Ideal granites and models of their metasomatic transformations: Theory, experience, and current problems

A definition of the ideal granite and its metasomatic transformation are discussed and the corresponding matrices of transition probabilities are given, along with guidelines for accepting models. Some results are presented using these models to examine granitic massifs and to search for high-temperature ore deposits. Current problems in investigation of granitic massifs on the basis of stochastic models are reviewed.     

都庞岭岩体环斑花岗岩的特征

都庞岭复式岩体内包括有志留纪、晚三叠世、中侏罗世等三个时代花岗岩。通过岩石学研究发现晚三叠世三个单元的花岗岩均具斜长环斑结构，为黑云母奥长环斑花岗岩。环斑花岗岩酸碱性程度高，富钾及稀土元素，铕亏损明显，富铷贫锶，似钾质花岗岩；Nd，Sr，Pb等同位素具低级上地壳物质特征，εNd(t)值—8．2，t2DM值1660Ma，可能为中元古代沉积物及其内所夹火山物质熔融物；多数特征介于复式岩体内的志留纪花岗岩和中侏罗世花岗岩之间，但最富钾，明显不同于秦岭地区沙河湾环斑花岗岩体，部分特征相似或比较近似于华北地台沙厂环斑花岗岩和芬兰Ahvenisto地块黑云母环斑花岗岩。     

Western dome of the Arga-Ynnykh-Khaya granitic intrusion and its white granites

Adamellites, granite porphyries, and white granites of the western dome of the Arga-Ynnykh-Khaya Massif (Yakutiya) have been studied with emphasis on the white granites. The main features of its magmatic crystallization and initial transformation have been obtained by investigating the intensity of transformations using stochastic Markov models. The most widespread transformation is albitization. Two varieties exist. First, and most common, is development of a secondary albite crystal between two primary grains of potassium feldspar. Second, secondary albite has developed between primary quartz grains. Both variants have definite probabilities of occurrence. Less commonly, but with a constant probability, a secondary quartz grain forms as an intergrowth between two grains of potassium feldspar.     

西藏当惹雍错吉松环斑花岗岩的发现及地质意义

在1∶25万区域地质调查工作中,于当惹雍错中部的吉松一带发现环斑花岗岩,获同位素K-Ar法年龄45.3Ma,时代为始新世。吉松环斑花岗具特殊的卵状环斑结构,K2O Na2O含量较高,K2O>Na2O;稀土元素含量高,具明显的铕亏损,为造山型环斑花岗岩,形成于青藏高原陆内碰撞造山晚期的后碰撞环境。     

Age,origin, and tectonic implications of Palaeozoic rapakivi granites in the North Qaidam orogen,Northwest China

Palaeozoic rapakivi granites occur in the western segment of the China Central Orogenic System. Exhibiting typical rapakivi texture, these granites contain magmatic microgranular enclaves of intermediate compositions. SHRIMP zircon U–Pb ages for the granites and enclaves are 433 ± 5 Ma and 433 ± 3 Ma, respectively. The rapakivi granites are magnesian to ferroan, calc-alkalic to alkalic, and are characterized by high FeO_t/(FeO_t + MgO) (0.74–0.91) and Ga/Al ratios, and SiO₂, Na₂O + K₂O and rare earth element (apart from Eu) contents, but low CaO, Ba, and Sr contents. These are typical A-type granite geochemical features. The granites and enclaves exhibit a uniform decrease in TiO₂, CaO, Na₂O, K₂O, FeO, and MgO with increasing SiO₂, and both lithologies have similar trace element patterns. Whole-rock ?_Nd(t) values vary from??9.2 to??8.7 for the granites and from??9.0 to??8.4 for the enclaves, but zircon ?_Hf(t) values vary more widely from??5.8 to??0.2 and??4.6 to +5.1, respectively. Our data suggest that the granites and enclaves have crystallized from different magmas. The granites appear to have been derived from old continental crust, whereas the enclaves required a source having a juvenile component. The spherical shape and undeformed nature of the granites and their geochemical characteristics, coupled with the (ultra)-high pressure metamorphism and evolution of Palaeozoic granitoid magmatism in the North Qaidam orogen, indicate that the rapakivi granites were generated in a post-collisional setting. These rocks are therefore an example of Palaeozoic rapakivi granites emplaced in a post-collisional, extensional orogenic setting.     

