Enriched Isotopic Compositions Research Articles

Discovery of multi-crustal rejuvenations for the formation of the Lincang granitic batholith, Southwest China: magmatism relating to Changning–Menglian Paleo–Tethyan termination

ABSTRACT The consumption of the Changning–Menglian Palaeo-Tethys ocean in the Triassic formed the Lincang granitic batholith in the southwestern China, once the oceanic supra-subduction zone. The batholith provides us a critical window to understand the nature of the crustal basements and the process of Palaeo-Tethyan termination. Here we report zircon U-Pb ages and Hf isotopes, and whole-rock elements and Sr-Nd isotopes of granitic samples from the central part of the Lincang batholith. The geochemical results reveal two genetic types of granites indicated by low-K and high-K affinities, respectively. The Group 1 low-K granitoids dated at 237–229 Ma are low-K calc-alkaline and metaluminous to peraluminous (A/CNK = 0.75–1.29), with K2O contents of 0.13–1.33 wt%, K2O/Na2O ratios of 0.03–0.30, Rb/Sr ratios < 0.18, and initial 87Sr/86Sr = 0.71693 and εNd(t) = −10.8. In contrast, the Group 2 high-K granitoids dated at 239–200 Ma are calc-alkaline and peraluminous (A/CNK = 1.02–1.20), with higher K2O contents (3.15–4.7 wt%), K2O/Na2O ratios (1.15–2.00) and Rb/Sr ratios (0.77–2.62), and more enriched isotopic compositions (initial 87Sr/86Sr = 0.73129 to 0.74037, εNd(t) = −13.1 to −11.0) as well as highly negative zircon εHf(t) values of −16.8 to −6.7. The formation time of these granitoids is compatible with that of the post-collision of the Palaeo-Tethys, subsequently to the magmatic gap (250–240 Ma) coupling with the syn-collision stage. The magmatic source discriminations suggest that the low-K granitoids were derived from partial melting of the dehydrated low-K amphibolite source and the high-K granitoids were dominantly from the psammitic source with minor mantle-derived magma involvement. Using the geochemical variations of the granitic rocks related to the Palaeo-Tethyan closure, we reveal melting of multiple crustal sources rather than a sole source to construct the Lincang batholith.

Aplicación de Isótopos estables e hidroquímica para la comprensión del sistema hidrológico en Laguna de Tiscapa. Managua, Nicaragua

Isótopos estables (Deuterio y Oxígeno 18) e hidroquímica fueron utilizados como trazadores para determinar el origen del agua en el cráter de la Laguna de Tiscapa y sus interacciones con el agua subterránea y Lago Xolotlán, para esto se utilizaron 253 resultados de isótopos estables y 56 resultados hidroquímicos de las fuentes: precipitación, agua subterránea, Laguna de Tiscapa y Lago Xolotlán. Los isótopos estables indican que Laguna de Tiscapa es recargada en zona oeste por agua subterránea donde hay una composición media de -7‰ en δ18O, los procesos de evaporación en laguna hacen que tenga una composición media de -5.4‰ en δ18O, reflejando este aporte de Laguna de Tiscapa zona noreste del agua subterránea con enriquecimiento de -6‰ en δ18O. La composición hidroquímica en Laguna de Tiscapa tiene común relación con el agua subterránea donde rigen los procesos de disolución de material geológico y no el meteórico, esta disolución hace predominen los iones HCO-3, Ca+2, Na+, Mg+2. Los procesos de disolución se dan principalmente por la presencia de silicatos seguidos de calcitas, que es característico de la composición del acuífero Las Sierras. Se puede descartar aporte de agua del Lago Xolotán a Laguna de Tiscapa, puesto que este presenta una composición isotópica altamente enriquecido, a lo que se suma los procesos de salinización que experimenta este cuerpo de agua.

Stable isotopes of water and specific conductance reveal complimentary information on streamflow generation in snowmelt-dominated, seasonally arid watersheds

