Alteration of mélange-hosted chromitites from Korydallos, Pindos ophiolite complex, Greece: evidence for modification by a residual high-T post-magmatic fluid

2014 ◽  
Vol 64 (4) ◽  
pp. 473-494 ◽  
Author(s):  
Argyrios N. Kapsiotis
Keyword(s):  
Small Volume ◽  
Spinel Phase ◽  
Magmatic Fluid ◽  
Textural Features ◽  
Ophiolite Complex ◽  
Ferrian Chromite ◽  
Compositional Variations ◽  
Back Arc ◽  
Compatible Elements ◽  
Mantle Section

Abstract The peridotites from the area of Korydallos, in the Pindos ophiolitic massif, crop out as deformed slices of a rather dismembered sub-oceanic, lithospheric mantle section and are tectonically enclosed within the Avdella melange. The most sizeable block is a chromitite-bearing serpentinite showing a mesh texture. Accessory, subhedral to euhedral Cr-spinels in the serpentinite display Cr# [Cr/(Cr + Al)] values that range from 0.36 to 0.42 and Mg# [Mg/(Mg+ Fe2+ )] values that vary between 0.57 and 0.62, whereas the TiO2 content may be up to 0.47 wt.%. The serpentinite fragment is characterized by low abundances of magmaphile elements (Al2 O3 : 0.66 wt.%, CaO: 0.12 wt.%, Na2 O: 0.08 wt.%, TiO2 : 0.007 wt.%, Sc: 4 ppm) and enrichment in compatible elements (Cr: 2780 ppm and Ni: 2110 ppm). Overall data are in accordance with derivation of the serpentinite exotic block from a dunite that was formed in the mantle region underneath a back-arc basin before tectonic incorporation in the Korydallos melange. Two compositionally different chromitite pods are recognized in the studied serpentinite fragment, a Cr-rich chromitite and a high-Al chromitite, which have been ascribed to crystallization from a single, progressively differentiating MORB/IAT melt. Although both pods are fully serpentinized only the Al-rich one shows signs of limited Cr-spinel replacement by an opaque spinel phase and clinochlore across grain boundaries and fractures. Modification of the ore-making Cr-spinel is uneven among the Al-rich chromitite specimens. Textural features such as olivine replacement by clinochlore and clinochlore disruption by serpentine indicate that Cr-spinel alteration is not apparently related to serpentinization. From the unaltered Cr-spinel cores to their reworked boundaries the Al2 O3 and MgO abundances decrease, being mainly compensated by FeOt and Cr2 O3 increases. Such compositional variations are suggestive of restricted ferrian chromite (and minor magnetite) substitution for Cr-spinel during a short-lived but relatively intense, low amphibolite facies metamorphic episode (temperature: 400-700 °C). The presence of tremolite and clinochlore in the interstitial groundmass of the high-Al chromitite and their absence from the Cr-rich chromitite matrix imply that after chromitite formation a small volume of a high temperature, post-magmatic fluid reacted with Cr-spinel, triggering its alteration.

scholarly journals Study of the chromite mineralization associated to ophiolites from Tinos Island, Attico-Cycladic Massif

2019 ◽  
Vol 55 (1) ◽  
pp. 185
Author(s):  
Maria Kokkaliari ◽  
Karen St. Seymour ◽  
Stylianos F. Tombros ◽  
Eleni Koutsopoulou
Keyword(s):  
Subduction Zone ◽  
Mantle Wedge ◽  
Binary Classification ◽  
Mineral Chemistry ◽  
Textural Features ◽  
Ophiolite Complex ◽  
Depleted Mantle ◽  
Back Arc ◽  
Host Rocks ◽  
Petrographic Study

This paper aims to study the chromitites, as well as their host rocks (meta-peridotites, meta-dunites and serpentinites) of the ophiolite complex of Mount Tsiknias, in Tinos Island. Recognition of their mineralogy and their textural features was carried out through detailed petrographic study. The mineral chemistry analysis contributed to the evaluation of the analyzed chromites, the chemical composition of which provides important information about the petrogenetic evolution of the chromitite ores. The chromites were in equilibrium with boninite melts derived from Supra-Subduction Zone, e.g., a depleted mantle wedge. In the binary classification diagram for spinels, the Tinos samples extend in the fields of Mg-chromite and chromite sensu strictu. In the TiO2 vs Al2O3 diagram, the chromites plot in the field of Supra-Subduction Zone (SSZ) peridotites and partly overlap the field of chromites in Back-Arc Basalts (BABB), however the same samples plot in the field of chromite of boninites. In the Al2O3 vs Cr2O3 diagram both groups of Tinos chromites plot in the field/extremity of “mantle chromites”.

