crustal contamination
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2022 ◽  
Author(s):  
S Matte ◽  
M Constantin ◽  
R Stevenson

The Kipawa rare-earth element (REE) deposit is located in the Parautochton zone of the Grenville Province 55 km south of the boundary with the Superior Province. The deposit is part of the Kipawa syenite complex of peralkaline syenites, gneisses, and amphibolites that are intercalated with calc-silicate rocks and marbles overlain by a peralkaline gneissic granite. The REE deposit is principally composed of eudialyte, mosandrite and britholite, and less abundant minerals such as xenotime, monazite or euxenite. The Kipawa Complex outcrops as a series of thin, folded sheet imbricates located between regional metasediments, suggesting a regional tectonic control. Several hypotheses for the origin of the complex have been suggested: crustal contamination of mantle-derived magmas, crustal melting, fluid alteration, metamorphism, and hydrothermal activity. Our objective is to characterize the mineralogical, geochemical, and isotopic composition of the Kipawa complex in order to improve our understanding of the formation and the post-formation processes, and the age of the complex. The complex has been deformed and metamorphosed with evidence of melting-recrystallization textures among REE and Zr rich magmatic and post magmatic minerals. Major and trace element geochemistry obtained by ICP-MS suggest that syenites, granites and monzonite of the complex have within-plate A2 type anorogenic signatures, and our analyses indicate a strong crustal signature based on TIMS whole rock Nd isotopes. We have analyzed zircon grains by SEM, EPMA, ICP-MS and MC-ICP-MS coupled with laser ablation (Lu-Hf). Initial isotopic results also support a strong crustal signature. Taken together, these results suggest that alkaline magmas of the Kipawa complex/deposit could have formed by partial melting of the mantle followed by strong crustal contamination or by melting of metasomatized continental crust. These processes and origins strongly differ compare to most alkaline complexes in the world. Additional TIMS and LA-MC-ICP-MS analyses are planned to investigate whether all lithologies share the same strong crustal signature.


Author(s):  
Kata Molnár ◽  
Pierre Lahitte ◽  
Stéphane Dibacto ◽  
Zsolt Benkó ◽  
Samuele Agostini ◽  
...  

AbstractLate Miocene to Pleistocene volcanism within the Vardar zone (North Macedonia) covers a large area, has a wide range in composition, and is largely connected to the tectonic evolution of the South Balkan extensional system, the northern part of the Aegean extensional regime. The onset of the scattered potassic to ultrapotassic volcanism south from the Scutari-Peć transverse zone occurred at ca. 8.0 Ma based on this study. Here, we focused on three volcanic centers located on deep structures or thrust faults along the western part of the Vardar zone, for which there is none to very little geochronological and geochemical data available. Pakoševo and Debrište localities are represented as small remnants of lava flows cropping out at the southern edge of Skopje basin and at the western edge of Tikveš basin, respectively. Šumovit Greben center is considered as part of the Kožuf-Voras volcanic system, and it is located on its westernmost side, at the southern edge of Mariovo basin, which is largely composed of volcaniclastic sediments. We present new eruption ages applying the unspiked Cassignol-Gillot K–Ar technique on groundmass, as well as petrological and geochemical data, supplemented with Sr and Nd isotopes to complement and better understand the Neogene-Pleistocene volcanism in the region. Eruption ages on these rocks interlayered between sedimentary formations allow to better constrain the evolution of those sedimentary basins. Rocks from the three volcanic centers belong to the high-K calc-alkaline–shoshonitic series based on their elevated K content. The oldest center amongst these three localities, as well as other Late Miocene centers within the region, is the trachyandesitic Debrište, which formed at ca. 8.0 Ma, and exhibits the highest Nd and lowest Sr isotopic ratios (0.512441–0.512535 and 0.706759–0.706753, respectively). The basaltic trachyandesite Pakoševo center formed at ca. 3.8 Ma and its Nd and Sr isotopic ratios (0.512260 and 0.709593, respectively) bear the strongest signature of crustal contamination. The rhyolitic Šumovit Greben center is a composite volcanic structure formed at ca. 3.0–2.7 Ma. Its youngest eruption unit has a slightly higher Nd and lower Sr isotopic ratios (0.512382 and 0.709208, respectively) representing a magma with a lesser extent of crustal assimilation than the other samples from this center. The overall trend through time in the Sr and Nd isotopic ratios of the Late Miocene to Pleistocene mafic volcanic centers in the region implies an increasing rate of metasomatism of the lithospheric mantle.


