Peridotite of the Gros Morne area, western Newfoundland

--- mantle rock in root of an ophiolite complex

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Figure 1. View of the serpentine barrens of the Tablelands. This area lies within Gros Morne National Park, and is a UNESCO-designated World Heritage Site. Here is a view up Winterhouse Brook, near the end of an easy 4-km, there-and-back trail walk through the unusual scenery, part of the Humber zone of the Appalachian geological belt. Rocks of the Humber zone underlie most of the western coastal area of Newfoundland (see beautiful field guide by Hild, 2016, pp.24-27 and [Tablelands] 74-77).

"Rock of the Month #210, posted for December 2018" ---

Peridotite, Mantle Rock, exposed in the ophiolite complex at Gros Morne National Park, western coast of Newfoundland, Canada:

A serpentinised peridotite underlies a strangely barren, apparently "deforested" landscape (but, see under "Ecology", below) on the west coast of Newfoundland, just south of the attractive village of Woody Point on Bonne Bay. The rock of this Tablelands area is the northernmost of four massifs that comprise the large Bay of Islands ophiolite complex, a well-preserved slice through the Earth's oceanic crust and underlying mantle (there is also another, smaller ophiolite near the northern extremity of the Great Northern Peninsula, close to St. Anthony, known as the White Hills peridotite). The Bay of Islands complex itself is the largest and best-exposed ophiolite in the Appalachian orogen. From south to north, evenly spaced about the actual Bay, are the Lewis Hills, Blow Me Down Mountain, North Arm Mountain and Table Mountain massifs. It is regarded as one of the most complete ophiolite sequences in the world, outcropping over an area of roughly 100x25 km (Malpas, 1987; Calon et al., 1988; Jenner et al., 1991; Bedard, 2014). As such, the Bay of Islands displays iron and magnesium-rich ultramafic plutonic rocks (peridotite and related rocks, such as lherzolite and harzburgite, names assigned primarily on the proportions of minerals in each sample), dykes and volcanic rocks.

Where land meets sea

The Bay of Islands complex has been dated at 484±5 Ma, at or just above the Cambrian-Ordovician boundary (Jenner et al., 1991). This was the time of ancient seas known as the Iapetus Ocean, remnants of which are found from from North America and Greenland to Britain and Scandinavia. Since the development of the theory of plate tectonics, there has been debate over the setting of individual ophiolites: was each one formed in the open ocean basin, or toward the ocean margin, where ocean crust slides beneath an island arc or continental margin to be destroyed (recycled) as it merges back into the Earth's upper mantle (?). Based on the ages and chemistry of rocks in the Bay of Islands complex and the Little Port complex (which forms a western and northern fringe to the Bay of Islands massifs), Jenner et al. (1991) affirmed that the Bay of Islands massifs formed above a subduction zone in a back-arc basin (a "suprasubduction-zone setting"). The Little Port complex represent somewhat older rocks formed along a volcanic arc, and - in this interpretation - the Bay of Islands ophiolite formed marginal to, rather than deep within, the Iapetus Ocean itself. Still geologically young, the ophiolites were then slid onto the east margin of the adjacent continent of Laurentia in the mid-Ordovician Taconic orogeny (nicely summarised in Fensome et al., pp.114-116). This obduction of oceanic rocks onto a shallow marine sequence of Cambrian-Ordovician carbonate sediments and older (Precambrian) basement rocks was part of a major mountain-building episode along the edge of the Iapetus Ocean. It also led to their preservation, rather than recycling (one of the key signatures of active plate tectonics is the virtual absence of ocean crust more than 200 million years old).

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Figures 2-3. Outcrop (at left) and a loose boulder of serpentinised ultramafic rock, showing the flat surface (left) and cross section of thin veins cross-cutting the parent igneous rock. The serpentine family of minerals are sheet silicates, just like micas and clay minerals. They typically form during the cooling of magnesium-rich igneous rocks. Cracking of the rock as it cools generates fractures (think of the columnar joints in lavas, as at the famed Giant's Causeway in Ireland) which are infilled by "secondary" (post-magmatic) minerals. Conversely, the hydrous alteration of olivine and in part clinopyroxene leads to expansion in the bulk, as much as 30 volume percent (Komor et al., 1985). Names such as serpentine and lizardite (one variety) allude to the lustrous, snakeskin-like appearance of coarse flakes and fibres of these minerals.

