Importance of water for Archaean granitoid petrology: a comparative study of TTG and potassic granitoids from Barberton Mountain Land,South Africa

A new model for Archaean granitoid magmatism is presented which reconciles the most important geochemical similarities and differences between tonalite–trondhjemite–granodiorite (TTG) and potassic granitoids. Trace element abundances reveal a strong arc magmatism signature in all studied granitoids from Barberton Mountain Land. Characteristic features include HFSE depletion as well as distinct enrichment peaks of fluid-sensitive trace elements such as Pb in N-MORB normalisation, clearly indicating that all studied granitoids are derived from refertilised mantle above subduction zones. We envisage hydrous basaltic (s.l.) melts as parental liquids, which underwent extensive fractional crystallisation. Distinctive residual cumulates evolved depending on initial water content. High-H₂O melts crystallised garnet/amphibole together with pyroxenes and minor plagioclase, but no olivine. This fractionation path ultimately led to TTG-like melts. Less hydrous basaltic melts also crystallised garnet/amphibole, but the lower compatible element content indicates that olivine was also a liquidus phase. Pronounced negative Eu-anomalies of the granitic melts, correlating with Na, Ca and Al, indicate plagioclase to be of major importance. In the context of our model, the post-Archaean disappearance of TTG and concomitant preponderance of granites (s.l.), therefore, is explained with secular decrease of aqueous fluid transport into subduction zones and/or efficiency of deep fluid release from slabs.     

Sources and formation conditions of Early Proterozoic granitoids from the southwestern margin of the Siberian craton

Three stages of Early Proterozoic granitoid magmatism were distinguished in the southwestern margin of the Siberian craton: (1) syncollisional, including the formation of migmatites and granites in the border zone of the Tarak massif; (2) postorogenic, postcollisional, comprising numerous granitoid plutons of diverse composition; and (3) intraplate, corresponding to the development of potassic granitoids in the Podporog massif. Rocks of three petrological and geochemical types (S, I, and A) were found in the granitoid massifs. The S-type granites are characterized by the presence of aluminous minerals (garnet and cordierite), and their trace element distribution patterns and Nd isotopic parameters are similar to those of the country paragneisses and migmatites. Their formation was related to melting under varying H₂O activity of aluminous and garnet—biotite gneisses at P ≥ 5 kbar and T < 850°C with a variable degree of melt separation from the residual phases. The I-type tonalites and dioritoids show low relative iron content, high concentrations of CaO and Sr, fractionated REE distribution patterns with (La/Yb)_n = 11–42, and variable depletion of heavy REE. Their parental melts were derived at T ≥ 850°C and P > 10 and P < 10 kbar, respectively. According to isotopic data, their formation was related to melting of a Late Archean crustal (tonalite-diorite-gneiss) source with a contribution of juvenile material ranging from 25–55% (tonalites of the Podporog massif) to 50–70% (dioritoids of the Uda pluton). The most common A-type granitoids show high relative iron content; high concentration of high-field-strength elements, Th, and light and heavy REE; and a distinct negative Eu anomaly. Their primary melts were derived at low H₂O activity and T ≥ 950°C. The Nd isotopic composition of the granitoids suggests contributions to the magma formation processes from ancient (Early and Late Archean) crustal (tonalite-diorite-gneiss) sources and a juvenile mantle material. The contribution of the latter increases from 0–35% in the granites of the Podporog and Tarak massifs to 40–50% for the rocks of the Uda and Shumikha plutons. The main factors responsible for the diversity of petrological and geochemical types of granitoids in collisional environments are the existence of various fertile sources in the section of the thickened crust of the collisional orogen, variations in magma generation conditions $(\alpha _{H_2 O} , T, and P)$ during sequential stages of granite formation, and the varying fraction of juvenile mantle material in the source region of granitoid melts.     

