Ground electrical and electromagnetic studies in Koyna-Warna region,India

The geochemistry of the mafic xenoliths from Baspa valley of Himachal Pradesh, India has been investigated to characterize their protoliths on the basis of immobile elements, especially trace elements including REE. The mafic xenoliths occur within the Kinnaur Kailash granite (KKG) and their geochemistry show that they have tholeiitic nature with basaltic composition. Compositionally, they range from ‘depleted’ to ‘enriched’ MORB as observed on the binary diagrams of Ti vs V and Zr vs Ti and on ternary diagrams of Zr-Ti-Y and Th-Zr-N. Likewise, they match with various enriched or ‘transitional’ MORB types as evident from their Zr vs Nb binary plot. Their enriched character when compared with N-MORB, E-MORB and OIB rocks on chondrite and primordial mantle normalized plots reveals that it is intermediate to that of E-MORB and OIB. The geochemistry of the rocks suggest that the enriched components are probably derived by melting of a mantle source with E-MORB or OIB rather than due to the crustal contamination. The study carried out emphasize that the mafic xenoliths have developed in rift environment, and that they are not volcanic rocks of island arc related to subduction tectonics. It is visualized that the mafic xenoliths were formed as cumulate rocks from the tholeiitic magmas that were rising to lower crust levels in a rift environment, which at a later stage got entrapped as restitic material in the host Kinnaur Kailash granite formed in a collision environment, and propose a change of regime from rift related to collision environment prior to Palaeozoic period.     

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.     

Geochemistry of eclogites from the Dabie–Sulu terrane, eastern China: New insights into protoliths and trace element behaviour during UHP metamorphism

The major and trace element compositions of nine eclogites from the Dabie–Sulu ultrahigh pressure (UHP) metamorphic terrane in eastern China were determined for both whole rock and the main constituent minerals, garnet and clinopyroxene. The results indicate that the eclogite protoliths originated from a basaltic magma, which formed in a continental setting as shown by isotopic and immobile element data. Based on the garnet REE characteristics, the eclogites can be roughly divided into two groups. Group 1 has LREE enrichment with no Eu anomaly for whole rock, and smooth LREE depletion but HREE enrichment pattern for garnet, whereas group 2 shows a depletion of LREE with a pronounced positive Eu anomaly and flat HREE pattern for both whole rock and garnet. From these features, we suggest that the protoliths for group 2 are Fe–Ti–gabbros with relatively high cumulus plagioclase and Fe–Ti oxide, whereas the group 1 eclogites are probably from basalts. Therefore, the unusual garnet REE pattern observed in group 2 can be considered as an important signature for identifying gabbro protoliths for eclogites. The identification of gabbro protoliths from the eclogites in the Dabie–Sulu terrane provides evidence for Neoproterozoic rift magmatism in the northern margin of the Yangtze craton. During ultrahigh pressure metamorphism in the Dabie–Sulu terrane, LILEs (including Ba, Rb, Th, U, K) had high mobility, but REEs and HFSEs were immobile, and trace element distribution equilibrium was approached between garnet and clinopyroxene. An estimate of mass balance indicates that garnet and clinopyroxene host the majority of HREEs and Y, and clinopyroxene is a significant host for Sr, but minor and accessory minerals predominantly account for LREEs, Th, U, and Zr.     

Intra-oceanic island arc origin for Iratsu eclogites of the Sanbagawa belt,central Shikoku,southwest Japan

