Geology of the Plutonic and Hypabyssal Rocks of the Betts Cove Ophiolite Complex, Newfoundland

• Main Author: Idleman, Bruce D.
• Format: Text
• Language: unknown
• Published: Scholars Archive 1981
• Subjects: Betts Cove Ophiolite Complex; Ordovician Snooks Arm Group; Geology; Sedimentology; Stratigraphy; Tectonics and Structure; Betts Cove; Cape St. John; Snooks Arm; Newfoundland
• Online Access: https://scholarsarchive.library.albany.edu/cas_daes_geology_etd/40; https://scholarsarchive.library.albany.edu/cgi/viewcontent.cgi?article=1039&context=cas_daes_geology_etd

The Betts Cove ophiolite complex lies within the Dunnage Zone island arc terrain of central Newfoundland. It forms the base of the lower Ordovician Snooks Arm Group, which consists of mafic igneous rocks of the ophiolite and a thick sequence of volcanic and volcaniclastic rocks which conformably overlies it. The Snooks Arm Group is unconformably overlain by sub-aerial sediments and volcanics of the presumed Silurian Cape St. John Group. The base o£ the ophiolite complex consists predominantly of ultramafic rocks. These are interlayered on a variety of scales, and show mesoscopic igneous structures which suggest that they were produced by magma chamber processes. They consist largely of olivine, orthopyroxene, and clinopyroxene, with minor plagioclase and chromite. These phases are generally quite altered, but original igneous textures are well-preserved in most samples. A thin sequence of interlayered ultramafic and gabbroic rocks overlies these layered ultramafic rocks. This sequence is similar to the underlying one, but contains a significantly greater proportion of gabbroic rocks. Most of the mesoscopic features of these interlayered rocks are similar to those of the layered ultramafic rocks. In some places interlayered rocks pass upward into a thin zone of homogeneous gabbro, which is largely devoid of compositional variation, igneous layering, and other mesoscopic features. This gabbro consists primarily of variably altered plagioclase, clinopyroxene, orthopyroxene and rare olivine. A sheeted diabase dike complex overlies the plutonic portion of the ophiolite. In some places the contact between the dikes and the plutonic rocks is quite sharp, while in others dikes related to the sheeted complex extend downward as far as the upper portion of the layered ultramafic sequence. The sheeted complex consists almost entirely of altered diabase and picritic diabase dikes, with rare screens of gabbroic and ultramafic plutonic lithologies. The ophiolite is capped by altered, basic volcanic rocks, occurring as pillows, massive flows, and volcanic breccia. Dikes of the sheeted complex extend upward into these volcanics in places. Many of the original petrologic and structural characteristics of the Betts Cove ophiolite have been obscured by deformation and metamorphism, which occurred at the same time as or soon after the formation of the ophiolite. Despite this, detailed mapping has shown that the ophiolite consists of a conformable sequence of igneous rocks similar to those seen in other ophiolite complexes. It differs somewhat from many ophiolites, however, in that the thickness of its gabbroic sequence is quite small. Field relationships, augmented by geochemical and petrologic data, suggest that the ophiolite complex was produced from a picritic primitive melt by igneous processes occurring within and adjacent to a high-level, sill-like magma chamber, Structural relationships observed within the complex provide some constraints for the size and geometry of this chamber. Regional relationships and geochemical data suggest that the Betts Cove ophiolite was formed during Early Ordovician time as a marginal or rear-arc basin above an east-dipping subduction zone.