Major Tectono-thermal Events in the Yangtze Craton: Insights from U-Pb-Lu-Hf Isotope Records in Zircons from End-Permian Volcanic Interlayers in Southwest China

The in situ zircon U-Pb-Lu-Hf isotope records from end-Permian volcanic interlayers in southwest China,integrated with previous studies,restructure the evolutionary history of the Yangtze Craton from Precambrian to Late Paleozoic.This includes early continental crust formation before 3.0 Ga and massive juvenile crustal growth at 2.6-2.4 Ga;large-scale crustal reworking at 2.1-1.7 Ga;Neoproterozoic crust addition at 1.1 to 0.7 Ga;collision and subduction along the craton margin between 700-541 Ma;Early Ordovician to Late Silurian magmatism;and large tectono-thermal events in the Middle Carboniferous to end-Permian.Some zircons with T(MD2)ages from 4.40 to 4.01 Ga and lower initial176Hf/177Hf values of 0.280592 to 0.280726 may imply the existence of Hadean crust relics beneath the Yangtze Craton and their provenances could be associated with Hadean crustal remelting.This study further clarifies that the Precambrian-age zircons between the end-Permian volcanic interlayers,the complexes in the western margin of the Yangtze Craton,and the sedimentary Kangdian Basin,may share an affinity based on similar U-Pb age spectra and Hf isotope features.It also shows that the Neoproterozoic tectono-thermal event may be associated with large-scale tectono-rifting activity,which is different from the Grenville-age continental collision between Yangtze and Cathaysia blocks in South China.The above findings support the inference of a widespread Archean basement extending to the western Yangtze Craton and a provenance in the Kangdian Basin that is derived from the weathering and erosion of Paleoproterozoic continental crust.

Sao Francisco-Congo Craton break-up delimited by U-Pb-Hf isotopes and trace-elements of zircon from metasediments of the Aracuaí Belt

Detrital zircon U-Pb geochronology combined with Hf isotopic and trace element data from metasedimentary rocks of the Aracuai Belt in southeastern Brazil provide evidence for break-up of the Congo-Sao Francisco Craton. The U-Pb age spectra of detrital zircons from metasediments of the Rio Doce Group(RDG) range from 900-650 Ma and define a maximum depositional age of ca. 650 Ma. Zircon trace element and whole rock data constrain an oceanic island arc as source for the deposition setting of the protoliths to the metasediments. Zircon ε_(Hf)(t) values from these rocks are positive between +1 and +15, supporting previous evidence of a Neoproterozoic extensional phase and oceanic crust formation in a precursor basin to the Aracuai Belt. Recrystallization of detrital zircon at ca. 630 Ma is compatible with a regional metamorphic event associated with terrane accretion to the Paleoproterozoic basement after transition from an extensional to a convergent regime. The juvenile nature, age spectra and trace element composition recorded in detrital zircons of metasediments from the Aracuai Belt correspond with zircons from metasedimentary rocks and oceanic crust remnants of other orogenic belts to its south. This suggests that rifting and oceanic crust formation of the entire orogenic system, the so-called Mantiqueira Province, was contemporaneous, most likely related to the opening of a large ocean. It further indicates that the cratonic blocks involved in the orogenic evolution of the Mantiqueira Province were spatially connected as early as 900 Ma.

Zircon U-Pb age, Lu-Hf Isotopic Characteristics and Origin of the Banshanping Granitoid Rocks in East Qinling Orogenic Belt

