Anticlockwise P-T Path and Zircon U-Pb Dating of HP Mafic Granulites in Yushugou Granulite-Peridotite Complex, Southern Tianshan Mountains, China

The Yushugou HP granulite-peridotite complex is located at east of northern margin of southern Tianshan mountains,China,which consists of granulite unit and peridotite unit mainly.Because of the rare association of

Spinel + quartz-bearing ultrahigh-temperature granulites from Xumayao,Inner Mongolia Suture Zone,North China Craton:Petrology,phase equilibria and counterclockwise p-T path

The Khondalite Belt within the Inner Mongolia Suture Zone （IMSZ） in the North China Craton is a classic example for Paleoproterozoic ultrahigh-temperature （UHT） metamorphism. Here we report new spinel-bearing metapelitic granulites from a new locality at Xumayao within the southern domain of the IMSZ. Petrological studies and thermodynamic modeling of the spinel -- quartz-bearing assemblage shows that these rocks experienced extreme metamorphism at UHT conditions. Spinel occurs in two textural settings：（1） high Xzn（Zn/（Mg ＋ FeH -- Zn） = 0.071--0.232） spinel with perthitic K-feld- spar, sillimanite and quartz in the rock matrix; and （2）low Xzn （0.045-0.070） spinel as inclusions within garnet porphyroblasts in association with quartz and sillimanite. Our phase equilibria modeling indicates two main stages during the metamorphic evolution of these rocks：（l） near-isobaric cooling from 975 ℃ to 875 ℃ around 8 kbar, represented by the formation of garnet porphyroblasts from spinel and quartz; and（2）cooling and decompression from 850 ℃, 8 kbar to below 750 ℃- 6.5 kbar, represented by the break-down of garnet. The spinel ＋ quartz assemblage is considered to have been stable at peak metamorphism, formed through the break-down of cordierite, indi- caring a near isothermal compression process. Our study confirms the regional extent of UHT metamor- phism within the IMSZ associated with the Paleoproterozoic subduction-collision process.

P-T path of metamorphism and U-Pb monazite and zircon age of the Kitoy terrane: Implication for Neoarchean collision in SW Siberian Craton

The first data on P-T metamorphic conditions coupled with U-Pb monazite and zircon age obtained for the Neoarchean Kitoy granulite-gneiss terrane(SW Siberian Craton).Alumina gneisses of the Kitoy terrane indicate two-staged metamorphic evolution.The first stage of regional metamorphism(M1)occurred at high-amphibolite facies conditions at T=780-800℃ and P=8-9 kbar.The second stage(M2)belongs to MT-HT/LP type of metamorphism with the wide temperature interval 600-750℃ and pressure 2-4 kbar.Two age peaks were established on the basis of U-Pb monazite and zircon dating in garnet-anthophyllite gneisses.Both of them correspond to the Neoarchean age:the age of M1 falls into the interval of ca.2489-2496 Ma,the age of M2-ca.2446-2456 Ma.The high-temperature metamorphism of the Kitoy block and nearly coeval granitoid magmatism can be an evidence for the Neoarchean collision in SW Siberian craton.

Integrated P-T Paths of the High-Pressure Rocks and Their Tectonic Implications for the Mountain-Building of the North Qilian, China

After the integration of petrographic study, geothermobarometry and Gibbs method, the synthetic P-T paths for the rocks from different geological profiles in the North Qilian, China, have been derived. The composite P-T paths from different methods indicate that all the high-pressure rocks in the Qilian area recorded P-T paths with clockwise loops starting at the blueschist facies, later reaching peak metamorphism at the blueschist facies, eclogite fades or epidote-amphibolite facies and ending up with the greenschist facies. The incremental Ar-Ar dating shows that the plateau ages for the high-pressure rocks range from 410 to 443 Ma. The plateau ages could be used as a minimum age constraint for the subduction that resulted in the formation of these high-pressure rocks in the Qilian area. It is proposed that the late-stage decompressional and cooling P-T paths with ends at the greenschist facies for these high-pressure rocks probably reflect the uplift process which could occur after shifting the arc-trench tectonic system to the system of continental orogenic belts. The retrograde paths for the high-pressure rocks in the North Qilian tectonic belt are characterized by dramatic decompression with slight cooling, which suggests very rapid exhumation. Petrography supports that the mountain-building for the Qilian mountain range could undergo a very fast process which caused rapid uplift and denudation.

