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1.
南岭地区中生代两种成矿花岗质岩的岩石学和矿物学特征探讨 总被引：1，自引：0，他引：1
南岭地区是我国重要的有色金属矿产资源集中区,中生代多金属成矿与本区广泛分布的花岗质岩石具有密切的成因关系.本文在总结南岭地区与花岗质岩石有关的钨锡钼铋和铜(钼)铅锌金银两类多金属矿床时空分布规律、岩体特征的基础上,探讨并对比了这两种成矿花岗质岩的岩石学和矿物学特征,指出:①与钨锡钼铋成矿有关的岩浆岩以壳源物质重熔形成的黑云母花岗岩为主,但不少岩体也带有幔源物质信息.造岩矿物组合为石英+钾长石+斜长石+黑云母+白云母±角闪石,石英、长石、黑云母包含结构十分发育.斜长石以钠长石为主,An平均值为8;黑云母为富铁黑云母—铁叶云母—铝铁叶云母,MF值为0.001～0.5311;个别岩体中含角闪石,Mg# =0.21～0.53.②与铜(钼)铅锌金银成矿有关的岩浆岩以壳幔混源的花岗闪长岩为主.造岩矿物组合为石英+斜长石+钾长石+黑云母+角闪石±白云母,包含结构不发育.斜长石以更长石为主,An平均值为22;黑云母为富镁黑云母,MF值为0.5120～0.7216;普遍见有角闪石,Mg# =0.48～1.00.两类成矿岩浆岩的岩石学和矿物学差异可以为南岭花岗质岩石地区找矿提供初步的指示意义. 相似文献
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杜尔基花岗岩基位于大兴安岭中南段,且处在大兴安岭一个大型多金属矿集区(Au-Ag-Cu-Zn-Sn-Fe)中。区内岩浆活动复杂,成矿元素多样,其中,杜尔基花岗岩是本地区出露面积较大(约190 km2)的岩体之一。为厘清区内不同花岗质岩石的源区特征及其与成矿的关系,文章对杜尔基地区花岗岩的主要岩性单元二长花岗岩和正长花岗岩进行了系统的矿物学和Hf同位素组成研究。结果表明:二长花岗岩的主要矿物为斜长石(32%)、钾长石(45%)、石英(20%),次要矿物为黑云母、角闪石和辉石等暗色矿物(3%);正长花岗岩的主要组成矿物为石英(10%~15%)、钾长石(60%~70%)和斜长石(30%),次要矿物为黑云母(5%),在这两种花岗岩中均广泛发育条纹长石。黑云母主要为铁质黑云母和铁叶云母,角闪石为韭闪石和普通角闪石,辉石为普通辉石。矿物学特征均指示杜尔基花岗岩主要为I型花岗岩。二长花岗岩εHf为-1.6~17.6,正长花岗岩εHf为-3.3~12.2。杜尔基花岗岩Hf同位素特征指示其源区为新生的地壳物质,可能是来自地幔的底侵玄武质岩浆发生重熔的结果。 相似文献
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江西香炉山大型钨矿床位于长江中下游成矿带南侧新识别的钨矿带西段,为一矽卡岩型白钨矿床.矽卡岩矿体主要产于中寒武统杨柳岗组含炭质灰岩和香炉山花岗岩接触部位.香炉山花岗岩具有细粒至中粗粒结构,主要由石英、钾长石、斜长石和黑云母组成,副矿物以钛铁矿为主.香炉山花岗岩的SiO2含量为66.37％～67.76％,并具有较高的Na... 相似文献
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《地学前缘》2017,(5):76-92
