scholarly journals Mineral Chemistry and metamorphic evolution of the Late Neoproterozoic metabasites of Do-Chah metamorphic - igneous complex (SE Shahrood)

2020 ◽  
Vol 28 (2) ◽  
pp. 341-356
Author(s):  
Mojgan Rezaei ◽  
mahmood Sadeghian ◽  
habiballah Ghasemi ◽  
Papadopaelo Lambrini ◽  
◽  
...  
Keyword(s):  
Mineral Chemistry ◽  
Metamorphic Evolution ◽  
Igneous Complex ◽  
Late Neoproterozoic

scholarly journals Mineral chemistry and thermobarometry of the Late Neoproterozoic metabasites from Do-Chah metamorphic-igneous complex (SE Shahrood)

2020 ◽  
Vol 28 (1) ◽  
pp. 233-248
Author(s):  
Mojgan Rezaei ◽  
mahmood Sadeghian ◽  
habiballah Ghasemi ◽  
Papa Lambrini ◽  
◽  
...  
Keyword(s):  
Mineral Chemistry ◽  
Igneous Complex ◽  
Late Neoproterozoic

Sulfide mineral chemistry and platinum-group minerals of the UG-2 chromitite in the northern limb of the Bushveld Igneous Complex, South Africa

2021 ◽  
Vol 59 (6) ◽  
pp. 1339-1362
Author(s):  
Malose M. Langa ◽  
Pedro J. Jugo ◽  
Matthew I. Leybourne ◽  
Danie F. Grobler
Keyword(s):  
Platinum Group Element ◽  
Mineral Chemistry ◽  
Sulfide Mineral ◽  
Sulfide Minerals ◽  
Group Element ◽  
Igneous Complex ◽  
Northern Limb ◽  
Platinum Group Minerals ◽  
Platinum Group ◽  
Bushveld Igneous Complex

ABSTRACT The UG-2 chromitite layer, with its elevated platinum-group element content, is a key marker horizon in the eastern and western limbs of the Bushveld Igneous Complex and the largest platinum-group element chromite-hosted resource of its kind in the world. In contrast, much less is known about its stratigraphic equivalent in the northern limb, the “UG-2 equivalent” (UG-2E) chromitite. Recent studies on chromite mineral chemistry show similarities between the UG-2 and sections of the UG-2E, but also that the UG-2E was partially contaminated by assimilation of local metasedimentary rocks. Here, we provide a detailed characterization of sulfide minerals and platinum-group minerals in a suite of samples from the UG-2E and compare the results with data obtained from a reference suite of samples from the UG-2. Results from petrographic observations, electron probe microanalysis, laser ablation-inductively coupled plasma-mass spectrometry, quantitative evaluation of materials by scanning electron microscopy, and δ34S isotopes show that: (1) sulfide minerals in the UG-2E and UG-2 consist mainly of pentlandite-chalcopyrite-pyrrhotite, but pyrrhotite is significantly more abundant in the UG-2E and almost absent in the UG-2; (2) iron contents in pentlandite from the UG-2E are significantly higher than in the UG-2; (3) platinum-group element contents within sulfide minerals are different between the two chromitites; (4) UG-2E platinum-group minerals are dominated by arsenides and bismuthotellurides, and by alloys and platinum-group element-sulfide minerals in the UG-2; (5) sulfide mineral chemistry and δ34S values indicate some crustal contamination of the UG-2E; and (6) sulfide mineral and secondary silicate mineral textures in both the UG-2E and UG-2 are indicative of minor, millimeter- to centimeter-scale, hydrothermal alteration. From our observations and results, we consider the UG-2E chromitite in the northern limb to be the equivalent to the UG-2 in the eastern and western limbs that has been contaminated by assimilation of Transvaal Supergroup footwall rocks during emplacement. The contamination resulted in UG-2E sulfide mineral elemental contents and platinum-group mineral types and abundances that are distinct from those of the UG-2 in the rest of the Bushveld.

scholarly journals Metamorphic Evolution of Garnet-Bearing Ultramafic Rocks in the Hujialin Area, Sulu Ultrahigh-Pressure Orogenic Belt, Eastern China

Minerals ◽  
2020 ◽  
Vol 10 (3) ◽  
pp. 225
Author(s):  
Zeli Wang ◽  
Shukai Zhang ◽  
Xu-Ping Li ◽  
Songjie Wang ◽  
Dan Wang ◽  
...  
Keyword(s):  
Mineral Assemblage ◽  
Eastern China ◽  
Stage Iv ◽  
Mineral Chemistry ◽  
Orogenic Belt ◽  
Ultrahigh Pressure ◽  
Retrograde Metamorphism ◽  
Metamorphic Evolution ◽  
The North ◽  
Convex Upward

