Mineralogy and Petrology最新文献

For a long time, it has been implicitly believed that oxygen isotopes of hydrothermally altered rocks and/or minerals were only elevated by the heavy water enriched in ¹⁸O from the modern geothermal and/or fossil hydrothermal systems around the world. While it is logically likely, there is no any previous attempt to argue for the elevation of oxygen isotopes of hydrothermally altered rocks and/or minerals by a light water depleted in ¹⁸O under appropriate natural conditions. Based on a novel procedure recently proposed for dealing with thermodynamic reequilibration of oxygen isotopes between constituent minerals and water, the initial oxygen isotopes of water (i.e., (delta {^{18}{text{O}}}_{text{W}}^{text{i}}) value hereafter) prior to the hydrothermal alteration are theoretically inverted from the early Cretaceous postcollisional granitoid and Triassic gneissic country rock across the Dabie orogen in central-eastern China. The oxygen isotopes of hydrothermally altered rock-forming minerals were concurrently elevated by the magmatic water with moderate to high ({delta ^{18}{text{O}}}_{text{W}}^{text{i}}) values ranging from 4.21 ± 0.04 (one standard deviation, 1SD) to 6.57 ± 0.05‰ in the course of postmagmatic processes. By contrast, oxygen isotopes of the susceptible alkali feldspar from a gneissic country rock could be preferentially elevated by the ancient meteoric water with low (delta {^{18}{text{O}}}_{text{W}}^{text{i}}) values down to -8.52 ± 0.56‰ during exhumation processes of the retrograde metamorphism. These fossil hydrothermal systems could kinetically sustain from a short duration of less than 12 thousand years (Kyr) via the surface-reaction oxygen exchange up to 1 million years (Myr) through the diffusive oxygen exchange, respectively, in this study. Cooling rates are further quantified for rock-forming minerals sequentially blocked and/or isolated from the magmatic water. Hereby, oxygen isotopes of constituent minerals can be hydrothermally elevated by diverse sources of water with paradoxical (delta {^{18}{text{O}}}_{text{W}}^{text{i}}) values, especially for the metamorphic rocks with anomalous oxygen isotopes. There is no doubt that more unexpected findings will be scientifically and methodologically decoded and/or unlocked worldwide in the coming decade(s).

This paper addresses the study of a pillow lava interbedded with Late Albian-Early Cenomanian sediments that crops out in Armintza (Bizkaia, Northern Spain). The lava flow is an alkaline basalt with abundant macrocrysts of clinopyroxene, kaersutite, Ca-rich plagioclase (50-86% An) and ilmenite, which display a variety of textures and complex zoning patterns indicative of open-system magmatic behaviour. Macrocryst cores are likely to be inherited antecrysts that underwent complex processes under deep pre-eruptive conditions (≈ 700-800 MPa). Microcrysts and macrocryst rims formed during magmatic ascent and emplacement at shallower levels (≈ 35 MPa). Hypothetical melts in equilibrium with clinopyroxenes and amphiboles have trace element compositions like metasomatic vein melts containing amphibole, and their patterns overlap with those of the Armintza pillow lava. This suggests a metasomatised lithospheric mantle with amphibole-rich veins as a potential source for the alkaline basaltic melt. It is even conceivable that the Armintza pillow lava and other alkaline volcanic manifestations of the Basque-Cantabrian Basin were part of the same magma plumbing system through which a series of time-limited eruptions of different batches of magma ascended from the lithospheric mantle to the upper crust during the Albian to the Santonian.

As a tectonic window into the Lesser Himachal Himalaya, India, a group of metasediments and gneissic rocks, known as the Jutogh Group and Wangtu Gneissic Complex (WGC), occurs near the Jhakri thrust to the west and Wangtu to the east. In the Jutogh Group, chlorite-mica schist, garnet-staurolite schist and sillimanite-schist develop successively. The formation of chemically zoned garnet, which destabilized low-temperature assemblages, is predicted to be at 550–650 °C and 0.8–0.9 GPa by phase equilibria modelling. The retrograde segment consists of exhumation and cooling, yielding a tight clockwise P–T path. Moreover, textural observations and in-situ U-Th-Pb chemical dating indicate that metasedimentary rocks contain Cambrian monazites. These monazites have ages that cluster around 500 Ma. The Ɛ_Nd[1.8Ga] of Jutogh rocks ranges from − 1.0 to -8.1, with depleted mantle-model ages between 3.07 and 2.25 Ga. The garnet core and its leachates yield an Sm-Nd isochron age of 472 Ma. Another Sm-Nd isochron age of 454 Ma is obtained from biotite, garnet rim, and garnet rim leachate. According to phase equilibrium modelling, Sm-Nd dating, and monazite geochronology, the Jutogh Group experienced metamorphism along the northeast margin of Gondwana during the Cambro-Ordovician accretion.

