与超大型锡矿床相关的花岗岩熔融实验结果及其成矿学意义

在６００～８５０℃，７５～３００ＭＰａ条件下对与个旧超大型锡（多金属）矿床密切相关的个旧复式花岗岩体，和与大厂长坡－铜坑、龙头山２个超大型锡（多金属）矿床密切相关的龙箱盖复式花岗岩体进行系统的实验研究。其结果表明，不仅形成这２个著名复式花岗岩的岩浆性质及岩浆的演化规律有明显的差异，而且形成同一个复式岩体的岩浆的性质及其演化特点亦有明显的不同。个旧花岗岩浆中，形成老卡岩体的岩浆固相线位置低，具有共结岩浆性质，而形成马松、龙岔河岩体的岩浆固相线位置稍高，具有近共结花岗岩浆的特点；形成龙箱盖复式花岗岩的岩浆在性质上有所不同，其固相线位置较高。这些研究结果表明，２类超大型锡矿床的成矿花岗岩浆的性质、起源与演化规律存在明显的差异，这些差异可能是造成这２个复式花岗岩体在超大型锡矿床形成过程中所起的作用不同的重要原因之一。     

花岗岩研究进展

花岗岩的起源、形成的物理化学过程、结晶和分异、矿物-化学成分多样性、时空演变等是当前花岗岩研究的主题.矿物微区原位同位素分析,特别是锆石原位微区U-Pb和Hf-O同位素分析,以及岩浆作用的物理过程实验模拟是重要的研究手段.近年来,不同类型花岗岩起源和演化过程认识的深入,得益于对与花岗岩及其伴生的其它相关岩石研究的突破,以及锆石原位U-Pb和Hf-O同位素分析技术的应用.本文结合国内外花岗岩研究进展,综述了我国东南部华夏陆块上花岗岩研究中的一些问题.     

近代花岗岩研究的回顾

近代花岗岩实验和地球化学的研究使花岗岩研究走向一个最终成因和在地构造环境变迁结合的动力学方向。     

南岭中西段若干复式花岗岩体的成因模式研究

岩石学、岩石地球化学、黑云母矿物化学和副矿物特征等研究表明，南岭中西段3个复式花岗岩体的演化规律表现为两种方式：一种是以花山复式花岗岩体为代表的不同母岩浆演化方式，其补体美华花岗岩可能经历了与花山主体岩浆相似的分离结晶作用；另一种是以金鸡岭复式花岗岩体和大东山复式花岗岩体为代表的相同母岩浆演化方式，它们的补体螃蟹木花岗岩和猪蹄石花岗岩分别为金鸡岭主体和大东山主体花岗质母岩浆经过分离结晶作用形成。两种不同产出方式的补体花岗岩可能表明它们与主体岩浆的形成机制不同。     

The fluid regime of crystallization of water-saturated granitic and pegmatitic magmas: a physicochemical analysis

Miarolitic granite pegmatites are a unique natural object that makes it possible to study magmatic processes that lead to the formation of ore-forming media and systems. This paper summarizes modern views on phase transformations in aqueous silicate systems at parameters close to those of the transition from magmatic to hydrothermal crystallization. Comparison of phase diagrams and the results of study of pegmatite-forming media permits making conclusions about the crystallization of the water-saturated magmas of miarolitic granite pegmatites. The fluid regime of aqueous granite systems of simple composition, not enriched in fluxing components, is determined mainly by magma degassing or the supply of volatiles with flows of transmagmatic fluids. These processes cause the separation of essentially carbon dioxide or essentially hydrous fluid. During the evolution of such magmas, crystallization from silicate melt is separated in PT-space and, possibly, in time from the crystallization from aqueous or mixed carbon dioxide-aqueous super- and subcritical solutions. The evolution of chambers of water-saturated granitic and pegmatitic magma enriched in F, B, and alkali metals presupposes the formation of a heterogeneous mineral-forming medium in which crystallization occurs in the magmatic melt at high-temperature stages; as temperature decreases, crystallization can proceed in hydrous fluid, hydrosilicate, and/or hydrosaline liquids simultaneously. Hydrothermal crystallization can also take place in a heterogeneous medium consisting of aqueous solutions of different salinities and vapor or vapor-carbon dioxide gas mixture. The relationship between different fluid regimes during the evolution of volatile-saturated granitic and pegmatitic magmas determines the variety of postmagmatic rocks accompanying granite massifs.     

