Pelitic Rocks Research Articles

Petrogenesis of U-rich two-mica granite from the Mesozoic Reshui pluton (Nanling Range, South China) with implications for uranium mineralization

Muscovite-bearing granites are the dominant rock type associated with metallic deposits in South China. Some of these granites are rich in U and considered potential sources for subsequent hydrothermal uranium mineralization. Although widely studied, the petrogenesis of muscovite-bearing granites remains obscure. The Reshui pluton contains fine-grained biotite granite and coarse grained two-mica granite (CGTG). CGTG exhibits high U concentrations (5.07–40.15 ppm, average 15.94 ppm) and is typical of U-rich granite in South China. We conducted detailed petrographic, geochemical and biotite mineralogical analyses of the CGTG from the Reshui pluton. Our findings provide new insights into the petrogenesis of U-rich granites in South China and their significance for uranium mineralization. LA-ICP-MS zircon U–Pb dating shows that the CGTG were formed at 167.3 ± 1.7 Ma. Integrated petrological, whole-rock geochemical and biotite geochemical data, the negative and limited variation of the εNd(t) values (from − 10.8 to − 9.6) and a two-stage Paleoproterozoic model age (1.72 to 1.83 Ga), suggest the U-rich S-type CGTG formed by partial melting of Paleoproterozoic clay-rich pelitic rocks and underwent extensive fractional crystallization. The U-rich pelitic magma sources characterized by relatively low oxygen fugacity and temperature, relatively high degree of magmatic differentiation and F content are the main factors controlling U enrichment in the Reshui pluton CGTG. The petrologic and whole-rock geochemical characteristics, together with the biotite chemical compositions and disseminated U-rich accessory minerals including uraninite, indicate that the CGTG in the Reshui pluton has a high uranium mineralization potential and should be given more attention in further explorations.

The Styrian Basin: an overview with new insights from seismic data

The Neogene Styrian Basin is the westernmost subbasin of the Pannonian Basin System. The last comprehensive studies on the basin were published in the last millennium. This overview of the current understanding of basin evolution is based on the re-interpretation of partly unpublished reflection seismic data and a wealth of sedimentological and stratigraphic studies performed during the last 25 years. Reflection seismic data indicate the presence of a major E to SE dipping detachment horizon within the pre-Neogene basement reflecting high-strain upper crustal extension during the early Miocene syn-rift stage of basin evolution. Fault block rotations caused subsidence in several subbasins, which developed often with marked asymmetric geometry. The maximum basin depth exceeds 4 km (Gnas Basin). Syn-rift deposits include thick alluvial fan and fan delta sediments overlain in the basin center by deeper marine pelitic rocks. Subduction-related magmatic activity commenced during the syn-rift stage and continued into the middle to late Miocene post-rift stage. Syn- and post-rift sediments are separated by the middle Miocene “Styrian Unconformity”. Shallow marine sedimentation, controlled by third- and fourth-order sea-level variations, prevailed during the early post-rift stage. The final stage of basin evolution was characterized by differential surface uplift, erosion, and Pliocene-Pleistocene basaltic magmatism.

CARACTERIZAÇÃO MINERALÓGICA DOS SEDIMENTOS DO IGARAPÉ DIABINHO (FORMAÇÃO SOLIMÕES) NO ESTADO DO ACRE – PROVENIÊNICA E IMPLICAÇÕS AMBIENTAIS

