Geology and geochemistry of the Mendejin plutonicrocks, Mianeh, Iran

The Mendejin pluton is located in the Mianeh area, NW Iran, 550 km from Tehran. This pluton is probably of Oligo-Miocene age and is the result of extensive magmatism which occurred during and after the Alpine Orogeny. Similar plutons are common in the Alborz–Azarbaijan structural zone of Iran, and it is likely that there are concealed plutons related to this extensive Cenozoic magmatism, but due to their youth and low rates of erosion they have not yet been exposed. The Mendejin pluton is a composite body made up of four types of plutonic rocks: pink tonalite, grey tonalite, diorite and aplite. The pink tonalite is porphyritic and contains phenocrysts of plagioclase, K-feldspar and hornblende in a groundmass consisting of quartz, plagioclase, K-feldspar, hornblende, zircon, monazite, leucoxene, apatite and hematite. The grey porphyritic tonalite has more biotite, pyroxene and pyrite and less accessory phases compared with the pink tonalite. The diorite has a microporphyritic texture with phenocrysts of plagioclase, hornblende and augite. This rock also occurs as xenoliths in the Mendejin pluton. The aplitic dykes are the youngest magmatic products at Mendejin. The Mendejin tonalite contains more Cl, As, S, Cu, Ni and Zn than the global granite. These rocks are of I-type, peraluminous and calc-alkaline, with medium to high potassium, and were formed as part of a volcanic arc. The Mendejin pluton contains up to 8 ppb gold and could potentially have been the source of an economic gold deposit by leaching of Au from wall rocks and deposition in extensive hydrothermally altered marginal zones.     

The influences of incremental pluton growth on magma crystallinity and aureole rheology: numerical modeling of growth of the Papoose Flat pluton,California

The Papoose Flat pluton in the White-Inyo Range, California, is one of the best examples of forcefully emplaced plutons within an arc crust, having internal fabrics and a contact aureole that deformed in a ductile manner. A 2-D numerical model for the thermo-rheological evolution of the pluton–wall rock system is proposed. We explore how the frequency of magma input, from instantaneous, episodic to continuous, affects magma chamber crystallinity and rheology of both the pluton and its contact aureole. We model pluton growth in the depth range of 10–13 km, which is at the brittle–ductile transition of the crust, and in the 7–4 km depth range, where the host rocks are initially brittle. For incremental growth (episodic and continuous), the pluton begins as a sill. With subsequent injections to the bottom, the pluton grows into a laccolith. Results of mid-crustal models show that the ductile region above the Papoose Flat pluton is related to thermal weakening. The ductile region during incremental growth is 100–150 m thick, matching the observed thickness. It is ten times thinner than in the instantaneous growth model. In episodic and continuous models, the upper part of the pluton is fully or quasi-crystalline throughout growth. During continuous growth, it is likely to remain ductile with potential for the development of solid-state fabrics. During episodic growth, strain rates within the pluton during each injection may become sufficiently high to cause embrittlement of magma. In no case a ductile aureole develops above the pluton at the upper-crustal level, but may develop below the pluton, which serves as thermal insulator. Thus, the pluton’s floor may sag. During incremental growth, most of the pluton is below the solidus and brittle. The results suggest that large volcanic eruptions are unlikely to occur by slow pressurization of magma chambers; instead they require rapid injections of large melt volumes.     

Crystallization of the West Farrington Pluton, North Carolina, U.S.A.

The West Farrington pluton in the North Carolina Piedmont isconcentrically zoned from gabbro-diorite near the chilled marginsto leucogranodiorite in the center. A crystallization modelfor the West Farrington pluton has been derived utilizing chemical,petrographic, field, and experimental data. The model involvessimple in situ fractional crystallization from the margins inward,with minimal contamination, crystal settling and floating, ormetasomatism. Rocks of the pluton can be considered as mixturesof early crystallizing minerals (liquidus or near-liquidus phases)and crystallized interstitial liquids. Relative percentage ofentrapped pore liquid increased with increasing degree of crystallization. The original tonalite magma began crystallizing Fe-Ti spinels,plagioclase, and hornblende within a short temperature interval.Crystallization of these minerals controlled fractionation trends.The initial water content in the magma was probably 2–3per cent; under such conditions water saturation would havebeen reached after about 60 per cent of the magma crystallized,assuming crystallization in the lower epizone at about 2000bars total pressure.     

