Quartz Alkali Feldspar Syenite Research Articles

Two-stage, U-mineralization of A-type granites from the Huangmeijian complex, eastern China

Parts of A-type granites are closely related to some critical mineral resources. The study of origin and geochemical properties of those A-type granites are of great significance in revealing the metallogenic potential. The Early Cretaceous Huangmeijian (HMJ) complex is located in the Lower Yangtze River Belt (LYRB), eastern China, which is proposed to be associated with uranium deposits. Here, we present whole-rock major and trace elements, and zircon U–Pb geochronology to reveal the formation of the two-stage, uranium mineralization of A-type granites. The high Zr + Nb + Ce + Y concentrations (598−1181 ppm) and Ga/Al ratios (2.7−4.2) suggest an A-type geochemical affinity. On the ternary discriminant diagrams, they can be further divided into A1-type. In-situ LA-ICP-MS U–Pb dating on the zircon grains from the HMJ complex yielded weighted mean ages of 129.7 ± 1.6 Ma (quartz alkali feldspar syenite), 131.3 ± 1.0 Ma (quartz alkali feldspar syenite), and 118.7 ± 0.7 Ma (alkali feldspar granite), respectively. The former two age results are consistent within error, and similar to other A-type granites in the LYRB. The latter one is newly identified, and is obviously younger than most A-type granites developed in the LYRB. Geochemical data suggest that these A-type granites are formed under high temperature and low oxygen fugacity conditions in an extensional setting. The presence of huttonite favors for the hydrothermal event and the uranium mineralization. It is important to note that, compared to the ca. 130 Ma quartz alkali feldspar syenites, the ca. 118 Ma alkali feldspar granites have obviously higher U (21.3–79.2 ppm) and Th (81.2–104 ppm) concentrations, which are also higher than those of other A-type granites developed in the LYRB. Moreover, the U concentration is even higher than some uranium mineralized granites of South China. Thus, besides the previously reported ca. 130 Ma A-type granite, the ca. 118 Ma A-type granite is of great probability to be a main source of uranium for the formation of uranium deposits in the LYRB, eastern China.

Evolution of amphiboles of the Serra do Catu intrusive suite: Crystallization conditions of Ediacaran shoshonitic rocks in the sergipano orogenic system, NE Brazil

The Ediacaran shoshonitic Santa Maria, Monte Pedral, Bom Jardim, Niterói stocks, and the Curituba batholith, belonging to the Serra do Catu Intrusive Suite, occur in the Canindé and Poço Redondo domains, in the northern region of the Sergipano Orogenic System, located in the northeast of Brazil. These intrusions occur over a wide region, have different shapes and sizes, and consist of quartz-monzonite, monzogranite, quartz alkali feldspar syenite, alkali feldspar granite, and dioritic enclaves. Calcic amphibole is the dominant mafic mineral in these rocks. Magmatic amphibole corresponds to magnesio-hornblende, edenite, magnesio-hastingsite, hastingsite, and tschermakite, while post-magmatic amphibole consists of actinolite. The amphiboles in these intrusions have similar compositions as those crystals formed in alkaline and subalkaline magmas, a typical characteristic of shoshonitic series magmas. Geothermobarometers were tested and the most convenient, those that consider a greater number of chemical elements, one was selected for the type of magma/rock studied. It was inferred that the primary crystals were formed from mantle magmas and mixed magmas (mantle + crustal). These crystals were formed under high oxygen fugacity. The bodies of the studied suite were emplaced under different temperatures, pressures, and depth conditions: Santa Maria stock (808 °C; 470 MPa; 14.6 km), Niteroi stock (801 °C; 550 MPa; 17 km), Curituba batholith (879 °C; 740 MPa; 22.9 km), Bom Jardim enclaves stock (877 °C; 860 MPa; 26.6 km), and Curituba batholith enclaves (912 °C; 970 MPa; 30 km).

Characteristics and significance of aegirine and arfvedsonite in Boziguoer Nb-Ta-Zr-Rb-REE deposit related to alkaline granite, Xinjiang

