Metamorphic Research Articles

Enhanced structural analysis through a hybrid analogue-digital mapping approach: Integrating field and UAV survey with microtomography to characterize metamorphic rocks

This study focused on collecting structural data orientations of a crustal-scale shear zone (Palmi Shear Zone, PSZ, southern Calabria, Italy) by integrating various analytical and field-based techniques. The PSZ consists of deformed metamorphic rocks (migmatitic biotitic paragneiss, marbles, and skarns) showing multiple folding phases, and Hercynian tonalites and pegmatites (306-290 Ma), crosscut by Late Hercynian leucocratic dykes (ca. 290 Ma). Multi-sized clasts composed of different lithologies are preserved on clean outcrop surfaces, and are sheared into both σ - and δ -type objects that collectively suggest opposing senses of shear. The study incorporates structural analysis of folds, field and aerial surveys (UAV), digital mapping, and microcomputed tomography. Various kinematic indicators were observed in the PSZ, indicating a mix of factors influencing the shear strain patterns (e.g. fold interference patterns, different rock types with high viscosity contrast). The findings suggest a clear consistency between structural data inferred from 3D VOM (Virtual Outcrop Model) and those collected directly in the field, confirming the occurrence of both sinistral and dextral shear in the PSZ, providing important insights into the tectonic evolution of the Calabrian-Peloritani Terrane.

Precious and Base Metal Minerals in Black Sands of the Egyptian Mediterranean Coast: Mineralogical and Geochemical Attributes

This paper investigates the mineralogical and geochemical characteristics, as well as the possible sources, of gold, silver, platinum group elements (PGE), copper, and lead found in the beach sands along Egypt’s Mediterranean coast. Using scanning electron microscopy and electron probe micro-analysis, this study determines the morphology and micro-chemistry of separated grains to assess their economic potential and how various minerals respond to different transport distances. The analysis reveals that gold grains are of high purity (94.11 to 98.55 wt.%; average 96 wt.% Au) and are alloyed with Ag (1.28–2.32 wt.%) and Cu (0.16–3.15 wt.%). Two types of gold grains were identified, indicating differences in transport distances. Variations in morphology, surface features, inclusion types, rims, and chemistry of the native metals, including gold grains, suggest differences in composition, weathering degree, transport distance, deposit types, and host rocks. The average Ag concentration in gold grains (1.86 wt.%) suggests a link to mesothermal or supergene deposits. Most silver, copper, and lead grains are spherical, with some variations in shape. Silver grains have 71.66–95.34 wt.% Ag (avg. 82.67 wt.%). Copper grains have 92.54–98.42 wt.% Cu (avg. 94.22 wt.%). Lead grains contain 74.22–84.45 wt.% Pb (avg. 79.26 wt.%). The identified platinum group minerals (PGM) belong to the Pt–Fe alloys and sperrylite, both of which are PPGE-bearing minerals. These metals likely originate from the weathering of upstream Nile tributaries surrounded by igneous and metamorphic rocks from Ethiopian and Central African regions, with a minor contribution from the Egyptian Eastern Desert Mountains.

Coesite-bearing garnet xenocrysts from diatexite in the Nordøyane domain, Western Gneiss Region, Norway: Implications for eclogite-melt interaction at ultra-high pressure

