Late Neoarchean reworking of the Mesoarchean crustal remnant in northern Liaoning,North China Craton: A U–Pb–Hf–O–Nd perspective

How the earth's crust formed and evolved during the Precambrian times is one of the key questions to decipher the evolution of the early Earth. As one of the few cratons containing well-preserved Eoarchean to Neoarchean basement on Earth, the North China Craton is an ideal natural laboratory to unravel the early crustal evolution. It is controversial whether the Archean tectonothermal events in this area represents reworking or growth of the continental crust. To solve this issue, we have compelled field-based mapping, zircon U–Pb dating by SHRIMP RG and LA–ICP–MS U–Pb, zircon SHRIMP SI oxygen and LA–MC–ICP–MS Hf isotope, and whole-rock Nd–O isotope analyses from the Archean granitoids in northern Liaoning, North China Craton. On the basis of zircon U–Pb isotopic dating and measured geological section investigation, two distinct magmatic suites as enclaves in the Jurassic granites are recognized, viz. a newly discovered 3.0 Ga crustal remnant and a 2.5 Ga granitoid. The Mesoarchean zircons from the 3.0 Ga granodioritic gneisses exhibit heterogeneous Hf isotopic compositions, with the most radiogenic analysis (ε_Hf(t) = +3.8) following the depleted mantle evolution array and the most unradiogenic ε_Hf(t) extending down to −3.4. This implies that both ancient continental crust at least as old as 3.4 Ga and depleted mantle contributed to the magma source of the protoliths of the Mesoarchean gneisses. The ε_Hf(t) values of the Neoarchean zircons from these gneisses overlap the 3.4–3.0 Ga zircon evolution trend, indicating that the ancient crustal materials have been reworked during the late Neoarchean. The Neoarchean zircons from the 2.5 Ga granitoids have a relatively small variation in the Hf isotope and are mainly plotted in the 3.0–2.8 Ga zircon evolution field. However, taking all the ε_Hf(t) values of the Neoarchean zircons into the consideration, we find that the Hf model age of the Neoarchean zircon does not represent the time of crustal growth or reworking but are artifacts of magma mixing. The interaction between the magmas derived from the ancient crustal materials and the depleted mantle is also supported by zircon O isotopic data and Hf–O isotopic modeling of the Neoarchean granitoids. Both Mesoarchean and late Neoarchean tectonothermal events involved synchronous crustal growth and reworking, which may be applicable to other parts of the world.     

Nd–Sr–Hf–O isotope provinciality in the northernmost Arabian–Nubian Shield: implications for crustal evolution

Multi-isotope study including whole-rock Nd–Sr, single zircon Hf, and SIMS δ¹⁸O analyses of zircons sheds light on magma sources in the northernmost Arabian–Nubian Shield (ANS) during ~820–570 Ma. Reconnaissance initial Nd and Sr isotope data for the older rocks (~820–740 Ma) reaffirms previous estimates that early crustal evolution in this part of the shield involved some crustal contamination by pre-ANS material. Prominent isotope provinciality is displayed by post-collisional calc-alkaline and alkaline igneous rocks of ~635–570 Ma across a NW-SE transect across basement of the Sinai Peninsula (Egypt) and southern Israel. Silicic rocks of the NW-region are characterized by lower εNd(T)–εHf(T) and higher Sr_i and δ¹⁸O compared with rocks of the SE-region, and the transition between the regions is gradual. Within each region isotope ratios are independent of the extent of magma fractionation, and zircon cores and rims yield similar δ¹⁸O values. Comparison with southern segments of the ANS shows that the source for most ~635–570 Ma rocks can be modeled as the isotopically aged lower-intermediate crust in the ANS core (SE-region) and its northern, more contaminated ANS margins (NW-region). Nevertheless, Nd–Sr isotope enrichment of the lithospheric mantle is indicated by some basic magmas of the NW-region displaying the most enriched Nd–Sr isotope compositions. Comparison of Nd and Hf depleted mantle model ages for rocks of the SE-region may indicate that crustal formation events in the ANS geographical core took place at 1.1–1.2 Ga and were followed by crustal differentiation starting at ~0.9 Ga.     

The isotope composition of Hf in zircon from Paleoarchean plagiogneisses and plagiogranitoids of the Sharyzhalgai uplift (southern Siberian craton): Implications for the continental-crust growth

This paper presents results of U–Pb dating (SHRIMP-II) and Lu–Hf (LA–ICP MS) isotope study of zircon from Paleoarchean plagiogneisses and plagiogranitoids of the Onot and Bulun blocks of the Sharyzhalgai uplift. Magmatic zircons from the Onot plagiogneiss and Bulun gneissic trondhjemite are dated at 3388±11 and 3311±16 Ma, respectively. Magmatic zircons from plagiogneisses and plagiogranitoids of the studied tonalite–trondhjemite–granodiorite (TTG) complexes are characterized mainly by positive values of εHf indicating that felsic melts were generated mainly from juvenile (mafic) sources, which are derived from a depleted mantle reservoir. The variable Hf isotope composition in magmatic zircons and the lower average εHf values in comparison with the depleted mantle values suggest the contributions of both mafic and more ancient crustal sources to magma formation. Metamorphic zircons from the gneissic plagiogranite and migmatized plagiogneiss either inherited the Hf isotope composition from magmatic zircon or are enriched in radiogenic Hf. The more radiogenic Hf isotope composition of metamorphic zircons from the migmatized plagiogneisses is due to their interaction with melt during partial melting. Variations in the Lu–Hf isotope composition of zircon from the Bulun rocks in the period 3.33–3.20 Ga are due to the successive melting of mafic crust or the growing contribution of crustal material to their genesis. Correlation between the Lu–Hf isotope characteristics of zircon and the Sm–Nd parameters of the Onot plagiogneisses points to the contribution of ancient crustal material to their formation. The bimodal distribution of the model Hf ages of zircons reflects two stages of crustal growth in the Paleoarchean: 3.45–3.60 and ~ 3.35 Ga. The isotope characteristics of zircon and rocks of the TTG complexes, pointing to recycling of crustal material, argue for the formation of plagiogneisses and plagiogranitoids as a result of melting of heterogeneous (mafic and more ancient crustal) sources in the thickened crust.     

