Sapphirine parageneses from the Caraiba complex, Bahia, Brazil: the influence of Fe2+–Fe3+ distribution on the stability of sapphirine in natural assemblages

Abstract Sapphirine-bearing rocks occur in three conformable, metre-size lenses in intrusive quartzo-feldspathic orthogneisses in the Curaçà valley of the Archaean Caraiba complex of Brazil. In the lenses there are six different sapphirine-bearing rock types, which have the following phases (each containing phlogopite in addition): A: Sapphirine, orthopyroxene; B: Sapphirine, cordierite, orthopyroxene, spinel; C: Sapphirine, cordierite; D: Sapphirine, cordierite, orthopyroxene, quartz; E: Sapphirine, cordierite, orthopyroxene, sillimanite, quartz; F: Sapphirine, cordierite, K-feldspar, quartz. Neither sapphirine and quartz nor orthopyroxene and sillimanite have been found in contact, however. During mylonitization, introduction of silica into the three quartz-free rocks (which represent relict protolith material) gave rise to the three cordierite and quartz-bearing rocks. Stable parageneses in the more magnesian rocks were sapphirine–orthopyroxene and sapphirine–cordierite. In more iron-rich rocks, sapphirine–cordierite, sapphirine-cordierite–sillimanite, cordierite–sillimanite, sapphirine–cordierite–spinel–magnetite and quartz–cordierite–orthopyroxene were stable. The iron oxide content in sapphirine of the six rocks increases from an average of 2.0 to 10.5 wt % (total Fe as FeO) in the order: C,F–A,D–B,E. With increase in Fe there is an increase in recalculated Fe₂O₃ in sapphirine. The four rock types associated with the sapphirine-bearing lenses are: I: Orthopyroxene, cordierite, biotite, quartz, feldspar tonalitic to grandioritic gneiss; II: Biotite, quartz, feldspar gneiss; III: Orthopyroxene, clinopyroxene, hornblende, plagioclase meta-norite; IV: Biotite, orthopyroxene, quartz, feldspar, garnet, cordierite, sillimanite granulite gneiss. The stable parageneses in type IV are orthopyroxene–cordierite–quartz, garnet–sillimanite–quartz and garnet–cordierite–sillimanite. Geothermobarometry suggests that the associated host rocks equilibrated at 720–750°C and 5.5–6.5 kbar. Petrogenetic grids for the FMASH and FMAFSH (FeO–MgO–Al₂O₃–Fe₂O₃–SiO₂–H₂O) model systems indicate that sapphirine-bearing assemblages without garnet were stabilized by a high Fe³⁺ content and a high X_Mg= (Mg/ (Mg+Fe²⁺)) under these P–T conditions.     

Low-pressure granulite facies metapelitic assemblages and corona textures from MacRobertson Land, east Antarctica: the importance of Fe2O3 and TiO2 in accounting for spinel-bearing assemblages

Low-pressure granulite facies metasedimentary gneisses exposed in MacRobertson Land, east Antarctica, include hercynitic spinel-bearing metapelitic gneisses. Peak metamorphic mineral assemblages include spinel + rutile + ilmenite + sillimanite + garnet, spinel + ilmenite + sillimanite + garnet + cordierite, ortho-pyroxene + magnetite + ilmenite + garnet, spinel + cordierite + biotite + ilmenite and orthopyroxene + cordierite + biotite, each with quartz, K-feldspar and melt. The presence of garnet + biotite- and cordierite + orthopyroxene-bearing assemblages implies crossing tie-lines in AFM projection for the K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O (KFMASH) system. This apparent contradiction, and the presence of spinel, rutile and ilmenite in the assemblages, is acounted for by using the KFMASH-TiO₂-O₂ system, i.e. AFM + TiO₂+ Fe₂O₃. We derive a petrogenetic grid for this system, applicable to low-pressure granulite facies metamorphic conditions. Retrograde assemblages are interpreted from corona textures on hercynitic spinel and Fe-Ti oxides. The relative positions of the peak and retrograde metamorphic assemblages on the petrogenetic grid suggest that corona development occurred during essentially isobaric cooling.     

