The geology, petrology and petrogenesis of the White Hills peridotite, St. Anthony complex, northwestern Newfoundland

Talkington, Raymond W. (Raymond Willis) (1981) The geology, petrology and petrogenesis of the White Hills peridotite, St. Anthony complex, northwestern Newfoundland. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The White Hills Peridotite is located at the northernmost tip of the Great Northern Peninsula of Newfoundland. The peridotite is the highest structural slice of the Hare Bay Allochthon and outcrops as three massifs (two large and one small) which may represent dissected parts of a once complete ophiolite complex tectonically emplaced during the Mid-Ordovician as a result of the closing of the Iapetus Ocean. -- The peridotite is made up of an interlayered sequence (1 mm to ½ m) of spinel lherzolite and harzburgite (~ 85%), and dunite that occurs as layers, dikes and lenses which either parallel or are oblique to the lithological layering. High-alumina pyroxenite veins (orthopyroxenite, websterite, wehrlite, and clinopyroxenite) are ubiquitous, whereas gabbro veins are restricted to the eastern section of the eastern massif. Chromite-related lithologies (low-alumina orthopyroxenite, low-alumina "websterite", and chromitite) are common to the eastern massif. The vein material is concordant with or discordant to the lithological layering. -- At least two major deformational events have affected the peridotite, F₁ and F₂. F₁ is tentatively interpreted to be the result of high temperature plastic flow of the peridotite during its ascent below an accreting ridge and during this event the orthopyroxene-foliation may have developed. F₂ is interpreted to have formed during the tectonic emplacement of the peridotite. An F₃ deformational event only slightly affects the peridotite and is post-emplacement, probably an Acadian feature. -- The bulk rock chemistry for the White Hills Peridotite is similar to other ophiolites and subcontinental upper mantle material. There are, however, several internal geochemical differences which distinguish the various White Hills lithologies, especially the pyroxenite (all types) and gabbro veins. These differences are predominantly in the TiO₂ and Al₂O₃ contents of the constituent minerals. On the basis of mineral and bulk rock geochemical differences, at least two geochemical trends are inferred, and at least two distinct, yet genetically related, liquids are required to explain these differences. Most of the minerals of the peridotite show the effects of subsolidus mineral-bulk rock equilibration, although to various degrees. -- The petrogenetic model proposed for the White Hills Peridotite envisages at least two major partial melting events one at moderate to high pressures, the other at low pressure. The first liquid, an olivine tholeiite (first-stage liquid), is derived by approximately 20 percent partial melting of the spinel lherzolite source at high pressure (~ 20 kb). The second liquid, a magnesian-quartz tholeiite (second-stage liquid), is derived by approximately 5 to 10 percent partial melting of the previously melted spinel lherzolite source at low pressure (≲ 8 kb). The partial melting residua produced after the first- and second-stage melting are a clinopyroxene-poor lherzolite and a highly refractory harzburgite, respectively. Polybaric crystallization, from approximately 20 kilobars to approximately 10 kilobars, of the first-stage liquid results in the formation of some dunite and high-alumina pyroxenite (orthopyroxenite, websterite, wehrlite, clinopyroxenite) and gabbro veins. Crystallization of the second-stage liquid at low pressure produces the chromite-related lithologies (low-alumina orthopyroxenite, low-alumina "websterite", chromitite). -- The White Hills Peridotite represents a sample of upper mantle that preserves a variety of features and processes resulting from a hypothesized off-axis ascent trajectory.

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