Pyroxene in enclaves and syenites of the Red Hill complex,New Hampshire: an ion and electron microprobe study

The Red Hill complex, New Hampshire, contains both silica-undersaturated and silica-saturated to oversaturated syenites. Ion microprobe analyses of pyroxene from the Nepheline Sodalite Syenite (NSS) and its enclaves reveal that the REE abundances increase in a systematic manner from low values in the enclave pyroxenes to higher values in the NSS host rock pyroxenes. This variation is interpreted to have resulted from differentiation and coupled with previously published mineral and bulk-rock compositions, suggests that the enclaves represent samples of NSS parental liquids that intruded into residual, syenitic liquids in a zoned magma chamber. Pyroxene analyses of the Garland Peak Syenite (GPS) and its enclaves indicate that the enclaves are of several populations: some may be related to the GPS, others are not. The GPS itself is heterogeneous and pyroxene trace element zoning is difficult to explain by fractionation processes. The silica-oversaturated Outer Coarse Syenite (OCS) contains pyroxenes with trace element characteristics that are distinct from the NSS. The low V concentrations suggest that the OCS exprienced magnetite fractionation prior to pyroxene growth. It is proposed that high f O₂in the OCS magma caused Fe-Ti oxide crystallization, which in turn, influenced magmatic silica activity. The chondrite normalized REE patterns of OCS pyroxenes are also suggestive of titanite crystallization, another indication of high f O₂. In contrast to the influence of magnetite crystallization, the high and rimwardly increasing Y and Yb concentrations in both the NSS and the OCS pyroxenes suggest that amphibole fractionation was not the major influence on silica activity. Therefore, it is unlikely that the OCS was derived by amphibole fractionation from a NSS precursor magma.     

Amphibole zoning in the Garland Peak Syenite,Red Hill complex,New Hampshire: camptonitic parental magmas and differentiation to silica-oversaturated syenites

The Garland Peak Syenite (GPS) of the Red Hill complex, New Hampshire, consists predominantly of amphibole, oligoclase, perthite, and quartz; amphiboles have homogeneous kaersutite cores with strongly zoned rims ranging in composition from pargasite to hastingsite to hornblende. The thin section scale association of kaersutite, an amphibole that typically crystallizes in silica-undersaturated magmas, with quartz suggests that the GPS magma experienced substanital changes in magmatic composition, including silica activity, during its crystallization history. Kaersutite-bearing camptonites are also associated with the Red Hill complex. The camptonite amphiboles are very similar in composition to the core kaersutites in the GPS, suggesting that the earliest GPS liquid may have had camptonitic affinities. In order to elucidate the process where-by silica-undersaturated magmas differentiate to saturation, amphiboles in these rocks were analyzed by electron and ion microprobe techniques. Amphiboles show a progressive increase in REE abundances from the camptonites to the GPS kaersutite cores to the GPS pargasite/hastingsite/hornblende rims. The systematic change in REE concentrations, and the variations in V, Ti, Sr versus Zr, Eu/Eu⁰ and La/Yb versus Ce, suggest a possible differentiation relationship for the amphiboles and imply that the GPS was derived from magmas similar to camptonites. Rimward depletions in Sr, Ti, V, and Eu/Eu⁰, and the increase in La/Yb values suggest that parental camptonites fractionated plagioclase, magnetite, and amphibole to produce the silica-oversaturated GPS. Bulk-rock modelling agrees with the trace element record preserved in the amphiboles, that plagioclase, magnetite, and amphibole fractionation caused silica saturation. Minor pegmatitic patches occur in the GPS. Ferrohornblendes in the pegmatites have REE abundances distinct from the other GPS amphiboles, and this difference may be due to open system processes.     

Mineralogy and Petrology of the Red Hill Alkaline Igneous Complex, New Hampshire, U.S.A.

