Pub Date : 2022-03-26DOI: 10.12789/geocanj.2022.49.183
D. Kellett, A. Zagorevski
The Laberge Group is an Early to Middle Jurassic sequence of mostly siliciclastic sedimentary rocks that were deposited in a marginal marine environment in the northern Canadian Cordillera. It forms a long narrow belt with a total thickness of 3–4 km extending for more than 600 km across southern Yukon and northwestern British Columbia. These sedimentary rocks overlap the Yukon-Tanana, Stikinia and Cache Creek terranes that form the main components of the Intermontane superterrane. The Laberge Group contains a record of the erosion of some of these terranes, and also offers some constraints on the timing of their amalgamation and accretion to the Laurentian margin. The Laberge Group was deposited with local unconformity on the Late Triassic Stuhini Group (in British Columbia) and correlative Lewes River Group (in Yukon), both of which are volcanic-rich, and assigned to the Stikinia terrane. The Laberge Group is in turn overlain by Middle Jurassic to Cretaceous clastic rocks, including the Bowser Lake Group in BC and the Tantalus Formation in Yukon. Clast compositions and detrital zircon populations within the Laberge Group and between it and these bounding units indicate major shifts in depositional environment, basin extent and detrital sources from Late Triassic to Late Jurassic. During the Early Jurassic clast compositions in the Laberge Group shifted from sediment- and volcanic-dominated to plutonic-dominated, and detrital zircon populations are dominated by grains that yield ages that approach or overlap their inferred depositional ages. This pattern is consistent with progressive dissection and unroofing of (an) active arc(s) to eventually expose Triassic to Jurassic plutonic suites. Detrital rutile and muscovite data from the Laberge Group indicate rapid cooling and then exhumation of adjoining metamorphic rocks during the Early Jurassic, allowing these to contribute detritus on a more local scale. The most likely source for such metamorphic detritus is within the Yukon-Tanana terrane, and its presence in the Laberge Group may constrain the timing of amalgamation and accretion of the Yukon-Tanana and Stikinia terranes. Thermochronological data also provide new insights into the evolution of the Laberge Group basin. Results from the U–Th/(He) method on detrital apatite suggest that most areas experienced post-depositional heating to 60°C or more, whereas U–Th/(He) results from detrital zircon show that heating to more than 200°C occurred on a more local scale. In detail, Laberge Group cooling and exhumation was at least in part structurally controlled, with more strongly heated areas situated in the footwall of an important regional fault system. The thermochronological data are preliminary, but they suggest potential to eventually constrain the kinematics and timing of inversion across the Laberge Group basin and may also have implications for its energy prospectivity. In summary, the Laberge Group is a complex package of sedimentary rocks
{"title":"The Jurassic Laberge Group in the Whitehorse Trough of the Canadian Cordillera: Using Detrital Mineral Geochronology and Thermochronology to Investigate Tectonic Evolution","authors":"D. Kellett, A. Zagorevski","doi":"10.12789/geocanj.2022.49.183","DOIUrl":"https://doi.org/10.12789/geocanj.2022.49.183","url":null,"abstract":"The Laberge Group is an Early to Middle Jurassic sequence of mostly siliciclastic sedimentary rocks that were deposited in a marginal marine environment in the northern Canadian Cordillera. It forms a long narrow belt with a total thickness of 3–4 km extending for more than 600 km across southern Yukon and northwestern British Columbia. These sedimentary rocks overlap the Yukon-Tanana, Stikinia and Cache Creek terranes that form the main components of the Intermontane superterrane. The Laberge Group contains a record of the erosion of some of these terranes, and also offers some constraints on the timing of their amalgamation and accretion to the Laurentian margin. The Laberge Group was deposited with local unconformity on the Late Triassic Stuhini Group (in British Columbia) and correlative Lewes River Group (in Yukon), both of which are volcanic-rich, and assigned to the Stikinia terrane. The Laberge