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1.
The Wolf River Batholith is an anorogenic rapakivi massif in central and northeastern Wisconsin with an age of 1.5 Ga. The Batholith has alkaline affinities and consists of biotite granite and biotite-hornblende adamellite with minor occurrences of quartz syenite and older monzonite and anorthosite. The batholith is part of a major Late Precambrian (1.4–1.5 Ga) magmatic event of continental proportions, represented by separate intrusions extending from Labrador to southern California (Silver et al., 1977).The major and trace element composition (Li, Rb, Sr, Ba, and REE) of 40 samples from the anorthosite, monzonite, and rapakivi granite and adamellite plutons precludes a comagmatic (although not cogenetic) model between all three rock units. However, the monzonite may be related to the anorthosite alone by fractional crystallization of plagioclase, orthopyroxene, clinopyroxene, and apatite. Alternatively, the monzonite may be a separate parent melt or a hybrid associated with the granite and adamellite plutons. The high REE content of the monzonite precludes it from being related to the rapakivi granite and adamellite plutons as a source material, a residuum, or a cumulate.A major portion of the Batholith is an undifferentiated intrusive sequence ranging from older rapakivi granite to younger adamellite. The compositions of these plutons suggest a crustal fusion origin at intermediate to lower levels of the crust (25–36 km). The trace element data are consistent with partial fusion of tonalitic to granodioritic source material.During crystallization and emplacement into the upper crust (less than 4 km), 55–70% fractionation of two feldspars, biotite and hornblende from one of the granite plutons produced a small volume of differentiated granitic melt high in Si, Fe/Mg, Rb, Li, and REE (except Eu), and low in Ca, Mg, Al, Ca/Na, Sr, Ba, and K/Rb and with a large negative Eu anomaly. Presumed associated cumulate material ranges from silica-poor quartz monzonite and quartz syenite.The chemical and mineralogical similarity between the Wolf River Batholith and younger magmatic analogs associated in continental break-up (Nigerian younger granites, White Mountain magma series, and the peralkaline volcanics of the Red Sea Region) are suggestive but not conclusive of an extensional tectonic setting. A preliminary tectonic model suggests that the 1.4–1.5 Ga event is in response to thermal doming in an extensional regime leading to continental separation in the western Cordillera (pre-Belt) and extensive crustal fusion with no rifting or separation across the North American Craton.  相似文献   

2.
Early Proterozoic Dongargarh granite complex of Central India, intruding the tonalitic to granodioritic Amgaon gneisses and the Nandgaon Group bimodal volcanic suite, comprises three different textural and compositional types, viz., porphyritic granodiorite (PG), coarse equigranular granite (EG) and microgranite (MG). Synplutonic mafic dykes are common in the granite complex. The PG is characterised by rapakivi texture and the EG is the dominant facies and exhibits sporadically developed rapakivi texture. Microgranular enclaves are common in the EG while they are rare in PG. Major and trace element geochemistry of PG shows marked I- type and some occasional A-type granite characters unusual for a rapakivi granite while the EG shows A-type granite signatures. The field, petrographic, chemical and isotopic data of these granites suggest their derivation by mixing of mantle derived basic magma with a crustal-derived partly crystalline granitic magma. Episodic mafic magma underplating caused the anatexis of the Archaean lower continental crust in a continental margin tectonic setting resulting first in the formation of the I-type granodiorite followed by A-type granite. The I-type granodiorite is mixed with the basic magma (synplutonic dykes) while the EG is formed by mingling of A- type granite magma and the intruding basic magma.  相似文献   

