Concentrations Of Large Ion Lithophile Elements Research Articles

Petrological and geochemical constraints on the magmatic evolution at the Ampato-Sabancaya compound volcano (Peru)

In order to gain insights into continental arc magmatic processes, we have conducted a petrological and geochemical study of major and trace elements and Sr, Nd, and Pb isotopes of the Ampato-Sabancaya compound volcano, which belongs to the Andean Central Volcanic Zone (CVZ). Whole-rock compositions for Ampato and Sabancaya range from andesites to dacites (56.7–69.3 wt% SiO2) and both belong to a medium- to high-K calk-alkaline magmatic series. Ampato-Sabancaya samples are characterized by high contents of large-ion lithophile elements (LILE; e.g., K, Rb, Ba, Th), low concentrations of high field strength elements (HFSE; e.g., Nb, Zr) and heavy rare earth elements (HREE; e.g., Yb), with consequently high La/Yb and Sr/Y ratios. An increase in these ratios is usually interpreted as a result of magmatic differentiation in the presence of garnet in the deep crust. A detailed analysis reveals that the rocks of Ampato-Sabancaya display three different compositional groups. (1) The first, composed mainly of andesites (56.7–59.8 wt% SiO2), corresponds to lavas from the early stage of the Ampato Basal edifice, as well as pyroclastic deposits from the Ampato Upper edifice. (2) The second group corresponds to andesitic and dacitic compositions (60.0–67.3 wt% SiO2) from the Ampato Basal edifice (Moldepampa stage), the Ampato Upper edifice, and the Sabancaya edifice. (3) The third group corresponds to dacitic compositions (65.0–69.3 wt% SiO2) associated with the Corinta Plinian fallout and pyroclastic flow deposits from the Ampato Upper edifice. This last group of dacites, erupted during the Ampato Upper edifice stage, have drastically different compositions from the other groups with Sr/Y (<27) and Sm/Yb (<4.7) ratios lower than other lavas and lacking evidence of amphibole and/or garnet fractionation during their genesis. As a whole, Sr, Nd, Pd isotopic ratios suggest that mantle-derived magmas are significantly affected by assimilation processes during their evolution, due to the thick (65–70 km) continental crust beneath the CVZ in southern Peru. In summary, the magmatic evolution of group 1 and 2 can be explained by a two-step model in which primitive magmas evolved in the deep crust in the so-called melting-assimilation-storage-homogenization (MASH)-type reservoirs by assimilation-fractional crystallization (AFC) processes involving garnet and/or amphibole. Then, amphibole-dominated upper crustal AFC processes and magma mixing are responsible for the geochemical diversity of the main ASCV trend. In contrats, the group 3 dacites followed an upper crustal AFC process (without amphibole) from a different primitive magma, which did not suffer the high pressure, garnet-dominated AFC processes. This evolution highlights the complexities associated to magma genesis and differentiation at continental arcs contructed on a thick crust.

Pb and O isotopic characteristics of single minerals in the Jianggang radial sand ridges of the southwestern Yellow Sea: Implications for provenance and pathways of silt-size sediments

Understanding the sediment provenance of the Jianggang radial sand ridges (RSRs) along the Jiangsu coast in the southwestern Yellow Sea is crucial for sustainable coastal development and land resource management. In this study, the provenance and transport patterns of silt-size sediments in the Jianggang RSRs were explored using Quartz oxygen (O) and K-feldspar lead (Pb) isotopic compositions and their large ion lithophile elements (LILEs) concentrations. The Pb-O isotopic compositions and concentrations of most LILEs in RSRs sediments were between those of the Yangtze River Mouth (YTZ), Old Yellow River Delta (OYR), and Modern Yellow River Mouth (MYR). The Pb-O isotopic compositions and typical elemental ratios were similar between the onshore and northwest offshore RSRs sediments, implying shoreward transport of offshore silt-size sediments. Through multidimensional scaling and graphic methods, it was found that the sediments of onshore and offshore RSRs originate mainly from the YTZ and OYR. Furthermore, the MixSIAR model showed that the contributions of the YTZ to onshore and offshore RSRs reached 33 ± 4 % and 36 ± 3 %, respectively. Followed by the OYR with contributions of 36 ± 3 % and 25 ± 8 %, respectively, and the contributions of the MYR and Korean Peninsula were <21 % and 8 %, respectively. Meanwhile, contributions from the Northern Chinese Deserts (approximately 10 %) deserved attention. Through the distribution of indicators, the transport patterns of silt-size sediments were proposed and compared with those of other fractions for the first time. According to the correlation analysis, the change in the area of central Jiangsu coast was mostly affected by terrestrial river supply and coastal mariculture. Therefore, it was necessary to control the scale of river reservoir construction and strengthen mariculture for sustainable land development and management. In future, comprehensive interdisciplinary and large temporal–spatial scales are suggested to further understand the coastal development.

