Structural and Magmatic Evolution of a Magma Chamber: The Newark Island Layered Intrusion, Nain, Labrador

The Newark Island Layered Intrusion occurs in the ProterozoicNain anorthosite complex of Labrador. It contains an exceptionalsuite of cumulates ranging from troctolites and gabbros to quartzmonzonites and intermediate hybrid rocks. These layered rocksformed in a chamber that was periodically fed by a wide rangeof basic and acid magmas, the compositions of which are preservedin numerous feeder dikes. Where basic magmas commingled withcooler granitic magma, they commonly formed chilled pillows.Because of periodic injections of both acid and basic magmasthe magma chamber was compositionally stratified for much ofits existence. At times, granitic cumulates formed along thechamber walls while mafic to intermediate hybrid cumulates formedon the floor. Stratigraphic and structural relations indicatethat the magma chamber grew upward during deposition, and thatit evolved from a west-dipping sheet to a north-plunging synform.Three major episodes of expansion can be linked to injectionsof large (e.g., 20km³) volumes of acid magma. The entry of thisacid magma into the chamber disrupted previously formed cumulates,creating enlarged feeders down which resident basic magma collapsed.The resultant structures (troughs) contain strongly chilledpillows of resident basic magma that existed near the bottomof the chamber at the time of acid replenishment.     

Interactions between dioritic and granodioritic magmas in mingling zones: plagioclase record of mixing,mingling and subsolidus interactions in the Gęsiniec Intrusion,NE Bohemian Massif,SW Poland

Dioritic and granodioritic rocks coexist in the Gęsiniec Intrusion in SW Poland showing typical relationships in many mafic–felsic mingling zones worldwide, such as dioritic syn-putonic dykes and microgranular enclaves within granodioritic host. Plagioclase zonation from granodioritic rocks suggests late stage mixing probably with dioritic magma, whereas no magma mixing is recorded in plagioclase from dioritic rocks. The diorites seem to show effects of interaction with evolved, leucocratic melts derived from granodiorite, not with the granodioritic melt itself. We conclude that the diorites’ compositions were modified after their emplacement within the granodioritic host, when the diorites were essentially solidified and injection of evolved melt from granodiorite did not involve marked modification of plagioclase composition. Compositional zoning patterns of plagioclase in diorites can be modeled by closed system fractional crystallization interrupted by resorption induced probably by decompression. Granodioritic plagioclase seems to be affected by the same resorption event. Plagioclase that crystallized in dioritic magma before the resorption does not record interaction between dioritic and granodioritic magmas, suggesting that both magmas evolved separately. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The Pleasant Bay Layered Gabbro--Diorite, Coastal Maine: Ponding and Crystallization of Basaltic Injections into a Silicic Magma Chamber

The Pleasant Bay layered gabbro–diorite intrusion, locatedon the coast of Maine between Bar Harbor and Machias, is roughlyoval in plan, measuring 12 km by 20 km. Gravity data, contactrelations, and internal layering suggest that it is basinformin structure with a maximum thickness of {small tilde}3 km.Its roof and upper parts have been lost through erosion. Whereit is in contact with underlying granite, the base of the intrusiontypically consists of strongly chilled gabbro with convex-downwardlobate forms, suggesting that the granite was incompletely solidifiedwhen the gabbro was emplaced. Roughly 90% of the exposed rocksare weakly layered gabbro and mafic diorite, both of which varywidely in grain-size and texture. Layers and lenses of medium-grainedleucocratic diorite to granodiorite are widely intercalatedwith the chilled mafic rocks and commonly contain partly digestedmafic inclusions; they also commonly contain zones of pillow-likebodies of gabbro chilled on all margins. The dioritic rocksare consistently topped by gabbroic layers with chilled lobatebases and commonly appear to feed granitic pipes and diapirsinto overlying gabbro. Much of the intrusion can be subdividedinto hundreds of macrorhythmic units (from 1 to 100 m thick)consisting of basally chilled gabbro that grades upward to dioriteor highly evolved leucocratic silicic cumulates. Basaltic dikesare abundant both in the underlying granite and in the layeredgabbro–diorites; they have appropriate compositions tobe feeders for chilled gabbroic layers in the Pleasant Bay intrusion. The layered rocks of the Pleasant Bay intrusion record hundredsof basaltic injections into a chamber with resident silicicmagma. Small injections produced chilled gabbroic layers andpillows within silicic cumulates. Larger infusions of basalticmagma produced temporary compositional stratification and episodesof double-diffusive convection within the chamber. Althoughfractional crystallization produced compositional variationin much of the gabbro, units that grade from chilled gabbroat the base to highly silicic cumulates at the top provide cumulaterecords of magma stratification and hybridization along a double-diffusiveinterface between basaltic and silicic magmas. The intrusionprovides a superb plutonic record of events that have oftenbeen inferred for silicic eruptive centers. Mafic–siliciclayered intrusions comparable with the Pleasant Bay are morewidespread than has generally been appreciated.     

