Crustal Contamination Research Articles

Geochemical and thermodynamic constraints on Archean comagmatic volcanic and cumulate rocks from southern West Greenland

The extensive exposure of the Archean continental crust in southern West Greenland makes it an important window into the tectonic evolution of early Earth. Here, we provide a comprehensive geochemical data set for tholeiitic amphibolites (meta-basalts), calc-alkaline leucoamphibolites (meta-andesites), and ultramafic rocks (meta-cumulates) for the Mesoarchean Bjørneøen Supracrustal Belt, Nuuk region, SW Greenland. This data helps constrain the geodynamic setting in which these rocks formed. The volcanic rocks display two distinct geochemical trends in terms of their La/Sm ratios. Nonetheless, both tholeiitic amphibolites and leucoamphibolites have negative Nb-Ta-Ti anomalies and thus geochemical features associated with apparent island arc or crust contamination processes. Uranium-lead zircon dating of a leucoamphibolite yields an age of 3077 ± 6 Ma, which is older than regional orthogneisses. A series of models for both major element variation (thermodynamics-based) and trace element variation (partition coefficient-based) implies that fractional crystallization of tholeiitic basalt can effectively produce the observed ultramafic rocks. Such ultramafic cumulates had low degrees of crystallinity, reflecting open system magmatic process at shallow depths likely representing magma conduits in a volcanic pile. The geochemical features of the andesites are distinct from the basalts and our modeling excludes a connection via fractional crystallization or crustal assimilation of the two suites. Instead, the andesites formed via high degrees of mixing between basaltic and felsic endmember magmas, requiring elevated temperatures in the mid- to lower-crust. The introduction of felsic components could be derived from partial melting of mafic lower crust, for example by mafic underplating or via some other process that achieves such anatexis, or alternatively by the addition of rhyolitic melt from extensive fractional crystallization. Mixing and homogenization of basaltic and felsic endmembers to produce andesites may occur in modern-style subduction environments, although this could also be feasible in other geodynamic settings in a hotter early Earth.

Constraints on martian depleted shergottite volcanism from the petrogenesis of olivine-phyric shergottites NWA 2046 and NWA 4925

The 1.1Ma ejection-paired depleted shergottites are sourced from a long-lived volcanic centre, likely located on the Tharsis plateau. Here we present bulk-rock geochemistry, mineral major- and trace-element concentrations, and in-situ maskelynite and pyroxene 87Sr/86Sr data for two depleted olivine-phyric shergottites, Northwest Africa (NWA) 2046 and NWA 4925, from this 1.1 Ma ejection event. The earliest formed olivine megacryst cores in NWA 2046 (Fo83) and NWA 4925 (Fo81) are among the most Mg-rich recorded in martian meteorites and are also in Mg–Fe equilibrium with their respective bulk-rock compositions. We infer the olivine megacrysts in NWA 2046 and NWA 4925 are phenocrysts and suggest that both samples represent melt compositions. CI chondrite-normalised rare earth element patterns for melts calculated to be in equilibrium with pyroxene in NWA 2046 and NWA 4925 are parallel to those of melts in equilibrium with maskelynite and merrillite, which suggests that both samples crystallised from continuously fractionating melts unaffected by crustal contamination. Oxygen fugacity estimates for NWA 2046 (QFM -2.7) and NWA 4925 (QFM -2.9), derived from the V-in-olivine oxybarometer, are comparable to other depleted olivine-phyric shergottites, including Tissint, Larkman Nunatak (LAR) 12095/12240, Dar Al Gani (DaG) 476 and Dhofar (Dho) 019.Effects of terrestrial alteration are observed in bulk-rock and mineral compositions from NWA 2046 and NWA 4925. The degree of terrestrial alteration is most pronounced in NWA 4925, consistent with an older reported exposure age relative to NWA 2046. The low Ni-contents of bulk-rock and olivine megacrysts in NWA 2046 and NWA 4925 suggests that both samples are not primary mantle-derived melt, but rather have undergone 8–10% and 10–13% fractional crystallisation, respectively, in a staging chamber at depth. Pyroxenes Ti/Al for NWA 2046 and NWA 4925, suggest initial crystallisation at ∼ 10 kbar, consistent with other ejection-paired olivine-phyric shergottites, indicating either a common or multiple staging chambers near the base of the martian crust. The 87Sr/86Sr compositions for unaltered maskelynite grains in NWA 2046 (0.70130 ± 7) and NWA 4925 (0.70128 ± 10) suggest that both samples originate from a similar mantle source to other depleted olivine-phyric shergottites, particularly DaG 476, and Sayh al Uhaymir (SaU) 005 and Yamato 980459 (Y-980459).

