Archean Boninite-like Rocks of the Northwestern Youanmi Terrane, Yilgarn Craton: Geochemistry and Genesis

2019 ◽  
Vol 60 (11) ◽  
pp. 2131-2168 ◽  
Author(s):  
Jack R Lowrey ◽  
Derek A Wyman ◽  
Tim J Ivanic ◽  
R Hugh Smithies ◽  
Roland Maas
Keyword(s):  
Trace Element ◽  
Crustal Contamination ◽  
Mantle Metasomatism ◽  
Chemical Compositions ◽  
Yilgarn Craton ◽  
Fine Grained ◽  
Mafic Intrusions ◽  
Field Evidence ◽  
Isotopic Analyses ◽  
Geochemical Analyses

Abstract Rocks with chemical compositions similar to Cenozoic boninites occur in many Archean cratons (boninite-like rocks), but they are rarely well-preserved, well-sampled, or presented within chrono- and chemo-stratigraphic context. This study provides a detailed description of the most extensive and well-preserved Archean boninite-like rocks reported to date. Within the 2820 to 2740 Ma magmatic suites of the northwest Youanmi Terrane, Yilgarn Craton, boninite-like rocks occur as two distinct units. The first boninite-like unit is thinner (several 10 s of m thick), occurs close to the base of the 2820–2800 Ma Norie Group and includes both volcanic flows and subvolcanic intrusions. The second boninite-like unit is thicker (locally several 100 s m), occurs near the base of the 2800–2740 Ma Polelle Group and consists of mainly fine-grained volcanic flows with local cumulate units. On average, major and trace element compositions for Youanmi Terrane boninite-like rocks are marginal between basalt, picrite and boninite and they have asymmetrically concave REE patterns, and Th–, Zr–Hf enrichments, similar to many Phanerozoic low-Si boninite suites, but at generally higher MREE–HREE contents. We report over 300 new whole-rock geochemical analyses, and 16 new Sm–Nd isotopic analyses, and associated petrographic evidence, including representative mineral compositions, which we support with published geochemical analyses and several decades of fieldwork in our study area. Comparison between Archean boninite-like rocks and Cenozoic boninites shows that most Archean examples had less depleted sources. We consider two possible petrogenetic models for the Youanmi Terrain examples: (1) they reflect variably contaminated komatiites, or (2) they reflect melts of metasomatised refractory mantle, analogous to Phanerozoic boninites. Trace element modelling indicates that crustal contamination could potentially produce rocks with boninite-like compositions, but requires an Al-enriched komatiitic parent liquid, for which there is no field evidence in our study area. Initial εNdT values in pre-2800 Ma rocks (εNdT -0·4 to +1·2) are on average slightly higher than those in 2800–2733 Ma examples (εNdT -3·2 to +1·2), compatible with increasing mantle metasomatism involving recycling of ≥ 2950 Ma crust. Integration of trace element and Nd isotopic data demonstrates that significant direct crustal assimilation was restricted to felsic magmas. The Th–Nb and Ba–Th systematics of mafic-intermediate rocks reflect fluid- and sediment-derived processes in the mantle, with boninite-like examples being linked primarily to fluid metasomatism. We compare the well-preserved igneous textures and mineralogy of Youanmi Terrane boninite-like rocks with those of their Phanerozoic counterparts, and based on studies of the latter, suggest that former had similarly hot, H2O-rich parent magmas. The association of boninite-like rocks in the Norie and Polelle Groups with coeval high-Mg andesites, sanukitoids and hydrous mafic intrusions of the Narndee Igneous Complex strongly suggests a metasomatised mantle source and subduction operating in the Yilgarn between 2820 and 2730 Ma.

