Showing papers on "Petrography published in 2020"

The formation of the ore-bearing dolomite marble from the giant Bayan Obo REE-Nb-Fe deposit, Inner Mongolia: insights from micron-scale geochemical data

Abstract: The genesis of Earth’s largest rare earth element (REE) deposit, Bayan Obo (China), has been intensely debated, in particular whether the host dolomite marble is of sedimentary or igneous origin. The protracted (Mesoproterozoic to Paleozoic) and intricate (magmatic to metasomatic) geological processes complicate geochemical interpretations. In this study, we present a comprehensive petrographic and in situ, high-spatial resolution Sr-Pb isotopic and geochemical investigation of the host dolomite from the Bayan Obo marble. Based on petrographic evidence, the dolomite marble is divided into three facies including coarse-grained (CM), fine-grained (FM), and heterogeneous marble (HM). All carbonates are ferroan dolomite with high SrO and MnO contents (> 0.15 wt.%), consistent with an igneous origin. Trace element compositions of these dolomites are highly variable both among and within individual samples, with CM dolomite displaying the strongest LREE enrichment. In situ 206Pb/204Pb and 207Pb/204Pb ratios of the dolomite are generally consistent with mantle values. However, initial 208Pb/204Pb ratios define a large range from 35.45 to 39.75, which may result from the incorporation of radiogenic Pb released from decomposition of monazite and/or bastnasite during Early Paleozoic metasomatism. Moreover, in situ Sr isotope compositions of dolomite indicate a large range (87Sr/86Sr = 0.70292–0.71363). CM dolomite is characterized by a relatively consistent, unradiogenic Sr isotope composition (87Sr/86Sr = 0.70295–0.70314), which is typical for Mesoproterozoic mantle. The variation of 87Sr/86Sr ratios together with radiogenic 206Pb/204Pb signatures for dolomite within FM and HM possibly represents recrystallization during Early Paleozoic metasomatism with the contribution of radiogenic Sr and Pb from surrounding host rocks. Therefore, our in situ geochemical data support a Mesoproterozoic igneous origin for the ore-bearing dolomite marble in the Bayan Obo deposit, which subsequently underwent intensive metasomatism during the Early Paleozoic.

The Deccan Volcanic Province (DVP), India: A Review: Part 1: Areal extent and distribution, compositional diversity, flow types and sequences, stratigraphic correlations, dyke swarms and sills, petrography and mineralogy

