Study of oxygen fugacity during magma evolution and ore genesis in the Hongge mafic-ultramafic intrusion，the Panxi region，SW China

Mingyang Liao·Yan Tao·Xieyan Song·Yubang Li·Feng Xiong

Study of oxygen fugacity during magma evolution and ore genesis in the Hongge mafic-ultramafic intrusion，the Panxi region，SW China

Mingyang Liao1，2·Yan Tao1·Xieyan Song1·Yubang Li1，2·Feng Xiong1，2

DOI 10.1007/s11631-015-0064-4

Economic concentrations of Fe-Ti oxides occurring as massive layers in the middle and upper parts of the Hongge intrusion are different from other layered intrusions（Panzhihua and Baima）in the Emeishan large igneous province，SW China.This paper reports on the new mineral compositions of magnetite and ilmenite for selected cumulate rocks and clinopyroxene and plagioclase for basalts.We use these data to estimate the oxidation state of parental magmas and during ore formation to constrain the factors leading to the abundant accumulation of Fe-Ti oxides involved with the Hongge layered intrusion.The results show that the oxygen fugacities of parental magma are in the range of FMQ-1.56 to FMQ+0.14，and the oxygen fugacities during the ore formation of the Fe-Ti oxides located in the lower olivine clinopyroxenite zone（LOZ）and the middle clinopyroxenite zone（MCZ）of the Hongge intrusion are in the range of FMQ-1.29 to FMQ-0.2 and FMQ-0.49 to FMQ+0.82，respectively. The MELTS model demonstrates that，as the oxygen fugacity increases from the FMQ-1 to FMQ+1，the proportion of crystallization magnetite increases from 11%to 16%and the crystallization temperature of the Fe-Ti oxides advances from 1134 to 1164°C.The moderate oxygen fugacities for the Hongge MCZ indicate that the oxygen fugacity was not the only factor affecting the crystallization of Fe-Ti oxides.We speculated that the initial anhydrous magma that arrived at the Hongge shallow magma chamber became hydrous by attracting the H2O of the strata.In combination with increasing oxygen fugacities from the LOZ（FMQ-1.29 to FMQ-0.2）to the MCZ（FMQ-0.49 to FMQ+0.82），these two factors probably account for the large-scale Fe-Ti oxide ore layers in the MCZ of the Hongge intrusion.

Oxygen fugacity·Fe-Ti oxide deposit· Basalts·Emeishan large igneous province·Hongge layered intrusion

1 Introduction

Magmatic Fe-Ti oxide ores are normally associated with layered mafic-ultramafic intrusions or Proterozoic anorthosite complexes（Cawthorn 1996；Force 1991；Lister 1966）.The oxygen fugacity（f O2）of a melt is a critical controlling parameter of the magmatic processes，as it controls the iron redox state of the melt（Kilinc et al.1983；Botcharnikov et al.2005）and strongly inf l uences the crystallization sequences and composition of the crystallizing minerals.However，several models，which include gravitational differentiation（Charlier et al.2006，2009；Wager and Brown 1968）and increasing oxygen fugacity（Botcharnikov et al.2008；Toplis and Carroll 1995）have been proposed for the formation of the Fe-Ti oxide ore layers located in the mafic-ultramafic intrusions.

The Panxi（full name:Panzhihua-Xichang）area，which contains world-class Fe-Ti-V oxide deposits，is the most important Fe-Ti-V oxide ore district in China（Panxi Geological Unit 1984）.The major thick V-Ti-magnetite ore layers are commonly concentrated in the lower and middle parts of the Panzhihua and Baima intrusions.However，the economic Fe-Ti oxide ore layers occur in the middle zone of the Hongge intrusion，instead of the lower zone（Figs.1，2）.Therefore，we speculated that the Hongge intrusion most likely has its own characteristics of magmatic evolution and metallogenesis in comparison to the other two typical intrusions（Panzhihua and Baima）.With such a special phenomenon occurring in the Panxi area，many researchers have proposed that the different oxidation state of the parental magma can account for this diversity（Pang et al.2008；Bai et al.2012a）.Gannio et al.（2008）demonstrated that the CO2-rich liquid，which was produced by the interaction between the basaltic parental magma and dolomitic limestone，elevated the oxygen fugacity of the magma chamber and caused the Fe-Ti oxides to crystallize relatively early and settle down into the lower parts of the Panzhihua intrusion.Similarly，it has been suggested that the extensive accumulation of the Fe-Ti oxides and the formation of the massive ore layers in the middle zone of the Hongge intrusion was due to the early crystallization of the Fe-Ti oxides，which resulted from increased oxygen fugacity due to the assimilation of the footwall limestone（Bai et al.2012a）.