Age and sources of the Paleoproterozoic premetamorphic granitoids of the Goloustnaya block of the Siberian craton: Geodynamic applications

This paper reports the results of geological, geochronological, and isotope geochemical investigations of two premetamorphic granite massifs of the Goloustnaya block of the Baikal salient of the basement of the Siberian craton and granite gneisses from the migmatite–gneiss sequence of this block. The U–Pb zircon age of the granites of the Khomut massif is 2153 ± 11 Ma. The age of the Elovka massif was previously determined by us as 2018 ± 28 Ma. The Khomut and Elovka granites underwent structural and metamorphic transformations accompanied by migmatization. An age of 1.98–1.97 Ga was obtained for the structural and metamorphic processes in the Goloustnaya block from the analysis of margins of zircon grains from the Khomut granites and zircon from the granite gneisses. The biotite granites of the Khomut massif show transitional I–S-type geochemical characteristics, which allowed us to suggest that they were derived by melting of a crustal source of intermediate–acid composition. The Khomut granites show positive ε_Nd(T) values from +2.0 to +2.2 and a Nd model age of 2.4 Ga, which may indicate their formation owing to the reworking of the Paleoproterozoic juvenile continental crust. The combined isotope geochemical data are consistent with collision of island arcs as a possible environment for the formation of the Khomut granites. The formation of these granites was not related to the development of the structure of the Siberian craton, similar to a few other anorogenic magmatic complexes of the margin of the Chara–Olekma terrane of the Aldan shield with ages of ~2.2–2.1 Ga, including the granites of the Katugin complex. The biotite–amphibole granites of the Elovka massif with an age of ~2.02 Ga are geochemically similar to I-type granites. The geochemical characteristics of these granites, including elevated Sr and Ba and low Nb and Ta contents, were inherited from a subduction-related source. Negative ε_Nd(T) values from–0.9 to–1.8 and rather high contents of K₂O and Th allow us to suppose a metamagmatic crustal source for the granites of the Elovka massif. The combined isotope geochemical characteristics of the Elovka granites suggest that a mature island arc or an active continental margin is the most probable environment of their formation. The estimates of the age of structural and metamorphic processes affecting the Goloustnaya block (1.98–1.97 Ga) coinciding with the time of similar transformations in the central part of the Aldan shield and eastern Anabar shield (1.99–1.96 Ga) indicate wide occurrence of collisional events of similar age in the Siberian craton and allow us to consider this age interval as an early large-scale stage of the formaiton of the structure of the Siberian craton.     

1.8 Ga magmatism in southern Finland: strongly enriched mantle and juvenile crustal sources in a post-collisional setting

Whole-rock and isotope geochemistry of six ～1.8 Ga post-kinematic intrusions, emplaced along the ～1.9 Ga Southern Svecofennian Arc Complex (SSAC) and in the SW part of the Karelian Domain in Finland, was studied. The intrusive age [U–Pb secondary ion mass spectrometer (SIMS)] of one of these, the Petravaara Pluton, was determined as 1811 ± 6 Ma.

Basic-intermediate rocks are alkali-rich (K₂O?+?Na₂O > 4 wt.%) and typically shoshonitic, strongly enriched in large ion lithophile elements and light rare earth elements, but relatively depleted in high field strength elements and heavy rare earth elements. The enrichment is much higher than can be accounted for by crustal contamination and requires previously melt-depleted mantle sources, subjected to variable metasomatism by carbonate-rich fluids and sediment-derived melts. These sources are inferred to consist of phlogopite ± amphibole-bearing peridotites from depths below the spinel–garnet transition, as shown by the high Ce/Yb ratios. ⁸⁷Sr/⁸⁶Sr(1.8 Ga) ratios in the range 0.7027–0.7031 and ‘mildly depleted’ ?_Nd(1.8 Ga) values (+0.1 to?+1.4), with T _DM values <2.1 Ga, suggest that mantle enrichment was associated with the previous Svecofennian subduction–accretion process, when enriched sub-Svecofennian mantle sections developed, dominantly characterized by ¹⁴⁷Sm/¹⁴⁴Nd ratios of 0.14–0.17.

