Nb Anomalies Research Articles

Geochemical evolution of the Mangalwar Complex, Aravalli Craton, NW India: Insights from elemental and Nd-isotope geochemistry of the basement gneisses

The Banded Gneissic Complex (BGC) of the Aravalli Craton is divided into BGC-I and BGC-II; the BGC-II (central Rajasthan) is comprised of the Sandmata Complex and the Mangalwar Complex. We report elemental and Nd-isotope geochemistry of basement gneisses of the Mangalwar Complex and constrain its origin and evolution. Geochemically, the basement gneisses have been classified as low-SiO2 gneisses (LSG) and high-SiO2 gneisses (HSG). Both the LSG and HSG are potassic, calc-alkaline and peraluminous in nature. The LSG are enriched in incompatible (K, Sr, Ba, large ion lithophile elements) and compatible elements (MgO, Cr, and Ni). They display fractionated rare earth element patterns (avg. LaN/YbN = 12.1) with small Eu-anomaly (δEu = 0.9), and exhibit negative anomalies of Nb and Ti in primitive mantle-normalized multi-element diagram. In terms of Nd-isotope geochemistry, the LSG are characterized by εNd(t) = −4.2 and depleted mantle model age of 3.3 Ga. To account for these geochemical characteristics we propose a three-stage petrogenetic model for the LSG: (1) fluids released from dehydration of subducting slab metasomatised the mantle-wedge; (2) the subducting slab underwent slab-breakoff causing upwelling and decompression melting of the asthenosphere during waning stage of subduction; and (3) upwelling asthenosphere provided the requisite heat for partial melting of the metasomatised mantle-wedge leading to generation of the LSG parental magma. Asthenospheric upwelling also contributed in the LSG petrogenesis which is evident from its high Mg# (avg. 0.53). The LSG formed in this way are contemporary and chemically akin to sanukitoids of the BGC-I and Archean sanukitoids reported elsewhere. This provides a basis to consider the LSG as a part of the BGC-I. Contrary to the LSG, the HSG are depleted in compatible elements (MgO = avg. 1.1 wt.%; Cr = avg. 8 ppm; Ni = avg. 6 ppm) but enriched in incompatible elements (Sr = avg. 239 ppm, Ba = avg. 469 ppm). Its εNd(t) values vary from −9.5 to −5.4. These chemical features of the HSG are akin to potassic granitoids found elsewhere. In this backdrop, we propose that the HSG suite of the Mangalwar Complex was derived from re-melting (partial) of an older crust (TTG?) occurring within the BGC-II.

Volcanology and geochemical study of the volcanic rocks of the Bafmeng area (Mount Oku, Cameroon Volcanic Line)

Detailed volcanological and petrographic descriptions, and petrogenetic studies of lavas from Bafmeng were performed, with the aim to establish their origin, magmatic evolution, and the tectonic environment. Bafmeng area is located on the NW flank of Mount Oku, an important volcanic massif along the Cameroon Volcanic Line in Central Africa. Four types of eruptive dynamisms are successively manifested; (1) an effusive dynamism (basaltic flows); (2), an extrusive dynamism (rhyolitic and trachytic domes); (3) a basaltic explosive dynamism (Strombolian cones) and (4) hydromagmatic eruption (maar). The lavas form a sodic alkaline series with three distinctive types: ultra-basic type (basanite), an intermediate type (benmoreite and trachyte), and an acid type (rhyolite). The mineralogical composition is made up of olivine, augite, plagioclase and opaque minerals for basic lavas. Acidic lavas are made up of sanidine, quartz, clinopyroxene, opaque minerals, and aegyrine. The peridotite has as minerals; plagioclases, spinel, olivine, and pyroxene The lavas show enrichment in LREE as compared to HREE with negative anomalies in Ba, Eu, Sr in trachyte and rhyolite and a positive anomalies in Nb, La, Zr, Dy. The major process responsible for the emplacement of the Bafmeng felsic lavas is partial melting of spinel-garnet rich peridotite, followed by contamination and assimilation. Mafic lavas are derived from the partial melting of the asthenospheric mantle. The tectonic context based on geochemistry indicates that, the regional development is related to within plate setting. Keywords : Cameroon Volcanic Line, Mount Oku, Bafmeng, Peridotite, Eruptive dynamism.

