Mineralogy, whole-rock and Sr-Nd isotope geochemistry of mafic microgranular enclaves in Cretaceous Dagbasi granitoids, Eastern Pontides, NE Turkey: Evidence of magma mixing, mingling and chemical equilibration

Rocks of the Late Cretaceous Dagbasi Pluton (88-83 Ma), located in the eastern Pontides, include mafic microgranular enclaves (MMEs) ranging from a few centimetres to metres in size, and from ellipsoidal to ovoid in shape. The MMEs are composed of gabbroic diorite, diorite and tonalite, whereas the felsic host rocks comprise mainly tonalite, granodiorite and monzogranite based on both mineralogical and chemical compositions. MMEs are characterized by a fine-grained, equigranular and hypidiomorphic texture. The common texture of felsic host rocks is equigranular and also reveals some special types of microscopic textures, e.g., oscillatory-zoned plagioclase, poikilitic K-feldspar, small lath-shaped plagioclase in large plagioclase, blade-shaped biotite, acicular apatite, spike zones in plagioclase and spongy-cellular plagioclase textures and rounded plagioclase megacrysts in MMEs. Compositions of plagioclases (An₃₃-An₆₀), hornblendes (Mg#=0.77-1.0) and biotites (Mg#=0.61-0.63) of MMEs are slightly distinct or similar to those of host rocks (An_12-57; hbl Mg#=0.63-1.0; Bi Mg#=0.50-0.69), which suggest partial to complete equilibration during mafic-felsic magma interactions.The felsic host rocks have SiO₂ between 60 and 76 wt% and display low to slightly medium-K tholeiitic to calc-alkaline and peraluminous to slightly metaluminous characteristics. Chondrite-normalized rare-earth element (REE) patterns are fractionated (La_cn/Lu_cn=1.5-7.3) with pronounced negative Eu anomalies (Eu/Eu*=0.46-1.1). Initial ε_Nd(i) values vary between −3.1 and 1.6, initial ⁸⁷Sr/⁸⁶Sr values between 0.7056 and 0.7067.Compared with the host rocks, the MMEs are characterized by relatively high Mg-number of 22-52, low contents of SiO₂ (53-63 wt%), low ASI (0.7-1.1) and low to medium-K tholeiitic to calc-alkaline, metaluminous to peraluminous composition. Chondrite-normalized REE patterns are relatively flat [(La/Yb)_cn=1.4-3.9; (Tb/Yb)_cn=0.9-1.5] and show small negative Eu anomalies (Eu/Eu*=0.63-1.01). Isotope signatures of these rocks (⁸⁷Sr/⁸⁶Sr_(i)=0.7054-0.7055; ε_Nd(i)=-1.0 to 1.9) are largely similar to the host rocks. Gabbroic diorite enclaves have relatively low contents of SiO₂, ASI; high Mg#, CaO, Al₂O₃, TiO₂, P₂O₅, Sr and Nb concentrations compared to dioritic and tonalitic enclaves.The geochemical and isotopic similarities between the MMEs and their host rocks indicate that the enclaves are of mixed origin and are most probably formed by the interaction between the lower crust- and mantle-derived magmas. All the geochemical data suggest that a basic magma derived from an enriched subcontinental lithospheric mantle, interacted with a crustal melt that originated from dehydration melting of the mafic lower crust at deep crustal levels. The existence of compositional and textural disequilibrium and the nature of chemical and isotopic variation in these rock types indicate that magma mixing/mingling between an evolved mafic and a granitic magma was involved in their genesis. Microgranular enclaves are thus interpreted to be globules of a more mafic magma probably from an enriched lithospheric mantle source. Al-in-amphibole estimates the pluton emplacement at ca. 0.3-3.8 kbar, and therefore, magma mixing and mingling must have occurred at 3.8 kbar or below this level.     

