Whole-rock Sr Research Articles

Source diversity in controlling the compositional diversity of Cenozoic granites in the Tethyan Himalaya

Cenozoic granites in the Himalayan orogen show compositional diversity. Yet the importance of source diversity in causing the compositional diversity is underestimated. Here we studied whole-rock Sr–Nd isotope geochemistry and relict zircon U–Pb geochronology of three suites of granites in the Himalayan orogen. Combing our new data and previous data, a new source component, namely metasedimentary rocks from the Triassic Langjiexue Group in the Tethyan Himalaya, was identified. Such a component is characterized by 200–400 Ma-aged zircons and relatively less enriched Sr–Nd isotopic compositions. These signatures have been imprinted on both the Eocene and Miocene granites from the Yalaxiangbo area. As a comparison, geochemical features and relict zircon U–Pb age spectrum of the Miocene Cuona granites both suggested they were derived from partial melting of metapelites of the Higher Himalayan Sequence. Phase equilibrium modeling results suggest that both the Langjiexue metapelite and the Higher Himalayan metapelite have comparable fertility. Given their similar lithology and major-trace element compositions, the resultant melts will have no significant distinction in lithochemistry. This explains why many reported granites in the Tethyan Himalaya have undistinguishable major-trace elemental compositions to the Higher Himalayan leucogranites, but have less enriched Sr–Nd isotopic compositions. However, if metamafic rocks were involved, the resultant melts will depart from the Higher Himalayan leucogranites in both elemental and isotopic compositions, exemplified by the Miocene adakites in the Tethyan Himalaya. This study highlights the importance of source diversity in causing the compositional diversity of the Cenozoic granites in the Tethyan Himalaya.

Imaging the Late Triassic lithospheric architecture of the Yidun Terrane, eastern Tibetan Plateau: Observations and interpretations

The present-day lithospheric architecture of modern and ancient orogens can be imaged by geophysical techniques. For ancient orogens, unravelling their architecture at the time of formation is hindered by later tectono-magmatic events. In this paper, we use spatial variations in radiogenic isotopic compositions of Late Triassic magmatism from the Yidun Terrane, eastern Tibetan Plateau, to establish its lithospheric architecture during the Triassic. Comprehensive geochemical and isotopic data of Late Triassic magmatic rocks from four transects across the Yidun Terrane document eastward enrichment in whole-rock Nd, Sr, and zircon Hf isotopic compositions. Mafic and felsic rocks of major plutons show coherent and nonlinear trends in the Zr and P2O5 systematics and have limited variation of isotopic compositions. This indicates that Late Triassic magmatic differentiation was dominated by fractionation of mantle-derived mafic magmas. The spatial isotopic trends result from changing mantle sources, including variable contributions of isotopically depleted asthenospheric mantle and isotopically enriched subcontinental lithospheric mantle (SCLM) to magma sources. The spatial variation of mantle sources suggests a westward thinning of the SCLM during the Triassic. We propose that this architecture is most likely associated with eastward subduction of oceanic lithosphere of the Jinshajiang Ocean located at the west of the Yidun Terrane, immediately prior to the Late Triassic magmatism.

Geochemical Distinction Between Altered Oceanic Basalt- and Seafloor Sediment-Derived Fluids in the Mantle Source of Mafic Igneous Rocks in Southwestern Tianshan, Western China

AbstractThe role of subducting oceanic crust-derived fluids in generating mafic arc magmatism has been widely documented. However, the subducting oceanic crust is generally composed of basaltic igneous crust and seafloor sediment, which may give rise to different compositions of liquid phases causing metasomatism of the mantle wedge. Because of the similarity in enrichment of fluid-mobile incompatible elements in the two sources of subduction zone fluids, it has been a challenge to distinguish between them when studying the products of mafic arc magmatism. This difficulty is overcome by a combined study of whole-rock Li isotopes and zircon O isotopes in addition to whole-rock major-trace elements and Sr–Nd–Hf isotopes in Late Paleozoic mafic igneous rocks from southwestern Tianshan in western China. Zircon U–Pb dating yields consistent ages of 313 ± 3 Ma to 305 ± 1 Ma for magma crystallization. The mafic igneous rocks exhibit arc-like trace element distribution patterns and depleted whole-rock Nd–Hf isotopes but slightly high (87Sr/86Sr)i ratios of 0·7039– 0·7057. They also show positive zircon εHf(t) values and slightly higher zircon δ18O values of 5·2–7·6‰. There are covariations of whole-rock Sr isotopes with Th/La and Rb/Nb ratios, indicating a contribution from terrigenous sediment-derived fluids to their mantle source in addition to basaltic igneous crust-derived fluids. Based on the slightly higher zircon δ18O values but variably lower whole-rock δ7Li values of -0·8 to 3·5‰ for the target rocks than those of mantle respectively, both altered oceanic basalt- and terrigenous sediment-derived fluids are identified in the mantle source of these mafic igneous rocks. Model calculations for trace elements and Sr–Nd–Li isotopes further confirm that the geochemical compositions of these mafic igneous rocks can be explained by chemical reaction of depleted MORB mantle peridotite with the mixed fluids to generate ultramafic metasomatites at subarc depths. Therefore, chemical metasomatism of the mantle wedge is a key mechanism for the incorporation of crustal components into the source of arc-like mafic igneous rocks above oceanic subduction zones.

