Geochronology and geochem istry of the f ivemagmatic rocks in the Ningzhen region,China

Shunfu Lu·Xiaoqing Zhu·Xiaofen Li

Abstract The Ningzhen region of China is located in the easternmostpartof them iddle-lower Yangtze River Cu—Fe polymetallic metallogenic belt.Fromwest to east,it comprises f ive main intermediate—acidic intrusive complexes:the Qilinmen,Anjishan,Xiashu—Gaozi,Shima,and Jianbi complexes.Geochemical investigations show that these f ive intrusive complexes exhibit high contents of SiO2,at64.74—73.40w t%,A l2O3,at14.15—17.37w t%,and K2O+Na2O,at 6.49—8.68 w t%.The majority of the samples belong to the high-K calc-alkaline series,w ith a few samples plotting in the calc-alkaline and tholeiitic series.Trace element analysis shows that the samples are enriched in large ion lithophile elements(LILE)and are depletedinhighf ieldstrengthelements(HFSE).The chondrite-normalized rare earth element(REE)patterns are characterized by right-inclined curves,show ing light rare earth element(LREE)enrichment.In addition,the(La/Yb)N ratios are high at 15.02—37.28,w ith an average of 29.13,and slightly negative or none Eu anomalies are present.In the(La/Yb)N—δEu diagram,the samples plot w ithin the crust-mantle type f ield.Laser ablation—inductively coupled plasma—mass spectrometry(LA—ICP—MS)zirconU—Pbdatingyieldedages of 122.0±1.0Ma,106.1±0.8Ma,108.7±1.4 Ma,103.5±1.9 Ma,and 96.8±1.7 Ma for the Qilinmen,Anjishan,Xiashu—Gaozi,Shima,and Jianbi complexes,respectively.On the basis of this research and know ledge of several known metal deposits related to these complexes,we suggest that theMesozoic large-scale diagenesis and metallogenesis in the Ningzhen region may have ceased at 100 Ma or about95 Ma.

Keywords Intermediate—acidic intrusive complexes·LA—ICP—MS zircon U—Pb dating·Geochem istry·Geodynam ic setting·Ningzhen region

1 Introduction

The middle—lower Yangtze River belt constitutes one of the most important metallogenic belts in China.This metallogenic belt hosts a large number of polymetallic deposits and includes several main types of ore systems including Cu—Mo porphyry,Cu—Fe skarn,and strata-bound deposits(Pan and Dong 1999;Mao etal.2006,2011).The Ningzhen region in Jiangsu Province is located in the easternmost part of themiddle-lower Yangtze River belt,where Mesozoic intermediate—acidic intrusive complexes areextensivelydeveloped.Theseintermediate—acidic intrusive complexes,listed in succession from west to east,form the Qilinmen,Anjishan,Xiashu—Gaozi,Shima,and Jianbi intrusive complexes(Fig.1).Many previous petrological and geochem ical studies have been performed on the complexes of the Ningzhen region(Mao and Zhao 1990;Xu et al.2001).In recent years,studies on the Qilinmen(Chen etal.2017),Anjishan(Zeng etal.2013;Liu etal.2014;Wang etal.2014a,b;Guan etal.2015),Gaozi(Sun et al.2013),Shima(Sun et al.2013;Guan et al.2015),and Jianbi(Chen et al.2017)complexes haveobtained highly precise and accurate ages.In this study,we use laser ablation—inductively coupled plasma—mass spectrometry(LA—ICP—MS)zircon U—Pb dating of the f ive main intermediate—acidic intrusive complexes combined w ith known highly precise and accurate ages of the other metal deposits in this region to constrain the age of the large-scalem ineralization in this region.The geochronology is the key to understanding thegeodynam ic setting and tectonic evolution as well as the Mesozoic metallogenic processes and is used to conf irm the relationship of the ore deposits and intrusivemagmatic activity in this region.By analyzing the major and trace element contents in the complexes,this paper presents prelim inary discussions on the regionalmagma sources and evolution processes that occurred in the Ningzhen region and offers an in-depth discussion on the regional geodynam ic setting and its geological signif icance.

Fig.1 a Tectonicmap and b distribution of Mesozoic complexes in the Ningzhen region(Xia 2000;Zeng etal.2013).1.Quaternary.2.Upper Cretaceous.3.Cretaceous Yangchong Formation.4.Cretaceous Shangdang Formation.5.Sinian to Jurassic.6.Diorite.7.Diorite porphyrite.8.Granodiorite.9.Quartz diorite.10.Quartz diorite porphyrite.11.Quartz monzonite.12.Conjectural and actual fractures.13.Lithofacies boundary.14.Sampling location.15.Ningzhen region

2 Geological setting

TheNingzhenregionis locatedalongtheeastern boundary of the North China Craton at the northern margin of the Yangtze Block,where Yanshanian magmatism is extensively developed(Fig.1a).The stratigraphy in this region is divided into three units(Bureau of Geology and M ineral Exploration of Jiangsu Province 1989):early and m iddle Proterozoic to Sinian basement strata that have undergone slightm igmatization and lowgrademetamorphism,SiniantoTriassicsedimentary cover comprising amarine to terrestrial facies succession,and Jurassic to Cretaceous superimposed cover comprising a thick terrestrial succession of red clastic and volcanicintrusiverocks.Thestructural frameworkis dom inated by folds and faults and is comparatively complex in this region.The compound fold structure consists of a series of parallel folds w ith E—W axes and has an E—W orientation.The fault structures are characterized mainly by E—W thrust faults on both sides of the anticline and include NW-and NE-trending conjugate shear faults.Magmatic activity in the Ningzhen region occurred mainly in the Yanshanian followed by the Himalayan,which included only a small amount of basic magmatic activity.The regionalmagmatic activity in the Yanshanian was intense as is characterized by multi-stage and multi-period evolution.The intrusive rocks are located mainly along the fault zones and in the cores of the folds,and their lithology ismainly intermediate—acidic.

