Petrogenesis of the Kangjinla peridotite in the Luobusa ophiolite, Southern Tibet

The Kangjinla peridotite in the eastern part of the Luobusa ophiolite consists mainly of fresh harzburgite with less abundant lherzolite and dunite. The generally depleted nature of the rocks and minerals suggests that they are the residue of partial melting of MOR-like mantle. However, all of the peridotites show some degree of LREE enrichment which is attributed to modification by subduction-related fluids. The principal minerals in the peridotites typically show two stages of mineral growth. Early stage olivine and pyroxene typically form large grains with kink banding and wavy extinction; the pyroxene also shows kinking of twin lamellae. Later stage minerals are normally smaller in size and occur along fractures and cleavage planes within or between the early stage minerals. The late stage minerals show no evidence of deformation. Early stage orthopyroxene and clinopyroxene have higher Al 2 O 3 and Cr 2 O 3 than the later varieties. Fo values of olivine are 90–92 in harzburgite and lherzolite and 92–94 in dunite. The late-stage olivine has higher Cr 2 O 3 and NiO than the early stage grains. The Cr numbers (Cr# = (Cr × 100)/(Cr + Al)) of chrome spinel in the mantle peridotites are between 30 and 77, being lowest in the lherzolites and highest in the dunites. On the Cr# vs. Mg# (=(Mg × 100)/(Mg + Fe 2+ )) diagram the lherzolites and clinopyroxene harzburgites plot in the abyssal peridotite field, whereas the harzburgites and dunites plot in the island-arc peridotite field. Our preferred explanation for these compositional features is that the mantle peridotite formed in a MOR environment, then was modified altered by later-stage melts and fluids in a suprasubduction zone (SSZ) setting. The Kangjinla peridotites host a number of small podiform chromitites with a wide range of textures, including massive, disseminated, layered, nodular and anti-nodular. The chromitites have uniformly high Cr# (75.6–82.7) and moderately high Mg numbers (56.4–74.1), similar to that elsewhere in the Luobusa ophiolite. Many of the textures suggest precipitation from mafic magma but the chromitites contain a variety of ultrahigh pressure (UHP) minerals that indicate crystallization at depths >120 km. Keywords Peridotite Ophiolite Chromitite Yarlung Zangbo suture Tibet plateau 1 Introduction Mantle sections of ophiolites have a complex history typically involving deformation, partial melting, melt-rock reaction and refertilization. Some ophiolites appear to have originated at mid-ocean spreading centers and been modified later in a suprasubduction zone (SSZ) environments whereas others appear to have originated directly in the mantle wedge above subduction zones. Many ophiolites host podiform chromitites in upper mantle peridotites and the origin of these ore deposits and their relationship to the host peridotites has long been debated. Various models have been proposed for the formation of podiform chromitites ( Lago et al., 1982; Wang et al., 1983; Wang and Bao, 1987; Paktunc, 1990; Prichard and Lord, 1990; Mcelduff and Stumpfl, 1990; Zhou et al., 1994, 1996; Pei, 1995 ), but most workers agree that they form from SSZ magmas in the upper mantle. However, this view must be reconsidered in light of the discovery of ultrahigh pressure (UHP) minerals in both the chromitites and peridotites of the Luobusa ophiolite of Tibet. Diamonds and serpentine pseudomorphs after octahedral olivine were discovered many years ago in podiform chromitite of the Luobusa ophiolite ( IGCAGS, 1981; Yang et al., 1981 ) and have been confirmed by subsequent workers ( Hu, 1999; Bai et al., 2000a; Robinson et al., 2004; Yang et al., 2007, 2008, 2009 ). Similar UHP and highly reduced minerals have also been found recently in both the chromitites and peridotites of Luobusa, including coesite (possibly pseudomorphing stishovite) ( Yang et al., 2007 ), kyanite, a diamond inclusion in Os–Ir alloy ( Yang et al., 2007 ), diamond in peridotite ( Yang et al., 2008 ), inclusions of silicon spinel in Os–Ir alloy ( Bai et al., 2005 ), and reduced phases such as moissanite, wüstite, native elements and base-metal alloys ( Hu, 1999; Bai et al., 2000b, 2006; Robinson et al., 2004; Fang et al., 2009; Shi et al., 2009; Xu et al., 2009 ). Diamond and moissanite have now been confirmed as in-situ phases in the chromitites ( Yang et al., 2009 ). The presence of these UHP and highly reduced minerals has raised important questions regarding the origin of the Luobusa chromitites and their relationship to the host peridotites. The podiform chromitites of the Luobusa ophiolite consist of a number of small pods and irregular bodies that are grouped into three districts from west to east ( Fig. 1 ): the Luobusa, Xiangkashan and Kangjinla districts ( Zhang et al., 1996 ). Earlier investigators focused on orebodies (Cr-31 and -74) of the Luobusa district, which resulted in the recovery of nearly 100 unusual mineral species ( Bai et al., 2000a, 2003; Robinson et al., 2004 ). The current investigation was undertaken to determine whether the same unusual minerals occur in the chromitites of the Kangjinla district, and if