The origin of the c. 1.7 Ga gabbroic intrusion in the Hekou area, SW China: constraints from SIMS U–Pb zircon geochronology and elemental and Nd isotopic geochemistry

2016 ◽  
Vol 154 (2) ◽  
pp. 286-304 ◽  
Author(s):  
WEI-GUANG ZHU ◽  
ZHONG-JIE BAI ◽  
HONG ZHONG ◽  
XIAN-TAO YE ◽  
HONG-PENG FAN
Keyword(s):  
Trace Element ◽  
Crustal Contamination ◽  
Central Zone ◽  
Coarse Grained ◽  
Yangtze Block ◽  
Sw China ◽  
Mafic Magmatism ◽  
Fine Grained ◽  
Mafic Intrusions ◽  
Gabbroic Intrusion

AbstractThe late Palaeoproterozoic to early Mesoproterozoic igneous rocks of southwestern China are characterized by a number of mafic intrusions and dykes. However, the origin and tectonic implications of these mafic intrusions and dykes remain unclear. The Hekou mafic intrusion, intruding into the Hekou Group in the Hekou area, SW China, is the biggest and most representative one. The intrusion is mainly composed of coarse-grained in the central zone (CZ) and medium- to fine-grained gabbroic rocks in the outer zone (OZ). Cameca secondary ion mass spectroscopy (SIMS) U–Pb zircon ages, and geochemical and Nd isotopic results for the intrusion are reported in this paper. SIMS U–Pb zircon ages indicate that the gabbroic rocks from the CZ and OZ were emplaced at 1735±6.5 Ma and 1736±4.0 Ma, respectively. This suggests that the Hekou intrusion originated from c. 1.7 Ga mafic magmatism in the southwestern Yangtze Block. The coarse-grained rocks in the CZ of the intrusion show fairly homogeneous major- and trace-element compositions. In contrast, the medium- to fine-grained rocks from the OZ display slightly evolved compositions, with relatively lower Mg nos, MgO, Al2O3, Cr and Ni contents, and higher SiO2, CaO and Zr concentrations than those of the rocks from the CZ. Although the gabbroic rocks of the intrusion have low total rare earth element (REE) contents (REE = 29.3–40.2 ppm) with slightly light REE (LREE)-enriched and heavy REE (HREE)-depleted patterns, they exhibit distinct trace-element and Nd isotopic features. The rocks from the CZ are characterized by slightly LREE-enriched and ‘convex upwards’ incompatible trace-element patterns with significant Th depletion and insignificant Nb and Ta depletion relative to La. However, the rocks from the OZ have relatively flatter REE patterns than those of the rocks from the CZ. In addition, the rocks from the OZ are slightly enriched in Th and depleted in Nb and Ta relative to La. The εNd(T) values of the CZ and the OZ rocks are +0.70 to +2.3 and −0.30 to +0.24, respectively. The parental magma for the Hekou gabbroic intrusion exhibits affinity with a subalkaline basaltic magma, which was possibly generated by relatively high degrees of partial melting of a slightly depleted asthenospheric mantle source. Their geochemical and isotopic variations were due to slight crystal fractionation with varying degrees of crustal contamination. The Hekou intrusion was therefore supposed to form in an anorogenic extensional environment. It is further suggested that c. 1.7 Ga is an important onset timing of widespread anorogenic magmatism in the southwestern Yangtze Block. We interpret the late Palaeoproterozoic gabbroic intrusion to represent anorogenic mafic magmatism, which was most likely related to the break-up of the Columbia supercontinent.

scholarly journals Three types of mantle eclogite from two layers of oceanic crust: a key case of metasomataically-aided transformation of low-to-high-magnesian eclogite

2021 ◽  
Author(s):  
Nester Korolev ◽  
Larisa P Nikitina ◽  
Alexey Goncharov ◽  
Elena O Dubinina ◽  
Aleksey Melnik ◽  
...  
Keyword(s):  
Trace Element ◽  
Major And Trace Elements ◽  
Major Elements ◽  
Mantle Metasomatism ◽  
Coarse Grained ◽  
High Alumina ◽  
Chemical Compositions ◽  
Depth Range ◽  
Fine Grained ◽  
Ocean Ridge

