High Large Ion Lithophile Element Research Articles

Petrogenesis of the Paleogene potassium-rich volcanic rocks in the western Yangtze Craton, southeastern Tibetan Plateau

Paleogene potassium-rich volcanic rocks (PVRs) are widespread in the southeastern Tibetan Plateau, providing an opportunity to illustrate the architecture of the lithosphere and further constrain the development of the SE Plateau. Their petrogenesis, together with the geodynamic mechanism, remains in dispute. Mg isotopic compositions of the PVRs in western Yangtze Craton, along with new elemental and Sr-Nd-Pb isotopic data, are utilized to elucidate their petrogenesis and reveal the plausible metasomatic and genetic mechanisms. The PVRs give a compositional spectrum from basaltic trachyandesite to trachyte and are characterized by high large ion lithophile elements concentrations, negative anomalies in high field strength elements, and enriched Sr-Nd-Pb isotopes, suggesting an origin of enriched lithospheric mantle. The correlations among varying major oxides and fractional crystallization simulation results jointly reveal fractional crystallization of olivine and clinopyroxene in the magmatic process. The less-evolved PVRs present similar major oxide contents (e.g., Ti, Na, K) to those melts originated from carbonated peridotite + silicate sediment. Apart from enriched Sr-Nd-Pb isotopes, they also show high La/Yb ratios (11.73–58.95), low Ti/Ti* (0.08–0.71), Hf/Sm (mostly <0.70), and varying δ 26 Mg values (−0.13 to −0.36‰). Combined with the Sr-Nd-Mg isotope numerical simulations, it points out that the mantle source of the PVRs was replenished by a hybrid carbonate-silicic sediment, which was plausibly caused by the Neoproterozoic seafloor subduction. These volcanic rocks also present relatively high K 2 O (3.29–7.50 wt%) and K 2 O/Na 2 O ratios (1.00–2.85). Besides, varying elevated Rb/Sr and low Ba/Rb ratios, together with the La/Yb-Yb non-modal batch melting result, suggest that the enriched mantle source recorded by these PVRs is predominantly constituted by a hybrid phlogopite-bearing garnet-spinel lherzolite. Constrained by petrology studies and seismic tomography, the generation of the PVRs is proposed to be closely associated with the delamination of the thickened lithosphere and thereafter asthenosphere upwelling. • Paleogene K-rich volcanic rocks in the western Yangtze Craton are variably evolved. • These volcanic rocks are sourced from phlogopite-bearing garnet-spinel lherzolite. • Recorded mantle source was replenished by a hybrid carbonate-silicic sediment.

Geochemistry and Sr-Nd isotopic studies of Precambrian gneisses from central Aravalli Craton, NW India: Implications for crustal evolution and reworking

The Aravalli Craton of the Indian shield constitutes heterogeneous basement lithologies (Banded Gneissic Complex; BGC), and among them, the granitoids are the most voluminous lithology. The BGC comprises two lithotectonic units, viz., BGC-I and BGC-II. The BGC-II has been further classified as amphibolite facies Mangalwar and granulite facies Sandmata Complexes. In the present study, the gneisses of the Mangalwar Complex are geochemically categorized into (i) low-and high-pressure sodic gneisses and (ii) potassic gneisses. The sodic gneisses are metaluminous and characterized by high Sr/Y and LaN/YbN ratios; and exhibit subduction-related negative anomalies of Nb and Ti. The εNd (t = 2992 Ma) ranges from +2.3 to +3.1, with an average Nd-depleted mantle model age (TDM) of 3.06 Ga. The whole-rock Sm-Nd isochron age is ∼3.0 Ga (2992 ± 340 Ma). Genetically, the sodic gneisses originated from the melting of an enriched precursor (oceanic plateau) in an arc environment. These gneisses show strong correlations with the gneisses from BGC-I depicting similar geochemical signatures. In contrast, the potassic gneisses are characterized by slightly higher SiO2 along with high K2O and high large-ion lithophile elements and negative Eu anomalies along with negative εNd (t = 1.7 Ga) (−13.2 to −3.9), higher initial 87Sr/86Sr isotopic ratios and average TDM = 2.87 Ga. These geochemical features of the potassic gneisses indicate that they were derived from the reworking of the pre-existing TTG-like (sodic gneisses) crust during the Paleoproterozoic Era.

Evolved magmatic arcs of South Borneo: Insights into Cretaceous slab subduction

Combined U-Pb zircon data of granitic, dioritic, and gabbroic suites reveal development of arc-related magmatism in South Borneo throughout the Late Mesozoic but dominantly in the Cretaceous (144–72 Ma). Cretaceous igneous suites constitute the Schwaner-Singkawang, Kuching-LongPahangai, and Meratus-West Java magmatic arc zones. Arc-related felsic suites are low-K tholeiitic, and calc-alkaline to high-K calc-alkaline. Most of them are metaluminous (A/CNK 0.7–1.1), I-type, and have high large-ion lithophile element (LILE) contents, low high-field strength element (HFSE) contents, and low crystallization temperatures (688–769 °C). A few rocks have adakite characteristics, such as high Sr/Y and low Y. Late Cretaceous granitic suites from South Schwaner, which evolved at elevated temperature (761–852 °C), have diverse compositions that range from I-type transitional to A-type. The highly fractionated chemistry of these samples is attributed to an intraplate stretching tectonic setting. This regime is consistent with tectonic controls of the synchronous tholeiitic suites within North Schwaner. Two subduction systems beneath Sundaland converged and interacted during the Cretaceous. The Paleo-Pacific subduction in the north built arc-trench architectures, as shown by the Early Cretaceous Lupar Line mélange, the Schwaner to Singkawang magmatic arc, and the Late Cretaceous Lupar Line mélange and Kuching-LongPahangai arc. The Meso-Tethys subduction in the southeast produced the Cretaceous Schwaner-Singkawang magmatic arc, Meratus to Luk-Ulo mélanges, and Meratus-West Java magmatic arc. When entered into Late Cretaceous, slab rollback led to crustal stretching in the Schwaner to Singkawang regions, with the production of A-type transitional and tholeiitic suites. Though debatable, we infer the SW Borneo block to be a Sundaland continuation; Semitau shows an affinity for South Cathaysia. Both of the terranes would accrete onto Sundaland during the Indosinian orogeny.