Variation in Trace Element and Isotope Composition of Neoarchean Mafic Granulites of the Southwest Siberian Craton: A Consequence of Various Mantle Sources or Crustal Contamination

The paper presents geochemical and isotopic characteristics of Neoarchean (2.7–2.66 Ga) mafic granulites of the Sharyzhalgay uplift in the southwestern Siberian craton. Mafic and predominant felsic granulites compose fragments of the metamorphic complex among the Neoarchean and Paleoproterozoic granitoids. Mafic granulites are characterized by the mineral association Cpx + Pl ± Hbl ± Opx ± Qz and include two types with different major and immobile trace element contents. The dominant rocks of the first type have a wide range of Mg# and concetrations of TiO2 and immobile trace elements (REE, Zr, Nb, and most positive εNd(Т) va-lues. The first type of mafic granulites show elevated (La/Sm)n and enrichment in Th and LREE relative Nb which is typical of basalts of subduction origin or crustal contaminated basalts. The absence of negative correlation between (La/Sm)n and εNd(Т) and a clear positive correlation of TiO2 with Nb testify against the effect of crustal contamination on the composition of the mafic granulites. The magmatic protoliths of first type of mafic granulites are suggested to form by the melting of depleted peridotites of the subcontinental mantle which metasomatized by melts formed from basalts or terrigenous sediments of the subducting plate. Mafic granulites of the second type have a narrower range of Mg#, TiO2 content, positive εNd(Т), flat rare earth patterns and no subduction signatures, which indicates an asthenospheric depleted mantle source. Ma-fic granulites contaminated by the Paleoarchean crust are characterized by increased (La/Sm)n, depletion of Nb relative to Th and LREE, and negative εNd(Т) values. Post magmatic influence of granitoids lead to the enrichment of mafic granulites in biotite and apatite, an increased in concentrations of K2O, P2O5, a signi-ficant enrichment of Zr, Nb, Th, LREE, and negative εNd(Т) values. The difference between mafic granulites of the first and second types is not resulted from crustal contamination, but is due to the melting of two types of sources: asthenospheric and subcontinental lithospheric mantle. The subcontinental lithospheric mantle of the Irkut block was isotopically depleted for the Neoarchean time (∼2.7 Ga), and its enrichment in incompatible trace elements, presumably by felsic melts generated from the rocks of subducting plate, immediately preceded mafic magmatism.

Geochemistry, Age, and Geodynamic Setting of the Volcanic Rocks of the Indigirka Section of the Uyandina-Yasachnaya Volcanic Belt (Northeast Asia)

The Uyandina–Yasachnaya volcano-plutonic belt (UYVB) is one of the largest structures of this type in Northeast Asia. It strikes in the north-west direction for 900 km from the upper reaches of the Kolyma River to the upper reaches of the Selenyakh River. The belt is characterized by the strong facies variability and zoning, which is expressed in a change from mafic volcanic rocks (Ilin’-Tass zone) in the northeast to felsic volcanic rocks (Darpir zone) in the southwest. Poor knowledge of the UYVB results in an ambiguous interpretation of its geodynamic nature and volcanic evolution. The paper presents new detailed geochemical, isotopic, and geochronological data on the volcanic rocks of the Indigirka section of the Darpir zone of the UYVB. The U-Th-Pb SIMS zircon dating showed that volcanic rocks of the section previously attributed to the Oxfordian–Tithonian stages have the younger age within 150 ± 2–152 ± 2 Ma, which corresponds to the Tithonian. Andesites from the section base show Ta–Nb depletion, indicator element ratios Th/Nbpm, La/Nbpm, and La/Smpm 1, and extremely low εNd = (–8), which may indicate a crustal contamination of mantle sources. Felsic volcanic rocks overlying the andesites have the postcollisional signatures. They are also characterized by the wide variations of εNd from –2.4 to –6.5 and model ages. The systematic position of the rocks in the section as well as upward increase of alkalinity and silica content from basaltic andesites to rhyolites in the upper parts of the section, which is typical of island-arc buildups, suggest that suprasubduction melts were generated beneath the collisional crust. Source of these melts could be a mantle domain that was enriched in fluid during previous subduction event.

Neoproterozoic Aksu Diabase Dyke, Chinese South Tianshan: Magma Sources and Implications for Regional Gold Metallogeny