Abstract Reconstructed whole-rock and mineral major- and trace-element compositions, as well as new oxygen isotope data, for 22 mantle eclogite xenoliths from the Catoca pipe (Kasai Craton) were used to constrain their genesis and evolution. On the basis of mineralogical and major-element compositions, the Catoca eclogites can be divided into three groups: high-alumina (high-Al) (kyanite-bearing), low-magnesian (low-Mg#), and high-magnesian (high-Mg#) eclogites. The high-Al Catoca eclogites contain kyanite and corundum; high Al2O3 contents in rock-forming minerals; rare earth element (REE) patterns in garnets showing depleted LREEs, positive Eu anomalies (1.03–1.66), and near-flat HREEs; and high Sr contents in garnets and whole-rock REE compositions. All of these features point to a plagioclase-rich protolith (probably gabbro). Reconstructed whole-rock compositions (major elements, MREEs, HREEs, Li, V, Hf, Y, Zr, and Pb) and δ18O of 5.5–7.4‰ of the low-Mg# Catoca eclogites are in good agreement with the compositions of picrite basalts and average mid-ocean ridge basalt (MORB). The depleted LREEs and NMORB-normalised Nd/Yb values of 0.07–0.41 indicate that the degree of partial melting for the majority of the low-Mg# eclogites protolith was ≥30%. The narrow δ18O range of 5.5–7.4‰ near the ‘gabbro–basalt’ boundary (6‰) obtained for the high-Al and low-Mg# Catoca eclogites reflects the influence of subduction-related processes. This case shows that mantle eclogites represented by two different lithologies and originating from different protoliths — plagioclase-rich precursor, presumably gabbro (for high-Al eclogites), and basalt (low-Mg# eclogites) — can provide similar and overlapping δ18O signatures on account of the influence of subduction-related processes. Chemical compositions of the high-Mg# eclogites indicate a complicated petrogenesis, and textural signatures reveal recrystallisation. The presence of Nb-rich rutile (8–12 wt% of Nb2O5) enriched with HFSE (Zr/Hf of 72.6–75.6) and multiple trace-element signatures (including reconstructed whole-rock NMORB-normalised Ce/Yb of 3.9–10.6 and Sr/Y of 5.8–9.6, MgO contents of 15.7–17.9 wt%, and high Ba and Sr) provide strong evidence for deep metasomatic alteration. High Cr contents in clinopyroxene (800–3740 ppm), garnet (430–1400 ppm), and accessory rutile (700–2530 ppm), together with extremely low Li contents of 1.0–2.4 ppm in clinopyroxene, may indicate hybridisation of the eclogites with peridotite. Comparison of the chemical compositions (major and trace elements) of (1) unaltered fresh cores of coarse-grained garnets from the low-Mg# eclogites, (2) secondary garnet rims (ubiquitous in the low-Mg# eclogites), (3) proto-cores in the coarse-grained garnet (high-Mg# eclogites), and (4) homogeneous recrystallised fine-grained garnets (high-Mg# eclogites) suggests that the high-Mg# eclogites formed through recrystallisation of low-Mg# eclogite in the presence of an external fluid in the mantle. Four of the five high-Mg# samples show that mantle metasomatism inside the Kasai craton mantle beneath the Catoca pipe occurred at a depth range of 145–160 km (4.5–4.8 GPa).

Age and Geochemical Studies of the Snyder Breccia, Coastal Labrador

1975 ◽  
Vol 12 (3) ◽  
pp. 361-370 ◽  
Author(s):  
Jackson M. Barton Jr. ◽  
Erika S. Barton
Keyword(s):  
Trace Element ◽  
North Atlantic ◽  
Layered Intrusion ◽  
Basement Complex ◽  
Fine Grained ◽  
The North ◽  
Isotopic Analyses ◽  
The Matrix ◽  
The North Atlantic ◽  
Geochemical Studies