The Red Hill intrusion, New Hampshire is one of the alkalineintrusions making up the White Mountain Magma Series. Earlierwork has shown that it consists of several units with ring-or plug- like form, in order of intrusion: Outer Coarse Syenite(OCS) plus Nepheline Sodalite Syenite (NSS Fire Tower Sycnite(FTS); Garland Peak Syenite (GPS) Watson Ledge Quartz Syenite(WLQS) Interior Fine Granite (IFG). New studies have revealedtrends of increasing alkalinity in both the OCS (OCS-AOCS-B)and NSS (NSS-A-B-C). Conventional K-Ar and Rb-Sr age datingon separated minerals and bulk rocks show that the OCS, NSS,FTS, and GPS have indistinguishable ages at 198.5?1.5 Ma whilethe IFG formed about 10 Ma later. These values are believedto represent intrusion ages. Amphiboles from the NSS vary from ferro-pargasites through taramitesto katophontes and pyroxenes from ferrosalites to aegirine-augitesthese trends follow the increasing degree of under- saturationin the NSS-A-C series. The NSS-A contain mafic syenite xenolithsas well as partially resorbcd salitic pyroxene cores withinamphiboles. Mineral and isotope data are consistent with theseinclusions being cognate suggesting derivation from more-basicparental magmas. Pyroxenes and amphiboles from saturated andoversaturated rocks vary from ferro-salites to ferro-hedenbergitesand hastingsites to ferro-edenites, respectively, but OCS-Bshow fractionation towards aegirine-augites and katophorites. Mineral assemblages crystallized close to the quartz-fayalite-magnetitebuffer at a pressure of about 1–1.5 kb under essentiallyfluid-saturated conditions. It seems likely that the complexformed by emplacement of fractionated magmas derived from alower-level magma chamber which initially contained a basalticparent magma. The first pulse of magma had an overall compositionsimilar to OCS plus NSS and was intruded as a sheet-like bodywhich differentiated in situ to give a central unit of highlyundersaturated magma (NSS-C). NSS-C samples with relativelyradiogenic Sr isotope compositions were modified by introductionof country-rock Sr via circulating fluids. Fractional crystallizationof alkali-rich amphiboles from critically undersaturated magma(NSS-A?) in the lower- level chamber led to the developmentof saturated and oversaturated magmas. These were intruded intothe OCS/NSS unit along ring fractures to form the FTS and GPSunits. Rock and mineral compositions, including isotope data,are consistent with the IFG forming by partial melting of countryrock and being intruded along pre-existing planes of weaknesssome 10 Ma after the main complex was formed.     

Petrogenesis of carbonate-bearing nepheline syenites and carbonatites from Southern Victoria Land, Antarctica: origin of carbon and the effects of calcite-graphite equilibrium

Chemically-evolved carbonate-bearing nepheline syenites are intruded into basement metasediments of the Koettlitz Group on Dismal and Radian Ridges in the Pipecleaner Glacier region of Southern Victoria Land, Antarctica. Whole rock XRF data from the Dismal Nepheline Syenite defines a broad trend which is consistent with the removal of a cumulate fraction of approximate composition 70% hedenbergite, 15% nepheline, 10% titanite and 5% apatite. Stable isotope, major and trace element and mineralogical characteristics of the syenites are very similar to those of cross-cutting calcite-rich dykes indicating derivation from closely-related source magmas. The general association of carbonatites and nepheline syenites with extensional environments, suggests that the Dismal and Radian Ridge nepheline syenites and carbonatite dykes indicate a period of early Paleozoic (531 Ma) rifting or intrusion into localised tensional structures in an overall compressional regime. Assimilation of marble by the syenite magmas is evidenced by abundant rafts and xenoliths within the Dismal Nepheline Syenite, however, carbon and oxygen isotopic ratios from syenite and carbonatite calcites are distinctly lighter than values from the marble country rock indicating a magmatic source. Graphite and calcite commonly occur as aggregates in the Dismal Nepheline Syenite suggesting equilibrium between these two carbon-bearing phases. Isotopic fractionation between calcite and graphite, via the equilibrium: C + O₂ = CO₂ has resulted in enrichment of the ¹³C isotope in calcites from the Dismal Nepheline Syenite.     