3 Sam ples and petrography

The Mesozoic intermediate—acidic intrusive complexes are extensively developed in the Ningzhen region(Fig.1b).The Qilinmen complex covers an area of～14 km2trending NNW and dipping NEE.Themain rock types in the Qilinmen complex are granite porphyry,diorite porphyrite,and quartz diorite porphyry.The granite porphyry samples,whichwerecollectedinNiutoushanat 32°03′44.3′′Nand118°55′42.′′E,have not undergone weathering.They contain a few melanocraticm ineralsand exhibit a porphyritic texture and blocky structure.The phenocryst composition is 15—20%plagioclase,8—10%quartz,and 4—6%K-feldspar.The matrix has a granitic m icrostructure and consists of K-feldspar,plagioclase,quartz,and a small amount of biotite.The accessory m inerals include zircon,magnetite,and apatite(Fig.2a).

The Anjishan complex has been the subjectof previous geochemical studies(Xu et al.2001,2002;Zeng et al.2013).Itsmain rock types are granodiorite porphyry andquartz diorite porphyrite.The studied granodiorite porphyrysamples,situatedat32°06′19.4′′Nand 119°04′40.3′′E,exhibit no weathering,and they have a porphyritic texture and blocky structure.The phenocrysts composition is 10—15%plagioclase,5—8%hornblende,2—5%K-feldspar,and 1—3%quartz.The matrix has a granitic m icrostructure and consists of K-feldspar,plagioclase,quartz,and a small amountof biotite.The accessory minerals include sphene,zircon,and apatite(Fig.2b).

Fig.2 Representative photomicrographs of the f ive intrusive rocks from the Ningzhen region.Pl plagioclase,Kfs potash feldspar,Qtz quartz,Am amphibole,Bt biotite

The main rock types in the Xiashu—Gaozi complex include quartz diorite porphyrite,granodiorite,monzonitic granite,quartz diorite,and quartz monzonite.The quartz diorite porphyrite samples,situated at 32°09′42.9′′N and 119°07′10.7′′E,exhibit slight alteration and have a porphyritic texture and blocky structure.The phenocrysts are dom inated by plagioclase,followed by hornblende,and biotite.Thematrix has an anhedral to subhedral structure and consists of quartz,hornblende,biotite,and a small amount of pyroxene.The accessory m inerals include apatite,sphene,and magnetite(Fig.2c).

The Shima complex is located to the southwest of Zhenjiang City and covers an area of～34 km2;itsmain rock types are granodiorite porphyry,diorite porphyrite,and syenite granite.The granodiorite porphyry samples,situated at32°08′14.9′′N and 119°17′40.7′′E,exhibit slight alteration and have a porphyritic texture and blocky structure.The phenocrysts are dom inated by plagioclase and K-feldspar,followed by hornblende,and biotite.The matrixhas a crystal microstructure andconsists of K-feldspar,plagioclase,quartz,and small amounts of hornblende and biotite.The accessory minerals include apatite,sphene,zircon,andmagnetite(Fig.2d).

The Jianbicomplex trends E—W and is located in Jianbi Town,Zhenjiang City.The bedrock area is～46 km2.Themain rock types in the Jianbicomplex aremonzonitic granite,quartz diorite porphyry,and quartz monzonite porphyry.Themonzonitic granite isgenetically related to a W—Mo deposit.The samples,situated at32°10′38.3′′N and 119°33′06.4′′E,exhibit no weathering and have a hypidiomorphic granular texture and blocky structure.Themain minerals are 30—40%plagioclase,25—30%K-feldspar,and 15—20%quartz.Themelanocraticm inerals are hornblende and biotite,each at 5%.The accessory m inerals include sphene,zircon,and apatite,w ith a total content of～5%(Fig.2e).