碱性造岩矿物能够记录碱性岩源区特征、岩浆演化以及晚期成矿的重要信息，是开展碱性岩成岩成矿研究的有效手段。波孜果尔碱性花岗岩型铌-钽-锆-铷-稀土矿床位于塔里木北缘-中亚南天山晚古生代造山带，是塔里木地块北缘铌成矿带中典型的碱性岩型矿床。本文通过对含矿岩体中的霓石和钠铁闪石开展矿物学研究，结合全岩成分揭示波孜果尔稀有-稀土金属矿床含矿岩体的岩石类型、演化特征、构造背景及成矿条件。研究发现，含矿碱性岩体由霓石钠铁闪石碱长花岗岩、霓石钠铁闪石石英碱长正长岩、霓石钠铁闪石碱长正长岩组成。不同岩相岩石均表现出相似的稀土和微量元素配分模式，以富集轻稀土元素和高场强元素，亏损大离子亲石元素和重稀土元素为特征，具有显著的负Eu异常，指示三种岩石类型是同源岩浆演化的产物。含矿岩体中的辉石为霓石-霓辉石，角闪石为钠铁闪石。霓石和钠铁闪石稀土和微量元素配分模式相似且含量都较低，富集重稀土、亏损轻稀土，具有显著的负Eu异常，并富集Zr、Hf等高场强元素，亏损Ba、Sr等大离子亲石元素。霓石和钠铁闪石微量元素特征指示三个岩相单元的演化程度由低到高依次为霓石钠铁闪石碱长花岗岩→霓石钠铁闪石石英碱长正长岩→霓石钠铁闪石碱长正长岩，且结晶分异作用控制了岩体的形成。同时，早期霓石钠铁闪石碱长花岗岩的钠铁闪石呈现Ce正异常，晚期岩相中则无异常，指示演化的早期阶段氧逸度较高，随着磁铁矿等氧化物的结晶熔体趋于还原。波孜果尔含矿岩体成矿元素除Rb外分布并不均匀，表现为演化早期的岩相富集Nb，而演化晚期的岩相更富集稀土和Zr。进一步研究发现，塔里木北缘碱性岩带发育的铌、稀土、钽、锆、铷、铀等稀有稀土金属矿化与地幔柱引起的幔源岩浆底侵有关，其构造背景可能为地幔柱对造山带的叠置。;Alkaline minerals can record the important information of source area characteristics, magmatic evolution and late mineralization of alkaline rocks, which is an effective means for the study of diagenesis and mineralization of alkaline rocks. Boziguoer Nb-Ta-Zr-Rb-REE deposit is a typical deposit related to alkaline rocks, which is located in the Late Paleozoic orogenic belt of the southern Tianshan-the northern margin of Tarim block. Based on mineralogical study of aegirine and arfvedsonite in the ore-bearing intrusion, combining with its whole rock compositions, this paper reveals the rock types, evolution characteristics, tectonic setting and metallogenic conditions of the intrusion in the Boziguer rare-earth metal deposit. The rock types of ore-bearing intrusion are aegirine arfvedsonite alkali feldspar granite, aegirine arfvedsonite quartz alkali feldspar syenite and aegirine arfvedsonite alkali feldspar syenite. All of the three types of rocks show similar REE and trace element distributions, with enriched LREE and HFSE, depleted HREE and LILE, and a significant negative Eu anomaly, indicating that they are the products of homologous magmatic evolution. The pyroxene group minerals are aegirine, and the amphibole group minerals are arfvedsonite. Aegirine and arfvedsonite show similar distribution patterns of trace and rare earth elements, which are characterized by enriched HREE and depleted LREE with a significant Eu negative anomaly. They all show enrichment in HFSE (Zr, Hf), depletion in LILE, such as Ba and Sr. Trace element and REE distributions of these minerals indicate that the degree of their evolution from low to high is: aegirine arfvedsonite alkali feldspar granite→aegirine arfvedsonite quartz alkali feldspar syenite→aegirine arfvedsonite alkali feldspar syenite, showing the formation of the intrusion is controlled by crystallization and differentiation. The arfvedsonite of aegirine arfvedsonite alkali feldspar granite shows a positive anomaly of Ce, indicating a high oxygen fugacity environment at the early stages of its evolution. The crystallization of magnetite in the early lithofacies make the evolution environment of the melt tend to be reduced. The Rb in all rock types is equally distributed, and the rocks of early evolution stage is more enriched in Nb, while that of the late-stage more enriched in REE and Zr. The mineralization of rare earth metals such as Nb, REE, Ta, Zr, Rb and U developed in the alkaline rock belt in the northern margin of Tarim basin is related to the mantle magma underplating caused by mantle plume, and its tectonic setting may be the superposition of mantle plume to orogenic belt.

The importance of in situ crystallisation and loss of interstitial melt during formation of the Kærven Syenite Complex, Kangerlussuaq, East Greenland

The Kærven Syenite Complex (KSC) is one of the oldest felsic intrusions in the Tertiary East Greenland province. Here we update our previous description of the KSC and supply a greatly expanded and comprehensive geochemical dataset. New data allow us to present a more detailed petrogenetic model for the evolution of the KSC and to investigate the geochemical characteristics of igneous cumulates subjected to loss and, occasionally, replacement of residual liquid. The KSC comprises eleven mappable units that generally young westwards. Rock types range from quartz syenite to quartz alkali feldspar syenite and alkali feldspar granite. Individual intrusive units are relatively narrow and steep-sided and are collectively suggested to represent a ring dyke complex. Basement gneiss and gabbro host rocks have locally contaminated the oldest quartz syenite KSC unit, but most of the main part of the complex escaped significant influence from host rocks. A late suite of E–W to NE–SW striking peralkaline dykes of trachytic to phonolitic compositions intrude the KSC. Compositions of the KSC rocks span a considerable range in SiO2, 59–73 wt%. Concentrations of several elements vary widely for a given SiO2 (especially at SiO2 < 66 wt%), and variation diagrams do not suggest a single model for the evolution of the units of the complex. A cumulative origin is envisaged for several KSC units. Geochemical modelling suggests that KSC magmas were derived from more than one primary magma, and that the complex evolved through a four-stage process: fractional crystallisation in precursory magma chambers was followed by final emplacement of each unit, establishment of a crystal/melt mush, expulsion of part of the residual melt and, finally, crystallisation of the remaining melt. Trace element disequilibria between alkali feldspar and host rocks in two closely associated quartz alkali feldspar syenite units indicate that highly evolved residual melt was replaced by a less evolved melt phase. Modelling of potential parent melt compositions to the Kærven magmas suggests an origin not in the Iceland plume asthenosphere, but rather in a moderately enriched source, possibly in the continental lithosphere. The course of melt evolution by fractional crystallisation is indicated to have taken place in magma chambers at depth, and repeated rise of magma into the upper crustal magma chambers and crystallisation there formed the KSC. Based on our survey of published geochemical data, the inferred parental magmas seem to have few equivalents in the North Atlantic Igneous Province and may have been generated mainly from melting of enriched dry lithospheric mantle of possibly Archaean age.

