Radioactive element distribution and rare-metal mineralization in anorogenic acid volcano-plutonic rocks of the Neoproterozoic Malani Felsic Province,western Peninsular India

The Anorogenic Malani Felsic Province (MFP) of western Peninsular India consists of peralkaline, metaluminous to mildly peraluminous A-type granites-acid volcanics with minor basic volcanics and dykes. The suite is bimodal in nature that characterized by volcano-plutonic ring structures and radial dykes. The granitoids of Siwana and Kundal areas of MFP are traversed by numerous quartz veins with fluoride, iron encrustations, druses and knots of pegmatite phases. Petrographically, they show cloudy, patchy perthitic textures; spherulite form of alkali amphibole and alkali pyroxenes; alteration of K-Na-feldspar to kaolin/sericite, magnetite to haematite; growth of granophyres/perthite/rapakivi like textures. They are enriched in SiO₂, Na₂O+K₂O, Fe/Mg, Rb, Zr, Y, Ga, REE (except Eu) and depleted in MgO, CaO, Mg#, P, Ti, Sr, Ni, Cr, Co and V. Uniform REE patterns, parallel to sub-parallel, LREE enriched over HREE and prominent negative Eu-anomalies are the characteristics of these granitoids. Geochemical parameters satisfy the A-type nature of granitoids and crustal origin of these rocks. These granitoids are high heat producing granitoids because of their high content of radioactive elements (U, Th, K), and can be classified as granite (Type I) (avg. 7.18 μWm⁻³), rhyolite and trachyte (Type II) (avg. 4.47 μWm⁻³) and acid dyke (Type III) (avg. 14.53 μ Wm⁻³). The average total heat generation unit (HGU) of Type I (17.10 HGU), Type II (10.64 HGU) and Type III (35.31 HGU) are much higher than the average value of continental crust (3.8 HGU), which imply a possible linear relationship among the surface heat generations in the MFP. Field, petrography and whole rock geochemical characteristics suggest potentiality for rare metals and rare earth elements mineralization in the studied granitoids of the MFP.     

Geochemistry and petrogenesis of Neoproterozoic Mylliem granitoids,Meghalaya Plateau,northeastern India

The Mylliem granitoids of the Meghalaya Plateau, northeastern India, represent one of the disharmonic Neoproterozoic igneous plutons, which are intrusive into low-grade Shillong Group of metasediments. Field studies indicate that the Mylliem granitoids cover an area of about 40 km² and is characterized by development of variable attitude of primary foliations mostly marked along the margin of the pluton. Xenoliths of both Shillong Group of metasediments and mafic rocks have been found to occur within Mylliem granitoids. Structural study of the primary foliation is suggestive of funnel-shaped intrusion of Mylliem granitoids with no appreciable evidence of shearing. Petrographically, Mylliem granitoids are characterized by pink to white phenocrysts of prismatic microcline/perthite and lath-shaped plagioclase (An₂₀–An₂₉). Groundmass material is characterized by quartz, microcline, plagioclase, muscovite and biotite. Sphene and apatite occur as accessory minerals. Petrographically Mylliem granitoids have been discriminated as granite and granodiorite according to IUGS system of classification.     

Significance of selective crystal entrainment and differential crystal-melt separation in petrogenesis of granites from the Tongbai orogen

Partial melting has been shown to be an important mechanism for intracrustal differentiation and granite petrogenesis. However, a series of compositional differences between granitic melt from experiments and natural granites indicate that the processes of crustal differentiation are complex. To shed light on factors that control the processes of crustal differentiation, and then the compositions of granitic magma, a combined study of petrology and geochemistry was carried out for granites (in the forms of granitic veins and parautochthonous granite) from a granulite terrane in the Tongbai orogen, China. These granites are characterized by high SiO₂ (>72 wt%) and low FeO and MgO (<4 wt%) with low Na₂O/K₂O ratios (<0.7). Minerals in these granites show variable microstructures and compositions. Phase equilibrium modelling using P–T pseudosections shows that neither anatectic melts nor fractionated melts match the compositions of the target granites, challenging the conventional paradigm that granites are the crystallized product of pure granitic melts. Based on the microstructural features of minerals in the granites, and a comparison of their compositions with crystallized minerals from anatectic melts and minerals in granulites, the minerals in these granitoids are considered to have three origins. The first is entrained garnets, which show comparable compositions with those in host granulites. The second is early crystallized mineral from melts, which include large plagioclase and K-feldspar (with high Ca contents) crystals as well as a part of biotite whose compositions can be reproduced by crystallization of the anatectic melts. The compositions of other minerals such as small grained plagioclase, K-feldspar and anorthoclase in the granites with low Ca contents are not well reconstructed, so they are considered as the third origin of crystallized products of fractionated melts. The results of mass balance calculation show that the compositions of these granites can be produced by mixing between different proportions of crystallized minerals and fractionated melts with variable amounts of entrained minerals. However, the calculated modal proportions of different crystallized minerals (plagioclase, K-feldspar, biotite and quartz) in the granites are significantly different from those predicted by melt crystallization modelling. Specifically, some rocks have lower modes of biotite and plagioclase, whereas others show lower K-feldspar modes than those produced by melt crystallization. This indicates that the crystallized minerals would be differentially separated from the primary magmas to form the evolved magmas that produce these granites. Therefore, the crystal entrainment and differential melt-crystal separation make important contributions to the composition of the target granites. Compared with leucogranites worldwide, the target granites show comparable compositions. As such, the leucogranites may form through the crystal fractionation of primary granitic magmas at different extents in addition to variable degrees of partial melting.     

