Young Hf Model Ages Research Articles

Arc-arc amalgamation during accretionary orogenesis: Insights from mid-Paleozoic tectono-magmatic records in eastern Junggar, NW China

Abstract Arc-arc amalgamation occurs during the evolution of composite orogens at convergent plate margins and plays a critical role in controlling accretionary patterns and processes. The eastern Junggar terrane in the southern Central Asian Orogenic Belt underwent a long-lived subduction-accretion process in the Paleozoic, but whether and how the Yemaquan and Dananhu-Harlik arcs were amalgamated remain debatable. A systematic U-Pb–Hf-O isotopic study was conducted on zircons from Silurian granitic rocks in the Yemaquan arc. The U-Pb dating results suggest that these rocks were emplaced at 433–422 Ma and inherited abundant 536–435 Ma zircons representing a predominant magmatic episode in the Yemaquan arc. Their positive εHf(t) values and young Hf model ages indicate that the Yemaquan arc is dominated by juvenile basement with significant crustal growth during the Neoproterozoic to early Paleozoic. The variations in zircon Eu/Eu*, εHf(t), and δ18O values reveal that the Yemaquan arc experienced remarkable crustal thickening and remobilization at ca. 450 Ma, similar to the northern Dananhu-Harlik arc, and this was followed by extension that initiated at ca. 420 Ma. These features support the amalgamation of these two arcs occurring ca. 450–420 Ma. Integrated with regional data, we correlated this amalgamation event in the eastern Junggar terrane with the orogenic event in the Chinese Altai terrane, and we propose a middle Paleozoic tectonic evolution model in the eastern Junggar–Altai area from arc assembly to dispersal in association with a transition in accretionary mode. This scenario probably took place as a response to plate reorganization during the breakup of the northern margin of Gondwana.

Ore Genesis and the Magmatism of the Yuhaixi Mo(Cu) Deposit in Eastern Tianshan, NW China: Constraints from Geology, Geochemistry, Zircon U-Pb and Molybdenite Re-Os Dating

The Yuhaixi Mo(Cu) deposit is a new discovery in the eastern section of the Dananhu-Tousuquan island arc, Eastern Tianshan. However, the genesis of the Yuhaixi Mo(Cu) deposit is still not fully understood. The Yuhaixi intrusion is composed of monzonitic granites, diorites, granites, and gabbro dikes, among which disseminated or veinlet Mo and Cu mineralization is mainly hosted by the monzonitic granites. The LA-ICP-MS zircon U-Pb dating yields emplacement ages of 359.4 ± 1.6 Ma for the monzonitic granite, 298.8 ± 1.8 Ma for the diorite, and 307.0 ± 2.3 Ma for the granite. The Re-Os dating of molybdenite hosted by monzonitic granite yields a well-constrained 187Re-187Os isochron age of 354.1 ± 6.8 Ma (MSWD = 1.7) with a weighted average age of 344.5 ± 3.1 Ma. The Mo mineralization is closely associated with the Yuhaixi monzonitic granite. The Yuhaixi monzonitic granite rocks are characterized by high silica (SiO2 > 70 wt.%), low MgO (0.23–0.36), Ni, Cr contents, and they are enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs: e.g., K, Ba, Pb and Sr), and depleted in heavy rare earth elements (HREEs) and high field-strength elements (HFSEs: e.g., Nb, Ta and Ti). They are weak peraluminous and have high εHf(t) (11.37–17.59) and εNd(t) (1.36–7.75) values, and varied initial 87Sr/86Sr (0.7037–0.7128) values. The Yuhaixi post-ore granites exhibit similar geochemical and isotopic signatures to the Yuhaixi monzonitic granite. These characteristics suggest that the Yuhaixi felsic rocks are likely sourced from the partial melting of the juvenile lower crust. The Yuhaixi diorite has low SiO2, and K2O contents, relatively high Na2O, MgO (Mg# = 45–53) contents, and depletions in HFSE (e.g., Nb, Ta, and Ti). These geochemical features, coupled with isotopic data such as low initial 87Sr/86Sr (≤0.7043), high εNd(t) (2.5 to 3.0) and εHf(t) (≥11.6) values, and young Hf model ages, suggest that their parental magmas possibly originated from the partial melting of the depleted lithospheric mantle that was metasomatized by hydrous melts or fluids from the subducting oceanic plate. Integrating our new results with previous works on the Dananhu-Tousuquan island arc belt, we suggest that the Yuhaixi Mo(Cu)deposit is likely sourced from the juvenile lower crust, which was formed in an arc setting, where the bipolar subduction of the North Tianshan oceanic slab forms the Dananhu Tousuquan belt to the north and the Aqishan-Yamansu belt to the south. The eastern section of the Dananhu-Tousuquan island arc is a promising target for late Paleozoic porphyry Mo(Cu) deposits.

Genetic link between orogenic Au and porphyry Cu ([sbnd]Au) mineralization in the Dananhu Arc, NW China: Evidence from geochronology, geochemistry, and Sr–Nd–Hf isotopes of the Tudunbei Au deposit

