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1.
The Qinghai–Tibet plateau is a composite continental fragment formed by collision of multiple terranes and island arcs.The Lhasa terrane,which is located in the central part of the plateau,is bounded by the Yarlung–Zangbo suture to the south and Bangong-Nujiang suture to the north.An E–W–trending belt of(ultra)-high pressure eclogite was discovered in the Sumdo region of the Lhasa terrane.Careful field studies combined with petrological,geochemical and isotopic analyses show that the Sumdo eclogites mark a Carboniferous–Permian suture zone,at least 100 km long,containing ophiolite fragments,eclogites and Indosinian post–orogenic granitoids.This suture divides the Lhasa block into a northern and southern segment.Sumdo eclogite occurs about 200 km east of Lhasa city,and extends over 100km in an E–W direction.Sumdo eclogites were accompanied by garnet amphibolite and plagioclaseamphiboliteformedbyretrograde metamorphism of the eclogites.The eclogites were derived from oceanic basalts.LA–ICPMS U–Pb dating of zircon from the Sumdo eclogites indicates a Permian metamorphic age(260–270 Ma)and a Carboniferous protolith age of 303±4.8 MaThe ophiolite fragments in the Sumdo suture zone are composed of the ultramafic rocks,MORB–type basalt,OIB–type basalt and island arc basaltic andesite,some of which are intruded by post-collisional granites.The ultramafic body,a typical tectonic block in the suture zone,is completely serpentinized.Its geochemical features suggest that it is composed of harzburgite,typical of depleted mantle peridotite.The MORB and OIB–type basalts crop out in the Chasagang Formation,and the basaltic andesite crops out in the Leilongku Formation,both of which make up the Sumdo Group.Zircons from the OIB–type basalt with typical magmatic characteristics yield an average U–Pb age of306(95%)Ma,suggesting formation in a Paleo–Tethyan basin in the Carboniferous.U–Pb dating of zircon from the basaltic andesites yielded a concordant age of 265±3.1Ma,similar to the metamorphic age(266–270 Ma)of the eclogites,suggesting formation during subduction of the oceanic crust.Indosinian granodiorite with an age of194±4.3 Ma crops out north of the Sumdo suture.These granodiorites are similar to the late Indochina granites in the Lhasa block,and most likely formed during continent-arc collision or during closure of the Paleo-Tethyan Ocean.A four–stage model for the evolution of the Sumdo Paleo–Tethyan suture is proposed:1)From the Ordovician to the Devonian Gondwanaland was part of an epicontinental sea;2)In the Carboniferous,continental rifting produced a local basin,which then evolved into a Paleo-Tethyan ocean basin,dividing the Lhasa block into two segments;3)From the Permian to the early Triassic,Paleo-Tethyan oceanic crust was subducted northward and;4)In the middle Triassic and early Jurassic,the two fragments collided to form the modern Lhasa block.  相似文献   

2.
Muscovite 40Ar-39Ar dating of muscovite-quartz schist, eclogite and retrograde eclogite indicates an Indosinian orogenesis occurred at 220–240 Ma in the Lhasa terrane, which is caused by the closure of Paleo-Tethyan ocean basin and the following collision of the northern Lhasa terrane and southern Gondwana land. This Indosinian orogenesis is further confirmed by the regional sedimentary characteristics, magmatic activity and ophiolite mélange. This evidence suggests that the Indosinian orogenic belt in the Lhasa terrane is widely distributed from the Coqen county in the west, and then extends eastward through the Ningzhong and Sumdo area, finally turning around the eastern Himalayan syntaxis into the Bomi county. Based on the evolutionary process, the geological development of Lhasa terrane from early Paleozoic to early Mesozoic can be divided into seven stages. All of the seven stages make up a whole Wilson circle and reveal a perfect evolutionary process of the Paleo-Tethys ocean between the northern Lhasa terrane and southern Gondwana land. The Indosinian orogenisis is a significant event for the evolution of the Lhasa terrane as well as the Tibetan Plateau.  相似文献   

