Lu Hf Isotopic Compositions Research Articles

Continuity of the North Qilian and North Altun orogenic belts of NW China: evidence from newly discovered Palaeozoic low-Mg and high-Mg adakitic rocks

AbstractIn this study, the petrology, zircon U–Pb ages, Lu–Hf isotopic compositions, whole-rock geochemistry and Sr–Nd isotopes for newly recognized low-Mg and high-Mg adakitic rocks from the North Altun orogenic belt were determined. The results will provide important insights for understanding the continuities of the North Qilian and North Altun orogenic belts during early Palaeozoic time. The low-Mg adakitic granitoids (445 to 439 Ma) are characterized by high SiO2(69–70 wt %), low Mg no. (43–48) and low Cr and Ni contents. In contrast, the high-Mg adakitic granitoids (425 to 422 Ma) have relatively lower SiO2(65–67 wt %), higher Mg no. (60–62) and higher Cr and Ni contents. The low-Mg adakitic rocks have high initial87Sr/86Sr ratios (0.7073–0.7084), negative εNd(t) (−1.9 to −4.0) and εHf(t) values (−6.8 to −2.0), and old zircon Hf model ages (1.4–1.7 Ga). In contrast, the high-Mg adakitic rocks show lower initial87Sr/86Sr ratios (0.7044–0.7057), higher εNd(t) (−0.7 to 3.1) and positive εHf(t) values (2.0 to 6.9), with younger zircon Hf model ages (0.9–1.2 Ga). These results suggest that the low-Mg adakitic rocks were probably generated by the partial melting of thickened crust, whereas the high-Mg adakitic rocks were derived from the anatexis of delaminated lower crust, which subsequently interacted with mantle magma upon ascent. The data obtained in this study provide significant information about the geological and tectonic processes after the closure of the Altun Ocean. The continent–continent collision and thickening probably occurred during 450–440 Ma with the formation of low-Mg adakitic rocks, and the transition of the tectonic regime from compression to extension probably occurred at 425–422 Ma with the formation of high-Mg adakitic rocks. The geochemical, geochronological and petrogenetic similarities between the North Altun and North Qilian adakitic rocks suggest that these two orogenic belts were subjected to similar tectonomagmatic processes during early Palaeozoic times.

Delamination of lithospheric mantle evidenced by Cenozoic potassic rocks in Yunnan, SW China: A contribution to uplift of the Eastern Tibetan Plateau

New zircon U–Pb ages, mineral chemical data, whole-rock geochemistry and Sr–Nd isotopes from the potassium-rich intrusions in the Yunnan area, SW China, were determined to provide constraints on the uplift of the Eastern Tibetan Plateau. The intrusive rocks consist of shoshonitic syenites (high-Mg syenites, low-Mg syenites and syenite porphyries) and potassic granitoids (granite porphyries). Zircon LA-ICP-MS U–Pb dating indicates coeval emplacement ages of ~35Ma. The shoshonitic syenites have alkaline affinities and the enrichment in LILEs and LREEs (e.g. La, Sr, U, Pb), with depletion of HFSEs (e.g. Nb, Ti, Ta) and weak Eu anomalies. They display uniform Sr–Nd–Lu–Hf isotopic compositions with similar initial 87Sr/86Sr ratios (0.7073–0.7079), enriched εNd(t) values (−6.8 to −4.3) and mostly negative zircon εHf(t) values ranging from −4.6 to +0.1. The high-Mg syenites have high MgO, Fe2O3T, TiO2, CaO, Cr, Ni concentrations and relatively high Mg# (60–68), indicating an origin from enriched lithospheric mantle. The low-Mg syenites and syenite porphyries are geochemically distinct with the high-Mg syenites, but the insignificant variations in major elements, linear trends of La against (La/Yb)N and similar Sr–Nd isotopic compositions to the high-Mg syenites suggest that they were produced by different degrees of partial melting of the same enriched mantle source. The potassic granitic intrusions are sub-alkaline with a strongly peraluminous character. They display an S-type granite affinity, with high Al2O3/TiO2 and low CaO/Na2O and K2O/Al2O3 ratios, suggesting a pelitic source. They are LREE-enriched and have relatively flat HREE patterns with weakly negative Eu anomalies and positive Rb, U, and Pb anomalies and negative Nb, Ta, and Ti anomalies. They have relatively high initial 87Sr/86Sr ratios (0.7143) and enriched Nd isotopic compositions [εNd(t)=−4.1]. Their zircon εHf(t) values (−4.0 to +0.09) and old two-stage Hf model ages (TDMc=1.16–1.36Ga) suggest that they were generated by partial melting of an ancient K-rich crust. The petrogenesis of the potassic intrusive rocks with their uniform Nd model ages (TDM=968–1168Ma) indicates that the lithospheric mantle beneath Yunnan was enriched by metasomatism related to ancient oceanic subduction. Considering the widespread and coeval nature of potassium-rich rocks in Eastern Tibet, we consider that delamination of metasomatized lithospheric mantle beneath Yunnan in the Eocene was a result of the India–Asia collision, which triggered partial melting of both the lithospheric mantle and the K-rich crust, and contributes to uplift of the Eastern Tibetan Plateau.

