Stage Of Regional Metamorphism Research Articles

The Age of Gold Mineralization of the Unglichikan Deposit (Russian Far East): Results of 40Ar/39Ar Dating

Abstract —The isotope-geochronological studies of ore metasomatites from the Unglichikan gold deposit have shown their age of 136–140 Ma. There are no data on magmatism of this age within the study area; therefore, it is impossible to associate the ore mineralization of the Unglichikan deposit with magmatic processes. The thermal event superposed on the host rocks of the Zlatoustovsk Formation beyond the ore zone is dated at 140 ± 2 Ma. Thus, the last stage of regional metamorphism and deformation and the formation of ore metasomatites are of the same age. We believe that the orogen deformation processes accompanied by hydrothermal activity played a significant role in the mobilization and redistribution of ore matter and in the formation of the Unglichikan deposit.

P-T path of metamorphism and U-Pb monazite and zircon age of the Kitoy terrane: Implication for Neoarchean collision in SW Siberian Craton

The first data on P-T metamorphic conditions coupled with U-Pb monazite and zircon age obtained for the Neoarchean Kitoy granulite-gneiss terrane (SW Siberian Craton). Alumina gneisses of the Kitoy terrane indicate two-staged metamorphic evolution. The first stage of regional metamorphism (M1) occurred at high-amphibolite facies conditions at T ​= ​780–800 ​°C and P ​= ​8–9 ​kbar. The second stage (M2) belongs to MT-HT/LP type of metamorphism with the wide temperature interval 600–750 ​°C and pressure 2–4 ​kbar. Two age peaks were established on the basis of U-Pb monazite and zircon dating in garnet-anthophyllite gneisses. Both of them correspond to the Neoarchean age: the age of M1 falls into the interval of ca. 2489–2496 ​Ma, the age of M2 – ca. 2446–2456 ​Ma. The high-temperature metamorphism of the Kitoy block and nearly coeval granitoid magmatism can be an evidence for the Neoarchean collision in SW Siberian craton.

Regional, Contact Metamorphism, and Autometamorphism of the Olkhon Terrane (West Baikal Area)

The rocks of the Olkhon terrane experienced high-grade regional metamorphism reaching granulite facies. In addition to regional metamorphism, they were subjected to contact metamorphism and autometamorphism (mafic dikes). Three types of regional metamorphism have been distinguished. A terrane collage north of the Orso Zone revealed two stages of regional metamorphism: early-stage granulite metamorphism (Т = 750–900°С, Р = 8–9 kbar, around 500 Ma) and late-stage amphibolite and low–amphibolite facies (Т = 550–710°С, Р = 4–6 kbar, 460–470 Ma). The Orso microterrane separating the Krestovsky island-arc subterrane from a collage of other microterranes contains high-pressure mineral assemblages (up to 10 kbar), which are characterized by the presence of high-calcium garnet in metapelites. The Orso microterrane and Krestovsky subterrane are characterized by relatively high-pressure path of metamorphism. An extended zone of contact high-temperature metamorphism related to the emplacement of gabbro of the second phase of the Ustkrestovsky Complex has been mapped in the Krestovsky subterrane. The temperature of formation of two-pyroxene hornfels after metaporphyrites of the Birkhin volcanoplutonic association is estimated at 750–850°С. Beerbachites were formed by autometamorphism of subvolcanic mafic bodies, which compose the northern part of the Tazheran composite massif (syenite, Ne-syenite, subalkaline gabbro), dikes in the Birkhin gabbro massif, as well as dikes and separate blocks in marble melange. The temperature of autometamorphism during formation of beerbachites is estimated from two-pyroxene geothermometer as 700–1000°С.

Nanodiamond Formation at the Lithogenesis and Low-Stages of Regional Metamorphism

Samples of gilsonite from Adzharia, anthraxolite and graphite of coal from Taimyr, shungite from Karelia, and anthracite from Donbass are studied using Raman spectroscopy. Peaks at 1600 cm−1, indicating the presence of nanographite, are recorded in all samples. The anthracite sample from Donbass, 1330 cm−1, corresponds to the sp3-line of carbon hybridization conforming to a nanodiamond. It is concluded that in nature diamonds can be formed at late stages of lithogenesis (catagensis, metagenesis), and for coals, it can occur at the zeolite stage of regional metamorphism of rocks, before the green schist stage.