Fault-controlled pluton emplacement in the Sevier fold-and-thrust belt of southwest Montana,USA

Problems associated with syncompressional pluton emplacement center on the need to make room for magma in environments where crustal shortening, not extension, occurs on a regional scale. New structural data from the Pioneer and Boulder batholiths of southwest Montana, USA, suggest emplacement at the top of frontal thrust ramps as composite tabular bodies at crustal depths between 1 and 10 km. Frontal thrust facilitated pluton emplacement was accommodated by: (1) a magma feeder zone created along the ramp interface; (2) providing ‘releasing steps’ at ramp tops that serve as initial points of emplacement and subsequent pluton growth; and (3) localizing antithetic back-thrusts that assist in pluton ascent. A model of magma emplacement is proposed that involves these elements. This model for syntectonic ramp-top emplacement of plutons helps explain how space is made for plutons within fold-and-thrust belts.     

Geophysical characterisation of a blind I-type pluton emplaced within the Bundarra Suite S-type granites of the New England Batholith

Geophysical data are presented that characterise a blind pluton, the Mountain Home Pluton (MHP), which intrudes the southern portion of the Bundarra Suite (BS), 30 km northeast of Bendemeer, New South Wales. A positive magnetic anomaly within the non-magnetic granites of the BS (Banalasta and Pringles Monzogranites) was previously identified as a sub-surface intrusion. Interpretation of new gravity data and analysis of aeromagnetic data are used to infer the depth, size, density, magnetic susceptibility and likely petrology of the pluton. The best-fit model indicates that the MHP is very similar to the Looanga Monzogranite, a felsic member of the Moonbi Suite of the New England Batholith (NEB) that intrudes the BS 5–7 km southeast of the MHP. The top of the MHP is inferred to lie about 1 km beneath the surface and the pluton extends to a depth of at least 6 km. Our model furthermore suggests that the southwestern margin of the MHP is subvertical, whereas a shallower dip (<45°) towards the north is proposed for the northeastern surface of the pluton. A north-trending dyke swarm, identified on the basis of linear positive magnetic anomalies, may be related to the MHP. This swarm of more than 20 relatively magnetic dykes extends out to about 10 km north from the pluton. Magnetic modelling of the dykes indicates that susceptibility values of the dykes are probably very similar to the range of the MHP, and also suggests the width of individual dykes (also not known to be exposed at the surface) to be at most a few tens of metres. A petrographic examination of the intruded BS granites at the surface suggests that metamorphic zoning as seen in mineralogical characteristics may be related to the underlying pluton.     

Thermochronology for the Granitic Pluton Related to Lead-Zinc Mineralization in Tsushima, Japan

Abstract: K–Ar datings and oxygen isotope analyses revealed a cooling history of the Uchiyama granitic pluton, which is genetically related to the Pb-Zn deposits (Taishu mine) in the Tsushima Islands, Japan. The pluton intrudes into the Paleogene Taishu Group to form the biotite-hornfels zone, while the Taishu vein-type Pb–Zn deposits occur in fissures developed in the non-hornfels zone about 1 to 3 km westward from the contact. Amphibole and biotite K-Ar ages of the pluton have a wide range from 19 to 13 Ma. Oxygen isotopes of the biotite and coexisting quartz grains indicate that isotopic exchange reactions have occurred under subsolidus conditions, and that the K-Ar ages are affected by various cooling rates in the pluton. The mineralization age of the Taishu ore deposits is obtained for the first time to be 15.40.8 Ma by a K-Ar age of 2 M ₁–muscovite in a calcite–quartz–muscovite–chlorite veinlet of the Shintomi orebody. Whole rock K-Ar ages of biotite-hornfels near the pluton represent similar ages to the ore deposits. Using blocking temperature calculated from reported diffusivity for argon, the pluton was cooled from 560 to 350C in the period of 17 to 14 Ma. The vein formation took place after the time when temperature in wall rocks of the pluton had dropped below the brittle-plastic transition (about 400C). These results imply that the cooling of the pluton has caused injection of magmatic fluids into meteoric hydrothermal systems, and the Pb-Zn mineralization has occurred due to this mixing at the age of about 15 Ma.     