Late Mesozoic high-K calc-alkaline magmatism in Southeast China: the Tongling example

ABSTRACTWe report new zircon U–Pb ages, Hf isotopic and geochemical results for the Tongling granitic plutons of Southeast China. SHRIMP U–Pb ages for the Miaojia quartz monzodiorite porphyrite，the Tianebaodan and Tongguanshan quartz monzodiorites, the Xinqiaotou granodiorite porphyry, and the Shatanjiao and Nanhongchong granodiorite are 143 ± 2, 141 ± 1 and 142 ± 1, 147 ± 1, and 145 ± 1 and 139 ± 1 Ma, respectively. Combined with previous geochronological data, our results indicate that the porphyritic rocks are older than rocks of the same type lacking porphyritic texture. Geochemically, these high-K calc-alkaline intrusive rocks are characterized by arc-like trace element distribution patterns, with significant enrichment in LILE and LREE but depletion in HFSE. Lu–Hf isotopic compositions of zircons from the high-K calc-alkaline (HKCA) rocks have εHf(t) values of magmatic 139–147 Ma zircons from −8.1 to −25.6, with two-stage model ages (tDM2) of 1.71–2.67 Ga, whereas εHf(t) values of inherited 582–844 Ma zircons range from 5.4 to −9.5, with tDM2 of 1.39–2.22 Ma, younger than tDM2 values of igneous zircon, indicating that newly formed mantle material was added to the continental crust of the Yangtze Block. Moreover, εHf(t) values of inherited zircon cores older than 1000 Ma are from −7.8 to −26, similar to magmatic zircons, and the tDM2 values are all greater than 3.0 Ga (3.16–3.75 Ga), reflecting partial melting of ancient sialic material. We conclude that the plutonic melts were derived from both the enriched mantle and the ancient crust. The HKCA Tongling intrusions coincide temporally with the J3–K1 magmatic event that was widespread in Southeast China. This igneous activity may have accompanied sinistral slip along the Tan-Lu fault due to oblique subduction of the Palaeo-Pacific plate.

Zircon U-Pb ages and Lu-Hf isotope compositions from clastic rocks in the Hutuo Group: Further constraints on Paleoproterozoic tectonic evolution of the Trans-North China Orogen

The Hutuo Group, as one of the classic examples of Paleoproterozoic strata, plays an important role in establishing evolutionary processes in the Trans-North China Craton. In this contribution, we present petrologic, detrital zircon U-Pb ages and Lu-Hf isotopes from three subgroups of the Hutuo Group. Sandstones in the Doucun and Dongye Subgroups are dominated by Q (monocrystalline and polycrystalline quartz) and F (K-feldspar and plagioclase) with minor lithic fragments, suggesting that the detrital components were mainly derived from both the continental block and recycled orogen. In contrast, clastic components in the Guojiazhai Subgroup are mainly Q and lithic fragments, indicating they were derived predominantly from the recycled orogen. The ages of detrital zircons from sandstones in the Doucun and Dongye Subgroups are mainly concentrated at ca. 2.5Ga and 2.2–2.1Ga with minor 2.7Ga zircons also present which indicates they were dominantly sourced from 2.5Ga Wutai, Fuping and Zanhuang Complexes, and Paleoproterozoic intrusives. The Guojiazhai Subgroup displays a different zircon age population of ca.2.4Ga, 2.2–2.1Ga and 2.0–1.9Ga, which indicates likely derivation from Paleoproterozoic intrusives in the Wutai, Lüliang and Hengshan areas. Based on the observation that sandstone clastic components in the Doucun and Dongye Subgroups are dominantly quartz, feldspar and sedimentary lithic fragments, but not volcanic lithics, we propose that they were deposited in a rift-related setting. Zircons from the lower sequence of the Hutuo Group yielded a young population of 2140Ma. Considering the volcanic rocks of 2140±14Ma at the base of the group and 2.2–2.0Ga magmatism along the TNCO, we propose that Doucun and Dongye Subgroups formed at 2.2–2.0Ga. The youngest, ca. 1.9Ga, zircons in the Guojiazhai Subgroup indicate this Group was deposited during closure of the rift at 1.9–1.8Ga. The two stage model ages of the detrital zircons mainly range from 2.6 to 2.9Ga with a minor ∼2.5Ga contribution. Therefore, we infer that 2.6–2.9Ga represents a period of intensive crustal growth in the Trans-North China Orogen, but that some degree of crustal growth continued to ∼2.5Ga.