Geochemistry of Apatite from the Apatite-rich Iron Deposits in the Ningwu Region, East Central China

Four types of apatite have been identified in the Ningwu region.The first type of apatite is widely distributed in the middle dark colored zones(i.e.iron ores) of individual deposits.The assemblage includes magnetite,apatite and actinolite(or diopside).The second type occurs within magnetite-apatite veins in the iron ores.The third type is seen in magnetite-apatite veins and (or) nodules in host rocks(i.e.gabbro-diorite porphyry or gabbro-diorite or pyroxene diorite).The fourth type occurs within apatite-pyrite-quartz veins filling fractures in the Xiangshan Group.Rare earth elements (REE) geochemistry of apatite of the four occurrences in porphyry iron deposits is presented.The REE distribution patterns of apatite are generally similar to those of apatites in the Kiruna-type iron ores,nelsonites.They are enriched in light REE,with pronounced negative Eu anomalies.The similarity of REE distribution patterns in apatites from various deposits in different locations in the world indicates a common process of formation for various ore types,e.g. immiscibility.Early magmatic apatites contain 3031.48-12080×10~(-6) REE.Later hydrothermal apatite contains 1958×10~(-6) REE,indicating that the later hydrothermal ore-forming solution contains lower REE.Although gabbro-diorite porphyry and apatite show similar REE patterns,gabbro-diorite porphyries have no europium anomalies or feeble positive or feeble negative europium anomalies, caused both by reduction environment of mantle source region and by fractionation and crystallization(immiscibility) under a high oxygen fugacity condition.Negative Eu anomalies of apatites were formed possibly due to acquisition of Eu~(2+) by earlier diopsite during ore magma cooling. The apatites in the Aoshan and Taishan iron deposits yield a narrow variation range of ~(87)Sr/~(86)Sr values from 0.7071 to 0.7073,similar to those of the volcanic and subvolcanic rocks,indicating that apatites were formed by liquid immiscibility and differentiation of intermediate and basic magmas.     

Mineralogy and Petrology of Nepheline Syenites from the Coldwell Alkaline Complex, Ontario, Canada

Nepheline natrolite syenites of Center 2 of the Coldwell alkalinecomplex exhibit rare wispy mafic-rich modal layering, extensivexenolith-rich zones and a wide variety of textural types, thelatter resulting from the imposition of high temperature shearingand recrystallization on consolidated syenite. The texturesdeveloped range from allotriomorphic granular to porphyroclasticto mosaic granulob-lastic. The nepheline syenites are pyroxene-poor.Pyroxenes occur most commonly as corroded diopside to diopsidichedenbergite cores surrounded by amphibole and less commonlyas acmitic hedenbergite overgrowths upon cores of iron-richamphiboles. Amphiboles are the dominant mafic phase and rangefrom magnesian hastingsitic hornblende to hastingsite to hastingsitichornblende to ferroedentic hornblende. Nephelines contain excesssilica and have not equilibrated to compositions characteristicof low temperatures. Feldspars lack microcline twinning andperthites and have undergone extensive ion exchange at highsub-solidus temperatutes with sodium-rich fluids. Formationof late stage primary and replacement natrolite, muscovite andthomsonite is characteristic. The nepheline syenites are consideredto be a part of a cycle of continental rift magmatism and tohave been emplaced by cauldron subsidence as a hot hydrous magma.The rocks did not undergo long term subsolidus re-equilibrationas the high temperature mineral assemblage has been preservedby uplift during post-intrusive regional block faulting. Thenepheline syenites were probably dervied by extensive fractionalcrystallization of alkali basaltic magmas.     

微花岗岩类包体与岩浆的扩散作用和熔离作用——以诸广山桂东、上堡岩体为例

诸广山加里东期桂东及上堡黑云母花岗闪长岩体中的微花岗岩类包体包括同源包体和析离体。同源包体与寄主岩的暗色矿物组成不同,主要含角闪石;析离体与寄主岩中的暗色矿物组成相同,主要含黑云母。包体与寄主岩的矿物、岩石化学、地球化学及产状特征对比表明,这两种包体的形成涉及到岩浆内部组分的相互扩散、岩浆的熔离、对流及派生岩浆的相互混合。形成微花岗岩类包体的偏基性岩浆大多数是中酸性岩浆自身演化的产物。微花岗岩类包体的形成是岩浆结晶过程的记录。     

