scholarly journals Microstructural evolution of metabasalts within a contact metamorphic aureole : a preliminary quantitative bi-dimensional approach

2003 ◽  
Vol 174 (2) ◽  
pp. 107-114 ◽  
Author(s):  
Ronan Hebert ◽  
Jean-Claude Guézou ◽  
Christine Souque ◽  
Michel Ballevre
Keyword(s):  
Static Recrystallization ◽  
Regional Scale ◽  
Preferred Orientation ◽  
Qualitative Description ◽  
Contact Metamorphism ◽  
Contact Aureole ◽  
Shape Factors ◽  
Thin Sections ◽  
Textural Evolution ◽  
Host Rocks

Abstract The emplacement of the Saint-Brieuc diorite (533 ± 12 Ma ; Brittany, France) developed a narrow contact aureole in metabasaltic host-rocks which were previously submitted to a regional-scale deformation and metamorphism at around 590-570 Ma. This study aims to qualify and quantify the microstructural changes of rocks that occur within the contact aureole and that are the result of the static recrystallization due to the thermal effect of the diorite. In order to quantify the textural evolution of rocks, an image analysis has been performed on thin sections. It focused on the measurements of morphological features of hornblende and opaque phases. They are anisotropy shape factors (stretching and elongation) of minerals, the preferred orientation of minerals and the distribution size (areas) of minerals. The quantification of these different parameters shows that the static recrystallization, which increases when approaching the contact with the intrusive, (i) is responsible of a reduction of the anisotropy shape and elongation parameters of grains, (ii) causes coarsening and (iii) is responsible for the disappearance of preferred orientation of minerals. Both together, the qualitative description and quantitative measurements show that the solid-state transformations due to contact metamorphism tend to make the rocks isotropic and equigranular.

Magma and hydrothermal fluids exploiting similar crustal traps at Gavorrano (Tuscany)

2021 ◽  
Author(s):  
Luca Tinagli ◽  
Simone Vezzoni ◽  
Sergio Rocchi ◽  
Andrea Dini
Keyword(s):  
Vertical Section ◽  
Thick Layer ◽  
Hydrothermal Fluids ◽  
Contact Metamorphism ◽  
Contact Aureole ◽  
The West ◽  
Ore Bodies ◽  
Base Metal Sulfides ◽  
Host Rocks

<p>The 3D reconstruction of magmatic, metasomatic and/or ore bodies plays a major role in understanding the emplacement mechanisms for magmas and hydrothermal fluids in the upper crust.</p><p>The Gavorrano Intrusive-Hydrothermal Complex (GIHC, Tuscany, Italy) is an excellent case study in which intrusive and hydrothermal rocks, as well as sulphides ore bodies are spatially associated.</p><p>The evolution of the GIHC starts in the early Pliocene with the sequential emplacement, at the contact between the Paleozoic basement (metapelites) and the overlying Mesozoic limestone-dolostone formations, of a cordierite-biotite monzogranite and a tourmaline microgranite. The monzogranite is highly porphyritic with megacrysts of K-feldspar and phenocrysts of quartz, plagioclase, biotite, and cordierite. The microgranite is characterised by a huge number of euhedral microliths (10-500 µm) of black tourmaline set in a quartz-feldspars groundmass. The small size of the Gavorrano intrusion (ca. 3 x 1 km) and its shallow emplacement level (ca. 5 km) resulted in a thin contact aureole (< 100 m thick) made of phlogopite-olivine marble and biotite-andalusite pelitic hornfels. Isoclinal folds in marble are indicative of dynamic crystallization during contact metamorphism and point out an outward sense of movement of the aureole rocks with respect to the granite intrusion. At the contact with the intrusion, marbles were overprinted by a discontinuous (0.1-10 m thick) layer of vesuvianite-garnet exoskarn. Exoskarn, contact aureole and undisturbed host rocks, were subsequently affected by hydraulic brecciation. The closing stage of the evolution of the complex is characterized by mineralizing fluid circulation, producing widespread chloritization-silicification and decametric pyrite bodies (with adularia, fluorite, and base metal sulfides). </p><p>Surface and underground mapping integrated by mining reports and drill logs allow us gave way to the reconstruction of the attitude and shape of magmatic and hydrothermal bodies. The NW-SE elongated intrusion is characterised by a pronounced asymmetry: the eastern part is made of sub-horizontal multiple bodies, locally with both roof and bottom contacts exposed; the western part has an overall sub-vertical, west-dipping attitude. Such an asymmetry is shown by each of the two intrusive units and highlighted by second order features: the monzogranite unit reaches its maximum thickness (0.8 km) in the central-western subvertical zone while in the subhorizontal eastern branches is few hundred meters thick, and the subhorizontal microgranite bodies display steep west-dipping offshoots. The GIHC asymmetry is also exhibited by the hydrothermal system: the pyrite orebodies mantle the top and the western flank of the intrusion, with the two main masses displaying, in vertical section, a sigmoidal shape with a steep west-dipping thick portion connecting upper and lower tails gently dipping to the west.</p><p>The collected data indicate the west side of the GIHC as the focus zone for both magmas and hydrothermal fluids. The overall geometries of the intrusive units and pyrite bodies suggest a sense of movement top-down-to-the-west. This close spatial and shape relationship between intrusive rocks and hydrothermal bodies suggests a common extensional tectono-magmatic regime capable to produce asymmetric crustal traps (dilational structures) for magmas and fluids.</p>

