西藏冈底斯带始新世曲水岩基的岩浆混合作用:来自斜长石阴极发光特征和成分变化的证据

斜长石作为主要造岩矿物,是研究岩石成因、示踪岩浆演化和岩浆混合过程的有效工具.对冈底斯带曲水岩基始新世花岗闪长岩、二长花岗岩、闪长岩脉和暗色包体中的斜长石进行了阴极发光图像结构特征、电子探针主量元素和LA-ICP-MS微量元素成分的分析,揭示了斜长石复杂环带的成因和相关的岩浆过程.该区斜长石的阴极发光图像呈现出多种颜色且与其An值相对应,随着An值降低依次为绿色、蓝色和暗灰色或暗红色等,并发育补丁状环带、筛状环带、韵律环带等.花岗闪长岩、二长花岗岩中斜长石的An值具有相似的变化范围(20~55),而闪长岩脉和暗色包体中An值的变化范围较大(25~85),表明曲水岩基经历了复杂的开放过程.微量元素结果表明:花岗闪长岩与闪长岩脉和暗色微粒包体具有相同的Sr含量范围(600×10^-6~1 100×10^-6);而二长花岗岩的Sr含量(1 000×10^-6~2 400×10^-6)整体高于前者.以上研究表明,花岗闪长岩中阴极发光呈现绿色的核部或幔部是偏中性岩浆注入寄主岩岩浆混合的结果;具有高Sr含量的二长花岗岩认为是高Sr含量的岩浆结晶形成的;闪长岩脉和暗色微粒包体中的筛状结构斜长石为寄主岩捕掳晶.     

Field evidence,mineral chemical and geochemical constraints on mafic-felsic magma interactions in a vertically zoned magma chamber from the Chotanagpur Granite Gneiss Complex of Eastern India

The Nimchak granite pluton (NGP) of Chotanagpur Granite Gneiss Complex (CGGC), Eastern India, provides ample evidence of magma interaction in a plutonic regime for the first time in this part of the Indian shield. A number of outcrop level magmatic structures reported from many mafic-felsic mixing and mingling zones worldwide, such as synplutonic dykes, mafic magmatic enclaves and hybrid rocks extensively occur in our study domain. From field observations it appears that the Nimchak pluton was a vertically zoned magma chamber that was intruded by a number of mafic dykes during the whole crystallization history of the magma chamber leading to magma mixing and mingling scenario. The lower part of the pluton is occupied by coarse-grained granodiorite (64.84–66.61?wt.% SiO₂), while the upper part is occupied by fine-grained granite (69.80–70.57?wt.% SiO₂). Field relationships along with textural and geochemical signatures of the pluton suggest that it is a well-exposed felsic magma chamber that was zoned due to fractional crystallization. The intruding mafic magma interacted differently with the upper and lower granitoids. The lower granodiorite is characterized by mafic feeder dykes and larger mafic magmatic enclaves, whereas the enclaves occurring in the upper granite are comparatively smaller and the feeder dykes could not be traced here, except two late-stage mafic dykes. The mafic enclaves occurring in the upper granite show higher degrees of hybridization with respect to those occurring in the lower granite. Furthermore, enclaves are widely distributed in the upper granite, whereas enclaves in the lower granite occur adjacent to the main feeder dykes.Geochemical signatures confirm that the intermediate rocks occurring in the Nimchak pluton are mixing products formed due to the mixing of mafic and felsic magmas. A number of important physical properties of magmas like temperature, viscosity, glass transition temperature and fragility have been used in magma mixing models to evaluate the process of magma mixing. A geodynamic model of pluton construction and evolution is presented that shows episodic replenishments of mafic magma into the crystallizing felsic magma chamber from below. Data are consistent with a model whereby mafic magma ponded at the crust-mantle boundary and melted the overlying crust to form felsic (granitic) magma. The mafic magma episodically rose, injected and interacted with an overlying felsic magma chamber that was undergoing fractional crystallization forming hybrid intermediate rocks. The intrusion of mafic magma continued after complete solidification of the magma chamber as indicated by the presence of two late-stage mafic dykes.     