Petrological and zircon evidence for the Early Cretaceous granulite-facies metamorphism in the Dabie orogen, China

An integrated study of petrology, mineralogy, geochemistry, and geochronology was carried out for contemporaneous mafic granulite and diorite from the Dabie orogen. The results provide evidence for granulite-facies reworking of the ultrahigh-pressure (UHP) metamorphic rock in the collisional orogen. Most zircons from the granulite are new growth, and their U-Pb ages are clearly categorized into two groups at 122–127 Ma and 188±2 Ma. Although these two groups of zircons show similarly steep HREE patterns and variably negative Eu anomalies, the younger group has much higher U, Th and REE contents and Th/U ratios, much lower εHf(t) values than the older group. This suggests their growth is associated with different types of dehydration reactions. The older zircon domains contain mineral inclusions of garnet + clinopyroxene ± quartz, indicating their growth through metamorphic reactions at high pressures. In contrast, the young zircon domains only contain a few quartz inclusions and the garnet-clinopyroxene-plagioclase-quartz barometry yields pressures of 4.9 to 12.5 kb. In addition, the clinopyroxene-garnet Fe-Mg exchange thermometry gives temperatures of 738–951 °C. Therefore, the young zircon domains would have grown through peritectic reaction at low to medium pressures. The younger granulite-facies metamorphic age is in agreement not only with the adjacent diorite at 125±1 Ma in this study but also the voluminous emplacement of coeval mafic and felsic magmas in the Dabie orogen. Mineral separates from both mafic granulite and its adjacent diorite show uniformly lower δ18O values than normal mantle, similar to those for UHP eclogite-facies metaigneous rocks in the Dabie orogen. In combination with major-trace elements and zircon Lu-Hf isotope compositions, it is inferred that the protolith of mafic granulites shares with the source rock of diorites, both being a kind of mafic metasomatites at the slab-mantle interface in the continental subduction channel. The spatial and temporal distribution of Early Cretaceous granulite-facies metamorphic rocks in this region is associated with the bimodal magmatism within a short period of 120–130 Ma in the postcollisional stage. This provides a direct link in petrogenesis between the granulitic, migmatic and magmatic rocks in the collisional orogen to active continental rifting, whereby high heat flow was transferred from the asthenospheric mantle into the thinned orogenic lithosphere for partial melting.

Zircon U–Pb age and Hf isotope evidence for an Eoarchaean crustal remnant and episodic crustal reworking in response to supercontinent cycles in NW India

Scattered T DM2 (3.8–3.2 Ga) for 3.28–2.99 Ga zircons from the Proterozoic Delhi Supergroup in northwestern India provide evidence for generation of juvenile crust and reworking of older crust. Depleted mantle-like ε Hf( t ) values (+7.2 to +5.6) for 2.86–2.71 Ga zircons indicate that generation of juvenile crust occurred during this period and ceased at 2.71 Ga. Extensive magmatism at 2.66–2.34, 2.11–2.01 and 1.60–1.37 Ga was dominated by reworking of pre-existing crust with variable ages, and the last two periods were accompanied by formation of juvenile crust. An Eoarchaean age of 3671 ± 15 Ma represents the oldest age found in NW India. Zircons formed during supercontinent assembly have positive to negative ε Hf( t ) values, suggesting involvement of juvenile and ancient crust, whereas largely positive ε Hf( t ) values for zircons crystallized subsequent to supercontinent amalgamation suggest involvement of predominantly juvenile crust. Correlation of detrital age patterns and tectonomagmatic events indicates a conjugate position for northern Indian and the Cathaysia Block of South China during the assembly of Nuna. The South China Block remained juxtaposed to India until its separation from Pangaea in the late Palaeozoic. Supplementary material: Supplementary data, including detailed metadata related to laboratory and sample preparation methods, U-Pb and Lu-Hf isotopic compositions of the analyzed samples and standards are available at https://doi.org/10.6084/m9.figshare.c.3711847

Source characteristics and provenance of metasedimentary rocks from the Kangxiwa Group in the Western Kunlun Orogenic Belt, NW China: Implications for tectonic setting and crustal growth

