Petrology of the Basistoppen Sill, East Greenland: A Calculated Magma Differentiation Trend

The 660 m thick Basistoppen sill is an Eocene, tholeiitic, layeredintrusion emplaced in the upper part of the Skaergaard complexshortly after solidification of the Skaergaard magma. Despiteits small size, the Basistoppen sill has one of the most extensivedifferentiation sequences known. The ranges of the solid solutionsin olivine, plagioclase, and pyroxene from the Basistoppen arecomparable to those in the Skaergaard and Bushveld intrusions.The rocks of the sill are orthocumulates composed of approximately35% trapped liquid and 65% cumulus minerals and can be dividedinto zones based on changes in the cumulus mineral assemblage.From the base upward those zones are: a Gabbro Picrite Zonecontaining cumulus olivine, Fe-Cr spinel, and minor biotite;a Bronzite Gabbro Zone containing cumulus orthopyroxene, Ca-richclinopyroxene, plagioclase, and minor Fe-Cr spinel; a PigeoniteGabbro Zone containing cumulus plagioclase, Ca-rich clinopyroxene,pigeonite, magnetite, and minor ilmenite; and a Fayalite DioriteZone containing cumulus plagioclase, Ca-rich clinopyroxene,magnetite, ilmenite, apatite, and olivine. The Basistoppen isoverlain by a zoned granophyre sill that was most likely derivedin part from the Basistoppen magma and in part from melted Precambriangneiss. The excellent exposure, uncomplicated structure, goodchilled margin, and lack of strong modal layering facilitatethe calculation of a differentiation trend for the Basistoppensill. During crystallization the Basistoppen magma became progressivelyricher in Fe, P, Na, K, Zn, Rb, Zr, La, Sm, and Th, became progressivelypoorer in Mg, Ca, Al, Cr, and Ni, and remained relatively unchangedin Si, Sc, and Sr through at least the first 90% of crystallization.     

Glass-bearing gabbro inclusions in hyaloclastites from Tindfjallajökull,Iceland

Gabbro inclusions from Tindfjallajökull are divided into two types: I. Panidiomorphic gabbros of non-cumulative origin composed of plagioclase + olivine ± clinopyroxene and interstitil vesicular glass. They have formed in equilibrium with the host magma and may either represent a marginal facies or a highly solidified magma body. In the latter case the host magma or part of it could be mobilized interstitial liquid. II. Allotriomorphic-hypidiomorphic tholeiitic olivine gabbro and diorite xenoliths with scarce Ti-pargasite which have undergone less than 10% partial melting in the host magma forming melts of alkali basaltic or Hekla andesite-like compositions dependent on the original mineral assemblage. Such liquids, enriched in K₂O and possibly other incompatible elements, may contaminate basaltic magmas rising slowly through a gabbroic lower crust. Large scale production of andesites by partial melting of such rocks is not possible but would need more hydrous or differentiated source rocks.     

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.     

阿拉善地块南缘野芨里镁铁-超镁铁质岩体锆石U-Pb年龄及其地质意义

阿拉善地块南缘北大山地区野芨里镁铁-超镁铁质岩体主要由单辉橄榄岩、二辉橄榄岩、橄榄辉长岩组成。岩体局部发现了硫化物,主要由磁黄铁矿、镍黄铁矿与黄铜矿组成,指示岩浆结晶过程中发生了硫化物不混溶作用。岩石地球化学分析显示野芨里岩体属于铁质超基性岩,富集大离子亲石元素,相对亏损高场强元素,具有明显的Nb,Ta负异常,微量元素比值指示岩浆在上升过程中受到了中上地壳物质混染。橄榄辉长岩中锆石SHRIMP U-Pb年龄为286.6±2.5 Ma,属于早二叠世。结合区域构造演化,认为野芨里岩体形成背景可能与古亚洲洋的闭合或俯冲之后的伸展作用相关。     

