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.     

秦岭-昆仑造山型环斑花岗岩与世界典型环斑花岗岩的对比

通过秦岭-昆仑造山带中的环斑花岗岩同世界元古宙环斑花岗岩的岩石学、岩相学、岩石地球化学和构造环境等方面的对比研究发现,二者具有相同或一致的特征:具环斑结构,属准铝、高钾、富碱岩浆,具双峰式岩浆组合,形成于后碰撞环境,但其地球化学的某些指标、岩浆形成时代和出露的大地构造位置等有一定差异.世界元古宙环斑花岗岩的岩石化学及暗色矿物明显富铁,w(FeT)/w(FeT Mg)较高,多数在0.9以上,岩石成因类型多数是A型花岗岩,产在稳定地台区的边缘,而昆仑地区多数环斑花岗岩的w(FeT)/w(FeT MgO)＞0.8,亦较富Fe,且多数是A型花岗岩;秦岭地区的岩体铁指数相对较低,只有0.62,岩石成因类型的地球化学判据既有A型也有Ⅰ型花岗岩特征.秦岭-昆仑造山带中环斑花岗岩的显著特征是都产在造山带中,与板块缝合带关系密切,时代从元古宙到古生代直到中生代都有发现,具多旋回性.它们出现在每一个大的造山旋回晚期,即向另一个构造旋回的转折期,这在世界造山带中是十分罕见的,反映出世界上造山带与稳定区元古宙和显生宙的地幔与地壳状态是不一样的,有着不同的构造演化历史和动力学过程,表明秦岭-昆仑地区的环斑花岗岩是一种有别于元古宙稳定区的造山型环斑花岗岩.     

Petrology of calc-alkaline/adakitic basement hosting A-type Neoproterozoic granites of the Malani igneous suite in Nagar Parkar,SE Sindh,Pakistan

The Nagar Parkar area contains three distinct groups of rocks, from oldest to youngest, (1) basement rocks ranging in composition from mafic to (quartz)diorite, tonalite, granite, and younger granodiorite, (2) granite plutons similar in general features to those of the Malani Igneous Suite of Rajasthan, and (3) abundant mafic, felsic and rhyolitic dykes. The basement rocks show strong brittle and local plastic deformation, and epidote amphibolite/upper greenschist facies metamorphic overprint. The chemistry of the basement rocks contrasts the commonly agreed within plate A-type character of the Neoproterozoic granites (group 2) that are emplaced into them. The basement rock association is calc-alkaline; the granodiorite displays the compositional characteristics of adakites, whereas the tonalite has intermediate composition between typical adakite and classical island arc rocks. This paper presents detailed petrography of the basement rocks and compares their geochemistry with those of the group 2 granites as well as with rocks from other tectonic environments. It is proposed that the Nagar Parkar basement is part of a 900–840 Ma magmatic arc that was deformed before it was intruded 800–700 Ma ago by the A-type continental granitic rocks followed by mafic to felsic dykes.     

阿尔金造山带南缘岩浆混合作用:玉苏普阿勒克塔格岩体岩石学和地球化学证据

阿尔金造山带南缘玉苏普阿勒克塔格岩体中的似斑状中粗粒黑云钾长花岗岩发育有岩浆成因的暗色包体,并且该花岗岩被花岗细晶岩呈脉状侵入。该岩体含有丰富的岩浆混合作用特征: 如暗色包体中的碱性长石斑晶、针状磷灰石、长石的环斑结构、石英/斜长石主晶和榍石眼斑等。暗色包体、寄主花岗岩和花岗细晶岩代表了岩浆混合演化过程中不同端元比例混合的产物。地球化学特征上,钾长花岗岩和暗色包体的主要氧化物含量在Harker图解中多呈线性变化。暗色包体主要为闪长质,MgO、K₂O含量高,为钾玄岩系列,总体上高场强元素不亏损,显示了岩浆混合中的基性端元信息,可能为幔源熔体结晶分异或壳幔物质的混合产物。寄主花岗岩均为准铝质,富碱,为高钾钙碱性系列,亏损Nb、Ta、Sr、P、Ti等高场强元素,高K₂O/Na₂O,富集高不相容元素,Ga含量高,显示了A型花岗岩的特征,Th/U 和Nb/Ta比值分别介于为6.67~10.96、8.99~11.94,代表了下地壳源区。花岗细晶岩均为钠质、过铝质,TiO₂、MgO含量低, Na₂O和CaO含量高,具有混合岩浆侵位后分异的特征。岩相学和地球化学特征说明岩浆混合作用对于环斑结构花岗岩的形成起到重要作用。花岗细晶岩中环斑长石的斜长石外环与钾长石内核的厚度比大于钾长花岗岩中的环斑长石,指示混合岩浆在一定的减压条件下更有利于环斑结构的形成。玉苏普阿勒克塔格岩体中的钾玄质暗色包体、高钾钙碱性花岗岩和中钾钙碱性花岗细晶岩代表了岩浆演化不同阶段的产物,反映了一个幔源岩浆和下地壳不断相互作用,引起地壳连续伸展减薄的过程,指示阿尔金南缘在早古生代末期存在造山后伸展背景下的幔源岩浆底侵作用。同一岩体中两种不同时代岩性的环斑结构显示了该岩体形成历史中的一定时空演化关系,代表了伸展过程中不同阶段的产物。     