Early–Middle Jurassic magmatic rocks along the coastal region of southeastern China: Petrogenesis and implications for Paleo-Pacific plate subduction

• The 200–160 Ma arc magmatic belt is identified along the coastal region of SE China. • This study firstly reports the ancient basement-derived granitoids in SE China. • The Paleo-Pacific Plate subduction underneath southeast China started at ca. 200 Ma. This study presents new zircon U–Pb ages and whole-rock geochemical data for the Fanyindong (FYD) and Jincheng (JC) granitoid intrusions in coastal Zhejiang and Fujian Provinces, respectively, southeast China, to investigate their petrogenesis and provide new constraints on the regional tectonic evolution and crustal growth. Zircon U–Pb ages indicate that the FYD and JC intrusions were emplaced at c. 170 Ma and 193 Ma, respectively. Both granitoids belong to the high-K calc-alkaline series, and display arc-like trace element patterns, with enrichment of large ion lithophile elements and Nb-Ta depletion. The FYD granitoids show adakitic geochemical characteristics, with high radiogenic Sr isotope ratios, and the most enriched bulk-rock Nd and zircon Hf isotopic compositions (crustal Nd–Hf model ages = ca. 2.35–2.75 Ga) ever reported in the Mesozoic granitoids within southeast China. Geochemical characteristics suggest that the FYD granitoids were generated by partial melting of mostly Paleoproterozoic basement rocks in a thickened lower crust. However, whole-rock Sr–Nd and zircon Hf isotopes of the JC granites show depleted source signatures. Geochemical signatures suggest the JC granites originated from the partial melting of a mixed source, consisting of the pre-existing, ancient, lower crust and juvenile, mantle-derived, basaltic rocks. The data from this study, in conjunction with previously published data, indicates that there exists a NE-trending, Early–Middle Jurassic, arc-related, magmatic rock belt along the southeast coast of China, and the Paleo-Pacific Plate subduction most likely started at c. 200 Ma. The early stage subduction of the Paleo-Pacific plate and the induced reactivation and extension of pre-existing E–W striking faults in the Nanling region of southeastern China gave rise to the generation of the JC granites. Ongoing subduction of the Paleo-Pacific plate resulted in a thickening of the continental margin, and subsequent crustal anatexis of basement materials produced the FYD granitoids.

Magmatic Evolution of Zoned and Unzoned Ignimbrites: Evidence for a Complex Crustal Architecture Feeding four Rapid-sequence, Caldera-forming Eruptions in the San Juan Mountains, Colorado

Abstract The last four caldera-forming ignimbrites in the central San Juan caldera cluster, Colorado, erupted 1400 km3 in ≤80 kyr and alternated between zoned crystal-poor rhyolite to crystal-rich dacite and unzoned, crystal-rich dacite. The zoned 150 km3 Rat Creek Tuff (26·91 Ma), unzoned 250 km3 Cebolla Creek Tuff, and zoned 500 km3 Nelson Mountain Tuff (26·90 Ma) formed the nested San Luis caldera complex with slightly offset calderas, and the unzoned 500 km3 Snowshoe Mountain Tuff (26·87 Ma) formed the Creede caldera to the south. The Rat Creek Tuff, Nelson Mountain Tuff, and Snowshoe Mountain Tuff have similar mineral assemblages of plagioclase, sanidine, quartz, biotite, hornblende, clinopyroxene, Fe–Ti oxides, and accessory zircon, titanite, and apatite. The Cebolla Creek Tuff differs from the other three ignimbrites with more abundant hornblende and a lack of quartz and sanidine. Trace element compositions of interstitial glass are unique to each ignimbrite, correlating with mineral assemblages and inferred crystallization depths. Glass, feldspar, hornblende, and clinopyroxene thermobarometry calculations provide evidence for vertically extensive crustal magma reservoirs with a common magmatic zone at ∼435–470 MPa (∼16–17 km) showing a transition into shallow pre-eruptive reservoirs between ∼110 and 340 MPa (∼4–13 km), similar to the estimated magma reservoir architecture of the Altiplano Puna Volcanic Complex. The upper portions of the eruptible parts of the magma reservoirs of the Rat Creek Tuff (215 ± 50 MPa, ∼810–820 °C), Cebolla Creek Tuff (340 ± 20 MPa, ∼860–880 °C), Nelson Mountain Tuff (215 ± 20 MPa, ∼745–800 °C), and Snowshoe Mountain Tuff (110 ± 40 MPa, 825 ± 10 °C) occupied shallow levels in the crust similar to other magma reservoirs of the central San Juan caldera cluster. Trace element modelling correlates with a deep crystallization signature in the unzoned Cebolla Creek Tuff and Snowshoe Mountain Tuff, typified by a flat trend in Ba versus Sr whole-rock and glass chemistry. The zoned Rat Creek Tuff and Nelson Mountain Tuff are typified by a steep trend in Ba versus Sr chemistry interpreted as a shallower crystallization signature. Similarly, the unzoned Cebolla Creek Tuff and Snowshoe Mountain Tuff have flatter slopes in FeO versus An space of plagioclase chemistry interpreted as more abundant deep plagioclase crystallization and a difficulty in physically mixing with Fe-rich mafic recharge magma owing to higher viscosity. The zoned Rat Creek Tuff and Nelson Mountain Tuff have higher slopes in FeO versus An space of plagioclase chemistry interpreted as more abundant shallow plagioclase crystallization and more feasible mixing with Fe-rich mafic recharge magma owing to lower viscosity. The eruption of the Rat Creek Tuff was probably triggered by mafic injection, but the other three ignimbrites lack mingling textures in pumice, suggesting that other mechanisms were important in causing such large eruptions. After a prolonged period of mantle-derived magma injection and crustal heating (∼25 000 km3 Conejos Formation erupted during ∼35–29 Ma), the San Juan magmatic body became a robust and versatile producer of diverse eruptible magmas in short time periods during its Oligocene ignimbrite flare-up.