The Banshanping granitoid rocks distribute in the east of the North Qinling orogenic belt.It is a diorite-quartz diorite-granodiorite-granite series,spreading in a NW-SE direction,and intrudes into the Erlangping Group.The SiO2 content ranges from 57.04% to 76.56%,Na2O from 2.05% to 4.65%,K2O from 0.84% to 3.40%.Major element characteristics indicate that Banshanping granitoid rocks have properties of I type granotoids.SREE ranges from 36.51 ppm to 473.25 ppm,and LREE/ HREE ratios lie between 3.95 and 22.18.Negative Eu anomalies are not obvious in most samples,though there are obvious Nb,P and Ti positive anomalies.The zircon LA-ICP-MS ages of Banshangping granitoid rocks are 496.0±8.1 Ma-486.9±9.3 Ma.Hf isotope shows that 176Hf/177Hf ratios range from 0.282721 to 0.282876,εHf（t） values from 8.5 to 14,all positive,and corresponding modal ages （TDM2） range from 559 Ma to 908 Ma.Based on Hf isotope characteristics and existing SmNd and Rb-Sr isotope data,we consider that the Banshanping granitoid rocks originate from mantlederived material,i.e.the igneous rocks that formed in Neoproterozoic,and there may be a certain amount of crust-derived material during the formation of Banshanping granitoid rocks.

Late Mesoproterozoic to Early Neoproterozoic Tectonic Evolution of the SW Yangtze Block, South China: Evidence from U-Pb Geochronology and Lu-Hf Isotopes of Detrital Zircons from Sedimentary Rocks

In situ zircon U-Pb geochronological and Lu-Hf isotope studies of detrital zircons from Late Mesoproterozoic to Early Neoproterozoic sedimentary units on the southwestern margin of the Yangtze Block have important implications for the tectonic evolution of the Yangtze Block.The Huili Group contains zircons whose ages are mainly Late Archean to Mesoproterozoic(2650-2450,2100-1800,and 1350-1150 Ma).The Dengxiangying Group has one major age population of 1900-1600 Ma,and two subordinate age populations of 1350-1100 and 2300-2000 Ma.Yanbian Group sedimentary rocks have a zircon age population mainly in the range of 970-850 Ma,contemporaneous with the ages of widespread arc-related magmatism in the western Yangtze Block.Combining these results with previous work,the Huili and Dengxiangying groups were most likely deposited during ca.1160 to 1000 Ma in an intra-continental rift basin setting,while the Yanbian Group accumulated during>920 to 782 Ma in a back-arc basin setting at the southwestern margin of the Yangtze Block.In addition,all these results further suggest a tectonic transition from a continental rift basin to a convergent environment at the southwestern margin of the Yangtze Block at 1000-970 Ma.

苏鲁地体胡家林和梭罗树三叠纪变质超基性岩U-Pb、Lu-Hf和O同位素地球化学研究

胡家林超基性岩单矿物氧同位素组成基本上接近正常地幔值（石榴石4．7‰～5．4‰）。梭罗树超基性岩及其共生榴辉岩具有氧同位素正异常（超基性岩中石榴石9.8‰～10．4‰；榴辉岩中石榴石9．0‰～12．1‰），指示超基性岩和榴辉岩原岩可能在板块俯冲之前，在浅层或地表环境中经历过蛇绿岩套顶部相似的低温热液蚀变。胡家林超基性岩样品中的变质锆石年龄227．2±2．4Ma，可能代表俯冲板块经峰期超高压变质后，折返初期从金刚石榴辉岩相降压至柯石英榴辉岩相过程中流体活动和锆石结晶的年龄。梭罗树超基性岩样品中变质锆石的一组变质年龄240．4±2．4Ma代表了板块俯冲至超高压峰期变质之前流体活动过程中锆石的结晶年龄，而另一组变质年龄217．1±3．3Ma代表了板块折返过程中高压榴辉岩相重结晶条件下的锆石结晶年龄。从变质锆石到重结晶锆石，其Th／U和^176Lu／^177Hf呈现出相似的变化趋势，可能指示了变质作用对锆石中的U-Th-Pb同位素体系和Lu-Hf同位素体系产生相似的地球化学效应。梭罗树超基性岩可能来源于俯冲地壳内部的“残留地幔”。胡家林超基性岩重结晶锆石中继承的放射成因铅或者来源于俯冲地壳与仰冲盘底部石榴辉石岩相地幔发生的壳幔相互作用而导致碎屑锆石的重结晶；或者来源于超基性岩岩浆在板块俯冲前，通过底侵作用上升侵位与地壳物质发生混染形成岩浆锆石，这种岩浆锆石在超高压变质过程中经历了重结晶。