Whether high oxygen fugacity was required during the evolution of the basaltic magma is one of the most critical factors needed to better understand the accumulation of the Fe-Ti oxides in the Panxi area.In this paper，we present the new Fe-Ti oxides and silicates mineral data to estimate the oxygen fugacity of different periods during the magmatic evolution of the basaltic magma and to distinguish the factors controlling the crystallization of Ti-magnetite for the Hongge intrusion.In addition，we adopted the traditional method to reconstruct the original Fe-Ti oxide composition in order to retrace the f O2and T related to the Fe-Ti oxides located in the lower and middle zone of the Hongge mafic-ultramafic intrusion，and further constrain the ore-forming conditions of whether high oxygen fugacity is required for the formation of the Fe-Ti oxide ore layers in the middle zone of the Hongge intrusion.

Fig.1 Geological map showing the distribution of the Emeishan large igneous province（a；ELIP），the distribution of the major Fe-Ti-V oxidebearing intrusions in the Panxi region（b；modified after Pang et al.2008a），and geological map of the Hongge intrusion（c；modified after Sichuan Geological Survey，2010）

Fig.2 Simplified stratigraphy of the Panzhihua，Hongge，Baima and Xinjie（modified after Pang et al.2010）intrusions.FW footwall，HW hangingwall，LZ lower zone，MZ middle zone，UZ upper zone，LOZ lower olivine clinopyroxenite zone，MCZ middle clinopyroxenite zone，UGZ upper gabbro zone and OGZ olivine gabbro zone

2 Geological background and Petrography of the Hongge intrusion

The Emeishan large igneous province（ELIP）in Southwest China consists of the Late Permian continental f l ood basalts associated with mafic-ultramafic intrusions（Fig.1）；it is believed to have originated from a mantle plume（Chung and Jahn 1995；Xu et al.2001；Song et al. 2001；Zhong and Zhu 2006；Zhong et al.2002，2006）.

The Panxi area is located in the central part of the ELIP，where several mafic-ultramafic layered intrusions host the most economically important Fe-Ti oxide deposits in China（Fig.1）.The exposure of these layered intrusions are controlled by major N-S trending faults，including the Hongge（259±1.3 Ma，Zhong and Zhu 2006）and Xinjie（259±3 Ma，Zhou et al.2002）mafic-ultramafic intrusions，and the Panzhihua（263±3 Ma，Zhou et al.2005），Baima（262±3 Ma，Zhou et al.2008）and Taihe（262±3 Ma，Guo et al.2004）mafic intrusions.These five intrusions contain a total ore reserve of～7209 Mt Fe2O3，～559 Mt TiO2，and～17.4 Mt V2O3with grades of 27 wt% FeO，10.6 wt%TiO2，and 0.24 wt%V2O3（Ma et al.2003； Zhong et al.2005）.The ore-bearing intrusions are spatially associated with contemporaneous f l ood basalts（Zhang et al. 1999）and their parental magma is probably genetically related to the high-Ti basalts of the ELIP（e.g.，Hongge，Xinjie，Baima and Panzhihua；Wang et al.2008；Zhong et al.2002，Zhong et al.2003，2004；Zhou et al.2008）.

The Hongge layered intrusion is a NNE-striking elongated lopolith，sized 15 km-long，3-5 km-wide，and 1.2 kmthick，located to the northeastofPanzhihua City（Fig.1）.The geology ofthe Hongge intrusion has been described in detail by Zhong et al.（2002，2003），Bai et al.（2012a）and Wang et al.（2013）.A brief description is given below.The～260 Ma ELIP（Chung and Jahn 1995）is coeval with the Hongge mafic-ultramafic intrusion（259±1.3 Ma，Zhong and Zhu 2006），which indicates that the regional tectonics of ELIP is related to the mine geology of the Hongge intrusion. The Hongge intrusions have wall-rocks of dolomitic limestone from the Sinian Dengying Formation and granitic gneisses of the Neoproterozoic Kangding Complex.The Dengying Formation is composed of dolomitic limestone，which hasmetamorphosed to marble in places adjacentto the layered intrusion.The west and north contact zones of theintrusion were intruded by the late Permian alkaline granites and alkaline syenites，as shown in Fig.1（Zhang etal.1999）. Part of the intrusion at the northeast corner is overlain by a～180 m-thick basaltic sequence of the Emeishan basalts（Fig.1b，c）.Based on the cumulus minerals and lithological textures（Zhong et al.2002），the Hongge intrusion was divided into a lower olivine clinopyroxenite zone（LOZ），a middle clinopyroxenite zone（MCZ）and an upper gabbro zone（UGZ）.The LOZ and MCZ are characterized by the appearance and disappearance of olivine，whereas the UGZ is defined by the appearance of abundant euhedral apatite. The massive Fe-Ti oxide bodies mainly occur in the upper part of the LOZ and the lower part of the MCZ（Zhong et al. 2002）.