Intra-oceanic subduction in the southeastern Paleo-Asian ocean: Evidence from early Permian gabbro of the Diyanmiao ophiolite in central Inner Mongolia, North China

Investigation of intra-oceanic subduction can improve our understanding of plate tectonic processes and the history of continental growth. Evidence for intra-oceanic subduction in the Paleo-Asian Ocean has recently become an important focus of research, including the Diyanmiao ophiolite in central Inner Mongolia, North China. Here, we report a newly discovered occurrence of early Permian gabbro in the Diyanmiao ophiolite zone. The gabbro yields a weighted mean zircon U–Pb age of 294.4 ± 2.2 Ma. The gabbro samples are characterized by moderate SiO2 (47.32–50.51 wt%), low TiO2 (0.26–0.54 wt%) and K2O (0.04–0.75 wt%), and high Na2O (1.84–4.52 wt%) contents, high Na2O/K2O ratios (2.92–58.29), and depleted chondrite-normalized light rare Earth element patterns that are similar to N-MORB. The gabbros show slightly lower contents of high-field-strength elements (e.g., Nb, Ta, and Ti) and slightly higher contents of large-ion lithophile elements (e.g., K, Rb, Ba, and U) relative to N-MORB. In addition, the gabbros show high εNd (t) values (8.0–9.8) that are similar to those of forearc basalt in the Diyanmiao ophiolite and N-MORB. Integrating these new data with available results for ophiolite and arc-magmatic rocks of central Inner Mongolia, we propose that the studied gabbro was formed during the initial stage of intra-oceanic subduction and that the Paleo-Asian Ocean was still in a subduction setting during the early Permian.

Oligo-Miocene subduction-related volcanism of the Loyalty and Three Kings ridges, SW Pacific: a precursor to the Tonga-Kermadec arc

The SW Pacific region contains several ridges and basins that are inferred to represent pre-Quaternary volcanic arcs and back-arc basins. The geology of these features is less well characterized than that of the active Tonga-Kermadec and Vanuatu arcs. We report new major and trace element, and Pb, Hf, Sr and Nd isotope data for 27 lavas dredged from the Loyalty and Three Kings ridges during the 2015 VESPA cruise of R/V l'Atalante. Low-K basalts were dredged from the seabed deeper than 3300 m, and high-K to shoshonitic suites from shallower ridge crests at 2000–3300 m. The samples are mainly basalts, with lesser trachybasalts, basaltic andesites, trachyandesites andesites, dacites, and one granite (anhydrous SiO2 and K2O + Na2O range from ∼47 to 64 and 1.5 to 11 wt% respectively). Trace element patterns allow discrimination of three geochemical signatures, identified as i) depleted, ii) transitional and iii) enriched, based on their light to heavy rare earth element (REE) ratios (with La/Sm ranging from 0.4 to 8). Depleted and transitional samples are basalts, featuring REE concentrations similar to MORB, but with high field strength element and large ion lithophile element contents, typical of back-arc basin basalts. The most enriched samples are basaltic andesites, andesites, trachyandesites and trachytes with island arc magma trace element signatures. Pb isotope ranges are limited (208Pb/204Pb ∼38 to 39.8, 207Pb/204Pb ∼15.51 to 15.64 and 206Pb/∼17.9 to 20.1), while Hf isotopes display more diverse compositions (εHf ranging from +7.7 to +14). Both Nd (εNd = 2.8–9.3) and Sr (87Sr/86Sr = 0.7026–0.7048) isotopes are correlated with Hf data. Trace element and isotopic compositions can be explained in terms of mixing between three distinct geochemical endmembers in the mantle resembling DMM, HIMU and EM-2 sources. Our study confirms voluminous subduction-related magmatism on the Loyalty and Three Kings ridges, mostly of Late Oligocene – Early Miocene age. The issue of polarity of subduction to generate these rocks remains open, but the composition-space-time distribution of the igneous rocks can be explained in the context of SW Pacific geodynamics using a west-dipping Pacific slab model.

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.