Juvenile vs. recycled crust in NE China: Zircon U–Pb geochronology,Hf isotope and an integrated model for Mesozoic gold mineralization in the Jiaodong Peninsula

The continental crust of the North China Craton (NCC) is a major reservoir of mineral resources with imprints of secular changes in tectonics and metallogeny. The Jiaodong Peninsula, located in the eastern margin of the North China Craton (NCC), is currently one of the largest gold producers over the globe, and preserves the records of multiple magmatic and metamorphic events. Here we characterize the timing and tectonics of the major Mesozoic magmatism and the associated gold metallogeny in this region through a comprehensive U–Pb geochronological and Hf isotope investigation of zircons in a suite of granitoids, mafic magmatic enclaves, melanocratic dikes and melted basement rocks.The Linglong granite, hosting one of the major gold deposits in Jiaodong, shows emplacement ages between 150 and 160 Ma, and the dominantly negative ε_Hf (t) values (− 34.0 to − 23.8) of zircons from this intrusion suggest magma derivation from recycled components in the Archean basement. The Guojialing granodiorite and its mafic magmatic enclaves show similar ages between 123 and 127 Ma, with negative ε_Hf (t) values (− 19.3 to − 16.8), corresponding to crustal magma source. The melanocratic dikes, belonging to pre- and syn-mineralization stages, with U–Pb age range of 126 to 166 Ma display large variation in their zircon ε_Hf (t) values (− 25.7 and 2.3) suggesting the involvement of both recycled crustal and juvenile mantle components. Zircons in the melted basement rocks with ages in the range of ca. 127–132 Ma also display both positive and negative ε_Hf (t) values (− 44.6 and 9.8) indicating a mixture of recycled ancient crust and juvenile magmas. Our study shows that although the peak of gold metallogeny coincided with the tectonics associated with Pacific plate subduction which mobilized and concentrated the ores, the source materials of gold mineralization and magmatism had multiple origins including from the Precambrian basement rocks, Mesozoic granitoids and mantle-derived mafic magmas with extensive mixing of crustal, lithosphere mantle and asthenospheric components. A combination of delamination, mantle upwelling, subduction-related metasomatic enrichment and recycling of ancient components facilitated the gold metallogeny in this region. Our study provides a typical case of juvenile and recycled components in the formation and evolution of continental crust and associated mineral resources.     

Subduction of the Indian lower crust beneath southern Tibet revealed by the post-collisional potassic and ultrapotassic rocks in SW Tibet

New major and trace elemental, Sr–Nd–Pb isotope, and zircon U–Pb geochronological and Hf–O isotope data of post-collisional potassic and ultrapotassic volcanic rocks (PVRs and UPVs, respectively) along with geochemical data of PVRs, UPVs, and Mg-rich potassic rocks (MPRs) in the literature are used to constrain their mantle source and genesis. The PVRs, UPVs, and MPRs share similar geochemical features but with some discrepancies, suggesting that they were derived from subcontinental lithospheric mantle (SCLM) with isotopic heterogeneity resulting from the varying contributions of subducted Indian lower crust into the mantle source (ca. 6–20%, ca. 8–30%, and ca. 9–30%, respectively). The zircon Hf–O isotopic compositions of these rocks can be classified into two groups, including Group I rocks with high δ¹⁸O (6.7–11.3‰), low ε_Hf(t) (− 17.0 to − 12.0), and old Hf crustal model ages (1.87–2.19 Ga) that indicate an ancient SCLM source, and Group II rocks with δ¹⁸O values of 6.8–10.7‰, ε_Hf(t) values of − 11.8 to − 6.3, and younger Hf crustal model ages (1.50–1.86 Ga). The negative correlation defined by δ¹⁸O and ε_Hf(t) of Group II samples suggests a two-component mixing between mantle- and crust-derived melts, in which the latter would be the subducted Indian lower crust as indicated by the similar negative ε_Hf(t) values between Group II samples (− 11.8 to − 6.3) and the High Himalayan gneiss (− 14.2 to + 0.3). Thus we propose two enrichment events to account for the Hf–O isotopic compositions of the PVRs and UPVs/MPRs: the first involves the enrichment of the overlying SCLM that was metasomatized by fluids derived from dehydration of the subducted Indian lower crust, and the second invokes the enrichment of the overlying SCLM metasomatized by melts of the already dehydrated different proportions of the Indian lower crust. We argue that break-off of the northwards subducted Indian Plate in the early Miocene caused the asthenospheric upwelling under the Indian plate through slab window, resulting in varying degrees of partial melting of the overlying metasomatized heterogeneous SCLM to produce the primitive magmas of the PVRs, UPVs, and MPRs in an extensional setting. These observations and interpretations imply that the Indian lower crust was subducted beneath the Lhasa terrane in the Early–Middle Miocene.     