^40Ar／^39Ar Plateau Age Spectra of Feldspars and Their Geological Implications

The paragenic minerals plagioclase,perthite,biotite,hornblende and pyroxene in acid-granite alkali-granite,monzonite and volcanic rocks collected from seven areas different in thermal history have been determined.On the basis of 14 plateau age spectra and isochron ages of paragenic miner-als in conjunction with the observed mineral textures ,the suitability of plateau age spectra of plagioclase and perthite and their thermo-chronological significance are discussed in this paper.The results indicate that undisturbed feldspar gives satisfactory^40Ar-^39Ar plateau ages in consis-tence with those of paragenic minerals.This means that feldspars from the undisturbed area are suita-ble for ^40Ar-^39Ar dating .On the other hand,the age spectra of feldspars as well as of biotite,pyroxene and hornblende affected by tectonic or thermodynamic events appear unsmooth at varying temperatures,thus complicating their interpretation.Feldspars may give an emplacement age of a rock and /or that of the latest thermodynamic event,depending on the intensity of the event and the retentivity of Ar in the mineral.     

Petrology,geochemistry and genesis of Kuiqi granite batholith

The Kuiqi granite batholith outcrops in the vicinity of Fuzhou City, Fujian Province and constitutes one of the typical alkali granitic complexes in the “Belt of Miarolitic Granites” extending along the southeast coast of China. The complex is believed to have been emplaced at higher levels of the crust in a tensional fault environment. Petrographically it is composed mainly of aegirine-arfvedsonite granites with early biotite granites scattered. Miarolitic structure and granophyric texture are commonly observed. The Rb-Sr isochron age of the complex is 107.65 m.y. Both petrological and petrochemical studies show that the Kuiqi granite is of A-type. Data on chemical composition, REE pattern and transition elements reveal that there is a close genetic connection between granites and associated volcanic rocks. Thus, syntexistype (I-type) granite, A-type granite and volcanic rocks form a cogenetic “trinity”, in which the A-type granite is usually the latest member of the volcanic-intrusive series.     

Geochemistry of the uranium-thurium-bearing granitic rocks and pegmatites of wadi haleifiya area, southeastern Sinai, Egypt

There are two main granitic rocks cropping out in the study area:1) the syn-orogenic granites are moderately weathered,jointed,exfoliated and characterized by low relief.These rocks are subdivided into tonalite and granodiorite.They are essentially composed of plagioclase,quartz,biotite,hornblende and potash feldspar;and 2) the post-orogenic granites,characterized by high relief terrain and represented by monzogranite,syenogranite and alkali granite.The monzogranites suffered hydrothermal alteration in particular along joints,faults,shear zones and fractures,which recorded the highest values of radioactivity,reflecting the role of post-magmatic alteration processes in the enhancement of radioactivity.The hydrothermal alteration(desilicification and hematitization) resulted in the formation of mineralized(altered) granites.The altered granites are enriched in TiO 2,Al 2 O 3,FeO T,MnO,MgO,Na 2 O,Rb,Sr,Y,Zr,Zn,Ga and Co and depleted in SiO 2,CaO,P 2 O 5,Nb,Pb,Cu,Ni and Cr relative to the fresh monzogranite.The investigated granites contain basic xenoliths as well as pockets of pegmatites.Perthites,quartz,plagioclase and sometimes biotite,represent the essential constituents.Some accessory minerals like zircon are metamicted reflecting their radiogenic nature.The alkali granites are characterized by the presence of aegirine,rebeckite and arfvedsonite.Both syn-and post-orogenic granites show some variations in their bulk chemical compositions.The older granitoids are metaluminous and exhibit characteristics of I-type granites and possess an arc tectonic environment.On the other hand,the younger granites are peraluminous and exhibit the characteristics of post-collisional granites.It is interpreted that radioactivity of the studied rocks is mainly controlled by both magmatic and post-magmatic activities.Frequently,the post-orogenic granites host zoned and unzoned pegmatite pockets.Some of these pockets anomalously attain high radioactivity.The syenogranites and the pegmatites are characterized by high contents of SiO 2 and K 2 O and low CaO and MgO.They have transitional characters from highly fractionated calc-alkaline to alkaline.The alkali granites related to A2-subtype of A-type granites.The post-orogenic granites were originated from magma of dominant crustal source materials and related to post-collisional setting under extensional environment.     