Petrological and geochemical characteristics of REE mineralization in the A-type French Creek Granite, New Zealand

The French Creek Granite, New Zealand, is an alkaline intrusion with enrichment in rare earth elements (REE). Petrography and whole-rock geochemical signatures demonstrate that the c. 82–84-Ma French Creek Granite is a composite granitoid, dominated by a ferroan, weakly peraluminous biotite alkali feldspar granite to syenogranite, with subordinate quartz-alkali feldspar (QAF) syenite. Maximum total REE + Y contents are higher in the granite and felsic dikes relative to the marginal QAF syenite, cogenetic mafic Hohonu Dike Swarm, and pegmatites. Our findings suggest that REE and high-field-strength element enrichment in the unaltered French Creek Granite is a function of partial melting from an enriched HIMU-like lithospheric mantle source, minor crustal assimilation, and extensive magmatic differentiation. Primary (magmatic) REE enrichment is defined by the occurrence of allanite, zircon, apatite, fergusonite, monazite, perrierite, and loparite, which are commonly associated with interstitial biotite. French Creek Granite samples from a phyllic-sericitic alteration zone in the Eastern Hohonu River are strongly elevated in REE relative to unaltered French Creek Granite, indicating remobilization and secondary REE enrichment by hydrothermal fluids. The REE are hosted in bastnasite group minerals, monazite, xenotime, and zircon. Quartz protuberances, dilatational microfractures, and dike emplacement indicate that this alteration is structurally controlled. The δ13C and δ18O values of secondary carbonate veinlets are consistent with mixed low-temperature (~ 250–260 °C) magmatic hydrothermal fluids containing mantle-derived carbon. These fluids were likely part of a late-stage porphyry-type system operating during the same mantle degassing and extensional episode that was associated with the emplacement of the French Creek Granite and lamprophyric dikes of the Hohonu Dike Swarm, and initial Tasman Sea spreading. Results from this study provide an important insight into the complex processes responsible for REE enrichment in alkaline igneous systems.

Relationships between magmatism and deformation in northern Yorke Peninsula and southeastern Proterozoic Australia

ABSTRACTThe ca 1600–1580 Ma time interval is recognised as a significant period of magmatism, deformation and mineralisation throughout eastern Proterozoic Australia. Within the northern Yorke Peninsula in South Australia, this period was associated with the emplacement of multiple phases of the Tickera Granite, an intensely foliated quartz alkali-feldspar syenite, a leucotonalite and an alkali-feldspar granite. These granites belong to the broader Hiltaba Suite that was emplaced at shallow crustal levels throughout the Gawler Craton. Geochemical and isotopic analysis suggests these granite phases were derived from a heterogeneous source region. The syenite and alkali-feldspar granite were derived from similar source regions, likely the underlying ca 1850 Ma Donington Suite and/or the ca 1750 Ma Wallaroo Group metasediments with some contamination from an Archean basement. The leucotonalite is sourced from a similar but more mafic/lower crustal source. Phases of the Tickera Granite were emplaced synchronously with deformation that resulted in development of a prominent northeast-trending structural grain throughout the Yorke Peninsula region. This fabric is associated with composite events resulting from folding, shearing and faulting within the region. The intense deformation and intrusion of granites within this period resulted in mineralisation throughout the region, as seen in Wheal Hughes and Poona mines. The Yorke Peninsula shares a common geological history with the Curnamona Province, which was deformed during the ca 1600–1585 Ma Olarian Orogeny, and resulted in development of early isoclinal and recumbent folds overprinted by an upright fold generation, a dominant northeast-trending structural grain, mineralisation, and spatially and temporally related intrusions. This suggests correlation of parts of the Gawler Craton with the Curnamona Province, and that the Olarian Orogeny also affected the southeastern Gawler Craton.

Petrochemistry of Granitoids in Sibolga and its Surrounding Areas, North Sumatra, Indonesia

