The nature and distribution of fluids during amphibolite facies metamorphism, Naxos (Greece)

On the basis of fluid inclusion evidence, pervasive influx of deep-seated CO₂-rich fluids has been invoked to account for mid- to upper amphibolite facies (M2_B) metamorphism on the island of Naxos (Cyclades, Greece). In this paper, mineral devolatilization and melt equilibria are used to constrain the composition of both syn- and post-peak-M2_B fluids in the deepest exposed levels of the metamorphic complex. The results indicate that peak-M2_B fluids were spatially and compositionally heterogeneous throughout the high-grade core of the complex, whereas post-peak-M2_B fluids were generally water-rich. The observed heterogeneities in syn-M2_B fluid composition are inconsistent with pervasive CO₂-flushing models invoked by previous workers on the basis of fluid inclusion evidence. It is likely that few CO₂-rich fluid inclusions on Naxos preserve fluids trapped under peak metamorphic conditions. It is suggested that many of these inclusions have behaved as chemically open systems during the intense deformation that accompanied the uplift of the metamorphic complex. A similar process may explain the occurrence of some CO₂-rich fluid inclusions in granulite facies rocks.     

西藏洛巴堆矽卡岩型铁多金属矿床石榴子石和锆石U-Pb测年及地质意义

石榴子石是矽卡岩型矿床中最常见的蚀变矿物之一,因此,对石榴子石进行年代学研究能够准确限定矽卡岩型矿床的成矿时代.青藏高原冈底斯成矿带中部发育众多矽卡岩型多金属矿床,由于缺乏精确的成矿年代学数据,制约着对这些矿床成因和动力学背景的深入认识.因此,文章以该成矿带具有代表性的洛巴堆矽卡岩型铁多金属矿床为研究对象,通过对赋矿矽...     

西藏拉隆穹窿淡色花岗岩中石榴子石矿物学研究及对岩浆-热液过程的指示

为建立淡色花岗岩演化和稀有金属成矿的矿物学指标,本文选取了北喜马拉雅拉隆淡色花岗岩的石榴子石为研究对象,对其开展电子探针分析和矿物原位LA-ICP-MS微量分析,结果表明MnO含量从白云母花岗岩(12.42%~13.48%)到钠长石花岗岩(16.83%~22.09%)逐渐增高,白云母花岗岩石榴子石主要为铁铝榴石,钠长石花岗岩中石榴子石主要为锰铝榴石,其均为典型岩浆成因的石榴子石。石榴子石微量元素结果显示白云母花岗岩和钠长石花岗岩石榴子石稀土均呈现HREE富集、LREE亏损,Eu负异常的特征。从白云母花岗岩到钠长石花岗岩,石榴子石中Zn含量增加,Sc、Y和HREE等元素含量降低,特别是当HREE含量小于1000×10^-6时,稀有金属元素Be、Nb和Ta含量增加,标志着岩浆演化从正岩浆阶段进入了岩浆-热液过渡阶段。形成于岩浆-热液过渡阶段的锰铝榴石可以作为拉隆淡色花岗岩Be-Nb-Ta稀有金属矿化的矿物学指标,此外,石榴子石中Sc、Y和HREE等元素的变化也可以作为淡色花岗岩稀有金属矿化的判别标志。     

福建云霄晶洞花岗岩及含石榴子石伟晶岩的硅氧同位素研究

在运用常规方法和激光烧蚀同位素分析方法分别对云霄晶洞花岗岩和伟晶岩(长石、石英和石榴子石)的硅、氧同位素组成进行研究的基础上,探讨了晶涧花岗岩及其含石榴子石伟晶岩的物质来源及形成条件.伟晶岩中的石英和长石的氧、硅同位素组成分别较之花岗岩中的石英和长石的氧、硅同位素组成,均未发生明显变化,表明二者岩浆来源一致.云霄县的乌...     

Zircon in amphibolites from Naxos,Aegean Sea,Greece: origin,significance and tectonic setting

