Zircon Hf Isotope Research Articles

Mid-Cretaceous intra-oceanic arc-continent collision recorded by the igneous complex in central Myanmar

Magmatic rocks from intra-oceanic arcs are critical for understanding the formation of continental crust and tectonic evolution. The early tectonic evolution of the Neo-Tethyan Ocean before the final Indo-Asia collision remains mysterious, and the geodynamic processes that triggered the Cretaceous magmatism in central Myanmar is still debated. The Cretaceous magmatic complex in the Banmauk-Kawlin area (BKC), west Myanmar terrane (WMT) is composed of the Kanza Chaung granitoid batholith, the Mawgyi Volcanic rocks, and the Pinhinga plutonic complex. Zircon U-Pb dating results of various rocks from the Kanza Chaung batholith suggest magmatism lasted from ca. 110 to ca. 94 Ma, roughly overlapping with new geochronological data for the Mawgyi Volcanics. Mafic rocks, including basalts from the Mawgyi Volcanics and gabbros from the Kanza Chaung Batholith, have geochemical features resembling intra-oceanic arc magmas, characterized by high large-ion-lithophile elements (LILEs) and low high-field-strength elements (HFSEs) and flat trace element patterns. They have depleted Sr (initial 87Sr/86Sr = 0.7035–0.7054) and Nd (εNd(t) = 0.39–6.71) isotopic compositions, with zircon εHf(t) values ranging from +5.8 to +16.1, probably derived from partial melting of the mantle wedge. Diorites formed by differentiation of basaltic magma have similar trace element patterns and Sr-Nd isotopes. The granitic rocks were likely originated from partial melting of juvenile arc lower-crust, indicated by their high SiO2 (>65.0 wt%), low MgO (<2.50 wt%) and depleted Nd and zircon Hf isotopes. The εNd(t) values of the BKC shift markedly (from ~ + 7 to 0) from 105 to 94 Ma, which correlates with a temporal increase of Th/Nb, La/Ta, and La/Sm. Given the juvenile characteristics of the WMT crust, this can be explained by exotic isotopically enriched crustal components subducted into the mantle source, rather than steady-state sediment subduction and crustal contamination. Given the Albian unconformity in the WMT and recent paleomagnetic data, such continent crustal components were likely introduced by collision followed by subduction of Greater India-derived continental sliver beneath the WMT. Thus crust with an Indian continent affinity was possibly accreted to an intra-oceanic arc (WMT) during the mid-Cretaceous.

From the southern Gangdese Yeba arc to the Bangong-Nujiang Ocean: Provenance of the Upper Jurassic-Lower Cretaceous Lagongtang Formation (northern Lhasa, Tibet)

The stratigraphic records of sedimentary basins situated on the Lhasa block provide crucial evidence to reconstruct the early history of the Tibetan Plateau associated with the successive closure of oceanic branches and related magmatic activities. The Upper Jurassic to Lower Cretaceous Lagongtang Formation of the Nagqu Basin located in the northern part of the Lhasa block chiefly consists of fine-grained quartzose to litho-quartzose sandstones, siltstones and shales deposited in continental shelf to bathyal environments. Provenance data, including sandstone petrography, U-Pb ages and in situ Hf isotopes of detrital zircon, together with paleocurrent indicators, suggest that the Lagongtang Formation was mainly fed by recycling of quartz-rich siliciclastic strata of the central-southern Lhasa block. Fresh felsitic volcanic rock fragments, plagioclase prevailing among detrital feldspars, and zircon grains yielding the same age as that indicated by biostratigraphic analysis testify to penecontemporaneous magmatic activity. Volcanic detritus may have been derived from the Middle Triassic-Lower Cretaceous Yeba and/or Sangri volcanics exposed in the southern Lhasa block to the south as well as from Middle Triassic-Lower Cretaceous magmatic rocks of the central Lhasa block. Although the Nagqu Basin is adjacent to the Bangong–Nujiang suture, no debris from Bangong–Nujiang suture and Qiangtang was found in the Lagongtang Formation here. Our provenance data rule out the hypothesis that the Lagongtang Formation could have been deposited in a forearc basin associated with the southward subduction of the Bangong-Nujiang Ocean or in a basin related to the supposed Shiquanhe-Namco Ocean.

Detrital zircon provenance of the Archean Moodies Group, Barberton Greenstone Belt, South Africa and Eswatini

Sandstones of the 3.22 Ga Archean Moodies Group represent one of the world9s oldest quartz-rich sedimentary sequences. The provenance of this unit is unresolved because its quartz and common microcline can be sourced both from either now eroded or covered granitoid plutons outside the Barberton Greenstone Belt (BGB) or, alternatively and perhaps more controversially, (rhyo-)dacitic (sub-)volcanic rocks within the BGB. We compiled 31 detrital zircon data sets (n = 2588) from sandstones, reworked tuffs and conglomerate of the Moodies Group in order to constrain its age and provenance. After selection using quality criteria, the remaining zircons (n = 1621) in nearly all samples show a distribution corresponding to the four known major pulses of felsic magmatism in the BGB: 1) the <i>ca.</i> 3550 to 3530 Ma Theespruit and Sandspruit Formations at the base of the Onverwacht Group; 2) the <i>ca.</i> 3440 to 3410 Ma rhyo-dacites of the upper Hooggenoeg Formation, Onverwacht Group; 3) the <i>ca.</i> 3300 to 3280 Ma thin felsic tuffs in the Mendon Formation; and 4) the <i>ca.</i> 3260 to 3215 Ma felsic volcanic and shallow intrusive rocks of the Auber Villiers, Bien Venue and Schoongezicht Formations of the upper Fig Tree Group and the Moodies Group as well as their co-magmatic plutonic counterparts. Almost all data sets also contain near-concordant younger zircons as young as 2820 Ma, which can be attributed to one of six tectonic or magmatic events affecting the young Kaapvaal Craton in post-BGB time, causing the partial or complete resetting of the U-Pb system in some grains. The youngest (near-)concordant zircon clusters yield ages of <i>ca.</i> 3220 Ma in most locations, and the youngest discordant group of zircons from a reworked tuff near the top of the Moodies Group at 3212 ± 13 Ma agrees well with previous estimates of the maximum depositional age. With very few exceptions, the oldest zircons (<i>ca.</i> 3564 Ma) are only slightly older than the oldest rocks in the BGB stratigraphy (<i>ca.</i> 3550 Ma). Subtle regional and stratigraphic differences in age spectra may indicate localized or nearby sediment sources within a synorogenic setting. Preliminary age spectra along vertical stratigraphic profiles show little systematic variation, possibly indicating that intrabasinal recycling was dominant over considerable time periods of the basin9s evolution. Extra-basinal plutonic sources of similar age and composition as the intra-BGB sources appear to be required to provide quartz and some feldspar to Moodies Group sandstones, although zircon age spectra, limited zircon Hf isotope data, sandstone petrography, facies analysis, and the high variability in Moodies conglomerate clast composition are consistent with uplift, deformation and erosion of several intra-BGB sources, but in particular from the region of the Onverwacht Anticline. Zircon populations, conglomerate clasts, and sandstone composition show no evidence that high-grade metamorphic rocks from the adjacent Ancient Gneiss Complex (AGC) contributed significantly to the Moodies Group.

