HREE Depletion Research Articles

Geochemistry and Sm-Nd isotopic characteristics of the Paleoarchean Komatiites from Singhbhum Craton, Eastern India and their implications

Komatiites near Kapili village, located in Palaeo-Mesoachean Gorumahishani-Badampahar greenstone belt of Singhbhum Craton, Eastern India preserve excellent igneous textures and exhibit co-association of Al-depleted (ADK) and Al-undepleted komatiites (AUK). The Kapili komatiites exhibit repeated spinifex textured flows (cooling units) overlying massive cumulate zone. The basal cumulate part of Kapili komatiite is chemically similar to ADK which is characterized by subchondritic Al2O3/TiO2 (9.38–10.5) and HREE depleted nature (average (Gd/Yb)PM∼1.35) elucidating presence of majorite in residuum whereas compound upper spinifex lava typically display superchondritic Al2O3/TiO2 (∼22–28.5) and HREE enriched nature (average (Gd/Yb)PM∼0.81) demonstrating majorite free residuum.Calculated primary magma compositions of Kapili ADK and AUK reveal that generation of primary liquid of Kapili ADK or basal cumulate took place at greater depth (>300km) or at higher pressure (∼9GPa) at around 30% melting of source in contrast with primary liquid of AUK (spinifex lava), which was generated at shallower depth (∼150km) or at lower pressure (∼6GPa) beyond majorite stability and at higher degree of melting (∼30–40%). ‘Hump-shaped’ pattern of primitive mantle (PM) normalized REE spectra of Kapili ADKs and their primary liquid is caused by depletion of both LREE and HREE with respect to MREEs. Depletion in LREE and HREE is explained by depletion of source prior melting and sequestration of HREE in residual majorite respectively. However, Kapili AUK or spinifex lava samples show flat, near primitive mantle (REE/PM∼1) pattern exclusive of any significant depletion and implying higher degree of melting (between 30 and 40%) of a PM like source.Sm-Nd isotopic studies of Kapili ADK and AUK reveal that they both were derived from a mantle source with long term depletion (εNd∼+2 to +4). Except differences in LREE/HREE ratio featured due to differences in melting conditions, similarity in overall immobile HREE and high field strength element (HFSE) concentrations of both the types and their corresponding primary magmas suggest a single plume source rather than multiple sources, whose composition was closer to primitive mantle and eventually segregated chemically different komatiitic melts (Al-depleted and undepleted) at different depths. Whole rock Th/Yb, Th/Nb, Zr/Nb, Nb/Y, (La/Nb)PM ratios of both Kapili AUK and AUKs are comparable to oceanic island basalt (OIB) and mid oceanic ridge basalt (MORB) similar to typical greenstone belt volcanics from oceanic provenance. In addition, presence of copious pillow basalt and oceanic chert associated with komatiites, absence of negative δNb values recommend emplacement of komatiitic lava in oceanic rather than continental or arc setting.

Palaeoproterozoic granitic magmatism in the northern segment of the Jiao-Liao-Ji Belt: implications for orogenesis along the Eastern Block of the North China Craton

ABSTRACTAs the northern segment of the Jiao-Liao-Ji Belt (JLJB), the Palaeoproterozoic Liaoji Belt is a key region for deciphering the formation and evolution of the North China Craton (NCC). In this study, we present the geochronology, geochemical, and isotopic studies on the monzogranitic gneiss, which is one of the major lithotectonic elements of the Liaoji Belt. LA-ICP-MS zircon U–Pb dating reveals that the studied monzogranitic gneisses were formed in the period of 2213–2178 Ma. They are in tectonic contact with the Palaeoproterozoic volcano-sedimentary rocks in the field. The monzogranitic gneisses belong to the high-K calc-alkaline series, and are metaluminous to peraluminous. They have 10,000 Ga/Al ratios of 2.63–3.14 with an average of 2.90, and are thus classified as aluminous A-type granites. Their εNd(t) values vary from −3.4 to +2.5, indicating heterogeneous source region. The monzogranitic gneisses are characterized by enrichment in LREE and LILE (e.g. Rb, Ba, Th, and K) and depletion in HREE and HFSE (such as Nb, Ta, and Ti), and are typical to magmatism in active continental margins formed in a subduction-related tectonic setting. Taking into account their A-type affinity and regional geological data, we suggest that the monzogranitic gneisses were most probably generated in a local extensional back-arc environment during subduction.

Multiple magmatism in an evolving suprasubduction zone mantle wedge: The case of the composite mafic–ultramafic complex of Gaositai, North China Craton

