WHERE THE TRAINED EYE BEATS THE PSEUDOSECTION: PETROGRAPHIC IDENTIFICATION OF INDICATORS OF EXTREME METAMORPHISM (Invited Presentation)

Modern petrology, through its integration of advances in the fields of microanalysis, thermodynamic databases and modelling, is in a better position than ever before to quantify ancient and modern geodynamic processes. However, geoscience teaching has also incorporated other expanding fields such that less time is left to cover a fundamental aspect required for understanding Earth – namely the characterisation of rocks, their minerals and reaction microstructures both in nature and by optical microscopy. The field of ultrahigh pressure (UHP) metamorphism, unheard of 30 years ago, has revolutionised our understanding of geodynamics. This area of research started with the optical observation of indicator minerals coesite and /or microdiamond in rocks of the continental crust that have been subducted to, and exhumed from, depths of 100 km or more. This is at least double the depths previously accepted for often well studied, and dated, metamorphic terranes thus implying rates of burial and exhumation at least twice as fast. UHP metamorphism occurs at conditions of the eclogite facies albeit a facies extended to much greater depths. Conventional P-T estimation in metabasites (intersection of the Gt-Cpx Fe-Mg thermometer with the Ab=Jd(in Cpx)+Qz barometer), or the pseudosection approach delimiting a huge Grt+Cpx field, are unsuitable for confirming UHP conditions. Identifying coesite optically is based on two features: radially-oriented palisade quartz around a higher relief, lower birefringence coesite relic and a high density of radial fractures in the host phase (matrix coesite is only known from one regional metamorphic rock location). Recognising microdiamond requires reflected light microscopy as high reflectivity diamond, and its retrograde transformation product graphite, are thus readily determinable. These simple petrographic observations, confirmable by micro-Raman, have enormous implications as they allow pressure estimations by the simple Qz=Coe and Dia=Gr univariant curves in rocks where typically the key mineral compositions in major minerals allowing definition of UHP conditions by conventional or pseudosection methods have been modified. Cpx exsolution from majoritic garnet or K-feldspar exsolution from K-bearing Cpx are further examples where petrography provides the answer.