Rb-Sr and K-Ar ages on minerals at temperatures of 300°–400° C from deep wells in the Larderello geothermal field (Italy)

Rb-Sr and K-Ar ages have been obtained on six biotites, two muscovites and one hornblende from samples of micaschist, gneiss and amphibolite of Lower Paleozoic to Precambrian age at a depth exceeding 2,000 m in basement rocks of the Larderello-Travale geothermal region. Most of the data cluster in the range 2.5–3.7 Ma, revealing the existence of a Pliocene thermal event to which the origin of the field may be attributed. The resulting duration of the Larderello geothermal field is unexpectedly long. In the basement levels of the two wells examined, unstabilized minimum temperatures of 290° and 380° C were measured. All the biotites show almost complete ⁴⁰Ar and ⁸⁷Sr retention at the measured well temperatures. Petrologic evidence (stilpnomelane stability) and experimental data (activation energies and diffusion coefficients) also favour a closure temperature above 400° C for Rb-Sr and K-Ar in biotites, in agreement with recent direct experimental determinations.For the last 3 Ma mean geothermal gradients of 120°–150° C/km have been evaluated in the first 2–3 km, and 60°–65° C/km in the underlying 2 km. A rough estimate of total cooling in the last 3 Ma gives a value of 120° C at 2,500 m depth and 50° C at 4,000 m depth in Sasso 22 well. A mean uplift rate of about 0.2 mm/year is calculated independently.Research conducted under a collaboration agreement between the Italian National Research Council (CNR) and the Italian National Electricity Board (ENEL)     

Decompressed eclogites in the Sveconorwegian (–Grenvillian) orogen of SW Sweden: petrology and tectonic implications

Relict eclogites and associated high-pressure rocks are present in the Eastern Segment of the SW Swedish gneiss region (the tectonic counterpart of the Parautochthonous Belt of the Canadian Grenville). These rocks give evidence of Sveconorwegian eclogite facies metamorphism and subsequent pervasive reworking and deformation at granulite and amphibolite facies conditions. The best-preserved eclogite relics suggest a clockwise P–T –t history, beginning in the amphibolite facies, progressing through the eclogite facies, decompressing and partially reequilibrating through the high- and medium-pressure granulite facies, before cooling through the amphibolite facies. Textures demonstrate the former coexistence of the plagioclase-free assemblages garnet+clinopyroxene+quartz+rutile+ilmenite, garnet+clinopyroxene+ kyanite+rutile, and garnet+kyanite+quartz+rutile. The former existence of omphacite is evidenced by up to 45 vol.% plagioclase expelled as small grains within large clinopyroxene. Matrix plagioclase is secondary and occurs expelled from clinopyroxene or in fine-grained, granulite facies reaction domains formed during resorption of garnet and kyanite. Garnet shows preserved prograde growth zoning with rimward increasing pyrope content, decreasing spessartine content and decreasing Fe/(Fe+Mg) ratio, but is partly resorbed and reequilibrated at the rims. P–T estimates from microdomains with clinopyroxene+plagioclase+quartz+garnet indicate pressures of 9.5–12 kbar and temperatures of 705–795 °C for a stage of the granulite facies decompression. The preservation of the prograde zoning suggests that the rocks did not reside at these high temperatures for more than a few million years, and chemical disequilibrium and ‘frozen’ reaction textures indicate heterogeneous reaction progress and overstepping of reactions during the decompression through the granulite facies. Together these features suggest a rapid tectonic exhumation. The eclogite relics occur within a high-grade deformation zone with WNW–ESE stretching and associated oblique normal-sense, top-to-the-east (sensu lato) displacement, suggesting that extension was a main cause for the decompression and exhumation. Probable tectonic scenarios for this deformation are Sveconorwegian late-orogenic gravitational collapse or overall WNW–ESE extension.     

