共查询到20条相似文献，搜索用时 31 毫秒
1.
Residence of REE, Y, Th and U in Granites and Crustal Protoliths; Implications for the Chemistry of Crustal Melts 总被引：36，自引：8，他引：28
A systematic study with laser ablation—ICP-MS, scanningelectron microscopy and electron microprobe revealed that 70–95wt% of REE (except Eu), Y, Th and U in granite rocks and crustalprotoliths reside within REEYThU-rich accessories whose nature,composition and associations change with the rock aluminosity.The accessory assemblage of peraluminous granites, migmatitesand high-grade rocks is composed of monazite, xenotime (in low-Cavarieties), apatite, zircon, Thorthosilicate, uraninite andbetafite-pyrochlore. Metaluminous granites have allanite, sphene,apatite, zircon, monazite and Thorthosilicaie. Peralkaline graniteshave aeschinite, fergusonite, samarskite, bastnaesite, fluocerite,allanite, sphene, zircon, monazite, xenotime and Th-orthosilicate.Granulite-grade garnets are enriched in Nd and Sm by no lessthan one order of magnitude with respect to amphibolite-gradegarnets. Granulitegrade feldspars are also enriched in LREEwith respect to amphibolite-grade feldspars. Accessories causenon-Henrian behaviour of REE, Y, Th and U during melt—solidpartitioning. Because elevated fractions of monazite, xenotimeand zircon in common migmatites are included within major minerals,their behaviour during anatexis is controlled by that of theirhost. Settling curves calculated for a convecting magma showthat accessories are too small to settle appreciably, beingseparated from the melt as inclusions within larger minerals.Biotite has the greatest tendency to include accessories, therebyindirectly controlling the geochemistry of REE, Y, Th and U.We conclude that REE, Y, Th and U are unsuitable for petrogeneticalmodelling of granitoids through equilibrium-based trace-elementfractionation equations. KEY WORDS: accessory minerals; geochemical modelling; granitoids; REE, Y, Th, U 相似文献
2.
稀土元素地球化学对太古宙花岗岩类成因的判别 总被引：1，自引：0，他引：1
太古宙花岗岩类的成因是地学界争论颇久的问题。本文以稀土元素地球化学理论论证了北京、辽吉地区某些花岗岩为岩浆成因而非混合岩构成的“地层”。文章论述了主元素特征为钾质花岗岩的岩体实是钾化的TTG岩体 ,论述了主元素成分相同的TTG岩石具不同的稀土图谱 ;被长英质细脉注入的TTG岩石受混染作用改造稀土图谱也发生了变化 ;各种各样非TTG成分的岩石由于硅质的渗透被改造为TTG质岩石。这些实例说明 ,必须进行岩石学、矿物学和地球化学的综合研究才能判定太古宙形形色色的花岗质岩石。 相似文献
3.
《Geochimica et cosmochimica acta》1999,63(7-8):1133-1153
This study is aimed at understanding the behavior of monazite, xenotime, apatite and zircon, and the redistribution of Zr, REE, Y, Th, and U among melt, rock-forming and accessory phases in a prograde metamorphic sequence, the Kinzigite Formation of Ivrea-Verbano, NW Italy, that may represent a section from the middle to lower continental crust. Metamorphism ranges from middle amphibolite to granulite facies and metapelites show evidence of intense partial melting and melt extraction. The appearance of melt controls the grain size, fraction of inclusions and redistribution of REE, Y, Th, and U among accessories and major minerals. The textural evolution of zircon and monazite follows, in general, the model of Watson et al. (1989). Apatite is extracted from the system dissolved into partial melts. Xenotime is consumed in garnet-forming reactions and is the first source for the elevated Y and HREE contents of garnet. Once xenotime is exhausted, monazite, apatite, zircon, K-feldspar, and plagioclase are progressively depleted in Y, HREE, and MREE as the modal abundance of garnet increases. Monazite is severely affected by two retrograde reactions, which may have consequences for U-Pb dating of this mineral. Granulite-grade metapelites (stronalites) are significantly richer in Ti, Al, Fe, Mg, Sc, V, Cr, Zn, Y, and HREE, and poorer in Li, Na, K, Rb, Cs, Tl, U, and P, but have roughly the