富铝泥质岩混合岩化深熔作用的热力学相平衡模拟研究

基于岩石相平衡,对富铝泥质岩K2O-Al2O3-SiO2一H2O(KASH)和K2O-FeO—MgO—A12O3-SiO2-H2O(KFMASH)体系的混合岩化深熔作用相关系进行了模拟计算,得到泥质岩深熔作用的成岩格子、熔体成分变化特征、熔体含水量及其温压条件、石榴石变斑晶成分演化趋势和泥质岩进变质、退变质矿物组合特征、各种压力条件下S型花岗质熔体特征等,并进一步将模拟结果应用于内蒙古固阳等地的泥质岩,根据相关岩石的矿物组合及结构特征,获得了变质反应历史和P—T轨迹。     

塔河油田溶洞充填物的元素地球化学特征及环境意义

塔河油田位于塔里木盆地塔北隆起阿克库勒凸起的西南部,加里东-海西期强烈抬升导致的大气淡水岩溶作用、早石炭世发生的广泛海侵以及二叠纪大规模的火山活动,对奥陶系岩溶储层产生了不同程度的改造。对塔河地区的地质演化特征以及塔河油田地区溶洞砂泥质充填物镓、铷、锶、钡等微量元素和稀土元素的研究表明,大部分样品表现出淡水岩溶特征,仅T615井等4口井的溶洞充填物却表现出海水成因特征,充填物的元素地球化学特征表现出与早期大气淡水岩溶作用的叠加效应;此外,还有少量样品具有能反映热液作用特征的Eu正异常,说明该区碳酸盐岩地层也受热水溶蚀作用的影响。通过溶洞方解石和溶洞方解石胶结物碳、氧同位素值分布特征的分析也证实了塔河地区除了发育大气淡水岩溶作用外,还存在海水-淡水混合水溶蚀作用和热液溶蚀作用。     

胶北日庄石墨结晶与寄主麻粒岩的变质演化关系

日庄镇含石墨泥质麻粒岩出露于莱西石墨成矿带, 详细的岩相学、地球化学、拉曼光谱、年代学等研究显示岩石经历了超高温变质作用。麻粒岩变质演化过程包括三个阶段, 其中峰前M₁阶段矿物仅部分保留; 峰期M₂阶段和峰后M₃阶段矿物组合分别为Grt+Qz+Pl+Kfs+Sil+Rt+Liq和Grt+Qz+Bt+Pl+Kfs+Sil+Rt+Liq; 相平衡模拟结合二长石温度计和石英中Ti温度计研究结果显示峰期M₂阶段的温压条件为1025~1100℃、1.05~1.2GPa。研究样品中石墨结晶程度较高, 石墨拉曼光谱特征与麻粒岩相变质条件下形成的石墨一致, 岩相学特征指示高晶度石墨结晶于M₂向M₃转变过程中。麻粒岩中获得变质锆石年龄为~1.85Ga, 该年龄是寄主岩石峰期变质作用后熔体冷却时间的记录。岩相学和年代学研究表明, 优质高晶度石墨的形成和生长可能与胶-辽-吉活动带区域上碰撞折返过程中发生的深溶事件或混合岩化作用密切相关。

     

K–Ar age–chemistry–fabric relations of phengite from the Sanbagawa high-pressure schists, Japan

The Sanbagawa high-pressure schists from central Shikoku in Southwest Japan have experienced high-strain ductile deformation during exhumation and cooling. This study examines the effects of high-strain ductile deformation on K–Ar ages of phengites on the basis of fabric, chemistry and K–Ar ages of phengites from the pelitic, psammitic and quartzose (or albitic) schists collected from the same outcrop in the albite–biotite zone. Phengites in the pelitic and psammitic schists generally occur forming aggregates consisting of fine-grained phengite crystals and are extremely fine-grained in domains close to relatively rigid garnet and albite porphyroblasts, indicating that deformation-induced grain-size reduction had taken place in phengite during the ductile deformation accompanying the exhumation of host schists. We suggest that the grain-size reduction of phengite is due to strain-induced recrystallization or dynamic recrystallization. The matrix phengites in schists are chemically heterogeneous on the thin-section scale but the phengites from pelitic and psammitic schists from the same outcrop have similar chemical range. Phengite included in garnet has a high Si value and its Na/(Na + K) and Mg/(Mg + Fe) ratios are significantly low in comparison with those in matrix. The phengite included in garnet records the chemistry in equilibrium with other major silicate phases during the higher pressure stage of the P–T–t history of the schists. In contrast, the matrix phengites having low Si values are likely to have been formed during retrograde metamorphism in extremely restricted equilibrium domains. The two or three different types of schists from the same outcrop, which have a similar grain size of phengite, have similar K–Ar ages, suggesting that the closure temperature does not depend on chemistry. However, the hematite-rich quartzose schist with strong grain-size reduction of both phengite and quartz has a significantly younger K–Ar phengite age than the pelitic and quartzose schists in the same outcrop that do not show grain-size reduction. We suggest that the exhumation tectonics of the schists, which have experienced strong ductile deformation at temperatures less than ~350°C, played an important role resulting in the observed variation in age.     

