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1 Chemical Geology 259 (2009) Contents lists available at ScienceDirect Chemical Geology journal homepage: The role of Fe Ti oxide crystallization in the formation of A-type granitoids with implications for the Daly gap: An example from the Permian Baima igneous complex, SW China J. Gregory Shellnutt a,b,, Mei-Fu Zhou b, Georg F. Zellmer a a Academia Sinica, Institute of Earth Science, 128 Academia Road Sec. 2, Nankang, Taipei 11529, Taiwan b Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong SAR article info abstract Article history: Received 16 May 2008 Received in revised form 26 September 2008 Accepted 31 October 2008 Editor: B. Bourdon Keywords: Emeishan large igneous province Fe Ti V deposit Layered mafic intrusion A-type granitoid SW China Daly gap Permian The Baima igneous complex (BIC) is a member of the Permian Emeishan large igneous province, SW China, and is composed of a layered gabbroic unit and an isotropic syenitic unit. The gabbroic unit consists of four distinct zones, 1) lower cumulate zone, 2) massive Fe Ti V oxide ore zone, 3) olivine gabbro zone and 4) upper gabbro zone. SHRIMP zircon U Pb dating results yield an age of 261±2 Ma for the gabbroic unit, contemporaneous with the spatially associated syenitic unit. The gabbroic rocks show slightly LREE enriched patterns with (La/ Yb) N values between 1.6 and 12.1 and positive Eu anomalies (Eu/Eu = ). In contrast, the syenites display stronger enrichments of LREE (La/Yb N = ) and negative Eu anomalies (Eu/Eu = ). Rocks of the syenitic unit are chemically similar to the ferroan alkalic A-type granitoids (K 2 O/Na 2 O= , FeOt/(FeOt+ MgO)= ) and have a within-plate geochemical signature. Both units have similar trace element ratios (Zr/Nb, Th/U, Ni/Co, Cu/Ni, Hf/Ta and Zn/Ga) resembling an ocean island basalt source. The εnd (T) values for the gabbroic unit (εnd (T) = ) and syenitic unit (εnd (T) = ) are within the range of the high-ti Emeishan flood basalts. The geochemical and geochronological data indicate that the gabbroic and syenitic units are comagmatic and cogenetic. Geochemical modeling suggests that the two units originated by fractional crystallization of a common parental magma that resembles the high-ti Emeishan flood basalts. The formation of the two units by fractional crystallization was likely responsible for the enrichment of Fe Ti V oxide minerals in the gabbroic unit and thus the development of the Daly gap. The results of this study indicate that silica saturated peralkaline A-type granitoids can be formed by fractionation of mantle derived mafic magmas Elsevier B.V. All rights reserved. 1. Introduction The concentration of oxide and sulphide minerals in layered mafic intrusions is a longstanding problem in petrology as layered mafic ultramafic intrusions commonly host large deposits of Ni, Cu, platinum group elements (PGEs), Cr, Fe, Ti and V (Lee, 1996). Typically, Ni, Cu and PGE sulphide deposits are hosted within high-mg volcanic rocks or their intrusive equivalents and are formed through the concentration of sulphides in dynamic magmatic systems (Hawkesworth et al., 1995; Eales and Cawthorn, 1996; McCallum, 1996). Magmatic Fe Ti oxide deposits on the other hand are not well understood because the process whichenrichesthemagmainmetalsisnotentirelyknown(lee, 1996; Eales and Cawthorn, 1996). Models have been used to explain the concentration of oxide minerals including fractional crystallization, magma mingling, silicate liquid immiscibility, separate magma systems, Corresponding author. Academia Sinica, Institute of Earth Science, 128 Academia Road Sec. 2, Nankang, Taipei 11529, Taiwan. Tel.: x618; fax: address: jgshelln@earth.sinica.edu.tw (J.G. Shellnutt). and periodic changes in ƒo 2 (Philpotts, 1967; Kolker, 1982; Reynolds, 1985; von Gruenewaldt et al., 1985; Klemm et al., 1985; Harney and von Gruenewaldt, 1995; Duchesne, 1999; Higgins, 2005). Many oxide bearing layered mafic intrusions are temporally and spatially associated with A-type granitic rocks which may be petrologically related to the enrichment of metals in the mafic intrusions (Yang et al., 1997; Bonin, 2007). A-type granitic rocks are considered to form by fractional crystallization of mantle derived, transitional to mafic alkaline magmas (Loiselle and Wones, 1979; Bonin, 2007). In theory, fractional crystallization of silicate magmas should produce a series of compositions with evolving compositions from mafic to intermediate to felsic (Clague, 1978). However the continuous rock series seems to be uncommon and a compositional or silica gap (e.g. the Daly gap) is often observed (Yoder, 1973; Chayes, 1977; Peccerillo et al., 2003). Rocks of intermediate compositions are rarely found or reported in association with peralkaline A-type granitoids and layered mafic/ultramafic intrusions and it is not clear if the fractionation model can satisfactorily explain the Daly gap (Bonin, 2007). In SW China, layered mafic intrusions hosting magmatic oxide deposits are members of the Permian (260 Ma) Emeishan large igneous /$ see front matter 2008 Elsevier B.V. All rights reserved. doi: /j.chemgeo