The Snyder breccia is composed of angular to subrounded xenoliths of migmatites and amphibolites in a very fine grained matrix. It is apparently intrusive into the metasediments of the Snyder Group exposed at Snyder Bay, Labrador. The Snyder Group unconformably overlies a migmatitic and amphibolitic basement complex and is intruded by the Kiglapait layered intrusion. K–Ar ages indicate that the basement complex is Archean in age (> 2600 m.y. old) and that the Kiglapait layered intrusion was emplaced prior to 1280 m.y. ago. Major and trace element analyses of the matrix of the Snyder breccia indicate that while it was originally of tonalitic composition, later it locally underwent alteration characterized by loss of sodium and strontium and gain of potassium, rubidium and barium. Rb–Sr isotopic analyses show that this alteration occurred about 1842 m.y. ago, most probably contemporaneously with emplacement of the breccia. The Snyder Group thus was deposited sometime between 2600 and 1842 m.y. ago and may be correlative with other Aphebian successions preserved on the North Atlantic Archean craton.

Isotopic analyses of clinopyroxene in mantle xenoliths demonstrate the effects of kimberlite melt metasomatism upon the lithospheric mantle

2020 ◽  
Author(s):  
Angus Fitzpayne ◽  
Andrea Giuliani ◽  
Janet Hergt ◽  
Jon Woodhead ◽  
Roland Maas
Keyword(s):  
Trace Element ◽  
Lithospheric Mantle ◽  
Mantle Metasomatism ◽  
Mantle Xenoliths ◽  
Isotopic Compositions ◽  
Isotopic Analyses ◽  
Cretaceous Magmatism

<p>As clinopyroxene is the main host of most lithophile elements in the lithospheric mantle, the trace element and radiogenic isotope systematics of this mineral have frequently been used to characterise mantle metasomatic processes. To further our understanding of mantle metasomatism, both solution-mode Sr-Nd-Hf-Pb and in situ trace element and Sr isotopic data have been acquired for clinopyroxene grains from a suite of peridotite (lherzolites and wehrlites), MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside), and PIC (Phlogopite-Ilmenite-Clinopyroxene) rocks from the Kimberley kimberlites (South Africa). The studied mantle samples can be divided into two groups on the basis of their clinopyroxene trace element compositions, and this subdivision is reinforced by their isotopic ratios. Type 1 clinopyroxene, which comprises PIC, wehrlite, and some sheared lherzolite samples, is characterised by low Sr (~100–200 ppm) and LREE concentrations, moderate HFSE contents (e.g., ~40–75 ppm Zr; La/Zr < 0.04), and restricted isotopic compositions (e.g., <sup>87</sup>Sr/<sup>86</sup>Sr<sub>i</sub> = 0.70369–0.70383; εNd<sub>i</sub> = +3.1 to +3.6) resembling those of their host kimberlite magmas. Available trace element partition coefficients can be used to show that Type 1 clinopyroxenes are close to equilibrium with kimberlite melt compositions, supporting a genetic link between kimberlites and these metasomatised lithologies. Thermobarometric estimates for Type 1 samples indicate equilibration depths of 135–155 km within the lithosphere, thus showing that kimberlite melt metasomatism is prevalent in the deeper part of the lithosphere beneath Kimberley. In contrast, Type 2 clinopyroxenes occur in MARID rocks and coarse granular lherzolites, which derive from shallower depths (<130 km), and have higher Sr (~350–1000 ppm) and LREE contents, corresponding to higher La/Zr of >~0.05. The isotopic compositions of Type 2 clinopyroxenes are more variable and extend from compositions resembling the “enriched mantle” towards those of Type 1 rocks (e.g., εNd<sub>i</sub> = -12.7 to -4.4). To constrain the source of these variations, in situ Sr isotope analyses of clinopyroxene were undertaken, including zoned grains in Type 2 samples. MARID and lherzolite clinopyroxene cores display generally radiogenic but variable <sup>87</sup>Sr/<sup>86</sup>Sr<sub>i</sub> values (0.70526–0.71177), which might be explained by the interaction between peridotite and melts from different enriched sources with the lithospheric mantle. In contrast, the rims of these Type 2 clinopyroxenes trend towards compositions similar to those of the host kimberlite and Type 1 clinopyroxene from PIC and wehrlites. These results are interpreted to represent clinopyroxene overgrowth during late-stage (shortly before/during entrainment) metasomatism by kimberlite magmas. Our study shows that an early, pervasive, alkaline metasomatic event caused MARID and lherzolite genesis in the lithospheric mantle beneath the Kimberley area, which was followed by kimberlite metasomatism during Cretaceous magmatism. This latter event is the time at which discrete PIC, wehrlite, and sheared lherzolite lithologies were formed, and MARID and granular lherzolites were partly modified.</p>

