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The Cobequid Highlands lie north of the Cobequid – Chedabucto fault zone, the term applied to a series of faults, with a wide range of ages, which mark the upper crustal expression of the crustal scale Minas Geofracture Keppie between the Avalon and Meguma terranes. Late Paleozoic strike-slip motion on this system within and adjacent to the Cobequid Highlands formed the Cobequid Shear Zone.

Bibliothèque LMV catalog › Details for: Microtextures des roches magmatiques et métamorphiques

The Cobequid Fault Fig. Within the highlands, the Rockland Brook Fault separates two contrasting blocks of Neoproterozoic rocks, the Bass River block to the south and the Jeffers block to the north Fig. The northern margin of the Cobequid Highlands is largely fault-bound, with the Spicers Cove Fault in the west and a series of unnamed faults in the east Durlingwith minor overstep of upper Carboniferous rocks of the Cumberland Basin Fig.

Display large image of Figure 1. These two highland areas are separated by Carboniferous rocks of the Stellarton Basin Fig.

Much of the Cobequid Highlands volcanlques underlain by latest Devonian — earliest Carboniferous granite and gabbro plutons and their extrusive equivalents, the Fountain Lake Group. Terrestrial fluvial and lacustrine sediments accumulated in small basins of mid Devonian to early Carboniferous age. The Neoproterozoic units include a wide range of plutonic and volcanic rocks, the tectonic setting of which can be well defined.

These Neoproterozoic rocks form a geographic link between that part of the Avalon terrane outcropping in southern New Brunswick and the Avalonian rocks of the Antigonish Highlands and southern Cape Breton Island Fig. The Cobequid Highlands include the only thick early Carboniferous basalt succession in Plutoniquse Canada, overlying several kilometres of latest Devonian rhyolite. The abundance of granitic and gabbroic plutons of similar age points to a major role for the Cobequid Shear Zone in controlling the locus of igneous activity.

The geology of this area is summarized in Figure 2, showing Upper Paleozoic rocks volcaniquex Figure 3, showing Neoproterozoic and Lower Paleozoic rocks.

We also briefly synthesize key information previously published in a dispersed manner over the past twenty years.

Microtextures des roches magmatiques et métamorphiques

The stratigraphic section in the Cobequid Highlands is summarized in Table 1. We have in general followed their stratigraphic nomenclature. Donohoe and Wallace provided a synthesis and bibliography of earlier work, including Geological Survey of Canada work in the late ‘s by Kelley and Eisbacher, The field notes and plutohiques sheets of Donohoe and Wallace are available through the N.

Information on mineral localities was presented by Donohoe and by Ervine Detailed studies of selected areas are reported in fourteen B. Display large image of Figure 2. Display large image of Figure 3. Display large image of Table 1. Note that mappable units listed in Table 1 and shown on our 1: Not included in either block is the Economy River Gneiss, a granodioritic orthogneiss that has yielded a U-Pb zircon age of Ma Doig et al.

The previously reported “West Moose River anorthosite” Pe-Piper and Dessureau consists of rocks trucked from western Newfoundland and dumped in an abandoned gravel pit. Pe-Piper and Murphyand intruded by rochew series of plutonic volcankques Table 1, Fig. Individual plutons are difficult to define because of the tectonic dismemberment of the Bass River block, but three principal plutonic rock types are distinguished: The first two units occur principally as sheets tens to hundreds of metres thick within the Gamble Brook Plutoniqurs, with structures indicating emplacement during shearing Pe-Piper et al.

Poor quality U-Pb and Rb-Sr ages of ca. In addition, small bodies of the geochemically distinct Glen Road and Gully Brook granite units have been recognised in rches Mount Thom area Pe-Piper et al.