Perogenesis of granites, Sharm El-Sheikh area, South Sinai, Egypt: petrological constrains and tectonic evolution

Precambrian granites of the Sharm El-Sheikh area in south Sinai, Egypt belong to collisional and post-collisional Magmatism (610–580 Ma). The granites are widely distributed in the northern part of the Neoproterozoic Arabian-Nubian Shield. South Sinai includes important components of successive multiple stages of upper crust granitic rocks. The earliest stages include monzogranite and syenogranites while the later stages produced alkali feldspar granites and riebeckite-bearing granites. Numerous felsic, mafic dikes and quartz veins traverse the study granites. Petrographically, the granitic rocks consist mainly of perthite, plagioclase, quartz, biotite and riebeckite. Analysis results portray monzogranites displaying calc-alkaline characteristics and emplaced in island-arc tectonic settings, whereas the syenogranites, alkali-feldspar granites and the riebeckite bearing-granites exhibit an alkaline nature and are enriched in HFSEs similar to granites within an extensional regime. Multi-element variation diagrams and geochemical characteristics reinforce a post-collision tectonic setting. REEs geochemical modeling reveals that the rocks were generated as a result of partial melting and fractionation of lower crust basaltic magma giving rise to A₁ and A₂ subtype granites. They were subsequently emplaced within an intraplate environment at the end of the Pan-African Orogeny.     

Late Neoproterozoic alkaline magmatism in the Arabian–Nubian Shield: the postcollisional A-type granite of Sahara–Umm Adawi pluton, Sinai, Egypt

The Sahara–Umm Adawi pluton is a Late Neoproterozoic postcollisional A-type granitoid pluton in Sinai segment of the Arabian–Nubian Shield that was emplaced within voluminous calc-alkaline I-type granite host rocks during the waning stages of the Pan-African orogeny and termination of a tectonomagmatic compressive cycle. The western part of the pluton is downthrown by clysmic faults and buried beneath the Suez rift valley sedimentary fill, while the exposed part is dissected by later Tertiary basaltic dykes and crosscut along with its host rocks by a series of NNE-trending faults. This A-type granite pluton is made up wholly of hypersolvus alkali feldspar granite and is composed of perthite, quartz, alkali amphibole, plagioclase, Fe-rich red biotite, accessory zircon, apatite, and allanite. The pluton rocks are highly evolved ferroan, alkaline, and peralkaline to mildly peraluminous A-type granites, displaying the typical geochemical characteristics of A-type granites with high SiO₂, Na₂O + K₂O, FeO*/MgO, Ga/Al, Zr, Nb, Ga, Y, Ce, and rare earth elements (REE) and low CaO, MgO, Ba, and Sr. Their trace and REE characteristics along with the use of various discrimination schemes revealed their correspondence to magmas derived from crustal sources that has gone through a continent–continent collision (postorogenic or postcollisional), with minor contribution from mantle source similar to ocean island basalt. The assumption of crustal source derivation and postcollisional setting is substantiated by highly evolved nature of this pluton and the absence of any syenitic or more primitive coeval mafic rocks in association with it. The slight mantle signature in the source material of these A-type granites is owed to the juvenile Pan-African Arabian–Nubian Shield (ANS) crust (I-type calc-alkaline) which was acted as a source by partial melting of its rocks and which itself of presumably large mantle source. The extremely high Rb/Sr ratios combined with the obvious Sr, Ba, P, Ti, and Eu depletions clearly indicate that these A-type granites were highly evolved and require advanced fractional crystallization in upper crustal conditions. Crystallization temperature values inferred average around 929°C which is in consistency with the presumably high temperatures of A-type magmas, whereas the estimated depth of emplacement ranges between 20 and 30 km (upper-middle crustal levels within the 40 km relatively thick ANS crust). The geochronologically preceding Pan-African calc-alkaline I-type continental arc granitoids (the Egyptian old and younger granites) associated with these rocks are thought to be the crustal source of f this A-type granite pluton and others in the Arabian–Nubian Shield by partial melting caused by crustal thickening due to continental collision at termination of the compressive orogeny in the Arabian–Nubian Shield.     