Slow,dense replenishments of a basic magma chamber: the layered series of the Newark Island layered intrusion,Nain, Labrador

The Newark Island layered intrusion is a composite layered intrusion within the Nain anorthosite complex, Labrador. The intrusion comprises a lower layered series (LS) dominated by troctolites, olivine gabbros and oxide-rich cumulates and an upper hybrid series (HS) characterized by a wide range of mafic, granitic and hybrid cumulates and discontinuous layers of chilled mafic rocks (Wiebe 1988). The HS crystallized from a series of replenishments of both silicic and basic magmas. The LS crystallized from periodically replenished basic magmas. The LS has a lower zone that consists mainly of olivine-plagioclase cumulates and contains minor cryptic reversals in mineral compositions that resulted from replenishments of relatively primitive magma. An upper zone is dominated by olivine-plagioclaseaugite-ilmenite cumulates. Cumulus titanomagnetite and pyrrhotite occur within some oxide-rich cumulates, and the stratigraphically highest layers contain cumulus apatite. At intermediate levels in the sequence, cumulus inverted pigeonite occurs in place of olivine. Several prominent regressions in the stratigraphy of the upper zone are marked by fine-grained troctolitic layers with much higher Mg no. [100 MgO/(MgO+FeO)] and anorthite than underlying cumulates. These layers coarsen upward and grade back to oxide-bearing olivine gabbros within thicknesses ranging from 10 cm to 15 m. Dikes that cut the LS have major- and trace-element compositions that strongly suggest that they are feeders for the replenishments. In the lower zone when olivine and plagioclase were the only cumulus phases, replenishments were less dense than the resident magma and rose as plumes and mixed with it. Precipitation of cumulus oxides in the upper zone lowered the density of resident magma so that subsequent replenishments were more dense than resident magma. Replenishments that occurred after oxides began to precipitate had small injection velocities. These post-oxide injections flowed along the interface between resident magma and the cumulate pile and precipitated flow-banded, fine-grained troctolites.     

Isotopic responses to basaltic injections into silicic magma chambers: a whole-rock and microsampling study of macrorhythmic units in the Pleasant Bay layered gabbro-diorite complex, Maine, USA

The Pleasant Bay layered gabbro-diorite complex (420 Ma) formed via repeated injections of mafic magma into a felsic magma chamber. It is dominated by repeating sequences (macrorhythmic units) with chilled gabbroic bases which may grade upward into medium-grained gabbro, diorite and granite. Each unit represents an injection of mafic magma into the chamber followed by differentiation. Increases in Sr_i and decreases in )_Ndi with stratigraphic height indicate open-system isotopic behaviour and exchange between the mafic and felsic magmas. Isotopic variations of whole-rock samples in individual macrorhythmic units do not conform to bulk mixing or AFC models between potential parental magmas. Sr isotopic studies of single feldspar crystals from one macrorhythmic unit indicate that exchange of crystals between the resident felsic magma and mafic influxes was important, that some of the rocks contain feldspar xenocrysts, and that the rocks are isotopically heterogeneous on an intercrystal scale. Xenocryst abundance increases with stratigraphic height, suggesting that crystal exchange occurred in situ. The lack of disequilibrium textures in the xenocrystic feldspar indicates the evolved macrorhythmic magma and resident silicic magma were of a similar composition and likely in thermal equilibrium at the time of crystal transfer. Mafic chilled margins are enriched in alkalis and isotopically evolved compared with mafic dikes (representing the parental melts) and suggest rapid in-situ diffusional exchange following emplacement of individual mafic replenishments.     

