High Radiogenic Sr Research Articles

Origin and composition of dissolved and particulate matter in mountain rivers of the Nepal Himalayas: Insights from Sr and Nd isotopes and elemental ratios

The Ganga River exhibits elevated concentrations of dissolved strontium (Sr) and a higher radiogenic Sr (87Sr/86Sr) among global rivers, which impacts oceanic Sr composition. This has implications for understanding weathering and sediment flux into open oceans. The Nepal Himalayan section is a significant part of the upstream Ganga River catchment. We analyzed Sr and Nd isotopes and major element concentrations in water and sediments to trace compositional and seasonal variability in dissolved and particulate matter. The Sr and Nd isotopes, in particular, are crucial in this study as they provide unique signatures that can be used to trace the origin and composition of the dissolved and particulate matter. Representative rivers draining monolithological terrains were selected to investigate the isotopic and elemental provenance and weathering intensity. The elemental ratios (Ca/Na, Mg/Na) indicate the watersheds are carbonate-rich terrains. The Lesser Himalaya (LH) rivers transport radiogenic Sr with high values compared to rivers draining the Tethyan Sedimentary Series (TSS), Higher Himalayan Crystalline (HHC), Mahabharat, and Siwalik Hills. The dry season records a higher 87Sr/86Sr ratio than the monsoon. Due to the faster dissolution of carbonates compared to silicates, monsoon waters transport less radiogenic Sr. The significant correlation of 87Sr/86Sr in dissolved and particulate phases signifies that short-term interactions between sediment and water may alter the 87Sr/86Sr composition. Notably, sediments originating from continental rocks exhibit an inverse correlation between 87Sr/86Sr and εNd. In conclusion, the Sr and Nd isotopic records in this study categorize the fluvial catchments into geological clusters aligned with the TSS, LH, and Siwaliks, which advance our understanding regarding the provenance besides providing crucial insights into geological processes, weathering, landscape evolution, and sediment flux.

Metal remobilization from country rocks into the Jiaodong-type orogenic gold systems, Eastern China: New constraints from scheelite and galena isotope results at the Xiadian and Majiayao gold deposits

Orogenic gold deposits in the Jiaodong Peninsula contain >5000 t of known gold resources. Precambrian metamorphic rocks and Mesozoic granite represent the main host rocks for these deposits, but gold deposition (~120 Ma) is significantly younger and it remains unclear whether some of the metals are sourced from the local country rock to these mineral systems. This study uses in situ geochemistry and isotope signatures of scheelite (Sr isotopes) and galena (Pb isotopes) at the Xiadian and Majiayao gold deposits to determine the role, if any, of country rock compositions on the formation of gold ore systems in the Jiaodong Peninsula. Scheelite from the two gold deposits occurred as part of a syn-mineralization stage with quartz, siderite, ankerite, and pyrite in the main orebody and/or its footwall. New results show that scheelite yields high Sr (>200 ppm) and low Mo (<40 ppm) and As (<5 ppm) concentrations, bell to flat REE profiles, and variable Eu anomalies. These scheelite compositions are very similar to previously published results from orogenic-style gold deposits in greenstone belts, indicating fluids with similar geochemical properties. The Xiadian scheelite with negative Eu anomalies may be controlled by REE substitution mechanisms and the composition of parent fluids that may have interacted with graphitic formations of the Jingshan Group, while the Majiayao scheelite with positive Eu anomaly is most likely the result of crystallographic controls on REE partition. New scheelite Sr isotope results for the Xiadian (87Sr/86Sr: 0.71258–0.71540) and Majiayao (87Sr/86Sr: 0.71568–0.71930) deposits further support interaction between hydrothermal fluids and local to distal country rocks. The high radiogenic Sr isotope signature of scheelite from the Majiayao deposit is primarily derived from the regional Jingshan and/or Penglai Group meta-sedimentary rocks. Galena in the Xiadian deposit, which occurs with pyrite in the syn-mineralization stage, has Pb isotope signatures of 206Pb/204Pb: 17.185 ± 0.004 that may reflect mixing between the Linglong granite and/or metamorphic rocks. The Majiayao galena yields the least radiogenic Pb isotope results (206Pb/204Pb: 16.554 ± 0.040), which are consistent with sulfide in the Jiaodong Group meta-volcanic rocks (206Pb/204Pb: 16.270–16.718) and suggest remobilization of Pb from sulfide within the previously metamorphosed formation. New geochemistry and isotope results document a mixed metal contribution that likely occurred as a consequence of fluid-rock interaction between ore-forming fluids and wall rocks at shallow crustal depths (<10 km). Such processes may have played a critical role in generating similar metallogenic features of the Jiaodong gold deposits as well as syn-metamorphic orogenic gold deposits in other greenstone belts. Our study highlights that the remobilization and precipitation of metals (Pb, Sr, W, and possible Au) scavenged from country rocks are perhaps ubiquitous processes, whether in syn- and/or post-metamorphism orogenic gold systems.

