Whole-rock Sr Research Articles

Late Eocene Two-Pyroxene Trachydacites from the Southern Qiangtang Terrane, Central Tibetan Plateau: High-Temperature Melting of Overthickened and Dehydrated Lower Crust

AbstractOrthopyroxene-bearing granitic rock (e.g. charnockite) is relatively rare but provides an excellent opportunity to probe the thermal and tectonic evolution of deep orogenic crust because of its distinct mineral assemblage. Here we present petrological, mineralogical, elemental, and Sr–Nd–Hf–O isotopic data for late Eocene (ca. 36 Ma; zircon U–Pb ages) volcanic rocks exposed in the Ejiu region in the southern Qiangtang Terrane to investigate how the central Tibetan crust evolved to its modern thickness and thermal state. The Ejiu volcanic rocks (EVRs) are trachydacites with anhydrous mineral assemblages (i.e. two pyroxenes, sanidine, plagioclase, and ilmenite, without amphibole and biotite) and geochemical characteristics (e.g. high P2O5 and TiO2) that resemble those of charnockite-type magmatic rocks. Mineral and whole-rock thermometry and hygrometry suggests that the parent magma crystallized under hot (~1000°C) and dry (H2O < 2 wt.%) condition. Besides, the EVRs display adakitic affinities according to their high SiO2 and Al2O3 contents, high Sr/Y, La/Yb, and Gd/Yb ratios, and low Y and Yb contents, without marked negative Eu anomalies. The calculated melts in equilibrium with pyroxenes also display adakitic compositions (e.g. high Sr/Y and La/Yb ratios), indicating that the adakitic compositions of the EVRs did not result from late-stage magmatic evolution. In addition, the melts of the EVRs were saturated in TiO2, as inferred from the high TiO2 contents of these rocks and the presence of ilmenite. An integrated analysis of the geochemical, petrological, and mineralogical data suggests that the EVRs were neither evolutional products nor partial melts of hydrous mafic materials at normal crustal pressures, but were formed by fusion of an eclogitized mafic protolith with residue containing garnet and rutile but lacking amphibole and plagioclase. The whole-rock Sr–Nd and zircon Hf isotope compositions of the EVRs [(87Sr/86Sr)i = 0.7053 to 0.7066; εNd(t) = −1.40 to −0.99; zircon εHf(t) = +1.08 to +5.31] indicate that the parental protolith was relatively juvenile in nature, but also contained some supracrustal materials given the high zircon δ18O values [zircon δ18O = +8.21‰ to +11.00‰]. The above arguments lead us to propose that of partial melting of a previously dehydrated—but chemically undepleted—mafic lower continental crust at high pressure (>1.5 GPa) and high temperature (>1000°C) generated the EVRs. Based on a synthesis of independent geological and geophysical data, we further suggest that the southern Qiangtang Terrane crust of the central Tibetan Plateau was thick, dry, and elevated during the Late Cretaceous to early Eocene time, and that it became abnormally hot owing to the ascending asthenosphere after lithospheric foundering during the middle Eocene.

Cumulate Mush Hybridization by Melt Invasion: Evidence from Compositionally Diverse Amphiboles in Ultramafic–Mafic Arc Cumulates within the Eastern Gangdese Batholith, Southern Tibet

Abstract Amphibole plays an important role in the petrogenesis and evolution of arc magmas, but its role is not completely understood yet. Here, a field, petrological, geochronological and geochemical study is carried out on ultramafic–mafic arc cumulates with textural and chemical heterogeneities and on associated host diorites from the eastern Gangdese Batholith, southern Tibet to explore the problem. The cumulates occur as a large body in diorite host-rocks. The core of the body consists of coarse-grained Cpx hornblendite with a porphyritic texture. Towards the contact with the host diorite, the coarse-grained Cpx hornblendite grades to relatively homogeneous fine-grained melagabbro. Zircon U–Pb dating indicates that they all crystallized at 200 ± 1 Ma. Textural features and whole-rock and mineral chemical data reveal that both the Cpx hornblendite and the melagabbro are mixtures of two different mineral assemblages that are not in equilibrium: (1) brown amphibole and its clinopyroxene inclusions; (2) matrix clinopyroxene + green amphibole + plagioclase + quartz + accessory phases. Clinopyroxene and brown amphibole from the first assemblage are enriched in middle rare earth elements (MREE) relative to light REE (LREE) and heavy REE (HREE), and are weakly depleted in Ti, whereas clinopyroxene and green amphibole from the second assemblage are characterized by LREE enrichment over MREE–HREE and more marked Sr and Ti depletion. The higher Mg#, MgO and Cr of the late-formed green amphibole than the early formed brown amphibole suggest that the two assemblages are not on the same liquid line of descent. Given the close relations of the three rock types in the exposed crustal section, the cumulates are interpreted to have formed in an open system, in which an ultramafic cumulate body consisting of the first assemblage reacted with the host dioritic melt to form new clinopyroxene and amphibole of the second assemblage. The melt calculated to be in equilibrium with the first mineral assemblage resembles an average continental arc basalt, which is less evolved than the host dioritic melt, responsible for the second mineral assemblage. On the basis of whole-rock Sr–Nd–Hf isotopic similarity of the cumulates and a host diorite sample, we argue that the host diorites were formed through crystal fractionation from the parent melt of the first assemblage. Results of least-squares mass-balance calculations suggest that the quantities of the host dioritic melts, involved in the generation of these modified cumulates, vary from ~25 to ~44 %. The presence of magmatic epidote in the host diorites and Al-in-Hb geobarometry indicate that the reaction that occurred when the dioritic melts percolated through the cumulate body was at ~6 kbar. Both the brown and green amphiboles are enriched in MREE relative to HREE, and can impart to residual melts a strong geochemical signature of amphibole fractionation (low Dy/Yb). Thus, we conclude that fractional crystallization and melt–rock reaction are two mechanisms by which amphibole controls arc magma petrogenesis and evolution.