九龙脑岩体位于南岭成矿带东段崇余犹矿集区内,是由四个期次花岗岩组成的复式岩体,从早到晚分别为中粗粒黑云母花岗岩(γ2-1a5)、中粗粒斑状(含白云母)黑云母花岗岩(γ2-1b5)、细-中细粒斑状黑云母花岗岩(γ2-2a-2b5)、细-中细粒(含黑云母、石榴石)花岗岩(γ25),以其为中心,钨锡、金银铜铅锌、铀、铌钽等多矿种、多期次、多成因矿床分带产出。通过对花岗岩的矿物学研究,确定九龙脑花岗岩中的钾长石以正长石为主,斜长石为钠-更长石,黑云母为富铁黑云母-铁叶云母-铝铁叶云母,原生白云母较次生白云母具有高铁、锰、镁、氟、氯和低铝特征,石榴石属于锰铝榴石-铁铝榴石,绿泥石为鲕绿泥石-蠕绿泥石(铁绿泥石)-铁镁绿泥石,副矿物中常含微量的稀有金属元素。原生白云母和钛铁矿的存在以及黑云母矿物化学特征指示九龙脑花岗岩为S型花岗岩。第一期次花岗岩结晶时具有较高的氧逸度,其黑云母结晶温度为550~600℃;第二、三、四期次花岗岩结晶时具有相对较低的氧逸度,其黑云母结晶温度分别为600~700℃、600~700℃、550℃。第一、三、四期次花岗岩中绿泥石的形成温度分别为385~400℃、300~370℃、359~397℃,显示九龙脑花岗岩经历了中高温热液流体的影响。矿物化学特征表明,九龙脑花岗岩与南岭地区成钨锡钼铋矿的花岗岩具有相似的矿物组成,第一期次、第二期次、第四期次花岗岩可能与矿田内丰富的钨、锡、铌钽、铀矿化密切相关。 相似文献
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北大别位于大别造山带的核部，分布着大量的造山带垮塌时期形成的混合岩，其于理解大别造山带的形成和演化有着重要的意义。北大别混合岩的原岩为TTG（D）岩石，因黑云母和角闪石的脱水熔融诱发深熔作用产生。顺层产出的为富斜长石浅色体，主要矿物组成为斜长石+石英+黑云母±钾长石±角闪石。伟晶岩脉或团块为富钾长石浅色体，主要矿物组成为钾长石+石英±斜长石±黑云母±角闪石。暗色体为变晶结构，主要矿物组成为角闪石+黑云母+斜长石+石英±单斜辉石；其中，暗色矿物角闪石和黑云母常常定向排列，具有明显的溶蚀结构；暗色体中浅色矿物颗粒较小，以斜长石和石英为主，指示部分熔融的残余产物。全岩地球化学特征表明，碱金属元素（Na、K等）、大离子亲石元素（Ba、K、La等）和LREE等优先进入酸性熔体，而相容元素和中-重稀土元素等残留在残余体中。浅色体与本区花岗岩相比，二者都有右倾的稀土配分模式，富集LREE，亏损HREE。但浅色体具有明显的Eu正异常，δEu值为2.48~6.55，而花岗岩则有弱的Eu负异常，并且浅色体中大颗粒斜长石相互构成框架结构，含量明显高于正常花岗岩熔体，表明浅色体更可能是熔体早期结晶的产物。 相似文献
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安徽金寨县沙坪沟钼矿成矿岩体矿物学特征及其成岩成矿意义 总被引：1，自引：0，他引：1
安徽金寨县沙坪沟钼矿成矿岩体主要包括二长花岗斑岩、正长斑岩及花岗斑岩,主要矿物组成为石英、钾长石、斜长石、黑云母。本文利用电子探针技术对主要矿物的矿物学特征进行了研究,结果表明,岩石中斜长石为钠长石;钾长石以条纹长石为主;黑云母属于镁质黑云母,富Mg、Al、K,贫Ca,MF值变化于0.65~0.72之间。黑云母化学成分显示本区岩体为I型花岗岩,属于造山带钙碱性花岗岩类,其成岩物质具壳幔混合的特点。黑云母矿物温压计计算结果显示,岩体固结温度约500~580℃,氧逸度变化范围是10-14~10-15.8,其含铁指数对寻找钼矿具较好的指示意义。 相似文献
7.