The late Paleozoic Chinese Tianshan is characterized by large volumes of arc-related magmatic rocks, and numerous associated orogenic Au and porphyry Cu (Au) deposits. The link between these two deposit styles, however, is ambiguous. Here, we present zircon U–Pb–Hf isotopes, bulk-rock geochemistry, and sericite 40Ar/39Ar geochronology from the newly discovered Tudunbei Au deposit located east of the Tuwu–Yandong porphyry Cu (Au) ore district in the Dananhu Arc, Chinese North Tianshan, and combine this with previously published geochronological and geochemical data to characterize the genetic relationship between orogenic Au and porphyry Cu (Au) mineralization. Our new UPb zircon ages indicate that the wall rocks of the Tudunbei Au deposit formed during the Carboniferous between 337 and 314 Ma (K-feldspar granite: 334.8 ± 1.9 Ma, basaltic tuff: 336.6 ± 4.0 Ma, quartz diorite: 316.0 ± 1.8 Ma, gabbroic diorite: 318.3 ± 1.8 Ma, and gabbro: 314.3 ± 1.8 Ma). These wall rocks are enriched in light rare earth elements and large ion lithophile elements (e.g., Ba, U, and Pb), and depleted in high field strength elements (e.g., Nb, Ta, and Ti). Given the absence of Precambrian basement rocks in the Chinese North Tianshan, and the depleted NdHf isotope compositions (εHf(t) = 12.12–14.98; εNd(t) = 5.77–7.06) of the wall rocks, their young Hf model ages (350–510 Ma), the lack of inherited zircons, and their enrichment in Na2O (2.84–4.73 wt%) relative to K2O (0.43–2.40 wt%), it is suggested that they formed in an oceanic arc environment. One sericite sample selected from an auriferous quartz vein yielded 40Ar/39Ar plateau ages of 260.6 ± 1.1 Ma, which is representative of the gold mineralization age. Given the abundance of gold in the Carboniferous wall rocks of the Tuwu–Yandong porphyry Cu (Au) ore district, the similarity in the timing of its magmatism with Tudunbei, and the geochemical–isotopic (Sr–Nd–Hf) similarity of their wall rocks, it is inferred that the gold in the Tudunbei deposit was remobilized from pre-existing gold in the Carboniferous wall rocks during regional metamorphism and deformation.

Paleo-Mesoproterozoic magmatic-sedimentary events in eastern Bainaimiao micro-block: Implications for Precambrian tectonic evolution of a fragment from Columbia supercontinent

Identifying tectonic positions and origins of Precambrian micro-blocks within the Central Asian Orogenic Belt (CAOB) is essential for revealing global paleogeography and evolution of the early Earth. The Bainaimiao micro-block in the southeastern CAOB preserves multistage geologic records which can better reveal Precambrian tectonic history of micro-blocks within the Phanerozoic accretionary orogenic belt. This study presents integrated research of geochronology, geochemistry, and zircon Hf isotopes on the Chenjiatun metamorphic complex in the eastern Bainaimiao micro-block, which consists of meta-sedimentary rocks (plagioclase gneisses and mica schists), amphibolites and two-mica granites. The protoliths of amphibolites, two-mica granites and meta-sedimentary rocks were formed at 1.75 Ga, 1.38 Ga and 1.39–1.34 Ga, respectively, suggesting that they represent Paleo-Mesoproterozoic basement of the Bainaimiao micro-block. The detrital zircon age spectra of meta-sedimentary rocks show two representative age peaks at 1.80 Ga and 1.40 Ga, with zircon Hf isotopes varying from −11.11 to +10.67. Characteristic mostly positive zircon Hf isotopes and geochemical features of the Chenjiatun complex indicates that the Bainaimiao micro-block probably has tectonic affinity with the Laurentia-Baltica continent. Paleoproterozoic amphibolites with positive εHf(t) (+5.67 to +15.42) were derived from metasomatized mantle wedge mixed with sediment melts. They are characterized by arc-like geochemistry such as high Th/Yb (0.73–2.77), enrichment in LILEs and depletion in HFSEs (eg. Nb and Ta), similar to coeval arc magmatism in the Laurentia-Baltica continent, which indicate the Bainaimiao micro-block was possibly in a subduction convergent setting in the late Paleoproterozoic. By contrast, Mesoproterozoic two-mica granites, with mostly positive εHf(t) (−0.29 to +11.53), belong to typical S-type granites and could be generated from partial melting of meta-pelites with participation peritectic garnets. These two-mica granites, together with ∼1.40 Ga juvenile A-type grantioids from nearby and other Precambrian micro-blocks in the southern CAOB, exhibit relatively high alkalis and HFSEs contents, young Hf model ages (1.50–1.40 Ga) and were generated at high-temperature environment. These findings reveals that the Bainaimiao micro-block was in an extensional setting at 1.40 Ga coeval with the breakup evolution of the Columbia supercontinent.

Palaeozoic tectonic evolution and magmatism in the Kalatag area, East Tianshan, NW China: Evidence from the geochronology and geochemistry of intrusive rocks

The Kalatag area contains many intrusive rocks of Late Ordovician to Permian age and hosts polymetallic deposits that are associated with magmatism. It is a key area for revealing the nature and genesis of the Palaeozoic rocks in East Tianshan. The intrusive rocks of different periods preserve useful information regarding the tectonomagmatic history of the Kalatag area. Zircon U–Pb geochronological, Hf isotopic, Sr–Nd isotopic, and whole‐rock geochemical analyses are reported for the intrusions from the Meiling ore district, Yudai ore district, and Kalatag pluton to reveal their petrogenesis and geodynamic setting. The LA–ICP–MS zircon U–Pb analyses suggest that the quartz diorite from the Kalatag pluton formed at 420–432 Ma, and the quartz diorite porphyry from the Yudai ore district formed at 436–439 Ma. The zircons from the quartz diorite and quartz diorite porphyry mainly yield high εHf(t) values (12.2–16.2) and very young Hf model ages (TDM = 0.41–0.56 Ga; TDMC = 0.49–0.56 Ga), which are consistent with a depleted mantle origin. The geochemical characteristics of the granites and quartz diorites from the Kalatag pluton and diorite porphyry from the Meiling ore district exhibit typical subduction‐related features such as enrichment in LILEs and LREEs and depletion in HFSEs. These diorites have moderate Mg#(30–47), positive εNd(t) values (4.2–5.6), and young Nd model ages (0.62–0.83 Ga), which are consistent with a depleted mantle origin that is accompanied by limited crustal assimilation. Based on the regional geology and geochemical evidence and previous studies, the Ordovician–Carboniferous rocks from the Kalatag area are considered to have formed in an island‐arc setting and were probably formed by a subduction event.