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.

Formation and genesis of Paleozoic sediment-hosted barite deposits in Turkey

Most barite deposits in Turkey are sediment-hosted deposits which are located in different lithologies within passive margin sediments from the Paleozoic. The Şarkikaraağaç, Hüyük and Tordere deposits occur in the western part of the Tauride-Anatolide tectonic unit, whereas the Şekeroba and Önsen region deposits located in the eastern part of Turkey occur on both Arabian and Tauride-Anatolide microplates. While the former deposits occur as both concordant and discordant bodies within Early Paleozoic carbonates, the latter deposits are found as discordant, fracture filling and replacement-type ore bodies.O, D, S, Sr and Pb isotope studies were used to determine and correlate the formation conditions of these deposits. δ18OSMOW mean values of the Tordere barite deposit are slightly higher (mean 16.2‰) than the mean values for the Şekeroba (10.8‰) and Önsen (11.3‰) deposits. The δ18OSMOW (mean 15.83‰) and δDSMOW (mean −67‰) values obtained from fluid inclusions in quartz veins in the Şekeroba barite deposit correspond to metamorphic or deep circulation hydrothermal waters. Mean δ34 SCDT values from Şekeroba, Tordere and Önsen deposits are 31.7, 33.8 and 20.6‰, respectively, which are compatible with the early Paleozoic seawater, except for the Önsen deposit.87Sr/86Sr mean values for the barite deposits of Turkey are defined as follows (‰); Şekeroba: 0.713, Önsen: 0.711, and Tordere: 0.711, which are higher 87Sr/86Sr isotope ratios than Paleozoic seawater. The Pb isotope values in Şekeroba and Tordere deposits are similar and mean is 206Pb/204Pb = 18.28, 207Pb/204Pb = 15.67 and 208Pb/204Pb = 38.50. High radiogenic Sr and Pb isotope data from the deposits indicate that Ba was generated from thick Paleozoic clastics or the older crust beneath them.Sr, S, and O isotopes and the positive Gd anomaly in the barites are very similar to those of Paleozoic stratiform cold-seep barites. The primary stratiform-stratabound barite deposits, like Şarkikaraağaç-Hüyük deposit, could have formed either by cold seep or diagenetic processes at the sediment–seawater interface or within the sediment related to sulfate-methane boundary. Although there are similar Sr and Pb isotopic signatures between the western and eastern region deposits, the eastern region Şekeroba, Önsen and Tordere barite deposits show discordant ore geometry and highly radiogenic isotope values indicating that primary stratabound-stratiform barite was dissolved and remobilized along deep-seated fault zones during the Hercynian orogenesis and deposited in different Paleozoic lithologies related to cooling and/or increase in Eh.

Early Cretaceous Wulong intermediate-mafic dike swarms in the Liaodong Peninsula: Implications for rapid lithospheric delamination of the North China Craton

