We present new whole-rock major, trace, and platinum group element (PGE) and mineral chemistry data from the Kalaymyo peridotite massif in the central part of the Indo-Myanmar Ranges (western Myanmar) and discuss its mantle melt evolution. The Kalaymyo peridotites consist mainly of harzburgites, which show typical porphyroclastic or coarse-grained equigranular textures. They are composed of olivine (forsterite, Fo = 89.8–90.5), orthopyroxene (enstatite, En86–91, wollastonite, Wo1–4, ferrosilite, Fs8–10; Mg# = 89.6–91.9), clinopyroxene (En46–49Wo47–50Fs3–5; Mg# = 90.9–93.6), and spinel (Mg# = 67.1–78.9; Cr# = 13.5–31.5), and have relatively homogeneous whole-rock compositions with Mg#s of 90.1–90.8 and SiO2 (41.5–43.65 wt%), Al2O3 (1.66–2.66 wt%), and CaO (1.45–2.67 wt%) contents. They display light rare earth element (LREE)–depleted chondrite-normalized (CN) REE patterns with (La/Yb)CN = 0.04–0.21 and (Gd/Yb)CN = 0.40–0.84, and show a slight enrichment from Pr to La with (La/Pr)CN in the range of 0.98–2.36. The Kalaymyo peridotites are characterized by Pd-enriched chondrite-normalized PGE patterns with superchondritic (Pd/Ir)CN ratios (1.15–2.36). Their calculated oxygen fugacities range between the quartz-fayalite-magnetite (QFM) oxygen buffers, QFM–0.57 and QFM+0.90. These mineralogical and geochemical features collectively suggest that the Kalaymyo peridotites represent residual upper mantle rocks after low to moderate degrees (5%–15%) of partial melting at a mid-oceanic ridge environment. The observed enrichment in LREE and Pd was a result of their reactions with enriched mid-oceanic ridge basalt–like melts percolating through these already depleted residual peridotites. The Kalaymyo and other ophiolites in the Indo-Myanmar Ranges therefore represent mid-oceanic ridge–type Tethyan oceanic lithosphere derived from a downgoing plate and accreted into a westward-migrating subduction-accretion system along the eastern margin of India.
Mesozoic ophiolites crop out discontinuously in the Indo-Myanmar Ranges in northeast India and Myanmar, and represent the remnants of the Neo-Tethyan oceanic lithosphere (Sengupta et al., 1990; Mitchell, 1993). These ophiolites are the southern continuation of the Neo-Tethyan ophiolites occurring along the Yarlung Zangbo suture zone (YZSZ) in southern Tibet farther northwest (Mitchell, 1993; Fareeduddin and Dilek, 2015), as indicated by their coeval crystallization ages and geochemical compositions (Yang et al., 2012; Liu et al., 2016).
The ophiolites within the YZSZ have been extensively studied for their formation ages, crustal rock geochemistry, and upper mantle peridotite compositions. Recent geochronological data reveal that the ophiolites in the YZSZ formed between 130 and 120 Ma in the Early Cretaceous (e.g., Xia et al., 2008; Z. Liu et al., 2011; F. Liu et al., 2015a, 2015c; Xiong et al., 2011; Zhang et al., 2016). Recent systematic petrological and geochemical studies on the ophiolitic crustal rocks and upper mantle peridotites suggest a two-stage tectonomagmatic evolution process for these ophiolites: early mid-oceanic ridge igneous construction, followed by subduction-influenced magmatism in the upper plate of an intraoceanic subduction zone (e.g., Hébert et al., 2003; Dubois-Côté et al., 2005; Dupuis et al., 2005; Zhou et al., 2005; Dilek et al., 2007, 2008; C. Liu et al., 2010; Bezard et al., 2011; Dai et al., 2011; Xu et al., 2011; Yang et al., 2011; F. Liu et al., 2015b; Lian et al., 2016; Feng et al., 2017).