Petrogenesis of the Barcroft pluton, northern White-Inyo Mountains, east-central California

The White-Inyo Range lies within the regional transition from Paleozoic-Precambrian North American continental basement to outboard Mesozoic and younger accreted terranes and a superimposed Andean-type arc. In the central White Mountains, the metaluminous Barcroft granodiorite invaded a major NE-striking, SE-dipping high-angle reverse fault—the Barcroft break. Because it is a relatively isolated igneous body and is well exposed over an elevation range of 1,500–4,000 m, its thermal history and that of the surrounding superjacent section are clearer than those of nearly coeval, crowded plutons emplaced in the hotter Sierra Nevada belt. The Barcroft pluton was emplaced as a compositionally heterogeneous series of areally scattered melt pulses episodically injected over the approximate interval 167–161 Ma. The oldest dated rocks are relatively quartzofeldspathic, whereas the youngest is more ferromagnesian, suggesting progressive partial fusion of a relatively mafic protolith. Heavy rare earth-enriched zircons indicate that Barcroft melts were derived at mid-crustal depths from a previously emplaced metabasaltic protolith containing plagioclase but lacking garnet. Granodioritic magma genesis involved the possible mixing of mafic and felsic melts, as well as very minor assimilation of country rocks, but mainly by fractional fusion and crystallization. Bulk chemical, rare earth, and isotopic data suggest that analyzed Barcroft rocks are members of a single suite. Granodioritic rocks are slightly more magnetite-rich at higher elevations on the NE, nearer the roof of the pluton. Earlier thermobarometry chronicled cooling and re-equilibration of the Barcroft pluton from its margins inward, as well as from mid-crustal generation depths of ~25 km through ascent and stalling at ~10–12 km. Refractory phase assemblages crystallized along the pluton margins, whereas subsolidus minerals in the interior of the of body continued to exchange with upper crustal deuteric and surficial aqueous fluids during exhumation and cooling.     

Mg-Al Sapphirine- and Ca-Al Hibonite-bearing Granulite Xenoliths from the Chyulu Hills Volcanic Field, Kenya

Basanites of the Chyulu Hills (Kenya Rift) contain mafic Mg–Aland Ca–Al granulite xenoliths. Their protoliths are interpretedas troctolitic cumulates; however, the original mineral assemblageswere almost completely transformed by subsolidus reactions.Mg–Al granulites contain the minerals spinel, sapphirine,sillimanite, plagioclase, corundum, clinopyroxene, orthopyroxeneand garnet, whereas Ca–Al granulites are characterizedby hibonite, spinel, sapphirine, mullite, sillimanite, plagioclase,quartz, clinopyroxene, corundum, and garnet. In the Mg–Algranulites, the first generation of orthopyroxene and some spinelmay be of igneous origin. In the Ca–Al granulites, hibonite(and possibly some spinel) are the earliest, possibly igneous,minerals in the crystallization sequence. Most pyroxene, spineland corundum in Mg–Al and Ca–Al granulites formedby subsolidus reactions. The qualitative P–T path derivedfrom metamorphic reactions corresponds to subsolidus cooling,probably accompanied, or followed by, compression. Final equilibrationwas achieved at T 600–740°C and P <8 kbar, inthe stability field of sillimanite. The early coexistence ofcorundum and pyroxenes (± spinel), as well as the associationof sillimanite and sapphirine with clinopyroxene and the presenceof hibonite, makes both types of granulite rare. The Ca–Alhibonite-bearing granulites are unique. Both types enlarge thespectrum of known Ca–Al–Mg-rich granulites worldwide. KEY WORDS: granulite xenoliths; corundum; sapphirine; hibonite; Kenya Rift     

