The petrogenesis and tectonic implications of the granitoid gneisses from Xingxingxia in the eastern segment of Central Tianshan

As part of Central Asian Orogenic Belt (CAOB), the Central Tianshan zone plays a crucial role in the reconstruction of the tectonic evolution of the CAOB. Furthermore, it is bordered by the Tarim Craton to the south, and the comparable evolutionary history between them enables the Central Tianshan zone to provide essential information on the crustal evolution of the Tarim Craton. The eastern segment of the Central Tianshan tectonic zone is characterized by the presence of numerous Precambrian metamorphic rocks, among which the Xingxingxia Group is the most representative one. The granitoids gneisses, intruded into the Xingxingxia Group, consist of two major lithological assemblages: (1) biotite-monzonitic gneisses and (2) biotite-plagioclase gneisses. These metamorphosed granitoid rocks are characterized by enrichment in SiO₂, Al₂O₃ and K₂O and depletion in MgO and FeO_T. The Rittmann index (σ) spreads between 1.44 and 2.21 and ACNK (Al₂O₃/(CaO + Na₂O + K₂O)) ranges from 1.03 to 1.08, indicating that these granitoid gneisses are high-K calc-alkaline and peraluminous. Trace element data indicate that the studied samples are enriched in LREE with moderate REE fractionated patterns ((La/Yb)_N = 10.5–75.3). The concentrations of HREE of the garnet-bearing gneisses are significantly higher than those of garnet-free gneisses. The former show pronounced negative Eu anomalies (Eu/Eu^* = 0.32–0.57), while the latter are characterized by negligible negative Eu anomalies to moderate positive Eu anomalies (Eu/Eu^* = 0.80–1.35). In addition, the enrichment of LILE (Rb, Th, K, Pb) and depletion of HFSE (Ta, Nb, P, Ti) of the examined granitoid gneisses are similar to typical volcanic-arc granites. Zircons U–Pb dating on the biotite monzonitic gneiss yields a weighted mean ²⁰⁶Pb/²³⁸U age of 942.4 ± 5.1 Ma, suggesting their protoliths were formed in the early Neoproterozoic, which is compatible with the time of the assembly of supercontinent Rodinia. The zircons have a large ε_Hf(t) variation from −5.6 to +3.2, suggesting that both old crust-derived magmas and mantle-derived juvenile materials contributed to the formation of their protoliths. Based on field observation, and petrological, geochemical and geochronological investigations, we infer that the granitoid gneisses from Xingxingxia were probably formed on a continental arc that resulted from the interaction of Australia and the Tarim Craton during the assembly of the Rodinia supercontinent, and that the Central Tianshan zone was a part of the Tarim Craton during that time. Besides, the Grenvillian orogenic events may have developed better in the Tarim Craton than previously expected.     

Geochronology and geochemistry of the late Neoproterozoic A-type granitic clasts in the southwestern Tarim Craton: petrogenesis and tectonic implications

ABSTRACT

Due to sparse data for deciphering the late Neoproterozoic tectonic history, there is still considerable debate on whether long-lasting superplume-related or long-duration subduction-related dynamics dominated the Tarim Craton. In this contribution, our field investigations detail the late Neoproterozoic siliciclastic successions, and we report the first granitic conglomerates with zircon U–Pb ages of 753.9 ± 3.7 Ma in the SW Tarim Craton. Importantly, detrital zircons from the thick Cryogenian sedimentary basin also contain a major zircon population at ca. 750 Ma. Together with seismic data, this suggests a large ca. 750 Ma magmatic event in the SW Tarim Craton. Geochemically, the granitic clasts exhibit A-type granite features with high SiO₂, high alkali but extremely low K, high FeO^T/MgO and Ga/Al, and high high-field strength elements (HFSEs) (i.e. Nb, Hf, and Ta) with significant depletion in Rb, K, Sr, P, Eu, and Ti, and significant negative Eu anomalies (Eu* = 0.13–0.36), showing ferroan granite affinities. Including the detrital zircons, the ca. 750 Ma zircons have a large range of negative εHf(t) values (?19.46 to ?1.16). Elemental and zircon Hf isotope data suggest that the granites were derived from Palaeoproterozoic reworked continental crust and are probably related to crustal thinning and extension. By comparison with previous studies, we conclude that Rodinia breakup was diachronous in the outer parts of the supercontinent.     

http://www.sciencedirect.com/science/article/pii/S1674987114000322

The Central Hebei Basin （CHB） is one of the largest sedimentary basins in the North China Craton, extending in a northeast-southwest direction with an area of 〉350 km2. We carried out SHRIMP zircon dating, Hf-in-zircon isotopic analysis and a whole-rock geochemical study on igneous and metasedi- mentary rocks recovered from drill holes that penetrated into the basement of the CHB, Two samples of gneissic granodiorite （XG1-1） and gneissic quartz diorite 048-1） have magmatic ages of 2500 and 2496 Ma, respectively. Their zircons also record metamorphic ages of 2.41-2.51 and ～2.5 Ga, respec- tively. Compared with the gneissic granodiorite, the gneissic quartz diorite has higher REE contents and lower Eu/Eu＊ and （La/Yb）n values. Two metasedimentary samples （MG1, H5） mainly contain ～2,5 Ga detrital zircons as well as late Paleoproterozoic metamorphic grains. The zircons of different origins have eHf （2.5 Ga） values and Hf crustal model ages ranging from 0 to 5 and 2.7 to 2,9 Ga, respectively, Therefore, ～2.5 Ga magmatic and Paleoproterozoic metasedimentary rocks and late Neoarchean to early Paleoproterozoic and late Paleoproterozoic tectono-thermal events have been identified in the basement beneath the CHB. Based on regional comparisons, we conclude that the early Precambrian basement beneath the CHB is part of the North China Craton.     