Tianshan is one of the world’s largest gold provinces; however, the relationship between gold mineralization and metasomatized subcontinental lithospheric mantle (SCLM) remains poorly understood. To improve our understanding, we present new bulk-rock geochemistry and platinum group element (PGE) concentrations of the SCLM-sourced Aksu Neoproterozoic diabase dykes in Chinese South Tianshan. These data, combined with in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of hydrothermal pyrite grains in the diabase dykes, are used to discuss the SCLM source characteristics in the region and their potential links to formation of gold deposits. The diabase dykes exhibit high Th/Yb (0.47–0.62) and low Nb/U (13.4–16.3) ratios, indicating that magma evolution involves subduction-related fluid metasomatism and limited contamination of the continental crust. This is consistent with little variation in whole-rock Pd/Zr, Cu/Zr, and Ni/MgO ratios, suggesting that no sulfide segregation was caused by crustal contamination and magma mixing. In addition, the diabase dykes show low PGE and Au contents, with high Cu/Pd (>105) and low Cu/Zr (<0.5) ratios, indicating that magmas were derived from low-degree partial melting of the SCLM under S-saturated conditions. Such source characteristics indicate residual sulfides and chalcophile elements (e.g., PGEs, Au, and Cu) were concentrated at the SCLM reservoir in South Tianshan. Hydrothermal pyrite in the studied dykes has similar Au/Ag ratios and trace element distribution patterns to gold-bearing pyrite of lode gold deposits in Chinese South Tianshan, indicating that metasomatized SCLM may have contributed ore metals during the formation of these gold deposits. Adding to the available data, our study highlights that the SCLM may be a potential metal source reservoir, and it may have contributed to formation of the lode gold deposits in Chinese South Tianshan.

Geochemistry, geochronology, and tectonic setting of the Cretaceous volcanic rocks in east Mongolia

Late Mesozoic volcanic rocks in East Mongolia exhibit high-K calc-alkaline and shoshonitic series and display features of bimodal-like volcanic with a mafic and felsic members. The mafic member is composed mainly of trachybasalt and basaltic trachyandesite, with a minor amount of trachyandesite. The mafic rocks have elevated incompatible trace element concentrations and significantly negative Nb-Ta and Ti anomalies, features of typical subduction-related magmas, distinguished from OIBs. The mafic magmas have undergone fractional crystallization dominated by pyroxene and/or olivine and crustal contamination. The mafic magma was derived from low-degree partial melting of an enriched lithospheric mantle source that might have been metasomatized by subduction-derived fluids. The felsic member consists of trachydacite and rhyolite. The geochemical data indicate they produced from partial melting of a crustal source that is dominated by juvenile mafic rocks. Our new K-Ar dating and previous age data demonstrated that the late Mesozoic volcanism in East Mongolia took place during Late Jurassic-Early Cretaceous at between 156-99 Ma. The model of the arc‒back-arc extension possibly induced by slab roll-back of the westward (paleo) Pacific-subduction can explain the geodynamic setting and the eastward young trend of the late Mesozoic volcanism in East Mongolia and its adjacent NE China.

Age and petrogenesis of Ni-Cu-(PGE) sulfide-bearing gabbroic intrusions in the Lausitz Block, northern Bohemian Massif (Germany/Czech Republic)

The Lausitz Block, located in the northernmost part of the Bohemian Massif, hosts a large number of dike- to stock-shaped gabbroic intrusions that mainly comprise brown hornblende-poor (Group I; i.e. olivine gabbronorite, olivine gabbro, gabbro and diorite) and subordinately brown hornblende-rich lithologies (Group II; i.e. olivine-hornblende gabbro and hornblende gabbro). Several of these intrusions host small-scaled magmatic Ni-Cu-(PGE) sulfide accumulations. The intrusions are part of interconnected mafic–(ultramafic) plumbing systems that intruded Cadomian granodiorites of Lausitz Block in the Middle to Late Devonian during the early stages of the Variscan Orogeny. The previously inferred Devonian age of the intrusions is refined by biotite Ar-Ar dating that yield ages between 389.1 ± 3.9 Ma and 372.2 ± 3.7 Ma (2σ). Group I and Group II lithologies differ in their mineralogical and geochemical composition. Compared to the Group I lithologies those of Group II are characterized by higher modal contents of primary brown hornblende, Fe-Ti oxides and apatite, by Ti- and Al-enriched clinopyroxene and by lower contents of SiO2 and increased contents of TiO2, P2O5, LILE, HFSE and LREE. The differences suggest at least two different magmatic series where Group I rocks are linked to tholeiitic basaltic magmas with low to moderate Ti and volatile contents, whereas Group II rocks are derived from Ti- and volatile-enriched alkaline basaltic magmas. The magmas experienced clinopyroxene fractionation during their crustal ascent and storage, but were only minor affected by crustal contamination (< 5%) according to Sr-Nd-Pb isotope systematics. The tholeiitic and alkaline basaltic magmatism could be explained by a tectonic environment between a subduction and intraplate setting. Group II rocks have an intraplate chemical signature and their parental magmas are clearly originated from a deeper-seated mantle source with melting of garnet peridotite, likely associated with an asthenospheric upwelling. By contrast, precursor magmas of Group I rocks are originated from partial melting of a less-enriched source, presumably spinel peridotite, from a subduction-related environment. Trace elements systematics suggest that both groups at least partly resulted from an interaction (mixing) of the alkaline and tholeiitic magmas. Partial melting estimates for both groups vary between 5 and 20%. The proposed tectonic setting could have been related to a subduction slab retreat within the framework of the Variscan orogeny, which led to an upwelling of the asthenospheric mantle. Cu/Zr ratios <1 of gabbroic rocks from several intrusions suggest a previous segregation of magmatic sulfides in other sections of the magmatic plumbing system and give rise for a vertical and lateral Ni-Cu exploration potential.

