Origin of Reverse Zoned Cr-Spinels from the Paleoproterozoic Yanmenguan Mafic – Ultramafic Complex in the North China Craton

We conducted petrological and mineral chemistry investigations of Cr-spinel in ultramafic rocks of the Yanmenguan mafic–ultramafic complex in the North China Craton. The Cr-spinel grains occur as inclusions in enstatite, tschermakite, phlogopite, and olivine, or as interstitial grains among the aforementioned silicate minerals, and show concentric or asymmetrical textures. Back-scattered electron and elemental images and compositional profiles of the spinel grains indicate the presence of Crand Fe-rich cores and Aland Mg-rich rims. The host silicate minerals display a decrease in Al and Mg contents accompanied by an increase in Cr and Fe away from the spinel. These textures and compositional variations suggest that subsolidus elemental exchange more likely gave rise to the compositional zonation, resulting in the transfer of Al and Mg from the silicate minerals to the spinel. The Mn, Ni, and Ti contents in spinel and the major elements of olivine-hosted spinel are relatively stable during subsolidus elemental diffusion and thus are more reliable tracers of primary high-temperature processes. The temperature estimates reveal that the subsolidus diffusion might have occurred at 600–720 ◦C, which could be linked to the regional metamorphic event.

In this study, we report reverse zoning texture in Cr-spinels from ultramafic rocks of the Yanmenguan mafic-ultramafic complex.The zoned-textured Cr-spinel occurs as inclusions in a variety of silicate minerals or as interstitial grains among different silicate minerals, and the texture is variable from concentric to asymmetrical.Mineral morphology, elemental mapping images, and compositional profile analyses were employed to determine the mechanism and origin of the reverse zoning texture and its implications.

Geology of the Yanmenguan Mafic-Ultramafic Complex
The North China Craton is traditionally divided into two major continental blocks, the Western and Eastern Blocks, which are separated by a collision zone referred to as the Trans-North China Orogen (TNCO), and are composed of TTG (tonalite-trondhjemite-granodiorite) gneisses and granite-greenstone belts [38] (Figure 1a).The Hengshan-Wutai-Fuping area located in the central segment of this orogen is identified as an extensive exposure in the NCC (Figure 1b).Thereinto, abundant Neoarchean mafic-ultramafic boudins are distributed in the Wutai Group in a NE-SW direction, and separated by the Fuping terrane and Hengshan gneiss complex.All of these complexes are scattered within strongly deformed flysch-like metabasic and metapelitic rocks, which record the amphibolite-to granulite-facies metamorphism in the region [38].Because of intensive metamorphic overprint [39], the origin and tectonic setting of these mafic-ultramafic rocks are disputed.Li et al. [40,41] interpreted these mafic-ultramafic complexes as remnants of paleo-oceanic lithosphere.Wang et al. [39] identified them as a mélange of Neoarchean ophiolite, whereas Polat et al. [42] suggested that some of them formed in a forearc setting.Recently, Yanmenguan complex, one of these mafic-ultramafic complexes which was akin to Alaskan-type complex in aspects of mineral chemistry, greatly depended on the compositions of Cr-spinel and silicate minerals [43,44].
The Yanmenguan mafic-ultramafic complex is located in the northwest of the Hengshan-Wutai -Fuping area (Figure 1b).It cuts through the Neoarchean gneisses, and has a formation age of 2.2 Ga (zircon U-Pb dating; [43]).The complex occurs as lenses with a distinctive zoning pattern of peridotite, olivine pyroxenite, pyroxenite, hornblendite, and gabbro, in the order from center to margin (Figure 1c).The peridotite shows granoblastic or porphyroclastic textures and is mainly composed of olivine, enstatite, diopside, Cr-spinel, and tschermakite, with minor phlogopite, magnetite, and sulfides.Olivine grains occur as mainly cumulate minerals in variable sizes from 0.2 to 1 mm (Figure 2a,b), and occasionally display granular and equilibrated texture with nearly 120 • triple junctions (Figure 2c).All other minerals show subhedral or anhedral texture.Enstatite is the dominant intercumulus mineral, forming oikocrysts in the range of 0.1-0.5 cm in diameter.Diopside and tschermakite occur as large oikocrysts or small anhedral grains at the boundary of olivine grains.Olivine pyroxenite shows gradational contacts with the peridotite and consists mainly of enstatite, tschermakite, diopside, olivine, and Cr-spinel, with accessory phlogopite, chlorapatite, and magnetite (Figure 2d-h).Enstatite and diopside are oikocrysts with sizes in the range of 0.2-1 cm, whilst tschermakite is relatively small in size and less abundant.Olivine occurs as interstitial grains or as inclusions in enstatite, diopside, or tschermakite (Figure 2d-g).The radial fractures in enstatite are exhibited for the differential contraction between encapsulated olivine and host enstatite during the cooling process (Figure 2h) [45].Intercumulus tschermakite is present as the latest intercumulus voids or resorption-induced rim around the diopside (Figure 2i).Phlogopite in many places occurs with chlorapatite as inclusions in enstatite (Figure 2j).Cr-spinel is the most common accessory mineral in the ultramafic rocks and rarely occurs in the mafic rocks.It is euhedral to subhedral, ranging in size from 0.05 to 0.2 mm, and occurs within or among silicate minerals.Apart from the opaque or green spinel (Figure 2k), zoning texture is commonly emphasized by the light brown core and greenish rim (Figure 2l).[43]).(c) Geological sketch map of the Yanmenguan complex, showing the major rock units of peridotite, olivine pyroxenite, pyroxenite, hornblendite, gabbro, and the surrounding Archean gneiss as a country rock.Note that the lithological units are zonal patterns in the vertical section (modified after [43]).