same average concentration of Cu, Sr, Pb, Zr, Ba, LREE, and Th as amphibolite-grade metapelites (kinzigites). The kinzigite-stronalite transition is marked by the sudden change of Th/U from 5–6 to 14–15, the progressive increase of Nb/Ta, and the decoupling of Ho from Y. Leucosomes were saturated in zircon, apatite, and (except at the lowest degree of partial melting) monazite. Their REE patterns, especially the magnitude of the Eu anomaly, depend on the relative proportion of feldspars and monazite incorporated into the melt. The presence of monazite in the source causes an excellent correlation of LREE and Th, with nearly constant Nd/Th ≈ 2.5–3. The U depletion and increase in Th/U characteristic of granulite facies only happens in monazite-bearing rocks. It is attributed to enhancement of the U partitioning in the melt due to elevated Cl activity followed by the release of a Cl-rich F-poor aqueous fluid at the end of the crystallization of leucosomes. Halide activity in partial melts was buffered by monazite and apatite. Since the U (and K) depletion does not substantially affect the heat-production of metapelites, and mafic granulites maintain similar Th/U and abundance of U and Th as their unmetamorphosed equivalents, it seems that geochemical changes associated to granulitization have only a minor influence on heat-production in the lower crust. 相似文献
4.
千里山花岗岩体地质地球化学及与成矿关系 总被引：51，自引：6，他引：51
千里山花岗岩由似斑状黑云母花岗岩、等粒黑云母花岗岩和花岗斑岩组成。前两期岩体分别与两期钨多金属矿化有关，后一期与铅锌银成矿密切相联。两套花岗岩虽然均来自地壳，但取于不同源地。该岩体既为富Ｆ、Ｌｉ，Ｒｂ，Ｂｅ，Ｇａ的ＢＥＬＩＦ花岗岩，又是富Ｕ，Ｔｈ的高热花岗岩。 相似文献
5.
The relationships between mineralization and magmatism during the formation of the Early Mesozoic West Transbaikal beryllium province are exemplified in the Urma helvite-bertrandite deposit. The deposit is drawn toward granitoids of elevated alkalinity, which belong to the Tashir Complex. Mineralization is related to leucogranite and characterized by patched distribution controlled by localization of metasomatic alteration. The latter is identified owing to replacement of feldspar with microcline and albite followed by silicification related to fracture zones. Helvite and bertrandite are the major Be minerals at the deposit. The Be grade of the ore is nonuniform and varies from 740 to 25000 ppm. Zircon, malacon, monazite, allanite, bastnaesite, columbite, and xenotime occur in metasomatic rocks together with Be minerals. Geochemical characteristics of alkali granites and metasomatic rocks are similar in a wide range of incompatible elements. Both are characterized by lowered Ba, Sr, P, and Eu contents and enriched in Th, U, Pb, Zr, and Hf. The degree of enrichment is the highest in the ore. The Be content in the ore correlates with concentrations of a number of other rare metals typical of host granite, which form their own mineralization against the background of metasomatic alteration, including Zr and REE minerals. Similarity in geochemistry of granitic rocks and Be ore indicates that the Urma deposit was related to the evolution of magmatic melt. Regional correlation shows that the ore-magmatic system of the Urma deposit is close to that of the Orot deposit, one of the largest in the central segment of the West Transbaikal metallogenic province. Both deposits are characterized by a similar composition of granitoids and comparable localization of ore zones in the structure of plutons. This similarity supports the high ore resource potential of Early Mesozoic alkali granites in the western Transbaikal region. Taking into account that these granitoids are widespread in the West Transbaikal Rift Zone that controls the metallogenic province, one can expect the discovery of new deposits therein. 相似文献
6.
