变斑晶晶内微构造应用研究进展

变斑晶晶内包体径迹在变质地质学和构造地质学中具有广泛的用途。尤其在造山带研究、PTt轨迹、变质与变形关系及历史、变形机制及褶皱和剪切带运动学、变质变形程度、变斑晶生长率、应变量、应变速率等方面的应用取得许多重大进展。其中所有变斑晶都是同运动的、未旋转的“固定论”新观点、新应用,值得重视和深入综合研究。另外,在任何应用之前都宜首先确定变斑晶旋转与否。     

雅江北部热隆扩展系的变形-变质作用

对雅江北部热隆变质体的变质作用、成因机制有过不少的研究,提出过不同的认识。作者应用微观构造与宏观构造相结合的方法,再次对该变质体变形-变质作用进行分析研究,详细阐述了变质体变形-变质作用的特征、相互关系、变质压力-温度条件及成因机制等问题,认识到热隆变质体的形成主要是推覆滑脱使地壳局部熔融引热隆扩展而产生,它们曾经历过复杂的变质-变形过程,最终形成了现今的变质分带和构造面貌。     

超高压变质作用研究现状及问题

本文从超高压变质作用的矿物学、Ｐ－Ｔ－ｔ轨迹和超高压地体抬升、保存机制三个方面总结了近年来超高压变质作用研究所取得的重要进展和存在的问题。超高压变质作用的矿物学研究正在向深入发展，其Ｐ－Ｔ－ｔ轨迹具有三种成因类型。而其超高压地体的抬升、保存机制则是争论的焦点，其中增生楔模式（Ｐｌａｔｔ，１９８７）和连续俯冲─仰冲机制较好地解释了抬升过程，但也存在一些问题需进一步研究。     

四川石棉草科穹状岩浆核杂岩构造特征

位于扬子陆块西缘的石棉草科穹状变形变质体,据近年来的研究表明,该穹隆体经历了三次变形变质时期：早期为收缩滑脱变形的区域动力变质、中期热隆伸展动热变质和后期岩浆热接触变质。对主期变质划分出黑云母带、石榴石带、红柱石－十字石带和矽线石带,确定为低压相系,利用变质反应、矿物地质温压计及相关的同位素年龄资料,建立了草科穹状变形变质体演化的ｐ－Ｔ－ｔ－Ｄ轨迹。轨迹图呈顺时针形式,具碰撞造山带环境的特点,变形变质过程受变质体前缘西油房韧性剪切带逆冲－推覆作用和后缘碰撞晚期岩浆大规模上侵的双重制约,为深源岩浆热动力变质成因,属穹状岩浆核杂岩构造。     

变形对变质作用压力—温度径迹的影响

以褶皱和逆冲断层为主构造区的压力-温度(P－T)径迹可能是复杂的,在一个变质带内可能变化很大.P—T径迹的复杂变化,是由于褶皱作用和逆冲作用造成的热水平对流、变形期间及变形后发生的热传导,以及褶皱和逆冲断层内相对短时间的热驰豫所造成的.在褶皱区,可以观察到背斜与邻近向斜具有不同的P－T径迹,背斜内是冷却径迹,而向科内是加热径迹,这些不同P—T径迹是由于变形期间发生等温线的褶皱作用,变形后又发生等温线验豫所造成的.在多期变形褶皱区,非共轴褶皱作用造成变形带短距离(＜15km)内复杂的P－T径迹,这是因为在每个褶皱作用事件中,重复发生等温线的褶皱作用以及驰豫过程.在逆冲断层为主的构造区,大型逆冲席体的P一T径迹之间存在差异,因为在变形期间从较热的逆掩席体向较冷的下盘发生热传递.逆冲作用期间的热传递引起上盘冷却,下盘加热.在发育多重逆冲席体的地区,P－T径迹可能是复杂的,它将取决于逆冲席体间的热传递速率与逆冲断层发育的相对同步性.本文讨论的褶皱和逆冲断层地区所观察到的P一T径迹,记录了10~5~10~6年的相对短时间间隔.由于小尺度的热扰动会引起矿物平衡,在一个变质带的短距离内,P－T径迹可能变化很大.因此,将P－T径迹用于恢复构造历史之前,必须进行构造及岩石学的双重研究.     

