Crustal Contamination Processes Research Articles

Stratigraphy, geochronology, geochemistry and Nd isotopes of the Ouarzazate Group, Anti-Atlas, Morocco: Evidence of a Late Neoproterozoic LIP in the northwestern part of the West African Craton

In the eastern and central Anti-Atlas of Morocco, the Late Neoproterozoic Ouarzazate Group (OG) forms a ∼ 2 km-thick section of volcano-sedimentary rocks deposited during intermittent magmatic activity, spanning a time period between 590 and 540 Ma. In the eastern Anti-Atlas, the stratigraphy of the OG, termed IMS (Imiter Mine succession), includes four units that reflect a gradual change from fluvial to lacustrine depositional sedimentary environments. In the central Anti-Atlas, the deposition of the OG occurs within a caldera environment, consisting of five units referred to as BMS (Bou Azzer Mine succession). UPb dating on zircon from the rhyolite lava flow at the top of the IMS constrains the latest lava flow of the IMS to 570.7 ± 2.1 Ma. In the Bou Azzer inlier, the stratigraphically correlated basal andesite and top dacite lava flows were emplaced at 590.6 ± 4.2 Ma and 555.9 ± 20.4 Ma, respectively, suggesting that the OG was constructed through successive pulses within a long-lived magmatic episode. The magmatic rocks display geochemical signatures typical of continental arc magmas, including high-K calc-alkaline to shoshonite composition, enrichment in LILE, and negative Nb, Sr, Ti and P anomalies. Nd isotopes indicate that magmas supplying the plumbing systems of the IMS and BMS constitute transitional products resulting from a combination of orogenic and intraplate volcanism, involving deep crustal recycling. The compositions of these magmas evolved through processes of crustal contamination, bulk assimilation, and minor fractional crystallization of the parent melt. High εNd values of the BMS rocks argue for a juvenile origin, whereas lower εNdt recorded in the IMS likely reflect old crustal protoliths and an overall less contribution of juvenile material in their source. The magmatic rocks of the OG are interpreted as products of a large igneous province (LIP) developed by a post-collisional delamination model that initiated tens of millions of years after the subduction-collision process stage of the Pan-African orogeny in the northwestern part of the West African Craton (WAC).

OXYGEN ISOTOPE FRACTIONATION IN THE SILICATE-CARBONATE SYSTEM DURING THE ROCK FORMATION OF THE KOVDOR MASSIF (KOLA PENINSULA)

For the first time the δ18O values both in carbonate and non-carbonate (Ol, Phl, Mt) components of the rocks (calcite carbonatites, phoscorites, ijolites, turyaites) of the Kovdor massif were studied to clarify the role of contamination processes, retrograde exchange, and high-temperature fractionation in the igneous system. It has been established that the processes of crustal contamination are not recorded in the isotopic characteristics of the silicate part of the rocks, and the δ18О values of olivine, phlogopite, and magnetite are lowered compare to the compositions attributed to equilibrium with mantle peridotite. At the same time, the carbonate component of the rocks shows complementary enrichment in the 18O isotope with respect to the composition of derivatives of mantle melts. Estimates of the δ18О closure temperature of the olivine and the rates of rock cooling do not support the hypothesis of retrograde isotopic exchange. A hypothesis is proposed about the presence of a stage of redistribution of oxygen isotopes between the silicate and carbonate components of melts in the magmatic system, which took place before the formation of the Kovdor massif and its cooling. According to oxygen isotope data, the carbonate component could sharply predominate (1.5–4.5 times) over the silicate component during the equilibration.

Petrology, mineralogy, and geochemistry of the Carboniferous Katbasu Au-Cu deposit, western Tianshan, Northwest China: Implications for petrogenesis, ore genesis, and tectonic setting

