High Zircon δ18O Values Research Articles

The giant tin polymetallic mineralization in southwest China: Integrated geochemical and isotopic constraints and implications for Cretaceous tectonomagmatic event

The Gejiu-Bozushan-Laojunshan W–Sn polymetallic metallogenic belt (GBLB) in southeast Yunnan Province is an important part of the southwestern Yangtze Block in South China. Tin polymetallic mineralization in this belt includes the Niusipo, Malage, Songshujiao, Laochang and Kafang ore fields in the Gejiu area which are spatially and temporally associated with the Kafang - Laochang and Songshujiao granite plutons. These granites are characterized by variable A/CNK values (mostly ​> ​1.1, except for two samples with 1.09), high contents of SiO2 (74.38–76.84 ​wt.%) and Al2O3 (12.46–14.05 ​wt.%) and variable CaO/Na2O ratios (0.2–0.65) as well as high zircon δ18O values (7.74‰–9.86‰), indicative of S-type affinities. These rocks are depleted in Rb, Th, U, Ti, LREE [(La/Yb)N ​= ​1.4–20.51], Ba, Nb, Sr, and Ti and display strong negative Eu and Ba anomalies. The rocks possess high Rb/Sr and Rb/Ba ratios, relatively low initial 87Sr/86Sr ratios (0.6917–0.7101), and less radiogenic εNd(t) values (−8.0 to −9.1). The zircon grains from these rocks show negative εHf(t) values in the range of −3.7 to −9.9 with mean TDM2 (Nd) and TDM2 (Hf) values of 1.57 ​Ga and 1.55 ​Ga. They show initial 207Pb/204Pb ranging from 15.69 to 15.71 and 206Pb/204Pb from 18.36 to 18.70. Monazite from Songshujiao granites exhibits higher U and lower Th/U ratios, lower δ18O values and higher εHf(t) values than those of the zircon grains in the Kafang - Laochang granites. The geochemical and isotopic features indicate that the Laochang - Kafang granites originated by partial melting of Mesoproterozoic crustal components including biotite-rich metapelite and metagraywacke, whereas the Songshujiao granites were derived from Mesoproterozoic muscovite-rich metapelite crustal source. Most zircon grains from the Songshujiao, Laochang and Kafang granites have high-U concentrations and their SIMS U–Pb ages show age scatter from 81.6 Ma to 88.6 ​Ma, 80.7 Ma to 86.1 ​Ma and 82.3 Ma to 87.0 ​Ma, suggesting formation earlier than the monazite and cassiterite. Monazite SIMS U–Pb ages and Th–Pb ages of three same granite samples are consistent and show yielded 206Pb/238U ages of 83.7 ​± ​0.6 Ma, 83.7 ​± ​0.6 Ma, and 83.4 ​± ​0.6 ​Ma, and 208Pb/232Th ages of 83.2 ​± ​0.5 Ma, 83.8 ​± ​0.4 Ma, and 83.5 ​± ​0.9 ​Ma, which are within the range of the SIMS zircon U–Pb ages from these rocks. The data constrain the crystallization of the granites at ca. 83 Ma. In situ U–Pb dating of two cassiterite samples from the cassiterite-sulfide ore in the Songshujiao ore field and Kafang ore field, and two from the cassiterite - oxide ​+ ​cassiterite bearing dolomite in the Laochang ore field yielded weighted mean 206Pb/238U ages of 83.5 ​± ​0.4 Ma (MSWD ​= ​0.6), 83.5 ​± ​0.4 Ma (MSWD ​= ​0.5), 83.6 ​± ​0.4 Ma (MSWD ​= ​0.6) and 83.2 ​± ​0.7 ​Ma (MSWD ​= ​0.6), respectively. Combined with geological characteristics, the new geochronological data indicate that the formation of the granites and Sn polymetallic deposits are coeval. We correlate the magmatic and metallogenic event with lithospheric thinning and asthenosphere upwelling in continental extension setting in relation to the eastward subduction of the Neo-Tethys beneath the Sanjiang tectonic domain during Late Cretaceous.

Petrogenesis of Late Jurassic Pb–Zn mineralized high δ18O granodiorites in the western Nanling Range, South China

Amphibole-bearing granites are commonly considered to be I-type granites, derived by partial melting of igneous rocks. However, the contribution of sedimentary rocks to the source of such rocks is difficult to identify and remains debated. Here, we present a study of amphibole-bearing Pb-Zn mineralized granodiorites in the western Nanling Range of South China. LA–ICP–MS zircon U–Pb ages show that these granodiorites were generated during the Late Jurassic (ca. 161 Ma). They have variable SiO2 contents of 61.8–70.7 wt% and have high-K or shoshonitic characteristics, with varied Mg# (0.28–0.54). These granodiorites are enriched in Th and U, and depleted in Nb, Ta and Ti. They display enriched light rare earth element (LREE) and flat heavy rare earth element (HREE) patterns, with slightly negative to negligible Eu anomalies (δEu = 0.74–0.95). Given their high zircon δ18O values (8.8‰–11.9‰), as well as negative εNd(t) (−7.5 to −5.1) and zircon εHf(t) (−13.9 to −7.5) values, we suggest that these Pb-Zn mineralized granodiorites in the western Nanling Range were derived mainly by partial melting of Middle Neoproterozoic mafic volcaniclastic rocks from the Nanhua rift basin, South China, in the lower crust. This study provides an important insight into the genesis of amphibole-bearing granites. We suggest that the partial melting of crustal metasedimentary and metaigneous rocks is an important mechanism for the Mesozoic crustal reworking of South China, and granodiorites generated by such a mechanism should also favor Pb–Zn mineralization.