The associated granitoids are diversified. One group is marginally peraluminous, transitional between I (volcanic-arc) and S (syn-collisional) types, and was derived from mixed igneous and sedimentary, but juvenile Svecofennian source rocks, as supported by near-chondritic ?_Nd(1.8 Ga) and somewhat elevated ⁸⁷Sr/⁸⁶Sr(1.8 Ga). The other group is transitional between I and A (within-plate) types in character and had dominantly igneous protoliths. The whole-rock geochemistry and isotopes suggest that the compositional variation between ～50 and 70 wt.% SiO₂ may be explained by hybridization between strongly enriched mantle-derived magmas and anatectic granitic magmas from the juvenile Svecofennian crust. One intrusion in the east contains a significant portion of Archaean, mostly igneous protolithic material (?_Nd(1.8 Ga)?=?–2.8 and ?_Hf(t) for zircons between?+2.8 and??11.9, with an average of??4.9).

The ～1.8 Ga post-kinematic intrusions were emplaced within the SSAC subsequent to the continental collision with the Volgo-Sarmatia craton from the SE, during a shift from contraction to extension, that is, in a post-collisional setting.     

Post-collisional potassic granitoids from the southern and northwestern parts of the Late Neoproterozoic East African Orogen: a review

Potassic metaluminous granitoids with enrichments of HFS elements constitute part of widespread post-collisional magmatism related to the Late Neoproterozoic Pan-African orogeny in northeastern Africa (Sudan, Ethiopia, Somalia) and Madagascar. The plutons were emplaced between 580 and 470 Ma and comprise both subsolvus and hypersolvus biotite–granite, biotite–hornblende–granite, quartz–monzonite and quartz–syenite. Pyroxene-bearing granitoids are subordinate. Basic dikes and enclaves of monzodioritic composition are locally associated with the granitoid plutons. Granitoids emplaced in pre-Neoproterozoic crust have Sr_i-ratios between 0.7060 and 0.7236 and _Nd(t) values between −15.8 and −5.6 while those emplaced in, or close to the contact with, juvenile Neoproterozoic crust have lower Sr_i-ratios (0.7036–0.7075) and positive _Nd(t) values (4.6). However, it is unlikely that the potassic granitoids represent products of crustal melting alone. The association with basic magmas derived from subduction-modified enriched mantle sources strongly suggests that the granitoids represent hybrid magmas produced by interaction and mixing of mantle and crust derived melts in the lower crust. The most intense period of this potassic granitoid magmatism occurred between 585 and 540 Ma, largely coeval with HT granulite facies metamorphism in Madagascar and with amphibolite facies retrogression in northeastern Africa (Somalia, Sudan). Granitoid magmatism and high-grade metamorphism are probably both related to post-collisional lithospheric thinning, magmatic underplating and crustal relaxation. However, the emplacement of potassic granites continued until about 470 Ma and implies several magmatic pulses associated with different phases of crustal uplift and cooling. The potassic metaluminous granites are temporally and spatially associated with post-collisional high-K calc-alkaline granites with which they share many petrographical, geochemical and isotopical similarities, except the incompatible element enrichments. The resemblance indicates a strongly related petrogenesis of both granite associations.     

Late Paleoproterozoic basites of the northern Baikal area: composition and melt sources

We present results of isotope-geochemical studies of Late Paleoproterozoic basites from intrusions located in different parts of a dike swarm traceable for more than 200 km within the Baikal marginal salient of the Siberian craton basement (northern Baikal area). The basites of the southern (Khibelen site) and northern (Chaya site) parts of the dike swarm show both similarity and difference in their sources and formation conditions. For example, the Khibelen basites correspond in chemical composition to basalts and trachybasalts, and the Chaya basites, to basalts and andesite-basalts. Based on petrographic and petrochemical data, the basites of both sites can be referred to as medium-alkali (subalkalic) series. All analyzed basites show distinct negative Nb–Ta and Ti anomalies on element spidergrams, negative ε_Nd(t) values, and indicative geochemical ratios Th/Nb_pm, La/Nb_pm, and La/Sm_n > 1. All this points to the formation of basites of both sites from mantle sources contaminated with continental crust. Contamination might have occurred in intermediate magma chambers localized in crust. Differentiated basic varieties of both sites resulted from fractionation of clinopyroxene. For the Khibelen basites, the mantle source (probably, with geochemical parameters close to those of IAB) might have been initially contaminated with middle-crust rocks and then, with lower/upper-crust material.The source of the Chaya basites was probably produced during the interaction of mantle components similar in composition to IAB and N-MORB with a crustal component. The performed studies testify to the heterogeneous composition of the upper mantle beneath different sites of the Siberian craton basement.     

