Geodynamic evolution of the southern Abitibi and Pontiac terranes: evidence from geochemistry of granitoid magma series (2700–2630 Ma)

1992 ◽  
Vol 29 (10) ◽  
pp. 2266-2286 ◽  
Author(s):  
Rui Feng ◽  
Rob Kerrich
Keyword(s):  
Rare Earth ◽  
Partial Melting ◽  
Alkali Feldspar ◽  
Quartz Syenite ◽  
Southern Volcanic Zone ◽  
Suprasubduction Zone ◽  
Depleted Mantle ◽  
Quartz Monzonite ◽  
Ree Patterns ◽  
Low K

Four distinct granitic series developed during the evolution of the Archean Abitibi Southern Volcanic Zone (SVZ): (1) A minor synvolcanic tonalite–trondhjemite–granodiorite (TTG) series was emplaced at ~2700 Ma, which has extremely low K2O (<1.5 wt.%), Rb (<50 ppm), and Sr (<200 ppm), higher Nb, Y, and Sc, flat rare-earth-element (REE) patterns, and negative Eu anomalies. (2) A voluminous tonalite – granodiorite – granite – quartz monzonite (TGGM) series developed syntectonically at ~2695–2685 Ma, and displays low K2O (1–3 wt.%), Rb (10–50 ppm), Ba (<1000 ppm), and U, enhanced Sr, depletion of Ta, Nb, and Ti, and strongly fractionated REE patterns (La/Ybn = 49–21). (3) A late-tectonic quartz syenite – quartz monzonite – granite (SMG) series was emplaced from ~2685 to 2675 Ma, and is grossly similar to the TGGM but has lower CaO/(K2O + Na2O) and greater concentrations of Rb, Ba, Th, and U. (4) A late-tectonic to posttectonic alkali feldspar syenite – alkali feldspar quartz syenite (SS) series was emplaced from 2680 to 2670 Ma, and occurs along regional strike-slip structures. The primitive rocks (SiO2 ≤ 65 wt.%) exhibit coenrichment of large-ion lithophile elements (LILE) and mafic elements (Cr, Co, and Ni) and strongly fractionated REE patterns, whereas evolved phases (SiO2 ≥ 65 wt.%) display lower contents of compatible and incompatible elements stemming from differentiation.In the adjacent Pontiac Subprovince and the Lacorne block within the SVZ, two granitic series predominate: (1) A syntectonic to late-tectonic monzodiorite–monzonite–granodiorite–syenite (MMGS) series (2690–2670 Ma) is comparable to the Abitibi SMG and SS series in most major-element, LILE, and REE contents but is distinguished by high MgO contents, extremely high Ba/Th ratios, and coenrichment of Cr, Co, and Ni with light rare-earth elements (LREE), Li, and Cs. (2) A garnet–muscovite–granite (GMG) series (2644 ± 13 Ma) displays K2O/Na2O ≥ 1, restricted SiO2 range (69–75 wt.%), pronounced enrichments of Rb, Li, Cs, Ta, Nb, Th, and U, and moderately fractionated REE's (La/Ybn = 16–0.9), with prominent negative Eu anomalies.The synvolcanic TTG series is interpreted to have formed by differentiation of low-K mafic magmas of the Blake River Group type in suprasubduction-zone environments. Geochemical compositions of the TGGM, SGM, SS, and MMGS series resemble those of Phanerozoic granitoids in island-arc settings and reflect a transition from partial melting of the subducted or subcreted slab to melting of the metasomatized depleted mantle wedge assisted by LILE- and LREE-enriched fluids released from the slab. The GMG, which formed by partial melting of the Pontiac metasediments when the Pontiac Sub-province collided with and underthrust the Abitibi SVZ at ~2670 Ma, is similar to Himalayan collisional leucogranites.

scholarly journals Vendian and Permian-Triassic plagiogranite magmatism of the Ust’-Belaya Mountains, West-Koryak fold system, Northeastern of Russia

2019 ◽  
pp. 87-114
Author(s):  
A. V. Moiseev ◽  
M. V. Luchitskaya ◽  
I. V. Gul’pa ◽  
V. B. Khubanov ◽  
B. V. Belyatsky
Keyword(s):  
Rare Earth ◽  
Partial Melting ◽  
Rare Earth Element ◽  
Fractional Crystallization ◽  
Earth Element ◽  
Island Arc ◽  
Active Margin ◽  
Fold System ◽  
Icp Ms ◽  
Low K

