Burial Metamorphism Research Articles

Barrovian metamorphism in nominally lowermost Alpujarride Complex units: tectonic implications for the development of the orogenic wedge in the Western Mediterranean

ABSTRACT The sedimentary successions of the Permian-Triassic rifting related to the Alpine Tethys in the easternmost Lower Alpujarride units (Internal Betic Zone, south Spain) host shallow-intrusive basic bodies of Mesozoic age. These gabbroic bodies experienced low-grade Alpine metamorphic overprint, singularly recorded in three outcrops nearby Alicante city in southeast Spain: Callosa and Orihuela Ranges and Nueva Tabarca Island. Overall, the metabasic bodies still preserve magmatic ophitic/subophitic texture and relicts of magmatic augite and ferropargasite-ferroedenite-edenite and kaersutite. In contrast, metamorphic minerals include magnesioriebeckite, actinolite, albite, stilpnomelane, phengite, and chlorite. Major and trace element geochemical data suggest crystallization of the igneous protoliths from subalkaline (tholeiitic) to alkaline within-plate (continental) basaltic magmas slightly enriched in incompatible elements. These magmas with E-MORB geochemical affinity very likely intruded at shallow crustal levels in an extensional-synrift-geodynamic setting related to the breakoff of Pangea in the region. Pressure-Temperature (P-T) conditions estimated by average P-T method and pseudosection calculations suggest metamorphic peak at ca. 6 kbar at 340°C consistent with burial metamorphism down to ca. 18 km depth with intermediate thermal gradients of ca. 18°C/km. These conditions are different to the typical subduction-related blueschist facies conditions of the Lower Alpujarride units and suggest, instead, a collisional (Barrovian) event. This can be explained by a late-stage accretion of the studied units after the main subduction-related event, pointing to the dispersion of the corresponding Lower Alpujarride sections along the margin in different palaeogeographic positions before deformation and movement to their actual structural position. The metamorphic correlation of the studied units with the accreted passive margin of South Iberia (Subbetic zone) is also discussed.

Evolution of the neoarchean Kola alkaline granites, northeastern Fennoscandian Shield: insights from SHRIMP-II titanite and zircon U–Pb isotope and rare earth elements data

Abstract An U–Pb isotopic investigation combined with rare earth element data for titanites, zircons and coexisting accessories has been undertaken to gain insight into the formation of the Archean peralkaline granites of the northeastern Fennoscandian Shield and to test the stability of titanite during metamorphic and hydrothermal processes. The obtained set of isotope data shows that whereas more stable zircon retains a memory of the major episodes of granite evolution, the coexisting titanite provides additional information on crystallization, subsequent growth, cooling and alteration of the plutonic complexes. In addition to titanites formed at the magmatic stages of c. 2710 and c. 2650 Ma, the peralkaline granites contain titanite populations which have undergone major resetting at c. 1870 Ma during the burial metamorphism related to the Svecofennian orogeny and the 1760 Ma hydrothermal alteration near contemporaneous with regional metamorphism of c. 1780 Ma. The peralkaline granites which contain c. 2710 Ma titanites also include inherited titanite grains of an age of 2795 Ma. These data support the concept that the titanite can remain a closed system to Pb diffusion at temperatures of peralkaline granite melt (<800°C) as long as the crystals escape magmatic or metamorphic recrystallization.

Phase equilibria modelling and geochemistry of high‐grade gneiss from the Chhotanagpur Granite Gneiss Complex, eastern India: Implications for tectono‐metamorphic evolution

We investigated high-grade gneiss from the Daltonganj region of the Chhotanagpur Granite Gneiss Complex (CGGC) in eastern India. EPMA monazite dating from the Daltonganj region revealed three age domains at ca. 1434, ca. 978, and ca. 850 Ma, which have been interpreted as the protolithic age and two metamorphic stages of high-grade gneisses, respectively. The Grt1 + Chl + Amp1 + Bt assemblages are thought to represent the first metamorphism as a pre-peak stage of high-grade gneiss, which could have occurred after ∼1434 Ma. The peak metamorphic stage is defined by relict porphyroblasts of orthopyroxene and garnet2 formed during ∼978 Ma in a burial process followed by an isothermal decompression or post-peak stage represented by a Crd + Grt3 + Amp3 assemblage at ∼850 Ma. The P-T conditions estimated for the pre-peak, peak, and post-peak stages using the NCKFMASH model system are 6.6-6.8 kbar/635-645°C, 8.65-9.42 kbar/772-788°C, and 5.71-6.18 kbar/745-762°C, respectively. The geochronology and metamorphic conditions define a clockwise P-T-t path for the high-grade gneiss, indicating that rocks experienced high-pressure conditions during burial metamorphism at subduction-related tectonic activity, followed by an isothermal decompression condition after the gneissic unit was exhumed. The geochemical composition of the high-grade gneiss reveals that the protolith is calc-alkaline basaltic magma formed in an island arc setting and that the restitic features of the gneisses developed after granitic melt segregation.