本文报道了南岭地区两个花岗岩(大东山和千里山)的全岩以及主要造岩矿物(黑云母、斜长石和石英)的硼含量和硼同位素组成。结果显示,黑云母含有最高的硼含量,斜长石次之,石英中的硼含量则非常低。花岗岩中的硼可能主要以晶格替换方式赋存在黑云母和斜长石中,而石英中的微量硼则来自其包裹的流体包裹体。同时,黑云母具有最低的硼同位素组成,斜长石次之,而石英则具有最高的硼同位素组成。本次研究首次发现花岗岩中的主要造岩矿物间存在着较大的硼同位素分馏:大东山花岗岩中黑云母与斜长石之间存在着-9.3‰的分馏,黑云母与石英之间存在-9.9‰的分馏;千里山花岗岩中黑云母与斜长石之间存在着-6.6‰的分馏,黑云母与石英之间存在着-10.2‰的硼同位素分馏。结合镜下观察和氧同位素研究发现,造成不同矿物间这一大的硼同位素分馏的原因很可能是岩浆后期的热液蚀变作用。综合以往所发表的研究数据,得到含四次配位硼为主的硅酸盐矿物与中酸性热液流体之间的硼同位素分馏方程:1000lnα硅酸盐-流体=-11.19×(103/T[K])+5.09,该分馏是由于硼的四次配位和三次配位之间的转换引起的。 相似文献
8.
苏北东海地区主侵入花岗岩类岩石属Ⅰ型花岗岩类。其主要造岩矿物为阳起角闪石、富镁黑云母、更长石、钾长石和石英。主要副矿物为榍石、磷灰石、锆石、磁铁矿等。主侵入岩体的形成温度为910—690℃,logfo_2=-12,fH_2O=(1.3—1.8)×10~8Pa,log(fH_2O/f_(HF))=3—4。第一类过渡元素富集于角闪石、黑云母和榍石中,而在更长石、钾长石和石英中明显亏损。由于榍石、角闪石、黑云母和更长石对全岩REE含量的贡献最大,因此它们控制了本区花岗岩类岩石的稀土配分模式。前三者富重稀土,负Eu异常较强,斜长石相对富轻稀土,并表现为正Eu 异常。因此可以认为随着花岗质岩浆中铁镁矿物和斜长石的分离,将会导致残余岩浆中第一类过渡元素、重稀土元素和Eu的亏损。 相似文献
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10.
粤西大金山钨锡多金属矿床地质特征及成岩成矿年代学研究 总被引：3，自引：2，他引：1
粤西大金山钨锡多金属矿是一个近年新发现的与花岗岩有关的石英脉型钨锡多金属矿,目前估算的资源量已达中型,并具有大型矿床的找矿潜力。矿体形态简单,主要以石英脉的形式产出,由石英脉、云英岩脉和多金属硫化物石英脉等组成。钨锡多金属矿化的主要类型为细脉状和网脉状,围岩蚀变主要有硅化、云英岩化和绿泥石化等。本文在详细介绍矿床地质特征的基础上,对矿床进行了成岩成矿年代学研究。采用LA-MC-ICP-MS锆石U-Pb测年技术,得到了花岗岩的成岩年龄:中细粒黑云母花岗岩形成于82.89±0.35Ma~85.6±0.52Ma,似斑状黑云母花岗岩形成于75.01±0.16Ma~84.17±0.34Ma。通过对与中细粒黑云母花岗岩有关的5件石英脉型辉钼矿进行Re-Os同位素分析,获得其模式年龄为80.07±1.19Ma~84.93±1.42Ma。以上年代学测试结果说明大金山钨锡多金属矿成岩成矿时代为晚白垩世,成岩成矿作用基本同时。本文认为大金山钨锡多金属矿成岩成矿作用发生在华南晚中生代岩石圈拉张-伸展的构造背景下,是华南晚中生代大规模成岩成矿作用的产物。 相似文献
11.
