Primitive Mantle-normalized Trace Element Research Articles

Ordovician appinites in the Wugongshan Domain of the Cathaysia Block, South China: Geochronological and geochemical evidence for intrusion into a local extensional zone within an intracontinental regime

Abstract Palaeozoic mafic igneous rocks are potentially significant in constraining the tectonic nature and evolution of the Kwangsian Orogeny in the eastern South China Block, yet they have received little attention because of their limited outcrop. Geochemistry and geochronology was carried out on newly identified Ordovician ultramafic–mafic appinites in the Wugongshan Domain of the Cathaysia Block. Seven appinite samples yielded 206Pb/238U crystallisation ages ranging from 452 ± 4 Ma to 473 ± 3 Ma. Abundant 480–500 Ma zircon xenocrysts and/or inherited zircons were found in the appinites, possibly indicating an earlier magmatism episode in the early Palaeozoic period. The Wugongshan appinites are ultramafic to mafic in composition, and the ultramafic rocks display features of cumulates (high concentrations of MgO, Fe2O3t, Cr, Ni, and low concentrations of total alkali and total rare earth elements [REE]). The appinite geochemistry displays: relatively flat chondrite normalised REE patterns with slight enrichment in light REE and weak negative Eu anomalies; enrichment in large-ion lithophile elements (such as Rb, K), and weak depletion in Nb–Ta in primitive mantle normalised trace element patterns. We suggest that the Wugongshan appinites likely originated from an ancient metasomatised mantle, and that crustal assimilation, fractional crystallisation (AFC), magma mingling and hydration were involved in the petrogenetic process, based on the combination of geochemistry, crust-like bulk Sr, Nd and zircon Hf isotopic compositions (eNd (t) = − 8.2 to − 3.2, initial 87Sr/86Sr ratios of 0.7067–0.7144, zircon eHf (t) values peaking at − 9 to − 3) and regional geological data. Further considering the alignment and chronology of the appinites, we suggest that the appinitic magmas probably were emplaced along the Jiangshan–Shaoxing Fault in a local extensional zone in an intracontinental regime in the early Palaeozoic.

Neoarchean continental growth through arc magmatism in the Nilgiri Block, southern India

The formation and growth of continental crust in the Archean have been evaluated through models of subduction–accretion and mantle plume. The Nilgiri Block in southern India exposes exhumed Neoarchean lower crust, uplifted to heights of ∼2500m above sea level along the north western margin of the Peninsula. Major lithologies in this block include charnockite with or without garnet, anorthosite-gabbro suite, pyroxenite, amphibolite and hornblende-biotite gneiss (TTG). All these rock types are closely associated as an arc magmatic suite, with diffuse boundaries and coeval nature. The charnockite and hornblende-biotite gneisses (TTG) show SiO2 content varying from 64 to 73wt.%. The hornblende-biotite gneisses (TTG) are high-Al type with Al2O3 >15wt.% whereas the charnockites show Al2O3 <15wt.%. The composition of charnockite is mainly magnesian and calcic to calc-alkaline. The mafic-ultramafic rocks show composition close to that of tholeiitic series. The low values of K2O (<3wt.%), (K/Rb)/K2O (<500), Zr/Ti, and trace element ratios like (La/Yb)n/(Sr/Y), (Y/Nb), (Y+Nb)/Rb, (Y+Ta)/Rb, Yb/Ta indicate a volcanic arc signature for these rocks. The geochemical signature is consistent with arc magmatic rocks generated through oceanic plate subduction. The primitive mantle normalized trace element patterns of these rocks display enrichment in large ion lithophile elements (LILE) and comparable high field strength elements (HFSE) in charnockite and hornblende-biotite gneisses (TTG) consistent with subduction-related origin. Primitive mantle normalized REE pattern displays an enrichment in LREE in the charnockite and hornblende-biotite gneisses (TTG) as compared to a flat pattern for the mafic rocks. The chondrite normalized REE patterns of zircons of all the rock types reveal cores with high HREE formed at ca. 2700Ma and rims with low HREE formed at 2500–2450Ma. Log-transformed La/Th–Nb/Th–Sm/Th–Yb/Th discrimination diagram for the mafic and ultramafic rocks from Nilgiri displays a transition from mid-oceanic ridge basalt (MORB) to island arc basalt (IAB) suggesting a MORB source. The U–Pb zircon data from the charnockites, mafic granulites and hornblende-biotite gneisses (TTG) presented in our study show that the magma generation during subduction and accretion events in this block occurred at 2700–2500Ma. Together with the recent report on Neoarchean supra-subduction zone ophiolite suite at its southern margin, the Nilgiri Block provides one of the best examples for continental growth through vertical stacking and lateral accretion in a subduction environment during the Neoarchean.