The role of apatite in mantle enrichment processes and in the petrogenesis of some alkali basalt suites

Analyses of Sr and REE in apatites from a variety of mantle-derived parageneses are used in conjunction with trace element data from the literature to investigate relationships between alkali basalts and apatite-rich materials in upper-mantle source regions. Despite difficulties in interpretation, positive P-anomalies in the hygromagmatophile element abundance patterns of some continental primary alkali basalts suggest either P-enrichment of their source or assimilation of P-rich material, or both. Amphibole- and apatite-rich xenoliths occur in several alkali-basalt provinces, and by virtue of the P and LREE enrichment represent a probable source of the P anomalies and part of the other trace element enrichments of these magmas. Incorporation of such apatite-rich materials by later primary magmas would be enhanced by the high P₂O₅ concentrations required to achieve apatite saturation in basaltic liquids.In the early stages of mantle diapirism an undersaturated magma, produced by slight partial melting of garnet peridotite, might fractionate as it rises to the range of amphibole stability. Hygromagmatophile element patterns of clinopyroxenite xenoliths indicate that clinopyroxene fractionation could produce P-enriched liquids which might subsequently crystallize amphibole- and apatite-rich materials now represented by xenoliths. During generation of later primary magma, apatite-rich materials might preferentially contaminate the liquids, to yield positive P-anomalies. This model requires that magmas undergo prolonged fractionation at considerable depth (~ 100 km), a process which is apparently most probable in subcontinental environments.An apatite- and zircon-bearing mica-clinopyroxenite xenolith from Matsoku provides a link between the S. African MARID suite and amphibole and apatite-rich xenoliths from various alkali basalt provinces. Unusual REE patterns ($\text{La}{}_{\mathrm{N}}\text{<}\text{Ce}{}_{\mathrm{N}}\text{<}\text{Nd}{}_{\mathrm{N}}\text{,}\text{Ce}{}_{\mathrm{N}}\text{/Y}{}_{\mathrm{N}}\text{?10}$) of apatites in this xenolith suggest a link between the MARID suite xenoliths and postulated pre-Karroo mantle metasomatism.     

Trace element and Nd isotope analyses of apatite in granitoids and metamorphosed granitoids from the eastern Central Asian Orogenic Belt: Implications for petrogenesis and post-magmatic alteration

We present in situ trace element and Nd isotopic data of apatites from metamorphosed and metasomatized (i.e., altered) and unaltered granitoids in the Songnen and Jiamusi massifs in the eastern Central Asian Orogenic Belt, with the aim of fingerprinting granitoid petrogenesis, including both the magmatic and post-magmatic evolution processes. Apatites from altered granitoids (AG) and unaltered granitoids (UG) are characterized by distinct textures and geochemical compositions. Apatites from AG have irregular rim overgrowths and complex internal textures, along with low contents of rare earth elements (REEs), suggesting the re-precipitation of apatite during epidote crystallization and/or leaching of REEs from apatite by metasomatic fluids. ε_Nd(t) values of the these apatites are decoupled from zircon ε_Hf(t) values for most samples, which can be attributed to the higher mobility of Nd as compared to Sm in certain fluids. Apatites from UG are of igneous origin based on their homogeneous or concentric zoned textures and coupled Nd-Hf isotopic compositions. Trace element variations in igneous apatite are controlled primarily by the geochemical composition of the parental melt, fractional crystallization of other REE-bearing minerals, and changes in partition coefficients. Sr contents and Eu/Eu* values of apatites from UG correlate with whole-rock Sr and SiO₂ contents, highlighting the effects of plagioclase fractionation during magma evolution. Apatites from UG can be subdivided into four groups based on REE contents. Group 1 apatites have REE patterns similar to the host granitoids, but are slightly enriched in middle REEs, reflecting the influence of the parental melt composition and REE partitioning. Group 2 apatites exhibit strong light REE depletions, whereas Group 3 apatites are depleted in middle and heavy REEs, indicative of the crystallization of epidote-group minerals and hornblende before and/or during apatite crystallization, respectively. Group 4 apatites are depleted in heavy REEs, but enriched in Sr, which are features of adakites. Some unusual geochemical features of the apatites, including the REE patterns, Sr contents, Eu anomalies, and Nd isotopic compositions, indicate that inherited apatites are likely to retain the geochemical features of their parental magmas, and thus provide a record of small-scale crustal assimilation during magma evolution that is not evident from the whole-rock geochemistry.     