Zr, Hf, U, Th and REE-Fertile Lower Proterozoic Potassic Granite from Parts of Andhra Pradesh, South India 总被引：1，自引：0，他引：1
Yamuna SINGH 《《地质学报》英文版》2004,78(4):921-930
The medium- to coarse-grained and porphyritic granitoid of Dharmawaram, Karimnagar district, Andhra Pradesh, south India is a biotite-hornblende granite with notable contents of rare metal (Zr, Hf, Th) and rare earth (including Y) minerals like zircon, thorite, allanite, monazite and xenotime. Chemically, it is metaluminous (average A/ C+N+K = 0.95)-type, potassic (av. 5% K2O) granite, with dominantly sub-alkaline characters. It shows up to 8 times enrichment of rare metals (Zr, Hf, U, Th) and rare earths (including Y, Sc), with reference to their abundances in normal unevolved granite, and hence, fertile for some of these elements. Field, petrological, geochemical and isotopic data of potassic granite (PG) indicate involvement of silica-rich metasedimentary-basic crustal rocks (amphibole-quartzite, amphibolite, hornblende-biotite gneiss, etc.) in its genesis, at a depth range of 30 km. Further, chondrite-normalized REE patterns demonstrate that low-degree partial melting of source rocks is the major con 相似文献
7.
8.
华南是我国重要的战略性矿产资源基地,以花岗岩相关的稀有和稀土金属成矿作用而举世瞩目。其中,铌的成矿作用一般与过铝质高分异花岗岩有关,稀土元素则随岩浆演化程度增强而富集程度降低,而江西铁木里含黑云母碱长花岗岩体同时富集铌和稀土元素,矿化组合极具特色。本文在详细的矿物岩相学研究基础上,利用电子探针、飞秒激光电感耦合等离子质谱对铌和稀土矿物进行了矿物地球化学分析,借此对铁木里碱长花岗岩中铌和稀土元素的富集机制进行探讨。铁木里岩体由肉红色含黑云母碱长花岗岩(r-G)和灰白色含黑云母碱长花岗岩(g-G)组成,发育暗色包体。r-G中的铌矿物主要为岩浆期形成的铌铁金红石,稀土矿物包括岩浆期形成的硅钛铈矿、独居石、磷灰石和热液期形成的独居石和氟碳(钙)铈矿。g-G中的铌矿物包括岩浆期形成的铌铁金红石和热液期形成的铌铁金红石、易解石、铌铁矿,稀土矿物包括岩浆期磷灰石和热液期磷灰石、独居石、氟碳(钙)铈矿。暗色包体为岩浆混合成因,内含磷灰石、独居石和零星的硅钛铈矿、金红石。矿物组合特征显示,铁木里碱长花岗岩中的铌和稀土元素经过了岩浆和热液两个时期的富集。应用金红石、磷灰石、绿泥石等矿物成分特征约束了岩浆-... 相似文献
9.
南岭稀土花岗岩、钨锡花岗岩及其成矿作用的对比 总被引：15，自引：3，他引：15
南岭地区的钨锡和稀土矿床都与花岗岩类有直接成因联系,但二者的成矿作用有许多不同之处.钨锡是典型的热液成矿,而稀土则主要形成于风化作用.随着花岗岩类的分异演化,岩石中的W、Sn等元素含量逐渐增加,因此钨锡等矿床主要与高度分异演化的晚阶段小岩体有关;但是稀土的表现与钨锡不同,由于花岗岩类的分异演化导致稀土栽体黑云母及许多副矿物的减少,因此稀土元素含量在晚阶段岩体中反而降低.赣南的五里亭-大吉山岩体、桂东北的花山-姑婆山岩体等提供了很好的范例.因此,南岭地区与风化壳型稀土矿床有关的岩石主要有:印支期准铝质花岗岩,燕山期A型花岗岩,燕山中-晚期黑云母二长花岗岩等. 相似文献
10.
