Sediment metamorphosed

The basement complex for the Patagonian desert and arid northeastern Brazil is formed by metamorphosed Precambrian rocks. Landscapes are characterized by level erosion surfaces of different ages. The landscape is dissected by a large number of valleys. Large depressions are filled with marine and continental beds of sedimentary rocks. Rocks in the Andean system, that stretches the entire length of the west side of the continent, vary greatly. Many depressions are filled with sediments. In addition, many active volcanoes are responsible for periodic lava flows and the deposition of volcanic ash. East of the Andes, the land surface is level and slopes towards the Atlantic Ocean. Broad depressions contain saline or sodic soils. 

Caro G, Bourdon B, Birck JL, Moorbath S (2003) Sm- Nd evidence from Isua metamorphosed sediments for early differentiation of the Earth s mantle. Nature 423 428-432 Chen JH, Wasserburg GJ (1981) The isotopic composition of uranium and lead in Allende inclusions and meteoritic phosphates. Earth Planet Sci Lett 52 1-15... 

Three MER diagrams (Figs. 2, 3, 4) collectively illustrate the mineralogical controls observed in each of the Meguma Supergroup formations. Because these metamorphosed rocks derive from proximal and distal flysch sediments, they likely once contained quartz, K-feldspar, albite, muscovite, illite, smectite (montmorillonite-beidellite), chlorite... 

Almost all larvae that survived to day 15 eventually 32 metamorphosed successfully to postlarvae No deaths of larvae in 22 days when sediments 34... 

The plant precursors that eventually formed coal were compacted, hardened, chemically altered, and metamorphosed by heat and pressure over geologic time. It is suspected that coal was formed from prehistoric plants that grew in swamp ecosystems. When such plants died, their biomass was deposited in anaerobic, aquatic environments where low oxygen levels prevented their reduction (rotting and release of carbon dioxide). Successive generations of this type of plant growth and death formed deep deposits of unoxidized organic matter that were subsequently covered by sediments and compacted into carboniferous deposits such as peat or bituminous or anthracite coal. Evidence of the types of plants that contributed to carboniferous deposits can occasionally be found in the shale and sandstone sediments that overlie coal deposits. 

Models for the formation of Precambrian sediments suggest that the chemical sediments (such as cherts) of the Isua supracrustal belt have formed as shallow water deposits. This is in agreement with structures locally preserved in the metacherts of the sequence. After deposition, the supracrustals were folded and metamorphosed. Finally, the metamorphism reached lower amphibolite facies and in consequence, most of the primary minerals became recrystallized. As a result all chert now appears as quartzite. But apparently metacherts, magnetite iron formation and quartz carbonate rocks have retained their major element chemistry largely unaltered during metamorphism (Nutman et al., 1984) 119). 

More extreme subterranean processes (especially heat) cause substantial changes in the sediments and lead to the formation of metamorphic rock. Thus, marble is heat metamorphosed limestone and slate is pressure metamorphosed shale. 

The BIF mainly are metamorphosed chemogenic sediments which were normal deposits in ancient geosynclinal zones. 

The association hematite -I- iron silicate, encountered in sediments, is not characteristic of metamorphosed iron formations. 

Typical BIF are metamorphosed chemogenic cherty iron sediments, deposition of which was typical only of the Precambrian and ceased in subsequent geologic epochs. The formation of these rocks reflects a certain stage in the irreversible process of evolution of the Earth s crust, atmosphere, hydrosphere and biosphere. The period of intensive deposition apparently was preceded by a long period of accumulation of dissolved iron and silica in the waters of the original sedimentary basins. 

Wollastonite is common in thermally metamorphosed impure limestone and can occur in contact-altered calcareous sediments. In most occurrences, it has been formed as a result of the reaction... 

Ague J. J. (1997b) Compositional variations in metamorphosed sediments of the Littleton Formation, New Hampshire (Discussion). Am. J. Sci. 297, 440-449. 

Perry E. C., Jr. and Ahmad S. N. (1977) Carbon isotope composition of graphite and carbonate minerals from the 3.8-AE metamorphosed sediments, Isukasia, Greenland. Earth Planet. Sci. Lett. 36, 281-284. 

Schoenberg R., Kambeer B. S., Collerson K. D., and Moorbath S. (2002) Tungsten isotope evidence from 3.8-Gyr metamorphosed sediments for early meteorite bombardment of the E th. Nature 418, 403 -405. 

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