房山变质核杂岩基底拆离断层韧性剪切变形构造及环境分析

在房山岩体南北缘出露有太古代(基底) 官地杂岩, 官地杂岩与上覆盖层不同地层之间发育一条基底韧性拆离断层.宏观及微观尺度上拆离断层运动学标志均指示SE; 剪切带内发育区域动力变质作用下的矿物组合角闪石-斜长石及硅值较高的白云母, 对其进行电子探针分析, 计算出拆离断层韧性剪切变形的温压条件为: 温度492~555 ℃, 压力0.33 Gpa左右, 达到低角闪岩相.按正常的静岩压力计算, 该韧性剪切作用发生于地表以下12.9 km左右, 代表了中地壳韧性流变的变形环境.野外观察发现房山侵入体与官地杂岩及该韧性剪切带间均呈明显的侵入接触关系, 在侵入岩体南北边缘有大量的片麻岩等捕虏体, 沿杂岩的片麻理或韧性构造面理, 发育大量的石英二长闪长岩脉, 岩脉成分与房山岩体一致, 因此该韧性剪切带的形成应早于房山岩体侵位.如对房山岩体的侵入和改造进行复原和恢复, 该韧性剪切带代表了早期的伸展作用, 可能与房山伸展穹隆体的韧性变形同期.      

Corundum-group minerals in rocks of the Khibiny alkaline pluton,Kola Peninsula

Five minerals of the corundum group have been identified in the Khibiny pluton with certainty. Corundum proper and karelianite occur only in hornfels after volcanic and sedimentary rocks. Xenoliths of hornfels mark the ring faults that bound foidalite within the field of foyaite. Hematite occurs in hydrothermally altered nepheline syenite and crosscutting hydrothermal veins related to the ring faults. Minerals of the ilmenite-pyrophanite series are present in all rocks of the pluton, including veins. Accessory ilmenite in foyaite varies from the manganese variety and pyrophanite in the inner and outer parts of the pluton to manganese-free ilmenite in zone of the Main Ring Fault. In xenoliths of volcanic rocks and alkaline ultramafic rocks, ilmenite is enriched in magnesium. The zoning in distribution of the above-mentioned minerals and the character of variation in their compositions from margins of the pluton to its center are consistent with the petrochemical zoning formed as a result of foyaite alteration of near ring faults.     

Incremental emplacement and syn-tectonic deformation of Late Triassic granites in the Qinling Orogen: Structural and geochronological constraints

Granitoids are important components of major orogenic belts, and provide important information about the regional geodynamic evolution. The emplacement mechanism of granite plutons and its relationship with regional tectonics has long been discussed, although it still remains debated. The Qinling Orogen within the Central China Orogen was marked by the emplacement of numerous Late Triassic granitic plutons. Although the petrology, geochemistry and geochronology of these intrusions have been addressed in various studies, their tectonic setting remains controversial, particularly since the structural aspects not been evaluated in detail. In this study, we attempt to reconstruct the emplacement process of the Late Triassic Dongjiangkou pluton in the South Qinling Belt. Field observations show extensive syn-plutonic deformations both in the pluton and its contact zones. Microstructural observations demonstrate that fabrics in the pluton were mainly acquired during submagmatic flow to high-T solid-state deformation. Zircon U–Pb ages reveal that the pluton is a composite intrusion which is composed of two juxtaposed small plutons with distinct ages (~210 Ma and ~200 Ma). Al-in-hornblende thermobarometer indicates that the pluton was formed at depths ranging from 4.7 km to 8.8 km, with an increasing depth trend from the inner unit to the outer unit. Distribution of the internal fabrics shows two concentric patterns which are concordant with pluton margins at the pluton scale and were probably induced by the regional sinistral transpression. Integrating these analyses, an incremental emplacement model is proposed for the syn-tectonic pluton. This model not only solves the ‘room problem’ but also accounts for the zoned petrological features of the pluton. Combined with previous studies, we suggest that the Late Triassic granite plutons in the Qinling Orogen were emplaced under a syn-collisional convergence setting, and that the granite magmatism was probably controlled by regional tectonics. Additionally, the incremental emplacement model may be a common mechanism for the Late Triassic granite plutons.     

Space accommodation by roof lifting during pluton emplacement at Amiata (Italy)

The Amiata area is characterized by an uplift attributed to pluton emplacement during the Pliocene epoch. The shape of the buried pluton is constrained by gravity data. Fieldwork was carried out to identify possible deformations within the overburden, in order to evaluate the mechanism of emplacement. The attitudes of Lower Pliocene marine sediments, deposited immediately before the uplift, were measured also. The overall pattern of the deposits defines a dome, approximately 2.5 km high and 35 × 50 km wide. Consistency between these field data and the existing geophysical data suggests that the dome results from the emplacement of a laccolith-like intrusion which was responsible for lifting and flexuring the overburden. The Amiata case shows that roof lifting is a viable process by which space in the upper crust, to a depth of 7 km, can be accommodated during pluton emplacement.     

