New evidence for two sharp replacement fronts during albitization of granitoids from northern Aravalli orogen,northwest India

We present new evidence of infiltration metasomatism in granitoids that were albitized in a process that produced two sharp replacement fronts, both of which are clearly visible in the field. The two fronts advanced through the original granite simultaneously, but at different rates. Here we focus mainly on the Ajitgarh intrusive in the northern Aravalli orogen of northwest India. This intrusion shows geographically well-defined metasomatic zones on the outcrop scale as well as a large volume of original ferroan granite, both of which were poorly preserved in most of the previously studied Khetri granites. Stage I metasomatism transformed the grey original granite to pink microcline–albite granite, and stage II converted the microcline–albite granite to white albite granite. Both these reaction fronts are sharp and are easily recognized in the field by their different colours. The mineralogical and chemical changes during the first stage are expressed by transformation of original oligoclase to albite, biotite (annite-rich) and hastingsite (amphibole) to hastingsite with low X_Fe values, dehydration, gain in Na, and losses in Fe and Rb. The second stage of metasomatism caused almost complete conversion of microcline to albite and complete or nearly complete disappearance of amphibole. Chemically, these changes are manifested by substantial gain in Na and extreme losses in K, Rb, Ba, Ca, Sr, Fe, and Mg. Depending on the modal abundances of amphibole, stage II albitized rocks are depleted in light rare earth elements or heavy rare earth elements or both, signifying that rare earth elements are principally hosted by mafic phases. The disparity in whole-rock δ¹⁸O values during both stages of albitization is related to the variations in modal amounts of Si-bearing phases. The replacement microstructures are in accord with the fluid-mediated phase transformations by a coupled dissolution–precipitation mechanism. The albitizing event took place at low temperatures of 350–400 °C and the fluid was metamorphic in nature.     

Metasomatism of ferroan granites in the northern Aravalli orogen,NW India: geochemical and isotopic constraints,and its metallogenic significance

The late Palaeoproterozoic (1.72–1.70 Ga) ferroan granites of the Khetri complex, northern Aravalli orogen, NW India, were extensively metasomatised ~900 Ma after their emplacement, at around 850–830 Ma by low-temperature (ca. 400 °C) meteoric fluids that attained metamorphic character after exchanging oxygen with the surrounding metamorphic rocks. Albitisation is the dominant metasomatic process that was accompanied by Mg and Ca metasomatism. A two-stage metasomatic model is applicable to all the altered ferroan intrusives. The stage I is represented by a metasomatic reaction interface that developed as a result of transformation of the original microcline–oligoclase (An_12–14) granite to microcline–albite (An_1–3) granite, and this stage is rarely preserved. In contrast, the stage II metasomatic reaction front, where the microcline-bearing albite granite has been transformed to microcline-free albite granite, is readily recognisable in the field and present in most of the intrusives. Some of them lack an obvious reaction interface due to the presence of stage II albite granites only. When studied in isolation, these intrusives were incorrectly classified and their tectonic setting was misinterpreted. Furthermore, our results show that the mafic mineralogy of metasomatised granites has a significant impact on the characterisation of such rocks in the magmatic classification and discrimination diagrams. Nevertheless, the stage I metasomatised granites can be appropriately characterised in these diagrams, whereas the characterisation of the stage II granites will lead to erroneous interpretations. The close spatial association of these high heat producing ferroan granites with iron oxide–copper–gold (IOCG), U and REE mineralisation in the region indicates a genetic link between the metasomatism and the mineralisation. World-class IOCG, U and REE deposits are associated with metasomatised ferroan granites, suggesting that such a relationship may act as a critical first-order exploration target for undiscovered mineral deposits.     