4 Analyticalmethods

The zircon crystals were separated by using the conventional heavy liquid and magnetic techniques described in detail by Dube′et al.(1996).The original rock samples were crushed into powder.Then,heavym ineral separation was performed using an electromagnetic separator,and a binocularm icroscopewas used to select zircon grainsw ith good crystallinity and transparency(Romeo et al.2006).The zircons were set in an epoxy mount,which was polished and buffed.Prior to LA—ICP—MS zircon U—Pb isotope analysis,the morphology and internal structures of these zircons were studied using a binocular m icroscope and cathodo luminescence(CL)imaging to determ ineappropriate points for theanalysis.The CL observation and photography aswell as LA—ICP—MS zircon U—Pb isotope analysiswere conducted at the State Key Laboratory ofOre Deposit Geochem istry(SKLODG),Institute of Geochemistry,Chinese Academy of Sciences.Laser sampling was performed by using a GeoLas Pro 193 nm ArF excimer laser.An Agilent7500×ICP—MS instrumentwasused to acquire ion-signal intensities.He was applied as a carrier gas and wasmixed w ith Argon via a T-connector before entering the ICP—MS.Each analysis incorporated a background acquisition of approximately 30 s gas blank followed by 60 sof data acquisition from the sample.Off-line selection and integration of background and analyte signals,time-drift correction,and quantitative calibration for traceelementanalysesand U—Pb datingwere performed by using ICPMSDataCal software(Liu etal.2008,2010a,b).Zircon 91500 was used as the external standard for U—Pb dating and was analyzed twice every six to eightanalyses(i.e.,two zircon 91500+six—eight samples+two zircon 91500).Uncertainty of the preferred values for the external standard 91500 was propagated to the f inal results of the samples.Concordia diagrams and weighted mean calculations were developed by using the Isoplot program(Ludwig 2003).Because207Pb/206Pb is sensitive to the common Pb corrections,the206Pb/238U age is generally used for rocks younger than about1000 Ma(Black etal.2003).No common Pb correctionwasapplied to the samplesow ing to the very low204Pb countsand itspooranalytical precision.The trace amount of common Pb did not affect the206Pb/238U ages.Therefore,206Pb/238U ages,which are precise and stable,wereused to calculate the crystallization ages for the samples.

Fresh parts of all original rock samples were carefully selected to avoid the weathered sections,and the samples were then were crushed to 200-mesh size.Themajor elementabundances in whole rockswere determ ined on fused glass discs by using an AXIOS-PW 4400 X-ray f luorescence(XRF)instrument at SKLODG,Institute of Geochem istry,Chinese Academy of Sciences.The accuracy wasmonitored by using Chinese national standard GSR-3.The accuracy for major elements was better than±2%.The loss-on-ignition(LOI)was determ ined by the weight loss of a powered sample after 1 h at 1000°C(Sun et al.2017a,b,c).

The trace element concentrationswere analyzed using a Perkin-Elmer Sciex Elan DRC-e quadrupole ICP—MS at SKLODG.For each sample,50mg of powder was dissolved using am ixture of HF and HNO3in a Tef lon bomb for 48 h at 190°C.Rh was used to monitor signal drift during data acquisition.International standards GBPG-1 and OU-6 and ChinesenationalstandardsGSR-1 and GSR-3 were used to monitor accuracy.The difference between our results and the recommended values for the standards was generally better than±10%(Sun et al.2017a,b,c).

5 Results

The resultsof LA—ICP—MS zircon U—Pb dating of the f ive typical complexes from the Ningzhen region,including common Pb correction,are shown in Table 1.The errors foreach data analysispoint,also listed in Table 1,were all 1σ.The CL imagesare shown in Fig.3,and the concordia plots are shown in Fig.4.

The zircon grains from the Qilinmen granite porphyry are mostly large,long-columnar,and idiomorphic with lengths of～110—200μm and w idths of～50—80μm.Their internal structure was largely uniform and clear and showed well-developed oscillatory zoning,which is characteristic ofmagmatic zircons(Fig.3a).The contentsof Th and U were high,at 338—1187 ppm and 447—1482 ppm,respectively.The ratio of Th/U ranged between 0.40 and 0.89(Table 1),slightly larger than 0.50,w ith an average of 0.64(n=15).Positive correlation is present between the Th and U,which is also consistentw ith the general characteristics ofmagmatic zircons(Hoskin and Black 2000).The dates of 15 pointswere in agreementw ith each other,show ing no large age f luctuations.This indicates that the U—Th—Pb isotopic system of the zircons was not affected by latermagmatic activity,tectonic activity,ormetamorphism and that the system was closed.The 15 data points were clustered in the concordia plots(Fig.4a),and the weightedaverageagewas 122.0±1.0 Ma(n=15,MSWD=0.15),which dates the emplacement age of the Qilinmen complexes to the Early Cretaceous.

The zircon grains from the Anjishan granodiorite porphyry weremostly large,long-columnar,and idiomorphic,although a few were incomplete;their lengths and widths were～130—190μmand～40—70μm.Their internal structure was largely uniform and clear and showed welldeveloped oscillatory zoning,which is a characteristic of magmatic zircons(Fig.3b).The contentsof Th and U were high,at977—3530 ppm and 1006—2043 ppm,respectively.The ratio of Th/U was0.97—1.73(Table 1)w ith an average of 1.32(n=15)and was typically greater than 0.50.A positive correlation exists between the Th and U,which is also consistentw ith thegeneral characteristicsofmagmatic zircons.The dates of 15 points were in agreement with each other,w ith no large f luctuations in age.This indicates that the U—Th—Pb isotopic system of the zircons was not affected by later magmatic activity,tectonic activity,or metamorphism and that the system was closed.The15 data pointswere clustered in the concordia plots(Fig.4b),and the weighted average age was 106.1±0.8 Ma(n=15,

MSWD=0.90),which dates the emplacement age of the Anjishan complexes to the Early Cretaceous.