2 J.G. Shellnutt et al. / Chemical Geology 259 (2009) province (ELIP) and are commonly associated with peralkaline granitic plutons (Shellnutt and Zhou, 2007). Linking the mafic and felsic intrusions to the formation of the ore deposits has not previously been attempted. However, it will have significant economic implications if these rock types are related to the formation of the ore deposits. The Baima igneous complex (BIC) in the central part of the ELIP consists of an oxide ore-bearing layered gabbroic unit and a peralkaline syenitic unit of A-type affinity (Shellnutt and Zhou, 2007). Thus the BIC provides an opportunity to examine the genetic relationship between the spatially and temporally associated mafic and felsic rocks of the ELIP. Bulk-rock major and trace element data, radiogenic isotopic data and radiometric dating for the BIC are reported here in order to determine if the two units are comagmatic and cogenetic and to provide insight into the cause of metal enrichment and the formation of peralkaline felsic plutons of the ELIP. 2. Geological background 2.1. Regional geology Southwestern China includes the western margin of the Yangtze Block to the east and the eastern most part of the Tibetan Plateau to the west (Fig. 1). The Yangtze Block consists of Mesoproterozoic granitic gneisses and metasedimentry rocks which have been intruded by Neoproterozoic (~800 Ma) granites (Li et al., 1999; Zhou et al., 2002b). The granites are overlain by a series of marine and terrestrial strata from the late Neoproterozoic (~600 Ma) to the Early Permian. The eastern part of the Tibetan Plateau, known as the Songpan-Ganze terrane, is composed primarily of late Triassic early Jurassic marine sediments (Bruguier et al., 1997; Yan et al., 2003). The Late Middle Permian ELIP is a comparatively small large igneous province, covering an area of km 2 in SW China and northern Vietnam and consists of flood basalts, felsic plutons and layered mafic ultramafic intrusions, some of which host giant Fe Ti V oxide deposits (Ali et al., 2005; Zhou et al., 2008). The ELIP has been modified by Mesozoic and Cenozoic faulting associated with the formation of the Songpan Ganze terrane and the Indo-Eurasian collision (Chung and Jahn, 1995). The flood basalts range in thickness from 1.0 to 5.0 km and are subdivided into high- and low-ti groups (Xu et al., 2001). The age of the ELIP is geologically constrained to the Late Permian having an emplacement age of ~260 Ma as shown from zircon U Pb age dates of the layered, mafic ultramafic intrusions and felsic plutons (Zhou et al., 2002a, 2005, 2006; Fan et al., 2004; Guo et al., 2004; Shellnutt and Zhou, 2007). Previous studies of the flood basalts suggest the ELIP was mantle plume-derived (Chung et al., 1998; Song et al., 2001, 2004; Xu et al., 2001) Field relationships of the Baima igneous complex Within the Panxi region of the ELIP, layered gabbroic intrusions are always spatially associated with granitic and syenitic plutons (Fig. 1). Among the grabbroic intrusions, the Panzhihua, Baima, Xinjie and Taihe intrusions host magmatic oxide deposits and are temporally associated with peralkaline granitic rocks whereas the Hongge intrusion is associated with a metaluminous granitoid (Shellnutt and Zhou, 2007). The BIC is located in the central part of the Panxi region, ~100 km northeast of Panzhihua, and consists of a gabbroic unit and a syenitic unit (Figs. 1 and 2) (Yang et al., 1997). The layered gabbroic unit lies to the east of the syenitic unit and is dipping ~25 to the west. The two Fig. 1. Simplified geological map of the Panxi region showing the distribution of mafic and felsic plutonic rocks of the ELIP. The Baima igneous complex is outlined in the boxed area (modified from Zhou et al., 2005).

3 206 J.G. Shellnutt et al. / Chemical Geology 259 (2009) Fig. 2. Geological map of the Baima igneous complex: gabbro (grey), ore zone (black) and syenite (crosses) and the sampling locations (A to G). units cover an area of similar size, but their respective volumes are unknown. Situated between the gabbroic unit and the syenitic unit is a magmatic breccia zone. The mafic portion of the breccia has textures and mineralogies similar to the upper gabbro zone whereas the felsic portion appears to be assimilated country rock. The syenitic unit is structurally above the gabbroic unit and contains abundant ellipsoidal mafic enclaves varying in size from a few centimeters to 10s of centimetres in length. The syenitic unit is granular except for a few small bands of finer grained textures which appear to be more siliceous in composition. Intruding the western part of the BIC is the ~252 Ma, metaluminous Huangcao pluton and to the east and southeast is the ~260 Ma metaluminous Woshui pluton (Shellnutt and Zhou, 2007; 2008). The Woshui pluton intrudes the lowermost zone of the gabbroic unit and is thus slightly younger than the gabbroic unit. Cutting all of the plutons in the region are mafic alkaline dykes dated at ~242 Ma (Shellnutt et al., 2008). 3. Petrography 3.1. Gabbroic unit The gabbroic unit can be divided into four lithologic zones: 1) a lower cumulate zone; 2) an oxide ore zone; 3) an olivine gabbro zone; and 4) an upper gabbro zone (Chen, 1990) (Fig. 2). The gabbroic unit consists of coarse grained cumulate olivine, plagioclase, clinopyroxene and interstitial Fe Ti oxide minerals with minor amounts of sulphide minerals, spinel (pleonaste) and apatite. Olivine is typically equant with rounded edges, ranging in size from 5 mm to b0.1 mm with ubiquitous serpentine alteration. In a few cases olivine is completed