赣东北高压变质岩的岩石类型、矿物组成与变质过程

赣东北高压变质岩包括含硬玉霓辉石钠长角闪片岩、含硬玉霓辉石石英钠长石岩、含霓辉石角闪石英钠长石岩、含霓辉石钠长角闪片岩、蓝透闪石石英钠长石岩、镁钠闪石石英钠长石岩等岩石类型，主要组成脏矿物为硬玉、霓辉石、镁钠闪石、蓝透闪石、镁角闪石、阳起石、石英、钠长石、金红石和榍石。研究表明，高压变质峰期后经历了近等温降压退变质过程。     

湖南湘东钨矿区花岗岩地质地球化学特征及意义

通过对湖南湘东钨矿三类花岗岩系统的岩矿鉴定和探针分析,确定燕山早期花岗岩中的长石为碱性长石,钠长石An5,大多数样品An3;而印支期花岗岩的斜长石An=10~15,为更长石。因此,确定邓阜仙复式岩体的岩性,分别为印支期的粗粒斑状黑云母二长花岗岩、燕山早期的中粒斑状白云母碱长花岗岩和细粒碱长花岗岩。系统的常量、微量元素地球化学研究也显示出燕山早期岩浆作用明显不同于印支期花岗岩,具有富硅、偏铝、富钙碱等特点,燕山早期花岗岩为碱长花岗岩,和南岭地区与脉型钨矿有关碱长花岗岩一致。成矿与燕山早期花岗岩浆活动有关。     

东昆仑祁漫塔格伊涅克阿干花岗岩特征及构造意义

伊涅克阿干花岗岩是含白云母过铝质花岗岩，由二长花岗岩一钾长花岗岩分3次侵入组成的复式岩体。岩石呈灰白色，富含钾长石和钠一更长石，普遍含原生白云母，富含浅源捕虏体和残留体；A/CNK=0．99～1．34；ISr=0．71199，Rb—Sr全岩等时线年龄为435.7Ma，是早古生代柴达木板块与塔里木板块碰撞导致地壳加厚，并在地幔热流注入参与作用下，地壳深部发生熔融作用形成花岗岩浆侵位形成的同碰撞花岗岩。     

Geochemical characteristics of the niobium-rich arfvedsonite granites, Younger Granites province of Nigeria

Arfvedsonite granites are most prevalent in the northern sector of the Nigerian anorogenic ring-complex province wherein they form the main granitic rocks at Kudaru and Fagam and are important components of Kila-Warji, Ririwai and Dutsen-Wai ring-complexes. The albitized variety of these rocks hosts pyrochlore to varying extents depending on the degree of albitization and are, therefore, important targets for niobium investigation. Geochemical data of the granites reveal that niobium has a mean concentration of 111 ppm in the arfvedsonite granite, increasing to 168 ppm in the aegirine arfvedsonite granite and reaching 1568 ppm in the albite arfvedsonite granite. Niobium is thus enriched in the albite arfvedsonite granite by a factor of 8-11 relative to its mean value in the aegirine arfvedsonite and arfvedsonite granites, respectively. Uranium contents show a sympathetic trend with niobium, being also enriched in the albite arfvedsonite granite relative to its abundance in both the aegirine arfvedsonite granite and arfvedsonite granite by a factor of 15. The uranium abundance in the albite arfvedsonite granite is more than 48 times higher than the mean background values in low-calcium granite.The REE fractionation patterns in all three arfvedsonite granite varieties are characterized by enrichment of both the light (La-Sm) and heavy (Gd-Lu) rare earth elements and a significant negative Eu anomaly. The albite arfvedsonite granite is, however, preferentially more enriched in the heavy REE relative to the aegirine arfvedsonite and the arfvedsonite granites. A plot of the ∑REE against Na₂O and niobium reveals positive correlation in the arfvedsonite granites. There is also a linear relationship and strongly positive correlation between Nb and Na₂O because the pyrochlore is most abundant in the most extensively albitized variety of the arfvedsonite granites.     