The utility of a particular tracer to perform hydrograph separations depends on the dominant watershed properties combined with meteorological patterns; therefore, drawing conclusions from one tracer can be misleading. Combining information from multiple tracers can reveal complimentary insights that advance our knowledge of runoff generating processes. We performed hydrograph separations during spring snowmelt and for one summer rain event at two seasonally arid, montane watersheds in southeastern Wyoming using two independent tracer systems: Stable water isotopes (18O and 2H; 1–4 h resolution) and specific conductance (SC; 15 min resolution). Event water dominated streamflow generation during snowmelt using both tracers, but much lower uncertainty in hydrograph separations were achieved during this period with SC than with stable isotope tracers; the stable isotopic composition of pre-event and event water were too similar which led to large degrees of uncertainty. Stable isotopes of stream water did not vary as much as SC throughout the year and indicated the dominance of stream water derived from snowmelt. During the main snowmelt period, high frequency stream water isotopic composition was remarkably consistent, despite considerable variability measured in snowpack and snowmelt samples. Stable isotope measurements were useful for partitioning streamflow during a summer rain event when source waters were more isotopically distinct and suggested the dominance of pre-event water at generating streamflow. Despite the inability to partition event and pre-event streamflow during snowmelt, relationships between isotopic composition, SC, and watershed properties helped to understand mechanisms for streamflow generation. For instance, line conditioned excess (lc-excess) on the rising limb of the main snowmelt period was significantly positively correlated to the amount of rapid diurnal snowmelt contributions, the fastest moving compartment of event flow, supporting evidence that freshly melted snow, with relatively higher lc-excess, preferentially contributed to rapid diurnal snowmelt contributions. During fall low flow stream conditions, the isotopic value of streamflow was significantly positively correlated to elevation, where the highest-elevation watersheds had the highest delta values. This relationship was contradictory to what was expected (i.e. “elevation effect”) and may be explained by extensive snowpack fractionation occurring later in the season in deeper snowpacks located at higher elevations, or by a larger amount rainfall, with enriched isotopic composition relative to snowmelt, occurring at higher elevations. Through synthesizing patterns in two different natural tracers in conjunction with watershed properties, insights into the dominant controls on streamflow generation were gained from seasonally arid, snowmelt-dominated, headwater watersheds.

Petrology, Geochemistry, and Sr-Nd-S Isotopic Compositions of the Ore-Hosting Biotite Monzodiorite in the Luanjiahe Gold Deposit, Jiaodong Peninsula, China

The Jiaodong Peninsula is one of the most important Au ore provinces in China. There is an ongoing debate on the correlation between ore formation and magmatism in this province, because few intrusive rocks exhibit a clear association with ore deposits. A mineralized biotite monzodiorite (BM) stock, with disseminated ore, pervasive phyllic alteration, and no deformation, was found in a borehole in the footwall of the Zhaoping fault within the Luanjiahe Au deposit, which may shed light on this debate. The biotite monzodiorite contains explosion breccias, miarolitic cavities, skeletal and dendritic quartz, and late-stage evolved aplite dikes, and the in-situ δ34S values of the disseminated pyrite which is associated with Au mineralization are -1.7‰ to 7.3‰ (mean=3.5‰), indicative of a magmatic-hydrothermal system. These findings, combined with the reported age of 123 Ma, show that the intrusion has close spatial, temporal, and geochemical relationships with Au mineralization in the area. The biotite monzodiorite is metaluminous, high-K calc-alkaline and shoshonitic, with enrichment in light rare earth elements (REEs) and large-ion lithophile elements (LILEs), depletion in high-field-strength elements (HFSEs), and enriched Sr-Nd isotopic compositions. The intrusion may be the product of partial melting of enriched lithospheric mantle with a small lower crustal component. The hydrous, Au-bearing, enriched mantle source, and the strongly oxidized magma that was generated, created favorable conditions for Au mineralization.

Oblique continental collision and the formation of syn-collisional A-type granites: insights from the Early Jurassic Baoji granite suite in the Qinling orogenic belt, central China

ABSTRACT A-type granites are conventionally interpreted to be formed in late post-collisional or anorogenic settings. However, we firstly identified an Early Jurassic syn-collisional A-type granite suite in the Qinling orogenic belt: the Baoji granite suite. A comprehensive investigation into its mineralogical, petrological, geochronological, and geochemical characteristics can shed light on the genesis of syn-collisional A-type granites and geodynamic evolution of collisional orogeny. The Baoji granite suite contains two types of rocks: (1) A-type granites including biotite syenite, coarse-grained, porphyritic, and fine-grained syenogranites, and alkali-feldspar granite, and (2) magnesian monzodiorite; both types are coeval, with zircon U–Pb ages of ca. 190 Ma. The A-type granites yield A1-type trace element signatures and large variations in trace element contents, with Ba contents of 16.9–762 ppm, Sr of 11.0–211 ppm, Rb of 193–645 ppm, and Eu of 0.08–1.24 ppm. Their radiogenic isotope compositions (87Sr/86Sri = 0.70631–0.70637; εNd = −5.18 to −3.84; εHf = −5.01 to +2.76) are similar, and overlap those of Neoproterozoic OIB-like basic volcanics from the South Qinling belt. These characteristics show that the A-type granites originated from the anatexis of OIB-like lower crust and underwent fractional crystallization in a silicic magma reservoir. The monzodiorite has low SiO2 contents (49.82–53.29 wt.%) and enriched radiogenic isotopic compositions (87Sr/86Sri = 0.70554–0.70638; εNd = −11.0 to −5.28) contains abundant hornblende and biotite (~40%), and exhibits evidence of magma mingling. It also contains two groups of zircon: pristine and unzoned zircon that yields negative Hf isotope compositions (εHf = −9.42 to −3.14), and xenocrystic zircon that yields similar Hf isotope compositions (εHf = −3.54 to +1.85) to the A-type granites. These characteristics demonstrate that the monzodiorite was derived from the differentiation of basaltic partial melts of an enriched mantle source, and mingled with the A-type granitic magma during its ascent and emplacement into a silicic magma reservoir. Along with regional records of the Early Jurassic magmatism, deformation, metamorphism, and sedimentation, the genetic models for the Baoji A-type granites and monzodiorite suggest that the Middle Triassic to Early Jurassic oblique continental collision (ca. 230–190 Ma) produced coupled EW-trending strike-slip and NE-trending extensional faults at crustal levels and Rayleigh–Taylor (gravitational) convective instabilities at mantle depths in the Qinling orogenic belt, which controlled the production of the syn-collisional A-type granite suite. Our work also indicates that the A-type granites can be formed in syn-collisional setting in response to oblique continental collision and the resultant slow foundering of a high-density lithospheric root; therefore, we suggest that the syn-collisional A-type granites are evidence for oblique continental collision.