The North China Craton (NCC) underwent a dramatic change in the lithospheric architecture, and more than 100 km of the ancient and refractory lithospheric mantle was removed. However, the timing and mechanism of the lithospheric thinning remain controversial. New zircon UPb ages and whole-rock/in-situ geochemical and isotopic data of the Early Cretaceous intermediate-mafic dike swarms in the Liaodong Peninsula have been carried out to decipher the timing and mechanism of lithospheric thinning of the NCC. The dike swarms are chemically subdivided into the ca. 126 Ma diorite porphyry and coeval (ca. 119 Ma) low-Ti (TiO2 < 1.20 wt%, Ti/Y < 375) and high-Ti (TiO2 > 1.90 wt%, Ti/Y > 580) diabase dikes. The diorite porphyries exhibit a moderate silica (56.71–61.13 wt%) and high-K calc-alkalic signature, with low TiO2 and total Fe2O3 and high Cr and Ni compositions. They are enriched in large ion lithophile elements (LILEs) (e.g., Rb, K, Pb, and Sr) and depleted in high field strength elements (HFSEs) (e.g., Nb, Ta, Ti, and P). They have high radiogenic Sr isotopes (whole-rock (87Sr/86Sr)i = 0.711553–0.714284, in-situ (87Sr/86Sr)i of plagioclase = 0.71392), low εNd(t) values (−19.9 to −14.5), and old Nd model ages (1.81–2.25 Ga). Meanwhile, some plagioclase phenocrysts have reverse zonings with Na-rich cores (An: 35–48) and Ca-rich rims (An: 48–57). These features imply that the diorite porphyries were generated by mixing between magmas from the Archean–Paleoproterozoic lower crust and the lithospheric mantle. The low-Ti diabase dikes are characterized by medium to high-K calc-alkaline series with high Al2O3 but low TiO2 contents. They show arc-like trace element signatures, such as enrichment in LILEs (Rb, K, Pb, and Sr) and depletion in HFSEs (Nb, Ta, Zr, Hf, Ti, and P). They also have relatively lower initial 87Sr/86Sr ratios (0.706331–0.708315 and 0.70837 for the whole-rock and in-situ Sr isotopes, respectively) compared to the porphyries and negative εNd(t) values (−13.6 to −5.0), indicating that they were derived from partial melting of an enriched lithospheric mantle. Their variable Th/Yb and Th/Nb ratios with constant Sr/Nd and U/Th values further suggest that the mantle source might have been previously modified by the subduction-related melt. In contrast, the high-Ti diabase dikes belong to the high-K calc-alkaline to shoshonitic series, with higher TiO2 and total Fe2O3 but lower Al2O3 contents. They have OIB-like trace elements (e.g., slight enrichments of Nb and Ta with no HFSE depletion, high Nb/U values (42 ± 5)) and SrNd isotopic compositions (e.g., positive whole-rock εNd(t) values (+3.1 to +4.3)), suggesting that they were generated from an asthenospheric mantle source. The coexisting Wulong low-Ti and high-Ti mafic dikes with similar formation ages (ca. 119 Ma) may indicate a transition from lithospheric to asthenospheric mantle source. In combination with previous studies on the Mesozoic–Cenozoic mafic rocks in the Eastern Block of the NCC, we propose that the lithospheric thinning beneath the Liaodong Peninsula initiated at ca. 119 Ma and this process beneath the NCC lasted for a narrow duration (ca. 13 Ma). The unsteady mantle flows due to the rapid subduction and rollback of the Paleo-Pacific plate might trigger the thickened, modified lithosphere to lose its gravitational stability, eventually leading to lithospheric delamination and thinning of the NCC.

Sr isotopic characteristics and fractionation during weathering of a small granitic watershed system in the Jiuhua Mountains (eastern China)

To determine the influence of the Mesozoic Yanshanian orogeny in East Asia on contemporaneous Sr isotopic evolution of seawater, we systematically investigated the weathering profile, riverbed sediment and stream water in mono-lithological small granitic watersheds of the Jiuhua Mountains, Anhui, eastern China. Analysis based on 190 samples from 1 to 2 samplings per month during an entire hydrological year, spanning July 2014 to June 2015, shows that the intra-annual change of Sr concentration is 10–70%, but 87Sr/86Sr ratios only exhibit a slight change (0.709148–0.710427). This result indicates that using single sampling data to evaluate the influence of chemical weathering on the 87Sr/86Sr ratio evolution of seawater may cause some deviations. The 87Sr/86Sr ratio in the small granitic watersheds of the Jiuhua Mountains is 0.709148–0.710427 with an average of 0.710021, which is significantly higher than the lowest value (0.7068, ∼160 Ma) of seawater in the Phanerozoic. During and after this period, the East Asian continent experienced a strong tectonic event — Yanshanian orogeny and formed widespread Jurassic–Cretaceous igneous rocks, such as the Jiuhua Mountains granite in the Yangtze Block. The Yanshanian granites in several main tectonic units in China exhibit high radiogenic Sr characteristics. Combined with the evidence of enhanced chemical weathering during Late Jurassic-Early Cretaceous, the notable increase of the seawater 87Sr/86Sr ratio after 160 Ma may be related to the Yanshanian orogeny in East Asia.

