吉林桦甸太古代杂岩的矿物化学和变质作用温压条件

本文重点研究了桦甸杂岩的主要造岩矿物黑云母、角闪石、辉石和石榴石以及它们之间的平衡共生关系。各主要造岩矿物的化学成分及相互关系都说明桦旬杂岩的变质作用高峰达麻粒岩相。文中用斜方辉石-单斜辉石、石榴石-黑云母、石榴石-普通角闪石、石榴石-斜方辉石和石榴石-斜方辉石-斜长石-石英等矿物对和矿物组合作了变质作用温度、压力估测。桦甸杂岩的变质作用温压条件约为6.4—7.1kbar,700—750℃,桦甸杂岩变质时的埋藏深度约25km,当时的地热梯度大约30℃/km。     

Petrologic evidence of a complex plumbing system feeding the July–August 2001 eruption of Mt. Etna,Sicily, Italy

After the major 1991–1993 eruption, Mt. Etna resumed flank activity in July 2001 through a complex system of eruptive fissures cutting the NE and the S flanks of the volcano and feeding effusive activity, fire fountains, Strombolian and minor phreatomagmatic explosions. Throughout the eruption, magmas with different petrography and composition were erupted. The vents higher than 2,600 m a.s.l. (hereafter Upper vents, UV) erupted porphyritic, plagioclase-rich trachybasalt, typical of present-day summit and flank activity. Differently, the vents located at 2,550 and 2,100 m a.s.l. (hereafter Lower vents, LV) produced slightly more primitive trachybasalt dominated by large clinopyroxene, olivine and uncommon minerals for Etna such as amphibole, apatite and orthopyroxene and containing siliceous and cognate xenoliths. Petrologic investigations carried out on samples collected throughout the eruption provided insights into one of the most intriguing aspects of the 2001 activity, namely the coeval occurrence of distinct magmas. We interpret this evidence as the result of a complex plumbing system. It consists in two separate magma storage systems: a shallow one feeding the activity of the UV and a deeper and more complex storage related to the activity of LV. In this deep storage zone, which is thermally and compositionally zoned, the favourable conditions allow the crystallization of amphibole and the occurrence of cognate xenoliths representing wall cumulates. Throughout 2001 eruption, UV and LV magmas remain clearly distinct and ascended following different paths, ruling out the occurrence of mixing processes between them. Furthermore, integrating the 2001 eruption in the framework of summit activity occurring since 1995, we propose that the 2001 magma feeding the vents lower than 2,600 m a.s.l. is a precursor of a refilling event, which reached its peak during the 2002–2003 Etna flank eruption.     

鄂东南矿集区鸡冠嘴矽卡岩型金铜矿床含矿岩体中辉石和角闪石成分变化特征及其对岩浆演化和成矿的指示意义

为了解长江中下游地区与金铜矿化有关的岩体在浅部的演化过程及其对成矿的作用,本文对鄂东南矿集区鸡冠嘴矽卡岩型金铜矿成矿岩体石英二长闪长斑岩进行了详细的岩相学观察,并利用电子探针(EMPA)分析了其中辉石和角闪石的主量成分。岩相学特征显示辉石形成时间早于角闪石。辉石成分变化较小,而角闪石成分变化较大。角闪石可根据Al的含量分为高低两组,即高铝含量的自形角闪石以及低铝含量的半自形及他形角闪石。高铝角闪石又不同程度的被低铝角闪石交代。通过矿物温压计估算辉石形成的温度和压力为1055~1071℃(平均1060℃)和224~312MPa(平均255MPa)。高铝角闪石形成的温度和压力为809~864℃(平均833℃),108~193MPa(平均137MPa),低铝角闪石形成的温度和压力为721~766℃(平均741℃),48~67MPa(平均56MPa)。岩浆具有较高的氧逸度,并且从高铝角闪石到低铝角闪石,熔体的氧逸度从△NNO+0.6升高到△NNO+1.9。岩浆经历了连续的侵位历史,从9.6km到5.2km的岩浆房再到2.1km的浅地壳处就位。岩浆在9.6km处经历了辉石的分离结晶,在5.2km处经历了角闪石及少部分斜长石的分离结晶,在2.1km处经历由于降压引起的流体出溶。由于深部的分离结晶作用,熔体中的水含量增加,氧逸度升高。较高的水含量使岩浆更容易演化出成矿热液,较高的氧逸度使岩浆演化早期没有硫化物的分离结晶,从而阻止了成矿元素在岩浆演化早期亏损。鸡冠嘴岩体较浅的就位深度也更有利于成矿流体的析出。这些因素共同作用形成了鸡冠嘴金铜矿床。     