Table 1 LA—ICP—MS zircon U—Pb ages of the f ive complexes in the Ningzhen region

Table 1 continued

Table 1 continued

Table 1 continued

The zircon grains from the Xiashu—Gaoziquartz diorite porphyriteweremostlylarge,long-columnar,and idiomorphic,although a few were incomplete;their lengths andw idthswere～100—180μmand～40—80μm,respectively.Their internalstructurewasuniform and clear and showed well-developed oscillatory zoning,which is characteristic ofmagmatic zircons(Fig.3c).The contents of Th and U werehigh,at384—747 ppm and 350—706 ppm,respectively.The ratio of Th/U ranged between 0.91 and 1.27 with an average of 1.09(n=15)and was typically greater than 0.50(Table 1).A positive correlation was present between the Th and U,which was also consistent w ith the general characteristics of magmatic zircons.The dates of 15 pointswere in agreementw ith each other,w ith no large f luctuations in age.This indicates that the U—Th—Pb isotopic system of the zirconswas notaffected by later magmatic activity,tectonic activity,ormetamorphism and that the system was closed.The 15 data pointswere clustered in the concordia plots(Fig.4c),and the weighted averageagewas108.7±1.4Ma(n=15,MSWD=0.45),which dates the emplacement age of the Xiashu—Gaozi complexes to the Early Cretaceous.

The zircon grains from the Shima granodiorite porphyry were large,long-columnar,and idiomorphic,although a few were incomplete and asymmetric;their lengths and w idths were～120—160μm and～50—80μm,respectively.Their internal structure was largely uniform and clearand showedwell-developed oscillatory zoning,which is characteristic of magmatic zircons(Fig.3d).The contentsof the Th and U are 387—707 ppm and 288—592 ppm,respectively.The ratio of Th/U ranged between 0.93 and 1.41(Table 1)w ith an average of 1.09(n=14)and was typically greater than 0.50.A positive correlation ispresent between the Th and U,which is also consistent w ith the general characteristics of magmatic zircons.At 394 Ma,the age of one zircon was obviously older than that of the other fourteen.We speculate that the early zircon crystallization was inherited by early magmatic activity;therefore,theageof thiszirconwaseliminatedwhen calculating the diagenetic age.In this case,the 14206Pb/238U dates obtained by the LA—ICP—MSmethod were distributed on or near the U/Pb harmonic line in the concordia plots(Fig.4d),andtheweightedaverageagewas 103.5±1.9 Ma(n=14,MSWD=1.20),which dates the emplacement age of the Shima complexes to the Early Cretaceous.The scattered207Pb/235U numerical values in the sample concordia plotsmay be attributed to the inaccuracy of the207Pb count.

Fig.3 CL images of selected zircons from the f ive intrusive rocks

Fig.4 Zircon LA—ICP—MSU—Pb concordia plots for the f ive complexes

The zircons from the Jianbi monzonitic granite were long-columnar,euhedral,and subhedral,exhibiting lengths of～140—200μmand widths of～50—90μm.Their internal structure was relatively uniform and clear and showedwell-developedoscillatoryzoning,whichischaracteristic ofmagmatic zircons(Fig.3e).The contents of Th and U are 114—402 ppm and 89—379 ppm,respectively.The ratio of Th/U ranges between 0.65 and 2.17(Table 1)w ith an average of 1.44(n=15)and was typically greater than 0.50.Apositive correlation exists between the Th and U,which is also consistent w ith the general characteristics ofmagmatic zircons.These results indicate that the zircons from the Jianbi complexes are magmatic in origin.The 15206Pb/238U dates obtained by the LA—ICP—MSmethod were distributed on or near the U/Pb harmonic line in the concordia plot(Fig.4e),and the weightedaverageagewas96.8±1.7 Ma(n=15,MSWD=0.40),which suggests that the Jianbi complexes were emplaced in the Early Cretaceous.

6 Lithogeochem istry

6.1 M ajor element geochem ical characteristics

In this paper,lithogeochem ical research was conducted on 31 representative samples;their major(w t%)and trace element(ppm)analysis results and relevantparameters are listed in Table 2.

Asshown in Table 2 and Fig.5,the intermediate—acidic intrusive complexes had high SiO2and relatively high MgO contentsatmaximum valuesof 73.40 and 2.25 w t%,respectively,and displayed signif icant positive Sr anomalies and negative Ta,Nb and Ti anomalies,resembling those of adakitic rocks(Martin 1999;Xu etal.2002).The total alkali(K2O+Na2O)content was generally high,ranging from 6.49 to 8.68 w t%,with an average of 7.62 w t%.In the K2O versus SiO2major element diagram(Fig.5a),most of the intermediate—acidic rock samples plotted w ithin the high-K calc-alkaline series,w ith a few situated w ithin the calc-alkaline series.In the MgO versus SiO2major element diagram(Fig.5b),most samples of Anjishan,Xiashu—Gaozi,and Shima fell in the overlap region between subducted oceanic slab-derived adakites and thickened lower crust-derived adakites.Moreover,most samples of Qilinmen and Jianbi fell in the region of metabasaltic and eclogite experimental melts.As illustrated in Harker diagrams(Fig.6),each oxide shows a good correlation w ith SiO2.The SiO2is positively correlated w ith K2O;however,it is negatively correlated w ith Na2O,TiO2,Al2O3,FeOT,MgO,CaO,and P2O5.

