Rock Assemblages Research Articles

Two-Stage Origin of K-Enrichment in Ultrapotassic Magmatism Simulated by Melting of Experimentally Metasomatized Mantle

The generation of strongly potassic melts in the mantle requires the presence of phlogopite in the melting assemblage, while isotopic and trace element analyses of ultrapotassic rocks frequently indicate the involvement of subducted crustal lithologies in the source. However, phlogopite-free experiments that focus on melting of sedimentary rocks and subsequent hybridization with mantle rocks at pressures of 1–3 GPa have not successfully produced melts with K2O >5 wt%–6 wt%, while ultrapotassic igneous rocks reach up to 12 wt% K2O. Accordingly, a two-stage process that enriches K2O and increases K/Na in intermediary assemblages in the source prior to ultrapotassic magmatism seems likely. Here, we simulate this two-stage formation of ultrapotassic magmas using an experimental approach that involves re-melting of parts of an experimental product in a second experiment. In the first stage, reaction experiments containing layered sediment and dunite produced a modally metasomatized reaction zone at the border of a depleted peridotite. For the second-stage experiment, the metasomatized dunite was separated from the residue of the sedimentary rock and transferred to a smaller capsule, and melts were produced with 8 wt%–8.5 wt% K2O and K/Na of 6–7. This is the first time that extremely K-enriched ultrapotassic melts have been generated experimentally from sediments at low pressure applicable to a post-collisional setting.

El registro lítico en fuentes de aprovisionamiento de sitios procedentes de El Ranchito, provincia de Córdoba.

En este trabajo se presenta un estudio tecno-morfológico sobre conjuntos líticos de diversas materias primas, entre ellas silcretes, rocas volcánicas/subvolcánicas, cuarzos y metacuarcitas, procedentes de siete cuadrículas de recolección de los sitios canteras taller de silcrete El Ranchito 1 y 2 (Dpto. Ischilín, Córdoba, Argentina). Los resultados demuestran la existencia de distintas trayectorias para estas materias primas, en las que se realizaron tareas de obtención, manufactura y uso mayormente de silcretes. La forma geométrica de los nódulos de esta roca silícea, a saber, tabular, laminar y esférica, ha sido una opción para la selección de formas base para su posterior manufactura final y uso en otros lugares, como así también para su manufactura y uso en las canteras.

Amphibolites of the metamorphic basement of Wrangel island: age, nature of the protolite and conditions of metamorphism

New data on petrography, geochemistry and geochronology of metamorphic rocks sampled from Neoproterozoic basement of Wrangel Island are presented. Oceanic affinity for mafic rocks underwent greenschist to amphibolite facies conditions (350600C) has been established by geochemical data. Protolith of this rock belongs to products of the evolution of magmatic melts parental for MORB. Protoliths age is 61713 and 69812 Ma was determined by U-Pb isotope dating of Zircons from these rocks (SHRIMP II and La-ICP MS). There are two main stage of metamorphism could be established: Neoproterozoi Early Paleozoi (amphibolite and greenschist facies conditions) and Middle Cretaceous (greenschist facies condition only), Last stage of metamorphism related to Chukcha deformation events accompanied by formation of folded nappes structure and consequent extension. An comparison of studied rocks with samples dredged at Chukcha Borderland evidenced for certain similarity existing between these rock assemblages judging by their petrography and chemistry.

Geodynamic nature of magmatic sources of North-West Pacific: an interpretation data on isotope composition of Sr and Nd in rocks dredged at Stalemate ridge, Ingenstrem depression, and Shirshov Rise

First data on isotope composition of Sr and Nd in rocks dredged at different areas belong to lithosphere of the NW Pacific are present. All samples examined were obtained from NW termination of Stalemate Ridge (NW Pacific) and Central part of Shirshov Rise (Western Bering Sea). Results of conducted study allow sure enough to judge on geodynamic affinity of the central segment of Shirshov Rise. Mafic-ultramafic rocks dredged here originated due evolution of magmatic melt formed by partial melting of source parental for MORB belongs to mantle wedge perhaps. Thus, this interpretation means that Shirshov Rise is remnant Back-Arc Spreading Center. Data on petrology and isotope chemistry of rocks from Stalemate magmatic assemblage demonstrate geochemical heterogeneity of their possible magmatic sources. The presented data allow to assume participation in magmatism of this region of NW Pacific source that responsible for formation of most older volcanic seamounts from NW Termination of Hawaiian-Emperor volcanic chain. There is petrographic similarity between rock assemblage recovered at NW Stalemate and plutonic rocks composed of xenoliths from volcanic effusions of Aleutian Island Arc exists. Considering the scarcity of existing information about the structure of the lithosphere in the NW Pacific it is possible to assume with caution the participation in the construction of the oceanic slope of the Aleutian Trench and the adjacent segment of the Stelmate Ridge fragments of Aleutian Arc basement.

