An overview of the Canadian Cordilleran lithospheric mantleThis article is one of a series of papers published in this Special Issue on the theme Lithoprobe — parameters, processes, and the evolution of a continent.

2010 ◽  
Vol 47 (4) ◽  
pp. 353-368 ◽  
Author(s):  
Don Francis ◽  
William Minarik ◽  
Yuliana Proenza ◽  
Lang Shi
Keyword(s):  
Oxidation State ◽  
Lithospheric Mantle ◽  
Mantle Xenolith ◽  
Geothermal Gradient ◽  
Primitive Mantle ◽  
Canadian Cordillera ◽  
S Wave ◽  
The North ◽  
Melting Event ◽  
North American Craton

Recent alkaline basalts have brought xenoliths of the underlying lithospheric mantle to the surface at more than 20 localities along the strike length of the Canadian Cordillera. The populations of 13 of these xenolith suites display a common mode at 39–40 wt.% MgO and 3.0–3.5.0 wt.% Al2O3, corresponding to a relatively fertile lherzolite whose composition could reflect 8%–10% melting of primitive mantle. The present oxidation state of the Cordillera lithospheric mantle obtained from olivine–spinel equilibria is ∼1 log unit below the fayalite–magnetite–quartz (FMQ) buffer, which is essentially the same as the oxidation state at the time of the melting event that stabilized the Cordilleran lithospheric mantle in the mid-Proterozoic, as constrained by the relative variation of Sc and V. Two xenolith suites near the Yukon – British Columbia border exhibit a second stronger mode, corresponding to relatively refractory spinel harzburgite with significantly higher Mg contents (45–46 wt.% MgO) and lower Al contents (0.5–1.0 wt.% Al2O3). These bi-modal mantle xenolith suites overlie a teleseismic S-wave slowness anomaly in the underlying asthenospheric mantle, and the harzburgites appear to have been produced by a more recent, localized partial melting (∼15%) of the lherzolite lithosphere. The temperatures estimated from clinopyroxene–orthopyroxene equilibria indicate that the lithospheric mantle beneath the Canadian Cordillera is significantly hotter than that beneath the adjacent Archean of the North American craton, with temperatures at the Moho on the order of 800 °C, a minimum geothermal gradient of ∼10 °C/km, and a thickness of <∼65 km.

Paleomagnetic and geobarometric study of the mid-Cretaceous Whitehorse Pluton, Yukon Territory

1997 ◽  
Vol 34 (10) ◽  
pp. 1379-1391 ◽  
Author(s):  
M. J. Harris ◽  
D. T. A. Symons ◽  
W. H. Blackburn ◽  
C. J. R. Hart
Keyword(s):  
United States ◽  
British Columbia ◽  
Crystallization Temperature ◽  
Remanent Magnetization ◽  
Average Rate ◽  
Yukon Territory ◽  
Canadian Cordillera ◽  
The North ◽  
North American Craton ◽  
High Level

This is the first of several Lithoprobe paleomagnetic studies underway to examine geotectonic motions in the northern Canadian Cordillera. Except for one controversial study, estimates for terranes underlying the Intermontane Belt in the Yukon have been extrapolated from studies in Alaska, southern British Columbia, and the northwestern United States. The Whitehorse Pluton is a large unmetamorphosed and undeformed tonalitic body of mid-Cretaceous age (~112 Ma) that was intruded into sedimentary units of the Whitehorse Trough in the Stikinia terrane. Geothermobarometric estimates for eight sites around the pluton indicate that postmagnetization tilting has been negligible since cooling through the hornblende-crystallization temperature and that the pluton is a high-level intrusion. Paleomagnetic measurements for 22 of 24 sites in the pluton yield a well-defined characteristic remanent magnetization (ChRM) direction that is steeply down and northwards. The ChRM direction gives a paleopole of 285.5°E, 81.7°N (dp = 53°, dm = 5.7°). When compared with the 112 Ma reference pole for the North American craton, this paleopole suggests that the northern Stikinia terrane has been translated northwards by 11.0 ± 4.8° (1220 ± 530 km) and rotated clockwise by 59 ± 17°. Except for an estimate from the ~70 Ma Carmacks Group volcanics, this translation and rotation estimate agrees well with previous estimates for units in the central and southern Intermontane Belt. They suggest that the terranes of the Intermontane Belt have behaved as a fairly coherent unit since the Early Cretaceous, moving northward at a minimum average rate of 2.3 ± 0.4 cm/a between ~140 and ~45 Ma.

