Protolith natures and U-Pb sensitive high mass-resolution ion microprobe (SHRIMP) zircon ages of the metabasites in Hainan Island, South China: Implications for geodynamic evolution since the late Precambrian

Abstract Metabasites within the Paleozoic volcanic‐clastic sedimentary sequences in Hainan Island, South China, show large differences not only in the nature of protoliths, but also in zircon U‐Pb sensitive high mass‐resolution ion microprobe (SHRIMP) ages. The protoliths for the Tunchang area metabasites have intraoceanic arc geochemical affinities. In the east‐central island gabbroic to diabasic rocks and pillow lavas are also present, while the Bangxi area metabasites with back‐arc geochemical affinities in the northwest island consist of basaltic, gabbroic and/or picritic rocks. Three types of zircon domains/crystals in the Tunchang area metabasites are defined. Type 1 is comagmatic and yields concordant to approximately concordant 206 Pb/238 U ages ranging from 442.1 ± 13.7 Ma to 514.3 ± 30.2 Ma with a weighted U‐Pb mean age of 445 ± 10 Ma. Type 2 is inherited and yields a weighted 207 Pb/206 Pb mean age of 2488.1 ± 8.3 Ma. Type 3 is magmatic with a 207 Pb/206 Pb age of ca 1450 Ma. Magmatic zircons in the Bangxi area metabasites yield a weighted U‐Pb mean age of 269 ± 4 Ma. We suggest 450 Ma is the minimum age for crystallization of protoliths of the Tunchang area metabasites, because the age range of ca 440–514 Ma probably corresponds to both the time of igneous crystallization and the high‐temperature overprint. The presence of abundant inherited zircons strongly favors derivation of these rocks from a NMORB‐like mantle proximal to continental crust. A protolith age of ca 270 Ma for the Bangxi area metabasites probably records expansion of an epircontinental back‐arc basin and subsequent generation of a small oceanic basin. The presence of ophiolitic rocks with an age of ca 450 Ma, not only in Hainan Island, but also in the Yangtze block, highlights the fact that the South China Caledonian Orogeny was not intracontinental in nature, but characterized by an ocean‐related event.     

SHRIMP U–Pb zircon dating of the Kinshozan Quartz Diorite from the Kanto Mountains,Japan: Implications for late Paleozoic granitic activity in Japanese Islands

The new result of SHRIMP U–Pb zircon dating of the Kinshozan Quartz Diorite from the Kanto Mountains, Japan, provides 281.5 ± 1.8 Ma. The age is 30 m.y. older than the available age of the Kinshozan Quartz Diorite obtained by hornblende K–Ar method. The new U–Pb zircon age represents the time of crystallization of the Kinshozan Quartz Diorite. The hornblende K–Ar age indicates the time that the Kinshozan Quartz Diorite cooled down to 500 °C which is the closure temperature of the systematics. Permian granites are found in small exposures in Japan, and frequently referred to as 250 Ma granites. The Kinshozan Quartz Diorite is considered as a type of the 250 Ma granites, and the age was influential in establishing a model of Paleozoic tectonic evolution for the Japanese Islands. The new age of the Kinshozan Quartz Diorite provides the opportunity to re‐examine the model. The Kinshozan Quartz Diorite and other Permian granites in the south of the Median Tectonic Line of Japan were constituents of the Paleo‐Ryoke Belt. The geochemical characteristics of the granitic rocks in the Paleo‐Ryoke Belt indicated that the granitic rocks were formed in a primitive island arc environment, and the new trace element data also support this interpretation. Examination of the available data and results of the present study suggests the late Paleozoic granitic activity in Japan as follows. At about 310–290 Ma, arc magmatism generated adakitic granites and other granites in the South Kitakami Belt. Quartz diorite and tonalites of primitive characteristic, such as the Kinshozan Quartz Diorite and granites in the Maizuru Belt appear to have been formed at the immature island arc, and accreted to the Japanese Islands at the end of Paleozoic or early Mesozoic era. During 260–240 Ma, granitic activity took place in the Hida and Maizuru Belts as a part of the Asian continent.     

