新疆新源县城南石炭纪火山岩岩石学和元素地球化学研究

新疆新源县南部那拉提山北坡出露的石炭纪火山岩主要由玄武岩、玄武质粗面安山岩、粗面安山岩、安山岩、流纹岩和火山碎屑岩组成。该火山岩中玄武岩属于钙碱性系列,安山质岩石和流纹岩属于高钾钙碱性系列,其中轻稀土轻微富集而重稀土相对亏损,玄武岩富集大离子亲石元素、U、Th和Pb,亏损高场强元素。研究表明,该火山岩岩浆可能是由俯冲板片脱水产生的流体交代地幔楔后,地幔楔发生部分熔融的结果。微量元素模拟计算表明,该玄武岩岩浆可以由石榴石二辉橄榄岩经3％～6％的部分熔融得到;安山质岩浆可由玄武岩岩浆经15％-28％的分离结晶形成。     

Geochemistry of Late Proterozoic volcanic rocks from Tassendjanet area (N.W. Hoggar,Algeria)

The Late Proterozoic calc-alkaline andesitic rocks of the Tassendjanet volcanic complex, north-western Hoggar, Algeria resemble the continental margin rocks and also are closely comparable to the Archaean andesites. Most Tassendjanet rocks were affected to various extents by low grade hydrous metamorphism which led to an increase of Na, P, Fe, Ti, and V and a decrease of Mg, Ca, Sr, Cr, and Ni. Although K, Rb, Ba, and Li are enriched in the majority of altered rocks, these elements are strongly depleted in the albitized Na-rich volcanics. REE are enriched in albitized andesites but their fractionation patterns remain unchanged. The trace element data are consistent with the derivation of the Tassendjanet andesites by partial melting of an upper mantle source enriched in LILE and presumably overlying the subduction zone.     

Accessory phases and the generation of LREE-Enriched basalts — A test for disequilibrium melting

The REE contents of tholeiitic rocks can be derived by simple mantle melting models. However this type of model has been less successful in accounting for rocks of the alkali basalt-kimberlite suite with strong LREE enrichment. Rocks from these associations have similar La/Sm ratios over a range of total REE concentrations and their generation by simple melting models requires an enriched source. These requirements conflict with the conclusions reached from Nd-Sm studies.An alternative hypothesis involves the disequilibrium melting of a LREE-rich accessory phase in the mantle. Such a phase will be among the first to melt and thus will dominate the REE patterns of early-formed liquids, even if subsequent partial re-equilibration occurs. This model could account for liquids with strong LREE enrichment and similar La/Sm ratios at different levels of total REE enrichment by partial melting of a simple chondritic mantle. Since the model predicts the existence of an accessory phase in the mantle with a REE pattern parallel to that found in strongly LREE-enriched magma, it provides a sensitive test for disequilibrium melting.     

Geochemistry and geochronology of mafic rocks from the Vespor suite in the Juruena arc,Roosevelt-Juruena terrain,Brazil: Implications for Proterozoic crustal growth and geodynamic setting of the SW Amazonian craton

