Differentiation of tholeiitic Karroo magma at Birds River,South Africa

Strongly fractionated residua derived by partial crystallization of tholeiitic gabbroic magma were emplaced at shallow depth within sediments of the uppermost part of the Karroo succession. Field evidence suggests that these residua were tapped from a deeper intrusion during cauldron subsidence, but were subsequently engulfed by later intrusion of olivine gabbro on a much larger scale. Ferrogabbro and pegmatoids were produced by differentiation in place of the younger gabbro, but the older intrusion was already chemically evolved when emplacement occurred. The product, a quenched, porphyritic rock containing ferroaugite-ferrohedenbergite, fayalitic olivine and sodic plagioclase in a microcrystalline to glassy matrix, is chemically comparable with the coarse-grained, iron- and alkali-enriched facies of other well known differentiated bodies. Regular trends in the contents of Rb, Sr, Y, Zr and Nb correlate well with fractionation indices based on major-element chemistry.     

新疆北山地区罗东镁铁质-超镁铁质层状岩体岩石成因

罗东镁铁质-超镁铁质岩体位于塔里木板块东北部的新疆北山地区,岩体平面形态为眼球状,出露面积约2.1 km2.由纯橄岩、单辉橄榄岩、斜长二辉橄榄岩、橄榄二辉岩、方辉辉石岩、橄长岩、橄榄辉长岩、辉长岩、苏长辉长岩和淡色辉长岩组成,堆晶结构和堆晶韵律发育,属于层状岩体.岩浆演化过程中主要分离结晶/堆晶相是橄榄石和单斜辉石,此...     

新疆阿尔泰东南缘哈拉乔拉镁铁质岩体地球化学特征与地质意义

哈拉乔拉镁铁质岩体位于新疆阿尔泰造山带东南缘, 主要岩石类型有辉长岩、辉长苏长岩、含长辉石岩、橄榄辉长岩和橄长岩.哈拉乔拉岩体具有低TiO₂(0.09%~1.28%)、低碱(Na₂O+K₂O=0.37%~0.78%)的特点, 属于拉斑玄武岩系列; 富集大离子亲石元素和轻稀土元素, 亏损高场强元素(Nb、Ta等), ε_Nd(t)为-2.62~-0.78, ε_Sr(t)为37.49~45.28.元素地球化学和Nd-Sr同位素组成表明, 岩浆源区为被消减板片交代改造过的富集型岩石圈地幔.原生岩浆为高镁拉斑玄武岩浆, 岩浆演化过程中主要发生了橄榄石、单斜辉石以及斜长石的分离结晶/堆晶, 后期有钛铁矿的析出.      

Geochemistry, Petrogenesis and Metallogenesis of the Panzhihua Gabbroic Layered Intrusion and Associated Fe-Ti-V Oxide Deposits, Sichuan Province, SW China

The Panzhihua gabbroic layered intrusion is associated withthe 260 Ma Emeishan Large Igneous Province in SW China. Thissill-like body hosts a giant Fe–Ti–V oxide depositwith 1333 million ton ore reserves, which makes China a majorproducer of these metals. The intrusion has a Marginal zoneof fine-grained hornblende-bearing gabbro and olivine gabbro,followed upward by Lower, Middle, and Upper zones. The Lowerand Middle zones consist of layered melanogabbro and gabbrocomposed of cumulate clinopyroxene, plagioclase, and olivine.These zones also contain magnetite layers. The Upper zone consistschiefly of leucogabbro composed of plagioclase and clinopyroxenewith minor olivine. Most rocks in the body show variable-scalerhythmic modal layering in which dark minerals, primarily clinopyroxene,dominate in the lower parts of each layer, and lighter minerals,primarily plagioclase, dominate in the upper parts. The oxideores occur as layers and lenses within the gabbros and are concentratedin the lower parts of the intrusion. Ore textures and associatedmineral assemblages indicate that the ore bodies formed by verylate-stage crystallization of V-rich titanomagnetite from animmiscible oxide liquid in a fluid-rich environment. The rocksof the Panzhihua intrusion become more evolved in chemistryupward and follow a tholeiitic differentiation trend with enrichmentin Fe, Ti, and V. They are enriched in light rare earth elementsrelative to heavy rare earth elements, and exhibit positiveNb, Ta, and Ti anomalies and negative Zr and Hf anomalies. Thesilicate rocks and oxide ores of the Panzhihua intrusion formedfrom highly evolved Fe–Ti–V-rich ferrobasaltic orferropicritic magmas. The textures of the ores and the abundanceof minor hydrous phases indicate that addition of fluids fromupper crustal wall-rocks induced the separation of the immiscibleoxide melts from which the Fe–Ti–V oxide ore bodiesin the lower part of the intrusion crystallized. KEY WORDS: magnetite; Fe–Ti-rich gabbro; layered intrusion; Panzhihua; SW China     