东昆仑造山带祁漫塔格地区白干湖含钨锡矿花岗岩:岩石学、年代学、地球化学及岩石成因

白干湖钨锡矿床发育大量与成矿关系密切的花岗岩.LA-ICPMS锆石U-Pb年代学研究表明含矿更长花岗岩形成于(429.5±3.2) Ma,二长花岗岩形成于(430.5±1.2) Ma.锆石Hf同位素组成表明,二长花岗岩的εHf(t)为-2.31～5.57,T2DM主体为1188～1390Ma;含矿更长花岗岩εHf(t)...     

Episodic crustal growth in the Bundelkhand craton of central India shield: Constraints from petrogenesis of the tonalite–trondhjemite–granodiorite gneisses and K-rich granites of Bundelkhand tectonic zone

Tonalite–trondhjemite–granodiorite gneisses (TTG) and K-rich granites are extensively exposed in the Mesoarchean to Paleoproterozoic Bundelkhand craton of central India. The TTGs rocks are coarse- grained with biotite, plagioclase feldspar, K-feldspar and amphibole as major constituent phases. The major minerals constituting the K-rich granites are K-feldspar, plagioclase feldspar and biotite. They are also medium to coarse grained. Mineral chemical studies show that the amphiboles of TTG are calcic amphibole hastingsite, plagioclase feldspars are mostly of oligoclase composition, K-feldspars are near pure end members and biotites are solid solutions between annite and siderophyllite components. The K-rich granites have biotites of siderophyllite–annite composition similar to those of TTGs, plagioclase feldspars are oligoclase in composition, potassic feldspars have \(\hbox {X}_{\mathrm{K}}\) ranging from 0.97 to 0.99 and are devoid of any amphibole. The tonalite–trondhjemite–granodiorite gneiss samples have high \(\hbox {SiO}_{2}\) (64.17–74.52 wt%), \(\hbox {Na}_{2}\hbox {O}\) (3.11–5.90 wt%), low Mg# (30–47) and HREE contents, with moderate \((\hbox {La/Yb})_{\mathrm{CN}}\) values (14.7–33.50) and Sr/Y ratios (4.85–98.7). These geochemical characteristics suggest formation of the TTG by partial melting of the hydrous basaltic crust at pressures and depths where garnet and amphibole were stable phases in the Paleo-Mesoarchean. The K-rich granite samples show high \(\hbox {SiO}_{2}\) (64.72–76.73 wt%), \(\hbox {K}_{2}\hbox {O}\) (4.31–5.42), low \(\hbox {Na}_{2}\hbox {O}\) (2.75–3.31 wt%), Mg# (24–40) and HREE contents, with moderate to high \((\hbox {La/Yb})_{\mathrm{CN}}\) values (9.26–29.75) and Sr/Y ratios (1.52–24). They differ from their TTG in having elevated concentrations of incompatible elements like K, Zr, Th, and REE. These geochemical features indicate formation of the K-granites by anhydrous partial melting of the Paleo-Mesoarchean TTG or mafic crustal materials in an extensional regime. Combined with previous studies it is interpreted that two stages of continental accretion (at 3.59–3.33 and 3.2–3.0 Ga) and reworking (at 2.5–1.9 Ga) occurred in the Bundelkhand craton from Archaean to Paleoproterozoic.     

南岭锡钨多金属矿区碱长花岗岩的厘定及其意义

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。     

The Durulgui rare-metal granite‒pegmatite system in the eastern Transbaikal region: Petrological and geochronological aspects

The Durulgui granite?pegmatite system unites the Dedova Gora granite massif and pegmatite field with the Chalotskoe beryl deposit. New geochronological data on micas from porphyric biotite granites, fine-grained biotite granites, two-mica granites, and Be-bearing pegmatites are discussed. The plateau age of 128.5(±1.5)–131.2(±1.5) should be considered as indicating the formation time of the granite?pegmatite system as a whole. The age of the system implies the possibility of its formation owing to several magmatic pulses. This assumption concerns porphyric and fine-grained biotite granites and two-mica and muscovite granites, the contact between which is locally sharp. At the same time, the succession “two-mica granites → muscovite granites → granite?pegmatites → microcline pegmatites → microcline?albite pegmatites → albite pegmatites” demonstrates gradual facies transitions between rocks, which indicates their emplacement during a single magmatic pulse.     