The West Kunlun Orogenic Belt is a result of the closure of Proto– and Paleo–Tethys Ocean and subsequent continental collision. In order to better understand the tectonic evolution of this orogenic belt, whole-rock geochemistry of major and trace elements, detrital zircon U–Pb geochronology and Lu–Hf isotopic composition were determined for metasediments of the Kangxiwa Group (KG) in the South Kunlun Terrane (SKT). These rocks have low Chemical Index of Alteration and high Index of Compositional Variability values, indicating relatively weak chemical weathering and a source compositionally dominated by immature material that lacks alumina-rich minerals. The rocks are all enriched in light rare earth element and display relatively flat heavy rare earth element patterns. Their low contents of ferromagnesian (e.g. Cr, Co and Ni) and high field strength elements (e.g. Zr, Hf, Nb, Ta and Y) are similar to those of graywackes from active continental margins. Detrital zircons from three samples of the KG metasediments can be divided into igneous and metamorphic grains. Zircon U–Pb dating of the igneous grains shows similar age spectra dominated by Early Paleozoic to late Paleozoic ages. The Precambrian igneous grains yield Neoproterozoic 206Pb/238U ages and a few early Mesoproterozoic to Paleoproterozoic 207Pb/206Pb ages. The youngest igneous zircons define the maximum depositional age at ~307Ma for the KG metasediments. The U–Pb dating of all the metamorphic grains yield 206Pb/238U ages at ~245Ma, suggesting the metasediment rocks were deposited prior to the early Triassic. These results indicate that the KG metasediments were deposited on an active continental margin during the time between late Carboniferous and late Permian. Lu–Hf isotopic analyses of the igneous detrital zircons from the three samples show large variations of εHf(t) values mostly between −21 and +12. Combined with their U–Pb ages, two respective crustal growth events are identified in the SKT at the Early Paleozoic and the late Carboniferous.

In situ U–Pb and Lu–Hf isotopic studies of zircons from the Sancheong–Hadong AMCG suite, Yeongnam Massif, Korea: Implications for the petrogenesis of ∼1.86 Ga massif-type anorthosite

Isotopic and geochemical characteristics of Proterozoic anorthosite-mangerite-charnockite-granite (AMCG) suite have long been used for tracing the mantle–crustal source and magmatic evolution. We analyzed Lu–Hf isotopic compositions of zircon from the Sancheong–Hadong AMCG complex, Yeongnam Massif, Korea, in order to understand tectonomagmatic evolution of the Paleoproterozoic AMCG suite occurring at the southeastern margin of the North China Craton (NCC). The anorthositic rocks in this complex, associated with charnockitic and granitic gneisses, were recrystallized to eradicate magmatic features. In situ SHRIMP (sensitive high-resolution ion microprobe) U–Pb analyses of zircon from a leuconorite and an oxide-bearing gabbroic dyke yielded weighted mean 207Pb/206Pb ages of 1870±2Ma and 1861±6Ma, respectively. Charnockitic, granitic, and porphyroblastic gneisses yielded weighted mean 207Pb/206Pb zircon ages of 1861±6Ma, 1872±6Ma, and 1873±4Ma, respectively. These crystallization ages, together with our previous geochronological data for anorthosites (1862±2Ma), are indicative of episodic AMCG magmatism over an ∼10Ma interval. Initial εHf(t) values of zircon analyzed from five anorthositic rocks and four felsic gneisses range from +2.1 to −6.1 and −0.3 to −5.4, respectively. Zircon Hf isotopic data in combination with available whole rock Sr–Nd isotopic data suggest that anorthositic parental magma was most likely derived from a mantle source and variably affected by crustal contamination. This crustal component is also reflected in charnockitic-granitic magmas produced primarily by the melting of lower crust. Taken together, the AMCG magmatism at 1.87–1.86Ga in the Yeongnam Massif is most likely a late orogenic product of Paleoproterozoic NCC amalgamation tectonically linked to assembly of the Columbia supercontinent.

Изотопный Lu-Hf состав циркона и геохимия палеопротерозой-ских гранитоидов Бирюсинского блока (юго-запад Сибирского кратона)

Изотопный Lu-Hf состав циркона и геохимия палеопротерозой-ских гранитоидов Бирюсинского блока (юго-запад Сибирского кратона)

Early Mesozoic intracontinental orogeny and stress transmission in South China: evidence from Triassic peraluminous granites

The Early Mesozoic tectonic and magmatic events in South China are considered to represent intracontinental orogeny, although the mechanism and geodynamic setting remain debated. Here we report zircon U–Pb geochronology, Hf isotopes and whole-rock geochemistry from four granitic plutons in the Nanling and Yunkai domains. The zircon data define seven groups of weighted mean 206 Pb/ 238 U ages as 218 ± 2, 222 ± 4, 225 ± 3, 227 ± 3, 221 ± 2, 224 ± 3 and 231 ± 3 Ma, representing the magmatic crystallization ages. These intrusions are characterized by a peraluminous nature, with high A/CNK values (>1.1), negative Eu, Ba, Sr, Nb and Ti anomalies, and enrichment in LREE, Rb, Th, U and Pb. Zircon grains from all the samples show variably negative ε Hf ( t ) values (−3.8 to −18.6) with two-stage Hf model ages clustered in the range 1.4–2.5 Ga. Our data indicate that the Early Mesozoic granitic magmas were generated by the partial melting of crustal rocks with insignificant input of mantle materials. In conjunction with other geological evidence, we suggest that Triassic intracontinental deformation was triggered by far-field stress propagation from the plate boundary of South China. Supplementary materials: U–Pb geochronological data, zircon Lu–Hf isotopic compositions, major and trace element data, and 40 Ar/ 39 Ar ages are available at https://doi.org/10.6084/m9.figshare.c.3672838 .

