Geochemical and geochronological studies of the Aketas granite from Fuyun County，Xinjiang：the implications of the petrogenesis and tectonic setting

Xiaofeng Wei·Xin Zhang·Jiuhua Xu·Rufu Ding· Guorui Zhang·Yong Tang

Geochemical and geochronological studies of the Aketas granite from Fuyun County，Xinjiang：the implications of the petrogenesis and tectonic setting

Xiaofeng Wei1，2·Xin Zhang3·Jiuhua Xu1·Rufu Ding2· Guorui Zhang1·Yong Tang3

As the wall rock of the Aketas gold deposit，the Aketas granite is about 45 km away from Fuyun County，Xinjiang Province.The zircon weighted mean U—Pb age of the Aketas granite is 309.0±4.7 Ma，indicating that the Aketas granite was emplaced during the late Carboniferous.The Aketas granite belongs to the High-K calcalkaline series，with SiO2content from 63.00 to 68.20%，K2O content from 3.06 to 4.49%and Na2O content from 4.14 to 6.02%.The Alkaline Ratio（AR）of the Aketas granite is high，from 1.89 to 3.47，and is 2.95 on average. The Aketas granite has lowPREE（92.42—122.73 ppm）and highPLREE/PHREE ratios（6.54—11.88）.For the trace elements，the Aketas granite is enriched in LILE（Rb，U，Th，K）and incompatible elements，and marked depleted in HFSE（Nb，Ta，P，Ti）.The geochemical characteristics of the Aketas granite suggest that it is a typical I-type and volcanic arc granite，and that the crystallization of clinopyroxene and hornblende is notable during the magmatic evolution.In combination with the regional tectonic studies，we propose that the emplacement of the Aketas granite implies the Altai and East Junggar area was still dominated by a subduction system at～309 Ma.

The Central Asian Orogenic Belt·Volcanic arc granite·Subduction·Zircon U—Pb age

1 Introduction

As one of the largest Phanerozoic orogenic belts in the world，the Central Asian Orogenic Belt is located among the Siberia，Eastern Europe，Tarim and North China Cratons（Sengo¨r et al.1993；Badarch et al.2002；Jahn et al. 2004；Yakubchuk 2004；Xiao et al.2008）.From the Neoproterozoic to the late Paleozoic，the Central Asian Orogenic Belt had experienced subduction，terrane accretion，craton collision and post-collisional extension.Such a long-term，complex tectonic evolution not only contributed to its massive continental crust growth in the Phanerozoic，but also played a crucial role in the formation of the Central Asian metallogenic province（Sengo¨r et al. 1993；Xiao et al.2004，2009，2010；Chen and Jahn 2004；Windley et al.2007）.

The Siberia-Altai block and Kazakhstan-Junggar block are the important parts of the Central Asian Orogenic Belt，and are located in Russia，Kazakhstan，China and Mongolia-Kazakhstan（Fig.1a，Chen and Jahn 2004）.The late Paleozoic tectonic evolution of the Siberia-Altai block and Kazakhstan-Junggar block has drawn extensive attention for the past years（Filippova et al.2001；Li et al.2003；Zhang et al.2003；Buslov et al.2004；Windley et al.2007；Xiao et al.2009，2010，2014）.Recent studies show that the Altai orogen was developed as an island arc within the Paleo-Asian Ocean during the early Paleozoic，and was accreted onto the southern margin of the Siberia block during the middle Paleozoic（Sun et al.2008；Cai et al. 2011a，b，c；Xiao and Santosh 2014）.Collision between the Siberia and the Kazakhstan blocks probably occurredduring the late Paleozoic，and this collision acted directly between the Altai orogenic belt and the East Junggar（Filippova et al.2001；Li et al.2003；Zhang et al.2003；Buslov et al.2004；Windley et al.2007；Xiao et al.2009，2010，2014）.

Fig.1 a Tectonic sketch map of the Junggar-Altai area；b Geological sketch map of the studied area；c geological sketch map of the Aketas granite（modified after Wang and Xu 2006；Pan et al.2012）

However，there is a considerable debate on the age of the subduction-collision of the Altai and Junggar blocks：from Ordovician to Silurian（Kheraskova et al.2003），from Devonian to Carboniferous（Hendrix et al.1996），and from Carboniferous to Permian（Filippova et al.2001；Li et al.2003；Zhang et al.2003；Buslov et al.2004）. After the final collision between the Siberia and Kazakhstan plates，the Altai and East Junggar region was dominated by a post-collision extensional tectonic setting（Wang et al.2003，2010；Chen 2011；Ren et al.2011；Lu¨ et al.2012a，b）.