In situ LA-ICP-MS zircon U-Pb age of ultrahigh-pressure eclogites in the Yukahe area,northern Qaidam Basin

Zircon grains were selected from two types of ultrahigh-pressure (UHP) eclogites, coarse-grained phengite eclogite and fine-grained massive eclogite, in the Yukahe area, the western part of the North Qaidam UHP metamorphic belt. Most zircon grains show typical metamorphic origin with residual cores in some irregular grains and sector, planar or misty internal textures on the cathodoluminescence (CL) images. The contents of REE and HREE of the core parts of grains range from 173 to 1680 μg/g and 170 to 1634 μg/g, respectively, in phengite eclogite, and from 37 to 2640 μg/g and 25.7 to 1824 μg/g, respectively, in massive eclogite. The core parts exhibit HREE-enriched patterns, representing the residual zircons of protolith of the Yukahe eclogite. The contents of REE and HREE of the rim parts and the grains free of residual cores are much lower than those for the core parts. They vary from 13.1 to 89.5 μg/g and 12.5 to 85.7 μg/g, respectively, in phengite eclogite, and from 9.92 to 45.8 μg/g and 9.18 to 43.8 μg/g, respectively, in massive eclogite. Negative Eu anomalies and Th/U ratios decrease from core to rim. Positive Eu anomalies are shown in some grains. These indicate that the presence of garnet and the absence of plagioclase in the peak metamorphic mineral assemblage, and the zircons formed under eclogite facies conditions. LA-ICP-MS zircon U-Pb age data indicate that phengite eclogite and massive eclogite have similar metamorphic age of 436±3Ma and 431±4Ma in the early Paleozoic and magmatic protolith age of 783–793 Ma and 748–759 Ma in the Neo-proterozoic. The weighted mean age of the metamorphic ages (434±2 Ma) may represent the UHP metamorphic age of the Yukahe eclogites. The metamorphic age is well consistent with their direct country rocks of gneisses (431±3 Ma and 432±19 Ma) and coesite-bearing pelitic schist in the Yematan UHP eclogite section (423–440 Ma). These age data together with field observation and lithology, allow us to conclude that the Yukahe eclogites were Neo-proterozoic igneous rocks and may have experienced subduction and UHP metamorphism with continental crust at deep mantle during the early Paleozoic, therefore the metamorphic age of 434±2 Ma of the Yukahe eclogites probably represents the continental deep subduction time in this area.

     

Early Cretaceous I‐type granites in the southwest Fujian Province: new constraints on the late Mesozoic tectonic evolution of southeast China

Zircons from two samples of the Sukeng pluton in the southwest Fujian Province, China, were analyzed by LA–ICP–MS with the aim of determining the timing of formation. The zircons from the two samples yield similar U–Pb ages of 100.47 ± 0.42 and 102.46 ± 0.69 Ma, indicating that the Sufeng pluton is contemporaneous with the Sifang and Luoboling plutons, all of which are also related to Cu–Au–Pb–Zn–Mo mineralization within the study area. All three plutons have geochemical features of I‐type granites, are high‐ to mid‐K calc‐alkaline metaluminous rocks, and have average molar Al₂O₃/ (CaO+Na₂O+K₂O) values of 0.95, initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70465–0.70841, εNd(t) values at 101 Ma from –1.72 to –7.26, and two‐stage Nd model ages (T_2DM) from 1.16 to 1.60 Ga. Zircons within these plutons have εHf(t) values at 101 Ma from –3.5 to 6.25 and T_2DM ages from 0.74 to 1.46 Ga, suggesting these I‐type granites formed from magmas generated by partial melting of Mesoproterozoic to Neoproterozoic continental crust that mixed with mantle‐derived magmas. The magmatism was associated with thickening of the lower crust caused by collisions between microcontinents in the Cathaysian Block, which were driven by Early Cretaceous subduction of the Pacific Plate.     