Lu-Hf isotope composition of zircon as an indicator of the sources for Paleoproterozoic collisional granites (Sharyzhalgai uplift, Siberian craton)

We present geochemical characteristics of rocks and results of local dating and Lu-Hf isotopic analysis of zircons from two plutons of Paleoproterozoic collisional granitoids in the northwest of the Sharyzhalgai uplift. The rocks of the Alar pluton in the Bulun terrane correspond in major- and trace-element composition to I-type potassic granites. The Alar granites formed at ~780 °C and <5-8 kbar through melting of predominantly graywacke (volcanosedimentary) source rocks with the contribution of melt from plagiogneisses of tonalite-trondhjemite complex. The age and Lu-Hf isotopic similarity between inherited zircon core (3.3-3.0 and 2.85-2.6 Ga) in these granites and zircons from the Paleo- and Mesoarchean rocks of the Bulun terrane suggests that the latter are the most likely crustal sources of the granites. The more radiogenic isotope composition of the Paleoproterozoic (1.85 Ga) magmatic zircons from the granites as compared with the Archean crustal rocks of the Bulun terrane testifies to the contribution of juvenile material to the granite formation. Highly ferroan granodiorites and granites of the Shumikha pluton in the Onot terrane are enriched in HFSE and correspond to A-type granites. They probably derived by the melting of crustal sources of intermediate-felsic (tonalitic) and mafic composition at >860 °C. The Hf isotope composition of magmatic (1.86 Ga) and inherited (ca. 2.5 Ga) zircons indicates that the granites formed from ancient crustal source (model Hf age is >3.0 Ga) with the contribution of Neoarchean juvenile, probably mafic material.

Paleoproterozoic granulite-facies metamorphism and anatexis in the Oulongbuluke Block, NW China: Respond to assembly of the Columbia supercontinent

The Oulongbuluke Block, which is located in the northeastern margin of the Tibet Plateau, has traditionally been considered to be a fragment of the Tarim Craton. Here we present a systematic petrologic, geochemical, and zircon U-Pb and Hf isotopic investigation on mafic granulite and migmatite in the Oulongbuluke Block. The mafic granulite is mainly composed of clinopyroxene, orthopyroxene, plagioclase, amphibole and quartz, with peak metamorphic P-T contions of 6.5–8.8kbar, 745–770°C. Macroscopic and microscopic observations provide strong evidence for in situ partial melting of the felsic gneiss involving breakdown of biotite within the Oulongbuluke Block. The Pl-rich leucosomes with positive Eu anomalies and higher Sr contents were generated as the early-formed feldspar cumulate, and the Kfs-rich pegmatite with negative Eu anomalies and lower Sr contents may represent percolating fractionated melt that was trapped during cooling. Zircon U–Pb dating and Hf isotopic analyses on the mafic granulite and migmatite of the Oulongbuluke Block reveal two distinct age populations: the early Paleoproterozoic (∼2.37Ga) and late Paleoproterozoic (1.93–1.92Ga). The ∼2.37Ga magmatic zircon cores of the migmatite have εHf(t) values between −4.3 and 0.4, with two-stage Hf model ages (TDMC) mainly between 2.82Ga and 3.05Ga. The age of 1.93–1.92Ga obtained from the mafic granulite and migmatite is interpreted as the age of Late Paleoproterozoic metamorphism and anatexis. Most of the 1.93–1.92Ga metamorphic and anatectic zircons have significantly lower 176Lu/177Hf ratios but higher 176Hf/177Hf (t) values than the inherited magmatic zircon cores, which demonstrates that both the zircon U–Pb and Lu–Hf isotope compositions were significantly reset during metamorphism and anatexis. The late Paleoproterozoic metamorphic and anatectic event coincided with global orogenic events that are recorded in many continental fragments, which suggests their link to the Columbia supercontinent.