Petrographic and mineralogical features of amblygonite- and spodumene-carrying granites

Granites carrying amblygonite and spodumene are characterized by 1) little if any, biotite; 2) strong alteration of rock-forming minerals; 3) development of potash feldspar over plagioclase in amblygonite granites; 4) development of plagioclase over potash feldspar + an epidote-garnet accessory mineral association in spodumene granite; 5) concentration of rubidium in amblygonite granites rich in potash feldspar. Amplygonite granite is most typical of greisenized granites and spodumene, of albitized granites. Chemically spodumene granites are of sodium type (Na/K>1), the amblygonite varieties are of potassium type (Na/K<1), and accessory epidote and tourmaline of spodumene and amblygonite granites are richer in lithium than those in common granites. Fluorine in the greisenizing solutions promoted transfer and deposition of lithium as amblygonite. Altered leucocratic granites are most likely to contain lithium minerals. The criterion for lithium presence is its higher content in pneumatolytic and hydrothermal minerals, tourmaline and epidote. — R.M. Hutchinson     

High heat producing radioactive granites of malani igneous suite at northeast of Jodhpur,northwestern India

The Malani igneous suite, a terrain showing crustal formation as late as in Neoproterozoic, shows some pink and grey granites in the northeast of the desert city of Jodhpur, in northwestern, India. The average heat generation value of 15.925 HGU for these granites that is much higher than the average known value (3.8 HGU) for the continental crust has been reported here. The concentration of uranium determined is four times higher than the average continental crust. Thorium is still higher than U and K. The radioelement concentration (Ur) varies from 15.58 to 73.48 in the granites with an average of 45.671, clearly indicates a ‘hot crust’. Hence it is favourable for the formation of mineralization of HFS elements like, Nb, Ce, REE and U and Th, which need to be explored in the terrain as an economic deposit.     

Geochemistry and petrogenesis of Proterozoic granitic rocks from northern margin of the Chotanagpur Gneissic Complex (CGC)

This study presents the geochemical characteristics of granitic rocks located on the northern margin of Chotanagpur Gneissic Complex (CGC), exposed in parts of Gaya district, Bihar and discusses the possible petrogenetic process and source characteristics. These granites are associated with Barabar Anorthosite Complex and Neo-proterozoic Munger–Rajgir group of rocks. The granitic litho-units identified in the field are grey, pink and porphyritic granites. On the basis of geochemical and petrographic characteristics, the grey and pink granites were grouped together as GPG while the porphyritic granites were named as PG. Both GPG and PG are enriched in SiO₂, K₂O, Na₂O, REE (except Eu), Rb, Ba, HFSE (Nb, Y, Zr), depleted in MgO, CaO, Sr and are characterised by high Fe* values, Ga/Al ratios and high Zr saturation temperatures (GPG_avg～ 861 ^°C and PG_avg～ 835 ^°C). The REE patterns for GPG are moderately fractionated with an average (La/Yb)_N～ 4.55 and Eu/Eu* ～ 0.58, than PG which are strongly fractionated with an average (La/Yb)_N～ 31.86 and Eu/Eu* ～ 0.75. These features indicate that the granites have an A-type character. On the basis of geochemical data, we conclude that the granites are probably derived from a predominant crustal source with variable mantle involvement in a post-collisional setting.     