Ultra-high-pressure (UHP) metamorphic rocks in the Western Gneiss Region (WGR) of Norway formed in subducted Baltican crust during the Scandian phase of the Caledonian orogeny. In the Nordøyane UHP domain, eclogites locally preserve coesite and microdiamond. Along the north coasts of Haramsøya and Flemsøya, in situ orthopyroxene-eclogite bodies are surrounded by dioritic "diatexite selvages" containing abundant eclogitic enclaves and xenocrysts. In one location eclogite and diatexite are associated with a quartz diorite dyke that is highly contaminated with eclogitic material. The diatexite matrix and dyke have similar plagioclase + quartz + biotite assemblages with abundant embayed and fragmented garnet and clinopyroxene xenocrysts. Coesite is present in some garnet xenocrysts but has not been found in the adjacent eclogite bodies. Pressure-temperature (P-T) estimates from in situ eclogites range from 2.2-3.2 GPa and 810-840 °C, spanning the quartz-coesite transition, with P-T estimates from xenocrysts spanning a wider range. Diorites and retrogressed eclogites record amphibolite-facies conditions, ca. 1.0-1.5 GPa and 700-800 °C. The wide range and large uncertainties in P-T estimates reflect high-temperature re-equilibration and a relative lack of P-sensitive assemblages. Differences in texture, composition, and included assemblages indicate that the xenocrysts were not derived from the nearby eclogite bodies. By implication, their dioritic hosts were not locally derived but must have originated from a different, probably deeper, source. The results are integrated with a numerical model for WGR tectonic evolution to assess the sources of the melts and their possible effects on the exhumation of UHP rocks in western Norway.

Extension during the Liebig Orogeny: Revised tectonic setting of Paleoproterozoic central Australia

Magnetotelluric imaging of a lithospheric-scale boundary in central Australia has been used to support a model of collision between the North Australian Craton and an exotic continental ribbon referred to as the Warumpi Province during the late Paleoproterozoic Liebig Orogeny (c. 1640 Ma). This idea of collision has become an important element to reconstructions of late Paleoproterozoic Australia. In this study, we present mineral equilibria forward models from c. 1640 Ma metamorphic rocks to delineate the thermal regime associated with the Liebig Orogeny. Liebig-aged cordierite–orthopyroxene–biotite–quartz–plagioclase–K-feldspar–ilmenite–magnetite assemblages constrain peak metamorphic conditions to 4–5 kbar and 770–800 °C. These conditions are indicative of a high geothermal gradient regime and the mineral assemblage evolution shows no evidence for significant pressure changes that would generally support a crustal thickening model. Detrital zircon age spectra for the metasediments in the assumed exotic crust are broadly similar to the age record in the Aileron Province in the southern part of the North Australian Craton. Furthermore, εNd and εHf isotopic data of Liebig-aged granitic rocks make it permissible that they are derived from melting of North Australian Craton crust. This suggests the Warumpi Province was not exotic to the North Australian Craton and was instead a part of it. On the basis of the shared history between the Warumpi and Aileron Provinces and the newly constrained thermal character of c. 1640 Ma metamorphism, we suggest the Liebig Orogeny was characterised by extensional deformation of the North Australian Craton in the upper plate above a southward retreating subduction system. Tectonically, the Liebig Orogeny is interpreted to record a back-arc regime characterised by sedimentation, mafic and felsic magmatism, and predominantly contact metamorphism, and represents the continuation of a long-lived (c. 200 Ma) upper plate system that dictated the evolution of the North Australian Craton during the late Paleoproterozoic.

Heavy Minerals Distribution and Provenance in Modern Beach and Fluvial Sands of the Betic Cordillera, Southern Spain

This study uses heavy detrital minerals to determine actualistic fluvial and beach sand provenance across the Betic Cordillera (Spain), along the coast from Almeria to Marbella. The Betic Cordillera, primarily composed of metamorphic rocks to the east, supply an assemblage dominated by almandine and graphite, with a longshore dispersal from Almeria to Malaga. Buergerite and hypersthene indicate the provenance of calcalkaline lavas east of Cabo de Gata. The western part of the Betic Cordillera, which comprises the Ronda Peridotite Complex, supplies a chromite and diopside assemblage, with a dispersal from Marbella to Algeciras. Considering these mineralogical suites, the effects of source rock compositions and weathering are evaluated. The heavy mineral species mirror the mineralogy of the source rocks of local outcrops and wider source terranes. The fluvial heavy mineral suites do not differ significantly from those in the beaches except for some unstable species. Unstable species such as olivine, pyroxene, and amphibole do not show evidence of loss because of elevated topography and semiarid climate, which do not affect heavy minerals. This contribution also evaluates the potential of some heavy detrital species as ideal pathfinders in searching for diamonds.