Geochronology and geochemistry of the PiaOac granites: Implication for Late Cretaceous magmatism and metallogeny in NE Vietnam

Sn-W deposits have recently been found in NE Vietnam, which has been considered as southwestern extension of the South China Block. The timing and relationships between the Sn-W mineralization and associated granitic magmatism in this area have not been well understood. This study presents new zircon and cassiterite U-Pb geochronology, whole-rock geochemistry, and zircon Hf-isotope data for the PiaOac granites, greisens, and cassiterite veins in NE Vietnam. The PiaOac granites are characterized by high Na2O + K2O, and K2O/Na2O, and low Fe2O3T and Mg#. The presence of primary muscovite, and high A/CNK (1.23–1.26), Rb/Sr (20–25), and P2O5 (0.33–0.35%) suggest that they are strongly peraluminous S-type granites. Low K/Rb, Zr/Hf, Nb/Ta, and Eu/Eu* indicate that the granitic magma experienced strong fractional crystallization. The PiaOac granites have homogeneous zircon Hf-isotope compositions with average εHf(t) values of −9.6 to −8.7 and two-stage Hf model ages of 1.76–1.71 Ga. They probably were sourced from Neoproterozoic metasedimentary rocks with late Paleoproterozoic model ages in NE Vietnam. Low CaO/Na2O (<0.3) and high Rb/Sr and Rb/Ba ratios further suggest a clay-rich pelitic source. Zircons from greisen have significantly unradiogenic Hf-isotope compositions (εHf (t) = -12.2) and large variations, implying strong interaction with wall rocks in late fluid-rich stage during magmatic evolution. Extended magmatic differentiation was followed by interaction with wall rocks, which has altered the redox environment, facilitating the Sn-W mineralization.Zircon U-Pb dating results reveal that the PiaOac granites formed in the Late Cretaceous (89–88 Ma), coincident with the ∼ 89 Ma mineralization constrained by cassiterite U-Pb dating, suggesting that the Sn-W mineralization is closely related to the granitic magmatism and greisenization. Late Cretaceous magmatism and mineralization in NE Vietnam are similar to those in SW South China in terms of formation age, rock association, deposit type, source composition, and tectonic setting, all together constituting an E-W trending Late Cretaceous metallogenic belt. The Late Cretaceous granites and associated deposits in the belt were probably formed in an intracontinental orogenic setting in response to the combined action of the subduction of Pacific and Neo-Tethys Plates.

Diverse Anatexis in the Main Central Thrust Zone, Eastern Nepal: Implications for Melt Evolution and Exhumation Process of the Himalaya

Abstract Sitting between the Greater Himalayan sequence (GHS) and Lesser Himalayan sequence (LHS), the Main Central Thrust zone (MCTZ) has experienced multiple episodes of anatexis, which presents an opportunity to explore the nature of partial melting and its response to Himalayan orogenic processes. A series of deformed rocks, including migmatites, gneisses, and leucosomes were collected across the MCT at Arun Valley, eastern Nepal. We investigated the bulk rock major and trace elements, Sr-Nd isotopes, mineral chemistry, zircon geochronology and Hf isotopes, and conducted phase equilibria modeling. The protolith boundary between the GHS and LHS is recognized on the basis of Sr–Nd isotopes with εNd(0) of −16.7 to −8.0 for the GHS and −31.2 to −23.9 for the LHS. Samples from both the GHS and LHS have undergone partial melting, as revealed by in situ leucosomes at outcrops and melt inclusions at thin-section scale. Leucosomes separated from their host rocks are divided into four groups: those derived from hydration melting, muscovite dehydration melting, amphibole dehydration melting, and feldspar accumulation. Phase equilibria modeling results for the GHS migmatite show isothermal decompression from peak P–T conditions of 11 kbar and 795°C, accompanied by muscovite dehydration melting evolving into biotite dehydration melting. In contrast, rocks from the LHS are modeled to have undergone hydration melting at P–T conditions of 9 kbar and 685°C. Zircon U–Pb geochronology suggests that long-lived partial melting (35–13 Ma) occurred in the MCTZ. Moreover, anatectic zircon Hf isotopes show that the protoliths for partial melting changed from the GHS to the LHS with εHf(t) of −19.4 to −5.7 during the early Miocene, and lower values of −42.5 to −16.7 during the middle to late Miocene. These zircon geochemical results indicate that hydrous metasediments from the LHS were progressively accreted to the base of the GHS, resulting in hydration melting of both the GHS and LHS assisted by MCT. The timing of activity of the MCT is constrained to 25–13 Ma, coeval with movement of the South Tibetan detachment system. Integration of petrogenetic modeling, the chronology of partial melting, and metamorphic P–T paths allows us to propose that thickened Himalayan crust was heated from the middle to late Eocene, and widespread anatexis occurred during the Oligocene to middle Miocene, forming a large-scale melt channel. The hot GHS channel flow moved upward in association with the synchronous activity of the MCT system, triggered intense dehydration of LHS metasediments, resulting in fluid-present melting in both the GHS and LHS during middle to late Miocene, and the formation of leucogranite with mixture features of GHS and LHS. Furthermore, with the cooling of the melt channel, duplexing has gradually operated since the middle to late Miocene in the shallow crust.