Neoproterozoic thermal events and crustal growth in the Zambezi Belt, Zambia: New insights from geothermobarometry, monazite dating, and detrital zircon geochronology of metapelites

The Zambezi Belt located in southern Zambia and northern Zimbabwe forms part of the E–W-trending Kuunga orogen associated with the amalgamation of Gondwana supercontinent. The belt consists of a crystalline basement unconformably overlain by a supracrustal sequence of meta-pelites/psammites, calc-silicates, meta‑carbonates, and magmatic rocks. Monazite-bearing meta-pelites are distinguished by petrography, geothermobarometry, monazite chemistry, and detrital provenance. The studied samples include six garnet-bearing schists with biotite, plagioclase, quartz, muscovite ± staurolite ± kyanite from the Nega Formation, and three kyanite-mica schists with chlorite, quartz, and accessory rutile, ilmenite, tourmaline from the Chipongwe Formation. Phase equilibria modelling and isopleth geothermobarometry identified Barrovian-type medium- T / medium- P facies peak conditions of 570–665 °C/ 6.5–9 kbar on garnet-bearing schists, and 600–663 °C/ ~8.8–12 kbar on kyanite-mica schists. Evidence from detrital geochronology suggests that the two formations are stratigraphically equivalent, but electron microprobe dating on monazite suggests they underwent different tectonothermal histories. Monazite ages indicate metamorphism and crustal growth from ~650–579 Ma in 3 of the garnet-bearing schists of the Nega Formation, and ~ 550–500 Ma in one of the kyanite-mica schists of the Chipongwe Formation. Dating of detrital zircon by LA-ICP-MS indicates local and distal sediment sources including Neoarchean to Paleoproterozoic (2633–1800 Ma) and Meso- to Neoproterozoic (1700–852 Ma). A tectonic cycle of burial metamorphism to exhumation is indicated by clockwise P-T-t paths along different geothermal gradients. The rock units were brought together during collision between the Congo and Kalahari Cratons and final assembly of Gondwana. • The Zambezi Belt is part of the Kuunga Orogen formed during Gondwana assembly • Metapelites are distinguished by monazite chemistry and detrital provenance • P-T conditions of 570–665 °C/6–9 kbar and 600–663 °C/8.8–12 kbar were obtained • The rocks underwent metamorphism at 〜650–579 Ma and 550–500 Ma respectively • The rocks evolved along different geotherms prior to final Gondwana assembly

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).

Tectonic Evolution of the Nevado‐Filábride Complex (Sierra de Los Filábres, Southeastern Spain): Insights From New Structural and Geochronological Data

AbstractThe high‐pressure metamorphic Nevado‐Filábride Complex (NFC) in the Betics mountain range of southeastern Spain exhibits continental and ocean‐derived tectonic units, which are key for understanding the geodynamic evolution of the Western Mediterranean. We address the current debate in the definition of tectonic units, the emplacement of (ultra)mafic rocks, and the timing of burial metamorphism by conducting a structural study combined with single grain fusion 40Ar/39Ar dating of white micas in structurally critical outcrops of the eastern Sierra de Los Filábres. One older 40Ar/39Ar age population (38–27 Ma) is found at distance from the main shear zones in the relics of an early foliation, while a younger 40Ar/39Ar population (22–12 Ma) is dominant in the vicinity of these shear zones, where the early foliation is obliterated. Both age groups are interpreted as the record of deformation or fluid‐induced recrystallization during distinct fabric‐forming events, while alternative scenarios are discussed. A key observation is the presence of an ophiolitic mélange, which—together with new and published geochronological data—allows for a new tectonic hypothesis. This considers Paleogene subduction beneath a Jurassic oceanic lithosphere, followed by the continued subduction of NFC and overlying ophiolites below the Alpujárride Complex. Exhumation during westward slab roll‐back led to the formation of an extensional detachment system that obliquely cut nappe contacts. Although the timing constraints for high pressure‐low temperature (HP‐LT) metamorphism in the NFC remain inconclusive, the new tectonic hypothesis provides a solution that can account for both Paleogene and Miocene ages of HP‐LT metamorphism.