Tin-carrier minerals in metaluminous granites of the western Nanling Range (southern China): Constraints on processes of tin mineralization in oxidized granites

Huashan, Guposhan and Qitianling are three similar and representative metaluminous A-type tin granites in the western Nanling Range, China. They all have a high oxidization state with magnetite as the dominant Fe–Ti oxide. This study presents an understanding of systematic mineralogy of Sn-bearing minerals (biotite, titanite, magnetite and cassiterite) in the three granites. Biotite has an annite composition and both electron-microprobe and LA-ICP-MS analyses indicate trace amounts of tin in biotite (approximately 100–20 ppm). Chloritization of biotite is accompanied by formation of Sn-rich rutile and cassiterite. Titanite has a long history of crystallization from the early-magmatic stage through the late-magmatic stage to the hydrothermal stage. Owing to its solid-solution relationship with malayaite (CaSnSiO₅), titanite always contains tin to various extents. Early-magmatic titanite contains about 0.5 wt.% SnO₂, while the late-magmatic titanite is markedly enriched in tin (on average 14.8 and 3.4 SnO₂ in titanite from the Qitianling and Huashan granites, respectively). Magnetite grains typically display a trellis structure with ilmenite lamellae, where microinclusions of cassiterite (<1 μm in size) are present. This is likely consistent with features of the “oxy-exsolution” process of Sn-bearing titanomagnetite precursor. Cassiterite may be observed as late-magmatic phase, but most commonly appears as an alteration product of other primary minerals. All tin-bearing minerals in the three granites record a complete process of tin mineralization in granite. The features of tin in primary biotite, titanite and magnetite reflect an initial enrichment during the early stage of magmatic crystallization of the Huashan, Guposhan and Qitianling granites. Association of interstitial Sn-titanite and cassiterite suggests further tin enrichment related to fractional crystallization of granitic magmas. Fluids and alteration of primary minerals play an important role in the leaching, concentration and transportation of Sn during hydrothermal processes, which favors vein-type Sn mineralization.     

REE Mineralization of Weathered Crust and Clay Sediment on Granitic Rocks in the Sanyo Belt,SW Japan and the Southern Jiangxi Province,China

Rare earth element (REE) geochemistry and mineralogy have been studied in the weathered crusts derived from the Early Yanshanian (Jurassic) biotite granites of Dabu and Dingnan, as well as in the Indosinian (Permian) muscovite–biotite granite of Aigao in southern Jiangxi province, China, and the weathered crusts and clay sediments on biotite granites in the Sanyo belt, SW Japan, that is, Okayama, Tanakami, and Naegi areas. In all of the weathered crusts, biotite and plagioclase commonly tend to decrease toward the upper part of the profile, whereas kaolinite and residual quartz and K‐feldspar increase. The weathered crusts of the Dingnan granites and some Naegi granites, which are characterized by the enrichment in light REE (LREE) in C horizons, have higher total REE (ΣREE) content than the parent REE‐enriched granites. Weathering of LREE‐bearing apatite and fluorocarbonates in the Dingnan granites and allanite and apatite in some Naegi granites may account for the leaching of LREE at the B horizons. The leached LREE must result in subsequent enrichment of LREE in the C horizons. The enrichment is probably associated with mainly adsorption onto kaolinite and partly formation of possible secondary LREE‐bearing minerals. In Japan it was found that REE mineralization occurs not in the weathered granitic crusts but in reworked clay sediments, especially kaolinite‐rich layers, derived mainly from the weathering materials of REE‐enriched granitic rocks. The clay sediments are more enriched in LREE, which likely adsorbed onto kaolinite. Concentration of heavy REE within almost all the weathered crusts and clay sediments, however, may reflect mainly residual REE‐bearing minerals such as zircon, which originated in the parent granitic rocks. The findings of the present study support the three processes for fractionation of the REE during weathering: (i) selective leaching of rocks containing both stable and unstable REE‐bearing minerals; (ii) adsorption onto clay minerals; and (iii) presence of possible secondary LREE‐bearing minerals.     

Co-development of Jurassic I-type and A-type granites in southern Hunan,South China: Dual control by plate subduction and intraplate mantle upwelling

