Sustainable Development Goals

The diversity and evolution of late-Archean granitoids: Evidence for the onset of “modern-style” plate tectonics between 3.0 and 2.5 Ga

In general, Archean rocks exhibit rather ordinary moderate-P-high-T facies series metamorphism; neither blueschists nor any record of deep continental subduction and return are documented. However, the abundance and scale of ultrahigh-temperature (UHT) metamorphic belts from the Neoarchean to the Cambrian imply a significant change in geodynamics during the Neoarchean Era, after which transient sites of high heat flow were available at intervals throughout this period of Earth evolution. Many Neoproterozoic-Cambrian UHT metamorphic belts appear to have developed in settings analogous to modern backarcs that were closed and inverted during crustal aggregation and formation of the Gondwana supercontinent. If backarcs were the general setting for UHT metamorphism, then on a hotter Earth the cyclic formation of supercratons (in the Neoarchean Era) and supercontinents (in the Proterozoic Eon) required the destruction of oceans floored by thinner lithosphere that may have generated hotter backarcs than those as...

Metamorphic Conditions in Orogenic Belts: A Record of Secular Change

Nickel-copper sulfide ore deposits are found at the base of mafic and ultramafic bodies. All their host rocks, except the Sudbury Igneous Complex, are thought to be mantle derived melts. The Sudbury Igneous Complex is thought to be the product of complete melting of continental crust.
In the case of mantle-derived magmas, a high degree of partial melting of the mantle serves to enrich the
silicate magma in Ni and platinum group elements (PGE). This magma must then be transported to the crust
by an efficient process in order to reduce the possibility that Ni is removed from the magma by crystallization of olivine. Once the magma is emplaced into the crust, S from some source must be added to bring about saturation of the base metal sulfide liquid. An ideal site for all of these processes is where a mantle plume intersects a continental rift. The plume provides a large volume of magma, produced by a high degree of partial melting. The normal faults of the rift provide easy access to the crust so that the magma is transported efficiently.
In many cases rifts contain sedimentary rocks rich in S, thus providing an ideal source of S for sulfide
saturation. The heat from the plume can lead to melting of a large volume of rift sediments and release of S from the sediments to the Ni-PGE–rich primary magma. In the case of the Sudbury Igneous Complex, a very large volume of superheated magma formed by flash melting of the crust. This melting event was the result of the impact of shock waves from the explosion of a large meteor in the atmosphere.
In both the case of mantle-derived magma and the case of the Sudbury Igneous Complex, once sulfide liquid formed as suspended droplets in the silicate magma it must have interacted with a large volume of mafic magma in order to become enriched in Ni, Cu, and PGE in the sulfide. This enrichment occurred when the droplets were transported or when they were suspended in eddies. The magma from which the Sudbury Igneous Complex formed was superheated and base metal sulfide liquid formed at approximately 200°C above the magma liquidus. Thus the Sudbury sulfide liquid had more time to equilibrate with the silicate magma than
a sulfide droplet in a mantle-derived magma. This extra time and the huge volume of silicate magma in the melt sheet allowed the sulfide liquid to maximize the concentration of Ni, Cu, and PGE. The efficiency of the
metal collection step in the case of the Sudbury Igneous Complex counterbalanced the fact that this crustalderived
magma had lower Ni and PGE contents than most mantle-derived magmas.
The sulfide droplets collected at the base of intrusions and lava flows because they are denser than the silicate
magma. The largest concentrations are typically found in locations where there are changes in the geometry
of the contacts between intrusions or flows and the country rock. In some cases the accumulated sulfide
liquid fractionated to form an Fe-rich monosulfide solid-solution (mss) cumulate and a Cu-rich sulfide liquid
which later crystallized as an intermediate solid solution (iss). As a result of crystal fractionation of mss many Ni sulfide orebodies show a strong zonation with respect to Cu and PGE. During mss fractionation Os, Ir, Ru, and Rh concentrated in the mss cumulate and Cu, Pt, Pd, and Au concentrated in the Cu-rich sulfide liquid.
The partition coefficient for Ni into mss is close to 1; thus, mss fractionation would not have caused large variations in Ni concentrations. The silicate magma solidified at or above 1,000°C whereas the Cu-rich sulfide liquid solidified at ~900°C. Thus, at many localities the Cu-rich sulfide liquid appears to have migrated into dilatent spaces in the footwall or the hanging wall to form veins that extend into the country rock for up to 2 km.
At subsolidus temperatures a number of processes modify the orebodies. Both the mss and iss are not stable
below 600°C. As the sulfides cooled mss exsolved to form pyrrhotite and pentlandite (±pyrite), and iss exsolved
to form chalcopyrite and pyrrhotite (±cubanite, ±pyrite). Most of the PGE and chalcophile elements that originally partitioned into mss or iss are not readily accommodated in the structure of pyrrhotite, pentlandite, and chalcopyrite; therefore, they exsolve from the mss and iss at low temperature and form a wide variety
of platinum group minerals (PGM).
During deformation stress may focus in the structurally incompetent massive sulfide units, which are generally
located at the lower contact of the mafic or ultramafic host rock. In this situation the massive sulfides
may then be displaced relative to the host rocks. Finally, during greenschist to amphibolite metamorphism,
olivine is unstable and Ni released from the olivine will partition into disseminated sulfides, thereby upgrading
the sulfides

Formation of Magmatic Nickel Sulfide Deposits and Processes Affecting Their Copper and Platinum Group Element Contents

http://www.i-scholar.in/index.php/JGSI/article/download/70750/61283

Records of the Geological Survey of India

Eclogite facies metabasites from the Paleoproterozoic Nyong Group, SW Cameroon: Mineralogical evidence and implications for a high-pressure metamorphism related to a subduction zone at the NW margin of the Archean Congo craton