Revisiting problem of chilled margins associated with marginal reversals in mafic–ultramafic intrusive bodies

Numerous observations on mafic–ultramafic layered intrusions, sills and dykes show that chilled margins always develop as an integral part of their marginal reversals and possess the following features: (a) they are commonly much more evolved or primitive than bulk intrusion compositions, (b) evolved chilled margins are composed of the low temperature cotectic assemblages of relevant magmatic systems and (c) tend to be compositionally similar in intrusions formed from different parental magmas, (d) fine-grained chilled margins are notably absent in many intrusions, with contact rocks being represented by medium- to coarse-grained cumulates. The anomalous features of chilled margins can be partly attributed to contamination, intratelluric inhomogeneity of magma, changes in composition of intruding magma, loss of magma from the chamber, supercooling, etc. A major process still remains, however, illusive, but appears to be universally operating along the cooling margins of magmatic bodies in a liquid state, being gravity-independent and temperature gradient-driven. We recognize this not yet specified process as Soret fractionation and explain the above observations in the following way. Primary chilled margins do not commonly survive because of intensive remelting by heat flux from the interior of the chamber. The subsequently formed “secondary chilled margins” represent cumulates that crystallized from liquids produced by temperature gradient-driven Soret fractionation. At high temperature gradients the process tends to produce similar cotectic liquids crystallizing gabbronorite (or gabbro) from all parental magmas of a given magmatic system, resulting in compositionally similar “secondary chilled margins” that are more evolved than bulk compositions. At low temperature gradients the process produces liquids that are only slightly more fractionated than the parental magma and form “secondary chilled margins” that are more primitive than bulk compositions. This interpretation suggests that, apart from the rare cases of chilled margins that survived remelting, they should not be used as monitors for parental magma compositions of intrusive bodies, even if all conventional complicating factors were not operative.     

Petrology of basic and intermediate orogenic granitoids from the Sila Massif (Calabria,southern Italy)

Hercynian gabbroic, dioritic and tonalitic rocks crop out in the neighbourhood of Rovale (Sila Grande, Calabria). They make up a crude rectangular outcrop with the western part consisting of gabbroic rocks and the eastern of dioritic and tonalitic rocks. They come into contact with medium to high grade metapelites on the western side and with heterogeneous granodiorites on the other sides. In the gabbroic body both opx ± ol bearing cumulates and amphibole differentiates occur and are characterized by the widespread presence of brown pargasite. Sporadic magmatic to subsolidus corona textures between olivine and plagioclase or orthopyroxene and plagioclase can be observed and their preservation clearly suggests a post-tectonic emplacement for the gabbroic magma. Diorites and tonalites display hypidiomorphic textures free of olivine and orthopyroxene and bearing green Mg-hornblende. The granitoids, on the basis of chemical data, display orogenic features of the continent-continent collision type. The gabbroic rocks have high Al tholeiitic composition and fractionation of orthopyroxene and plagioclase played an important part in their evolution. The Rb/Sr isochron method did not give a precise emplacement age for the granitoids as a whole. Initial ⁸⁷Sr/⁸⁶Sr ratios (at 290 Ma) are higher in the gabbroic body (0.7091–0.7095) than in diorites and tonalites (0.7083–0.7092). Thus gabbroic rocks appear more displaced than diorites and tonalites towards crustal isotopic composition. The eNd data seem to confirm this feature, thus suggesting that the gabbroic rocks and diorites derived from distinct mantle magma batches. Interestingly, small isotropic gabbroic masses occur within the diorites and show general features that allow them to be considered as possibly parental with respect to the host diorites. The evolution to the dioritic composition might have occurred through fractionation and minor mixing with a more acidic component such as the northern granodiorites. Geochemical, Sr and Nd isotopic data indicate a scenario of a composite plutonic body formed by distinct magma batches of mixed crust and mantle origin.     