Evolution of the Kap Edvard Holm Complex: a Mafic Intrusion at a Rifted Continental Margin

The Kap Edvard Holm Layered Series forms part of the East GreenlandTertiary Province, and was emplaced at shallow crustal level(at depths corresponding to a pressure of 1–2 kbar) duringcontinental break-up. It consists of two suites: a gabbro suitecomprising olivine and oxide gabbros, leucocratic olivine gabbrosand anorthosites, and a suite of wehrlites that formed fromthe intrusion of the gabbros during their solidification bya hydrous, high-MgO magma. Ion microprobe analyses of clinopyroxenereveal chemical contrasts between the parental melt of the wehrlitesuite and that of the gabbro suite. Thin sills (1–2 mthick) of the wehrlite suite, however, have clinopyroxene compositionssimilar to the gabbro suite, and were formed by interactionwith interstitial melts from the host layered gabbros. All evolvedmembers of the gabbro suite have elevated Nd, Zr and Sr concentrationsand Nd/Yb ratios, relative to the melt parental to the gabbrosuite. These characteristics are attributed to establishmentof a magma chamber at depths corresponding to a pressure of10 kbar, where melts evolved before injection into the low-pressuremagma chamber. Anorthosites of the gabbro suite are believedto have crystallized from such injections. The melts becamesupersaturated in plagioclase by the pressure release that followedtransportation to the low-pressure magma chamber after initialfractionation at 10 kbar. The most evolved gabbros formed bysubsequent fractionation within the low-pressure magma chamber.Our results indicate that high-pressure fractionation may beimportant in generating some of the lithological variationsin layered intrusions. KEY WORDS: fractionation; ion microprobe; layered intrusions; rift processes; trace elements ^*Corresponding author.     

Petrology and Emplacement of Reversely Zoned Gabbro-Diorite Plutons in the Smartville Complex, Northern California

Gabbroic plutons are part of the intrusive substructure of theSmartville Complex, a late Jurassic, rifted, ensimatic arc locatedin the northern Sierra Nevada of California. The plutons rangefrom unzoned, equant bodies of olivine gabbro less than 1 kmin diameter to elongate intrusions up to 25 km in length thatare reversly zoned from olivine gabbro cores to quartz dioriterims. The felsic rocks dip inward beneath the mafic core, indicatingthat this zoning reversal continues to depth. The zoned plutonshave relatively shallow keels. We interpret the reversed zoningas an emplacement feature, analogous to the compositional zoningin a zoned tephra sheet. It formed as a result of tapping analready zoned, deeper level magma chamber. Whether the originalzoning of the magma was concentric or stratiform cannot be readilydeduced. During emplacement, considerable amounts of cumulaterocks were mobilized. The mineralogy and geochemistry of the reversely zoned plutonsindicate that they contain two suites of rocks: a cumulate suiterepresented by olivine gabbro and olivine clinopyroxenite anda differentiated suite of non-cumulate olivine gabbros, gabbronorites,and diorites that lie along a compositional continuum and approximateliquid compositions. Plagioclase and olivine compositions inthe Smartville Complex cumulate suite are identical to thosein modern arc cumulates and are characteristic of the arc cumulatesuite. The differentiated rocks form a compositionally continuousseries that is geochemically very similar to a differentiatedsuite of arc tholeiitic basalts and andesites. Fractionationmodeling indicates that removal of mineral phases found in thecumulate gabbros from the mafic members of the differentiatedsuite can produce the lithologic variation seen in the zonedplutons. Plutons such as those in the Smartville Complex indicatethat there is a genetic link between cumulate rocks and a basalt-andesitefractionation trend in arcs, supporting the hypothesis thatarc andesites form by crystal fractionation. The gabbroic plutonsand related Alaska-type ultramafic complexes contain ultramaficcumulates that can rectify the discrepancy between the cumulatemode predicted by fractionation models and the observed modeof gabbroic cumulates in arcs.     

