Zircon Hf Isotopic Compositions Research Articles

Tectonic transition from subduction to retreat of the palaeo-Pacific plate: new geochemical constraints from the late Mesozoic volcanic sequence in eastern Fujian Province, SE China

AbstractThe widespread occurrence of late Mesozoic volcanic rocks in SE China is associated with widespread mineralization. Most geologists have accepted the model of Pacific plate subduction beneath the eastern Asian continent, but there are still controversies, especially the initial timing of the tectonic transition. In order to understand the tectonic evolution of the palaeo-Pacific plate subduction, an integrated study of zircon U–Pb, Lu–Hf isotope compositions and whole-rock geochemistry was carried out for Mesozoic volcanic rocks in eastern Fujian Province. According to the field observations and zircon U–Pb ages, these volcanic rocks can be divided into three phases: an early stage of Late Jurassic volcanic rocks (Phase 1, 159–153 Ma), a late stage of Late Jurassic volcanic rocks (Phase 2, 152–146 Ma) and an early stage of Early Cretaceous volcanic rocks (Phase 3, 143–139 Ma). Overall, these volcanic rocks are composed of intermediate–acid pyroclastic rocks and lava, with high SiO2, Na2O, K2O and Al2O3 contents, belonging to the high-K calc-alkaline and peraluminous series, with enrichment in large ion lithophile and light rare earth elements but depletion in high-field-strength elements and heavy rare earth elements. However, the Phase 1 and 2 volcanic rocks contain different zircon Hf isotopic compositions and whole-rock geochemistry to the Phase 3 volcanic rocks, implying that they have a different petrogenesis. Our study combined with previous research shows that the decreasing zircon ϵHf(t) values of the Middle–Late Jurassic volcanic rocks indicate a decreasing mantle-derived material contribution. On the contrary, the majority of the Early Cretaceous volcanic rocks have variable ϵHf(t) values, requiring varied degrees of involvement of juvenile components in their origin. Thus, we consider that the Late Jurassic volcanic rocks were generated in a compressional tectonic environment during the early stage (> 146 Ma) of palaeo-Pacific subduction. In contrast, the Cretaceous volcanic rocks were formed in an extensional tectonic setting during a later stage (< 143 Ma) of subduction.

Widespread Permian granite magmatism in Lower Austroalpine units: significance for Permian rifting in the Eastern Alps

The Grobgneis complex, located in the eastern Austroalpine unit of the Eastern Alps, exposes large volumes of pre-Alpine porphyric metagranites, sometimes associated with small gabbroic bodies. To better understand tectonic setting of the metagranites, we carried out detailed geochronological and geochemical investigations on the major part of the porphyric metagranites. LA–ICP–MS zircon U–Pb dating of three metagranites sampled from the Grobgneis complex provides the first reliable evidence for large volumes of Permian plutonism within the pre-Alpine basement of the Lower Austroalpine units. Concordant zircons from three samples yield ages at 272.2 ± 1.2 Ma, 268.6 ± 2.3 Ma and 267.6 ± 2.9 Ma interpreted to date the emplacement of the granite suite. In combination with published ages for other Permian Alpine magmatic bodies, the new U–Pb ages provide evidence of a temporally restricted period of plutonism (“Grobgneis”) in the Raabalpen basement Complex during the Middle Permian. Comparing the investigated basement with that of the West Carpathian basement, we argue that widespread Permian granite magmatism occurred in the Lower Austroalpine units. They belong to the high-K calc-alkaline to shoshonitic S-type series on the base of geochemical data. Zircon Hf isotopic compositions of the Grobgneis metagranites show εHf(t) values of − 4.37 to − 0.6, with TDM2 model ages of 1.31–1.55 Ga, indicating that their protoliths were derived by the recycling of older continental crust. We suggest that the Permian granitic and gabbroic rocks are considered as rifted-related rocks in the Lower Austroalpine units and are contemporaneous with cover sediments.

Detrital Zircon U-Pb Geochronology and Hf Isotope Geochemistry of the Hayang Group, SE Korea and the Himenoura and Goshoura Groups, SW Japan: Signs of Subduction-Related Magmatism after a Long Resting Period

There was a hiatus in magmatism in Korea and Japan, located on the eastern continental margin of Asia, during a period of about 40 Ma from 160 Ma to 120 Ma. The cause of the resumption of magmatism since then is not yet well understood. In this study, we analyzed the Hf isotope composition of detrital zircons in the Cretaceous sediments of Korea (Hayang Group) and Japan (Goshoura and Himenoura groups) to investigate the tectonic evolution of eastern Asia in the Early Cretaceous period. εHf(t) in Cretaceous zircons from Japanese samples values from +8.2 to +0.1, suggesting that magmatism was sourced from the depleted juvenile materials, which is compatible with ridge subduction and subsequent melting of the young oceanic crust. εHf(t) values from Cretaceous zircons in the Hayang Group are negative, except for the Jindong Formation, which had a sediment supply from Japan, indicating that the old continental crust material of the Korean Peninsula was included in the magma generation. The detrital zircons of this study exhibit a depleted isotopic character at the beginning of subduction-related magmatism in Permian and Early Cretaceous, and then gradually change to a more enriched composition. This trend may be a typical example of the Pacific-type orogenic cycle.