The Age of Granulite-Facies Metamorphism in the Ivrea–Verbano Zone (NW Italy) Determined Through In Situ U–Pb Dating of Garnet

Abstract Rates of melt production, extraction and crystallization, as well as scales of melt transfer and interaction with their residuum change continuously in migmatite and granulite, affecting the behavior of monazite and zircon as time capsules. Therefore, accessory mineral chronometers may be ambiguous and incomplete in providing an overview of the temperature–time evolution of high-grade metamorphic rocks. In this study, we applied the novel technique of in situ U–Pb dating of garnet to the archetypal lower continental crust of the Ivrea–Verbano Zone (IVZ), NW Italy. In the IVZ, the temporal relationship between granulite-facies metamorphism and mafic underplating has long been debated, because of the interplay between tectonic, magmatic, metamorphic and metasomatic processes over a period of more than a hundred million years. Garnet from mafic and pelitic granulites yielded U–Pb ages between 287.4 ± 4.9 Ma and 280.1 ± 12.4 Ma, overlapping within uncertainty the time proposed for the emplacement of the Mafic Complex (286–282 Ma). These results indicate that the thermal climax in granulitic rocks was caused by mafic underplating and concomitant asthenospheric upwelling, rather than being inherited from the post-Variscan Carboniferous evolution. Providing robust dating of garnet with as low as 4 ng/g U, this study demonstrates the strength of garnet petrochronology in resolving complex tectono-metamorphic histories of high-grade terranes. It also represents a further step forward towards establishing garnet as part of the in situ U–Pb geochronology repertoire.

Assessment of groundwater hydrogeological processes, isotopic composition, and associated health risks of the Banaz (Usak) Basin, Turkey.

The study area covers the Banaz (Usak) basin located in the Aegean Region in the western part of Turkey. Metamorphic, sedimentary, ultramafic, and volcanic rocks are dominant in the basin. The groundwaters in the study area are used for domestic, irrigational, and industrial purposes. Hence, the groundwater chemistry and major geochemical processes in the region were determined. The dominance of major elements was of the order of Ca2+ > Mg2+ > Na+ > K+ and HCO3- > CO3- > Cl- > SO42-. Piper, Durov, Chadha, and Radial plots identified generally Ca2+-Mg2+-HCO3- type waters as the dominant types of water in this area. In terms of physical parameters in the basin, the waters are suitable for drinking. However, arsenic content in Yesilyurt and Corum settlements exceeds the limit values of drinking water standards. In addition, the ammonium value is high in the water sample in the Corum region. Isotope contents in water samples from 2008 to 2023 were evaluated in the study area. The waters in the basin are of meteoric origin according to their stable isotope content. Tritium content in the plain waters indicates recent recharge. Additionally, for children, As and U elements were identified as risky with oral intake and As with dermal contact.

Effects of foliation angle on mechanical characteristics of carbonaceous slate using uniaxial compression tests

Carbonaceous slate is one kind of metamorphic rocks with developed foliation, which is frequently encountered during tunnel construction in Western China. The foliation plays a crucial role in the stability of tunnels. For this, we conducted uniaxial compression tests, acoustic emission (AE) monitoring and scanning electron microscope (SEM) tests on carbonaceous slate. The results show that the strength, failure mode, and AE characteristics exhibit marked anisotropy with the angle between the axial and the foliation (β). As β increases, the ultrasonic wave velocity decreases monotonically, whereas the uniaxial compressive strength (UCS) displays a distinctive U-shaped trend. The elastic modulus initially decreases and then increases. The cumulative AE counts curve and energy curve show a stepped growth when β ≤ 45°. The AE events are active during the crack compaction phase and remain calm during the linear elastic deformation phase when β > 45°. Upon failure, the energy release accounts for the highest proportion (67%) when β = 45°, while the proportions in other cases are less than 37%. The maximum percentage (31%) of shear cracks is reported when β = 60°, which is six times greater than that at β = 0°. Moreover, Kernel density estimation analysis reveals that the high concentration area with low AF (AE counts / duration time) and high RA (rise time / amplitude) increases initially, and then decreases when β > 60°. In addition, nine types of cracks and seven modes of failure were identified. The foliation angle has a pronounced impact on shear failure modes in comparison with tensile failure modes. The supports could suffer larger deformation when β ≥ 60° compared to other cases. The failure behaviors correspond well with field observations.