The Aketas granite is located at the contact zone between the Altai orogen and East Junggar，which is obviously an ideal place to study the tectonic evolution of Altai orogen and East Junggar（He et al.1994；Li et al. 2003，2006；Xiao et al.2004；Yang et al.2010；Li et al. 2010；Wang et al.2010；Dong et al.2012）.However，little is known about the age and petrology of the Aketas granite.Thus，we carriedoutthe geochemicaland geochronological studies of Aketas granite in this work，in order to unravel the petrogenesis and the tectonic setting of the granite.

2 Regional geological background

Aketas granite is located at the contact zone of the NWW-trending Sarbulak-Aketas and NNW-trending Kayierte faults（Fig.1b）.Theexposedstratainthestudiedareaarethe Middle Devonian Beitashan group（D2b），Yundukala group（D2y）andlateCarboniferousNanmingshuigroup（C1n）.The Beitashan group（D2b）is mainly composed of andesite breccia，andesitic volcanic，limestone and tuffaceous sandstone，and the Nanmingshui group（C1n）is comprised of carbonaceoussandstoneandlimestone.Extensivemagmatic activities occurred within this region，and can be subdivided into two phases：during the Devonian，this region was dominated by mafic magmatism，including the rock types of diabase，diorite，dioritic porphyrite；and during the Carboniferous，the leading magmatism was mafic-acidicbimodal，producing the Kalatongke gabbro-norite and the Aketas granite（Fig.1b）.

Fig.2 Field and microscope photographs for the Aketas granite：a field photograph of the Aketas granite；b hand specimen picture of the Aketas granite；c—f microscope pictures of the Aketas granite.Pl plagioclase，Q quartz，Ser sericite，Pr perthite，Mic microcline perthite

Fig.3 The cathodoluminescence（CL）images of the zircons from the Aketas granite

Table 1 Zircon U—Pb dating of the Aketas granite

3 Petrological characteristics of Aketas granite

The Aketas granite has two oval-shaped rock bodies.The northwestern rock body is defined by a cropping of 400×300 m2，while the other one on the southeast has an area of 400×100 m2（Fig.1c）.The Aketas granite intruded into the middle Devonian Beitashan group（D2b）（Fig.1b）.The alteration of the wall rocks mainly shows phyllite sericitization，silicification，epidotization，potassic alteration.

The Aketas granite is light red and light gray in color，with a subhedral granular texture and massive structure in the hand specimen.The rock-forming minerals mainly includeplagioclase（40%—50%），K-feldspar（30%—40%），quartz（10%—15%）and biotite（1%—2%），with a small amount of accessory minerals of zircon，apatite，and magnetite.Under the microscope，the plagioclase is grey in color，and is a hypidiomorphic column，with the size of 0.2—3.0 mm.The polysynthetic twins are developed，with sericitization and clayization alteration.The K-feldspar with a size of 0.3—3.0 mm is a hypidiomorphic-euhedral column in shape.The quartz with the size of 0.5—3.5 mm is a xenomorphic granular（Fig.2）.

4 Analytical samples

4.1 Samples characteristics

We collected a total of 30 outcrop samples and core samples of the Aketas granite in this study，and tried to avoid contact zones and alteration zones in order to ensure that the samples were fresh.After careful microscopic identification，we picked out six samples on which to conduct the major and trace elements geochemical analysis.Samples for the LA-ICP-MS zircon U—Pb geochronological analysis were all taken from the drill-core，and the coordinates of the sampling location was 46°44′01′N，89°48′04′E.

4.2 Analytical methods

Major and trace elements analyses of the Aketas granite were carried out in the analytical laboratory of Beijing Research Institute of Uranium Geology.Major elements were carried out with the X-ray fluorescence（XRF）spectrometry method，XRF was implemented in accordance with the national standard GB/T 14506.28-1993，with RSD＜2.5%.The FeO contents were determined by wet chemical analysis separately，and the loss on ignition（LOI）was obtained by baking it in the oven at 1000°C high temperature with 90 min，then weighing.Analysis for trace elements was performed by Element I high-resolutioninductively coupled plasma mass spectrometer（HR-ICPMS）made by Finnigan MAT，with RSD＜3%.The analytical processes were performed with room temperature of 20°C and relative humidity of 30%.