浙西南遂昌—大柘地区八都岩群印支期变质变形序列

浙西南遂昌-大柘地区八都岩群在印支期变质事件影响下发生变质变形,通过详细野外调查和岩相学研究,可将其划分为3期变质变形序列：S₁变形期,NW向片麻理记录的残留紧闭褶皱,共生矿物组合为石榴子石变斑晶及其内部定向分布的包裹体矿物,石榴子石+黑云母+石英（泥质）和石榴子石+角闪石+斜长石+石英（长英质）;S₂变形期,区域性宽缓褶皱及NE向缓倾透入性片麻理,共生矿物组合为石榴子石变斑晶及定向分布的基质矿物,矽线石+石榴子石+黑云母+石英+斜长石±钾长石（泥质）和石榴子石+钾长石+斜长石+黑云母+石英（长英质）;S₃变形期,NE向陡倾透入性片麻理及韧脆性断裂大部分被花岗斑岩脉填充,共生矿物组合为石榴子石变斑晶及其周围退变矿物,石榴子石+矽线石+堇青石+斜长石+黑云母+石英±钾长石（泥质）和角闪石+斜长石+黑云母+钛铁矿（长英质）。结合前人研究成果,八都岩群印支期变质事件峰期变质程度达到麻粒岩相,显示顺时针近等温降压（ITD）型的p-T演化轨迹,S₁-S₃变质变形反映出从俯冲碰撞到快速折返冷却的演化过程,伴随S₃同期侵位的花岗斑岩锆石U-Pb定年结果,将该演化过程完成时间约束在229.7 Ma,可能是浙西南地区对印支期古特提斯洋域内印支-华南-华北板块之间俯冲-碰撞过程的响应。     

Stable isotope constraints on the Al2SiO5'triple-point' rocks from the Proterozoic Priest pluton contact aureole, New Mexico, USA

The Priest pluton contact aureole in the Manzano Mountains, central New Mexico preserves evidence for upper amphibolite contact metamorphism and localized retrograde hydrothermal alteration associated with intrusion of the 1.42 Ga Priest pluton. Quartz–garnet and quartz–sillimanite oxygen isotope fractionations in pelitic schist document an increase in the temperatures of metamorphism from 540 °C, at a distance of 1 km from the pluton, to 690 °C at the contact with the pluton. Comparison of calculated temperature estimates with one‐dimensional thermal modelling suggests that background temperatures between 300 and 350 °C existed at the time of intrusion of the Priest pluton. Fibrolite is found within 300 m of the Priest pluton in pelitic and aluminous schist metamorphosed at temperatures >580 °C. Coexisting fibrolite and garnet in pelitic schist are in oxygen isotope equilibrium, suggesting these minerals were stable reaction products during peak metamorphism. The fibrolite‐in isograd is coincident with the staurolite‐out isograd in pelitic schist, and K‐feldspar is not observed with the first occurrence of fibrolite. This suggests that the breakdown of staurolite and not the second sillimanite reaction controls fibrolite growth in staurolite‐bearing pelitic schist. Muscovite‐rich aluminous schist locally preserves the Al₂SiO₅ polymorph triple‐point assemblage – kyanite, andalusite and fibrolite. Andalusite and fibrolite, but not kyanite, are in isotopic equilibrium in the aluminous schist. Co‐nucleation of fibrolite and andalusite at 580 °C in the presence of muscovite and absence of K‐feldspar suggests that univariant growth of andalusite and fibrolite occurred. Kyanite growth occurred during an earlier regional metamorphic event at a temperature nearly 80 °C lower than andalusite and fibrolite growth. Quartz–muscovite fractionations in hydrothermally altered pelitic schist and quartzite are small or negative, suggesting that late isotopic exchange between externally derived fluids and muscovite, but not quartz, occurred after peak contact metamorphism and that hydrothermal alteration in pelitic schist and quartzite occurred below the closure temperature of oxygen self diffusion in quartz (<500 °C).     