花岗岩混合问题:与玄武岩对比的启示——关于花岗岩研究的思考之一

最近,花岗岩混合成了花岗岩研究的热点,国内外许多学者探讨了花岗岩混合问题,并尝试用不同端元组分不同比例的混合来解释花岗岩的地球化学变化.本文从花岗岩与玄武岩的对比出发,探讨了花岗岩混合的可能性和局限性.作者认为,花岗岩混合的现象是普遍存在的,但是次要的和局部的.岩浆混合的能力或能干性(competence of mixing)主要取决于岩浆的黏性和温度,而黏性又与硅氧四面体有关.相对于玄武岩,花岗岩的SiO2含量高,温度低,因此,花岗质岩浆的混合能干性很低.玄武质岩浆的混合是mixing(以化学混合为主),而花岗质岩浆的混合通常只是mingling(以机械混合为主),只有在少数情况下才能达到mixing的程度,例如,埃达克岩与地幔混合形成的高镁安山岩或高镁埃达克岩.许多人认为,花岗岩中的暗色微粒包体是花岗质岩浆混合作用最显著、最直接证据.研究表明,花岗岩中的暗色微粒包体大多是闪长质成分的,其初始成分大多是玄武质的.因此,暗色微粒包体不是花岗质岩浆混合作用最显著、最直接证据,而是玄武质岩浆混合能力强过花岗质岩浆的证据.与玄武质岩浆的起源比较,花岗质岩浆从一开始熔融就是不均一的,这源于源区的不均一及熔融过程的复杂性.花岗质岩浆原始均一性的假定是不可能的.花岗岩成分的变化以及在哈克图解中成分点的"连续谱系",主要是由源区不均一性引起的,混合和分异可能有一定的作用,但毕竟是次要的.花岗质岩浆从源区生成、迁移、直至在地表喷出或在浅部定位的全过程,是一个不断均一化和不均一化的过程.但是,由于花岗质岩浆的黏性大,上述过程及岩浆演化的程度和规模都受到限制,也限制了岩浆混合的程度和规模.许多人仅从花岗岩地球化学成分的变化来研究花岗岩的成因,而很少考虑花岗岩物理性质对岩浆演化的制约.对比玄武岩与花岗岩,我们认为,地球化学方法在花岗岩中应用的范围和程度可能远远不及玄武岩,我们应当重新考虑花岗岩的地球化学应用问题.     

Geochemical and thermodynamic modeling of the petrogenesis of A1-type granites and associated intermediate rocks: A case study from the central Fennoscandian Shield

The origin of ferroan A-type granites in anorogenic tectonic settings remains a long-standing petrological puzzle. The proposed models range from extreme fractional crystallization of mantle-derived magmas to partial melting of crustal rocks, or involve combination of both. In this study, we apply whole-rock chemical and Sm-Nd isotopic compositions and thermodynamically constrained modeling (Magma Chamber Simulator, MCS) to decipher the genesis of a suite of A1-type peralkaline to peraluminous granites and associated intermediate rocks (monzodiorite-monzonite, syenite) from the southwestern margin of the Archean Karelia craton, central Finland, Fennoscandian Shield. These plutonic rocks were emplaced at ca. 2.05 Ga during an early stage of the break-up of the Karelia craton along its western margin and show trace element affinities to ocean island basalt-type magmas. The intermediate rocks show positive ε_Nd(2050 Ma) values (+1.3 to +2.6), which are only slightly lower than the estimated contemporaneous depleted mantle value (+3.4), but much higher than average ε_Nd(2050 Ma) of Archean TTGs (–10) in the surrounding bedrock, indicating that these rocks were essentially derived from a mantle source. The ε_Nd(2050 Ma) values of the peralkaline and peraluminous granite samples overlap (–0.9 to +0.6 and –3.2 to +0.9, respectively) and are somewhat lower than those in the intermediate rocks, suggesting that the mafic magmas parental to granite must have assimilated some amount of older Archean continental crust during their fractionation, which is consistent with the continental crust-like trace element signatures of the granite members. The MCS modeling indicates that fractional crystallization of mantle-derived magmas can explain the major element characteristics of the intermediate rocks. The generation of the granites requires further fractional crystallization of these magmas coupled with assimilation of Archean crust. These processes took place in the middle to upper crust (∼2–4 kbar, ∼7–15 km) and involved crystallization of large amounts of clinopyroxene, plagioclase and olivine. Our results highlight the importance of coupled FC-AFC processes in the petrogenesis of A-type magmas and support the general perception that magmas of A-type ferroan granites become more peraluminous by assimilation of crust. They further suggest that variable fractionation paths of the magmas upon the onset of assimilation may explain the broad variety of A-type felsic and intermediate igneous rocks that is often observed emplaced closely in time and space within the same igneous complex.     