The Katbasu Au-Cu deposit in the Chinese western Tianshan is hosted in the ca. 355 Ma granite intrusion. This paper reports whole-rock geochemical and zircon Hf-O isotopic data for the granites and the mafic enclaves as well as in situ S isotopic data for the sulfides from the Katbasu Au-Cu deposit. Our results provide constraints on the petrogenesis and ore genesis of the Katbasu Au-Cu deposit. Geochemical characteristics suggest that the Katbasu granites are metaluminous rocks, exhibiting mineral assemblages and geochemical characteristics of I-type granites. The granites have positive zircon εHf(t) values of 7.1–10.2 and zircon δ18O values of 6.2–7.2‰, suggesting a juvenile crustal origin. Mixing processes between felsic and mafic magmas were involved in their generation. The mafic enclaves are enriched in LILEs and depleted in HFSEs, and have zircon εHf(t) values of 7.5–15.8 and zircon δ18O values of 5.4–6.9‰, suggesting that they were derived from a subduction-modified mantle and underwent subsequent crustal contamination and/or magma mixing processes. In situ S isotopic analyses show that pyrite grains from the pre-ore stage and post-ore stage have positive δ34S values of 7.0–8.5‰ and 8.2–9.0‰, respectively. Pyrite and chalcopyrite grains from an early magmatic-hydrothermal Cu-Au mineralization stage show δ34S values of 8.8–11.1‰, and pyrite grains from the main orogenic Au mineralization stage have positive δ34S values of 5.9–8.4‰. The heavy S isotope characteristics of magmatic-hydrothermal Cu-Au mineralization indicate that the magma source might be metasomatized by subduction materials, whereas the heavy S isotope characteristics of the late orogenic Au mineralization suggest that the ore-forming materials mainly originated from the metamorphic devolatilization of the source rocks. The ca.352–349 Ma early magmatic-hydrothermal Cu-Au mineralization in the Katbasu deposit formed in a subduction tectonic setting, whereas the ca. 323–311 Ma late orogenic Au mineralization formed in an orogenic environment during the final stage of subduction.

Oxygen Isotopic Fractionation in a Silicate–Carbonate System during the Formation of Rocks of the Kovdor Pluton (Kola Peninsula)

The δ18О values of carbonates and silicates (Ol, Phl, Mt) of calcite carbonatites, phoscorites, ijolites, and turjaites of the Kovdor pluton have been studied for the first time to elucidate the role of processes of contamination, retrograde exchange, and high-temperature fractionation in a magmatic system. It is established that no processes of crustal contamination are registered in the isotopic characteristics of silicates and that the δ18О values of olivine, phlogopite, and magnetite are lower relative to the compositions typical of equilibrium with mantle peridotite. At the same time, the carbonates show a complementary enrichment in the 18О isotope relative to the composition of derivative mantle melts. The estimated closure temperature of the O isotopic composition of olivine and the cooling rates of rocks do not support a hypothesis on retrograde isotope exchange. A redistribution of O isotopes between silicate and carbonate constituents of melts in the magmatic system before the origination of the Kovdor pluton and its cooling is suggested. According to the O isotope data, the carbonate constituent at that moment could strongly be dominant (in 1.5–4.5 times) over the silicate one.

Large-Scale Cretaceous Adakitic Magmatism Induced by Water-Fluxed Melting of Continental Crust during the North China Craton Destruction

ABSTRACT The mechanism behind the destabilization of the North China Craton (NCC) remains a contentious topic among researchers. Large-scale Cretaceous adakitic magmatism in the NCC offers insights into the decratonization process. This study focuses on the Huashan and Laoniushan plutons located in the Lesser Qinling on the southern margin of the NCC and compiles published data for coeval adakitic rocks to investigate the role of water in adakitic rock petrogenesis during the peak destruction of NCC. Both the Huashan and Laoniushan plutons exhibit adakitic signatures, including high Sr (193–1080 ppm), low Yb (<14.8 ppm) and Y (<1.24 ppm) concentrations, as well as high Sr/Y (18–100) and La/Yb (24–58) ratios. The zircon Hf–O isotope compositions suggest that the primary source for the Huashan and Laoniushan plutons is the mafic lower crust of NCC. Nevertheless, there are significant differences in trace element characteristics between the two plutons. Specifically, the Huashan pluton displays higher Na2O/K2O ratios, lower levels of Rb, Rb/Sr, Nb, Ta content, and a weak Eu anomaly in comparison to the Laoniushan pluton. These variations in geochemical attributes cannot be accounted for by mechanisms like mantle-derived magma mixing, crustal contamination, or fractional crystallization processes. Instead, these disparities are attributed to distinct modes of crustal anatexis, involving both water-fluxed and dehydration melting. Subsequently, we conducted thermodynamic simulations of the melting process of mafic lower crust under different pressure (0.5–1.5 GPa) and water content conditions (1–3 wt.%). The simulation results suggest that the Huashan pluton is most likely formed through water-fluxed melting in a scenario with normal crustal thickness (1 GPa). On the other hand, the Laoniushan pluton might have originated from dehydration melting under normal crustal thickness and pressure conditions. Notably, high pressure (>1.5 GPa) is not necessary for the formation of intracontinental adakitic rocks. The release of water from metasomatized lithospheric mantle and subsequent hydration of the lower continental crust triggers extensive adakitic magmatism in the NCC. These findings emphasize the significance of deep water cycling in understanding large-scale magmatic events and illuminate the decratonization mechanism.