so, to document carefully the mineralogical and geochemical compositions of the host peridotites. In this manner, it was hoped to provide a clearer picture of the genesis of the chromitites themselves. Orebody 11 of the Kangjinla district was selected because it is well exposed, is undergoing active mining that provides access to deeper levels and because of the well-developed textures and structures of the exposed chromitite. A collection of unusual minerals, similar to those of orebody 31 was discovered in the Kangjinla chromitite and host peridotites and have been reported elsewhere ( Xu et al., 2008, 2009; Xu, 2009 ). Some of these minerals must have formed in the deep mantle under highly reducing conditions, raising many questions about their origin and preservation in the upper mantle. In this paper, we present new data on the lithology, mineralogy, mineral compositions and geochemistry of the peridotites hosting the Kangjinla chromitite and use these data to place constraints on their petrogenesis. 2 Field occurrence of the Kangjinla peridotite The Luobusa ophiolite is a remnant of Neo-Tethyan oceanic lithosphere that lies in the Yarlung Zangbo suture zone between India and Asia ( Wang et al., 1987; Xiao and Wang, 1984 ). It is located about 200 km ESE of Lhasa where it extends east–west along the Yarlung Zangbo River for ∼37 km and ranges from 2 to 3.7 km in width ( Fig. 1 ). On the south, the ophiolite is separated from Triassic flysch by a steep reverse fault whereas to the north it is thrust over the Gangdise batholith and clastic sedimentary rocks of the Tertiary Luobusa Formation. The ophiolite is a tectonic slab composed chiefly of mantle peridotites and dunites with minor crustal cumulates and mafic dikes. The mantle rocks comprise about 93% of the exposed area and are composed mainly of harzburgite, with lesser amounts of dunite and minor lherzolite ( Fig. 1 ). The sparse cumulate rocks consist of wehrlite, lherzolite, pyroxenite and gabbro ( Zhang et al., 1996 ). All of the chromitite bodies are hosted in the mantle rocks near the crust-mantle boundary ( Wang et al., 1983 ) where they are locally cut by gabbro-diabase dikes. Orebodies cropping out in the Kangjinla district include Cr-7, Cr-9, Cr-62, Cr-11, Cr-53 and Cr-69 ( Zhang et al., 1996 ). Most of these are vein-like or lenticular bodies with envelopes of dunite, grading outward into harzburgite. Orebody Cr-11, the largest chromitite in the district, crops out in the eastern part of the ophiolite where it is hosted in generally fresh harzburgite. It extends along strike for about 200 m and ranges in thickness from 0.3 to 10.5 m (average = 3.3 m). It strikes about N75°W and dips 50–72°SW. The harzburgite is underlain by about 200 m of dunite, which is in fault contact with the underlying Tertiary Luobusa Formation ( Fig. 2 ). At this location, the Luobusa Formation consists of grayish-green conglomerate containing rounded clasts of peridotite and granite up to 10 cm across. Many of the clasts are flattened and elongated, presumably due to emplacement of the ophiolite along a thrust fault. The steep reverse fault on the south side of the ophiolite is marked by a band of highly resistant listwanite produced by hydrothermal activity ( Fig. 3 A) ( Robinson et al., 2005 ). Like the harzburgite, the associated dunite is quite fresh, with only a thin yellowish-brown weathered crust ( Fig. 3 B). At this site, the boundary between the dunite and harzburgite is sharp but elsewhere it is typically gradational, suggesting formation of the dunite by melt-rock reaction ( Zhou et al., 2005 ). Textures in the ore body are highly variable, ranging from compact and massive to disseminated, layered, nodular, anti-nodular, irregular and deformed. Massive ore is the most common but nodular, irregular and disseminated varieties are also abundant ( Fig. 4 A and B). Such nodular textures are characteristic of Alpine and ophiolitic chromitite ( Wang et al., 1983; Thayer, 1964; Zhou and Bai, 1992; Malpas and Robinson, 1987 ). Contacts between banded ore and dunite are commonly sharp and fine veins of compact ore are locally present ( Xu et al., 2008, 2009; Xu, 2009 ). The shapes and distribution of individual chromite grains are also quite variable and several varieties are present: (1) euhedral to subhedral, octahedral grains, generally <0.1–1 mm across, with straight crystal edges ( Fig. 5 A); (2) subhedral-anhedral grains, generally 0.5–2 mm across with curved margins ( Fig. 5 B). This type is particularly common in densely disseminated and massive ore where the grain size is somewhat larger (2–5 mm); (3) broken grains with fractures, cracks and tension gashes filled with gangue minerals ( Fig. 5 C); and (4) inclusion-bearing grains ( Fig. 5 D). The latter generally contain round olivine grains, some of which may themselves contain small inclusions of chrome spinel ( Fig. 5 D). 