Orthomagmatic quartz and post-magmatic carbonate veins in a reported porphyry copper deposit,Andean Intrusive Suite,Livingston Island,South Shetland Islands

A previously reported porphyry Cu + Mo deposit in an Eocene pluton within the South Shetland Island magmatic arc has been re-interpreted as three distinct hydrothermal assemblages. The oldest assemblage (1) exsolved under confinement from the deep (～6 km?) cooling magma whereas assemblages (2) and (3) formed during tectonic ± magmatic episodes at depths of < 1.5 km in the late Cenozoic. The three assemblages occur over the 5 × 11 km mapped in Barnard Point tonalite pluton. Assemblage (1) comprises shallowly dipping sheets of aplite, biotite + tourmaline pegmatite, massive ‘grey’ quartz, and quartz + tourmaline + bornite + chalcopyrite + molybdenite veins. Magnetite + tourmaline + chalcopyrite breccias have associated biotite, K-feldspar and muscovite alteration. Fluid inclusions indicate formation from hot (～600°C), saline (40 equivalent weight % NaCl + CaCl₂) aqueous-carbonic fluids that exsolved from the partly consolidated magma. The primary control on solution chemistry and nature of fracturing was the depth of pluton emplacement. Assemblage (2) consists of steep, vuggy veins and country-rock breccias, with thick propylitic alteration selvages, cemented by microcrystalline quartz, complex inter-growths of FeMg carbonate, bladed barite and trace amounts of bornite and chalcopyrite. These rocks, previously described as breccia (sensu ‘pebble’) dykes in the porphyry complex, are reinterpreted as an influx of moderately hot (175–330°C), weak to moderately saline (2–21 EWP NaCl), aqueous-carbonic fluids that underwent isobaric boiling at 0.8 to 1.3 km depth. Assemblage (3) consists of thin, hematitic fault infillings formed during a second episode of brittle faulting.     

Relations between the emplacement and fabric-forming conditions of the Kapitan-Dimitrievo pluton and the Maritsa shear zone (Central Bulgaria): magnetic and visible fabrics analysis

The Kapitan-Dimitrievo pluton was emplaced within the 15 km wide Maritsa shear zone during the Late Cretaceous. It has well-known U–Pb zircon age (78.54 ± 0.13 Ma) and appears as a late-syntectonic intrusion that marked the last ductile deformation in the Maritsa shear zone. Magnetite is believed to be the main carrier of the magnetic fabric in this pluton, and crystallized mainly late, after the main rock-forming minerals. Two fabrics are recorded, a visible syn-magmatic fabric (due to magma flow) and magnetic late-magmatic fabric (related to regional stresses). Although different, both are mainly related to the shearing along this shear zone. These results constrain in age the dextral strike-slip controlled emplacement and evolution of the Late Cretaceous plutons from Central Bulgaria.     

Crystal Accumulation and Shearing in a Megacrystic Quartz Monzonite: Bodoco Pluton, Northeastern Brazil

The Bodocó pluton, typical of numerous felsic intrusionsin northeastern Brazil that are characterized by blocky megacrystsof K-feldspar, consists mainly of porphyritic coarse-grainedquartz monzonite (SiO₂ 58–70 wt %) and is reversely zonedfrom a granitic margin to a quartz monzodioritic core. Thereis little variation in mineral composition throughout the pluton,despite a range of variation in mineral proportions. Isotopiccharacteristics also are homogeneous, with ¹⁸O_quartz between+9·3 and +9·8 and initial ⁸⁷Sr/⁸⁶Sr within limitsof 0·7056–0·7063. Petrogenetic modellingindicates that in situ crystal accumulation processes, accompaniedby the upward migration of a crystal-poor felsic melt, can accountfor many of the observed chemical and isotopic features, petrographictextures, and spatial relationships of rock types. Localizedshearing associated with regional ductile deformation producedextensive kilometre-wide bands of strongly foliated megacrysticquartz monzonite intruded by mafic dykes. Shear-related magmamingling and/or mixing were localized post-emplacement differentiationprocesses, particularly at the upper level of the intrusionand in quartz monzonite border units along the southeast margin. KEY WORDS: accumulation; Brazil; megacryst; petrogenesis; shearing     