Mafic Microgranular Enclaves Formed by Gas-driven Filter Pressing During Rapid Cooling: an Example from the Gangdese Batholith in Southern Tibet

AbstractMafic microgranular enclaves (MMEs), widespread in intermediate to felsic arc plutons, carry significant information on the genesis and evolution of arc magmas, yet their origin remains debatable. Here, we examine MME-host diorite pairs from the c.200 Ma Cuijiu Igneous Complex in the eastern Gangdese Batholith, southern Tibet, to constrain the petrogenesis of MMEs and the evolution of arc magmas. Within the complex, MMEs are essentially similar to their host diorites with similar emplacement ages (∼200 Ma), mineral assemblages and mineral compositions, as well as whole-rock Sr–Nd–Hf and zircon Hf isotopic compositions. However, MMEs have higher modal contents of hornblende and biotite, and are enriched in compatible elements and depleted in incompatible elements. Zircons from some MME samples are characterized by dark cathodoluminescence (CL) cores overgrown by light-CL rims of varying thickness. The dark-CL cores show higher Th, U and rare earth elements (REE) abundances than the light-CL rims. Based on comparison with co-genetic mafic melts and mass-balance calculations, we propose that the MMEs were early-crystallized cumulates (autoliths) related to their host diorites. The chilled textures, flow microstructures and pillow shapes suggest that the MMEs experienced rapid cooling before being captured by the host magmas. The rapid cooling may result from contact between ascending diorite magmas and cooler wall rocks. As the magmas quickly crystallized, they reached second boiling and vesiculation, and separated into fine-grained crystal-rich margins and melt-rich centres. Gradients in crystallinity and pressure expelled interstitial melts from the crystal-rich margins to the crystal-poor centres, leading to crystal-liquid separation (gas-driven filter pressing). The dark-CL zircon cores with high Th and U abundances may crystallize from highly evolved interstitial melts within the crystal-rich margins. The fine-grained crystal-rich margins were subsequently captured and dragged as MMEs before their complete crystallization by later ascending host magmas. This differentiation process could have occurred over several kilometres of magma ascent, and have played an important role in the polybaric fractional crystallization of the Cuijiu Igneous Complex, feeding more differentiated andesitic magmas to upper crustal mushes.

Large-scale Late Triassic to Early Jurassic high εHf(t)–εNd(t) felsic rocks in the Ergun Massif (NE China): implications for southward subduction of the Mongol–Okhotsk oceanic slab and lateral crustal growth

The Manzhouli–Xin Barag right banner are located in the southwest of the Ergun Massif (NE China), the tectonic evolution of these areas are still controversial. In this study, a series of rocks comprising rhyolites (LA–ICP–MS U–Pb zircon ages of 206 ± 2 Ma and 195 ± 2 Ma), granodiorites (196 ± 1 Ma and 190 ± 1 Ma), and monzogranites (206 ± 5 Ma and 196 ± 1 Ma) are newly identified. Whole-rock major and trace element analyses show that these felsic rocks and granites belong to the metaluminous to peraluminous, high-K calc-alkaline series, and can be classified as I-type granites. The zircon in-situ Lu–Hf isotope analyses on rhyolite, granodiorite, and monzogranite, yielded the eHf(t) values that are ranging from of 9.7–12.1, and two-stage model age are in the range of 0.5 Ga to 0.6 Ga, eHf(t) values of 4.4–7.5, 0.8–1.0 Ga, eHf(t) values of 6.0–10.5, 0.6–0.9 Ga, respectively. The whole-rock Sr–Nd isotope analyses on rhyolite, granodiorite, and monzogranite, yielded lower initial 87Sr/86Sr ratios (ISr) of 0.703374–0.703586, 0.704503–0.704577, 0.704936–0.704379, with medium eNd(t) values of 4.32–4.38, 1.54–2.96, − 0.68 to 2.73, respectively, suggesting that they are derived from the partial melting of a juvenile depleted lower crust. 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), indicating that they are generated in an Andean-type arc setting and are metasomatized by subduction-slab released fluids or melts with minor crustal contamination. Mainly three magmatic phases: ca. 283–225 Ma, ca. 225–166 Ma, and ca. 166 Ma–114 Ma are identified in the Ergun Massif, and above Late Triassic to Early Jurassic magmatic rocks are related to the second phase, which attribute to the Mongol–Okhotsk ocean southward subduction and lateral crustal growth.

Boron and molybdenum isotopic fractionation during crustal anatexis: Constraints from the Conadong leucogranites in the Himalayan Block, South Tibet