新疆阿尔泰造带南缘萨勒巴斯推覆体变形的变质作用

萨勒巴斯推覆体中发育－套深层次变形构造组合和例转递增高质带，其中糜棱岩、混合岩、混合岗岩的成因关系对研究挤压造山背景下，地壳深层次变形作用和成岩作用具有特殊的意义。研究表明：在大型滑脱推覆系统中，存在摩棱岩－混合岩－混合花岗岩成岩系列；成岩过程为：韧性变形－剪切加热－部分熔融；控制成岩过程的主导因素为构造动力条件。这一成岩过程能导致稀土元素发生分异，出现重稀土元素有规律地亏损。变质作用ｐＴｔ轨迹显     

剪切带中变斑晶的生长及包裹体痕迹的演化

韧性剪切带中，由于变形分解作用的存在，岩石发生递进变形过程中，产于共轴或非共轴递进缩短带内的变斑晶不发生旋转，而变斑晶内的包裹体痕迹是递进变形过程中遗留在变斑晶内的变形变质痕迹。利用未旋转斑晶中的包裹体痕迹可以确定早期面理的取向，寻找构造演化的时间标志，确定变形变质的关系及其演化史。对北祁连托勒牧场大型走滑韧性剪切带中石榴石、黑云母等变斑晶及包裹体痕迹的研究，揭示了变斑晶的生长和包、裹体痕迹与褶劈理的演化有着重要联系以及剪切变形过程中变形变质演化史、应变速率的变化。递进变形相应地发生递增变质，但两者存在着一定的差异性。     

麻粒岩相变质作用的PTt演化及其地质动力学意义

麻粒岩地体和基性火山岩中的麻粒岩包体是研究过去和现在下部地壳的窗口,通过它们的研究可了解下地壳的物质组成,物理和化学状态,构造变形,岩浆作用和变质作用的特征,并进而探讨大陆地壳形成和改造的地质动力学机制。当前研究中涉及许多重要问题,如麻粒岩相变质作用的PTt演化和成因模式及其构造环境、变质流体的含量、成分和作用,岩浆作用的性质和机制及其对麻粒岩相变质作用的贡献,构造变形作用的样式及其所反映的构造体制,麻粒岩中LILE亏损等地球化学特征的成因意义等等。这些问题互相联系,都是麻粒岩地体成因的约束条件。本文将讨论麻粒岩相变质作用的PTt演化特征,阐述大地构造环境和地质动力学机制对其所起的约束作用。     

中条山西南段与变基性岩有关的铜矿地质特征及成因浅析

与变基性岩有关的铜矿，是在中条山西南段新发现的一种新的铜矿类型，其形成是在原始基性岩浆侵入或喷发作用形成的矿源岩或矿化体基础上，经过区域变质和印支－燕山期构造－岩浆热液的改造加富而成，其成因应属与变基性岩有关的变质－热液改造型铜（镍、铁）矿床。     

江西金山金矿床成矿流体地球化学

江西金山金矿床成矿流体地球化学研究表明，1）含金硅化糜棱岩（或石英脉）中石英主要分布主矿化期受韧性剪切裂隙控制的流体包裹体。2）成矿流体温度变化于230－370℃范围, 表明成矿历经多阶段构造－热液脉动作用。3）流体包裹体成分特征、氢氧同位素组成及其他地质－地球化学证据表明，成矿流体具多源性：①来自地球深部高温、高压深源流体；②再循环大气降水；③变质－变形过程中产生的变质热流体。此外，还有前述3种成因流体派生的有机流体。     

Interpretation of porphyroblast inclusion trails: limitations imposed by growth kinetics and strain rates