The Chilled Marginal Gabbro and Other Contact Rocks of the Skaergaard Intrusion

Rocks in the outer selvage of the Skaergaard intrusion havea range of textures and compositions, and among these are materialsrepresenting quenched Skaergaard magma. Pristine chilled marginalgabbro (CMG), however, is not ubiquitous at the intrusive contact,because many of the "contact" rocks have been hydrothermallyor metasomatically altered, contaminated with gneiss or olivinexenocrysts, while others contain accumulated minerals. Materialrepresenting quenched magma appears to be restricted to contactrocks that are texturally and mineralogically similar to diabase,and free of accumulated minerals. Where it exists, the CMG isfound within one to three meters of the exposed intrusive contactexcept at the roof of the intrusion where its thickness is greater.CMG was distinguished from the diverse group of contact rocksby petrographic and geochemical screening of over 80 specimens.Samples of CMG from the eastern and western margins and fromthe roof of the intrusion have relatively uniform compositionsimilar to that of ferrobasalt, and are noticeably richer iniron (mg-number=0?51-0?54), TiO₂ K₂O, and P₂O₅ than other unmodifiedcontact rocks. CMG's also have trace element compositions distinctfrom most other rocks in the outer Marginal Border Series (MBS).They have incompatible element contents up to 3–6 timesgreater than in LZa-type cumulates, negligible Eu anomalies,and Ni and Cr contents and Ni/Cr ratios that are among the lowestof rocks in the outer MBS. The results of melting experiments corroborate selection ofthis material as CMG. The composition of glasses obtained frompartial melting experiments of LZa-type cumulates are essentiallyidentical to those of the CMG. The 1-atm. liquidus phase relationsfor one of the CMG samples (KT-39) is largely consistent withthe sequence and composition of cumulus minerals observed withdistance inward through the MBS and upward through the LayeredSeries. Solidification of magma at the outer margin of the intrusionis interpreted to have involved locally efficient quench crystallizationfollowed by initial primocryst growth in an undercooled transitionzone a short distance inward that finally extended into regionsof near equilibrium crystallization. The similarity in composition between samples of chilled marginalgabbro from the exposed roof and sides of the intrusion, andthose of reconstituted trapped liquid from early cumulates inthe outer MBS suggests that a single magma, similar in compositionto ferrobasalt, was parental to the Skaergaard intrusion. Thisinterpretation corroborates geophysical evidence of a significantlysmaller mass for the intrusion than that estimated by Wager,and provides a basis for revision of models of its chemicalevolution. Samples chosen by Wager as chilled marginal gabbrobelong spatially, texturally, and compositionally to the groupof LZa-type cumulates in the MBS, and should no longer be regardedas chilled marginal gabbro.     

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...     

Petrogenesis of the Piqiang mafic-ultramafic layered intrusion and associated Fe-Ti-V oxide deposit in Tarim Large Igneous Province,NW China

ABSTRACT

The Piqiang intrusion is one of the two important ma?c-ultrama?c layered intrusions that host giant Fe-Ti-V oxide deposits in the Permian Tarim Large Igneous Province, NW China. The intrusion mainly consists of gabbro, anorthosite and minor plagioclase-bearing clinopyroxenite in the marginal zone. Disseminated to massive Fe-Ti oxide ores occur as layers and lenses within the gabbro. SHRIMP zircon U-Pb results from both a gabbro from the Piqiang intrusion and a granite from the surrounding granitic dyke yield ages of ~270 Ma. Geochemically, the Piqiang silicate rocks are enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE), moderately depleted in high ?eld strength elements (HFSE), and have a limited range of Sr-Nd-Hf isotopic compositions. The similar mineralogy, mineral compositions, and trace element characteristics of the layered units suggest that all the rocks are co-magmatic. The parental magma is Fe-Ti-rich and is akin to the most primitive diabasic dyke which is associated with the Piqiang intrusion. Partial melting of the Tarim mantle plume with involvement of a subduction-metasomatized lithospheric mantle source best explains the geochemistry and petrogenesis of the parental magmas of the Piqiang intrusion. We propose that the lithospheric mantle source may have been metasomatized by subduction-related materials and the metasomatic enrichment of this source region which may be correlated with oceanic sediment recycling during southward subduction of the South Tianshan oceanic slab during the Early-Middle Paleozoic. Crystal settling and mechanical sorting is the predominant process responsible for the formation of the massive Fe-Ti oxide ores in the Piqiang intrusion. Central to ore formation is a combination of the protracted differentiation history of a Fe-Ti-enriched parental magma and the later addition of external H₂O from the country rocks to the slowly cooling magma chamber.     