The Karavansalija Mineralized Center at the Rogozna Mountains in SW Serbia: Magma Evolution and Time Relationship of Intrusive Events and Skarn Au ± Cu–Pb–Zn Mineralization

The Karavansalija Mineralized Center (KMC) with its Au–Cu skarn mineralization associated with the Rogozna Mountains magmatic suite in southwestern Serbia belongs to the Oligocene Serbo-Macedonian magmatic and metallogenic belt (SMM-MB). Samples from intrusive and volcanic rocks at the KMC show typical arc signatures of subduction-derived magmas through enrichment in large-ion lithophile elements (LILE) and depletion of high–field strength elements (HFSE). The magmas developed a high-K (calc-alkaline) fractionation trend and evolved toward shoshonitic compositions. Whole-rock trace element data suggest plagioclase-absent, high-pressure amphibole ± garnet fractionation that generates adakite-like hydrous magmas during evolution in lower crustal magma chambers. Zircon LA–ICP–MS and high-precision CA–ID–TIMS dating together with zircon trace elements and Hf isotope measurements were carried out in order to couple the geochronologic and geochemical evolution of the KMC. The results suggest that magmatism starts around 29.34 Ma with granitic to rhyodacitic subvolcanic intrusions followed by a more evolved magmatic intrusion that was emplaced into Cretaceous limestone, generating a widespread skarn alteration at ca. 28.96 Ma. After a period of quiescence of about 1.2 My, either another magma body evolved or the same upper crustal magma chamber was recharged and also likely partly reactivated older plutonic rocks as indicated by xenocrysts. The REE ratios shift from apatite, titanite ± amphibole-dominated fractionation of the older magmatic event to crystallization of allanite, efficiently depleting the LREE and Th/U in the younger upper crustal magma. After a lamproite-like melt was injected, the increased heat and fluid pressure led to the expulsion of a quartz-monzonite porphyritic stock at ca. 27.72 Ma, strongly interacting with the skarns and established a fertile hydrothermal system. Soon after a non-mineralized second pulse of some porphyry dykes cut the previous phenocryst-rich “crowded” porphyries and skarns at ca. 27.60 Ma, thus bracketing the maximum timespan of ore mineralization to about 112 ± 45 Ka. Increased contribution of a lamproite-like melt is inferred from the presence of phlogopite micro-phenocrysts, phlogopitization of biotite, and diopside clusters in the latest porphyry dykes. There is a trend of increased crustal assimilation from the oldest volcanic phase to the emplacement of the youngest porphyry dykes recorded by ɛ-Hf of the zircons. Oligocene occurrences of significant base metal mineralization within Serbia, northern Macedonia, and Greece, e.g., Crnac, Rudnik, Veliki Majdan, Stratoniu, or the Cu–Au porphyry at Buchim (northern Macedonia), are all associated with trachy-andesitic (quartz latitic) porphyry dykes, which originated through post-collisional tectonic settings or upper plate extension involving reworking of crustal arc-derived rocks and partial melting of the mantle wedge. This study demonstrates that on the basis of field relationships and the application of high-precision CA-ID-TIMS zircon age data, pulses of porphyry dykes of a 10ka age range can be distinguished, and the timing of mineralization can be parenthized.

Elevation of zircon Hf isotope ratios during crustal anatexis: Evidence from migmatites close to the eastern Himalayan syntaxis in southeastern Tibet

The initial Hf isotope composition of zircon in high-grade metamorphic rocks is an important tracer for protolith origin as well as for the growth and evolution of continental crust. However, less attention has been paid to the behavior of Lu-Hf isotopes in peritectic reactions during partial melting of the continental crust. In this paper, we present a combined study of zircon U-Pb ages, Lu-Hf isotopes and trace elements as well as petrology and whole-rock geochemistry for migmatites close to the eastern Himalayan syntaxis in southeastern Tibet. The results show that peritectic zircons in the migmatites have significantly higher 176Hf/177Hf ratios than their protolith zircons. More interestingly, the 176Lu/177Hf ratios and the contents of high field strength elements (such as Nb, Ta, and Hf) and heavy rare earth elements are significantly elevated in the peritectic zircons compared to the protolith zircons. This suggests that garnets with high Lu/Hf ratios and other minerals (such as titanite, ilmenite, amphibole, and biotite) with moderate Lu/Hf ratios were decomposed during crustal anatexis, contributing to the Hf isotope composition of peritectic zircons. This is confirmed by petrographic observations that garnet and biotite occur as residues or remnants in mesosomes (biotite gneiss) and some leucosomes (felsic veins), but are absent in the melanosomes of the target migmatites. Therefore, the peritectic zircons were produced together with anatectic melts through peritectic reactions and acquired the elevated 176Lu/177Hf ratios compared to the inherited zircons from the protoliths if no peritectic garnet was coevally produced during the crustal anatexis. In this regard, the greatly elevated Hf isotope compositions of peritectic zircons in anatectic granites cannot faithfully reflect the Hf isotope composition of parental rocks. As such, great care must be taken when using Hf isotope ratios to trace the nature of parental rocks with respect to magmatic processes and crustal evolution.

