Neoarchean Magmatic Events in the Western Songnen Massif,Central Asian Orogenic Belt:Timing and Tectonic Significance

Whether a Neoarchean basement existing in the Songnen massif is currently debated.Identification of Archean magmatism from the Songnen Massif is helpful to resolve this issue.Here,we report newly discovered Neoarchean Shanquan pluton in the Western Songnen Massif.These Neoarchean Shanquan pluton are mainly composed of granites that are exposed near the town of Shanquan in Heilongjiang Province.LA-ICP-MS zircon U-Pb dating reveals that the sample 2015TW1 has an upper intercept age of 2801±69 Ma and a weighted mean age of 2708±18 Ma,while samples LJ27QY1 and LJ27QY2 have upper intercept ages of 2677±57 Ma and 2653±18 Ma,and weighted mean ages of 2649±10 Ma and 2653±15 Ma,respectively.This indicates that these granites were formed at~2.7 Ga.Most of the~2.7 Ga zircons have older TDM2 ages of 2762–3326 Ma with positiveεHf(t)values ranging from 0 to 6.4,while a few of the zircons have negativeεHf(t)values ranging from-8.1 to-11.5 and older TDM2 ages varying from 3158 to 3264 Ma.The zircon Hf isotopes indicate that Paleo-Mesoarchean crusts might once existed in the Songnen Massif,and the studied Neoarchean magmas were principally derived from partial melting of these Paleo-Mesoarchean ancient crust.Based on the geochronological spectrum of magmatic and detrital zircons,the Songnen,Erguna,Jiamusi,Bureya massifs may have a common basement prior to the Neoproterozoic and may even be linked with the Triam Craton.

Neoarchean granite-greenstone belts and related ore mineralization in the North China Craton:An overview

Tectonic processes involving amalgamations of microblocks along zones of ocean closure represented by granite-greenstone belts(GGB) were fundamental in building the Earth's early continents. The crustal growth and cratonization of the North China Craton(NCC) are correlated to the amalgamation of microblocks welded by 2.75-2.6 Ga and ~2.5 Ga GGBs. The lithological assemblages in the GGBs are broadly represented by volcano-sedimentary sequences, subduction-collision related granitoids and bimodal volcanic rocks(basalt and dacite) interlayered with minor komatiites and calc-alkalic volcanic rocks(basalt, andesite and felsic rock). The geochemical features of meta-basalts in the major GGBs of the NCC display affinity with N-MORB, E-MORB, OIB and calc-alkaline basalt, suggesting that the microblocks were separated by oceanic realm. The granitoid rocks display arc signature with enrichment of LILE(K,Rb, Sr, Ba) and LREE, and depletion of HFSE(Nb, Ta, Th, U, Ti) and HREE, and fall in the VAG field. The major mineralization includes Neoarchean BIF-type iron and VMS-type Cu-Zb deposits and these,together with the associated supracrustal rocks possibly formed in back-arc basins or arc-related oceanic slab subduction setting with or without input from mantle plumes. The 2.75-2.60 Ga TTG rocks,komatiites, meta-basalts and metasedimentary rocks in the Yanlingguan GGB are correlated to the upwelling mantle plume with eruption close to the continental margin within an ocean basin. The volcanosedimentary rocks and granitoid rocks in the late Neoarchean GGBs display formation ages of 2.60-2.48 Ga, followed by metamorphism at 2.52-2.47 Ga, corresponding to a typical modern-style subduction-collision system operating at the dawn of Proterozoic. The late Neoarchean komatiite(Dongwufenzi GGB), sanukitoid(Dongwufenzi GGB and Western Shandong GGB), BIF(Zunhua GGB) and VMS deposit(Hongtoushan-Qingyuan-Helong GGB) have closer connection to a combined process of oceanic slab subduction and mantle plume. The Neoarchean cratonization of the NCC appears to have involved two stages of tectonic process along the 2.75-2.6 Ga GGB and ~2.5 Ga GGBs, the former involve plume-arc interaction process, and the latter involving oceanic lithospheric subduction, with or without arcplume interaction.

