Origins of Xenolithic Eclogites and Pyroxenites from the Central Slave Craton, Canada

Major- and trace-element and Sr–Nd–Hf isotopic compositionsof garnet and clinopyroxene in kimberlite-borne eclogite andpyroxenite xenoliths were used to establish their origins andevolution in the subcontinental lithospheric mantle beneaththe central Slave Craton, Canada. The majority of eclogitescan be assigned to three groups (high-Mg, high-Ca or low-Mgeclogites) that have distinct trace-element patterns. Althoughpost-formation metasomatism involving high field strength element(HFSE) and light rare earth element (LREE) addition has partiallyobscured the primary compositional features of the high-Mg andhigh-Ca eclogites, trace-element features, such as unfractionatedmiddle REE (MREE) to heavy REE (HREE) patterns suggestive ofgarnet-free residues and low Zr/Sm consistent with plagioclaseaccumulation, could indicate a subduction origin from a broadlygabbroic protolith. In this scenario, the low REE and smallpositive Eu anomalies of the high-Mg eclogites suggest moreprimitive, plagioclase-rich protoliths, whereas the high-Caeclogites are proposed to have more evolved protoliths withhigher (normative) clinopyroxene/plagioclase ratios plus trappedmelt, consistent with their lower Mg-numbers, higher REE andabsence of Eu anomalies. In contrast, the subchondritic Zr/Hfand positive slope in the HREE of the low-Mg eclogites are similarto Archaean second-stage melts and point to a previously depletedsource for their precursors. Low ratios of fluid-mobile to lessfluid-mobile elements and of LREE to HREE are consistent withdehydration and partial melt loss for some eclogites. The trace-elementcharacteristics of the different eclogite types translate intolower _Nd for high-Mg eclogites than for low-Mg eclogites. Withinthe low-Mg group, samples that show evidence for metasomaticenrichment in LREE and HFSE have lower _Nd and _Hf than a samplethat was apparently not enriched, pointing to long-term evolutionat their respective parent–daughter ratios. Garnet andclinopyroxene in pyroxenites show different major-element relationshipsfrom those in eclogites, such as an opposite CaO–Na₂Otrend and the presence of a CaO–Cr₂O₃ trend, independentof whether or not opx is part of the assemblage. Therefore,these two rock types are probably not related by fractionationprocesses. The presence of opx in about half of the samplesprecludes direct crystallization from eclogite-derived melts.They probably formed from hybridized melts that reacted withthe peridotitic mantle. KEY WORDS: eclogites; pyroxenite xenoliths; mantle xenoliths; eclogite trace elements; eclogite Sr isotopes; eclogite Hf isotopes; eclogite Nd isotopes     

Ultrahigh pressure metamorphic rocks from the Chinese Continental Scientific Drilling Project: I. Petrology and geochemistry of the main hole (0–2,050 m)

The main hole (MH) of the Chinese Continental Scientific Drilling Project (CCSD) in southern Sulu has penetrated into an ultrahigh-pressure (UHP) metamorphic rock slice which consists of orthogneiss, paragneiss, eclogite, ultramafic rock and minor schist. Recovered eclogites have a UHP metamorphic mineral assemblage of garnet + omphacite + rutile ± phengite ± kyanite ± coesite ± epidote. Ultramafic rocks contain garnet + olivine + clinopyroxene + orthopyroxene ± Ti-clinohumite ± phlogopite. Gneisses and schists contain an amphibolite-facies paragenesis, but their zircons have coesite, garnet, omphacite (or jadeite) and phengite inclusions, indicating that eclogites and gneisses have been subjected to in situ UHP metamorphism. Using available geothermobarometers, P–T estimates of 3.1–4.4 GPa and 678–816°C for eclogites were obtained. If surface outcrops and neighboring shallow drill holes are considered together, we suggest that a huge supracrustal rock slab (> 50 km long × 100 km wide × 5 km deep) was subducted to a depth > 100 km and then exhumed to the surface. The depth interval (0–2,050 m) of the CCSD-MH can be divided into six lithological units. Unit 1 consists of alternating layers of quartz-rich and rutile-rich eclogites, with thin interlayers of gneiss and schist. Eclogites of unit 1 are characterized by Nb, Ta, Sr and Ti depletions, low Mg number and general LREE enrichment. Unit 2 comprises rutile- and ilmenite-rich eclogite and minor “normal” eclogite and is characterized by high TiO₂, total Fe, V, Co and Sr, and very low SiO₂, alkali, Zr, Ba, Nb, Ta and total REE contents, and LREE-depleted REE patterns with slightly positive Eu anomalies. Unit 3 contains ultramafic rock and minor MgO-rich eclogite. Protoliths of UHP rocks from units 1, 2 and 3 represent a layered mafic to ultramafic intrusion at crustal depth. Units 4 and 6 consist of interlayered eclogite and paragneiss; the eclogites are characterized by Th, U, Nb, Ta and Ti depletion and K enrichment and LREE-enriched REE patterns. Paragneisses show Nb, Ta, Sr and Ti depletions and LREE-enriched REE patterns occasionally with slightly negative Eu anomalies, indicating that their protoliths represent metamorphic supracrustal series. Unit 5 consists mainly of orthogneisses, showing distinct Nb, Ta, Sr and Ti depletions, and LREE-enriched REE patterns with pronounced negative Eu anomalies, suggesting granitic protoliths. In conclusion it is proposed that the southern Sulu UHP belt consists of a series of meta-supracrustal rocks, a layered mafic–ultramafic complex and granites.     

