Mechanisms of Archean crust formation inferred from high-precision HFSE systematics in TTGs

It has been proposed that Archean tonalitic-trondhjemitic-granodioritic magmas (TTGs) formed by melting of mafic crust at high pressures. The residual mineralogy of the TTGs (either (garnet)-amphibolite or rutile-bearing eclogite) is believed to control the trace element budget of TTGs. In particular, ratios of high-field-strength elements (HFSE) can help to discriminate between the different residual lithologies. In order to place constraints on the source mineralogy of TTGs, we performed high-precision HFSE measurements by isotope dilution (Nb, Ta, Zr, Hf) together with Lu-Hf and Sm-Nd measurements on representative, ca. 3.85-2.8 Ga TTGs and related rock types from southern West Greenland, W-India and from the Superior Province. These measurements are complemented by major and trace element data for the TTGs. Texturally homogeneous early Archean (3.85-3.60 Ga old) and Mesoarchean (ca. 3.1-2.8 Ga old) TTGs have both low Ni (<11 ppm) and Cr contents (<20 ppm), indicating that there was little or no interaction with mantle peridotite during ascent. Ratios of Nb/Ta in juvenile Eoarchean TTGs range from ca. 7 to ca. 24, and in juvenile Mesoarchean TTGs from ca. 14 to ca. 27. Even higher Nb/Ta (14-42) were obtained for migmatitic TTGs and intra-crustal differentiates, most likely mirroring further fractionation of Nb from Ta as a consequence of partial melting, fluid infiltration and migmatisation. In the juvenile TTGs, positive correlations between Nb/Ta and Gd/Yb, La/Yb, Sr/Y, Zr/Sm and Zr/Nb are observed. These compositional arrays are best explained by melting of typical Isua tholeiites in both, the rutile-bearing eclogite stability field (>15 kbar, high Nb/Ta) and the garnet-amphibolite stability field (10-15 kbar, low Nb/Ta). With respect to the low end of Nb/Ta found for TTGs, there is currently some uncertainty between the available experimental datasets for amphibole. Independent of these uncertainties, the TTG compositions found here still require the presence of both endmember residues. A successful geological model for the TTGs therefore has to account for the co-occurrence of both low- and high-Nb/Ta TTGs within the same geologic terrane. An additional feature observed in the Eoarchean samples from Greenland is a systematic co-variation between Nb/Ta and initial εHf(t), which is best explained by a model where TTG-melting occured at progressively increasing pressures in a pile of tectonically thickened mafic crust. The elevated Nb/Ta in migmatitic TTGs and intra-crustal differentiates can shed further light on the role of intra-crustal differentiation processes in the global Nb/Ta cycle. Lower crustal melting processes at granulite facies conditions may generate high-Nb/Ta domains in the middle crust, whereas mid-crustal melting at amphibolite facies conditions may account for the low Nb/Ta generally observed in upper crustal rocks.     

TTGs and adakites: are they both slab melts?

Although both high-Al TTG (tonalite–trondhjemite–granodiorite) and adakite show strongly fractionated REE and incompatible element patterns, TTGs have lower Sr, Mg, Ni, Cr, and Nb/Ta than most adakites. These compositional differences cannot be easily related by shallow fractional crystallization. While adakites are probably slab melts, TTGs may be produced by partial melting of hydrous mafic rocks in the lower crust in arc systems or in the Archean, perhaps in the root zones of oceanic plateaus. It is important to emphasize that geochemical data can be used to help constrain tectonic settings, but it cannot be used alone to reconstruct ancient tectonic settings.

Depletion in heavy REE and low Nb/Ta ratios in high-Al TTGs require both garnet and low-Mg amphibole in the restite, whereas moderate to high Sr values allow little, if any, plagioclase in the restite. To meet these requirements requires melting in the hornblende eclogite stability field between 40- and 80-km deep and between 700 and 800 °C.

Some high-Al TTGs produced at 2.7 Ga and perhaps again at about 1.9 Ga show unusually high La/Yb, Sr, Cr, and Ni. These TTGs may reflect catastrophic mantle overturn events at 2.7 and 1.9 Ga, during which a large number of mantle plumes bombarded the base of the lithosphere, producing thick oceanic plateaus that partially melted at depth.     

太古宙TTG的Nb/Ta变化特征:对“Nb-Ta悖论”的启示

英云闪长岩-奥长花岗岩-花岗闪长岩(TTG)是地球早期大陆地壳最重要的组成部分.TTG的Nb/Ta比值变化不仅与它的成因相关,而且与早期构造环境和地壳分异过程关系紧密.本文选择阴山地块出露的TTG片麻岩及下地壳斜长角闪岩/麻粒岩包体作为研究对象,开展了寄主花岗闪长岩和同源镁铁质包体中的角闪石和黑云母的原位微区矿物的微量...     