As plutpniques by Volcwniques et al. Metamorphism of the Bass Plutoniqyes block is of greenschist facies, with sparse evidence locally of higher grades adjacent to mafic and intermediate plutons. The principal deformation and metamorphism took place during Late Neoproterozoic transpressional motion on the Rockland Brook Fault, when the rocks underwent thermo-mechanical softening as a consequence of emplacement of the Frog Lake gabbro and Debert River granodiorite Pe-Piper et al.

Murphy showed from sedimentary geochemistry that the Gamble Brook Formation formed in a rifted arc environment. Jeffers Group rocks are also recognised in the River Philip area Fig. Here, they are tectonically intercalated with Carboniferous sedimentary and igneous rocks in the hanging wall of the New Annan Thrust Piper and Pe-Piper The Jeffers Group can be readily recognized because rohces volcanic rocks are geochemically distinct from younger Devonian—Carboniferous volcanic rocks Pe-Piper et al.

Doig, in Murphy et al. Smaller intrusive rocjes in this area have been termed the River John gabbro, tonalite and granodiorite units Pe-Piper et al.

Elsewhere, such as in Bailey Brook, strata are rohces undeformed. The Jeffers Brook Pluton shows synmagmatic deformation features near its southwestern margin, much of the pluton shows a weak ductile solid-state foliation, and mafic enclaves tend to be flattened.

Such rapid changes in degree of deformation suggest that deformation was related to regional shear. In the Mount Thom area, the Jeffers block shows a major shear zone contact with the Bass River block that has been unaffected by late Paleozoic deformation Pe-Piper et al.

The great thickness of mylonitic granite suggests that this shear zone may have been synchronous with the Ma Gunshot Brook Pluton.

To the west, the contact between the two blocks along the Rockland Brook fault experienced major late Paleozoic reactivation. Comparisons of the Avalonian tectonic and igneous events Fig. In most of these areas, there is a long and complex history of Late Neoproterozoic igneous activity. No volcanic rocks are known from the Cobequid Highlands of similar age to the Ma Stirling Group of the Mira terrane.

The oldest volcanic rocks are the Ma Jeffers Group, which are a little older than the ca. In the Bass River block, the Ma Frog Lake gabbro is of similar age to widespread dioritic to granodioritic plutons in the Caledonia and Mira terranes.

Similar plutonism in the Antigonish Highlands is regarded as being a little younger — Ma and the — Ma Debert River granodiorite of the Bass River block is younger still. The — Ma plutons of the Jeffers block are of similar age to the Debert River granodio-rite.

The Bass River block, which lacks volcanic rocks related to plutonism, appears to be at a deeper crustal level than either of these blocks or the Caledonia and Mira terranes. From its present geometry, it may have been the most inboard of these peri-Gondwanan terranes.

Donohoe and Wallaceidentified two possible Ordovician rock units. The Cobequid Highlands thus differ from Avalonian successions in eastern Newfoundland, Cape Breton Island, the Antigonish Highlands and southern New Brunswick in lacking Cambrian—Ordovician overstep shelfal sequences with characteristic Avalonian fossils.

Whether this is the result of non-deposition or later erosion is unknown. The widespread preservation of Silurian strata suggests that either Cambrian — Ordovician strata were not deposited, or they were eroded prior to the Silurian. Display large image of Figure 4.

These rocks were re-interpreted by Piper as Horton Group. We follow Murphy et al. The Wilson Brook Formation passes upwards conformably into the Portapique River Formation, of red unfossiliferous siltstone and minor sandstone, which Donohoe and Wallace interpreted as early Devonian based on lithologic correlation with the Knoydart Formation near Arisaig. Both the Wilson Brook and Portapique River formations are folded and penetratively cleaved to a varying degree. Both formations are known only within the Jeffers Block, but are widely distributed in relatively small inliers from north of Parrsboro in the west to Earltown in the west.

