Abstract

Results of total-fusion 40Ar/39Ar dating (eight biotites, four muscovites, and one hornblende) of magmatic and hydrothermal stages of the high-silica, chemically zoned Ackley Granite indicate three distinct episodes of magmatic activity, viz. ≥ 410, 378–374, and 355 Ma, and that the metallogenic system (W–Sn, Mo) evolved synchronously with only one of the magmatic phases.The oldest suite is indicated by two biotite dates at 410.4 ± 4.4 and 392.5 ± 7.6 Ma, which are themselves considered as representing partial resetting related to a tectonothermal event of Acadian age. Thus, the 410 Ma age offers the best estimate of intrusion, although it must be considered a minimum. The main magmatic pulse for the Ackley Granite is indicated by dates between 378 and 374 Ma, with most of these phases occurring in the southern part of the complex. Concordant hornblende–biotite ages (374.8 ± 3.8 and 372.3 ± 4.8 Ma, respectively) for one of the southern phases (Rencontre Lake granite) suggests rapid cooling for this part of the complex. In contrast, the data for a contemporaneous intrusion (Kepenkeck granite) in the northern part of the Ackley Granite are discordant, with a muscovite–biotite pair yielding ages of 378.4 ± 4.8 and 367.7 ± 4.3 Ma, respectively. Four remaining biotite dates, widely distributed within the southern part of the Ackley Granite, gave similar ages of ca. 368 Ma.Three hydrothermal muscovites from mineralized greisen zones along the soumern margin of the granite are dated at 371.3 ± 4.5, 371.3 ± 5.4, and 373.5 ± 4.0 Ma, essentially coeval with the main magmatic event of the Ackley Granite. Since these paragenetically late muscovites give slightly older ages than the magmatic biotites of the second pulse, it is suggested that the ambient temperature for a large part of the Ackley Granite remained in excess of the 250–300 °C biotite blocking temperature for several million years after initial intrusion.A third and presumably magmatic event, presently of unknown dimensions, occurred at 355 Ma. The coincidence of this age with previously published Rb–Sr whole-rock isochron dates of 355 Ma for the Ackley Granite might indicate that this younger magmatic event was in part responsible for resetting the Rb–Sr systematics.

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