Vendian and Permian-Triassic plagiogranite magmatism is distinguished for Ust’-Belsky and Algansky terranes of West-Koryak fold system. U–Pb zircon ages from Vendian and Permian-Triassic plagiogranites are 556 ± 3 Ma (SIMS), 538 ± 7 Ma (LA–ICP–MS) and 235 ± 2 Ma (SIMS) consequently. It is revealed, that Vendian and Permian-Triassic plagiogranites are mainly low-K and low-Al. Sr–Nd isotopy and rare-earth element patterns allow supposing their formation by partial melting of primarily mantle substrate or by fractional crystallization of basic magma. Vendian plagiogranites formed within active margin in ensimatic island arc simultaneously with deposition of lower part of volcanic-sedimentary complex of Otrozhninskaya slice. We suggest the Permian-Triassic plagiogranites were being formed within the limits of Ust’-Belsky segment of Koni-Taigonos arc during partial melting of melanocratic ophiolite material build up as fragments in accretionary structure of that arc or by fractional crystallization of basic magmas melted from the similar substrate.

scholarly journals Experimental melting of hydrous low-K tholeiite: evidence on. the origin of Archaean era tons

1991 ◽  
Vol 39 ◽  
pp. 213-228
Author(s):  
K. Tobias Winther ◽  
R.C. Newton
Keyword(s):  
Partial Melting ◽  
Zone Melting ◽  
Calc Alkaline ◽  
Shallow Subduction ◽  
Alkaline Magmas ◽  
The Stability ◽  
Ree Patterns ◽  
Low K ◽  
Experimental Melting ◽  
Grey Gneiss

Experimental melting studies were performed on a natural high-Al basalt and a synthetic average Archaean tholeiite (AAT) composition (0.3 wt.% K20) with variable amounts of H20. Microprobe analyses of quenched melts (glass) from runs at 5-30 kbar and 750°-l100°C showed that typical Archaean tonalitic and trondhjemitic "grey gneiss" compositions were produced from the average Archaean tholei-ite over the entire experimental range, with 15% to less than 1 % H20. The high-Al basalt produced liquids too high in Al203 (18--23%) for realistic grey gneiss compositions. The persistent generation in our experiments of low-K calc-alkaline magmas directly by vapor-undersaturated partial melting of low-K Archaean tholeiite strongly suggests this mechanism for the origin of early continents. Temperatures of 850°-l000°C and pressures around 15 kbar are appropriate melting conditions. Ton­alitic magmas are favored by higher temperatures, lower pressures, and higher H20 contents in the source. Trondhjemitic magmas are favored by lower temperatures, higher pressures, and lower H20 contents. Heavy REE depletion of magmas would be possible for partial melting above 15 kbar because of the stability at higher pressures of residual garnet. Unfractionated REE patterns of magmas could result from melting at lower pressures, where garnet does not coexist with liquid. The low-K trends of melts are maintained by very refractory amphibole (up to 0.7 wt. % KzO) which coexists with liquid for bulk H20 contents of 2 wt. % or more. Shallow subduction-zone melting of amphibolite with magma extraction, and partial melting of amphibolite under deep-crustal metamorphic conditions are models for early crustal evolution which appear to satisfy the experimental constraints.

scholarly journals From Decompression Melting to Mantle-Wedge Refertilization and Metamorphism: Insights from Peridotites of the Alag Khadny Accretionary Complex (SW Mongolia)

Minerals ◽  
2020 ◽  
Vol 10 (5) ◽  
pp. 396 ◽  
Author(s):  
Marina Gornova ◽  
Anas Karimov ◽  
Sergei Skuzovatov ◽  
Vasiliy Belyaev
Keyword(s):  
Partial Melting ◽  
Mantle Wedge ◽  
Spreading Center ◽  
Accretionary Complex ◽  
Depleted Mantle ◽  
Basaltic Melt ◽  
Arc Setting ◽  
Late Neoproterozoic ◽  
Back Arc ◽  
Ree Patterns