花岗质岩浆起源和多次岩浆混合的标志：包体：—以北京周口店岩体为例

北京周口店岩株,主要由两次侵入的石英二长闪长岩和花岗闪长岩组成,其中包体广泛分布。在岩体南缘的关坻变质岩中,零星出露了一种块状的闪长质岩石。关坻闪长岩与主岩体同期侵入的岩浆结晶产物,角闪石质包体的形成是二长辉长岩浆结晶—再平衡的结果,富云包体系花岗闪长岩浆源区的耐熔残余。通过二长辉长岩与花岗闪长岩两种岩浆简单混合可形成细粒石英二长质包体,由二长辉长岩、关坻闪长岩和花岗闪长岩3种岩浆混合形成了微粒闪长质包体。主岩体的成分变异,也与岩浆混合有关。     

Magmatic evolution of the Mantos Blancos copper deposit, Coastal Range of northern Chile: insight from Sr–Nd isotope, geochemical data and silicate melt inclusions

The Mantos Blancos copper deposit (500 Mt at 1.0% Cu) was affected by two superimposed hydrothermal events: (i) phyllic alteration related to a rhyolitic dome emplacement and brecciation at ca 155 Ma; and (ii) potassic, sodic and propylitic alteration at ca 142 Ma, coeval with stocks and sills emplacement of dioritic and granodioritic porphyries, that locally grade upwards into polymictic magmatic hydrothermal breccias. Major hypogene copper sulfide mineralization is related to the second event. A late‐ore mafic dike swarm cross‐cuts all rocks in the deposit. Two types of granodioritic porphyries can be distinguished from petrographic observations and geochemical data: granodiorite porphyry I (GP I) and granodiorite porphyry II (GP II), which resulted from two different trends of magmatic evolution. The concave shape of the rare earth element (REE) distribution pattern together with the weak or absence of negative Eu anomalies in mafic dikes, dioritic and GP I porphyries, suggest hornblende‐dominated fractionation for this magmatic suite. In contrast, distinct negative Eu anomalies and the flat REE patterns suggest plagioclase‐dominated fractionation, at low oxygen fugacity, for the GP II porphyry suite. But shallow mixing and mingling between silicic and dioritic melts are also likely for the formation of the GP II and polymictic breccias, respectively. Sr‐Nd isotopic compositions suggest that the rhyolitic dome rocks were generated from a dominantly crustal source, while the GP I has mantle affinity. The composition of melt inclusions (MI) in quartz crystals from the rhyolitic dome is similar to the bulk composition of their host rock. The MI analyzed in quartz from GP II and in the polymictic magmatic hydrothermal breccia of the deposit are compositionally more evolved than their host rocks. Field, geochemical and petrographic data provided here point to dioritic and siliceous melt interaction as an inducing mechanism for the release of hydrothermal fluids to form the Cu mineralization.     