Whole-rock geochemical and zircon Hf–O isotopic constraints on the origin of granitoids and their mafic enclaves from the Triassic Mishuling pluton in West Qinling, central China

The origin of mafic microgranular enclaves (MMEs) in granitoids bears significant information on their petrogenesis. An integrated study including petrology, mineralogy and geochemistry was carried out for monzogranites and the hosted MMEs as well as biotite granites from the Mishuling pluton in the West Qinling, central China. LA-ICPMS zircon U–Pb dating yields similar emplacement ages of 212 ± 2 Ma to 217 ± 3 Ma for the monzogranites, biotite granites and MMEs. All of them display similar whole-rock Sr–Nd–Hf–O isotope and in-situ zircon Hf–O isotope compositions, suggesting their origination from common crustal sources, most likely meta-igneous rocks.MMEs have igneous textures, and contain acicular apatite and amphibole. Plagioclase with distinct core-rim compositions and clinopyroxene with replacement texture by amphibole have also been observed in MMEs. These features suggest that MMEs were formed by mixing of an intermediate magma with a felsic magma. Biotite granites are weakly peraluminous, and have high zircon δ18O values of 7.6–8.8‰ and low zircon saturation temperatures, indicating their derivation from partial melting of crustal rocks at relatively low temperatures. Compared with biotite granites, monzogranites are metaluminous, have high zircon saturation temperatures, and contain amphibole, reminiscent of typical I-type granites. However, they also show high in-situ zircon δ18O values of 7.9–9.2‰. These geochemical features indicate that monzogranites were derived from crustal rocks at elevated temperatures. The sources themselves have experienced low-temperature interaction with surface waters. Therefore, partial melting of crustal rocks at different temperatures plays an important role in dictating the geochemical diversity of the resulted granitoids.

Origin of peraluminous A-type granites from appropriate sources at moderate to low pressures and high temperatures

Four Triassic plutons from South China exhibit compositional features of S-type granites (high A/CNK values, enriched Sr–Nd isotopic compositions and high zircon δ18O values), but also of A-type granites (alkali-rich and ferroan compositions, high Zr + Nb + Ce + Y contents and high Ga/Al ratios). The S-type-like features suggest a metasedimentary source. However, the exact anatectic processes are poorly known. Here, phase equilibrium modelling on the melting of five representative metasedimentary rocks with contrasting compositions at 0.4–1.2 GPa and FMQ − 1 to FMQ + 1 indicate that the most likely source should have a composition of Mg#<52, A/CNK>1.14, K2O/Na2O<1.90, and K2O + Na2O>5. Two Neoproterozoic volcanic-sedimentary rocks adjacent to the PAGs have compositions that match these requirements, and modelled melts at P≤0.8 GPa and T≥850 °C match the main compositional variables of the plutons. This study highlights that source composition and anatectic P–T conditions are the main factors controlling the generation of peraluminous A-type granites.

Late Cretaceous Neo-Tethyan slab roll-back: Evidence from zircon U-Pb-O and whole-rock geochemical and Sr-Nd-Fe isotopic data of adakitic plutons in the Himalaya-Tibetan Plateau

AbstractLate Cretaceous (ca. 100–80 Ma) magmatism in southern Lhasa subterrane records critical geological events, which can provide important insights into the regional tectonic evolution and geodynamic process of South Tibet. This study presents new zircon U-Pb ages, whole-rock geochemistry and Sr-Nd-Fe and zircon U-Pb-O isotopic data for two dioritic plutons in the southern Lhasa subterrane. Secondary ion mass spectrometry U-Pb dating on magmatic zircons from these rocks yielded a consistent age at ca. 90 Ma. The rocks exhibit variable SiO2 contents (52–59 wt%), high Fe2O3T contents (7.1–10.0 wt%), and low K2O/Na2O ratios (0.18–0.48). Most samples have high Al2O3 (17.0–19.5 wt%) and Sr (493–678 ppm), but low Yb (0.9–2.4 ppm) and Y (9–25.2 ppm) concentrations, and thus high Sr/Y (23–74) ratios, typical of adakite-like geochemical features. The adakitic rocks have relatively uniform initial 87Sr/86Sr isotopic ratios (0.7043–0.7046) and εNd(t) values (+3.67 to +4.16), indicating derivation from similar parental magmas. The δ56Fe values of whole-rock samples vary from 0.011 to 0.091‰ with an average of 0.045 ± 0.046‰ (two standard deviations), reflecting a homogeneous Fe isotopic composition, which is associated with melt-mantle interaction. In addition, the rocks are characterized by relatively high zircon δ18O values of 5.72–7.19‰, indicating the involvement of an 18O-enriched component during magma formation. The calculation of Al-in-hornblende barometer indicates that the adakitic rocks were emplaced at pressures of 6.4–9.8 kbar. Therefore, it is proposed that the adakitic rocks were most likely generated by partial melting of mantle wedge that had been previously modified by slab-melts at a relatively shallow depth, followed by minor fractional crystallization of hornblende. Taking into account previously published data in the southern Lhasa subterrane, we suggest that the ca. 90 Ma magmatism could be related to a period of Neo-Tethyan oceanic slab roll-back, which can provide new insights into the revolution process of the Neo-Tethyan ocean realm and the accretion of the Himalaya-Tibetan Plateau.

Late early Cretaceous peraluminous biotite granites along the Bangong–Nujiang suture zone, Central Tibet: Products derived by partial melting of metasedimentary rocks?

The petrogenesis of biotite granites, which may be metaluminous or peraluminous in composition, is a subject of debate. In this study, we present new zircon U-Pb geochronology and O-Hf isotopic, and whole-rock major and trace element, and Sr-Nd isotopic data for biotite granites in the Langla–Laqinpu area of northern Lhasa Terrane, central Tibet. The rocks are distributed along the eastern segment of the Bangong–Nujiang Tethys Ocean suture, and were emplaced during the late Early Cretaceous (ca.119 Ma). The biotite granites are peraluminous, and have high SiO2 (67.5–72.4 wt%) and low MgO (0.57–1.20 wt%) and CaO (2.36–4.38 wt%) contents with variable K2O/Na2O ratios (0.77–1.32), and show geochemical affinity with the upper-continental crust, e.g., enrichment of Rb, Th, U, K and Pb, depletion of Nb, Ta, Ti, P, Eu, and Sr. They are characterized by relatively high initial 87Sr/86Sr ratios ((87Sr/86Sr)i = 0.7098–0.7137) and low εNd(t) values (−8.4 to −10.3), and high zircon δ18O values (8.29–13.47‰) and variable εHf(t) values (−0.02 to −21.36), indicating they were derived through partial melting of a crustal source consisting of metagreywackes with or without minor metaigneous rocks. Taking into account all available regional geological data, we suggest that the Bangong–Nujiang Tethys Ocean may undergo diachronous closure–although the eastern segment may have been closed by the late Early Cretaceous, its western and western–middle segments may still have been at least partially open and active at that time. The Langla–Laqinpu biotite granites were probably generated in post-collisional setting, following slab break-off after closure of the Bangong–Nujiang Tethys Ocean.