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.     

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。     

钦-杭成矿带东段村前含矿斑岩地球化学特征及其构造环境与成矿意义

村前铜多金属矿床位于钦杭成矿带东段,为一具有矽卡岩型矿化和斑岩型矿化的铜多金属矿床,含矿岩体为燕山早期花岗闪长斑岩,岩石具有富硅、富铝、富碱的特点,属于偏铝-过铝质钙碱性花岗岩类。岩体具有从深部向浅部蚀变增强,大部分组分活动性不明显,而成矿元素Cu-Mo-Fe-Pb-Zn-Au-Ag含量明显增加,Na2O、Sr含量降低,REE元素除Eu少量丢失外,其余均呈一致的迁入特征。岩体属Ⅰ型花岗质岩石,由具角闪石+石榴子石残留相的火成岩部分熔融形成的熔浆,混合或混染了地壳重熔型岩浆上侵就位而成。钦杭结合带东段,燕山期中酸性岩浆活动具有从176~150Ma的埃达克岩或具岛弧花岗岩特征的Ⅰ型花岗岩,至150~140Ma的S型花岗岩,向140~110Ma的A型花岗岩演化趋势,显示了地壳由厚减薄的过程,暗示其大地构造背景为岩石圈的伸展减薄环境,而形成于169.3±1.1Ma的村前斑岩体正处于伸展阶段早期。综合岩体成矿特征表明,钦杭成矿带东段及邻近地区,176~160Ma主要形成与Ⅰ型花岗质岩石有关的以Cu为主的多金属矿床;160~150Ma主要形成与Ⅰ型花岗质岩石有关的Cu-Mo矿床与W-Sn矿床;150~140Ma主要形成与S型花岗质岩石有关的以W-Sn-Mo为主的多金属矿床,以及以Ag-Pb-Zn为主的多金属矿床;140~110Ma主要形成与A型花岗质岩石有关的以W-Sn-Mo为主的多金属矿床,少量与Ⅰ型花岗质岩石有关的Pb-Zn矿床。     

辽西新太古代钓鱼台花岗岩成因及演化过程:来自岩石组构的证据

辽西兴城钓鱼台地区分布一套花岗质杂岩,是新太古代"绥中花岗岩"的重要组成部分。花岗质杂岩以似斑状花岗闪长岩和石英闪长岩为主,少量细粒黑云闪长岩(包体)及脉状花岗岩,各类岩石接触关系明确,本文定义为"钓鱼台花岗岩"。锆石U-Pb同位素测试结果显示似斑状花岗闪长岩、石英闪长岩、脉状花岗岩的形成年龄分别为2538±20Ma、2476±56Ma、2470±18Ma,同为新太古代末期热事件的产物。通过该花岗岩组合的宏观-微观组构解析表明,似斑状花岗闪长岩表现为均匀块状构造,具有深熔花岗质岩浆的典型堆晶结构;细粒黑云闪长岩为细粒结构,呈小型暗色包体分布在似斑状花岗闪长岩中,包体的塑性变形、捕掳晶、淬冷边及反向脉等组构发育,具有铁镁质基性岩浆加入同深熔花岗闪长岩并快速冷却的特征;暗色的石英闪长岩主要分布在似斑状花岗闪长岩之下,接触带附近似斑状花岗闪长岩中的钾长石变斑晶明显增多,显示闪长质岩浆"底垫"侵位加热的特征;脉状花岗岩同时穿切似斑状花岗闪长岩和石英闪长岩,具有熔体富集脉体的结构特征。各类岩石中变形组构均不发育。钓鱼台花岗岩记录了新太古代末期地壳深熔和壳幔相互作用过程,岩石组构研究表明新太古代地壳再造作用是一个"静态"多期次的缓慢深熔过程,伴有同期幔源基性物质加入并混合,以及随后大规模的基性岩浆底侵。由此推断钓鱼台花岗岩形成的构造背景为幔源岩浆垂向底侵过程,可能是与俯冲带关联不明显的岩浆弧环境。     

