# The Barrachina suevite and suevite-like breccias

### Suevite

In a polymictic breccia quarried out as large blocks in a temporary outcrop at the northeastern roadside in the Pancrudo valley (see Fig. 1), silicate melt was found. Apart from the melt, the breccia is composed of matrix-supported subrounded to angular limestone, sandstone and quartzite clasts having various grain sizes.

Fig. 1. Temporary quarry set up in the Barrachina megabreccia. Here, large blocks of suevite were uncovered.

The dominantly carbonate matrix is peppered with fragmented, predominantly quartz and calcite grains, and frequently shows flow texture with the larger clasts preferentially adjusted to the flow (see images below). In some cases, clasts are themselves brecciated (breccia-within-breccia). In thin sections, shock metamorphism in the form of quartz grains with multiple sets of PDFs and diaplectic quartz is observed. Strongly kinked mica are abundant. The melt occurs in elliptic whitish clasts with diameters between several millimeters and 2-3 cm. The soft clasts are extremely fine-grained and can easily be carved by a small spatula to get material for analysis. From x-ray powder diffraction analysis, these clasts are not pure melt, but consist of a mixture of amorphous material (typical glass hump) together with dolomite, calcite and minor amounts of quartz, muscovite and gypsum. This mixture is also reflected by the chemical bulk composition (see bulk-5 analysis in Table 1). High contents of MgO and CaO and the distinct LOI values reflect dominating carbonates, mainly dolomite and subordinate calcite. The bulk-5 composition of the clasts closely matches a mixture of 50% pure dolomite, 10% pure calcite and 25% of the white glass (mean composition) in Table 1. A dominant amount of carbonates and a subordinate portion of melt is also estimated from the x-ray diffractogram. According to the current classification and nomenclature of impact rocks (IUGS Subcommission on the Systematics of Metamorphic Rocks, Study Group for Impactites), this polymictic impact breccia composed of shocked and melt clasts is termed a suevite or suevite breccia.

Figs. 2-4. Suevite breccias from the Barrachina megabreccia. Silicate melt is concentrated in the whitish, porous clasts.

Fig. 3. Rubielos de la Cérida suevite.

Fig. 4. Rubielos de la Cérida suevite.

 wt.% white white white white white white mean wt.% bulk-1 bulk-2 bulk-3 bulk-4 bulk-5 SiO2 59,95 59,72 59,38 57,19 59,95 59,18 59,23 SiO2 56,06 58,13 53,45 54,47 19,78 TiO2 0,24 0,24 0,21 0,20 0,23 0,20 0,22 TiO2 0,33 0,34 0,38 0,45 0,24 Al2O3 20,75 19,53 19,88 21,30 23,16 18,63 20,54 Al2O3 20,91 19,76 20,40 20,96 6,34 MgO 7,26 7,49 7,42 6,14 6,45 8,21 7,16 MgO 5,81 4,77 5,24 6,14 12,62 CaO 0,88 1,04 0,92 0,99 1,09 1,17 1,02 CaO 1,48 1,56 1,72 0,98 22,56 FeO 1,61 1,77 1,62 1,89 1,85 1,73 1,75 FeO 2,00 2,70 2,76 2,49 2,68 Na2O 1,92 1,87 1,82 1,63 1,56 1,66 1,74 Na2O 0,48 1,20 0,29 0,48 0,02 K2O 0,23 0,28 0,27 0,21 0,18 0,26 0,24 K2O 0,65 1,34 0,45 0,57 1,82 Total 92,84 91,94 91,52 89,55 94,47 91,04 91,89 LOI 10,30 9,24 14,02 11,70 32,91 Total 98,02 99,04 98,71 98,24 98,97 wt.% grey grey grey grey grey mean ppm SiO2 56,45 56,89 58,05 59,54 57,12 57,61 V 14 21 27 23 TiO2 0,27 0,21 0,26 0,22 0,25 0,24 Zn 36 46 68 81 Al2O3 20,81 19,88 19,66 15,99 22,74 19,82 Ga 35 38 30 33 MgO 6,77 6,34 7,18 6,90 5,93 6,62 Rb 16 38 5 7 CaO 1,14 1,17 1,23 1,24 1,14 1,18 Sr 492 363 327 364 FeO 1,68 2,18 1,63 1,51 1,79 1,76 Y 43 37 32 38 Na2O 1,42 1,19 1,49 0,79 1,31 1,24 Zr 493 475 491 522 K2O 0,21 0,28 0,24 0,23 0,19 0,23 Nb 56 50 47 53 Total 88,75 88,14 89,74 86,42 90,47 88,70 Ba 1250 171 48 1034 Pb 79 238 29 31 Th 68 59 64 59

Table 1. Electron microprobe analyses of white and grey glass particles separated from the suevite silicate glass, and mean compositions. X-ray fluorescence bulk analyses of four samples of the silicate glass (bulk-1 to bulk-4) and one melt-containing inclusion of the suevite (bulk-5).

### Suevite-like breccia

Along the road between Barrachina and Torre Los Negros, exposed limestones are strongly deformed and frequently show allochthonous material intercalated in the form of megablocks and dikes composed of multicolored marls and shales and Paleozoic Pelarda Fm. facies. At UTM 6 57 900, 45 28 300, black shales have been injected into the bedded limestones exposing a peculiar macroscopic breccia zone (image below).

Fig. 5. Dikes of a suevite-like breccia cut through heavily destroyed limestones.

Fig. 6. The suevite-like breccia under the microscope (the field is 7 mm wide). The cotton-like white material is assumed to be relics of carbonate melt.

On a smaller scale, fragmented black shales are intensively mixed with a white material to form a fine-grained breccia. Frequently, cotton-like scraps are interspersed. In thin section, flow texture can be observed within the dark brownish to black, extremely fine-grained matrix. This matrix contains clasts of quartz and subordinate feldspar, and sometimes very fine-grained aggregates of light minerals. From x-ray powder diffraction analysis, the rock consists of quartz, kaolinite and illite, carbonate and an amorphous phase, the latter defined and documented by a typical “glass hump” (see Fig. 7). Backscattered electron images of electron microprobe investigations show an extremely fine-grained mixture of medium grayish and darker grayish components on a scale of few microns. The analysis of the medium grayish parts reproducibly yields illite mixed with other clay minerals and a composition of about 53 wt.% SiO2, 25% Al2O3, 5% FeO, 3% K2O and, 2.5% each, CaO and MgO. The darker grayish parts are rich in SiO2 and contain variable, but subordinate Al2O3 content. Most probably, this fine-grained mixture reflects strongly alterated relics of the amorphous (glass) phase which in an uncorroded state could not be detected by microprobe analysis. By corrosion, a glass may be transformed to clay minerals passing through an intermediate gel stage with typically enhanced Si-contents (Rösch et al., 1977). No clear shock-metamorphic features have so far been identified in mineral clasts of this glass-bearing breccia. Therefore, we chose the term suevite-like breccia for this peculiar rock.

Fig. 7. X-ray powder diffractogram of the suevite-like breccia. Next to the sharp diffraction peaks of low quartz (q) and calcite (c), more or less broadened peaks of mica phases (kaolinite, illite, montmorillonite) are observed. In comparison to the silicate melt rock, a less pronounced glass “hump” is displayed due to the superimposed strong and sharp quartz and calcite peaks. The enlarged part of the diffractogram with a logarithmic scale for the intensity clearly reveals the “hump”.