4 J.G. Shellnutt et al. / Chemical Geology 259 (2009) Syenitic unit The syenites are coarse-grained and consist of about 70% alkali feldspar, 10% ferrorichterite, 10% quartz, 5% aegirine and about 5% of accessory minerals (i.e. apatite, titanite, zircon, plagioclase, biotite, Fe Ti oxides and pyrite). The alkali feldspar crystals are large (N1.0 cm), subhedral to anhedral and commonly separated by an intergranular mixture of subrounded grains of microcline, quartz and plagioclase. Plagioclase is usually an accessory mineral although one large, subhedral phenocryst of plagioclase surrounded by alkali feldspar was observed. Ferrorichterite crystals are euhedral to subhedral, rhombic, up to 5 mm long, and commonly associated with subhedral to rounded grains of aegirine. Titanite typically has the characteristic wedge shape although there are many equant crystals. Other less abundant accessory minerals are euhedral biotite, zircon and apatite and anhedral magnetite, hematite and ilmenite. The presence of magnetite, quartz and titanite suggests that the magma was comparatively oxidizing (c.f. Wones, 1989). Fig. 3. Concordia plot of the SHRIMP zircon U/Pb dating results for gabbroic unit (GS05-056B). 4. Analytical methods 4.1. SHRIMP zircon analyses enclosed by plagioclase or in rarer cases by clinopyroxene. Olivine decreases in abundance from ~30% in the lower cumulate zone to b5% in the upper gabbro zone. Plagioclase has euhedral to anhedral shapes and ranges in size from ~5 mm to b0.1 mm. Most crystals are elongate although there are a few rounded equant crystals. Many of the feldspars have saussurite alteration or have alteration rims of brown hornblende or biotite when in contact with oxide minerals. Clinopyroxene crystals are similar in size and shape to the plagioclase crystals. They have light-red pleochroism and extensive ilmenite exsolution lamellae. Oxide minerals, titanomagnetite, ilmenite and spinel, are interstitial to the silicate minerals. Magnetite has undergone significant sub-solidus re-equilibration as evident by extensive oxidation exsolution lamellae of ilmenite and spinel and 120 triple junction grain boundaries. Ilmenite is the most abundant oxide mineral in the upper gabbro zone and spinel is absent. Accessory apatite is euhedral, 0.1 cm across, and typically surrounded by oxide minerals. Apatite is significantly more abundant in the upper gabbro zone than other parts of the gabbroic unit. Accessory sulphide minerals are common in the olivine gabbro zone. Separation of zircon crystals was accomplished by conventional heavy liquid and magnetic techniques. The individual crystals were mounted in epoxy, polished, coated with gold, and photographed in transmitted and reflected light to identify the best crystals for analysis. U Pb isotopic ratios of zircon crystals were measured using the SHRIMP II at the Chinese Academy of Geological Sciences, Beijing, China. The measured isotopic ratios were reduced off-line using standard techniques and calibrated to the TEMORA 1 standard which was repeatedly analyzed after every three zircon analyses (Claoue- Long et al., 1995; Black et al., 2003a,b). Common Pb was corrected using the methods of Compston et al. (1984). The and 207 Pb/ 235 U data were corrected for uncertainties associated with the measurements of the TEMORA 1 standard Major and trace elemental analyses Major elements were analyzed by X-ray fluorescence spectroscopy at The University of Hong Kong using fused glass discs. Trace elements were analyzed by inductively coupled plasma mass spectrometry (ICP- Table 1 SHRIMP zircon analytical data of sample GS05-056B from the upper gabbro zone of the BIC (1) (2) (3) (1) Spot U Th 206 Pb c 206 Pb 208 Pb/ 232 Th (ppm) (ppm) (%) (ppm) Age Age Age Age GS05-056B ± ± ± ±4.0 GS05-056B ± ± ± ±6.5 GS05-056B ± ± ± ±5.6 GS05-056B ± ± ± ±8.4 GS05-056B ± ± ± ±6.4 GS05-056B ± ± ± ±7.8 GS05-056B ± ± ± ±5.5 GS05-056B ± ± ± ±4.5 GS05-056B ± ± ± ±5.0 GS05-056B ± ± ± ±5.3 GS05-056B ± ± ± ±5.8 GS05-056B ± ± ± ±4.7 GS05-056B ± ± ± ±4.9 GS05-056B ± ± ± ±4.5 GS05-056B ± ± ± ±4.2 Errors are 1-sigma; Pb c and Pb indicate the common and radiogenic portions, respectively. Error in standard calibration was 0.64% (not included in above errors but required when comparing data from different mounts). (1) Common Pb corrected using measured 204 Pb. (2) Common Pb corrected by assuming 207 Pb/ 235 U age-concordance. (3) Common Pb corrected by assuming 208 Pb/ 232 Th age-concordance.

5 208 J.G. Shellnutt et al. / Chemical Geology 259 (2009) MS) using the technique of Qi et al. (2000) also at The University of Hong Kong. Standard reference materials for the major elements were BHVO-2 (basalt), JGB-2 (gabbro) and GSR-1 (granite). For the trace element analysis, the standard reference material was AMH-1 (Mount Hood andesite), GBPG-1 (garnet biotite plagiogneiss) and OU-6 (Prerhyn slate) (Thompson et al., 1999; Potts et al., 2000, 2001). The precisions for the major and trace element results are better than 5% and 10%, respectively. The comparison between the certified reference standard values for trace elements and the measure reference values can be found in Shellnutt and Zhou (2007) Rb Sr and Sm Nd isotopic analyses Isotopic ratios were measured using a Finnigan MAT-262 thermal ionization mass spectrometer (TIMS) in the Laboratory for Radiogenic Isotope Geochemistry, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing. Approximately 50 mg of whole rock power was decomposed using a Hf-HClO 4 mixture with a additional 150 Nd 149 Sm tracer. The samples were heated at 180 C in PTFE bombs for six days to ensure complete decomposition. Strontium, Rb and REE were separated on quartz columns with a 5 ml resin bed of AG 50 W- X12, mesh. Neodymium was separated from Sm on quartz columns using 1.7 ml Teflon powder with HDEHP as the cation exchange medium. Raw data was processed using the Isoplot program which calculated 2σ error (Ludwig, 2001). Total procedural blanks were b200 pg for Sr and ~30 pg for Nd and ~10 pg for Sm. 143 Nd/ 144 Nd ratios were normalized to 146 Nd/ 144 Nd= and 87 Sr/ 86 Sr ratios to 86 Sr/ 88 Sr = Technique details on chemical separation and measurement are described in Chen et al. (2000, 2002). 