6.2 Trace element geochem ical characteristics

The REE contentwas generally high in the intermediateacidic intrusive complexes of the Ningzhen region;the ΣREE ranged from 70.68 to 184.59 ppm w ith an average of 148.60 ppm.The chondrite-normalized REE patternsof the f ive complexes were essentially sim ilar.The REE patterns also exhibited right-inclined curves.They are characterized by enrichment in light rare earth elements(LREE)and depletioninheavyrare earthelements(HREE),show ing high(La/Yb)Nratios of 15.02—37.28 and slightly negative Eu anomalies(Fig.7a,Table 2).These characteristics indicate that themagmatic evolution process in the Ningzhen region was consistent.The numerical values ofδEu in the f ive complexes ranged between 0.69 and 1.13,w ith an average of 0.90;the majority show slightly negative Eu anomalies,and the m inority show slightly positive Eu anomalies.The positive Eu anomalies suggest that noplagioclasefractional crystallization occurred during the formation process of the intrusive rocksor that the crystallization timewas short.In addition,the(La/Yb)Nratio in the intermediate—acidic intrusive complexes ranged from 15.02 to 37.28,which is generally higher than that in both the lower crustand upper crust,at(La/Yb)N=5.30 and 15.50,respectively(Rudnick and Gao 2003).This suggests that the complexes in the Ningzhen region experienced intense REE differentiation.

The prim itive-mantle-normalized trace element patterns(Fig.7b)show that all f ive complexes are enriched in LILE,including Rb,Ba,and K,and are strongly depleted in HFSE,including Nb,Ta,P,Zr,and Ti.Therefore,the trace element curve gradually decreases toward the right.The trace element geochemical characteristics show that fractional crystallization in themagmatic evolution process may have involved apatite,Ti—rich m inerals,and hornblende.This result corroborates the REE analysis results.In the(La/Yb)N—δEu diagrams(Fig.8),the samplesallplot in the crust-mantle type f ield.The crust-mantle m ixing(crust-mantle type)has been used by many researchers recently to study the genesis of granitoids(Castro et al.1991;Liu and Li2001;Nguyen and Kozo 2003;Chen etal.2009).The genesis of granitoids has a close relationship with crust-mantle interaction.The granitoid components may be derived from the mantle as well as the crust.Moreover,it isbelieved thatcrust-mantlem ixing in various degrees m ight form many types of granitoids.Granitic magma is formed by them ixing of crustalmaterials with mantlematerials by subduction,detachment,underplating,mantle pluming,or other processes.The petrological,geochemical,isotopic,and geochronological characteristics of granitoids could be used to determine whether the genesisof a granitoid is related to crust-mantle interaction.

Table 2 Whole-rock geochemistry results including major(wt%) and trace element(ppm) compositions of the five complexes

Table 2 continued

Table 2 continued

Fig.5 Selectedmajor elementdiagramsof a K2O versus SiO2(Peccerillo and Taylor 1976;M iddlemost1985)and b MgO versus SiO2(Defant and Drummond 1990;Atherton and Petford 1993;Ding et al.2016).Data sources:Qilinmen complexes(Chen 1984;Xia 2000);Anjishan complexes(Chen etal.1980,1984;Ning and Chen 1989;Xia 2000;Xu etal.2001;Zeng etal.2013);Xiashu—Gaozicomplexes(Chen 1984;Xia 2000;Sun 2012);Shima complexes(Chen 1984;Xia2000;Sun 2012);Jianbicomplexes(Yang etal.1985;Zhu 1987;Zhen and Chen 1988;Xia 2000);and others(Table 2).The reference data are identif ied by black symbols;our data aremarked by red symbols

7 Discussion

7.1 Tim ing of diagenesis and metallogenesis

The Ningzhen region is an important part of the m iddlelower Yangtze River Fe—Cu—Au polymetallicmetallogenic belt.Many previous studieshave investigated the geology,geochronology,geochemistry,and isotope geochemistry of thesemagmatic rocks(Xia 2000;Zeng et al.2013;Wang etal.2014a,b).The geological and geochronological data show that the Ningzhen Mountainsarean importantpartof the calc-alkaline series in them iddle-lower Yangtze River belt,which formed 122—96 Ma.From west to east,this region is composed mainly of six homologous complexes that gradually change in composition from intermediate—acidic to acidic:the Qilinmen,Anjishan,Xiashu—Gaozi,Xinqiao,Shima,and Jianbi complexes(Mao and Zhao 1990).Geochem ical research on the trace elements and REE of the f ive complexes in this study show that the magma source is crust-mantle type in the Ningzhen region,which has the same evolutionary trend.

The age ofmagmatic intrusion in the Ningzhen region reported in literature is variable(Table 3).Regardless of these age differences,the existing geochronological data indicate that the complexes in this region formed during the Early Cretaceous,belonging to the Late Yanshanian.In the present study,LA—ICP—MS zircon U—Pb dating yielded agesof122.0±1.0Ma,106.1±0.8 Ma,108.7±1.4 Ma,103.5±1.9 Ma,and 96.8±1.7Ma for the Qilinmen,Anjishan,Xiashu—Gaozi,Shima,and Jianbi complexes,respectively.The published emplacement ages of the f ive complexes are～102,～106—109,～109,～102—103,and～109Ma,respectively(Table 3).The formationageoftheintermediate—acidicintrusive complexes is bounded by the above ages.Therefore,it is believed thatmagmatic emplacement occurred during the Early Cretaceous.

