by R. W. Shepherd
In 1905, German geologist Gustav Steinmann identified a succession of basal serpentinite, basalt, and deep water sedimentary rocks at a continental location known as Steinmann’s Trinity. Layered cumulate ultramafics and layered gabbros have been added to Steinmann’s original association of rocks, now known by the term “ophiolite” and derived from the Greek word “ophis“ meaning snake. Ophiolites are interpreted to be fragments of ocean floor sheared off from subducting crust, caught between colliding continents, and elevated to a continental location. Ophiolites have great geological significance for two reasons:
The Betts Cove ophiolite was identified in the 1960s by J.F. Dewey and J.M. Bird of the State University of New York, and by graduate student Robert Stevens of Memorial University, amongst others, as the remains of ancient ocean crust. This ophiolite was one of the first confirmations of the 1966 hypothesis of the University of Toronto’s J. Tuzo Wilson relevant to plate tectonic activity, seafloor spreading, and the Wilson cycle.
Figure 1: Exposed crust - lithosphere contact at Table Mountain, Gros Morne National Park, Newfoundland.
The supercontinent Rodinia broke apart over the period from 750 million to 600 million years ago, in the Precambrian, as mantle heat built up under the supercontinental mass, increased elevation, and created a rift and spreading centre. The proto-North American continent, Laurentia, moved away from the spreading centre and away from the continents of Africa, South America, Baltica, and Siberia, creating a new basin which was filled by the Iapetus Ocean. Over a period of 100 million years, the Iapetus spread to a maximum width of approximately 5,000 kilometers before a subduction zone and associated volcanic arc developed off the Laurentian continental margin. In the late Cambrian, about 480 million years ago, the Iapetus began to close as ocean crust was subducted beneath the volcanic arc. By 420 million years ago, in the late Silurian, the Iapetus was almost closed as a prelude to the continental re-assembly phase of a Wilson cycle forming the supercontinent Pangea. As re-assembly proceeded, four terranes comprising present day Newfoundland collided and were accreted to the continental margin of Laurentia over the period 600 million years ago to 200 million years ago, in order, as follows:
Humber Terrane: continental margin of the proto-North American continent, Precambrian in age
Dunnage Terrane: deformed relics of metamorphosed ocean crust, thrust over the Humber Terrane, includes obducted remnants comprising the Betts Cove Ophiolite
Gander Terrane: fragment of the Moroccan-European continent
Avalon Terrane: unidentified continental fragments with similarities to the geology of Morocco and to the Precambrian geology of central England
The foregoing history is supported by the discovery of Cambrian age trilobites in western Newfoundland which are faunally and chronologically similar to trilobites in northern Scotland. Also, in eastern Newfoundland, trilobite fossil suites are closely aligned in age and type with those in England.
By the late Ordovician-early Silurian, the Taconic Orogeny resulted in significant crustal shortening. The Dunnage Terrane, comprised of crustal and arc remnants from the Iapetus Ocean, including the Betts Cove Ophiolite suite, collided with and was compressed against the Humber Terrane. Accretion to the Laurentian margin deformed intervening rocks into a melange. The weight of emplaced oceanic crust comprising the ophiolite suite on the underlying continental crust, and the shearing stresses of emplacement, resulted in metamorphic alteration at the contact between the ophiolite and underlying rocks of the Humber Terrane. The mélange and an ophiolitic sole of serpentinite developed at this contact. Subsequently, the Gander Terrane was deformed and faulted against the Dunnage Terrane, later followed by accretion of the Avalon Terrane.
The Betts Cove Ophiolite is Ordovician in age, included within the Dunnage Terrane, and located on the southeast shore of Newfoundland’s Baie Verte Peninsula, at the north end of the Appalachian mountain range.
Figure 4: Correlation of typical ophiolite suite to Betts Cove Ophiolite suite.
Although ophiolites represent ocean crust generated at a spreading centre, ophiolitic geochemistry differs from that of mid-ocean ridge basalts. The Betts Head Formation has boninitic affinity and is conformably overlain by tholeiitic pillow lavas and pillow breccias of the Mount Misery Formation. Tholeiitic magmas have arc affinities and are not generated in the fore-arc environment indicated for magmas of boninitic affinity. The tholeiitic affinity of the Mount Misery and other rocks overlying the Betts Head assists in defining the top of the ophiolitic suite.
The contact between mantle peridotites and overlying ocean crust, encountered at a typical depth of 4.75 kilometers, represents the petrologic Mohorovicic discontinuity. The seismic Mohorovicic discontinuity is shallower, within the basal portion of the ocean crust, at a depth which correlates to an increase in seismic wave velocity as olivine crystal structure changes to the denser spinel structure.
Figure 5: Geological location and ophiolitic units at Betts Cove, Newfoundland.