Generation of syn-collisional S-type granites in collision zones: An example from the Late Triassic Tanggula Batholith in northern Tibet

Mechanisms of syn-collision magma generation are unclear due to the uncertainty in defining the term collision. We address this issue by presenting detailed petrographic, chronological, and geochemical data for the Tanggula Batholith located immediately to the north of the Longmu Tso–Shuanghu suture zone (LSSZ) in northern Tibet. Zircon LA–ICPMS U–Pb dating suggests that the Tanggula Batholith was emplaced at 235–226 Ma. Samples from the batholith show variable SiO2 of 66.6–80.6 wt% and are characterized by high normative corundum (1.09–4.35 wt%) and high aluminum saturation index (A/CNK = 1.05–1.40). These samples exhibit high initial 87Sr/86Sr (0.7165–0.7214), negative whole-rock εNd(t) (−10.1 to −8.5), εHf(t) (−8.7 to −6.3), enriched Pb isotope compositions, and variable zircon εHf(t) values (−29.9 to +5.6), as well as variable zircon δ18O value (6.90–9.22‰). These signatures are characteristic of S-type granites, and were derived from anataxis of metasedimentary rocks from the Qiangtang Terrane with input from mantle-derived components. We propose that the Tanggula Batholith was emplaced in a syn-collisional setting following closure of the Longmu Tso–Shuanghu Ocean along a north dipping subduction zone based on: the timing of high-pressure, low-temperature metamorphism at 244–221 Ma marking collision between Southern and Northern Qiangtang; the timing of exhumation of metamorphic rocks at ~223–204 Ma; and, bimodal volcanism (225–202 Ma) associated with slab breakoff at 220–215 Ma.

Chapter 2.1b Ferrar Large Igneous Province: petrology

Abstract The Lower Jurassic Ferrar Large Igneous Province consists predominantly of intrusive rocks, which crop out over a distance of 3500 km. In comparison, extrusive rocks are more restricted geographically. Geochemically, the province is divided into the Mount Fazio Chemical Type, forming more than 99% of the exposed province, and the Scarab Peak Chemical Type, which in the Ross Sea sector is restricted to the uppermost lava. The former exhibits a range of compositions (SiO 2 = 52–59%; MgO = 9.2–2.6%; Zr = 60–175 ppm; Sr i = 0.7081–0.7138; ε Nd = −6.0 to −3.8), whereas the latter has a restricted composition (SiO 2 = c. 58%; MgO = c. 2.3%; Zr = c. 230 ppm; Sr i = 0.7090–0.7097; ε Nd = −4.4 to −4.1). Both chemical types are characterized by enriched initial isotope compositions of neodymium and strontium, low abundances of high field strength elements, and crust-like trace element patterns. The most basic rocks, olivine-bearing dolerites, indicate that these geochemical characteristics were inherited from a mantle source modified by subduction processes, possibly the incorporation of sediment. In one model, magmas were derived from a linear source having multiple sites of generation each of which evolved to yield, in sum, the province-wide coherent geochemistry. The preferred interpretation is that the remarkably coherent geochemistry and short duration of emplacement demonstrate derivation from a single source inferred to have been located in the proto-Weddell Sea region. The spatial variation in geochemical characteristics of the lavas suggests distinct magma batches erupted at the surface, whereas no clear geographical pattern is evident for intrusive rocks.