This study presents new geochemical data on rocks from the Vespor suite, an important mafic unit from the Juruena arc, Roosevelt-Juruena terrain, SW Amazonian craton, northwest Mato Grosso, Brazil, attempting to define their tectonic setting and type of mantle source. The Juruena arc may be part of a magmatic belt (Jamari and Juruena arcs) at the southwestern Amazonian craton during assembly of the Columbia supercontinent. The investigated rocks represent a Paleoproterozoic subduction-related mafic suite of sigmoidal bodies, composed mainly of gabbro, norite, gabbronorite and diorite, that underwent amphibolite facies metamorphism. Here we present also preliminary petrology aspects and zircon U–Pb geochronology. Geochemical character and variation trends of major and trace elements as well as selected trace element ratios suggest that Vespor suite rocks have a tholeiitic lineage of arc affinity controlled by fractional crystallization with a prominent iron enrichment trend. Gabbros, norites and gabbronorites are characterized by enrichment of LILE and weakly to moderately differentiated HFSE patterns, suggesting their deviation from an enriched heterogeneous lithospheric mantle source. Vespor suite rocks are characterized by depletion of Nb–Ta, P and Ti, with flat distribution of HFSE, markedly large variations in most of the LILE, positive anomalies displayed by Ba, K, Th, Sr, Pb and weak negative anomalies of Hf–Zr. These features reflect limited degrees of crustal contamination associated with a subduction-related magma process where the mantle wedge was chemically modified. In addition, the enrichment in LILE and Pb, low values of the ratios (La_n/Sm_n – 0.83 to 4.58) and (Nb_n/La_n – 0.04 to 0.45), but high Th/Yb ratios, gently to moderately sloping REE profiles (La/Yb_n = 2.53–7.37), negative anomalies in HFSE (Ta, Zr, Hf, and Ti), and positive anomalies in LILE (Th, Ba, Sr), suggest derivation from a metasomatized lithospheric mantle source above a subduction zone with weak crustal contamination. Both the composition of the mantle source and the degree of partial melting that produced the parental magmas of these rocks, determined by using REE abundance and ratios, indicate that gabbroic/dioritic melts were generated at different degrees of melting of the source: about 5–20% partial melting of a garnet-spinel lherzolite, around 1–10% partial melting of spinel lherzolite source, and approximately 1–5% partial melting of intermediate source composition, and crystallizing between 1.773 and 1.764 Ma.     

U-Pb Zircon Geochronology and Geochemistry of Granitoids in the Douling Group in the Eastern Qinling

LA-ICPMS U-Pb zircon dating of the Sanpinggou, Gangou and Fengzishan granitoids in the Douling Group of the Eastern Qinling yields ages of 760-685 Ma, which represents a strong tectono-magmatic event in the southern Qinling during the late Neoproterozoic. Geochemical data show that these intrusions have wide compositions ranging from minor gabbros through diorites to granodiorites. They are relatively enriched in LILE, poor in HFSE and strongly depleted in Nb and Ta, displaying affinities of Ⅰ-type granites formed in an active continental margin with oceanic subduction. In contrast to granitoids, gabbros and enclaves in the granitoids have higher REE abundances, relatively flat REE patterns, lower LILE, slightly higher HFSE and more depletion in Nb and Ta. All these suggest that the gabbros were formed by partial melting of the upper mantle above the subduction zone and the granitoids by the partial melting of the lower crust. Combined with regional geological data, the subduction-related granitoids in     

Ar/Ar age and geochemistry of the post-collisional Miocene Yamadağ volcanics in the Arapkir area (Malatya Province), eastern Anatolia, Turkey

