Approaches to equilibrium in the distribution of trace elements among the principal minerals in a high-grade metamorphic terrane

X-ray fluorescence, instrumental neutron activation, and particle-induced X-ray emission methods were used to determine the distribution of numerous trace elements among garnet (Grt), Ca-pyroxene (Cpx), hornblende (Hbl), biotite (Bt), plagioclase (Pl) and K-feldspar (Kf) in a high-grade metamorphic terrane within the Grenville Province of the Canadian Shield. Results are presented as distribution formulae, e.g. Sr: Kf 1.1 Pl 16 Hbl 2.2 Cpx 1.0 Bt 1.2 Grt Sc: Hbl 1.1 Cpx 1.0 Grt 7.8 Bt 22 Pl 2.6 Kf V: Hbl 1.15 Bt 2.07 Cpx 6.0 Grt (1.4% CaO)>1 (Pl, Kf) Zn: Bt 1.6 Hbl 1.62 Cpx 2.9 Grt 10 Pl Ga: Bt 1.2 Hbl 1.2 Pl 2.5 Cpx 1.3 Grt where numbers are distribution ratios, e.g. ppm Sr in Hbl/ppm Sr in Cpx=2.2. Examples of inter-element similarities and differences are (a) both Rb and Cs are concentrated in biotite relative to K-feldspar, but for Rb the ratio is 2.3 and for Cs it is 16, (b) the distribution formulae for seven lanthanides are similar except for the position of garnet, e.g. Ce: Hbl 2.7 Cpx 2.8 Pl 1.1 Bt 11 Kf 16 Grt Yb: Grt 2.8 Hbl 2.7 Cpx 9 Pl 1.0 Bt 7 Kf and (c) all of Sr, eight lanthanides, Zr, V and Cr are concentrated in hornblende relative to Ca-pyroxene by a factor that lies in the narrow range of 2.2–3.1. There is a larger variation (departure from the mean) in some distribution ratios than in others. Thus the mean ratios (Hbl/Cpx) for each of six elements and in parentheses the percentage relative standard deviation are Zn 1.62 (8.6), V 2.38 (12), Cr 2.42 (18), Sr 2.7 (28), Ba 2.9 (36) and Ni 1.66 (38). We suggest that variation of this kind is the result of differences from place to place in the magnitude of deformation and recrystallization (which facilitated the rearrangement of atoms), combined with rates of lattice and crystal-boundary diffusion that are unique for the various elements, thus permitting some trace elements to approach equilibrium more closely than others.     

Petrological characterization and tectonic significance of retrogressed garnet peridotites, Raobazhai area, North Dabie Complex, east-central China

The Raobazhai ultramafic body of the North Dabie Complex is re-interpreted as a mantle-derived peridotitic slice enclosed in, and isofacially metamorphosed with, surrounding granulite-to-amphibolite facies gneisses. The ultramafic sheet consists mainly of metaharzburgite, but includes subunits of metadunite and mylonitic lherzolite. The rocks contain spinel but neither garnet nor plagioclase. However, in the mylonitic lherzolite, fine-grained intergrowths of spinel, orthopyroxene and clinopyroxene outline domains resembling the habit of garnet in two dimensions; broad-beam microprobe analyses imply pseudomorphs after a pyropic garnet precursor. The mineral assemblage of the metadunite and metaharzburgite is: olivine (Fo₉₂)+orthopyroxene (En₉₂)+tremolitic-to-magnesiohornblende+Mg–Al-chromite, indicating amphibolite facies recrystallization. The mineral assemblage of the mylonitic lherzolite is: olivine (Fo₉₀)+orthopyroxene (En₉₀)+clinopyroxene+Cr-bearing spinel+pargasitic amphibole, indicative of granulite-to-amphibolite facies metamorphism. Phase equilibria and geothermometric estimations show that the Raobazhai meta-ultramafics have undergone at least three stages of recrystallization: (I) 950–990 °C, (II) 750–860 °C, and (III) 670–720 °C, assuming equilibrium in the spinel peridotite stability field ( c. 6–15 kbar), although an early, high-pressure stage (≥18 kbar) is probable, based on the inferred garnet pseudomorphs. Petrochemical and geothermobarometric data suggest that the ultramafic slice represents a fragment of the mantle wedge, tectonically incorporated into subducted continental crust and re-equilibrated at granulite-to-amphibolite facies conditions while being exhumed to shallow levels.     