Timing and Duration of Hydrothermal in the Oligocene Hamad Cauldron, SW Japan: Evidence from K-Ar Ages of Sericites

Abstract: Crystallinity, chemical compositions and K-Ar ages of sericites in highly-sericitized granites and associated fissure-filling veins were examined to delineate the timing and duration of the hydrothermal activity in the Oligocene Hamada cauldron in the San-in district, SW Japan. Sericite separates (>2 μm) from the highly-sericitized granites consist mainly of 2M₁ polytype having high crystallinity and low Kübler indices of 0.22–0.35, while those in the fissure-filling veins have lower 2M₁/1Md ratios and crystallinity, and high Kübler indices of 0.29–0.35. This suggests that the sericites in highlysericitized granites were formed at a higher temperature than the vein sericites.
Sericites from the highly-sericitized granites of the Kumogi pluton give K-Ar ages of 30.0±0.7, 30.4±0.7, 30.6±0.7, 30.6±0.7, 32.1±0.7, 32.3±0.7 and 33.0±0.7 Ma (1), while those of the central plutons, 33.8±0.7 and 33.8±0.7 Ma. Sericites in the fissure-filling veins of the Kumogi granite give K-Ar ages of 31.0±0.7, 31.5±0.7, 31.6±0.7, 31.7±0.7 and 32.3±0.7 Ma. Biotite separates from the fresh Kumogi granite give K-Ar ages of 31.7±0.8, 32.0±0.8, 32.7±0.7 and 33.5±0.7 Ma. The K-Ar age data revealed that the hydrothermal alteration began at about 33 Ma and ended by about 30 Ma and that the period of sericite alteration was nearly synchronous with the cooling of the granite intrusions in the Hamada cauldron.
Despite intense hydrothermal alteration, the Oligocene granitoids have not accompanied with any economic base metal mineralization. The bulk chemical analyses of sericite separates in the veins indicate that the post-magmatic fluids were originally barren in heavy metals.     

Differential behaviour of the Rb-Sr and K-Ar systems of spilitic flows and interbedded metasediments: the spilite group of Erquy (Brittany,France). Paleomagnetic implications

The Erquy series (Côtes du Nord, France) consists, in its upper part, of spilitic pillow lavas with some interbedded volcano-sedimentary horizons.The Rb-Sr system of the pillows allowed the construction of a whole-rock isochron at 482±10 M.a. with an initial⁸⁷Sr/⁸⁶Sr ratio of 0.7055±0.0002. These rocks and the associated keratophyres give, on the other hand, K-Ar ages of 285±16 M.a. interpreted as the consequence of late-hercynian tectonism.A volcano-sedimentary horizon interbedded with such pillow flows has been studied from petrographic, mineralogical, geochemical and isotopic (Rb-Sr and K-Ar) points of view. The sequence keeps a sedimentary memory. Its clay fractions <2 m and corresponding whole-rocks fit an isochron which is identical to that of the volcanic rocks: 494±11 M.a. with an initial⁸⁷Sr/⁸⁶Sr ratio of 0.7052±0.0005. The clay fractions give K-Ar data at about 450 M.a., but those which contain important amounts of volcanic glass, at the top of the horizon, have K-Ar values as low as 400 M.a., and those which contain almost no glass have a K-Ar age close to the Rb-Sr age at 480 M.a.This study emphasizes the possibility of a complete reset of the K-Ar system of spilitic rocks by a tectonic event without notice-able temperature increase. This result may have important implications on combined paleomagnetic and K-Ar studies: it seems that a least for spilites and keratophyres, the Curie point and Ar blocking temperature can be very different.     

Newly identified Caledonian gabbro (450 Ma) in the eastern Guangxi,South China: Production of intraplate orogenic process

1.Objectives As one of three major tectonic blocks of China,the South China Block has undergone multiple periods of tectonic-magmatic-metamorphic events.Among t...     

Eruption of the Continental Flood Basalts at -259 Ma in the Emeishan Large Igneous Province, SW China： Evidence from Laser Microprobe ^40Ar/^39Ar Dating

A suite of continental flood basalts sampled over a vast exposure and stratigraphic thickness in the Emeishan large igneous province (LIP), SW China was investigated for laser microprobe 40Ar/39Ar dating. There are two 40Ar/39Ar age groups for these basalts, corresponding to 259-246 Ma and 177-137 Ma, respectively. A well-defined isochron gives an eruption age of huge quantities of mafic magmas at 258.9±3.4 Ma, which is identical to previous dating and paleontological data. Much younger 40Ar/39Ar ages for some basalts with low-greenschist metamorphic facies probably recorded a late thermo-tectonic event caused by collision between the Yangtze and Qiangtang continental blocks during the Mesozoic, which resulted in the reset of argon isotope system. The 40Ar/39Ar age data, we present here, combined with previous dating and paleontological data, suggest relatively short duration (about 3 Ma) of mafic volcanism, which have important implication on mantle plume genesis of the Emeishan continental flood basalts in the LIP.     