New geochemical and Sr–Nd isotopic data for the Iratsu eclogite and surrounding metamorphic rocks of the Sanbagawa belt, Japan, show that, while the protoliths of the metamorphic rocks formed in a variety of tectonic settings, the Iratsu body represents a deeply subducted and accreted island arc. The igneous protoliths of eclogites and garnet amphibolites were probably generated from a mantle source that had components of both a depleted mantle modified by slab-released fluid (as seen in a negative Nb anomaly) and an enriched mantle, similar to that of ocean island basalts (OIB). Fractional crystallization modeling indicates that the protoliths of some garnet clinopyroxenites from the Iratsu body are cumulates from a basaltic magma that crystallized under high O₂ and H₂O fugacities in the middle to lower crust. The source characteristics and crystallization conditions suggest that the protoliths of the Iratsu rocks formed in an oceanic island arc. Quartz eclogites from the marginal zone of the Iratsu body have geochemical signatures similar to turbidites from the Izu–Bonin island arc (as seen in a negative Nb anomaly and a concave REE pattern). The protoliths might be volcaniclastic turbidites that formed in a setting proximal to the oceanic island arc. Geochemical and isotopic signatures of the surrounding mafic schists are similar to normal (N-) and enriched (E-) mid-ocean-ridge basalt (MORB), and distinct from the rocks from the Iratsu body. The protoliths of the mafic schists likely formed in a plume-influenced mid-ocean ridge or back-arc basin. Pelitic schists from the surrounding rocks and pelitic gneisses from the marginal zone of the Iratsu body have evolved, continental geochemical signatures (as seen in a negative ε_Nd(t) value (~?5)), consistent with their origin as continent-derived trench-fill turbidites.     

Geochemical evidence for arc-type volcanism in the Aegean Sea: the blueschist unit of Siphnos, Cyclades (Greece)

Blueschists, eclogites, chlorite–actinolite rocks and jadeite-gneisses of the blueschist unit of Siphnos have been investigated for their geochemical composition. Their protolith nature is characterised and a geodynamic model for the pre-metamorphic evolution of these metavolcanic rocks is proposed on the basis of immobile elements, especially trace elements and rare earth elements (REE).

The protoliths of the eclogites are characterised as calc-alkaline basalts, andesites and Fe-rich tholeiites evolving in an island-arc setting. Trace element data indicate that subducted marine sediments were assimilated in the magma chamber, enriching the protoliths in LILE and Pb. Produced in the early stage of back-arc basin opening, a protolith with affinities to both island-arc and MORB formed the precursor of the chlorite–actinolite rocks. They were created by low degrees of partial melting of very primitive magmas, akin to spinel-peridotites and have affinities to boninites, probably through melting of the peridotitic mantle wedge. Tholeiitic basalts and andesites with N-MORB affinity, especially in their REE-patterns, were then produced by partial melting, possibly in an embryonic back-arc basin. These rocks were the protoliths of the blueschists of Siphnos. Their enrichment in some LILE and Pb indicates a N-MORB source contaminated by marine sediments, probably shales or other Pb-rich sediments. Because the jadeite-gneisses show affinities to MOR-granites and volcanic arc granites, intrusion of their protoliths in a back-arc environment is likely. The protoliths of the quartz-jadeite gneisses are rhyodacites/dacites and rhyolites, those of the glaucophane-jadeite gneisses were andesites.

The proposed geodynamic model, solely based on geochemical data, is consistent with geochemical data from neighbouring islands, though those rock units show much higher chemical variability. Consistent with geotectonic models, which are based on structural and geophysical data, the volcanic protoliths of the Siphnos blueschist unit reflect the transition from subduction to spreading environment and record in detail: subduction, formation of an island-arc, and the evolution of a back-arc basin.     

Composition and pre-metamorphic geodynamic setting of the ultrahigh-pressure metabasic rocks from Dabie Shan,E-China

As part of the Yangtze plate, segments of the Dabie Shan terrane of Central China underwent ultra-high pressure metamorphism during Triassic subduction. We studied the geochemistry of the abundant eclogites to evaluate the nature of the protoliths and their geodynamic setting. Although some previous geochemical work exists, the analyses and interpretation herein are based on a new subdivision of the ultra-high pressure sequence into basement and cover units (Changpu and Ganghe Unit), revealing new and important results. In addition, eclogites of the so-called HP Unit south of the UHP units were studied. Whereas the large ion lithophile elements indicate postmagmatic, metasomatic changes of some samples, the high-field strength elements and the rare earth elements display original magmatic trends. The geochemical characteristics of the eclogites of the ultra-high pressure areas display a strong dependence on their “structural” and geographic position. The eclogites of the basement and the Changpu Unit indicate melt intrusion and extrusion in a continental rift system, i.e. during extensional tectonics. In contrast, the Ganghe Unit is characterized by a pronounced chemical homogeneity. The composition of the eclogites indicates generation from a mantle source highly influenced by slab-derived fluids. Those of the HP Unit show similar characteristics. Magmatism of the Ganghe and HP Unit probably occurred in a continental arc setting. A similar age for both units, geographically and/or tectonically separated, is possible. The geodynamic interpretation based on the geochemistry of the four units points to a Neoproterozoic scenario in which the protoliths of the HP and the cover units could have been of similar age and deposited in one evolving geological system. A rift-related larger-scale basin might have formed, e.g. a continental back-arc basin behind a magmatic arc after or simultaneous to sedimentation and magmatism in the magmatic arc. Alternatively, magmatism occurred in independent geodynamic settings, distinct in time and space. The units were juxtaposed during exhumation, after subduction to varying depths.     