Partial melts derived from crustal anatexis exhibit large variations in field characteristics, composition, and isotope geochemistry. The relative influence of melting process and magmatic differentiation on such heterogeneity is a subject of ongoing debate. Boron and molybdeum isotopes have the potential to shed light on the genesis of crustal melts, but the behaviors of B and Mo isotopes during crustal anatexis and differentiation are unclear. Here, we present geochemical and isotopic data, including whole-rock Sr–Nd–B–Mo and mineral B–Mo data, for the Conadong leucogranites in the Himalayan Block, to provide new insights into these issues. The studied samples include two-mica granites (biotite + muscovite), muscovite leucogranites (muscovite ± garnet), and biotite-rich granites. The two-mica granites were derived from muscovite dehydration melting of metasedimentary rocks. Their initial Sr and Nd isotopic compositions show positive correlation, implying non-modal partial melting associated with dissolution of accessory phases. The B contents (3.44–21.2 ppm) of the two-mica granites decrease but their δ11B (−15.80‰ to −13.25‰) and δ98Mo (−0.15‰ to 0.34‰) values increase with increasing Sr and Ba contents. This may have been controlled by different phases involved in non-modal melting (i.e., muscovite and feldspar). The muscovite leucogranites display features of residual liquids. Their δ11B (−15.38‰ to −11.90‰) and δ98Mo (−0.15‰ to 0.17‰) values negatively correlate with Sr and Ba contents, which can be ascribed to shallow-crustal crystal–liquid separation and melt–fluid interaction. The biotite-rich granites are least evolved, with large variations in their initial Sr–Nd isotopic compositions. They show relatively heavy whole-rock B isotopic compositions of −11.97‰ to −9.00‰. These may indicate that biotite-rich granites may be contaminated by entrained restitic minerals (e.g., biotite, feldspar and accessory minerals). Overall, this study demonstrates that compositional diversity of anatectic melts is controlled primarily by melting processes in the source and secondarily by emplacement-level crystal–liquid separation. Both non-modal partial melting and crystal–liquid separation could result in B–Mo isotopic fractionation in anatectic melts.

Geochronology and geochemistry of Devonian magmatism in the Frontal cordillera (Argentina): geodynamic implications for the pre-Andean SW Gondwana margin

ABSTRACT Whereas north of 33°S Devonian calcalkaline magmatism is notably absent, a Devonian arc is well developed to the south of 33°S. The Carrizalito, Pampa de los Avestruces, and Papagayos plutons are located in the Frontal Cordillera between 34° and 35°S. U-Pb zircon SHRIMP and LA-MC-ICP-MS data from the plutons yield Early Devonian ages of 389 ± 3 and 409 ± 3 for Carrizalito, 413 ± 2 Ma for Pampa de los Avestruces, and 346 ± 3, 393 ± 5 and 412 ± 3 Ma for Papagayos. The younger zircon age from the Papagayos pluton suggests that the Devo nian magmatism continued during the Early Carboniferous. Carrizalito and Pampa de los Avestruces plutons display metaluminous to slightly peraluminous granodiorite composition, with SiO2 ranging from 65.1 to 67.8 wt.% and from 67.9 to 69.7 wt.%, respectively, and define a magnesian calc-alkalic trend. The Papagayos pluton is metaluminous to slightly peraluminous and slightly more evolved in composition (felsic granodiorite to monzogranite), with a SiO2 content that varies between 68.4 to 70.5 wt.%, defining a magnesian with ferroan enrichment and calc-alkalic to alkali-calcic trend. These geochemical characteristics are similar to those reported for the Carboniferous arc granitoids in the Frontal Cordillera and Western Sierras Pampeanas at 27°–30°S and the archetypical calc-alkaline granitoids of the Peruvian Coastal batholith, indicating an Early Devonian magmatic arc. Rare earth element patterns ([La/Yb]N = 10–20) with mostly slight to moderate negative Eu anomalies (Eu/Eu* = 0.62–0.80) and geochemical ‘spider-diagrams’ are similar to arc granitoids. Combined whole-rock Nd and Sr isotope data from the largest Carrizalito and Pampa de los Avestruces plutons (εNdt = −4.3 and −4.6; 87Sr/86Srt = 0.70704 and 0.70762; TDM = ca. 1.5 Ga) together with in situ U–Pb and Hf isotope data from magmatic zircon (εHft ranging from −16.5 to −5.4; TDM = ca. 1.7 Ga), suggest a common older continental lithosphere source. The presented data indicate tectonic segmentation in the pre-Andean margin controlling the presence or absence of Devonian subduction-related magmatism. This segmentation is best explained by changes in the tectonic plate configuration occurred in a long-lived convergent margin.

Multi-isotope tracing of the 1.3–0.9 Ga evolution of Fennoscandia; crustal growth during the Sveconorwegian orogeny

Magmatism between 1.3 and 0.9 Ga at the southwestern margin of Fennoscandia, comprising mainly granitic batholiths and subordinate bimodal volcanic rocks, provides a nearly continuous magmatic record of the Fennoscandian tectonic evolution. Here, we present new and published zircon Hf, K-feldspar Pb and whole-rock Sr isotopic data from the granitic rocks. The εHf isotopic evolution since 1300 Ma starts out as relatively juvenile, with a flat superchondritic trend at 1300–1130 Ma followed by a steeper trend towards lower, but still superchondritic values at 1070–1010 Ma. During the 1000–920 Ma period, the trend flattens out at near-chondritic values. The variations between flat and steep εHf trends correspond to previously documented extensional and compressional periods, respectively. Although the change to a steeper εHf trend at ca. 1100 Ma may indicate the emergence of a new isotopic reservoir (i.e. a colliding continent), there is no corresponding change in the K-feldspar Pb or whole-rock Sr isotopic composition. We argue that the trends are better explained by varying proportions of isotopically evolved crust and juvenile mantle in the magma source regions, similar to Nd and Hf isotopic pull-downs and pull-ups observed in many accretionary orogenic systems. We therefore conclude that continuous accretionary processes without involvement of exotic sources is the best explanation for the isotopic evolution before and during the Sveconorwegian orogeny, and that the orogeny involved generation of significant volumes of new crust to the SW margin of the Fennoscandia.

