Phase equilibrium, geothermobarometric and xenotime age dating constraints on the Alpine metamorphism recorded in chloritoid schists from the southern part of the Tisia Mega-Unit (Slavonian Mts., NE Croatia)

The chloritoid schists from the Slavonian Mts., which are attributed to the basal part of Devonian to Permian “Hercynian Semimetamorphic Complex,” represent a very rare lithology, not only in the Tisia Mega-Unit outcrops in Croatia, but also in the wider area. The investigated outcrop in the Kutjeva?ka Rijeka transect (Mt. Papuk) encompasses chloritoid-bearing metapelitic and metapsammitic lithologies. Both contain K-white mica, chlorite, chloritoid (10–15 vol.%), quartz and minor K-feldspar, plagioclase (albite), opaque minerals and pyrophyllite, together with accessory zircon, rutile, xenotime. The Th–U–Pb age dating on xenotime grains within the K-white mica + chlorite + quartz matrix and on inclusions found inside the chloritoids gave an average age 120 ± 36 Ma. Peak metamorphic conditions during the Alpine chloritoid-forming event reached 3.5–4 kbar and 340–380 °C, based on phengite barometry, chlorite–chloritoid thermometry and intersection of chlorite and chloritoid isopleths in the KFMASH quantitative phase diagram. The post-tectonic character of lath- and rosette-shaped chloritoids with respect to two foliations in the rock, together with the older age of 219 ± 81 Ma obtained on Yb-rich xenotime core domain(s), implies a possible existence of older low-grade metamorphic phase(s). The chemistry of the chloritoid schists bears the signature of upper continental crustal felsic rocks as potential protoliths, probably the felsic rocks of the nearby Papuk Complex of Slavonian Mts. The evidence presented here for the chloritoid-bearing low-grade metamorphic rocks from the Slavonian Mountains clearly show that the prograde Alpine metamorphic event had a more significant influence on the evolution of the southern part of Tisia Mega-Unit than previously considered.     

加强极低级变质作用研究

本文主要论述了4个与极低级变质作用研究相关的内容:(1)极低级变质作用研究的主要内容和标志;(2)加强极低级变质作用研究的重要意义和必要性;(3)当前国内外研究现状和存在问题;(4)加强极低级变质作用研究需要考虑的4个方面,一是立项,二是稳定的经费支持,三是人才培养,四是先进仪器设备的配备。     

Alpine metamorphism of pelitic schists in the Nufenen Pass area, Lepontine Alps

Abstract The effects of Tertiary Alpine metamorphism on pelitic Mesozoic cover rocks have been studied along a cross-section in the central Lepontine Alps in the Nufenen Pass area, Switzerland. Greenschist facies to amphibolite facies conditions are indicated by the formation of the index minerals chloritoid, garnet, staurolite and kyanite in pelitic rocks. Regional metamorphism reached maximum conditions during the interkinematic period between a main Alpine penetrative (D2) and a late Alpine (D3) crenulation type deformation phase or synchronous with the late Alpine deformation. Based on AFM phase relationships four different metamorphic zones can be distinguished: (1) chloritoid zone; (2) staurolite + chlorite zone; (3) staurolite + biotite zone; and, (4) kyanite zone. The isograds that separate these zones can be modelled by univariant reactions in the KFMASH system. The conditions of metamorphism calculated from geological ther-mobarometers for the maximum post-D2 por-phyroblast stage are from North to South: 500° C at 5-6 kbar and 600° C at 7-8 kbar. Detailed thermobarometry of garnet por-phyroblasts with complex textures suggests that maximum temperature was reached later than maximum pressure. Early garnet growth occurred along a prograde P-T-path, post-D2 rims grew with increasing temperature but decreasing pressure, and finally post-D3 garnet formed along a retrograde P-T-path. It may be concluded from the calculated pressure and temperature difference over a short distance (3 km) across the mapped area that the isogradic surfaces of the post-D2 metamorphism are steeply oriented. The data also suggest that isobaric and isothermal surfaces are parallel. Much of the observed metamorphic pattern can be explained as the result of a significant post-D2 differential uplift of the hot Pennine area relative to the Helvetic area along a tectonic contact zone. The closely spaced isograds (isotherms) in the North may then be interpreted as a thermal effect owing to the emplacement of the hot Pennine rocks against the Got-thard massif with its cover. Whereas, in the Pennine metasediments, post-D2 porphyroblast formation can be related to the decompression path which was steep enough for dehydration reactions to proceed. It is also remarkable that late kyanite porphyroblasts probably formed with decreasing pressure. The interpretation given here for the Nufenen Pass area may also apply to the Luk-manier Pass area where similar metamorphic patterns have been reported by Fox (1975). The formation of the ‘Northern Steep Belt’;, as denned by Milnes (1974b), and the associated late Alpine fold zones may, therefore, have significantly modified the metamorphic pattern of the Helvetic-Penninic contact zone.     