Magmatic evolution of the Calc-alkaline Middle Jurassic igneous rocks in the eastern pontides, NE Turkey: insights from geochemistry, whole-rock Sr-Nd-Pb, in situ zircon Lu-Hf isotopes, and U-Pb geochronology

ABSTRACT The Eastern Pontides in NE Turkey hosts a very limited number of plutonic rocks from the Jurassic period. Here we submit new geochemical, Sr-Nd-Pb and zircon Lu-Hf isotope data, and zircon U-Pb ages for plutonic rocks in this region. Zircon U-Pb dating for the Alemdar and Işıkdere plutons yielded an age of 175–170 Ma (Middle Jurassic). The Alemdar and Işıkdere plutons have I-type, medium to high-K calc-alkaline series, and metaluminous to slightly peraluminous characteristics. On PM-normalized diagrams, the plutons are enriched in large-ion lithophile elements (LILEs) with pronounced depletion of high field strength elements (HFSEs, Ta, Nb, Ti). Chondrite-normalized plots for the plutons show variable enrichment in light rare earth elements (LREEs) with depletion of heavy rare earth elements (HREEs). The rocks of the Alemdar Pluton have homogeneous ISr values (0.70585 to 0.70673), low εNd(i) values (−3.9 to −1.6), pozitif εHf(i) values (+3.3 to +9.1), and Hf single-stage model ages (563–712 Ma), whereas those of Işıkdere have high ISr values (0.70666 to 0.70673), low εNd(i) values (−3.9 to −3.8), εHf(i) values (−3.1 to +1.1), and Hf single-stage model ages (791–969 Ma). These values imply that they have different sources. These results suggest that the magma of the Alemdar was formed by the partial melting of a relatively depleted lithospheric mantle wedge interaction of the OIB-like melts, while those of Işıkdere have derived from the partial melting of an enriched mantle metasomatized by slab-derived sediments and then modified by fractional crystallization and crustal contamination in crustal magma chambers. Considering the available data, we conclude that the studied plutons were related to extensional tectonic events during the Middle Jurassic in response to the roll-back of the Paleotethys oceanic slab in the final stage of oceanic closure. These rocks are an example of the formation of deep subduction products following Early Jurassic subduction.

Petrogenesis and tectonic setting of Early Cretaceous mafic dykes in the North Qinling Orogenic Belt, central China: constraints on the lithospheric lower crust delamination

Late Mesozoic mafic dykes, which are widely developed in the North Qinling Orogenic Belt (NQOB), include abundant geodynamic information. This paper describes the mafic dykes that intrude the Late Jurassic granite in the Dayu and Kuyu areas, and reports important petrological constraints for the late Mesozoic tectonic transition from compression to extension in the NQOB. Three zircon U–Pb results show that the minimum ages of the mafic dykes are 139.8 ± 1.4 Ma, 137.4 ± 1.7 Ma and 133.4 ± 0.9 Ma, indicating that the emplacement age of the Dayu and Kuyu mafic dykes is 140–133 Ma. Petrogeochemical analyses suggest that the mafic dykes belong to the high-K calc-alkaline shoshonite series with low SiO2 (46.93–56.73 wt%), MgO (1.88–9.10 wt%) and TiO2 (1.17–1.82 wt%), and high Al2O3 (13.98–17.46 wt%), TFe2O3 (7.81–10.92 wt%) and K2O (1.28–4.78 wt%). The mafic dykes are enriched in large ion lithophile elements (e.g. Rb, Ba, K, La, Sr) and depleted in high-field-strength elements (e.g. Nb, Ta, Zr, Ti). These samples have the right-sloping chondrite-normalised rare earth element patterns, which suggest light rare earth element enrichment and heavy rare earth elements depletion with no obvious Eu anomalies (δEu = 0.94–1.11). The I Sr, ε Nd(t), ε Hf (t) and T DM2(crust) values are 0.7056–0.7060, −10.60 to −5.98, −14.1 to −2.8, and 1382.4 ± 25.1 to 2081.9 ± 47.6 Ma, respectively. Both elemental and isotopic geochemistry show that the formation of Dayu and Kuyu mafic dykes is due to the partial decompression melting of previously enriched lithospheric mantle during a delamination process. The mafic dykes have undergone fractionation crystallisation of Mg–Fe phase minerals during magma ascent, accompanied by some crustal contamination. Combined with the regional tectonic setting, we suggested that the NQOB experienced intra-continental extension during the Early Cretaceous. KEY POINTS Early Cretaceous (140–133 Ma) mafic dykes have been discovered in the middle part of the North Qinling Orogenic Belt. The remote effect of the Paleo-Pacific Plate subduction has reached the middle of the North Qinling Orogenic Belt. The North Qinling Orogenic Belt entered the extensional stage in the Early Cretaceous (140–133 Ma).