Minerals ◽  
2021 ◽  
Vol 11 (12) ◽  
pp. 1410
Author(s):  
Pavel A. Serov

This paper continues the Sm-Nd isotope geochronological research carried out at the two largest Paleoproterozoic ore complexes of the northeastern Baltic Shield, i.e., the Cu-Ni-Cr Monchegorsk and the Pt-Pd Fedorovo-Pansky intrusions. These economically significant deposits are examples of layered complexes in the northeastern part of the Fennoscandian Shield. Understanding the stages of their formation and transformation helps in the reconstruction of the long-term evolution of ore-forming systems. This knowledge is necessary for subsequent critical metallogenic and geodynamic conclusions. We applied the Sm-Nd method of comprehensive age determination to define the main age ranges of intrusion. Syngenetic ore genesis occurred 2.53–2.85 Ga; hydrothermal metasomatic ore formation took place 2.70 Ga; and the injection of additional magma batches occurred 2.44–2.50 Ga. The rock transformation and redeposited ore formation at 2.0–1.9 Ga corresponded to the beginning of the Svecofennian events, widely presented on the Fennoscandian Shield. According to geochronological and Nd-Sr isotope data, rocks of the Monchegorsk and the Fedorovo-Pansky complexes seemed to have an anomalous mantle source in common with Paleoproterozoic layered intrusions of the Fennoscandian Shield (enriched with lithophile elements, εNd values vary from −3.0 to +2.5 and ISr 0.702–0.705). The data obtained comply with the known isotope-geochemical and geochronological characteristics of ore-bearing layered intrusions in the northeastern Baltic Shield. An interaction model of parental melts of the Fennoscandian layered intrusions and crustal matter shows a small level of contamination within the usual range of 5–10%. However, the margins of the Monchetundra massif indicate a much higher level of crustal contamination caused by active interaction of parental magmas and host rock.


Minerals ◽  
2021 ◽  
Vol 11 (12) ◽  
pp. 1361
Author(s):  
Ewa Krzemińska ◽  
Leszek Krzemiński ◽  
Paweł Poprawa ◽  
Jolanta Pacześna ◽  
Krzysztof Nejbert

The U–Pb measurements of youngest, coherent group of zircons from the Mielnik IG1 dolerite at the Teisseyre-Tornquist margin (TTZ) of East European Craton (EEC) in Poland yielded age of 300 ± 4 Ma. Zircon dated an evolved portion of magma at the late stage crystallization. It is shown that this isolated dyke from the northern margin of the Lublin Podlasie basin (Podlasie Depression) and regional dyke swarms of close ages from the Swedish Scania, Bornholm and Rügen islands, Oslo rift, Norway, and the Great Whine Sill in northeastern England, were coeval. They have been controlled by the same prominent tectonic event. The Mielnik IG1 dolerite is mafic rock with Mg-number between 52 and 50 composed of the clinopyroxene, olivine-pseudomorph, plagioclase, titanite, magnetite mineral assemblage, indicating relatively evolved melt. This hypabyssal rock has been affected by postmagmatic alteration. The subalkaline basalt composition, enrichment in incompatible trace elements, progressive crustal contamination, including abundance of zircon xenocrysts determines individual characteristics of the Mielnik IG1 dolerite. The revised age of dolerite, emplaced in vicinity of TTZ provides more evidences documenting the reach of the Permo-Carboniferous extension and rifting accompanied by magmatic pulses, that were widespread across Europe including the margin of the EEC incorporated that time into the broad foreland of the Variscan orogen.