Geochemical evidence for incorporation of subducting sediment-derived melts into the mantle source of Paleozoic high-Mg andesites from northwestern Tianshan in western China

Abstract Although high-Mg andesites (HMA) have attracted increasing attention due to their unique geochemical composition and important geological significance, there is no consensus on their petrogenesis. The present study indicates that the subducting terrigenous sediment-derived hydrous melts were incorporated into the mantle source of Paleozoic HMA in northwestern Tianshan, western China. These HMA are composed of basaltic andesite and andesite. They generally exhibit arc-type trace element distribution patterns and weakly enriched Sr-Nd-Hf isotope compositions. Some of them show remarkably higher Ba/La, Ba/Th, Rb/Nb, and U/Th ratios than normal mid-ocean ridge basalts (MORB). Furthermore, most of them are characterized by higher Th/Nb, Th/Yb, and Th/Nd ratios but lower Nb/U ratios relative to normal MORB, similar to those of terrigenous sediments. Whole-rock Nd isotopes covary with Rb/Nb and Th/Yb ratios, indicating contributions from both oceanic crust-derived aqueous solutions and terrigenous sediment-derived hydrous melts. Together with their high zircon δ18O values, it appears that the mantle source of the target HMA contains terrigenous sediment-derived hydrous melts in addition to subducting oceanic crust-derived aqueous solutions. In the studied HMA, the andesite generally exhibits higher contents of large-ion lithophile elements and light rare earth elements and more enrichments in Sr-Nd-Hf isotopes than the basaltic andesite. These differences indicate that the mantle source of andesite would contain more subducting sedimentderived hydrous melts than that of basaltic andesite. As a consequence, relatively Si-rich and Si-poor pyroxenite sources were respectively generated as the mantle sources of the andesite and basaltic andesite. This qualitative interpretation is verified by quantitative modeling of the geochemical transfer from subducting oceanic crust into the mantle wedge. Model calculations indicate that the addition of ~3% oceanic crust-derived aqueous solutions and 4%–12% terrigenous sediment-derived hydrous melts into the mantle wedge peridotite can account for the geochemical compositions of the target HMA. Therefore, the HMA in northwestern Tianshan provide the geochemical evidence for the crust-mantle interaction during the oceanic subduction in the Paleozoic. As such, the subducting terrigenous sediment-derived hydrous melts play a dominant role in the composition of the lithochemically fertile, geochemically enriched mantle sources and thus in the origin of HMA above oceanic subduction zones.

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.

Petrochemical features of tholeiites from the Shaka ridge (South Atlantic)

The article presents original data of chemical composition of tholeiitic basaltoids and andesites, dredged from the Shaka Ridge (South Atlantic) in the course of field research in spring 2016 on the scientific expedition vessel “Akademik Fedorov”. The analytical part of the work on estimating the contents of petrogenic, trace and rare-earth elements was carried out using the classical method (“wet chemistry”), X-ray fluorescence analysis (XRF) and inductively coupled plasma mass spectrometry (ICP-MS). The studied samples demonstrate elevated concentrations of large-ion lithophile elements, or LILE, (Ba, Rb, Pb) and light rare earth elements, or LREE, (La, Ce, Nd, Sm) relative to high field strength elements, or HFSE, (Nb, Ta) and heavy rare earth elements, or HREE, (Dy, Yb, Lu). The specifics of trace element geochemistry suggest a significant contribution of crustal or subduction components to the magmas of the Shaka Ridge. Discrimination diagrams of basaltoids and allied rocks with fields of different geodynamic settings indicate that they were formed in the setting of the mid-ocean ridge basalt (MORB). The reason behind the appearance of subduction and crustal marks in the rocks is possibly associated with assimilation of crustal matter by magmas or lies in their inheritance from the mantle source.

O Batólito Catolé do Rocha (RN-PB): Um magmatismo granítico do tipo-A2 reduzido no Domínio Rio Piranhas-Seridó, Província Borborema, Nordeste do Brasil

O Batólito Catolé do Rocha, situado no Domínio Rio Piranhas-Seridó da Província de Borborema (Nordeste (NE) do Brasil), é um importante representante do magmatismo ediacarano pós-colisional que acometeu essa região. Ele inclui sienogranitos e quartzo sienitos metaluminosos a ligeiramente peraluminosos, rochas básico-intermediárias e, subordinadamente, diques e/ou bolsões de microgranitos menores. Esse batólito é tido como representante da suíte cálcio-alcalina de alto K, que inclui principalmente granitos de afinidade química próxima aos de tipo-I caledonianos. Contudo, são diversas as evidências litoquímicas que aproximam o batólito aos magmas de tipo-A. As rochas graníticas são álcali-cálcicas a alcalinas, possuem caráter ferroano, concentrações significativas de elementos litófilos de raio grande (LILE) e de alto potencial iônico (HFSE) e enriquecimento de terras-raras leves (LREE) sobre pesados (HREE). Suas assinaturas químicas permitem classificá-los como granitos de tipo‑A2 pós-colisionais. Estimativas geotermobarométricas apontam cristalização sob pressões de 4,6 – 6,3 kbar (~16 – 24 km de profundidade) e temperaturas entre 950 – 750°C, em condições de baixa fugacidade de oxigênio (-4 &lt; ΔQFM &lt; -1). Evidências químicase distintas condições redox estimadas para os granitos e rochas básico-intermediárias do batólito sugerem que se trata de magmas distintos, com misturas do tipo mixing e mingling locais. A origem do batólito parece estar relacionada a fontes enriquecidas (metassomatizadas), com diferenciação controlada principalmente por processos de cristalização fracionada.

