Regional-low

Low-grade regional metamorphic rocks. Examples from Cornwall that show the first tectonic and metamorphic fabrics in metasediments, plus a variety of sedimentary and igneous rock types in outcrop and hand specimen.
  • 118-1816 IMG Slate and sandstone of the Porthtowan Formation, near Hayle, Cornwall. Sedimentary layering disrupted by a vertical tectonic fabric axial-planar to the synformal fold. Field of view ca. 1.5 m wide.
  • c98-07 Distal turbidites near Porthleven, Cornwall. These rocks have a tectonic foliation parallel to the compositional banding which has been deformed into tight folds, and a new horizontal axial-planar foliation has begun to develop (seen best at top right).
  • c97-09 Turbidite succession of the Portscatho Formation, Cornwall, shows original bedding overprinted by a slaty cleavage that is refracted across the sandy layers, where it has a more vertical orientation, curving into a shallower orientation in the slaty layers.
  • c97-10 Asymmetric folds in a sandy layer with brittle rupture and quartz vein fillings. Slaty cleavage in the intervening fine-grained rock is approximately axial-planar to the folds. Cornwall.
  • IMG 6038 Banded metasediments at Porthleven, Cornwall. Note how a new striping, horizontal in the photograph, has developed parallel to a cleavage in the fine-grained layers. This shows how local chemical transfer by dissolution of material in cleavage zones can lead to a new compositional banding in low-grade metasediments.
  • IMG 5318 Sandstone layer with a spaced cleavage at a high angle to bedding. Quartz veins follow this new cleavage. Quartz vein systems are common in low-grade rocks. Loe Bar, Cornwall.
  • IMG 6042 First signs of metamorphic porphyroblasts (albite) in fine-grained metasediments at Porthleven, Cornwall.
  • IMG 3854 Low-grade metagreywacke, Western Alps, cut by several generations of quartz + feldspar veins.
  • IMG 3993 Foliation surface surface of slate, Western Alps. The white mineral is pyrophyllite (allegedly). Field of view ca. 5 cm.
  • IMG 4604 Greenschist-facies phyllite, showing muscovite-rich foliation surfaces spotted with small albite porphyroblasts, and crossed by kink-style microfolds. SW British Columbia, Canada.
  • met1a Hand specimen of black slate, showing pyrite cubes and a crenulation on the foliation surfaces. The original rock was rich in both organic material (now graphite) and sulphide (originally disseminated).
  • met2b Hand specimen of chlorite-muscovite phyllite, folded and containing foliation-parallel quartz veins. Note also the fine crenulation in the foliation surface, running at an angle across the fold axis.
  • IMG 6367a Limestone breccia, deformed so that original clasts have been flattened and stretched, and subsequently folded. Polished slab in stone merchant's yard.
  • IMG 6357 Strongly-deformed marble: light and dark bands derived from variable purity of the original carbonate, cut by slightly discordant veins of coarse white calcite, some asymmetrically folded. Polished slab in stone merchant's yard.
  • IMG 3428 Augen gneiss formed from a granitic rock, by ductile deformation at low metamorphic grade. K-feldspar phenocrysts in the original granite survive as eye-shaped lenses aligned in a foliation defined by greenschist-facies minerals. Saas-Fee, Switzerland.
  • c00-01 Metabasic pillow lavas near Levant Mine, West Cornwall. Lower greenschist facies regional metamorphism overprinted by contact metamorphism from the Lands End Granite, but original structures still visible.
  • IMG 5429 Top half of a metabasaltic pillow at St Ives, Cornwall. The rock has a greenschist-facies mineral assemblage but you can still distinguishoriginal features: the pale pillow rind below the tape measure, a zone of larger vesicles near the pillow margin; the pillow centre at the bottom, with smaller vesicles; dark shaly material between pillows, right and left of the tape measure.
  • IMG 5944 Metagabbro vein cutting serpentinite of the Lizard ophiolite at Kennack Sands, Cornwall. The igneous assemblage has been replaced by metamorphic minerals but in the absence of penetrative deformation the igneous texture is preserved.