The trace element composition of silicate inclusions in diamonds: a review

On a global scale, peridotitic garnet inclusions in diamonds from the subcratonic lithosphere indicate an evolution from strongly sinusoidal REE_N, typical for harzburgitic garnets, to mildly sinusoidal or “normal” patterns (positive slope from LREE_N to MREE_N, fairly flat MREE_N–HREE_N), typical for lherzolitic garnets. Using the Cr-number of garnet as a proxy for the bulk rock major element composition it becomes apparent that strong LREE enrichment in garnet is restricted to highly depleted lithologies, whereas flat or positive LREE–MREE slopes are limited to less depleted rocks. For lherzolitic garnet inclusions, there is a positive relation between equilibration temperature, enrichment in MREE, HREE and other HFSE (Ti, Zr, Y), and decreasing depletion in major elements. For harzburgitic garnets, relations are not linear, but it appears that lherzolite style enrichment in MREE–HREE only occurs at temperatures above 1150–1200 °C, whereas strong enrichment in Sr is absent at these high temperatures. These observations suggest a transition from melt metasomatism (typical for the lherzolitic sources) characterized by fairly unfractionated trace and major element compositions to metasomatism by CHO fluids carrying primarily incompatible trace elements. Melt and fluid metasomatism are viewed as a compositional continuum, with residual CHO fluids resulting from primary silicate or carbonate melts in the course of fractional crystallization and equilibration with lithospheric host rocks.

Eclogitic garnet inclusions show “normal” REE_N patterns, with LREE at about 1× and HREE at about 30× chondritic abundance. Clinopyroxenes approximately mirror the garnet patterns, being enriched in LREE and having chondritic HREE abundances. Positive and negative Eu anomalies are observed for both garnet and clinopyroxene inclusions. Such anomalies are strong evidence for crustal precursors for the eclogitic diamond sources. The trace element composition of an “average eclogitic diamond source” based on garnet and clinopyroxene inclusions is consistent with derivation from former oceanic crust that lost about 10% of a partial melt in the garnet stability field and that subsequently experienced only minor reenrichment in the most incompatible trace elements. Based on individual diamonds, this simplistic picture becomes more complex, with evidence for both strong enrichment and depletion in LREE.

Trace element data for sublithospheric inclusions in diamonds are less abundant. REE in majoritic garnets indicate source compositions that range from being similar to lithospheric eclogitic sources to strongly LREE enriched. Lower mantle sources, assessed based on CaSi–perovskite as the principal host for REE, are not primitive in composition but show moderate to strong LREE enrichment. The bulk rock LREE_N–HREE_N slope cannot be determined from CaSi–perovskites alone, as garnet may be present in these shallow lower mantle sources and then would act as an important host for HREE. Positive and negative Eu anomalies are widespread in CaSi–perovskites and negative anomalies have also been observed for a majoritic garnet and a coexisting clinopyroxene inclusion. This suggests that sublithospheric diamond sources may be linked to old oceanic slabs, possibly because only former crustal rocks can provide the redox gradients necessary for diamond precipitation in an otherwise reduced sublithospheric mantle.     

Petrogenesis of Maobei rutile eclogites from the southern Sulu ultrahigh-pressure metamorphic belt, eastern China

The Maobei complex in the southern Sulu ultrahigh‐pressure (UHP) metamorphic belt, eastern China, mainly consists of layered eclogites, garnet peridotites and orthogneisses. Based on the modal mineral and whole‐rock compositions, eclogites from the Maobei complex are divided into quartz eclogite, quartz‐rich eclogite, rutile eclogite, rutile‐rich eclogite and eclogite. The distinct spatial changes in the lithology and related chemical compositions indicate that this complex includes 10 rhythmic layers. The rutile eclogites have high TiO₂ (2.4–5.9 wt%), commonly coupled with high P₂O₅ (up to 4.1 wt%) contents; most show fractionated REE patterns with slight positive Eu anomalies. The rutile‐rich eclogites have very high TiO₂ (3.3–5.7 wt%), FeO^T (17.5–25.3 wt%), V (126–1163 ppm) and Co (14–132 ppm), and very low SiO₂ (38.0–42.3 wt%), Zr (24–85 ppm), Nb (0.3–6.9 ppm), Ta (<0.1–0.6 ppm) and total REE (10.7–334.0 ppm) contents, variable degree of LREE depletion, and positive Eu anomalies (Eu/Eu* = 1.1–2.9), and the Ti is decoupled from other high‐field‐strength elements. These characteristics are consistent with Fe‐Ti gabbros of typical layered intrusions, implying a cumulate of plagioclase, clinopyroxene and abundant accessory magnetite in an evolved basaltic magmatic chamber. Based on a normal stratigraphic sequence, the Maobei complex shows an iron‐enrichment trend, followed by alkaline enrichment with increasing fractionated crystallization and stratigraphic height. These facts, together with SHRIMP U‐Pb zircon ages of 773.7 ± 8.0 Ma, indicate that the protolith of the Maobei complex is a Neoproterozoic layered intrusion consisting of a base of peridogabbro, a main body of gabbro and minor granodiorite. Unusually high Ti, V and P contents in three rutile eclogite layers suggest that they are potential economic ore deposits.     