Peraluminous magmatism in the Southernmost Dom Feliciano Belt (Brazil): Magmatic evolution process, sources and tectonic implications

This paper presents new petrological and geochronological constraints on the evolution of the Três Figueiras Granite, the main occurrence of peraluminous granitic rocks in the Southernmost Dom Feliciano Belt, Brazil. The main goal of this work is to constrain the time frame and possible crustal melting processes and sources for this magmatism and its tectonic implications for the evolution of the Dom Feliciano Belt during the Ediacaran, based on field and structural data, petrography (optical microscopy), mineral chemistry (electron proble microanalysis), isotope geochemistry (whole-rock Sr–Nd) and geochronology (U–Pb zircon SHRIMP). The granite is composed of quartz, orthoclase (Or91), oligoclase (An14- Ab86), primary muscovite and biotite, with garnet, tourmaline, zircon, monazite and apatite as accessory minerals. This magmatism is synkinematic to the Ayrosa Galvão-Arroio Grande ductile shear zone system, developing a mylonitic fabric with a subvertical (>70°) southeastward dipping tectonic foliation, and a subhorizontal, ENE plunging stretching lineation marked by elongated mica crystals and aggregates of quartz and feldspar, with dextral kinematics. The granite shows intermediate 87Sr/86Sr(i) ratios (0.7096–0.7123), negative εNd(t) values (−2.61 to −6.54) and Nd model ages between 1.47 and 1.78 Ga. We suggest that the main sources for this magmatism include metagreywackes and others rocks from the Arroio Grande Complex, as a result of dehydration melting, in a transpressional regime, which is a common feature in oblique collisional orogens. The oblique collision is possibly related to the closure of the South Adamastor paleo-ocean. The new U–Pb zircon ages reported here, between 609 and 560 Ma, are similar to those of peraluminous granitic rocks associated to the Dorsal de Canguçu shear zone in the central domain of the Dom Feliciano Belt, as well as to those reported for the Otazo-Cerro Amaro shear zone system, which is the Uruguayan counterpart of the Ayrosa Galvão-Arroio Grande shear zone system.

Sr–Nd–Pb–Hf Isotopic Constraints on the Mantle Heterogeneities beneath the South Mid-Atlantic Ridge at 18–21°S

In an attempt to investigate the nature and origin of mantle heterogeneities beneath the South Mid-Atlantic Ridge (SMAR), we report new whole-rock Sr, Nd, Pb, and Hf isotopic data from eight basalt samples at four dredge stations along the SMAR between 18°S and 21°S. Sr, Nd, and Pb isotopic data from SMAR mid-ocean ridge basalts (MORBs) at 18–21°S published by other researchers were also utilized in this study. The SMAR MORBs at 18–21°S feature the following ratio ranges: 87Sr/86Sr = 0.70212 to 0.70410, 143Nd/144Nd = 0.512893 to 0.513177, 206Pb/204Pb = 18.05 to 19.50, 207Pb/204Pb = 15.47 to 15.71, 208Pb/204Pb = 37.87 to 38.64, and 176Hf/177Hf = 0.283001 to 0.283175. The 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, and 176Hf/177Hf ratios of these MORBs varied considerably along the SMAR axis. The variable compositions of the Sr–Nd–Pb–Hf isotopes, combined with the corresponding whole-rock major and trace elemental abundances reported in previous studies, suggest that the SMAR MORBs at 18–21°S were probably derived from a heterogeneous mantle substrate related to a mixture of depleted mantle (DM) materials with a small amount (but variable input) of HIMU (high-μ, where μ = 238U/204Pb)- and enriched (EMII)-type materials. The HIMU-type materials likely originated from the proximal St. Helena plume and may have been transported through “pipe-like inclined sublithospheric channels” into the SMAR axial zone. The EMII-type materials possibly originated from a recycled metasomatized oceanic crust that may have been derived from the early dispersion of other plume heads into the subcontinental asthenosphere prior to the opening of the South Atlantic Ocean. In addition, the contributions of subducted sediments, continental crust, and subcontinental lithospheric mantle components to the formation of the SMAR MORBs at 18–21°S may be nonexistent or negligible.

Petrogenesis of Cretaceous granitoids in the Bengbu–Wuhe area, southeastern North China Craton: Implications for gold mineralization