Cold plumes trigger contamination of oceanic mantle wedges with continental crust-derived sediments: Evidence from chromitite zircon grains of eastern Cuban ophiolites

The origin of zircon grains, and other exotic minerals of typical crustal origin, in mantle-hosted ophiolitic chromitites are hotly debated. We report a population of zircon grains with ages ranging from Cretaceous (99 Ma) to Neoarchean (2750 Ma), separated from massive chromitite bodies hosted in the mantle section of the supra-subduction (SSZ)-type Mayarí-Baracoa Ophiolitic Belt in eastern Cuba. Most analyzed zircon grains (n = 20, 287 ± 3 Ma to 2750 ± 60 Ma) are older than the early Cretaceous age of the ophiolite body, show negative ε_Hf(t) (−26 to −0.6) and occasional inclusions of quartz, K-feldspar, biotite, and apatite that indicate derivation from a granitic continental crust. In contrast, 5 mainly rounded zircon grains (297 ± 5 Ma to 2126 ± 27 Ma) show positive ε_Hf(t) (+0.7 to +13.5) and occasional apatite inclusions, suggesting their possible crystallization from melts derived from juvenile (mantle) sources. Interestingly, younger zircon grains are mainly euhedral to subhedral crystals, whereas older zircon grains are predominantly rounded grains. A comparison of the ages and Hf isotopic compositions of the zircon grains with those of nearby exposed crustal terranes suggest that chromitite zircon grains are similar to those reported from terranes of Mexico and northern South America. Hence, chromitite zircon grains are interpreted as sedimentary-derived xenocrystic grains that were delivered into the mantle wedge beneath the Greater Antilles intra-oceanic volcanic arc by metasomatic fluids/melts during subduction processes. Thus, continental crust recycling by subduction could explain all populations of old xenocrystic zircon in Cretaceous mantle-hosted chromitites from eastern Cuba ophiolite. We integrate the results of this study with petrological-thermomechanical modeling and existing geodynamic models to propose that ancient zircon xenocrysts, with a wide spectrum of ages and Hf isotopic compositions, can be transferred to the mantle wedge above subducting slabs by cold plumes.     

The late- to postorogenic transition in the Apiaí domain,SE Brazil: Constraints from the petrogenesis of the Neoproterozoic Agudos Grandes Granite Batholith

A remarkable change in the typology of granite magmatism occurred during the late evolution of the Agudos Grandes Batholith (Apiaí domain, SE Brazil), marked by the appearance of A-type granites some 10–15 Myr after the last manifestations of the compressional period. The elemental and Sr–Nd isotope geochemistry of the approximately 600 Ma, “late-orogenic”, moderately peraluminous Piedade, Serra dos Lopes, and Pilar do Sul granites shows that the middle crust was heated to approximately 820–850 °C during the latest stages of the compressional period and generated large amounts of melt through biotite dehydration–melting of gneiss protoliths. These crustal magmas either formed independent intrusions or mixed in varied proportions with raising high-K calc-alkaline magmas, as suggested by continuous trends toward more mafic granites (65–70 wt% SiO₂), which have the geochemical signature of contaminated calc-alkaline magmas. The postorogenic granites occur as two subgroups with different age (∼585 and ∼565 Ma), both with the chemical fingerprints typical of A-type granites (low mg#, Ba, and Sr; high Zr, Hf, Nb, and Y), interpreted as a result of crustal melting at lower P, fO₂, and a(H₂O) and higher T (up to 870 °C, indicated by zircon saturation). Significant trace-element and radiogenic isotope contrasts (e.g., εNd_T = −16 vs. −11) show that the two postorogenic subgroups derived from different sources. A Sr–Nd isotope signature nearly identical to the “end-member” late-orogenic crustal melts suggests that the older postorogenic granites derived from a similar (but probably less fertile) source. The less negative εNd_T of the young subgroup is unmatched by known crustal sources in the Apiaí domain and may have resulted from either an infracrustal component or mobilization of an unexposed section with less crustal residence time.     

The formation and rejuvenation of continental crust in the central North China Craton: Evidence from zircon U–Pb geochronology and Hf isotope

The Trans-North China Orogen (TNCO) along the central part of the North China Craton (NCC) is considered as a Paleoproterozoic suture along which the Eastern and Western Blocks of the NCC were amalgamated. Here we investigate the Precambrian crustal evolution history in the Fuping segment of the TNCO and the subsequent reactivation associated with extensive craton destruction during Mesozoic. We present zircon LA-ICP-MS U–Pb and Lu–Hf data on TTG (tonalite–trondhjemite–granodiorite) gneiss, felsic orthogneiss, amphibolite and granite from the Paleoproterozoic suite which show magmatic ages in the range of 2450–1900 Ma suggesting a long-lived convergent margin. The ε_Hf(t) values of these zircons range from −11.9 to 12 and their model ages suggest magma derivation from both juvenile components and reworked Archean crust. The Mesozoic magmatic units in the Fuping area includes granite, diorite and mafic microgranular enclaves, the zircons from which define a tight range of 120–130 Ma ages suggesting a prominent Early Cretaceous magmatic event. However, the ε_Hf(t) values of these zircons show wide a range from −30.3 to 0.2, indicating that the magmatic activity involved extensive rejuvenation of the older continental crust.     

