Tectonic constraints from thin granitoid sheets in H.U. Sverdrupfjella, Western Dronning Maud Land, Antarctica

Abstract

This work is a study of granitoid sheets in H.U. Sverdrupfjella. The objectives were to measure the orientations and take samples of the granitoid sheets to produce orientation, geochronological and geochemical data from the granitoids. This data is intended to reveal the nature and timing of emplacement for the granitoid sheets and to provide constraints regarding the tectonic history of the study area.

This work identified three suites of granitoid sheets, namely the Pre-existing Granitoids, Salknappen Pegmatites and Dalmatian Granites: 1) The oldest of these suites is the Pre-existing Granitoids, as is evident from field relationships. Pre-existing Granitoids were only seen at a handful of outcrops at the Rootshorga nunatak, which limited the extent of study possible. 2) Salknappen Pegmatites are weakly deformed, very coarse grained and white in colour. From cross-cutting relationships, Salknappen Pegmatites are older than Dalmatian Granite. SHRIMP geochronology gives ages of 517 ± 3.5 Ma, 507 ± 3.1 Ma and 513 ± 3.7 Ma for the three samples which were dated. Geochemically, Salknappen Pegmatites have two distinct groups, one characterised by positive Eu anomalies (+Eu Salknappen Pegmatites), and another characterised by negative Eu anomalies (-Eu Salknappen Pegmatites). These rocks have a flat REE profile with LREE enrichment. Isotope geochemistry gives a younger model age and higher 143Nd/144Nd than Dalmatian Granites and show a similarity to the Rootshorga Complex country rock. 3) Dalmatian Granites crosscut the granitoid suites described above and structural features in the country rock. No deformation is evident in competent country rocks. In the field Dalmatian Granite can be identified by course to very coarse-grained texture and pink colour. SHRIMP dating gives ages of 492 ± 2.1 Ma, 483 ± 2.8 Ma and 483 ± 3.1 Ma for the three samples analysed. The geochemistry of Dalmatian Granite is consistent with an upper crustal source. The granitoids are interpreted to record the earlier stages of orogeny (Salknappen Pegmatites) followed by the late to post-orogenic stage (Dalmatian Granites). Owing to the ~30 m.y. gap between the emplacements of the two suites, a model involving a diachronous metamorphic event is favoured.