Carboniferous ridge subduction in the Xingmeng Orogenic Belt: Constraints from geochronological, geochemical, and Sr-Nd-Hf isotopic analysis of strongly peraluminous granites and gabbro-diorites in the Xilinhot micro-continent

The Xingmeng Orogenic Belt evolved through a long-lived orogeny involving multiple episodes of subduction and accretion. However, there is a debate on its tectonic evolution during the Late Paleozoic. Here, we report geochemical, geochronological, and isotopic data from strongly peraluminous granites and gabbro-diorites from the Sunidzuoqi–Xilinhot region. Zircon U–Pb ages suggest that the intrusive rocks were emplaced during the Early Carboniferous (333–322 Ma). The granites exhibit geochemical characteristics similar to S-type granites, with high SiO2 (72.34–76.53 wt.%), Al2O3 (12.45–14.65 wt.%), and A/CNK (1.07–1.16), but depleted Sr, Nb, and Ta contents. They exhibit positive εNd(t) and εHf(t) values (−0.3 to 2.8 and 2.7–5.7, respectively) and young Nd and Hf model ages (TDM2(Nd)=853–1110 Ma and TDM2(Hf)=975–1184 Ma), suggesting that they may be the partial melting products of heterogeneous sources with variable proportions of pelite, psammite, and metabasaltic rocks. The meta-gabbro-diorites from the Maihantaolegai pluton have low SiO2 (47.06–53.49 wt.%) and K2O (0.04–0.99 wt.%) contents, and demonstrate slight light rare earth element (REE) depletion in the chondrite-normalized REE diagrams. They have high zircon εHf(t) values (14.41–17.34) and young Hf model ages (TDM2(Hf)= 230–418 Ma), indicating a more depleted mantle source. The variations of the Sm/Yb and La/Sm ratios can thus be used to assess the melting degree of the mantle source from 5% to 20%, suggesting a quite shallow mantle melting zone. We propose that the petrogenesis and distribution of the strongly peraluminous granites and gabbro-diorites, as well as the tectonic architecture of the region, can be explained by a ridge subduction model. Based on these results, and previous studies, we suggest a southward ridge subduction model for the Sunidzuoqi–Xilinhot region.

Silurian alkaline magmatism in the Saur area, northern West Junggar: Evidence for the Middle Palaeozoic amalgamation of the Kazakhstan Block at the south‐west of the Central Asian Orogenic Belt

Although it is generally accepted that the Kazakhstan Block is an assemblage of Precambrian microcontinents and Early Palaeozoic island arcs, the details and timing constraint of the collage process need to be precisely studied in local regions. Thus, we conducted combined studies of geochemistry, zircon U–Pb geochronology and Sr–Nd–Hf isotopes upon the Awusiqi alkali‐feldspar granite and volcanic rocks from the Saur area in northern West Junggar. The Awusiqi volcanic rocks are mainly composed of andesites, rhyolites, and pyroclastic rocks. The alkali‐feldspar granite directly intrudes into the surrounding volcanic strata. Zircon U–Pb ages of the Awusiqi andesites, rhyolites, and alkali‐feldspar granite are 431 ± 3 Ma, 428 ± 5 Ma, and 426 ± 6 Ma, respectively. These new age data imply for the existence of Silurian magmatism in the Saur area. The alkali‐feldspar granite and rhyolites with high silica and alkali contents belong to metaluminous to weakly peraluminous and alkaline series and show seagull‐type REE patterns with strongly negative Eu anomalies. They are relatively enriched in the light rare earth elements (LREEs), K, Rb, Ba, Cs and other large‐ionic lithophile elements (LILEs), and are poor in P, Sr, and Eu, geochemically similar to those of A2‐type granites. The alkali‐feldspar granites have positive zircon εHf(t) values varying from +11.03 to +16.87 and young Hf model ages (TDM2 = 686 – 425 Ma). These features suggest that the Awusiqi alkali‐feldspar granite and rhyolites (except sample 18XJSF22C) were generated by partial melting of the juvenile lower crust with fractional crystallization, whereas sample 18XJSF22C shows mantle‐derived features. Awusiqi andesites show typical sanukitic high‐Mg andesites characteristics(Mg# = 48.08 – 65.52). The andesites are enriched in LREEs and LILEs, and are depleted in Nb, Ta, and Ti. They also have relatively high initial 87Sr/86Sr ratios (0.70543 – 0.70547), positive εNd(t) values (+2.5 – +5.7), and ancient Nd model ages (TDM1 = 1,093 – 815 Ma). These features suggest the andesites were likely derived from partial melting of a depleted mantle source previously metasomatized by slab‐derived fluids and melts. The discovery of Silurian magmatism in the Saur area has further confirmed the existence of Early Palaeozoic Tarbagtay arc basement and widespread Middle Palaeozoic A2‐type granitoids. We favor a post‐collisional setting in the Silurian in northern West Junggar, which also is the local embodiment of the Middle Palaeozoic amalgamation of the Kazakhstan Block.