Igarapé Diabinho, situated in the central region of the state of Acre, is a tributary on the right bank of the Envira river, belonging to the Juruá river basin. Its U-shaped grooved channel is carved in sediments known as the Solimões Formation, of Late Miocene–Pleistocene age. The rocky exposures along Diabinho stream demonstrate a succession of massive silt-clay sediments, covered by other compact sandy silts cemented by carbonates. These sediments are partly overlain by alluvial (fluvial-lacustrine) sediments from the Quaternary, in the region where this stream dominates. For the study, samples collected in three profiles along a ravine on the right bank of Igarapé Diabinho were investigated, in an area used for picnics during the dry season, that is, with slight human disturbance. To assist in identifying the probable source areas of these sediments, and determine their weathering and transport conditions, granulometric analyzes were carried out, investigating the mineralogical composition of the sediments, their heavy minerals, and the mineralogy of the sediments’ clay fraction. The sediments are predominantly silts with little clay, being composed of quartz, feldspars (albite and microcline), clay minerals (smectite, illite, and kaolinite), muscovite, and chlorite. As a result, the sediments were deposited in mild paleoclimatic conditions under weak chemical weathering, indicated by the predominance of unstable minerals, such as amphiboles, feldspars, biotite, epidote, garnet, apatite, and titanite. The dominant presence of aluminosilicates (kyanite, andalusite, sillimanite, garnet, and micas) and epidote indicates that these sediments had as their primary source metamorphosed pelitic rocks. A second assemblage composed of tourmaline, zircon, apatite, titanite, and even ilmenite points to possible contributions from basic and intermediate igneous rocks that are currently observed in Serra do Divisor on the borders of Brazil and Peru. In conclusion, these findings contribute to implications in the paleoenvironmental context by providing insights into the probable source areas of the sediments, determining the weathering and transport conditions, and identifying the mineralogy of the sediments.

Geochemical Study of the Osumi Granodiorite, Southwestern Japan

The Osumi Granodiorite, located on the Osumi Peninsula in southwest Japan, is an example of outer zone granites that were formed during a limited period (13–15 Ma) in response to the subduction of the Philippine Sea Plate. This event, which is linked to the separation of southwest Japan from continental Asia, resulted in unique igneous activity. The Osumi Granodiorite is the largest Miocene granite body in the region. It intrudes into the Mesozoic to Paleogene accretionary complex of the Shimanto Belt and affects contact metamorphism. Despite considerable research on the Osumi Granodiorite, limited geochemical studies, especially on trace and rare earth element (REE) analyses, have been conducted. Furthermore, there are insufficient data on the Rb–Sr isotopic system, leaving the formation process unclear. This study presents whole-rock geochemical and Rb-Sr isotopic data to investigate the petrogenesis of the Osumi Granodiorite. The results suggest a common magma origin for this pluton, as indicated by linear trends on the Harker diagrams and similar REE patterns. The presence of a relatively large Eu anomaly implies formation under a reducing environment. The AKF diagram indicates predominant contamination by pelitic rocks of the Shimanto Belt during magma formation. The Rb–Sr whole-rock isochron diagram and SrI–1000/Sr diagram suggest that the Osumi Granodiorite body was formed by heterogeneous assimilation of magma into the Shimanto Belt. Furthermore, the whole-rock isochron age is 64.3 Ma, which differs by approximately 50 My from the previously reported biotite K–Ar age (14–22 Ma). This age is considered to be a pseudo-isochron age, rather than the consolidation age. During the middle Miocene, the compressive stress field in the outer zone south of the Butsuzo Tectonic Line made it difficult for magma to rise. As a result, it reacted with the sedimentary rocks of the Shimanto Belt to various degrees. The Osumi Granodiorite underwent magma differentiation upon intrusion into the Shimanto Belt. It subsequently ascended, cooled, and interacted with pelitic rocks under stable geological conditions.

Pedogenesis of pelitic rocks of the Serra da Saudade Formation - Bambuí Group

ABSTRACT Serra da Saudade Formation corresponds to the upper part of the stratigraphic column of the Bambuí Group. Few studies have addressed the soil properties and pedogenesis of the pelitic rocks rich in potassium minerals of this formation. This study analyzed siltstone-derived soils, some of which are glauconitic (green siltstone; “verdete”), to understand the role of the main pedogenetic factors and processes in the landscape of the Central-West region of the Minas Gerais State, covered by Cerrado vegetation. Nine soil profiles were described, and their morphological, physical, chemical and mineralogical properties were analyzed. Soils were classified as Neossolos Litólicos (P1, P3, P6, P8 and P9), Cambissolos Háplicos (P2 and P5), Argissolo Vermelho-Amarelo (P4) and Argissolo Acinzentado (P7). The main active pedogenetic processes identified in the study area are melanization, goethization, argiluviation and elutriation. These are essentially controlled by the nature of the parent material and position of the soil in the relief. Soils are typically shallow, dystrophic, highly Al-saturated and contain essentially quartz and micas in the coarse fractions (sand and silt) and illite/glauconite and kaolinite in the clay fraction. In soil environments with siltstone and green siltstone under “dry forests”, the soil water pH was higher and high levels of exchangeable calcium and magnesium, a eutrophic character and high-activity clay were observed. Barium, chromium, lead and zinc contents were high in all studied soils. Green siltstone-derived soils have peculiar physical and chemical properties, divergent from those developed from other glauconitic rocks on the Earth surface. However, greenish tones in horizons are common in all these soils.