Mesoproterozoic and Paleozoic hydrothermal metasomatism in the giant Bayan Obo REE-Nb-Fe deposit: Constrains from trace elements and Sr-Nd isotope of fluorite and preliminary thermodynamic calculation

Fluorite of various occurrences could portray the hydrothermal metasomatism occurred in the giant Bayan Obo REE-Nb-Fe ore deposit. Two Mesoproterozoic and Paleozoic hydrothermal events with distinguished fluid origin and geochemical properties have been studied. In Mesoproterozoic, discrete fluorite from the dolomite-dominating REE-Nb ores and banded fluorite aggregates from the banded REE-Nb-Fe ores are extremely enriched in LREE and depleted in HFSE compared with the primitive mantle. In Paleozoic, fluorite from veined-breccia REE-Nb ores varies significantly in trace element components, ranging from LREE-enriched HFSE-depleted fluorite to fluorite with “flat” REE pattern and extremely depletion of Zr. The veined fluorites have variable and higher radiogenic Sr isotopic components (initial 87Sr/86Sr ratios: 0.703464–0.708208) than the discrete and banded fluorite (0.703183–0.705047), indicating crustal contamination in the Paleozoic hydrothermal fluids from Precambrian basement rocks and Bayan Obo Group. Fluorites of different occurrences and associated bastnäsite and monazite have similar Nd isotopic components and Nd evolution history, which is mostly consistent with whole rock of the ore-hosting dolomite. The Sm-Nd system of some fluorite and bastnäsite were reset during the Paleozoic alteration triggered by the closure of Paleo-Asian Ocean. In such geological setting, according to the δ34S of the dominating sulfur-bearing mineral, Paleozoic veined sulfides (∼0 ± 3‰), the prime sulfur source was oceanic crust/MORB. Relatively high δ34S of barite (∼13–14‰) was caused by fractionation of sulfur isotopes at an equilibrium temperature of 400–430 °C, rather than contribution from ancient seawater. The trace element, Sr-Nd isotope of fluorite and sulfur isotope characters of sulfides indicate carbonatitic origin of the Mesoproterozoic hydrothermal fluids, while the Paleozoic fluids, derived from subducted Paleo-Asian oceanic crust, carried sulfur components from MORB, scavenged high radiogenic Sr components from Precambrian basement rocks or Bayan Obo group, may leached a deep-seated carbonatite stock and finally altered the Mesoproterozoic mineralized ore-hosting dolomite.Both the Mesoproterozoic and Paleozoic hydrothermal fluids had initial temperature of ∼400–500 °C, and gangue minerals (fluorite, sulfide and sulfate) precipitated at the temperature of ∼320–440 °C. Preliminary thermodynamic calculations of the REE-F-C-Ca system constrained that the pressure of both fluids would not exceed 2 kbar. The drop of temperature and increase of pH are two critical factors to REE mineralization. Paragenesis of Fe-bearing minerals indicates an increase of fO2 in the Mesoproterozoic hydrothermal fluids and a drop of fS2 in the Paleozoic fluids in the Fe-O-S system. Furthermore, in the Ca-F-C-O system, the Mesoproterozoic hydrothermal fluids evolved with increasing aCa2+ and decreasing aF−, while the Paleozoic fluids present characteristic of mixing origin again with significantly variable aCa2+ and aF−.

Zircon U–Pb geochronological, geochemical, and Sr–Nd isotope data for Early Cretaceous mafic dykes in the Tancheng–Lujiang Fault area of the Shandong Province, China: Constraints on the timing of magmatism and magma genesis

The timing and source of magmatism that formed Early Cretaceous dolerite dykes in the Tancheng–Lujiang (Tan–Lu) Fault area of the southeastern North China Craton was determined using geochronological, geochemical, and whole-rock Sr–Nd isotopic data. Laser ablation–inductively coupled plasma–mass (LA–ICP–MS) spectrometry U–Pb analysis of zircon yielded consistent ages of 129.6±0.7, 126.8±0.7, 125.5±0.7, 124.9±0.9, 126.4±0.7, and 125.5±0.7Ma for six samples of the mafic dykes within the NCC. The K2O+Na2O concentrations (5.02–5.21wt.%) of the dykes indicate they are alkaline and these dykes have K2O concentrations (2.35–2.48wt.%) that indicate they are shoshonitic. These dolerites are also characterized by high and wide ranging (La/Yb)N (14.5–36.0), have slightly negative Eu anomalies (δEu=0.70–0.91) and positive Ba, U, K, and Pb anomalies, and are depleted in the high field strength elements (Nb, Ta, P, and Ti). In addition, these mafic dykes are characterized by high radiogenic Sr [(87Sr/86Sr)i=0.7099–0.7100] and negative εNd (t) values (−14.4 to −13.7). These data suggest that the magmas that formed the dykes were derived through the partial melting (12.0–15.0%) of an enriched region of the mantle that was hybridized during interaction with subducted sedimentary rocks from the Yangtze Craton. The parental magmas then fractionated olivine and Fe–Ti oxides during ascent and underwent negligible crustal contamination during magma emplacement. These mafic magmas were finally emplaced as dyke swarms associated with lithospheric extension.