Sinistral slip along the Tancheng–Lujiang Fault Zone (TLFZ) during the Mesozoic resulted in an offset of approximately 400 km between the Dabie and Sulu orogens. The Bengbu–Wuhe gold belt (BWGB) represents an extension of the Zhaoyuan–Yexian gold belt to the west of the TLFZ. Early Cretaceous intrusions are widely distributed throughout the BWGB. Gold deposits in the BWGB are commonly associated with intermediate acid dykes that are roughly contemporaneous with mineralization, and some orebodies are hosted in granitoids. This paper investigates granitoids within the BWGB to constrain their petrogenesis, tectonic setting, and metallogenic significance using systematic whole-rock elemental geochemistry, apatite geochemistry, zircon U–Pb dating, zircon Lu–Hf isotopes, and whole-rock Sr, Nd, and Pb isotopes. The timing of Au mineralization (134–113 Ma) in the area is consistent with the age of intrusive adakite-like rocks (130–115 Ma) and normal calc-alkaline granitoids (114–109 Ma). The adakite-like samples are characterized by pronounced enrichment in light rare earth elements (REE), depletion in heavy REE ((La/Yb)N = 30.5–66.2), an absence of Eu anomalies, high Sr contents and Sr/Y ratios, low Y and Yb contents, and fractionated REE patterns ((Dy/Yb)N = 1.22–1.69). They have relatively low Mg# values (38–42) and variable Sr–Nd–Pb–Hf isotopic compositions (87Sr/86Sr(t) = 0.7083–0.7111; εNd(t) = −22.2 to −14.9; 206Pb/204Pb(t) = 17.219–18.010; zircon εHf(t) = −21.0 to −12.3). These features indicate that the parental magmas of the adakites were derived from partial melting of thickened lower crust of the South China Plate and/or the North China Craton. The normal calc-alkaline granitoids have low Sr/Y (6.0–40.5), La/YbN (6.8–24.7), and Dy/YbN (0.90–1.28) ratios, exhibit negative Eu anomalies, and have variable Sr–Nd–Pb–Hf isotopic compositions (87Sr/86Sr(t) = 0.7082–0.7085; εNd(t) = −22.7 to −17.7; 206Pb/204Pb(t) = 16.686–16.713; zircon εHf(t) = −24.9 to −19.0), indicating they were derived from the middle to lower crust of the North China Craton. Compared with the normal calc-alkaline granitoids, the adakite-like rocks have higher zircon Ce4+/Ce3+, higher apatite SO3 and lower apatite MnO contents, lower and more variable zircon crystallization temperatures, and higher apatite molar OH/F and Cl/F ratios, indicating that the adakitic magmas were more enriched in volatiles and more highly oxidized. Taken together, field observations and geochemical data suggest that the adakite-like rocks related to the subducted South China Block are the ore-causative rocks.

Mineralogy, geochemistry, and Sr–Pb and in situ S isotopic compositions of hydrothermal precipitates from the Tangyin hydrothermal field, southern Okinawa Trough: Evaluation of the contribution of magmatic fluids and sediments to hydrothermal systems

The Okinawa Trough (OT), is a back-arc basin where rifting is in its initial stages, and where hydrothermal fields (HTFs) have developed above terrigenous sediments and volcaniclastics layers of variable thickness. The source of ore-forming materials within the hydrothermal systems of the OT is complex, and may include seawater, magmatic rock, magmatic fluid, and sediments. The contribution of magmatic fluids and sediments to the hydrothermal systems of the OT is still uncertain. At least 10 modern seafloor HTFs have been reported from the OT, including the Jade, Hakurei, Iheya North Knoll, CLAM, Tangyin, Yonaguni Knoll IV, Hatoma, and Irabu. Here, we report results of mineralogical, geochemical, and whole-rock Sr–Pb and in situ S isotopic analyses of hydrothermal precipitates from the Tangyin HTF in the southern OT. This field was discovered during the HOBAB3 cruise in 2014 and contains four types of hydrothermal precipitates: Fe–Zn- and Zn–Pb-rich massive sulfides, silica-rich precipitates, and native sulfur-rich samples. Sulfates in Zn–Pb-, Fe–Zn-, and silica-rich samples have 87Sr/86Sr ratios of 0.71068–0.71750, which are higher than those of modern seawater and magmatic rocks, and indicate that the Sr is derived mainly from seawater and sediments. The Zn–Pb-, Fe–Zn-, and silica-rich samples have 206Pb/204Pb (18.5129–18.5307), 207Pb/204Pb (15.6374–15.6489), and 208Pb/204Pb (38.8759–38.9113) ratios that are intermediate between sediments and magmatic rocks, suggesting the Pb is a mixture of sediment- and magmatic-rock-derived Pb. The in situ S isotopic data indicate that the ore-forming fluids associated with all sample types had low δ34S values, with the Fe–Zn- (mean δ34S = 1.27‰ ± 1.38‰; SD, n = 12) and Zn–Pb-rich (mean δ34S = 0.93‰ ± 0.32‰; n = 36) massive sulfides, and silica-rich precipitates (mean δ34S = 2.85‰ ± 1.14‰; n = 58) having lower δ34S values than those of native sulfur-rich samples (mean δ34S = 4.23‰ ± 0.72‰; n = 5). The Zn–Pb- and Fe–Zn-rich massive sulfides, and silica-rich precipitates represent the medium–high temperature (>120 °C) early–middle stages of mineralization, formed under reducing conditions at low sulfur fugacity (fS2), based on the occurrence of low-fS2 mineral assemblages (e.g. high-Fe sphalerite ± isocubanite ± pyrrhotite). The low δ34S values of these three types of samples, and of the Hakurei and CLAM HTFs with low-fS2 mineral assemblages and low concentrations of Cu, Bi, and Au, most likely reflect the incorporation of biological sulfur previously formed by MSR within the sediments. The native sulfur-rich samples of the Tangyin HTF formed at low temperatures (<120 °C), and have slightly higher δ34S values than the Fe–Zn-, Zn–Pb-, and silica-rich samples, may reflect the incorporation of biological sulfur formed mainly by MSR during hydrothermal flow in a seawater recharge zone. Some sulfates in the Jade and Iheya North Knoll HTFs have δ34S values lower than that of modern seawater; this, in combination with high-fS2 oxidizing hydrothermal fluids, and high concentrations of Cu and Au in sulfide samples, indicates a magmatic fluid contribution to these HTFs.