AbstractPetrochemical studies were conducted on granitoids in Sibolga and surrounding areas in North Sumatra, Indonesia. The granitoids presented the characteristics of A‐ and I‐type ilmenite series. The syenogranites from Sarudik, the syenogranite, quartz syenite, and quartz alkali feldspar syenite from Sibuluhan Sihaporas, and the monzogranites from Sibolga Julu are A‐type granitoids that have high SiO2, CaO, Al2O3, Na2O + K2O, and Rb content, a high FeO/MgO ratio, and low Ba, Sr, and Zr content. The alkali feldspar syenite from Sarudik, the quartz alkali feldspar syenite from Tukka, and the quartz syenite from Adian Koting show I‐type characteristics. I‐type granitoids have lower SiO2, Na2O + K2O, Rb, Yb, and total REE (ΣREE) content and low FeO/MgO ratio. However, quartz syenite from the Sibolga Julu, which exhibits I‐type characteristics, has high ΣREE content but a low FeO/MgO ratio.The granitoids consisted mainly of K‐feldspar megacrysts, quartz, plagioclase, and biotite with hornblende in some cases. Accessory minerals were zircon, apatite, allanite, and titanite. Cordierite and corundum occurred as xenocrysts and inclusions in K‐feldspar in syenogranite (Sarudik) and quartz syenite (Adian Koting). SiO2 and ΣREE content and the Rb/Sr ratio are positively correlated to each other.The REE are enriched in quartz alkali feldspar syenites from Sibolga Julu and Sarudik, quartz syenite from Tarutung, and quartz alkali feldspar syenite, syenogranite, and quartz syenite from Sibuluhan Sihaporas. These highly‐differentiated granitoids were formed within the plate settings. In contrast, the ΣREE content of hornblende‐bearing granitoids that were formed in volcanic arc settings is low. The coexistence of cordierite and corundum with biotite and hornblende indicates that the metaluminous magma was contaminated by metasedimentary basement rocks.

Mianningite, ([], Pb, Ce, Na) (U4+, Mn, U6+) Fe3+ 2(Ti, Fe3+)18 O38, a new member of the crichtonite group from Maoniuping REE deposit, Mianning county, southwest Sichuan, China

Mianningite (IMA 2014-072), ideally (□,Pb,Ce,Na)(U4+,Mn,U6+)Fe3+2(Ti,Fe3+)18O38, is a new member of the crichtonite group from the Maoniuping REE deposit, Mianning county, Sichuan province, China. It was found in fractures of lamprophyre veins and in the contact between lamprophyre and a later quartz–alkali feldspar syenite dyke with REE mineralization, and is named after its type locality. Associated minerals are microcline, albite, quartz, iron-rich phlogopite, augite, muscovite, calcite, baryte, fluorite, epidote, pyrite, magnetite, hematite, galena, hydroxylapatite, titanite, ilmenite, rutile, garnet-group minerals, zircon, allanite-(Ce), monazite-(Ce), bastnasite-(Ce), parisite-(Ce), maoniupingite-(Ce), thorite, pyrochlore-group minerals and chlorite. Mianningite occurs as opaque subhedral to euhedral tabular crystals, up to 1–2 mm in size, black in color and streak, and with a submetallic luster. Mianningite is brittle, with a conchoidal fracture. Its average micro-indentation hardness is 83.8 kg/mm2 (load 0.2 kg), which is equivalent to ~6 on the Mohs hardness scale. Its measured and calculated densities are 4.62 (8) g/cm3 and 4.77 g/cm3, respectively. Under reflected light, mianningite is grayish white, with no internal reflections. It appears isotropic and exhibits neither bireflectance nor pleochroism. The empirical formula, calculated on the basis of 38 O atoms per formula unit ( apfu ), is [□0.322(Pb0.215Ba0.037Sr0.036Ca0.010)Σ0.298(Ce0.128La0.077Nd0.012)Σ0.217 (Na0.127K0.036)Σ0.163]Σ1.000(U4+0.447Mn0.293U6+0.112Y0.091Zr0.023Th0.011)Σ0.977(Fe3+1.224Fe2+0.243Mg0.023P0.008Si0.006 □0.496)Σ2.000(Ti12.464Fe3+5.292V5+0.118Nb0.083Al0.026Cr3+0.017)Σ18.000O38. Mianningite is trigonal, belongs to the space group ![Formula][1] , and has unit-cell parameters a = 10.3462(5) A, c = 20.837(2) A, V = 1931.65 (20) A3, and Z = 3. The structure was solved ( R 1 = 0.070) using reflections with I > 2σ( I ) on a heated crystal; it is isostructural with the other members of the crichtonite group. Mianningite can be considered as the analogue of mapiquiroite with the M0 site preferentially vacant. Its eight strongest X-ray powder-diffraction lines [ d in A( I / I )( hkl )] are 2.627(100) (125), 2.144 (100) (135), 3.065(75) (025), 2.254 (70) (028), 1.545(60) (336), 2.883(55) (116), 2.476 (55) (027), and 1.705 (55) (146). [1]: /embed/mml-math-1.gif

Petrogenesis of cogenetic silica-oversaturated and -undersaturated syenites of Abu Khruq ring complex, South Eastern Desert, Egypt

