Melting Of Ancient Lower Crust Research Articles

Apatite geochemistry as a proxy for porphyry-skarn Cu genesis: a case study from the Sanjiang region of SW China

In view of the importance of the magmatic oxidation state and volatile composition (H2O-S-Cl) in porphyry Cu mineralization, identification of magmatic information has become the basis for the evaluation of the Cu-mineralization potential of porphyries. Apatite, as a common accessory mineral in porphyry, is recognized as an effective petrogenetic and metallogenic indicator. In this study, we analyze the in situ major and trace elements and Nd isotope content of apatite from four granitic plutons in the Yidun Terrane, SW China, in order to provide new insights into the identification of the magmatic features of Cu-mineralized plutons. The plutons investigated in this study are a Cu-mineralized pluton (Hongshan) and three non-mineralized plutons (Cilincuo, Rongyicuo, and Hagela). Chemical composition and textural analysis indicate that the apatites are all of magmatic origin and have not been altered. Our results show that apatite δCe values rather than δEu and Ga values are more valid indicators of the magma oxidation state of the four plutons. Based on apatite δCe values and zircon compositions, the parental magma of the Hongshan pluton is identified as having been more oxidized than those of the other three plutons. Moreover, the higher Cl content (0.06–0.23 wt%) in the Hongshan apatite compared to that in apatite from the other three plutons (0.01–0.09 wt%), indicates that the parental magma of the former pluton was more enriched in Cl. Apatite from the Hongshan pluton contains more SO3 (average 0.05 wt%) than that from the other three plutons (SO3 content mostly lower than the detection limit of electron probe micro-analysis), which may be related to higher magmatic oxygen fugacity of the Hongshan pluton. In addition, the Nd isotopic composition of apatite [εNd(t): −13.6−5.9] demonstrates that the Cu-mineralized and non-mineralized plutons were derived from the melting of ancient lower crust. Furthermore, based on the geochemical signature of adakite revealed by apatite Sr/Y and δEu values, the Hongshan pluton can be considered to have been produced as the result of partial melting of the thickened lower crust. Finally, an index system consisting of apatite δCe, Sr/Y, and F/Cl values is established to distinguish between the Cu-mineralized and non-mineralized plutons.

Petrogenesis, W metallogenic and tectonic implications of granitic intrusions in the southern Great Xing’an Range W belt, NE China: insights from the Narenwula Complex

AbstractExtensive magmatism in NE China, eastern Central Asian Orogenic Belt, has produced multi-stage granitic plutons and accompanying W mineralization. The Narenwula complex in the southwestern Great Xing’an Range provides important insights into the petrogenesis, geodynamic processes and relationship with W mineralization. The complex comprises granodiorites, monzogranites and granite porphyry. Mafic microgranular enclaves are common in the granodiorites, and have similar zircon U–Pb ages as their host rocks (258.5–253.9 Ma), whereas the W-bearing granitoids yield emplacement ages of 149.8–148.1 Ma. Permian granodiorites are I-type granites that are enriched in large-ion lithophile elements and light rare earth elements, and depleted in high field strength elements and heavy rare earth elements. Both the mafic microgranular enclaves and granodiorites have nearly identical zircon Hf isotopic compositions. The results suggest that the mafic microgranular enclaves and granodiorites formed by the mixing of mafic and felsic magmas. W-bearing granitoids are highly fractionated A-type granites, enriched in Rb, Th, U and Pb, and depleted in Ba, Sr, P, Ti and Eu. They have higher W concentrations and Rb/Sr ratios, and lower Nb/Ta, Zr/Hf and K/Rb ratios than the W-barren granodiorites. These data and negative ϵHf(t) values (–6.0 to –2.1) suggest that they were derived from the partial melting of ancient lower crust and subsequently underwent extreme fractional crystallization. Based on the regional geology, we propose that the granodiorites were generated in a volcanic arc setting related to the subduction of the Palaeo-Asian Ocean, whereas the W-bearing granitoids and associated deposits formed in a post-orogenic extensional setting controlled by the Mongol–Okhotsk Ocean and Palaeo-Pacific Ocean tectonic regimes.