Tracking a continental deep subduction and exhumation from granulitized kyanite eclogites in the South Altyn Tagh, northern Qinghai-Tibet Plateau, China

Kyanite eclogites can provide insights into global orogenic processes within subduction-collision zones. Here we present an integrated study of the petrography, mineral chemistry, zircon U-Pb geochronology, and whole-rock geochemistry of granulitized kyanite eclogites from South Altyn Tagh, located at the northern margin of the Qinghai-Tibet plateau, China. Our results suggest that the rocks experienced eclogite-facies metamorphism (27–37 kbar, 1030–1068 °C), followed by a high pressure-ultra-high temperature (HP-UHT) stage (17–20 kbar, 945–1033 °C), isothermal decompression, and a medium pressure-(ultra)-high temperature (MP-(U)HT) (10–12 kbar, 810–950 °C) granulite-facies overprint.Zircon U-Pb dating yielded a weighted mean U-Pb age of ca. 500 Ma for the metamorphic domains, which is interpreted as the timing of HP-UHT metamorphism. In contrast, the age of ca. 800 Ma recorded by the inherited cores of zircon grains is considered to represent the protolith formation age. High Al2O3 and CaO and relatively low MgO and FeO contents, LREE enrichment and nearly flat HREE patterns, and enriched Sr-Nd isotopic compositions suggest that the protoliths of the granulitized kyanite eclogites were likely derived from an enriched mantle source affected by crustal contamination. These results, together with previous research on these associations suggested that the protoliths of granulitized kyanite eclogites and associated garnet peridotite in the Bashiwake area are derived from mafic-ultramafic intrusions composed of plagioclase-rich gabbroic rocks and olivine-rich cumulates. These mafic-ultramafic intrusions were possibly generated by plume-induced melting related to the rifting of the Rodinia supercontinent during the Neoproterozoic, and were later subjected to early Paleozoic metamorphism. The geodynamic model proposed envisages continental subduction followed by relamination of felsic and mafic-ultramafic associations at the base of the lower crust, which were subsequently exhumed, forming diapir-like bodies. Our results, combined with those from previous studies, not only provide insights into the processes of continental collision to exhumation, but also provide significant constraints on the geotectonic processes leading to the formation of HP rocks within subduction-collision orogens.

High-Mg# Adakitic Rocks Formed by Lower-crustal Magma Differentiation: Mineralogical and Geochemical Evidence from Garnet-bearing Diorite Porphyries in Central Tibet

AbstractModern arc adakites with high Mg# values (molar 100 × Mg/(Mg + Fe) ratio) are generally considered products of interaction between melts derived from subducted oceanic crust and/or eroded forearc crust and peridotite in the mantle wedge. An alternative model, in which high-Mg# adakitic rocks are produced by garnet fractionation of mantle-derived magmas, has been proposed based on whole-rock geochemical variations; however, magmatic garnet has not been found in high-Mg# adakitic rocks, and little is known about the physical conditions required for this magmatic differentiation. Here we report geochronological, mineralogical and geochemical data for Late Triassic garnet-bearing high-Mg# (Mg# = 45–56) adakitic diorite porphyries and garnet-free non-adakitic diorite porphyries with Mg# &amp;gt; 62 from central Tibet. Consistent compositional correlation between Ca-rich garnet crystals, their host rocks and zircon autocrysts suggests that the garnet crystals grew in their host magmas. Amphibole, garnet, zircon and the host rocks display increasing Dy/Yb ratios with increasing magma differentiation. Pristine magmas in equilibrium with amphibole that crystallized prior to garnet are not adakitic. The garnet-bearing high-Mg# adakitic rocks were probably generated by the fractionation of pyroxene, amphibole and garnet at ∼1 GPa from a primitive andesitic parent that was geochemically similar to the garnet-free diorite porphyries. The primitive andesitic magmas with enriched isotope compositions ([87Sr/86Sr]i &amp;gt; 0·709, ɛNd[t] &amp;lt; −3·4) may be derived from shallow melting of subduction-enriched lithospheric mantle in a post-collisional, extensional setting resulting from oceanic slab breakoff. The most likely scenario for garnet crystallization is that mantle-derived hydrous (H2O &amp;gt;5 wt %) magmas stalled, cooled isobarically and differentiated at the base of the crust. This study provides direct mineralogical evidence for the generation of high-Mg# adakitic rocks by crystal fractionation involving garnet, rather than by interaction between crust-derived melt and the mantle, although the latter is potentially a frequent occurrence in the mantle wedge.