Detrital Zircon Records of the Banxi Group in the Western Jiangnan Orogen:Implications for Crustal Evolution of the South China Craton

The Neoproterozoic evolution of the Jiangnan Orogen is important for understanding the tectonic history of South China.As a volcanic-sedimentary sequence developed in the Nanhua rift,the Banxi Group preserves the records of important magmatic and tectonic events linked to the assembly and breakup of the Rodinia supercontinent.In this study,we report the results from whole-rock major-and trace-element concentrations,with zircon LA-(MC)-ICP-MS U-Pb ages,trace elements and Lu-Hf isotopic compositions of sandstones from the Banxi Group.The rocks are characterized by high SiO_(2)(65.88%–82.76%,with an average of 75.50%)contents,moderate(Fe_(2)O_(3)^(T)+MgO)(1.81%–7.78%,mean:3.79%)and TiO_(2)(0.39%–0.54%,mean:0.48%),low K_(2)O/Na_(2)O(0.03–0.40,mean:0.10)ratios and low Al_2O_(3)/SiO_(2)(0.11–0.24,mean:0.15)ratios.The sandstones have highΣREE contents(mean:179.1 ppm),with chondrite-normalized REE patterns similar to the upper crust and PAAS,showing enriched LREE((La/Yb)_N mean:14.85),sub-horizontal HREE curves and mild Eu(Eu/Eu^(*):0.75–0.89,mean:0.81)negative anomalies.Their geochemical characteristics resemble those of passive continental margin sandstones.Most of the zircons are magmatic in origin and yield a U-Pb age distribution with three peaks:a major age peak at 805 Ma and two subordinate age peaks at 1990 Ma and 2470 Ma,implying three major magmatic sources.The Neoproterozoic zircons haveε_(Hf)(t)values ranging from-47.4 to 12.4(mostly-20 to 0),suggesting a mixture of some juvenile arc-derived material and middle Paleoproterozoic heterogeneous crustal sources.The Hf model ages of middle Paleoproterozoic zircons(~1990 Ma)with negativeε_(Hf)(t)values(-12.65 to-6.21,Ave.=-9.8)concentrated around the Meso-Paleoarchean(mean T_(DM)^(C)=3.3–3.1 Ga).For late Neoarchean detrital zircons(~2470 Ma),ε_(Hf)(t)values are divided into two groups,one with negative values(-9.16 to-0.6)with model ages of 3.5–2.9 Ga,the other featuring positive values(1.0 to 3.9)with model ages of 2.9–2.7 Ga,recording a crustal growth event at~2.5 Ga.Neoproterozoic zircons show volcanic arc affinities with partly intraplate magmatic features.We propose that the Banxi Group formed in a rift basin within a passive continental margin setting,which derived detritus from felsic to intermediate rocks from the Yangtze Block and a small amount of arc volcanic rocks.The middle Paleoproterozoic detrital zircon data suggest Columbia-aged basement lies beneath the western Jiangnan orogen.

Early Cenozoic diorite and diabase from Doumer Island, Antarctic Peninsula: zircon U-Pb geochronology, petrogenesis and tectonic implications

In order to understand the petrogenesis and tectonic setting of diorite and diabase units on Doumer Island, Antarctic Peninsula, this paper reports new laser ablation-inductively coupled plasma-mass spectrometry(LA-ICP-MS) zircon U-Pb, geochemical, and Lu-Hf isotopic data for the magmatism. The diorite and diabase samples yielded zircon U-Pb ages of 55.4 ± 0.3 and 52.8 ± 0.4 Ma, respectively. These samples are enriched in the large ion lithophile elements and the light rare earth elements, and are depleted in the high field strength elements. The zircons in these samples yield eHf(t) values from 9.03 to 11.87 and model ages(TDM2) of 342–524 Ma. The major, trace, rare earth element(REE), and Hf isotopic data for the diorites indicate that these units were formed by the mixing of magmas generated by(a) the partial melting of mantle wedge material that experienced fluid-metasomatism in a subduction zone setting, and(b) the melting of juvenile crustal material induced by the upwelling of mantle-derived magmas in a subduction–collision setting. The diabase units contain higher total REE concentrations than the diorite, indicating they were derived from a different source region. These samples also have higher Mg~# values and contain lower concentrations of Cr and Ni than the diorites, and have weakly negative Nb and Ta anomalies with Nb/Ta values of <3. The zircons in these samples yield eHf(t) values from 9.08 to 11.11 and model ages(TDM2) of 389–503 Ma. The major, trace, REE, and Hf isotopic compositions of the diabase units indicate that that they were derived from the mixing of depleted mantle-derived magmas with magmas generated by the melting of juvenile crustal material which was induced by the upwelling of the mantle into the crust. Overall the Cenozoic diorite and diabase on Doumer Island is related to subduction environment.

