西秦岭温泉花岗岩体岩石学特征及岩浆混合标志

温泉花岗岩体由酸性端元的寄主岩石和暗色微细粒镁铁质包体群及基性岩墙群组成。无岩浆混合作用或岩浆混合作用较弱区段，寄主岩石以似斑状二长花岗岩为主．显示正常的花岗岩结构构造岩浆混合作用强烈区段。岩石的异常结构构造十分发育．矿物之间自形程度差异显著．常见包晶反应、包含结构、交代边、熔蚀边、交代蚕食的港湾状结构构造及交代缝合线、矿物镶边、斜长石异常环带和矿物残留等，多见指示岩浆混合的标志性矿物针状磷灰石。暗色微粒包体中多见寄主二长花岗岩中的捕掳晶。包体的形态、结构构造以及与寄主岩石强烈地成分交换等均是岩浆混合作用的标志。     

Trace Element Partitioning and Accessory Phase Saturation during H2O-Saturated Melting of Basalt with Implications for Subduction Zone Chemical Fluxes

Experimental phase equilibrium and trace element partitioningdata are reported for H₂O-saturated mid-ocean ridge basalt at2·5 GPa, 750–900°C and oxygen fugacities atthe nickel–nickel oxide buffer. Garnet, omphacite andrutile are present at all temperatures. Amphibole and epidotedisappear as residual phases above 800°C; allanite appearsabove 750°C. The Na–Al-rich silicate glass presentin all run products is likely to have quenched from a supercriticalliquid. Trace element analyses of glasses demonstrate the importantcontrol exerted by residual minerals on liquid chemistry. Inaddition to garnet, which controls heavy rare earth elements(HREE) and Sc, and rutile, which controls Ti, Nb and Ta, allanitebuffers the light REE (LREE; La–Sm) contents of liquidsto relatively low levels and preferentially holds back Th relativeto U. In agreement with previous experimental and metamorphicstudies we propose that residual allanite plays a key role inselectively retaining trace elements in the slab during subduction.Experimental data and analyses of allanite-bearing volcanicrocks are used to derive a model for allanite solubility inliquids as a function of pressure, temperature, anhydrous liquidcomposition and LREE content. The large temperature dependenceof allanite solubility is very similar to that previously determinedfor monazite. Our model, fitted to 48 datapoints, retrievesLREE solubility (in ppm) to within a factor of 1· 40over a pressure range of 0–4 GPa, temperature range of700–1200°C and for liquids with anhydrous SiO₂ contentsof 50–84 wt %. This uncertainty in LREE content is equivalentto a temperature uncertainty of only ± 27°C at 1000K, indicating the potential of allanite as a geothermometer.Silicic liquids from either basaltic or sedimentary protolithswill be saturated in allanite except for Ca-poor protolithsor at very high temperatures. For conventional subduction geothermsthe low solubility of LREE (+ Th) in liquids raises questionsabout the mechanism of LREE + Th transport from slab to wedge.It is suggested either that, locally, temperatures experiencedby the slab are high enough to eliminate allanite in the residueor that substantial volumes of H₂O-rich fluids must pass throughthe mantle wedge prior to melting. The solubility of accessoryphases in fluids derived from subducted rocks can provide importantconstraints on subduction zone thermal structure. KEY WORDS: subduction; experimental petrology; allanite; solubility; supercritical liquid; eclogite     

Phase Relations and Melting of Anhydrous K-bearing Eclogite from 1200 to 1600{degrees}C and 3 to 5 GPa