Magma dynamics at the base of an evolving mafic magma chamber: Incompatible element evidence from the Partridge River intrusion, Duluth Complex, Minnesota, USA

A characteristic feature of the Partridge River intrusion of the Keweenawan Duluth Complex is the approximately fivefold to ninefold increase in the concentrations of incompatible elements in the lower zone compared with cumulates stratigraphically higher. The concentrations of incompatible elements decrease from the lower zone upward to steady state values, which is ascribed to variations in the proportions of trapped liquid rather than variable degrees of fractional crystallization of a single parental magma. The calculated average composition of trapped liquid using our algorithm is similar to typical Keweenawan low-alumina, high Ti---P basalts associated with the Duluth Complex but is different from the leading edge ferrodioritic liquid quenched in the chilled margin of the intrusion. This difference suggests that the chilled margin does not represent the original (parental) magma composition from which the whole intrusion solidified, and that the enrichment of incompatible elements may be related to the local flotation of magmatic suspensions. To test the latter hypothesis numerically, we have used heat-mass transfer models, assuming a sheet-like magma chamber, to calculate the parameters of the model that best reproduce the observed distribution of incompatible elements in a mush zone at the base of the Partridge River intrusion. The results indicate that a mush zone enriched in the incompatible elements is produced if the velocity of movement of the lower solidification front into the magma body was less than the floating velocity of the bulk crystal mush. The dynamic parameters that best reproduce the observed distribution of incompatible elements include a magma emplacement pressure of 2 kbar, critical crystallinities of 50–68% in the mush zone from which the liquid is being expelled, and an emplacement temperature of 1160°C for the initial magma.     

Controls on disseminated PGE–Cu–Ni sulfide mineralization within the Rietfontein deposit,Eastern Limb,Bushveld Complex,South Africa: Implications for the formation of contact-type magmatic sulfide deposits

The Rietfontein platinum group element (PGE)–Cu–Ni sulfide deposit of the Eastern Limb of the Bushveld Complex hosts disseminated contact-style mineralization that is similar to other economic magmatic sulfide deposits in marginal settings within the complex. The mineralization at Rietfontein consists of disseminated PGE-bearing base metal sulfides that are preferentially located at the contact between a distinct package of marginal norites overlain by a thick heterogeneous unit dominated by gabbronorites with lesser norites and ultramafic rocks. Down-hole composite data and metal scatterplots indicate that the PGE correlate well with Ni, Cu and S and that only minor metal remobilization has taken place within the basal norite sequence. Plots of (Nb/Th)_PM vs. (Th/Yb)_PM indicate that the melts that formed the Rietfontein intrusive sequence were strongly crustally contaminated prior to emplacement at Rietfontein, whereas inverse relationships between PGE tenors and S/Se ratios indicate that these magmas assimilated crustal S, causing S-saturation and the formation of immiscible sulfides under high R-factor conditions that generated high PGE tenor sulfides. Reverse zoning of cumulus minerals at Rietfontein suggests that fresh primitive melts were introduced to a partially fractionated staging chamber. The introduction of new magmas into the chamber caused overpressure and the forced evacuation of the contents of the chamber, leading to the emplacement of the existing magmas within the staging chamber at Rietfontein in two separate pulses. The first pulse of magma contained late-formed cumulus phases, including low Mg# orthopyroxene and plagioclase, was emplaced between footwall unreactive and S-poor Pretoria Group quartzites and a hangingwall sequence of Rooiberg Group felsites, and was rapidly chilled to form the basal norite sequence at Rietfontein. The second pulse of magma contained early formed cumulus phases, including olivine, chromite, and high Mg# orthopyroxene, and was emplaced above the chilled norite sequence as a crystal mush to form gabbronorites and ultramafic rocks. This second pulse of magma also contained PGE-bearing base metal sulfides that accumulated at the contact between this second batch of magma and the already chilled basal norite sequence. The formation of Platreef-type mineralization outside of the Northern Limb of the Bushveld Complex confirms there are a number of areas within the Bushveld Complex that are prospective for this style of mineralization.     