The Neogene Yamadağ volcanics occupy a vast area between Sivas and Malatya in eastern Anatolia, Turkey. These volcanic rocks are characterized by pyroclastics comprising agglomerates, tuffs and some small outcrops of basaltic–andesitic–dacitic rocks, overlain upward by basaltic and dacitic rocks, and finally by basaltic lava flows in the Arapkir area, northern Malatya Province. The basaltic lava flows in the Arapkir area yield a ⁴⁰Ar/³⁹Ar age of 15.8 ± 0.2 Ma, whereas the dacitic lava flows give ⁴⁰Ar/³⁹Ar ages ranging from 17.6 through 14.7 ± 0.1 to 12.2 ± 0.2 Ma, corresponding to the Middle Miocene. These volcanic rocks have subalkaline basaltic, basaltic andesitic; alkaline basaltic trachyandesitic and dacitic chemical compositions. Some special textures, such as spongy-cellular, sieve and embayed textures; oscillatory zoning and glass inclusions in plagioclase phenocrysts; ghost amphiboles and fresh biotite flakes are attributable to disequilibrium crystallization related to magma mixing between coeval magmas. The main solidification processes consist of fractional crystallization and magma mixing which were operative during the soldification of these volcanic rocks. The dacitic rocks are enriched in LILE, LREE and Th, U type HFSE relative to the basaltic rocks. The basaltic rocks also show some marked differences in terms of trace-element and REE geochemistry; namely, the alkaline basaltic trachyandesites have pronounced higher HFSE, MREE and HREE contents relative to the subalkaline basalts. Trace and REE geochemical data reveal the existence of three distinct magma sources – one subalkaline basaltic trachyandesitic, one alkaline basaltic and one dacitic – in the genesis of the Yamadağ volcanics in the Arapkir region. The subalkaline basaltic and alkaline basaltic trachyandesitic magmas were derived from an E-MORB type enriched mantle source with a relatively high- and low-degree partial melting, respectively. The magmatic melt of dacitic rocks seem to be derived from an OIB-type enriched lithospheric mantle with a low proportion of partial melting. The enriched lithospheric mantle source reflect the metasomatism induced by earlier subduction-derived fluids. All these coeval magmas were generated in a post-collisional extensional geodynamic setting in Eastern Anatolia, Turkey.     

念青唐古拉中基性侵入岩年代学、地球化学及岩石成因

念青唐古拉早始新世中基性侵入岩由角闪辉长岩及二长闪长岩组成,其中角闪辉长岩锆石LA-MC-ICPMS U-Pb年龄为(49.92±0.24)Ma。中基性侵入岩属于高钾钙碱系列至钾玄岩系列;稀土总量较高且轻稀土富集;富集大离子亲石元素(Rb,Th,K),亏损高场强元素(Nb,Ta,Ti),具有弧岩浆岩的微量元素特征。中基性侵入岩具有较高的初始锶((87Sr/86Sr)i为0.708 77～0.713 71)、低的εNd值(-6.9～-6.1)、古老的亏损地幔模式年龄(TDM为1 329～1 995Ma),以及εNd、Nb/La与MgO无明显相关性,表明其源区为受古老地壳交代的富集地幔。稀土元素模拟表明中基性侵入岩的成岩过程受地幔中轻程度部分熔融作用所控制。动力学背景分析表明其与新特提斯洋板片的断离及软流圈物质的上涌密切相关。     

赣东北蛇绿混杂岩岩石地球化学特征及构造意义

赣东北蛇绿混杂岩是中国为数不多的新元古代蛇绿岩,对该蛇绿混杂岩带中的方辉橄榄岩和玄武岩类岩石进行元素地球化学研究,以探讨新元古代赣东北地区的构造动力学背景.研究表明,方辉橄榄岩具有低TiO₂(0.02%~0.37%)、低K₂O(0~0.02%)、低Na₂O(0.02%~0.10%)和富MgO(40.81%~44.58%)特征,烧失量普遍较高(10.09%~13.47%),表明样品普遍遭受蚀变;稀土元素配分模式图和微量元素蛛网图显示,橄榄岩样品具有亏损地幔源区特征,同时兼有俯冲带流体交代的特征.玄武岩类SiO₂含量主要为52.63%~56.19%,平均为55.00%,属安山玄武岩范围.岩石具低MgO(3.66%~6.46%),中等TiO₂(1.14%~2.66%),高Al₂O₃(13.07%~15.27%)、Na₂O(3.42%~5.03%)的特征.安山玄武岩轻稀土富集((La/Yb)_N=1.84~2.61),无明显Eu异常,稀土配分曲线呈右倾型;微量元素蛛网图中显示大离子亲石元素Th、Ba、U、Pb富集、高场强元素Nb、Ti亏损,可能系俯冲板片脱水产生的流体交代地幔楔所形成,TNT(Ta、Nb和Ti)的负异常指示赣东北蛇绿混杂岩的形成过程中可能遭受了陆源物质的混染,反映了消减作用的影响.方辉橄榄岩和安山玄武岩的地球化学特征表明赣东北蛇绿混杂岩的形成可能经历了多期演化,早期可能系古华南洋发生洋内俯冲作用,俯冲过程中释放的流体交代了俯冲带上的地幔楔,使地幔橄榄岩富集轻稀土.随后的弧后盆地扩张阶段,形成具有岛弧性质和MORB特征的玄武岩类,但由于弧后盆地发育不成熟,玄武岩类在地球化学上显示有大陆物质的特征,表现出高Pb、低Ti和轻稀土强烈富集.      

