Genesis and evolution of chromitites in ophiolite complexes from a mineralogical perspective

Abstract

[eng] The finding of a suite of unusual minerals associated to ophiolitic chromitites questions traditional model for the formation of these rocks. These findings include minerals typically considered indicators of ultra-high pressure and super-reducing conditions (i.e., diamond). Several authors proposed new genetic models for ophiolitic chromitites which involved their formation and/or evolution in the deep mantle (>410 km), opening an intense debate that is still active. This Ph.D. thesis contributes to this debate by studying four examples of ophiolitic chromitites from different geodynamic settings (fore-arc, back-arc, and supra-subduction mantle interacted with mantle plume) in order to evaluate the origin of these “exotic” minerals in ophiolitic chromitites and their possible meaning into the evolutionary history of these enigmatic rocks. The studied examples of chromitites formed in fore-arc settings include chromitite veins from Havana-Matanzas ophiolite (Cuba) and chromitite pods from Loma Las Cabirmas hill in Loma Caribe peridotite (Dominican Republic). The chromitites from Havana-Matanzas are characterized by abundant platinum-group mineral (PGM) inclusions and by being associated with orthopyroxenite in ophiolitic mantle section. Chromite studies reveal that the chromitites and orthopyroxenites formed coetaneously from Si- and Mg-rich andesitic melts with boninitic affinity originated in the fore-arc mantle of an intra-oceanic arc. The melts infiltrated the pre-existing harzburgite, dissolving olivine while precipitating orthopyroxene and chromite. The abundance of PGM is interpreted to result from the “collector” effect of chromite during the chromitite formation, which was enhanced due to the small volume of chromitites formed. The Loma Las Cabirmas chromitites display unusually high platinum-group elements (PGE) contents and hydrothermal PGE mineralizations. The study of these chromitites suggests that hydrothermal fluids infiltrated the chromitites during serpentinization of the rocks and gave place to the in situ alteration of magmatic PGM in the chromitites and the precipitation of hydrothermal Pt-rich mineralization in uvarovite- and chromian clinochlore-filled fractures, within the thermal range of 350–150 ºC. Therefore, the PGE bulk geochemistry and mineralogy of these chromitites was largely impacted by post-magmatic processes. Microdiamonds were found in situ in ophiolitic chromitites from Tehuitzingo serpentinite (Mexico), interpreted as an ophiolitic mantle section formed in a back-arc geodynamic setting. Micrometric diamonds were located within healed fractures crosscutting the chromite grains. These diamonds were associated with amorphous C bearing O-species and with a low-pressure and low-temperature mineral assemblage. Thermodynamic calculations show that the chromite from the healed fractures recrystallized at 670–510 ºC. Consequently, the diamonds are inferred to be metastable and form from C undersaturated CO2–H2O–CH4 fluids infiltrating the chromitite during serpentinization processes. The diamond formed due to the super-reducing conditions achieved within the sealed inclusions. The formation of microdiamonds in (ultra)reduced low-pressure and low-temperature environments during serpentinization challenges the idea that diamond in ophiolitic rocks constitutes an unequivocal indicator of ultra-high pressure conditions. Chromitites from the northern and central part of the Loma Caribe peridotitic belt (Dominican Republic) were studied as an example of chromitites hosted in an ophiolite with geochemical fingerprints of deep-seated plume activity. These chromitites display unusual compositions, different from other ophiolitic chromitites. Instead, the analyzed chromite grains exhibit major and trace element composition that overlaps the field of komatiitic chromite. Moreover, the composition of parental melts of these chromitites is similar to plume derived products of the Dominican Republic. Therefore, I inferred that these chromitites formed in the lithospheric mantle after parental melts derived from an enriched peridotite source influenced by the Caribbean mantle plume. Altogether, my results show the importance of differentiating features resulting from magmatic primary processes from those resulting from alteration processes. Only then it will be possible to reconstruct the genetic story and evolution of the chromitites and propose accurate models.

[cat] En els darrers anys s’han trobat una sèrie de minerals considerats “exòtics” en cromitites ofiolítiques que han fet replantejar a la comunitat científica internacional els models de formació d’aquestes roques. Aquests minerals inusuals són sovint considerats indicadors de condicions d’ultra-alta pressió i súper reductores (i.e. diamant), de manera que els models proposats a partir d’aquestes troballes inclouen un origen i/o una evolució de les cromitites a nivells profunds mantèl·lics (>410 km), contraposant-se a la formació en el mantell litosfèric proposada pels models tradicionals. Aquesta tesi doctoral estudia quatre exemples de cromitites ofiolítiques que pertanyen a diferents contexts geodinàmics amb l’objectiu d’identificar minerals o trets inusuals en les cromitites i/o en les seves roques associades per tal d’avaluar-ne el seu origen i el seu possible significar en el marge de la historia evolutiva d’aquestes roques. Els casos estudiats són: (1) les venes de cromitita de l’ofiolita d’Habana-Matanzas (Cuba), pertanyents a un context de avant-arc; (2) els pods de cromitites de Loma Las Cabrimas (peridotita de Loma Caribe, República Dominicana), també pertanyents a un context d’avant-arc i caracteritzades per les seves mineralitzacions hidrotermals de baixa temperatura de Pt; (3) les cromitites de la serpentina de Tehuitzingo (Mèxic), interpretada com una secció mantèl·lica ofiolítica formada en un context de rere-arc; per últim, (4) les cromitites del nord i del centre del cinturó peridotític de Loma Caribe (República Dominicana), que constitueixen un exemple de cromitites encaixades en una ofiolita amb evidències geoquímiques d’activitat de ploma mantèl·lica. En conjunt, els resultats demostren la importància de diferenciar els efectes de processos magmàtics primaris d’aquells derivats de processos d’alteració per tal de fer una interpretació correcta de la història evolutiva d’aquestes roques.