LA‐ICP‐MS zircon U–Pb age, whole‐rock geochemistry, Sr–Nd–Pb isotope, and zircon Hf isotope constraints on the origin and dynamic setting of Mesozoic mafic dykes, Henan Province, southeastern North China Craton

Mafic dykes from the Henan Province within the southeastern North China Craton (NCC) were sampled. Herein, we present new zircon laser ablation‐inductively coupled plasma‐mass spectrometry U–Pb age, whole‐rock geochemical, Sr–Nd–Pb isotopic, and zircon Hf isotopic data for the five areas where these mafic dykes outcrop. The mafic dykes are all dolerite and were emplaced between 121.6 ± 1.1 Ma and 131.7 ± 1.2 Ma. These are characterized by enrichment in some of the large‐ion lithophile elements (e.g., Rb, Ba, Th, U, K, and Sr), light rare earth elements, and depletion in high‐field‐strength elements (Nb, Ta, Hf, and Ti) and Eu (Eu/Eu* is between 0.62 and 0.96) relative to chondrite‐ and primitive mantle‐values. The dykes have high initial Sr isotopic ratios ([87Sr/86Sr]i = 0.7056–0.7058), negative εNd (t) ratios (−12.3 to −12.1), relatively constant Pb isotopic ratios ([206Pb/204Pb]i = 16.771–16.778; [207Pb/204Pb]i = 15.461–15.467; [208Pb/204Pb]i = 36.850–36.857), negative εHf (t) (−14.9 to −3.5), as well as old Nd (TDM2; 2.22–2.54 Ga) and Hf (TDM2; 2.99–4.34 Ga) model ages. These geochronological, geochemical, and isotopic data indicate that the investigated Henan Province dykes were likely derived from magmas generated by low to moderate degree partial melting (1.0–10%) of an EM1‐like garnet lherzolite mantle source. Moreover, the mafic melts fractionated olivine, clinopyroxene, plagioclase, Ti‐bearing oxides (e.g., ilmenite, titanite, apatite, etc.) prior to emplacement, and assimilated only minimal amounts of crustal material. Several possible models have previously been proposed to explain the origin of Mesozoic magma activities in Henan Province. However, here we propose a foundering model for the mafic dykes studied, involving the foundering of eclogite from thickened lower crust due to the collision between the Yangtze Block, Qinling ocean crust, and southeastern NCC.

Formation of the Late Cretaceous-Paleogene Sn-rich granites in the Tengchong block: The magma origin and differentiation conditions

腾冲地块锡成矿作用主要与晚白垩世-古近纪岩浆活动相关，但仅矿区花岗岩体具有较高的锡含量（平均25×10^-6），区域上同时代的非成矿花岗岩体并未发生锡的富集。本文通过搜集分析现有研究数据，总结了富锡成矿岩体和区域非成矿岩体在岩浆源区、演化条件和结晶分异程度的异同：区域非成矿岩体锆石ε_Hf（t）值（-15.1~+3.39）表明，自东北向西南幔源物质加入有升高的趋势，但区域玄武岩中低含量的Sn （1.61×10^-6）表明幔源物质混入不利于岩浆中锡的富集。晚白垩世部分非成矿岩体与成矿岩体具有相同ε_Hf（t）（-9.7）值，表明其皆源于古老地壳物质的部分熔融，但岩体均表现为准铝质-弱过铝质特征，且锆石Hf同位素（t_DM2=1724Ma）和全岩Nd同位素（t_DM2=1836Ma）二阶段模式年龄基本一致，因此其源区可能并非富锡的高黎贡山群变质沉积岩，而可能是其中未经风化的变质花岗岩。根据腾冲地块地层厚度（28km）和莫霍面深度（47~35km）推断岩浆源区至少位于地下30km （8.4kbar），由于仅靠地温梯度（25℃/km）无法达到初始熔融温度（>1066℃），源区部分熔融过程很可能受地幔热的影响。根据Fe₂O₃/FeO比值，非成矿岩体（0.59）与成矿岩体（0.48）均具有较低的氧逸度，属钛铁矿系列，但成矿岩体的结晶分异程度明显高于非成矿岩体，且成矿岩体富含挥发分，高含量的挥发分降低了岩浆固结温度（650~550℃），延长了结晶分异时间，促进了锡在晚期岩浆中的富集。因此腾冲地块富锡花岗岩主要是普通岩浆在低氧逸度环境下发生高度结晶分异的结果。;Tin metallogenesis in Tengchong block is associated with the Late Cretaceous and Paleocene magmatism. However, only the ore-forming granitoids are characterized by high Sn content (25×10^-6 on average). The regional contemporary unmineralized granitoids hardly show Sn enrichment. By collecting and analyzing available research material, the differences between the Sn-rich ore-forming rocks and regional unmineralized rocks are summarized in terms of their magma origins, evolution conditions and differentiation degrees. Zircon ε_Hf(t) values (-15.1~+3.39) of the regional unmineralized rocks indicate that the magmas in this area are added with increasing mantle material from northeast to southwest, while the addition of the mantel material is disadvantageous to tin enrichment in the mixing magma according to the Sn content (1.61×10^-6) of the regional basalt. Part of the regional unmineralized granitoids formed in Late Cretaceous have the same ε_Hf(t) values of -9.7 with the Sn-rich ore-forming rocks, suggesting both of them were derived from partial melting of ancient crustal material. Considering these rocks are all metaluminous or weak peraluminous, and the crustal model age (t_DM2) calculated from zircon Hf isotopes (1724Ma) are nearly identical to that from the whole-rock Nd isotopes (1836Ma), the magma may not be derived from the Sn-rich Gaoligongshan Group consisting of metasedimentary rocks, but probably the inner metamorphic granitoids. The metamorphic granitoids are at least 30km underground (corresponding to 8.4kbar) based on the regional strata thickness (28km) and the Moho depth (47~35km). The initial melting temperature (1066℃) is unable to be reached according to geothermal gradient (25℃/km), therefore, the partial melting process is likely influenced by mantle heat. According to Fe₂O₃/FeO values, both the unmineralized granitoids (0.59) and the tin mineralized granitoids (0.48) have low oxygen fugacity, and belong to ilmenite series. While the fractionation of Sn-rich granitoids is much higher than that of unmineralized granitoids, moreover, the Sn-rich granitoids contain abundant volatiles, which decrease the magma consolidation temperature (650~550℃), prolong magma differentiation process, and promote Sn gathering in the late magma. In conclusion, the Sn-rich granitoids in the Tengchong block is mainly resulted from the high fractionation of normal magma under a low oxygen fugacity.