Metasomatic transformations of carbonate rocks observable in quarries of Riverside, California, United States

The results of skarn-forming processes at contacts of the multiphase Southern California Batholith with carbonate rocks accessible to study in quarries in Riverside, California, involve prograde metasomatic transformations of marmorized dolomites and calcareous rocks in contact with granitic melts and contaminated magmas. The processes of contact assimilation are proved to have been controlled by the emplacement of granitic melts overheated relative to subliquidus melts (with the overheated melts prone to approach the composition of granodiorite, syenite, and gabbro) into skarnified marbles. The degree of magma overheating was evaluated based on G.F. Smith’s data on linear melting temperature variations for anhydrous intrusive rocks with various SiO₂ concentrations (<750°C for granites and >1100°C for contaminated rocks, ΔT 350°). This corresponds to the thermal regime of the development of mineralogically contrasting hypabyssal skarn aureoles: magnesian at contacts with granite magmas and calcic at contacts with melts of high basicity. The peripheral parts of the aureoles ubiquitously contain preserved zones of forsterite calciphyres and periclase marbles, whereas skarns at mafic intrusions consist of high-temperature silicates of decreasing Mg contents: monticellite, merwinite, melilite, and spurrite. Prograde and retrograde mineralforming processes in the metasomatic rocks and their facies affiliation are analyzed, and the chemical composition of the minerals are examined. The Riverside skarn aureoles are compared with other compositionally contrasting skarn aureoles that developed in contacts with granite magmas and melts of increasing basisity.     

Concentric zoning in the Tunk Lake pluton,coastal Maine

The Tunk Lake pluton of coastal Maine, USA is a concentrically zoned granitic body that grades from an outer hypersolvus granite into subsolvus rapakivi granite, and then into subsolvus non-rapakivi granite, with gradational contacts between these zones. The pluton is partially surrounded by a zone of basaltic and gabbroic enclaves, interpreted as quenched magmatic droplets and mushes, respectively, as well as gabbroic xenoliths, all hosted by high-silica granite. The granite is zoned in terms of mineral assemblage, mineral composition, zircon crystallization temperature, and major and trace element concentration, from the present-day rim (interpreted as being closer to the base of the chamber) to the core (interpreted as being closer to the upper portions of the chamber). The ferromagnesian mineral assemblage systematically changes from augite and hornblende with augite cores in the outermost hypersolvus granite to hornblende, to hornblende and biotite, and finally, to biotite only in the subsolvus granite core of the pluton. Sparse fine-grained basaltic enclaves that are most common in the outermost zone of the pluton suggest that basaltic magma was present in the lower portions of the magma chamber at the same time that the upper portions of the magma chamber were occupied by a granitic crystal mush. However, the slight variations in initial Nd isotopic ratio in granites from different zones of the pluton suggest that contamination of the granitic melt by basaltic melt played little role in generating the compositional gradation of the pluton. The zone of basaltic and gabbroic chilled magmatic enclaves, and gabbroic xenoliths, hosted by high-silica granite, that partially surround the pluton is interpreted as mafic layers at the base of the pluton that were disrupted by invading late-stage high-silica magma. These mafic layers are likely to have consisted of basaltic lava layers and basalt that chilled against granitic magma to produce coarse-grained gabbroic mush. Basaltic and gabbroic magmatic enclaves and gabbroic xenoliths are hornblende-bearing, suggesting that their parent melts were relatively hydrous. The water-rich nature of the underplating mafic magmas may have prevented extensive invasion of the granitic magma by these magmas, owing to the much greater viscosity of the granitic magma than the mafic magmas in the temperature range over which magma interaction could have occurred.     