Trace and Rare Earth Elements, and Sr Isotopic Compositions of Fluorite from the Shihuiyao Rare Metal Deposit, Inner Mongolia: Implication for Its Origin

Abundant fluorites occur in the Shihuiyao rare metal (Nb-Ta-Rb) deposit in Inner Mongolia of NE China, and they can be classified by their occurrence into three types. Type I occurs disseminated in greisen pockets of albitized granite. Type II occurs in the skarn zone between granite and carbonate host rocks, and it can be subdivided into different subtypes according to color, namely dark purple (II-D), magenta (II-M), green (II-G), light purple (II-P), and white (II-W). Type III are the fluorite-bearing veins in the silty mudstones. On the basis of petrography of the fluorites and their high contents of HFSEs (high field strength elements) and LILEs (large ion lithophile elements), strong negative Eu anomalies, and tetrad effects, we suggest that Type I fluorites crystallized in a late-magmatic stage with all the components derived from the granite. The high Y/Ho ratios suggest that the Type II fluorites crystallized in the early- or late-hydrothermal stage. The rare earth elements (REEs) characterized by various Eu anomalies of the Type II fluorites indicate a mixed origin for ore-forming metals from granite-related fluids and limestones, and the oxygen fugacity increased during fluid migration and cooling. Compared to the Type II fluorites, the similar trace element contents of the Type III suggest a similar origin, and remarkable positive Eu anomalies represent a more oxidizing environment. The Sr isotopic composition (87Sr/86Sr)i = 0.710861) of the Type I fluorites may represent that of the granite-derived fluids, whereas the (87Sr/86Sr)i ratios of the Type II (0.710168–0.710380) and Type III (0.709018) fluorites are lower than that of the Type I fluorites but higher than those of the Late Permian-Early Triassic seawater, suggesting a binary mixed Sr source, i.e., granite-derived fluids and marine limestones. Nevertheless, the proportion of limestone-derived Sr in the mixture forming the Type III fluorites is much higher than that of Type II. The rare metal Nb and Ta get into the granite-derived F-rich fluids by complexing with F and precipitate in the form of columbite-group minerals after the Type I fluorites crystallize. Most of Nb and Ta may have deposited as columbite-group minerals during the magmatic stage, resulting in no Nb-Ta mineralization in the hydrothermal stage when the Type II and III fluorites formed. Hence, the Type I fluorites in the Shihuiyao mining area can be used as an important exploration tool for the Nb-Ta mineralization.

Tracing multiple sources of sediments using trace element and Nd isotope geochemistry: provenance of the Mesozoic succession in the Kutch Basin, western India

AbstractAn integrated approach involving Sr–Nd isotope, trace and rare earth element analyses tracks multiple sources of the Mesozoic sediments of the Kutch Basin at the western continental margin of India. High (87Sr/86Sr)t (ratio at time of deposition), negative εNd and high concentrations of large-ion lithophile elements (LILEs) indicate the upper continental source. Ratios of Nb/Ta and Zr/Hf suggest sedimentary and felsic igneous sources of sediments. The moderate to high concentration of La, Th and Sc, light rare earth elements (LREE-) enrichment, weak negative Eu anomalies and the relationship between εNd(0) and Th/Sc indicate the dominantly felsic composition of source rocks. However, low contents of Th, low values of (87Sr/86Sr)t and depleted mantle model age TDM &lt; 1600 Ma indicate input from a younger mafic source. Increasing concentrations of Zr, Hf and Nd isotopes and a gradual increase in mean TDM from the older to the younger formations indicate erosional unroofing at the source terrain. The increasing (87Sr/86Sr)t through time relates to increased weathering of the source rock. The overwhelmingly southwesterly palaeocurrent direction of current-generated sedimentary structures, and the mean TDM ages trace suggest source areas of the Kutch Basin to Precambrian rocks in the north and NE of this basin. The TDM ages highlight the dominance of late Palaeoproterozoic source rocks. Nd isotope composition indicates that Proterozoic rocks of Marwar Supergroup and Erinpura Granite, in particular, served as main sediment contributors for the Mesozoic sediments in Kutch. We therefore conclude that the Mesozoic sediments in the Kutch Basin are predominantly of late Palaeoproterozoic age with lesser inputs from rocks of early Mesoproterozoic and early Palaeoproterozoic age.