新疆东天山黄山东岩体橄榄石成因意义探讨

黄山东岩体位于新疆东天山造山带中段,由四次岩浆侵入形成:第一次侵入形成了岩体上部的橄榄辉长岩、角闪辉长岩和闪长岩,构成岩体的主体;第二次侵入形成辉长苏长岩,呈岩墙状分布于岩体西端和西北部;第三次侵入岩石为斜长二辉橄榄岩,位于岩体下部,为主要的赋矿岩石;第四次侵入岩石为底部角闪辉长岩。橄榄石为第三次侵入的斜长二辉橄榄岩和第一次侵入的橄榄辉长岩主要造岩矿物之一,橄榄石的镁橄榄石牌号(Fo)值介于68.5～82.5之间。其中含硫化物斜长二辉橄榄岩中的橄榄石具有较高的Fo值(79.7～82.5);斜长二辉橄榄岩中橄榄石的Fo值为78.3～79.9;而基性程度较低的橄榄辉长岩中橄榄石具有较低的Fo值(68.5～72.2)。利用橄榄石矿物成分计算得出黄山东岩体母岩浆Mg#(Mg2+/(Mg2++Fe2+))为0.59,为原生玄武质岩浆经历结晶分异作用形成。模拟计算结果显示黄山东岩体不含矿岩石中橄榄石是母岩浆经过2%的橄榄石结晶分异且硫达到饱和后,在硫化物熔离的同时岩浆发生橄榄石结晶而形成,并且橄榄石︰硫化物≈50︰1,部分橄榄石成分投点在橄榄石结晶和硫化物熔离的模拟曲线右下侧,指示它们可能受到晶间硅酸盐熔浆作用的影响。含硫化物斜长二辉橄榄岩中Fo值与Ni含量呈负相关关系,说明橄榄石与硫化物熔体之间发生了Fe-Ni交换反应。     

The petrology of the layered gabbro intrusion, eastern gabbro, Coldwell alkaline complex, Northwestern Ontario, Canada: evidence for multiple phases of intrusion in a ring dyke

The Coldwell alkaline complex is a large (> 350 km²) gabbro and syenite intrusion on the north shore of Lake Superior. It was emplaced at 1108 Ma during early magmatic activity associated with the formation of the Mid-Continent Rift of North America. The eastern gabbro forms a partial ring dyke on the outer margin of the complex and consists of at least three discrete intrusions. The largest of these is the layered gabbro that comprises a 300 m thick fine- to medium-grained basal unit overlain by up to 1100 m of variably massive to layered gabbroic cumulates which vary from olivine gabbro to anorthosite. Several xenoliths of Archaean metamorphic rocks that range in size from 10's to 100's of meters are present in the central part of the intrusion. Within discrete horizons in the layered gabbro are many centimeter- to meter-scale, gabbroic xenoliths. The main cumulus minerals, in order of crystallization, are plagioclase, olivine and clinopyroxene ± Fe-Ti oxides. Biotite and Fe-Ti-oxide are the dominant intercumulus phases. Orthopyroxene occurs not as a cumulus phase but as peritectic overgrowths on cumulus olivine. A detailed petrographic and mineral chemical study of samples from two stratigraphically controlled traverses through the layered gabbro indicates that the stratigraphy cannot be correlated along the 33 km strike of the ring dyke. Mineral compositions show both normal and reversed fractionation trends. These patterns are interpreted to record at least three separate intrusions of magma into restricted dilatant zones within the ring dyke possibly associated with ongoing caldera collapse. Calculations of parental melt composition using mineral — melt equilibria show that even the most primitive gabbros crystallized from an evolved magma with mg# of 0.42-0.49. The presence of orthopyroxene overgrowths on cumulus olivine suggests rising silica activity in the melt during crystallization and implies a subalkaline parentage for the layered gabbro.     