The petrogenesis and implications of deformed late Caledonian–early Hercynian granites in the Wuyi area,South China

Precambrian basement rocks have been affected by Caledonian thermal metamorphism. Caledonian‐aged zircon grains from Precambrian basement rocks may have resulted from thermal metamorphism. However, Hercynian ages are rarely recorded. Zircon U–Pb Sensitive High Resolution Ion Microprobe (SHRIMP) dating reveals that zircon ages from the Huyan, Lingdou, and Pengkou granitic plutons can be divided into two groups: one group with ages of 398.9 ±5.3 Ma, 399 ±5 Ma, and 410.2 ±5.4 Ma; and a second group with ages of 354 ±11 Ma, 364.6 ±6.7 Ma, and 368 ±14 Ma. The group of zircon U–Pb ages dated at 410–400 Ma represent Caledonian magmatism, whereas the 368–354 Ma ages represent the age of deformation, which produced gneissosity. The three plutons share geochemical characteristics with S‐type granites and belong to the high‐K calc‐alkaline series of peraluminous rocks. They have (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.710 45–0.724 68 and εNd(t) values of ?7.33 to ?10.74, with two‐stage Nd model ages (T_DM2) ranging from 1.84 Ga to 2.10 Ga. Magmatic zircon εHf(t) values range from ?3.79 to ?8.44, and have T_DMC ages of 1.65–1.93 Ga. The data suggest that these granites formed by partial melting of Paleoproterozoic to Mesoproterozoic continental crust. A collision occurred between the Wuyi and Minyue microcontinents within the Cathaysia Block and formed S‐type granite in the southwest Fujian province. The ca 360 Ma zircon U–Pb ages can represent a newly recognized period of deformation which coincided with the formation of the unified Cathaysia Block.     

SHRIMP U-Pb zircon age,geochemistry and Nd isotope of the Guandaoshan pluton in SW Sichuan: Petrogenesis and tectonic significance

SHRIMP U-Pb zircon age, geochemical and Nd isotopic data are reported for the Neo-proterozoic Guandaoshan pluton in the Yanbian region, SW Sichuan. This pluton is of typical I-type granite and emplaced at (857 ± 13) Ma. Geochemical and Nd isotopic characters suggest that the pluton was generated by partial melting of pre-existing, young (late Mesoproterozoic to early Neo-proterozoic) low-K tholeiitic protolith within an intraplate anorogenic setting. The Guandaoshan pluton probably records the earliest magmatism induced by the proposed ca. 860–750 Ma mantle superplume beneath the supercontinent Rodinia.     

Hafnium isotopes in Jack Hills zircons and the formation of the Hadean crust

New bulk Hf and Pb isotope data were obtained for 63 leached single zircons from Jack Hills (JH), Western Australia, using solution chemistry and, respectively, MC-ICP MS and ICP-MS. With the exception of one “young” zircon at 3.32 Ga, the remainder of the selected grains were previously dated at > 3.9 Ga by ion-microprobe. This work extends and complements the solution chemistry data of Harrison et al. [Harrison, T.M., Blichert-Toft, J., Müller, W., Albarède, F., Holden, P., Mojzsis, S.J., 2005. Heterogeneous Hadean hafnium: evidence of continental crust at 4.4 to 4.5 Ga. Science 310, 1947–1950.] but uses bulk rather than in situ Pb–Pb ages to interpret the Hf isotope data. This larger data set is used to explore whether the host rocks of the JH zircons formed as a succession of pulses or rather as a single event, and to calculate the age and assess the nature of their crustal protolith. We find that the parent granites of the JH zircons analyzed here formed during a single pulse 4.1 ± 0.1 Ga ago by the remelting of a 4.30–4.36 Ga old protolith. Monte Carlo modeling indicates that the ¹⁷⁶Lu/¹⁷⁷Hf ratios of this material (< 0.01) are unlike the ratios of modern-type oceanic crust and island arc rocks but rather fit a tonalite–trondhjemite–granodiorite (TTG) source. TTGs themselves derived their inordinately enriched character from a basaltic progenitor which corresponds to the missing enriched reservoir identified by the ¹⁴³Nd–¹⁴⁴Nd, ¹⁴²Nd–¹⁴⁴Nd, and ¹⁷⁶Hf/¹⁷⁷Hf systematics of Archean rocks. We speculate that crystallization of the magma ocean in the presence of garnet left the upper mantle and an early basaltic crust enriched in incompatible elements. Reaction of this early crust with the overlying hydrosphere and subsequent foundering into the mantle gave rise at ～ 4.3 Ga to the TTG protolith of the JH granites. Dating the onset of plate tectonics therefore depends on whether TTGs can be considered as subduction zone magmas or not.     