L'intrusion ultramafique de Jinchuan, dans la partie occidentale de la Chine, a une expression allongee en surface, environ 6000 par 350 metres. C'est un parmi plusieurs massifs intrusifs de composition mafique a ultramafique dans le socle faille en emergence de Longshoushan, d'âge proterozoique, situe en bordure sud-ouest de la plateforme sino-koreenne. Trois trous de forage, representatifs des parties ouest, centrale et est de l'intrusion, ont ete echantillonnes. Les roches sont des cumulats a olivine -orthopyroxene - spinelle chromifere, avec orthopyroxene, clinopyroxene, plagioclase et phlogopite dans les interstices. En moyenne, les sulfures des metaux de base (pyrrhotite, pentlandite et chalcopyrite) constituent 5% (poids) des roches. Les roches ont subi les effets d'un metamorphisme de faible intensite, une alteration hydrothermale et une oxydation pres de la surface. Une fraction mineure du spinelle chromifere et de l'orthopyroxene ont precede l'olivine dans la sequence de cristallisation. L'olivine varie entre Fo 7 9 et Fo 8 5 . Cette variation montre une correlation negative avec la quantite de liquide silicate piege dans ces roches, et serait due plutot aux effets subliquidus et metamorphiques. Les structures lithologiques, complexes et fortement variables, la variation limitee de la teneur de l'olivine en Fo, et la taille des grains, font penser que l'intrusion s'est mise en place par injection d'une emulsion visqueuse de cristaux + liquide a limite elevee d'elasticite, contenant des proportions variables de liquides sulfure et silicate (composition basaltique) dans une fissure ou une faille. Au cours de la mise en place de cette emulsion a Jinchuan, une differenciation par epanchement a regi la partie centrale, proximale, de l'intrusion, ce qui a mene a une zonation subconcentrique des unites lithologiques dans ce regime, tandis qu'un epanchement plus chaotique etait important dans les parties plus distales, a l'est et a l'ouest, avec comme resultat un rubanement lithologique et une interdigitation. La mise en place originale et les structures d'epanchement ne se sont pas effondrees, a cause de la viscosite et le seuil d'elasticite eleves de ces emulsions, suffisamment fortes pour contrecarrer les forces dues a la gravitation imposees a mesure que cessait l'epanchement. D'apres la sequence de cristallisation observee dans ces roches, les emulsions de cristaux se sont formees dans une chambre transitoire a une profondeur entre 4 et 9 km. Dans cette chambre, le magma a faible teneur en olivine s'est accumule vers le sommet, et les emulsions a olivine, contenant orthopyroxene et spinelle chromifere accessoires, et des quantites variables de liquides silicate et sulfure, se sont accumulees vers la base a cause de la gravite. Les materiaux contenant liquides + cristaux stratifies ont par la suite subi une extrusion hors de cette chambre par compression, et une injection dans une fracture ou une faille a un niveau superieur, pour former une serie d'intrusions mafiques et ultramafiques, possiblement dues a l'effondrement du toit. Un tel effondrement a probablement ete declenche par une extension regionale. L'extraction precoce du magma a faible teneur en cristaux de la partie superieure de la chambre est responsable des roches a faible teneur en sulfures pres de l'intrusion de Jinchuan. L'extraction subsequente de fractions enrichies en sulfures pres de la base de cette chambre a forme l'intrusion de Jinchuan. Le rapport relativement eleve de sulfures a silicates est le resultat d'une preconcentration de la fraction sulfuree par stratification de la fraction liquide dans la chambre transitoire.

Emplacement of viscous mushes in the jinchuan ultramafic intrusion, western china

The geochronology of the Limpopo Complex: A controversy solved

Al-Mg granulites, with cordierite, garnet, sapphirine, orthopyroxene, sillimanite, spinel, phlogopite, K-feldspar, plagioclase and variable quartz from Ihouhaouene (In Ouzzal, Algeria), display a range of decompression textures involving the breakdown of orthopyroxene and sillimanite, and of garnet. The succession of parageneses suggests that the P–T–t evolution corresponds to decompression with cooling from peak conditions of about 950°C and 10 kbar. This decompression path is obtained from the paragenetic analysis in the FMAS system. However, according to current KFMASH grids, this P–T–t evolution should take place outside the stability field of phlogopite+quartz; yet this assemblage is probably stable during most of the P-T evolution, notably during peak metamorphism. This discrepancy is interpreted as the effect of the high content of F in phlogopite which should shift its stability limit towards higher temperature. The consequences of this shift on the phase relationships in the KFeMASH system are investigated and it is concluded that a topological inversion could exist in the F-bearing system.

Petrology of phlogopite‐sapphirine‐bearing Al‐Mg granulites from Ihouhaouene, In Ouzzal, Hoggar, Algeria: an example of phlogopite stability at high temperature

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Tectono-metamorphic evolution and timing of the melting processes in the Svecofennian Tonalite-Trondhjemite Migmatite Belt: An example from Luopioinen, Tampere area, southern Finland

Hassina Mouri

Papers

Eclogite facies metabasites from the Paleoproterozoic Nyong Group, SW Cameroon: Mineralogical evidence and implications for a high-pressure metamorphism related to a subduction zone at the NW margin of the Archean Congo craton

Emplacement of viscous mushes in the jinchuan ultramafic intrusion, western china

The geochronology of the Limpopo Complex: A controversy solved

Petrology of phlogopite‐sapphirine‐bearing Al‐Mg granulites from Ihouhaouene, In Ouzzal, Hoggar, Algeria: an example of phlogopite stability at high temperature

Tectono-metamorphic evolution and timing of the melting processes in the Svecofennian Tonalite-Trondhjemite Migmatite Belt: An example from Luopioinen, Tampere area, southern Finland