Petrochemistry, tectonic history, and Sr–Nd systematics of the Liscomb Complex, Meguma Lithotectonic Zone, Nova Scotia

1993 ◽  
Vol 30 (3) ◽  
pp. 449-464 ◽  
Author(s):  
D. B. Clarke ◽  
A. K. Chatterjee ◽  
P. S. Giles
Keyword(s):  
Crustal Contamination ◽  
Granitic Rocks ◽  
Low Grade ◽  
South Mountain Batholith ◽  
Mafic Intrusions ◽  
Depleted Mantle ◽  
Isotopic Analyses ◽  
Wide Range ◽  
Granitoid Rocks ◽  
Lithotectonic Zone

The Liscomb Complex (area ca. 240 km2), located in the Meguma Lithotectonic Zone of the Canadian Appalachians, consists of three main lithological components: high-grade gneisses, mafic plutons, and peraluminous granitoid bodies. Field relations and 40Ar/39Ar dating (369–377 Ma) embracing all three lithological groups show that the complex is post-Acadian. The gneisses occur as a domal uplift and represent a mixed volcano-sedimentary package that is structurally, metamorphically, and chemically distinct from the surrounding low-grade metawackes and metapelites of the Meguma Group. The mafic intrusions (quartz gabbro to quartz diorite) have major and trace element compositions (e.g., Ti–Zr–Y, Nb–Zr–Y, Th/Yb – Ta/Yb, rare earth elements) typical of within-plate or volcanic arc materials. The peraluminous granitoid rocks range from two-mica granodiorites to leucomonzogranites, and are mineralogically and chemically very similar to granitic rocks elsewhere in the Meguma Zone. Neodymium and strontium isotopic analyses show that (i) the gneisses have a wide range of εNd and initial Sr isotopic ratios, with Nd model ages that are generally younger than those of the Meguma Group; (ii) the mafic intrusive rocks represent magmas derived from slightly depleted mantle sources (εNd +3.3 to +1.4), in part modified by crustal contamination (εNd +0.5 to −5.0); and (iii) the granitoid rocks isotopically overlap both the South Mountain Batholith and the intermediate gneisses of the Liscomb Complex. The combined field, petrological, and chemical evidence suggests that underplating by mafic magmas, followed by thermal doming of the gneisses, diapirism through the Meguma Group, anatexis, and multiple intrusion of both mafic and felsic magmas best explain the observed relationships in the Liscomb Complex. This mechanical model may also apply to granite generation throughout the Meguma Zone.

Layered rhyolite bands in a thick North Mountain Basalt flow: the products of silicate liquid immiscibility?

1992 ◽  
Vol 56 (384) ◽  
pp. 309-318 ◽  
Author(s):  
John D. Greenough ◽  
J. Dostal
Keyword(s):  
Liquid Immiscibility ◽  
Immiscible Liquid ◽  
Chemical Compositions ◽  
Forming Process ◽  
Basalt Flow ◽  
Immiscible Liquids ◽  
Fine Grained ◽  
Mafic Intrusions ◽  
Bulk Chemical ◽  
Rich Liquid