诸广山南体是中国重要的铀资源基地之一,复式岩体内与铀矿床关系密切的花岗岩均为S型,I型花岗岩一般无铀成
矿潜力。三江口岩体位于诸广山南体西部,与产有铀矿床的燕山期长江岩体相邻,岩性也与长江岩体相似。目前对三江口
岩体研究程度相对薄弱,缺乏高精度年代学研究,岩石成因类型也未明确。文章对三江口岩体东部（简称三江口东岩体）
进行U-Pb年代学和岩石地球化学研究,并与产铀的长江岩体进行对比。LA-ICP-MS锆石U-Pb年龄为161.9±2.1 Ma,属于
燕山早期花岗岩。岩体富SiO2 （73.5%~76.1%）,贫FeOT （1.12%~3.25%）、MgO（0.07%~0.83%）、CaO（0.64%~1.27%）,具
高分异指数DI（86.4~93.6）,A/CNK值为1.00~1.35,属过铝-强过铝质花岗岩;微量元素Ba、Sr、Nb、Eu、Ti亏损,Rb、
Th、U富集,属于典型的低Ba、Sr花岗岩;稀土总量中等（ΣREE = 119×10-6 ~268×10-6）,稀土配分模式为右倾的轻稀土富
集型;(87Sr/86Sr)i值为0.70789~0.71488,εNd(t )值较低（-10.8~-9.6）,两阶段Nd模式年龄为1.73~1.83 Ga。三江口东岩体与长
江岩体年龄相近,具有相似的岩石地球化学特征和同位素组成,均为S型花岗岩。结合两岩体形成年龄和区域构造背景,
认为其形成与华南燕山早期陆内伸展作用有关,是由华南基底麻源群泥质岩、砂质岩部分熔融形成,在成岩过程中有少量
幔源物质参与。通过与产铀的长江岩体对比研究认为,三江口东岩体具有较强的产铀潜力。 相似文献
11.
12.
REE Mineralization of Weathered Crust and Clay Sediment on Granitic Rocks in the Sanyo Belt,SW Japan and the Southern Jiangxi Province,China 总被引：1，自引：0，他引：1
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. 相似文献
13.
Differential Fractionation of Rare Earth Elements in Oxidized and Reduced Granitic Rocks: Implication for Heavy Rare Earth Enriched Ion Adsorption Mineralization 
下载免费PDF全文
下载免费PDF全文 Ion adsorption rare earth element (REE) deposits in southern China are the exclusive source of heavy REEs (HREEs) in the world, and this HREE‐enriched character of the deposits is inherited from the REE compositions of the underlying granitic rocks. Such HREE‐enriched rocks form from heavy fractionation of reduced granitic magmas. We explore why reduced granitic magmas are enriched in HREEs during the fractionation, based on the REE geochemistry of granitic rocks and abundance of REEs in their constituent minerals in the southwestern Japan arc of Cretaceous to Paleogene age. The compilation of the whole rock geochemistry and REE compositions of the granitic rocks of the Sanin (oxidized), Sanyo (reduced) and Ryoke (reduced) belts in the southwestern Japan arc indicates that: (i) light REEs (LREEs) decease with fractionation of the granitoids in the Sanin belt but this trend is not clear in the granitoids in the Sanyo belt and LREEs rather increase in the Ryoke granitoids; (ii) Eu decreases with fractionation in all the belts; and (iii) HREEs slightly, but steadily decrease in the Sanin belt but enrich significantly in the Sanyo and Ryoke belts with fractionation. Analytical results of REE concentrations by scanning electron microscope with energy dispersive X‐ray spectroscope and laser ablation‐inductively coupled plasma mass spectrometer in the constituent minerals in a granodiorite sample from the Sanin belt show a moderate concentration of REEs in hornblende (577 ppm) in addition to high concentrations in allanite (~20 %), britholite (~30 %), primary titanite (8922 ppm), apatite (4062 ppm), and zircon (1693 ppm). Because primary titanite and allanite are commonly present in the oxidized granitoids but not in the reduced ones, the REE depletion in the fractionated, oxidized granites is attributed to the crystallization of these minerals. In contrast, scarcity of these minerals in the reduced granitoids enriches REEs, in particular HREEs in the fractionated magmas, which finally precipitate REEs in the granites and pegmatites. Both positive, but different correlation ratios between the Nb and Dy concentrations in the granitoids of the Sanin and Sanyo‐Ryoke belts suggest that columbite–pyrochlore‐group and fergusonite‐group minerals are the major HREE host in the oxidized and reduced granites, respectively. 相似文献
14.