The upper Cretaceous Abu Khruq ring complex (ARC) is located in the South Eastern Desert of Egypt displays concentric zonation of syenitic rocks from quartz-rich syenite at the margin, through alkali feldspar syenite to nepheline syenite in the centre. The syenitic rocks occur with nepheline monzogabbro, volcanic rocks (phonolite and trachyte) and the quartz- and nepheline-bearing pegmatites. Rocks of contrasting composition (mafic and salic) exhibit sophisticate geometric relationships. The nepheline monzogabbroic rocks have pillowy xenoliths forms within the salic (nepheline syenite and quartz alkali feldspar syenite) rocks, suggesting synchronous emplacement of the mafic and salic magmas. Clinopyroxene analysis of mafic and salic plutonic rocks of the ARC revealed that the overall pyroxene trend suggesting that fractionation involved a late, progressive increase in Na, in a reaction of the type Ca Mg Fe2+↔Na Fe3+. The chemistry of the analysed amphiboles are compositionally similar to those from typical differentiated peralkaline suites. Geochemically, the complex is enriched in the LILE, HFSE and REE. The concentrations of the compatible elements (V, Sr and Ba) generally decrease with increasing silica, consistent with fractional crystallization. A generalised increase in the Nb/Ta from the nepheline monzogabbro to nepheline syenite compositions is attributed to titanite fractionation. All the rock samples show relative increment of the LREE content than the HREE indicating weak to steep fractionated REE patterns (La/Yb) from 9.43 to 10.86, and thus retaining the geochemical characteristics of anorogenic suites. The magma sources of ARC are not derived from normal primitive mantle. The early stages of differentiation involved extensive olivine and pyroxene fractionation, the fractionation of amphibole, titanite, magnetite, apatite and feldspar may have been involved in the genesis of the salic differentiated compositions. The deviation towards silica-oversaturated members attributed to crustal assimilation and associated with fractional crystallization of the basic magma at a relatively early stage during the development of the complex. The whole complex is deduced to have originated from alkali basaltic to basanitic magma of OIB-like character. The general increasing of the amphibole and biotite among the different rocks of the complex reflects the hydrous nature of ARC magmas.

The Geochemical and Zircon Trace Elements Characteristics of A‐type Granitoids in Boziguoer, Baicheng County, Xinjiang

AbstractThe Boziguoer A‐type granitoids in Baicheng County, Xinjiang, belong to the northern margin of the Tarim platform as well as the neighboring EW‐oriented alkaline intrusive rocks. The rocks comprise an aegirine or arfvedsonite quartz alkali feldspar syenite, an aegirine or arfvedsonite alkali feldspar granite, and a biotite alkali feldspar syenite. The major rock‐forming minerals are albite, K‐feldspar, quartz, arfvedsonite, aegirine, and siderophyllite. The accessory minerals are mainly zircon, pyrochlore, thorite, fluorite, monazite, bastnaesite, xenotime, and astrophyllite. The chemical composition of the alkaline granitoids show that SiO2 varies from 64.55% to 72.29% with a mean value of 67.32%, Na2O+K2O is high (9.85%–11.87%) with a mean of 11.14%, K2O is 2.39%–5.47% (mean = 4.73%), the K2O/Na2O ratios are 0.31–0.96, Al2O3 ranges from 12.58% to 15.44%, and total FeOT is between 2.35% and 5.65%. CaO, MgO, MnO, and TiO2 are low. The REE content is high and the total SREE is (263–1219) ppm (mean = 776 ppm), showing LREE enrichment and HREE depletion with strong negative Eu anomalies. In addition, the chondrite‐normalized REE patterns of the alkaline granitoids belong to the “seagull” pattern of the right‐type. The Zr content is (113–1246) ppm (mean = 594 ppm), Zr+Nb+Ce+Y is between (478–2203) ppm with a mean of 1362 ppm. Furthermore, the alkaline granitoids have high HFSE (Ga, Nb, Ta, Zr, and Hf) content and low LILE (Ba, K, and Sr) content. The Nb/Ta ratio varies from 7.23 to 32.59 (mean = 16.59) and the Zr/Hf ratio is 16.69–58.04 (mean = 36.80). The zircons are depleted in LREE and enriched in HREE. The chondrite‐normalized REE patterns of the zircons are of the “seagull” pattern of the left‐inclined type with strong negative Eu anomaly and without a Ce anomaly. The Boziguoer A‐type granitoids share similar features with A1‐type granites. The average temperature of the granitic magma was estimated at 832–839°C. The Boziguoer A‐type granitoids show crust–mantle mixing and may have formed in an anorogenic intraplate tectonic setting under high‐temperature, anhydrous, and low oxygen fugacity conditions.

중국 신장 위그루자치구 바이청현 보즈구얼의 A형화강암류의 지화학 및 지르콘 미량원소특징에 대한 연구

The Boziguoer A-type granitoids in Baicheng County, Xinjiang, belong to the northern margin of the Tarim platform as well as the neighboring EW-oriented alkaline intrusive rocks. The rocks comprise an aegirine or arfvedsonite quartz alkali feldspar syenite, an aegirine or arfvedsonite alkali feldspar granite, and a biotite alkali feldspar syenite. The major rock-forming minerals are albite, K-feldspar, quartz, arfvedsonite, aegirine, and siderophyllite. The accessory minerals are mainly zircon, pyrochlore, thorite, fluorite, monazite, bastnaesite, xenotime, and astrophyllite. The chemical composition of the alkaline granitoids show that varies from 64.55% to 72.29% with a mean value of 67.32%, is high (9.85~11.87%) with a mean of 11.14%, is 2.39%~5.47% (mean

The 2.74 Ga Likamännikkö complex in Suomussalmi, East Finland: Lost between sanukitoids and truly alkaline rocks?

