麻豆传媒

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B. Sc. Honours Thesis

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A study of ⁴⁰Ar/³⁹Ar age relationships of various structural levels of the St. Anthony Complex dynarnothermal aureole was undertaken. Units of the aureole dated include a small jacupirangite suite metamorphosed with the base of the peridotite, the Long Ridge Metagabbro below the peridotite contact and the Green Ridge Arnphibolite structurally below the rnetagabbro and furthest from the peridotite contact.

Only the jacupirangite suite gives a geologically reasonable age of 495 +/- 6 Ma. Excess argon effects and low potassium contents of the Long Ridge Metagabbro and the Green Ridge Amphibolite preclude a geologically consistent interpretation of their apparent ages (450 +/- 10 Ma and 900 Ma respectively).

A date of 495 Ma is a minimum age for the formation of the dynamothermal aureole. It is, at least, 5 Ma older than Dallmeyer's (1977)听⁴⁰Ar/³⁹Ar ages (480 +/- 10) for amphibolites lower .in the aureole. An age discrepancy between the top of the aureole and lower structural levels may be implied. Such a pattern is also recognized in the Bay of Islands and Samail- ophiolites.

An emplacement model to account for an age difference suggests that successively shallower crustal rocks were accreted onto the base of an overriding peridotite tectonite. The progressive displacement of the zone of the shear heating to the base of each new slab allowed the cooling of structurally higher and earlier incorporated units. A previously hot peridotite is therefore not necessary

Pages: 39

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