The first data on U–Pb (SHRIMP) dating of zircon from metamorphic rocks of the crystalline base of the Taigonos-Paren Uplift (southern margins of the Omolon Massif)

U–Pb SHRIMP dating of zircons from metamorphic rocks of the crystalline basement of the Omolon-Taigonos area (northeastern margins of Asia) has been used to determine the periods of early endogenic activity of the crust. These periods correlate to the time of formation of the magmatic rock protolith (3.2–3.3 Ga), the stage of regional metamorphism (2.6–2.8 Ga), and the superimposed granitization. The resulting data can be used to reconstruct the history of development of the crust in the Precambrian.

Age, sources, and provenances of protoliths of metasedimentary rocks of the Dzheltulak group, Dzheltulak suture

The results of Sm–Nb isotopic–geochemical studies of metasedimentary and metavolcanic rocks of the Dzheltulak Group of the central part of the Dzheltulak suture, as well as geochronological U–Th–Pb (LA ICP MS) studies of detrital zircons from metasedimentary rocks, which are considered as Paleoproterozoic in current stratigraphic schemes, are presented. The age of the youngest zircons is 170–190 Ma, whereas the age of the last stage of regional metamorphism is 140–150 Ma. Thus, the Dzheltulak Group hosts metasedimentary rocks, the age of the protolith of which ranges from 140–150 to 170–190 Ma. The detrital zircons derived from intrusive and metamorphic rocks of the Selenga–Stanovoi and Dzhugdzhur–Stanovoi superterranes.

Metamorphic-Hydrothermal Ag–Pd–Pt Mineralization In the Noril'sk Sulfide Ore Deposit, Siberia

Abstract The 251 Ma Noril9sk magmatic sulfide ores, together with igneous rocks of trap association and underlying sequences, were affected by three stages of regional metamorphism under conditions of early zeolite, middle prehnite-pumpellyite, and late zeolite facies. The Rb-Sr age (apophyllite, calcite, metabasalts) of metamorphism varies from 232 Ma (onset) to 122 Ma (completion). The metamorphosed ores contain remobilized metamorphic-hydrothermal silver mineralization (Au-free silver, Hg-rich silver, Ag sulfides and selenides, chalcopyrite–lenaite solid solution, Ag-rich bornite, argentopentlandite), palladium (Sb-free paolovite, Ag-rich paolovite, vysotskite, palladoarsenide, vincentite, Pd-breithauptite, malyshevite, palladium, etc .), and platinum (kharaelakhite, cooperate, platinum). A metamorphic trend of dearsenization is outlined: initial majakite PdNiAs → palladoarsenide Pd 2 As → vincentite Pd 3 As → palladium Pd. The mineral assemblages and compositions of metamorphic-hydrothermal PGM and Ag minerals markedly differ from pneumatogenic PGM, Ag, and Au–Ag minerals. The low-grade metamorphism of the Noril9sk sulfide ores was accompanied by appreciable mobilization of Ag, however mobilization of Pd, and especially Pt, was less intense, and no Au mobilization was seen at all.

Permo-Triassic and Paleoproterozoic metamorphism related to continental collision in Yangpyeong, South Korea