Post-collisional granitoids from the Dabie orogen: New evidence for partial melting of a thickened continental crust

The geological implications of granitoid magmas with high Sr/Y and La/Yb are debated because these signatures can be produced by multiple processes. This study presents comprehensive major and trace element compositions and zircon SHRIMP U-Pb age data of 81 early Cretaceous granitoids and 4 mafic enclaves from the Dabie orogen to investigate partial melting of the thickened lower continental crust (LCC). On the basis of Sr/Y ratios, granitoids can be grouped into two magma series: (i) high Sr/Y granitoids (HSG) and (ii) normal granitoids with low Sr/Y. Relative to normal granitoids, HSG display the following distinct chemical features: (1) at given SiO₂ and CaO contents, the HSG have significantly higher Sr than normal granitoids, defining two different trends in Sr versus SiO₂, CaO diagrams; (2) highly depleted heavy rare earth element (REE) relative to middle and light REE with (Dy/Yb)_N and (La/Yb)_N up to 3.2 and 151, respectively; (3) variable and higher Nb/Ta; and (4) positive correlations among Sr/Y, (Dy/Yb)_N, (La/Yb)_N, and Nb/Ta.High Sr/Y, (La/Yb)_N, (Dy/Yb)_N, and Sr/CaO of HSG do not correlate with major elements (e.g., SiO₂). Large variations in these ratios at a given SiO₂ content indicate that these features do not reflect magma mixing or fractionation. HSG have higher Sr at a given CaO content and larger variation of (Dy/Yb)_N than old crustal rocks (including exposed basement, global mafic LCC xenoliths, high Sr/Y TTG suites, and adakites in modern arcs). This precludes inheritance of the HSG chemical features from these source rocks. Instead, the chemical features of the HSG are best explained by partial melting of the thickened LCC with garnet-dominant, plagioclase-poor, and rutile-present residual lithologies. The coupled chemical features of the HSG are not observed in post-collisional granitoids younger than ca.130 Ma, indicating removal of the eclogitic source and/or residuum of HSG underneath the orogen. These characteristic chemical relationships in the Dabie HSG may be applied to distinguish partial melts of thickened LCC from high Sr/Y intermediate-felsic magmatic rocks which do not show clear indications for melting depth.     

TTG岩系Nb-Ta-La分馏特征的地球化学模拟:对太古宙板块俯冲与大陆地壳生长机制的约束

TTG的Nb/Ta比值以及Nb、Ta相对于La(代表LILE)的亏损取决于部分熔融体系中金红石、角闪石作为残留相矿物存在与否.本研究采用金红石和低Mg#角闪石的微量元素分配系数模拟部分熔融过程中Nb-Ta-La的分馏.模拟结果表明:如果与TTG熔体平衡的残留相是不含金红石的石榴角闪岩,熔体Nb/Ta比值低于源岩但Ta含...     

Multi-stage crustal growth and Neoarchean geodynamics in the Eastern Dharwar Craton,southern India