Tholeiitic volcanic rocks of the late Archean Blake River Group, southern Abitibi greenstone belt: origin and geodynamic implications

1992 ◽  
Vol 29 (7) ◽  
pp. 1448-1458 ◽  
Author(s):  
M. R. Laflèche ◽  
C. Dupuy ◽  
J. Dostal
Keyword(s):  
Greenstone Belt ◽  
Volcanic Rocks ◽  
Incompatible Trace Element ◽  
Progressive Change ◽  
Abitibi Greenstone Belt ◽  
Mid Ocean Ridge ◽  
Compositional Variations ◽  
Arc Setting ◽  
Back Arc ◽  
Ocean Ridge

The late Archean Blake River Group volcanic sequence forms the uppermost part of the southern Abitibi greenstone belt in Quebec. The group is mainly composed of mid-ocean-ridge basalt (MORB)-like tholeiites that show a progressive change of several incompatible trace element ratios (e.g., Nb/Th, Nb/Ta, La/Yb, and Zr/Y) during differentiation. The compositional variations are inferred to be the result of fractional crystallization coupled with mixing–contamination of tholeiites by calc-alkaline magma which produced the mafic–intermediate lavas intercalated with the tholeiites in the uppermost part of the sequence. The MORB-like tholeiites were probably emplaced in a back-arc setting.

scholarly journals Chromitite petrogenesis in the mantle section of the Ballantrae Ophiolite Complex (Scotland)

Lithos ◽  
2019 ◽  
Vol 344-345 ◽  
pp. 51-67 ◽  
Author(s):  
E.J. Derbyshire ◽  
B. O'Driscoll ◽  
D. Lenaz ◽  
A. Zanetti ◽  
R. Gertisser
Keyword(s):  
Ophiolite Complex ◽  
Mantle Section

scholarly journals Mineralogy and Geochemistry of Titaniferous Gabbros of Ophiolitic Fanouj Zone (Sistan & Baluchestan, Iran)

2016 ◽  
Vol 10 (4) ◽  
pp. 189 ◽  
Author(s):  
Majid Falaknazi ◽  
Mehrdad Karimi
Keyword(s):  
Crystallization Process ◽  
The West ◽  
Ophiolite Complex ◽  
Tholeiitic Magma ◽  
Incompatible Elements ◽  
South West ◽  
West Part ◽  
Main Host ◽  
Uplift Zone ◽  
Compatible Elements

<p><span lang="EN-US">Ophiolite complex in the west of Fanuj is 200 Km south west of Iranshahr in Sistan and Baluchestan province. This ophiolite complex lies in the uplift zone of the oceanic crust of Oman between Makran and Fanuj faults. Ophiolite of the west part of Fanuj is consisted of three parts including gabbro, diabase dikes and small quantity microdiorite masses. Ilmenite is the main mineral of titanium which along with magnetite has been formed independently or inter-crystalline way after crystallization of plagioclase, pyroxene and often along with amphibole in gabbro rocks. The formation of the broad gabbro masses which is associated with plagioclase and pyroxene crystallization in High Oxygen fugacity condition formed a fluid rich in iron and titanium during the formation of ferro gabbro rocks as the main host of the ilminite reserves. Gradual crystallization process and decrease in compatible elements such as </span><span lang="EN-US">Cr, Ni, Mg and increase in incompatible elements such as Mn</span><span lang="FA" dir="RTL">،</span><span lang="EN-US"> Na</span><span lang="FA" dir="RTL">،</span><span lang="EN-US"> Ti from the bottom to the upper parts of ophilite complex shows that </span><span lang="EN-US">the formation of the complex has been occurred through the process of crystal fractionation from a tholeiitic magma which is rich in titanium.</span></p>