The Kamusite A2-type granites in the Karamaili tectonic belt, Xinjiang (NW China): tracing staged postcollisional delamination in the eastern Junggar

AbstractThe Kamusite pluton is located in the eastern Junggar area, the westernmost segment of the Karamaili structural belt, and is predominantly composed of medium granite and microgranite with an exposure area of 30 km2. The U–Pb zircon geochronology of the Kamusite granites indicates that they crystallized in the late Carboniferous period (328–321 Ma). These granites exhibit high contents of SiO2 (76.09–77.85 wt %) and K2O + Na2O (8.01–9.06 wt %), but low MgO (0.01–0.14 wt %), CaO (0.07–0.32 wt %) and TiO2 (0.01–0.13 wt %) contents, showing alkalic–calcic, weakly peraluminous and ferroan features. They are depleted in Ba, Sr, Ti and P and enriched in Rb and some high-field-strength elements (Hf, Nd, Ta and Y); their rare earth element patterns are slightly right-leaning with strongly negative Eu anomalies, high 10 000 * Ga/Al (3.4–5.36, >2.6) and especially high Y/Nb ratios (1.61–10.33, >1.2), showing the geochemical characteristics of A2-type granite. These granites were produced by partial melting in a high-temperature, low-pressure, reduced and anhydrous environment and experienced extensive fractional crystallization, which concomitantly resulted in tin mineralization. Combining the high positive zircon ϵHf(t) values of +10.9 to +15.76 with young Hf (TDM2) model ages (638–330 Ma), it can be suggested that underplating-related mantle-derived materials were the original source of the Kamusite A2-type granites; namely, these granites formed by the partial melting of juvenile crust. The record of large-scale magmatism indicates that the whole tectonic belt was in a postcollisional extensional setting induced by staged delamination from west to east during the late Carboniferous to early Permian periods.

Geology and genesis of the Cihai mafic intrusions in Beishan Terrane, Xinjiang, Northwest China: Implication for iron mineralization and tectonic setting

The Cihai Fe deposit (100 Mt @ 48% Fe) represents an economically important iron deposit in Beishan Terrane, on the southern margin of the Central Asian Orogenic Belt, Xinjiang, northwest China. Many Late Carboniferous to Permian Fe(-Cu) deposits (e.g., Yamansu, Shaquanzi, and Weiya) are also present in adjacent Eastern Tianshan orogenic belt. The Cihai deposit is hosted in mafic dykes (i.e., diabase) and in the contact zone between the mafic intrusion and Mesoproterozoic schist. New zircon U–Pb isotope dating in this study gives a crystallization age of 306.9 ± 3.2 Ma for the ore-bearing diabase, which is the first age-constrained Late Carboniferous mafic magmatic activity at Cihai district. In combination with previous Ar–Ar amphibole dating (281.9 ± 2.2 Ma) for the massive ores, it is suggested that the main iron mineralization and associated mafic magmatic activities occurred at ca. 306–282 Ma, coeval with the large-scale iron mineralization epoch (ca. 324–280 Ma) in Eastern Tianshan. The Cihai diabase samples are LILEs-rich (e.g., K, Rb, and Sr) and HFSEs-depleted (e.g., Nb, Ta, and Ti) calc-alkaline series, and exhibit high Mg# values (51–63), as well as low compatible element concentrations (Ni = 14–53 ppm; Co = 18–43 ppm; and Cr = 26–219 ppm), indicating a mantle wedge source. They have elevated Ba/La and Sr/Nd values with limited Th/Yb ratios (<1), and display slightly Eu anomalies (0.90–1.19). Isotopically, they are characterized by positive εHf(t) values (+13.0 to + 16.1) and young Hf model ages (300 to 420 Ma), which are broadly similar to the isotopic compositions of the depleted mantle and within the ranges of most other mafic–ultramafic intrusions in Beishan-Tianshan orogen. These features suggest that the Cihai diabase intrusions were most likely formed by partial melting of a mantle wedge with some degree of metasomatism by slab-derived fluids. Combined our new geochemical and isotopic results with previous regional investigations, we suggest that the earliest generation of mafic dykes formed in an arc setting related to southward subduction of the paleo-Tianshan Ocean at ca. 306 Ma, and the main iron mineralization and intensive mafic magmatism occurred in a subsequent post-collisional regime during the Early Permian.

Origin of Paleozoic granitoids in the Yuhai Cu–Mo deposit, Eastern Tianshan, NW China and implications for regional metallogeny

The Yuhai porphyry Cu–Mo deposit is located at the eastern section of the Dananhu–Tousuquan Arc in Eastern Tianshan, NW China. Cu–Mo mineralization generally occurs as disseminations and veins in potassic and phyllic alteration zones, and is predominantly hosted in the Yuhai quartz diorite, granodiorite and minor syenogranite intrusions. New LA–ICP–MS zircon U–Pb dating indicates that the acidic syenogranite was emplaced at 318–320 Ma, younger than the intermediate rocks (i.e., quartz diorite and granodiorite) with Silurian ages. The acidic to intermediate intrusions exhibit geochemical affinities with normal arc rocks (e.g. high Y and Yb contents, and low Sr/Y ratios), in contrast with the subducted slab-derived adakites hosting large porphyry Cu deposits (e.g. Tuwu–Yandong) in the middle section of the Dananhu–Tousuquan Arc. The Yuhai acidic syenogranite rocks have high SiO2 (≥74.90 wt%), and K2O (≥3.48 wt%), relatively low Mg# and compatible element contents (e.g. Cr = 0.65–3.37 ppm; Ni = 0.63–1.02 ppm), and strong depletions in HFSE (e.g. Nb, Ta, and Ti). These geochemical features, coupled with isotopic data such as low initial 87Sr/86Sr (≤0.7033), positive εNd(t) (4.0 to 5.9) and εHf(t) (≥11) values, young Hf model ages, and variable 206Pb/204Pb ratios (18.549 to 19.465) reported in this study, suggest that their parental magmas originated from the partial melting of a juvenile lower crust and mixed with some old crustal components beneath the Dananhu–Tousuquan Arc. In contrast, the Yuhai intermediate intrusions exhibit relatively low SiO2 and K2O contents, high Mg# values (40–56), and a wide range of εHf(t) values, suggesting a source of sub-continental lithospheric mantle metasomatized by slab-derived fluids and melts. Zircon trace element and plagioclase compositions of the Yuhai granitoids further imply that the Silurian intermediate intrusion-related magma was moderately oxidized (mostly EuN/EuN* > 0.6) and water-rich (~9 wt%), favorable for the copper mineralization, whereas the Carboniferous syenogranite-related magma was weakly oxidized (mostly EuN/EuN* = 0.3–0.6) but also hydrous (~6 wt%). Combined with the regional tectono-magmatic activities and metallogenic processes, we suggest that the Yuhai Cu–Mo deposit and associated granitoids were formed in an arc setting, and that the magma nature and its source components are the key factors constraining the distribution, scale and style of the porphyry copper mineralization in Eastern Tianshan.