Analytical Methods
Polished thin sections of all the samples from the ultramafic rock suits were investigated by a FEI Nova nano450 scanning electron microscope (SEM), equipped with energy dispersive spectrometry (EDS) at the State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS), Beijing, China.High-resolution elemental mapping images were obtained at the working conditions of 20 kV accelerating voltage and 3.5 nA beam current, with a working distance of 4 mm from the polepiece to the sample surface.
For major elements, compositional profiles of minerals as well as back-scattered electron images were obtained by wavelength-dispersive spectrometry (WDS) using a JEOL JXA8100 electron probe at the State Key Laboratory of Lithospheric Evolution, IGGCAS, Beijing, China, operating at an accelerating voltage of 15 kV, 12 nA beam current, 5 µm beam spot size, and 10-30 s dwell time as well as 11 mm working distance.The elements were acquired using three crystals as follows: LIFH for Cr, Mn, Fe, and Ni, PETJ for K, Ca, and Ti, and TAP for Na, Mg, Al, and Si.The standards used were albite for Na, diopside for Si, Ca, and Mg, haematite and synthetic Cr 2 O 3 for Cr, synthetic TiO 2 for Ti, orthoclase for K, synthetic Al 2 O 3 for Al, synthetic MnO for Mn and synthetic NiO for Ni.K(α) lines were selected for analysis, and the detection limits were in the range of 0.008~0.02wt % (1σ).The spacing of points in the profiles was within 10~15 µm and traversed the Cr-spinel and paired silicates.Matrix effects were corrected by a program based on the ZAF procedure.Fe 2+ -Fe 3+ redistribution from electron microprobe analyses was carried out using the general equation for estimating Fe 3+ defined by Droop [46].The profiles of reverse-zoned Cr-spinel in and among different silicate minerals are provided in supplementary materials.

Zoning Texture of Cr-Spinel
Reverse zoning texture is commonly found in Cr-spinel inclusions in enstatite (Figure 3a,b), tschermakite (Figure 3c,d), phlogopite (Figure 3e) and olivine (Figure 3f), which are all from the ultramafic rocks in the Yanmenguan complex.Specifically, the concentric-reverse zonation shows a gradual variation from bright core to dark rim in back-scattered electron images, except for the uniqueness of olivine-hosted Cr-spinels with very thin rims relative to broad cores (Figure 3f).Interstitial Cr-spinel grain that occurs amongst different silicate minerals is also zoned with concentric texture (Figure 3g,h), except that the Cr-spinels among the olivine-Cr-spinel-enstatite (or tschermakite) arrangements display asymmetrical zonation (Figure 3i-l), with their cores usually restrict close to the direction of olivine.In addition, the occurrence of a cluster of Cr-spinel indicates that zoning textures are well developed in larger grains, whilst some of the relatively small Cr-spinel grains hardly preserve any zoning texture (Figure 3m-p).The clear distribution of elements in reverse-zoned Cr-spinel and host silicate minerals is evident in elemental mapping images (Figure 4) and representative compositional profile (Figure 5).In general, the Cr-spinel displays a bell-like distribution of the major elements: the cores are enriched in heavy elements such as Cr and Fe, whereas the rims show conjugated enrichment in light elements such as Al and Mg.The host silicate minerals are characterized by a general decrease in Al and Mg contents accompanied by increasing Cr and Fe contents with the distances close to Cr-spinel.With respect to the minor elements, the elemental mapping images could hardly detect any non-accidental elemental variation.The limited variation in the contents of Ni, Mn, and Ti in both Cr-spinel and host silicate minerals is revealed by the compositional profiles.

