A comparison of observed and thermodynamically predicted phase equilibria and mineral compositions in mafic granulites

Recently published activity–composition (a–x) relations for minerals in upper amphibolite‐ and granulite facies intermediate and basic rocks have expanded our ability to interpret the petrological evolution of these important components of the lower continental crust. If such petrological modelling is to be reliable, the abundances and compositions of phases calculated at the interpreted conditions of metamorphic equilibration should resemble those in the sample under study. Here, petrological modelling was applied to six granulite facies rocks that formed in different tectonic environments and reached different peak metamorphic pressure–temperature (P–T) conditions. While phase assemblages matching those observed in each sample can generally be calculated at P–T conditions that approximate those of peak metamorphism, a consistent discrepancy was found between the calculated and observed compositions of amphibole and clinopyroxene. In amphibole, Si, Ca and A‐site K are underestimated by the model, while Al and A‐site Na are overestimated; comparatively, in clinopyroxene, Mg and Si are generally underestimated, while Fe²⁺ and Al are typically overestimated, compared to observed values. One consequence is a reversal in the Fe–Mg distribution coefficient (K_D) between amphibole and clinopyroxene compared to observations. Some of these mismatches are attributed to the incorrect partitioning of elements between the predicted amphibole and clinopyroxene compositions; however, other discrepancies are the result of the incorrect prediction of major substitution vectors in amphibole and clinopyroxene. These compositional irregularities affect mineral modal abundance estimates and in turn the position and size (in P–T space) of mineral assemblage fields, the effect becoming progressively more marked as the modal abundance of hornblende increases; hence, this study carries implications for estimating P–T conditions of high‐temperature metabasites using these new a–x relations.     

Metasomatic Stages and Scapolitization Effects on Chemical Composition of Pasveh Pluton,NW Iran

Pasveh gabbros are mafic component of a plutonic complex in the northwest Sanandaj-Sirjan Zone.These cumulative rocks are composed of plagioclase and calcic clinopyroxene(Cpx) ,which yield unusually high CaO(>19 wt.%) in whole-rock chemistry.Petrographical and geochemical data suggest that Pasveh gabbros can be divided into two groups:free scapolite and scapolite-bearing gabbros.The second group has higher Na2O,K2O,and P2O5 relative to free scapolite ones and is enriched in LIL(large ion lithophile) and HFS...     

黔中三叠系台地边缘相碳酸盐岩中的玛瑙纹构造及其成因

 本文通过一条剖面的重点解剖,研究了广泛分布于上扬子台地边缘相碳酸盐岩中的玛瑙纹构造的成因。研究表明,这是一种早期的成岩组构,组成成分为纤状方解石和粒状方解石。沉淀介质为溶解了部分地表藻类的淡水同孔隙和裂隙海水发生不同程度混合的混合水溶液。被玛瑙纹充填的围岩裂隙主要由岩溶坍塌作用造成,但也有牵引拉伸裂隙和暴露收缩裂隙的叠加。由于介质溶液以地下潜水方式侧向运动,因而随潜水面波动形成了间隔的多层状含玛瑙纹岩层。     

Prograde and retrograde metamorphism of hematitebearing basic schists in the Sanbagawa belt in central Shikoku

The mineral assemblages of hematite-bearing basic schists in intermediate high-pressure metamorphism are temperature dependent. For assemblages with excess hematite, albite, muscovite and quartz, the paragenetic relations can be dealt with in terms of a four-component system, without omitting or grouping major components.
In the Sanbagawa belt in central Shikoku, the dominant amphibole in the hematite-bearing basic schists changes from winchite, via crossite and barroisite to hornblende. The stability of amphibole is described chemographically within a pseudoternary system with another excess phase, epidote. Many amphiboles are chemically heterogeneous owing to retrograde reactions which produced low- T/P amphibole around the prograde amphibole. The examination of amphibole zoning makes it possible to draw a retrograde P-T trajectory which passes on the lower pressure side of the prograde one.     

New Compositional and Structural Constraints on the Smithsonian Microanalytical Reference Materials: Amphiboles from Kakanui and Arenal

The amphiboles from Kakanui and Arenal are two natural minerals that have been used worldwide as microanalytical reference materials, but their compositions and crystal structures are still poorly constrained. In this paper, we report new data on H₂O and trace element mass fractions and single-crystal structural refinement of these two amphiboles. H₂O mass fractions of the Kakanui and Arenal amphiboles determined via Karl-Fischer titration are 0.92 ± 0.18 (2s) and 1.56 ± 0.22% m/m (2s), respectively; these values estimated based on crystal-structure refinement are 0.86 and 1.46% m/m, respectively. Trace element mass fractions measured via LA-ICP-MS in two laboratories are in good agreement, and spots from five fragments for both Kakanui and Arenal amphiboles are generally consistent within reproducibility precision (2s). Our measurements indicate a better homogeneity for the amphiboles from Kakanui than that from Arenal. According to the latest scheme for amphibole classification and nomenclature (Hawthorne et al. 2012), the sample from Arenal is a (partially dehydrogenated) pargasite, and that from Kakanui is a kaersutite. The significant amount of oxo-component and that ^CTi⁴⁺ content is strongly ordered at the M(1) site for both amphiboles indicate crystallisation under high fO₂ conditions.     