3 Analytical methods From a large collection of carefully selected samples of chromite, peridotite and dunite, we obtained 260 polished thin sections. After detailed petrographic examination, 34 thin sections were selected for electron microprobe analysis. The analyses were performed in the Key Laboratory of Nuclear Resources and Environment (East China Institute of Technology), Ministry of Education using a JEOL JXA-8100 electron microprobe with an Inca energy-dispersive spectrometer. The microprobe was set to operate at a voltage of 15 kV, a beam current of 20 amps and a spot diameter of 2 μm. We selected the freshest samples available for whole-rock geochemical analysis. The samples were carefully cleaned, crushed and then ground in an agate mortar to pass a 200-mesh screen. Major elements were determined on fused glass beads by X-ray fluorescence (XRF) spectrometry. The analytical accuracy is estimated to be 1% relative for SiO 2 and 2% relative for the other oxides. Trace elements, including the REE were determined by inductively coupled mass spectrometry (ICP-MS). Two national standard samples (GSR3 and GSR5) and three internal standards were measured simultaneously to ensure consistency of the analytical results. Analytical uncertainties are estimated to be 10% for trace elements with abundances <10 ppm, and around 5% for those >10 ppm. Water was determined by gravimetric techniques in which the sample is heated in a closed container and the water vapor is collected in a separate tube, condensed and then weighed. CO 2 was also determined gravimetrically. The detection limit for the H 2 O and CO 2 is 0.01 wt%. 4 Petrography of the Kangjinla peridotite Orebody Cr-11 is hosted mainly in harzburgite with minor dunite and lherzolite. All of the rocks are very fresh with generally less than 5% serpentinization and their original textures and structures are well preserved. The harzburgite is a medium- to coarse-grained, greenish-gray rock that has a thin, buff-colored weathering rind. Large orthopyroxene crystals are prominent on the weathered surface and show a crude foliation, making it easy to distinguish these rocks from dunite ( Fig. 3 B). The harzburgite consists chiefly of forsteritic olivine (75–90 modal%), Mg-rich orthopyroxene, (7–25 modal%), minor clinopyroxene (<5 modal%) and accessory minerals such as chrome spinel and magnetite (1–2 modal%). Sparse secondary minerals include serpentine, brucite, magnesite and chlorite. Small amounts of lherzolite are also locally present, being characterized by 5–10 modal% clinopyroxene. The clinopyroxene grains are small (<1 mm) and interstitial to the granular olivine. Orthopyroxene typically forms short, prismatic grains with many clinopyroxene exsolution lamellae and undulating extinction. Dunite occurs chiefly as an envelope around orebody Cr-11 or as patches and zones interspersed with the chromite ore. This dunite is separate from the thick dunite sequence at the base of the ophiolite, and shows very complicated relationships with the ore, commonly being broken into blocks that both contain abundant chromite nodules and are surrounded by disseminated ore ( Robinson et al., 2004 ). The blocks have different sizes and shapes and generally display sharp contacts with the ore. In thin section, the dunite generally displays granular textures, sometimes with evidence of plastic deformation and recrystallization. Typical samples consist of >95 modal% olivine, 1–2 modal% chrome spinel and traces of orthopyroxene and clinopyroxene. The podiform chromitites of the Luobusa ophiolite have yielded a wide variety of unusual minerals, including UHP phases such as diamond and coesite, highly reduced minerals such as moissanite, and native elements, carbides, alloys, oxides, sulfides (arsenides) and silicates ( IGCAGS, 1981; Zhu et al., 1981; Bai et al., 1993, 2000a,b, 2001, 2003, 2005, 2006; Robinson et al., 2004; Yang et al., 2007, 2008 ). A detailed investigation of chromitite orebody Cr-11, located in the Kangjinla district has revealed many of the same minerals ( Xu, 2009 ). This orebody is particularly rich in diamonds, with over 1000 grains recovered from a single 1100-kg sample of chromitite ( Xu et al., 2009 ). In all, more than 40 mineral species were discovered in the Kangjinla chromite deposit, including moissanite, rutile, native iron, and numerous metal alloys. 5 Mineral chemistry 5.1 Olivine Olivine in the peridotites and dunites hosting orebody Cr-11 is mostly fresh with only traces of serpentine and magnesite. Two stages of olivine formation are recognized in the peridotite. The early grains are large (up to 8 mm) and most show evidence of plastic deformation such as kink banding and wavy extinction. Later grains are much smaller (0.05–0.5 mm), and have a well-developed crystalloblastic textures with triple junctions between grains. The fine-grained olivine occurs along fractures and cleavages in the pyroxene or between the large, early olivine grains. It lacks evidence of plastic deformation and is interpreted as the product of recrystallization. All of the olivine in the harzburgites and lherzolites is relatively uniform in composition. In the harzburgites it ranges from Fo 90.2 to Fo 92.4 (average = Fo 90.8 ) and in the lherzolites from Fo 90.8 to Fo 92.3 (average Fo 91.4 ) ( Table 1 ; Fig. 6 ). No zoning was detected in individual crystals. These grains are also characterized by having relatively low NiO, averaging 0.39 wt% for both harzburgite and lherzolite, and