2021 ◽  
Author(s):  
◽  
William Robert Hackett

<p>Ruapehu Volcano is an active, multiple-vent, andesite composite volcano at the southern terminus of the Taupo Volcanic Zone, central North Island, New Zealand. The present-day volume of Ruapehu is estimated at 110 km3, and construction of the massif probably occurred during the past 0.5 m.y. Geologic mapping and stratigraphic studies have led to the recognition of four periods of cone construction, each occurring over 104-105 year time intervals. On the basis of lithologic/petrographic differences, and conspicuous unconformities which separate the deposits of each cone-building period, four new formations are defined, comprises the Ruapehu Group. Te Herenga formation (new formation name) comprises the oldest deposits of Ruapehu (upper lavas ca. 0.23 Ma) and is exposed as planeze surfaces and aretes on N and NW Ruapehu. The formation includes lava flows, tuff breccias, and small intrusive bodies surrounded by zones of hydrothermal alteration. There is little petrographic and compositional diversity; most lavas are porphyritic titanomagnetite- augite- hypersthene- plagioclase basic andesites. Wahiance Formation (new formation name) is younger than Te Herenga Fm,. but of unknown age. It is well exposed on SE Ruapehu, and comprises mostly lava flows and tuff breccias. The lavas comprise acid and basic andesites. Mangewhero Formation (new formation name) is well exposed everywhere except SE Ruapehu, and the upper lavas and pyroclastics (ca. 0.02 Ma) form the present high peeks and main cone of Ruapehu. The lavas are petrographically and geochemically diverse, ranging from basalt to decite in bulk composition. Some of the lower lavas are olivine-beering andesites of hybrid orgin. Whakapapa Formation (new formation name; ca 15,000 years to present) comprises conspicuously young lava flows, tuff breccias, airfall pyroclastics and minor pyroclastic flows of acid- and basic andesite. The deposits of these post-glacial summit and flank eruptions are subdivided into the lwikau, Rangataua, Tama and Crater Lake Members. 'Related vents' produced Heuhungatahi Andesite Fm. (> 0.5 Ma?), and Holocene deposits of basalt and basic andesite at isolated, monogenetic centres comprising Ohakune Andesite Fm., Pukeonake Andesite Fm., and Waimarino Basalt Fm. (new formation name). Most Ruapehu lavas are medium-K acid and basic andesites (mean of 144 bulk rock analyses is 57.8 wt % SiO2), but rare basalt and minor decite are present. Nearly all lavas are porphyritic in plagioclase, augite and hypersthene [plus or minus] olivine, with titanomagnetite micro- phenocrysts, and contain abundant metamorphic and igneous rock inclusions. Petrography, mineral chemistry and bulk rock chemistry indicate fractional crystallization series from parental basalts (52-53 % SiO2, Q-normative, low-alumina) to medium-K basic- and acid andesites (58-59 % SiO2). Early fractionating minerals are olivine and clinopyroxene with minor chrome spinel and plagioclase, followed by plagioclase, orthopyroxene, clinopyroxene and minor titanomagnetite in later stages of differentiation. Thus, basalt differentiation to produce andesites involves 'POAM-type' (Gill, 1981) fractional crystallization. Three second-order differentiation processes operate concurrently with frational crystallization: (1) Crystal accumulation involves addition of co-genetic plutonic rock fragments and crystals derived from them. These inclusions are common and few rocks represent liquid compositions. (2) Magma mixing involves mingling of magmas in repeatedly-occupied conduits. End members are as diverse as basalt and decite, yielding petrogaphically and chemically distinctive high-Mg andesites of the upper cone complex and parasitic centres. (3) Selective crustal assimilation is suggested by partially fused metamorphic inclusions, positive correlation of 87Sr/86Sr with SiO2, and failure of simple 'POAM' fractionation to explain decites (63-65 % SiO2). Petrogenesis of Ruapehu andesites takes place under open-system condition, involving production of parental Q-normative basalts in the mantle wedge, concurrent fractional crystallization and crustal contamination, entrainment of co-genetic plutonic rocks, and mixing of magmas in common conduits.</p>