5. Results 5.1. SHRIMP zircon U Pb dating Zircon crystals of sample GS05-056B, collected from the upper gabbro zone at N, E, show typical igneous zonation and have fragmented, anhedral and euhedral textures. Analyses of fifteen individual zircon crystals form a single age group and yield a mean 206 U/ 238 Pb age of 261±2 Ma with a mean square of weighted deviates (MSWD) of 0.66 (Fig. 3) (Table 1). Their Th/U ratios do not show much variation and range from 0.24 to 0.51 with an average of 0.46 and a standard deviation of Two syenitic samples, GS and GS04-016, were selected for zircon U Pb dating. Sample GS was collected at N, E and sample GS was collected at N, E. Zircon grains from sample GS have Th/U ratios ranging from 0.31 to 1.0 with an average of 0.70 and a standard deviation of Analyses from 21 individual zircon crystals from sample GS yield a mean 206 U/ 238 Pb age of 259±5 Ma (Fig. 4a) (Table 2). Sample GS has euhedral to anhedral zircon crystals. Their Th/U ratios vary from 0.31 to 9.8 with an average of 2.2 and a standard deviation of 2.2. Analyses on 27 zircon crystals yield three groups: one group of high 238 U/ 206 Pb ratios (N26.5) and one with very low 238 U/ 206 Pb ratios (7.9) were rejected. The remaining twenty-two analyses comprise the third group and produce a mean 206 U/ 238 Pb age of 258±4 Ma (Fig. 4b) (Table 3) Whole rock geochemistry Gabbroic unit The low loss on ignition (LOI) (b2.00 wt.%) for all the samples indicates limited alteration and thus minimal element mobility (Supplementary Table 1). There are a few obvious trends in the layered gabbroic unit, for example, Fe 2 O 3 t increases from the lower cumulate zone to the oxide ore zone and decreases thereafter. The concentration of TiO 2 and MnO follow a similar trend as Fe 2 O 3 t. Fig. 4. Concordia plot of the SHRIMP zircon U/Pb dating results for syenitic unit, (a) sample GS and (b) sample GS The two samples from the lower cumulate zone have the highest Mg# (57.1 and 59.2) and Fe 2 O 3 t/tio 2 ratios (Fe 2 O 3 t/tio 2 =7.5 and 8.6) in the entire sequence due to a comparatively minor amount of oxide minerals. The SiO 2 ( wt.%) and alkali ( wt.%) contents of the lower cumulate zone are low and resemble samples from the olivine gabbro zone (Fig. 5a). Samples from the upper part of the oxide ore zone have variable concentrations of Fe 2 O 3 t ( wt.%), MgO ( wt.%), TiO 2 ( wt.%), SiO 2 ( wt.%), Na 2 O ( wt.%), CaO ( wt.%) and Al 2 O 3 ( wt.%) (Fig. 5a d). The major elements vary as a function of the amount silicate minerals within the ore samples (Fig. 5b). The olivine gabbro and upper gabbro show continuous decreasing concentration of Fe 2 O 3 t, MgO and TiO 2 and increasing concentrations of SiO 2,Na 2 O, CaO and Al 2 O 3 (wt.%). The upper gabbros overlap with the olivine gabbros in particular with TiO 2,Al 2 O 3 and MnO, however they have higher SiO 2, CaO, Na 2 O and P 2 O 5 and lower Fe 2 O 3 and MgO. Because the compositions of all the gabbroic rocks (except the ore samples) overlap, it can be difficult to distinguish olivine gabbros from the upper gabbros, however the most effective discriminator is the Fe: Ti ratio which reflects the amount of magnetite to ilmenite and thus the proportion of these elements. Potassium tends to remain relatively constant throughout the gabbroic unit ( wt.%). The gabbros contain variable Sc (8 54 ppm), Ni ( ppm), Co ( ppm), Cu ( ppm), V ( ppm), Cr (12

6 J.G. Shellnutt et al. / Chemical Geology 259 (2009) Table 2 SHRIMP zircon analytical data for sample GS from the BIC syenite (1) (2) (3) (1) Spot U Th 206 Pb c 206 Pb 208 Pb/ 232 Th (ppm) (ppm) (%) (ppm) Age Age Age Age GS ± ± ±12 221±14 GS ± ±11 245±17 341±22 GS ±11 261±11 418± ±13 GS ± ± ± ±10.0 GS ± ± ± ±9.4 GS ±10 276±10 277± ±21 GS ± ± ± ±9.3 GS ± ± ± ±9.9 GS ±10 271±10 268±11 304±25 GS ±11 253±11 251±11 356±31 GS ±10 265±10 269±13 251±12 GS ± ± ± ±8.9 GS ± ±10 268± ±15 GS ± ± ± ±9.0 GS ± ± ±11 364±27 GS ±13 249±12 248±24 276±21 GS ± ± ±11 270±14 GS ± ± ± ±8.9 GS ± ± ± ±11 GS ± ± ± ±9.3 GS ± ± ±11 251±10 GS ± ± ±13 243±13 Errors are 1-sigma; Pb c and Pb indicate the common and radiogenic portions, respectively. Error in standard calibration was 0.64% (not included in above errors but required when comparing data from different mounts). (1) Common Pb corrected using measured 204 Pb. (2) Common Pb corrected by assuming 207 Pb/ 235 U age-concordance. (3) Common Pb corrected by assuming 208 Pb/ 232 Th age-concordance. 446 ppm), and Sr ( ppm) and have low contents of Rb ( ppm), Zr (6 17 ppm) and Nb ( ppm). The primitive mantle normalized incompatible trace element patterns show distinct positive anomalies of Ba, Sr and Ti and negative anomalies of Rb, Th, U, Ta, Nb, Hf and Zr (Fig. 6). The rocks of the layered gabbroic unit have similar LREE-rich patterns with (La/Sm) N and (La/Yb) N ratios between 0.8 and 3.3 and 1.6 and 12.1 (Fig. 7). The positive Eu anomalies (Eu/ Eu =([2 Eu N /(Sm N +Gd N )]) of the gabbroic unit are caused by the abundant cumulate plagioclase (Eu/Eu =1.1 to 3.6) Syenitic unit Rocks of the syenitic unit are mildly peralkaline (ANCK= , Na+K/Al= ) with moderately high Fe (FeOt/(FeOt+MgO) = ) values and are chemically similar to ferroan alkalic A-type Table 3 SHRIMP zircon analytical data of sample GS from the BIC syenite (1) (2) (3) (1) Spot U Th 206 Pb c 206 Pb 208 Pb/ 232 Th (ppm) (ppm) (%) (ppm) Age Age Age Age GS ± ±10 259±12 231±26 GS ±11 265±11 266±12 182±40 GS ±11 271±11 267± ± 21 GS ± ±10 261± ± 30 GS ±12 257± ± ± 57 GS ± ± ± ± 15 GS ± ± ± ±22 GS ± ± ± ±34 GS ± ± ± ± 21 GS ± ± ± ±31 GS ± ± ± ± 22 GS ±10 242±10 243± ± 20 GS ±11 267±11 268±13 237±26 GS ±19 246±11 243± ± 130 GS ± ± ± ± 25 GS ±13 243±13 247±15 188±34 GS ±11 264± ± ± 30 GS ± ± ± ±55 GS ± ± ± ±60 GS ± ±10 257±12 311±30 GS ± ± ± ±41 Errors are 1-sigma; Pb c and Pb indicate the common and radiogenic portions, respectively. Error in standard calibration was 0.64% (not included in above errors but required when comparing data from different mounts). (1) Common Pb corrected using measured 204 Pb. (2) Common Pb corrected by assuming 207 Pb/ 235 U age-concordance. (3) Common Pb corrected by assuming 208 Pb/ 232 Th age-concordance.