High-Al gabbros in the Laramie Anorthosite Complex,Wyoming: implications for the composition of melts parental to Proterozoic anorthosite

High-Al gabbro represents one of the latest phases of magmatism in the 1.43 Ga Laramie anorthosite complex (LAC) in southeastern Wyoming. This lithology, which is mineralogically and geochemically the most primitive in the LAC, forms dikes and small intrusions that cross cut monzonitic and anorthositic rocks. High-Al gabbro is characterized by high Al₂O₃ (15–19 wt%), REE patterns with positive europium anomalies (Eu/Eu^*=1.2–3.8), and the lowest initial ⁸⁷Sr/⁸⁶Sr (as low as 0.7033) and highest initial _Nd (up to +2) in the LAC. Their Sr and Nd isotopic characteristics indicate a mantle origin followed by crustal assimilation during ascent. Intermediate plagioclase (An_50–60) and mafic silicate (Fo_54–63) compositions suggest that they are not primary mantle melts and that they differentiated prior to final emplacement. High-Al gabbros of the LAC are similar compositionally to gabbros from several other Proterozoic anorthosite complexes, including rocks from the Harp Lake complex and the Hettasch intrusion in Labrador and the Adirondack Mountains of New York. These gabbros are considered to be parental to their associated anorthositic rocks, a theory that is supported by recent experimental work. We interpret LAC high-Al gabbros to represent mantle-derived melts produced by the differentiation of a basaltic magma in an upper mantle chamber. Continued evolution of this magma eventually resulted in the formation of plagioclase-rich diapirs which ascended to mid-crustal levels and formed the anorthositic rocks of the LAC. Because these gabbros intrude the anorthositic rocks, they do not represent directly the magma from which anorthosite crystallized and instead are younger samples of magma formed by identical processes.     

Geochemical and Petrological Studies on the Early Carboniferous Sidingheishan Mafic–Ultramafic Iintrusion in the Southern Margin of the Central Asian Orogenic Belt, NW China

The Sidingheishan mafic-ultramafic intrusion is located in the eastern part of the Northern Tianshan Mountain, along the southern margin of the Central Asian Orogenic Belt in northern Xinjiang autonomous region of China. The Sidingheishan intrusion is mainly composed of wehrlite, olivine websterite, olivine gabbro, gabbro and hornblende gabbro. At least two pulses of magma were involved in the formation of the intrusion. The first pulse of magma produced an olivine-free unit and the second pulse produced an olivine-bearing unit. The magmas intruded the Devonian granites and granodiorites.An age of 351.4±5.8 Ma(Early Carboniferous) for the Sidingheishan intrusion has been determined by U-Pb SHRIMP analysis of zircon grains separated from the olivine gabbro unit. A U-Pb age of 359.2±6.4 Ma from the gabbro unit has been obtained by LA-ICP-MS. Olivine of the Sidingheishan intrusion reaches 82.52 mole% Fo and 1414 ppm Ni. On the basis of olivine-liquid equilibria, it has been calculated that the MgO and FeO included in the parental magma of a wehrlite sample were approximately10.43 wt% and 13.14 wt%, respectively. The Sidingheishan intrusive rocks are characterized by moderate enrichments in Th and Sm, slight enrichments in light REE, and depletions in Nb, Ta, Zr and Hf. The ε_(Nd)(t) values in the rock units vary from +6.70 to +9.64, and initial ~(87)Sr/~(86)Sr ratios range between 0.7035 and0.7042. Initial ~(206)Pb/~(204)Pb, ~(207)Pb/~(204)Pb and ~(208)Pb/~(204)Pb values fall in the ranges of 17.23-17.91,15.45-15.54 and 37.54-38.09 respectively. These characteristics are collectively similar to the Heishan intrusion and the Early Carboniferous subduction related volcanic rocks in the Santanghu Basin, North Tianshan and Beishan area. The low(La/Gd)_(PM) values between 0.26 and 1.77 indicate that the magma of the Sidingheishan intrusion was most likely derived from a depleted spinel-peridotite mantle.(Th/Nb)_(PM)ratios from 0.59 to 20.25 indicate contamination of the parental magma in the upper crust.Crystallization modeling methods suggest that the parental magma of the Sidingheishan intrusion was generated by flush melting of the asthenosphere and subsequently there was about 10 vol%contamination from a granitic melt. This was followed by about 5 vol% assimilation of upper crustal rocks. Thus, the high-Mg basaltic parental magma of Sidingheishan intrusion is interpreted to have formed from partial melting of the asthenosphere during the break-off of a subducted slab.     

