Engebøfjellet
Mineralogy
RutilNett
By Muriel Erambert, Univ. Oslo. Chapter 4 in Korneliussen et al (1997)
The Engebøfjellet gabbroic protolith was metamorphosed under eclogite-facies P and T conditions of 15-17 kbar and ca. 600oC. Eclogitisation corresponds to a complete mineralogical change. No relict of magmatic silicate minerals was found in the Engebøfjellet eclogite. The degree of textural equilibration of the eclogites is dependent on their deformational history during the eclogite-facies metamorphism (D1 and D2 phases). The leucocratic rocks show a textural evolution from coronites to foliated eclogites. In coronitic leuco eclogites (e.g. "massive leuco eclogite"), albite, together with phengite, zoisite and quartz, forms microcrystalline pseudomorphs after plagioclase. Small granular amphibole and clinopyroxene has replaced magmatic mafic phases. Coronas of garnet separate felsic and mafic domains, underlining the former grain boundaries and the relict gabbroic texture. In more deformed rocks, segregations after plagioclase have become coarser-grained and taken an elongate shape; they contain paragonite, phengite and clinozoisite. The mafic matrix surrounding them is recrystallised into a fine-grained assemblage of garnet, clinopyroxene and amphibole. Completely recrystallized leuco eclogites are found at the deformed margins of the massive leuco eclogite body. The melanocratic rocks, with Ti ore potential, are generally completely recrystallized into foliated eclogites. On average, eclogites from Engebøfjellet are fine-grained rocks, either as a result of strong deformation (ferro eclogites) or lack of complete recrystallisation (coronitic leuco eclogite). Ferro eclogites are generally granular, with a grain size less than 0.5 mm.
Eclogite paragenesis comprises garnet, omphacite, amphibole, phengite, clinozoisite, quartz, dolomite, rutile and pyrite. Accessory minerals are apatite, allanite and zircon. Ti is now mainly residing in rutile, mostly as matrix grains (0.5 mm); a minor amount forms numerous but tiny inclusions within silicates. Discrete ilmenite grains are recorded in one sample. Modal as well as mineral chemical variations are found from ferro- to transitional and leuco- eclogites. Ferro eclogites are usually rich in garnet (from ca. 25% to 55%) and rutile (3-6%). Within this group, bulk-rock chemical variations are expressed by modal variations in garnet, clinopyroxene and amphibole as well as in minor phases like quartz, clinozoisite, phengite, rutile and apatite. Transitional eclogites show an increase in clinopyroxene (omphacite); large amphibole porphyroblasts are common. Within this group, coarser-grained eclogites contain omphacite porphyroclasts sheared within a fine-grained matrix of omphacite, garnet, amphibole, carbonate and mica. Leucocratic rocks are enriched in mica, clinozoisite, and often in amphibole and quartz; garnet forms generally less than 20% of the rock. Omphacite characteristically forms elongate porphyroblasts, up to 2-3 cm long. Some leuco eclogites are layered on the mm-cm scale and contain numerous phengite (carbonate-bearing quartz-rich layers. Concurrently with the modal changes, mineral compositions also evolve. The almandine garnet become richer in Mg (pyrope component), from Alm 56-65 Spe 1-5 Pyr 9-17 Gro 17-24 And 1-7 in ferro- and transitional eclogites to Alm 52 Spe 2 Pyr 24 Gro 21 And 2 in one leuco eclogite. Both clinopyroxene and amphibole show an increase in Al and Na, evolving from chloromelanitic omphacite (Jd 21-35) to omphacite (Jd 45) and from actinolitic hornblende to barroisite.
Fluid-rock interaction was frequent at all stages in the history of these rocks. The abundance of volatile-bearing minerals (mainly amphibole, phengite, clinozoisite, dolomite and apatite) characterizes eclogite-facies parageneses from Engebøfjellet. Amphibole is ubiquitous, in amounts ranging from trace to being the main Na- and Ca-bearing phase (40-50%). Dolomite is common, varying in abundance from a few scattered grains to a major phase. Abundant eclogite-facies veining (quartz, omphacite, garnet, carbonate, phengite...) indicate that the fluid pressure was high during this event.
Retrogression of the eclogites is often seen dependant on both deformation and fluid infiltration and occurred predominantly along shear zones and margins of the lens. Garnet amphibolites represent the first retrogression stage (D3). Amphibolites within major shear zones (D4) contain amphibole (hornblende type), epidote and plagioclase; garnet breaks down. Static retrogression of eclogites to symplectitic assemblages of the same minerals is observed in undeformed areas near these shear zones. The mineral assemblages indicate that the D4 deformation occured under conditions characteristic of the epidote-amphibolite facies. Local coronitic retrogression along late fractures and mineral filling in the veins (actinolite, epidote, chlorite, calcite, quartz, magnetite, ilmenite and/or titanite) represent a greenschist facies overprint (D5). During retrogression, rutile is replaced by ilmenite along grain rim and fractures. This transformation generally amounts to less than 10% in eclogites with minor retrogression but can reach more than 95% in totally amphibolitized samples. Only trace amounts of titanite occur in a few late D5 veins or as thin coronas around rutile/ilmenite grains in extensively retrograded eclogites.