7 210 J.G. Shellnutt et al. / Chemical Geology 259 (2009) Fig. 5. Measured and modeled major element data variation for the layered gabbroic unit and syenitic unit of the BIC. LCZ lower cumulate zone, OXZ oxide ore zone, OGZ olivine gabbro zone, UGZ upper gabbro zone and SYE syenitic unit. (a) Total alkalis (Na 2 O+K 2 O) vs. SiO 2. (b) Al 2 O 3 vs. MgO showing the modeled compositions of the major lithologic zones of the gabbroic unit and the syenitic unit. The arrow shows the whole rock chemical evolution of the pure oxide ore zone (modeled) with increasing silicate incorporation. (c) FeOt vs. Na 2 O with least squares mixing line (solid line) calculated using the modeled whole rock compositions of the LCZ, OXZ, OGZ and UGZ plotted with the measured data. (d) TiO 2 vs. FeOt. granitoids (Frost et al., 2001) (Supplementary Table 1). Three samples have a metaluminous composition (Na+K/Al= ) however the remaining samples are peralkaline (Na+K/AlN1) and thus we refer to the entire pluton as weakly peralkaline. The syenites belong to the post-orogenic (anorogenic) suite of granitic rocks and are classified as within-plate granitoids based on the geochemical classification Fig. 6. Primitive mantle normalized trace element plots of the syenitic and gabbroic units (symbols as in Fig. 5) normalized to values of Sun and McDonough (1989). The inset shows the statistical mean of the syenites and the gabbros.

8 J.G. Shellnutt et al. / Chemical Geology 259 (2009) Fig. 7. Chondrite normalized rare earth element plots of the syenitic and gabbroic unit (symbols as in Fig. 5) normalized to values of Sun and McDonough (1989). The inset shows the statistical mean of the syenites and the gabbros. diagrams of Batchelor and Bowden (1985) and Pearce et al. (1984). The bulk compositions correspond to the description of Loiselle and Wones (1979) for A-type granites although the rocks have low K 2 O/ Na 2 O ratios ( ). There are only a few major element trends observed including weakly negative trends of Fe 2 O 3 t, MgO, CaO and P 2 O 5 against SiO 2 and a weakly positive trend of K 2 O against SiO 2.A rock from a felsic layer in the gabbroic unit, between the upper part of the ore zone and the olivine gabbro zone exhibits a composition similar to the syenitic unit of the BIC. It has high Fe (FeOt/(FeOt+ MgO) =0.98) and high total alkalis (N10.0 wt.%). In contrast to the gabbroic unit, the syenites contain much lower Sc (10 13 ppm), Ni (1 10 ppm), Co ( ppm), Cu (2 12 ppm), V ( ppm), Cr (1 15 ppm), moderate Rb ( ppm), Sr ( ppm) and Nb ( ppm) and variably high Zr ( ppm). They show chondrite normalized REE patterns enriched in LREE relative to HREE with (La/Yb) N values between 6.8 and 51.3 and negative Eu anomalies (Eu/Eu = ) (Fig. 7). In the primitive mantle-normalized trace element diagrams they show distinct depletion of Cs, Sr and Ti with variable Ba anomalies (Fig. 6). The light REEs, Ta, Nb, Th and U are elevated to values of 10 to 100 times over primitive mantle values Rb Sr and Sm Nd isotopic compositions Gabbroic unit Whole rock radiogenic Sr and Nd isotopes were determined for two samples from the oxide ore zone, three samples from the olivine gabbro zone and three samples from the upper gabbro zone (Table 4). The εnd (T) values and 87 Sr/ 86 Sr initial have been calculated to their SHRIMP zircon U Pb age. The layered gabbros have similar εnd (T) values, ranging from +1.6 to Sample GS has the highest value (εnd (T) =+4.2) and Table 4 Rb Sr and Sm Nd radiogenic isotope data for the BIC gabbroic and syenitic units Sample Rb Sr 87 Rb/ 86 Srm 87 Sr/ 86 Srm 87 Sr/ 86 Sr initial Sm Nd 147 Sm/ 144 Nd m 143 Nd/ 144 Nd m εnd (T) (ppm) (ppm) (ppm) (ppm) GS GS GS GS GS GS GS GS GS GS GS GS GS GS GS GS BCR ( 87 Rb/ 86 Sr)m, ( 87 Sr/ 86 Sr) m,( 147 Sm/ 144 Nd) m and ( 143 Nd/ 144 Nd) m measured isotopic ratios, respectively. 87 Sr/ 86 Sr initial and εnd (T) are age calculated to 260 Ma for the two gabbroic unit. The 87 Sr/ 86 Sr initial is calculated to the Rb Sr isochron age of the syenitic unit and the εnd (T) is calculated to 260 Ma. Concentrations of Nd, Sm, Rb and Sr were determined by thermal ionization mass spectrometer. All measured isotope values of 87 Sr/ 86 Sr and 143 Nd/ 144 Nd have 2σ of or less except GS (2σ= ). BCR-1 is standard reference material analyzed for this study (Raczek et al., 2003).

