Role of P-T conditions and bulk rock composition in the mineralogical variations in millimeter scale: a study from South Maharashtra Shear Zone, western India 

2021 ◽  
Author(s):  
Pallavi Praharaj ◽  
Sukumari Rekha
Keyword(s):  
Shear Zone ◽  
Dharwar Craton ◽  
Western India ◽  
Bulk Rock ◽  
Rock Composition ◽  
Garnet Porphyroblasts ◽  
Bulk Rock Composition ◽  
Layer 2 ◽  
Layer 4 ◽  
Garnet Porphyroblast

<p>The pressure-temperature conditions are transient in time and space during tectonic processes. To understand the complete P-T history of crustal domains examining the mineral paragenetic sequences and zoning profiles of minerals from diverse lithologies in the domain is necessary. But in highly tectonised crustal domains establishing time equivalence between far-spaced samples is difficult. To overcome this, a mylonite sample with closely spaced layers of different mineralogy collected from the South Maharashtra Shear Zone located along the north of Western Dharwar Craton (Rekha and Bhattacharya, 2014) was studied. The mylonite has four mineralogically distinct layers of few millimeters width containing garnet porphyroblasts of distinct zoning pattern separated by quartz layers. Layer-1 has two domains on the basis of the relative abundance of quartz; Layer-1A with more quartz and less flaky minerals and Layer-1B with less quartz and more flaky minerals. Layer-1A is composed of quartz>biotite>plagioclase>chlorite>K-feldspar with syn- to post-tectonic garnet porphyroblasts and the fabric is defined by shape preferred biotite-chlorite aggregates, recrystallized plagioclase and quartz ribbons.Layer-1B is relatively quartz poor and plagioclase>biotite>chlorite>K-feldspar aggregates rich domain as compared to L1A with biotite-chlorite aggregates and recrystallized plagioclase defined fabric.Prehnite elongated parallel to schistosity present but not very common. Layer-2 is very thin with amphibole-biotite±chlorite defined foliation and consists of plagioclase-K-feldspar-quartz with large garnet porphyroblast ofsyn to post-tectonic origin. Chlorites are mainly present near to garnet. Layer-3 is composed of biotite-calcite-plagioclase-chlorite-quartz with syn/post-tectonic garnet porphyroblast and the foliation is defined by biotite-chlorite aggregates, recrystallize plagioclase, calcite grains aligned parallel to the foliation and elongated quartz grains.Layer-3 is separated from the quartz layers on both sides by the formation of thin hornblende layers arranged parallel to the foliation. Very few hornblende grains found within the layer aligned parallel with the fabric defining minerals. Large pre-tectonic muscovite grains are preserved in Layer-3 and are altered to epidote along the margins of the grain. Layer-4 consists of hornblende, calcite, quartz with few plagioclase, K-feldspar and post tectonic garnet porphyroblast. The fabric is defined by the long axis of amphibole and calcite grains aligned parallel to it. Later biotite-prehnite grains formed at high angle to the fabric defining minerals. Conventional geothermobarometers were used for P-T estimation and it varies from 450-560°C and 6 kbar for Layer-1A, 445-550°C and 7 kbar for Layer-1B, 475-570°C and 6 kbar for Layer-2, 450-575°C and 7-8 kbar for Layer-3 and 450-5500°C and 7-9 kbar for Layer-4 at reference temperature of 500°C and pressure of 6kbar. Though different layers have distinctly different mineral assemblages there is hardly any variation in the P-T conditions which implies the original bulk rock composition was different for different layers not the P-T conditions of deformation.</p><p><strong>Keywords:</strong> Mylonite, Western Dharwar Craton, Geothermobarometry</p>

Shackanite and related analcite-bearing lavas in British Columbia

1978 ◽  
Vol 15 (10) ◽  
pp. 1669-1672 ◽  
Author(s):  
B.N. Church
Keyword(s):  
British Columbia ◽  
Great Depth ◽  
Bulk Rock ◽  
Rock Composition ◽  
Bulk Rock Composition ◽  
South Central ◽  
Early Tertiary ◽  
Broad Field ◽  
New Localities