Petrogenesis of macrocrystic and aphanitic intrusions in Mesoproterozoic diamondiferous pipe 2 kimberlite, Wajrakarur kimberlite field, eastern Dharwar craton, southern India

Mesoproterozoic Pipe 2 kimberlite intruding granitic gneisses in the Wajrakarur Kimberlite Field (WKF) of Eastern Dharwar craton, southern India, is one of the first discovered pipes from this field. The two different intrusions belonging to coherent facies are observed in this kimberlite which are texturally distinct i.e., macrocrystic (with abundant macrocrysts) as well as aphanitic (devoid of or very fewer macrocrysts). Petrographic observations give evidence of presence of veined metasomatic mantle in the source region. Distinct geochemical variation is observed between macrocrystic (Mg# 75.4-76.6, SiO 2 : 34.3-35.4 wt%, MgO: 18.7-20.4 wt%) and aphanitic samples (Mg# 74.7-75.5, SiO 2 : 30.4-31.8 wt%, MgO: ∼21 wt%), furthermore aphanitic samples are relatively more enriched in trace and rare earth elements. Geochemical studies point out that aphanitic and macrocrystic varieties of Pipe 2 kimberlite to be products of the same mantle source but have experienced different evolutionary histories. The inferred primary magma of Pipe 2 kimberlite (for both aphanitic and macrocrystic) is suggested to have a composition of ∼33 wt% SiO2, Mg# ∼75.6 and ∼460 ppm Ni. Pipe 2 primary magma can be derived by using forward melting model assuming ∼1% partial melting of a source enriched in light REE by a factor of ∼11 x chondrite and almost chondritic heavy REE with 2% residual garnet. Differences between aphanitic and macrocrystic varieties in their chondrite normalized REE abundance patterns can be explained by about 5% crystal fractionation of primary magma and not by variations in the degree of partial melting. The primitive mantle normalized trace element patterns exhibit significant negative K, Sr, P, Ti and Hf anomalies that are interpreted to be characteristic of the primary magma. Combined petrology and whole rock geochemistry including compatible and incompatible element abundances and their ratios confirm that Pipe 2 kimberlite intrusions are archetypal Group I kimberlites similar to other kimberlites of EDC and not lamproites.

Plume-lithosphere interaction in the generation of the Tarim large igneous province, NW China: Geochronological and geochemical constraints

The magmatism in the early Permian Tarim large igneous province (TLIP) in NW China is represented by basaltic lava flows in Keping and ultramafic-mafic-felsic intrusions and mafic dikes in Bachu, northwestern Tarim Craton. This paper reports new Ar-Ar dating results and chemical compositions of Keping basalts and Bachu dikes, with aims of better characterizing the timing of and mantle/crustal contribution to the TLIP. The Keping basalts yield two well-defined ⁴⁰Ar/³⁹Ar plateau ages of 287.3 ± 4.0 Ma and 287.9 ± 4.1 Ma, which, together with age data from the literature, define a magmatic event at ∼289 Ma. The intrusions and dikes in Bachu are believed to have formed at ∼279 Ma based on screened literature data. Thus, they together define two magmatic episodes. The Keping basalts, representing the earlier episode, have alkaline affinity (SiO₂ = 44.0-47.9 wt.%, Na₂O + K₂O = 3.7-4.9 wt.%), low MgO (4.3-5.9 wt.%) and high TiO₂ (3.8-5.1 wt.%) contents, showing fractionated chondrite-normalized LREE and nearly flat HREE patterns [(La/Yb)_N = 6.27-7.71; (Dy/Yb)_N = 1.36-1.48] with noticeable negative Nb and Ta anomalies in the primitive mantle-normalized trace element diagram. They have negative and relatively uniform ε_Nd(t) (−2.3 to −3.8) and low (²⁰⁶Pb/²⁰⁴Pb)_i (17.43-17.57). We argue that these “crustal signatures” cannot be attributed to crustal assimilation because neither ε_Nd(t) nor (²⁰⁶Pb/²⁰⁴Pb)_i correlates with SiO₂; rather they are more likely derived from a sub-continental lithospheric mantle (SCLM) source metasomatized by subduction-related processes. The Bachu dikes, representing the later episode and confined to the margins of the Tarim Craton, have similar MgO (3.6-5.4 wt.%) and TiO₂ (3.1-4.7 wt.%) contents to the Keping basalts, and display more fractionated REE patterns [(La/Yb)_N = 10.1-14.0; (Dy/Yb)_N = 1.79-1.99]. They have variable isotope compositions [ε_Nd(t) = −0.3-4.8, (²⁰⁶Pb/²⁰⁴Pb)_i = 17.50-18.11] and display OIB-like trace element signatures. Correlations between isotopic and trace element ratios indicate that some dikes with low ε_Nd(t) and low initial Pb isotope ratios could have been subjected to crustal assimilation. We propose a model involving plume-lithosphere interaction to account for the two discrete magmatic episodes with distinct mantle sources in the TLIP. The earlier episode was formed in response to the impact of a sub-lithospheric mantle plume at the base of the SCLM. Partial melting of the metasomatized lithospheric mantle was triggered by temperature increase due to conductive heating of the impregnating mantle plume. The later episode was generated by decompression melting of the mantle plume, as a result of deflection of the plume towards the margins of the Tarim Craton with thinner lithosphere.