Mineralogy,geochemistry and petrogenesis of the Khopoli mafic intrusion,Deccan Traps,India

The Khopoli intrusion, exposed at the base of the Thakurvadi Formation of the Deccan Traps in the Western Ghats, India, is composed of olivine gabbro with 50–55 % modal olivine, 20–25 % plagioclase, 10–15 % clinopyroxene, 5–10 % low-Ca pyroxene, and <5 % Fe-Ti oxides. It represents a cumulate rock from which trapped interstitial liquid was almost completely expelled. The Khopoli olivine gabbros have high MgO (23.5–26.9 wt.%), Ni (733–883 ppm) and Cr (1,432–1,048 ppm), and low concentrations of incompatible elements including the rare earth elements (REE). The compositions of the most primitive cumulus olivine and clinopyroxene indicate that the parental magma of the Khopoli intrusion was an evolved basaltic melt (Mg# 49–58). Calculated parental melt compositions in equilibrium with clinopyroxene are moderately enriched in the light REE and show many similarities with Deccan tholeiitic basalts of the Bushe, Khandala and Thakurvadi Formations. Nd-Sr isotopic compositions of Khopoli olivine gabbros (εNd_t?=??9.0 to ?12.7; ⁸⁷Sr/⁸⁶Sr?=?0.7088–0.7285) indicate crustal contamination. AFC modelling suggests that the Khopoli olivine gabbros were derived from a Thakurvadi or Khandala-like basaltic melt with variable degrees of crustal contamination. Unlike the commonly alkalic, pre- and post-volcanic intrusions known in the Deccan Traps, the Khopoli intrusion provides a window to the shallow subvolcanic architecture and magmatic processes associated with the main tholeiitic flood basalt sequence. Measured true density values of the Khopoli olivine gabbros are as high as 3.06 g/cm³, and such high-level olivine-rich intrusions in flood basalt provinces can also explain geophysical observations such as high gravity anomalies and high seismic velocity crustal horizons.     

Monteregian Hills I. Petrography, Major and Trace Element Geochemistry, and Strontium Isotopic Chemistry of The Western Intrusions: Mounts Royal, St. Bruno, and Johnson

The Monteregian Hills petrographic province of southwesternQuebec, Canada, consists of a series of alkaline intrusionsemplaced along faults associated with the St. Lawrence graben.The intrusions are crudely cylindrical in shape, show verticalcontacts, and apparently extend to great depths. Where observed,igneous foliation is generally steeply dipping. The western intrusions consist of two petrographically distinctgroups. One group is composed of slightly undersaturated tocritically saturated pyroxenites and gabbros, largely of cumulateorigin, and associated slightly quartz-saturated syenites. Thesecond group is composed of strongly to moderately undersaturateddiorites, monzonites, and syenites which contain significantamounts of feldspathoidal minerals. The Oka carbonatite complexbelongs to the latter group. Available age data indicate that these two petrographic groupsrepresent separate periods of igneous activity. The slightlyundersaturated to critically saturated series has a mean ageof 136 Ma, while the strongly to moderately undersaturated serieshas a mean age of 118 Ma. Mounts Royal and St. Bruno are largely composed of gabbros andpyroxenites which belong to the slightly undersaturated to criticallysaturated series. These units consist of variable amounts ofcumulus pyroxene and olivine and intercumulus minerals. Someof the finer-grained gabbros approximate liquid compositions.Major and trace element rock and mineral chemistry demonstratethat the evolution of these magmas was largely controlled bypyroxene and olivine fractionation, with plagioclase appearingon the liquidus late in the crystallization history. The quartz-bearingsyenites at Mt. St. Bruno represent a late stage differentiatewhich was contaminated by siliceous crustal material. The strongly to moderately undersaturated series is representedby the essexites and pulaskites at Mount Johnson and the nepheline-bearingdiorites and feldspathoidal monzonites and syenites at MountRoyal. The petrogenetic relationships between these rocks arecomplex and apparently involve a number of processes includingliquid immiscibility, contamination, and alkali transport. Low initial Sr isotope ratios (0.7032 to 0.7035) for both ofthese rock series indicate a mantle origin. Calculated initialmelts are alkali picrites for the slightly undersaturated tocritically saturated series and basanites for the strongly tomoderately undersaturated series. The alkali picrites can beproduced by an 8 per cent melt of a light rare-earth enrichedgarnet lherzolite source. The basanites require a much morelimited degree of melting (1–2 per cent) of a spinel lherzolitesource. In the case of the basanites, CO₂ may have played animportant role in determining the nepheline-normative characterof the magmas.     