The Dharwar Craton is a composite Archean cratonic collage that preserves important records of crustal evolution on the early Earth. Here we present results from a multidisciplinary study involving field investigations, petrology, zircon SHRIMP U–Pb geochronology with in-situ Hf isotope analyses, and whole-rock geochemistry, including Nd isotope data on migmatitic TTG (tonalite-trondhjemite-granodiorite) gneisses, dark grey banded gneisses, calc-alkaline and anatectic granitoids, together with synplutonic mafic dykes along a wide Northwest – Southeast corridor forming a wide time window in the Central and Eastern blocks of the Dharwar Craton. The dark grey banded gneisses are transitional between TTGs and calc-alkaline granitoids, and are referred to as ‘transitional TTGs’, whereas the calc-alkaline granitoids show sanukitoid affinity. Our zircon U–Pb data, together with published results, reveal four major periods of crustal growth (ca. 3360-3200 Ma, 3000-2960 Ma, 2700-2600 Ma and 2570-2520 Ma) in this region. The first two periods correspond to TTG generation and accretion that is confined to the western part of the corridor, whereas widespread 2670-2600 Ma transitional TTG, together with a major outburst of 2570–2520 Ma juvenile calc-alkaline magmatism of sanukitoid affinity contributed to peak continental growth. The transitional TTGs were preceded by greenstone volcanism between 2746 Ma and 2700 Ma, whereas the calc-alkaline magmatism was contemporaneous with 2570–2545 Ma felsic volcanism. The terminal stage of all four major accretion events was marked by thermal events reflected by amphibolite to granulite facies metamorphism at ca. 3200 Ma, 2960 Ma, 2620 Ma and 2520 Ma. Elemental ratios [(La/Yb)_N, Sr/Y, Nb/Ta, Hf/Sm)] and Hf-Nd isotope data suggest that the magmatic protoliths of the TTGs emplaced at different time periods formed by melting of thickened oceanic arc crust at different depths with plagioclase + amphibole ± garnet + titanite/ilmenite in the source residue, whereas the elemental (Ba–Sr, [(La/Yb)_N, Sr/Y, Nb/Ta, Hf/Sm)] and Hf-Nd isotope data [εHf_(T) = −0.67 to 5.61; εNd_(T) = 0.52 to 4.23; ] of the transitional TTGs suggest that their protoliths formed by melting of composite sources involving mantle and overlying arc crust with amphibole + garnet + clinopyroxene ± plagioclase + ilmenite in the residue. The highly incompatible and compatible element contents (REE, K–Ba–Sr, Mg, Ni, Cr), together with Hf and Nd isotope data [εHf_(T) = 4.5 to −3.2; εNd_(T) = 1.93 to −1.26; ], of the sanukitoids and synplutonic dykes suggest their derivation from enriched mantle reservoirs with minor crustal contamination. Field, elemental and isotope data [εHf_(T) = −4.3 to −15.0; εNd_(T) = −0.5 to −7.0] of the anatectic granites suggest their derivation through reworking of ancient as well as newly formed juvenile crust. Secular increase in incompatible as well as compatible element contents in the transitional TTGs to sanukitoids imply progressive enrichment of Neoarchean mantle reservoirs, possibly through melting of continent-derived detritus in a subduction zone setting, resulting in the establishment of a sizable continental mass by 2700 Ma, which in turn is linked to the evolving Earth. The Neoarchean geodynamic evolution is attributed to westward convergence of hot oceanic lithosphere, with continued convergence resulted in the assembly of micro-blocks, with eventual slab break-off leading to asthenosphere upwelling caused extensive mantle melting and hot juvenile magma additions to the crust. This led to lateral flow of hot ductile crust and 3D mass distribution and formation of an orogenic plateaux with subdued topography, as indicated by strain fabric data and strong seismic reflectivity along an E-W crustal profile in the Central and Eastern blocks of the Dharwar Craton.     

变质玄武岩体系相平衡及矿物 熔体微量元素分配：限定TTG/埃达克岩形成条件和大陆壳生长模型

埃达克质岩石是高Na、Al和Sr、低Y和HREE以及Nb、Ta亏损的钠质花岗质岩石,奥长花岗岩-英云闪长岩-花岗闪长岩(TTG)是早期(太古宙)大陆壳主要组分,成分与埃达克质岩石相似,这些成分独特的岩石总体上认为是俯冲洋壳、下地壳和拆沉的下地壳中变质玄武岩部分熔融的产物。文中综述我们近年来在变质玄武岩体系相平衡和矿物-熔体微量元素分配实验研究成果:相平衡实验和熔体微量元素特征研究表明,变质玄武岩部分熔融过程中金红石是导致TTG/埃达克岩浆Nb、Ta亏损的必要残留矿物,从而否定了前人“TTG由无金红石的角闪岩熔融产生”的观点;证实金红石仅仅在压力1.5GPa以上才能稳定存在,从而限定TTG/埃达克岩熔体必定产生在大约50km以上,表明TTG/埃达克岩是在相对较深的含金红石榴辉岩相条件下熔融产生的。矿物(石榴子石、角闪石,单斜辉石和金红石)-熔体微量元素分配系数测定和部分熔融模拟结果进一步限定俯冲洋壳和下地壳起源的TTG/埃达克岩浆由含金红石角闪榴辉岩熔融产生,而拆沉下地壳起源的埃达克岩浆的产生要求软流圈地幔高温,由无水或含有少量含水矿物的榴辉岩熔融产生。     

河南嵩山地区新太古代TTG质片麻岩的成因及其地质意义：来自岩石学、地球化学及同位素年代学的制约

河南嵩山地区位于华北克拉通南缘,是我国记录前寒武纪地质的典型地区之一。该区广泛出露新太古代TTG质片麻岩套,主要以英云闪长岩类为主,闪长岩类次之,测得的锆石SHRIMP年龄在2600~2500Ma。该套TTG质片麻岩富Na₂O(3%~7%)、SiO₂(>67%)、贫铁、镁,高的Na₂O/K₂O比值（多在1.5~5.2）,Al₂O₃=13.72%~16.37%,A/CNK=0.97~1.21,属英云闪长岩-奥长花岗岩系列,显示新太古代富铝型TTG岩石特征。岩石富Sr (平均433×10^-6),Rb/Sr比值(<0.5) 较低,Sr/Y比值高(平均165),Nb、Ta和Ti负异常,∑REE偏低,强烈分异((La/Yb) _N=27~150),基本无Eu异常,低的Nb/Ta(14左右)、La/Nb (平均约为7)比值及其它微量元素特征表明其与岛弧或大陆边缘弧玄武质岩石特征相似。全岩Nd同位素和锆石Hf同位素数据显示岩石源区是来自亏损地幔的约2.66Ga的初生地壳;Mg^#值变化较大反映存在地幔楔不同程度的混染。地球化学特征指示该岩浆是在较高温度和压力(约700~1000℃,>1.5Gpa)下由俯冲的含水玄武质洋壳部分熔融形成,残留相中有石榴石和角闪石而不含斜长石。嵩山地区TTG片麻岩的这种成因机制表明当时陆壳以水平方式增生,也说明在随后的2.5Ga左右微陆块碰撞拼合事件之前不同陆块之间很可能被一个古大洋所分隔。     