Development of late Paleozoic volcanic arcs in the Canadian Cordillera: an example from the Klinkit Group, northern British Columbia and southern Yukon

2003 ◽  
Vol 40 (7) ◽  
pp. 907-924 ◽  
Author(s):  
Renée-Luce Simard ◽  
Jaroslav Dostal ◽  
Charlie F Roots
Keyword(s):  
British Columbia ◽  
North America ◽  
Volcanic Rocks ◽  
Late Paleozoic ◽  
Canadian Cordillera ◽  
Alkali Basalts ◽  
The North ◽  
Pacific Margin ◽  
North American Craton ◽  
Arc Setting

The late Paleozoic volcanic rocks of the northern Canadian Cordillera lying between Ancestral North America to the east and the accreted terranes of the Omineca belt to the west record early arc and rift magmatism along the paleo-Pacific margin of the North American craton. The Mississippian to Permian volcano-sedimentary Klinkit Group extends discontinuously over 250 km in northern British Columbia and southern Yukon. The two stratotype areas are as follows: (1) in the Englishman Range, southern Yukon, the English Creek Limestone is conformably overlain by the volcano-sedimentary Mount McCleary Formation (Lower Clastic Member, Alkali-Basalt Member and Volcaniclastic Member), and (2) in the Stikine Ranges, northern British Columbia, the Screw Creek Limestone is conformably overlain by the volcano-sedimentary Butsih Formation (Volcaniclastic Member and Upper Clastic Member). The calc-alkali nature of the basaltic volcaniclastic members of the Klinkit Group indicates a volcanic-arc setting ((La/Yb)N = 2.77–4.73), with little involvement of the crust in their genesis (εNd = +6.7 to +7.4). Alkali basalts in the Mount McCleary Formation ((La/Yb)N = 12.5–17.8) suggest periodic intra-arc rifting events. Broadly coeval and compositionally similar volcano-sedimentary assemblages occur in the basement of the Mesozoic Quesnel arc, north-central British Columbia, and in the pericratonic Yukon–Tanana composite terrane, central Yukon, suggesting that they all represent pieces of a single long-lived, late Paleozoic arc system that was dismembered prior to its accretion onto Ancestral North America. Therefore, Yukon–Tanana terrane is possibly the equivalent to the basement of Quesnel terrane, and the northern Quesnel terrane has a pericratonic affinity.

Petrogenesis of pleistocene basalts from the Western snake river plain, Idaho

2020 ◽  
Author(s):  
Tiffany A Rivera ◽  
Craig M White ◽  
Mark D Schmitz ◽  
Brian R Jicha
Keyword(s):  
Fractional Crystallization ◽  
Mantle Source ◽  
Lithospheric Mantle ◽  
Snake River Plain ◽  
Snake River ◽  
Isotopic Ratios ◽  
Isotopic Signatures ◽  
The North ◽  
North American Craton ◽  
River Plain

Abstract We present new geochemical, Sr, Nd, and Pb isotope, and 40Ar/39Ar data from Pleistocene basalts of the Western Snake River Plain (WSRP), Idaho, USA to explore their petrogenesis and to investigate the nature of the lithosphere at the western boundary of the North American craton. The basalts are divided into three groups based on their geochemical and isotopic characteristics. Prior to ∼1 Ma, volcanoes in the WSRP erupted iron-rich tholeiites (FeB1), but subsequent volcanism was dominated by concurrent eruptions of mildly alkaline, alumina-rich lavas (AlB) and iron-rich tholeiites (FeB2) with isotopic signatures similar to the AlB lavas. New 40Ar/39Ar dates of AlB and FeB2 basalts range from 0.920 ± 0.049 Ma to 0.287 ± 0.014 Ma. MELTS models of FeB1 differentiation trends indicate that the range of compositions in this suite can be produced by 10–15% crystallization of olivine and plagioclase at low pressure using the least evolved FeB1 composition as a parental magma; isotopic ratios can be produced via combined assimilation of a Miocene rhyolite and fractional crystallization. Additional modeling suggests that parental magmas at AlB centers were produced by 3–12% equilibrium melting of a garnet-spinel enriched mantle source, slightly different to that proposed for the youngest mildly alkaline lavas of the eastern and central Snake River Plain. Our new geochemical, isotopic, and geochronologic data of the FeB2 basalts suggests they are related to AlB-type magmas via a combination of fractional crystallization and assimilation of evolved mafic crust. MELTS models suggest that crystallization of an AlB parental melt at a depth of 6–8 km (2.5 kb) could produce residual liquids having many of the major oxide characteristics of FeB2 ferrobasalts. Sr-Nd-Pb isotopic signatures of these three suites indicate a dominant contribution from an enriched plume source. FeB1 lavas are likely products of mixing between melts of an enriched plume mantle source (represented by Imnaha and Steens Basalts of the Columbia River Basalt Group) and isotopically heterogeneous sub-continental lithospheric mantle (SCLM) that has been isolated from the convecting mantle since the Archean. Isotopic ratios of FeB2 and AlB lavas capture mixing between enriched plume mantle and a more isotopically homogeneous ancient SCLM domain characteristic of the eastern and central Snake River Plain, with a coupled decrease in lithospheric contribution and degree of partial melting through time to the present. Mixtures of enriched asthenospheric reservoirs with lithospheric mantle have been proposed for neighboring volcanic fields to the east along the strike of the Yellowstone-SRP hotspot track, and to the west due to differences in the mantle underlying the boundary of the North American craton and accreted terranes. Our petrogenetic model for the Pleistocene WSRP basalts suggests that there is also a lateral, across strike gradient in the geometry and interaction of enriched plume mantle and ancient lithosphere. We reiterate suggestions that the WSRP is a lithosphere-scale conduit connecting initial plume head impingement in east-central Oregon with the subsequent Yellowstone-SRP hotspot plume tail track.