To investigate eclogite melting under mantle conditions, wehave performed a series of piston-cylinder experiments usinga homogeneous synthetic starting material (GA2) that is representativeof altered mid-ocean ridge basalt. Experiments were conductedat pressures of 3·0, 4·0 and 5·0 GPa andover a temperature range of 1200–1600°C. The subsolidusmineralogy of GA2 consists of garnet and clinopyroxene withminor quartz–coesite, rutile and feldspar. Solidus temperaturesare located at 1230°C at 3·0 GPa and 1300°C at5·0 GPa, giving a steep solidus slope of 30–40°C/GPa.Melting intervals are in excess of 200°C and increase withpressure up to 5·0 GPa. At 3·0 GPa feldspar, rutileand quartz are residual phases up to 40°C above the solidus,whereas at higher pressures feldspar and rutile are rapidlymelted out above the solidus. Garnet and clinopyroxene are theonly residual phases once melt fractions exceed 20% and garnetis the sole liquidus phase over the investigated pressure range.With increasing melt fraction garnet and clinopyroxene becomeprogressively more Mg-rich, whereas coexisting melts vary fromK-rich dacites at low degrees of melting to basaltic andesitesat high melt fractions. Increasing pressure tends to increasethe jadeite and Ca-eskolaite components in clinopyroxene andenhance the modal proportion of garnet at low melt fractions,which effects a marked reduction in the Al₂O₃ and Na₂O contentof the melt with pressure. In contrast, the TiO₂ and K₂O contentsof the low-degree melts increase with increasing pressure; thusNa₂O and K₂O behave in a contrasted manner as a function ofpressure. Altered oceanic basalt is an important component ofcrust returned to the mantle via plate subduction, so GA2 maybe representative of one of many different mafic lithologiespresent in the upper mantle. During upwelling of heterogeneousmantle domains, these mafic rock-types may undergo extensivemelting at great depths, because of their low solidus temperaturescompared with mantle peridotite. Melt batches may be highlyvariable in composition depending on the composition and degreeof melting of the source, the depth of melting, and the degreeof magma mixing. Some of the eclogite-derived melts may alsoreact with and refertilize surrounding peridotite, which itselfmay partially melt with further upwelling. Such complex magma-genesisconditions may partly explain the wide spectrum of primitivemagma compositions found within oceanic basalt suites. KEY WORDS: eclogite; experimental petrology; mafic magmatism; mantle melting; oceanic basalts     

Sedimentary petrology and geochemistry of siliciclastic rocks from the upper Jurassic Tordillo Formation (Neuquén Basin, western Argentina): Implications for provenance and tectonic setting

The Upper Jurassic Tordillo Formation is exposed along the western edge of the Neuquén Basin (west central Argentina) and consists of fluvial strata deposited under arid/semiarid conditions. The pebble composition of conglomerates, mineralogical composition of sandstones and pelitic rocks, and major- and trace-element geochemistry of sandstones, mudstones, and primary pyroclastic deposits are evaluated to determine the provenance and tectonic setting of the sedimentary basin. Conglomerates and sandstones derived almost exclusively from volcanic sources. The stratigraphic sections to the south show a clast population of conglomerates dominated by silicic volcanic fragments and a predominance of feldspathic litharenites. This framework composition records erosion of Triassic–Jurassic synrift volcaniclastic rocks and basement rocks from the Huincul arch, which was exhumed as a result of Late Jurassic inversion. In the northwestern part of the study area, conglomerates show a large proportion of mafic and acidic volcanic rock fragments, and sandstones are characterised by a high content of mafic volcanic rock fragments and plagioclase. These data suggest that the source of the sandstones and conglomerates was primarily the Andean magmatic arc, located west of the Neuquén Basin. The clay mineral assemblage is interpreted as the result of a complex set of factors, including source rock, climate, transport, and diagenesis. Postdepositional processes produced significant variations in the original compositions, especially the fine-grained deposits. The Tordillo sediments are characterised by moderate SiO₂ contents, variable abundances of K₂O and Na₂O, and a relatively high proportion of ferromagnesian elements. The degree of chemical weathering in the source area, expressed as the chemical index of alteration, is low to moderate. The major element geochemistry and Th/Sc, K/Rb, Co/Th, La/Sc, and Cr/Th values point to a significant input of detrital volcanic material of calcalkaline felsic and intermediate composition. However, major element geochemistry is not useful for interpreting the tectonic setting. Discrimination plots based on immobile trace elements, such as Ti, Zr, La, Sc, and Th, show that most data lie in the active continental margin field. Geochemical information is not sufficiently sensitive to differentiate the two different source areas recognized by petrographic and modal analyses of conglomerates and sandstones.     