冈底斯岩浆弧东段变泥质岩的逆时针P-T-t轨迹与构造机制

青藏高原南部冈底斯岩浆弧东段出露的中-高级变质岩代表岩浆弧的中-下地壳,是研究岩浆弧深部组成与形成演化的理想载体。本文对冈底斯弧东段布久地区的变泥质岩,即石榴石夕线石云母片岩进行了岩石学、相平衡模拟和地质年代学研究。岩石学和相平衡模拟表明,片岩由黑云母、白云母、石榴石、夕线石、斜长石、石英及少量的钛铁矿、金红石组成,含或不含蓝晶石,经历了早期的低压麻粒岩相变质与部分熔融,变质温压条件为～7kbar和～770℃,中期的高压麻粒岩相变质与深熔作用,变质温压条件为～10. 5kbar和～775℃,以及晚期的角闪岩相退变质与熔体结晶作用,退变质温压条件为～7. 5kbar和～705℃,具有逆时针型的变质作用P-T-t轨迹。本文和现有年代学研究表明,片岩的早期的低压麻粒岩相变质与部分熔融发生在～70Ma,后续高压麻粒岩相变质与深熔发生在50～45Ma,晚期角闪岩相退变质与熔体结晶开始于45Ma,持续到25Ma。大体积幔源岩浆底侵导致的地壳加热与加厚,以及大陆碰撞导致的地壳缩短与加厚是冈底斯弧表壳岩在晚中生代至早新生代被埋藏到下地壳,经历长期持续高温、高压变质与部分熔融的构造机制,而加厚岩石圈地幔在渐新世的拆沉导致了岩浆弧加厚下地壳的折返。

佛子岭群逆时针P-T轨迹对大别碰撞造山带构造演化的探究

佛子岭群位于大别造山带最北缘,处于华北和扬子两大板块结合部位,构造位置十分关键,是解析两大板块精细耦合过程和探究大别造山带晚古生代构造事件的重要单元。然而,由于长期以来佛子岭群一直被视为"浅变质"复理石沉积单元,因而对其变质演化过程的解析较为薄弱,制约了对大别造山带形成过程的全面理解。本次通过对老虎场-龚店地质剖面细致的观测,石榴黑云母片麻岩精细的岩相学、矿物化学分析、热力学评价和年代学研究,建立了佛子岭群一个逆时针P-T变质演化型式。研究显示,佛子岭群记录了Ⅰ、Ⅱ、Ⅲ和Ⅳ四个变质阶段,温压变化依次为:(1)T=658~663℃、P=1.08~1.16GPa→(2)T=755~762℃、P=1.21~1.45GPa→(3)T=549~554℃、P=1.08~1.19GPa→(4)P标定=0.3~0.6GPa、T=477~499℃,(1)至(3)为近等压升温和降温过程,(3)至(4)则表现为近等温降压型式。锆石U-Pb定年显示了1484±64Ma^1180±192Ma、995±35Ma^606±14Ma和370±14Ma^331±8Ma三组年龄,前两组代表了碎屑锆石年龄;第三组共计7颗锆石,年龄集中,Th/U比值0.01~0.03,加权平均年龄为344±11Ma,代表了主期变质年龄。结合此次地质剖面结构特征和岩石类型,以及前人白云母40Ar-39Ar的270~255Ma变质年龄,表明佛子岭群记录了两期变质事件,早期为晚古生代的拉张背景下的岛弧根部变质,晚期为晚古生代末的较为快速抬升构造热事件,并据此推测大别造山带可能为增生和碰撞复合型造山带。