The Likamännikkö quartz alkali feldspar syenite complex is located in eastern Finland, in the Suomussalmi district, within the Archean Karelian Craton. The intrusive complex covers an area of 2km2 and is cut by both Neoarchean leucogranite and Paleoproterozoic dolerite dykes. Three zircon fractions define a concordant U–Pb age of 2741±2Ma for the quartz alkali feldspar syenite. The syenitic complex also contains carbonatite patches and angular fragments of ultrabasic rock less than 50cm across. However, the paucity of outcrop has prevented direct observation of the relationship between carbonatites and other rock types. The quartz alkali feldspar syenite is not alkaline sensu stricto, but has alkaline affinities, with major element compositions in the range SiO2=62.6–70.3wt.%, K2O=1.09–5.73wt.%, Na2O=4.41–7.19wt.% and MgO=0.65–2.06wt.%. The ultrabasic fragments are interpreted as cumulates containing clinopyroxene (generally altered to actinolite), apatite, allanite, epidote, and albite; these minerals occur in varying proportions, leading to considerable whole-rock compositional heterogeneity, with major elements in the range SiO2=31.4–48.9wt.%, CaO=16.35–30.74wt.% and MgO=2.00–14.07wt.%, as well as notable enrichment in P2O5 (3.2–13.5wt.%). The carbonatites are calcitecarbonatites displaying mantle-like stable isotope values with δ18OVSMOW from 5.2 to 6.9‰ and δ13CVPDB values from −3.8 to −5.5‰, also trace element characteristics are consistent with a magmatic origin. Based on Sm–Nd model ages, the carbonatites are approximately coeval with the host quartz alkali feldspar syenite and thus possibly among the oldest known carbonatites in the world. All three rock types are interpreted to have originated from metasomatized mantle and the intrusive complex consists of two or more silicate magma pulses. Positive εNd values and their small scatter from −0.2 to 2.0 at 2741Ma suggest only limited assimilation of older crust or Paleoproterozoic disturbance of Sm–Nd systematics.

Postorogenic carbonatites at Eden Lake, Trans-Hudson Orogen (northern Manitoba, Canada): Geological setting, mineralogy and geochemistry

The Eden Lake pluton in the Trans-Hudson Orogen is the first known occurrence of carbonatites in Manitoba. The pluton is largely made up of modally and geochemically diverse syenitic rocks derived from postorogenic magma(s) of shoshonitic affinity. Their diversity can be accounted for by a combination of crystal fractionation and fluid release in the final evolutionary stage (crystallization of quartz alkali-feldspar syenite). At Eden Lake, carbonatites, represented predominantly by coarse-grained massive to foliated sövite, occur as branching veins and lenticular bodies up to 4 m in thickness showing crosscutting relations with respect to all of the syenitic units. The host rocks are intensely fenitized at the contact, and there is also abundant mineralogical and textural evidence for assimilation of silicate material by carbonatitic magma through wallrock reaction and xenolith fragmentation and digestion. The bulk of the carbonatites are composed of (in order of crystallization): Sr–REE-rich fluorapatite, aegirine–augite, and coarse calcite crystals surrounded by fine-grained calcite (on average, ∼ 90 vol.% of the rock). Noteworthy accessory constituents are celestine, bastnäsite-(Ce) (both as primary inclusions in calcite), Nb–Zr–rich titanite, low-Hf zircon, allanite-(Ce) and andradite. The calcite is chemically uniform (Sr-rich, Mg–Mn–Fe-poor and low in 13C), but shows clear evidence of ductile deformation and syndeformational cataclasis. Geochemically, the carbonatites are enriched in Sr, Ba, light rare-earth elements, Th and U, but depleted in high-field-strength elements (particularly, Ti, Nb and Ta). The stable-isotope composition of coarse- and fine-grained calcite from the carbonatites and interstitial calcite from syenites is remarkably uniform: ca. − 8.16 ± 0.27‰ δ 13C (PDB) and + 8.04 ± 0.19‰ δ 18O (SMOW). The available textural and geochemical evidence indicates that the Eden Lake carbonatites are not consanguineous with the associated syenites and may have been derived from a Nb–Ti-retentive and 13C-depleted source such as the subducted crustal material underlying the Eden Lake deformation corridor.

Petrogenesis of silicic magmatism related to the ∼ 2.44 Ga rifting of Archean crust in Koillismaa, eastern Finland

Early Paleoproterozoic extension in the Archean craton of the Fennoscandian shield led to the emplacement of several 2.44 Ga layered gabbroic intrusive complexes in northern Finland and adjacent Russia. Closely associated with them are felsic rocks of similar age: (1) the Sirniö Group volcanic rocks on top of the Koillismaa layered complex; (2) a quartz alkali feldspar syenite at Kynsijärvi near the Koillismaa complex; and (3) an aluminous A-type granite at Nuorunen near the Oulanka layered complex. In the Koillismaa area, the ruptured Archean crust consists of ortho- and paragneisses that were intruded and migmatized by somewhat younger granites. U–Pb zircon data indicate that the gneisses are at least ∼2.8 Ga old and that the granites were crystallized at ∼2.7 Ga. Both rock types show a common monazite age of 2695 Ma that registers the peak of granulite facies metamorphism and, possibly, the intrusion of the granites. The local Neoarchean crust has ɛ Nd(at 2440 Ma) values between − 5 and − 8.5. The mafic rocks of the Koillismaa complex show initial ɛ Nd(at 2440 Ma) values around − 1.5 and those of the Oulanka complex range from − 2.1 to 0. The ɛ Nd value (− 4.8) and T DM model age (2.9–3.0 Ga) of the Kynsijärvi quartz alkali feldspar syenite are within the limits of the evolution path of the local Archean crust. The corresponding values for the Nuorunen granite are − 2.0 and 2.76 Ga and are thus closer to those of the mafic rocks. The volcanic rocks of the Sirniö Group show more scatter with initial ɛ Nd(at 2440 Ma) values of − 1.1 to − 5.3; the lowest ɛ Nd values probably reflect later disturbance–magmatic values cluster around − 2. Major and trace element modeling shows that fractional crystallization of the Koillismaa complex parental magma or partial melting of the Archean crust cannot account for the ∼ 2.44 Ga silicic rocks of Koillismaa. The geochemical and Nd isotope characteristics of the volcanic rocks and the Kynsijärvi quartz alkali feldspar syenite are best explained by a combined assimilation–fractional crystallization model involving a contaminated komatiitic parental magma and intermediate-mafic lower crustal assimilant. Differences in the initial Nd isotope composition of the 2.44 Ga rocks reflect varying source components and evolutionary history for the Archean lithosphere in eastern Fennoscandia.