Our knowledge of the ophiolites along the Indo-Myanmar Ranges is poor by comparison. Some have interpreted them as segments of an island-arc system that were emplaced as nappes during the India-Burma collision (Sengupta et al., 1990; Mitchell, 1993, and references therein). The Manipur ophiolites, located in the northern part of the Indo-Myanmar Ranges (northeast India), have been studied in detail in recent years. However, their origin and tectonic setting are still debated. The existing tectonic models include (1) an island-arc origin (Bhattacharjee, 1991); (2) a mid-oceanic ridge origin (Singh, 2013; Khogenkumar et al., 2016); and (3) a mid-oceanic ridge–generated oceanic lithosphere, that underwent high-pressure metamorphism and deformation in a subduction channel and then was incorporated into an accretionary prism in the upper plate (Fareeduddin and Dilek, 2015). Most of the previous studies in the Indo-Myanmar ophiolites and these existing tectonic models for their evolution are almost entirely based on the geochemistry and geochronology of crustal rocks. The peridotite suites in these ophiolites have been largely untouched, although recent systematic geochemical, petrological, and structural studies of upper mantle peridotites in many ophiolites around the world have provided significant clues for their melt evolution and tectonic setting of their igneous construction (e.g., Pearce et al., 2000; Zhou et al., 2005; Arai et al., 2007; Dilek et al., 2007, 2008; Dilek and Furnes, 2009; Dilek and Thy, 2009; Liu et al., 2010; Dai et al., 2011; Uysal et al., 2012; Dokuz et al., 2015; Saccani et al., 2017; Wu et al., 2017).
In this paper we document the mineralogy and the major, trace, and platinum group element geochemistry of upper mantle peridotites in the Cretaceous Kalaymyo ophiolite in the central part of the Indo-Myanmar Ranges (Myanmar) and discuss their mantle melt evolution and origin. Our results indicate that the Kalaymyo and other peridotite massifs that are discontinuously exposed along the eastern periphery of the Indian subcontinent are mid-oceanic ridge type with no subduction zone influence in their magmatic evolution. They are therefore significantly different, in terms of their tectonic evolution within the same Neo-Tethyan realm, from their contemporaneous counterparts along the YZSZ to the northwest.
REGIONAL GEOLOGY AND STRUCTURE OF THE KALAYMYO OPHIOLITE
Myanmar is divided into three north-south–trending tectonic belts that include, from east to west, the Shan Plateau, Shan scarp, and western Myanmar (Fig. 1). Western Myanmar is part of the Burma plate, which is bound on the east by the Sagaing dextral strike-slip fault, separating it from the Shan scarp and the Shan Plateau on the Indochina block (Fig. 1; Mitchell, 1993, 2007). The ophiolite occurrences in Myanmar have been traditionally grouped into the Eastern belt (Tagaung-Myitkyina belt) and the Western belt (Kabaw-Kalaymyo belt) ophiolites. The Eastern belt is northwest of the Shan Plateau. The north-south–trending Western belt is west of western Myanmar and along the eastern Indo-Myanmar Ranges (Sengupta et al., 1990).
The Western belt ophiolites consist of highly disrupted mafic-ultramafic rock units and mélanges, and include, from south to north, the Chin Hills, Kalaymyo, and Manipur-Nagaland ophiolites (Fig. 1). These ophiolites are mainly composed of serpentinized peridotites, amphibolites, gabbros, dolerites, and pillow basalts, tectonically overlying mica schists, gneisses, and pegmatites (Mitchell, 1993; Singh, 2013; Fareeduddin and Dilek, 2015).
The Kalaymyo ophiolite is located in the central part of the Western belt (Fig. 1), and tectonically overlies the Triassic Pane Chaung turbiditic sandstones (Fig. 2). It comprises serpentinized peridotites and ultramafic and mafic rocks spatially associated with oceanic marine sedimentary-volcanic sequences of siliceous limestone, radiolarian chert, shale, ocean island basalt (OIB)–like basalt, and tuffaceous rocks (Fig. 2). The Kalaymyo peridotites are composed largely of harzburgites. Locally, rodingites occur as irregular pods within the serpentinized peridotites. U-Pb zircon dating of rodingitic gabbros has revealed a crystallization age of 127 Ma (Liu et al., 2016), indicating that the Kalaymyo ophiolite is coeval in age with the YZSZ ophiolites in southern Tibet. These new age data clearly show that the Western belt ophiolites are younger than those in the Eastern belt (e.g., Myitkyina ophiolite; Fig. 1) that have U-Pb zircon ages of 176–166 Ma (Yang et al., 2012) and 173–171 Ma (Liu et al., 2016). The Eastern belt ophiolites in Myanmar thus appear to be contemporaneous with the Bangong-Nujiang suture zone (BNSZ) ophiolites in south-central Tibet (Fig. 1).