福建南平花岗伟晶岩型稀有金属矿田Nb-Ta-Sn地球化学研究

福建南平是我国重要的花岗伟晶岩型矿田之一,伟晶岩主要产出在中-新元古代变质岩系中.伟晶岩脉的形成和加里东期西芹花岗岩具有密切的成因联系.根据伟晶岩主要矿物成分和所含Nb、Ta、Sn等元素可分为白云母-钾长石-早期钠长石型（Ⅰ）、白云母-钠长石-钾长石型（Ⅱ）、白云母-钾长石-钠长石型（Ⅲ）和白云母-钠长石-锂辉石型（Ⅳ）.对矿田中的中-新元古代变质岩系、西芹花岗岩,花岗伟晶岩中的Nb、Ta、Sn地球化学做了详细研究.认为南平伟晶岩是在区域地层和相关花岗岩中Nb、Ta、Sn含量具有高背景值的环境中产生的.伟晶岩在形成过程中,Nb、Ta、Sn元素伴随伟晶熔体的分异、演化及交代作用,其含量向晚期趋于增高,在白云母-钠长石-锂辉石伟晶岩中,Nb、Ta、Sn构成了工业矿体,而且主要以独立矿物形式存在.     

新疆可可托海地区花岗岩成因的研究

新疆可可托海地区有三种花岗岩,它们形成于造山运动不同阶段:片麻状黑云母花岗岩是海西造山运动早期的产物;似斑状花岗岩形成于海西晚期;二云母花岗岩是印支一燕山期侵入而成。三者的岩石、岩石化学、CMF值和QAP图解、亲合度、微量无素、稀土元素、铅和氧同位素组成等研究表明,它们是陆壳交代重熔成因的一个连续成岩系列。     

An example of trondhjemite genesis by means of alkali metasomatism: Rockford Granite,Alabama Appalachians

A model for trondhjemite genesis is proposed where granite is transformed to trondhjemite via infiltration by a Na-rich metamorphic fluid. The Rockford Granite of the Northern Alabama Piedmont serves as the case example for this process and is characterized as a synmetamorphic, peraluminous trondhjemite-granite suite. The major process operative in the conversion of granite to trondhjemite involves cation exchange of Na for K in the feldspar and mica phases through a volatile fluid medium. Whole-rock ¹⁸O values for the trondhjemites are negatively correlated with atomic proportion K/Na ratio indicating a partial reequilibration of the altered granitoids with a Na- and¹⁸O-rich metamorphically derived fluid. Biotite decomposition to an Al-epidote-paragonitic muscovite-secondary quartz assemblage is also associated with the sodium metasomatism, as are apatite replacement by Al-epidote and secondary zircon crystallization. Rare albitization of primary magmatic plagioclase and discontinuous grossularite reaction rim growth on magmatic garnet are present in the trondhjemites indicating the mobility of Ca during alkali metasomatism. The replacement of magmatic phases by me tasomatic phases exemplifies the chemical changes produced during infiltration metasomatism where the trondhjemites are depleted in P₂O₅, Th, Rb, U, K₂O, V, Sn, F, MgO, Pb, TiO₂, FeO^* and Li and enriched in CaO, Na₂O, Zr and Sr relative to the granites. Other elements, such as Cr, MnO, Cu, Zn, Co, Ba, SiO₂, Ni, Al₂O₃, are shown to be relatively immobile during the metasomatism. The infiltration metasomatism probably occurred during prograde regional metamorphism, when a discrete fluid phase was produced in the surrounding amphibolite-grade metasediments. Foliation planes in the granite apparently served as conduits for fluid flow with reaction-enhanced permeability accompanying the 8% molar volume reduction during Na-for-K exchange in the feldspars. A source for the Na and Sr in the metamorphic fluid may have been paragonitic muscovite in the metasedimentary country rocks. Rubidium and K were probably retained in metasedimentary biotite. The Silent Lake pluton in southeastern Ontario is a possible analogue to the alkali metasomatic processes affecting the Rockford Granite.     

