Marathon dikes: Rb–Sr and K–Ar geochronology of ultrabasic lamprophyres from the vicinity of McKellar Harbour, northwestern Ontario, Canada

1983 ◽  
Vol 20 (6) ◽  
pp. 961-967 ◽  
Author(s):  
R. Garth Platt ◽  
Roger H. Mitchell ◽  
Paul M. Holm
Keyword(s):  
Upper Mantle ◽  
Lake Superior ◽  
Alkaline Rock ◽  
Alkaline Complex ◽  
Northwestern Ontario ◽  
Isotopic Studies ◽  
Mantle Sources ◽  
Ar Geochronology ◽  
Magmatic Event

The ultrabasic lamprophyre dikes from the vicinity of McKellar Harbour, northwestern Ontario, are composed essentially of a mixture of phlogopite, olivine, primary carbonate, spinel, and apatite with associated perovskite and (?) melilite. Petrologically and chemically they have strong affinities with alnöites and magmas derived from upper mantle sources in stable continental regimes.Rb–Sr isotopic studies give an isochron age of 1.65 ± 0.12 Ga (2σ) with an initial 87Sr/86Sr ratio of 0.70167 ± 0.00036 (2σ). This age is considerably older than the Neohelikian Keweenawan magmatic event associated with the development of the Lake Superior Basin (e. g., 950–1170 Ma). K–Ar data indicate a mean whole-rock age for the dikes of 1144 Ma and a pseudoisochron age of 1120 ± 34 Ma (2σ). These ages are thought to reflect partial degassing of phlogopite and incomplete updating of the rocks under thermal influences induced by the intrusion of the Coldwell alkaline complex at 1044 Ma.Ages of carbonatitic and alkaline rock intrusions in the general vicinity of the McKellar Harbour – Marathon region closely approximate those associated with the intrusion of similar magmas in the Kapuskasing structural zone, i.e., 1650–1700 and 1020–1050 Ma. By analogy with the Kapuskasing zone, it is suggested that periodic activation of a deep-seated, long-lived structural zone in the Marathon region has lead to the tapping of upper mantle magmas. The McKellar Harbour ultrabasic lamprophyres are the product of one such event.

Rb–Sr geochronology of the Coldwell Complex, northwestern Ontario, Canada

1982 ◽  
Vol 19 (9) ◽  
pp. 1796-1801 ◽  
Author(s):  
R. Garth Platt ◽  
Roger H. Mitchell
Keyword(s):  
Upper Mantle ◽  
Lake Superior ◽  
Parental Magma ◽  
Initial Ratio ◽  
North Shore ◽  
The North ◽  
Northwestern Ontario ◽  
Alkaline Intrusion ◽  
Intrusive Rocks ◽  
Igneous Activity

The Coldwell Complex of Northwestern Ontario is North America's largest structurally and petrologically complex alkaline intrusion. Situated on the north shore of Lake Superior, it consists of at least three intrusive centres and is cross-cut by a diverse suite of coeval–cogenetic dikes. The main intrusive rocks range from gabbros to ferroaugite syenites, nepheline syenites, and quartz syenites. The dikes are predominantly lamprophyric. A seventeen point whole rock Rb–Sr isochron (MSWD 2.22) gives an age of 1044.5 ± 6.2 Ma (2σ) and an initial ratio of 0.70354 ± 0.00016 (2σ). The age is late Neohelikian and is younger than the bulk of igneous activity (Keweenawan activity) prevalent in the Lake Superior Basin during the Neohelikian. The low initial ratio indicates an upper mantle origin for the parental magma of the complex.

Transition metal rutiles and titanates from the Deadhorse Creek diatreme complex, northwestern Ontario, Canada

1996 ◽  
Vol 60 (400) ◽  
pp. 403-413 ◽  
Author(s):  
R. Garth Platt ◽  
Roger H. Mitchell
Keyword(s):  
Transition Metals ◽  
Upper Mantle ◽  
Order Transition ◽  
Alkaline Solutions ◽  
Alkaline Complex ◽  
Northwestern Ontario ◽  
Crystallographic Shear ◽  
Second Order Transition ◽  
Mineralized Zone ◽  
A Site