广西大厂地区黑云母花岗岩中电气石的化学组成及其对岩浆热液演化的指示

广西大厂地区笼箱盖黑云母花岗岩与区内晚白垩世锡多金属成矿作用在时空上密切相关。岩相学特征表明,笼箱盖黑云母花岗岩中的电气石可以分为三类:1)浸染状电气石; 2)石英-电气石囊; 3)电气石-石英脉。本文利用电子探针和激光剥蚀等离子体质谱系统测定三种不同产状电气石的化学组成。分析结果显示,三种产状的电气石均具有高的Fe/(Fe+Mg)和Na/(Na+Ca)比值,主体属于碱基亚类铁电气石。浸染状电气石为岩浆晚期结晶,其Fe/(Fe+Mg)比值变化于0. 85～0. 94,随着岩浆分异,电气石逐渐富集Li、F、Fe和Sn等元素。与浸染状电气石相比,石英-电气石囊中早阶段电气石具有低的Fe/(Fe+Mg)比值,高的V、Co和Sr含量,可能反映了岩浆演化晚期出现的不混溶富硼熔/流体对早期黑云母和长石的交代作用,从而使囊中早阶段电气石继承部分被交代矿物的化学组成特征;石英-电气石囊中晚阶段电气石的化学组成变化较大(如Li、F、Mg、Al、V、Fe和Zn),与热液成因电气石的推论一致。与浸染状和囊状电气石相比,石英脉中的电气石具有高的Fe/(Fe+Mg)和Na/(Na+Ca)比值;微量元素组成与囊状电气石相似。就成矿元素锡而言,三种产状的电气石均具有相对高的锡含量,与其他地区锡成矿花岗岩中电气石的成分特征相似。但是,从岩浆晚期到热液阶段,大厂地区电气石的锡含量并没有显著升高,可能反映了早期岩浆热液流体对熔体锡有限的萃取作用。     

A calculated petrogenetic grid for the system K2O-FeO-MgO-Al2O3-SiO2-H2O, with particular reference to contact-metamorphosed pelites

A petrogenetic grid is presented for the system KFMASH (K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O), including biotite, muscovite, K-feldspar, chlorite, chloritoid, staurolite, cordierite, garnet, orthoamphibole, orthopyroxene, spinel, andalusite, sillimanite, kyanite, quartz and corundum with H₂O in excess, which was calculated using the computer program THERMOCALC and the Powell and Holland internally consistent thermodynamic dataset. By removing the normal constraint of having quartz in excess, both quartz-bearing and quartz-absent equilibria are shown. Quartz-absent equilibria are particularly relevant at high- T and low- P conditions, because of their common occurrence at these conditions. The calculated mineral assemblage and mineral compositional variations in terms of FeMg-1 and (Fe, Mg)SiAl_-2 exchange vectors are broadly compatible with observations on natural rocks, particularly when non-KFMASH components are taken into account.     

Orthomagmatic quartz and post-magmatic carbonate veins in a reported porphyry copper deposit,Andean Intrusive Suite,Livingston Island,South Shetland Islands

A previously reported porphyry Cu + Mo deposit in an Eocene pluton within the South Shetland Island magmatic arc has been re-interpreted as three distinct hydrothermal assemblages. The oldest assemblage (1) exsolved under confinement from the deep (～6 km?) cooling magma whereas assemblages (2) and (3) formed during tectonic ± magmatic episodes at depths of < 1.5 km in the late Cenozoic. The three assemblages occur over the 5 × 11 km mapped in Barnard Point tonalite pluton. Assemblage (1) comprises shallowly dipping sheets of aplite, biotite + tourmaline pegmatite, massive ‘grey’ quartz, and quartz + tourmaline + bornite + chalcopyrite + molybdenite veins. Magnetite + tourmaline + chalcopyrite breccias have associated biotite, K-feldspar and muscovite alteration. Fluid inclusions indicate formation from hot (～600°C), saline (40 equivalent weight % NaCl + CaCl₂) aqueous-carbonic fluids that exsolved from the partly consolidated magma. The primary control on solution chemistry and nature of fracturing was the depth of pluton emplacement. Assemblage (2) consists of steep, vuggy veins and country-rock breccias, with thick propylitic alteration selvages, cemented by microcrystalline quartz, complex inter-growths of FeMg carbonate, bladed barite and trace amounts of bornite and chalcopyrite. These rocks, previously described as breccia (sensu ‘pebble’) dykes in the porphyry complex, are reinterpreted as an influx of moderately hot (175–330°C), weak to moderately saline (2–21 EWP NaCl), aqueous-carbonic fluids that underwent isobaric boiling at 0.8 to 1.3 km depth. Assemblage (3) consists of thin, hematitic fault infillings formed during a second episode of brittle faulting.     

