VERY LOW-GRADE AND LOW-GRADE METAMORPHISM IN THE TRINITY PENINSULA GROUP (PERMO-TRIASSIC) OF NORTHERN GRAHAM LAND, ANTARCTIC PENINSULA

The Permo-Triassic Trinity Peninsula Group is a widespread, regionally metamorphosed metasedimentary sequence in northern Graham Land, Antar ctica, which forms the local 'basement' to the mainly Jurassic-Cretace ous Antarctic Peninsula magmatic are. The metamorphic grade, thermal e volution and pressure series of this major tectono-stratigraphical uni t are largely unknown. Determining the nature of the metamorphism has relied hitherto on conventional optical identifications of the major p hases, mainly in rare volcanic beds. However, diagnostic mineral parag eneses are generally absent and the precise metamorphic grade is unkno wn or has to be inferred over large areas. Using white mica (illite) c rystallinity of interbedded mudrocks, the Trinity Peninsula Group is n ow shown to have been pervasively altered mainly at anchizonal and epi zonal grades. Conditions ranged from upper anchizonal in the northeast to thoroughly epizonal in the southwest. Outwith thermal aureoles nea r plutonic intrusions, the alteration temperatures ranged mainly from 250 to 325 degrees C, exceeding 300 degrees C in the highest-grade (ep izone/greenschist facies) parts a of the sequence. The facies series, K-white mica b cell dimension measurements and mineral phases present are characteristic of an intermediate pressure series altered under mo derate geothermal gradients (< 35 degrees C/km), corresponding to buri al depths of c. 7-10 km. Unroofing and substantial erosion of the Trin ity Peninsula Group took place during polyphasal vertical tectonic mov ements linked to the development of the magmatic are in northern Graha m Land. The geological setting of the Trinity Peninsula Group is ambig uous and could have been a foreland (or back-are) basin or the mid- to upper levels of an accretionary prism.