The Paleo-Mesoarchaean Gondpipri Mafic-Ultramafic Intrusions, Western Bastar Archaean Craton, Central India: Insights from Bulk-Rock Geochemistry and Sm-Nd and S Isotope Studies on the Formation of Ni-Cu-PGE Mineralization

Abstract Magmatic Ni-Cu-platinum group element (PGE)-Te mineralization in the Gondpipri mafic-ultramafic layered intrusion of ca. 3323 ± 74 Ma age, western Bastar craton, central India, is one of the most prospective exploration targets for magmatic sulfides in India. The Gondpipri layered intrusion is divided into two distinct groups of rocks based on their mineralization potential, which include (1) mineralized layered gabbro and pyroxenite and (2) a barren olivine gabbro intrusion. The host rocks show Cu + Ni concentrations up to 5,000 ppm with a Cu/Ni ratio &amp;lt;1 and all PGE values between 0.1 and 1.1 ppm. Mineralization occurs in two modes: type I mineralization occurring as blebs, specks, and dissemination and type II mineralization occurring as stringers and minor veins. The geochemical data suggest that the parental magma of the host rock was generated at depths between spinel and garnet peridotite mantle source regions and subsequently modified by assimilation fractional crystallization (AFC) of the continental crust. High large ion lithophile elements, Th/Yb ratios of the studied rocks, and Sm-Nd isotope studies are consistent with a depleted mantle source. The geochemical proxies such as Th versus Ba/Th and (Ta/La)PM versus (Hf/Sm)PM and higher Sr/Nd (2.21–82.58) ratios indicate involvement of fluid-related subduction metasomatism and enrichment processes in an island-arc tectonic setting. Mineral assemblages and textural relationships between platinum group minerals (PGMs) and base metal sulfides suggest that sulfide-silicate liquid immiscibility was brought about by the precipitation of magnetite/Cr magnetite resulting in sulfide saturation in the melt by decreasing S solubility. Sulfur isotope compositions (δ34S: 1.61–3.30‰) and Sm-Nd geochemistry suggest that the sulfur was added in the tholeiitic magma by magmatic process. Crustal contamination played a significant role in sulfide saturation and in bringing about PGE and Te, As, Bi, Sb, Se (TABS) mineralization. PGM-NiTeBi developed at relatively low temperatures, whereas moncheite (PtPd)Te2 and merenskyite (PdTe) were formed at 650°C. The identification of Ni-Cu-PGM-Te in the margin of the western Bastar craton boosts deeper subsurface exploration.

Identifying deep recycled carbonates through Miocene basalts in the Maguan area, SE Tibetan Plateau

Cenozoic mantle-derived magmas are widespread on the Tibetan Plateau, and provide evidence for the evolution of deep mantle and its influence on the Plateau development. Miocene basalts in the Maguan area on the southeastern Plateau have high MgO (9.13–13.10 wt%) and Mg# (0.60–0.70) with high Ce/Pb (10.6–32.5) and Nb/U (43.7–52.9) ratios, similar to those of oceanic basalts. Distinct from Eocene-Oligocene mantle-derived potassic magmas in Western Yunnan and Cenozoic basic volcanic rocks in Tengchong, these Maguan basalts are characterized by high large ion lithophile elements (LILEs) concentrations, positive anomaly in high field strength elements (HFSEs) and depleted SrNd isotopes, consistent with the melt of an asthenospheric mantle origin. The high Ce/Pb and Nb/Y (1.80–2.68) ratios together with low Ba/Y and Ba/Th ratios indicate a significant input of slab-derived melt into the asthenospheric source. Besides, Hf/Hf* and Ti/Ti* ratios are significantly lower than those of ocean island basalt (OIB), which are proportional to the lighter δ26Mg (-0.6‰ to -0.4‰) values. Based on the results of experimental petrology, the SrMg isotope mixing model suggests that the asthenospheric mantle beneath the Maguan area had undergone the significant metasomatism of recycled carbonates prior to the late Miocene. The above petrological and geochemical understanding, together with the geophysical data, allows us to propose that the mantle metasomatism is most probably associated with the Neo-Tethys seafloor subduction, which is further testified by the decoupling between depleted SrNd isotopes and elevated LILE concentrations.