REE-HFSE distribution/partitioning between garnetiferous restites and TTG from Nademavinapura area,Western Dharwar craton

The major part of the Peninsular Gneiss in Dharwar craton is made up of Trondjhemite-Tonalite-Granodiorite (TTG) emplaced at different periods ranging from 3.60 to 2.50 Ga. The sodic-silicic magma precursors of these rocks have geochemical features characteristic of partial melting of hydrated basalt. In these TTGs, enclaves of amphibolites (± garnet) are abundant. These restites are considered to be the residue of a basaltic crust after its partial melting. A detailed study of these (residue) enclaves reveals textures formed due to the process of partial melting. Major, trace and REE analysis of these residue enclaves and the melt TTGs and microprobe analysis of the coexisting minerals show partitioning of REE and HFSE between the precursor melt of TTGs and the upper amphibolite facies residues. Formation of garnetiferous amphibolites with biotite, Cpx and plagioclase consequent to melting, has squeezed the original MORB type of basaltic crust and given rise to the TTGs, depleted in Y, Yb, K₂O, MgO, FeO, TiO₂ and enriched in La, Th, U, Zr and Hf. Coevally during the process of melting, the hydrated basalt was depleted in Na₂O, Al₂O₃, LREE, Th, U and enriched in K₂O, MgO, Nb, Ti, Yb, Y, Sc, Ni, Cr and Co. Mineral chemistry of co-existing garnet-biotite and amphibole-plagioclase in these amphibolitic (restite) enclaves indicates an average temperature of 700 ± 50° C and pressure of 5 ± 1 Kbar. These data are inferred to indicate that during the garnet stability field metamorphism, effective fractionation of HREE and HFSE has taken place between the restites having Fe-Mg silicates, ilmenites and the extracted melt generated from the MORB type of hydrated basalt. These results are strongly substantiated by the reported melting experiments on hydrated basalts.     

Nb–Ta fractionation by partial melting at the titanite–rutile transition

During the evolution of the Earth, distinct geochemical reservoirs with different Nb/Ta ratios have developed. Archean granitoids of the tonalite–trondhjemite–granodiorite (TTG) suite, which represent the Earth’s early continental crust, show larger Nb/Ta variations than any other Earth reservoir. This implies that significant Nb–Ta fractionation must have occurred during early crust formation, while the underlying mechanism behind is still unclear. Here, we present a new model on how Nb may be fractionated from Ta during partial melting of subducted oceanic crust. Our data show that Nb/Ta ratios in melts derived from rutile- and titanite-bearing eclogite are largely controlled by the modal relative abundances of rutile and titanite in the source. High modal ratios of titanite over rutile generate melts with very high Nb/Ta (>60), whereas low modal titanite/rutile produces melts with much lower Nb/Ta (≤30). Very low Nb/Ta (<16) occur when all Ti-phases are consumed at very high degrees of melting. As the modal ratio of titanite to rutile is a function of pressure, the Nb/Ta of melts is a function of melting depth. Our new model helps to explain the extreme variation of Nb/Ta observed in many TTGs and thus how Nb and Ta were fractionated during the early evolution of the Earth. Furthermore, the model also indicates that simple one-stage melting models for mafic crust are not sufficient to explain the formation of TTGs.     

Paleoarchean tonalite-trondhjemite complex in the northwestern part of the Sharyzhalgai uplift (southwestern Siberian craton): results of U-Pb and Sm-Nd study