Iron Isotope Systematics of the Skaergaard Intrusion and Implications for its Liquid Line of Descent

Abstract The Skaergaard intrusion is one of the most thoroughly studied layered mafic intrusions on Earth and an exceptional example of (near) closed-system magmatic differentiation. We report new Fe isotope data for whole rocks, and magnetite and ilmenite separates through the layered series (LS) and upper border series (UBS) of the intrusion. δ56Fe values for gabbroic rocks range from 0.033‰ to 0.151‰ with an abrupt step increase at the base of Lower Zone c (LZc) within LS with the appearance of cumulus magnetite and subsequent decline accompanying FeTi oxide fractionation. The lowest δ56Fe values are found near the Upper Zone b (UZb) to c (UZc) boundary followed by a sharp rise across UZc approaching the Sandwich Horizon. Magnetite–ilmenite separates straddle bulk rock compositions with fractionation factors (Δ56Femt-ilm) of 0.081‰ to 0.239‰, consistent with subsolidus equilibration. Granophyric rocks occurring as pods, sheets and wispy layers from the upper zone and UBS equivalents and having unradiogenic Sr similar to gabbroic rocks of Skaergaard, are isotopically heavier than their host ferrodiorites (Δ56Fegranophyre-ferrodiorite ≥ 0.1‰) reaching a maximum δ56Fe of 0.217‰ in UBS. A fused xenolith from UBS has δ56Fe = 0.372‰. This range in δ56Fe spans much of that reported for terrestrial igneous rocks, and like the global dataset, shows a pronounced increase in δ56Fe with inferred silica content of modeled Skaergaard liquids. Forward modeling of closed system fractional solidification was undertaken to account for Fe isotope systematics, first by testing published liquid lines of descent (LLD), and then by exploring improvements and considering the impacts of liquid immiscibility, crustal contamination, fluid exsolution and diffusional processes. Our modeling relies on published Fe+2 and Fe+3 force constants for magmatic minerals and silicate glasses, and the most reliable estimates of the average bulk composition and mass proportions of the well-defined subzones of the intrusion. We show that the increase in δ56Fe across the LZb–LZc boundary is readily explained by the increased incorporation of Fe+3 into the crystallizing solid including magnetite. We further demonstrate that the classic Fenner LLD, involving strong Fe enrichment at nearly constant silica, does not lead to a rise in δ56Fe toward the end stages of evolution, while a Bowen-like LLD, with little Fe enrichment and strong Si enrichment, also underestimates enrichment in heavy Fe isotopes in the ferrodiorites of UZc. A LLD following an intermediate path involving modest Fe and Si enrichment, followed by Fe depletion best explains the observations. We predict ~3.5% (by mass) residual liquid after crystallization of UZc having a composition similar to felsic segregations in pegmatitic bodies found in the intrusion. While liquid immiscibility may have been encountered within fractionating mush at the margins of the intrusion, the Fe isotope systematics do not support liquid phase separation of the bulk magma. Crustal contamination, fluid exsolution, hydrothermal alteration and thermal diffusion are also shown to have no resolvable effect on the Fe isotope composition of the gabbroic and granophyric rocks. We conclude that the Fe isotope systematics documented in the Skaergaard intrusion reflect the dominant role of fractionating Fe-rich minerals from gabbroic through ferrodioritic to rhyolitic liquids. The success of our model to account for the observed Fe isotope systematics for Skaergaard demonstrates the utility of Fe+2 and Fe+3 force constants determined at ambient conditions to model magmatic conditions and gives critical insights into plutonic processes fractionating Fe isotopes complementary to the volcanic record.

Petrology of continental, OIB-like, basaltic volcanism in Saudi Arabia: Constraints on Cenozoic anorogenic mafic magmatism in the Arabian Shield

