Skarn type grandite garnet in oceanic lower crust of the Troodos Ophiolite, Cyprus. Grandite growth from a new view?

Skarn type grandite garnet in oceanic lower crust of the Troodos Ophiolite, Cyprus. Grandite growth from a new view?

 

Anna Hagen ¹, Romain Bousquet ¹, Colin Devey ², Thor Hansteen ²

 

¹ Kiel University, Institute of Geosiences, Kiel – Germany    ² GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel – Germany

 

The Troodos ophiolite on Cyprus allows us to have a detailed view in the stratigraphy of a late cretaceous oceanic lithosphere, as this oceanic floor including a former spreading axis (mid oceanic ridge) is rotated and uplifted since then. Both structural and petrological, the Troodos ophiolite is a great study area to describe and understand former magmatic processes during the formation of new oceanic crust as well as for the comparison with today’s spreading ridges (Robinson et al., 2003).

 

Within the stratigraphy of oceanic crust as found in the Troodos ophiolite, several sequences of plagiogranites occurred from the lower crustal gabbroic complex until the sheeted dyke complex (Marien et al., 2019). In addition to these plagiogranites we now find a single plagiogranite complex incorporating a large amount of epidote and grandite, the later one up to fist size, that has not described yet. Epidote and grandite crystals show partially intergrown patterns within this special type of plagiogranite.

 

The grandite type garnets show features similar to other known skarn type associated grandites including an onion like, really fine and sharp oscillatory chemical zoning and both isotropic and anisotropic features when investigated under polarized light. The sieve texture observable at the outer rim of the grandite minerals is made up of mainly quartz inclusions, which itself incorporate many highly saline fluid inclusions. Due to the high content of Ca and Fe³⁺ in both epidote and grandite, we assume a highly oxidizing environment with a high involvement of saline fluids, boiling at some point and enhancing the mobility of iron.

 

In conclusion we can state that the grandite we find here in this plagiogranite in the Troodos ophiolite complex, looks like other skarn type associated grandite but here, no sedimentary rock type is included in the forming process. Further we assume the growth of this grandite to be associated with very high volatile activity, either during a late stage of hydrothermal alteration or metasomatic process or even earlier in a magmatic stage indicating the activity of magmatic volatiles in a system with very unique chemical composition.

 

 

 

Chris S. Marien, · J. Elis Hoffmann, · C.‐Dieter Garbe‐Schönberg and · Carsten Münker, 2019, Petrogenesis of plagiogranites from the Troodos Ophiolite Complex, Cyprus, Contributions to Mineralogy and Petrology, 174:35.

 

Paul T. Robinson, John Malpas and Costas Xenophontos , 2003, The Troodos Massif of Cyprus: Its role in the evolution of the ophiolite concept, Geological Society of America Special Papers, 373,  295-308.