Accessory phase petrogenesis in relation to major phase assemblages in pelites from the Nelson contact aureole, southern British Columbia     

H. S. TOMKINS  D. R. M. PATTISON 《Journal of Metamorphic Geology》2007,25(4):401-421

Monazite petrogenesis in the Nelson contact aureole is the result of allanite breakdown close to, but downgrade and therefore independent of, major phase isograds involving cordierite, andalusite and staurolite. The development of garnet downgrade of the staurolite and andalusite isograds does not appear to affect the onset of the allanite-to-monazite reaction but does affect the textural development of monazite. In lower pressure, garnet-absent rocks, allanite breakdown results in localized monazite growth as pseudomorphous clusters. In higher pressure, garnet-bearing rocks, allanite breakdown produces randomly distributed, lone grains of monazite with no textural relationship to the original reaction site. Fluids liberated from hydrous phases (chlorite, muscovite) during garnet formation may have acted as a flux to distribute light rare earth elements more widely within the rock upon allanite breakdown, preventing the localized formation of monazite pseudomorphs. Despite these textural differences, both types of monazite have very similar chemistry and an indistinguishable age by electron microprobe chemical dating (157 ± 6.4 Ma). This age range is within error of isotopic ages determined by others for the Nelson Batholith. Garnet from the garnet, staurolite and andalusite zones shows euhedral Y zoning typified by a high-Y core, low-Y collar and moderate-Y annulus, the latter ascribed to allanite breakdown during garnet growth in the garnet zone. The cause of the transition from high-Y core to low-Y collar, traditionally interpreted to be due to xenotime consumption, is unclear because of the ubiquitous presence of xenotime. Accessory phase geothermometry involving monazite, xenotime and garnet returns inconsistent results, suggesting calibration problems or a lack of equilibration between phases.  相似文献   

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轨迹的指示意义     

卢良兆  靳是琴  胡大千  董永胜 《矿物学报》1998,(1)

集安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。经微区化学成分详细研究后,正确运用Grt－Bt温度计获得进变质阶段,变质高峰期和修期后Fe－Mg交换反应停止时的温度条件。按石榴子石和斜长石的微区钙含量变化确定了峰期和退变质阶段压力。据此建立起逆时针形式的变质作用p－T－t轨迹,反映该区早元古代陆壳内裂陷带的闭合过程。  相似文献   

蔓箐沟一带石榴基性麻粒岩变质反应与矿物演化     

刘福来 《吉林大学学报(地球科学版)》1998,(1)

蔓箐沟地区石榴基性麻粒岩发育多期变质反应结构。矿物组合演化可划分三个阶段：①峰期阶段（Ｍ１）,以包裹于石榴石中的Ｏｐｘ＋Ｃｐｘ±Ｈｂ＋Ｐｌ±Ｑｚ和由Ｇｔ＋Ｏｐｘ＋Ｃｐｘ±Ｈｂ＋Ｐｌ±Ｑｚ组成基质矿物组合为代表;②退变早期近等温减压阶段（Ｍ２）,形成蠕虫状后成合晶Ｏｐｘ＋Ｈｂ＋Ｐｌ±Ｍｔ;③退变晚期阶段（Ｍ３）,则形成细粒、黄绿色Ｈｂ＋中酸性Ｐｌ的较低温组合。并揭示该区石榴基性麻粒岩从中下部地壳的麻粒岩相变质,到快速抬升至中浅部地壳发生的近等温减压及晚期降温退变的复杂ｐＴ演化历史。  相似文献   

Chloritoid–glaucophane schist in the north Qilian orogen, NW China: phase equilibria and P–T path from garnet zonation     