2021 ◽  
Author(s):  
◽  
William Robert Hackett

<p>Ruapehu Volcano is an active, multiple-vent, andesite composite volcano at the southern terminus of the Taupo Volcanic Zone, central North Island, New Zealand. The present-day volume of Ruapehu is estimated at 110 km3, and construction of the massif probably occurred during the past 0.5 m.y. Geologic mapping and stratigraphic studies have led to the recognition of four periods of cone construction, each occurring over 104-105 year time intervals. On the basis of lithologic/petrographic differences, and conspicuous unconformities which separate the deposits of each cone-building period, four new formations are defined, comprises the Ruapehu Group. Te Herenga formation (new formation name) comprises the oldest deposits of Ruapehu (upper lavas ca. 0.23 Ma) and is exposed as planeze surfaces and aretes on N and NW Ruapehu. The formation includes lava flows, tuff breccias, and small intrusive bodies surrounded by zones of hydrothermal alteration. There is little petrographic and compositional diversity; most lavas are porphyritic titanomagnetite- augite- hypersthene- plagioclase basic andesites. Wahiance Formation (new formation name) is younger than Te Herenga Fm,. but of unknown age. It is well exposed on SE Ruapehu, and comprises mostly lava flows and tuff breccias. The lavas comprise acid and basic andesites. Mangewhero Formation (new formation name) is well exposed everywhere except SE Ruapehu, and the upper lavas and pyroclastics (ca. 0.02 Ma) form the present high peeks and main cone of Ruapehu. The lavas are petrographically and geochemically diverse, ranging from basalt to decite in bulk composition. Some of the lower lavas are olivine-beering andesites of hybrid orgin. Whakapapa Formation (new formation name; ca 15,000 years to present) comprises conspicuously young lava flows, tuff breccias, airfall pyroclastics and minor pyroclastic flows of acid- and basic andesite. The deposits of these post-glacial summit and flank eruptions are subdivided into the lwikau, Rangataua, Tama and Crater Lake Members. 'Related vents' produced Heuhungatahi Andesite Fm. (> 0.5 Ma?), and Holocene deposits of basalt and basic andesite at isolated, monogenetic centres comprising Ohakune Andesite Fm., Pukeonake Andesite Fm., and Waimarino Basalt Fm. (new formation name). Most Ruapehu lavas are medium-K acid and basic andesites (mean of 144 bulk rock analyses is 57.8 wt % SiO2), but rare basalt and minor decite are present. Nearly all lavas are porphyritic in plagioclase, augite and hypersthene [plus or minus] olivine, with titanomagnetite micro- phenocrysts, and contain abundant metamorphic and igneous rock inclusions. Petrography, mineral chemistry and bulk rock chemistry indicate fractional crystallization series from parental basalts (52-53 % SiO2, Q-normative, low-alumina) to medium-K basic- and acid andesites (58-59 % SiO2). Early fractionating minerals are olivine and clinopyroxene with minor chrome spinel and plagioclase, followed by plagioclase, orthopyroxene, clinopyroxene and minor titanomagnetite in later stages of differentiation. Thus, basalt differentiation to produce andesites involves 'POAM-type' (Gill, 1981) fractional crystallization. Three second-order differentiation processes operate concurrently with frational crystallization: (1) Crystal accumulation involves addition of co-genetic plutonic rock fragments and crystals derived from them. These inclusions are common and few rocks represent liquid compositions. (2) Magma mixing involves mingling of magmas in repeatedly-occupied conduits. End members are as diverse as basalt and decite, yielding petrogaphically and chemically distinctive high-Mg andesites of the upper cone complex and parasitic centres. (3) Selective crustal assimilation is suggested by partially fused metamorphic inclusions, positive correlation of 87Sr/86Sr with SiO2, and failure of simple 'POAM' fractionation to explain decites (63-65 % SiO2). Petrogenesis of Ruapehu andesites takes place under open-system condition, involving production of parental Q-normative basalts in the mantle wedge, concurrent fractional crystallization and crustal contamination, entrainment of co-genetic plutonic rocks, and mixing of magmas in common conduits.</p>


2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Jing-Yao Xu ◽  
Andrea Giuliani ◽  
Qiu-Li Li ◽  
Kai Lu ◽  
Joan Carles Melgarejo ◽  
...  

AbstractOxygen isotope ratios in mantle-derived magmas that differ from typical mantle values are generally attributed to crustal contamination, deeply subducted crustal material in the mantle source or primordial heterogeneities. Here we provide an alternative view for the origin of light oxygen-isotope signatures in mantle-derived magmas using kimberlites, carbonate-rich magmas that assimilate mantle debris during ascent. Olivine grains in kimberlites are commonly zoned between a mantle-derived core and a magmatic rim, thus constraining the compositions of both mantle wall-rocks and melt phase. Secondary ion mass spectrometry (SIMS) analyses of olivine in worldwide kimberlites show a remarkable correlation between mean oxygen-isotope compositions of cores and rims from mantle-like 18O/16O to lower ‘crustal’ values. This observation indicates that kimberlites entraining low-18O/16O olivine xenocrysts are modified by assimilation of low-18O/16O sub-continental lithospheric mantle material. Interaction with geochemically-enriched domains of the sub-continental lithospheric mantle can therefore be an important source of apparently ‘crustal’ signatures in mantle-derived magmas.


