花岗岩中原生与次生白云母的鉴别特征及其地质意义——以赣南富城强过铝质花岗岩体为例

通过显微镜下观察和电子探针成分分析,发现赣南富城强过铝质花岗岩中存在3种类型白云母,即原生白云母、交代型白云母和次生白云母,其平均晶体化学式分别为:K1.62Na0.06Fe0.32Mg0.39Ti0.02Al4.89Si6.54O10(OH)4(原生白云母)、K1.55Na0.07Fe0.43Mg0.24Ti0.03Al4.96Si6.50O10(OH)4(交代型白云母)、K1.51Na0.07Fe0.27Mg0.21Ti0.00Al4.98Si6.65O10(OH)4(次生白云母)。根据富城花岗岩主要造岩矿物的结晶顺序(斜长石→钾长石→黑云母→白云母→石英),结合白云母、黑云母稳定曲线及合成花岗岩初融曲线对比分析,富城强过铝质花岗岩中交代型白云母是在花岗岩结晶过程中交代较早晶出的黑云母形成的,其形成温度高于花岗岩熔体的固相线温度(～650℃),故应归属于原生白云母。本文提出根据岩石学宏观特征、岩石化学特征及岩相学微观特征区分花岗岩中原生白云母与次生白云母的综合鉴别方法。     

Petrology and Li-Be-B geochemistry of muscovite-biotite granite and associated pegmatite near Yellowknife,Canada

Prosperous granite (Rb-Sr 2520±25 Ma) occurs as several plutons (1–380 km² outcrop area) in a thick succession of metamorphosed greywacke-mudstone of the Yellowknife Supergroup. The average mineral content of the Sparrow pluton (in vol.%) is quartz (32), plagioclase (31), K-feldspar (24), muscovite (9), biotite (3), and apatite (<1). Average trace-element concentrations (in ppm) are Li (140), Be (4), B (28), Zn (47), Rb (250), Sr (76), Zr (75) and Ba (360). The central portion of the pluton is slightly richer in K, Sr, and Ba than the margin. Li is concentrated in mica (Li in biotite/Li in muscovite=4.7), and Be and B in muscovite and plagioclase. Countless pegmatite dikes occur in the Sparrow pluton and in schist-hornfels to the east; the outer limit is marked by the cordierite isograd, 9 km from the granite contact. Dikes vary greatly in size (1 km to a few cm in length), in mineral content (quartz, albite, K-feldspar, muscovite, tourmaline, beryl, spodumene), in major element composition (especially the NaK ratio), and in trace-element content (Li 18–5000 ppm, Be 5–260 ppm, B 20–150 ppm). Compared with Prosperous granite, the pegmatite bodies are richer in P and Rb, and poorer in Ti, Fe, Mg, Zr, and Ba. Dikes rich in tourmaline, beryl, and spodumene occur in overlapping zones situated progressively farther from the centre of the Sparrow pluton. The composition of tourmaline is related to host rock; the highest concentrations of Fe and Zn occur in crystals from pegmetite and the highest concentrations of Mg and V occur in crystals from tourmalinized schist, while those from granite and quartz veins occupy on intermediate position. Complex compositional zoning is present in some tourmaline crystals in pegmatite. Estimates of temperature (500°–600° C) and pressure (2–4 kb) of granite emplacement, based on the distribution of andalusite and sillimanite in the contact rocks, suggest that the final stage of granite emplacement occurred at sub-solidus conditions. A vaportransport model is proposed to explain the widespread distribution of the pegmatite dikes and their extreme compositional variability. Some of the pegmatite constituents, including Li, Be, and B, were possibly derived from Yellowknife graywacke and mudstone.     

青海巴颜喀拉山隆升的热年代学研究

对巴颜喀拉山年宝玉则花岗岩岩体样品的锆石、磷灰石进行了裂变径迹（FT）分析。结果显示该区的隆升可以分为三大阶段：①从低绿片岩相到锆石时钟启动：从200MaBP到105．57MaBP,去顶量约为2km,样品埋深上升到距地表约6km深度。去顶速率约为0．02mm／a;②从锆石时钟启动到磷灰石时钟启动：由于磷灰石在此时已抬升到距地表约6km的深度。由此到达时钟启动需距地表约3km时,去顶量应为3km,此阶段的持续时间为101．53MaBP,去顶速率约为0．03mm／a：③从磷灰石时钟启动至达到地表（至今）：由于磷灰石在前一阶段也已抬升到距地表约3km的深度。即去顶量为3km。此阶段的持续时间为4．04MaBP,去顶速率约为0．74mm／a。     

Geochemistry and evolution of the Wolf River Batholith,a late Precambrian Rapakivi Massif in North Wisconsin,U.S.A.