Metamorphic Evolution of the Amphibolites from Bundelkhand Craton, Central India: P-T Constraints and Phase Equilibrium Modelling

The amphibolites from the Mauranipur and Babina regions are located in the central part of the Bundelkhand Craton (BuC), northern India. During the geodynamic evolution of the BuC, these amphibolites underwent medium-grade metamorphism. This study combines textural observations of amphibolites from two distinct regions (Mauranipur and Babina) with mineral chemistry and phase equilibrium modelling. Observations suggest that the amphibolites of both areas have gone through three stages of metamorphism. The pre-peak stage in the amphibolites from the Mauranipur and Babina regions is marked by the assemblages Ep-Amp Cpx-Pl-Ilm-Ru-Qz and Ep-Amp-Cpx-Pl-Ab-Ilm-Qz respectively; the peak metamorphic stage is characterized by the mineral assemblages Amp-CpxPl-Ilm-Ru-Qz and Amp-Cpx-Pl-Ilm-Qz-H2O, which is formed during the burial process, and the post-peak stage is represented by the assemblages Amp-Pl-Ilm-Ru-Qz and Amp-Pl-Ilm-Qz-H2O respectively, which is formed by exhumation event. By applying the phase equilibria modelling in the NCFMASHTO system, the P-T conditions estimated from pre-peak, peak to post-peak stages are characterized as 6.7 kbar/510 ºC, 7.3 kbar/578 ºC and > 3.0 kbar/> 585 ºC, respectively, for the Mauranipur amphibolites; and 6.27 kbar/520 ºC, 5.2 kbar/805 ºC and > 3.0 kbar/> 640 ºC respectively for Babina amphibolites. The textural association and P-T conditions of both amphibolites suggest that these rocks were affected by burial metamorphism followed by an exhumation process during subduction tectonism in the BuC.

Hellandite-(Y)–hingganite-(Y)–fluorapatite retrograde coronae: a novel type of fluid-induced dissolution–reprecipitation breakdown of xenotime-(Y) in the metagranites of Fabova Hoľa, Western Carpathians, Slovakia

AbstractTwo contrasting reaction coronae were developed around rare earth element (REE) accessory phosphates in Variscan metagranitic rocks, which have been overprinted by Alpine blastomylonitisation from the Fabova Hol'a Massif, in the Veporic Unit, Western Carpathians, Central Slovakia. The Th–U–Pb total EPMA age determination of primary magmatic monazite-(Ce) from the metagranite indicates a Carboniferous (Mississippian, Tournaisian) age of 355 ± 1.9 Ma. Monazite-(Ce) breakdown resulted in impressive, though common, fluorapatite ± Th-silicate + allanite-(Ce) + clinozoisite coronae. The alteration of xenotime-(Y) produced a novel type of secondary coronal micro-texture consisting of a massive fluorapatite mantle zone and tiny satellite crystals of hellandite-(Y) [(Ca,REE)4Y2Al□2(B4Si4O22)(OH)2] and hingganite-(Y) [Y2□Be2Si2O8(OH)2] of ~1–5 μm, and rarely ≤10 μm in size. The localised occurrence of Y–B–Be silicates, which are associated closely with other secondary minerals, suggests the involvement of B and Be during the metasomatic alteration transformation of xenotime-(Y). General reactions for monazite-(Ce) and xenotime-(Y) decomposition, including the fluids involved, can be written as follows: Mnz + (Ca, Fe, Si, Al and F)-rich fluid → FAp + Ht + Aln + Czo; Xtm + (Ca, Fe, Si, Al, F, B and Be)-rich fluid → FAp + Hld + Hin + Czo.The granitic rocks underwent Early Cretaceous burial metamorphism under greenschist- to lower amphibolite-facies P–T conditions. Subsequently, Alpine post-collisional uplift and exhumation of the Veporic Unit, starting from the Late Cretaceous epoch, was accompanied by a retrograde tectono-metamorphic overprint; the activity of external fluids, caused the formation of secondary coronae minerals around monazite-(Ce) and xenotime-(Y). A portion of B (± Be) should have been liberated from the metagranite feldspars, micas, or xenotime-(Y) enriched in (Nb,Ta)BO4 (schiavinatoite or béhierite) components. However, the principal source of B and Be in fluids necessary for the production of hellandite and hingganite, was probably of external origin from adjacent magmatic, metamorphic, or sedimentary rocks (Permian granites, rhyolites and sedimentary rocks, and Palaeozoic metapelites).