New constraints on the origin and evolution of the Thomson Orogen and links with central Australia from isotopic studies of detrital zircons

Interpretation of the Thomson Orogen and its context within the Tasmanides of eastern Australia is hampered by vast areas of deep sedimentary cover which also mask potential relationships between central and eastern Australia. Within covered areas, basement drill cores offer the only direct geological information. This study presents new detrital zircon isotopic data from these drill cores and poorly understood outcropping units to provide new age and provenance information for sedimentary rocks from the Thomson Orogen. Two distinct detrital zircon signatures are revealed. One is dominated by Grenvillian-aged (1300–900Ma) zircons with a significant peak at ~1180Ma and lesser peak at ~1070Ma. These age peaks, along with Lu–Hf isotopic compositions (median εHf(t)=+1.5), dominantly mantle-like δ18O values (median=5.53‰) and model ages of ~1.89Ga, support a Musgrave Province (central Australia) source. The dominance of Grenvillian-aged material additionally points to deposition during the Petermann Orogeny (570–530Ma) when the Musgrave Province was uplifted, shedding abundant material to the Centralian Superbasin. Comparable age spectra suggest that parts of the Thomson Orogen were connected to the Centralian Superbasin during this period. We use the term ‘Syn-Petermann’ to describe this signature which is observed in two drill cores adjacent to the North Australian Craton and scattered units in the outcropping Thomson Orogen. The second signature marks a significant provenance shift and is remarkably consistent throughout the Thomson Orogen. Age spectra exhibit dominant peaks at 600–560Ma, lesser 1300–900Ma populations and maximum depositional ages of ~495Ma. This pattern is termed the ‘Pacific Gondwana’ detrital zircon signature and is recognised throughout eastern Australia, Antarctica and central Australia. Lu–Hf isotope data for Thomson Orogen rocks with this signature are highly variable with εHf(t) values between ‐49 and +10 and dominantly supracrustal δ18O values suggesting input from different and more diverse source regions relative to those exhibiting the Syn-Petermann signature.

Isotope Lu–Hf composition of detrital zircon from paragneisses of the Sharyzhalgai uplift: evidence for the Paleoproterozoic crustal growth

We present results of study of the trace-element and Lu–Hf isotope compositions of zircons from Paleoproterozoic high-grade metasedimentary rocks (paragneisses) of the southwestern margin of the Siberian craton (Irkut terrane of the Sharyzhalgai uplift). Metamorphic zircons are represented by rims and multifaceted crystals dated at ~1.85 Ga. They are depleted in either LREE or HREE as a result of subsolidus recrystallization and/or synchronous formation with REE-concentrating garnet or monazite. In contrast to the metamorphic zircons, the detrital cores are enriched in HREE and have high (Lu/Gd)n ratios, which is typical of igneous zircon. The weak positive correlation between 176Lu/177Hf and 176Hf/177Hf in the zircon cores evidences that their Hf isotope composition evolved through radioactive decay in Hf = the closed system. Therefore, the isotope parameters of these zircons can give an insight into the provenance of metasedimentary rocks. The Paleoproterozoic detrital zircon cores from paragneisses, dated at ~2.3–2.4 and 2.0–1.95 Ga, are characterized by a wide range of εHf values (from +9.8 to –3.3) and model age TC 2.8–2.0 Ga. The provenance of these detrital zircons included both rocks with juvenile isotope Hf parameters and rocks resulted from the recycling of the Archean crust with a varying contribution of juvenile material. Zircons with high positive εHf values were derived from the juvenile Paleoproterozoic crustal sources, whereas the lower εHf and higher TC values for zircons suggest the contribution of the Archean crustal source to the formation of their magmatic precursors. Thus, at the Paleoproterozoic stage of evolution of the southwestern margin of the Siberian craton, both crustal recycling and crustal growth through the contribution of juvenile material took place. On the southwestern margin of the Siberian craton, detrital zircons with ages of ~2.3–2.4 and 1.95–2.0 Ga are widespread in Paleoproterozoic paragneisses of the Irkut and Angara–Kan terranes and in terrigenous rocks of the Urik–Iya graben, which argues for their common and, most likely, proximal provenances. In the time of metamorphism (1.88–1.85 Ga), the age of Paleoproterozoic detrital zircons (2.4–2.0 Ga), and their Lu–Hf isotope composition (εHf values ranging from positive to negative values) the paragneisses of the southwestern margin of the Siberian craton are similar to the metasedimentary rocks of the Paleoproterozoic orogenic belts of the North China Craton. In the above two regions, the sources of detrital zircons formed by both the reworking of the Archean crust and the contribution of juvenile material, which is evidence for the crustal growth in the period 2.4–2.0 Ga.