扬子地块北缘新元古代望江山层状岩体矿物成分和铂族元素特征:对岩浆演化过程和构造环境的制约

望江山层状岩体位于扬子地块北缘新元古代汉南杂岩带中,岩体从底部到顶部由超镁铁质岩过渡为中性岩:底部主要由辉石岩和橄长岩组成;中部为辉长苏长岩和辉长岩;上部为辉长岩和闪长岩。研究以中部岩相带橄榄辉长苏长岩、辉长苏长岩和辉长岩为对象,通过主要矿物的主微量元素和全岩主微量元素的分析,查明望江山岩体来源于尖晶石二辉橄榄岩组成的大陆下岩石圈地幔,并且地幔源区受到了来自俯冲板片流体的交代,岩体中部带的母岩浆为拉斑玄武质岩浆。钛铁矿—磁铁矿矿物对成分计算表明,母岩浆在形成时具有较高氧逸度。通过单斜辉石压力计得到岩体的侵位深度约为12.9~18 km。对岩体母岩浆橄榄石分离结晶过程的模拟计算表明,中部带橄榄石为母岩浆经过~28%分离结晶的产物。此外,铂族元素（PGE）组成暗示岩体并未经历过大规模的硫化物熔离,可能与缺乏地壳物质混染有关。岩体中单斜辉石与岛弧环境堆晶岩中单斜辉石成分相似,不同于裂谷环境中堆晶单斜辉石的成分;同时,全岩Th/Yb和Nb/Yb比值也与岛弧玄武岩比值相似,因此矿物和全岩成分均说明望江山层状岩体应形成于岛弧环境。研究认为扬子北缘在新元古代长期的俯冲过程中,大洋板片断离导致软流圈上涌,提供热源使交代大陆下岩石圈地幔部分熔融形成具有岛弧特征的镁铁质岩浆,在局部伸展环境中上升侵位形成汉南杂岩带中镁铁—超镁铁质层状岩体。      

Mullite-corundum-spinel-cordierite-plagioclase xenoliths in the Skaergaard Marginal Border Group: multi-stage interaction between metasediments and basaltic magma

Metapelitic country rocks were contact- and pyro-metamorphosed by the Tertiary Skaergaard Intrusion, East Greenland. In an initial stage of heating, while they were probably still in place within the host rock contact aureole, they overstepped a range of equilibrium and disequilibrium melting reactions and produced both a granitic melt and very refractory spinel+cordierite+plagioclase±corundum residuals. Parts of these refractory rocks were then subjected to another melting event after being entrained as xenoliths into the Skaergaard Marginal Border Group, where they experienced a temperature of about 1,000°C at a pressure of about 650 bars and at an oxygen fugacity about 0.2–0.5 log units below the FMQ buffer. Here, they underwent bulk melting, but did not mix with the Skaergaard magma, presumably because of the high viscosity contrast. The Al-rich melts crystallized to an assemblage of corundum+mullite+sillimanite+ plagioclase+spinel+rutile±tridymite±cordierite and they reacted with the surrounding basalt producing a strongly cryptically zoned rim of plagioclase (An₅₅ close to the basalt to An₉₀ close to the Al-rich melt). The assemblage in the inner parts of the xenoliths provides textural evidence for disequilibrium growth due to slow diffusivities in the highly viscous, probably water-free Al-rich melt. Later interaction of lower temperature ferrobasaltic to granophyric melts with the xenoliths along their margins and along cracks led to consumption of corundum and mullite and to the stable assemblage of spinel+cordierite+plagioclase+quartz+K-feldspar +magnetite+ilmenite at about 800°C.     

