Hf Isotope Research Articles

Hf-Nd isotope decoupling in S-type granitoids caused by disequilibrium melting of metasedimentary rocks: Evidence from the North Qinling Terrane, Central China

The intrinsic assumption of radiogenic isotope tracing is that isotope systems maintain equilibrium between melt and source. However, this assumption is not naturally correct, particularly for S-type granites, which are formed by partial melting of metasedimentary rocks. The S-type metagranites from the North Qinling Terrane show various degrees of HfNd isotope decoupling and discrepancies between whole-rock and zircon Hf isotopes. Zircon UPb dating yields emplacement ages from 946 to 890 Ma. Their high δ18O values, (87Sr/86Sr)i ratios, and negative εNd(t) values overlapped with metasedimentary rocks in the Qinling Group imply that they are derived from partial melting of the Qinling Group. The low Rb/Ba, Rb/Sr, Al2O3/TiO2, and high CaO/Na2O ratios indicate that their sources are metagraywackes. The negative correlation between decoupling degrees of HfNd isotopes and Zr concentrations suggests that the incomplete dissolution of zircon during partial melting results in the decoupling. The discrepancy between whole-rock and zircon Hf isotopes can be attributed to the combined effect of incomplete dissolution of zircons and the release of various Hf isotope compositions from partly dissolved inherited zircons to local melts. The compiled literature data suggest that whole-rock HfNd isotope decoupling of most metagraywacke-derived S-type granites is caused by incomplete melting of zircons, whereas those of metapelite-derived S-type granites may be controlled by both incomplete melting of zircons and source inheritance.

Connection between tectonothermal events of the Yakutian kimberlite province and assembly of the Siberian craton

The tectonic framework of the Siberian craton is still under debate. It is generally assumed that the Markha and Daldyn terranes collided at 2.2–2.1 Ga and the final assembly of the Siberian craton occurred at 1.9–1.8 Ga. However, previous study on zircons from xenoliths of the Zapolyarnaya pipe, which is located in the Upper Muna kimberlite field and close to the boundary between the Markha and Daldyn terranes, do not show tectonothermal evidence younger than 2.7 Ga. To clarify the tectonothermal evolution of the Siberian craton, we present U-Pb ages and Hf isotope data of zircons from crustal xenoliths in the Novinka and Komsomolskaya Magnitnaya kimberlite pipes in the Upper Muna field. The zircon ages confirm only one major tectonothermal event at 2.7 Ga in the Upper Muna field, which doubts the existence of the collision zone between the Markha and Daldyn terranes. The middle-lower crust beneath the Upper Muna field is mainly composed of mafic and intermediate granulites and experienced long-term cooling. The negative values of εHf(2.7) suggests ignorable addition of juvenile component during partial melting of the Paleoarchean crust at 2.7 Ga. Compared with crustal xenoliths from other kimberlite pipes in the Yakutian diamondiferous province, the crust of the Anabar province shows vertical and lateral heterogeneity and the absence of a relationship between the crustal reworking degree and the main collision zones. We propose that the widespread 2.7 and 1.9 Ga tectonothermal events in the Anabar tectonic province were associated with the episodic rise of superplumes, which not only caused the crustal growth and reworking but also facilitated collision and amalgamation of terranes.

Continental Arc Accumulation Mafic Rocks in the Mid-Upper Crust: Constraints From the Early Eocene Hornblende Gabbro–Diorite in the Tengchong Block, Southeastern Extension of Tibet

Abstract Magmatic activity in the syn-collision stage is key for net crustal growth. To understand the mechanism of accretion–differentiation and compositional change of the continental crust, it is important to focus on the magmatic activity during the syn-collision stage. Early Eocene mafic–ultramafic rock assemblages found in the western part of the Tengchong Block resulted from a continuous series of arc magmatic evolution, thoroughly recording the continental arc magmatic system during the subduction of the Neo-Tethys Ocean and syn-collision of the Indian-Asian continents. Early Eocene hornblende gabbro–diorite in the Tengchong Block formed at 53 Ma, and the primitive magma was derived from an enriched mantle source due to the enriched Nd–Hf isotopes. The amphibole and biotite thermobarometer measurements indicate that the mafic magma reservoirs in the Tengchong Block occurred at a mid-upper crust. Petrography, amphibole Fe/Mg exchange coefficient (KD), Rayleigh fractionation, and equilibrium melt calculation indicate that the Early Eocene hornblende gabbro–diorite in the Tengchong Block was created due to plagioclase-dominated accumulation at the mid-upper crust level. Based on the calculation, the corresponding amphibole equilibrium melt is more silicic (dacitic–rhyolitic in composition) than the bulk rocks, indicating a more evolved composition in the mid-upper crust. Three types of plagioclases reveal the multi-recharging and dissolution–reprecipitation promoting the further evolution of these mafic rocks. Therefore, this study concludes that magma recharge and plagioclase-dominated accumulation processes may be important mechanisms for the formation and evolution of mafic magma and the further crustal differentiation at the mid-upper crust level in a continental margin arc.