Felsic dyke swarms from central Inner Mongolian: Implications for the Triassic tectonic setting in the southeast Central Asian Orogenic Belt

This work presents the whole-rock elemental geochemistry and Sr − Nd isotopes and zircon Hf − O isotopes for dikes from Tagaolemiao and Tuilamamiao, the southern margin of the southeast Central Asian Orogenic Belt (SE CAOB), to explore their origin and dynamic setting. Zircon U − Pb dating reveals that the Tagaolemiao dikes emplaced at ca. 242 Ma and the Tuilamamiao dikes emplaced at ca. 250 Ma, which postdated the Late Permian magmatic lull and convergent events in the SE CAOB. Dikes from the two areas are I-type rhyolitic rocks and exhibit high SiO2, low MgO, and low Cr, Co, and Ni contents. The Tagaolemiao dikes have depleted zircon Hf (εHf(t) = +3.75 to +9.82), whole-rock Sr − Nd (ISr(t) = 0.704340–0.704855; εNd(t) = +1.3 to +2.1) isotopic composition and variable zircon δ18O values (4.88–7.29‰), and were derived from the mixing melts of the juvenile crust and depleted mantle. In contrast, the dikes from Tuilamamiao show negative zircon εHf(t) (−2.33 to −10.85) and whole-rock εNd(t) (−6.7 to −15.5) values, low ISr(t) (0.706640–0.708208) and high δ18O (6.38–7.61‰) values, which originated from the partial melting of the old continental lower crust. The Triassic dyke swarms from this work indicate the SE CAOB switched into an intra-continental extension tectonic setting after the Paleozoic long-term accretion-convergence.

In-situ U–Pb dating of zircon coronas, Sr–Nd–Hf isotopes and petrological constraints of the Daxigou anorthosite complex, NW China

Massif-type anorthosites, mainly Proterozoic in age, have long been recognized as a signature of crust-mantle interactions. The Daxigou Anorthosite Complex (DAC) is one such massif situated in the Kuluketage block, a tectonically important domain between the Tarim Craton and the Central Asian Orogenic Belt. In this contribution, a combination of geochronological and geochemical analyses of fine-grained DAC diabase are used to constrain the characteristics of the parental magma and the initiation time of DAC magmatism. We then pinpoint the DAC’s closure time by in-situ petrographic thin section U–Pb dating of zircon coronas around Fe-Ti oxides identified in a gabbroic anorthosite. Within data uncertainty, we find that the closing age of 1813 ± 9 Ma, is indistinguishable from the 1804 ± 7 Ma crystallization age of its last episode of parental magma. MELTS simulations of the parental diabase magma suggest a pressure of 8 kbar at a fayalite-magnetite-quartz oxygen fugacity leads to broadly observed mineral compositions and mineral phases. The bulk rock major and trace elements, integrated with the whole-rock Sr–Nd and Hf isotopes in zircon, indicate that the DAC was formed by partially melting of a metasomatized sub-continental lithospheric mantle, which was likely induced by a post-collisional slab break-off with minor crustal assimilation. Finally, we propose that the DAC magmatism can be ascribed to a series of Paleoproterozoic tectono-thermal events along the northern and southern margins of the Tarim Craton, which could have provided important implications for the Tarim Craton and the North China Craton associated with the assembly of Columbia supercontinent.

Unravelling the hidden evidences of magma mixing processes via combination of in situ Sr isotopes and trace elements analyses on plagioclase crystals

Granites from the ~580 Ma oxidized A-type Salto Rapakivi Granite (Itu Granitic Province, SE Brazil) bear abundant felsic microgranular enclaves (FMEs) and scarce cm-sized mafic microgranular enclaves (MMEs) with whole-rock Sr and Nd isotope signatures identical to those of their host rapakivi granites. In order to unravel the sources involved in the formation of both enclave types we conducted in situ trace elements and Sr isotopic analyses on plagioclase megacrysts from representative rocks of rapakivi granites, porphyry granites and both microgranular enclave varieties. Plagioclase from host rapakivi granites have 87Sr/86Sr ~0.7068, and are relatively more radiogenic than those from the enclaves. Plagioclase xenocrysts from the FMEs typically register a rimward decrease in 87Sr/86Sr ratios, with core 87Sr/86Sr up to 0.7069, whereas rims are usually in equilibrium with whole rock (~0.7060–0.7063). Crystals with U-shaped Sr isotopes profiles (less radiogenic cores with rimward increase in 87Sr/86Sr ratios) are rare, as are crystals with strongly radiogenic signatures observed in partially digested xenoliths found within the enclaves (87Sr/86Sr ~0.709–0.710). Plagioclase from MMEs show more primitive signatures (87Sr/86Sr 0.7055–0.7058). Crystals from the MMEs are distinguished by higher Sr (~1000 ppm) and lower Eu (< 0.5 ppm) and light rare earth elements (LREE, 6–12 ppm) compared to crystals from FMEs, porphyry and granite, which have Sr = 200–600 ppm, and LREE = 20–80 ppm. Our results allowed us to infer that FMEs were formed by mixing between a resident melt similar to that of rapakivi granites with a less radiogenic melt similar to that which originated the MMEs. Therefore the combined use of Sr isotopes and trace element contents in plagioclase crystals from A-type granites might shed light on the sources of rock varieties that have similar whole-rock isotope signatures.

