Large Lithophile Elements Research Articles

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

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

Anatexis of late Neoarchean-Early Paleoproterozoic and late Paleoproterozoic garnet-bearing leucogranite from the Khondalite Belt in the North China Craton

ABSTRACT Two episodes of late Neoarchean-early Paleoproterozoic and late Paleoproterozoic garnet-bearing leucogranite, named the Baotou and Jining–Liangcheng garnet-bearing leucogranite are recognized from the Khondalite Belt in the North China Craton. Geochemical studies show that garnet-bearing leucogranite has high Al2O3 content and FeOT/MgO ratio, with a large variation in the CaO content and K2O/Na2O ratio. Additionally, the garnet-bearing leucogranite is enriched in light rare earth elements and large ion lithophile elements and depleted in Nb, Ta, P, and Ti. However, there are some variations in Eu anomalies, as indicated by the Eu-enriched and Eu-depleted patterns. The sensitive high-resolution ion microprobe (SHRIMP) zircon U-Pb dating of the Baotou garnet-bearing leucogranite and Jining–Liangcheng garnet-bearing leucogranite show that their anatectic ages were 2.35–2.43 Ga and 1.90–1.92 Ga, respectively, with zircon εHf (t) values of −0.01–3.93 and −4.36–1.99, and two-stage model ages (TDM2) of 2.76–3.02 Ga and 2.57–2.83 Ga, respectively. U-Th-Pb SHRIMP analyses on zircon reveal ages of ca. 2.35–2.47 Ga for the Baotou metapelite gneiss and ca. 1.90–1.91 Ga for the Jining–Liangcheng metapelite gneiss, which represent the ages of tectonic metamorphic events related to the anatexis. Our results, combined with geological and petrographical observations, geochemistry, and geochronology, suggest that (1) Garnet-bearing leucogranites formed as a result of anatexis of metapelite gneisses; (2) They are mixtures of leucosomes, residual minerals and mantle-derived materials; (3) Two episodes of garnet-bearing leucogranites have been identified; (4) Both two episodes of garnet-bearing leucogranites corresponded to late Neoarchean-early Paleoproterozoic and late Palaeoproterozoic tectonothermal events, respectively.

Magmatic response to arc-arc amalgamation: Insights from latest Paleozoic igneous rocks from the Gangou section of the Eastern Tianshan

Multiple arc systems were developed in response to Neoproterozoic to Mesozoic consumption of the Paleo-Asian Ocean and their magmatic evolution is crucial for understanding the arc-arc amalgamation in the Central Asian Orogenic Belt. Here, we report whole-rock geochemical data as well as simultaneous in situ zircon U-Pb and Lu-Hf isotope results for the late Paleozoic magmatic rocks associated with the shear zones in the Gangou section of the Eastern Tianshan to constrain their petrogenesis and arc-arc amalgamation processes. We obtained ∼307 Ma and ∼270 Ma zircon U-Pb ages for the Late Carboniferous diorites and Middle Permian andesites, respectively. These rocks have high Mg# (∼65 and ∼47, respectively) values, and are enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs), with depleted zircon εHf(t) values (+12.1 to +18.1), suggesting an origin from partial melting of the depleted mantle with various degree of differentiation. Furthermore, our results also provided ∼276 Ma zircon U-Pb ages for the Middle Permian granitic porphyries and ∼256 Ma zircon U-Pb ages for the Late Permian diabase rocks. The granitic porphyries are geochemically similar to A2-type granites, i.e., high K2O + Na2O, FeOT/MgO, Ga/Al and low CaO, Sr and Ba. They have high zircon εHf(t) (+9.0 to +13.7) values, indicating a possible derivation from juvenile lower crust. The diabase rocks show depletion of light rare earth elements (LREEs), resembling normal mid-ocean ridge basalt. These rocks have depleted zircon Hf isotopic compositions (εHf(t) = +8.9 to +15.8), demonstrating a possible derivation from partial melting of the depleted mantle. The available geochemical data from the Eastern Tianshan show that the Permian mafic rocks along the shear zones possess higher Nb/La and lower Ba/La ratios than their counterparts in the Carboniferous. These contrasting features imply that the Carboniferous mafic rocks originated from a metasomatic mantle wedge, while the Permian mafic rocks were likely derived from the depleted mantle with addition of enriched asthenosphere components. The high zircon saturation temperatures (mostly > 800 °C) of the Permian A-type granitoids and diverse magma sources of coeval mafic rocks also imply possible asthenosphere upwelling. Therefore, we propose that the Late Carboniferous magmatic rocks were likely resulted from subduction of the North Tianshan oceanic plate, while the spatial and temporal evolution of the Permian magmatic rocks were probably attributed to large-scale dextral transcurrent tectonics associated with arc-arc amalgamation.