The Kalaymyo peridotites are mainly harzburgites. They are composed of olivine (55–80 vol%), orthopyroxene (20–40 vol%), clinopyroxene (1–8 vol%), and spinel (1–6 vol%). Most of our peridotite samples show porphyroclastic textures, characterized by millimeter-sized porphyroclasts of orthopyroxene and small granoblastic grains of olivine (Figs. 3A, 3C, 3D). Less commonly they show weakly deformed, coarsely equigranular textures (Fig. 3B). In highly deformed peridotites, olivine is generally recrystallized to relatively fine grained aggregates occurring as small neoblasts (Fig. 3E). Orthopyroxene shows a much smaller degree of recrystallization, forming large porphyroclasts, and exhibits embayments generally filled with olivine neoblasts (Figs. 3E, 3F). Orthopyroxene porphyroclasts also contain clinopyroxene exsolution lamellae. Both the orthopyroxene and olivine grains display internal deformation. Orthopyroxene shows undulatory extinction or distorted lamellae and elongated shape fabrics (Fig. 3E), whereas the olivine displays undulatory extinction or kink bands (Figs. 3D, 3F). Clinopyroxene occurs as porphyroclasts or irregular interstitial grains, or locally as small lamellae exsolved from orthopyroxene. Spinel grains are brown to reddish-brown, and occur as euhedral or vermicular crystals (Figs. 3G, 3H). These spinels locally contain small olivine inclusions.
Major elements of olivine, orthopyroxene, clinopyroxene, and spinel were measured on JXA-8100 electron microprobe at the Key Laboratory of Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, using an accelerating voltage of 15 kV, beam current of 10 nA, and spot diameter of 1 mm. The PRZ method was used for correction, and standard samples used in analysis are the 53 kinds of minerals produced by Structure Probe Inc. (http://www.2spi.com). Precision is better than 1%.
Whole-Rock Major and Trace Elements
Whole-rock major and trace element measurements were done at the National Research Center of Geoanalysis (Beijing, China). Major elements were determined by X-ray fluorescence spectrometry using fused glass disks with analytical uncertainty <1%. Trace elements were measured using inductively coupled plasma–mass spectrometry (ICP-MS). Analytical uncertainties are 10% for elements with abundances <10 ppm and ∼5% for those >10 ppm.
Whole-Rock Platinum Group Elements
Whole-rock platinum group element (PGE) analyses were carried out at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (Guangzhou, China), using the nickel sulfide fire assay and Te-coprecipitation method of Jackson et al. (1990) and Sun et al. (1993). The PGE concentrations were measured by ICP-MS. The detailed chemical separation, purification procedures, and test accuracy are the same as described by Sun and Sun (2005) and Dai et al. (2011).
Olivine grains in the Kalaymyo peridotites are uniform in composition. Their Fo values [100 × molar Mg/(Mg + Fe2+)] range from 89.8 to 90.5 with FeO contents of 9.09–9.80% and MgO contents of 48.00–48.62%. Their NiO and MnO values are in the range of 0.28–0.38% and 0.13–0.15%, respectively.
Spinels have Mg# values [100 × molar Mg/(Mg + Fe2+)] from 67.1 to 78.9 and Cr# values [100 × molar Cr/(Cr + Al)] from 13.5 to 31.5. Their major oxide compositions are represented by MgO = 15.91–19.89%, FeO = 11.87–18.32%, Al2O3 = 39.23–54.26%, and Cr2O3 = 12.67–26.89%. The NiO and TiO2 contents range from 0.17% to 0.31% and from 0.01% to 0.23%, respectively. Variations in spinel Cr# and Mg#s are interpreted to reflect different degrees of partial melting and melt-rock reaction processes in the host peridotites (Arai, 1994; Dick and Bullen, 1984; Hirose and Kawamoto, 1995). The Kalaymyo spinel compositions suggest low to moderate degrees of partial melting of the host peridotites, and plot in the compositional fields for abyssal peridotites (Figs. 4A, 4B; Dick and Bullen, 1984).
Orthopyroxenes are in the range of En86–91Wo1–4Fs8–10. They have Mg# values [100 × molar Mg/(Mg + Fe2+)] between 89.6 and 91.9, with MgO = 30.84–33.64%, and FeO = 6.01–6.55%. Their Al2O3 and Cr2O3 contents range from 3.14% to 4.94% and from 0.41% to 0.67%, respectively. The Kalaymyo orthopyroxene compositions are similar to those of orthopyroxenes in abyssal peridotites (Figs. 4C, 4D).
Clinopyroxenes (including those clinopyroxene exsolution lamellae in the orthopyroxenes) are diopside with a composition of En46–49Wo47–50Fs3–5. They show relatively low Mg# values [100 × molar Mg/(Mg + Fe2+)] ranging from 90.9 to 93.6, and have the following major oxide compositions: MgO = 14.78–16.46%, FeO = 2.51–3.20%, high Al2O3 contents ranging from 3.29% to 6.49%, and moderate Cr2O3 contents ranging from 0.87% to 1.09%. These geochemical features of the Kalaymyo clinopyroxenes are also similar to the compositions of clinopyroxene in the abyssal peridotites (Figs. 4E, 4F). Their Na2O and TiO2 contents are in the range of 0.22–0.56% and 0.22–0.43%, respectively.