Fig.4 U—Pb concordia diagrams of zircons from the Aketas granite

Table 2 Major element contents of the Aketas granite（wt%）

The LA-ICP-MS zircon U—Pb dating was carried out at the State Key Laboratory of Continental Dynamics of the Northwest University.The instrument for the ICP-MS is a combination of the Geo Las 200 M laser ablation system with the Elan 6100DRC ICP-MS made by German Micronas.During the analysis，a laser frequency of 10 Hz， with a laser diameter of 30 μm and laser energy of 32—36 mJ were adopted，with the ablated depths of 20—40 μm.International standard zircon 91500 was used as the external standard.Details of the analytical methods and data processing approach are referred to Yuan et al.2003. The apparent and field dia U—Pb ages were calculated by using the ISOPLOT program（Ludwig 1991）.Uncertainty of individual analysis was reported with 1σ and the weighted mean206Pb/238U age was calculated at the 2σ level.

5 Analytical results

5.1 Zircon U-Pb field dialog

The zircons of the Aketas granite are long columnar，euhedral，pale yellow and transparent crystals with a length of 0.05—0.15 mm and a width of 0.05—0.10 mm.

A total of 18 spots of zircons from the Aketas granite are selected for the determination of the U and Pb isotope.The concentric oscillatory zones of the zircons are observed clearly in the cathodoluminescence（CL）images，showing the typical characteristics of the magmatic zircons（Fig.3）. However，some zircons have a dark rim，which was probably produced by the late hydrothermal activity，such as the spots of AG-01-01，AG-01-03，AG-01-05，AG-01-08，or AG-01-16（Fig.3）.The U—Pb isotopic compositions of the 18 analyzed zircons are presented in Table 1，showing the features of magmatic zircons with high Th/U ratios of 0.63—1.47，with an average of 0.89（Hoskin and Black 2000）.

In the diagram，16 analyzed spots are very close to the concordantline，withaweightedaverageageof 309.0±4.7 Ma（MSWD=1.4）（Fig.4）.Meanwhile，two spots of AG-01-05 and AG-01-16 are deviated from the concordant line，and have younger U—Pb ages of 225.8 and 232.7 Ma.In the CL image，these two spots are quite near the rim of the zircon，and reach the dark edge.In addition，their Th/U ratios are 0.63 and 0.69，lower than the average value of 0.89 of the 18 spots.Therefore，these two measurement points are likely to have been affected by late hydrothermal metasomatism，and they can not represent the native zircon ages（Fig.3）.

5.2 Major element geochemistry

Fig.5 Classification diagrams of the Aketas granite：a QAP diagram；b TAS diagram；c K2O versus SiO2diagram；d A/NK versus A/CNK diagram（after Maniar and Piccoli 1989；Wilson 1989）

The Aketas granite's SiO2content ranged from 63.00%to 68.20%，K2O content from 3.06%to 4.49%and Na2O content from 4.14%to 6.02%（Table 2）.The major elements show features of high FeO and Al2O3（with FeO content of 3.18%—5.98%and Al2O3content of 15.97%—17.45%），low MgO，TiO2and CaO（with MgO content of 0.22%—1.07%，TiO2content of 0.21%—0.36%and CaO content of 1.14%—2.85%）.In the QAP and TAS diagrams，the plots of the Aketas granite fall mainly within the‘quartz monzonite diorite''and the‘quartz monzonite''fields.Combined with the microscopic observations，we named it as quartz monzonite（Fig.5a，b）.

In the SiO2—K2O diagram，the samples of the Aketas granite are distributed in the high-K calc-alkaline series（Fig.5c）.In the A/CNK-A/NK diagram，most of the samples fall into the‘metaluminous''or weakly‘peraluminous''area，showing features on a transition from calcalkaline to alkaline（Fig.5d）.In addition，the Rittman index（σ）of the Akrtas granite is from 2.82 to 3.89.

5.3 Trace elements

In the primitive mantle normalized spider diagram（Fig.7），samples from the Aketas granite are enriched in LILE（Rb，U，Th，K）and incompatible elements，and are relatively depleted in high field strength elements（Nb，Ta，P and Ti），illustrating the participation of a large number of crust source material during the magmatic processes.