Petrogenesis of ultramafic-mafic to felsic plutonic rock associations from southern portion of Chhotanagpur Gneissic Complex in Central India: A multi-stage crust-mantle interaction process

A granite-granodiorite-gabbro-ultramafic rock association occurs in the southern sector of Chhotanagpur Gneissic Complex in Central India. Field relations show mingling and mixing of mafic and granodioritic magmas along the contacts of the intrusives. Petrographic studies, coupled with analyses of phase compositions and bulk rock major and trace element compositions favor origin of mafic magma from partial melting of sub-continental hybridized lithospheric mantle and subsequent two stage emplacement. Initial ponding of mafic magma at basal crust elevated the geothermal gradient so as to cause partial melting of lower crustal materials and generation of granodioritic melt. Simultaneous emplacement of granodioritic and mafic magmas tapped from basal crustal reservoir at mid-crustal depth resulted in restricted mingling-mixing along the contacts of the contrasting magma types locally producing rocks of dioritic composition. The mode of evolution of this cogenetic mafic-felsic association, when combined with available geochronological data, has important implications in demarcation of the extent of Grenvellian orogen that resulted in amalgamation of the Southern Crustal Province of India (SCP) with the Bundelkhand Craton or Northern Crustal Province (NCP).     

Magma mixing in the subvolcanic environment: petrology of the Gerena interaction zone near Seville,Spain

The mechanisms by which felsic and mafic magmas interact and approach a uniform hybrid composition through the processes of mingling and mixing have been studied in a high-level subvolcanic setting in the Spanish Hercynian at Gerena, near Seville. The compositions involved are calc-alkaline and the situation is one of tonalite-quartz diorite synplutonic dykes injected into a granitic magma chamber. The resulting hybrids include dykes, pillows and globules of tonalite with chilled margins which are variously disrupted and homogenised with the host granite. The present investigation is based on field and petrographic observations of hybridization textures, the identification of different stages in the crystallisation history of the tonalite through mineral textures, and the characterization of mineral compositions at these various stages. Proportions of the end-member magmas involved were obtained by major-oxide mixing models and tested satisfactorily with trace elements. A mechanistic model is presented to account for these observations which involves the early quenching of the tonalite when it was emplaced into the granite magma chamber. After high temperature crystallization had occurred the two magmas attained thermal equilibrium and disruption of the tonalite in the high energy regime of this subvolcanic complex resulted in dispersion of fragments and crystals through the granite giving rise to hybrid granodiorite compositions. It is argued that such high-energy flow conditions are a necessary requirement for effective hybridization in this environment in contrast to most large-scale magma chamber settings where mixing is driven by thermal and buoyancy contrasts.     

Origin of composite dikes in the Gouldsboro granite, coastal Maine

Composite dikes, consisting of aphyric basaltic margins and phenocryst-rich rhyolitic interiors, cut the Gouldsboro granite of coastal Maine at many localities. Limited hybridization (exchange of crystals, commingling, and mixing) occurs in most of the dikes and indicates that the two magmas were contemporaneous with emplacement of rhyolitic magma following closely in time the initial emplacement of the basaltic dike. Petrographic characteristics and geochemistry indicate that the source of the rhyolite was resident magma in the Gouldsboro granite magma chamber. The composite dikes formed when basaltic dikes ruptured the Gouldsboro magma chamber, permitting partly crystallized magma from the margin of the chamber to flow outward into the center of the basaltic dikes. Field relations of similar composite dikes in other areas (e.g., Iceland, Scotland) are consistent with this model. A second type of composite dike (silicic margins with chilled basaltic pillows) commonly cuts mafic intrusions along the Maine coast and probably formed when a granitic dike ruptured an established chamber of mafic magma, permitting resident mafic magma to collapse downward into the still Liquid granitic dike. Most composite dikes have probably formed when a magma chamber was disrupted by a dike of contrasting magma rather than by tapping a stratified magma chamber.     