Gneisses and migmatites exposed in the Yangpyeong area in the northern Gyeonggi Massif provide insight into the Paleoproterozoic and Triassic metamorphic events in South Korea. Garnet–biotite gneiss and sillimanite–garnet–biotite gneiss in the western part of the area reveal Paleoproterozoic metamorphism (1888–1871 Ma) at P–T conditions of 760–820 °C and 8–10 kbar and 710–750 °C and 5–7 kbar, respectively. These rocks were overprinted by low-P/T type metamorphism (590–650 °C, 3–4 kbar) during the Triassic (ca. 237 Ma). In contrast, a cordierite-rich migmatite near the post-collisional Triassic igneous complex in the eastern part of the area was strongly metamorphosed during the Triassic (ca. 235 Ma) at 750–790 °C and 7–8 kbar. The similar Triassic ages in the western and eastern areas suggest that low-P/T type metamorphism occurred as a second stage of regional metamorphism, which is characterized by the production of cordierite with an irregularly shaped garnet. The metamorphic grade of the Triassic metamorphism decreases spatially towards the west from granulite facies to amphibolite facies metamorphic conditions, and the Paleoproterozoic metamorphism is well preserved in the western part with low grade Triassic metamorphism. The new discovery of the Triassic metamorphic event in the Yangpyeong area, in addition to the previously reported Triassic post-collision igneous event, supports the idea that the continental collision belt between the North and South China blocks extends from the Hongseong area into the Odesan area through the Yangpyeong area in South Korea.

The age of the last episode of the Precambrian regional metamorphic event in south Ulutau (Central Kazakhstan): Results of U-Pb geochronological studies of granites from the Aktas complex

The basement of the south Ulutau sialitic massif, which is located in the western part of Central Kazakhstan, comprises metamorphized volcanogenic-sedimentary and plutonic complexes of Proterozoic age. The upper boundary of the metamorphism age corresponds to the age of nonmetamorphized syenites from the Karsakpai massif (673 ± 2 Ma, Late Riphean). U-Pb geochronological studies of accessory zircon were made, and a Late Riphean age of biotite alkali granite from the Aktas massif (the youngst metamorphic Precambrian igneous units in South Ulutau) was found. The obtained age estimate of 791 ± 7 Ma can be considered as the lower age limit of metamorphism. Thus, the last stage of regional metamorphism in South Ulutau took place in the second half of the Late Riphean, in the time interval of 790–670 Ma.

Iron mobility during synmetamorphic alteration in the Selwyn Range area, NW Queensland: implications for the origin of ironstone-hosted Au-Cu deposits

The Proterozoic metamorphic rocks of the Selwyn Range area in western Queensland contain a variety of styles of Au + Cu deposit including some (e.g. Selwyn mine and Osborne prospect) that are spatially associated with magnetite-rich ironstones. The area is characterized by extensive alkali-(chloride) metasomatism that occurred during the later stages of regional metamorphism. Existing experimental data indicate that hot saline fluids will dissolve and transport iron as they move through temperature/pressure gradients in the vicinity of the amphibolite-greenschist facies transition and predict that circulation cells will move iron and concentrate it in relatively cool, dilute and/or oxidized parts of such systems. New geological mapping coupled with geochemical data from the amphibolite facies zone of the Selwyn Range area suggest such processes occurred on a large scale and that large masses of Fe were mobilized, particularly in association with Na-metasomatism of biotite-bearing metasediments and high Fe-tholeiitic dolerites. Field and textural features of ironstones and other Fe-rich lithologies demonstrate that some of them at least formed by replacement of other rocks during metamorphism and deformation. Mineralization localized within such iron stones must itself be epigenetic.

Contact aureoles as constraints on regional P‐T trajectories: an example from the Northern Alabama Piedmont, USA

Abstract Contact aureoles of plutons emplaced into regionally metamorphosed terranes can provide indicators of physical conditions along a portion of regional metamorphic P‐T trajectories, thereby allowing reconstruction of more complete P‐T loops than would be otherwise possible. In the Northern Alabama Piedmont of the southern Appalachians, Wedowee Group metapelites preserve evidence for two regional metamorphic phases overprinted by contact metamorphism adjacent to the Blakes Ferry Trondhjemite. Textural evidence indicates that an early bt+st+grt assemblage was replaced by bt+chl+grt during the latter stages of regional metamorphism. Changes in AKFM topology, complex Fe‐Mg‐Ca garnet zoning, and the latestage appearance of epidote indicate that a sequence of continuous reactions (bt+st = grt+ ms followed by chl+ms+Ca‐grt+Ca‐pl=bt+ (Fe+Mg)‐grt+ep) occurred in response to increasing pressure and resulted in the observed changes in mineral assemblage. Pl‐ms‐bt‐grt thermobarometry indicates conditions of 580° 65°C, 8.5±0.8 kbar for equilibration of grt+ bt+chl. Pluton emplacement, subsequent to penetrative deformation, caused textural annealing and mineral re‐equilibration by the continuous reaction bt+(Fe+Mg)‐grt+ep = chl+ms+Ca‐grt+Ca‐pl within 50 m of the pluton. Conditions of 510±65°C, 5‐7 kbar are inferred. A reconstructed P‐T trajectory for this area is characterized by (1) early moderate‐T, moderate‐P metamorphism; (2) an increase in P to approximately 8.5 kbar; and (3) decompression and slight cooling prior to pluton emplacement. The compressional phase of this path is interpreted to result from underthrusting of the Wedowee metasediments to mid‐crustal levels during Palaeozoic crustal thickening. Late‐stage decompression prior to intrusion records uplift of these rocks in response to movement on structurally lower thrusts.

