昌宁-孟连缝合带干龙塘-弄巴蛇绿岩地球化学及Sr-Nd-Pb同位素组成研究

本文对三江古特提斯昌宁-孟连带中段弄巴-干龙塘蛇绿混杂岩进行了详细的主量、微量元素及Sr-Nd-Pb同位素地球化学研究。结果表明,弄巴玄武岩包括拉斑系列和碱性系列,弄巴拉斑玄武岩具有高TiO2和低K2O的特征,(La/Yb)N介于1.87～2.38之间,岩石的Sr-Nd-Pb同位素组成和典型MORB十分相似,结合岩石较高的Th/Yb和低的Zr/Nb值,可以认为弄巴拉斑玄武岩具有富集型洋脊玄武岩(E-MORB)的特征,可能起源于富集的地幔源区或是亏损地幔源区和地幔柱发生交代作用的结果。弄巴碱性玄武岩具有较高的TiO2(2.38%)和K2O(2.37%)含量,(La/Yb)N=11.19,富集轻稀土,表现出典型的碱性OIB的特征,可能是大洋板内热点浅部熔融的产物。干龙塘拉斑玄武岩具有高TiO2、Mg#,低K2O和亏损轻稀土等特征,表现出N-MORB的地球化学特征,岩石的Sr-Nd-Pb与MORB相似,表明岩石起源于亏损的地幔源区。     

Geochemistry and origin of metabasites from the granulite-gneiss complex of the Angara-Kan block,southwestern Siberian craton

In this paper, we present data on major and trace elements in highly metamorphosed mafic rocks from the granulite-gneiss complex of the Angara-Kan block (southwestern Siberian craton), identify igneous protoliths of the metabasites, and assess the mobility of elements during metamorphism. Two types of rocks with different geologic relations and compositions were recognized. Garnet-bearing two-pyroxene granulites (Cpx + Pl + Grt + Opx) occur as sheet- and boudin-like bodies, which were folded and deformed with their host paragneisses. Dikes, which in most cases underwent only brittle deformation, are composed of metabasites characterized by the assemblage Cpx + Hbl + Pl + Grt. The major element compositions of igneous protoliths for the mafic granulites and metabasite dykes correspond to variously differentiated basaltic magmas. The protoliths of the metabasites are depleted in K₂O, LILE, Zr, Nb, and LREE and were derived from a depleted mantle source. The major and trace element compositions of the dike metabasites are similar to those of low-K tholeiitic basalts of oceanic island arcs. Continental intraplate basalts derived from an enriched mantle source are possible igneous protoliths for the mafic granulites enriched in Ba, LREE, Nb, Ta, Zr, and Hf. It is assumed that lower Rb, Th, and U contents in the mafic granulites compared with continental flood basalts, high K/Rb and La/Th, and moderate Th/U ratios reflect the loss of Rb, Th and U during granulite-facies metamorphism.     

Trace-Element Modeling and Source Constraints for Tholeiitic and Cale-alkaline Basalts from a Depleted Asthenospheric Mantle Source,Mt. Erciyes Stratovolcano,Turkey

The Mt. Erciyes stratovolcano was built up in an intraplate tectonic environment as a consequence of Eurasian and Afro-Arabian continental collision. However, the volcanic products generally exhibit a calc-alkaline character; minor amounts of tholeiitic basalts are also present. Tholeiitic basalts show high Fe₂O₃, MgO, CaO, low K₂O, and depleted Ba, Nb, and especially Rb (2.3-5.97 ppm) contents, low ⁸⁷Sr/⁸⁶Sr (0.703344-0.703964), and high ¹⁴³Nd/¹⁴⁴Nd (0.512920-0.512780) isotopic ratios. These compositional features show that they were derived from a depleted asthenospheric mantle source, possibly a MORB-like source component. In contrast, calc-alkaline basaltic rocks exhibit relatively high large-ion-lithophile and high-field-strength elements, high ⁸⁷Sr/⁸⁶Sr (0.704591-0.70507) and low ¹⁴³Nd/¹⁴⁴Nd (0.51272-0.512394) isotopic ratios.

The bulk-rock chemistry of the tholeiitic basalts reflects the chemical composition of the extracted source component. Furthermore, trace-element concentrations may be calculated from an accepted mantle source component (starting composition) for different degrees of partial melting. These calculations also provide a sensitive approach to the origin of tholeiitic basalts. Modeled trace-element compositions of tholeiitic basalts are calculated from a primitive mantle composition. Calculated trace-element compositions imply that tholeiitic basalts are derived by minor fractional melting (1-1.5 %), in the absence of assimilation or deep-crustal melting. The calc-alkaline basalts were subsequently produced from initially tholeiitic basalts by the way of an AFC (assimilation-fractional crystallization) process, with a crustal assimilation of 10-15 %.

The geochemical data, partial melting, and AFC modeling all indicate that basaltic products have a complex evolutionary history involving partial melting from a MORB-like mantle source. The assimilation and fractional crystallization processes are considered as providing an example for the chemical evolution of basaltic products, from tholeiitic to calc-alkaline, in an intraplate environment.     