Geochemical and Sr–Nd isotopic constraints on the genesis of the Soheyle-PaKuh granitoid rocks (central Urumieh-Dokhtar magmatic belt, Iran)

ABSTRACT Soheyle-Pakuh granitoid rocks, with a variety of quartz diorite, quartz monzodiorite, granodiorite, tonalite, and granite, have been emplaced into the Tertiary volcanic rocks in the Urumieh-Dokhtar magmatic arc in central Iran. Zircon U–Pb dating yields an age of 39.63 ± 0.93 Ma for the crystallization of this body. Whole-rock compositions show that SiO2 changes from 52.31 to 65.78 wt.% and Al2O3 varies from 15.54 to 18.24 wt.%, as well as high concentrations of large-ion lithophile elements (LILE, e.g. Cs, Rb, Ba, and K) and quite low contents of high field strength elements (HFSE, e.g. Nb, Ti, P), as expected in I-type arc granitoids formed in an active continental margin setting. Initial ratios of 87Sr/86Sr and 143Nd/144Nd exhibit ranges 0.7043–0.7047 and 0.51284 to 0.51287, respectively, with positive εNd(t) from +4.9 to +5.5 with a young TDM1 age (483–674 Ma); this tracer isotopic data suggesting that the SPG originated from juvenile basaltic crust derived from depleted mantle (~90%) with variable contributions from undepleted mantle and approximately 10% old lower crust, despite diverse processes (e.g. magma mixing and fractional crystallization) during their evolution and emplacement into a local extensional setting within the continental margin arc. The isotopic data are similar to those of other Phanerozoic granitoids of the Central Asian Orogenic Belt and corroborate melting of predominantly mantle-derived juvenile crustal protoliths and indicating extensive addition of new continental crust, during Cambrian-Neoproterozoic time, in the suprasubduction zone beneath the central Urumieh-Dokhtar magmatic arc. Generation of these types of granitoids favours a model whereby rollback and (or) break-off of a subducted slab with subsequent lithospheric extension triggered by mantle upwelling, heat advection, and underplating resulting in melting of the central UDMA mantle-derived juvenile lower continental crust in the Late Eocene.

Petrologic nature of the active subarc crust-mantle boundary: Mixed magmatic-metasomatic processes recorded in xenoliths from Sabtang island, Luzon arc

Some arc magmas reside in the uppermost mantle and the lower crust. Their deep-seated behavior determines the composition of magmas that erupt at the surface. Mafic-ultramafic xenoliths newly found in Sabtang island, Batanes group of islands of the Luzon arc record subarc processes. The xenolith suite is comprised mainly of dunites, orthopyroxenites, clinopyroxenites, hornblendites, and gabbros, all hosted in basaltic to andesitic lavas. Petrographic characteristics suggest the metasomatic formation of orthopyroxenites and hornblendites from dunites and clinopyroxenites, respectively. The apparently primary minerals are homogeneous in composition. Olivine is relatively magnesian (Fo82–90) and chromian spinel is rich in Cr# (=Cr/[Cr + Al], around 0.7) in dunites. Clinopyroxene is relatively magnesian (Mg# = Mg/[Mg + Fe2+] = 0.73–0.93) in clinopyroxenites and gabbros, and plagioclase is highly anorthitic (An89–98) in the gabbros. The primary mineral assemblage reflects crystallization of olivine and spinel followed by clinopyroxene all occurring in the uppermost mantle and lower crust of the Luzon arc. The orthopyroxenes and amphiboles were metasomatically produced at the expense of olivine and clinopyroxene, respectively. Clinopyroxene in the xenoliths is in equilibrium with the magmas that formed the Sabtang volcanics. They have relatively elevated contents of large-ion lithophile elements and light-rare-earth elements, which suggest derivation from an enriched mantle. The Sabtang xenoliths evidence the very active modification of the subarc mantle-crust boundary zone by mantle-derived magmas and slab-derived melts/fluids so that the mineral assemblage of the resultant rocks is similar to that of the predominant recent magma.