The Wolf River Batholith is an anorogenic rapakivi massif in central and northeastern Wisconsin with an age of 1.5 Ga. The Batholith has alkaline affinities and consists of biotite granite and biotite-hornblende adamellite with minor occurrences of quartz syenite and older monzonite and anorthosite. The batholith is part of a major Late Precambrian (1.4–1.5 Ga) magmatic event of continental proportions, represented by separate intrusions extending from Labrador to southern California (Silver et al., 1977).The major and trace element composition (Li, Rb, Sr, Ba, and REE) of 40 samples from the anorthosite, monzonite, and rapakivi granite and adamellite plutons precludes a comagmatic (although not cogenetic) model between all three rock units. However, the monzonite may be related to the anorthosite alone by fractional crystallization of plagioclase, orthopyroxene, clinopyroxene, and apatite. Alternatively, the monzonite may be a separate parent melt or a hybrid associated with the granite and adamellite plutons. The high REE content of the monzonite precludes it from being related to the rapakivi granite and adamellite plutons as a source material, a residuum, or a cumulate.A major portion of the Batholith is an undifferentiated intrusive sequence ranging from older rapakivi granite to younger adamellite. The compositions of these plutons suggest a crustal fusion origin at intermediate to lower levels of the crust (25–36 km). The trace element data are consistent with partial fusion of tonalitic to granodioritic source material.During crystallization and emplacement into the upper crust (less than 4 km), 55–70% fractionation of two feldspars, biotite and hornblende from one of the granite plutons produced a small volume of differentiated granitic melt high in Si, Fe/Mg, Rb, Li, and REE (except Eu), and low in Ca, Mg, Al, Ca/Na, Sr, Ba, and K/Rb and with a large negative Eu anomaly. Presumed associated cumulate material ranges from silica-poor quartz monzonite and quartz syenite.The chemical and mineralogical similarity between the Wolf River Batholith and younger magmatic analogs associated in continental break-up (Nigerian younger granites, White Mountain magma series, and the peralkaline volcanics of the Red Sea Region) are suggestive but not conclusive of an extensional tectonic setting. A preliminary tectonic model suggests that the 1.4–1.5 Ga event is in response to thermal doming in an extensional regime leading to continental separation in the western Cordillera (pre-Belt) and extensive crustal fusion with no rifting or separation across the North American Craton.     

REE Abundance and REE Minerals in Granitic Rocks in the Nanling Range,Jiangxi Province,Southern China,and Generation of the REE‐rich Weathered Crust Deposits

Studies of Mesozoic granites associated with rare earth element (REE)‐rich weathered crust deposits in southernmost Jiangxi Province indicate that they have high‐K to shoshonite compositions and belong to ilmenite‐series I‐type granites. Of the studied rocks at 59–292 ppm of bulk REE content, the highest are seen in the biotite granites of Dingnan (358, 429 ppm) and mafic biotite granite of the Wuliting Granite (344 ppm) near the Dajishan tungsten mine, both areas where weathered‐crust REE deposits occur. REE‐bearing accessory minerals in these granites are mainly zircon, apatite and allanite, and REE‐fluorocarbonates are common. REE enrichment occurs in the rims of apatite crystals, and in fluorocarbonates that occur along grain boundaries of and cracks in major silicate minerals, and in fluorocarbonates that replaced altered biotite. It is therefore thought that a major part of the REE content of these granites was concentrated during deuteric activity, rather than during magmatic crystallization. The crack‐filling REE‐fluorocarbonates could subsequently have been easily leached out and deposited in weathered crust developed during a long period of exposure.     