Evidence for probable temporal and thermal trends among Devonian granitic magmas in central Victoria, and the composition of the Selwyn Block

This study provides new constraints on the causes of the chemical and isotopic variability in the central Victorian, Devonian, silicic, magmatic rocks and relates these to crustal architecture in the region. Synthesising present and previous work, it is concluded that the Selwyn Block, which forms the main source region for the Devonian silicic magmas (granitic and silicic volcanic rocks), is heterogeneous in three dimensions, on scales of 1 km and less. The sources of both the I- and S-type magmas were formed in Paleoproterozoic to Mesoproterozoic times, in the distal back-arc region of an Andean-type margin, in a basin that was extending and deepening eastward with time. The prominent rock types are high-grade metamorphosed greywackes, potassic andesites and dacites, with smaller volumes of pelitic rocks. The metasediment-dominated source rocks generally lie at deeper levels and reached higher metamorphic grades than the sources of the I-type magmas. This means that the I-type magmas were generally at lower temperatures and were also more hydrous than the S-type magmas. Heat for the metamorphism and partial melting of the source rocks of the silicic magmas was advected into the crust by mantle-derived magmas. These probably formed an underplate, to drive the regional metamorphism, but numerous, scattered, sill-like mafic bodies caused local contact anatexis and silicic magma production. Along with the emplacement of mafic magma bodies, the ascent of the silicic magmas to the upper crust rendered the deeper parts of the Selwyn Block denser and more mafic.

Hydraulic parameters of Submenilite Formation of Ždánice Unit

This study aims to re-asses hydrodynamic tests of the Submenilite Unit of the Outer Carpathian Flysch Zone sediments, exposed at Přítlucká hora located near Rakvice village in South Moravian region, Czech Republic. We focused on four shallow, 6.0–7.6 m deep hydrogeological wells, drilled by Aqua Enviro s. r. o. company in 2005. The purpose of the drilling was a feasibility study for drainage of western part of Rakvice village. The sediments of the Submenilite Formation of the Ždánice unit that were drilled are dominated by compressed clays, claystones and siltstones, containing thin layers of sand and weathered sandstones. For the purpose of determining hydraulic parameters of underlying aquifer, the company conducted pumping and recovery tests on each of the wells. In this work, we used Jacob’s semilogarithmic method to interpret the measured groundwater level data. Hydraulic conductivity and transmissivity parameters were consequently calculated. The resulting transmissivity values range from 1.6 × 10–2 m2/d to 3.7 × 10–1 m2 /d and hydraulic conductivity value ranges from 3.6 × 10–3 m/d to 1.0 × 10–1 m/d. The calculated index of transmissivity Y corresponds to 4.32–4.57 and index of conductivity Z ranges between 5.80 and 6.27. The values of index of transmissivity Y from this study are generally comparable with results presented by previous authors and demonstrate very low transmissivity of the rocks of this unit mostly built by non-permeable pelitic rocks.

Interpretation of Physical Weathering and Deterioration Mechanism for Thermal Altered Pelitic Rocks: Ulju Cheonjeon-ri Petroglyph

Interpretation of Physical Weathering and Deterioration Mechanism for Thermal Altered Pelitic Rocks: Ulju Cheonjeon-ri Petroglyph

Chemostratigraphy of early Ediacaran carbonate rocks of the Cachoeirinha Group (Northeastern Brazil): Implications for paleonvironmental conditions and atmospheric oxygenation