攀枝花大田地区康定群咱里组变质杂岩P—T—t轨迹及其构造意义

扬子板块西缘的康定杂岩为一套由侵入岩和变质岩共同组成的岩浆—变质混合杂岩,是研究罗迪尼亚超大陆裂解的重要窗口。长期以来对康定杂岩中的岩浆杂岩研究较多,但对变质杂岩研究较为薄弱。为全面了解康定杂岩形成的构造背景及成因机制,笔者等对扬子板块西缘的攀枝花大田地区康定群咱里组石榴夕线黑云斜长片麻岩进行了详细的岩相学、矿物地球化学、变质P—T条件估算研究,同时对同一层位的石榴二云片岩进行了锆石LA-ICP-MS U-Pb年代学分析。结果表明:石榴夕线黑云斜长片麻岩可识别出三期变质矿物组合:进变质阶段矿物组合(M1)为石榴子石变斑晶核部及其核部包裹体(石榴子石核部+第一期黑云母+第一期夕线石+钛铁矿+石英),P—T条件为609~690℃—0.2~0.3 GPa;峰期变质阶段矿物组合(M2)为石榴子石变斑晶幔部及变基质矿物(石榴子石幔部+第二期黑云母+第二期夕线石+斜长石+石英),P—T条件为643~732℃—0.20~0.51 GPa;退变质阶段矿物组合(M3)为石榴子石变斑晶边部及变基质矿物(石榴子石边部+第二期黑云母+第二期夕线石+斜长石+石英),P—T条件为602~687℃—0.27~0.39 GPa。石榴夕线黑云斜长片麻岩记录了具近等压冷却型(IBC)的逆时针P—T演化轨迹;石榴二云片岩锆石U-Pb定年结果表明区内的变质时代为842±10 Ma,综合分析表明:扬子板块西缘的变质杂岩形成于拉张环境,结合区内的同期岩浆活动特征,认为区内新元古代变质事件可能与超级地幔柱引发的岩浆活动有关。

海南岛屯昌晨星地区变基性岩体变质特征及构造意义

海南岛屯昌晨星地区变基性岩体原岩可能包括辉长岩(粗粒的斜长角闪岩)、辉长-辉绿岩(中细粒和中粗粒的斜长角闪岩)和枕状熔岩(细粒或块状斜长角闪岩)。由于受多期变质和蚀变作用的影响,组成这些岩石的多数矿物如角闪石、斜长石和绿泥石等常表现两个或两个以上的世代。角闪石环带矿物化学分析表明,从核到边,钛、铝、(钠+钾)含量增长,反映了压力和温度同时升高的进变质演化过程;相反,角闪石环带从核部到边部钛、铝和钾+钠含量降低,则暗示温度和压力同时降低而进入另一个退变质演化阶段。所获得的 Sm-Nd 全岩及角闪石的内部等时线年龄为128±12 Ma、Rb-Sr 全岩等时线年龄分别为131.8±6.2 Ma和344±11Ma。根据角闪石环带、矿物共生组合、微结构以及变质温压条件的半定量计算和同位素定年,屯昌晨星地区变基性岩体的变质作用可能经历了三个演化阶段。第一阶段(～527～450 Ma)可能记录了与洋底变质作用有关的(亚)绿片岩相至角闪岩相条件,早期的阳起石、钙质角闪石及镁铁质角闪石残余核为该阶段的产物;第二阶段(～330～240 Ma)进变质作用达到高峰的P=～0.9 GPa、T=～700℃,显示了过渡的高角闪石相-麻粒岩相变质条件。广泛的剪切变形及韭闪质普通角闪石-韭闪石边是该阶段的产物;第三阶段(～130 Ma～90 Ma)显示一个绿片岩相变质条件(P=～0.1～0.3 GPa、7=～300～400℃),可能与区域/热接触变质有关。因而屯昌晨星地区变基性岩体的变质作用演化显示了一个逆时针 P-T-t 轨迹,最可能与古太平洋板块俯冲导致的大陆边缘裂解及洋壳化和随后的年轻洋壳消亡、陆(华南大陆)-陆(印支板块)碰撞事件有关。

内蒙石板井—小黄山与蛇绿岩相伴的变质岩及其演化

在内蒙古石板井 -小黄山蛇绿岩带发现了具典型热变质特征的红柱石堇青石角岩和以兰绿色角闪石、硬绿泥石、黑硬绿泥石等矿物代表的中高压区域变质角闪片岩。据变质作用时序和相拓朴图两个方面的研究 ,表明石板井和小黄山两地的变质岩属同一构造混杂岩 ,其中的变质岩先经过与火成作用有关的热变质作用 ,后又叠加了中高压变质作用 ,即经历过一反时钟PT轨迹。它的热高峰为 6 2 0℃ ,压力高峰近 0 .6 5GPa。

Petrological Investigations and Zircon U-Pb Dating of High Pressure Felsic Granulites from the Yushugou Complex, South Tianshan, China