Petrology and geochemistry of late archaean granitoids in the northern part of Eastern Dharwar Craton,Southern India: Implications for transitional geodynamic setting

The results of field, petrographic and geochemical work of the granitoids of Hutti-Gurgunta area in the northern part of Eastern Dharwar Craton (EDC) is presented in this paper. This crustal section comprises polyphase banded to foliated TTG gneisses, middle amphibolite facies Gurgunta schist belt and upper greenschist facies Hutti schist belt and abundant granite plutons. The focus of the present study is mainly on basement TTG gneisses and a granite pluton (∼ 240 sq km areal extent), to discuss crustal accretion processes including changing petrogenetic mechanism and geodynamic setting. The TTGs contain quartz, plagioclase, lesser K-feldspar and hornblende with minor biotite while the granite contain quartz, plagioclase, K-feldspar and hornblende. Late stage alteration (chloritisation, sericitisation and epidotisation) is wide spread in the entire area. A huge synplutonic mafic body which is dioritic to meladioritic in composition injects the granite and displays all stages of progressive mixing and hybridization. The studied TTGs and granite show distinct major and trace element patterns. The TTGs are characterized by higher SiO₂, high Al₂O₃, and Na₂O, low TiO₂, Mg#, CaO, K₂O and LILE, and HFS elements compared to granite. TTGs define strong trondhjemite trend whilst granite shows calc-alkaline trend. However, both TTGs and granite show characteristics of Phanerozoic high-silica adakites. The granite also shows characteristics of transitional TTGs in its high LILE, and progressive increase in K₂O with differentiation. Both TTGs and granite define linear to sub-linear trends on variation diagrams. The TTGs show moderate total REE contents with fractionated REE patterns (La/Yb_N =17.73–61.73) and slight positive or without any significant Eu anomaly implying little amount of amphibole or plagioclase in residual liquid. On the other hand, the granite displays poor to moderate fractionation of REE patterns (La/Yb_N = 9.06–67.21) without any significant Eu anomaly. The TTGs have been interpreted to be produced by low-K basaltic slab melting at shallow depth, whereas the granite pluton has been formed by slab melting at depth and these melts interacted with peridotite mantle wedge. Such changing petrogenetic mechanisms and geodynamic conditions explain increase in the contents of MgO, CaO, Ni and Cr from 2700 Ma to 2500 Ma granitoids in the EDC.     

Orosirian (ca. 1.96 Ga) mafic crust of the northwestern São Francisco Craton margin: Petrography,geochemistry and geochronology of amphibolites from the Rio Preto fold belt basement,NE Brazil

The bulk of the Atlantic shield basement in the eastern South American platform is made of Rhyacian calc-alkaline magmatic rocks (∼2.2–2.15 Ga) affected by granulite-facies metamorphism at around 2.06 Ga, as part of a Paleoproterozoic orogenic system which joined Archean crustal fragments together. Orosirian (2050–1800 Ma) rocks, on the other hand, are scarce in all four major provinces of eastern South America (the Borborema, São Francisco, Tocantins and Mantiqueira provinces). Recently, however, sparse occurrences of Orosirian-aged rocks have been described, particularly in the northern São Francisco Craton margin and adjoining fold belts that compose the Borborema Province further north. In this paper, we present new geochemical, geochronological and isotopic data of amphibolites which compose the basement of the Rio Preto fold belt, in the northwestern São Francisco Craton margin. Geochemical data suggest tholeiitic gabbro protoliths intruded in a subduction-related setting, as suggested by tectonic discrimination diagrams and differentiation trends. U–Pb analyses of magmatic zircon crystals yielded a 1958.3 ± 16 Ma Concordia age. Slightly negative to positive εNd_(1.96) = (−0.3 to +1.0) and εHf_(1.96) from −0.76 to −4.55 suggest variable mixing of a depleted mantle source and older continental crust. We interpret the amphibolite body as a remnant of a subduction-related crustal fragment developed in the Orosirian, around 1.96 Ga ago, possibly in a continental back-arc setting. This crustal fragment was further reworked as part of the basement of the Rio Preto fold belt during the Brasiliano Orogeny (∼600–550 Ma). The Cristalândia do Piauí Complex gneisses further north might represent fragments of a coeval continental magmatic arc. These rocks could be the first well documented representatives of an Orosirian tectono-magmatic event which has been, up to now, broadly ignored within this portion of West Gondwana, and thus present major implications for the Proterozoic crustal evolution of South America and for Paleoproterozic supercontinent reconstructions in general.     

Episodic crustal growth in the Tanzania Craton: evidence from Nd isotope compositions

The analysis of available Nd isotope data from the Tanzania Craton places important constraints on the crust-mantle separation ages, and events marking juvenile crustal addition and crustal recycling. Nd model ages date the oldest crust extraction to 3.16 Ga in the Tanzania Craton, although a rock record of such antiquity is yet to be found there. The most significant period of juvenile crustal addition as well as crustal recycling is 2.7–2.6 Ga. The Nd isotopes of mafic samples show that chemical heterogeneity existed in the mantle beneath the Tanzania Craton, with some samples originating from significantly depleted mantle, and most samples originating from the mixture of primitive mantle and depleted mantle. The Nd isotope section reveals significant differences in Nd isotopes between the north craton and central craton; compared to the north craton, the central craton yields a Nd model age that is approximately 100 Ma older, and its ε_Nd(t) values are more negative, indicating that the two parts of the craton have different mantle source regions. Different types of granitoids are distributed in the Tanzania Craton, such as high-K and low-Al granite, calc-alkaline granite, peraluminous granite and transitional types of tonalite-trondhjemite-granodiorites (TTGs). Most of the granitoids formed later than the mafic rocks in syn-collision and post-collision events.     