Alpine tectonic evolution of a Jurassic subduction-accretionary complex: Deformation,kinematics and Ar/Ar age constraints on the Mesozoic low-grade schists of the Circum-Rhodope Belt in the eastern Rhodope-Thrace region,Bulgaria-Greece

Deformation of the Circum-Rhodope Belt Mesozoic (Middle Triassic to earliest Lower Cretaceous) low-grade schists underneath an arc-related ophiolitic magmatic suite and associated sedimentary successions in the eastern Rhodope-Thrace region occurred as a two-episode tectonic process: (i) Late Jurassic deformation of arc to margin units resulting from the eastern Rhodope-Evros arc–Rhodope terrane continental margin collision and accretion to that margin, and (ii) Middle Eocene deformation related to the Tertiary crustal extension and final collision resulting in the closure of the Vardar ocean south of the Rhodope terrane. The first deformational event D₁ is expressed by Late Jurassic NW-N vergent fold generations and the main and subsidiary planar-linear structures. Although overprinting, these structural elements depict uniform bulk north-directed thrust kinematics and are geometrically compatible with the increments of progressive deformation that develops in same greenschist-facies metamorphic grade. It followed the Early-Middle Jurassic magmatic evolution of the eastern Rhodope-Evros arc established on the upper plate of the southward subducting Maliac-Meliata oceanic lithosphere that established the Vardar Ocean in a supra-subduction back-arc setting. This first event resulted in the thrust-related tectonic emplacement of the Mesozoic schists in a supra-crustal level onto the Rhodope continental margin. This Late Jurassic-Early Cretaceous tectonic event related to N-vergent Balkan orogeny is well-constrained by geochronological data and traced at a regional-scale within distinct units of the Carpatho-Balkan Belt. Following subduction reversal towards the north whereby the Vardar Ocean was subducted beneath the Rhodope margin by latest Cretaceous times, the low-grade schists aquired a new position in the upper plate, and hence, the Mesozoic schists are lacking the Cretaceous S-directed tectono-metamorphic episode whose effects are widespread in the underlying high-grade basement. The subduction of the remnant Vardar Ocean located behind the colliding arc since the middle Cretaceous was responsible for its ultimate closure, Early Tertiary collision with the Pelagonian block and extension in the region caused the extensional collapse related to the second deformational event D₂. This extensional episode was experienced passively by the Mesozoic schists located in the hanging wall of the extensional detachments in Eocene times. It resulted in NE-SW oriented open folds representing corrugation antiforms of the extensional detachment surfaces, brittle faulting and burial history beneath thick Eocene sediments as indicated by 42.1–39.7 Ma ⁴⁰Ar/³⁹Ar mica plateau ages obtained in the study. The results provide structural constraints for the involvement components of Jurassic paleo-subduction zone in a Late Jurassic arc-continental margin collisional history that contributed to accretion-related crustal growth of the Rhodope terrane.     

桐柏山商丹断裂带东延段石榴云母片岩中石榴子石显微构造特征及变质—变形环境分析

韧性剪切带上岩石变质—变形条件的研究是探讨造山带造山过程及造山机制的重要环节.岩石中矿物显微、超微构造特征是造山带形成环境的真实体现,是造山带变形研究的基础.本文通过对商丹断裂带(即商南—丹凤断裂带)东延段桐柏山地区石榴云母片岩中石榴子石的位错特征及动态重结晶型式等显微构造特征,分析研究石榴子石塑性变形方式、变形机制及...     