C. J. WEI  S. G. SONG 《Journal of Metamorphic Geology》2008,26(3):301-316

Chloritoid–glaucophane‐bearing rocks are widespread in the high‐pressure belt of the north Qilian orogen, NW China. They are interbedded and cofacial with felsic schists originated from greywackes, mafic garnet blueschists and low‐T eclogites. Two representative chloritoid–glaucophane‐bearing assemblages are chloritoid + glaucophane + garnet + talc + quartz (sample Q5‐49) and chloritoid + glaucophane + garnet + phengite + epidote + quartz (sample Q5‐12). Garnet in sample Q5‐49 is coarse‐, medium‐ and fine‐grained and shows two types of zonation patterns. In pattern I, X_grs is constant as X_py rises, and in pattern II X_grs decreases as X_py rises. Phase equilibrium modelling in the NC(K)MnFMASH system with Thermocalc 3.25 indicates that pattern I can be formed during progressive metamorphism in lawsonite‐stable assemblages, while pattern II zonation can be formed with further heating after lawsonite has been consumed. Garnet growth in Q5‐49 is consistent with a continuous progressive metamorphic process from ～14.5 kbar at 470 °C to ～22.5 kbar at 560 °C. Garnet in sample Q5‐12 develops with pattern I zonation, which is consistent with a progressive metamorphic process from ～21 kbar at 540 °C to ～23.5 kbar at 580 °C with lawsonite present in the whole garnet growth. The latter sample shows the highest P–T conditions of the reported chloritoid–glaucophane‐bearing assemblages. Phase equilibrium calculation in the NCKFMASH system with a recent mixing model of amphibole indicates that chloritoid + glaucophane paragenesis does not have a low‐pressure limit of 18–19 kbar as previously suggested, but has a much larger pressure range from 7–8 to 27–30 kbar, with the low‐pressure part being within the stability field of albite.  相似文献   

Micro-zircon: origin and evolution during metamorphism     

T. J. DEMPSTER  D. C. HAY  S. H. GORDON  N. M. KELLY 《Journal of Metamorphic Geology》2008,26(5):499-507

The size and distribution of zircon within a garnet-mica-schist from the Scottish Highlands were assessed using scanning electron microscopy. The study reveals that abundant 0.2–3.0 μm sized zircon is preferentially concentrated within garnet and biotite porphyroblasts. The micro-zircon has grown during regional metamorphism and represents >90% of the total number of zircon in the schist. It is texturally distinct from a few larger detrital zircon grains in the schist that commonly preserve evidence of dissolution, and more rarely, small metamorphic outgrowths. The sequential incorporation of zircon in porphyroblasts allows prograde changes in the morphology of the zircon population to be identified. Zircon is reactive and soluble, and responds to medium-grade metamorphism in a series of dissolution and crystallization events, linked to possible changes in fluid composition. Deformation also has a significant influence on the distribution of zircon, allowing inclusions previously trapped within biotite to react. About 8 × 10⁶ micro-zircon occur as inclusions within a typical individual 5-mm garnet porphyroblast and their presence must be considered prior to trace-element or isotopic analysis of such metamorphic phases.  相似文献   

Metamorphic and structural evolution of the Early Proterozoic Puolankajärvi Formation, Finland – I. Structural and textural relations     