AbstractThe main mineralized zone of the West subcomplex of the Deadhorse Creek diatreme complex, northwestern Ontario possesses an exotic mineralogy. Mineralization involves the first-order transition metals (principally Sc, Ti, V, Cr, Mn, and Fe), the second-order transition metals (principally Zr and Nb), the lanthanides, the actinides (principally Th and U), Be, Ba and Sr. Minerals include phenacite, zircon, uraninite, thorite, monazite-(Ce), xenotime-(Y), barylite, thortveitiite, hollandite, tyuyamunite, a number of unknown and as yet undescribed species, and those minerals more specifically described in this paper. These are Cr-V-Nb rutile, V-rich members of the crichtonite series, and a titanate of general composition (Cr,V3+,Fe3+)2(Ti,V4+,Nb)O5.Similar to rutiles reported from alkaline rocks in general, the Deadhorse Creek rutiles are enriched in Cr and Nb, with the latter element attaining some of the highest recorded values. V contents are also unusually high and this element is thought to exist in both the tri- and tetravalent states.The V-rich crichtonites are essentially vanadium analogues of crichtonite and lindsleyite. M-site Nb and V are the highest yet recorded. A-site cations are dominated by Ba and Sr with an inverse relationship together with lesser but significant amounts of Ca and Pb. Although not of upper mantle origin, they plot in the upper mantle LIMA quadrant of TiO2vs. FeO + Fe2O3 + MgO (Haggerty, 1991).(Cr,V3+,Fe3+)2(Ti,V4+,Nb)O5 is thought to be a member of an homologous series of type (Cr,V3+,Fe3+)2p(Ti,V4+,Nb)2p+qO5p+4q with p = 1 and q = 0 and a V3O5-type structure. Whether this structure is ultimately derived from that of rutile or from α-PbO2 by crystallographic shear is not known.The rutiles and titanates discussed here are thought to have formed from hydrous alkaline solutions which have scavenged the necessary elements from a mafic/ultramafic source. The origin of the solutions is not specifically known although the magmatic activity associated with the spatially related Coldwell alkaline complex and/or the Prairie Lake complex are both potential sources. Both complexes contain the necessary mafic/ultramafic rocks.

Potassium–argon geochronology of the Poohbah Lake alkaline complex, northwestern Ontario

1976 ◽  
Vol 13 (10) ◽  
pp. 1456-1459 ◽  
Author(s):  
Roger H. Mitchell
Keyword(s):  
Alkaline Rock ◽  
Strike Slip ◽  
The Other ◽  
Alkaline Complex ◽  
Geological Evidence ◽  
Thermal Event ◽  
Alkaline Rocks ◽  
Northwestern Ontario ◽  
Tight Cluster ◽  
Minimum Age

The Poohbah Lake complex is an Archean undersaturated potassic pluton consisting of porphyritic syenite, malignite, amphibole syenite, and biotite–pyroxenite, intrusive into Couchiching metasediments. Apparent K–Ar ages for biotites from the alkaline rocks fall into two groups, one (2 samples) with ages greater than 2700 m.y., the other (8 samples) forming a tight cluster of ages at 2556 ± 36 m.y. This latter group defines a K–Ar isochron age of 2706 ± 23 m.y. The data are interpreted to imply that the isochron age and the ca. 2700 m.y. apparent ages record the cooling age of the complex and that the majority of samples were subjected to a later thermal event which resulted in loss of equal amounts of Ar from each sample, this event being correlated with epeirogenic strike-slip faulting. Algoman granite and pegmatite (3 samples), which on geological evidence was emplaced later than the alkaline rocks, gives apparent K–Ar ages of 2550–2630 m.y. Algoman mica samples plot on the alkaline rock isochron giving 2702 ± 24 m.y. as a minimum age for plutonism in this area.

Emplacement age and isotopic composition of the Prairie Lake carbonatite complex, Northwestern Ontario, Canada

2016 ◽  
Vol 154 (2) ◽  
pp. 217-236 ◽  
Author(s):  
FU-YUAN WU ◽  
ROGER H. MITCHELL ◽  
QIU-LI LI ◽  
CHANG ZHANG ◽  
YUE-HENG YANG
Keyword(s):  
Nepheline Syenite ◽  
Lake Superior ◽  
Alkaline Rock ◽  
Main Stage ◽  
Carbonatite Complex ◽  
Midcontinent Rift ◽  
Northwestern Ontario ◽  
Depleted Mantle ◽  
Two Samples ◽  
Age Determinations