Paleoproterozoic crustal evolution of the Tarim Craton: Constrained by zircon U–Pb and Hf isotopes of meta-igneous rocks from Korla and Dunhuang

The Tarim Craton is one of three large cratons in China. Presently, there is only scant information concerning its crustal evolutionary history because most of the existing geochronological studies have lacked a combined isotopic analysis, especially an in situ Lu–Hf isotope analysis of zircon. In this study, Precambrian basement rocks from the Kuluketage and Dunhuang Blocks in the northeastern portion of the Tarim Craton have been analyzed for combined in situ laser ablation ICP-(MC)-MS zircon U–Pb and Lu–Hf isotopic analyses, as well as whole rock elements, to constrain their protoliths, forming ages and magma sources. Two magmatic events from the Kuluketage Block at ∼2.4 Ga and ∼1.85 Ga are revealed, and three stages of magmatic events are detected in the Dunhuang Block, i.e., ∼2.0 Ga, ∼1.85 Ga and ∼1.75 Ga. The ∼1.85 Ga magmatic rocks from both areas were derived from an isotopically similar crustal source under the same tectonic settings, suggesting that the Kuluketage and Dunhuang Blocks are part of the uniform Precambrian basement of the Tarim Craton. Zircon Hf model ages of the ∼2.4 Ga magmatism indicate that the crust of the Tarim Craton may have been formed as early as the Paleoarchean period. The ∼2.0 Ga mafic rock from the Dunhuang Block was formed in an active continental margin setting, representing an important crustal growth event of the Tarim Craton in the mid-Paleoproterozoic that coincides with the global episode of crust formation during the assembly of the Columbia supercontinent. The ∼1.85 Ga event in the Kuluketage and Dunhuang Blocks primarily involved the reworking of the old crust and most likely related to the collisional event associated with the assembly of the Columbia supercontinent, while the ∼1.75 Ga magmatism in the Dunhuang Block resulted from a mixture of the reworked Archean crust with juvenile magmas and was most likely related to a post-collisional episode.     

Petrology and Geochemistry of the Banded Iron Formation of the Kuluketage Block,Xinjiang, NW China: Implication for BIF Depositional Setting

The Kuluketage block, located in the northeast Tarim craton, is one of the largest Precambrian blocks in the Xinjiang province. Recently, many banded iron formation (BIF)‐type (Superior‐type) deposits have been discovered in the western part of the Kuluketage block. These deposits occurred in the Paleoproterozoic Shayiti Formation, Xingditage Group, which has a nearly E–W distribution in the southern Xinger and Xingdi faults. Tremolite biotite schist and quartzite are the main wall rocks. The geochemical characteristics of schist indicate that the BIFs occurred in a passive continental margin environment. The LA–ICP–MS zircon ²⁰⁶Pb/²³⁸U ages of BIF and late syenite are 1945 ± 10 Ma(MSWD = 0.77) (weighted average age) and 1974 ± 27 Ma(MSWD = 1.05) (upper intercept age), respectively, indicating that the BIFs occurred in the Paleoproterozoic. In addition, the approximately 1.9 Ga magmatic and metamorphic events are consistent with the global‐scale 2.1–1.8 Ga collisional orogen events which are associated with the assembly of the Columbia supercontinent. The geochemical characteristics show that magnetite and quartz are dominant components (total content, 91.65–98.22 wt.%), and the Zr(Nb) and TiO₂, Zr(Nb) and Al₂O₃ and Zr and Y/Ho display strongly positive correlations, illustrating the addition of crustal materials into the chemical precipitate of the original BIFs. The higher Zr, Nb and Al₂O₃ contents and a lower Y/Ho ratio of the Kuluketage BIFs indicate a higher terrigenous detrital component contaminant compared to BIFs of North China Craton (NCC). The rare earth and yttrium (REY) distribution patterns show a slight LREE enrichment and weak Eu positive anomaly features, indicating that the source of Fe and Si of the Kuluketage BIFs is mainly from the contribution of low‐temperature hydrothermal alteration of the oceanic crust. In addition, along with the decreasing BIF depositional age, the declining of Eu anomaly values reflects the increasing importance of low‐temperature hydrothermal solutions relative to high‐temperature hydrothermal solutions. Moreover, no Ce anomalies in studied BIFs, NCC and Xinyu BIFs are attributed to relative reducing environmental condition when the original BIFs precipitated.     