Zeolite Minerals from Wat Ocheng, Ta Ang, Ratanakiri Province, Cambodia – Occurrence, Composition, and Paragenesis

ABSTRACT Located in the southwest corner of the Ratanakiri Volcanic Province, the Wat Ocheng basalt is the first known zeolite locality in Cambodia. The basalt is a fine-grained, vesicular to amygdaloidal, subalkaline to transitional alkaline intraplate tholeiite comprised of 30% lath-like plagioclase (average: Ab51An45Or4), 35% interstitial augite (average: Wo44En35Fs21), 25–30% zeolites after plagioclase and volcanic glass, and minor skeletal ulvöspinel. It contains mineralized amygdales ranging in size from 0.5 × 0.5 cm to 5 × 9 cm. Nine zeolite species occur at Wat Ocheng, including analcime, chabazite-Ca, gonnardite, natrolite, phillipsite-Ca, and thomsonite-Ca, along with clays, aragonite, calcite, and pyrite. All the zeolite species are being described from Cambodia for the first time. The zeolite and secondary mineral assemblages observed at Wat Ocheng are similar to those reported from other alkaline basalt localities, including those in neighboring Vietnam, and are a product of alteration of the primary Ca-Na minerals and volcanic glass as a result of burial metamorphism and infiltration of heated meteoric waters. The mineral assemblage is not homogeneous across amygdales within the exposed lava flow, suggesting localized closed systems, likely the result of early precipitation of clay minerals and fine-grained zeolites. Decreased porosity and differences in fluid geochemistry would account for the diversity in the observed assemblages. Four stages of hydrothermal alteration and zeolitization have been defined based on mineral textures and chemistry. Zeolite formation began with fine-grained Ca- (chabazite-Ca and phillipsite-Ca) and Na- (analcime) dominant, high TSi (Si/Si+Al) species in Stage II following deposition of clay minerals in Stage I. Stage III is characterized by increasing Na+K contents and decreasing TSi. Crystallization of coarse-grained chabazite-Ca and phillipsite-Ca with increasing Na+K contents in the rims of the crystals followed the development of natrolite with a later-stage epitaxial overgrowth of thomsonite-Ca. The final stage of mineralization (Stage IV) included late-stage calcite, pyrite, and termination of growth of acicular sprays of thomsonite-Ca. Post-magmatic cooling and circulation of meteoric water and fluids derived from alluvial sediments overlying the basalts were involved in zeolitization. Thermal sources include an underlying basaltic andesite flow as well as regional deep-seated, extensional pull-apart structures, the result of a thinned lithosphere and injection of fertile mantle following the collision of the Eurasian and Indochina plates during the Himalayan Orogeny.

The Barzavand and Neysian Copper Deposits, NW Naein, Central Iran zone: Constraints on styles mineralization and geochemical signatures of hydrothermal alterations.

Field observations in the northwest of Naein (Central Iran zone) indicate various types of alteration products have extended into pyroclastic and volcanic units with Oligo- miocene age. It seems the Zefreh (with NW- SE trend) and Kachumesqal (with E-W trend) faults are effective factors in the formation of Neysian and Barzavand copper deposits respectively. Alteration in Barzavand copper deposit (with stratabound form) includes pyritization, propylitization, zeolitization, saussuritization, uralitization and silicification, while in Neysian (with oval form) include phyllic, advanced argillic, argillic and propylitic. Geochemical studies in Barzavand show the positive correlation between (La/Lu)N, (La/Yb)N and (La/Sm)N and CaO (r= 0.70 to 0.96) related to propylitization of host rock basalt and increase in pH of fluids responsible for mineralization that play important role in differentiation of lanthanides in study area. The Barzavand copper deposit has submarine volcanism, diagenesis, burial metamorphism and weathering stages during its evolution. Remarkable geochemical features in Neysian include high (La/Lu)N, Ba and Sr values than La, Ce and Y. It seems that extensive alteration occurred along with hypogene and supergene processes in Neysian. According to alteration properties, mineralogy and the whole-rock geochemistry, the Barzavand and Neysian copper deposits are most similar to Manto- type and porphyry copper deposits respectively.

Evolution of the Garmab-e-Paein native copper-rich volcanogenic massive sulfide deposit from northeast Iran: Insights from sulfur isotope and chlorite chemistry

The Garmab-e-Paein Cu-Ag volcanogenic massive sulfide (VMS) deposit occurs as stratiform and stratabound orebody within a specific stratigraphic horizon in a Late Cretaceous volcano-sedimentary sequence of the Sabzevar zone, northeast Iran. The host rocks to mineralization are andesitic-trachyandesitic volcanic and volcaniclastic rocks. Based on textural and mineralogical studies, the VMS mineralization is comprised of four ore facies: 1) vein-veinlets (stringer) containing chalcopyrite, pyrite and minor magnetite, 2) massive ore, dominated by pyrite and minor chalcopyrite, 3) bedded ore containing laminated pyrite, and 4) exhalite containing Fe-Mn oxide-hydroxides such as hematite, psilomelane and pyrolusite. Wall rock alteration styles are chloritic, minor silicic and secondary argillic. There is a distinct metal zonation in the massive sulfide orebody; Au, Pb, As, and Ag contents and the Cu/Zn ratio increase vertically from the stringer to the massive facies, but decrease laterally from the stringer to the bedded facies. Zinc and Mn increase from the stringer to the bedded ore facies. Sulfur isotope values for pyrite range from −1.7 to +5.1‰, with average +1.4‰. The δ34S values of massive ore facies (−1.7 and +2.7‰) increase downward toward the stockwork (+2.2 to +4.3‰) and laterally outward toward the bedded (+5.1‰) zones. The near-zero sulfur isotope values indicate that most of the sulfur was derived from leaching of underlying host volcanic rocks by hydrothermal fluids circulating in the high temperature reaction zone.Extensive disseminations and vein-veinlets of native copper mineralization formed within the hanging wall Late Cretaceous volcanic rocks and younger Paleocene conglomerates. The native copper mineralization is accompanied by chlorite and zeolite alteration, and both formed after formation of the Garmab-e-Paein VMS deposit during diagenesis, burial metamorphism and uplift. The deposit is now potentially economic due to the native copper mineralization.Two generations of chlorite are observed in the deposit: Chl-I is accompanied pyrite and chalcopyrite, in the VMS mineralization system, and the second Chl-II occurs along with zeolite and native copper mineralization. The EPMA results show that Chl-I is more Fe-rich than Chl-II.