Two types of spatially and temporally associated Jurassic granitic rocks, I-type and A-type, occur as pluton pairs in several locations in southern Hunan Province, South China. This paper aims to investigate the genetic relationships and tectonic mechanisms of the co-development of distinct granitic rocks through petrological, geochemical and geochronological studies. Zircon LA-ICPMS dating results yielded concordant U–Pb ages ranging from 180 to 148 Ma for the Baoshan and Tongshanling I-type granodiorites, and from 180 to 158 Ma for the counterpart Huangshaping and Tuling A-type granites. Petrologically, the I-type granodiorites consist of mafic minerals such as hornblende whereas the A-type granites are dominated by felsic minerals (e.g., quartz, K-feldspar and plagioclase). Major and trace element analyses indicate that the I-type granodiorites have relatively low SiO₂ (64.5–71.0%) and relatively high TiO₂ (0.28–0.51%), Al₂O₃ (13.8–15.5%), total FeO (2.3–4.7%), MgO (1.3–2.6%) and P₂O₅ (0.10–0.23%) contents, and the A-type granites are characterized by high concentrations of Rb (212–1499?ppm), Th (18.3–52.6?ppm), U (11.8–33.6?ppm), Ga (20.0–36.6?ppm), Y (27.1–134.0?ppm) and HREE (20.3–70.0?ppm), with pronounced negative Eu anomalies (Eu/Eu*?=?0.01–0.15). Moreover, the I-type granodiorites are classified as collision-related granites emplaced under a compressional environment, whereas the A-type granites are within-plate granites generated in an extensional setting. Zircon Hf isotopic compositions vary substantially for these granitic rocks. The I-type granodiorites are characterized by relatively young Hf model ages (T_DM1?=?1065–1302 Ma, T_DM^C =1589–2061 Ma) and moderately negative εHf(t) values (–5.9 to –11.5), whereas the A-type granites have very old model ages (T_DM1?=?1454–2215 Ma, T_DM^C?=?2211–2974 Ma) and pronounced negative εHf(t) values (–15.8 to –28.3). These petrochemical and isotopic characteristics indicate that the I-type granodiorites may have been derived from a deep source involving mantle-derived juvenile (basaltic) and crustal (pelitic) components, whereas the A-type granites may have been sourced from melting of meta-greywacke in the crust. This study proposes that the pressure and temperature differences in the source regions caused by combined effects of intra-plate mantle upwelling and plate subduction are the major controlling factors of the co-development of the two different types of magmas. Crustal anatexis related to lithospheric delamination and upwelling of hot asthenosphere under a high pressure and temperature environment led to the formation of the I-type magmas. On the other hand, the A-type magmas were formed from melting of the shallower part of the crust, where extensional stress was dominant and mantle-crust interaction was relatively weak. Rifts and faults caused by mantle upwelling developed from surface to depth and successively became channels for the ascending I- and A-type magmas, resulting in the emplacement of magmas in adjacent areas from sources at different depths.     

华北板块北缘中段早石炭世构造属性:察哈尔右翼后旗高分异Ⅰ型花岗岩地球化学的制约

察哈尔右翼后旗二长花岗岩岩体位于华北板块北缘中段.岩体富SiO₂、富K、富碱、低Ca和P、贫Fe和Mg,w(P₂O₅)与w(SiO₂)呈负相关,铝指数(A/CNK)为0.96~1.15,分异指数为90.36~92.96;主要造岩矿物为条纹长石、斜长石和石英,其铁镁矿物主要为黑云母,未出现碱性铁镁矿物和富铝矿物,副矿物为锆石、磁铁矿、磷灰石、榍石和钛铁矿;可见察哈尔右翼后旗二长花岗岩属高分异钙碱性I型花岗岩类.结合较低的w(∑REE)((46.8~94.4)×10^-6)、w(Th)((1.74~2.39)×10^-6)、w(U)((0.27~0.39)×10^-6)和微量元素判别图解,二长花岗岩岩浆源区可能为下地壳,源岩可能为岩石圈地幔.岩石轻重稀土分馏较强 ((La/Yb)_N= 6.52~28.39),δEu以正异常为主(0.83~3.51),富集大离子亲石元素(LILEs,Cs、Rb、Ba和K),亏损高场强元素 (HFSEs,Nb、Ta、P和Ti).地球化学特征反映了二长花岗岩具有火山弧岩石特征,且岩体侵入新元古界埃迪卡拉系什那干群,表明岩体形成于活动大陆边缘弧环境,其侵位反映了古亚洲洋的俯冲岩浆事件.     

Geology and petrography of the Nagar Parkar igneous complex,southeastern Sindh,Pakistan: The Kharsar body

Kharsar hill is one of many granitic plutons comprising the Nagar Parkar igneous complex. The eastern part of the hill is occupied by grey-pink granite (earlier) and the western part by pink granite (later). They are composed of perthite, quartz, and plagioclase, with minor opaque oxide, biotite, titanite, local amphibole, and secondary chlorite, epidote, leucoxene/titanite. The pink granite is characterized by the presence of mafic clots. Both the granitoids are intruded by microgranite/aplite, and porphyritic mafic and rhyolite dykes, locally in swarms. These are abundant in a NE trending 200 m wide zone cutting the entire granite hill. The dykes may extend over 1 km in length and >10 m in thickness, but most are < 100 m in length. The felsic dykes are of several generations; some are associated with the two varieties of granite, others are contemporaneous with the rhyolite and mafic dykes. The mafic dykes can be grouped into two types one of which contains hornblende and the other augite as the principal mafic mineral. Major element analyses suggest that the granitic rocks are metaluminous. The Kharsar granites, like the others in Nagar Parkar, may be an extension of the Malani igneous suite of Rajasthan. The occurrence of bimodal mafic-felsic dykes and petrographic variation in the mafic dykes are briefly discussed.     