AbstractAlkaline rock and carbonatite complexes, including the Prairie Lake complex (NW Ontario), are widely distributed in the Canadian region of the Midcontinent Rift in North America. It has been suggested that these complexes were emplaced during the main stage of rifting magmatism and are related to a mantle plume. The Prairie Lake complex is composed of carbonatite, ijolite and potassic nepheline syenite. Two samples of baddeleyite from the carbonatite yield U–Pb ages of 1157.2±2.3 and 1158.2±3.8 Ma, identical to the age of 1163.6±3.6 Ma obtained for baddeleyite from the ijolite. Apatite from the carbonatite yields the same U–Pb age of ~1160 Ma using TIMS, SIMS and laser ablation techniques. These ages indicate that the various rocks within the complex were synchronously emplaced at about 1160 Ma. The carbonatite, ijolite and syenite have identical Sr, Nd and Hf isotopic compositions with a 87Sr/86Sr ratio of ~0.70254, and positive εNd(t)1160 and εHf(t)1160 values of ~+3.5 and ~+4.6, respectively, indicating that the silicate and carbonatitic rocks are co-genetic and related by simple fractional crystallization from a magma derived from a weakly depleted mantle. These age determinations extend the period of magmatism in the Midcontinent Rift in the Lake Superior area to 1160 Ma, but do not indicate whether the magmatism is associated with passive continental rifting or the initial stages of plume-induced rifting.

The effects of solid-solid phase equilibria on the oxygen fugacity of the upper mantle

2020 ◽  
Vol 105 (10) ◽  
pp. 1445-1471
Author(s):  
Edward M. Stolper ◽  
Oliver Shorttle ◽  
Paula M. Antoshechkina ◽  
Paul D. Asimow
Keyword(s):  
Phase Equilibria ◽  
Oxygen Fugacity ◽  
Upper Mantle ◽  
Phase Changes ◽  
Primitive Mantle ◽  
Spinel Lherzolite ◽  
Garnet Lherzolite ◽  
Constant Composition ◽  
Al Content ◽  
Mantle Sources

Abstract Decades of study have documented several orders of magnitude variation in the oxygen fugacity (fO2) of terrestrial magmas and of mantle peridotites. This variability has commonly been attributed either to differences in the redox state of multivalent elements (e.g., Fe3+/Fe2+) in mantle sources or to processes acting on melts after segregation from their sources (e.g., crystallization or degassing). We show here that the phase equilibria of plagioclase, spinel, and garnet lherzolites of constant bulk composition (including whole-rock Fe3+/Fe2+) can also lead to systematic variations in fO2 in the shallowest ~100 km of the mantle. Two different thermodynamic models were used to calculate fO2 vs. pressure and temperature for a representative, slightly depleted peridotite of constant composition (including total oxygen). Under subsolidus conditions, increasing pressure in the plagioclase-lherzolite facies from 1 bar up to the disappearance of plagioclase at the lower pressure limit of the spinel-lherzolite facies leads to an fO2 decrease (normalized to a metastable plagioclase-free peridotite of the same composition at the same pressure and temperature) of ~1.25 orders of magnitude. The spinel-lherzolite facies defines a minimum in fO2 and increasing pressure in this facies has little influence on fO2 (normalized to a metastable spinel-free peridotite of the same composition at the same pressure and temperature) up to the appearance of garnet in the stable assemblage. Increasing pressure across the garnet-lherzolite facies leads to increases in fO2 (normalized to a metastable garnet-free peridotite of the same composition at the same pressure and temperature) of ~1 order of magnitude from the low values of the spinel-lherzolite facies. These changes in normalized fO2 reflect primarily the indirect effects of reactions involving aluminous phases in the peridotite that either produce or consume pyroxene with increasing pressure: Reactions that produce pyroxene with increasing pressure (e.g., forsterite + anorthite ⇄ Mg-Tschermak + diopside in plagioclase lherzolite) lead to dilution of Fe3+-bearing components in pyroxene and therefore to decreases in normalized fO2, whereas pyroxene-consuming reactions (e.g., in the garnet stability field) lead initially to enrichment of Fe3+-bearing components in pyroxene and to increases in normalized fO2 (although this is counteracted to some degree by progressive partitioning of Fe3+ from the pyroxene into the garnet with increasing pressure). Thus, the variations in normalized fO2 inferred from thermodynamic modeling of upper mantle peridotite of constant composition are primarily passive consequences of the same phase changes that produce the transitions from plagioclase → spinel → garnet lherzolite and the variations in Al content in pyroxenes within each of these facies. Because these variations are largely driven by phase changes among Al-rich phases, they are predicted to diminish with the decrease in bulk Al content that results from melt extraction from peridotite, and this is consistent with our calculations. Observed variations in FMQ-normalized fO2 of primitive mantle-derived basalts and peridotites within and across different tectonic environments probably mostly reflect variations in the chemical compositions (e.g., Fe3+/Fe2+ or bulk O2 content) of their sources (e.g., produced by subduction of oxidizing fluids, sediments, and altered oceanic crust or of reducing organic material; by equilibration with graphite- or diamond-saturated fluids; or by the effects of partial melting). However, we conclude that in nature the predicted effects of pressure- and temperature-dependent phase equilibria on the fO2 of peridotites of constant composition are likely to be superimposed on variations in fO2 that reflect differences in the whole-rock Fe3+/Fe2+ ratios of peridotites and therefore that the effects of phase equilibria should also be considered in efforts to understand observed variations in the oxygen fugacities of magmas and their mantle sources.