Triggers for the generation of post–collisional porphyry Cu systems in the Kerman magmatic copper belt, Iran: New constraints from elemental and isotopic (Sr–Nd–Hf–O) data

Cenozoic tectono–magmatic activities and metallogenic processes in the Kerman magmatic copper belt (KMCB) led to the formation of two contrasting porphyry copper systems in a post–collisional setting: (1) early to late Oligocene (i.e., 33.9–23.03 Ma) Jebal Barez–type intrusive rocks comprising small porphyry copper deposits (PCDs) of normal calc–alkaline and locally tholeiitic affinity, and (2) early Miocene to early Pliocene (i.e., 17–3.6 Ma) Kuh Panj–type granitoids hosting moderate to giant PCDs, such as the world–class Sarcheshmeh deposit with potassic and adakite–like calc–alkaline features. Here I present new Sr–Nd–Hf–O isotopic data from the both economic and sub–economic porphyries. The Kuh Panj ore–bearing granitoids exhibit low ISr (<0.706), high εNd(t) (>−4.6), positive zircon εHf(t) (>+6.1), low δ18O (<6.76‰), and TDMC (Hf model ages) values of 425 to 705 Ma (mean TDMC = 571 Ma). These isotopic data imply that the KMCB economic porphyry systems originated mainly from dehydration melting of a composite juvenile underplated thick lower crust derived from a depleted mantle source.In contrast, the sub–economic porphyries at Jebal Barez show comparatively older Hf model ages (mean TDMC = 1274 Ma), low εNd(t) (<−4.1), and higher ISr (>0.706) values suggesting a source dominated by lower crust. A wide range of εHf(t) values (+2.8 to −10.6) and high zircon δ18O values (>7.31‰) for the Jebal Barez granitoids, are higher than pristine mantle values indicating involvement of the supracrustal recycling materials during zircon growth. These isotopic data, coupled with geochemical characteristics of the Kuh Panj granitoids, such as high SiO2 (>69.5%), K2O (>2.8%) contents, low MgO contents (<1.4), high Sr/Y (>60.9), and La/Yb (>20) ratios, relatively low Mg# (<42.6), and compatible element contents (e.g., Cr ≤ 21.6 ppm) reported in this study, indicate a magma mixing model for the generation of high–K adakite–like porphyries within the garnet amphibolite stability field in the sub–continental lithospheric mantle (SCLM). In new magma mixing model, ore–forming high Sr/Y magmas first involved partial melting of highly metasomatized SCLM that was triggered by post–collisional lithospheric thinning and asthenospheric mantle counterflow, which produced potassic magmas. Underplating of these mantle–derived magmas beneath the lower part of the thickened juvenile lower crust (~45–55 km) would have led to dehydration melting of the lower crust with excess water (>10 wt%).

Nature and origin of Mesozoic granitoids and associated mineralization in the Sanjiang Tethys Orogeny, SW China: the Xiuwacu complex example

ABSTRACTThe magmatic generation for the Late Triassic–Early Jurassic (~215–200 Ma) and Early Cretaceous–Late Cretaceous (~108–79 Ma) post-collisional granites in the Sanjiang Tethys orogeny remain enigmatic. The Xiuwacu complex, located in the southern Yidun Terrane, consists of biotite granite with a weight mean 206Pb/238U age of 199.8 ± 2.5 Ma, aplite granite of 108.2 ± 2.3 Ma, monzogranite porphyry of 80.8 ± 1.0 Ma, and diorite enclaves of 79.2 ± 0.9 Ma and 77.9 ± 0.8 Ma. The Late Triassic biotite granites show I-type granite affinities, with high SiO2 contents, high Mg# values, high zircon δ18O values, and negative whole-rock ƐNd(t) values, indicating a predominant ancient crustal source with the input of juvenile materials. Their fractionated REE patterns and concave-upward middle-to-heavy REE patterns require garnet-bearing amphibolite as the melt source. The Cretaceous highly fractionated aplite granites and monzogranite porphyries have relatively high SiO2 contents, high (Na2O + K2O)/CaO ratios, high zircon δ18O values, and enriched whole-rock Sr–Nd isotopic signatures, suggesting that their parent magmas were likely originated from the ancient middle- to lower crust. Their significant negative Eu anomalies and obvious depletions in Nb, Sr, and Ti demonstrate that the Cretaceous granitic magmas had experienced more fractionation than the Late Triassic felsic magmas. The Late Cretaceous diorite enclaves show low SiO2 contents, high Mg# values, and high zircon δ18O values, suggesting that they were probably derived from the partial melting of subcontinental lithospheric mantle enriched by the Late Triassic subduction. The Late Triassic–Early Jurassic and Early Cretaceous–Late Cretaceous magmatism witnessed the post-collisional setting and intraplate extensional setting in response to the slab break-off and lithospheric-scale transtensional faulting, respectively. The partial melting of subduction-modified lithospheric mantle or/and residual sulphide cumulates within the lower crust during the origination of Late Cretaceous magmas could have provided metals for the formation of Xiuwacu deposit.

