Radial cracks around α-quartz inclusions in almandine: constraints on the metamorphic history of the Oman mountains

Details Radial cracks around α-quartz inclusions in almandine: constraints on the metamorphic history of the Oman mountains Radial cracks around α-quartz inclusions in almandine: constraints on the metamorphic history of the Oman mountains

Feb 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993e%26psl.114..449w&link_type=abstract

Earth and Planetary Science Letters, Volume 114, Issue 4, p. 449-461.

Mathematics

Logic

16

Scientific paper

Radiating tensional cracks around α-quartz inclusions in almandine have been observed in metapelite samples from the southeastern Saih Hatat tectonic window, northeastern Oman Mountains. These almandines show an inclusion-rich (glaucophane + epidote) and strongly deformed core with inclusions of different mineral phases. The rim of the same almandines is inclusion-poor and shows only quartz, apatite, zircon, rutile and Ba-Al phosphates as inclusions. Quartz and apatite inclusions in the rim are single crystals often surrounded by radial cracks. These radial cracks developed during uplift by the dilation of α-quartz (4-5 vol%) without a phase transformation. Subsequently, these cracks were filled with kaolinite, phengite (Si content 3.4 per formula unit, p.f.u.), chlorite and Fe oxides.
We calculated the appearance of radial cracks without phase transformation using the mathematical procedure of Van der Molen and Van Roermund [1]. This calculation involves terms for thermal expansion, isothermal compressibility and shear modulus for the example of α-quartz and almandine for the same P and T interval during a retrograde path. Published geothermobarometric estimates give pressures of between 1.0 and 2.0 GPa and temperatures of between 450 and 600°C for the peak conditions for these rocks of the Saih Hatat tectonic window. On the basis of these P-T data we calculated different retrograde P-T paths in the α-quartz domain. Initiation of garnet fracturing is dependent on the P-T starting conditions and the component of isothermal compression of the retrograde path. The calculations yield a set of smooth monotonic curves whose exact position on the P-T plane between 0.1 and 0.6 GPa and 40 and 500°C depends on the initial P-T conditions and the component of isothermal compressibility of the retrograde P-T paths. This model can be used in general terms to estimate pressure and temperature for the following cases: (1) If independent evidence (such as petrological data) allow the determination of the final pressure at which radial cracks appeared, the initial inclusion pressure can be recalculated. (2) If the initial inclusion pressure is known (e.g. from petrological data), the conditions of radial cracking can be calculated, and the pair initial pressure-final pressure leads to an estimate of the shape of the retrograde P-T path as a function of its component of isothermal decompression. In the example from the northeastern Saih Hatat tectonic window the late syntectonic growth of albite + phengite + kaolinite suggests that the final pressure for fracturing ranged between 0.4 GPa and 0.5 GPa at temperatures of 300°C. These values correspond to high initial pressures of at least 2.0 GPa at a temperature of 550°C.
The following geodynamic model is suggested: A regionally extended metamorphism led to the growth of inclusion-rich garnets in the rocks from the northeastern Saih Hatat tectonic window at depths of about 30 km ( < 0.1 GPa, about 450°C). Continuing prograde metamorphism at a depth of more than 60 km with P < 2.0 GPa and T ~ 550°C affected a metapelite unit that is only exposed immediately south of As Sifah village. In this area, clear rims of almandine grew around the older garnets and entrapped mainly quartz and apatite. During uplift along a retrograde P-T path with a large component of isothermal decompression radial cracks around α-quartz inclusions developed in the rims of almandines at a depth of about 12 km (0.4-0.5 GPa, >= 300°C).

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