云南保山东缘早古生代高Si花岗岩的成因及构造意义

对保山地块东缘高Si花岗岩开展矿物化学、岩石地球化学及锆石U Pb Hf系统研究,结果表明该高Si花岗岩为具钙碱性、强过铝质特征的S型花岗岩。锆石U Pb同位素分析表明,高Si花岗岩侵位于454 Ma,并含有800~1 100 Ma的继承锆石。锆石Hf同位素分析表明其岩浆锆石具有与青藏高原及东南缘同时代长英质侵入体相似的Hf同位素组成,暗示其相似的岩浆起源。矿物化学、同位素组成及Melts模拟计算结果表明,保山东缘高Si花岗岩为一系列复杂作用的结果：高硅花岗岩母岩浆起源于该区沉积岩部分熔融;熔体形成后经高度分异演化,在侵位过程中同化混染围岩;岩浆冷凝至固相线下部分矿物再平衡。保山东缘高Si花岗岩体与平河花岗岩体具相似年龄和地球化学特征,暗示它们之间可能存在类似的成因机制。     

Insights from quartz cathodoluminescence zoning into crystallization of the Vinalhaven granite,coastal Maine

The Vinalhaven intrusive complex provides field and petrographic evidence for multiple replenishments of mafic and silicic magmas, mingling and limited mixing, and rejuvenation of granite. Quartz in granitic rocks preserves a record of those processes, in the form of cathodoluminescence (CL) zoning, which is related to concentration of titanium, and to temperature of crystallization using the new TitaniQ (Ti in quartz) geothermometer. Injection of mafic melts into partly crystalline Vinalhaven granite resulted in partial quartz resorption followed by higher-temperature growth from H₂O-undersaturated melt. This is shown by steep, rimward increases in CL intensity and Ti content across discordant boundaries that truncate older growth zones. Quartz zoning in granite affected by mafic magmas displays large rimward jumps in Ti content, whereas quartz in granitic feeders and in granite far from mafic rocks typically displays broad rims with decreasing Ti contents, consistent with slow cooling without thermal disruptions due to mafic recharge.     

内蒙古沙德盖花岗岩岩浆混合作用:岩相学、矿物化学和年代学证据

华北地台北缘乌拉山地区的沙德盖钾长花岗岩体中普遍发育以二长岩为主的暗色微粒包体,包体具塑性流变特征,与寄主岩的接触界线或为截然或为渐变过渡。岩相学观察表明,包体中发育多种反映岩浆混合作用的典型组构,如石英眼斑、环斑长石、镁铁质团块、钾长石巨晶的溶蚀、磷灰石的针柱状形貌、长石中的包体带以及钙长石的"针尖"结构等。造岩矿物的电子探针分析表明,岩浆混合在沙德盖岩体的形成中起了重要作用,寄主花岗岩浆主要来自下地壳,而暗色包体岩浆则主要为地幔来源。锆石LA-ICP-MS U-Pb同位素定年结果显示,沙德盖花岗岩及其暗色微粒包体的形成时代基本一致,分别为233.4±2.3Ma和229.7±1.5Ma(中三叠世),进一步佐证了该岩体是岩浆混合作用的产物。研究认为,当铁镁质岩浆与长英质岩浆混合时,早期基性岩浆的快速淬冷形成了边界清楚、具明显冷凝边且暗色矿物含量较高的包体;随着两种不同成分岩浆之间温差的减小以及组分的交换,进一步形成了颜色较浅、边界渐变过渡和无明显冷凝边的包体。     

The 401–409 Ma Xiaodonggou granitic intrusion: implications for understanding the Devonian Tectonics of the Northwest China Altai orogen