Metamorphosed banded iron formation (BIF) in granulite-amphibolite facies, tonalitic orthogneisses from a series of locations in the Kolli Massif of southern India are described and analysed with regard to their lithologies, whole rock chemistry, mineral reaction textures, and mineral chemistry. On the basis of their mineral reaction textures along magnetite-quartz grain boundaries these BIFs are grouped according to their predominant silicate mineralogy: 1) amphibole; 2) orthopyroxene; 3) orthopyroxene–clinopyroxene; 4) orthopyroxene-clinopyroxene-garnet; 5) clinopyroxene-garnet-plagioclase; and 6) Fe-Mg silicates are absent. Two-pyroxene and garnet-pyroxene Fe-Mg exchange thermometry, coupled with thermodynamic pseudo-section modelling of whole rock data from one of the magnetite-quartz-orthopyroxene-clinopyroxene-bearing lithologies, indicates that the magnetite-quartz-orthopyroxene-clinopyroxene-garnet assemblages formed at ~900 to 1200 MPa and 750 to 900 °C under relatively low H₂O activities. Magnetite-quartz-orthopyroxene reaction textures were experimentally replicated at 800 and 900 °C and 1000 MPa in a synthetic BIF using isolated magnetite grains in a quartz matrix to which was added a hypersaline Mg- and Al-bearing fluid (approximately 1% by mass), which permeated along all the grain boundaries. The fact that Fe-Mg silicate reaction textures did not form in one of the BIF samples, which had experienced the same P-T conditions as the other BIF samples, suggests that, unless a BIF initially incorporated Mg, Al, and Ca during formation with or was infiltrated from the surrounding rocks by Mg-, Al-, and Ca-bearing saline fluids, these silicate minerals could not and would not have formed from the inherent magnetite and quartz during granulite-facies and amphibolite-facies metamorphism.

The Tirodi Gneissic Complex (TGC) represents the basement sequence of the Central Indian Tectonic Zone (CITZ), underlying the Proterozoic supracrustal sequences of the Sausar and Betul Groups of rocks. Lithologically, the TGC constitutes a combination of pink and grey granitic gneiss assemblages, characterised by biotite-rich, hornblende-biotite-rich, and muscovite-biotite-rich granite gneiss. Compositionally, the TGC granitoids represent tonalite-trondhjemite-granodiorite to granite, and have calc-alkaline lineage with metaluminous to peraluminous characteristics. Geochemically, they dominantly belong to A2-type granitoids. Chondrite normalised REE ratios of La/Sm, La/Yb, La/Gd, and Gd/Yb indicate diverse LREE/HREE enrichment. Multi-element patterns for the TGC granitoids are characterised by light rare earth elements (LREE) and large ion lithophile elements (LILE) enrichment and depletion of high field strength elements (HFSE: Nb, P, and Ti) and strong positive Pb and Th anomalies. The observed negative anomalies for HFSE are attributed to diverse crustal/lithospheric sources, with some influence from K-feldspar, plagioclase and Ti-oxide fractionation. Sm–Nd data presents initial ¹⁴³Nd/¹⁴⁴Nd (t = 1.7 Ga) ratios (0.509898 to 0.510508), and ε_Nd (t = 1.7 Ga) is (+ 0.58 to -10.59), with T_DM model ages ranging from 2.11 to 2.95 Ga. Such a wide range of ε_Nd (t = 1.7 Ga), indicates heterogeneous crustal/lithosphere sources, which have probably experienced longer crustal residence times. Zircon U–Pb ages for individual TGC samples are 1506 ± 11 Ma (TG-01), 1534 ± 26 Ma (MU-5), 1675 ± 9 Ma (BT-4), 1724 ± 11 Ma (BT-3), 1730 ± 13 Ma (BT-4), and 1960 ± 2 Ga (Ms-2), respectively. These ages have probably recorded the key periods of the Columbia supercontinent's assembly, growth, and breakup. Geochemical and geochronological results suggest that the TGC granitoids have a crustal/lithospheric origin and are formed by partial melting of felsic sources in dominantly VAG (volcanic arc granite) and, to some extents, WPG (within-plate granite) settings.