New localities of shackanite and related analcite-bearing lavas have been discovered in a broad field of early Tertiary phonolite and mafic phonolite in south-central British Columbia. The development of primary and secondary analcite in these rocks is probably the result of cooling lava during and shortly after extrusion.The possibility of leucite to analcite transformation in Daly's shackanite is unlikely because of lack of petrographic evidence and a preponderance of Na2O over K2O in bulk rock composition. It is also unlikely that analcite, and particularly groundmass analcite, crystallized at great depth and was transported to surface during eruption.

scholarly journals STUDY OF THE METAMORPHIC EVOLUTION OF A CARBONATE-BEARING METAPERIDOTITE FROM THE SIDIRONERO COMPLEX (CENTRAL RHODOPE, GREECE) USING P-T AND P(T)-XCO2 PSEUDOSECTIONS

2017 ◽  
Vol 43 (5) ◽  
pp. 2667
Author(s):  
E. Mposkos ◽  
I. Baziotis
Keyword(s):  
Phase Diagram ◽  
Fluid Phase ◽  
Bulk Rock ◽  
Prograde Metamorphism ◽  
Metamorphic Evolution ◽  
Rock Composition ◽  
Bulk Rock Composition ◽  
Hp Metamorphism ◽  
T Path

The carbonate-bearing metaperidotite from Sidironero Complex, north of the Xanthi town is composed primarily of olivine and orthopyroxene megacrysts and of Ti-clinohumite, tremolite, chlorite, dolomite, magnesite, talc, antigorite and spinel group minerals. The metaperidotite underwent a prograde HP metamorphism probably isofacial with the neighboring amphibolitized eclogites. Calculated P-T and P(T)-XCO2 phase diagram sections (pseudosections) for the bulk rock composition showed that XCO2 in the fluid phase was extremely low (≤0.008) at the first stages of the metamorphism and increased up to 0.022 at the peak P-T conditions ~1.5 GPa and 690 0C. The prograde metamorphism probably started from a hydrated and carbonated assemblage including talc+chlorite+magnesite+dolomite and proceeded with tremolite and antigorite formation before olivine growth, and orthopyroxene formation after olivine growth (Ol-1). Matrix dolomite, breakdown of chlorite (Chl-1) to Cr spinel+olivine and of Ti-clinohumite to olivine+Mg-ilmenite occurred during decompression. The P-T path is constrained by the absence of clinopyroxene in the metaperidotite.

Phase equilibrium and trace-element modeling of the partial melting of basaltic rocks under low pressure: Implications for plagiogranite generation

2021 ◽  
Author(s):  
Xiao-Fei Xu ◽  
Long-Long Gou ◽  
Xiao-Ping Long ◽  
Yu-Hang Zhao ◽  
Feng Zhou
Keyword(s):  
Trace Element ◽  
Partial Melting ◽  
Low Pressure ◽  
Bulk Rock ◽  
Hydrous Mineral ◽  
Rock Composition ◽  
Bulk Rock Composition ◽  
Element Modeling ◽  
Anhydrous Mineral ◽  
Ree Patterns

Abstract Phase equilibria and trace-element modeling using two previously reported basaltic bulk-rock compositions (samples D11 and 104-16), were carried out in this study, in order to better understand mechanism of low-pressure (LP) partial melting of mafic rocks and associated melt compositions. The T–MH2O pseudosections for both samples at three pressures (i.e. 0.5, 1.0 and 2.0 kbar) display that the H2O-stability field gradually increased with decreasing pressure within the T–MH2O range of 600–1100 °C and 0–12 mol.%. The H2O contents of 10, 5.0, and 0.5 mol.% were selected on the basis of the T–MH2O pseudosections to calculate P–T pseudosections over a P–T window of 0.1–3 kbar and 600–1100 °C, so that the reactions of both the H2O-fluxed and -absent meltings at LP conditions can be investigated. The solidus displays a negative or near-vertical P–T slope, and occurs between 710 and 900 °C at pressure between 0.1 and 3.0 kbar. LP melting of metabasites is attributed to the reactions of the hydrous mineral (hornblende and/or biotite) melting and anhydrous mineral (plagioclase, orthopyroxene, and augite) melting. The hydrous mineral melting is gradually replaced by anhydrous mineral melting as pressure decreasing, as the stability of hornblende decreases with falling pressure. With increasing temperature at a given pressure, the modeled melt compositions are expressed as progressions of the granite-granodiorite-gabbroic diorite fields for sample D11and granite-quartz monzonite-monzonite-gabbroic diorite fields for sample 104-16 on the total alkali–silica diagram. The modeled melts produced through the H2O-fluxed melting display higher Al2O3, CaO, MgO, and lower SiO2 and K2O than those formed by H2O-absent melting at the same P–T conditions. Furthermore, the modeled melts formed by H2O-absent melting, become richer in Al2O3, CaO, MgO, FeO, Na2O, but poorer in SiO2 and K2O as increasing water content. The results of trace-element modeling suggests that the nearly flat REE patterns of modeled bulk-rock composition are inherited by all the modeled melts, and the negative Eu anomalies and Sr depletion of the modeled melts gradually decrease as melting degree increases. Combined with the geochemical characteristics of natural oceanic plagiogranites, which have low K2O contents and flat or slightly LREE-depleted REE patterns, our results imply that a bulk-rock composition with low K2O (<0.17 wt.%) and slightly LREEs depletion is the most likely protolith composition (e.g. basalt D11) for plagiogranites, and the compositions of modeled melts formed by LP H2O-absent partial melting of the basalt D11 at relatively high temperatures (1000–1025 °C) are coincident with those of 1256D tonalites.