Genesis and ore-forming process of the Benqu mesothermal gold deposit in the Jiapigou ore cluster, NE China: Constraints from geology, geochronology, fluid inclusions, and whole-rock and isotope geochemistry

The Jiapigou ore cluster (JOC) located on the northeastern margin of the North China Craton (NCC) is a major producer of gold in China and is famous for its quartz vein-type gold deposits. It currently contains a dozen gold deposits with total proven reserves of >180 t Au, among which the Benqu deposit is the earliest discovered and most representative. The deposit is hosted within Neoarchean granitic gneisses and metamorphosed volcanic-sedimentary sequences. Gold orebodies mainly occur as gold-bearing quartz veins controlled by NE–NEE-trending ductile–brittle faults. Hydrothermal mineralization can be divided into early (quartz–pyrite), main (quartz–polymetallic sulfide–native gold), and late (quartz–carbonate) stages, with gold precipitation occurring in the main stage. Three types of primary fluid inclusions (FIs) are identified in quartz formed at different stages: CO2–H2O–NaCl (C-type), H2O–NaCl (W-type) and pure CO2 (PC-type). The early-stage quartz contains C- and W-type FIs, which have homogenization temperatures of 269–298 °C and salinities of 8.8–16.7 wt% NaCl equiv. The main-stage quartz contains all three types of FIs, with homogenization temperatures of 197–268 °C and salinities of 5.1–15.2 wt% NaCl equiv. The late-stage quartz contains only W-type FIs with homogenization temperatures of 145–198 °C and salinities of 3.2–12.4 wt% NaCl equiv. The ore-forming fluid system evolved from a CO2–H2O–NaCl system in the early stage to a H2O-dominated system in the late stage. The addition of meteoric water led to fluid immiscibility, facilitating the precipitation of gold and associated sulfides. The H–O–S–Pb–Sr isotope results indicate that the ore-forming fluids were derived from magmatic water and that the ore-forming materials were mainly extracted from a deep magmatic reservoir sourced mostly from the lower crust, with trace mantle components. Rb–Sr dating of the gold-bearing sulfides yields an isochron age of 175.8 ± 4.6 Ma (MSWD = 1.9). Zircon U–Pb dating of the Benqu diorite yields a weighted mean 206Pb/238U age of 176.5 ± 2.4 Ma (MSWD = 1.2). The Benqu diorite is high-K calc-alkaline, with enriched LREEs and LILEs (e.g., Rb, Pb, and K) and depleted HFSEs (e.g., Nb, Ta, and Ti), which is typical of the geochemical signature of arc magmas. The zircons have negative εHf(t) values of −11.3 to −8.9, indicating strong involvement of old crustal materials. These data suggest that the primary magma originated mainly from the partial melting of ancient lower crust that mixed with minor underplated mantle-derived magma. The above results systematically reveal that the Benqu is a mesothermal magmatic–hydrothermal lode gold deposit that formed in an active continental margin environment in the late Early Jurassic. Comparative studies of Benqu and other gold deposits in the JOC confirm that a significant magmatic–hydrothermal event occurred during the conversion from compression to extension that followed the first subduction of the Paleo-Pacific Plate beneath the Eurasian Plate.

Geochronology, Geochemistry and Hf Isotope of the Late Mesozoic Granitoids from the Lushi Polymetal Mineralization Area: Implication for the Destruction of Southern North China Craton

The North China Craton (NCC) is the best example of an Archean craton that has lost its stability in the Late Mesozoic. Although the cratonic destruction is generally considered to have occurred in the Eastern Block and reached a peak in the Early Cretaceous, the exact areal extent of cratonic destruction is debated, especially the southern and northern margin of the NCC. Here we report geochronology, geochemical and Hf isotopic data of the Late Mesozoic granitoids from Lushi polymetal mineralization area (LPMA) in the southern margin of NCC. These results provide new insights into the destruction in the southern margin of the NCC during the Late Mesozoic. Zircon U-Pb dating indicates that eight granitoids intruded in the Late Jurassic to Early Cretaceous (136.8-154.1 Ma), respectively. Geochemical signatures define these granitoids being A-type or I-type granite that formed in an extension setting. In addition, Hf isotopic compositions of zircons from these granitoids vary in a relatively large range, with eHf(t) values and TDM2 ages ranginge from -26.1 to +15.2 and 215 to 2 849 Ma, respectively. The parental magmas were likely derived from diverse sources, including materials of the partial melting of ancient lower crust and mantle-derived mafic magmas in various proportions. Combining with previous studies on the contemporaneous magma-tectonic activities in circum of NCC, we suggest that the rim of NCC was already unstabilized from the Late Jurassic in the LPMA. The subduction of the Paleo-Pacific Plate was the main trigger to the destruction of the southern margin of NCC, which was responsible for the lithospheric extension and thinning, extensive magmatism and mineralization.