very low Cr 2 O 3 , ranging from an average of 0.04 wt% in the lherzolites to 0.03 wt% in the harzburgites ( Table 1 ; Fig. 6 ). Olivine in the dunites is typically coarse to very coarse grained, and includes some crystals up to 4 cm across. Individual grains are irregular to subhedral, and the rocks typically have a granular texture. Most olivines from dunite show no indication of plastic deformation as seen in the harzburgites and lherzolites. These olivines are significantly more magnesian than those in the harzburgites, ranging from Fo 91.9 to Fo 94.3 ( Table 1 ; Fig. 6 ); the highest Fo values (Fo 96–98 ) are from grains included in Cr-spinel. Olivine in the dunites is also characterized by having relatively high NiO (max. 0.48 and aver. 0.39 wt%), whereas the Cr 2 O 3 contents in olivine are similar to those of olivine in the lherzolite and harzburgite (up to 0.24 wt%). Olivine is the main gangue mineral in the chromitites and a total of 150 analyses from 20 samples were obtained by EPMA. Seven representative analyses are given in Table 1 . Most of the olivine has Fo contents between 95.5 and 97.9, significantly more magnesian than olivine in the associated peridotites ( Fig. 6 ; Table 1 ). Such high-Mg olivines are generally attributed to subsolidus equilibration between olivine and chromite ( Roeder et al., 1979; Lehmann, 1983 ). A few grains have compositions between Fo 94 and 94.3, overlapping with olivine from the dunites ( Fig. 6 ). These are from samples of dunite at the margins of the ore body, in close contact with the host harzburgites. Nickel contents are relatively high in the olivine, mostly between 0.4 and 0.9 wt% and they increase with increasing Fo content ( Fig. 6 ). Chrome values are mostly low but a few grains have Cr 2 O 3 contents as high as 0.73 wt%. 5.2 Orthopyroxene Orthopyroxene is abundant in the harzburgites, sparse in the lherzolites and very rare in the dunites. Like the olivine, the orthopyroxene exhibits two stages of growth. In the harzburgites and lherzolites early orthopyroxene occurs as stubby prisms, many of which show deformational features such as bending, wavy extinction and twin gliding. Most of these grains also have numerous exsolution lamellae of clinopyroxene. Some grains, particularly those near the boundary between peridotite and dunite, show evidence of melt-rock reaction, such as partial replacement of orthopyroxene by granular olivine. Extensive replacement can result in small, isolated grains with uniform extinction, indicating that they were once part of a larger, single crystal. Some early orthopyroxene grains also occur as inclusions in early olivine. The late-stage orthopyroxene grains are small and irregular, with little or no evidence of deformation. They generally occur along fractures in the olivine crystals. As expected the orthopyroxene from the dunite, lherzolite and harzburgite is all highly magnesian but compositions vary somewhat depending on the host rock. The most magnesian variety occurs in the dunite, with En values ranging from 90.1 to 92.1 (average = 91.1). In the harzburgites, the orthopyroxene composition ranges from En 88.6 to 90.8 with an average of 89.8, and in the lherzolites, it ranges from 86.1 to 90.8 with an average of 89.3 ( Table 2 ; Fig. 7 ). However, the Mg# (=(Mg × 100)/(Mg + Fe 2+ )) of orthopyroxene in dunite (93) is higher than that in harzburgite and lherzolite ( Fig. 7 ). Alumina contents of orthopyroxene in dunite are very low, whereas those in harzburgite and lherzolite range up to 3.7 wt%. Nickel contents in orthopyroxene are uniformly low in all the lithologies ( Fig. 7 ), whereas Cr 2 O 3 contents can range up to 0.92 wt%. The En values do not differ significantly between the early and later varieties of orthopyroxene, however Al 2 O 3 , NiO and Cr 2 O 3 do show some variations. The early orthopyroxene has higher Cr 2 O 3 content (0.6–0.8 wt%) than the later stage grains (0–0.3 wt%), and slightly higher NiO and Al 2 O 3 . 5.3 Clinopyroxene Most of the harzburgites in the Kangjinla district contain small amounts of clinopyroxene (1–3 modal%) whereas the lherzolites typically have 5–10 modal%. Only traces of clinopyroxene are found in the dunites. As is the case for the other minerals, two stages of clinopyroxene growth can be recognized particularly in the lherzolites and harzburgites. The earliest grains are relatively large (more than 1 mm) and occur between orthopyroxene and olivine grains, or as inclusions in these minerals. Second-stage grains are much smaller (100 μm), and occur along cracks in olivine and orthopyroxene crystals. The Mg number of the clinopyroxene ranges from about 93 to 96.5, being consistently highest in the dunites ( Table 2 ; Fig. 8 ). Clinopyroxenes in the harzburgites and lherzolites have relatively uniform in composition Mg# (93–95), and show similar ranges of Al 2 O 3 , NiO, and Cr 2 O 3 , but grains in the dunites have consistently lower values for all three oxides ( Fig. 8 ). Generally, the early clinopyroxenes have slightly lower Mg# (up to 93.9) and higher Al 2 O 3 (2–3 wt%) and Cr 2 O 3 (>1 wt%) contents than the late-stage grains (up to 94.7, 1–2 wt%, and 0.4–0.7 wt%, respectively). 