Abstract

Palaeozoic granitoids in the Chinese Altai are important for understanding the evolution of the Central Asian Orogenic Belt (CAOB). The Xiaodonggou granitic intrusion, situated in the Chinese Altai (southern CAOB), is composed of two intrusive phases, medium-grained granite intruded by porphyritic granite. Zircon LA-ICP-MS U–Pb analyses of medium-grained granite and porphyritic granite yield ages of 409 ± 2 Ma and 400 ± 1 Ma, respectively, indicating that these formed in Early Devonian time. Medium-grained granite and porphyritic granite have similar geochemical features and Nd–Hf isotopic compositions. Arc-like geochemical characteristics (e.g. enrichment of LILEs and negative anomalies of Nb, Ta, Ti, and P) show that both phases are volcanic arc granites (VAGs). Geochemical and isotopic characteristics suggest that these magmas originated from melting older crust. Based on their near-zero or negative ε_Nd(t) values (?1.4to 0) and positive ε_Hf(t) values (+1.4 to +7.8), together with Nd model ages of 1.15–1.26 Ga and zircon Hf model ages of 0.90–1.30 Ga, we suggest that the Xiaodonggou granites were derived from a mixture of juvenile and old crustal components. Some other Devonian granitic intrusions were recently identi?ed in the Chinese Altai with ages between 416 and 375 Ma. These Devonian granites have similar geochemical characteristics and petrogenesis as Xiaodonggou granites. The formation of these Devonian granites was in response to subduction processes, suggesting that Chinese Altai was an active continental margin in Early Devonian time.     

Isotopic evidence for multiple contributions to felsic magma chambers: Gouldsboro Granite, Coastal Maine

The Gouldsboro Granite forms part of the Coastal Maine Magmatic Province, a region characterized by granitic plutons that are intimately linked temporally and petrogenetically with abundant co-existing mafic magmas. The pluton is complex and preserves a felsic magma chamber underlain by contemporaneous mafic magmas; the transition between the two now preserved as a zone of chilled mafic sheets and pillows in granite. Mafic components have highly variably isotopic compositions as a result of contamination either at depth or following injection into the magma chamber. Intermediate dikes with identical isotopic compositions to more mafic dikes suggest that closed system fractionation may be occurring in deeper level chambers prior to injection to shallower levels. The granitic portion of the pluton has the highest Nd isotopic composition (ε_Nd = + 3.0) of plutons in the region whereas the mafic lithologies have Nd isotopic compositions (ε_Nd = + 3.5) that are the lowest in the region and similar to the granite and suggestive of prolonged interactions and homogenization of the two components. Sr and Nd isotopic data for felsic enclaves are inconsistent with previously suggested models of diffusional exchange between the contemporaneous mafic magmas and the host granite to explain highly variable alkali contents. The felsic enclaves have relatively low Nd isotopic compositions (ε_Nd = + 2 – + 1) indicative of the involvement of a third, lower ε_Nd melt during granite petrogenesis, perhaps represented by pristine granitic dikes contemporaneous with the nearby Pleasant Bay Layered Intrusion. The dikes at Pleasant Bay and the felsic enclaves at Gouldsboro likely represent remnants of the silicic magmas that originally fed and replenished the overlying granitic magma chambers. The large isotopic (and chemical) contrasts between the enclaves and granitic dikes and granitic magmas may be in part a consequence of extended interactions between the granitic magmas and co-existing mafic magmas by mixing, mingling and diffusion. Alternatively, the granitic magmas may represent an additional crustal source. Using granitic rocks such as these with abundant evidence for interactions with mafic magmas complicate their use in constraining crustal sources and tectonic settings. Fine-grained dike rocks may provide more meaningful information, but must be used with caution as these may also have experienced compositional changes during mafic–felsic interactions.     