Geochemistry of eclogites of the Tso Morari complex, Ladakh, NW Himalayas: Insights into trace element behavior during subduction and exhumation

Whole rock major and trace element compositions of seven eclogites from the Tso Morari ultra-high pressure (UHP) complex, Ladakh were determined with the aim of constraining the protolith origins of the subducted crust. The eclogites have major element compositions corresponding to sub-alkaline basalts. Trace element characteristics of the samples show enrichment in LILE's over HFSEs (Rb, Th, K except Ba) with LREE enrichments ((La/Lu)n = 1.28–5.96). Absence of Eu anomaly on the Primitive Mantle normalized diagram suggests the absence of plagioclase fractionation. Positive correlation between Mg# with Ni and Cr suggests olivine fractionation of mantle melts. Narrow range of (La/Yb)n (2.1–9.4) and Ce/Yb (6.2–16.2) along with Ti/Y (435–735) ratios calculated for the Tso Morari samples is consistent with generation of melts by partial melting of a garnet free mantle source within the spinel peridotite field. Ternary diagrams (viz. Ti–Zr–Y and Nb–Zr–Y) using immobile and incompatible elements show that the samples range from depleted to enriched and span from within plate basalts (WPB) to enriched MORB (E-MORB) indicating that the eclogite protoliths originated from basaltic magmas. Primitive Mantle normalized multi element plots showing significant Th and LREE enrichment marked by negative Nb anomalies are characteristic of continental flood basalts. Positive Pb, negative Nb, high Th/Ta, a narrow range of Nb/La and the observed wide variation for Ti/Y indicate that the Tso Morari samples have undergone some level of crustal contamination. Observed geochemical characteristics of the Tso Morari samples indicate tholeiitic compositions originated from enriched MORB (E-MORB) type magmas which underwent a limited magmatic evolution through the process of fractional crystallization and probably more by crustal contamination. Observed geochemical similarities (viz. Zr, Nb, La/Yb, La/Gd, La/Nb, Th/Ta ratios and REE) between Tso Morari eclogites and the Group I Panjal Traps make the trap basalt the most likely protoliths for the Tso Morari eclogites.

Paleoproterozoic Nb–enriched meta-gabbros in the Quanji Massif, NW China: Implications for assembly of the Columbia supercontinent

Diverse models have been proposed for the role of the Tarim Craton within the Paleoproterozoic Columbia supercontinent assembly. Here we report a suite of ∼1.71 Ga Nb-enriched meta–gabbro lenses in the eastern Quanji Massif, within the Tarim Craton in NW China. The meta–gabbroic rocks have Nb contents of 11.5–16.4 ppm with Nb/La ratios varying from 0.84 to 1.02 ((Nb/La)N = 0.81–0.98) and Nb/U ratios from 38.0 to 47.2. They show low SiO2 (45.1–48.5 wt.%) and MgO (5.96–6.81 wt.%) and Mg# (Mg# = Mg/(Mg + Fe) = 43.5–47.7), high FeOt (13.0–15.7 wt.%) and moderate TiO2 (1.70–2.51 wt.%), with tholeiitic affinities. These rocks possess low fractionated REE patterns without obvious Eu anomalies (Eu/Eu* = 0.87–1.02). Their primitive mantle-normalized elements patterns display significant Zr–Hf troughs, positive Nb anomalies, weak negative Ti and P anomalies, and high contents of Rb and Ba, resembling Nb-enriched basalts generated in arc-related tectonic settings. Their arc–like geochemical signatures together with whole rock ɛNd(t) values of 0.4–2.1 and corresponding old TDM (2.22–2.37 Ga) as well as (143Nd/144Nd)t and (87Sr/86Sr)t (t = 1712 Ma) values of 0.5104–0.5105 and 0.7030–0.7058, respectively, suggest that their precursor magma originated from mantle wedge peridotite metasomatised by subduction-derived melts. The results from our study reveal subduction along the eastern periphery of the Tarim Craton and marginal outgrowth continuing to ∼1.7 Ga within the Columbia supercontinent.

Shoshonitic- and adakitic magmatism of the Early Paleozoic age in the Western Kunlun orogenic belt, NW China: Implications for the early evolution of the northwestern Tibetan plateau