SHRIMP U-Pb zircon age,geochemistry and Nd isotope of the Guandaoshan pluton in SW Sichuan: Petrogenesis and tectonic significance

SHRIMP U-Pb zircon age, geochemical and Nd isotopic data are reported for the Neo-proterozoic Guandaoshan pluton in the Yanbian region, SW Sichuan. This pluton is of typical I-type granite and emplaced at (857 ± 13) Ma. Geochemical and Nd isotopic characters suggest that the pluton was generated by partial melting of pre-existing, young (late Mesoproterozoic to early Neo-proterozoic) low-K tholeiitic protolith within an intraplate anorogenic setting. The Guandaoshan pluton probably records the earliest magmatism induced by the proposed ca. 860–750 Ma mantle superplume beneath the supercontinent Rodinia.

     

Sr–Nd isotopes and geochemistry of the infrastructural rocks in the Meatiq and Hafafit core complexes, Eastern Desert, Egypt: Evidence for involvement of pre-Neoproterozoic crust in the growth of Arabian–Nubian Shield

Abstract Meatiq and Hafafit core complexes are large swells in the Eastern Desert of Egypt, comprising two major tectono‐stratigraphic units or tiers. The lower (infrastructure) unit is composed of variably cataclased gneissose granites and high‐grade gneisses and schists. It is structurally overlain by Pan–African ophiolitic mélange nappes (the higher unit). The two units are separated by a low‐angle sole thrust, along which mylonites are developed. Major and trace element data indicate formation of the gneissose granites in both volcanic arc and within‐plate settings. Nevertheless, all analyzed gneissose granites and other infrastructural rocks, exhibit low initial ratios (Sr_i) (<0.7027), positive ε_Nd(t) (+4.9 to +10.3) and Neoproterozoic Nd model age (T_DM) (592–831 Ma for the gneissose granite samples). Although these values are compatible with other parts of the Arabian– Nubian Shield considered to be juvenile, the ε_Nd(t) values and several incompatible element ratios of the gneissose granites are too low to be derived from a mantle source without contribution from an older continental crust. Our geological, Sr–Nd isotopic and chemical data combined with the published zircon ages indicate the existence of a pre‐Neoproterozoic continent in the Eastern Desert that started to break up at ca 800 Ma. Rifting and subsequent events caused the formation of oceanic crust and emplacement within‐plate alkali basalts in the hinterland domains of the old continent. The emplacement of basaltic magma might have triggered melting of lower crust in the old continent and resulted in emplacement of the within‐plate granite masses between 700 Ma and 626 Ma. The granite masses and other rocks in the old continent have been subjected to deformation during the over‐thrusting of Pan–African nappes, probably because of the oblique convergence between East and West Gondwanaland. Rb–Sr isotopes of the gneissose granites in both Meatiq and Hafafit core complexes defines an isochron age of 619 ± 25 Ma with Sr_i of 0.7009 ± 0.0017 and mean squares of weighted deviates = 2.0. We interpret this age as the date of thrusting of the Pan–African nappes in the Eastern Desert. Continued oblique convergence between East and West Gondwanaland could have resulted in the formation northwest–southeast‐trending Meatiq and Hafafit anticlinoriums.     