The Cachoeirinha Group, situated in northeastern Brazil, primarily consists of meta-pelites, metagraywackes, meta-conglomerates, banded iron formations, carbonate deposits, metavolcanics, and metatuffs. This group comprises two formations: Serra Olho d'Água and Santana dos Garrotes. Within the Serra Olho d'Água Formation, polymictic meta-conglomerates and associated carbonate rocks have been dated to a maximum depositional U–Pb age of 880 ± 8 Ma. The presence of negative δ13C values ranging from −2.83‰ to −0.71‰ is associated with glaciomarine depositional environments, while low Y/Ho ratios (≤26) indicate a nearshore/restricted basin environment. The δ53Crauth values in carbonate samples range from 0.0605‰ to 0.3248‰, and the absence of Ce/Ce* and Eu/Eu* anomalies suggest deposition under low oxygenation conditions. The 87Sr/86Sr ratios ranging from 0.70776 to 0.70862 indicate a possible deposition during the Cryogenian-Ediacaran period. On the other hand, the Santana dos Garrotes Formation predominantly consists of pelitic rocks with interbedded felsic metavolcanics, banded iron formations, and associated carbonate rocks. Detrital zircons from this formation were dated (U–Pb in zircon) and indicated an age of 630–610 Ma. High Y/Ho ratios (>45) and positive δ13C values between +3.1‰ and +5.5‰, associated with organic matter (OM) buried in marine sediments, together with positive δ53Crauth values ranging from 0.058‰ to 0.487‰, suggest the existence of a seawater pool with high 53Cr/52Cr ratios, related with a well-established marine environment. The 87Sr/86Sr ratios ranging from 0.70799 to 0.71226 are consistent with an early Ediacaran interglacial period. The analysis of isotopic and elemental systems in Cachoeirinha Group carbonates provides evidence of long-lasting seawater stratification and an isotopically stratified water column during climatic changes associated with the “Snowball Earth” scenarios linked to the Neoproterozoic Oxidation Event (NOE).

Melting Processes of Pelitic Rocks in Combustion Metamorphic Complexes of Mongolia: Mineral Chemistry, Raman Spectroscopy, Formation Conditions of Mullite, Silicate Spinel, Silica Polymorphs, and Cordierite-Group Minerals

Melted rocks (clinkers and paralavas) of the Mongolian combustion metamorphic (CM) complexes were formed during modern and ancient (since the Quaternary) wild-fires of brown coal layers in the sedimentary strata of the Early Cretaceous Dzunbain Formation. According to XRD, Raman spectroscopy, and SEM-EDS data, cordierite, sekaninaite, indialite, ferroindialite, silica polymorphs, mullite, Fe-mullite, anhydrous Al-Fe-Mg silicate spinel (presumably new mineral), and other phases were identified. It has been established that isomorphic impurity of potassium in the cordierite-group minerals does not correlate with their crystal structure (hexagonal or orthorhombic). Indialite and ferroindialite retained their hexagonal structure in some fragments of the CM rocks, possibly due to the very fast cooling of local zones of sedimentary strata and the quenching of high-temperature K-rich peraluminous melt. Clinkers, tridymite–sekaninaite, and cristobalite–fayalite ferroan paralavas were produced by partial melting of Fe-enriched pelitic rocks (mudstone, siltstone, and silty sandstone) in a wide temperature range. The formation of mullite, Fe-mullite, and Al-Fe-Mg silicate spinel in clinkers developed from dehydration–dehydroxylation and incongruent partial melting of Fe-enriched pelitic matter (Al-Mg-Fe-rich phyllosilicates, ‘meta-kaolinite’, and ‘meta-illite’). Large-scale crystallization of these minerals in the K-rich peraluminous melts occurred, presumably, in the range of T > 850–900 °C. The subsurface combustion of coal layers heated the overburden pelitic rocks from sedimentary strata to T > 1050 °C (judging by the formation of cordierite-group minerals) or locally till the melting point of detrital quartz grains at T > 1300 °C and, possibly, till the stability field of stable β-cristobalite at T > 1470 °C. Ferroan paralavas were formed during the rapid crystallization of Fe-rich silicate melts under various redox conditions. From the analysis of the liquidus surface in the Al2O3–FeO–Fe2O3–SiO2 major-oxide system, it follows that the least high-temperature (<1250 °C) and the most oxidizing conditions occurred during the crystallization of mineral assemblages in the most-enriched iron silicate melts parental for cristobalite–fayalite paralava.