The thickness of the LOZ is about 340 m.It is composed of medium-to fine-grained rocks containing cumulus olivine，magnetite and ilmenite and minor chromite，and，in its lower part，intercumulus clinopyroxene（cpx）and hornblende.In some samples，magnetite and ilmenite are also present as small inclusions in the olivine and cpx crystals；this type of magnetite is thought to be formed contemporaneously with olivine.The LZ rocks are characterized by plenty of hornblende and are comprised of hornblende clinopyroxenite and hornblende olivine clinopyroxenite.The hornblende clinopyroxenite contains 70-75 modal%cpx，8%-20%Fe-Ti oxides，7%-10% hornblende，less than 5%olivine and minor plagioclase（pl）and apatite.

The MCZ is comprised of lherzolite and olivine clinopyroxenite at the bottom and clinopyroxenite at the top，which contains cumulus pl.Compared to the LOZ，the MCZ contain less hornblende.The massive ore consists of more than 85%Fe-Ti oxides and less than 15%olivine or cpx.The magnetite olivine clinopyroxenite contains 20%-50%Fe-Ti oxides，20%-30%olivine and 30%-35% cpx.The magnetite-bearing gabbro consists of 20%-28% Fe-Ti oxides，30%-45%cpx and 25%-37%pl.Hornblende in the MCZ is interstitial，generally lower than 3% and locally up to 5%-8%.The MCZ has more interstitial magnetite and ilmenite than the LOZ.Minor amounts of Cr-spinel and olivine are located in the rocks beneath the massive Fe-Ti oxide layers.The abundance of pl increases progressively from the base to the top of the MCZ and UGZ.Some cpx crystals at the base of the MCZ contain exsolved Fe-Ti oxides.The MCZ contains the largest and richest economic Fe-Ti-V oxide layers.

The 527-to 1346-m-thick UGZ is composed of pl，cpx，and minor olivine at its base.The UGZ is characterized by an abundance of apatite，and consists of apatite gabbro with a few interlayers of apatite magnetite gabbro.The apatite gabbro generally contains＜15%Fe-Ti oxides，20%-45%cpx，40%-50%pl，5%-7%apatite and＜3%-5% hornblende（locally up to 10%）.The general crystallization order inferred from cumulus phases in the LOZ and MCZ is:olivine→olivine+chromite+Fe-Ti oxides→olivine+clinopyroxene+Fe-Ti oxides→clinopyroxene+Fe-Ti oxides.In addition，the order inferred from cumulus stratigraphy of the UGZ is: clinopyroxene+plagioclase+Fe-Ti oxides+apatit→clinopyroxene+plagioclase+apatite.

3 Sampling and analyses

The samples analyzed in this study were collected from the surfaces located in the south and northwest parts of the Hongge intrusion（Fig.1c）.Thirty-one samples（HG0-6，8-28，30-32），including olivine clinopyroxenite，clinopyroxenite，gabbro and massive ores from the lower zone to upper zone，were collected from the surface of Tongshan and fifteen basaltic rocks（BC1-4 and BFQ1-11）were collected from Baicao and Banfangqing，respectively.

The compositions of magnetite，ilmenite，cpx and pl in the samples were determined by wavelength-dispersive X-ray analysis using an EPMA-1600 electron microprobe（EMP）at the SKLODG.Major elements were analyzed using a 10 μm beam with a current of 10 nA and peak counting time of 20 s.The analytical conditions included a beam current of 25 nA and an acceleration voltage of 25 kV.A small beam size of 1 μm was used to determine the composition of exsolution lamellae.A large beam size of 30 μm was used to ablate both the magnetite and its ilmenite exsolution to avoid the effects of subsolidus exsolution on the magnetite.The accuracy of the analysis was monitored using mineral standards:pyrope for Mg，Al，Cr and Mn，rutile for Ti，magnetite for Fe and olivine for Ni.The detection limits for these elements are 0.01%，with errors within 2%.The compositions for each sample of magnetite and ilmenite for intrusive rocks and cpx and pl for basalts are listed in Tables 1 and 2.