塔里木板块东北部笔架山东基性-超基性岩体地球化学特征与岩石成因

笔架山东基性—超基性岩体位于塔里木板块东北部的新疆北山地区,是笔架山岩带中最东边的一个岩体,岩体形态为椭圆状,出露面积约5 km2.由含长单辉橄榄岩、橄榄辉长岩、含橄辉长岩、辉长岩以及辉绿岩组成.含橄榄石较多的岩石属拉斑玄武岩系列,而辉长岩属钙碱性系列.岩相学、岩石化学和微量元素地球化学特征表明岩浆演化过程中经历了橄榄石、辉石、斜长石的分离结晶作用.Nd、Sr、Pb同位素组成和岩石地球化学特征反映出岩浆与围岩之间存在物质交换.笔架山东岩体的岩浆源区为OIB型地幔源区,原生岩浆为高镁拉斑玄武岩浆[w(MgO)=11.2％],这些性质与塔里木板块东北部其他基性-超基性岩体的特征相似,表明塔里木板块东北部二叠纪基性—超基性岩体的源区主要为OIB型地幔,可能与地幔柱有关.     

An Anorthosite Suite in a Ring-Complex: Crystallization and Emplacement of an Anorogenic Type from Abontorok, A?r, Niger

Laminated anorthosite grading outwards into leucogabbro, gabbro,and monzogabbro occurs in a 2.6-km-diameter funnel-shaped intrusion,cut by a quartz alkali syenite plug and concentric syenite andgranite ring-dykes. The anorthosite-gabbro series is laminatedbut not modally or otherwise texturally layered. The lamination,defined by large tabular plagioclase crystals, forms a set ofinwarddipping cones, the dips of which decrease from 60–45?in the central anorthosite to < 25? in the outer gabbros.Rocks close to the outer contact are medium-grained isotropicgabbros. Plagioclase, forming >80% of the series, generallyhas homogeneous labradorite cores (An_62–58 in the wholeseries) and thin strongly zoned rims, which follow progressivelylonger solidus paths from the anorthosites to the gabbros. Allrocks contain a late-magmatic alkali feldspar. Plagioclase isthe main or only cumulus phase, the anorthosites being ad- tomesocumulates and the gabbros orthocumulates. Olivine (FO_49–41)is more abundant than clinopyroxene in most of the series. Dependingon quartz content, the syenites and granites are hypersolvusor subsolvus and the depth of crystallization was calculatedto be 5 ? 2 km. A Rb/Sr isochron for the syenites and granites gave an age of399 ? 10 Ma with an initial strontium isotopic ratio of 0.7084? 0.0005. Ten samples from the anorthosite-gabbro scries havean average calculated initial ratio of 0.70582 ? 0-00004 at– 400 Ma, showing that the two series are not comagmatic.The anorthosite-gabbro series has parallel REE trends (La_N/Yb_N{small tilde} 7–10) with decreasing positive Eu anomaliesand increasing total REE contents from anorthosite to gabbro;two monzogabbros have almost no Eu anomaly. The liquid calculatedto be in equilibrium with the lowest anorthosite has almostno Eu anomaly and its normalized REE pattern lies just abovethose for the monzogabbros. The syenites and granites have complementaryREE patterns with negative Eu anomalies. The inferred parental magma was alkalic and leucotroctoliticwith high TiO₂ P₂O₅, Sr and K/Rb and with low MgO, very similarto parental magmas in the Gardar province, South Greenland.It was probably produced at depth by settling of olivine andclinopyroxene but not of plagioclase, which accumulated by flotation.It is suggested that plagioclase crystals from this lower chamberwere progressively entrained (from 0% in the gabbros to 30–40%in the anorthosites), giving rise to the flow lamination inthe upper chamber. The magma in the lower chamber may have beenlayered, because the plagioclase cores in the anorthosite areconsiderably richer in Or than those in the leucogabbros orgabbros. Overall convection did not occur in the upper chamber,whereas compositional convection occurred in the more slowlycooled central anorthositic adcumulates.     