In the northwestern part of the Sharyzhalgai uplift of the Siberian craton (Bulun block), the earliest sialic crust (grey-gneiss complex) is composed of plagiogneisses, their migmatized varieties, and subordinate plagiogranitoids. The petrochemical, trace-element, and Sm-Nd isotope compositions of rocks were studied, and U-Pb dating of zircons (SHRIMP II) was performed. Plagiogneisses and plagiogranitoids of trondhjemite and, more seldom, tonalite compositions are predominant; their compositions are typical of rocks of Archean tonalite-trondhjemitegranodiorite (TTG) complexes (Al₂O₃ ≥ 15%, Mg# = 28–38, (La/Yb)_n = 23–66, Sr/Y = 27–135, Eu/Eu^? = 0.7–1.1). Plagiogneisses of meta-andesite-rhyodacite association are subordinate (SiO₂ = 59–69%, (La/Yb)_n = 7–32, Sr/Y = 11–24, Eu/Eu^? = 0.5–0.7). Cathodoluminescent study of zircons revealed “magmatic cores” and metamorphic rims; most of the rims differ from the cores in U and Th contents and low or greatly varying Th/U ratios. In migmatized plagiogneisses of trondhjemite composition, two zircon generations of different morphologies have been recognized. The protoliths of the grey-gneiss complex rocks formed in the Paleoarchean as a result of two discrete magmatic events, at ～3.3 and 3.25 Ga, and their metamorphism and migmatization took place at ～3.2 Ga. The isotopic and geochemical features of rocks evidence that the primary melts were produced mainly through the melting of metabasic sources at different depths of the thickened crust. Plagiogneisses of trondhjemite composition apparently resulted from magma generation involving ancient sialic material.     

Trace element partitioning between silicate minerals and carbonatite at 25 kbar and application to mantle metasomatism

Summary Experiments at 25 kbar and 1000°C, on a model trace element-enriched carbonatite-eridotite mix, produced augite + pargasite ± garnet ± dolomite coexisting with a carbonatite melt. Proton microprobe analysis of the phases showed that key trace elements (Rb, Ba, Sr, Nb, Ta, Zr, Y and REE) all partitioned strongly into the melt (with the exception of Y, Ho and Lu in garnet), verifying that carbonatite is potentially a highly effective metasomatizing agent. The data also indicate that carbonatitic metasomatism will impart higher Ba/Rb, Ba/Nb, Nb/Ta, Sr/Ta, La/Ta, and lower Zr/Y, with little change to Sr/Nb, in affected mantle.

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Spurenelementverteilung zwischen Silikatmineralen und Karbonatit bei 25 kbar: Anwendung für die Mantel-Metasomatose

Zusammenfassung Experimente mit einer Modell-mischung von Karbonatit-Peridotit, angereichert mit Spurenelementen, produzierten bei 25 kbar und 1000°C Augit + Pargasit ± Granat ±Dolomit coexistierend mit einer Karbonatitschmelze. Protonmikrosonden-Analyse der Phasen zeigte, dass alle Schlüsselspurenelemente (Rb, Ba, Sr, Nb, Ta, Zr, Y and REE) stark in der Schmelze angereichert werden (mit der Ausnahme von Y, Ho und Lu in Granat), was beweist, dass Karbonatit potentiell ein sehr effektives Agens für Metasomatose ist. Die Daten zeigen weiterhin, dass karbonatitische Metasomatose in betroffenen Mantel höhere Ba/Rb, Ba/Nb, Nb/Ta, Sr/Ta, La/Ta und niedrigere Zr/Y produziert, mit geringen Äderungen für Sr/Nb.

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冀东-辽西中-晚三叠世柏杖子花岗质侵入岩地球化学、Sr-Nd-Hf同位素特征及岩石成因

关于冀东-辽西中-晚三叠世具有埃达克质岩石特征的花岗质岩石的源区性质,存在较大争议,且以往对位于其北东向隆起区内的柏杖子岩体的成因研究程度低.对柏杖子花岗质侵入岩进行了系统的年代学、地球化学、Sr-Nd-Hf同位素研究,结果显示:柏杖子岩体结晶年龄为233±3 Ma,岩石具有低MgO、Mg#和Co、Ni、Cr含量;富集大离子亲石元素,亏损高场强元素,显示明显的Pb、Th正异常和Nb、Ta、Ti、P负异常;轻、重稀土元素分馏明显;87Sr/86Sr (t) 为0.704 45~0.705 24,ε_Nd(t) 值为-7.3~-1.7;锆石ε_Hf(t) 为-13.4~-5.9.综合分析认为,高Sr/Y比值和低Y含量的柏杖子岩体属于华北克拉通正常厚度镁铁质下地壳熔融的产物,岩石高Sr/Y比值、低Y含量和Nb-Ta负异常特征为继承的华北克拉通下地壳内在性质,并具有与太古宙-古元古代TTG岩石系列相一致的Nb-Ta解耦特征,表明柏杖子岩体源区物质有TTG岩石的贡献;岩石Sr-Nd-Hf同位素组成和低相容元素含量特征同样表明岩浆来源于受改造的下地壳熔融.早中生代玄武岩底侵过程中幔源组分的加入对华北克拉通下地壳形成改造,同时提供热源诱发下地壳物质熔融,形成的熔体向浅表侵位,经历一定程度的斜长石分离结晶,并最终形成柏杖子岩体.      

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.     