Abstract: The Deccan Volcanic Province (DVP), covering presently an area of 0.5 million km2, and estimated to be 2–3 times larger during the Upper Cretaceous-Paleocene, is one of the largest continental flood basalt provinces of the world. Its formation has been linked to the foundering of the Gondwanaland and Greater India’s northward drift, passing over the Reunion plume and eruption of over a million km3 of lava that apparently led to a mass extinction of global proportions. The DVP has thus been a major domain of scientific interest and study the world over. It had received attention since the 1830s, first from the army and civil service men of the British Raj and subsequently from the officers of the Geological Survey of India (GSI) founded in 1851, and academicians from a number of Indian Universities and research Institutions, often in collaboration with geologists from countries such as the UK, USA, Russia, France, Japan, Italy and others. Thus, studies of the DVP conducted for over 170 years, and especially in the last five decades, have provided a very large database that has led to a better understanding of the genesis and evolution of the this province and similar flood basalt provinces of the world. The DVP is thickest in the Western Ghats, forming many individual 400 m to 1650 m thick sections over some 700 km. The structural evolution of the lava sequence envisages a pre-uplift, thick (c. 2–3 km), lensoid pile of dense basalt that gradually sank into the crust by the end of the eruptive phase, followed by an uplift of the western margin of the Deccan due to both denudational-isostasy reasons and the associated geomorphological and structural evolution of the lava pile from Tertiary uplift and coastal flexure formation (the Panvel structure). Such an evolution has led to stresses that get accommodated along fractures in the pre-Deccan basement at varying depths and apparently provide the loci for seismicity observed over the province. The DVP is predominantly composed of quartz- and hypersthene-normative tholeiitic basalts in the plateau regions (Western Ghats and adjoining central and eastern parts, Malwa and Mandla). However, along the ENE-WSW-trending Narmada-Tapi rift zones, the N-S to NNW-SSE-trending Western coastal tract, the Cambay rift zone, and the Saurashtra and Kutch regions, the DVP shows considerable diversity in terms of structures, dyke swarms and dyke clusters, and intrusive and extrusive centres with diverse rock types. These include: primary picrite basalts and their differentiates (e.g., Botad, Dhandhuka, Wadhwan Jn., Pavagadh), granophyre-rhyolite intrusive ring-complexes and mixed basalt-rhyolite associations (e.g., Alech, Barda, Osham, Chogat-Chamardi, Mumbai Island, and others), carbonatite-nephelinite associations (Amba Dongar-Kawant), gabbro-anorthosite-nepheline syenite-syenite ring/layered complexes(e.g., Mt. Girnar, Mundwara, Phenai Mata), mantle-derived spinel peridotite-hosting melanephelinites and basanites (e.g. Dhrubya, Vethon and others in Kutch), besides scores of alkaline and lamprophyre dykes. Some of these complexes are associated with high gravity anomalies indicating dense plutonic bodies at depths. Flow morphological studies of the DVP have led to the recognition of two main types of flows, namely ‘a’a (typically forming simple, sheet flows) and pâhoehoe (typically forming compound, pahoehoe lobate flows) with transitions between them that result in mixed types. The ‘a’a types are largely single units found in the peripheral parts where thicknesses of the flow sequence range from a few meters to a few tens of meters. The compound pâhoehoe flows contain many units or lobes and are largely found in the thicker sections of Western Ghats, and also in the central parts of the province. The flow sequences of the Western Ghats (c. 400 m to 1650 m thick and spread over an area of400 km × 100 km along the N-S tract from north of Nasik to Belgaum) have been mapped and correlated using flow morphology, petrology and selected trace elements (Sr, Ba, Zr, Y and Ti) and Sr- isotopes. Such a combination of geochemical characters, constrained further by altitude and magnetic polarity (chrons 30N-29R-29N) have led to the delineation of the flow sequences from north to south into a Deccan Basalt Group, comprising three Subgroups from the base to the top, namely the Kalsubai, Lonavala and Wai Subgroups, with twelve (12) formations in total, each formation containing many flows. Giant plagioclase basalts (GPBs) and bole beds of diverse origin (intertrappean sediments, weathered basalt or tuffs with baking effects) have been found in many flow sequences of the DVP, especially in the Western Ghats and contiguous plateau regions, and these interflow units help in subregional-scale mapping and also provide insights into magma chamber processes and eruptive breaks in the volcanic cycles. Field and geochemical studies of some twenty-three (23) flow sequences (10 from Western Ghats, five from central India and eight from eastern India) by several groups have enabled correlation of some formations of the Western Ghats such as the Ambenali (crustally uncontaminated) and Poladpur (contaminated) over long distances (c. 400–700 km) to Toranmal, Mhow, Chikaldara, Jabalpur and other sections. However, these formations occur at different stratigraphie elevations at these places and also differ in some isotopic characters (e.g. 206Pb/204Pb). Such features have cast doubts on long distance travel of flows from a single source and led to suggestions of multiple source areas (vents and dykes) as also inferred from the two zones of compositional diversity mentioned above. Based on detailed field, petrological and geochemical characters including isotopic data and Ar-Ar ages, dyke swarms and clusters in the Narmada-Tapi and western coastal tracts have been shown to belong to two groups: (1) The randomly oriented group between Pune and Nasik as possible feeders to the lava flow sequences of the Western Ghats and (2) Some of the dykes from the east-west-oriented Narmada-Tapi swarm, attributed to active N-S extension during the flood basalt episode, and showing chemical affinities to the lower and middle formations (Jawhar, Igatpuri, Neral, Thakurwadi, Bhimashankar, Khandala). A wide variety of petrographic types of basalts have been observed in the DVP attesting to the diverse crystallisation and differentiation of the different magma types during transport and in magma chambers. Based on petrographic and mineralogical data from a number of thick sections, it has been inferred that minerals such as olivine (Fo90–Fo20), clinopyroxenes (diopsidic augite, augite, subcalcic augite and pigeonite), plagioclase (An84–An30) and opaque oxides including spinels show considerable variations depending upon the tholeiitic or alkaline character of the host magma and its degree of evolution. Secondary minerals, especially zeolites such as heulandite and stilbite, are found in all the ten formations of the Western Ghats whereas merlionite and analcite are only found in the Khandala Formation. Other ten species are of variable abundance are found in the ten formations of the Western Ghats and other areas. Zeolite zonation in DVP suggested earlier has not been substantiated by recent studies, instead multigeneration of secondary minerals in cavities is attributed to late hydrothermal activity from Paleocene to early Miocene.