中国西部柴北缘—阿尔金的超高压变质榴辉岩及其原岩性质探讨

中国西部祁连山柴北缘地区和南阿尔金地区存在一条被阿尔金断裂错开 4 0 0km ,但构造上相连的早古生代超高压变质带。通过对柴北缘地区大柴旦、锡铁山、都兰和南阿尔金地区且末一带榴辉岩的岩石地球化学研究 ,发现榴辉岩原岩主要由玄武岩和苦橄岩两类岩石组成 ,进一步分为高Ti型 (w(TiO2 ) =2 %～ 5 % ) ,中Ti型 (1%～ 2 % )和低Ti型 (<1% ) 3种类型 ,识别出榴辉岩的原岩类型有洋脊玄武岩、岛弧拉斑玄武岩和洋岛玄武岩类等产在不同环境的岩石类型。榴辉岩的Nd同位素组成与现代洋脊玄武岩类相似 ,ε(Nd ,0 )主要为正值 ,少量为轻微负值 ,表明榴辉岩的原岩曾是海底玄武岩 ,并且经过了消减俯冲作用 ,混入了部分的地壳物质。榴辉岩的超高压变质年龄为 5 0 0～ 4 4 0Ma,原岩年龄分别为 80 0～ 75 0Ma和～ 10 0 0Ma。研究表明 ,柴北缘滩涧山群中存在两套时代不同的基性超基性岩 ,一套为产在绿梁山的新元古代时期形成的蛇绿岩组合 ,新获得的年龄值为 (76 8±39)Ma(Rb Sr)和 (780± 2 2 )Ma(Sm Nd) ,另一套主要为产在赛什腾山的晚寒武世岛弧火山岩 ,形成时代约在 5 15～ 4 86Ma。榴辉岩的岩石化学成分和Nd同位素组成 ,以及 80 0～ 75 0Ma的原岩时代与其中的新元古代基性岩类可以对比。初步认为它们是同一套岩石?     

西天山高压变质岩的形成环境

西天山高压变质岩呈不连续岩块、凸镜体、条带、薄层或夹层岩片产于绿片岩相围岩中。蓝片岩和榴辉石的原岩有 N- MORB、E- MORB、OIB、辉长岩、基性火山碎屑岩、硬砂岩等类型。变质作用和常量、微量、稀土元素地球化学研究表明高压变质岩形成于塔里木板块和伊犁中天山板块间的早古生代南天山洋北缘的 B型俯冲带     

Hydrogen and oxygen isotope evidence for fluid–rock interactions in the stages of pre- and post-UHP metamorphism in the Dabie Mountains

Hydrogen and oxygen isotope studies were carried out on high and ultrahigh pressure metamorphic rocks in the eastern Dabie Mountains, China. The δ¹⁸O values of eclogites cover a wide range of −4.2 to +8.8‰, but the δD values of micas from the eclogites fall within a narrow range of −87 to −71‰. Both equilibrium and disequilibrium oxygen isotope fractionations were observed between quartz and the other minerals, with reversed fractionations between omphacite and garnet in some eclogite samples. The δ¹⁸O values of −4 to −1‰ for some of the eclogites represent the oxygen isotope compositions of their protoliths which underwent meteoric water–rock interaction before the high to ultrahigh pressure metamorphism. Heterogeneous δ¹⁸O values for the eclogite protoliths implies not only the varying degrees of the water–rock interaction before the metamorphism at different localities, but also the channelized flow of fluids during progressive metamorphism due to rapid plate subduction. Retrograde metamorphism caused oxygen and hydrogen isotope disequilibria between some of the minerals, but the fluid for retrograde reactions was internally buffered in the stable isotope compositions and could be derived from structural hydroxyls dissolved in nominally anhydrous minerals.     