Open magma chamber processes in the formation of the Permian Baima mafic–ultramafic layered intrusion, SW China

The Baima mafic–ultramafic layered intrusion of the 260-Ma Emeishan Large Igneous Province (ELIP) hosts the second largest Fe–Ti–(V) oxide deposit in the Panxi region, SW China. It is a ~1600-m-thick layered body intruded by slightly younger syenitic and granitic plutons. The intrusion includes the Lower and Upper Zones. Troctolite and olivine pyroxenite of the Lower Zone contains conformable oxide ore layers, whereas the Upper Zone consists of olivine gabbro and gabbro with abundant apatite in the higher level. The crystallization order of the silicates in the Baima intrusion is olivine→plagioclase→clinopyroxene. Fe–Ti oxides (titanomagnetite and ilmenite) crystallized after olivine, and possibly plagioclase. The oxide ores in the Lower Zone show slightly LREE enriched patterns with (La/Yb)N values between 2.0 and 6.4, and positive Eu anomalies (Eu/Eu⁎) of 1.0 to 2.7. In contrast, olivine gabbros in the Lower Zone display stronger LREE enrichments (La/YbN=7.7–14.0) and positive Eu anomalies (Eu/Eu⁎=2.8–3.3). Gabbros in the Upper Zone have REE profiles characterized by intermediate LREE enrichments with (La/Yb)N values of 3.2 to 11.2 and positive Eu anomalies of 2.1 to 3.0. Primitive mantle-normalized trace element patterns are characterized by negative La–Ce, Th, Sm and positive Nb–Ta, Ba and Ti anomalies in oxide ores and negative Th–U, Zr–Hf and positive Ba, Sr and Ti anomalies in olivine gabbro and gabbro. Fo of olivine and An of plagioclase remain roughly constant from 0 to ~90m in the Lower Zone, indicating that the magma chamber was continuously filled by compositionally similar magmas during the initial stage. Three magma replenishments occurred afterwards in the upper part of the Lower Zone and the Upper Zone based on compositional reversals of plagioclase, olivine and Sr isotope. Mass balance calculations show that the Baima parental magma can produce all oxide ores under closed system conditions. A wide range of An values of plagioclase within thin sections and disequilibrium Sr isotopic compositions of plagioclase both along the stratigraphic profile (87Sr/86Sri=0.70312–0.70510) and within thin sections indicate convection and co-accumulation of cumulus plagioclase that had crystallized from different magmas. We propose that the ~1600m-thick Baima intrusion formed in an open and isotopically heterogeneous magma chamber that was periodically recharged by compositionally similar or more primitive magmas. In each replenishment cycle, the magma underwent progressive crustal contamination and fractional crystallization. Convection and sorting of crystals based on density differences result in igneous layering characterized by intervals of oxide ores, troctolite and olivine gabbro.

Sr–Nd isotope geochemistry and tectonomagmatic setting of the Dehsalm Cu–Mo porphyry mineralizing intrusives from Lut Block, eastern Iran

The Dehsalm Cu–Mo-bearing porphyritic granitoids belong to the Lut Block volcanic–plutonic belt (central eastern Iran). These rocks range in composition from gabbro-diorite to granite, with dominance of monzonites and quartz monzonites, and have geochemical features of high-K calc-alkaline to shoshonitic volcanic arc suites. Primitive mantle-normalized trace element spider diagrams display strong enrichment in large-ion lithophile elements such as Rb, Ba and Cs and depletions in some high-field strength elements, e.g., Nb, Ti, Y and HREE. Chondrite-normalized plots display significant LREE enrichments, high LaN/YbN and a lack of Eu anomaly. High Sr/Y and La/Yb ratios of Dehsalm intrusives reveal that, despite their K-rich composition, these granitoids show some resemblances with adakitic rocks. A Rb–Sr whole rock–feldspar–biotite age of 33 ± 1 Ma was obtained in a quartz monzonite sample and coincides, within error, with a previous geochronological result in Chah-Shaljami granitoids, further northwest within the Lut Block. (87Sr/86Sr)i and eNdi isotopic ratios range from 0.70481 to 0.70508 and from +1.5 to +2.5, respectively, which fits into a supra-subduction mantle wedge source for the parental melts and indicates that crustal contribution for magma diversification was of limited importance. Sr and Nd isotopic compositions together with major and trace element geochemistry point to an origin of the parental magmas by melting of a metasomatized mantle source, with phlogopite breakdown playing a significant role in the geochemical fingerprints of the parental magmas; small amounts of residual garnet in the mantle source also help to explain some trace element patterns. Geochemical features of Dehsalm porphyries and its association with Cu–Mo mineralization agree with a mature continental arc setting related to the convergence of Afghan and Lut plates during Oligocene.

Geochemistry of Huyanshan Alkaline Complex, Shanxi Province, China: Evidences for the Sources of Magma

The Huyanshan alkaline complex outcrops in the middle part of the eastern wing of the Lvliang Mountain in Shanxi Province. Due to its typical alkaline rock type, the Huyanshan alkaline complex has attracted much attention from the geological field all the time. This paper describes the characteristic of trace elements of the Huyanshan alkaline complex and discussed the scientific problems such as magma sources and substance composition. The primitive mantle-normalized trace element spider diagram shows that the Huyanshan alkaline complex are enriched in Pb, Hf, Sr, Ba, Cs, Th, U and depleted in Cr, Ni and Co. The data presented indicates that the complex has deep source characteristic and derives from enriched mantle controlled by spinel peridotite partial melting process.