周口店岩体矿物学、年代学、地球化学特征及其岩浆起源与演化

周口店岩体由三次侵入的中酸性岩石组成, 本次测得石英闪长岩锆石U-Pb年龄为131.6±2.1 Ma, 闪长玢岩锆石U-Pb年龄为128.1±1.4 Ma.周口店岩体各种类型岩石属高钾钙碱性系列、偏铝质, Mg#较高, 重稀土元素和Ta、Nb、P以及Ti明显亏损, 轻稀土元素和Ba、K以及Sr相对富集, Eu没有异常, Yb元素含量小于2×10-6, (La/Yb)_N和Sr/Y比值较高.斜长石复杂环带能谱线扫描表明, 花岗闪长岩中的斜长石核部牌号高, 完整的幔部由内向外由反环带和正环带组成, 微粒包体中的斜长石核部牌号低, 幔部以尘状环带开始, 然后演变为正环带, 这揭示存在多期基性岩浆的注入作用, 结合暗色微粒包体的形态、大小、数量、反向脉、矿物含量统计、矿物成分、地球化学和各类环带包体、岩墙状包体群等特征, 说明暗色微粒包体是在花岗闪长岩浆冷凝过程的不同阶段, 多期幔源基性岩浆注入并与酸性岩浆在围绕包体周缘的局部范围内发生不均一机械混合作用的结果.周口店中酸性岩石体现埃达克质岩的地球化学特征, 岩浆成分主要受源区控制, 形成于加厚下地壳环境.由石英闪长岩-花岗闪长岩至中酸性岩脉, 岩石(Er/Lu)_N和Nb/Ta比值升高, 说明源区残留相矿物组合由角闪石+石榴石向石榴石+金红石变化, 岩浆源区不断变深.      

西秦岭德乌鲁岩体成因及地质意义——来自岩石地球化学的证据

西秦岭德乌鲁岩体位于夏河-合作断裂和力士山-围当山断裂之间,寄主岩石以花岗闪长岩为主,石英闪长岩和石英二长闪长岩次之,并含有大量暗色微粒包体(MMEs).对德乌鲁岩体开展了年代学研究,获得的花岗闪长岩的LA-ICP-MS锆石U-Pb年龄为225.9±1.3 Ma,表明该岩体形成于晚三叠世早期,可能与扬子板块与华北板块陆...     

Zircon U–Pb geochronology,major-trace elements and Sr–Nd isotope geochemistry of Mashhad granodiorites (NE Iran) and their mafic microgranular enclaves: evidence for magma mixing and mingling

ABSTRACT

Mashhad granitoids and associated mafic microgranular enclaves (MMEs), in NE Iran record late early Mesozoic magmatism, was related to the Palaeo-Tethys closure and Iran-Eurasia collision. These represent ideal rocks to explore magmatic processes associated with Late Triassic closure of the Palaeo-Tethyan ocean and post-collisional magmatism. In this study, new geochronological data, whole-rock geochemistry, and Sr–Nd isotope data are presented for Mashhad granitoids and MMEs. LA–ICP–MS U–Pb dating of zircon yields crystallization ages of 205.0 ± 1.3 Ma for the MMEs, indicating their formation during the Late Triassic. This age is similar to the host granitoids. Our results including the major and trace elements discrimination diagrams, in combination with field and petrographic observations (such as ellipsoidal MMEs with feldspar megacrysts, disequilibrium textures of plagioclase), as well as mineral chemistry, suggest that MMEs formed by mixing of mafic and felsic magmas. The host granodiorite is a felsic, high K calc-alkaline I-type granitoid, with SiO₂ = 67.5–69.4 wt%, high K₂O (2.4–4.2 wt%), and low Mg# (42.5–50.5). Normalized abundances of LREEs and LILEs are enriched relative to HREEs and HFSEs (e.g. Nb, Ti). Negative values of whole-rock εNd_(t) (?3 to ?2.3) from granitoids indicate that the precursor magma was generated by partial melting of enriched lithospheric mantle with some contributions from old lower continental crust. In the MMEs, SiO₂ (53.4–58.2 wt%) is lower and Ni (3.9–49.7 ppm), Cr (0.8–93.9 ppm), Mg# (42.81–62.84), and εNd_(t) (?2.3 to +1.4) are higher than those in the host granodiorite, suggesting a greater contribution of mantle-derived mafic melts in the genesis of MMEs.     