3.
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.  相似文献   

4.
Three Paleoproterozoic A-type rapakivi granite suites (Jamon, Serra dos Carajás, and Velho Guilherme) are found in the Carajás metallogenic province, eastern Amazonian craton. Liquidus temperatures in the 900–870 °C range characterize the Jamon suite, those for Serra dos Carajás and Velho Guilherme are somewhat lower. Pressures of emplacement decrease from Jamon (3.2±0.7 kbar) through Serra dos Carajás (2.0±1.0 kbar) to Velho Guilherme (1.0±0.5 kbar). Oxidizing conditions (NNO+0.5) characterized the crystallization of the Jamon magma, the Velho Guilherme magmas were reducing (marginally below FMQ), and the Serra dos Carajás magmas were intermediate between the two in this respect. The three granite suites have Archean TDM model ages and strongly negative Nd values (−12 to −8 at 1880 Ma), and they were derived from Archean crust. The Jamon granite suite may have been derived from a quartz dioritic source, and the Velho Guilherme granites from K-feldspar-bearing granitoid rocks with some sedimentary input. The Serra dos Carajás granites either had a somewhat more mafic source than Velho Guilherme or were derived by a larger degree of melting. Underplating of mafic magma was probably the heat source for the melting. The petrological and geochemical characteristics of the Carajás granite suites imply considerable compositional variation in the Archean of the eastern Amazonian craton. The oxidized Jamon suite granites are similar to the Mesoproterozoic magnetite-series granites of Laurentia, and they were derived from Archean igneous sources that were more oxidized than the sources of the Fennoscandian rapakivi granites. The Serra dos Carajás and Velho Guilherme granites approach the classic reduced rapakivi series of Fennoscandia and Laurentia. No counterparts of the Mesoproterozoic two-mica granites of Laurentia have been found, however. Following the model of Hoffman [Hoffman, P., 1989. Speculations on Laurentia's first gigayear (2.0 to 1.0 Ga). Geology 17, 135–138], the origin of the 1.88 Ga Carajás granites is related to a mantle superswell beneath the Trans-Amazonian supercontinent. This caused breakup of the continent and was associated with magmatic underplating and resultant crustal melting and generation of A-type granite magmas. The Paleoproterozoic continent that included the Archean and Trans-Amazonian domains of the Amazonian craton was assembled at 2.0 Ga; its disruption was initiated at 1.88 Ga, at least 200 Ma earlier than in Laurentia and Fennoscandia. The Carajás granites were related to the breakup of the supercontinent, not to subduction processes.  相似文献   

5.
This study of the Pikes Peak batholith includes the mineralogy and petrology of quartz syenite at West Creek and of fayalite-bearing and fayalite-free biotite granite near Mount Rosa; major element chemistry of the batholith; comparisons with similar postorogenic, intracratonic, sodic to potassic intrusives; and genesis of the batholith.The batholith is elongate in plan, 50 by 100 km, composite, and generally subalkalic. It was emplaced at shallow depth 1,040 m. y. ago, sharply transects its walls and may have breached its roof. Biotite granite and biotite—hornblende granite are predominant; quartz syenite, fayalite granite and riebeckite granite are present in minor amounts.Fayalite-bearing and fayalite-free quartz syenite, fayalite-biotite granite and riebeckite granite show a well-defined sodic differentiation trend; the less sodic fayalite-free granites exhibit a broader compositional range and no sharp trends.Crystallization was largely at PH2O < Ptotal; PH2O approached Ptotal only at late stages. Aplite residual to fayalite-free biotite granite in the north formed at about 1,500 bars, or 5 km depth. Feldspar assemblages indicate late stages of crystallization at about 720°C. In the south ilmenite and manganian fayalite indicate fO2 of 10?17 or 10?18 bars. Biotite and fayalite compositions and the ‘granite minimum’ imply completion of crystallization at about 700°C and 1,500 bars. Nearby fayalite-free biotite granite crystallized at higher water fugacity.All types of syenite and granite contain 5–6% K2O through a range of SiO2 of 63–76%. Average Na2O percentages in quartz syenite are 6.2, fayalite granite 4.2, and fayalite-free granite 3.3 MgO contents are low, 0.03–0.4%; FeO averages 1.9–2.5%. FeO/Fe2O3 ratios are high. Fluorine ranges from 0.3 to 0.6%.The Pikes Peak intrusives are similar in mode of emplacement, composition, and probably genesis to rapakivi intrusives of Finland, the Younger Granites of Nigeria, Cape Ann Granite and Beverly Syenite, Mass., and syenite of Kungnat, Greenland, among others — allowing for different levels of erosion. A suite that includes gabbro or basalt, anorthosite, quartz syenite, fayalite granite, riebeckite granite, and biotite and/or hornblende granites is of worldwide occurrence.A model is proposed in which mantle-derived, convecting alkali olivine basaltic magma first reacts with K2O-poor lower crust of granulite facies to produce magma of quartz syenitic composition. The syenitic liquid in turn reacts with granodioritic to granitic intermediate crust of amphibolite facies to produce the predominant fayalite-free biotite and biotite-hornblende granites of the batholith. This reaction of magma and roof involves both partial melting and the reconstitution and precipitation of refractory phases, as Bowen proposed. Intermediate liquids include MgO-depleted and Na2O-enriched gabbro, which precipitated anorthosite, and alkali diorite. The heat source is the basaltic magma; the heat required for partial melting of the roof is supplied largely by heats of crystallization of phases that settle out of the liquid — mostly olivine, clinopyroxene and plagioclase.  相似文献   