Petrogenesis and Mo prospecting significance of Sadaigoumen granites on the northern margin of the North China Craton

The Sadaigoumen porphyry Mo deposit lies on the northern margin of the North China Craton. Molybdenum mineralization occurs mainly as veins and veinlets, or is disseminated within alkali feldspar granite and granite porphyry, which yield weighted-mean 206Pb/238U ages (±2σ) of 243.7 ± 1.6 to 242.2 ± 2.0 Ma and 240.0 ± 1.7 Ma, respectively. ReOs dating of molybdenite from Stage 3 mineralization yields an isochron age of 238.0 ± 1.9 Ma. The Sadaigoumen ore-related granites have petrological, mineralogical, and geochemical characteristics of I-type granite. Ore-related granites exhibit low (87Sr/86Sr)i ratios of 0.710256–0.711463, negative εNd(t) values of −16.4 to −15.4 with corresponding TDM2 (Nd) ages of 2061–1870 Ma, and negative εHf(t) values of −21.2 to −11.2 with corresponding TDM2 (Hf) ages of 2614–1983 Ma, indicating that the primary magma was derived by partial melting of ancient lower crust. In chondrite- and primitive-mantle-normalized trace element diagrams, the Sadaigoumen ore-related granites are characterized by moderate enrichment of light rare earth elements (REE) relative to heavy REE, moderate to strong negative Nb, Ta, Zr, Hf, and Ti anomalies, and increasing Rb/Sr ratios with decreasing Sr content, all attributed to fractional crystallization. Fractionation of amphibole, muscovite, and possibly minor biotite and garnet (and/or zircon) produced the variations in most elements. Together with the regional geology, the data imply that the Sadaigoumen ore-related granites and corresponding mineralization formed in a syn-collision to post-collision transitional setting during the Early–Middle Triassic. These fractionated Triassic alkaline granites or syenogranites potentially contain new porphyry Mo deposits.

Magmatic and structural controls on the tonnage and metal associations of collision-related porphyry copper deposits in southern Tibet

Porphyry Cu deposits (PCDs) in continental collision settings are new targets for modern mineral exploration, and the relationships among locations, tectonic environments, tonnages, and metal associations are not yet understood. The Gangdese PCD belt formed in a typical continental collision setting between the Indian and Asian plates. In the present study, new elemental and isotopic data from fertile porphyries (Cu–Mo, Cu–Mo–Pb–Zn, and Cu–W–Mo) and barren coeval and pre-ore porphyries of the Gangdese PCD belt were combined with previously published data to provide new insights into the metallogenesis of PCDs in continental collision settings.Factors that contributed to the formation of collision-related PCDs in the Lhasa terrane, Tibet, include the nature of the magmatic sources, magmatic evolution during ascent through the crust, and regional tectonics. These PCDs were not formed by partial melting of ancient lower crust. Instead, attributed to the subduction of Neo-Tethyan oceanic slab, arc magmas and slab-derived fluids resulted in the accumulation of sulfides and hydrous minerals at the base of the continental crust. Partial melting of this refertilized juvenile lower crust produced oxidized water- and Cu-rich magmas with the potential to produce large PCDs, although only under favorable regional tectonic conditions. Regional compressional stress is not conducive to the formation of large magma chambers, so the collision-related adakite-like magmas that developed during the early stages of India–Asia collision commonly lack alteration and associated mineralization. In contrast, relatively large magma chambers can develop in extensional settings within the continental collision orogen, which facilitates the formation of large and giant PCDs. Small Cu–W–Mo deposits formed during the transition between compressional and extensional settings. Thus, the regional tectonic setting and structural framework of the host orogen influenced the tonnage of PCDs. The metal associations of collision-related PCDs in the Lhasa terrane were controlled mainly by the extent of assimilation of the upper crust and the degree of fractional crystallization of the ore-forming magmas. Lead–zinc mineralization in Cu–Mo deposits is attributed to the assimilation of metapelites by fertile magmas in the upper crust, and highly fractionated fertile magmas are required to produce W mineralization.

Reconciling Orogenic Drivers for the Evolution of the Bangong‐Nujiang Tethys During Middle‐Late Jurassic

AbstractIdentifying arc‐trench systems along with spatial and temporal variations in their record of tectono‐magmatic events is crucial for determining the orogenic divers and evolution of orogenic systems. New geochronological and geochemical data of Jurassic igneous rocks, as well as detrital zircon data from contemporaneous sedimentary units, within the eastern Bangong‐Nujiang suture in central Tibet indicate the existence of an approximately 1,200‐km Middle‐Late Jurassic magmatic arc system. This arc system can be divided into two distinct along‐strike segments, which are characterized by magmatic activity extending from 166 to 160 Ma in the east and 170–148 Ma in the west, followed by magmatic gaps at 160–120 and 148–125 Ma, respectively. An accretionary prism, magmatic arc, and retro‐arc sedimentary units are identified from south to north in the eastern segment. The 166–160 Ma arc includes high‐K calc‐alkaline granitoids, and high‐Mg andesites, dacites, and rhyolites, which collectively can be interpreted to originate from partial melting of ancient lower crust and mélange diapirs above a north dipping subduction zone. Our analysis reveals the existence of an overall compressional arc‐trench system along strike, which overlaps with a phase of 170–160 Ma ophiolite generation and a rock association of 160–148 Ma slab‐derived adakites and oceanic island basalt‐type rocks, and is followed by an overall magmatic gap during 148–125 Ma with subsequent 125–105 Ma extensive magmatism. We infer that these records may reflect sequential tectonic events, including subparallel ridge‐trench collision (170–160 Ma), slab window formation (160–148 Ma), subsequent subduction termination (148–125 Ma), and final Lhasa‐Qiangtang amalgamation (125–105 Ma).