Chemical-Abrasion U-Pb zircon geochronology reveals 150 Myr of partial melting events in the Archean crust of the São Francisco Craton

Field observations and CA-LA-ICP-MS U–Pb zircon ages and Hf isotope compositions obtained from migmatitic orthogneisses and granitoids from the Belo Horizonte Complex,southern São Francisco Craton,indicate a major period of partial melting and production of felsic rocks in the Neoarchean.Our observations show that the complex is an important site for studying partial melting processes of Archean crystalline crust.Much of the complex exposes fine-grained stromatic migmatites that are intruded by multiple leucogranitic veins and sheeted dikes.Both migmatites and leucogranite sheets are crosscut by several phases of granitoid batholiths and small granitic bodies;both of which are closely associated with the host banded gneisses.Chemical abrasion followed by detailed cathodoluminescence imaging revealed a wide variety of zircon textures that are consistent with a long-lived period of partial melting and crustal remobilization.Results of U-Pb and Hf isotopes disclose the complex as part of a much wider crustal segment,encompassing the entire southern part of the São Francisco Craton.Compilation of available U-Pb ages suggests that this crustal segment was consolidated sometime between 3000 Ma and 2900 Ma and that it experienced three main episodes of partial melting before stabilization at 2600 Ma.The partial melting episodes took place between 2750 Ma and 2600 Ma as a result of tectonic accretion and peeling off the lithospheric mantle and lower crust.This process is likely responsible for the emplacement of voluminous potassic granitoids across the entire São Francisco Craton.We believe that the partial melting of Meso-Archean crystalline crust and production of potassic granitoids are linked to a fundamental shift in the tectonics of the craton,which was also responsible for the widespread intrusion of large syenitic bodies in the northern part of the craton,and the construction of layered mafic–ultramafic intrusions to the south of the BHC.

Fluid effect on zircon O and U-Pb isotopes during ultrahigh-pressure metamorphism:Insights from the Dora-Maira Massif of the Western Alps

Zircon geochemistry such as U-Pb and O isotopes have been widely used in dating and tracing complex geological processes.However,it still remains unclear how fluid action affects zircon geochemistry during metamorphic and metasomatic processes in subduction zones.Here a systematic study on zircon U-Pb dating,O isotopes and trace elements as well as whole-rock O isotopes was carried out for the coesite-bearing whiteschists,jadeite quartzites and granitic gneisses from the Dora-Maira Massif,Western Alps.Whole-rock and zircon geochemistry supports a common protolith,i.e.,Permian S-type granites,for the above three types of rocks and an intense fluid metasomatism during the Alpine orogeny to form whiteschists and jadeite quartzites.Zircon cores in all samples have nearly identicalδ^(18)O values(9‰–11‰),whereas their apparent^(206)Pb/^(238)U ages show a greater variability due to Pb loss during metamorphism.Zircon rims formed in the late Eocene to early Oligocene can be categorized into two types.Type-Ⅰrims occur in granitic gneisses and jadeite quartzites.They have highδ^(18)O values consistent with zircon cores,but much lower contents of P and Y as well as lower Th/U ratios than the cores.Their growth can be attributed to internal metamorphic fluid action at the UHP metamorphic stage.Type-II rims occur in whiteschists and jadeite quartzites.They have remarkably lowerδ^(18)O values(5‰–8‰)and Th/U ratios(<0.01),compared with zircon cores and Type-Ⅰrims.Their growth can be ascribed to external fluids during the metasomatic process.Some zircon domains in whiteschists and jadeite quartzites show a positive correlation betweenδ^(18)O values and apparent^(206)Pb/^(238)U ages,which suggest the simultaneous impacts on U-Pb-O isotopes during external fluid metasomatism.This process can be attributed to the fluidassisted dissolution and recrystallization of protolith zircons.Especially,coesite inclusions that would have been expected to occur only in young zircon rims formed during UHP metamorphism are also observed in the relict magmatic zircon cores,indicating that the fluid-related metasomatism at the UHP metamorphic conditions also affected these pre-existing protolith-related cores.Therefore,fluid action in subduction zones reveals significant impacts on both the U-Pb and O isotope systems of zircon,especially when external metasomatic fluids are involved.Therefore,a detailed study on zircon,including microstructure,mineral inclusion and geochemical data of different growth and recrystallization domains,is needed in order to unravel continental crustal evolution based on zircon U-Pb ages and O isotope compositions.