K. LAAJOKI  P. TUISKU 《Journal of Metamorphic Geology》1990,8(3):357-374

The amphibolite facies Puolankajärvi Formation (PjF) occupies the western margin of the Early Proterozoic Kainuu Schist Belt (KSB) of northern Finland. The lower and middle parts of the PjF consist of turbiditic psammites and pelites and tempestitic semipelites. This report concentrates on the pelitic lithologies which include quartz–two-mica–plagioclase schists with variable amounts of garnet, staurolite, andalusite and biotite porphyroblasts as well as sillimanite and cordierite segregations. The KSB forms a major north–south-trending synclinorium between two Archaean blocks. It contains both autochthonous and allochthonous units and is cut by faults and shear zones. The PjF lies on the western side of the KSB and is probably allochthonous. The formation has undergone six major deformation phases (D₁, D₂, D_3a, D_3b, D₄ and D₅). During D_3a-D₅ the maximum principal stress (σ₁) changed in a clockwise direction from south-west to north-east. Between D₂ and D₃ the intermediate principal stress (σ₂) changed from horizontal to vertical and the interval between D₂ and D₃ marks a transition from thrust to strike-slip tectonics. Relict structures in the porphyroblasts indicate the following mineral growth–deformation evolution in the PjF. (1) Throughout the PjF there was a successive crystallization of garnet (syn-D₁), poryphyroblastic biotite (inter-D_3/4) and staurolite (inter-D_3/4) during the pre-D₄ stage. (2) A syn-D₄-inter-D_4/5 crystallization of kyanite, sillimanite (fibrolite), porphyroblastic tourmaline, magnetite, rutile, cordierite and muscovite–biotite–plagioclase pseudomorphs after staurolite was most localized at and near D₄ shear zones. (3) A syn- to post-D₅ generation of andalusite, ilmenohematite and sheet silicates after staurolite and after cordierite occurred near D₅ faults. The evolution outlined here permits the relative dating of the PjF parageneses, which is used in the second part of the study (Tuisku & Laajoki, 1990), and, together with the knowledge of the pressure–temperature conditions during various growth events, makes it possible to compile pressure–temperature–deformation paths for the PjF.  相似文献   

Synchronous peak Barrovian metamorphism driven by syn-orogenic magmatism and fluid flow in southern Connecticut, USA     

P. J. LANCASTER  E. F. BAXTER  J. J. AGUE  C. M. BREEDING  T. L. OWENS 《Journal of Metamorphic Geology》2008,26(5):527-538

Recent work in Barrovian metamorphic terranes has found that rocks experience peak metamorphic temperatures across several grades at similar times. This result is inconsistent with most geodynamic models of crustal over‐thickening and conductive heating, wherein rocks which reach different metamorphic grades generally reach peak temperatures at different times. Instead, the presence of additional sources of heat and/or focusing mechanisms for heat transport, such as magmatic intrusions and/or advection by metamorphic fluids, may have contributed to the contemporaneous development of several different metamorphic zones. Here, we test the hypothesis of temporally focussed heating for the Wepawaug Schist, a Barrovian terrane in Connecticut, USA, using Sm–Nd ages of prograde garnet growth and U–Pb zircon crystallization ages of associated igneous rocks. Peak temperature in the biotite–garnet zone was dated (via Sm–Nd on garnet) at 378.9 ± 1.6 Ma (2σ), whereas peak temperature in the highest grade staurolite–kyanite zone was dated (via Sm–Nd on garnet rims) at 379.9 ± 6.8 Ma (2σ). These garnet ages suggest that peak metamorphism was pene‐contemporaneous (within error) across these metamorphic grades. Ion microprobe U–Pb ages for zircon from igneous rocks hosted by the metapelites also indicate a period of syn‐metamorphic peak igneous activity at 380.6 ± 4.7 Ma (2σ), indistinguishable from the peak ages recorded by garnet. A 388.6 ± 2.1 Ma (2σ) garnet core age from the staurolite–kyanite zone indicates an earlier episode of growth (coincident with ages from texturally early zircon and a previously published monazite age) along the prograde regional metamorphic T–t path. The timing of peak metamorphism and igneous activity, as well as the occurrence of extensive syn‐metamorphic quartz vein systems and pegmatites, best supports the hypothesis that advective heating driven by magmas and fluids focussed major mineral growth into two distinct episodes: the first at c. 389 Ma, and the second, corresponding to the regionally synchronous peak metamorphism, at c. 380 Ma.  相似文献   