Triassic shoshonites and andesites,Lakmon Mountains,western continental Greece: Differences in primary geochemistry and sheet silicate alteration products

Shallow marine lavas and pyroclastic rocks, several tens of metres thick, outcrop at the base of thrust slices in the Pindos nappe in the Lakmon Mountains of western continental Greece. These volcanic rocks are basalts and andesites formed during mid-Triassic subduction. Geochemically, on the basis of the potassium content, shoshonitic and calc-alkalic groups are distinguished. Principally the shoshonitic rocks contain primary K-feldspar. The calc-alkalic rocks contain complexly zoned feldspars and are depleted in intermediate REE: this depletion could reflect a small degree of partial melting of amphibole, or possibly phlogopite, during petrogenesis. These two characteristics are found in other examples of minor calc-alkalic rocks associated with shoshonites described in the literature. A petrogenetic model is developed involving deeper mantle enrichment in LILE and partial melting of this enriched mantle rock containing amphibole. Calc-alkalic rocks are the result of initial partial melting and may be mixed with small amounts of magma of deeper origin; further partial melting produces the more voluminous shoshonites. Celadonite developed as a secondary mineral during burial beneath about 1 km of Mesozoic to early Tertiary sediments. During nappe emplacement, vermiculite formed in the calc-alkalic rocks; in contrast, smectite developed in the shoshonites, because the potassium was sufficient to immobilize all available Fe in celadonite.     

Collision‐related granite magma genesis,potential sources and tectono‐magmatic evolution: comparison between central,northwestern and western Anatolia (Turkey)

The central, northwestern and western Anatolian magmatic provinces are defined by a large number of late Mesozoic to late Cenozoic collision‐related granitoids. Calc‐alkaline, subalkaline and alkaline intrusive rocks in central Anatolia are mainly metaluminous, shoshonitic, I‐ to A‐types. They cover a petrological range from monzodiorite through quartz monzonite to granite/syenite, and are all enriched in LILE. Their geochemical characteristics are consistent with formation from a subduction‐modified mantle source. Calc‐alkaline plutonic rocks in northwestern Anatolia are mainly metaluminous, medium‐ to high‐K and I‐types. They are monzonite to granite, and all are enriched in LILE and depleted in HFSE, showing features of arc‐related intrusive rocks. Geochemical data reveal that these plutons were derived from partial melting of mafic lower crustal sources. Calc‐alkaline intrusive rocks in western Anatolia are metaluminous, high‐K and I‐types. They have a compositional range from granodiorite to granite, and are enriched in LILE and depleted in HFSE. Geochemical characteristics of these intrusive rocks indicate that they could have originated by the partial melting of mafic lower crustal source rocks.     

Origin and geodynamic significance of Upper Cretaceous lamprophyres from the Villány Mts (S Hungary)

Summary In the Villány Mts of southern Hungary, ocelli-bearing porphyritic lamprophyre dykes and sills of Upper Cretaceous age occur sporadically, intruding Mesozoic carbonate rocks. They at places contain metasomatised mantle xenoliths and quartz xenocrysts of crustal origin. They are moderately fractionated with significant LILE and LREE enrichments and a notable Nb–Ta negative anomaly. Trace elements indicate that they formed in an intraplate environment by very low degree partial melting of a metasomatised garnet lherzolite mantle source that was enriched by earlier subduction. Based on petrography, geochemistry and age constraints, they differ from other Mesozoic basic rocks of the Tisza block (Mecsek Mts and Slavonian basalts); however, they show a significant geochemical similarity to the Upper Cretaceous lamprophyre dyke swarm from NE Transdanubia (northwestern Hungary) situated on the Alcapa microplate. Thus we suggest that lamprophyres from the Villány Mts and NE Transdanubia could have originated from the same or similar enriched asthenospheric mantle sources.     