Continental basaltic volcanism in the Arabian Shield of Saudi Arabia has distinctive mineralogical and geochemical features important for understanding the composition of its mantle source and melting conditions. The studied Cenozoic basaltic rocks (19.3–15 Ma) occur at Jabal Al Gharib and Jabal Al Ghuraybayn areas within the Al Qasr quadrangle and southwest of Hail City in Saudi Arabia. They show similar chondrite-normalized REE patterns, suggesting that they were derived from a common mantle source. Their OIB-like features, silica undersaturated nature, and incompatible trace element ratios all reveal an asthenospheric mantle source and argue against crustal contamination process, subduction contribution, and interaction with a lithospheric mantle. Moreover, the elevated compatible trace element concentrations of Ni, Cr, and Co and the low and relatively narrow range of FeOtotal/MgO ratios argue for their nearly primitive nature and indicate that the role of crystal fractionation processes was minor during the evolution of the studied rocks. The estimated temperatures indicate that the sequence of crystallization likely began with olivine at 1,378–1,475°C, then clinopyroxene at 1,137–1,214°C, and soon after followed by plagioclase at 1,096–1,108°C. Plagioclase geo-hygrometers reveal 0.63–1.41 wt% water contents, suggesting crystallization under hydrous conditions. The basalts have geochemical features such as high (K2O + Na2O)/TiO2, Zr/Hf, and Nb/Ta ratios and negative K anomalies which suggest a carbonated peridotite mantle source. Their bulk-rock compositions are consistent with <5% CO2 in their peridotite melts. Highly incompatible elemental ratios supported by REE modeling indicate that they were generated by low degrees (4–10%) of partial melting of a garnet-bearing lherzolite mantle source. The garnet signature in the source region suggests a deeper origin exceeding ∼85 km. The basanite compositions fall within the range of alkaline OIB and intracontinental basalts formed in the rifted region. Moreover, they show geochemical characteristics typical of Cenozoic rift-related Oman basanites and other Arabian intraplate volcanic rocks. The basaltic volcanism in Saudi Arabia seems to be the result of melting asthenospheric mantle source in response to the lithospheric extension that is spatially and temporally linked to Red Sea rifting and triggered anorogenic mafic magmatism due to passive mantle upwelling beneath the Arabian Shield.

Tournaisian volcanism associated with transtensional basin development in western Newfoundland during the amalgamation of Pangea

During the Carboniferous, the assembly of the supercontinent Pangea periodically reactivated strike-slip faults and shear zones that transect the northern Appalachian Orogen. These strike-slip deformation zones represent crustal-scale zones of structural weakness that have long-lived and complex kinematic histories. Their periodic reactivation controlled the evolution of the transtensional sedimentary basins of the region. The continuous interplay between the lithospheric plates, their subcontinental lithospheric mantle, and surficial processes (i.e. oceanic/riverine, biological, atmospheric, gravitational, erosional) controlled the development of the Deer Lake Basin in western Newfoundland. The Saltwater Cove Formation of the Deer Lake Basin is a late Tournaisian lacustrian-deltaic succession interrupted by minor eruption of basalt flows and pyroclastic deposits. The basalts have E-MORB (enriched-mid ocean ridge basalt) to OIB (ocean island basalt) chemical affinities with LREE enriched profiles, lack negative Nb and Ti anomalies and εNd(t=350 Ma) values between +2.8 to +6.4. Pyroclastic deposits include mafic lapilli tuff and intermediate tuff, and have compositions similar to CAB (continental arc basalt), having steep LREE relative to HREE patterns, negative Nb and Ti anomalies, and εNd(t=350 Ma) ranging from −6.1 to −1.0. Geochemical variation within the mafic rocks is explained by an E-MORB source variably influenced by crustal contamination processes. Lithofacies analysis coupled with geochemical and isotopic data support their eruption in a transtensional, intra-orogenic basin setting that formed during the assembly of Pangea. Their moderately primitive chemistry supports the interpretation that faulting, associated with transtensional basin development, facilitated asthenosphere upwelling via edge-driven convection.

Partial Melting of Lithospheric Mantle and Formation of the Early Cretaceous Alkaline Rocks in the Guandimiao REE Deposit, Luxi Terrane, Eastern China

The Luxi Terrane (eastern China) exposes widespread Early Cretaceous alkaline rocks, whereas their petrogenesis remains controversial, including fractional crystallization, partial melting and crustal contamination regime. Here, we present petrology, geochemistry, sphene U-Pb geochronology and trace element data from the syenogranite, quartz syenite and quartz monzonite of the Guandimiao alkaline complex rocks to investigate their petrogenesis. Geochemical data suggest that these alkaline rocks show alkalic and peralkaline characters, and high Ga/Al ratios, SiO2, light rare-earth element (LREE), Zr and Nb, and low MgO, CaO, Eu contents, corresponding to A-type granites. Sphene trace elements in syenogranite and quartz monzonite show obvious fractionation between LREE and heavy rare-earth element (HREE) and high Th/U ratios, indicating a magmatic origin. They yield U-Pb lower intercept ages of 128 ± 2.3 Ma and 127 ± 1.3 Ma, representing the crystallization ages of these alkaline rocks. The negative correlations between CaO, Fe2O3 (Total), MgO, P2O5, TiO2, MnO and the pronounced depletion in Nb, Ta and Ti suggest that the alkaline rocks were formed by fractional crystallization. Additionally, the positive correlation between La/Hf and La, Th and Th/V, Ce/Yb and K2O, and Tb/Yb and Yb suggest that the alkaline melts are generated by partial melting. Such high Rb/Nb, (Th/Nb)N and Nb/Th ratios indicate crustal contamination during the magma emplacement. We, therefore, propose the magma source of the alkaline rocks in the Guandimiao complex originated by partial melting of lithospheric mantle, which experienced fractional crystallization and crustal contamination processes during its emplacement. Such complex alkaline rocks were probably formed in an extensional back-arc setting induced by the retreat of the subducting Izanagi plate.