Petrology of mafic inclusions from Itinome-gata,Japan

Various types of mafic inclusions up to 30 cm in size occur in lapilli tuff of alkali basalt at Itinome-gata crater, northeastern Japan. They are divided into the following four groups: amphibolite, hornblendite—hornblende gabbro, leucogabbro, and pyroxene gabbro. Also occurring with the mafic inclusions are lherzolite and websterite inclusions and megacrysts of Mg-rich olivine and chromian diopside. New analyses are presented for twenty five representative mafic inclusions, eight clinopyroxenes, six orthopyroxenes, and fifteen brown hornblendes. There are conspicuous chemical differences between the mafic inclusions and lherzolite and websterite inclusions: the former have higher TiO₂, Al₂O₃, total FeO, CaO, Na₂O, and K₂O, and lower MgO than the lherzolites and higher TiO₂, Al₂O₃, total FeO, and alkalis, and lower MgO than the websterites. The petrographic and chemical gradations among these three are not easily recognized. It is indicated that the Moho in this region is a boundary between mafic and ultramafic phases. The mineral assemblages of the mafic inclusions and the compositions of their essential minerals show that all of them recrystallized or crystallized under approximately the same temperature — pressure conditions, within the range of 600–1000° C and 6–9 kb. The following is hypothetically considered. The old and thick tholeiites or high-alumina basalts (may be pre-Silurian) making up the basement of the Japanese Islands had been subjected to the high T/P type metamorphism during Cretaceous time, and changed to amphibolites. In the cataclastic stage, complete melting of the lower part of the amphibolites occured locally and formed a gabbro magma. This gabbro magma moved upward slightly and produced hornblendite, hornblende gabbro and leucogabbro magmas by differentiation under wet conditions and a pyroxene gabbro magma under less wet conditions. Namely, the mafic inclusions are thought to be of fragments of the lower crust.     

新疆北山地区旋窝岭镁铁-超镁铁岩体的年代学、岩石矿物学和地球化学研究

新疆北山地区的旋窝岭岩体侵位于下石炭统红柳园组变质砂岩中,辉长岩的锆石SIMSU-Pb定年结果为260.7±2.0Ma,晚于区域上其它岩体的成岩年龄。该岩体由橄榄岩、橄长岩、橄榄辉长岩和辉长岩组成,岩石较新鲜,具有典型的包橄结构和辉长结构,各岩性中均可见斜长石,而斜方辉石仅在辉长岩中出现。橄榄石的Fo值在80～85之间,NiO含量介于0.1%～0.2%之间;单斜辉石主要为普通辉石和透辉石,斜方辉石为古铜辉石。这些镁铁质矿物和斜长石成分与全岩的地球化学特征具有很好的相关性,说明该岩体母岩浆的演化过程主要受控于结晶分异作用。全岩的MgO含量在9.5%～21.8%之间,具有平坦的稀土配分型式和正Eu异常以及微量元素Nb、Zr、Hf的负异常等。因此,旋窝岭岩体可能是在北山裂谷形成过程中由于岩石圈地幔高程度部分熔融形成的高镁玄武质岩浆分异演化的产物,具有一定的铜镍硫化物矿床的成矿潜力。     

Petrology of the Proterozoic Potato River Layered Intrusion, Northern Wisconsin, USA

The Potato River intrusion is a Keweenawan (1100 Ma) mafic plutonemplaced in Keweenawan volcanics and earlier Proterozoic metasedimentaryrocks along the southeastern flank of the Lake Superior syncline.It comprises the following lithostratigraphic zones: a thinto absent Border zone of altered olivine gabbro; a Lower zoneof olivine gabbro; a Picritic zone of picrite and troctolite;a Middle zone of olivine gabbro and leucogabbro; an Upper zoneof quartz leucogabbro and ferrogabbro; and a Roof zone of granophyricand granitic rocks. Fractional crystallization is evident fromcompositional changes in the rocks and cumulus minerals withstratigraphic height. Elements concentrated in the cumulus mineralsolivine and plagioclase (Mg, Fe²⁺, Al, Ca, Ni, Co, Cr, Sr) decreasewith height; elements concentrated in the trapped liquid (Na,K, La, Y, Zr, Nb, Rb, Ba) increase with height; and other elements(Ti, Fe³⁺, P, Ga, V, Sc, Cu, Zn) show complicated behavior relatedto the appearance of additional cumulus phases such as clinopyroxene,Fe-Ti oxides, and apatite. Lower zone rocks contain some sulfide,probably from sulfur derived from the country rock, and theUpper zone has sulfides probably precipitated from an immisciblesulfide liquid. The sulfide-bearing rocks have similaritiesto those of other intrusions, such as Bushveld, Stillwater,and Skaergaard. The picritic and troctolitic rocks of the Picritic zone indicatethat the intrusion was open to additional injections of maficmagma. Roof zone granophyric rocks are residual liquids intrudedalong the upper margin of the intrusion during regional tilting,but Roof zone granitic rocks are probably melted country rock.An attempt is made to estimate by reverse stratigraphic summationthe compositional path of the magma that solidified above thePicritic zone. The first compositions are highly aluminous,which suggests that the upper part of the intrusion has beenenriched in plagioclase by convection-aided crystal sorting.A complementary unit of mafic rocks is not exposed, but it couldbe present down dip. Some of the later compositions are similarto typical Keweenawan high-Al tholeiites. The magma did notundergo extreme iron enrichment, probably because of oxygenfugacity buffering.     