18O/16O ratios of anorthosites and related rocks from the Rogaland Complex (SW Norway)

The South Rogaland Complex (South Western Norway) consists of several anorthositic intrusions emplaced in granulite facies metamorphic rocks. The anorthosites and related norites and jotunites have δ ¹⁸O values of 5.2 to 7‰ suggesting a mantle origin for these rocks, in agreement with the strontium isotopic evidence. The acidic rocks, mostly charnockitic, associated with the anorthosites have similar δ ¹⁸O values and thus a comagmatic relation between these two rock types is inferred. Small departures from mantle values are explained in terms of crustal contamination by surrounding gneisses that have δ ¹⁸O values between 4.3 and 10‰ Locally, this corresponds to important anatexis as has been suggested for the Farsund charnockite on the basis of strontium isotope and REE geochemistry. The isotopic temperatures calculated from the isotopic fractionations are in the range 500°–700° C, lower than the orthomagmatic temperatures and probably due to subsolidus isotopic exchange during the slow cooling of these plutonic rocks, either during a late magmatic deuteric stage or during a slow, postorogenic ascent under wet conditions.     

拉萨地块北缘早白垩世晚期地壳生长:来自改则亚多～106Ma侵入岩的证据

北部拉萨地块晚中生代的地壳生长时间和机制存在争论。本文报道了北部拉萨地块的改则亚多侵入体的年代学、地球化学资料。改则亚多侵入体形成于早白垩世晚期(~106 Ma),其岩石类型包括二长闪长岩、闪长岩、花岗闪长斑岩、花岗斑岩。岩石属于钙碱性系列岩石,显示轻稀土富集,Nb和重稀土亏损,其中花岗闪长斑岩、花岗斑岩显示了埃达克质岩的地球化学特征。主体岩石样品具有一致的εNd(t)(2.65~1.42)和(~(87)Sr/~(86)Sr)i(0.7045~0.7049)。二长闪长岩、闪长岩由俯冲流体交代的地幔橄榄岩熔融产生的玄武质岩浆经过地壳混染和分离结晶作用形成。花岗闪长斑岩、花岗斑岩由增厚的新底侵玄武质下地壳熔融形成。早白垩世晚期(118~105 Ma),俯冲的班公湖–怒江特提斯洋岩石圈板片后撤过程中,诱发软流圈上涌,导致其上覆地幔熔融或其自身发生减压熔融,来自亏损地幔的岛弧岩浆连续底侵加入到北部拉萨地块的地壳或喷出地表,导致了该区在晚中生代的地壳生长。     

Evolution of feldspars at the amphibolite-granulite-facies transition in augen gneisses (SW Norway): geochemistry and Sr isotopes

The Sveconorwegian Augen Orthogneisses of Rogaland — Vest-Agder (SW Norway) were emplaced as amphibole- and biotite-bearing granodiorites at 1040 Ma (concordant Rb/Sr and zircon U/Pb ages). They underwent prograde metamorphism which increased from lower amphibolite-facies in the eastern zone to granulite-facies in the western zone, close to the Rogaland anorthosite complex. K-feldspar megacrysts initially crystallised as phenocrysts and were chemically equilibrated during metamorphism, as shown by the flat Ba concentration profiles and the increase of the anorthite content from An_1.1 in the amphibolitefacies to An_2.6 in the granulite-facies. This increase of the An content suggests an increase in metamorphic temperature. The REE content of the megacrysts is related to the associated accessory minerals which depend upon the metamorphic grade: sphene + allanite + apatite + zircon and rarely thorite in amphibolite-facies, and apatite + zircon + monazite ± thorite in lower amphibolite-and granulite-facies. Amphibole and biotite inclusions in megacrysts were also equilibrated during metamorphism. Groundmass K-feldspar and plagioclase experienced late-metamorphic changes during uplift. An internal Rb/Sr mineral isochron (plagioclase, apatite, K-feldspar) defines an age of 870 Ma, which represents the closure of the Rb/Sr isotopic system in minerals of the augen gneisses. This age also represents a K-feldspar cooling age in regionally distributed augen gneiss samples. The K-feldspar cooling age appears to be similar to or slightly older than the biotite cooling age.     