Neoarchean(2.5-2.8 Ga) crustal growth of the North China Craton revealed by zircon Hf isotope:A synthesis

The crustal growth of the North China Craton （NCC） during the Neoarchean time （2.5--2.8 Ga） is a hotly controversial topic, with some proposing that the main crustal growth occurred in the late Neoarch- ean （2.5--2.6 Ga）, in agreement with the time of the magmatism, whereas others suggest that the main crustal accretion took place during early Neoarchean time （2.7-2.8 Ga）, consistent with the time of crustal- formation of other cratons in the world. Zircon U-Pb ages and Hf isotope compositions can provide rigorous constraints on the time of crustal growth and the evolution and tectonic division of the NCC. In this contri- bution, we make a comprehensive review of zircon Hf isotope data in combination with zircon U-Pb geochro- nology and some geochemistry data from various divisions of the NCC with an aim to constrain the Neoarchean crustal growth of the NCC. The results suggest that both 2.7--2.8 Ga and 2.5-2.6 Ga crustal growth are distributed over the NCC and the former is much wider than previously suggested. The Eastern block is characterized by the main 2.7-2.8 Ga crustal growth with local new crustal-formation at 2.5-2.6 Ga, and the Yinshan block is characterized by ~2.7 Ga crustal accretion as revealed by Hf-isotope data of detrital zircons from the Zhaertai Group. Detrital zircon data of the Khondalite Belt indi- cate that the main crustal growth period of the Western block is Paleoproterozoic involving some ~ 2.6 Ga and minor Early- to Middle-Archean crustal components, and the crustal accretion in the Trans-North China Orogen （TNCO） has a wide age range from 2.5 Ga to 2.9 Ga with a notable regional discrepancy. Zircon Hf isotope compositions, coupled with zircon ages and other geochemical data suggest that the southern margin may not be an extension of the TNCO, and the evolution and tectonic division of the NCC is more complex than previously proposed, probably involving multi-stage crustal growth and subduction processes. However, there is no doubt that 2.7--2.8 Ga magmatism and crustal-formation are more widely distributed than previ- ously considered, which is further supported by the data of zircons from Precambrian lower crustal rocks, overlying sedimentary cover, modern river sediments and Late Neoarchean syenogranites.

Early Neoarchean Magmatic and Paleoproterozoic Metamorphic Events in the Northern North China Craton:SHRIMP Zircon Dating and Hf Isotopes of Archean Rocks from the Miyun Area,Beijing

The Miyun area of Beijing is located in the northern part of the North China Craton （NCC） and includes a variety of Archean granitoids and metamorphic rocks. Magmatic domains in zircon from a tonalite reveal Early Neoarchean （2752±7 Ma） ages show a small range in εHf（t） from 3.1 to 7.4 and tDM1（Hf） from 2742 to 2823 Ma, similar to their U-Pb ages, indicating derivation from a depleted mantle source only a short time prior to crystallization. SHRIMP zircon ages of granite, gneiss, amphibolite and hornblendite in the Miyun area reveal restricted emplacement ages from 2594 to 2496 Ma. They also record metamorphic events at ca. 2.50 Ga, 2.44 Ga and 1.82 Ga, showing a similar evolutionary history to the widely distributed Late Neoarchean rocks in the NCC. Positive eHf（t） values of 1.5 to 5.9, with model ages younger than 3.0 Ga for magmatic zircon domains from these Late Neoarchean intrusive rocks indicate that they are predominantly derived from juvenile crustal sources and suggest that significant crustal growth occurred in the northern NCC during the Neoarchean. Late Paleoproterozoic metamorphism developed widely in the NCC, not only in the Trans-North China Orogen, but also in areas of Eastern and Western Blocks, which suggest that the late Paleoproterozoic was the assembly of different micro-continents, which resulted in the final consolidation to form the NCC, and related to the development of the Paleo-Mesoproterozoic Columbia or Nuna supercontinent.