Petrogenesis of a calc-alkaline lamprophyre (minette) from Thanewasna, Western Bastar Craton, Central India: insights from mineral, bulk rock and in-situ trace element geochemistry

AbstractThe lamproites and kimberlites are well known from the Eastern Bastar Craton, Central India. However, a Proterozoic lamprophyre dyke is discussed here, from the Western Bastar Craton (WBC). The field geology, petrographic, mineralogical and whole-rock and in-situ trace element geochemistry of biotite are described to understand the petrogenesis and lithospheric evolution in the WBC. The Thanewasna lamprophyre (TL) is undeformed and unmetamorphosed, intruded into c. 2.5 Ga charnockite and metagabbro but closely associated with c. 1.62 Ga undeformed Mul granite. The TL has a characteristic porphyritic texture, dominated by phenocrysts of biotite, microphenocryst of amphibole, clinopyroxene and a groundmass controlled by feldspar. Mineral chemistry of biotite and amphibole suggest a calc-alkaline (CAL) type, and pyroxene chemistry reveals an orogenic setting. The TL is characterized by high SiO2 and low TiO2, MgO, Ni and Cr, consistent with its subcontinental lithospheric origin. The presence of crustal xenolith and ocelli texture followed by observed variations in Th/Yb, Hf/Sm, La/Nb, Ta/La, Nb/Yb, Ba/Nb indicate substantial crustal contamination. Whole-rock and in-situ biotite analysis by laser ablation inductively coupled plasma mass spectrometry show low concentrations of Ni (30–50 ppm) and Cr (70–150 ppm), pointing to the parental magma evolved nature. Enrichment in H2O, reflected in magmatic mica dominance, combined with high large ion lithophile element, Th/Yb ratios, and striking negative Nb–Ta anomalies in trace element patterns, is consistent with a source that was metasomatized by hydrous fluids corresponding to those generated by subduction-related processes. Significant Zr–Hf and Ti anomalies in the primitive mantle normalized multi-element plots and the rare earth element pattern of the TL, similar to the global CAL average trend, including Eastern Dharwar Craton lamprophyres. Our findings provide substantial petrological and geochemical constraints on petrogenesis and geodynamics. However, the geodynamic trigger that generated CAL magmatism and its role in Cu–Au metallogeny in the WBC, Central India, is presently indistinct in the absence of isotopic studies. Nevertheless, the lamprophyre dyke is emplaced close to the Cu–(Au) deposit at Thanewasna.

Transformation from oxidized to reduced alkaline magmas in the northern North China Craton

The oxygen fugacity of a magma controlling element distribution and mobility is influenced by multiple factors including source composition and magmatic processes (e.g., assimilation and degassing). Here, whole-rock and in situ mineral analyses of the alkaline complex from the northern North China Craton are reported to investigate the influence of source composition and magmatic processes on the evolution of the melt oxidation state. The alkaline complex comprises orthoclase pyroxenites, pyroxene syenites, and syenites with similar mineral assemblages and trace element patterns. Consistent zircon and titanite U-Pb age of ~234 Ma is presented for the alkaline rocks. Their high large ion lithophile elements content, negative high field strength element anomalies, and high 87Sr/86Sr ratio suggest a metasomatized mantle origin by recycled crustal material. Magnetite and hematite inclusions were found in the early-crystallized minerals including clinopyroxene and apatite. Fluorapatites are characterized by high S (SO3 up to 0.64 wt%) and low Mn contents, with estimated ΔFMQ higher than +3. These features together indicate a primary oxidized magma inherited from the metasomatized mantle.However, inclusion-rich fluorapatites with decreasing S and increasing Mn content from the core to the rim were found in some pyroxene syenites. Moreover, the S content of inclusion-rich fluorapatite from these pyroxene syenites is lower than that of other rock types in the complex, which may correspond to a decrease in oxygen fugacity of ~2.5 log units. These features suggest that the oxidized magma was later transformed into reduced magma. Oriented Cu-rich sulfide in inclusion-rich fluorapatite formed during the reducing event. δ65Cu of the pyroxene syenite with inclusion-rich fluorapatite (−0.06 to −0.24‰) and sulfides in fluorapatite (−0.26‰) are lighter than those of other rock types (+0.30 to +0.43‰), suggesting that the sulfides were derived from extraneous materials via magma assimilation. Variable and high 87Sr/86Sr ratios (0.7071–0.7085) in titanites and high-207Pb/206Pb rims of zoned K-feldspar in the pyroxene syenite imply that the reduced extraneous materials were derived from ancient crust. Overall, in situ mineral analyses and Cu isotopes indicate a direct link between the transformation of alkaline magma from an oxidized to a reduced state with source composition and magmatic processes, and improve our understanding of element mobility and distribution.

Generation of a potassic to ultrapotassic alkaline complex in a syn-collisional setting through flat subduction: Constraints on magma sources and processes (Otjimbingwe alkaline complex, Damara orogen, Namibia)