Neoproterozoic nascent island arc volcanism from the Nubian Shield of Egypt: Magma genesis and generation of continental crust in intra-oceanic arcs

The Neoproterozoic Wadi Ranga metavolcanic rocks, South Eastern Desert of Egypt, constitute a slightly metamorphosed bimodal sequence of low-K submarine tholeiitic mafic and felsic volcanic rocks. The mafic volcanic rocks are represented by massive and pillow flows and agglomerates, composed of porphyritic and aphyric basalts and basaltic andesites that are mostly amygdaloidal. The felsic volcanic rocks embrace porphyritic dacites and rhyolites and tuffs, which overlie the mafic volcanic rocks. The geochemical characteristics of Wadi Ranga volcanic rocks, especially a strong Nb depletion, indicate that they were formed from subduction-related melts. The clinopyroxene phenocrysts of basalts are more akin to those crystallizing from island-arc tholeiitic magmas. The tholeiitic nature of the Wadi Ranga volcanics as well as their LREE-depleted or nearly flat REE patterns and their low K₂O contents suggest that they were developed in an immature island arc setting. The subchondritic Nb/Ta ratios (with the lowest ratio reported for any arc rocks) and low Nb/Yb ratios indicate that the mantle source of the Wadi Ranga mafic volcanic rocks was more depleted than N-MORB-source mantle. Subduction signature was dominated by aqueous fluids derived from slab dehydration, whereas the role of subducted sediments in mantle-wedge metasomatization was subordinate, implying that the subduction system was sediment-starved and far from continental clastic input. The amount of slab-derived fluids was enough to produce hydrous magmas that follow the tholeiitic but not the calc-alkaline differentiation trend. With Mg# > 64, few samples of Wadi Ranga mafic volcanic rocks are similar to primitive arc magmas, whereas the other samples have clearly experienced considerable fractional crystallization.The low abundances of trace elements, together with low K₂O contents of the felsic metavolcanic rocks indicate that they were erupted in a primitive island arc setting. The felsic volcanic rocks are characterized by lower K/Rb ratios compared to the mafic volcanic rocks, higher trace element abundances (~ 2 to ~ 9 times basalt) on primitive arc basalt-normalized pattern and nearly flat chondrite-normalized REE patterns, which display a negative Eu anomaly. These features are largely consistent with fractional crystallization model for the origin of the felsic volcanic rocks. Moreover, SiO₂-REE variations for the Wadi Ranga volcanic rocks display steadily increasing LREE over the entire mafic to felsic range and enriched La abundances in the felsic lavas relative to the most mafic lavas, features which are consistent with production of the felsic volcanic rocks through fractional crystallization of basaltic melts. The relatively large volume of Wadi Ranga silicic volcanic rocks implies that significant volume of silicic magmas can be generated in immature island arcs by fractional crystallization and indicates the significant role of intra-oceanic arcs in the production of Neoproterozoic continental crust. We emphasize that the geochemical characteristics of these rocks such as their low LILE and nearly flat REE patterns can successfully discriminate them from other Egyptian Neoproterozoic felsic volcanic rocks, which have higher LILE, Zr and Nb and fractionated REE patterns.     

Continuous supply of recycled Pacific oceanic materials in the source of Cenozoic basalts in SE China: the Zhejiang case

Various enriched recycled oceanic components in the source of Cenozoic intra-plate alkaline basalts from eastern China were identified by previous studies. Due to the existence of a stagnant subducted Pacific slab in the mantle transition zone beneath eastern China, it is logical to connect the stagnant slab to the recycled oceanic materials. However, the recycled oceanic materials could also result from ancient subduction events (e.g., Paleo-Tethyan, Paleo-Asian or Izanagi plate subduction) because enriched geochemical signatures of a recycled slab can be preserved in the mantle for longer than 1 Gyr. Investigating the temporal variations of the recycled oceanic materials in the mantle source is a useful way to trace the origin of the basalts. In this article, we have conducted a detailed geochemical study, including major and trace elements and Sr–Nd–Pb isotopes, on two alkaline basalt groups from Zhejiang, SE China, which erupted 26–17 Ma and after 11 Ma, respectively. In particular, we recovered the H₂O content of the initial magmas based on the H₂O content of the clinopyroxene (cpx) phenocrysts and the partition coefficients of H₂O between cpx and basaltic melts. The H₂O contents of the Zhejiang basalts range from 1.3 to 2.6 (wt.%), which fall within the range of back-arc basin or island arc basalts. The older basalts are more alkaline and have lower Si and Al contents; higher trace element concentrations; higher La/Yb, Ce/Pb and Nb/La ratios; lower H₂O/Ce and Ba/Th ratios; and stronger negative K, Pb, Hf and Ti anomalies than the younger ones. The co-relationships between Ba/La, H₂O/Ce, Nb/La, Ce/Pb and Ba/Th in the two groups of the Zhejiang basalts indicate that a recycled dehydrated oceanic alkaline basalt component is needed in the source of the older rocks, along with a depleted mantle component. Meanwhile, an additional recycled dehydrated sediment component was required in the source of the younger rocks. The temporal change in the recycled oceanic materials in the mantle sources of Zhejiang Cenozoic basalts demonstrates that the recycled components can only originate in the stagnant Pacific slab that is the only plate subducted since 100 Ma in this area.     

