O EOL GIC G A D D A E D C E I H C I L E O S F u n 2 d 6 la serena octubre 2015 ada en 19 Detrital zircon geochronology of Palaeozoic siliciclastic rocks from the Ellsworth Mountains, West Antarctica Paula Castillo* and C. Mark Fanning Research School of Earth Sciences, The Australian National University, Canberra, Australia. Rodrigo Fernández University of Texas Institute for Geophysics, The University of Texas at Austin, Austin, Texas, USA. Fernando Poblete Geosciences Rennes, Université de Rennes 1, Rennes, France. Departamento de Geología, Universidad de Chile, Santiago, Chile. *Contact email: [email protected] Abstract. In the Ellsworth Mountains there is an extensive record of sedimentation from early Cambrian to Permian times. However, the tectonic history and the palaeogeographic significance remain enigmatic. Nine sandstone samples were analysed for their U-Pb detrital zircon age spectra using SHRIMPII and RG. They belong to the early Cambrian to Carboniferous-Permian sequences and record typical Gondwana margin signatures. Variations up section/sequence in zircon provenance suggest restricted basinal deposition during the Cambrian,! with likely sources in the Namaqua-Natal and Mozambique/Maud Belts. There are little or no contributions from older cratons and so the Ellsworth basin evolved as a separate basin to that in the Transantarctic Mountains.! This basin configuration changed after the Devonian and deposition continued Figure 1. Reconstruction of part of Gondwana at ca. 500 Ma. during the late Palaeozoic, when the Ellsworth Mountains South America: SFC - Sao Francisco craton; PPC - basin only received detritus from the Ross/Pan-African Paranapanema craton; RPC - Río de la Plata craton. In southern orogenic belts. Comparison of the glaciogenic strata in Africa: Na-Na - Namaqua-Natal belt. In Antarctica: Ma - Maud the Ellsworth Mountains with others in southern belt. The simplified sketch is adapted from Jacobs and Thomas Gondwana suggests deposition related to that in the (2004), Boger (2011) and Rapela et al. (2011) and references Transantarctic Mountains. therein. Keywords: detrital zircon, U-Pb, West Antarctica. Nine recently collected sandstone samples from the Ellsworth Mountains were analysed for their U-Pb detrital zircon age spectra using SHRIMPII and RG at 1.! Introduction The Australian National University. They are from the early Cambrian to Carboniferous-Permian sequences The Ellsworth-Whitmore Mountains block (EWM) is (Fig. 2). one of the most isolated fragments forming West Antarctica. The best exposures are in the Ellsworth Mountains (Fig. 1), where there is an extensive record of 2.! Results sedimentation from early Cambrian to Permian times. However, the tectonic history and the palaeogeographic The active continental rift sediments of the Cambrian significance of the EWM remain enigmatic. Gondwana Heritage Group are dominated by igneous zircons with reconstructions suggest that the EWM originated in a Grenville ages (1.0-1.2 Ga). Detrital zircons from two position adjacent to southern Africa and the Coats Land different localities of the Middle Cambrian Liberty coast of Antarctica (Curtis, 2001; Randall and Mac Formation document a change in provenance up- Niocaill, 2004). This position implies that the EWM stratigraphy (Fig. 3). The lower level yields younger would have a record of the Cambro-Ordovician orogenic detrital zircons of ca. 850 Ma whilst the upper level event that occurred along the palaeo-Pacific margin of (close to the contact with the Upper Cambrian Minaret Gondwana. However, the EWM lacks evidence for this Formation), records younger zircons at ca. 530 Ma. orogenic event; rather, there appears to be signs of Samples from the Upper Cambrian Springer Peak continental rifting during the Cambrian (Curtis, 2001; Formation and the upper part of the Liberty Formation Curtis et al., 1999). have a second igneous and metamorphic zircon component with Pan-African ages (530-650 Ma). 805 ST 4 ESTRATIGRAFÍA, ANÁLISIS DE CUENCAS Y PROCESOS SEDIMENTARIOS Figure 3. Summary and comparison of the detrital zircon Figure 2. Stratigraphic column for the Ellsworth Mountain components. Relative probability density plot (orange area) and succession after Curtis (2001). Circled numbers within the kernel density estimator (black line). column indicate sample localities within the stratigraphy. At upper stratigraphic levels, the passive margin 3.! Conclusions sediments of the Upper Cambrian to Devonian Crashsite Group are conformable. Sandstone samples from the From the continuous record of early Cambrian to base of this Group yield dominantly igneous zircon Permian sedimentation in the Ellsworth Mountains, new components with a Ross/Pan-African provenance (500- detrital zircon dating