Small-scale coexistence of island-arc- and enriched-MORB-type basalts in the central Vanuatu arc

We report here major, trace element and Sr–Nd–Pb isotopic data for a new set of basaltic lavas and melt inclusions hosted in Mg-rich olivines (Fo86–91) from Mota Lava, in the Banks islands of the Vanuatu island arc. The results reveal the small-scale coexistence of typical island-arc basalts (IAB) and a distinct type of Nb-enriched basalts (NEB) characterized by primitive mantle-normalized trace element patterns without high-field-strength element (HFSE) depletion. The IAB show trace element patterns with prominent negative HFSE anomalies acquired during melting of mantle sources enriched with slab-derived, H2O-rich components during subduction. In contrast, the NEB display trace element features that compare favourably with enriched-mid-ocean ridge basalt (MORB) and the most enriched basalts from the Vanuatu back-arc troughs. Both their trace element and Nd–Sr isotopic compositions require partial melting of an enriched-MORB-type mantle source, almost negligibly contaminated by slab-derived fluids (~0.2 wt%). The coexistence of these two distinct types of primitive magma, at the scale of one volcanic island and within a relatively short span of time, would reflect a heterogeneous mantle source and/or tapping of distinct mantle sources. Direct ascent of such distinct magmas could be favoured by the extensive tectonic setting of Mota Lava Island, allowing decompression melting and sampling of variable mantle sources. Significantly, this island is located at the junction of the N–S back-arc troughs and the E–W Hazel Home extensional zone, where the plate motion diverges in both direction and rate. More broadly, this study indicates that crustal faulting in arc contexts would permit basaltic magmas to reach Earth’s surface, while preserving the geochemical heterogeneity of their mantle sources.

Trace element, PGE, and Sr–Nd isotope geochemistry of the Panzhihua mafic layered intrusion, SW China: Constraints on ore-forming processes and evolution of parent magma at depth in a plumbing-system

The Panzhihua intrusion (260Ma) is part of the Emeishan Large Igneous Province (ELIP), SW China. Layered intrusions related to the ELIP have recently received much attention due to their economic potential for Ni–Cu–PGE and Fe–Ti oxide ore deposits. ELIP layered intrusions vary in composition from ultramafic to mafic, which are interpreted to have crystallised from two distinct magma series through plume-related partial melting of the mantle: (1) high-Ti basalt and (2) low-Ti basalt.We present here new trace element, PGE, and Sr–Nd isotope data for the Panzhihua intrusion in order to constrain the evolution of the parent magma from source to current chamber. REE profiles are generally characterised by flat LREE profiles with low (La/Sm)N<2 and steeper HREE with (Gd/Yb)N – general<2.5. Primitive mantle-normalised trace element profiles are characterised by negative Th–U, La–Ce and Zr–Hf anomalies. The Panzhihua intrusion is PGE-depleted relative to related volcanic rocks, with total PGE concentrations generally≪5ppb. Cu/Pd (24,000–250,000) and Pd/Pt (0.3–8.3). (87Sr/86Sr)i and εNdi value ranges are 0.7043–0.7054 and −0.61–3.71 respectively with distinct enriched values for marginal zone rocks.The Panzhihua magma is likely related to a picritic parent magma. Sr–Nd isotope data indicate a common source for the gabbroic and Fe–Ti oxide ore rocks with only minor or localised crustal contamination. The highly PGE-depleted and elevated Cu/Pd nature of the Panzhihua rocks suggests early segregation of a sulphide liquid. Positive correlation of Cr and Ni with MgO suggest fractionation of olivine and chromite. Enriched Sr–Nd isotopic ratios indicate a proportion of assimilation of crustal material at depth, which would aid S-saturation. S-saturation was likely caused by a combination of fractional crystallisation and assimilation of crustal material. Modelled fractionation of picritic parent magma, using Pt, Pd and Cr indicate that approximately 25% fractional crystallisation of 96% olivine, 4% chromite and 0.15% sulphide is required to produce the PGE and Cr depletion observed.The Panzhihua intrusion represents a feeder/plumbing system related to ELIP volcanism. Picritic magmas fractionate at depth forming the parental basaltic magmas of the Panzhihua intrusion. Furthermore, the main Fe–Ti oxide ore layer, within the section of the intrusion, is not in situ and interpreted to be emplaced along the footwall contact as a result of gravitational instability and slumping of crystal-rich slurries.

Geochronological, geochemical and Sr–Nd–Hf isotopic constraints on the origin of the Cretaceous intraplate volcanism in West Qinling, Central China: Implications for asthenosphere–lithosphere interaction

This paper reports new zircon LA-ICP-MS UPb age and Lu-Hf isotopes, laser-fusion 40Ar/39Ar age, mineral chemistry, bulk-rock major and trace element, and SrNd isotope data from intraplate volcanic rocks in the Duofuntun Area, West Qinling. It was dated at 105.8±0.9Ma (UPb Zircon) and 103±2Ma (matrix glass laser-fusion 40Ar/39Ar dating). The Duofuntun Volcanic Rocks (DVR) are dominantly composed of alkali basalts with minor basanites and basaltic andesites. Some evolved samples of the DVR underwent a variable fractionation of olivine, clinopyroxene, plagioclase, FeTi oxide, Cr-rich spinel and apatite. Chondrite-normalized rare earth element and primitive mantle-normalized trace element patterns of the DVR remarkably resemble those of oceanic island basalts (OIB). Nearly all the alkali basalts show positive Nb, Ta and Ti anomalies and negative Rb, Th and P anomalies, whereas basanites do not display positive Ti anomalies. Nearly all the samples show marked negative Rb and K anomalies relative to their neighbouring elements on a primitive mantle normalized trace element diagram. 87Sr/86Sr(i) and 143Nd/144Nd(i) ratios for all the mafic samples also show an OIB affinity, covering a range of 0.703264–0.705032 and 0.512718–0.512873, respectively. Several lines of evidence indicate that both garnet and amphibole play a key role for the genesis of the DVR. The melt segregation PT conditions in the source region, around 1371°C at 1.98–3.25GPa, combined with trace element collectively suggest a polybaric melting process during the formation of the DVR. The non-modal batch partial melting modelling result indicates the DVR were generated by small-degree polybaric partial melting of amphibole-bearing spinel peridotites (ca. 4%) and amphibole-bearing garnet peridotites (ca. 0.7%). We suggest the DVR were not formed in a plume. Rather, they originated by the interaction between the upwelling asthenosphere, which might provide both materials and heat flux, and hydrous mineral-bearing (amphibole) lithosphere during lithospheric extension.

