Origin of C type adakite magmas in the NE Xing’an block,NE China and tectonic implication

Changzhou Deng•Guangyi Sun•Deyou Sun•Hu Huang•Jianfeng Zhang•Jun Gou

1 Introduction

As initially defined(Defant and Drummond 1990;Martin 1999),adakites are defined as a kind of intermediate to felsic volcanic rocks formed in the arc setting,that show geochemical characteristics of SiO2≥56 wt%,high Na2O contents(3.5 wt%≤Na2O≤7.5 wt%),high La/Yb and Sr/Y ratios,and a strong depletion in high field strength elements(HFSE),Yb and Y.They are further classified into high-SiO2adakites and low-SiO2adakites by Martin et al.(2005),and are thought to be generated from the partial melting of metabasaltic rocks at relatively high pressure,with eclogites or amphibolites in the residual(see e.g.Moyen and Stevens 2006;Rapp and Watson 1995;Wolf and Wyllie 1994).Subsequently,continental(C)type(high Sr/Y)adakite was defined by Zhang et al.(2001)to describe intermediate to felsic magmatic rocks generated in continental settings in eastern China,which have similar geochemical characteristics to the typical adakite.This kind of adakite-like igneous rocks is widely reported in cratons(Zhang et al.2001;Gao et al.2004;Xu et al.2002,2006,2008;Wang et al.2007)and active continental margins(He et al.2011;Yang et al.2016;Zhou et al.2016).

Though the petrogenesis of high Sr/Y type magmas is still open to debate,C-type adakite can preserve some imprints about the lower crust,which can be used to trace the composition of the lower crust and discuss the crustmantle process(He et al.2011).Because of their unique temperature and pressure formation conditions,C-type adkitic rocks can be potentially utilized as tectono-magmatic process indicators,which can be of vital importance in reconstructing past regional tectonic regimes(e.g.Yang et al.2016;Zhou et al.2016).

Northeast(NE)China is located in the eastern part of Central Asian Orogenic(CAOB),and its tectonic transformation and overprinting in the Mesozoic is highly controversial(Xu et al.2013;Wu et al.2011).There are different proposals on the timing of the transformation from the Paleo-Asian regime to the Circum-Pacific regime:1)in the Triassic(HBGMR 1993;Zhao et al.1996);2)in the Early-Middle Jurassic(Sun et al.2005;Wu et al.2007;Pei et al.2008;Zhou et al.2009);3)in the Early Cretaceous(He et al.1998).In recent years,based on the studies of precise age data and whole-rock geochemistry from igneous rocks,the influence of the spatial and temporal extents of the tectonic regime in the Mesozoic have constrained to a certain degree.e.g.,the volcanic rocks in the Erguna massif are related to the subduction of the Mongol-Okhotsk oceanic plate,and those in the eastern Jilin–Heilongjiang provinces are related to the subduction of the Paleo-Pacific plate,the effect of the Mongol-Okhotsk suture belt on NE China was mainly in the Early Jurassic,Middle-late Jurassic and Early Cretaceous(Xu et al.2013).The generation of the massive Triassic-Jurassic granitoids in the Erguna massif is related to the evolution of the Mongol-Okhotsk ocean(Wu et al.2011).In general,Triassic-Jurassic granitoids in the Erguna Massif resulted from the southward subduction of the Mongol-Okhotsk ocean.Early-Middle Jurassic granitoids,with an age range of 210–155 Ma,were considered to be formed under the tectonic setting of subduction and collision between the Jiamusi Massif and Songliao Terrane(Wu et al.2011).However,many studies up to now have focused on the normal(generally low Sr/Y)igneous rocks,while few studies have been paid attention on the C-type(high Sr/Y)granitoids in Xing’an block,NE China.Importantly,the Early Jurassic geodynamic setting of the Xing’an block are poorly constrained.