As a window of insight into the lower crust, high pressure granulite has received much attention since last decade. Yushugou high pressure granulite-peridotite Complex was located in the northeast margin of Southern Tianshan, NW China. Previous ideas agreed that the peridotite unit in Yushugou, combined with the ultramafic rocks in Tonghuashan and Liuhuangshan, represent an ophiolite belt. However, the metamorphic evolution and tectonic mechanism of the Yushugou high pressure （HP） granulite remain controversial. Petrological investigations and phase equilibrium modelling for two representative fclsic granulitc samples suggest two stages metamorphism of the rocks in Yushugou Complex. Granulite facies metamorphism （Stage I） with P-T conditions of 9.8-10.4 kbar at 895-920℃ was recorded by the porphyroblastic garnet core; HP granulite facies metamorphism （Stage II） shows P-T conditions of 13.2-13.5 kbar at 845-860℃, based on the increasing grossular and decreasing pyrope contents of garnet rims. The Yushugou HP felsic granulites have recorded an anti- clockwise P-T path, characterized by the temperature decreasing and pressure increasing simultaneously. The LA-ICP-MS isotopic investigations on zircons from the felsic granulite show that the protolith ages of the granlulites are -430 Ma, with two age groups of-390 Ma and 340-350 Ma from the metamorphic rims of zircon, indicating the Stage I and II metamorphic events, respectively. A tectonic model was proposed to interpret the processes. The investigated felsic granulite was derived from deep rooted hanging wall, with Stage I granulite facies metamorphism of -390 Ma, which may he related to the Devonian arc magmatic intrusion; Stage II HP granulite facies metamorphism （340-350 Ma） may due to the involvement of being captured into the subducting slab and experienced the high pressure metamorphism.

Recognition of Ophiolite Belt and Granulite in Northern Area of Mian－Lue， Southern Qinling， China and Their Implication

A ophiolite belt associated with the tectonic melange is recoghzed in northern area of Mian-Lue, southern Qinling. The ophiolite is considered to originate in a island arc. The occurrence of the ophiolite indicates that a paleo-ocean or finite oceanic basin existed POSt-Prot6rozoic in southern Qiuling, implying the difference of the continental basement of Southern Qinling from the Yangtze craton. The ophiolitc and themelange may mark the paleo-suture zoic between the two plates. The basic granulite is found in eastern area (Anzishan) of the ophiolite belt. The p-T path for metamorphism of the granulite demonstrates a process of continental collision.

Phase Equilibria of Hornblende-Bearing Eclogite in the Western Dabie Mountain,Central China

The high-pressure （HP） eclogite in the western Dabie Mountain encloses numerous hornblendes, mostly barroisite. Opinions on the peak metamorphic P-T condition, PT path and mineral paragenesis of it are still in dispute. Generally, HP eclogite involves garnet, omphacite, hornblendes and quartz, with or without glaucophane, zoisite and phengite. The garnet has compositional zoning with XMg increase, XCa and XMn decrease from core to rim, which indicates a progressive metamorphism. The phase equilibria of the HP eclogite modeled by the P-T pseudosection method developed recently showed the following： （1） the growth zonation of garnet records a progressive metamorphic PT path from pre-peak condition of 1.9-2.1 GPa at 508~C-514~C to a peak one of 2.3-2.5 GPa at 528~C-531~C for the HP eclogite; （2） the peak mineral assemblage is garnet＋omphacite＋glaucophane＋quartz_＋phengite, likely paragenetic with lawsonite; （3） the extensive hornblendes derive mainly from glaucophane, partial omphacite and even a little garnet due to the decompression with some heating during the post-peak stage, mostly representing the conditions of about 1.4-1.6 GPa and 580~C-640~C, and their growth is favored by the dehydration of lawsonite into zoisite or epidote, but most of the garnet, omphacite or phengite in the HP eclogite still preserve their compositions at peak condition, and they are not obviously equilibrious with the hornblendes.

