¹Dailto Silva, ²Cristiano Lana,¹Carlos Roberto de Souza Filho
¹Department of Geology and Natural Resources, University of Campinas, Sao Paulo, Brazil
²Department of Geology (DEGEO), Federal University of Ouro Preto (UFOP), Minas Gerais, Brazil

Petrographic and geochemical data obtained on the Araguainha impact crater (Goiás/Mato Grosso States, Brazil) indicate the existence of several molten products that originated during impact-induced congruent melting of an alkali-granite exposed in the inner part of the central uplift of the structure. Although previous studies have described these melts to some extent, there is no detailed discussion on the petrographic and geochemical variability in the granite and its impactogenic derivatives, and therefore, little is known about the geochemical behavior and mobility of trace elements during its fusion in the central part of the Araguainha crater. This paper demonstrates that the preserved granitoid exposed in the core of the structure is a magnesium-rich granite, similar to postcollisional, A-type granites, also found in terrains outside the Araguainha crater, in the Brasília orogenic belt. The molten products are texturally distinct and different from the original rock, but have very similar geochemical composition, making it difficult to separate these lithotypes based on concentrations of major and minor elements. This also applies for trace and rare earth elements (REE), thus indicating a high degree of homogenization during impact-induced congruent melting under high pressure and postshock temperature conditions. Petrographic observations, along with geochemical data, indicate that melting occurs selectively, where some of the elements are transported with the melt. Simultaneously, there is an effective dissolution of the rock (granite), which leads to entrainment of the most resistant solid phases (intact or partially molten minerals) into the melt. Minerals more resistant to melting, such as quartz and oxides, contribute substantially to a chemical balance between the preserved granite and the fusion products generated during the meteoritic impact.

Reference
Silva D, Lana C, de Souza Filho CB (2016) Petrographic and geochemical characterization of the granitic rocks of the Araguainha impact crater, Brazil. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12601]
Published by arrangement with John Wiley & Sons

^1,2Winfried H. Schwarz, ^1,2Mario Trieloff, ¹Klemens Bollinger, ¹Niklas Gantert, ^1,3Vera A. Fernandes, ¹Hans-Peter Meyer, ⁴Hal Povenmire,⁵Elmar K. Jessberger, ^1,2Massimo Guglielmino, ^6,7Christian Koeberl
¹Institut für Geowissenschaften, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 234-236, 69120 Heidelberg, Germany
²Klaus-Tschira-Labor für Kosmochemie, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 234-236, 69120 Heidelberg, Germany
³Museum für Naturkunde – Berlin, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Invalidenstrasse 43, D-10115 Berlin, Germany
⁴7560 Greenboro Drive, #4, West Melbourne, FL 32904
⁵Institut für Planetologie, Universität Münster, Wilhelm Klemm Straße 10, 48419 Münster,Germany
⁶Department of Lithospheric Research, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria
⁷Natural History Museum, Burgring 7, 1010 Vienna, Austria

High resolution 40Ar-39Ar step heating dating of australites and indochinites, representing a large area of the Australasian strewn field, and more recently discovered tektite like glasses from Central America (Belize) and Western Canada, were analysed. Precise plateau ages were obtained in all cases, yielding indistinguishable ages of 789 ± 9 ka for four australites, 783 ± 5 ka for four indochinites, 783 ± 17 ka for one Western Canadian and 769 ± 16 ka for one Belize impact melt glass. Concerning major elements and REEs, australites and the Western Canadian impact melt glass are indistinguishable. If the Western Canadian sample was transported by impact ejection and belongs to the Australasian strewn field, this implies extremely far ballistic transport of 9000 km distance assuming a source crater in southern Asia. The distinct major element and REE composition of the Belize impact melt glass suggests formation in another separate impact event. We conclude that the Australasian/Western Canadian impact melt glasses formed 785 ± 7 ka ago and Belize impact melt glass 769 ± 16 ka ago. The two impact events forming these two strewn fields occurred remarkably closely related in time, i.e., separated by <30 ka.

Reference
Schwarz WH, Trieloff M, Bollinger K, Gantert N, Fernandes VA, Meyer H-P, Povenmire H, Jessberger EK, Guglielmino M, Koeberl (2016) Coeval ages of Australasian, Central American and Western Canadian tektites reveal multiple impacts 790 ka ago. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.12.037]
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