Late Carboniferous – Middle Permian arc/forearc‐related basin in Central Asian Orogenic Belt: Insights from the petrology and geochemistry of the Shuangjing Schist in Inner Mongolia,China

The Solonker Suture Zone is thought to record the terminal evolution of the Central Asian Orogenic Belt (CAOB) in Inner Mongolia. However, two contrasting interpretations of the timing of suturing of the Solonker Suture Zone exist: (i) Permian to Early Triassic; and (ii) Middle Devonian or Late Devonian to Carboniferous. The Shuangjing Schist is exposed in the Linxi area along the Xar Moron Fault Zone, which marks the southern boundary of the Solonker Suture Zone in the eastern section of the CAOB, and thus provides insight into the timing of suturing of the Solonker Suture Zone. Detailed and systematic analysis of the petrology and geochemistry of the Shuangjing Schist shows that the Shuangjing Schist developed by greenschist facies prograde metamorphism of a volcanisedimentary rock series protolith. The volcanic parts of the Shuangjing Schist are a calc‐alkaline series with large volumes of intermediate members and subordinate acidic members. Volcanism occurred in a magmatic arc on the continental margin and was induced by subduction‐related magmatism resulting from mantle metasomatism. The sedimentary parts of the Shuangjing Schist reflect a transition from continental shelf to abyssal plain sedimentation. The formation of the Shuangjing Schist is suggested to be related to closure of an arc/forearc‐related ocean basin. The timing is constrained by a laser ablation inductively coupled plasma–mass spectrometry (LA‐ICP–MS) U–Pb magmatic zircon age of 298 ± 2 Ma from a carbonaceous biotite–plagioclase schist that was intruded by granite at 272 ± 2 Ma. In the Linxi area, southward subduction of the arc/forearc basin led to uplift, thickening, collapse, and erosion of the overriding continental crust. Collapse induced extension and widespread magmatism along the volcanic arc at the northern margin of the North China Craton. The closure of the arc/forearc‐related oceanic basin led to the formation of Late Permian to Middle Triassic collisional granites and the subsequent end of the collision of the Solonker Suture Zone.     

Petrogenesis and tectonic implications of Late-Triassic high εNd(t)-εHf(t) granites in the Ailaoshan tectonic zone(SW China)

High εNd(t)-εHf(t) granites are robust evidence for crustal growth. In this paper we report results of petrologic, geochronological and geochemical investigations on the Huashiban granites from the Ailaoshan tectonic zone in western Yunnan(SW China). Zircon grains separated from the two samples(10HH-119 A and 10HH-120A) yield the weighted mean 206Pb/238 U ages of 229.9 ± 2.0 Ma and 229.3 ± 2.3 Ma, respectively, interpreted as the crystallization ages of the granites. Based on our results, in combination with the existing U-Pb geochronological data for the Ailaoshan metamorphic rocks, we propose that the Ailaoshan Group might be a rock complex composed of the Mesoproterozoic, Neoproterozoic, Hercynian, Indosinian and Himalayan components, rather than a part of the crystalline basement of the Yangtze block. The zircon grains show highly depleted Lu-Hf isotope compositions, with positive εHf(t) values ranging from 8.4 to 13.1. The Huashiban granites have high SiO2(72.66 wt%–73.70 wt%), low Mg#(0.28–0.34) with A/CNK=1.01–1.05, and can be classified as peralumious high-K calc-alkaline I-type granites. A synthesis of these data indicates that the Ailaoshan tectonic zone had evolved into a post-collisional setting by the Late-Triassic(229 Ma). Genesis of the Huashiban high εNd(t)-εHf(t) granites involved into two processes:(1) underplating of the sub-arc mantle into the lower crust, and(2) remelting of the juvenile crustal materials in response to the upwelling of the asthenospheric mantle in the post-collisional setting.     

Petrogensis and tectonic setting of the Yemaquan granite from the iron‐polymetallic ore area of Qimantag,Eastern Kunlun Mountains,Qinghai–Tibet Plateau