Nb/Ta fractionation observed in eclogites from the Chinese Continental Scientific Drilling Project

This paper reports detailed analyses of Nb and Ta concentrations of 19 eclogite samples and their principal mineral constituents from the main drill hole of the Chinese Continental Scientific Drilling Project (CCSD) and nearby outcrops. We observe highly fractionated and overall suprachondritic Nb/Ta values in minerals, e.g., rutile (4.8–87), titanite (12–62) and amphibole (2.0–67). Amphiboles in amphibolites (retrograded from eclogite) can be classified into two groups: a low Nb/Ta group that bears higher Al contents and is thus of higher pressure origin, and a high Nb/Ta, lower pressure group. The former group was likely formed during subduction; the latter may have formed during exhumation in the presence of rutile and titanite. The significant Nb/Ta fractionation in rutile and other minerals may reflect early dehydration of the subducted slab at shallow depths before the formation of rutile, which occurs at depths ≥50 km. The dehydration, with amphiboles existing as the main Nb–Ta-bearing phase, would lead to Nb/Ta fractionation, i.e., forming subchondritic Nb/Ta ratios in the released fluids and, complementarily, suprachondritic Nb/Ta ratios in the residual phases. While a large proportion of the fluids may escape from the slab to the mantle wedge, considerable amounts of the fluids can be retained in hydrous minerals within the descending slab, thus forming hydrated cold eclogites with subchondritic Nb/Ta characteristics. As subduction continues to depths over 50 km, rutile appears and consequently controls the Nb–Ta budget. In the presence of rutile, melting of the hydrated cold eclogites with very low Nb/Ta ratios would form magmas with negative Nb, Ta anomalies and subchondritic Nb/Ta. Further dehydration of the continuously descending slab results in even more fractionated Nb/Ta ratios in subsequently released fluids and residues, providing a feasible explanation for the large Nb/Ta variation observed in the modern arc magmas and residual eclogites.     

The Serra da Graciosa A-type Granites and Syenites, southern Brazil. Part 3: Magmatic evolution and post-magmatic breakdown of amphiboles of the alkaline association

Amphiboles are the main mafic minerals in most metaluminous to peralkaline alkali-feldspar granites and syenites, and they usually preserve an important record of the compositional evolution of the melts from which they crystallize. In the alkaline association of the Serra da Graciosa A-type Granites and Syenites (southern Brazil), amphibole compositions span a large range, including calcic, sodic–calcic, and sodic amphiboles. Calcic amphiboles are typically observed in the metaluminous rocks, while sodic amphiboles are characteristic of the more strongly peralkaline rocks; sodic–calcic amphiboles are found in intermediate varieties. Compositional variations record the differentiation trends within two petrographic series of the alkaline association. The overall evolution of amphibole compositions is similar in both: they reveal a progressive increase in Na and Fe³⁺ with differentiation (increase in alkalinity of the magmas), a characteristic shared by undersaturated peralkaline (or agpaitic) differentiation trends. In detail, however, the evolutions of the amphibole compositions in the two series are distinct. In Alkaline series 1, the cores of the crystals form a continuum from calcic to sodic compositions, with the exception of a small compositional gap within the sodic–calcic amphiboles. The rims, however, show compositions that diverge from this main trend; this divergence results from increasing amounts of the oxy-amphibole component, and reflects more oxidizing conditions at the final stages of magmatic crystallization. In Alkaline series 2, these oxidation trends are much more subtle and a reverse trend is observed in the sodic amphiboles. Sodic–calcic amphiboles are in several cases replaced by intergrowths of post-magmatic sodic amphibole and Al-poor (“tetrasilicic”) biotite.     