Abstract Microprobe analyses of feldspars in granite mylonites containing flame perthite give compositions that invariably plot as three distinct clusters on a ternary feldspar diagram: orthoclase (Or92–97), albite and oligoclase-andesine. The albite occurs as grains in the matrix, as flame-shaped lamellae in orthoclase, and in patches within plagioclase grains. We present a metamorphic model for albite flame growth in the K-feldspar in these rocks that is related to reactions in plagioclase, rather than alkali feldspar exsolution. Flame growth is attributed to replacement and results from a combination of two retrograde reactions and one exchange reaction under greenschist facies conditions. Reaction 1 is a continuous or discontinuous (across the peristerite solvus) reaction in plagioclase, in which the An component forms epidote or zoisite. Most of the albite component liberated by Reaction 1 stays to form albite in the host plagioclase, but some Na migrates to form the flames within the K-feldspar. Reaction 2 is the exchange of K for Na in K-feldspar. Reaction 3 is the retrograde formation of muscovite (as ‘sericite’) and has all of the chemical components of a hydration reaction of K-feldspar. The Si and Al made available in the plagioclase from Reaction 1 are combined with the K liberated from the K-feldspar, to produce muscovite in Reaction 3. The muscovite forms in the plagioclase, rather than the K-feldspar, as a result of the greater mobility of K relative to Al. The composition of the albite flames is controlled by both the peristerite and the alkali feldspar miscibility gaps and depends on the position of these solvi at the pressure and temperature that existed during the reaction. Using an initial plagioclase composition of An20, the total reaction can be summarized as: 20 oligoclase + 1 K-feldspar + 2 H2O = 2 zoisite + muscovite + 2 quartz + 15 albiteplagioclase+ 1 albiteflame. This model does not require that any additional feldspar framework be accreted at replacement sites: Na and K are the only components that must migrate a significant distance (e.g. from one grain to the next), allowing Al to remain within the altering plagioclase grain. The resulting saussuritization is isovolumetric. The temperature and extent of replacement depends on when, and how much, water infiltrates the rock. The fugacity of the water, and therefore the pressure of the fluid, may have been significantly lower than lithostatic during flame growth. 相似文献
12.
南岭地区锡钨多金属矿成矿作用主要与燕山早期花岗岩岩浆活动有关,这些花岗岩一直被认为是黑云母花岗岩、二长花岗岩、细粒二长花岗岩等,少量矿区岩体顶部出现钠长石花岗岩。通过对湖南瑶岗仙钨矿岩体的深入解剖,以及对杮竹园、黄沙坪、锡田、邓阜仙、栗木、梅子窝等矿区花岗岩全面的岩矿鉴定和电子探针分析,确定钨矿区致矿花岗岩的长石主要为碱性长石,其中绝大部分样品中钠长石An<5,因此确定这些花岗岩均属于碱长花岗岩。与成矿有关的由浆液过渡态流体形成的云英岩包体中的钠长石更加富Na,An<3。碱长花岗岩的成分以及钠长石成分在岩体顶部约1 000 m深度范围内无明显垂向变化。致矿花岗岩体中部分早期花岗岩包体、晚期花岗斑岩,部分花岗岩基以及印支期花岗岩、加里东期花岗闪长岩等,钠(斜)长石An值明显高,很多属更长石、中长石甚至基性长石。这种包含有两种碱性长石的碱长花岗岩由富挥发分的岩浆形成,在岩体顶部附近广泛发生液态不混溶作用,是导致锡钨多金属矿富集成矿的主要分异方式。钨锡矿区碱长花岗岩钠长石An值明显低于区域大花岗岩基或不致矿花岗岩,可作为花岗岩的钨锡成矿评价标志之一。 相似文献
13.