Discovery of the Plagiogranites in the Diyanmiao Ophiolite,Southeastern Central Asian Orogenic Belt,Inner Mongolia,China and Its Tectonic Significance

In this study, plagiogranites in the Diyanmiao ophiolite of the southeastern Central Asian Orogenic Belt(Altaids) were investigated for the first time. The plagiogranites are composed predominantly of albite and quartz, and occur as irregular intrusive veins in pillow basalts. The plagiogranites have high SiO_2(74.37–76.68 wt%) and low Al_2O_3(11.99–13.30 wt%), and intensively high Na_2O(4.52–5.49 wt%) and low K_2O(0.03–0.40 wt%) resulting in high Na_2O/K_2O ratios(11.3–183). These rocks are classified as part of the low-K tholeiitic series. The plagiogranites have low total rare earth element contents(∑REE)(23.62–39.77 ppm), small negative Eu anomalies(δEu=0.44–0.62), and flat to slightly LREE-depleted chondrite-normalized REE patterns((La/Yb)N=0.68–0.76), similar to N-MORB. The plagiogranites are also characterized by Th, U, Zr, and Hf enrichment, and Nb, P, and Ti depletion, have overall flat primitivemantle-normalized trace element patterns. Field and petrological observations and geochemical data suggest that the plagiogranites in the Diyanmiao ophiolite are similar to fractionation-type plagiogranites. Furthermore, the REE patterns of the plagiogranites are similar to those of the gabbros and pillow basalts in the ophiolite. In plots of SREE–SiO_2, La–SiO_2, and Yb–SiO_2, the plagiogranites, pillow basalts, and gabbros show trends typical of crystal fractionation. As such, the plagiogranites are oceanic in origin, formed by crystal fractionation from basaltic magmas derived from depleted mantle, and are part of the Diyanmiao ophiolite. LA–ICP–MS U–Pb dating of zircons from the plagiogranites yielded ages of 328.6±2.1 and 327.1±2.1 Ma, indicating an early Carboniferous age for the Diyanmiao ophiolite. These results provide petrological and geochronological evidence for the identification of the Erenhot–Hegenshan oceanic basin and Hegenshan suture of the Paleo-Asian Ocean.     

Metabasalts and metagabbros from the Llano Uplift,Texas: Petrologic and geochemical characterization with emphasis on tectonic setting

The 1.0-1.2 b.y. old rocks of the southeastern Llano Uplift, Texas include a 7 km thick sequence of amphibolite-grade, stratified, mafic metasedimentary rock (Packsaddle Schist) which is intruded by a varied suite of syntectonic and late-kinematic intrusions. The metasediments contain large blocks of serpentinized peridotite (Coal Creek serpentinite) and coarse hornblendite and metagabbro. Prior to the end of maximum deformation, the sequence was intruded by low to medium-K₂O tholeiitic basalts (0.40-0.72% K₂O). Late-kinematic low-K₂O tholeiitic basalts (0.38-0.40% K₂O) were intruded as dikes into the folded rocks. The Coal Creek serpentinite contains both syntectonic and late-kinematic low-K₂O tholeiitic metabasalts (0.13–0.36% K₂O). The Llano metabasalts and metagabbros are characterized by low Cr (67–378 ppm) and Ni (36–170 ppm), variable Rb (1.5–14.7 ppm), Sr (140–1229 ppm), TiO₂ (0.40–2.20%), P (568–2707 ppm), and Zr (18–230 ppm), and Y (16–45 ppm), Co (40–57 ppm), and Sc (36–49 ppm) similar to modern MORB. The metabasalts have La abundances from 7 to 39 times chondritic and exhibit light REE enrichment with ¦La/Sm¦_N from 1.13 to 1.45 and ¦La/Yb¦_N from 1.12 to 2.99. The metabasalts show a strong correlation of increasing Zr, Ti, and Y and decreasing Eu/Eu^* (1.56–1.00), CaO/TiO₂, Al₂O₃/TiO₂, and MgO/ MgO+FeO^* with increasing REE enrichment (La_N). The Llano metabasalts and metagabbros have initial ⁸⁷Sr/⁸⁶Sr=0.7029±0.0001. A likely petrogenetic model for these metabasalts is an island arc in which events from early arc development to final late-kinematic intrusion were dominated by tholeiitic volcanism and intrusion. The chemical systematics of the Llano mafic metaigneous rocks suggest they are products of fractionation of olivine, clinopyroxene, and plagioclase from more primitive basaltic magmas generated beneath the island arc complex.     