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.     

Petrology, geochemistry and isotopic ages of eclogites from the Dulan UHPM Terrane, the North Qaidam, NW China

The Dulan eclogite–gneiss region is located in the eastern part of the North Qaidam eclogite belt, NW China. Widespread evidence demonstrates that this region is a typical ultrahigh-pressure (UHP) metamorphic terrane. Eclogites occur as lenses or layers in both granitic and pelitic gneisses. Two distinguished sub-belts can be recognized and differ in mineralogy, petrology and geochemistry. The North Dulan Belt (NDB) has tholeiitic protoliths with high TiO₂ and lower Al₂O₃ and MgO contents. REE patterns and trace element contents resemble those of N-type and E-type MORB. In contrast, eclogites in the South Dulan Belt (SDB) are of island arc protoliths with low TiO₂, high Al₂O₃ and show LREE-enriched and HFSE-depleted patterns. Sm–Nd isotope analyses give isochron ages of 458–497 Ma for eclogite-facies metamorphism for the two sub-belts. The ages are similar to those of Yuka and Altun eclogites in the western extension of the North Qaidam-Altun eclogite belt. The Dulan UHP metamorphic terrane, together with several other recently recognized eclogite-bearing terrenes within the North Qaidam-Altun HP-UHP belt, constitute the key to the understanding of the tectonic evolution of the northern Tibetan Plateau. The entire UHP belt extends for more than 1000 km from the Dulan UHP terrane in the southeast to the Altun eclogite–gneiss terrane in the west. This super-belt marks an early Paleozoic continental collision zone between the Qaidam Massif and the Qilian Massif.     

Mineral phase compositions in silica-undersaturated ‘leucite’ lamproites from the Bucak area, Isparta, SW Turkey

Ultrapotassic rocks in the Bucak area of Isparta Angle, SW Turkey, show unusually low SiO₂ (46.8–49.2 wt.%) and high MgO (10.4–11.6 wt.%) contents, and lamproitic affinity (K/Na, > 2.5; Mg#, 73–75; Al₂O₃, 9.2–11 wt.%, CaO 7.4–10.6 wt.%, Cr, 525–675 ppm; Ni, 442–615 ppm). They are made up by phlogopite (30–40 vol.%), leucite (25–30 vol.%), olivine (5–20 vol.%), which rarely contain Cr-spinel, clinopyroxene (5–10 vol.%), sanidine (5 vol.%) and richterite, with accessory apatite, magnetite and ilmenite. One sample also include negligible sodalite in groundmass, which is unusual mineral in lamproites. Mineral phase variation and textures record discrete phases of pre-eruptive crystallization: (1) early appearance of (Cr-spinel-bearing) olivine, Ti poor phlogopite ± apatite at pressures of ca. 1.0–2.0 GPa, at or close to the lithospheric Mechanical Boundary Layer (MBL), and (2) later appearance of Ti rich phlogopite, clinopyroxene, richterite, leucite, sanidine, and other minor phases, at pressures of ca. 0.1–1.0 GPa, indicating discrete, pressure-specific fractionation events. The Bucak silica poor ‘leucite’ lamproites were probably generated by partial melting of phlogopite-bearing, refractory peridotite at pressures of ca. 1.5–2 GPa, higher than those proposed for SiO₂-saturated ‘phlogopite’ lamproites (ca. 1–1.5 GPa) from Afyon, to the North. The depth (total pressure) of melt segregation probably dominates over volatile partial pressures (e.g. of CO₂, F, H₂O) in determining the SiO₂-undersaturated character of Bucak magmas.     