Evidence for continental rifting from two episodes of mid-Neoproterozoic silicic magmatism in the northeastern Yangtze Block, China

Mid-Neoproterozoic (850–720 Ma) magmatism around the periphery of the Yangtze Block records an intense extensional event related to the breakup of Rodinia. However, the detailed geodynamic mechanisms of this extensional tectonism are still poorly constrained. To address this issue, we undertook a whole-rock geochemical and Sr–Nd isotopic, and zircon U–Pb–Hf–O isotopic study of mid-Neoproterozoic silicic meta-igneous rocks from the Feidong Complex and Zhangbaling Group in the northeastern Yangtze Block. These rocks can be classified into two groups: Group A (granitoids) and Group B (volcanic rocks). Zircon U–Pb dating revealed that Group A (808–786 Ma) pre-dates Group B (762–750 Ma). All these rocks are characterized by A-type granite features, with high heavy REE, Zr, and Hf contents, (K2O + Na2O)/CaO ratios, Fe index values, and zircon saturation temperatures, and formed during a mid-Neoproterozoic extensional event. However, the extremely enriched whole-rock Sr–Nd and zircon Hf isotopic compositions (ISr = 0.7024–0.7068, εNd(t) = −19.3 to −14.1, and εHf(t) = −25.7 to −11.8) of Group A suggest derivation by reworking of ancient crustal basement. Group B is characterized by relatively depleted Sr–Nd–Hf isotopic compositions (ISr = 0.7015–0.7063, εNd(t) = −1.6 to +3.6, and εHf(t) = −4.8 to +4.1), indicating a significant contribution from juvenile-crust-derived material. Group A contains zircons with δ18O = 4.1‰–7.1‰, whereas Group B contains zircons with δ18O = 1.6‰–3.1‰. These chemical and isotopic data, along with the gradual change in residual mineralogy from garnet to plagioclase (Group A to B), indicate that magmatism transitioned from an ancient lower crustal source to a juvenile middle–upper crustal source that had experienced intense high-temperature meteoric water–rock interactions. Our results provide new insights into the tectonic evolution from early extension to peak rifting driven by upwelling asthenosphere, due to rifting of the Yangtze Block from Rodinia during the mid-Neoproterozoic.

Three-phased Middle Permian–Mesozoic magmatism in the Great Xing'an Range: implications for episodic southern subduction of the Mongol–Okhotsk Ocean

The Erguna Massif is located in the southwestern portion of the Great Xing'an Range and is adjacent to the Mongol–Okhotsk suture zone. It has not been determined whether the tectonic evolutionary processes of the Erguna Massif belonged to the Mongol–Okhotsk tectonic regime during the Middle Permian–Mesozoic. In this study, a suite of rocks comprising Mesozoic S-type monzogranite (U–Pb zircon age of 248 ± 1.2 Ma), highly fractionated I-type rhyolite (204 ± 1.1 Ma), gabbro (196 ± 1.9 Ma), A2-type volcanic rocks (190 ± 0.9 Ma), A1-type trachydacite (167 ± 0.8 Ma) and Early Cretaceous A1-type alkaline rhyolite are newly identified and geochemically studied. The rhyolite, gabbro, trachydacite and alkaline rhyolite whole-rock Sr–Nd isotope analyses obtained initial values 87 Sr/ 86 Sr ratios ranging from 0.7044 to 0.7058 and ε Nd ( t ) values of −0.68 to +2.73. These samples show ε Hf ( t ) values ranging from +5.3 to +11.2 and T DM2 ranging from 0.48 to 0.90 Ga. The 248 Ma monzogranites were produced by the partial melting of greywackes. The 204 Ma rhyolites were derived from the partial melting of lower mafic crust. The 196 Ma gabbros originated from the partial melting of an enriched mantle metasomatized by subduction-slab-released fluids. The 190 Ma volcanic rocks, 167 Ma trachydacite and Early Cretaceous alkaline rhyolite were mainly formed by the partial melting of the basaltic rocks. They all show enrichment in the large-ion lithophile elements (e.g. Rb, Ba, and K) and depletion in the high-field-strength elements (e.g. Nb, Ta, and Ti), suggesting they formed in an active continental margin setting. The features of these igneous rocks indicate the southward subduction of the Mongol–Okhotsk oceanic plate. Based on compiled age data, three phases of middle Mesozoic magmatism were identified in the Erguna Massif at c. 275–225, 215–165, and 150–110 Ma. In addition, three similar magmatic phases were found in the Xing'an Massif. However, a hysteresis of c. 15–20 Ma exists between the two massifs. These magmatic rocks may record the three stages of the southward subduction of the Mongol–Okhotsk oceanic plate, and two periods of slab rollback occurred during the Middle Permian to Early Cretaceous. Supplementary material: Analytical details, radiometric ages and Tables S1–S5 are available at: https://doi.org/10.6084/m9.figshare.c.5459285

Pb-Nd-Sr Isotope Geochemistry of Metapelites from the Iberian Pyrite Belt and Its Relevance to Provenance Analysis and Mineral Exploration Surveys

Abstract The Iberian Pyrite Belt is a world-class metallogenic district developed at the Devonian-Carboniferous boundary in the Iberian Variscides that currently has seven active mines: Neves Corvo (Cu-Zn-Sn) and Aljustrel (Cu-Zn) in Portugal, and Riotinto (Cu), Las Cruces (Cu), Aguas Teñidas (Cu-Zn-Pb), Sotiel-Coronada (Cu-Zn-Pb), and La Magdalena (Cu-Zn-Pb) in Spain. The Iberian Pyrite Belt massive sulfide ores are usually hosted in the lower sections of the volcano-sedimentary complex (late Famennian to late Visean), but they also occur in the uppermost levels of the phyllite-quartzite group at the Neves Corvo deposit, stratigraphically below the volcano-sedimentary complex. A Pb-Nd-Sr isotope dataset was obtained for 98 Iberian Pyrite Belt metapelite samples (from Givetian to upper Visean), representing several phyllite-quartzite group and volcano-sedimentary complex sections that include the footwall and hanging-wall domains of ore horizons at the Neves Corvo, Aljustrel, and Lousal mines. The combination of whole-rock Nd and Sr isotopes with Th/Sc ratios shows that the siliciclastic components of Iberian Pyrite Belt metapelites are derived from older quartz-feldspathic basement rocks (–11 ≤ εNdinitial ≤ –8 and (87Sr/86Sr)i up to 0.727). The younger volcano-sedimentary complex metapelites (upper Tournaisian) often comprise volcanic-derived constituents with a juvenile isotopic signature, shifting the εNdi up to +0.2. The Pb isotope data confirm that the phyllite-quartzite group and volcano-sedimentary complex successions are crustal reservoirs for metals found in the deposits. In Neves Corvo, where there is more significant Sn- and Cu-rich mineralization, the higher (206Pb/204Pb)i and (207Pb/204Pb)i values displayed by phyllite-quartzite group and lower volcano-sedimentary complex metapelites (up to 15.66 and 18.33, respectively) suggest additional contributions to the metal budget from a deeper and more radiogenic source. The proximity to Iberian Pyrite Belt massive sulfide ore systems hosted in metapelite successions is observed when (207Pb/204Pb)i &amp;gt;15.60 and Fe2O3/TiO2 or (Cu+Zn+Pb)/Sc &amp;gt;10. These are important criteria that should be considered in geochemical exploration surveys designed for the Iberian Pyrite Belt.