P-T Paths Derived from Garnet Growth Zoning in Danba Domal Metamorphic Terrain, Sichuan Province, West China

Danba （丹巴） domal metamorphic terrain belongs to Songpan （松潘）-Ganze （甘孜） orogenic belt, where typical Barrovian and Buchan metamorphic zones are preserved. The former included chlorite, biotite, garnet, staurolite, kyanite and sillimanite zones, while the latter only developed silimanite＋muscovite and sillimanite＋K-feldspar zones. Integrated study has been carried on metamorphic reactions of garnet production and consumption, P-T paths and P-T-X-M phase relation and thermal tectonic model for Danba metamorphic zones. Petrological textures in thin sections show that garnet production and consumption in kyanite-sillimanite zone is mainly attributed to ChI＋Ms＋PI＋Q= Grt＋Bt＋H20 and kyanite=sillimanite respectively. Based on mineral compositions, the geothermo- barometry gives an average P, T condition of （4.9±0.3）×10^8 Pa, 543±30 ℃ for the first growth stage of the garnet and （5.8±0.3）×10^8 Pa, 534±29 ℃ for the second stage of garnet growth respectively. Anti-counter clockwise P-T paths were drawn using Gibbs method by NCMnKFMASH system for sample G98686 in the kyanite zone. The P-T-X-M modeling for the first mineral assemblages shows that the prediction is similar to the measured values in gossular, almandine and spessartine but mole fraction of pyrope and Fe/（Fe＋Mg） deviated far from the contours; while that for the second mineral assemblages exhibits that the prediction is consistent with the measured value of pyrope, grossular content and Fe/（Fe＋Mg） of garnet. A thermal tectonic model that there are at least three structure levels across the thrust-decollement zones is presented according to the P-T paths, metamorphic grades and deformation styles for the staurolite-kyanite zone of the Barrovian type metamorphism, which will provide some constraints for the evolution of the nappe complex.

Petrology and Metamorphic P-T Paths of Metamorphic Zones in the Huangyuan Group,Central Qilian Block,NW China

The Central Qilian Block is a Precambrian block in the Qilian Orogen,which has long drawn international attention for the study of orogeny and continental dynamics.The Huangyuan Group in the Datong area is one of the Precambrian metamorphic basement units in the Central Qilian Block and reflects metamorphism in the Barrovian garnet zone and sillimanite zone from south to north.Based on detailed fieldwork,this study presents a systematic study of petrography,mineral chemistry and phase equilibria of schists and gneisses from the two metamorphic zones.The garnet metamorphic zone is composed of micaschist,garnet-bearing micaschist and felsic leptynite,with interlayered plagioclase amphibolite.The sillimanite metamorphic zone consists of garnet-bearing biotite micaschist,sillimanite-bearing biotite-plagioclase gneiss and felsic leptynite.Garnet from the garnet metamorphic zone shows growth zoning with increasing almandine and pyrope and decreasing spessartine from core to rim.Garnet from the sillimanite metamorphic zone is almost homogeneous.Towards the outer rim,the contents of almandine and pyrope slightly decrease and grossular slightly increase.Biotite in both metamorphic zones is ferro-biotite.Plagioclase is oligoclase in garnet metamorphic zone and andesine in sillimanite metamorphic zone.Phase equilibrium modeling of a sample from garnet metamorphic zone resulted in a clockwise P-T path with a prograde stage(4.5–5.0 kbar,520–530℃),a peak P stage(9.8–10.2 kbar,560–570℃),a stage of thermal relaxation(8.0–8.5 kbar,580–590℃)and finally a retrograde stage(6.8–7.0 kbar,560–580℃).Thermodynamic modeling of a sample from the sillimanite metamorphic zone indicates a prograde stage(5.5–6.0 kbar,540–550℃)and a peak stage(7.8–8.5 kbar,660–690℃).The results indicate that the Huangyuan Group experienced medium-pressure amphibolite-facies metamorphism,which resulted from continental-continental collision between the Qaidam Block and the Central Qilian Block.

Metamorphism of the East Sector of the Southern Qinling Orogenic Belt and Its Geological Significance

The east sector of the southern Qinling belt is, lithologically, composed mainly of metapelites, ***qüartzites, marbles and small amount of metabasites and gneisses, whose protoliths are the Silurian, Devonian and less commonly the Sinian and Upper Palaeozoic. They have been subjected at least to two epochs of metamorphism. The early epoch belongs to progressive metamorphism which is centered on high amphibolite-granulite fades in the Fuping area and changed outwards into low amphibolite facies (staurolite-kyanite zone), epidote amphibolite facies (garnet zone) and greenschist facies (chlorite and biotite zones), the metamorphic age of which is about 220–260 Ma. This early-epoch metamorphism belongs to different pressure types: the rocks from greenschist to low amphibolite facies belong to the typical medium-pressure type which shows geothermal gradients of about 17–20 ***C/km and was probably produced by a crustal thickening process related to continental collision, and the high amphibolite-granulite facies belongs to the low-pressure type which shows geothermal gradients of about 25–38 ***C/km and was probably affected by some magmatic heats. Based on the basic characteristics of the P-T paths of the different facies calculated from the garnet zonations, it can be deduced that the metamorphism of medium-pressure facies series took place during an imbricated thickening process, rather than during the uplifting process after thickening. The late-epoch metamorphism belongs to dynamic metamorphism of greenschist facies which is overprinted on the early-epoch metamorphic rocks and is Yanshanian or Himalayan in age, probably related to intracontinental orogeny.