Granulite-facies regional and contact metamorphism of the Tasiuyak paragneiss, northern Labrador: textural evolution and interpretation

2007 ◽  
Vol 44 (10) ◽  
pp. 1413-1437 ◽  
Author(s):  
Tanya Tettelaar ◽  
Aphrodite Indares
Keyword(s):  
Bulk Composition ◽  
Regional Metamorphism ◽  
Granulite Facies ◽  
Contact Metamorphism ◽  
Contact Aureole ◽  
Metamorphic Overprint ◽  
Textural Evolution ◽  
Progressive Development ◽  
Nain Plutonic Suite ◽  
Pelitic Rocks

The Tasiuyak paragneiss at the western margin of the Nain Plutonic Suite has been subjected to two granulite-facies metamorphic events: (i) regional metamorphism during the Paleoproterozoic Torngat orogeny, and (ii) contact metamorphism due to emplacement of the Mesoproterozoic Nain Plutonic Suite. Regional metamorphism led to partial melting of pelitic rocks and the development of a locally well-preserved sequence of prograde and retrograde textures. These textures are partly controlled by bulk composition and formed in the pressure–temperature (P–T) field of the continuous reaction: biotite + sillimanite + plagioclase + quartz  = garnet + K-feldspar + melt, along a hairpin P–T path with peak conditions of ~8–10 kbar (0.8–1.0 GPa) and up to 870 °C in the NaKFMASH (Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O) system. These textures controlled the development of the contact metamorphic assemblages. Contact metamorphism of the pelitic rocks between the Tessiarsuyungoakh intrusion and the Makhavinekh Lake pluton led to growth of orthopyroxene-cordierite symplectite after garnet–biotite, and cordierite–spinel symplectite after garnet–sillimanite. These phase associations attest to reactions in specific microtextural settings, some of which produced a second generation of partial melt. Maximum temperatures were above ~750 °C and pressures were lower than those of the regional metamorphism. The aureole around the Makhavinekh Lake pluton is ~4 km wide and shows a progressive development of the contact metamorphic assemblages toward the pluton. In contrast, the contact metamorphic overprint is incipient around the Tessiarsuyungoakh intrusion, which developed a ~20 m wide contact aureole and is most prominent in screens of paragneiss within that intrusion.