Neoproterozoic tectonic evolution and exhumation history of transpressional shear zones in the East African orogen: implications from kinematic analysis of Meatiq area, Central Eastern Desert of Egypt

The origin, structural history, and tectonic evolution of core complexes and associated shear zones in the East African orogen are fundamental issues in the recent geological studies. Meatiq dome is one of the best exposed complexes in the Arabian–Nubian shield. The dome is occupied by metamorphic rocks with amphibolite-facies grade, tectonites, and deformed granites which are structurally overlaid by low-grade metamorphosed ophiolitic and island-arc rock assemblage. This tectonic pile is intruded by several granite intrusions through their evolution history. Microstructural, strain, and vorticity data from deformed tectonites were used to investigate the kinematics of rock flow in the Meatiq and Atalla shear zones and to reconstruct their deformation and exhumation history. The strain and vorticity analyses of Atalla shear zone (ASZ) indicate that this zone experienced general shear transpression with a monoclinic symmetry. The mean vorticity number (Wm) estimated with the porphyroclast-based methods ranges from 0.65 to 0.72 which indicate a high pure shear contribution in the shear zone. The numerical estimates allow the kinematic modelling of ASZ which introduces valuable implication about the tectonic framework of the Central Eastern Desert during the collision of East and West Gondwana. The results of satellite image interpretation and structural analysis of Meatiq area reveal a poly-deformational structural history including five stages of deformation during the successive convergence and collision of East and West Gondwana continents at around 600 Ma. The first stage is represented by compressional structures and high-grade metamorphism in the amphibolite enclaves preserved in Um Baanib granite. The reconstructed structural history of the shear zones started with the second structural stage. It involves the initiation of Meatiq shear zone and associated ductile structures and fabrics. The third stage was related to the final oblique convergence and collision of East and West Gondwana and represented by the NW–SE thrust faults, NW–SE folds, and the initiation of Atalla shear zone. The fourth and fifth stages were evolved after the crustal thickening. They involve the initiation of the strike-slip faults and the NW–SE extensional structures. The tectonic and regional implications of the structural and kinematic analysis were discussed.

Trace element geochemistry of magnetite and pyrite and sulfur isotope geochemistry of pyrite and barite from the Thanewasna Cu-(Au) deposit, western Bastar Craton, central India: Implication for ore genesis

The Thanewasna copper belt in Bastar Craton of central India is well-known for Cu ± Au mineralization. In this study, we use major and trace element chemistry of magnetite, hematite, and pyrite, as well as the sulfur isotope composition of chalcopyrite, pyrite and barite from the deposit to characterize the mineralization and to constrain its petrogenesis. The mineralization is hosted in quartz-chlorite veins associated with 2.5 Ga and 1.6 Ga granitoids. The mineralization is focused along the NW-SE trending Thanewasna brittle-ductile shear zone and comprises disseminated and vein-hosted Cu-sulfides, mainly chalcopyrite and pyrite intergrown with minor magnetite, hematite, and sulfide-free barite. The associated hydrothermal alteration is characterized by pervasive potassic-sodic alteration and chloritization. Calcite and sulfide-bearing barite veins postdate the Fe-oxide and Cu-sulfide ores. At least two generations of magnetite, hematite, and pyrite were identified based on their textural and trace element characteristics.Texturally early magnetite (Mag-I) displays oxy-exsolution of ilmenite and is of magmatic origin, representing the pre-mineralization igneous assemblage of the host rocks. Texturally later magnetite (Mag-II) is of hydrothermal origin and replaces Mag-I. It has high Co and Mg concentrations and shows dissolution and re-precipitation texture as well as evidence for high-temperature annealing. Martitisation of Mag-I and II associated with chloritization and Cu-Au mineralization produced hematite.The δ34SVCDT of syn-mineralization pyrite and chalcopyrite (−0.77‰ to −4.28‰) is suggestive of magmatic sources for the sulfur. Post-mineralization barite veins have δ34SVCDT between +10.31‰ and +17.55‰ which indicate that the sulfur was derived from seawater sulfate without going through an intermediate stage of reduction. Variation in the sulfur isotopic compositions of sulfide and sulfate was the result of dilution and cooling of the metalliferous fluid after interaction with meteoric fluids, which triggered the deposition of Cu ± Au along structural weak planes.The widespread presence of iron-rich breccias, association with crustal-scale shear zone, pervasive K-Na alteration, chloritic and advanced argillic alteration, the involvement of magmatic and non-magmatic fluid and Cu-Fe-Au-Ag-Ba-REE metal association are all suggestive of IOCG-style mineralization at Thanewasna. This supported by the Ti, Al + Mn, Ti + V concentrations and Ti/V, Ni/Cr, Ni/(Cr + Mn) ratios of Mag-II and hematite. The Co/Ni ratios of pyrite further supports a hydrothermal origin of the deposit, unrelated to skarns or copper porphyry, having similarity with known IOCG deposits. The identification of IOCG-type deposits in the Thanewasna region gives a boost to deeper sub-surface exploration in this belt as well as in similar geological setting elsewhere.