Petrogenesis of the Early Paleozoic S-type granites in southern Alxa, Northwest China and its implications on fluid flourishing in the middle and lower crust

Fluids play a key role in the partial melting of crustal materials which leads to the formation of the andesitic continental crust. However, the origin of the aqueous fluid in the lower and middle crust remains unclear. Here, we present three early Paleozoic S-type granites of the Yabulai Mountain in southern Alxa in an attempt to clarify the origin of the fluid and its implications for the geodynamic processes of the Late Silurian to Early Devonian. The coarse-grained two-mica granite and fine-grained two-mica granite formed during 423–417 Ma, whereas the muscovite granite was relatively younger (408 Ma). The fine-grained two-mica granites contain high SiO2 (72.56–75.77 wt%) and are metaluminous to weakly peraluminous (A/CNK = 0.97–1.07), while the coarse-grained two-mica granites contain comparable SiO2 (72.18–74.56 wt%) and are strongly peraluminous (A/CNK = 1.10–1.16). The muscovite granites (SiO2 = 72.50–76.04 wt%) are different from the two-mica granites in having higher Na2O (3.38–4.58 wt%) and lower K2O (3.70–4.90 wt%) with higher Na2O/K2O ratios (0.71–1.24). In addition, they have higher Ba (1071–2152 ppm) and Sr (640–801 ppm) contents and lower Rb (66.7–107 ppm) and U (0.23–0.74 ppm) contents and lower Rb/Sr (0.09–0.17) ratios and higher Eu/Eu* (0.95–3.92) ratios than those of the two-mica granites. These geochemical features imply that the two-mica granites were derived from fluid-absent partial melting and the muscovite granites were formed by fluid-present partial melting and their magma sources were plagioclase-rich metapsammite. The whole-rock Sr–Nd and zircon Hf isotopic compositions suggest that the metapsammite sources originated from parts of the Mesoproterozoic basement in the Alxa block. It is proposed that a tectonic shift from southward advancing to northward retreating of the Paleo-Asian slab subduction took place during 423 to 397 Ma and this caused a progressive back-arc extension in southern Alxa. Such a tectonic transformation may promote fluids release from the mantle wedge and upward infiltration through the crustal fractures resulting in intense fluid-present melting in the back-arc setting.

Geochronology, petrography and Sr-Nd-Hf isotopes of Mbalizi carbonatite, southwestern Tanzania

The Mbalizi carbonatite is located in the middle of the Paleoproterozoic Ubendian Mobile Belt and the western branch of East Africa Rift, southwestern Tanzania. Calcite, dolomite, phlogopite, pyrochlore and apatite are found in the sample. Mineral chemistry studies have shown that the carbonatite phlogopite is linked to mantle-derived magmatism. The apatite is fluorapatite, means they are of magmatic origin. The analyses on two crystals of pyrochlore show high concentrations of Nb2O5, and therefore the Nb-oxide is classified as pyrochlore subspecies. Three types of zircon have been obtained from the Mbalizi carbonatite, including xenocrysts zircon, igneous zircon and metamorphic zircon. Zircon in-situ LA-ICP-MS U–Pb dating in this contribution indicates that the Mbalizi carbonatite was crystallized at ca. 116.0 ± 1.8 Ma. The εHf(t) values of igneous zircon ranging from −13.9 to +5.7, indicates that the carbonatite parental magma was originated from the sub-continental lithospheric mantle, and evolves toward HIMU and EM. The whole-rock Sr–Nd isotopic data suggest more contribution of the HIMU and EMⅠ material. We propose that the complex evolutionary history of the Ubendian Mobile Belt has stored the subduction oceanic crust which has the EMⅠ and HIMU components, forming the compositional heterogeneity mantle beneath the Ubendian Mobile Belt. At 116.0 ± 1.8 Ma, with the extension stress field, deep faults cause the pressure reduction, resulting in reactive of the upwelling of the HIMU and EMⅠ components. This provides the metamorphic conditions to induce the isotopic resetting and may result in large scatter of initial 176Hf/177Hf ratios of carbonatite melts.