滇西腾冲—梁河地区花岗岩的年代学、地球化学及其构造意义

滇西腾冲-梁河地区位于喜马拉雅东构造结的东侧,区域内广泛分布的中、新生代花岗岩（简称腾梁花岗岩）由古永岩群、宾榔江岩群的若干个花岗岩体组成,以岩基、岩株、岩墙状态产出。花岗岩呈现带状沿着一系列北北东向弧形断裂平行分布,展示明显的同构造剪切被动侵位和岩墙扩展侵位特征。岩浆锆石SHRIMP U-Pb定年结果显示,东侧的古永岩群花岗岩结晶年龄为白垩世晚期（76~68Ma）;而西侧的槟榔江岩群花岗岩结晶年龄为稍晚的始新世（53Ma）。腾梁花岗岩主要为中、粗粒黑云母二长花岗岩、黑云母条纹长石花岗岩、伟晶花岗岩,缺少典型的富铝矿物。地球化学特征表明腾梁花岗岩是起源于中下地壳的过铝-强过铝高钾钙碱性花岗岩,源岩是富含泥质的硬砂岩,并具有岛弧-后碰撞花岗岩特征。由于喜马拉雅新特提斯封闭及印度陆块与亚洲陆块的陆陆碰撞发生于65Ma, 进一步推测腾梁花岗岩是新特提斯封闭到陆陆碰撞造成陆壳增厚所引起的中下地壳部分熔融的产物。腾梁花岗岩是冈底斯的东延部分,但在形成机制上,与冈底斯花岗岩具有明显的差别。     

河北武安坦岭多斑斜长斑岩中基质矿物特征及其研究意义

河北武安坦岭斜长斑岩具有多斑斑状结构,基质为显微晶质结构。岩相学观察表明,斜长石斑晶有一个宽广的核部和一个宽度可变的条纹长石反应边,个别核部包含有角闪石、黑云母等矿物。基质矿物主要由蓝透闪石、条纹长石(An0Ab8.4Or91.5～An0.1Ab57.3Or42.6)、石英、钾长石(An0.3Ab5.9Or93.7～An0.3Ab4.7Or95.2)、钠长石(An0.2Ab98.3Or1.5～An0.1Ab99.2Or0.7)、磁铁矿、赤铁矿、钛铁矿、磷灰石、榍石和锆石等11种矿物组成。角闪石温压计计算结果得出,基质角闪石核部的结晶压力高于边部,核部为34.05 MPa,对应的结晶温度为660.35℃,结晶深度为1.29km;边部的结晶压力为24.32MPa,结晶温度为598.32℃,结晶深度为0.92km;而斜长石斑晶中的角闪石形成时压力为159.51～178.19MPa,温度为817.68～819.79℃,对应的形成深度为6.03～6.73km。基质角闪石在Al2O3-TiO2图上落在壳源区,而斑晶中的角闪石和黑云母都落在壳幔混合区。斜长石、条纹长石、磁铁矿和磷灰石的微量和稀土元素测试数据显示,其都具有相对富集LILE、亏损HFSE的特点,暗示了基质矿物的形成有流体参与。ICT三维扫描结果显示,斜长斑岩基质中的孔隙体积含量约为3.428%,铁质体积含量为4.371%,且铁氧化物和孔隙具弱连通性。通过讨论分析,笔者得出:(1)坦岭斜长斑岩中斜长石斑晶具有明显的交代结构,且晶体本身没有明显熔蚀现象,这些特征表明大量的斜长石斑晶快速上升,即"冻结岩浆房"的活化机制与流体密切相关;(2)斜长斑岩中基质矿物有十一种,且矿物类型复杂,不符合平衡系统矿物相律,应属于流体晶矿物组合;(3)坦岭斜长斑岩的基质"岩浆"可能是一种富Fe、K、P、Si、Na等元素的熔体-流体流;(4)多斑斜长斑岩的形成经历了(1)深度6～7km的深部岩浆房形成斜长石堆晶→(2)富Fe、K、P、Si、Na等元素的熔体-流体流加入深部岩浆房,冻结岩浆房活化→(3)由于流体超压,含大量斜长石斑晶的熔体-流体在地壳浅部(0.9～1.2km)呈小岩株状或岩脉状就位。多斑斜长斑岩为深部找矿提供了有力的线索。     

