西藏改则县北亭贡南部地区埃达克岩的发现

在西藏改则县北亭贡南部地区发现一套岩石组合以辉绿玢岩-细粒闪长岩一细粒石英闪长岩-安山岩-凝灰岩组成的埃达克岩,根据岩石学、岩石化学、岩石地球化学及产出构造环境与典型埃达克岩对比,其完全符合埃达克岩类型,埃达克岩的发现为研究区火成岩成因问题和铜矿的找矿方向提供新思路。     

Neoproterozoic adakitic plutons in the northern margin of the Yangtze Block, China: Partial melting of a thickened lower crust and implications for secular crustal evolution

Numerous Neoproterozoic felsic and mafic–ultramafic intrusions occur in the Hannan region at the northern margin of the Yangtze Block. Among these, the Wudumen and Erliba plutons consist of granodiorites and have SHRIMP zircon U–Pb ages of  735 Ma. The rocks have high K₂O (0.8–3.6 wt.%) and Na₂O (4.4–6.4 wt.%) and low MgO (0.4–1.7 wt.%). They also have high Sr/Y (32–209) and (La/Yb)_n ratios (4.4–38.6). Their εNd values range from − 0.41 to − 0.92 and zircon initial ¹⁷⁶Hf/¹⁷⁷Hf ratios from 0.282353 to 0.282581. These geochemical features are similar to those of adakitic rocks produced by partial melting of a thickened lower crust. Our new analytical results, combined with the occurrence of voluminous arc-related mafic–ultramafic intrusions emplaced before 740 Ma, lead us to propose that the crustal evolution in the northern margin of the Yangtze Block during Neoproterozoic involved: (1) rapid crustal growth and thickening by underplating of mafic magmas from the mantle which was modified by materials coming from the subducting oceanic slab from  1.0 to  0.74 Ga, and (2) partial melting of the thickened lower crust due to a thermal anomaly induced by upwelling of asthenosphere through an oceanic slab window, producing the  735 Ma adakitic Wudumen and Erliba plutons. Our model suggests that the crustal thickness was more than 50 km at the northern margin of the Yangtze Block at  735 Ma, and rule out the possibility of a mantle plume impact causing the > 735 Ma magmatism in the region.     

Metasomatism of sub-arc mantle peridotites below southernmost South America: reduction of<Emphasis Type="Italic"> f</Emphasis>O<Subscript>2</Subscript> by slab-melt

Quaternary basalts in the Cerro del Fraile area contain two types of mantle xenoliths; coarse-grained (2–5 mm) C-type spinel harzburgites and lherzolites, and fine-grained (0.5–2 mm) intensely metasomatized F-type spinel lherzolites. C-type xenoliths have high Mg in olivine (Fo = 90–91) and a range in Cr# [Cr/ (Cr + Al) = 0.17–0.34] in spinel. Two C-type samples contain websterite veinlets and solidified patches of melt that is now composed of minute quenched grains of plagioclase + Cr-spinel + clinopyroxene + olivine. These patches of quenched melts are formed by decompression melting of pargasitic amphibole. High Ti contents and common occurrence of relic Cr-spinel in the quenched melts indicate that the amphibole is formed from spinel by interaction with the Ti-rich parental magma of the websterite veinlets. The fO₂ values of these two C-type xenoliths range from ΔFMQ −0.2 to −0.4, which is consistent with their metasomatism by an asthenospheric mantle-derived melt. The rest of the C-type samples are free of “melt,” but show cryptic metasomatism by slab-derived aqueous fluids, which produced high concentrations of fluid-mobile elements in clinopyroxenes, and higher fO₂ ranging from ΔFMQ +0.1 to +0.3. F-type lherzolites are intensely metasomatized to form spinel with low Cr# (∼0.13) and silicate minerals with low MgO, olivine (Fo = ∼84), orthpyroxene [Mg# = Mg/(Mg + ΣFe) = ∼0.86] and clinopyroxene (Mg# = ∼0.88). Patches of “melt” are common in all F-type samples and their compositions are similar to pargasitic amphibole with low TiO₂ (<0.56 wt%), Cr₂O₃ (<0.55 wt%) and MgO (<16.3 wt%). Low Mg# values of silicate minerals, including the amphibole, suggest that the metasomatic agent is most likely a slab melt. This is supported by high ratios of Sr/Y and light rare earth elements (REE)/heavy REE in clinopyroxenes. F-type xenoliths show relatively low fO₂ (ΔFMQ −0.9 to −1.1) compared to C-type xenoliths and this is explained by the fusion of organic-rich sediments overlying the slab during the slab melt. Trench-fill sediments in the area are high in organic matter. The fusion of such wet sediments likely produced CH₄-rich fluids and reduced melts that mixed with the slab melt. High U and Th in bulk rocks and clinopyroxene in F-type xenoliths support the proposed interpretation.     