青海南山黑马河岩体岩浆混合成因的岩石地球化学和年代学证据及其构造意义

黑马河岩体位于西秦岭北缘青海南山构造带内,岩体中发育大量闪长质包体,但对其成因却有一定的争议.从岩石学、矿物学、地球化学和年代学等方面对黑马河岩体花岗闪长岩及其中的闪长质包体进行了详细研究.包体的野外产出状态、形态、结构构造和矿物学特征均显示出,它们是基性岩浆注入到中酸性岩浆中快速冷凝结晶的产物.在主量元素协变图解中,花岗闪长岩和闪长质包体显示出壳幔岩浆混合作用的趋势.另外,两者稀土总量和相似的稀土元素配分模式及Eu负异常程度,也显示二者具有岩浆混合的特征.LA-ICPMS锆石U-Pb定年结果显示,花岗闪长岩形成于246 Ma,闪长质包体形成于245 Ma,两者具有几乎一致的结晶年龄,排除了包体为源区难熔残余或围岩捕掳体的可能性,也排除了基性岩浆在花岗质岩浆固结后再侵入的可能性.结合区域地质资料,认为黑马河岩体形成于西秦岭北缘有限小洋盆向南的俯冲阶段,青海南山构造带与天峻南山一带具有相同的构造岩浆演化历史.      

东昆仑东段三叠纪岩浆混合作用:以香加南山花岗岩基为例

香加南山花岗岩基位于东昆仑造山带东段,岩基主要岩石类型为花岗闪长岩。千瓦大桥-加鲁河一带花岗岩体为香加南山岩基的重要组成部分。香加南山花岗岩基含大量暗色微粒包体,包体中捕掳晶丰富。千瓦大桥-加鲁河一带花岗岩体寄主岩中斜长石和暗色微粒包体中捕掳晶斜长石具正常环带,An值震荡变化,角闪石和黑云母Mg O含量和Mg#值较低,具壳源特征;暗色微粒包体中基质斜长石具核边结构,核部和边部An值存在间断,角闪石和黑云母Mg O含量和Mg#值较高,具幔源特征。LA-ICP-MS锆石U-Pb同位素定年结果显示千瓦大桥花岗闪长岩、暗色微粒包体和加鲁河辉长岩的结晶年龄分别为251.0±1.9Ma、252.8±3.0Ma和221.4±3.3Ma。千瓦大桥花岗闪长岩和加鲁河花岗闪长岩富集轻稀土元素(LREE)和大离子亲石元素(LILE),亏损高场强元素(HFSE),具较低的Mg#和Nb/Ta比值;从千瓦大桥到加鲁河花岗闪长岩呈现出由准铝质中钾钙碱性系列向准铝-弱过铝质中钾-高钾钙碱性系列演化;暗色微粒包体和加鲁河辉长岩轻重稀土元素分异程度相对较低,具较高的Mg#和Nb/Ta比值。千瓦大桥花岗闪长岩和加鲁河花岗闪长岩分别为古特提斯演化俯冲阶段和后碰撞阶段幔源岩浆底侵新生地壳使其部分熔融产物。镁铁质岩浆注入长英质岩浆的混合作用形成了暗色微粒包体。岩浆混合过程中,如果岩浆不完全混合,混合岩浆中混入物质除了长英质岩浆的残留岩浆和捕掳晶,还应该有镁铁质岩浆与长英质岩浆之间的元素梯度差导致的物质扩散;如果岩浆为近完全混合,混合岩浆近似为镁铁质岩浆和长英质岩浆以一定比例二元混合。东昆仑东段晚古生代-早中生代幔源岩浆对花岗质岩浆的影响是一个持续的过程,从俯冲阶段早期流体交代地幔熔融,到俯冲阶段后期板片断离,然后同碰撞阶段板片断离的持续影响,再到后碰撞阶段加厚地壳的拆沉作用,由于地球动力学体制不同,导致幔源岩浆影响的大小和特征不同。     

Identification of Magma Mixing: A Case Study of the Daocheng Batholith in the Yidun Arc