Geochemistry, Nd Isotopic Characteristics of Metamorphic Complexes in Northern Hebei： Implications for Crustal Accretion

The middle segment of the northern margin of the North China Craton （NCC） consists mainly of metamorphosed Archean Dantazi Complex, Paleoproterozoic Hongqiyingzi Complex and unmetamorphosed gabbro-anorthosite-meta-alkaline granite, as well as metamorphosed Late Paleozoic mafic to granitoid rocks in the Damiao-Changshaoying area. The -2.49 Ga Dantazi Complex comprises dioritic-trondhjemitic-granodoritic-monzogranitic gneisses metamorphosed in amphibolite to granulite facies. Petrochemical characteristics reveal that most of the rocks belong to a medium- to high-potassium calc-alkaline series, and display Mg^# less than 40, right-declined REE patterns with no to obviously positive Eu anomalies, evidently negative Th, Nb, Ta and Ti anomalies in primitive mantlenormalized spider diagrams, εNd（t）=＋0.65 to -0.03, and depleted mantle model ages TDM=2.78-2.71 Ga. Study in petrogenesis indicates that the rocks were formed from magmatic mixing between mafic magma from the depleted mantle and granitoid magma from partial melting of recycled crustal mafic rocks in a continental margin setting. The 2.44-2.41 Ga Hongqiyingzi Complex is dominated by metamorphic mafic-granodioritic-monzogranitic gneisses, displaying similar petrochemical features to the Dantazi Complex, namely medium to high potassium calc-alkaline series, and the mafic rocks show evident change in LILEs, negative Th, Nb, Ta, Zr anomalies and positive P anomalies. And the other granitiod samples also exhibit negative Th, Nb, Ta, P and Ti anomalies. All rocks in the Hongqiyingzi Complex show right-declined REE patterns without Eu anomaly. The metamorphic mafic rocks with εNd（t） = -1.64 may not be an identical magmatic evolution series with granitoids that have εNd（t） values of ＋3.19 to ＋1.94 and TDM ages of 2.55-2.52 Ga. These granitic rocks originated from hybrid between mafic magma from the depleted mantle and magma from partial melting of juvenile crustal mafic rocks in an island arc setting. All the -311 Ma Late Paleozoic metamorphic mafic rocks and related granitic rocks show a medium-potassium calc-alkaline magmatic evolution series, characterized by high Mg^#, obviously negative Th, Nb, Ta anomalies and positive Sr anomalies, from no to strongly negative Ti anomalies and flat REE patterns with εNd（t） = ＋8.42, implying that the maflc magma was derived from the depleted mantle. However the other granitic rocks are characterized by right-declined REE patterns with no to evidently positive Eu anomalies, significantly low εNd（t） = -13.37 to -14.04, and TDM=1.97-1.96 Ga, revealing that the granitoid magma was derived from hybrid between maflc magma that came from -311 Ma depleted mantle and granitoid magma from Archean to Early Paleoproterozoic ancient crustal recycling. The geochemistry and Nd isotopic characteristics as well as the above geological and geochronological results indicate that the middle segment of the northern margin of the NCC mainly experienced four crustal growth episodes from Archean to Late Paleozoic, which were dominated by three continental marginal arc accretions （-2.49, -2.44 and 311 Ma）, except the 1.76-1.68 Ga episode related to post-collisional extension, revealing that the crustal accretion of this segment was chiefly generated from arc accretion and amalgamation to the NCC continental block.     

Switching from advancing to retreating subduction in the Neoproterozoic Tarim Craton,NW China: Implications for Rodinia breakup

Geodynamic drivers for the supercontinent cycle are generally attributed to either top-down(subduction-related)or bottom-up(mantle-related)processes.Compiled geochemical data and U-Pb ages and Hf isotopic signatures for magmatic and detrital zircons from the Tarim Craton reveal a distinct change in subduction style during the Neoproterozoic.The subduction cycle is recorded in increasing and decreasing intensity of subduction-related magmatic rocks and time-equivalent sedimentary successions,and converse trends ofεHf(t)values and corresponding changes in crustal incubation time.These trends are consistent with a switch from advancing to retreating subduction.The switch likely occurred at ca.760 Ma when zirconεHf(t)values increase and crustal incubation times decrease following a transitional shift between 800 Ma and 760 Ma.A switch at this time is consistent with Rodinia breakup and may have resulted in the late Neoproterozoic Tarim rift basin.The long-lived(ca.500 Ma)subduction recorded in the Tarim Craton suggests the predominance of a top-down process for Rodinia breakup on this part of its margin.     

Geochemical features of the Matomb alluvial rutile from the Neoproterozoic Pan-African belt,Southern Cameroon