Fluorides and Fluorcarbonates in Rocks of the Katugin Complex,Eastern Siberia: Indicators of Geochemical Mineral Formation Conditions

The paper discusses the chemical composition and parageneses of fluorides and fluorcarbonates in rocks of the Katugin Complex, with which a unique deposit of REE–Nb–Ta ore with cryolite is associated. In mineralogy and chemical composition, the rocks correspond to biotite, biotite–amphibole, arfvedsonite, and aegirine–arfvedsonite granites, which were regarded in earlier publications as granite-like metasomatic rocks. Aegirine–arfvedsonite granite contains a cryolite–gagarinite assemblage, which reflects depletion of Ca in the mineral-forming medium and enrichment in Na and F. Arfvedsonite granite is characterized by intergrowth of yttrofluorite with fluocerite and gagarinite, which indicates a relative enrichment in Ca and low CO₂ content. Biotite granite is characterized by an assemblage of fluorite with titanite, apatite, and monazite as evidence for an elevated Ca concentration along with moderate F and P contents in the system. Neighborite, coulsellite, gagarinite, fluocerite, and tveitite-(Y) appear in biotite–amphibole granite along with replacement of annite with riebeckite and development of albite after microcline. All this indicates that a moderately alkaline Na-fluoride solution with a low Ca concentration affects biotite granite.     

Characterisation of the Dabla Granitoids, North Khetri Copper Belt, Rajasthan, India: Evidence of Bimodal Anorogenic Felsic Magmatism

The Palaeoproterozoic Dabla granitoid pluton of the North Khetri Copper Belt is located to the east of a NNE-SSW trending lineament with numerous albite-rich intrusives, the intraplate ‘albitite line’. The Dabla pluton is essentially made up of calcic amphibole-bearing granitoids, displaying a concentric bimodal distribution of alkali-feldspar granites, comprising a microcline-albite granite and an albite-granite. The dominant rock type is pink-coloured granite, which is characterised by quartz, microcline, albite and hastingsitic hornblende, and occurs in the marginal parts of the pluton. The volumetrically subordinate albite-granite in the central part of the pluton is invariably white in colour, non-foliated and is mainly composed of quartz, albite and amphibole of actinolite to ferro-actinolite composition. The albite-granite is characterised by low K₂O (0.06-0.09%), Rb (<5 ppm) and Ba (<20 ppm), high Na₂O (7.19-7.36%) and high Na/K ratios (122.4-185.2) as compared to the granite. These rocks are not subjected to any metamorphic overprint, especially the albite-granite, which shows pristine abundances of major and trace elements. The rocks are highly evolved as reflected in their high SiO₂ (72 to 78%) contents and high DI (89.5-97) values. The Dabla granitoids are characterised by similar REE and spider patterns, displaying LREE enriched slopes, flat HREE profiles and strong negative Sr, P, Ti and Eu anomalies suggesting their comagmatic nature. Nevertheless, the granite is relatively more fractionated [(La/Yb)_N = 3.89-8.19] and show higher REE abundances (466-673 ppm) as compared to the albite-granite [(La/Yb)_N = 1.97-2.96; REE = 220-277 ppm]. Distinctive features of these rocks are their low Ca (0.21-1.53%), Mg (<0.02-0.19%), Al (11.84-12.96%) and Sr (12-46 ppm) abundances, high Zr (155-631 ppm), Y (67-156 ppm), Nb (14-91 ppm), and Ga (20-31 ppm) concentrations and high Fe*-number, high Ga/Al ratio and high agpaitic index (AI) values. These features, coupled with their ferroan, alkaline and metaluminous nature, are typical of within-plate aluminous A-type granites. The geochemical data further indicate that the Dabla magma was generated at fairly high temperature, apparently in an upper mantle region, under relatively low H₂O activities and reduced conditions and emplaced at a shallow depth in an extensional tectonic environment.     