The Western Kunlun orogenic belt in the northwestern margin of the Tibetan plateau contains two magmatic belts; early Paleozoic belt in the northern part of Western Kunlun Terrane (WKT), and early Mesozoic belt in the southern part of WKT. Both formed from northward subduction of the Paleo-Tethys. The early Paleozoic belt contains large Datong and Qiukesu igneous complexes and many smaller plutons. The Datong complex is mainly composed of dark-colored porphyritic syenite and monzonite with minor light-colored dykes of granite and monzonite. The dark-colored rocks are characterized by moderate SiO2 (58.2–69.3wt.%), and high Al2O3 (15.3–17.1wt.%), total alkali (Na2O+K2O=8.07–10.2wt.%) and ratios of K2O/Na2O (0.77–1.83). They plot in “shoshonite” field, and show high abundances of LILE including LREE ((La/Yb)n=15.4–26.2; mean 20.2) with pronounced negative anomalies of Nb–Ta–P–Ti in normalized trace elemental patterns and weak negative anomalies of Eu (δEu=2Eun/(Smn+Gdn)=0.68–0.80). The light-colored rocks contain slightly higher concentrations of SiO2 (60.3–72.0wt.%), similar Al2O3 (14.7–17.6wt.%), and slightly lower total alkalis (6.57–9.14wt.%) than dark-colored rocks. They show adakitic geochemical signatures with low Y (5.80–17.2ppm) and Yb (0.63–1.59ppm), and high Sr/Y (>40). U–Pb zircon dating indicates that shoshonitic rocks and adakitic dykes formed at ~444Ma to ~443Ma, and a separate small adakitic plug at ~462Ma. The mean εHf(t) values of zircon range from −1.6 to −0.94 (n=14) with TDM2 of ~1.5Ga for shoshonitic rocks and εHf(t) values from −1.8 to +0.72 (n=12) with TDM2 of 1.4 to 1.5Ga for adakitic rocks. Shoshonitic rocks show initial 87Sr/86Sr and εNd(t) of 0.7092–0.7100 and −3.9 to −3.2, respectively, and adakitic rocks yield initial 87Sr/86Sr and εNd(t) of 0.7099–0.7134 and −3.6 to −3.1, respectively. Similar Sr, Nd, and Hf isotope compositions for the shoshonitic and adakitic rocks suggest similar ancient rocks as their sources.Combined with the geological development of the area, we propose that a local extension in the WKT over the subducting Paleo-Tethys has induced the upwelling of asthenospheric mantle and partial melting in the lower crust during mid-Ordovician to early Silurian. The primary shoshonitic melt evolved into the parental magmas for the shoshonitic rocks and adakitic rocks through fractional crystallization of hornblende.

Ultramafic to mafic granulites from the Larsemann Hills, East Antarctica: Geochemistry and tectonic implications

The Larsemann Hills area is part of a reworked early Neoproterozoic metamorphic terrain in southwestern Prydz Bay, East Antarctica. Ultramafic and mafic granulites, whose origins remain controversial, occur as lenses, boudins or layered bodies within the para- and ortho-gneiss in the region. The ultramafic and mafic granulites show spinel-olivine-bearing and two-pyroxene-bearing mineral assemblages recrystallized at 860–900°C. Their bulk rock analyses indicate an origin as igneous cumulates, with high Mg# (molar MgO/(MgO+FeO)) from 0.73 to 0.84 for ultramafic granulite and from 0.46 to 0.78 for mostly mafic granulite as well as high Cr and Ni contents ([Cr] and [Ni] up to 1826ppm and 1400ppm respectively for ultramafic granulite and [Cr] of 1460ppm for mafic granulite). Trace element patterns show pronounced negative Nb anomalies, suggesting a subduction-related tectonic setting for their precursors, consistent with derivation from arc basalts, also suggested by low TiO2 and K-enrichment in mafic granulites. The ultramafic to mafic granulites may have been formed in a subduction-related continental back-arc basin environment simultaneously with peak metamorphism associated with arc-continent collision during the early Neoproterozoic (990–900Ma) Rayner orogeny.

Exotic island arc Paleozoic terranes on the eastern margin of Gondwana: Geochemical whole rock and zircon U–Pb–Hf isotope evidence from Barry Station, New South Wales, Australia

Early Paleozoic intra-oceanic terranes crop out along the Peel-Manning Fault System, in the southern New England Orogen, NSW Australia. These are the Cambrian ophiolitic Weraerai terrane and the Siluro-Devonian island arc Gamilaroi terrane. There has been debate whether these terranes formed at the Gondwana margin or if they are intra-oceanic, and were accreted to Gondwana later in the Paleozoic. Major-trace-REE elemental data indicate Weraerai terrane formed in a supra-subduction environment. Rare zircons extracted from Weraerai terrane gabbro-plagiogranite suites at Barry Station yield a U–Pb zircon date of 504.9±3.5Ma with initial εHf values of +11.1 indicating a juvenile source. Amphibole-bearing felsic dykes and net-vein complexes are also found within the gabbro with a U–Pb zircon date of 503.2±5.7Ma and initial εHf values of +11.6. These are coeval in age with their host rocks and we propose they represent partial melts of the mafic crust during the circulation of seawater. The Gamilaroi trondhjemites of prehnite-pumpellyite–greenschist metamorphic grade terrane yielded very few zircons with an age of 413±8.7Ma. Zircon initial εHf values range from +5.0 to +2.9, indicating an input from an evolved crustal source, unlike the purely oceanic Weraerai terrane. Gamilaroi terrane trondhjemites are enriched in LREE have low K2O and K2O/Na2O ratios and strong negative Nb anomalies consistent with supra-subduction zone environments. Multiple subduction zones may well have existed within the Panthalassa Ocean during the early-mid Paleozoic with the Weraerai-Gamilaroi being accreted onto the Gondwanan margin during the latest Devonian.

