Abstract

Rocks buried in the upper part of a doubly thickened crust during orogeny can show prograde reactions with transitional features from lower grade rocks up to granulite facies. They can be exposed at the earth's surface within this one erogenic cycle. For example, the granulites exposed in British Columbia have concordant U/Pb zircon ages of 65–85 Ma. In contrast, rocks buried in the lower part of a thickened crust will crystallize granulites that typically show only retrograde relations with younger, lower grade rocks. At the cessation of orogeny, such rocks will undergo near-isobaric cooling from the peak metamorphism and can have very long residence times at the base of a crust of normal thickness. These granulites require a second orogeny to uplift and expose them at the surface. As an example, the Archean granulites of Enderby Land, Antarctica, were metamorphosed at 3070 Ma at 1000 °C at 8–10-kbar pressure. They then followed at least a 400 °C isobaric cooling path. They also had a prolonged residence time of 2000 m.y. near the base of the crust before uplift.

No doubt a continuum exists between these extremes, but it is proposed that many intermediate- and high-pressure granulites form in the lower parts of thickened crust and therefore require a two-stage cycle of tectonism to be uplifted and exposed at the earth's surface. Although most granulites probably form in thickened crust at active plate margins, underplating of normal (30–40-km thickness) crust by mantle melts during extension can also produce granulites. These granulites would also undergo isobaric cooling and require a later orogeny to expose them. An example is the Paleozoic Lachlan Fold Belt of eastern Australia, which has undergone only minor uplift since the major ultrametamorphism at about 400 Ma. However, isotopic, petrologic, and geochemical differences are to be expected between these extremes of granulite formation.

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