Geochemistry of Precambrian gneisses: relevance for the evolution of the East Antarctic Shield

Archaean granulite-facies orthogneisses of the Napier Complex in Enderby Land, metamorphosed 3070 Maago, comprise two chemically distinct suites. The more abundant, mainly of tonalitic to granodioritic composition, shows strong Y depletion, explicable by hydrous partial melting of a garnet-bearing source (garnet amphibolite or possibly eclogite); it apparently represents new continental crust. Other gneisses (predominantly of trondhjemitic to granitic (s.s) composition) do not show Y depletion, and have higher TiO₂, Zr, Nb, La, Ce and Ga/Al, and lower CaO, Sr and Mg/(Mg + total Fe); they probably originated by relatively dry melting of predominantly felsic crystal rocks. Both suites show evidence for loss of Rb (relative to K), Th, and U during metamorphism. Late Archaean (−2800 Ma) amphibolite-facies gneisses of MacRobertson Land are of ‘undepleted’ type and may be representative of a higher crustal level than those of Enderby land. Late Proterozoic (1000 Ma) granulite-facies gneisses of Enderby Land (Rayner Complex) are to a large extent remetamorphosed Napier Complex rocks of igneous derivation; in contrast, gneisses of similar age in MacRobertson Land include a much higher proportion derived, either directly or by partial melting, from sedimentary protoliths.     

Geochemistry of amphibolitized eclogites and cross-cutting tonalitic–trondhjemitic dykes in the Metamorphic Kimi Complex in East Rhodope (N.E. Greece): implications for partial melting at the base of a thickened crust

In the ultra-high pressure Metamorphic Kimi Complex widespread tonalitic–trondhjemitic dykes, with an intrusion age ca. 65–63 Ma, cross-cut boudins and layers of amphibolitized eclogites. Geochemical investigation proclaims the tied genetic relationship of the amphibolitized eclogites and the associated tonalitic–trondhjemitic dykes. The major and trace element contents and rare earth element patterns of the amphibolitized eclogites indicate formation of their protoliths by fractional crystallization of tholeiitic magmas in a back-arc environment. The tonalites and trondhjemites are characterized by moderate to high Sr contents (>130 ppm), and low Y (<8.2 ppm) and heavy rare earth element contents (Yb content of 0.19–0.88 ppm). The chemical composition of the tonalitic and trondhjemitic dykes are best explained by partial melting of a tholeiitic source like the amphibolitized eclogites with residual garnet and amphibole, at the base of a thickened crust during Early Tertiary subduction/accretion at the southern margins of the European continent.     

Subducted seamounts in an eclogite-facies ophiolite sequence: the Andean Raspas Complex, SW Ecuador

The metamorphic Raspas Complex of southwest Ecuador consists of high-pressure mafic, ultramafic, and sedimentary rocks. The Lu–Hf ages of a blueschist, a metapelite, and an eclogite overlap at around 130 Ma and date high-pressure garnet growth. Peak metamorphic conditions in the eclogites reached 1.8 GPa at 600°C, corresponding to a maximum burial depth of ~60 km. The geochemical signatures of the eclogites suggest that their protoliths were typical mid-ocean ridge basalts (MORB), whereas the blueschists exhibit seamount-like characteristics, and the eclogite-facies peridotites seem to represent depleted, MORB-source mantle. That these rocks were subjected to similar peak PT conditions contemporaneously suggests that they were subducted together as an essentially complete section within the slab. We suggest that this section became dismembered from the slab during burial at great depth—perhaps as a consequence of scraping off the seamounts. The spatially close association of MORB-type eclogite, seamount-type blueschist, serpentinized peridotite, and metasediments points to an exhumed high-pressure ophiolite sequence.     

Geochemical signatures for eclogite protolith from the Maksyutov Complex, South Urals

We conducted a geochemical study of eclogites (40 samples) from a boudin of the Lower Unit of the Maksyutov Complex in the South Urals in order to determine their protolith nature. The eclogites have major element compositions corresponding to quartz-bearing hypersthene basalts. Trace-element characteristics of the eclogites further suggest that they resemble enriched-type of tholeiites such as E-MORB. The compositional variation of eclogites was likely caused by fractional crystallization of parental melt under hypabyssal conditions, during its intrusion in thinned continental crust shortly before subduction. The high-pressure metamorphism has not affected significantly the major- and trace-element signatures of the protoliths. The compositions of co-existing minerals from the distinguished rock groups do not show significant distinctions. The considerable scatter of P–T estimates of metamorphic conditions does not depend on whole-rock composition. Therefore, the eclogitization was preceded by a chemical differentiation of an initial magmatic source, which is responsible for co-existence of rocks of variable composition in the same boudin. Dikes or sills of tholeiite basalts having geochemical characteristics of E-MORB could be the protoliths for the Maksyutov eclogites.     