Early Carboniferous High Ba‐Sr Granitoid in Southern Langshan of Northeastern Alxa: Implications for Accretionary Tectonics along the Southern Central Asian Orogenic Belt

Voluminous granitoids are widely distributed in the Langshan region, northeast of the Alxa block, and record the evolutionary processes of the southern Central Asian Orogenic Belt. The Dabashan pluton was emplaced into the Paleoproterozoic Diebusige complex. Early Carboniferous zircon LA‐ICP MS U‐Pb ages were from 327 Ma to 346 Ma. The Dabashan pluton can be classified as monzogranite and syenogranite, and exhibits high K2O contents and K2O/Na2O ratios, which reveal a high‐K calc‐alkaline nature. The samples display strongly fractionated REE patterns, and are enriched in large ion lithophile elements (LILE) relative to high field strength elements (HFSE). The Dabashan plutons display unusually high Ba (823–2817 ppm) and Sr (166–520 ppm) contents and K/Rb ratios (315–627), but low Rb/Ba ratios (0.02–0.14), and exhibit fertile zircon Hf isotopic compositions [εHf(t)=–14 to –20], which are comparable to those of typical high Ba–Sr granitoids. Based on the geochemical compositions of the samples, we suggest that subducted sediments and ancient crustal materials both played important roles in their generation. Basaltic melts were derived from partial melting of subcontinental lithophile mantle metasomatized by subducted sediment‐related melts with residual garnet in the source, which caused partial melting of ancient lower crust. Magmas derived from underplating ascended and emplaced in the middle–upper crust at different depths. The resultant magmas experienced some degree of fractional crystallization during their ascent. Given these geochemical characteristics, together with regional tectonic, magmatic, and structure analysis data, an active continental margin environment is proposed for the generation of these rocks.

Geochronology, Geochemistry, and Hf Isotopic Compositions of Monzogranites and Mafic-Ultramafic Complexes in the Maxingdawannan Area, Eastern Kunlun Orogen, Western China: Implications for Magma Sources, Geodynamic Setting, and Petrogenesis

This paper presents zircon U-Pb-Hf isotopic compositions and whole-rock geochemical data for monzogranites and mafic-ultramafic complexes of the Maxingdawannan area in the western end of the east Kunlun orogenic belt, western China. The data are used to determine the ages, petrogenesis, magma sources, and geodynamic setting of the studied rocks. U-Pb zircon dating indicates that monzogranites and gabbros of the complexes were emplaced at 399 and 397 Ma, respectively. The monzogranites are shoshonitic, with high SiO2, Al2O3 and total-alkali contents, and low TFeO, MgO, TiO2 and P2O5 contents. The mafic-ultramafic complexes are characterized by low SiO2 contents. The monzogranites display enrichment in light rare-earth elements (LREE) and large-ion lithophile elements (LILE), depletion in heavy REEs (HREE) and high-field-strength elements (HFSE), and negative Eu anomalies (Eu/Eu*=0.36-0.48). The mafic-ultramafic complexes are also enriched in LREEs and LILEs, and depleted in HREEs and HFSEs, with weak Eu anomalies (Eu/Eu*=0.84-1.16). Zircon eHf(t) values for the monzogranites and mafic-ultramafic complexes range from -6.68 to 1.11 and -1.81 to 6.29, with zircon model ages of 1 812-1 319 Ma (TDM2) and 1 087-769 Ma (TDM1), respectively. Hf isotopic data indicate that primary magmas of the monzogranites are originated from partial melting of ancient lower crust during the Paleo-Mesoproterozoic, with a juvenile-crust component. Primitive magmas of the mafic-ultramafic complexes are likely originated from a depleted-mantle source modified by slab-derived fluids and contaminated by crustal components. Geochemical data and the geological setting indicate that Devonian intrusions in the Maxingdawannan area are related to northward subduction of the ProtoTethys oceanic lithosphere.