Late Neoarchean volcanic rocks in the southern Liaoning Terrane and their tectonic implications for the formation of the eastern North China Craton

The late Neoarchean metamorphosed volcanic rocks in the southern Liaoning Terrane(SLT) of the eastern North China Craton(NCC) are mainly composed of amphibolites and felsic gneisses and can be chemically classified as basalt(Group#1),basaltic andesite(Group#2),dacite(Group#3) and rhyodacite(Group#4).LA-ICP-MS zircon U-Th-Pb dating reveals that they formed at ~2.53-2.51 Ga.Group#1 samples are characterized by approximately flat chondrite-normalized rare earth element(REE) patterns with low(La/Yb)_N ratios and a narrow range of(Hf/Sm)N ratios,and their magmatic precursors were generated by partial melting of a depleted mantle wedge weakly metasomatized by subducted slab fluids.Compared to Group#1 samples,Group#2 samples display strongly fractionated REE patterns with higher(La/Yb)_N ratios and more scattered(Hf/Sm)N ratios,indicative of a depleted mantle wedge that had been intensely metasomatized by slab-derived melts and fluids.Group#3 samples are characterized by high MgO and transition trace element concentrations and fractionated REE patterns,which resemble typical high-Si adakites,and the magmatic precursors were derived from partial melting of a subducted oceanic slab.Group#4 samples have the highest SiO_2 and the lowest MgO and transition trace element contents,and were derived from partial melting of basaltic rocks at lower crust levels.Integrating these tholeiitic to calcalkaline volcanic rocks with the mass of contemporaneous dioritic-tonalitic-trondhjemitic-granodioritic gneisses,the late Neoarchean volcanic rocks in the SLT were most likely produced in an active continental margin.Furthermore,the affinities in lithological assemblages,metamorphism and tectonic regime among SLT,eastern Hebei to western Liaoning Terrane(EH-WLT),northern Liaoning to southern Jilin Terrane(NL-SJT),AnshanBenxi continental nucleus(ABN) and Yishui complex(YSC) collectively indicate that an integral and much larger continental block had been formed in the late Neoarchean and the craton-scale lateral accretion was a dominantly geodynamic mechanism in the eastern NCC.

Geochemical systematics of the Mauranipur-Babina greenstone belt,Bundelkhand Craton,Central India:Insights on Neoarchean mantle plume-arc accretion and crustal evolution