9 212 J.G. Shellnutt et al. / Chemical Geology 259 (2009) Fig. 8. Isotopic results from the BIC gabbroic unit and syenitic unit. Isotopic values of εnd (T) and 87 Sr/ 86 Sr initial for the gabbroic unit are calculated to 260 Ma (symbols as in Fig. 5). The syenitic unit samples are shown for reference only as the 87 Sr/ 86 Sr initial values are calculated to their Rb Sr isochron age (228 Ma). The Emeishan high-ti (solid line) and low-ti (dashed line) basalt ranges are shown for reference (modified after Ali et al., 2005). also the highest 2σ value ( ) which is approximately four times higher than the other samples. The 87 Sr/ 86 Sr initial ratios of the layered gabbros have a narrow range from to (Fig. 8) Syenitic unit Whole rock radiogenic Sr and Nd isotopes were analyzed for eight samples from the syenitic unit (Table 4). The εnd (T) values are calculated to 260 Ma and range from +2.5 to The 87 Sr/ 86 Sr initial values are very low when calculated to 260 Ma. The Rb Sr isotopic system usually does not produce reliable results for peralkaline rocks and thus is problematic when discussing any petrogenetic implications (Smith and Johnson, 1981). When the 87 Sr/ 86 Sr initial results are calculated to their Rb Sr isochron age (228 Ma) the values range from to , similar to the layered gabbroic unit (Fig. 8). 6. Discussion 6.1. Age of the BIC and relationship to the ELIP Chen (1990) first suggested that the age of the BIC was Devonian Carboniferous ( Ma). Since the mafic and felsic units were thought to be comagmatic, the age was inclusive of both units (Chen, 1990; Yang et al., 1997). The U Pb SHRIMP zircon ages of 258±4 Ma and 259±5 Ma of the syenitic unit are in agreement with the 261± 2 Ma of the gabbroic unit. These results indicate for the first time that a spatially associated peralkaline pluton and layered gabbroic intrusion in the Panxi area is also temporally linked. The implication is that the syenitic unit may be petrogenetically related to the formation of the giant magmatic Fe Ti V oxide deposit in the layered gabbroic unit. The age of the BIC is identical to other mafic and felsic plutonic rocks of the ELIP in the Panxi region (Zhou et al., 2002a, 2005; Guo et al., 2004; Luo et al., 2006; Zhong and Zhu, 2006; Shellnutt and Zhou, 2007; Zhong et al., 2007) Cogenetic origin of the gabbroic and syenitic units The trace element and isotopic data indicate that both units are derived from a common parental magma and thus cogenetic. Isotopically, the syenites have positive εnd (T) values ( ), similar to the gabbroic unit (εnd (T) = ) indicating they may have originated from the same source (Fig. 8). The Th/U ( ), Zr/ Nb ( ), Ni/Co ( ), Cu/Ni ( ), Hf/Ta ( ) and Zn/ Ga ( ) ratios of both units are similar and resemble an ocean island basalt source, although samples from the lower cumulate zone have higher Th/U ratios (12.1 and 15.5) (Pearce and Norry, 1979; Sun and McDonough, 1989; Weaver, 1991). The negative Ba, Sr and Ti anomalies of the syenites in the primitive mantle normalized trace element diagram mirror the positive anomalies of the gabbros (Fig. 7). Enrichment of Rb, Th, U, Ta, Nb, REE (except Eu), Hf and Zr in the syenites contrasts the depletions of these elements in the gabbros. Also, the chondrite normalized REE patterns of the two units show a similar reciprocal relationship between the Eu anomalies (Fig. 7). The reciprocal normalized trace element plots are the most compelling evidence which geochemically link the two units. Major and trace element trends in particular FeOt (FeOt= Fe 2 O 3 t) and Na 2 O as well as TiO 2 and FeOt indicate that there is a geochemical link between the two rock types (Fig. 5a d). The ages, similar isotopic compositions, conjugate incompatible trace element profiles and reciprocal chondrite normalized Eu anomalies provide strong evidence for a comagmatic and cogenetic origin of the syenitic and gabbroic units. There are only a few possible mechanisms such as silicate immiscibility and fractional crystallization which may explain the formation of bimodal igneous suites Possible genetic relationship: immiscibility or fractional crystallization? Iron-rich mafic magmas may enter into immiscibility given the proper temperature and pressure conditions (Philpotts, 1976, 1982). Two-silicate immiscibility will produce two end-members, Fe-rich and Si-rich members, of approximately equal proportions (Roedder, 1979). Since the Si-rich end-member is compositionally similar to granitic rocks in general, it is difficult to recognize a granitic rock which was derived from silicate immiscibility (Veksler et al., 2006). However, experimental work by Waston (1976) showed that nearly all incompatible elements (e.g., Zr, Hf, Ti, Nb, Ta, and REEs) partition into the Fe-rich end-member. It is clear that the BIC syenites contain most of the incompatible elements, which is the opposite of what would be expected if the syenitic unit was the Si-rich immiscible end member. Therefore it is unlikely that the BIC gabbroic unit and syenitic unit formed by silicate immiscibility. Fractional crystallization is hence the most likely explanation for the formation of the syenitic unit given the geologic, temporal and geochemical evidence. The gabbroic unit contains abundant cumulate olivine, plagioclase and clinopyroxene with interstitial oxide minerals while the syenitic unit contains alkali feldspar, ferrorichterite, aegirine, quartz, titanite and zircon. If the mafic cumulate minerals fractionated from an evolved mafic parental magma, the residual magma should be depleted in Eu, Sr, Ni, Co, Sc, and Cr due to the preferential partitioning of these elements for olivine, plagioclase, and clinopyroxene. The syenites are depleted in all of these elements and enriched in Rb, Th, U, Ta, Nb, REE (except Eu), Hf and Zr which can partition into titanite, zircon, ferrorichterite and aegirine (Linnen and Keppler, 2002; Marks et al., 2004; Prowatke and Klemme, 2005). Crystal fractionation and magma convection can explain the formation of bimodal mafic felsic igneous complexes and en masse oxide mineral crystallization (Clague, 1978; Sparks et al., 1984; Turner and Campbell, 1986; MacDonald, 1987; Brophy, 1991; Peccerillo et al., 2003). En masse formation of oxide minerals is the likely reason why there is a jump in the silica concentration from mafic to felsic and why no intermediate magmas were produced in the BIC, thus explaining the observed Daly gap (Clague, 1978; MacDonald, 1987). The syenitic layers between the uppermost part of the oxide ore zone and olivine gabbro zone are more gradational than mingled, intruded or