This study reports on mineral and bulk rock compositions of metaperidotites from the Alag Khadny accretionary complex in SW Mongolia, to reveal their nature and relationships with associated eclogites. The peridotites preserved original porphyroclastic textures and are composed of olivine, orthopyroxene relics, Cr-spinel, interstitial (not residual) clinopyroxene, and secondary chlorite, tremolite, olivine, Cr-magnetite, clinopyroxene, and antigorite. Cr-spinel has Cr# of 0.3–0.5, and primary olivine shows Mg# of 0.90–0.92. The pyroxenes are high-magnesian with low Al2O3 and Cr2O3. The bulk rocks have U-shaped normalized trace-element patterns with enrichment in LILE, L-MREE relative to HREE, and weak Pb–Sr peaks and Nb–Zr–Hf minima. Interstitial clinopyroxene exhibits V- and U-shaped normalized REE patterns with (La/Yb)N > 1 (Yb = 1.2–3 of chondritic values) and enrichment in fluid-mobile elements and Zr. HREE abundances of clinopyroxene can be simulated by 23–26% partial melting of depleted mantle starting at garnet-facies (6–8%) depths, followed by hydrous or anhydrous melting at spinel-facies depths L-MREE characteristics of clinopyroxenes can be simulated by further interaction of harzburgites with an island-arc basaltic melt in a supra-subduction environment. The association of hydrous secondary minerals in the Alag Khadny peridotites suggests their retrograde metamorphism at 1.6–2.0 GPa and 640–720 °C, similar to P–T conditions reported earlier for the spatially associated eclogites. This supports metamorphism of the Alag Khadny peridotites in a mantle wedge, followed by joint exhumation of peridotites and eclogites. Given the findings above and implying the regional geological background, we advocate for a sequential Neoproterozoic evolution the Alag Khadny harzburgites from (1) their formation by decompression partial melting in an Early Neoproterozoic or older spreading center of a mid-ocean or back-arc setting, and (2) refertilization by supra-subduction melts, followed by (3) Late Neoproterozoic–Early Cambrian hydrous-fluid metamorphism and juxtaposition with eclogites.

Rare-earth and other trace element data bearing on the origin of Archaean granitic rocks from Yellowknife, Northwest Territories

1979 ◽  
Vol 16 (4) ◽  
pp. 809-815 ◽  
Author(s):  
S. A. Drury
Keyword(s):  
Rare Earth ◽  
Trace Element ◽  
Partial Melting ◽  
Northwest Territories ◽  
Granitic Rocks ◽  
Trace Element Data ◽  
Sr Isotope ◽  
Isotope Data ◽  
Element Data ◽  
Ree Patterns

Trace element data, including rare-earth elements, for six granitic rocks and two metasediments from the Archaean granite–greenstone terrain of Yellowknife, Northwest Territories are presented. Three granodiorites from the synkinematic Western and South-east plutons show similar Rb, Sr, Th, Rb/Sr, and K/Rb to many high level granodiorites, but are enriched in Ba. Their rare-earth element (REE) patterns show enrichment of light REE relative to heavy REE, and lack Eu anomalies. These features, together with Sr isotope data, are compatible with their origin by partial melting of mantle depth garnet-bearing basaltic source rocks, and little if any detectable fractionation of the rising magma. Potassic granites from post-kinematic plutons, such as the Prosperous Lake granite, are enriched in Th, depleted in Sr and Eu, and have very high Rb/Sr ratios and low K/Rb ratios. Their REE patterns, except for Eu, are very similar to those of metasediments which they intrude. These features, together with Sr isotope data, suggest that high degrees of partial melting of metasediments, leaving a plagioclase-rich residue, are the most likely origin for the post-kinematic granites. The REE patterns of the Archaean metasediments, in particular their high CeN/YbN ratios, suggest that they contain a high proportion of material derived from earlier sialic crust, and that local metavolcanic rocks are not clearly reflected in the composition of the stratigraphically younger metasediments.