The Neoarchean Bundelkhand greenstone sequences at Mauranipur and Babina areas within the Bundelkhand Gneissic Complex preserve a variety of magmatic rocks such as komatiitic basalts, basalts,felsic volcanic rocks and high-Mg andesites belonging to the Baragaon, Raspahari and Koti Formations.The intrusive and extrusive komatiitic basalts are characterized by low SiO_2(39-53 wt.%), high MgO(18-25 wt.%).moderately high Fe_2O_3(7.1-11.6 wt.%), Al_2O_3(4.5-12.0 wt.%), and TiO_2(0.4-1.23 wt.%)with super to subchondritic(Gd/Yb)N ratios indicating garnet control on the melts. The intrusive komatiitic suite of Ti-enriched and Al-depleted type possesses predominant negative Eu and positive Nb, Ti and Y anomalies. The chemical composition of basalts classifies them into three types with varying SiO_2, TiO_2, MgO, Fe_2O_3, Al_2O_3 and CaO. At similar SiO_2 content of type Ⅰ and Ⅲ basalts, the type II basalts show slightly high Al_2O_3 and Fe_2O_3 contents. Significant negative anomalies of Nb, Zr, Hf and Ti, slightly enriched LREE with relatively flat HREE and low ∑REE contents are observed in type Ⅰ and Ⅱ basalts. TypeⅢ basalts show high Zr/Nb ratios(9.8-10.4), TiO_2(1.97-2.04 wt.%), but possess strikingly flat Zr, Hf, Y and Yb and are uncontaminated. Andesites from Agar and Koti have high SiO_2(55-64 wt.%), moderate TiO_2(0.4-0.7 wt.%), slightly low Al_2O_3(7-11.9 wt.%), medium to high MgO(3-8 wt.%) and CaO contents(10-17 wt.%). Anomalously high Cr, Co and Ni contents are observed in the Koti rhyolites. Tholeiitic to calc alkaline affinity of mafic-felsic volcanic rocks and basalt-andesite dacite-rhyolite differentiation indicate a mature arc and thickened crust during the advanced stage of the evolution of Neoarchean Bundelkhand greenstone belt in a convergent tectonic setting where the melts were derived from partial melting of thick basaltic crust metamorphosed to amphibolite-eclogite facies. The trace element systematics suggest the presence of arc-back arc association with varying magnitudes of crust-mantle interaction. La/Sm, La/Ta,Nb/Th, high MgO contents(>20 wt.%), CaO/Al_2O_3 and(Gd/Yb)_N > 1 along with the positive Nb anomalies of the komatiite basalts reflect a mantle plume source for their origin contaminated by subductionmetasomatized mantle lithosphere. The overall geochemical signatures of the ultramafic-mafic and felsic volcanic rocks endorse the Neoarchean plume-arc accretion tectonics in the Bundelkhand greenstone belt.

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 TiO_(2)(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 zirconε_(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 zirconε_(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.

Geochemical Characteristic of Charnockites in North Margin of North China Craton: Indicating the Significiant of the Neoarchean Tecnotic Event

The Neoarchean charnockites of North margain of North China Craton(NCC) has become a hot topic into understanding the Early Precambrian basement. Although there is a broad consensus that charnockite is usually related to granulite facies metamorphism, whether its petrogenesis and tectonics characteristics remains controversial. Inclusions within hypersthene and garnet in charnockite are used to identify the peak granulite facies mineral assemblage, with the formation of Magnesian-charnockite attributed to anatexis of the protolith associated with this granulite facies metamorphism. The distribution of major and trace elements in charnockite is very uneven, significant depleted in LILEs(eg. Cs, U, Th) and HFSEs(eg. Nb, Ta, P and Ti), riched in Sr. Raising to the coexistence of Eu-enrichment and Eu-depletion type of REE patterns that influenced by the content of plagioclase and the remnants minerals of zircon and apatite. Comparative the petrography, geochemistry and geochronology data of Magnesian-charnockite indicate that the ratios of mafic pellites and basalts involved in anatectic melting are different by the upwelling of mantle magma, also resulting in the Eu anormals characteristics. The formation of the Magnesian-charnockite is closely connected with the subduction of the NCC oceanic crust(About ~2.5 Ga). However, Ferroan-charnockite may be the formed by the crystallization differentiation of the upwelling of mantle-derived shoshonitic magma(About ~2.45 Ga), with the lower crust material addition.

P-T path of metamorphism and U-Pb monazite and zircon age of the Kitoy terrane: Implication for Neoarchean collision in SW Siberian Craton

The first data on P-T metamorphic conditions coupled with U-Pb monazite and zircon age obtained for the Neoarchean Kitoy granulite-gneiss terrane(SW Siberian Craton).Alumina gneisses of the Kitoy terrane indicate two-staged metamorphic evolution.The first stage of regional metamorphism(M1)occurred at high-amphibolite facies conditions at T=780-800℃ and P=8-9 kbar.The second stage(M2)belongs to MT-HT/LP type of metamorphism with the wide temperature interval 600-750℃ and pressure 2-4 kbar.Two age peaks were established on the basis of U-Pb monazite and zircon dating in garnet-anthophyllite gneisses.Both of them correspond to the Neoarchean age:the age of M1 falls into the interval of ca.2489-2496 Ma,the age of M2-ca.2446-2456 Ma.The high-temperature metamorphism of the Kitoy block and nearly coeval granitoid magmatism can be an evidence for the Neoarchean collision in SW Siberian craton.