2021 ◽  
Vol 59 (6) ◽  
pp. 1543-1570
Author(s):  
Yiguan Lu ◽  
C. Michael Lesher ◽  
Liqiang Yang ◽  
Matthew I. Leybourne ◽  
Wenyan He

ABSTRACT The ∼259 Ma Baimazhai Ni-Cu-(platinum-group element) deposit is located in the Ailaoshan-Red River fault zone on the southwest margin of the Yangtze Plate in the Jinping area of southeastern Yunnan Province. The intrusion is lenticular (∼530 m long × 190 m wide × 24–64 m thick) and concentrically zoned (margin to core) from gabbro through pyroxenite to peridotite. It contains ∼50 kt of Ni-Cu-(platinum-group element) mineralization, concentrically zoned (margin to core) from disseminated through net-textured to massive sulfides with an average grade of 1.03 wt.% Ni, 0.81 wt.% Cu, and 0.02∼0.69 ppm Pd+Pt. The sulfide assemblage comprises pyrrhotite, chalcopyrite, and pentlandite, with lesser magnetite, violarite, galena, and cobaltite. The mineralization is enriched in Ni-Cu-Co relative to the platinum-group elements and the host rocks are enriched in highly incompatible lithophile elements relative to moderately incompatible lithophile elements with high Th/Yb and intermediate Nb/Yb ratios. These host rocks, and those at most other Ni-Cu-platinum-group element deposits in the Emeishan Large Igneous Province, have high γOs and intermediate εNd values, indicating that they crystallized from a magma derived from a subduction-modified pyroxenite mantle source and modified by crustal contamination. The initial concentrations of metals in the primary magma are estimated to have been on the order of 200 ppm Ni and 100 ppm Cu, but only 0.4 ppb Pd, 0.2 ppb Pt, 0.005 ppb Rh, 0.02 ppb Ru, and 0.01 ppb Ir. The δ34S values of ores and separated sulfides range from 5.8‰ to 8.6‰, between the ∼10‰ value of sulfides in the metasedimentary country rocks and the 0 ± 0.5‰ value expected for magmas derived from MORB-type mantle, or the –2.5 ± 0.3‰ value expected for subduction-modified mantle, consistent with equilibration at magma:sulfide mass ratios (R factors) of 100–1000. Variations in Ir100 and Pd100 (metals in 100% sulfide) are consistent with 40–60% fractional crystallization of monosulfide solid solution to form Ni-Co-intermediate platinum-group element (Ru, Os, Ir)-rich massive ores and Cu-palladium/platinum-group elements (Pt, Pd, Rh)-Au-rich residual sulfide liquids. This process is also recorded by magnetite: Type I (early magmatic), type II (late magmatic), and type III (secondary) magnetites exhibit progressively lower Cr-Ti-V concentrations. The platinum-group element contents in base-metal minerals are low, and only pentlandite, violarite, and cobaltite contain detectable concentrations of Pd, Rh, and Ru. There is abundant textural evidence for metamorphic-hydrothermal alteration of sulfides in the Baimazhai intrusion, with secondary violarite, chalcopyrite, and pentlandite being enriched (Ag, Sb, Au, Pb) or depleted (Sn) in more mobile chalcophile elements. The different tectonic and petrogenetic settings of the Baimazhai and other deposits in China highlight the potential of Ni-Cu-platinum-group element deposits to occur in subduction or post-subduction settings and demonstrate that the key controls are magma flux and access to crustal S. Exploration potential remains for the Ailaoshan orogenic belt to host additional magmatic Ni-Cu deposits.


2021 ◽  
Vol 34 (04) ◽  
pp. 1200-1214
Author(s):  
Abdolreza Soleimani ◽  
Shahrooz Haghnazar ◽  
Mansour Vosoughi Abedini ◽  
Saeed Hakimi Asiaber

This study was performed on the outcrops of lamprophyric lavas found in the north of Jirandeh and east of Lushan in the mountain of Alborz (north of Iran). These lavas has been placed discordantly on the middle Eocene lime..Petrographic observation indicates olivine phenocrysts, green-core alkaline clinopyroxenes, nepheline, abundant biotite, and apatites with flakes. and in the matrix it also contains biotite, olivine, clinopyroxene and plagioclase.The presence of carbonates, plagioclase and xenocrystals with rounded margins asserts the contamination with continental crust Petrologically, these rocks classify as alkaline lamprophyres of comptonite variety.These rocks can be subsumed under alkaline sodic categories at K2O/Na2O<1 ratio. The rare elements patterns in the rocks, normalized with the primitive mantle, causing partial negative Nb anomalies and showing no blades at the surface. It, therefore, can be indicative of the evidence for an intraplate magmatism with the different degree in the crustal contamination. Geochemistry states the first cause of asthenospheric flow can be occurred at La/Nb<1 and La/Ta 13 ratios, and the presence of garnet can be assumed at 1/8< (Tb/Yb) N ratio in the rocks origin area. In tectonic discrimination diagrams, these rocks fall in the range of intra-continental rift zones. Geochemical analyses indicate that these lamprophyres originate from partial (1%) melting of an OIB-like asthenospheric mantle source of lherzolite garnet nature.


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