pelitic

Pelites are metamorphic rocks derived from protoliths rich in aluminum (shales and turbidite muds). Metamorphic rocks with sandstone parents are termed quartzo-feldspathic. Whereas pelites are derived from muds or clays, psammites are derivatives of micaceous sandstones. 'In unmetamorphosed turbidite sequences, psammite layers consist predominantly of quartz and feldspar, and pelite layers consist predominantly of phyllosilicates, quartz and feldspar. The predominance of relatively weak phyllosilicates in unmetamorphosed pelite units makes them weaker than interlayered psammite units during layer-parallel shearing at low to intermediate temperatures. However, during prograde metamorphism of pelitic rocks, the relative abundance of phyllosilicates can decrease due to the increased abundance of effectively rigid porphyroblasts, which leads to changes in the relative strengths of pelitic and psammitic rocks.'[pdf] Mineral assemblages in pelites can be employed as indicators of P, T, or PT ratios: andalusite low P metamorphism / kyanite high P / sillimanite high T (first grows as fine-grained acicular fibrolite) / chlorites low T / biotite moderate T / garnets moderate T / staurolite moderate-high P / chloritoid moderate-high P / cordierite low P / paragonite high P / see chart of stability fields of pelitic minerals.

links: images: hand-specimens: pelite, 2; psammite: psammite interveined with calcite and galena; psammite, laminated, Grampian Group; close-ups: deformed psammite; formations: pelites: contact between pelite and marble; alternating layers of pelite (mud) and psammite (sand); metapelite from Namaqualand with compositional layering that represents former graded bedding, the neosomes developed around large pre-existing garnets in the more aluminous layers; metapelitic rock (Damara belt, Namibia), pink garnet-bearing neosomes are arranged as if they occupy or nucleate around tensional fractures; Wechsellagerung of distal turbidite and dark grey pelite, 2, Portugal, German (Engl); Folded limestone and pelite, Otter Lake, Quebec; Ailnack gorge - black schists (graphitic pelite to semi-pelite); Burrator Quarry exposing the irregular granite-pelite contact; folded pelite; Folded limestone and pelite, Otter Lake, Quebec; migmatitic semipelite in Ruthven Semipelite Formation, Grampian Group, Scotland; Psammite/pelite turbidites of the Longstaff Bluff Formation, 2; psammites: climbing ripple lamination (mica) in a psammite (quartzose bed); psammite with thin pelite (Claudy Formation), Kildoag Quarry, Co. Londonderry; amphibolite grade psammite with well preserved cross bedding, Gaick Psammite Formation, Grampian Group, and recumbent folds in Gaick Psammite Formation; Deeply weathered psammite, Gaick plateau; sheared psammites at Portvasgo; shear fold developed in Moine psammites; fold pair in highly sheared Moine psammite; undeformed; highly flattened conglomerate within the Moine metasedimentary sequence; webpages: Pelitic Rocks; related: pelite axehead