A flow-foliated ignimbrite related to the Åland rapakivi granite in SW Finland

A flow-foliated felsic ignimbrite constitutes the uppermost lithological unit of the 1.58 Gyr anorogenic magmatic rocks in SW Finland. The ignimbrite is derived from an explosive eruption of hot (≅ 950 °C) phenocryst-bearing A-type (rapakivi-type granite magma.
The ignimbrite is close in composition to subvolcanic rapakivi granites that occur in the margins of the kand rapakivi batholith. The subvolcanic granites crystallized under a pressure of ≅ 1 kbar and at temperatures of about 650–700 °C. However, both major and rare earth elements show that the ignimbrite- forming magma was more fractionated than the magma forming the subvolcanic varieties.
Supported by evidence of mafic-felsic magma mingling, it is suggested that injection of hot mafic magma into a shallow magma chamber produced the high temperature of the ignimbrite-forming magma. This injection increased the magmatic and the volatile pressure that caused the eruption of the dry felsic magma.     

Geochemistry and U-Pb zircon dating constraints of some plutonic rocks along Bir Tawilah shear zone,central Saudi Arabia: Implication for magma peterogenesis and age of gold mineralization

The study area covered by this work is located along the Bir Tawilah fault zone which encompasses the Arabian Shield between Afif terrane and western oceanic terranes. The rocks are dominantly ophiolite assemblages, island arc metavolcanic and metasedimentary rocks, and dioritic to granitic intrusions. The diorite and granodiorite rocks are I-type granitoids, calk-alkaline, metaluminous to peraluminous, formed in a volcanic arc setting, whereas the monzogranite is classified as A-type granite, alkaline and highly fractionated calc-alkaline, generated in within-plate tectonic setting. Nb and Y relationships indicated that the diorites and granodiorites were generated by a mafic parental magma contaminated with crustal materials, and controlled by fractional crystallization, whereas the monzogranites were generated from a magma characterized by an enriched mantle (EM) source.Mineralization including gold is hosted by the carbonatized serpentinite (listvenite) and the syn-tectonic granodiorite along Bir Tawilah thrust zone. U-Pb zircon geochronology indicates that the granodiorite at Jabal Ghadarah is emplaced at ca. 630 ± 12 Ma, probably suggests that the metallic minerals associated with the granodiorite along Bir Tawilah thurst zone are the result of remobilization of pre-existing gold mineralization associated with listevenite that is related to arc accretion.     

甘肃天水地区早古生代黄门川花岗闪长岩体LA-ICP-MS锆石U-Pb定年及构造意义

甘肃天水地区黄门川花岗闪长岩体位于北祁连造山带东端,侵位于晚奥陶世陈家河群中酸性火山岩系中,岩石学和地球化学特征表明其具有壳幔岩浆混合的特点。对黄门川花岗闪长岩体进行了LA-ICP-MS锆石U-Pb定年,结果表明该花岗闪长岩体的年龄为440.5Ma±4.4Ma,形成于早志留世。地球化学特征表明,黄门川花岗闪长岩体属于中钾钙碱性系列,具I型花岗岩的特征。构造环境判别表明其形成于安第斯型大陆边缘弧。综合区域地质背景,认为祁连造山带东端在早古生代期间发育有有限洋盆,洋盆向北俯冲消减产生大量弧岩浆岩。对进一步研究祁连与秦岭造山带构造交接部位早古生代的构造格局、演化等大陆动力学问题具有重要意义。     

