Age and cooling history of the kiglapait Intrusion from an 40 Ar / 39 Ar study

Details Age and cooling history of the kiglapait Intrusion from an 40 Ar / 39 Ar study Age and cooling history of the kiglapait Intrusion from an 40 Ar / 39 Ar study

Jun 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992gecoa..56.2471y&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 56, Issue 6, pp.2471-2485

Physics

Scientific paper

40 Ar / 39 Ar age spectra of hornblende, biotite, and feldspars from the Kiglapait layered intrusion, Labrador, give closure ages that define the subsolidus cooling history of the intrusion and its surroundings. Biotites give very good plateaus, but U-shaped age spectra are typical for hornblende, antiperthite, and plagioclase, and a spectrum of multi-plateau segments is found in mesoperthites. Isochron studies reveal an age range from 1298 ± 14 Ma for hornblende to 1217 ± 28 Ma for plagioclase. Multiple linear arrays or clusters appear on the isochron diagrams of perthitic feldspars, indicating the effect of excess argon. For antiperthite, this excess argon resides mainly in the plagioclase domains, based on its inverse correlation with K/Ca. With the highest closure temperature, the hornblende age is very close to the published Sm/Nd rockmineral isochron age of 1305 ± 22 Ma and gives a good approximation to the crystallization age. Calculated closure temperatures of different feldspars, combined with published values for hornblende and biotite, show that this intrusion cooled rapidly down to an ambient temperature of about 240°C in the first 20 Ma after its crystallization, then underwent extremely slow cooling for a long time. Combining these results with the published pre-intrusion ambient temperature of about 350°C, a regional slow cooling is revealed, and this implies a late uplift of the region during and after the Kiglapait magmatism.

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