The Matomb region constitutes an important deposit of detrital rutile. The rutile grains are essentially coarse (> 3 mm), tabular and elongated, due to the short sorting of highly weathered detritus. This study reports the major, trace, and rare-earth element distribution in the bulk and rutile concentrated fractions. The bulk sediments contain minor TiO₂ concentrations (1–2 wt%), high SiO₂ contents (∼77–95 wt%) and variable contents in Al₂O₃, Fe₂O₃, Zr, Y, Ba, Nb, Cr, V, and Zn. The total REE content is low to moderate (86–372 ppm) marked by high LREE-enrichment (LREE/HREE ∼5–25.72) and negative Eu anomalies (Eu/Eu^* ∼0.51–0.69). The chemical index of alteration (CIA) shows that the source rocks are highly weathered, characteristic of humid tropical zone with the development of ferrallitic soils. In the concentrated fractions, TiO₂ abundances exceed 94 wt%. Trace elements with high contents include V, Nb, Cr, Sn, and W. These data associated with several binary diagrams show that rutile is the main carrier of Ti, V, Nb, Cr, Sn, and W in the alluvia. The REE content is very low (1–9 ppm) in spite of the LREE-abundance (LREE/HREE ∼4–40). The rutile concentrated fractions exhibit anomalies in Ce (Ce/Ce^* ∼0.58 to 0.83; ∼1.41–2.50) and Eu (Eu/Eu^* ∼0.42; 1.20–1.64). The high (La/Sm)_N, (La/Yb)_N and (Gd/Yb)_N ratios indicate high REE fractionation.     

Geochronology and geochemistry of Grenville-aged (1063 ± 16 Ma) metabasalts in the Shennongjia district,Yangtze block: implications for tectonic evolution of the South China Craton

To better understand the evolution of the South China Craton (SCC), we have determined the geochronological and geochemical compositions of newly recognized Grenville-aged metabasalts in the Shennongjia region of the northern Yangtze block. LA-ICP-MS U-Pb dating of zircons indicates that the metabasalts formed at 1063 ± 16 Ma. The rocks are calc-alkaline, are characterized by SiO₂ contents (50.50 to 55.62 wt.%), and have moderate-to-high MgO contents (7.25–9.60 wt.%). They display light rare earth element enrichment ((La/Yb)_N = 7.0–9.8) with slightly negative Eu anomalies (Eu/Eu* = 0.82–0.90) and have pronounced depletion in high-field strength elements as well as positive Pb anomalies in the primitive mantle-normalized trace element pattern. They possess high initial Sr isotopic ratios of 0.7092–0.7107, large negative ε_Nd(t) values of ?12.1 to ?11.0, and a relatively narrow range of initial Pb isotope ratios (²⁰⁶Pb/²⁰⁴Pb = 16.503–17.019, ²⁰⁷Pb/²⁰⁴Pb = 15.259–15.452, and ²⁰⁸Pb/²⁰⁴Pb = 36.169–36.994). These isotopic characteristics are typical of basalts derived from an EM2 source region and suggest a subcontinental lithospheric mantle source that was metasomatized by subducted components (fluids and melts). Integrating our new data with documented igneous and metamorphic events during late Mesoproterozoic to early Neoproterozoic time in the region and western segments of the Yangtze block, we suggest that the Shennongjia area might be a microcontinent that was independent of the continental nucleus of the Yangtze block and was accreted to the Yangtze block at the end of Mesoproterozoic time. This study thus argues against the traditional view that the SCC was formed simply by Yangtze–Cathaysia collision and supports a hypothesis in which the Yangtze block was a collage of microcontinents accreted during the Grenvillian period accompanying the assembly of the Rodinia supercontinent.     

A review of Palaeoarchaean felsic volcanism in the eastern Kaapvaal craton:Linking plutonic and volcanic records

In the Kaapvaal craton of southern Africa, as well as other Archaean cratons worldwide, the progression from dominant tonalite-trondhjemite-granodiorite(TTG) to granite-monzogranite-syenogranite(GMS)rock types is interpreted to reflect progressive reworking and differentiation of the continental crust.Here we re-evaluate the early Archaean evolution of the Kaapvaal craton and propose a unified view of the plutonic and volcanic records based on elemental and isotopic(Nd, Hf) data and zircon U-Pb ages.We also report new whole-rock major and trace element analyses, zircon U-Pb ages and Hf-in-zircon analyses of igneous clasts from a conglomerate of the 3.2 Ga Moodies Group of the Barberton Greenstone Belt. Many of these clasts are derived from shallow intrusive rocks of granitic composition, which are scarcely represented in outcrop. Despite alteration, the volcanic rocks can be classified based on their trace element contents into two main groups by comparison with plutonic rocks. One group has characteristics resembling TTGs: relatively low and fractionated rare earth element concentrations with no Eu anomaly and relatively low concentrations of high field strength elements(Nb mostly ≤12 ppm). The second group has GMS-like characteristics: less fractionated REE, marked negative Eu anomalies and HFSE-increasing trends with progressing fractionation(Nb ≤ 50 ppm or more, Th up to 30-40 ppm). In addition, igneous clasts of Moodies Group conglomerate have chemical, mineralogical and isotopic characteristics that link them to GMS. New analyses of some of these clasts indicate elevated high field strength elements(Nb up to 20 ppm) and_(εHf)(t)of zircon down to -3.5. These rocks imply the presence of an already differentiated felsic crust at 3.5 Ga, which has Nd and Hf model ages indicating mantle extraction ages extending back to the Eoarchaean. The combined record of plutonic and volcanic rocks of the Kaapvaal craton provides a more complex scenario than previously suggested and indicates that TTG and GMS-like felsic magmas were emplaced broadly coevally in multiple pulses between ~3.5 Ga and 3.2 Ga.     