5.4 Chrome spinel Chrome spinel is ubiquitous in the peridotites and dunites but rarely exceeds 5 modal% of the rock. It is unevenly distributed and is generally associated with orthopyroxene in the harzburgites. The chrome spinels show a wide range of composition, typical of ophiolitic mantle and Alpine peridotites ( Irvine, 1967 ). Variations in the Cr number (Cr# = [Cr × 100/(Cr + Al)]) are believed to reflect variations in the degree of partial melting of the host peridotites and their level of depletion ( Dick and Bullen, 1984 ). Two major compositional groups of spinel are recognized in the peridotites and dunites ( Table 3 ; Fig. 9 ). All of the lherzolites and some of the harzburgites have spinels with low Cr#s ranging from 29.8 to 41.2 and a narrow range of Mg#s ranging from 64.4 to 67.0. All of these plot in the field of abyssal peridotite ( Fig. 9 , Irvine, 1967; Cameron et al., 1980 ). These values suggest relatively low degrees of partial melting, an interpretation supported by the common presence of clinopyroxene. The remaining harzburgites have spinels that form a tight cluster in the island arc basalt field with Cr#s of about 67–70 and Mg#s of 53–60 ( Fig. 9 ). These rocks are clearly more depleted than the lherzolites, with lower modal percentages of clinopyroxene, consistent with higher degrees of partial melting. There is some overlap in the compositions of Cr-spinel in the harzburgites and dunite but the dunites generally have higher Cr#s (66.4–76.9) and slightly lower Mg#s (39.4–59) ( Fig. 9 ). One unusual sample has a much higher Mg# than the others (70) and a Cr# of 70 ( Fig. 9 ). As shown in Fig. 9 , there is a negative correlation between Cr# and Mg# for the Kangjinla peridotites, which is typical of most Alpine ultramafic rocks ( Leblanc et al., 1980 ). The wide range in Cr#s (29.8–76.9) in the chrome spinels implies a big difference in the amount of melt extraction or degree of melt-rock reaction. The harzburgites and lherzolites with low Cr# of 30–40 correspond to the abyssal peridotite range whereas the harzburgites and dunites with high Cr#s of 66–77 fall in the island-arc peridotite range. The latter is interpreted to form by partial melting of the former during the intra-ocean subduction, which is also suggested by the geochemical characteristics (see below). 5.5 Chromitite The chromitite ores in Kangjinla range from massive to nodular, anti-nodular, layered and disseminated, with varying proportions of chromite. Most of the ore is quite fresh and the chromite compositions are remarkably uniform regardless of their textures. A total of 136 chromite grains were analyzed by electron microprobe in 20 samples and they consist of 54.6–63.1 wt% Cr 2 O 3 , 8.9–12.2% Al 2 O 3 and 11.74–15.6% MgO The grains have very high Cr#s (75.6–82.7) and moderately high Mg#s (56.4–74.1) ( Table 3 ). These values are consistent with those from orebody 31 of the Luobusa district and with those from the Dongqiao ophiolite in central Tibet. All of the minerals in the peridotites and dunites exhibit two stages of growth. Interestingly, the first stage minerals are the least mafic and most evolved; first-stage olivine has relatively low Fo values, orthopyroxene and clinopyroxene have high Al 2 O 3 and Cr 2 O 3 contents, and chrome spinel has low Cr#s, whereas second-stage minerals show the opposite trend. The second-stage minerals are thought to be related to reaction between the peridotites and mafic SSZ melts. 6 Whole-rock chemistry 6.1 Major elements Whole-rock geochemical analyses were performed for 12 samples including four dunites, six harzburgites and two lherzolites ( Table 4 ). Most of the peridotites and dunites in Kangjinla are fresh with LOI typically <3 wt%. One dunite is significantly altered (LOI = 10 wt%) and 3 of the harzburgites have LOI between 4.2 and 7.2 wt% ( Table 4 ). All data were normalized on an anhydrous basis before plotting in variation diagrams. The lherzolites and harzburgites overlap in composition, ranging from about 43 to 47 wt% SiO 2 on the Harker diagrams. Both show relatively linear increases in CaO, Al 2 O 3 , and SiO 2 with decreasing MgO ( Fig. 10 ). Total iron as FeO ∗ and MnO show little change with decreasing MgO whereas NiO decreases slightly ( Fig. 10 ). Three fresh dunite samples have normalized MgO values of approximately 50–53 wt%. Silica, CaO and Al 2 O 3 are essentially constant over this compositional range but both FeO ∗ and MnO increase steeply whereas NiO decreases with decreasing MgO. The one highly altered dunite sample plots close to the harzburgites and lherzolites. All of the samples are significantly depleted relative to the primitive mantle ( Fig. 10 ). These compositional variations of the Kangjinla peridotites are very similar to those for Alpine ophiolites ( Lugovic et al., 1991; Parlak and Delaloye, 1999; Melcher et al., 2002 ). Dupuis et al. (2005) recognized several different types of peridotites in the ophiolites of the Yarlung Zangbo suture zone, from lherzolite and Cpx-bearing harzburgite, through transitional harzburgite to harzburgite and dunite and suggested that these variations reflect different degrees of partial melting. The Luobusa lherzolites and Cpx-bearing harzburgites are similar to the less-depleted varieties described by Dupuis et al. (2005) . 