Crustal growth during Western Gondwana amalgamation and onset of the Brasiliano orogeny: Insights from geochemistry and Pb–Sr–Nd–O isotopes from granites in northeastern Brazil

The lithological units of the eastern Pernambuco–Alagoas superterrane of the Borborema Province, northeastern Brazil, includes the Santana do Ipanema (SI) batholith, whose framework is defined by a large multi-intrusive plutonic complex. The results from field, petrologic, geochemical, whole-rock Sr–Nd–O isotopes, and zircon U-Pb geochronological studies on a suite of three porphyritic granites from this batholith are reported here. The studied granites carry biotite, amphibole, titanite, and ± magmatic epidote as major mafic phases. These rocks have high Mg# (up to 52), and are metaluminous to slightly peraluminous, high-K calc-alkaline, and magnesian akin to Cordilleran granitic batholiths. All of the studied granitic rocks are enriched in LREE- and LILE-enriched (e.g., Rb, Ba, K, and Th), and HFSE-depleted (e.g., Nb, Ta, and Ti), typical of subduction-related volcanic arc rocks. Estimated intensive parameters suggest crystallization pressure from 3 to 4 kbar, temperature > 1000 °C (near-liquidus) to 640 ± 35 °C (near-solidus), and magmatic oxidizing conditions between ΔNNO+0 to ΔNNO+1. SHRIMP UPb zircon data point to crystallization ages from 643 to 625 Ma, while zircon xenocrysts are 800 to 1230 Ma old. Nd isotopes reveal depleted mantle model ages from 0.96 to 1.32 Ga, and εNd(t), initial 87Sr/86Sr, and δ18O[zircon] values from −3.35 to +0.73, 0.7054 to 0.7086, and 6.36 to 8.62‰, respectively. The geochemical and isotopic signatures of the studied granites, coupled with those from the Santana do Ipanema batholith available in the literature, suggest magma derivation from mixed juvenile Neoproterozoic (late Cryogenian to Early Ediacaran) high-K basalts and reworked Mesoproterozoic to late-Tonian components modified (metasomatized) by subduction-related processes during the amalgamation of the São Francisco Craton and Borborema Province. These data evidence crustal growth during Western Gondwana amalgamation, associated with the onset of the Brasiliano/Pan African orogeny.

Mush, Melts and Metasediments: a History of Rhyolites from the Okataina Volcanic Centre, New Zealand, as Captured in Plagioclase

Abstract The Okataina Volcanic Centre (OVC), located in the Taupo Volcanic Zone, New Zealand, is a dominantly rhyolitic magmatic system in an arc setting, where eruptions are thought to be driven by mafic recharge. Here, Sr–Pb isotopes, and compositional and textural variations in plagioclase phenocrysts from 10 rhyolitic deposits (two caldera, one immediately post-caldera, four intra-caldera, and three extra-caldera) are used to investigate the OVC magmatic system and identify the sources and assimilants within this diverse mush zone. Plagioclase interiors exhibit normal and reverse zoning, and are commonly in disequilibrium with their accompanying glass, melt inclusions, and whole-rock compositions. This indicates that the crystals nucleated in melts that differed from their carrier magma. In contrast, the outermost rims of crystals exhibit normal zoning that is compositionally consistent with growth in cooling and fractionating melts just prior to eruption. At the intra-crystal scale, the total suite of 87Sr/86Sr ratios are highly variable (0·7042–0·7065 ± 0·0004 average 2SE); however, the majority (95 %) of the crystals are internally homogeneous within error. At whole-crystal scale (where better precision is obtained), 87Sr/86Sr ratios are much more homogeneous (0·70512–0·70543 ± 0·00001 average 2SE) and overlap with their host whole-rock Sr isotopic ratios. Whole-crystal Pb isotopic ratios also largely overlap with whole-rock Pb ratios. The plagioclase and whole-rock isotopic compositions indicate significant crustal assimilation (≥20 %) of Torlesse-like metasediments (local basement rock) by a depleted mid-ocean ridge mantle magma source, and Pb isotopes require variable fluid-dominant subduction flux. The new data support previous petrogenetic models for OVC magmas that require crystal growth in compositionally and thermally distinct magmas within a complex of disconnected melt-and-mush reservoirs. These reservoirs were rejuvenated by underplating basaltic magmas that serve as an eruption trigger. However, the outermost rims of the plagioclase imply that interaction between silicic melts and eruption-triggering mafic influx is largely limited to heat and volatile transfer, and results in rapid mobilization and syn-eruption mixing of rhyolitic melts. Finally, relatively uniform isotopic compositions of plagioclase indicate balanced contributions from the crust and mantle over the lifespan of the OVC magmatic system.

Geochronology and geochemistry of an early Silurian plagiogranite from Xiarihamu area, East Kunlun, northwest China: implications for Proto-Tethyan evolution

Oceanic plagiogranites are directly or indirectly related to ophiolites. A small plagiogranite vein has been discovered in the No. III mafic–ultramafic complex of the Xiarihamu ophiolite from the eastern margin of Qiman Tagh, East Kunlun Orogen, northwest China. This plagiogranite yielded a weighted mean zircon U–Pb age of 435.7 ± 4.3 Ma (MSWD = 0.073) and is characterized by high SiO2 (73.08–76.24 wt.%) and Na2O (6.42–8.34 wt.%) contents and high Na2O/K2O ratios (3.06–12.36) with metaluminous and calc-alkaline affinity. It is enriched in Th and U, depleted in Nb, Sr, Ti, and Ba, and has low ΣREE contents (7.64–11.35 ppm), negative Eu anomalies (δEu = 0.31–0.71), and light REE enrichment. These features are typical of oceanic plagiogranites. Whole-rock Sr–Nd isotopic data for the plagiogranite show obvious EMII mantle characteristics. The petrographic, geochemical, and Sr–Nd isotopic characteristics suggest the plagiogranite is part of an ophiolite formed by partial melting of oceanic crust in the presence of hydrothermal fluids. Given this, other parts of the Xiarihamu ophiolite and other ophiolites in Qiman Tagh area, we propose the plagiogranite formed in a back-arc basin setting in the early Silurian, probably the youngest known intrusion related to the Proto-Tethys Ocean.