Jadeite–K-feldspar rocks and jadeitites from northwest Turkey

1997 ◽  
Vol 61 (409) ◽  
pp. 835-843 ◽  
Author(s):  
Aral I. Okay
Keyword(s):  
Grain Size ◽  
Debris Flows ◽  
Bulk Rock ◽  
Rock Composition ◽  
Bulk Rock Composition ◽  
Fine Grained ◽  
Fine Grain ◽  
The Matrix ◽  
Mineral Pair

AbstractBlueschist-facies rocks with jadeite-K-feldspar-lawsonite paragenesis occur as exotic blocks in Miocene debris flows in the blueschist belt of northwest Turkey. The jadeite-K-feldspar rocks have a very fine grain size and although recrystallized locally retain a relict porphyritic volcanic texture. The former nepheline microphenocrysts, recognized from their characteristic shapes, are pseudomorphed by jadeite and K-feldspar, while the relict magmatic aegirine has rims of jadeite. The matrix of the rock consists of very fine-grained aggregates of jadeite, K-feldspar and lawsonite. In some blocks, jadeite makes up >60% of the mode. Jadeite, K-feldspar and lawsonite in the blocks are essentially pure end-member in composition. P-T estimates for these rocks are 8 ± 2 kbar and 300 ± 50°C. The preserved volcanic texture, relict aegirine and the bulk rock composition indicate that these rocks represent metamorphosed phonolites. The paragenesis in these rocks shows that jadeite-K-feldspar is a stable mineral pair in blueschist-facies P-T conditions.

scholarly journals Metamorphism of metachert from the Southern Alps, New Zealand. A thermodynamic forward modelling study

2021 ◽  
Author(s):  
◽  
Jill Fernandes
Keyword(s):  
Mineral Assemblage ◽  
Bulk Composition ◽  
Carbonaceous Material ◽  
Southern Alps ◽  
Forward Modelling ◽  
Bulk Rock ◽  
Rock Composition ◽  
Bulk Rock Composition ◽  
Garnet Composition ◽  
Garnet Core