Group is in turn overlain by Middle Jurassic to Cretaceous clastic rocks, including the Bowser Lake Group in BC and the Tantalus Formation in Yukon. Clast compositions and detrital zircon populations within the Laberge Group and between it and these bounding units indicate major shifts in depositional environment, basin extent and detrital sources from Late Triassic to Late Jurassic. During the Early Jurassic clast compositions in the Laberge Group shifted from sediment- and volcanic-dominated to plutonic-dominated, and detrital zircon populations are dominated by grains that yield ages that approach or overlap their inferred depositional ages. This pattern is consistent with progressive dissection and unroofing of (an) active arc(s) to eventually expose Triassic to Jurassic plutonic suites. Detrital rutile and muscovite data from the Laberge Group indicate rapid cooling and then exhumation of adjoining metamorphic rocks during the Early Jurassic, allowing these to contribute detritus on a more local scale. The most likely source for such metamorphic detritus is within the Yukon-Tanana terrane, and its presence in the Laberge Group may constrain the timing of amalgamation and accretion of the Yukon-Tanana and Stikinia terranes. Thermochronological data also provide new insights into the evolution of the Laberge Group basin. Results from the U–Th/(He) method on detrital apatite suggest that most areas experienced post-depositional heating to 60°C or more, whereas U–Th/(He) results from detrital zircon show that heating to more than 200°C occurred on a more local scale. In detail, Laberge Group cooling and exhumation was at least in part structurally controlled, with more strongly heated areas situated in the footwall of an important regional fault system. The thermochronological data are preliminary, but they suggest potential to eventually constrain the kinematics and timing of inversion across the Laberge Group basin and may also have implications for its energy prospectivity. In summary, the Laberge Group is a complex package of sedimentary rocks","PeriodicalId":55106,"journal":{"name":"Geoscience Canada","volume":" ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-03-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48211529","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2022-03-26DOI: 10.12789/geocanj.2022.49.186
N. Van wagoner, L. Fyffe, D. Lentz, K. Dadd, W. McNeil, D. Baldwin
This field trip is an excursion through the exquisite, nearly pristine exposures of a Silurian, felsic-dominated bimodal volcanic and sedimentary sequence exposed in the Passamaquoddy Bay area of southwestern, New Brunswick (Eastport Formation). These rocks form the northwest extension of the Coastal Volcanic Belt that extends from southwestern New Brunswick to the southern coast of Maine. The sequence is significant because it is part of a large bimodal igneous province with evidence for supervolcano-scale eruptions that began to form during the close of the Salinic Orogeny (about 424 Ma), and continued into the Acadian Orogeny (421–400 Ma). The geochemical characteristic of the rocks can be explained by extension related volcanism but the specific drivers of the extension are uncertain. The Passamaquoddy Bay sequence is 4 km thick and comprises four cycles of basaltic-rhyolitic volcanism. Basaltic volcanism typically precedes rhyolitic volcanism in Cycles 1–3. Cycle 4 represents the waning stages of volcanism and is dominated by peritidal sediments and basaltic volcanics. A spectrum of eruptive and emplacement mechanisms is represented ranging from the Hawaiian and Strombolian-type volcanism of the basaltic flows and pyroclastic scoria deposits, to highly explosive sub-Plinian to Plinian rhyolitic pyroclastic eruptions forming pyroclastic density currents (PDC) and high grade rheomorphic ignimbrites. During this field trip we will examine key exposures illustrating this spectrum of eruptive and emplacement processes, and their diagnostic characteristics, along with evidence for the interaction between mafic and felsic magmas and a variety of peperitic breccias formed as a result of emplacement of flows on wet peritidal sediments. The constraints the depositional setting and voluminous bimodal volcanism places on tectonic models will also be considered.