Petrogenesis and tectonic setting of the Early Permian gabbro–granite complex in the southeastern Central Asian Orogenic Belt, central Inner Mongolia, China

Gabbro–granite complexes formed in active continental margins provide critical constraints on magmatic processes, crust–mantle interactions, and tectonic evolution. The petrogenesis and tectonic setting of the Early Permian igneous rocks in the southeastern Central Asian Orogenic Belt have been a subject of dispute for a long time. This contribution presents an integrated study on the field occurrence, petrology, petrography, zircon U–Pb ages, in situ zircon Hf isotopes, whole-rock geochemistry, and Sr–Nd isotopes of the Early Permian Yiheshaorong gabbro–granite complex, Xi Ujimqin Banner, central Inner Mongolia. Lithologically, the Yiheshaorong complex consists predominantly of gabbro, pyroxene diorite, and alkali feldspar granite. Results of LA–ICP–MS zircon U–Pb dating suggest that crystallization ages of the gabbro, pyroxene diorite, and alkali feldspar granite are 281–280, 278, and 281–280 Ma, respectively. Silica (SiO2) content varies from 41.26 to 79.64 wt% and exhibits a gap between 56.84 and 75.45 wt%, displaying a good bimodal geochemical distribution. Geochemically, the gabbros and pyroxene diorites are characterized by enrichment of large ion lithophile elements (LILEs; e.g., Ba and K) and depletion of high field strength elements (HFSEs; e.g., Nb, Ti, and P). These rocks show similar homogeneous zircon Hf isotopic (positive εHf (t) values from +7.8 to +10.1) and whole-rock Sr–Nd isotopic (low initial 87Sr/86Sr ratios from 0.7044 to 0.7051, positive εNd (t) values from +3.7 to +4.7) compositions. The gabbros and pyroxene diorites originated from partial melting of depleted asthenospheric mantle metasomatized by subduction slab fluids; and they experienced various degrees of crystallization differentiation during ascension. The alkali feldspar granites are ferroan, alkali-calcic–calc-alkalic series rocks marked by high SiO2, K2O + Na2O, and FeOT/MgO; and low Al2O3, CaO, and MgO contents. Their trace elements are marked by enrichment of LILEs (such as K and Rb); depletion of HFSEs (such as Nb and Ti), Ba, Sr, and P; and strongly negative Eu anomalies (Eu/Eu* = 0.17–0.57), with coincident distribution patterns forming a seagull-type distribution form, showing an A-type granite affinity. The alkali feldspar granites are characterized by positive εHf (t) values (+8.6 to +10.3, young TDM2 ages, 645–758 Ma) and εNd (t) values (+3.5 to +3.8 as well as young TDM2 ages, 667–687 Ma). All these geochemical characteristics indicate that the alkali feldspar granites originated from the partial melting of juvenile crust under high temperature and low pressure condition. The Yiheshaorong complex was most likely formed in a back-arc spreading basin. Such an extensional regime can likely be attributed to the upwelling of asthenospheric mantle caused by the rollback and break-off of a subducted Paleo-Asian Ocean slab in the Early Permian, which induced large-scale extension and magmatism in the study area.

Petrogenesis of early Eocene granites and associated mafic enclaves in the Gangdese batholith, Tibet: Implications for net crustal growth in collision zones

Granitoids and related mafic microgranular enclaves (MMEs) in continental collision zones may provide crucial insights into the formation and evolution of continental crust. In this paper, we report an investigation into the mineralogy, zircon U–Pb geochronology, whole-rock major and trace elements, and Sr–Nd–Hf isotopic geochemistry of granodiorites and hosted MMEs, as well as monzogranites, from the Zhaxiding intrusive complex in the Gangdese batholith, southern Tibet. Zircon U–Pb dating suggests that these rocks were emplaced during the early Eocene (49 to 47 Ma). Field investigation and petrological observations indicate that the MMEs represent globules of a mafic magma that was injected into the host felsic magma. On the basis of depleted zircon Hf (εHf(t) values of +6.5 to +9.2) and whole-rock Sr–Nd (εNd(t) values of +1.6 to +2.6) isotopic compositions, along with their whole-rock compositions, the MMEs are interpreted as having resulted from mixing of mantle-derived magma with juvenile crust-derived melts. The granodiorites and monzogranites are metaluminous and have positive zircon εHf(t) (+6.3 to +8.9 for granodiorites and +1.7 to +10.2 for monzogranites) and whole-rock εNd(t) (+1.6 to +1.9 for granodiorites and +0.5 to +1.1 for monzogranites) values. The granodiorites further exhibit continental crust-like chemical compositions (e.g., enrichment in Rb, K, and Pb and depletion in Nb and Ta) and high Mg# values, suggesting that they originated from partial melting of juvenile crust with the involvement of mantle-derived melts. However, the monzogranites have lower P, Ti, and total rare earth element contents, similar Rb, Ba, and Sr contents, and uniform Eu anomalies compared with the granodiorites, and were likely generated by fractional crystallization of amphibole and Fe–Ti oxides from juvenile crust-derived magma. We propose that underplating of mantle-derived magma likely triggered coeval crustal magmatism and that mixing of the mantle- and crust-derived magmas and subsequent crystallization differentiation formed the intermediate to felsic Zhaxiding intrusive complex in the Gangdese batholith. The continental crust-like bulk compositions of the intermediate to felsic magmatic rocks represent a net addition of juvenile material to continental crust. Our study thus indicates that magma mixing and differentiation play a significant role in the formation of andesitic to dacitic continental crust in collision zones.