Discussion
Clear reverse zoning in Cr-spinel within the ultramafic rocks of the Yanmenguan complex and the origin(s) of the reverse zonation have been the subject of continuous debate.In this study, therefore, we look at all the possible mechanisms to enable us to establish the most likely process responsible for the observed zonation in the Yanmenguan complex.

Reaction with Non-Homologous Melt
The reaction between crystals and non-homologous melt may generate reverse zoning in spinels [34,35,47].The most common is the inherited or captured spinel grains, exhibiting sharp variation from Cr-rich core to Al-rich rim.This kind of reverse zoning has been reported in arc volcanic rocks [48] and those in greenstone belts [49], suggesting marked disequilibrium between spinel core and host silicate liquid.However, the limited number of inherited spinel is inconsistent with the abundance of zoned spinel in the Yanmenguan complex.
Magma mixing or assimilation with crust-derived wall rock can also result in significant change in the melt composition as well as the disequilibrium between the previously formed core and the evolved silicate liquid.Relevant studies have been reported in seamount lavas [35] and tholeiitic basaltic magmas [36].This mechanism can explain the localized asymmetric zonation in Cr-spinel hosted in olivine and shielded from reaction with mixed magma (Figure 3i-l).However, the evolved melt can be excluded by the contradiction of increasing Cr concentrations in silicates and increasing Al concentrations in spinel (Figure 5).Thus, the reaction with non-homologous melt could not be the fundamental cause of the reverse zonation observed in the Cr-spinel from the Yanmenguan complex.

Reaction with Homologous Melt
It has been suggested that abundant [34] or rapid [21,28] growth of spinels could lead to localized Cr-depletion in homologous melt, and the continued growth of accumulated spinel grains gives rise to the massive reverse zoning texture in the postcumulus stage.The former is well-documented in the large, layered mafic-ultramafic complex, in which the extensive accumulation of chromitite as well as the absence of supplementary magma could also induce the dramatic change in melt composition and generate reverse zoning chromite (Rhum-layered mafic-ultramafic complex [33]; Panton Sill-layered mafic-ultramafic complex [34]).However, it is clearly different from the sparse Cr-spinel in the Yanmenguan complex, which can hardly lead to the large-scale variation in the melt at the post cumulus stage.
The latter is currently the dominant origin for reverse zoning spinels which, combined with the relative difficulty of nucleating crystals of Al-rich silicate minerals [20,29,47,50], results in the formation of extremely Al-rich melt.The increasing Al with decreasing Cr could result in progressive changes in crystal chemistry of the spinels from Cr-and Fe-rich crystal cores to Al-and Mg-rich rims and leading to the formation of reverse zonation [28,30,[51][52][53][54]. Related observation was first described by Sigurdsson [25] to explain the intensely-zoned spinels in olivine-spinel pairs in basalts collected during the ocean drilling program, marked by a decrease in Cr# (Cr/(Cr + Al) × 100) from core to rim with re-equilibrated zoning of Mg# (Mg/(Mg + Fe 2+ ) × 100) [55].Subsequently, the reverse-zoned spinel grains through the same mechanism have been reported in alkali basaltic glasses from Iceland [21,31], quenched basaltic lavas from Hawaii [56], and MORB-type greenstones in East Takayama [32], and confirmed by thermodynamic experiments [26].
This mechanism is well-documented to explain the gradual elemental change in spinel core and rim by continuous crystallization (e.g., Figure 3a) and the larger crystals display more distinct zoning texture than the small ones (Figure 3m-p) caused by the different nucleation period [33,34,50].The asymmetric zonation (Figure 3i-l) most likely resulted from local melt heterogeneity due to disequilibrium induced by crystallization of various phases with different compositions, which could affect the diffusion rates of ions into the growing crystals [33,50,57].However, this type of reverse zonation is usually generated in the olivine-spinel paired samples, accompanied by the absence of Al-rich silicate minerals [50,57].On the contrary, zoned Cr-spinel grains are preferentially distributed in the enstatite (Figure 3a,b), tschermakite (Figure 3c,d), and phlogopite (Figure 3e) in the Yanmenguan complex, where the abundant silicate minerals could buffer the Cr/Al ratio in melt to some extent [32].More importantly, the compositional variation in spinel from core to rim cannot decrease temperature and increase oxygen fugacity (Figure 6a), suggesting that the zonation could not be the result of fractional crystallization [58].In addition, the marked increase in Cr and decrease in Al in silicate minerals cannot conform to the elemental variation of magma differentiation (Figure 5).Therefore, the aforementioned mechanisms could not explain the reverse zonation in the Yanmenguan complex.