小兴安岭伊春地区环斑花岗岩组合特征及其地质意义

通过1∶25万鹤岗市幅区域地质调查,首次在小兴安岭中南段摩天岭一带发现了具环斑结构的晚石炭世二长花岗岩.环斑长石多呈自形晶宽板状或宽板柱状,少数为圆球状.大小不一.2～3.5 cm,内核为肉红色碱性长石,外壳为宽1～2 mm的灰白色斜长石,有时也见无环的卵圆形斑晶.其岩相学、岩石地球化学特征研究表明,摩天岭环斑花岗岩及其岩石组合为不同于前寒武纪典型非造山环斑花岗岩的造山带型环斑花岗岩,为形成于造山带与板块俯冲体制有关的岛弧或活动大陆边缘环境下的Ⅰ型花岗岩.     

Rapakivi granites in the geological history of the earth. Part 1, magmatic associations with rapakivi granites: Age, geochemistry, and tectonic setting

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     

北京密云环斑花岗岩的锆石SHRIMP U-Pb 年龄及其构造意义

密云环斑花岗岩与华北克拉通的破坏、燕辽裂谷的启动同期,其年代学研究一直令人关注,已获得的一些数据虽然具有参考意义,但缺乏高质量的SHRIMP定年的制约.利用西澳大利亚科庭大学离子探针中心进行远程测试,首次在该花岗岩体中获得锆石SHRIMP U-Pb加权平均年龄1685Ma±15Ma.这一年龄表明燕辽裂谷最早启动于1700Ma,为准确地标定长城系底界的年龄提供了重要的参考依据.     

Sn-polymetallic greisen-type deposits associated with late-stage rapakivi granites, Brazil: fluid inclusion and stable isotope characteristics

Tin-polymetallic greisen-type deposits in the Itu Rapakivi Province and Rondônia Tin Province, Brazil are associated with late-stage rapakivi fluorine-rich peraluminous alkali-feldspar granites. These granites contain topaz and/or muscovite or zinnwaldite and have geochemical characteristics comparable to the low-P sub-type topaz-bearing granites. Stockworks and veins are common in Oriente Novo (Rondônia Tin Province) and Correas (Itu Rapakivi Province) deposits, but in the Santa Bárbara deposit (Rondônia Tin Province) a preserved cupola with associated bed-like greisen is predominant. The contrasting mineralization styles reflect different depths of formation, spatial relationship to tin granites, and different wall rock/fluid proportions. The deposits contain a similar rare-metal suite that includes Sn (±W, ±Ta, ±Nb), and base-metal suite (Zn–Cu–Pb) is present only in Correas deposit. The early fluid inclusions of the Correas and Oriente Novo deposits are (1) low to moderate-salinity (0–19 wt.% NaCl eq.) CO₂-bearing aqueous fluids homogenizing at 245–450 °C, and (2) aqueous solutions with low CO₂, low to moderate salinity (0–14 wt.% NaCl eq.), which homogenize between 100 and 340 °C. In the Santa Bárbara deposit, the early inclusions are represented by (1) low-salinity (5–12 wt.% NaCl eq.) aqueous fluids with variable CO₂ contents, homogenizing at 340 to 390 °C, and (2) low-salinity (0–3 wt.% NaCl eq.) aqueous fluid inclusions, which homogenize at 320–380 °C. Cassiterite, wolframite, columbite–tantalite, scheelite, and sulfide assemblages accompany these fluids. The late fluid in the Oriente Novo and Correas deposit was a low-salinity (0–6 wt.% NaCl eq.) CO₂-free aqueous solution, which homogenizes at (100–260 °C) and characterizes the sulfide–fluorite–sericite association in the Correas deposit. The late fluid in the Santa Bárbara deposit has lower salinity (0–3 wt.% NaCl eq.) and characterizes the late-barren-quartz, muscovite and kaolinite veins. Oxygen isotope thermometry coupled with fluid inclusion data suggest hydrothermal activity at 240–450 °C, and 1.0–2.6 kbar fluid pressure at Correas and Oriente Novo. The hydrogen isotope composition of breccia-greisen, stockwork, and vein fluids (δ¹⁸O_quartz from 9.9‰ to 10.9‰, δD_H2O from 4.13‰ to 6.95‰) is consistent with a fluid that was in equilibrium with granite at temperatures from 450 to 240 °C. In the Santa Bárbara deposit, the inferred temperatures for quartz-pods and bed-like greisens are much higher (570 and 500 °C, respectively), and that for the cassiterite-quartz-veins is 415 °C. The oxygen and hydrogen isotope composition of greisen and quartz-pods fluids (δ¹⁸O_qtz-H2O=5.5–6.1‰) indicate that the fluid equilibrated with the albite granite, consistent with a magmatic origin. The values for mica (δ¹⁸O_mica-H2O=3.3–9.8‰) suggest mixing with meteoric water. Late muscovite veins (δ¹⁸O_qtz-H2O=−6.4‰) and late quartz (δ¹⁸O_mica-H2O=−3.8‰) indicate involvement of a meteoric fluid. Overall, the stable isotope and fluid inclusion data imply three fluid types: (1) an early orthomagmatic fluid, which equilibrated with granite; (2) a mixed orthomagmatic-meteoric fluid; and (3) a late hydrothermal meteoric fluid. The first two were responsible for cassiterite, wolframite, and minor columbite–tantalite precipitation. Change in the redox conditions related to mixing of magmatic and meteoric fluids favored important sulfide mineralization in the Correas deposit.     