The origin and compositions of Mesoarchean oceanic crust: Evidence from the 3075 Ma Ivisaartoq greenstone belt, SW Greenland

The Mesoarchean (ca. 3075 Ma) Ivisaartoq greenstone belt contains well-preserved primary magmatic structures, such as pillow lavas, volcanic breccias, and clinopyroxene cumulate layers (picrites), despite the isoclinal folding and amphibolite facies metamorphism. The belt also includes variably deformed gabbroic to dioritic dykes and sills, actinolite schists, and serpentinites. The Ivisaartoq rocks underwent at least two stages of post-magmatic metamorphic alteration, including seafloor hydrothermal alteration and syn- to post-tectonic calc-silicate metasomatism, between 3075 and 2961 Ma. These alteration processes resulted in the mobilization of many major and trace elements. The trace element characteristics of the least altered rocks are consistent with a supra-subduction zone geodynamic setting and shallow mantle sources. On the basis of geological similarities between the Ivisaartoq greenstone belt and Phanerozoic forearc ophiolites, and intra-oceanic island arcs, we suggest that the Ivisaartoq greenstone belt represents a relic of dismembered Mesoarchean supra-subduction zone oceanic crust. This crust might originally have been composed of a lower layer of leucogabbros and anorthosites, and an upper layer of pillow lavas, picritic flows, gabbroic to dioritic dykes and sills, and dunitic to wehrlitic sills.

The Sm–Nd and U–Pb isotope systems have been disturbed in strongly altered actinolite schists. In addition, the U–Pb isotope system in pillow basalts appears to have been partially open during seafloor hydrothermal alteration. Gabbros and diorites have the least disturbed Pb isotopic compositions. In contrast, the Sm–Nd isotope system appears to have remained relatively undisturbed in picrites, pillow lavas, gabbros, and diorites. As a group, picrites have more depleted initial Nd isotopic signatures (ε_Nd = + 4.23 to + 4.97) than pillow lavas, gabbros, and diorites (ε_Nd = + 0.30 to + 3.04), consistent with a variably depleted, heterogeneous mantle source.

In some areas gabbros include up to 15 cm long white inclusions (xenoliths). These inclusions are composed primarily (> 90%) of Ca-rich plagioclase and are interpreted as anorthositic cumulates brought to the surface by upwelling gabbroic magmas. The anorthositic cumulates have significantly higher initial ε_Nd (+ 4.8 to + 6.0) values than the surrounding gabbroic matrix (+ 2.3 to + 2.8), consistent with different mantle sources for the two rock types.     

赣东北蛇绿岩带新元古代(~800Ma)高镁安山岩的发现及其意义

在赣东北蛇绿岩带中的樟树墩地区新发现新元古代高镁安山岩,其LA-ICP-MS锆石U-Pb法年龄为794.8±6.0 Ma。它们的Si O2介于54.90%~58.45%,Al2O3介于15.31%~16.77%(平均小于16%);Ca O为2.46%~6.73%,Fe OT/Mg O变化于0.87~1.20,具有高Mg O(6.39%~8.76%)、高Mg#(64~71)特点;富集轻稀土元素且具弱负Eu异常,明显富集LILE而亏损HFSE,Sr含量(普遍200×10-6)和Sr/Y比值(4.11~7.29)均较低,具有典型高镁安山岩的地球化学特征,类似于日本Setouchi火山带的Sanukitoids。锆石εHf(t)值介于10.23~17.79之间,反映它们起源于亏损地幔。主、微量及同位素特征均表明,本区高镁安山岩是被俯冲板片释放的含水流体交代的地幔楔部分熔融的产物,形成于大洋岛弧(洋内弧)环境,指示赣东北地区在约800Ma前时仍存在洋壳俯冲,双溪坞弧尚未增生到扬子东南缘,暗示扬子和华夏两大陆块此时尚未碰撞拼合。     