Structural anisotropy: Using image analysis to quantify block-in-matrix fabrics

Block-in-matrix rock assemblages — such as mélanges — are structurally complex units whose fabric is primarily defined, not by planar or linear features as most rock units are, but by the orientation and aspect ratio of irregularly shaped blocks. Despite this, previous attempts to quantitatively characterize mélange fabrics have predominantly focused on measuring the foliation using traditional techniques. Here we introduce a method of characterizing block-in-matrix fabrics using image analysis of field photos to discern the aspect ratios and orientations of blocks and define the structural anisotropy of the rock unit. We also include the software to calculate structural anisotropy and the trend of the fabric from image analysis data.

Tracing deep fluids in the Bozhong Sag: Evidence from integrated petrography and geochemistry data

Abstract The Bozhong Sag is a mature exploration area for petroleum in the Bohai Bay basin, China, which is a typical oil-bearing basin and hosts the Penglai (PL) 19-3, and Qinhuangdao (QHD) 32-6 giant oil fields (Xue, 2018). Despite 60 years of hydrocarbon exploration in the basin, only few large gas fields have been discovered. The discovery of the Bozhong (BZ) 19-6 gas field not only indicated that the Bozhong Sag has great potential for natural gas exploration, but also raised questions regarding the formation of large-scale natural gas fields in typical oil-prone basins and in sapropelic source rocks. The present study therefore performs a detailed investigation of the sources and migration pathways of the deep fluids in the Bozhong Sag area as a means of shedding light into this scientific challenge. The Bozhong Sag is located in the thinnest crust and tectonic transition zone of the Bohai Bay basin. Intense extension and thinning of lithosphere led to the upward ejection of deep crust and mantle materials under diapirism. Improvements in this tectonic setting and the fluid migration path occurred as a result of early faults that were formed by compressive thrust in the Indosinian-Yanshan orogenies in addition to a process of extensional development and reactivation of Himalayan faults. We obtained cores from 13 wells in the Bozhong Sag, and subsequently observed their petrography and analyzed samples for fluid inclusions. Also, volcanic rock samples were analyzed for major and trace elements, and carbonate rock samples were analyzed for δ13C, δ18O, and 87Sr/86Sr. Previously reported δ13CCO2 and He isotopic data were are also discussed. Our results showed that petrography was characterized by thermal fading and hydrothermal breccia around fractures filled with quartz and calcite. Deep fluid activity was mainly reflected by petrographic assemblages of quartz, ankerite, pyrite, plus calcite and volcanic eruptive rocks that were i) enriched with large ion-lithophile elements (LILE), ii) enriched with light rare-earth elements (LREE), iii) relatively deficient in high-field strength elements (HFSE), and iv) relatively deficient heavy rare-earth elements (HREE). Thermal anomaly events were reflected by the homogenization temperature of fluid inclusions, and characteristics of magmatic hydrothermal fluids were exhibited by δ13C, δ18O, and 87Sr/86Sr. Our findings indicated that deep fluids were mainly derived from the deep parts of the upper mantle, and that they were slightly contaminated by crustal materials. Combining our results with those of previous studies on deep xenoliths and xenolithic fluid compositions in eruptive rocks of eastern China (Zhang et al., 1999; Han et al., 2008), we interpreted that deep fluid properties in the study region were characterized by high H2 and low total volatile gases. We conclude that the deep fluids in the Bozhong Sag was mainly derived from deep material in the upper mantle, and that the fluid activity pattern was aided by the deep fracture-fault systems in addition to central and fissure-type wide-area eruption activities.

Amphibolites of the Metamorphic Basement of Wrangel Island: Age, Nature of the Protolith, and Conditions of Metamorphism

The paper presents newly acquired data on the petrography, geochemistry, and geochronology of metamorphic rocks sampled at the Neoproterozoic basement of Wrangel Island. Geochemical data on the mafic rocks metamorphosed at greenschist to amphibolite facies parameters (350–600°C) indicate their oceanic affinity. As follows from the geochemical data, the protolith of these rocks was made up of products of the evolution of magmatic melts parental for MORB. The U–Pb zircon age (SHRIMP II and LA-ICP MS) of the protoliths is 617 ± 13 and 698 ± 12 Ma. The rocks were affected by two episodes of metamorphism: one in the Neoproterozoic–Early Paleozoic (to the amphibolite and greenschist facies) and the other in the Middle Cretaceous (greenschist facies), with the latter episode related to the Chukchi deformation phase, which was accompanied by the development of the folded nappe structure and consequent extension. Comparison of the studied rocks with samples dredged at the Chukchi Borderland reveals certain petrographic and chemical similarities between these rock assemblages.