3.
Based on the deformation characteristics of the ductile shear zones in Sumdo (松多) Group, the quartz fabric by EBSD (electron backscatter diffraction), the data of muscovite 40Ar-39Ar geochronology (220-230 Ma) from ductile shear zones and the zircon SHRIMP U-Pb chronology (190 Ma) of granites in Snmdo region, Lhasa (拉萨) terrane is thought to have experienced an important Indosinian orogenic event at 220-230 Ma, which caused the closure of the paleo-Tethys Ocean along the tectonic zone of eclogite and the collision between northern part and southern part of the Lhasa terrane. The zircon SHRIMP U-Pb chronology of 190 Ma for biotite adamellite, with the distributing characteristics of the granite massif intruding in Sumdo Group, indicates that the biotite adamellitc should be the late orogenic or post-orogenic granite resulting from the lndosinian orogenesis. The discovery of Indosinian orogenic belt in Lhasa terrane expansed the southern boundary of lndosinian orogenic belt in Qinghai (青海)-Tibet plateau to Lhasa terrane from Qiangtang (羌塘) terrane, which changed the understanding about the distribution of Indosinian orogenic belt in Qinghai-Tibet plateau and extended the "T" type lndosinian orogenic belt in China. The study is very important for the formation and distribution of paleo-Tethys Ocean in Tibet. The ancient terrane framework and evolution of Qinghai-Tibet plateau need further research.  相似文献   

4.
Diffusion modeling of zoning profiles in eclogite garnets from three different tectonic units of Mt. Dabie, UHPM unit, HPM unit and northern Dabie, was used to estimate the relative time span and cooling rates of these rocks. Modeling result for the Huangzhen eclogite garnet shows that the maximal time span for the diffusion-adjustment process is about 22 Ma since the peak-temperature metamorphism, which is the maximum time span from amphibolite facies metamorphism to greenschist facies metamorphism. The Bixiling eclogites had subjected to a cooling process at a rate of - 10℃/Ma from 750℃ to 560℃ during 20 Ma. The second cooling stage of the Raobazhai eclogite following granulite-facies metamorphism is an initial fast cooling process at a rate of about 25℃/Ma and then slowed down gradually. All these belong to a coherent Dabie collision orogen with differences in subduction depth and exhumation/uplifting path.  相似文献   

5.
The West Kunlun orogenic belt(WKOB) along the northern margin of the Tibetan Plateau is important for understanding the evolution of the Proto-and Paleo-Tethys oceans. Previous investigations have focused on the igneous rocks and ophiolites distributed mostly along the Xinjiang-Tibet road and the China-Pakistan road, and have constructed a preliminary tectonic model for this orogenic belt. However, few studies have focused on the so-called Precambrian basement in this area. As a result, the tectonic affinity of the individual terranes of the WKOB and their detailed evolution process are uncertain. Here we report new field observations, zircon and monazite U-Pb ages of the "Precambrian basement" of the South Kunlun terrane(SKT) and the Tianshuihai terrane(TSHT), two major terranes in the WKOB. Based on new zircon U-Pb age data, the amphibolite-facies metamorphosed volcanosedimentary sequence within SKT was deposited during the late Neoproterozoic to Cambrian(600-500 Ma), and the flysch-affinity Tianshuihai Group, as the basement of the TSHT, was deposited during the late Neoproterozoic rather than Mesoproterozoic. The rock association of the volcano-sedimentary sequence within SKT suggests a large early Paleozoic accretionary wedge formed by the long-term lowangle southward subduction of the Proto-Tethys Ocean between Tarim and TSHT. The amphibolitefacies metamorphism in SKT occurred at ca. 440 Ma. This ca. 440 Ma metamorphism is genetically related to the closure of the Proto-Tethys Ocean between Tarim and the Tianshuihai terrane, which led to the assembly of Tarim to Eastern Gondwana and the final formation of the Gondwana. Since the late Paleozoic to early Mesozoic, the northward subduction of the Paleo-Tethys Ocean along the HongshihuQiaoertianshan belt produced the voluminous early Mesozoic arc-signature granites along the southern part of NKT-TSHT. The Paleo-Tethys ocean between TSHT and Karakorum closed at ca. 200 Ma, as demonstrated by the monazite age of the paragneiss in the Kangxiwa Group. Our study does not favor the existence of a Precambrian basement in SKT.  相似文献   

6.
For the first time, we apply different geospeedometric models to garnet zoning patterns that were obtained in this study from detailed EMP analyses for garnets from eclogites and granulite in the Dabie-Sulu orogen. Various zonings of cation diffusion were preserved in the garnets, enabling the acquirement of average cooling rates for the high-to ultrahigh-pressure rocks without using geochronological approaches. The coesite-bearing hot eclogites yield fast cooling rates of about 20 to 30℃/Ma subsequent to peak metamorphic temperatures, whereas the cold eclogite gives a relatively slow cooling rate of 8℃/Ma at its initial exhumation. A very slow cooling rate of <0.3℃/Ma is obtained for the granulite at Huangtuling, suggesting that the granulite may not be involved in the continental deep subduction.  相似文献   