Genesis and tectonic setting of Late Jurassic-Early Cretaceous granites in Nachatang area, Central Lhasa Terrane: Constraints from geochemistry, chronology and Hf isotopes

西藏中拉萨地块东段大规模侏罗纪-白垩纪花岗岩类的成因类型及构造背景尚未得到有效约束，该时期岩浆作用的时空分布、岩石成因以及深部动力学机制等问题亟需新的深入研究。本文对中拉萨地块东段南缘那茶淌地区的花岗岩类进行了系统的岩相学、元素地球化学、年代学和锆石Hf同位素研究。LA-MC-ICPMS锆石U-Pb定年结果显示，那茶淌地区黑云母二长花岗岩成岩年龄为147±1.4Ma，花岗闪长岩年龄为140.6±1.3Ma，系晚侏罗世-早白垩世中酸性岩浆活动的产物。在元素地球化学组成上，黑云母二长花岗岩和花岗闪长岩的主量元素组成具有富SiO₂（分别为71.02%~71.81%和65.17%~66.73%）、Al₂O₃（分别为13.45%~13.57%和14.43%~15.20%）、碱金属（Na₂O+K₂O）（分别为6.79%~7.48%和6.55%~7.37%），贫TiO₂（分别为0.15%~0.21%和0.10%~0.13%）等特征，显示Ⅰ型准铝-弱过铝质（A/CNK=0.98~1.14）的高钾钙碱性系列特征。微量元素以富集K、Rb、Ba、Th等大离子亲石元素，亏损Ta、Nb、P、Ti等高场强元素为特征。花岗闪长岩稀土元素总量（ΣREE=263.9×10^-6~313.8×10^-6）高于黑云母二长花岗岩（ΣREE=116.3×10^-6~168.8×10^-6），均表现为轻稀土元素富集，重稀土元素相对亏损，具弱的负Eu异常。两种岩石锆石Hf同位素均以负的ε_Hf（t）值和古老的地壳模式年龄为特征，黑云母二长花岗岩ε_Hf（t）值为-21.64~-7.66，地壳模式年龄（t_DM^C）为1.68~2.56Ga，花岗闪长岩锆石ε_Hf（t）值为-11.95~-8.15，地壳模式年龄（t_DM^C）为1.71~1.95Ga。结合前人研究资料，认为中拉萨地块东段那茶淌地区晚侏罗世-早白垩世花岗岩类形成的构造背景与班公湖-怒江洋南向俯冲有关，岩浆来源于俯冲作用诱发古老下地壳物质的重熔。;The genetic types and tectonic settings of the large-scale Jurassic-Cretaceous granites in the eastern part of the Central Lhasa Terrane in Tibet have not been well constrained, and the temporal and spatial distribution of their magmatism, petrogenesis and deep dynamic mechanism in this period are in urgent need of further research. In this paper, the lithography, chronology, geochemistry and zircon Hf isotope of the biotite monzogranite and granodiorite from the Nachatang area in the southern margin of the eastern segment of the Central Lhasa Terrane are systematically studied. LA-MC-ICPMS zircon U-Pb dating results show that the biotite monzogranite and granodiorite are 147±1.4Ma and 140.6±1.3Ma, respectively, showing they are the products of magmatism in Late Jurassic-Early Cretaceous. In terms of geochemical compositions, the biotite monzogranite and granodiorite have low TiO₂(0.15%~0.21% and 0.10%~0.13%), high SiO₂ (71.02%~71.81% and 65.17%~66.73%), Al₂O₃ (13.45%~13.57% and 14.43%~15.20%) and alkali (Na₂O+K₂O) (6.79%~7.48% and 6.55%~7.37%) contents, showing the characteristics of Ⅰ-type peraluminum to weak peraluminum calc-alkaline series (A/CNK=0.98~1.14). Trace element compositions of the measured samples are characterized by enrichment of large ion lithophile elements such as K, Rb, Ba and Th, and depletion of high field strength elements such as Ta, Nb, P and Ti. The total amount of REE in the granodiorite (ΣREE=263.9×10^-6~313.8×10^-6) is higher than that in the biotite monzonite (Σ REE=116.3×10^-6~168.8×10^-6), which shows the enrichment of LREE and relative depletion of HREE, with weak negative Eu anomaly. The zircon Hf isotopes are characterized by negative ε_Hf(t) values and ancient crustal model ages. The ε_Hf(t) values of the biotite monzogranite range from -21.64 to -7.66, and the crustal model ages (t_DM^C) range from 1.68Ga to 2.56Ga. Zircon ε_Hf(t) values of the granodiorite vary from -11.95 to -8.15, and the crustal model ages (t_DM^C) from 1.71Ga to 1.95Ga. Finally, we conclude that the tectonic setting of Late Jurassic to Early Cretaceous granites in the Nachatang area is related to the southward subduction of the Bangong-Nujiang Ocean seafloor, and the magma is derived from the remelting of ancient lower crust material induced by subduction.

Multiple crustal melting pulses and Hf systematics in zircons

This paper focuses on the U–Pb ages and Lu–Hf isotopes in zircons in a long-lived anatectic terrane, to understand both the nature of melting events and controls on Hf-isotope ratios in the zircon record. We focus on rocks from the Wuyi-Yunkai orogeny, South China, that mainly comprises Proterozoic basement rocks that generated numerous granitoids during Early Paleozoic granulite facies metamorphism. We investigate zircons from three anatectic granites and one migmatite. Zircon U–Pb dates for individual samples vary between ~465 and ~410 Ma, covering the duration of the orogeny. U–Pb dates for two samples define three normal distributions with populations spaced by ~15–20 Myr, indicating protracted and pulsed melting events. The inherited pre-orogenic zircon cores yield a wide spread of εHf(435) values (−39.0 to −2.8), whilst all Early Paleozoic, magmatic syn-orogenic cores and rims have a narrower range shifted to higher values ranging between −14.3 and + 5.5 and no systematic variation with the date of the analytical spot. The data suggest that the magmatic syn-orogenic zircons grew from isotopically heterogeneous melts with ~9 to 16 ε units variation for individual samples. The narrower ranges compared to the inherited cores suggest that some magma homogenization must have occurred, and the upward shift of the εHf(435) values is likely due to the influence of non-zircon Hf rather than due to mixing with an external less-evolved magma. Thus, the samples record a ~55 Myr, multi-pulsed melting event, during which heterogeneous crustal magmas had their Hf isotopic composition controlled by inherited zircon dissolution modified by contributions from non-zircon phase.