Zircon U-Pb Ages and Hf Isotopes of Neoproterozoic Meta-Igneous Rocks in the Liansandao Area,Northern Sulu Orogen,Eastern China,and the Tectonic Implications

The Sulu Orogen preserves the Neoproterozoic tectonic-magmatic events,corresponding to the breaking up of the Rodinia supercontinent.The ages and petrogenesis of meta-igneous rocks in the Liansandao area in the northern Sulu Orogen are not well-constrained.This study reports zircon U-Pb ages and Hf isotopes of these rocks from the Liansandao area.Three meta-igneous rock samples give similar weighted mean 206 Pb/238 U ages of 744±11,767±12,and 762±15 Ma,respectively,indicating the Neoproterozoic crystallization ages.These rocks formed coevally with the Wulian and Yangkou intrusions that located along the Yantai-Qingdao-Wulian fault zone.The Neoproterozoic ages indicate that the meta-igneous rocks from the Liansandao area have affinity to the Yangtze Block.The three samples haveεHf(t)values of-7.2–-10.5,-6.0–-17.5,and-6.8–-12.0,respectively.These negativeεHf(t)values indicate a primarily crustal source.However,the widely variousεHf(t)values that are higher than the continental crust,suggesting magma mixing between mantle-derived materials and the continental crust or source heterogeneity.Combined with the Hf model ages and geochemical characteristics,the monzodiorite(sample LSD-2)is most likely to be mantle-derived magma then interacted with ancient continental crust,and the granitic protolith(samples LSD-1 and LSD-3)in the Liansandao area might derive from the re-melting of a Paleoproterozoic continental crust at^750 Ma,resulting from the upwelling and underplating of mantle-derived magma formed in an extensional setting due to the break-up of the Rodinia supercontinent.

Detrital Zircon U-Pb Geochronology and Lu-Hf Isotopic Compositions of the Wuliangshan Metasediment Rocks in SW Yunnan(China)and Its Provenance Implications

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

Neoproterozoic Crustal Reworking and Growth in the Zhangbaling Uplift, Tan-Lu Fault Zone: Evidence from the Feidong Complex and Zhangbaling Group