Magma mixing/mingling in the Eocene Horoz (Nigde) granitoids, Central southern Turkey: evidence from mafic microgranular enclaves

Mafic microgranular enclaves (MMEs) are widespread in the Horoz pluton with granodiorite and granite units. Rounded to elliptical MMEs have variable size (from a few centimetres up to metres) and are generally fine-grained with typical magmatic textures. The plagioclase compositions of the MMEs range from An₁₈?CAn₆₄ in the cores to An₁₇?CAn₂₉ in the rims, while that of the host rocks varies from An₁₇ to An₅₅ in the cores to An₀₇ to An₃₃ in the rims. The biotite is mostly eastonitic, and the calcic-amphibole is magnesio-hornblende and edenite. Oxygen fugacity estimates from both groups?? biotites suggest that the Horoz magma possibly crystallised at fO₂ conditions above the nickel?Cnickel oxide (NNO) buffer. The significance of magma mixing in their genesis is highlighted by various petrographic and mineralogical characteristics such as resorption surfaces in plagioclases and amphibole; quartz ocelli rimmed by biotite and amphibole; sieve and boxy cellular textures, and sharp zoning discontinuities in plagioclase. The importance of magma mixing is also evident in the amphiboles of the host rocks, which are slightly richer in Si, Fe³⁺ and Mg in comparison with the amphiboles of MMEs. However, the compositional similarity of the plagioclase and biotite phenocrysts from MMEs and their host rocks suggests that the MMEs were predominantly equilibrated with their hosts. Evidence from petrography and mineral chemistry suggests that the adakitic Horoz MMEs could be developed from a mantle-derived, water-rich magma (>3 mass%) affected by a mixing of felsic melt at P >2.3?kbar, T >730°C.     

Petrography and major elements geochemistry of microgranular enclaves and neoproterozoic granitoids of south Khasi,Meghalaya: Evidence of magma mixing and alkali diffusion

Neoproterozoic (690±19 Ma) felsic magmatism in the south Khasi region of Precambrian northeast Indian shield, referred to as south Khasi granitoids (SKG), contains country-rock xenoliths and microgranular enclaves (ME). The mineral assemblages (pl-hbl-bt-kf-qtz-mag) of the ME and SKG are the same but differ in proportions and grain size. Modal composition of ME corresponds to quartz monzodiorite whereas SKG are quartz monzodiorite, quartz monzonite and monzogranite. The presence of acicular apatite, fine grains of mafic-felsic minerals, resorbed maficfelsic xenocrysts and ocellar quartz in ME strongly suggest magma-mixed and undercooled origin for ME. Molar Al₂O₃/CaO+Na₂O+K₂O (A/CNK) ratio of ME (0.68–0.94) and SKG (0.81–1.00) suggests their metaluminous (I-type) character. Linear to sub-linear variations of major elements (MgO, Fe₂O₃ ^t, P₂O₅, TiO₂, MnO and CaO against SiO₂) of ME and SKG and two-component mixing model constrain the origin of ME by mixing of mafic and felsic magmas in various proportions, which later mingled and undercooled as hybrid globules into cooler felsic (SKG) magma. However, rapid diffusion of mobile elements from felsic to mafic melt during mixing and mingling events has elevated the alkali contents of some ME.     

