Abstract

Intermediate composition orthogneisses of the southward extension of the Flin Flon greenstone belt surround the ultramafic host rock of the Namew Lake Ni–Cu deposit in northern Manitoba. The gneisses began cooling from elevated temperatures of approximately 700 °C at 1860 Ma when growth of new zircons ceased, after a protracted period of formation starting at ca. 1890 Ma. Zircons in the ultramafic rock itself indicate that it was intruded into the gneisses at 1847 Ma. Subsequently, the temperature decreased very slowly, with a relaxation time of 160 Ma, cooling through the blocking temperature for Ar loss in hornblende at 1799 Ma and for that in biotite at 1735 Ma. Using published values for blocking temperatures, we find that the temperature–time data are consistent with simple exponential decrease in temperature and do not require discreet pulses of thermal activity during the first 150 Ma of cooling. There is no indication of temperature as high as the blocking temperature for biotite, subsequent to 1735 Ma.Data from sphenes yield a range of ages between 1828 and 1820 Ma, correlated with the color of the sphene grains. This age range is intermediate between that of zircon and hornblende and is consistent with a blocking temperature of about 525 °C, whereas apatite gives an age of ca. 1810 Ma and therefore a blocking temperature between that of sphene and hornblende. The good precision on all the ages obtained provides rather precise relative blocking temperatures for zircon, sphene, apatite, hornblende, and biotite and provides a detailed thermal history for this region of the Flin Flon belt.

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