocks. Differences that can be recognised are generally higher Ti/Al ratios in the low-pressure amphiboles, and a very poorly developed compositional gap between actinolite and hornblende compared with a well-developed gap at medium pressures. These features, together with the relatively low-grade appearance of calcic plagioclase at low pressures, provide the best means of distinguishing metabasites from the two facies series.All three features can be explained by the configuration of cation-exchange equilibria at the greenschist/amphibolite facies boundary. Enrichment in Ti at low-pressures is due to the positive slope of reactions partitioning Ti into the amphibole. The composition gap in amphiboles at medium-pressure is due to overstepping of the tschermakite-enriching equilibrium. At low pressures this overstepping still occurs, but the equilibrium tschermakite-content in the amphibole is much lower for a given amount of overstepping. The relatively low-grade appearance of oligoclase at low pressures is due to convergence of the tschermakite and anorthite-enriching equilibria with decreasing pressure.

Reconstructing the ancestral Yellowstone plume from accreted seamounts and its relationship to flat-slab subduction

Recent geodynamic analyses have emphasized the relationship between modern flat-slab subduction zones and the overriding of buoyant oceanic crust. Although most models for the evolution of the Late Mesozoic–Cenozoic Laramide orogeny in the southwestern United States involve flat-slab subduction, the mechanisms proposed are controversial. An examination of the geological evolution of the 60–50-Ma Crescent terrane of the Coast Ranges indicates that it was formed in a shallowing-upward Loihi-type oceanic setting culminating in the eruption of subaerial lavas. Plate reconstructions indicate that the Crescent terrane was emplaced into ca. 20-Ma crust, and the presence of subaerial lavas implies an uplift due to the plume of ca. 4.2 km, which we use to calculate a minimum buoyancy flux of 1.1 Mg s � 1 , similar to that of the modern Yellowstone plume. Published paleomagnetic data indicate that the Crescent terrane was formed at a paleolatitude similar to that of the Yellowstone plume. The Crescent seamount was accreted within 5 My of the cessation of plume magmatism. Plate reconstructions indicate that it would have originated about 750 km to the west of the North American plate margin if it developed above a fixed Yellowstone plume, and are therefore consistent with the recorded very short interval between its formation and tectonic emplacement. We interpret the Crescent terrane as due to the ancestral Yellowstone plume. Such a plume would have generated an elongate swell and related plateau that would have been overridden by the North American margin. Taken together, the relationship between flat-slab and overriding of oceanic plateau in Laramide times would have been analogous to the relationship between modern Andean flat-slab subduction zones and the Juan Fernandez and Nazca oceanic plateaus. D 2003 Elsevier Science B.V. All rights reserved.

Subduction of continental margins and the uplift of high-pressure metamorphic rocks

Abstract The mechanism by which high-pressure metamorphosed continental material is emplaced at high structural levels is a major unsolved problem of collisional orogenesis. We suggest that the emplacement results from partial subduction of the continental margin which, because of its high flexural rigidity, produces a rapid change in the trajectory of the descending slab. We assume a two-fold increase in effective elastic thickness of the lithosphere as the continental margin approaches the subduction zone, and calculate the flexural profile of a thin plate for progressive downward migration of the zone of increased rigidity. We assess the effect of changes in the flexural profile on the overlying accretionary prism and mantle wedge as the continent approaches by estimating the extra stresses that are imposed on the wedge due to the bending moment exerted by the continental part of the plate. The wedges overlying the subduction zones, and the subducting slab itself, experience substantial extra compressional stress at depths of around 100 km, and extensional stress at shallower depths, as the continental margin passes through the zone of maximum curvature. The magnitudes of such extra stresses are probably adequate to effect significant deformation of the wedge and/or the descending plate, and are experienced in a time interval of less than 5 m.y. for typical subduction rates. The spatial variation of yield stresses in the region of the wedge and descending slab indicates that much of this deformation may be taken up in the crustal part of the descending slab, which is the weakest region in the deeper parts of the subduction zone. This may result in rapid upward migration of the crust of the partially subducted continental margin, against the flow of subduction. High-pressure metamorphosed terranes emplaced by the mechanism envisaged in this paper would be bounded by thrust faults below and normal faults above. Movement on the faults would have been coeval, and would have resulted in rapid unroofing of the high-pressure terranes, synchronous with arrival of the continental margin at the subduction zone and, therefore, relatively early in the history of a collisional orogen.