Slab roll-back and crustal growth in the Eastern Junggar terrane, NW China: evidence from Carboniferous A-type granitoids and adakitic rocks

ABSTRACT Revealing the formation mechanism of granitic plutons is vital for understanding crustal growth of Earth. Here, we present an integrated study of geochronological, elemental, whole rock Nd and zircon Hf isotopic data for a granitic pluton system that formed in the Eastern Junggar, NW China. Zircon U–Pb dating suggests the seven granitic plutons were emplaced at 356–312 Ma. The monzogranites have high alkaline and Zr contents, and high Ga/Al and TFe2O3/MgO ratios, which are geochemically similar to A-type granitoids. They display positive ɛNd(t) (+4.3 to +5.4) and ɛHf(t) (+7.6 to +12.4) values, and young Nd and Hf model ages, suggesting a lower arc crust source. The quartz monzodiorites have high Sr, low Y and Yb contents, yielding relatively high Sr/Y ratios that are similar to modern adakites. These rocks also have high positive ɛNd(t) (+7.6 to +8.0) and ɛHf(t) (+13.9 to +15.5) values, and young Nd and Hf model ages, indicating a subducted oceanic crustal source. In combination with data from other plutons in this area, we suggest that they were likely generated by roll-back of the subducted Junggar oceanic slab, which resulted in back-arc extension, asthenosphere upwelling, and partial melting of oceanic crust and lower arc crust.

Petrogenesis of early Cretaceous granitoids in the southern Great Xing’an Range, NE China: Constraints from the Haliheiba pluton

Early Cretaceous granitoids are widespread in the southern Great Xing’an Range. However, the petrogenesis of these rocks is poorly understood. We therefore present field investigations, petrological observations, zircon U–Pb ages, whole-rock geochemistry and Hf isotope compositions for the Haliheiba pluton. The pluton comprises fine-grained biotite granite, medium- to coarse-grained biotite granite and porphyritic biotite granite, and mafic microgranular enclaves (MMEs) are unevenly distributed within the host granites. Zircon LA-ICP-MS U–Pb dating yields emplacement ages of ∼139 Ma for the three units. The host granites are characterized by high SiO2 contents (73.16–76.55 wt%) and alkali concentrations (7.92–8.62 wt%), enrichment in light rare earth elements (LREEs), Rb, U, Th, K, and Pb, depletion in heavy rare earth elements (HREEs), Sr, Ba, Ti, and P, and intensely negative Eu anomalies with δEu values from 0.14 to 0.47. The zircons in the host granites yield positive εHf(t) values of +5.0 to +11.2 and fairly young Hf model ages of 408 Ma to 731 Ma. These results indicate that the host granites have high-K calc-alkaline, metaluminous to weakly peraluminous A-type affinity and were mainly derived from partial melting of juvenile crustal materials originated from the depleted mantle, and fractional crystallization was involved in magma evolution. The MMEs have SiO2 contents of 63.45–64.14 wt% and have higher MgO, CaO, tFe2O3 and TiO2 contents than the host granites. The MMEs are enriched in LREEs relative to HREEs with slightly negative Eu anomalies (δEu values of 0.77–0.79) and are slightly enriched in Rb, U, Th, K, Sr, Pb and strongly to slightly depleted in Nb, Ta, Ti and P. These results, together with field investigations and petrological observations, indicate that crust-mantle magma mixing played an important role during the petrogenesis. Furthermore, the MMEs were intermediate members after magma mixing between mafic end-member magmas derived from lithospheric mantle and felsic end-member magmas derived from juvenile crustal materials. Combined with regional geology, our results suggest that a substantial crustal accretion in the southern Great Xing’an Range that occurred during the Neoproterozoic–Carboniferous, and the formation of the Early Cretaceous granitoids in this region was closely related to the lithospheric delamination triggered by the subduction of the Paleo-Pacific Ocean beneath Eurasia.

Late Ordovician to early Silurian calc-alkaline magmatism in the Xiemisitai Mountains, northern West Junggar: a response to the subduction of the Junggar-Balkhash Ocean

ABSTRACTThe Early Palaeozoic tectonic evolution of northern West Junggar is essential for understanding the tectonic framework and accretionary processes in the southwestern segment of the Central Asian Orogenic Belt. However, its evolutionary history is controversial due to insufficient pre-late Silurian magmatic data. In order to clarify this issue, we conducted geochronological, geochemical, and Sr-Nd-Hf isotope analyses on samples from the Dengdeer and Maodun complexes in the Xiemisitai Mountains of Boshchekul-Chingiz (BC) arc, northern West Junggar. LA-ICP-MS zircon U-Pb dating yielded ages ranging from 445 Ma to 429 Ma, revealing the Late Ordovician to early Silurian magmatic events in northern West Junggar. The dioritic and metaluminous Dengdeer complex has high Mg# (49.5–52.9), moderate La/Yb (11.0–12.3), and low initial 87Sr/86Sr (0.70324–0.70410) and εNd(t) (+3.7 to +4.5). This complex exhibits variable zircon εHf(t) (+11.31 to +17.62), young Hf model ages (470–440 Ma), and appears to have been produced by differentiation of melts from metasomatized mantle. The intermediate to felsic, metaluminous to weakly peraluminous Maodun complex has abundant enclaves and inherited zircons. This complex seems to have fractionated from mantle- and crustal-derived magma mixing with some crustal contamination, based on its high Mg# (36.6–49.5), low Sr/Y (8.7–29.6), Nb/Ta (8.3–13.9), initial 87Sr/86Sr (0.70377) and εNd(t) (+4.0 to +4.2), and variable zircon εHf(t) (+11.59 to +15.21). The Dengdeer and Maodun samples are high-K calc-alkaline, rich in hydrous minerals, Rb, Th, and LREEs, depleted in Nb, Ta, and Ti, and show decoupling Hf-Nd isotopic signatures. These characteristics indicate that their source had been modified by melts from slab sediments. After combining our data with previous observations, we conclude that a trench-ward migration of an arc-back-arc system occurred during the Late Ordovician to early Silurian in northern West Junggar, while the Dengdeer and Maodun complexes were related to subduction-accretionary processes of the Junggar-Balkhash oceanic slab beneath the BC arc.