Structural Relationships along a Neoarchean Arc-Continent Collision Zone, North China Craton

The Archean North China Craton is composed of the Western Block,Eastern Block and the intervening Central Orogenic Belt.A 4-10 km wide and 85 km long tectonic mélange belt informally called the Zanhuang tectonic

U-Pb-Hf isotopes of Neoarchean Shuimowan tonalites in southern margin of North China Craton: constraints on petrogenesis

This study presents zircon LA-ICP-MS U-Pb geochronology,trace elements characteristics,whole rock geochemistry composition and zircon Hf isotopic data of Shuimowan tonalites in Dengfeng City.The results show that zircons from Shuimowan tonalites have oscillatory growth zoning,relatively high Th/U ratios(0.8--1.7)and upturned zircon rare earth elements(REE),indicating that they are magmatic origin.Shuimowan tonalites yield a weighted mean 207 Pb/206 Pb age of 2522±9 Ma,indicating they were formed in the latest Neoarchean.The tonalites are characterized by relatively low SiO 2 contents,high MgO and Na 2O contents,high REE(∑REE=322×10-6-354×10-6),obvious fractionation of REE,enrichment in LREE and feeble negative Eu anomaly(Eu/Eu*=0.97--0.99).The samples are also enriched in large ion lithophile elements and depleted in high field strength elements,with negative Nb,Ta,P and Ti anomalies.TheεHf(t)values of zircon grains from Shuimowan tonalites range between 3.5 and 5.3.Combined with regional tectonic evolution,it is suggested that Shuimowan tonalites were derived from partial melting of mantle peridotite metasomatized by the slab-derived felsic melt and formed in the arc magma tectonic setting related to plate subduction.Research on the Neoarchean Shuimowan tonalites provides a constraint for the study of the Pre-Cambrian tectonic evolution in the southern margin of North China Craton.

A Deep Mantle Source for the Late Neoarchean Metamorphosed Basalts in Eastern Hebei,North China Craton:Insights from Whole-Rock Geochemistry and Sm-Nd Isotopes,and Zircon U-Pb-Hf Isotopes

The basalts within the greenstone belt worldwide serve as an ideal target to decipher the nature of Archean mantle sources and further to extend the understanding of the early stages of Earth's evolution.To provide important insights into the issues,we carried out a detailed investigation of whole-rock geochemistry and Sm-Nd isotopes,and zircon U-Pb-Hf isotopes for the Late Neoarchean metamorphosed basalts in eastern Hebei,North China Craton.U-Pb isotopic dating using the LA-ICPMS on zircons reveals that the basalts in eastern Hebei erupted at ca.2.48-2.51 Ga and subsequently experienced multiple regional metamorphic events at 2477 and 1798 Ma,respectively.The metamorphosed basalts are featured by low SiO_(2),MgO,K_(2)O+Na_(2)O,and high Fe O contents,endowed with the subalkaline and high-Fe tholeiitic affinities.The radiogenic initial Nd and Hf isotope values and correlations among V,Ni and Cr contents strongly imply that the basalts experienced significant clinopyroxene and olivine fractionation and minor crustal contamination during magma evolution.They are also characterized by the relatively low total REE contents and exhibit significant depletions to moderate enrichments in the LREE contents,indicating the derivation from a deep mantle source in an Archean proto-mantle plume setting.