The emplacement sequence of the magmatic rocks is consistent across the Ningzhen region.The results show that the emplacement sequence,from early to late,is diorite porphyrite,quartz diorite porphyrite,granodiorite porphyry,monzonitic granite,and granite(Ning and Chen 1989).The granitoid complex zonation shows a single complex comprised from the edge to the interior phase of quartz diorite porphyrite,quartz monzonite porphyry(granodiorite porphyry),and monzonitic granite(Mao and Zhao 1990).The geological,geochemical,and isotopic characteristics of the intrusive complexes in the Ningzhen region indicate that the diagenetic sequence is augite diorite,diorite or quartz diorite,granodiorite ormonzonitic granite,and granite(Chen et al.1987).The above observations are consistent w ith the evolution sequence of Bowen's reaction series.According to the emplacement agesof the f ive complexes,they decrease in age from west to eastand gradually change in composition from neutral to acidic.

Severalm ineralization ageshave been obtained recently in the Ningzhen region.For example,the Anjishan and Tongshan Cu(Mo)deposits show a direct relationship w ith the Anjishan and Xiashu—Gaozi complexes,respectively(Wang etal.1997),and the Cishantou skarn iron deposit is related directly to Xiashu—Gaozicomplex(Sun etal.2013).Similarly,themetallogenic age of theWeigang magnetite deposit was reported to be about 102.5 Ma,which is related directly to the Shima complex(Guan et al.2015).Moreover,the Sm—Nd isochron age of the f luorite of the hydrothermal II stage is 93.7±3.1 Ma,which represents the main ore-form ing epoch/stage of the Lunshan golddeposit located in the m iddle of the Anjishan and Shima complexes(Lu etal.2017).In addition,the Jianbiporphyry Mo(W)deposit is related directly to the Jianbimonzonitic granite(Ma and Wang 2003).Thus far,however,no deposit has shown a direct relationship w ith related the Qilinmen complex.

Fig.6 Harker diagrams for samples of the f ive complexes.The data sources and symbol explanation are the same as those given in Fig.5

The zircon U—Pb dates indicate that Mesozoic intermediate—acidic intrusive complexes in the Ningzhen region were emplaced during the Early Cretaceous,ranging from 109 Ma to 97 Ma.These ages suggest that crystallization oftheintermediate—acidicintrusivecomplexesand initiation of them ineralization are temporally and possibly genetically related.Therefore,the aforementioned metal deposits belong to one set of the Early Cretaceous polymetallic metallogenic events in them iddle-lower Yangtze Rivermetallogenic belt.Thus,we suggest that the Mesozoic large-scale diagenesis and metallogenesis in the Ningzhen regionmay have ceased 100Maorabout95 Ma.

Fig.7 Chondrite-normalized REE patterns(a)and prim itive-mantle-normalized trace elementpatterns(b).The normalized values are based on Boynton(1984)and Sun and M cDonough(1989).Data sources:Qilinmen complexes(Chen etal.1991);Anjishan complexes(Chen etal.1991;Xia 2000;Xu et al.2001;Zhang et al.2010;Zeng et al.2013);Xiashu—Gaozi complexes(Chen et al.1991;Xia 2000;Sun 2012);Shima complexes(Chen etal.1991;Xia 2000;Sun 2012);Jianbi complexes(Zhen and Chen 1988;Chen et al.1991;Xia 2000);others(Table 2)

Fig.8(La/Yb)N—δ(Eu)diagram for the Ningzhen complexes.The data sourcesare the same as those given in Fig.7.The reference data are represented by black symbols;our data are shown by red symbols

7.2 Petrogenesis

The Harker diagrams for the f ive complexes in the Ningzhen region(Fig.6)indicate that SiO2is positively correlated w ith K2O.Conversely,SiO2is negatively correlated w ith Na2O,TiO2,A l2O3,FeOT,MgO,CaO,and P2O5,which ref lects that the f ive complexes have a close genetic relationship and cognatemagma evolution.During themagmatic evolution,fractional crystallization ofmaf ic minerals such as pyroxene and hornblende played amajor role.The trace element and REE geochem ical characteristics also showsim ilarity across the f ive complexes(Fig.7),which indicates a common magma source and magmatic evolution.

The REE patterns of the intermediate—acidic complexes are characterized by enrichment in LREE and depletion in HREE in addition to slightly negative or none Eu anomalies(Fig.7a);the REEpatterns exhibit right-inclined curves.The REE patterns also show high(La/Yb)Nratios,which are in accord w ith the geochemical signature of adakitic rocks.In summary,the intermediate—acidic complexes in the Ningzhen region are part of the Early CretaceousadakitesuccessionineasternChina.The intermediate—acidic complexes show chondrite-normalized REE patterns sim ilar to those in samples from Wushan,Lishui,and Ningwu in them iddle-lower Yangtze Riverbelt(Gao et al.2007;Jiang et al.2008).

In the(La/Yb)N—δ(Eu)diagram(Fig.8),the studied samples all plot in the crust—mantle-type f ield.This preliminary study indicates that the initial magma formed through interaction of the uppermantle and the crust and then underwent fractional crystallization in the lower crust before being emplaced as intrusive complexes.Therefore,the samples belong to multi-stage homologous intrusive complexes,show ing a clear trend in composition from neutral to acidic during their evolution.Ning and Chen(1989)found picotite,anatase,and homologous basic xenoliths in complexes in the Ningzhen region;the87Sr/86Sr ratio in apatite was 0.7066±0.0020.This evidence also excludes the possibility of the initial magma being directly sourced from the uppermantle or from remelting of crustal sediments.