Geochemistry, in-situ Sr-Nd-Hf-O isotopes, and mineralogical constraints on origin and magmatic-hydrothermal evolution of the Yulong porphyry Cu[sbnd]Mo deposit, Eastern Tibet

The giant Yulong porphyry CuMo deposit was formed in postsubduction setting in eastern Tibet. Origin of the ore-related Yulong intrusion remains a matter of debate. This study presents new whole-rock major and trace element geochemistry, in-situ apatite SrNd and zircon HfO isotopes, and mineralogical chemistry of the Yulong intrusion. Least-altered samples from the Yulong intrusion have high SiO2 (66.3–69.5 wt%) and Al2O3 (14.9–15.5 wt%) contents, high La/Yb (36.4–68.0) and Sr/Y (46.0–76.3) ratios, and low MgO (0.63–1.24 wt%) and Cr (<30 ppm) contents, similar to adakitic rocks deriving from thick juvenile lower crust. They are enriched in large ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs), and show listric REE patterns. In-situ apatite SrNd isotopes show limited variations ((87Sr/86Sr)i = 0.7060–0.7068, εNd(t) = −4.8–0.2), which plot between Paleo-Tethys ocean-related arc magmas and the ancient crust in eastern Tibet. Zircon grains from this study and published studies have mostly positive yet variable εHf(t) values (−20.6 to +12.2) and young Hf model ages that overlap those of the Paleo-Tethys ocean-related arc magmas. The above Sr-Nd-Hf isotopes, together with the elevated zircon δ18O values (6.4 to 9.3‰) and arc-like trace element patterns, collectively suggest that the Yulong intrusion may have originated from partial melting of juvenile lower arc crust related to the subduction of the Paleo-Tethys ocean, with incorporation of a small amount of ancient crustal materials.Two generations of amphibole were recognized at Yulong. Their compositions are used to calculate crystallization depths, magmatic oxidation states, and water contents. The caculated results show that the early-stage euhedral high-Al (5.87–8.51 wt%) amphibole phenocrysts may have crystallized in the underlying magma chamber (7.1–12.5 km in depth), whereas the late-stage xenomorphic low-Al (3.47–4.87 wt%) amphibole grains may have crystallized in the porphyritic stock (4.0–5.6 km). Magmatic water contents decrease from early- (3.5–4.6 wt%) to late-stage (2.8–3.5 wt%) amphibole, which is interpreted to indicate fluid exsolution from the magma chamber during emplacement of the Yulong intrusion. Caculated oxidation states increase from early- (ΔNNO = 0.6–1.5) to late-stage (ΔNNO = 1.9–2.3) amphibole. Plagioclase phenocrysts show periodic or reverse core-to-rim zonation of An contents (range up to 25 mol%), which are coupled by FeO contents, probably suggesting magma recharge events. Collectively, we propose that the magma chamber beneath Yulong was recharged by a less evolved magma, and was saturated in fluids to produce intensive alteration and mineralization. The relatively high oxidation states allow the metals to be enriched in the evolving magma, and to be deposited in the hydrothermal alteration stage.

Sources of Sediment Clasts and Depositional Environment of the Upper Paleozoic Shazagaitui and Zhipkhoshi Formations of the Chiron Basin, Eastern Transbaikalia

The article presents the results of comprehensive geological and geochemical study of the sedimentary rocks of the Upper Paleozoic Shazagaitui and Zhipkhoshi formations filling the upper part of the Chiron Basin, as well as U–Pb geochronological and Lu–Hf isotopic studies of zircons from these formations. The geochemical features of the terrigenous rocks of the Shazagaitui and Zhipkhoshi formations indicate that they were formed in a subduction zone environment. The presence of conglomerates and gravelstones and the predominance of unrounded and subrounded fragments in the rocks of the Shazagaitui and Zhipkhoshi formations also indicate their sedimentation in tectonically active zones. The abundant detrital zircons of Paleoproterozoic age, as well as the Paleoproterozoic and Archean Hf model ages of most of the zircons in the sandstones of the Shazagaitui and Zhipkhoshi formations, suggest that the main source of sediment clasts for these formations are the magmatic and metamorphic rocks of the southern surrounding of the North Asian Craton. However, the presence of Devonian–Carboniferous zircons with relatively young (mostly Neoproterozoic) Hf model ages is evidence that these zircons in the sedimentation basin were derived through erosion of mature (?) island arcs, but the contribution of this source is minor. The obtained results in combination with our regional geological data from previous studies suggest that the sediments of the Shazagaitui and Zhipkhoshi formations were accumulated in a basin setting on the southern framing of the North Asian Craton facing the Paleozoic Ocean.