Petrogenesis of the quartz diorite from the Lietinggang-Leqingla Pb-Zn-Fe-Cu-(Mo) deposit in southern Tibet: Implications for the genesis of a skarn-type polymetallic deposit in the Tibetan-Himalayan collisional orogen

The Lietinggang-Leqingla deposit is a representative skarn-type polymetallic deposit in the eastern of the central Lhasa subterrane. It comprises Pb-Zn and Fe-Cu-(Mo) mineralization that are part of the same metallogenic system. Garnet-skarn hosting Fe-Cu-(Mo) mineralization distributed in the contact zone between the Jubuzhari complex intrusions and carbonate strata of the Mengla Formation. The Pb-Zn mineralization is hosted in distal garnet-pyroxene and pyroxene-actinolite skarns, which formed in the strata of Mengla Formation. Here, we investigate quartz diorites (61.06 ± 0.93 Ma) that are related to Fe-Cu mineralization in the northern part of the mining area. The quartz diorites are characterized by low SiO2 (61.57–65.88 wt%), low K2O (2.53–3.07 wt%), and high total alkali (Na2O + K2O) contents (5.69–6.35 wt%). They are enriched in large-ion lithophile elements and light rare earth elements, but depleted in high field strength elements and heavy rare earth elements. The quartz diorites yield variably low (La/Yb)N ratios (4.29–7.07, the average value is 6.36) with moderately negative Eu anomalies (Eu/Eu* = 0.49–0.72). Furthermore, they exhibit a narrow range in (87Sr/86Sr)i ratios (0.706355–0.706807) and weakly negative εNd (t) (−2.08 to −1.57), and positive εHf (t) (+0.14 to +3.46) values. Based on the geochemical characteristics presented in this study and previously published data, we propose that the ore-forming rocks related to Fe-Cu and Pb-Zn mineralization were formed at the same time. The intermediate intrusions associated with Fe-Cu mineralization were derived from partial melting of lithospheric mantle mixed with the rejuvenated lower crustal melts, after which the mixed magma underwent minor fractional crystallization during upwelling. The felsic intrusions associated with Pb-Zn mineralization were formed by melting of mantle-derived basaltic magmas with old lower crust component contributions and which underwent extensive fractional crystallization.

Late Paleozoic Shoshonitic Magmatism in the Southwestern Middle Tianshan (Tajikistan) of the Southwestern Altaids: Implications for Slab Roll-Back With Extensional Arc-Related Basins After Flat Subduction

Paleozoic magmatic rocks are widespread in the western Middle Tianshan. Their petrogenesis can provide important insights into the geodynamic evolution of the southwestern Altaids. Here, we present an integrated study of U–Pb zircon geochronology and geochemical and Lu–Hf isotopic compositions for the Late Paleozoic shoshonitic Chorukhdairon pluton and genetically and spatially related quartz porphyry in the southern Chatkal–Kurama terrane, western Middle Tianshan. The Chorukhdairon pluton mainly comprises monzodiorite and quartz monzodiorite (first phase), quartz monzonite (second and main phase), monzogranite (third phase), and leucomonzogranite (fourth phase). LA–ICP–MS zircon dating yielded magma crystallization ages of 294–291 Ma and 286 Ma for the Chorukhdairon pluton and quartz porphyry, respectively. All the rocks possess high K2O content (3.29–5.90 wt.%) and show an affinity with shoshonite series rocks. They display similar trace element compositions characterized by the enrichment of large ion lithophile elements (e.g., Rb, Th, U, and K) and depletion of high-field strength elements (e.g., Nb, Ta, P, and Ti), compatible with typical arc magmatism. Combined with zircon Lu–Hf isotopic data, we suggest that the Chorukhdairon pluton was produced by partial melting of the enriched mantle, followed by fractional crystallization of pyroxenes, amphibole, plagioclase, biotite, and accessory Fe–Ti oxides, apatite, and zircon. The quartz porphyries are similar to highly fractionated I-type granitic rocks, and their parental magma could result from the mixing of different batches of mantle-derived magmas or magmas derived from the mantle and juvenile lower crust. Considering the continuousness of the Middle Carboniferous to Early Permian magmatism in the western Middle Tianshan and other regional geological data, we suggest that the Chorukhdairon pluton and related quartz porphyry probably formed in an oceanic subduction setting. Furthermore, the temporal and spatial evolution of the Paleozoic magmatism imply that the flat-slab subduction that was induced by the subduction/accretion of seamounts probably occurred beneath the Middle Tianshan during the Middle Devonian to Early Carboniferous, after which the southeastward slab roll-back occurred during the Middle Carboniferous to Early Permian. The late slab roll-back was responsible for the southeastward arc magmatism migration and magmatic flare-up in the Chatkal–Kurama terrane, western Tianshan, and led to the formation of arc-related extensional basins and significant crustal growth in the southwestern Altaids.

Geochronology, geochemistry, and petrogenesis of Early Cretaceous highly differentiated I-type granites in the central Great Xing’an Range, northeastern China