Table 3 Trace element compositions of the Aketas granite（10-6）

6 Discussion

6.1 Zircon U-Pb age

The zircon LA-ICP-MS U—Pb dating of the Aketas granite gives a weighted mean age of 309.0±4.7 Ma，indicating a late Carboniferous age for the emplacement of the granite.

Fig.6 Chondrite-normalized REE pattern diagram of the Aketas granite（after Sun and McDonough 1979）

Fig.7 Primitive mantle normalized spider diagram of the Aketas granite（after Sun and McDonough 1979）

6.2 Petrogenesis

6.2.1 Rock classification

In the 10000Ga/Al versus Ce diagram and the 10000Ga/Al versus Zr diagram，the samples of the Aketas granite are plotted into the I-，S-，M-type granite area（Fig.8a，b）.In addition，sample spots of the Aketas granite in the SiO2versus Zr diagram and the SiO2versus P2O5diagram show a good match with I-type granite，ruling out the possibility of a S-type granite（Fig.8c，d）.Therefore，the Aketas granite can be classified as an I-type granite.

Fig.8 Geochemical classification diagrams of the Aketas granite：a 10000Ga/Al versus Ce；b 10000Ga/Al versus Zr；c SiO2versus Zr；d SiO2versus P2O5（after Collins et al.1982；Huang et al.2013）

6.3 Magmatic evolution

The analytic data of the Aketas granite are consistent with the trend of crystallization differentiation in the Zr versus Zr/Nb diagram，illustrating that its magmatic composition is mainly controlled by crystallization differentiation rather than partial melting during the magmatic evolution（Fig.9a）.As shown in the Sr versus Rb/Sr diagram，compared with plagioclase and biotite，the fractional crystallization of orthopyroxene and amphibole is much stronger，which is compatible with the slight Eu depletion feature in the REE diagram（Fig.9b）. Likewise，in the Eu/Eu*versus Ba diagram，the sample plots of the Aketas granite show that the fractional crystallization of plagioclase and K-feldspar is obviously limited（Fig.9c）. Moreover，as shown in Fig.9d，sample plots from the Aketas granite are parallel to the crust contamination line，indicating a great deal of involvement from the crustal materials.

In the primitive mantle normalized spider diagram，samples of the Aketas granite show a negative anomaly of Nb and Ta，indicating a significant contamination of crustal materials（Fig.7，Rudnick and Gao 2003）.Commonly，the crustal material is accepted to be sourced from either the partial melting of ancient crust（Nelson et al.1986；Bernard-Griffiths et al.1991）or the subduction-related fluids and sediments（Tatsumi et al.1986；Donnelly et al.2004）. Previous studies suggested that the subduction-related fluids are featured by enrichment in LILE（Rb，Ba，Sr），U and Pb（Seghedi et al.2001），hence the high Ba/Nb，Sr/Th andBa/Thratios（46.75—128.18，60.84—250.18and 117.92—271.27，respectively）of the Aketas granite indicate an involvement of subduction-related fluids during the magmatic evolution.In addition，the depletion of Th of the Aketas granite may imply that the involvement of oceanic sediments is limited（Guo et al.2006）.

Fig.9 Diagrams for magmatic evolution of the Aketas granite：a Zr versus Zr/Nb；b Sr versus Rb/Sr；c Eu/Eu*versus Ba；d Th/Nb versus Zr（after Eby 1990；Geng et al.2009；Zhong et al.2013）

6.4 Tectonic environment

As shown in the Zr versus Zr/Nb diagram and Sr versus Rb/ Sr diagram，the spots of the Aketas granite are plotted within the island-arc-granite area，indicating that the granite was originated from an island arc tectonic setting（Fig.10a，b）.In the（La/Sm）Nversus Ba/Th diagram and the Al2O3+Fe2O3+MgO+TiO2versus Al2O3/（Fe2-O3+MgO+TiO2）diagram，the plots of the Aketas granite match with the trend lines of‘fluid added''and‘high pressure''，indicating that a great amount of fluids were involved in its magma source and that it was formed in a high pressure tectonic environment，which was well accordant with the island-arc magmatism（Fig.10c，d）.