Intensive mid-Neoproterozoic magmatism is the salient feature of the Yangtze Block,preserving abundant information about crustal reworking and growth.Zircon U-Pb-Lu-Hf isotope analysis was performed on material from the Feidong Complex(FDC)and Zhangbaling Group(ZBLG)of the Zhangbaling Uplift,in order to determine the age and magmatic source of the Neoproterozoic igneous rocks as well as the detrital provenance for the sedimentary rocks,to further provide important data for understanding the mid-Neoproterozoic crustal evolution of the Northeast Yangtze Block.The amphibolite and gneissic granites in the Feidong Complex(FDC)gave similar protolith ages of 782-776 Ma.The synmagmatic zircons exhibited variable negativeεHf(t)values of-26.9 to-8.3.Early(ca.2.4 Ga)to late Paleoproterozoic(ca.2.0-1.9 Ga)inherited zircons were found in the gneissic monzogranite,with negativeεHf(t)values of-11.2 to-7.2,indicating strong reworking of the ancient crustal materials of the Northeast Yangtze Block.Whereas the amphibolites represent minor crustal growth through emplacement of continental rifting-related mafic magmas.The quartz-keratophyres in the Xileng Formation of the ZBLG in contrast systematically yield young protolith crystallization ages of 754-727 Ma with highεHf(t)values of-2.0 to+5.6,indicating their derivation from the reworking of juvenile crustal materials.The detrital zircons from the metasiltstone in the Beijiangjun Formation yield variable^(206)Pb/^(238)U ages(871-644 Ma)with a peak age at 741±11 Ma andεHf(t)values of-4.3 to+5.3,which is consistent with those of the Xileng Formation,but distinct from the FDC,indicating that the provenance of the metasiltstone is primarily the underlying Xileng Formation.The mid-Neoproterozoic igneous and sedimentary rocks of the Zhangbaling Uplift were products from continental rifting zones along the northern margin of the Yangtze Block,situated in different positions from the Susong Complex and the Haizhou Group.The transition from ancient to juvenile crustal sources for felsic magmatic rocks is attributed to gradually increased crustal extension during continental rifting.

Evolution of the 3.65-2.58 Ga Mairi Gneiss Complex,Brazil:Implications for growth of the continental crust in the São Francisco Craton

The composition and formation of the Earth’s primitive continental crust and mantle differentiation are key issues to understand and reconstruct the geodynamic terrestrial evolution,especially during the Archean.However,the scarcity of exposure to these rocks,the complexity of lithological relationships,and the high degree of superimposed deformation,especially with long-lived magmatism,make it difficult to study ancient rocks.Despite this complexity,exposures of the Archean Mairi Gneiss Complex basement unit in the São Francisco Craton offer important information about the evolution of South America’s primitive crust.Therefore,here we present field relationships,LA-ICP-SFMS zircon U-Pb ages,and LA-ICP-MCMS Lu-Hf isotope data for the recently identified Eoarchean to Neoarchean gneisses of the Mairi Complex.The Complex is composed of massive and banded gneisses with mafic members ranging from dioritic to tonalitic,and felsic members ranging from TTG(Tonalite-Trondhjemite-Granodiorite)to granitic composition.Our new data point to several magmatic episodes in the formation of the Mairi Gneiss Complex:Eoarchean(ca.3.65–3.60 Ga),early Paleoarchean(ca.3.55–3.52 Ga),middle-late Paleoarchean(ca.3.49–3.33 Ga)and Neoarchean(ca.2.74–2.58 Ga),with no records of Mesoarchean rocks.Lu-Hf data unveiled a progressive evolution of mantle differentiation and crustal recycling over time.In the Eoarchean,rocks are probably formed by the interaction between the pre-existing crust and juvenile contribution from chondritic to weakly depleted mantle sources,whereas mantle depletion played a role in the Paleoarchean,followed by greater differentiation of the crust with thickening and recycling in the middle–late Paleoarchean.A different stage of crustal growth and recycling dominated the Neoarchean,probably owing to the thickening of the continental crust by collision,continental arc growth,and mantle differentiation.

Early Permian A-type Granites in the Zhangdaqi Area, Inner Mongolia, China and Their Tectonic Implications