安徽铜陵狮子山矿田岩浆岩锆石SHRIMP定年及其成因意义

铜陵狮子山矿田发育大量岩浆岩,且与矿田中的铜金多金属成矿关系密切。锆石SHRIMP同位素精确定年表明,矿田中的岩浆侵位年龄在132.4～142.9Ma之间,即晚侏罗世—早白垩世,属燕山早期晚阶段。矿田岩浆岩体是在同期岩浆活动中多次侵位形成的,岩浆侵入活动可以划分为分别起始于140Ma前后和约136Ma的早晚两次。从岩浆上升侵位到冷却结晶的时间间隔均较短,但其中白芒山辉石二长闪长岩冷却史相对较长,且经历了早期深部岩浆房中的分离结晶作用和后期构造脉动、岩浆上升侵位、减压受热、早期晶体再熔蚀及冷却结晶的过程。结合主量元素和微量元素地球化学研究认为,狮子山矿田岩浆演化的后期,即起源于上地幔或下地壳的原生岩浆在同化了壳源物质并聚集到岩浆房中以后,在滞留的过程中发生了一定程度的分离结晶作用,但尚未固结,成分上显示了一定的带状分布,在区域构造应力松弛及构造事件诱发下,随机地沿发育的构造裂隙先后上升侵位,冷凝结晶。     

The ultramafic cumulate series,Gardiner complex,East Greenland

The ultramafic cumulate series of the ultramafic, alkaline and carbonatite bearing Gardiner complex in East Greenland is divided in: 1) Contact zone of plagio-clase-bearing alkaline rocks chilled to the surrounding gneisses and alkaline lavas; 2) a banded sequence of dunites to mt-pyroxenites; 3) a zoned dunite — cpx-dunite ring and 4) in the centre of the complex ol-pyroxenites and mt-pyroxenites.The zones and bands are superimposed with gradational contacts and are increasingly younger towards the centre of the complex. Primocrysts and intercumulus phases, which are equivalent to phenocryst phases in magmatic liquids show that these rocks accumulated from nephelinitic to nepheline-hawaiitic magmas and the contact rocks from less alkaline basanitic magma types similar to the regional alkaline magmas.The cumulates apparently formed in a magma chamber of a nephelinitic volcano, resting on the regional basalts of the Kangerdlugssuaq area.     

Late Mesozoic Ore‐forming Events in the Ningwu Ore District,Middle‐Lower Yangtze River Polymetallic Ore Belt,East China: Evidence from Zircon U‐Pb Geochronology and Hf Isotopic Compositions of the Granodioritic Stocks

Late Mesozoic volcanic-subvolcanic rocks and related iron deposits, known as porphyry iron deposits in China, are widespread in the Ningwu ore district (Cretaceous basin) of the middle–lower Yangtze River polymetallic ore belt, East China. Two types of Late Mesozoic magmatic rocks are exposed: one is dioritic rocks closely related to iron mineralization as the hosted rock, and the other one is granodioritic (-granitic) rocks that cut the ore bodies. To understand the age of the iron mineralization and the ore-forming event, detailed zircon U-Pb dating and Hf isotope measurement were performed on granodioritic stocks in the Washan, Gaocun-Nanshan, Dongshan and Heshangqiao iron deposits in the basin. Four emplacement and crystallization (typically for zircons) ages of granodioritic rocks were measured as 126.1±0.5 Ma, 126.8±0.5 Ma, 127.3±0.5 Ma and 126.3±0.4 Ma, respectively in these four deposits, with the LA-MC-ICP-MS zircon U-Pb method. Based on the above results combined with previous dating, it is inferred that the iron deposits in the Ningwu Cretaceous basin occurred in a very short period of 131–127 Ma. In situ zircon Hf compositions of εHf(t) of the granodiorite are mainly from ?3 to ?8 and their corresponding 176Hf/177Hf ratio are from 0.28245 to 0.28265, indicating similar characteristics of dioritic rocks in the basin. We infer that granodioritic rocks occurring in the Ningwu ore district have an original relationship with dioritic rocks. These new results provide significant evidence for further study of this ore district so as to understand the ore-forming event in the study area.     

