Tectono-metamorphic history recorded in high-grade rocks from Filchnerfjella:Further evidence for Pan-African reworking of the Grenville-aged crust in central Dronning Maud Land, East Antarctica

摘要

The bedrock exposed in Filchnerfjella comprises migmatitic gneissic biotite - hornblende granite with amphibolite boudins, intruded by peraluminous garnet-bearing alaskite. The garnet-bearing alaskite contains enclaves of deformed mafic granulite layers and migmatitic gneissic granite. The migmatitic gneissic granite shows contact with a sheet-like foliated charnockite and rare layers of garnet-sillimanite-cordierite-bearing metapelitic granulite occur at the contact of foliated charnockite and migmatitic gneissic granite.The rocks record imprints of high-grade tectono-metamorphic events, D_1/M_1 and D_2/M_2 followed by D3 and D4 deformation events. The dominant layering and foliation in the migmatitic gneissic granite represents a composite fabric (S_1/ /S_2) which trends E to ENE with moderate dips to the south and was the form surface deformed during D_2. The D_2 deformation resulted in formation of map-scale N- to NNW-vergent F_2 folds overturned to the south and development of mesoscopic-scale S_2 foliation in foliated charnockite and garnet-bearing alaskite. All the lithounits were later folded by regional F_3 folds formed during the D_3 deformation, which are upright, nearly cylindrical and have axial trace trending E-W resulting in the present-day map-pattern. N-S-trending F_4 folds have weakly deformed the F_3 axial trace.The gneissic granite and its amphibolite boudins preserve an amphibolite-facies imprint, whereas the retrograde P-T path inferred for the high-grade rocks comprises a near-isothermal decompression segment from near-peak granulite-facies conditions of 7-8 kbar and >=700 degC (M_1 -stage) to ~4.5 kbar and ~650-700 degC (M_2 -stage), recorded in metapelitic granulite and in boudins of mafic granulite.The protolith of the gneissic granite probably was emplaced around 1.1 Ga, as they are similar to the dated granodioritic rocks of adjacent Conradgebirge and Miihlig-Hofmann Mountains. The foliated charnockite yields a Rb-Sr whole-rock errorchron corresponding to an age of 536±56 Ma (MSWD=4.4) with Sr_i=0.7338±0.0007 and is interpreted broadly as time of cooling post-dating the syn-D_2 amphibolite-facies recrystallisation. Thus, it is very likely that this Pan-African timing for the major D_1/M_1 and M_2/D_2 events, with interleaved amphibolite-facies and granulite-facies rocks, links M_1 and M_2 metamorphic stages into a continuous path. This reworking of the Grenville-aged rocks, from the central Dronning Maud Land, is interpreted to have occurred due to collision of the ~1.1 Ga Coats Land-Maud-Grunehogna block with the East Antarctic craton followed by subsequent uplift.