Compressional origin of the Naxos metamorphic core complex, Greece: Structure, petrography, and thermobarometry

Abstract: The island of Naxos, Greece, has been previously considered to represent a Cordilleran-style metamorphic core complex that formed during Cenozoic extension of the Aegean Sea. Although lithospheric extension has undoubtedly occurred in the region since 10 Ma, the geodynamic history of older, regional-scale, kyanite- and sillimanite-grade metamorphic rocks exposed within the core of the Naxos dome is controversial. Specifically, little is known about the pre-extensional prograde evolution and the relative timing of peak metamorphism in relation to the onset of extension. In this work, new structural mapping is presented and integrated with petrographic analyses and phase equilibrium modeling of blueschists, kyanite gneisses, and anatectic sillimanite migmatites. The kyanite-sillimanite–grade rocks within the core complex record a complex history of burial and compression and did not form under crustal extension. Deformation and metamorphism were diachronous and advanced down the structural section, resulting in the juxtaposition of several distinct tectono-stratigraphic nappes that experienced contrasting metamorphic histories. The Cycladic Blueschists attained ∼14.5 kbar and 470 °C during attempted northeast-directed subduction of the continental margin. These were subsequently thrusted onto the more proximal continental margin, resulting in crustal thickening and regional metamorphism associated with kyanite-grade conditions of ∼10 kbar and 600–670 °C. With continued shortening, the deepest structural levels underwent kyanite-grade hydrous melting at ∼8–10 kbar and 680–750 °C, followed by isothermal decompression through the muscovite dehydration melting reaction to sillimanite-grade conditions of ∼5–6 kbar and 730 °C. This decompression process was associated with top-to-the-NNE shearing along passive-roof faults that formed because of SW-directed extrusion. These shear zones predated crustal extension, because they are folded around the migmatite dome and are crosscut by leucogranites and low-angle normal faults. The migmatite dome formed at lower-pressure conditions under horizontal constriction that caused vertical boudinage and upright isoclinal folds. The switch from compression to extension occurred immediately following doming and was associated with NNE-SSW horizontal boudinage and top-to-the-NNE brittle-ductile normal faults that truncate the internal shear zones and earlier collisional features. The Naxos metamorphic core complex is interpreted to have formed via crustal thickening, regional metamorphism, and partial melting in a compressional setting, here termed the Aegean orogeny, and it was exhumed from the midcrust due to the switch from compression to extension at ca. 15 Ma.

Rock classification in petrographic thin section images based on concatenated convolutional neural networks

Abstract: Rock classification plays an important role in rock mechanics, petrology, mining engineering, magmatic processes, and numerous other fields pertaining to geosciences. This study proposes a concatenated convolutional neural network (Con-CNN) method for classifying geologic rock types based on petrographic thin sections. Plane polarized light (PPL) and crossed polarized light (XPL) were used to acquire thin section images as the fundamental data. After conducting the necessary pre-processing, the PPL and XPL images as well as their comprehensive image developed by principal component analysis were sliced into small patches and were put into three CNNs, comprising the same structure for achieving a preliminary classification. Subsequently, these patches classification results of the CNNs were concatenated by using the maximum likelihood method to obtain a comprehensive classification result. Finally, a statistical revision was applied to fix the misclassification due to the proportion differences of minerals that were similar in appearance. In this study, there were 92 rock samples of 13 types giving 106 petrographic thin sections and 2208 petrographic thin section images, and finally 238,464 sliced image patches were used for the training and validation of the Con-CNN method. The 5-folds cross validation showed that the proposed method provides an overall accuracy of 89.97% and a kappa coefficient of 0.86, which facilitates the automation of rock classification in petrographic thin section images.