Titanian clinohumite–garnet–pyroxene rock from the Su-Lu UHP metamorphic terrane, China: chemical evolution and tectonic implications

A garnet–pyroxene rock containing abundant Ti-clinohumite (ca. 40 vol.%) occurs along with eclogites as small blocks in quartzo-feldsparthic gneiss in the southern end of the Chinese Su-Lu ultrahigh-pressure (UHP) metamorphic terrane. It consists of three aggregates: (1) Ti-clinohumite-dominated aggregate with interstitial garnet and pyroxene, (2) garnet+pyroxene aggregate with Ti-clinohumite inclusions, and (3) Ti-clinohumite-free aggregate dominated by garnet. Apatite, phlogopite, zircon, hematite, pentlandite, and an unknown Ni-Fe-volatile-Si (NFVS) mineral, which is replaced by Ni-greenalite, occur as accessories. Serpentine is the major secondary mineral. Garnet (Prp_63.9–64.6Alm_25.8–26.9Grs_1.4–7.9Uva_0.5–7.6Sps_1.0) in all three aggregates is pyrope-rich with very low grossular component, with that in the aggregate (2) most enriched in Cr (Cr₂O₃=2.55 wt.%). Orthopyroxene is depleted in Al (Al₂O₃=0.16 wt.% in the cores) and Ca (CaO=0.06–0.09 wt.% in the cores), with X_Mg (Mg/(Mg+Fe)) values at ca. 0.900. Clinopyroxene is chromian diopside with Fe³⁺≥Fe²⁺. Matrix clinopyroxene has a lower X_Mg (0.862) than that (0.887) included in Ti-clinohumite. The rock contains modest amount of heavy rare earth elements (HREE) (10 to 12×C1 chondrite), with significant enrichment in Cr, Co, Ni, V, Sr, and light rare earth elements (LREE) (22 to 33×C1 chondrite). The clinopyroxene is very enriched in Cr (Cr₂O₃ is up to 2.09 wt.% in the cores) and Sr (ca. 350 ppm) and LREE (Ce_N/Yb_N=157.7). Ti-clinohumite is enriched in Ni (1981 ppm), Co (123 ppm), and Nb (85 ppm).

While it is possible to enrich ultramafites in incompatible elements in a subducted slab, the high Al, Fe, Ti, and low Si, Ca, and Na contents in the Ti-clinohumite rock are difficult to account for by crustal metasomatism of an ultramafite. On the other hand, the similarity in major and trace element compositions and their systematic variations between the Ti-clinohumite-garnet-pyroxene rock of this study and those of Mg-metasomatised Fe–Ti gabbros reported in the literature suggest that crustal metasomatism occurred in a gabbroic protolith, which resulted in addition of Cr, Co, Ni, and Mg and removal of Si, Ca, Na, Al, and Fe. This implies that the rock was in contact with an ultramafite at low pressure. During subsequent subduction, the metagabbro was thrust into the country gneiss, where gneiss-derived hydrous fluids caused enrichment of Sr and LREE in recrystallised clinopyroxene. P–T estimates for the high-pressure assemblage are ca. 4.2 GPa and ca. 760 °C, compatible with those for the eclogites and gneisses in this terrane. It is possible that the Ti-clinohumite-garnet-pyroxene rock and associated eclogites represent remnants of former oceanic crust that was subducted to a great depth.     

Ultrahigh-pressure metamorphism in the forbidden zone: the Xugou garnet peridotite, Sulu terrane, eastern China