Sr‐Nd‐Hf‐O isotope geochemistry of the Ertaibei pluton, East Junggar, NWChina: Implications for development of a crustal‐scale granitoid pluton and crustal growth

AbstractTo better understand the compositional diversity of plutonic complexes and crustal growth of the Central Asian Orogenic Belt (CAOB), we conducted an integrated study of the Ertaibei pluton, which obtained geochronological, petrological, geochemical, and isotopic (including whole rock Sr‐Nd, in situ zircon Hf‐O) data. The pluton (ca. 300 Ma) is composed of granodiorites that contain mafic microgranular enclaves (MMEs), dolerite dikes, and granite dikes containing quartz‐tourmaline orbicules. The dolerite dikes were possibly generated by melting of an asthenospheric mantle source, with discrete assimilation of lower crustal components in the MASH (melting, assimilation, storage, and homogenization) zone. The MMEs originated from hybridization between mantle and crust‐derived magmas, which spanned a range of melting depths (∼25–30 km) in the MASH zone and were episodically tapped. Melting of the basaltic lower crust in the core of the MASH zone generated magmas to form the granodiorites. The granite dikes originated from melting of an arc‐derived volcanogenic sedimentary source with a minor underplated basaltic source in the roof of the MASH zone (∼25 km). The compositional diversity reflects both the magma sources and the degree of maturation of the MASH zone. Although having mantle‐like radiogenic isotope compositions, the Ertaibei and other postcollisional granitoids show high zircon δ18O values (mostly between +6 and +9‰), indicating a negligible contribution to the CAOB crustal growth during the postcollisional period.

Zircon U–Pb, molybdenite Re–Os geochronology and Sr–Nd–Pb–Hf–O–S isotopic constraints on the genesis of Relin Cu–Mo deposit in Zhongdian, Northwest Yunnan, China

The Relin Cu–Mo deposit is located in Zhongdian, southern portion of the Yidun Terrane, which was spatially and temporally associated with the Late Cretaceous intrusions. Orebodies at Relin are mainly hosted by monzogranite porphyry and hornfels with mineralization styles of dissemination and quartz-sulfide vein. LA–ICP–MS zircon U–Pb dating of four monzogranite porphyries yields 206Pb/238U ages varying from 83.9 to 79.0Ma, with four weighted mean 206Pb/238U ages of 80.6±0.3Ma, 82.8±0.3Ma, 80.9±0.5Ma and 81.6±0.5Ma. Re–Os isotope analyses of molybdenite separates from quartz-vein and altered granite ores yield model ages ranging from 84.3 to 81.3Ma. Geochemically, the ore-related monzogranite porphyries are metaluminous to slightly peraluminous, with an aluminum saturation index of 0.95–1.10. They are moderately rich in alkalis with K2O contents of 3.22–5.47wt% and Na2O of 2.97–3.93wt%, belonging to the high-K calc-alkaline to shoshonite series. Low MgO (0.68–1.85wt%), Cr (8.14–16.67ppm) and Ni (4.46–12.94ppm), negative zircon εHf(t) values (−9.1 to −2.7) and whole-rock εNd(t) values (−7.7 to −5.8), and relatively high zircon δ18O values (5.8–7.3‰) and radiogenic Pb isotopic compositions suggest that the Relin monzogranite porphyries were most likely generated by partial melting of the ancient lower crust. The calculated Hf isotope crustal model ages of 1.5–1.2Ga and Nd-isotope depleted mantle ages of 1.2–1.1Ga indicate that the ancient lower crust might belong to the Mesoproterozoic era. In addition, the Relin intrusions display fractionated rare earth element patterns with high (La/Yb)N and Sr/Y ratios, but low (Dy/Ho)N ratios, indicating a garnet-bearing amphibolite source. Breaking down of amphibole during partial melting of lower crust would generate hydrous and high fO2 magmas, which are favorable magmas to form the porphyry Cu–Mo deposit. Positive δ34S values (3.6–4.7‰) of sulfides point to a magmatic source of sulfur and ore-forming fluids in the Relin deposit. These results collectively show that the Relin deposit is a Late Cretaceous porphyry Cu–Mo deposit. Intraplate extension along NNW-SSE-trending sinistral faults and shear zones triggered asthenospheric upwelling and underplating of mafic magma, thus induced the partial melting of ancient lower crust. This explanation might have revealed the mechanisms of ore-forming and magma-emplacement for the porphyry deposits occurred under intra-continental extensional settings.

Paleoproterozoic S-type granites from the Helanshan Complex in Inner Mongolia: Constraints on the provenance and the Paleoproterozoic evolution of the Khondalite Belt, North China Craton

The Khondalite Belt is a Paleoproterozoic tectonic belt in the Western Block of the North China Craton. It consists mainly of high-grade metamorphosed Al-rich sedimentary rocks, also named as the ‘Khondalite Series’. The vast majority of detrital zircons from the Khondalite Series rocks have ages dominantly at 2.2–2.0Ga, but their provenances become an enigma due to lack of available synchronous magmatic rocks. In this paper, detailed geochronological and geochemical studies were done on Mid-Paleoproterozoic granites from the Helanshan Complex in the westernmost part of the Khondalite Belt to constrain the provenance of the Khondalite Series rocks and the tectonic setting of this belt. These granites can be divided into two types: the garnet-bearing granite and the two-mica granite. Both kinds of granite contain peraluminous minerals (i.e., garnet, muscovite), and are characterized by variable SiO2 (72.1–77.5wt%), high Al2O3 (11.79–18.93wt%) and A/CNK values (1.09–1.27) as well as high zircon δ18O values (7.30–11.49‰), indicating an origin of typical S-type granites. They are enriched in Rb, Th, U, LREE ((La/Yb)N=19.2–202) and display pronounced negative Nb-Ta-Ti and Eu (Eu/Eu∗=0.28–0.94) anomalies, similar to geochemical characteristics of magmatic rocks formed in a subduction zone. Zircon SIMS U-Pb dating yielded consistent Paleoproterozoic intrusive ages of 2045±17Ma and 2069±25Ma for the garnet-bearing granite and the two-mica granite, respectively. These magmatic zircons have variable zircon εHf(t) values mostly ranging between −1.7 and +6.7, corresponding to Hf model ages (TDMC) of 2.2–2.8Ga. The geochemical and isotopic features indicate that these S-type granites have been produced by partial melting of the ancient crust with addition of juvenile crustal or mantle-derived components. Combined with previous geological data of the Khondalite Belt, it is speculated that the ∼2.0Ga magmatism was mostly likely generated in an active continental margin, providing robust evidence for a provenance for the clastic materials of the Khondalite Belt.