Igneous garnet and amphibole fractionation in the roots of island arcs: experimental constraints on andesitic liquids

To evaluate the role of garnet and amphibole fractionation at conditions relevant for the crystallization of magmas in the roots of island arcs, a series of experiments were performed on a synthetic andesite at conditions ranging from 0.8 to 1.2 GPa, 800–1,000°C and variable H₂O contents. At water undersaturated conditions and fO₂ established around QFM, garnet has a wide stability field. At 1.2 GPa garnet + amphibole are the high-temperature liquidus phases followed by plagioclase at lower temperature. Clinopyroxene reaches its maximal stability at H₂O-contents ≤9 wt% at 950°C and is replaced by amphibole at lower temperature. The slopes of the plagioclase-in boundaries are moderately negative in space. At 0.8 GPa, garnet is stable at magmatic H₂O contents exceeding 8 wt% and is replaced by spinel at decreasing dissolved H₂O. The liquids formed by crystallization evolve through continuous silica increase from andesite to dacite and rhyolite for the 1.2 GPa series, but show substantial enrichment in FeO/MgO for the 0.8 GPa series related to the contrasting roles of garnet and amphibole in fractionating Fe–Mg in derivative liquids. Our experiments indicate that the stability of igneous garnet increases with increasing dissolved H₂O in silicate liquids and is thus likely to affect trace element compositions of H₂O-rich derivative arc volcanic rocks by fractionation. Garnet-controlled trace element ratios cannot be used as a proxy for ‘slab melting’, or dehydration melting in the deep arc. Garnet fractionation, either in the deep crust via formation of garnet gabbros, or in the upper mantle via formation of garnet pyroxenites remains an important alternative, despite the rare occurrence of magmatic garnet in volcanic rocks.     

班公湖-怒江缝合带西段洞错石榴石麻粒岩岩石学特征及其构造意义

班公湖-怒江缝合带是青藏高原中部一条重要的构造边界,将北部的羌塘地块与南部的拉萨地块分隔开,当前对班公湖-怒江洋壳的俯冲过程知之甚少,本文报道班公湖-怒江缝合带西段洞错地区的石榴石麻粒岩,将有助于提升班公湖-怒江缝合带构造演化过程的认识。石榴石麻粒岩呈5.0~_10mm的透镜体产出于斜长角闪岩内,后者呈构造岩片产出于班公湖-怒江缝合带内,与超基性岩片呈断层接触。石榴石麻粒岩的石榴石呈变斑晶产出,具有不规则的成分环带,石榴石核部成分较均匀,以富Ca和Mg,贫Fe和Mn为特征,石榴石边缘及内部裂隙成分则与之相反,富Fe和Mn,与绿泥石和黝帘石组成的细脉共生,反映了石榴石遭受后期流体的交代作用。单斜辉石为透辉石,在其内部发现早期进变质阶段的细小石榴石与角闪石颗粒,石榴石粒径_10μm,该石榴石相对富Fe和Mn,贫Ca和Mg。角闪石均为钙质系列的角闪石,早期角闪石以相对富Na和Fe~(3+),含有微量Ba,而与晚期角闪石区分开。早期角闪石常产出于石榴石和透辉石变斑晶内部,脱水熔融生成富Ca、贫Fe~(3+)和Na的透辉石和以钠长石为主的熔融囊体,囊体内部发育斜长石和富Ba冰长石的包体/出溶体。晚期角闪石常与斜长石呈细小的后成合晶产出于石榴石边部或呈变斑晶产出于基质之中,表明斜长角闪岩系石榴石麻粒岩退变质的产物。此外,金红石斑晶常变成细粒、不规则的贫Al榍石和钛铁矿,基质中有相对富Al榍石产出。因此该麻粒岩进变质阶段矿物组合由早期石榴石(Grt__1)、早期角闪石(Amp_1)、相对富Al的榍石(Spn_1)等矿物组成。峰期矿物组合包括:核部石榴石(Grt2)+透辉石+斜长石(Pl_1)+石英+金红石+富Na和Ba的熔体,采用传统温/压计估算此峰期矿物组合形成条件为870℃、11.7kbar。退变质矿物组合有:边部石榴石(Grt3)+晚期贫Na和Fe~(3+)的角闪石(Amp_1)+黝帘石+绿泥石+斜长石(Pl_2)+白云母+贫Al榍石(Spn2)+钛铁矿。岩相学观察与地球化学示踪、变质温压估计表明洞错石榴石麻粒岩系热洋壳俯冲的产物,为前人提出的"班公-怒江洋盆包含次级小洋盆"之观点提供了变质岩石学的证据。从峰期到退变质阶段,角闪石的Fe~(3+)含量明显下降,铁铝榴石含量明显升高,指示该麻粒岩经历了还原反应,早期角闪石脱水熔融时被还原,生成了高氧逸度的埃达克质岩浆,促进了Cu、Au等成矿元素从俯冲热洋壳的活化与迁移至弧岩浆之中,因此本文的研究提供了一个探讨俯冲热洋壳与上覆地幔楔之间化学反应的研究案例,这有助于理解班公-怒江成矿带内众多斑岩型铜、金矿床的形成。     