西藏甲玛铜多金属矿床暗色包体岩石成因:对岩浆混合和成矿的启示

岩浆混合作用的研究对揭示壳幔相互作用,探讨成岩成矿过程具有重要意义。甲玛矿区位于冈底斯成矿带东段,为超大型斑岩-矽卡岩型铜多金属矿床,矿区内的中酸性岩浆岩中普遍发育暗色包体,对其中的暗色包体中的闪长质包体开展详细的岩相学、岩石地球化学、Hf同位素地球化学及U-Pb同位素地质年代学等方面研究以期查明岩石成因,为岩浆混合作用和成矿作出启示,完善甲玛成岩成矿模型。岩相学观察表明,闪长质包体及寄主岩浆岩中存在多种反映岩浆混合作用的典型组构,如长石-石英熔蚀结构、石英镶边结构、长石交代筛状结构、长石反环带结构、磷灰石针柱状结构等,锆石LA-ICP-MS UPb同位素定年结果显示,包体形成时代(15. 3±0. 3Ma)与中酸性寄主岩石在误差范围内一致,也符合了岩浆混合作用的存在。闪长质包体化学成分上类似高Mg埃达克岩(MgO=3. 53%～6. 62%,Sr/Y=20～57,(La/Yb)N=51～64),具有低SiO_2(52. 44%～59. 45%),高K_2O(3. 19%～5. 62%),高相容元素(Ni=86×10~(-6)～146×10~(-6); Cr=102×10~(-6)～228×10~(-6))的特征,∑REE高于中酸性寄主岩浆岩,且轻重稀土分异明显((LREE/HREE)N=21～23),富集LILE(Rb=189×10~(-6)～284×10~(-6),Sr=498×10~(-6)～658×10~(-6),Ba=1247×10~(-6)～1378×10~(-6)),相对亏损HFSE(Nb、Ta、Ti),在稀土元素配分图及微量元素蛛网图中闪长质包体介于冈底斯带碰撞后时期的超钾镁铁质岩(来源于富集的岩石圈地幔)与甲玛中酸性寄主岩浆岩(主要来源于加厚新生下地壳)之间,Hf同位素(ε_(Hf)(t)=-0. 9～4. 6)同样也介于超钾镁铁质岩与花岗闪长斑岩(代表中酸性寄主岩浆)之间。这些特征说明闪长质包体是富集的岩石圈地幔部分熔融形成的镁铁质岩浆与加厚新生下地壳部分熔融形成的中酸性岩浆发生混合的产物,同时指示了东冈底斯带中新世时期也存在岩石圈地幔伸展对流减薄事件,以及证实了南拉萨地体广泛分布的高钾埃达克质岩在形成过程中,伴随着与富集岩石圈地幔来源的超钾镁铁质岩浆发生不同程度混合。此外,富集的岩石圈地幔部分熔融形成的镁铁质岩浆的混入,将会为中酸性岩浆系统加入大量的水和金属物质,这也是控制甲玛超大型斑岩-矽卡岩型矿床形成的关键因素。     

班公湖-怒江缝合带西段阿翁错复式岩体的岩浆混合成因:地球化学、年代学和暗色微粒包体证据

阿翁错复式岩体位于班公湖-怒江缝合带西段,是班公湖-怒江特提斯洋俯冲消减,造山过程中岩浆响应的重要组成部分,以广泛发育暗色微粒包体和岩浆混合、不协调现象为特征。本文以阿翁错复式岩体为研究对象,对寄主岩和暗色微粒包体开展了系统的地质学、地球化学和锆石U-Pb年代学研究,探讨了阿翁错复式岩体的岩浆混合成因。暗色微粒包体塑性变形特征明显,与寄主岩呈截然或渐变接触,偶见反向脉发育,包体具细-中粗粒结构,含斜长石、钾长石、角闪石、暗色镶边石英等斑晶,偶见角闪石斑晶横跨包体和寄主岩,在包体及包体周围寄主岩中见长柱状斜长石、角闪石和针状磷灰石等结构特征,表明暗色微粒包体为岩浆混合作用的产物。寄主岩与包体均为准铝质、钙碱性-高钾钙碱性系列岩石,主要氧化物含量在Harker图解上具有良好的线性关系,稀土元素配分曲线图和微量元素蛛网图具有高度一致性,表明二者具有强烈的地球化学亲源关系,且经历了相似的岩浆演化过程。寄主岩和暗色微粒包体的成岩年龄分别为109. 1±1. 0Ma和107. 4±0. 7Ma,岩浆混合作用发生在早白垩世晚期,处于班公湖-怒江特提斯洋由弧-陆碰撞向陆陆碰撞的转换阶段即软碰撞阶段。研究表明,在班公湖-怒江特提斯洋向北向羌塘地块之下俯冲的背景下,洋壳脱水,引起上覆地幔楔发生部分熔融,形成镁铁质岩浆,镁铁质岩浆向上运移,并底侵于壳-幔边界,引发下地壳物质发生部分熔融,形成长英质岩浆,当镁铁质岩浆从底部注入长英质岩浆房时,镁铁质岩浆快速冷凝,形成部分色率高、粒度细,具冷凝边的包体,与寄主岩呈截然型接触,随着端元岩浆之间的温差逐渐降低,包体色率降低,粒度变大,与寄主岩呈渐变过渡。     