Pathways and distances of fluid flow during low-grade metamorphism: evidence from pyrite deposits of the Cameros Basin, Spain

New geochemical and sulphur isotopic data are presented for a number of pyrite deposits from the Late Jurassic–Early Cretaceous Cameros Basin, Spain. The deposits were formed at, or close to, the peak of metamorphism and are always related to sandstone units in the mainly metapelite sequence. Iron remained immobile and conservative, pyrite iron being derived by sulphidation of chlorite in the host metapelites. Reduced sulphur, however, was supplied from two external sources: thermochemical reduction of sulphate and release of S during metamorphism of sedimentary sulphides. These sources provided isotopically heavy and light S, respectively, with variation in pyrite isotopic composition between different deposits resulting from differences in their relative importance at each site. During metamorphism, the sandstone units acted as aquifers, carrying the sulphidic pore waters to locations where permeability provided by syn-depositional fractures on a scale of 0.5–5  m allowed its interaction with the metapelites. Transport distances for sulphide during metamorphism were of the order of hundreds of metres.     

A record of pre-Variscan Barrovian regional metamorphism in the eastern part of the Slavonian Mountains (NE Croatia)

Summary Petrological investigations and monazite dating are carried out on medium-grade metamorphic rocks (micaschist, gneiss and amphibolite) from the Kutjevačka Rijeka transect in the Slavonian Mts., Tisia Unit (NE Croatia). Field, mesoscopic and microstructural observations, as well as the preserved mineral chemistry, point to a single metamorphic event during peak assemblage growth reaching amphibolite facies conditions of ca. 600–650 °C and 8–11 kbar. Th, U and Pb contents of yttrium-rich accessory monazites indicate a pre-Variscan, i.e. Ordovician-Silurian age (444 ± 19 and 428 ± 25 Ma) for the medium-grade metamorphism of garnet-bearing micaschist.     

Very low-grade metamorphism of the Taveyanne greywacke, Glarus Alps, Switzerland

Detailed textural and chemical data for mineral assemblages on a regional scale are presented for the metaandesitic Eocene-Oligocene Taveyanne greywacke of the Glarus Alps, Eastern Switzerland. Presented data indicate an increase of metamorphic grade from zeolite facies to prehnite-pumpellyite and pumpellyite-actinolite facies. Low-grade outcrops contain laumontite, minor corrensite and pumpellyite (assemblage type 1), whereas outcrops of higher metamorphic grade contain prehnite and two populations of pumpellyite (type 2), prehnite—pumpellyite-(Al)—white mica (type 3), a single outcrop shows pumpellyite-actinolite facies (type 4). From the zeolite to prehnite-pumpellyite/pumpellyite-actinolite facies there are indications for an increase of the chemical equilibrium domain size for the critical paragenesis from a single detrital grain ≤1 mm) in type 1, to a few millimetres in type 2, and to a whole thin section in type 3. Metamorphic P - T conditions were determined by a combination of chlorite thermometry, fluid inclusion and vitrinite reflectance data. Peak temperatures range from 170-190 C for zeolite facies to 270-310 C for prehnite-pumpellyite and pumpellyite-actinolite facies. For the higher temperature range, pressures of 2-3 kbar are derived indicating a geothermal gradient of 24-32 C km^-1. The well-constrained temperature estimations derived for the assemblages provide a useful test of the different empirical calibrations of chlorite thermometers recently proposed. The best correspondence to the temperatures determined here is for the Cathelineau calibration. In addition, in the lower grade samples differences in textures and calculated temperatures provide a mean to distinguish between detrital and newly formed chlorites.     

Low-grade metamorphism of the Mikabu and northern Chichibu belts in central Shikoku, SW Japan: implications for the areal extent of the Sanbagawa low-grade metamorphism