In this study, we undertook zircon U–Pb dating and Hf isotopic and whole-rock geochemical analysis of Mesozoic granites from the Zhalantun and Arxan areas of the Great Xing’an Range, northeastern China. Zircon U–Pb dating of four samples indicates they formed in the Early Cretaceous (ca. 140.7–130.0 Ma). Geochemically, the samples have high silica (74.00–77.29 wt.%) and total alkali (8.52–8.90 wt.%) and low CaO (0.21–0.71 wt.%), MgO (0.06–0.48 wt.%), and P2O5 (0.01–0.06 wt.%) contents and exhibit the weakly peraluminous (A/CNK = 0.98–1.06) and high-K calc-alkaline characteristics, with highly differentiated I-type granite affinities (differentiation index = 93.19–96.93). The studied granites are enriched in light rare earth and large ion lithophile elements and depleted in heavy rare earth and high field strength elements. Their zircon εHf(t) values are mainly positive (+2.49 to +11.64) and yield two-stage Hf model ages of 1023–445 Ma. The geochemical and isotopic data indicate they might have been derived from the partial melting of medium- to high-K basaltic crust material during the Mesoproterozoic to Phanerozoic. They underwent fractional crystallization in the magma evolution process, and K-feldspar and plagioclase were the major crystallization phases. Combined with the temporal–spatial migration of the Mesozoic magmatic events in the whole northeastern China, we propose that the generation of Early Cretaceous highly differentiated I-type granites in the central Great Xing’an Range was associated with the delamination effect caused by the roll-back of the Paleo-Pacific plate.

Deep origin of mantle peridotites from the Aladağ ophiolite, Turkey: Implication from trace element geochemistry of pyroxenes and mineralogy of ophiolitic diamonds

The Aladağ peridotites mainly consist of harzburgites with subordinary dunites. High olivine Fo [100 × Mg/(Mg + Fe2+) = 90.8–92.0] and spinel Cr# [100 × Cr/(Cr + Al) = 61.5–71.6] values indicate the Aladağ harzburgites have experienced high degrees of partial melting. Significant depletion of middle rare earth element in the clinopyroxene and orthopyroxene implies that the partial melting of mantle source initiated in the garnet stability field. Occurrences of the interstitial clinopyroxene and spinel-hosted hydrous mineral inclusions, as well as variable enrichments of light rare earth element (LREE) and large-ion lithophile element in the clinopyroxene and orthopyroxene, suggest that the harzburgites suffered pervasive melt-rock interaction. The modeled interacting melts show enrichments of LREE and have a boninite-like affinity. In the dunites, olivines have higher contents of Fo (92.3–92.7), Ca (476–1830 ppm), and Ti (3.4–9.8 ppm) than those in the harzburgites (90.8–92.0, 50–319 ppm, and 0.6–6.4 ppm, respectively). Spinels in dunites have high Cr# (73.4–77.4) values and Ti (569–722 ppm) concentrations. Such mineral chemical characteristics imply extensive melt-rock interaction. We propose that the dunite was produced by early-stage interaction of harzburgite with migrating Si-undersaturated melt in asthenosphere. Subsequently, dunite and harzburgite together experienced late-stage interaction with boninite-like melt as incorporated into thermal boundary layer above a supra-subduction zone. An unusual mineral assemblage of ophiolitic diamond, moissanite, zircon, corundum and rutile, separated from the Aladağ harzburgites, indicates that relatively deep processes involved in the formation of the Aladağ mantle peridotites.

Petrogenesis of Neoproterozoic magmatic rocks in the Songnen Massif (northeastern China): Implications for basement composition and crustal growth

This paper reports new zircon U–Pb geochronological, whole-rock geochemical, and zircon Hf isotopic data for Neoproterozoic intrusive rocks in the Songnen Massif in the eastern Central Asian Orogenic Belt (CAOB), with the aim of revealing the petrogenesis of the Neoproterozoic intrusive rocks, and constraining the basement composition and crustal growth process of the Songnen Massif. Dating results identify three episodes of Neoproterozoic magmatism at ca. 950, ca. 856, and ca. 790 Ma in the eastern Songnen Massif. The ca. 950 Ma magmatic rocks consist of syenogranites, the ca. 856 Ma magmatic rocks comprise mainly gneissic monzogranites, and the ca. 790 Ma magmatic rocks consist of gneissic monzonites and monzogranites. The Neoproterozoic granitoids have high contents of SiO2 and K2O, low contents of total Fe2O3 and MgO, and show characteristics of enrichment in large-ion lithophile elements (e.g., K and Rb), and depletion in high-field-strength elements, implying that their primary magmas could have been derived from partial melting of the lower crust. However, compared with the ca. 790 Ma monzogranites [εHf(t) = +7.3 to +9.5] in the south of the Songnen Massif, the ca. 950 and ca. 856 Ma granitoids in the north have much lower zircon εHf(t) values (−2.0 to −0.2 and −10.6 to −8.9, respectively), suggesting that the ca. 950 and ca. 856 Ma magmas were generated by the reworking of ancient mafic lower crust, whereas the ca. 790 Ma granitic magma was generated by the reworking of juvenile mafic lower crust. In contrast, the ca. 790 Ma gneissic monzonites were formed by mixing of the mantle-derived and crust-derived melts. Considering the Precambrian magmatism, detrital and captured zircons, and zircon Hf model ages together, the Precambrian basement of the Songnen Massif includes not only the Neoproterozoic crustal materials, but also the Neoarchean–Mesoproterozoic components. Zircon εHf(t) values of the Neoproterozoic granitoids in the Songnen Massif appear to progressively decrease from south to north, reflecting a lateral heterogeneity of basement composition of the eastern Songnen Massif, and indicating that crustal growth occurred mainly during the late Mesoproterozoic and Neoproterozoic in the south, while during the late Paleoproterozoic in the north.

