In the Rubielos de la Cérida impact basin, strong shock-metamorphic effects are observed to occur in the silicate impact melt rocks and the suevite breccias from the Barrachina megabreccia.
In the basal suevite breccia, shock metamorphism may be related with amorphous SiO2 discussed for the Azuara basal breccia, and with crystallized carbonate melt possibly contributing to the matrix of the basal breccia. Silicate rock and mineral fragments in the basal breccia are in general rare (except for the many Buntsandstein components in the basal breccia from the southern basin and rim region), and related with this scarcity, clear shock effects are not very abundant.
Moderate shock effects are regularly observed in samples from silicate rocks, mostly Cretaceous and Permotriassic sandstones from the Rubielos de la Cérida central-uplift chain and the rim zone. They also occur in sandstones of the small Buntsandstein hill emerging from the Quaternary near Singra, and the Permotriassic rocks exposed in the surroundings of Visiedo (erroneously mapped as Malmian there, see ITGE , IGME ). As in the Azuara sedimentary rocks, moderate shock is indicated by diaplectic quartz crystals (relatively rare), PDFs (relatively rare), PFs and kink bands in quartz, strong kink banding in mica, and microtwinning in calcite. In some thin sections, up to 100% of the micas are kinked.
Fig. 1. Locations where shock metamorphism has so far been established.
While according to current knowledge, diaplectic crystals, diaplectic glass and PDFs can form only by strong impact shock, some of the before-mentioned features have been reported from both impact shock metamorphism and endogenetic processes. Shock deformation of moderate intensity creates planar fractures (PFs, cleavage) in quartz, which belong to the regular shock inventory of impact structures. Cleavage is commonly unknown to occur in quartz. In rare cases, however, planar fractures may be produced tectonically in zones of extreme regional metamorphism. Very strong tectonic deformations may also cause kink banding in mica, which also is a typical low-grade shock effect.
The kinked micas and the multiple sets of PFs in quartz in the Mesozoic rocks cannot possibly have resulted from Alpidic tectonics. The minerals cannot originate from reworked Paleozoic rocks either, because they are not statistically distributed in the thin sections. Moreover, the heavily kinked micas would not have survived reworking.
Typical shock effects observed in Rubielos de la Cérida thin sections
In the following photomicrographs, the width of the fields is between 200 µm and 500 µm. Generally crossed polarizers; two photomicrographs additionally taken with one polarizer.
Fig 2. PDFs and diaplectic quartz. Polymictic breccia, northeastern rim region.
Fig. 3. Melt glass, plane light and xx nicols. Suevite from the Barrachina megabreccia.
Fig. 4. Diaplectic feldspar (the long grain). Silicate melt rock, Barrachina megabreccia. xx nicols and plane polarized light.
Fig. 5. Kink bands in quartz. Permotriassic sandstone; central-uplift chain.
Fig. 6. Kink bands in mica. Buntsandstein sandstone, near Singra.
Fig. 7. Conjugate sets (NNW – SSE and NNE – SSW trending) of kink bands in mica. Cretaceous sandstone, northeastern rim zone.
Fig. 8. Multiple sets of microtwinning in calcite, polymictic breccia, northeastern rim region.
Fig. 9. PDFs in quartz. Cretaceous sandstone, near Portalrubio, northeastern rim region.
Fig. 10. PDFs in quartz. Sandstone, Buntsandstein hill near Singra.
Fig. 11. PDFs in quartz. Sandstone, Buntsandstein hill near Singra.
Fig. 12. PDFs and kink bands in quartz. Buntsandstein hill near Singra.
Fig. 13. Planar fractures (PFs) in quartz. Buntsandstein hill near Singra.
Fig. 14. Kink bands in quartz. Quartzite cobble from the Barrachina megabreccia.
Fig. 15. PDFs in quartz. Cretaceous sandstone, near Portalrubio.
Fig. 16. Kink bands in quartz. Paleozoic quartzite, northwestern rim region.
Fig. 17. PDFs and diaplectic glass in feldspar. Barrachina megabreccia.
Fig. 18. PDFs in quartz; quartzite cobble, Buntsandstein conglomerate, central-uplift chain near Caudé.
Fig. 19. PDFs in quartz; basal suevite breccia near Celadas.
Fig. 20. PDFs in quartz; Buntsandstein sandstone, southern Rubielos de La Cérida basin near Caudé.
Fig. 21. Faint PDFs in muscovite, NW – SE and SW – NE trending, not to be confused with more pronounced basal planes; Buntsandstein sandstone, southern Rubielos de La Cérida basin near Caudé.
Fig. 22. PDFs in quartz; basal suevite breccia, northeastern basin rim.