Hf-Nd isotope evidence for contemporaneous subduction processes in the source of late Archean arc lavas from the Superior Province, Canada

Volcanic suites from Wawa greenstone belts in the southern Superior Province comprise an association of typical late Archean arc volcanic rocks including adakites, magnesian andesites (MA), niobium-enriched basalts (NEB), and ‘normal’ tholeiitic to calc-alkaline basalts to rhyolites. The adakites represent melts from subducted oceanic crust and all other suites were derived from the mantle wedge above the subducting oceanic lithosphere. The magnesian andesites are interpreted to be the product of hybridization of adakite melts with arc mantle wedge peridotite. The initial ?_Hf values of the ∼2.7 Ga Wawa adakites (+3.5 to +5.2), magnesian andesites (+2.6 to +5.1), niobium-enriched basalts (+4.4 to +6.6), and ‘normal’ tholeiitic to calc-alkaline arc basalts (+5.3 to +6.4) are consistent with long-term depleted mantle sources. The niobium-enriched basalts and ‘normal’ arc basalts have more depleted ?_Hf values than the adakites and magnesian andesites. The initial ?_Nd values in the magnesian andesites (+0.4 to +2.0), niobium-enriched basalts (+1.4 to +2.4), and ‘normal’ arc tholeiitic to calc-alkaline basalts (+1.6 to +2.9) overlap with, but extend to lower values than, the slab-derived adakites (+2.3 to +2.8). The lower initial ?_Nd values in the mantle-wedge-derived suites, particularly in the magnesian andesites, are attributed to recycling of an Nd-enriched component with lower ?_Nd to the mantle wedge. As a group, the slab-derived adakites plot closest to the 2.7 Ga depleted mantle value in ?_Nd versus ?_Hf space, additionally suggesting that the Nd-enriched component in the mantle wedge did not originate from the 2.7 Ga slab-derived melts. Accordingly, we suggest that the enriched component had been added to the mantle wedge at variable proportions by recycling of older continental material. This recycling process may have occurred as early as 50-70 Ma before the initiation of the 2.7 Ga subduction zone. The selective enrichment of Nd in the sources of the Superior Province magmas can be explained by experimental studies and geochemical observations in modern subduction systems, indicating that light rare earth elements (e.g., La, Ce, Sm, Nd) are more soluble than high field strength elements (e.g., Zr, Hf, Nb, Ta) in aqueous fluids that are derived from subducted slabs. As a corollary, we suggest that the recycled Nd-enriched component was added to the mantle source of the Wawa arc magmas by dehydration of subducted sediments.     

Geochemistry and Sr–Nd isotope composition of Carboniferous volcanic rocks of the Jueluotage Orogenic Belt: implications for the tectonic evolution of Eastern Tianshan,China

The major and trace element, and Sr–Nd isotopic compositions of the Carboniferous Qi’eshan, Wutongwozi, and Yamansu volcanic rocks from the northern and southern parts of the Jueluotage Orogenic Belt in East Tianshan, China, were analysed to understand their genesis and geodynamic implications. The early Carboniferous Qi’eshan basalts are characterized by high Al₂O₃, with La/Sm (1.38–1.79) and Ba/La (27.06–58.76) values higher than those of typical normal mid-ocean ridge basalt. They are relatively enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), and depleted in high field strength elements. Overall, their initial Nd–Sr isotopic compositions are ε_Nd(t) = (5.6–7.0) and Isr = 0.70397–0.70429, implying the magma originated from a mantle wedge source that was metasomatized by subduction-related fluids. In contrast, the late Carboniferous Wutongwozi basalts have lower Ba/La (4.86–12.82), La/Nb (0.87–2.45), and LILE concentrations. They have the isotopic characteristics of depleted asthenosphere, relatively high and heterogeneous ε_Nd(t) (9.3–9.4), and high Isr (0.70471–0.70533). Thus, the late Carboniferous Wutongwozi basalts may have been derived from the partial melting of mantle sources during asthenospheric upwelling. The early Carboniferous Yamansu acid volcanic rocks are characterized by high Mg# (46–48) and Lu/Y (~0.15), and low K₂O/Na₂O (0.01–0.20), similar to M-type granites. However, their ε_Nd(t) (5.0–5.5) and Isr (0.70642–0.70768) values are lower than those of depleted mantle, indicating they were contaminated by lower crustal material. The magma source originated from a mantle-derived magma that was contaminated by middle Tianshan massif in a continental margin arc setting. Based on the results and previous field-based studies, we conclude that the Carboniferous volcanics in the Jueluotage Orogenic Belt formed in a complex trench–arc–basin setting in the Kuguertage–Aqikuduke Suture Zone.     

Potassium and Associated Elements in Tholeiitic Basalts

Selected average analyses of tholeiitic basalts, which havemarkedly differing contents of K₂O, show a broad coherence betweenK, P, Ti, Ba, Rb, Sr, and Zr. The content of K₂O in the averageanalyses of 43 tholeiitic provinces shows a wide range (0.07–1.61per cent) and has a positively skewed frequency distribution.K₂O in individual tholeiitic basalts and dolerites has a similardistribution pattern. The relationship between K₂O and MgO in the average analysesindicates that the range of K₂O content cannot be solely theresult of near-surface differentiation, nor, indeed, any differentiationprocess which is indexed by the MgO content of the differentiates. Processes which could effect an enrichment of K and associatedelements in tholeiites are assessed and it is concluded thatdegree of melting, eclogite fractionation at high pressure,and mantle wall-rock reaction are likely to be significant factors.Crustal contamination is considered not to be a generally significantfactor in the development of continental tholeiites. The concept that all other tholeiites are derived from a parentalmagma with the composition of low-K deep oceanic tholeiitesis rejected.     