Three episodes of Precambrian mafic magmatism in the southern Central Tianshan Block (NW China): Insight into an evolving geodynamic model

Precambrian microcontinents are an essential component in the tectonic evolution and architecture of the Central Asian Orogenic Belt (CAOB). However, the early crustal evolution and origin of Precambrian microcontinents remain poorly constrained due to the scarcity of early Precambrian rocks. The Chinese Central Tianshan Block (CTB) constitutes a microcontinent in the western CAOB with Precambrian basement rocks along its southern margin. The basement rocks, comprising amphibolite-facies supracrustal assemblages of possible volcano-sedimentary origin, are exposed in the Wuwamen Complex, which occurs as tectonic slivers in the late Paleozoic Wuwamen ophiolitic mélange. Here we present in situ zircon U–Pb and Lu–Hf isotopic data and the whole-rock geochemistry of three suites of (meta-)mafic rocks from the Wuwamen Complex and an associated Neoproterozoic gabbro body. The zircon dating revealed that the Wuwamen Complex contains two suites of meta-mafic rocks with protolith formation at ca. 2.55 Ga and 2.21 Ga followed by an amphibolite-facies metamorphic overprint at ca. 1.83 Ga. In addition, an individual gabbro body adjacent to the Wuwamen Complex yielded an emplacement age of ca. 0.69 Ga, reflecting a mid-Neoproterozoic mafic magmatic event in the CTB. Furthermore, zircon U–Pb dating of a crosscutting diorite dyke and a gneissic diorite sourced from a composite batholith in the CTB yielded weighted mean intrusive ages of ca. 320 Ma and 454 Ma, respectively. In conjunction with the field observation, the emplacement ages suggest that the Wuwamen Complex represents a part of the continental arc basement of the CTB. The three suites of (meta-)mafic rocks show geochemical characteristics similar to typical arc-related basalts. Along with the evolved zircon Hf isotopic compositions, it is suggested that these rocks originated from partial melting of a metasomatized sub-continental lithospheric mantle during different events. In conjunction with previous studies, the here presented new data further suggest that the CTB and other microcontinents in the western CAOB record similar Precambrian magmatic and metamorphic episodes, which are comparable to those of the northern Tarim during Proterozoic times. The here outlined new model indicates that the microcontinents of the western CAOB and the Tarim Craton may have shared a common geodynamic evolution.

Late Paleozoic Mantle Source Nature of Tianshan Orogen, Northwest China: Evidence from the Geochemistry, Zircon U‐Pb Dating, Hf and Whole Rock Sr‐Nd‐Pb Isotopes of the Mafic Dykes

Late Paleozoic Mantle Source Nature of Tianshan Orogen, Northwest China: Evidence from the Geochemistry, Zircon U‐Pb Dating, Hf and Whole Rock Sr‐Nd‐Pb Isotopes of the Mafic Dykes

Lu–Hf Isotope Composition of Zircon and Magma Sources of the Vendian–Early Paleozoic Granitoids in Tuva (by the Example of the Kaa-Khem and East Tannu-Ola Batholiths)

Abstract —We present results of geochemical and Sr–Nd isotope studies of rocks and of local dating and determination of the Lu–Hf isotope composition of zircons from late Vendian–early Cambrian and Cambrian–Ordovician intrusive associations (granitoids and gabbroids) of the Kaa-Khem and East Tannu-Ola batholiths in Eastern Tuva. The wide ranges of the eNd values (6.9 to 0.5) of rocks and the εHf values of magmatic and inherited zircons reflect the diversity of the magma sources of late Vendian–early Paleozoic intrusive associations formed at the island arc and accretion–collision stages. Late Vendian (572–562 Ma, Kopto and Buren massifs) and early Cambrian (522–518 Ma, East Tannu-Ola batholith) island arc tholeiitic and calc-alkalic plagiogranitoids resulted from the melting of the Vendian–early Cambrian island arc crust without the contribution of a more ancient crustal material. The subalkalic gabbro–monzodiorite–granosyenite association of the Zubovka massif (510 Ma) formed from a mantle source depleted isotopically but enriched in incompatible elements, with the participation of an island arc crust material; this process took place in the early phase of plume activity at the accretion–collision stage. Island arc complexes were the main source of Cambrian–Ordovician accretion–collision calc-alkalic plagiogranitoids (500–450 Ma, Terektyg-Cheder, Karaos, Tapsa, Baisyut, and other massifs). Variations in their composition were due to the melting of thick crust, whose isotopic heterogeneity was caused by the different contributions of a more ancient crustal source. The crust of the Tuva–Mongolian terrane made the main contribution to the formation of the potassic granitoids of the Bren’ massif (450 Ma), marking the completion of accretion–collision processes in this region. The isotope parameters of the Vendian–early Paleozoic granitoids are indicators of the crust formation and evolution in the course of subduction and accretion–collision processes.