Evolution and tectonic setting of the Malani – Nagarparkar Igneous Suite: A Neoproterozoic Silicic-dominated Large Igneous Province in NW India-SE Pakistan

The Neoproterozoic Malani Igneous Suite (MIS) in NW India, along with analogous magmatic rocks from the adjoining Nagarparkar region in SE Pakistan can be collectively classified as a Silicic-dominated Large Igneous Province (SLIP). This magmatic event includes bimodal (predominantly felsic) volcanism, granite emplacement and felsic and mafic dyke intrusions. Felsic rocks have typical A-type affinity as indicated by high abundance of silica, alkali, high field strength and large ion lithophile element concentrations and low CaO and MgO contents. Their Nb negative anomalies and Zr-saturation parameters indicate significant crustal input and high temperature melting. Mafic volcanics and dykes show geochemical homogeneity and derivation from a depleted continental mantle source without any significant crustal contamination (low U and Th contents and no visible Nb anomaly). The region extending from the Mount Abu batholith in the east to Jaswantpura in the west (2700 km2), representing a transition from the metamorphic Sirohi terrane to the undeformed MIS, was evaluated through an integrated structural (including satellite image analysis), geochemical and geochronological study. During the initial stage (prior to 760 Ma) the granitic basement (Erinpura granites) and overlying Sirohi metasediments behaved in a brittle manner that led to development of linear fractures and NNE trending rift structures, and bimodal volcanic activity. Emplacement of voluminous granitic bodies in response to progressive extension of the crust is inferred during the more evolved second stage (younger than 760 Ma). Mirpur Granite, a representative of this younger granitic suite (Jalor type pink granite) has yielded 753 ± 9 Ma zircon, U-Pb, crystallization age. Granitic plutons mark regions of crustal extension, as seen in parallel alignment of plutonic bodies (Jaswantpura granitic belt) and parallel mafic dyke swarms (340°) transecting the granites. Structural analysis further identified an episode of crustal convergence which is documented in folding and faulting of the Sindreth Basin sequence and in tectonic overprint of early stage mafic rocks. Rifting and bimodal magmatic activity in MIS is coeval with similar rock types in Nagarparkar in SE Pakistan, further traceable into the Seychelles microplate and Central Madagascar. Considering the Neoproterozoic paleogeography and our observations, an extensional setting and an active continental margin position for MIS is inferred.

Geochemistry and Sr–Nd isotope composition of Carboniferous volcanic rocks of the Jueluotage Orogenic Belt: implications for the tectonic evolution of Eastern Tianshan, China

ABSTRACTThe major and trace element, and Sr–Nd isotopic compositions of the Carboniferous Qi’eshan, Wutongwozi, and Yamansu volcanic rocks from the northern and southern parts of the Jueluotage Orogenic Belt in East Tianshan, China, were analysed to understand their genesis and geodynamic implications. The early Carboniferous Qi’eshan basalts are characterized by high Al2O3, with La/Sm (1.38–1.79) and Ba/La (27.06–58.76) values higher than those of typical normal mid-ocean ridge basalt. They are relatively enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), and depleted in high field strength elements. Overall, their initial Nd–Sr isotopic compositions are εNd(t) = (5.6–7.0) and Isr = 0.70397–0.70429, implying the magma originated from a mantle wedge source that was metasomatized by subduction-related fluids. In contrast, the late Carboniferous Wutongwozi basalts have lower Ba/La (4.86–12.82), La/Nb (0.87–2.45), and LILE concentrations. They have the isotopic characteristics of depleted asthenosphere, relatively high and heterogeneous εNd(t) (9.3–9.4), and high Isr (0.70471–0.70533). Thus, the late Carboniferous Wutongwozi basalts may have been derived from the partial melting of mantle sources during asthenospheric upwelling. The early Carboniferous Yamansu acid volcanic rocks are characterized by high Mg# (46–48) and Lu/Y (~0.15), and low K2O/Na2O (0.01–0.20), similar to M-type granites. However, their εNd(t) (5.0–5.5) and Isr (0.70642–0.70768) values are lower than those of depleted mantle, indicating they were contaminated by lower crustal material. The magma source originated from a mantle-derived magma that was contaminated by middle Tianshan massif in a continental margin arc setting. Based on the results and previous field-based studies, we conclude that the Carboniferous volcanics in the Jueluotage Orogenic Belt formed in a complex trench–arc–basin setting in the Kuguertage–Aqikuduke Suture Zone.

SHRIMP U–Pb zircon ages and whole-rock geochemistry for the Şapçı volcanic rocks, Biga Peninsula, Northwest Turkey: implications for pre-eruption crystallization conditions and source characteristics