AbstractThe upper 35 m of a thick (≤175 m) Early Jurassic North Mountain Basalt flow at KcKay Head contains 25 cm thick differentiated layers that are separated by 130 cm sections of basalt. The lower layers are mafic, pegmatitic, and contain thin (2 cm), fine-grained 'rhyolite' bands. Evidence that the rhyolite represents a Si-rich immiscible liquid includes: (1) textures such as fiine-grained globules of Ferich pyroxene (once Fe-rich liquid) bordering pegmatite feldspar grains; (2) structureless, microcrystalline, interstitial, polygonal patches of Si-rich minerals and similar areas of Fe-rich stilpnomelane surrounding skeletal Fe-Ti oxide grains, with bulk chemical compositions (to a first approximation), relative proportions and total modat percentages suggesting they were once Si-rich and Fe-rich glasses respectively; (3) basalt and pegmatite compositions (particularly their Fe, and Ti contents) similar to rocks known to contain immiscible liquids; (4) rhyolite major element compositions generally consistent with formation from an immiscible Si-rich liquid; (5) mineral compositions and temperature of pegmatite formation compatible with immiscibility; (6) the inability of mass balance calculations (crystal fractionation) to explain rhyolite formation unless mesostasis stilpnomelane (representing the Fe-rich liquid) is included in the caculations. If, as we suggest, these rocks are the result of immiscibility, they shed light on the incipient formation of granophyres in mafic intrusions and support liquid immiscibility as an important rock-forming process.

scholarly journals Age, Geochemistry and Origin of the Ardara Appinite Plutons, Northwest Donegal, Ireland

2019 ◽  
pp. 31-48 ◽  
Author(s):  
J.Brendan Murphy ◽  
R.Damian Nance ◽  
Logan B. Gabler ◽  
Alexandra Martell ◽  
Douglas A. Archibald
Keyword(s):  
Trace Element ◽  
Crustal Contamination ◽  
Magma Ascent ◽  
Mafic Rocks ◽  
Element Abundances ◽  
Isotopic Analyses ◽  
Two Samples ◽  
De Se ◽  
Continental Lithospheric Mantle ◽  
Asthenospheric Upwelling

In northwest Donegal, Ireland, a large number of coeval appinitic (hornblende-plagioclase-rich) plutons and lamprophyre dykes occur around the Ardara pluton, a granitic satellite body and one of the oldest phases of the ca. 428–400 Ma composite Donegal Batholith. The appinite units form a bimodal (mafic–felsic) suite in which hornblende is the dominant mafic mineral and typically occurs as large prismatic phenocrysts within a finer grained matrix. Lamprophyre dykes are mafic in composition with a geochemistry that is very similar to that of the mafic appinite bodies. Both mafic rocks are subalkalic, with calc-alkalic and tholeiitic tendencies, and show trace element abundances indicating that the mantle source was contaminated by subduction zone fluids. 40Ar/39Ar analysis of hornblende separated from two samples of appinite yield mid-Silurian (434.2 ± 2.1 Ma and 433.7 ± 5.5 Ma) cooling ages that are interpreted to closely date the time of intrusion. Hence, according to the available age data, the appinite bodies slightly predate, or were coeval with, the earliest phases of the Donegal Batholith. Sm–Nd isotopic analyses yield a range of initial εNd values (+3.1 to –4.8 at t = 435 Ma) that, together with trace element data, indicate that the appinitic magmas were likely derived from melting of metasomatized sub-continental lithospheric mantle and/or underplated mafic crust, with only limited crustal contamination during magma ascent. The appinitic intrusions are interpreted to have been emplaced along deep-seated crustal fractures that allowed for mafic and felsic magma to mingle. The magmas are thought to be the products of collisional asthenospheric upwelling associated with the closure of Iapetus and the ensuing Caledonian orogeny, either as a result of an orogen-wide delamination event or as a consequence of more localized slab break-off.RÉSUMÉDans le nord-ouest du Donegal, en Irlande, un grand nombre de plutons appinitiques (riches en hornblendes ou en plagioclases) et de dykes de lamprophyres contemporains se retrouvent autour du pluton d’Ardara, un corps satellite granitique et l’une des phases les plus anciennes du batholite composite de Donegal, âgé d’environ 428–400 Ma. Les unités de l’appinite forment une suite bimodale (mafique–felsique) dans laquelle la hornblende est le minéral mafique dominant et se présente généralement sous forme de grands phénocristaux prismatiques au sein d’une matrice à grains plus fins. Les dykes de lamprophyres ont une composition mafique dont la géochimie est très similaire à celle des corps d’appinite mafique. Les deux roches mafiques sont subalcaliques, avec des tendances calcoalcalines et tholéiitiques, et elles montrent des teneurs en éléments traces indiquant que la source du manteau a été contaminée par des fluides de zone de subduction. L'analyse 40Ar/39Ar des hornblendes provenant de deux échantillons d'appinite donne des âges de refroidissement du Silurien moyen (434,2 ± 2,1 Ma et 433,7 ± 5,5 Ma) qui sont interprétés comme étant proches de la date de l’intrusion. Par conséquent, selon les données d’âge disponibles, les corps d’appinite sont légèrement antérieurs ou contemporains des toutes premières phases du batholite de Donegal. Les analyses isotopiques Sm–Nd aboutissent à une gamme de valeurs εNd initiales (+3,1 à -4,8 à t = 435 Ma) qui, associées aux données des éléments traces, indiquent que les magmas appinitiques sont probablement dérivés de la fusion d'un manteau lithosphérique souscontinental métasomatisé et / ou d’une croûte mafique sousplaquée, avec une contamination crustale limitée lors de l'ascension du magma. Les intrusions appinitiques sont interprétées comme s'étant mises en place le long de fractures profondes de la croûte qui ont permis au magma mafique et au magma felsique de se mélanger. On pense que les magmas sont les produits de la remontée (upwelling) asthénosphérique collisionnelle associée à la fermeture de l’océan Iapetus et à l'orogenèse calédonienne qui s'ensuit, soit à la suite d'un délaminage à l'échelle de l'orogène, soit à la suite d'une rupture plus localisée de la plaque.