6.
Grain sequences of Precambrian rapakivi granites of the Vyborg and Salmi Massifs have been compared with the stochastic model for ideal granite. These sequences show that classical rapakivi granites correspond to metasomatically weakly altered granites with a simple loss of Markov transitions from quartz and plagioclase. Observed parameters of the model indirectly indicate rapakivi magma had small volatile content and large viscosity which is also characteristic of many other Precambrian granites.  相似文献   

7.
D Field  S.A Drury  D.C Cooper 《Lithos》1980,13(3):281-289
High-Fe, intermediate-acid, charnockitic gneisses in the Arendal-Tromøy area of the Svecofennian terrain of southeast Norway comprise two chemically contrasting zones - one with normal large-ion-lithophile (LIL) element characteristics, and the other IIL-deficint. The noramal -LiLL-deficient varitties also have low ΣEEE, commonly with positive Eu anomalies. The normal-LIL rocks are enriched in ?REE, exhibit fractionated patterns and have negative Eu anomalies. Modelling shows that both the LIL and REE patterns are consistent with an essentially primary fractionation process involving the separation of cumulus (LIL-deficient) phases from andesitic-dacitic magma emplaced directly under the high-grade conditions, with the normal-LIL rocks crystallising from the residual melt. This process is interpreted as a deep-seated component of the magma system which culminated in the emplacement of some higher level rapakivi granite late in the Svecofennian event. The model presented does not require anorthosite to be part of the same magma system.  相似文献   

8.
答“对秦岭奥长环斑花岗岩质疑”   总被引：8，自引：1，他引：8  
环斑花岗岩是一种特殊结构的花岗岩类，并且多数产在元古宙克拉通中。笔者曾报道了在秦岭造山带中发育有印支期具有环斑结构的花岗质岩石。“对秦岭奥长环斑花岗岩质疑”一文认为它们不是环斑花岗岩，并引用Ramo的图表来说明自己的观点。本文将从以下几方面进行讨论：秦岭环斑花岗岩的研究历史；环斑花岗岩的定义；世界上环斑花岗岩的成因类型；秦岭环斑花岗岩的副矿物及铁镁含量和环斑钾长石特征；秦岭环斑花岗岩与基性岩共存等。本文还论证了秦岭环斑花岗岩不同于元古宙非造山环斑花岗岩，而是一种造山型的环斑花岗岩，其形成于后造山环境，是挤压（造山）向拉张（稳定）转折时期的产物。最后对研究秦岭环斑花岗岩的几个理论问题进行了探讨。  相似文献   

9.
Ilmari Haapala  Sari Lukkari 《Lithos》2005,80(1-4):347-362
The 6×3 km Kymi monzogranite stock represents the apical part of an epizonal late-stage pluton that was emplaced within the 1.65 to 1.63 Ga Wiborg rapakivi batholith. The stock has a well-developed zonal structure, from the rim to the center: stockscheider pegmatite, equigranular topaz granite, porphyritic topaz granite. The contact between the two granites is usually gradational within a few centimeters, but local inclusions of the porphyritic granite in the equigranular granite indicate that the latter solidified later. Hydrothermal greisen and quartz veins, some of which contain genthelvite, beryl, wolframite, cassiterite, and sulfides, cut the granites of the stock and the surrounding country rocks. The equigranular granite contains 1 to 4 vol.% topaz, and its biotite is lithian siderophyllite; the porphyritic granite has 0 to 3 vol.% topaz, and the mica is siderophyllite. The equigranular granite is geochemically highly evolved with elevated Li, Rb, Ga, Ta, and F, and very low Ba, Sr, Ti, and Zr. The REE patterns show deep negative Eu anomalies and tetrad effects indicating extreme magmatic fractionation and aqueous fluid–rock interaction. The zonal structure of the stock is interpreted as a result of differentiation within the magma chamber. Internal convection in the crystallizing magma chamber and upward flow of residual melt as a boundary layer along sloping contacts resulted in accumulation of a layer of highly evolved, volatile-rich magma in the apical part of the chamber. Crystallization of this apical magma produced the stockscheider pegmatite and the equigranular granite; the underlying crystal mush solidified as the porphyritic granite. Much of the crystallization took place from volatile-saturated melt, and episodic voluminous degassing expelled fluids into opened fractures where they or their derivatives reacted with country rocks and caused alteration and mineralization.  相似文献   