Middle Paleoproterozoic tectonic evolution of the Trans-North China Orogen, North China Craton: Constraint from the intermediate-acid magmatism in the Lüliang area

The excellent middle-Paleoproterozoic tectono-thermal activities from the Lüliang Complex can provide pivotal constraints on the tectonic evolution of the Trans-North China Orogen between Eastern and Western blocks of North China Craton. LA-ICP-MS zircon dating gives emplacement ages of 2189 ± 16 Ma for gneissic diorite, 2188 ± 11 Ma for gneissic quartz monzonite, 2153 ± 9 Ma for gneissic granite in Chijianling area, respectively. The gneissic diorites have high Al2O3 (15.89–16.69 wt%), MgO (3.35–4.96 wt%), low Sr/Y (11.3–16.6), (La/Yb)N (7.40–8.44), enrichment of LILE and depletion in HFSE, especially Nb, Ta, indicating partial melting of mantle wedge fluxed by the fluids. The gneissic quartz monzonites and gneissic granites belong to high K calc-alkaline and shoshonitic series and show right inclined REE patterns, consisting with I-type granite. Zircon Hf and whole-rock Nd isotopic compositions (εHf(t) = +0.1 to +5.3 and − 7.8 to −1.7; εNd(t) = −0.4 to +0.4 and − 1.8 to −0.6) of the gneissic quartz monzonites and gneissic granites are identical to those of dominant Neoarchean basement materials with addition of Mesoarchean vestiges to a certain extent. The diversity of source material (including continental and volcanic arc materials) resembles to magmatism in normal continental arc margins. The spatial distribution, rock assemblage and decreasing rock formation pressure further hint the tectonic transformation from compression to extension regime in local areas. Taking into account the prevalence magmatic records and contemporaneous deposition of middle-Paleoproterozoic in the Trans-North China Orogen, it suggests an active continental arc setting at ~2.2 Ga and local tectonic extension regime with back-arc basin at ~2.1 Ga for a consequence of the Paleoproterozoic subduction between the Eastern block and Western block of the NCC.

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The excellent middle-Paleoproterozoic tectono-thermal activities from the Lüliang Complex can provide pivotal constraints on the tectonic evolution of the Trans-North China Orogen between Eastern and Western blocks of North China Craton. LA-ICP-MS zircon dating gives emplacement ages of 2189 ± 16 Ma for gneissic diorite, 2188 ± 11 Ma for gneissic quartz monzonite, 2153 ± 9 Ma for gneissic granite in Chijianling area, respectively. The gneissic diorites have high Al2O3 (15.89–16.69 wt%), MgO (3.35–4.96 wt%), low Sr/Y (11.3–16.6), (La/Yb)N (7.40–8.44), enrichment of LILE and depletion in HFSE, especially Nb, Ta, indicating partial melting of mantle wedge fluxed by the fluids. The gneissic quartz monzonites and gneissic granites belong to high K calc-alkaline and shoshonitic series and show right inclined REE patterns, consisting with I-type granite. Zircon Hf and whole-rock Nd isotopic compositions (εHf(t) = +0.1 to +5.3 and − 7.8 to −1.7; εNd(t) = −0.4 to +0.4 and − 1.8 to −0.6) of the gneissic quartz monzonites and gneissic granites are identical to those of dominant Neoarchean basement materials with addition of Mesoarchean vestiges to a certain extent. The diversity of source material (including continental and volcanic arc materials) resembles to magmatism in normal continental arc margins. The spatial distribution, rock assemblage and decreasing rock formation pressure further hint the tectonic transformation from compression to extension regime in local areas. Taking into account the prevalence magmatic records and contemporaneous deposition of middle-Paleoproterozoic in the Trans-North China Orogen, it suggests an active continental arc setting at ~2.2 Ga and local tectonic extension regime with back-arc basin at ~2.1 Ga for a consequence of the Paleoproterozoic subduction between the Eastern block and Western block of the NCC.