Detailed internal structure and along-strike variability of the core of a plate boundary fault: the Highland Boundary fault, Scotland

The Highland Boundary fault near Stonehaven, NE Scotland, provides a rare opportunity to study the internal fault structure of a well-exposed, along-strike section of an ancient plate boundary fault. As in many plate boundaries, serpentinite juxtaposes quartzo-feldspathic crustal rocks of distinct terranes. We report, for the first time, the complex internal structure of the Highland Boundary fault core, comprised of four structurally and chemically distinct clay-rich units that remain unmixed. Despite the evidence for internal strain, relatively intact clasts of wall rock and microfossils are preserved within the clay. The fault core clay minerology is consistent with a shallow, low-temperature authigenesis derived from shear-enhanced chemical reactions between wall rocks of contrasting chemistry during sinistral strike-slip. The observed structure is comparable to those of other major weak-cored plate boundaries (e.g. the San Andreas fault). Through detailed mapping, we demonstrate that the internal structure of a plate boundary fault core can vary in thickness and composition along-strike over centimetre- to metre-length scales. Earthquake rupture mechanics critically depend on the physical properties of fault rock assemblages. Therefore, models that investigate rupture propagation at active plate boundaries should incorporate, or else assess tolerance and sensitivity to, variable fault core thickness and composition. Supplementary material : Further X-ray diffraction (XRD) analytical data are available at https://doi.org/10.6084/m9.figshare.c.4663640

Classic Rock Tours 2. Exploring a Famous Ophiolite: A Guide to the Bay of Islands Igneous Complex in Gros Morne National Park, Western Newfoundland, Canada

Ophiolites are complex assemblages of ultramafic and mafic igneous rocks that are now widely considered to be pieces of ancient oceanic crust that were emplaced on to the continents courtesy of global plate tectonics. However, most examples were originally considered parts of enormous layered mafic intrusions and so were interpreted in that light. The new understanding of ophiolites in the late 1960s and early 1970s was a crucial part of the global Earth Science revolution, and they are now central to all plate tectonic models developed for ancient orogenic belts. Although their equivalence to oceanic crust is now well established, many ophiolites may not be ‘typical’ examples of such, and not all examples are identical. Most ophiolites likely formed in subduction-influenced environments rather than at mid-ocean ridges. Ophiolites remain important foci for research in the 21st century, and many questions remain about their environments of formation and especially their mechanisms of emplacement onto the continents. Although it was not the first to be seen as a relic of a vanished ocean, the Bay of Islands Igneous Complex in western Newfoundland is one of the best preserved and most easily accessible ophiolites in the world. In the late 20th century, research work in this area proved highly influential in understanding the oceanic crust, and in unravelling the diachronous events involved in the progressive destruction of an ancient stable continental margin as arcs and microcontinental blocks were accreted along it. Parts of the Tablelands Ophiolite lie within Gros Morne National Park, which is a UNESCO world heritage site because of its importance to our understanding of global tectonics. The wider region around the park also includes the Cabox Aspiring Geopark Project, now also in the process of seeking recognition through UNESCO. This article provides background information on ophiolites and the development of our ideas about them, and links this material to four self-guided field excursions that allow examination of many classic features. These excursions range from a collection of roadside outcrops, to some relatively easy hiking excursions on official National Park trails, and eventually to a more challenging off-trail hike that ascends to the summit plateau of the Tablelands to visit rare exposures of the Moho (the Mohorovicic Discontinuity, i.e. the lower boundary of the Earth’s crust) and the underlying upper mantle rocks. Collectively, the field stops should allow geologically-minded visitors to experience some amazing geology in a spectacular and sometimes surreal landscape.

An Early Ordovician fossil seamount of the Hongguleleng–Balkybey Ocean in the northern West Junggar terrane (NW China) and its implications for the ocean evolution

Seamounts are common constituents in the ocean basins today, and can be accreted in the accretionary prism or carried into the subduction zone together with the slab. Here, we report a typical rock assemblage of a fossil seamount in the northern West Junggar terrane (NW China), which is composed of ocean island basalt (OIB), marble, siliceous mudstone, and chert. The OIB yields a zircon SHRIMP U–Pb age of 475 ± 4 Ma, and the detrital zircons of siliceous mudstone give a weighted average age of 472 ± 2 Ma (n = 50, MSWD = 1.09), showing the fossil seamount formed in the Early Ordovician. Geochemically, all the basalt samples manifest as high concentrations of TiO2 (mostly within 1.51–3.22 wt%), Na2O + K2O (4.6–10.65 wt%), and high εNd(t) values (+3.01–+3.65), with LREE enrichment ((La/Yb)N = 11–20.6) and no Nb, Ta negative anomalies, which suggests the signature of OIB. Moreover, the OIBs are inferred to have originated from a plume-related mantle source and to have been produced by ca. 1%–3% garnet + spinel lherzolite partial melting in an oceanic intraplate setting. The seamount from the Hebukesaier ophiolitic mélange (HOM) is suggested to have accreted in the accretionary complex with oceanic fragments during the Middle–Late Ordovician.