The Xugou garnet peridotite body of the southern Sulu ultrahigh‐pressure (UHP) terrane is enclosed in felsic gneiss, bounded by faults, and consists of harzburgite and lenses of garnet clinopyroxenite and eclogite. The peridotite is composed of variable amounts of olivine (Fo₉₁), enstatite (En_92?93), garnet (Alm_20?23Prp_53?58Knr_6?9Grs_12?18), diopside and rare chromite. The ultramafic protolith has a depleted residual mantle composition, indicated by a high‐Mg number, very low CaO, Al₂O₃ and total REE contents compared to primary mantle and other Sulu peridotites. Most garnet (Prp_44?58) clinopyroxenites are foliated. Except for rare kyanite‐bearing eclogitic bands, most eclogites contain a simple assemblage of garnet (Alm_29?34Prp_32?50Grs_15?39) + omphacite (Jd_24?36) + minor rutile. Clinopyroxenite and eclogite exhibit LREE‐depleted and LREE‐enriched patterns, respectively, but both have flat HREE patterns. Normalized La, Sm and Yb contents indicate that both eclogite and garnet clinopyroxenite formed by high‐pressure crystal accumulation (+ variable trapped melt) from melts resulting from two‐stage partial melting of a mantle source. Recrystallized textures and P–T estimates of 780–870 °C, 5–7 GPa and a metamorphic age of 231 ± 11 Ma indicate that both mafic and ultramafic protoliths experienced Triassic UHP metamorphism in the P–T forbidden zone with an extremely low thermal gradient (< 5 °C km^?1), and multistage retrograde recrystallization during exhumation. Develop of prehnite veins in clinopyroxenite, eclogite, felsic blocks and country rock gneiss, and replacements of eclogitic minerals by prehnite, albite, white mica, and K‐feldspar indicate low‐temperature metasomatism.     

Petrography and geochemistry of eclogites from the Mir kimberlite,Yakutia, Russia

 Diamond-bearing eclogites are an important component of the xenoliths that occur in the Mir kimberlite, Siberian platform, Russia. We have studied 16 of these eclogite xenoliths, which are characterized by coarse-grained, equigranular garnet and omphacite. On the basis of compositional variations in garnet and clinopyroxene, this suite of eclogites can be divided into at least two groups: a high-Ca group and a low-Ca group. The high-Ca group consists of high-Ca garnets in equilibrium with pyroxenes that have high Ca-ratios [Ca/(Ca+Fe+Mg)] and high jadeite contents. These high-Ca group samples have high modal% garnet, and garnet grains often are zoned. Garnet patches along rims and along amphibole- and phlogopite-filled veins have higher Mg and lower Ca contents compared to homogeneous cores. The low-Ca group consists of eclogites with low-Ca garnets in equilibrium with pyroxenes with a low Ca-ratio, but variable jadeite contents. These low-Ca group samples typically have low modal% of garnet, and garnets are rarely compositionally zoned. Three samples have mineralogic compositions and modes transitional to the high- and low-Ca groups. We have arbitrarily designated these samples as the intermediate-Ca group. The rare-earth-element (REE) contents of garnet and clinopyroxene have been determined by ion microprobe. Garnets from the low-Ca group have low LREE contents and typically have [Dy/Yb]_n < 1. The high-Ca group garnets have higher LREE contents and typically have [Dy/Yb]_n > 1. Garnets from the intermediate-Ca group have REE contents between the high- and low-Ca groups. Clinopyroxenes from the low-Ca group have convex-upward REE patterns with relatively high REE contents (ten times chondrite), whereas those from the high-Ca group have similar convex-upward shapes, but lower REE contents, approximately chondritic. Reconstructed bulk-rock REE patterns for the low-Ca group eclogites are relatively flat at approximately ten times chondrite. In contrast, the high-Ca group samples typically have LREE-depleted patterns and lower REE contents. The δ¹⁸O values measured for garnet separates range from 7.2 to 3.1‰. Although there is a broad overlap of δ¹⁸O between the low-Ca and high-Ca groups, the low-Ca group samples range from mantle-like to high δ¹⁸O values (4.9 to 7.2‰), and the high-Ca group garnets range from mantle-like to low δ¹⁸O values (5.3 to 3.1‰). The oxygen isotopic compositions of two of the five high-Ca group samples and four of the eight low-Ca group eclogites are consistent with seawater alteration of basaltic crust, with the low-Ca group eclogites representative of low-temperature alteration, and the high-Ca group samples representative of high-temperature hydrothermal seawater alteration. We interpret the differences between the low- and high-Ca group samples to be primarily a result of differences in the protoliths of these samples. The high-Ca group eclogites are interpreted to have protoliths similar to the mid to lower sections of an ophiolite complex. This section of oceanic crust would be dominated by rocks which have a significant cumulate component and would have experienced high-temperature seawater alteration. Such cumulate rocks probably would be LREE-depleted, and can be Ca-rich because of plagioclase or clinopyroxene accumulation. The protoliths of the low-Ca group eclogites are interpreted to be the upper section of an ophiolite complex. This section of oceanic crust would consist mainly of extrusive basalts that would have been altered by seawater at low temperatures. These basaltic lavas would probably have relatively flat REE patterns, as seen for the low-Ca group eclogites. Received: 10 July 1995 / Accepted: 17 May 1996     

Mineral geochemistry of the Sangan skarn deposit,NE Iran: Implication for the evolution of hydrothermal fluid