Early Paleozoic intracontinental granites in the Guangzhou region of South China: Partial melting of a metasediment-dominated crustal source

Whether the formation of early Paleozoic granites in the South China Block was related to intracontinental orogenic processes or oceanic subduction continues to be debated. Here, we present whole-rock major element, trace element, and SrNd isotopic data, as well as secondary ion mass spectrometry and laser ablation–inductively coupled plasma–mass spectrometry zircon UPb geochronological and HfO isotopic data, from biotite, two-mica, and muscovite granites in the Baiyunshan and Maofengshan areas of Guangzhou City, South China. Zircon UPb ages reveal that these rocks were formed during the early Paleozoic (446–438 Ma). These muscovite- and biotite-bearing granites have high SiO2 (69.0–80.4 wt%) and K2O (3.19–5.32 wt%) contents, and low MgO (0.46–1.17 wt%) contents. The granites are enriched in Rb, Th, U, and Pb, and depleted in Ba, Sr, Nb, and Ti. The Baiyunshan muscovite granites and Maofengshan biotite granites are slightly to moderately enriched in light rare-earth elements [LREEs; (La/Yb)CN = 2.27–15.1] with slight to pronounced negative Eu anomalies (Eu/Eu* = 0.27–0.72), whereas the Baiyunshan two-mica granites are significantly enriched in LREEs [(La/Yb)CN = 19.6–32.1] with moderate negative Eu anomalies (Eu/Eu* = 0.42–0.58). These muscovite- and biotite-bearing granites have enriched whole-rock Nd and zircon Hf isotopic compositions (εNd(t) = −12.1 to −7.6; εHf(t) = −11.4 to −0.8), and high zircon δ18O values (8.5‰–10.0‰). We suggest that the Baiyunshan granites were formed by partial melting of metasedimentary rocks, as they contain primary muscovite and have high zircon δ18O values. The Maofengshan granites were formed by partial melting of a hybridized crustal source containing metasedimentary rocks with subordinate amphibolites, as they have high zircon δ18O values, variable whole-rock SiO2 contents, and biotite compositions similar to those crystallized in I- and S-type magmas. These granites, along with the widespread early Paleozoic S-type granites in the Wuyi–Yunkai orogen, are distributed across a broad area rather than in a narrow belt. In addition, early Paleozoic S-type granites in the South China Block generally do not show younging trends from east to west or south to north. Moreover, the Baiyunshan–Maofengshan granites show geochemical characteristics similar to syn- or post-collisional granites and different from volcanic arc granites. We therefore propose that these muscovite- and biotite-bearing granites were formed in an intracontinental orogenic setting.

Oxygen-Hafnium-Neodymium Isotope Constraints on the Origin of the Talnakh Ultramafic-Mafic Intrusion (Norilsk Province, Russia)

Abstract The ultramafic-mafic Talnakh intrusion in the Norilsk province (Russia) hosts one of the world’s major platinum group element (PGE)-Cu-Ni sulfide deposits. This study employed a multitechnique approach, including in situ Hf-O isotope analyses of zircon combined with whole-rock Nd isotope data, in order to gain new insights into genesis of the Talnakh economic intrusion. Zircons from gabbrodiorite, gabbroic rocks of the layered series, and ultramafic rocks have similar mantle-like mean δ18O values (5.39 ± 0.49‰, n = 27; 5.64 ± 0.48‰, n = 34; and 5.28 ± 0.34‰, n = 7, respectively), consistent with a mantle-derived origin for the primary magma(s) parental to the Talnakh intrusion. In contrast, a sulfide-bearing taxitic-textured troctolite from the basal part of intrusion has high δ18O (mean of 6.50‰, n = 3), indicating the possible involvement of a crustal component during the formation of sulfide-bearing taxitic-textured rocks. The Hf isotope compositions of zircon from different rocks of the Talnakh intrusion show significant variations, with ɛHf(t) values ranging from –3.2 to 9.8 for gabbrodiorite, from –4.3 to 11.6 for unmineralized layered-sequence gabbroic rocks, from 2.3 to 12 for mineralized ultramafic rocks, and from –3.5 to 8.8 for mineralized taxitic-textured rocks at the base of the intrusion. The significant range in the initial 176Hf/177Hf values is ascribed to interaction of distinct magma sources during formation of the Talnakh intrusion. These include (1) a juvenile source equivalent to the depleted mantle, (2) a subcontinental lithospheric source, and (3) a minor crustal component. Initial whole-rock Nd isotope compositions of the mineralized taxitic-textured rocks from the base of the intrusion (mean ɛNd(t) = –1.5 ± 1.8) differ from the other rocks, which have relatively restricted ranges in initial ɛNd (mean ɛNd = 0.9 ± 0.2). The major set of ɛNd values around 1.0 at Talnakh is attributed to limited crustal contamination, presumably in deep magma chambers, whereas the smaller set of negative ɛNd values in taxitic-textured rocks is consistent with greater involvement of a crustal component and reflects an interaction with the wall rocks during emplacement.