4 Estimation of oxygen fugacity

4.1 Estimation of oxygen fugacity（f O2）from basalts

The recent study of France et al.（France et al.2010）indicated that the oxygen fugacity（expressed as the ΔFMQ）of basaltic magma could be estimated with the microprobe analyses of the two most common minerals，pl and cpx，in basalts.The method is based on the different partitioning behavior of ferric and ferrous iron between cpx and melt，as well as pl and melt.The links between the Fe2O3/FeO ratio of the melt and the partitions coefficients for the cpx and pl to the melt can be expressed as:

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With a=0.207；b=12，980；c=-6.115；dSiO2= -2.368；dAl2O3=-1.622 and dCaO=2.073.With T as the temperature in K，and Xibetween 0 and 1 calculated from oxides（in wt%）.As the partition coefficients have been derived from experiments performed under shallow pressures ranging between 0.1 and 0.5 GPa，we employed the interval value of 0.3 GPa（Table 2）when using this model.Besides，for a given composition at a fixed temperature，ln（Fe203/FeO）meltis always linear with log（f O2），with a slope of 1/2（Kress and Carmichael，1991），so we calculated temperature and f O2with the formula（2）above. The redox state of a melt can be better understood by comparing the f O2to oxygen buffers.We recall here the relations corresponding to the FMQ from Ballhaus et al.（1993）:

Based on the method above，we obtained the oxygen fugacities（FMQ-4.53 to FMQ+0.14）when the basalts crystallized，which are listed in Table 2.The results show that the high-Ti basalts have relatively higher oxygen fugacities than that of the low-Ti basalts，the former has ΔFMQ in range from-1.56 to+0.14，whereas the latter has a range from-4.53 to-2.06.

4.2 Estimation of oxygen fugacity（f O2）from the Fe-Ti oxide of the intrusion

Several methods are effective in estimating the redox state of a melt（Herd 2008）.The most popular and traditional method we employed was the oxybarometer，which was based on the coexisting magnetite and ilmenite in the basaltic series（Buddington and Lindsley 1964；Andersen and Lindsley 1985；Ghiorso and Sack 1991；Ghiorso and Evans 2008）.However，the processes that re-equilibrated between the magnetite and ilmenite and the solid solution（e.g.UspMt or IlmHem）separation during magma slowcooling commonly masked the real information about the compositions of the primary Fe-Ti oxides，which are expressed by the equilibrium，respectively:

the effects in many cases can be compensated for by reintegrating the ilmenite lamellae Hematite-rich ilmenite=ilmenite+magnetite-ulvospinel+O2into the magnetite composition（e.g.，Buddington and Lindsley 1964；Bohlen and Essene 1977）.Therefore the compositional recovering work should be done before the calculation.

Subsolidus oxidation of the ulvospinel component commonly results in two types of exsolution textures:the ribbon exsolution ilmenite and the granule exsolution ilmenite；the former usually appears as a sandwich or trellis type ilmenite exsolution lamellae in magnetite（Fig.4b）and the latter was not found in our samples.The subsolidus reduction of the hematite commonly appears as small grains of magnetite surrounding the ilmenite（Fig.4c）.We chose the inclusions of the Fe-Ti oxide in the olivine and cpx（Fig.3a，b）from the Hongge intrusion to reconstruct the original homogeneous oxide composition as a simple exsolution situation comparable to the samples from the massive ore.Then we applied similar methods（Buddington and Lindsley 1964；Bohlen and Essene 1977；Bowles 1977）to calculate the T and f O2，which the Ti-magnetite and ilmenite last equilibrated.We obtained the composition of primary magnetite and ilmenite（C in Table 1）through the composition（A in Table 1）of magnetite（host）/ilmenite（exsolution）and ilmenite（host）/magnetite（exsolution）by EMP and their area proportion（B in Table 1）by the backscattered electron and ref l ected light microscope images.Based on C，we calculate the primary end-member of ulvospinel-magnetite and hematite-ilmenite solid solution（D in Table 1）.The interpretation proposed by Bohlen and Essene（1977）and Bowles（1977）that‘‘exsolved'' ilmenite was once ulvospinel solid solution in magnetite in equilibrium with primary ilmenite，so it is reasonable to use the reintegrating composition of magnetite and ilmenite directly or combined with composition of individual ilmenite（Fig.4f）and magnetite（Fig.4e）to curves byBuddington and Lindsley（1964）to get the temperature（814-1118°C）and oxygen fugacities（FMQ-1.29 to FMQ+0.82，where FMQ is the fayalite-magnetite-quartz buffer，E in Table 1）corresponding to the time when Fe and Ti was no longer able to exchange between magnetite and ilmenite.