大陆板片-地幔相互作用:来自大别造山带碰撞后安山质火山岩的地球化学证据

俯冲到地幔深度的地壳物质不可避免地在板片-地幔界面与地幔楔发生相互作用,由此形成的超镁铁质交代岩就是造山带镁铁质火成岩的地幔源区.因此,造山带镁铁质火成岩为研究俯冲地壳物质再循环和壳-幔相互作用提供了重要研究对象.为了揭示俯冲陆壳物质再循环的机制和过程,对大别造山带碰撞后安山质火山岩开展了元素和同位素地球化学研究.这些安山质火山岩的SIMS锆石U-Pb年龄为124±3~130±2 Ma,表明其形成于早白垩世.此外,残留锆石的U-Pb年龄为中新元古代和三叠纪,分别对应于大别-苏鲁造山带超高压变火成岩的原岩年龄和变质年龄.它们具有岛弧型微量元素特征、富集的Sr-Nd-Hf同位素组成,以及变化的且大多不同于正常地幔的锆石δ18O值.这些元素和同位素特征指示,这些安山质火山岩是交代富集的造山带岩石圈地幔部分熔融的产物.在三叠纪华南陆块俯冲于华北陆块之下的过程中,俯冲华南陆壳来源的长英质熔体交代了上覆华北岩石圈地幔楔橄榄岩,大陆俯冲隧道内的熔体-橄榄岩反应产生了富沃、富集的镁铁质地幔交代岩.这种地幔交代岩在早白垩世发生部分熔融,就形成了所观察到的安山质火山岩.因此,碰撞造山带镁铁质岩浆岩的地幔源区是通过大陆俯冲隧道内板片-地幔相互作用形成的,而加入地幔楔中长英质熔体的比例决定了这些镁铁质岩浆岩的岩石化学和地球化学成分.      

Rare-earth elements in high-alumina basaltic rocks from Sardinia

High-alumina basalts and basic andesites, which represent the most “primitive” magma types of the Cenozoic andesitic series of Sardinia, show a spatial chemical zonation with respect to REE. The basaltic rocks from the northern and south-central part of the island have REE patterns typical of calc-alkaline rocks with an enrichment of light REE and fractionation of heavy REE. In contrast, those from the southernmost part have a pattern similar to typical continental tholeiites with only a small light-REE enrichment and unfractionated heavy REE.The present data suggest that basaltic rocks may be formed by anatexis of upper-mantle material with mineral assemblages containing either garnet (calc-alkaline rocks) or spinel (rocks of tholeiitic affinities). The presence of garnet or spinel could merely reflect mineral phase transformation and indicates a different depth of fusion for the various types of basaltic rocks with those of tholeiitic affinities originating at a shallower depth than the calcalkaline rocks. The REE data are consistent with the generation of the basaltic rocks by partial melting of mantle peridotite overlying a subducted plate.     

Petrological and Geochemical Characteristics of Archean Gneisses and Granitoids from Bastar Craton, Central India – Implication for Subduction Related Magmatism

The gneisses and granitoids of Bastar craton (with rock suites up to 3.5 Ga) show calc-alkaline trondhjemitic characteristics. The rocks are enriched in both LILE and HFSE than primordial mantle. They have also relatively higher abundances of LILE and strong depletion at P and Ti in the multielement diagram. The depletion of Ti and P indicates retention of these elements by titanite and/or apatite during partial melting. It is proposed that subduction of an oceanic slab and its consequent melting led to the formation of the protoliths of the gneisses without much interaction with the mantle wedge. The granitoids represent temporally distinct suites formed in response to further melting of slab at greater depth and interaction of magma with the mantle wedge during their transport to the crust.     