Petrogenesis of a nepheline syenite from parts of the Chotanagpur Granite Gneissic Complex: implications for Neoproterozoic crustal extension in the East Indian Shield

AbstractThe North Purulia Shear Zone that dissects the granulite basement of the Chotanagpur Granite Gneissic Complex of the East Indian Shield exposes a deformed and metamorphosed nepheline syenite. The studied ‘foid-monzosyenite’ shows high abundances of large ion lithophile elements and high field strength elements with low abundances of compatible elements. Trace-element signatures show negative U, Th, Zr, Ti and Pb and positive Sr, Ba and Eu anomalies with respect to the primitive mantle. The chondrite-normalized diagram shows strongly fractionated rare earth element patterns ((La/Lu)N ∼23–87). Geochemical fingerprints suggest that the basanitic protolith was formed by low-degree partial melting of garnet peridotite in the sub-continental lithospheric mantle. The enriched large ion lithophile, high field strength element and light rare earth element concentrations (relative to primitive mantle) can be explained by a mixed mantle source with components from a previously deformed alkaline rock/carbonatite. Geochemical data do not support any significant crustal contamination and suggest variable fractionation of clinopyroxene, ilmenite, titanite and apatite from the parental melt. Petrological data are consistent with the view that the nepheline syenite magma was emplaced in a rift setting with a minimum temperature of 800–900°C, low fO2 conditions (below the fayalite–magnetite–quartz buffer) at a mid-crustal depth between 950 and 900 Ma. The continental rift zone, however, did not lead to the formation of an open ocean basin. Subsequently, the studied rock and its basement was deformed and metamorphosed in a continent–continent collisional setting at ∼900 Ma. Combining information from the other Indian occurrences with this study, it is demonstrated that the deformed alkaline rocks and carbonatite are potentially valuable for tracing the birth and demise of the palaeo-supercontinents.

Episodic magmatism in the Dananhu arc, eastern Tianshan: Implications for the Late Palaeozoic tectonic evolution of the North Tianshan Ocean

ABSTRACT The North Tianshan Ocean represents a critical southwestern segment of the Paleo-Asian Ocean and is crucial for understanding accretionary and collisional processes of the southwestern Central Asian Orogenic Belt. The Dananhu arc belt, an important porphyry Cu deposit metallogenic belt in the eastern North Tianshan, records Paleozoic episodic magmatism. However, the tectonic affinity and evolution processes of the Dananhu arc belt are still open to debate. Here, geochronology, major and trace elements, and Sr-Nd-Hf isotope compositions for magmatic rocks are conducted to reveal the ages, petrogenesis, and tectonic settings of these magmatic rocks in the Dannanhu arc belt. The Carboniferous dacite (328 Ma) and gneissic granite (310 Ma) show high-K calc-alkaline, enriched in large ion lithophile elements (LILEs; e.g. Rb, Th, and U), depleted in high field strength elements (HFSEs; e.g. Nb, Ta, and Ti), depleted isotopic compositions (εNd(t) = 1.3-3.4; εHt(t) = 0.5-12.5) and juvenile Nd-Hf model ages. Combining these Carboniferous magmatic rocks with the features of high Sr-Ba granitoid and Sr-Nd isotopic decoupling, we suggest that these Carboniferous rocks were probably formed through fractional crystallization of partial melting of mantle wedge that had been metasomatized by melts from subducted sediments in a forearc setting. The early Permian granites (289-276 Ma) show high K2O+Na2O (6.9-8.9 wt.%), low MgO (0.06-0.15 wt.%) contents, high positive zircon εHt(t) (10.5 to 12.1) and whole-rock εNd(t) (4.4-5.5) values, suggesting that the early Permian granites were derived from juvenile lower crust in a post-collisional setting. Our geochronological and geochemical data, together with published data, reveal that high flux arc magmatism with age peaks at ca. 431 and 332 Ma is consistent with porphyry copper mineralization in the Dananhu arc belt, implying a close genetic relationship. The final closure of the North Tianshan Ocean was constrained to the late Carboniferous and early Permian.

Neoproterozoic tectonic shift from collisional orogenesis to intraplate extension in the Yili Block, southern Central Asian Orogenic Belt

Understanding the tectonic evolution of the many micro-continents in the Central Asian Orogenic Belt is a complex and controversial issue. One such micro-continent, the Yili Block is characterized by Neoproterozoic magmatic rocks, which can provide important constraints on the tectono-magmatic history of the region. Herein, we present a systematic investigation of the petrology, whole-rock geochemistry, zircon U-Pb geochronology and Lu-Hf isotopes on newly identified leucogranite intrusions and gabbroic dikes in the Wenquan district of the northern Yili Block. Two leucogranite samples yielded zircon U-Pb ages of 983 ± 9 Ma and 905 ± 9 Ma, respectively. Both of muscovite-bearing leucogranites have S-type affinities and show distinctive geochemical compositions similar to peraluminous granites. Furthermore, the leucogranites exhibit low Rb/Sr ratios (0.10–0.49) and elevated zircon εHf(t) values (+3.04 - +13.59), indicating that they were derived from partial melting of supracrustal pelitic and mafic rocks. In contrast, one gabbroic dike sample yielded a zircon U-Pb age of 779 ± 5 Ma. The gabbroic samples have distinctly high Th/Yb (1.10–1.68) and low Nb/La (0.37–0.46) ratios, with enrichment in large ion lithophile elements and depletion in Nb, Ta and Ti, implying that they were sourced from a metasomatic subcontinental lithospheric mantle. Integrated with other available data, we propose that the Yili Block underwent continuously collisional orogenesis during the early Neoproterozoic (ca. 983–780 Ma), with a subsequent tectonic shift from collisional to an intraplate extensional setting at ca. 779–776 Ma. A comparable magmatic evolution also occurred in adjacent micro-continents of the Kazakhstan region and northern Tarim Craton, indicating that these micro-continents may have shared a common Neoproterozoic geodynamic evolution.

