集安岩群黑云母的化学成分及意义

黑云母系集安岩群主要变质矿物之一，其化学组成ＴｉＯ＿２平均含量３．３１％，Ａｌ＿２Ｏ＿３１８．２９％，Ａｌ￣Ⅵ０．３１、Ｘ＿Ｍｇ值０．５４，形成于高角闪岩相。黑云母微区成分不均匀，周围与长英质矿物相接触的黑云母，内部成分受变质峰期后扩散作用的影响甚微，与石榴石相邻的黑云母边缘具有扩散环带。因此，通过黑云母和石榴石晶体中心成分及两者的边缘成分，可以分别获得接近变质峰期和Ｆｅ－Ｍｇ交换反应封闭时的温度。     

高温变泥质岩石中石榴石-黑云母地质温度计的应用——以胶北荆山群富铝岩石为例

在对胶北荆山群麻粒岩相富铝岩石中石榴石、黑云母的成分环带进行深入研究基础上,选取不同粒径、与不同矿物相邻的石榴石、黑云母各微区点成分,利用石榴石-黑云母温度计分别进行了温度估算。确定在黑云母含量较高的岩石(V_(Grt)/V_(Bt)≤1)中,利用大颗粒石榴石(d≥1500μm)晶体核部(或靠近长英质矿物一侧的晶体幔部)成分与基质中远离石榴石等镁铁矿物处于长英质矿物之间的黑云母核部成分配合。通过石榴石-黑云母温度计可以获得相当可信的变质峰期温度。但是对于黑云母含量极低的岩石(V_(Grt)/V_(Bt)≥6),由于黑云母的成分普遍遭到了强烈改造。使得温度估算结果异常偏低,因此不适合采用石榴石-黑云母温度计估算峰期温度。同一岩石中,采用不同的相邻石榴石-黑云母矿物对晶体边缘成分获得的温度值差异较大,反映它们在峰期后发生Fe-Mg交换反应并达到封闭温度平衡状态的程度不同,因此利用石榴石-黑云母温度计难以获得准确的封闭温度。通过热力学计算,建立了一个新的石榴石-黑云母温度计公式。确定胶北荆山群所经历的变质峰期温度为720～770℃,峰期后最低相对封闭温度为480～500℃。     

北祁连南缘右行韧性走滑剪切带糜棱岩矿物的化学成分及其对形成环境的响应

长英质糜棱岩是北祁连南缘右行韧性走滑剪切带内的主要构造岩类型,其中主要矿物都发生不同程度的塑性变形。石榴石内矿物包裹体的痕迹、长英质矿物的定向分布及矿物间的接触关系等表明糜棱岩中的矿物为同构造生长的产物。石榴石、黑云母、白云母和斜长石的矿物化学成分特点表明糜棱岩的变质程度达角闲岩相,形成于高温环境。石榴石微区成分存在两种类型：一种为环带型,从核部到边缘,MnO含量从中心向边缘逐渐降低,FeO和MgO加含量逐渐增加,反映石榴石形成于递进变质环境;另一种为均一型,显示剪切带内局部变质作用的强度继续增大,温度进一步升高(>600℃),组分扩散能力的提高使石榴石内的环带部分或全部均一化。矿物化学成分温压计的计算结果表明矿物形成温度为651～763℃．压力为6.2×10⁸～8.4×10⁸Pa。据此,推测该韧性剪切带形成于地下17～19km．后随祁连加里东造山带一起隆升出露地表。     

通化集安群石榴子石和黑云母微区化学成分特征及其对变质作用p—T—t轨

集巡群片麻岩中石榴子石和黑云母接触带两侧边缘广泛发育Ｆｅ－Ｍｇ交换反应所形成的扩散环带,但晶体内部各自保持变质峰期的成分,中温带石榴子石可保留生长环带。     

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。     

花岗岩拓扑学的反思

介绍了石榴石成分环带 (剖面 )的现代测试技术 ,强调了二维成分填图在当代成分环带研究中的重要性。根据石榴石成分环带 (剖面 )的变化特征 ,将其划分为匀变连续型成分环带和突变间断型成分环带。匀变连续型环带形成受平衡生长控制 ;突变间断型成因复杂 ,其中的台阶型可与多期变质、变质条件大幅度改变、变质反应性质改变或深熔作用干扰等过程有关 ;脉冲型又可分生长型环带(剖面 )和扩散型环带 (剖面 ) ,后者是变质顶峰后石榴石与包体矿物反应导致石榴石某种成分的晶内扩散有关。东昆仑小庙岩群石榴石的岩相学、主量微量元素环带和稀土元素配分环带特征表明其是两期变质作用的产物。     