The suprasubduction zone mantle wedge of active convergent margins is impregnated by melts and fluids leading to the formation of a variety of magmatic and metasomatic rock suites. Here we investigate a composite mafic–ultramafic intrusion at Gaositai, in the northern margin of the North China Craton (NCC). The hornblende gabbro–serpentinite–dunite–pyroxenite–gabbro–diorite suite surrounded by hornblendites of this complex has long been considered to represent an “Alaskan-type” zoned pluton. We present petrologic, mineral chemical, geochemical and zircon U–Pb and Lu–Hf data from the various rock types from Gaositai including hornblende gabbro, serpentinite, dunite, pyroxenite, diorite and the basement hornblendite which reveal the case of multiple melt generation and melt–peridotite interaction. Our new mineral chemical data from the mafic–ultramafic suite exclude an “Alaskan-type” affinity, and the bulk geochemical features are consistent with subduction-related magmatism with enrichment of LILE (K, Rb, and Ba) and LREE (La and Ce), and depletion of HFSE (Nb, Ta, Zr, and Hf) and HREE. Zircon U–Pb geochronology reveals that the hornblendites surrounding the Gaositai complex are nearly 2billionyears older than the intrusive complex and yield early Paleoproterozoic emplacement ages (2433–2460Ma), followed by late Paleoproterozoic metamorphism (1897Ma). The serpentinites trace the history of a long-lived and replenished ancient sub-continental lithospheric mantle with the oldest zircon population dated as 2479Ma and 1896Ma, closely corresponding with the ages obtained from the basement rock, followed by Neoproterozoic and Phanerozoic zircon growth. The oldest member in the Gaositai composite intrusion is the dunite that yields emplacement age of 755Ma, followed by pyroxenite formed through the interaction of slab melt and wedge mantle peridotite at 401Ma. All the rock suites also carry multiple population of younger zircons ranging in age from Paleozoic to Mesozoic, suggesting continuous mantle metasomatism through melts and fluids associated with prolonged subduction, which is also substantiated by the pervasive hydration of all the ultramafic units. Zircon Lu–Hf isotopic data from the basement rock (hornblendite) on concordant grains yield εHf(t) values in the range of −23.8 to −5.2 with TDM of 1979–2424Ma and TDMC between 2754 and 2899Ma, suggesting Mesoarchean to Neoarchean reworked and juvenile sources. Concordant grains in the serpentinite also display a large range of εHf(t) values (−5.0 to 5.8) suggesting multiple sources, whereas the concordant magmatic zircon grains in the dunite have a tight range of εHf(t) values between −2.3 and 0.1 indicating primitive source. Those from the pyroxenite are characterized by highly negative εHf(t) values of −21.5 to −18.6 suggesting reworked ancient components. The diverse ages, lack of typical geochemical imprints, and magmas derived from multiple sources including Mesoarchean to Neoarchean reworked and primitive components within the same mafic–ultramafic complex exclude an “Alaskan-type” affinity and suggest multiple magmatism in an evolving and metasomatized suprasubduction zone mantle wedge. We correlate the tectonics with the prolonged subduction regime of the Paleo-Asian Ocean with melt–peridotite interaction and geologic history spanning through compression to extension.

Early Cretaceous high-Mg adakites associated with Cu-Au mineralization in the Cebu Island, Central Philippines: Implication for partial melting of the paleo-Pacific Plate

Early Cretaceous arc volcanic rocks, diorite intrusions and an associated large porphyry deposit occur in the Cebu Island, Central Philippines. In this paper, we studied the diorite porphyries associated with Cu-Au mineralization in the Kansi region, where Early Cretaceous arc volcanic rocks are widely distributed. Zircon U-Pb age reveals that the diorites were formed at ca. 110Ma, close to the formation age of Lutopan diorites in the famous Atlas porphyry Cu-Au deposit (109–101Ma), and younger than those of the arc volcanics in this region (126–118Ma). The Kansi diorites and Lutopan diorites are both calc-alkaline high-Mg adakites with high Sr/Y ratios. Their major elements define similar variation trends in Harker diagrams, suggesting that they were probably generated from a uniform source but experienced different degree of partial melting or fractional crystallization. The Kansi diorites are characterized by LREE enrichment, HREE depletion, no Eu negative anomaly, with enrichment of Pb, Sr, Zr and Hf and depletion of Nb, Ta, and Ti. They are probably generated by the partial melting of subducted oceanic crust, followed by a certain degree of mantle interaction and crustal contamination. The highly depleted Sr-Nd-Pb-Hf isotopes of the Kansi diorites are close to the Amami Plateau basalt and tonalite, indicating the dominance of a Pacific-type MORB in the source. The Cu-Au mineralization-related Kansi diorites are characterized by high oxygen fugacities more than ΔFMQ +2, indicating quite a good potential for porphyry Cu-Au mineralization in the region. Finally, in our tectonic model, the successive generation of arc volcanic rocks and adakites in the Cebu Island are responses to the subduction and rollback of the paleo-Pacific Plate to the proto-Philippine Sea Plate (PSP) in the Early Cretaceous.

Neoarchean granite-greenstone belts and related ore mineralization in the North China Craton: An overview

Tectonic processes involving amalgamations of microblocks along zones of ocean closure represented by granite-greenstone belts (GGB) were fundamental in building the Earth's early continents. The crustal growth and cratonization of the North China Craton (NCC) are correlated to the amalgamation of microblocks welded by 2.75–2.6 Ga and ∼2.5 Ga GGBs. The lithological assemblages in the GGBs are broadly represented by volcano-sedimentary sequences, subduction-collision related granitoids and bimodal volcanic rocks (basalt and dacite) interlayered with minor komatiites and calc-alkalic volcanic rocks (basalt, andesite and felsic rock). The geochemical features of meta-basalts in the major GGBs of the NCC display affinity with N-MORB, E-MORB, OIB and calc-alkaline basalt, suggesting that the microblocks were separated by oceanic realm. The granitoid rocks display arc signature with enrichment of LILE (K, Rb, Sr, Ba) and LREE, and depletion of HFSE (Nb, Ta, Th, U, Ti) and HREE, and fall in the VAG field. The major mineralization includes Neoarchean BIF-type iron and VMS-type Cu-Zb deposits and these, together with the associated supracrustal rocks possibly formed in back-arc basins or arc-related oceanic slab subduction setting with or without input from mantle plumes. The 2.75–2.60 Ga TTG rocks, komatiites, meta-basalts and metasedimentary rocks in the Yanlingguan GGB are correlated to the upwelling mantle plume with eruption close to the continental margin within an ocean basin. The volcano-sedimentary rocks and granitoid rocks in the late Neoarchean GGBs display formation ages of 2.60–2.48 Ga, followed by metamorphism at 2.52–2.47 Ga, corresponding to a typical modern-style subduction-collision system operating at the dawn of Proterozoic. The late Neoarchean komatiite (Dongwufenzi GGB), sanukitoid (Dongwufenzi GGB and Western Shandong GGB), BIF (Zunhua GGB) and VMS deposit (Hongtoushan-Qingyuan-Helong GGB) have closer connection to a combined process of oceanic slab subduction and mantle plume. The Neoarchean cratonization of the NCC appears to have involved two stages of tectonic process along the 2.75–2.6 Ga GGB and ∼2.5 Ga GGBs, the former involve plume–arc interaction process, and the latter involving oceanic lithospheric subduction, with or without arc-plume interaction.