Devonian alkaline magmatism in the northern North China Craton: Geochemistry,SHRIMP zircon U-Pb geochronology and Sr-Nd-Hf isotopes

The Wulanhada pluton is among the rare suite of Devonian alkaline plutons occurring along the northern margin of the North China Craton(NCC).The intrusion is mainly composed of quartz-monzonite.Here we report zircon SHRIMP U-Pb data from this intrusion which shows emplacement age of ca.381.5 Ma.The rock is metaluminous with high(Na_2O + K_2O) values ranging from 8.46 to 9.66 wt.%.The REE patterns of the rocks do not show any Eu anomaly whereas the primitive-mantle-normalized spider diagram shows strong positive Sr and Ba anomalies.The Wulanhada rocks exhibit high initial values of(~(87)Sr/~(86)Sr)_t = 0.70762-0.70809,low ε_(Nd)(t) =-12.76 to-12.15 values and negative values of ε_(Hf)(t) =-23.49 to-17.02 with small variations in(~(176)Hf/~(177)Hf),(0.281873-0.282049).These geochemical features and quantitative isotopic modeling results suggest that the rocks might have been formed through the partial melting of Neoarchean basic rocks in the lower crust of the NCC.The Wulanhada rocks,together with the Devonian alkaline rocks and mafic-ultramafic complex from neighboring regions,constitute a post-collisional magmatic belt along the northern NCC.     

Geochemistry of 1.78 Ga A-type granites along the southern margin of the North China Craton: implications for Xiong'er magmatism during the break-up of the supercontinent Columbia

The Palaeoproterozoic Luoling granites occur along the southern margin of the North China Craton. They are rich in silica and total alkalis with SiO₂ contents ranging from 65.18 to 72.72 wt.%, K₂O from 4.68 to 6.62 wt.%, and Na₂O from 1.35 to 4.88 wt.%. They have high Fe*[FeO_t/(FeO_t + MgO)] ranging from 0.84 to 0.95 wt.% and low MnO (0.03–0.09 wt.%), MgO (0.27–1.55 wt.%), CaO (0.36–2.04 wt.%), TiO₂ (0.4–1.12 wt.%), and P₂O₅ (0.04–0.36 wt.%). Geochemically, they show typical characteristics of A-type granites, such as high contents of alkalis (i.e. high K₂O + Na₂O, with K₂O/Na₂O > 1), Rb, Y, Nb, and REEs (except for Eu); high FeO_t/MgO and Ga/Al ratios; and low CaO, Al₂O₃, and Sr contents. New secondary ion mass spectroscopy (SIMS) zircon U–Pb ages reveal that the Luoling granites were emplaced at 1786 ± 7 Ma and thus were approximately coeval with Xiong'er volcanic rocks in the area. Their negative bulk-rock initial Nd and zircon initial Hf isotopic ratios suggest that they have affinities to EM-I-type mantle and both are the products of Xiong'er magmatism during the Palaeoproterozoic. We regard them as produced under a continental rift setting during the Palaeoproterozoic, genetically related to the break-up of the Columbia supercontinent.     

Geochronology and geochemistry of the Donglufang porphyry-skarn Mo-Cu deposit in the southern Yidun Terrane and their geological significances

The newly-discovered Donglufang Moe Cu porphyry-skarn deposit is located in the southern Yidun Terrane, southeast Tibet, with more than 80 million tonnes(Mt) of reserves(grading 0.15 wt.% Mo and0.48 wt.% Cu) hosted in Triassic strata and Late Cretaceous granodiorite porphyry. Ree Os dating of molybdenum ore yielded a weighted mean age of 84.9 ± 1.0 Ma and an isochron age of 85.2 ± 0.6 Ma.LA-ICP-MS Ue Pb dating of zircons from the granodiorite porphyry yielded206 Pb/238 U ages ranging from 87.4 Ma to 84.2 Ma with a weighted mean206 Pb/238 U age of 85.1 ±0.5 Ma, indicating a temporal linkage between granitic magmatism and Moe Cu mineralization. Geochemical analyses show that the granodiorite porphyries are I-type granites with Si O_2 contents of 64.3 -66.7 wt.%. These rocks are typically metaluminous with high K_2 O/Na_2 O ratios, low Mg O(1.32 -1.56 wt.%), Cr(5.6 -12.9 ppm), Ni(3.79 -10.81 ppm), Mg#(43 -52) values, and high Sr(304 -844 ppm), Sr/Y(21.2 -50.8) and La/Yb ratios(37.0 -60.1). They are enriched in light rare-earth elements(LREE) relative to heavy rare-earth elements(HREE), with slightly negative Eu anomalies, and are enriched in Th, U and large ion lithophile elements(LILE, e.g., K and Rb), and depleted in high field strength elements(HFSE, e.g., Nb, Ta, P and Ti). They also show negative zircon εHf(t) values(-6.7 to -2.3) and negative whole rock εNd(t) values(à5.2 to-4.3), as well as old Hfe Nd model ages, indicating the magmas were derived from a thickened ancient lower crust within the garneteamphibolite facies. Considering the tectonic evolution of the Yidun Terrane, geochemical characteristics of granodiorite porphyry, and the ages of mineralization obtained in this study. We suggest that the Donglufang deposit was formed in a post-collisional setting, which has a genetic relationship with the emplacement of the granodiorite porphyry. The present study provide key information for the exploration of the Late Cretaceous metallogeny in the Yidun Terrane.     