Current Literature Survey

The Imiter inlier (eastern part of the Moroccan Anti-Atlas) is located on the northwestern border of the West African Craton (WAC) and exhibits a range of Pan-African granitoids. Three massifs that crosscut the Imiter Saghro Group were targeted in this work: the Igoudrane granodiorite, Bou Teglimt granodiorite and Bou Fliou granite. We present here additional geochemical analyses (major and trace elements) and Sm-Nd isotopic data, which define two distinct groups: (i) the Igoudrane massif (677 Ma) and (ii) the Bou Teglimt granodiorite (576 Ma) and the Bou Fliou granite (550 Ma).Geochemical data confirm the calc-alkaline signature of the studied granitoids. Both groups of granitoids are slightly peraluminous and show strong negative anomalies in Nb, Ta and Ti in multi-element plots normalized to the primitive mantle. The granitoids have low 143Nd/144Nd initial ratios (0.5116–0.5117), with TDM model ages ranging from 1.73 to 1.52Ga. The εNd(t) values are negative, decreasing from the Igoudrane samples (−1.1 to −3.1) to the Bou Teglimt granodiorite (−3.0 to −3.3) and Bou Fliou (−4.2 to −4.8). All these data suggest a mixed magmatic origin involving a juvenile mantle source and an old, at least Paleoproterozoic crust. Given Mesoproterozoic rocks are lacking or very scarce in the Anti-Atlas, these results confirm the existence of an old cratonic basement beneath the eastern Anti-Atlas, and therefore suggest that the northern border of the West African Craton must be placed further to the north, as suggested by previous work in this region.

Petrology of foiditic and meymechitic volcanism in the Maimecha-Kotui province (Polar Siberia)

Comparative analysis of ultramafic meymechites of the Maimecha Suite and alkaline volcanics of the Ary-Dzhang Suite (foidites (nephelinites, analcimites, limburgites, etc.) and melilitites) has shown their consanguinity, which indicates their relationship with the same magmatic system periodically producing large amounts of alkaline ultramafic melts. We have studied the petrogeochemical and mineralogical compositions of rocks and melt inclusions in the hosted olivines. The rocks of the Maimecha and Ary-Dzhang Suite differ considerably in MgO content, which is well explained by the accumulation of olivine. The inclusions in olivines from the meymechites and the rocks of the Ary-Dzhang Suite correspond in composition to foidites. The trace and rare-earth element patterns are similar both in the foidites and meymechites and in the melt inclusions: They show negative anomalies of Rb and K and positive anomalies of Nb and Ta. The ratios of indicator elements (Nb/Ta, Ba/La, Ta/La, etc.) in the rocks of the Maimecha and Ary-Dzhang Suite are constant and almost independent of their Mg# values. The La/Yb ratio in the foidites is significantly higher than that in the meymechites and in the melt inclusions from their olivines, which indicates that the rocks of the Ary-Dzhang Suite resulted from the fractionation of highly magnesian alkaline picritoid melt.

Petrology, geochemistry and zircon U–Pb geochronology of the Jurassic porphyry dykes in the Dehua gold field, Southeast China: Genesis and geodynamics

The widespread tectonic and magmatic events in Southeast China during the Mesozoic were accompanied by large‐scale metallogenesis. Here, we investigate the petrology, geochemistry, and zircon U–Pb age of the porphyry dykes in relation to the gold mineralization at Dongyang deposit in the Fujian Province. The geochemical data show silica‐rich and Mg‐ and Ca‐poor A‐type granite affinity for the dykes. The rocks have high concentrations of Light Rare Earth Elements (LREE) and Large Ion Lithophile Elements (LILE) and relatively low contents of Heavy Rare Earth Elements (HREE) and High Field Strength Element (HFSE). They display (La/Yb)N and (La/Sm)N values in the range of 5.7 to 31.4 and 6.0 to 8.3, respectively, with negative anomalies of Ba, Nb, Ta, Sr, Zr, Eu, and Ti and positive Ce anomaly. The source magma for the dykes was derived from a mixture of partially melted Al‐rich crustal material, with an input of mantle components. We present sensitive high‐resolution ion microprobe zircon U–Pb data from the dykes, which show emplacement ages in the range of 158 to 162 Ma, coinciding with the onset of the Yanshanian magmatism in Southeast China. Integrating the data obtained from studies on geology, geochronology and petro‐geochemistry, we suggest that the dykes played role in controlling the gold mineralization, and the magmatism and gold mineralization are the response of continental lithosphere thinning in Southeast China triggered by the subduction of the ancient Pacific Ocean Plate.