In this contribution,we focus on the Early Jurassic intrusive rocks at the northeast(NE)margin of the Xing’an block,NE China,and present zircon U–Pb and geochemical data,with the aim of constraining the petrogenesis of C type adakite in this area.We also use this data to discuss the tectonic implication of the C type magma generation.

2 Geological setting and sample descriptions

2.1 Geological setting

The tectonic evolution of the Xing’an–Mongolia orogenic belt(the eastern part of the CAOB)is characterized by the amalgamation of multiple microcontinental blocks during the Phanerozoic(Sengör et al.1993;Xiao et al.2003;Li 2006;Wu et al.2011,2012).The northeastern part of this belt is divided into the Erguna,Xing’an,Songnen,and Jiamusi blocks from northwest to southeast,separated by the Tayuan–Xiguitu fault zone,the Hegenshan–Heihe fault zone,and the Mudanjiang faultzone,respectively(Fig.1a).The Erguna block has remained stable since the Early Paleozoic(Ge et al.2005;Wu et al.2005).During the Middle Paleozoic,the Xing’an block accreted onto the Erguna block along the Tayuan–Xiguitu fault zone(She et al.2012).The Songnen block collided with the joined Xing’an and Erguna blocks along the Hegenshan–Heihe fault zone during the late Paleozoic(Wu et al.2002),and the Jiamusi terrane collided with these blocks along the Mudanjiang fault zone in the Early Mesozoic(Ge et al.2007).Our studied region is located in the northeast section of Xing’an block(Fig.1a),and the southeast section of the Tayuan–Xiguitu fault zone.

The studied area has experienced multiple tectonic movements,and is characterized by a series of E–W,NNE–SSW and NNW–SSE to NW–SE trending faults.Outcropping strata in the region consist mainly of Neoproterozoic,Cambrian,upper Silurian-Middle Devonian and Cretaceous units.The Neoproterozoic Xinghuadukou Group is composed of metamorphic intermediate-mafic submarine volcanic rocks,amphibolites,felsic gneisses,and marble(Zhou et al.2011).The Cambrian Hongshenggou Formation consists of submarine limestone and light metasedimentary rocks.The Silurian–Devonian Niqiuhe Formation consists of marine facies volcanic rocks, clastic rocks, carbonates and radiolarian-bearing siliceous formations.The Mesozoic outcrops are mainly Early Cretaceous volcanic formations consisting of a suit of subducted-related basalt-andesiterhyolite volcanic rocks.

2.2 Occurrence and petrograp hy

Fig.1 Tectonic location map(after Hu et al.2014)(a),F1 Tayuan–Xiguitu fault;F2 Hegenshan–Heihe fault;F3 Chifeng–Kaiyuan fault;F4 Dunhua–Mishan fault;F5 Yitong–Yilan fault;F6 Jiayin–Mudanjiang fault.And simplified regional geological map of NE Xing’an block

Lacking precious age data,the intrusion rocks in Xing’an block were mapped as Late Proterozoic to Paleozoic granites(HBGMR 1993).However,recent U–Pb isotopic studies indicate that Proterozoic rocks are quite restricted and many so-called Precambrian granites are actually Early Paleozoic granitoids(Sui et al.2007;Wu et al.2011).But by far the most components of intrusion rocks in this region are granites that emplaced during the Late Triassic to Early Jurassic(Wu et al.2011).

In this paper,samples from the Early Jurassic Diyingzi batholith in the NE Xing’an block were collected.The Diyingzi batholith,with an area of～1000 km2,crops out in the northern section of the Xinglong town(Fig.1b),and is dominated by monzogranites(sample DGS19,N51°54′20′,E126°09′15′),with minor amounts of diorite veins and porphyry granite.It intrudes into the Neoproterozoic,Cambrain,and Ordovician strata,and is overlain by Early Cretaceous volcanic rocks. The monzogranite shows coarse-medium grained crystalline texture,massive structure,and consists of 20%–30%plagioclase,35%–40%K-feldspar,25%–30%quartz,and 5%biotite.Anhedral quartz grains show undulatory extinction,the plagioclase and K-feldspar is generally automorphic and subhedral crystal,and the former is twinned(Fig.2).Accessory minerals include zircon and magnetite.