北京密云元古宙环斑花岗岩古风化壳及其与长城系常州沟组的关系

华北地区长城系在中国元古宙研究中占有相当重要的地位,其底界是目前划分古、中元古界的重要界线标志。长期以来,由于在靠近长城系底部缺少可定年的岩层或地质体,其底界年龄一直难以确定。近年来笔者在北京密云地区,发现了长城系常州沟组砂岩层直接覆盖在发育有古风化壳的环斑花岗岩(脉)之上。风化壳物质的组成主要为环斑花岗岩的原地风化岩石,以及向上过渡为貌似环斑花岗岩的粗碎屑岩。已获得古风化壳环斑花岗质碎屑岩的碎屑锆石U Pb年龄值为（1 682±20）Ma (SHRIMP) 和（1 708±56）Ma (LA ICP MS)等,与相邻的密云环斑花岗岩年龄相同。初步分析表明,密云环斑花岗岩及其同期岩脉在侵位之后,曾随着区域地壳抬升暴露出地表。经过一定时期的风化和部分剥蚀,在其表面形成残积物和接近原地堆积的环斑花岗质碎屑物质。从形成时间来看,这套岩石应属于“前常州沟期(组)”。它的存在和测年结果显示,长城系常州沟组的底界年龄应小于1 682 Ma,而不是以前划定的1 800 Ma。根据环斑花岗质沉积岩碎屑锆石以及已有的上覆地层相关年龄数据的分析,我们初步提出长城系底界年龄约为1 665~1 670 Ma。     

A stochastic approach to optimum decomposition of cyclic patterns in sedimentary processes

This paper examines the issues of sedimentary cycles by means of reversible Markov chains. Two types of cyclic patterns in sedimentary processes are considered in terms of symmetric cycles (ABCDCBA) and asymmetric cycles (ABCDABCD). By introducing concepts of reversibility and unidirectionality, a general solution is given for decomposing all possible cyclic patterns of these two types existing in a sedimentary sequence. On the basis of two new measures f_R and f_U, the most probable trends in a sequence can be identified in an optimum way. Effective and reliable use of the technique proposed here is demonstrated by a case study.     

Transbaikal Mesozoic granites: Mode of crystallization and subsequent metasomatic transformations

Crystallization conditions and early post-magmatic rock alteration have been studied for three orebearing granite massifs using the method of stochastic modeling. These Mesozoic granites were crystallized from a relatively dry melt, whose barren (ore-free) parts were crystallized from a volatile-enriched magma under sealed conditions. Surface zones of weak rock alteration over subsurface mineralization reflect the subsurface structural patterns controlling the mineralization.     

Geochemistry and Origin of Early Proterozoic Dongargarh Rapakivi Granite Complex, Central India - An Example for Magma Mixing and Differentiation

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.     

北京密云元古宙常州沟组之下环斑花岗岩古风化壳岩石的发现及其碎屑锆石年龄

在北京密云地区,最近发现环斑花岗岩(脉)上发育有古风化壳,并被长城系常州沟组砂岩覆盖。风化壳物质的组成主要为来自环斑花岗岩的原地风化残留物和粗碎屑岩,采用SHRIMP和LA-ICP-MS方法,分别获得环斑花岗岩古风化壳碎屑岩的碎屑锆石U-Pb年龄值为(1682±20)Ma、(1708±6)Ma等,与相邻的密云环斑花岗岩年龄相同。这套古风化壳碎屑岩的存在和测年结果显示,长城系常州沟组的底界年龄应小于1682Ma。     

Features of the geological history of the Iultin Massif determined by stochastic modeling

Petrographic data from the Iultin Massif (Chuckotka) were analyzed for crystallization and postmagmatic transformation using a Markov stochastic model. The goal was to ascertain whether this model can be used to divide the massif into distinctive portions. A comparison was made between the stochastic approach and conventional geological methods for understanding the historical development of granitic massifs.     