A model of reverse differentiation at Dikii Greben' Volcano,Kamchatka: progressive basic magma vesiculation in a silicic magma chamber

Dikii Greben' Volcano is the largest modern volcano with silicic rocks in the Kurile-Kamchatka island arc. It consists of many domes and lava flows of rhyodacite, dacite and andesite which were erupted in a reverse differentiation sequence. Non-equilibrium phenocryst assemblages (quartz + Mg-rich olivine, An-rich + An-poor plagioclase etc.), abundance of chilled mafic pillows in the dacites and andesites, and linear variations of rock compositions in binary plots are considered as mineralogical, textural and geochemical evidence for mixing. Mafic pillows in volcanics have a lower density (because of high porosity) and contain the same non-equilibrium phenocryst assemblages as the host rocks. Their groundmass contains skeletal microlites of plagioclase and amphibole proving that the groundmass as well as the pillows themselves formed from a water-rich basaltic magma at depth. They are considered as supercooled, vesiculated floating drops of a hot hybrid layer in the magma chamber which formed after refilling. The lower density of the inclusions allows them to float in the host magma and to concentrate at the top of the chamber prior to eruption. Magma mingling was effected by mechanical disintegration of the inclusions in the host magma during eruption. The rhyodacitic and basic end-members of the mixing series cannot be linked by low-P fractionation though high-P, amphibole-rich fractionation is not excluded.     

The pattern of Ni and Co abundances in lunar olivines

Near liquidus experiments on peridotite and other olivine normative compositions from 1.7 to 6 GPa confirm the applicability of exchange-based empirical models of Ni and Co partitioning between olivine and silicate liquids with compositions close to the liquidus of peridotite. Given that most estimates of lunar bulk composition are peridotitic, the partitioning models thus lend themselves to calculation of olivine compositions produced during the early stages of magma ocean crystallization. Calculation of olivine compositions produced by fractional crystallization of a model lunar magma ocean, initially 700 km deep, reveals a prominent maximum in Ni concentration versus fraction crystallized or Mg’ (molar MgO/(MgO + FeO)), but a pattern of monotonically increasing Co concentration. These patterns qualitatively match the puzzling patterns of Ni and Co concentrations observed in lunar rocks in which forsteritic olivines in magnesian suite cumulates have lower Ni and Co abundances than do less magnesian olivines from low-Ti mare basalts, and olivines from the ferroan anorthosite suite (FAS) have lower Ni, but similar Co to mare basalt olivines.The Ni and Co abundances in olivines from the magnesian suite cumulates can be reconciled in terms of fractional crystallization of a deep magma ocean which initially produces a basal dunite comprised of the hottest and most magnesian olivine overlain by an olivine-orthopyroxene (harzburgite) layer that is in turn overlain by an upper zone of plagioclase-bearing cumulates. The ultramafic portion of the cumulate pile overturns sending the denser harzburgite layer, which later becomes a portion of the green glass source region, to the bottom of the cumulate pile with Ni- and Co-rich olivine. Meanwhile, the less dense, but hottest, most magnesian olivines with much lower Ni and Co abundances are transported upward to the base of the plagioclase-bearing cumulates where subsequent heat transfer leads to melting of mixtures of primary dunite, norite, and gabbronorite with KREEP (a K-REE-P enriched component widely believed to be derived from the very latest stage magma ocean liquid). These hybrid melts have Al₂O₃, Ni, and Co abundances and Mg’ appropriate for parent magmas of the magnesian suite. Ni and Co abundances in the FAS are consistent with either direct crystallization from the magma ocean or crystallization of melts of primary dunite-norite mixtures without KREEP.     

安徽钱桥-象山角闪石堆晶岩岩石学和地球化学特征

安微钱桥-象山角闪石堆晶岩是由相当于低硅闪长岩中暗色微粒包体的玄武质岩浆在9.24～12.77km深度和高水压条件下结晶分离作用堆晶形成的。残余的低硅闪长质熔体将它们携带到浅部侵位。结晶分离形成这种堆晶岩的岩浆是玄武质的,可能是由地幔岩中等程度部分熔融形成的拉斑玄武质岩石经再次熔融形成的。