Cristobalite Clinker and Paralavas of Ferroan and Melilite–Nepheline Types in the Khamaryn-Khural–Khiid Combustion Methamorphic Complex, East Mongolia: Formation Conditions and Processes

Abstract —Rock samples from the Khamaryn-Khural–Khiid combustion metamorphic (CM) complex, including cristobalite clinker, ferroan tridymite–sekaninaite and cristobalite–fayalite paralavas, which are rock types new to the complex, as well as clinker xenoliths in melilite–nepheline paralava, have been studied in terms of chemistry and mineralogy. The obtained data on rock-forming, minor, accessory, and rare phases (silica polymorphs, cordierite-group minerals, fayalite, Fe and Ti oxides, ferrosilite, etc.) have implications for the formation conditions and processes of the CM rocks. The Raman spectra of sekaninaite, indialite, ferroindialite, mullite, and anhydrous Fe–Ca–Mn phosphate, presumably from the graftonite group, have several specific features. The diversity of mineral assemblages in the CM rocks is due to heterogeneous lithology of the sedimentary protolith and to local effects in the multistage history of the Khamaryn-Khural–Khiid complex. According to geochemical data, all CM rocks of the complex are derived from the Early Cretaceous Dzunbain Formation, their protolith molten to different degrees. The cristobalite clinker and tridymite–sekaninaite and cristobalite–fayalite paralavas were produced by partial melting of pelitic rocks containing different amounts of iron in a wide temperature range. The formation of mullite developed from dehydration–dehydroxylation and incongruent partial melting of amorphous pelitic matter. Large-scale crystallization of mullite in clinker, occurred from the high-silica potassic aluminosilicate melt at >850 °C. Combustion of subsurface coal seams heated the overburden to >1050 °C or locally to >1300–1400 °C (melting point of detrital quartz) or even, possibly, to >1470 °C corresponding to the stability field of β-cristobalite. Melilite–nepheline paralava was formed by incongruent melting of silicate (pelitic) and carbonate (calcite) components of marly limestone under elevated CO2 partial pressure. Oxygen fugacity (fO2) during combustion metamorphism changed from strongly reducing conditions favorable for crystallization of Fe phosphides (barringerite, schreibersite) and metallic iron from silica-undersaturated melts parental to melilite–nepheline paralava to high fO2 values that can maintain the formation of hematite in Fe-rich CM rocks.

Petrology and Mineralogy of Metamorphic Rocks in The Pringsewu District, Lampung Province, Indonesia

The origin of metamorphic rock is greatly influenced by the temperature and pressure changes in every tectonic setting, especially in active subduction margins. One of the wide outcrops of metamorphic rock occurs in the Pringsewu District, Lampung, and it is a part of the Palaeozoic Gunung Kasih Complex. The presence of metamorphic rocks in Pringsewu has raised several questions and debates for some time due to the lack of research and field evidence found in this area, especially, since the origin of metamorphic rock in Lampung has been rarely studied. This research aims to determine the metamorphic rock facies and the tectonic setting underlying the formation of metamorphic rocks in the study area.
 Petrographic analysis on 19 thin-section samples shows that metamorphic rock in the research area can be divided into two regions i.e., western and eastern regions. The Eastern part is characterized by muscovite-epidote schist and greenschist which consist of quartz, muscovite, actinolite, epidote, and garnet as index minerals. Whereas the western part is characterized by quartzite and biotite-epidote schist that consist of quartz, biotite, and muscovite. Based on the mineral index, metamorphic rock’s protoliths are pelitic rock, mafic rock, and quartz-feldspathic sandstone. The metamorphic rock zonation shows the created temperature is from 280-550oC.
 The foliation structure such as schistose and porphyroblastic texture in the whole rocks sample indicated that metamorphic rocks are created in medium-grade metamorphism and are characterized by the greenschist facies to epidote-amphibolite facies. The abundance of quartz, k-feldspar, and labradorite minerals show that metamorphic rocks came from the microcontinent which was caused by a collision between the intra-oceanic Woyla plate and West Sumatra microcontinent in the Cretaceous. Indications of tectonic activity that create the lithology in Lampung Province need more geological study, especially to determine the absolute age of the metamorphic rock.