Petrography and geochemistry of Archaean greywackes from northern part of the Dharwar-Shimoga greenstone belt,western Dharwar craton: Implications for nature of provenance

Greywackes (Dharwar greywackes) are the most abundant rock types in the northern part of the Dharwar-Shimoga greenstone belt of the western Dharwar craton. They are distinctly immature rocks with poorly-sorted angular to sub-angular grains, comprising largely quartz, plagioclase feldspar and lithic fragments of volcanics (mafic+felsic), chert and quartzite, with subordinate biotite, K-feldspar and muscovite. They are characterized by almost uniform silica (59.78-67.96 wt%; av. 62.58), alkali (4.62-7.35 wt%; av. 5.41) contents, SiO₂/Al₂O₃ (3.71-5.25) ratios, and compositionally are comparable to the andesite and dacite. As compared to Ranibennur greywackes, located about 100 km south of Dharwad in the Dharwar-Shimoga greenstone belt, the Dharwar greywackes have higher K₂O, CaO, Zr, Y, ΣREE, Th/Sc, Zr/Cr, La/Sc and lower Sr, Cr, Ni, Sc, Cr/Th values. The chondrite normalized patterns of Dharwar greywackes are characterized by moderately fractionated REE patterns with moderate to high LREE enrichment, with almost flat HREE patterns and small negative Eu anomalies, suggesting felsic dominated source rocks in the provenance. The frame work grains comprising felsic and mafic volcanics, feldspars and quartz suggest a mixed source in the provenance. The moderate CIA values ranging between 57 and 73, indicate derivation of detritus from fresh basement rocks and from nearby volcanic sources.The mixing calculations suggest that the average REE pattern is closely matching with a provenance having 40% dacite, 30% granite, 20% basalt and 10% tonalite. These greywackes were deposited in a subduction related forearc basin than a continental margin basin. Their La/Sc ratios are high (av. 4.07) compared to the Ranibennur greywackes (1.79), suggesting that the greywackes of the northern part of the basin received detritus from a more evolved continental crust than the greywackes of the central part of the Dharwar-Shimoga basin.     

Comparison of 2.1-1.6 Ga fault-line alkaline metasomatic rocks and granitoids from regional fault zones on the southern framing of the Siberian craton

The geologic position, development stages, age, and geochemical features of metasomatic and felsic igneous rocks along the southern edge of the Siberian craton are compared. The comparison shows that all the studied metasomatic rocks are confined to the faults feathering the main suture zone of the craton. From Biryusa zone in the southwest and farther northeast, from Primor’e zone to Davan shear zone and Katugino-Ayan zone in the Aldan area, the metasomatic rocks are of similar composition but show higher mineralization. The process begins with blastocataclasis (barren stage). During the second stage, ore-bearing (Nb, Zr, Hf, and REE) potassic solutions circulate along the blastocataclastic zones. They form metasomatic potassic rocks of the early alkaline stage, expressed in subalkaline granitization. The next (acid) stage is marked by the formation of greisens with Sn, Be, Th, U, and W mineralization. The igneous stage might precede or follow the metasomatism. At the time of ongoing tectonic movements, it produces rapakivi-like granites rich in the same elements. Also, a huge volcanoplutonic belt develops along the craton edge during this time. The geochemical features of its felsic volcanics are close to those of the metasomatic rocks and granites. The age of all these rocks is within 2.1-1.6 Ga.     