Elemental Exchange with Coexisting Silicate Minerals
Elemental exchange with coexisting silicate minerals is another important factor modifying the composition of spinel [61][62][63][64][65], and is commonly induced by the variation of physical conditions [66][67][68][69].Generally, diffusion-induced zonation is caused by the long-term exchange between crystals subjected to continuous, high subsolidus temperatures [23,[70][71][72].In the study of the Red Mountain Alpine-type mafic-ultramafic complex in South Island, New Zealand [22,[72][73][74], Sinton [22] discovered significant reverse zoning in spinel grains in a spinel-enstatite pair.It is believed that the continuous reaction with enstatite led to the enrichment of Al and Mg in the Cr-spinel, whereas the enstatite became enriched in Cr and Fe.Similar interpretations were assigned to reverse zonation in spinel grains from the Rhum-layered mafic-ultramafic complex in Scotland [37] and also to spinel grains from the Gabbro Akarem Alaskan-type mafic-ultramafic complex in Egypt [24,75,76].
The aforementioned compositional variations between spinel and silicate minerals are consistent with the result of elemental mapping images (Figure 4) and compositional profiles (Figure 5) of the Yanmenguan complex.The asymmetrical zonation (Figure 3i-k) is interpreted to be caused by various degrees of elemental exchange with different silicate minerals, which are also characterized by variable degrees of solubility and different capacities to accommodate elements [22,[77][78][79].Thus, tschermakite, phlogopite, and enstatite with higher contents of Al and high Cr-bearing capacity impose greater influence on spinel than olivine, which only contains limited concentrations of Cr [72,80,81] and Al [82,83]), and acts as a relatively perfect barrier to elemental diffusion [84].In addition, the characteristic of large Cr-spinel grains preserving well-developed zonation (Figure 3m-p) suggests that the relatively small grains are completely controlled or equilibrated by the surrounding silicate minerals, thereby destroying the zoned texture [85].Obviously, the elemental exchange with coexisting silicate minerals can reasonably explain the reverse zonation in Cr-spinel.