Geochemical and petrophysical characterization of mantle metasomatism beneath the North Tanzanian Divergence, East African rift.

2021 ◽  
Author(s):  
Adeline Clutier ◽  
Fleurice Parat ◽  
Michel Gregoire ◽  
Benoit Gibert ◽  
Stéphanie Gautier ◽  
...  
Keyword(s):  
Wave Velocity ◽  
Lithospheric Mantle ◽  
East African Rift ◽  
Mantle Xenoliths ◽  
P Wave ◽  
S Wave ◽  
East African ◽  
The North ◽  
Alkaline Magmas ◽  
African Rift

&lt;p&gt;The North Tanzanian Divergence (NTD) is the prolongation of the eastern branch of the East African Rift and is a place of intense volcanism. Numerous volcanoes erupted deep subalkaline to highly alkaline magmas, including the particular active natrocarbonatite Oldoinyo Lengai. On the North-South axis (Natron to Manyara basins), three highly alkaline volcanoes, Pello Hills, Lashaine and Labait, erupted melilite magmas that originated from low degree of partial melting of asthenospheric mantle (depth &gt; 120 km). The particularity of these volcanoes is that they sampled numerous mantle xenoliths during ascent. This represents a unique opportunity to study the composition and the rheology of lithospheric mantle. Mantle xenoliths are deep garnet-bearing peridotites (120 km depth), amphibole and phlogopite peridotites and phlogopitites. They contain abundant hydrous minerals as isolated crystals or veins that attest to an important metasomatism beneath the NTD. Previous geochemical and petrological studies have highlighted interactions of alkaline magmas and the thick cratonic lithosphere as metasomatic agent. However, the presence and composition of magmas, the degree of metasomatism, and the role of metasomatism on mantle rheology below the NTD is still debated.&lt;/p&gt;&lt;p&gt;To characterize these previous parameters, in this study we performed geochemical and petrophysical analyses on metasomatized, fertile and refractory mantle xenoliths from Labait (on-craton volcano) and Pello Hills (in-rift volcano). Using mineral compositions and thermobarometer calibrations, we estimated the depth of mantle xenoliths between 40 and 140 km (14 to 47 kbar) and temperatures from 930 to 1340&amp;#176;C. EBSD analysis on thin sections indicate that peridotites and amphibole/phlogopite-bearing mantle xenoliths display a moderate to strong deformation induced crystal preferred orientation. In contrast, weak mineral orientations have been observed in phlogopite-amphibole-clinopyroxene-bearing veins. Calculation of seismic properties using MTEX program show that peridotites are seismically anisotropic, up to 12.4% for P-wave velocity (Vp) and 6.8% for S-wave velocity (Vs). The Vp and Vs in hydrous veins are lower than in peridotites (Vp: 7.5-7.9 and 8.3-9.6 km/s; Vs: 4.4-4.6 and 5.0-5.3 km/s respectively) and therefore the Vp and Vs velocities decrease with the increasing proportion of metasomatic minerals. We estimate that a peridotite with 20 vol.% metasomatic vein has a velocity decrease of 3.5% for Vp and 2.9% for Vs, compared to a fertile peridotite.&lt;/p&gt;&lt;p&gt;These geochemical and petrophysical approaches are important to understand P- and S-wave propagation in the lithospheric mantle beneath the NTD and more specifically in metasomatized lithospheric mantle. The new in situ data and models from mantle xenoliths will be compared to tomographic acquisition and discussed in term of temperature, presence of melt or metasomatism processes. Both petrophysical and geophysical data will help to precisely determine the structure and rheology of the lithospheric mantle, which may control the propagation of the rift at early stage rifting between the Tanzanian craton and the mobile Proterozoic belts.&lt;/p&gt;