滇西马厂箐铜钼多金属矿床成矿流体来源及演化过程——来自石榴子石原位微量元素及H- O同位素组成的限定     

吉彦冰  杜丽娟  陈军  黄智龙  李波  李鑫正  刘林林  杨再风 《地质学报》2023,97(4):1140-1156

滇西马厂箐铜钼多金属矿床位于三江特提斯成矿域,是一个与喜马拉雅期富碱斑岩侵入有关的多金属矿床。前人研究表明,马厂箐铜钼多金属矿床形成于斑岩-矽卡岩成矿系统,但由于缺乏系统矿物学研究,目前对矽卡岩矿化过程和成矿效应仍不清楚,限制了对该矿床成矿过程的全面认识。因此,本文以马厂箐矿床矽卡岩型矿化中的石榴子石为研究对象,利用H-O同位素、电子探针(EPMA)以及LA-ICP-MS原位微区技术开展了同位素及成分分析,限定成矿流体来源以及反演成矿流体演化过程。石榴子石可分为早期自形石榴子石(Grt I)和晚期他形石榴子石(Grt II)。Grt I与辉石共生,并伴有黄铁矿、黄铜矿等金属矿物组合,属于钙铝-钙铁榴石固溶体(And_{49.37～99.58}Gro_0～49.79);Grt II更富Fe,属钙铁榴石系列(And_{67.50～99.85}Gro_0～31.84)。两期石榴子石均富含Th、U、Nd,亏损Ba、Sr、Hf、Nb,富轻稀土。基于石榴子石矿物化学特征认为,Grt I可能是在弱酸性、氧化、低水岩比(W/R)条...  相似文献   

Evaluation of a Quality Control Monitor Material for the Routine Electron Probe Microanalysis of Kimberlite Exploration Garnets     

Deon De Bruin 《Geostandards and Geoanalytical Research》2010,34(3):257-264

Election probe microanalysis of indicator minerals is extensively used in the exploration for kimberlite deposits, the evaluation of specific kimberlite occurrences for their diamond bearing potential and to classify grains into different chemical and lithological mantle associations. Kimberlite exploration programmes can involve several tens of thousands of indicator mineral analyses. Procedures for monitoring data quality and consistency of analyses across large data sets are commonly absent. Suitable monitor minerals should be used to verify the data quality of kimberlite exploration and evaluation data sets. This material should have a suitable composition, be homogenous, be available in sufficient quantities and have a similar appearance to the unknown samples. Garnet P1, a megacryst garnet from the Premier kimberlite, was found to have a suitable composition as a monitor for kimberlite garnet analyses. Data were collected on the monitor material at regular intervals during routine analyses, over an extended period, both as a fixed grain mounted on the sample holder and as separate grains set within batches of routine samples. The data were evaluated to assess the quality and consistency in the analyses of large data sets over time. The monitor material was also analysed at independent laboratories using their routine analytical set-up and calibration procedures for comparative purposes. Values are given for the mean ± 2s range, which can serve as guide values for acceptable analyses for all elements.  相似文献   

郯庐断裂带(安徽段)内磁铁石榴角闪岩的形成条件、年代学及构造归属的探究   总被引：5，自引：5，他引：0  

聂峰  石永红  王娟  康涛  曹晟 《岩石学报》2014,30(6):1718-1730

通过对肥东群南缘特征性岩石——磁铁石榴角闪岩的构造地质学和岩相学研究表明,磁铁石榴角闪岩及其围岩以构造透镜体形式产出,该变质岩具有两个阶段变质矿物组合,热力学评价显示①阶段的温压范围为：T=616~700℃和P=0.66~0.85GPa,平均温压为T=653±35℃和P=0.75±0.09GPa。②阶段的温压范围为：T=597~643℃和P=0.51~0.94GPa,平均温压为T=620±15℃和P=0.73±0.15GPa,并表现出一个快速等压降温P-T演化特征。锆石U-Pb定年和拉曼光谱分析表明,该磁铁石榴角闪岩的变质年龄为2469±49Ma。结合地质背景和前人的研究,可以判定该类岩石可能源于华北板块。由于郯庐断裂的左旋走滑作用被构造并置与扬子板块中。从而进一步推测,郯庐断裂（安徽段）横向宽度可达到10~15km。  相似文献