Evolution of weathering patterns in the Indo‐Burman Ranges over the last 280 kyr: Effects of sediment provenance on 87Sr/86Sr ratios tracer

A high‐resolution study of mineralogy and major element geochemistry combined with Sr and Nd isotopes has been conducted on high sedimentation rate cores collected off the Irrawaddy River mouth in the Andaman Sea and the Bay of Bengal to reconstruct the erosional and weathering history of the Irrawaddy River basin. In both cores, ɛNd(0) values imply that both glacial and interglacial sediments share a common crustal source: the Irrawaddy River. Strong glacial/interglacial cycles are recorded by 87Sr/86Sr: interglacial periods yield values between 0.713 and 0.717, whereas glacial periods show higher values between 0.717 and 0.719. Variations of the pedogenic clays (smectite and kaolinite) to primary mineral (feldspar, quartz, illite, and chlorite) ratios show strong precessional cycles, suggesting a control by past changes in the summer monsoon intensity. Each increase in pedogenic clays content is also associated with a net loss of labile elements (Na, K, and Ca) from the detrital minerals under chemical weathering. Wet periods of summer monsoon reinforcement correspond to an increase in weathering of the Irrawaddy plain soils and a decrease of 87Sr/86Sr ratio. Plotting 87Sr/86Sr versus 87Rb/86Sr gives a pseudo‐isochrons interpreted as a mixing line representing the strength of chemical weathering. During glacial stages, enhanced physical erosion induced by glacier scour and frost action in the highland of the Irrawaddy River basins produced high volumes of unaltered, Rb‐rich minerals. The low sea level of glacial times constricted the river to the main channel in the lower reaches and permitted an efficient transport of unaltered Rb‐rich minerals with high radiogenic Sr composition from the high relief of the Indo‐Burman Ranges and the Tibetan plateau to the Indian Ocean.

Petrological, geochemical, and stable isotope constraints on the genesis of the Miocene igneous rocks of Chetaibi and Cap de Fer (NE Algeria)

Miocene igneous rocks (diorites, andesites, dacites, rhyolites and microgranites) of Chetaibi and Cap de Fer massif, NE Algeria, are high-K calc-alkaline to shoshonitic rocks. Fresh diorites have δ 34S and δ 18O values ranging between −2.5‰ and +5.9‰, +6.5‰ and +6.7‰ respectively, indicating a mantle origin. The relatively low δ 34S values (−5.4‰ to −12.2‰) and high δ 18O (+8.3‰ to +9.0‰) of altered diorites indicate the input of a crustal component to the initial magma. The microgranites’ I-type signature is indicated by the geochemical data and the δ 34S and δ 18O values of −1.2‰ and −3.6‰, and +7.8‰ to +10.4‰ respectively. The andesites show a large variation of δ 34S, between −33.2‰ and +25.7‰. Massive andesites with δ 34S between +6.8‰ and +7.6‰ preserve a 34S-enriched mantle signature. The δ 34S of the lava flows between +25.7‰ and +25.8‰ are attributed to open system magma degassing, whereas the low δ 34S of two andesitic dyke samples (−13.7‰ and −33.2‰) strongly suggest a crustal sulphur input. High δ 18O (+9.2‰ to +15.7‰) of andesites indicate post-magmatic alteration (mainly silicification); the flyschs with δ 18O between of +13.3‰ and +21.7‰ are most likely the contaminant. Quartz veins within the andesites gave a δ 18O value of +23.0‰ while silica-filling vesicles yielded a value of +13.8‰. Initial Sr-isotope data are rather high for all the rocks (diorites: 0.707–0.708, andesites: 0.707–0.710, and microgranites and rhyolites: 0.717–0.719), and because geochemical and stable isotope data do not indicate a substantial amount of crustal assimilation, an extensive enrichment of the mantle source by subducted sediments is called for. A metasomatized-mantle source, characterized by high radiogenic Sr and relatively high δ 18O, has also been indicated for the genesis of similar Tertiary igneous rocks in the Western Mediterranean basin, e.g. the Volcanic Province of southeasten Spain [Benito, R., Lopez-Ruiz, J., Cebria, J.M., Hertogen, J., Doblas M., Oyarzun, R., Demaiffe, D., 1999. Sr and O isotope constraints on source and crustal contamination in the high-K calc-alkaline and shoshonitic neogene volcanic rocks of SE Spain. Lithos 46, 773–802] and some plutons of northeastern Algeria [Ouabadi, A., 1994. Pétrologie, géochimie et origine des granitoïdes peralumineux à cordiérite (Cap Bougaroun, Béni-Touffout et Filfila), Algérie nord-orientale. Thèse de Doctorat, Université de Rennes I, France, 257p; Fourcade, S., Capdevila, R., Ouabadi, A., Martineau, F., 2001. The origin and geodynamic significance of the Alpine cordierite-bearing granitoids of northern Algeria. A combined petrological, mineralogical, geochemical and isotopic (O, H, Sr, Nd) study. Lithos 57, 187–216].

