Coldwell Complex Research Articles

Zircon fission track ages from the Coldwell Complex Ontario Canada.

Fission track ages are reported for the mineral zircon from the Coldwell Complex, Ontario, Canada using both the external detector and the population methods. Results for zircon yield a spread of fission track ages from 956.7±40 to 978.6±41 Ma. The geological history of the area is discussed in the light of fission track ages uncovered.

U–Pb geochronology of the Coldwell Complex, northwestern Ontario, Canada: Discussion

U–Pb geochronology of the Coldwell Complex, northwestern Ontario, Canada: Discussion

U–Pb geochronology of the Coldwell Complex, northwestern Ontario, Canada

The Coldwell Complex gives a U–Pb zircon age of 1188 ± 56 Ma, a U–Pb tritomite age of 1126 ± 6 Ma, and a Pb–Pb potash feldspar age of 1139 ± 55 Ma. These ages are all significantly older than the previously published Rb–Sr and K–Ar ages. We interpret these ages as follows: (1) emplacement of the complex at 1188 Ma as part of the initial stages of the Keweenawan rifting event at depth; (2) slow cooling leading to closure of the U and Pb systems in tritomite and potash feldspar between 1139 and 1126 Ma; and (3) continued slow cooling leading to closure of the Rb–Sr system in the complex at an average age of 1053 ± 10 Ma and of the K–Ar system at ~1035 Ma.

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

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

A gravity study of the Coldwell Complex, northwestern Ontario and its petrological significance

Gravity studies of the Coldwell alkaline complex show that the complex is associated with a positive gravity anomaly. The anomaly indicates that a significant volume of dense material occurs below the current levels of exposure of principally felsic rocks. A new high-resolution gravity profile used to model the infrastructure of the complex shows that the felsic rocks form a 3–5 km thick layer over a differentiated basic intrusion. This intrusion is interpreted to consist of a layer of gabbro 3–5 km in thickness underlain by 3 km of peridotite and (or) pyroxenite. Intrusion of the basic magma was associated with Keeweenawan continental rifting. The presence of large volumes of basic magma supports the hypothesis that some of the Coldwell felsic rocks were derived by differentiation of a basic parent.

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

Platt and Mitchell have recently presented the results of a Rb-Sr geochronological study of the Coldwell Complex of northwestern Ontario. On the basis of a 17 point whole-rock isochron, they suggest an age for the complex of ? 6.2 Ma, a value that they imply is statistically different from our earlier (Bell and Blenkinsop 1980) determination of the age of the same body. They also state that and Blenkinsop (1980) provided little or no information on the nature of the samples studied from the complex nor the exact nature of their analytical procedure. We find the latter part of this assertion difficult to understand. We give in Bell et al. (1979) some information about our analytical procedure, and refer the reader to another publication (Bell et al. 1977) in which, we clearly state, further details can be found. In the later paper (Bell and Blenkinsop 1980) we provide additional information about analytical procedures, including estimates of the uncertainties of both the ~ ' b / ~ % r and a7Sr/86~r ratios. We agree that descriptions of the samples are lacking; we judged it impractical to supply such information for the 67 samples analyzed isotopically and listed in the 1980 report. Platt and Mitchell also appear uncertain about the number of samples we used to obtain our isochron, and in a summary of our data (their Table 3) they give a figure of 1 I?, implying that it was difficult for them to determine how many data points we had used. This value, however, represents the number of samples that we analyzed for Rb and Sr contents. In both our 1979 and 1980 reports we list the isotopic analyses. and it can be seen from the appropriate tables that five samples were used to determine the isochron. We even published a diagram of the isochron for the Coldwe11 Complex in Bell er al. ( 2979). We now wish to consider the question of whether our age of 1070 ? 15 Ma is statistically different from Platt and Mitchell's value of 1044.5 + 6.2 Ma. In the course of reviewing their data, we recomputed their standardized isochrons, and were puzzled by our inability to

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

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

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.

Mineralogy and Petrology of Nepheline Syenites from the Coldwell Alkaline Complex, Ontario, Canada

Mineralogy and Petrology of Nepheline Syenites from the Coldwell Alkaline Complex, Ontario, Canada

Mafic Mineralogy of Ferroaugite Syenite from the Coldwell Alkaline Complex, Ontario, Canada

Mafic Mineralogy of Ferroaugite Syenite from the Coldwell Alkaline Complex, Ontario, Canada