The contribution of geochronology to determination of burial and exhumation rates of (U)HP rocks
Abstract
The seemingly extraordinary rapid and deep subduction of buoyant continental crust and its return to the surface is proving rather more the rule than the exception in continental collision zones, notwithstanding the patchy preservation of such rocks. Modelling and buoyancy contrasts suggest that following rapid subduction and ductile de-coupling from the downgoing slab, the rate of exhumation is first very rapid within the mantle but slower as the buoyancy contrast is reduced in the crust. One test of models is the quantitative determination of the P-t path of real samples. Geochronology is pivotal in this regard and with rapid exhumation (∼1 cm/a), it is paramount to produce precise and robust mineral growth and cooling ages of a variety of pressure- and temperature-sensitive minerals, ideally in their petrographic context, using a variety of chronometers, so that the geochronology can resolve the rapid rates. Issues of high relevance to this application of geochronology are (1) resolving and applying the ~0.5-1.0% decay constant bias between (40K)40Ar-39Ar dates and U-Pb dates, especially critical for older orogen exhumation rates (i.e. Caledonides); (2) correctly interpreting U-Pb ages when complex U-Pb systematics are likely to be present; (3) using prograde mineral chronology during cool subduction with minerals of high closure temperature (allanite-epidote, garnet, monazite, zircon, titanite); (3) determining P-T conditions of growth of these minerals using metamorphic modelling and chemical mapping; dating minerals in their petrographic context wherever possible; (5) making the most of minerals like zircon through innovative dating and inclusion petrology in very thin overgrowths; (5) and assessing growth v. closure temperature interpretation of dated minerals using experimental and empirical diffusion data. These points will be illustrated by examples of studies from Kaghan Himalayan, Caledonide and Chinese UHP rocks to suggest fruitful multi-technique approaches as a way forward.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.T32C..07P
- Keywords:
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- 1100 GEOCHRONOLOGY;
- 1115 Radioisotope geochronology;
- 3654 Ultra-high pressure metamorphism;
- 8104 Continental margins: convergent