An experimental study of trace element distribution during partial melting of mantle heterogeneities

Trace elements are widely used to interpret the origin of mantle-derived magmas, yet we lack detailed understanding of how trace elements behave during melting of mantle source components. Here, we present new data on trace element distribution and partitioning between phases from high pressure (3.0 to 5.0GPa), high-temperature (1230 to 1550°C) melting experiments on starting compositions that represent altered oceanic crust and metasedimentary protoliths. These compositions are expected to be recycled into the mantle via subduction or delamination to form heterogeneous mantle domains that are implicated in the genesis of intraplate and/or ocean floor magmas.In most of the experiments, the investigated trace elements behave incompatibly, expect for HREE and Y, which are compatible in garnet, and V, Cr and Zn, which partition into both garnet and clinopyroxene. Relative to Nd, P is more compatible in garnet than clinopyroxene, leading to fractionation of P/Nd with melting in some cases. Melt compositions in some experiments with low melt fractions feature distinctive negative anomalies for Nb, and for Sr, Ba and Eu, due to retention of these elements in minor/accessory rutile and feldspar, respectively. We also show that highly incompatible trace element (e.g., Cs, Th, U, LREE) concentrations in melts are strongly controlled by melt fraction, whereas moderately incompatible (M-HREE, Zr) to compatible (Cr, V) element concentrations are controlled by temperature and/or phase composition. Pressure has relatively little influence on trace element behaviour at the investigated conditions.Based on our results, we suggest that partial melting of eclogitic components of mantle domains may ultimately produce magmas with trace element compositions that are unlike peridotite-sourced magmas. Therefore, the trace element systematics of mantle-derived magmas should not only be interpreted in terms of mantle source compositions, but also with consideration to source petrology (e.g., mineral compositions and accessory phase stability) and melting conditions (e.g., melt fraction, pressure, temperature).

Adakite-like signature of porphyry granitoid stocks in the Meiduk and Parkam porphyry copper deposits, NE of Shahr-e-Babak, Kerman, Iran: Constrains on geochemistry

The Middle Miocene porphyry granitoid stocks of Meiduk and Parkam porphyry copper deposits are intruded in the north-western part of the Dehaj-Sarduiyeh volcano-sedimentary belt in the south-eastern extension of the Urumieh-Dukhtar Magmatic Arc (UDMA) in Iran. The porphyritic to microgranular granitoids are mainly consist of quartz diorite, granodiorite and diorite. The whole rock geochemical analyses of these rocks reveals sub-alkaline, calc-alkaline, meta-peraluminous and I-type characteristics. Their geochemical characteristics such as Al2O3 content of 13.51–17.05wt%, high Sr concentration (mostly >400ppm), low Yb (an average of 0.74ppm) and Y (an average of 9.02ppm) contents, strongly differentiated REE patterns (La/Yb≥20), lack of Eu anomaly (Eu/Eu∗≥1) are indicative of adakitic signature. Their enrichment in low field strength elements (LFSE) and conspicuous negative anomalies for Nb, Ta and Ti are typical of subduction related magmas. Detailed petrological studies and geochemical data indicated that Meiduk and Parkam porphyry granitoids were derived from amphibole fractionation of hydrous melts at a depth of >40km in a post-collisional tectonic setting.

Petrogenesis and tectonic implications of gabbro and plagiogranite intrusions in mantle peridotites of the Myitkyina ophiolite, Myanmar