The ~545 Ma-old syn-collisional Otjimbingwe alkaline complex is composed of pyroxene-amphibole-biotite-bearing, mildly nepheline-normative to quartz-normative rocks ranging in composition from monzogabbro to monzonite, syenite and granite. The alkaline rocks have moderate to high SiO2 (50.5–73.0 wt%) and Na2O + K2O (5.1–11.5 wt%) and moderate to low MgO (6.6–0.2 wt%) concentrations. All samples have high large ion lithophile element (LILE: Ba up to 4600 ppm) and high-field-strength element contents (HFSE; Zr: 155–1328 ppm; Nb: 16–110 ppm; Ta: 1.4–7.1 ppm and Hf: 4–24 ppm) and have strongly fractionated LREE patterns ((La/Yb)N = 14–51). The most primitive members lack significant negative Eu anomalies. Mantle-normalized multi-element diagrams show depletion in Ba, Rb, Nb (Ta), P and Ti. The alkaline rocks have moderate radiogenic initial 87Sr/86Sr ratios (0.7061–0.7087) and unradiogenic initial ɛNd values (−3.9 to −6.1). This isotope signature, associated with high LREE/HFSE ratios indicates that the parental melts were generated in enriched portions of the shallow lithospheric mantle, which was probably affected by previous subduction zone processes. In addition, correlations between Sr and Nd isotopes indicate that some of these variations result from combined crustal assimilation and fractional crystallization (AFC) processes. A new model of flat subduction is presented that explains most of the unsolved problems in the orogenic evolution of the Damara orogen, namely (i) the absence of early intrusive rocks with a clear subduction zone setting, (ii) the absence of high-pressure rocks such as blueschists and eclogites, (iii) the unusual distribution of igneous rocks with a clear predominance of granite and granodiorite and (iv) the need for a asthenospheric window during a classical subduction to explain the high T/moderate P granulite facies conditions in the overriding plate.

Tectonic Transformation and Metallogenesis of the Yanshan Movement during the Late Jurassic Period: Evidence from Geochemistry and Zircon U-Pb Geochronology of the Adamellites in Xingcheng, Western Liaoning, China

The Yanshan Movement occurred mainly during the Middle-Late Jurassic, and gave rise to NE trending structures, magmatic events, volcanism and mineral resources. The transformation and evolution of the movement during the Middle-Late Jurassic were investigated from the rock assemblage, geochemistry, and chronology in adamellites which were exposed in the Xingcheng area, western Liaoning. Two types of adamellites were recognized—biotite adamellites with the formation age of 172–168 Ma and garnet-bearing adamellites of 158–152 Ma. All the samples of the two types of adamellites displayed enriched characteristics with high content of SiO2 (66.86–75.55 wt.%) and total alkali (Na2O + K2O = 7.56–8.71 wt.%), high large ion lithophile element (LILE: K, Rb, Sr), and low high field strength element (HFSE: Ce, Ta, P, Ti). The biotite adamellites belong to metaluminous-peraluminous I-type granites, and show volcanic arc granite characteristics, and were formed by partial melting of the ancient crust in the compressional setting that resulting from the subduction of the Paleo-Pacific plate beneath the north margin of the North China Craton (NCC). The garnet-bearing adamellites are also metaluminous-peraluminous I-type granites, with characteristics of both the compressional and extensional regimes, which were formed at the middle-late stages of the continuing subduction of the Paleo-Pacific plate, while simultaneously, the frontal side of the subduction slab began to roll back, leading to an extensional environment. Combining with regional geophysical studies and our petrological and geochemical studies, we propose that the eastern segment of the northern margin of NCC may have been controlled by the Paleo-Pacific tectonic domain at the latest in the Middle Jurassic, while the initiation of the tectonic regime from a compressional to an extensional environment was during the Late Jurassic (158–152 Ma) as a response of the Yanshan Movement. Simultaneously, geochronological statistics of the ore deposits in western Liaoning show that the Mesozoic endogenetic metalliferous deposits formed in a compressive environment influenced by the subduction of the Paleo-Pacific plate, similar to the magma events in ages, and the magmatism provided the thermodynamic condition and the source of metallogenic hydrothermal fluid for mineralization.

Tectonic Implications of the Carboniferous Volcanic Rocks in the Eastern Junggar — Evidence from Zircon U‐Pb Ages and Geochemistry

AbstractWell Drilling shows that the volcanic rocks from the Carboniferous Batamayineishan Formation in the Eastern Junggar basin are mainly composed of volcaniclastic rocks (av. 52%) and volcanic lavas (32%), with a small amount of volcanic pyroclastic lavas (av. 11%). The volcanic lavas are basalt‐basaltic andesite‐andesite‐dacite assemblage. The LA‐ICP‐MS zircon U‐Pb dating of the andesite and the dacite yielded 325∼321 Ma and 310 Ma ages, respectively, which is of high agreement with the published age (300 Ma) of basalts from this Formation, it is implied that an important volcanic activity occurred in Junggar basin in the late Carboniferous. The lavas have low TiO2 and high Na2O, indicating a calc‐alkaline series. Geochemical data show that they are characterized by LREE‐enriched patterns with slightly negative Eu anomalies. The rocks have high large ion lithophile element (LILE), and low high field strength element (HFSE) concentrations, with strong negative Nb, Ta and Ti anomalies. From basic through intermediate to felsic, the depletions in Sr, Ti and P of the studied volcanic rocks increase gradually. These geochemical characteristics indicate that the volcanic rocks are magmatic evolution products attributed to partial melting of mantle‐derived spinelle lherzolite related to oceanic subduction in an island‐arc setting. In combination with the LA‐ICP‐MS zircon U‐Pb dating, it is inferred that subduction of the Junggar Ocean in eastern Junggar basin lasted to the Late Carboniferous. Consequently, the final closure of the Junggar Ocean occurred most likely after 310 Ma.