{"title":"Volcanism of the Late Silurian Eastport Formation of the Coastal Volcanic Belt, Passamaquoddy Bay, New Brunswick","authors":"N. Van wagoner, L. Fyffe, D. Lentz, K. Dadd, W. McNeil, D. Baldwin","doi":"10.12789/geocanj.2022.49.186","DOIUrl":"https://doi.org/10.12789/geocanj.2022.49.186","url":null,"abstract":"This field trip is an excursion through the exquisite, nearly pristine exposures of a Silurian, felsic-dominated bimodal volcanic and sedimentary sequence exposed in the Passamaquoddy Bay area of southwestern, New Brunswick (Eastport Formation). These rocks form the northwest extension of the Coastal Volcanic Belt that extends from southwestern New Brunswick to the southern coast of Maine. The sequence is significant because it is part of a large bimodal igneous province with evidence for supervolcano-scale eruptions that began to form during the close of the Salinic Orogeny (about 424 Ma), and continued into the Acadian Orogeny (421–400 Ma). The geochemical characteristic of the rocks can be explained by extension related volcanism but the specific drivers of the extension are uncertain. The Passamaquoddy Bay sequence is 4 km thick and comprises four cycles of basaltic-rhyolitic volcanism. Basaltic volcanism typically precedes rhyolitic volcanism in Cycles 1–3. Cycle 4 represents the waning stages of volcanism and is dominated by peritidal sediments and basaltic volcanics. A spectrum of eruptive and emplacement mechanisms is represented ranging from the Hawaiian and Strombolian-type volcanism of the basaltic flows and pyroclastic scoria deposits, to highly explosive sub-Plinian to Plinian rhyolitic pyroclastic eruptions forming pyroclastic density currents (PDC) and high grade rheomorphic ignimbrites. During this field trip we will examine key exposures illustrating this spectrum of eruptive and emplacement processes, and their diagnostic characteristics, along with evidence for the interaction between mafic and felsic magmas and a variety of peperitic breccias formed as a result of emplacement of flows on wet peritidal sediments. The constraints the depositional setting and voluminous bimodal volcanism places on tectonic models will also be considered.","PeriodicalId":55106,"journal":{"name":"Geoscience Canada","volume":" ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-03-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44177297","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2022-03-26DOI: 10.12789/geocanj.2022.49.182
Andrew Kerr
Over the years, it has become a tradition that the first issue of Geoscience Canada contains some sort of editorial piece. When the deadline looms in March, I regret that this precedent was ever established. What can I possibly write that has relevance and interest to readers? We are still here, obviously, and we plan to continue as best we can and serve our Geoscience Community in Canada. Surviving as a small scientific journal in a large pond has more than its fair share of challenges, but our long-term goal is to grow and prosper, not just to persist. Our ongoing efforts would not be possible without the support of volunteers and GAC members, and of course the invaluable work of managing editor Cindy Murphy. So let my first statement this year be one of sincere thanks to Cindy and to all who assist us every year in smaller ways to produce the journal. In previous editorials, I have outlined some of the challenges that we face, and especially the need for the submission of good papers on diverse topics. This is the only viable route towards raising our profile and impact in a world dominated by corporate publishing. I have discussed the open-access concept, and its possible benefits to journals like us, even with the additional fiscal challenges that it implies. In 2020, I even ventured into the impact of the Covid-19 pandemic on the lives and work of Earth Scientists, mostly in an effort to find silver linings in a large bank of clouds. I doubt that many readers really want to hear more on that subject after two more years, as it is all too familiar. All of these topics are important to Geoscience Canada, and some are clearly vital, and many will come back in future years. Hopefully, Covid will not be in that latter group. So, the search for topics suited to a 2022 editorial seemed fruitless for quite some time. In the end, I decided to avoid all the obvious but well-worn subjects and will spend a few pages to instead contemplate the past. Not the recent past, or even some historical past, but the distant and mysterious geological past that lies at the very heart of our chosen calling. Those who read to the end of this might well feel that this is no more than an escapist flight into imagination, and perhaps just a diversion from the many serious issues confronting our world in the spring of 2022. There may be indeed some truth in this perspective. The two technical papers featured in this first issue for 2022 have much in common, although this is certainly not by our design. Both articles focus on the use of detrital zircon U– Pb geochronology to solve geological problems, but they also share a deeper theme. Superficially, they include statistics, probability density charts and tables of data, but they are in the end delving into something more fundamental. Both papers seek to recreate vanished worlds places that existed tens to hundreds of millions of years ago on an Earth that was simultaneously familiar and alien. Earth Scientists are uniq