Xenoliths in recent basaltic andesite flows from Arenal Volcano,Costa Rica: inference on the composition of the lower crust

Basaltic andesite flows erupted between 1973 and 1980 from Arenal Volcano contain abundant inclusions of anorthosite, olivine gabbro, and pyroxenites, and megacrysts of olivine and anorthite. The anorthosites with large (20 mm) anorthite grains (An_96-92) exhibit deformation twinning and granulation between grain boundaries. Some olivine gabbros have angular clasts of anorthite with bent twins, pyroxene, and olivine in a finer-grained matrix which is distinctly foliated. These textural features suggest that these inclusions were deformed. An exotic (xenolithic) origin is supported in part by the mineral compositions and the estimated temperatures of equilibration: a temperature of about 975° C is obtained by two-pyroxene and Fe-Ti oxide geothermometers for the gabbros, but two-pyroxene temperatures are higher (1064 to 1120° C) for the basaltic andesite host. The olivine gabbro is thought to have crystallized at a pressure between 8.5 and 9.5 kb; whereas the lava phenocrysts crystallized at a much lower pressure of less than 5 kb. These xenoliths probably represent fragments of the lower crust below Arenal volcano. The lava flows show evidence for some contamination especially from fragments of anorthite broken apart from the larger megacrysts and xenoliths. A few phenocrysts of plagioclase in the lava samples have deformation twins. The unusually high Al₂O₃ content (19.4 to 23.2 wt%) of the lava samples can be attributed directly to the addition of anorthite; in fact the observed chemical variation in the lava flows (the increasing alumina and lime contents with decreasing silica) can be explained by this contamination.     

The Composition and Age of the Mesoarchean Gabbro in the South Vygozersky and Kamennoozersky Greenstone Structures,Karelia

The geochemical and zircon geochronological (U-Pb, SHRIMP-II) study of Mesoarchean gabbros of the South Vygozersky and Kamennoozersky greenstone structures of Central Karelia made it possible to distinguish four gabbro types: (1) Fe–Ti gabbro, 2869 ± 12 Ma, (2) gabbro compositionally close to tholeiitic basalts, 2857 ± 7 Ma, (3) leucogbabbro, 2840 ± 5 Ma; and (4) melanogabbro, 2818 ± 14 Ma. From the early to late gabbros, the rocks are depleted in Ti, Fe, V, Y, Zr, Nb, Hf, REE and enriched in Mg, Ca, Cr, Ni. According to the systematics (Condie, 2005), the Nb/Y, Zr/Y, Zr/Nb ratios in the studied Late Archean gabbros are close to those of primitive mantle, while the gabbros in composition are similar to those of plumederived ocean-plateau basalts. Their magma sources were derived from different mantle reservoirs. The leucogabbro and melanogabbro with similar εNd = +4 were derived from a depleted mantle source (DM). The gabbro close in composition to tholeiitic basalts and having the elevated positive ε_Nd (+4.9) was derived from a strongly depleted mantle source. Insignificant admixture of crustal material or lithospheric mantle is inferred in a source of the Fe–Ti gabbro (with lowest ε_Nd = +2.1).     

胶东海阳所堆积辉长岩的变质反应结构、石榴石形成及P—T演化

海阳所堆积辉长岩由橄长岩、橄榄辉长岩和辉长岩组成。在橄长岩和橄榄辉长岩中发育有典型的变质反应结构：主要为橄榄石与斜长石之间形成由斜方辉石、尖晶石、角闪石和石榴石等矿物组成的多期次次变边，并有三个不同世代变质矿物，早期Cpx＋Opx＋Spl，中期Amp，晚期Grt；期次是钛铁矿与斜长石之间形成石榴石次变边，相对比较简单，只有一个世代变质矿物，为Grt＋Amp＋Rut或Grt＋Rut岩中石榴石是通过斜长石与角闪石或斜长石与钛铁矿之间的变质反应形成的，虽为峰值变质作用的产物，但变质反应的期次及类型不同导致了所形成石榴石的温度和压力有所不同。堆积辉长岩形成演化的温压计算表明，堆积辉长岩在经过近等压降温的岩浆作用之后的变质作用早期，仍为近等压降温，而晚期则表现为近等温升压。这一特殊的P－T演化可能反映了堆积辉长冷侵位与深俯冲特征。     