Geochemistry and Petrogenesis of Volcanic Rocks in the Yeba Formation on the Gangdise Magmatic Arc, Tibet

The Early Jurassic bimodal volcanic rocks in the Yeba Formation, situated between Lhasa, Dagzê and Maizhokunggar, composed of metabasalt, basaltic ignimbrite, dacite, silicic tuff and volcanic breccia, are an important volcanic suite for the study of the tectonic evolution of the Gangdise magmatic arc and the Mesozoic Tethys. Based on systematic field investigations, we carried out geochemical studies on representative rock samples. Major and trace element compositions were analyzed for these rock samples by XRF and ICP-MS respectively, and an isotope analysis of Rb-Sr and Sm-Nd was carried out by a MAT 262 mass spectrograph. The results show that the SiO2 contents in lava rocks are 41 %-50.4 % and 64 %-69 %, belonging to calc-alkaline basalt and dacite. One notable feature of the basalt is its low TiO2 content, 0.66 %-1.01 %, much lower than those of continental tholeiite. The ΣREE contents of basalt and dacite are 60.3-135 μg/g and 126.4-167.9 μg/g respectively. Both rocks have similar REE and other trace element characteristics, with enriched LREE and LILE relative to HREE and HFS, similar REE patterns without Eu anomaly. The basalts have depleted Ti, Ta and Nb and slightly negative Nb and Ta anomalies, with Nb*＝0.54-1.17 averaging 0.84. The dacites have depleted P and Ti and also slightly negative Nb and Ta anomalies, with Nb*＝0.74-1.06 averaging 0.86. Major and trace elemental and isotopic studies suggest that both basalt and dacite originated from the partial melting of the mantle wedge at different degrees above the subduction zone. The spinal lherzolite in the upper mantle is likely to be their source rocks, which might have been affected by the selective metasomatism of fluids with crustal geochemistry. The LILE contents of both rocks were affected by metamorphism at later stages. The Yeba bimodal volcanic rocks formed in a temporal extensional situation in a mature island arc resulting from the Indosinian Gangdise magmatic arc.     

Chemical discontinuities in Archean metavolcanic terrains and the development of Archean crust

Most large Archean greenstone belts ( 2.7 Ga), comprise thick (12–15 km) mafic to felsic metavolcanics sequences which exhibit consistent but discontinuous geochemical patterns resulting from mantle-crust processes. In a typical Archean metavolcanic sequence, thick (5–8 km) uniform tholeiitic basalt is followed by geochemically evolved rock units (4–7 km thick) containing intermediate and felsic calc-alkaline rocks. This major geochemical discontinuity is marked by a change from LIL-element depleted basalts which show unfractionated REE abundance patterns, to overlying andesites with higher LIL-element contents, fractionated REE patterns and relatively depleted HREE. A less well marked discontinuity separates andesitic rocks from still later more felsic dacite-rhyolite extrusive assemblages and their intrusive equivalents, and is identified by a further increase in LIL element content and REE fractionation. The major geochemical discontinuity apparently separates rocks derived by partial melting of mantle (either directly or through shallow fractionation processes) from those which originated either by partial melting of mantle material modified by crustal interactions or by partial melting of crustal material.We suggest that accumulation of a great thickness of mantle derived volcanic rocks can lead to sagging and interaction of the lower parts of the volcanic piles with upper mantle material. The resulting modified mantle acts as a source for some of the geochemically evolved rocks observed in volcanic successions. Subsequent direct melting of the volcanic pile produces the felsic magmas observed in the upper parts of Archean volcanic successions. This process, termed sag-subduction, is the inferred tectonic process operating in the comparatively thin, hot Archean crustal regime. By this process, large masses of ultimately mantle-derived material were added to the crust.     