Mid-Neoproterozoic crustal evolution of the northeastern Yangtze Block: Evidence from the felsic-gneiss xenoliths hosted in the Donghai Cenozoic basalts

Crustal xenoliths can provide new insights into the unexposed crust, and those from the northeastern Yangtze Block have rarely been studied. This paper reports U–Pb–Hf isotopes and trace-element compositions of zircons from six felsic xenoliths hosted by the Neogene alkali basalts in the Donghai region (i.e. Anfengshan and Pingmingshan) of the Sulu orogen in central eastern China. The xenoliths are mainly composed of orthoclase and quartz, or orthoclase and natrolite, with accessory minerals of Fe–Ti oxides and zircon. Most zircon grains show core-rim structures, with the cores and rims being magmatic and metamorphic in origin, respectively. The zircon cores mainly yield ages of ca. 827–794 Ma, while the zircon rims give ages of ca. 232–212 Ma. We interpret the zircon core ages as the time of an early Mid-Neoproterozoic magmatic event in the northeastern Yangtze Block and the zircon rim ages as the time of collision between the Yangtze and North China Blocks. Our data suggest that much more ca. 830–800 Ma magmatic records are possibly preserved in the unexposed deep crust, and the early Mid-Neoproterozoic is an important era for the crust evolution of the northeastern Yangtze Block. The new zircon Hf isotopic analyses show that the Anfengshan sample (south of Donghai) has zircon ε_Hf (820 Ma) values ranging from −15.3 to −9.4, and two-stage Hf model ages of 2.66–2.30 Ga; the Pingmingshan sample (southeast of Donghai) has zircon ε_Hf (820 Ma) values ranging from −1.4 to +3.8, and two-stage Hf model ages of 1.80–1.47 Ga. These data suggest that ancient crust as old as Neoarchean to Mesoproterozoic was involved in the early Mid-Neoproterozoic magmatism. Combined with the previously reported zircon U–Pb–Hf results of the exposed rocks, it is highlighted that crustal recycling was dominant in the early Mid-Neoproterozoic (ca. 830–800 Ma) magmatism, whereas both crustal recycling and addition of mantle-derived melts were significant in the late Mid-Neoproterozoic (ca. 800–720 Ma) magmatism in the northeastern Yangtze Block.     

Zircon U-Pb and Hf isotope constraints on crustal melting associated with the Emeishan mantle plume

SHRIMP zircon U-Pb dates, combined with in-situ Hf isotopic data, provide new constraints on the petrogenesis and protolith of peralkaline, metaluminous and peraluminous intrusions and rhyolitic tuffs in the Emeishan large igneous province, with significant bearing on crustal melting associated with mantle plumes. Syenite and A-type granitic intrusions from Huili, Miyi and Taihe in the center of this large igneous province yield U-Pb dates at ∼260 Ma, consistent with the ages obtained for mafic layered intrusions in the same province. Zircon from these rocks exhibits a wide range of initial Hf isotope ratios (ε_Hf(t) = −1.4 to +13.4), with corresponding T_DM1 of 400-900 Ma. The highest ε_Hf(t) value is only marginally lower than that of depleted mantle reservoir at 260 Ma, suggesting that their source is primarily juvenile crust added during Emeishan volcanism, with incorporation of variable amounts of Neoproterozoic crust. The trigger of crustal melting is most likely related to advective heating associated with magmatic underplating. In contrast, the 255-251 Ma peraluminous granites from Ailanghe and 238 Ma rhyolitic tuff from Binchuan, have negative initial ε_Hf values of −1.3 to −4.4, and of −7.7 to −14, respectively. Hf isotopic model ages and presence of inherited zircons indicate their derivation from Mesoproterozoic and Paleoproterozoic crust, respectively. Given the time lag relative to the plume impact (∼260 Ma) and insignificant mantle contribution to 255-238 Ma magmatism, conductive heating is suggested as the trigger of crustal melting that resulted in formation of delayed felsic magmas. The involvement of older crust in younger felsic magmas is consistent with upward heat transfer to the lithosphere during plume impregnation, if the age of crust is inversely stratified, i.e., changes from Paleoproterozoic to Mesoproterozoic to Neoproterozoic to Permian with increasing depth. Such crust may have resulted from episodic, downward crustal growth during the evolution of the western Yangtze Craton.     

Geochronological and petrogeochemical constraints on the skarn deposits in Tongshanling ore district,southern Hunan Province: Implications for Jurassic Cu and W metallogenic events in South China

Southern Hunan Province, South China, is located in the central part of the Qin–Hang metallogenic belt and is characterized by abundant Cu–Pb–Zn and W–Sn polymetallic ore deposits. The Cu–Pb–Zn deposits are associated with Jurassic granodiorite porphyries whereas the W–Sn deposits occur within Jurassic granite porphyries. Here we present geochronologic and geochemical data for the Tongshanling Cu–(Mo)–Pb–Zn deposit and the Weijia W deposit in the district of Tongshanling, southern Hunan. Zircon U–Pb dating and molybdenite Re–Os geochronology indicate that the emplacement of the Tongshanling granodiorite porphyry and the associated Cu mineralization occurred at 162–160 Ma, slightly earlier than the formation of the Xianglinpu granite porphyry and associated W mineralization at 159–158 Ma. The Tongshanling granodiorite is high-K calc-alkaline, weakly peraluminous, and weakly fractionated with zircon ε_Hf(t) values of − 15.1 to − 5.6. In contrast, the Xianglinpu granite is alkaline, peraluminous, and highly fractionated, with ε_Hf(t) values of − 9.5 to 0.9. Our data indicate that the Tongshanling granodiorite is relatively oxidized and was formed by the partial melting of Paleoproterozoic crustal material with inputs of mafic magma which was derived from a subduction-modified lithospheric mantle. In contrast, the Xianglinpu granite porphyry is relatively reduced and was formed by direct interaction between the crust and asthenospheric mantle. The difference in magma generation and tectonics is considered to have resulted in the different types of mineralization associated with these two intrusive bodies.     