TEM/AEM characterization of fine-grained clay minerals in very-low-grade rocks: Evaluation of contamination by empa involving celadonite family minerals

1996 ◽  
Vol 54 ◽  
pp. 694-695
Author(s):  
Gejing Li ◽  
D. R. Peacor ◽  
D. S. Coombs ◽  
Y. Kawachi
Keyword(s):  
Electron Microscopy ◽  
Volcanic Rocks ◽  
Analytical Electron Microscopy ◽  
Metamorphic Rocks ◽  
New Mineral ◽  
Chemical Compositions ◽  
Low Grade ◽  
Fine Grained ◽  
Depth Analysis ◽  
Mineral Aggregates

Recent advances in transmission electron microscopy (TEM) and analytical electron microscopy (AEM) have led to many new insights into the structural and chemical characteristics of very finegrained, optically homogeneous mineral aggregates in sedimentary and very low-grade metamorphic rocks. Chemical compositions obtained by electron microprobe analysis (EMPA) on such materials have been shown by TEM/AEM to result from beam overlap on contaminant phases on a scale below resolution of EMPA, which in turn can lead to errors in interpretation and determination of formation conditions. Here we present an in-depth analysis of the relation between AEM and EMPA data, which leads also to the definition of new mineral phases, and demonstrate the resolution power of AEM relative to EMPA in investigations of very fine-grained mineral aggregates in sedimentary and very low-grade metamorphic rocks.Celadonite, having end-member composition KMgFe3+Si4O10(OH)2, and with minor substitution of Fe2+ for Mg and Al for Fe3+ on octahedral sites, is a fine-grained mica widespread in volcanic rocks and volcaniclastic sediments which have undergone low-temperature alteration in the oceanic crust and in burial metamorphic sequences.

scholarly journals Mineralogy, Geochemistry and Provenance of Coastal Sands from Greece: New Insights on the REE Content of Black Coastal Sands from Aggelochori Area, N.-Greece

Minerals ◽  
2021 ◽  
Vol 11 (7) ◽  
pp. 693
Author(s):  
Argyrios Papadopoulos ◽  
Stylianos Lazaridis ◽  
Afroditi Kipourou-Panagiotou ◽  
Nikolaos Kantiranis ◽  
Antonios Koroneos ◽  
...  
Keyword(s):  
Major And Trace Elements ◽  
Heavy Minerals ◽  
Sea Waves ◽  
Complex Flow ◽  
Fine Grained ◽  
Microscopy Analysis ◽  
Beach Sands ◽  
Geochemical Analyses ◽  
The One ◽  
Black Color