Porphyroblast inclusion trails provide important information about the tectonometamorphic evolution of a metamorphic rock. However, there remains considerable controversy over whether porphyroblasts rotate during bulk non-coaxial deformation.
With reference to an area of the Scandinavian Caledonides and utilizing existing data from theoretical and experimental modelling, this study demonstrates that both 'straight' and 'S-shaped' inclusion trails are consistent with an interpretation in terms of syndeformational porphyroblast growth in a regime approximating to Newtonian simple shear. At crustal strain rates of 10^-14 s^-1 and porphyroblast growth times of 0.1–1.0 Ma, it is shown that a maximum of 5-9 angular rotation would occur during growth. At faster strain rates of 10^-12 s^-1 (e.g. those in a shear zone) porphyroblast angular rotations of 90 are shown to occur in 0.1–0.25 Ma (i.e. times comparable with or faster than porphyroblastesis). In view of this, 'S-shaped' inclusion trails are to be expected in porphyroblasts growing in active shear zones or other situations of high shear strain, whereas 'straight' inclusion trails can be interpreted as static overgrowth of an existing fabric or as syndeformational porphyroblastesis at low strain rates.     

Lack of porphyroblast rotation in the Otago schists, New Zealand: implications for crenulation cleavage development, folding and deformation partitioning

Porphyroblasts of garnet and plagioclase in the Otago schists have not rotated relative to geographic coordinates during non-coaxial deformation that post-dates their growth. Inclusion trails in most of the porphyroblasts are oriented near-vertical and near-horizontal, and the strike of near-vertical inclusion trails is consistent over 3000 km². Microstructural relationships indicate that the porphyroblasts grew in zones of progressive shortening strain, and that the sense of shear affecting the geometry of porphyroblast inclusion trails on the long limbs of folds is the same as the bulk sense of displacement of fold closures. This is contrary to the sense of shear inferred when porphyroblasts are interpreted as having rotated during folding.
Several crenulation cleavage/fold models have previously been developed to accommodate the apparent sense of rotation of porphyroblasts that grew during folding. In the light of accumulating evidence that porphyroblasts do not generally rotate, the applicability of these models to deformed rocks is questionable.
Whether or not porphyroblasts rotate depends on how deformation is partitioned. Lack of rotation requires that progressive shearing strain (rotational deformation) be partitioned around rigid heterogeneities, such as porphyroblasts, which occupy zones of progressive shortening or no strain (non-rotational deformation). Therefore, processes operating at the porphyroblast/matrix boundary are important considerations. Five qualitative models are presented that accommodate stress and strain energy at the boundary without rotating the porphyroblast: (a) a thin layer of fluid at the porphyroblast boundary; (2) grain-boundary sliding; (3) a locked porphyroblast/matrix boundary; (4) dissolution at the porphyroblast/matrix boundary, and (5) an ellipsoidal porphyroblast/shadow unit.     

Rotated staurolite porphyroblasts in the Littleton Schist at Bolton, Connecticut, USA

Staurolite porphyroblasts, 1.5–8cm in length and 0.3–2cm in width, in the Littleton Schist at Bolton, Connecticut, contain curved quartz inclusion trails which document synkinematic rotations of at least 135°. The orientations of long axes of these staurolite crystals define a weak preferred orientation in a plane approximately parallel to the external foliation. Serial sections of four differently orientated crystals and U-stage measurements of the orientations of their inclusion trails demonstrate that the inflection hinge line and the statistical 'symmetry axis' characterizing the foliation within a porphyroblast are unrelated to the orientations of external crenulations and are, in all cases, parallel to the long axis of the porphyroblast. The cumulative rotation reflected in the curvature of the inclusion trails is a maximum in a c -axis section through the initial core of a crystal. The amount of rotation about the c -axis decreases linearly along the length of the crystal away from the nucleation site.
The sense and amount of rotation recorded by a porphyroblast is related to its orientation. A tightly constrained transition from clockwise to anticlockwise rotation defines a slip direction that coincides with the preferred orientation of the staurolite c -axes. The total rotation reflected by the inclusion trails increases as a function of the angle between the c -axes of the staurolite crystals and the slip direction.
Initially random staurolite porphyroblasts rotated during growth, as a consequence of laminar shear in the surrounding viscous matrix. This interpretation is quantitatively consistent with: the staurolite preferred orientation; its coincidence with the apparent slip direction; the correlation between both the sense and the amount of rotation and the orientation of the long axis of the porphyroblast; and the twisted conical shape of the family of surfaces defined by the inclusion trails.     