Evolution of the Mesozoic Granites in the Xiong＇ershan-Waifangshan Region, Western Henan Province, China, and Its Tectonic Implications

Based on the new data of isotopic ages and geochemical analyses, three types of Mesozoic granites have been identified for the Xiong'ershan-Waifangshan region in western Henan Province: high-Ba-Sr I-type granite emplaced in the early stage (～160 Ma), I-type granite in the middle stage (～130 Ma) and anorogenic A-type granite in the late stage (～115 Ma).Geochemical characteristics of the high-Ba-Sr I-type granite suggest that it may have been generated from the thickened lower crust by partial melting with primary residues of amphibole and garnet. Gradual increase of negative Eu anomaly and Sr content variations reflect progressive shallowing of the source regions of these granites from the early to late stage. New 40Ar/39Ar plateau ages of the early-stage Wuzhangshan granite (156.0±1.1 Ma, amphibole) and middle-stage Heyu granite (131.8±0.7 Ma, biotite) are indistinguishable from their SHRIMP U-Pb ages previous published, indicating a rapid uplift and erosion in this region. The representative anorogenic A-type granite, Taishanmiao pluton, was emplaced at ～115 Ma. The evolution of the granites in this region reveals a tectonic regime change from post-collisional to anorogenic between ～160 Ma and ～115 Ma. The genesis of the early- and middle-stage I-type granites could be linked to delamination of subducted lithosphere of the Qinling orogenic belt, while the late-stage A-type granites represent the onset of extension and the end of orogenic process. In fact, along the Qinling -Dabie-Sulu belt, the Mesozoic granitoids in western Henan, Dabieshan and Jiaodong regions are comparable on the basis of these temporal evolutionary stages and their initial 87Sr/86Sr ratios,which may suggest a similar geodynamic process related to the collision between the North China and Yangtze cratons.     

The Petrological and Geochemical Evolution of Ediacaran Rare-Metal Bearing A-type Granites from the Jabal Aja Complex, Northern Arabian Shield, Saudi Arabia

New fieldwork, mineralogical and geochemical data and interpretations are presented for the rare-metal bearing A-type granites of the Aja intrusive complex(AIC) in the northern segment of the Arabian Shield. This complex is characterized by discontinuous ring-shaped outcrops cut by later faulting. The A-type rocks of the AIC are late Neoproterozoic post-collisional granites, including alkali feldspar granite, alkaline granite and peralkaline granite. They represent the outer zones of the AIC, surrounding a core of older rocks including monzogranite, syenogranite and granophyre granite. The sharp contacts between A-type granites of the outer zone and the different granitic rocks of the inner zone suggest that the AIC was emplaced as different phases over a time interval, following complete crystallization of earlier batches. The A-type granites represent the late intrusive phases of the AIC, which were emplaced during tectonic extension, as shown by the emplacement of dykes synchronous with the granite emplacement and the presence of cataclastic features. The A-type granites consist of K-feldspars, quartz, albite, amphiboles and sodic pyroxene with a wide variety of accessory minerals, including Fe-Ti oxides, zircon, allanite, fluorite, monazite, titanite, apatite, columbite, xenotime and epidote. They are highly evolved(71.3–75.8 wt% SiO_2) and display the typical geochemical characteristics of post-collisional, within-plate granites. They are rare-metal granites enriched in total alkalis, Nb, Zr, Y, Ga, Ta, REE with low CaO, MgO, Ba, and Sr. Eu-negative anomalies(Eu/Eu* = 0.17–0.37) of the A-type granites reflect extreme magmatic fractionation and perhaps the effects of late fluid-rock interactions. The chemical characteristics indicate that the A-type granites of the AIC represent products of extreme fractional crystallization involving alkali feldspar, quartz and, to a lesser extent, ferromagnesian minerals. The parent magma was derived from the partial melting of a juvenile crustal protolith with a mantle contribution. Accumulation of residual volatile-rich melt and exsolved fluids in the late stage of the magma evolution produced pegmatite and quartz veins that cut the peripheries of the AIC. Post-magmatic alteration related to the final stages of the evolution of the A-type granitic magma, indicated by alterations of sodic amphibole and sodic pyroxene, hematitization and partial albitization.     