ABSTRACTPalaeogene and Neogene volcanic rocks are widespread on the Biga Peninsula of Northwest Turkey. These rocks were formed during the Eocene, Oligocene–Miocene, and late Miocene, and the early Miocene Şapçı volcanic rocks in the Balıkesir area consist of andesitic lava flows and associated pyroclastics. Temperatures, pressures, and oxygen fugacities calculated for the hornblendes in these andesitic rocks are 903–930°C, 3.3–4.8 kbar, and –9.91 to –11.88, respectively, and for the biotites they are 755–788°C, 1.30–1.74 kbar, and –14.88 to –13.98, respectively. SHRIMP U–Pb dating of zircons from three andesite samples gave ages of 22.72 ± 0.19, 22.97 ± 0.23, and 18.72 ± 0.17 Ma (early Miocene), and these are regarded as crystallization ages. Geochemical analyses show that the volcanic rocks are mainly high-K and calc-alkaline, and have high contents of large-ion lithophile elements and low contents of high-field strength elements, revealing that they evolved from parental magmas that were derived from an enriched subcontinental lithospheric mantle source. The chondrite-normalized rare earth element patterns of the rocks are concave upwards with LaCN/LuCN = 11.9–21.2 and EuCN/Eu* = 0.84–0.92, implying significant fractional crystallization of hornblende during their evolution. According to the petrological data with regional geology, Neogene magmatic activity on the Biga Peninsula has a post-collisional feature, and was closely related to slab break-off geodynamic model after collision of Tauride–Anatolide Block and Sakarya continent.

Early Cretaceous bimodal volcanic rocks in the southern Lhasa terrane, south Tibet: Age, petrogenesis and tectonic implications

Limited geochronological and geochemical data from Early Cretaceous igneous rocks of the Gangdese Belt have resulted in a dispute regarding the subduction history of Neo-Tethyan Ocean. To approach this issue, we performed detailed in-situ zircon U–Pb and Hf isotopic, whole-rock elemental and Sr–Nd isotopic analyses on Late Mesozoic volcanic rocks exposed in the Liqiongda area, southern Lhasa terrane. These volcanic rocks are calc-alkaline series, dominated by basalts, basaltic andesites, and subordinate rhyolites, with a bimodal suite. The LA–ICPMS zircon U–Pb dating results of the basaltic andesites and rhyolites indicate that these volcanic rocks erupted during the Early Cretaceous (137–130Ma). The basaltic rocks are high-alumina (average>17wt.%), enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depleted in high field strength elements (HFSEs), showing subduction-related characteristics. They display highly positive zircon εHf(t) values (+10.0 to +16.3) and whole-rock εNd(t) values (+5.38 to +7.47). The silicic suite is characterized by low Al2O3 (<15.4wt.%), Mg# (<40), and TiO2 (<0.3wt.%) abundances; enriched and variable concentrations of LILEs and REEs; and strongly negative Eu anomalies (Eu/Eu*=0.08–0.19), as well as depleted Hf isotopic compositions (εHf(t)=+4.9 to +16.4) and Nd isotopic compositions (εNd(t)=+5.26 to +6.71). Consequently, we envision a process of basaltic magmas similar to that of MORB extracted from a source metasomatized by slab-derived components for the petrogenesis of mafic rocks, whereas the subsequent mafic magma underplating triggered partial melting of the juvenile crust to generate acidic magma. Our results confirm the presence of Early Cretaceous volcanism in the southern Lhasa terrane. Combined with the distribution of the contemporary magmatism, deformation style, and sedimentary characteristics in the Lhasa terrane, we favor the suggestion that the Neo-Tethyan oceanic lithosphere was flat-lying beneath the Lhasa terrane during the Early Cretaceous.

Late Cretaceous lithospheric extension in SE China: Constraints from volcanic rocks in Hainan Island

Petrological, geochemical and in-situ zircon U–Pb dating and Hf-isotope analyses have been carried out on a suite of basalt–andesite–rhyolite volcanic rocks exposed in the Liuluocun area, Hainan Island, SE China. Zircon analyses show that these volcanic rocks crystallized in the Early Cretaceous (ca. 102Ma). The basalts are characterized by low MgO contents and mg-numbers but high rare earth element, high field strength element and large ion lithophile element contents and Nb–Ta negative anomalies. They have relatively uniform Sr–Nd isotope compositions with εNd(t) values of −4.09 to −3.63. The andesites show enrichment of high field strength element and rare earth element with negligible Eu anomalies. They have εNd(t) values of −2.35 to −3.88 and εHf(t) values of −9.73 to −1.13. The rhyolites have high K2O and SiO2 contents. They are characterized by prominent Eu, P and Ti negative anomalies and enrichment in large ion lithophile element, and show εHf(t) values of −7.51 to +0.47 and εNd(t) values of −2.49 to −2.69. Petrogenetic analysis indicates that the Liuluocun volcanic rocks were produced by incomplete reaction of the mantle wedge peridotite with felsic melts derived from partial melting of subducted sediment. All these characteristics, combined with geological observations, suggest that their formation was related to regional lithospheric extension in the South China Craton during the Early Cretaceous, which may have been caused by subduction of the Paleo-Pacific plate beneath the continental plate of China.