{"title":"The Allure of Vanished Worlds","authors":"Andrew Kerr","doi":"10.12789/geocanj.2022.49.182","DOIUrl":"https://doi.org/10.12789/geocanj.2022.49.182","url":null,"abstract":"Over the years, it has become a tradition that the first issue of Geoscience Canada contains some sort of editorial piece. When the deadline looms in March, I regret that this precedent was ever established. What can I possibly write that has relevance and interest to readers? We are still here, obviously, and we plan to continue as best we can and serve our Geoscience Community in Canada. Surviving as a small scientific journal in a large pond has more than its fair share of challenges, but our long-term goal is to grow and prosper, not just to persist. Our ongoing efforts would not be possible without the support of volunteers and GAC members, and of course the invaluable work of managing editor Cindy Murphy. So let my first statement this year be one of sincere thanks to Cindy and to all who assist us every year in smaller ways to produce the journal. In previous editorials, I have outlined some of the challenges that we face, and especially the need for the submission of good papers on diverse topics. This is the only viable route towards raising our profile and impact in a world dominated by corporate publishing. I have discussed the open-access concept, and its possible benefits to journals like us, even with the additional fiscal challenges that it implies. In 2020, I even ventured into the impact of the Covid-19 pandemic on the lives and work of Earth Scientists, mostly in an effort to find silver linings in a large bank of clouds. I doubt that many readers really want to hear more on that subject after two more years, as it is all too familiar. All of these topics are important to Geoscience Canada, and some are clearly vital, and many will come back in future years. Hopefully, Covid will not be in that latter group. So, the search for topics suited to a 2022 editorial seemed fruitless for quite some time. In the end, I decided to avoid all the obvious but well-worn subjects and will spend a few pages to instead contemplate the past. Not the recent past, or even some historical past, but the distant and mysterious geological past that lies at the very heart of our chosen calling. Those who read to the end of this might well feel that this is no more than an escapist flight into imagination, and perhaps just a diversion from the many serious issues confronting our world in the spring of 2022. There may be indeed some truth in this perspective. The two technical papers featured in this first issue for 2022 have much in common, although this is certainly not by our design. Both articles focus on the use of detrital zircon U– Pb geochronology to solve geological problems, but they also share a deeper theme. Superficially, they include statistics, probability density charts and tables of data, but they are in the end delving into something more fundamental. Both papers seek to recreate vanished worlds places that existed tens to hundreds of millions of years ago on an Earth that was simultaneously familiar and alien. Earth Scientists are uniq","PeriodicalId":55106,"journal":{"name":"Geoscience Canada","volume":" ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-03-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42071415","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2022-03-26DOI: 10.12789/geocanj.2022.49.184
J. Sears, L. Beranek
The idea of a great pre-glacial river that drained much of North America into the Arctic waters of modern Canada was first suggested in 1895 by Robert A. Bell. In the 1970s, petroleum exploration in Hudson Strait and the Labrador Sea located the massive, submerged delta of what is now known as the Bell River. Reconstructions suggest that three main branches of the Bell River joined up near modern Hudson Bay. The eastern branch largely drained the Canadian Shield, but the central and western branches had headwaters in the Cordilleran orogenic belt and its foreland in the present-day U.S. and northwestern Canada, respectively. We present new detrital zircon U–Pb data from Lower Oligocene and Lower Miocene sand from an exploration well in the Saglek delta of the northern Labrador Sea. In conjunction with other detrital zircon results from the Labrador Sea (and elsewhere) these data record the configuration and history of this continental-scale drainage basin in more detail. Mesozoic and younger detrital zircon grains (< 250 Ma) are subordinate to Precambrian age groupings, but Cenozoic populations become more abundant during the Oligocene, suggesting that the basin had expanded into areas now occupied by the Colorado Plateau and the Basin-and-Range Province. Proterozoic and Phanerozoic detrital zircon grain populations in Saglek delta sediments are similar to those of the Pliocene Colorado River. The results support an earlier idea that initial incision of the Grand Canyon and denudation of the Colorado Plateau were associated with a north-flowing paleo-river that fed into the Bell River basin. This contribution continued until the Pliocene capture of this ancestral river by the Gulf of California basin, after which the excavation of the modern Grand Canyon was completed. The Bell River drainage basin was later blocked by the expansion of Pleistocene ice sheets.