The Daocheng batholiths, located in the east of the Yidun arc, consist of granite, granodiorite and K-feldspar granite. Abundant massive mafic microgranular enclaves (MMEs) mainly developed within the granodiorite and K-feldspar granite, and they have clear contacts with the hosted granites. The MMEs are characterized by the quartz eye structure, quenched apatite, and plagioclases phenocrysts with obvious oscillatory zones. Petrographical studies on MMEs and host granites, zoned plagioclase and whole-rock geochemical analysis were carried out to identify the presence of magma mixing. Combined with the previous studies on the whole-rock Sr-Nd-Hf isotopic signatures, the petrogenesis of Daocheng batholith was discussed. The zoned plagioclases from MMEs have An contents varying between 29 and 44, while those from the host granites have An contents of 21~50. The compositional variations and corrosion structure of plagioclase are probably related to magma mixing. Geochemically, the MMEs have relatively low SiO₂ contents (56.34~60.91wt%), high Al₂O₃ contents of 16.06~17.98wt%, and are enriched in magnesium and iron, belonging to metalumnious series (A/CNK=0.82~0.98). The Daocheng batholith belongs to high-K calc-alkaline series, which have high alkaline contents (Na₂O+K₂O=6.25~7.79wt%) and low CaO contents (1.40~3.22wt%). Furthermore, both the MMEs and hosted granites are enriched in LILEs (K, Rb and Pb) and LREEs and depleted in HFSEs (Nb, Ta, Zr, Hf, P and Ti), showing affinities of typical arc magmas. Compared with the host granites, the MMEs are characterized by lower (La/Yb)_N ratios of 1.99 to 2.46, and much more obvious Eu depletions (Eu/Eu^*=0.30~0.50). The host granites have Rb/Sr ratios ranging from 1.0 to 1.9, and they are consistent with the crust-derived materials (Rb/Sr>0.5). Their Zr/Hf ratios range from 27.5 to 36.9, which are close to the transitional Zr/Hf ratios between mantle-and crust-derived materials. This indicates that the formation of Daocheng batholith is genetically related to the mixing between mantle-and crust-derived materials. In addition, the relatively low silica contents and high Mg# values, and the linear patterns of MgO, Al₂O₃ and Fe₂O₃ with SiO₂ contents from the MMEs and host granites, show that the formation of MMEs is genetically related to magma mixing. Overall, the parent magmas of Daocheng granites are derived from the partial melting of Late Triassic arc lower crust, with the input of minor mantle-derived materials. The MMEs are generated by the mixing of the mafic magma with felsic magma.     

东昆仑沟里地区暗色包体及其寄主岩石地球化学特征及成因

通过青海东昆仑东部沟里地区阿斯哈岩体中寄主闪长岩和暗色微粒包体的岩相学、全岩地球化学研究,确定了岩石成因及其构造属性。阿斯哈岩体中暗色包体广泛分布,包体岩性主要为角闪辉长岩。包体具有岩浆结构,部分包体具有塑性流变特征,包体中可见寄主岩石矿物的捕掳晶和针状磷灰石,表现出岩浆混合的岩相学特征。主岩及暗色包体同属准铝质、高钾钙碱性-钾玄岩系列过渡岩石,主量元素在Harker图解及Al₂O₃/K₂O-CaO/K₂O和SiO₂/CaO-K₂O/CaO的共分母协变图上具良好的线性关系,反映两者成分的变化与岩浆混合作用有关。两者的稀土元素配分模式总体一致,显示二者密切的成因联系。两者都富含大离子亲石元素（Rb、K）,相对亏损高场强元素（Nb、Ta、P、Ti）。暗色包体具有贫硅（w(SiO₂)=50.70%~53.88%）和富镁、铁、钙的地球化学特征,其Mg^#值较高（Mg^#=0.52~0.59）,暗示其来源于俯冲带流体交代地幔楔的部分熔融。主岩的Rb/Sr值为0.22~0.27,接近地壳平均值,Nb/Ta值为14.5~15.2,介于地幔平均值与地壳平均值之间,表明寄主岩石岩浆具有壳源岩浆的性质并经历了幔源岩浆的混合作用。结合区域构造演化及构造判别,认为阿斯哈岩体形成于安第斯型活动大陆边缘的构造环境。早三叠世,阿尼玛卿洋向北俯冲,俯冲带流体交代地幔楔,导致其部分熔融形成基性岩浆,底侵的幔源基性岩浆诱发下地壳部分熔融并与之发生混合形成本区闪长岩,而其中的暗色包体为幔源岩浆混合不彻底的产物。     