Fractional crystallization and magma mixing: evidence from porphyritic diorite-granodiorite dykes and mafic microgranular enclaves within the Zhoukoudian pluton,Beijing

The Zhoukoudian pluton in the North China craton is a circular granodiorite intrusion containing porphyritic diorite dykes (PDDs), porphyritic granodiorite dykes (PGDs) and abundant mafic microgranular enclaves (MMEs), which provide an excellent opportunity to study fractional crystallization and magma mixing. The PDDs and PGDs are located in the western part of the pluton with the PDDs intruded by the PGDs. The dykes have similar mineral assemblages although plagioclase in the PDDs has higher anorthite content than the PGDs. Linear relationships between the SiO₂ and most major and trace element contents, as well as a positive trend of initial ⁸⁷Sr/⁸⁶Sr ratios and a negative trend of epsilon Nd values with increasing SiO₂ contents for the dykes suggest that both types were formed by assimilation and fractional crystallization of a common parental magma. Major oxide mass balance and trace element Rayleigh fractionation modeling points to early separation of garnet (11 %), clinopyroxene (27 %), orthopyroxene (16 %), plagioclase (25 %), biotite (19 %), and apatite (2 %) and late fractionation of hornblende (25 %), plagioclase (46 %), biotite (25 %), apatite (1 %), and magnetite (3 %). Most MMEs occur within the transitional granodiorite of the Zhoukoudian pluton. Zoned MMEs, dyke-like MME swarms, local presence of concave margins, veins and enclaves of host granodiorite within some MMEs, and several MMEs surrounded by the biotite-rich granodiorite support their formation by multiple magma mixing events, which finally resulted in different whole-rock major oxides and compatible elements, but homogeneous mineral major oxides (except zoned plagioclase), whole-rock incompatible elements and Sr-Nd isotopes between the MMEs and their host granodiorite. We suggest that multiple magma mixing events might also cause complexly zoned plagioclase in the Zhoukoudian pluton. Relative calcic, irregular or patchy cores and dusty zoned mantles from the zoned plagioclase crystals and their relatively low anorthite content indicate multiple mixing events between mafic/intermediate and felsic magmas. The mafic/intermediate end-members could be represented by the diabase dykes and the PDDs. Therefore, the dykes and MMEs in the Zhoukoudian pluton are genetically linked.     

Magma mixing in the Sithonia Plutonic Complex, Greece: evidence from mafic microgranular enclaves

Summary ¶Mafic microgranular enclaves occur in most calc-alkaline granitoids, and it is widely accepted that they represent the remnants of basic magmas that interacted with more acid magmas. In this work we present new data on mafic microgranular enclaves occurring in the granodiorites of the Sithonia Plutonic Complex (Northern Greece). Enclave properties have been studied using different methods. Quantitative textural analysis has been carried out in order to decipher the crystallization history of enclaves once they have been entrained in the more acid and cooler host magma. In particular, the nucleation density (C), the mode (M) and the crystal index (n) of enclaves has been measured. Along with textural analysis, the size of enclaves has also been estimated using a method that, based on two-dimensional sections of enclaves, allows the estimation of volume of enclaves. Geochemical analyses have been performed to investigate the degree of chemical interaction that enclaves suffered from the host acid magma. The different data sets have been utilized to furnish a general evolutionary model of the magmatic interaction process between the basic and the acid magmas that led to the formation of the granodioritic host rock and related mafic microgranular enclaves. It is concluded that, as the magmatic interaction process proceeded, the crystallization of enclaves involved the nucleation of apatite and epidote (first stage of crystallization) followed by biotite, ± hornblende, plagioclase, and titanite (second stage of crystallization); the last minerals that nucleate were quartz and K-feldspar. During crystallization enclaves underwent contamination by the host acid magma through flow channels opened during the transfer of mineral phases from the host magma to the enclaves. When the two magmas attained similar rheological behaviour a two-end member mixing process was favoured inducing progressively more vigorous mixing dynamics. Volumetric analysis of enclaves indicates that the smaller ones suffered a more intense geochemical interaction compared to the larger ones. We interpret this evidence as being strictly related to the kinematics of the mixing process, the latter governed by chaotic dynamics. Enclaves are viewed as portions of the basic magma that did not mix completely with the acid host magma and survived the mixing process. Host rocks are considered as volumes of the magmatic system where the more efficient mixing dynamics produced different, generally higher, degrees of hybridisation.Received May 22, 2002; revised version accepted November 5, 2002 Published online February 24, 2003     