{"title":"The Great Preglacial “Bell River” of North America: Detrital Zircon Evidence for Oligocene–Miocene Fluvial Connections Between the Colorado Plateau and Labrador Sea","authors":"J. Sears, L. Beranek","doi":"10.12789/geocanj.2022.49.184","DOIUrl":"https://doi.org/10.12789/geocanj.2022.49.184","url":null,"abstract":"The idea of a great pre-glacial river that drained much of North America into the Arctic waters of modern Canada was first suggested in 1895 by Robert A. Bell. In the 1970s, petroleum exploration in Hudson Strait and the Labrador Sea located the massive, submerged delta of what is now known as the Bell River. Reconstructions suggest that three main branches of the Bell River joined up near modern Hudson Bay. The eastern branch largely drained the Canadian Shield, but the central and western branches had headwaters in the Cordilleran orogenic belt and its foreland in the present-day U.S. and northwestern Canada, respectively. We present new detrital zircon U–Pb data from Lower Oligocene and Lower Miocene sand from an exploration well in the Saglek delta of the northern Labrador Sea. In conjunction with other detrital zircon results from the Labrador Sea (and elsewhere) these data record the configuration and history of this continental-scale drainage basin in more detail. Mesozoic and younger detrital zircon grains (< 250 Ma) are subordinate to Precambrian age groupings, but Cenozoic populations become more abundant during the Oligocene, suggesting that the basin had expanded into areas now occupied by the Colorado Plateau and the Basin-and-Range Province. Proterozoic and Phanerozoic detrital zircon grain populations in Saglek delta sediments are similar to those of the Pliocene Colorado River. The results support an earlier idea that initial incision of the Grand Canyon and denudation of the Colorado Plateau were associated with a north-flowing paleo-river that fed into the Bell River basin. This contribution continued until the Pliocene capture of this ancestral river by the Gulf of California basin, after which the excavation of the modern Grand Canyon was completed. The Bell River drainage basin was later blocked by the expansion of Pleistocene ice sheets.","PeriodicalId":55106,"journal":{"name":"Geoscience Canada","volume":" ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-03-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47064098","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-12-08DOI: 10.12789/geocanj.2021.48.178
Courtney Onstad
Geology Outreach at the University of Saskatchewan was initiated during the 2018/19 academic year as a free and informal education opportunity for K–12 educators and their students in Saskatchewan. The program was 100% volunteer-run by undergraduate and graduate students in the Department of Geological Sciences at the University of Saskatchewan. We estimate reaching more than 1000 students in Saskatoon and surrounding areas following two years of outreach offerings. Hands-on activities offered included ‘Rocks and Minerals’, ‘Fossils’, ‘Meteorite Impacts’ and ‘Volcanoes’ and also involved a tour of the Museum of Natural Sciences when completed on campus. The overall intent of these activities was to foster excitement about the Earth Sciences. Typically, Educators who booked our program taught grades 4–7, where the Earth Sciences are strongly represented in Saskatchewan’s science curriculum. Most outreach offerings occurred on the University of Saskatchewan campus, but some were offered remotely at elementary schools and various Girl Guides of Canada events. During the 2019/20 academic year, we booked every outreach event planned for that year within two days and had a waiting list of more than 30 teachers across the province. The demand for geoscience outreach in Saskatchewan is high, and we hope to continue providing engaging, relevant, and fun educational outreach opportunities. University departments across Canada should allocate funds for community and school outreach initiatives and hire science communicators to oversee programs such as this.