WHOLE-ROCK MAJOR AND TRACE ELEMENT GEOCHEMISTRY
We present the major and trace element contents of the Kalaymyo peridotites in Table 5. The Kalaymyo peridotites are moderately serpentinized, showing small to intermediate loss on ignition values of 1.26–6.88 wt%. They have relatively homogeneous whole-rock compositions with Mg# values [Mg/(Mg + Fetotal) varying from 90.1 to 90.8 and MgO = 38.88–41.54 wt%, Fe2O3 = 1.14–2.38 wt%, and FeO = 5.44–7.02 wt%. Compared with the compositions of forearc peridotites, they have relatively high contents of Al2O3 (1.66–2.66 wt%, with an average of 2.15 wt%) and CaO (1.45–2.67 wt%, with an average of 2.08 wt%), and they plot in the composition field of abyssal peridotites (Fig. 5). Their SiO2, TiO2 and Na2O contents are 41.5–43.65 wt%, 0.04–0.07 wt%, and 0.11–0.26 wt%, respectively.
Chondrite-normalized rare earth element (REE) patterns and primitive mantle–normalized multi-element diagrams of the Kalaymyo peridotites are shown in Figure 6. We compare the patterns of the Kalaymyo peridotites in these diagrams with those of Izu-Bonin-Mariana forearc peridotites (IBM; Parkinson and Pearce, 1998), depleted mid-oceanic ridge basalt (MORB) mantle (DMM; Workman and Hart, 2005), and OIB, enriched-MORB, and normal-MORB types (Sun and McDonough, 1989).
The Kalaymyo peridotites exhibit light (L) REE–depleted patterns with (La/Yb)CN = 0.04–0.21, (La/Gd)CN = 0.06–0.32, and (Gd/Yb)CN = 0.40–0.84 (Fig. 6A; CN is chondrite normalized; normalizing values are from Boynton, 1984). There is a slight enrichment from Pr to La with (La/Pr)CN mainly in the range of 0.98–2.36. The Kalaymyo peridotites have total REE concentrations ranging from 0.91 to 1.62 ppm (LREE = 0.19–0.41 ppm, heavy [H] REE = 1.28–2.36 ppm, HREE/LREE = 3.76–9.09; Table 5), which are lower than those of DMM (Workman and Hart, 2005) and chondrite (Boynton, 1984), but are higher than those of the IBM forearc peridotites (Parkinson and Pearce, 1998).
Similarly, the Kalaymyo peridotites show depleted patterns in the primitive mantle–normalized multielement diagrams (Fig. 6B). Their element concentrations are mostly below those of DMM (Workman and Hart, 2005), and are also higher than those of IBM forearc peridotites (Parkinson and Pearce, 1998). However, compared with the moderately incompatible trace elements, which display a continuous pattern of depletion from Lu to Pr, the Kalaymyo peridotites show relative enrichment in the most incompatible trace elements (Ba, Th, U, Nb, Ta, and Pb), with peaks of U, Ta, and Pb. The Kalaymyo peridotites are also characterized by high contents of compatible trace elements (Cr = 1203–2145 ppm, Co = 96.9–111 ppm, Ni = 1822–2054 ppm, and V = 47.6–70.6 ppm; Table 5). In MgO versus trace element variation diagrams, the Kalaymyo peridotites also plot in the compositional field of abyssal peridotites (Fig. 5).
The whole-rock PGE concentrations of the Kalaymyo peridotites are listed in Table 6. The peridotites have variable total PGE contents ranging from 10.98 to 32.95 ppb, with an average value of 22.02 ppb, which is compatible with the primitive mantle values (23.5 ppb; McDonough and Sun, 1995). Their Os contents are between 1.58 and 4.88 ppb (3.04 ppb on average), Ir contents are between 1.46 and 4.33 ppb (2.98 ppb on average), Ru contents are between 2.54 and 7.22 ppb (4.98 ppb on average), Rh contents are between 0.46 and 1.39 ppb (0.95 ppb on average), Pt contents are between 2.71 and 8.7 ppb (5.54 ppb on average), and Pd contents are between 2.23 and 7.34 ppb (4.52 ppb on average). The average iridium (I) PGE and palladium (P) PGE abundances and IPGE/PPGE ratios of the peridotites are 11.01 ppb, 11.02 ppb, and 1.01, respectively. These PGE features of the Kalaymyo peridotites are also comparable with those of the primitive mantle (11.6 ppb, 11.9 ppb, and 0.97, respectively; McDonough and Sun, 1995).