Whole Rock Geochemistry and Geodynamic Evolution of Paleoproterozoic Gneisses, Ako’ozam-Njabilobe Area, Southwestern Cameroon

The Ako’ozam–Njabilobe area, which is part of the Nyong group, is characterized mainly by highly deformed gneisses that host amphibolites, and metadolerites as enclaves. The gneisses within the Ako’ozam-Njabilobe area were investigated to constrain their geotectonic environment as well as understand their geodynamic evolution within the Nyong group. Three litho-types of gneisses outcrop within the Ako’ozam-Njabilobe area, consisting of horblende-biotite, quartzofeldspathic, and garnet gneisses. They are strongly peraluminous, with low Yb<SUB>N</SUB>, Rb/Sr, K₂O/Na₂O, and enriched in large ion lithophile elements (LILE). The gneisses within the Ako’ozam-Njabilobe area have a trachy-andesite parentage. The negative anomalies in high field strength elements (HFSE) and enrichments in LILE and light rare earth elements (LREE) suggest a subduction setting for this magmatism. The geochemical signatures including, low K₂O/Na₂O ratio, silica content (67.18 to 73.65 wt%), Yb and Y contents less than 1 and 10 ppm respectively, and low heavy REE values of the studied gneisses are akin to chemical signatures of archean crustal rocks. The Nb/Th < 1 and Th/Yb >1 of the studied gneisses indicate that the magma that sourced their protolith is mantle-derived with some degree of crustal contamination. The similarity in whole rock data of the three lithotypes of gneisses within the Ako’ozam-Njabilobe area suggests a possible mechanical mixing during emplacement. The chemistry of the gneisses indicates a possible Archean inheritance during the emplacement of their protoliths, as such could provide insights into the petrogenetic processes controlling crustal growth and aid in understanding the Archean-Proterozoic transition within the Nyong group.

Crustal Contamination Versus Subducted Components: An Example from The West Java Arc, Indonesia and Its implications in Magma Genesis

New Sr isotope and K2O data are presented for Papandayan and Cikuray Volcanoes in West Java. The data are combined with published Sr isotope and K2O data, and compared with arc that has a similar geological setting, namely Northeast Japan Arc (NJA, N 38° ~ 41°) to constrain the relative importance of crustal assimilation and subducted input of crustal material in magma genesis beneath West Java Arc (WJA). New strontium isotope and K2O data from fifty-four Quaternary volcanic rocks from WJA were collected and compared to forty-six Quaternary volcanic rocks from NJA. The increasing K2O and decreasing of 87Sr/86Sr ratios with distance from trench have been found in NJA, but there are rough and no across arc variation of K2O and Sr isotopic ratios in WJA. This study shows that the across arc variation of magma chemistry on the WJA is attributed to the crustal assimilation and the involvement of subducted sediments and slab fluids from altered oceanic crust.

Petrological, geochemical and geochronological evolution of massif type charnockite from the Eastern Ghats Province, India: Implications on the regional tectonics of the Rayner-Eastern Ghats orogeny

Massif type charnockite was emplaced in the lower crust of the Eastern Ghats Province and is now exposed over a large geographic area. The rock shows clear evidence of magmatic emplacement in lower crustal metasedimentary rocks, which occur as enclaves within the charnockite. Textural data suggest the magma underwent subsolidus cooling and subsequently metamorphosed to granulite facies (up to ∼ 910 °C, 9 kbar). Geochemical data show that the charnockite magma had variable chemistry which was acquired by differentiation and possible crustal contamination. The rock shows both high- and low-SiO2 types, with weakly peraluminous to metaluminous characters. Trace and REE fractionation trends suggest the magma had calcic to calc-alkaline affinities and was emplaced in a continental arc type collisional setting. Theoretical modelling suggests that such a magma could be generated by melting of a hydrated basaltic slab under CO2-rich fluid. U-Pb analysis on oscillatory zoned zircon domains from eight samples yields crystallization ages for the magma. While the majority of the samples show crystallization ages within ca. 980–940 Ma (978 ± 16 Ma, 968 ± 22 Ma, 951 ± 9 Ma, 954 ± 8 Ma, 951 ± 13 Ma and 939 ± 27 Ma), two samples yield crystallization ages of 1002 ± 13 Ma and 1020 ± 16 Ma. This implies two-phase emplacement of the charnockite magma which can be correlated with the tectonometamorphic evolution of the province. While the earlier pulse of charnockite magmatism is broadly synchronous with the first cycle (M1) of metamorphism, the later pulse followed when the lower crust was still hot. The two pulses of charnockite magmatism are broadly synchronous with those of the Mawson charnockite of the Rayner Province, East Antarctica. It is argued that the charnockite magmatism in the combined Rayner-Eastern Ghats Province was extensive and resulted from arc-continent accretion and collision between the India and East Antarctica during ca. 1030–900 Ma.