Early Jurassic monzogranite-tonalite association from the southern Zhangguangcai Range: Implications for paleo–Pacific plate subduction along northeastern China

The initiation time and tectonic responses of paleo–Pacific plate subduction beneath the Eurasian continent remain controversial. In this paper we report on Early Jurassic (201–198 Ma) monzogranite-tonalite association from the southern Zhangguangcai Range, northeastern China. Zircon laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb dating indicates that the monzogranite and tonalite have identical 206Pb/208U ages of 201 ± 2 (mean square of weighted deviates, MSWD = 1.2, 2σ) and 198 ± 3 Ma (MSWD = 3.2, 2σ), respectively. The monzogranite and tonalite display different geochemical features, suggesting that they were derived from two distinct source regions. The monzogranite displays high SiO2, K2O, and Rb contents, as well as depleted whole-rock Sr-Nd-Pb isotopic compositions, i.e., εNd(t) = +8.3 to +11.7, with single-stage Nd model ages of 0.30–0.05 Ga. Zircons from the monzogranite also have depleted Lu-Hf isotopic compositions, and these contradictory geochemical features suggest that the monzogranite may be derived from melting of mid-oceanic ridge basalt (MORB)–sediment mélanges in subduction zone. The Na-rich tonalite has lower SiO2 and higher TiO2 contents. In combination with their relatively evolved Sr-Nd-Pb isotopic compositions and zircon Lu-Hf isotopic compositions, it can be considered that the tonalite was derived from juvenile basaltic crust in an active continental margin. Considering other Triassic to Jurassic mafic intrusive rocks and I-type granites in the Zhangguangcai Range, we propose that the Early Jurassic monzogranite-tonalite association in the Shihe area was caused by the westward subduction of the paleo–Pacific plate beneath northeastern China.

Detrital zircon U-Pb geochronology and Lu-Hf isotopic compositions of the Wuliangshan metasediment rocks in SW Yunnan (China) and its provenance implications

The Wuliangshan Group occurs to the east of the Lancang giant igneous zone in SW Yunnan, and is mainly composed of low-grade metamorphosed sedimentary rocks. The group has been considered as the syn-orogenic product of the Baoshan with Simao-Indochina blocks. However, its depositional time and provenance remain to be poorly constrained. This paper presents zircon U-Pb dating and Lu-Hf-isotopic data for five representative sandstone samples from the Wuliangshan Group. The detrital grains yield a major age-peak at ∼259 Ma, and four subordinary age-peaks at ∼1 859, ∼941, ∼788, and ∼447 Ma, respectively. Our results suggest that the Wuliangshan metasedimentary sequence was deposited after Middle Triassic rather than previously-thought Cambrian. The detrital zircon age spectrum, along with in-situ Lu-Hf isotopic data suggest that the Wuliangshan Group might be a syncollisional sedimentary product related to the collision of Baoshan with Simao-Indochina blocks. It is inferred that the provenance of the Wuliangshan Group is mainly from the Simao/Yangtze blocks to the east rather than the Baoshan Block or Lancang igneous zone to the west.

Pikes Peak batholith (Colorado, USA) revisited: A SIMS and LA-ICP-MS study of zircon U–Pb ages combined with solution Hf isotopic compositions

The Lake George ring complex (LGRC) is an epizonal intrusion that is part of seven alkalic stocks distributed throughout the Mesoproterozoic Pikes Peak batholith (PPB) in central Colorado. Despite having been studied for more than a century, details about the emplacement history of the PPB lithologies have remained elusive. To parse out the relative sequence of intrusive events and magma petrogenesis, we analyzed zircons from different LGRC intrusive phases for their U–Pb ages and Lu–Hf isotopic compositions. We found that the LGRC is composed of a 1428±8Ma syenomonzonite at its center, which has a Hf isotopic composition indistinguishable from that of coeval wall rocks related to the Berthoud plutonic suite. We hence interpret it as a roof pendant. The three ring dikes that surround the LGRC center have ages of 1085±5Ma, 1115±12Ma, and 1078±11Ma from innermost to outermost, respectively. The local phase of Pikes Peak granite that surrounds LGRC was emplaced at 1066±10Ma. Therefore, LGRC represents an early stage in PPB emplacement, and evidently formed over a protracted period of time. The LGRC zircon εHf values (+4.8±0.2 at 1115Ma, +0.4±0.9 at 1078Ma, and −0.8±0.2 at 1066Ma) indicate input of shallow mantle-derived melts and direct involvement of the Proterozoic Colorado Front Range (CFR) crust with the latter gradually prevailing over time. The results presented here provide new insights into the petrogenesis of the PPB and call for a general reassessment of emplacement ages and source characteristics of the other alkalic intrusive centers in the batholith.

Using detrital zircons from river sands to constrain major tectono-thermal events of the Cathaysia Block, SE China