浙西南八都群泥质麻粒岩的变质演化与pT轨迹

浙西南古元古界八都群是目前华夏地块最古老的变质基底,以往研究认为其变质程度仅达角闪岩相。近来在对遂昌地区八都群富铝片麻岩的研究过程中,发现了具有"石榴石+夕线石+正/反条纹长石+黑云母"特征组合的泥质麻粒岩,表明该地体曾经历麻粒岩相变质改造。通过岩相学与矿物化学分析,确定该岩石经历了3个阶段的演化过程,即:早期进变质阶段(M1),形成"石榴石+黑云母+白云母+夕线石+斜长石+石英"的矿物组合;变质峰期阶段(M2-3),形成"石榴石+夕线石+三元长石+黑云母+石英"的矿物组合;峰期后降压冷却阶段(M4),形成"黑云母+白云母+新生斜长石+石英"的矿物组合。岩石中石榴石普遍发育与降温过程有关的扩散成分环带和与降压过程有关的斜长石后生合晶。通过石榴石-黑云母温度计和GASP压力计估算变质峰期的温压条件为800~850℃、0.6~0.7 GPa,峰期后退变质阶段的温压条件为560~590℃、0.25~0.33 GPa,具有顺时针样式的pT演化轨迹,反映一种陆壳碰撞增厚、后又拉伸减薄的动力学过程。     

Growth and deformation of porphyroblasts in the Foothills terrane, central Sierra Nevada, California: negotiating a microstructural minefield

Abstract The main porphyroblastic minerals in schists and phyllites of the Foothills terrane, Western Metamorphic Belt, central Sierra Nevada, California, are cordierite and andalusite (mostly chiastolite). Less commonly, biotite, muscovite, chlorite, garnet or staurolite are also present as porphyroblasts. The variety of porphyroblast and matrix microstructures in these rocks makes them suitable for testing three modern hypotheses on growth and deformation of porphyroblasts: (1) porphyroblast growth is always syndeformational; (2) porphyroblasts nucleate only in low-strain, largely coaxially deformed, quartz-rich (Q) domains of a crenulation foliation and are dissolved in active high-strain, non-coaxially deformed, mica-rich (M) domains, the spacing between which limits the size of the porphyroblasts; and (3) porphyroblasts generally do not rotate, with respect to geographical coordinates, during deformation, provided they do not deform internally, so that they may be used as reliable indicators of the orientation of former regional structural surfaces, even on the scale of orogenic belts. Some porphyroblast–matrix relationships in the Foothills terrane are inconsistent with hypotheses 1 and 2, and others are equivocal. For example, in many rocks it cannot be determined whether the porphyroblasts grew where the strain had already been partitioned into M and Q domains, whether the porphyroblasts caused this partitioning, or both. Although most porphyroblasts appear to be syndeformational, as predicted by hypothesis 1, observations that do not support the general application of hypotheses 1 and 2 to rocks of the Foothills terrane include: (a) lack of residual crenulations in many strain-shadows and alternative explanations where they are present; (b) absence of porphyroblasts smaller than the distance between nearest mica-rich domains; (c) nucleation of crenulations on existing porphyroblasts, rather than nucleation of porphyroblasts between existing crenulations; (d) presence of micaceous ‘arcs’in an undifferentiated matrix against some porphyroblasts, suggesting static growth; (e) absence of crenulations in porphyroblastic rocks showing sedimentary bedding; and (f) porphyroblasts with very small, random inclusions, which are probably pre-deformational. Similarly, porphyroblasts that have overgrown sets of crenulations and porphyroblasts with micaceous ‘arcs’are probably post-deformational, at least on the scale of a large thin section and probably over much larger areas, judging from mesoscopic structural evidence. Some porphyroblasts in rocks of the Foothills terrane do not appear to have rotated, with respect to geographical coordinates, during matrix deformation, in accordance with hypothesis 3, at least on the scale of a large thin section. However, other porphyroblasts evidently have rotated. In some instances, this appears to be due to mutual interference, but many apparently rotational porphyroblasts are too far apart to have interfered with each other, which indicates that the rotation was associated with deformation of the matrix. The occurrence of planar bedding surfaces adjacent to porphyroblasts about which bedding and/or foliation surfaces are folded suggests rotation of the porphyroblasts during non-coaxial flow parallel to bedding, rather than crenulation of the matrix foliation around static porphyroblasts. It appears that porphyroblasts may rotate during deformation if the matrix is relatively homogeneous, so that the strain is effectively non-coaxial. This may occur after homogenization of a matrix in response to the strongest degree of crenulation folding, whereas the same porphyroblasts may have been inhibited from rotating previously, when strain accumulation was partitioned in the matrix.     