The tetrad effect and geochemistry of apatite from the Altay Koktokay No. 3 pegmatite, Xinjiang, China: implications for pegmatite petrogenesis

In order to better constrain the evolution and petrogenesis of pegmatite, geochemical analysis was conducted on a suite of apatite crystals from the Altay Koktokay No. 3 pegmatite, Xinjiang, China and from the granitic and amphibolitic wall rocks. Apatite samples derived from pegmatite zones show convex tetrad effects in their REE patterns, extremely negative Eu anomalies and non-chondritic Y/Ho ratios. In contrast, chondritic Y/Ho ratios and convex tetrad effects are observed in the muscovite granite suggesting that different processes caused non-chondritic Y/Ho ratios and lanthanide tetrad effects. Based on the occurrence of convex tetrad effects in the host rocks and their associated minerals, we propose that the tetrad effects are likely produced from immiscible fluoride and silicate melts. This is in contrast to previous explanations of the tetrad effect; i.e. surface weathering, fractional crystallization and/or fluid-rock interaction. Additionally, we put forward that extreme negative Eu and non-chondritic Y/Ho in apatite are likely caused by the large amount of hydrothermal fluid exsolved from the pegmatite melts. Evolution of melt composition was found to be the primary cause of inter and intra-crystal major and trace element variations in apatite. Mn entering into apatite via substitution of Ca is supported by the positive correlation between CaO and MnO. Different evolution trends in apatite composition imply different crystallization environments between wall rocks and pegmatite zones. Based on the geochemistry of apatite samples, it is likely that there is a genetic relationship between the source of muscovite granite and the source of the pegmatite.     

Ocellar lamprophyre dyke bearing mineralization,Wadi Nugrus,Eastern Desert,Egypt: Geology,mineralogy and geochemical implications

The ocellar lamprophyre dyke (ENE-WSW) is recorded at Wadi Nugrus, Eastern Desert, Egypt. It cuts porphyritic biotite granites and varies in thickness from 0.5 to 1.5 m and up to 3 km in length. The lamprophyre dyke has been altered, and it is characterized by porphyritic and panidiomorphic textures with plagioclase, olivine, and augite constituting the porphyritic phase in a fine groundmass of the same composition. Rutile, titanite, apatite, fluorite, graphite, calcite, allanite, autunite and Fe-Ti oxides are accessory minerals. Kaolinite, chlorite and epidote are secondary minerals. Carbonitization and hematitization are common. Rounded to sub-rounded porphyritic and zoned ocelli with radiate or brush-like shapes are generally common and represent physical traps for mineralization. The ocellar features are interpreted to represent the late stage of magmatic segregation or magmatic crystallization involving two immiscible magmatic liquids.     

Lithospheric Structure of the Southeast Australian Lachlan Orogen along the Victorian Global Geoscience Transect

Upper-crustal elements of the ～35 km thick crust of the southern portion of the Lachlan Orogen consist of a chevron-folded and faulted turbidite package (15 to 17 km structural thickness) overlying imbricated Cambrian metabasites and cherts (～5 km structural thickness). These are intruded by both Early and Late Devonian granites and are overlain by Upper Silurian (?) marine to continental clastics (Grampians Group) in the west, and Upper Devonian-Early Carboniferous silicic volcanics and continental redbed elastics in the east. The turbidites show a general younging to the east, as well as eastward vergence apart from a local reversal (Tabberabbera zone). The region has been relatively stable since the mid-Paleozoic with apatite fission-track data recording cooling below ～100°C at 340-330 Ma in the west and 300 to 280 Ma in the east. Younger fission-track ages to the south, approaching the present coastline, reflect denudation during the opening of Bass Strait and the formation of the Cretaceous Otway and Gippsland basins. The major crustal discontinuities, the Woorndoo-Moyston and Mount Wellington fault zones, show significant Mesozoic reactivation and juxtapose regions of younger against older apatite fission-track ages. The nature of the lower crust remains unclear, but there is increasing evidence that it is not underlain by thinned Proterozoic continental crust. The Lachlan Orogen is an example of mid-Paleozoic tectonic accretion in a Southwest Pacific-style oceanic setting. Subduction-related oceanic thrusting produced the deformed and imbricated turbidite packages, and subduction-related magmatic underplating (perhaps during “rollback”) produced the large volumes of granite and volcanic rocks, and the localized high-T/low-P metamorphism.     

