The Pontiac problem, Quebec–Ontario, in the light of gravity data

1987 ◽  
Vol 24 (9) ◽  
pp. 1916-1919 ◽  
Author(s):  
J. Kalliokoski
Keyword(s):  
Gravity Anomaly ◽  
Greenstone Belt ◽  
Gravity Data ◽  
Volcanic Rocks ◽  
Sedimentary Rocks ◽  
Iron Formation ◽  
Bouguer Gravity ◽  
The West ◽  
Abitibi Greenstone Belt ◽  
Uplifted Block

A belt of Archean quartzose metasedimentary gneisses with minor mafic volcanic rocks (the Pontiac Group) lies south of the Blake River and older Archean mafic volcanic rocks of the Abitibi Greenstone Belt, and is separated from them by the Larder Lake – Cadillac Break. To the west of the Pontiac Group, on strike, is the Archean Larder Lake Group of turbidite conglomerate, argillite, limestone, and iron formation with abundant mafic flows and intrusions. These strata also lie south of the Larder Lake – Cadillac Break and south of the Blake River and older Archean mafic volcanic rocks. The western contact between the Pontiac and Larder Lake groups is covered by a narrow north–south strip of Proterozoic Cobalt sedimentary rocks. On the basis of gravity work that compares the Bouguer gravity anomaly gradient across the Cadillac Break with that across the west margin of the Pontiac Group, it is proposed that the Larder Lake and Pontiac groups are separated by a north–south fault and that the Pontiac Group represents a lithologically distinct uplifted block. The Pontiac block may be an Archean terrane.

Petrology of the Archean Pontiac and Kewagama sediments and implications for the stratigraphy of the southern Abitibi belt

1984 ◽  
Vol 21 (11) ◽  
pp. 1305-1314 ◽  
Author(s):  
Jean Lajoie ◽  
John Ludden
Keyword(s):  
Earth Element ◽  
Volcanic Rocks ◽  
Sedimentary Rocks ◽  
Igneous Rocks ◽  
Present Position ◽  
The West ◽  
Abitibi Greenstone Belt ◽  
Sedimentary Sequences ◽  
Bed Thickness ◽  
Water Facies

We present an interpretation of the petrology of two Archean sedimentary groups in the southeastern portion of the Abitibi greenstone belt: (1) the Pontiac sediments, which represent an assemblage of quartz-rich sandstones, and (2) the Kewagama sediments, which are quartz poor and rich in volcanic fragments. The Pontiac sediments display a north–south decrease in bed thickness, sand/shale ratio, and grain size, suggesting derivation from a northern source relative to their present position. Petrographic and rare earth element (REE) analysis indicates that three provenances contributed to the Pontiac: (1) a mature craton, (2) light/heavy REE fractionated igneous rocks probably of tonalite–trondhjemite composition, and (3) ultramafic–mafic volcanics.In contrast to the Pontiac sediments, the Kewagama sediments were derived from acid to mafic volcanic rocks (ratio of 3:1), with minor contributions from plutonic rocks and from graded sedimentary rocks (possibly the Pontiac). The REE data indicate a source fractionated in light/heavy REE relative to the adjacent Blake River Group volcanic rocks, and it is suggested that a suitable source may be the upper Skead volcanics 60 km to the west or a volcanic assemblage similar to the upper Skead that has been eroded from a location above the Blake River Group.The Pontiac Group has characteristics similar to Phanerozoic sedimentary sequences that have accumulated at a passive continental margin, whilst the Kewagama sediments were derived from uplifted volcamc terrane.Our study indicates that the Pontiac Group is stratigraphically below the Blake River Group and that it is not a lateral deep-water facies of the Timiskaming Group. In addition, we consider the possibility that the Kewagama is younger than the Pontiac.

Tholeiitic volcanic rocks of the late Archean Blake River Group, southern Abitibi greenstone belt: origin and geodynamic implications

1992 ◽  
Vol 29 (7) ◽  
pp. 1448-1458 ◽  
Author(s):  
M. R. Laflèche ◽  
C. Dupuy ◽  
J. Dostal
Keyword(s):  
Greenstone Belt ◽  
Volcanic Rocks ◽  
Incompatible Trace Element ◽  
Progressive Change ◽  
Abitibi Greenstone Belt ◽  
Mid Ocean Ridge ◽  
Compositional Variations ◽  
Arc Setting ◽  
Back Arc ◽  
Ocean Ridge

The late Archean Blake River Group volcanic sequence forms the uppermost part of the southern Abitibi greenstone belt in Quebec. The group is mainly composed of mid-ocean-ridge basalt (MORB)-like tholeiites that show a progressive change of several incompatible trace element ratios (e.g., Nb/Th, Nb/Ta, La/Yb, and Zr/Y) during differentiation. The compositional variations are inferred to be the result of fractional crystallization coupled with mixing–contamination of tholeiites by calc-alkaline magma which produced the mafic–intermediate lavas intercalated with the tholeiites in the uppermost part of the sequence. The MORB-like tholeiites were probably emplaced in a back-arc setting.