西南三江甭哥与富碱侵入岩有关金矿床成矿热史的磷灰石裂变径迹年代学证据

甭哥金矿位于哀牢山-金沙江富碱斑岩成矿带的北段,属于西南三江地区与富碱侵入岩有关的典型金矿床,发生于喜马拉雅期的印-亚板块碰撞造山事件使其具有多期复合成矿的特点,矿床的成矿年代学研究较为薄弱,制约着成矿机制的研究。磷灰石裂变径迹热年代学技术是近年来研究矿床热史演化的有效手段之一,本文通过甭哥含矿岩体的磷灰石裂变径迹年龄测试,获得三组裂变径迹年龄峰值:30Ma、25Ma及15Ma。利用裂变径迹热史模拟分析,揭示了甭哥金矿自喜马拉雅期以来主要经历的成矿活动热史:42～32Ma温度降幅20～30℃,冷却速率为3～3. 8℃/Ma;32～16Ma温度降幅20～10℃,冷却速率为1. 5～0. 71℃/Ma;16～6Ma发生了快速降温活动,温度降幅50～45℃,冷却速率为11～6. 25℃/Ma;前两个时期为缓慢降温阶段说明处于挤压缩短的环境,最后一个时期是快速冷却阶段,暗示这个阶段是以拉张的构造活动为主。裂变径迹热史模拟反映出甭哥金矿区经历了两个阶段的成矿过程,第一个阶段处于挤压缩短的环境与当时西南三江地区处于压扭性的构造背景相吻合,裂变径迹年龄30Ma代表了主要金矿体的形成时间;第二个阶段反映了拉张的构造环境与西南三江地区处于伸展张裂的演化机制相同,代表了锑-金矿体的形成过程,15Ma是成矿作用的具体时限;表明成矿构造活动经历了由挤压到拉伸的演化机制。通过磷灰石裂变径迹数据的模拟分析,构建了甭哥金矿在喜马拉雅期的成矿热事件过程,结果表明甭哥金矿的演化机制与西南三江地区富碱岩浆带内的成矿事件大致相同,显示甭哥金矿属于该成矿系统的一部分,两者之间具有相似的地球动力学机制。     

Diagenetic and thermal history of the Jurassic-Tertiary succession of the Zagros Mountains in the Dezful Embayment (SW Iran): constraints from fluid inclusions

To constraint the diagenetic and thermal history in the Mesozoic-Tertiary succession of the Zagros Fold-Belt, a study was performed on fluid inclusions trapped in intergranular, vug, and fracture-filling cements in Jurassic to Miocene outcrop samples collected along the Anneh and Fahliyan Valleys of Khuzestan Province, southwest Iran. Petrographic observations showed at least two systems of fractures that postdate intergranular cementation. Two different types of oil-filled fluid inclusions occur in the intergranular cements and in the first fracture network, but they are absent in the second fracture network. Microthermometry of fluid inclusions was used to determine the temperature and salinity of fluids responsible for mineral precipitation. Within intergranular cements and calcite fillings the oldest fractures, precipitation occurred from high saline fluids in a temperature range of 60–120°C, depending on the stratigraphic depth. The strong correlation between fluid inclusions temperature and stratigraphic depth suggests that the intergranular cementation and the first fracture-filling event occurred prior to structural deformation. Furthermore, this correlation indicates a relatively constant paleogeothermal gradient at the time of cement precipitation of about 28°C/Km. On the contrary, fluid inclusions trapped in cements within the second fracture network lack oil and contain freshwater trapped at low temperatures that do not correlate to the stratigraphic position. This suggests second fracturing event occurred after oil migration and during or after deformation of the Zagros Fold-Belt, when the samples were in a near-surface position.     

The early Caledonian (Finnmarkian) event reassessed in Finnmark: Ar/Ar cleavage age data from NW Varangerhalvøya, N. Norway