华北克拉通新太古代晚期岩浆作用:对构造体制和克拉通化的启示

在对华北克拉通太古宙基底作简要介绍的基础上,本文总结了华北克拉通新太古代晚期(主要为2.55~2.5 Ga期间)岩浆岩的锆石年龄分布模式、地球化学和Nd-Hf-O同位素组成特征。华北克拉通新太古代晚期变质基底的主要特征如下:(1)新太古代晚期岩石在华北克拉通广泛分布,但许多地区都存在中太古代晚期—新太古代早期地质记录。(2)岩浆锆石年龄主要变化于2.55~2.5 Ga,年龄峰值约为2.52 Ga。(3)与新太古代早期以前(>2.6 Ga)的英云闪长岩-奥长花岗岩-花岗闪长岩(TTG)相比,新太古代晚期英云闪长岩和花岗闪长岩比例明显增大。富钾花岗岩、闪长-辉长岩(包括闪长岩、辉长岩及两者之间过渡岩石)、赞岐岩(Sanukite,主体为富镁闪长岩)分布范围和规模也明显增大。大规模富钾花岗岩主要分布于华北克拉通东部,构成新太古代晚期双岩浆岩带的富钾花岗岩带。(4)在新太古代晚期变质基底分布区,几乎都有变质表壳岩存在。它们以较小规模存在于TTG和富钾花岗岩中。岩石类型包括变玄武质岩石、变质安山质-英安质火山岩/火山碎屑岩和变质碎屑沉积岩。一些地区存在变质超基性岩。(5)总体上,新太古代晚期地质事件存在如下演化规律:首先是表壳岩形成,然后是TTG侵入,最后是变质变形和富钾花岗岩形成。2.6~2.55 Ga为华北克拉通岩浆构造的“寂静期”。(6)新太古代晚期TTG岩石的Sr/Y和La/Yb比值存在很大变化,中-高压TTG岩石大量形成表明新太古代晚期陆壳规模、厚度发生了明显增大。至少部分富钾花岗岩在形成过程中有沉积物参与。(7)不同类型TTG岩石具有类似的全岩Nd同位素和岩浆锆石Hf同位素组成,Nd-Hf同位素亏损地幔模式年龄主要分布在3.0~2.5 Ga,与中太古代晚期—新太古代早期岩石的模式年龄相近或稍偏年轻。富钾花岗岩Nd-Hf同位素组成特征受物源区早期形成演化历史制约。岩浆锆石O同位素组成与全球太古宙岩浆锆石类似,但显示更大的变化范围。结合其他研究,上述证据表明:(1)中太古代晚期—新太古代早期是华北克拉通陆壳增生最重要时期,这与全球其他许多克拉通类似,不同之处在于华北克拉通遭受了新太古代晚期构造岩浆热事件强烈改造;(2)在华北克拉通,类似于现代板块运动的构造体制在新太古代晚期开始启动;(3)规模最大的BIF(条带状铁建造)沿华北克拉通东部古老陆块西缘的双岩浆岩带分布,鞍本和冀东之间、冀东和鲁西之间是BIF找矿重要靶区;(4)华北克拉通在新太古代晚期完成初始克拉通化。

辽宁本溪地区新太古代晚期钾质花岗岩的发现及其地质意义

辽宁本溪地区位于华北克拉通东部陆块东北部,区内广泛发育新太古代深成侵入岩和变质表壳岩,尚无钾质花岗岩的报道。钾质花岗岩的形成过程是早期陆壳发育成熟和稳定的重要标志,本次钾质花岗岩的发现对系统的研究华北克拉通早期陆壳的形成与演化具有重要的价值。本文对本溪地区马家沟的钾质花岗岩进行了岩石学、成岩年代学、岩石地球化学的研究,探讨了岩石成因和形成的构造背景。马家沟岩体为片麻状中细粒黑云母二长花岗岩。锆石U-Pb测年数据显示,岩石^(207)Pb/^(206)Pb加权平均年龄为(2490±21)Ma,形成于新太古代晚期。该岩体属弱过铝质、高钾钙碱性系列钾质岩石;主要富集高场强元素La、Zr、Th及大离子亲石元素K、Nd等元素,亏损P元素、高场强元素Nb和大离子亲石元素Sr、Ti等;REE曲线为右倾型,负Eu异常,属于S型花岗岩。研究区钾质花岗岩岩浆主要来源于地壳,是变质泥岩部分熔融的产物,形成于活动大陆边缘碰撞造山后的伸展环境,代表华北太古宙克拉通固结基底形成之前的最后一次岩浆活动,标志着在新太古代晚期本溪地区微陆块已拼贴完成,并与其他地区一起构成稳定的华北太古宙克拉通。