广东阳春锡山钨锡矿床成岩成矿年代学研究 总被引：4，自引：0，他引：4
为了精确厘定锡山钨锡矿床成岩成矿时代,本文分别对锡山岩体斑状二长花岗岩和矿体开展了锆石LA-ICP-MS U-Pb和石英Rb-Sr年代学研究,获得的锆石U-Pb年龄为103±3Ma,石英矿物Rb-Sr等时线年龄为94±12Ma,两者在测定误差内一致,表明锡山岩体晚阶段形成的花岗岩在时间和空间上与成矿作用有明显的耦合关系,其成岩成矿作用发生在早白垩世晚期。锡山成岩成矿的动力学背景应与早白垩世晚期南岭地区的岩石圈伸展拉张引发的大规模岩浆活动有关。 相似文献
14.
Robert J. Stull 《Lithos》1978,11(3):243-249
Mantled feldspars that formed by resorption, development of skeletal plagioclase crystals, and filling with alkali feldspar are common in the Golden Horn batholith, Washington. Subhedral plagioclase mantles have weak normal zoning from An17 to An10. Plagioclase zoning and twinning are crosscut by resorption channels. Resorption cavities and channels are coated with albite (An10). Anhedral, perthitic orthoclase within the plagioclase is optically continuous with orthoclase in channels and on the mantle exterior.This texture resulted from resorption of calcic cores of plagioclase as pressure decreased when water-undersaturated granite magma intruded to a shallow crustal level. At shallow level, only alkali feldspar and quartz crystallized and were available to fill the skeletal plagioclase. 相似文献
15.
The Xishan deposit, located in the western Guangdong Province in South China, is a quartz-vein type W-Sn deposit with an average Sn grade of 0.1–0.4 wt%. The deposit is temporally and spatially associated with Xishan alkali feldspar granite. The W–Sn mineralization is present mainly as veins that are hosted by the granite. In this paper we present new zircon U–Pb age, whole-rock geochemical data, Sr–Nd–Pb–Hf isotopic data and Re–Os age in order to constrain the nature and timing of magmatism and mineralization in the Xishan mining district with implications on geodynamic settings. LA–ICP–MS zircon U–Pb analyses yielded an age of 79.14 ± 0.31 Ma for the alkali feldspar granite, consistent with the molybdenite Re–Os age of 79.41 ± 1.11 Ma. The alkali feldspar granite shows high contents of SiO2 (71.52–76.25 wt%), high total alkalis (Na2O + K2O = 9.35–13.51 wt%), high field strength elements (e.g. Zr = 95.4–116 ppm, Y = 97.1–138 ppm, Nb = 36.1–55.5 ppm, Ga = 97.1–138 ppm), and rare earth elements (total REE = 171.8–194.0 ppm) as well as high Ga/Al ratios (10,000 × Ga/Al = 3.23–3.82) suggesting that it has the geochemical characteristics of A-type granite and shows an A2 subtype affinity. Sr–Nd isotopes of the alkali feldspar granite show that (87Sr/86Sr)i values range from 0.7111 to 0.7183, and the εNd(t) values and Nd model ages (T2DM) vary from −6.8 to −6.5 and 1414 to 1433 Ma, respectively. The Pb isotopic compositions are variable, with 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values ranging from 18.783 to 18.947, 15.709 to 15.722 and 38.969 to 39.244, respectively, indicating that the alkali feldspar granite was derived from a mantle-crust mixed source. In situ Hf isotopic analyses reveal that the alkali feldspar granite has εHf(t) values ranging from −9.69 to −0.04 and two-stage Hf model ages from 1145 Ma to 1755 Ma, indicating that the alkali feldspar granite was formed by the partial melting of Mesoproterozoic crusts of the Cathaysia Block with additions of mantle-derived materials. These results, together with previously presented regional geological relationships, suggest that the formation of the Xishan granite and associated W–Sn mineralization is related to lithospheric extension and asthenospheric upwelling that are attributed to a directional change of Pacific plate motion. 相似文献
16.