Distribution of Fe and Mg between coexisting garnet and hornblende in synthetic and natural systems: an empirical calibration of the garnet–hornblende Fe–Mg geothermometer

Multiple regression analysis of a compilation of the Fe²⁺–Mg distribution between garnet and hornblende from experimental runs on basaltic to intermediate compositions (n=22) and coexisting garnet–clinopyroxene–hornblende from natural (intermediate to basaltic) rocks (n=43) has been performed to define ln K_{D(Fe²⁺/Mg)}^Grt–Hbl as a function of temperature and garnet composition. The regression of data covering a large span in pressure (5–16 kbar), temperature (515–1025°C) and composition yields the ln K_{D(Fe²⁺/Mg)}^Grt–Hbl–P–T compositional relationship (r²=0.93):

where

Application of this expression to natural garnet–hornblende pairs in intermediate to basaltic and semipelitic rock types from various settings gives temperatures that are consistent with other methods.     

沙坪沟钼矿区橄榄角闪石岩岩石成因——矿物学特征约束

对位于秦岭-大别造山带的沙坪沟钼矿矿区出露的橄榄角闪石岩开展了系统的岩石学、岩相学及电子探针原位分析,厘定了其成岩物理化学条件,并探讨了岩石成因及形成的构造环境。橄榄角闪石岩主要造岩矿物为橄榄石、角闪石、金云母和辉石;副矿物为磷灰石、铬铁尖晶石、镁铁尖晶石、磁铁矿、磁黄铁矿、镍黄铁矿等。电子探针原位分析结果显示,橄榄石属于贵橄榄石系列,角闪石主要是韭闪石,辉石为透辉石,云母为金云母。造岩矿物结晶顺序为橄榄石-辉石-角闪石,结晶温度介于1 111. 64～1 223. 30℃之间,形成压力为350～740 MPa,形成于较高的氧逸度环境(角闪石结晶时的氧逸度范围为NNO+0. 59～NNO+2. 27)。角闪石和金云母的矿物学特征显示它们均是与幔源岩浆作用相关的原生矿物,表明该套基性-超基性岩与幔源岩浆作用有关。综合分析认为,在大别造山带中生代地壳减薄的伸展构造背景下,该橄榄角闪石岩母岩浆源于地幔,是经过地幔7. 92%部分熔融的玄武质岩浆经结晶分异作用形成的幔源岩浆底侵下地壳的产物。     

Crystallogenetic models for metasomatic replacement in zircons: implications for U–Pb geochronology of Precambrian rocks

Recrystallization of zircons under the influence of fluids was studied using examples from Precambrian rocks (microcline granites, metasedimentary, and mafic rocks) of the Kola Peninsula. All zircon crystals showed complex internal textures visible by cathodoluminescence and backscattered electron (BSE) imaging. Detailed mineralogical and geochemical studies with subsequent secondary ion mass spectrometer U–Pb dating of different zircon domains show that secondary texture formation can be interpreted in terms of metasomatic replacement of zircon crystals on the base of crystallogenetic experimental models. Mechanisms of zircon replacement and interpretation of U–Pb ages for secondary zircon domains are dependent on the degree of damage of the zircon structure and the fluid composition. The recrystallization of metamict zircon without additional supply of new zircon substance (Zr, SiO₂) goes with the dissolution of amorphous domains and precipitation of new polycrystalline zircon, which preserves the U–Pb initial age, but loses radiogenic lead, and the lower intercept of Discordia lines with the Concordia curve determines the time of fluid influence. The recrystallization of metamict zircon or crystalline zircon with high contents of impurities with additional supply of Si and Zr forms monocrystalline replacements. Dissolution of primary zircon is accompanied by growth of new zircon domains differing in the composition of isomorphic impurities and zones of transitional composition, whose ages have no geological sense. The study is of particular importance for zircons from Precambrian rocks with long and complex histories.     