浙江安吉铅锌多金属矿区细粒花岗岩的岩石化学、年代学及成矿意义探讨

文章在详细的野外地质调查、岩石学、岩石化学和锆石U-Pb定年结果基础上,对浙江安吉矿区细粒花岗岩的岩浆起源、侵位时代与成矿的关系及找矿指示意义等进行了较为深入的探讨.细粒花岗岩侵位时代日于矿区的黑云母二长花岗岩、正长花岗岩和花岗闪长岩,侵位时代约在134 Ma左右,为中国东部东侏罗世一早白垩世岩石圈活化及大规模成岩成矿作用的产物.岩石化学研究结果表明,细粒花岗岩属高钾钙碱性钾玄岩、过铝质岩石系列,轻稀士元素相对富集,重稀土元素相对亏损,富集大离子亲石元素Rb、Th、U、K、轻稀土元素和Pb,明显亏损高场强元素Nb、Ta、Ti和P,显示较高的分异程度.据根锆饱和温度,计算其母岩浆结晶温度约在834.6 ～ 870.6℃.结合矿区已知的矿化和蚀变与细粒花岗岩的空间关系、细粒花岗岩岩石化学特征以及细粒花岗岩与矿石铅同位素研究结果,笔者认为,安吉铅锌多金属矿区铅锌钼矿化与细粒花岗岩具有密切的成因联系,细粒花岗岩与寒武系灰岩接触部位控制了矽卡岩型铅锌矿化的产出,这一成果对该区的进一步找矿勘查具有重要的指导意义.     

东昆仑古特提斯后碰撞阶段伸展作用:来自晚三叠世岩浆岩的证据

通过对东昆仑造山带晚三叠世岩浆岩的岩石类型、形成时代、岩石地球化学和同位素地球化学资料综合分析,对岩浆岩的岩石组合、分布特征和岩石成因进行研究,探讨东昆仑造山带晚三叠世构造演化的地球动力学背景。东昆仑造山带晚三叠世是古特提斯演化过程中重要的构造转换期,岩浆岩岩石类型多样,主要包括辉长岩、花岗闪长岩、二长花岗岩和正长花岗岩,并且广泛出露具埃达克质特征的岩浆岩和A型花岗岩。晚三叠世岩浆岩的出露规模与俯冲阶段相比,规模较小,一般以小岩体、岩株和岩脉侵入于早期岩体和地层中。东昆仑晚三叠世岩浆岩主体为准铝-弱过铝质高钾钙碱性-钾玄岩系列,轻重稀土元素具有一定分异,富集大离子亲石元素,亏损高场强元素,岩石类型不同时分异程度、富集和亏损程度有一定差异。大部分晚三叠世花岗质岩浆岩的同位素特征与晚二叠世-三叠纪镁铁质岩浆岩近似,部分具有更高的εNd(t)和εHf(t)值。镁铁质岩浆岩、普通花岗岩、埃达克质岩浆岩在东昆仑各个构造带皆有分布,A型花岗岩主要分布在祁漫塔格构造带(东昆北)的阿牙克库木湖-香日德断裂附近。东昆仑晚三叠世镁铁质岩浆岩具有弧岩浆岩特征,为俯冲流体交代的地幔楔部分熔融产物。普通花岗岩和埃达克质岩浆岩多为新生下地壳部分熔融产物,少量埃达克质岩浆岩由于与地幔的交代作用,具有幔源特征。A型花岗岩为残留下地壳部分熔融的产物。部分普通花岗岩、埃达克质岩浆岩和A型花岗岩由于岩浆混合作用,具幔源特征。构造环境研究表明,东昆仑在晚三叠世进入古特提斯演化的后碰撞阶段。巴颜喀拉地块同东昆仑地块的持续碰撞导致地壳加厚,密度增大,使岩石圈重力不稳定发生拆沉作用,引发岩石圈地幔减压熔融,产生大量的镁铁质岩浆岩;镁铁质岩浆底侵不同类型地壳熔融及拆沉地壳部分熔融而形成的岩浆交代地幔,以及岩浆混合和岩浆后期演化,形成了东昆仑造山带晚三叠世丰富多样的岩浆岩。     

Evidence for hybridisation in the Tynong Province granitoids,Lachlan Fold Belt,eastern Australia