We present the chondrite-normalized and primitive mantle–normalized PGE patterns for the Kalaymyo peridotites in Figure 7. Our samples display superchondritic (Pd/Pt)CN and (Pd/Ir)CN ratios (1.23–1.85 and 1.15–2.36, respectively), and show a regular enrichment from Rh to Pt to Pd in Figure 7A, and from Pt to Pd in Figure 7B. From Os to Rh, their patterns are relatively flat, similar to those of the primitive mantle. The overall PGE patterns of the Kalaymyo peridotites are also comparable to those of sulfide-rich abyssal harzburgites (Luguet et al., 2003).
Residual Origin and Partial Melting Degrees of the Kalaymyo Peridotites
Our mineral chemistry and whole-rock geochemistry results demonstrate that the Kalaymyo peridotites represent residual upper mantle rocks after variable degrees of basaltic magma extraction. This is the common occurrence of ultramafic rocks in most ophiolites (e.g., Dupuis et al., 2005; Uysal et al., 2012; Dokuz et al., 2015).
The well-developed porphyroclastic textures with millimeter-sized porphyroclasts of orthopyroxene and millimeter-sized granoblastic grains of olivine (Fig. 3) in the Kalaymyo peridotites represent the typical high-temperature deformation fabrics of upper mantle rocks beneath an oceanic spreading center (Vernon, 2004; Dupuis et al., 2005). In addition, the olivine and orthopyroxene compositions of our peridotite samples span a narrow range, and their Mg# values are similar (89.8–90.5 and 89.6–91.9, respectively), characteristic of residual mantle peridotites that underwent partial melting (Dick, 1977; Komor et al., 1990). Clinopyroxene is generally considered to be the most rapidly consumed mineral during the process of partial melting in spinel peridotites. The Kalaymyo harzburgites have 1–8 vol% of clinopyroxene. However, compared with the clinopyroxene compositions of igneous ultramafic rocks (e.g., clinopyroxenes in the Fanshan ultramafic rocks with MgO = 9.43–14.26%, FeO = 6.08–13.3%, TiO2 = 0.14–2.52%, and Na2O = 0.23–1.25% contents; Niu et al., 2012), the Kalaymyo clinopyroxenes have higher MgO (14.78–16.46%), but lower FeO (2.51–3.20%), TiO2 (0.22–0.43%), and Na2O (0.22–0.56%) contents, supporting our interpretation that they are residual minerals. Accordingly, the Kalaymyo peridotites have depleted whole-rock composition with higher MgO (38.88–41.54 wt%), and lower SiO2 (41.5–43.65 wt%), FeO (5.44–7.02 wt%), Al2O3 (1.66–2.66 wt%), CaO (1.45–2.67 wt%), and Na2O (0.11–0.26 wt%) contents than those of the primitive mantle (MgO = 37.8 wt%, SiO2 = 45.0 wt%, FeO = 8.05 wt%, Al2O3 = 4.45 wt%, CaO = 3.55 wt%, Na2O = 0.36 wt%; McDonough and Sun, 1995). The moderately incompatible trace element abundances below the depleted MORB mantle and primitive mantle values (Fig. 6), combined with the high contents of compatible trace elements, also suggest that the Kalaymyo peridotites are residual upper mantle rocks.
The spinel and olivine compositions are useful in estimating the degree of partial melting undergone by their host peridotites (Arai, 1994; Hirose and Kawamoto, 1995). The plots of spinel Cr# versus olivine Fo and spinel Cr# versus spinel Mg# suggest low to moderate degrees of partial melting of our samples (<10%–15%; Figs. 4A, 4B). We can further constrain the degrees of partial melting by comparing the HREE contents of the samples with the modeled HREE contents of the depleted MORB mantle at different degrees of partial melting, because HREE are less mobile and are considered to be only slightly affected during metasomatic processes (Hellebrand et al., 2001). We infer that ∼5%–15% partial melting of a depleted MORB mantle source could have produced residual peridotites with HREE contents similar to those of the Kalaymyo peridotites (Fig. 6A).
Equilibrium Temperatures and Oxygen Fugacities
We have calculated the equilibrium temperatures of the Kalaymyo peridotites using the two-pyroxene geothermometers of Wood and Banno (1973), Wells (1977), and Brey and Köhler (1990). The oxygen fugacity of the Kalaymyo peridotites was also determined using the oxygen geobarometer introduced by Ballhaus et al. (1991). We have taken a pressure of 1 GPa throughout these calculations (Dare et al., 2009), and have adopted T3 (equilibrium temperatures using the geothermometer of Brey and Köhler, 1990) in oxygen fugacity calculations. The results are displayed in Table 7, and the oxygen fugacities are given as deviations [ΔlogfO2(QFM)] from the QFM (quartz-fayalite-magnetite) oxygen buffer in log units.