New insights from an emplacement model for the Santa Eulália Plutonic Complex (SW Iberian Peninsula)

The Santa Eulália Plutonic Complex is formed by two main granites: G0 and G1. The G0 granite hosts metasedimentary carbonate and pelitic rocks (roof pendants) and elongated masses of mafic–intermediate rocks (the M-group). The host rocks form a diverse sequence of igneous, metasedimentary and metamorphic rocks. The study area is constrained by Variscan structures formed in a transpressional/transtensional sinistral tectonic regime (the Tomar–Badajoz–Córdoba sinistral shear zone) and is cut by the Alter do Chão and Assumar faults. The geological complexity of the area makes it difficult to determine the emplacement mechanism of the pluton. We propose a model for the ascent and emplacement of the pluton that can also be applied to similar post-collision Variscan granites. The gravity anomalies suggest that the pluton is slightly asymmetrical and extends to the SSE beneath the host rocks; its main root is >8 km thick in its deepest areas. The available radiometric data for an extended area embracing several regional plutons suggest a west–east magmatic alignment. The Variscan structures probably formed efficient crustal discontinuities enabling the generation and ascent of magma. Our model involves a west–east magmatic axis for magma spreading along extensional fractures (T-fractures) related to the Tomar–Badajoz–Córdoba shear zone. The opening movement along these fractures created divergent forces that allowed the ascent and emplacement of the plutonic rocks. The importance of these fractures is represented well by outcrops of porphyritic biotite granites (the Ervedal, Fronteira and G1 granites).

Protolith age and metamorphic temperature of the Yokokawagawa metamorphic rocks in Nagano Prefecture, central Japan, and comparison with the Sanbagawa metamorphic rocks

We estimated the protolith age and peak metamorphic temperature of the Yokokawagawa metamorphic rocks (YMR) east of the Itoigawa–Shizuoka Tectonic Line using detrital zircon U–Pb dating and Raman carbonaceous material geothermometry, respectively. U–Pb dating of a psammitic rock yielded a youngest age of ∼100 Ma, which corresponds to the protolith age. Raman carbonaceous material geothermometry results for five pelitic rock samples give metamorphic temperatures of ∼350–380 °C. The protolith age is consistent with those of the Sanbagawa metamorphic rocks (SMR), strongly indicating that the YMR are an extension of the SMR. The increase in peak temperature toward intrusive rocks along the eastern margin of the YMR may indicate that the relatively young ages yielded by previous K–Ar dating of the YMR reflect thermal resetting due to contact metamorphism.

Modeling of Geochemical Variations and Weathering Depth on the Surface of Pelitic Rocks in Periodical Submerging Zone: Bangudae Petroglyphs

Modeling of Geochemical Variations and Weathering Depth on the Surface of Pelitic Rocks in Periodical Submerging Zone: Bangudae Petroglyphs

Petrogenesis and Geodynamic Implications of Early Cretaceous (∼130 Ma) Magmatism in the Baingoin Batholith, Central Tibet: Products of Subducting Slab Rollback

AbstractThe Baingoin batholith is one of the largest granitic plutons in the North Lhasa terrane. Its petrogenesis and tectonic setting have been studied for decades, but remain controversial. Here we report data on geochronology, geochemistry and isotopes of Early Cretaceous granitoids within the Baingoin batholith, which provide more evidence to uncover its petrogenesis and regional geodynamic processes. The Early Cretaceous magmatism yields ages of 134.4–132.0 Ma and can be divided into I‐type, S‐type and highly fractionated granites. The I‐ and S‐type granites exhibit medium SiO2, high K2O/Na2O with negative εNd(t) and εHf(t) values, whereas, the albite granites have very high SiO2 (79.04%–80.40%), very low K2O/N2O, negative εNd(t) and a large variation in εHf(t). Our new data indicate that these granitoids are derived from unbalanced melting in a heterogeneous source area. The granodiorites involved had a hybrid origin from partial melting of basalt‐derived and Al‐rich rocks in the crust, the porphyritic monzogranites being derived from partial melting of pelitic rocks. The albite granites crystallized from residual melt separated from K‐rich magma within the ‘mush’ process and underwent fractionation of K‐feldspar. We believe that the Early Cretaceous magmatism formed in an extensional setting produced by the initial and continuous rollback of a northward‐subducting slab of the NTO.