Stratoid granites of Madagascar: structure and position in the Panafrican orogeny

Abstract

Stratoid granites constitute a major feature of the Precambrien basement of Madagascar. A detailed structural study was carried out NNW of Antananarivo. New zircon isotopic data on a typical alkaline granite ascertain their Panafrican age (585 Ma). The sheets of granites metric to kilometric of thickness, are interlayered with migmatitic gneisses and amphibolites. Their internal structures, determined by anisotropy of magnetic susceptibility measurements, everywhere yield foliations gently dipping to the west, and lineations striking WSW-ENE. These structures were mostly acquired at the magmatic stage in the granites, in the country-rocks they resulted from high-temperature plastic deformation.

The very constant structural pattern, interpreted in terms of shear deformation of a section of the crust, as well as the low P (P = 4 - 5 kb) - high T (T # 750°C) conditions, suggest that the emplacement of the stratoid granites was coeval with a late-orogenic stage in the Panafrican Mozambique belt, and possibly linked to the thinning of the lithosphere.     

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.     

原特提斯洋在滇西南地区的俯冲记录：来自保山地块奥陶纪花岗岩的年代学、地球化学以及Hf-Nd同位素的支持

中国西南地区在早古生代期间位于冈瓦纳大陆北缘, 该地区保存有早古生代岩浆岩记录, 它们的形成与原特提斯洋的演化密切相关。本文报道了滇西南地区保山地块平达乡附近的早古生代花岗岩的岩石学、全岩主-微量地球化学、LA-ICP-MS锆石U-Pb年龄、锆石Hf同位素和磷灰石Nd同位素组成, 以探讨保山地块早古生代花岗岩的岩石成因、岩浆源区性质以及岩体形成的地球动力学机制。通过对保山地块平达花岗岩中岩浆锆石开展LA-ICP-MS U-Pb同位素测试, 结果显示花岗岩形成于465.9~454.9Ma, 是中-晚奥陶世岩浆作用的产物。平达岩体主体岩性为二长花岗岩, 主要矿物组成为斜长石、石英和碱性长石(条纹长石和正长石), 次要矿物为黑云母和白云母。本文报道的花岗岩样品显示出高的SiO₂(72.16%~76.87%)和Al₂O₃(12.50%~14.36%)含量, 低的MgO(0.12%~0.64%)、Mg^#(20.9~35.9)、Cr和Ni含量以及较高的Rb/Sr和Rb/Ba比值; 结合花岗岩较高的A/CNK值(1.06~1.45), 显示弱过铝质至强过铝质的特征, 表明其属于S型花岗岩。此外, 平达岩体二长花岗岩样品具有低的Al₂O₃/TiO₂比值和变化的CaO/Na₂O、Rb/Sr、Rb/Ba比值, 表明其岩浆源区包含泥质岩石和砂质岩石, 显示混合源区的特征。结合滇西南早古生代二长花岗岩变化较大的锆石ε_Hf(t)值(-11.10~-4.05)、磷灰石ε_Nd(t)值(-8.76~-6.96)和Hf同位素二阶段模式年龄(2140~1704Ma), 表明它们的岩浆源区为古元古代地壳中泥质岩石和砂质岩石混合源区。综合滇西南地区广泛发育的早古生代岩浆岩以及同时期的蛇绿混杂岩记录, 本文认为保山地块早古生代S型花岗岩是地壳沉积组分部分熔融的产物, 形成于原特提斯洋西向俯冲相关的活动大陆边缘背景。

     