桂北—湘南中生代玄武质岩石及其深源包体的地球化学性质和岩石成因探讨

桂北-湘南中生代玄武质岩石中含有丰富的深源包体，它们分别为橄榄岩、变形的辉长岩和中酸性片麻岩三大类，本文在论述上述岩石地球化学性质的基础上，探讨了它们之间的成因关系；寄主的中生代玄武质岩石为地幔楣榄岩部分熔融的产物，与辉长岩和中酸性片麻岩并无成因联系，后者属偶然包体，值得注意的是，深源包体中的变形辉长岩与片麻岩之间为分离结晶的成因关系，它们均为元古宙壳、幔间底侵玄武质岩浆的演化产物，其中辉长岩为底侵岩浆的堆积相，而片麻岩则为底侵岩浆经历分离结晶的堆积作用之后所剩下的残余岩浆的变质产物。     

Supersaturation and crystal growth in the roof-zone of the Skaergaard magma chamber

The coarse-grained Upper Border Series rocks of the Skaergaard intrusion contain abundant skeletal crystals of magnetite and ilmenite, skeletal and hopper crystals of apatite, and less abundant sector-zoned augite crystals and hopper zircon crystals. In addition, the melanogranophyres which occur as pods and lenses in the lower part of the Upper Border Series and the upper part of the Layered Series are characterized by very coarse-grained dendritic ferrohedenbergite crystals. Skeletal, hopper, and sectorzoned crystals are not present in the Layered Series gabbros. The development of these unusual crystal morphologies in the Upper Border Series requires that the roof-zone magma was intermittently supersaturated and indicates that the Skaergaard magma chamber was compositionally zoned and that heat loss through the roof maintained a temperature gradient in the magma that was greater than the adiabatic gradient. It is suggested that supersaturation developed in the roof-zone of the intrusion as a result of convective overturn and magma mixing during the early stages of crystallization, and as a result of sudden volatile loss during the later stages of crystallization when the Upper Border Series rocks became rigid enough to fracture.     

西准噶尔萨吾尔地区科克托别基性岩体岩石成因:地球化学和锆石U-Pb年代学证据

萨吾尔地区位于西准噶尔东北缘,广泛发育晚古生代中酸性侵入岩和火山岩以及少量基性侵入岩,这些岩浆岩的年代学研究对于限制西准噶尔地区石炭纪构造环境具有重要的意义。本文通过研究萨吾尔地区科克托别岩体的岩相学特征、锆石SHRIMP U-Pb年龄以及地球化学特征,探讨该岩体构造背景以及成因机制,为进一步论证西准噶尔地区石炭纪构造环境提供佐证。科克托别岩体包括中粗粒辉长岩、细粒辉长岩和闪长岩,在野外露头显示细粒辉长岩以脉状侵入中粗粒辉长岩中,细粒辉长岩中包裹有中粗粒辉长岩包体,中粗粒辉长岩与闪长岩之间呈渐变过渡接触关系,说明科克托别岩体是不同期次岩浆侵位形成的杂岩体,早期岩浆侵入形成中粗粒辉长岩和闪长岩,晚期岩浆上侵就位于中粗粒辉长岩构造裂隙中形成细粒辉长岩。科克托别岩体中细粒辉长岩锆石SHRIMP U-Pb年龄为323.2±6.2Ma,表明岩体形成于早石炭世晚期。该岩体成岩年龄晚于该地区蛇绿岩套岩石年龄,也晚于岛弧火山岩年龄以及含斑岩矿床侵入岩年龄,与该地区I型花岗岩年龄相似,而明显早于碰撞后A型花岗岩和双峰式火山岩的形成年龄,说明科克托别岩体可能形成于同碰撞构造环境中。不同岩相的岩石主量元素之间的相关关系以及微量元素配分型式相似性说明它们为同源岩浆结晶分异的产物。岩相学和地球化学特征表明岩体初始岩浆可能为软流圈地幔与上覆交代地幔相互作用形成,板片断离可能为软流圈地幔的上涌起到重要作用。     