7.
The Qinling-Dabie-Sulu high-pressure and ultra-high pressure metamorphic belt wasformed by subduction and collision between the North China and Yangtze plates. The study ofthe eclogite belt is very important in understanding the evolution of the Qinling Dabie orogen. Inthe present paper the geology, petrology, minerology and chronology of the eclogites in the Dabieand Sulu areas are described. The principal conclusions of this work are as follows: (1) Based up-on the field occurrence and the P-T conditions of the eclogites, two types of eclogite can be dis-tinguished: Type 1—the low-temperature and high-pressure eclogite in the mid-late Proterozoicmetamorphic series, and Type 2—the ultra-high pressure eclogite in the late Archaean to earlyProterozoic metamorphic complex. In the Dabie area, the ultra-high-pressure eclogite,high-pressure eclogite and epidote-blueschist units are nearly parallel to each other and stretchintermittently from north to south. (2) The P-T conditions of the high-pressure eclogites and ul-tra-high pressure eclogites have been estimated. The former are formed at 450-550℃ and1.4-1.6 GPa; while the latter at 650-870℃ and >2.7-2.9 GPa in the Dabie area and at820-1000℃ and >2.8-3.1 GPa in the Sulu area. The metamorphic temperatures of the eclogitesincrease progressively from west to east. (3) The ultra-high pressure eclogites were subjected to 5stages of metamorphism: pre-eclogite epidote amphibolite facies, peak coesite eclogite facies,post-eclogite amphibolite facies, epidote-blueschist facies or epidote amphibolite facies andgreenschist facies. The general features of the PTt path of the ultra-high pressure eclogite are:clockwise pattern, progressive metamorphism being a process of slow increasing temperature andrapid increasing pressure, and the retrogressive section with nearly isothermal decompression atthe early stage, isobaric cooling at the middle stage and nearly isothermal decompression at thelate stage. (4) At least two stages of high-pressure metamorphism occurred in the orogenic belt:the high-pressure eclogite and ultra-high pressure eclogite were formed by the subduction of theoceanic crust northward beneath the North China plate or the Dabie block during theCaledonian; while the epidote-blueschist belt came into being by subdution and collision be-tween the two continental plates during the Indosinian. (5) Due to the continuous sequentialsubduction of the cold plate, the ultra high-presssure metamorphic rocks were uplifted to thecrust by the underplating processes. They can be preserved just because of the "frozen effect" re-sulting from the continuous subduction of the cold plate. (6) The carbonates, such as magnesite,breunnerite, aragonite and dolomite, and the H_2O-bearing minerals, such as phengite, epidoteand zoisite, were stable during the high-pressure and/or ultra-high pressure metamorphism.  相似文献   

8.
Coesite inclusions are found in kyanite from the Lanshantou eclogite in the Sulu ultrahigh-pressure (UHP) metamorphic belt. This discovery extends the stable region of kyanite to over 2.4 GPa. As an important UHP metamorphic belt in China, the Sulu eclogite belt is the product of A-subduction induced by strong compression of the Yellow Sea terrane to the Jiaodong-northereastern Jiangsu terrane during the interaction of the Eurasian plate and Palaeo-Pacific plate in the Indosinian. It stretches about 350 km and contains over 1000 eclogite bodies. Most eclogites in this belt belong to Groups B and C in the classification of Coleman et al., and commonly contain kyanite, while the Lanshantou eclogite belongs to Group A and contains coesite. The MgO, CaO and FeO contents in garnet and pyroxene show regular variation from the core to the rim, which reveals the PTt paths of progressive metamorphism during the Early Mesozoic (240-200 Ma) and retrogressive metamorphism during the Late Mesozoic and Cenozoic exhum  相似文献   