Metallogenetic epoch and material source of Be-pegmatite differentiated from biotite granite in Mufushan complex

The Mufushan Complex (MFSC) is a significant rare metal mineralized in the central China district and is renowned for its marked economic value of rare metal deposits. While the main rare metal mineralization (RMM) stage in the MFSC relates to the magmatic event of two-mica monzogranite, the mineralization event related to biotite monzogranite remains unknown. To fill this gap, this study focuses on porphyritic biotite monzogranite and adjacent Be-rich pegmatite samples obtained from Changqing Pegmatite No. 1, which is situated in the southeastern part of the MFSC. We subjected these samples to zircon U–Pb dating and Hf isotope analysis using laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) and utilized an electron probe X-ray microanalyzer (EPMA) and LA–ICP–MS to examine the major and trace elements present in the muscovite samples. Our findings revealed precise chronological data for the geological history of Changqing, pinpointing the intrusion age of biotite monzogranite to 145.8 ± 1.1 Ma and the crystallization age of Be-rich pegmatite to 144.9 ± 1.1 Ma, firmly situating these events within the Late Jurassic to Early Cretaceous epoch. Regarding isotopic composition, zircon εHf (t) for granite ranged from −9.29 to −5.09, and their corresponding TDM2 ranged from 1788 to 1520 Ma. Similarly, zircon εHf (t) for pegmatite ranged from −7.89 to −5.47, and their corresponding TDM2 ranged from 1699 to 1544 Ma. These isotopic signatures strongly indicate a shared origin for the initial magma of granite and pegmatite. The partial melting of the thick Lengjiaxi Group tasedimentary strata serves as a material source for RMM. The transition from granite to pegmatite exhibits remarkable constancy in the primary components of muscovite. By examining its geochemical attributes, we observed synchronous decreases in FeO, MgO, SiO2, MnO, Be, Rb, and Cs, coupled with synchronous increases in Nb and Ta contents, along with Ta/Nb values. These findings strongly imply that the Changqing biotite monzogranite and Be-rich pegmatite originate from the continuous differentiation and evolution of a common magmatic source, establishing a clear genetic relationship between them. The MFSC has its origins in three distinct stages of magmatic activity and RMM spanning from 160 Ma to 120 Ma. Considering the distribution patterns of similar pegmatite veins, the metallogenic potential observed during the biotite granite stage may serve as an external indicator of marked mineralization within large batholiths.

Zircon provenances of the late Paleoproterozoic metasedimentary rock in the South Yangtze Block: Implications for paleogeographic reconstruction

ABSTRACT The zircon provenances of the late Paleoproterozoic metasedimentary rocks are important for assembling Proto-Yangtze and its reconstruction in the Nuna supercontinent. We present a dataset involving 363 detrital zircon U-Pb dating and 165 Hf isotope analyses from the Laochanghe Formation of the Dahongshan Group in the southwestern Yangtze Block. New detrital zircon U-Pb ages, combined with the heterogeneity of sedimentary strata and available magmatic age data, suggest that the maximum depositional age of the Laochanghe Formation cloud be younger than 1.81 Ga and older than 1.74 Ga. The zircon U-Pb-Hf isotope compositions of the volcanic and metamorphic rocks overlap with those of the late Paleoproterozoic metasedimentary rock, suggesting the southwestern Yangtze Block provided provenance at 1.9, 2.3, and 2.7 Ga. This characteristic is significantly different from the northern Yangtze Block only provided partial provenance at 1.9 and 2.7 Ga. The northern and southwestern of the Yangtze Block jointly provide material for the late Paleoproterozoic metasedimentary rocks, combining synchronous late Paleoproterozoic magmatism and metamorphism, indicative of the formation of the proto-Yangtze Block in response to the accumulation of the Nuna supercontinent. Additionally, the potential provenance areas of the late Paleoproterozoic metasedimentary rock in the southwestern Yangtze include the northwestern Laurentia and southern Siberia, excluding the northern Australian, and Cathaysia. These findings indicate that the southwestern Yangtze may have been located between northwestern Laurentia and southern Siberia during the partial fragmentation of the Nuna supercontinent, enhancing our understanding of changes in the Nuna supercontinent over time.

Petrogenesis of newly identified mantle-plume-related ca. 3.3 Ga tholeiitic-komatiitic rocks in the Anshan area, North China Craton

The geodynamic regime that governed the crustal evolution of Earth during the early Archean remains intensely debated. The North China Craton (NCC) preserves a geological history spanning 3.8–2.5 Ga, particularly in the Anshan area, making it an ideal natural laboratory for probing the formation and evolution of the NCC and its associated geodynamic regime. Here, we document new geochronology, geochemistry, and zircon Hf-O isotope data from newly discovered Paleoarchean meta-mafic rocks in the Anshan area. Based on their whole-rock geochemistry, these unit can be divided into tholeiitic and komatiitic series. Of which, the komatiitic rocks are characteristic by high MgO contents (up to 20.6 wt%). Zircon U-Pb dating shows that all meta-mafic rocks formed contemporaneously, with crystallization ages of ca. 3.3 Ga. Most of the studied meta-mafic rocks display radiogenic zircon Hf isotopes, with TDM1 ages ranging from 3592 to 3346 Ma and εHf(t) values from −1.94 to + 3.49. They also exhibit mantle-like zircon δ18O values (+4.92 ‰ to + 5.98 ‰). These zircon Hf-O isotope data indicate that the protoliths of the ca. 3.3 Ga meta-mafic rocks originated from the partial melting of a depleted mantle without significant crustal contamination. Furthermore, the parental magmas of the komatiitic rocks and some of tholeiitic rocks were generated from 20 % and 5–10 % partial melting of spinel-lherzolite, respectively, whereas other tholeiitic rocks were the result of a low degree (1–5 %) of partial melting of spinel-garnet lherzolite. The ca. 3.3 Ga tholeiitic-komatiitic rocks were most likely formed in an intraplate rifting setting, associated with a mantle plume.