The Sangan iron skarn deposit is located in the Sabzevar-Dorouneh Magmatic Belt of northeastern Iran. The skarn contains zoned garnet, clinopyroxene and magnetite. Cores and rims of zoned garnets are generally homogeneous, having a relatively high ΣREE, low ΣLREE/ΣHREE ratios, and positive Eu anomalies. The cores of the zoned clinopyroxenes are exceptionally HREE-rich, with relatively high ΣREE and HREE/LREE ratios, as well as positive Eu anomalies. Clinopyroxene rims are LREE-rich, with relatively low ΣREE contents and HREE/LREE ratios, and do not have Eu anomalies. Magnetite grains are enriched in LREEs in comparison with the HREEs and lack Eu anomalies. Variations of fluid composition and physicochemical conditions rather than YAG-type substitution mechanism are considered to have major control on incorporating trace elements, including REE, into the skarn mineral assemblage. Based on baro-acoustic decrepitation analysis, the calc-silicate and magnetite dominant stages were formed at similar temperatures, around 350–400 °C. In the Sangan skarns, hydrothermal fluids shifted from near-neutral pH, reduced conditions with relatively high ΣREE, low LREE/HREE ratios, and U-rich characteristics towards acidic, oxidized conditions with relatively low ΣREE, high LREE/HREE ratios, and U-poor characteristics.     

华中超高压变质带中榴辉岩的稀土元素地球化学

根据ＲＥＥ含量及其分配模式,华中超高压变质带中榴辉岩可大体分为六种类型,即ＬＲＥＥ富集型、ＬＲＥＥ富集＋Ｅｕ正异常型、ＬＲＥＥ富集＋Ｅｕ负异常型、ＲＥＥ平坦型、ＭＲＥＥ（中稀土）富集型和ＨＲＥＥ富集型,主要为ＬＲＥＥ富集型和ＬＲＥＥ富集＋Ｅｕ正异常型,不同地区榴辉岩ＲＥＥ类型可进行对比,不同地区同ＲＥＥ类型榴辉岩的ＲＥＥ特征相似。原岩恢复结果表明榴辉岩的原岩主要为来源于相对富集地幔的大陆拉斑玄武岩     

Continental rift and oceanic protoliths of mafic–ultramafic rocks from the Kechros Complex,NE Rhodope (Greece): implications from petrography,major and trace-element systematics,and MELTS modeling

The whole-rock chemistry of eclogites, partially amphibolitized eclogites, and dyke amphibolites from the metamorphic Kechros complex in the eastern Rhodope Mountains preserves evidence of the geodynamic framework for the origin of their protoliths. Major and trace-element concentrations define two distinct protolith groups for the eclogites. The low-Fe–Ti (LFT) eclogites have low-TiO₂ content (<0.67 wt%), negative high field strength element anomalies, and variable enrichments in large ion lithophile elements (LILE). The rare earth element (REE) patterns are characterized by strong light-REE (LREE) enrichment and heavy-REE (HREE) depletion. The high-Fe–Ti (HFT) eclogites have small to moderate LILE enrichment and lack Nb anomalies. The REE patterns of the HFT eclogites are characterized by LREE depletion and relatively flat MREE–HREE patterns. The rock compositions and petrographic features of the LFT eclogites resemble gabbros formed in a continental rift environment with minor to moderate contamination of a mantle-derived mafic magma by continental crust, whereas the HFT eclogites resemble mafic rocks formed in extensional oceanic environments. We interpret the HFT suite to represent a later stage in an evolution from continental rift to open ocean, following the origin of the LFT suite. Dyke amphibolite compositions, except for probable SiO₂ loss associated with metamorphic dehydration reactions, appear to represent liquid compositions quenched in conduits through the lower crust. MELTS modeling shows that dyke amphibolite compositions can be related to each other by fractional crystallization under strongly oxidizing conditions at ~0.5 GPa pressure, and all can be derived from a low-degree melt of modified fertile peridotite from around 1.7 GPa. Cumulates crystallized from the parental liquids of the amphibolites under oxidizing conditions may have yielded the protoliths of the HFT suite.     

南苏鲁超高压变质带东海地区富钛榴辉岩及金红石矿的成因

苏鲁超高压变质带南部的东海地区有富Fe—Ti的钛铁矿榴辉岩和富Ti的金红石榴辉岩。富Fe—Ti榴辉岩的次要矿物主要由钛铁矿和钛磁铁矿组成，可舍少量金红石，具有很低的Si02（38．0％-42．3％）、Na20＋K20（0．48％～2．13％）和Zr、Nb、Ta、Ba、1Kb、REE含量，很高的FeO（18．24％-25．33％）、V和Co含量，并具有不同程度的LREE亏损、明显的正Eu异常；富Ti金红石榴辉岩舍有丰富的金红石和磷灰石，SiO2含量范围（38．0％-54．8％）较宽，FeO含量较低，P2O5含量较高（可达4．1％），而X-TiO2与P205含量具正相关性。相对于原始地慢岩．富Ti金红石榴辉岩具有Rb、Th、K、Nb、Pr、Nd、Eu、Ti、Lu的负异常和Ba、Sr、Zr的正异常．并具相对较高的IKEE含量，LREE富集和HREE亏损的1KEE分配型式。岩石学和地球化学特征揭示，Fe—Ti榴辉岩的原岩是变质的Fe—Ti辉长岩，是基性岩浆强烈分离结晶作用形成的超基性-基性-中酸性层状侵入体的特征组成部分。     

