Geochronological study of post-metamorphic granite from Kasumi Rock, Lutzow-Holm Complex, East Antarctica

Kasumi Rock is situated in the amphibolite-facies metamorphic zone in the Lutzow-Holm Complex, East Antarctica. In this area, granitic rocks occur as steeply dipping linear dykes with sharp intrusive contacts, and cross-cut the foliation of layered metamorphic rocks. A geochronological study of this post-metamorphic granite (PMG) has been performed. An Rb-Sr whole rock isochron for five granitic rocks defines an age of 492.1+-23.4 Ma with an initial ^87Sr/^86Sr ratio of 0.70535+-0.00027 (MSWD=0.08). In the Sm-Nd isochron diagram, four rock samples yield an age of 498.4+-90.6 Ma with an initial ^143Nd/^144Nd ratio of 0.511782+-0.000101 (MSWD=0.05). The whole rock isochron ages and mode of occurrence suggest that the Kasumi PMGs were intruded after regional cooling of the area, and probably affected local reequilibrium for isotopic systems of surrounding metamorphic rocks. The PMGs might have been slowly cooled and crystallized after the intrusion. The Kasumi PMGs have no genetic relation to the Oku-iwa PMGs as regards Rb-Sr and Sm-Nd isotopic systems, because the initial ratios of both PMGs are quite different from each other. This is interpreted to mean that the PMGs in the Lutzow-Holm Complex were originated from different source materials or had different chemical reaction processes with crustal materials.