Nature of the Late Cretaceous mantle source beneath the western Lhasa terrane, southern Tibet: Insights from the newly discovered mafic intrusion

Recent studies indicate that the mantle source of eastern Lhasa terrane was still depleted during the late stage of oceanic subduction (100–50 Ma). However, the mantle source beneath the western Lhasa terrane during this period was poorly constrained mainly due to the rare exposure of mafic rocks in this region. Here, we report the detailed petrological, geochronological, geochemical, and Sr–Nd–Hf–Pb isotopic data for recently identified gabbroic rocks in the Taruocuo area, western Lhasa terrane, southern Tibet. The zircon U-Pb dating results indicate that the Taruocuo gabbroic rocks were emplaced during the Late Cretaceous (~90 Ma). These gabbroic rocks have low SiO2 (50.9–55.0 wt%), and moderately high MgO (4.10–5.60 wt%) contents with arc-like trace-element patterns, such as enrichment in large ion lithophile elements, light rare earth elements, and depletion in high field strength elements. Meanwhile, they have high Th/La, low Ba/La values, and exhibit relatively enriched and uniform Sr–Nd–Hf–Pb isotopic compositions (87Sr/86Sr(i) = 0.7048–0.7058, εNd(t) = −1.8 to −1.2, εHf(t) = +3.1 to +3.8, and 206Pb/204Pb = 18.60 to 18.66). Such geochemical features of the Taruocuo gabbroic rocks suggest that they would originate from an enriched and fertile mantle source that was generated by the metasomatic reaction between the mantle wedge peridotite and the subducted Neo-Tethyan oceanic sediment-derived melts. Taking the regional tectonic evolution into consideration, we favor the model of Neo-Tethyan oceanic-ridge subduction having taken place to account for the Late Cretaceous (~90 Ma) magmatic event in the Lhasa terrane. Under such geological background, partial melting of this enriched mantle would have produced the Taruocuo gabbroic rocks in the western Lhasa terrane. Given that the Late Cretaceous Taruocuo gabbroic rocks have the relatively enriched Sr–Nd–Hf isotopic compositions, distinctive from the contemporaneous mafic rocks in the eastern Lhasa terrane that have the depleted Sr–Nd–Hf isotopic compositions, we propose that the mantle source beneath the Lhasa terrane was probably isotopically segmented during the Late Cretaceous (~90 Ma).

Age and petrology of Eocene-Oligocene calc-alkaline volcanism in Biga Peninsula (NW Turkey): Implications for magma origin and geodynamic evolution

In NW Anatolia, Eocene-Oligocene post-collisional volcanism is represented by K-rich volcanic rocks, including high K calc-alkaline products. Magmatic rocks in the Dededağ area (Biga Peninsula, NW Turkey) can be classified into 6 different groups with lithostratigraphic and chronostratigraphic data using 40Ar/39Ar methods (Eocene-early Oligocene, 42.08 ± 0.09 to 29.02 ± 0.29 Ma). From the Eocene period, there are calc-alkaline Laledağ granodiorite, Beyçayır volcanics with andesite and dacite composition, and also Kızıldam volcanics with alkaline and basic character (basaltic trachyandesite, trachybasalt). Furthermore, in the Oligocene period, calc-alkaline volcanism started with Dededağ volcanics with dacite and rhyolite intermediate-acidic composition, continued with Kırcalar volcanics consisting of basalt and basaltic andesite, and Harmancık volcanics ranging from basalt to rhyolite. The calc-alkaline Çamyayla pluton with quartz monzonite composition intruded into Oligocene Dededağ and Harmancık volcanics in the region. Alkaline lavas in the region are classified as high-K and shoshonitic, and calc-alkaline lavas are medium-high-K. On N-type mid-ocean ridge basalt and chondrite-normalized trace element diagrams, enrichment was observed for all volcanic units with regard to large ion lithophile elements and light rare earth elements. High field strength elements and heavy rare earth elements were depleted in all volcanic units. These compositions reveal the participation of a mantle source that was enriched by subduction components. Using geochemical data, all the volcanic rocks differentiated as a result of magma mixing/replenishment and contamination/assimilation processes. Consequently, the source of volcanism is thought to derive from the compressional regime associated with collision during the Eocene-Oligocene, and from a previously-enriched lithospheric mantle source with subsequent effects of the extensional regime.