岩浆混合作用与火成岩多样性的耦合关系:以东昆仑造山带白日其利长英质岩体为例

壳-幔岩浆相互作用如何影响长英质火成岩的岩石学多样性是当前岩石学研究的焦点问题之一.以岩石类型丰富的东昆仑白日其利长英质岩体和暗色微粒包体为研究对象,开展系统的锆石U-Pb年代学、矿物学、全岩元素地球化学和Sr-Nd-Hf同位素研究,探讨和解析这一重要科学问题.LA-ICPMS锆石U-Pb年代学研究表明,暗色微粒包体（247.8±2.0 Ma）与二长花岗岩（247.5±1.4 Ma）、花岗闪长岩（248.8±2.1 Ma）和石英闪长岩（248.8±1.5 Ma）均侵位结晶于早三叠世.岩相学和矿物学研究表明,白日其利长英质岩石与包体的成因机制与壳-幔岩浆的机械或化学混合作用密切相关.元素地球化学和Sr-Nd-Hf同位素组成研究揭示,幔源镁铁质岩浆端元起源于受俯冲板片流体交代的富集地幔熔融,而壳源长英质岩浆端元则起源于东昆仑古老的变质杂砂岩基底.岩石成因分析揭示,幔源镁铁质岩浆侵入长英质晶粥岩浆房,促使长英质晶粥发生活化,随后壳-幔岩浆端元以不同比例和不同方式发生机械和化学混合等相互作用,从而形成镁铁质岩墙、包体、石英闪长岩和花岗闪长岩等多种岩石类型.晶粥状态下壳-幔岩浆相互作用是控制东昆仑长英质火成岩多样性和大陆地壳生长演化的重要方式.      

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.     

http://www.sciencedirect.com/science/article/pii/S1674987113001151

Magma mixing process is unusual in the petrogenesis of felsic rocks associated with alkaline complex worldwide. Here we present a rare example of magma mixing in syenite from the Yelagiri Alkaline Comp...     

Magmatic history during late Carboniferous to early Permian in the North of the central Xing’an-Mongolia Orogenic Belt: a case study of the Houtoumiao pluton,Inner Mongolia

We undertake zircon U–Pb dating, Hf isotopes, and geochemical analyses of the Houtoumiao pluton in the Xilinhot microcontinent (XLMC) in the central Inner Mongolia with an aim of determining their ages, petrogenesis, and sources, which are important for understanding the late Palaeozoic tectonic evolution of the Xing’an-Mongolia Orogenic Belt. The Houtoumiao pluton consists of medium-grained granodiorite, coarse- and medium-grained syenogranite. Mafic microgranular enclaves (MMEs) are common in the Houtoumiao pluton. Zircon U–Pb dating has yielded ages of 303 ± 2 and 301 ± 2 Ma for the granodiorite, 295 ± 2 Ma for the syenogranite, and 292 ± 1 Ma for the MMEs. The granodiorite and syenogranite have features of high-K high silica content, rich in Rb, U, and Th, but low content of HFSE, belong to calc-alkaline series. The P₂O₅ concentration decreases with the increasing SiO₂ content, suggesting I-type affinity. The MMEs, which are characterized by low SiO₂, relatively high and variable TiO₂, Al₂O₃, FeO^T, MgO, CaO, Ni, and Cr contents, also have much higher total rare earth element concentrations that the REE patterns are subparallel to those of the host rocks. Zircons from the host rocks have ε_Hf(t) values from +3.91 to +7.73 and T_DM2 values of 820–1067 Ma, suggesting that the granitoids were probably dominated by remelting of juvenile crust materials. The MMEs are of ε_Hf(t) value ranging from +6.23 to +11.04 and T_DM1 values from 490 to 693 Ma, suggesting that the primary magma probably was derived from partial melting of a depleted lithospheric mantle, the mafic mineral fractional crystallization and crustal contamination occurred during the magma evolution. Combined with previous studies on the contemporaneous magma-tectonic activities in the Uliastai Continental Margin and XLMC, we suggest that the Houtoumiao pluton formed in a post-orogenic setting.     

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.     

Grain to outcrop-scale frozen moments of dynamic magma mixJing in the syenite magma chamber,Yelagiri Alkaline Complex,South India

