Journal of Conference Abstracts

Dearth of Late Burial Signatures in Early-Stabilised Platform Dolomites

Gordon Coy (gcoy@badley.ashton.co.uk)¹, Tony Dickson (jadd1@esc.cam.ac.uk)² & Jim Marshall (isotopes@liv.ac.uk)³

¹ Badley Ashton & Associates Ltd, Winceby House, Winceby, Horncastle, Lincolnshire. LN9 5LJ, UK

² Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK

³ Department of Earth Sciences, University of Liverpool, Brownlow Hill, PO Box 147, Liverpool, L69 3BX, UK

It is well documented that many ancient dolomites have undergone re-crystallisation leading to changes in texture, structure and composition (Machel, 1997). Therefore, ancient dolomites tend to be near-stoichiometric and better ordered, have more negative oxygen isotopes (i.e. Mazzullo, 1992) and be more coarsely crystalline than their modern counterparts.The largely pervasively dolomitised, Permo-Triassic Akhdar Group of Oman, which forms the basal 1000 m of an extensive sequence of shallow marine platform carbonates, and the overlying partially dolomitised Sahtan Group provide an ideal laboratory for studying the differential effects of burial on the textural, structural and compositional signatures of dolomite. The groups outcrop in the Jebel Akhdar range of the Oman Mountains and subcrop in the Interior of Oman. The Akhdar and Sahtan Groups exposed in the mountains has been buried in excess of 15 km beneath the Semail Ophiolite which was obducted onto the platform margin during the Late Cretaceous, whereas in the interior have been buried to less than 4 km. Petrographic, major and trace element geochemical and stable isotope studies show that while vertical trends can be correlated across the platform, variations between the interior and mountains are not significant. In particular:

* Akhdar Group pervasive replacement dolomites are near stoichiometric and moderately well ordered with no significant difference between the interior of the platform and the Oman Mountains. Sahtan Group dolomites from the mountains remain non-stoichiometric (44 to 46 mole% MgCO₃).

* Textural analysis of pervasive replacement dolomites from the Akhdar Group show that the crystal size and degree of preservation of the primary depositional fabric does not significantly vary between the mountains and interior.

* The ¹⁸O values of the pervasive replacement dolomites range from +2 to -8‰ and can be interpreted as having resulted from the progressive recrystallisation in a meteoric environment of dolomites formed initially from Permo-Triassic seawater.

* The ¹³C signature of the Akhdar Group echos published curves for seawater ¹³C variation in the Permo-Triassic, but show no significant lateral variation.

* Mn and Fe concentrations are generally low across the platform, except in regions where the dolomite has been recrystallised by late metasomatic fluids.

It is therefore probable that the ordering and stoichiometry of the Akhdar Group pervasive replacement dolomites were stabilised by recrystallisation before the emplacement of the ophiolite. Once having stabilised as near-stoichiometric, the Akhdar Group dolomites were not affected by deep burial under the ophiolite.

Machel HG, Sedimentary Geology, 113, 161-168, (1997).

Mazzullo SJ, Carbonates and Evaporites, 7, 21-37, (1992).

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