Petrogenesis and tectonic implications of ∼130 Ma diabase dikes in the western Tethyan Himalaya (western Tibet)

Early Cretaceous mafic dikes are widely exposed in the eastern Tethyan Himalaya. However, coeval mafic rocks are limited in the western Tethyan Himalaya. We report the major and trace elements and zircon U–Pb ages of the newly discovered diabase dikes in the Zhongba area in the western Tethyan Himalaya. The ICP-MS zircon U–Pb analyses from one representative diabase dike yields a crystallization age of 129.8±1.3Ma (MSWD=6.7). These mafic dikes are characterized by high concentrations of TiO2 (2.81–3.27%), Fe2O3T (10.79–12.14%), and MgO (5.11–6.71%). The chondrite-normalized diagram indicates the enrichment of LREE and the depletion of HREE. The primitive mantle-normalized trace element patterns do not display negative anomalies of Nb, Ta, and Ti. These characteristics imply an OIB affinity. Trace element modelling (Sm/Yb vs. Sm) suggests that the dikes originated from an enriched mantle source region, composed primarily of garnet and spinel (spinel<garnet), with a low degree of partial melting (4–8%). Comparing with coeval OIB-affinity mafic rocks from the areas of Gyangze, Kangma, Cona, and Laguila, we propose that the Cretaceous mafic rocks were generated in an extensional setting along a passive continental margin and were derived from similar magma sources with some crustal contamination in the case of the Zhongba intrusives. Because of the similar geochemical characteristics, ages and tectonic settings, of the Zhongba diabase rocks and other volcanic rocks from the Kerguelen LIP, we suggest that the Zhongba diabase rocks are likely also part of the Kerguelen LIP.

Skarn-mineralized porphyry adakites in the Harlik arc at Kalatage, E. Tianshan (NW China): Slab melting in the Devonian-early Carboniferous in the southern Central Asian Orogenic Belt

The geodynamic control of mineralization in the accretionary evolution of the Central Asian Orogenic Belt (CAOB) has long been controversial. Here we report new field, geochemical and geochronological data on recently defined porphyry and skarn-type ore deposits (Devonian-Early Carboniferous) in the Kalatage area in the middle of the Harlik-Dananhu arc, Eastern Tianshan, NW China in the southern CAOB, with the aim of better understanding the accretionary tectonics and genesis of porphyry and skarn-type mineralization. The Yudai porphyry Cu-(Au) deposits and the Xierqu skarn Cu-Fe-(Au) deposits are closely associated with Middle Devonian adakitic diorite porphyries (382–390Ma), which are calc-alkaline and characterized by high Na2O/K2O ratios and Sr contents (310–1020ppm), strong depletion of HREE (e.g., Yb=0.80–1.44ppm) and Y (7.68–14.50ppm), and all enriched in Rb, Sr, Ba, K and depleted in Nb and Ti. They are characterized by distinctive Eu positive anomalies, high Na2O contents and MORB-like Sr and Nd isotope signatures (high εNd(t)=+6.1to+7.0 and low (87Sr/86Sr)i=0.70412–0.70462). These adakites most likely formed by melting of a young/hot subducted oceanic slab, and adakites in general are important carriers of porphyry Cu±(Au) deposits. Early Carboniferous adakites in the Tuwu area south of Kalatage are known to have similar features. Therefore, skarn-mineralized porphyry adakites get younger from north to south, suggesting southward migration of the Harlik-Dananhu arc from 390Ma to 322Ma. These data indicate that partial melting of hot (and/or young) oceanic crustal slabs were an important mechanism of accretionary crustal growth and mineralization in the southern CAOB.

“Normal” to adakite-like arc magmatism associated with the El Abra porphyry copper deposit, Central Andes, Northern Chile

The El Abra porphyry copper deposit belongs to the Late Eocene—Early Oligocene metallogenic belt of northern Chile, which host several world-class porphyry copper deposits. Our previous geochronological work done on this deposit provides the temporal framework for petrological data interpretation. The magmatic history of the El Abra deposit lasts for ~8.6 Ma and can be divided into two stages. An early period, from about 45 to ~38.7 Ma, dominated by diorites and quartz monzodiorites with “normal” (non-adakite) arc geochemistry and a late period, with rocks younger than ~38.7 Ma that developed adakite-like geochemistry, where equigranular granodiorites are the volumetrically dominant rock type (e.g., Clara granodiorite ~38 Ma). These granodiorites are then intruded by leucocratic porphyry dikes and aplites. Most copper mineralization is associated with multiple intrusions of these younger porphyritic rocks, described as the El Abra porphyry unit, and emplaced over a ~1.4 Ma period, from ~37.5 to 36.1 Ma. The adakite-like geochemistry of the younger rock units (<38.7 Ma) is attested by a significant depletion in REE contents, particularly MREE and HREE (concave MREE distribution patterns), high La/Yb and Sr/Y ratios, and Na2O and Al2O3 contents, along with the absence of the Eu anomaly in normalized REE distribution patterns. The evolution of this large, long-lived magmatic system from “normal” to adakite-like arc magmatism is discussed in a tectonic context of crust overthickening due to a major orogenic episode (Incaic compressive phase). This tectonic setting may have promoted higher pressure conditions at the lower crust “hot zone” and increased the crustal residence time of derivative melts favoring extensive differentiation leading to water-rich (and oxidized?) felsic melts, where amphibole fractionation played an important role. Strontium, Nd, and Pb isotope data suggest a common mantle source for both the non-adakite and adakite-like rocks. This implies that these two groups of rocks from the El Abra porphyry copper deposit may have largely resulted from the different crustal conditions under which they have evolved.