A geological wonderland

The ultramafic massifs of the Bay of Islands have been studied intensively (the MINLIB bibliography, a partial sampling, has about 100 references describing them). They were soon noted to include good examples of layered igneous rocks (Wager and Brown,1968), podiform chromitites (chromite-rich rocks: Kacira, 1982; Dahl and Watkinson, 1986), cumulate rocks (Elthon and Casey, 1985; Komor and Elthon, 1990), serpentinization (Komor et al., 1985) and anorthosites (Ashwal, 2010). The chromitite occurrences appear subeconomic, even though, as is common in such rocks, they contain traces of minerals of platinum-group elements such as osmium, iridium and ruthenium (e.g., laurite and irarsite). The highly-reduced serpentinites may also contain flecks of a natural nickel-iron alloy, the mineral awaruite.

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Figures 4-5. Two more images of peridotite boulders depicting the alteration of ultramafic, iron and magnesium- rich rocks, leading to destruction of olivine and consequent expansion, with fracture development and deposition of magnesium-rich minerals (which include serpentine, talc, and possibly others such as carbonate (magnesite) and hydroxide (brucite)).

Ecology

Unlike much of North America and the wider world, the Tablelands landscape is not bare and devoid of trees due to logging and settlement. Rather, the combination of a harsh maritime climate and - especially - the peculiar nature of the soil dictates the flora that can thrive here. This turns out to be a rather restricted cast of characters - plants and shrubs that are tolerant of the thin soil and rock developed on the weathered surface of the peridotite massif. The soil is key: even the rounded summits of the much higher Long Range Mountains, just to the east, display more low, shrubby vegetation than are seen here. The landscape has been sculpted by Ice-Age glaciers (Osborn et al., 2007). Osborn et al. constructed a digital elevation model for the Tablelands plateau, which rises to a maximum elevation of 711 m, south of the Bonne Bay fjord system. The southwest margin is another fjord valley, infilled by Trout River Pond. This valley held an active glacier until very late in Pleistocene time, while Winterhouse Brook itself occupies a glaciated gorge.

The peridotite is richer than most rocks in a number of elements, most notably magnesium, but also minor elements such as nickel and chromium. Weathering of the olivine-rich rock generates secondary minerals such as serpentine and other silicates and carbonates. The resulting waters seeping from the massif are extremely alkaline (Hild, 2016), which is why the environment proves so inhospitable for most flora. Stevens (1988) reported the presence of such groundwaters in hyperalkaline springs, sometimes with calcareous tufa deposits, as evidence of serpentinization active to this day.

Some dwarf trees, shrubs and flowers do appear, such as juniper, harebell, moss campion, serpentine sandwort and pitcher plant (see exhibit at the visitor centre, below). A few others, noted on the Winterhouse Brook trail in September, include small examples of alder and tamarack, asters (including pearly everlasting) and goldenrod, red clover and yarrow. These small but hardy inhabitants of the serpentine barrens are as intriguing to botanists as are the rocks to a geologist!


MUSEUM MOMENT #6

Gros Morne National Park of Canada Discovery Centre, 21 September 2018.

For a clear and attractive explanation of the local geology and ecology, visitors should not miss the Gros Morne National Park of Canada Discovery Centre, located just outside of Woody Point, beside the road to Trout River, and not far from the car park for the hiking trail at Winterhouse Brook. The building is bright and spacious, and the staff friendly and helpful. A popular guidebook for Gros Morne was published some time ago (Burzynski and Marceau, 1990). The wider geological context (all geology, all Canada!) is summarized in an excellent guide to the geology of Canada (Fensome et al., editors, 2014). The Discovery Centre has large samples of local rocks, from the ophiolite sequence and adjacent geological terranes: gneiss, limestone conglomerate and quartzite, pillow basalt and peridotite. There are biographical notes on eminent scientists whose work has involved this rugged coast, from plate tectonics pioneer J. Tuzo Wilson to Newfoundland geologists Bob Stevens and Hank Williams, and marine biologist Bob Hooper. There are even droll miniature sculptures, representing the various kinds of scientist that work along these shores and across the mountains!