Geochemistry of eclogites of the Tso Morari complex,Ladakh, NW Himalayas:Insights into trace element behavior during subduction and exhumation

Whole rock major and trace element compositions of seven eclogites from the Tso Morari ultra-high pressure(UHP) complex, Ladakh were determined with the aim of constraining the protolith origins of the subducted crust. The eclogites have major element compositions corresponding to sub-alkaline basalts. Trace element characteristics of the samples show enrichment in LILE's over HFSEs(Rb, Th, K except Ba) with LREE enrichments((La/Lu)n = 1.28-5.96). Absence of Eu anomaly on the Primitive Mantle normalized diagram suggests the absence of plagioclase fractionation. Positive correlation between Mg# with Ni and Cr suggests olivine fractionation of mantle melts. Narrow range of(La/Yb)n(2.1-9.4) and Ce/Yb(6.2-16.2) along with Ti/Y(435-735) ratios calculated for the Tso Morari samples is consistent with generation of melts by partial melting of a garnet free mantle source within the spinel peridotite field. Ternary diagrams(viz. Ti-Zr-Y and Nb-Zr-Y) using immobile and incompatible elements show that the samples range from depleted to enriched and span from within plate basalts(WPB)to enriched MORB(E-MORB) indicating that the eclogite protoliths originated from basaltic magmas.Primitive Mantle normalized multi element plots showing significant Th and LREE enrichment marked by negative Nb anomalies are characteristic of continental flood basalts. Positive Pb, negative Nb, high Th/Ta, a narrow range of Nb/La and the observed wide variation for Ti/Y indicate that the Tso Morari samples have undergone some level of crustal contamination. Observed geochemical characteristics of the Tso Morari samples indicate tholeiitic compositions originated from enriched MORB(E-MORB) type magmas which underwent a limited magmatic evolution through the process of fractional crystallization and probably more by crustal contamination. Observed geochemical similarities(viz. Zr, Nb, La/Yb, La/Gd,La/Nb, Th/Ta ratios and REE) between Tso Morari eclogites and the Group I Panjal Traps make the trap basalt the most likely protoliths for the Tso Morari eclogites.     

REE concentrations of garnet and omphacite in eclogites from the Dabie Mountain, central China

Distributions of the rare-earth elements (REE) in omphacite and garnet and REE behaviors during metamorphic processes were discussed. The REE concentrations of garnet and omphacite in six eclogite samples from the Dabie Mountain, central China, were measured by inductively coupled plasma-mass spectrometry (ICP-MS). The correlation of δEu ratios between garnet and omphacite indicated that chemical equilibrium of REE distribution between garnet and omphacite could be achieved during ultra-high pressure (UHP) metamorphism. Most of the partition coefficients (Kd=CiOmp/CiGrt) of light rare-earth elements (LREE) are higher than 1. However the partition coefficients of heavy rare-earth elements (HREE) are lower than 1. This indicated that the LREE inclined to occupy site M2 in omphacite, but the HREEs tended to occupy eightfold coordinated site in garnet during the eclogite formation. The REE geochemistry of the eclogites indicated that LREE could be partially lost during the prograde metamorphic process of protolith, but be introduced into the rocks during the symplectite formation. LREE are more active than HREE during the UHP metamorphism. The results are favorable to highlighting the REE behavior and evolution of UHP metamorphic rocks.     

华北地块东南缘中生代侵入杂岩中所含榴辉岩类包体矿物微量元素地球化学特征及其意义

华北地块东南缘中生代侵入杂岩中所含榴辉岩类包体中矿物的分析结果表明, 组成榴辉岩类包体的各矿物之间元素的分配在榴辉岩相变质阶段已经达到了化学平衡;石榴石中Al、Fe、Mn, 单斜辉石中Na、Mg、Ca, 角闪石中Na、Mg、Fe等主量元素含量与其全岩之间均呈良好的正相关, 受全岩成分制约;石榴石明显富集HREE、贫LREE, 单斜辉石相对富集LREE、MREE、贫HREE, 两者轻、重稀土含量呈相互消长的互补关系, 石榴石和单斜辉石中分别富集Y、Sr, Ga和过渡族元素Sc、Ti、V、Cr、Co等含量较高, 其含量受全岩成分控制;金红石中稀土元素的含量很少, Fe、Al、Mg、Ca、Mn、Nb、Ta、Cr和Zr含量相对较高, Rb、Sr、Ba含量极低, 部分微量元素(如Nb、Cr、Fe、V、W等)含量与其原岩有继承和对应关系;部分原生或/和退变角闪石的REE配分模式分别与单斜辉石和石榴石相似, 过渡族金属元素等的含量与石榴石、单斜辉石和全岩具有一定的相关性, 角闪石容纳Na、K、Sr、Rb、Ba等元素的能力较强, 其成分除受全岩成分的制约外, 石榴石、单斜辉石对其形成也有影响。榴辉岩类包体与其寄主中生代侵入杂岩中部分高价态/高场强元素(HFSE)和重稀土元素(HREE)呈相互消长的关系。榴辉岩的形成与扬子地块和华北地块之间的俯冲-碰撞作用有关。     