3 Analytical methods

We selected 7 monzogranite samples for whole rock geochemical study,and the sample DGS19 was also used for zircon U–Pb dating analysis.All samples representextremely rare examples of unaltered,fresh and undeformed igneous rocks.The sampling locations are shown in Fig.1.

Fig.2 Microphotographs of representative monzogranite samples in NE Xing’an block.Pl Plagioclase;Kfs K-feldspar;Q Quartz;Bt Biotite

All whole-rock major elements were analyzed using X-ray fluorescence at the State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences,Wuhan.The analytical uncertainty is generally＜5%.Trace element analyses were performed in Wuhan Sample Solution Analytical Technology Co.,Ltd(WSSATCL),and trace element concentrations were determined using a Thermo-Finnigan element inductivity coupled plasma mass spectrometer(ICP-MS)after HNO3–HF digestion of about 50 mg of the sample powder in a Teflon vessel at 190°C.Analytical accuracy and precision were monitored by Chinese national reference GSR1(granite),GSR2(andesite)and GSR3(basalt).The precision is 5%of the quoted values for elements present less than 1 ppm.Accuracy is estimated to be better than 5%for the reported values.The detailed sample-digesting procedure for ICP-MS analyses and analytical precision and accuracy for trace elements are the same as described by Liu et al.(2008).

Zircon grains were separated using conventional magnetic and density techniques.Zircon grains were handpicked under a binocular microscope,mounted in an epoxy resin 1 inch in diameter,and polished to expose the centers of zircon grains.Prior to analytical work,all zircon grains were examined under a microscope with transmitted and reflected light,as well as Cathodoluminescene(CL)images using a scanning electron microscope to reveal their internal structures at WSSATCL.LA-ICP-MS zircon U–Pb dating and trace element were analyzed at WSSATCL.Detailed operating conditions for the laser ablation system and the ICP-MS instrument and data reduction are the same as the description by Liu et al.(2010).Zircon 91,500 was used as an external standard for U–Pb dating,and was analyzed twice every 5 analyses.Preferred U–Th–Pb isotopic ratios used for 91,500 are from Wiedenbeck et al.(1995).Uncertainty of preferred values for the external standard weighted mean calculations were made using Isoplot/Ex_ver3(Ludwig 2003).

4 Results

4.1 U–Pb zircon age

Sample DGS19 from the NE Xing’an block were selected for LA-ICP-MS zircon U–Pb dating.The results are listed in the Table 1.Representative zircon CL images and their U–Pb Concordia plots are shown in Fig.3.Zircons from the magmatic rocks are characterized by light yellow to colorless,euhedral,and long to short prismatic forms.The crystals range in length from 65 to 200 μm,with length/width ratios ranging from 2:1 to 3:1.Most zircons show obvious oscillatory zoning on the CL images(Fig.3a).Zircon grains have variable U and Th contents of 73.7–1122 and 39.8–702 ppm,respectively.The relatively

high Th/U ratios(0.27–1.84),together with the clear oscillatory zoning,indicate a magmatic origin(Williams and Claesson 1987;Kinny et al.1990;Koschek 1993;Corfu et al.2003).

Table 1 LA-ICP-MS zircon U–Pb dating for the monzogranite samples in the NE Xing’an block

Fig.3 Representative zircon CL images(a),and concordia diagram of zircon U–Pb ages(b)for monzogranite samples from the NE Xing’an block.Dashed ellipses are not included in the age calculation.Ellipses represent 1-sigma uncertainty for individual analyses

Twenty-three spots yield206Pb/238U ages between 174 and 185 Ma,with a weighted mean age of 180±1 Ma(MSWD=0.83)(Fig.3b),which could be interpreted to be the emplacement age of the granite intrusion in our study area.Two other spots give slightly old206Pb/238U ages 198 and 203 Ma,which could be interpreted as the xenocrystic grains incorporated into the granite magma.