Being a part of the Paleo‐Tethys Ocean, closing of the Buqingshan‐Anyemaqen oceanic basin left a rich geologic record in the East Kunlun Orogenic Belt. The genesis and tectonic setting of the granites including quartz monzodiorite, granodiorite and mozogranite is discussed in light of the geochemical and U–Pb chronological data obtained. U–Pb dating studies on zircon from the quartz monzodiorite and monzogranite of the research area yielded ages of 220.11 ± 0.49 Ma ((Mean Square Weighted Deviates) MSWD = 0.046) and 223.33 ± 0.54 Ma (MSWD = 0.14), respectively, by Laser Ablation Multiple Collector Inductively Coupled Plasma Mass Spectrometry (LA–MC–ICP–MS) method. According to sedimentological and structural investigations, the Paleo‐Tethys Ocean in the Qimantag region began to close at about 235 Ma, and completely disapperared at about 220 Ma. The three types of granites in this study are considered to intrude the syn‐ to post‐collisional stages. The quartz monzodiorite and granodiorite belong to the I‐type granite whereas the monzogranite is of the S‐type granite. These two types of granites were formed by different ways of partial melting: first, partial melting of the lower crust took place as a result of asthenosphere upwelling triggered by break‐up of the leading edge or tearing of the descending oceanic slab. Subsequently partial melting of the middle–lower crust was caused by the underplating of basaltic magma formed by partial melting of the mantle wedge fluxed by fluids liberated by the oceanic slab dehydration. The magma responsible for the formation of S‐type granites appears to have originated from partial melting of the upper crustal material at a shallower level with a clear signature of continental crust.     

Geochemical characteristics and Sr‐Nd‐Hf isotope compositions of Late Triassic post‐collisional A‐type granites in Sarudik,SW Sumatra,Indonesia

Late Triassic A‐type granites are identified in this study in Sarudik, SW Sumatra. We present new data on zircon U–Pb geochronology, whole‐rock major and trace elements and Sr‐Nd‐Hf isotope geochemistry, aiming to study their petrogenesis and tectonic implications. LA‐ICP‐MS U–Pb dating of zircon separated from one biotite monzogranite sample yields a concordia age of 222.6 ±1.0 Ma, indicating a Late Triassic magmatic event. The studied granites are classified as weakly peralumious, high‐K calc‐alkaline granites. They exhibit high SiO₂, K₂O + Na₂O, FeO/(FeO + MgO) and Ga/Al ratios and low Al₂O₃, CaO, MgO, P₂O₅ and TiO₂ contents, with enrichment of Rb, Th and U and depletion of Ba, Sr, P and Eu, showing the features of A‐type granites. The granites have zircon ε_Hf(t) values from ?4.6 to ?0.4 and whole‐rock ε_Nd(t) values from ?5.51 to ?4.98, with Mesoproterozoic T_DM2 ages (1278–1544 Ma) for both Hf and Nd isotopes. Geochemical and isotopic data suggest that the source of these A‐type granites is the Mesoproterozoic continental crust, without significant incorporation of mantle‐derived component, and their formation is controlled by subsequent fractional crystallization. The Sarudik A‐type granites are further assigned to A2‐type formed in post‐collisional environment. Combined with previous knowledge on the western SE Asia tectonic evolution, we conclude that the formation of the Late Triassic A‐type granites is related to the post‐collisional extension induced by the crustal thickening, gravitational collapse, and asthenosphere upwelling following the collision between the Sibumasu and the East Malaya Block.     

Dating the lower crust by ion microprobe

Ion microprobe UThPb ages of zircons from granulite facies lower crustal xenoliths from north Queensland, Australia, correlate well with the ages of major orogenic episodes manifest at the earth's surface. About half of the xenoliths contain Proterozoic zircons which are similar in age to the episodes of high-grade metamorphism of the older surface rocks. All the xenoliths contain late Paleozoic zircons which show a real 100 Ma range in²⁰⁶Pb²³⁸/U ages (from 320 to 220 Ma), which is attributed to granulite facies metamorphism followed by slow cooling in the deep crust. The Paleozoic zircon ages coincide in time with the prolonged episode of eruption of voluminous felsic ash-flows and intrusion of high-level granites in this region (320-270 Ma). Mineral and melt inclusions in the zircons provide clues to the origin of some of the xenoliths, and coupled with the age information, can be used to infer the geological processes operating in the lower crust. The zircons from two mafic xenoliths contain felsic and intermediate melt inclusions implying at least a two-stage history for these rocks, involving either partial melting of a more felsic protolith or crystal accumulation from an evolved melt. Some of the zircons from the felsic xenoliths contain CO₂-rich fluid inclusions, indicating that those zircons grew during high-grade metamorphism. The isotopic and chemical data for the whole rock xenoliths show that they originate from a segment of the lower crust which is a heterogeneous mixture of supracrustal and mafic, mantle-derived, lithologies. The major orogenic event responsible for the formation of that crust occurred in the late Paleozoic, when Proterozoic supracrustal rocks were emplaced into the lower crust, possibly along thin-skinned thrust slices. This was accompanied by intrusion of high-temperature, mantle-derived melts which caused partial melting of pre-existing crust. The most likely setting for such tectonism is a continental margin subduction zone.     

