Very low- and low-grade metamorphism in the Trinity Peninsula Group (Permo-Triassic) of northern Graham Land, Antarctic Peninsula

Abstract

The Permo-Triassic Trinity Peninsula Group is a widespread, regionally metamorphosed metasedimentary sequence in northern Graham Land, Antarctica, which forms the local ‘basement’ to the mainly Jurassic–Cretaceous Antarctic Peninsula magmatic arc. The metamorphic grade, thermal evolution and pressure series of this major tectono-stratigraphical unit are largely unknown. Determining the nature of the metamorphism has relied hitherto on conventional optical identifications of the major phases, mainly in rare volcanic beds. However, diagnostic mineral parageneses are generally absent and the precise metamorphic grade is unknown or has to be inferred over large areas. Using white mica (illite) crystallinity of interbedded mudrocks, the Trinity Peninsula Group is now shown to have been pervasively altered mainly at anchizonal and epizonal grades. Conditions ranged from upper anchizonal in the northeast to thoroughly epizonal in the southwest. Outwith thermal aureoles near plutonic intrusions, the alteration temperatures ranged mainly from 250 to 325 °C, exceeding 300 °C in the highest-grade (epizone/greenschist facies) parts 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 moderate geothermal gradients (<35 °C/km), corresponding to burial depths of c. 7–10 km. Unroofing and substantial erosion of the Trinity Peninsula Group took place during polyphasal vertical tectonic movements linked to the development of the magmatic arc in northern Graham Land. The geological setting of the Trinity Peninsula Group is ambiguous and could have been a foreland (or back-arc) basin or the mid- to upper levels of an accretionary prism.

Petrographic evidence for regional burial metamorphism of the sedimentary rocks in the Lower Benue rift

“…It is most unlikely that the grade exceeded the temperature of 325°C, which is the upper limit of lower epizone (Smellie et al, 1996). The stability ranges of the diagnostic minerals presented in the foregoing suggest that the P -T conditions of the metamorphism observed in this study are 220-325°C and 1-2 Kb.…”

“…The diagnostic minerals--pyrophyllite, paragonite and phlogopite in most of the samples suggest that the rocks have been altered up to the anchizonal or the prehnite-pumpellyite facies of metamorphism, otherwise referred to as very-low-grade regional metamorphism (Frey, 1978;Winkler, 1979;Smellie et al, 1996;Livi et al, 1997). The presence of chloritoid in two of the samples indicates a low epizonal, low-grade or lower greenschist metamorphism of a pelitic rock of a special composition.…”

“…This sodic mica forms at very-low-grade metamorphic conditions (Frey, 1978;Winkler, 1979;Smellie et al, 1996) Winkler (1979, p. 116), this is the first mineral to form during the metamorphism of siliceous carbonate rocks in the presence of high CO 2 concentration in the very low-grade. It is formed from the following reaction:…”

“…This boundary occurs around a temperature of 300°C (Smellie et al, 1996). The mineral possibly occurs through the following reaction between minerals in the very-low-grade of metamorphism: …”

Recognition and implication of tectonic loading-induced reheating in the northern Variscan front (Belgium and northern France), based on an illite K�bler index and oxygen isotope study

“…Geological processes such as regional burial, tectonic overburden and local contact metamorphism are the main geological causes of such illite KI (e.g. Schaer and Persoz 1976;Fortey et al 1993;Smellie et al 1996). The diagenetic alteration of oxygen isotopes, which occurs during carbonate cementation, replacement and recrystallization (Marshall 1992), hinders the precise temperature determination of palaeooceans, but can be applied to diagenetic/metamorphic studies (e.g.…”