Co-development of Jurassic I-type and A-type granites in southern Hunan, South China: Dual control by plate subduction and intraplate mantle upwelling

Two types of spatially and temporally associated Jurassic granitic rocks, I-type and A-type, occur as pluton pairs in several locations in southern Hunan Province, South China. This paper aims to investigate the genetic relationships and tectonic mechanisms of the co-development of distinct granitic rocks through petrological, geochemical and geochronological studies. Zircon LA-ICPMS dating results yielded concordant U–Pb ages ranging from 180 to 148 Ma for the Baoshan and Tongshanling I-type granodiorites, and from 180 to 158 Ma for the counterpart Huangshaping and Tuling A-type granites. Petrologically, the I-type granodiorites consist of mafic minerals such as hornblende whereas the A-type granites are dominated by felsic minerals (e.g., quartz, K-feldspar and plagioclase). Major and trace element analyses indicate that the I-type granodiorites have relatively low SiO2 (64.5–71.0%) and relatively high TiO2 (0.28–0.51%), Al2O3 (13.8–15.5%), total FeO (2.3–4.7%), MgO (1.3–2.6%) and P2O5 (0.10–0.23%) contents, and the A-type granites are characterized by high concentrations of Rb (212–1499 ppm), Th (18.3–52.6 ppm), U (11.8–33.6 ppm), Ga (20.0–36.6 ppm), Y (27.1–134.0 ppm) and HREE (20.3–70.0 ppm), with pronounced negative Eu anomalies (Eu/Eu* = 0.01–0.15). Moreover, the I-type granodiorites are classified as collision-related granites emplaced under a compressional environment, whereas the A-type granites are within-plate granites generated in an extensional setting. Zircon Hf isotopic compositions vary substantially for these granitic rocks. The I-type granodiorites are characterized by relatively young Hf model ages (TDM1 = 1065–1302 Ma, TDMC =1589–2061 Ma) and moderately negative εHf(t) values (–5.9 to –11.5), whereas the A-type granites have very old model ages (TDM1 = 1454–2215 Ma, TDMC = 2211–2974 Ma) and pronounced negative εHf(t) values (–15.8 to –28.3). These petrochemical and isotopic characteristics indicate that the I-type granodiorites may have been derived from a deep source involving mantle-derived juvenile (basaltic) and crustal (pelitic) components, whereas the A-type granites may have been sourced from melting of meta-greywacke in the crust. This study proposes that the pressure and temperature differences in the source regions caused by combined effects of intra-plate mantle upwelling and plate subduction are the major controlling factors of the co-development of the two different types of magmas. Crustal anatexis related to lithospheric delamination and upwelling of hot asthenosphere under a high pressure and temperature environment led to the formation of the I-type magmas. On the other hand, the A-type magmas were formed from melting of the shallower part of the crust, where extensional stress was dominant and mantle-crust interaction was relatively weak. Rifts and faults caused by mantle upwelling developed from surface to depth and successively became channels for the ascending I- and A-type magmas, resulting in the emplacement of magmas in adjacent areas from sources at different depths.

Petrogenesis and Ore Genesis of the Late Yanshanian Granites and Associated Porphyry-Skarn W-Mo Deposits from the Yunkai Area of South China: Evidence from the Zircon U-Pb Ages, Hf Isotopes and Sulfide S-Fe Isotopes

There are a wide range of magmatism and mineralization in the Yunkai area of South China during the Late Yanshanian Period, including the newly discovered Michang, Youmapo, Sanchachong and Songwang porphyry-skarn W-Mo deposits. In this study, we obtained zircon U-Pb ages of the ore-bearing biotite granites and their mafic enclaves from 88±1 to 110±1 Ma. Zircons from the granites show Hf isotopic compositions with negative eHf(t) values of -5.9 to -0.6 and calculated Hf model ages (TDM2) of 1.5-1.2 Ga; indicating that the Middle Proterozoic crustal materials may have provided an important source for the magmatic rocks in this district during the Late Yanshanian Period, whereas zircons from the mafic enclaves show positive eHf(t) values of 1.3 to 10.1 with younger Hf model ages (TDM2) of 0.5-1.1 Ga, suggesting a mantle component may have involved in the granitic magma generation. Sulfur isotope study of the sulfide minerals from the W-Mo deposits show a narrow δ34S distribution with most data ranging from -4.2‰ to 5.2‰. In addition, this study reports the first Fe isotopic compositions of pyrite in the W-Mo deposits, which show a uniform distribution range with the values near zero (δ56Fe=0.16‰-0.58‰, average 0.35‰; δ57Fe=0.02‰-0.54‰, average 0.48‰). These data indicate that the ore-forming materials may come from the deep-sourced granitic magma, and the mineralizations show a close relationship with the granitic magmatism during the Late Yanshanian Pe-riod. Combining with previous results, we suggest that there is a widespread porphyry-skarn W-Mo mineralization in the Yunkai area during the Late Cretaceous (80-110 Ma), which has a close relation-ship with the Late Yanshanian magmatism that may have formed during the rollback of the subducted Pacific Plate.