Geochemistry,geochronology and Hf isotope of granitoids in the Chinese Altai: Implications for Paleozoic tectonic evolution of the Central Asian Orogenic Belt

The Chinese Altai in northwestern Xinjiang has numerous outcrops of granitoids which provide critical information on accretionary orogenic processes and crustal growth of the Central Asian Orogenic Belt.Zircon U-Pb ages, Hf-isotopic compositions and whole-rock geochemistry of monzogranite and granodiorites in the Qinghe County are employed to elucidate Paleozoic tectonics of the Chinese Altai. Granodiorites have crystallization ages of 424.6 ± 3.1 Ma(MSWD = 0.23) and 404.0 ± 3.4 Ma(MSWD = 0.18);monzogranite was emplaced in the early Permian with a crystallization age of 293.7 ± 4.6 Ma(MSWD = 1.06). Both granodiorites and monzogranite are I-type granites with A/CNK ratios of 0.92 -0.97 and 1.03 -1.06, respectively. They also show similar geochemical features of high HREE and Y contents, low Sr contents and Sr/Y ratios, as well as enrichment of Cs, Rb, Th and U, and depletion of Nb, Ta, P and Ti.These geochemical features indicate that the monzogranite and granodiorites were formed in an arc setting related to subduction. The gneissic monzogranites display high SiO_2 and K_2 O contents, and belong to the high-K calc-alkaline series. In the chondrite normalized REE distribution pattern, the monzogranite samples exhibit enrichment of LREE with strong negative Eu anomalies(σE u =0.44 -0.53), zircon εHf(t) values from +7.24 to +12.63 and two-stage Hf model ages of 463 -740 Ma. This suggests that the monzogranite was generated from the mixing of pelitic and mantle material. The granodiorite samples are calc-alkaline granites with lower contents of Si O_2 and Na_2 O + K_2 O, higher contents of TiO_2, Fe_2O_3~t, MgO and CaO compared to the monzogranite samples. They also show enrichment of LREE and moderate negative Eu anomalies(σE u= 0.54 =0.81), as well as slightly higher differentiation of LREE than that of HREE. The425 Ma granodiorite has zircon εHf(t) values from -0.51 to +1.98 and two-stage Hf model ages of 1133 -1240 Ma, whereas the 404 Ma granodiorite displays those of +2.52 to +7.50 and 816 -1071 Ma.Geochemistry and zircon Hf isotopic compositions indicate that granodiorites were formed by partial melting of juvenile lower crust. Together with regional geology and previous data, the geochemical and geochronological data of the monzogranite and granodiorites from this study suggest long-lived subduction and accretion along the Altai Orogen during ca. 425 -294 Ma.     

Petrogenesis of Carboniferous adakites and Nb-enriched arc basalts in the Alataw area,northern Tianshan Range (western China): Implications for Phanerozoic crustal growth in the Central Asia orogenic belt

Carboniferous volcanic rocks in the Alataw area, Northern Tianshan Range (Xinjiang), consist of early Carboniferous (ca. 320 Ma) adakites and Nb-enriched arc basalts and basaltic andesites (NEBs), and late Carboniferous (ca. 306–310 Ma) mainly high-K calc-alkaline andesites, dacites and rhyolites. The adakites are calc-alkaline, and characterized by high Na₂O/K₂O (1.52–3.32) ratios, negligible to positive Eu anomalies, strong depletion of heavy rare earth elements (e.g., Yb = 0.74–1.47 ppm) and Y (6.7–14.9 ppm), positive Sr and Ba but negative Nb and Ti anomalies, and relatively constant ε_Nd(T) values (+ 3.4–+ 6.6) and (⁸⁷Sr/⁸⁶Sr)_i ratios (0.7035–0.7042). Some andesitic and dacitic adakite samples exhibit high MgO contents similar to magnesian andesites. The NEBs are sodium-rich (Na₂O/K₂O = 2.03–8.06), and differ from the vast majority of arc basalts in their higher Nb, Zr, TiO₂ and P₂O₅ contents and Nb/Th, Nb/La and Nb/U ratios, and minor negative to positive anomalies in Ba, Nb, Sr, Zr and Ti. They have the highest ε_Nd(T) values (+ 6.4–+ 11.6) but varying (⁸⁷Sr/⁸⁶Sr)_i ratios (0.7007–0.7063). The high-K calc-alkaline suite is similar to typical ‘normal’ arc volcanic rocks in terms of moderately fractionated rare earth abundance and distinctly negative Eu, Nb, Sr and Ti anomalies. They have ε_Nd(T) values (+ 1.2–+ 6.4) and (⁸⁷Sr/⁸⁶Sr)_i ratios (0.7018–0.7059). Geochemically, they are similar to coeval I-type granitoids in the Alataw area. Given the presence of early Carboniferous ophiolites in the Northern Tianshan Range, and the isotopically inappropriate compositions of Proterozoic metamorphic basement in the Alataw area, we argue that the Alataw adakites were most probably related to the melting of young subducted crust of the Northern Tianshan Ocean. The NEBs likely originated from mantle wedge peridotites metasomatized by adakites and minor slab-derived fluids. The later high-K calc alkaline suite was generated by AFC processes that acted on melts derived from a mantle wedge metasomatized by hydrous fluids. The larger range of isotopic compositions exhibited by both the NEB and high-K suite, relative to the adakites, suggests that the mantle wedge was heterogeneous prior to slab- or fluid-mediated metasomatism.Continental crustal growth of the Central Asian orogenic belt was dominated by contributions of the juvenile materials from the depleted mantle prior to 270 Ma and possibly afterwards. The results of this study suggest that other Carboniferous Nb-enriched basalts in the Tianshan Range were generated by subduction processes rather than by intraplate tectonics as previously proposed.     