拉萨地块松多榴辉岩的变质演化过程:NCKMnFMASHTO体系中的相平衡关系

拉萨地块松多榴辉岩主要矿物组合为石榴子石、绿辉石、角闪石、多硅白云母、绿帘石、金红石。石榴子石环带不明显,核部成分均一,从核部到边部,镁铝榴石和钙铝榴石含量降低,可能分别记录了榴辉岩峰期及退变质过程信息。绿辉石显示微弱的成分环带,硬玉含量从核部到边部略有升高,部分绿辉石边部发育韭闪石退变质边,反映了在减压过程中外来流体进入体系的过程。多硅白云母具有高的Si含量(3.5~3.6),其中石榴石包体中的多硅白云母相对基质中的白云母有更高的Si值。本文利用Thermocalc变质相平衡模拟软件,结合详细岩相学观察,在NCKMn FMASHTO体系下,模拟松多含多硅白云母榴辉岩的变质演化过程。其中,榴辉岩峰期矿物组合为g+o+law+phn+ru,石榴石核部最大镁铝榴石值和石榴子石包体中多硅白云母最大Si值确定的榴辉岩峰期温压条件约为620℃,32×105Pa,榴辉岩经历了近等温降压的退变质过程。相平衡模拟结果表明拉萨地块松多榴辉岩经历了超高压变质作用过程,并经历了相对快速的折返过程到中部地壳层次。     

西大别造山带红安高压榴辉岩的变质演化:岩相学与Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-O(Fe2O3)体系中相平衡关系

西大别造山带红安高压榴辉岩主要矿物为石榴石、绿辉石、冻蓝闪石、石英和绿帘石,有时可见蓝闪石、多硅白云母和钠云母.石榴石具有生长环带且边缘成分变化大,可分为代表峰期的Ⅰ型边(XMg高、Grs低)和受退变质改造的Ⅱ型边(XMg低、Grs高).石榴石内蓝闪石包体发育冻蓝闪石退变边,说明包体不能完全反映进变质条件.基质绿辉石比包体绿辉石Jd含量低,在一个晶体内成分有明显变化和沿解理缝发育冻蓝闪石,显示峰后绿辉石有成分变化和退变质改造.基质中冻蓝闪石晶体较大,核部见有蓝闪石残留,说明二者有成因联系.冻蓝闪石和绿辉石都发育后成合晶结构,石榴石有韭闪石的反应冠状体.在THERMOCALC程序计算的P-T视剖面图中,石榴石Ⅰ型边反映的峰期P-T条件为2.4～2.6GPa、570～585℃,和基质中多硅白云母Si含量等值线限定范围一致,对应硬柱石蓝闪石榴辉岩组合.石榴石Ⅱ型边P-T范围为1.9～2.4GPa、530～570℃,低于峰期条件.在可能的峰后降压过程中,岩石先后主要经历了硬柱石脱水生成绿帘石和蓝闪石、绿辉石退变为冻蓝闪石的反应阶段.绿辉石、冻蓝闪石发育的后成合晶说明晚期退变过程缺乏流体,石榴石的韭闪石冠状体也可能在该阶段产生,都受局部成分域控制.红安高压榴辉岩中各矿物与成分代表不同变质阶段,称其为冻蓝闪石榴辉岩只是对现有主要组成矿物的描述,不是基于共生关系的严格岩石学命名.     