A new interpretation of the structure of the Sept Iles Intrusive suite, Canada

The layered mafic intrusion at Sept Iles, Canada, is one of the largest intrusions in the world. A new interpretation of its structure is proposed, based on a review of its geology and a comparison with the Skaergaard intrusion, Greenland. Several different magmatic components are recognized; hence the name Sept Iles Intrusive suite (SIIS) is proposed. Emplacement of the suite may have been preceded by eruption of flood basalts. The first magmas of the suite rose in the crust to accumulate beneath the density filter afforded by the basalts. The largest component is the Sept Iles Mafic intrusion (SIMI). The Lower series of the SIMI is dominated by leucotroctolites and leucogabbros. Above it lie the Layered series, which is largely comprised of gabbro and troctolite. Both these units are unchanged from earlier interpretations. The anorthosites (s.l.), gabbros and monzogabbros, formerly called the Transitional series, are now considered to be the Upper Border series, developed by floatation of plagioclase. Common autoliths in the Layered series are parts of the hydrothermally altered Upper Border series from towards the interior of the intrusion, which have foundered and settled through the magma. The contamination of the magma that accompanied this event oxidised iron in the magma and led to the precipitation of magnetite around the periphery of the intrusion. The subsequent depletion of Fe³⁺ and/or increase in SiO₂, CaO and P₂O₅ may have induced apatite saturation and accumulation to form two layers rich in apatite, near the base and at top of the Layered series. Granitic magma was developed by fractional crystallisation and was emplaced along the roof of the chamber, where it acquired large quantities of xenoliths. These were probably derived from the flood basalts, their evolved members and fragments of mafic dykes chilled by the granitic magma. Accumulations of monzonite pillows in this unit testify to another magmatic event and a floor to the granitic magma chamber, indicating lateral transport of magma. Chemically distinct syenites in the upper part of the intrusion are part of the Point du Criade intrusion, a large, late composite sill. Diabase and leucogabbro components show a close link with the SIMI and all the acidic magmas may have originally formed by differentiation of the main magma in cupolas towards the centre of the intrusion. A series of late gabbro intrusions that cut the SIMI may represent a rejuvenation of magmatism. The Border zone is a mass of fine-grained rocks that occurs along the border of the SIMI: it may be another magmatic component, or just the lateral border series of the SIMI.     

新疆西准噶尔地区两类蛇绿岩的地质特征及其成因研究

本文将西准噶尔地区蛇绿岩分为两类:一类是变质橄榄岩 橄长岩 辉长岩岩石组合(简称PTG系列);另一类则是变质橄榄岩 辉石岩 辉长岩岩石组合(简称PPG系列)。前者以达拉布特、和布克赛尔蛇绿岩带为代表,后者以唐巴勒、玛依勒山蛇绿岩带为代表。PTG系列遵循富Al的演化趋势,而PPG系列则遵循富Ca的演化趋势;从而造成两类蛇绿岩之间在岩石组合、矿物学、岩石化学、稀土元素等地球化学以及所含铬铁矿床的种属上均有明显的差异。 两类蛇绿岩中变质橄榄岩的成因机制是上地幔岩部分熔融,两者之间的差异则是由部分熔融程度决定的。而壳层岩石(指堆积杂岩、岩墙杂岩、熔岩)则是由岩浆结晶作用形成的;堆积杂岩中出现辉石岩 辉长岩和橄长岩 辉长岩的不同岩石组合则与堆积岩岩浆房出露的深度和氧化状态有关。     

Assimilation and fractionation in adjacent parts of the same magme chamber: Vandfaldsdalen macrodike,East Greenland

The Vandfaldsdalen macrodike, which lies in the Skaergaard region of East Greenland, is a remarkably zoned fossil magma chamber, with a granophyric cap overlying cumulate gabboros. The intrusion is distinctly bimodal, with a large compositional discontinuity at the contact between the gabbro and granophyre. Although the exposed part of the macrodike is in contact with Tertiary basalts and sediments, the granophyre originated by assimilation of xenoliths derived from the underlying Archean basement. Sr and Nd isotopic ratios throughout the cumulate sequence are remarkably similar, indicating insignificant contamination of the gabbro by the granophyre. Modelling of the compositional effects of cooling and crystallization indicate that the cumulate pile resulted from fractional crystallization, with the complicating effects of trapped liquid and post-cumulus fractionation. The uppermost rocks in the mafic part, of the chamber (SiO₂=62%; FeO^*=12.4%) resulted from about 85% fractional crystallization. A transgressive sill of strongly fractionated magma (SiO₂=67%; FeO^*=8.8%) formed from extracted intercumulus liquid that was the result of 90% fractional crystallization of the original magma. Mass-balance indicates that typical granophyre is made up of about 75% dissolved xenoliths, by weight, and 25% mantle-derived basaltic magma. The magmas were not measurably affected by material exchange across the interface between the gabbro and granophyre. This magma chamber evolved by both assimilation and fractional crystallization, but the residual liquids formed by fractional crystallization were unaffected by assimilation. Heat exchange between were unaffected by assimilation. Heat exchange between the two parts of the chamber was obviously important, but there was insignificant material exchange. The inability of fractional crystallization and assimilation to affect the same liquid is related to the dynamic behavior of this particular magma chamber, particularly the buoyancy of granophyre relative to evolving tholeiitic magma.     

