Mineralogy and metamorphic evolution of Jabal Ash Shumt skarn deposits, Saudi Arabia : an Example of superimposed metamorphism

Abstract EN

Marble and skarn deposits were studied at five occurrences at Jabal Ash Shumt.

These deposits are members of the Afna Formation which is a part of the so-called "Hulayfah Group" that are dominated by a metamorphosed volcano-sedimentary succession rich in marbles and skarn rocks.

Integrated field and microscopic evidence indicate that transformation of marbles to calc-silicate skarn is not only attributed to regional metamorphism but also to a thermal event contemporaneous to the injection of post-Hulayfah intrusions as well.

Fabrics shown by some minerals (specifically diopside & garnet) on both mesoscopic and microscopic scales indicate complexity of the metamorphic history that the impure limestone precursor practiced.

The formation of calc-silicate assemblage started during a synkinematic event during regional metamorphism followed by postkinematic event contemporaneous with felsic igneous intrusions that were responsible for overprints of thermal metamorphism.

Due to post-Hulayfah intrusions (granophyre, monzo- & syeno-granites), the regionally metamorphosed calc-silicates and marbles were recrystallized due to heat source of the felsic intrusions that were accompanied by migration of fluids mainly rich in Fe, Al & Si.

Most of the studied skarn samples show distortion of regional foliation and development of annealing texture which are exclusive of a thermal event.

Some garnet-group minerals were identified (mostly grossular and andradite) in the skarn samples and composition of resultant garnet is controlled by the original carbonate composition, in addition to the nature of interacting fluid and its composition.

Several garnet crystals in the garnetiferous skarn of Ash Shumt area are anisotropic either partly or completely.

This is very characteristic of thermal garnet that appears replacing both calcite and epidote.

Anisotropism and XRD analyses suggest that this garnet can be considered as "hydrogarnet".