wAbstract
@Besshi-type Cu deposits are strata-bound volcanogenic massive
sulfide deposits usually associated with mafic volcanic rocks
or their metamorphic equivalents. Although there are numerous
Besshi-type deposits in the Sanbagawa metamorphic belt, Japan,
their tectonic settings and depositional environments remain controversial
because of a lack of depositional age constraints. We report Re-Os
data for the Iimori deposit, one of the largest Besshi-type deposits
in western Kii Peninsula, in order to examine the robustness of
the Re-Os isotope system for dating sulfide minerals in the high-P/T
metamorphic belt and to elucidate the primary depositional environment
of the Iimori sulfide ores. An 11-point Re-Os isochron plot yields
an age of 156.8}3.6 Ma. As this Re-Os isochron age is significantly
older than the timing of the Sanbagawa peak metamorphism (110-120
or `90 Ma) and a well-defined isochron was obtained, the Re-Os
are determined here is most likely the primary depositional age.
Despite high-grade metamorphism at up to 520}25 and 8-9.5 kbar,
the Re-Os isotope system of the Iimori sulfides was not disturbed.
Hence, we consider that the whole-rock Re-Os closure temperature
for the Iimori sulfide ores was probably higher than 500. As
the accretion age of the Sanbagawa metamorphic belt is considered
to be 120-130 or 65-90 Ma on the basis of radiolarian and radiometric
ages, we estimated the time from the Iimori sulfide deposition
on the paleo-seafloor to its accretion at the convergent plate
boundary to be greater than 25 Myr. Consequently, the depositional
environment of the Iimori sulfide ores was not in the marginal
sea, but was truly pelagic.x
1. Introduction
2. Geological background
3. samples
4. Analytical procedures
5. Results and discussion
@5.1. Re-Os geochemistry and timing of sulfide deposition
@5.2. Depositional environment of the Iimori deposit
6. Conclusions
Acknowledgments
References