西藏新达多地区榴辉岩的变质过程研究及其对古特提斯俯冲带演化的限定意义

位于拉萨地块中部松多榴辉岩带西端的新达多地区出露两种类型榴辉岩:含蓝闪石榴辉岩和双矿物榴辉岩。含蓝闪石榴辉岩主要矿物组合为石榴石、绿辉石、蓝闪石、多硅白云母及少量的绿帘石、角闪石、石英、金红石。石榴石不具有成分环带结构,蓝闪石存在于基质中,边部大多退变为冻蓝闪石并普遍发育有角闪石和石英的后成合晶。双矿物榴辉岩的主要矿物组合为石榴石、绿辉石、石英及少量的绿帘石、角闪石、金红石、钛铁矿、榍石。石榴石具有典型的进变质环带特征,从核部到边部镁铝榴石和钙铝榴石组分先升高后降低,铁铝榴石组分变化与之耦合,石榴石边部发育角闪石和钠长石的冠状体,推断石榴石记录了进变质的生长过程后又受到了退变质改造。结合传统温压计和变质相平衡模拟两种温压计算方法对榴辉岩的峰期变质条件进行限定,得到含蓝闪石榴辉岩的峰期温压条件为:615±5℃,33±0.5kbar;双矿物榴辉岩的峰期变质温度为630±10℃,压力不超过27kbar。变质相平衡模拟计算结果显示:(1)含蓝闪石榴辉岩经历了退变初期近等温减压过程,这一过程以硬柱石和少量滑石的脱水反应生成蓝闪石和绿帘石为主要特征;中晚期退变质阶段以大范围硬柱石消失后局部富余流体的消耗在蓝闪石边部形成冻蓝闪石以及蓝闪石边部发育后成合晶为特征,部分石榴石边部的韭闪石冠状体大致也发生在这一过程;(2)双矿物榴辉岩则记录了从进变质生长阶段到峰期变质阶段,最后再到退变质演化阶段的完整变质过程。结合前人对松多榴辉岩的工作,对新达多地区新近发现的两类榴辉岩的岩石学研究表明:拉萨地块内部的榴辉岩为典型的大洋俯冲带产物,代表了古特提斯洋盆的存在。俯冲折返过程中复杂的构造机制使得不同类型榴辉岩在同一地区出露共生。     

松树沟榴闪岩中的石榴石和角闪石成分环带特征及岩石变质过程

陕西商南松树沟超镁铁质岩体周缘的榴闪岩中的石榴石和角闪石保存着良好的成分环带．这些成分环带及其矿物世代的成分变异表明,榴闪岩经历了区域绿片岩相和高压角闪榴辉岩相两期变质作用．高压变质作用的Ｐ－Ｔ趋势呈特征的发荚型轨迹,这种样式的Ｐ－Ｔ轨迹可为晋宁期间秦岭带局部存在板块构造体制提供重要的变质作用证据．     

Zoning patterns in orthopyroxene and garnet in granulites: implications for geothermometry

Compositional maps of orthopyroxene and garnet of contrasting grain size and in contact with different minerals were made from two paragneiss granulites from the Minto terrane of northern Quebec. The compositional maps provide clear evidence of late exchange of Fe/(Fe + Mg) after Ca in garnet and Al in orthopyroxene had been quenched-in. The extent of late Fe-Mg exchange was controlled by neighbouring minerals, with negligible Fe-Mg gradients against plagioclase and quartz, and substantial gradients against exchangeable Fe-Mg minerals. Cores of grains in contact with exchangeable Fe-Mg neighbours are progressively more reset in Fe/(Fe + Mg) as grain size decreases, whereas cores of even small grains surrounded by only plagioclase and quartz are not significantly different in Fe/(Fe + Mg) than cores of the largest grains. Gradients of Ca in garnet and of Al in orthopyroxene in grains of uniform Fe/(Fe + Mg) preserve a high-temperature retrograde history during which intergranular exchange effected compositional uniformity of mineral rims and intragranular Fe-Mg diffusion in garnet and orthopyroxene was rapid enough to homogenize Fe/(Fe + Mg). The transition from efficient intergranular exchange at relatively high temperatures to local Fe-Mg exchange at lower temperatures may have been controlled by loss of an intergranular exchange medium in the rock, possibly an internally generated dehydration melt phase. Implications for geothermometry of granulites include the following (numerical values are particular to this study): (1) core compositions of garnet and orthopyroxene grains in contact with exchangeable neighbours may be reset in Fe/(Fe + Mg) relative to the most refractory compositions by an amount equivalent to 120d? C; (2) Fe-Mg exchange thermometry using even the most refractory Fe/(Fe + Mg) compositions may not record peak granulite conditions, possibly recording instead the temperature at which an intergranular exchange medium was lost from the rock; and (3) temperature-sensitive net transfer equilibria involving Al solubility in orthopyroxene yield temperatures up to 150d? C higher than maximum Fe-Mg exchange temperatures, even in grains with flat Fe/(Fe - Mg) compositional profiles, making them a better means of estimating peak granulite temperatures than Fe-Mg exchange thermometry.     

