Cargill Township carbonatite complex, District of ... - Geology Ontario
Cargill Township carbonatite complex, District of ... - Geology Ontario
Cargill Township carbonatite complex, District of ... - Geology Ontario
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
CARBONATITE - ALKALIC ROCK COMPLEXES: CARGILL TOWNSHIP<br />
pyroxenites to have resulted solely from the metasomatic effects <strong>of</strong> the car-<br />
bonatitic fluids. Amphibole replacement <strong>of</strong> pyroxene, in part at least, was likely<br />
the result <strong>of</strong> interstitial deuteric fluids and thus an autometamorphic effect related<br />
to the magmatic event that formed the <strong>complex</strong>. Some <strong>of</strong> the amphibole may be<br />
related to reaction with metasomatic fluids derived from the <strong>carbonatite</strong>.<br />
GEOCHRONOLOGY<br />
Gittins e t al. (1967) obtained the first isotopic age on the <strong>Cargill</strong> <strong>Township</strong><br />
Carbonatite Complex. K-Ar isotopic ages obtained from biotite were 1790, 1820,<br />
and 1860 Ma.<br />
In 1986, Kwon obtained a U-Pb isotopic age <strong>of</strong> 1906 30 Ma on zircon. He<br />
noted that the U-Th-Pb did not behave as a closed system. Kwon (1986) plotted<br />
2oepb7204pb - 207pb7204pb isotope ratios and obtained an errorchron age <strong>of</strong> 1890<br />
100 Ma. He also obtained an Sm-Nd errorchron age <strong>of</strong> 1903 100 Ma<br />
(Kwon 1986). He interpreted his isotopic data as suggesting that a thermal distur<br />
bance had occurred at approximately 500 Ma. The lead isotopic data indicates<br />
that the <strong>carbonatite</strong> magma has not been contaminated by Kapuskasing gneissic<br />
rocks. Kwon (1986) compared the lead isotopes at <strong>Cargill</strong> <strong>Township</strong> with those at<br />
the nearby Borden <strong>Township</strong> <strong>carbonatite</strong> which may be 30 Ma younger and con<br />
cluded that the differences can be explained only by a heterogeneous mantle<br />
source. Kwon (personal communication, 1987) revised the <strong>Cargill</strong> zircon U-Pb<br />
isotopic age to 1907 4 Ma.<br />
Sharpe (1987) reported an Rb-Sr whole rock-mineral isochron age <strong>of</strong><br />
1891 15 Ma with an initial 87Sr7^Sr ratio <strong>of</strong> 0.70185 0.00004. Sharpe<br />
(1987) indicated that variation in the 87Sr786Sr ratio exceeds analytical uncer<br />
tainty which she suggested was a reflection <strong>of</strong> a heterogeneous mantle source. The<br />
ratio is consistent with a mantle source uncontaminated by crustal rocks (Sharpe<br />
1987).<br />
STRUCTURAL GEOLOGY<br />
REGIONAL SETTING<br />
The <strong>Cargill</strong> <strong>Township</strong> <strong>complex</strong> lies within the Kapuskasing Subprovince (Struc<br />
tural Zone) <strong>of</strong> the Superior Province. This subprovince is characterized geophy-<br />
sically as a northeast-trending zone <strong>of</strong> gravity highs and pronounced linear<br />
aeromagnetic trends (Innes 1960; ODM-GSC 1970). This anomalous gravity<br />
zone has been interpreted as reflecting an upwarp in the Conrad discontinuity<br />
caused by major regional faulting and the formation <strong>of</strong> a <strong>complex</strong> horst structure<br />
(Wilson and Brisbin 1965; Bennett et al. 1967). This prominent regional struc<br />
ture extends southward from the south end <strong>of</strong> Hudson Bay, becoming broader<br />
and more ill-defined as it approaches the Lake Superior basin. The <strong>Cargill</strong> Town<br />
ship Carbonatite Complex has been emplaced into this structure, which also con<br />
tains many other <strong>carbonatite</strong>-alkalic rock <strong>complex</strong>es north and south <strong>of</strong> <strong>Cargill</strong><br />
<strong>Township</strong>.<br />
Recent studies <strong>of</strong> the Kapuskasing Structural Zone indicate it is an oblique<br />
section through about 20 km <strong>of</strong> Archean crust uplifted along a northwest-dipping<br />
thrust fault (Percival and Card 1983). To the east, high-grade gneissic rocks are<br />
in sharp fault contact with low-grade rocks and to the west high-grade rocks<br />
grade westward to low-grade rocks over a distance exceeding 100 km (Percival<br />
and Card 1983). The author concurs with Percival and Card (1983) in that the<br />
Kapuskasing Structural Zone is likely a tilted upthrust segment <strong>of</strong> Archean crust.<br />
24