Formation of the Qiyugou porphyry gold system in East Qinling, China: insights from timing and source characteristics of Late Mesozoic magmatism

The Qiyugou gold deposit, located in the East Qinling Orogen, is characterized by porphyry, breccia pipe-hosted and fracture-controlled hydrothermal mineralization. Four intrusive phases are identified in this deposit: pre-mineralization quartz porphyry, syn-mineralization granite porphyry associated with breccia pipe-hosted mineralization, syn-mineralization hornblende monzogranite associated with porphyry mineralization and post-mineralization monzogranite porphyry. These granitoids are metaluminous, alkalic to calc-alkalic, shoshonitic to high-K series, and belong to highly fractionated I-type granitoids. Most of the Qiyugou granitoids show relatively lower Sr/Y (<40) and (La/Yb) N (average = 12.0) than the typical adakite-like rocks. The granitoids are strongly enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE) and depleted in heavy rare earth elements (HREE) with slightly negative europium anomalies (except for hornblende monzogranite) and moderate depletion of high field strength elements (HFSE), mostly plotting in the range of the basement Taihua Group rocks. Zircon U–Pb ages of granite porphyry, hornblende monzogranite and monzogranite porphyry from the Qi189 pluton, and hornblende monzogranite beneath the J4 breccia pipe are 133.4 ± 0.8, 131.3 ± 0.9, 128.0 ± 0.7 and 129.9 ± 0.9 Ma, respectively. The ε Hf ( t ) values and T DM2 ages show a variation trend from −37.4 to −11.4 and from 3530 to 1912 Ma, consistent with Hf isotopic features of the Taihua Group. The ε Hf ( t ) values have a positive correlation with MgO and negative correlation with SiO 2 , indicating sustained involvement of mantle-derived materials. Notably, these granitoids have comparable δ 18 O zircon values ranging from 5.7 to 5.9‰. The covariations of geochemical and isotopic data indicate that these granitoids were produced by mixing of the partially melted Taihua Group basement with mantle-derived components. The Qiyugou granitoids were generated at different crustal levels by a lithospheric thinning process linked to the tectonic transition from compression to extension at 133–128 Ma. The hornblende monzogranite from the Qi189 pluton was probably more hydrous and resulted from a highly evolved magma after hornblende fractionation, which was favourable for porphyry mineralization. Our study provides insights into the formation of a porphyry gold system associated with a prolonged and active magmatic–hydrothermal system, upwelling of hot asthenosphere and intense mantle–crust interaction. Supplementary material : Tables giving compositions of the granitoids are available at https://doi.org/10.6084/m9.figshare.c.5835494

Highly differentiated felsic granites linked to Mo mineralization in the East Kunlun Orogenic Belt, NW China: Constrains from geochemistry, and Sr-Nd-Hf isotopes of the Duolongqiarou porphyry Mo deposit

The Mo deposits in the Kunlun-Qinling-Dabie orogen are mainly associated with porphyry or porphyry-skarn mineralisation in a collision or post-collision setting. However, collision-type Mo deposits that are related to granite and mineralization produced in magmatic systems remain poorly constrained. To better define the genetic relationship between the intrusions and Mo mineralization in collision setting, the geochronology, geochemistry, and Sr-Nd-Hf isotopes of the granodiorite and monzogranite in Duolongqiarou Mo deposit, East Kunlun Orogenic Belt (EKOB), NW China, are presented. Zircon U-Pb dating shows granodiorite and monzogranite ages of 237.6 ± 1.5 Ma and 236.8 ± 1.8 Ma, respectively, which is consistent with the molybdenite Re-Os weighted mean model age of 235.9 ± 1.4 Ma. This suggests that the granitic magma related to Mo mineralization at the Duolongqiarou formed during the Triassic (ca. 236 Ma). The ore-bearing monzogranites with high SiO2 contents (76.84–77.92 wt%) are peraluminous high-K calc-alkalic rocks, while the ore-barren granodiorites with low SiO2 contents (66.89–69.90 wt%) are metaluminous Na-rich calc-alkalic rocks. Both are enriched in large ion lithophile elements and light rare earth elements but depleted in high-field strength elements, which is characteristic of I-type granites. Ore-bearing and barren granites have similar εNd(t), εHf(t), and TDM2 age, indicating a similar magma source derived from partial melting of ancient Mesoproterozoic lower crustal components with involvement of mantle materials. The amphiboles of monzogranite show higher values of calculated pressures (average 122 MPa) and melt H2O content (average 5.7 wt%) than granodiorites, suggesting that monzogranites and granodiorites crystallized at different depths of the same magma chamber. Ore-bearing monzogranites show much lower contents of transition metals (e.g., Sc, V, and Cr), negative Eu anomalies (δEu = 0.32–0.46), and higher differentiation index values (94.52–96.92) compared to barren granodiorites. This suggests that high differentiation and fractionation promote Mo metal enrichment during magmatic evolution and subsequent formation of the magmatic-hydrothermal Duolongqiarou Mo deposit. The Duolongqiarou Mo deposit was formed in a collisional setting in response to the slab break-off of the EKOB Paleo-Tethys ocean. The results of this study combined with published data suggest that at least two generations of Mo polymetallic metallogenic events were observed in the Paleo-Tethys of the EKOB. This is very helpful to target porphyry Mo mineralization and guide regional exploration.