{"title":"Earth Science Education #6. Lessons Learned: Organizing a Geoscience Outreach Program at the University of Saskatchewan","authors":"Courtney Onstad","doi":"10.12789/geocanj.2021.48.178","DOIUrl":"https://doi.org/10.12789/geocanj.2021.48.178","url":null,"abstract":"Geology Outreach at the University of Saskatchewan was initiated during the 2018/19 academic year as a free and informal education opportunity for K–12 educators and their students in Saskatchewan. The program was 100% volunteer-run by undergraduate and graduate students in the Department of Geological Sciences at the University of Saskatchewan. We estimate reaching more than 1000 students in Saskatoon and surrounding areas following two years of outreach offerings. Hands-on activities offered included ‘Rocks and Minerals’, ‘Fossils’, ‘Meteorite Impacts’ and ‘Volcanoes’ and also involved a tour of the Museum of Natural Sciences when completed on campus. The overall intent of these activities was to foster excitement about the Earth Sciences. Typically, Educators who booked our program taught grades 4–7, where the Earth Sciences are strongly represented in Saskatchewan’s science curriculum. Most outreach offerings occurred on the University of Saskatchewan campus, but some were offered remotely at elementary schools and various Girl Guides of Canada events. During the 2019/20 academic year, we booked every outreach event planned for that year within two days and had a waiting list of more than 30 teachers across the province. The demand for geoscience outreach in Saskatchewan is high, and we hope to continue providing engaging, relevant, and fun educational outreach opportunities. University departments across Canada should allocate funds for community and school outreach initiatives and hire science communicators to oversee programs such as this.","PeriodicalId":55106,"journal":{"name":"Geoscience Canada","volume":" ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2021-12-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45533435","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-12-08DOI: 10.12789/geocanj.2021.48.180
E. Wohl
{"title":"River Planet: Rivers from Deep Time to the Modern Crisis","authors":"E. Wohl","doi":"10.12789/geocanj.2021.48.180","DOIUrl":"https://doi.org/10.12789/geocanj.2021.48.180","url":null,"abstract":"","PeriodicalId":55106,"journal":{"name":"Geoscience Canada","volume":" ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2021-12-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45982593","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-12-08DOI: 10.12789/geocanj.2021.48.177
Alexandria Littlejohn-Regular, J. Greenough, K. Larson
Rocks in the Late Proterozoic Horsethief Creek Group at Quartz Creek in British Columbia display rare ‘pinolitic’ textures resembling those described in some sparry magnesite deposits elsewhere in the world. Elongated white magnesite crystals up to 30 cm long occur in a contrasting, dark, fine-grained matrix of dolomite, chlorite, organic material, clay minerals and pyrite. The rocks are aesthetically appealing for use in sculpture and as dimension stone. The term ‘pinolite’ is derived from the superficial similarities between these unusual textures and pinecones. Petrographic examination indicates that these textures formed when metasomatic fluids replaced primary sedimentary dolomite with magnesite. Fluids moved along fractures and bedding planes with repeated fracturing yielding magnesite crystals oriented in opposite directions on either side of annealed fractures, and broken magnesite crystals adjacent to later fractures. Magnesite contains dolomite microinclusions and has elevated Ca contents that are consistent with its formation by replacement of dolomite. Low concentrations of Cr, Ni, Co, Ti, Sr, and Ba in magnesite also imply formation in a metasomatic rather than a sedimentary environment. The rare earth element (REE) concentrations in the Quartz Creek