On the interpretation of peak metamorphic temperatures in light of garnet diffusion during cooling

Abstract Finite difference models of Fe-Mg diffusion in garnet undergoing cooling from metamorphic peak conditions are used to infer the significance of temperatures calculated using garnet-biotite Fe-Mg exchange thermometry. For rocks cooled from high grades where the garnet was initially homogeneous, the calculated temperature (T_calc) using garnet core and matrix biotite depends on the size of the garnet, the ratio of garnet to biotite in the rock (V_garnet/V_biotite) and the cooling rate. For garnets with radii of 1 mm and V_garnet/V_biotite<1, T_calc is 633, 700 and 777°C for cooling rates of 1, 10 and 100°C/Ma. For V_garnet/V_biotite= 1 and 4 and a cooling rate of 10° C/Ma, T_calc is approximately 660 and 610° C, respectively. Smaller and larger garnets have lower and higher T_calc, respectively. These results suggest that peak metamorphic temperatures may be reliably attained from rocks crystallized at conditions below T_calc of the garnet core, provided that V_garnet/V_biotite is sufficiently small (<0.1) and that the composition of the biotite at the metamorphic peak has not been altered during cooling. Numerical experiments on amphibolite facies garnets with nominal peak temperatures of 550–600° C generate a ‘well’in Fe/(Fe + Mg) near the rim during cooling. Maximum calculated temperatures for the assemblage garnet + chlorite + biotite + muscovite + plagioclase + quartz using the Fe/(Fe + Mg) at the bottom of the ‘well’with matrix biotite range from 23–43° C to 5–12° C below the peak metamorphic temperature for cooling rates of 1 and 100° C/Ma, respectively. Maximum calculated temperatures for the assemblage garnet + staurolite + biotite + muscovite + plagioclase + quartz are approximately 70° C below the peak metamorphic temperature and are not strongly dependent on cooling rate. The results of this study indicate that it may be very difficult to calculate peak metamorphic temperatures using garnet-biotite Fe-Mg exchange thermometry on amphibolite facies rocks (T_max > 550° C) because the rim composition of the garnet, which is required to calculate the peak temperature, is that most easily destroyed by diffusion.     

集安岩群石榴石成分特征及其动力学意义

集安岩群石榴石以富铁为特征，均属铁铝榴石。形成于高角闪岩相的石榴石成分比较稳定，＜ＦｅＯ＞平均含量３２．２０％，ＭｇＯ５．１７％，Ｘ＿（Ｍｇ）平均值０．２３；结晶于低角闪岩相的石榴石（ＦｅＯ＋ＭｇＯ）含量３３．５０％，（ＭｎＯ＋ＣａＯ）６．９１％。石榴石微区成分不均匀，晶体内具有一些微区成分环带，这种环带不是各进变质作用阶段ｐ－Ｔ条件平衡或近平衡的产物。石榴石边缘存在着明显的扩散环带，表现为晶体边缘相对晶体核部，Ｃａ含量和＜ＦｃＯ＞含量增高，ＭｇＯ含量降低，Ｘ＿（Ｍｇ）值变小，反映本区变质峰期后经历的是一个近等压冷却的地质动力学过程。     

Genetic Mechanism of Mineral Inclusions in Zircons from the Khondalite Series, Southeastern Inner Mongolia

The early Precambrian khondalite series is widely distributed in the Jining-Zhuozi-Fengzhen-Liangcheng area, southeastern Inner Mongolia. The khondalite series mainly consists of sillimanite garnet potash feldspar (or two-feldspar) gneiss and garnet biotite plagioclase gneiss. These gneissic rocks have commonly experienced granulite-facies metamorphism. In zircons separated from sillimanite garnet potash feldspar gneisses, many mineral inclusions, including Sil, Grt, Ky, Kfs, Qtz and Ap, have been identified by the Laser Raman spectroscopy. Generally, prograde metamorphic mineral inclusion assemblages such as Ky + Kfs + Qtz + Ap and Ky + Grt + Kfs + Qtz are preserved in the core of zircon, while peak granulite-facies metamorphic minerals including Sil + Grt + Kfs + Qtz and Sil + Grt + Kfs + Qtz + Ap are identified in the mantle and rim of the same zircon. However, in some zircons are only preserved the peak metamorphic minerals such as Sil + Grt + Kfs + Qtz and Sil + Grt + Kfs + Qtz + Ap from core to ri     

橄榄岩中尖晶石化学成分分带的机理及其地质意义

橄榄岩中尖晶石的化学成分分带显示，在由温度降低引起的亚固相平衡过程中，与橄榄石及斜方辉石相邻的尖晶石边缘贫Mg和Al、富Fe和Cr，而与单斜辉石相邻的尖晶石边线相反。尖晶石的Fe和Mg分带为其他矿物的粒间Fe－Mg离子扩散所致；尖晶石中的Cr和Al分带则是尖晶石晶体内部Cr－Al离子的交互扩散所致。本文从热力学理论角度说明了尖晶石晶体内部的Cr－Al交互扩散受其Fe－Mg分带的控制；从热力学计算证明了尖晶石内Al必须从富Fe处向富Mg处扩散，解释了尖晶石中Mg和Al及Fe和Cr分带的一致性。     