High-Mg Diorite from Qulong in Southern Tibet: Implications for the Genesis of Adakite-like Intrusions and Associated Porphyry Cu Deposits in Collisional Orogens

We have investigated a suite of Miocene high-Mg diorite porphyries from Qulong in southern Tibet, the largest porphyry Cu–Mo deposit in China. Laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating shows that the high-Mg diorite porphyry was emplaced at 15.7±0.2 Ma, which is slightly younger than the Qulong adakite-like Rongmucuola pluton (c. 19.5–16.4 Ma). The Qulong high-Mg diorites have phenocrysts showing disequilibrium textures and include high-Mg# clinopyroxene (0.91–0.97). These porphyry rocks exhibit both ultrapotassic and adakite-like features, and are characterized by high contents of MgO (4.2–5.1 wt %), K2O (3.2–3.6 wt %), and compatible trace elements (e.g. Ni: 115–142 ppm; Cr: 214–291 ppm), as well as by high Sr/Y and La/Yb ratios. The rocks have highly radiogenic isotopic compositions with (87Sr/86Sr)i = 0.707004–0.707198, eNd(t)=-5.1 to -5.5, 207Pb/204Pb = 15.697–15.704, and 208Pb/204Pb = 39.082 – 39.116, as well as variable zircon eHf values of -3.0 to +5.9. Petrographic, elemental, and isotopic evidence suggests that the Qulong high-Mg diorites were formed by mixing between ultrapotassic and adakite-like melts, derived from metasomatized Tibetan lithospheric mantle and juvenile lower crust, respectively. In contrast, the Qulong pre-ore Rongmucuola pluton is characterized by high SiO2 (66.3–68.9 wt %) and Al2O3 (16.4–17.0 wt %) contents, high Sr/Y ratios (121–151), low compatible element contents (e.g. Ni = 16.0 – 17.4 ppm; Cr=14.5–20.2 ppm), low Mg# values (0.44–0.52), positive large-ion lithophile element (LILE) anomalies, marked negative high field strength element (HFSE) anomalies, positive eNd(t) values (+0.4 to +2.5), and low (87Sr/86Sr)i values (0.704847–0.705237). These features indicate that the Rongmucuola pluton was formed by partial melting of subduction-modified juvenile lower crust within the garnet stability field. The newly identified Qulong high-Mg diorite allows us to propose a mixing model for the origin of the Gangdese high-K, adakite-like rocks. In this model, the formation of these rocks occurred in two stages: (1) partial melting of highly metasomatized lithospheric mantle that generated ultrapotassic mafic melts; (2) underplating of such melts beneath thickened juvenile lower crust, which resulted in melting of the lower crust and the generation of adakite-like magmas. Mixing of the adakite-like melt with ultrapotassic magmas elevated the K2O, MgO, and other LILE (e.g. Rb and Ba) contents of the adakite-like melt. Exogenous water necessary for formation of the Gangdese porphyry Cu systems was mainly added during mixing of ultrapotassic magma with adakite-like melt at lower and/ or upper-crustal depths, reflecting the large decrease in the H2O solubility of the ultrapotassic mafic melt upon ascent and decompression. Upper-crustal fractionation of the Rongmucuola magma could also possibly increase the water content of mineralization-related, adakite-like porphyry intrusions at Qulong. Fluid exsolution from the ultrapotassic magma is likely to have been a key process in the generation of the Gangdese porphyry Cu deposits, as well as other porphyry Cu deposits in the Tibetan collisional orogens.

Sm–Nd and Rb–Sr isotope geochemistry and petrology of Abu Hamamid intrusion, Eastern Desert, Egypt: An Alaskan-type complex in a backarc setting

Alaskan-type complexes are uplifted fragments of the deep levels of island arcs. These complexes usually reveal chemical diversity that cannot be assigned to a unique setting within arcs. Here we report the petrology, Rb–Sr and Sm–Nd isotopic data of the Abu Hamamid intrusion from the Eastern Desert of Egypt as an example of Neoproterozoic backarc Alaskan-type complex. The intrusion comprises clinopyroxene dunite, olivine clinopyroxenite, amphibole clinopyroxenite and gabbro. All rock units represent cumulates crystallized from a high-MgO hydrous magma with geochemical characteristics intermediate between mid-ocean ridge basalt and island arc magmas. Slab signatures of the parental magma comprise elevated H2O, 87Sr/86Sr and large ion lithophile element contents and relative depletion in Nb. High positive initial ɛNd (+6.2 to +11.0) is consistent with the Abu Hamamid magma being extracted from a highly depleted mantle source. The parental magma was generated by 10% partial melting of a mantle source modified by 10% MORB (ecolgite) derived fluid. The Abu Hamamid Alaskan-type complex and the surrounding related volcanic rocks were formed in a Neoproterozoic backarc setting. The magmas forming the lower crust of the Arabian-Nubian shield were liberated from a depleted mantle before 900Ma.