大兴安岭中部柴河地区钾长花岗岩的成因及构造背景:岩石地球化学、锆石U-Pb同位素年代学的制约 总被引：6，自引：3，他引：3
对大兴安岭中部柴河林场地区钾长花岗岩进行了系统的地球化学及锆石U-Pb同位素年代学研究,并对其岩石成因及构造意义进行讨论.研究结果表明,柴河林场地区钾长花岗岩中锆石具有典型的岩浆振荡生长环带和较高的Th/U比值(0.44 ～ 1.42),反映了岩浆成因特征.LA-ICP-MS锆石U-Pb定年结果为133±3Ma.岩石具有富硅、稀土元素含量较高、相对富集轻稀土元素亏损重稀土元素的特征,Eu负异常较为明显,稀土元素配分图解具有右斜“V”字型的特征,并相对富集高场强元素和大离子亲石元素.以上特征表明,柴河林场地区钾长花岗岩为铝质A型花岗岩,是地壳岩石部分熔融的产物,具有典型A1型(裂谷或板内)花岗岩的特征,代表了伸展的构造环境. 相似文献
15.
The Sakarya Zone is intruded by several Late Paleozoic granitoids, of which the Sar?cakaya intrusive rocks in the central Sakarya (Eski?ehir) region, is the least-studied. The Sar?cakaya intrusive rocks consist mainly of quartz diorite-granodiorite, granodiorite and granite. They are, geochemically, divided into two groups: diorites and granites. The former is medium-K and calc-alkaline (mainly I-type), whereas the latter is high-K to shoshonite and calcic (I-type). Typical minerals for both rock types are plagioclase, K-feldspar, quartz, biotite, hornblende and Fe–Ti oxides. Chondrite-normalized REE patterns for the Sar?cakaya intrusive rocks are moderately fractionated and have small negative Eu anomalies. They are enriched in LILE and LREE relative to HFSE showing characteristics of arc-related granitoids. Geochemical characteristics of the Sar?cakaya intrusive rocks indicate a hybrid origin through partial melting of lower crustal source rocks. 相似文献
16.
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. 相似文献
17.
Dewashish Upadhyay Kamal Lochan Pruseth 《Contributions to Mineralogy and Petrology》2012,164(2):303-316
Primary igneous monazite from the Polokongka La granite of the Tso Morari complex in the western Himalayas has been partially replaced by a three-layered corona of metamorphic fluor-apatite, allanite + U- and Th-bearing phases (huttonite + brabantite), and epidote. The alteration is related to high-pressure amphibolite-facies (10–11 kbar and 587–695 °C) fluid-induced retrogression of the ultra-high-pressure granite during exhumation after India–Asia collision. The corona textures can be explained by pseudomorphic partial replacement of the original monazite to apatite and allanite via a fluid-mediated coupled dissolution–reprecipitation process. Mass balance calculations using the volume proportions and compositions of coronal minerals show that the REE, U, Th, Pb, Ba and P were conserved and not transported outside the alteration corona. The formation of fluor-apatite, allanite, huttonite and coffinite from monazite and the immobility of REE, U and Th require an influx of alkali- and F-bearing, Ca-rich fluid having high Ca/Na into the corona. We are aware of only two other occurrences of such alteration textures, and these have several similarities in terms of geodynamic setting and P–T histories of the host rocks. We suggest that there may be a common mechanism of exhumation style, and source and composition of fluids during retrogression of granitoid rocks in collisional orogens and that such breakdown textures can be used to identify metagranites that have experienced high-P metamorphism in continental collision zones, which is otherwise difficult to constrain due to the high variance of the mineral assemblages in these rocks. 相似文献
18.
