engleski

Kelyphitic microstructures from orogenic garnet-bearing rocks of the Dinaride ophiolite zone

The Inner Dinarides, a part of the Alpine-Dinaride mountain chain in south-eastern Europe, comprise ultramaficmassifs and related metamorphic rocks that are relics of a partially dismembered ophiolite which originatedalong the contact of the European plate and the Adria microplate. The Late Jurassic to Early Cretaceous regionalgeodynamic processes involved intra-oceanic NE-dipping subduction that preceded the final emplacement of theophiolite at the continental margin. During subduction mafic cumulates and other rocks came in contact to the hotupper mantle (spinel lherzolite) resulting in the formation of high- to medium-grade metamorphic rocks.Such rocks were sampled mainly representing garnet-bearing amphibolite and metaperidotite varieties. Thesamples comprise various proportions of amphibole, plagioclase, garnet, corundum, sapphirine, spinel, pyroxenes(diopside and hypersthene), TiO2phases and occasionally quartz. Pyrope-almandine garnet typically occurs aslarge grains either with or without fine-grained radial and fibrous symplectitic mineral intergrowth forming acorona structure (kelyphite). The chemical composition of garnet does not show a significant change from thecore to the rim. The kelyphite consists of orthopyroxene and spinel with Ca-amphibole and plagioclase. The bulkcomposition of the kelyphite differs from the original garnet composition pointing to a reaction between garnetand its surrounding including H2O possibly from an external source. As such a reaction depends on the physicalconditions during kelyphite formation, this, besides reaction kinetics, puts constraints on the exhumation historyof the host rocks.P-T pseudosections were constructed in the MnNCFMASHTO system and contoured by isopleths for the modeand chemical composition of minerals. On this basis combined with conventional cation-exchange geothermo-barometry, a clockwise P-T path with maximum pressure conditions at ca. 20 kbar (ca. 65-70 km depth) withtemperatures over 830◦C was reconstructed. These conditions were followed by a significant pressure decreaseto medium-pressure values at 11 kbar (depth of ca. 35 km) and temperatures of 780◦C and a final setting atlow-pressure, low-temperature conditions of ca. 4 kbar (∼14 km depth) and 550◦C.A plausible explanation for the observed reaction microstructure is that the kelyphite was formed around garnetduring the uplift of deep-seated rocks that were significantly decompressed but without drastic change of temper-ature supported with hornblende-plagioclase thermometry and the occurrence of orthopyroxene as component ofthe kelyphite. Such conditions driven by large tectonic movements enable infiltration of hydrous fluid necessaryto form amphibole and to trigger the genesis of kelyphitic microstructures. The so far unknown details of theDinaride geodynamic evolution can be partially unraveled with the aid of this peculiar microstructure inasmuchhot mantle rocks, being part of the ophiolite, were proved to be uplifted for at least 50 km accompanied by fluidinfiltration.Support by the Croatian Science Foundation (IP-2014-09-9541) is acknowledged.

kelyphite ; microstructures ; garnet ; Dinarides

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