Late Indosinian granitoids in the Zhacanggou, Guide Basin, NW China: Whole‐rock geochemistry, zircon U–Pb geochronology, Lu–Hf isotopes, and implications for magma mixing

The Guide Basin, located in the West Qinling orogen of central China, is a typical geothermal field. However, the granodiorites occurring in the Zhacanggou area have been poorly restricted. Herein, we report new isotopic compositions of whole‐rock major‐trace elements, zircon U–Pb, and Lu–Hf, on the host granodiorites and their microgranular enclaves (MEs) in the Zhacanggou area. Zircon U–Pb dating for the host rocks (porphyritic granodiorite and granodiorite) revealed ages of 230.5 ± 1.4 Ma and 229.0 ± 1.6 Ma, similar to those (230.5 ± 1.5 Ma and 231.4 ± 1.4 Ma) of the MEs. Zircon Hf isotope analyses for the granodiorite and MEs yielded εHf(t) values from −11.0 to −1.5 and from −7.8 to −0.7, with corresponding TDM2 ages of 1,355–1,959 and 1,304–1,755, respectively. The MEs possess plagioclase phenocrysts, acicular apatites, and chilled borders. They were characterized by relatively low SiO2 (58.39–65.24 wt%) and high Cr, Ni, and MgO contents, leading to relatively high Mg# values (44–59) and low Sr/Y (14.23–19.84) ratios. The host granodioritic rocks show similar adakitic geochemical characteristics with SiO2 = 64.23–68.57 wt%, MgO = 1.29–2.22 wt%, Mg# = 48–53, Sr = 340–596 ppm, Y = 11.3–13.7 ppm, Sr/Y = 28.39–47.89, and no obvious Eu anomalies. These geochemical and isotopic data, combined with the mineralogical features, indicate that the granodiorites in the Zhacanggou area were produced by the mixing of high‐Mg# melts originating from an enriched lithospheric mantle source, and felsic magmas deriving by the partial melting of the normal lower continental crust with the tectonic regime transition from a syn‐collision to a post‐collision setting.

Petrogenesis of the Early Silurian Renda appinite suite in the southeastern Qilian Orogen, NW China: Implications for the evolution of a Proto-Tethys magmatic arc

ABSTRACT Early Paleozoic subduction beneath peri-Gondwana microcontinents formed a major Proto-Tethys magmatic arc system. Appinite suite rocks constitute important components within that magmatic arc. Herein, we report hornblende 40Ar/39Ar and zircon U–Pb geochronology data, zircon Hf isotope, mineral chemistry, whole-rock geochemistry, and Sr–Nd–Pb isotope data for the rocks of hornblende gabbro-diorite-quartz diorite-quartz monzodiorite from the Renda appinite suite and adjacent granite porphyry in the southeastern Qilian Orogen. Hornblende 40Ar/39Ar and zircon U–Pb isotopic data constrain ages of the intrusions of the Renda appinite suite to 439–435 Ma. Rocks in this suite represent the calc-alkalic to alkali-calcic series, and they are enriched in large-ion lithophile elements and depleted in high-field-strength elements. The hornblende gabbros exhibit moderate initial 87Sr/86Sr ratios (0.7046–0.7055) and higher whole-rock εNd(t) (–1.3 to +1.7) and zircon εHf(t) (+7.2 to +11.2) values relative to the other rocks of the suite (0.7047–0.7064, – 7.7 to – 1.1, and – 6.6 to +6.8, respectively) as well as Pb isotopic features similar to EM2 lithospheric mantle. These geochemical characteristics suggest that the Renda appinite suite originates from partial melting of an enriched lithospheric mantle, with subsequent fractional crystallization and hybridization with crustal components. The granite porphyry formed by partial melting of the crust coeval with the emplacement of juvenile magma. This study suggests emplacement occurred during a short-lived collision with slab failure (breakoff) after latest stage (cessation) of Proto-Tethys oceanic subduction along the northern side of the southeastern Qilian Orogen. The heat supplied by the asthenospheric upwelling through a gap in the subducted slab induced melting in lithospheric mantle and deep crust producing Renda appinite suite rocks and granite porphyry.

Tectonic transition from Paleo- to Neo-Tethyan Ocean in Tangjia-Sumdo area, Southern Tibet: Constraints from Early Jurassic magmatism

The history Paleo-Tethyan Ocean basin in Tangjia-Sumdo area of Lhasa block has not been investigated in detail, particularly the tectonic evolution after its closure. To better understand this issue, we investigated U-Pb zircon chronology, zircon Hf isotopes, and whole rock geochemistry of Early Jurassic granitic rocks in Tangjia-Sumdo area on Lhasa Block. Three types of samples were obtained in this study, diorites (199.8 ± 1.1 Ma), monzogranites (194.3 ± 1.1 Ma and 200.5 ± 1.2 Ma), and granites (184.5 ± 2.1 Ma, 185.5 ± 4.0 Ma and 185.6 ± 2.1 Ma), which can be divided into two stages according to their eras. The whole rock geochemical characteristics show that all the samples are I-type granitoids. The geochemical analyses show that the first stage of magmatism was derived from partial melting of crustal material mixed with mantle material, and the second stage of magmatism was derived from partial melting of the crust. Based on the zircon Hf isotopic characteristics (the calculated εHf(t) values of the diorite are − 16.7 to − 7.7, monzogranites are + 1.8 to + 5.2, granites are + 1.6 to + 9.9), we conclude that diorites were derived from the ancient continental crust, and the other two groups of samples were derived from juvenile crust. Based on a comparison with previous research results and regional tectonic development, we suggest that the first stage of magmatism was the result of slab break-off of subducting Tangjia-Sumdo Paleo-Tethyan oceanic crust. The second stage of magmatism was the result of northward subduction of Neo-Tethyan Ocean, and it may represent tectonic transition from Paleo- to Neo- Tethys Ocean in Early Jurassic in Lhasa Block.