An early Paleozoic Subduction Channel in the North Qilian Mountains,Northwestern China: Structural,Petrological and Chronological Constraints

Detailed geological mapping, structural, petrological and chronological investigation allow us to place new constraints on the tectono‐thermal evolution of the North Qilian high pressure/low temperature (HP/LT) metamorphic belt. The North Qilian HP/LT metamorphic belt manly consists of eclogite, blueschist, metasedimentary rocks and serpentinite. Most of eclogites and mafic blueschists occur as lenses within metasedimentary rocks, and minor eclogites within serpentinite. Petrological and geochemistical data indicate that the protoliths of eclogite and mafic blueschist includes E‐, N‐MORB, OIB and arc basalt. Geochronology and Lu‐Hf isotope of detrital zircons from metasedimentary rocks indicate the detritus materials are derived from Qilian block and likely deposit in continental margin or fore‐arc basin. Zircon U‐Pb datings show that the protolith ages of eclogites vary between 500 Ma and 530 Ma, and the metamorphic age of eclogite between 460 and 489 Ma. The detrital zircon ages of metasedimentary rocks distribute between 532 and 2700 Ma. The structural data show that the deformation related to the subduction during prograde is recorded in eclogite blocks. In contrast, the dominant deformation structures are characterized by tight fold, sheath fold and penetrative foliation and lineation, which are recorded in various rocks, reflecting a top‐to‐the‐south shear sense and representing the deformation related to the exhumation. The petrological data suggest that the different rocks in the North Qilian HP/LT metamorphic belt equilibrated at different peak metamorphic conditions and recorded different P‐T path. Synthesizing the structural, petrological, geochemical and geochronological data suggest a subduction channel model related to oceanic subduction during Paleozoic in the North Qilian Mountains. The different HP/LT metamorphic rocks formed in different settings with various protolith ages were carried by the subducted oceanic crust into different depth in subduction channel, and experienced independent tectono‐thermal evolution inside subduction channel. The North Qilian HP/LT mélange reflects a fossil oceanic subduction channel.     

The subducted oceanic crust within continental-type UHP metamorphic belt in the North Qaidam, NW China: Evidence from petrology, geochemistry and geochronology

Three types of eclogite, together with a serpentinized harzburgite, coexist as blocks within granitic and pelitic gneisses along the Shaliuhe cross section, the eastern part of the North Qaidam continental-type ultrahigh-pressure (UHP) metamorphic belt, NW China. The olivine (Ol¹) and orthopyroxene in the harzburgite are compositionally similar to present-day abyssal peridotites. The kyanite–eclogite is derived from a troctolitic protolith, whereas the epidote–eclogite from a gabbroic protolith, both having distinct positive Eu anomalies, low TiO₂, and high Al₂O₃ and MgO. The kyanite–eclogite shows inherited cumulate layering. The phengite–eclogite has high TiO₂, low Al₂O₃ and MgO with incompatible trace elements resembling enriched-type MORB. Sr–Nd isotope data indicate that the protoliths of both kyanite–eclogite and epidote–eclogite ([⁸⁷Sr/⁸⁶Sr]_i ~ 0.703–0.704; ε_Nd(T) ~ 5.9–8.0) are of mantle origin (e.g., ocean crust signatures). On the other hand, while the lower ε_Nd(T) value (1.4–4.1) of phengite–eclogite is more or less consistent with an enriched MORB protolith, their high [⁸⁷Sr/⁸⁶Sr]_i ratio (0.705–0.716) points to an additional enrichment in their history, probably in an subduction-zone environment. Field relations and geochemical analyses suggest that the serpentinized harzburgite and the three types of eclogite constitute the oceanic lithological section of an ophiolitic sequence from mantle peridotite, to cumulate, and to upper basaltic rocks. The presence of coesite pseudomorphs and quartz exsolution in omphacite plus thermobarometric calculations suggests that the eclogites have undergone ultrahigh pressure metamorphism (i.e., peak P ≥ 2.7 GPa). The harzburgite may also have experienced the same metamorphism, but the lack of garnet suggests that the pressure conditions of ≤ 3.0 GPa. Zircon U–Pb SHRIMP dating shows that the eclogites have a protolith age of 516 ± 8 Ma and a metamorphic age of 445 ± 7 Ma. These data indicate the presence of a Paleo-Qilian Ocean between Qaidam and Qilian blocks before the early Ordovician. The ophiolitic assemblage may be the relics of subducted oceanic crust prior to the subduction of continental materials during Ordovician–Silurian times and ultimate continent collision. These rocks, altogether, record a complete history of ocean crust subduction, to continental subduction, and to continental collision.     