Single‐Stage Synchronous India‐Asia Collision Model Revealed by Himalayan High‐Pressure Metamorphic Rocks

AbstractDespite a half‐century of intense research, the timing and diachroneity of initial collision between India and Asia remain highly debated, largely due to different definitions of “initial collision” and correspondingly different methods adopted. This study focuses on high‐pressure pelitic granulites of the Eastern Himalayan Syntaxis (EHS) to elucidate their metamorphic evolution and provide new constraints on the timing of initial India‐Asia collision. Petrological examination and phase equilibria modeling show that high‐pressure pelitic granulites have undergone four metamorphic stages, with the peak assemblage of garnet + K‐feldspar + kyanite + biotite ± plagioclase ± rutile + ilmenite + quartz at P‐T conditions of 13.1–15.7 kbar and 790–850°C. Clockwise P‐T paths suggest that the Indian continent underwent tectonometamorphic processes of initial collision and subsequent continent subduction. Zircon and monazite dating results indicate that the metamorphic ages of pelitic granulites range from 60 to 15 Ma, with the oldest ones clustered at 60–58 Ma. The oldest metamorphic ages of high‐ to ultrahigh‐pressure Himalayan metamorphic rocks can provide an upper age limit of the initial collision. Therefore, the initial India‐Asia collision must have occurred before 60–58 Ma in the EHS, roughly consistent with ca. 57 Ma in the western Himalaya and 63–60 Ma in the central Himalaya. Collectively, we conclude that the northern margin of India most likely underwent a single‐stage synchronous collision with the southern margin of Asia initially at around 60 Ma along the entire Yarlung‐Tsangpo suture zone.

Application of Geophysical Methods in the Identification of Mineralized Structures and Ranking of Areas for Drilling as Exemplified by Alto Guaporé Orogenic Gold Province

Mineral exploration works conducted in the Alto Guaporé Gold Province (AGGP), situated in the southwest region of the Amazon Craton in Brazil, faces the challenges of many gold provinces around the world, i.e., declines in the discoveries of new economic deposits and increases in exploration costs. Ground geophysical methods, combined with structural analyses and geological mapping, are valuable tools that have potential to improve accuracy in selecting exploration targets and in determining drilling locations. AGGP deposits are primarily associated with regional N20°–W50° inverse faulting and sheared geologic contacts between Meso-Neoproterozoic siliciclastic metasedimentary rocks and Mesoproterozoic basement (granite and volcano–sedimentary sequences). Mining currently occurring in the central portion of the province drives exploration works towards the many existing targets at the area. Among them, the ABP target is one of the most promising for being located few kilometers north of the Pau-a-Pique mine. At the ABP target, gold is associated with hydrothermal alteration located in the sheared contacts and in the hinge zone of folded metasedimentary sequence. Hydrothermal phases include Fe-oxides, sulfide (py), muscovite and quartz veins. In this study, we use magnetic and geoelectric (induced polarization) surveys coupled with structural and geological mapping to identify potential footprints within the ABP target. The results from induced polarization (IP) profiles successfully mapped the shape and orientation of the main structures down to approximately 350 m at the ABP target, indicating potential locations for hydrothermal alteration hosting gold. Additionally, 3D magnetic data inversions illustrated the distribution of magnetic susceptibilities and magnetization vectors associated with shear zone structures and isolated magnetic bodies. Magnetic data highlighted fault zones along the contacts between metamorphic rocks and granites, while IP data identified areas with high chargeability, correlating with sulfidation zones mineralized with gold. These findings suggest a metallogenic model where gold deposits are transported through deep structures connected to regional faults, implying significant tectonic and structural control over gold deposition. The results underscore the potential of multiparameter geophysics in identifying and characterizing deposits in both deep and strike, thereby advancing our understanding of mineral occurrences in the region and enhancing the search for new mineralized zones.