Geochemical constraints on the petrogenesis of the Proterozoic granitoid gneisses from the eastern segment of the Central Tianshan Tectonic Zone, northwestern China

2007 ◽  
Vol 144 (2) ◽  
pp. 305-317 ◽  
Author(s):  
QIUGEN LI ◽  
SHUWEN LIU ◽  
ZONGQI WANG ◽  
QUANREN YAN ◽  
ZHAOJIE GUO ◽  
...  
Keyword(s):  
Partial Melting ◽  
Minor Amount ◽  
Tectonic Zone ◽  
Depleted Mantle ◽  
Eastern Segment ◽  
Tectonic Zones ◽  
Granitoid Gneisses ◽  
A Minor ◽  
T Values ◽  
Ree Patterns

The Tianshan orogen is divided into the Northern, Central and Southern Tianshan tectonic zones by the northern and southern sutures on both sides of the Central Tianshan Tectonic Zone. The eastern segment of the Central Tianshan Tectonic Zone is characterized by the presence of numerous Precambrian metamorphic blocks and is unconformably overlain by Ordovician–Silurian and late Palaeozoic strata. The Precambrian Kumishi and Pargantag metamorphic blocks are the largest older blocks in the eastern segment of the Central Tianshan Tectonic Zone, consisting mainly of metamorphic granitoids and sedimentary rocks in greenschist to amphibolite facies. There are two major lithological assemblages of the metamorphic granitoids: (1) quartz dioritic gneisses, and (2) granodioritic–monzogranitic gneisses with a minor amount of tonalitic and syenogranitic gneisses in both the Kumishi and Pargantag blocks. The quartz dioritic gneisses are characterized by low Sr/Ce (<5.3) and Sr/Y (<28), relatively high Mg no. (51.0–57.0), K2O (2.65–4.04 wt %) contents and εNd(t) values (−2.37–5.84), and negative Nb and Zr–Hf anomalies, as well as relatively flat chondrite-normalized REE patterns with slightly negative Eu anomalies, suggesting that the quartz dioritic gneisses were derived from partial melting of a depleted mantle source enriched by fluids and sedimentary melts from the subducted slab. However, most of granitic gneiss samples display high K2O contents, low Al2O3/(FeO* + MgO + TiO2) values, and relatively flat chondrite-normalized REE patterns with intensively negative Eu anomalies. Integrated low εNd(t) values and older TDM model ages suggest that crustal materials played a significant role in the petrogenesis of these granitoid gneisses and that they were mainly derived from the partial melting of calc-alkaline mafic to intermediate rocks in the crust. Also, variations in geochemical features between the Kumishi–Gangou and Pargantag regions, such as Zr and Hf, may reflect geographic variability in the development of coeval granitic magmas. Tectonic discrimination for granitoid, using trace elements, together with Nd isotopic data, demonstrates that these granitoid gneisses in the eastern segment of the Central Tianshan Tectonic Zone formed in a continental margin arc during late Mesoproterozoic times.

Crustal assimilation in the Burnt Lake metavolcanics, Grenville Province, southeastern Ontario, and its tectonic significance

1997 ◽  
Vol 34 (9) ◽  
pp. 1272-1285 ◽  
Author(s):  
T. E. Smith ◽  
P. E. Holm ◽  
N. M. Dennison ◽  
M. J. Harris
Keyword(s):  
Rare Earth ◽  
Trace Element ◽  
Partial Melting ◽  
Continental Crust ◽  
Fractional Crystallization ◽  
Volcanic Rocks ◽  
Lake Area ◽  
Depleted Mantle ◽  
Mafic Magmas ◽  
Back Arc

Three intimately interbedded suites of volcanic rocks are identified geochemically in the Burnt Lake area of the Belmont Domain in the Central Metasedimentary Belt, and their petrogenesis is evaluated. The Burnt Lake back-arc tholeiitic suite comprises basalts similar in trace element signature to tholeiitic basalts emplaced in back-arc basins formed in continental crust. The Burnt Lake continental tholeiitic suite comprises basalts and andésites similar in trace element composition to continental tholeiitic sequences. The Burnt Lake felsic pyroclastic suite comprises rhyolitic pyroclastics having major and trace element compositions that suggest that they were derived from crustal melts. Rare earth element models suggest that the Burnt Lake back-arc tholeiitic rocks were formed by fractional crystallization of mafic magmas derived by approximately 5% partial melting of an amphibole-bearing depleted mantle, enriched in light rare earth elements by a subduction component. The modelling also suggests that the Burnt Lake continental tholeiitic rocks were formed by contamination – fractional crystallization of mixtures of mafic magmas, derived by ~3% partial melting of the subduction-modified source, and rhyolitic crustal melts. These models are consistent with the suggestion that the Belmont Domain of the Central Metasedimentary Belt formed as a back-arc basin by attenuation of preexisting continental crust above a westerly dipping subduction zone.