4.2 Major and trace elements

The monzogranite samples contain SiO2of 67.49 wt%–72.31 wt%.Their Na2O and K2O contents range from 4.20 wt%to 5.24 wt%and from 2.88 wt%to 3.66 wt%,respectively,with K2O/Na2O ratios of 0.55–0.85.The samples contain Al2O3of14.30 wt%–16.40 wt%,relatively low CaO of 1.62 wt%–2.53 wt%,showing moderately peraluminous signatures(Fig.4a).They have Fe2O3T contents of 1.1 wt%–2.6 wt%and MgO of 0.39 wt%–0.96 wt%,with Mg#values of40–46.They plot in or near the filed of high-K calc-alkline in the SiO2–K2O diagram(Fig.4b).

The samples are low in total REEs(96.29–143.22 ppm),and exhibit LREE enrichment and HREE depletion in chondritenormalized REE patterns[(La/Yb)N=29.75–41.53,(Dy/Yb)N=1.31–1.78],with negligible Eu(Eu/Eu*=0.77–1.08,with an average value of 0.94)anomalies(Fig.5a).These samples show strong enrichments in large ion lithophile elements(LILE),such as Rb, Ba, Th and U, and depletion in high field strength elements (HFSE), such as Nb, Ta and Ti (Fig. 5b),which exhibit typical subduction-related geochemical characteristics(Innocenti et al.2005).

5 Discussion

5.1 Petrogenesis

The seven samples are characterized by prominently high SiO2(≥67.49),and Sr(461–759 ppm),butlow Y(4.63–8.06 ppm)and HREE(∑HREE=3.83–6.49 ppm,Yb=0.5–0.77 ppm)contents,with therefore high Sr/Y(67.2–119)and (La/Yb)N(29.7–41.5)ratios,showing geochemical characteristics of C type adakitic granite.In the two widely used(La/Yb)Nversus YbN(Martin 1999),and Sr/Y versus Y(Defant and Drummond 1990)discrimination diagrams,all samples plot into the field of adakite(Fig.6).Several models have been proposed for the petrogenesis of the granitoid rocks with high Sr/Y geochemical signature:assimilation fractional crystallization(AFC)processes of mantle-derived magmas(Castillo et al.1999;Liu et al.2003;Macpherson et al.2006),partial melting of subducted oceanic slabs(Defant and Drummond 1990;Rapp et al.1999),magmas mixing between felsic and basaltic magmas(Streck et al.2007),partial melting of delaminated(Kay and Kay 1993;Xu et al.2002)or thickened(Chung et al.2003;Long et al.2015;Zhou et al.2016)lower continental crust(LCC).

Adakitic rocks generated by AFC processes of basaltic magmas usually contain amphibole or clinopyroxene minerals and require volumes of basaltic to dacitic rocks(Castillo et al.1999;Macpherson et al.2006).In the filed,no coeval mafic to intermediate rocks have been found in the NE Xing’an block,combined with the absence of amphibole and clinopyroxene minerals in the samples,and the lack of mafic xenoliths,precluding their derivation from the basaltic magma by AFC processes or magma mixing.Moreover,the lack of strong correlations between Sr/Y,La/Sm,(La/Yb)N,Zr/Nb and Nb/Ta ratios and SiO2contents preclude fractionation.