The Rizhao Hujialin area is located in the central Sulu ultrahigh-pressure orogenic belt, where garnet clinopyroxenite is exposed in the upper part of an ultramafic rock complex and serpentinized dunite is exposed in its lower part. Based on textural criteria, the garnet clinopyroxenites were divided into three types: Equigranular garnet, porphyroclastic garnet, and megacrystic garnet pyroxenites. The garnet clinopyroxenites have convex-upward chondrite-normalized rare earth element patterns, large positive Pb anomalies, and depletion of high-field-strength elements (e.g., Nb, Zr, and Ti), suggesting a mantle source protolith overprinted by fluid metasomatism. Petrographic, mineral chemistry, phase equilibrium modeling, and zircon U–Pb geochronology data show that the evolutionary stages of the Hujialin garnet clinopyroxenites were as follows: Stage I: formation of the magmatic protoliths; stage II: formation of megacrystic garnet pyroxenite accompanying subduction; stage III: formation of porphyroclastic or equigranular garnet clinopyroxenite with a mineral assemblage of garnet + clinopyroxene + ilmenite + humite accompanying initial exhumation at ~215.0 ± 5.7 Ma; stage IV = progressive cooling and decompression associated with the crystallization of water-bearing minerals such as clinochlore and pargasite at 206 Ma; and Stage V = late epidote amphibolite-facies retrograde metamorphism producing a mineral assemblage of garnet + clinopyroxene + amphibole + chlorite + epidote + ilmenite at ~180–174 Ma associated with fluid activity in shear–tensional fractures and/or pores. The P-T conditions of the peak metamorphism were estimated at 4.5 ± 0.5 GPa and 800 ± 50 °C. Retrograde metamorphism recorded conditions of 1.0 GPa and 710 ± 30 °C during the exhumation and cooling process. The mineral transformation from early high-Al clinopyroxene to garnet and to late diopside records the general metamorphic evolution during subduction and exhumation, respectively. One zircon U–Pb analysis presents the Palaeoproterozoic age of 1817 ± 40 Ma, which is coeval with widespread magmatic and metamorphic events in the North China Craton.

Precise U–Pb zircon ages from Alderney, Channel Islands: growing evidence for discrete Neoproterozoic magmatic episodes in northern Cadomia

2001 ◽  
Vol 138 (6) ◽  
pp. 719-726 ◽  
Author(s):  
R. S. D'LEMOS ◽  
B. V. MILLER ◽  
S. D. SAMSON
Keyword(s):  
Small Groups ◽  
Quartz Diorite ◽  
Channel Islands ◽  
Igneous Complex ◽  
Plutonic Complex ◽  
Northern Channel Islands ◽  
Cadomian Orogeny ◽  
Late Neoproterozoic ◽  
Magmatic Event ◽  
Deformational Event

The northernmost exposures of rocks formed during the Late Neoproterozoic Cadomian orogeny in the Channel Islands–northern France region occur on Alderney. The island mainly comprises foliated quartz diorite, once considered to be 2 Ga, pre-Cadomian basement, and an undeformed basic to intermediate plutonic complex. A precise age of 610±2 Ma, based on U–Pb analyses of single and small groups of zircons, for the foliated Fort Tourgis quartz diorite demonstrates that the oldest rocks were emplaced and deformed during a Cadomian magmatic event. The age is virtually identical to ages from similar, foliated syntectonic quartz diorite bodies on the islands of Guernsey and Sark and at La Hague (north Normandy), indicating that this magmatic and deformational event was regional in extent. Discordant zircon xenocrysts define an upper intercept age of c. 2 Ga indicating the presence of Palaeoproterozoic basement at depth. Single zircons from the undeformed Bibette Head granodiorite give a precise U–Pb age of 572±1 Ma. This age is closely similar to that for the emplacement of the Northern Igneous Complex of Guernsey. The emerging data indicate that Cadomian magmatism in the northern Channel Islands region was not a protracted continuum, but occurred during two distinct, short-lived events separated by c. 30–40 my.

scholarly journals Mineral chemistry (biotite, muscovite, garnet, and plagioclase) in the Kathmandu and Gosainkund regions, central Nepal Himalaya

2004 ◽  
Vol 30 ◽  
Author(s):  
S. M. Rai ◽  
S. Guillot ◽  
B. N. Upreti ◽  
A. Pecher ◽  
P. Le Fort
Keyword(s):  
Granulite Facies ◽  
Mineral Chemistry ◽  
Systematic Variation ◽  
Main Central Thrust ◽  
Metamorphic Evolution ◽  
Nepal Himalaya ◽  
Central Nepal ◽  
Compositional Field ◽  
Southward Extension ◽  
Bottom To Top