Geochemistry and petrogenesis of anorogenic(?) granitoids of west Garo Hills,Meghalaya

The granite in Samingiri — Dilsekgiri area occurs as discordant, isolated pluton within the migmatitic terrain of West Garo Hills district, Meghalaya. The pluton is exposed over 140 sq km (18 km × 8 km). It exhibits structures of solid state and piecemeal stoping effect proximal to the contact and enveloped by a contact metamorphic aureole of albite-epidote-hornfels facies. Modally, it is biotite-monzogranite and biotite-syenogranite with minor biotite, chlorite, epidote and sericite and accessories like zircon, apatite, allanite, pyrite, magnetite and sphene. Geochemically, it is marked by restricted composition (69–76 wt% SiO₂), high alkalies, low Ca, metaluminous to strongly peraluminous (Molar Al₂O₃/CaO+Na₂O+K₂O = 0.95 ? 1.54), high FeO/MgO, high Ga/Al, high contents of Rb, Sr, Ba, Y, Zr and Ce and depleted in Ti and P. The field observation, mineralogical and geochemical aspects indicate the post-tectonic nature of West Garo pluton more like as A-type granite formed by partial melting of lower crustal blocks followed by low to moderate degree of fractional differentiation. Low Ca, alkaline nature and peraluminous character point to A-type nature of West Garo granite significantly different from other granites of Meghalaya Plateau. Rb-Sr age (616±86 Ma) of granite, however, corresponds to widespread Middle to Upper Pan African activity, a thermal event prevailed during Late Proterozoic — Early Palaeozoic (500–800 Ma) period, manifested in the form of several granitic intrusions in the basement gneissic complex and the overlying Proterozoic metasediments of the Shillong Group in Meghalaya Plateau.     

Occurrence of uraniferous iron and manganese oxides in biotite granite North East Gabal El Sela area, South Eastern Desert, Egypt

Optical microscopy, X-ray diffraction (XRD), and back-scattered electron imaging (BSE) have been used to determine the mineralogical composition of the uraniferous iron and manganese oxides and the associated U-minerals hosted in biotite granite that occurred north east Gabal El Sela area south Eastern Desert, Egypt. These mineralizations were found as veinlets fractures filling associated with strongly kaolinitic alteration of the coarse-grained biotite granite. XRD determined that the geothite mineral form the main constituent of uraniferous iron oxide in addition to tapiolite, and kaolinite minerals, where as uraniferous manganese oxide composed of pyrolusite, ramsdellite, and cryptomelane. BSE confirmed that the associated uranium minerals represented by uranothorite, kazolite, and zentime in addition to columbite-bearing minerals. Uranothorite and columbite-bearing minerals are the most abundant minerals in this mineralization. Petrographically, biotite granite is composed mainly of quartz, in addition to K-feldspars, biotite and muscovite with minor zircon, garnet, apatite, uranium-rich thorite and iron oxide. Petrochemical studies and tectonic discrimination diagrams for this granite reveal that they are classified as granite to alkali feldspar granite, originated from calc-alkaline magma having peraluminous nature and developed in within-plate tectonic environment. Field radiometric measurements revealed the localization of two high radiometric anomalies associated with iron and manganese oxides, within this anomaly uranium content range from 65 to 85 ppm. Alpha Track-etch Detectors of radon gas registrations revealed high track density reach up to 15,448.7 Bq/m³.     

Evidence for distinct stages of magma history recorded by the compositions of accessory apatite and zircon

Accessory minerals contain a robust and accessible record of magma evolution. However, they may reflect relatively late-stage conditions in the history of the host magmas. In the normally zoned Criffell granitic pluton (Scotland), whole-rock (WR) compositions reflect open system assimilation and fractional crystallisation at depths of >11 km, whereas amphibole barometry and the absence of inherited zircon suggest that the observed mineral assemblages crystallised following emplacement of magmas with little or no crystal cargo at depths of 4–6 km. The crystallisation history is documented by large trace-element variations amongst apatite crystals from within individual samples: decreasing LREE and Th concentrations in apatite crystals from metaluminous samples reflect broadly synchronous crystallisation of allanite, whereas lower LREE and Th, and more negative Nd anomalies in apatites from peraluminous samples reflect the effects of monazite crystallisation. WR evolution is likely to have occurred within a deep crustal hot zone where H₂O-rich (~6 wt%), low-viscosity magmas segregated and ascended adiabatically in a super-liquidus state, leading to resorption of most entrained crystals. Stalling, emplacement and crystallisation resulted from intersection with the H₂O-saturated liquidus at ~4 km. H₂O contents are as important as temperature in the development of super-liquidus magmas during ascent, blurring distinctions between apparently ‘hot’ and ‘cold’ granites. The trace-element contents of most accessory minerals are controlled by competitive crystallisation of other accessory minerals in small melt batches, consistent with the incremental assembly of large granitic plutons.     