Geochemical and isotopic constraints on the role of juvenile crust and magma mixing in the UDMA magmatism,Iran: evidence from mafic microgranular enclaves and cogenetic granitoids in the Zafarghand igneous complex

International Journal of Earth Sciences - The Zafarghand Igneous Complex is composed of granite, granodiorite, diorite, and gabbro that contain many mafic microgranular enclaves. This complex was...     

Plagioclase transfer from a host granodiorite to mafic microgranular enclaves: diverse records of magma mixing

Chemical and structural zoning in plagioclase can develop in response to a number of different magmatic processes. We examine plagioclase zonation formed during the transfer of plagioclase from a granodioritic host to a monzodioritic enclave to understand the development of different zonation patterns caused by this relatively simple magma mixing process. The transferred plagioclase records two stages of evolution: crystallization of oscillatory plagioclase in the host granodioritic magma and crystallization of high An zones and low An rims in the hybrid enclave magma. High An zones (up to An₇₂) are formed only in the hybrid enclaves after plagioclase transfer. Plagioclase from a primitive enclave, showing no or only minimal interaction with the host, is An_30–43. The implication is that high An zones crystallize only from the hybrid magma and not from the primitive one, probably because of an increase in water content in the hybrid magma. Complex interactions between the two magmas are also recorded in Sr content in plagioclase, which indicates an initial increase in Sr concentration in the melt upon transfer. This is contrary to what is expected from the mixing of low Sr enclave magma with a high Sr granodiorite one. Such Sr distribution in the plagioclase implies that the transfer of the plagioclase took place before the onset of plagioclase crystallization in the enclave magma. Therefore, the mixing between high Sr granodiorite magma and low Sr enclave magma was recorded only in plagioclase rims and not in the high An zones.     

U-Pb ages and morphologies of zircon in microgranitoid enclaves and peraluminous host granite: evidence for magma mingling

 Granites of the S-type Wilson's Promontory Batholith (Lachlan Fold Belt, Australia) contain zircons which are euhedral and relatively large; their age is 395 Ma, which can be considered as the best available estimate of the crystallysation age of the granites. Contrary to their dominance in other S-type granites of the Lachlan Fold Belt, very few zircon cores give inherited ages, varying between 500 and 1700 Ma. Microgranitoid enclaves contained within the granites contain a zircon population that is dominated by relatively small, anhedral or elongated crystals. These give ages that are indistinguishable from the crystallisation age of the granite. Some enclaves, which are characterised by the presence of megacrysts, contain a proportion of larger, euhedral zircons. These zircons give inherited ages similar to the zircons from the granitic host rocks. The data are in agreement with a magma mingling origin for the microgranitoid enclaves. The large euhedral zircons are interpreted to have been introduced into the “enclave magma” during a hybridisation event which also introduced quartz and plagioclase megacrysts into the magma. The relatively high proportion of inherited cores within the “large” zircon population of the enclaves is related to the timing of mixing between “enclave” and host magma. This mixing event took place before the majority of the magmatic zircons crystallised in the granitic magma. The small, anhedral zircons within the enclaves crystallised during quenching of the globules of enclave magma against the cooler granitic magma. Received: 21 August 1995 / Accepted: 9 October 1995     

Enclaves in granitoids of north of Jonnagiri schist belt,Kurnool district,Andhra Pradesh: Evidence of magma mixing and mingling