Late Neoarchean crustal growth under paired continental arc-back arc system in the North China Craton

The late Archean (~3.0–2.5 Ga) was a key period of continental growth globally, which is widely considered to reflect the onset of vigorous plate tectonic activity, although related continental growth modes remain contentious. Here we investigate a suite of late Neoarchean metavolcanic rocks from the southwest Qixia area of the Jiaobei terrane in the North China Craton. The rocks in this suite include amphibolites, clinopyroxene amphibolites, and hornblende plagioclase gneisses. We present zircon U-Pb isotopic data which indicate that the protoliths of these rocks formed during ~2549–2511 Ma.The (clinopyroxene) amphibolites correspond to meta-basaltic rocks, with some containing high modal content of titanite. These rocks show moderate to high FeO_T (8.96–13.62 wt.%) and TiO₂ (0.59–1.59 wt.%), flat to less fractionated REE patterns, and mildly negative Th, Nb, and Ta anomalies, resembling those of Fe-tholeiites. In addition, they display positive zircon ?_Hf(t) values (+2.6 to +8.7), and are devoid of crustal contamination or fractional crystallization. Combined with the low Nb/Yb (mostly < 1.60) and (Hf/Sm)_N (mostly < 0.95), low to moderate Th/Yb (0.08–0.54), and low V/Sc (5.53–9.19) ratios, these basaltic rocks are interpreted to have been derived from a relatively reduced and depleted mantle source that was mildly metasomatized by hydrous fluids. The hornblende plagioclase gneisses are meta-andesitic rocks, and occur interlayered with the basaltic rocks. They are transitional between tholeiitic and calc-alkaline rock series, and show fractionated REE patterns with evidently negative Th, Nb, and Ta anomalies. The depleted zircon ?_Hf(t) values (+2.4 to +8.4) and quantitative chemical modeling suggest that the andesitic rocks were most likely generated by injection and mixing of juvenile felsic magmas with the tholeiitic basaltic magmas.In general, the chemical features and genesis of late Neoarchean meta-basaltic rocks in our study area resemble those of Mariana back-arc basin basalts. Combined with regional geological data, it is proposed that the Jiaobei terrane witnessed late Neoarchean crustal growth under a paired continental arc-back arc setting. On a regional context, we propose two distinct geodynamic mode of late Neoarchean continental growth across North China Craton (particularly the Eastern Block), i.e., (1) arc-continent accretion along northwestern part of the Eastern Block; and (2) paired continental arc-back arc system surrounding the ~3.8–2.7 Ga continental nuclei to the southeast.     

Petrogenesis of adamellites from eastern Shandong Province: geochronological,geochemical, and Sr–Nd–Pb isotopic evidence

Geochronology, geochemistry, and whole-rock Sr–Nd–Pb isotopes were studied on a suite of Mesozoic adamellites from eastern China to characterize their ages and petrogenesis. Sensitive high-resolution ion microprobe U–Pb zircon analyses were done, yielding consistent ages of 123.2 ± 1.8 to 122.1 ± 2.1 Ma for the samples. These rocks belong to the alkaline magma series in terms of K₂O + Na₂O contents (8.45–9.58 wt.%) and to the shoshonitic series based on their high K₂O contents (5.23–5.79 wt.%). The adamellites are further characterized by high light rare earth element contents [(La/Yb)_N = 14.96–45.99]; negative Eu anomalies (δEu = 0.46–0.75); positive anomalies in Rb, Th, Pb, and U; and negative anomalies in Sr, Ba, and high field-strength elements (i.e. Nb, Ta, P, and Ti). In addition, all of the adamellites in this study display relatively low radiogenic Sr [(⁸⁷Sr/⁸⁶Sr)_i = 0.7081–0.7089] and negative ?_Nd(t) values from –16.70 to –17.80. These results suggest that the adamellites were derived from low-degree partial melting of an enriched lithospheric mantle below the North China Craton (NCC). The parent magmas likely experienced fractional crystallization of potassium feldspar, plagioclase and Fe–Ti oxides (e.g. rutile, ilmenite, and titanite), apatite, and zircon during the ascent of alkaline rocks without crustal contamination.     