Geochemistry,geochronology and Sr–Nd–Pb–Hf isotopic compositions of Middle to Late Jurassic syenite–granodiorites–dacite in South China: Petrogenesis and tectonic implications

In situ zircon U–Pb ages and Hf isotope data, major and trace elements and Sr–Nd–Pb isotopic compositions are reported for coeval syenite–granodiorites–dacite association in South China. The shoshonitic syenites are characterized by high K₂O contents (5.9–6.1 wt.%) and K₂O/Na₂O ratios (1.1–1.2), negative Eu anomalies (Eu/Eu* = 0.65 to 0.77), enrichments of Rb, K, Nb, Ta, Zr and Hf, but depletion of Sr, P and Ti. The adakitic granodiorite and granodiorite porphyry intrusions are characterized by high Al₂O₃ contents (15.0–16.8 wt.%), enrichment in light rare earth elements (LREEs), strongly fractionated LREEs (light rare earth elements) to HREEs (heavy rare earth elements), high Sr (438–629 ppm), Sr/Y (29.2–53.6), and low Y (11.7–16.8 ppm) and HREE contents (e.g., Yb = 1.29–1.64 ppm). The calc-alkaline dacites are characterized by LREE enrichment, absence of negative Eu anomalies, and enrichment of LILEs such as Rb, Ba, Th, U and Pb, and depletion of HFSEs such as Nb, Ta, P and Ti.Geochemical and Sr–Nd–Hf isotopic compositions of the syenites suggest that the shoshonitic magmas were differentiated from parental shoshonitic melts by fractional crystallization of olivine, clinopyroxene and feldspar. The parent magmas may have originated from partial melting of the lithospheric mantle with small amount contribution from crustal materials. The adakitic granodiorite and granodiorite porphyry have Sr–Nd–Pb isotopic compositions that are comparable to that of the mafic lower crust. They have low Mg# and MgO, Ni and Cr contents, abundant inherited zircons, low ε_Nd(t) and ε_Hf(t) values as well as old whole-rock Nd and zircon Hf model ages. These granodiorites were likely generated by partial melting of Triassic underplated mafic lower crust. The Hf isotopic compositions of the dacites are relatively more depleted than the Cathaysia enriched mantle, suggesting those magmas were derived from the partial melting of subduction-modified mantle sources. The coeval shoshonitic, high-K calc-alkaline and calc-alkaline rocks in Middle to Late Jurassic appear to be associated with an Andean-type subduction. This subduction could have resulted in the upwelling of the asthenosphere beneath the Cathaysia Block, which induced partial melting of the mantle as well as the mafic lower crust, and formed an arc regime in the coastal South China during Middle to Late Jurassic.     

Whole-rock Nd–Hf isotopic study of I-type and peraluminous granitic rocks from the Chinese Altai: Constraints on the nature of the lower crust and tectonic setting

The nature of the lower crust and tectonic setting of the Chinese Altai in the early to middle Paleozoic are still hotly debated. Decoupling between zircon Hf and whole-rock Nd isotopic systems for granites results in different interpretations for the above issues. In order to solve the problem, whole-rock Nd–Hf isotopic analyses were conducted on representative early to middle Paleozoic I-type granite and strongly peraluminous granites and rhyolites from the Chinese Altai. The I-type granites show metaluminous to weakly peraluminous feature and have ε_Nd(t) values ranging from − 2.2 to + 0.8 and ε_Hf(t) from + 3.9 to + 12.9, respectively. The strongly peraluminous granites and rhyolites have similar ε_Nd(t) and ε_Hf(t) values ranging from − 3.0 to + 1.7 and from + 2.1 to + 10.4, respectively. All samples plot above the Terrestrial Array on Nd–Hf isotopic diagram, indicating significant Nd–Hf isotopic decoupling in the magma sources. These samples show flatten HREE pattern and have Lu/Hf ratios similar to the average crust, suggesting that Nd–Hf isotopic decoupling was not originated from an ancient basement with elevated Lu/Hf ratios. The observed isotopic decoupling is similar to those modern island arcs, such as the Lesser Antilles and Sunda, where Nd selectively enriched over Hf due to metasomatism in the mantle wedge and consequently resulted in decoupling between the Sm–Nd and Lu–Hf isotopic systems. Our results, combined with the available data, show that prolonged subduction and crust–mantle interaction caused the Nd–Hf isotopic decoupling in the lithospheric mantle beneath the Chinese Altai. The crust of the Chinese Altai was extracted from the lithospheric mantle and inherited the Nd–Hf isotopic decoupling feature. Therefore, the Hf, rather than Nd, isotopic data more faithfully reflect the nature of the lower crust that was quite juvenile in the Paleozoic, and the Chinese Altai represents an early Paleozoic magmatic arc possibly built near western Mongolia.     