<p>Scattered, scarce occurrences of garnet- and quartz-rich metamorphic rock, probably derived from Mn- and Fe-rich chert, occur within metamorphosed greywacke sequences worldwide. The metamorphism of such garnetiferous metacherts has not previously been investigated using modern thermodynamic forward modelling techniques due to the lack of appropriate, internally-consistent activity-composition (a–x) models for Mn-bearing minerals. The present study applies thermodynamic forward modelling using the recently-proposed a–x models of White et al. (2014) to investigate the metamorphism of garnetiferous metachert samples from the Southern Alps, New Zealand.  Pressure-temperature (P–T) pseudosections are used in combination with results from petrography, element composition mapping using micro X-ray fluorescence (µXRF) and scanning electron microscope (SEM) methods, and garnet composition data from analytical transects by electron probe microanalysis (EPMA), to study metachert metamorphism. All the samples are compositionally layered, so the possibility exists that an input bulk rock composition might not match the effective bulk composition at the site of garnet growth. If a mineral assemblage stability field in a calculated P–T pseudosection matched the mineral assemblage in the rock, this was taken as an initial indication of a permissible input bulk rock composition. In that case, refined constraints on the P–T conditions were sought by comparing calculated and measured garnet compositions. The studied rocks include samples that are carbonate-bearing, which require consideration of the effects of fluid composition in mixed H₂O–CO₂ fluids, as well as a sample in which the garnet is strongly zoned, texturally-complex, and inferred to be of polymetamorphic origin. The effects of element fractionation by that garnet were investigated by recalculating the P–T pseudosection using a new bulk rock composition with the garnet core content removed. In none of the samples did the calculated and observed composition isopleths for the garnet cores match, suggesting that initial garnet nucleation in these Mn-rich rocks was locally controlled. For most samples in which the calculated and observed mineral assemblages matched, successful estimates of the peak metamorphic conditions were obtained. A garnet chert (A12E) from the mylonite zone of the Alpine Fault at Vine Creek, near Hokitika, gave a tight intersection of composition isopleths, indicating peak metamorphic conditions of 510 °C/5.5 kbar, after recalculation to correct for element fractionation by the strongly-zoned garnet. This tight, modern constraint is within error of previously-reported results from traditional geothermobarometry (420–600 °C/5.9–13 kbar) and Raman spectroscopy of carbonaceous material (RSCM T = 556 °C) from nearby sites. A peak metamorphic estimate of 520–550 °C/7–10 kbar was obtained from a dolomite-bearing sample from the garnet zone near Fox Glacier (J34), in good comparison with published temperatures from Raman spectroscopy of carbonaceous material in nearby metagreywacke samples (526–546 °C). The prograde metamorphic P–T path was probably steep, based on growth of the garnet core at ~475535 °C/5–9 kbar. The successful results for these garnet chert samples show that the new a-x models for Mn-bearing minerals extend the range of rock types that are amenable to pseudosection modelling.  Results obtained in this study also serve to highlight several possible concerns: a) garnet nucleation and initial growth in very Mn-rich rocks may be subject to local compositional or kinetic controls; b) bulk rock compositions may not always mimic the effective bulk composition; c) the existing a–x models for Mn-bearing minerals and white micas may need refining; and d) some rocks may simply be ill-suited to thermodynamic forward modelling. Items a) and b) may be indicated by the common observation of a mismatch between predicted and measured garnet composition isopleths for garnet cores, and by a mismatch between garnet composition isopleths and the appropriate mineral assemblage field for sample AMS01, from the mylonite zone, Hari Hari, Southern Alps. For item c) every P–T pseudosection calculated using the new a–x models for Mn-bearing minerals showed garnet stable to very low temperatures below 300 °C. In addition, the P–T pseudosection for an oligoclase-zone metachporphyroblasts of Fe-Ti oxides (magnetitert (Sample J36) from Hari Mare stream, Franz Josef - Fox Glacier, indicated that the white mica margarite should be present instead of plagioclase (oligoclase), for a rock in which oligoclase is present and margarite is absent, a problem previously noted elsewhere. Item d) is exemplified by a very garnet-rich ferruginous metachert sample (J35, garnet zone, headwater region, Moeraki River, South Westland) which proved impossible to model successfully due to its complex mineral growth and deformation history. This sample contained multiple generations of carbonate with differing compositions, amphibole (not incorporated for modelling with the new a–x models for Mn-bearing minerals), large e associated with smaller, possibly later-formed ilmenite), and the garnet bands were offset by late deformation.  The garnetiferous metachert samples studied here preserve in their textures and compositions clues to their growth mechanism and metamorphic history. The textures in at least two of the samples are consistent with the diffusion controlled nucleation and growth model for garnet. This research has successfully used state of the art thermodynamic modelling techniques in combination with the latest internally consistent a-x models on Mn-rich metachert, for the first time, extracting P–T conditions of the metamorphism of garnetiferous metachert from the Southern Alps.</p>

scholarly journals Bulk rock composition and geochemistry of olivine-hosted melt inclusions in the Grey Porri Tuff and selected lavas of the Monte dei Porri volcano, Salina, Aeolian Islands, southern Italy

2012 ◽  
Vol 4 (2) ◽  
Author(s):  
Angela Doherty ◽  
Robert Bodnar ◽  
Benedetto Vivo ◽  
Wendy Bohrson ◽  
Harvey Belkin ◽  
...  
Keyword(s):  
Basaltic Andesite ◽  
Mineral Assemblage ◽  
Melt Inclusions ◽  
Southern Italy ◽  
Compositional Variation ◽  
Bulk Rock ◽  
Rock Composition ◽  
Aeolian Islands ◽  
Bulk Rock Composition ◽  
Mineral Phases