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The composition of Fe-Ti oxide minerals in the Hongge intrusion is variable.Magnetite grains in the olivine have higher Fe2O3and lower TiO2than magnetite host ilmenite lamellae.The ilmenite grains in the olivine have higher FeO but lower Fe2O3than the ilmenite in host reductive exsolution phase of magnetite.The TiO2and end-member of Hem of the reintegrated ilmenite are relative lower than the ilmenite grains，which probably attributed to its high proportion of ilmenite in the primitive end-member（Table 1）.The calculated results of oxygen fugacities are shown in Fig.8，FMQ-1.29 to FMQ-0.2 and FMQ-0.49 to FMQ+0.82，respectively，for the Fe-Ti oxides located in the LOZ and MCZ of the Hongge intrusion.Carefully reconstruction and twenty-seven systematic analyses have been done to enhance the reliability of our results.

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5 Discussion

5.1 Oxygen fugacity（f O2）variation during magma evolution

f O2is a traditionalmonitorofoxidation state ofmelts，which largely controls the evolution processes ofmagmatic system（Kress and Carmichael 1991；Ottonello et al.2001）.As the chilled margins are absent in the Hongge area，we failed to constrain the oxidation state of the Hongge parental magma in a traditionalway.However，the recently study ofBaietal.（2014）indicated that the average compositions of the Longzhoushan high-Ti basalts can be used to represent the compositions of the parental magmas for the Hongge Fe-Ti oxide-bearing intrusion.In the results of the MELTS modeling shown in Fig.9a and b，at～1260 and 1155°C，the compositions of residual liquid are similar to our observed high-Ti（BFQ-2）and low-Ti（BC-1）basaltic samples（Liao et al.2015），and we conclude that the parental magma of the Hongge intrusion（suggested by Bai et al.2014），the Banfangqing high-Ti（BFQ-2）and Baicao low-Ti（BC-1）basalts are the product of magma evolution from the same primary magma.Besides，the petro-geochemistry in this study shows that the Baicao and Banfangqing and Longzhoushan basalts have similar characteristic in primitive mantle-normalized trace element patterns（Fig.5a）and chondrite-normalized REE patterns（Fig.5b）.So it is reasonable to presume that the Baicao and Banfangqing basalts，outcropped in the northeast of Hongge（Fig.1），represent the parental magma of the intrusion.

Fig.3 a Euhedral titanomagnetite（Mt）inclusion is enclosed in olivine（Ol），sample HG-6.b Titanomagnetite（Mt）inclusion is enclosed in clinopyroxene（Cpx）and the Mt grain association interstitial to clinopyroxenes（Cpx），sample HG-9.c Hornblende（Hbl）rim of clinopyroxene（Cpx）in the ore-bearing rock，sample HG-26.d Banfangqing basalt，sample BFQ-4.All the images is photoed under crossed polarized light

The basalts in Banfangqing and Baicao could be divided into high-Ti and low-Ti types（Table 2）.The calculation shows that the high-Ti basalts have relatively higher oxygen fugacities in values from FMQ-1.56 to FMQ+0.14，whereas the low-Ti basalts have relatively lower oxygen fugacities in values from FMQ-4.53 to FMQ-2.06.Xu et al.（2003）suggested that the relatively high oxygen fugacities for Longzhoushan basalts comparing with the Guizhou basalts were caused by crustal contamination.The Th/Ta ratios for Banfangqing high-Ti basalts（BFQ-4 and BFQ-7 located near the line of Th/Ta=4.6，Fig.6a）were higher than those in the low-Ti basalts（BFQ-5，BFQ-6 and BC-3，Fig.6a）.This difference was attributed to the fact that they evolved to different degree of upper crustal component of Yangzta craton（Th/Ta=13，Gao et al. 1998）during magmatic evolution.Moreover，the negative correlation between SiO2and Nb/La（Fig.6b），further indicates that the crustal contamination was most likely the key factor in increasing the Th/Ta ratios for high-Ti basalts.As shown in Fig.7，the high-Ti basalts of Banfangqing experienced higher degree of crustal contamination than the low-Ti basalts.This also confirmed the description above.However，the relatively low Th/Ta ratios and the low oxygen fugacity for the Banfangqing and Baicao low-Ti basalts indicated they probably experienced minor or little crustal contamination.Therefore，we speculate that the low oxygen fugacities（FMQ-4.53 to FMQ-2.06）of low-Ti basalts in the Hongge area indicating that they originated from the reduced mantle source，whereas the relative high oxygen fugacities（FMQ-1.56 to FMQ+0.14）of high-Ti basalts represent the oxidation state of the parental magma for the Hongge shallow magma chamber.It is known that the crystallization of magnetite from the basaltic magma will consume the oxygen and decrease the oxygen fugacity of the magmatic system. However，our calculated oxygen fugacity for the LOZ（FMQ-1.29 to FMQ-0.2）were relatively lower than the Hongge parental magma and the Fe-Ti oxide in interstitial association to cpx and pl（Fig.3d），which indicates that minor crystallization of magnetite at the deep magma chamber probably led the oxygen fugacity decreased before the parental magma arrived at the Hongge shallow magma chamber.The calculated oxygen fugacities for the MCZ（FMQ-0.49 to FMQ+0.82）indicated that there was some mechanism to elevate the f O2of the magmatic system（see discuss below）.