Mineral assemblages formed at low oxygen fugacity are commonly confined to the lithologies of extraterrestrial or deep Earth origin. The occurrences of reduced mineral phases in upper crustal rocks [formed under oxygen fugacity conditions below the iron-wüstite (IW) buffer] are rare. However, they are important for understanding the chemical drivers of natural redox processes. Here, we present detailed studies of reduced mineral assemblages, which were found in situ in superficial combustion metamorphic (CM) rocks of west-central Jordan and compare them to reduced mineral assemblages found in situ in the CM rocks of south-central Israel. The studied assemblages contain a suite of exotic phases more typical of meteorites: native iron, phosphides (schreibersite, Fe₃P; allabogdanite, Fe₂P; transjordanite, Ni₂P; murashkoite, FeP; halamishite, Ni₅P₄; zuktamrurite, FeP₂; polekhovskyite, MoNiP₂), and sulphides (daubréelite, FeCr₂S₄; oldhamite, CaS; troilite, FeS), part of which (native iron, allabogdanite, halamishite, polekhovskyite, daubréelite) have not previously been discovered in the CM rocks of west-central Jordan. The mineralogical diversity of terrestrial phosphides and the occurrence of Ni- / Mo-rich phases can be explained by (1) high P, Ni, and Mo content in the sedimentary protolith, (2) transformations of primary Fe₃P / Fe₂P, (3) extreme disequilibrium of the processes, and (4) crystal-chemical control of Ni- / Mo- speciation.

Tourmaline is a valuable forensic mineral that contains a wide range of elemental components capable of reconstructing its geologic evolution. In this study, we developed detailed petrographic and geochemical research on tourmaline-bearing rocks and tourmalinites found surrounding the ca. 3.30 Ga rhyolite of the Gavião Block, northern São Francisco Craton. Two distinct types of mineral assemblages in the tourmaline-bearing rocks are recognized based on the presence of sulfides, oxide minerals, and carbonaceous matter. Along with these tourmaline-bearing rocks, tourmalinite samples are found. The mineral chemistry of tourmalines from the two types of tourmaline-bearing rocks reveals mainly schorl-dravite as tourmaline from a mineral assemblage containing quartz, tourmaline, pyrite, and hematite, and a diverse composition ranging from schorl-dravite, feruvite-uvite, and schorl-feruvite solid solutions for tourmalines present in the mineral assemblage that contains quartz, pyrite, and carbonaceous matter. The tourmaline composition is essentially foitite-schorl in tourmalinite samples. Most of the metasedimentary rocks studied have felsic and mafic source compositions. The interaction of hydrothermal fluid was crucial for the formation of sulfide and tourmaline minerals. However, distinct tourmaline origins are recognized in tourmaline-rich rocks, pointing out the relevance of sedimentary and metamorphic processes.

Juína is the second-largest diamond-producing municipality in Brazil and is globally known for its outstanding sublithospheric diamond occurrences in both placer and kimberlite-hosted deposits. However, the scarcity of petrological data for Juína kimberlite pipes hampers understanding the nature and mantle source of these primary diamond sources in this region. Here, we present a textural and mineralogical study of ten kimberlite pipes from the Juína area. Based on petrographic features and mineral compositions, we interpret the studied Juína pipes as archetypal kimberlites with pyroclastic emplacement styles filled with resedimented volcaniclastic kimberlite and Kimberley-type pyroclastic kimberlite variants. The composition and texture of the magmatic phases, particularly spinel and phlogopite, suggest crystallisation from kimberlite sensu stricto magmas. The presence of high-Na eclogitic garnets and the absence of high-Cr low-Ca G10 garnets within the mantle xenocryst suite suggest the likelihood of eclogitic diamonds among Juína's lithospheric diamond populations. The Zr and Y contents, Ti/Eu and Zr/Hf ratios in the peridotite garnets, and Zr contents, Ca/Al, La_N/Yb_N (primitive-mantle normalised), Ti/Eu, and Zr/Hf ratios in the clinopyroxenes suggest a solid connection to kimberlite melt-related mantle metasomatism. Thermobarometry calculations indicate a relatively narrow stability window (825–936 ºC and 32–36 kbar) for lithospheric diamonds in the Juína region. Our findings have important implications for regional diamond exploration programs, shedding light on the primary sources of Juína's diamonds and contributing to understanding the deep geological processes in the underlying lithospheric mantle beneath the Amazonian craton.

Veloso is one of numerous and poorly documented auriferous deposits of Ouro Preto, the town that symbolises the gold rush in Brazil at the turn of the seventeenth century. We present the results of underground geological mapping, combined with a boron (B) isotopic study of tourmaline, an elusive mineral in the auriferous quartz lode of the historical Veloso deposit. Its lode is characteristically brecciated in a host rock that shows no cataclastic fabric. The host rock is itabirite, a metamorphosed banded iron formation. Tourmaline is essentially dravite and locally occurs as abundant crystals in breccia-cementing pockets of goethite, formed from the oxidation of sulfide minerals. Gold is spatially associated with tourmaline in the goethite-rich pockets. In situ measurements for B isotopes yielded δ¹¹B values in the range of −21 to −9‰. This range is similar to that reported for tourmaline of the Passagem de Mariana deposit, the best documented auriferous lode deposit at the south-eastern edge of the Quadrilátero Ferrífero. The tourmaline B isotopic data reflect auriferous fluids of crustal origin that sourced B from metasedimentary rocks, which may include a non-marine evaporitic component.