Dolomite genesis and reservoir-cap rock assemblage in carbonate-evaporite paragenesis system

Regarding to the problem on the reservoir-cap rock assemblage evaluation in the carbonate-evaporite paragenesis system, this study examined the dolomite and reservoirs genesis and the characteristics of reservoir-cap rock assemblage. Based on the literature research of the global carbonate reservoirs and the case study on four profiles of carbonate-evaporite succession, together with geological and experimental work, three aspects of understandings are achieved. (1) Lithology of carbonate-evaporite paragenesis system is mainly composed of microbial limestone/bioclastic limestone, microbial dolomite, gypsum dolomite and gypsum salt rock deposited sequentially under the climatic conditions from humid to arid, and vice versa, and an abrupt climate change event would lead to the lack of one or more rock types. (2) There developed two kinds of dolomite (precipitation and metasomatism) and three kinds of reservoirs in the carbonate-evaporite system; and the carbon dioxide and organic acid generated during early microorganism degradation and late microbial dolomite pyrolysis process, and early dolomitization are the main factors affecting the development of microbial dolomite reservoirs with good quality. (3) In theory, there are 14 types of reservoir-cap rock assemblages of six categories in the carbonate-evaporite system, but oil and gas discoveries are mainly in four types of reservoir-cap rock assemblages, namely “microbial limestone/bioclastic limestone – microbial dolomite – gypsum dolomite – gypsum salt rock”, “microbial limestone/bioclastic limestone – gypsum salt rock”, “microbial dolomite – gypsum dolomite – gypsum salt rock” and “gypsum dolomite – microbial dolomite – tight carbonate or clastic rock”. These four kinds of reservoir-cap rock assemblages should be related with the climate change rules in the geologic history, and have good exploration prospects.

First geochronological constraint for the Palaeoproterozoic Lomagundi-Jatuli δ13C anomaly in the Tandilia Belt basement (Argentina), Río de la Plata Craton

Geochronological data of Palaeoproterozoic carbonates with δ13C values between +5‰ and +16‰ suggest that the positive carbon isotope excursion of the global Lomagundi-Jatuli event (LJE) occurred mostly between 2220 and 2060 Ma.In the southern portion of the Río de la Plata Craton, the LJE was recorded in calcite-diopside marble of the Tandilia Belt basement. The marble was found at the San Miguel area as part of a metamorphic rock assemblage, mainly composed of gneisses and migmatites, formed during the Transamazonian Orogeny. The latter represents the maximum magmatic and metamorphic activity in the Tandilia Belt basement. The marble, derived from a calcareous-siliciclastic sedimentary marine sequence, was intruded by leucogranites that provoked the San Miguel skarn formation.First U-Pb LA-ICP-MS age of 2206 ± 14 Ma, obtained from titanites of the skarn, also placed the metasomatic processes into the Transamazonian Orogeny. In addition, this age corresponds to the minimum age for the marble’s protolith deposition which matches with the beginning of the LJE worldwide. These new data would favor the correlation between the San Miguel marble and the Montevideo Formation marble of the Tandilia Terrane (Uruguay), within the Río de la Plata Craton, and not with the Paso Severino Formation of the Piedra Alta Terrane, which is at least 60 Ma younger.New δ13C determinations in calcite crystals from the marble allowed the lowest and the highest carbon values (+3.19‰ and +6.90‰, respectively) to be obtained. The lowest values are within the range of normal marine carbonates (between −5‰ and +5‰), despite having been altered by contact metamorphism, while the highest values are typical of the LJE carbonates. In this sense, the early deposition of the carbonate protolith within the LJE, together with the effect of post-depositional metamorphic processes would justify the wide range of values shown by the isotopic ratios of C.