Dynamic melting of the Precambrian mantle: evidence from rare earth elements of the amphibolites from the Nellore–Khammam Schist Belt, South India

The Nellore–Khammam Schist Belt (NKSB) in South India is a Precambrian greenstone belt sited between the Eastern Ghats Mobile Belt (EGMB) to the east and the Cratonic region to the west. The belt contains amphibolites, granite gneisses and metasediments including banded iron formations. Amphibolites occurring as dykes, sills and lenses—in and around an Archaean layered complex—form the focus of the present study. The amphibolites are tholeiitic in composition and are compositionally similar to Fe-rich mafic rocks of greenstone belts elsewhere. The NKSB tholeiites show highly variable incompatible trace element abundances for similar Mg#s, relatively constant compatible element concentrations, and uniform incompatible element ratios. Chondrite-normalized REE patterns of the tholeiites range from strongly LREE depleted ((La/Yb) _N = 0.19) to LREE enriched ((La/Yb) _N = 6.95). Constant (La/Ce) _N ratios but variable (La/Yb) _N values are characteristic geochemical traits of the tholeiites; the latter has resulted in crossing REE patterns especially at the HREE segment. Even for the most LREE depleted samples, the (La/Ce) _N ratios are > 1 and are similar to those of the LREE enriched samples. There is a systematic decrease in FeO^t, K₂O and P₂O₅, as well as Ce and other incompatible elements from the LREE enriched to the depleted samples without any variation in the incompatible element ratios and Mg#s. Neither batch and fractional melting, nor magma chamber processes can account for the non-correlation between the LREE enrichment and HREE concentrations. We suggest that dynamic melting of the upper mantle is responsible for these geochemical peculiarities of the NKSB tholeiites. Polybaric dynamic melting within a single mantle column with variable mineralogy is the likely mechanism for the derivation of NKSB tholeiitic melts. It is possible that the NKSB tholeiites are derived from a source with higher FeO/MgO than that of present day ridge basalts.     

五大连池等地钾质碱性玄武岩、幔源包体的微量元素特征及成因信息

黑龙江省五大连池、科洛、二克山火山岩的成因,是一个重要而又复杂的问题.作者根据岩相学、地球化学、模式计算等一系列工作,认为这组钾质玄武岩是含金云母的尖晶石二辉橄榄岩低度局部熔融的产物。岩浆在上升过程中,经历了结晶分异和同化混染,从而形成一套具成因联系的新生代大陆板内钾质碱性玄武岩。本文试从微量元素方面对此结论提供证据,并显示钾质熔岩及幔源包体的微量元素特征,由此说明地幔交代作用在钾质系列岩浆形成过程中的重要性。     

Geochemistry of lavas from Mohns Ridge,Norwegian-Greenland Sea: implications for melting conditions and magma sources near Jan Mayen

 Mohns Ridge lavas between 71 and 72°30′N (∼360 km) have heterogeneous compositions varying between alkali basalts and incompatible-element-depleted tholeiites. On a large scale there is a continuity of incompatible element and isotopic compositions between the alkali basalts from the island Jan Mayen and Mohns Ridge tholeiites. The variation in isotopes suggests a heterogeneous mantle which appears to be tapped preferentially by low degree melts (∼5%) close to Jan Mayen but also shows its signature much further north on Mohns Ridge. Three lava types with different incompatible element compositions [e.g. chondrite-normalized (La/Sm)_N<1 to >2] occur in the area at 72°N and were generated from this heterogeneous mantle. The relatively depleted tholeiitic melts were mixed with a small degree melt from an enriched source. The elements Ba, Rb and K of the enriched melt were probably buffered in the mantle by residual amphibole or phlogopite. That such a residual phase is stable in this region of oceanic mantle suggests both high water contents and low mantle temperatures, at odds with a hotspot origin for Jan Mayen. Instead we suggest that the melting may be induced by the lowered solidus temperature of a “wet” mantle. Mohns MORB (mid ocean ridge basalt) and Jan Mayen area alkali basalts have high contents of Ba and Rb compared to other incompatible elements (e.g. Ba/La >10). These ratios reflect the signature of the mantle source. Ratios of Ce/Pb and Rb/Cs are normal MORB mantle ratios of 25 and 80, respectively, thus the enrichments of Ba and Rb are not indicative of a sedimentary component added to the mantle source but were probably generated by the influence of a metasomatizing fluid, as supported by the presence of hydrous phases during the petrogenesis of the alkali basalts. Geophysical and petrological models suggest that Jan Mayen is not the product of hotspot activity above a mantle plume, and suggest instead that it owes its existence to the unique juxtaposition of a continental fragment, a fracture zone and a spreading axis in this part of the North Atlantic. Received: 3 May 1995 / Accepted: 6 November 1995     

The Pleistocene-Recent Tonga-Kermadec Arc Lavas: Interpretation of New Isotopic and Rare Earth Data in Terms of a Depleted Mantle Source Model