Back-arc basin development: Constraints on geochronology and geochemistry of arc-like and OIB-like basalts in the Central Qilian block (Northwest China)

The Lajishan belt of the Central Qilian block was a back-arc basin during Early Paleozoic. The basaltic magmatism and temporal evolution in this basin provide an opportunity to study the development of back-arc basin in an active continental margin. In this study, we carry out an integrated study of geochronological, geochemical and Sr-Nd isotopic compositions for the Early Paleozoic arc-like and OIB-like basalts. The Lajishan arc-like basalts are enriched in large ion lithophile element (LILE) and show negative Nb and Ta anomalies whereas the OIB-like basalts have high LILE abundances and show positive Nb and Ta anomalies. The arc-like basalts have initial 87Sr/86Sr values of 0.7050–0.7054 and εNd(t) values of +0.51–+2.63, and the OIB-like basalts have initial 87Sr/86Sr values of 0.7049–0.7050 and εNd(t) values of +0.66–+1.57. The geochemical and Sr-Nd isotopic compositions suggest that the arc-like basalts are derived from partial melting of a depleted mantle source metasomatized by slab-derived components at shallow depth levels, and the OIB-like basalts also originated from a metasomatized mantle wedge source. U-Pb zircon dating yielded the ages of ~494 ± 4 Ma for the arc-like basalts and ~468 ± 6 Ma for the OIB-like basalts. We argue that the arc-like basalts are products of back-arc extension before the back-arc rifting initiated in earlier stage, resulting from the northward subduction of the Qaidam-West Qinling oceanic slab, while the OIB-like basalts represent products of further back-arc spreading in response to rollback of the Qaidam-West Qinling oceanic lithospheric slab. The association of arc-like and OIB-like basalts in the Lajishan belt records the development of back-arc basin from initial rifting to subsequent spreading, offering insight into how basaltic magmatism generates in the formation of back-arc basin in subduction zone setting.

Evidence for a chondritic impactor, evaporation-condensation effects and melting of the Precambrian basement beneath the ‘target’ Deccan basalts at Lonar crater, India

The ∼1.88km diameter Lonar impact crater formed ∼570ka ago and is an almost circular depression hosted entirely in the Poladpur suite of the ∼65Ma old basalts of the Deccan Traps. To understand the effects of impact cratering on basaltic targets, commonly found on the surfaces of inner Solar System planetary bodies, major and trace element concentrations as well as Nd and Sr isotopic compositions were determined on a suite of selected samples composed of: basalts, a red bole sample, which is a product of basalt alteration, impact breccia, and impact glasses, either in the form of spherules (<1mm in diameter) or non-spherical impact glasses (>1mm and <1cm). These data include the first highly siderophile element (HSE) concentrations for the Lonar spherules. The chemical index of alteration (CIA) values for the basalts and impact breccia (36.4–42.7) are low while the red bole sample shows a high CIA value (55.6 in the acid-leached sample), consistent with its origin by aqueous alteration of the basalts. The Lonar spherules are classified into two main groups based on their CIA values. Most spherules show low CIA values (Group 1: 34.7–40.5) overlapping with the basalts and impact breccia, while seven spherules show significantly higher CIA values (Group 2: >43.0). The Group 1 spherules are further subdivided into Groups 1a and 1b, with Group 1a spherules showing higher Ni and mostly higher Cr compared to the Group 1b spherules. Iridium and Cr concentrations of the spherules are consistent with the admixture of 1–8wt% of a chondritic impactor to the basaltic target rocks. The impactor contribution is most prominent in the Group 1a and Group 2 spherules, which show higher Ni/Co, Ni/Cr and Cr/Co ratios compared to the target basalts. In contrast, the Group 1b spherules show major and trace element compositions that overlap with those of the impact breccia and are characterized by high EFTh (Enrichment Factor for Th defined as the Nb-normalized concentration of Th relative to that of the average basalt) as well as fractionated La/Sm(N), and higher large ion lithophile element (LILE) concentrations compared to the basalts. The relatively more radiogenic Sr and less radiogenic Nd isotopic composition of the impact breccia and non-spherical impact glasses compared to the target basalts are consistent with melting and mixing of the Precambrian basement beneath the Deccan basalt with up to 15wt% contribution of the basement to these samples. Variations in the moderately siderophile element (MSE) concentration ratios of the impact breccia as well as all the spherules are best explained by contributions from three components – a chondritic impactor, the basaltic target rocks at Lonar and the basement underlying the Deccan basalts. The large variations in concentrations of volatile elements like Zn and Cu and correlated variations of EFCu-EFZn, EFPb-EFZn, EFK-EFZn and EFNa-EFZn, particularly in the Group 1a spherules, are best explained by evaporation-condensation effects during impact. While most spherules, irrespective of their general major and trace element composition, show a loss in volatile elements (e.g., Zn and Cu) relative to the target basalts, some spherules, mainly of Group 1, display enrichments in these elements that are interpreted to reflect the unique preservation of volatile-rich vapour condensates resulting from geochemical fractionation in a vertical direction within the vapour cloud.