Progress on Formation Mechanism of the Fibrous Veins in Mudstone and Its Implications to Hydrocarbon Migration

The fibrous mineral veins are widespread in mudstones. According to the different microscopic morphology of the minerals, the fibres can be divided into stretched crystals, elongate-blocky crystals and the very fibrous crystals. Veins can also be classified according to the growth direction of these crystals into stretched veins, syntaxial veins and antitaxial veins. The resulting texture in the vein depends on the morphology of the fracture surface, the width of the fracture and the growth habit of the vein forming mineral. The crack-seal mechanism can only interpret the formation of the stretched crystal veins and the elongate-blocky crystal veins, and the antitaxial well-developped fibrous veins form without fracturing and the growth competition is inhibited during the rock deformation, which implies that the nutrient transport is by diffusional flow transport. Horizontal crack is the primary condition of the formation of the antitaxial bedding-parallel fibrous calcite veins in muddy hydrocarbon source rocks. The formation of the horizontal crack which caused by the abnormal high pressure can enhance the hydrocarbon generation efficiency and has a great effect on the rate and direction of hydrocarbon migration in the local. The presence of the veins indicates the hydrocarbon fluid can migrate laterally along the layers. The antitaxial bedding-parallel fibrous calcite veins in muddy hydrocarbon source rocks can be the sign of the generation and migration of hydrocarbon under the abnormal high pressure condition in petroleum-bearing basins.     

龙游角闪橄榄岩的富流体相岩浆成因—矿物化学和微量元素地球化学制约

龙游角门橄榄岩山露于古老变质岩中．与围岩间的接触关系尚不清楚,但根据岩相学上矿物的平衡共生关系、岩石的矿物化学组成和角门橄榄岩及与之相关的花岗岩、金云母岩的微量元素地球化学特征．该橄榄岩很可能是富含流体组分的岩浆侵人而形成的,其形成的构造背景为具俯冲带性质的活动大陆边缘,富流体相的岩浆是由于其地幔源区受到了来自俯冲带的流体所交代的结果。     

豫南高压麻粒岩中富钛角闪石类的成因和意义

作者用光学方法、电子探针分析方法、红外吸收光和Ｘ射线粉晶衍射方法对豫南高压麻粒岩中的富钛角闪石类进行了详细。结果表明这些富钛角闪石可分别属于钛角闪石类和成分的宇镁绿钙闪石的富钛角闪石类。它们系幔源偏碱生镁铁质岩类成分相当的原岩子高压麻粒岩相变质条件下再平衡后的变质残余矿物和重结晶矿物。     

东秦岭秦岭岩群中角闪石环带所记录的pTt轨迹

东秦岭秦岭岩群是一套中深变质结晶岩系，主要由片麻岩、斜长角闪岩、变粒岩和大理岩组成。秦岭岩群中角闪石多具成分环带，由角闪石成分环带确定的ｐＴｔ轨迹由三段组成：①等压升温轨迹，②等压降温轨迹，③降温降压轨迹。同位素年代学表明东秦岭秦岭岩群曾经历晋宁和加里东两个变质旋回，在晋宁旋回晚期形成等压升温轨迹，在加里东旋回早期形成等温降压轨迹，晚期形成降温降压轨迹。     

Metasomatism of the peridotites from southern Mariana fore-arc:Trace element characteristics of clinopyroxene and amphibole

Modal composition and mineral composition of harzburgites from the southern Mariana fore-arc show that they are highly refractory. There are a few modals of clinopyroxene (0.7 vol %) in harzburgites. Two types of amphibole are found in these harzburgites: magnesiohornblende accompanied by clinopy-roxene with higher Al2O3 content (>7%) and lower Mg#; tremolite around orthopyroxene with lower Al2O3 content (< 2%) and higher Mg#. Trace element of clinopyroxene and two types of amphibole are ana-lyzed. Primitive mantle-normalised REE patterns for clinopyroxene and magnesio hornblende are very similar and both show HREE enrichment relative to LREE,while magnesiohornblende has higher con-tent of trace element than clinopyroxene. The contents of trace element of tremolite are much lower than those of magnesiohornblende. Clinopyroxene shows enrichment of most of the trace element except HREE and Ti relative to clinopyroxene in abyssal peridotites. Petrology and trace element characteristic of clinopyroxene and two types of amphibole indicate that southern Mariana fore-arc harzburgites underwent two stages of metasomatism. The percolation of a hydrous melt led to mobility of Al,Ca,Fe,Mg,Na,and large amounts of trace element. LILE and LREE can be more active in hydrous melt than HREE and Ti,and the activities of most of the trace element except some of LILE are influ-enced by temperature and pressure.