Petrogenesis and tectonic implications of Early Cretaceous high‐K calc‐alkaline volcanic rocks in the Laiyang Basin of the Sulu Belt, eastern China

Abstract Early Cretaceous high‐K calc‐alkaline volcanism occurring in the Laiyang Basin north of the Sulu high‐pressure to ultrahigh‐pressure (HP‐UHP) Metamorphic Belt, eastern China, comprises a wide spectrum of rock types, ranging from trachybasalts to trachydacites. The basaltic–andesitic rocks erupted at 107–105 Ma, spanning an SiO2 range of 50.1–59.6% and an MgO range of 2.6–7.2%, and are characterized by large ion lithophile element (LILE; e.g. Ba and K) and light rare earth element (LREE) enrichment, high field strength element (HFSE) depletion and highly radiogenic Sr but non‐radiogenic Nd isotopic compositions (87Sr/86Sr(i) = 0.70750–0.70931; ɛNd(t) = −17.9 − −15.6). The geochemical similarities between these rocks and the earlier Sulu Belt lamprophyres suggest that both types of mafic rocks were derived from similar mantle sources with LILE and LREE enrichment. Thus, the Wulian–Qingdao–Yantai Fault that separates the two terranes at the surface should not be considered as a lithospheric boundary between the North China and Yangtze blocks. The felsic lavas erupted at 93–91 Ma, spanning an SiO2 range of 61.6–67.0% and an MgO range of 1.1–2.6%, and show a trace element geochemistry similar to the basaltic rocks, but with higher radiogenic Sr and even lower Nd isotopic compositions (87Sr/86Sr(i) = 0.70957–0.71109; ɛNd(t) = −19.1 − −17.5), similar to I‐type granitoids in the Sulu Belt. A crustal origin was proposed to explain their compositions (which are comparable to those of experimental slab melts), the &gt;10 Ma eruption interval and the compositional gaps in some elements (e.g. P, Ti and Sr) between them and the older basaltic–andesitic rocks. These melts were derived from predominant metaigneous protoliths containing mafic accumulative counterparts of the basaltic–andesitic and/or lamprophyric magmas. The extensive extrusion of Early Cretaceous high‐K calc‐alkaline rocks in the Laiyang Basin favored an extensional regime in response to the progressive attenuation of the thickened lithosphere and orogenic collapse, as reflected in the development of the basin from a foreland basin (before the end of the Jurassic period) to a fault basin (since the Early Cretaceous period).

Sr and 87Sr/ 86Sr in the Yamuna River System in the Himalaya: sources, fluxes, and controls on sr isotope composition