Crustal basement controls granitoid magmatism, and implications for generation of continental crust in subduction zones: A Sr–Nd–Hf–O isotopic study from the Paleozoic Tongbai orogen, central China

Ascertaining the petrogenesis of granitoid rocks in subduction zones holds the key for understanding the processes of how continental crust is produced. The synchronous Taoyuan and Huanggang plutons occur in two different geological units of the Paleozoic Tongbai orogen of central China. They provide an optimal opportunity for a study to address the role of the crustal basement in generating voluminous granitoid magmatism in subduction zones. The Taoyuan and Huanggang plutons have identical U–Pb zircon crystallization ages of 440–444Ma, which are temporally related to northward subduction of the Paleotethyan Ocean. The Taoyuan samples show high SiO2 (73.36–79.16%) and low Al2O3 (12.00–13.45%) contents, Mg numbers (20.6–38.2), and Sr/Y (2.04–10.1) and (La/Yb)N (2.34–7.32) ratios with negative Eu anomalies (Eu/Eu*=0.33–0.93). They yielded positive εNd(t) (+3.0 to +6.7) and εHf(t) (+11.8 to +13.2) values, elevated initial Sr isotopic ratios (0.7040–0.7057) and relatively low zircon δ18O values of 4.62–5.39‰. These suggest that they were produced through partial melting of hydrothermally altered lower crust of the accreted Erlangping oceanic arc. In contrast, the Huanggang samples exhibit variable whole-rock geochemical and isotopic compositions with SiO2 contents of 57.01–64.42wt.%, initial Sr isotopic ratios of 0.7065–0.7078, and εNd(t) values of −5.7 to −9.4. Additionally, they have high zircon δ18O values of 7.57–8.45‰ and strongly negative zircon εHf(t) values of −14.4 to −10.5. They were suggested to have been mainly derived from ancient continental crust of the Kuanping crustal unit with the addition of 20–40% juvenile, mantle-derived material. Accordingly, the granitoids in both oceanic and continental arcs are likely to be mainly derived from intracrustal melting of their crustal basement. It is revealed by the Huanggang pluton that little net continental crust growth occurs in continental arcs, and addition of new volume of continental crust may be balanced by the loss of old mafic lower continental crust. Continental growth was likely achieved through the accretion and subsequent differentiation of oceanic arcs, as manifested by the Taoyuan pluton.

Two contrasting late Paleozoic magmatic episodes in the northwestern Chinese Tianshan Belt, NW China: Implication for tectonic transition from plate convergence to intra-plate adjustment during accretionary orogenesis

Late Carboniferous to Early Permian is a critical period for the final amalgamation of the Central Asian Orogenic Belt (CAOB). However, as most of the accreted terranes of the CAOB are unclear in tectonic nature and origin, the timing and processes of their mutual amalgamation have been poorly constrained. To understand assembly of the West Junggar Terrane with the Yili Block, a suite of the late Paleozoic magmatic rocks, including ignimbrite, rhyolite and granite, in northwestern Chinese Tianshan Belt were studied for their petrogenesis and tectonic implications. Our new results of secondary ion mass spectrometry (SIMS) zircon U-Pb dating reveal two separate magmatic episodes, ca. 300Ma volcanism (ignimbrite and rhyolite) and ca. 288Ma plutonsim (biotite granite). Geochemically, for the ca. 300Ma volcanism, the ignimbrites have low SiO2 (65.8–71.5wt.%) and Mg# (6–13) values, and exhibit arc affinity with significantly enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE) such as Nb, Ta and Ti. The whole-rock εNd(t) and zircon εHf(t) values range from +6.9 to +7.0 and +9.9 to +14.1 respectively, indicating a juvenile basaltic lower crustal origin. Rhyolites have slightly high SiO2 (72.7–74.0wt.%) and K2O (3.86–4.53wt.%) contents, high zircon δ18O (11.67–13.23‰) values, and low whole-rock εNd(t) (+2.9 to +3.8) and zircon εHf(t) (+2.8 to +10.0) values, which may suggest sediment involvements during magma generation. In contrast, for the ca. 288Ma plutonism, the biotite granites have obviously higher SiO2 (74.7–75.5wt.%) contents and whole-rock εNd(t) (+7.7 to +8.8), zircon εHf(t) (+9.8 to +12.7), and lower zircon δ18O (5.99–6.84‰) values, than those of the ca. 300Ma volcanic rocks, which are consistent with signatures of juvenile magma source. According to our estimates of zircon saturation temperatures, together with their contrasting genesis, we attribute the formation of ca. 300Ma high temperature (815–938°C) volcanism to oceanic slab break-off during assembly of the West Junggar Terrane with the Yili Block, and relate the generation of ca. 288Ma low temperature (723–735°C) plutonism to subsequent strike-slipping of North Tianshan Fault that facilitated introduction of water-fluxes triggering hydrous partial melting of juvenile lower crust. The sequential magmatic episodes in the northwestern Chinese Tianshan Belt may provide a crucial clue to a tectonic transition from plate convergence to intra-plate adjustment during the formation of the Kazakhstan Orocline in the late Paleozoic.