We report U–Pb zircon ages of c. 700–550 Ma, 262–220 Ma, 47–38 Ma and 15–14 Ma from amphibolites on Naxos Island in the Aegean extensional province of Greece. The zircon has complex internal structures. Based on cathodoluminescence response, zoning and crosscutting relationships a minimum of four zircon growth stages are identified: inherited core, magmatic core, inner metamorphic (?) rim and an outer metamorphic rim. Trace element compositions of the amphibolites suggest igneous differentiation and crustal assimilation. Zircon solubility as a function of saturation temperatures, Zr content and melt composition indicates that the zircon did not originally crystallize in the mafic bodies but was inherited from felsic precursor rocks, and subsequently assimilated into the mafic intrusives during emplacement. Zircon inheritance is corroborated by the complex, xenocrystic nature of the zircon in one sample. Ages of c. 700–550 Ma and 262–220 Ma are assigned to inherited zircon. Available geochemical data suggest that the 15–14 Ma metamorphic rims grew in situ in the amphibolites, corresponding to a high‐grade metamorphic event at this time. However, the geochemical data cannot conclusively establish if the c. 40 Ma zircon rims also grew in situ, or whether they were inherited along with the xenocrystic cores. Two scenarios for emplacement of the mafic intrusives are discussed: (i) Intrusion during late‐Triassic to Jurassic ocean basin development of the Aegean realm, in which case the 40 Ma zircon rims would have grown in situ, and (ii) emplacement in the Miocene as a result mafic underplating during large‐scale extension. In this case, only the 15–14 Ma metamorphic outer rims would have formed in situ in the amphibolitic host rocks.     

Geochemical characteristics and age of metamorphic sole rocks within a Neotethyan ophiolitic mélange from Konya region (central southern Turkey)

Palaeo- and Neo-Tethyan-related magmatic and metamorphic units crop out in Konya region in the south central Anatolia. The Neotethyan assemblage is characterized by mélange and ophiolitic units of Late Cretaceous age. They tectonically overlie the Middle Triassic–Upper Cretaceous neritic to pelagic carbonates of the Tauride platform. The metamorphic sole rocks within the Konya mélange crop out as thin slices beneath the sheared serpentinites and harzburgites. The rock types in the metamorphic sole are amphibolite, epidote-amphibolite, garnet-amphibole schist, plagioclase-amphibole schist, plagioclase-epidote-amphibole schist and quartz-amphibole schist. The geochemistry of the metamorphic sole rocks suggests that they were derived from the alkaline (seamount) and tholeiitic (E-MORB, IAT and boninitic type) magmatic rocks from the upper part of the Neotethyan oceanic crust. Four samples from the amphibolitic rocks yielded ⁴⁰Ar/³⁹Ar isotopic ages, ranging from 87.04 ± .36 Ma to 84.66 ± .30 Ma. Comparison of geochemistry and geochronology for the amphibolitic rocks suggests that the alkaline amphibolite (seamount-type) cooled below 510 ± 25 °C at 87 Ma whereas the tholeiitic amphibolites at 85 Ma during intraoceanic thrusting/subduction. When all the evidence combined together, the intraoceanic subduction initiated in the vicinity of an off-axis plume or a plume-centered spreading ridge in the Inner Tauride Ocean at 87 Ma. During the later stage of the steady-state subduction, the E-MORB volcanics on the top of the down-going slab and the arc-type basalts (IAT/boninitic) detached from the leading edge of the overriding plate, entered the subduction zone after ~2 my and metamorphosed to amphibolite facies in the Inner Tauride Ocean. Duration of the intraoceanic detachment (~87 Ma) and ophiolite emplacement onto the Tauride-Anatolide Platform (Tav?anl? Zone), followed by subsequent HP/LT metamorphism (~82 Ma) spanned ~5 my in the western part of the Inner Tauride Ocean.     

Influence of dissolution/reprecipitation reactions on metamorphic greenschist to amphibolite facies mica 40Ar/39Ar ages in the Longmen Shan (eastern Tibet)

Linking ages to metamorphic stages in rocks that have experienced low‐ to medium‐grade metamorphism can be particularly tricky due to the rarity of index minerals and the preservation of mineral or compositional relicts. The timing of metamorphism and the Mesozoic exhumation of the metasedimentary units and crystalline basement that form the internal part of the Longmen Shan (eastern Tibet, Sichuan, China), are, for these reasons, still largely unconstrained, but crucial for understanding the regional tectonic evolution of eastern Tibet. In situ core‐rim ⁴⁰Ar/³⁹Ar biotite and U–Th/Pb allanite data show that amphibolite facies conditions (~10–11 kbar, 530°C to 6–7 kbar, 580°C) were reached at 210–180 Ma and that biotite records crystallization, rather than cooling, ages. These conditions are mainly recorded in the metasedimentary cover. The ⁴⁰Ar/³⁹Ar ages obtained from matrix muscovite that partially re‐equilibrated during the post peak‐P metamorphic history comprise a mixture of ages between that of early prograde muscovite relicts and the timing of late muscovite recrystallization at c. 140–120 Ma. This event marks a previously poorly documented greenschist facies metamorphic overprint. This latest stage is also recorded in the crystalline basement, and defines the timing of the greenschist overprint (7 ± 1 kbar, 370 ± 35°C). Numerical models of Ar diffusion show that the difference between ⁴⁰Ar/³⁹Ar biotite and muscovite ages cannot be explained by a slow and protracted cooling in an open system. The model and petrological results rather suggest that biotite and muscovite experienced different Ar retention and resetting histories. The Ar record in mica of the studied low‐ to medium‐grade rocks seems to be mainly controlled by dissolution–reprecipitation processes rather than by diffusive loss, and by different microstructural positions in the sample. Together, our data show that the metasedimentary cover was thickened and cooled independently from the basement prior to c. 140 Ma (with a relatively fast cooling at 4.5 ± 0.5°C/Ma between 185 and 140 Ma). Since the Lower Cretaceous, the metasedimentary cover and the crystalline basement experienced a coherent history during which both were partially exhumed. The Mesozoic history of the Eastern border of the Tibetan plateau is therefore complex and polyphase, and the basement was actively involved at least since the Early Cretaceous, changing our perspective on the contribution of the Cenozoic geology.     