Magma mixing in the genesis of the Kalatongke dioritic intrusion: Implications for the tectonic switch from subduction to post-collision, Chinese Altay, NW China

The Kalatongke complex, located in south Chinese Altay of the Central Asian Orogenic Belt (CAOB), consists of the ~308Ma dioritic suite intruded by the 287Ma noritic suite. The noritic suite is composed of olivine norite and hornblende norite and hosts sulfide mineralization. The dioritic suite is composed of ferrodiorite, diorite and quartz diorite.Ferrodiorites have 43.6 to 51.8wt.% SiO2, 8.2 to 12.1wt.% Fe2O3T, 2.87 to 4.86wt.% MgO, with Mg# ranging from 42 to 53. They have highly variable Al2O3 (17.4–20.7wt.%), Na2O (2.78–4.50%), K2O (0.58–3.70%), Sr (280–1300ppm) and Ba (219–3100ppm). Diorites and quartz diorites have 15.2–19.6wt.% Al2O3, 239–890ppm Sr and170–1100ppm Ba, 11.2–25.5ppm Y and1.2–2.2ppm Yb. All rocks have LREE enriched REE patterns and do not display Eu anomalies. They have primitive mantle-normalized trace element patterns enriched in large ion lithosphile elements (LILE) (e.g. Ba and Sr) and depleted in Ti, Nb and Ta.Both ferrodiorites and diorites have relatively constant initial 87Sr/86Sr ratios of 0.7037 to 0.7040 but highly variable εNd(t) values of +4.5 to +11.6, whereas quartz diorites have higher initial 87Sr/86Sr ratios (0.7056) and lower εNd(t) values (+0.5). Plagioclase from ferrodiorites, diorites and quartz diorites has similar initial 87Sr/86Sr ratios of 0.7015 to 0.7055.The dioritic suite formed from a mixed magma of an evolved mantle-derived basaltic magma by partial melting of subduction-modified mantle source in response to slab breakoff and a crustal-derived granitic magma at ~308Ma. Before mixing with granitic magmas, basaltic magmas underwent fractional crystallization of olivine to form ferrodiorite. The mixed magma also formed diorite and quartz diorite combining fractionation clinopyroxene, amphibole and plagioclase. The noritic suite formed from a mantle-derived high-Mg magma in a post-collisional environment. Both the dioritic and noritic suites in Kalatongke suggest the switch from subduction to extension and the final collision between the Junggar Block and Chinese Altay at ~308Ma.

Formation age and tectonic environment of the Gantaohe Group, North China Craton: Geology, geochemistry, SHRIMP zircon geochronology and Hf-Nd isotopic systematics

The Gantaohe Group is an important early Precambrian unit in the Trans-North China Orogen, North China Craton, and is mainly composed of greenschist-facies metabasalt, meta-sandstone and dolomitic marble. We report whole-rock geochemical compositions and SHRIMP zircon ages as well as LA-ICP-MS Hf-in-zircon isotopeic analyses for metabasalts from the Gantaohe Group. SHRIMP dating yielded a weighted mean 207Pb/206Pb age of 2087±16 Ma (MSWD=1.3) for magmatic zircons, but there are also abundant ca. 2.5 Ga inherited zircon xenocrysts. The magmatic zircons shows a large ɛ Hf(t) variation in ɛ Hf(t) from −7.17 to +0.45, suggesting an isotopically highly heterogeneous source for the metabasalt. Chemically all samples show no distinct Zr or Hf anomalies, and some samples show no Nd or Ta anomalies in a primitive mantle-normalized trace element variation diagram, and their whole-rock ɛ Nd(t) values range from 4.0 to −0.8. We suggest that the basalt is formed by partial melting of a depleted mantle source, followed by significant crustal contamination. Field observations, the presence of abundant inherited zircon, as well as isotope and trace elements geochemistry support formation of the Gantaohe Group on top of a continental basement. These data and the regional geology lead us to conclude that the Trans-North China Orogen constituted an intracontinental rift during the Paleoproterozoic that was connected to the Eastern Block since the end of the Archean.