Contact metamorphism of pelitic,psammitic and calcareous sediments in the Southern Highlands of New South Wales

Contact metamorphism has been recognized along a 4 km wide belt adjacent to the shallow‐dipping eastern margin of the Arthursleigh Tonalite, an Early Devonian pluton of the Marulan Batholith, eastern New South Wales. In Ordovician psammitic and pelitic rocks three zones of progressive contact metamorphism range from muscovite + biotite + chlorite assemblages in the outer zone to K‐feldspar + cordierite assemblages adjacent to the pluton and in metasedimentary xenoliths. Retrograde phenomena include extensive replacement of metamorphic minerals by ‘sericite’ and chlorite. Calcareous metasediments adjacent to the tonalite typically contain assemblages of quartz + calcic plagioclase + ferrosalite + sphene, or wollastonite + calcite + diopside with minor grossularite and vesuvianite. Thermal effects in volcanic rocks along the western margin of the pluton are confined to recrystallization of the groundmass.

The regional geology indicates confining pressures of approximately 1 kbar at the time of emplacement of the tonalite. Contact metamorphic temperatures were estimated from two‐feldspar geothermometry to attain a maximum of approximately 590°C for rocks in the innermost zone of the aureole and 700°C for the xenoliths. Fluid compositions attending progressive contact metamorphism were water‐rich (Xco₂<0.2) and, during cooling, these fluids probably account for the extensive retrograde hydration observed in the aureole.     

An occurrence of grandidierite,kornerupine, and tourmaline in southeastern Ontario,Canada

Grandidierite, kornerupine, and tourmaline occur in high-grade pelitic gneisses from southeastern Ontario, Canada. The kornerupine occurs in quartz-bearing layers associated with biotite, cordierite, garnet, ilmenite, K-feldspar, magnetite, quartz, and, less commonly, sillimanite. Grandidierite is found in quartz-poor, cordierite+sillimanite segregations in contact with biotite, cordierite, ilmenite, K-feldspar, magnetite, sillimanite, and, more rarely, garnet. Tourmaline is sporadically distributed in all compositional layers, but is not in contact with the other borosilicates. There is no textural evidence for a reactive relationship among the three borosilicates. Neither chemical or textural equilibrium has been achieved on the scale of a thin section.It is proposed that the granite, K-feldspar-rich leucosomes, and different borosilicate assemblages in adjacent compositional layers evolved along a path of decreasing pressure and increasing temperature. The P-T path intersected a series of dehydration and melting reactions. This P-T path indicates that uplift had occurred before cooling had started and before the maximum temperature was reached. Corona and symplectite textures developed at various times during uplift both before and after cooling had started.     

二长石地质温度计在估算乌拉山金矿碱性长石形成温度中的应用

内蒙古乌拉山金矿田内主要出露晚太古代乌拉山群区域变质岩和规模不一的花岗岩体以及不同时代、不同种类的脉状地质体。含金矿脉中主要矿物共生组合为碱性长石、石英、斜长石、碳酸盐矿物(方解石、白云石)和少量金属硫化物。矿床的显著特征为碱性长石交代作用强烈，碱性长石也广泛产于该地区其他各种类型的岩石中。本文采用电子显微探针分析了共生碱性长石和斜长石的化学成分，并采用三元二长石温度模型估计了碱性长石的平衡温度。结果表明，第一成矿阶段的碱性长石一石英含金矿脉中碱性长石的形成温度为353℃，第二成矿阶段石英含金矿脉中碱性长石的形成温度为281℃，矿脉碱性长石形成压力约为5kbar。这些结果与同类矿石中平衡共生的碳酸盐矿物和云母类矿物的地质温度计估计的形成温度以及共生石英中流体包裹体的均一温度非常一致。因此，乌拉山金矿床形成和富集的温度可估测为260～380℃，压力约为5kbar。此外，应用二长石温度计计算了本地区区域变质片麻岩和花岗岩中碱性长石的平衡温度，所得温度比采用共生铁铝榴石和黑云母温度计估计的温度要低约250℃。这表明共生的铁铝榴石和黑云母的平衡温度可能代表其寄主变质岩变质期温度及寄主花岗岩原生温度，而区域变质岩和花岗岩中的碱性长石在经历了随后多次热液作用后，可能重新平衡再生，这也与前人对乌拉山金矿的矿床地质和同位素研究的结果一致。     

The upper thermal stability of chlorite + quartz: an experimental study in the system MgO-Al2O3-SiO2-H2O