6.2 Rare earth elements The mantle peridotites in Kangjinla have ∑REE of 0.195–0.819 ppm, which is well below primitive mantle values, indicating significant depletion, presumably by high degrees of partial melting. When normalized to primitive mantle values, the rocks display two distinct patterns ( Fig. 11 ). All but one of the dunites has essentially U-shaped patterns with relative enrichment in both LREE and HREE and depletion in MREE ( Fig. 11 A). All of the other samples, including the altered dunite, have ‘spoon-shaped’ patterns with a relative smooth decrease from Lu to Nd followed by a small increase in the LREE ( Fig. 11 B). These patterns suggest some LREE enrichment of previously depleted peridotite, and are consistent with patterns for MORB-type mantle that has been refertilized in a SSZ environment. A similar interpretation of refertilized MOR-type peridotites was made for the Yungbwa ophiolite, located in the western part of the Yarlung Zangbo suture zone. ( Miller et al., 2003; Liu et al., 2010 ). Similar mid-ocean ridge and suprasubduction geochemical signatures for spinel peridotites have also been reported from the Neotethyan ophiolites in SW Turkey ( Aldanmaz et al., 2009 ). 6.3 Trace elements Most of the REE and other trace elements, especially Hf, Ta, Pb, Th, U are very low in the Kangjinla peridotites and dunites, so we have selected only a few of the more abundant elements, such as Cr, V, Ni and Y, to plot against MgO ( Fig. 12 ). Chromium values show two patterns on the Cr vs. MgO plot. The fresh dunites and MgO-rich harzburgites show rapid depletion in Cr with decreasing MgO, whereas the other samples show little change. Ni contents also decrease rapidly in the most MgO-rich samples and then remain relatively constant whereas V shows a regular linear increase with decreasing MgO ( Fig. 12 ). Yittrium also shows a small but discernable increase with decreasing MgO ( Fig. 12 ). The variations in Ni and Cr probably reflect variations in the modal abundances of spinel and olivine in these rocks ( Dick and Bullen, 1984 ). The low and relatively uniform contents of Zr and Y suggest depletion of incompatible elements by partial melting with no replenishment by late-stage fluids or melts ( Paulick et al., 2006 ). The peridotites are strongly depleted in lithophile trace elements, with concentrations well below those of abyssal peridotites ( Fig. 13 ). The peridotites have slightly U-shaped, primitive mantle-normalized trace element patterns, with no significant variation in trends from one rock type to the other ( Fig. 13 ). These patterns reflect a strong depletion of the most incompatible REE, as well as Zr and Hf, relative to the HREE. Zr is slightly more depleted than Hf. There is also selective enrichment in highly incompatible elements (Rb to Ta) relative to the LREE. The peridotites are enriched in Nb and Ta relative to LREE, and strongly depleted in Th. ( Fig. 13 ). Generally, the spider diagrams show that the incompatible elements are depleted in the Kangjinla peridotite and the patterns are similar to those of abyssal peridotites and Oman harzburgites. 7 Discussions 7.1 Origin of the Kangjinla peridotites Peridotite tectonites in ophiolites are generally considered to be residues of the primary mantle after extraction of basaltic magma ( Coleman, 1977; Ringwood, 1981; Moores and Vine, 1971; Moores and Jackson, 1974; Wang and Bao, 1987; Hawkins, 2003 ). Hydrous melting of such depleted mantle in SSZ environments can produced island arc to boninitic magmas depending on the degree of melting and the original composition of the peridotites. In the eastern Mirdita ophiolite of Albania, orthopyroxenite veins are recognized as a reaction product between migrating silica-rich, hydrous melt and the host peridotite in the upper mantle, whereas the harzburgite is likely to be the residual, depleted peridotite of the partial melts that produced the orthopyroxenites ( Dilek and Morishita, 2009 ). The lherzolitic peridotites, which are structurally lower in the mantle sequence, probably represent the residue after MORB extraction ( Dilek and Morishita, 2009 ). Such a process is also indicated in the Kangjinla peridotites by the Cr content of the spinel in these rocks ( Fig. 9 ), which probably indicates variable degrees of partial melting rather than different tectonic settings. In the Cr# vs. Mg# diagram for spinel all of the lherzolites and some of the Cpx-bearing harzburgites plot in the field of abyssal peridotites. This is consistent with the relative abundance of clinopyroxene in these rocks and their relatively high contents of Al 2 O 3 and CaO, indicating low degrees of partial melting in the mantle wedge. A second group of harzburgites, containing little or no clinopyroxene, plots in the island arc basalt field ( Fig. 9 ). As expected, these rocks also show greater depletion in Al 2 O 3 and CaO ( Fig. 10 ), consistent with higher degrees of partial melting. U-shaped, chondrite-normalized REE patterns provide good evidence that the original MORB-type mantle was refertilized in a SSZ environment ( Miller et al., 2003 ). An abundance of dunite provides strong evidence for extensive melt-rock reaction in the peridotites ( Zhou et al., 1996, 2005 ). Thus, our preferred explanation is that the mantle peridotites first formed in a MOR environment, and were then modified by later-stage melts and fluids in a SSZ setting during intra-oceanic subduction ( Malpas et al., 2003 ). 