Felsic xenoliths in Triassic quartz-monzodiorite dykes from the central Jebilet (Morocco): Detailed petrography and geochemistry, with a critical assessment of the so-called “granulitic” samples

Felsic xenoliths occur locally in the Early Triassic quartz-monzodiorite dykes that crosscut the Sarhlef basin in Central Jebilet, Morocco. Irrespective of the secondary features linked to entrapment, three types of xenoliths are distinguished on the basis of textural evidence: 1) isotropic rocks of broad granitic appearance; 2) foliated samples whose planar fabric undoubtedly corresponds to a ghost foliation, 3) a specific xenolith with a blastomylonitic structure.Geochemically, samples with Na > K and moderately negative εNd values (-1.5 to - 3.8), reflecting a hybrid origin, depart from samples with K > Na and strongly negative εNd values (-9.1 to - 11.7), pointing to a stronger or even pure crustal derivation. The blastomylonitic sample differs drastically from the rest by its plagiogranitic composition and elevated Nb content, associated with a highly radiogenic Nd isotope signature (εNd240 = + 6.3). Altogether, these features and available radiometric data suggest a connection of the xenoliths with the Variscan igneous bodies that crop out in the central Jebilet. The potassic xenoliths with unradiogenic Nd are broadly similar to the granodiorite plutons. The xenoliths with Na > K and mildly negative εNd and the blastomylonitic plagiogranite are reminiscent of the bimodal association of microgranites and gabbros of the Sarhlef basin.From a structural point of view, samples with a gneissic fabric may reflect the possible occurrence of a metamorphic, migmatitic complex at somewhat deeper crustal level. However, based on detailed mineralogical and geochemical data, the samples previously known as “mid-crustal granulites” turn out to be highly differentiated, aplo-pegmatitic rocks, interpreted as early, incipient partial melts. The presence of high-grade metamorphic rocks at depth is only documented by one small, HT garnet-biotite xenolith, while peraluminous lenses (fibrolite + spinel ± corundum) dispersed in the quartz-monzodiorite dykes suggest that a metapelitic component was involved in the melting process generating these lavas. In summary, this set of felsic xenoliths does not provide much information on the crustal composition under the Jebilet, apart from fairly superficial levels.

Geotechnical assessment of the rock massif in the area of the medieval cultural monument Asen’s Fortress, Bulgaria

The medieval fortress Asenova Krepost (Asen’s Fortress) is built on a rocky massif of metamorphic rocks in south-central Bulgaria. The rock hill, on which the fortress was built, stands about 100 m above the Chepelarska River and the road Asenovgrad–Bachkovo Monastery, which passes at the foot of the slope. The slopes of the hill are steep and overhanging the road, making them completely inaccessible. A characteriza tion of this area was made in order to assess the state of the rock massif in the area of the Asen’s Fortress. Rock samples (marbles and gneisses) were collected and examined. Laboratory tests and field studies were carried out to determine physical and strength parameters of the studied rocks. An assessment of the cracking of the rock massif was made. The results show that the cracking of the marbles and the direction and inclina tion of the schistosity in the gneisses are of essential importance for the stability of the massif.