汝城地区晚中生代火山岩地球化学特征及其对源区属性的指示

湘东南汝城地区发育一套由基性玄武岩和中酸性安山质-英安质岩石组成的火山岩建造,属于低钾拉斑系列,该火山岩系中两个玄武岩的K-Ar年龄分别为124.5±2.5Ma和127.6±1.9Ma,属晚侏罗—早白垩世产物。在主、微量元素上两者成分存在明显差异。其中安山质-英安质岩石具有高MgO特征,属高MgO岩石,LILE富集、Nb-Ta、Sr-P亏损强烈,(La/Yb)N=6.7～7.9,Eu*/Eu=0.74～0.85,具岛弧型微量元素配分型式,87Sr/86Sr(t)=0.71079～0.71118,εNd(t)=-7.64～-8.16,与adakites高Mg岩石有着明显的差别,可能是富集岩石圈地幔熔融后直接分异的产物;玄武岩LILE富集,Nb-Ta富集,(La/Yb)cn=4.0～4.3,Eu*/Eu=1.00～1.16,具OIB型微量元素配分型式,87Sr/86Sr(t)=0.70812～0.70832,εNd(t)=0.48～1.03,其源区具二元混合趋势,其源区可能是富集型岩石圈地幔端员与亏损的软流圈地幔端员的混合产物。汝城地区晚中生代玄武岩和高Mg安山质-英安质岩石源区属性的限定及其相互的空间依存关系表明该区晚中生代时有着较薄的岩石圈厚度,处于岩石圈伸展减薄的大地构造背景。     

Major and trace element geochemistry of plutonic rocks from Francistown, NE Botswana: evidence for a Neoarchaean continental active margin in the Zimbabwe craton

The Neoarchaean Tati granite–greenstone terrane occurs within the southwestern part of the Zimbabwe craton in NE Botswana. It comprises 10 intrusive bodies forming part of three distinct plutonic suites: (1) an earlier TTG suite dominated by tonalites, trondhjemites, Na-granites distributed into high-Al (Group 1) and low-Al (Group 2) TTG sub-suite rocks; (2) a Sanukitoid suite including gabbros and Mg-diorites; and (3) a younger high-K granite suite displaying I-type, calc-alkaline affinities.

The Group 1 TTG sub-suite rocks are marked by high Sr/Y values and strongly fractionated chondrite-normalized rare earth element (REE) patterns, with no Eu anomaly. The Group 2 TTG sub-suite displays higher LREE contents, negative Eu anomaly and small to no fractionation of HREE. The primordial mantle-normalized patterns of the Francistown TTGs are marked by negative Nb–Ti anomalies. The geochemical characteristics of the TTG rocks are consistent with features of silicate melts from partial melting of flat subducting slabs for the Group 1 sub-suite and partial melting of arc mafic magmas underplated in the lower crust for the Group 2 sub-suite. The gabbros and high-Mg diorites of the Sanukitoid suite are marked by Mg#>0.5, high Al₂O₃ (>>16%), low TiO₂ (<0.6%) and variable enrichment of HFSE and LILE. Their chondrite-normalized REE patterns are flat in gabbros and mildly to substantially fractionated in high-Mg diorites, with minor negative or positive Eu anomalies. The primordial mantle-normalized diagrams display negative Nb–Ti (and Zr in gabbros) anomalies. Variable but high Sr/Y, Sr/Ce, La/Nb, Th/Ta and Cs/La and low Ce/Pb ratios mark the Sanukitoid suite rocks. These geochemical features are consistent with melting of a sub-arc heterogeneously metasomatised mantle wedge source predominantly enriched by earlier TTG melts and fluids from dehydration of a subducting slab. Melting of the mantle wedge is consistent with a steeper subduction system. The late to post-kinematic high-K granite suite includes I-type calc-alkaline rocks generated through crustal partial melting of earlier TTG material. The Neoarchaean tectonic evolution of the Zimbabwe craton is shown to mark a broad continental magmatic arc (and related accretionary thrusts and sedimentary basins) linked to a subduction zone, which operated within the Limpopo–Shashe belt at 2.8–2.65 Ga. The detachment of the subducting slab led to the uprise of a hotter mantle section as the source of heat inducing crustal partial melting of juvenile TTG material to produce the high-K granite suite.     