The Younger Granites of Yahmid-Um Adawi area, located in the southeastern part of Sinai Peninsula, comprise two coeval Late Neoproterozoic post-collisional alkaline (hypersolvous alkali-feldspar granites; 608–580?Ma) and calc-alkaline (transsolvous monzo- and syenogranites; 635–590?Ma) suites. The calc-alkaline suite granitoids are magnesian and peraluminous to metaluminous, whereas the alkaline ones are magnesian to ferroan alkaline to slightly metaluminous. Both granitoid suites exhibit many of the typical geochemical features of A-type granites such as enrichment in Nb (>20?ppm), Zr (>250?ppm), Zn (>100?ppm) and Ce (>100?ppm) and high 10000*Ga/Al2O3 ratios (>2.6) and Zr?+?Nb?+?Y?+?Ce (>350?ppm). Accessory mineral saturation thermometers demonstrated former crystallization of apatite at high temperatures prior to zircon and monazite separation from the magma for both granitoid suites. The mild zircon saturation temperatures of the studied Younger Granites (around 800?°C) imply low-temperature crustal fusion and incomplete melting of the largely refractory zircon. The two Younger Granite suites were semi-synchronously evolved during the post-collisional stage of the Arabian-Nubian Shield subsequent to the collision between the juvenile shield crust and the older pre-Neoproterozoic continental blocks of west Gondwana. Their parental magmas has been generated by melting of crustal source rocks with minor involvement from mantle, which might participated chiefly as a source of heat necessary for fusion of the crustal precursor. Extensive in-situ gamma-ray spectrometry revealed anomalously high radioactivity of some Younger Granite exposures along Wadi Um Adawi (eU; 388–746?ppm and eTh; 1857–2527?ppm) and pegmatitic pockets pertaining to the calc-alkaline suite (equivalent U and Th; 212–252?ppm and 750–1757?ppm, respectively). The radioactivity of the syngenetic pegmatites arises from the primary radioactive minerals uranothorite and thorite together with the U- and/or Th-bearing minerals zircon, columbite, samarskite and monazite. The anomalously high radioactivity of some Younger Granite exposures in Wadi Um Adawi stem from their appreciable enclosure of the epigenetic uranium minerals metatorbenite and uranophane. 相似文献
19.
Pei-Shan Hsieh Cheng-Hong Chen Huai-Jen Yang Chi-Yu Lee 《Journal of Asian Earth Sciences》2008,33(5-6):428-451
The widespread Mesozoic granitoids in South China (135,300 km2) were emplaced in three main periods: Triassic (16% of the total surface area of Mesozoic granitoids), Jurassic (47%), and Cretaceous (37%). Though much study has been conducted on the most abundant Jurassic Nanling Mountains (NLM) granites, their rock affinities relative to the Triassic Darongshan (DRS) and Cretaceous Fuzhou–Zhangzhou Complex (FZC) granites which are typical S- and I-type, respectively, and the issue of their petrogenetic evolution is still the subject of much debate. In this study, we discuss the petrogenesis of NLM granites using apatite geochemistry combined with whole-rock geochemical and Sr–Nd isotope compositions. Sixteen apatite samples from six granite batholiths, one gabbro, and three syenite bodies in the NLM area were analyzed for their major and trace element abundances and compared with those collected from DRS (n = 7) and FZC (n = 6) granites. The apatite geochemistry reveals that Na, Si, S, Mn, Sr, U, Th concentrations and REE distribution patterns for apatites from DRS and FZC granites basically are similar to the S and I granite types of the Lachlan Fold Belt (Australia), whereas those from NLM granites have intermediate properties and cannot be correlated directly with these granite types. According to some indications set by the apatite geochemistry (e.g., lower U and higher Eu abundances), NLM apatites appear to have formed under oxidizing conditions. In addition, we further found that their REE distribution patterns are closely related to aluminum saturation index (ASI) and Nd isotope composition, rather than SiO2 content or degree of differentiation, of the host rock. The majority of apatites from NLM granites (ASI = 0.97–1.08 and εNd(T) = −8.8 to −11.6) display slightly right-inclined apatite REE patterns distinguishable from the typical S- and I-type. However, those from few granites with ASI > 1.1 and εNd(T) < −11.6 have REE distribution patterns (near-flat) similar to DRS apatites whereas those from granites with ASI < 1.0 and εNd(T) > −6.6 and gabbro and syenite are similar to FZC apatites (strongly right-inclined). In light of Sr and Nd isotope compositions, magmas of NLM intrusives, except gabbro and syenite, and few granites with εNd(T) > −8, generally do not involve a mantle component. Instead, they fit with a melt derived largely from in situ melting or anatexis of the pre-Mesozoic (mainly Caledonian) granitic crust with subordinate pre-Yanshanian (mainly Indosinian) granitic crust. We suggest that an application, using combined whole-rock ASI and εNd(T) values, is as useful as the apatite geochemistry for recognizing possible sources for the NLM granites. 相似文献
20.
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. 相似文献