Late Precambrian Metamorphic Complexes of the Ulutau Massif (Central Kazakhstan): Age, Composition, and Formation Settings of Protoliths

This work presents the results of a study of metamorphic rocks of the Bekturgan and Balazhezdy groups of the Precambrian Ulutau massif (Central Kazakhstan), which have been traditionally identified with the most ancient Early Precambrian complexes, comprising the massif’s basement. The protoliths of the metamorphic rocks, represented by lavas and tuffs of basalt and rhyolite–trachyrhyolite composition, sandstones, and tuffaceous sandstones, comprise a contrasted volcanic-sedimentary sequence. The obtained U‒Th–Pb zircon ages (LA–ICP MS, SHRIMP II) demonstrate that the formation of this sequence occurred during a narrow (762–788 Ma) interval during the second half of Tonian (Neoproterozoic). The metamorphic transformations of the rocks were completed before an intrusion of the alkaline syenites of the Karsakpai complex (673 ± 2 Ma), which were not subject to metamorphism. The whole-rock Nd isotopic compositions of the metamorphic rocks indicate the origin of their protoliths due to erosion and partial melting of the more ancient complexes of the Precambrian continental crust of the Ulutau massif. The Hf isotopic compositions of zircons indicate that the main stages of formation and transformation of the Ulutau continental crust occurred during the Neoarchean–Neoproterozoic and coincided with the main stages of the Precambrian magmatism in the western part of the Central Asian Orogenic Belt.

Early Paleozoic magmatism in northern Kontum Massif, Central Vietnam: Insights into tectonic evolution of the eastern Indochina Block

The Chu Lai and Dai Loc granitic complexes are distributed on two sides of the early Paleozoic Tam Ky – Phuoc Son suture zone (TPSZ) in the northern Kontum Massif (KTM), central Vietnam. Studies of their petrogeneses and tectonic setting provide new insights into the tectonic evolution of the eastern Indochina Block and its affinity with the South China Block (SCB). Zircon U-Pb dating indicates that the Chu Lai and Dai Loc granites formed at 445–452 Ma and 415–420 Ma, respectively. The Chu Lai granites have high ASI, K2O/Na2O and Rb/Sr as well as strongly negative zircon εHf(t) (−14.6 to −2.9) and complex inherited zircon components, characteristics of typical S–type granites. The Chu Lai granites may be subdivided into two groups based on their geochemistry. However, both groups have similar zircon Hf isotopes and inherited–zircon age spectra, suggesting that they probably were derived from different degrees of partial melting of the same late Neoproterozoic meta–greywacke source at different crustal levels. The Dai Loc granites have variable major– and trace–element contents, ASI, K2O/Na2O, Rb/Sr, and zircon Hf–isotope compositions, indicating that most of them are S–type granites and some are I–type granites, suggesting derivation from heterogeneous sources. The petrology, geochemistry, geochronology and formation temperatures suggest that the Chu Lai granites formed in a syn–collisional setting between the KTM and Truong Son Belt (TSB) and the Dai Loc granites in a post–collision extensional setting triggered by the upwelling asthenosphere mantle. Similarities of age spectra and Hf isotope compositions of inherited zircons from the Chu Lai granites with detrital zircons from the Neoproterozoic sedimentary rocks in Western Cathaysia Block and Tethyan Himalaya suggest they are adjacent each other during the late Neoproterozoic. The ages and zircon Hf–isotope compositions of early Paleozoic granites between the KTM and TSB are consistent with those in the SCB, but different from many other microcontinents from the northern margin of East Gondwana, suggesting that the SCB was close to the KTM and TSB and experienced similar early Paleozoic tectonothermal events. It can be inferred that there was a concealed early Paleozoic subduction zone south of the SCB, connecting with the TPSZ in central Vietnam, and that the far–field effects of the subduction of an unknown oceanic plate triggered the early Paleozoic intracontinental orogeny in the SCB.

Late Cambrian to Early Silurian Granitic Rocks of the Gemuri Area, Central Qiangtang, North Tibet: New Constraints on the Tectonic Evolution of the Northern Margin of Gondwana

AbstractThe Paleozoic tectonic framework and paleo–plate configuration of the northern margin of Gondwana remain controversial. The South Qiangtang terrane is located along the northern margin of Gondwana and records key processes in the formation and evolution of this supercontinent. Here, we present new field, petrological, zircon U‐Pb geochronological, and Lu‐Hf isotopic data for granitic rocks of the Gemuri pluton, all of which provide new insights into the evolution of the northern margin of Gondwana. Zircon U‐Pb dating of the Gemuri pluton yielded three concordant ages of 488.5 ± 2.1, 479.9 ± 8.9, and 438.5 ± 3.5 Ma. Combining these ages with the results of previous research indicates that the South Qiangtang terrane records two magmatic episodes at 502–471 and 453–439 Ma. These two episodes are associated with enriched zircon Hf isotopic compositions (∊Hf(t) = –10.1 to –3.9 and –16.6 to –6.5, respectively), suggesting the granites were formed by the partial melting of Paleoproterozoic–Mesoproterozoic metasedimentary rocks (Two–stage Hf model ages (TDMC) = 2094–1704 and 2466–1827 Ma, respectively). Combining these data with the presence of linearly distributed, contemporaneous Paleozoic igneous rocks along the northern margin of Gondwana, we suggest that all of these rocks were formed in an active continental margin setting. This manifests that the two magmatic episodes within the Gemuri area were associated with southward subduction in the Proto‐ (Paleo‐) Tethys Ocean.