Evidence for a Neoproterozoic carbonate ramp on the northern edge of the Central African craton: relations with late Proterozoic intracratonic troughs

During Late Proterozoic times, the Archaean Central African craton was affected by trough faulting which led to the formation of grabens, the Sangha aulacogen being the main structure of this type in the studied area. This transverse basin connects with other basins on the northern and south-western borders of the craton. During the Cryogenian, this network of basins was filled with fluvio-deltaic and lacustrine periglacial deposits. The glacio-eustatic transgression in Neoproterozoic III (end-Proterozoic) times flooded extensive areas of shelf on the northern edge of the craton, leading to the development of carbonate sedimentation in a broad outer shelf environment associated with nearshore barriers and evaporitic lagoons. These facies are similar to those developed in the West Congolian Schisto-calcaire (shale-limestone) ramp succession. The North-Central African ramp succession (sediment slope) contains an example of tidal rhythmites in vertical accretion, which occurs beneath the barrier deposits on the subtidal outer shelf. Mathematical analysis of the bedding pattern yields a period of 29–30 days for the lunar month, a result which is in agreement with astrophysical evidence for this epoch (i.e. 650 Ma ago). Major subsidence and seismic activity on this gently sloping platform, associated with the proximity of the Sangha aulacogen, caused the triggering of carbonate turbidites and mass flow deposits. The proliferation of microbial mats under euphotic conditions on an extensive shelf led to the build-up of a carbonate platform. During early Neoproterozoic III times, the West Congolian and North-Central African ramps prograded northwards and southwards, respectively, into the Sangha aulacogen. The sea at that time was restricted to a long graben-like basin, while a remaining area of marine sedimentation persisted into the Palaeozoic. Thus the pattern of end-Proterozoic carbonate sedimentation on the borders of the Central African craton can be interpreted in terms of an overall gently sloping ramp model with progradation converging towards the Sangha aulacogen.     

华北克拉通北缘中段古元古代强过铝质花岗岩地球化学特征及其构造意义

华北克拉通北缘中段沿集宁—凉城—千里山一线分布着大量的强过铝质花岗岩。与一般强过铝质花岗岩相比,其SiO,含量、A120卵i02比值（小于100）、Rb／Sr比值和Rb／Ba比值低,但CaO／Na2O比值高（大于0．3）。稀土元素复杂,正Eu异常、负Eu异常、Eu异常不明显均发育,大致可以分成2种类型：第一类具有中等程度轻稀土富集、重稀土平缓;第二类轻稀土特征与第一类一致,但其重稀土变化大。稀土元素特征不一致主要是由于其源岩不同。LILE（K、Rb、Ba）相对富集,HSFE（Nb、Ti、P）亏损,这种地球化学特征暗示该区强过铝质花岗岩的源区成分为杂砂岩,熔融温度较高,来源较深,其构造环境与澳大利亚拉克伦造山带—致,属高温型碰撞带,应为华北克拉通西部陆块和东北陆块古元古代碰撞峰期后岩石圈伸展的产物。     

中蒙边界亚干变质核杂岩糜棱状钾质花岗岩—早中生代收缩与伸展构造体制的转换标志

中、晚中生代大型推覆和伸展构造的发育是华北地块及北缘构造的一大特色。本文在典型伸展构造亚干变质核杂岩中识别出早期具拉张环境的糜棱状花岗岩,岩石类型为钾质花岗岩及钾玄岩系列,具造山后及板内拉张构造环境特点,锆石U-Pb定年为228±7Ma,这些岩石强烈变形,并发育同侵位岩浆流动和高温固态流动变形。这些特征表明在晚中生代该核杂岩成型之前即早中生代中下地壳层就已经历调整、减薄和伸展变形。它们的组合揭示了一种特有的构造动力学机制和背景。     

鲁中隆起北缘地热区岩溶热储水化学特征及形成机理

通过研究鲁中隆起北缘岩溶热储地热水水化学特征,分析岩溶热储地热水的形成机理。以鲁中隆起北缘地热区内32个地热井为研究对象,采用Piper图解、Schoeller图解、离子组分比率特征、同位素特征、Na−K−Mg平衡图解、矿物饱和指数计算与评价等方法,研究地热水的补给来源、水岩作用过程、运移途径、循环演化特征和形成机理。结果表明：在岩溶地下水向地热田径流方向上,岩溶地下水的水化学类型与地热水的明显不同,岩溶水的TDS含量呈增加趋势,而${\rm{HCO}}_3^{-}$含量百分比呈现降低趋势;同一地热田区段的地热水水化学成分相似、水化学类型相同。研究区地下热水中微量组分F、Li、Sr、偏硅酸等含量表现出增加的趋势。通过使用玉髓地热温标计算的地热区岩溶热储温度为39~70 ℃,地热水的循环深度为856~1 877 m,结合测温曲线特征分析,确定研究区地热系统为对流−传导型。水文地球化学演化、离子组分比率、同位素等多种分析方法,揭示出岩溶热储地热水的补给来源为地热区南部泰山北麓山区晚更新世寒冷气候条件下古大气降水的入渗;大气降水入渗补给岩溶含水层后,经地下水深循环径流运移,在大地热流、地下水运移传导−对流加热作用下形成地热水。     

