Petrographic and geochemical characterization of the granitic rocks of the Araguainha impact crater, Brazil

Abstract

Abstract-Petrographic and geochemical data obtained on the Araguainha impact crater (Goi as/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 Bras ılia 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.

Linking shock textures revealed by BSE, CL, and EBSD with U‐Pb data (LA‐ICP‐MS and SIMS) from zircon from the Araguainha impact structure, Brazil

“…; Silva et al. ; Erickson et al. ) and from shocked sandstone from the Furnas Formation, similar to the one studied here (Hippertt et al.…”

“…5). This texture has been reported previously in zircon from impact melt rock derived from granite at Araguainha (e.g., Tohver et al 2012;Silva et al 2016;Erickson et al 2017a) and from shocked sandstone from the Furnas Formation, similar to the one studied here (Hippertt et al 2014).…”

“…; Preuss, ; Fischer ; Silva et al. ). They showed that the impact melt rocks and the granite are very similar in composition to each other.…”

“…von Engelhardt et al (1992) also alleged that the impact melt rocks formed at least two large, allegedly continuous bodies, the main remnants of which were exposed on the top of hills in the southern and central parts of the central uplift. By field observation and petrographic studies, the impact melt rocks have been classified variably by different authors (von Engelhardt et al 1992;Machado et al 2009;Silva et al 2011Silva et al , 2016Preuss 2012;Yokoyama et al 2012;Fischer 2015). Yokoyama et al (2012) and UnB-IG (2012) reported individual occurrences of impact melt rock within the larger areas mapped by earlier workers and demonstrated that these do not represent continuous bodies.…”

“…Several petrographic and geochemical studies have been carried out on the granite and impact melt rocks (von Engelhardt et al 1992;Machado et al, 2009;Yokoyama et al 2012;Preuss, 2012;Fischer 2015;Silva et al 2016). They showed that the impact melt rocks and the granite are very similar in composition to each other.…”