9.
The North Qaidam orogenic belt(NQOB) is generally considered to be an early Paleozoic ultrahigh pressure metamorphic belt,but increasing reports of the Neoproterozoic magmatic and metamorphic events indicate that the NQOB probably also experienced the assembly of the Rodinia.However,the Neoproterozoic evolution of the NQOB is not well constrained due to the sparse records and ambiguous nature of the Neoproterozoic metamorphism.In order to reveal the multi-orogenic history of the NQOB,an integrated study of petrology,phase equilibrium modelling and geochronology was conducted on an epidote eclogite and host garnet mica schist from the Yuka–Luofengpo terrane.New zircon and monazite U–Pb ages show that the protolith of the garnet mica schist was deposited during 994–920 Ma and experienced Neoproterozoic(920–915 Ma) and early Paleozoic(451–447 Ma) polyphase metamorphism together with the enclosed eclogite.Relic omphacite inclusions were first identified in garnet and early Paleozoic zircon domains from the garnet mica schist,which provide solid evidence for the early Paleozoic eclogite facies metamorphism of the mica schist.Similar early Paleozoic peak P–T conditions of 27.4 kbar/613–670 ℃ and 30.2–30.8 kbar/646–655 ℃ were obtained for the garnet mica schist and enclosed eclogite,respectively,indicating that eclogites and their host paragneisses in this region underwent continental deep subduction as a coherent metamorphic terrane in early Paleozoic.The peak P–T conditions of the Neoproterozoic metamorphism were roughly constrained at 7.7–12.0 kbar and 634–680 ℃ for the garnet mica schist,based on stability field of mineral inclusions in Neoproterozoic zircons domains in P–T pseudosection,the relic garnet core composition and Ti-in-zircon thermometer.The high thermal gradients(16–37 ℃/km) defined by presently our and previously reported P–T conditions indicate that the Neoproterozoic metamorphism likely occurred in continental collision setting at 945–890 Ma.Since the Grenvillian syn-orogenic granitic magmatism and metamorphism(ca.1.0–0.9 Ga) in the NQOB are much younger than the Grenvillian orogeny in the central part of Rodinia,the Qaidam Block was probably located at the north margin of Rodinia in Neoproterozoic.  相似文献   

10.
Eclogite lenses in marbles from the Dabie-Sulu ultrahigh-pressure (UHP) terrane are deeply subducted meta-sedimentary rocks. Zircons in these rocks have been used to constrain the ages of prograde and UHP metamorphism during subduction, and later retrograde metamorphism during exhumation. Inherited (detrital) and metamorphic zircons were distinguished on the basis of transmitted light microscopy, cathodoluminescence (CL) imaging, trace element contents and mineral inclusions. The distribution of mineral inclusions combined with CL imaging of the metamorphic zircon make it possible to relate zircon zones (domains) to different metamorphic stages. Domain 1 consists of rounded, oblong and spindly cores with dark-luminescent images, and contains quartz eclogite facies mineral inclusion assemblages, indicating formation under high-pressure (HP) metamorphic conditions of T = 571-668℃and P = 1.7-2.02 GPa. Domain 2 always surrounds domain 1 or occurs as rounded and spindly cores with white-luminescent images. It contains coesite edogite facies mineral inclusion assemblages, indicating formation under UHP metamorphic conditions of T = 782-849℃and P > 5.5 GPa. Domain 3, with gray-luminescent images, always surrounds domain 2 and occurs as the outermost zircon rim. It is characterized by low-pressure mineral inclusion assemblages, which are related to regional amphibolite facies retrograde metamorphism of T = 600-710℃and P = 0.7-1.2 GPa. The three metamorphic zircon domains have distinct ages; sample H1 from the Dabie terrane yielded SHRIMP ages of 245±4 Ma for domain 1, 235±3 Ma for domain 2 and 215±6 Ma for domain 3, whereas sample H2 from the Sulu terrane yielded similar ages of 244±4 Ma, 233±4 Ma and 214±5 Ma for Domains 1, 2 and 3, respectively. The mean ages of these zones suggest that subduction to UHP depths took place over 10-11 Ma and exhumation of the rocks occurred over a period of 19-20 Ma. Thus, subduction from～55 km to > 160 km deep mantle depth took place at rates of approximately 9.5-10.5 km/Ma and exhumation from depths >160 km to the base of the crust at～30 km occurred at approximately 6.5 km/Ma. We propose a model for these rocks involving deep subduction of continental margin lithosphere followed by ultrafast exhumation driven by buoyancy forces after break-off of the UHP slab deep within the mantle.  相似文献   

11.
在藏北丁固地区,变质岩广泛分布,主要为区域变质岩,少量接触变质岩、混合岩化变质岩及动力变质岩。变质类型、变质岩相及变质岩石组合的空间分布等,受地层时代、原岩性质、地质构造及中酸性侵入岩体等多重因素控制。本文初步建立其综合模式。  相似文献   