Triassic magmatism in the eastern part of the South China Block: Geochronological and petrogenetic constraints from Indosinian granites

The granitic dykes in the Badu Group,Zhejiang Province,South China provide important insights on tectonic setting and crustal evolution of the South China Block(SCB) and the Indochina Block during Triassic.Here we report LA-ICP-MS U-Pb data of granitic rocks from the Hucun and Kengkou which show early Triassic ages of 242 ± 2 and 232 ± 3 Ma,respectively,representing their timing of emplacement.The dyke rocks are enriched in K,Al,LREE,Rb,Th.U,and Pb.and are depleted in Nb,Ta,Sr,and Ti.The rocks are characterized by highly fractionated REE patterns with(La/Yb)N ratios of 28.46-38.07 with strong negative Eu anomalies(Eu/Eu* = 0.65-0.73).In situ Hf isotopic analyses of zircons from the Hucun granite yielded ε_(Hf)(t) values of-13.9 to-6.4 and two-stage depleted mantle Hf model ages of 1.68-2.15 Ga,which indicate that the magma was formed by partial melting of the Paleoproterozoic metasedimentary protoliths in the Cathaysia Block.The zircons from the Kengkou granite have ε_(Hf)(t) values ranging from 40.7 to 31.5 and yield two-stage depleted mantle Hf model ages of 0.99-2.49 Ga,indicating magma origin from a mixed source.The Hucun and Kengkou dykes,together with the Triassic A-type granites in SE China were probably generated during magmatism associated with crust-mantle decoupling along the convergent plate boundary between SCB and the Indochina Block.     

Diversity of late Neoarchean K-rich granitoid rocks derived from subduction-related crust/mantle interactions in the Jiaobei terrane,North China Craton

The appearance of voluminous K-rich granitoids within nearly all ancient cratons represents one major characteristic of late Archean Earth, which hold the key to understand the transitional geodynamic regimes globally during this period. The genetic regimes and links among different K-rich granitoids and their implications for crustal growth and evolution remain controversial. A series of late Neoarchean K-rich granitoids, including quartz dioritic, granodioritic, and monzogranitic gneisses, occur in the Jiaobei terrane of North China Craton. Zircon U-Pb isotopic data reveal that they emplaced during ~2544–2494 Ma, coeval with regional ~2530–2470 Ma high-grade metamorphism.The quartz dioritic-granodioritic gneisses are magnesian rocks, and they show low Si and Ti, but high K and Mg, similar to Archean low-Ti sanukitoids. The Sr/Y and (La/Yb)_N are high (mostly 59.99–119.32 and 8.56–61.42), with moderate to high Nb (up to 11.79 ppm). Geochemical modeling, combined with depleted zircon εHf(t₂) (+0.5 − +7.2) and the presence of minor xenocrystic zircons, indicate that these low silica samples were derived from a metasomatized depleted mantle source with inputs of slab-derived fluids and melts, and minor contamination by ancient crustal materials. The monzogranitic rocks are ferroan rocks showing high Si, K, and Fe, but low Mg. They are divided into two subgroups: the first displays low TREE of 44.00–127.00 ppm and positive Eu anomalies (Eu_N/Eu*_N = 1.06–1.60), whereas the second shows high TREE of 85.76–819.02 ppm but negative Eu anomalies (Eu_N/Eu*_N = 0.51–0.62). Geochemical modeling and depleted zircon εHf(t₂) of +2.6 − +8.4 suggest their formation by partial melting of juvenile crustal sources involving tonalitic and some metasedimentary rocks at diverse crustal levels.Combined with regional geological data, these late Neoarchean K-rich granitoids were generated by coupled melting of metasomatized depleted mantle and dominantly juvenile crustal materials during crustal stabilization. Furthermore, the Jiaobei terrane experienced ~2.6–2.5 Ga crustal growth under a subduction-accretion setting.     

The Neoproterozoic Hongliujing A-type granite in Central Tianshan (NW China): LA-ICP-MS zircon U–Pb geochronology,geochemistry, Nd–Hf isotope and tectonic significance

Located between the Turpan-Hami, Junggar and Tarim blocks, the Central Tianshan zone is an important component of the Central Asian Orogenic Belt (CAOB) and crucial linkage between the Siberian, Kazakhstan, Junggar, Turpan-Hami and Tarim blocks. The Hongliujing granite associated with Nb–Ta mineralization in the Central Tianshan zone, dated at ca. 740 Ma using zircon LA-ICP-MS dating, is the first reported Neoproterozoic intrusion with a reliable and precise age in the Chinese Central Tianshan. The Hongliujing granite shares all the characteristics of A-type granites. It contains predominant alkali feldspar, and is characterized by high contents of SiO₂, Na₂O + K₂O, K₂O and high field strength elements (such as Nb, Ta, Zr, Ga and Y), and low contents of CaO, MgO, Ba and Sr, with high FeO^t/(FeO^t + MgO) and Ga/Al ratios typical of A-type granites. Based on the geochemistry and zircon Hf isotope data, we propose that the Hongliujing granite was most likely produced by partial melting of basic rocks in the lower crust which may have been derived from mantle magmas. The Hongliujing granite belongs to A1-type granites, which indicate a rifting formation environment, suggesting that like the Tarim Block, the Central Tianshan zone recorded Neoproterozoic rift-related igneous events related to the breakup of the Rodinia supercontinent. Our study verifies that not only the Tarim Block is related to the breakup of the Rodinia supercontinent, but also it is true for some key blocks in CAOB such as the Central Tianshan. Our new geochemical and geochronologic data also support and strengthen the notion that the Central Tianshan zone may be a part of the Tarim Block.