6.5 Petrogenetic model

The geochemical characteristics of the Aketas granite suggest that it is an I-type island arc granite with intensive crystallization of orthopyroxene and amphibole.As to island-arc magmatism，the Aketas granite was probably derived from the partial melting of the mantle wedge and/ or oceanic curst with the contamination of slab materials，which corresponds to the feature of fluid involvement，as shown in Fig.10c（Pearce et al.1984）.Therefore，the petrogenesis of the Aketas granite can be divided into the following three stages：（1）the subducted slab dehydrates due to high pressure；（2）the fluid from the subducted slab triggers the mantle wedge and/or oceanic crust to partial melt；（3）the magma moves upward and crystallizes into granite after a long evolution（Fig.11）.

Fig.10 Tectionic discrimination diagrams of the Aketas granite：a Y+Nb versus Rb；b Rb/30-Hf—Ta*3；c（La/Sm）Nversus Ba/Th；d Al2O3+Fe2O3+MgO+TiO2versus Al2O3/（Fe2O3+MgO+TiO2）（after Pearce et al.1984；Geng et al.2009）

Fig.11 Sketch map of the tectonic setting of the Altay-East Junggar area during the late Carboniferous（after Xiao et al. 2008；Shen et al.2011）

RecentzirconU—Pbdatingresultsforigneousrocksindicate that the magmatism was widespread in the Altai-Junggar area，continuously from early Paleozoic to early Mesozoic（Zhu et al.2006；Briggs et al.2007；Yuan et al.2007；Long et al.2007；Sun et al.2008；Cai et al.2011a，b，c；Shen et al. 2011）.It is noted that the subduction-related magmatism lasted from 497 Ma to 313 Ma，and the first record of postcollision magmatism of Saertielieke granite in the East Junggar was dated at 308 Ma（Long et al.2007；Sun et al. 2008；Cai et al.2011a，b，c；Shen et al.2011；Wang et al. 2011）.ThegeochemicalcharacteristicsoftheAketasgranite indicate that it was probably derived from the island-arc magmatism in a subduction tectonic setting（Fig.11）.In combination with the geochronology of the rock，it can beproposed that the Altai-Junggar area was still controlled by subduction at～309 Ma.More recently，after systematic discussions of the petrogenesis and tectonic setting of the granites in the Altai-Junggar area，a late Carboniferous to Early Permian collision is becoming more acceptable to an increasing number of authors（Xiao et al.2008，2009，2010，2014；Chen 2011；Han et al.2010，2011；Xu et al.2014；Zhang et al.2014；Zhang and Zhang 2014；Chen et al.2015；Muhetaer et al.2015；Yang et al.2015）.Therefore，the emplacement of this island-arc granite of the Aketas granite demonstrates that the collision of the East Junggar and the Altai orogen probably occurred after～309 Ma，which provides a new evidence for further studies on the exact collision time between the Siberia and Kazakhstan blocks.

7 Conclusions

（1） The zircon U—Pb isotope data demonstrate that the Aketas granite was emplaced at 309.0±4.7 Ma.

（2） The Aketas granite belongs to the High-K cal-calkaline series，with a SiO2content from 63.00%to 68.20%，K2O content from 3.065%to 4.49%and Na2O content from 4.14%to 6.02%.Combined with microscopic observation，we named it as a quartz monzonite.

（3） The geochemical characteristics reveal that the Aketas granite is an I-type island arc granite.During the magmatic evolution of the Aketas granite，the fractionalcrystallizationofclinopyroxeneand amphibole was intense，and a great deal of crustal materials have involved in its magma source.

（4） Geochronological studies of the Aketas granite suggest that Altai-East Junggar area was still dominated by a subduction tectonic setting at～309 Ma.

AcknowledgmentsThis research project was jointly financially supported by the Mineral Prospecting and Assessment project，CGS（1212011085020）and the National Nature Science Foundation of China（40972066）.

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10.1007/s11631-015-0071-5

25 November 2014/Revised：17 April 2015/Accepted：25 August 2015/Published online：12 September 2015

✉ Xin Zhang

zhangxin869@126.com

1University of Science and Technology Beijing，Beijing 100083，People's Republic of China

2Beijing Institute of Geology for Mineral Resources，Beijing 100012，People's Republic of China

3Key Laboratory of High-temperature and High-pressure Study of the Earth's Interior，Institute of Geochemistry，Chinese Academy of Sciences，Guiyang 550002，People's Republic of China

©Science Press，Institute of Geochemistry，CAS and Springer-Verlag Berlin Heidelberg 2015