Centimeter-size intrusions of gabbros and plagiogranites occur in mantle peridotites of the Myitkyina ophiolite, Myanmar. The gabbros mainly consist of plagioclase and clinopyroxene, whereas orthopyroxene occasionally occurs. The plagiogranites are mainly composed of plagioclase, quartz and amphibole, with small amount of accessory minerals, such as zircon, apatite and rutile. Plagioclase in the gabbros varies from andesine to anorthite (An37–91), whereas plagioclase in the plagiogranites is less calcic (An1–40). Clinopyroxene in the gabbros is pervasively altered to hornblende. The gabbros contain 42.97–52.88wt% SiO2, which show negative correlations with Al2O3, CaO and MgO, but positive correlations with Na2O, P2O5 and TiO2. Microtextural relations reveal the crystallization of clinopyroxene prior to plagioclase in the Myitkyina gabbros. This suggests that the gabbros were crystallized from hydrous melts, which is also supported by the occurrence of orthopyroxene and anorthitic plagioclase in some gabbros. The gabbros have slightly enriched Sr-Nd isotopes, with initial 87Sr/86Sr ratios of 0.703938–0.706609 and εNd(t) values of +2.4–+7.2, and relatively variable Hf isotopes, with εHf(t) values of +13.4–+24.9. A subduction component is required to explain the decoupled Nd-Hf isotopes of the gabbros. Binary mixing suggests that addition of ca 2% subducted sediments to a depleted mantle can account for the Nd-Hf decoupling. Therefore, both petrological and geochemical data of the gabbros support that the Myitkyina ophiolite was originated in a supra-subduction zone setting. The plagiogranites have compositions of tonalites and trondhjemites, containing 56.93–77.93wt% SiO2, 1.27–10.79wt% Na2O and 0.05–0.71wt% K2O. They are slightly enriched in LREE over HREE and display positive anomalies in Eu, Zr, Hf but negative Nb anomalies. Very low TiO2 contents (0.03–0.2wt%) of the plagiogranites suggest that they were not products of fractional crystallization of MORB but were generated by hydrous melting of gabbros. This process is also supported by the nearly constant contents of both La and Yb with increasing SiO2 of the plagiogranites. The Sr-Nd-Hf isotopes of the plagiogranites indicate the involvement of crustal component in their genesis. We proposed that the Myitkyina plagiogranites were generated by partial melting of the mafic protoliths in the subducted slab.

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.

Geochemistry and zircon-apatite U-Pb geochronology of mafic dykes in the Shuangxiwu area: Constraints on the initiation of Neoproterozoic rifting in South China

The Shuangxiwu area of northwestern Zhejiang Province preserves the most complete magmatic records of the Neoproterozoic orogenic cycle at the eastern segment of the Jiangnan Orogen, South China. However, the transition from orogenesis to post-orogenic extension or rifting still remains debated and this led to great confusion on the breakup of Rodinia supercontinent. In this study, we carried out LA-ICP-MS zircon and apatite U-Pb dating and whole-rock geochemical analyses for the mafic dykes that intruded the Shuangxiwu arc volcanic rocks and the Heshangzhen Group in northwestern Zhejiang Province. The U-Pb dating results indicate that the mafic dykes formed at ∼760Ma, rather than ca. 860–850Ma as previously suggested. All the rocks are tholeiitic in compositions and can be divided into three subgroups based on their geochemistry. Group-1 has the lowest TiO2 contents (0.7–1.0wt%) and intermediate Mg# values (40–47), and they show enrichments in large ion lithophile elements and depletions in high field strength elements, resembling typical continental arc basalts. Group-2 has the highest TiO2 contents (1.5–3.6wt%) and Mg# values (50–55), and they do not show anomalies in P, Zr and Hf, similar to ocean island basalts. Group-3 has elevated SiO2 of 58.6–61.2wt%, intermediate TiO2 contents of 1.2–1.5wt%, the lowest Mg# values of 21–30, and negative anomalies in Nb, Ta, Ti and Y. The synthetic studies of Sr-Nd isotopes indicate two end-members for the formations of the three types of mafic dykes: depleted asthenospheric mantle and metasomatized lithospheric mantle. The upwelling of asthenosphere at ca. 760Ma led to partial melting of the pre-existed metasomatized lithospheric mantle to generate the Group-1 rocks with arc-like geochemical features. Continued extension resulted in low-degree partial melting of asthenospheric mantle and subsequent interaction with Group-1 magmas form the Group-2 rocks. The Group-3 rocks were resulted from AFC processes of the Group-2 magmas. Based on our new data and the available U-Pb ages of Neoproterozoic magmatic rocks, the rifting of South China from Rodinia should have not happened before ca. 820Ma.