The Western Amazonia Igneous Belt

The Western Amazonia Igneous Belt (WAIB) occupies the northern part of South American continent, and a total area close to 125,000 km2. The belt is composed dominantly by volcano-plutonic felsic rocks (Juruena supersuite, Teles Pires suite and Colíder group) and has dominant alkali-calcic, metaluminous to peraluminous, ferrous, geochemical characteristics, similar to A-type granites. The Silicic members are represented by granites and rhyolitic-rhyodacitic volcanic rocks, mafic members by gabbroic rocks and diabase dykes. Intermediate rocks are rare. Local magma mingling and hybridization are present in several areas. The assemblage of silicic and basic rocks of WAIB was formed between c. 1825 and 1757 Ma. The presence of inherited zircons, from 1875 to 2050 Ma, as well as some Archean ages in the granitic and volcanic rocks from the WAIB, are suggestive of derivation by melting of pre-existing crustal basement (Ventuari-Tapajós Province). This hypothesis is also corroborated by bimodal magmatism and by several model-ages of granites, felsic volcanic rocks and mafic plutonic rocks, with TDM ranging from 2.0 to 2.3 Ga, and εNd value, varying from −3.90 to +2.52.The preferred model of formation of the WAIB envisages the emplacement of large volumes of hot mantle-derived melts into the lower crust. These processes resulted in partial melting of lower-crustal material, mixing with the mantle-derived melts and formation of granitic melts. The timing and spatial features and interpreted petrogenetic mechanisms of WAIB are suggestive of a continental rift setting. In addition, this volcano-plutonic province is similar to silicic igneous provinces (Silicic Large Igneous Province-SLIP) in areal extent, eruptive volume and petrogenetic characteristics.

Building the core of a Paleoarchean continent: Evidence from granitoids of Singhbhum Craton, eastern India

A significant part of many Archean cratons formed during Paleoarchean. Yet, the mechanism and tectonic setting of formation of Paleoarchean continental crust remain highly debated. In this contribution, we present field, petrographic, geochemical, zircon U-Pb age and Hf isotope data on Paleoarchean granitoids from west-central part of the Singhbhum Craton (Champua-Hat Gamharia corridor), India. The whole process starting from nucleation of a juvenile continent to its evolution and final stabilization is documented. The core of the craton nucleated with formation of 3.45–3.40 Ga TTGs showing juvenile character (zircon ɛHft = +0.6 to +7.1). These rocks show slightly depleted HREE and Y, negligible Eu-anomaly (Eu/Eu* = 0.90 to 1.00) and moderate Sr/Y (25–64), consistent with derivation from a low-K mafic crust at a pressure near the lower end of the garnet stability field, causing subordinate garnet retention in the residue and negligible role of plagioclase. A second generation of TTG formation took place at 3.32 Ga in the area by deeper melting of a juvenile mafic crust (zircon ɛHft = +1.3 to +5.7) as suggested by strongly depleted HREE and Y, and high Sr/Y (52–155) implying significant amount of garnet retention in the residue. Subsequently, the area witnessed intracrustal melting at 3.28 and 3.25 Ga which tapped moderately old to juvenile (zircon ɛHft = −1.9 to +4.5), mostly TTG sources at variable depths generating potassic, LILE-enriched, high-silica granites. Intrusion of these potassic granites marks the final cratonization of the Singhbhum Craton. The sequence of events is interpreted in terms of repeated crustal melting and granitoid generation in a gradually thickening oceanic plateau with a progressive change in granitoid source from mafic to felsic in composition. A synthesis of rock assemblage, regional geological setting and structural pattern also supports intraplate nature of the magmatism in Singhbhum Craton, which might have been a significant mechanism of crustal growth worldwide during Paleoarchean. Further, a comparison of juvenile crustal growth and crustal reworking events of the Singhbhum and other Indian cratons show that these cratons record distinct evolutionary histories and were probably nucleated at different sites.

Stratigraphic update, paleotectonic, paleogeographic, and depositional environments of the Crixás Greenstone Belt, Central Brazil