Systematic and Temporal Geochemical Changes in the Upper Deccan Lavas: Implications for the Magma Plumbing System of Flood Basalt Provinces

AbstractLarge Igneous Provinces (LIPs) are unusual volcanic events in which massive amounts of melt (∼106 km3) erupt in relatively short time periods (&lt;106 years). Most LIP magmas have undergone extensive fractional crystallization and crustal contamination, but the crustal magmatic plumbing systems and the processes triggering eruptions are poorly understood. We present new major and trace element and radiogenic isotope data for 43 individual lava flows from a continuous 1,200 m thick stratigraphic profile through the upper, most voluminous part of the Deccan LIP (Bushe to Mahabaleshwar Formations). Eruption rates for this section are constrained by published paleomagnetic directions and absolute U‐Pb ages for zircons from weathered flow tops exposed in the profile. We find four magmatic sequences each lasting ∼104–∼105 years during which major and trace element compositions change systematically, followed by an abrupt change in geochemistry at the start of a new sequence. Within each sequence, the MgO content and proportion of crustal contamination decrease progressively, indicating a continuous replenishment of the associated magma reservoirs with less contaminated but more evolved melts. These geochemical signatures are best explained by repeated episodes of melt recharge, mixing, and eruption of variably evolved magmas originating from relatively small magma reservoir located in different crustal levels.

Petrogenesis of the Carboniferous Intrusive Rock in the Xiaobaishitou District of East Tianshan, Northwest China: Magma Evolution and Tectonic Significance

AbstractThe Xiaobaishitou gabbro‐diorite pluton comprises a medium‐grained gabbro‐diorite suite and a fine‐grained diorite suite, which intrude the Kawabulag Group in the East Tianshan Orogen of the Central Asian Orogenic Belt (CAOB). A combination of mineral chemistry, zircon U‐Pb age, whole‐rock geochemistry, Sr‐Nd isotopes, and in situ zircon Hf isotopes for newly found gabbro‐diorite from the Xiaobaishitou district in the Central Tianshan Terrane (CTT) is presented to investigate the petrogenesis and tectonic or even crustal evolution of the East Tianshan Orogen. Laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) zircon U‐Pb analyses indicate that the gabbro‐diorite was formed at 324.7 ± 2.4 Ma. The isolated clinopyroxene formed under higher P‐T‐ƒO2 melt conditions (10.7–14.6 kbar; 1199–1269°C; high ƒO2) than those for the hornblende, plagioclase, and zircon (557–687°C; moderate ƒO2) in the gabbro‐diorite, which reveals a multilevel, magmatic storage system. The gabbro‐diorite is characterized by fractioned REE patterns, enriched LILEs (e.g., Ba and Pb), negative anomalies of HFSEs (e.g., Nb and Ta), and low La/Yb and Sr/Y ratios, which are typically indicative of crustal contamination and accounted for by subduction‐related fluids. The rock also characterized by typical features of high compatible elements (MgO = 3.14–11.65 wt%, Cr = 1–157 ppm, Ni = 6–830 ppm), high Mg# (47 –74), positive ɛHf(t) values (+5.1 to +10.3) and ɛNd(t) values (+2.3 to +4.4). These features suggest that the Xiaobaishitou gabbro‐diorite was most likely derived from metasomatic mantle and contaminated minor crustal components. Taking into account the spatial and temporal distribution of the Carboniferous magmatic rocks in the CTT, we suggest the formation of the Xiaobaishitou gabbro‐diorite was attributed to southward subduction of the Kangguer Ocean.

Two million years of evolution of the southern central andes retroarc, Payenia Volcanic Province, Argentina, from the study of the Cerro Nevado Volcanic Complex

New K–Ar ages, major and trace element analyses, and Sr and Nd isotopic data for the Cerro Nevado Volcanic Complex (CNVC) coupled with those previously published allow us to better document the hot thermal anomaly which affected the upper mantle beneath the Andean back-arc Payenia Volcanic Province and generated alkaline magmatism. These data, along with previously published ones from this and neighbor volcanic centers, suggest that this anomaly stagnated there, during at least the last 2 million years. Geochemical features from the CNVC basaltic magmas indicate that they were sourced within an OIB-type mantle with an EMI signature, similar to that underlying the neighboring Auca Mahuida Shield Volcano to the south, and attributed to the rise of a mantle thermal anomaly. We identify at least five events of partial melting and seven eruptive phases from 1.88 to 0.75 Ma. Continental crust contamination in the San Rafael Block crustal basement occurred coupled with fractional crystallization processes, probably in magma chambers and across several cycles, producing the trachybasaltic, trachyandesitic and trachytic magmas of the Cerro Nevado volcanic cone (1.34–1.22 Ma).