indicates typical Gondwana margin 620 Ma) and scattered older ages. Detrital zircons from signatures; however, variations up section/sequence in the Devonian Mt Wyatt Earp Formation are principally zircon provenance suggest restricted basinal deposition in the range of 480-610 Ma, with major peaks at ca. 500 during the Cambrian. and 530 Ma. The Heritage Group samples have U-Pb detrital zircon The Whiteout Conglomerate conformably overlays the patterns similar to those from the Pensacola Mountains Crashsite Group (Fig. 2). It is a diamictite that marks the (Goodge et al., 2004) and South Africa (Fourie et al., Permo-Carboniferous Gondwana glaciation in the 2011; Frimmel et al., 2013; Naidoo et al., 2013), with Ellsworth Mountains. Although sampled from widely likely sources in the Namaqua-Natal and different areas (north-upper and south-lower parts Mozambique/Maud Belts. There are little or no respectively) their detrital zircon age patterns are very contributions from older cratons and so evolved in a similar. They are dominated by a 500-650 Ma zircon separate basin to that recorded in the Transantarctic component, lacking detrital zircons younger than Mountains. Cambrian (Fig. 3). This separated basin configuration changed after the Devonian and continued during the late Palaeozoic, when the South African basin had an influx of detritus from Ordovician to Devonian exotic sources (Fourie et al., 2011), whereas the EWM only received detritus from the Ross/Pan-African orogenic belts. Comparison of the Whiteout Conglomerate with other glaciogenic strata in southern Gondwana suggests deposition related to that in the Transantarctic Mountains. Acknowledgements 806 This work was supported by the Anillo Antártico Project (ACT-105). Special thanks to Bernabé López, INACH, Figure 3. Summary and comparison of the detrital zircon Figure 2. Stratigraphic column for the Ellsworth Mountain components. Relative probability density plot (orange area) and succession after Curtis (2001). Circled numbers within the kernel density estimator (black line). column indicate sample localities within the stratigraphy. At upper stratigraphic levels, the passive margin 3.! Conclusions sediments of the Upper Cambrian to Devonian Crashsite Group are conformable. Sandstone samples from the From the continuous record of early Cambrian to base of this Group yield dominantly igneous zircon Permian sedimentation in the Ellsworth Mountains, new components with a Ross/Pan-African provenance (500- detrital zircon dating indicates typical Gondwana margin 620 Ma) and scattered older ages. Detrital zircons from signatures; however, variations up section/sequence in the Devonian Mt Wyatt Earp Formation are principally zircon provenance suggest restricted basinal deposition in the range of 480-610 Ma, with major peaks at ca. 500 during the Cambrian. and 530 Ma. The Heritage Group samples have U-Pb detrital zircon The Whiteout Conglomerate conformably overlays the patterns similar to those from the Pensacola Mountains Crashsite Group (Fig. 2). It is a diamictite that marks the (Goodge et al., 2004) and South Africa (Fourie et al., Permo-Carboniferous Gondwana glaciation in the 2011; Frimmel et al., 2013; Naidoo et al., 2013), with Ellsworth Mountains. Although sampled from widely likely sources in the Namaqua-Natal and different areas (north-upper and south-lower parts Mozambique/Maud Belts. There are little or no respectively) their detrital zircon age patterns are very contributions from older cratons and so evolved in a similar. They are dominated by a 500-650 Ma zircon separate basin to that recorded in the Transantarctic component, lacking detrital zircons younger than Mountains. Cambrian (Fig. 3). This separated basin configuration changed after the Devonian and continued during the late Palaeozoic, when the South African basin had an influx of detritus from Ordovician to Devonian exotic sources (Fourie et al., 2011), whereas the EWM only received detritus from the Ross/Pan-African orogenic belts. Comparison of the Whiteout Conglomerate with other glaciogenic strata in southern Gondwana suggests ATdeposition 1 Geolo Gíarelated ReGional to y Geodinámicathat in theandina Transantarctic Mountains. Acknowledgements This work was supported by the Anillo Antártico Project (ACT-105). Special thanks to Bernabé López, INACH, the Chilean Army Navy and ALE for logistical support during the field work. F.P thanks additional funding from CONICYT and IRD. References Boger, S.D., 2011. Antarctica - Before and after Gondwana. Gondwana Research, 19(2): 335-371. Curtis, M.L., 2001. Tectonic history of the Ellsworth Mountains, West Antarctica: Reconciling a Gondwana enigma. Geological Society of America Bulletin, 113(7): 939- 958. Curtis, M.L., Leat, P.T., Riley,