Back-arc and post-collisional volcanism in the Palaeoproterozoic Granites-Tanami Orogen, Australia

Pillow basalt, dolerite sills, and doleritic dykes are the major products of volcanism in the Granites-Tanami Orogen, and provide a window for understanding the magmatism and tectonic evolution of the orogen. The pillow basalt and dolerite sills from the Stubbins Formation have similar geochemical features, suggesting that they are genetically related. The pillow basalt and dolerite sills (ca. 1864Ma) have tholeiitic affinity, with high TiO2 contents (0.9–1.88wt.%), low Mg# (40–55), and positive ɛNd(t) values (+0.6 to +2.8). Enriched mid-ocean ridge basalt (E-MORB)-normalized REE and primitive mantle-normalized trace element patterns for these rocks show relatively flat REE distribution, slight enrichment of LILE (Rb, Th, U, and K) and weak negative anomalies of HFSE (Nb and P), which are the characteristics similar to those of the enriched back-arc basin basalt (E-BABB), but different from those of modern normal-MORB (N-MORB), E-MORB, oceanic island basalt (OIB), depleted back-arc basin basalt (D-BABB), and island arc basalt (IAB). The pillow basalt and dolerite sills are interpreted to be the results of a high-degree decompressional melting (5–15%) of the asthenosphere source in the spinel stability field. The mantle source of these rocks have primitive mantle signature with input of 3–4% subduction material. Convective flow operating during subduction, circulating the enriched material from wedge corner into the asthenospheric mantle beneath the Granites-Tanami back-arc basin and generated the mild enriched mafic magma that was emplaced as dolerite sills or erupted as basalt.“Late” (ca. 1800Ma) doleritic dykes in the region have high contents of MgO (7.15–14.0wt.%), Cr (262–757ppm), K2O (2.25–4.85, wt.%), with high Mg# (57–72), K2O/Na2O>2, and negative ɛNd(t) values range from −4.5 to 0.3. Their REE and trace element characteristics, such as enrichment of Rb, Ba, Th, U, K, LREE, and negative Nb–Ta anomalies, are similar to those of the upper (UCC) and middle continental crust (MCC), but more enriched than those of the lower continental crust (LCC). The interpreted continental–continental collision between the Granites-Tanami and Arunta orogens during the Tanami Orogeny (DGTO2; ca. 1800Ma) resulted in the thickening of the lower crust and lithospheric mantle root along the Willowra Lineament. The thickened lithospheric mantle root has higher density than the upper asthenospheric mantle leading to gravitational instability and resulting in convective removal. The detached denser lithospheric mantle root sank into asthenosphere triggered upwelling of the hot asthenospheric mantle material and extension. It is proposed here that melts derived from partial melting of the previously subduction-metasomatized lithospheric mantle with crustal assimilation generated the doleritic dykes, and modelling of geophysical data supports this interpretation. This post-collisional magmatism is approximately synchronously with the ca. 1800Ma gold mineralization and extensive granitic magmatism in the Granites-Tanami Orogen.

Age and tectonic setting of the Bavanat Cu–Zn–Ag Besshi-type volcanogenic massive sulfide deposit, southern Iran

The Bavanat Cu–Zn–Ag Besshi-type volcanogenic massive sulfide (VMS) deposit occurs within the Surian volcano-sedimentary complex in the Sanandaj–Sirjan zone (SSZ) of southern Iran. The Surian complex is comprised of pelite, sandstone, calcareous shale, basalt, gabbro sills, and thin-bedded limestone. Mineralization occurs as stratiform sheet-like and tabular orebodies hosted mainly by greenschist metamorphosed feldspathic and quartz feldspathic sandstone, basalt, and pelites. The basalts of the Surian complex show predominantly tholeiitic to transitional affinities, with a few samples that are alkalic in composition. Primitive mantle-normalized trace and rare earth element (REE) patterns of the Surian basalts display depletions in light REE, negative anomalies of Nb, Ta, and Ti, and positive anomalies of P. Positive P anomalies are indicative of minor crustal contamination. Furthermore, Th enrichments in the mid-ocean ridge basalt-normalized patterns of the Surian basalts are characteristic of rifted arc basalts emplaced in continental margin subduction zones. The high MgO content (>6 wt.%) of most Surian basalts and low TiO2 content of two samples (0.53 and 0.62 wt.%) are characteristic of boninites. The aforementioned features of the basalts indicate arc tholeiites emplaced in intra-arc rift environments and continental margin subduction zones. U–Pb dating by laser ablation- inductively coupled plasma mass spectrometry of detrital zircons extracted from the host feldspathic and quartz feldspathic sandstone yields various ages that are predominantly Permian and Triassic; however, the youngest zircons give a mean Early Jurassic concordant U–Pb age of 191 ± 12 Ma. This age, together with geological and petrochemical data, indicate that VMS mineralization formed in the Early Jurassic in pull-apart basins within the SSZ. These basins and the VMS mineralization may be temporally related to an intra-arc volcano–plutonic event associated with Neo-Tethyan oblique subduction.

Sr and Nd isotope ratios and trace element concentrations in kimberlites from Shandong and Liaoning (China) and the Kimberley area (South Africa)

Strontium-Neodymium isotopes and 28 elements for Kimberlites from China (Shandong and Liaoning) and from South Africa (Kimberley area) have been measured to estimate their chemical characteristics and magma sources. The eSr and eNd values for Chinese kimberlites were, respectively, +1.0 to +93.0 and ‐4.3 to +0.2. The isotopic signature for all Shandong samples revealed very similar to those of Group I kimberlites (South African samples), whereas the Liaoning samples were categorized as Transitional. Although primitive mantle-normalized trace element patterns of the mean values of Group I Shandong and South African samples all resembled those of other Group I kimberlites, their HREE concentrations differed slightly. Shandong samples were depleted in HREEs. Our model calculation demonstrated that the kimberlites would be produced by partial melting with degree of less than 1%. Furthermore, Shandong samples were possibly generated from source rock in which the mineral contents differed slightly (specifically the proportion of garnet) from those of other regions.