Petrogenesis of Early Cretaceous volcanic rocks of the northeastern North China Craton: Constraints from elemental and Sr–Nd–Pb isotope geochemistry

This study examines the geochemical and whole-rock Sr–Nd–Pb isotopic compositions of Early Cretaceous volcanic rocks of the southern Jilin Province (Tonghua and Huanren areas) and Liaodong Peninsula (Xiuyan, Dandong, and Guiyunhua areas), northeastern North China Craton (NCC). Two groups of volcanic rocks are recognized in the northeastern NCC. The geochemistry and isotopic compositions of the intermediate-acid volcanic rocks indicate that a systematic decrease in whole-rock εNd(t) is accompanied by an increase of Zr/Nb. Modeling results suggest that those intermediate-acid volcanic rocks experienced the assimilation-fractional crystallization (AFC) process with different lower continental crust (LCC) components, indicating the heterogeneity of LCC in the northeastern NCC. As for basic volcanic rocks, they are characterized by “weakly enriched” isotopic signatures, i.e., moderately radiogenic Sr (0.7062–0.7082), unradiogenic Nd (−5.29 to −1.14), and highly radiogenic Pb compositions. The narrow scope of Sr–Nd–Pb isotopic compositions of whole-rock indicates insignificant crustal assimilation. Instead, the high Th/Yb ratios were largely inherited from source enrichment through the input of subducted sediment. Furthermore, element-isotopic modeling suggests that the “weakly enriched” mantle-derived rocks have resulted from a depleted mantle source metasomatized via the sediment-derived melt. Our results provide powerful petrological and geochemical evidence on the Early Cretaceous subduction of the Paleo-Pacific plate beneath the northeastern NCC and suggest the metasomatic process by subducted sediment-derived melt may play an important role in depleted mantle enrichment.

Petrogenesis of the late Mesozoic Bashan complex in the Lower Yangtze River Belt, eastern China: Implications for the definition and significance of A-type granite

Many late Mesozoic granites within the Lower Yangtze River Belt (LYRB), eastern China, are defined as A-types based on their geochemical characteristics, although their petrogenesis is unclear. We present an integrated study combining geochronology, whole-rock geochemistry, Sr, Nd, Pb and zircon Hf isotopes and mineral chemistry for a lithologically-diverse suite of A-type granitoids at Bashan. The Bashan complex in the Chizhou area is composed of quartz monzonite, quartz syenite, syenogranite and alkali-feldspar granite. Zircon UPb geochronology indicates that these intrusive rocks formed at 126–123 Ma. The quartz monzonites have intermediate SiO2 (60.5–63.1 wt%) and high Na2O + K2O (8.66–9.83 wt%) contents, arc-like trace element compositions, enriched whole-rock Sr, Nd and zircon Hf isotopes ((87Sr/86Sr)i = 0.7082–0.7091; εNd(t) = −6.9 to −7.1; εHf(t) = −5.3 to −8.2) and high radiogenic Pb isotopes (206Pb/204Pb(t) = 18.581–18.792). They are inferred to have been derived from partial melting of an enriched lithospheric mantle source, followed by fractional crystallisation and limited crustal contamination. The quartz syenites show high SiO2 (65.9–69.8 wt%) and Na2O + K2O (11.3–12.3 wt%) contents, low MgO (0.14–0.23 wt%) contents, high 104 * Ga/Al (2.34–3.61) values, arc-like trace element compositions, and whole-rock Nd and Pb and zircon Hf isotopic compositions (εNd(t) = −6.8 to −7.0; εHf(t) = −7.3 to −12.3; 206Pb/204Pb(t) = 18.559–18.970) similar to the quartz monzonite, suggesting that they were derived through fractional crystallisation of the quartz monzonite, with some crustal assimilation. The quartz monzonite has higher Ti-in-zircon temperatures (TTi-in-Zrn = 696–832 °C) and lower zircon saturation temperatures (TZr = 772–818 °C) and oxygen fugacities (ΔFMQ = +1.8 to +2.8) than the quartz syenite (TTi-in-Zrn = 623–805 °C; TZr = 856–909 °C; ΔFMQ = +3.5 to +4.8), indicating that the zircons from the quartz syenite crystallised at lower temperatures, and that the oxygen fugacity significantly elevated as the magmatic temperature decreases. The syenogranites display high SiO2 (74.6–75.5 wt%) contents, high TTi-in-Zrn (671–871 °C), TZr (799–822 °C), and low oxygen fugacities (ΔFMQ = +0.9 to +3.7), and thus did not evolve from the quartz syenite. They show high 104 * Ga/Al values (2.67–2.95), low MgO (0.1–0.17 wt%) contents and (La/Yb)N (7.60–10.19) ratios, pronounced negative Eu anomalies (Eu/Eu* = 0.28–0.38) and enriched Sr, Nd and zircon Hf isotopic compositions (εNd(t) = −7.2 to −7.5; εHf(t) = −5.1 to −14.0), indicating that they were derived from fluid-absent partial melting of Neoproterozoic calc-alkaline igneous rocks under low-pressure–high-temperature conditions. The alkali-feldspar granites are characterised by high SiO2 (76.5–78.0 wt%) and Na2O + K2O (8.34–9.02 wt%) contents, but low MgO (0.03–0.08 wt%) contents. They show less enriched Nd isotopes (εNd(t) = −5.7), a wide range of zircon εHf(t) values (−1.9 to −11.2) and high oxygen fugacities (ΔFMQ = +2.9 to +4.3). They were produced by mixing of highly-fractionated alkaline basaltic magmas and melts derived from partial melting of Mesoproterozoic crust. The inferred petrogenesis of the Bashan complex magmatic rocks suggests that the A-type granites within the LYRB have multiple magmatic sources and crystallised under a wide range of temperatures, water contents and oxygen fugacities. During the Mesozoic, subduction and rollback of the Palaeo-Pacific slab induced crustal extension and intense crust–mantle interaction, which played a dominant role in the formation of the A-type granites within the LYRB.