Sr and 87Sr/ 86Sr have been measured in the Yamuna river headwaters and many of its tributaries (YRS) in the Himalaya. These results, with those available for major ions in YRS rivers and in various lithologies of their basin, have been used to determine their contributions to riverine Sr and its isotopic budget. Sr in the YRS ranges from 120 to 13,400 nM, and 87Sr/ 86Sr from 0.7142 to 0.7932. Streams in the upper reaches, draining predominantly silicates, have low Sr and high 87Sr/ 86Sr whereas those draining the lower reaches exhibit the opposite resulting from differences in drainage lithology. 87Sr/ 86Sr shows significant co-variation with SiO 2/TDS and (Na * + K)/TZ + (indices of silicate weathering) in YRS waters, suggesting the dominant role of silicate weathering in contributing to high radiogenic Sr. This is also consistent with the observation that streams draining largely silicate terrains have the highest 87Sr/ 86Sr, analogous to that reported for the Ganga headwaters. Evaluation of the significance of other sources such as calc-silicates and trace calcites in regulating Sr budget of these rivers and their high 87Sr/ 86Sr needs detailed work on their Sr and 87Sr/ 86Sr. Preliminary calculations, however, indicate that they can be a significant source to some of the rivers. It is estimated that on an average, ∼25% of Sr in the YRS is derived from silicate weathering. In the lower reaches, the streams receive ∼15% of their Sr from carbonate weathering whereas in the upper reaches, calc-silicates can contribute significantly (∼50%) to the Sr budget of rivers. These calculations reveal the need for additional sources for rivers in the lower reaches to balance their Sr budget. Evaporites and phosphorites are potential candidates as judged from their occurrence in the drainage basin. In general, Precambrian carbonates, evaporites, and phosphorites “dilute” the high 87Sr/ 86Sr supplied by silicates, thus making Sr isotope distribution in YRS an overall two end member mixing. Major constraints in quantifying contributions of Sr and 87Sr/ 86Sr from different sources to YRS rivers are the wide range in Sr and 87Sr/ 86Sr of major lithologies, limited data on Sr and 87Sr/ 86Sr in minor phases and on the behavior of Sr, Na, and Ca during weathering and transport. The Ganga and the Yamuna together transport ∼0.1% of the global Sr flux at the foothills of the Himalaya which is in the same proportion as their contribution to global water discharge. Dissolved Sr flux from the Yamuna and its mobilization rate in the YRS basin is higher than those in the Ganga basin in the Himalaya, a result consistent with higher physical and chemical erosion rates in the YRS.

The Finero phlogopite-peridotite massif: an example of subduction-related metasomatism

The Finero peridotite massif is a harzburgite that suffered a dramatic metasomatic enrichment resulting in the pervasive presence of amphibole and phlogopite and in the sporadic occurrence of apatite and carbonate (dolomite)-bearing domains. Pyroxenite (websterite) dykes also contain phlogopite and amphibole, but are rare. Peridotite bulk-rock composition retained highly depleted major element characteristics, but was enriched in K, Rb, Ba, Sr, LREE (light rare earth elements) (LaN/YbN = 8–17) and depleted in Nb. It has high radiogenic Sr (87Sr/86Sr(270) = 0.7055–0.7093), low radiogenic Nd (ɛNd(270) = −1 to −3) and EMII-like Pb isotopes. Two pyroxenite – peridotite sections examined in detail show the virtual absence of major and trace element gradients in the mineral phases. In both rock types, pyroxenes and olivines have the most unfertile major element composition observed in Ivrea peridotites, spinels are the richest in Cr, and amphibole is pargasite. Clinopyroxenes exhibit LREE-enriched patterns (LaN/YbN ∼16), negative Ti and Zr and generally positive Sr anomaly. Amphibole has similar characteristics, except a weak negative Sr anomaly, but incompatible element concentration ∼1.9 (Sr) to ∼7.9 (Ti) times higher than that of coexisting clinopyroxene. Marked geochemical gradients occur toward apatite and carbonate-bearing domains which are randomly distributed in both the sections examined. In these regions, pyroxenes and amphibole (edenite) are lower in mg## and higher in Na2O, and spinels and phlogopite are richer in Cr2O3. Both the mineral assemblage and the incompatible trace element characteristics of the mineral phases recall the typical signatures of “carbonatite” metasomatism (HFSE depletion, Sr, LILE and LREE enrichment). Clinopyroxene has higher REE and Sr concentrations than amphibole (amph/cpxDREE,Sr = 0.7–0.9) and lower Ti and Zr concentrations. It is proposed that the petrographic and geochemical features observed at Finero are consistent with a subduction environment. The lack of chemical gradients between pyroxenite and peridotite is explained by a model where melts derived from an eclogite-facies slab infiltrate the overhanging harzburgitic mantle wedge and, because of the special thermal structure of subduction zones, become heated to the temperature of the peridotite. If the resulting temperature is above that of the incipient melting of the hydrous peridotite system, the slab-derived melt equilibrates with the harzburgite and a crystal mush consisting of harzburgite and a silica saturated, hydrous melt is formed. During cooling, the crystal mush crystallizes producing the observed sequence of mineral phases and their observed chemical characteristics. In this context pyroxenites are regions of higher concentration of the melt in equilibrium with the harzburgite and not passage-ways through which exotic melts percolated. Only negligible chemical gradients can appear as an effect of the crystallization process, which also accounts for the high amphibole/clinopyroxene incompatible trace element ratios. The major element refractory composition is explained by an initially high peridotite/melt ratio. The apatite, carbonate-bearing domains are the result of the presence of some CO2 in the slab-derived melt. The CO2/H2O ratio in the peridotite mush increased by crystallization of hydrous phases (amphibole and phlogopite) locally resulting in the unmixing of a late carbonate fluid. The proposed scenario is consistent with subduction of probably Variscan age and with the occurrence of modal metasomatism before peridotite incorporation in the crust.