Final Closure of the Paleo‐Asian Ocean and Onset of Subduction of Paleo‐Pacific Ocean: Constraints From Early Mesozoic Magmatism in Central Southern Jilin Province, NE China

AbstractWhen the Paleo‐Asian Ocean closed and the Paleo‐Pacific subduction started in NE China remains controversial. To constrain these key issues, we carried out zircon U‐Pb‐Hf isotopic and whole‐rock geochemical studies of Early Mesozoic igneous rocks in central southern Jilin Province, NE China. Early Mesozoic igneous rocks in region were formed at three stages: the middle Triassic (246 Ma), the late Triassic (219–220 Ma), and the early Jurassic (172–194 Ma). Middle Triassic trachyandesites in southern Jilin Province were solely generated through partial melting of ancient lower crust of the North China Craton (NCC), implying ongoing southward subduction of Paleo‐Asian Ocean Plate beneath the NCC at this time. Late Triassic trachyandesites in southern Jilin Province were derived from the magma mixing/mingling of the NCC lithospheric mantle and juvenile eastern Central Asian Orogenic Belt crustal sources. Late Triassic hornblende gabbros in central Jilin Province were derived from depleted lithospheric mantle previously metasomatized by subduction‐related fluids. Coeval adakitic granodiorites were likely produced through the partial melting of thickened juvenile lower crust. Spatially, these rocks occurred on both sides of the Solonker‐Xra‐Moron‐Changchun Suture Zone and record N–S directed extension accompanied by the final closure of the Paleo‐Asian Ocean. Early Jurassic andesites were produced through mixing/mingling between partial melts derived from eastern Central Asian Orogenic Belt lithospheric mantle and ancient NCC crust. In contrast, Early Jurassic felsic rocks, including A‐type granites, were derived from the partial melting of Neoproterozoic accreted lower crust, jointly implying an extensional setting as the result of the initial subduction from Paleo‐Pacific Plate at this time.

Petrogenesis of the Dasuji porphyry Mo deposit at the northern margin of North China Craton: Constrains from geochronology, geochemistry and isotopes characteristics

The Dasuji porphyry Mo deposit is located at the northern margin of North China Craton. Mo mineralization occurs mainly within the intrusions of alkali feldspar granite, granite porphyry, and quartz porphyry, which yield weighted mean 206Pb/238U ages (2σ) of 227.6 ± 1.5, 225.7 ± 1.5, and 224.3 ± 1.8 Ma, respectively. These ages are indistinguishable to the molybdenite ReOs isochron age of 223.9 ± 1.5 Ma (2σ). The Dasuji granites show the petrological, mineralogical, geochemical, and geotectonic characteristics of A-type granites. They exhibit low negative εNd(t) values (−16.7 to −12.6) with corresponding TDM2 (Nd) ages of 1705–2000 Ma, variable but mainly negative εHf(t) values (mostly −21.0 to −7.9) with corresponding TDM2 (Hf) ages of 1715–2728 Ma, and Pb isotopic compositions indicating that the primary magmas were dominantly derived from partial melting of ancient lower crust. In chondrite- and mantle-normalized trace element diagrams, all the granitic rocks show negative anomalies in Ba, Sr, and Eu, and a positive anomaly in Pb. The ratios of LREE/HREE, and (La/Yb)N and δEu values decrease from the alkali feldspar granite to the granite porphyry and quartz porphyry. These data are attributed to fractional crystallization processes. Fractionation of feldspar, biotite, and possibly K-feldspar produced the variations in major elements and Rb, Sr and Ba, and the distribution of REE are controlled mainly by fractionation of zircon, allanite, minor apatite, and possibly a small quantity of monazite. Together with the regional geology, the data infer that the Dasuji porphyry Mo deposit formed in a post-collision extensional setting during the Late Triassic.

Long-lived melting of ancient lower crust of the North China Craton in response to paleo-Pacific plate subduction, recorded by adakitic rhyolite