Calc-alkaline, metaluminous granitoids in the north of Jonnagiri schist belt (JSB) are associated with abundant mafic rocks as enclave. The enclaves represent xenoliths of the basement, mafic magmatic enclaves (MME) and synplutonic mafic dykes. The MME are mostly ellipsoidal and cuspate shape having lobate margin and diffuse contact with the host granitoids. Sharp and crenulated contacts between isolated MME and host granitoids are infrequent. The MME are fine-grained, slightly dark and enriched in mafic minerals compare to the host granitoids. MME exhibits evidences of physical interaction (mingling) at outcrop scale and restricted hybridization at crystal scale of mafic and felsic magmas. The textures like quartz ocelli, sphene (titanite) ocelli, acicular apatite inclusion zone in feldspars and K-feldspar megacrysts in MME, megacrysts across the contact of MME and host and mafic clots constitute textural assemblages suggestive of magma mingling and mixing recorded in the granitoids of the study area. The quartz ocelli are most likely xenocrysts introduced from the felsic magma. Fast cooling of mafic magma resulted in the growth of prismatic apatite and heterogeneous nucleation of titanite over hornblende in MME. Chemical transfer from felsic magma to MME forming magma envisage enrichment of silica, alkalis and P in MME. The MME show low positive Eu anomalies whereas hybrid and host granitoids display moderate negative Eu-anomalies. Synplutonic mafic dyke injected at late stage of crystallising host felsic magma, display back veining and necking along its length. The variable shape, dimensions, texture and composition of MME, probably are controlled by the evolving nature and kinematics of interacting magmas.     

Field Evidence of Magma Mixing from Microgranular Enclaves Hosted in Palaeoproterozoic Malanjkhand Granitoids, Central India

The Malanjkhand granitoids (MG) pluton (about 1500 sq km) occurs in the Balaghat district of Madhya Pradesh. The MG (~2400 Ma) represent an episode of Palaeoproterozoic felsic magmatism in Central India and hosts potential Cu (±Mo±Au) deposits. The enclaves hosted in MG can be broadly classified into two categories: microgranular enclaves (dark-coloured, fine-grained magmatic) and xenoliths of country rocks. The microgranular enclaves (ME) may be rounded, ellipsoidal, discoid, elongated, lenticular or tabular, and their size commonly reaches up to 2 metres across. The ME have sharp and in places, diffuse contacts with their host granitoids. The shape and size of ME indicate contemporaneous flow and mingling of partly crystalline felsic-mafic magmas. Some ME exhibit dark crenulated margins giving them a pillow-like form that has been attributed to undercooling of a ME magma as globules intruded into a granitoid magma. The presence of corroded felsic and mafic minerals (xenocrysts) in ME is interpreted as the result of mechanical transfer during the mafic-felsic magma interaction and mixing event. Mafic minerals (biotite) rim the quartz xenocrysts giving rise to ocellar texture, which exhibit signatures of resorption under hybrid (enclave) magma conditions. All these features suggest an origin for the calc-alkaline intermediate granitoid magma in Malanjkhand involving a magma mixing process.     

Mafic microgranular enclaves in Late Paleozoic granitoids in the Burgasy quartz syenite massif, western Transbaikalia: Composition and petrogenesis

The paper presents original data on the inner structure, mineralogy, and geochemistry of the Late Paleozoic Burgasy quartz syenite massif in western Transbaikalia and mafic microgranular enclaves (MME) in its rocks. The composition of the mafic microgranular enclaves is close to that of phase-1 monzonitoids of this pluton, but the enclaves are not xenoliths of these rocks but were produced by the crystallization of an individual portion of dispersed hybridized basalt melt. The basaltoid nature of the enclaves follows, first of all, from the relict assemblage of calcic plagioclase (An _73–60) and clinopyroxene and from the magmatic dolerite and microgabbro textures of the rocks. The monzonitoid composition of the enclaves was caused by hybridism, which was responsible for the crystallization of quartz, potassic feldspar, and sodic plagioclase due to the introduction of silica, potassium, and some other components. Hybridism was restricted to a boundary crystallization layer in the deep portion of the magmatic chamber (near its bottom). The scatter of the enclaves throughout the whole volume of the pluton is explained by the density inversion of the hybrid layer and material transfer by convective flows. The mafic enclaves crystallized from basaltic melt of within-plate geochemical type. In spite of intense hybridism, the enclaves preserved typical compositional signatures of mafic magma related to the generation of granites in western Transbaikalia in the Late Paleozoic. The basaltoid nature of the mafic enclaves of the Burgasy Massif testifies that magma was simultaneously generated in the mantle and crust during the development of the Late Paleozoic province in the area.     