Geologists divided adakitic rocks into four origin types as follows:(1)adakitic rock produced by direct partial melting of the lower crust(Atherton and Petford 1993);(2)adakitic rock generated by partialmelting of delaminated lower crust(Xu etal.2002;Wang etal.2004);(3)adakitic rock by differentiation ofabasalticmagmaathigh pressure condition(Castillo et al.1999;Xiong 2006);(4)adakiticrock bymagmam ixing(Guo etal.2007;Chen etal.2013).In this study,themajor element geochemical characteristicsof the complexeswerehigh SiO2contentand relatively high MgO and Al2O3contents.These complexes also display signif icant positive Sr anomalies and negative Ta,Nb,and Ti anomalies.In addition,the geochem ical characteristicsof the trace elements in the f ive complexeswere as follows:the chondrite-normalized REE patterns were basically sim ilar,exhibiting right-inclined curves.The patterns had high(La/Yb)Nratios and slightly negative Eu anomalies.The primitive-mantle-normalized trace element patterns were also sim ilar in that they were rich in LILE including Rb,Ba,and K and low in HFSE such as Nb,Ta,P,and Ti.The geochemical characteristics of thesemajor and trace elements indicate that the process of magmatic evolution is consistent in the Ningzhen region.The complexes from this region have high Mg#(＞50)(Wang etal.2014a,b;Xu et al.2014),high Sr/Y ratios coupled w ith low Y(Xu et al.2002;Wang et al.2014a,b),lowεNd(t)values(26.8—29.7)and143Nd/144Nd ratios＜0.5123(Xu et al.2002),and high87Sr/86Sr ratios(0.7053—0.7066)(Ning and Chen 1989;Xu et al.2002).These characteristicsare inconsistentw ith origins from slabmelting or plate subduction.Previous studies have suggested that the lithosphere in eastern China was thick in the Mesozoic,measuringmore than～40 km in Ningzhen region before theunderplating ofbasalticmagmaand delam ination of the lower crustoccurred(Xu and Ma2003;Wu etal.2007).At the latter stage,the crust began to thin gradually before reaching the present thickness of approximately 30 km as measured by geophysical techniques(Wang and Li1992).Thus,we infer that the lower crust subsided into the underlying mantle,and the lower crust which entered the mantle partially melted to form adakitic melt.As this adakitic melt then ascended,the MgO content and Mg#value of the adakitic rocks increased from interaction w ith mantle rocks.This led us to conclude that the adakitic rocksmost likely originated from the partialmelting of the delam inated lower crust.

Table 3 Ages of the complexes in the Ningzhen region

7.3 Geodynam ic setting

The difference in crustal thickness indicates that the lithosphere of eastern China has been thinned since the Mesozoic.A thick overriding plate is favorable for the formation of magmatic rocks over volcanic rocks.Moreover,it favorsmagma evolution and prevents the leakage of magmatic f luids,both of which promote porphyry deposits(Sun etal.2017a,b,c).A summary by Mao etal.(2003)suggests that large-scalem ineralization in the North China Craton and its adjacent areas occurred during three periods:200—160,140,and 130—110 Ma.It is proposed that the geodynam ic setting of the third period is related to the thinning of the lithosphere,which is directly related to our discussion.A lthough the occurrence time and mechanism of the lithospheric thinning events are still under debate,mostgeologistsbelieve that the interaction between the Paleo-Pacif ic Plate and the overlying continental crust was the main driving force of lithospheric thinning in eastern China(Mao etal.2003;Wu etal.2003;Sun etal.2007).The lithospheric thinning events should have a starting time,amaximum thinning time,and an end time;thus,the time constraint is indirect(Wu et al.2003).The peak periodsof lithospheric thinningmay correspond to the most intense periods of diagenesis and m ineralization.Although dating of the lithospheric thinning events remainscontroversial,the zircon U—Pb dating of the f ivemagmatic rocksareinthethirdlarge-scalem ineralization(130—110 Ma),and the lithosphere of Ningzhen region thinned during the formation of f ivemagmatic rocks.

We suggest that the Late Mesozoic in eastern China,which includes the Ningzhen region,was a period of tectonic regime transformation.The areawas inf luenced by a change in the tectonic regime of the Pacif ic Plate,resulting in a transformation from a compressional to extensional setting and the associated reduction in crustal thickness(Wu et al.2003).After thinning,the gravitational instability of the lithosphere increased and led to deep lithospheric mantle and lower crust delam ination followed by large-scale asthenospheric upwelling.The basic magma then intruded into the base of the lower crust,leading to partial melting and magma mixing in the lower crust.Eventually,themagma was emplaced and formed a crustmantle type complex.During the process of lower crust delam ination,if granitic magma was generatedand emplaced into the upper crust,the resulting granitic rocks could have trace element signatures.For example,during the crystallization of partial melting relics under highpressure conditions,the ratios of La/Yb in magma should be high owing to the remnants of garnet and hornblende(Xu and Qiu 2010);this corresponds to the high ratios of(La/Yb)Nfound in the f ive complexes in this study.