Precise Location and Mapping of the Main Central Thrust Zone in Reference to Micro-Structures and Deformation along Khudi-Tal Area of Marsyangdi Valley

Geological mapping was carried out along Marsyangdi valley in the Khudi - Dahare -Tal area on a scale of 1: 50,000 covering about 142 square kilometers. Recent study aims to locate the Main Central Thrust (MCT) precisely based on lithostratigraphy, micro-structures, deformation, and metamorphism. Several thin sections were observed to study the metamorphism, deformation, and micro-structures developed in the rocks. The rocks sequences in both the Higher Himalaya and the Lesser Himalaya have undergone polyphase metamorphism and deformation. The Lesser Himalaya experienced first burial metamorphism (M1) followed by garnet grade inverted metamorphism related to the MCT activity (M2) followed by retrograde metamorphism (M3) whereas the Higher Himalaya has undergone regional high-pressure/ high-temperature kyanite/ sillimanite- grade prograde regional metamorphism (M1) followed by the (M2) related to ductile sharing which in turn is overprinted by the later post-tectonic retrograde garnet to chlorite grade metamorphism during exhumation. The polyphase deformation is indicated by the cross-cutting foliation and many other features. The deformation phase D1 is associated with the development of the bedding parallel foliation due to burial in both the Higher Himalaya and the Lesser Himalaya. Isoclinal folds and crenulation cleavage were developed before the collision is categorized as D2. Development of nearly N- S trending mineral and stretching lineation, south vergent drag folds, folded S2 cleavage and microscopic shear sense indicators, rotated syn- tectonic garnet grains, etc. were developed during the deformation D3 related to the ductile shearing through the MCT. Various brittle faults and shear zones cross-cutting all earlier features were developed during D4 during the upheaval. The rocks in the MCT zone are affected by intense sharing and mylonitization as indicated by the presence of many mylonitic structures in the thin sections throughout the Lesser Himalaya in the area. Features like polygonization and ribbon quartz with evidence of sub-grain rotation, mica fish, syn-tectonic rotated garnet grains indicate the ductile shearing in the MCT area suggesting the dynamic recrystallization in the MCT zone whereas rocks of the Higher Himalaya show the evidence of recrystallization under static condition. The MCT zone was mapped precisely based on the microstructures and deformation.

A Comparative Study on the Wettability of Two Coal Samples during Deep Burial Metamorphism

In order to investigate the effect of deep burial metamorphism on the wettability of coal during deep burial metamorphism, a superficial coal sample (∼90 m) and a deep coal sample (∼490 m) collected from two main mining seams were selected to simulate the deep burial metamorphism process. The wettability of two coal samples during deep burial metamorphism was investigated by X-ray photoelectron spectroscopy (XPS), FTIR, zeta potential, and contact angle measurements. Besides, comprehensive DLVO interaction analyses between two coal samples from different mining depths were carried out through the zeta potentials measurements under different pH values. The XPS results demonstrate that the content of surface oxygen atom and oxygen-containing functional group for superficial coal tends to be higher than that of deep coal. The FTIR results indicate that the peak intensity of oxygen-containing functional groups for the superficial is higher than that of deep coal, implying the stronger surface hydrophobicity of deep coal compared to superficial coal. The contact angle of superficial coal is lower than that of deep coal. The zeta potential and DLVO theory analyses of superficial particles and deep coal particles indicate that deep burial metamorphism process might be favorable for bubble-particle collision and attachment, while might be unfavorable for the wetting process oppositely. Based on the surface chemistry characterization and theory analyses, this study is expected to give a theoretical insight into the efficient processing or dedusting process of coals experiencing different deep burial metamorphism processes in the future.