Shale gas leakage in lower Ecca shales during contact metamorphism by dolerite sill intrusions in the Karoo Basin, South Africa

2021 ◽  
Author(s):  
V. Nengovhela ◽  
B. Linol ◽  
L. Bezuidenhout ◽  
T. Dhansay ◽  
T Muedi ◽  
...  
Keyword(s):  
South Africa ◽  
Grain Boundaries ◽  
Shale Gas ◽  
Elevated Temperatures ◽  
Contact Metamorphism ◽  
Microscopy Imaging ◽  
Gas Leakage ◽  
Karoo Basin ◽  
Maximum Temperatures ◽  
Host Rocks

Abstract Contact metamorphism along widespread dolerite sills and dykes, emplaced at 182 to 183 Ma through the sedimentary host rocks of the Karoo Basin, triggered devolatilization of carbon-rich shales of the Lower Ecca Group. Hornfel samples collected from drill cores that intersect dolerite sills were analyzed for mineral phase equilibria, chemistry and porosity to characterize thermal aureoles at various distances from sill intrusions. Andalusite-chiastolite and cordierite porphyroblasts with biotite and muscovite occur within 10 to 20 m of many intrusive contacts. These metamorphic minerals crystallized when host shales attained maximum temperatures ranging between 450 and 600°C. Scanning electron microscopy imaging confirms that the hornfels are compact and that their metamorphic minerals limit porosity along grain boundaries. In few cases intra-mineral porosity occurs within individual crystals such as calcite, andalusite and cordierite. Disequilibrium metamorphic textures such as irregular grain boundaries, and inclusions in andalusite and cordierite reveal that the elevated temperatures were too short-lived to accomplish complete (re)crystallization. Thermal modeling results are consistent with the observed metamorphic mineral assemblages. Gas leakage calculations along a 7 m and a 47 m thick dolerite sill that intrude toward the top of the Whitehill Formation suggest that methane volumes ranging between 8 to 15 Tcf were generated during the sill emplacement. Methane was likely released into the atmosphere through hydrothermal vent complexes that are well preserved in the western Karoo Basin. If such loss was widespread across the entire basin, the implications for paleo-climate change and preserved shale gas reserves in the Karoo Basin of South Africa would be significant.

Geology, Structural Analysis, and Paragenesis of the Arrow Uranium Deposit, Western Athabasca Basin, Saskatchewan, Canada: Implications for the Development of the Patterson Lake Corridor

2020 ◽  
Author(s):  
Sean Hillacre ◽  
Kevin Ansdell ◽  
Brian McEwan
Keyword(s):  
Structural Analysis ◽  
Uranium Deposit ◽  
Regional Scale ◽  
Structural Features ◽  
White Mica ◽  
Uranium Mineralization ◽  
Fault System ◽  
Thin Sections ◽  
Athabasca Basin ◽  
Quartz Veins

Abstract Recent significant discoveries of uranium mineralization in the southwestern Athabasca basin, northern Saskatchewan, Canada, have been associated with a series of geophysical conductors along a NE- to SW-trending structural zone, termed the Patterson Lake corridor. The Arrow deposit (indicated mineral resource: 256.6 Mlb U3O8; grade 4.03% U3O8) is along this trend, hosted exclusively in basement orthogneisses of the Taltson domain, and is the largest undeveloped uranium deposit in the basin. This study is the first detailed analysis of a deposit along this corridor and examines the relationships between the ductile framework and brittle reactivation of structures, mineral paragenesis, and uranium mineralization. Paragenetic information from hundreds of drill core samples and thin sections was integrated with structural analysis utilizing over 18,000 measurements of various structural features. The structural system at Arrow is interpreted as a partitioned, strike-slip–dominated, brittle-ductile fault system of complex Riedel-style geometry. The system developed along subvertical, NE- to SW-trending dextral high-strain zones formed syn- to post-D3 deformation, which were the focus of extensive premineralization metasomatism (quartz flooding, sericitization, chloritization), within the limb domain of a regional-scale fold structure. These zones evolved through post-Athabasca dextral and sinistral reactivation events, creating brittle fault linkages and dilation zones, allowing for hydrothermal fluid migration and resulting uraninite precipitation and associated alteration (white mica, chlorite, kaolinite, hematite, quartz veins). This study of the structural context of Arrow is important as it emphasizes that protracted reactivation of deep-seated structures and their subsidiaries was a fundamental control on uranium mineralization in the southwestern Athabasca basin.

scholarly journals U-Pb monazite ages of the Kabanga mafic-ultramafic intrusions and contact aureoles, central Africa: Geochronological and tectonic implications