The role of mafic–felsic magma mixing in the formation of granites is controversial. Field evidence in many granite plutons undoubtedly implies interaction of mafic (basaltic–intermediate) magma with (usually) much more abundant granitic magma, but the extent of such mixing and its effect on overall chemical features of the host intrusion are unclear. Late Devonian I-type granitoids of the Tynong Province in the western Lachlan Fold Belt, southeast Australia, show typical evidence for magma mingling and mixing, such as small dioritic stocks, hybrid zones with local host granite and ubiquitous microgranitoid enclaves. The latter commonly have irregular boundaries and show textural features characteristic of hybridisation, e.g. xenocrysts of granitic quartz and K-feldspars, rapakivi and antirapakivi textures, quartz and feldspar ocelli, and acicular apatite. Linear (well defined to diffuse) compositional trends for granites, hybrid zones and enclaves have been attributed to magma mixing but could also be explained by other mechanisms. Magmatic zircons of the Tynong and Toorongo granodiorites yield U–Pb zircon ages consistent with the known ca 370 Ma age of the province and preserve relatively unevolved ?_Hf (averages for three samples are +6.9, +4.3 and +3.9). The range in zircon ?_Hf in two of the three analysed samples (8.8 and 10.1 ?_Hf units) exceeds that expected from a single homogeneous population (～4 units) and suggests considerable Hf isotopic heterogeneity in the melt from which the zircon formed, consistent with syn-intrusion magma mixing. Correlated whole-rock Sr–Nd isotope data for the Tynong Province granitoids show a considerable range (0.7049–0.7074, ?_Nd +1.2 to –4.7), which may map the hybridisation between a mafic magma and possibly multiple crustal magmas. Major-element variations for host granite, hybrid zones and enclaves in the large Tynong granodiorite show correlations with major-element compositions of the type expected from mixing of contrasting mafic and felsic magmas. However, chemical–isotopic correlations are poorly developed for the province as a whole, especially for ⁸⁷Sr/⁸⁶Sr. In a magma mixing model, such complexities could be explained in terms of a dynamic mixing/mingling environment, with multiple mixing events and subsequent interactions between hybrids and superimposed fractional crystallisation. The results indicate that features plausibly attributed to mafic–felsic magma mixing exist at all scales within this granite province and suggest a major role for magma mixing/mingling in the formation of I-type granites.     

Two different magma series imply a Palaeogene tectonic transition from contraction to extension in the SE Korean Peninsula

Late Cretaceous–early Tertiary granites in the Gyeongsang Basin have distinctly different bulk-rock compositions. Calc-alkaline I-type metaluminous granites display petrographic features implying magma mixing, whereas A-type granites are hypersolvus and peralkaline. I-type plutons mainly consist of enclave-rich granodiorites and enclave-poor porphyritic granites typified by abundant plagioclase phenocrysts; these granitoids contain various mafic clots and magmatic/microgranular enclaves (MMEs). A-type bodies are perthitic alkali-feldspar granites characterized by interstitial annite + riebeckite-arfvedsonite. New SHRIMP-RG zircon U–Pb age dating of an I-type enclave-poor porphyritic granite and an A-type alkali-feldspar granite yielded ages of 65.7 ± 0.7 and 53.9 ± 0.3 million years, respectively. Based on prior geochronologic data and these contrasting ages of granitic magma genesis, SE Korea may have evolved tectonically from latest Cretaceous compression to late Palaeocene extension (i.e. orogenic collapse). The later part of the 66–54 Ma magmatic gap apparently includes the time of tectonic inversion in the SE Korean Peninsula, a far-field effect of the collision of the Indian subcontinent with Eurasia. This process is also reflected in the 69–52 Ma NNE-trending Eurasian apparent polar wandering path.     