7.2 Tectonic setting of the Luobusa ophiolite Based on Nd and Pb isotope data from gabbro dikes in the Luobusa peridotites, Zhou et al. (2002) suggested that the ophiolite has an Indian Ocean MORB affinity. On a Cr# vs. Mg# diagram spinels from all of the lherzolites and some of the Cpx-bearing harzburgites plot in the field of abyssal peridotites, supporting a MORB affinity for the peridotites ( Zhou et al., 1994, 1996 ). In addition, sparse volcanic rocks in Luobusa have compositions typical of MORB and OIB ( Ye et al., 2006 ). However, other tectonic settings for the ophiolite have also been proposed. For example, a few basalts in the northern part of the ophiolite have been identified as island arc and marginal basin lavas. These rocks are enriched in large ion lithophile elements (LILE), such as Rb, K, Ba, and depleted in high field strength (HFS) elements, particularly in Nb and Ta, features typical of SSZ lavas. Based on this discovery Zhong et al. (2006a) suggested that the basalts and andesitic basalts of the Luobusa ophiolite formed in an island arc setting. Island arc tholeiites and boninites are typical rock types in SSZ tectonic settings, e.g., in the Cretaceous Kizildag ophiolite (Turkey), the Troodos ophiolite (Cyprus) and the Semail ophiolite (Oman) ( Dilek and Thy, 2009 ). The multistage evolution of SSZ magmatism in Tethyan subduction factories has been explored by Dilek et al. (2007), Dilek and Thy (2009) and Caran et al. (2010) , and has been shown to reflect a geochemical progression through time and space, generally beginning with MORB and moving sequentially to IAT and boninitic compositions. This evolution is considered to be the result of variable degrees of melting of a highly heterogeneous and repeatedly depleted mantle source, which had been previously modified by slab- derived fluids and melts ( Dilek et al., 2007 ). Although boninite has not been reported from Luobusa Zhou et al. (1996) speculated that such melts may have passed through the mantle section, creating dunite dikes, pods and envelopes associated with the podiform chromitites. Interestingly, boninitic lavas have been reported from the Daji ophiolite in the middle segment of the Yarlung Zangbo suture ( Chen et al., 2003 ). Geochronologic studies of the Luobusa ophiolite support the interpretation of a two-stage process of formation. The first stage involving Indian Ocean MORB has ages ranging from 177 ± 31 Ma (Sm–Nd age on gabbro; Zhou et al., 2002 ) to 175 ± 20 Ma (Sm–Nd isochron age of basalt from Zedang; Wei et al., 2006a,b ) to 163 ± 3 Ma (SHRIMP U–Pb zircon date from diabase dikes in Luobusa; Zhong et al., 2006b ). The second stage of magmatic activity, interpreted as being subduction related, has U–Pb zircon ages of 126 ± 2 Ma, 123 ± 2 Ma and 125 ± 3 Ma all from ophiolites in the middle segment of the suture zone ( Malpas et al., 2003; Wang et al., 2006; Xia et al., 2008 ). In addition a single U–Pb zircon age of 122 ± 2 Ma has been reported from an ophiolitic diabase in the far western segment of the YZSZ ( Wei et al., 2006a,b ). The island arc tholeiites and boninites in the YZSZ are believed to have formed by partial melting of depleted peridotite that had already experienced previous MORB-type melt extraction during the early stages of ophiolite formation in the Tethyan subduction rollback systems. Rapid slab rollback and associated extension in the arc-forearc region can cause asthenospheric diapirism resulting in shallow partial melting of the highly refractory harzburgites to produce boninitic magmas ( Dilek and Furnes, 2009 ). 7.3 Origin of the chromitites in the Kangjinla peridotites Significant podiform chromitites are known only in ophiolites where they are hosted chiefly in depleted harzburgite. Most ophiolites are considered to have formed, or been modified, in mantle wedges above subducted slabs and the associated chromitites have been interpreted as being co-genetic with the ophiolites. Current models for the genesis of podiform chromitites focus on the generation of island-arc and boninitic magmas by hydrous melting of depleted peridotites in SSZ environments, and the interaction of these melts with the overlying harzburgites ( Zhou and Bai, 1992; Zhou et al., 1994, 1996; Arai and Yurimoto, 1995; Arai, 1997; Ballhaus, 1998; Robinson et al., 1998, 2004; Edwards et al., 2000; Dilek and Morishita, 2009 ). However, the discovery of UHP and highly reduced phases in the chromitites of the Luobusa and other ophiolites raises serious doubts regarding these models. Abundant diamonds and moissanite have been recovered from mineral separates of the chromitites ( Hu, 1999; Bai et al., 2001; Robinson et al., 2004; Xu et al., 2009 ). Diamonds and moissanite also occur in-situ as inclusion in chromite grains ( Yang et al., 2009 ). Coesite rimming a grain