Mineral chemistry of late Variscan gabbros from central Spain: constraints on crystallisation processes and nature of the parental magmas

Major and trace element chemistry has been determined in the main minerals (olivine, orthopyroxene, clinopyroxene, plagioclase, amphibole, phlogopite) from the Talavera and La Solanilla gabbroic intrusions (Spanish Central System). The results show a heterogeneous mineral composition illustrative of the chemical evolution of calc-alkaline basic magmas during crystallization. The formation of incompatible element-rich interstitial minerals, such as amphibole, phlogopite and accessory phases (e.g., zircon, apatite), and the presence of zoning patterns in major minerals towards a more evolved composition, point to a progressive differentiation of the parental melt. The chemical variation depicted by trace elements usually incompatible with clinopyroxene (e.g., rare earth elements (REE), Th, Zr, Nb, Ti) implies co-precipitation with phases displaying a compatible behaviour with respect to these elements (e.g., apatite, zircon, amphibole). However, the very high LILE (large ion lithophile elements) and LREE (light REE) shown by clinopyroxene can not be explained exclusively by a closed-system in-situ crystallization process. Several features of the mineral chemistry (e.g. positive correlation of (La/Sm)N with La and Na in clinopyroxene and plagioclase, respectively) support the involvement of an external component (wall-rock assimilation or hybridization with an evolved melt). The petrography and mineral chemistry of the gabbros, in conjunction with bulk rock data from previous studies, favour a calc-alkaline affinity for the parental melts. Thus, metasomatism in the mantle sources related with this basic magmatism might be associated with recycling of crustal components during the Variscan orogeny.

Chapter 5 The magmatism of the Azores islands

Abstract Magmas in the Azores (Portugal) were erupted from both fissure zones and central volcanoes during overlapping periods. Fissure zones follow extensional trends oriented accordingly with the regional tectonics, erupting basalts and hawaiites. Central volcanoes are characterized by transtensive tectonics and erupted basaltic to trachytic magmas. Highly porphyritic magmas, consisting of cumulitic olivines and clinopyroxenes, can be also erupted during flank eruptions. Volcanism occurs because of the different direction of the plate movement at the triple junction where islands are located. Magma production and its ascent occur in response to the stresses of the lithosphere over a geochemically heterogeneous mantle, enriched in incompatible trace elements and water. The variability in the radiogenic isotopes results from the interaction of the differently enriched mantle sources with a ubiquitous and slightly depleted Azores mantle. Finally, a contribution of recycled oceanic crust is evidenced by a progressive east–west variation in Pb isotope ratios across the island of São Miguel. Primitive melts were generated by 1–7% partial melting of a garnet-bearing peridotite. Overall, basalts show relatively high large-ion lithophile element (LILE) abundances and LILE/high-field-strength element ratios that point to chemical heterogeneities in the mantle sources.

Episodic magmatism at 105 Ma in the Kinki district, SW Japan: Petrogenesis of Nb-rich lamprophyres and adakites, and geodynamic implications

Cretaceous episodic magmatism produced Nb-rich lamprophyres and adakitic granitoids in the Kinki district of SW Japan. K–Ar dating of minerals from the lamprophyres, adakites, and hornblende peridotite xenoliths yielded ages of 109–99Ma, indicating a short-lived episodic magmatism. The lamprophyres generally display primitive high-Mg basaltic to basaltic andesite compositions with high Mg# and high Cr and Ni contents that preclude substantial differentiation. Some high-Nb basalt (HNB) and Nb-enriched basalt (NEB) compositions also occur. The lamprophyres have high large-ion lithophile element (LILE) and high field-strength element (HFSE) contents and variable (La/Yb)n ratios, and can be divided into high-(La/Yb)n (12.5–22.1) and low-(La/Yb)n (3.6–6.1) groups. The former contains nepheline-normative rocks with positive initial εNd(T) values, whereas the latter contains hypersthene-normative subalkaline rocks with negative initial εNd(T) values. The adakitic granitoids have relatively high TiO2, Nb, and Ta contents compared to more typical high-silica adakites elsewhere, indicating that they were produced by high temperatures (ca. 920 to 970°C) during slab melting.Early Cretaceous slab rollback and the accompanying asthenospheric upwelling at 105Ma could form a transitory thermal anomaly, and hence induce melting of the subducted slab to form adakitic granitoids, and produce metasomatized wedge mantle to form the lamprophyres. The high-(La/Yb)n lamprophyres originated from small degrees of partial melting of an enriched metasomatized mantle wedge within the garnet stability field at depths of ≥70km, whereas the low-(La/Yb)n lamprophyres originated from a different mantle source by a comparatively larger degree of partial melting in a relatively shallow part of the mantle wedge. The magmatic diversity of the Kyoto lamprophyres thus derives primarily from a heterogeneous mantle source that has been variably affected by the results of subduction.

Zircon U–Pb ages, trace elements and Nd–Hf isotopic geochemistry of Guyang sanukitoids and related rocks: Implications for the Archean crustal evolution of the Yinshan Block, North China Craton