Neoproterozoic granitoids from the Phan Si Pan belt, Northwest Vietnam: Implication for the tectonic linkage between Northwest Vietnam and the Yangtze Block

Several Neoproterozoic granitic intrusions, including the Phin Ngan intrusion and some smaller ones from the Sin Quyen Cu mine, have been identified in the Phan Si Pan belt, Northwest Vietnam. Whole-rock geochemical and Sm-Nd isotopic data and zircon U-Pb age and Hf-O isotopes have been determined for these intrusions in order to constrain their ages, petrogenesis and tectonic implications. The Phin Ngan intrusion, mainly composed of syenogranite, was emplaced at 824±4Ma. Rocks from this intrusion have high SiO2 (72.1–73.5wt.%), and K2O (5.14–5.52wt.%), with A/CNK values ranging from 1.02 to 1.06. They have negative whole-rock εNd(t) (−8.4 to −5.2) and zircon εHf(t) values (−8.1 to −5.5), and high zircon δ18O values (9.7–10.9‰). These geochemical features suggest that the Phin Ngan intrusion was derived from ancient, K-rich crustal sources. Granitic intrusions from the Sin Quyen mine occur as stocks or dykes intruding ores and ore-hosting rocks. They are composed of monzogranite and granodiorite, emplaced between 736±8 and 758±6Ma. They have high SiO2 (68.2–76.7wt.%) and (K2O+Na2O) (6.93–8.54wt.%), and low MgO (0.19–0.87), with A/CNK values ranging from 0.91 to 1.14. Their whole-rock εNd(t) values range from −6.6 to 0, mostly clustered between −6.6 and −4.3. Their zircon εHf(t) values vary from −6.7 to +11.1, mostly lower than −2.0. Such geochemical data indicate that these intrusions were produced mainly through partial melting of ancient crustal materials, coupled with subordinate involvement of mantle-/juvenile crust-derived components. The Phin Ngan intrusion and intrusions from the Sin Quyen mine are sub-alkaline in nature, and have arc-like trace-element compositions. Compiled with other broadly synchronous igneous rocks with arc-like affinities in the region, the Neoproterozoic magmatism in the Phan Si Pan belt was generated in a subduction-related setting. The Neoproterozoic intrusions in the Phan Si Pan belt are geochronologically and geochemically correlated with the Neoproterozoic subduction-related igneous rocks along the western Yangtze Block. Thus, the Phan Si Pan belt represents the southern most part of the giant arc system of the western Yangtze Block.

Geochemical constraints on the source nature and melting conditions of Triassic granites from South Qinling in central China

The Qinling orogen marks the continental collision between the North China Block and the South China Block in the Triassic, but the Triassic collisional orogeny is only prominent in the South Qinling zone with the occurrence of voluminous granites. In order to understand the petrogenesis of these granites, a combined study of petrology, geochronology and geochemistry was carried out for them from outcrops at Guangtoushan (GTS) and Huayang (HY) in South Qinling. The results indicate that the studied granites were derived from partial melting of metasedimentary rocks rather than metaigneous rocks as suggested by previous studies. LA-ICPMS zircon U–Pb dating yields concordant ages of 207±3Ma to 212±4Ma for their magma emplacement. The GTS granites are weakly to strongly peraluminous with A/CNK ratios of 1.05 to 1.15 and K2O/Na2O ratios of 0.5 to 1.32 and show high whole-rock δ18O values of 9.52 to 11.20‰. The HY granites are peraluminous with high A/CNK values of 1.04 to 1.14 and K2O/Na2O ratios of 1.01 to 2.82, and have high zircon δ18O values of 10.71 to 11.43‰ and whole-rock δ18O values of 12.42–13.77‰. On the other hand, the GTS granites exhibit low Rb/Sr ratios but high Sr/Ba ratios, variable Eu anomalies (Eu/Eu⁎=0.29–1.05), in association with low zircon saturation temperatures. In contrast, the HY granites have high Rb/Sr ratios but low Rb/Ba ratios, conspicuous negative Eu anomalies (Eu/Eu⁎=0.52–0.77) and relatively high zircon saturation temperatures. An integrated interpretation of all these results is that the GTS granites were produced by fluid-present partial melting of a mixture source of metapelite and metagreywacke at low temperatures, whereas the HY granites were generated by fluid-absent partial melting of a metagreywacke at relatively higher temperatures. Furthermore, the two granite plutons show variably high whole-rock (87Sr/86Sr)i ratios of 0.7047 to 0.7115, negative whole-rock εNd (t) values of −9.8 to −6.8 and zircon εHf (t) values of −13.1 to −2.0, with two-stage Nd-Hf model ages of Mesoproterozoic to Paleoproterozoic. Their εNd (t) values are comparable with those for sedimentary rocks in the southern margin of the South Qinling zone and the northern margin of the South China Block, suggesting a genetic link between them. Therefore, the GTS and HY granites originated from the sedimentary rocks rather than igneous rocks, and thus belong to S-type rather than I-type. It is inferred that these sedimentary rocks were subducted to the lower crustal depth during the Late Triassic collisional orogeny in the Qinling orogen for partial melting to produce the S-type granites.

Xenoliths in ultrapotassic volcanic rocks in the Lhasa block: direct evidence for crust–mantle mixing and metamorphism in the deep crust