The Crixás Greenstone Belt is a supracrustal rock sequence of the allochthonous Archean-Paleoproterozoic Crixás-Goiás Terrane of the Neoproterozoic Tocantins Province of Central Brazil. The belt occurs between Mesoarchean (2.9-2.8 Ga) primitive orthogneiss complexes intruded by Siderian intracratonic mafic dikes and stocks. The rock succession of the greenstone belt consists of metakomatiites followed by metabasalts, both of greenschist to low amphibolite facies, underlying metasedimentary rocks of low greenschist facies. The greenstone belt is divided into the southern, central, and northwestern sectors, with different rock associations. The northwestern sector tectonically underlies a small exotic block known as the Mina Inglesa Sequence. The sequence consists of a low greenschist facies volcano-sedimentary package similar to a greenstone belt, but of unknown origin. The latest stratigraphic nomenclature for the belt labels its rock package as the Crixás Group. From base to top, the Group comprises the Córrego Alagadinho, Rio Vermelho, and Ribeirão das Antas Formations, made up of metakomatiites, metabasalts, and metasedimentary rock, respectively. The Mina Inglesa Sequence is an independent unit of the Crixás Group, in spite of the similar rock assemblage of both units. Protoliths of the Ribeirão das Antas Formation comprise shallow water black shales with dolomite lenses, laterally wedge-shaped with impure turbiditic sandstones (wakes). Published U/Pb and Sm/Nd whole-rock isotopic data indicate that the metakomatiites and metabasalts erupted during the Mesoarchean (3.0 Ga). Detrital zircon grains from the sedimentary protoliths suggest a mixed Mesoarchean and Rhyacian source-area (3.2 and 2.2 Ga). The U/Pb age from hydrothermal zircon grains of a gold-bearing quartz vein indicates that the sedimentation ended just before 2.1 Ga. Field, drilling, and the isotopic data led the authors to review the stratigraphic content of the Crixás Group by subdividing it into an Upper and Lower Subgroups. The Lower Subgroup comprises the Mesoarchean volcanic section subdivided, from base to top, into the currently adopted Córrego Alagadinho and Rio Vermelho Formations. The Upper Subgroup rests ubnconformable on the Lower Subgroup, and comprisess the Rhyacian metasedimentary section. The Upper Subgroup contains two rock units deposited in different environments. The currently used Ribeirão das Antas Formation is the lower unit, which is redefined as composed of carbonaceous phyllites with dolomite lenses. The upper unit comprises the new proposed Córrego Geral Formation, composed of metagraywackes. The wedge-shaped contact between both sedimentary units indicates that they were coevally deposited. The Paleoproterozoic sedimentary basin consisted of a small depression formed during the sagduction of the volcanic pile, triggered by the diapiric rise of the adjacent orthogneiss protoliths. The sediment deposition started with black shales in response to a slow rise of the seawater and paleoclimatic conditions. An increasing input of siliciclastic turbidites occurred in response to the uplift of a Rhyacian/Mesoarchean source-area that provided a mixed clastic load. Anomalous positive δ13C data from the dolomite lenses indicate that they deposited during the 2.2-2.1 Ga Lomagundi Event, which is compatible with the dominant Rhyacian zircon grains of the graywackes. The Crixás-Goiás Terrane lacks geologic evidence for the existence of an internal Rhyacian crustal block to justify the youngest detrital zircon grains of the graywackes. These observations render uncertainty about the geographic position of the rising crust. Paleoproterozoic crustal shortening of the belt resulted in stacked thin-skinned thrust-faults that host gold orebodies. An eastward age decrease of the crustal shortening from Crixás to an easterly belt leads the authors to propose that the clastic load source could occur to the West of the Terrane. The Mina Inglesa Sequence is an allochthonous block with lithologic and isotopic features that distinguish it from the greenstone belt. The authors also describe that there is no isotopic evidence for Meso- to Neoproterozoic source-areas, neither for the Córrego Geral Formation nor for the Mina Inglesa Sequence. These findings raise a suggestion that the granite-greenstone belt association of the Terrane may represent a craton border that remained stable before its amalgamation to the province during the Neoproterozoic.

Tectonic Transformation and Metallogenesis of the Yanshan Movement during the Late Jurassic Period: Evidence from Geochemistry and Zircon U-Pb Geochronology of the Adamellites in Xingcheng, Western Liaoning, China

The Yanshan Movement occurred mainly during the Middle-Late Jurassic, and gave rise to NE trending structures, magmatic events, volcanism and mineral resources. The transformation and evolution of the movement during the Middle-Late Jurassic were investigated from the rock assemblage, geochemistry, and chronology in adamellites which were exposed in the Xingcheng area, western Liaoning. Two types of adamellites were recognized—biotite adamellites with the formation age of 172–168 Ma and garnet-bearing adamellites of 158–152 Ma. All the samples of the two types of adamellites displayed enriched characteristics with high content of SiO2 (66.86–75.55 wt.%) and total alkali (Na2O + K2O = 7.56–8.71 wt.%), high large ion lithophile element (LILE: K, Rb, Sr), and low high field strength element (HFSE: Ce, Ta, P, Ti). The biotite adamellites belong to metaluminous-peraluminous I-type granites, and show volcanic arc granite characteristics, and were formed by partial melting of the ancient crust in the compressional setting that resulting from the subduction of the Paleo-Pacific plate beneath the north margin of the North China Craton (NCC). The garnet-bearing adamellites are also metaluminous-peraluminous I-type granites, with characteristics of both the compressional and extensional regimes, which were formed at the middle-late stages of the continuing subduction of the Paleo-Pacific plate, while simultaneously, the frontal side of the subduction slab began to roll back, leading to an extensional environment. Combining with regional geophysical studies and our petrological and geochemical studies, we propose that the eastern segment of the northern margin of NCC may have been controlled by the Paleo-Pacific tectonic domain at the latest in the Middle Jurassic, while the initiation of the tectonic regime from a compressional to an extensional environment was during the Late Jurassic (158–152 Ma) as a response of the Yanshan Movement. Simultaneously, geochronological statistics of the ore deposits in western Liaoning show that the Mesozoic endogenetic metalliferous deposits formed in a compressive environment influenced by the subduction of the Paleo-Pacific plate, similar to the magma events in ages, and the magmatism provided the thermodynamic condition and the source of metallogenic hydrothermal fluid for mineralization.