Fig.4 Backscatter images of Ti-magnetite and ilmenite exsolution microtextures within the cumulate rocks and Fe-Ti oxide ore layers from the Tongshan section.a Ilmenite grains forming along Ti-magnetite margins which distributed with clinopyroxenes.b Extensive development of ilmenite lamellae in a Ti-magnetite grain.c The reductive exsolution of granule magnetite in ilmenite.d The Fe-Ti oxide inclusions in olivine. e and f The Ti-magnetite and ilmenite grain wrapped in olivine.Abbreviations:Mt-magnetite，Ilm-ilmenite，exso-exsolution，indiv-individual

5.2 Ore-forming constrained from H2O

Pang et al.（2010）proposed that the Panzhihua Fe-Ti oxide deposits in the Panxi region represent a new type of magmatic Fe-Ti oxide deposit in comparison to other worldwide intrusions（e.g.Bushveld and skaergaard）.The Fe-Ti oxide ore layers mainly occur at the base of the Panzhihua intrusion，which is mainly composed of gabbroic rocks（Shellnutt and Pang 2012）.However，the Hongge intrusion is mainly composed of ultramafic clinopyroxenite with olivine（Bai et al.2012b）.The Fe-Ti oxide ore layers are hosted within clinopyroxenite rather than in gabbros and are only present within the middle and upper zone of the intrusion.This indicated that the Hongge intrusion，also located in the same region，probably has its own characteristics in magma evolution processes，comparing with the Panzhihua intrusion.Here，we employed the MELTS package（Ghiorso and Sack，1995），combination of the calculated results of oxygen fugacity above to explore the factors affecting the crystallization and accumulation of Fe-Ti oxides in the Hongge intrusion.

As the common range of f O2from plutonic and volcanic tholeiitic rock is normally between FMQ+1 and FMQ-2（FMQ=fayalite-magnetite-quartz）（Frost et al.1988；Kress and Carmichael 1991；Thy et al.2009）and the H2O content of mantle derived magmas is usually less than 0.51 wt%（Sobolev and Chaussidon 1996），we assumed the fractional crystallization of Hongge parental magma in thedeep magma chamber was under f O2of FMQ，0.5wt%H2O and pressure of 5 kbar（～16 km）.The melt inclusion（M8 62）hosted in olivine phenocrysts of the high-Ti picrite（Kamenetsky et al.2012）contain 12.29 wt%FeOt，2.25 wt%TiO2，19.68 wt%MgO and 46.38 wt%SiO2.This composition was chosen to represent the primary magma composition for modeling the concealed crystallization path of the Hongge intrusion.Bai et al.（2014）used the experimentally determined partition coefficient of 0.45 for TiO2between cpx and basaltic magma from Hauri et al.（1994）and the Fe-Mg exchange coefficient of 0.27 between cpx and melt from Be´dard（2010）to figure out the concentrations of TiO2and the MgO/FeO ratios for the parental magma of the Hongge intrusion.They obtained values of 3.9-4.9 and 0.63，respectively.MELTS calculations（Figs.8，9a，b）indicate that when the temperature decreased to 1235°C，the TiO2content and the MgO/FeO ratios of the residual liquid increased to 4.58 and decreased to 0.62，respectively.Therefore，the residual liquid composition at 1235°C was chosen to represent the parental magma for modeling the crystallization processes at the Hongge shallow magma chamber.Field relationships suggested that the layered intrusions in the Panxi area are emplaced at shallow depth，e.g.the Hongge and Xinjielayered intrusions are in contact with Emeishan f l ood basalts（Fig.2）.We therefore assumed that the pressure at the time of emplacement of the Hongge intrusion was no more than 1.5 kbar.The oxygen fugacity is still FMQ，based on the calculated results above.As the roof rocks of the Hongge intrusion are basalts and syenites，which are unlikely to contain significant H2O，and the carbonates are absent in the Hongge area，we presumed the parental magma arriving at the Hongge intrusion were anhydrous magma.

