New gravitational evidence for the subsurface extent of oceanic crust in north-central Newfoundland

1976 ◽  
Vol 13 (3) ◽  
pp. 459-469 ◽  
Author(s):  
H. G. Miller ◽  
E. R. Deutsch
Keyword(s):  
Oceanic Crust ◽  
Coastal Areas ◽  
Crustal Material ◽  
Two Dimensional ◽  
Ophiolite Complex ◽  
Southern Boundary ◽  
North Central ◽  
Density Data ◽  
Bouguer Anomalies ◽  
Plate Margin

Gravity has been measured at 226 locations having a mean spacing of 2.5 km in the area of western Notre Dame Bay, in north-central Newfoundland. The data reveal that the observed steep seaward positive gradients of the Bouguer anomalies previously found on eastern Notre Dame Bay continue to coastal areas of western Notre Dame Bay. This belt of large gradients correlates spatially with the trace of the Chanceport – Lobster Cove Fault, which is the southern boundary of this belt of ophiolitic and island arc rocks (Belts Cove Ophiolite Complex) on the Burlington peninsula, which forms the basement of this belt. The observed anomalies may represent the gravity signature of a Paleozoic lithospheric plate margin, but a test of this possibility must await data from the bay itself.Interpretation of four profiles, using density data from rock samples and two-dimensional models, leads to the conclusion that oceanic crustal material similar to that found on Notre Dame Bay, zone D, continues inland beneath parts of zones C and E to 5–10 km maximum depth. The interpretation also suggests the desirability of establishing a subzone on the Burlington peninsula.

scholarly journals The Petrogenesis of the Permian Podong Ultramafic Intrusion in the Tarim Craton, Western China: Constraints from C-He-Ne-Ar Isotopes

Geofluids ◽  
2019 ◽  
Vol 2019 ◽  
pp. 1-14 ◽  
Author(s):  
Mingjie Zhang ◽  
Pengyu Feng ◽  
Tong Li ◽  
Liwu Li ◽  
Juerong Fu ◽  
...  
Keyword(s):  
Oceanic Crust ◽  
Mantle Plume ◽  
Mantle Source ◽  
Noble Gas ◽  
Western China ◽  
Crustal Material ◽  
Sulfide Mineralization ◽  
Hydrocarbon Gases ◽  
Isotopic Compositions ◽  
Ultramafic Intrusion

The Podong Permian ultramafic intrusion is only one ultramafic intrusion with massif Ni-Cu sulfide mineralization in the Pobei layered mafic-ultramafic complex, western China. It is obviously different in sulfide mineralization from the nearby coeval Poyi ultramafic intrusion with the largest disseminated Ni-Cu sulfide mineralization and mantle plume contribution (Zhang et al., 2017). The type and addition mechanism of the confirmed crustal contaminations and possible mantle plume involved in the intrusion formation require evidences from carbon and noble gas isotopic compositions. In the present study, we have measured C, He, Ne, and Ar isotopic compositions of volatiles from magmatic minerals in the Podong ultramafic intrusion. The results show that olivine, pyroxene, and plagioclase minerals in the Podong intrusion have variable δ13C of CO2 (-24.5‰ to -3.2‰). The CH4, C2H6, C3H8, and C4H10 hydrocarbon gases show normal or partial reversal distribution patterns of carbon isotope with carbon number and light δ13C1 value of CH4, indicating the hydrocarbon gases of biogenic origin. The δ13C of CO2 and CH4 suggested the magmatic volatile of the mantle mixed with the volatiles of thermogenic and crustal origins. Carbon and noble gas isotopes indicated that the Podong intrusion could have a different petrogenesis from the Poyi ultramafic intrusion. Two types of contaminated crustal materials can be identified as crustal fluids from subducted altered oceanic crust (AOC) in the lithospheric mantle source and a part of the siliceous crust. The carbon isotopes for different minerals show that magma spent some time crystallizing in a magma chamber during which assimilation of crustal material occurred. Subduction-devolatilization of altered oceanic crust could be the best mechanism that transported large proportion of ASF (air-saturated fluid) and crustal components into the mantle source. The mantle plume existing beneath the Poyi intrusion could provide less contribution of real materials of silicate and fluid components.