Microstructural features of albite porphyroblasts as indicators of sequential Barrovian metamorphic mineral growth in the Caledonides of the SW Scottish Highlands

Inclusion – porphyroblast and porphyroblast – porphyroblast relationships show that abundant albite in mica schists in the Caledonides of the SW Scottish Highlands are part of the Barrovian metamorphic assemblage. Growth early in the D2 deformational phase of porphyroblast cores followed the growth of Mn‐rich garnet but preceded the growth of porphyroblasts of the index mineral almandine. Two sets of inclusion trails in the albite correspond to the regionally expressed S1 and S2. Straight trails of muscovite, chlorite, quartz, epidote and the earliest growth of biotite make up S1. Crenulated trails express deformation of S1 early in D2 with muscovite, chlorite, biotite, quartz, epidote and the Mn‐rich garnet associated with the development of S2 crenulation cleavage. The geometries of these trails uniquely record early stages of D2 deformational history. An _0?3 growth is related to the temporal coincidence of the formation of S1–S2 crenulation cleavage hinges as favourable sites for nucleation and the release of large amounts of water from prograde reactions during tectonothermal reconstitution of first cycle immature sediments with a volcanic component. The main characteristics of the regionally expressed D2 schistosity were developed during the major grain coarsening that followed both albite and almandine porphyroblast growth. Essentially inclusion‐free An _4?19 rims grew on the inclusion‐containing cores in the almandine zone in the later stages of schistosity growth and unoriented porphyroblasts of muscovite, biotite and chlorite indicate that mineral growth extended from the later stages of D2 to post‐D2. Previous interpretations of the albite porphyroblast growth having been during D4 to post‐D4 contemporaneous with retrogression are inconsistent with the microstructural evidence.     

Shear zone vs folding origin for spiral inclusion trails in the Canton Schist

The three-dimensional geometry of spiral inclusion trails from the Canton Schist were measured to determine whether the spirals were a product of porphyroblast rotation within a shear zone, or porphyroblast growth during a series of overprinting fold events. The spiral inclusion trails are composed of three separate, sub-planar inclusion trail surfaces occupying texturally distinct parts of the porphyroblasts. These surfaces are correlated across a >10 km² area using textural criteria and relative timing. Measured patterns of inclusion trail orientation and asymmetry suggest they did not form by porphyroblast rotation within a non-coaxial shear zone. Rather, the porphyroblasts grew during three successive overprinting fold events (F₂–F₄), and the spiral inclusion trails represent the accumulated curvature associated with folding of successive axial plane foliations. The data show that spiral garnets are not peculiar to shear zones, and can form by overprinting crenulations and folds. This is consistent with the common occurrence of spiral garnets in multiply-deformed, regionally metamorphosed fold belts.     

Episodic porphyroblast growth in the Fleur de Lys Supergroup, Newfoundland: timing relative to the sequential development of multiple crenulation cleavages

Inclusion trails in garnet and albite porphyroblasts in the Fleur de Lys Supergroup preserve successive generations of microstructures, some of which correlate with equivalent microstructures in the matrix. Microstructure–porphyroblast relationships provide timing constraints on a succession of seven crenulation cleavages (S1–S7) and five stages of porphyroblast growth. Significant destruction and alteration of early fabrics has occurred during the microstructural development of the rock mass. Garnet porphyroblasts grew episodically through four growth stages (G1–G4) and preserve a succession of five fabrics (S1–S5) as inclusion trails. Garnet growth during each of the four growth phases did not occur on all pre-existing porphyroblasts, resulting in contrasting growth histories between individual garnet porphyroblasts from the same outcrop. Albite porphyroblasts grew during a single stage of growth and have overgrown microstructures continuous with the matrix. The garnet and albite porphyroblast inclusion trails record a succession of crenulation cleavages without any rotation of the porphyroblasts relative to other porphyroblasts in the population.
Complex microstructural histories are best resolved by preparing multiple oriented thin sections from a large number of samples of different rock types within the area of study. The succession of matrix foliations must be understood, as it provides the most useful time-frame against which to measure the relative timing of phases of porphyroblast growth. Comparable microstructures must be identified in different porphyroblasts and in the rock matrix.     