magnesite are higher than those in most evaporitic magnesite and REE patterns lack the Ce and Eu anomalies that characterize carbonate rocks from sedimentary environments. Enrichment in light REE relative to heavy REE, and the similarities between dolomite, chlorite, and magnesite REE profiles, imply that metasomatic fluids modified the original sedimentary geochemical signature of the dolostones during formation of the pinolite rocks. A Late Ordovician to Early Silurian U–Pb age (433 ± 12 Ma), for titanite in the black matrix surrounding the sparry magnesite is younger than the local host rocks, and also younger than the Mesoproterozoic to Middle Cambrian stratigraphic ages of the host rocks for nearby magnesite deposits. The ca. 433 Ma titanite overlaps the ages for numerous fault-associated diatremes and volcaniclastic deposits in the area. Possibly the igneous activity furnished heat for, and/or was the source for, metasomatic fluids that produced the pinolite deposits.
{"title":"Heritage Stone 8. Formation of Pinolitic Magnesite at Quartz Creek, British Columbia, Canada: Inferences from Preliminary Petrographic, Geochemical and Geochronological Studies","authors":"Alexandria Littlejohn-Regular, J. Greenough, K. Larson","doi":"10.12789/geocanj.2021.48.177","DOIUrl":"https://doi.org/10.12789/geocanj.2021.48.177","url":null,"abstract":"Rocks in the Late Proterozoic Horsethief Creek Group at Quartz Creek in British Columbia display rare ‘pinolitic’ textures resembling those described in some sparry magnesite deposits elsewhere in the world. Elongated white magnesite crystals up to 30 cm long occur in a contrasting, dark, fine-grained matrix of dolomite, chlorite, organic material, clay minerals and pyrite. The rocks are aesthetically appealing for use in sculpture and as dimension stone. The term ‘pinolite’ is derived from the superficial similarities between these unusual textures and pinecones. Petrographic examination indicates that these textures formed when metasomatic fluids replaced primary sedimentary dolomite with magnesite. Fluids moved along fractures and bedding planes with repeated fracturing yielding magnesite crystals oriented in opposite directions on either side of annealed fractures, and broken magnesite crystals adjacent to later fractures. Magnesite contains dolomite microinclusions and has elevated Ca contents that are consistent with its formation by replacement of dolomite. Low concentrations of Cr, Ni, Co, Ti, Sr, and Ba in magnesite also imply formation in a metasomatic rather than a sedimentary environment. The rare earth element (REE) concentrations in the Quartz Creek magnesite are higher than those in most evaporitic magnesite and REE patterns lack the Ce and Eu anomalies that characterize carbonate rocks from sedimentary environments. Enrichment in light REE relative to heavy REE, and the similarities between dolomite, chlorite, and magnesite REE profiles, imply that metasomatic fluids modified the original sedimentary geochemical signature of the dolostones during formation of the pinolite rocks. A Late Ordovician to Early Silurian U–Pb age (433 ± 12 Ma), for titanite in the black matrix surrounding the sparry magnesite is younger than the local host rocks, and also younger than the Mesoproterozoic to Middle Cambrian stratigraphic ages of the host rocks for nearby magnesite deposits. The ca. 433 Ma titanite overlaps the ages for numerous fault-associated diatremes and volcaniclastic deposits in the area. Possibly the igneous activity furnished heat for, and/or was the source for, metasomatic fluids that produced the pinolite deposits.","PeriodicalId":55106,"journal":{"name":"Geoscience Canada","volume":" ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2021-12-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43083285","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-08-18DOI: 10.12789/geocanj.2021.48.175
S. Johnston
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