Hydrothermal origin of early Permian saddle dolomites in the Junggar Basin, NW China

Hydrothermal-related saddle dolomites widely exist in the lower Permian Fengcheng Formation in the northwestern Junggar Basin, and this formation is a set of carbonate-rich deposits in an alkaline saline basin. Core and thin section observations, cathodoluminescence (CL) imaging, and carbon and oxygen isotope composition analyses have been used to study the origins of saddle dolomites with various occurrences. Hydrothermal fluids include Si-rich and Si-poor fluids, and saddle dolomites occur, along with siliceous bands, as debris and assemblages in fragmented rocks and as cements filling pores and microfractures. Different types of dolomites in the Fengcheng Formation have similar δ13C values, while the δ18O values differed from each other, which demonstrated that the dolomites originated from the same primary carbonates and underwent multiple stages of recrystallization. The δ18O values were extremely negative, which was caused by fractionation at high temperatures from hydrothermal fluids. Dolomites are directly or indirectly affected by hydrothermal diagenetic fluids. According to the diagenetic stages, the types of migration paths for hydrothermal fluids, and the patterns of hydrothermal fluids acting on the dolomites, a model was proposed to illustrate the origins and evolution of the dolomites, and three paths were utilized. Path I occurred during the contemporaneous diagenetic stage, when the carbonate particles were still free and could be captured by high-density bottom hydrothermal flows and distorted. Path II occurred during the long diagenetic history after the carbonate-bearing sediments had consolidated. Saddle dolomite assemblages and saddle dolomite debris occurred with the aid of compressive tectonic forces. Path III also occurred in the long diagenetic history, and the host rocks were not fragmented. The saddle dolomites reprecipitated as cements in the pores, stylolites, fractures/microfractures or laminated seams.

The physical-chemical regime of a sulfide-bearing semi-graphite mineral assemblage in metabasic rocks (SE Germany) – A multidisciplinary study of the missing link between impsonite and graphite

Along the western edge of the Bohemian Massif, SE Germany, graphitic carbon occurs in metabasic rocks plus calcsilicates, metabiolites, and paragneisses (graphite I), in pegmatites (graphite II) and in mineralized structure zones (semigraphite and impsonite) The current studies unveiled these graphitic carbon compounds are strikingly different with regard to their age and temperature of formation: Graphite I (≥324 Ma, 570 to 625 °C), graphite II (317 ± 3 Ma, >400 °C), semi-graphite (305 ± Ma, 225 to 400 °C), impsonite (<298 ± 4 Ma, 100 to 363 °C). Semi-graphite takes a special position among these graphitic carbon compounds because it links the different carbon modifications with regard to its age of formation, its structural position and its S- and C isotopes that point to a mantle and crustal influence on its formation in contrast to graphite (graphite I: crustal, graphite II: mantle) and impsonite (mantle). Semi-graphite precipitated in a fault zone which evolved from a zone of strong felsic mobilization in metabasic rocks spawning KNa feldspar-quartz pegmatoids/aploids. During conversion of a pre-existing zone of felsic mobilization into a brittle shearzone not only carbonaceous matter but also Ni-, Pb-, Cu-, Zn-, As-, Fe-, Hg- and Mo sulfides were concentrated. The semi-graphite-bearing mineralized zone is located near rare element pegmatites hosting graphite flakes. The C-bearing systems are useful pathfinders to locate structurebound mineral deposits hosting U or base metals in the Variscan orogen.The structurebound metalliferous semi-graphite mineralization in metabasic rocks can be taken as a reference type of dual-source hydrocarbon immigration into fault zones, syn- to postkinematically relative to the fault movement. The model can be applied to host rocks undergoing retrograde medium- to very-low-grade stage dynamo-metamorphic conditions.To elucidate the complex history of the various types of graphite and metamorphosed bitumen a multidisciplinary approach has been taken involving petrographic and geological field mapping combined with drill core examination, petrographic and ore microscopy supplemented by electron microprobe, X-ray diffraction and scanning electron microscopy with EDX, micro-Raman spectroscopy in addition to classical coal petrographic studies, and inorganic geochemistry of major and minor elements and isotope (carbon and sulfur) chemical analysis followed up by a statistical treatment of the various chemical datasets.