Advantages and Limitations of Combined Diffusion-Phase Equilibrium Modelling for Pressure–Temperature–Time History of Metamorphic Rocks

ABSTRACT This paper presents and discusses the results of phase diagram (Perple_X) and diffusion modelling (CZGM, or Compositional Zoning and its Modification by diffusion) to constrain the P–T path of metamorphism. The approach is based on the best fits between the zoning profile in measured garnet and that obtained by the intersections of garnet isopleths calculated by phase diagram modelling using whole rock bulk composition. The model was applied to garnets in natural rocks of various metamorphic grades, which were formed within different geotectonic environments. To compare the sequence of compositional change during Barrovian-type metamorphism, well-studied pelitic rocks from garnet–staurolite, kyanite–sillimanite, and sillimanite-K-feldspar metamorphic zones were selected. Garnets with two-stepped core and rim profiles that were formed during two different metamorphic stages or events were used for pressure–temperature (P–T) path constraint of each stage or event. For high-grade rocks, in which the original zoning profile in garnet was severely modified, the diffusion of the initial zoning profile was quantified to estimate the timescale of the metamorphic event. These rocks include high- to ultra-high-pressure rocks, which were subjected to thermal overprinting during collisional orogenesis. The results of the application of this approach allow for deciphering the reason why the calculated profile by phase diagram modelling does not fit with that of the measured garnet from low-grade rocks, in which garnet has preserved the original compositional zoning. This includes garnets whose nucleation was shifted from the garnet-in boundary to higher temperatures and pressures, as well as garnet crystallised during different metamorphic stages or events. Finally, the P–T paths in high-grade rocks were constrained after the multicomponent diffusion in garnet was quantified, and this was used for further P–T-time path constraint of metamorphism in the rocks.

Role of aqueous fluids during low pressure partial melting of pelites in the Adamello pluton contact aureole (Italy)

The contact aureole of the Adamello pluton (Italy) exhibits a rare combination of pelitic rocks anatexis around a shallow metaluminous pluton and formation of interconnected leucosomes, aplites and small lithium and cesium-enriched pegmatitic bodies. Contact metamorphic mineral assemblages are described in the non-graphitic pelites and psammites hosting the pluton in the central sector of the Adamello Massif. Three metamorphic zones are identified on the field: I – Muscovite-Biotite, II – Andalusite-Sillimanite-K feldspar, III – Melt zone. In the melt zone, the presence or absence of high temperature Crd defines two types of migmatites, a Crd-bearing metatexite characterized by orbicular textures and a Crd-absent stromatic metatexite. Peak P-T metamorphic conditions in the Adamello pluton thermal aureole were calculated at 690–700 °C and 3 kbar on the basis of phase equilibria and geothermobarometry. Phase equilibria modelling reveals that Crd-bearing migmatites, which produced around 20 vol% melt fraction, were generated through internal fluids-buffered partial melting conditions while Crd-absent stromatic metatexites, which produced ca. 35 vol% melt fraction, originated through local fluid present partial melting. Different types of leucosomes were produced by these two partial melting conditions. In-situ leucosomes linked to Crd-bearing migmatites are compositionally similar to primary liquid predicted by phase equilibria modelling (calculated at peak P-T conditions). Leucosomes within the Crd-absent stromatic metatexites show residual mineral compositions extremely enriched in Kfs and Bt and are connected to extraction structures. Their whole rock composition does not represent a primary melt while being compatible with Kfs cumulate textures. The extracted melt from the stromatic metatexites was calculated to be compatible with the spatially related Li-rich pegmatites mean composition, which are hosted by the same geological formation. The studied area shows the small-scale variability in intraformational fluids participation during metamorphic reactions while demonstrating the hybrid nature of partial melting in anatectic terranes. • Differential fluids influx created different migmatites from one protolith. • Extracted anatectic melt closely fits the self-hosted LCT pegmatites composition. • Fluid circulation in metamorphism causes hybrid fluid absent - present anatexis.