Inducement of Elemental Exchange
According to the empirical model supplied by Sack and Ghiorso [58], the crystallization temperature of the Cr-spinel in the Yanmenguan complex approximates 1050 • C (Figure 6a).However, the calculated temperature of the spinel-olivine pair from the Yanmenguan complex measured by the thermodynamics thermometer from Ballhaus [59] yielded a range of 600-720 • C, which is similar to the medium-and high-grade metamorphic temperature (Figure 6b).The calculated temperature may be associated with the complex metamorphic history recorded in the Hengshan-Wutai-Fuping area.
Previous studies have shown that a pre-existing ocean was completely consumed by subduction between the Eastern and Western Blocks of the North China Craton [38,86,87].Consequently, the continent-arc-continent collision [88] introduced those rocks into lower crustal levels or the upper mantle [88], where granulite to eclogite facies [89][90][91][92] or greenschist to lower amphibolite facies [93][94][95][96] metamorphism occurred.This metamorphic history may provide long-lived dynamics and thermal status for elemental exchange in the Yanmenguan complex.
In fact, the occurrence of the coexistence of both reddish-brown and green spinel grains in the Nikubuchi peridotite in Japan has been reported [97].The textural relationships and mineral chemistry of the Nikubuchi rocks indicate that the spinel from the peridotite was equilibrated at granulite facies conditions before being emplaced into the Sanbagawa metamorphic belt [97].Similar occurrences are also recorded in ultramafic peridotite rocks metamorphosed under granulite facies conditions from the Rio de Las Tunas Belt in Argentina [98].In Skye Island, the peridotite is also characterized by the occurrence of red and green spinel grains.Lenaz et al. [99] suggested that the green pleonaste is a by-product of the metasomatism of previously Cr-rich spinel, resulting in the mobilization of Cr during a localized thermal event.
Interestingly, the elemental exchange is commonly mentioned in Mg-Fe 2+ exchange without the equilibrium of Cr-Al-Fe 3+ , because the diffusion rates of divalent ions are several magnitudes faster than those of trivalent ions [24].However, all the compositional profiles in this study show that elemental exchange is more pronounced in Cr-Al-Fe 3+ than in Mg-Fe 2+ (Figure 5 and Supplementary Materials).We assume the discrepancy could be associated with the relatively faster rate of diffusion of divalent ions in Cr-spinel, during which the well-developed zonation preferentially preserves trivalent ions instead of divalent ions.The blocking temperatures of divalent ions are also lower than those of trivalent ions [23], and therefore, the increased frequency and prolonged exchange of divalent ions between the core and rim of Cr-spinel can destroy the already formed zonation texture.It is suggested that the re-equilibration of spinel with adjacent silicate restricts the application of spinel to investigate igneous processes [15,23,100,101].Thus, the trivalent ions may be relatively appropriate, especially for olivine-hosted spinel (Figure 6a), to trace the original composition of spinel (Figure 7a,b).There are limited variations in the contents of Ni, Mn, and Ti in Cr-spinel (Figure 5).In this respect, much attention should be paid to these minor elements in their application to trace geologic processes (Figure 7c), since these elements are not affected by elemental exchange and are therefore more reliable and valid than the traditional index [64].

Conclusions
The widespread reverse zoning in Cr-spinel from rocks of the Yanmenguan complex is compositionally characterized by Al-and Mg-rich rims and Cr-and Fe-rich cores.The zoning texture is discriminative in the different host silicate minerals and the sizes of Cr-spinel grains.These features rule out fractional crystallization and the reaction with melt as responsible for the zoning.The compositional profiles traversing the Cr-spinel and host silicate mineral were generated to investigate the elemental exchange reaction that gave rise to the zonation.Based on the calculated temperature of 600-720 • C, we postulate that the metamorphic event was the origin of elemental exchange in the Hengshan-Wutai-Fuping area.S1: Data of compositional profiles; Table S2: Supplementary coupled olivine and spinel compositions for temperature estimations.

Fig. 1 Figure 1 .
Fig.1 Figure 1.(a) Subdivisions of the North China Craton and location of the Yanmenguan mafic-ultramafic complex (modified after [38]).(b) Simplified geological map of the Hengshan-Wutai-Fuping area (modified after[43]).(c) Geological sketch map of the Yanmenguan complex, showing the major rock units of peridotite, olivine pyroxenite, pyroxenite, hornblendite, gabbro, and the surrounding Archean gneiss as a country rock.Note that the lithological units are zonal patterns in the vertical section (modified after[43]).

Figure 5 .
Figure 5. Compositional profile analyses of zoned Cr-spinel located between enstatite and tschermakite.The profile shows in Figure 3k.

Fig. 6
Fig.6Figure6.(a) Cr-spinel empirical thermometer after Sack and Ghiorso[58].The equipotential curves and Fe 3+ /Fe 2+ of melt are from Irvine[1] andAgata and Adachi [33].Equipotential curves shifting to the left represent magmatic differentiation, and the paralleled arrow denotes the trend of elemental exchange.(b) Olivine-spinel thermometer after Ballhaus[59].The compositions for temperature estimates based on the rim of spinel and co-existing olivine.The presupposed pressures (1.2 and 1.4 GPa) are from Wang et al.[39].Data are available in supplementary material.P-T diagram with eight principal metamorphic facies is from Spear[60].