U-Pb geochronology and Hf isotopes of the Grenvillian Río Hondo gneisses, Puebla: redefining the western edge of Oaxaquia

In situ Río Hondo (Puebla, southern Mexico) gneiss samples, as well as clastic samples recovered from the nearby-outcropping latest Paleozoic Matzitzi Formation, a fluvial unit mainly sourced by Grenvillian rocks, were historically interpreted as representative of the northernmost exposures of the Oaxacan Complex, the largest outcrop of ortho and metasedimentary units, made up of mostly 1.0–1.3 Ga protoliths, affected cfby local migmatization (ca. 1.1 Ga) and granulite facies metamorphism at ca. 0.98 Ga, constituting the most prominent outcrop of the Oaxaquia microcontinent.
 U-Pb geochronology and Lu-Hf isotopic determinations in zircon by LA-(MC)-ICP-MS were performed on both in situ and clast samples. In situ basement samples record a ca. 1.2 crystallization event, together with a younger one at ca. 1.02 Ga. both lacking inherited >1.3 Ga components. The clasts have an unimodal zircon U-Pb age distribution, recording a crystallization event at ca. 1.2–1.27 Ga. Scarce inherited zircon cores between ca. 1.4–1.6 Ga were found, with only a few samples with a broader age distribution, suggesting a detrital protolith with zircon cores as old as ca. 1.8 and 2.4 Ga. No zircon overgrowths or geochemical-petrographic evidences are indicative of granulite metamorphism. Furthermore, all the studied metaigneous samples show discordant zircon ages produced by Pb loss events barely constrained between the latest Paleozoic to the Mesozoic.
 Hf isotopes reveal that zircon crystals from clasts have a range of εHf (1.25 Ga) ≈+1 to +5 and yield Hf model ages from 1.7 to 1.9 Ga. On the other hand, the The zircon Hf isotopes of one analyzed basement sample reveal a higher range of εHf (1.25 Ga) ≈+7 to +9, and Hf model ages from 1.5 to 1.6 Ga.
 Both ≈1.2 Ga and 1.02 Ga events are consistent with magmatic ages previously documented elsewhere in Oaxaquia, interpreted as indicating portions of the NW Amazonia-Oaxaquia arc system with cratonic influence or to slices of Baltica thrust over Oaxaquia during the Grenville orogeny. However, the absence of granulite facies indicators, such as zircon metamorphic ages and/or granulite paragenesis (typically, in other Oaxaquia samples, orthopyroxene and garnet) are interpreted as prime evidence that the studied samples didn’t undergo such high grade of metamorphism. Río Hondo gneisses, as this sequence is informally named, must belong to a source that had the influence of an older continental crust and can be tentatively associated either with rocks recently described in the Sierra de Juárez, or those belonging to the central basement of the Maya block, currently exposed farther to the SE in Chiapas.

Neoproterozoic low-T/P metamorphism in the Yangtze Block manifests a long-lived subduction girdle around Rodinia

Identifying long-lived subduction at the periphery of supercontinents provides insights into the processes of block assembly and dispersal within the supercontinent cycle. The Yangtze Block of South China stands as a key component during the transition from Rodinia to Gondwana, characterized by active tectono-magmatic processes throughout the early to middle Neoproterozoic. However, the lack of metamorphic records associated with accretionary orogenesis results in ambiguity about the tectonic regime and its evolution in Rodinia during this period. Here, we present evidence of low-temperature/pressure ratio (low-T/P) metamorphism in phengite-bearing orthogneisses from the western margin of the Yangtze Block (WYB). Through petrographic investigations and zircon U–Pb–Hf–O isotopic analysis in core and rim domains, we identify two distinct metamorphic rock types, with protolith ages of 1419 ± 19 Ma and 831 ± 9 Ma. Both rock types record metamorphism at ca. 830–820 Ma, accompanied by infiltration of externally-derived melts with high-δ18O and unradiogenic Hf isotopes. Our phase equilibria modeling, combined with mineral thermometry and Si-in-phengite content, shows peak metamorphic conditions of ∼550 °C and ∼8.2–8.6 kbar, corresponding to a cold geothermal gradient (19.0–20.5 °C/km). The low-T/P metamorphism is interpreted to occur either in a subduction zone accretionary wedge or in an overriding forearc continental crust, providing robust evidence for Neoproterozoic orogenesis of the WYB within a subduction framework. Based on petrological, isotopic, and paleomagnetic data, we conclude that the WYB represents part of a long-lived subduction girdle (at least ca. 970–750 Ma) at the periphery of Rodinia. Since the early stage of Rodinia breakup, this girdle was involved in alternating advance and retreat accretionary orogenesis as the Yangtze Block continuously drifted away from a mantle upwelling toward the degree-2 girdle of mantle downwelling. Our study provides a snapshot of the evolution of a complete long-lived circum-Rodinian subduction girdle during the transition from Rodinia to Gondwana.