Magma mixing in the subvolcanic environment: petrology of the Gerena interaction zone near Seville,Spain

The mechanisms by which felsic and mafic magmas interact and approach a uniform hybrid composition through the processes of mingling and mixing have been studied in a high-level subvolcanic setting in the Spanish Hercynian at Gerena, near Seville. The compositions involved are calc-alkaline and the situation is one of tonalite-quartz diorite synplutonic dykes injected into a granitic magma chamber. The resulting hybrids include dykes, pillows and globules of tonalite with chilled margins which are variously disrupted and homogenised with the host granite. The present investigation is based on field and petrographic observations of hybridization textures, the identification of different stages in the crystallisation history of the tonalite through mineral textures, and the characterization of mineral compositions at these various stages. Proportions of the end-member magmas involved were obtained by major-oxide mixing models and tested satisfactorily with trace elements. A mechanistic model is presented to account for these observations which involves the early quenching of the tonalite when it was emplaced into the granite magma chamber. After high temperature crystallization had occurred the two magmas attained thermal equilibrium and disruption of the tonalite in the high energy regime of this subvolcanic complex resulted in dispersion of fragments and crystals through the granite giving rise to hybrid granodiorite compositions. It is argued that such high-energy flow conditions are a necessary requirement for effective hybridization in this environment in contrast to most large-scale magma chamber settings where mixing is driven by thermal and buoyancy contrasts.     

花岗岩结晶分离作用问题——关于花岗岩研究的思考之二

岩浆结晶分离作用是一个古老的话题,很早就有学者指出,地球内部生成的岩浆大多是玄武质岩浆,大多数花岗岩是由玄武岩结晶分离形成的.本文在考察了岩浆结晶分离作用的制约因素、比较了不同性质岩浆结晶分离作用的特征之后指出：玄武质岩浆可以发生结晶分离作用,因为有与其相关的堆晶岩产出;安山质岩浆也可以发生结晶分离作用,因为也有与其相关的堆晶岩产出.但是,花岗质岩浆似乎不大可能发生结晶分离作用,因为,很少见到有与（富硅的）花岗质岩浆相伴的堆晶岩产出.花岗质岩浆之所以不大可能发生结晶分离作用的原因在于：（1）岩浆的黏性大,它不仅阻滞了矿物的结晶作用（使斜长石不能发育为自形晶）,而且阻止了密度大的矿物（例如角闪石）下沉;（2）主要造岩矿物（例如斜长石）的密度与花岗质岩浆的密度相差无几,使结晶分离作用难以进行.本文详细考察了花岗质岩浆中斜长石的行为,指出在花岗质岩浆中斜长石结晶分离几乎是不可能的.那么,文献中大量充斥的花岗岩结晶分离作用的说法是依据什么呢？作者认为,文献中的许多说法可能主要是根据哈克图解得出的,而不是根据实际观察和理论研究得出的.作者认为,玄武岩和花岗岩不仅来源不同,成分不同,而且解释也不同.哈克图解中许多适合玄武岩的解释未必适合花岗岩.由于鲍文反应原理是结晶分离作用的理论基础,因此,文中也对鲍文反应原理进行了评述,并指出文献中存在的一些需要认真对待的问题,例如,从玄武岩-安山岩-英安岩-流纹岩的连续演化序列是不可能的;单元-超单元填图方法是不科学的;中国东部中生代大规模花岗岩不可能是玄武质岩浆结晶分离形成的等等.本文还以Ajaji et al.（1998）报道的摩洛哥Tanncherfi花岗岩为例,指出结晶分离作用的解释是不可能的.作者认为,花岗岩类的成分变化大,主要可能与源区组成、温度、压力、挥发分、部分熔融程度和过程、混合作用、岩浆分异及结晶分离作用有关.其中,源区组成可能是花岗岩多样性的最重要的原因,而结晶分离作用的影响可能是微乎其微的.本文认为,花岗岩结晶分离作用对于花岗岩成因的意义已经被大大地夸大了,我们应当重新思考结晶分离作用对于花岗质岩浆的意义.由于花岗岩的极端复杂性,许多问题还得不到比较合理的解释,本文的认识只是初步的.     

Successive mixing and mingling of magmas in a plutonic complex of Northeast Brazil

Field and petrographic evidence together with major element geochemistry suggest that mixing and mingling of magmas of contrasting compositions were important petrogenetic processes in the Fazenda Nova/Serra da Japeganga plutonic complex of Northeast Brazil. The complex was emplaced at pressures of 300–500 MPa in amphibolite facies metamorphic rocks of Neoproterozoic age and consists of three main rock types: (1) coarse-grained granite; (2) porphyritic granite and (3) diorite to quartz-monzodiorite. The latter two make up the Fazenda Nova batholith which is located on the northwestern side of the sinistral, NE-trending, Fazenda Nova strike-slip shear zone. NE-plunging stretching lineations in the shear zone suggest that this batholith represents an uplifted, and therefore deeper, portion of the complex. The structure of the complex reflects the stratigraphy in a magma chamber, with the porphyritic granite above the diorite and below the coarse-grained granite.

