¹Jean-Guillaume Feignon,^2,3Sietze J. de Graaff,⁴Ludovic Ferrière,^2,3Pim Kaskes,^2,3Thomas Déhais,²Steven Goderis,²Philippe Claeys,¹Christian Koeberl
Meteoritcs & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13705]
¹Department of Lithospheric Research, University of Vienna, Althanstrasse 14, Vienna, A-1090 Austria
²Research Unit: Analytical, Environmental & Geo-Chemistry, Department of Chemistry, Vrije Universiteit Brussel, AMGC-WE-VUB, Pleinlaan 2, Brussels, 1050 Belgium
³Laboratoire G-Time, Université Libre de Bruxelles, Av. F.D. Roosevelt 50, Brussels, 1050 Belgium
⁴Natural History Museum, Burgring 7, Vienna, A-1010 Austria
Published by arrangement with John Wiley & Sons

The IODP-ICDP Expedition 364 drilling recovered a 829 m core from Hole M0077A, sampling ˜600 m of near continuous crystalline basement within the peak ring of the Chicxulub impact structure. The bulk of the basement consists of pervasively deformed, fractured, and shocked granite. Detailed geochemical investigations of 41 granitoid samples, that is, major and trace element contents, and Sr–Nd isotopic ratios are presented here, providing a broad overview of the composition of the granitic crystalline basement. Mainly granite but also granite clasts (in impact melt rock), granite breccias, and aplite were analyzed, yielding relatively homogeneous compositions between all samples. The granite is part of the high-K, calc-alkaline metaluminous series. Additionally, they are characterized by high Sr/Y and (La/Yb)N ratios, and low Y and Yb contents, which are typical for adakitic rocks. However, other criteria (such as Al2O3 and MgO contents, Mg#, K2O/Na2O ratio, Ni concentrations, etc.) do not match the adakite definition. Rubidium–Sr errorchron and initial 87Sr/86Srt=326Ma suggest that a hydrothermal fluid metasomatic event occurred shortly after the granite formation, in addition to the postimpact alteration, which mainly affected samples crosscut by shear fractures or in contact with aplite, where the fluid circulation was enhanced, and would have preferentially affected fluid-mobile element concentrations. The initial (ɛNd)t=326Ma values range from −4.0 to 3.2 and indicate that a minor Grenville basement component may have been involved in the granite genesis. Our results are consistent with previous studies, further supporting that the cored granite unit intruded the Maya block during the Carboniferous, in an arc setting with crustal melting related to the closure of the Rheic Ocean associated with the assembly of Pangea. The granite was likely affected by two distinct hydrothermal alteration events, both influencing the granite chemistry: (1) a hydrothermal metasomatic event, possibly related to the first stages of Pangea breakup, which occurred approximately 50 Myr after the granite crystallization, and (2) the postimpact hydrothermal alteration linked to a long-lived hydrothermal system within the Chicxulub structure. Importantly, the granites sampled in Hole M0077A are unique in composition when compared to granite or gneiss clasts from other drill cores recovered from the Chicxulub impact structure. This marks them as valuable lithologies that provide new insights into the Yucatán basement.

^1,2Ming Chen,³Christian Koeberl,^2,4Dayong Tan,^1,2Ping Ding,^2,4Wansheng Xiao,^1,2Ning Wang,^1,2Yiwei Chen,^2,4Xiande Xie
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13711]
¹State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640 China
²CAS Center for Excellence in Deep Earth Science, Guangzhou, 510640 China
³Department of Lithospheric Research, University of Vienna, Althanstrasse 14, Vienna, A-1090 Austria
⁴Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640 China
Published by arrangement with John Wiley & Sons

The Yilan crater is 1.85 km in diameter and is located in the northeast of China’s Heilongjiang Province. The crater is exposed in the Early Jurassic granite of the regional Paleozoic–Mesozoic granite complexes. The southern third of the crater rim is missing, but other rim sections are well preserved, with a maximum elevation above the present crater floor of 150 m. A drillcore from the center of the structure shows that the crater fill consists of 110 m thick lacustrine sediments underlain by a 319 m thick brecciated granite unit mainly composed of unconsolidated granite clasts and fragments. Melt products derived from the target granite, which include melted (and recrystallized) granite clasts, vesicular glass, and teardrop-shaped glass, were found in the brecciated granite unit at 218–237 m depth. Petrographic investigations of unmelted granite clasts in the brecciated granite unit from this depth interval show the presence of multiple sets of planar deformation features (PDFs) in quartz. Orientation measurements for 79 PDF sets in 38 quartz grains with a U-stage indicate the dominance of the ω{103} and π{102} orientations with a relative frequency of 39% and 18%, respectively. Only 7.6% of the observed PDFs remain unindexed. The observations of PDFs with the appropriate orientations are clear evidence of shock metamorphism and thus of an impact origin of the Yilan structure. Crystallite aggregates of coesite embedded in silica glass were found in the impact-melted granite clasts. The carbon-14 dates of possibly impact-produced charcoal and lacustrine sediments from the crater fill suggest a young age for the impact event of 0.0493 ± 0.0032 Ma.