November 10, 2005 Handout: Sedimentary Rocks Beginning of metamorphic petrology: sedimentary protoliths Sedimentary protoliths Siliclastics – by size, categorized as conglomerates, sandstones, or shales; by chemistry, categorized as graywackes, lithic sandstones, arkoses (“red beds”), or quartzites Carbonates + Evaporites Shales are the dominant type of sedimentary rock on earth (70% of sedimentary rock mass), full of aluminous clays Diagenesis – chemical alteration of sediment at low T See handout for background on sedimentary rocks. Metamorphic rocks Oldest crustal rocks: Acasta granitic gneisses 4.0 B.y. old Isua greenstone belt (granite gneisses, metaboninites) 3.8 B.y. old Focus on solid state changes in rocks Metaigneous, metasedimentary Interactions with H2O or CO2 rich fluids Concept of isochemical reactions (not correct) Metasomatism – adding or removing elements by action of metasomatizing agent Å fluid Concept of metamorphic grade Low T – diagenesis 7 in between, metamorphism High T – melting (anatexis) – melting T depends on protolith melting shales 600-700 ºC basalts 600-900 ºC marble 1000-1300 ºC Types of metamorphic environments • Contact metamorphism – near igneous plutons, result of heat of intrusive magma Sometimes intrusions are fluid-rich Æ set up hydrothermal circulation systems that also influence the surrounding rocks Skarns – calcite and qtz protoliths, calc-silicate minerals Fluids carry metals, make ore deposits• Shock metamorphism – meteorite impacts impose very high P-T conditions Example minerals are stishovite (VI fold coordination), wadsleyite (sorosilicate), ringwoodite (spinel) • Regional metamorphism o Burial metamorphism – P + T imposed by burial o Anorogenic metamorphism – burial in sed. Basins o Orogenic – deformation @ plate margins, subduction zones and continent-continent collision • Pyrometamorphism – by fire, where coal seams burn underground Also possible to metamorphose rocks at lightning strikes Metamorphic variables T most important var, requires a heat source – in contact metamorphism, it’s intrusion, while in burial, it’s radioactive decay of elements like K, U, and Th P usually lithostatic pressure, ρgh Granite ρ = 2700 kg/m3 Basalt 3000 Peridotite 3300 Fluid Bulk composition of protolith – SiO2, Al2O3, K2O, FeO, MgO, H2O Pelites – quartzofeldspathic High P, low T – subduction zones High T, low P – ocean floor spreading