The Yinshan Block is a unique coherent Neoarchean basement terrane of the Western Block of the North China Craton. In this paper, three Archean plutons, namely the Dajitu, Banshentu and Haolaigou plutons, have been investigated. Based on the field observations, the major difference among these plutons is that many ultramafic–mafic enclaves are enclosed in the Dajitu and Banshentu plutons, whereas none occurs in the Haolaigou pluton. Geochemically, the Dajitu and Banshentu plutons are characterized by low SiO2 contents (56–67wt%), high MgO (1.9–7.1wt%), Cr (10–947ppm), Ni (2–274ppm) and Mg# (44–64) contents, negative Nb, Ta and Ti anomalies, high large ion lithophile elements (LILEs), with no or slightly negative Eu anomalies, similar to a typical Archean sanukitoid. New geochronological data indicate that the Dajitu and Banshentu plutons were emplaced at 2523±7Ma and 2480±20Ma, with inherited ages of 2606±19Ma and 2647±19Ma, respectively. In contrast, the Haolaigou pluton shows high SiO2 (68–71wt%), MgO, Mg# (46–55), Cr, and Ni contents, negative Nb, Ta and Ti anomalies, and high LILEs, with strongly positive Eu anomalies, high Sr and Ba concentrations and Sr/Y and La/Yb ratios, and low Y and HREEs abundances. It was emplaced at 2466±15Ma, with an inherited age of 2638±21Ma. The whole-rock ɛNd(t) and zircon ɛHf(t) values of the Haolaigou pluton are positive. The whole-rock TDM(Nd) and zircon TDM1(Hf) values range between 2.6 and 2.8Ga with a peak at ∼2.7Ga, indicating that the melt was generated by the partial melting of a subducted slab interacting with the overlying mantle wedge. The Dajitu pluton has Nd and Hf isotope characteristics identical to those of the Haolaigou pluton, implying that the two plutons have similar sources. It should be noted, however, that the Dajitu pluton displays sanukitoid characteristics, which are the result of interaction with ultramafic–mafic enclaves. The whole-rock ɛNd(t) and zircon ɛHf(t) values of the Banshentu pluton are mostly negative. The whole-rock TDM(Nd) and zircon TDM1(Hf) values of the pluton range between 2.7 and 2.9Ga with a peak at ∼2.8Ga, indicating that the pluton was generated from the melting of a lower crust. The sanukitoid characteristics of the pluton resulted from interactions with an ultramafic enclave. Taken together, these results with the geochronological geochemical data for other Archean rocks in this area, suggest that the Yinshan Block experienced significant crustal growth at 2.8–2.6Ga and further crustal reworking that led to strong magmatism at ∼2.5Ga.

Volcanism from fissure zones and the Caldeira central volcano of Faial Island, Azores archipelago: geochemical processes in multiple feeding systems

AbstractMagmas in Faial Island, Azores (Portugal), were mostly erupted from two fissure zones and the Caldeira central volcano during overlapping periods. The fissure zones follow extensional trends oriented WNW and ESE and erupted nepheline- to hypersthene-normative basalts and hawaiites. The Caldeira central volcano builds the central part of the island, which is cut by the fissure zones. Ne-normative basalts show similar high-field-strength element (HFSE) concentrations but higher large ion lithophile element (LILE) concentrations than hy-normative equivalents. Primitive melts were generated by small (3–5%) degrees of partial melting of garnet-bearing peridotite, variably enriched in incompatible elements. Overall, basalts from Faial show relatively higher LILE abundances and LILE/HFSE ratios than those of the other islands of the Azores and of many other volcanoes in the Atlantic area. This feature indicates the existence of chemical heterogeneities in the mantle sources characterized by variable degrees of metasomatism, both at local and regional scales. Hawaiites evolved from basalts through 30–40% fractional crystallization of mafic phases plus some plagioclase, in deep reservoirs, at about 430–425 MPa (~ 15 km). The Caldeira central volcano rocks range from basalts to trachytes. Basalts, produced under similar conditions as fissure basalts, evolved to trachytes through large degrees of polybaric fractional crystallization (100–760 MPa; i.e. ~ 3.6–26 km), involving olivine, clinopyroxene, feldspar and minor quantities of amphibole, biotite, apatite and oxides. In contrast, mafic magmas from the fissure zones were erupted directly onto the surface from magma reservoirs mainly located at the crust–mantle boundary.

Geochemical evolution of basaltic volcanism within the tertiary basins of southeastern Korea and the opening of the East Sea (Sea of Japan)

We determined the geochemical and Sr–Nd–Hf–Pb isotopic compositions of volcanic rocks within the Tertiary basins of southeastern Korea to better understand the petrogenetic evolution of local volcanism. The volcanic rocks in this region are divided into two major groups based on their geochemical characteristics and spatiotemporal distributions: Eoil from the Yangnam Basin, and Daljeon from the Pohang Basin.The Eoil rocks (16–22Ma K–Ar ages) are primarily subalkaline basalt to basaltic andesite, whereas the Daljeon samples (13–15Ma K–Ar ages) are sodic alkaline phonotephrites. The Eoil basaltic rocks exhibit light rare earth element (LREE)-enrichment patterns with chondrite-normalized (La/Yb)N ratios of 3.6–5.0. Daljeon phonotephrites are typified by significant LREE-enrichment [(La/Yb)N=32.0–33.2] and slight negative Sm and Eu anomalies on the chondrite-normalized REE patterns. The Eoil samples show an arc signature on the primitive mantle-normalized multi-element variation diagrams, with high large ion lithophile element (LILE) enrichment and the depletion of high field strength elements (HFSE). In contrast, the Daljeon samples are dominated by high ratios of Nb and Ta to LILE and HREE, resembling typical oceanic island basalt (OIB) but distinguished by a significant trough in K, Rb, Sm, and Ti, and a positive anomaly in P. In Sr–Nd isotope space, all samples fall in the depleted quadrant, but the Eoil basaltic rocks are characterized by lower 143Nd/144Nd ratios at a given 87Sr/86Sr ratio than those of the Daljeon phonotephrites. The Eoil rocks have elevated 176Hf/177Hf ratios compared with the Daljeon samples, exhibiting a decoupling of Nd and Hf isotopes in Hf–Nd space. These observations, together with the Eoil volcanic rocks showing more radiogenic Pb isotopic compositions than the Daljeon samples, indicate two distinct mantle sources for the volcanism. The source of Eoil magmatism might be mantle wedge metasomatized by the addition of subducted pelagic sediment melts, whereas hydrous mineral-bearing sub-continental lithospheric mantle could be the dominant source for the Daljeon volcanism. The difference may be related to changes in the geodynamic setting induced by a back-arc opening in the East Sea, which culminated at around 15Ma. Lithospheric thinning caused by rifting produced the East Sea and the resultant thermal effect of asthenospheric upwelling may have initiated partial melting of the metasomatized lithospheric mantle, producing the Daljeon volcanism.