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

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.

Genesis of oceanic island basalt in ophiolitic m&amp;eacute;lange from core area of Central Asian Orogenic Belt

The Central Asian Orogenic Belt (CAOB) is one of the largest orogens in the world and comprises island arcs, seamounts, accretionary wedges, oceanic plateaux and microcontinents accreted during the closure of the Paleo-Asian Ocean which is a long-lived ocean that existed for ~800 Ma from the late Mesoproterozoic to the Mesozoic. The CAOB extends from the Urals in the west, through Kazakhstan, northern China, and Mongolia to the eastern Russian coast. The core area, which extends from central Kazakhstan to Xinjiang (northwestern China), are divided into three main geological domains. To the northeast, (1) the Altai range is composed by Early and Late Palaeozoic units that were accreted and docked to the Siberian margin and affected by high-grade metamorphism. To the south, the convergence between the Tarim Block and several micro continents such as Yili and Central Tianshan formed the (2) Palaeozoic Tianshan Orogen. The central and northwestern parts of Central Asia display a horseshoe shape that can be followed from North Tianshan to West Junggar around the Balkash Lake area. This megastructure is termed the (3) Kazakh Orocline. Until now so many studies focused on this area. However, the architecture and orogenic processes of the CAOB are still hotly debated. Remnants of oceanic island basalts (OIB) in association with ophiolites have been identified in several accretionary complexes in core area of CAOB. The nature of occurrence, geochemical characteristics of these OIBs and ophiolites has been taken to indicate extensive crust-mantle cycling magmatism. Therefore, in this paper, we provide an overview of the salient features of twelve OIB-hosting with age of Late Neoproterozoic to Early Carboniferous in core area of CAOB, and distinguish rock association of typical seamount from ophiolitic melanges, including pillow basalt, volcanic breccia, reef limestone, olistostromes rocks, and terrestrial clastic rocks. Combined with unique structural features of seamount, we preliminary reconstruct stratigraphic framework of paleoseamount. Geochemically, the volcanic rocks from paleoseamount are mainly alkaline basalts with litte tholeiitic basalts. The alkaline basalts are characterized by LREE enrichment and HREE depletion, and no obvious Nb, Ta and Ti negative anomalies, suggesting typical OIB affinity. The Nd, Sr and Pb isotopic features indicate that the magma source have enriched components and show fluid metasomatism. In short, this are consistent with geochemical features of Louisville, Ontong Java and Hawaiian seamounts which are related to mantle plume, but not develop high magnesium picrite and komatitic basalt. The OIB in core area of CAOB are at least two possible sources: (1) seamount fragments were scraped and retained in the accretionary wedge; (2) partial melting of mantle metasomatism rocks. However, we can not rule out the possibility of partial melting of subducted oceanic crust and terrigenous sediments. Therefore, the source of OIB in core area of CAOB are very complex and show crust-mantle interaction. This is similar to mantle plume activity and seamounts evolution in modern ocean.

Paleoproterozoic arc basalt-boninite-high magnesian andesite-Nb enriched basalt association from the Malangtoli volcanic suite, Singhbhum Craton, eastern India: Geochemical record for subduction initiation to arc maturation continuum

The Singhbhum Craton of eastern India preserves distinct signatures of ultramafic-mafic-intermediate-felsic magmatism of diverse geodynamic affiliations spanning from Paleo-Mesoarchean to Proterozoic. Here we investigate the 2.25Ga Malangtoli volcanic rocks that are predominantly clinopyroxene- and plagioclase-phyric, calc-alkaline in nature, display basalt-basaltic andesite compositions, and preserve geochemical signatures of subduction zone magmatism. Major, trace and rare earth element characteristics classify the Malangtoli volcanic rocks as arc basalts, boninites, high magnesian andesites (HMA) and Nb enriched basalts (NEB). The typical LILE enriched-HFSE depleted geochemical attributes of the arc basalts corroborate a subduction-related origin. The boninitic rocks have high Mg# (0.8), MgO (>25wt.%), Ni and Cr contents, high Al2O3/TiO2 (>20), Zr/Hf and (La/Sm)N (>1) ratios with low (Gd/Yb)N (<1) ratio, TiO2, and Zr concentrations. The HMA samples are marked by moderate SiO2 (>54wt.%), MgO (>6wt.%), Mg# (0.47) with elevated Cr, Co, Ni and Th contents, depleted (Nb/Th)N, (Nb/La)N, high (Th/La)N and La/Yb (<9) ratio, moderate depletion in HREE and Y with low Sr/Y. The NEBs have higher Nb contents (6.3–24ppm), lower magnitude of negative Nb anomalies with high (Nb/Th)pm=0.28–0.59 and (Nb/La)pm=0.40–0.69 and Nb/U=2.8–34.4 compared to normal arc basalts [Nb=<2ppm; (Nb/Th)pm=0.10–1.19; (Nb/La)pm 0.17–0.99 and Nb/U=2.2–44 respectively] and HMA. Arc basalts and boninites are interpreted to be the products of juvenile subduction processes involving shallow level partial melting of mantle wedge under hydrous conditions triggered by slab-dehydrated fluid flux. The HMA resulted through partial melting of mantle wedge metasomatized by slab-dehydrated fluids and sediments during the intermediate stage of subduction. Slab-melting and mantle wedge hybridization processes at matured stages of subduction account for the generation of NEB. Thus, the arc basalt-boninite-HMA-NEB association from Malangtoli volcanic suite in Singhbhum Craton preserves the signature of a complete spectrum of Paleoproterozoic active convergent margin processes spanning from subduction initiation to arc maturation.