Fig.5 a Primitive mantle-normalized trace element patterns and b Chondrite-normalized REE patterns for Baicao and Banfangqing and Longzhoushan basalts.The normalization values are from Sun and McDonough（1989）.Data for the Longzhoushan basalts are from Qi et al.（2008）and data for Baicao and Banfangqing are from our unpublished results

Fig.6 a Plots of Th versus Ta and b Nb/La versus SiO2in the basalts of Banfangqing and Baicao area，which comparable with Guizhou basalts. Data for the Guizhou basalts are from Xu et al.（2003）and data for Baicao and Banfangqing are from our unpublished results

Fig.7 Binary plots of εNd259Maversus（87Sr/86Sr）259Mafor the Banfangqing and Baicao basalts located in the northwest of the Hongge intrusion.The values of εNd259Maand（87Sr/86Sr）259Mafrom our unpublished data

Fig.8 Projection diagram of T-f O2（a，modified after Buddington and Lindsley 1964）.The oxygen fugacities（-log f O2）were calculated into FMQ based on the equation of Ballhaus（1993）and shown on b.The black circle spots were calculated by the samples in the MCZ of the Hongge intrusion，and the gray square spots represent the T-f O2crystallization condition of Fe-Ti oxide in the LOZ

Fig.9 Modeling results（a and b）of MELTS using a starting composition of melt inclusion（M8 62）of the high-Ti picrite（Kamenetsky et al. 2012），and assuming a pressure of 5000 bars，starting temperature of 1400°C，ending temperature of 1100°C，H2O=0.5 wt%，FMQ.PM（Hongge parental magma）are after Bai et al.（2014）.BC-1 and BFQ-2 are low-Ti and high-Ti basalts are from our unpublished data..Modeling results（c and d）of MELTS under FMQ，starting temperature of 1300°C，ending temperature of 1000°C，under the dry system，1500 bars，using a liquid composition at 1235°C（a and b）to represent the Hongge parental magma

Toplis and Carroll（1995）and Botcharnikov et al.（2008）emphasized that the timing of Ti-magnetite crystallization and the degree of Fe-Ti oxide saturation were largely controlled by the oxygen fugacity of the magmatic system. The MELTS calculations indicates that，from the FMQ-1 to FMQ+1，the proportion of crystallization magnetite increased from 11%to 16%and were largely affected by the f O2（Fig.10c），whereas the crystallization temperature of Fe-Ti oxides just increased from 1134 to 1164°C（Fig.10d）.The experimental work of Toplis and Carroll（1995）suggested that titanomagnetite-dominated oxide assemblages commonly crystallized at high f O2conditions nearly FMQ+1.5，whereas those containing cumulus Timagnetite and ilmenite crystallize at f O2at FMQ，which are consistent with our calculated results of oxygen fugacity for MCZ（FMQ-0.49 to FMQ+0.82）and both magnetite and ilmenite are present in the MCZ.Although the oxygen fugacities increased about two log units from the LOZ（FMQ-1.29 to FMQ-0.2）to the MCZ（FMQ-0.49 to FMQ+0.82），it is reasonable to demonstrate that our calculated oxygen fugacities for MCZ were high enough for extensive crystallization of magnetite and ilmenite and the f O2was not the key factor leading the abundant Fe-Ti oxides crystallization for the Hongge intrusion，which was inconsistent with recently studies of Bai et al.（2012a）and Pang et al.（Pang et al.2010）who argued that the relatively high oxygen fugacities（FMQ+1 to FMQ+2 and FMQ to FMQ+1.5）promoting the crystallization of Fe-Ti oxides in Hongge and Panzhihua intrusion，respectively.