Genesis of the Xiarihamu Magmatic Ni-Co Sulfide Deposit in the East Kunlun Orogen, Northern Tibetan Plateau: In Situ Oxygen Isotope and Geochemical Perspectives

Abstract Convergent margins are commonly deemed to have poor potential to form giant magmatic Ni-Cu-(platinum group element) sulfide deposits, due to the possibly low lithospheric geothermal gradient and the small basaltic magma fluxes. This view, however, is challenged by the recent discovery of the Xiarihamu Ni-Co deposit in the East Kunlun orogen (northern Tibetan Plateau), which contains ~157 million metric tons (Mt) of sulfide ores at 0.65% Ni, 0.013% Co, and 0.14% Cu, making it the world’s 20th largest magmatic sulfide deposit. To investigate the mantle source nature, magmatic processes, and their genetic linkage to Ni-Co mineralization in orogenic belts, we presented new in situ orthopyroxene oxygen isotope and pyroxene trace element data from the mafic-ultramafic cumulate rocks at Xiarihamu. Our results show that orthopyroxene crystals have much higher δ18O (6.40–8.58‰) than those in unmodified mantle peridotite (5.72 ± 0.14‰). Orthopyroxene grains in the primitive harzburgite cumulates have the lowest δ18O values (6.40–7.71‰) and the highest Mg# contents (88.7–83.9). From the harzburgite to the evolved lithologies (e.g., websterite and gabbronorite), the δ18O values of orthopyroxene increase, whereas the Mg# values, Ni contents, and heavy/light rare earth element (HREE/LREE) ratios of orthopyroxene decrease, implying crustal contamination during magma fractionation. The δ18O variations and sharp increases in V/Sc and LREE/HREE ratios from the primitive orthopyroxene crystals (with the highest Ni and Mg#) to the evolved ones (with low Ni and Mg#) indicate that crustal contamination played a critical role in sulfide saturation. The absences of any trend in orthopyroxene δ18O values versus Mg# contents, Ni concentrations, and La/Yb ratios toward unmodified mantle values suggest an enriched mantle source. On the mid-ocean ridge basalt (pyroxene)-normalized multielement plots, both orthopyroxene, clinopyroxene, and calculated parental magma show LREE and large ion lithophile element (LILE) enrichments and depletions in Nb-(Ta), Sr, and Zr-(Hf). The predicted primitive magma trace element compositions and the high-δ18O characteristics of the high-Mg# orthopyroxene grains suggest that the Xiarihamu primitive magma was partly originated from the pyroxenite mantle metasomatized by subducted slab-derived melts and fluids. Given the lower solidus temperature of clinopyroxene relative to olivine, pyroxenites in the mantle wedge may generate voluminous high-Ni magmas during partial melting. The high-Ni magmas are likely conducive to magmatic Ni-Cu sulfide mineralization in orogenic belts.

Element Geochemical Characteristics and Provenance Conditions of the 1st Member of Jurassic Zhongjiangou Formation in Wudun Sag, Dunhuang Basin

In order to clarify the provenance of the 1st member of the Zhongjiangou formation in Wudun sag, Dunhuang basin, the structural attributes, weathering and sedimentary characteristics of the provenance area were analyzed by means of element geochemistry, so as to determine the differences of sediment sources in different well areas. The results show that the higher the Al2O3 and K2O contents, the higher the enrichment of large ion lithophile elements and high field strength elements, while the iron and magnesium elements are relatively deficient, and there are characteristics of medium degree differentiation of light and heavy rare earth elements in Well XC1 and Well D2. The lower the Al2O3 content and the higher the SiO2 content, a loss of large ion lithophile elements and high field strength elements are observed, while the ferrophilic magnesium elements show serious loss, as shown in the characteristics of the high degree of differentiation of light and heavy rare earth elements in Well D1. In the UCC-normalized element spidergrams, the trend of Well XC1 and Well D2 is similar, which is different from that of well D1, indicating that the sediments of Well XC1 and Well D2 come from the same provenance area, while the sediment of Well D1 comes from a different provenance area. The provenance area of Well XC1 and Well D2 shows strong tectonic activity and strong weathering, while the provenance area of well D1 exhibits relatively weak tectonic activity and weathering. Combined with previous research results, Wudun sag is mainly characterized by a faulted lacustrine basin controlled by the southern boundary fault in the Jurassic layer. Therefore, the sediments of Well XC1 and Well D2 mainly come from the southern Sanweishan uplift provenance area, with strong tectonic activity; the sediments of Well D1 mainly come from the northern Beishan provenance area, with relatively weak tectonic activity.

Two-stage exhumation of deeply subducted continental crust: Insight from zircon, titanite, and apatite petrochronology, Sulu belt of eastern China