Carboniferous–Early Permian volcano-sedimentary units from the northern margin of the North China Craton recorded subduction initiation in the Paleo-Asian Ocean

The precise timing of subduction initiation of the Paleo-Asian Ocean (PAO) in the northern margin of the North China Craton (NCC) remains controversial. In this study, we have conducted zircon UPb dating, whole-rock major and trace elements, and zircon Hf isotope analyses on metadacites and metamorphic greywackes from the Changde volcano-sedimentary rocks, feldspar quartz sandstones and argillaceous siltstones from the Luquantun Formation and the Sidao sedimentary rocks in central-eastern Jilin Province (NE China), to constrain the initial closure of the PAO. We obtained Early Carboniferous zircon UPb ages (345 ± 2 Ma and 351 ± 7 Ma) from the Changde volcano-sedimentary rocks. The metadacites are characterized by the enrichment in light rare earth elements and large ion lithophile elements, depletion in heavy rare earth elements and high field strength elements, and rare earth element ratios similar to those of the continental crust, indicating that these rocks originated from crustal melting. The metadacites exhibit affinity with the peraluminous A-type granite, with high zircon εHf(t) values (+3.46 − +12.21) and TDM2 of 1132–571 Ma, suggesting that they formed in a post-collisional extensional setting. Detrital zircon UPb ages from the Luquantun Formation and Sidao sedimentary rocks constrain their maximum depositional ages to the Late Carboniferous (319 ± 5 Ma) and Early Permian (284 ± 4 Ma), respectively. Most detrital zircons have age peaks at 1800 Ma with negative zircon εHf(t) values, and 2500 Ma with positive zircon εHf(t) values, suggesting that the Carboniferous–Early Permian sediments from these two formations were mainly sourced from the NCC. Moreover, the chemical index of alteration, index of compositional variability, and plagioclase index of alteration values suggest that the sediments of the Changde volcano-sedimentary rocks originated from a mafic igneous source with a low degree of weathering and a sedimentary recycling history, while the samples from the Luquantun Formation and Sidao sedimentary rocks were sourced from an intermediate-felsic igneous rock with a moderate to intense degree of weathering and mainly influenced by source composition. The crystallization age curves and summary plots of detrital zircons indicate that the Early Carboniferous metamorphic greywackes from the Changde Formation were mainly deposited in a passive margin, while the Carboniferous–Early Permian sediments of the Luquantun Formation and Sidao sedimentary rocks were mainly deposited in an active margin. We suggest that the tectonic transition from passive to active margin occurred at ∼333 Ma, marking subduction initiation in the PAO close to the northern margin of the NCC.

North Lhasa terrane in the Precambrian originated from Western Australia

Constraining the origin and Paleogeographic positions of ancient continental fragments plays a crucial role in reconstructing and validating supercontinent cycles. The Lhasa terrane in southern Tibet is the key to revealing the early formation and evolution of the Himalayan–Tibetan orogen. Although previous origin models of the Lhasa terrane are supported by Phanerozoic evidence, Precambrian geological records have not been comprehensively considered. The U–Pb age and Hf isotope data on detrital zircons from newly discovered Precambrian basement in the North Lhasa terrane define two distinctive age populations of ca. 1170 Ma with εHf(t) values identical to the coeval detrital zircons from Western Australia, and ca. 950 Ma with a similar εHf(t) range to the contemporaneous detrital zircons from South Qiangtang and Tethyan Himalaya terranes. The ca. 1170 Ma detrital zircons were most likely derived from Western Australia, whereas the ca. 950 Ma detrital zircons might have been sourced from the norther Indian margin. Compiled detrital zircon data shows that in the early Neoproterozoic, the affinity between northern Lhasa and Western Australia decreased, while the predilection between northern Lhasa and northern India increased. Combined with ca. 930–870 Ma rift, ca. 870–700 Ma arc igneous rocks and ∼650 Ma high-pressure granulites, we propose that the North Lhasa terrane was a ribbon continent that rifted from Western Australia in the early Neoproterozoic (ca. 930–870 Ma), drifted westward along the northern edge of Rodinia in the middle Neoproterozoic (ca. 870–700 Ma), and was emplaced in the Northern East African Orogen during the late Neoproterozoic (∼650 Ma).