On the occurrence of Bérem dolerites dyke swarms (north east adamawa plateau, Cameroon)

Petrography and geochemical outline studied carried out on Bérem dolerites have shown that they crosscut the local granitoıds of the basement toward N100-120, EW and N160E directions. Individual dyke may have 5 m to 50 m wide and extend along strike on 200 m to 3 km. Microscopic observations have distinguished the lavas of doleritic textures of ophitic to sub ophitic types. ICP-AES and ICP-MS geochemical analyses of representative samples have distinguished the lavas of basaltic trachyandesite of normative quartz-hypersthene compositions. All lavas belong to the same lavas series of continental tholeiites affinity which have undergone the fluids circulation and crustal contamination processes. Mantle of Bérem dolerites should be E-MORB mantle component which have experienced the relatively high partial melting rate. Bérem dolerites should be considered as imprints of late Pan African relaxation phase which cracks should have been filled by dolerite lavas.

The parental magma composition, crustal contamination process, and metallogenesis of the Shitoukengde Ni‐Cu sulfide deposit in the Eastern Kunlun Orogenic Belt, NW China

AbstractShitoukengde is an important magmatic Ni–Cu sulfide deposit in the Eastern Kunlun Orogenic Belt (EKOB). It comprises several mafic–ultramafic complexes and contains different kinds of mafic–ultramafic rocks. Lherzolite and olivine websterite are the most significant Ni–Cu‐hosted rocks. The No. I complex hosts six Ni–Cu ore bodies, and the depth of the intrusion has great exploration potential. Therefore, geochronology, geochemistry, and mineral chemistry of the Shitoukengde deposit were studied to constrain its mineralization time, parental magma composition, and crustal contamination process. Zircon U–Pb dating of olivine websterite shows the magmatic origin (Th/U = 0.40–1.05) and an age of 418.1 ± 8.7 Ma (MSWD = 0.01), which is coeval with the Xiarihamu, Akechukesai, and other Cu–Ni deposits in the EKOB. Geochemically, the mafic–ultramafic rocks are characterized by low SiO2, TiO2, and Na2O + K2O and high MgO (9.49–36.02%), with Mg# values of 80–87. They are relatively enriched in LREE and LILEs (e.g., K, Rb, and Th), with weakly positive Eu anomalies (δEu = 0.83–2.26), but depleted in HFSEs (e. g., Ta, Nb, Zr, and Ti). Based on the electron microprobe analyses, all of the olivines are chrysolite (Fo = 81–86), and the pyroxenes are dominated by clinoenstatite (En = 80–84) and augite (En = 49–55) in the mafic–ultramafic rocks. Therefore, the composition of parental magma is estimated to be picritic basaltic magma with SiO2 and MgO concentrations of 54.47 and 13.95%, respectively. The zircon εHf(t) values of olivine websterite vary from −0.8 to 4.6, with a TDM1 of 0.84–1.06 Ga, indicating that the parental magma was derived from relatively high degree partial melting (about 13.4%) of a depleted mantle source and experienced significant crustal contamination (about 12–16%). We propose that crustal assimilation, rather than fractional crystallization, played a key role in triggering the sulfide saturation of the Shitoukengde deposit, and the metallogenesis of “deep liquation–pulsing injection” is the key mechanism underlying its formation. The parental magma, before intruding, underwent liquation and partial crystallization at depth, partitioning into barren, ore‐bearing, and ore‐rich magma and ore pulp, and was then injected multiple times, resulting in the formation of the Shitoukengde Ni–Cu deposit.

Zircon petrochronology in large igneous provinces reveals upper crustal contamination processes: new U\u2013Pb ages, Hf and O isotopes, and trace elements from the Central Atlantic magmatic province (CAMP)