Experiments up to water pressures of 21 kbar have been undertaken to bracket the reactions chlorite + quartz = talc + kyanite + H₂O, chlorite + quartz = talc + cordierite + H₂O, and talc + kyanite + quartz = cordierite ± H₂O by reversed runs in the system MgO-Al₂O₃-SiO₂-H₂O (MASH). These reaction curves intersect at an invariant point (IP1) at PH₂O = 6.4 ± 0.2 kbar and a temperature of 624 ± 4°C. The curve of the chlorite + quartz breakdown to talc + kyanite + H₂O at water pressures above 6.4 kbar shows a negative dP/dT, with the slope decreasing with rising pressure, whereas the slope of the breakdown curve to talc + cordierite + H₂O at water pressures is clearly positive. The composition of the chlorite solid solution reacting with quartz has been estimated to be approximately Mg_4.85Al_1.15[Al_1.15Si_2.85O₁₀](OH)₈ over the entire pressure range investigated. The composition of the talc solid solution forming by the breakdown of chlorite + quartz appears to be Mg_2.94Al_0.06[Al_0.06Si_3.94O₁₀](OH)₂ at PH₂O = 2kbar. With increasing pressure, the Al content of talc decreases, reaching a value of about 0.06 atoms per formula unit at P,H₂O = 21 kbar. As a consequence of the new experimental data, the existing phase topologies of the MASH-system and K₂O-MASH-system have been revised. For example, the invariant point IP1 and the univariant reaction curve kyanite + talc + H₂O = chlorite + cordierite are stable. For this reason, the development of medium- to high-temperature metamorphic rocks compositionally approximating the MASH-system must be reconsidered. The whiteschists from Sar e Sang, Afghanistan, are treated as an example. The application of the present experimental data to metamorphic rocks of more normal composition requires the examination of the influence of further components. This leads to the conclusion that the introduction of Fe²⁺ into magnesian chlorite extends its stability field in the presence of quartz by 10°-15°C in comparison with pure Mg-chlorite.     

Progressive sequence of reactions of the Ryoke metamorphism in the Yanai district, southwest Japan: the formation of cordierite

The compositions of biotite and muscovite were examined in terms of the paragenesis and the metamorphic grade in low- to medium-grade pelitic rocks of the Ryoke metamorphism in the Yanai district, southwest Japan. The biotite and muscovite that coexist with K-feldspar have a higher K component in an A'KF diagram than those in rocks lacking K-feldspar. This fact reflects an increase in the K₂O content in muscovite, but in biotite it reflects an increase of not only the K₂O content but also of the octahedral vacancy.
At higher metamorphic grade beyond the cordierite isograd, where cordierite coexists with neither chlorite nor K-feldspar, the biotite shows an increase in illite, K Aliv □xii−1 Si−1, and Tschermak components, Alvi Aliv R+−1 Si−1, where □xii and R+ denote the interlayer vacancy and (Fe+Mg+Mn), respectively. A reaction to define the cordierite isograd is proposed by treating this chemical change as being responsible for the first appearance of cordierite, i.e. K,Al-poor biotite+phengitic muscovite=K,Al-rich biotite+cordierite+quartz+water .By treating this as a key reaction in medium-grade metamorphism, a set of reaction in a progressive metamorphism is established for the Ryoke metamorphism, a typical low-pressure type metamorphism. Some textures in one of the high-grade areas, the K-feldspar-cordierite zone, suggest that a further two prograde reactions have taken place, i.e. andalusite+biotite+quartz=cordierite+K-feldspar+water
and   andalusite=sillimanite.quartz=cordierite+K-feldspar+water
This implies that this zone probably has a P–T  path involving isobaric heating.     

阳春盆地锡山钨锡矿床钾长花岗岩的矿物学特征及地质意义

锡山钨锡多金属矿是粤西阳春盆地内一个中型石英脉型矿床,成矿岩体岩性为钾长花岗岩,主要矿物组成为钾长石、斜长石、石英、黑云母,副矿物包括锆石、磷灰石、金红石、钛铁矿和铌铁矿等。文章利用电子探针技术对主要矿物的矿物学特征进行了研究,结果表明花岗岩中斜长石为钠长石,钾长石为透长石,黑云母属于铁叶云母。黑云母化学成分显示其w(Ti)介于0.19~0.39,具有低的氧逸度(log f(O2)Ni-Ni O),指示其有利于钨锡矿床的形成且仍具有较好的找矿潜力。结合已有的区域地质资料,文章认为锡山钨锡多金属矿床是岩石圈伸展背景下的产物。     