Geochemistry, petrology, and mineralization in volcanic rocks located in south Neyshabour, NE Iran

This paper presents the new geochemical isotopes Sr and Nd, and the mineralization data for the south Neyshabour volcanic rocks located in NE Iran. Based on the chemical classifications, the studied rocks are basaltic trachy andesite, trachy andesite, trachyte, and trachy dacite in composition. All the analyzed volcanic rocks display enrichment in light rare earth elements (LREE) relative to the heavy rare earth elements (HREE), have significant negative Ti and Nb anomalies, and have a positive U anomaly. The tectonic discrimination diagrams for the volcanic rocks in the studied area show a post-collisional arc environment. These characteristics are the specifications of the subduction-related volcanic rocks generated in a post-collisional setting. The initial 87Sr/86Sr ratios ranging from 0.70408 to 0.70593and the eNdi values between +3.34 and +5 for the four samples analyzed indicate that the studied rocks are derived from a lithospheric mantle source. Finally, it is concluded that these volcanic rocks should have formed in a post-collisional environment that followed the Neo-Tethys subduction. There are strong evidence for copper mineralization in these volcanic rocks. The main copper oxide minerals are malachite and atacamite. The copper sulfide minerals such as chalcocite, minor bornite, and covellit are also present. Chalcocite is the most abundant sulfide ore mineral present in this area. This mineralization is observed as open space filling and thin veinlets, and it is partially controlled by linear structures and fault zones. Based on the identified characteristics, this ore deposit is hydrothermal. Carbonate alteration is frequently seen in the area but argillic alteration is very low, and this issue displays a hydrothermal solution with an alkaline pH.

Petrogenesis of Late Cretaceous Volcanism in Kazhaba Area and its relationship with mantle plume activity of Reunion hotspot

Basaltic pillow lavas near the Kazhaba village in Balochistan are found in the tectonic slivers of the Bagh complex: the melange zone beneath the Muslim Bagh ophiolite complex. These vol-canics are mainly represented by alkali basalts. The petrography and chemistry suggest that these volcanic rocks belong to mildly to strongly alkaline, intra-plate volcanic rock series. Their low Mg# and low Cr, Ni and Co contents suggest that the parent magma of these volcanics was not directly derived from a partially melted mantle source, but resulted fractionation in an upper level magma chamber, en-route to eruption. Their LILE and HFSE, enriched primordial mantle-normalized patterns with marked positive Nb anomalies further confirm their within-plate geochemical signatures and are consistent with an enriched mantle source. Their highly enriched LREE patterns and high (La/Yb)N and (Ce/Yb)N ratios suggest a partially melted garnet-lherzolite parent magma source. The Zr versus Zr/Y studies suggest that these volcanics were derived from about 15% partially melted enriched mantle source. It is suggested that these Late Cretaceous intra-plate volcanics may represent the mantle plume activity of the Reunion hotspot, and were erupted during the passage of Ceno-Tethys Ocean floor prior to the passage of Indian Plate over it.

Geochemistry and Zircon U–Pb age of the Yao’an pseudoleucite porphyry, Yunnan Province, China

The Yao’an Pb–Ag deposit, located in the Chuxiong Basin, western Yangtze Block, is an important component of the Jinshajiang–Ailaoshan alkaline porphyry–related polymetallic intrusive belt. This complex suite of rock bodies includes a vein of pseudoleucite porphyry within deposits of syenite porphyry and trachyte. The pseudoleucite is characterized by a variable greyish, greyish-white, and greyish-green porphyritic texture. Phenocrysts are mainly pseudoleucite with small amounts of alkali feldspar and biotite. In an intense event, leucite phenocrysts altered to orthoclase, kaolinite, and quartz. Both the pseudoleucite porphyry and the syenite porphyry samples were typical alkali-rich, K-rich, al-rich rocks with high LaN/YbN ratios; enriched in light rare earth elements and large-ion lithophile elements, and depleted in high field strength elements; and with strongly negative Ta, Nb, and Ti (TNT) anomalies and slightly negative Eu anomalies—all characteristics of subduction-zone mantle-derived rock. We obtained a LA-ICP-MS zircon U–Pb age of 34.1 ± 0.3 Ma (MSWD = 2.4), which is younger than the established age of the Indian and Eurasian Plate collision. The magma derived from a Type-II enriched mantle formed in a post-collisional plate tectonic setting. The geochemical characteristics of the Yao’an pseudoleucite porphyry are powerful evidence that the porphyry’s development was closely linked to the Jinshajiang–Ailaoshan fault and to the Indian-Eurasian collision.