Kyanite-eclogite to amphibolite fades evolution of hydrous mafic and pelitic rocks, Adula nappe, Central Alps

In the southern Adula nappe (Central Alps), two stages of regional metamorphism have affected mafic and pelitic rocks. Earlier eclogite facies with a regional zonation from glaucophane eclogites to kyanite-hornblende eclogites was followed by a Tertiary overprint which varied from greenschist to high-grade amphibolite facies. Despite a common metamorphic history, contrasting equilibration conditions are often recorded by high-pressure mafic eclogite and adjacent predominantly lower-pressure pelite assemblages. This pressure contrast may be explained by different overprinting rates of the two bulk compositions during unloading. The rates are controlled by a mechanism in which dehydrating metapelites provide the H2O required for simultaneous overprinting of enclosed mafic eclogites by hydration.

An Unusual Gabbroic Intrusion from Northern Rhodesia

AbstractGabbro is intruded near the contact of gneiss and schist groups in the Southern Province of Northern Rhodesia. The petrography of normal and altered rocks is described. The rocks may have originated as an intrusion into an original reversed drag fold which it has emphasized and distorted. Intrusion took place at a late stage of regional metamorphism and folding.

Present Activity and Petroleum Possibilities in the Arkansas Valley: ABSTRACT

During the past year the Arkansas Valley of west-central Arkansas, which has produced dry gas from the Atoka formation (Pennsylvanian) for the past fifty years, has had its most intense period of activity. An estimated one million acres of leases have been taken and at least six oil companies now have surface parties working in the area. This new activity is the result of a combination of factors: (1) new and better markets for gas, (2) new discoveries with initial potentials of as much as seventy million cubic feet per day from the Atoka formation, and (3) recognition of the possibilities for production from pre-Atoka beds following discoveries of gas in the Hale formation of the Morrow group (Pennsylvanian). This last factor was extremely important as it had previously been considered by many that the pre-Atoka sands had been silicified in the first stage of regional metamorphism, and therefore they could not be considered as potential reservoir rocks. There are now at least two gas fields, Cecil and Clarksville, producing from the Morrow group. The Atoka gas possibilities are great in the area, which contains roughly fifty untested anticlines. Tests on at least three of these structures are either staked or drilling at the present time. The oil possibilities are still a question mark. In the Morrow and pre-Boone Mississippian outcrops to the north, there are three well-developed sandstone sections (Hale, Wedington, and Batesville), all potential reservoir rocks in the Valley. An oolitic member of the Boone limestone (Osage, Mississippian) is also present on the outcrop and may have porosity in parts of the region, as may the basal Mississippian sand, where present. In the less than a dozen scattered wells in the region that cut the older rocks, the Hale sandstone is the only formation in which appreciable porosity was found. This information is too fragmentary to properly evaluate the productive possibilities of the section involved. Eight wells have tested the Wilcox-St. Peter (Ordovician) section in the region, but in each case the porosity present at the outcrop was absent in the subsurface. These wells also tested the Silurian-Devonian section, which is cut out at many places on the outcrop by unconformities, but again, porosity was missing. End_of_Article - Last_Page 1674------------