The late Neoarchean transition from low- to high-K granitoids in western Liaoning Province,NE China: Clues from geochemistry and zircon U–Pb–Hf–O isotopes

High-K granitoids are among the most abundant rock types in many Archean cratons. Late Neoarchean monzogranitic to syenogranitic gneisses with high-K affinities are widely distributed in the Anshan, Suizhong, Qinhuangdao, and Aolaishan areas on the northeastern margin of the North China Craton (NCC). In this contribution, we present an integrated study of zircon U–Pb–Hf–O isotopic compositions and whole-rock elemental compositions of amphibolites and trondhjemitic and monzo–syenogranitic gneisses of the Jinzhou area in the metamorphic basement of eastern Hebei–western Liaoning, with the aim of constraining their petrogenesis and geodynamic setting. Emplacement ages of the amphibolites and trondhjemitic–monzogranitic gneisses are 2543 ± 27, 2532 ± 19, and 2513 ± 7 Ma, respectively. The amphibolites are tholeiitic in composition with SiO₂ contents of 49.7–50.8 wt%, variable degrees of light rare-earth-element (LREE) enrichment and high-field-strength element (HFSE) depletion, and high zircon εHf(t) values of +2.6 to +6.3, suggesting a depleted lithospheric mantle origin. The major- and trace-element compositions of the trondhjemitic gneisses are similar to those of the low-pressure tonalite–trondhjemite–granodiorite (TTG) suite. The zircon Hf (εHf(t) = +1.6 to +3.9) and O (δ¹⁸O = +3.76‰ to +5.73‰) isotopic compositions of the trondhjemitic gneisses indicate a juvenile basaltic source at the base of a thickened magmatic arc. The monzogranitic gneisses differ from their TTG counterparts in that they have lower SiO₂ and higher MgO, K₂O, and incompatible-element (especially Ba, Th, Sr, P, and LREE) contents. They also have slightly evolved zircon εHf(t) values (+0.6 to +3.8) and higher δ¹⁸O values (+4.69‰ to +6.13‰). These features suggest that the monzogranitic gneisses represent sanukitoid-type rocks, with a mantle source modified by crust-derived melts. The weakly deformed syenogranitic gneisses are characterized by high SiO₂ and K₂O, and very low MgO, Cr, and Ni contents, suggesting that they were formed by partial melting of local TTG rocks. Our results, together with those of previous investigations, suggest that the 2554–2513 Ma low- to high-K magmatism in the Jinzhou area most likely originated in an arc–back-arc tectonic setting on the northern margin of the NCC. The large volumes of high-K granitoids in eastern Hebei–western Liaoning are related to extensive mantle–crust interactions and crustal reworking in such a setting.     

Zircon Lu-Hf systematics and the evolution of the Archean crust in the southern Superior Province,Canada

A combined Lu-Hf and U-Th-Pb isotopic study was made of 25 zircons and 2 whole rocks from the late Archean crust (2,888-2,668 Ma) in the southern Superior Province, Canada. The relative abundances of U, Th, Lu and Hf in zircons from the low grade Michipicoten and Gamitagama greenstone belts show variable patterns which in part reflect the bulk compositional differences of their parent rocks. Zircons from the high grade lower crustal regions adjacent to these belts (Kapuskasing Structural Zone) are distinguished from the low grade zircons by their strong depletions of Lu and Hf. The low Hf contents imply that the growth of metamorphic zircon involves a significant fractionation of the Zr/Hf ratio.Initial Hf isotope ratios for Hf in zircons from the low grade rocks are correlated with silica enrichment of their host rocks. e _Hf varies from +9.2 to –1.3 and data from similar rock types exhibit correlations of e _Hf with time. Whole rock basalt analyses yield e _Hf values of +8.7 and +11.3 suggesting their derivation from a depleted mantle. The basalt data fall on an evolution trend which implies that differentiation from a chondritic mantle occurred at 3,100-2,900 Ma. Low e _Hf values (–1.3 to +1.4) for rhyolites and granites are consistent with a derivation involving remelting of old crust similar to a 2,888 Ma granite with e _HF of +0.5. Significantly higher values (+1.4 to +3.9) are found in zircons from 2,748-2,682 Ma dacites and tonalites suggesting that their parent rocks had higher Lu/Hf ratios. This may indicate that their parent rocks were mafic. However, there is some evidence that the possible lower crustal source reservoirs of these rocks may have undergone processes early in their histories which increased their Lu/ Hf ratios. This would give rise to the higher e _Hf values observed in their derivatives.     

Magmatism and metallogenic mechanisms of the Baoshan Cu-polymetallic deposit from the Lesser Xing’an Range,NE China: Constraints from geology,geochronology, geochemistry,and Hf isotopes