2019 ◽  
Vol 131 (11-12) ◽  
pp. 1857-1870 ◽  
Author(s):  
Jian-Wei Zi ◽  
Birger Rasmussen ◽  
Janet R. Muhling ◽  
Wolfgang D. Maier ◽  
Ian R. Fletcher
Keyword(s):  
Central Africa ◽  
Layered Intrusion ◽  
Thermal Anomaly ◽  
Contact Metamorphism ◽  
Contact Aureole ◽  
East African ◽  
Metamorphic History ◽  
History Of ◽  
Rodinia Assembly ◽  
Contact Aureoles

AbstractMafic-ultramafic rocks of the Kabanga-Musongati alignment in the East African nickel belt occur as Bushveld-type layered intrusions emplaced in metasedimentary sequences. The age of the mafic-ultramafic intrusions remains poorly constrained, though they are regarded to be part of ca. 1375 Ma bimodal magmatism dominated by voluminous S-type granites. In this study, we investigated igneous monazite and zircon from a differentiated layered intrusion and metamorphic monazite from the contact aureole. The monazite shows contrasting crystal morphology, chemical composition, and U-Pb ages. Monazite that formed by contact metamorphism in response to emplacement of mafic-ultramafic melts is characterized by extremely high Th and U and yielded a weighted mean 207Pb/206Pb age of 1402 ± 9 Ma, which is in agreement with dates from the igneous monazite and zircon. The ages indicate that the intrusion of ultramafic melts was substantially earlier (by ∼25 m.y., 95% confidence) than the prevailing S-type granites, calling for a reappraisal of the previously suggested model of coeval, bimodal magmatism. Monazite in the metapelitic rocks also records two younger growth events at ca. 1375 Ma and ca. 990 Ma, coeval with metamorphism during emplacement of S-type granites and tin-bearing granites, respectively. In conjunction with available geologic evidence, we propose that the Kabanga-Musongati mafic-ultramafic intrusions likely heralded a structurally controlled thermal anomaly related to Nuna breakup, which culminated during the ca. 1375 Ma Kibaran event, manifested as extensive intracrustal melting in the adjoining Karagwe-Ankole belt, producing voluminous S-type granites. The Grenvillian-aged (ca. 990 Ma) tin-bearing granite and related Sn mineralization appear to be the far-field record of tectonothermal events associated with collision along the Irumide belt during Rodinia assembly. Since monazite is a ubiquitous trace phase in pelitic sedimentary rocks, in contact aureoles of mafic-ultramafic intrusions, and in regional metamorphic belts, our study highlights the potential of using metamorphic monazite to determine ages of mafic-ultramafic intrusions, and to reconstruct postemplacement metamorphic history of the host terranes.

Cellular Automata Simulation of Textural Evolution during Primary Static Recrystallization of IF Steel

2011 ◽  
Vol 702-703 ◽  
pp. 615-618
Author(s):  
Dong Kyu Kim ◽  
K.H. Jung ◽  
H.W. Lee ◽  
Yong Taek Im
Keyword(s):  
Cellular Automata ◽  
Static Recrystallization ◽  
Grain Morphology ◽  
Interstitial Free ◽  
If Steel ◽  
Probabilistic Cellular Automata ◽  
Cellular Automata Model ◽  
Textural Evolution ◽  
Annealing Texture ◽  
Curvature Driven

A two dimensional probabilistic cellular automata model is used to simulate primary static recrystallization of interstitial free (IF) steel. The present study is to investigate the effect of curvature-driven pressure that is induced by protrusions/retrusions of recrystallization fronts on the microstructural and textural evolution during recrystallization. It was found that local interface migration of protrusions/retrusions of recrystallization fronts could significantly affect the kinetics, grain morphology and annealing texture according to the present investigation.

