Jurassic Reconstruction of the Gulf of Mexico Basin |
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| Abstract: | Octahedral, tetrahedral, and cubic forms of graphite, interpreted here as pseudomorphs after diamond, have been discovered in situ in crustal metamorphic rocks from central Macedonia, northern Greece. Several types of rocks, mainly of sedimentary origin, including eclogite, phyllite, quartzite, schist, and amphibolite, have been identified as hosts to inferred diamonds. All assemblages are invariably graphitic and retrograded under greenschist-facies metamorphism. The graphitized diamonds themselves occur as inclusions in garnet, quartz, amphibole, and graphite, and range in size from approximately 2 to 300 μm. In marked contrast with previously published Raman spectra of graphitized diamonds from crustal metamorphic rocks, the Raman spectra of the Greek specimens indicate very poor carbon crystallinity. This probably resulted from a rapid phase transition induced by high contact compressive stress (i.e., non-hydrostatic pressure) at ultradeep shear zones and subsequent rapid pressure release. The presence of former microdiamonds invalidates previous models on the geotectonic evolution of the Internal Hellenide zones, and demarcates a new ultrahigh-pressure zone, the width of which is currently uncertain, and which probably represents a Late Paleozoic suture marking the collision of individual continental blocks of unknown provenance. |
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