Microstructures and Phase Transition in Omphacite: Constraints on the P-T Path of Shuanghe Eclogite (Dabie Orogen)

Omphacite microstructures are important to decipher the metamorphic pressuretemperature(P-T)conditions of eclogite during subduction and exhumation processes.Here we present a systematic microstructural investigation of omphacite using transmission electron microscopy(TEM)in an ultrahigh-pressure(UHP)eclogite sample from Shuanghe,Dabie orogen,China.The omphacite can be divided into two phases:(1)Omp-1,exhibits as the main part of omphacite with P2/n space group containing small amount of dislocations and numerous antiphase domains(APDs).(2)Omp-2,shows as banded subgrains with C2/c space group containing large amount of dislocations.These two phases of omphacite have almost the same crystallography orientation and bounded by dislocation walls.Along these dislocation walls,we found some barroisite and albite microcrystals.The barroisite microcrystals contain large amount of dislocations and show a topological relation with host omphacite.While the albite microcrystals contain small amount of dislocations and does not show topological relation with host.These microstructures could give us the following metamorphic information to constrain the P-T path of eclogite:Firstly,this two omphacite space groups should be evolved from the same precursor C2/c omphacite,which had been underwent strongly plastic deformation during the syn-to late-peak metamorphic stage.Secondly,the precursor C2/c omphacite began to be retrograded and altered by small amount of barroisite microcrystals along its dislocation walls under the P-T condition of 2.2–2.6 GPa and 650–700℃ at the early amphibole eclogite stage.Thirdly,large amount of Na and other elements were exsolved from some precursor C2/c omphacite subgrains and crystallized numerous albite microcrystals at their boundaries,which is the necessary for the space group of C2/c surviving under the lower P-T conditions(1.7–1.9 GPa and 630–690 C)during the middle amphibole eclogite stage.Lastly,the rest of precursor C2/c omphacite subgrains were transformed into P2/n polymorph and formed the APDs structures under the P-T condition of^1.5 GPa and 650–680℃ during the late amphibole eclogite stage.

Metamorphic gradient modification in the Early Cretaceous Northern Andes subduction zone:A record from thermally overprinted high-pressure rocks

New field observations and petrological data from Early Cretaceous metamorphic rocks in the Central Cordillera of the Colombian Andes allowed the recognition of thermally overprinted high-pressure rocks derived from oceanic crust protoliths.The obtained metamorphic path suggests that the rocks evolved from blueschist to eclogite facies towards upper amphibolite to high-pressure granulite facies transitional conditions.Eclogite facies conditions,better recorded in mafic protoliths,are revealed by relic lawsonite and phengite,bleb-to worm-like diopside-albite symplectites,as well as garnet core composition.Upper amphibolite to high pressure granulite facies overprinting is supported by coarse-grained brown-colored Ti-rich amphibole,augite,and oligoclase recrystallization,as well as the record of partial melting leucosomes.Phase equilibria and pressure-temperature(P-T)path modeling suggest initial high-pressure metamorphic conditions M1 yielding 18.2-24.5 kbar and 465-580℃,followed by upper amphibolite to high pressure granulite facies overprinting stage M2 yielding 6.5-14.2 kbar and 580-720℃.Retrograde conditions M3 obtained through chlorite thermometry yield temperatures ranging around 286-400℃at pressures below 6.5-11 kbar.The obtained clockwise P-T path,the garnet zonation pattern revealing a decrease in X_(grs)/X_(prp)related to Mg#increment from core to rim,the presence of partial melting veins,as well as regional constraints,document the modification of the thermal structure of the active subduction zone in Northern Andes during the Early Cretaceous.Such increment of the metamorphic gradient within the subduction interface is associated with slab roll-back geodynamics where hot mantle inflow was triggered.This scenario is also argued by the reported trench-ward magmatic arc migration and multiple extensional basin formation during this period.The presented example constitutes the first report of Cretaceous roll-back-related metamorphism in the Caribbean and Andean realms,representing an additional piece of evidence for a margin-scale extensional event that modified the northwestern border of South America during the Early Cretaceous.