Petrology and geochemistry of mafic and ultramafic cumulate rocks from the eastern part of the Sabzevar ophiolite (NE Iran): Implications for their petrogenesis and tectonic setting

The Late Cretaceous Sabzevar ophiolite represents one of the largest and most complete fragments of Tethyan oceanic lithosphere in the NE Iran. It is mainly composed of serpentinized mantle peridotites slices; nonetheless, minor tectonic slices of all crustal sequence constituents are observed in this ophiolite. The crustal sequence contains a well-developed ultramafic and mafic cumulates section, comprising plagioclase-bearing wehrlite, olivine clinopyroxenite, olivine gabbronorite, gabbronorite, amphibole gabbronorite and quartz gabbronorite with adcumulate, mesocumulate, heteradcumulate and orthocumulate textures. The crystallization order for these rocks is olivine ​± ​chromian spinel → clinopyroxene → plagioclase → orthopyroxene → amphibole. The presence of primary magmatic amphiboles in the cumulate rocks shows that the parent magma evolved under hydrous conditions. Geochemically, the studied rock units are characterized by low TiO₂ (0.18–0.57 ​wt.%), P₂O₅ (<0.05 ​wt.%), K₂O (0.01–0.51 ​wt.%) and total alkali contents (0.12–3.04 ​wt.%). They indicate fractionated trends in the chondrite-normalized rare earth element (REE) plots and multi-element diagrams (spider diagrams). The general trend of the spider diagrams exhibit slight enrichment in large ion lithophile elements (LILEs) relative to high field strength elements (HFSEs) and positive anomalies in Sr, Pb and Eu and negative anomalies in Zr and Nb relative to the adjacent elements. The REE plots of these rocks display increasing trend from La to Sm, positive Eu anomaly (Eu/Eu1 ​= ​1.06–1.54) and an almost flat pattern from medium REE (MREE) to heavy REE (HREE) region [(Gd/Yb)_N ​= ​1–1.17]. Moreover, clinopyroxenes from the cumulate rocks have low REE contents and show marked depletion in light REE (LREE) compared to MREE and HREE [(La/Sm)_N ​= ​0.10–0.27 and (La/Yb)_N ​= ​0.08–0.22]. The composition of calculated melts in equilibrium with the clinopyroxenes from less evolved cumulate samples are closely similar to island arc tholeiitic (IAT) magmas. Modal mineralogy, geochemical features and REE modeling indicate that Sabzevar cumulate rocks were formed by crystal accumulation from a hydrous depleted basaltic melt with IAT affinity. This melt has been produced by moderate to high degree (~15%) of partial melting a depleted mantle source, which partially underwent metasomatic enrichment from subducted slab components in an intra-oceanic arc setting.     

Petrogenesis and tectonic implications of Indosinian granitoids from Western Qinling Orogen,China:Products of magma-mixing and fractionation

The Western Qinling Orogen(WQO) is characterized by voluminous distribution of Indosinian granitoids,the formation of which provides an important window to unravel the geochemical and geodynamic evolution and associated metallogeny.Here we investigate a group of intrusions termed "Five Golden Flowers" based on petrological,geochemical,zircon U-Pb geochronological and Lu-Hf isotopic studies on the granitoids and their mafic microgranular enclaves(MMEs).Our results show that these intrusions are genetically divided into two types,namely,magma-mixing and highly fractionated.The Jiaochangba,Lujing,Zhongchuan,and Luchuba granitoids are biotite monzogranites(220±0.8 Ma to 217±2.6 Ma) with abundant coeval MMEs(220±.1 Ma to 217±2.7 Ma).The rocks contain moderate to high SiO_2,high MgO,Rb,Sr,Ba,and Th contents,but low TiO_2,P_2 O_5,and Sc values,A/CNK of 1.1,and a range of ε_(Hf)(t) values of-11.7 to +2.23 with corresponding T_(DM2)values of 1967-1228 Ma.The MMEs possess K-feldspar megacrysts,abundant acicular apatites,and show lopsided textures.They have lower SiO_2,Al_2 O_3,and Th contents,but higher MgO,TiO_2,and Sc,with ε_(Hf)(t) values of-18.0 to +3.18 and T_(DM1) of 849-720 Ma.The data indicate that the MMEs were derived from a magma sourced from the enriched lithospheric mantle.We suggest that these host granitoids were produced by partial melting of latePaleoproterozoic to early-Mesoproterozoic lower crust with the involvement of Neoproterozoic SCLM-derived mafic magmas.The Baijiazhuang pluton is dominantly composed of leucogranite(muscovite granite and twomica monzogranite,216±1.5 Ma) without MMEs.The rocks are peraluminous with high A/CNK(1.06-1.27).Compared with the other four granitoids,the Baijiazhuang leucogranite shows higher SiO_2 content,markedly lower concentrations of TiO_2,MgO,Al_2 O_3,CaO,and Fe_2 O_3~T,and lower LREE/HREE and(La/Yb)N values.These leucogranites are also rich in Rb,Th,and U,and display marked depletions in Ba,Sr,Ti,and Eu,indicating that they experienced significant fractionation.Zircon ε_(Hf)(t) values(-10.2 to-3.27) and T_(DM2)(1868-1424 Ma),as well as the Nb/Ta and K_2 O/Na_2 O values are similar to the other four granitoids,indicating that they are likely to have been derived from a similar source;with sediments playing only a minor role in the magma generation.The low contents of Yb and Y suggest that their partial melting was controlled by garnets and micrographic texture of K-feldspar reflects high-temperature melting through undercooling.Based on the above features,we infer that the Baijiazhuang leucogranite likely represents the product of high degree fractionation of the I-type biotite monzogranite magma which generated the other four granitoids at relatively high temperatures,within magma chambers at mid-crust depths.We propose that the granitoid suite was formed in the transitional setting from synto post-collision during the collisional orogeny between the SCB and NCB,following break-off of the subducted South China Block lithosphere during 220-216 Ma.     