新疆西南天山霍什布拉克碱长花岗岩体岩石学及地球化学特征——岩石成因及其构造与成矿意义

位于中国西南天山南缘的霍什布拉克岩体,由碱长花岗岩和碱长花岗斑岩组成。岩石化学成分比较均一,具有富SiO2、Al2O3,贫Fe2O3T、MgO、TiO2、P2O5等特点。AR-SiO2图解中,样品均落入碱性区间,在A/CNK-A/NK图解中,样品主要为准铝质。微量元素总体表现为高场强元素(HFSE)相对于大离子亲石元素的富集以及高场强元素P、Ti和大离子亲石元素Sr、Ba的明显亏损。稀土元素中轻稀土元素相对于重稀土元素富集,Eu的负异常十分明显。岩体含副矿物萤石,并具有高FeOT/MgO、低CaO等特征,10000Ga/Al的比值和Zr+Ni+Sr+Y的含量亦较高,岩石学和地球化学特征表明它为典型的A1型花岗岩,是铁镁质下地壳在低压条件下部分熔融的产物。岩体具有极低的Ti含量是由于源区部分熔融过程中大量磁铁矿的残留所致,而较高的Nb-Ta含量则与岩浆中富含F有关。岩体产出于板内环境,在构造岩体的形成上受到塔里木板块内部裂谷体系的影响。岩体对霍什布拉克铅锌矿成矿意义不大,但具有Sn元素的成矿潜力。     

Petrogenesis of A-type granites and origin of vertical zoning in the Katharina pluton, Gebel Mussa (Mt. Moses) area, Sinai, Egypt

The central pluton within the Neoproterozoic Katharina Ring Complex (area of Gebel Mussa, traditionally believed to be the biblical Mt. Sinai) shows a vertical compositional zoning: syenogranite makes up the bulk of the pluton and grades upwards to alkali-feldspar granites. The latters form two horizontal subzones, an albite–alkali feldspar (Ab–Afs) granite and an uppermost perthite granite. These two varieties are chemically indistinguishable. Syenogranite, as compared with alkali-feldspar granites, is richer in Ca, Sr, K, Ba and contains less SiO₂, Rb, Y, Nb and U; Eu/Eu* values are 0.22–0.33 for syenogranite and 0.08–0.02 for alkali-feldspar granites. The δ¹⁸O (Qtz) is rather homogeneous throughout the pluton, 8.03–8.55‰. The δ¹⁸O (Afs) values in the syenogranite are appreciably lower relative to those in the alkali–feldspar granites: 7.59–8.75‰ vs. 8.31–9.12‰. A Rb–Sr isochron (n = 9) yields an age of 593 ± 16 Ma for the Katharina Ring Complex (granite pluton and ring dikes).

The alkali–feldspar granites were generated mainly by fractional crystallization of syenogranite magma. The model for residual melt extraction and accumulation is based on the estimated extent of crystallization ( 50 wt.%), which approximates the rigid percolation threshold for silicic melts. The fluid-rich residual melt could be separated efficiently by its upward flow through the rigid clusters of crystal phase. Crystallization of the evolved melt started with formation of hypersolvus granite immediately under the roof. Fluid influx from the inner part of the pluton to its apical zone persisted and caused increase of P_H2O in the magma below the perthite granite zone. Owing to the presence of F and Ca in the melt, P_H2O of only slightly more than 1 kbar allows crystallization of subsolvus Ab–Afs granite. Abundance of turbid alkali feldspars and their ¹⁸O/¹⁶O enrichment suggest that crystallization of alkali-feldspar granites was followed by subsolvus fluid–rock interaction; the δ¹⁸O (Fsp) values point to magmatic origin of fluids.

The stable and radiogenic isotope data [δ¹⁸O (Zrn) = 5.82 ± 0.06‰, I_Sr = 0.7022 ± 0.0064, ε_Nd (T) values are + 3.6 and + 3.9] indicate that the granite magma was generated from a ‘juvenile’ source, which is typical of the rocks making up most of the Arabian–Nubian shield.     