赣-杭带早白垩世A型花岗岩成因: 浅部地壳岩浆储库活化的产物

A型花岗岩是一类特殊的花岗岩,其独特的矿物学和地球化学特征使其有别于与俯冲作用相关的钙碱性花岗岩。虽然前人的研究已经提出了多种A型花岗岩成因模型,但是它的成因仍然存在很大争议。本文总结了赣-杭带早白垩世火山-侵入杂岩、A型花岗岩及相关花岗岩类岩石的数据（年代学、地球化学、同位素）,试图通过建立火山岩与侵入岩之间的成因联系,从而限定这些A型花岗岩的成因。对于硅质火山-侵入杂岩（例如相山火山-侵入杂岩）,我们的研究表明硅质火山岩较硅质侵入岩具有更高的分异程度。硅质火山岩代表了从岩浆储库中抽离的高硅熔体,硅质侵入岩代表了岩浆储库中的残余堆晶。对于赣-杭带早白垩世A型花岗岩及相关岩石,本文根据岩石结构,将其划分为斑状-似斑状花岗岩和等粒花岗岩。岩相学及地球化学的数据表明,两类花岗岩均由来源于古老地壳物质与地幔来源基性岩浆混合所形成。而且二类花岗岩具有相似的形成时代、相似的矿物组成及类似的Sr-Nd-Hf同位素组成和连续的主微量元素变化趋势,这些特征表明它们的原始岩浆来自于同一岩浆储库,而它们岩石特征的差别是由岩浆储库内的晶体-熔体分异所主导。斑状-似斑状花岗岩以低SiO₂含量和Rb/Sr比值,高Sr、Ba含量和Eu/Eu^*值为特征,代表残余的岩浆储库;等粒花岗岩具有A型花岗岩特征,并且具有极高的SiO₂含量和Rb/Sr比值,极低的Sr、Ba含量以及Eu/Eu^*比值,表明它们是从岩浆储库中抽离至浅部地壳的高硅熔体。斑状-似斑状花岗岩中广泛发育暗色包体,而且等粒花岗岩中广泛发育晶洞构造,这些特征表明,幔源基性岩浆的注入和粒间熔体中挥发份的出溶作用导致先前存在的岩浆储库活化,使得高硅熔体抽离岩浆储库并向上迁移,到达地壳浅部后形成了具有A型花岗岩特征的等粒花岗岩,而残余堆晶在岩浆储库底部结晶形成了斑状-似斑状花岗岩。我们的研究表明,A型花岗岩可以通过浅部岩浆储库内晶体-熔体分离形成。

     

Oxidized, magnetite-series, rapakivi-type granites of Carajás, Brazil: Implications for classification and petrogenesis of A-type granites

The varying geochemical and petrogenetic nature of A-type granites is a controversial issue. The oxidized, magnetite-series A-type granites, defined by Anderson and Bender [Anderson, J.L., Bender, E.E., 1989. Nature and origin of Proterozoic A-type granitic magmatism in the southwestern United States of America. Lithos 23, 19–52.], are the most problematic as they do not strictly follow the original definition of A-type granites, and approach calc-alkaline and I-type granites in some aspects. The oxidized Jamon suite A-type granites of the Carajás province of the Amazonian craton are compared with the magnetite-series granites of Laurentia, and other representative A-type granites, including Finnish rapakivi and Lachlan Fold Belt A-type granites, as well as with calc-alkaline, I-type orogenic granites. The geochemistry and petrogenesis of different groups of A-types granites are discussed with an emphasis on oxidized A-type granites in order to define their geochemical signatures and to clarify the processes involved in their petrogenesis. Oxidized A-type granites are clearly distinguished from calc-alkaline Cordilleran granites not only regarding trace element composition, as previously demonstrated, but also in their major element geochemistry. Oxidized A-type granites have high whole-rock FeO_t/(FeO_t + MgO), TiO₂/MgO, and K₂O/Na₂O and low Al₂O₃ and CaO compared to calc-alkaline granites. The contrast of Al₂O₃ contents in these two granite groups is remarkable. The CaO/(FeO_t + MgO + TiO₂) vs. CaO + Al₂O₃ and CaO/(FeO_t + MgO + TiO₂) vs. Al₂O₃ diagrams are proposed to distinguish A-type and calc-alkaline granites. Whole-rock FeO_t/(FeO_t + MgO) and the FeO_t/(FeO_t + MgO) vs. Al₂O₃ and FeO_t/(FeO_t + MgO) vs. Al₂O₃/(K₂O/Na₂O) diagrams are suggested for discrimination of oxidized and reduced A-type granites. Experimental data indicate that, besides pressure, the nature of A-type granites is dependent of ƒO₂ conditions and the water content of magma sources. Oxidized A-type magmas are considered to be derived from melts with appreciable water contents (≥ 4 wt.%), originating from lower crustal quartz-feldspathic igneous sources under oxidizing conditions, and which had clinopyroxene as an important residual phase. Reduced A-type granites may be derived from quartz-feldspathic igneous sources with a metasedimentary component or, alternatively, from differentiated tholeiitic sources. The imprint of the different magma sources is largely responsible for the geochemical and petrological contrasts between distinct A-type granite groups. Assuming conditions near the NNO buffer as a minimum for oxidized granites, magnetite-bearing granites formed near FMQ buffer conditions are not stricto sensu oxidized granites and a correspondence between oxidized and reduced A-type granites and, respectively, magnetite-series and ilmenite-series granites is not always observed.