New REE data, and new Nd, O, Sr, and Pb isotopic data are presentedand integrated with previous data for this low-K intra-oceanicarc suite. Geochemically, the arc tholeiites and basaltic andesitesrange from extremely HFS element depleted (northern Tonga) tonear N-MORB-like HFS element abundances in L'Esperance (southernKermadecs). LIL elements (Sr, Rb, K, Rb, Ba, Th) show the characteristicselective enrichment generally recognized in arc magmas, andthus indicate decoupling of the HFS and LIL elements. Modellingsuggests a compositionally variable source (mantle wedge) alongthe arc, ranging from restite after remelting an N-type MORBsource (northern end), to progressively less depleted, MORB-likesources southwards. Thus, the low HFS/LIL element ratios areinterpreted in terms of HFS depletion followed by LIL elementenrichment associated with subduction; broad correlations occurbetween Zr/Ba and Sr/Nd ratios (fractionation corrected) and87Sr/86Sr and ¹⁴³Nd/¹⁴⁴Nd ratios. Derivation of the arc magmas from depleted peridotote requiressuperimposed fractional crystallization, which has been modelledthermodynamically using SILMIN (Ghiorso, 1985), utilizing experimentallyproduced partial melts from depleted lherzolite (Jaques &Green, 1980). It is shown that the arc tholeiites and basalticandesites (and also high Mg-andesites) are potentially developedat low pressures 5 kb), from parental magmas also generatedat relatively low pressure ( 10 kb). These data further suggestthat a southward increasing depth of magma segregation (correlatingwith Benioff Zone geometry) could account for differences inchemistry between the Tonga and Kermadec arc segments. The mechanism of LILE enrichment is still highly problematic,but it is suggested that the model of Tatsumi et al. (1986)may account for much of the geochemical data; this involvesrelatively shallow release, via fluids, of LIL elements intooverlying peridotite beneath the fore-arc region. Induced convectionin the mantle wedge moves the metasomatized mantle into thezones of magma generation. The development of the inferred, variably depleted mantle wedgesource is here related to active back-arc spreading which isslightly older and more rapid behind the northern region ofthe arc. The lherzolite restite from this spreading processis interpreted to undergo further partial melting in the metasomatizedmantle wedge overlying the subduction zone, involving inducedconvection. The back-arc island of Niua fo'ou is geochemically quite distinctfrom the arc magmas, being similar to N-MORB in its trace elements,but to OIB in its isotope ratios.     

钦-杭结合带南段新元古代俯冲作用:来自粤西贵子混杂岩变基性岩年代学和地球化学的证据

新元古代以来,扬子地块和华夏地块拼贴形成钦-杭结合带。粤西贵子混杂岩为揭示钦-杭结合带南段地质演化提供了一个重要的窗口。贵子混杂岩构造岩块为强烈变形-变质的变基性岩、深海硅质岩,基质为石英岩、云母片岩以及具鲍马序列富锰质硅泥质岩等。对混杂岩中变基性岩年代学和元素地球化学分析显示,锆石U-Pb年龄为948±11Ma,除少部分属低钾(拉斑)外,其余均为钙碱性系列,CIPW标准矿物成分属石英拉斑玄武岩-橄榄拉斑玄武岩组合。变基性岩具有高TiO_2(平均1.85%)、P_2O_5(平均0.21%)的特点;无Eu异常或轻微负异常,配分曲线右倾(球粒陨石标准化(La/Yb)N=2.65~3.89),大离子亲石元素(Rb、Ba、Th、U)富集,高场强元素(Nb、Zr、Hf)轻微亏损,表现为洋壳消减作用下岛弧岩浆岩特征。高的Nb(7.56×10~(-6)~16.11×10~(-6),均大于7×10~(-6))、Nb/U(18.39~25.65)、(Nb/La)N(0.76~0.94,原始地幔标准化)显示为岛弧环境富Nb玄武岩。本研究认为,云开地块出露的这套构造混杂岩,是新元古代古华南洋俯冲在钦-杭结合带南段的记录,可为该带的南东边界提供线索。     