大别山地区石榴石橄榄岩的岩石学和地球化学

本文首次对我国石榴石橄榄岩类岩石进行系统的岩石学和地球化学研究。通过对大别山地区两个不同类型岩体中的石榴石橄榄岩的岩石学、岩石化学、矿物化学和稀土元素地球化学研究,表明这类岩石属钙碱性系列超基性岩;与地幔成分相比,富集易熔元素,亏损难熔元素;利用矿物地质温压计估算其形成压力为2×10~9pa,成岩温度为600～700℃具有正铕异常和轻稀土元素富集特征 推测为地壳下部的超基性岩经深成变质而成。     

海拉尔盆地乌尔逊凹陷与松辽盆地孤店CO2气田含片钠铝石砂岩的岩石学特征

海拉尔盆地乌尔逊凹陷南屯组与松辽盆地孤店CO₂气田泉头组发育大量含片钠铝石砂岩。通过偏光显微镜、扫描电镜、茜素红-S染色、X射线衍射、电子探针与INCA能谱分析等,对含片钠铝石砂岩的骨架碎屑组分、胶结物与自生矿物、成岩共生序列等岩石学特征进行了系统研究。研究表明,含片钠铝石砂岩的岩石类型为长石砂岩和岩屑长石砂岩,粒度以细粒、细-中粒为主,分选差-中等。砂岩中胶结物主要为次生加大石英、自生石英、片钠铝石、铁白云石和粘土矿物。其中,片钠铝石最高可达砂岩总体积的22％在砂岩中或以放射状、束状、菊花状、杂乱毛发状、毛球状、板状等集合体充填孔隙,或呈束状和板状交代长石和岩屑。电子探针与INCA能谱综合分析表明,片钠铝石主要由Na、Al、O、C等组成。在含片钠铝石砂岩中,成岩共生序列依次为粘土矿物包壳-次生加大石英、自生石英、自生高岭石-油气充注-CO₂充注-片钠铝石-铁白云石。其中,CO₂注入前形成的自生矿物组合主要为次生加大石英、自生石英和自生高岭石,CO₂注入后形成的自生矿物组合主要为片钠铝石和铁白云石。     

Zircon LA-ICP-MS U-Pb age, Ce^4＋/Ce^3＋ ratios and the geochemical features of the Machangqing complex associated with copper deposit

1IntroductionQuite a number of porphyry copper deposits havebeen found in the Cenozoic alkali-intrusions along theJinshajiang-Ailaoshan-Red River fault zone,whichconstitute the Cenozoic ore belt in China.The Cenozo-ic alkali-intrusive rocks are similar in…     

An experimental study on the shallow-level migmatization of ferrogabbros from the Fuerteventura Basal Complex, Canary Islands

Spectacular shallow-level migmatization of ferrogabbroic rocks occurs in a metamorphic contact aureole of a gabbroic pluton of the Tierra Mala massif (TM) on Fuerteventura (Canary Islands). In order to improve our knowledge of the low pressure melting behavior of gabbroic rocks and to constrain the conditions of migmatization of the TM gabbros, we performed partial melting experiments on a natural ferrogabbro, which is assumed as protolith of the migmatites. The experiments were performed in an internally heated pressure vessel (IHPV) at 200 MPa, 930–1150 °C at relatively oxidizing conditions. Distinct amounts of water were added to the charge.

From 930 to 1000 °C, the observed experimental phases are plagioclase (An_60–70), clinopyroxene, amphibole (titanian magnesiohastingsites), two Fe–Ti oxides, and a basaltic, K-poor melt. Above 1000 °C, amphibole is no longer stable. The first melts are very rich in normative plagioclase (>70 wt.%). This indicates that at the beginning of partial melting plagioclase is the major phase which is consumed to produce melt. In the experiments, plagioclase is stable up to high temperatures (1060 °C) showing increasing An content with temperature. This is not compatible with the natural migmatites, in which An-rich plagioclase is absent in the melanosomes, while amphibole is stable. Our results show that the partial melting of the natural rocks cannot be regarded as an “in-situ” process that occurred in a closed system. Considerable amounts of alkalis probably transported by water-rich fluids, derived from the mafic pluton underplating the TM gabbro, were necessary to drive the melting reaction out of the stability range of plagioclase. A partial melting experiment with a migmatite gabbro showing typical “in-situ” textures as starting material supports this assumption.

Crystallization experiments performed at 1000 °C on a glass of the fused ferrogabbro with different water contents added to the charge show that generally high water activities could be achieved (crystallization of amphibole), independently of the bulk water content, even in a system with very low initial bulk water content (0.3 wt.%). Increasing water contents produce plagioclase richer in An, reduces the modal proportion of plagioclase in the crystallizing assemblage and extends the melt fraction. High melt fractions of >30 wt.% could only be observed in systems with high bulk water contents (>2 wt.%). This indicates that the migmatites were generated under water-rich conditions (probably water-saturated), since those migmatites, which are characterized as “in-situ” formations, show generally high amounts of leucosomes (>30 wt.%).     