The equilibrium temperatures calculated for the Kalaymyo peridotites are between 814 °C and 1662 °C based on the two-pyroxene geothermometers, and in the range of 945 °C to 1229 °C based on the orthopyroxene geothermometer (Table 7). The variations between these two sets of equilibrium temperatures obtained from the same samples are anticipated because of the different geothermometers we utilized. The calculated temperatures show no regular correlation with the Cr# of spinels. Calculated oxygen fugacities for the Kalaymyo peridotites are between QFM–0.57 and QFM+0.90, and are positively correlated with the Cr# of spinels (Fig. 8A).
Studies of the oxidation state of typical upper mantle rocks and mantle-derived primitive melts have revealed the following trends (Ballhaus et al., 1991; Parkinson and Arculus, 1999). Undepleted, fertile mantle rocks are moderately reduced (∼QFM–2). Depleted MORB and abyssal peridotites plot at ∼QFM–1. Enriched MORB and metasomatized harzburgites are more oxidized between QFM and QFM+1. Mantle wedge peridotites above subduction zones are the most oxidized (∼QFM+2). In the fO2-Cr# discrimination diagram (Fig. 8A), all of the Kalaymyo peridotite samples plot within the field of mid-oceanic ridge harzburgites, with the exception of two samples plotting in the field of suprasubduction zone peridotites. A V-Yb plot can also be used to estimate oxygen fugacities and hence to discriminate between a ridge and subarc origin of harzburgites, due to the different mineral-melt partition coefficients for V under different oxygen fugacity conditions (Pearce et al., 2000). The Kalaymyo peridotites plot close to the QFM melting curve (Fig. 8B), consistent with the calculated oxygen fugacities based on mineral compositions (Table 7). The nature and the degree of partial melting of the Kalaymyo peridotites have also been quantified using the V-Yb plot (Parkinson and Pearce, 1998); our peridotite samples are on the curve at a value of ∼7%–12%, consistent with an origin of low to moderate degrees of partial melting of a depleted MORB mantle source.
The PGEs are classified into two groups based on their different melting temperatures: the IPGE (Os, Ir, and Ru, with melting temperatures higher than 2000 °C) and the PPGE (Rh, Pt, and Pd, with melting temperatures lower than 2000 °C) (Woodland et al., 2002). In upper mantle peridotites, IPGEs generally occur in the lattice of sulfide phases that are enclosed in silicate minerals, whereas the PPGE mainly occur in interstitial sulfides (Alard et al., 2000). Thus, the IPGE are more compatible during partial melting processes, and their abundances should increase in residual upper mantle rocks (e.g., Rehkämper et al., 1999; Becker et al., 2006). The PPGEs are comparatively less compatible, and the residual peridotites should have subchondritic Pd/Ir ratios (Barnes et al., 1985). However, many upper mantle rocks display superchondritic Pd/Ir ratios, and refertilization processes leading to the precipitation of metasomatic sulfides may significantly enhance Pd concentrations of peridotites while marginally affecting Pt and Rh (Luguet et al., 2003; Lorand et al., 2003). These observations and inferences are consistent with the interpretation that silicate-enclosed sulfides are the residues of melting processes, whereas interstitial sulfides are the crystallization products of sulfide-bearing (metasomatic) fluids (Alard et al., 2000).
The Kalaymyo peridotites have IPGE patterns similar to those of the primitive mantle (Fig. 7); however, they display different PPGE patterns and are characterized by Pt depletion relative to Rh and Pd (Fig. 7B). Accordingly, they show high-Pd mantle signatures with superchondritic Pd/Ir ratios (1.16–2.38) and (Pd/Ir)CN ratios (1.15–2.36) (Table 6). Partial melting processes can cause lower Pd/Ir ratios, which would generally be below, rather than above, the chondritic value (e.g., Lorand et al., 2003). Therefore, the PPGE patterns, as well as the high Pd/Ir ratios of the Kalaymyo peridotites, suggest that these upper mantle rocks might have been affected by melt-rock interactions. This interpretation is also supported by the similarity of the PGE patterns (especially the PPGE patterns) of the Kalaymyo peridotites to those of (1) the interstitial sulfides of metasomatic origin (Alard et al., 2000), (2) the sulfide-rich abyssal harzburgites modified by shallow-level refertilization processes (Luguet et al., 2003), and (3) the abyssal peridotites modified by suprasubduction-related melts (Dai et al., 2011). The IPGE concentrations of the Kalaymyo peridotites are lower than those of the Zhongba harzburgites (Fig. 7). This phenomenon may be explained by the relatively lower degrees of partial melting (5%–15%) of the Kalaymyo peridotites in comparison to the Zhongba harzburgites (13%–24%; Dai et al., 2011).