Magmatism in eastern China in response to paleo-Pacific plate subduction during the Mesozoic was complex, and it is unclear how and when exactly the magmas formed via thinning and partial destruction of the continental lithosphere. To better understand this magmatism, we report the results of a geochronological and geochemical study of Early Cretaceous adakitic rhyolite (erupted at 125.4±2.2Ma) in the Xintaimen area within the eastern North China Craton (NCC). In situ zircon U-Pb dating shows that this adakitic rhyolite records a long (~70Myrs) and complicated period of magmatism with concordant 206Pb/238U ages from 193Ma to 117Ma. The enriched bulk rock Sr-Nd isotopic compositions of the Xintaimen adakitic rhyolite, as well as the enriched zircon Hf and O isotopic compositions, indicate that the magmas parental to the adakitic rhyolite were derived from partial melting of the Paleoproterozoic mafic lower crust, heated by mafic melts derived from the mantle during the paleo-Pacific plate subduction. A minor older basement component is indicated by the presence of captured Neoarchean to Early Paleoproterozoic zircons. The Mesozoic zircons have restricted Hf and O isotopic compositions irrespective of their ages, suggesting that they formed from similar sources at similar melting conditions. The Xintaimen adakitic rhyolite offers an independent line of evidence that the ancient lower crust of eastern China underwent a long period (~70Myrs) of destruction, melting or remelting, from ~193 to ~120Ma, related to the subduction of the paleo-Pacific plate beneath eastern China.

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.

Neoproterozoic granitoids along the Ailao Shan-Red River belt: Zircon U-Pb geochronology, Hf isotope analysis and tectonic implications

The Ailao Shan-Red River belt (ASRRB) is a complex tectonic collage of Archean to Miocene geological units, located between two major tectonic domains, i.e., the South China domain to the east and the Tethys domain to the west. Although general agreements have been reached on that the Ailao Shan suture zone along the ASRRB represents a Silurian to middle Triassic subsidiary ocean in the eastern Paleo-Tethyan system and the Ailao Shan-Red River shear zone within the belt occurred as the northeastern boundary of the Cenozoic extrusion of the Indochina block during the Indian-Eurasian collision, the Precambrian tectonic attributes of the belt remain controversial. This study presents new zircon U–Pb ages, whole-rock geochemical and Hf isotopic data of Neoproterozoic granitoids along the ASRRB.Neoproterozoic granitoids along the ASRRB are located to the east of the Jinshajiang-Ailao Shan suture zone. Samples from the granitoids have zircon 206U/238Pb ages between 799Ma and 724Ma, implying the existence of extensive Neoproterozoic granitic magmatism along the ASRRB. Their basement rocks share similar characteristics (e.g. Paleo- to meso-Proterozoic protolith ages) to those of the western Yangtze block. Evidences suggest that the Neoproterozoic intrusions and their metamorphic basement rocks along the ASRRB were derived from the southwest margin of the Yangtze block.Zircon εHf (t) values of the granitoid samples range from −10.22 to 0.64, with Hf crustal model ages of 1632.2–2335.9Ma. It is suggested, combining the present results and regional geology data, that the Neoproterozoic granitic magmatism along the ASRRB is dominantly of crustal origin, and contributions from mantle sources are common. They are possibly attributed to partial melting of ancient lower crust and addition of mantle fluids related to oceanic subduction. The results support a scenario that arc magmas were generated by continued oceanic subduction beneath the southwestern margin of the Yangtze block, instead of the northeastern margin of the Indochina block, lasted from 799Ma to 724Ma.

Geochronology and Petrogenesis of Mesozoic Granitoids in the Geological Corridor of Western Liaoning Province, China

Geochronology and Petrogenesis of Mesozoic Granitoids in the Geological Corridor of Western Liaoning Province, China

Coexisting Early Cretaceous High-Mg Andesites and Adakitic Rocks in the North China Craton: the Role of Water in Intraplate Magmatism and Cratonic Destruction

High-Mg andesites (HMAs) and adakitic rocks are purported to occur exclusively in subduction zones in the modern Earth. In the North China Craton, early Cretaceous HMAs and adakitic dacites were erupted in a continental setting, apparently unrelated to subduction given their location distal (>1000 km) to the trench at that time. Here we report petrological, mineralogical and geochemical data for these rocks with the aim of constraining their petrogenesis and elucidating the role of water in intraplate magmatism and cratonic destruction. The HMAs can be subdivided into olivine (Ol-)HMAs and clinopyroxene (Cpx-)HMAs. The former have high MgO (>9·8 wt %) and Mg# (>71), with rare high-Fo (up to 91) olivine phenocrysts, corresponding to (near-)primary magmas that equilibrated with mantle peridotite. The latter have moderate MgO (7·8–8·8 wt %) and Mg# (mostly <70) and low-Fo (mostly < 83) olivine phenocrysts. The Cpx-HMAs are interpreted as magmas differentiated from the Ol-HMAs by olivine-dominated fractionation at lower-crust levels. P–T–XH2O estimations show that the primary HMAs are melts of shallow (1·1–1·2 GPa), hot (∼1250°C) and wet (H2O > 3 wt %) lithospheric mantle. The coexisting adakitic dacites are hydrous (H2O ≥ 5 wt %) magmas with high SiO2 (>63 wt %), Sr/Y ratios (≥39) and YbSN (source-normalized), low (Sm/Yb)SN, and negligible Eu anomalies. They also have unradiogenic whole-rock Nd [εNd(t) = –19 to –9] and zircon Hf [εHf(t) = –23 to –21] isotopic compositions consistent with derivation by melting of ancient lower crust at depths < 40 km. Melting may have been induced by heating and addition of H2O from underplated HMAs. Mixing between Cpx-HMAs and low-Mg adakitic dacites in magma chambers produced high-Mg adakitic rocks. The petrogenetic model presented here explains the occurrence of intraplate HMAs and adakitic magmas elsewhere in the North China Craton. The P–T–XH2O conditions inferred for HMA generation imply that the subcontinental lithospheric mantle beneath the craton was hot and hydrous in the early Cretaceous, which may have triggered the destruction of the cratonic root. The occurrence of young HMAs and adakitic rocks in an intraplate extensional environment also casts doubts on the common use of a similar igneous rock association as an indicator of subduction processes in Archean time.