The Baoshan Cu-polymetallic deposit is a recently discovered skarn deposit in the northern Lesser Xing’an Range, NE China. The orebodies are mainly hosted in the contact zone between granitic intrusions and Lower Cambrian dolomitic crystalline limestones or skarns. We present here zircon U–Pb and molybdenite Re–Os age data, whole-rock geochemistry, and zircon Hf isotopic data to constrain the geodynamic mechanisms of igneous activity and metallogenesis within the Baoshan Cu–polymetallic deposit. LA–ICP–MS zircon U–Pb dating suggests that a hornblende–quartz monzonite and porphyritic biotite granite were emplaced at 252.45 ± 0.70 Ma and 251.10 ± 0.98 Ma, respectively. Molybdenite separated from ore-bearing quartz veins or skarn-type ores yields a weighted mean model age of 250.3 ± 3.4 Ma, which coincide with the emplacement of the igneous rocks. These data suggest that the Late Permian-Early Triassic magmatic and mineralization event led to the formation of the Baoshan Cu–polymetallic deposit. Granitic intrusions are closely associated with this mineralization and have high contents of SiO₂ (60.90–68.98 wt.%), Al₂O₃ (15.15–16.98 wt.%) and K₂O (2.77–4.17 wt.%), with A/CNK ratios of 0.86–0.96. These granites are classified as metaluminous and high-K calc-alkaline I-type granites, and are enriched in Rb, Th, U, and K, and depleted in Nb, Ta, P, and Ti. Moreover, Moreover, the hornblende–quartz monzonite and porphyritic biotite granite have geochemical characteristics similar to adakites and island arc calc-alkaline rocks, respectively. In situ zircon Hf isotope data on the hornblende–quartz monzonite samples show ε_Hf(t) values from +0.1 to +3.1, and porphyritic biotite granite samples exhibit heterogeneous ε_Hf(t) values from −5.4 to +1.1. The geochemical and isotopic data for the Baoshan intrusions indicate that the Late Permian–Early Triassic continental–continental collision caused over thickening and delamination of the lower crust. Partial melting of delaminated lower crust formed the primary adakitic magmas, which may have reacted with surrounding mantle peridotite during ascent. Hornblende–quartz monzonite was formed by the emplacement of the adakitic magmas, whereas the formation of the porphyritic biotite granite was caused by the mixing of adakitic magmas with ancient crustal materials during ascent. Moreover, ore-forming materials were typically derived from the adakitic magmas with high oxygen fugacity, which incorporated significant amounts of ore-forming elements. Based on the regional geological history and the new geochemical and isotopic data from intrusions, we suggest that diagenesis and mineralization of the Baoshan Cu–polymetallic deposit took place in a transitional tectonic setting from collisional orogeny to extension, after collision of the North China Plate and Songnen Block, during the latter stages of the Xingmeng orogeny.     

Tectono-magmatic evolution of the Gaoligong belt,southeastern margin of the Tibetan plateau: Constraints from granitic gneisses and granitoid intrusions

The Gaoligong belt is located in the southeastern margin of the Tibetan plateau, and is bound by the Tengchong and Baoshan blocks. This paper presents new data from zircon geochronology, geochemistry, and whole-rock Sr–Nd–Pb–Hf isotopes to evaluate the tectonic evolution of the Gaoligong belt. The major rock types analysed in the present study are granitic gneiss, granodiorite, and granite. They are metaluminous to peraluminous and belong to high-K, calc-alkaline series. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) analyses of zircons from nine granitic rocks yielded crystallization ages of 495–487 Ma, 121 Ma, 89 Ma, and 70–63 Ma. The granitoids can be subdivided into the following four groups. (1) Early Paleozoic granitic gneisses with high ε_Nd(t) and ε_Hf(t) values of − 1.06 to − 3.45 and − 1.16 to 2.09, and model ages of 1.16 Ga to 1.33 Ga and 1.47 Ga to 1.63 Ga, respectively. Their variable ⁸⁷Sr/⁸⁶Sr and Pb values resemble the characteristics of the Early Paleozoic Pinghe granite in the Baoshan block. Our data suggest that the rocks were derived from the break-off of the Proto-Tethyan oceanic slab between the outboard continent and the Baoshan block, which induced the partial melting of Mesoproterozoic pelitic sources mixed with depleted mantle materials. (2) Early Cretaceous granodiorites with low ε_Nd(t) and ε_Hf(t) values of − 8.92 and − 4.91 with Nd and Hf model ages of 1.41 Ga and 1.49 Ga, respectively. These rocks have high initial ⁸⁷Sr/⁸⁶Sr (0.711992) and lower crustal Pb values, suggesting that they were derived from Mesoproterozoic amphibolites with tholeiitic signature, leaving behind granulite residue at the lower crust. (3) Early Late Cretaceous granites with low ε_Nd(t) and ε_Hf(t) values of − 9.58 and − 4.61 with Nd and Hf model ages of 1.43 Ga and 1.57 Ga, respectively. These rocks have high initial ⁸⁷Sr/⁸⁶Sr (0.713045) and lower crustal Pb isotopic values. These rocks were generated from the partial melting of Mesoproterozoic metapelitic sources resulting from the delamination of thickened lithosphere, following the closure of the Bangong–Nujiang Ocean and collision of the Lhasa–Qiangtang blocks. (4) Late Cretaceous to Paleogene granitic gneisses with low ε_Nd(t) and ε_Hf(t) values of − 4.41 to − 10 and − 5.95 to − 8.71, Nd model ages ranging from 1.08 Ga to 1.43 Ga, and Hf model ages from 1.53 Ga to 1.67 Ga, respectively. These rocks show high initial ⁸⁷Sr/⁸⁶Sr (0.713201 and 714662) and lower crustal Pb values. The data suggest that these rocks are likely related to the eastward subduction of the Neo-Tethyan Oceanic slab, which induced partial melting of Mesoproterozoic lower crustal metagreywacke. The results presented in this study from the Gaoligong belt offer important insights on the evolution of the Proto-Tethyan, Bangong–Nujiang, and Neo-Tethyan oceans in the southeastern margin of the Tibetan Plateau.