Geochemistry, petrogenesis and radioactivity of El Hudi I-type younger granites, South Eastern Desert, Egypt

The present study deals with geochemical characteristics and petrogenesis of three younger granite varieties (coarse-grained biotite-muscovite granites (CBG), garnetiferous muscovite granites (GMG) and Abu Aggag biotite granites (AAG)) in El-Hudi area, east of Aswan, southeastern desert of Egypt. Mineral chemistry and whole rock chemistry data revealed that all granites have high SiO₂ (70.8-74.7 wt.%), Al₂O₃ (12.8-14.3 wt.%), Na₂O and K₂O (>3.2 wt.%) contents with high Na₂O/K₂O ratios (～>1). Plagioclase feldspars range in composition from albite to oligoclase (An_9-27) in CBG, oligoclase (An_13-18) in GMG and albite (An_2-6) in AAG. Potash feldspars are mainly perthitic microcline and exhibit chemical formulae as (Or_93-96 Ab_7-4 An₀) in CBG, (Or_95-98 Ab_5-2 An₀) in GMG and (Or_82-98 Ab_18-2 An₀) in AAG. Biotites from CBG and GMG are enriched in (Mg and Ti) and depleted in (Al, Fe, Mn and K) compared with those of AAG. Biotites from CBG and GMG had been derived from calc-alkaline magma, whereas those from AAG had been derived from peraluminous magma. Chlorites from CBG and GMG are Mg-Fe bearing, while that from AAG is Fe-rich chlorite (chamosite). The CBG and GMG are Mg-rich monzogranites originated from high-K calc-alkaline magma with metaluminous to mildly peraluminous nature. The AAG are Fe-rich monzogranites to syenogranites generated from high-K calc-alkaline peraluminous magma. Both CBG and GMG are late- to post-orogenic granites, while the AAG are post-orogenic granites. All three granite varieties are considered as evolved I-type granites, formed under low to moderate water pressures (～ 0.5-7 kbars) and relatively high ranges of crystallization temperatures (～700-890°C). They were generated from partial melting of crustal materials at lower (CBG >30 km depth) and intermediate (GMG & AAG ～20-30 km depth) levels. The crystal fractionation was the predominant process during differentiation of parent magmas of these granites. Geochemical characteristics manifest that AAG represent the highly fractionated member of magma cycle differs from that produced CBG and GMG. The CBG are relatively enriched in both U and Th existing only within the accessory minerals such as zircon, sphene, and allanite.     

Geology and Pb-Pb Geochronology of Paleoproterozoic Volcanic and Granitic Rocks of Pitinga Province,Amazonian Craton,Northern Brazil