中祁连西段花岗岩类的地球化学特征及构造意义

中祁连西段石板墩地区北部及南部发育有辉长岩、闪长岩及花岗岩。北带岩体的LA-ICPMS锆石U-Pb年龄为(469.3±2.8)Ma,(461.2±3.3)Ma和(470.0±2.5)Ma;岩石Si O2质量百分含量为53.2%~66.11%,高Al,Ti,Mg,Fe,Ca,K,Na,A/CNK1.1;富集大离子亲石元素Rb,K和Pb,亏损高场强元素Nb,Ta,Ce,Sr,Hf,Ti;REE总量较高,LREE富集、HREE近平坦型分布,中等的负Eu异常,δEu=0.6;此外,在构造判别图中,样品落于岛弧环境。南带岩体LA-ICP-MS锆石U-Pb年龄为(470.9±2.8)Ma和(472.3±4.2)Ma,岩石Si O2质量百分含量较高(67.13%~70.73%),高Al,Mg,Fe,Ca,富Na贫K,A/CNK1.1;岩石表现为明显的高Sr低Yb,Y的特点,富集大离子亲石元素R,K,Sr等,亏损高场强元素Nb,Ta,P,Ti等;REE总量较低,LREE富集、HREE亏损,轻重稀土分异明显,轻微的负Eu异常至正异常,δEu=0.74~1.18。研究表明,北带岩体为地幔楔部分熔融产生的,形成于岛弧环境;南带岩体为消减的大洋岩石圈板块部分熔融产生的埃达克岩,是北祁连向南俯冲导致的岩浆作用的产物,"中祁连西段"是早古生代时期在"残留的微陆块"基础上形成的一个岛弧增生杂岩地体。     

Crustal growth processes as illustrated by the Neoarchaean intraoceanic magmatism from Gadwal greenstone belt, Eastern Dharwar Craton, India

Geochemical studies on metavolcanic rocks of the Gadwal greenstone belt (GGB), eastern Dharwar craton, have documented several rock types that are indicative of subduction zone tectonics reflecting on the crustal growth processes in the Dharwar craton. The dominance of komatiites in the western Dharwar craton (WDC) and the arc volcanics in the eastern Dharwar craton (EDC) is an indication for the predominance of plume magmatism in the WDC and the intraoceanic subduction zone processes in EDC which together played a significant role in the growth and evolution of continental crust in the Dharwar craton. Boninites of GGB are high calcic type with high MgO (13–24 wt.%) and a characteristic MREE depleted U-shaped REE patterns whereas the basalts have flat REE patterns with no Eu anomalies. Nb-enriched basalts exhibit slightly fractionated REE patterns with high Nb (8–26 ppm) content compared to arc basalts. Adakites of GGB are Sr depleted with highly fractionated REE patterns and no Eu anomaly compared to rhyolites. The occurrence of boninites along with arc basalts, Nb-enriched basalts–basalt–andesite–dacite–rhyolites and adakites association in Gadwal greenstone belt indicate the intraoceanic subduction zone processes with a clear cut evidence of partial melting of metasomatized mantle wedge (boninites), melting of subducting slab (adakites) and residue of adakite–wedge hybridization (Nb-enriched basalts) which have played a significant role in the growth of continental crust in the Dharwar craton during the Neoarchaean.     