山东汤头盆地钾质及钠质火山岩的年代学与地球化学:对华北克拉通岩石圈减薄的启示

山东汤头盆地位于沂沭断裂带南段,盆地内广泛发育以粗安质岩石为主体的晚中生代火山岩,这套岩石主要可归为碱性系列,按化学组成可进一步区分为钾质和钠质二种类型。钾质火山岩的主要岩性为黑云母粗安质火山碎屑岩和潜火山岩,钠质火山岩主要为辉石粗安质潜火山岩,其中钾质火山岩是盆地内火山岩的主体。锆石LA-ICP-MS U-Pb定年获得钾质和钠质火山岩的成岩年龄分别为124.0±1.3Ma和106.4±4.0Ma,表明钠质火山岩较钾质火山岩形成晚。在化学组成上,钾质火山岩较钠质火山岩全碱含量更高,二者的K2O+Na2O含量分别为11.02%～11.37%和8.75%～8.93%。它们均富轻稀土和大离子亲石元素,但钾质火山岩较钠质火山岩稀土总量更高,且更富轻稀土,二者的∑REE值分别为360.1×10-6～417.0×10-6和232.3×10-6～291.0×10-6,(La/Yb)N比值分别为62.02～64.66和40.32～40.52。钾质火山岩的Cs、Rb、Ba、Th、U、Pb等大离子亲石元素和Zr、Hf等高场强元素均较钠质火山岩富集,但Sr、Ti的含量偏低。钾质与钠质火山岩均具有富集的Sr-Nd同位素组成特征,但钠质火山岩的ISr值偏低、而εNd(t)值偏高,二者的ISr和εNd(t)值分别为0.7107～0.7119和-15.48～-16.96,以及0.7098和-10.03。元素和Sr-Nd同位素组成的系统分析表明浅部地壳混染对火山岩地球化学特征未产生显著影响,二类火山岩地球化学特征的变异应主要受控于岩浆源区组成的不同,而不是岩浆演化过程的差异所致。二元混合模拟指示二类火山岩均最可能起源因华北克拉通下地壳拆沉而形成的富集地幔的熔融,但钠质火山岩源区含有较高比例的亏损软流圈地幔组分。根据对火山岩地质与地球化学特征的综合分析,表明郯庐断裂持续的引张促使岩石圈减薄,并诱发深部软流圈熔体上涌,这一上涌的软流圈熔体随后又与原先富集的岩石圈地幔混合,从而导致晚期的钠质火山岩源区中含有较高的亏损软流圈地幔组分。火山岩成分由钾质向钠质演化,是软流圈地幔上涌并置换原有岩石圈地幔,最终导致华北克拉通减薄的直接响应。     

Geochemistry and isotope systematics of calc-alkaline volcanic rocks from the Saar-Nahe basin (SW Germany) – implications for Late-Variscan orogenic development