An example of Phanerozoic continental crustal growth: The West Junggar Orogenic Belt, Northwest China

It is generally accepted that continental crustal growth occurred principally during Precambrian, whereas the Phanerozoic growth seems insignificant. However, the West Junggar Orogenic Belt (WJOB), a key part of Central Asian Orogenic Belt in NW China, underwent significant crustal growth during Phanerozoic. A compilation of 683 whole-rock Sr–Nd isotopic data shows that WJOB has different crustal structures and/or compositions from adjacent areas. All igneous rocks in WJOB are characterized by depleted Sr–Nd isotopes, with initial 87Sr/86Sr ratios from 0.7022 to 0.7060, εNd (t) values from +3.0 to +9.2, and Nd model ages from 0.9 to 0.3 Ga. These features are obviously different from the large variations in Sr–Nd isotopes in adjacent Tianshan Orogenic Belt (Tianshan) to the south and Altay Orogenic Belt (Altay) to the north. The continental crustal growth in WJOB resulted from the transformation of pre-existing juvenile crust and mantle wedge during the successively southward subduction of oceanic lithosphere beneath the Ediacaran to Ordovician Tangbale-Mayile arc in southern WJOB, the Silurian to Devonian Boshchekul-Chingiz arc and Early Carboniferous Zharma-Saur arc in northern WJOB, followed by the addition of mantle-derived materials in a post-collisional setting throughout WJOB during Late Carboniferous to Permian, with little or no involvement of Precambrian crust. Such a pattern of continental crustal growth in WJOB is significantly different from those of the adjacent Tianshan and Altay as well as the Hercynian Orogenic Belt in French Massif Central, where Precambrian crustal materials are largely involved in. Thus, WJOB would probably be a good example of Phanerozoic continental crustal growth around the world.

Magma recharge processes of the Yandangshan volcanic-plutonic caldera complex in the coastal SE China: Constraint from inter-grain variation of Sr isotope of plagioclase

Abstract In situ Sr-isotope of plagioclase can be used to elucidate the details of magmatic processes. Here we report in situ Sr isotopic compositions of plagioclase from the intra-caldera porphyritic quartz syenite and mafic microgranular enclave of the Yandangshan caldera, SE China. Plagioclase phenocrysts from porphyritic quartz syenite show consistent initial 87Sr/86Sr ratios (0.7088–0.7091). Plagioclases in groundmass of mafic microgranular enclave have low initial 87Sr/86Sr ratios (0.7080–0.7081). In contrast, the plagioclase phenocrysts of mafic microgranular enclave show relatively variable initial 87Sr/86Sr ratios of 0.7081–0.7088, which exhibit distinct inter-grain variation with high values similar to the plagioclase phenocrysts in porphyritic quartz syenite, and low values similar to the plagioclase in groundmass. However, the initial 87Sr/86Sr ratios of individual plagioclases from phenocrysts and groundmass show insignificant intra-grain variation. Furthermore, initial 87Sr/86Sr ratios (0.7080–0.7088) of plagioclase from phenocrysts and groundmass of mafic microgranular enclave are within and lower than the whole-rock initial 87Sr/86Sr ratios (0.7082–0.7088), while the values of plagioclase phenocrysts from the porphyritic quartz syenite are within and higher than the whole-rock initial 87Sr/86Sr ratios (0.7086–0.7089). Therefore, we propose that the inter-grain Sr isotope variation of plagioclase is related to magma recharge events, which induce chemical and physical exchanges between the recharge magma and the host magma and the remobilization of crystal mush in magma reservoir. Furthermore, our study shows that whole-rock Sr isotopic compositions could not represent the end-members and they may be a result of the assemblage of rock-forming minerals such as plagioclase with different initial Sr isotopic ratios.

Geochronological, Geochemical and Sr–Nd–Pb Isotope Characteristics of the Meydan Ophiolite, SE Turkey: Petrogenesis and Implications for Mesozoic Tectonic Evolution

Gabbros, sheeted dikes and volcanic rocks of the Meydan ophiolite in the South-east Anatolian orogenic belt were examined to determine their U–Pb zircon ages, whole-rock, mineral chemistry and Nd‒Sr–Pb isotope compositions. The new U–Pb dating of zircon separates yielded a 88.3 ± 1.3 Ma crystallization age for gabbroic rocks of Meydan ophiolite. The ophiolite includes tectonites, ultramafic-mafic cumulates, isotropic gabbros, isolated mafic dikes, sheeted dikes, plagiogranites and rare basalts. The sheeted dikes and volcanics of the Meydan ophiolite exhibit near-flat rare earth element (REE) patterns, flat high field strength element trends and negative Nb anomalies that are similar to other SSZ–type eastern Mediterranean ophiolites. The whole-rock and mineral chemistry of the Meydan ophiolite/volcanic rocks suggests that they formed a supra-subduction zone and the dismembered ophiolite metamorphic sole formed beneath the ophiolite in a subduction trench setting. Isotopic ratios, i.e., 87Sr/86Sr versus 206Pb/204Pb and 143Nd/144Nd versus 87Sr/86Sr, show only limited variations, whereas 143Nd/144Nd versus 206Pb/204Pb, 87Sr/86Sr ratios are much more variable. The isotopic ratios are indicative of mixed magmatic contributions from DMM/MORB and EMII reservoirs that probably reflect the overall recycling of mantle wedge, pelagic sediment, oceanic crust and continentally derived sediment. The Meydan ophiolite, inferred arc volcanics and accretionary melange are all intruded by Late Cretaceous (87.7 ± 0.47 to 80.1 ± 0.46 Ma; zircon U–Pb) granitic rocks which are evaluated as the product of continental margin arc magmatism. The available regional comparisons suggest that the rocks studied were derived from the Berit Ocean to the north of the Bitlis Puturge continent, in which the Komurhan, Ispendere and Goksun ophiolites also formed during the Late Cretaceous.