Geology, mineralogy and evolution of iron skarn deposits in the Zanjan district, NW Iran: Constraints from U-Pb dating, Hf and O isotope analyses of zircons and stable isotope geochemistry

The Zanjan iron skarn deposits are located in the northwest Central Iranian terrane, along the Soltaniyeh mountains. The deposits are hosted by late Proterozoic to late Cambrian carbonate rocks (Soltaniyeh, and Mila Formations) and by Permian (Ruteh Formations), intruded by the Arjin granite, the Khakriz granodiorite and the Inche-Rahbari quartzmonzonite (53±1Ma to 55±0.4Ma, early Eocene). These granitoids show high-K calc-alkaline, metaluminous affinity and have similar geochemical features, including strong enrichment of LILEs and LREEs, and depletion in high field strength elements (HFSEs) and HREE. The Arjin, Khakriz and Inche-Rahbari granitoid samples show zircon εHf(t) values of −0.7 to +5.8 (TDM2=1052–1640Ma) and δ18O values of +5.55‰ to +7.42‰ suggesting partial melting of older crustal material then mixing with juvenile mantle-derived melts. The thermal anomaly induced by underplating mafic melts resulted in partial melting of continental crust and formation of Arjin, Khakriz and Inche-Rahbari magmas emplaced during several magmatic pulses at 53–55Ma.The Zanjan iron skarn deposits are magnesian (Arjin and Inche-Rahbari) and calcic (Khakriz) with endoskarn and exoskarn facies. In these skarn facies forsterite, garnet and pyroxene comprise the main prograde skarn minerals, whereas magnetite, serpentine, amphiboles of the tremolite-ferroactinolite series (sometimes asbestiform), epidote, talc and calcite are the principal retrograde skarn minerals. Iron exoskarns formed at the contacts of the intrusions parallel to the fabric of the host rocks and in networks of crosscutting veins and veinlets. The assemblage of ore minerals consists of magnetite, hematite (specularite), pyrite, chalcopyrite and bornite; in addition, minor galena and sphalerite occur in the Khakriz deposit. The sulfur isotopic compositions in the Arjin deposit range from +15.99 to +18.35‰ and in the Inche-Rahbari and Khakriz deposits from +2.96 to 5.30‰. The oxygen and carbon isotope ratios of the fresh or partly altered limestones from Soltaniyeh and Mila Formations reveal δ13C values of −0.64 to +0.03‰ and δ18O composition of +13.12 to +24.41‰ typical of marine carbonate. The isotopic compositions of the hydrothermal calcite (skarn calcite) range from −5.32 to −2.01‰ for carbon isotope and 3.29 to 20.67‰ for oxygen isotope. Complex water-rock interaction between the ore-bearing magmatic fluids and sedimentary wall rocks was responsible for sulfur, oxygen and carbon mixing. Field observations, mineral paragenesis, and stable isotopic signatures in these deposits are consistent with ore mineralization formed by the interaction of magmatic fluids with both cooler meteoric water and carbonate host rocks.

Sr-Nd-Pb isotopic compositions of the lower crust beneath northern Tarim: insights from igneous rocks in the Kuluketage area, NW China

The composition of lower crust of the Tarim Craton in NW China is essential to understand the petrogenesis of the ~290–275 Ma Tarim basalts and associated intermediate-felsic rocks. However, it remains poorly constrained because extremely sparse granulite terrains or granulite xenoliths have been found in the Tarim Craton. New trace element and Sr-Nd-Pb isotopic data are reported for the Neoarchean and Neoproterozoic igneous rocks widely distributed in the northern margin of the Tarim Craton. The Neoarchean granitic gneisses show fractionated REE (rare earth element) patterns [(La/Yb) N = 12–58, YbN = 10.6–36] with pronounced negative Nb-Ta and Ti anomalies. These features, together with negative eNdi (−0.7 to −3.2) suggest that they were derived from melting of mafic lower crust. The Neoproterozoic biotite granodiorites are strongly depleted in HREE with (La/Yb) N up to 55. They are characterized by high Sr (671–789 ppm) but very low Y (7.10–8.06 ppm) and Yb contents (0.47–0.58 ppm), showing typical features of adakitic rocks. The samples with different SiO2 contents display identical 87Sr/86Sri (0.7101–0.7103), eNdi (−14.1 to −15.7) and Pb isotopes (208Pb/204Pbi = 36.94–37.07). These features together with arc-like trace element patterns suggest that they were derived from melting of thickened lower crust. In comparison, the Neoproterozoic hornblende-biotite granodiorites have similar trace element compositions except for weaker depletion in HREE and have lower 87Sr/86Sri (0.7078) and initial Pb isotopes, and higher eNdi (−12.3 to −12.7). This suggests that they were formed by melting of old lower continental crust at a shallower depth than the biotite granodiorites. These rocks were derived from the lower crust, thus providing valuable information on the nature of the lower crust beneath northern Tarim. Combined with published data, the 87Sr/86Sri, eNdi, 206Pb/204Pbi and eHfi of the northern Tarim lower crust ranges from 0.7055 to 0.7103, from −12 to −17, from 16.20 to 16.65, and from −7 to −19, respectively, at ~785 Ma. These data also suggest vertical compositional heterogeneity of the northern Tarim lower crust.