Experimental evidence for diamond-facies metamorphism in the Dora-Maira massif

The peak metamorphic conditions of subducted continental crust in the Dora-Maira massif (Western Alps) have been revised by combining experimental results in the KCMASH system with petrologic information from whiteschists. Textural observations in whiteschists suggest that the peak metamorphic assemblage garnet+phengite+kyanite+coesite±talc originates from the reaction kyanite+talc↔garnet+coesite+liquid. In the experimentally determined petrogenetic grid, this reaction occurs above 45 kbar at 730 °C. At lower pressures, talc reacts either to orthopyroxene and coesite or, together with phengite, to biotite, coesite and kyanite. The liberated liquid contains probably similar amounts of H₂O and dissolved granitic components. The composition of the liquid in the whiteschists at peak metamorphic conditions, a major unknown in earlier studies, was probably very similar to the liquid composition produced in the experiments. Therefore, the experimentally determined petrogenetic grid represents a good model for the estimation of the peak metamorphic conditions in whiteschists. Experimentally determined Si-isopleths for phengite further constrain peak pressures to 43 kbar for the measured Si=3.60 of phengite in the natural whiteschists. All these data provide evidence that the whiteschists reached diamond-facies conditions.

The fluid-absent equilibrium 4 kyanite+3 CELADONITE=4 coesite+3 muscovite+pyrope has been calibrated on the basis of garnet and phengite compositions in the experiments and serves as a geothermobarometer for ultra-high-pressure (UHP) metapelites. For graphite-bearing metapelites and kyanite–phengite eclogites, forming the country rocks of the whiteschists, peak metamorphic pressures of about 44±3 kbar were calculated from this barometer for temperatures of 750 °C estimated from garnet–phengite thermometry. Therefore, the whole ultra-high-pressure unit of the Dora-Maira massif most likely experienced peak metamorphic conditions in the diamond stability field. While graphite is common in the metapelites, diamond has not been found so far. The absence of metamorphic microdiamonds might be explained by the low temperature of metamorphism, the absence of a free fluid phase in the metapelites and a short residence time in diamond-facies conditions resulting in kinetic problems in the conversion of graphite to diamond.     

Petrological and geochronological constraints on high pressure, high temperature metamorphism in the Snowbird tectonic zone, Canada

The upper deck of the East Athabasca mylonite triangle (EAmt), northern Saskatchewan, Canada, contains mafic granulites that have undergone high P–T metamorphism at conditions ranging from 1.3 to 1.9 GPa, 890–960 °C. Coronitic textures in these mafic granulites indicate a near‐isothermal decompression path to 0.9 GPa, 800 °C. The Godfrey granite occurs to the north adjacent to the upper deck high P–T domain. Well‐preserved corona textures in the Godfrey granite constrain igneous crystallization and early metamorphism in the intermediate‐pressure granulite field (Opx + Pl) at 1.0 GPa, 775 °C followed by metamorphism in the high pressure granulite field (Grt + Cpx + Pl) at 1.2 GPa, 860 °C. U–Pb geochronology of zircon in upper deck mafic granulite yields evidence for events at both c. 2.5 Ga and c. 1.9 Ga. The oldest zircon dates are interpreted to constrain a minimum age for crystallization or early metamorphism of the protolith. A population of 1.9 Ga zircon in one mafic granulite is interpreted to constrain the timing of high P–T metamorphism. Titanite from the mafic granulites yields dates ranging from 1900 to 1894 Ma, and is interpreted to have grown along the decompression path, but still above its closure temperature, indicating cooling following the high P–T metamorphism from c. 960–650 °C in 4–10 Myr. Zircon dates from the Godfrey granite indicate a minimum crystallization age of 2.61 Ga, without any evidence for 1.9 Ga overgrowths. The data indicate that an early granulite facies event occurred at c. 2.55–2.52 Ga in the lower crust (c. 1.0 GPa), but at 1.9 Ga the upper deck underwent high P–T metamorphism, then decompressed to 0.9–1.0 GPa. Juxtaposition of the upper deck and Godfrey granite would have occurred after or been related to this decompression. In this model, the high P–T rocks are exhumed quickly following the high pressure metamorphism. This type of metamorphism is typically associated with collisional orogenesis, which has important implications for the Snowbird tectonic zone as a fundamental boundary in the Canadian Shield.