There is a controversy regarding the amalgamation of Xing’an and Songnen Blocks along the Hegenshan-Heihe Suture(HHS)in the eastern Central Asian Orogenic Belt(CAOB).To solve this problem,we performed detailed study on the granites from the Zhangdaqi area,adjacent to the north of the HHS in the northern part of the Great Xing’an Range,NE China.Geochemically,the granites in the study area are metaluminous-weak peraluminous and high-K calc-alkaline series.Trace elements of the granites show that LREEs are relatively enriched,while HREEs are relatively deficient and obvious REE fractionation.The granites are characterized by obvious negative Eu anomalies,meanwhile,they are relatively enriched in Rb,K,Th and depleted in Ba,Nb,Sr,P,Ti.All the geochemical features suggest that the granites in the Zhangdaqi area are aluminum A-type granites.The zircon LA-ICP-MS U-Pb ages of these granites are 294-298 Ma,indicating that they formed in the Early Permian.These granites also have positiveεHf(t)values(8.4-14.2)and a relatively young two-stage model age between 449 Ma and 977 Ma,implying that the magma was derived from the re-melting of the Early Paleozoic-Neoproterozoic juvenile crust.Combined with geochemical characteristics(Nb/Ta ratios of 9.0-22.2,and Zr/Hf ratios of 52.3-152.0),we believe that the magmatic source area is a mixture of partial melting of the lower crust and depleted mantle.A-type granites and bimodal volcanic rocks along the Hegenshan-Heihe Suture formed during the Late Carboniferous-Early Permian,indicating that the HHS between Xing’an and Songnen Blocks closed in the late EarlyCarboniferous.Subsequently,the Zhangdaqi area was in a post-orogenic extensional environment from Late Carboniferous to Early Permian and resulted in the formation of the A-type granites.

Timing of Anatexis within the Berere HTHP Complex Belt of Maevatanana Area,North-Central Madgascar,and its Geological Significance

The Berere HTHP Complex belt in Maevatanana area of north–central Madagascar formed in the^2.5 Ga orogeny and underwent high temperature(up to 1050℃)and high pressure(up to 11.5 kbar)granulite facies metamorphism.Then a widespread anatexis took place and numerous widely distributed felsic leucosomes formed.The majority of these leucosomes are parallel to the schistosity of the complex or are present as stockworks,as thin layers,or as lenses at different scales in the host rocks.Here,we report new petrographic data,zircon LA-ICP-MS U-Pb ages,and Lu–Hf isotopic data for felsic leucosomes within this complex.Anatexis,as identified by the petrological study of felsic leucosomes in the field and in thin sections,involved initial ternary feldspar exsolving to produce antiperthite and a quartz+plagioclase±K-feldspar+sericite mineral assemblage around feldspar grain boundaries.Dissolution is apparent along muscovite grain boundaries,and residual sericite is present around the margins of feldspar and quartz,all suggesting that anatexis was driven by reactions involving muscovite.Zircon U–Pb dating indicates that the felsic leucosomes within the complex formed at 2467–2369 Ma.The majority of samples have positiveεHf(t)values,although a few have negative values,suggesting their formation from magmas predominantly sourced from the depleted mantle,possibly with the involvement of minor amounts of crustal materials.Two-stage Hf model ages andεHf(t)values for these samples are consistent with those for gneisses of the basement,indicating that the felsic leucosomes were formed by the anatexis of gneisses and both of their protolith formed during the formation of continental crust in Meso-Neoarchean(ca.3.1–2.7 Ga).As such,the crystallization age of the felsic leucosome(~2.4 Ga)represents the timing of regional anatexis and a change to post-orogenic tectonism.And this anatexis is also corresponds to the thermal event in Dharwar craton in India which has a pronounced similar Precambrian geology with Madagascar,providing an important constraints on the correlation of the two continental fragments.

Extensional Setting of Hainan Island in Mesoproterozoic:Evidence from Granitic Intrusions in the Baoban Group

