Abstract

Xenotopic texture, which is commonly observed in pre-Cenozoic rocks, is defined here as a mosaic of anhedral crystals with irregular or curved intercrystalline boundaries and, usually, undulatory extinction. Xenotopic dolomite texture is similar in appearance to neomorphic limestone textures. Idiotopic dolomite texture (euhedral to subhedral crystals with straight, intercrystalline boundaries) contrasts with xenotopic texture and is commonly observed in both Cenozoic and more ancient dolomites. Texture may be controlled by the temperature at which crystals grow. Crystal growth theory predicts that at low temperature a smooth crystal surface is energetically favored, and atoms are added to crystal faces layer by layer with dislocations acting as nucleation sites. This results in faceted crystals and euhedral to subhedral crystal mosaics. Above a "critical roughening temperature" (CRT), a rough surface is energetically favored, surface nucleation does not require dislocations, and atoms are randomly added to the crystal surface, resulting in nonfaceted growth and an anhedral crystal mosaic. It is hypothesized that a "critical roughening temperature" exists for dolomite above 50 degrees C. Xenotopic dolomites are produced by dolomitization of limestone and/or neomorphic recrystallization of dolomite at elevated temperature (above the CRT) after burial. Idiotopic dolomites are produced below the CRT by near-surface processes and are occasionally produced above CRT if growth-inhibiting effects of the solution or impurities stabilize crystal faces. Calcite has a CRT below 25 degrees C and, therefore, develops anhedral crystal mosaics (characteristic of neomorphic texture) both at near-surface and at elevated temperatures. Synthetic xenotopic dolomite was produced in the laboratory at 250 degrees C and 300 degrees C by dolomitization of aragonite and calcite skeletal fragments and by recrystallization (neomorphism) of nonstoichiometric Cenozoic dolomites. Xenotopic dolomite resulted from the metamorphic recrystallization of the idiotopic Hueco dolomite (Permian, Texas) near the Marble Canyon intrusion, at temperatures between 350 degrees C and 600 degrees C. Hydrothermal dolomitization of periclase-calcite marble near the intrusion also resulted in a xenotopic texture. Xenotopic dolomite in the Galena Group (Ordovician, Wisconsin) was produced by neomorphism of a preexisting dolomite during the emplacement of lead-zinc sulfides at temperatures between 80 degrees C and 227 degrees C. In the Trenton Limestone (Ordovician, Michigan), xenotopic dolomite replaced limestone during the migration of hot (> 50 degrees C) fluids along fracture systems. Xenotopic dolomite was not observed in Cenozoic dolomites that were subjected to temperatures near surface values only.

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