Major and Trace Element Characteristics of Apatites in Granitoids from Central Kazakhstan: Implications for Petrogenesis and Mineralization

This paper presents abundances of major and trace elements of apatites in granitic rocks associated with different types of ore deposits in Central Kazakhstan on the basis of electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry. Our results demonstrate that the concentrations and ratios of elements in apatites from different granitoid rocks show distinct features, and are sensitive to magma evolution, petrogenetic and metallogenetic processes. Apatites in the rocks associated with Mo‐W deposits have high content of F and MnO, low content of Cl, which may be indicative of sedimentary sources, while apatites from a Pb‐Zn deposit show relatively high content of Cl and low F content, which possibly suggest a high water content. In these apatites, Sr contents decrease, while Mn and Y contents increase with magma evolution. This relationship reflects that these elements in apatites are related with the degree of magmatic differentiation. Four types of REE patterns in apatites are identified. Type 1 character of highest (La/Yb)_N in apatites of Aktogai porphyry Cu‐Mo deposit, Sayak‐I skarn Cu deposit and Akzhal skarn Pb‐Zn depposit is likely produced by the crystallization of heavy REE‐enriched minerals. Type 2 character of upward‐convex light REE in apatite of Aktogai porphyries likely results from La‐enriched mineral crystallization. Type 3 feature of Nd depletion in apatites of East Kounrad and Zhanet deposits both from Mo‐W deposits primarily inherits the character of host‐rock. Type 4 apatites of Aktogai deposit and Akshatau W‐Mo deposit with wide range of REE contents may suggest that apatites crystallize under a wide temperature range. Three types of apatite with distinct redox states are identified based on Eu anomaly. The Aktogai apatite with slight negative Eu anomaly displays the most oxidized state of the magma, and the apatites of other samples at Aktogai, East Kounrad and Akzhal with moderate negative Eu anomaly show moderate oxidizing condition of these rocks, while the remaining apatites with strong En anomaly indicate a moderate reductive state of these rocks.     

Apatite as an indicator mineral for mineral exploration: trace-element compositions and their relationship to host rock type

Over 700 apatite grains from a range of rock types have been analysed by laser-ablation microprobe ICPMS for 28 trace elements, to investigate the potential usefulness of apatite as an indicator mineral in mineral exploration. Apatites derived from different rock types have distinctive absolute and relative abundances of many trace elements (including rare-earth elements (REE), Sr, Y, Mn, Th), and chondrite-normalised trace-element patterns. The slope of chondrite-normalised REE patterns varies systematically from ultramafic through mafic/intermediate to highly fractionated granitoid rock types. (Ce/Yb)_cn is very high in apatites from carbonatites and mantle-derived lherzolites (over 100 and over 200, respectively), while (Ce/Yb)_cn values in apatites from granitic pegmatites are generally less than 1, reflecting both HREE enrichment and LREE depletion. Within a large suite of apatites from granitoid rocks, chemical composition is closely related to both the degree of fractionation and the oxidation state of the magma, two important parameters in determining the mineral potential of the magmatic system. Apatite can accept high levels of transition and chalcophile elements and As, making it feasible to recognise apatite associated with specific types of mineralisation. Multivariate statistical analysis has provided a user-friendly scheme to distinguish apatites from different rock types, based on contents of Sr, Y, Mn and total REE, the degree of LREE enrichment and the size of the Eu anomaly. The scheme can be used for the recognition of apatites from specific rock types or styles of mineralisation, so that the provenance of apatite grains in heavy mineral concentrates can be determined and used in geochemical exploration.     