10.
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.  相似文献   

11.
王超  刘良  张安达  杨文强  曹玉亭 《岩石学报》2008,24(12):2809-2819
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。  相似文献   

12.
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.  相似文献   

13.
鹰峰环斑花岗岩地球化学特征及其构造意义   总被引：3，自引：0，他引：3  
邢作云  卢欣祥 《地球科学》2005,30(2):153-158
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.   相似文献   

14.
黑龙江宝山地区在构造上位于兴蒙造山带东部的松嫩地块和佳木斯地块之间的伊春-延寿花岗岩带北段,区内分布大面积的古生代-中生代花岗岩.其中海西晚期花岗岩,岩性主要为碱长花岗岩、二长花岗岩和花岗闪长岩等,锆石U-Pb法 LA-ICP MS测年结果为252.6±3 Ma.其主量元素表现出富Si、略富Al、富碱质和低Mg、低Ca的特点;微量元素表现出富集Rb、Nd、K、Pb、U和亏损Nb、Ta、P、Ti等高场强元素的特点,并且Sr、Ba呈明显的负异常;稀土元素具有明显的轻稀土元素富集、重稀土元素相对亏损的特征,轻重稀土元素分馏程度较高.岩石总体上属于高钾钙碱性花岗岩,是岩浆经历了高度结晶分异作用的产物.矿物化学和岩石地球化学特征表明其特征类似于S型花岗岩,源岩物质来自于地壳.  相似文献   

15.
对秦岭奥长环斑花岗岩的质疑   总被引：13，自引：2，他引：13       下载免费PDF全文
赵玉平 《地质论评》2001,47(5):487-491
本文介绍了环斑结构的含义及奥长环斑花岗岩的地质与地球化学特征。在此基础上对所谓的秦岭奥长环斑花岗岩带提出质疑，并提出秦岭中的一些花岗岩虽然具有环斑结构，但不是奥长环斑花岗岩。  相似文献   

16.
新疆可可托海是世界著名的伟晶岩型稀有金属矿床,该矿床的成因长期存在争议,资源已枯竭多年,其成矿理论及找矿工作均亟待突破。我们对矿区中—上奥陶统哈巴河群变质岩和三叠纪稀有金属花岗岩进行了地质和地球化学研究,并开展了矿区遥感数据解译和地球物理测量工作。结果表明,哈巴河群变质岩以云母片岩为主,与大陆上地壳微量元素含量相似,若以此作为花岗质岩浆的源岩,很难通过部分熔融直接形成含矿熔体;三叠纪稀有金属花岗岩由白云母钠长花岗岩和少量钠长花岗岩组成,其岩浆源于地下深处,在向上运移过程中,经过结晶和流动分异作用,形成富挥发分的含矿岩浆,异地侵位形成稀有金属花岗岩岩枝,矿区存在花岗岩-伟晶岩成矿系统。遥感数据解译显示,矿区发育多个环形影像,这些影像是深部环形构造在浅部的反映,已知的伟晶岩脉及稀有金属花岗岩均赋存其中,指示矿区含矿岩浆活动可能与深部环形构造有关。大地电磁测深显示,在矿区15km以下深处发育低电阻率异常体,反映深部可能存在残余的岩浆房或局部熔融带,矿区花岗质岩浆源于此处;音频大地电磁测深显示,在矿区南北环形构造的深部均存在低电阻率异常。基于上述地质-遥感-地球物理的研究结果,我们预测在矿区之...  相似文献   