Plagioclase zonation styles in hornblende gabbro inclusions from Little Glass Mountain,Medicine Lake volcano,California: implications for fractionation mechanisms and the formation of composition gaps

The rhyolite of Little Glass Mountain (73–74% SiO₂) is a single eruptive unit that contains inclusions of quenched andesite liquid (54–61% SiO₂) and partially crystalline cumulate hornblende gabbro (53–55% SiO₂). Based on previous studies, the quenched andesite inclusions and host rhyolite lava are related to one another through fractional crystallization and represent an example of a fractionation-generated composition gap. The hornblende gabbros represent the cumulate residue associated with the rhyolite-producing and composition gap-forming fractionation event. This study combines textural (Nomarski Differential Interference Contrast, NDIC, imaging), major element (An content) and trace element (Mg, Fe, Sr, K, Ti, Ba) data on the style of zonation of plagioclase crystals from representative andesite and gabbro inclusions, to assess the physical environment in which the fractionation event and composition gap formation took place. The andesite inclusions (54–61% SiO₂) are sparsely phyric with phenocrysts of plagioclase, augite and Fe-oxide±olivine, +/–orthopyroxene, +/–hornblende set within a glassy to crystalline matrix. The gabbro cumulates (53–55% SiO₂) consist of an interconnected framework of plagioclase, augite, olivine, orthopyroxene, hornblende and Fe-oxide along with highly vesicular interstitial glass (70–74% SiO₂). The gabbros record a two-stage crystallization history of plagioclase+olivine+augite (Stage I) followed by plagioclase+orthopyroxene+ hornblende+Fe-oxide (Stage II). Texturally, the plagioclase crystals in the andesite inclusions are characterized by complex, fine-scale oscillatory zonation and abundant dissolution surfaces. Compositionally (An content) the crystals are essentially unzoned from core-to-rim. These features indicate growth within a dynamic (convecting?), reservoir of andesite magma. In contrast, the plagioclase crystals in the gabbros are texturally smooth and featureless with strong normal zonation from An₇₄ at the core to around An₃₀. K, and Ba abundances increase and Mg abundances decrease steadily towards the rim. Ti, Fe, and Sr abundances increase and then decrease towards the rim. The trace element variations are fully consistent with the two-stage crystallization sequence inferred from the gabbro mineralogy. These results indicate progressive closed-system in situ crystallization in a quiescent magmatic boundary layer environment located along the margins of the andesite magma body. The fractional crystallization that generated the host rhyolite lava is one of inward solidification of a crystallizing boundary layer followed by melt extraction and accumulation of highly evolved interstitial liquid. This mechanism explains the formation of the composition gap between parental andesite and rhyolite magma compositions.     

Petrological and source region characteristics of ophiolitic hornblende gabbros from the Aksaray and Kayseri regions, central Anatolian crystalline complex, Turkey

Many Neo-Tethyan ophiolitic bodies are well exposed as thrust-slices in Central Anatolia and are predominantly represented by massive hornblende gabbros, most of which are cut by Supra Subduction Zone (SSZ) plagiogranites. The allochthonous gabbros are distinct from their autochthonous counterparts, with their mineralogy including both igneous hornblende, relict diopside rimmed by replacement hornblende and their chemical composition corresponding mostly to gabbro rather than diorite.The results of major and trace element analyses of forty-two samples, and REE analyses of nine samples, indicate that the hornblende gabbros are SSZ-type and formed from a wet magma by high-degree partial melting of peridotite possibly coupled with contamination by predominantly neighbouring-slab derived fluids within an intra-oceanic back-arc basin. The mafic magmas then underwent high-level fractional crystallization involving titaniferous magnetite, diopside, tschermakite and possibly olivine. Emplacement was followed by extensive ocean–floor metamorphism, which has induced crystallization (or recrystallization) of chlorite, biotite, amphiboles and mobilisation of most of the major elements such as alkali and alkali earth elements, and some LREE.