Titanite: A potential solidus barometer for granitic magma systems

Constraining crystallization pressure and thus intrusion depth of granites in various geodynamic settings remains challenging, yet important to further our understanding of magma system and crustal evolution. We propose that titanite, which is a common accessory in metaluminous and weakly peraluminous granites, can be used as a barometer if it crystallized in magmatic, near-solidus conditions and in equilibrium with amphibole, plagioclase, K-feldspar, quartz, biotite, and magnetite ± ilmenite. Titanite Al₂O₃ increases with pressure (P) according to: P (in MPa) = 101.66 × Al₂O₃ in titanite (in wt%) + 59.013 (R² = 0.83) with estimated uncertainties of ~±60 to ~±100 MPa for crystallization between ~150 and 400 MPa. We highlight that the current calibration dataset is limited, and that systematic experimental studies are needed to rigorously quantify the relation. The most important use of this empirical barometer will be for rocks in which amphibole is present but significantly altered, or in combination with amphibole barometry, as titanite can be easily dated by LA-ICP-MS.     

苏北桃林岩基与苏胶造山带燕山期隆升机制的探讨

苏北桃林岩基主要单元角闪石、黑云母的矿物化学研究揭示,苏胶造山带在印支期陆内双向对冲碰撞造山后,于燕山期末Ａ型花岗岩体侵位之前,由挤压向拉伸转化时期,曾发现急剧隆升。早期隆升快,为０１３９ｍｍ／ａ,中期隆升变慢,为００２３ｍｍ／ａ。山根拆沉,均衡抬升,地幔上隆,上地壳伸展造盆成山,是造就苏胶造山带现今盆山构造和（超）高压带燕山期折返的主要原因     

Intermediate high-pressure exhumation of the northern segment of the Sanbagawa belt, Saruta-gawa area, central Shikoku, Japan

The retrograde chemical zonal structure of amphibole in hematite-bearing basic and quartz schists from the higher grade zone in the Saruta-gawa area of the Sanbagawa belt was studied to investigate the relationships between the prograde and retrograde P–T paths of the Sanbagawa metamorphism. This amphibole coexists with chlorite, epidote, muscovite, albite, quartz and hematite, and is composed of Al-rich core and Al-poor mantle. The core is fairly homogeneous and has a barroisitic composition. In the mantle part, ^[B]Na increases with decreasing ^[4]Al towards the margins, which have winchite–magnesioriebeckite compositions. The barroisite–winchite–magnesioriebeckite composite crystal is sometimes rimmed by actinolite and/or winchite with low ^[4]Al and ^[B]Na. The Al-rich core and Al-poor mantle are regarded as prograde and retrograde products, respectively. The retrograde mantle in the Saruta-gawa area: (1) is systematically richer in ^[B]Na [0.40–1.73 per formula unit (pfu; for O=23)] than that from the same grade zone in the Asemi-gawa area (0.19–0.78 pfu), about 8 km south of the studied area; (2) tends to be ^[B]Na-poorer (less than 1.73 pfu) than prograde sodic amphibole (up to 1.93 ^[B]Na pfu) produced in the peak temperature stage from the lower grade zone in the same and other areas; and (3) extends its compositional range towards higher ^[B]Na and lower ^[4]Al than prograde-formed amphibole from the same grade zone in the same area. These zonal characteristics imply that (1) the Saruta-gawa samples experienced retrograde metamorphism under higher P/T conditions than the Asemi-gawa samples, (2) the retrograde P–T path of the Saruta-gawa area passes on the lower pressure side of the metamorphic field gradient, and (3) the Saruta-gawa samples underwent retrograde metamorphism under higher P/T conditions than the prograde metamorphism. The higher P/T conditions of the retrograde metamorphism suggests an increasing dP/dT of the geotherm during exhumation. Retrograde P–T conditions during the formation of magnesioriebeckite can be roughly estimated at 7–8 kbar, 400–450°C based on semi-quantitative phase relations of actinolite–winchite–magnesioriebeckite–barroisite series associated with chlorite, epidote, muscovite, albite, quartz and hematite.