Fig.4 a A/NK(molar Al2O3/[Na2O+K2O])versus A/CNK(molar Al2O3/[CaO+Na2O+K2O])diagram(Maniar and Piccoli 1989)for monzogranite samples in NE Xing’an block.b SiO2versus K2O diagram(Peccerillo and Taylor 1976)for monzogranite samples in NE Xing’an block

Fig.6 a Sr/Y versus Y diagram(Defant and Drummond 1990)and b(La/Yb)Nversus YbNdiagram(Martin 1986)for monzogranite samples in NE Xing’an block

Due to the subsequent interaction with overlying mantle peridotite during the magma ascent,delaminated lower crust or oceanic slab-derived adakitic rocks generally have the geochemical feature of high Cr(≥20 ppm)and Ni(≥12 ppm)values(Martin 1999;Xu et al.2002;Martin et al.2005).However,the NE Xing’an adakitic granites are characterized by dominantly low Cr(1.27–3.94 ppm),and Ni(1.28–3.33 ppm)contents.In addition,they have Rb/Sr ratios of 0.08–0.23,higher than those(＜0.05)of slabderived adakites(Huang et al.2009),and show high SiO2contents.These geochemical features rule out an origin from the partial melting of a subducted ocean slab or delaminated lower crust.

The studied samples with low Mg#(40.3–46.2),MgO(0.39 wt%–0.96 wt%)(Fig.7a,b)values,as well as low Rb/Ba(0.05–0.17),and Rb/Sr(0.08–0.27)ratios(Fig.7c),have typical features of lower continental crust(Rudnick and Gao 2003).In addition,adakitic rocks in the present study exhibit diagnostic characteristics of low Ni and Cr contents(Fig.7d,e),indicating that its magmatic protolith was most probably derived from the partial melting of lower crustal materials.

5.2 Source characteristics

The source of C type adakitic granitoid rocks are also highly debated because these geochemical signatures can be produced by multiple lower continental crust sources.Several studies suggest that C type(high Sr/Y)adakitic can be derived from amphibolites,ecogites,gneisses or granulites(Zhang et al.2001;Ge et al.2002;Xu et al.2006,2013;Wang et al.2007).The geochemical feature of the C type adakite in this contribution indicate that the Early Jurassic monzogranites in NE Xing’an block were generated by the partial melting of amphibolite and garnet amphibolite in a thickened lower continental crust setting,as discussed below.

Fig.7 a Mg#versus SiO2diagram(Long et al.2015),b–e MgO versus SiO2,Th versus Rb/Sr,Ni versus SiO2,and Cr versus SiO2diagrams(Huang et al.2009),f Nb/Ta versus Zr/Sm diagrams(Long et al.2015)for adakitic rocks from NE Xing’an block.Major oxides are in wt%,and trace elements are in ppm

Many studies have proposed that both amphibole and garnet as residuals in the magma source can generate high Sr/Y and La/Y ratio melts(Davidson et al.2007;Macpherson et al.2006).The fact that HREE and Y have high garnet-melt partition coefficients means that residual garnet generally result in geochemical features of depletion in HREE and Y.And previous studies show that partial melting of a thickened lower continental crust with residual garnet will generate melts with (Ho/Yb)N＞1.2,Y/Yb＞10 and high(Dy/Yb)Nvalue(Ge et al.2002;Hu et al.2012).The geochemical feature that amphibole preferentially incorporates,MREE,means that hornblende in the residual will generate melts with flat HREE patterns with Y/Yb≈10,(Ho/Yb)N≈1 and relatively low(Dy/Yb)Nvalue(Ge et al.2002;Hu et al.2012).The Early Jurassic C type adakites have Y/Yb,(Ho/Yb)Nand(Dy/Yb)Nratios of 9.56–12.59(with an average value of 11.07),1.02–1.49(with a average value of 1.16),and 1.31–1.77(with a average value of 1.47),respectively.This indicates that both amphibole and garnet were residual within the source of these adakitic magmas,consistent with their low Nb/Ta(11.7–24.2,with an average value of 15.9)and high Zr/Sm(40.9–78,with an average value of 55.5)ratios(Fig.7f).Compared with the high Sr/Y group I granite in the SW Alxa block,Southern CAOB,which was generated by fluid-absent partial melting of mafic to intermediate rocks(Zhou et al.2016),samples in this study have lower(Dy/Yb)N,(La/Yb)Nand LREE values(Fig.8a–c),indicating relatively minor garnet in the residual.Meanwhile,the studied samples have higher zircon saturation temperatures(Tzr,calculated by Watson and Harrison 1983;the results show in Table 2)than high Sr/Y group II granite in SW Alxa block,which was derived from the partial melting of the crustal basement materials under fluid-present conditions(Zhou et al.2016),suggesting a fluid-absent melting condition(Fig.8d).In addition,the presence of residual garnet in magma sources also indicates that partial melting occurred in a thickened LCC(＞50 km;Atherton and Petford 1993;Wang et al.2007;Xu et al.2013).