KV01 zircon—A potential New Archean reference material for microbeam U-Pb age and Hf-O isotope determinations

To meet the rapid development of zircon U-Pb geochronology and Hf-O isotope geochemistry, it is imperative to develop well-characterized zircon reference materials for microbeam analysis. Here, zircon KV01 separated from the Kaap Valley pluton, South Africa is introduced as a potential Archean reference material for microbeam U-Pb age and Hf-O isotope determination. SIMS and LA-ICP-MS U-Pb isotopic measurements gave grand mean 207Pb/206Pb ages of 3233±16 Ma (2s, n=90) and 3230±26 Ma (2s, n=52), which are consistent with reported CA-ID-TIMS results. Slight heterogeneity of oxygen isotopes is revealed by SIMS analyses that yielded a relatively large δ18O range of 4.61–6.51‰. Among them, the zircon grains with clear oscillatory zones give a grand mean δ18O value of 6.17±0.33‰ (2s), which is suggested to record the primary O isotopic compositions. The homogeneity in the Hf isotopic compositions of KV01 zircon was checked by the laser ablation and solution MC-ICP-MS methods. They yielded consistent results with a recommended 176Hf/177Hf ratio of 0.280810±0.000013 (2s). The results reported here demonstrate that KV01 zircon could be a reliable reference material for microbeam 207Pb/206Pb age and Hf isotope determinations of ancient zircons, and be suitable as a quality monitor for SIMS oxygen isotopic analysis.

Exotic origin of Pingtan Island in the Pingtan-Dongshan Metamorphic Belt (SE China): Zircon U-Pb age and Hf isotope evidences

Cathaysia Block in South China had accreted tremendous amounts of Mesozoic magmatic rocks. Recent zircon UPb age results revealed existence of some Early Jurassic gneissic granites (200–190 Ma) in the Pingtan-Dongshan Metamorphic Belt (PDMB), coastal SE China which corresponds one stage of magmatic quiescence in the rest part of East Cathaysia. To better understand Mesozoic thermal evolution of the PDMB, UPb geochronology and Hf isotope composition of zircon separates (LA-ICPMS) and inclusions (NanoSIMS, age only) are examined for two granitic rocks and one meta-sandstone in Pingtan area (northern PDMB) and three granites in Matsu Islands further north. For Pingtan rocks, ages of discrete zircons give two Early Jurassic ages (196 Ma and 173 Ma) and prossess in dominance positive ɛHf(t) values (−0.2 to 7.5). On the other hand, zircon inclusions within rock-forming minerals yield a wide age span from Permo-Triassic to Early Cretaceous in six clusters, namely 270–206 Ma, 191–170 Ma, 168–151 Ma, 146–132 Ma, 126–119 Ma and 109–100 Ma that basically coincide the major Mesozoic thermal episodes in Cathaysia except the second cluster. For Matsu granites, discrete zircons exhibit Late Jurassic (162–158 Ma) and Early Cretaceous (137 Ma) ages with ɛHf(t) values of −9.1 to −1.6 and −4.9 to −0.8, respectively. Nevertheless, zircon inclusions show ages corresponding to Pingtan samples in the last four clusters. With reference to Mesozoic magmatic rock units in the neighboring Southeast China Magmatic Belt (SCMB), our data suggest irrelevance of Pingtan samples to the Cathaysia and linkage of off-shore Matsu granites to the PDMB. Noting that there are similarities of zircon ages and ɛHf(t) values between the granite-schist associations in Tananao Metamorphic Belt (TMB) of eastern Taiwan and Pingtan island of northern PDMB, these Early Jurassic granites were probably originated from a same source. Three consistent Early Cretaceous age clusters of zircon inclusions over the studied samples match two well-known magma prevailing episodes in the coastal SE China, with one tectonic episode (128–119 Ma) in the PDMB that is often invisible from the zircon separates remains unexplainable. In response to a plausible tectonic evolution, we support the earlier proposition for a now-concealed microcontinent that had impinged on Cathaysia around 125 Ma, but further suggest that it left Pingtan Island behind as a remnant fragment when drifted away at later time.