Two episodes of subduction and collision events at the northern foot of Dabie Mountains: Evidence from petrology and structural geology of granitoid rocks

The intrusive bodies studied include Mafan diorites ((462.7±1.5) Ma,⁴⁰Ar/³⁹Ar amphibole plateau age), Duhudian granites ((293±12) Ma, U-Ph zircon age) and Suxianshi granites ((146.2±0.9) Ma) in Beihuaiyang area at the northern foot of Dabie Mountains, central China. Petrological studies indicate that all of them belong to I-type granitoid rocks. Among them, the Mafan and Duhudian stocks were formed by arc magmatism, while the Suxianshi pluton is a post-collisional granitic body. Three intrusive bodies have distinctive characteristics of structural deformation. The Mafan stock has a rather complicated structure pattern resulting from polyphase deformation during the Caledonian and Mesozoic, the Duhudian stock has been pronouncedly deformed during the Hercynian-Yanshanian events, while regional foliation is not pronounced within the Yanshanian Suxianshi stock. Combination of regional stratigraphic, regional structural and geochronological data shows that the Yangtze plate has experienced two episodes of subduction northward beneath the North China plate during the Paleozoic and following collisional events. The first phase of collision at about 400 Ma resulted in the formation of the Beihuaiyang crystalline basement and the Caledonian high-pressure metamorphism in Dabie orogenic zone, and a late phase of continent-continent collision (~230 Ma) is responsible for the Triassic ultrahigh- and high-pressure metamorphism in Dabie Mountains and for orogenic uplift of the Dabie Mountains. It is suggested that the Beihuaiyang tectonic belt at the northern foot of the Dabie Mountains is a multicyclic suture.     

Zircon SHRIMP U-Pb ages and trace element geochemistry of the Kuhai gabbro and the Dur’ngoi diorite in the southern east Kunlun tectonic belt,Qinghai, Western China and their geological implications

Timing of the intermediate-basic igneous rocks developed in the area of Kuhai-A’nyêmaqên along the southern east Kunlun tectonic belt is a controversial issue. This paper presents new zircon SHRIMP U-Pb dating data for igneous zircons from the Kuhai gabbro and the Dur’ngoi diorite in the Kuhai-A’nyemaqen tectonic belt, which are 555±9 Ma and 493±6 Ma, respectively. The trace element geochemical features of the Kuhai gabbro and the Dur’ngoi diorite are similar to those of ocean island basalts (OIB) and island arc basalts (IAB), respectively. Thus, the Kuhai gabbro with the age of 555±9 Ma and OIB geochemical features is similar to the Yushigou oceanic ophiolite in the North Qilian orogen, whereas the Dur’ngoi diorite with the age of 493±6 Ma and IAB geochemical features is similar to the island arc volcanic rocks developed in the north Qaidam. The Late Neoproterozoic to Early Ordovician ophiolite complex in the area of Kuhai-A’nyêmaqên suggests that the southern margin of the “Qilian-Qaidam-Kunlun” archipelagic ocean in this period was located in the southern east Kunlun tectonic belt. Therefore, the southern east Kunlun tectonic belt in the early Paleozoic is not comparable to the Mianlüe tectonic belt in the Qinling orogenic belt.