A Cretaceous forearc ophiolite in the Shyok suture zone, Ladakh, NW India: Implications for the tectonic evolution of the Northwest Himalaya

The northwestern Himalaya in India contains critical evidence of the convergent margin and collision tectonics between the Ladakh arc and the Karakoram block. Here we present new petrochemical and geochronological data from a forearc ophiolite at the Shyok‐Nubra river confluence. Whole-rock geochemical data show relatively high large-ion lithophile elements (LILE), light rare earth elements (LREE), and Ba, U and Pb anomalies, and depletions in La, Ce and Zr, particularly, the high-field strength elements (e.g. Nb, Ta); these geochemical characteristics are similar to those in modern ophiolites that formed in arc-related environments. Meta-volcanic greenschists that contain spinel layers have high MgO, Ni, Co, and Cr contents. In contrast, they have low contents of TiO2, very low Nb and Zr that are diagnostic of high‐Ca boninitic magmas in modern forearc settings, as in Izu–Bonin–Mariana (IBM) and Tonga. The spinels have high Cr# [Cr/(Cr+Al)] and Mg#[Mg/(Mg+Fe2+)], which are characteristic of spinels in forearc boninite‐type of melts. The spinel-free meta-volcanic samples have incompatible trace element abundance patterns similar to those of MORB. However, their enrichments in Cs, Rb, Pb, U and depletion in HFSE may reflect an input of subduction fluids that are different from MORB; these MORB-like basalts are suggestive of a forearc complex that erupted prior to the formation of boninitic rocks.Our data from the Shyok ophiolite indicate the existence of supra-subduction rocks on the southern Karakoram margin. Albite porphyroblasts in greenschists yield a K–Ar age of 104.4±5.6Ma that represents the time of early exhumation of the greenschists. The boninite‐type melts formed prior to 104Ma, thereby indicating that the time of initiation of subduction on the southern margin of the Karakoram block was before 104Ma. These geochemical signatures together with the spatial and temporal distribution of the arc rocks on the southern margin of Karakoram block suggest northward subduction of NeoTethys in the Early Cretaceous. The subsequent collision between the Ladakh arc and the Karakoram block thrusted/obducted the forearc ophiolite onto the southern edge of the Karakoram block probably between 74Ma and 97Ma.

Permian-Carboniferous arc magmatism and basin evolution along the western margin of Pangea: Geochemical and geochronological evidence from the eastern Acatlan Complex, southern Mexico

In the Acatlan Complex of southern Mexico, a late Paleozoic assemblage, consisting of a gabbro-diorite-tonalite-trondhjemite suite (Totoltepec pluton) and clastic-calcareous metasedimentary rocks (Tecomate Formation), postdates collisional orogeny that resulted in the amalgamation of Pangea. This region offers a rare opportunity to examine assemblages developed at different crustal levels along the periphery of Pangea at the critical stage between amalgamation and breakup. The Totoltepec pluton consists of minor mafic-ultramafic rocks (306 ± 2 Ma; concordant U-Pb zircon analysis) that are marginal to the main mafic-felsic intrusion (289 ± 2 Ma). Geochemistry of the marginal rocks indicates an arc tholeiitic to calc-alkaline character with high large ion lithophile elements (LILEs)/high field strength elements (HFSEs), flat rare earth element (REE) patterns, and initial ɛ Nd values of +1.3 to +3.3. The younger Totoltepec phase exhibits a calc-alkaline trace-element geochemistry with flat to moderately fractionated light (L) REE–enriched patterns and initial ɛ Nd values of –0.8 to +2.6, which are also consistent with an arc environment. The Sm-Nd isotopic signature is more primitive compared to contemporaneous arc-related igneous rocks in southern Mexico, suggesting the pluton was emplaced in a less mature, outboard part of the arc, and/or along a fault conduit. The Tecomate Formation, as currently defined, is a composite of lithologically similar strata deposited in several fault-bounded basins ranging from Carboniferous to Early Permian in age. To the south of the Totoltepec pluton, the depositional age of the Tecomate Formation is tightly constrained in one section to ca. 300 Ma, but in another section, it is between ca. 288 and ca. 263 Ma. The Tecomate Formation rocks are interpreted to have been derived from a late Paleozoic arc based on (1) arc-related geochemistry, (2) ɛ Nd (t) values ranging from –5.6 to +0.3 (t = 288 Ma) that overlap those of the Totoltepec pluton, and (3) detrital zircons with predominantly Carboniferous–Permian ages. The Totoltepec and Tecomate units in the study area form part of a continental arc extending from Guatemala to California, which necessitates subduction of the paleo-Pacific oceanic lithosphere beneath the western margin of a Pangea-A configuration.