| Popis: |
The IODP-ICDP Expedition 364 drilling recovered a 829 m core from Hole M0077A, sampling ~600 meters 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 forty-one granitoid samples, i.e., 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 in situ 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 adakites. However, other criteria (such as Al2O3 and MgO contents, Mg#, K2O/Na2O ratios, Ni concentrations, etc.) do not match the adakite definition. In addition, initial 87Sr/86Srt=326 Ma is indicative of an hydrothermal fluid metasomatic event occurring shortly after the granite formation, affecting mainly 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 εNd (t=326 Ma) values range from -4.0 to 3.2 for the samples which were less affected by the hydrothermal overprint and indicate that a minor Grenville basement component may be 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 with the closure of the Rheic ocean associated with the assembly of Pangea. Then, an hydrothermal metasomatic event, possibly related to the initial breakup of Pangea, occurred ca. 50 Myr after the granite crystallization. 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 insight into the Yucatán basement. |