Detrital zircons from the Minjiang and Zhujiang Rivers in SE China have been analyzed for U/Pb ages and Lu–Hf isotopic compositions to constrain their provenance and the growth history of the continental crust of the Cathaysia Block. Zircon U/Pb ages show five major populations at 90–250Ma, 400–500Ma, 0.7–1.2Ga, 1.6–2.0Ga and 2.3–2.6Ga. Proterozoic zircons have Hf isotopic signatures consistent with the remelted ancient crust and the involvement of juvenile crust. However, Phanerozoic zircons have Hf isotopic signatures indicative of reworked or recycled ancient crust. Crustal growth rates based on the two-stage Hf model ages of the detrital zircons indicate that 30% and 90% of present crust in the northeastern Cathaysia Block was formed by 2.5Ga and 1.6Ga, respectively, whereas <20% continental crust was formed by 2.5Ga, and 80% by 1.6Ga in the southwestern Cathaysia Block. Therefore, Neoarchean and Paleoproterozoic were two major periods of crustal growth of the Cathaysia Block. Our results also reveal that the tectono-thermal events at ∼370Ma and ∼117Ma may have occurred to the Wuyishan terrane in the northeastern Cathaysia Block. Jurassic zircon grains have Hf isotopic compositions more variable than Cretaceous grains, which may have been derived from different source rocks. The 140–120Ma tectono-thermal events were likely related to the change of the subduction direction of the Paleo-Pacific plate from northward to northwestward at 140Ma. The 112–90Ma tectono-thermalevents may be correlated with the rollback of the subducted paleo-Pacific plate at ∼110Ma.

The early-stage evolution of the Neo-Tethys ocean: Evidence from granitoids in the middle Gangdese batholith, southern Tibet

The Gangdese magmatic belt, located along the southern margin of the Lhasa terrane, plays a critical role in understanding the tectonic framework associated with the Indian-Eurasian collision and the crustal growth of the southern Tibet. In this paper, we present a series of results from new petrological, geochemical and geochronological investigations of the granitoid rocks. The granitoids mainly have sub-alkaline compositions and show medium K calc-alkaline affinities, as well as I-type granitoid characteristics. Significant depletions of Nb and Ta, combined with other geochemical features including enrichments of LILEs and LREEs confirm that the parental magmas of these rocks were generated in a subduction-related active continental margin (continental arc environment). Lu-Hf isotopic compositions and relatively low MgO contents indicate that the granitoids might be generated from partial melting of juvenile crust and basaltic lower crust, which is caused by the underplating of mantle materials, and the mantle materials that have been involved in this process. The 191.2–169.2Ma zircon U-Pb ages of the granitoids reveal middle-early Jurassic magmatic events. Combined with published data in the Gangdese magmatic belt, our study suggests that the northward subduction of the Neo-Tethys oceanic crust beneath the southern margin of the Lhasa terrane probably started no later than 191.2Ma. Zircons from the granitoids suite display positive ɛHf(t) values between 10.1 and 15.4 (mean value is 12.7), which correspond to the two-stage model ages (tDM2) in the range of 198–415Ma, attesting to crustal growth in the southern Lhasa terrane associated with the subduction of the Neo-Tethys oceanic crust. Our study is a systematic report of the granitoid suite in the Gangdese magmatic belt and strengthens the concept that the Neo-Tethys oceanic crust might have experienced a long evolution history.

Crustal evolution of the Eastern Block in the North China Craton: Constraints from zircon U–Pb geochronology and Lu–Hf isotopes of the Northern Liaoning Complex

The Northern Liaoning Complex in northeastern China constitutes an important component of the Eastern Block in the North China Craton. The major lithologies consist of Archean granitoid gneisses with minor supracrustal rocks occurring as tectonic lenses. This study presents zircon Lu–Hf isotopic data for the first time as well as new SHRIMP zircon U–Pb data of the major lithologies from the Northern Liaoning Complex, in order to elucidate the crustal evolution of the complex and provide new constraints on the Neoarchean crustal evolution of the Eastern Block in the North China Craton. Magmatic zircon U–Pb data from this study show that the protolith magmas of the supracrustal rocks and granitoid gneisses were generated during ∼2.55–2.50Ga. Metamorphic zircons document consistent metamorphic ages at 2.49–2.48Ga, suggesting a regional metamorphic event immediately after the magmatism at the end of the Neoarchean in the Northern Liaoning Complex. Inherited/detrital zircons of 2.79–2.60Ga suggest possible existence of ancient crust in this region. Zircon Lu–Hf isotopic compositions show that the magmatic zircons have variable ɛHf(t) values from −4.0 to +9.0 with depleted mantle model ages of 3.6–2.5Ga, of which most ɛHf(t) values are positive with a model age peak at 2.9–2.7Ga. These zircon Hf signatures reveal major juvenile crustal growth with additions of older crustal materials during 2.9–2.7Ga, and a crustal reworking event with minor juvenile additions at 2.6–2.5Ga in the studied area. Integrated with previous data from other Neoarchean complexes in the Eastern Block, both the major juvenile crustal growth during 2.9–2.7Ga and the strong crustal reworking at 2.6–2.5Ga contribute to the extensive Neoarchean crust formation of the Eastern Block in the North China Craton. The North China Craton share similar Neoarchean continental crustal evolution to other cratons in the world, though it is distinctively featured by intensive tectonothermal overprinting at the end of the Neoarchean.