The geodynamic evolution of Mesoarchean anorthosite complexes inferred from the Naajat Kuuat Complex, southern West Greenland

Layered anorthosite complexes are typical components of Archean crustal domains. However, the geodynamic settings in which they were emplaced are still discussed as geological relationships are often ambiguous. Here we report major, trace element and high-precision high-field-strength-element (HFSE) data for the recently discovered well preserved Naajat Kuuat Anorthosite Complex from the inner Ameralik fjord region, southern West Greenland. The dataset is complemented by the first combined Hf–Nd isotope analyses for Archean layered anorthosite complexes and U–Pb zircon geochronology. The data contribute to the small database on Archean layered anorthosite complexes and are used to unravel the origin of these complexes and the tectonic regime involved. Fractional crystallisation of olivine, pyroxene, plagioclase and possibly amphibole controls major and trace element variations in the layered intrusion. There are two groups of amphibolites: (1) a group with primitive mantle normalized trace element patterns are similar to those of MORB-like basalts and (2) typical island-arc tholeiites (IAT), apparently indicating an island-arc setting. Lu–Hf regression lines yield an age of 2985±59Ma (MSWD 4) within the error of the Sm–Nd regression age of 2929±110Ma (MSWD 17). The initial ɛHf(2985) for the Naajat Kuuat rocks range from +1.6 to +5.8 and the initial ɛNd(2985) range from +0.4 to +3.9, either indicating variably depleted mantle sources or variable degrees of crustal contamination. In contrast to most mafic assemblages, ratios of Nb/Ta are highly variable (7.85 to 18.6), reflecting fractionation and accumulation of amphibole, ilmenite and pyroxene. The MORB-like parental liquids have the highest Nb/Ta of ca. 18, consistent with a mantle source overprinted by melt-like components from subducting oceanic crust with high Nb/Ta. The accumulation of plagioclase forming the anorthosites and the primary fractionation of amphibole as well as the occurrence of high-Al basalts within the Naajat Kuuat complex argue for hydrous parental liquids in support of an island-arc related setting. Zircon U–Pb geochronology from the anorthosite and adjacent tonalites reveal major tonalite intrusion into the complex at ca. 2802Ma and a second regional event at ca. 2710Ma, in accord with crustal heating due to micro-continent amalgamation and crustal thickening. Altogether, the geochemical data can be interpreted with a geodynamic model, where anorthosite-complex associated rocks intrude into tectonically thickened island-arc crust. Crustal thickening is possibly triggered by island-arc accretion, leading to the emplacement of TTG bodies that further thickened the crustal pile. Further collision and amalgamation with other proto-crustal assemblages might have led to enhanced crustal magmatism and granulite facies metamorphism.

Genesis of the Chehugou Mo-bearing granitic complex on the northern margin of the North China Craton: geochemistry, zircon U–Pb age and Sr–Nd–Pb isotopes

AbstractThe Chehugou granite-hosted molybdenum deposit is typical of the Xilamulun metallogenic belt, which is an important Mo–Ag–Pb–Zn producer in China. A combination of major and trace element, Sr and Nd isotope, and zircon U–Pb isotopic data are reported for the Chehugou batholith to constrain its petrogenesis and Mo mineralization. The zircon SIMS U–Pb dating yields mean ages of 384.7 ± 4.0 Ma and 373.1 ± 5.9 Ma for monzogranite and syenogranite and 265.6 ± 3.5 Ma and 245.1 ± 4.4 Ma for syenogranite porphyry and granite porphyry, respectively. The Devonian granites are calc-alkaline with K2O/Na2O ratios of 0.44–0.52, the Permian granites are alkali-calcic with K2O/Na2O ratios of 1.13–1.25, and the Triassic granites are calc-alkaline and alkali-calcic rocks with K2O/Na2O ratios of 0.78–1.63. They are all enriched in large-ion lithophile elements (LILEs) and depleted in high-field-strength elements (HFSEs) with negative Nb and Ta anomalies in primitive mantle-normalized trace element diagrams. They have relatively high Sr (189–1256 ppm) and low Y (3.87–5.43 ppm) concentrations. The Devonian granites have relatively high initial Sr isotope ratios of 0.7100–0.7126, negative ɛNd(t) values of −12.3 to −12.4 and 206Pb/204Pb ratios of 16.46–17.50. In contrast, the Permian and Triassic granitoids have relatively low initial 87Sr/86Sr ratios (0.7048–0.7074), negative ɛNd(t) values of −10.1 to −13.1 and 206Pb/204Pb ratios of 17.23–17.51. These geochemical features suggest that the Devonian, Permian and Triassic Chehugou granitoids were derived from ancient, garnet-bearing crustal rocks related to subduction of the Palaeo-Asian Ocean and subsequent continent–continent collision between the North China and Siberian plates.