Zircon occasionally crystallizes in evolved melt pockets in mafic large igneous province (LIP) magmas, and in these cases, it is used to provide high-precision age constraints on LIP events. The precision and accuracy of high-precision ages from LIPs are crucially important, because they may be implicated in mass extinctions. However, why zircon crystallizes in these magmas is not clearly understood, since their mafic compositions should limit zircon saturation. Here, we investigate the occurrence of zircon (and baddeleyite) in intrusive and extrusive mafic rocks from Central Atlantic Magmatic Province (CAMP) using petrography, trace-element analysis, Ti temperatures, Hf and oxygen isotopes, and high-precision U–Pb geochronology, along with petrological and thermal modeling. We provide new ages for CAMP sills that intruded into Paleozoic sediments in Brazil, indicating that the high and low Ti magmatism in this area occurred synchronously over 264 ± 57 ka. We show that upper crustal assimilation, especially of shales, during the emplacement of the CAMP likely led to zircon saturation. Assimilation of upper crustal sediments is also supported by high δ18O values and some rare negative εHf values in the zircon crystals. The only extrusive sample analyzed was the North Mountain basalt in Nova Scotia, Canada. This sample contains a large age variation in its zircon crystals (up to 4 Ma), and the older crystals have slightly more negative εHf values suggesting the presence of small (micron scale) xenocrystic cores associated with very late-stage sediment assimilation. However, the CAMP dataset as a whole suggests that the presence of xenocrystic cores is rare. Assuming no xenocrystic cores, and considering the zircon undersaturated nature of LIP mafic melts, the oldest zircon age clusters in a population should record the magma emplacement (or time when assimilation occurred), and the younger ages in a population are more likely to reflect Pb loss, especially given the high U concentrations of LIP zircon. Our identification of heterogeneous isotopic and elemental compositions in LIP zircon indicates that zircon in these magmas saturate in isolated minute melt pockets just before the system cools below its solidus.

Generation of andesite through partial melting of basaltic metasomatites in the mantle wedge: Insight from quantitative study of Andean andesites

Continental crust in average exhibits a similar composition in both major and trace elements to andesites along active continental margins. For this reason, andesitic magmatism above oceanic subduction zones is considered to have played a key role in the generation of continental crust along convergent plate boundaries. With respect to the origin of andesites themselves, however, there is still a hot debate on how they have acquired their geochemcial compositions. The debate is mainly centralized on the relative contributions of crustal contamination, magma differentiation and source mixing, which reaches an impasse in the past decades. The essential reason for this kind of debates is that these three mechanisms only can account for some of the geochemical observations for andesites, leading to insufficient discrimination among them. Nevertheless, the geochemical features of andesites are primarily controled from early to late by the composition of their source rocks in addition to partial melting and magma differentiation processes. If source mixing and partial melting processes in the early stage of andesite magmatism can account for the first-order geochemical features of andesites, there is no need to invoke the late processes of magma differentiation and crustal contamination for andesite petrogenesis. This is illustrated by quantitative forward modeling of the geochemical data for Quaternary andesites from the Andean arc in South America based on an integrated interpretation of these data. The modeling has run with four steps from early to late: (1) dehydration of the subducting oceanic crust at forearc depths; (2) partial melting of the subducting terrigenous sediment and altered oceanic basalt at subarc depths to produce hydrous felsic melts; (3) the generation of basaltic metasomatites (e.g., Si-excess pyroxenite) in the mantle wedge through reaction of the mantle wedge peridotite with large amounts of the hydrous felsic melts; (4) the production of andesitic melts by partial melting of the basaltic metasomatites. The results not only testify the hypothesis that the trace element and radiogenic isotope compositions of andesites can be directly produced by the source mixing and mantle melting but also demonstrate that partial melting of the basaltic metasomatites can reproduce the lithochemical composition of andesites. The compositional variations of Andean andesites within a single volcanic zone and among different volcanic zones can be explained by incorporating different amounts of heterogeneous hydrous felsic melts into their mantle sources, followed by different degree of partial melting under different pressures and temperatures. Therefore, the source mixing and partial melting processes at subarc depths can account for the first-order geochemical features of Andean andesites. In this regard, it may be not necessary for andesite petrogenesis to invoke the significant contributions from the processes of magma differentiation and crustal contamination.

The Deccan Volcanic Province (DVP), India: A Review

The Deccan Volcanic Province (DVP), India: A Review

The Kandalaksha-Kolvitsa gabbro-anorthosite complex: Nd-Sr isotope-geochronological evidence of its affinity to the East-Scandinavian Large Igneous Province

The article provides new Sm-Nd and Nd-Sr isotope-geochronological data on rocks of the Paleoproterozoic Kandalaksha-Kolvitsa gabbro-anorthosite complex. The isotope Sm-Nd dating on metamorphic minerals (apatite, garnet, sulfide) and rock of the Kolvitsa massif yielded the age of 1887±37 Ma (high-temperature metasomatic transformation) and 1692±71 Ma (regional fluid processing). The model Sm-Nd age of metagabbro is 3.3 Ga with a negative value of εNd=-5.5, which indicates processes of crustal contamination or primary enriched mantle reservoir of initial magmas. According to geochronological and Nd-Sr isotope data, rocks of the Kandalaksha-Kolvitsa complex seem to have a common anomalous mantle source with Paleoproterozoic layered intrusions in the Baltic Shield.