Melt-Rock Interactions in Kalaymyo Upper Mantle Evolution
Mantle peridotites in ophiolites represent depleted, refractory oceanic lithosphere mantle melt residues, and are formed by different degrees of partial melting. However, during their evolution and exhumation processes, most ophiolitic peridotites appear to have undergone melt-rock interactions that significantly modified their textures and chemical compositions (e.g., Piccardo et al., 2007; Dilek and Furnes, 2009, 2014; Morishita et al., 2011; Uysal et al., 2012; Dokuz et al., 2015). We infer that melt-rock interactions might have also played an important role in the formation of the Kalaymyo peridotites, as evidenced by their textural and geochemical features.
The microtextural features that we have observed in the Kalaymyo peridotites in support of melt-rock interactions include (1) the existence of olivine neoblasts, which recrystallized at high temperatures, surrounding and replacing large, corroded porphyroclasts of orthopyroxene (Fig. 3E); (2) olivine occurring as inclusions in the orthopyroxene porphyroclasts and spinels (Figs. 3E–3G); and (3) the presence of vermicular spinels (Fig. 3H) (e.g., Piccardo et al., 2007; Dilek and Morishita, 2009).
In addition, the LREE contents of the Kalaymyo peridotites are higher than the calculated LREE contents (Fig. 6A), and the high field strength elements (HFSEs; e.g., Th, U, Nb, Ta) show relative enrichment in the primitive mantle–normalized multielement diagrams (Fig. 6B). These geochemical features indicate that the Kalaymyo peridotites underwent melt-rock interactions after partial melting. This inference is also consistent with their relatively high oxygen fugacities (QFM–0.57 to QFM+0.90) and Pd-enriched PGE patterns (Fig. 7).
Spinel compositions of the Kalaymyo peridotites indicate that the melts that reacted with the Kalaymyo peridotites might have been island-arc tholeiitic (IAT) or MORB-like in their geochemical affinities (Fig. 9). However, an IAT affinity is less likely, considering that IATs and mantle wedge peridotites above subduction zones commonly display much higher oxygen fugacities (∼QFM+2; Ballhaus et al., 1991) than those we have obtained from the Kalaymyo peridotites. In addition, the Kalaymyo peridotites have whole-rock and mineral chemistries consistent with those of abyssal peridotites (Figs. 4 and 5). These findings and comparisons allow us to rule out a suprasubduction origin of the Kalaymyo peridotites. Depleted MORB magmas can also be precluded, due to their LREE-depleted chondrite-normalized REE patterns (Fig. 6). Enriched MORBs have oxygen fugacities (∼QFM+1; Ballhaus et al., 1991) similar to those of the Kalaymyo peridotites. They also have LREE-enriched chondrite-normalized REE patterns (Fig. 6; Sun and McDonough, 1989), suggesting that LREE-enriched MORB melts might have percolated through the Kalaymyo peridotites, resulting in the LREE, HFSE, and PPGE additions. Figure 9 shows that the Kalaymyo peridotites underwent low degrees (10%–15%) of partial melting, in contrast to the Dongbo and Zhongba peridotites located at the western part of the YZSZ in southern Tibet. This comparison is important in that the Dongbo and Zhongba peridotites were produced by interactions between typical boninitic melts and an upper mantle, which had undergone significant (>20%) degrees of prior partial melting (Dai et al., 2011; Niu et al., 2015).
Tectonic Setting of the Formation of the Kalaymyo Peridotites
Peridotites originating in different tectonic settings display distinct mineralogy, textures, and geochemistry, due to the varying degrees of partial melting and melt-rock interaction processes under variable physical-chemical conditions (Dilek and Newcomb, 2003). Upper mantle peridotites exposed in ophiolites commonly have their origin in a mid-oceanic ridge environment (abyssal peridotites; e.g., Snow and Reisberg, 1995) or in suprasubduction settings, particularly in a forearc or a backarc setting (SSZ peridotites; e.g., Parkinson and Pearce, 1998; Dubois-Côté et al., 2005; Dilek and Thy, 2009; Morishita et al., 2011). However, studies of some ophiolites have shown that upper mantle peridotites occurring in those ophiolites might have undergone multiple stages of partial melting episodes (e.g., Guilmette et al., 2009; Uysal et al., 2012, 2015). In these cases, the ophiolitic peridotites first underwent a melt extraction event beneath a mid-oceanic ridge axis and then were involved in a second phase of a partial melting event in a forearc or backarc setting, following their emplacement in the upper plate of an intraoceanic suprasubduction zone (Dilek and Furnes, 2014). Such peridotites commonly show the geochemical fingerprints of subduction influence as a result of their interactions with subduction-derived fluids and melts.