     

Petrogenesis and tectonic setting of the Queershan composite granitic pluton,eastern Tibetan Plateau: Constraints from geochronology,geochemistry and Hf isotope data

The Queershan composite granitic pluton is located in the north of the late Paleozoic Yidun arc collision-orogenic belt, eastern Tibetan Plateau. The main rock types are coarse-grained porphyritic alkalic-monzonite granite with minor fine-grained porphyritic monzogranite and granodiorite distributed in the eastern and southwestern regions. Here we report their zircon U-Pb ages and geo- chemical data. The intrusive contact relations indicate that granodiorite was formed earlier than the alkalic-monzonite granite(105.9±1.3 Ma) and monzogranite(102.6±1.1 Ma). These suggest that the Queershan composite granitic pluton was formed through three-stage magmatic events. The alkalic-monzonite granite(105.9±1.3 Ma) and monzogranite(102.6±1.1 Ma) are characterized by high SiO2(73.5%–77.7%), K2O+Na2O(6.9%–8.5%), Ga/Al ratios(2.6–3.4) and low Al2O3(11.8%–14.5%), CaO(0.25%–1.5%), MgO(0.18%–0.69%), negative Ba, Sr and Eu anomalies, showing A-type granite affinities. The granodiorite exhibits lower SiO2, P2O5 and K2O+Na2O contents, but higher Al2O3, CaO and MgO contents than alkalic-monzonite granite and monzogranite, showing I-type granite affinity. 176Hf/177 Hf ratios of the alkalic-monzonite granite and the monzogranite are 0.282692–0.282749 and 0.282685–0.282765, respectively, and with similar ?Hf(t) values(?0.56 to 1.43 and ?0.87 to 1.90 respectively). They also present similar TDM2 model ages(1.04–1.22 and 1.07–1.2 Ga respectively), indicating they may be sourced from a similar rock source, mostly like Kangding Complex. The homogeneity of the Hf isotopic compositions and the absence of the MMEs demonstrate that little depleted mantle materials have contributed to the source. We propose that the Mesoproterozoic crust materials of the Yangtze Craton exist beneath the Yidun arc terrane and support it was a dismembered part of the Yangtze Craton. The A-type granites of Queershan composite granitic pluton are most probably related to the closure of the Bangong-Nujiang Tethys ocean.     

Role of mantle-derived magma in genesis of early Yanshanian granites in the Nanling Range,South China: <Emphasis Type="Italic">in situ</Emphasis> zircon Hf-O isotopic constraints

Although a number of petrographic observations and isotopic data suggest that magma mixing is common in genesis of many granite plutons, it is still controversial whether the mantle-derived magmas were involved in granites. We carried out in this study a systematic analysis of in situ zircon Hf-O isotopes for three early Yanshanian intrusions dated at ca. 160 Ma from the Nanling Range of Southeast China. The Qinghu monzonite has very homogeneous zircon Hf-O isotopic compositions, εHf(t) =11.6±0.3 and δ18O=5...     

SHRIMP ziron U-Pb geochronology of early Mesozoic felsic igneous rocks from the southern Lancangjiang and its tectonic implications

The SHRIMP zircon U-Pb geochronology of three typical samples, including two monzonitic granites from the Lincang batholith and a rhyolite from the Manghuai Formation are presented in the southern Lancangjiang, western Yunnan Province. The analyses of zircons for the biotite monzonitic granites from the northern (02DX-137) and southern (20JH-10) Lincang batholith show the single and tight clusters on the concordia, and yield the weighted mean ²⁰⁶Pb/²³⁸U ages of 229.4 ± 3.0 Ma and 230.4 ± 3.6 Ma, respectively, representing the crystallized ages of these granites. The zircons for the rhyolitic sample (02DX-95) from the Manghuai Formation give a weighted mean ²⁰⁶Pb/²³⁸U age of 231.0 ± 5.0 Ma. These data suggest that the igneous rocks from the Lincang granitic batholith and Manghuai Formation have a similar crystallized age. In combination with other data, it is inferred that both were generated at a narrow age span (∼230 Ma) and were originated from the postcollisional tectonic regime. An early Proterozoic ²⁰⁶Pb/²³⁸U apparent age of 1977±44 Ma is additionally obtained from one zircon from the biotite monzonitic granite (southern Lincang batholith), indicative of development of the early Proterozoic Yangtze basement in the region. These precisely geochronological data provide important constraints on better understanding the Paleozoic tectonic evolution of the Tethys, western Yunnan Province.