Magma mixing process is unusual in the petrogenesis of felsic rocks associated with alkaline complex worldwide. Here we present a rare example of magma mixing in syenite from the Yelagiri Alkaline Complex, South India. Yelagiri syenite is a reversely zoned massif with shoshonitic (Na₂O + K₂O=5–10 wt.%, Na₂O/K₂O = 0.5–2, TiO₂ <0.7 wt.%) and metaluminous character. Systematic modal variation of plagioclase (An_11–16 Ab_82–88), K-feldspar (Or_27–95 Ab_5–61), diopside (En_34–40Fs_11–18Wo_46–49), biotite, and Ca-amphibole (edenite) build up three syenite facies within it and imply the role of in-situ fractional crystallization (FC). Evidences such as (1) disequilibrium micro-textures in feldspars, (2) microgranular mafic enclaves (MME) and (3) synplutonic dykes signify mixing of shoshonitic mafic magma (MgO = 4–5 wt.%, SiO₂ = 54–59 wt.%, K₂O/Na₂O = 0.4–0.9) with syenite. Molecular-scale mixing of mafic magma resulted disequilibrium growth of feldspars in syenite. Physical entity of mafic magma preserved as MME due to high thermal-rheological contrast with syenite magma show various hybridization through chemical exchange, mechanical dilution enhanced by chaotic advection and phenocryst migration. In synplutonic dykes, disaggregation and mixing of mafic magma was confined within the conduit of injection. Major-oxides mass balance test quantified that approximately 0.6 portions of mafic magma had interacted with most evolved syenite magma and generated most hybridized MME and dyke samples. It is unique that all the rock types (syenite, MME and synplutonic dykes) share similar shoshonitic and metaluminous character; mineral chemistry, REE content, coherent geochemical variation in Harker diagram suggest that mixing of magma between similar composition. Outcrop-scale features of crystal accumulation and flow fabrics also significant along with MME and synplutonic dykes in syenite suggesting that Yelagiri syenite magma chamber had evolved through multiple physical processes like convection, shear flow, crystal accumulation and magma mixing.     

Eocene I-type magmatism in the Eastern Pontides,NE Turkey: insights into magma genesis and magma-tectonic evolution from whole-rock geochemistry,geochronology and isotope systematics

ABSTRACT

The Eastern Pontides orogenic belt in NE Turkey hosts numerous I-type plutons of Eocene epoch. Here, we report new U–Pb SHRIMP zircon ages and in situ zircon Lu-Hf isotopes along with bulk-rock geochemical and Sr-Nd-Pb-O isotope data from the Kemerlikda??, Ayd?ntepe and Pelitli plutons and mafic microgranular enclaves (MMEs) to constrain their parental melt source(s) and evolutionary processes. U-Pb SHRIMP zircon dating yielded crystallization ages between 45 and 44 Ma for the studied plutons and their MMEs. The plutons range from gabbro to granite and have I-type, medium to high-K calc-alkaline, and metaluminous to slightly peraluminous characteristics. On the primitive mantle-normalized multi-trace-element variations, the plutons and their MMEs are characterized by signi?cant enrichment in LILE/HFSE. Chondrite-normalized REE patterns of the plutons and their MMEs are close to each other and show moderate enrichment with variable negative Eu anomalies. The studied plutons have fairly homogeneous isotope composition (⁸⁷Sr/⁸⁶Sr_(i) = 0.70502 to 0.70560; εNd_(i) = +0.9 to – 1.4; δ¹⁸O = +5.0 to +8.7‰, εHf_(i) = – 2.2 to +13.5). The MMEs show medium to high-K calc-alkaline and metaluminous character. Although the isotope signatures of the MMEs (⁸⁷Sr/⁸⁶Sr_(i) = 0.70508 to 0.70542; εNd_(i) = +0.9 to ?1.1; δ¹⁸O = +5.8 to +8.0, εHf_(i) = +4.3 to +10.4) are very similar to those of the host rocks. Fractionation of plagioclase, amphibole, pyroxene and Fe-Ti oxides played an important role in the evolution of the plutons. The isotopic composition of the studied plutons and MMEs are similar to I-type plutons derived from mantle sources. The MMEs show incomplete magma mixing/mingling, representing small bodies of mafic parental magma. The parental magma(s) of the studied plutons were generated from the enriched lithospheric mantle and then modified by fractional crystallisation, and lesser assimilation and mixing/mingling in the crustal magma chambers.     

