ABSTRACT
Detailed mapping (1:15,800) of the northern half of North Arm
Massif shows the area to be underlain with a complete, although
thin, ophiolite assemblage consisting, from top to base, of
basalts, sheeted diabase dikes, isotropic and layered gabbroic
rocks, layered ultramafic rocks and harzburgite tectonite. The
assemblage represents a preserved piece of Cambrian oceanic crust
analogous to present-day oceanic crust. Sheeted dikes and
isotropic gabbros comprise the majority of the rocks exposed, with
this sequence at present flat-lying to gently warped into open,
upright but nonsystematic folds. Basalts occupy a shallow
discontinuous N-S trending trough within the west-central part of
the area. The remaining lithologies are exposed in lesser amounts
along the western, eastern and northeastern edges of the surveyed
area.
Volcanics consist of pillowed lavas, massive flows and breccias
plus intercalated red sediment, all crudely layered, with layer
attitudes intersecting the lower horizon of the unit with large
angles. Volcanics are fed by underlying diabase dikes, and grade
into sheeted dikes by downward increase in numbers of dikes across
a transition generally less than 50 m thick. The sheeted dikes
unit consists entirely of multiple diabase dikes of 0.9 m mean
thickness, subparallel throughout and trending NNW-SSE across the
massif. The dike intrusion process includes a mode wherein several
dikes each intrude the previous dikes, with the position of these
very narrow, short term spreading axes episodically shifting
within a wider but still narrow overall zone of intrusion and
spreading. Dikes intersect the upper and lower contacts of the
unit with mean angles of 60º and 75º respectively, and
dip away from the spreading center, located to the west of the
present position of the section on the basis of dikes attitudes
and chill margin analysis. The sheeted dikes unit ranges between
400 and 600 m thick. An estimated 40% of the diabase shows
extensive in situ fracturing in distinct zones subparallel to
dikes trends. Major zones of fracturing extend downward into
gabbro metamorphosed to foliated and lineated amphibolite. This
fracturing is interpreted to reflect oceanic crustal fissuring and
faulting. Sheeted diabase dikes grade downward into isotropic
gabbro across a complex transition wherein numbers of dikes
decrease downward with corresponding increase of thickness of
intervening screens of gabbro, with complete transition commonly
occurring over as little as several meters, and wherein gabbro
locally intrudes and stopes overlying sheeted diabase. Largely
homogeneous throughout, isotropic gabbros near the base of the
dikes unit vary widely in texture and locally show extensive
development of ductile shear zones.
The lithology and igneous structure of the upper units of the
ophiolite and the sense of tilting of the sheeted dikes suggest
that upper oceanic crust, viewed as forming as a floating roof
zone, or lid, to a spreading center magma chamber, behaves as a
rigid beam and forms by accretion along a narrow central zone with
subsequent subsidence and rotation away from the accretion axis as
a consequence of loading of the lid by extrusives. Continued
steady-state spreading of a lid forming in this manner generates a
lid surface topography and structural morphology identical to that
observed at present-day spreading centers and an internal
structure which is consistent with that of ophiolites. The model
indicates that the zone of dike intrusion and basalt extrusion is
narrow and remains so with continual spreading. Because the model
predicts a specific internal geometry of sheeted dike and volcanic
layering attitudes, it can be tested against other detailed
reconstructions of upper oceanic crustal structure as such
attitudes would affect magnetics inclinations in oceanic crust.
Rosencrantz, E.J., 1980. The geology of the northern part of
North Arm Massif, Bay of Islands Ophiolite Complex, Newfoundland:
with application to upper oceanic crust lithology, structure, and
genesis. Unpublished PhD dissertation, State University of New
York at Albany. 318pp., +xxii; 2 folded plates (maps)
University at Albany Science Library call number: SCIENCE
MIC Film QE 199 R67X
Copies of this PhD dissertation can be ordered
from Proquest UMI
Front matter (title,
table of contents, abstract, acknowledgements) - 0.4MB pdf
file
Photo pages in dissertation
(colour
and greyscale photos with captions): - 14.1MB pdf file
Plate 1 - Geological
map
of the North Arm massif, northern part - Bay of Islands
Ophiolite Complex, Newfoundland, Canada
(uncoloured geological outcrop map; scale
~1:15,840) 7.2MB pdf file
Plate 2 - Structural
sections,
North Arm massif, northern part
(uncoloured cross-sections; scale ~1:15,840)
1.2MB pdf file
see also: Casey, J.F., and Rosencrantz, E.J., 1981. Geological
map of the North Arm Massif, Bay of Islands Ophiolite Complex,
Newfoundland, Canada
(uncoloured geological map; scale 1:28,800)
3.8MB pdf file
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