Petrogenesis of the Kaikomagatake granitoid pluton in the Izu Collision Zone, central Japan: implications for transformation of juvenile oceanic arc into mature continental crust

The Miocene Kaikomagatake pluton is one of the Neogene granitoid plutons exposed in the Izu Collision Zone, which is where the juvenile Izu-Bonin oceanic arc is colliding against the mature Honshu arc. The pluton intrudes into the Cretaceous to Paleogene Shimanto accretionary complex of the Honshu arc along the Itoigawa-Shizuoka Tectonic Line, which is the collisional boundary between the two arcs. The pluton consists of hornblende–biotite granodiorite and biotite monzogranite, and has SiO₂ contents of 68–75 wt%. It has high-K series compositions, and its incompatible element abundances are comparable to the average upper continental crust. Major and trace element compositions of the pluton show well-defined chemical trends. The trends can be interpreted with a crystal fractionation model involving the removal of plagioclase, biotite, hornblende, quartz, apatite, and zircon from a potential parent magma with a composition of ~68 wt% SiO₂. The Sr isotopic compositions, together with the partial melting modeling results, suggest that the parent magma is derived by ~53% melting of a hybrid lower crustal source comprising ~30% Shimanto metasedimentary rocks of the Honshu arc and ~70% K-enriched basaltic rocks of the Izu-Bonin rear-arc region. Together with previous studies on the Izu Collision Zone granitoid plutons, the results of this study suggest that the chemical diversity within the parental magmas of the granitoid plutons reflects the chemical variation of basaltic sources (i.e., across-arc chemical variation in the Izu-Bonin arc), as well as a variable contribution of the metasedimentary component in the lower crustal source regions. In addition, the petrogenetic models of the Izu Collision Zone granitoid plutons collectively suggest that the contribution of the metasedimentary component is required to produce granitoid magma with compositions comparable to the average upper continental crust. The Izu Collision Zone plutons provide an exceptional example of the transformation of a juvenile oceanic arc into mature continental crust.     

Garnetization as a ground preparation process for copper mineralization: evidence from the Mazraeh skarn deposit, Iran

The Mazraeh Cu–Fe skarn deposit, NW Iran is the result of the intrusion of an Oligocene–Miocene granitic pluton into Cretaceous calcareous rocks. The pluton ranges in composition from monzonite to quartz monzonite, monzogranite, tonalite and granodiorite with I-type, calc-alkaline, and weakly peraluminous characteristics. The Mazraeh pluton was emplaced in a volcanic arc setting in an active continental margin at a depth of ~8 km. Pyroxene skarn, garnet skarn, and epidote skarn zones were formed during the intrusive phase. The garnet skarn developed as exoskarn and endoskarn from the calcareous wall rocks and the pluton, respectively, prior to mineralization. Garnet skarn from the exoskarn zone is identified by relict layering inherited from the precursor calcareous lithologies. Mass balance calculation of garnet skarn in the endoskarn zone indicates that hydrothermal fluids originating from the cooling magma introduced Si, Fe, Mn, Ca, Mg, P, Ag, Cu, Zn, La, Pb, Cd, Mo, and Y. The main mass loss in the garnet skarn was due to destruction of feldspars in the Mazraeh plutonic rocks and leaching of K₂O and Na₂O. Released Ca has been fixed in the andraditic garnet. Garnetization of the Mazraeh pluton was accompanied by mass and volume increase. The magnitude of these changes depends mainly on the degree of alteration and composition of the precursor. The brittle behavior of the endoskarn zone was increased due to formation of massive garnet which subsequently fractured. These fractures not only facilitated movement of hydrothermal fluids but also provided new locations for Cu mineralization. Therefore locating strongly garnetized zones may be a vector to ore in skarn deposits.     