CH4 Inclusions in High‐Pressure Metapelite: Revealing the Link Between Fluid Behavior and Redox Mechanisms for Subducting Sedimentary Carbon

Abstract(Ultra)high‐pressure metamorphic rocks provide valuable insights into the properties of slab‐derived fluids. Here, we report CH4‐rich fluid inclusions in garnet of a metapelite from the Zermatt‐Saas ophiolite, western Alps. Two types of metapelite, a CH4‐bearing pelitic schist and a calcareous pelitic schist, were investigated to unravel favorable P‐T‐fO2 conditions for preservation of CH4 in high‐pressure metapelite. In the CH4‐bearing pelitic schist, CH4‐rich fluid inclusions exclusively occur in the core of garnet (GrtI) rather than the rim (GrtII). GrtI records P‐T conditions of ∼2.85 GPa and ∼555°C, whereas GrtII records a prograde P‐T path from ∼1.75 GPa at 510°C to ∼2.0 GPa at 530°C. Compositional profile of garnet in the calcareous pelitic schist reflects a prograde metamorphic path from ∼1.9 GPa at 510°C to ∼2.12 GPa at 545°C. CH4‐rich fluid formation may primarily rise from graphite reduction at high‐pressure reduced conditions (ΔFMQ −3.5 to −4, 2.85 GPa, ∼550°C), while graphite and carbonates stabilize in a relatively oxidized environment (ΔFMQ ∼0, 2.12 GPa, 545°C). The initial redox budget of subducted sediments is primarily controlled by the amount of sedimentary carbonate and organic carbon, which plays the most important role in deciding the carbon speciation at different subduction depths. CH4 formation in COH fluids could primarily be attributed to the reduction of graphite. Subducted metasediments act as conduits for transporting non‐oxidized fluids to arc magmas, which provides crucial evidence to support the heterogeneity for slab‐derived COH fluids and offers new insights into the deep carbon cycle.

Comparison of Geological Structures and Satellite-based Tectonic Lineaments in Gülağaç (Aksaray) and its Surroundings

The study area covers the Gülagaç district of Aksaray and its surroundings in the Central Anatolia region of Turkey. The basement of the study area consists of metamorphic rocks belonging to the Paleozoic–Mesozoic-aged Kaman Group. Mesozoic-aged ophiolites overlie these rocks, and both are cut by Late Cretaceous-aged igneous rocks. These units were then overlain by Palaeocene-Quaternary-aged volcanic and sedimentary units. The region has important geological structures that have undergone polyphase deformation, especially in the Eocene units. As a result of these deformations, faults, fractures, folds and linearities were formed. . These structures were compared with satellite-based tectonic lineaments, and it was observed that the general orientations of the lines determined in the field and the satellite-based lineaments were largely compatible. The results of this study can make an important contribution to the exploration of mineral and geothermal resources in the region.

Zircon Internal Deformation and Its Effect on U-Pb Geochronology: A Case Study from the Himalayan High-Pressure Eclogites

Zircon, with a chemical formula of ZrSiO4, is a widely used mineral for determining the crystallization age of igneous rocks. It is also used to constrain the timing of metamorphic events from its overgrowth or recrystallized domains. Furthermore, detrital zircon grains can provide information on the sedimentary provenance. Due to the trace amounts of uranium (parent) which decays into its daughter element (Pb), it is a prime geochronometer for the majority of magmatic and metamorphic rocks. With high-precision analytical instruments, such as TIMS, SIMS, and LA-ICP-MS, huge amounts of geochronological and trace element data from zircon have been generated around the globe to date. Target domains within zircon grains are analyzed to extract geochemical and geochronological records using spatially resolved techniques such as ion probes or laser ablation coupled with mass spectrometry. Before any such analysis, the zircon grains are examined for internal structures, growth zoning, and the presence of tiny inclusions. However, many researchers analyze multiple domains within single zircon grains for U-Pb isotope analysis with little regard for their internal structures, particularly crystallographic orientations. Hence, they may obtain mixed ages with variable discordance, leading to imprecise interpretation especially when the growth domains are not well-identified. Particularly, zircon grains that contain multi-growth domains or have local internal deformations within a single grain may not produce geologically meaningful age results if the analyses are conducted on mixed domains. This study presents a brief review on zircon geochronology, how to identify and visualize micro-deformations in metamorphic zircons through the EBSD analysis, and the effects of micro-deformation on age results. Examples from a case study conducted on zircons hosted in the Himalayan high-pressure eclogites are presented that show intra-grain plastically deformed domains and their effects on the corresponding age results.