Pitinga Province is one of the main tin provinces of the Amazonian craton. The oldest unit in the studied area is the Iricoumé Group, which consists of rhyolites and rhyodacites with a ²⁰⁷Pb/²⁰⁶Pb zircon age of 1888 ± 3 Ma. This volcanic sequence is intruded by five A-type granite plutons. The studied portion of the Europa pluton is homogeneous, and composed of a peralkaline alkali-amphibole hypersolvus granite that yielded a ²⁰⁷Pb/²⁰⁶Pb zircon age of 1829 ± 1 Ma. The early facies of the Madeira pluton consists of a metaluminous amphibole-biotite syenogranite (rapakivi facies) with a ²⁰⁷Pb/²⁰⁶Pb zircon age of 1824 ± 2 Ma. It is intruded by a 1822 ± 1 Ma, mildly peraluminous biotite syenogranite. The later facies of this pluton consist of a porphyritic, hypersolvus, alkali-feldspar granite and an albite granite. Field relationships and an extensive drilling survey indicate that these two facies are sheet-shaped and were emplaced almost simultaneously. The hypersolvus alkali-feldspar granite has a ²⁰⁷Pb/²⁰⁶Pb zircon age of 1818 ± 2 Ma. Taking in account its field relationships with the albite granite, a similar age is assumed for the latter.

The albite granite intrudes the biotite granite and rapakivi granite facies of the Madeira pluton, which was emplaced by shallow-level cauldron subsidence. The albite granite is sheet shaped and consists of a magmatic peralkaline cryolite-bearing core facies partially surrounded by an autometa-somatic peraluminous fluorile-bearing border facies. Both albite granite facies are strongly tin-mineralized and display anomalous contents of Nb, Rb, Zr, and REE. A massive body of cryolite and pegmatitic rocks is associated with the albite granite.

The contrast in age between the Iricoume Group and the Europa + Madeira granites demonstrates that the plutons are not subvolcanic intrusions related to the extrusives. The ages of 1824 ± 2 Ma, 1822 ± 2 Ma, and 1818 ± 2 Ma obtained, respectively, for the amphibole + biotite syenogranite, biotite granite, and porphyritic hypersolvus granite of the Madeira pluton are consistent with the emplacement sequence inferred for these facies. These ages indicate that the Madeira pluton was emplaced in a relatively short time. Its facies are a little younger than the peralkaline granite of the Europa pluton, suggesting that the latter is not coeval with the Madeira peralkaline albite granite.     

Palaeoproterozoic A-type felsic magmatism in the Khetri Copper Belt, Rajasthan, northwestern India: petrologic and tectonic implications

Summary A number of small Palaeoproterozoic granitoid plutons were emplaced in the Khetri Copper Belt, which is an important Proterozoic metallogenic terrane in the northeastern part of Aravalli mountain range. Contiguous Biharipur and Dabla plutons are located about 15 km southeast of Khetri, close to a 170 km long intracontinental rift zone. The plutons are composed of amphibole-bearing alkali-feldspar granites, comprising microcline-albite granite, albite granite and late-stage microgranite. The albite granite in Biharipur is confined to the margins of the pluton, and shows extensive commingling with the synchronous mafic plutonics. Geochemically, the albite granites are characterised by low K₂O (∼0.5 wt.%) and elevated Na₂O (∼7.0 wt.%) abundances. By contrast, the microcline-albite granite does not show any significant mafic-granite interactions and shows normal concentrations of alkali elements. The granitoids display high concentrations of the rare earth (except Eu) and high field strength elements, high values of Ga/Al (>2.5), agpaitic index and Fe^*-number. These features together with their alkaline metaluminous and ferroan nature classify the rocks as typical A-type within-plate granites. All the granitoid facies display similar REE and incompatible element profiles indicating their cogenetic nature. These granitoids were emplaced in a shallow crustal chamber under relatively low pressures, high temperature (≥850 °C) and relatively oxidising conditions. The oxidised nature, HFSE concentrations and Nd isotope data (ɛNd = −1.3 to −2.9) favour derivation of these granitoid rocks from crustal protoliths. The generation of albite granite is attributed to the replacement of alkali feldspar and plagioclase of the original granite by pure albite as a consequence of pervasive infiltration of a high Na/(Na + K) fluid at the late-magmatic stage. This model may have wider significance for the generation of albite granites/low-K granites or albitites in other areas. The A-type plutonism under consideration seems to be an outcome of ensialic rifting of the Bhilwara aulacogen.