Late Paleozoic granitoids in western Transbaikalia: sequence of formation,sources of magmas,and geodynamics

The evolution of Late Paleozoic granitoid magmatism in Transbaikalia shows a general tendency for an increase in the alkalinity of successively forming intrusive complexes: from high-K calc-alkaline granites of the Barguzin complex (Angara–Vitim batholith) at the early stage through transitional from calc-alkaline to alkaline granites and quartz syenites (Zaza complex) at the intermediate stage to peralkaline granitoids (Early Kunalei complex) at the last stage. This evolution trend is complicated by the synchronous development of granitoid complexes with different sets and geochemical compositions of rocks. The compositional changes were accompanied by the decrease in the scales of granitoid magmatism occurrence with time. Crustal metaterrigenous protoliths, possibly of different compositions and ages, were the source of granitoids of the Angara–Vitim batholith. The isotopic composition of all following granitoid complexes points to their mixed mantle–crustal genesis. The mechanisms of granitoid formation are different. Some granitoids formed through the mixing of mantle and crustal magmas; others resulted from the fractional crystallization of hybrid melts; and the rest originated from the fractional crystallization of mantle products or the melting of metabasic sources with the varying but subordinate contribution of crustal protoliths. Synplutonic basic intrusions, combined dikes, and mafic inclusions, specific for the post-Barguzin granitoids, are direct geologic evidence for the synchronous occurrence of crustal and mantle magmatism. The geodynamic setting of the Late Paleozoic magmatism in the Baikal folded area is still debatable. Three possible models are proposed: (1) mantle plume impact, (2) active continental margin, and (3) postcollisional rifting. The latter model agrees with the absence of mafic rocks from the Angara–Vitim batholith structure and with the post-Barguzin age of peralkaline rocks of the Vitim province.     

Two types of Archaean supracrustal belts in the Bundelkhand craton,India: Geology,geochemistry, age and implication for craton crustal evolution

Groundwater sampling was carried out in fast growing Vijayawada urban agglomeration in Andhra Pradesh state with a view to generate base line data and to assess groundwater quality, its variations vis-à-vis urbanization and hydrochemical characteristics. The groundwaters are found to be fresh to brackish, hard to very hard, often enriched with nitrate, phosphate,and faecal colliform indicating anthropogenic influence on groundwater on account of urbanization. Manganese and iron of geogenic origin are also found to be in high concentrations in some localities. The quality deterioration is more in shallow aquifers, as also in core urban and red soil covered areas. Spatial variations in groundwater quality are discernible, with high concentrations of SO₄^-4–,NO₃^- in core urban area suggesting the impact of urbanization. A wide range of chemical constituents indicate water rock interactions as influenced by anthropogenic activities controlling the urban aquifers. The order of abundance of cations is Na>Mg>Ca>K, while the order of abundance of anions is Cl>HCO₃>SO₄>NO₃. The baseline study suggests the need to protect the groundwater resource through sound environmental protection measures for the welfare of the inhabitants in the city around which the new capital is proposed to be constructed for newly formed Andhra Pradesh state in India.     