Crustal electrical conductivity in north-central Saskatchewan: the North American Central Plains anomaly and its relation to a Proterozoic plate margin

1984 ◽  
Vol 21 (5) ◽  
pp. 533-543 ◽  
Author(s):  
S. Handa ◽  
P. A. Camfield
Keyword(s):  
Electrical Conductivity ◽  
Subduction Zone ◽  
North American ◽  
Central Plains ◽  
Period Range ◽  
Geological Evidence ◽  
North Central ◽  
The North ◽  
Plate Margin ◽  
Lake Zone

Seven recording magnetometers monitored time-varying fields at points on a northwest–southeast line 280 km long in north-central Saskatchewan during July 1981. The experiment was designed to test the hypothesis advanced in 1975 by Alabi, Camfield, and Gough that the electrical conductivity anomaly in the North American Central Plains links with the Wollaston Domain in the exposed Precambrian Shield of Saskatchewan. From clear reversals in the phase of vertical variations, it is evident that the conductor passes between two stations straddling the Rottenstone–La Ronge Magmatic Belt, to the immediate east of the Wollaston Domain. Enhanced horizontal variations transverse to the belt at a third, intermediate, station reinforce this interpretation. Vertical-field response arrows obtained from daytime events in the period range 1–40 min clearly indicate the existence of a major conductor that extends to lower crustal depths beneath the belt. To the northwest across the Cree Lake Zone, reversals in the direction of response arrows at short periods (up to 4 min) imply complex electrical structures in the shallow part of the crust.Lewry termed the Rottenstone–La Ronge Belt a Hudsonian "Cordillera-type" arc massif, and described strong geological evidence for collisional suturing and microplate interaction in this part of the Churchill Province. A similar scenario seems to apply in Wyoming, from the work of Hills and Houston. Thus the conductor appears to trace a Proterozoic plate margin 1500 km from a subduction zone in Wyoming along a transform fault to a subduction zone in northern Saskatchewan.

Geology of part of a long-lived dynamic plate margin: the coastal cordillera of north-central Chile, latitude 30°51′–31°S

1988 ◽  
Vol 25 (4) ◽  
pp. 603-624 ◽  
Author(s):  
James J. Irwin ◽  
Carlos García ◽  
Francisco Hervé ◽  
Maureen Brook
Keyword(s):  
South America ◽  
Tierra Del Fuego ◽  
First Episode ◽  
North Central ◽  
Central Chile ◽  
Igneous Complex ◽  
Plate Margin ◽  
Rock Formations ◽  
Facies Metamorphism ◽  
Basaltic Lavas

A complex composed of ultrabasic and basaltic lavas, chert, arkose, and conglomerate was assembled in the coast of north-central Chile (lat. 30°30′–31°S) prior to 200 Ma. The character of, and relationships between, the rock formations exposed here are consistent with an autochthonous evolution of this part of Chile in the last 200 Ma. Three major episodes of deformation and metamorphism have been observed in this area. The first episode (F1) produced a compositional layering (S1) and amphibolite-facies metamorphism coeval with the intrusion of an extensive igneous complex between 220 and 200 Ma. A second episode of deformation (F2) locally formed reverse faults and tight, recumbent folds in S1. Units in which F2 folds are well developed yield K–Ar and Rb–Sr ages between 163 and 140 Ma. At between 140 and 126 Ma, upright, open to tight folds (F3) formed with sharp hinges and axial planes that strike north and dip steeply east. Structures of similar age, style, and orientation have been reported as far south as Tierra del Fuego. The timing of the F3 deformation appears to correspond to the opening of the South Atlantic Ocean and accelerated westward motion of South America.

Late Permian mafic rocks identified within the Doba basin of southern Chad and their relationship to the boundary of the Saharan Metacraton

2015 ◽  
Vol 152 (6) ◽  
pp. 1073-1084 ◽  
Author(s):  
J. GREGORY SHELLNUTT ◽  
TUNG-YI LEE ◽  
CHIH-CHENG YANG ◽  
SHIN-TAI HU ◽  
JONG-CHANG WU ◽  
...  
Keyword(s):  
Isotope Composition ◽  
Central Africa ◽  
Late Permian ◽  
Southern Boundary ◽  
North Central ◽  
Nd Isotope ◽  
Mantle Sources ◽  
Basaltic Melt ◽  
Step Heating ◽  
Trace Elemental