The porphyritic granite has a uniform composition, intermediate in mafic mineral content, quartz, and majorelements between the coarse-grained granite and the diorite. It is free of disequilibrium mineral assemblages, and locally displays gradational contacts with the overlain coarse-grained granite. Most elements display linear correlation with SiO₂ in Harker diagrams. These features are interpreted as resulting from mixing of almost crystal-free felsic and intermediate magmas. Fluid dynamic calculations using the coarse-grained granite and the silica-poorest diorite as end-members in the mixing process show that mechanical mixing was possible, and thermal modelling suggests that the formation of an homogeneous hybrid may have been achieved in less than 50,000 yr.

The diorites contain corroded K-feldspar megacrysts, and range in composition from low to relatively high silica contents, partly overlapping with the porphyritic granite. This suggests that a new mixing event occurred during the crystallisation of the porphyritic granite, this time producing a heterogeneous, xenocryst-bearing, dioritic hybrid. Abundant enclaves of diorite in the porphyritic granite, despite their textural diversity, are typically devoid of chilled margins, and were therefore formed relatively early in the crystallisation history of the granite. They are interpreted as liquid droplets separated from the heterogeneous hybrid magma through convection currents and incorporated in the, crystallising granitic magma.

Subsequently, during the crystallisation of the porphyritic granite, mafic magma supply to the batholith continued at a declining rate, probably assisted by the development of the Fazenda Nova shear zone. This leads to the production of stromatitic-like structures, with alternating bands of mutually contaminated granite and diorite, then to the intrusion of contorted synplutonic dykes, and, finally, of late-stage dykes, some of which with chilled finer-grained margins.     

Intrusion of basaltic magma into a crystallizing granitic magma chamber: The Cordillera del Paine pluton in southern Chile

The Cordillera del Paine pluton in the southernmost Andes of Chile represents a deeply dissected magma chamber where mafic magma intruded into crystallizing granitic magma. Throughout much of the 10x15 km pluton, there is a sharp and continuous boundary at a remarkably constant elevation of 1,100 m that separates granitic rocks (Cordillera del Paine or CP granite: 69–77% SiO₂) which make up the upper levels of the pluton from mafic and comingled rocks (Paine Mafic Complex or PMC: 45–60% SiO₂) which dominate the lower exposures of the pluton. Chilled, crenulate, disrupted contacts of mafic rock against granite demonstrate that partly crystallized granite was intruded by mafic magma which solidified prior to complete crystallization of the granitic magma. The boundary at 1,100 m was a large and stable density contrast between the denser, hotter mafic magma and cooler granitic magma. The granitic magma was more solidified near the margins of the chamber when mafic intrusion occurred, and the PMC is less disrupted by granites there. Near the pluton margins, the PMC grades upward irregularly from cumulate gabbros to monzodiorites. Mafic magma differentiated largely by fractional crystallization as indicated by the presence of cumulate rocks and by the low levels of compatible elements in most PMC rocks. The compositional gap between the PMC and CP granite indicates that mixing (blending) of granitic magma into the mafic magma was less important, although it is apparent from mineral assemblages in mafic rocks. Granitic magma may have incorporated small amounts of mafic liquid that had evolved to >60% SiO₂ by crystallization. Mixing was inhibited by the extent of crystallization of the granite, and by the thermal contrast and the stable density contrast between the magmas. PMC gabbros display disequilibrium mineral assemblages including early formed zoned olivine (with orthopyroxene coronas), clinopyroxene, calcic plagioclase and paragasite and later-formed amphibole, sodic plagioclase, mica and quartz. The early formed gabbroic minerals (and their coronas) are very similar to phenocrysts in late basaltic dikes that cut the upper levels of the CP granite. The inferred parental magmas of both dikes and gabbros were very similar to subalkaline basalts of the Patagonian Plateau that erupted at about the same time, 35 km to the east. Mafic and silicic magmas at Cordillera del Paine are consanguineous, as demonstrated by alkalinity and trace-element ratios. However, the contemporaneity of mafic and silicic magmas precludes a parent-daughter relationship. The granitic magma most likely was derived by differentiation of mafic magmas that were similar to those that later intruded it. Or, the granitic magma may have been contaminated by mafic magmas similar to the PMC magmas before its shallow emplacement. Mixing would be favored at deeper levels when the cooling rate was lower and the granitic magma was less solidified.     