12.
新疆西天山高压变质带的变质矿物与变质作用演化   总被引：3，自引：2，他引：3       下载免费PDF全文
新疆西天山高压变质带主要由石榴石，角闪石，绿辉石，多硅白云母，钠云母，绿帘石，绿泥石，钠长石，石英，榍石和金红石等组成，石榴石主要含铁铝榴石组份，角闪石有蓝闪石，亚铁蓝闪石，青铝闪石，冻蓝闪石等类型，变质矿物组合显示高压变质带经历了由硬柱石蓝片岩相，榴辉岩相，绿帘蓝片岩相至绿片岩相的变质作用演化进程。  相似文献   

13.
The geochemistry of the Leisure Bay Formation, Natal Metamorphic Province suggests that its protoliths were greywackes, pelites and arkoses that were deposited in an oceanic island arc environment. These rocks contain the mineral assemblage biotite + hypersthene + cordierite (with hercynite inclusions) + garnet + quartz + feldspar. Numerous generations of garnet genesis are evident from which a long history of metamorphism can be interpreted. M1 involved syn-D1 high temperature/low pressure metamorphism (4kb and >850oC) and dehydration melting to produce essentially anhydrous assemblages particularly in the vicinity of, and probably related to the intrusion of the Munster Suite sills. The inclusions of hercynite in cordierite and the garnet + quartz symplectites after hypersthene + plagioclase (550oC and 5kb) suggests isobaric cooling after M1. This indicates an anticlockwise P-T loop related to the early intrusion of subduction related calc-alkaline magmatic rocks. M2 involved syn-D2 dehydration melting of hydrous assemblages possibly related to the emplacement of many A-type rapakivi charnockite granitoids, which provided heat and loading. The D2 tectonism post-dated all lithologies in the region, except for syn- to late-D2 granitoid plutons, and is interpreted as a transpressional tectonothermal reworking of pre-existing (Proterozoic) crust at 1030Ma.  相似文献   

14.
西藏拉轨岗日核杂岩盖层变质分带特征及其地质意义   总被引：5，自引：0，他引：5  
藏南拉轨岗日由一系列链状的热穹隆构成, 总体呈东西向延伸, 每一个热穹隆是一个变质核杂岩, 核部发育大量变质岩, 基底与盖层之间发育拆离断层.通过对拉轨岗日变质带及其特征变质矿物进行化学成分分析和温度压力估算, 得出拉轨岗日变质带的分带规律及矿物成分、变质温度、压力、深度的变化规律, 为拉轨岗日变质核杂岩的热活动提供了佐证.   相似文献   

15.
豫西东秦岭造山带低压变质带的变质变形和变质反应   总被引：4，自引：0，他引：4  
豫西西峡北部低压变质带受到４次变形改造。低压变质作用主要发生在Ｄ１和Ｄ２变形间期，形成黑云母－红柱石、石榴石－红柱石、十字石－红柱石和堇青石－红柱石４个递增变质带。由矿物变质反应识别出堇青石－红柱石带ｐ－Ｔ趋势具有减压增温特点，这一演化特征可能反映变质作用具有地壳拉张的地球动力学特点。  相似文献   

16.
闽北麻源群变质流体对变质反应温度缓冲作用研究   总被引：4，自引：0，他引：4  
闽北麻源群在空间上表现为巴罗式递增变质带：绿泥石带→黑云母带→石榴石带→十字石带→蓝晶石带→夕线石带。变质温度计算结果表明，区内蓝晶石带变质温度（４７０℃～５００℃）明显低于石榴石带和十字石带变质温度（５００℃～５５０℃），即蓝晶石带在石榴石带和十字石带之前出现。这显然与传统的递增变质带理论相悖。流体组成化学平衡计算和变质反应热力学分析研究表明，蓝晶石带较低的变质温度并非是压力或退化变质作用所致，而是低ＸＨ２Ｏ流体对形成蓝晶石的脱水变质反应温度缓冲的结果，缓冲温度值高达１００℃～２００℃。本文从热力学理论上解释了缓冲作用的机制并估算了缓冲的温度范围，据此讨论了变质流体在变质相带研究方面的重要意义。  相似文献   

17.
Norman Fry 《Lithos》1982,15(3):183-190
The formation of pressure solution stripes is discussed on the basis of previosly published observations and of local chemical equilibria between solution and solid phases. Chemically driven mineral reactions can be localized at early sites of nucleation and be enhanced by stresses to create patterns of cleavage stripes. These reactions cause transport of material over distances greater than single grains. They provide suitable sites and suitable chemistry for later incongruent pressure solution, potentially a much faster process than congruent pressure solution discussed in previous literature. Neither reaction nor straightforward incongruent pressure solution explains the solution of quartz, commonly observed at cleavage stripes. Quartz pressure solution is explicable on certain assumptions about diffusivities and the effects of stresses. This involves interaction between quartz and the established incongruent solution of other minerals, an overall process termed ‘interdependent pressure solution’. Electrical potential differences will be greatest during early mineral reactions, a feature which may provide a means to corroborate the chemical role in initiating ‘pressure solution stripes’.  相似文献   