Tracing the protolith, UHP metamorphism, and exhumation ages of orthogneiss from the SW Sulu terrane (eastern China): SHRIMP U–Pb dating of mineral inclusion-bearing zircons

Orthogneisses are the major country rocks hosting eclogites in the Sulu UHP terrane, eastern China. All of the analyzed orthogneiss cores from the main drilling hole of the Chinese Continental Scientific Drilling Project (CCSD-MH) have similar major and trace element compositions and a granite protolith. These rocks have relatively high LREE/HREE ratios, strong negative Eu anomalies (Eu/Eu*=0.20–0.39), and negative Ba anomalies (Ba/Ba*=0.25–0.64). Coesite and coesite-bearing UHP mineral assemblages are common inclusions in zircons separated from orthogneiss, paragneiss, amphibolite, and (retrograded) eclogite of the CCSD-MH. This suggests that the eclogite, together with its country rocks, experienced in situ ultrahigh-pressure (UHP) metamorphism. Laser Raman spectroscopy and cathodoluminescence (CL) images show that zircons from the orthogneisses are zoned and that they have distinct mineral inclusions in the different zones. Most zircons retain early magmatic cores with abundant low-pressure mineral inclusions, which are mantled with metamorphic zircon-containing inclusions of coesite and other UHP minerals. The outermost rims on these grains contain low-pressure mineral inclusions, such as quartz and albite. SHRIMP U–Pb dating of the zoned zircons gives three discrete and meaningful groups of ages: Proterozoic ages for the protolith, 227±2 Ma for the coesite-bearing mantles, and 209±3 Ma for the amphibolite facies retrograde rims. The widespread occurrence of UHP mineral inclusions in zircons from the Sulu metamorphic belt dated at about 227 Ma suggests that voluminous continental crust experienced late Triassic subduction to depths of at least 120 km and perhaps more than 200 km. Eighteen million years later, the terrane was rapidly exhumed to midcrustal levels, and the UHP rocks were overprinted by amphibolite facies metamorphism. The exhumation rate deduced from the zircon age data and previously obtained metamorphic P–T data is estimated to be 5.6–11.0 km/Ma. Such rapid exhumation of the Sulu UHP terrane may be due to the buoyancy forces produced by subduction of low-density continental material into the deep mantle.     

Petrochemistry, oxygen isotopes and U-Pb SHRIMP geochronology of mafic–ultramafic bodies from the Sulu UHP terrane, China

Two Rongcheng eclogite‐bearing peridotite bodies (Chijiadian and Macaokuang) occur as lenses within the country rock gneiss of the northern Sulu terrane. The Chijiadian ultramafic body consists of garnet lherzolite, whereas the Macaokuang body is mainly meta‐dunite. Both ultramafics are characterized by high MgO contents, low fertile element concentrations and total REE contents, which suggests that they were derived from depleted, residual mantle. High FeO contents, an LREE‐enriched pattern and trace‐element contents indicate that the bulk‐rock compositions of these ultramafic rocks were modified by metasomatism. Oxygen‐isotope compositions of analysed garnet, olivine, clinopyroxene and orthopyroxene from these two ultramafic bodies are between +5.2‰ and +6.2‰ (δ¹⁸O), in the range of typical mantle values (+5.1 to +6.6‰). The eclogite enclosed within the Chijiadian lherzolite shows an LREE‐enriched pattern and was formed by melts derived from variable degrees (0.005–0.05) of partial melting of peridotite. It has higher δ¹⁸O values (+7.6‰ for garnet and +7.7‰ for omphacite) than those of lherzolite. Small O‐isotope fractionations (ΔCpx‐Ol: 0.4‰, ΔCpx‐Grt: 0.1‰, ΔGrt‐Ol: 0.3–0.4‰) in both eclogite and ultramafic rocks suggest isotopic equilibrium at high temperature. The P–T estimates suggest that these rocks experienced subduction‐zone ultrahigh‐pressure (UHP) metamorphism at ～700–800 °C, 5 GPa, with a low geothermal gradient. Zircon from the Macaokuang eclogite contains inclusions of garnet and diopside. The 225 ± 2 Ma U/Pb age obtained from these zircon may date either the prograde conditions just before peak metamorphism or the UHP metamorphic event, and therefore constrains the timing of subduction‐related UHP metamorphism for the Rongcheng mafic–ultramafic bodies.     