Geochemistry of khondalites from the central portion of North China craton (NCC):implications for the continental cratonization in the Neoarchean

Within the high-grade metamorphic basement, the central portion of North China Craton (NCC), a group of Neoarchean khondalites (KS) is identified. They are characterized by large ion lithophile elements (LILE) enrichment, lower abundances of Zr, Hf and Sr. Their rare earth element (REE) distribution has significant LREE enrichment and negative Eu anomalies. The protoliths of KS are interpreted as feldspathic quartzite, shale or petite and carbonite, deposited in a shallow sea upon cratonic shelf distant from the land. KS’s source region might be dominated by granitic rocks, with a minor amount of TTG, underwent comparatively severe chemical weathering. Considering relevent tectonic constraints, we suggest that khondalites from central portion of NCC, an important metamophosed sedimentary cover, are the most significant exogenetic marker of Neoarchean continental cratonization for NCC.

Discovery of Neoarchean unconformity and its implication for continental cratonization of North China craton

An ancient unconformity event is identified within the granulite-facies province of North China, which is marked by the boundary between the overlying khondalite series (KS) and the basement grey gneiss complex (BGGC). REE patterns of KS suggest that their protoliths aM mature sediments. As documented by isotopic ages and metamorphic P-T-t paths, KS is originally deposited after the basement complex formed. Furthermore, the BGGC was overprinted by the metamorphic event associated with the KS. The binary structure of KS-BGGC is well preserved in the western part of North China craton (NCC), and it is structurally truncated and overlain by the Palaeoproterozoic Wntai rift, which suggests that the unconformity occurred in the Neoarchean, and represents the major tectono-thermal episodes of the Neoarchean continental cratonization of NCC.

Tectonothermal Records in Migmatite-Like Rocks of the Guandi Complex in Zhoukoudian, Beijing: Implications for Late Neoarchean to Proterozoic Tectonics of the North China Craton

Migmatite-like rocks transformed from strongly metamorphosed and deformed enclave- bearing felsic plutons usually make people confuse with the true migmatites and mistake in interpreta- tion of their petrogenesis and tectonic implications. Here we report a suite of rocks that have long been called as migmatites from the Guandi complex in Zhoukoudian region, southwest of Beijing. The rocks are dominated by felsic gneisses with garnet-free amphibolites. Field occurrence, petrography and geochemistry indicate that the felsic gneisses and amphibolites were metamorphosed from protoliths of intermediate-acid and basic igneous rocks, respectively. New LA-ICP-MS zircon U-Pb dating and geothermobarometry study further reveal that precursor magmas of the two types of rocks were emplaced at 2.54-2.56 Ga and the rocks subsequently underwent medium P/T-type metamorphism with upper amphibolite facies conditions of 0.55-0.90 GPa and 670-730℃ at -2.48-2.50 Ga. Geochemically, precursor magmas of the amphibolites were suggested to be derived from an enriched lithospheric mantle source in continental arc setting, and those of the felsic gneisses are characterized by tonalitic to trondhjemitic magmas that are usually considered to be generated by partial melting of hydrated, thickened metamorphosed mafic crust with garnet as residues, suggesting that the rock associations are not of true migmatites but migmatite-like rocks. Our study reveal that protoliths of the migmatite-like rocks from the Guandi complex, were likely formed via magmatism in a continental arc setting, followed by accretion and collision of the continental arc as well as the intro-oceanic arc terranes to the Eastern Block of the North China Craton in the transition from the Late Neoarchean to Early Paleoprnterozuic.

