66 research outputs found
A Study of Pressure Solution Effects
No full text A microscopic study of two samples of the Gowganda
Formation, both having undergone pure shear deformation at
greenschist f acies metamorphism has shown: Sample A -Reduction in quartz grain size, and pressure solution shadows were evidence for strong pressure solution activity. Pre-lithification fractures provided channelways for the removal of quartz and water out of the system. Matrix quartz was not recrystallized.
Sample B -Extensive local recrystallization of quartz due to pressure solution activity caused metamorphic segregation and the formation of a cleavage. Cleavage behaviour differs in the matrix from that observed in an area of contact strain, as produced by a buckled quartz vein. There is no evidence for removal of quartz from the system. A comparison between the two samples suggests that
(a) water, in this case at least, is necessary to produce metamorphic segregation, (b) the system has to be closed to reach metamorphic segregation and (c) original features may be preserved through limited metamorphism if the rock is dried out early in its history. ThesisBachelor of Science (BSc
Spaced Cleavage Development in the Metagreywackes of the Goldenville Formation, Meguma Group, Nova Scotia
No full text The greywackes of the Goldenville Formation, Meguma Group, Nova Scotia display a well developed spaced cleavage. The cleavage is characterized by zones of mica enrichment and associated quartz depletion and forms a network of anastomosing cleavage rhombs. Parallelism between cleavage and water escape structures suggests that the cleavage was emplaced perpendicular to bedding early in the deformational history of the rocks. Subsequent passive rotation of cleavage and water escape structures reduced the bedding-cleavage angle to its present values. XRF whole rock analyses of cleavage planes and lithons from two different sample locations show that the cleavage is depleted in SiO2, CaO, Na2O and enriched in Al2O3, Fe2O3, MgO, K2O, TiO2, MnO and P2O5 relative to lithons. Mass balance calculations based on the Gresens (1967) approach indicate that the cleavage with respect to the lithon represents a 40%-60% volume loss. Using the volume loss it can be shown that the cleavage lost SiO2, Na2O, CaO, MnO, P2O5, Pb, Sr, Nb, and Y and gained MgO, K2O, Ba, Rb, and W during its formation. TiO2, Al2O3, Fe2O3 and Zr remain relatively immobile during the cleavage forming processes. The behavior of Au, Sb, As, Ce, Nd, and La can not be discerned. The cleavage was initiated by pressure solution at grain contacts. However, several of the above mentioned chemical migrations increased the solubility of quartz and therefore greatly enhanced the development of the cleavage.ThesisMaster of Science (MSc
Equatorial layered deposits in Arabia Terra, Mars: Facies and process variability
No full textWe investigated the equatorial layered
deposits (ELDs) of Arabia Terra, Mars, in
Firsoff crater
and on the adjacent plateau.
We produced a detailed geological map that
included a survey of the relative stratigraphic
relations and crater count dating. We reconstructed
the geometry of the layered deposits
and inferred some compositional constraints.
ELDs drape and onlap the plateau materials
of late Noachian age, while they are unconformably
covered by early and middle
Amazonian units. ELDs show the presence
of polyhydrated sulfates. The bulge morphology
of the Firsoff crater ELDs appears to be
largely depositional. The ELDs on the plateau
display a sheet-drape geometry. ELDs
show different characteristics between the
crater and the plateau occurrences. In the
crater they consist of mounds made of breccia
sometimes displaying an apical pit laterally
grading into a light-toned layered unit
disrupted in a meter-scale polygonal pattern.
These units are commonly associated
with fissure ridges suggestive of subsurface
sources. We interpret the ELDs inside the
craters as spring deposits, originated by fluid
upwelling through the pathways likely provided
by the fractures related to the crater
formations, and debouching at the surface
through the fissure ridges and the mounds,
leading to evaporite precipitation. On the
plateau, ELDs consist of rare mounds, flat-lying
deposits, and cross-bedded dune fields.
We interpret these mounds as possible
smaller spring deposits, the flat-lying deposits
as playa deposits, and the cross-bedded
dune fields as aeolian deposits. Groundwater
fluctuations appear to be the major factor
controlling ELD deposition
Fluid-assisted grain boundary sliding in bedding-parallel quartz veins deformed under greenschist metamophic grade.