Small independent intrusives of the Ikh-Khairkhan area (Central Mongolia)

There is apparently a structural-geological connection between accumulations of tungsten ores and the relative abundance of Triassic dikes in the area. The early and the late stages of the dikes are here identified, respectively, as the series of diorites-granodiorite porphyries-granite porphyries (Mg  Fe) and diorite porphyrites-quartz porphyries (Fe > Mg). The granodiorite porphyries and granite porphyries strike in the same direction, on the whole, as the isodynamic lines of the areal magnetic anomaly, the source of which lies at about 2 km depth. The hypothetical source of the magma is associated with a major rupture in the southwestern part of the area. The dikes lie in fissures which feather that rupture. The larger indications of the ores are in exocontacts of the massifs. --V.P. Sokoloff     

大兴安岭扎兰屯地区四班岩体岩石成因及构造环境研究

大兴安岭扎兰屯地区四班岩体主要由正长花岗岩和二长花岗岩组成,内部发育细粒闪长质包体。二长花岗岩和正长花岗岩LA-ICP-MS锆石U-Pb定年结果分别为303±3Ma和291±3Ma,属于晚古生代岩浆活动的产物。四班花岗质岩石高硅(67.9~77.5 wt%)、富碱(K_2O+Na_2O=7.55~10.79 w%)、相对高铝(Al_2O_3=12.05~16.33 wt%),富集轻稀土元素(LREE)和大离子亲石元素(LILE),而亏损高场强元素(Nb、Ta、Ti和P等),属于高钾钙碱性I型花岗岩。四班花岗质岩石内部发育的闪长质微粒包体及花岗岩与其伴生的基性岩的"一锅粥"现象,表明四班花岗质岩石具有岩浆混合成因的特征,地球化学特征也支持上述观点。四班岩体显示后碰撞岩浆岩的岩石学及地球化学特征,为后碰撞阶段岩石圈地幔拆沉减薄壳幔相互作用的产物。     

Anorogenic metaluminous and peraluminous granite plutonism in the mid-proterozoic of Wisconsin,USA

A magmatic gap from 1.82 to 1.76 b.y. in the Lake Superior region represents the transition from synorogenic calc-alkaline igneous activity of the Penokean Orogeny to anorogenic potassic granophyric granite and ignimbrite. This paper deals with the petrogenetic evolution of 1.76 b.y. granites which represent a major change in source material and conceivably tectonic setting. Although perhaps related to a termination of the Penokean Orogeny by melting of a tectonically thickened crust during collision, these post-Penokean granites may represent the initial appearance of anorogenic, potentially rift-related igneous activity that was widespread throughout North America during late Precambrian time.These post-Penokean granites are too iron-rich and Al-poor to be considered calc-alkaline, a compositional feature shared with most anorogenic igneous activity of continental regions. Within this suite in central and northern Wisconsin, regional differences in composition indicate at least two different granite magma types: one a metaluminous suite of biotite and biotite-hornblende granite and a peraluminous suite of two-mica granite. The systematic compositional differences (Al, Fe/Mg, Ba/Sr, REE) in the two magma suites are likely the result of small differences in residue mineralogy and/or source composition. In general, the degree of fusion was small (10%) and probably of relatively young Penokean material. Both suites have a range of composition due to feldspar dominated fractional crystallization. Removal of the accessory minerals apatite, zircon, and allanite resulted in the REE depletion with differentiation of the two-mica granites.The granites intruded into the upper levels of the crust, and the appearance of primary celadonitic muscovite and subsolvus alkali feldspars (silicic members only) in the two mica granites indicate crystallization at depths of 10–11 km. The biotite granites contain both hypersolvus and subsolvus members and are intruded at depths less than 6 km with the more shallow members generating major volumes of ignimbrite. As a marked departure from the characteristics of most anorogenic granites, these melts crystallized at fairly oxidizing conditions (higher for the two-mica suite) as reflected in the composition of biotite, predominance of magnetite over ilmenite, and early appearance of the Fe-Ti oxides in the crystallization sequence.     