Petrogenesis of the diamondiferous Pipe-8 ultramafic intrusion from the Wajrakarur kimberlite field of Southern India and its relation to the worldwide Mesoproterozoic(～1.1 Ga) magmatism of kimberlite and related rocks

Detailed mineralogy,bulk rock major,trace and Sr-Nd isotope compositions,and ~(40)Ar/~(39)Ar dating of the Pipe-8 diamondiferous ultramafic intrusion in the Wajrakarur cluster of southern India,is reported.Based on the presence of Ti-rich phlogopite,high Na/K content in amphibole,Al-and Ti-rich diopside,a titanomagnetite trend in spinel and the presence of Ti-rich schorlomite garnet and carbonates in the groundmass,the Pipe-8 intrusion is here more precisely classified as an ultramafic lamprophyre(i.e.,aillikite).An aillikite affinity of the Pipe-8 intrusion is further supported by the bulk rock major and trace element and Sr-Nd isotope geochemistry.Sr-Nd isotope data are consistent with a common,moderately depleted upper mantle source region for both the Pipe-8 aillikite as well as the Wajrakarur kimberlites of southern India.A phlogopite-rich groundmass ~(40)Ar/~(39)Ar plateau age of 1115.8±7.9 Ma(2σ) for the Pipe-8 intrusion falls within a restricted 100 Ma time bracket as defined by the 1053-1155 Ma emplacement ages of kimberlites and related rocks in India.The presence of ultramafic lamprophyres,carbonatites,kimberlites,and olivine lamproites in the Wajrakarur kimberlite field requires low degrees of partial melting of contrasting metasomatic assemblages in a heterogeneous sub-continental lithospheric mantle.The widespread association of kimberlite and other mantle-derived magmatism during the Mesoproterozoic(ca.1.1 Ga) have been interpreted as being part of a single large igneous province comprising of the Kalahari,Australian,West Laurentian and Indian blocks of the Rodinia supercontinent that were in existence during its assembly.In India only kimberlite/lamproite/ultramafic lamprophyre magmatism occurred at this time without the associated large igneous provinces as seen in other parts of Rodinia.This may be because of the separated paleo-latitudinal position of India from Australia during the assembly of Rodinia.It is speculated that the presence of a large plume at or close to 1.1 Ga within the Rodinian supercontinent,with the Indian block located on its periphery,could be the reason for incipient melting of lithospheric mantle and the consequent emplacement of only kimberlites and other ultramafic,volatile rich rocks in India due to comparatively low thermal effects from the distant plume.     

Late Mesozoic magmatism in the Jiaodong Peninsula,East China:Implications for crust-mantle interactions and lithospheric thinning of the eastern North China Craton

A section from the Linglong gold deposit on the northwestern Jiaodong Peninsula,East China,containing Late Mesozoic magmatic rocks from mafic and intermediate dikes and felsic intrusions,was chosen to investigate the lithospheric evolution of the eastern North China Craton(NCC).Zircon U-Pb data showed that low-Mg adakitic monzogranites and granodiorite intrusions were emplaced during the Late Jurassic(～145 Ma) and late Early Cretaceous(112-107 Ma),respectively;high-Mg adakitic diorite and mafic dikes were also emplaced during the Early Cretaceous at～139 Ma and ～118 Ma,and 125-145 Ma and 115-120 Ma,respectively.The geochemical data,including whole-rock major and trace element compositions and Sr-Nd-Pb isotopes,imply that the mafic dikes originated from the partial melting of a lithospheric mantle metasomatised through hydrous fluids from a subducted oceanic slab.Low-Mg adakitic monzogranites and granodiorite intrusions originated from the partial melting of the thickened lower crust of the NCC,while high-Mg adakitic diorite dikes originated from the mixing of mafic and felsic melts.Late Mesozoic magmatism showed that lithosphere-derived melts showed a similar source depth and that crust-derived felsic melts originated from the continuously thickened lower crust of the Jiaodong Peninsula from the Late Jurassic to Early Cretaceous.We infer that the lower crust of the eastern NCC was thickened through compression and subduction of the Palaeo-Pacific plate beneath the NCC during the Middle Jurassic.Slab rollback of the plate from ～160 Ma resulted in lithospheric thinning and accompanied Late Mesozoic magmatism.