Metamorphic amphiboles in the Ironwood Iron-Formation, Gogebic Iron Range, Wisconsin: Implications for potential resource development

2020 ◽  
Vol 105 (8) ◽  
pp. 1259-1269
Author(s):  
Carlin J. Green ◽  
Robert R. Seal ◽  
Nadine M. Piatak ◽  
William F. Cannon ◽  
Ryan J. McAleer ◽  
...  
Keyword(s):  
Objective Assessment ◽  
Regional Scale ◽  
Resource Development ◽  
The United States ◽  
Contact Metamorphism ◽  
Iron Formation ◽  
Detailed Knowledge ◽  
Lower Grade ◽  
Iron Range ◽  
Wide Range

Abstract The Paleoproterozoic Ironwood Iron-Formation, a Superior-type banded iron formation located in the western Gogebic Iron Range in Wisconsin, is one of the largest undeveloped iron ore resources in the United States. Interest in the development of this resource is complicated by potential environmental and health effects related to the presence of amphibole minerals in the Ironwood, a consequence of Mesoproterozoic contact metamorphism. The presence of these amphiboles and their contact metamorphic origin have long been recognized; however, recent interest in this resource has highlighted the lack of detailed knowledge on their distribution, mineral chemistry, and morphology. Optical microscopy, X-ray diffraction, scanning electron microscopy, and electron microprobe analysis were utilized to investigate the origin, distribution, morphology, and chemistry of amphiboles in the Ironwood. Amphibole is present in the western portion of the study area due to regional-scale contact meta-morphism associated with the intrusion of the 1.1 Ga Mellen Intrusive Complex. Locally amphibole is also present, adjacent to diabase and/or gabbro dikes and sills in the lower-grade Ironwood in the eastern portion of the study area. In both localities, amphiboles in the Ironwood most commonly developed in massive and prismatic habits, and locally assumed a fibrous habit. Fibrous amphiboles were recognized locally in the two potential ore zones of the Ironwood but were not observed in the portion likely to be waste rock. Massive and prismatic amphiboles show a wide range of Mg# [molar Mg/(Mg+Fe2+)] values (0.06 to 0.87), whereas Mg# values of fibrous amphiboles are restricted from 0.14 to 0.35. Factors that influenced the compositional variability of amphiboles in the Ironwood may have included temperature of formation, morphology, bulk chemistry of the iron formation, and variations in prograde and retrograde metamorphism. The presence of amphiboles in the Ironwood is a known issue that will need to be factored into any future mine plans. This study provides an objective assessment of the distribution and character of amphiboles in the Ironwood to aid all decision-makers in any future resource development scenarios.

The western margin of the Coast Plutonic Complex on Hardwicke and West Thurlow Islands, British Columbia

1979 ◽  
Vol 16 (6) ◽  
pp. 1166-1175 ◽  
Author(s):  
Jo Anne Nelson
Keyword(s):  
Quartz Diorite ◽  
Contact Metamorphism ◽  
Contact Aureole ◽  
Western Margin ◽  
Late Mesozoic ◽  
The North ◽  
Early Mesozoic ◽  
Plutonic Complex ◽  
Coast Plutonic Complex ◽  
Maximum Contact

The western margin of the Coast Plutonic Complex, one of the major tectonic boundaries of the Canadian Cordillera, has been variously interpreted as an intrusive contact, a shear zone, and a suture zone joining the Early Mesozoic Insular Belt to the North American continent. A representative section of this boundary, exposed on islands in Johnstone Strait, is an intrusive contact along which a quartz diorite with peripheral mafic phases truncates Early Mesozoic sediments and volcanics of the Insular Belt. Concordant hornblende–biotite pairs and two whole rock biotite isochrons date the intrusion as Late Jurassic (151 Ma). Prior to intrusion the stratified units underwent prehnite–pumpellyite facies metamorphism and west-northwest block faulting.The contact aureole of the quartz diorite and its associated mafic phases involves greenschist and hornblende–hornfels facies assemblages. Total pressure in the upper Karmutsen Formation during contact metamorphism was less than 2.5 × 105 kPa. The maximum contact temperature was between 670 and 700 °C. Forcible emplacement of the intrusion caused penetrative deformation of wall rocks in the inner aureole. The maximum contact temperatures indicate that the plutonic bodies were at near-liquidus temperatures when emplaced.The contact on Hardwicke and West Thurlow Islands appears representative of most of the tectonic boundary between the southern Coast Plutonic Complex and the Insular Belt. The western margin of the Coast Plutonic Complex is thus a Late Mesozoic magmatic front, the western limit of the intense magmatism that generated the Coast Plutonic Complex. The formation of Georgia Depression over the province boundary was a later event, coeval with major uplift of the Coast Plutonic Complex.