Geochemical characteristics,cooling history and mineralization significance of Zhangtiantang pluton in South Jiangxi Province,P.R. China

The zircon SHRIMP dating of the Zhangtiantang granite gave an age of 159±7 Ma., which shows that the granite was produced at the early Late Jurassic. The Ar-Ar plateau ages of biotite and K-feldspar from the Zhangtiantang pluton are 153.2±1.1 Ma and 135.8±1.2 Ma, respectively. The Ar-Ar anti-isochrone ages of biotite and K-feldspar are 152.5±1.7Ma and 135.4±2.7Ma, respectively. The ages represent the isotopic closure ages of minerals in the pluton. The Zhangtiantang granites are regarded as peraluminous crust-derived type granites to possess the typical geochemical characteristics of calc-alkaline rocks on continental margin, with enriched Si, K, Al (average value of A/CNK as 1.18), HREE, Rb, U, and Th, heavily depleted V, Cr, Co, Ni, Ti, Nb-Ta, Zr, Sr, P, and Ba, strongly negative Eu and common corundum normative (average value of C as 1.84). The εNd_(t) values of the Zhangtiantang granite are −5.84 to −7.79, and t _2DM values are 1.69 to 1.83 Ga, which indicates partial melting of continental-crust metamorphic sedimentary rocks during the Middle Proterozoic. The cooling history of the Zhangtiantang granitic pluton indicates that the cooling velocity of pluton was faster (about 67°C/Ma) from zircon (158 Ma) to biotite (152 Ma), and was slower (about12°C/Ma) from biotite (152.5 Ma) to K-feldspar (135.8 Ma). It can be deduced that the temporal gap (about 10 Ma) between the granite formmation and W-Sn mineralization in South China may be related to ordinary magma-hydrothermal processes by the variational cooling curve of the pluton. The Zhangtiantang pluton was formed in a compressive setting, with differentiation evolution and mineralization occurring in a relative relaxation setting.     

Staurolite Stability in a Part of the System Fe-Al-Si-O-H

The following reactions, believed to be analogous to those whichdefine the maximum extent of staurolite-quartz compatibilityat moderate oxygen fugacity in metamorphic rocks, have beendetermined in terms of hydrous fluid pressure and temperature.The O: H composition ratio of the fluid was controlled withthe quartz-fayalite-magnetite (QFM) buffer assemblage. (I) Fe-staurolitequartz almandine+sillimanite+water. (II) Fe-staurolitequartz Fe-cordierite+sillimanite+water. (III) Fe-chloritoid+sillimanite Fe-staurolite+quartz+water. In addition, two reactions which delineate part of the stabilitylimits of Fe-cordierite have been investigated: (IV) Fe-cordierite almandine+sillimanite+quartz. (V) Fe-cordierite hercynite+sillimanite+quartz. The experimental information has been used to predict boundariesto the PT fields of all quartz and QFM-buffered fluid-bearingassemblages involving Fe-staurolite, Fe-cordierite, Fe-chloritoid,almandine, and sillimanite. Using information from this andother studies, three mineral assemblages are recognized whichare stable at similar temperatures but different fluid pressures.In order of decreasing pressure they are: (a) Above 5 kb: staurolite, quartz, kyanite, fluid; (b) Between 1.5 and 8.5 kb (outer limits; in natural rocks thisfield will have a much narrower pressure range) staurolite,quartz, cordierite, fluid. (c) Below 3.5 kb: Fe-cordierite, andalusite, fluid of oxygenfugacity equivalent to the quartz-fayalite-magnetite assemblage. These phase assemblages may be the equivalents of naturallyoccurring mineral facies, but this must be proven in the field.In addition the absence of cordierite from rocks of appropriatecomposition and temperature of formation betokens total pressuresgreater than 3–5 kb. ¹Present address: Grant Institute of Geology, West Mains Road, Edinburgh 9, Scotland.     