Felsic granulite xenoliths entrained in Miocene (~13 Ma) isotopically evolved, mantle-derived ultrapotassic volcanic (UPV) dykes in southern Tibet are refractory meta-granitoids with garnet and rutile in a near-anhydrous quartzo-feldspathic assemblage. High F–Ti (~4 wt.% TiO2 and ~3 wt.% F) phlogopite occurs as small inclusions in garnet, except for one sample where it occurs as flakes in a quartz-plagioclase-rich rock. High Si (~3.45) phengite is found as flakes in another xenolith sample. The refractory mineralogy suggests that the xenoliths underwent high-T and high-P metamorphism (800–850 °C, >15 kbar). Zircons show four main age groupings: 1.0–0.5 Ga, 50–45, 35–20, and 16–13 Ma. The oldest group is similar to common inherited zircons in the Gangdese belt, whereas the 50–45 Ma zircons match the crystallization age and juvenile character (eHf i +0.5 to +6.5) of Eocene Gangdese arc magmas. Together these two age groups indicate that a component of the xenolith was sourced from Gangdese arc rocks. The 35–20 Ma Miocene ages are derived from zircons with similar Hf–O isotopic composition as the Eocene Gangdese magmatic zircons. They also have similar steep REE curves, suggesting they grew in the absence of garnet. These zircons mark a period of early Miocene remelting of the Eocene Gangdese arc. By contrast, the youngest zircons (13.0 ± 4.9 Ma, MSWD = 1.3) are not zoned, have much lower HREE contents than the previous group, and flat HREE patterns. They also have distinctive high Th/U ratios, high zircon δ18O (+8.73–8.97 ‰) values, and extremely low eHf i (−12.7 to −9.4) values. Such evolved Hf–O isotopic compositions are similar to values of zircons from the UPV lavas that host the xenolith, and the flat REE pattern suggests that the 13 Ma zircons formed in equilibrium with garnet. Garnets from a strongly peraluminous meta-tonalite xenolith are weakly zoned or unzoned and fall into four groups, three of which are almandine-pyrope solid solutions and have low δ18O (+6 to 7.5 ‰), intermediate (δ18O +8.5 to 9.0 ‰), and high δ18O (+11.0 to 12.0 ‰). The fourth is almost pure andradite with δ18O 10–12 ‰. Both the low and intermediate δ18O groups show significant variation in Fe content, whereas the two high δ18O groups are compositionally homogeneous. We interpret these features to indicate that the low and intermediate δ18O group garnets grew in separate fractionating magmas that were brought together through magma mixing, whereas the high δ18O groups formed under high-grade metamorphic conditions accompanied by metasomatic exchange. The garnets record complex, open-system magmatic and metamorphic processes in a single rock. Based on these features, we consider that ultrapotassic magmas interacted with juvenile 35–20 Ma crust after they intruded in the deep crust (>50 km) at ~13 Ma to form hybridized Miocene granitoid magmas, leaving a refractory residue. The ~13 Ma zircons retain the original, evolved isotopic character of the ultrapotassic magmas, and the garnets record successive stages of the melting and mixing process, along with subsequent high-grade metamorphism followed by low-temperature alteration and brecciation during entrainment and ascent in a late UPV dyke. This is an excellent example of in situ crust–mantle hybridization in the deep Tibetan crust.

Phanerozoic amalgamation of the Alxa Block and North China Craton: Evidence from Paleozoic granitoids, U–Pb geochronology and Sr–Nd–Pb–Hf–O isotope geochemistry

The North China Craton (NCC) has been considered to be part of the supercontinent Columbia. The nature of the NCC western boundary, however, remains strongly disputed. A key question in this regard is whether or not the Alxa Block is a part of the NCC. It is located in the vicinity of the inferred boundary, and therefore could potentially resolve the issue of the NCC's relationship to the Columbia supercontinent. Some previous studies based on the Alxa Block's geological evolution and detrital zircon ages suggested that it is likely not a part of the NCC. The lack of evidence from key igneous rock units, however, requires further constraints on the tectonic affinity of the western NCC and Alxa Block and on the timing of their amalgamation.In this study, new zircon U–Pb age and Hf–O isotopes and whole-rock geochemical and Sr–Nd–Pb isotopic data for the Paleozoic granitoids in or near the eastern Alxa Block were used to constrain the petrogenesis of these rocks and the relationship between the Alxa Block and NCC. Secondary ion mass spectrometry (SIMS) U–Pb zircon dating indicates that the Bayanbulage, Hetun, Diebusige and South Diebusige granitoids were formed at ca. 423Ma, 345Ma, 345Ma and 337Ma, respectively. The Late Silurian (Bayanbulage) quartz diorites have variable SiO2 (58.0–67.9wt.%), and low Sr/Y (20–24) values, while the Early Carboniferous (Hetun, Diebusige and South Diebusige) monzogranites have high SiO2 (71.5–76.7wt.%) and Sr/Y (40–94) values. The Late Silurian quartz diorites display relatively homogeneous and high zircon δ18O (8.5–9.1‰) and εHf(t) (−8.6 to −5.3) values, high whole-rock εNd(t) values (−9.2 to −7.6) and highly radiogenic Pb isotopes (206Pb/204Pb=18.13–18.25), whereas the Early Carboniferous monzogranites exhibit relatively low and variable zircon δ18O (5.7–7.2‰) and εHf(t) (−23.1 to −7.4) values, low whole-rock initial 87Sr/86Sr (0.7043–0.7070) and εNd(t) (−19.1 to −13.5) values and variable Pb isotopes (206Pb/204Pb=16.06–18.22). The differences in whole rock Nd model ages and Pb isotope compositions of the Paleoproterozoic–Permian rocks in either side of the west fault of the Bayanwulashan–Diebusige complexes suggest that the Alxa Block is not a part of the NCC, and that the western boundary of the NCC is probably located on this fault. Furthermore, the linear distribution of the Early Paleozoic–Early Carboniferous granitoids, the high zircon δ18O values of the Late Silurian quartz diorites, the Early Devonian metamorphism and the foreland basin system formed during the collision between the Alxa Block and the NCC indicate that a Paleozoic cryptic suture zone likely existed in this area and records the amalgamation of the Alxa Block and North China Craton. Together with detrital zircon data, the initial collision was considered to have possibly occurred in Late Ordovician.

The source of Mesozoic granitoids in South China: Integrated geochemical constraints from the Taoshan batholith in the Nanling Range