鹰峰环斑花岗岩地球化学特征及其构造意义

柴达木北缘鹰峰环斑花岗岩出露于柴达木地块与南祁连地体之间的柴北缘造山带,是我国发现的又一元古宙环斑花岗岩体.初步研究表明,鹰峰环斑花岗岩是具环斑结构和A-型花岗岩特征的典型的元古宙环斑花岗岩体,且属于A1亚型,岩浆组合具双峰式特征.环斑结构主要由几个钾长石斑晶颗粒形成聚斑,中心有一斜长石内核,斑晶表面具不均匀高岭土化,条纹构造明显且有规律,基质由细粒-微粒的石英组成,有明显重结晶及定向构造; 岩石化学组成以高钾为特征,A/ NKC < 1,A/NK > 1,属准铝质; 微量元素组成上富集Ba、U、Th、Ce、Hf、Sm,亏损Sr、Ta、Nb、Zr、Y,Rb/Sr (0.17~0.6)和Rb/Ba (0.03~0.24) 很低,岩石分异演化程度不高; 稀土元素: REE、Ce、Zr含量高,Ga含量高达25×10-6以上,远远高出其他类型花岗岩,但Eu (0.75~4.3) ×10-6轻度亏损,属轻稀土富集型.通过对微量元素和稀土元素的地球化学行为分析,鹰峰环斑花岗岩是发生在板内的一种岩浆作用,是下地壳的麻粒岩受底侵或拆沉作用地幔上涌影响,发生部分熔融,然后经过分异演化形成了碱性的“干”岩浆,并在后碰撞的区域拉伸构造环境下侵位.同时伴随温度的降低,钠质的斜长石从钾长石中出溶,并迁移到钾长石的边沿,形成了具环斑结构的A1型花岗岩.      

Synplutonic mafic dykes from late Archaean granitoids in the Eastern Dharwar Craton,southern India

We present a first overview of the synplutonic mafic dykes (mafic injections) from the 2.56–2.52 Ga calcalkaline to potassic plutons in the Eastern Dharwar Craton (EDC). The host plutons comprise voluminous intrusive facies (dark grey clinopyroxene-amphibole rich monzodiorite and quartz monzonite, pinkish grey porphyritic monzogranite and grey granodiorite) located in the central part of individual pluton, whilst subordinate anatectic facies (light grey and pink granite) confined to the periphery. The enclaves found in the plutons include highly angular screens of xenoliths of the basement, rounded to pillowed mafic magmatic enclaves (MME) and most spectacular synplutonic mafic dykes. The similar textures of MME and adjoining synplutonic mafic dykes together with their spatial association and occasional transition of MME to dismembered synplutonic mafic dykes imply a genetic link between them. The synplutonic dykes occur in varying dimension ranging from a few centimeter width upto 200 meters width and are generally dismembered or disrupted and rarely continuous. Necking of dyke along its length and back veining of more leucocratic variant of the host is common feature. They show lobate as well as sharp contacts with chilled margins suggesting their injection during different stages of crystallization of host plutons in magma chamber. Local interaction, mixing and mingling processes are documented in all the studied crustal corridors in the EDC. The observed mixing, mingling, partial hybridization, MME and emplacement of synplutonic mafic dykes can be explained by four stage processes: (1) Mafic magma injected during very early stage of crystallization of host felsic magma, mixing of mafic and felsic host magma results in hybridization with occasional MME; (2) Mafic magma introduced slightly later, the viscosities of two magmas may be different and permit only mingling where by each component retain their identity; (3) When mafic magma injected into crystallizing granitic host magma with significant crystal content, the mafic magma is channeled into early fractures and form dismembered synplutonic mafic dykes and (4) Mafic injections enter into largely crystallized (>80% crystals) granitic host results in continuous dykes with sharp contacts. The origin of mafic magmas may be related to development of fractures to mantle depth during crystallization of host magmas which results in the decompression melting of mantle source. The resultant hot mafic melts with low viscosity rise rapidly into the crystallizing host magma chamber where they interact depending upon the crystallinity and viscosity of the host. These hot mafic injections locally cause reversal of crystallization of the felsic host and induce melting and resultant melts in turn penetrate the crystallizing mafic body as back veining. Field chronology indicates injection of mafic magmas is synchronous with emplacement of anatectic melts and slightly predates the 2.5 Ga metamorphic event which affected the whole Archaean crust. The injection of mafic magmas into the crystallizing host plutons forms the terminal Archaean magmatic event and spatially associated with reworking and cratonization of Archaean crust in the EDC.     