10 J.G. Shellnutt et al. / Chemical Geology 259 (2009) perturbed, indicating they may have been residua from the surrounding mafic melt. The presence of oxidation exsolution lamellae of spinel and ilmenite in the gabbroic unit and the titanite magnetite quartz association in the syenite leave little doubt that both magmas were oxidized and that appropriate conditions for the crystallization of oxide minerals were present (Buddington and Lindsley, 1964; Price and Putnis, 1979; Wones, 1989) Geochemical modeling of the BIC Major element modeling Basaltic rocks of the ELIP are sub-divided into high-ti and low-ti magma series (Xu et al., 2001; Zhou et al., 2008). The high-ti basaltic rocks are considered to be the parental magmas of the Fe Ti oxide bearing layered intrusions of the Panxi region, whereas the Ni Cu sulphide bearing intrusions belong to the low-ti series (Zhou et al., 2008). Therefore petrogenetic modeling employed a high-ti flood basalt (EM-81) as the parental magma composition. Mineral compositions from the layered gabbroic unit were fractionated from the parental magma composition to produce bimodal compositions that match those of the syenitic and gabbroic units of the BIC (Table 5). Removal of an assemblage consisting of 4.2% olivine, 31% clinopyroxene, 29% plagioclase, 1.9% ilmenite, 11.5% magnetite, 1% spinel and ~1% apatite from the parental high-ti basaltic magma, resulted in a residual magma with a composition similar to that of the syenitic unit and equaling ~19% of the initial magma mass. The modeled gabbroic unit can be subdivided into three parts equaling the lower cumulate zone (~47%), oxide ore zone (~7%) and the upper gabbro zone (~25%). In our model, the olivine gabbro zone represents a mixture of the pure silicate minerals and the pure oxide minerals and equals ~55% of the proportion of the lower cumulates (i.e. 26% total). For simplicity the olivine gabbros are summed together with the lower cumulate zone. Major element modeling indicates that the known compositions of the three rocks types of the BIC (e.g. syenitic, gabbroic and ore) can be satisfactorily reproduced through crystallization of the observed Table 6 Partition coefficients (D) of the gabbroic zones Zone LCZ OXZ OGZ UGZ Element BIC bulk D Literature value BIC bulk D Literature value BIC bulk D Literature value BIC bulk D Literature value Sr Ba Ni Co Nb Ta LCZ = lower cumulate zone, OXZ = oxide ore zone, OGZ = olivine gabbro zone, UGZ = upper gabbro zone. Percentages of olivine (ol), plagioclase (pl), clinopyroxene (px) and magnetite (mt) for the LCZ=28% ol, 35% pl, 30% px, 5% mt; OXZ=0% ol, 0% pl, 0% px, 78% mt; OGZ=6% ol, 35% pl, 28% px, 20% mt; UGZ=2.5% ol, 51% pl, 30% px, 8.4% mt. D = partition coefficient. D of Sr, Ba, Ni, Co, Nb and Ta in olivine=0.014, 0.010, 5.9, 6.6, 0.01, n/ a; in pyroxene=0.06, 0.026, 1.5, 2.0, 0.005, 0.013; in plagioclase=2.5, 0.5, n/a, n/a, 0.01, n/ a; in magnetite=n/a, n/a, 10.0, 7.4, 3.0, 3.0. Mineral melt partition coefficients taken from Rollinson (1993) and the Geochemical Earth reference model (GERM). mineral assemblages from a high-ti basalt, which is consistent with the results from studies of similar intrusions from the Panxi region (Pang et al., 2008). The major element modeling also shows that the silica gap between the two units is directly related to the en masse crystallization of the oxide minerals. For example, if the proportion of the magnetite fractionating change by a small amount (e.g. from 11.5% to 9%), the whole rock SiO 2 concentration will change significantly (from 66.9 wt.% to 59.1 wt.%) while CaO and MgO, Na 2 O and K 2 O will hardly change (Table 5). The MELTS program of Ghiorso and Sack (1995) was successfully applied using the starting composition equal to the whole rock composition of sample EM-81 of Xu et al. (2001) assuming FeO=12.81 wt.% and Fe 2 O 3 wt.%=2.13 wt.%. Under pressure conditions equal to 1000 bars, a starting temperature of 1200 C, final temperature of 800 C and oxygen fugacity of QFM+1, we were able to model bulk rock compositions similar to the BIC syenite at ~950 C. Table 5 Whole rock geochemical modeling of the BIC and high-ti Emeishan flood basalt Sample SiO 2 TiO 2 Al 2 O 3 FeOt MnO MgO CaO Na 2 O K 2 O P 2 O 5 Total Hi-Ti (EM-81) Hi-Ti Remaining Normalized Hi-Ti Normalized Syenite Normalized Hi-Ti Ore Remaining Normalized Ore Normalized Remaining Composition Olivine (Fo76) Cpx Plagioclase (An65) Plagioclase (An57) Plagioclase (An46) Ilmenite Magnetite Apatite Spinel SiO TiO Al 2 O FeOt MnO MgO CaO Na 2 O K 2 O P 2 O ol% cpx % pl65% pl57% pl46% il % mt % ap% sp% Total Hi-Ti Ore Ol olivine with forsterite (Fo) 76; cpx clinopyroxene; pl65 plagioclase anorthite (An) content 65; pl57 plagioclase anorthite (An) content 57; pl46 plagioclase anorthite (An) content 46; il ilmenite; mt magnetite; ap apatite; sp spinel. High-Ti Emeishan flood basalt sample EM-81 taken from Xu et al. (2001). Apatite composition taken from Deer et al. (1992). Hi-Ti composition is based on the proportion of minerals. Ore composition based on the proportion of minerals.

11 214 J.G. Shellnutt et al. / Chemical Geology 259 (2009) Trace element modeling To test if fractional crystallization is consistent with the observed trace element systematics of the BIC, partitioning of Ba, Sr, Ni, Co, Nb and Ta between fractionating crystals and evolving melt was Fig. 9. Trace element modeling results showing the evolution of the BIC parental magma using a) Ba (ppm) and Sr (ppm), b) Nb (ppm) and Ta (ppm), and c) Ni (ppm) and Co (ppm). The starting composition is an average of high-ti basalts from Xu et al. (2001) (EM-78, EM-81, EM-82, EM-84, EM-85). The residual liquid composition ( ) is shown evolving at the melt fraction percentages using the bulk partition coefficients of the LCZ, OXZ, OGZ and UGZ until reaching 19%. Fig. 10. Proposed petrogenetic model of the formation of the Baima igneous complex. (a) Emplacement of a parental magma of the Baima igneous complex with an initial amount of suspended crystals. (b) Settling and in situ crystallization of cumulate olivine, plagioclase and clinopyroxene. (c) After a period of fractionation, a lighter alkali-rich (white) residual magma migrates to the top of the magma chamber. At this point the magma chamber is likely oxygenated and crystallizes massive amounts of magnetite (black) in the lower part of the magma chamber while the syenite forms in the upper part of the magma chamber. (d) Solidification and stratification of the BIC gabbroic unit into four zones and formation of the syenitic unit.