Reference frame, angular momentum, and porphyroblast rotation

Rotation of small rigid objects in a deforming ductile matrix can produce two different types of microstructure: a shape fabric due to alignment of the principal axes of a population of elongate objects and the inclusion trail microstructure preserved in syntectonic porphyroblasts. We use numerical modeling to show that inclusion trails of elongate porphyroblasts are expected to be extremely complex. In contrast, snowball garnets are readily interpretable. But misuse of reference frame and kinematic misconceptions have obfuscated the discussion on the formation of porphyroblast inclusion trails in general and snowball garnet inclusion trails in particular. We clarify this point. Models for snowball garnet formation that are based on the notion of garnets being irrotational with respect to the earth can be rejected on a geometrical and kinematic basis. Further, the notion that rigid objects embedded in a deforming ductile matrix generally do not rotate is unsound—it violates the fundamental physical law of balance of angular momentum.     

Numerical simulations of spiral‐shaped inclusion trails: can 3D geometry distinguish between end‐member models of spiral formation?

Numerical 3D simulations of the development of spiral inclusion trails in porphyroblasts were conducted in order to test the proposals that (a) 3D spiral geometry differs between the rotation and nonrotation end‐member models of spiral formation proposed in the literature, and (b) 3D spiral geometry can be used as a criterion to distinguish between the two end‐member models in rocks. Four principal differences are identified between the two sets of simulations: smoothness of spiral curvature; spacing of foliation planes; alignment of individual foliation planes either side of the sphere representing the porphyroblast; and spiral asymmetry with respect to matrix shear sense. Of these differences, only spiral asymmetry and possibly the alignment of individual foliation planes are diagnostic criteria for distinguishing between the end‐member models. In the absence of a readily applied test to distinguish the end‐member models, interpretation of spiral inclusion trails is problematic. It is necessary to determine complementary evidence to distinguish porphyroblast rotation or nonrotation during spiral formation.     

Testing models for the development of spiral-shaped inclusion trails in garnet porphyroblasts: to rotate or not to rotate, that is the question

Abstract The formation of spiral-shaped inclusion trails (SSITs) is problematical, and the two viable models for their formation involve opposite shear senses along the foliation in which the porphyroblasts are growing. One model argues for porphyroblast rotation, with respect to a geographically fixed reference frame, whereas the other argues for no such porphyroblast rotation, but instead rotation of the matrix foliation around the porphyroblast. Thus, porphyroblasts with SSITs cannot be used as shear-sense indicators until it is conclusively determined which model best explains them.
Any successful model must explain features associated with SSITs, including: (1) foliation truncation zones, (2) smoothly curving SSITs, (3) millipede microstructure, (4) total inclusion-trail curvature in median sections, (5) porphyroblasts with SSITs that have grown together, (6) evidence for relative porphyroblast displacements, (7) shear-sense indicators inside and outside porphyroblasts; (8) crenulations associated with porphyroblasts and (9) geometries in sections subparallel to spiral axes (axes of rotation). A detailed study of these features suggests that most, if not all, can be explained by both the rotational and non-rotational models, in spite of these models involving diametrically opposed movement senses. Therefore, geometrical analysis of individual porphyroblast microstructures may not determine which model best explains SSITs until the kinematics required to form these microstructures are better understood, in particular the sense of shear along a developing crenulation cleavage. Specific tests for determining the shear sense along crenulation cleavages are proposed, and results of such tests may conclusively resolve the debate over how SSITs form.