The Kalaymyo peridotites have the mineral (olivine, spinel, orthopyroxene, and clinopyroxene) (Fig. 4) and whole-rock chemical compositions resembling those of abyssal peridotites. On whole-rock MgO versus Al2O3, TFe2O3, CaO, Sc, V, Ga, Ce, Yb, and Y diagrams, the Kalaymyo peridotites all plot within the abyssal peridotite field (Fig. 5). On chondrite-normalized and primitive mantle–normalized PGE diagrams, the PGE patterns of the Kalaymyo peridotites are similar to those of the sulfide-rich abyssal harzburgites (Luguet et al., 2003). In addition, the calculated oxygen fugacities for the Kalaymyo peridotites (QFM–0.57 to QFM + 0.90) are comparable to those of the abyssal harzburgites (between QFM–1 and QFM+1; Ballhaus et al., 1991). Therefore, we infer that the Kalaymyo peridotites represent the residues of upper mantle rocks after variable degrees of basaltic magma extraction at a mid-oceanic ridge environment.
The peridotites in the Manipur ophiolite occurring in the northern part of the Indo-Myanmar Ranges in northeast India also show abyssal peridotite mineralogy and geochemical affinities; they represent residual mantle rocks after low degrees of partial melting (2%–12%) in a mid-ocean-ridge setting (Singh, 2013), similar to the Kalaymyo peridotites. Thus, the upper mantle rocks of the Cretaceous ophiolites in the Indo-Myanmar Ranges are all mid-oceanic ridge related without any detectable subduction influence in their melt evolution. These peridotites and their crustal counterparts were accreted to the Indo-Myanmar Ranges from the downgoing Tethyan plate as part of subduction-accretion processes (Fareeduddin and Dilek, 2015). They are tectonically interleaved with and overlie metamorphosed accretionary prism rock assemblages, which show west-vergent imbricate thrust tectonics, synthetic to the modern subduction zone dipping eastward beneath the Indo-Myanmar Ranges (Fig. 1). The Tethyan ophiolites and the Indo-Myanmar Ranges largely represent, therefore, a westward-migrating subduction-accretion system, rather than a collisional suture zone, as in the YZSZ to the northwest in southern Tibet (Fig. 1). The coeval Tethyan ophiolites in southern Tibet are tectonically along south-vergent thrust faults on an ophiolitic mélange and the rifted passive margin sequences of the Tethyan Himalaya (Liu et al., 2015a; Xu et al., 2015). Thus, these ophiolites along the YZSZ are tectonically underlain by the continental crust of India, and they separate the downgoing Indian plate from Eurasia in the upper plate. They were derived from the upper plate of a subduction zone, and underwent a second-stage melt evolution in the presence of subduction-derived fluids and melts (Dai et al., 2011; Liu et al., 2012; Niu et al., 2015), reminiscent of the many other Tethyan ophiolites in Iran, Turkey, Cyprus, Greece, and Albania farther west (e.g., Dilek and Flower, 2003; Dilek et al., 2007, 2008; Dilek and Furnes, 2009; Uysal et al., 2012, 2015; Dokuz et al., 2015). The coeval peri-Indian ophiolites therefore appear to have different tectonomagmatic evolutionary histories (Fareeduddin and Dilek, 2015).
1. The harzburgites in the Kalaymyo ophiolite of the Indo-Myanmar Ranges represent residual upper mantle peridotites after low to moderate degrees (5%–15%) of partial melting at a mid-oceanic ridge setting.
2. They subsequently underwent melt-rock interactions, resulting in enrichment in LREEs and HFSEs and superchondritic (Pd/Ir)CN ratios.
3. Melts that reacted with the peridotites had an enriched MORB-like geochemical character, as suggested by the moderate oxygen fugacities recorded by these peridotites.
4. The Kalaymyo and other Cretaceous ophiolites exposed in the Indo-Myanmar Ranges have a tectonic evolutionary history different from their counterparts in the YZSZ in southern Tibet, thus providing important constraints for the Neo-Tethyan geodynamics around the Indian subcontinent during the late Mesozoic.
We thank H. Rong, G.Y. Feng, F.H. Xiong, and Y.Z. Tian for their assistance in the laboratory and during field work. This work is financially supported by grants from the China Geological Survey (DD20160023-01, DD20160022-01, and 2014DFR21270), the Ministry of Science and Technology of China (Sinoprobe-05-02, 201511022), and the National Natural Science Foundation of China (41541017, 41641015, 41672063). Dilek’s field work was supported by a research grant from the Chinese Academy of Geological Sciences, Beijing. We thank two journal referees for their critical and constructive reviews and Science Editor K. Stüwe for his editorial help and handling.