Mechanics of growth of some laccolithic intrusions in the Henry mountains, Utah, II: Bending and failure of overburden layers and sill formation

Deformation of host rocks during growth of a laccolithic intrusion is analyzed using the theory of bending a stack of thin elastic plates. The theoretical model suggests that magma spreading laterally in the form of a sill will eventually gain sufficient leverage on the overlying strata to deflect them upward and form a laccolith. The amount of bending increases as the fourth power of the distance the magma spreads, whereas the overburden resists bending as the third power of its effective thickness. Effective thickness is the thickness of a single layer which has the same resistance to bending as a multilayer of similar length and elastic modulus. The effective thickness of overburden in the Henry Mountains is estimated as between and of the actual thickness. The form of bending is similar for Newtonian, pseudoplastic, and Bingham magmas. The magnitude of the bending depends upon the total upward force and its distribution and is not simply related to magma viscosity as has been suggested by several previous investigators.After elastic bending strata should fail over the periphery of an intrusion, the site of maximum bending strain and differential stress predicted by the theory. Field observations described in Part I correlate well with these predictions. Because bending strains are proportional to layer thickness, strata of comparable strength but different thicknesses fail at different stages of laccolith development. This leads to the different cross-sectional forms of laccoliths observed in the field.The effect of host rocks on sill form and growth is analyzed using the elastic solution for an elliptical hole under uniform pressure. The theory suggests that sill thickness increases in proportion to length. The concentration of high stresses near the sill termination should induce permanent deformation and account for the blunt terminations described in Part I. This blunting is most likely to occur in relatively ductile rocks whereas sills simply split brittle rocks and maintain sharp terminations. The driving pressure in sills can be calculated from measurements of length and termination radius of curvature, if the yield strength of the host rocks can be estimated. This driving pressure must be greater than the overburden pressure, but sills apparently do not form or propagate by lifting their overburdens. Instead they propagate by locally deforming the host rock. After spreading over a distance about three times the effective overburden thickness, the overlying layers begin to bend upward significantly. This stage marks the transition from a sill to a laccolithic intrusion.     

Chronological and genetic relationships between intrusive rocks of the Berdyaush pluton,South Urals,in light of new U-Pb and Sm-Nd isotopic data

Geological observations in combination with previously published and new isotopic data allowed us to reconstruct the history of geological events that eventually gave rise to the formation of the Berdyaush pluton situated on the western slope of the South Urals: (1) emplacement of gabbro into Lower Riphean sedimentary rocks (1410–1390 Ma); an enriched mantle source of gabbro arising in the Archean or Paleoproterozoic; (2) formation of granitic melt in the lower crust; Archean TTG association subsequently enriched in K and correlative elements as a result of interaction with enriched mantle-derived magmas and related fluids was a magma source; mixing of mantle and crustal magmas in the course of their synchronous ascent with formation of hybrid intrusive rocks; injections of mafic and hybrid melts into incompletely solidified granite; fragmentation of such injections with the formation of melanocratic nodules; emplacement of basic dikes into the cooled granite—all these events took place 1410–1360 Ma ago; (3) discrete episodes of partial melting of enriched mantle source with waning intensity; formation of minor volumes of melt, which solidified under auto- and paraautochthonous conditions as local domains highly enriched in incompatible elements (1360–1270 Ma); (4) partial melting of those domains resulting in the formation of minor nepheline syenite intrusions (915–800 Ma), containing relict zircon grains dated at >1270 Ma; (5) injections of mantle-derived alkaline melt contaminated with crustal granitic material as microsyenite and syenite porphyry dikes (700–500 Ma ?). Thus, the Berdyaush pluton is a projection of a local domain of mantle and crustal magma generation, which periodically resumed its activity over almost a billion years.     