An Example of Consequent Mantle Metasomatism in Peridotite Inclusions from Nunivak Island, Alaska

Many of the coarse-grained peridotite inclusions in basanitesfrom Nunivak Island, Alaska, contain amphibole and a smallerfraction also contain phlogopite and apatite. All of these peridotiteshave light REE/heavy REE abundance ratios greater than chondritesand many have abundances of K, Rb, Sr, Ba and light REE whichexceed estimates for primitive mantle. On the basis of mineraltextures and compositions we infer that the clinopyroxene, amphibole,phlogopite and apatite equilibrated with a metasomatic fluid.Isotopic (Sr and Nd) ratios and parent-daughter abundance datafor the coarse-grained peridotites constrain the age of themetasomatism to be less than 200 million years. Associated amphibole pyroxenite inclusions are not metasomatized;these inclusions probably formed as crystal segregates froman alkalic magma. Both pyroxenites and coarse-grained peridotitesare isotopically similar to basalts from Nunivak Island. Usingthese data, we propose a model in which the metasomatized peridotiteswere wallrocks located adjacent to the pyroxenites, and thatmetasomatism of these peridotites was caused by the infiltrationof a residual silicate melt or volatile-rich fluid derived fromthe parental magma of the pyroxenites; i.e. the metasomatismwas a consequence of basaltic magmatism. Furthermore, the parentalmagma of the pyroxenites was probably petrogenetically relatedto the Nunivak volcanism. REE modelling of fluids in equilibriumwith clinopyroxenes from the coarse-grained peridotites is consistentwith this model.     

The geochemistry of the Dunedin Volcano,East Otago,New Zealand: Rare earth elements

A variety of alkaline lavas from the Dunedin Volcano have been analyzed for the rare earth elements (REE) La-Yb. The compositions analyzed were: basalt-hawaiite-mugearite-benmoreite; basanite, nepheline hawaiite, nepheline trachyandesite and nepheline benmoreite; trachyte; phonolite. The series from basalt to mugearite shows continuous enrichment in the REE, consistent with a crystal fractionation model involving removal of olivine and clinopyroxene. From mugearite to benmoreite there is a depletion in the REE which is explained by the appearance of apatite as a liquidus phase. The chondrite normalized REE patterns for the phonolites are characterized by strong enrichment and fractionation coupled with a sharp depletion in Eu. Removal of plagioclase from benmoreite magma is suggested for the derivation of the phonolites. The series basanite-nepheline hawaiite, and basanite-nepheline hawaiite-nepheline benmoreite appear to be high pH₂O analogues of the series basalt-ben-moreite, with enrichment of the REE being achieved by removal of clinopyroxene, kaersutite and olivine. Compared with other lavas the trachyte has low REE abundances and is characterized by a striking positive Eu anomaly.     

Rare earth element geochemistry of Late Cenozoic alkaline lavas of the McMurdo Volcanic Group, Antarctica

Rare earth element (REE) concentrations were determined in 16 Ross Island and northern Victoria Land alkaline lava samples which were representative of four lava lineages of the McMurdo Volcanic Group, Antarctica. A kaersutite and two feldspar mineral separates were also analysed.

Two of the lava lineages, a basanite to nepheline benmoreite and a basanite to phonolite, have similar chondrite-normalized REE fractionation patterns, with a continuous enrichment of light and heavy REE and depletion of middle REE. The patterns result from the fractionation of olivine, clinopyroxene, spinels, feldspar, kaersutite and apatite. Kaersutite is an important fractionated phase responsible for the middle REE depletion.

Another of the lava lineages is mildly potassic with trachyandesite to peralkaline K-trachyte lavas which have partly overlapping REE fractionation patterns. There is a depletion in REE from tristanite to K-trachyte. Fractionation of olivine, clinopyroxene, feldspar and apatite probably control the REE chemistry of the lineage, greater degrees of apatite fractionation deplete the K-trachyte in REE relative to the tristanite. Feldspar fractionation in the genesis of the peralkaline K-trachyte is shown by a large negative Eu anomaly (Eu/Eu* = 0.10).