Geology, mineralogy and geochemistry of the Shangxu orogenic gold deposit, central Tibet, China: Implications for mineral exploration

Shangxu is an orogenic gold deposit within the Bangong-Nujiang suture zone, central Tibet, China. It is hosted by turbidite sedimentary rocks of the Jurassic Mugagangri Group. The host rocks were metamorphosed to subgreenschist facies prior to being hydrothermally altered adjacent to mineralization. Hydrothermal minerals in wall rocks consist of muscovite, carbonate, sulfides and chlorite. Whole rock geochemistry indicates hydrothermal alteration is characterized by the introduction of K2O, CO2 and S, and leaching of Na2O. Increasing 3 K/Al and decreasing Na/Al alteration indexes approaching the ore correlate to muscovitization. CO2 concentration reflects the degree of carbonation, which in conjunction with molar (Mn + Fe)/(Fe + Ca + Mg + Mn) and CO2/CaO can be used to distinguish siderite from ankerite in carbonate alteration. Mass transfer analysis shows gains in As, Au, CO2, W, Ca, Cd, Ni, Cr, Sr, Rb, K and losses in Na, Pb, Cu during hydrothermal alteration. In the muscovite alteration zone, muscovite within shear zones (Ms1) shows an increase in Fe, Mg and Na towards the ore, whereas metamorphic muscovite along cleavage planes and in pressure shadows (Ms2), and hydrothermal muscovite (Ms3) change in composition from phengitic (Mg, Fe) distal to mineralization, to paragonitic (Na) adjacent to gold. In carbonate alteration, the composition in carbonate phases changes from early magnesian siderite to ankerite, and then to calcite and dolomite during the evolution of the hydrothermal system. Siderite spots in host rocks and ankerite in veins, wall rock or its replacement of siderite are all enriched in Fe, Mn and depleted in Mg from proximal to distal alteration zones. Pyrite is a dominant mineral in sulfide alteration of the Shangxu deposit. The earliest framboidal pyrite (Py1) formed during diagenesis, and overprinted by Py2, which is recrystallized pyrite associated with burial metamorphism. Py3 rims and enlarges previous pyrite aggregates, or recrystallizes into cubic grains with cracks and quartz pressure shadows during deformation. Py4 is ore-related euhedral pyrite, disseminated near gold, and formed in hydrothermal stage. Vein pyrite (Py5) coexists with marcasite, and fills cracks in quartz veins. Hydrothermal alteration minerals form overlapping alteration haloes surrounding the main auriferous lodes. Distal muscovitization, carbonatization and proximal sulfidation constitute a progressive alteration front from gold. The alteration envelope is tabular in shape, following the strike and dip of the mineralization, and tends to be the widest around the thickest parts of the ore, such that the occurrence and thickness of the alteration envelope can be used as an approximate guide to the size of gold mineralization. Shangxu shares many diagnostic features with turbidite-hosted orogenic gold deposits, such as Bendigo, Reefton and Meguma districts, in deposit geology, alteration mineralogy and lithogeochemistry.

The genesis of agates and amethyst geodes

AbstractPractically all aspects of agate genesis generate debate. The time is ripe to clarify the most important enigmas concerning the environments of formation of agates and the related famous amethyst geodes of Brazil and Uruguay. Agates form over a wide range of temperatures, from those of basaltic and andesitic melts (about 1100 °C) down to about 50 °C, and at rather low pressures. Their formation in liquid mafic magmas is indicated by a correlation between (1) the orientation of amygdules and the inclination of onyx banding in them and (2) the attitude of amygdules in the lava flow layers. The correlation arises because lava moves at a different rate close to and far from the upper and lower rims of a flow. The alkaline supercritical fluid fills gas vesicles in lavas and dissolves silica, mainly, from ambient lava or rock to produce a silica sol. If the pressure on the fluid causes percolation of water from amygdules, the sol coagulates on the walls of the vesicle to form a concentric lining. If the pressure in amygdules falls below the maximum osmotic pressure of a sol (about 0.1 MPa for a silica sol), percolation of fluid stops, and coagulation leads to the formation of horizontal onyx banding. Multiple repetitions of precipitation of various gel layers can be caused by overlapping fresh flows upon the cooling older agate-bearing lava flow. In a submarine setting, phase separation of the fluid and the formation of a film of gel between vapor (or diluted solution) and brine stimulate the osmotic processes, which result in growth of hollow membrane tubes and branching moss-like arrays at the bottom of amygdules. Some agates exhibit numerous channels as a result of repeated extrusion of fluid or gel from inner zones to the periphery of amygdules that were compressed under the burden of new flows. Previously, such channels were interpreted to be feeding channels for silica supply in amygdules. Periodic compression of amygdules after percolation of fluid from them requires no additional supply of silica because the volume of the amygdules is reduced in proportion to the loss of fluid. The concentric and horizontal banding and mossy textures of agates from the lithophysae of felsic volcanic rocks were created during active volcanism as well. The agates from dissolution-induced cavities in carbonate rocks and the famous amethyst druses of Brazil and Uruguay formed at the moderate temperatures associated with low-grade burial metamorphism, as indicated by the lack of moss textures and onyx banding.