Volcanic evolution of oceanic crust in a Late Ordovician back-arc basin: The Solund-Stavfjord Ophiolite Complex, West Norway

2003 ◽  
Vol 4 (10) ◽  
Author(s):  
Harald Furnes ◽  
Helge Hellevang ◽  
Bjarte Hellevang ◽  
Kjell Petter Skjerlie ◽  
Brian Robins ◽  
...  
Keyword(s):  
Oceanic Crust ◽  
Late Ordovician ◽  
Ophiolite Complex ◽  
Volcanic Evolution ◽  
Back Arc

scholarly journals ON THE METAMORPHIC MODIFICATION OF CR-SPINEL COMPOSITIONS FROM THE ULTRABASIC ROCKS OF THE PINDOS OPHIOLITE COMPLEX (NW GREECE)

2007 ◽  
Vol 40 (2) ◽  
pp. 781 ◽  
Author(s):  
A. Kapsiotis ◽  
B. Tsikouras ◽  
T. Grammatikopoulos ◽  
S. Karipi ◽  
H. Hatzipanagiotou
Keyword(s):  
Ultramafic Rocks ◽  
Cation Diffusion ◽  
Ophiolite Complex ◽  
Ferrian Chromite ◽  
The Common ◽  
Northwestern Greece ◽  
Common Association

Serpentinites and serpentinised ultramafic rocks from the Pindos ophiolite complex, northwestern Greece, contain Cr-spinel grains that are usually altered. The extent of alteration differs among Cr-spinels and two alteration trends can be distinguished. The most dominant is characterised by Cr-spinel overgrown by Cr-magnetite, while the second shows gradual replacement of Cr-spinel by ferrian chromite locally combined with Cr-magnetite development. Compared to cores, the altered rims are enriched in Fe and show elevated Cr# in both types of alteration, while they are impoverished in Mg and Al only at the second one. The common association of Crmagnetite with serpentine and ferrian chromite with chlorite provides insights to the metamorphic context of their formation through processes that include metasomatism by cation diffusion exchange

Meta-igneous rocks of the West-Carpathian basement, Slovakia: indicators of Early Paleozoic extension and shortening events

2009 ◽  
Vol 180 (6) ◽  
pp. 461-471 ◽  
Author(s):  
Marián Putiš ◽  
Peter Ivan ◽  
Milan Kohút ◽  
Ján Spišiak ◽  
Pavol Siman ◽  
...  
Keyword(s):  
Early Paleozoic ◽  
The South ◽  
Ophiolite Complex ◽  
The North ◽  
Late Cambrian ◽  
Collision Zone ◽  
Gondwana Margin ◽  
Group A ◽  
Arc Setting ◽  
Back Arc