The relative significance of early (Finnmarkian) and late (Scandian) Caledonian deformation in N. Norway is uncertain. Early studies suggested pervasive Finnmarkian deformation whilst later results indicated a restricted Finnmarkian domain. The present work suggests it was more widespread than accepted and that inter Finnmarkian–Scandian deformation occurred. ⁴⁰Ar/³⁹Ar dating of 2–6 and 6–11 μm pelitic fractions from the lower to mid-greenschist facies Tanahorn Nappe (five samples; base Middle Allochthon) and the epizone Løkvikfjellet and Barents Sea Groups (three samples; North Varanger Region) in the north Scandinavian Caledonides show slightly discordant spectra. Most spectra from the Tanahorn Nappe preserve possible evidence of an early Caledonian event in the high temperature steps, with recoil/excess Ar effects in the low temperature steps; no pre-Caledonian relict component has been recorded. The results indicate Finnmarkian deformation continued to 460 Ma, with Scandian reactivation at 425–415 Ma. From the North Varanger Region, a strongly crenulated sample yielded plateau ages (444–442 Ma); means of combined young steps from weakly to uncrenulated samples gave 470–450 Ma, suggesting penetrative strike-slip deformation occurred in the late Finnmarkian to inter-Finnmarkian–Scandian period. No Scandian ages were recorded in the North Varanger Region. Reassessment of published data from the Laksefjord Nappe and Gaissa Thrust Belt suggests they were affected by Finnmarkian deformation.     

Chemical and structural study of 1M- and 2M 1-phlogopites coexisting in the same Kasenyi kamafugitic rock (SW Uganda)

In the present work, crystal chemical variations between 1M and 2M ₁ phlogopites coexisting in the same rock sample from kamafugite of Kasenyi (southwest Uganda, west branch of the East African Rift) were explored by electron probe microanalyses, single crystal X-ray diffraction and M?ssbauer spectroscopy. Chemical analyses revealed close similarity both within and between the two polytypic arrangements as well as high TiO₂ (~4.9?wt%) and Al₂O₃ (~12.9?wt%), and low Cr₂O₃ (~0.8?wt%), F (~0.3?wt%) and BaO (~0.2?wt%) contents. Room temperature ⁵⁷Fe M?ssbauer investigation proved that the studied mica is a tetraferriphlogopite with: ^IVFe³⁺?=?19(1)?%, ^VIFe²⁺?=?58(1)?%, ^VIFe³⁺?=?23(1)?%. Single crystal refinement showed that both polytypes have narrow range of variation in terms of some relevant unit cell parameters and similar values in terms of mean bond lengths, mean atomic numbers and distortion parameters. Similar substitutions were active in the structure of the 1M and 2M ₁ studied phlogopites. However, in 2M ₁ polytypes the oxy-type substitutions were found to occur to a greater extent. Comparison of unit layer of 1M mica (in the 2M ₁ setting) with that of the 2M ₁ ones showed that the 2M ₁ polytypes are affected to different extent by relative shifts of the upper and lower triads of octahedral oxygens along the ±b directions. This effect did not cause any symmetry lowering in the T-O-T layer of the studied samples.     

滇西北拉巴燕山晚期花岗岩岩石成因及其成矿指示——黑云母和角闪石矿物化学证据

在滇西北香格里拉拉巴地区,近年通过钻探新发现了燕山晚期花岗岩体及伴生的超大型钼(-铜)-多金属矿床。调查发现,岩浆成因黑云母和角闪石记录了其形成时的岩浆温度、压力、氧逸度以及物质来源等岩石成因信息,这些物理化学条件制约了成矿元素在熔体相与流体相之间的分配,成为约束岩浆过程、岩石成因及成矿机制的重要因素。本文对拉巴矿区花岗岩中黑云母和角闪石进行了详细的矿相学和成分分析,据此厘定了岩石形成的物理化学条件,探讨其岩石成因及成矿效应。结果显示,花岗岩中黑云母的Fe2+/(Fe2++Mg)值较为均一,具有无钙或贫钙的特点,Ti阳离子数为0. 31～0. 52,属于岩浆成因;角闪石的Si阳离子数为6. 68～7. 20,Ti阳离子数为0. 09～0. 13,属于岩浆成因;计算获得岩浆结晶温度为705～903℃,结晶压力为59～449 MPa,侵位深度为2. 2～17. 0 km。黑云母和角闪石的矿物化学特征指示,寄主花岗岩体为I型花岗岩,具有幔源物质参与特点,形成于较高的氧逸度环境中;黑云母的卤族元素(F、Cl)含量为0. 17%～0. 58%,指示岩浆出溶流体为富含F、Cl的流体,利于Mo、Cu等元素的富集成矿,暗示本区具有很大的成矿潜力。     