Geochemistry and zircon U-Pb-Hf isotopes of Paleozoic intrusive rocks in the Damao area in Inner Mongolia, northern China: Implications for the tectonic evolution of the Bainaimiao arc

ABSTRACT The tectonic evolution of the Central Asian Orogenic Belt (CAOB) is related to the subduction and closure of the Paleo-Asian Ocean (PAO). Considerable controversy still surrounds whether, when and how the PAO subducted underneath the northern margin of the North China Craton (NCC). This study carries out whole rock geochemical and zircon U-Pb-Hf isotopic investigations on Paleozoic intrusive rocks in both the Bainaimiao arc and the northern margin of the NCC. Zircon U Pb dating of gabbroic diorite and diorite in the Bainaimiao arc and the granite and diorite at the northern margin of the NCC yielded crystallization ages of 458 ± 2 Ma, 428 ± 2 Ma, 326 ± 2 Ma, and 258 ± 2 Ma respectively. In the Bainaimiao arc, the middle Silurian diorite as the main body of the arc-related intrusive rocks there show geochemical characteristics of continental margin arc magma with negartive ƐHf(t) values and old Hf model ages. On the other hand, the middle Ordovician gabbroic diorite mainly shows typical calc-alkaline continental arc geochemical characteristics with positive ƐHf(t) values and relative young Hf model ages, indicative of the southward-propagating reworking and arc-accretion processes. Thus, we interpret these rocks as a consequence of southward subduction of the PAO beneath the Bainaimiao arc, here termed as Bainaimiao Arc terrane, BAT. The early Carboniferous granite and late Permian diorite at the northern margin of the NCC show geochemical characteristics similar to post-collisional adakites and typical continental arc rocks, respectively. Combined with previous work, the early Carboniferous post-collisional adakite shows limited distribution and corresponds to the subsequent effect of the amalgamation between the BAT and the NCC. Whereas most of the Carboniferous-Permian magmatic rocks (ca. 342–258 Ma) at the northern margin of the NCC are arc-related and resulted from the continued effect of the southward subduction of the PAO. Moreover, the southward subduction in the early Paleozoic was transformed from relative high angle subduction beneath the BAT to shallow subduction beneath the unity of the NCC and the BAT in the late Paleozoic, which lasted until the late Permian (ca. 258 Ma).

Zircon U–Pb geochronology, Hf isotopes and geochemistry of intrusive rocks in the Gazu copper deposit, Iran: Petrogenesis and geological implications

The Gazu Cu deposit is located at the boundary between the Lut and Tabas Blocks in Central Iranian Microcontinent. The intrusions in the Gazu area are characterized by SiO2 contents ranging from 52.6 to 66.4wt.%, K2O from 1.98 to 3.64wt.%, and Na2O from 2.5 to 5.2wt.%. The intrusions have features typical of high-K calc-alkaline rocks, are metaluminous to weakly peraluminous, and formed in a volcanic arc setting. Zircon U–Pb age data for hornblende monzonite porphyry, hornblende monzodiorite porphyry, pyroxene monzonite, hornblende granite, hornblende quartz monzonite porphyry, and hornblende diorite porphyry are 68.1±1.7Ma, 67.74±0.72Ma, 67.41±0.94Ma, 65.9±3.7Ma, 65.1±1.2Ma, and 60.2±1.2Ma, respectively. Copper mineralization is related to the hornblende granite and hornblende quartz monzonite porphyry. Field observations and the U–Pb dating indicated three stages for intrusive rock emplacement: pre-mineralization intrusions (hornblende monzonite porphyry, hornblende monzodiorite porphyry, and pyroxene monzonite), syn-mineralization intrusions (hornblende granite and hornblende quartz monzonite porphyry), and post-mineralization intrusions (hornblende diorite porphyry). Mantle-normalized, trace-element spider diagrams display enrichment in large ion lithophile elements, such as Rb, Ba, K, and Cs, and depletion in high field strength elements, e.g., Nb, Ti, Zr, Y, and heavy rare earth elements, with light rare earth element enrichment ((La/Yb)N=6.4–13.4) and a weak negative Eu anomaly (Eu/Eu*=0.87–1.01). Zircon Hf isotope data indicate that the Gazu intrusive rocks are characterized by low positive εHf(t) values, with an average of 4.23, and show young Hf model ages (789–188Ma), indicating derivation from juvenile crust. Relatively low positive εHf(t) values, the wide range for εHf(t), and the presence of zircon xenocrysts can be explained by involvement of a mantle component that mixed with older continental crust (Cadomian).

A genetic linkage between subduction- and collision-related porphyry Cu deposits in continental collision zones

The genesis of continental collision-related porphyry Cu deposits (PCDs) remains controversial. The most common hypothesis links their genesis with magmas derived from subduction-modified arc lithosphere. However, it is unclear whether a genetic linkage exists between collision- and subduction-related PCDs. Here, we studied Jurassic subduction-related Cu-Au and Miocene collision-related Cu-Mo porphyry deposits in south Tibet. The Jurassic PCDs occur only in the western segment of the Jurassic arc, which has depleted mantle-like isotopic compositions [e.g., ( 87 Sr/ 86 Sr) i = 0.7041–0.7048; e Nd(t) as high as 7.5, and e Hf(t) as high as 18]. By contrast, no Jurassic PCDs have been found in the eastern arc segment, which is isotopically less juvenile [e.g., ( 87 Sr/ 86 Sr) i = 0.7041–0.7063, e Nd(t) < 4.5, and e Hf(t) ≤ 12]. These results imply that incorporation of crustal components during underplating of Jurassic magma induced copper accumulation as sulfides at the base of the eastern Jurassic arc, inhibiting PCD formation at this time. Miocene PCDs are spatially confined to the Jurassic arc, and the giant Miocene PCDs cluster in its eastern segment where no Jurassic PCDs occur. This suggests that the arc segment barren for subduction-related PCDs could be fertile for collision-related PCDs. Miocene ore-forming porphyries have young Hf model ages and Sr-Nd-Hf isotopic compositions overlapping with those of the Jurassic rocks in the eastern segment, whereas contemporaneous barren porphyries outside the Jurassic arc have abundant zircon inheritance and crustlike Sr-Nd-Hf isotopic compositions. These data suggest that remelting of the lower crustal sulfide-bearing Cu-rich Jurassic cumulates, triggered by Cenozoic crustal thickening and/or subsequent slab break-off, led to the formation of giant Miocene PCDs. The spatial overlap and complementary metal endowment between subduction- and collision-related magmas may be used to evaluate the mineral potential for such deposits in other orogenic belts.