Magma mixing and mingling textures in granitoids: examples from the Galway Granite, Connemara, Ireland

Summary ?Many granitoid intrusions display textural evidence for the interaction of mafic and silicic magmas during their genesis. The ∼ 400 Ma Galway Granite exhibits excellent evidence for magma mixing and mingling both at outcrop/map scale (magma mingling and mixing zones), and at thin-section/crystal scale (mixing textures). These textures – quartz ocelli, rapakivi feldspars, acicular and mixed apatite morphologies, inclusion zones in feldspars, anorthite ‘spikes’ in plagioclase, sphene ocelli, K-feldspar megacrysts in mafic microgranular enclaves (MME), and mafic clots – constitute a textural assemblage whose origin can be explained in terms of magma mixing and mingling models. Furthermore, textures from this assemblage have been recorded throughout the Galway batholith indicating that magma mingling and mixing played a key role during its evolution. Received November 18, 2000; revised version accepted November 6, 2001     

骑田岭花岗岩体的岩浆混合成因:寄主岩及其暗色闪长质微细粒包体的锆石U-Pb年龄和Hf同位素证据

南岭骑田岭复式岩体仰天湖单元燕山期二长花岗岩中广泛分布有暗色闪长质微细粒包体,这些包体主要呈被打散的片云状或次圆状,大小不等,颜色较寄主岩深,粒度较细。本文在锆石内部结构的CL图像特征研究基础上,用LA-ICP-MS和MC-ICP-MS测定了锆石U-Pb年龄及Hf同位素组成,探讨了寄主二长花岗岩、闪长质微细粒包体的锆石U-Pb年代学及Hf同位素组成特征。获得寄主二长花岗岩、闪长质微细粒包体的结晶年龄为(166.0±2.0)Ma和(169.8±4.1)Ma,对应的εHf(t)值为-13.2～-1.9与-8.3～+6.7。研究表明,骑田岭复式岩体中的仰天湖岩体是以古老地壳物质熔融为主体的壳源岩浆与幔源岩浆高度混合的产物。     

Mafic to hybrid microgranular enclaves in the Ladakh batholith,northwest Himalaya: Implications on calc-alkaline magma chamber processes