Comparative Analysis of the Mineral Associations in the Sediments from Buor-Khaya Bay

According to the results of analysis of 99 samples of bottom sediments and submarine permafrost from wells 1D-14, 3D-14 and 1D-15 drilled in the Buor-Khaya Bay, differences in their mineral composition due to paleogeographic factors, namely Late Quaternary changes in climate and sea level, as well as regional hydrodynamics are shown. The basis of the light fraction of minerals was quartz and feldspar (mainly plagioclases), found in the form of grains of various dimensions and sorting degree, as well as fine grains. To a lesser extent, the presence of chlorites, kaolinite and serpentines is noted, illite and smectite are rare. 42 accessory minerals were identified in the heavy fraction (average yield 0.95%) concentrated in fine-grained sands. It mainly consists of pyroxenes, amphiboles, carbonatite, epidote, zoisite, magnetite, mica, garnet, limonite, sphene, leucoxene, ilmenite. Rutile, kyanite, sillimanite, zircon, tourmaline, apatite, and stavrolite were found in smaller quantities. In the studied strata, plant remnants and carbon-like particles (kerogen) are found, the contribution of which exceeds 5% by weight in a number of samples. The results of the study allowed to conclude that the basis of the petrofund of the studied deposits are most likely sedimentary rocks of the Kharaulakh ridge of the Verkhoyansk mountain system (sandstones, siltstones and mudstones). The presence of characteristic accessory minerals in the sediments marks the unloading of igneous and metamorphic rocks, but their contribution is subordinate. They probably also include rocks of the Verkhoyansk complex, common near the Tiksi.

Multidisciplinary Slope Stability Assessment in Weathered Metamorphic Rocks: A Case Study of the Highland Complex, Sri Lanka

Multidisciplinary Slope Stability Assessment in Weathered Metamorphic Rocks: A Case Study of the Highland Complex, Sri Lanka

Magmatic (Cr-Ni-PGE) and secondary/hydrothermal (emerald-peridot-rodingite-nephrite jade) mineralization associated with mafic-ultramafic rock complexes of Pakistan

Mafic magmatism has repeatedly taken place in Pakistan throughout its geologic history from the earliest Proterozoic to Recent. Much of it, however, does not have associated metallic mineralization. This paper concerns mineralization associated with mafic-ultramafic plutonic rock occurrences in the western- and northern suture zones of Pakistan, which demarcate the collision boundary between the northwestern Indian plate and Eurasian blocks. We discuss the petrology of the ophiolites and basaltic magma-related chromite deposits, showings of PGE and Ni minerals, and secondary mineral deposits (emerald and peridot gems, rodingite, and nephrite jade) associated with mafic-ultramafic complexes of the two suture zones. Chromite with high Cr#s (68 to 85) occurs in fair quantity and is mostly considered to be genetically related to basaltic magma of subduction-related environments. PGE and nickeliferous minerals are insignificant in quantity; PGE's are formed by a higher degree of partial melting and melt-rock reaction, whereas Ni-sulphides are the product of serpentinization of the ultramafic rocks. Emerald occurs in several localities in the ISZ, whereas the peridot has so far been reported from only one locality. Emerald is associated with Mg-carbonate ± quartz ± fuchsite ± talc ± serpentine, and peridot (Fo 89-92 ) is associated with chrysotile ± magnesite ± talc ± magnetite ± ludwigite. Both are formed via hydrothermal alteration of ultramafic rocks through the introduction of hydrous fluids incorporating CO 2 , Be, B, K, Al, and possibly Na. In view of the undeformed nature of the emerald and peridot and the 23 Ma age of emerald formation, the mineralization appears to post-date deformation and may be of Late Oligocene-Early Miocene age. The metasomatising fluids were either derived from the subducting Indian plate crust underneath the Kohistan arc or were related to 23-26 Ma silicic magmatism in this area of the NW Indian plate. The nephrite jade in the ophiolite mélange is hosted in serpentinites and has disseminated grains of chrome spinel with elevated Fe/(Mg + Fe) ratios (0.10–0.18), implying low-grade metamorphism of ultramafic rocks (protolith of serpentinite) with loss of Al and introduction of SiO 2 during accompanying metasomatism. The rodingite in Dargai ophiolite could have formed by the action of seawater channeled through fractures and cracks in rocks. However, their mineral assemblages indicate that they formed as a result of the influx of Ca, Al, and Na-bearing hydrothermal fluids related to the intrusion of gabbros/sheeted dykes in the serpentinized ultramafic rocks, resulting in Ca-metasomatism.