江西阳储岭两期岩浆活动与钨成矿作用的关系：来自黑云母地球化学的证据

阳储岭斑岩型W-Mo矿床位于江南造山带中部,是华南地区最早发现的斑岩型钨钼矿床。已探明WO₃资源储量6.13万吨(平均品位0.2%),Mo资源储量1.69万吨(平均品位0.03%～0.06%),其成矿作用与中生代花岗质岩浆活动密切相关。区内发育早期花岗闪长岩和晚期二长花岗斑岩,钨矿体以细脉状和浸染状产于二长花岗斑岩体内,而花岗闪长岩内未见矿化。两期岩浆活动与钨成矿的关系尚不明确,制约其含矿差异性的因素尚不清楚。本文以花岗闪长岩和二长花岗斑岩中的黑云母为研究对象,对比研究两类岩浆结晶分异程度、氧逸度、岩浆流体卤素浓度,探讨其对钨成矿的制约。黑云母主量元素分析结果显示,阳储岭两类岩浆岩均为壳源,但显著不同于S型花岗岩的Mg/(Fe+Mg)和Al^VI值,指示其具有I型花岗岩的特征。黑云母的微量元素信息显示,相对于花岗闪长岩中的黑云母(类型一),二长花岗斑岩中的黑云母(类型二)显示低的K/Rb、Nb/Ta比值,高的Rb、Cs、Nb和Ta含量,表明其分异程度较花岗闪长岩更高,更有利于钨的富集。两类黑云母所指示的岩浆氧逸度均在NNO缓冲线附近,表明其母岩...     

Rare earth element geochemistry and strontium isotopic composition of a massif-type anorthositic-charnockitic body: the Hidra Massif (Rogaland,SW Norway)

The Hidra Massif (Rogaland Complex, SW Norway) mainly consists of plagioclase cumulates (anorthosites and leuconorites), which grade progressively into a fine-grained (200 μm). locally porphyritic, jotunitic rock towards the contact with the granulite facies gneisses. The massif is cross-cut by thin (10 cm up to 1 m) charnockitic dykes.The petrographical and geochemical evolution of the Hidra Massif can be explained by fractional crystallization of a jotunitic parental magma. Major and trace element constraints indicate that mafic phases are underabundant in the exposed levels of the massif, most likely as a result of plagioclase flotation in the early stages of solidification. Partitioning into the cumulate minerals (mainly plagioclase and orthopyroxene) governs the trace element contents of the leuconoritic adcumulates. However, the trace element geochemistry of the apparently early formed anorthositic orthocumulates largely depends upon the amount of a trapped intercumulus liquid. On the basis of trace element abundances (high REE, Rb, Th, U; negative Eu anomalies) the silicic charnockitic dykes can be considered as the residual liquid of the anorthositic fractionation trend. The higher initial ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios (0.7086 ± 0.0006 vs 0.7055 ± 0.0004 for the plagioclase cumulates and jotunites) point to contamination of the charnockitic liquids by surrounding gneissic material.     

滇西北甭哥碱性岩有关金矿床多阶段剥蚀历史和保存示踪:磷灰石裂变径迹热年代学证据

造山带背景下矿床的矿后剥蚀历史及保存示踪研究,有助于理解区域成矿作用和指导找矿勘查与评价。滇西北甭哥金矿床是一个与印支期碱性岩有关的热液型金多金属矿床,金储量已达中型规模,但对其矿床成因和成矿潜力仍不明朗。该矿床处在西南三江造山带北段与扬子地台西缘的交汇地带,是香格里拉印支.燕山期巨型斑岩.矽卡岩型Cu-Au-Mo多金属矿集区的组成部分,是开展矿床剥蚀历史及保存示踪研究的理想地区。本文以甭哥金矿区含矿碱性岩体为研究对象,通过系统的剖面实测、样品采集和磷灰石裂变径迹测试,经热史模拟,估算其隆升历史及剥蚀深度。结果表明,甭哥金矿区自喜马拉雅期以来主要经历了42~32Ma、32~16Ma和16~6Ma三期隆升.剥蚀事件,三组年龄较好地限制了其构造热事件活动时限,该构造事件在西南三江地区具有普遍一致性;含矿岩体的隆升量及隆升速率分别为5852~6422m和0.21~0.39mm/a,剥蚀量和剥蚀速率分别为2130m和0.07~0.14mm/a;其最大剥蚀量远小于成矿深度,表明该矿床形成后的剥蚀程度弱,保存状况较好,尚具有较大的找矿前景。