Genesis and Mixing/Mingling of Mafic and Felsic Magmas of Back-Arc Granite: Miocene Tsushima Pluton, Southwest Japan

The Middle Miocene Tsushima granite pluton is composed of leucocratic granites, gray granites and numerous mafic microgranular enclaves (MME). The granites have a metaluminous to slightly peraluminous composition and belong to the calc‐alkaline series, as do many other coeval granites of southwestern Japan, all of which formed in relation to the opening of the Sea of Japan. The Tsushima granites are unique in that they occur in the back‐arc area of the innermost Inner Zone of Southwest Japan, contain numerous miarolitic cavities, and show shallow crystallization (2–6 km deep), based on hornblende geobarometry. The leucocratic granite has higher initial ⁸⁷Sr/⁸⁶Sr ratios (0.7065–0.7085) and lower ε_Nd(t) (?7.70 to ?4.35) than the MME of basaltic–dacitic composition (0.7044–0.7061 and ?0.53 to ?5.24), whereas most gray granites have intermediate chemical and Sr–Nd isotopic compositions (0.7061–0.7072 and ?3.75 to ?6.17). Field, petrological, and geochemical data demonstrate that the Tsushima granites formed by the mingling and mixing of mafic and felsic magmas. The Sr–Nd–Pb isotope data strongly suggest that the mafic magma was derived from two mantle components with depleted mantle material and enriched mantle I (EMI) compositions, whereas the felsic magma formed by mixing of upper mantle magma of EMI composition with metabasic rocks in the overlying lower crust. Element data points deviating from the simple mixing line of the two magmas may indicate fractional crystallization of the felsic magma or chemical modification by hydrothermal fluid. The miarolitic cavities and enrichment of alkali elements in the MME suggest rapid cooling of the mingled magma accompanied by elemental transport by hydrothermal fluid. The inferred genesis of this magma–fluid system is as follows: (i) the mafic and felsic magmas were generated in the mantle and lower crust, respectively, by a large heat supply and pressure decrease under back‐arc conditions induced by mantle upwelling and crustal thinning; (ii) they mingled and crystallized rapidly at shallow depths in the upper crust without interaction during the ascent of the magmas from the middle to the upper crust, which (iii) led to fluid generation in the shallow crust. The upper mantle in southwest Japan thus has an EMI‐like composition, which plays an important role in the genesis of igneous rocks there.     

A granite–gabbro complex from Madagascar: constraints on melting of the lower crust

The Ranomandry Complex is a Neoproterozoic, nested intrusion from central Madagascar composed of a gabbroic core within a coeval peraluminous granite ring intruding pelitic metasediments. Although xenocryst entrainment and magma mixing have both contributed to marginal phases of the granite, the primary melt is characterised by steep LREE/HREE ratios and negligible, or slightly positive, Eu anomalies. Both isotopic and trace element systematics preclude an origin from either partial melting of the metapelitic country rock or from fractional crystallisation of the gabbroic magma. However, trace-element modelling suggests an origin from the dehydration melting of a plagioclase-poor, garnet-bearing metagreywacke at temperatures of 850–900 °C and at lower crustal pressures (>10 kbar). Melting of an enriched subcontinental mantle generated gabbroic magmas that caused advective heating and anatexis at the base of thickened continental crust during their ascent. Transport of the resulting granite magma was facilitated by the pre-existing plumbing system that overcame thermal and mechanical problems associated with both diapirism and self-propagating dykes as mechanisms for long-distance transport of granite magmas. The nested geometry of the intrusions is an indication of a structurally homogeneous lower crust that contains no pre-existing shear zones or fault systems.