AbstractThe Doba gabbro was collected from an exploration well through the Cretaceous Doba basin of southern Chad. The gabbro is composed mostly of plagioclase, clinopyroxene and Fe–Ti oxide minerals and displays cumulus mineral textures. Whole-rock40Ar–39Ar step-heating geochronology yielded a Late Permian plateau age of 257 ± 1 Ma. The major and trace elemental geochemistry shows that the gabbro is tholeiitic in composition and has trace element ratios (i.e. La/YbN> 7; Sm/YbPM> 3.4; Nb/Y > 1; Zr/Y > 5) indicative of a basaltic melt derived from a garnet-bearing mantle source. The moderately enriched Sr–Nd isotopes (i.e. ISr= 0.70495 to 0.70839; ɛNd(T)= −1.0 to −1.3) fall within the mantle array (i.e. OIB-like) and are similar to other Late Permian plutonic rocks of North-Central Africa (i.e. ISr= 0.7040 to 0.7070). The enriched isotopic composition of the Doba gabbro contrasts with the more depleted compositions of the spatially associated Neoproterozoic post-Pan-African within-plate granites. The contrasting Nd isotope composition between the older within-plate granites and the younger Doba gabbro indicates that different mantle sources produced the rocks and thus may mark the southern boundary of the Saharan Metacraton.

Volcanic evolution of oceanic crust in a Late Ordovician back-arc basin: The Solund-Stavfjord Ophiolite Complex, West Norway

2003 ◽  
Vol 4 (10) ◽  
Author(s):  
Harald Furnes ◽  
Helge Hellevang ◽  
Bjarte Hellevang ◽  
Kjell Petter Skjerlie ◽  
Brian Robins ◽  
...  
Keyword(s):  
Oceanic Crust ◽  
Late Ordovician ◽  
Ophiolite Complex ◽  
Volcanic Evolution ◽  
Back Arc

Low-Potash Lush's Bight Tholeiites: Ancient Oceanic Crust in Newfoundland?

1972 ◽  
Vol 9 (5) ◽  
pp. 574-588 ◽  
Author(s):  
W. G. Smitheringale
Keyword(s):  
Oceanic Crust ◽  
Copper Sulfide ◽  
Alkali Basalt ◽  
Trace Element Content ◽  
Volcanic Origin ◽  
North Central ◽  
Ocean Ridges ◽  
Iron Copper ◽  
Volcanic Pile ◽  
Low K

The Lush's Bight Group of north-central Newfoundland consists mainly of regionally metamorphosed pillowed basalts. Mafic dikes and sills, comagmatic with the basalts, are abundant and dioritic stocks and silicic igneous rocks constitute a small portion of the group. Most rocks belong to a tholeiitic suite derived from low-K tholeiite that has a trace element content similar to oceanic tholeiite. A few rocks are slightly alkaline, implying that alkali basalt may have contributed to the volcanic pile. The thickness, chemical composition, and lithology of the group suggest it represents the upper (dominantly volcanic) part of layer two of oceanic crust. Alternatively, formation as a tholeiitic shield beneath a mid-ocean volcanic island is suggested by the alkaline affinities of some rocks and by a thick pyroclastic sequence that conformably overlies the group.The Lush's Bight Group contains many volcanogenic iron-copper sulfide occurrences.Mid-ocean ridges are recognized as magmatically dominated environments in which hydrothermal fluids of plutonic and volcanic origin might constitute ore fluids.

Two‐dimensional modeling of a towed in‐line electric dipole‐dipole sea‐floor electromagnetic system: The optimum time delay or frequency for target resolution

Geophysics ◽  
1988 ◽  
Vol 53 (6) ◽  
pp. 846-853 ◽  
Author(s):  
R. N. Edwards
Keyword(s):  
Time Delay ◽  
Half Space ◽  
Electric Dipole ◽  
Late Time ◽  
Crustal Material ◽  
Two Dimensional ◽  
Optimum Time ◽  
Electromagnetic System ◽  
Sea Floor ◽  
Dipole System

Towed in‐line transient electric dipole‐dipole systems are being used to map the electrical conductivity of the sea floor. The characteristic response of a double half‐space model representing conductive seawater and less conductive crustal material to a dipole‐dipole system located at the interface consists of two distinct parts. As time in the transient measurements progresses, two changes in field strength are observed. The first change is caused by the diffusion of the electromagnetic field through the resistive sea floor; the second is caused by diffusion through the seawater. The characteristic times at which the two events occur are measures of sea‐floor and seawater conductivity, respectively. Entirely equivalent responses are observed in a frequency‐domain measurement as frequency is swept from high to low. The simple double half‐space response is modified when the towed array crosses over a conductivity anomaly. I evaluate the magnitude of the anomalous response as a function of delay time and frequency using a two‐dimensional theory and a vertical, plate‐like target. If the ratio of the conductivity of the seawater to that of the sea floor is greater than unity, then an optimum time delay or frequency can be found which maximizes the response. For large conductivity contrasts, the optimum response is greater than the response at late time or zero frequency by a factor of the order of the conductivity ratio.

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