12 J.G. Shellnutt et al. / Chemical Geology 259 (2009) considered. The starting composition used for trace element modeling was an average of Ba, Sr, Ni, Co, Nb and Ta of high-ti basalts from Ertan (Xu et al., 2001). Bulk distribution coefficients were calculated for the four lithologic zones using reasonable bulk partition coefficients of Ba, Sr, Ni, Co, Nb and Ta. The results are very similar to values based on the observed mineral proportions of olivine, plagioclase, clinopyroxene, and magnetite in the gabbroic unit and published mineral melt partition coefficients (Table 6). The evolution of the trace element budget is illustrated for Ba Sr, Nb Ta and Co Ni (Fig. 9). For Ba and Sr, the removal of the lower cumulate, the oxide ore and olivine gabbro increases the amount of both elements in the melt. When the upper gabbroic unit is removed, Sr decreases along with a significant increase of Ba concentration. The calculations produce a modeled liquid composition of 401 ppm Sr and 1725 ppm Ba with ~19% of the original melt remaining. Fractionation (solid line) of alkali feldspar with titanite, quartz, amphibole and aegirine can explain the remaining trend which correlates with the actual syenite compositions (Fig. 9a). The trends of Ta Nb and Co Ni are also supportive of the fractionation model (Fig. 9b and c). The Ta Nb trend shows that most of the partitioning occurs in the syenite (as expected due to the presence of titanite) and the Co Ni trend shows a significant drop to nearly zero for both elements as the syenitic composition is approached. The results show that major and trace element modeling are consistent with each other. In both cases, the major and trace element modeling indicate the syenitic unit may be formed by a total of ~80% crystal fractionation from a high-ti flood basalt. The modeled concentrations of Ba, Sr, Co, Ni, Nb and Ta are very similar to the measured values in the syenite. Hence, the combined major and trace element modeling supports fractional crystallization as the principle process responsible for the formation of all units of the BIC Petrogenetic model of the BIC On the basis of the comagmatic and cogenetic relationship between the two units, we propose a three stage model outlining the possible evolution of the BIC Stage 1 A mafic magma resembling high-ti basalt from the ELIP was injected into Paleozoic sedimentary rocks of the Yangtze Block at ~260 Ma (Fig. 10a). During and after emplacement, the magma crystallized olivine, plagioclase and clinopyroxene, which controls the evolution of Ni, Co, Cr, Sc, Eu, Ba, and Sr within the melt. Internal differentiation (e.g. magma convection) may have contributed to the redistribution of earlier formed minerals (Fig. 10b) Stage 2 Fractionation of olivine, plagioclase and clinopyroxene, likely increased fo 2 and led to en masse crystallization of oxide minerals. The removal of plagioclase from the parental magma likely caused a plagioclase effect, which resulted in the residual magma to become peralkaline. In addition, the crystallization of oxide minerals was likely responsible for the SiO 2 gap between the two units. As crystallization continued, fo 2 decreased producing an ilmenite dominated upper gabbro near the boundary with the syenitic unit. The silicic residual liquid partitioned the REEs, Nb, Ta, Zr, Hf, Th and U (Fig. 10c) Stage 3 The solidification of the gabbroic unit resulted in the formation of a lower cumulate zone, an oxide ore zone containing partially resorbed cumulate crystals of olivine, plagioclase and clinopyroxene and syenitic layers, an olivine gabbroic zone with intermittent oxide-rich layers, and an ilmenite-dominated upper gabbroic zone with a noticeable amount of apatite (Fig. 10d) Implication for the formation of peralkaline A-type granitoids from mafic magmas Many A-type granitoids, in particular peralkaline granitoids, are considered to be generated by differentiation of mantle derived mafic magmas (MacDonald et al., 1975; Loiselle and Wones, 1979; Eby, 1990; Bonin, 2007). Geochemical and mineralogical data indicate that the BIC syenitic unit is similar to A-type granitoids and thus has significant implications for the genesis of these rocks. The original model for the formation of A-type granitoids suggested that fractional crystallization of mafic magmas was responsible for their unique chemistry although it is clear that A- type granitoids can be formed in different ways (Loiselle and Wones, 1979; Collins et al.,1982; Turner et al.,1992; Frost and Frost, 1997; King et al., 1997; Shellnutt and Zhou, 2007). Given the common association of layered gabbroic intrusions and granitic rocks of A-type affinity, it is difficult to dismiss their cogenesis (Barbarin, 1999). Bonin (2007) has suggested that A-types granitoids are derived from transitional to alkaline mafic to intermediate magmas of mantle origin. The observations from the BIC are consistent with the interpretation of Bonin (2007) and similar petrogenetic scenarios may apply to the other silica saturated peralkaline A-type granitoids in the Panxi region (i.e., Taihe and Panzhihua plutons). Considering there are metaluminous and peraluminous A-types granitic rocks in the Panxi region, which formed by processes different from the peralkaline granitoids, it would be prudent to suggest that fractionation of mantle derived, mildly alkaline mafic magmas may only produce the silica saturated, peralkaline granitoids (Shellnutt and Zhou, 2007). The magma conditions of the BIC formation (e.g., temperature, pressure and fo 2 ) are not well constrained but it is likely that the conditions were similar to the Panzhihua intrusion (e.g. significant crustal residency time, high temperature and shallow emplacement) (Pang et al., 2008). Metal-rich layers in the lower third of the BIC layered gabbroic unit are unusual. Typically, massive magnetite deposits occur in the upper parts of layered intrusions (e.g. Bushveld intrusion). Results presented in this paper suggest that fractional crystallization may explain why the BIC mafic and felsic magmas formed as well as a world class oxide deposit. The important implication is that granitic rocks with compositions similar to the BIC syenitic unit may be used as geological and geochemical indicators for finding layered gabbroic intrusions which host massive Fe Ti V oxide deposits. 7. Conclusions The whole rock geochemistry, radiometric dating and radiogenic isotope data of the BIC indicate that the mafic and felsic units are comagmatic and cogenetic. Major and trace element modeling based on published partition coefficients indicate that the two BIC units evolved by fractional crystallization from a common parental magma equivalent to an Emeishan high-ti flood basalt. Magma convection and crystal settling likely occurred and assisted in redistributing the early formed crystals. Convective fractionation coupled with en masse oxide mineral crystallization is responsible for the formation of the Daly gap. The upper part of the intrusion formed the syenitic unit and the lower part, mixed with cumulate minerals, formed the gabbroic unit and the oxide ore deposit. Acknowledgements We acknowledge the constructive reviews and comments of two anonymous reviewers and Editor Bernard Bourdon. We thank Professor Paul T. Robinson and Dr. Christina Y. Wang for reading earlier drafts of this manuscript. The authors would like to thank Professor Ma Yuxiao and Mr. Zhao Hao both from Chengdu University of Science and Technology for their field support and Mr. Liang Qi and Ms. Xiao Fu for their analytical support at the University of Hong Kong.