A combined study of zircon U–Pb ages and Lu–Hf isotopes, whole-rock major-trace elements and Sr–Nd isotopes as well as whole-rock and mineral O isotopes was carried out for Mesozoic granitic intrusions from the Taoshan batholith in the Nanling Range, South China. The results not only place constraints on the origin of granitoids but also provide insights into the effect of melting temperature on granitoid compositions. LA-ICPMS zircon U–Pb dating yields weighted 206Pb/238U ages of 230±2Ma for the Caijiang intrusion, 167±2Ma for the Huangpi intrusion, 152±3Ma for the Daguzhai intrusion, and 146±3Ma for the Luobuli intrusion. Relict zircon cores with older Mesozoic to Paleozoic U–Pb ages are present in some samples. These Mesozoic granitoids are weakly to strongly peraluminous with A/CNK ratios of 1.05 to 1.23. They exhibit arc-like trace element distribution patterns, with enrichment of LREE and LILE (e.g., Rb, K, Pb) but depletion of HFSE (e.g., Nb, Ta, Ti). They show high whole-rock initial 87Sr/86Sr ratios of 0.7101 to 0.7156 and low εNd(t) values of −11.2 to −8.8 as well as negative zircon εHf(t) values of −14.3 to −4.8 and Paleoproterozoic Nd-Hf model ages. These results suggest their derivation from partial melting of ancient continental crust. Furthermore, they have high zircon δ18O values of 8.6 to 10.6‰ and calculated whole-rock δ18O values of 10.8 to 12.5‰. Along with their peraluminous features, high K2O/Na2O ratios, arc-like trace element distribution patterns and enriched Sr–Nd–Hf isotope compositions, these Mesozoic granitoids are unambiguously derived from partial melting of metasedimentary rocks and thus are of S-type affinity. There are significant differences in both major and trace elements between the Caijiang intrusion of Triassic age and the Huangpi, Daguzhai and Luobuli intrusions of Jurassic age. While the Triassic granites exhibit high (Fe2O3)T+MgO, P2O5, Th, LREE and Zr+Nb+Ce+Y contents but low Al2O3/TiO2, Rb/Ba and Rb/Sr ratios in association with high zircon saturation temperatures, the Jurassic granites have low (Fe2O3)T+MgO, P2O5, Th, LREE and Zr+Nb+Ce+Y contents but high Al2O3/TiO2, Rb/Ba and Rb/Sr ratios in association with low zircon saturation temperatures. These correlations indicate that melting temperature and thus extent of partial melting have played an important role in dictating the compositions of granitoids. Therefore, the geochemical compositions of S-type granitoids are dictated not only by the composition of source rocks but also by the temperature of partial melting.

Latest Early Permian granitic magmatism in southern Inner Mongolia, China: Implications for the tectonic evolution of the southeastern Central Asian Orogenic Belt

Latest Early Permian (277–275Ma) magmatic episode is one of the most important tectono-magmatic events in southeastern Central Asian Orogenic Belt (CAOB), recording a tectonic transition from accretionary to post-accretionary evolution of the Paleo-Asian Ocean. The latest Early Permian granitoids along the Solonker suture zone in southern Inner Mongolia were emplaced between 275Ma and 277Ma as determined by SHRIMP zircon U–Pb dating. They are characterized by variable alkali contents, a range from calcic to slightly alkalic compositions, and are magnesian I-type granitoids. Their εNd(t) values from −0.4 to +3.1 record predominantly juvenile crustal sources with moderate involvement of old crustal components. Furthermore, these granitoids show positive zircon εHf(t) values of +7.6 to +10.7 that are also indicative of predominantly juvenile nature. Their high zircon δ18O values of 6.29‰ to 8.13‰, suggest significant involvement of supracrustal materials in their generation. These characteristics imply they were derived from an already hybrid andesitic magma in active continental margin. The resultant composite magmas were finally attached (relaminated) to the lower crust, where they produced segregated melt, to rise with minor assimilation of supracrustal materials, and to form these granitic plutons. The outboard migration of continental arc in response to slab roll-back, from the Late Carboniferous to the latest Early Permian, and tectonic switching from this prolonged extension to the short-lived contraction resulting from slab break-off and final amalgamation of the southeastern CAOB in the Middle-Late Permian lead to efficient continental growth in the southeastern CAOB.

Petrogenesis of Triassic granites from the Nanling Range in South China: Implications for geochemical diversity in granites

Abstract A combined study of whole-rock major-trace elements and Sr–Nd isotopes, zircon U–Pb ages, Hf and O isotopes as well as biotite geochemistry was carried out for Triassic granite intrusions from the Nanling Range in South China. The results provide insights into the effects of source composition and melting conditions on the geochemical diversity of granites. The granites of interest are peraluminous, and contain primary muscovite and tourmaline. They are characterized by high zircon δ18O values of > 9.0‰, high initial 87Sr/86Sr values of ~ 0.7200, and homogeneous eNd(t) values of − 11.3 to − 9.8, as well as variable zircon eHf(t) values of − 12.2 to − 5.8. Biotite geochemistry is similar to that of common peraluminous granites. An integrated interpretation of these petrological, mineralogical and geochemical data indicates that these granites were derived from partial melting of metasedimentary rocks under variable physicochemical conditions. The differences in whole-rock and biotite geochemistry between the intrusions are ascribed to the variable effects of source heterogeneity and melting temperature. The Luxi intrusion exhibits higher contents of MgO, FeOT, TiO2 and CaO than common melts derived from metasedimentary rocks, tight variations in major-trace elements and homogeneous Sr–Nd isotopic compositions, and homogeneous biotite composition with high Mg# [= Mg / (Mg + Fe) in molar] and lower whole-rock A/CNK values [= Al2O3 / (CaO + Na2O + K2O) in molar]. These can be explained by originating from a relatively mafic metasedimentary source. On the other hand, the geochemical diversity of granites can be caused by the difference in melting temperature in addition to the source heterogeneity. This is suggested by the Xiazhuang and Fucheng intrusions which exhibit similar range of SiO2. Nevertheless, the Fucheng intrusion is ferroan, and high in TiO2, (Na2O + K2O)/CaO, TiO2/MgO, Ga/Al and Zr + Nb + Ce + Y, but low in CaO, MgO and Mg#. Most of its major elements and their ratios are similar to those of A-type granites that were generated from F-rich rocks at high temperatures, consistent with the enrichment of Fe and F in its biotite. Thus, partial melting of a metasedimentary restite at reduced conditions after previous melt extraction is suggested to produce the geochemical feature of A-type granites in the Fucheng intrusion. In contrast, partial melting of fertile metasedimentary rocks would produce normal S-type granites like the Xiazhuang intrusion, with its geochemical variations ascrible to source heterogeneity and differential melting. Therefore, the geochemical composition of granites is primarily dictated by the composition of source rocks in addition to the physicochemical conditions of partial melting.