华北克拉通东南缘蚌埠隆起新太古代晚期片麻状二长花岗岩的年龄、成因及构造意义

华北克拉通东南缘前寒武纪下地壳经历幕式生长与多期改造,在新太古代晚期发生了大规模的构造热事件,并伴随新生陆壳增生。对华北克拉通东南缘蚌埠隆起新发现的片麻状二长花岗岩开展锆石U-Pb定年和岩石地球化学分析,探讨其成因及构造意义。结果表明,片麻状二长花岗岩的锆石发育明显的核幔结构和震荡生长环带,Th/U比值高,为岩浆成因锆石,其LA-ICP-MS U-Pb年龄为(2557±24)Ma,指示其形成时代为新太古代晚期;岩石具有高SiO_(2)(69.03%~70.05%)、高Al_(2)O_(3)(14.71%~15.63%)、低MgO(0.92%~1.43%)特征,K_(2)O/Na_(2)O比值为0.95~1.06,铝饱和指数为1.08~1.12,属于弱过铝质高钾钙碱性岩石;岩石富集Rb、K、Ba等大离子亲石元素,亏损Nb、Zr、P、Y等高场强元素,稀土元素含量较低(∑REE=81.25×10^(-6)~123.74×10^(-6)),轻重稀土元素强烈分异((La/Yb)N比值为52.71~72.13),显示Eu正异常(δEu=1.36~2.40),综合说明其属于中下地壳部分熔融形成的I型花岗岩。蚌埠隆起新太古代晚期片麻状二长花岗岩的发现,表明该地区在该时期发生过强烈的构造热事件,契合华北克拉通地壳生长和演化历史,佐证了2.5 Ga是华北克拉通陆壳生长重要阶段。

辽西勿兰乌苏磷矿床地质特征及控矿因素

勿兰乌苏磷矿是辽宁省西部重要的磷矿床之一,资源储量达到大型规模,同时伴生有益组分(磁铁矿).磷矿体赋存于新太古界小塔子沟岩组,控矿因素主要为绿岩型含磷磁铁建造.矿石矿物组合为磷灰石+磁铁矿+钛磁铁矿+角闪石+黑云母+斜长石.磷灰石含量与铁钛锰等暗色组分含量呈正相关,显示了磷的亲铁性.矿床成因属于变质型磷灰石矿床.

辽西新太古代片麻岩金矿中金与伴生元素的多重分形模型剖析

新太古代片麻岩在辽西地区分布面积广阔,其中发育的金矿矿产资源丰富。为了研究辽西地区新太古代片麻岩金矿中共伴生元素对于Au元素成矿的重要性,本文以辽西地区新太古代片麻岩中典型金矿床及矿化点矿体的组合分析数据为样本,运用多重分形理论进行Ag、Cu、Pb、Zn、S元素质量分数与金元素质量分数的相关性分析,进而建立了基于多重分形理论的金与共伴生元素多重分形模型。根据多重分形的拟合分析结果显示,Au、Cu、Pb、Zn和S元素的拟合曲线分布状态均呈明显的下凸型,其均匀程度介于正态分布和指数分布之间,广义分维值均大于1.3,说明它们之间存在明显的相关性。而银元素的拟合曲线分布状态明显不同于其它元素,说明银元素的富集不受或受到其它元素的影响很小。结合元素亲和性特征,以硫元素为核心,分析了Au、Ag、Cu、Pb、Zn的亲硫性,并将新太古代片麻岩金矿中金的演化过程划分成四个阶段,分别为①黄铁矿富集阶段,②自然金伴生阶段,③交代充填阶段,④变质氧化阶段。根据多重分形模型的拟合分析结果,并结合前人的理论研究成果表明:新太古代片麻岩金矿中共伴生元素组合对于金元素成矿极为重要,它为后期找矿的方向及隐伏矿床的发现提供了重要的指示。