Geology, geochemistry and petrogenesis of post-collisional adakitic intrusions and related dikes in the Khoynarood area, NW Iran

The Khoynarood area is located in the northwest of Iran, lying at the northwestern end of the Urumieh–Dokhtar volcano-plutonic belt and being part of the Qaradagh–South Armenia domain. The main intrusive rocks outcropped in the area have compositions ranging from monzonite–quartz monzonite, through granodiorite, to diorite–hornblende diorite, accompanied by several dikes of diorite–quartz diorite and hornblende diorite compositions, which were geochemically studied in order to provide further data and evidence for the geodynamic setting of the region. The SiO2, Al2O3 and MgO contents of these rocks are about 58.32–68.12%, 14.13–18.65% and 0.68–4.27%, respectively. They are characterized by the K2O/Na2O ratio of 0.26–0.58, Fe2O3+MnO+MgO+TiO2 content about 4.27–13.13%, low Y (8–17ppm) and HREE (e.g., 1–2ppm Yb) and high Sr contents (750–1330ppm), as well as high ratios of Ba/La (13.51–50.96), (La/Yb)N (7–22), Sr/Y (57.56–166.25), Rb/La (1.13–2.96) and La/Yb (10–33.63), which may testify to the adakitic nature of these intrusions. Their chemical composition corresponds to high-silica adakites, displaying enrichments of LREEs and LILEs and preferential depletion of HFSEs, (e.g., Ti, Ta and Nb). The REE differentiation pattern and the low HREE and Y contents might be resulted from the presence of garnet and amphibole in the solid residue of the source rock, while the high Sr content and the negative anomalies of Ti, Ta and Nb may indicate the absence of plagioclase and presence of Fe and Ti oxides in it. As a general scenario, it may be concluded that the adakitic rocks in the Khoynarood were most likely resulted from detachment of the subducting Neo-Tethyan eclogitic slab after subduction cessation between Arabian and Central Iranian plates during the upper Cretaceous–early Cenozoic and partial melting of the detached slab, followed by interactions with metasomatized mantle wedge peridotite and contamination with continental crust.

A Neoarchean subduction recorded by the Eastern Hebei Precambrian basement, North China Craton: Geochemical fingerprints from metavolcanic rocks of the Saheqiao-Shangying-Qinglong supracrustal belt

The Saheqiao-Shangying-Qinglong supracrustal belt (SSQB) in the northern Eastern Hebei Precambrian basement (EHPB) is located in the northern margin of the Eastern Block (EB) of the North China Craton (NCC). The Shangying terrane constitutes the middle segment of the SSQB and contains primarily metamorphic volcanics and plutonic tonalitic gneisses. The metamorphic volcanics mainly consist of pyroxene plagioclase amphibolites, garnet plagioclase amphibolites, biotite plagioclase amphibole gneisses, and amphibole plagioclase gneisses. Zircon U-Pb-Lu-Hf isotopic analyses reveal that the metavolcanic rocks from the Shangying terrane crystallized at ∼2506–2613Ma with TDM (Hf) values of ∼2541–2944Ma. These metamorphic volcanic rocks are subdivided into four groups based on their lithological and chemical features. Group I consists chiefly of tholeiites that are characterized by slightly light rare earth element (LREE) depleted patterns and flat multi-element spider diagrams, which are similar to back-arc basin basalt (BABB)-like rocks and were derived from the partial melting of the depleted mantle. The tholeiites in Group II have slightly fractionated rare earth element (REE) patterns without Nb anomalies, exhibit an affinity to Nb-enriched basalt (NEB)-like rocks, and were produced by the partial melting of HFSE-enriched mantle peridotites. Group III is composed of slightly LREE-enriched tholeiites with negative Nb-Ta anomalies that resemble island arc tholeiites. Group IV comprises calc-alkaline basalts and andesites with highly enriched LREEs and evident Nb, Ta and Ti depletions that are geochemically similar to the products of island arcs. The island arc tholeiites and calc-alkaline basalt-andesites originated from the partial melting of sub-arc mantle peridotites that were previously metasomatized by slab-derived fluids/melts with the fractional crystallization of ferromagnesian minerals. Collectively, the BABB-like rocks, the NEBs, arc tholeiites and calc-alkaline basalt-andesites recorded magmatism from a subduction regime. Similarly, NEB, arc tholeiite-andesite-dacites and back-arc related rocks were recognized in the Saheqiao and Qinglong areas in the western and eastern parts of the SSQB (R. Guo et al., 2013, 2015), which further supports the existence of a Neoarchean convergent margin in the Eastern Hebei Precambrian basement (EHPB). Compared to the northwestern EHPB, the southeastern EHPB underwent lower metamorphism, contains arc dacites/rhyolites and greater contributions of the ancient continental crust, implying that the southeastern EHPB might have been or near a back-arc setting. Thus, we conclude that the SSQB experienced a southeastward-dipping subduction, which is consistent with studies of the adjacent regions (e.g., Western Liaoning Province). When combining the ∼2.5–2.6Ga supracrustal rocks and plutonic granitoid gneisses with depleted mantle origins along the northern margin of the EB of the NCC, we concluded that ∼2.5–2.6Ga was a significant period for juvenile crustal growth and that lateral accretion played a dominant role in crustal growth.