Significance of an amorphous SiO2 phase in a pseudomorph after coesite enclosed in garnet from ultrahigh‐pressure eclogite,Su–Lu Belt,eastern China

Here, we report the first discovery of an amorphous SiO₂ phase (APSI phase) in a pseudomorph after coesite included in garnet from an ultrahigh‐pressure (UHP) eclogite from the Su–Lu metamorphic belt, eastern China. Using transmission electron microscopy, Raman spectroscopy and selected area electron diffraction, we show that the internal structure of the pseudomorph consists of an APSI phase with nano/submicrocrystalline particles of quartz and a polycrystalline K‐bearing fibrous sheet‐silicate phase (KFSS phase). The APSI phase‐bearing aggregates included in the garnet might have formed by reactions involving a supercritical fluid during exhumation by the following processes: (1) the development of radial cracks within the host garnet by the phase transition of coesite to quartz; (2) the decomposition of a part of the pseudomorph following infiltration of supercritical fluid; (3) the precipitation of the KFSS phase from the fluid phase during subsequent exhumation and cooling, which was likely promoted by a change in the metamorphic fluid from supercritical and/or subcritical to aqueous fluid; and (4) the rapid precipitation of the APSI phase under a metastable (non‐equilibrium) state, such as quenching, during a later stage of the exhumation. Whether the APSI phase generally formed during exhumation and survived widely throughout the Su‐Lu terrane is unknown. However, the presence of the APSI phase in a UHP eclogite provides new insight into the geodynamic phenomena occurring at continental collision zones.     

柴北缘阿日特克山铜钼矿床成矿岩体中黑云母和角闪石矿物学特征及其指示意义

阿日特克山铜钼矿床位于青海柴北缘成矿带中北部,为宗务隆构造带新发现的矽卡岩型-斑岩型矿床,矿床隐伏斑岩体中广泛发育黑云母,部分地段还发育角闪石。文章在野外地质工作的基础上,对花岗闪长（斑）岩进行了详细的岩相学研究和黑云母、角闪石成分的电子探针原位分析,初步探讨了成岩过程中的物理化学条件、成岩物质来源及其对成岩成矿的指示作用。电子探针结果显示,花岗闪长（斑）岩中黑云母主要为镁质黑云母,具有高Mg、Ti、K、Fe,低Ca、Na的特点;角闪石为镁质普通角闪石,表现为富Si、Mg、Ca,低Ti、Na、K的特征。黑云母和角闪石温度计算结果显示,花岗闪长（斑）岩中黑云母结晶温度为632～736℃（平均696℃）,角闪石结晶温度为737～764℃（平均749℃）;在黑云母和角闪石同时存在的花岗闪长岩中计算的岩体侵位压力为81～180 MPa,深度为3.07～6.82 km,为中深成斑岩体。黑云母和角闪石矿物化学特征指示花岗闪长（斑）岩属于钙碱性岩系I型花岗岩,具有壳幔混源的特征。岩体中黑云母、角闪石具有较高的结晶温度,其结晶的岩浆具有高的氧逸度及水含量,显示良好的成矿性,指示该地区具有寻找大型斑岩矿...     

福建平潭角闪辉长岩结晶过程的矿物学记录

福建平潭角闪辉长岩中具成分环带的斜长石由钙质斜长石核和膜状中长石两部分组成，它是钙碱性高铝玄武岩浆结晶过程中岩浆上升和吸回作用的记录。     

Transport of low-aH2O dehydration products to melt sites via reaction-zone networks, Milford Sound, New Zealand

The gabbroic/dioritic Pembroke Hornblende Granulite (PHG) of Milford Sound displays a geometrically simple mesoscopic network of sub‐planar garnet reaction zones (GRZ) in which the meta‐igneous hornblende granulite has been depleted of Na, Si, and H₂O, and c. 25 vol.% almandine‐rich garnet has formed. Some studies postulate the initial presence of melt along the centres of all GRZ, explaining the frequent absence of feldspathic veins by selective melt loss. A more parsimonious model is necessitated by structural evidence and, together with chemical data, suggests a relationship between mid‐range metasomatic transport and anatexis. The Pembroke outcrops show a process of incipient melting of gabbro/diorite in an environment of relatively low aH₂O in lithologies that have limited free quartz. A non‐equilibrium steady state is proposed, in which a sodic dehydration fluid moves some distance via the GRZ network towards areas of partial melting. Only in these areas are Na and Si reconstituted as albite, with more garnet as byproduct, having avoided the need for melt percolation. The combined structural and chemical evidence directs a focus on mass transport in low‐aH₂O gabbroic environments. In subsequent events of shearing and complete transposition, both sets of garnet – the atypical GRZ residue and partial melt melanosomes – were inherited by the Milford Gneiss ‘facies’ of the PHG.