17.
Rapakivi granites characteristic practically of all old platforms are greatly variable in age and irregularly distributed over the globe. Four types of magmatic associations, which include rapakivi granites, are represented by anorthosite-mangerite-charnockite-rapakivi granite, anorthosite-mangerite-rapakivi-peralkaline granite, gabbro-rapakivi granite-foidite, and rapakivi granite-shoshonite rock series. Granitoids of these associations used to be divided into the following three groups: (1) classical rapakivi granites from magmatic associations of the first three types, which correspond to subalkaline high-K and high-Fe reduced A2-type granites exemplifying the plumasitic trend of evolution; (2) peralkaline granites of the second magmatic association representing the highly differentiated A1-type reduced granites of Na-series, which are extremely enriched in incompatible elements and show the agpaitic trend of evolution; and (3) subalkaline oxidized granites of the fourth magmatic association ranging in composition from potassic A2-type granites to S-granites. Magmatic complexes including rapakivi granites originated during the geochronological interval that spanned three supercontinental cycles 2.7?1.8, 1.8?1.0 and 1.0?0.55 Ga ago. The onset and end of each cycle constrained the assembly periods of supercontinents and the formation epochs of predominantly anorthosite-charnockite complexes of the anorthosite-mangerite-charnockite-rapakivi granite magmatic association. Peak of the respective magmatism at the time of Grenvillian Orogeny signified the transition from the tectonics of small lithospheric plates to the subsequent plate tectonics of the current type. The outburst of rapakivi granite magmatism was typical of the second cycle exclusively. The anorthosite-mangerite-charnockite-rapakivi granite magmatic series associated with this magmatism originated in back-arc settings, if we consider the latter in a broad sense as corresponding to the rear parts of peripheral orogens whose evolution lasted from ～1.9 to 1.0 Ga. Magmatism of this kind was most active 1.8?1.3 Ga ago and represented the distal effect of subduction or collisional events along the convergent boundaries of lithospheric plates. An important factor that favored the emplacement of rapakivi granites and anorthosites in a huge volume was the thermal and rheologic state of the lithosphere inherited from antedating orogenic events, first of all from the event ～1.9 Ga ago, which was unique in terms of heat capacity transferred into the lithosphere. Anorthosite-mangerite-rapakivi granite-peralkaline granite magmatism is connected with activity of the mantle plums only. Degradation of the rapakivi granite magmatism toward the terminal Proterozoic was controlled by the general cooling of the Earth in the course of the steady dissipation of its endogenic energy, as these processes became accelerated since the Late Riphean  相似文献   

18.
Late Proterozoic rocks of Tanol Formation in the Lesser Himalayas of Neelum Valley area are largely green schist to amphibolite facies rocks intruded by early Cambrian Jura granite gneiss and Jura granite representing Pan-African orogeny event in the area. These rocks are further intruded by pegmatites of acidic composition, aplites, and dolerite dykes. Based on field observations, texture, and petrographic character, three different categories of granite gneiss (i.e., highly porphyritic, coarse-grained two micas granite gneiss, medium-grained two micas granite gneiss, and leucocratic tourmaline-bearing muscovite granite gneiss), and granites (i.e., highly porphyritic coarse-grained two micas granite, medium-grained two micas granite, and leucocratic tourmaline-bearing coarse-grained muscovite granite) were classified. Thin section studies show that granite gneiss and granite are formed due to fractional crystallization, as revealed by zoning in plagioclase. The Al saturation index indicates that granite gneiss and granite are strongly peraluminous and S-type. Geochemical analysis shows that all granite gneisses are magnesian except one which is ferroan whereas all granites are ferroan except one which is magnesian. The CaO/Na2O ratio (>0.3) indicates that granitic melt of Jura granite gneiss and granite is pelite-psammite derived peraluminous granitic melt formed due to partial melting of Tanol Formation. The rare earth element (REE) patterns of the Jura granite and Jura granite gneiss indicate that granitic magma of Jura granite and Jura granite gneiss is formed due to partial melting of rocks that are similar in composition to that of upper continental crust.  相似文献   

19.
花岗岩结晶分离作用问题——关于花岗岩研究的思考之二   总被引：11，自引：15，他引：11  
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 el ai.(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。  相似文献   

20.
宽甸环斑花岗岩的同化混染成因   总被引：3，自引：0，他引：3  
环斑花岗岩是发育于前寒武纪的一种特殊岩石。本文从环斑花岗岩的地质学、岩石学及矿物学等方面讨论了宽甸元古宙环斑花岗岩的成因,认为环斑花岗岩是由分异出石英二长岩的富钾质残余岩浆在深部同化围岩(类似盖县组的矽线石榴黑云斜长片麻岩及黑云母变粒岩等岩石)形成的含大量围岩捕虏晶的晶粥状岩浆,经侵位结晶而成。环斑长石的斜长石外壳是来自围岩的捕虏晶,它通过流动、吸附作用附着于钾长石巨晶的周围,构成环斑结构。  相似文献   

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