Previous geochronological studies conducted on the Yanshanian magmatic rocks in them iddle-lower Yangtze River belt concluded that three main periods of m ineralization occurred(Wu etal.2005;Xie etal.2008).The f irst period of magmatic activity occurred 145—136 Ma(Zhou etal.2007;Wu etal.2012;Wang etal.2015,2016;Duan and Jiang 2018)whenmagmatic rockswere formedmainly in the southeast regions of Hubei,Tongling,and Anqing.The second period occurred between 135 and 127Ma(Zhou et al.2008a,b,2013;Nie et al.2017;Sun et al.2017a,b,c;Liu et al.2018)when magmatic rocks were produced mainly in the Luzong and Ningwu basins.The third period occurred between 126 and 123 Ma(Lou and Du 2006;Fan etal.2008;Yuan etal.2010;Su etal.2013)when magmatic rockswere produced in areas of fault-related uplift and in fault-controlled basins,mainly in the Fanchang and Ningwu basins.These three periods of magmatismshowthat the Mesozoic magmatic rock emplacementages in themiddle-lower Yangtze River belt decrease in age from south to north.The Ningzhen region is located in the easternmost part of the m iddle-lower Yangtze River Cu—Fe polymetallic metallogenic belt.Thus,magmatic emplacementmay have occurred after the three periods described above.Zeng et al.(2013)dated samples from the Anjishan complex by using sensitive high—resolution ion microprobe(SHRIMP)zircon U—Pb dating.They found that the lithospheric thinning events in the Ningzhen region may have occurred～20 Ma later than that in the southeast Hubeiand Tongling regions and that lithospheric delam ination in the Ningzhen regionmay have continued until～107 Ma.Wang et al.(2014a,b)dated the Anjishan,Xiashu—Gaozi,and Shima complexes by using LA—ICP—MS zircon U—Pbmethods.Considering the tectonic evolution of the Pacif ic Plate and the age constraints on diagenesis and m ineralization in eastern China since the Cretaceous,Wang et al.(2014a,b)proposed that the lithospheric thinning in the m iddle-lower Yangtze River belt began～135 Ma and may have continued until～100 Ma.Through comprehensive consideration of the previous research resultsgiven above and the results of the present study,we agree that the diagenesis and m ineralization in the Ningzhen region belong to the f inalperiod ofmineralization in theMesozoic.Therefore,a new period of large-scale diagenetic and metallogenic events has been identif ied,which isof greatsignif icance to the study of the tectonic,magmatic,and m ineralization history of the m iddle-lower Yangtze River metallogenic belt.

Inthis paper,theJianbi complexwas datedto 96.8±1.7Ma using LA—ICP—MS zircon U—Pb methods.The ages of the Qilinmen complex,at～99—122Ma,the Anjishan complex,at～106—112 Ma,the Xiashu—Gaozi complex,at～106—109 Ma,and the Shima complex,at～82—109 Ma,have been reported.On the basis of these ages,we conclude that the magmatic emplacement coincided w ith the period of Cu—Fe m ineralization.As previously described,the third period ofmagmatic activity in the Ningzhen region ended at 123 Ma,w ith an interval of 17—23 Ma between the startof the period and the peak magmatism at 100—106Ma.Therefore,we propose that importantlarge-scalediagenesisandmineralization occurred after the three large-scale diagenetic and metallogenic periods,w ith emplacement times between 110 and 95 Ma.Large-scale diagenesis and m ineralization in the Mesozoic Ningzhen region of eastern China may have continued until 100 Ma or 95 Ma;however,further study is required to conf irm this f inding.

8 Conclusions

The results of this study are summarized below:

(1)We determ ined the zircon U—Pb agesw ith LA—ICP—MSfor the Qilinmen,Anjishan,Xiashu—Gaozi,Shima,andJianbicomplexestobe 122.0±1.0 Ma,106.1±0.8 Ma,108.7±1.4 Ma,103.5±1.9 Ma,and 96.8±1.7Ma,respectively.It is suggested that the intermediate—acidic intrusive complexes in the Ningzhen region were emplacedduring the Early Cretaceous.The major element characteristics of the f ive complexes show that the lithology changes from neutral to acidic from west to east.

(2)The SiO2values in the Ningzhen complexes positively correlate w ith K2O.Conversely,SiO2negatively correlates w ith Na2O,TiO2,A l2O3,FeOT,MgO,CaO,and P2O5,which indicates a close genetic relationship among the f ive complexes.The chondrite-normalized REEand primitive-mantlenormalized trace element patterns also show similarities.The trace elementgeochemical results show enrichment of LILE and depletion of HFSE,and samples from all f ive complexes are classif ied as crust-mantle type igneous rock.Therefore,we conclude that the adakitic rocks in the Ningzhen region are of crust-mantle type and can be classif ied as multi-stage homologous intrusive complexes.

(3)The ages of the intermediate—acidic intrusive complexes are as follows:Qilinmen,～99—117 Ma;Anjishan,～106—112 Ma,Xiashu—Gaozi,～106—109Ma,Shima,～82—109 Ma,and Jianbi,～100—102Ma.These ages coincide w ith the Cu—Fe m ineralization period.Therefore,we conclude that large-scale Mesozoic diagenesis and mineralization in the Ningzhen region in eastern China may have continued until 100 Ma or～95 Ma.

Acknow ledgem entsThe study was f inancially supported by the Major State Basic Research Development Program of China(973 Program)(No.2014CB440906).