Evolution of geodynamic processes in Neoarchean Kadiri greenstone belt, eastern Dharwar Craton, India: Implications on the migrating arc magmatism

Dharwar Craton of southern peninsular India is one of the significant Archean cratons in the world with well developed greenstone belt sequences representing plume-arc accretionary tectonics. The eastern Dharwar Craton (EDC) is endowed with excellent gold mineralization which is being mined and the greenstone belt lithounits among which majority of them represent the convergent margin processes. The Neoarchean Kadiri greenstone belt of EDC consists of an array of mafic, intermediate and felsic volcanic rocks among which the mafic rocks exhibit a wide range of silica (49.74–65.19 wt.%), high LILE, HFSE including the Nb contents (7−15 ppm) that are classified as Nb-enriched basaltic andesites (NEBA). Moderate composition of SiO2 (57−63 wt.%) along with higher MgO (3.96–7.58 wt.%), Cr (44−890 ppm) and Ni (22−491 ppm) contents distinguish some of the intermediate rocks as Mg-andesites. The petrographic characteristics along with higher silica (63−75 wt.%), Na2O (2.21–4.01 wt.%) and low K2O (0.7–3.35) classify the associated felsic volcanic rocks as sodic adakites. Abundance of K-feldspar phenocrysts and higher K2O content (1.19–7.0 wt.%) in some adakites identify them as potassic adakites. The geochemical characteristics of the calc-alkaline andesite-dacite-rhyolites (ADR) reflect on typical island arc environment involving the processes of fractional crystallization and crustal contamination in an active continental margin environment. The NEBA and Mg-andesites are the products of hybridized mantle wedge under the influence of slab fluids/melts whereas the Na-adakites are the slab melts erupted within the oceanic domain and the K-adakites were emplaced under the influence of continental crust during the migration of the island arc towards the continental margin. Beside the generation of various types of volcanic rocks, these convergent margin processes have played a significant role in the generation and mobilization of gold mineralized fluids, which are being mined from some greenstone belts of EDC.

The Paleoproterozoic Kandalaksha-Kolvitsa Gabbro-Anorthosite Complex (Fennoscandian Shield): New U–Pb, Sm–Nd, and Nd–Sr (ID-TIMS) Isotope Data on the Age of Formation, Metamorphism, and Geochemical Features of Zircon (LA-ICP-MS)

The paper provides new U–Pb, Sm–Nd, and Nd–Sr isotope-geochronological data on rocks of the Paleoproterozoic Kandalaksha-Kolvitsa gabbro-anorthosite complex. Rare earth element (REE) contents in zircons from basic rock varieties of the Kandalaksha-Kolvitsa area were analyzed in situ using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Plots of REE distribution were constructed, confirming the magmatic origin of zircon. Temperatures of zircon crystallization were estimated using a Ti-in-zircon geochronometer. The U–Pb method with a 205Pb artificial tracer was first applied to date single zircon grains (2448 ± 5 Ma) from metagabbro of the Kolvitsa massif. The U–Pb analysis of zircon from anorthosites of the Kandalaksha massif dated the early stage of the granulite metamorphism at 2230 ± 10 Ma. The Sm–Nd isotope age was estimated on metamorphic minerals (apatite, garnet, sulfides) and whole rock at 1985 ± 17 Ma (granulite metamorphism) for the Kolvitsa massif and at 1887 ± 37 Ma (high-temperature metasomatic transformations) and 1692 ± 71 Ma (regional fluid reworking) for the Kandalaksha massif. The Sm–Nd model age of metagabbro was 3.3 Ga with a negative value of εNd = 4.6, which corresponds with either processes of crustal contamination or primary enriched mantle reservoir of primary magmas.

The role of post-collisional mafic magmatism in the late stages of evolution of the northernmost Arabian-Nubian Shield (ANS): A case study from Isla Complex, Wadi Isla, South Sinai, Egypt

