Isotopic structure and tectonics of the central Transantarctic mountains

Abstract

Regional patterns of Nd, Sr, and O isotopic ratios of ∼500 Ma granitic rocks are used to identify the ages and areal extents of three crustal provinces in the central Transantarctic Mountains. One of the provinces is the edge of the East Antarctic Craton, which isotopic analyses show is composed of Archean rocks thrust over Proterozoic rocks. The other two provinces compose the Beardmore microcontinent, which we deduce was allochthonous to East Antarctica and was emplaced in late Precambrian or early Paleozoic time. Evidence for a former ocean basin between the Beardmore microcontinent and East Antarctica is provided by basalt and gabbro of mid‐ocean ridge character, dated by Sm‐Nd at ∼760 Ma, associated with marine sediments now located at the suture. The granitic rocks formed over a westward‐dipping subduction zone that was active at ∼500 Ma. The East Antarctic Craton is exposed in the Miller Range, which is a tectonic composite of reworked Archean and early Proterozoic material containing ∼500 Ma peraluminous granites with model ages (TDM) of 2.0 Ga, high δ18O (+11 to +12‰) and high initial 87Sr/86Sr (0.7324 to 0.7417). East of the Marsh Glacier the granitic rocks are metaluminous to weakly peraluminous with model ages of 1.3 to 1.8 Ga, high δ18O (+9 to +13‰) and lower 87Sr/86Sr (0.7068 to 0.7191). East of the Shackleton Glacier, gabbro, tonalite, diorite, and granodiorite have low δ18O (+6 to +7‰), low initial 87Sr/86Sr (0.7045 to 0.7059) and high ϵNd (+0.4 to +1.7). These isotopic provinces correspond to differences in age and composition of the middle and lower crust at the time of formation of the granitic magmas. The boundaries of the isotopic provinces also correspond to discontinuities in provenance, lithology, structural style, and grade of metamorphism of prebatholithic metasedimentary rocks. The isotopic data indicate that the granitic magmas were formed mostly by crustal anatexis in the areas west of the Shackleton Glacier. This is typical of early Paleozoic granitic batholiths elsewhere in the world and has led to speculation that subduction was not involved in granitic magmatism at that time of earth history. However, the granitic rocks located west of the Shackleton Glacier, by virtue of their mantle‐like isotopic compositions and their association with metavolcanic rocks, appear to be subduction‐related. The tectonic history deduced for the Gondwana margin, as represented in the central Transantarctic Mountains, began with deposition of sediments on an Atlantic‐type rifted margin at ∼760 Ma. The Beardmore microcontinent was most likely accreted in association with folding of the clastic sedimentary rocks before middle Early Cambrian time (550 Ma). Carbonate sedimentation and volcanism along the eastern margin of the Beardmore microcontinent commenced in Cambrian time. Folding and metamorphism of all older units occurred in late Cambrian time followed by emplacement of granitic rocks at ∼500 Ma.