Kilometre-scale folding in the Teslin zone, northern Canadian Cordillera, and its tectonic implications for the accretion of the Yukon-Tanana terrane to North America

1999 ◽  
Vol 36 (3) ◽  
pp. 479-494 ◽  
Author(s):  
Martin de Keijzer ◽  
Paul F Williams ◽  
Richard L Brown
Keyword(s):  
North America ◽  
Shear Zone ◽  
North American ◽  
Root Zone ◽  
Fault System ◽  
Canadian Cordillera ◽  
The West ◽  
The North ◽  
South Central ◽  
North American Craton

The Teslin zone in south-central Yukon has previously been described as a discrete zone with a steep foliation unique to the zone. It includes the Anvil assemblage and the narrowest portion of the Yukon-Tanana terrane (the Nisutlin assemblage), and is defined by post-accretionary faults: the Big Salmon fault to the west and the d'Abbadie fault system to the east. The zone was interpreted as a lithospheric suture or a crustal-scale transpression zone, and as the root zone of klippen lying on the North American craton to the east. We demonstrate that deformation and metamorphism are the same inside and outside the zone. The steep transposition foliation in the zone, in contrast to adjacent rocks to the east, coincides with the steep limb of a regional F3 structure. This fold has a shallow limb in the easternmost part of the zone and immediately east of the zone. Thus we reject earlier interpretations. If a suture exists between the obducted Anvil and Yukon-Tanana Nisutlin assemblages and North America, it is a shear zone that occurs at the base of the obducted rocks, which has been folded by the F3 fold. However, evidence that this thrust boundary is a lithospheric suture is lacking. A consequence of our interpretation is that North American rocks pass under the eastern Teslin zone and outcrop to the west of the Nisutlin and Anvil assemblages. This geometry precludes the possibility of the Teslin zone being the root zone of the klippen.

Interpretation of isotopic ages and 87Sr/86Sr initial ratios for plutonic rocks in the Whitehorse map area, Yukon

1979 ◽  
Vol 16 (10) ◽  
pp. 1988-1997 ◽  
Author(s):  
Gregg W. Morrison ◽  
Colin I. Godwin ◽  
Richard L. Armstrong
Keyword(s):  
British Columbia ◽  
Oceanic Crust ◽  
Late Cretaceous ◽  
Canadian Cordillera ◽  
Initial Ratio ◽  
Eastern Margin ◽  
The North ◽  
Plutonic Complex ◽  
Coast Plutonic Complex ◽  
North American Craton

Sixteen new K–Ar dates and four new Rb–Sr isochrons help define four plutonic suites in the Whitehorse map area, Yukon. The Triassic(?) suite, defined on stratigraphic evidence, is the southern extension of the Yukon Crystalline Terrane and is correlative with plutonic suites in the Intermontane Belt in British Columbia. The mid-Cretaceous (~100 Ma) suite in the Intermontane Belt in the Whitehorse map area is time equivalent to plutonic suites in the Omineca Crystalline Belt to the east. Late Cretaceous (~70 Ma) and Eocene (~55 Ma) suites include volcanic and subvolcanic as well as plutonic phases and are correlative with continental volcano–plutonic suites near the eastern margin of the Coast Plutonic Complex. The predominance of the mid-Cretaceous suite in the Intermontane Belt in Whitehorse and adjacent map areas in Yukon and northern British Columbia suggests that this area has undergone posttectonic magmatism more characteristic of the Omineca Crystalline Belt than of the Intermontane Belt elsewhere in the Canadian Cordillera.87Sr/86Sr initial ratio determinations suggest that the southern extension of the Yukon Crystalline Terrane in the western part of the Whitehorse map area and in northern British Columbia includes Precambrian crust separated from the North American craton by Paleozoic oceanic crust of the Intermontane Belt.

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