Felsic magmatisms in the north of Indus-Tsangpo Suture Zone (ITSZ) in Ladakh range of northwest Indian Himalaya, referred herein Ladakh granitoids (LG), and associated magmatic rocks constitute the bulk of the Ladakh batholith. They have been characterized as Andean-type, calc-alkaline, largely metaluminous (I-type) to a few peraluminous (S-type) granitoids derived from partial melting of subducting materials. The LG can be broadly classified into coarsegrained facies with abundant mafics (hbl-bt), medium-grained facies with low content of mafics, and fine-grained leucocratic facies with very low amount of mafics. Mesocratic to melanocratic, rounded to elliptical, fine to medium grained, mafic to hybrid microgranular enclaves (ME) are ubiquitous in medium to coarse-grained LG. ME are absent or rare in the leucocratic variety of LG. In this paper different types of ME, and their field relation and microstructures with respect to felsic host LG are documented from northwestern, central, southeastern parts of the Ladakh batholith. Rounded to elongate ME of variable sizes (a few cm to metres across, mostly d<30 cm) commonly having sharp, crenulate, and occasionally diffuse contacts of ME with felsic host LG suggest that several pulses of crystal-charged mafic and felsic magmas coexisted, hybridized, and co-mingled into subvolcanic settings. Occurrence of composite ME (several small mafic ME enclosed into large porphyritic ME) strongly point to multiple mafic to hybrid magma intrusions into partly crystalline LG magma chambers. Synplutonic mafic dykes disrupted to form subrounded to angular (brecciated) mafic ME swarms commonly disposed in strike-length suggest mafic magma injections at waning stage of felsic magma evolution with large rheological contrasts. Pillowing of mafic melt against leucocratic (aplitic) residual melt strongly suggests mafic magma intrusion in nearly-crystallized condition of pluton. Although common mineral asemblages (hblbt-pl-kfs-qtz-ap-zrn-mt±ilm) of ME (diorite, quartzdiorite) and host LG (granodiorite, monzogranite) may relate to their cogenetic relation, fine to medium grained porphyritic (hybrid) nature and lack of cumulate texture of ME strongly oppose cognate origin for ME. Presence of plagioclase xenocrysts, quartz ocelli and accicular apatite in porphyritic ME strongly indicate mingling and undercooling of hybridized ME globules into relatively crystal-charged cooler host LG magma. Grain size differences of some ME, except to those of porphyritic ones, appear related to varying degrees of undercooling of ME most likely controlled by their variable sizes. Several smaller ME, however, lack fine-grained chilled margin probably because of their likely disaggregation from a large size ME during the course of progressive hybridization (mingling to mixing) leaving behind trails of mafic schlieren. Field and microstructural evidences at least suggest that Ladakh granitoids and their microgranular enclaves are products of multistage magma mingling and mixing processes concomitant fractional differentiation of several batches of mafic and felsic magmas formed in open magma chamber(s) of subduction setting.     

新疆拜城县波孜果尔A型花岗岩类矿物学特征及岩浆形成的温度条件

新疆拜城县波孜果尔A型花岗岩类为富含铌、钽、锆等有用元素的含矿岩体。通过偏光显微镜、电子探针(EPMA)化学成分分析、电子探针背散射(BSE)对波孜果尔A型花岗岩类的矿物学特征进行了研究,并对岩浆形成的温度条件与构造背景进行了讨论。结果表明,波孜果尔A型花岗岩类包括霓石钠闪石英碱性长石正长岩、霓石钠闪碱性长石花岗岩、黑云母碱性长石正长岩3种岩石类型。主要造岩矿物包括石英、钠长石、钾长石、霓石、钠铁闪石和铁叶云母。副矿物包括锆石、烧绿石、钍石、萤石、独居石、氟碳铈镧矿、磷钇矿等。岩浆平均温度832~839℃,形成于非造山的板内构造环境,且具高温、无水、低氧逸度的成岩特点。     

义敦岛弧措交玛岩体岩浆混合成因:镁铁质微粒包体的证据

义敦岛弧北部的措交玛岩基岩体主要由黑云母二长花岗岩和边部的花岗闪长岩组成。在黑云母二长花岗岩中存在有少量镁铁质微粒包体,其成分为闪长质,与寄主岩石接触关系从渐变到截然。在包体周围的寄主岩石中存在黑云母、角闪石自身的包含结构,角闪石包含黑云母,斜长石发育明显的溶蚀结构,核部斜长石被溶蚀成筛状,边部环带状斜长石溶蚀不明显,是基性岩浆注入到酸性岩浆中导致岩浆混合的结果。黑云母二长花岗岩具有更高的轻重稀土分异系数,闪长质包体轻重稀土分异系数较低,黑云母二长花岗岩和暗色闪长质微粒包体具有明显相似性的微量元素特征。寄主岩黑云母二长花岗岩锆石U-Pb年龄为236±1.9Ma,闪长质包体为235±3.9Ma,二者形成年代在误差范围内基本一致,可能为甘孜-理塘洋向西俯冲过程中,俯冲洋壳部分熔融形成的玄武质岩浆上涌底侵于壳-幔边界导致地壳的部分熔融形成酸性的黑云母二长花岗岩岩基。