Magmatic Evolution following Damp Tholeiitic and Wet Calc-alkaline Liquid Lines of Descent: an Eastern Pontides (NE Turkey) Example

AbstractAssociations between tholeiitic and calc-alkaline arc magmatism with close spatial and temporal relationships can provide critical constraints on magma genesis and allow the reconstruction of subduction polarity at convergent margins. This study identifies two compositionally distinct intrusive series from the Yusufeli region in the Eastern Pontides arc, NE Turkey. The intrusive rocks from the Yusufeli intrusive complex were emplaced at 179–170 Ma and are dominated by the low- to medium-K tholeiitic series, with depleted Hf isotopic compositions. In contrast, the intrusive rocks from the Camlikaya intrusive complex were emplaced at 151–147 Ma and are characterized by the medium- to high-K calc-alkaline series, with relatively enriched Hf isotopic compositions. The Al-in-hornblende geobarometer reveals that the magmas of both intrusive complexes crystallized at upper crustal levels (∼150–250 MPa, ∼5–8 km). The presence of patchy-textured plagioclase and the widespread occurrence of coeval dykes and magmatic mafic enclaves indicate that the two intrusive complexes are derived from multiple magma pulses in open magmatic systems. The mineral crystallization order of amphibole and plagioclase, the trace elemental signatures (e.g. Sr/Y and Y), and rare earth element modeling collectively suggest that the Yusufeli intrusive complex was dominated by plagioclase and clinopyroxene fractionation with earlier plagioclase crystallization than amphibole, whereas the Camlikaya intrusive complex was dominated by the fractionation of amphibole accompanied by co-crystallization of plagioclase. Such significant differences in the fractionating mineral assemblages at comparable intrusion pressures can be attributed to different initial H2O contents of the Yusufeli and Camlikaya parental magmas, which ultimately control their distinct liquid lines of descent. In accord with thermodynamic modeling results derived using the Rhyolite-MELTS software, we propose that the Yusufeli intrusive rocks are derived from damp (∼1–2 wt% H2O) parental magmas formed dominantly by decompression melting of mantle wedge in a back-arc setting. In contrast, the wet parental magmas (&amp;gt;∼2 wt% H2O) of Camlikaya intrusive rocks are more hydrous and formed through flux melting of suprasubduction-zone mantle wedge. This conclusion, combined with the back-arc basin related Jurassic sedimentary and structural records previously determined in the Southern Zone of the Eastern Pontides, indicates that the geochemical compositions and spatial relationship of the Yusufeli and Camlikaya intrusive complexes are preferably explained by the southward subduction of the Paleotethys oceanic lithosphere in the Early to Late Jurassic.

Eocene arc magmatism and related Cu-Au (Mo) mineralization in the Shangalon-Kyungalon district, Wuntho-Popa Arc, northern Myanmar

The Shangalon-Kyungalon Cu-Au (Mo) ore district is located on the south-eastern margin of the Kanzachaung batholith in the Wuntho-Popa Arc, Myanmar. Porphyry and epithermal Cu-Au mineralization are associated with the Late-Eocene magmatic rocks. In this study, zircon U-Pb dating of gabbroic diorite, diorite porphyry, tonalite porphyry, and granodiorite yielded ages of 38.0 ± 0.35 Ma, 37.8 ± 0.35 Ma, 37.4 ± 0.27 Ma, and 36.9 ± 0.26 Ma, respectively. These rocks show sub-alkaline characteristics, LREE-enriched patterns with weakly positive/negative Eu anomalies, enrichment of LILEs and depletion of HFSEs. They have obviously higher initial Sr ratios and lower εNd(t) and εHf(t) values (Isr = 0.7066; εNd(t) = −0.5 to −4.8; εHf(t) = 0.7–9.7) than the mid-Cretaceous Kanzachaung batholith (Isr = 0.7046; εNd(t) = 2.4–4.5; εHf(t) = 10.9–15.1), indicating crustal materials were most likely added to the arc magma source. In particular, the gabbroic diorite has higher initial Sr, more enriched Nd-Hf isotopic compositions (Isr = 0.7081, εNd(t) = −4.6 to −4.8, εHf(t) = 0.7–2.3) and lower SiO2 content relative to the other magmatic rocks studied (Isr = 0.7068–0.7059; εNd(t) = −0.5 to −2.8, εHf(t) = 5.0–9.7). The results suggest that the gabbroic diorite was derived from melting of a mantle wedge, metasomatized by subducted oceanic sediment. Underplating on juvenile lower crust followed, accompanied by MASH processes, which led to increased εNd(t) (and SiO2 content) and decreased initial Sr ratios in later formed rocks. Moreover, the adakite-like features (Sr/Y = 19.3–53.4, Y = 7.54–18.8,) suggest fractional crystallization of hornblende in a hydrous magma, potentially leading to formation of the Shangalon-Kyungalon porphyry-epithermal Cu-Au system. In summary, the Late-Eocene magmas in Shangalon-Kyungalon formed from a mantle source metasomatized by subducted sediments during Neo-Tethyan subduction beneath Western Burma terrane, with the involvement of juvenile crustal materials.