scholarly journals The Transitional Gabbroic Rocks in Bayah Geological Complex, Western part of Java, Indonesia, Inferred from XRF, ICP-MS, and Microprobe Analysis

2021 ◽  
Vol 6 (4) ◽  
pp. 177-183
Author(s):  
Aton Patonah ◽  
Haryadi Permana ◽  
Ildrem Syafri
Keyword(s):  
Rare Earth ◽  
Rare Earth Elements ◽  
Partial Melting ◽  
Inductively Coupled Plasma ◽  
Tectonic Setting ◽  
Geochemical Composition ◽  
Inductively Coupled ◽  
Suprasubduction Zone ◽  
Icp Ms ◽  
Mantle Reservoir

Gabbro, is a fossil remnant of oceanic crust in western part of Java, found at Bayah Geological Complex (BGC) and Ciletuh Melange Complex (CMC), Indonesia. It has been studied by using petrographic, X-Ray Fluorescence (XRF), and inductively coupled plasma-mass spectrometry (ICP-MS) and mineralogical (microprobe) analyses. Mineral and geochemical composition of these rocks provide important clues to their origins since the rocks have been deformed and gone through auto metamorphism, beside they contain the economic mineral and or rare earth elements (REE). Gabbroic rocks in these two areas generally shows phaneritic to porphyritic texture, granular texture. These rocks in CMC are dominated by plagioclase (oligoclase to albite), hornblende, pyroxene, partly altered to tremolite, actinolite, chlorite, epidote, and sericite; meanwhile those of BGC dominantly consist of plagioclase, pyroxene, hornblende, some present of chlorite, actinolite, epidote and biotite as secondary minerals. In multi-element diagrams, gabbroic rocks in CMC show strong negative Sr and Zr, but positive Nb anomaly, while those of BGC show strong negative anomaly of Nb and Zr. In addition, based on rare earth elements (REE) diagrams, gabbroic rocks in CMC show depleted of light rare earth elements (LREE) with negative Eu anomaly, while gabbro’s in BGC show enrichment of LREE. These characteristics indicate that GBC’s and CMC’s gabbroic rocks came from different magma sources, one was formed by partial melting of depleted upper mantle reservoir while the other one was formed by partial melting of mantle wedge with active participation of subducted slab in an arc tectonic setting, suprasubduction zone which were formed at started Upper Cretaceous to Paleogene, and they had retrograde metamorphism to epidote amphibolite facies.

The Geochemistry and Ecotoxicity of Offshore New Zealand Phosphorites

2021 ◽  
Author(s):  
◽  
Grace Elizabeth Frontin-Rollet
Keyword(s):  
Rare Earth ◽  
New Zealand ◽  
Earth Element ◽  
Surficial Sediments ◽  
Site Specific ◽  
Sediment Disturbance ◽  
High Concentrations ◽  
Element Enrichment ◽  
Ree Patterns ◽  
Chatham Rise Phosphorite