东南亚巽他群岛埃达克岩的分布及斑岩型铜(金)矿成矿预测的地质准则

东南亚巽他群岛是新生代埃达克岩、类埃达克岩极其发育的地区。这些中酸性岩浆岩广泛见于几内亚岛上的中央山脉、巴布亚新几内亚的欧文-斯坦利推覆带、俾斯麦岛弧、布干维尔岛-所罗门岛弧、印度尼西亚苏拉威西、加里曼丹中部、班达岛弧,零星见于苏门答腊、爪哇等地。根据微量元素特征及REE曲线类型的特点,本区埃达克岩可以明显地划分为2种类型：第一种埃达克岩类型属于拉斑/钙-碱性系列,具有大洋岛弧的REE曲线特征（相当于O型埃达克岩）;第二种埃达克岩属于高钾钙-碱性系列,归于大陆埃达克岩（相当于C型埃达克岩）,形成于弧-陆碰撞带或碰撞后造山带。埃达克岩与浅成热液金属矿床和斑岩矿床的成矿作用有密切的关系,是世界级浅成热液和斑岩铜-金矿的容矿岩石。     

Late Neoproterozoic adakitic magmatism of the eastern Arabian Nubian Shield

Late Neoproterozoic adakitic magmatism within the Eastern Arabian Nubian Shield has been dated at633.2±9.0 Ma(2σ).These magnas intrude the forearc Ad Dawadimi Basin,which is composed of metapelitic schists and greywacke along with ophiolitic melanges of boninitic affinity which underwent inversion and deformation by~620 Ma.This adakitic magmatism,while intruding parts of the Ad Dawadimi Basin,predates this deformation,but is possibly coincident with basin closure.As adakitic magmatism requires melting of an amphibolite or eclogitic source,empirical and experimental constraints require anomalously hot supra-subduction zone mantle.Considering that this magmatism immediately predates basin inversion,these magmas possibly pinpoint the timing of the slab breaking,marking the terminal stages of arc magmatism,terrane accretion and the influx of hot oceanic asthenospheric mantle.This influx of hot asthenospheric mantle may also be responsible for postcolltsional A-type magmatism.     

Magmatic source enrichment by slab-derived melts in a young post-collision setting, central Mindanao (Philippines)

Central Mindanao was the locus of a Pliocene (4–5 Ma old) arc–arc collision event followed by basaltic to dacitic magmatism starting at 2.3 Ma, representing the most voluminous volcanic field in the Philippines. Lava compositions range from calc-alkaline to shoshonitic. Adakites and Nb-enriched basalts are among the magmatic products. All the lavas are Na-rich (up to 4.88%), with Na₂O/K₂O ratios from 2.5 to 6.5. Sr, Nd and Pb isotopic compositions are similar to MORB, except for some shoshonitic lavas that have slightly less radiogenic Nd ratios. K-enrichment in basalts can be related to both fractional crystallization (FC) at moderate pressures and to partial melting of an enriched source. Trace element systematics indicate that the sub-central Mindanao mantle is characterized by the presence of garnet, phlogopite, amphibole, and perhaps some titanate phase. The enrichment of this source is attributed to the interaction of slab-derived melts, i.e., adakites, with the arc mantle. This would explain the presence of Nb-enriched basalts, transitional adakites and high-magnesium andesites, as well as the bulk Na-enrichment and relatively unradiogenic character of the central Mindanao lavas. We envision an ion-exchange type of enrichment, in which the HFSE, LILE and LREE, mobilized during slab melting, are preferentially enriched in the metasomatized mantle, resulting in a diversity of post-collision magma compositions. The MORB-like isotopic signatures of the central Mindanao lavas preclude important contributions of slab-derived hydrous fluids, sediments, continental crust or an OIB-type contaminant. Slab melting after cessation of subduction is deemed possible by thermal rebound of previously depressed geotherms. Initial contributions to mantle enrichment in post-collision sites may thus come from slab melts. In most other cases of post-collision magmatism, however, this signature can be easily masked by enrichments coming from other sources, e.g., the continental lithosphere.     

埃达克岩与Cu-Au成矿作用：有待深入研究的岩浆成矿关系

研究表明，环太平洋地区新生代斑岩铜矿和浅成热液金矿与同期的埃达克质岩浆活动存在密切的时空与成因联系。埃达克岩是许多世界级的斑岩铜矿的容矿岩，也是许多浅成热液矿化系统的成矿母岩浆。根据目前的研究，二者成因联系可能在于埃达克质岩浆的富流体、高氧逸度和基性源岩等固有属性，有利于Cu、Au等深源金属元素的萃取与富集成矿一因此，这可能是一种潜在的岩浆成矿专属性关系，但对产生这种关系的原因与机制仍然不十分清楚。这有待于今后深入开展成矿与无矿的埃达克岩、成矿的埃达克岩与非埃达克岩、无矿的埃达克岩与非埃达克岩等方面的对比研究，以揭示这种成矿专属性的本质。