Late Carboniferous (300–290 Ma) calc-alkaline basalts, andesites, and rhyolites typical of volcanic arc settings occur in the intermontane Saar-Nahe basin (SW Germany) within the Variscan orogenic belt. The volcanic rock suite was emplaced under a regime of tensional tectonics during orogenic collapse and its origin has been explained by melting of mantle and crust in the course of limited lithospheric rifting. We report major, trace and rare-earth-element data (REE), and Nd-Pb-Sr-O isotope ratios for a representative sample suite, which are fully consistent with an origin closely related to plate subduction. Major and trace element data define continuous melt differentiation trends from a precursor basaltic magma involving fractional crystallization of olivine, pyroxene, plagioclase, and magnetite typical of magma evolution in a volcanic arc. This finding precludes an origin of the andesitic compositions by mixing of mafic and felsic melts as can be expected in anorogenic settings. The mafic samples have high Mg numbers (Mg# = 65–73), and high Cr (up to 330 ppm) and Ni (up to 200 ppm) contents indicating derivation from a primitive parental melt that was formed in equilibrium with mantle peridotite. We interpret the geochemical characteristics of the near-primary basalts as reflecting their mantle source. The volcanic rocks are characterized by enrichment in the large ion lithophile elements (LILE), negative Nb and Ti, and positive Pb anomalies relative to the neighboring REE, suggesting melting of a subduction-modified mantle. Initial _Nd values of −0.7 to −4.6, Pb, and ⁸⁷Sr/⁸⁶Sr_(t) isotope ratios for mafic and felsic volcanics are similar and indicate partial melting of an isotopically heterogeneous and enriched mantle reservoir. The enrichment in incompatible trace elements and radiogenic isotopes of a precursor depleted mantle may be attributed to addition of an old sedimentary component. The geochemical characteristics of the Saar-Nahe volcanic rocks are distinct from typical post-collisional rock suites and they may be interpreted as geochemical evidence for ongoing plate subduction at the margin of the Variscan orogenic belt not obvious from the regional geologic context. Received: 3 August 1998 / Accepted: 2 January 1999     

西秦岭甘青交界一带晚三叠世火山岩岩石成因及构造指示意义

西秦岭甘肃、青海交界一带中生代火山岩较为发育,火山岩组成以安山质、流纹质为主,均为高钾钙碱性火山岩,主、微量元素含量变化较大。SiO2的质量分数为5667%～7817%,平均6978%, Al2O3含量较高(1277%～1729%,平均1464%)。TiO2含量少(002％～075%,平均022%),Na2O/K2O比值较小(003％～268%,平均065%)。轻稀土元素中等富集,重稀土元素相对亏损,稀土元素配分曲线明显右倾,具Eu负异常。安山质火山岩与流纹质火山岩稀土元素球粒陨石配分曲线中重稀土型式明显不同,暗示二者来自不同的源区。微量元素中P、Nb、Ti、Ta等高场强元素(HFS)相对亏损,Th、 Ba、K、Rb等大离子亲石元素相对富集。研究区安山岩LA ICP MS锆石U Pb同位素定年结果为(236±12)Ma,代表了火山岩的喷发年龄。它们形成于隆务峡蛇绿岩洋壳向南俯冲的活动大陆边缘弧环境,为俯冲洋壳在地幔深部发生高程度部分熔融作用的产物,并在上升过程中受到陆壳物质作用。在岩浆演化过程受部分熔融和分异结晶作用的控制,但安山质火山岩主要受控于分异结晶作用,而流纹质火山岩受部分熔融作用较大。     

High-K calc-alkaline plutonism in Zouzan, NE of Lut block, Eastern Iran: An evidence for arc related magmatism in Cenozoic

The Zouzan pluton is one of the intrusive bodies in the NE of Lut block enclosed by Cenozoic volcanic and sedimentary rocks. It consists of two distinct mafic and felsic magmas which are genetically unrelated. All studied rocks are calc-alkaline in nature, with LILE/REE and HFSE/REE ratios compatible with arc related magmatism. Mafic phase has dioritic composition emplaced as small stocks in felsic rocks. Geochemical characteristics in dioritic rocks (relatively high contents of incompatible elements, low Na₂O and Mg#>44) suggest they were derived from partial melting of metabasalt sources in a subduction settings. Felsic phase composed of granodiorite to granite rocks with high-K calcalkaline metaluminous to slightly peraluminous signature. Major and trace element data exclude high pressure melting and metasedimentary parental in the formation of Zouzan felsic rocks. They have been formed by partial melting of mantle-derived mafic rocks. Field relation, petrographical evidences and chemical composition show that partial melting of a mantle wedge in conjunction with magma mixing and crystal fractionation would have led to generation of Zouzan pluton.