Evidence for two upper mantle sources driving volcanism in Central Kamchatka

2012 ◽  
Vol 321-322 ◽  
pp. 14-19 ◽  
Author(s):  
Alex Nikulin ◽  
Vadim Levin ◽  
Michael Carr ◽  
Claude Herzberg ◽  
Michael West
Keyword(s):  
Upper Mantle ◽  
Mantle Sources

Correlation of two Helikian peralkaline granite – volcanic centres in central Labrador

1983 ◽  
Vol 20 (5) ◽  
pp. 753-763 ◽  
Author(s):  
J. D. Hill ◽  
A. Thomas
Keyword(s):  
Red Wine ◽  
Lake Superior ◽  
Volcanic Complex ◽  
Continental Rifting ◽  
Alkaline Complex ◽  
Peralkaline Granite ◽  
Space And Time ◽  
Terrestrial Sediments ◽  
Grenvillian Orogeny ◽  
Intrusive Suite

Recent mapping in central Labrador has resulted in the recognition and correlation of two Neohelikian peralkaline silicic igneous centres. The Flowers River igneous suite is circular in shape, covers an area of approximately 1720 km2, and consists of undeformed comenditic granite in contact with extrusive equivalent porphyry, felsite, tuff, and breccia. The Letitia Lake volcanic complex has been deformed by the Grenvillian Orogeny into an elliptical structure that covers approximately 450 km2. The complex consists of comenditic granite and syenite of the Arc Lake intrusive suite and related porphyry, rhyolite, tuff, and volcanogenically derived sediments of the Letitia Lake Group. Undersaturated aenigmatite–nepheline gneisses and syenites of the Red Wine alkaline complex are associated in space and time with the peralkaline silicic rocks of the Letitia Lake complex. The two centres are separated by 175 km and are an integral part of a Neohelikian period of uplift and continental rifting that involved formation of plateau basalts, terrestrial sediments, diabase dikes, and peralkaline magmatic centres in a belt extending from south Greenland to Lake Superior.

scholarly journals Basalt derived from highly refractory mantle sources during early Izu-Bonin-Mariana arc development

2020 ◽  
Author(s):  
He Li ◽  
Richard Arculus ◽  
Osamu Ishizuka ◽  
Rosemary Hickey-Vargas ◽  
Gene Yogodzinski ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Philippine Sea Plate ◽  
Island Arcs ◽  
Element Abundances ◽  
Mid Ocean Ridge ◽  
Mantle Sources ◽  
Backarc Basins ◽  
Mariana Arc ◽  
Ocean Ridge ◽  
Aluminous Spinel

Abstract The character of magmatism associated with the early stages of subduction zone and island arc development is unlike that of mature systems, being dominated in the Izu-Bonon-Mariana (IBM) case by low-Ti-K tholeiitic basalts and boninites. Basalts recovered by coring the basement of the Amami Sankaku Basin (ASB), located west of the oldest remnant arc of the IBM system (Kyushu-Palau Ridge; KPR), were erupted at ~49 Ma, about 3 million years after subduction inception. The chain of stratovolcanoes defined by the KPR is superimposed on this basement. The basalts were sourced from upper mantle similar to that tapped following subduction inception, and represented by forearc basalt (FAB) dated at ~52-51 Ma. The mantle sources of the ASB basalt basement were more depleted by prior melt extraction than those involved in the vast majority of mid-ocean ridge (MOR) basalt generation. The ASB basalts are low-Ti-K, aluminous spinel-olivine-plagioclase-clinopyroxene-bearing tholeiites. We show this primary mineralogy is collectively distinct compared to basalts of MOR, backarc basins of the Philippine Sea Plate, forearc, or mature island arcs. In combination with bulk compositional (major and trace element abundances plus radiogenic isotope characteristics) data for the ASB basalts, we infer the upper mantle involved was hot (~1400oC), reduced, and refractory peridotite. For a few million years following subduction initiation, a broad region of mantle upwelling accompanied by partial melting prevailed. The ASB basalts were transferred rapidly from moderate pressures (1-2 GPa), preserving a mineralogy established at sub-crustal conditions, and experienced little of recharge-mix-tap-fractionate regimes typical of MOR or mature arcs.

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