18.
Metamorphic, Thermal, and Tectonic Evolution of Central New England   总被引：3，自引：0，他引：3  
A new, detailed tectonic model is presented for the Acadianorogenic belt of central New England (Vermont and New Hampshire)that accounts for a wide range of petrological and structuralobservations. Three belts are considered: the Eastern Vermont,Merrimack, and intervening Bronson Hill belts. Specific observationsin eastern Vermont that are accounted for in the model includethe following. P–T paths are clockwise with maximum pressuresnear the Athens, Chester, and Strafford domes of 8–11kbar, but with maximum pressures decreasing to 3–5 kbarat the boundary with the Bronson Hill belt. Differential exhumationof the Vermont domes relative to the rocks in easternmost Vermontis required by the recorded differences in maximum pressure(5–6 kbar; 15–20 km) and the present-day geographicalseparation (7–10 km). Specific observations in New Hampshirethat are explained include the following. P–T paths inthe Merrimack belt are counter-clockwise with maximum pressuresof 4–5 kbar and are related to high regional heat flowand heat transfer by early Acadian plutons. P–T pathsin the Bronson Hill belt are intimately associated with structuralposition. An early contact metamorphism is evidenced in theSkitchewaug and Fall Mountain nappes near contacts with theearly Acadian Bethlehem gneiss (  相似文献   

19.
Biotite‐muscovite‐garnet gneisses at Einasleigh contain quartz‐feldspar veins composed of the same minerals as found in the enclosing rock. The vein‐gneiss boundaries are commonly irregular and on a microscopic scale, gradational.

Certain amphibolite layers contain quartz‐feldspar veins composed of the same minerals as found in the amphibolite. Hornblende‐rich extraction zones surround these veins, and material balance calculations show that all or nearly all of the vein‐forming matter was locally derived. Variation in the abundance of hornblende and plagioclase in the amphibolite as a function of distance from a quartz‐feldspar vein can be expressed by error‐function curves, thus suggesting that the mineral‐segregation process was diffusion‐controlled. During the mineral rearrangement, the Na and Ca contents of plagioclase have evidently remained unchanged, but the vein hornblende has become slightly richer in Fe+3, Mg, and Ca, and poorer in Si and Al relative to hornblende in the adjacent amphibolite.

A certain biotite‐plagioclase rock forms layers and boudins in the gneisses and contains pegmatite veins composed of the same minerals as found in the host rock. The plagioclase in these veins is more sodic than that in the host rock while the biotite contains slightly more Ti and Fe+2 and less Si and Mg than the biotite of the enclosing rock.

The data indicate that significant portions of the vein‐forming matter at Einasleigh were locally derived. The chemistry of some minerals has changed slightly during the segregation process, resulting possibly from different diffusion rates for the different mineral‐forming constituents.  相似文献   

20.
In the Avnik area of the Bingöl massif the Lower Unit consists of basic to felsic metavolcanics (ca. 450 Ma), intruded by granitoids (ca. 350 Ma). These are unconformably overlain by an Upper Unit of micaschists and Permian marbles; both units have been deformed and metamorphosed in Alpine time. The metavolcanics and granitoids are extensively feldspathized and silicified. The granitoids, and basic-intermediate volcanics, are albitized, while felsic volcanics are K-feldspathized. Metasomatism has severely modified K/Rb and Rb/Sr ratios, but not REE patterns, and is inferred to have occurred at relatively low T. Nametasomatism of the Upper Unit micaschists has produced albite porphyroblasts. Metasomatism postdates formation of the Upper Unit sediments, and is probably related to reaction with sea water that infiltrated the basement of volcanics and granitoids during deposition of these sediments. Rb-Sr whole-rock dating of extensively feldspathized intermediate-felsic metavolcanics gives an age of ca. 90 Ma, which suggests that the most extensive reaction coincided with expulsion of the trapped sea water during the early stages of the Alpine orogeny. The distribution of albitization vs. K-felds-pathization suggests that the type of metasomatism was controlled on a local scale by permeability and grain size, rather than by T variations.  相似文献   

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