The greenschist- to granulite-facies rocks in the Kathmandu and Gosainkund regions are divided into three tectonic units on the basis of structure, lithology, and metamorphism. The Gosainkund Crystalline Nappe (GCN) corresponds to the southward extension of the Higher Himalayan Crystallines (HHC), which thrusts over the Kathmandu Crystalline Nappe (KCN) along the Main Central Thrust (MCT). The GCN and KCN thrust over the Lesser Himalaya (LH) along the MCT and the Mahabharat Thrust (MT), respectively. Systematic traverses with the microprobe of four minerals (i.e. biotite, muscovite, garnet, and plagioclase) from all the three units were carried out to study their chemical variations. Most of the biotite compositions from all units belong to the annite compositional field. There is an inverse relationship between the Ti and the Mg number (Mg2+/ (Mg2++Fe2+) and the latter increases from the top to bottom section of the KCN. The LH biotites resemble the bottom composition of the KCN whereas the GCN biotite compositions are quite scattered. However, the Ti-Mg number trend does not seem to be related directly to the grade of metamorphism. The muscovites from the KCN are rich in FeO while the muscovites from other two units are rich in Al2O3. In the KCN, the almandine and pyrope contents in garnet decrease from bottom to top section, while the grossular and spessartine contents in garnet increase. This trend is consistent with the prograde metamorphic evolution observed in the field. The composition of garnets from the bottom to the top section of the GCN does not show any systematic variation, but in the upper section, where sillimanite appears, the almandine con tent decreases and the spessartine content slightly increases. This variation in composition suggests a polyphase metamorphic evolution. The albite content of plagioclase decreases from the lower to upper section in the KCN while there is not any systematic variation in the GCN. The P-T conditions record the good preservation of inverse metamorphism in the LH below the MCT. The comparison of P-T results between the KCN and the GCN suggests that exhumation of the KCN was followed by the exhumation of the GCN.

Texturally Controlled U–Th–Pb Monazite Geochronology Reveals Paleoproterozoic UHT Metamorphic Evolution in the Khondalite Belt, North China Craton

2020 ◽  
Vol 61 (1) ◽  
Author(s):  
Shujuan Jiao ◽  
Ian C W Fitzsimons ◽  
Jian-Wei Zi ◽  
Noreen J Evans ◽  
Brad J Mcdonald ◽  
...  
Keyword(s):  
North China Craton ◽  
North China ◽  
Mineral Chemistry ◽  
Matrix Group ◽  
Metamorphic Evolution ◽  
The North ◽  
Group 5 ◽  
Group 2 ◽  
Uht Metamorphism ◽  
Khondalite Belt

Abstract Sapphirine-bearing UHT granulites from the Dongpo locality in the Khondalite Belt of the North China Craton have been comprehensively characterized in terms of petrology, mineral chemistry, metamorphic evolution and zircon geochronology. However, the precise timing of the peak-UHT metamorphism and other key stages in the P–T–t evolution remain controversial due to the complexity of multiple metamorphic overprints and the lack of petrographic context for zircon age data. In this study, monazite from four samples of the Dongpo granulite are divided into six groups based on chemical composition and textural context, and dated (in-situ SHRIMP and LA–ICP–MS U–Pb). An age population of 1·91–1·88 Ga was obtained from high-Y cores of monazite inclusions in garnet (Group 1) and on grains in the rock matrix (Group 2). The maximum age of c.1·91 Ga is interpreted as the minimum timing for prograde metamorphism before UHT metamorphism (M1). An age population of 1·90–1·85 Ga was obtained from low-Y domains of monazite inclusions (Group 3) and of matrix grains (Group 4). Combined with previous zircon dating results, the age population from low-Y Mnz constrains the timing and duration of the UHT metamorphism to 1·90–1·85 Ga and 50 (±15) million years, respectively. The large (50 m.y.) age spread is interpreted to reflect continuous monazite formation, and it is consistent with the slow post-peak near-isobaric cooling stage (M2). An age of c.1·86 Ga was obtained from monazite in textural contact with sapphirine/spinel + plagioclase intergrowths (Group 5), which is interpreted as the timing of the subsequent decompression–heating stage (M3). The younger age clusters at c.1·80 and 1·77 Ga, obtained from Th-rich monazite rims (Group 6) and one single Th-depleted monazite in textural contact with matrix biotite, respectively, indicate dissolution–reprecipitation and new monazite growth from fluid released by crystallizing anatectic melt during retrogression. These results, along with the previous 1·93–1·91 Ga data for UHT metamorphism, suggest that there was a very long-lived Paleoproterozoic UHT metamorphism (1·93–1·85 Ga) in the Khondalite Belt of the North China Craton. This supports the large hot orogeny model for the generation of Paleoproterozoic UHT metamorphism in the Khondalite Belt during the amalgamation of the Nuna supercontinent.

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