Volcanic rocks of the Blake River Group, Abitibi Greenstone Belt, Ontario, and their sulfur content

1977 ◽  
Vol 14 (4) ◽  
pp. 539-550 ◽  
Author(s):  
A. J. Naldrett ◽  
A. M. Goodwin
Keyword(s):  
Sulfur Content ◽  
Greenstone Belt ◽  
Geometric Mean ◽  
Volcanic Rocks ◽  
Smooth Curve ◽  
Arithmetic Mean ◽  
Arithmetic Means ◽  
Basaltic Rocks ◽  
Abitibi Greenstone Belt ◽  
Stratigraphic Height

Six hundred and ninety samples of volcanic rocks from the Blake River Group of the Abitibi Greenstone Belt have analysed for sulfur on a Leco sulfur analyser. Basaltic rocks have been subdivided into komatiites, Fe-rich tholeiites, Al-rich basalts, and intermediate basalts with more than 1% TiO2 and with less than 1% TiO2. Andesites have been subdivided into Fe-rich types, Al-rich types, and others. All dacites are grouped together as are all rhyolites. Rocks of many of these subdivisions occur at more than one level within the Blake River stratigraphy. Within a given rock subdivision, the sulfur content is distributed log normally. When the geometric mean of the sulfur content of each of the subdivisions outlined above is plotted against the arithmetic mean of the FeO content, a smooth curve is obtained, with sulfur increasing markedly with increase in FeO. The data give no indication of any change in sulfur content of a given rock subdivision with stratigraphic height. The arithmetic mean of the sulfur content of each rock subdivision also increases with the mean FeO content, although less smoothly than the geometric mean. The arithmetic means of sulfur content fall within the scatter of points obtained experimentally for the sulfur content of sulfur saturated basalts, supporting the contention that the Blake River rocks may have been saturated with sulfur at the time of their extrusion.

Correlation of gravity anomalies with Yellowknife Supergroup rocks, North Arm, Great Slave Lake

1980 ◽  
Vol 17 (11) ◽  
pp. 1506-1516 ◽  
Author(s):  
R. A. Gibb ◽  
M. D. Thomas
Keyword(s):  
Greenstone Belt ◽  
Gravity Data ◽  
Volcanic Rocks ◽  
Three Dimensional ◽  
Gravity Anomalies ◽  
Rock Density ◽  
The North ◽  
Great Slave Lake ◽  
Western Shore ◽  
Gravity Map

A gravity map compiled from observations made on the frozen surface of Great Slave Lake shows that the positive gravity anomaly associated with the Yellowknife greenstone belt extends offshore into the North Arm of the lake. On the western shore of Yellowknife Bay the axis of the anomaly coincides with mafic volcanic rocks of the Kam Formation. Offshore the axis continues southwards for about 10 km to the West Mirage Islands where it takes a dramatic turn to the southeast and continues for a further 60 km to the Outer Whaleback Rocks. Using the geology and rock density determinations on land for control, a three-dimensional geological model comprising a large number of prismatic blocks was derived from the gravity anomalies. In the model the simplifying assumption has been made that the greenstone belt is everywhere floored by granodiorite similar to the adjacent Western and South-east granodiorites. According to the model, mafic volcanic rocks of the Kam Formation are generally 1–3 km thick with a maximum thickness of 7 km at the mouth of Yellowknife Bay. Greywacke and mudstone of the Burwash Formation vary in thickness from 1 to 3 km. Locally these sedimentary rocks attain a thickness of 8 km but this is probably an overestimated value as they may very well be underlain by volcanic rocks of the Kam Formation. The presence of a third pluton of granodiorite flanking the belt to the southwest is also inferred from the gravity data. Previous seismic work indicated a greenstone basin with an average thickness of about 10 km. However, reexamination of the seismic records suggests that weak arrivals interpreted as originating from the base of the greenstone belt are more likely to be pulses associated with earlier arrivals.

Gravity and magnetic signatures of the Ackley Granite Suite, southeastern Newfoundland: implications for magma emplacement

1989 ◽  
Vol 26 (12) ◽  
pp. 2697-2709 ◽  
Author(s):  
H. G. Miller ◽  
J. Tuach
Keyword(s):  
Gravity Anomaly ◽  
Magma Chamber ◽  
Volcanic Rocks ◽  
Gravity Survey ◽  
Bouguer Gravity ◽  
Density Data ◽  
Bouguer Gravity Anomaly ◽  
Gravity And Magnetic ◽  
Magnetic Signatures ◽  
Granite Suite

A detailed gravity survey (255 stations over an area of 3000 km2) over the posttectonic, Devonian, Ackley Granite Suite in southeastern Newfoundland suggests that a set of northwest-trending geophysical lineations, interpreted as faults, controlled the bottom morphology of the magma chamber during emplacement.The main negative Bouguer gravity anomaly over the Ackley Granite Suite trends in a north–south direction across the trace of the Dover – Hermitage Bay Fault, the boundary between the Gander and Avalon terranes. Inversion of the gravity and density data indicates that the thickness of the granite suite varies between 2 and 8 km.Magnetic interpretation and modelling suggest that the southeast portion of the suite is underlain by mafic volcanic rocks typical of the Avalon Terrane. The magnetically determined depth to the top of these is consistent with that calculated from the gravity data.The combined geophysical models are indicative of a process of megablock stoping in which large crustal blocks were displaced downwards in midcrustal regions to permit the emplacement of what is now the Ackley Granite Suite and associated plutons.

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