Hot fluids, burial metamorphism and thermal histories in the underthrust sediments at IODP 370 site C0023, Nankai Accretionary Complex

Abstract Drilling during International Ocean Discovery Program (IODP) Expedition 370 at Site C0023 encountered instances of hydrothermal mineralization from 775 to 1121 m below seafloor. Fluid inclusion homogenization temperatures measured on barite veins within this interval indicate precipitation from fluids with temperatures up to 220 °C, and salinities ten times higher than interstitial water (16–25 c.f. 2.8–3.6 wt % NaCl). Patches of stratabound mineralization (rhodochrosite, calcite, barite and anhydride) are largely confined to the vicinity of veins and have vertical thicknesses and extents that can be explained by precipitation within the thermal aureoles of veins. Thermal maturities measured on petroleum biomarkers in underthrust sediments record a rise from pre-oil window to early oil window thermal maturities. Basin models show that increases in thermal maturity can be explained by burial metamorphism, and kinetic-based calculations suggest that hot fluids would only have had a minimal effect on hydrocarbon generation. However, the movement of hot fluids still has geochemical implications and creates a complex thermal history where both short-duration, localized heating within fracture zones at hot temperatures (~200 °C for less than 1 yr) and heating during burial over long durations (80–110 °C for 0.5–2 Myrs) need to be considered.

The geologic history of Vesta inferred from combined 207Pb/206Pb and 40Ar/39Ar chronology of basaltic eucrites

Eucrites represent samples of mafic crust of the parent body, likely Vesta, and record a complex geologic history involving magmatism, metamorphism, impact processing, and metasomatism. For better understanding of the geologic history, we present combined 207Pb/206Pb and 40Ar/39Ar chronology of four basaltic eucrites showing distinct petrologic features: Agoult, an equilibrated eucrite with granulitic textures; Camel Donga, a brecciated equilibrated eucrite enriched in metallic iron; DaG 380, a partially equilibrated monomict eucrite; and NWA 049, a metasomatized polymict eucrite comprising mostly unequilibrated clasts. The 207Pb/206Pb dating in combination with the acid washing technique was applied to pyroxene-rich and plagioclase-rich fractions, while the 40Ar/39Ar dating was performed mainly on plagioclase grains using the laser incremental heating method. The Pb isotopic data for acid leaching residues are more radiogenic than those for acid washes, reflecting efficient removal of non-radiogenic Pb components by the acid washing. Among the residues, however, only those of the Agoult and Camel Donga plagioclase-rich fractions yielded isochrons, rather than errorchrons, with 207Pb/206Pb ages of 4532.39 ± 0.87 Ma and 4515.43 ± 0.42 Ma, respectively. The trace element systematics further suggest that these residues essentially represent contaminant oxide minerals such as ilmenite and chromite rather than plagioclase. On the other hand, the Ar isotopic data for Agoult and Camel Donga yielded plateau ages with weighted means of 4494 ± 9 Ma and 3749 ± 25 Ma, in contrast to those for DaG 380 and NWA 049 that did not return any plateau. The 207Pb/206Pb and 40Ar/39Ar ages of Agoult are interpreted to reflect the time of the U–Pb and K–Ar system closure during a prolonged thermal metamorphism. By combining the ages reported here and in the literature with the calculated closure temperatures, the cooling rate of Agoult is determined to be 11 ± 2 °C/Ma. This cooling rate corresponds to a burial depth of 10–20 km, indicating that granulitic eucrites were originally crystallized near the surface and were subsequently buried and metamorphosed. The ∼20 Ma younger 207Pb/206Pb ages of Camel Donga than Agoult are considered to reflect delayed burial metamorphism in the source region, whereas the plateau 40Ar/39Ar age would record the time of an impact causing brecciation and efficient Ar-degassing of the buried crust. Furthermore, the model 207Pb*/206Pb* dates calculated for acid-washed pyroxenes imply that the burial metamorphism of Agoult and DaG 380 were broadly contemporaneous and also that NWA 049 was metasomatized by fluids within the uppermost crust later than 4.4 Ga.