Petrogenesis of early-middle jurassic igneous rocks in the Central Jilin Province, NE China: implications for the early subduction of the Paleo-Pacific ocean and crustal growth

ABSTRACT Crustal growth is closely associated with the oceanic subduction process, although the precise mechanism involved remains poorly constrained. Northeast (NE) China is an important component of the largest accretive orogenic belt worldwide – Central Asian orogenic belt. Its accretionary process was dominated by the Paleo-Asian and Paleo-Pacific Oceans; thus, NE China can provide a promising insight to explore this complex mechanism. We report new geochronological, geochemical and Hf isotopic compositions of the igneous rocks found in the Central Jilin Province, NE China. The geochronological results reveal that these igneous rocks were formed during 174.7 − 167.7 Ma (i.e. Early−Middle Jurassic). The quartz diorite and monzogranites from the northeastern margin of the North China Craton have high Sr (314 − 757 ppm) content, (La/Yb)N (7.11 − 29.9) and Sr/Y (41.50 − 157) ratios, low Y (2.03 − 12.97 ppm) and Yb (0.30 − 1.34 ppm) contents, exhibiting adakitic features. Coupling with their significant negative εHf(t) values (−32.51 to − 5.45), we propose that these igneous rocks were originated from the partial melting of an ancient thickened lower crust. In contrary, the syenogranite, granite porphyry, and porphyritic monzogranite from the Zhangguangcai and Jiamusi blocks are characterized by high K calc-alkaline, low MgO (0.03 − 0.88 wt.%), Zr + Nb + Ce + Y (141 − 462 ppm) contents and Sr/Y (0.93 − 17.40) ratios, indicating an affinity of I−type granite. In view of their positive εHf(t) values (5.41 to 9.47), we suggest that they were originated from the partial melting of a juvenile lower mafic crust. Despite they have different magma sources, all the Jurassic igneous rocks exhibit typical geochemical signatures associated with subduction, such as enriched in large ion lithophile elements and depleted in high field strength elements. Considering the spatial-temporal distribution of coeval igneous rocks within the region, we propose that the Early Jurassic is the early subduction of the Paleo-Pacific Ocean. The Jurassic igneous rocks can be divided into two categories, one with high Sr/Y ratios and negative Hf isotope, and the others with low Sr/Y ratios and positive Hf isotope, which may indicate that crustal architecture in subduction zone has a significant effect on continental crustal growth and reworking in NE China.

Vestiges of Earth’s earliest depleted mantle reservoir

Abstract There is a paucity of evidence preserved in the rock record regarding Earth’s earliest enriched crust and its complementary depleted mantle during the Hadean. In recent years, vestiges of these early reservoirs have been inferred by examination of Hf isotope systematics compiled from zircons. The Singhbhum craton of Eastern India, for example, preserves only the existence of an enriched (εHf <0) crustal reservoir during the Hadean–Eoarchean, with the notable absence of a depleted mantle reservoir signature (εHf >0) until ca. 3.5 Ga. Here we report a new Sm-Nd isochron for the Lower Lava greenstones of the western Iron Ore Group from the Singhbhum craton, confirming a 3.42 ± 0.14 Ga crystallization age with an initial εNd of +5.7 ± 2.5. This is the highest positive εNd value derived from an isochron of this age. We infer that this depleted mantle source is a vestige complementary to the primary crust following planetary differentiation. Furthermore, we present U-Pb zircon ages for a 3.39 ± 0.02 Ga tuff that lies stratigraphically above the Lower Lava and <30 cm below an extensive conformable banded iron formation (BIF). This age implies that the western Iron Ore Group’s BIF is the largest economic-grade iron formation of its Paleoarchean age, suggesting that free atmospheric oxygen existed as more than just whiffs at this time.

A special ore formation of the newly discovered Hamaling Au-Cu-Mo polymetallic deposit in the Tongling ore-cluster region: Constraints from geochronology and geochemistry

The Hamaling deposit represents the first discovery of a gold-copper-molybdenum deposit situated within the Ordovician strata of the Tongling ore-cluster region. It comprises three distinct types of ore, including porphyry-type Cu-Au ore found in granodiorite, skarn-type Cu-Au ore at the contact zone, and breccia-type Mo-Cu ore within the Ordovician strata. However, the exact mineralization age, ore genesis, and mineralization processes of the Hamaling deposit remain uncertain. To address these uncertainties, the present study employed LA-ICP-MS to date zircon in granodiorite and garnet in skarn, revealing ages of 139.3 ± 0.44 Ma and 142.3 ± 0.89 Ma, respectively. Additionally, the age of 140.0 ± 1.1 Ma was obtained through Re-Os dating of molybdenite from breccia-type ore. These findings suggest a connection between the Hamaling deposit and the early Cretaceous magmatic activity in the Tongling region. The granodiorite intrusion exhibits a high-potassium calc-alkaline affinity and formed under oxidized conditions at temperatures ranging from 721 to 832 °C (average 769 °C). The Hf isotope composition of the granodiorite implies its formation through the mixing of mantle-derived magma from an enriched lithospheric mantle with partially melted lower crustal source magma. The sulfur isotopes found in the ore minerals range from δ34S = 5.55 ‰ to 11.99 ‰, indicating a magmatic sulfur source. The H-O isotopic compositions of quartz during the quartz-sulfide stage suggest that the magmatic water was the primary component of the fluids responsible for ore formation, possibly with some contribution from meteoric water. The identification of the Hamaling deposit highlights the significant potential for mineral exploration within the Lower Paleozoic strata in the Tongling ore-cluster area and even the broader Middle-Lower Yangtze River Valley Metallogenic Belt (MLYB).