Phase equilibria modelling and zircon age dating of pelitic granulites in Zhaojiayao,from the Jining Group of the Khondalite Belt,North China Craton

The Jining Group occurs as the eastern segment of the Khondalite Belt, North China Craton and is dominated by a series of granulite facies rocks involving ‘normal’ pelitic granulites recording peak temperatures of ~850 °C and ultrahigh‐temperature (UHT) pelitic granulites recording peak temperatures of 950–1100 °C. The P–T paths and ages of these two types of granulites are controversial. Three pelitic granulite samples in the Jining Group comprising two sillimanite–garnet gneiss samples (J1208 and J1210) and one spinel–garnet gneiss sample (J1303) were collected from Zhaojiayao, where ‘normal’ pelitic granulites occur, for determination of their metamorphic evolution and ages. Samples J1208 and J1210 are interpreted to record cooling paths from the T_max stages with P–T conditions respectively of ~870–890 °C/7–8 kbar and >840 °C/>7.5 kbar constrained from the stability fields of the observed mineral assemblages and the isopleths of plagioclase, garnet and biotite compositions in pseudosections. Sample J1303 is interpreted to record pre‐T_max decompression from the kyanite‐stability fields to the T_max stage of 950–1020 °C/8–9 kbar and a post‐T_max cooling path revealed mainly from the stability field of the observed mineral assemblage, the plagioclase zoning and the biotite composition isopleth in pseudosections. The post‐T_max cooling stage can be divided into suprasolidus and subsolidus stages. The suprasolidus cooling may not result in an equilibrium state at the solidus in a rock. Therefore, different minerals may record different P–T conditions along the cooling path; the inferred maximum temperature is commonly higher than the solidus as well as different solidi being recorded for different samples from the same outcrop but experiencing different degrees of melt loss. Plagioclase compositions, especially its zoning in plagioclase‐rich granulites, are predicted to be useful for recording the higher temperature conditions of a granulite's thermal history. The three samples studied seem to record the temperature range covering those of the ‘normal’ and UHT pelitic granulites in the Jining Group, suggesting that UHT conditions may be reached in ‘normal’ granulites without diagnostic UHT indicators. LA‐ICP‐MS zircon U–Pb data provide a continuous trend of concordant ²⁰⁷Pb/²⁰⁶Pb ages from 1.89 to 1.79 Ga for sample J1210, and from 1.94 to 1.80 Ga for sample J1303. These continuous and long age spectrums are interpreted to represent a slow cooling and exhumation process corresponding to the post‐T_max cooling P–T paths recorded by the pelitic granulites, which may have followed the exhumation of deeply buried rocks in a thickened crust region resulted from a collision event at c. 1.95 Ga as suggested by the pre‐T_max decompression P–T path.     

Differential Evolution of High-Pressure and Ultrahigh-Pressure Metapelites from Habutengsu,Chinese Western Tianshan: Phase Equilibria Modelling and 40Ar/39Ar Geochronology

Metapelite is one of the predominant rock types in the high-pressure–ultrahigh-pressure(HP–UHP) metamorphic belt of western Tianshan, NW China; however, the spatial and temporal variations of this belt during metamorphism are poorly understood. In this study, we present comparative petrological studies and 40Ar/39 Ar geochronology of HP and UHP pelitic schist exposed along the Habutengsu valley. The schist mainly comprises quartz, white mica, garnet, albite and bluish amphibole. In the Mn O–Na2O–Ca O–K2O–Fe O–Mg O–Al2O3–Si O2–H2O(Mn NCKFMASH) system, P–T pseudosections were constructed using THERMOCALC 333 for two representative pelitic schists. The results demonstrate that there was a break in the peak metamorphic pressures in the Habutengsu area. The northern schist has experienced UHP metamorphism, consistent with the presence of coesite in the same section, while the southern one formed at lower pressures that stabilized the quartz. This result supports the previous finding of a metamorphic gradient through the HP–UHP metamorphic belt of the Chinese western Tianshan by the authors. Additionally, phengite in the northern schist was modelled as having a Si content of 3.55–3.70(a.p.f.u.) at the peak stage, a value much higher than that of oriented matrix phengite(Si content 3.32–3.38 a.p.f.u.). This indicates that the phengite flakes in the UHP schist were subjected to recrystallization during exhumation, which is consistent with the presence of phengite aggregates surrounding garnet porphyroblast. The 40Ar/39 Ar age spectra of white mica(dominantly phengite) from the two schists exhibit similar plateau ages of ca. 315 Ma, which is interpreted as the timing of a tectonometamorphic event that occurred during the exhumation of the HP–UHP metamorphic belt of the Chinese western Tianshan.