Heterogeneous mantle source and magma differentiation of quaternary arc-like volcanic rocks from Tengchong, SE margin of the Tibetan Plateau

The Tengchong volcanic field north of the Burma arc comprises numerous Quaternary volcanoes in the southeastern margin of the Tibetan Plateau. The volcanic rocks are grouped into four units (1–4) from the oldest to youngest. Units 1, 3 and 4 are composed of olivine trachybasalt, basaltic trachyandesite and trachyandesite, and Unit 2 consists of hornblende dacite. The rocks of Units 1, 3, and 4 form a generally alkaline suite in which the rocks plot along generally linear trends on Harker diagrams with only slight offset from unit to unit. They contain olivine phenocrysts with Fo values ranging from 65 to 85 mol% and have Cr-spinel with Cr# ranging from 23 to 35. All the rocks have chondrite-normalized REE patterns enriched in LREE and primitive mantle-normalized trace element patterns depleted in Ti, Nb and Ta, but they are rich in Th, Ti and P relative to typical arc volcanics. Despite minor crustal contamination, 87Sr/86Sr ratios (0.706–0.709), eNd values (−3.2 to −8.7), and eHf values (+4.8 to −6.4) indicate a highly heterogeneous mantle source. The Pb isotopic ratios of the lavas (206Pb/204Pb = 18.02–18.30) clearly show an EMI-type mantle source. The underlying mantle source was previously modified by subduction of the Neo-Tethyan oceanic and Indian continental lithosphere. The present heterogeneous mantle source is interpreted to have formed by variable additions of fluids and sediments derived from the subducted Indian Oceanic lithosphere, probably the Ninety East Ridge. Magma generation and emplacement was facilitated by transtensional NS-trending strike-slip faulting.

Petrologic, chemical, and isotopic evaluation of pitchstones in the Samho area, southwestern Okcheon Belt, South Korea

The Samho area in the southwestern Okcheon Belt, Korean Peninsula, contains spatially related eruptions of lapilli tuff, rhyolite, and pitchstone, considered to be of Late Cretaceous age. The pitchstones, which are the youngest rocks in this volcanic suite, are chemically highly evolved, strongly metaluminous to weakly peraluminous, and calc-alkaline. They have moderately fractionated chondrite-normalized rare earth element (REE) patterns, with relative enrichment of light REEs and Eu/Eu* values of 0.53–0.57. A distinct negative Nb anomaly is observed on a primitive mantlenormalized trace element diagram. These chemical features correspond to an I-type granite derived from a continental/oceanic arc. The pitchstones have Zr contents of 45–92 ppm, and zircon thermometry yields temperatures of 650–743 °C (mean value, 722 °C). The pitchstones have an initial 87Sr/86Sr isotopic ratio of 0.712, an initial 143Nd/144Nd ratio of 0.5120, and aNd(t) values of −10.6 to −12.0. The mean K-Ar whole-rock ages of the lapilli tuff, rhyolite, and pitchstone are 81.9 ± 2.4 Ma, 65.5 ± 1.9 Ma, and 12.7 ± 0.9 Ma, respectively. The depleted mantle model ages (TDM) range from 1.46 to 1.58 Ga. The 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb isotopic ratios are 18.477–18.569, 15.672–15.702, and 38.889–38.999, respectively. The Sr and Nd isotopic compositions of the pitchstones are relatively enriched compared with depleted MORB mantle (DMM), indicating the involvement of lower crustal components in their source. We conclude that the pitchstones in the Samho area formed in a continental arc setting by partial melting of a probable Mesoproterozoic parental source. The magma series, petrogenesis, and tectonic setting of Late Cretaceous magmatism in the Samho area can be correlated with those of the Haenam volcanic fields located east of the present study area.

Sr–Nd isotope geochemistry and petrogenesis of the Chah-Shaljami granitoids (Lut Block, Eastern Iran)

Chah-Shaljami porphyritic granitoids belong to the Lut Block volcanic–plutonic belt of central eastern Iran. These intrusive rocks are mostly quartz monzonites, granodiorites and diorites. Geochemical evidence reveals that they are co-genetic and that they have features typical of high-K calc-alkaline to shoshonitic rocks from volcanic arc setting. Primitive mantle-normalized trace element spider diagrams display strong enrichment in LILE, such as Rb, Ba, and Cs, and depletion in some HFSE, e.g. Nb, Ti, Y and HREE. Chondrite-normalized plots show a very marked REE fractionation, with significant LREE enrichment (23 ⩾ La N/Yb N ⩾ 14) and the lack of Eu anomaly. Sr/Y and La/Yb ratios of Chah-shaljami intrusives are respectively 20–67 and 21–34, which reveals that, despite their K-rich composition, these rocks also, have some adakitic affinity. Plots on the Sr/Y–Y and La/Yb–Yb diagrams show that the Chah-Shaljami intrusives may be subdivided into two distinct classes. A Rb–Sr age of 33.5 ± 1 Ma, mainly dependent on the Sr isotopic composition of biotite, was obtained in a quartz monzonite sample. Taking into account that this sample was almost unaffected by hydrothermal and meteoric alteration and that cooling was probably fast, the 33–34 Ma date is interpreted as the intrusion age. With the exception of two samples, initial 87Sr/ 86Sr ratios and C ̵ Nd values are clustered in the restricted ranges of 0.70470–0.70506 and +1.9–+2.7, which fits into a supra-subduction mantle wedge source for the parental melts and indicates that, in general, crustal contribution for magma diversification was not relevant; however, one sample shows higher 87Sr/ 86Sr and lower C ̵ Nd, revealing that, occasionally, crustal rock assimilation also contributed to geochemical variation; one other sample, strongly affected by hydrothermal alteration, departs from the main group only by higher 87Sr/ 86Sr, suggesting that its alteration involved crustal fluids. Sr and Nd isotope compositions together with major and trace element geochemistry points to the origin of the parental magmas by melting of a metasomatized mantle source, with garnet behaving as a residual phase, whilst phlogopite was an important contributor to the generated melts.