Fig.8 Variation of a(Dy/Yb)N,b(La/Yb)N,c LREE and d Eu/Eu*versus zircon saturation temperature(Tzr)(calculated by Watson and Harrison 1983).Major oxides are in wt%,and trace elements are in ppm

Most of the samples show relatively low Nb/Ta ratios(Fig.7f),which support amphibolite melting,but not eclogite melting.Besides,predominant amphiboles in the residual preclude the possibility of granulitic or gneissic source,which mainly consist of plagioclases and quartzs.Furthermore,the relatively low Rb/Ba and Rb/Sr ratios of the C-type adakitic samples indicate derivation from basaltic sources(Fig.9;Sylvester 1998).Thus,it appears plausible for the NE Xing’an block adakitic granite samples to have been formed by the partial melting of a lowercrustal metabasaltic source.

5.3 Tectonic implications

Magmatic-type zircons from the NE Xing’an block adakitic granite yield consistent ages of 180 Ma,representing the crystallization age of the adakitic rocks.They exhibit typical subduction-related geochemical characteristics(Fig.5)and dominantly plot into the volcanic arc granite(VAG)and late-orogenic fileds(Fig.10a–c,Pearce et al.1984;Batchelor and Bowden 1985).Combined with the occurrence of 180 Ma calc-alkaline volcanic arc granitoids in the Xing’an block(Sui et al.2007;Wu et al.2011),the NE Xing’an block most likely developed as a continental arc during the early Jurassic time,with the subduction-related geochemical features of the adakitic rocks inherited from lower-crustal sources.The formation of the early Jurassic adakitic rocks in the studied region can be reasonably explained by an oceanic subduction system located in the north of the NE Xing’an continental arc,which was possibly thickened in a compressional environment.

Table 2 Major element(wt%)and trace element(ppm)for samples from NE Xing’an block

Fig.9 Rb/Ba versus Rb/Sr discrimination diagrams(Sylvester 1998)for monzogranite samples in NE Xing’an block

As the NE Xing’an block was in a continental arc setting during the Early Jurassic time,the delamination of a thickened lower crust that typically occurs in post collisional extension settings seems unlikely.Ridge subductionrelated adakitic rocks are always associated with high temperature magmatism,such as igneous charnockites and alkali-feldspar granites(Geng et al.2009).Our calculation results shows that the NE Xing’an block rocks yield Tzr of 775–793 °C,much lower than temperatures(≥900 °C)of magmatism generated by ridge subduction(Geng et al.2009).Besides,all the Tzr values are slightly lower than the solidus(～820–900 °C)for dehydration melting of amphibolites at low pressure.However,the solidus decline sharply at higher pressures(～10–15 kbar,Moyen and Stevens 2006;Qian and Hermann 2013;Wyllie and Wolf 1993).All these features support a derivation of the monzogranite samples from a thickened lower continental crust,which suggests that there was a crust thickening event before Early Jurassic in NE Xing’an block,NE China.