Petrography and Geochemistry of the Carboniferous Ortokarnash Manganese Deposit in the Western Kunlun Mountains, Xinjiang Province, China: Implications for the Depositional Environment and the Origin of Mineralization

Abstract: The Upper Carboniferous Ortokarnash manganese ore deposit in the West Kunlun orogenic belt of the Xinjiang province in China is hosted in the Kalaatehe Formation. The latter is composed of three members: (1) the 1st Member is a volcanic breccia limestone, (2) the 2nd Member is a sandy limestone, and (3) the 3rd Member is a dark gray to black marlstone containing the manganese carbonate mineralization, which, in turn, is overlain by sandy and micritic limestone. This sequence represents a single transgression-regression cycle, with the manganese deposition occurring during the highstand systems tract. Geochemical features of the rare earth elements (REE+Y) in the Kalaatehe Formation suggest that both the manganese ore and associated rocks were generally deposited under an oxic water column with Post-Archean Australian Shale (PAAS)-normalized REE+Y patterns displaying characteristics of modern seawater (e.g., light REE depletion and negative Ce anomalies). The manganese ore is dominated by fine-grained rhodochrosite (MnCO3), dispersed in Mn-rich silicates (e.g., friedelite and chlorite), and trace quantities of alabandite (MnS) and pyrolusite (MnO2). The replacement of pyrolusite by rhodochrosite suggests that the initial manganese precipitates were Mn(IV)-oxides. Precipitation within an oxic water column is supported by shale-normalized REE+Y patterns from the carbonate ores that are characterized by large positive Ce (>3.0) anomalies, negative Y (~0.7) anomalies, low Y/Ho ratios (~20), and a lack of fractionation between the light and heavy rare earth elements ((Nd/Yb)PAAS ~0.9). The manganese carbonate ores are also 13C-depleted, further suggesting that the Mn(II) carbonates formed as a result of Mn(III/IV)-oxide reduction during burial diagenesis.

Metasomatism-induced wehrlite formation in the upper mantle beneath the Nógrád-Gömör Volcanic Field (Northern Pannonian Basin): Evidence from xenoliths

Abstract: Clinopyroxene-enriched upper mantle xenoliths classified as wehrlites are common (~20% of all xenoliths) in the central part of the Nograd-Gomor Volcanic Field (NGVF), situated in the northern margin of the Pannonian Basin in northern Hungary and southern Slovakia. In this study, we thoroughly investigated 12 wehrlite xenoliths, two from each wehrlite-bearing occurrence, to determine the conditions of their formation. Specific textural features, including clinopyroxene-rich patches in an olivine-rich lithology, orthopyroxene remnants in the cores of newly-formed clinopyroxenes and vermicular spinel forms all suggest that wehrlites were formed as a result of intensive interaction between a metasomatic agent and the peridotite wall rock. Based on the major and trace element geochemistry of the rock-forming minerals, significant enrichment in basaltic (Fe, Mn, Ti) and high field strength elements (Nb, Ta, Hf, Zr) was observed, compared to compositions of common lherzolite xenoliths. The presence of orthopyroxene remnants and geochemical trends in rock-forming minerals suggest that the metasomatic process ceased before complete wehrlitization was achieved. The composition of the metasomatic agent is interpreted to be a mafic silicate melt, which was further confirmed by numerical modelling of trace elements using the plate model. The model results also show that the melt/rock ratio played a key role in the degree of petrographic and geochemical transformation. The lack of equilibrium and the conclusions drawn by using variable lherzolitic precursors in the model both suggest that wehrlitization was the last event that occurred shortly before xenolith entrainment in the host mafic melt. We suggest that the wehrlitization and the Plio–Pleistocene basaltic volcanism are related to the same magmatic event.