The Kynsijärvi quartz alkali feldspar syenite, Koillismaa, eastern Finland—silicic magmatism associated with 2.44 Ga continental rifting

The Kynsijärvi quartz alkali feldspar syenite (KS) is a small A-type silicic pluton closely associated with the ∼2440 Ma Koillismaa complex, one of the Paleoproterozoic mafic layered intrusion complexes of central and northern Finland and Kola Peninsula. In the Koillismaa area, these mafic and felsic intrusions were emplaced into a late Archean (2900–2600 Ma) basement and stratigraphically underlie supracrustal rocks of the Paleoproterozoic Kuusamo schist belt. The KS is a medium-grained hypersolvus rock with mesoperthitic alkali feldspar, quartz, and ferro-edenite as the major rock forming minerals. Magnetite, titanite, zircon, fluorite, and secondary stilpnomelane are found in accessory amounts. The rock shows extensive subsolidus reactions, for example well-developed intergranular albite rims around alkali feldspar crystals. The KS is characterized by relatively high SiO 2 (70.1–71.7 wt.%), total alkalies (∼10 wt.%), Ba (>700 ppm), Zr (>500 ppm), Ga (>30 ppm), and Nb (>40 ppm). Chondrite-normalized REE patterns are moderately enriched in the LREE [(La/Yb) N∼12.65] and show a negative Eu-anomaly (Eu/Eu*∼0.3). Four zircon fractions from the KS yield a U–Pb upper intercept age of 2442±3 Ma. The KS was probably emplaced in an extensional environment, the extension being caused by a mantle plume, which also generated the mafic layered complexes in the region. Overall, the 2.44 Ga mafic and felsic plutonic rocks mark a ∼400-km-long zone of early Paleoproterozoic rifting of the Fennoscandian Archean craton.

Origin and Intensive Parameters in the Crystallization of Ultrapotassic Syenites: The Serra Do Man Pluton, Northeastern Brazil

The late Neoproterozoic ultrapotassic Serra do Man pluton intruded supracrustal schists and igneous rocks of the Alto Pajeu tectonostratigraphic terrane, in the Borborema province of northeastern Brazil. This quartz-alkali feldspar syenite is composed of microcline, albite, normally zoned aegirine-augite, winchite (formed after pyroxene), titanite and apatite, with the major-element geochemistry in consonance with early fractionation of pyroxene-titanite-apatite assemblage. Chondrite-normalized REE-patterns are LREE-enriched (La/Lu ∼ 20) with discrete negative Eu anomaly, and primitive mantle-normalized elemental concentration diagrams display enhanced negative Nb, P and Ti anomalies that are also observed in patterns of similar rocks in NE Brazil. Whole-rock d 18O values are rather high (avg. +8.7% SMOW) and in the +7.8 to +8.6% SMOW range when clinopyroxene-corrected. A five-point whole-rock Rb-Sr isochron yields an age of 580±28 Ma with high Sr o (0.7108), which together with eNd (0.58 Ga) value of -17 and t chur of 2.3 Ga allow hypothesizing a Paleoproterozoic enriched-mantle source for the syenites in the ultrapotassic province of the Alto Pajeu terrane. The aegirine-augite, titanite, and quartz assemblage attests to a high oxygen fugacity prevailing during magmatic crystallization. The almost complete absence of primary hydrated phases assures that the magma was rather anhydrous, and its viscosity, at the liquidus temperature, was relatively high. Density is slightly higher than expected for syenitic magmas (2.8 g/cm 3), being explained by a relatively high iron content of the magma. Geothermobarometry suggests shallow emplacement and temperatures of 700-750°C and 850-900°C, respectively, for the solidus and liquidus of the magma.

Structural setting, hydrothermal alteration, and gold mineralisation at the Archaean syenite-hosted Jupiter deposit, Yilgarn Craton, Western Australia