Obituary

Geological sections are still best drawn at natural scale. Sections with vertical exaggerations are rarely drawn correctly, and even when computed carefully give a false notion of the structural features. Many schematic sections which are not drawn to scale and are supposed to explain a proposed new tectonic interpretation give such a distorted and frequently absurd picture of the structural features that they cannot be regarded as valid.     

Garnet crystal plasticity in the continental crust,new example from south Madagascar

Garnet (10 vol.%; pyrope contents 34–44 mol.%) hosted in quartzofeldspathic rocks within a large vertical shear zone of south Madagascar shows a strong grain‐size reduction (from a few cm to ～300 μm). Electron back‐scattered diffraction, transmission electron microscopy and scanning electron microscope imaging coupled with quantitative analysis of digitized images (PolyLX software) have been used in order to understand the deformation mechanisms associated with this grain‐size evolution. The garnet grain‐size reduction trend has been summarized in a typological evolution (from Type I to Type IV). Type I, the original porphyroblasts, form cm‐sized elongated grains that crystallized upon multiple nucleation and coalescence following biotite breakdown: biotite + sillimanite + quartz = garnet + alkali feldspar + rutile + melt. These large garnet grains contain quartz ribbons and sillimanite inclusions. Type I garnet is sheared along preferential planes (sillimanite layers, quartz ribbons and/or suitably oriented garnet crystallographic planes) producing highly elongated Type II garnet grains marked by a single crystallographic orientation. Further deformation leads to the development of a crystallographic misorientation, subgrains and new grains resulting in Type III garnet. Associated grain‐size reduction occurs via subgrain rotation recrystallization accompanied by fast diffusion‐assisted dislocation glide. This plastic deformation of garnet is associated with efficient recovery as shown by the very low dislocation densities (10¹⁰ m^?3 or lower). The rounded Type III garnet experiences rigid body rotation in fine‐grained matrix. In the highly deformed samples, the deformation mechanisms in garnet are grain‐size‐ and shape‐dependent: dislocation creep is dominant for the few large grains left (>1 mm; Type II garnet), rigid body rotation is typical for the smaller rounded grains (300 μm or less; Type III garnet) whereas diffusion creep may affect more elliptic garnet (Type IV garnet). The P–T conditions of garnet plasticity in the continental crust (≥950 °C; 11 kbar) have been identified using two‐feldspar thermometry and GASP conventional barometry. The garnet microstructural and deformation mechanisms evolution, coupled with grain‐size decrease in a fine‐grained steady‐state microstructure of quartz, alkali feldspar and plagioclase, suggests a separate mechanical evolution of garnet with respect to felsic minerals within the shear zone.     

辽西—赤峰一带太古代变质杂岩中石石榴石的研究

石榴石是研究区内暗色镁铁质岩、麻粒岩及片麻岩的主要造岩矿物之一。成分分 析表明，它们是含钙铝榴石分子较多的镁铝一铁铝系列石榴石，红外光谱测定其近于铁铝榴石; 穆斯堡尔谱分析表明，Fe3+含量增多将引起石榴石内六配位多面体变形程度增大;:是胞参数ao 与Ca2+含量呈正相关。本区石榴石是麻粒岩相变质作用的产物，形成的温压条件为780-840℃. 0.85-1.10 GPa。形成石榴石的反应是复杂的，其中包括角闪石与斜长石的增温脱水反应以及 紫苏辉石+斜长石组合向石榴石+革斜辉石组合转化的滑动反应。