Late Jurassic Paleotethyan oceanic slab break-off revealed by Sr-Nd-Hf isotopes of Na-rich adakitic granites from northwestern Turkey

The Mesozoic geodynamic evolution of the Paleotethys Ocean is critical but a poorly known piece of the geological history of the Sakarya Zone, north Turkey. Here, we describe new integrated research of zircon U-Pb geochronology, bulk-rock major-trace element and Sr–Nd isotopes as well as Hf isotope of zircons from the granitic rocks of the Demirköy and Kızıldamlar plutons located in Bilecik area along the western Sakarya Zone, northwestern Turkey. Zircon U–Pb ages obtained by LA-ICP-MS reveal that the granitic magma intruded into the basement rocks during the Late Jurassic (ca. 162–157 Ma). The granites are tholeiitic and show slightly metaluminous to peraluminous geochemical affinities. The granite samples are adakitic, having relatively high Na2O (6.35–7.58 wt%) and Sr (172–580 ppm), and low K2O (0.39–1.13 wt%) and Y (3.2–5.3 ppm), and thus have high Sr/Y (70–250). The samples exhibit enrichment of light rare earth elements (REE) and large ion lithophile elements (LILE) and depletion in heavy REE and the high field strength elements (HFSE; e.g., Nb, Ti, Ta). The samples possess low 87Sr/86Sr(t) values of 0.70313 to 0.70372, and εNd(t) of −0.66 to +9.24. Zircon grains exhibit positive εHf (t) values of 5.0 to 12.8 and depleted mantle model ages of Hf (TDM1 = 0.24 to 0.68 Ga). These geochemical features and Sr–Nd–Hf radiogenic isotope systematics reveal that the adakitic rocks were likely formed by low degree melting (<10%) of the eclogitic part of an oceanic slab and subsequent melt fractionation and crustal contamination. An ongoing subduction scenario cannot explain the formation of the investigated adakitic rocks. We suggest that the adakitic plutons were generated through slab break-off after the final closure of the Paleotethys Ocean throughout the Late Jurassic (ca. 162–157 Ma). Considering the previous data, the continental back-arc basin system, leading to the opening of Neotethys Ocean at the southern margin of the continent, has been put forward to decipher the Late Jurassic tectonic evolution of the Paleotethys Ocean and the western Sakarya Zone.

Comparative Sm-Nd isotope behavior of accessory minerals: Reconstructing the Sm-Nd isotope evolution of early Archean rocks

Crustal growth and mantle differentiation through Earth’s history are often traced using two radiogenic isotope systems - 176Lu-176Hf and 147Sm-143Nd. Unlike most post-Archean igneous rocks that show correlated initial Hf and Nd isotopic compositions, many ancient rocks have broadly chondritic zircon initial εHf values but highly variable whole-rock initial εNd values. These features have classically been interpreted as differences in the behavior of the Lu-Hf and Sm-Nd isotope systems during either deep magma ocean crystallization, subduction zone processes, or post-crystallization metamorphism. To clarify the cause of early Archean Hf-Nd isotope relationships, which are essential for understanding early Earth’s evolution, we investigated the in situ U-Th-Pb and Sm-Nd isotope systematics of co-existing titanite, apatite, and allanite—the major Sm-Nd carriers in early Archean felsic rocks—in a representative early Archean (3.5–3.4 Ga) tonalite-trondhjemite-granodiorite (TTG) suite from the Minnesota River Valley (MRV) terrane, northern USA. These rocks exhibit multiple generations of closed-system zircon growth with chondritic initial zircon Hf isotope signatures, and apparent decoupled zircon initial Hf and whole-rock Nd isotopic compositions, and thus serve as an useful test of the role of accessory minerals in controlling the whole rock isotopic signatures.Our new U-Pb results show that the REE-rich accessory phases were reset during the 2.60–2.58 Ga Sacred Heart Orogeny, representing the accretion of the MRV terrane to the southern margin of the Superior Craton. Additionally, two samples contain apatite that yields significantly younger U-Pb dates of 1.9–1.8 Ga, suggesting that a portion of the MRV basement was thermally overprinted by the ∼1.85 Ga Penokean Orogeny. The ∼2.6 Ga Sm-Nd array recorded by apatite and titanite in these rocks suggest terrane-wide Sm-Nd isotopic re-equilibration in the MRV during the Sacred Heart Orogeny, with no Penokean disturbance of the Sm-Nd isotope systematics. Nevertheless, allanite in one sample has survived the post-crystallization 147Sm-143Nd resetting event, and yielded a Sm-Nd isochron date broadly consistent with the crystallization age of the host rock (∼3.4 Ga), with chondritic initial εNd. Therefore, allanite appears to be an important target in order to obtain the primary Nd isotopic signature of early Archean rocks in ancient terranes. In contrast to the ambiguous whole-rock data, the Nd isotopic compositions of the coexisting apatite, titanite and allanite collectively reconstruct a clear crustal evolution history in which the crust was repeatedly re-melted in a closed-system since formation at 3.5 Ga, initially from a source evolving with broadly chondritic Lu-Hf and Sm-Nd systematics. This suggests a minimally differentiated, or already rehomogenized, mantle at 3.5 Ga. Our study highlights the importance of using REE-rich accessory phases to obtain a clear Nd isotopic record to constrain the history of continent formation on the early Earth.