Rare earth elements of shallow-water articulated brachiopods: A bathymetric sensor

Variations in environmental physicochemical parameters result in corresponding changes in seawater rare earth element (REE) distributions. For an archive to be utilized in paleoceanographic and paleoenvironmental reconstructions, it should record these variations. Therefore, an archive's proxy potentiality should be assessed analogous to modern oceanic settings.Articulated brachiopods of several shallow water localities (<500m) of different water masses were evaluated for their REE contents. The affects of oceanographic factors such as depth, salinity, and temperature were investigated for REE uptake by the shells. Uptake of REE by dorsal and ventral valves of articulated shells are unequivocal, whereas shells of Thecideidines, the umbo region and primary layer of the Terebratulids are anomalous, and should be avoided. Water temperature and salinity appear to have minor to moderate impacts on the partition coefficients of REEs between seawater and brachiopod calcite.The mean REESN pattern of shallow-water brachiopods is significantly different from that of their deep-water counterparts, with lower enrichments in LREEs and MREEs, and depletion in HREEs. Depth exercises a major control on the LREE and MREE fractionation with a milder one on HREE fractionation. The Ce/Ce* anomaly is not affected by either of these parameters in shallow-water settings, but in deep settings the depth effect dominates. Due to the importance of depth on REE fractionation in the shells from shallow settings, their LREESN to HREESN ratios (L:H) were divided into six discrete intervals for the bathymetric sensor. The bathymetric sensor was applied to some fossils and whole rock in order to estimate their paleodepths. The evaluation was successful in placing the Permian Gyanyima, the Pennsylvanian Naco, Boggy and the Silurian Chicotte, Becscie and Jupiter Formations in shallow depths (<15–50m), which is consistent with their paleoenvironmental settings.

Petrogenesis of metamorphosed Paleoproterozoic, arc-related tonalites, granodiorites and coeval basic to intermediate rocks from southernmost Brazil, based on elemental and isotope geochemistry

In southern Brazil, three associations of metamorphosed tonalites and granodiorites that are compositionally similar to tonalite–trondhjemite–granodiorite (TTG) or adakitic associations have been identified in the Arroio dos Ratos Complex (ARC) Paleoproterozoic magmatism. The metatonalites of Association 1 (A1; 2148±33Ma) have a well-developed fabric, compatible with strong solid-state deformation. The metatonalites and metagranodiorites of Association 2 (A2; 2150±28Ma) are intrusive in A1 and have a similar composition, but are less deformed, and their primary structures are partly preserved. Both associations display contemporaneity relations with basic to intermediate magmas. Association 3 (A3; 2077±13Ma) is represented by tonalitic to granodioritic gneisses, without any associated basic to intermediate magmatism, and its main characteristic is the banding that resulted from strong solid-state deformation. Partial melting features are locally present in A3. The geochemical compositions of the three associations are similar and indicate sources related to a continental magmatic arc environment. The 87Sr/86Sr(i) ratios (between 0.701 and 0.703), positive ƐNd(t) values (+1.45 to +5.19), and TDM ages close to the crystallization ages indicate juvenile sources for the A1 and A2 associations. The A3 rocks have a 87Sr/86Sr(i) ratio of 0.715, an ƐNd(t) value of +0.47 and a TDM age that is close to the crystallization age, indicating a source composition different from those of the other associations. The Pb isotope ratios of A1 and A2 are similar and compatible with the evolution of mantle and orogen (208Pb/204Pb=37.3–37.6; 207Pb/204Pb=15.62–15.65; 206Pb/204Pb=18.0–18.2). The Pb isotope ratios of A3 differ from A1 and A2, indicating a more Th-poor source (208Pb/204Pb=37.1; 207Pb/204Pb=15.64; 206Pb/204Pb=18.5). The geochemistry of associations A1 and A2 suggests a juvenile source with contamination by crustal material. However, the Sr–Nd–Pb isotope signature of this contaminant is similar to that of the source material that originated these associations. This may be the crust generated in the magmatic arc, which is compatible with the geochronological results. The dataset points to the occurrence of self-cannibalism processes in the generation of the ARC rocks. The similar chemical composition and ƐNd(t) values of A3 relative to A1 and A2 indicate that the A3 source is similar to the one that generated the tonalitic and granodioritic rocks of A1 and A2. However, the slightly lower 208Pb/204Pb, and higher 206Pb/204Pb and 87Sr/86Sr(i) ratios indicate that the A3 association has also the addition of a distinct crustal source. The A3 association high values of the parameter 87Sr/86Sr(i) and its Pb isotope signature indicate a source with high Rb and U, and low Th contents. Such features, and moreover the depletion of HREE combined with TDM values near the igneous age, suggest that the source for A3 may be juvenile arc sediments. The greater degree of crustal contribution, the lack of associated basic to intermediate rocks, and the younger age possibly mark the more mature or late stages of the arc. The major and trace elements, as well as the isotope data obtained in this study suggest that melting of a metasomatized mantle wedge can be the process that generated the ARC basic to intermediate rocks (A1 and A2). The generation of tonalitic and granodioritic rocks with adakitic characteristics (i.e., the depletion of heavy rare earth elements in tonalitic and granodioritic rocks of A1, A2 and A3) may have arisen from the melting of a garnet-rich source or from fractional crystallization of ARC basic to intermediate magmas, which in time increased crustal assimilation under high-pressure conditions. The crustal garnet-rich source could be the basic rocks newly placed at the base of the crust, derived from partial melting of metasomatized mantle. The remobilization of this material by partial melting may have generated tonalitic and granodioritic liquids depleted in heavy REE due to the presence of garnet in the residue. The three associations display microstructures that indicate two episodes of recrystallization, one of a higher temperature and one of a lower temperature. The age of the high-temperature structure remains under discussion and may be attributed to a Paleoproterozoic metamorphic event. The low-temperature event is compatible with the temperature conditions observed in adjacent areas, in the host rocks of the Neoproterozoic post-collisional granitoids that have been emplaced along the Southern Brazilian Shear Belt (SBSB). Zircon crystals with Paleoproterozoic igneous cores exhibit a metamorphic overgrowth at 635±6Ma, compatible with the crystallization ages of the SBSB granitoids.