Quantitative Simulation of the Hydrothermal Systems of Crystallizing Magmas on the Basis of Transport Theory and Oxygen Isotope Data: An analysis of the Skaergaard Intrusion

Application of the principles of transport theory to studiesof magma-hydrothermal systems permits quantitative predictionsto be made of the consequences of magma intruding into permeablerocks. Transport processes which redistribute energy, mass,and momentum in these environments can be represented by a setof partial differential equations involving the rate of changeof extensive properties in the system. Numerical approximationand computer evaluation of the transport equations effectivelysimulates the crystallization of magma, cooling of the igneousrocks, advection of chemical components, and chemical and isotopicmass transfer between minerals and aqueous solution. Numerical modeling of the deep portions of the Skaergaard magma-hydrothermalsystem has produced detailed maps of the temperature, pressure,fluid velocity, integrated fluid flux, ¹⁸O-values in rock andfluid, and extent of nonequilibrium exchange reactions betweenfluid and rock as a function of time for a two-dimensional cross-sectionthrough the pluton. An excellent match was made between calculated¹⁸O-values and the measured ¹⁸O-values in the three principalrock units, basalt, gabbro, and gneiss, as well as in xenolithsof roof rocks that are now embedded in Layered Series; the latterwere evidently depleted in ¹⁸O early in the system's coolinghistory, prior to falling to the bottom of the magma chamber.The best match was realized for a system in which the bulk rockpermeabilities were 10^–13 cm² for the intrusion, 10^–11cm² for basalt, and 10^–16 cm² for gneiss; reaction domainsizes were 0.2 cm in the intrusion and gneiss and 0.01 cm inthe basalts, and activation energy for the isotope exchangereaction between fluid and plagioclase was 30 kcal/mole. The calculated thermal history of the Skaergaard system wascharacterized by extensive fluid circulation that was largelyrestricted to the permeable basalts and to regions of the plutonstratigraphically above the basalt-gneiss unconformity. Althoughfluids circulated all around the crystallizing magma, fluidflow paths were deflected around the magma sheet during theinitial 130,000 years. At that time, crystallization of thefinal sheet of magma and fracture of the rock shifted the circulationsystem toward the center of the intrusion, thereby minimizingthe extent of isotope exchange between rocks near the marginof the intrusion at this level. For comparison, similar calculationswere also made for pure conductive cooling; it was found thatthe rate of crystallization of the magma body was not changed.The solidified pluton cooled by a factor of about 2 faster inthe presence of a hydrothermal system. Transport rates of thermal energy out of the intrusion and oflow-¹⁸O fluids into the intrusion controlled the overall isotopeexchange process. During the initial 150,000 years, temperatureswere high and reaction rates were fast; thus, fluids flowinginto the intrusion quickly equilibrated with plagioclase. However,the temperature decreased between 120,000 and 175,000 yearsand caused a decrease in reaction rates and an increase in theequilibrium fractionation factor between plagioclase and fluid.Consequently, during this time period fluids in the intrusiontended to be out of equilibrium with plagioclase. After 175,000years temperatures had decreased sufficiently that reactionrates became insignificant, but convection rates were largeenough to redistribute fluid and enlarge the regions where fluidand plagioclase were out of equilibrium. By 400,000 years, thepluton had cooled to approximately ambient temperatures, andthe final ¹⁸O values were ‘frozen in’. Reactionsbetween hydrothermal fluid and the intrusion occurred over abroad range in temperature, 1000-200 °C, but 75 per centof the fluid circulated through the intrusion while its averagetemperature was >480 °C. This relatively high temperatureis consistent with the observation that only minor amounts ofhydrothermal alteration products were formed in the naturalsystem, even where several per mil shifts in ¹⁸O were detected. The relative quantities of fluid to rock integrated over theentire cooling history were 0.52 for the upper part of intrusion,0.88 for the basalt, 0.003 for the gneiss, and 0.41 for theentire domain. Almost all of the fluid flowed into the intrusionfrom the basalt host rocks that occur adjacent to the side contactsof the intrusion. Convection transferred about 20 per cent ofthe total heat contained in the gabbro upward into the overlyingbasalts; the remaining 80 per cent of the heat was transferredby conduction.