W. Johannes 《Contributions to Mineralogy and Petrology》1980,72(1):73-80
Time studies were performed in the quinary system Qz-Or-Ab-An-H2O at
kbars and T=665 ° and 660 ° C. Starting material was a mixture of quartz, alkali feldspar Or80 and plagioclase An31. The compositions of plagioclases of run products were determined and compared with the plagioclase of stable solidus conditions.The solidus of the granite system was fixed at P
HäO=5 kbars using various plagioclase — and appropriate alkali feldspar — compositions besides quartz in the starting mixture (Fig. 1).The results of time studies (Table 3 and Fig. 3) reveal metastable melting in the granite system Qz-Or-Ab-An-H2O. Plagioclase melts almost stoichiometrically. The new plagioclase compositions formed during melting of cotectic compositions approach the theoretically expected stable plagioclase compositions only extremely slowly. An extrapolation of the data achieved in run times of 5–1,500 h indicates attainment of equilibrium after 1014 years. Metastable melting of granitic compositions is not only considered as an experimental problem but also as a rock forming process in nature. 相似文献
17.
柿竹园钨锡多金属矿床地处南岭中段,矿种多、规模大、共生组分丰富,是世界罕见的超大型钨锡钼铋多金属矿床。区内发育有大量不同类型的脉体,包括酸性岩浆岩脉、云英岩脉、碱交代脉等,其中发育于矽卡岩中的碱交代脉是矿区规模最大、分布最广的矿体。本文通过详实的野外地质调研、显微镜下岩相学鉴定以及电子探针测试分析,发现大部分碱交代脉中以钾长石、斜长石、石英、萤石为主,白云母含量较少,一般小于3%,含少量黄玉、电气石等矿物,并非前人所称的"云英岩脉"或"云英岩网脉",脉体外侧发育较多的石榴子石、透辉石等矽卡岩类矿物,脉体及其两侧含有较多的白钨矿、磁铁矿等氧化物。从矿物组合和矿化特点上分析,碱交代脉总体碱质成分含量较高,发生了较强的碱交代作用,脉体的形成与矽卡岩演化关系密切,可进一步分为:早期阶段碱交代脉,富斜长石、黑钨矿,形成矽卡岩,对应矽卡岩演化的矽卡岩阶段,是黑钨矿的主成矿期;晚期阶段碱交代脉,富钾长石、白钨矿、磁铁矿,表现为交代矽卡岩,对应矽卡岩演化的退变质阶段,是白钨矿的主成矿期。综上,柿竹园矿床矽卡岩中碱交代脉,制约和影响着矽卡岩的形成与成矿,贯穿矽卡岩演化过程的矽卡岩阶段和退变质阶段,其形成的矿体为矽卡岩型矿体。 相似文献
18.
年代学研究是花岗岩型稀有金属矿床研究的重要内容,多种定年方法的联合有助于对岩浆演化及稀有金属元素富集成矿过程的精细刻画。论文选择栗木矿区具代表性的金竹源、老虎头和水溪庙花岗岩型钨锡铌钽矿床和（长石）石英脉型钨锡矿床,进行LA-ICP-MS锆石U-Pb和LA-MC-ICP-MS锡石U-Pb年代学研究。结果显示,老虎头细粒铁锂云母钠长石花岗岩锆石U-Pb年龄为（216.3±1.7）Ma,长石石英脉型钨锡矿床锡石UPb年龄为（210.1±4.9）Ma;两件金竹源花岗岩型钨锡铌钽矿床锡石U-Pb年龄分别为（219.6±1.5）Ma和（219.9±2.2）Ma,石英脉型钨锡矿床锡石U-Pb年龄为（217.3±3.2）Ma;水溪庙长石石英脉型钨锡矿床锡石U-Pb年龄为（210.7±2.5）Ma。本次研究可以看出,金竹源花岗岩的成岩年龄与花岗岩型钨锡铌钽矿床及石英脉型钨锡矿床的成矿年龄间隔较小,表明含矿花岗岩可能在较短时间内经历了岩浆冷凝结晶、富集成矿和热液锡矿化过程,是短暂矿化事件的产物,而非以往认为的岩浆-热液长时间演化的结果。同时,基于不同测年方法的对比结果显示,在含锡花岗岩中,锡石年龄既能代... 相似文献
19.