References (n=17)

Ashwal,LD (2010) The temporality of anorthosites. Can.Mineral. 48, 711- 728.

Bedard,JH (2014) Ophiolites: perspectives from fieldwork in the Appalachians. Elements 10 no.2, 87- 88.

Burzynski,M and Marceau,A (editors) (1990) Rocks Adrift: the Geology of Gros Morne National Park. Environment Canada Parks Service, 58pp.

Calon,TJ, Dunsworth,SD and Suhr,G (1988) The Bay of Islands Ophiolite. GAC/MAC Field Trip Guidebook B8, 92pp.

Dahl,R and Watkinson,DH (1986) Structural control of podiform chromitite in Bay of Islands ophiolite, Springer Hill area, Newfoundland. GSC Pap. 86-1B, 880pp. in 2 vols., 757- 766.

Edwards,SJ (1990) Harzburgites and refractory melts in the Lewis Hills massif, Bay of Islands ophiolite complex: the base metals and precious metals story. In `Advances in the Study of Platinum Group Elements' (Barnes,S J and Duke,JM editors), Can.Mineral. 28 part 3, 537- 552.

Elthon,D and Casey,JF (1985) The very depleted nature of certain primary mid ocean ridge basalts. Geochim. Cosmochim. Acta 49, 289- 298.

Fensome,R, Williams,G, Achab,A, Clague,J, Corrigan,D, Monger,J and Nowlan,G (editors) (2014) Four Billion Years and Counting: Canada's Geological Heritage. Nimbus Publishing / Canadian Federation of Earth Sciences, 402pp.

Hild,MH (2016) Geology of Newfoundland: Touring through Time at 48 Scenic Sites. Boulder Publications, Portugal Cove St. Philips, Newfoundland and Labrador, 256pp.

Jenner,GA, Dunning,GR, Malpas,J, Brown,M and Brace,T (1991) Bay of Islands and Little Port complexes, revisited: age, geochemical and isotopic evidence confirm suprasubduction zone origin. Can.J. Earth Sci. 28, 1635- 1652.

Kacira,N (1982) Chromite occurrences of the Canadian Appalachians. CIM Bull. 75 no.837, 73- 82.

Komor,SC and Elthon,D (1990) Formation of anorthosite gabbro rhythmic phase layering: an example at North Arm Mountain, Bay of Islands ophiolite. J.Petrol. 31, 1- 50.

Komor,SC, Elthon,D and Casey,JF (1985) Serpentinization of cumulate ultramafic rocks from the North Arm Mountain massif of the Bay of Islands ophiolite. Geochim. Cosmochim. Acta 49, 2331- 2338.

Malpas,J (1987) The Bay of Islands ophiolite: a cross section through Paleozoic crust and mantle in western Newfoundland. In `Centennial Field Guide Volume 5', Northeastern Section of the Geological Society of America (Roy,DC editor), GSA, 481pp., trip 96, 451- 456.

Osborn,G, Spooner,I, Gosse,J and Clark,D (2007) Alpine glacial geology of the Tablelands, Gros Morne national park, Newfoundland. Can.J. Earth Sci. 44, 819- 834.

Stevens,RK (1988) Ophiolite oddities: preliminary notices of nephrite jade, Sr-aragonite, troilite and hyperalkaline springs from Newfoundland. GAC/MAC Prog.w.Abs. 13, 118, St. John's.

Wager,LR and Brown,GM (1968) Layered Igneous Rocks. Oliver and Boyd Ltd, Edinburgh, 588pp.

Graham Wilson, 11,13,26 November-02 December 2018

See a very special rock, a chromitite from deep in the Earth's mantle, in the Luobusa ophiolite of southern Tibet,

or, visit the "Rock of the Month" Archives!