CCSD主孔100-1100m榴辉岩中单矿物的原位微区微量元素地球化学研究

本文通过对CCSD主孔100～1100m范围内榴辉岩中单矿物的LA-ICP-MS分析，探讨了榴辉岩中单矿物之间的微量元素分配，发现超高压变质作用中石榴石和绿辉石之间Ti和C0的分配显著受Mg控制（如DCo^Grt/Omp=3．43DMg^Grt/Omp-0．34），而REE、Sr和Y的分配则受Ca分配所控制。绿辉石中REE、Pb和Th的含量则明显受超高压副矿物磷灰石的出现与否所控制。结合岩石学特征，对角闪石和绿辉石中微量元素的研究表明角闪石主要是绿辉石退变质的产物。但退变质矿物的微量元素组成不仅受原矿物控制，而且受退变质矿物组合类型影响。绿帘石的出现会显著降低共生角闪石中LREE和Sr的含量，而多硅白云母的分解则会增加角闪石中的Rb、Ba含量。另外，退变质过程中的流体活动也会影响退变质矿物中的LREE、Sr和Pb等。结合REE在榴辉岩各主要矿物间分配系数随温度、压力的变化，我们推测部分石榴石边部MREE的富集特征可能反映榴辉岩在折返过程中经历了短时增温作用，这可能是引起苏鲁地区榴辉岩相向麻粒岩相转变叠加现象以及超高压岩石经历部分熔融作用的重要原因。此外，榴辉岩中金红石Nb和Ta组成的高度不均一性为金红石形成于超高压变质阶段富Ti磁铁矿相变作用的成因机制提供了佐证。     

Rare earth abundances in superferrian eclogites from the Voltri Group (Pennidic Belt,Italy)

Ten samples of blueschist facies eclogites occurring as dykes in the ultramafic rocks of the Voltri Group were analysed for eigth rare earth elements (REE). Their patterns are clearly different from those of cognate eclogites enclosed in ultramafic rocks and similar to those of the volcanic rocks dredged from the Mid-Atlantic Ridge (MAR). This similarity confirms both the tholeiitic nature of the magma injected into the Pennidic oceanic basement and metamorphosed to eclogite, and the immobile character of REE during metamorphism up to 10 Kbar and 450°C. Depletion of LREE vs HREE and a positive Eu anomaly in the coarse-grained central portions of the former dykes suggest their derivation from a mush consisting of pyroxene and plagioclase plus trapped superferrian liquid. The high total REE content and the flat pattern of the fine-grained eclogites occurring at the borders of the dykes are consistent with their derivation from a rapidly crystallized superferrian liquid.     

Mineral evolution of a garnet-pyroxenite nodule within eclogite, eastern Sulu ultrahigh-pressure metamorphic terrane, East China

Detailed microtextural observations and bulk chemical analysis were undertaken on a garnet‐pyroxenite nodule within retrograde eclogites from the NE Sulu ultrahigh‐pressure metamorphic (UHPM) terrane. The results suggest that the protolith was a cumulate from a gabbroic body. The nodule consists primarily of coarse clinopyroxene grains with a very high content of the Ca‐Tschermakite molecule. Microscopic observations and back‐scattered electron images (BSE) demonstrate a complicated intergrowth of clinopyroxene, garnet and ilmenite, which represents the peak metamorphic assemblage. The primary clinopyroxene grains are armoured with a thin garnet corona up to 0.5 mm wide that forms an interconnected network. Within the clinopyroxene grains, four sets of garnet lamellae are distributed along crystallographic planes; locally, a vermicular intergrowth of garnet and diopside is developed. Besides the garnet, parallel arrays of ilmenite blebs are common within the clinopyroxene. Hydrous minerals such as amphibole, zoisite and titanite formed at later stages, and replaced diopside, garnet and ilmenite respectively. The P–T conditions determined for the formation of the garnet lamellae indicate that the garnet pyroxenite experienced UHP metamorphism at the same peak P–T condition as its host eclogite. The very high Ca‐Tschermakite content (31–34 mol.%) of the primary clinopyroxene indicates crystallization at about 9–17 kbar and 1250–1450 °C, and together with the microtextural observations, suggests that the protolith of the garnet pyroxenite was a cumulate from a former gabbroic body, in which case, the host eclogite might represent the gabbroic body.     

1.5GPa、950℃下块状斜长角闪岩部分熔融微量元素的分配特征

在1.5GPa,950℃,恒温101h条件下对华北北缘太古宙地体中的斜长角闪岩块状样品进行了脱水部分熔融实验,实验产物组合为Hb+Cpx+Gt+Gl,获得的熔体为花岗闪长质成分。利用LA-ICP-MS测试了矿物和熔体的微量元素,获得该体系内各矿物/熔体的微量元素分配系数。角闪石、单斜辉石和石榴石的分配系数与前人在类似条件下的实验结果基本一致。这说明无论实验的初始物质是粉末状还是块状,对元素的分配没有太大的影响。各矿物的REE分配系数对离子半径的拟合曲线很好地符合晶格应变弹性模型。整体特征上,角闪石、单斜辉石和石榴石的LILE,LREE分配系数较低,而HREE的分配系数较高,石榴石具有强烈富集HREE的特征。由此,造成实验熔体表现出LILE、LREE富集而HREE亏损的特征。残留相中无金红石,使得熔体中没有明显的Nb、Ta负异常。熔体的主-微量元素特征符合华北北缘中生代埃达克质岩石的基本特征,进一步支持了该类岩石"可能起源于古老下地壳的部分熔融"的成因模式。