The Jupiter gold deposit in the northeastern Eastern Goldfields Province of the Yilgarn Craton of Western Australia is hosted in greenschist facies metamorphosed tholeiitic basalt, quartz–alkali-feldspar syenite, and quartz–feldspar porphyry. Syenite intrudes basalt as irregularly shaped dykes which radiate from a larger stock, whereas at least three E–W and NE–SW striking quartz–feldspar porphyries intrude both syenite and basalt. Brittle–ductile shear zones are shallow-dipping, NW to NE striking, or are steep-dipping to the south and west. Quartz ± carbonate veins that host gold at Jupiter occur in all lithologies and are divided into: (1) veins that are restricted to the shear zones, (2) discrete veins that are subparallel to shear zone-hosted veins, and (3) stockwork veins that form a network of randomly oriented microfractures in syenite wallrock proximal to shallow-dipping shear zones. The gold-bearing veins comprise mainly quartz, calcite, ankerite, and albite, with minor sericite, pyrite, chalcopyrite, galena, sphalerite, molybdenite, telluride minerals, and gold. Proximal hydrothermal alteration zones to the mineralised veins comprise quartz, calcite, ankerite, albite, and sericite. High gold grades (>2 g/t Au) occur mainly in syenite and in the hanging walls to shallow-dipping shear zones in syenite where there is a greater density of mineralised stockwork veins. The Jupiter deposit has structural and hydrothermal alteration styles that are similar to both granitoid-hosted, but post-magmatic Archaean lode-gold deposits in the Yilgarn Craton and intrusion-related, syn-magmatic, syenite-hosted gold deposits in the Superior Province of Canada. Based on field observations and petrologic data, the Jupiter deposit is considered to be a post-magmatic Archaean lode-gold deposit rather than a syn-intrusion deposit.

Fluid inclusion studies of the Rodeo de Los Molles REE and Th deposit, Las Chacras Batholith, Central Argentina

The Rodeo de Los Molles rare earth element (REE) and thorium deposit is located in granitic rocks of the Las Chacras-Piedras Coloradas Batholith, in the southern block of the Eastern Pampean Ranges, Central Argentina. Mineralization occurs within an elongate (2 km × 0.6 km) body of alkalifeldspar granite (alaskite) localized along the northeastern edge of a composite batholith. The surrounding lithology is predominantly a biotite monzogranite. Both the alaskite and localized areas of quartz alkalifeldspar syenite within the alaskite have been produced by hydrothermal alteration of a late-crystallizing phase of the monzogranite. REE minerals are primarily of the cerium group and include britholite and allanite, both partially replaced by bastnaesite or thorbastnaesite. These minerals occur as nodules with quartz, fluorite, aegirine-augite, sphene, and Fe-Ti oxides within aplitic to pegmatoidal quartz alkalifeldspar syenite. Uranothorite, along with a second generation of fluorite and minor amounts of MnBa oxides, occurs in the alaskite as nodules, or within quartz-lined miarolitic cavities, but is not found with the Ce-mineralization. Studies of fluid inclusions contained in quartz and fluorite indicate a complex history of open-system fluid migration and interaction with monzogranite host rocks. Fluids responsible for REE mineralization and quartz deposition, along with initial alteration of the monzogranite to alaskite and quartz alkalifeldspar syenite, were of relatively high temperature (T h of fluid inclusions in quartz = 356–535°C) and moderate salinity (15–25 eq. wt% NaCl). Mixed CO 2H 2O fluids (XCO 2 = .13–.07) found as both primary and secondary inclusions within fluorite are representative of fluids involved in the replacement of britholite-allanite by bastnaesite and sphene, aegirine-augite, and plagioclase by calcite. Minimum pressures of mineral deposition estimated from H 2OCO 2NaCl phase relations range from 1 to 2 kbars. Secondary aqueous fluid inclusions in quartz define a trend of low salinity-high temperature to high salinity-low temperature, thought to be a result of hydration reactions occurring in alaskite and quartz alkali-feldspar syenite. The highest salinity fluids (35–37 eq. wt% NaCl) detected in the area are associated with the formation of uranothorite and late fluorite. Multiple periods of hydrothermal fluid introduction are consistent with recent geological data that indicate that the batholith is composed of several stock-like bodies. The location of the mineralized area near the top of the magma chamber, the presence of numerous miarolitic cavities, and the bulk composition of inclusion fluids (Na ≥ K > Ca) suggest that the fluids responsible for REE and Th mineralization were of magmatic origin.

Jabal Hamra REE-mineralized silexite, Hijaz region, Kingdom of Saudi Arabia

The Jabal Hamra silexite, a crescent-shaped stock 300 m long by 100 m wide, averages over 6000 ppm combined REE and is the Kingdom's highest-grade resource of these elements. It is anomalously radioactive (total-count gamma radiation 1000–3000 cps), has high average contents of U (75 ppm) and Th (263 ppm) and is also enriched in Nb, Zr, Y, Sn and Ta. The silexite crystallized as a pressure-quenched rock resembling aplite, and was subsequently pervasively cataclased. It was derived by differentiation of a quartz alkali-feldspar syenite magma. Petrologic continuity can be demonstrated from quartz alkali-feldspar syenite through leucocratic and amphibole-bearing alkali-feldspar granite to silexite. Although the geochemical signature of the mineralization resembles that of mineralized Arabian alkali granites, the nature of the associated rocks and therefore the genesis of the deposit are significantly different.

Martabah gabbro—monzonite complex, Hijaz region, Kingdom of Saudi Arabia; petrography and structure

The Martabah complex consists of an almost circular gabbroic rim, the outer portion of which is intruded by ring dikes of alkali-feldspar granite, and a core of (quartz) monzonite intruded by arcuate lenses and dikes of (quartz) syenite. A central lens of kaolinized, porphyritic quartz alkali-feldspar syenite is possibly derived from monzonitic rocks. There is an intense aeromagnetic anomaly over the gabbro and a low-intensity radiometric anomaly over the core.