Tectonic and Crustal Processes Drive Multi-Million Year Arc Magma Evolution Leading up to Porphyry Copper Deposit Formation in Central Chile

Abstract Subduction zone magmatism is a major control of volcanism, the generation of modern continental crust and the formation of economically important porphyry Cu–(Mo–Au) deposits. Reading the magmatic record of individual arc segments and constraining the rates of magmatic changes are critical in order to fully understand and quantify the processes that drive magma evolution in subduction settings during arc growth. This study focuses on the San Francisco Batholith and the Rio Blanco-Los Bronces porphyry deposit cluster in central Chile, which provides an igneous rock record over ~13.5 Myr of arc evolution. We use whole-rock geochemistry, zircon geochronology and Hf isotope geochemistry to track changes in the crustal magmatic system of this arc segment during crustal thickening and porphyry Cu deposit formation. By combining the analytical dataset with Monte Carlo fractional crystallisation and assimilation fractional crystallisation modelling, we test a model for significant crustal involvement during magma evolution. Systematic and continuous increases in Dy/Yb, La/Yb, V/Sc and Sr/Y in the magmas over time indicate a transition in the main fractionation assemblage from plagioclase-dominated to amphibole-dominated that reflects deeper crystallisation and/or a higher meltwater content. Concomitant decreases in εHf and Th/La as well as increasing Ba/Th are best explained by assimilation of progressively deeper crustal lithologies from low (Chilenia) to high Ba/Th (Cuyania) basement terranes. Our study highlights that an increasingly hydrous magma and a deepening locus of crustal magma differentiation and assimilation, driven by crustal thickening contemporaneous with increased tectonic convergence and ingression of the aseismic Juan Fernandez ridge, can account for all investigated aspects of the multi-Myr magmatic evolution leading up to the formation of the Rio Blanco-Los Bronces porphyry Cu deposits. Our findings corroborate the importance of high-pressure differentiation of hydrous magma for the formation of Andean-style porphyry deposits. Once magmas favourable for porphyry Cu mineralisation were generated in the lower crust, multiple episodes of efficient magma migration into the upper crust fed several, discrete, shallow magmatic-hydrothermal systems over ~3.5 Myr to form the world’s largest known Cu resource at Rio Blanco-Los Bronces.

Controls on apatite compositional variations in magmatic rocks: A case study of Mesozoic mafic–felsic intrusions from the eastern North China Craton

Compositional variations of apatite are mainly determined by parameters such as partition coefficients and the composition of parental magma. However, the relative importance of these parameters in relation to apatite geochemistry in different lithologies of magmatic rocks is not well constrained. In this study, six Mesozoic (219–154 Ma) mafic–felsic intrusions from the eastern North China Craton, were selected for detailed apatite microanalyses (trace elements and Nd isotopes). Apatite from the studied intrusions has uniform textures in the cathodoluminescence image. They exhibit high abundances of rare earth elements (REE) and are characterized by light REE enriched patterns, indicative of a magmatic origin. Alongside compiled apatite data, these findings suggest that compositional variations of REE + Y in apatite from mafic to felsic magmatic rocks are controlled primarily by elemental partitioning between apatite and melts. The competitive crystallization of other accessory minerals prior to apatite can be responsible for compositional variations of Sr, Pb, Th, and U. New Nd isotope data for apatite are discussed and compared with whole-rock Nd and zircon Hf isotope data for the same sample. The Nd isotopic compositions between the whole-rock and apatite are identical. The integrated dataset provides an alternative interpretation for the isotopic decoupling between apatite Nd and zircon Hf isotope systems. In conclusion, apatite REE concentrations and Nd isotopes have the potential to serve as geological indicators.

New geochemical and geochronological insights on forearc magmatism across the Sanak-Baranof belt, southern Alaska: A tale of two belts

Abstract The Sanak-Baranof belt includes a series of near-trench plutons that intrude the outboard Chugach–Prince William terrane over ~2200 km along the southern Alaskan margin. We present new petrological, geochronological, and geochemical data for comagmatic microgranitoid enclaves and granitoid rocks from the Crawfish Inlet (ca. 53–47 Ma) and Krestof Island (ca. 52 Ma) plutons on Baranof and Krestof Islands, as well as the Mount Stamy (ca. 51 Ma) and Mount Draper (ca. 54–53 Ma) plutons and associated mafic rocks that intrude the Boundary block at Nunatak Fiord near Yakutat, Alaska, USA. These data suggest that intrusion of the Sanak-Baranof belt plutons is actually a tale of two distinct belts: a western belt with crystallization ages that young systematically from west to east (63–56 Ma) and an eastern belt with crystallization ages ranging from 55 to 47 Ma, but with no clear age progression along the margin. Hf isotope analyses of magmatic zircon from the western Sanak-Baranof belt become increasingly evolved toward the east with εHft = 9.3 ± 0.7 on Sanak Island versus εHft = 5.1 ± 0.5 for the Hive Island pluton in Resurrection Bay. The Hf isotope ratios of eastern Sanak-Baranof belt rocks also vary systematically with age but in reverse, with more evolved ratios in the oldest plutons (εHft = +4.7 ± 0.7) and more primitive ratios in the youngest plutons (εHft = +13.7 ± 0.7). We propose that these findings indicate distinct modes of origin and emplacement histories for the western and eastern segments of the Sanak-Baranof belt, and that the petrogenesis of eastern Sanak-Baranof belt plutons (emplaced subsequent to 57–55 Ma) was associated with an increasing mantle component supplied to the youngest eastern Sanak-Baranof belt magmas. These plutons reveal important information about offshore plate geometries and a dynamic period of plate reorganization ca. 57–55 Ma, but a clearer picture of the tectonic setting that facilitated these Sanak-Baranof belt intrusions cannot be resolved until the magnitude and significance of lateral translation of the Chugach–Prince William terrane are better understood.