Origin of Middle Cambrian and Late Silurian potassic granitoids from the western Kunlun orogen,northwest China: a magmatic response to the Proto-Tethys evolution

This paper presents new SHRIMP zircon U–Pb chronology, major and trace element, and Sr–Nd–Hf isotopic data of two Early Paleozoic granitic plutons (Yierba and North Kudi) from the western Kunlun orogen, in attempt to further constrain the Proto-Tethys evolution. SHRIMP zircon U–Pb dating shows that the Yierba pluton was emplaced in the Middle Cambrian (513?±?7 Ma) and the North Kudi pluton was emplaced in the Late Silurian (420.6?±?6.3 Ma). The Yierba pluton consists of quartz monzodiorite, quartz monzonite and granodiorite. These granitoids are metaluminous and potassic, with initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7072–0.7096, ε_Nd (T) of ?0.2 to ?1.6 and ε_Hf (T) (in-situ zircon) of ?1.2. Elemental and isotopic data suggest that they were formed by partial melting of subducted sediments, with subsequent melts interacting with the overlying mantle wedge in an oceanic island arc setting in response to the intra-oceanic subduction of Proto-Tethys. The North Kudi pluton consists of syenogranite and alkali-feldspar granite. These granites are metaluminous to weakly peraluminous and potassic. They show an affinity of A₁ subtype granite, with initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7077–0.7101, ε_Nd (T) of ?3.5 to ?4.0 and ε_Hf (T) (in-situ zircon) of ?3.9. Elemental and isotopic data suggest that they were formed by partial melting of the Precambrian metamorphic basement at a shallow depth (<30 km) during the post-orogenic regime caused by Proto-Tethyan oceanic slab break-off. Our new data suggest that the subduction of the Proto-Tethyan oceanic crust was as early as Middle Cambrian (～513 Ma) and the final closure of Proto-Tethys was not later than Late Silurian (～421 Ma), most probably in Middle Silurian.     

Constraints on Archaean crustal evolution of the Zimbabwe craton: a U-Pb zircon,Sm-Nd and Pb-Pb whole-rock isotope study

 The U-Pb ages of zircons from seven felsic volcanic and plutonic rocks from northern Zimbabwe combined with field data and Pb-Pb and Sm-Nd whole-rock isotope data, constrain the timespan of development of the Harare-Shamva granite-greenstone terrain and establish the relative involvement of juvenile mantle-derived and reworked crustal material. Basement-cover field relationships and isotope and geochemical data demonstrate that the greenstones were deposited onto 3.2–2.8 Ga basement gneisses, in ensialic, continental basins. Geodynamic models for the generation of the areally extensive bimodal magmatic products and growth of the pre-existing crustal nucleus consistent with our interpretations are rift-related: (1) intracontinental rifting related to mantle plume activity or; (2) rifting in a back-arc environment related to a marginal volcanic arc. The data, in conjunction with field evidence, do not indicate the presence and accretion of an older (ca. 2.70 Ga) and a younger (ca. 2.65 Ga) greenstone sequence in the Harare part of the greenstone belt, as was recently postulated on the basis of SHRIMP zircon ages. Zircon ages for basal felsic volcanics (2715±15 Ma) and a subvolcanic porphyry (2672±12 Ma) constrain the initiation and termination of deposition of the greenstone sequence. The timespan of deposition of the Upper Bulawayan part of the greenstone sequence corresponds well with radiometric ages for Upper Bulawayan greenstones in the central and southern part of the craton and supports the concept of craton-wide lithostratigraphic correlations for the late Archaean in Zimbabwe. Zircon ages for a syn-tectonic gneiss (2667±4 Ma) and a late syn-tectonic intrusive granodiorite (2664±15 Ma) pinpoint the age of deformation of the greenstone sequence and compare well with a Pb-Pb age of shear zone related gold mineralization (2659±13 Ma) associated with the latter intrusive phase. The intimate timing relation of greenstone deformation and granitoid emplacement, but also the metamorphic evidence for a thermal effect of the batholiths on the surrounding greenstone belts, and the structural and strain patterns in the greenstone sequence around and adjacent to the batholiths, imply that the intrusion of the granitoids had a significant influence on the tectono-thermal evolution of the greenstone belt. Prolonged magmatic activity is indicated by the zircon ages of small, post-tectonic plutons intrusive into the greenstone belt, with a mineralized granodiorite dated at 2649±6 Ma and an unmineralized tonalite at 2618± 6 Ma. The 2601±14 Ma crystallization age of an “external” Chilimanzi-type granite agrees well with existing radiometric ages for similar granites within the southern part of the craton, demonstrating a craton-wide event and heralding cratonization. The similarity between U-Pb zircon ages and T_DM model ages (2.65–2.62 Ga) and the positive ɛNd_T values (+3 to +2) for the late syn-tectonic and post-tectonic intrusive plutons within the greenstone belt indicate magmatism was derived directly from the mantle or by anatexis of lower crustal sources, with very short crustal residence times, and minor contamination with older crust. The rather high model μ₁ values (8.2–8.6) are unlikely to indicate the involvement of significant amounts of older crust and may be inherited from a high U/Pb mantle source, as was suggested by previous workers for the Archaean mantle beneath Southern Africa. The older T_DM ages for the felsic volcanics (3.0–2.8 Ga) and the porphyries (2.8–2.7 Ga) suggest that these felsic magmas were derived by partial melting of a source that was extracted from the mantle ca. 200 Ma prior to volcanism or may indicate interaction between depleted mantle-derived melts and older crustal material. Received: 15 August 1995 / Accepted: 12 January 1996     