Fig.10 Modelling-derived phase equilibria for Hongge parental magma（Fig.8a，b，1235°C）as a function of H2O（a）and FMQ（b）with temperature.The variation of crystallisation of Ti-magnetite（c）and FeOt content of residual liquid（d）with temperature at different oxidation state（FMQ-2 to FMQ+2）

Botcharnikovet al.（2008）showed experimentally that the crystallization temperature of pl is significantly affected by the H2O content of the parent magma，which was also confirmed by our results of MELTS calculation（Fig.10a）. The relative low crystallization temperature of pl，which formed the UGZ of the Hongge intrusion，may favor parental magma of the UGZ with high H2O，so that only olivine and cpx crystallized to form the lower and middle ultramafic zones when the parental magma arrived at the Hongge shallow magma chamber.However，we discussed above which considered the parental magma was anhydrous initially，so we argued the parental magma for LOZ and MCZ became hydrous after it joined the Hongge intrusion and before the abundant crystallization of pl.Also，the H2O joined the magma will promote Ti-magnetite crystallization and depress the crystallization temperature of pl.This is also confirmed by the microscope image（Fig.3c）that the rims of hornblende located in the massive ore rock in the MCZ and the recent study of Xing et al.（2012）indicated that the average bulk contents of volatiles（H2O and CO2）released from magnetite of the Hongge intrusion is 4891 mm3STP/g，much higher than those（H2O，H2and CO2）released from other minerals（cpx and pl）in total，382 mm3STP/g.Ganino et al.（2008）demonstrated the interaction of the parental magma with the footwall carbonates which result in increase in f O2for the formation of Fe-Ti oxide ore layers at the base of the Panzhihua intrusion.However，this is not necessarily case for the intrusions in Panxi region in general，for example，the Hongge Fe-Ti oxide ore bearing intrusion was contacted with limestone at the bottom but the major footwall rock is the meta-sandstone（Panxi Geological Survey 1984），which cannot provide enough oxygen to increase the oxygen fugacity from the FMQ-1.29 to-0.2 of the LOZ to the FMQ-0.49 to+0.82 of the MCZ to trigger the extensive crystallization of titanomagnetite.Therefore，we presume the multiple replenishment magma injection into the Hongge intrusion（Bai et al.2012b）combined with the evolved magma heating the strata around the magma chamber and extracting the H2O within it，which formed a water circulation system to supply the H2O for extensive crystallization and accumulation of Fe-Ti oxides in the MCZ of the Hongge intrusion.Howarth et al.（2013）demonstrated that magmas with low H2O generally followed the typical Fenner Fe-enrichment trend whereas those with higher H2O contents followed the Bowen Feloss trend.So we presumed that the anhydrous parental magma came into the Hongge shallow magma chamber which followed the Fenner trend and began to crystallize olivine，cpx and minor Fe-Ti oxides to form the Fe-Ti oxide free cumulate rocks at the base of the intrusion，and the magma was gradually enriched in Fe（Fig.9c）which are preparing for the onset of Ti-magnetite crystallization. Then the magma became hydrous combined with increasing of oxygen fugacity form LOZ to MCZ which change the magmatic system evolves along the Bowen trend and result in a sharply decrease in FeOt of the evolved magma，which also play a positive role for the occurrence of large scale Fe-Ti oxide ore layers in the MCZ of the Hongge intrusion.

6 Conclusions

（1）The oxygen fugacities（FMQ-1.56 to FMQ+0.14）calculated by Banfangqing high-Ti basalts represent the oxidation state of the parental magma for the Hongge intrusion.

（2）High oxygen fugacity does not necessarily trigger the abundant crystallization of Fe-Ti oxide layers for the intrusions in the Panxi region.For the Hongge intrusion，the moderate oxygen fugacities for the MCZ（FMQ-0.49 to FMQ+0.82），combined with the attracted water to the evolved magma，could account for the massive Fe-Ti oxide ore layers within the middle zone of the intrusion.

Acknowledgments This study was supported by the National 973 Program of China（2012CB416804 and 2014CB440906），CAS/ SAFEA International Partnership Program for Creative Research Teams（KZZD-EW-TZ-20），and National Natural Sciences Foundations of China（41473051）to Tao yan.We thank Zhou Guo-fu，Liu Shi-Rong and Zheng Wen-Qin for the microprobe analysis.

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Received:13 January 2015/Revised:25 March 2015/Accepted:1 July 2015/Published online:22 July 2015

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

✉Yan Tao taoyan@vip.gyig.ac.cn

1State Key Laboratory of Ore Deposit Geochemistry，Institute of Geochemistry，Chinese Academy of Sciences，Guiyang 550002，China

2University of Chinese Academy of Sciences，Beijing 100049，China