Petrogenesis and Tectonic Implications of the Oligocene Dalongtan Shoshonitic Syenite Porphyry in Central Yunnan, Southeastern Tibetan Plateau: Constraints from Geochronology, Geochemistry and Sr-Nd-Hf Isotopes

Shoshonitic rocks are widely distributed in post-collisional settings and provide key information on deep geodynamic mechanisms and magmatic evolution. In this paper, we present petrographic, zircon U-Pb age-related, trace elemental, Hf isotopic, bulk-rock elemental, and Sr-Nd isotopic data of the Dalongtan shoshonitic syenite porphyries (DSSPs) in central Yunnan, southeastern Tibet. The DSSPs formed at 33.2 ± 0.3 Ma in a post-collisional setting. They define linear trends on Harker diagrams, and they display similar trace element patterns and enriched bulk-rock Sr-Nd isotopes [(87Sr/86Sr)i = 0.70964–0.70968, εNd(t) = −12.9 to −12.7] and zircon Hf isotopes (εHf(t) = −15.7 to −13.1) to the coeval mantle-derived potassic mafic rocks. This suggests that the DSSPs were fractionated from the lithospheric mantle-derived mafic magmas. The DSSPs, along with the coeval felsic and mafic magmatic rocks (37.2–32.3 Ma), exhibit a planar distribution on the SE Tibet and predate the left-lateral shearing of the Ailaoshan–Red River shear zone (ARSZ) (32–22 Ma), suggesting that there are no genetic relationships between them. The DSSPs have geochemical characteristics similar to those of A-type granites, with high total alkalinity (10.39–11.17 wt.%), HFSE concentrations (Zr + Nb + Ce + Y = 890.2–1054.3 ppm), Ga/Al ratios (10,000 × Ga/Al = 2.95–3.46), whole-rock zircon saturation temperatures (906–947 °C), and oxygen fugacity (ΔFMQ = +3.30–+4.65), indicating that they are products of the high-temperature melting of the lithosphere as a result of asthenosphere upwelling in extensional settings. Based on our data and regional observations, it is proposed that the generation of the DSSPs may be linked to the convective thinning of the thickened lithospheric mantle following the India–Asia collision.

Crustal evolution of a continental magmatic arc from subduction to collision: A case study in the Gangdese arc, southern Tibetan Plateau

Magmatic arcs are the main environment where continental crust is created on the post-Archean Earth; however, how juvenile arc crust evolves into mature continental crust is still controversial. In this study, we report new bulk-rock major and trace elements, Sr-Nd isotopes, and zircon U-Pb ages and Hf isotopes from a large suite of granites collected from the eastern segment of the Gangdese arc, southern Tibetan Plateau, which record a complete history of arc crust evolution from Mesozoic subduction to Cenozoic collision. These new data show that Gangdese crust-derived granites generated during the subduction to collisional stages record significant geochemical changes with age, indicating that the bulk composition, lithological makeup, and thicknesses of the arc crust evolved over time. Here, we propose that the Gangdese arc had a thick juvenile crust with a small volume of ancient crustal components during late-stage subduction of the Neo-Tethys Ocean, a thin juvenile crust with heterogeneously distributed ancient crustal materials during early collision, and a thick juvenile crust with minor proportions of ancient rocks during late collision. This implies that the arc experienced episodes of crustal thickening during the Late Cretaceous and Eocene, interspersed by periods of thinning during the Paleocene and Miocene, and several discrete episodes of partial melting in the lower arc crust, and cycling or recycling of juvenile and ancient crustal materials within the arc crust and between the crust and mantle. We suggest that shallow subduction of the Neo-Tethys during the Late Cretaceous promoted tectonic thickening of the arc crust, partial melting of lower crust, and formation of high Sr/Y granites. After the onset of the Indo-Asian collision, breakoff of the subducted Neo-Tethyan oceanic slab during the Paleocene/early Eocene allowed thinning of the overlying arc crust and generation of granites derived from juvenile and ancient crustal sources. Continued underthrusting of the Indian continental crust and subsequent delamination of thickened lithospheric mantle led to thickening and thinning of the arc crust, respectively, and partial melting of thickened lower crust and generation of high Sr/Y granites during the Oligocene and Miocene. Using the Gangdese as an analogue for post-Archean continental margins, we suggest that the repeated thickening and thinning of arc crust, and associated multistage remelting of the lower arc crust, and material cycling or recycling within the crust and between the crust and mantle from subduction to collision are common processes that drive maturation of juvenile arc crust.