A Problem of High-Grade Polymetamorphism

A problem of evolution of high-grade polymeta- morphic terrain appeared in last decades:in worldwide complexes the identification of true stages of polymetamorphism encounters a problem of zircon zoning(or garnet etc.)that is the function of fluid activity and therefore the newly formed zone can not always be recorded.Structural geology allows discriminating between different stages of metamor-

Pressure-temperature Evolution of the Metapelites in the Motuo Area, the Eastern Himalayan Syntaxis

The Motuo area is located in the east of the Eastern Himalayan Syntaxis.There outcrops a sequence of high-grade metamorphic rocks,such as metapelites.Petrology and mineralogy data suggest that these rocks have experienced three stages of metamorphism.The prograde metamorphic mineral assemblages （M1） are mineral inclusions （biotite ＋ plagioclase ＋ quartz ± sillimanite ± Fe-Ti oxides） preserved in garnet porphyroblasts,and the peak metamorphic assemblages （M2） are represented by garnet with the lowest Xsps values and the lowest XFe# ratios and the matrix minerals （plagioclase ＋ quartz ± K-feldspar ＋ biotite ＋ muscovite ＋ kyanite ± siilimanite）,whereas the retrograde assemblages （M3） are composed of biotite ＋ plagioclase ＋ quartz symplectites rimming the garnet porphyroblasts.Thermobarometric computation shows that the metamorphic conditions are 562-714℃ at 7.3-7.4 kbar for the M1 stage,661-800℃ at 9.4-11.6 kbar for the M2 stage,and 579-713℃ at 5.5-6.6 kbar for the M3 stage.These rocks are deciphered to have undergone metamorphism characterized by clockwise P-T paths involving nearly isothermal decompression （ITD） segments,which is inferred to be related to the collision of the India and Eurasia plates.

Crystal Chemistry and Geochronology of Thorium-Rich Monazite from Kovela Granitic Complex,Southern Finland

Abundant porphyritic granites, including Grt-bearing and Bt-bearing porphyritic granites, and porphyritic potash-feldspar granite (trondhjemite-granitic composition) are widely distributed within the Kovela granitic complex Southern Finland, which associated with monazite-bearing dikes (strong trondhjemite composition). The investigated monazite-bearing dikes are dominated by a quartz + K-feldspar + plagioclase + biotite + garnet + monazite assemblage. The monazite forms complexly zoned subhedral to euhedral crystals variable in size (100 - 1500 μm in diameter) characterized by high Th content. The chemical zoning characterised as: 1) concentric, 2) patchy, and 3) intergrowth-like. Textural evidence suggests that these accessory minerals crystallized at an early magmatic stage, as they are commonly associated with clusters of the observed variations in their chemical composition are largely explained by the huttonite exchange , and subordinately by the cheralite exchange with proportions of huttonite (ThSiO4) and cheralite [CaTh(PO4)2] up to 20.4% and 9.8%, respectively. Textural evidence suggests that these monazites and associated Th-rich minerals (huttonite/thorite) crystallized at an early magmatic stage, rather than metamorphic origin. The total lanthanide and actinide contents in monazite and host dikes are strongly correlated. Mineral compositions applied to calculate P-T crystallization conditions using different approaches reveal a temperature range of 700°C - 820°C and pressure 3 - 6 kbars for the garnet-biotite geothermometry. P-T pseudo-section analyses calculated using THERMOCALC software for the bulk compositions of suitable rock types, constrain the PT conditions of garnet growth equilibration within the range of 5 - 6 kbars and 760°C - 770°C respectively. Empirical calculations and pseudo-section approaches indicate a clockwise P-T path for the rocks of the studied area. 207Pb/206Pb dating of monazite by LA-MC-ICPMS revealed a recrystallization period at around 1860 - 1840 Ma. These ages are related to the tectonic-thermal event associated with the intense crustal melting and intra-orogenic intrusions, constraining the youngest time limit for metamorphic processes in the Kovela granitic complex.