Magnesium and zinc isotope evidence for recycled sediments and oceanic crust in the mantle sources of continental basalts from eastern China

To investigate the origin of compositional variations in intraplate alkaline and tholeiitic basalts, this study reports a combined study of metal stable (i.e., Mg and Zn) and radiogenic (i.e., Sr and Nd) isotopes of 50 Cenozoic continental basalts from the South China Block, eastern China. These basalts are classified into high- and low-CaO basanites, trachybasalts, and tholeiites, and have variable Mg–Zn–Sr–Nd isotopic compositions (δ26Mg = −0.29 to −0.46‰, δ66Zn = 0.27 to 0.50‰, 87Sr/86Sr = 0.703233 to 0.704175, and εNd = 3.8 to 8.0), reflecting mantle heterogeneity. In detail, the high-CaO basanites and some trachybasalts have high Ca/Al and δ66Zn, low Hf/Hf* and δ26Mg, and depleted SrNd isotopic compositions, consistent with partial melts of carbonated mantle formed by reaction of the mantle with slab-derived carbonate melts. The low-CaO basanites, some trachybasalts, and tholeiites have low Ca/Al and δ26Mg, high Hf/Hf* and δ66Zn, and depleted SrNd isotopic compositions. These characteristics are related to recycled carbonated eclogites, rather than carbonates, which is consistent with the correlations between δ26Mg, δ66Zn, and Zn/Fe values. In addition, previously published data for the late Mesozoic to Cenozoic basalts from eastern China reveal another group of basalts with high Ba/Th ratios and enriched SrNd isotopic compositions, indicative of significant contributions from siliceous sediments. Our study demonstrates that the compositional variations of the late Mesozoic to Cenozoic alkaline and tholeiitic basalts from eastern China reflect variable proportions of oceanic slab components in their mantle sources.

Transition from the lithospheric to asthenospheric mantle-derived magmatism in the Early Jurassic along eastern Bangong–Nujiang Suture, Tibet: Evidence for continental arc extension induced by slab rollback

Abstract The transition of the geochemical signature in mafic rocks along the eastern Bangong–Nujiang suture in Tibet contains important information about geodynamic processes in the upper mantle. This study recognized two episodes of Early Jurassic gabbros from the Kaqiong terrane, a microblock within the Bangong–Nujiang suture zone. Early gabbros (ca. 197–191 Ma) appear as lenses in the basement complex and were overprinted by amphibolite/granulite-facies metamorphism at ca. 180 Ma. Later undeformed hornblende gabbros (ca. 177–175 Ma) occur as dikes intruding into the basement complex. The early metagabbros are characterized by arc-like geochemical features and enriched Nd-Hf isotopic compositions (whole rock ∑Nd(t) = –0.7 to +0.3; zircon ∑Hf(t) = –5.7 to –2.2), which suggests formation by partial melting of an enriched lithospheric mantle source. In contrast, the later hornblende gabbros have depleted Nd-Hf isotopic compositions (whole rock ∑Nd(t) = +6.1 to +7.1; zircon ∑Hf(t) = +10.7 to +16.8) and normal mid–oceanic–ridge basalt (N–MORB)-type rare earth element (REE) features. They also show variable enrichments of fluid mobile elements (e.g., Rb, U, Pb), indicative of the input of slab-derived fluids in their mantle source. Thus, the hornblende gabbros were most likely originated from the asthenospheric mantle metasomatized by subducted oceanic slab-derived fluids. The transition in geochemical and isotopic compositions of these mantle-derived magmas reveals a long-lasting lithosphere extension and thinning along the southern margin of the Qiangtang terrane in the Early Jurassic. Combined with geological observations, we propose that this transition has resulted from the southward rollback of the subducting Bangong–Nujiang Tethyan oceanic slab. The slab rollback could have initiated the overriding plate extension and the asthenosphere upwelling. Wider implications of this study are that an onset of slab rollback could be an important trigger for the transition of magmatic geochemistry in subduction zones.

Provenance changes across the mid-Cretaceous unconformity in basins of northeastern China: Evidence for an integrated paleolake system and tectonic transformation

Abstract Detrital zircon U-Pb dating and whole-rock Nd isotopic analyses were carried out on selected stratigraphic horizons across a major unconformity between synrift and postrift stages in the Songliao Basin and Dasanjiang basin group of NE China to constrain the crustal evolution of the source area providing detritus into these basins. The strata underlying the mid-Cretaceous unconformity in the Songliao Basin show regionally distinct detrital zircon age populations and Nd isotopic compositions, which generally are characterized by Phanerozoic age peaks and relatively depleted Nd isotopic compositions, indicating derivation from nearby highlands. In contrast, the overlying strata are dominated by Proterozoic zircon ages and enriched Nd isotopic compositions, which imply that the provenance source region shifted to the northern part of the North China craton. A coeval provenance change also affected the sedimentary architecture in the eastern Dasanjiang basin group, marking the migration of erosion centers from west to east. The contribution from Lesser Xing’an–Zhangguangcai ranges was pronounced during deposition of synrift strata but became negligible afterward in the Songliao and Dasanjiang areas, which is consistent with both basin complexes temporarily forming an extensive lake system during the early Late Cretaceous. This paleolake was likely responsible for transgressive events recorded in the Late Cretaceous strata of basins in NE China. Combining observations from seismic reflection profiles and the stratigraphic record of neighboring intracontinental sedimentary basins as well as widespread contemporaneous exhumation and denudation events, we suggest that the provenance variation in basin strata was controlled by large-scale tectonic transitions in East Asia. The switch from extension to contraction during the mid-Cretaceous is attributed to the docking of the Okhotomorsk block along the East Asian continental margin. The resulting lithospheric buckling might have been responsible for reshaping the basin-and-range configuration in NE Asia.