西准噶尔早泥盆世马拉苏组火山岩岩石成因研究

新疆西准噶尔北部地区的早泥盆世马拉苏组出露有少量富钠低钾的拉斑质中基性熔岩,这些分布于谢米斯台断裂北侧的玄武安山岩和玄武岩多呈夹层状断续产出于火山碎屑岩之中。马拉苏中基性熔岩的Mg#与主、微量元素协变关系及Th-Th/Nd图反映了其并非同源岩浆演化的结果。马拉苏火山岩中的玄武安山岩富集LILE、亏损HFSE,具有较高的Th含量及较低的Hf/Th和(Nb/Th)PM比值,显示出弧火山岩的地球化学特征。其中的玄武岩则具有略为平坦的稀土元素分配样式,较低的Th含量及较高的Hf/Th和(Nb/Th)PM比值,此同MORB地球化学特征极为相似;虽然其也显示有轻微的LILE富集、HFSE亏损,但是较高的La/Nb比值则暗示这同地壳或俯冲物质组分的卷入有关,且一系列构造环境判别图解也进一步印证了马拉苏组内的玄武岩应属似MORB基性熔岩。此外,两类岩石的高场强元素比值Zr/Nb、Hf/Ta同全球平均大洋中脊玄武岩的相应比值极为接近,反映了马拉苏组中基性火山岩的物质源区主体均为MORB地幔物质源区。La/Yb-Gd/Yb原始地幔标准化比值的模拟计算进一步显示了马拉苏组玄武安山岩与受改造(俯冲沉积物或地壳物质的混染)的石榴子石或尖晶石-石榴子石地幔橄榄岩物质源区的部分熔融作用有关,而似MORB型玄武岩则源自尖晶石地幔橄榄岩源区的部分熔融。结合区内同期的蛇绿岩、火山岩和碱性花岗岩的地球化学研究,我们可以进一步推断此类兼具有似MORB和弧火山岩地球化学特征的早泥盆世马拉苏火山岩应当是西准噶尔地块北部在早古生代受后期俯冲作用影响下经历弧后扩张形成的火山-岩浆地质记录。     

Petrology of basalts from the Mohns-Knipovich Ridge; the Norwegian-Greenland Sea

Major element compositions of submarine basalts, quenched glasses, and contained phenocrysts are reported for samples from 25 dredge stations along the Mohns-Knipovich Ridge between the Jan Mayen fracture zone and 77°30N. Most of the basalts collected on the Jan Mayen platform have a subaerial appearance, are nepheline normative, rich in incompatible elements, and have REE-patterns strongly enriched in light-REE. The other basalts (with one exception) are tholeiitic pillow basalts, many of which have fresh quenched glass rims. From the Jan Mayen platform northeastwards the phenocryst assemblage changes from olivine±plagioclase±clinopyroxene±magnetite to olivine +plagioclase±chrome-spinel. This change is accompanied by a progressive decrease in the content of incompatible elements, light-REE enrichments and elevation of the ridge that are similar to those observed south of the Azores and Iceland hotspots. Pillow basalts and glasses collected along the esternmost part of the Mohns Ridge (450 to 675 km east of Jan Mayen) have low K₂O, TiO₂, and P₂O₅ contents, light-REE depleted patterns relative to chondrites, and Mg/(Mg+Fe²⁺) ratios between 0.64 and 0.60. Pillow basalts and glasses from the Knipovich Ridge have similar (Mg/Mg+Fe²⁺) ratios, but along the entire ridge have slightly higher concentrations of incompatible elements and chondritic to slightly light-REE enriched patterns. The incompatible element enrichment increases slightly northward. Plagioclase phenocrysts show normal and reverse zoning on all parts of the ridge whereas olivines are unzoned or show only weak normal zoning. Olivine-liquid equilibrium temperatures are calculated to be in the range of 1,060–1,206° C with a mean around 1,180° C.Rocks and glasses collected on the Jan Mayen Platform are compositionally similar to Jan Mayen volcanic products, suggesting that off-ridge alkali volcanism on the Jan Mayen Platform is more widespread than so far suspected. There is also evidence to suggest that the alkali basalts from the Jan Mayen Platform are derived from deeper levels and by smaller degrees of partial melting of a mantle significantly more enriched in light-REE and other incompatible elements than are the tholeiitic basalts from the Eastern Mohns and Knipovich Ridge. The possibility of the presence of another hitherto unsuspected enriched mantle region north of 77° 30 N is also briefly considered.It remains uncertain whether geochemical gradients revealed in this study reflect: (1) the dynamics of mixing during mantle advection and magma emplacement into the crust along the Mid-Atlantic Ridge (MAR) spreading axis, (e.g. such as in the mantle plume — large-ion-lithophile element depleted asthenosphere mixing model previously proposed); or (2) a horizontal gradation of the mantle beneath the MAR axis similar to that observed in the overlying crust; or (3) a vertical gradation of the mantle in incompatible elements with their contents increasing with depth and derivations of melts from progressively greater depth towards the Jan Mayen Platform.     

An Archean arc basalt–Nb-enriched basalt–adakite association: the 2.7 Ga Confederation assemblage of the Birch–Uchi greenstone belt, Superior Province