花岗岩暗色微粒包体特征及其研究方向

通过调研近年来国内外学者对于花岗岩类中暗色微粒包体的研究成果,总结了其野外地质、岩相学、地球化学和同位素等特征以及成因研究中的主要问题。暗色微粒包体(MME)广泛分布于I型花岗岩中,一般呈随机分布,形态多样,但以塑性特征为主;一般认为是岩浆混合成因,其原始物源是玄武质岩浆,具典型的火成岩结构,发育针状磷灰石、环带结构和筛状结构的斜长石;相对于寄主岩石,暗色微粒包体富Fe,Mg,贫Si,Na,其锆石U-Pb年龄在误差范围内与寄主岩石年龄一致等,反映了壳幔源岩浆的混合事件。暗色微粒包体与岩浆动力学行为、矿化关系及区域地质演化等方面值得进一步研究、探索。     

Double injection events of mafic magma into supersolidus Yucheon granites to produce two types of mafic enclaves in the Cretaceous Gyeongsang Basin,SE Korea

Petrographic and geochemical features of the Cretaceous Yucheon granites and their mafic microgranular/magmatic enclaves (MMEs), SE Korea, reveal that the MMEs originated from magma mixing. Mesoscopic and microscopic features indicate that mechanical mixing operated heterogeneously to produce the MMEs with a wide range of sizes and textures. Chemical compositions of amphibole, biotite, and plagioclase rims of both the MMEs and host granites are almost identical, indicating that chemical homogenization took place to some extent after the mechanical mixing. Plagioclase cores, however, have various compositions depending on the host rocks and/or sampling locations, suggesting their sluggish re-equilibration. The MMEs are divided into Type A (low TiO₂, very fine-grained, chilled margins) and Type B (high TiO₂, fine- to medium-grained, no chilled margins). The lower TiO₂ MMEs cooled more rapidly and interacted with granitic magma for a shorter period of time than the higher TiO₂ MMEs. Additionally, the former are less enriched in HREEs than the latter. Zoned plagioclase has two zones of increased An content. These features are indicative of double injection events of mafic magma. A previous model explains the magma mixing as resulting from the generation of a slab window due to Kula-Pacific ridge subduction. The model cannot, however, explain the eastward younging of the granites in Korea, necessitating a new, more elaborate model of Cretaceous geodynamics and magmatism in East Asia.     

Eocene tonalite–granodiorite from the Havza (Samsun) area,northern Turkey: adakite-like melts of lithospheric mantle and crust generated in a post-collisional setting

ABSTRACT

Eocene intermediate to felsic plutons of different sizes and compositions are widespread in the Eastern Pontides Orogenic Belt in northern Turkey. Of these, the Ta?l?k Tepe pluton in the Havza (Samsun) area is fine-to-medium-grained, with granular, porphyritic, and micrographic textures, and include mafic microgranular enclaves (MMEs). LA-ICP-MS U-Pb zircon dating yielded emplacement ages of 42.9 (± 1.4) and 40.5 (± 1.3) Ma for the host granodioritic pluton and the dioritic MMEs, respectively. Petrochemically, the host pluton has I-type, high-K calc-alkaline, and metaluminous-to-slightly peraluminous features (A/CNK = 0.95–1.06). The host pluton also shows geochemical features of adakite-like rocks with high SiO₂ (67–68 wt%) and Al₂O₃ (15.5–16.0 wt%) content and Ba/La (17–23), Sr/Y (40.7–61.6), and La_N/Yb_N (14.4–23.7) ratios and low Y (8.2–9.9 ppm) and Yb_N (3.1–4.4) contents. Whole-rock major and trace element variations suggest that fractional crystallisation played a significant role in the pluton evolution. The N-MORB normalised trace element patterns of the pluton are similar to those of MMEs with enrichment in large-ion lithophile elements, Th and Ce, and negative Nb and Ti anomalies. Chondrite-normalised rare earth element plots show moderate-to-highly enriched concave patterns (La_N/Lu_N = 14.2–21.6) with insignificant negative Eu anomalies (Eu_N/Eu* = 0.86–1.14), all of which imply hornblende fractionation during magmatic evolution. The pluton samples have ⁸⁷Sr/⁸⁶Sr ratios of 0.704767 to 0.704927, ¹⁴³Nd/¹⁴⁴Nd ratios of 0.512767–0.512774, εNd values of (+2.52) – (+2.65), and δ¹⁸O values of 7.9–9.7‰. The isotopic compositions of the host pluton and MMEs are similar to I-type granitoids derived from mantle sources. The MMEs show incomplete magma mixing/mingling, representing small bodies of mafic parental magma. Combined with regional studies, these new data suggest that the parental magma of the studied adakite-like pluton was generated from the lithospheric mantle and then modified by fractional crystallisation and assimilation in a post-collisional setting.