赣南燕山期水头岩体的锆石铀-铅年代学研究及其含矿性评价

水头岩体是目前赣南地区缺少精确年龄数据的复式岩体之一。对赣南水头岩体中的锆石样品进行了U-Pb法年代学研究,测年结果显示该岩体主体年龄为(142.7±0.6)Ma,属于燕山期的产物,并不是前人认为的加里东期。但也存在前期(海西期和加里东期)的同位素年龄信息记录,说明该岩体是复式岩体,存在至少三期岩浆活动。对水头岩体进行了岩石学、地球化学研究,简要论述了岩石成因及含矿性。水头岩体主体为黑云母花岗岩,为过铝质钾玄岩系,轻稀土和重稀土中等分馏,负Eu异常明显。初步认为水头岩体是原地重熔交代作用形成的复式岩体。岩石化学资料数据显示,该岩体具备稀土成矿母岩的基本条件。     

Petrological and geochemical evolution of the Kymi stock, a topaz granite cupola within the Wiborg rapakivi batholith, Finland

The 6×3 km Kymi monzogranite stock represents the apical part of an epizonal late-stage pluton that was emplaced within the 1.65 to 1.63 Ga Wiborg rapakivi batholith. The stock has a well-developed zonal structure, from the rim to the center: stockscheider pegmatite, equigranular topaz granite, porphyritic topaz granite. The contact between the two granites is usually gradational within a few centimeters, but local inclusions of the porphyritic granite in the equigranular granite indicate that the latter solidified later. Hydrothermal greisen and quartz veins, some of which contain genthelvite, beryl, wolframite, cassiterite, and sulfides, cut the granites of the stock and the surrounding country rocks. The equigranular granite contains 1 to 4 vol.% topaz, and its biotite is lithian siderophyllite; the porphyritic granite has 0 to 3 vol.% topaz, and the mica is siderophyllite. The equigranular granite is geochemically highly evolved with elevated Li, Rb, Ga, Ta, and F, and very low Ba, Sr, Ti, and Zr. The REE patterns show deep negative Eu anomalies and tetrad effects indicating extreme magmatic fractionation and aqueous fluid–rock interaction. The zonal structure of the stock is interpreted as a result of differentiation within the magma chamber. Internal convection in the crystallizing magma chamber and upward flow of residual melt as a boundary layer along sloping contacts resulted in accumulation of a layer of highly evolved, volatile-rich magma in the apical part of the chamber. Crystallization of this apical magma produced the stockscheider pegmatite and the equigranular granite; the underlying crystal mush solidified as the porphyritic granite. Much of the crystallization took place from volatile-saturated melt, and episodic voluminous degassing expelled fluids into opened fractures where they or their derivatives reacted with country rocks and caused alteration and mineralization.     

Arc plutonism in a transtensional regime: the late Palaeozoic Totoltepec pluton,Acatlán Complex,southern Mexico

The ENE-trending, ca. 306–287 Ma, Totoltepec pluton is part of a Carboniferous–Permian continental magmatic arc on the western Pangaean margin. The 15 km?×?5 km pluton is bounded by two N–S Permian dextral faults, an E–W thrust to the south, and an E–W normal fault to the north. Thermobarometric data indicate that the main, ca. 289–287 Ma, part of the pluton was emplaced at ≤20 km depth and ≥700°C and was exhumed to 11 km and 400°C in 4 ± 2 million years. We have documented the following intrusive sequence: (1) the 306 Ma northern marginal mafic phase; (2) the 287 Ma main trondhjemitic phase; and (3) ca. 289–283 Ma sub-vertical dikes that vary from (a) N39E, undeformed with crystal growth perpendicular to the margins, through (b) ca. N50–73E, foliated and folded with sinistral shear indicators, to (c) N73–140E and boudinaged. The obliquity of the boundary between the folded and stretched dikes relative to the N–S dextral faults suggests sequential emplacement in a transtensional regime (with 20% E–W extension), followed by different degrees of clockwise rotation passing through a shortening field accompanied by sinistral shear into an extensional field. The ca. 289–287 Ma intrusion also contains a steep ENE-striking foliation and hornblende lineations varying from sub-horizontal to steeply plunging, probably the result of emplacement in a triclinic strain regime. We infer that magmatism ceased when some of the dextral motion was transferred from the western to the eastern bounding fault, causing thrusting to take place along the southern boundary of the pluton. This mechanism is also invoked for the rapid uplift and exhumation of the pluton between ca. 287 Ma and 283 Ma. The distinctive characteristics of the Totoltepec pluton should prove useful in identifying similar tectonic settings within continental arcs.