<p>The New Zealand offshore seabed hosts diverse resources including phosphate rich rocks. Phosphate rock deposits on the Chatham Rise have been the focus of previous investigations into their composition and mining potential; however, the diversity of the geochemistry of phosphate deposits, including their wider distribution beyond the Chatham Rise, their trace metal budget, and potential for ecotoxicity, remain poorly characterised. This study addresses some of these gaps by presenting a geochemical investigation, including trace metals, for a range of phosphate nodules from across the Chatham Rise, Bollons Seamount and offshore southeastern South Island. Elutriate and reconnaissance bioaccumulation experiments provide insights into the potential for ecotoxic trace metal release and effects on biota should sediment disturbance through mining activities occur.  The bulk chemistry of Bollons Seamount phosphorite nodules have been characterised for the first time, and show significant enrichment in first row transition metals; Co, Ni, Cu, Zn, in addition to Sr, Y, Mo, U, MnO, CaO and P2O5, and depletion in TiO2, Al2O3, MgO, K2O, FeO, SiO2, Sc, Cr, Ga, Rb, Cs, Hf, and Th relative to average upper continental crust. The cores of these nodules are dominated by apatite, quartz and anorthoclase phases, which are cross cut by Mn rich dendrites. The abundant presence of these minerals results in the significant differences in chemistry observed relative to Chatham Rise phosphorite nodules. The nodules also contain a secondary authigenic apatite phase, with a Mn crust rim. Significant rare earth element enrichment (REE) is most likely due to efficient scavenging by the Mn crust, resulting in seawater REE patterns characterised by negative Ce and Eu anomalies and heavy rare earth element enrichment.  The bulk geochemistry of the Chatham Rise and offshore South Island phosphorite nodules is characterised by enrichment in CaO, P2O5, Sr, U, Y, Mo and depletion in TiO2, Al2O3, MnO, MgO, FeO, K2O, Sc, Cr, Cu, Ga, Rb, Cs, Ba, Hf, Ta, Pb and Th relative to average upper continental crust. The low concentrations of Cd in Chatham Rise, offshore South Island, and Bollons Seamount phosphorites make them potentially suitable sources for direct application fertilizers.  The New Zealand marine phosphorite nodule deposits formed by repeated cycles of erosive bottom currents and phosphogenesis, resulting in the winnowing and concentration of the deposits. The iron pump model is proposed as a mechanism for the formation of apatite and associated mineral phases, giving the nodules their characteristic concentric zoning. The migration of the nodules through the oxic, suboxic, and anoxic zones of the sediment profile led to the formation of glaucony, apatite (suboxic zone), goethite (oxic zone), and pyrite with associated U enriched (anoxic zone) minerals. Rare earth elements (REE) in the Chatham Rise phosphorite nodules are associated with the glaucony rim minerals, and indicate that since the formation of the rims, very little diagenesis has occurred, preserving seawater REE patterns characterised by negative Ce and Eu anomalies and heavy REE enrichment. Site specific enrichments in trace elements Ba, V, Co, Ni, Cu, Zn, Y, Cd and Pb are attributed to either differences in incorporation of material into precursor carbonate e.g. volcanic materials, or higher fluxes of organic matter, delivering high concentrations of essential metals from biota, especially Cu and Zn.  Direct pore water measurements from surficial sediment of the Chatham Rise show high concentrations of dissolved Fe and Mn, along with Cu, indicating suboxic conditions. High Cu concentrations measured in sediment pore water suggest that Cu release requires monitoring should seafloor surficial sediments on the Chatham Rise be disturbed. However, the elutriate experiments were not able to resolve if Cu release by sediment disturbance would exceed Australian and New Zealand Environment Conservation Council (2000) environmental guideline trigger values.  The surrogate amphipod species Chaetocorophium c.f. lucasi shows promise as a biomonitor for disturbed marine sediments. Elements enriched in surficial sediments and phosphorite nodules, Hg, Pb, Fe, U and V, were not observed to bioaccumulate. Site specific differences in chemistry were observed, specifically in the different total relative bioaccumulation of Mo between amphipods exposed to sediments from two different sites. This suggests that future monitoring of chemical release during marine sediment disturbance requires the full geochemical characterisation of the substrate. Furthermore, fresh sediment and deep water should be used for future elutriate experiments, as storage of material by freeze-thawing and/or refrigeration causes mobilisation of some key trace metals such as U, V, Mo, Mn.</p>

scholarly journals Geochemical Characteristics and Tectonic Setting of Amphibolites in Ifewara Area, Ife-Ilesha Schist Belt, Southwestern Nigeria

2016 ◽  
Vol 6 (1) ◽  
pp. 43 ◽  
Author(s):  
Anthony Temidayo Bolarinwa ◽  
Adebimpe Atinuke Adepoju
Keyword(s):  
Rare Earth ◽  
Rare Earth Elements ◽  
Major Element ◽  
Tectonic Setting ◽  
Crustal Contamination ◽  
Geochemical Evolution ◽  
Schist Belt ◽  
Southwestern Nigeria ◽  
Eu Anomaly ◽  
Ree Patterns