Fig.10 Tectonic discrimination diagram for samples from the NE Xing’an block.Note that Nb versus Y and Rb versus(Y+Nb)diagrams are from Pearce et al.(1984),and R1–R2 diagram is from Batchelor and Bowden(1985).VAG volcanic arc granites,COLG collision granites,WPG within-plate granites,ORG ocean-ridge granites.1 Mantle fractionates;2 pre-plate collision;3 post-collision uplift;4 late-orogenic;5 anorogenic;6 Sy-collision;7 post-orogenic.R1=4Si-11(Na+K)-2(Fe+Ti);R2=6Ca+2 Mg+Al

In recent years,a growing number of evidence indicates that the Mesozoic magmatisms in the Great Xing’an Range(GXAR)are related to the subduction of the Mongol-Okhotsk ocean.Firstly,Xu et al.(2013)suggested that the Early Jurassic igneous rocks in the Erguna massif are related to the subduction of the Mongol-Okhotsk oceanic plate,which was developed during the Late Paleozoic(Tomurtogoo et al.2005)and closed in the Middle Jurassic(Kravchinsky et al.2002;Tomurtogoo et al.2005).Secondly,Zhang et al.(2010)proposed that Mesozoic igneous rocks show an Eastward-younging migration,coincident with the west to east closure direction of the Mongol-Okhotsk ocean(e.g.,Zhao et al.1990;Yin and Nie 1996;Tomurtogoo et al.2005;Ying et al.2010).Furthermore,previous research demonstrates that the Paleo-Pacific plate displayed Northward and North–North-Eastward subduction during the Jurassic-Early Cretaceous,and had negligible influence to East Asian continent until the Late Cretaceous(Maruyama and Send 1986).Thus,the subduction of the Paleo-Pacific ocean should have limited influence on the GXAR,which is located far away from the Paleo-Pacific margin in the east.Thus,we conclude that the igneous rocks are more plausibly related to the southward subduction of the Mongol-Okhotsk ocean.Meanwhile,another remarkable tectonic event is the collision between the Jiamusi Massif and Songliao Terrane in the Early Jurassic in the southeast section of the studied area(Zhou et al.2009).The Jiamusi Massif,which might have been part of Gondwana(Wilde et al.1997),is thought to be an accretionary Terrane,instead of a part of the CAOB,and have collided with the CAOB at～185–166 Ma along the Mudanjiang fault zone(Wu et al.2011).The amalgamation of the Jiamusi block with the CAOB might provide a sustained westerly-directed compressive tectonic setting,which overprint to the thickened crust in our studied area.

Brie fly,the subducted-related geochemical features and crust thickening events in this study imply that Xing’an block was influenced by both the Mongol-Okhostsk ocean tectonic regime and collision between the Jiamusi Massif and Songliao Terrane.The onset of the westerly-directed compression of the amalgamation of the Jiamusi block with the CAOB,associated with southward subduction of the Mongol-Okhotsk ocean,resulted in the formation of C type adakite in NE Xing’an block.

6 Conclusions

Based on the study of whole rock major,trace elements and zircon U–Pb age data from monzogranite samples in NE Xing’an block,we draw the following conclusions:

1. The monzogranites in NE Xing’an block intruded at 180 Ma and showed geochemical features of C type adakitic rocks.

2. These C type adakitic rocks were most likely derived from the partial melting of a thickened lower continental crust,with dominant amphibole and minor garnet in the residual.

3. The subducted-related geochemical features imply that the Xing’an block was influenced by the Mongol-Okhostsk ocean tectonic regime,and experienced a compressive setting by the amalgamation of the Jiamusi block with the CAOB in Early Jurassic.

AcknowledgementsWe are most grateful to the staff of the State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences,Wuhan and the Wuhan Sample Solution Analytical Technology Co.,Ltd for their assistance during the U–Pb dating and trace element analyses.Professor Yang Wen and engineer Yu Xihuan from the Hei Longjiang Institute of Geological survey provided great help in filed work.We especially thank anonymous reviewers for their insightful and constructive comments.This work was supported by the regional geology and mineralization research program of Heilongjiang province(HLJKD201417).

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