Reworking and maturation of continental crust in collision zones: Insights from Early Cretaceous compositionally diverse magmatic rocks in central Tibet

The reworking of continental crust in collision zones is a key process in the chemical maturation of crustal rocks. However, the mechanisms that cause crustal reworking remain highly controversial. The compositionally diverse magmatic rocks along the Lhasa–Qiangtang collision zone provide a favorable opportunity for studying the mechanism of crustal reworking and maturation in continental collision zones. Here we report zircon UPb ages and Hf isotopes, mineral major elements, and whole-rock geochemistry and SrNd isotopes for the Momu intrusive complex in the Lhasa–Qiangtang collision zone, central Tibet. Zircon UPb ages reveal that the intrusive complex formed during the late Early Cretaceous (115–113 Ma). Diorites from the intrusive complex have high Mg# values (49.7–51.2) and initial 87Sr/86Sr ratios (0.7082 to 0.7083), and low εNd(t) values (−7.3 to −6.9), with negative to slight positive zircon εHf(t) values (−3.8 to +0.8), suggesting that these rocks were likely generated by the partial melting of the enriched lithospheric mantle. Granodiorites from the intrusive complex have high SiO2 (64.4–67.9 wt%) and Al2O3 (14.8–16.1 wt%) and low MgO (1.4–1.8 wt%) contents. These rocks are also characterized by high initial 87Sr/86Sr ratios (0.7091–0.7098) and whole-rock negative εNd(t) (−9.9 to −8.4) and zircon εHf(t) (−7.9 to −2.2) values. The associated granites have the highest SiO2 (68.7–71.5 wt%) contents among the intrusive complex rocks, high initial 87Sr/86Sr (0.7089–0.7093) ratios, and negative whole-rock εNd(t) (−10.1 to −7.7) and zircon εHf(t) (−5.3 to −0.2) values. These geochemical characteristics indicate that the granodiorites were most likely derived from the partial melting of ancient continental crust and that the coeval granites were formed by the fractional crystallization of granodioritic magmas. Integrating our new results with data for the coeval magmatic rocks of the Lhasa–Qiangtang collision zone, we suggest that the Momu intrusive complex formed in a post-collisional slab breakoff setting. Asthenosphere upwelling caused by slab breakoff could have provided a substantial source of heat for crustal reworking, which gradually contributed to the maturation of continental crust in the Lhasa–Qiangtang collision zone.

Late Ediacaran post–collisional granite in Babouri–Figuil (Northwest Cameroon): Implication for crustal–mantle contributions in an extensional setting

The Babouri–Figuil Magmatic Complex (BFMC, adjacent to the central-south Chad domain) is located on the southern flank of the Saharan Metacraton. It is amongst the least geologically studied regions in Cameroon; only a few key research papers on geochronology and isotopes have been completed. The emplacement of granitic bodies is poorly constrained between the early and late–Neoproterozoic with inherited zircons of older Paleoproterozoic–Neoproterozoic ages. In this study, undeformed biotite granite massifs (Lombel and Badesi) were studied, and their geochronologic, geochemical and mineral chemical data, including Nd–Sr–Hf isotopes, are presented. The massifs are enriched in pink K-feldspar (orthoclase and microcline; 60–65 wt%) and plagioclase (8–10 wt%) anastomosing to intergranular quartz (18–20 wt%) and biotite (3–5 wt%) crystals. The Lombel massif yielded a Concordia age of 566 ± 2.5 Ma (MSWD = 1.6, zircon U–Pb), slightly older than the Concordia age of 556 ± 2.7 Ma (MSWD = 2.1, zircon U–Pb) obtained for the Badesi massif. These are interpreted as emplacement ages, which post–date arc magmatism by ∼ 45 Ma. The geochemistry of the massifs showed sub- to mid–alkaline, high–K alkali–calcic, and weakly peraluminous compositional characteristics. The calculated biotite temperatures ranged from 613 to 647 °C, pressures from 100 to 300 MPa, and H2O from 3.0 to 3.5 wt%. Whole–rock composition geothermobarometry revealed low pressure of 100–500 MPa, depths of 3.5–18 km and low oxygen fugacity (ΔQFM = −0.6), indicating reducing crystallization conditions. The estimated radiogenic isotopic values are similar for both massifs, falling in the range from initial εHf(t) = +2.6 to +8.4 and initial εNd(t) = +1.3 to +2.1, with a corresponding average initial Sr of 0.70212–0.70458. These radiogenic isotope values suggest a mantle–derived origin for this granite or the melting of a juvenile mafic crustal protolith, as confirmed by the two–stage model age obtained (TDM2 = ∼ 1.2 Ga). Given that biotite is interstitial in alkali feldspar and plagioclase in both massifs, this suggests that biotite chemistry records conditions of magma evolution. Overall, these results confirm that the studied massifs are post–collisional products emplaced at shallow crustal levels under evolved magmatic conditions. They are coeval with a few post–magmatic intrusions in the central–south Chad and emplaced after the Sao–Francisco Congo Craton collided with the Saharan Metacraton. The ages obtained enabled a possible correlation with several Neoproterozoic post–collisional granites in Cameroon, south-central Chad, Central African Republic and Borborema Province of Brazil. The ages also revealed that continental assembly coupled with regional late–magmatism occurred during the Proterozoic.