Geochemistry and petrogenesis of post-collisional alkaline and peralkaline granites of the Arabian-Nubian Shield: a case study from the southern tip of Sinai Peninsula,Egypt

The southern Sinai Peninsula, underlain by the northernmost extension of the Arabian-Nubian Shield, exposes post-collisional calc-alkaline and alkaline granites that represent the youngest phase of late Neoproterozoic igneous activity. We report a petrographic, mineralogical and geochemical investigation of post-collisional plutons of alkaline and, in some cases, peralkaline granite. These granites intrude metamorphosed country rocks as well as syn- and post-collisional calc-alkaline granitoids. The alkaline and peralkaline granites of the southern tip of Sinai divide into three subgroups: syenogranite, alkali feldspar granite and riebeckite granite. The rocks of these subgroups essentially consist of alkali feldspar and quartz with variable amounts of plagioclase and mafic minerals. The syenogranite and alkali feldspar granite contain small amounts of calcic amphibole and biotite, often less than 3%, while the riebeckite granite is distinguished by sodic amphibole (5–10%). These plutons have geochemical signatures typical of post-collisional A-type granites and were most likely emplaced during a transition between orogenic and anorogenic settings. The parental mafic magma may be linked to lithospheric delamination and upwelling of asthenospheric mantle material. Differentiation of the underplated basaltic magma with contributions from the juvenile crust eventually yielded the post-collisional alkaline granites. Petrogenetic modelling of the studied granitic suite shows that pure fractional crystallization cannot quantitatively explain chemical variations with the observed suite, with both major oxides and several trace elements displaying trends opposite to those required by the equilibrium phase assemblage. Instead, we show that compositional variation from syenogranite through alkali feldspar granite to riebeckite granite is dominated by mixing between a low-SiO₂ liquid as primitive or more primitive than the lowest-SiO₂ syenogranite and an evolved, high-SiO₂ liquid that might be a high-degree partial melt of lower crust.     

An Early Palaeozoic HP/HT granulite–garnet peridotite association in the south Altyn Tagh, NW China: P–T history and U-Pb geochronology

High‐pressure kyanite‐bearing felsic granulites in the Bashiwake area of the south Altyn Tagh (SAT) subduction–collision complex enclose mafic granulites and garnet peridotite‐hosted sapphirine‐bearing metabasites. The predominant felsic granulites are garnet + quartz + ternary feldspar (now perthite) rocks containing kyanite, plagioclase, biotite, rutile, spinel, corundum, and minor zircon and apatite. The quartz‐bearing mafic granulites contain a peak pressure assemblage of garnet + clinopyroxene + ternary feldspar (now mesoperthite) + quartz + rutile. The sapphirine‐bearing metabasites occur as mafic layers in garnet peridotite. Petrographical data suggest a peak assemblage of garnet + clinopyroxene + kyanite + rutile. Early kyanite is inferred from a symplectite of sapphirine + corundum + plagioclase ± spinel, interpreted to have formed during decompression. Garnet peridotite contains an assemblage of garnet + olivine + orthopyroxene + clinopyroxene. Thermobarometry indicates that all rock types experienced peak P–T conditions of 18.5–27.3 kbar and 870–1050 °C. A medium–high pressure granulite facies overprint (780–820 °C, 9.5–12 kbar) is defined by the formation of secondary clinopyroxene ± orthopyroxene + plagioclase at the expense of garnet and early clinopyroxene in the mafic granulites, as well as by growth of spinel and plagioclase at the expense of garnet and kyanite in the felsic granulite. SHRIMP II zircon U‐Pb geochronology yields ages of 493 ± 7 Ma (mean of 11) from the felsic granulite, 497 ± 11 Ma (mean of 11) from sapphirine‐bearing metabasite and 501 ± 16 Ma (mean of 10) from garnet peridotite. Rounded zircon morphology, cathodoluminescence (CL) sector zoning, and inclusions of peak metamorphic minerals indicate these ages reflect HP/HT metamorphism. Similar ages determined for eclogites from the western segment of the SAT suggest that the same continental subduction/collision event may be responsible for HP metamorphism in both areas.