浆液过渡态流体在矽卡岩型钨矿成矿过程中的作用——以湖南柿竹园钨锡多金属矿为例

湖南杮竹园是世界著名的大型矽卡岩型锡钨多金属矿床,产于千里山碱长花岗岩岩体南部接触带。矽卡岩中广泛发育网脉状碱交代脉和少量花岗岩脉、云英岩脉等各类脉体。碱交代脉主体由钾长石、萤石、少量石英、磁铁矿、黑钨矿、白钨矿及花岗岩构成,以往被统称为"云英岩脉"。其中早阶段碱交代脉中央发育花岗岩,边部为钾长石-萤石-黑钨矿,脉体两侧发育石榴子石透辉石矽卡岩化,对应矽卡岩阶段。晚阶段碱交代脉主要成分为钾长石、萤石,脉体及两侧出现大量阳起石、绿帘石、磁铁矿、白钨矿及辉钼矿、辉铋矿、自然铋等,对应退变质氧化物阶段。空间上,碱交代脉分布于矽卡岩和矽卡岩化大理岩中,不进入岩体。自花岗岩体→岩脉→碱交代脉→矽卡岩,Ca O、Ti O2、成矿元素W、Bi、Mo、Cu、Pb、Zn以及Sr、Ba等元素含量增高,显示出成矿元素向热液中富集,且岩浆和矽卡岩受到碳酸盐岩围岩的影响。碱交代脉的组构显示出其形成于富含成矿物质和挥发份流体的岩浆,其中广泛发育熔融包裹体和熔流包裹体,显示其浆液过渡态流体的成因性质。从岩浆晚期分异演化→热液阶段是连续演化的过程,块状云英岩和矽卡岩阶段,岩浆并未完全固结,成矿作用自岩浆固结之前已经开始。总结了杮竹园矿床成矿模型:碱长花岗岩岩浆演化晚期分异出的高度富含挥发份的熔浆,在岩体顶部聚集,部分形成似伟晶岩(壳)和块状云英岩以及条带状硅灰石符山石矽卡岩。进一步聚集以及矽卡岩化产生大量CO2引起大规模隐爆,富含挥发份的岩浆或浆液过渡态流体沿隐爆形成的碎裂裂隙进入碳酸盐岩围岩,与碳酸盐岩不断发生反应,在脉体边部形成钾长石化以及大范围的石榴子石透辉石矽卡岩化。至退变质氧化物阶段,随着岩浆冷凝和温度、压力的降低,地下水大范围参与,成矿流体逐渐转变为热液性质,形成大量阳起石、磁铁矿、白钨矿及钼、铋硫化物。硫化物阶段,大量的大气降水参与成矿,温度、盐度进一步降低,在矽卡岩及其外侧的碳酸盐岩中形成铅锌硫化物矿石。     

内蒙古浩尧尔忽洞金矿区岩体地球化学特征及成矿意义

内蒙古浩尧尔忽洞金矿位于华北地台北缘西段中元古代白云鄂博台缘凹陷带西部,矿区内有大面积花岗质岩体和岩脉出露,其岩性主要包括石英二长闪长岩、二长花岗岩和碱长花岗岩。通过岩相学特征和岩石地球化学特征分析,结合区域上同类型岩体的侵入时代,认为浩尧尔忽洞岩体形成于华北板块与西伯利亚板块同碰撞-后碰撞环境。其中二长花岗岩形成于同碰撞环境,该环境下由于板片持续俯冲,引起俯冲板片及地幔楔发生熔融,其上侵带来的热量致使下地壳物质部分熔融而形成该类型岩石的母岩浆;石英二长闪长岩形成于碰撞后隆起环境,是加厚下地壳熔融的产物;碱长花岗岩属于晚造山期A型花岗岩系列,该阶段地幔玄武质岩浆底侵,导致下地壳物质熔融,部分与其发生混染。结合浩尧尔忽洞金矿的成矿年龄、成矿流体特征及赋矿岩石有机地球化学特征,认为岩体的侵位提供了矿床形成必不可少的热量及部分成矿流体来源。     

Metastable growth of corundum adjacent to quartz in a spinel-bearing quartzite from the Archaean Napier Complex, Antarctica

In a granulite-facies spinel-bearing quartzite, corundum, orthopyroxene and sapphirine (and rarely cordierite and sillimanite) form partial rims separating spinel from quartz. Textures indicate the reactions:
spinel + quartz = orthopyroxene + corundum, and
spinel + quartz = orthopyroxene + sapphirine.
Thus, corundum and sapphirine are produced by reactions involving quartz. The low Al-content of the orthopyroxene (0.5–2.8 wt %) and low values for Mg–Fe distribution coefficient for spinel–sapphirine and spinel–orthopyroxene reflect low-temperature conditions during formation of the reaction products. Absence of zoning in spinel and a constant Mg–Fe distribution coefficient for spinel–sapphirine and spinel–orthopyroxene, over a compositional range, indicate Mg–Fe equilibration. It is suggested that stable reactions such as spinel + quartz = cordierite or spinel + quartz = garnet + sillimanite were over-stepped and that metastable reactions give rise to the anomalous juxtaposition of corundum + quartz.