Trace and Rare Earth Elements (REEs) data are used to constrain the geochemical evolution of the amphibolites from Ifewara in the Ife-Ilesha schist belt of southwestern Nigeria. The amphibolites can be grouped into banded and sheared amphibolites. Major element data show SiO2 (48.34%), Fe2O3 (11.03-17.88%), MgO (5.76-9.90%), CaO (7.76-18.6%) and TiO2 (0.44-1.77%) contents which are similar to amphibolites in other schist belts in Nigeria. The Al2O3 (2.85-15.55%) content is varied, with the higher values suggesting alkali basalt protolith. Trace and rare earth elements composition reveal Sr (160-1077ppm), Rb (0.5-22.9ppm), Ni (4.7-10.2ppm), Co (12.2-50.9 ppm) and Cr (2-7ppm). Chondrite-normalized REE patterns show that the banded amphibolites have HREE depletion and both negative and positive Eu anomalies while the sheared variety showed slight LREE enrichment with no apparent Eu anomaly. The study amphibolites plot in the Mid Oceanic Ridge Basalts (MORB) and within plate basalt fields on the Zr/Y vs Zr discriminatory diagrams. They are further classified as volcanic arc basalt and E-type MORB on the Th- Hf/3- Ta and the Zr-Nb-Y diagrams. The amphibolites precursor is considered a tholeiitic suite that suffered crustal contamination, during emplacement in a rifted crust.

Geology and geochemistry of Huronian rhyolites and low-Ti continental tholeiites from the Thessalon region, central Ontario

1987 ◽  
Vol 24 (7) ◽  
pp. 1360-1385 ◽  
Author(s):  
Wayne T. Jolly
Keyword(s):  
Rare Earth ◽  
Rare Earth Element ◽  
Earth Element ◽  
Magma Mixing ◽  
Mantle Metasomatism ◽  
Granitic Rocks ◽  
Heavy Rare Earth Element ◽  
Tonalitic Gneiss ◽  
Oceanic Basalts ◽  
Low K

Bimodal volcanism associated with early phases of Huronian rifting in central Ontario, dated about 2450 Ma, produced low-Ti tholeiitic basalts and two varieties of crustally derived calc-alkaline rhyolite. Early tholeiites are characteristically highly evolved, have Mg* values from 30 to 50, and display pronounced enrichment in large-ion lithophile elements (LILE) and light rare-earth element (LREE) in comparison with modern oceanic basalts, fractionated heavy rare-earth element (HREE) patterns, and low Ti, Zr, P, Nb, Ba, and K abundances. Ti/Zr ratios rise progressively in early basalts and associated basaltic andesite fractionates from about 35 in early flows to 55 in central units. Late basalts also carry enriched LILE and LREE, but, in contrast to early types, have average Mg* values greater than 50 and lower rare-earth element (REE) abundances with flat HREE patterns. They also display negative Ba, Nb, and P anomalies on chondrite-normalized distribution diagrams, but lack low K, Zr, and Ti contents. Their Ti/Zr ratios of about 80 approach chondritic levels. Melting models suggest the differences are explained by lower degrees of fusion (as low as 10%) in a hydrated, LILE- and LREE-enriched peridotite during generation of the early basalts, leaving a residue containing appreciable garnet, amphibole, Ti oxides, zircon, and apatite.Erupted simultaneously with the basalts were two distinctive rhyolite types: (1) a low-LILE, high-LREE group (25% of analysed specimens), derived by −20% melting of granulitic siliceous tonalitic gneiss, presumably at deep crustal levels, and (2) a high-LILE, low-LREE group (75%), derived, probably at shallower levels, by ≤ 30% melting in granitic rocks with pegmatitic or leucogranitic compositions. Mutual magma mixing of basalts and rhyolites during early stages of volcanism produced abundant hybrid andesites, but the frequency of contamination is much lower in later units.Hypothetical subcontinental source compositions, calculated from the Raleigh equation, suggest that the Huronian mantle had already undergone a complex history. Low Ba, Nb, P, Ti, and depleted HREE abundances compared with abundances for modern oceanic basalts suggest that a basaltic melt had already been withdrawn from this source during Archean time. Subsequently, an episode of hydrous metasomatism enriched the source in LILE and LREE. The latter event resulted from (1) subcontinental mantle metasomatism by previous Archean subduction, (2) mantle metasomatism during the terminal Archean Kenoran Orogeny, or (3) a wave of hydrous metasomatism accompanying Huronian mantle convection immediately preceding volcanism.

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