Evidence concerning the in situ genesis of rocks in the Betts Cove Ophiolite is provided by examining the geochemistry of the parent magmas. These magmas have a geochemistry characterized by high MgO, SiO2, and H2O content, TiO2 poor ultramafic minerals such as orthopyroxene and clinoenstatite, and depletion of trace elements relative to mid-ocean ridge basalts. This geochemistry is typical of the entire Betts Cove suite of ophiolitic rocks and indicates a boninitic affinity. Betts Cove boninites are geochemically indistinguishable from modern boninites which are restricted to fore-arc environments and, in rare cases, mature back-arc basins. Accordingly, generation of huge boninitic magma volumes requires seafloor spreading in a fore-arc environment showing subduction zone proximity and decompression.
As the Iapetus Ocean was closing, a narrow plume of magma rising from a subducting plate and mantle wedge was focused on overlying oceanic lithosphere thereby supporting a stable arc environment and depleting the underlying mantle. This magma flux uplifted the arc and thinned the oceanic crust. Extensional tectonic forces including gravity created a rift and a new seafloor spreading centre. Devolatization of water and large ion lithophile elements such as cesium, barium, rubidium, thorium, and uranium from the subducting slab and sediments, lowered the melting temperature in the convecting mantle wedge. Decompression coupled with effluxed volatiles and depleted mantle resulted in partial melting at shallower depths and production of magma of boninitic affinity, creating new ocean crust in a new fore-arc basin. Large volumes of boninitic magma, characteristic of the Betts Cove Ophiolite, were generated. The extensional forces exerted on the overriding plate due to seafloor spreading in the new basin resulted in trench rollback. Undepleted mantle migrated into the mantle wedge to occupy volume created by extension and slab rollback where melting occurred, creating the tholeiitic magmas of the overlying Snooks Arm Group.
As subduction proceeded to the southeast (present day), the leading edge of the Laurentian continent was partly subducted beneath ocean crust and Taconic arcs of the overriding plate, thus slowing convergence of terranes. As isostatic rebound of the continental crust occurred, a portion of the overriding oceanic crust was elevated to a location above sea level as the suite of rocks known as the Betts Cove Ophiolite
Ophiolites are important hosts for sedimentary exhalative and volcanogenic massive sulphide deposits of copper, zinc, and lead. At seafloor spreading sites, hydrothermal solutions circulated within volcanic rocks and became enriched in base metals which were then precipitated on or within pillow lavas. Other ore deposits bearing copper-nickel-cobalt-iron occur as tabular bodies within the deep plutonic portion of ophiolites with mineralogy dominated by harzburgites and dunites of the upper mantle.
A number of mineral occurrences have been identified within the ophiolite at Betts Cove, the most significant associated with the Tilt Cove Mine and the Betts Cove Mine. At Tilt Cove, sulphides occur in a 400 meter thick zone at the contact between boninitic lavas of the Betts Head Formation and overlying tholeiitic rocks of the Mount Misery Formation. From discovery in 1857, and through two productive periods ending in 1967, the Tilt Cove Mine yielded 7.4 million tons of ore with an average grade of 1.24% copper and associated production of 42,500 ounces of gold. The ore body at the Betts Cove Mine occurs between Betts Head Formation pillow lavas and the sheeted dyke unit. Over the period 1864 to 1886, 130,682 tons of ore was mined, grading to 10% copper with minor zinc, lead, and gold.
The only mine currently in production in the area of the Betts Cove Ophiolite is the Nugget Pond Mine which has produced since 1988 with a tonnage of 443,000 tons grading 0.357 ounces per ton gold. The Nugget Pond occurrence is associated with hydrothermal circulation and alteration by postophiolite granitoid intrusions which concentrated gold and other metals sourced within volcanic rocks of the Betts Cove Ophiolite in basal turbidites of the overlying Scrape Point Formation of the Snooks Arm Group.
The Betts Cove Ophiolite is an excellent example of obducted ocean floor providing evidence in support of geological premises which include the following:
Study of the Betts Cove Ophiolite and the Dunnage Terrane in which it is included has been instrumental in understanding the geologic history of the Iapetus Ocean and the northern portion of the Appalachian mountain chain. The Betts Cove Ophiolite has also been important in developing general concepts associated with continental drift, the Earth’s internal structure, seafloor spreading, subduction zones, plate tectonic mechanisms, and ore deposits, providing a focal point for the integration of many geological disciplines.
Figures 1, 2, 3, 5, and 7 are reprinted with the permission of Dr. Andrew Miall of the University of Toronto. Figure 6 is reprinted with the permission of Dr. J.H. Bedard of the Geological Survey of Canada. Cooperation in allowing the use of these figures is greatly appreciated.
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