二长石地质温度计在估算乌拉山金矿碱性长石形成温度中的应用 总被引：1，自引：0，他引：1
内蒙古乌拉山金矿田内主要出露晚太古代乌拉山群区域变质岩和规模不一的花岗岩体以及不同时代、不同种类的脉状地质体。含金矿脉中主要矿物共生组合为碱性长石、石英、斜长石、碳酸盐矿物(方解石、白云石)和少量金属硫化物。矿床的显著特征为碱性长石交代作用强烈，碱性长石也广泛产于该地区其他各种类型的岩石中。本文采用电子显微探针分析了共生碱性长石和斜长石的化学成分，并采用三元二长石温度模型估计了碱性长石的平衡温度。结果表明，第一成矿阶段的碱性长石一石英含金矿脉中碱性长石的形成温度为353℃，第二成矿阶段石英含金矿脉中碱性长石的形成温度为281℃，矿脉碱性长石形成压力约为5kbar。这些结果与同类矿石中平衡共生的碳酸盐矿物和云母类矿物的地质温度计估计的形成温度以及共生石英中流体包裹体的均一温度非常一致。因此，乌拉山金矿床形成和富集的温度可估测为260～380℃，压力约为5kbar。此外，应用二长石温度计计算了本地区区域变质片麻岩和花岗岩中碱性长石的平衡温度，所得温度比采用共生铁铝榴石和黑云母温度计估计的温度要低约250℃。这表明共生的铁铝榴石和黑云母的平衡温度可能代表其寄主变质岩变质期温度及寄主花岗岩原生温度，而区域变质岩和花岗岩中的碱性长石在经历了随后多次热液作用后，可能重新平衡再生，这也与前人对乌拉山金矿的矿床地质和同位素研究的结果一致。 相似文献
20.
Late-stage hydrous fluids, which evolved during the cooling of the Cadillac Mountain granite, Maine, USA, produced narrow veins that transect the pluton. The vein margins contain microstructures transitional between granite and the vein centre. They preserve the vestige shapes of original Na-rich alkali feldspar crystals that have been pseudomorphed by cordierite+quartz+K-rich alkali feldspar. Closer to the centres of the veins, the minerals change from an outer zone with cordierite, hercynitic–galaxite spinel, minor corundum, K-feldspar and plagioclase to an inner zone with remnant cordierite, hercynitic gahnite, strongly zoned almandine–spessartine garnet, chlorite and quartz. Three allochemical reactions are inferred to have occurred with the influx of hydrous fluids during the replacement process. Reaction (1) is Na-rich alkali feldspar+iron ions in solution=Fe-cordierite+quartz+K-feldspar+sodium ions in solution. Reactions (2) and (3) occurred during desilication. Reaction (2) is Fe-cordierite=hercynite+silica in solution, and reaction (3) is Fe-cordierite+water=corundum+iron hydroxide in solution+silicic acid in solution. Two independent techniques, experimental silica-solubility data and spinel–cordierite thermobarometry, constrain the conditions of vein formation to c. 1.0 kbar and both methods indicate that the progressive mineral reactions occurred during cooling of the hydrous fluids from c. 775° to 400–340 °C. This cooling trend is consistent with the petrographic evidence, which demonstrates that reactions occurred before desilication, during desilication, and then diminished with a final phase of resilication. Although the veins are minor features of the Cadillac Mountain granite, they provide insight into the conditions that prevailed during cooling of the pluton, and similar features may be important for constraining the cooling history of shallow-level plutonic complexes elsewhere. 相似文献