Beach sands from Aggelochori coast line are investigated for their geochemistry and REE content, mineralogy and their provenance. These fluvial sands bear heavy minerals enriched horizons (containing minerals such as magnetite, zircon, ilmenite, hematite, rutile and titanite) that can be distinguished due to their black color and are formed usually due to the action of sea waves that deposit the heavy minerals and remove the lighter ones. After a suitable processing (washing, sieving, drying and magnetic separation) of the samples, the mineral constituents and their presence (wt.%) were estimated by XRD. Among the samples, the one being simultaneously the more fine grained and the more zircon-enriched (as suggested by XRPD data and optical microscopy analysis) has been selected for further geochemical analyses. The major and trace elements contents were compared to previously studied REE enriched beach sands from Kavala and Sithonia. Beach sands from Aggelochori area appear to have relatively low REE contents. Considering the provenance of these sediments, we suggest that these sands, are a product of the erosion of multi-sources, including the near-by Monopigado granite, as well as metamorphic rocks, as indicated by the presence of rutile and both ilmenite and magnetite in some samples. Therefore, there are indications of a complex flow pattern that existed at the paleo-catchment area of the deposition.

scholarly journals Geochemical signatures of mineralizing events in the Juomasuo Au–Co deposit, Kuusamo belt, northeastern Finland

2021 ◽  
Author(s):  
Mikael Vasilopoulos ◽  
Ferenc Molnár ◽  
Hugh O’Brien ◽  
Yann Lahaye ◽  
Marie Lefèbvre ◽  
...  
Keyword(s):  
Trace Element ◽  
Sulfur Isotope ◽  
Mineral Chemistry ◽  
Chemical Compositions ◽  
Metasedimentary Rocks ◽  
Metavolcanic Rocks ◽  
Icp Ms ◽  
Stage 1 ◽  
Host Rocks ◽  
Relationship Of

AbstractThe Juomasuo Au–Co deposit, currently classified as an orogenic gold deposit with atypical metal association, is located in the Paleoproterozoic Kuusamo belt in northeastern Finland. The volcano-sedimentary sequence that hosts the deposit was intensely altered, deformed, and metamorphosed to greenschist facies during the 1.93–1.76 Ga Svecofennian orogeny. In this study, we investigate the temporal relationship between Co and Au deposition and the relationship of metal enrichment with protolith composition and alteration mineralogy by utilizing lithogeochemical data and petrographic observations. We also investigate the nature of fluids involved in deposit formation based on sulfide trace element and sulfur isotope LA-ICP-MS data together with tourmaline mineral chemistry and boron isotopes. Classification of original protoliths was made on the basis of geochemically immobile elements; recognized lithologies are metasedimentary rocks, mafic, intermediate-composition, and felsic metavolcanic rocks, and an ultramafic sill. The composition of the host rocks does not control the type or intensity of mineralization. Sulfur isotope values (δ34S − 2.6 to + 7.1‰) and trace element data obtained for pyrite, chalcopyrite, and pyrrhotite indicate that the two geochemically distinct Au–Co and Co ore types formed from fluids of different compositions and origins. A reduced, metamorphic fluid was responsible for deposition of the pyrrhotite-dominant, Co-rich ore, whereas a relatively oxidized fluid deposited the pyrite-dominant Au–Co ore. The main alteration and mineralization stages at Juomasuo are as follows: (1) widespread albitization that predates both types of mineralization; (2) stage 1, Co-rich mineralization associated with chlorite (± biotite ± amphibole) alteration; (3) stage 2, Au–Co mineralization related to sericitization. Crystal-chemical compositions for tourmaline suggest the involvement of evaporite-related fluids in formation of the deposit; boron isotope data also allow for this conclusion. Results of our research indicate that the metal association in the Juomasuo Au–Co deposit was formed by spatially coincident and multiple hydrothermal processes.

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