Blind reverse faulting system associated with the Mont Chenoua-Tipaza earthquake of 29 October 1989 (north-central Algeria)

A shallow and damaging earthquake struck the region of Tipaza located on the coast, 70 km to the west of Algiers (north-central Algeria). The main shock was felt as far as 200 km from the epicentral area and particularly in the urbanized zone of Algiers. The main shock epicentre determined by CRAAG is on Mt Chenoua, close to the coastline. Coseismic surface breaks with 4.0 km of fault length and 7.0 cm of vertical displacement appeared on the southern side of Mt Chenoua. They consist of cracks and fissures on vertical bedding planes that belong to a Neogene flexure. The aftershock activity was recorded for several weeks and comprised more than 100 seismic events per day, with 1 < M < 5. Aftershocks are distributed in a ENE-WSW to NE-SW zone extending offshore. A NW-SE cross-section indicates that seismic events affect the crust to a maximum depth of 20 km, showing a complex fault-plane geometry dipping to the NW. For the main shock, the focal mechanism solution obtained from the readings of first motion polarities of seismograms yields an ENE-WSW reverse fault dipping to the NNW, which is in good agreement with both field observations and the aftershock distribution at depth.
The occurrence of the Mt Chenoua-Tipaza moderate-sized event in a previously identified active zone improves the seismotectonic characterization of this part of the Tellan Atlas mountains. This thrust and fold geological domain also shows active folds capable of producing larger earthquakes. Since only a small portion of the Sahel anticline ridge has been reactivated, a serious seismic hazard must be urgently recognized in the Algiers region.     

湖南枞树板铅锌矿区构造特征及控矿条件研究

枞树板铅锌矿处于EW向山河复背斜和SN向西山复背斜复合部位。围岩为震旦系浅变质碎屑岩。NE向压扭性断裂是铅锌矿体的控矿及赋矿构造,富矿体产在控矿断裂较宽大且较破碎的部位,受构造透镜体控制;富矿体长轴向SW侧伏,侧伏角50°-60°,长宽比为2:1,出露标高向SW次第降低呈斜列之势;稀土元素、微量元素和硫、铅同位素特征显示成矿物质主要来自千里山花岗岩,成矿时代为燕山早期(155-170Ma),成矿温度168-244℃,属岩浆期后中低温热液矿床,是千里山岩体岩浆成矿系列(东坡矿田)的组成部分。花岗斑岩脉晚于铅锌矿脉侵位于NE向断裂中,局部穿切并破坏矿脉。区域上成矿、找矿有利部位是燕山早期小岩体周围或地下3-5km处并有隐伏岩体发育的地区。      

Trace‐element signatures of apatites in granitoids from the Mt Isa Inlier,northwestern Queensland

The concentrations of trace elements in apatite from granitoid rocks of the Mt Isa Inlier have been investigated using the laser‐ablation inductively coupled plasma‐mass spectrometry (ICP‐MS) microprobe. The results indicate that the distribution of trace elements (especially rare‐earth elements (REE), Sr, Y, Mn and Th) in apatite strongly reflects the chemical characteristics of the parental rock. The variations in the trace‐element concentrations of apatite are correlated with parameters such as the SiO₂ content, oxidation state of iron, total alkalis and the aluminium saturation index (ASI). The relative enrichment of Y, HREE and Mn and the relative depletion of Sr in the apatites studied reflect the degree of fractionation of the host granite. Apatites from strongly oxidised plutons tend to have higher concentrations of LREE relative to MREE. Manganese concentrations are higher in apatite from reduced granitoids because Mn²⁺substitutes directly for Ca²⁺. The La/Ce ratio of apatite is well‐correlated with the whole‐rock K²O and Na²O contents, as well as with the oxidation state and ASI. Because apatite trace‐element composition reflects the chemistry of the whole rock, it can be a useful indicator mineral for the recognition of mineralised granite suites, where particular mineralisation styles are associated with granitoids that have specific geochemical fingerprints.     