Geochronology and petrogenesis of Middle Permian S-type granitoid in southeastern Guangxi Province, South China: Implications for closure of the eastern Paleo-Tethys

The Bangxi-Chenxing suture zone is an important region to address the history of closure of the eastern Paleo-Tethys ocean. Among the widespread S-type granite batholiths in the SE Guangxi Province adjacent to this suture is the Nali granodiorite. Here we report zircon U–Pb ages, major and trace element geochemistry, and Sr–Nd–Hf isotope geochemistry of a newly found Middle Permian S-type granite in this region. LA-ICP-MS zircon U–Pb geochronology yields emplacement ages of 265±2 to 262±2Ma for the Nali granodiorite. Both the inner and outer phases are characterized by variable SiO2 (66.91–71.39wt.%), high Al2O3 (12.99–14.04wt.%), K2O+Na2O (4.78–5.98wt.%), and A/CNK values (1.11–1.50), resembling those of typical S-type granites. The rocks are enriched in Rb, Th, U and light rare earth elements with weak negative Eu anomalies, and show depletion in Nb, Ta, Ti, with typical arc-like affinity. They have relatively high (87Sr/86Sr)i ratios of 0.7228 to 0.7331, low εNd(t) values of −13.6 to −11.9, and low zircon εHf(t) values (−21.9 to −8.8). High whole-rock Nd isotopic model ages (2.00–2.02Ga) and zircon Hf isotopic model ages (1.59–2.20Ga), indicate that they might have been generated by partial melting of ancient lower crust with minor contribution from mantle sources. The granodiorite shows continental arc affinity and likely formed within a subduction setting. By combining previously extant data for Permo-Triassic arc-/collision-related magmatism in the Jinshajiang-Ailaoshan-Song Ma and Bangxi-Chenxing suture zones, we propose that the subduction of the eastern Paleo-Tethys might have started during Middle Permian triggered by the northward subduction of the Indochina Block (ICB) beneath the South China Block (SCB) and closure during Middle Triassic.

Timing of formation and origin of the Tongchanggou porphyry–skarn deposit: Implications for Late Cretaceous Mo–Cu metallogenesis in the southern Yidun Terrane, SE Tibetan Plateau

The newly-discovered Tongchanggou Mo–Cu porphyry–skarn deposit is located in the southern Yidun Terrane, SE Tibet, with more than 142.5metric ton resources (0.3metric ton @ 0.3% Mo, and 34kt @ 0.8% Cu) hosted in Permian to Triassic strata and Late Cretaceous granodiorite porphyry. Re–Os isotope analyses of molybdenite separates from skarn, granodiorite porphyry and the massive molybdenite orebody yield model ages ranging from 86.8 to 85.2Ma. LA-ICP-MS U–Pb dating of zircons from the ore-related granodiorite porphyry give 206Pb/238U ages ranging from 87.4 to 84.2Ma with a weighted mean 206Pb/238U age of 84.8±0.3Ma, indicating a temporal link between granitic magmatism and Mo–Cu mineralization. Geochemically, the ore-related granodiorite porphyries are metaluminous and have an adakite-like signature with relatively low MgO (1.32–1.58wt%), Cr (5.6–12.9ppm), Ni (3.79–10.81ppm), Mg# (43–52) values, and high Sr (304–844ppm), Sr/Y (21.2–50.8) and La/Yb ratios (37.0–60.1). They also display negative zircon εHf(t) values (−7.4 to −1.3) and negative whole-rock εNd(t) values (−5.3 to −4.2), as well as an older Hf–Nd model age, indicating that their magmas were derived from a thickened ancient lower crust within the garnet–amphibolite facies. Partial melting of ancient lower crust generated felsic magmas with high concentrations of Mo that accumulated within the ancient crustal components. Whereas, the copper mineralization is correlated to the remelting of residual sulfide cumulates in subduction-modified lower crust. The Late Cretaceous porphyry Mo–Cu metallogenesis in the southern Yidun Terrane is considered to be related to the transtensional faulting that triggered asthenospheric upwelling beneath the former sulfide-cumulated arc crust.