The Uchi subprovince of the Archean Superior Province is a series of greenstone belts extending 600 km east–west along the southern margin of the North Caribou Terrane protocontinent. The 2.7 Ga Confederation tectonostratigraphic assemblage of the Birch–Uchi greenstone belt, northwest Ontario, is dominated by volcanic suites of mafic, intermediate and felsic composition. Tholeiitic basalts range compositionally from Mg# 59–26 evolving continuously to greater REE contents (La=2–19 ppm; Th/La_pm˜1), with small negative Nb anomalies. Primitive tholeiites are similar to modern intraoceanic arc basalts, whereas evolved members extend to greater concentrations of Ti, Zr, V, Sc, and Y, and lower Ti/Zr, but higher Ti/Sc and Ti/V ratios characteristic of back arc basalts. Calc-alkaline basalts to dacites are characterised by more fractionated REE (La/Yb_n=1–8), high Th/Nb_pm ratios and deeper negative Nb anomalies; they plot with modern oceanic arc basalts and some may qualify as high magnesium andesites. The two suites are interpreted as a paired arc–back arc sequence. A third group of Nb-enriched basalts (NEB; Nb=9–18 ppm) extend to extremely high TiO₂, Ta, P₂O₅, Sc and V contents, with strongly fractionated REE and ratios of Nb/Ta and Zr/Hf greater than primitive mantle values whereas Zr/Sm ratios are lower. The most abundant rhyolitic suite has extremely enriched but flat trace element patterns and is interpreted as strongly fractionated tholeiitic basalt liquids. A second group are compositionally similar to Cenozoic adakites and Archean high-Al, high-La/Yb_n tonalites; they possess Yb ≤ 0.4 ppm, Y ≤ 6 ppm and Sc ≤ 8 ppm, with La/Yb_n of 19–30 and Zr/Sm of 50–59. They are interpreted as melts of ocean lithosphere basaltic crust in a hot shallow subduction zone. Adakites are associated with NEB in Cenozoic arcs where there is shallow subduction of young and/or hot ocean lithosphere, often with oblique subduction. Slab melt adakites erupt, or metasomatise sub-arc mantle peridotite to generate an HFSE-enriched source that subsequently melts during induced mantle convection. The Archean adakite–NEB association erupted during development of the tholeiitic to calc-alkaline arc and its associated back arc. Their coexistence in the Confederation assemblage of the Birch–Uchi greenstone belt implies convergent margin processes similar to those in Cenozoic arcs. Received: 2 June 1999 / Accepted: 29 December 1999     

Geochemistry and Petrogenesis of the Proterozoic Bandal Mafic Rocks,Himachal Pradesh,NW Himalaya

The Proterozoic Bandal mafic rocks, exposed in Kullu-Rampur window, Lesser Himalaya, Himachal Pradesh, indicate two distinct (high-Ti and low-Ti) magma types. The high-Ti basalts are characterised by high-TiO₂ (> 2 wt%), Ti/Y, Ti/Zr, TiO₂/K₂O and low Rb/Sr ratios. They are enriched in high field strength (HFS) elements (Nb, Zr, Ti) relative to low field strength (LFS) incompatible elements (K, Rb). The low-Ti basalts are charactersied by low TiO₂ (< 2 wt%), Ti/Y, Ti/Zr and high Rb/Sr and Rb/Ba ratios. Quartz-normative composition, continental tholeiite characteristics with Nb/La less than 1 are some of the common factors of the two groups of the Bandal mafic rocks. The trace element concentrations and their ratios of the two groups of the basalts indicate that they have been derived from the asthenosphere at different depths, low-Ti at shallow and high-Ti at deeper levels. Some of the chemical features like low Mg #, Cr, Ni, high incompatible element concentrations (especially Ba), light rare earth element (LREE) enriched patterns point towards assimilation and fractional crystallisation (AFC) process which may have played a significant role in the generation of these basalts.Furthermore, the Bandal mafic rocks, apart from field settings, are geochemically similar to other Proterozoic mafic bodies like the Rampur volcanics, Mandi-Darla volcanics, Garhwal volcanics and Bhimtal-Bhowlai volcanics of the Lesser Himalaya. This widespread Proterozoic continental tholeiitic magmatism over an area of 170,000 km² in the Lesser Himalaya provides an evidence of plume activity in the region.     

Petrogenesis of monzonoritic dykes in the Egersund-Ogna anorthosite (Rogaland,S.W. Norway): trace elements and isotopic (Sr,Pb) constraints

Two major monzonoritic dykes occur in the Egersund-Ogna anorthositic massif (S.W. Norway): the Lomland dyke, which varies from norite to monzonite, and the Vettaland ferronoritic dyke. They are characterized by high Fe, Ti and P contents, low SiO₂, variable K₂O (0.5%–4.5%) and high Fe/Mg ratios. Small variations in REE distribution are observed inside the Lomland dyke [La/Yb=12; LREE ca. 150–180 (chondrite-normalized values), neutral to slightly positive Eu anomaly]. Part of the Vettaland dyke is severely depleted in Zr, Rb and REE and shows a positive Eu anomaly. All rocks are depleted in U and Th, and show very low Cr and Ni contents, as well as high Sr contents (400–600 ppm). Variation within the Lomland dyke is satisfactorily ex-plained through subtraction of an apatite-bearing noritic cumulate. The role of apatite is predominant in controlling the REE behaviour. Eutectic partial melting of a Fe-rich noritic cumulate (containing apatite) under low pH₂O conditions is suggested for the Vettaland dyke by REE modelling. Sr isotopic initial ratios are different in Lomland (ca. 0.708) and Vettaland (ca. 0.706) dykes. These values preclude direct derivation from the mantle as well as comagmatic relationship with anorthosite. Contamination of a mantle derived magma by deep crustal material or anatexis of the LIL depleted lower crust is compatible with the low U and Th contents, and with the Pb isotopic ratios. The Vettaland partial melting process is extended to all monzonoritic types, the necessary heat being provided by the high temperature anorthositic crystal mush. Chargé de Recherches du F.N.R.S (Belgium)