Geochronology, geochemistry, and Hf isotopic compositions of Triassic igneous rocks in the easternmost segment of the East Kunlun Orogenic Belt, NW China: implications for magmatism and tectonic evolution

ABSTRACT The voluminous Triassic igneous rocks in the East Kunlun Orogenic Belt (EKOB) are important for investigations of the tectonic evolution of the Paleo-Tethys Ocean. This study focused on the geochronology and petrogenesis of igneous rocks in the Elashan area with the aim of constraining the regional tectonic evolution. The 238 Ma syenogranites are characterized by high SiO2 contents and marked depletions in Sr and Eu. We conclude that they are highly fractionated I-type granites derived from partial melting of Mesoproterozoic lower crust with significant fractional crystallization. Comparing with contemporaneous granites and mafic rocks, the 236 Ma quartz monzodiorites have moderate MgO (1.5–1.6 wt.%) and SiO2 (61.0–61.9 wt.%) contents and εHf(t) values (–1.2 to +1.9). They were probably generated by the mixing of Mesoproterozoic lower-crust-derived and enriched lithospheric mantle-derived melts. The 230 Ma appinites have high MgO contents (3.8–6.1 wt.%) and enriched Hf isotopic compositions, and were likely derived from partial melting of lithospheric mantle that had been metasomatized by subduction fluids. We infer that these rocks were emplaced in syn-collisional slab break-off and the later initial post-collisional extensional setting between the BayanHar-Songpan Ganze and East Kunlun terranes. The basement composition, magmatism, and tectonic evolution of the Elashan area are similar to those of the eastern EKOB.

Sodium-bicarbonate groundwaters in southeastern West Siberia, Russia: Compositions, types, and formation conditions

Sodium-bicarbonate HCO3–Na (sodic) groundwaters exist throughout southeastern West Siberia at approximate depths from 50–300 m to 1.0–2.3 km in Mesozoic-Cenozoic sediments. They belong to five main types of fresh (I), brackish (II), high-pH low-saline (III), coal-related saline (IV), and carbonated (V) waters that differ in composition, as well as in depth and lateral extent. Waters of types I and II are of regional extent and common chemistry, while those of three other types have specific compositions and are restricted to local areas. Isotope data (δ18O, δD, δ13C) indicate that waters of all five types originated by the infiltration mechanism; type IV water has an enriched oxygen isotope composition; all water types except V have biogenic carbon sources. As shown by thermodynamic calculations, all HCO3–Na waters are nonequilibrium with many primary aluminosilicate minerals bur are equilibrated with carbonates and clay minerals. The number of minerals equilibrated with these waters increases progressively from type I to IV with salinity and pH. The obtained data allow reconstructing the formation of sodic waters of different types in the context of the evolution in the system ‘water – rock – gas – organic matter’. The formation scenario is the same for all types of water: dissolution of sedimentary aluminosilicate minerals which are not in equilibrium with the waters and concurrent precipitation of carbonates. Waters in the zone of slow water exchange at depths from 100 to 300 m acquire the HCO3–Na compositions, with TDS >0.7–0.8 g/L and рH >7.6. The diversity of the waters results from difference in their residence time, even during the formation of HCO3–Na chemistry (types I and II), and from environment effects: presence of inorganic CO2 (V) and organic carbon (IV) sources or their absence (III).