Geochemical signatures of Variscan eclogites from the Saxonian Erzgebirge, central Europe

From the abundant metre to km-sized eclogite bodies in the Variscan crystalline complex of the Saxonian Erzgebirge we have investigated 19 samples from the ultrahigh pressure area at the Saidenbach reservoir. Twenty-two samples were from the south-western Erzgebirge, and from occurrences located only some km away from the reservoir. These samples were analysed for major and trace elements using X-ray fluorescence (XRF) spectrometry and inductively coupled plasma mass spectrometry (ICP-MS).The non-Saidenbach eclogites (SiO₂=49–53 wt%) can be derived from N-mid-ocean ridge basalts (MORBs) partially transitional to P-MORBs (e.g., (Nb)_N: 3–36; (Sr)_N: 4–17; (La/Sm)_N<1.5 (in most instances <0.7) and (Sm/Yb)_N around 1.2). Eclogites from the Saidenbach reservoir (SiO₂=49–61 wt%) are characterised by (Nb)_N: 20–170; (Sr)_N: 9–43; (La/Sm)_N: 1.2–3.0; (Sm/Yb)_N: 1.4–8.8, and a clear negative Eu anomaly for the Si-rich samples, thus, being significantly different from the other investigated eclogites. These signatures point to protoliths related to within plate igneous rocks. However, we also discuss the possibilities of (1) protoliths related to a magmatic arc along an active continental margin and (2) the formation by melting of crustal material in the deep mantle and final crystallisation in the lowermost continental crust similar to the adjacent diamondiferous quartzofeldspathic rocks.Due to the specific geochemical signatures of eclogites in the Saidenbach area including other facts, this ultrahigh pressure region is believed to represent a section of lowermost crust not outcropping in other portions of the Saxonian Erzgebirge.     

Variscan Sm-Nd and Ar-Ar ages of eclogite facies rocks from the Erzgebirge, Bohemian Massif

Abstract The Erzgebirge Crystalline Complex (ECC) is a rare example where both‘crustal’eclogites and mantle-derived garnet-bearing ultramafic rocks (GBUs) occur in the same tectonic unit. Thus, the ECC represents a key complex for studying tectonic processes such as crustal thickening or incorporation of mantle-derived material into the continental crust. This study provides the first evidence that high-pressure metamorphism in the ECC is of Variscan age. Sm-Nd isochrons define ages of 333 ± 6 (Grt-WR), 337± 5 (Grt-WR), 360± 7 (Grt-Cpx-WR) (eclogites) and 353 ± 7 Ma (Grt-WR) (garnet-pyroxenite). ⁴⁰Ar/³⁹Ar spectra of phengite from two eclogite samples give plateau ages of 348 ± 2 and 355 ± 2 Ma. The overlap of ages from isotopic systems with blocking temperatures that differ by about 300 ° C indicates extremely fast tectonic uplift rates. Minimum cooling rates were about 50° C Myr-¹. As a consequence, the closure temperature of the specific isotopic system is of minor importance, and the ages correspond to the time of high-pressure metamorphism. Despite textural equilibrium and metamorphic temperatures in excess of 800° C, clinopyroxene, garnet and whole rock do not define a three-point isochron in three of four samples. The metamorphic clinopyroxenes seem to have inherited their isotopic signature from magmatic precursors. Rapid tectonic burial and uplift within only a few million years might be the reason for the observed Sm-Nd disequilibrium. The ε_Nd values of the eclogites (+4.4 to +6.9) suggest the protoliths were derived from a long-term depleted mantle, probably a MORB source, whereas the isotopically enriched garnet-pyroxenite (ε_Nd–2.9) might represent subcontinental mantle material, emplaced into the crust prior to or during collision. The similarity of ages of the two different rock types suggests a shared metamorphic history.