Early Permian plutons from the northern North China Block: constraints on continental arc evolution and convergent margin magmatism related to the Central Asian Orogenic Belt

Recent zircon dating identified several late Carboniferous to early Permian hornblende gabbro–diorite–quartz diorite–granodiorite–tonalite–granite plutons in lithological assemblages at the northern margin of the North China Block (NCB) that were previously regarded as Archaean to Palaeoproterozoic. Our geochronological results indicate that emplacement of these plutons was a continuous process during the late Carboniferous to early Permian, from 324 ± 6 to 274 ± 6 Ma, and lasted for at least 50 Ma. In this paper, the early Permian components with compositions from gabbro to granite within the intrusive complex were studied. The early Permian plutons exhibit calc-alkaline or high-K calc-alkaline, metaluminous geochemical features and highly variable SiO2 contents. They have no significant Eu anomaly in their REE patterns, and in primitive-mantle-normalized spidergrams they display depletion in Th, U, Nb, Ta, P and Ti, and enrichment in Ba, K, Pb and Sr. The granitoid bodies within these plutons display I-type and adakitic geochemical signatures. The early Permian rocks exhibit low whole-rock initial 87Sr/86Sr ratios from 0.70520 to 0.70615 and have negative whole-rock e Nd(t) values ranging from −17.4 to −9.3 and zircon e Hf(t) values of −23.2 to −10.5. The gabbros exhibit higher e Nd(t) values from −11.1 to −9.3 and e Hf(t) values from −16.5 to −10.5, and one granodiorite exhibits an even lower e Nd(t) value of −17.4 and zircon e Hf(t) values of −23.2 to −15.1. Geochemical, Sr–Nd and in situ zircon Hf isotopic compositions suggest that the hornblende gabbros were derived from a metasomatized lithospheric mantle, and the diorite and quartz diorite were generated from a gabbroic magma by fractional crystallization, coupled with differential assimilation of ancient lower crustal material. The granodiorite was likely derived from partial melting of ancient lower crust with involvement of some mantle components. Involvement of both lithospheric mantle and ancient lower crust in the generation of the early Permian plutons indicates strong crust–mantle interaction in the northern NCB. Petrological associations as well as geochemical and Sr–Nd–Hf isotopic results show that the early Permian plutons were emplaced along an Andean-type active continental margin during southward subduction of the Palaeo-Asian oceanic plate beneath the NCB. Integration of our results with previously published data for late Carboniferous and late Permian to middle Triassic intrusions suggests that the continental arc on the northern margin of the NCB existed for at least 50 Ma during the late Palaeozoic, and final amalgamation of the Mongolian arc terranes with the northern NCB likely occurred during a period from ~270 to ~250 Ma, i.e, in the late Permian to earliest Triassic.

Derivation of Mesozoic adakitic magmas from ancient lower crust in the North China craton

Three Mesozoic plutons were intruded into the Archean granulite terrains in northern margin of the North China craton adjacent to the granulite xenolith-bearing Tertiary Hannuoba basalts. They are Triassic Guzuizi and Honghualiang granites and Early Cretaceous Zhuanzhilian diorite. Rocks of the three intrusions are characterized by high Sr (576–1216 ppm) and Na 2O (⩾4.0%), low Y (⩽18 ppm) and heavy rare-earth elements (Yb < 1.8 ppm). These features are similar to modern adakites (siliceous rocks with high sodium, aluminum and strontium and low heavy rare-earth element and yttrium contents) from island arcs and Archean high-Al tonalite–trondhjemite–granodiorite (TTG). However, they are distinctly potassium-rich and their evolved strontium, neodymium, and hafnium isotopic compositions and inherited zircon ages coincide with those of the Archean granulites and the Hannuoba granulite xenoliths. Such features cannot readily be explained with previous models of the origin of adakites and TTGs. We propose that these adakitic rocks are formed by partial melting of ancient lower crust, and that the restites are represented by some of the Hannuoba granulite xenoliths; we further argue that this could be one major mechanism to generate voluminous Mesozoic adakitic magmas in the eastern North China craton. Our hypothesis can also explain the discrepancies in ages and compositions between the granulite terrains and xenoliths, and implies that crustal anatexis may be one of the major processes controlling the chemical differentiation of the continental crust.