Genesis and tectonic setting of the giant Diyanqin'amu porphyry Mo deposit in Great Hingan Range, NE China: Constraints from U–Pb and Re–Os geochronology and Hf isotopic geochemistry

The Great Hingan Range is located in the eastern sector of the Central Asia Orogenic Belt, and is the second important Mo province in China, next to the Qinling–Dabie orogenic belt. Previous studies concluded that all the Mo deposits (Mo-only or Mo-dominated polymetallic) in the area were formed in the Mesozoic, and related either to the syn- to post-collisional setting following the closure of the Paleo-Asia Ocean, or to the westward subduction of the Pacific plate. However, the fluids forming the giant Diyanqin'amu porphyry Mo deposit in the area disagree with the features of collision-type Mo deposits, but accord with those of the subduction- or continental arc-type. The Mo mineralization is hosted in the Late Jurassic volcanic rocks and genetically related to the buried porphyritic granite and aplitic granite. Two porphyritic granite samples yield LA-ICP-MS zircon weighted mean 206Pb/238U ages of 158.7±0.8Ma (MSWD=0.64, 2σ) and 158.0±0.9Ma (MSWD=0.38, 2σ), respectively. The aplitic granite yields a zircon U–Pb weighted mean 206Pb/238U age of 156.9±1.1Ma (MSWD=0.48, 2σ), slightly younger than the porphyritic granite. Seven molybdenite samples from the ores yield Re–Os isotope ages of 156.1±2.2 to 158.1±4.3Ma. These ages constrain that the Diyanqin'amu Mo system was formed in the period of 159–156Ma. The granites at the deposit have high contents of SiO2, K2O and Al2O3, and low contents of TiO2, MgO and CaO, showing a peraluminous high-K calc-alkaline affinity. These granites and the host volcanic rocks are characterized by enrichment of K, Rb, U, Pb, Th and LREE, and depletion of Nb, Ta, Ti and HREE, with negative Eu anomaly, which are similar to magmatic rocks in the Andean arc. Zircon grains from the granites show positive εHf(t) values of 4.2–9.0, with TDM2(Hf) ages of 0.63–0.94Ga. These geochemical data imply that the granites at Diyanqin'amu are highly fractionated I-type, formed in a continental arc generated by the southeastward subduction of the Mongol–Okhotsk oceanic plate.

Petrogenesis and tectonic implications of early Carboniferous alkaline volcanic rocks in Karamay region of West Junggar, Northwest China

ABSTRACTZircon U–Pb geochronological and geochemical analyses are reported for a suite of the early Carboniferous volcanic rocks from West Junggar (Northwest China), southern Central Asian Orogenic Belt (CAOB), with the aim to investigate the sources, petrogenesis, and tectonic implications. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb analysis from an andesite yielded concordant weighted mean 206Pb/238U age of 345 ± 3 Ma, indicating the presence of early Carboniferous volcanic rocks in West Junggar. The early Carboniferous volcanic rocks consist of basalt, basaltic andesite, and andesite. Geochemically, all the samples bear the signature of ocean island basalt (OIB), and are characterized by alkaline affinity with minor variations in SiO2 compositions (45.13–53.05 wt.%), high concentrations of Na2O + K2O (5.08–8.89 wt.%) and TiO2 (1.71–3.35 wt.%), and LREE enrichment and HREE depletion ((La/Yb)N = 7.1–12.4), with weak Eu anomalies (Eu/Eu* = 0.9–1.1) and no obvious Nb, Ta, and Ti negative anomalies. These features suggest that the early Carboniferous volcanic rocks were derived from an OIB-related source that consists of oceanic lithosphere with ~1–3% degree partial melting of garnet lherzolite. From these observations, in combination with previous work, we conclude that the early Carboniferous alkaline volcanic rocks in Karamay region formed by upwelling of asthenospheric mantle through a slab window in a forearc setting during consumption of the West Junggar Ocean. Meanwhile, seamounts, which formed in the Late Devonian and were accreted and subducted in Karamay arc, also brought geological effects in the subduction zone.

Geochemistry, zircon U–Pb dating and Hf isotopies composition of Paleozoic granitoids in Jinchuan, NW China: Constraints on their petrogenesis, source characteristics and tectonic implication

Granitoids are widely distributed in Jinchuan at the southwestern margin of the North China plate, which is also an important area of mineral deposits. The research subject of this article are two Paleozoic granitoids, a cataclastic syenogranite and a granodiorite porphyry. This study presents whole rock geochemistry and zircon U–Pb–Hf isotope data for the two granitoids to determine their petrogenesis, source characteristics and tectonic significance. The cataclastic syenogranite is characterized by metaluminous composition with high potassium, and LaN/YbN from 39 to 48. The composition with strong negative Eu anomalies and Zircon saturation temperatures (TZr) from 947 to 1072°C classify this intrusion as an A-type granite. The granodiorite porphyry is metaluminous with high sodium, sub-alkaline, LaN/YbN ratios from 27 to 32. These I-type intrusions have no Eu anomalies and TZr ranges from 818 to 845°C. Both the cataclastic syenogranite and granodiorite porphyry show enrichment of LREE and LILE and depletion of HREE and HFSE, except Hf and Zr. Using single zircon LA-ICP-MS U–Pb dating, the emplacement age of the cataclastic syenogranite and granodiorite porphyry are determined at 433.4±3.7Ma and 361.7±4.6Ma, respectively. Zircons from the cataclastic syenogranits have uniform negative εHf(t) values (−11±0.5 to −9±0.5), implying the involvement of an old Palaeoproterozoic crustal source in magma genesis. The zircons from the granodiorite porphyry have εHf(t) values that range from −8±1.0 to +10±0.6, suggesting heterogeneous source materials involving both juvenile and ancient crust reworked crustal components. Based on the geological significance of granites at the southwestern margin of the North China plate, the closure of the North Qilian Ocean occurred at ∼444Ma. Geochemical features suggest that the cataclastic syenogranite and granodiorite porphyry formed in an intraplate extensional and compressional setting, respectively. Hence after a period of extensional post-collisional intraplate evolution, strong compressive forces affected the area.