Aptian bimodal volcanic associations and granitoids in the northern margin of the Amur microcontinent: Age, sources of material, and geodynamic environments

Data on the composition, age, and source of material of Aptian rocks composing a bimodal volcanic complex and related granitoids in the northern margin of the Amur microcontinent indicate that the granodiorites of the Talalinskii Massif and subalkaline granites of the Dzhiktandiunskii Massif crystallized at 117 ± 2 and 119 ± 2 Ma, respectively (⁴⁰Ar/³⁹Ar method), and their crystallization ages coincide with the age of volcanic rocks of the Gal’kinskii bimodal complex. These data make it possible to combine the rocks within a single volcano-plutonic association. Geochemical and isotopic-geochemical features of trachybasaltic andesites of the Gal’kinskii bimodal complex suggest that the parental melts were derived from such sources as PREMA (or DM) and an enriched source of the EMII type at a subordinate contribution of a crustal source. The parental melts of rhyolites of the Gal’kinskii Complex and granitoids of the Talalinskii and Dzhiktandinskii massifs were derived from crustal material with minor amounts of juvenile material. The bimodal volcanic association and related granitoids dated at 119–115 Ma were most likely formed in geodynamic environments implying the ascent of the asthenospheric mantle.     

华北克拉通中部早白垩世岩石圈地幔性质：来自狐偃山科头钾质正长岩年代学、同位素的限制

华北克拉通东部中生代岩石圈减薄已经取得了大量进展,相比之下对克拉通中部岩石圈演化认识不足。本文对华北克拉通中部狐偃山杂岩体中科头正长岩进行了锆石U-Pb年代学、主量和微量元素、Sr-Nd-Hf同位素地球化学研究。LA-ICP-MS锆石U-Pb定年结果显示,科头正长岩侵位于早白垩世晚期(～111Ma)。岩石样品均为钾质—超钾质,属于碱性系列岩石。这些岩石相对富集轻稀土,亏损重稀土和中稀土,具有明显的正Eu异常(Eu/Eu~*=1.22～1.96)、富集大离子亲石元素LILE(Rb、Sr、Ba),亏损高场强元素HFSE(Nb、Ta、Ti)。所有岩石样品具有相对高的初始~(87)Sr/~(86)Sr比值(0.7058～0.7062)和低的ε_(Nd)(t)(-10.4～—11.1),ε_(Hf)(t)介于—12.2～—5.2之间。详细的元素和同位素地球化学研究表明科头正长岩的原始岩浆可能来源于富集岩石圈地幔中富金云母的尖晶石二辉橄榄岩源区的部分熔融,岩浆演化过程中经历了地壳混染与结晶分异(AFC)过程。结合华北克拉通中部岩浆资料,将华北克拉通中部早白垩世岩浆作用分为早晚两期:早期(123～135Ma)为岩浆作用高峰期,晚期(～114Ma)为最后一期弱的岩浆期;华北克拉通中部陆下岩石圈地幔是富集的、不均一的,其至少在早白垩世(138～111Ma)一直处于减薄状态。