Detrital zircon geochronology and Hf isotope geochemistry of Mesozoic sedimentary basins in south-central Alaska: Insights into regional sediment transport, basin development, and tectonics along the NW Cordilleran margin

AbstractThe Jurassic–Cretaceous Nutzotin, Wrangell Mountains, and Wellesly basins provide an archive of subduction and collisional processes along the southern Alaska convergent margin. This study presents U-Pb ages from each of the three basins, and Hf isotope compositions of detrital zircons from the Nutzotin and Wellesly basins. U-Pb detrital zircon ages from the Upper Jurassic–Lower Cretaceous Nutzotin Mountains sequence in the Nutzotin basin have unimodal populations between 155 and 133 Ma and primarily juvenile Hf isotope compositions. Detrital zircon ages from the Wrangell Mountains basin document unimodal peak ages between 159 and 152 Ma in Upper Jurassic–Lower Cretaceous strata and multimodal peak ages between 196 and 76 Ma for Upper Cretaceous strata. Detrital zircon ages from the Wellesly basin display multimodal peak ages between 216 and 124 Ma and juvenile to evolved Hf compositions. Detrital zircon data from the Wellesly basin are inconsistent with a previous interpretation that suggested the Wellesly and Nutzotin basins are proximal-to-distal equivalents. Our results suggest that Wellesly basin strata are more akin to the Kahiltna basin, which requires that these basins may have been offset ∼380 km along the Denali fault. Our findings from the Wrangell Mountains and Nutzotin basins are consistent with previous stratigraphic interpretations that suggest the two basins formed as a connected retroarc basin system. Integration of our data with previously published data documents a strong provenance and temporal link between depocenters along the southern Alaska convergent margin. Results of our study also have implications for the ongoing discussion concerning the polarity of subduction along the Mesozoic margin of western North America.

Crustal Evolution in the New England Orogen, Australia: Repeated Igneous Activity and Scale of Magmatism Govern the Composition and Isotopic Character of the Continental Crust

AbstractThe generation of continental crust, its bulk composition and temporal evolution provide important records of plate tectonics and associated magma-generating processes. However, the long-term integrated effects of repeated magmatic events on crustal growth, composition and differentiation and, therefore, on crustal evolution are rarely considered. Here, we examine long-term (∼350 Myr) temporal compositional trends of granitic magmatism within a limited (∼200 km × 100 km) area in the Northern New England Orogen of Queensland, Australia to avoid lateral crustal variations in order to understand how temporal–compositional variations of silicic igneous rocks record crustal evolution. Long-term temporal compositional variations are tracked using whole-rock chemistry, zircon chronochemistry and zircon Hf isotopic compositions. We particularly focus on whole-rock U, Th and K abundances and calculated heat-production values as proxies for crustal evolution, and tracking crustal sources involved in granitic magmatism. We identified two major compositional groupings within the study area that were repeatedly produced over time: compositional Group 1 comprises voluminous I-type igneous rocks emplaced during the Permo-Carboniferous and Early Cretaceous; Group 2 represents mainly lower volume A-type igneous rocks of Triassic, Middle Cretaceous and Tertiary age. Importantly, these compositional groupings alternate over the 350 Myr history of granitic magmatism within the study area. Heat-production values over time exhibit a zigzag pattern and mirror zircon Hf isotopic signatures where rocks with elevated heat-production values exhibit unradiogenic (crustal) Hf isotopic compositions. We identify the composition of crustal sources, level of the crust undergoing partial melting, scale of magmatism and source crustal volume as important factors in understanding the compositional diversity of silicic igneous rocks. We interpret the two chemical groupings to reflect the following magma-generating conditions: Group 1 igneous rocks record large-scale magmatic systems triggered by extensive crustal melting of multiple lower to middle crustal sources, which produce more compositionally and isotopically uniform magma compositions that approach bulk crustal compositions. In contrast, Group 2 igneous rocks reflect smaller-scale magmatic systems generated from smaller-scale partial melting events of the middle to upper crust that produced A-type magmas. Over the long term, the successive large-scale magmatic events (recorded by Group 1 igneous rocks) through their concomitant basaltic underplating make the Hf composition of the lower crust more radiogenic, and tend to homogenize the isotopic composition of the continental crust. We consider three important coupled controls: (1) promotion of extensive crustal melting by large-scale magmatic systems, potentially blending multiple crustal sources that can also include a significant juvenile source contribution; (2) melt depletion, whereby older, and potentially more unradiogenic crustal materials become more refractory; (3) ‘crustal jacking’, where mantle-derived magmas are added as underplate to the crust (i.e. basification) and can shift older crustal materials to more shallow levels (potentially in concert with erosion and exhumation) and away from zones of crustal melting. Our findings highlight the importance of integrating the geological and intrusive history with whole-rock geochemical data and isotopic information, and have direct implications for continental regions that exhibit protracted igneous histories and where isotopic compositions may trend towards more juvenile compositions such as circum-Pacific or retreating accretionary orogens.