Exceptional Multi Stage Mineralization of Secondary Minerals in Cavities of Flood Basalts from the Deccan Volcanic Province, India

Flood basalts of the Deccan Volcanic Province erupted between about 67.5 to 60.5 Ma ago and reached a thickness of up to 3500 m. The main part consists of compound and simple lava flows with a tholeiitic composition erupted within 500,000 years at about 65 Ma. Within the compound lava flows, vesicles and cavities are frequent. They are filled by secondary minerals partly of well development and large size. This study presents data on the secondary mineralization including detailed field descriptions, optical, cathodoluminescence and SEM microscopy, X-ray diffractometry, fluid inclusions, C and O isotope analyses, and Rb-Sr and K-Ar geochronology. The investigations indicate a multistage precipitation sequence with three main stages. During stage I clay minerals and subsurface filamentous fabrics (SFFs), of probably biogenic origin, formed after the lava flows cooled down near to the Earth’s surface. In stage II, first an assemblage of calcite (I) and zeolite (I) (including mordenite, heulandite, and stilbite) as well as plagioclase was overgrown by chalcedony, and finally a second calcite (II) and zeolite (II) generation developed by burial metamorphism by subsequent lava flows. Stage III is characterized by precipitation of a third calcite (III) generation together with powellite and apophyllite from late hydrothermal fluids. Rb-Sr and K-Ar ages of apophyllite indicate a large time span for stage III. Apophyllite formed within different time intervals from the Paleogene to the early Miocene even within individual lava flows at certain localities. From the Savda/Jalgaon quarry complex, ages cluster at 44–48 Ma and 25–28 Ma, whereas those from the Nashik area are 55–58 Ma and 21–23 Ma, respectively.

Relations of Uranium Enrichment and Carbonaceous Debris within the Daying Uranium Deposit, Northern Ordos Basin

Carbonaceous debris (CD) is widely distributed in the sandstone of the Daying Uranium Deposit, northern Ordos Basin, and coexists with uranium minerals, which provides a favorable case for studying their relationship. Vitrinite reflectance (VR), macerals, moisture, volatile matter, ash, total sulfur (St) and uranium concentration of CD within the sandstone were studied. The results show that VR ranges from 0.372 %Ro to 0.510 %Ro with an average value of 0.438 %Ro, indicating that CD is in the stage of lignite. The contents of vitrinite (V), inertinite (I) and minerals range from 83.18%–99.48%, 0–7.70%, and 0.34%–15.72%, respectively, with the corresponding average value of 95.51%, 1.34%, and 3.15%, respectively which indicates that V is the major maceral. Moisture on air dried basis (Mad), volatile matter yield on dry, ash-free basis (Vdaf), ash yield on dried basis (Ad) and St mostly range from 7.95%–16.09%, 44.70%–66.54%, 4.84%–26.24% and 0.24%–1.12%, respectively, while their average values are 12.43%, 53.41%, 16.57% and 0.77%, respectively. It suggests that CD is of medium-high moisture, super-high volatile matter, low-medium ash and low sulfur. Uranium concentration ranges from 29 ppm to 92 ppm with an average value of 50 ppm, and uranium concentration increases with the decreased distance to CD. On the whole, Mad and Vdaf decrease with increasing burial depth, which indicates that CD experienced the burial metamorphism. However, Mad and Vdaf obviously decrease in uranium-rich areas whereas Ad and St noticeably increase. Comprehensive studies suggest that there is a certain relationship between uranium enrichment and CD. CD in the stage of lignite helps the adsorption of uranium. On one hand, radioactivity uranium enrichment makes organic matter maturation increase with a decrease in moisture and volatile matter. On the other hand, an increase in organic matter maturation, caused by radioactivity uranium enrichment, results in an increase in uranium minerals, which is instructive in the study of regional uranium mineralization and metallogenic regularity.

An Upper Crustal Ophiolite Remnant within the Feather River Ultramafic Belt, California: Tectonomagmatic Origins and Implications for Its Evolution

The 150-km-long Feather River ultramafic belt (FRB) of the northern Sierra Nevada, California, consists of serpentinized ultramafic rocks, gabbroic rocks, with lesser amounts of amphibolitic rocks. The gabbroic rocks contain metagabbro and dikes within it. Based on the electron microprobe analyses, the metagabbro consists of typical igneous composition for clinopyroxene and early hornblende, indicating that they bear low grade metamorphism. The gabbroic rocks show Nb and Ta depletion in primitive-mantle normalized plots that is similar to typical island arc rocks. We present that the metagabbro within the ultramafic rocks is forming in the suprasubduction zone setting. Tectonic models are presented for the metagabbro and amphibolitic rocks in FRB. The low P burial metamorphism of the upper plate metagabbro and the high grade amphibolitic rocks are typical of relationships of upper plate ophiolites to metamorphic soles. The amphibolitic rocks record as metamorphic sole forming during subduction initiation beneath the ultramafic rocks and metagabbro. The unusual and protracted tectonic history of the FRB is similar to other ophiolite belts of the Cordillera of California but differs in the lack of crustal rocks.