No full textIron formation rocks of Quadrilátero Ferrífero, Brazil, were deformed at greenschist facies. Quartz grains in bedding parallel veins were sheared and deformed by a combination of mechanisms assisted by aqueous fluids. Veins in the outcrop appear to be stretched parallel to the compositional layering. The overall vein shapes resemble those of boundinage and pinch and swell. In thin sections, veins show microstructures similar to those observed in hand samples, where domains of large quartz crystals are pulled apart for several millimeters. The voids between quartz fragments are filled with domains of polycrystalline quartz. The microstructural and orientation data show that the strain imposed on the vein as a rigid and competent layer was not accommodated in the quartz polycrystals exclusively by crystal plastic deformation or dynamic recrystallization. The new grains are strain-free, with straight boundaries and with weak to random crystallographic fabrics. We interpret these features to have resulted from a combination of processes, which included grain boundary sliding accomplished by solution transfer. We propose that the coeval operation of both mechanisms allows the aggregate to deform at higher strain rates without necking of the vein layer in a type of flow similar to those described in superplastic regimes
Sequence of infilling events in Gale crater, Mars: Results from morphology, stratigraphy, and mineralogy
No full textGale Crater is filled by sedimentary deposits including a mound of layered deposits,
Aeolis Mons. Using orbital data, we mapped the crater infillings and measured their
geometry to determine their origin. The sediment of Aeolis Mons is interpreted to be
primarily air fall material such as dust, volcanic ash, fine-grained impact products, and
possibly snow deposited by settling from the atmosphere, as well as wind-blown sands
cemented in the crater center. Unconformity surfaces between the geological units are
evidence for depositional hiatuses. Crater floor material deposited around Aeolis Mons
and on the crater wall is interpreted to be alluvial and colluvial deposits. Morphologic
evidence suggests that a shallow lake existed after the formation of the lowermost part of
Aeolis Mons (the Small yardangs unit and the mass-wasting deposits). A suite of several
features including patterned ground and possible rock glaciers are suggestive of periglacial
processes with a permafrost environment after the first hundreds of thousands of years
following its formation, dated to ~3.61 Ga, in the Late Noachian/Early Hesperian.
Episodic melting of snow in the crater could have caused the formation of sulfates and
clays in Aeolis Mons, the formation of rock glaciers and the incision of deep canyons and
valleys along its flanks as well as on the crater wall and rim, and the formation of a lake in
the deepest portions of Gale
Structural petrology along a transect across the Thompson Belt, Manitoba: dip slip at the western Churchill–Superior boundary
No full text Early Proterozoic rocks and Archean gneisses of the Thompson Belt, east of the Churchill Province – Superior Province boundary, record the history of the Hudsonian Orogeny in this area. A structural study has been undertaken along a 45 km long corridor cutting across the width of the belt.Three blocks are defined along this transect, each characterized by a specific structural style and metamorphic history, Foliation and lineation data and the analysis of kinematic indicators from these blocks show that (i) the Thompson Belt has been an area of predominantly dip-slip movement for its recognizable ductile deformation history; (ii) the sense of shear for most of that history corresponds to the Superior side moving up with respect to the Churchill side; (iii) the shearing emplaced rocks of progressively higher metamorphic grade towards the southeast, away from the Churchill–Superior boundary; and (iv) there appears to have been a period of movement at the later stages of the Hudsonian Orogeny during which the sense of movement was Churchill side up.Structural and metamorphic data support a tectonic model in which the Superior Province has been thrust over the Churchill Province, with a convergence direction approximately perpendicular to the Thompson Belt. </jats:p
Complex fringes around magnetite porphyroclasts: Growth and deformation history
No full textDeformed strain fringes in iron formation rocks show complex quartz fiber patterns that grew alongside magnetite porphyroclasts embedded in a matrix of quartz and iron oxides during coaxial to non-coaxial deformation. These rocks have been deformed by a combination of processes involving microfracturing, pressure solution and dislocation glide at temperatures of approximately 300 °C. Detailed microstructural observation and crystallographic analysis show that quartz fiber growth is not controlled by the crystal faces. Quartz c-axis orientations of fibrous quartz indicate that these grains initially grew with their c-axes paralle to the fiber length. Late solid-state deformation of crystallized fibers comprised dislocation glide along basal plane and subsequent recrystallization. This produces aggregates of recrystallized grains consisting of equant to elongated quartz grains with straight and orthogonal grain boundaries
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