Abstract The paper reviews the main West-Carpathian Early Paleozoic metamorphosed originally sedimentary-magmatic complexes, dated by SHRIMP on zircons, as indicators of crustal extension and shortening events. Igneous precursors of a Layered Amphibolite Complex (LAC) – fractionated upper mantle gabbros to diorites, dated at 503 ± 4 and 492 ± 4 Ma from the North-Veporic, or 480 ± 5 and 450 ± 6 Ma from the Tatric basement are contemporaneous with subaluminous to metaluminous I-type (507 ± 4 Ma, the South-Veporic basement), peraluminous S-type (497 ± 4 Ma, the South-Veporic basement; 516 ± 7, 485 ± 6 and 462 ± 6 Ma, the North-Veporic basement; 497 ± 6, 472 ± 6 and 450 ± 6 Ma, the Tatric basement), alkaline A-type (511 ± 6 Ma, South-Veporic basement) granitic orthogneisses and calcalkaline rhyolitic (482 ± 6 Ma) and dacitic (476 ± 7 Ma) metavolcanics (Gemeric basement), indicating a magmatic immature back arc setting. The ages point to Middle/Late Cambrian, Early and Late Ordovician magmatic phases, coeval with the extension in the northern Gondwana margin. Separation of an inferred Avalonian and/or Galatian terranes distal continental ribbon corresponds with the opening of a Medio-European Basin. A 430-390 Ma dated MP/HP metamorphic event, recorded in the LAC and associated orthogneisses, occurred in the area of thinned immature back arc basin crust due to closure of the Medio-European Basin. Thus a distal Gondwana continental ribbon north of this basin could be an eastward lateral pendant of Armorica, derived from Galatian terrane. Metaophiolites of the Pernek Group (a metagabbrodolerite dated at 371 ± 4 Ma) in the Tatric basement, analogous to island-arc tholeiites and back-arc basin basalts, indicate a back-arc basin setting north of a 430-390 Ma old northward dipping subduction/collision zone, dividing the northward drifting western Galatian terrane microplate from the Gondwana margin. Some metabasites of the Gemeric basement might indicate Late Devonian to Mississippian opening of a peri-Gondwanan Paleotethyan oceanic basin: a 383 ± 3 Ma old remelted metagabbro (482 ± 9 Ma) from the Klátov gneiss-amphibolite complex, ca. 385 Ma old porphyritic metabasite of the Zlatník ophiolite complex, as well as a 350 ± 5 Ma old HP metabasite as tectonic fragment within the Rakovec Group. The closure of Devonian-Mississippian basins, accompanied by medium-pressure (the Pernek Group) to high-pressure (blueschist to eclogitic tectonic fragments in greenschist facies rocks of the Rakovec Group) metamorphism, occurred in late Carboniferous to early Permian, when Paleotethyan realm complexes accreted to a Galatian terrane microplate, the latter represented by the older and the higher-grade Tatric and Veporic basement complexes.

Boninitic and tholeiitic dykes in the Lewis Hills mantle section of the Bay of Islands ophiolite: implications for magmatism adjacent to a fracture zone in a back-arc spreading environment

1995 ◽  
Vol 32 (12) ◽  
pp. 2128-2146 ◽  
Author(s):  
Stephen J. Edwards
Keyword(s):  
Partial Melting ◽  
Fracture Zone ◽  
Tholeiitic Magma ◽  
Suprasubduction Zone ◽  
Geochemical Study ◽  
Mid Ocean Ridge ◽  
Hill Area ◽  
Back Arc ◽  
Ocean Ridge ◽  
Mantle Section

A detailed, integrated field, petrographic, and geochemical study of the Springers Hill area of the Bay of Islands ophiolite exposed in the Lewis Hills was undertaken to explain the anomalously high abundance of veins and dykes of chromitite, orthopyroxenite, and clinopyroxenite, and their associated dunites, hosted by a refractory harzburgite–dunite mixture. A geodynamic situation is presented, which is constrained by previous studies requiring formation of the Springers Hill mantle section at a ridge–fracture zone intersection, and the whole of the Bay of Islands ophiolite within a back-arc spreading environment. The veins and dykes formed during magmatism at the ridge–fracture zone intersection and along the fracture zone, as progressively hotter, more fertile (richer in clinopyroxene) asthenosphere ascended and was channelled up and along the fracture zone wall. Shallow melting of refractory harzburgite in the presence of subduction-derived hydrous fluids produced light rare earth element (LREE)-enriched boninitic magma from which crystallized chromitites, some of their associated dunites, and orthopyroxenites. This melting event dehydrated much of the mantle in and around the zone of partial melting. Continued rise and shallow partial melting of hotter, more fertile mantle under conditions of variable hydration generated LREE-depleted, low-Ti tholeiitic magma. This magma crystallized olivine clinopyroxenite, some associated dunite, and clinopyroxenite. The final magmatic event may have involved partial melting of mid-ocean-ridge basalt-bearing mantle at depth, ascent of the magma, and formation of massive wehrlite–lherzolite bodies at the ridge–fracture zone intersection and along the fracture zone. Ridge–fracture zone intersections in suprasubduction-zone environments are sites of boninitic and tholeiitic magmatism because refractory asthenospheric mantle may melt as it is channelled with subduction-derived fluids to shallow depths by the old, cold lithospheric wall of the fracture zone. Heat for melting is provided by the ascent of hotter, more fertile mantle. Extremely refractory magmas do not occur along "normal" oceanic fracture zones because volumes of highly refractory mantle are much less, subduction-derived hydrous fluids are not present, and fracture zone walls extend to shallower depths.