Abstract The rates and mechanisms by which deeply subducted continental crust was exhumed back to the surface are not well understood, but can be better characterized using multimineral petrochronology. Here, we combine zircon, titanite, and apatite U-Pb ages from leucogranite and phengite gneiss with a pressure–temperature (P–T) path from eclogite to provide robust quantitative constraints on cooling and exhumation of the Sulu belt, a large ultrahigh-pressure metamorphic terrane in eastern China. The leucogranite, which formed during exhumation, is enriched in light rare earth elements (REE) relative to heavy REE and in large ion lithophile elements relative to high field strength elements, similar to hydrous crustal melts. Whole-rock Sr-Nd isotope compositions indicate that the leucogranite was not directly derived from the host phengite gneiss, but was more likely sourced from deeper in the exhuming crust. For the gneiss, mantles on inherited zircon yield an age of 230 ± 2 Ma and a temperature of 802 ± 36 °C based on a minimum pressure of 2.9 GPa, which records the minimum timing and P–T of initial decompression. Overgrowths on inherited zircon from the leucogranite constrain crystallization to 224 ± 1 Ma, coeval with the growth of zircon rims in the gneiss, at a temperature of 764 ± 42 °C and a pressure within the quartzeclogite facies. Titanite and apatite define single populations with lower concordia intercept ages of 222 ± 3 Ma and 198 ± 7 Ma, at temperatures of 720 ± 30 °C and ∼450 ± 100 °C, respectively, recording the timing of passage through the quartz-eclogite to the amphibolite facies and then the transition to the upper greenschist facies. Although the data yield a nearly constant cooling rate of 10.9−3.6+4.5 °C/m.y., exhumation was completed in two stages. The first stage from coesiteeclogite facies to ∼1.2 GPa, corresponding to the depth of the Moho, occurred at a rate of 7.5−2.6+5.8 km/m.y. Thereafter, exhumation into the mid-crust occurred at a much slower rate of 0.87−0.71+0.86 km/m.y. The first stage of faster exhumation was accompanied by migration of leucogranite melt along foliation in the gneiss, which would have decreased the average density and weakened the crust, enhancing the rate of return flow.

Petrogenesis and metallogenic implications of the Miocene granite porphyry in the Jiama Cu-polymetallic deposit, Gangdese belt, South Tibet

Geochronology, geochemical and isotopic studies were carried out on granite porphyry from the Jiama Cu-polymetallic deposit, Gangdese porphyry copper deposit belt, to constrain the genesis and metallogenic potential. LA-ICP-MS zircon UPb dating indicates that the granite porphyry was emplaced at 14.9 ± 0.3 Ma to 16.0 ± 0.3 Ma, which is consistent with other felsic porphyries in the deposit. The granite porphyry shows high-K calc-alkaline and peraluminous signatures. In addition, the granite porphyry shows enrichment in large ion lithophile elements and depletion in high field strength elements. However, it has low Sr (76.13–401.90 ppm) contents, which is different from other felsic porphyries with high Sr contents in the deposit. The initial 87Sr/86Sr (0.7055 to 0.7070) and εNd(t) (−0.53 to -2.37) values of the granite porphyry are close to those of other felsic porphyries in this deposit. The granite porphyry and diorite porphyry have almost identical zircon δ18O (5.71–5.82‰) and εHf(t) (4.1–3.97) values. These results suggest that all the felsic porphyries in the Jiama deposit were derived from the same source. Geochemical characteristics show that the magma of the granite porphyry experienced fractional crystallization of hornblende and plagioclase, whereas other felsic porphyries experienced hornblende-dominated fractional crystallization. Zircon Ce4+/Ce3+ values indicate the high oxygen fugacity of granite porphyries, which is key for CuMo mineralization.

In situ Sr-O isotopic and elemental compositions of apatite and zircon from Pengcuolin granodiorites: Implications for Jurassic metallogenic variation in the southern tibet

The Gangdese belt is among the most significant magmatic and metallogenic belts in the world. The multistage magmatism that generated the deposits in the Gangdese is a major subject of scientific interest and valuable economic resource. However, the metallogenic variability within the Jurassic arc system is still controversial. In this paper, we present a study of granodiorites in the Pengcuolin to investigate the magma source and crystallisation process in the western Gangdese belt. This work includes detailed descriptions of mineral textural features, major and trace elements of bulk rock and apatite, trace element of zircons and in situ Sr isotope of apatite and O isotope of zircon. The granodiorites yielded zircon U-Pb ages of 169 ± 0.7 Ma and 165 ± 0.8 Ma, and all samples are depleted in high-field-strength elements (HFSEs) and enriched in large-ion lithophile elements (LILEs). In situ Sr isotopic compositions of apatites exhibit low 87Sr/86Sr ratios (0.7036–0.7057). The zircon δ18O values range from 4.29‰ to 6.04‰. In addition, the granodiorites lack Eu anomalies. These results indicate that the Pengcuolin magmas were derived from asthenospheric mantle that was metasomatised by slab-derived fluids following intrusion into the active continental margin in the Middle Jurassic. By comparing previously published data for Jurassic arc rocks with the results of this study, we find apparent differences between the western segment (zircon Ce4+/Ce3+ and Eu/Eu* ratios: 40–2220 and 0.07–1.49, respectively; TTi-Zr: 587–804 °C; S and Cl contents in magma: 7–443 ppm and 0.030–1.264 wt%, respectively) and the eastern segment (zircon Ce4+/Ce3+ and Eu/Eu* ratios: 3–1380 and 0.08–0.66, respectively; TTi-Zr: 547–1053 °C; S and Cl contents in magma: 7–138 ppm and 0.006–0.093 wt%, respectively). The eastern segment is characterised by low water and volatile contents and a reduced oxidation state. In contrast, the western segment exhibits a more highly oxidised state with higher water and volatile contents, which provide ideal conditions for the formation of porphyry deposits.