Assessing crustal contamination in the 28 to 18 Ma Dulce dike swarm with Nd-Sr isotopic data, southwestern Colorado

Alkaline to subalkaline mafic dikes in the 28 to 18 Ma Dulce swarm were emplaced in a zone of incipient extension from southern Colorado into northern New Mexico on the northeastern boundary of the San Juan Basin. The 87Sr/86Sr ratios for the dikes are 0.70503 to 0.70584, akin to most post-28 Ma mafic rocks across the northern San Juan Basin. These data are consistent with melting of metasomatized subcontinental lithospheric mantle with little to no crustal contribution as revealed by the geochemical and Sr-Nd isotopic signatures of most 28–0.6 Ma mafic rocks in the region. Time-corrected εNd(t) values of −4.1 to −7.4 for rocks in the Dulce swarm, however, indicate that magma production involved the crust. A previous hypothesis for Dulce magmas was contamination of lithospheric mantle melts with up to 45% mafic lower crust ± 0.5% upper crust. In this investigation, six new whole-rock Sr and Nd isotopic analyses were combined with published data to further investigate the contamination of lithospheric mantle melts with different crustal reservoirs. The Nd isotope signatures of the Dulce swarm offer evidence for the long-term involvement (∼10 Ma) of lower crust in the production of rift-related mantle magmas. Isotopic mixing curves support previous hypotheses for the contamination of lithospheric mantle melts with 10 to 40 percent lower mafic crust. This provides further insight into regional variations in mantle magmas produced after 28 Ma in the Four Corners region that likely triggered crustal melting related to caldera complexes in the western San Juan Mountains.

Petrography and Geochemical Relationships of the Ultramafic Rocks in Galalah area within Erbil Governorate, NE Iraq

The current research to study the petrography, mineralogy and geochemical relationships of the ultramafic rocks in the Galalah area within Erbil Governorate, at the Unstable Shelf in the Imbricate Zone, to determine the origin of the ultramafic rocks. Dunite and Harzburgite are the most abundant rock bodies in the study area, and they represent most of the outer surface exposure of the ultramafic rock aggregates. The dunite rocks are characterized by pale brown color on the altered surface that is broken into olive green and dark green lithic. Dunite represents a homogenous lithologic feature that mainly consists predominately of olivine with accessory spinel and traces of orthopyroxene. While the harzburgite appears as a massive rock body consisting of olivine, orthopyroxene, and clinopyroxene. These rocks are mainly characterized by alteration to serpentinite, with accessory chromian spinel, secondary magnetite, chlorite, and talc. The studied ultramafic rocks are predominately rich in ferromagnesian minerals of less silica content, and other major elements affected by serpentinization are causing the deviation of elements. They have a strong decrease in sodium and calcium oxides relative to MgO and verified the silica and alumina with MgO and a slight increase in iron oxide. The low values of TiO2 in dunite and harzburgite are characteristic property of the Alpine type ophiolites during the primary process of the mantle in its upper part, before the tectonic emplacement of the ultramafic rocks, and also due to partial melting degree for the asthenospheric mantle. The MREE depletion relative to HREE and LREE may be attributed to the hornblende absence and the presence of olivine and pyroxenes minerals and/or to the source rock features. The U-shaped patterns of REE and their ratios are typical properties of ophiolitic ultramafic rocks, which are compatible with the supra-subduction zone. During the partial melting, the negative anomalies of Nb in ultramafic rocks with sequestration of the Nb concentration by sphene and ilmenite in various types of the source rocks have also been attributed to the crust contamination. These are also characterized by island arc conditions, and its depletion in the studied rocks may indicate the fore-arc tectonic environments

Archean Pb isotope variability tracks crust-mantle fractionation, granite production, and ore deposit formation

Various geological processes that affect Earth's crust may be encoded into isotopic tracers preserved in rocks and minerals. The enhanced sensitivity of U, Th, and Pb to crustal fractionation processes allows Pb isotopes to complement information from the Nd and Hf isotope systems. However, melt fractionation, crustal contamination and recycling, hydrothermal fluid flow and fluid-rock interaction, and other processes in the continental crust can lead to mixing of Pb isotopic signatures. Here, we report new Pb isotopic data from granite-K-feldspar and integrate these data with published Pb isotope ratios from granite K-feldspar and Pb-rich ores across the Yilgarn Craton in Western Australia. The aim of this study is to explore how the variability of Pb isotope ratios and derivative parameters can be used to gain information on specific geological processes occurring throughout the crustal column. We develop a model that subdivides different sampling media into chemical process groups and links their initial Pb signatures to Pb source regions and fractionation processes at various locations within the crust and upper mantle. Equilibration of Pb signatures with a primary mantle source reservoir (in part represented by volcanic-hosted massive sulfides) is contrasted with granite formation in the mid to lower crust (granite K-feldspar), and mineralization of ore deposits in the mid to upper crust (Pb-rich ores). Spatial trends similar to those in Nd and Hf isotopic data are recorded by Pb isotopic derivative parameters (μ = source 238U/204Pb, ω = 232Th/204Pb, and Δt - the difference between true sample age and Pb model age) calculated for komatiite-hosted Ni sulfide ores, granite K-feldspar, and volcanic-hosted massive sulfide (VHMS) ores. The significance of subtle differences in absolute values of derivative parameters is supported by the diversity of Pb isotope ratios, Pb model ages, and ∆t as tracked by a statistical metric, quantifying the variability of Pb sources involved in the formation of different chemical process groups. Generally greater variety in an older terrane (Youanmi) documents more ancient and recycled continental crust as compared with more homogeneous Pb isotopic signatures in a younger terrane (Eastern Goldfields Superterrane). The Pb signatures are interpreted, in part, to relate to the timing of source fractionation in the upper mantle with a legacy of this source signal preserved through various depths in the lithospheric section. The least radiogenic VHMS ore samples appear to provide a good approximation of mantle Pb signatures, indicated for example by a deficit in 206Pb and 208Pb relative to the other process groups. A significant heterogeneity recorded in Pb isotopic data from Pb-rich gold ores is explained by the interplay of hydrothermal fluids with diverse sources leading to the mineralization of gold deposits (e.g., leaching of Pb from surrounding rocks or fluid mixing). Such gold ore Pb signatures are distinct from other process groups, which together track sources less heterogeneous in age and/or U and Th.