Variations of Stable Oxygen and Deuterium Isotopes in River and Lake Waters During Flooding Season Along the Middle and Lower Reaches of the Yangtze River Regions

Stable isotope tracers have been widely applied to water sources and evolution, transforming relations, and pollution sources of various water bodies. This study analyzed the spatial variations of δ2H and δ18O in river and lake waters during flooding season, and revealed the factors underlying their variations along the middle and lower reaches of the Yangtze River based on a field sampling campaign in July 2018. Our results showed that δ2H and δ18O in the Yangtze River water were enriched from the Three Gorges reservoir region to the lower reaches of the Yangtze River, which was closely linked to isotopic variations in precipitation. There was no significant difference in δ2H and δ18O values in the mainstream river waters between the Three Gorges Reservoir Region and Yichang-Chenglingji. However, d-excess values in river water displayed a small variation range. In contrast, δ2H and δ18O values in the lake group from Dongting to Jianghan and Huayang to Poyang Lake were lower than in the lake group from Taihu to the Yangtze Delta. Negative d-excess values were observed in lake water from Taihu to the Yangtze Delta, suggesting the combined influence of enriched isotopic compositions in precipitation and strong evaporative enrichment. Of the lakes, the highest isotopic values were found in Dianshan Lake and Datong Lake, whereas the lowest isotopic values were recorded in Dongting Lake and Poyang Lake because of their direct connection with the Yangtze River. The water regimes of Dongting Lake and Poyang Lake were influenced by the Yangtze River, especially when a high water level of the Yangtze River occurred, and thus altered the isotopic compositions of Dongting Lake and Poyang Lake water. Hence, these findings will provide scientific data revealing the precipitation-river-lake interactions and investigating the rational utilization and management of water resources in the middle and lower reaches of the Yangtze River regions.

Granitic magmas with enriched isotopic compositions, from enriched mantle source: Implications for continental generation

Granitoids with depleted isotopic compositions are generally considered beneficial to continental generation. However, most granitoids exhibit enriched isotopic characteristics (high initial Sr isotopic ratios, negative ƐNd and ƐHf values) and their role in continental generation is scantly explored. In this contribution, we report petrological and geochemical data of the Permian (ca. 270 Ma) Yabulai granites and hornblende gabbros in the Alxa Block of western North China Craton. Compared to initial mantle-derived basalts, the hornblende gabbros show low MgO (3.74 to 5.68 wt%), Cr (34.9 to 66.2 ppm), Ni (16.6 to 45.6 ppm) contents and Mg# (0.46 to 0.54), with zircon ƐHf values of +3.5 to +6.6 and −8.4 to −4.8. Relative to heavy rare earth elements (HREEs), the hornblende gabbros are enriched in light rare earth elements (LREEs) with the absence of Eu anomalies. In primitive mantle-normalized spidergrams, the hornblende gabbros exhibit notably negative NbTa and positive Pb anomalies. Elemental and isotopic data indicate that they might have originated from a shallow and heterogeneous lithospheric mantle source modified by subducted materials. The granites are metaluminous to weakly peraluminous and can be classified as high-K calc-alkaline I-type, with negative zircon ƐHf values of −8.3 to −5.3. These granitic rocks were likely to be derived from a newly underplated mafic crustal source. Using our data and published data from the granitic rocks in the Alxa Block, we suggest that a Permian crustal generation event has occurred in the Nuru-Langshan belt of the Alxa Block. Our current results reveal that the crustal generation rates at the convergent plate margins might have been underestimated due to the unintended omission of the mantle contribution to the granitoids with enriched isotopic compositions.

The distribution of water in the early Cretaceous lithospheric mantle of the North China Craton and implications for its destruction

It has been suggested that the longevity of cratons is related in part to the low water content of their deep mantle roots, which leads to a much higher viscosity than that of the underlying asthenosphere. Consequently, removal of cratonic roots is thought to be closely related to hydration of the lowermost lithospheric mantle. The North China Craton (NCC) is generally considered to represent such a “destroyed craton”, but previous investigations of the water content have focused mainly on the marginal areas of the eastern part of the NCC, meaning that the state of hydration in the interior region remains unclear. Here, we present whole-rock geochemical and Sr-Nd-Pb isotope data for the early Cretaceous mafic intrusions in the interior of the eastern part of the NCC and calculate the water contents of their corresponding primary melts. We also estimate the water contents of the lithosphere mantle that produced these melts. The mafic rocks exhibit arc-like incompatible-trace-element signatures and enriched Sr-Nd isotopic compositions. The water contents of clinopyroxene phenocrysts, as measured by Fourier transform infrared spectrometry (FTIR), vary from 37 to 472 ppm. The water contents of the corresponding primary melts, which were calculated using the clinopyroxene water contents and the clinopyroxene/melt partitioning coefficient of water, range from 0.85 to 2.07 wt%. The estimated water contents of the mantle source are at least 571-1214 ppm and are considerably higher than the H2O contents (<10 ppm) of Kaapvaal peridotite xenoliths. Combining the new data with previous results, we show that the lowermost lithospheric mantle beneath the eastern part of the NCC in both its marginal and interior regions was hydrated during the early Cretaceous, which underlines the role of water in the thinning of the lithospheric mantle.

The potential of inductively coupled plasma mass spectrometry for measuring the isotopic composition of elements

Brief information about the development of enriched isotopic composition reference materials (RMs) is presented. The potential of inductively coupled plasma mass spectrometry for measuring the isotopic composition of elements are shown.