AbstractThe Aeolian Islands are an arcuate chain of submarine seamounts and volcanic islands, lying just north of Sicily in southern Italy. The second largest of the islands, Salina, exhibits a wide range of compositional variation in its erupted products, from basaltic lavas to rhyolitic pumice. The Monte dei Porri eruptions occurred between 60 ka and 30 ka, following a period of approximately 60,000 years of repose. The bulk rock composition of the Monte dei Porri products range from basaltic-andesite scoria to andesitic pumice in the Grey Porri Tuff (GPT), with the Monte dei Porri lavas having basaltic-andesite compositions. The typical mineral assemblage of the GPT is calcic plagioclase, clinopyroxene (augite), olivine (Fo72−84) and orthopyroxene (enstatite) ± amphibole and Ti-Fe oxides. The lava units show a similar mineral assemblage, but contain lower Fo olivines (Fo57−78). The lava units also contain numerous glomerocrysts, including an unusual variety that contains quartz, K-feldspar and mica. Melt inclusions (MI) are ubiquitous in all mineral phases from all units of the Monte dei Porri eruptions; however, only data from olivine-hosted MI in the GPT are reported here. Compositions of MI in the GPT are typically basaltic (average SiO2 of 49.8 wt %) in the pumices and basaltic-andesite (average SiO2 of 55.6 wt %) in the scoriae and show a bimodal distribution in most compositional discrimination plots. The compositions of most of the MI in the scoriae overlap with bulk rock compositions of the lavas. Petrological and geochemical evidence suggest that mixing of one or more magmas and/or crustal assimilation played a role in the evolution of the Monte dei Porri magmatic system, especially the GPT. Analyses of the more evolved mineral phases are required to better constrain the evolution of the magma.

Qualitative and quantitative mineralogical composition of the Rupelian Boom Clay in Belgium

Clay Minerals ◽  
2015 ◽  
Vol 50 (2) ◽  
pp. 249-272 ◽  
Author(s):  
E. Zeelmaekers ◽  
M. Honty ◽  
A. Derkowski ◽  
J. Środoń ◽  
M. De Craen ◽  
...  
Keyword(s):  
Clay Minerals ◽  
Clay Mineral ◽  
Quantitative Composition ◽  
Bulk Rock ◽  
X Ray Diffraction ◽  
Rock Composition ◽  
X Ray ◽  
Boom Clay ◽  
Bulk Rock Composition ◽  
Clay Formation

AbstractThe Boom Clay Formation of early Oligocene age, which occurs underground in northern Belgium, has been studied intensively for decades as a potential host rock for the disposal of nuclear waste. The goal of the present study is to determine a reference composition for the Boom Clay using both literature methods and methods developed during this work. The study was carried out on 20 samples, representative of the lithological variability of the formation. The bulk-rock composition was obtained by X-ray diffraction using a combined full-pattern summation and singlepeak quantification method. Siliciclastics vary from 27 to 72 wt.%, clay minerals with 25–71 wt.% micas, 0–4 wt.% carbonates, 2–4 wt.% accessory minerals (mainly pyrite and anatase) and 0.5–3.5 wt.% organic matter. This bulk-rock composition was validated independently by majorelement chemical analysis. The detailed composition of the clay-sized fraction was determined by modelling of the oriented X-ray diffraction patterns, using a larger sigma star (σ*) value for discrete smectite than for the other clay minerals. The <2 μm clay mineralogy of the Boom Clay is qualitatively homogeneous; it contains 14–25 wt.% illite, 19–39 wt.% smectite, 19–42 wt.% randomly interstratified illite-smectite with about 65% illite layers, 5–12 wt.% kaolinite, 4–17 wt.% randomly interstratified kaolinite-smectite and 2–7 wt.% chloritic minerals (chlorite, “defective” chlorite and interstratified chlorite-smectite). All modelled clay mineral proportions were verified independently using major-element chemistry and cation exchange capacity measurements. Bulkrock and clay mineral analysis results were combined to obtain the overall detailed quantitative composition of the Boom Clay Formation.

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