Paleomagnetism of the Keweenawan Chipman Lake and Seabrook Lake carbonatite complexes, Ontario

1992 ◽  
Vol 29 (6) ◽  
pp. 1215-1223 ◽  
Author(s):  
D. T. A. Symons
Keyword(s):  
Magnetic Field ◽  
Remanent Magnetization ◽  
Pole Position ◽  
Alteration Zone ◽  
Contact Aureole ◽  
Isothermal Remanent Magnetization ◽  
Abitibi Subprovince ◽  
Normal Polarity ◽  
Reversed Polarity ◽  
Host Rocks

The Chipman Lake complex crops out as a series of carbonatite and related alkalic mafic dikes in the Wabigoon Subprovince of the Superior Province, whereas the Seabrook Lake complex crops out as an alkalic syenite – carbonatite stock in the Abitibi Subprovince. Paleomagnetic analysis was done on specimens from 23 and 19 sites located in and around the Chipman Lake and Seabrook Lake complexes, respectively, using detailed alternating-field and thermal step demagnetization and isothermal remanent magnetization tests. Contact tests with adjacent Archean host rocks show that both complexes retain a primary characteristic remanence (ChRM). The Chipman Lake's ChRM is retained in 11 dikes with normal polarity and one dike with reversed polarity and at one site with normal polarity and one site with reversed polarity from the fenite alteration zone. Its ChRM gives a pole position at 186°E, 38°N (dp = 7°, dm = 11°), which corresponds to a Keweenawan age of 1098 ± 10 Ma, suggesting that younger K–Ar amphibole ages do not date emplacement. The ChRM of the host rock, the Chipman Lake diorite stock, gives a pole at 49°E, 51°N (dp = 8°, dm = 13°), showing that it is not part of the Keweenawan complex but may be a 2.45 Ga Matachewan intrusive. The Seabrook Lake complex's ChRM is found at six normal polarity sites from within the complex and at four normal and three reversed polarity sites from within the fenitized Archean granite and Matachewan diabase of the contact aureole. It gives a pole position at 180°E, 46°N (dp = 11°, dm = 17°), which corresponds to a Keweenawan age of 1103 ± 10 Ma, agreeing with K/Ar biotite ages. The paleomagnetic data indicate that no significant motion on the Kapuskasing Structural Zone occurred after emplacement of the complexes excluding minor vertical uplift of less than about 4 km, and that there were multiple polarity transitions of a symmetric Earth's magnetic field during Keweenawan time.

Cognate xenoliths in the Tertiary uhrabasic dykes of south-west Skye

1969 ◽  
Vol 37 (288) ◽  
pp. 504-514 ◽  
Author(s):  
Fergus G. F. Gibb
Keyword(s):  
Upper Mantle ◽  
High Viscosity ◽  
Preferred Orientation ◽  
Mantle Material ◽  
South West ◽  
Host Rocks ◽  
Axial Concentration ◽  
Shape And Size

SummaryA group of ultrabasic dykes occurring in south-west Skye is typified by often abundant and diverse ultrabasic xenoliths composed of essentially the same minerals as the dykes. The xenoliths, which vary considerably in shape and size, are in sharp contact with the host rocks and exhibit no evidence of remelting by, or reaction with, the dykes. Although extensive axial concentration of olivine occurred in the dykes during intrusion, the xenoliths appear to be randomly distributed throughout all but the extreme margins of the dykes and often exhibit a preferred orientation. The orientation and distribution of the xenoliths are attributed to the suppression of rotation and axial migration of the xenoliths during the emplacement of the dykes because of the relatively high viscosity of the suspending medium.The ultrabasic xenoliths, unlike many of those occurring in basalts, kimberlites, etc., are considered unlikely to represent primary upper mantle material and it is suggested that they were probably derived by the disintegration of layered ultrabasic rocks genetically related to the dykes and hence are of cognate origin.

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