Preferential dissolution of uranium-rich zircon can bias the hafnium isotope compositions of granites

Abstract Hafnium (Hf) isotopes in zircon are important tracers of granite petrogenesis and continental crust evolution. However, zircon in granites generally shows large Hf isotope variations, and the reasons for this are debated. We applied U-Pb geochronology, trace-element, and Hf isotope analyses of zircon from the Miocene Himalayan granites to address this issue. Autocrystic zircon had εHf values (at 20 Ma) of–12.0 to–4.3 (median =–9). Inherited zircon yielded εHf values (at 20 Ma) of–34.8 to +0.3 (median =–13); the majority of εHf values were lower than those of autocrystic zircon. The εHf values of inherited zircon with high U concentrations resembled those of autocrystic zircon. Geochemical data indicates that the granites were generated during relatively low-temperature (&amp;lt;800 °C) partial melting of metasedimentary rocks, which, coupled with kinetic hindrance, may have led to the preferential dissolution of high-U zircon that could dissolve more efficiently into anatectic melt due to higher amounts of radiation damage. Consequently, Hf values of autocrystic zircon can be biased toward the values of U-rich zircon in the source. By contrast, literature data indicate that granites generated at high temperatures (&amp;gt;820–850 °C) generally contain autocrystic and inherited zircons with comparable Hf isotope values. During higher-temperature melting, indiscriminate dissolution of source zircon until saturation is reached will result in near-complete inheritance of Hf isotope ratios from the source. Our results impose an extra layer of complexity to interpretation of the zircon Hf isotope archive that is not currently considered.

The source and tectonic setting of the Changhsingian K-bentonites in the Huaying Mountain region, South China

Numerous volcanic ash (or K-bentonite) beds are ubiquitous in the Permian-Triassic boundary (PTB) successions in South China and are conjectured to have been a critical factor in global events, such as the Permian-Triassic mass extinction. However, the precise location and tectonic setting of related volcanism remain debated. In this study, we present whole-rock geochemistry, zircon U Pb ages, trace element compositions and in situ Hf isotope analyses of K-bentonites found in the Late Permian Changxing Formation in the Huaying Mountain region, eastern Sichuan Province, northern South China. The two samples yield U Pb zircon ages of 253.7 ± 1.4 Ma (QJP-38-R-1 below) and 253.0 ± 2.0 Ma (QJP-38-R-2 above). Zircons with ages of 240–280 Ma from sample QJP-38-R-1 have negative ε Hf ( t ) values from −16.65 to −1.65 and two-stage model (T DM c ) ages from 1.38 to 2.34 Ga, whereas those from QJP-38-R-2 have mainly negative ε Hf ( t ) values from −7.28 to 0.13 and T DM c ages from 1.28 to 1.74 Ga. The low ε Hf ( t ) values and T DM c ages suggest that the original magmas mainly involved Paleoproterozoic-Mesoproterozoic crustal components and a few mafic mantle materials. Moreover, zircon trace element characteristics show that the two samples were sourced from a subduction-driven magmatic arc. The zircons from QJP-38-R-1 are primarily derived from S-type granites that formed in continental granitic rocks related to a continental arc tectonic setting, whereas the zircons from QJP-38-R-2 are mainly derived from S-type granites in oceanic granitic rocks related to an island arc tectonic setting. The K-bentonites near the PTB in South China may have originated from felsic volcanism as a result of the convergence of the Paleo-Tethys Ocean. The integration of zircon Hf isotope and trace element compositions indicates that the PTB K-bentonites in South China may have been derived from a series of volcanic eruptions occurring within a short interval at the continental margins around South China. Volcanic eruptions took place at the Ailaoshan-Sanjiang continental margins in or near southwestern South China and may be the major source of K-bentonites. Volcanic eruptions took place at the East Kunlun or West Qinling continental margins and may have provided a small number of original materials. This result indicates that these K-bentonites may have multiple sources. • Newly found PTB K-bentonites were reported weighted mean 206 Pb/ 238 U ages of 253.7 ± 1.4 Ma and 253.0 ± 2.0 Ma. • The PTB K-bentonites may be sourced from felsic volcanism as a result of the convergence of the Paleo-Tethys Ocean. • The PTB K-bentonites in South China may have multiple sources.

Mississippian southern Laurentia tuffs came from a northern Gondwana arc

Abstract A Paleozoic arc that formed by southward subduction of the Rheic oceanic plate beneath northern Gondwana has long been inferred, but its history and geochemical signatures remain poorly understood. New U-Pb ages, juvenile εHf signatures, and trace-element composition data of young zircons from tuffs at two southern Laurentia sites indicate their derivation from a continental arc that was active from ca. 328 to ca. 317 Ma and permit correlation of sedimentary sequences 800 km apart in southern Laurentia. These include the Stanley tuffs in the Ouachita Mountains of southeastern Oklahoma and southwestern Arkansas and the newly discovered Barnett tuff in the subsurface of the Midland Basin in west Texas (USA). The Barnett tuff has a zircon chemical abrasion–isotope dilution–thermal ionization mass spectrometry U-Pb date of 327.8 ± 0.8 Ma, similar to the oldest Stanley tuff in the Ouachita Mountains. Zircon Hf isotope depleted mantle model ages further suggest that the source was a continental arc on basement with both Grenville and Pan-African affinities, pointing to northern Gondwana or peri-Gondwana terranes. The new data link the tuffs to granitoids (326 Ma) of the Maya block in southern Mexico, which was part of northern Gondwana. Correlation of the Stanley-Barnett tuffs across southern Laurentia suggests the likely presence of Mississippian tuffs over a broad region in southern Laurentia, and their usefulness for constraining absolute ages of basin fills and characterizing the Gondwanan arc.

Zircons underestimate mantle depletion of early Earth

The mechanism and timing of crustal growth and differentiation on early Earth are debated. Evidence of crustal differentiation is detectable as deviations from Earth’s assumed chondritic uniform reservoir (CHUR) as crust is extracted from the mantle leading to a melt-depleted reservoir. For the long-lived zircon Lu-Hf system, no incontrovertible evidence of significant mantle depletion >3.8 Ga exists. We conduct combined U-Pb and Lu-Hf isotopic analyses for the detrital zircon from the Caozhuang supracrustal sequence in North China. The zircon Hf isotopic compositions are broadly scattered along the CHUR evolution line. However, given the possibility of potential systematic biases in zircon petrogenesis and the unique tectonic setting of early Earth, we posit that magmatism controlled by the nascent forms of plate tectonics during the Eoarchean could have likely hidden the degree of ancient crust-mantle differentiation. The non-depleted zircon Hf isotopes observed in North China and globally during early Earth may in verity imply the existence of ubiquitous depleted mantle domains at that time.