Precambrian geological history of the Tarim Craton (NW China) involving the assembly and fragmentation of the Columbia and Rodinia supercontinents: review and synthesis

The Tarim Craton (NW China) is a significant archive of the tectonic events that occurred during the assembly and break-up of the Precambrian supercontinents Columbia and Rodinia. It provides a comprehensive record of crustal development during the Proterozoic. We review and synthesize the magmatic, metamorphic and stratigraphic records of the Precambrian Tarim Craton and delineate the geochronology of significant geological events that led to the formation of this major cratonic block. The extant geophysical and geological data show that the Tarim Craton consists mainly of the South and North Tarim blocks. The record of late Paleoproterozoic tectonothermal events is displayed well in and across the craton, involving the amalgamation of these two blocks to form its unified crystalline basement during the build-up of Columbia. However, the record of mid-Neoproterozoic events during the assembly of Rodinia is only exposed around its periphery. The Proterozoic record of the craton includes Mesoproterozoic–early Neoproterozoic low-grade metamorphic rock units and a late Neoproterozoic sedimentary cover. The early Mesoproterozoic stratigraphy in the SW, NE and SE of the Tarim Basin suggests a period of tectonic stability following the amalgamation of the North and South Tarim blocks in the late Paleoproterozoic, indicating the completion of its cratonic build-up.

Channel evolution in the lower reach of palaeo Fen River: evidence from detrital apatite petrochronology and zircon geochronology

This study provides a comprehensive analysis combining multi-element (Sr, Y, Th, U and REE) geochemistry and U–Pb geochronology of single-grain apatite with zircon U–Pb ages from channel sand deposits of the palaeo Fen River, collected at the Chaizhuang and Weijiazhuang sites, corresponding to sediments from the Late Pleistocene and late Middle Pleistocene, respectively. The results unveil that the analysed apatites originated from a varied assemblage of source rocks, encompassing mafic–ultramafic rocks, intermediate to felsic I-type plutons, highly fractionated S-type granites and pegmatites, as well as assorted metamorphic rocks. U–Pb age dating of detrital zircons and apatites identifies three predominant age peaks at 2.4, 2.2 and 1.8 Ga, consistent with known tectonothermal episodes within the North China Craton (NCC). The similarity in U–Pb age spectra for both zircon and apatite, along with comparable apatite geochemical characteristics across both the Chaizhuang and Weijiazhuang sections, indicates stable provenance with little to no change from the late Middle Pleistocene to the Late Pleistocene. This continuity in provenance signatures suggests an absence of shift in drainage patterns or sediment sources, indicating that the current location and pattern of Fen River lower reach drainage was not formed at least by then. Consequently, it is inferred that the detrital material composing the palaeo Fen River sediments during this period predominantly originated from the NCC.