Miocene Olivine Leucitites in the Hoh Xil Basin, Northern Tibet: Implications for Intracontinental Lithosphere Melting and Surface Uplift of the Tibetan Plateau

Abstract The generation of Miocene–Pliocene post-collisional magmatic rocks in northern Tibet was coeval with surface uplift, meaning that understanding the petrogenesis of these rocks should provide clues to the mechanism of uplift of the Tibetan Plateau. However, the nature of the source(s) of Miocene–Pliocene post-collisional rocks is unresolved, especially for potassic–ultrapotassic rocks. This study focuses on 16 Ma olivine leucitites in the Hoh Xil Basin of northern Tibet, which display the lowest SiO2 (43·4–48·8 wt%) contents of all Miocene–Pliocene magmatic rocks in northern Tibet and have high MgO (4·85–8·57 wt%) contents and high K2O/Na2O (&amp;gt;1) ratios. Whole-rock geochemical compositions suggest that the olivine leucitites did not undergo significant fractional crystallization or crustal assimilation. All samples are enriched in large ion lithophile elements relative to high field strength elements, and they exhibit uniform whole-rock Sr–Nd isotope [(87Sr/86Sr)i = 0·7071–0·7077 and εNd(t) = −3·1 to −3·9] and olivine O isotope (5·8–6·6 ‰, mean of 6·2 ± 0·2 ‰, n = 21) compositions. We propose that the olivine leucitites were derived by low-degree partial melting of phlogopite-lherzolite in garnet-facies lithospheric mantle. Given the tectonic evolution of the Hoh Xil Basin and adjacent areas, we suggest that southward subduction of Asian (Qaidam block) lithosphere after India–Asia collision transferred potassium and other incompatible elements into the lithospheric mantle, forming the K-enriched mantle source of the Miocene–Pliocene potassic–ultrapotassic rocks. Removal of lower lithospheric mantle subsequently induced voluminous Miocene–Pliocene magmatism and generated &amp;gt;1 km surface uplift in the Hoh Xil Basin.

Magma plumbing system for the Millennium Eruption at Changbaishan volcano, China: Constraints from whole-rock U–Th disequilibrium

A caldera-forming eruption in 946 CE, known as the Millennium Eruption (ME), at Changbaishan (or Baekdusan) volcano, located on the border between China and North Korea, was one of the largest explosive eruptions on Earth in the last 2000 years. In this study, we obtained new whole-rock Sr, Nd, and Pb isotopic data and 238U–230Th disequilibrium data of representative volcanic products of the ME, as well as those of a pre-ME (4–5 ka) eruption, to understand the evolution of the magma plumbing system related to the caldera-forming eruption. The volcanic products of the ME have SiO2 contents ranging from ~54 to ~75 wt%, and the whole-rock (230Th/232Th) ratios tend to decrease systematically from basaltic trachy-andesite through trachyte to comendite. The relatively high (230Th/232Th) ratios of the felsic (trachytic and comenditic) volcanic products suggest that they were formed primarily from fractional crystallization of mafic magmas, rather than from partial melting of crustal materials. The basaltic trachy-andesite sample has a distinctly high (230Th/232Th) ratio and the least radiogenic Pb isotopic ratios among ME volcanic products, suggesting that the mafic magma was recently introduced into the main felsic magma system and is hypothesized to have triggered the eruption. The lower (230Th/232Th) ratios of the comendite samples than those of the trachyte samples, as well as published U-series disequilibrium data, suggest that the comendite magmas were produced by mixing old (> ~ 23 ka) and young (<~6 ka) comenditic magmas. The old comendite magma was the main component of ME volcanic products. Pre-ME pantelleritic magmas with distinctly low (230Th/232Th) and high 87Sr/86Sr ratios from those of ME magmas may have formed a short-lived magma chamber until ~4–5 ka, which was isolated from the long-lived, main comendite magma chamber.

Petrogenesis of the early Permian A-type granites in the Halajun region, southwest Tianshan, western Xinjiang, NW China: implications for geodynamics of Tarim large igneous province

ABSTRACT Formation of a large igneous province, an explosive magmatic event in earth, is intimately related to geodynamic processes. The Tarim large igneous province, a famous representative in China, contains A-type granites. However, the petrogenesis of the A-type granites is debated. Some researchers think that the A-type granites are developed through extensive fractionation of magma derived from partial melting of newly underplated or gabbroic rocks. Others insist that the A-type granites are developed through intensive fractional crystallization of plume-derived mafic magma. Here, we present zircon U–Pb ages and whole-rock geochemical and Sr–Nd isotopic data for the Kezilekezitage, Keziertuo, Tamu, and Halajun II intrusions in northwest Tarim to elucidate their petrogenesis and geodynamic implications. Zircon U–Pb dating yielded crystallization ages of the Kezilekezitage, Keziertuo, and Halajun II intrusions of 275.1 ± 3.1 Ma (MSWD = 1.14), 274.8 ± 1.8 (MSWD = 0.46) and 275.6 ± 0.7 (MSWD = 1.04), respectively. The whole-rock geochemistry of the four intrusions is characterized by REE and HFSE enrichment, Ba, Sr, P, Ti, and Eu depletion, εNd (t) values of – 1.2 to +0.3, two-stage Nd model ages (T2DM) of 1.14–1.02 Ga, and an A1-type granite affinity. Whole-rock zircon geothermometry indicates a high formation temperature of 721–924°C. It is concluded that these A-type granites were generated by partial melting of Neoproterozoic OIB-like rocks, coupled with extensive fractional crystallization and minor crustal contamination. The production of the A-type granites at ca. 275 Ma represented the termination and greatest degree of lithospheric thinning during large igneous province formation.