It is manifested by a fundamental change in style of sedimentation between the early and late Devonian. The Silurian Wilson Plutoniqeus and lfs Devonian Portapique River formations were deposited regionally over at least km of the Cobequid Highlands and almost identical facies are recognised 50 km farther east at Arisaig. In contrast, mid Devonian to early Carboniferous rocks of the Murphy Brook Formation, Fountain Lake Group, and Horton Group show rapid changes in facies and thickness, with common occurrence of conglomerate and lacustrine deposits, all suggesting deposition in fault-bound terrestrial basins.


Wilson Brook Formation at Wentworth is almost unde-formed. Wilson Brook Formation in the Portapique River area is strongly cleaved and folded, and locally overturned, but much of this deformation is associated with the latest Devonian emplacement of the Pleasant Hills Pluton Pe-Piper pltoniques al.

No untectonized contacts of Wilson Brook Formation with younger rocks are known. Falls Formation Donohoe and Wallacewhich unconform-ably overlies mid Tournaisian strata. The “Acadian orogeny” is probably represented by the onset of shear deformation in the Cobequid Shear Zone that continued throughout much of the late Devonian and Carboniferous.

Good biostratigraphic and geochronological data are sparse and lateral facies changes are common; hence, correlation between localities is difficult.

Plant fossils are common in this formation and have been identified as mid Devonian late Emsian to early Eifelian: Kasper in Donohoe and Wallace However, no suitable detrital sources of pebbles of rhyolite are known in mid Devonian or earlier rocks in this part of the Cobequid Highlands. The voluminous rhyolite in the Fountain Lake Group has been dated as latest Devonian or earliest Carboniferous and is widespread as clasts in the latest Devonian — earliest Carboniferous Nuttby Formation see below.

Sedimentologically, the Murphy Brook Formation resembles the lacustrine facies of the Nuttby Formation. We have failed to recover identifiable palynomorphs from the Murphy Brook Formation. Its assignment to the mid Devonian is thus based solely on macropaleontological evidence and is inconsistent with the presence of abundant rhyolite clasts. We suggest that a re-examination of the paleontological evidence is needed.

Its total thickness is generally less than volaniques. Note that our thickness estimates for the Fountain Lake Group are conservative and based on reasonably continuous sections in which large scale fault or fold repetition can be excluded: Donohoe and Wallacepp. The underling Byers Brook Formation consists of felsic pyroclastic rocks, lesser felsic and mafic flows, and minor interbedded sedimentary rocks, in total several kilometres thick Piper et al.

A rhyolite flow near the top of the formation has yielded a U-Pb zircon age of Ma Dunning et al. The conformably overlying 1. The last two facies were mapped together as the “Nabiscumb Brook Member” by Donohoe and Wallacebut Ryan and Boehner have shown that the type section is not part of the Diamond Brook Formation and we have therefore abandoned the term. Siltstone in the middle of the basaltic facies of the Diamond Brook Formation was reported as early Tournaisian on the basis of palynomorphs samples D, D of Donohoe and Wallacebut re-interpreted as Vlocaniques Famennian by Martel et al.

Plutoniquew Tournaisian early Tn3 spores were identified in the upper part of the formation in the Scotsburn 1 well Utting et al. These ages are consistent with a U-Pb age of Ma on a rhyolite in the lower part of the formation south of New Annan Dunning et al. In the western Cobequid Highlands, these rocks were included in the Rapid Brook and Greville River formations; in the eastern Cobequid Highlands, they were termed the Nuttby Formation, where they contain early Tournaisian spores.

Piper suggested that all of these occurrences represent the same formation, and can be mapped as such, distinguishing where appropriate a coarse facies principally conglomerate and medium sandstone and a fine facies principally argillite, siltstone, and fine sandstone. The Nuttby Formation in most areas contains conglomerate with sparse granitic llutoniques.

It is cut by minor late granite dykes at several localities: We therefore suggest that the Nuttby Formation is of similar age to the type Horton Group, namely latest Devonian to Tournaisian Martel et al. Plutonoques large image of Figure 5.