临沧花岗岩基中段花岗闪长岩类研究

临沧花岗岩基中段花岗闪长岩类属残留岩基，岩石学特征与临沧花岗岩基其它花岗岩明显不同。地球化学特征与二长花岗岩类相近，两类花岗岩之稀土特征与大勐龙岩群(Pt2d．)基本一致。加之花岗闪长岩类中含镁铁质微粒包体(MME)，认为花岗闪长岩类存在岩浆混合作用，岩浆向上运移过程中未发生同化混染和分离结晶作用。结合同位素测年结果及其它地质事件，确定是在二叠纪侵位于活动陆缘，是区内古特提斯洋向东俯冲消减作用的产物。     

Development of igneous layering during growth of pluton: The Tarçouate Laccolith (Morocco)

We discuss the significance of igneous layering with respect to pluton growth processes. The case study is the Tarçouate Laccolith (Morocco), whose core consists of modally layered hornblende granodiorites with high amount of monzodioritic enclaves, contrasting with peripheral, non-layered biotite granodiorites with low amount of enclaves. Rhythmic layering, with modal grading, cross-stratification and trough layering is associated with monzodioritic layers and wraps around mafic enclaves. Its steep dips ≥ 45° result from tilting that occurred above solidus conditions, as indicated by sub-vertical and synmagmatic granite, aplite and monzodiorite dykes cutting across the layering.The systematic association of igneous layering with mafic enclaves in calc-alkaline plutons suggests that layering originates from recurrent injection of mafic magma. Viscosity calculations suggest that the physicochemical properties of magma alone cannot account for the presence of layering in the central hornblende granodiorite and its coeval absence in the peripheral biotite granodiorite of the Tarçouate Laccolith. Intermittent pulses of hot mafic magma into crystallizing granodiorite likely produced thermal perturbations able to trigger local convection, formation of mafic enclaves and development of igneous layering through protracted crystallization.     

Late Neoproterozoic alkaline magmatism in the Arabian–Nubian Shield: the postcollisional A-type granite of Sahara–Umm Adawi pluton, Sinai, Egypt

The Sahara–Umm Adawi pluton is a Late Neoproterozoic postcollisional A-type granitoid pluton in Sinai segment of the Arabian–Nubian Shield that was emplaced within voluminous calc-alkaline I-type granite host rocks during the waning stages of the Pan-African orogeny and termination of a tectonomagmatic compressive cycle. The western part of the pluton is downthrown by clysmic faults and buried beneath the Suez rift valley sedimentary fill, while the exposed part is dissected by later Tertiary basaltic dykes and crosscut along with its host rocks by a series of NNE-trending faults. This A-type granite pluton is made up wholly of hypersolvus alkali feldspar granite and is composed of perthite, quartz, alkali amphibole, plagioclase, Fe-rich red biotite, accessory zircon, apatite, and allanite. The pluton rocks are highly evolved ferroan, alkaline, and peralkaline to mildly peraluminous A-type granites, displaying the typical geochemical characteristics of A-type granites with high SiO₂, Na₂O + K₂O, FeO*/MgO, Ga/Al, Zr, Nb, Ga, Y, Ce, and rare earth elements (REE) and low CaO, MgO, Ba, and Sr. Their trace and REE characteristics along with the use of various discrimination schemes revealed their correspondence to magmas derived from crustal sources that has gone through a continent–continent collision (postorogenic or postcollisional), with minor contribution from mantle source similar to ocean island basalt. The assumption of crustal source derivation and postcollisional setting is substantiated by highly evolved nature of this pluton and the absence of any syenitic or more primitive coeval mafic rocks in association with it. The slight mantle signature in the source material of these A-type granites is owed to the juvenile Pan-African Arabian–Nubian Shield (ANS) crust (I-type calc-alkaline) which was acted as a source by partial melting of its rocks and which itself of presumably large mantle source. The extremely high Rb/Sr ratios combined with the obvious Sr, Ba, P, Ti, and Eu depletions clearly indicate that these A-type granites were highly evolved and require advanced fractional crystallization in upper crustal conditions. Crystallization temperature values inferred average around 929°C which is in consistency with the presumably high temperatures of A-type magmas, whereas the estimated depth of emplacement ranges between 20 and 30 km (upper-middle crustal levels within the 40 km relatively thick ANS crust). The geochronologically preceding Pan-African calc-alkaline I-type continental arc granitoids (the Egyptian old and younger granites) associated with these rocks are thought to be the crustal source of f this A-type granite pluton and others in the Arabian–Nubian Shield by partial melting caused by crustal thickening due to continental collision at termination of the compressive orogeny in the Arabian–Nubian Shield.