of Ti–Fe alloy has also been recovered from the chromitites and its morphology suggests that it is pseudomorphic after stishovite ( Yang et al., 2007 ). Coesite has also been reported as exsolution lamellae in chromite from Luobusa ( Yamamoto et al., 2009 ). Diamonds form at about 3.3 GPa (∼120 km) and coesite forms at about 2.7 GPa (∼90 km). If stishovite was a precursor to the coesite, it would have formed at a pressure of at least 9 GPa (280 km). In addition to the UHP minerals listed above, numerous highly reduced phases have been recovered from the chromitites. These include moissanite (SiC), wüstite (FeO) and many native elements such as Si, Ti, Fe, Al, W, Cr, and Ni, along with a range of base-metal and PGE alloys ( Bai et al., 2000a, 2001, 2004, 2005, 2006; Robinson et al., 2004; Yang et al., 2003, 2007; Xu et al., 2008, 2009; Fang et al., 2009; Shi et al., 2009 ). There is no evidence, either in the ophiolite or surrounding rocks, of a meteorite impact, so we conclude that the UHP minerals must have came from minimum depths of 120–280 km, and possibly deeper. The presence of in-situ diamonds and moissanite in the chromitites indicates that at least some of the chromite crystallized at depth ( Bai et al., 2000a; Yang et al., 2007, 2008 ), an interpretation supported by the presence of coesite and clinopyroxene exsolution lamellae in the chromite grains. Such exsolution lamellae require the solubility of SiO 2 and CaO into the chromite grains, a process that could only happen at great depth ( Yamamoto et al., 2009 ). Yamamoto et al. (2009) suggest that the chromite formed from a precursor phase stable at depths of >380 km, and formation under such conditions would help explain some of the highly reduced phases found in the Luobusa chromitites. Zhao et al. (1997) recognized low-velocity anomalies in the mantle extending to depths of 400 km beneath the Lau back-arc spreading region, an environment conducive to ophiolite formation. They suggested that “geodynamic systems associated with back-arc spreading are related to deep processes” including convection and dehydration in the mantle wedge. Ye et al. (2007) found anomalous 3 He/ 4 He isotopic ratios (up to 32.66Ra) in the Yarlung Zangbo ophiolites that they interpreted as evidence of a deep mantle source. 7.4 Petrogenesis of the Kangjinla ophiolite Numerous diamonds have also been separated from the peridotites of the Kangjinla district ( Xu, 2009; Xu et al., 2009 ), indicating that these rocks also originated from depths of 120 km or more. Based on the abundant evidence of melt-rock reaction in the peridotites, we infer that the Kangjinla ophiolite formed in a tectonic setting, where enriched fluids or melts were derived from deeper and more fertile sources, most likely a subducting slab, and added to the overlying mantle wedge. Thus, we adopt the two-stage formation model of the ophiolite proposed by Malpas et al. (2003) in which late Jurassic Tethyan MORB-type oceanic lithosphere was trapped above an intra-oceanic subduction zone, where it was modified by rising fluids and melts. Melting in the mantle wedge occurred at relatively low pressures within the stability field of spinel and proceeded to high melt fractions in a hydrous environment. At this stage, it is uncertain how the diamond-bearing peridotites and chromitites were transported to shallow levels and emplaced in the upper mantle. It is possible that the surface outcrops are underlain by other chromitite bodies deeper in the mantle. The diamonds in the chromitites were preserved as inclusions within chromite grains but it is not yet clear how the diamonds are hosted in the peridotites, or how they were preserved in these rocks. 8 Conclusions The mantle peridotites hosting the Kangjinla chromitites are chiefly harzburgite with minor lherzolite and dunite. The dunite occurs as veins and pods in the harzburgite or as envelopes around the chromitites. The main rock-forming minerals in the peridotites and dunites are olivine, orthopyroxene and clinopyroxene with small amounts of Cr spinel. All of these minerals show at least two stages of formation; an early stage of relatively large, commonly deformed, crystals and a second stage of smaller, undeformed but recrystallized grains. The compositions of the minerals show some variations between the stages. Olivine in the chromitites is very Mg-rich (Fo = 96–97), significantly higher than that in the host peridotites (Fo = 90–92) and dunites (Fo = 92–94), presumably reflecting subsolidus re-equilibration between chromite and olivine. Chrome spinels in the mantle peridotites have Cr#s ranging from 29.8 to 76.9 and Mg#s ranging from 39.4 to 70, and plot in the abyssal and island-arc fields in the Cr# vs. Mg# diagram ( Dick and Bullen, 1984 ), probably reflecting different degrees of partial melting rather than formation in different tectonic environments. Chrome spinels in the chromitites have Cr#s between 75.6 and 82.7, and plot in the boninite field. Both the podiform chromitites and the host peridotites contain abundant UHP minerals ( Xu, 2009; Xu et al., 2009 ), indicating crystallization at depths >120 km. 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