The Mesoproterozoic Baoban Group is the oldest basement in Hainan Island and has played an important role in Columbia(Nuna)supercontinent reconstructions.The Mesoproterozoic granitic intrusions in the Baoban Group are the most widely-exposed Precambrian magmatic rocks and are the key to understanding the tectonic settings of Hainan Island and its relationship with the South China Block and the Columbia supercontinent.New LA-ICP-MS zircon U-Pb dating on three mylonitic granite samples from the Tuwaishan and Baoban areas yield ages ranging from 1447 Ma to 1437 Ma,representing the absolute timing of the emplacement of the granitic intrusions.Combined with previously published geochronological data for rocks from the Baoban Group and regional mafic intrusions,it is concluded that the Baoban Group formed at 1460-1430 Ma,coeval with the emplacement of the granitic and mafic intrusions.New in-situ zircon LuHf isotope analyses for the three mylonitic granite samples yielded positiveε(t)values,ranging from+0.49 to+8.27,with model ages(T~C)ranging from 2181 Ma to 1687 Ma,suggesting that the granitic intrusions originated from a mixed source of Paleoproterozoic crust with juvenile crust.New zircon trace element data show characteristics of high Th/U values of 0.24-1.50,steep slopes from LREE to HREE and negative Pr,Eu anomalies with positive Ce,Sm anomalies,representing typical magmatic zircons formed in continental crust.Compared with available magmatic and detrital zircon ages from Precambrian rocks in the Cathaysia Block,Yangtze Block and western Laurentia,it is inferred that Hainan Island was separated from both the Cathaysia Block and the Yangtze Block,instead being connected with western Laurentia in the Columbia supercontinent.Considering the decreasing tendency of basin deposition time along the western margin of Laurentia,it is proposed that Hainan Island was located to the north or northwest of the Belt-Purcell Supergroup,along the western margin of Laurentia,during the breakup of the Columbia supercontinent.

Multistage Formation of Neoarchean Potassic Meta-Granites and Evidence for Crustal Growth on the North Margin of the North China Craton

The North China Craton(NCC)is one of the most complex cratons in the world.It underwent a series of tectonothermal events during the Neoarchean-Paleoproterozoic.The petrogenesis of potassic granitoids,the timing,and the style of Archean crustal growth are still debated.Systematic field and petrological stdudies on the potassic meta-granites from the Guyang-Chayouzhongqi region were carried out.New U-Pb ages,zircon Lu-Hf isotopic analyses,and whole-rock geochemical data were obtained.Two groups(~2.7 Ga and~2.5 Ga)of potassic meta-granites were recognized.The~2.7 Ga meta-granites are mainly A2-type,with variableεHf(t)values(-8.4 to+3.3)and Archean one stage model ages(T_(DM)=~3.0 Ga),indicating that their source was derived from ancient anatectic TTG-like granite and depleted mantle,which suggests that thin crust had formed in the Guyang-Chayouzhongqi region by~3.0 Ga.Similar to the K-rich granites in the NCC,most of the~2.5 Ga potassic meta-granites are typical of A1-type granite,and are enriched in Sm and Gd and depleted in Nb,Ta,P,and Ti.The ages and isotopic data indicate that the~2.5 Ga meta-granites were generated from juvenile crustal sources with Neoarchean TTGs.The overall zircon U-Pb and Hf isotopic data furthermore suggested that the~2.7 Ga event is the most important stage of magmatic accretion in the NCC,similar to other cratons.In contrast,reworking or metamorphic alteration was the main crustal process in the NCC at~2.5 Ga.

Geochronology and Petrochemistry of Volcanic Rocks in the Xaignabouli Area, NW Laos

An integrated study of zircon U-Pb geochronology and petrochemistry, together with zircon Lu-Hf isotopes, has been carried out on the basaltic-andesitic tuff and volcanic breccia from the Nam Hang Formation and andesitic tuff from the Muang-Nan Formation in the Xaignabouli area, which had been mapped as the Permian–Early Triassic on the 1 : 1 000 000 geological map or Late Carboniferous on the 1 : 200 000 geological maps. Zircon U-Pb dating of three samples yielded weighted mean ages of 235±2.6, 232±1.4 and 278±2.8 Ma, respectively, suggesting a Late Triassic origin for the Nam Hang Formation and an Early Permian origin for the Muang-Nan Formation. Geochemically, they are characterized by depletions in HFSEs(e.g., Nb, Ta, Ti) and high LILE/HFSE ratios, and they have positive zircon ε_(Hf)(t) values of 8.7–15.9, which exhibits the continental arc volcanic affinity and partial melting of subducting oceanic slab in the magma source. Combined with spatial occurrence of the volcanic rock and existing geochronological and geochemical data, we suggest that the Xaignabouli-Luang Prabang volcanic belt can be linked to the Loei-Phetchabun belt. The Permian–Triassic volcanic rocks in this belt might be a product of the Nan back-arc basin eastward subduction.