Origin and evolution of volatiles in the Central Europe late Variscan granitoids,using the example of the Strzegom-Sobótka Massif,SW Poland

Mineralogy and Petrology - The results of the new Electron Microprobe Analysis of apatite, hornblende and biotite crystals of the hornblende-biotite variety of the Strzegom-Sobótka granite...     

浙江青田花岗岩中岩石包体特征及成因

浙江青田燕山晚期黑云母花岗岩中有许多石英闪长质岩石包体,它们大小不一,形态各异,以微细粒结构、具冷凝边构造、并发育针状磷灰石为特征。岩石包体与寄主花岗岩的主要氧化物、微量元素的变化趋势呈线性关系。岩石学、矿物学、岩石化学及地球化学等特征的研究表明,石英闪长质岩石包体属淬冷包体,由玄武质岩浆和花岗质岩浆通过不均匀的混合作用而形成。     

The Cretaceous Ofuku Pluton and Its Relation to Mineralization in the Western Akiyoshi Plateau,Yamaguchi Prefecture,Japan

The relationship between the magmatism of the Cretaceous Ofuku pluton and mineralization in and around the Akiyoshi Plateau, Yamaguchi Prefecture, Japan was investigated using a combination of field observation, petrographic and geochemical analyses, K–Ar geochronology, and fluid inclusion data. The Ofuku pluton has a surface area of 1.5 × 1.0 km, and was intruded into the Paleozoic accretionary complexes of the Akiyoshi Limestone, Ota Group and Tsunemori Formation in the western part of the Akiyoshi Plateau. The pluton belongs to the ilmenite‐series and is zoned, consisting mainly of early tonalite and granodiorite that share a gradational contact, and later granite and aplite that intruded the tonalite and granodiorite. Harker diagrams show that the Ofuku pluton has intermediate to silicic compositions ranging from 60.4 to 77.9 wt.% SiO₂, but a compositional gap exists between 70.5 to 73.4 wt.% SiO₂ (anhydrous basis). Modal and chemical variations indicate that the assumed parental magma is tonalitic. Quantitative models of fractional crystallization based on mass balance calculations and the Rayleigh fractionation model using major and trace element data for all crystalline phases indicate that magmatic fractionation was controlled mainly by crystal fractionation of plagioclase, hornblende, clinopyroxene and orthopyroxene at the early stage, and quartz, plagioclase, biotite, hornblende, apatite, ilmenite and zircon at the later stage. The residual melt extracted from the granodiorite mush was subsequently intruded into the northern and western parts of the Ofuku pluton as melt lens to form the granite and aplite. The age of the pluton was estimated at 99–97 Ma and 101–98 Ma based on K–Ar dating of hornblende and biotite, respectively. Both ages are consistent within analytical error, indicating that the Ofuku pluton and the associated Yamato mine belong to the Tungsten Province of the San‐yo Belt, which is genetically related to the ilmenite‐series granitoids of the Kanmon to Shunan stages. The aplite contains Cl‐rich apatite and REE‐rich monazite‐(Ce), allanite‐(Ce), xenotime and bastnäsite‐(Ce), indicating that the residual melt was rich in halogens and REEs. The tonalite–granodiorite of the Ofuku pluton contains many three‐phase fluid inclusions, along with daughter minerals such as NaCl and KCl, and vapor/liquid (V/L) volume ratios range from 0.2 to 0.9, suggesting that the fluid was boiling. In contrast, the granite and aplite contain low salinity two‐phase inclusions with low V/L ratios. The granodiorite occupies a large part of the pluton, and the inclusions with various V/L ratios with chloride daughter minerals suggest the boiling fluids might be related to the mineralization. This fluid could have carried base metals such as Cu and Zn, forming Cu ore deposits in and around the Ofuku pluton. The occurrence and composition of fluid inclusions in the igneous rocks from the Akiyoshi Plateau are directly linked to Cu mineralization in the area, demonstrating that fluid inclusions are useful indicators of mineralization.