A nepheline hawaiite to anorthoclase phonolite lava lineage from the Erebus Centre shows enrichment of REE, although minor overlapping in the middle REE does occur. Anorthoclase phonolite has a positive Eu anomaly (Eu/Eu* = 1.31), indicating possible accumulation of anorthoclase. The lineage resulted from fractionation of olivine, clinopyroxene, magnetite and apatite.     

Apatite Composition of Representative Magnetite‐series and Ilmenite‐series Granitoids in Japan

Apatites of representative magnetite‐series and ilmenite‐series granitoids were studied in the Japanese Islands. Concentrations of the volatile components F, Cl and SO₃ are differently distributed in apatites of these granitoid series. Apatites are always fluoroapatite. They have weakly higher F content in the ilmenite series than in the magnetite series. In contrast, Cl and SO₃, are significantly concentrated in apatites of the magnetite series compared to the ilmenite series. These characteristics reflect the original concentrations of these components in the host granitic magmas. A high fO₂ seems most important for the S‐concentration as sulfate in apatite of the magnetite series. REE and Y are only erratically high in the studied apatites.     

Some implications of xenolith glasses for the mantle sources of alkaline mafic magmas

The assumption that mafic alkaline magmas are derived from mantle sources with a lherzolite mineralogy has become entrenched in the petrologic literature. Although it is commonly assumed that highly alkaline magmas require metasomatised mantle sources, there is little understanding of the spatial relation of such sources with respect to those of associated more Si-rich transitional magmas. Glasses developed in mantle xenoliths represent natural experiments which may provide some insight on this problem. Highly silica undersaturated glasses developed in the amphibole-garnet clinopyroxenite portion of a composite xenolith from Nunivak Island, Alaska, become quartz normative where they penetrate adjacent spinel lherzolite. A comparison of glass compositions in mantle pyroxenite and lherzolite xenoliths reveals that glasses developed in amphibole pyroxenite xenoliths are in general more silica undersaturated than those in lherzolite xenoliths. This suggests that some highly silica undersaturated magmas such as nephelinites may in fact be derived by the preferential melting of amphibole or amphibole-garnet pyroxenite veins and that the spectrum from nephelinite to transitional alkaline basalt that characterizes many individual alkaline volcanic suites is produced by mixing with melt derived from the host lherzolite as the degree of partial melting increases.     

宁芜玢岩铁矿磷灰石的稀土元素特征

文章分析了宁芜玢岩铁矿 4种产状磷灰石的稀土元素组成 ,并与Kiruna型铁矿和斜长岩、苏长岩及钛铁霞辉岩中磷灰石的稀土元素组成进行了对比。结果表明产地和母岩不同的矿床中 ,它们的磷灰石稀土元素分布型式一致 ,以轻稀土富集和Eu负异常明显为特征 ,属陆相岩浆成因。前 3种产状磷灰石的ΣREE变化于 30 31.48×10 -6～ 12 0 80× 10 -6,第 4种产状磷灰石的ΣREE仅为 195 8× 10 -6,反映岩浆演化到热液的晚期阶段成矿溶液稀土元素含量较低。尽管辉长闪长玢岩与磷灰石的稀土元素分布型式一致 ,但辉长闪长玢岩无Eu异常或有弱Eu正异常 ,代表它们的地幔源区低氧逸度的还原环境 ,或反映氧逸度较高情况下的分离结晶作用。不混溶作用形成的矿浆在冷凝过程中 ,Eu2 + 优先被透辉石捕获 ,使得稍晚结晶的磷灰石产生负Eu异常     

Rare earth element evidence for differentiation of McMurdo volcanics,Ross Island,Antarctica