Late Neoarchean arc magmatism and crustal growth associated with microblock amalgamation in the North China Craton: Evidence from the Fuping Complex

The Fuping, Wutai, and Hengshan Complexes in the North China Craton preserve imprints of widespread late Neoarchean magmatism. Here, we report results from systematic petrology, mineral chemistry, whole-rock major, trace and platinum-group element geochemistry, zircon U–Pb geochronology and Hf–O isotopes from the Yangmuqiao mafic–ultramafic intrusion and coeval tonalite–trondhjemite–granodiorite (TTG) gneiss from the Fuping Complex. The mafic–ultramafic intrusion is composed of pyroxene hornblendites, hornblendites, and minor harzburgites. The salient geochemical features of the mafic–ultramafic intrusion and the Fuping TTG gneiss display subduction-related island arc signature, such as fractionated REE patterns with elevated LREE, enrichment of LILE (K, Rb, and Ba) and LREE (La and Ce), and depletion of HFSE (Nb, Ta, Zr, and Hf) and HREE. The chemistry of the clinopyroxene and chromite in the pyroxene hornblendites shows affinity with Alaskan-type mafic–ultramafic intrusions.Zircons from the pyroxene hornblendite yield weighted mean 207Pb/206Pb age of 2514±15 Ma, and those in the Fuping TTG gneiss show mean age of 2513±13Ma. Zircon Hf and O isotopic compositions are used as magma source and crustal evolution indicators. Zircon grains in the pyroxene hornblendite display positive εHf(t) values (2.6–6.7), Neoarchean TDM (2570–2723Ma), and their δ18O values vary from 3.8‰ to 7.0‰ (average 6.2‰). Zircons in the TTG gneiss show εHf(t) values in the range of −1.8 to 4.9, TDM of 2637–2888Ma, and δ18O values of 4.1‰–6.7‰ (average of 6.1‰). These results suggest that the parental magma of the late Neoarchean magmatism in the Fuping area was dominantly extracted from the depleted mantle and contaminated to different degrees by crustal components. The pyroxene hornblendites have obviously higher IPGE contents (ΣIPGE=1.69–2.39ppb) and lower Pd/Ir ratios (5.97–6.28) than those in the hornblendites (ΣIPGE=0.56–0.72ppb, Pd/Ir=6.48–15.25), suggesting different compatibility of IPGE and PPGE during the fractional crystallization. The study area is located at the western segment of the Fuping Complex and the eastern periphery of the Wutai greenstone belt. We propose that the late Neoarchean arc magmatism recorded by the Yangmuqiao mafic–ultramafic intrusion and the Fuping TTG gneiss is related to the subduction–collision process of the Ordos and the Qianhuai microblocks along the zone of ocean closure represented by the Wutai greenstone belt.

Dating prograde fluid pulses during subduction by in situ U–Pb and oxygen isotope analysis

The Dora-Maira whiteschists derive from metasomatically altered granites that experienced ultrahigh-pressure metamorphism at ~750 °C and 40 kbar during the Alpine orogeny. In order to investigate the P–T–time–fluid evolution of the whiteschists, we obtained U–Pb ages from zircon and monazite and combined those with trace element composition and oxygen isotopes of the accessory minerals and coexisting garnet. Zircon cores are the only remnants of the granitic protolith and still preserve a Permian age, magmatic trace element compositions and δ18O of ~10 ‰. Thermodynamic modelling of Si-rich and Si-poor whiteschist compositions shows that there are two main fluid pulses during prograde subduction between 20 and 40 kbar. In Si-poor samples, the breakdown of chlorite to garnet + fluid occurs at ~22 kbar. A first zircon rim directly overgrowing the cores has inclusions of prograde phlogopite and HREE-enriched patterns indicating zircon growth at the onset of garnet formation. A second main fluid pulse is documented close to peak metamorphic conditions in both Si-rich and Si-poor whiteschist when talc + kyanite react to garnet + coesite + fluid. A second metamorphic overgrowth on zircon with HREE depletion was observed in the Si-poor whiteschists, whereas a single metamorphic overgrowth capturing phengite and talc inclusions was observed in the Si-rich whiteschists. Garnet rims, zircon rims and monazite are in chemical and isotopic equilibrium for oxygen, demonstrating that they all formed at peak metamorphism at 35 Ma as constrained by the age of monazite (34.7 ± 0.4 Ma) and zircon rims (35.1 ± 0.8 Ma). The prograde zircon rim in Si-poor whiteschists has an age that is within error indistinguishable from the age of peak metamorphic conditions, consistent with a minimum rate of subduction of 2 cm/year for the Dora-Maira unit. Oxygen isotope values for zircon rims, monazite and garnet are equal within error at 6.4 ± 0.4 ‰, which is in line with closed-system equilibrium fractionation during prograde to peak temperatures. The resulting equilibrium ∆18Ozircon-monazite at 700 ± 20 °C is 0.1 ± 0.7 ‰. The in situ oxygen isotope data argue against an externally derived input of fluids into the whiteschists. Instead, fluid-assisted zircon and monazite recrystallisation can be linked to internal dehydration reactions during prograde subduction. We propose that the major metasomatic event affecting the granite protolith was related to hydrothermal seafloor alteration post-dating Jurassic rifting, well before the onset of Alpine subduction.