Multiple enrichment of subcontinental lithospheric mantle with Archean to Mesozoic components: Evidence from the Chicheng ultramafic complex, North China Craton

Numerous Late Paleozoic to Early Mesozoic mafic-ultramafic intrusions occur along the northern margin of the North China Craton (NCC). The timing, magma source, genetic types and tectonic evolution of these intrusions still remain controversial. Here we conduct a systematic petrological, mineral chemical, whole-rock geochemical, isotopic (zircon Lu–Hf and whole-rock Sr–Nd isotopes) and geochronological study on representative samples from the Chicheng ultramafic complex in the northern NCC, our results provide new insights into multiple melt-fluid interactions in the subcontinental lithospheric mantle through time. Zircon U–Pb data from the ultramafic rocks (pyroxenite and serpentinized dunite) show a wide range of ages (2721–129 Ma), indicating zircon growth within an evolving and metasomatized mantle wedge during multi-stage melt-fluid interactions. The youngest zircon population (145–129 Ma) possibly marks the timing of formation of the Chicheng ultramafic complex at the Early Cretaceous. Moreover, these ultramafic rocks also witnessed tectonic transition from Paleo-Asian Ocean to Paleo-Pacific Plate subduction during the Paleozoic to Early Mesozoic and subsequent progressive lithospheric thinning and craton destruction at the northern margin of the NCC, which is also attested by the newly discovered diorite porphyry dyke from this complex reported in this study with zircon U–Pb ages in the range of 447 to 128 Ma. Zircon Lu–Hf isotopic data show εHf(t) values in the range of −19.9 to +15.4 with TDM of 219–3012 Ma and TDMC of 240–3650 Ma. Whole-rock Sr–Nd isotopic data display εNd(t) values ranging from −12.3 to −1.6, indicating heterogenous sources involving both depleted mantle and reworked ancient crustal components. Whole-rock geochemical data show enrichment in LILEs (Rb, Sr, Ba, Pb) and depletion in HFSEs (Nb, Ta, Ti), with positive anomalies at Ba, K, Pb and Nd and strongly negative Zr and Ti anomalies, suggesting geochemical affinities related to subduction-related island arc setting within an active continental margin. Mineral chemical data on clinopyroxenes do not correspond to “Alaskan-type” affinities. Integrating geochemical, geochronological and isotopic studies from this study and those from previous studies related to the mafic-ultramafic complexes in the northern Hebei Province along the northern margin of the NCC, we propose that these complexes might be composite plutons formed during the Early Cretaceous, which were derived from the interaction of melts and fluids released from subducted slab with metasomatized mantle peridotite, triggering different degree of partial melting of mantle wedge.

Petrogenesis of the Middle–Late Triassic S- and I-type granitoids in the eastern Pamir and implications for the Tanymas–Jinshajiang Paleo-Tethys Ocean

A series of terranes in northwestern Tibet were progressively accreted to the southern margin of the Asian plate during the late Paleozoic to early Cenozoic. Numerous igneous rocks generated in the South Kunlun and Karakul–Mazar terranes help to elucidate the evolution of the northern active continental margin of the Tanymas–Jinshajiang Paleo-Tethys Ocean. In this study, zircon SHRIMP U–Pb data, zircon trace elements, hornblende major and trace elements, whole-rock major and trace elements, whole-rock Sr–Nd isotopes, and zircon Hf isotopes are analyzed to study the age, petrogenesis, and tectonic setting of the Qiate and Bulunkou plutons emplaced in the Karakul–Mazar terrane. Zircon U–Pb dating shows that the Qiate pluton was emplaced at ~ 240 Ma and the Bulunkou pluton at 221–219 Ma. Elemental and isotopic data indicate that the Qiate S-type leucogranite was generated by the partial melting of metasedimentary rocks in the thickened lower continental crust (~ 40–50 km). The Bulunkou I-type granodiorite was likely generated through the reworking of metasedimentary rocks by oxidized mantle-derived mafic magmas in the normal lower continental crust (< 40 km). Because of the injection of the oxidized mantle-derived magmas, the Bulunkou granodiorite is more oxidized than the pure crust-derived Qiate leucogranite. Integrated with previous findings, this study provides new data and interpretations to support that during the Middle–Late Triassic, in response to the northward subduction of the Tanymas–Jinshajiang Paleo-Tethys oceanic slab, the South Kunlun and Karakul–Mazar terranes developed into an active continental margin undergoing crustal thickening and lithospheric delamination.

Magmatic processes recorded in plagioclase and the geodynamic implications in the giant Shimensi W–Cu–Mo deposit, Dahutang ore field, South China

• The two magma chambers experienced chemically closed magmatic evolution. • The calcium needed for the scheelite mineralization may have been provided by the BG . • The highly evolved felsic magma chamber is critical to the formation of the giant Shimensi deposit. The Shimensi W–Cu–Mo deposit is one of the largest tungsten deposits in the world. Despite numerous geochemical studies conducted on ore-related granites in the district, few studies have concerned magma chambers processes. In this study, systematic in-situ major- and trace-element studies across plagioclase crystals from the ore-related Mesozoic granites as well as whole-rock Sr–Nd isotopic compositions of such granites in the Shimensi deposit were used to constrain the sources of calcium, the dynamics of the magmatic system and the metallogenic geodynamic setting. In-situ analyses of plagioclase showed no obvious positive correlations between An and FeO, while Sr was positively correlated with Ba, indicating that the magma chambers in the Shimensi deposit may have experienced chemically-closed evolution affected only by thermal mixing and/or decompression, without chemical mixing with mafic magma from the mantle. This conclusion was also supported by whole-rock Sr–Nd isotopic characteristics of high ( 87 Sr/ 86 Sr) i (0.71664–0.73689) and negative ε Nd (t) values (−9.81 to −5.07). It was found that the calcium needed for scheelite mineralization may have been predominantly provided by biotite granodiorite (BG) because of its high calcium content and large size, while ore-forming metals should mainly have been derived from the magma sources of pelites and basic volcanic rocks in the Shuangqiaoshan Group instead of the recharging of mafic magma. Moreover, change of the stress environment likely facilitated the formation of long-term stable, large-volume, highly evolved felsic magma chambers in the shallow crust, which would have been critical to the formation of the giant Shimensi W–Cu–Mo deposit.