Lithostratigraphy and metamorphism of the Khudi-Tal area, west central Nepal

The study is mainly focused on lithostratigraphy, petrography and geological structures to decipher the metamorphic facies and metamorphic history of the Khudi-Tal area along the Marsyangdi Valley, west central Nepal. Geological mapping was carried out covering 142 sq.km, on the scale of 1:50000. The Main Central Thrust (MCT) separates lithological succession of the Lesser Himalaya and Higher Himalaya around Bahundanda, Probhi and Naiche of the Khudi-Tal area. Lithological succession is dominated by metapelitic to psammatic schist, siliceous to dolomitic marble, quartzites, graphitic to garnet schist, kyanite garnet gneiss and banded gneiss from south to north. The kyanite isograd follow the MCT separating Kyanite zone with mineral assemblages Ky+Grt+Qtz+Ms+Bt+Pl of the Higher Himalaya and Garnet zone as Grt+Bt+Ms+Chl+K-Fel+Pl+Qtz in pelitic rocks, Bt+Ms+Chl+Plag+Qtz in Psammatic rocks and Cal/ Dol+Qtz+Pl+Bt+Ms in calcareous rock of the Lesser Himalaya. Based on the mineral assemblages, the Higher Himalaya belongs to Amphibolite Facies whereas the Lesser Himalaya belongs to Epidote-Amphibolite Facies. The lithological succession of the Higher and the Lesser Himalaya has undergone poly-phase metamorphism. In the Lesser Himalayan zone rocks, the regional burial metamorphism M1 (pre-MCT/ eo-Himalayan) is followed by barrovian type garnet grade inverted metamorphism M2 and later post-MCT retrogressive metamorphism M3. In the Higher Himalaya rocks, the early phase kyanite bearing medium temperature/ high pressure regional prograde metamorphism M1 (pre-MCT/ eo-Himalayan) is followed by barrovian type inverted metamorphism M3 (syn-MCT/ neo-Himalayan) and over printed by later phase retrogressive metamorphism M3 (post-MCT).

Post-collisional extension of the South Altun subduction-collision belt, northern Tibetan Plateau: Insight from phase equilibria modeling and zircon geochronology of pelitic migmatites

• Continental collision led to the burial of pelitic rocks in the South Altun. • Pelitic migmatites were formed under the conditions of ∼850–870 °C and 0.6–0.7 GPa. • High- T/P type metamorphism and migmatisation lasted for ∼10 Myr (457–447 Ma) • Continental subduction to post-collisional extension took place in ∼50 Myr. The South Altun subduction-collision belt in the northern Tibetan Plateau is one of the rare examples of ultradeep subduction (>350 km). In addition to subduction-related low-temperature/pressure ( T/P ) type metamorphic rocks, the belt also exposes high- T/P metamorphic rocks with well-developed migmatitic structures, which provide insight into the post-collisional process. Phase equilibria modeling of the Tula pelitic migmatites indicates peak conditions of ∼850–870 °C and 0.6–0.7 GPa for the high- T/P type metamorphism and migmatism. Phase equilibria modeling predicts this high temperature could result in the production of 10–20 vol% melt in metapelites, producing stromatic migmatites observed in multiple outcrops. Based on results of zircon U-Pb geochronology of granulite-facies migmatites, we interpret the supra-solidus conditions to last for ∼10 Myr (457–447 Ma), corresponding to short-lived granulite-facies metamorphism in post-collisional extension. The deposition of the protolith is inferred to occur immediately before the metamorphism on active continental arcs based on the youngest detrital and oldest metamorphic ages. Combined with studies on subducted-related metamorphic rocks, these findings suggest that the South Altun belt experienced the continental subduction (early collision) to the post-collisional extension within a period of ∼50 Myr. At the same time, the thermal regime of this orogenic belt evolved from low- T/P to high- T/P whereas the high- T/P regime predominates the orogenic belt towards the end of orogenic evolution, as evidenced by regional high- T/P type metamorphic rocks and overprinting on the ancient low- T/P regime of exhumed subduction-related rocks.