内蒙古卓资孔兹岩系中石榴石成分环带及其在温压估算中的应用

内蒙卓资出一了大面积太古宙矽线石榴黑云二长片麻岩，其中石榴石的Ｍｇ、〈Ｆｅ〉、Ｍｇ／（Ｍｇ＋Ｆｅ）等呈明显的环带特点。研究表明，石榴石既存在生长环带，也具有扩散环带。其成分环带与石榴石生长过程和变质条件有关。不同成分环带的热力学计算，可以确定变质作用过程中不同阶段的温压条件。     

Thermodynamic analysis of garnet growth zoning in eclogite facies granodiorite from M. Mucrone, Sesia Zone, Western Italian Alps

Samples of the metagranodiorite from M.  Mucrone (Sesia zone, Western Alps) show pseudomorphic and coronitic textures where the igneous minerals were partially replaced by high-pressure metamorphic assemblages. The original magmatic paragenesis consisted of quartz, plagioclase, K-feldspar, biotite and minor phases. During the eclogitic event the original plagioclase was fully replaced by zoisite, jadeite and quartz ± K-feldspar pseudomorphic symplectites and the biotite was in part replaced by phengitic mica. Moreover, a composite corona often developed around the biotite. This corona consists of a layer of phengite I and garnet and, where the igneous biotite and feldspars were in contact, of an outer layer of phengite II intergrown with quartz. Biotite, phengite and K-feldspar are homogeneous while garnet shows a strong composition zoning recording the kinetics of the metamorphic reactions. A numerical simulation of the observed garnet zoning is performed assuming that intercrystalline diffusion and plagioclase resorption were the slowest rate-determining processes during the prograde P-T path. The metamorphic paragenesis constrains the P-T path chosen in the simulation. The comparison between measured and calculated garnet zoning permits evaluation of the relative weights of interface kinetics, grain-boundary and lattice diffusion. Received: 26 November 1997 / Accepted: 6 August 1999     

Well-preserved garnet growth zoning in granulite from the Dabie Mountains, central China

Ultra-high pressure eclogites and granulites both occur in the Dabie Mountains, central China. A garnet porphyroblast from felsic granulite in the Dabie Mountains has been analysed for compositional zoning by electron microprobe. Two segments of the porphyroblast have opposite compositional variations. Segment I (from centre outward 9  mm to analytical point 18) has decreasing X_Sps and increasing X_Pyr, while Segment II (from analytical point 18, 1  mm outward to the rim) has increasing X_Sps and X_Alm and decreasing X_Pyr and X_Grs. The compositional zoning in segment I is considered as growth zoning and that in Segment II as diffusive retrograde zoning. Garnet growth zoning records a P–T  path prior to the peak granulite metamorphism. The minimum P – T  conditions are estimated to be 1.35  GPa and 850  °C for peak metamorphism, based on the highest Mg/(Fe+Mg) composition in the garnet (analytical point 18) and matrix hypersthene, biotite and plagioclase. A symplectitic corona surrounds the porphyroblast and appears to have formed at 0.6  GPa and 700  °C. The well-preserved growth zoning in garnet suggests a short residence time for the granulite at peak metamorphism and thus rapid tectonic uplift history. The P–T  path is consistent with that of ultra-high-pressure eclogite in the area. Tectonic movements during a collisional event could have brought both the granulite and the eclogite to their present positions.     

Garnet zoning and metamorphism of the Barrovian type area,Scotland

A microprobe investigation of the high grade metamorphic zones from the Barrovian type area in Angus, Scotland, shows the importance of local zones of retrograde cation exchange between garnet, staurolite and biotite. The interpretation of this zoning, established during a slow cooling history, is critical to any study of metamorphic reactions or conditions. The extent and intensity of these diffusion effects are dependent on a number of parameters including grainsize, fabric orientation, heating and cooling history, and the modal abundance of the phases. Increasing diffusion within garnets with metamorphic grade, and the subsequent retrograde effects are modelled using Temperature-Time-Transformation diagrams and provide information on the activation energy for Fe-Mg diffusion in garnet.