Carbonate Botryoids in Lower Devonian Amygdaloidal Basalts: Evidence for Precipitation of High-Magnesium Calcite from Heated and Volcanic CO2-Buffered Marine Waters

ABSTRACT Amygdaloidal basalts are found in the Lower Devonian Upper Gaspe Limestones in the Gaspe Peninsula. These basalts were extruded on a distal outer shelf, as evidenced by field relationships, embedded fragments of metazoans, and high vacuole content. The lava flows were fractured and brecciated; the network of fractures allowed connections of some vesicles with seawater. Vesicles in the nonfractured basalt are filled by silicates, whereas those opened to seawater are filled by carbonates. The latter comprise, in stratigraphic order, geopetal micrite, botryoidal calcite, and coarse anhedral calcite. Petrographic attributes, including microdolomite inclusions and trace-element distribution in the botryoids, indicate former high-magnesium calcite mineralogy. The d18OPDB (-9.2 to -10.0 and d13CPBD (-3.0 to -5.3) ratios support botryoid precipitation from heated marine waters that were contaminated by magmatic CO2 volatiles. High radiogenic Sr values originated from compaction waters expelled from the host succession. Submarine high-magnesium calcite agrees with the aragonite-inhibiting stage for the Early Devonian. The botryoid fabric should be the norm for early calcite in such an odd diagenetic setting.

Sr isotopes as tracers in volcanic-derived clay of Martinique Island (Lesser Antiles): hydrothermal vs. seawater alteration

Clay minerals in Tertiary volcano-sedimentary formations of Martinique result from the alteration of pyroclastic and volcanoclastic rocks by hydrous fluids. They consist exclusively of smectites of the beidellitic group. Their chemistry results from leaching of Rb, Sr and K from volcanic material. Even though they are intercalated with typical marine deposits, these minerals show no chemical or isotopic equilibration with seawater. However, they often contain more radiogenic Sr than arc volcanics of the same age. This radiogenic Sr is similar to that released during magmatic crustal contamination and was driven by fluids that formed the hydrothermal clays in the volcanic lava flows. Pseudoisochrons suggesting mixing lines with low to very high radiogenic Sr are recognized in clay minerals and volcanics, indicating participation of fluids of different origins.

Sr isotopic and trace element variations in Oligocene to recent igneous rocks from the Philippine island arc: evidence for recent enrichment in the sub-Philippine mantle

Oligocene-Miocene plutonic rocks from the Philippine island arc are characterized by uniform 87Sr/ 86Sr isotopic ratios of 0.7035–0.7039. In contrast, volcanics erupted in the past 5 Ma in Central Luzon, Marinduque, and Mindoro have higher radiogenic Sr (0.7042–0.7054) and show regional variations in their Sr isotopic compositions. Volcanics of the Pliocene-Recent Bicol volcanic chain of Southern Luzon have 87Sr/ 86Sr in the same range (0.7034–0.7040) as the Mid-Tertiary plutonics. Increases in radiogenic Sr are coupled with increases in Rb, Sr, Ba, and Th abundances in the Central Luzon-Mindoro-Marinduque Volcanic Belt, while no such correlation exists for the Mid-Tertiary plutonics. We consider the low values observed for Mid-Tertiary plutonics and Bicol volcanics to be controlled by fluids released from the subducting Philippine Sea plate basalt that have 87Sr/ 86Sr of the same range. Sr isotopic compositions of the volcanics in the northern part of the Central Luzon Volcanic Belt may be controlled by fluids generated from the subducted South China Sea crust. The highest values of radiogenic Sr (0.7051–0.7054) and highest LIL abundances, which are observed in the southern part of the Central Luzon volcanic arc, appear to be the result of source contamination by slivers of the continental crust of the North Palawan-Mindoro terrane, that collided with the Philippine arc in the Miocene.