Late Paleozoic tectonics of Southern Central Asian orogenic belt: Evidence from magmatic rocks in the northern Alxa, Northwest China

Late Paleozoic magmatic rock outcrops are common in the Northern Alxa, Southern Central Asian Orogenic Belt (CAOB), which is a key area for understanding tectonic processes and defining the final closure time of the Paleo-Asian Ocean (PAO). We present zircon U‒Pb chronology and whole-rock geochemistry data for late Paleozoic magmatic rocks from the Yagan area of northern Alxa. This paper reveals two periods of magmatism: syenogranite (374.8 Ma) and bimodal intrusive rocks, which consist of gabbro (261.4 Ma), diabase (262.9 Ma) and biotite monzogranite (263.4 Ma). The syenogranite is high-K calc-alkaline and shows enrichments in Th, Zr, Hf and LREEs; depletions in Sr, Nb, Ta, and Ti; and low Mg# values (6.9–13.2); the syenogranite was derived from partial melting of the crust and has volcanic arc characteristics. The gabbro and diabase have similar geochemical characteristics, such as enrichments in Pb, Rb, Sr, Zr, and Hf and depletions in Nb, Ta, and Ti, with positive εHf(t) values (+0.9–+2.7 and +2.6–+3.6, respectively), indicating that they originated from partial melting of depleted mantle and experienced crustal contamination during magma emplacement. The biotite monzogranite shows depletions in Nb, Ta, and Ti and εNd(t) values of -2.6 to −2.4 and resulted from partial melting of the lower crust caused by asthenospheric underplating. The bimodal intrusive rocks formed in a postcollision extensional setting. Combined with previous data, we conclude that northern Alxa was an active continental margin during the late Devonian and that the final closure of the Yagan branch ocean of the PAO occurred prior to the middle Permian.

Petrology and amphibole-plagioclase chemistry of amphibolites in the Ilesa area, Southwestern Nigeria

Amphibolites of Itagunmodi and Igun, within the Ilesa Area were studied in order to determine their petrochemical affinities, tectonic setting and conditions of formation; ultimately for an improved understanding of the evolution of the Ilesa Schist Belt. The amphibolites are massive in texture and occur as small to large lensoid boulders associated with quartz-biotite schist, quartzite and talc-chlorite schist. They comprise magnesio-hornblende, actinolite, labrodorite, albite and accessory titanite. Average elemental composition and ratios in the amphibolites were: Ba (872 ppm), Sr (609 ppm), Ni (89 ppm), Rb (72 ppm), Y (27 ppm), Zr (168 ppm), Nb (83 ppm), Cr (200 ppm), La (34 ppm), Cr/Ni (2.30), Y/Nb (0.63), Rb/Sr (0.10), Nb/La (2.55), and Zr/Nb (4.62). The major and trace element characteristics revealed that they are ortho-amphibolites with tholeiitic and calc-alkaline affinities, plotting in Within Plate Basalt field of the Zr vs Zr/Y diagram. The amphibolites exhibit enhanced mid-ocean ridge basalt (E-MORB) elemental characteristics with LREE > HREE enrichment and slight negative Eu anomaly. The calc-alkalic affinity of some of the amphibolites is a result of crustal contamination, and not an arc-related signature as earlier proposed. Elemental abundances and ratios revealed that the amphibolites were formed in an extensional tectonic setting where mantle plume activity and lithospheric contamination occurred. The mineralogical assemblage reflects amphibolite facies metamorphism, at temperature of formation of 450–687 °C and pressure of 0.5–4.3 kbar constrained by plagioclase-hornblende thermobarometry.