Isla complex is one of the late Neoproterozoic igneous complexes of the Arabian-Nubian Shield in southern Sinai. This research aims to examine the petrogenesis and tectonic setting of Isla complex, as well as inferring the P-T conditions of the parent magma and the depth of intrusion. A detailed field studies were carried out and representative samples for petrographic and geochemical studies were collected. Chemical analyses of major and trace elements for 18 samples and microprobe analyses for different mineral phases in olivine gabbro were performed. The Isla complex is distinguished into olivine gabbro, diorite and monzonite. The interrelationships between certain oxides and/or oxide ratios point to the important role of fractional crystallization process during the formation of the studied complex and the evolution of its mafic-intermediate rock units. The major fractionated phases were clinopyroxene, amphibole and plagioclase. The analyzed rocks show a gradual decrease in concentration of HFSE from olivine gabbro to monzonite via diorite suggesting overall co-magmatic origin. The low K content in plagioclase and low and constant Y/Nb, Nb/Zr, and K/Nb ratios in gabbroic rock relative diorites and monzonites suggest that during the formation of more evolved rock units, the crustal contamination process became more important. The rock varieties of the Isla complex are fresh and not deformed indicating post-collisional emplacement. The geochemical affinity of Isla complex is analogous to other within-plate post-collisional mafic magmatism of south Sinai. The subduction-related geochemical signature of the examined rocks can be interpreted by partial melting of an enriched lithospheric mantle source following an earlier subduction stage in the ANS. Based on hornblende-plagioclase thermometry, the gabbroic rocks of the Isla complex crystallized at a pressure of 5.5 kbar (~20 km depth), the crystallization temperature was estimated at 823 to 967 °C. The model of lithospheric delamination is supposed to form the studied melt since the delamination of the lithospheric mantle caused the asthenosphere to rise rapidly initiating partial melting and creating the studied gabbroic melt, followed by the fractionation of this melt to form different rock varieties of the Isla complex. The melts then penetrated the crust's base to create the Isla complex in southern Sinai. The estimated age of crystallization and emplacement of the examined complex is < 590–610 Ma, as it invades the old calc-alkaline granites.

Tracing crustal contamination of the Cenozoic basalts with OIB-affinity in northern marginal region of North China Craton: An Os perspective

The Cenozoic basalts with OIB-affinity in northern marginal region of the North China Craton are thought to experience minor even no crustal contamination during the magma evolution. The whole-rock Sr-Nd-Pb-Hf isotopes are attributed to a two-component mixing between depleted and enriched mantle sources, while the major element variations are controlled by the fractional crystallization of olivine and clinopyroxene. However, in this study, the new Os isotopic data proposes an opposite model for the Cenozoic basalts in northern marginal region of the North China Craton. In this model, the Jining basalts were contaminated by the Archean mafic rocks during the magma storage and ascent. The crustal contamination process is supported by (1) the highly radiogenic Os isotopic compositions, and (2) the positive correlation between 187Os/188Os and 1/Os of the Jining basalts. By modeling the Os isotopic composition of the basalts, an incorporation of < 10% mafic granulites/amphibolites to the parental magma can successfully explain the initial values of highly radiogenic Os. In contrast, the unradiogenic and uniform Os isotopic compositions of the Chifeng basalts suggest negligible crustal contamination. Os isotopic data acts as an indicator of crustal contamination during magma evolution, providing us a novel insight into the evolution of the intra-continental OIB-like basalts worldwide.©2021 China Geology Editorial Office.

Petrogenesis and tectonic implications of the late Carboniferous calc-alkaline and shoshonitic magmatic rocks in the Awulale mountain, western Tianshan

The widespread late Carboniferous calc-alkaline and shoshonitic magmatic rocks in the Awulale mountain provide crucial constraints on the tectonic evolution of the western Tianshan. Here, we perform detailed petrological investigations as well as zircon U-Pb chronological, whole-rock geochemical and Sr-Nd isotopic analyses on these magmatic rocks from two geological sections along the Duku road. Magmatic rocks in the section I with zircon SHRIMP U-Pb ages of 306.8 Ma and 306.4 Ma are composed of medium-K calc-alkaline to shoshonitic basalt, trachy-andesite and trachyte, while those in the section II consist of shoshonitic trachy-andesite, trachyte with a U-Pb age of 308.1 Ma, and monzonite with a U-Pb age of 309.6 Ma. All these magmatic rocks are characterized by strong enrichments in large iron lithophile elements with depletions of Nb, Ta and Ti, indicating the origination from subduction-modified lithospheric mantle. The εNd(t) values of the rock samples collected from the section I (2.80–5.45) and section II (3.34–5.37) are generally higher than those of the Devonian to early Carboniferous arc-type magmatic rocks in the Yili-central Tianshan, suggesting that depleted asthenosphere might also be involved in their generation. Based on these geochemical data and petrological observations, we suggest that the early-stage (308.1–309.6 Ma) shoshonitic monzonite, trachy-andesite and trachyte in the section II were generated by mixing between mafic magmas and trachytic melts, while the late-stage (306.4–306.8 Ma) medium-K calc-alkaline to shoshonitic basalt, trachy-andesite and trachyte in the section I were produced by partial melting of depleted asthenospheric and metasomatized lithospheric mantle, followed by the processes of fractional crystallization and crustal contamination. Taking into account the available regional geological data, the subduction of south Tianshan ocean was probably ceased at ∼310 Ma, and these calc-alkaline and shoshonitic magmatic rocks in the Awulale mountain formed in a post-collisional setting subsequent to slab break-off.