Eighteen samples of the McMurdo volcanics on Ross Island, Antarctica consisting of basanitoid, trachybasalt and phonolite have been analyzed for rare earth elements (REE) in order to determine the details of differentiation using quantitative trace element modeling. The basanitoids have REE patterns similar to those for alkali basalts or nephelinites from ocean islands. Since there is no correlation between REE and silica contents among five basanitoids, some of the variability in the REE contents must be related to the extent of partial melting, variation in the residual mineralogies of the mantle during melting, or to inhomogeneities in the REE composition of the mantle.In order to explain the data, more than one differentiation sequence is necessary. In each case a basanitoid melt is the parent which differentiates to trachybasalt upon separation of olivine, clinopyroxene, spinel, ±kaersutite±plagioclase±apatite. If clinopyroxene, kaersutite, anorthoclase, plagioclase and apatite separate from a trachybasalt melt, a mafic phonolite results.If, however, no kaersutite is involved, an anorthoclase phonolite results. A distinct type of mafic phonolite results if kaersutite is one of the minerals that separates from the anorthoclase phonolite. If the anorthoclase phonolite precipitates plagioclase and anorthoclase and if the melt reacts with plagioclase-rich continental rocks, a trachyte results.Formerly spelled: Shine Soon Sun     

Sugarloaf Mountain,central Arizona,USA: A small-scale example of Miocene basalt-rhyolite magma mixing to yield andesitic magmas

Sugarloaf Mountain is a 200-m high volcanic landform in central Arizona, USA, within the transition from the southern Basin and Range to the Colorado Plateau. It is composed of Miocene alkalic basalt (47.2–49.1?wt.% SiO₂; 6.7–7.7?wt.% MgO) and overlying andesite and dacite lavas (61.4–63.9?wt.% SiO₂; 3.5–4.7?wt.% MgO). Sugarloaf Mountain therefore offers an opportunity to evaluate the origin of andesite magmas with respect to coexisting basalt. Important for evaluating Sugarloaf basalt and andesite (plus dacite) is that the andesites contain basaltic minerals olivine (cores Fo_76-86) and clinopyroxene (～Fs_9-18Wo_35-44) coexisting with Na-plagioclase (An_48-28Or_1.4–7), quartz, amphibole, and minor orthopyroxene, biotite, and sanidine. Noteworthy is that andesite mineral textures include reaction and spongy zones and embayments in and on Na-plagioclase and quartz phenocrysts, where some reacted Na-plagioclases have higher-An mantles, plus some similarly reacted and embayed olivine, clinopyroxene, and amphibole phenocrysts.Fractional crystallization of Sugarloaf basaltic magmas cannot alone yield the andesites because their ～61 to 64?wt.% SiO₂ is attended by incompatible REE and HFSE abundances lower than in the basalts (e.g., Ce 77–105 in andesites vs 114–166?ppm in basalts; Zr 149–173 vs 183–237; Nb 21–25 vs 34–42). On the other hand, andesite mineral assemblages, textures, and compositions are consistent with basaltic magmas having mixed with rhyolitic magmas, provided the rhyolite(s) had relatively low REE and HFSE abundances. Linear binary mixing calculations yield good first approximation results for producing andesitic compositions from Sugarloaf basalt compositions and a central Arizona low-REE, low-HFSE rhyolite. For example, mixing proportions 52:48 of Sugarloaf basalt and low incompatible-element rhyolite yields a hybrid composition that matches Sugarloaf andesite well ? although we do not claim to have exact endmembers, but rather, viable proxies. Additionally, the observed mineral textures are all consistent with hot basalt magma mixing into rhyolite magma. Compositional differences among the phenocrysts of Na-plagioclase, clinopyroxene, and amphibole in the andesites suggest several mixing events, and amphibole thermobarometry calculates depths corresponding to 8–16?km and 850° to 980?°C. The amphibole P-T observed for a rather tight compositional range of andesite compositions is consistent with the gathering of several different basalt-rhyolite hybrids into a homogenizing ‘collection' zone prior to eruptions. We interpret Sugarloaf Mountain to represent basalt-rhyolite mixings on a relatively small scale as part of the large scale Miocene (～20 to 15 Ma) magmatism of central Arizona. A particular qualification for this example of hybridization, however, is that the rhyolite endmember have relatively low REE and HFSE abundances.     

Chloritoid stability in very iron-rich altered pillow lavas

Conclusions Our major conclusions are that the strong correlation of P₂O₅ and light REE abundances in basalts does not (a) require residual apatite or whitlockite during partial melting, or (b) imply that P₂O₅ and REE-rich magmas are derived from more refractory sources than the source of magmas with lower P₂O₅ and REE contents.