The origin and evolution of the Mexican Cordillera as registered in modern detrital zircons

Continents are thought to be built at convergent margins by mantle-derived mafic inputs, but the processes that transform them into more felsic and long-lasting constituents are still unclear. Here we use U-Pb geochronology, trace elements and Hf isotopic compositions of modern detrital zircons from western Mexico to illustrate how a typical mafic island-arc built on an oceanic basin can be rapidly transformed into a mature intermediate to felsic continent by the combined effects of subduction erosion and rear-arc sediment underthrusting, prompted by the closure of a back-arc basin. In western Mexico, the most significant mass transfer from mantle to crust occurred during the Early Cretaceous in the context of an intra-oceanic island arc, locally dubbed Alisitos-Guerrero. Subsequently, the process of “continentalization” was achieved by the deep reprocessing of the newly formed lithologies once they were dynamically incorporated into the melting source during the development of the Mexican Orogen. The Mexican continental crust did not only grow by arc-related inputs, however, as there was at least one magmatic episode in the Jurassic where extensional-driven mantle upwellings triggered melting of upper-crustal sediments. In this view, mafic island arcs and back-arc basins are ephemeral geologic features that can be easily consumed into trenches, but the geochemical evidence of their existence, and possibly even a significant portion of their mass, can be preserved in younger and more felsic continental igneous rocks.

Formation of early Archean Granite-Greenstone Terranes from a globally chondritic mantle: Insights from igneous rocks of the Pilbara Craton, Western Australia

The continental crust grows via juvenile additions from the mantle. However, the timing of initial continent stabilisation and the rate of subsequent continental growth during the first billion years of Earth history is widely debated, in part due to uncertainty over the composition of the mantle source of new crust. Well-preserved Archean granite-greenstone terranes, as present within the Pilbara Craton (Western Australia), provide insights into the sources of felsic magmas and the processes of continental growth and evolution in the distant geological past at the regional scale. Here, we present zircon U-Pb, O and Hf isotope data from ancient gneissic and granitic rocks of the Pilbara Craton, to decipher magma sources and the timing and processes of craton growth. There is no evidence for depleted mantle compositions, and the simplest interpretation is that the crust of the Pilbara Craton was generated from mantle with a chondritic Hf isotope composition. Our results indicate crustal addition at ~3.59 Ga, represented by emplacement of gabbroic to anorthositic rocks. We suggest that the formation of these igneous rocks, and the foundering of the complementary residues, triggered extensive melting of hot, upwelling mantle, leading to the subsequent accumulation of the >12 km thick greenstone belt eruptive sequences from 3.53 Ga, with emplacement of coeval felsic magmas at depth. This process shaped the initial crustal configuration of the proto-craton, which subsequently underwent gravitationally driven overturn and reworking to generate stable, cratonic continental crust with the distinctive dome and keel architecture. The zircon Hf and O isotope signatures of the Pilbara igneous rocks from ~3.59–3.4 Ga do not support remelting of an ancient (> 3.8 Ga) basement, and reinforce the overwhelmingly chondritic to near-chondritic zircon Hf isotope composition of Eoarchean meta-igneous rocks from a number of different Archean cratons. A corollary of this remarkable global consistency is that a significant volume of the mantle maintained a chondritic composition for the Lu-Hf system from the formation of the Earth into the Paleoarchean (up to 3.6–3.5 Ga), as would be the case if stabilised volumes of felsic continental crust prior to 3.5 Ga were relatively small. One implication is that the common assumption of a linear evolution of depleted mantle from 4.5 Ga to the present day is inappropriate for determining the timing and volume of continental crust extraction in the Archean. The nearly identical early evolution of the Pilbara and Kaapvaal cratons suggests a common process to generate Archean granite-greenstone terranes that does not require extensive reworking of ancient crust, but rather involves juvenile crustal addition above persistent zones of upwelling, chondritic mantle.

Crustal anatexis recorded by zircon grains from early Paleozoic granitic rocks in Southeast China

Episodic granitic magmatism occurred in Southeast China from the Paleoproterozoic to the Late Cretaceous, and some of the early Paleozoic granitic rocks in the region are inferred to have formed through crustal anatexis. However, the complex processes associated with upper amphibolite-facies metamorphism and anatexis remain unclear. We investigate the early Paleozoic anatexis by analyzing zircon grains from granitic rocks in the northern Wuyi Mountains of southern Jiangxi Province. By combining mineral assemblage, field relationship, and thermodynamic modeling data with observations of zircon internal structures, we infer that anatexis occurred via partial melting of Neoproterozoic metasedimentary rocks, immediately following amphibolite-facies regional metamorphism at ca. 442 Ma (recorded by rare metamorphic zircon overgrowths around inherited cores). We identify three types of zircon grains/domains in the analyzed samples, which are related to anatectic melt migration, extraction, and eventual crystallization. Large variations in zircon Hf isotopic compositions and homogenization trends indicate that melting was heterogeneous, and homogenization of the anatectic melt occurred later. Petrogenesis of the granitic rocks, including metamorphism, disequilibrium melting, and melt accumulation, occurred over a short period from ca. 447 ± 8 Ma to 437 ± 3 Ma. We demonstrate a strong relationship between Neoproterozoic metasediments and early Paleozoic granitic rocks in Southeast China and show that homogeneous granitic plutons can form through the ‘rehomogenization’ of continental crust.