scholarly journals Chemical Composition and Genesis Implication of Garnet from the Laoshankou Fe-Cu-Au Deposit, the Northern Margin of East Junggar, NW China

Minerals ◽  
2021 ◽  
Vol 11 (3) ◽  
pp. 334
Author(s):  
Pei Liang ◽  
Yu Zhang ◽  
Yuling Xie
Keyword(s):  
Rare Earth ◽  
Rare Earth Elements ◽  
Magmatic Fluid ◽  
Northern Margin ◽  
Fluid Evolution ◽  
Compositional Variations ◽  
Type 3 ◽  
Stage Stage

In order to reveal the formation mechanism of different garnets and its implications for the fluid evolution in the Laoshankou Fe-Cu-Au deposit in the northern margin of East Junggar (NW China), three types of garnet have been investigated in detail in this study. (1) Type 1 grossular, formed at Ca-silicate stage (stage I, the pre-mineralization stage), was replaced by Type 2 garnet and magnetite, and displays a compositional range of Grs44–53Adr44–53, which has relatively lower total REE (rare earth elements) contents (8.14–32.8 ppm) and markedly depleted LREE (light rare earth elements) with distinctive positive Eu anomaly (1.36–9.61). (2) Type 2 Al-rich andradite, formed at the early sub-stage of amphibole-epidote-magnetite stage (stage II, the main magnetite mineralization stage), can be divided into two sub-types, i.e., Type 2a and Type 2b. Type 2a garnets exhibit polysynthetic twinning and relatively narrow compositional variations of Adr63–66Grs31–34 with HREE-(heavy rare-earth elements) enrichment and positive Eu anomalies (3.22–3.69). Type 2b garnets own wide compositional variations of Adr55–77Grs21–43 with relatively higher REE contents (49.1–124 ppm), markedly depleted LREE and a distinctive positive Eu anomaly (2.11–4.61). (3) Type 3 andradite (Adr>91) associated with sulfide stage (stage III, the main copper-gold mineralization stage) is different from other types of garnets in Laoshankou, which are characterized by lowest total REE contents (1.66–91.1 ppm), flat HREE patterns, LREE-enrichment and the strongest positive Eu anomalies (3.31–45.48). Incorporation of REE into garnet is largely controlled by external factors, such as fluid chemistry, pH, ƒO2 and water-rock ratios as well as its crystal chemistry. Type 1 and 2 garnets mainly follow the creation of X2+ (e.g., Ca2+) site vacancy, e.g., [X2+]−3VIII[]+1VIII[REE3+]+2VIII. The REE3+ substitution mechanism for Type 3 garnet is the Na+-REE3+ coupled substitutions, e.g., [X2+]−2VIII[X+]+1VIII[REE3+]+1VIII, without the evaluation of the creation of site vacancy. The compositional variations from Type 1 to Type 3 garnet indicate significant differences of fluid compositions and physicochemical conditions, and can be used to trace the fluid–rock interaction and hydrothermal evolution of garnet. Type 1 grossular was formed by magmatic fluid under low water–rock ratios and ƒO2, and neutral pH environment by diffusion metasomatism in a nearly closed system with the preferential incorporation into the grossular of HREE. As the long fluid pore residence and continuing infiltration metasomatism under nearly closed-system conditions, fluids with high water/rock ratios were characterized by increased ƒO2, more active incorporation of Fe3+ and REE, and formed Type 2 Al-rich andradite. In contrast, Type 3 garnet formed by oxidizing magmatic fluid under a mildly acidic environment with highest ƒO2 and water–rock ratios, and was influenced by externally derived high salinity and Ca-rich fluids in an open system. Thus, the geochemical features of different types and generations of garnets in the Laoshankou deposit can provide important information of fluid evolution, revealing a transition from neutral magmatic fluid to oxidizing magmatic fluid with addition of external non-magmatic Ca-rich fluid from the Ca-silicate stage to the sulfide stage. The above proved the fluid evolution process further indicates that the Laoshankou deposit prefers to be an IOCG-like (iron oxide-copper-gold) deposit rather than a typical skarn deposit.

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