In central Shikoku, SW Japan, the Mikabu belt is bounded to the north by the Sanbagawa belt, and to the south by the northern (N) Chichibu belt. The N-Chichibu belt can be further subdivided into northern and southern parts. There is no apparent difference in the overall geology, structure, or fossil and radiometric ages between the Mikabu belt and the northern part of the N-Chichibu belt. Greenstones from the Mikabu belt and the northern part of the N-Chichibu belt show evidence for similar low-grade metamorphism, and include the following mineral assemblages with albite+chlorite in excess: metamorphic aragonite, sodic pyroxene+quartz, epidote+actinolite+pumpellyite, glaucophane+ pumpellyite+quartz, and lawsonite (not with actinolite or glaucophane). These similarities suggest that the Mikabu belt and the northern part of the N-Chichibu belt belong to the same geological unit (the MB-NNC complex). The mineral assemblages also indicate that the MB-NNC complex belongs to a different metamorphic facies from the low-grade part of the Sanbagawa belt, that is, the former represents lower temperature/higher pressure conditions than the latter. Structural and petrological continuity between the MB-NNC complex and Sanbagawa belt has not yet been confirmed, but both have similar radiometric ages. It is therefore most likely that the MB-NNC complex and Sanbagawa belt belong to the same subduction complex, and were metamorphosed under similar but distinct conditions. These two units were juxtaposed during exhumation. In contrast, the southern part of the N-Chichibu belt is distinct in lithology and structure, and includes no mineral assemblages diagnostic of the MB-NNC complex and the Sanbagawa belt. Thus, the southern part of the N-Chichibu belt may represent a different geological unit from the MB-NNC complex and Sanbagawa belt.     

Variscan relicts in Alpine high-P pelitic rocks from Samos (Greece): evidence from multi-stage garnet and its included minerals

Porphyroblastic garnet schists from northern Samos contain in their matrix the assemblage Ca‐rich garnet + phengite + paragonite ± chloritoid equilibrated at ～530 °C and ～19 kbar during early Tertiary metamorphism. These high‐pressure/low‐temperature (HP‐LT) metapelitic rocks also exhibit mineralogical and microstructural evidence of an older, higher temperature metamorphism. Large, centimetre‐sized Fe‐rich garnet showing growth zoning developed discontinuous, <0.5 mm thick, Ca‐rich and Mn‐poor overgrowths, compositionally matching small (<1 mm) high‐P matrix garnet. Because the discontinuous garnet rims are in textural and chemical equilibrium with Alpine high‐P minerals, the central parts of the garnet porphyroblasts were found to have formed prior to the Tertiary metamorphism. This is supported by electron microprobe U‐Th‐Pb dating of monazite inclusions yielding partly reset Variscan ages between 360 and 160 Ma. Monazite‐xenotime and garnet‐muscovite thermometry applied to inclusions in the pre‐Alpine garnet yielded temperatures of 600–625 °C (at 3–8 kbar). Prismatic Al‐rich pseudomorphs, possibly after kyanite/sillimanite, and inclusions in garnet composed of white K‐Na mica + quartz ± albite ± K feldspar, interpreted as possible replacements of an intermediate K‐Na feldspar, further support Variscan amphibolite facies conditions. The Samos metapelites thus experienced higher temperatures during the Variscan than during Alpine metamorphism. Diffusional relaxation was very limited between pre‐Alpine garnet and Alpine garnet; both were filled with Alpine garnet along overgrowths and fractures. Fluid‐mediated intergranular element transport, enhanced by deformation, appears crucial in transforming the Variscan garnet into a grossular richer composition during Alpine subduction‐zone metamorphism. At such conditions, dissolution–reprecipitation appears to be a much more effective mechanism for modifying garnet compositions than diffusion. Amphibolite facies conditions are typical for Variscan basement relics exposed in central Cycladic and Dodecanese islands as well as in eastern Crete. The Samos metapelites studied comprise a north‐eastern extension of these basement occurrences.     

Tectonically driven fluid flow and associated low-grade metamorphism during the Alpine compression in the eastern Iberian Chain (Spain)

Fluid inclusions and clay mineralogy of the Permo-Triassic rocks from the Espina and Espadà Ranges (SE Iberian Chain, Spain) have been investigated to establish their relationship with hydrothermal fluid circulation during the Alpine Orogeny. Primary fluid inclusions in quartz-filled tension gashes in Permo-Triassic sandstones reveal maximum temperatures around 230 °C and very constant salinities of 8.5% wt. eq. NaCl. Secondary fluid inclusions found in quartz from the Santonian Ba–Cu–Hg deposits show similar compositional and thermodynamic characteristics, denoting an Alpine recrystallization. Clay mineral composition of Permo-Triassic mudrocks is characterized by pyrophyillite, indicating low-grade metamorphic conditions. Field observations and experimental data suggest that the crystallization of quartz in tension gashes, the formation of secondary fluid inclusions and the development of the metamorphism are contemporaneous and related to fluid circulation during the Alpine compression. Fluid flow took place along the Hercynian fault system that was reactivated during the Mesozoic rift stage and inverted during the Alpine deformation.     

Diagenesis and very low-grade metamorphism of volcaniclastic sandstones from contrasting geodynamic environments, North Island, New Zealand: the Murihiku and Waipapa terranes

Abstract The Mesozoic Murihiku and Waipapa terranes are two accretionary wedges of linked forearc and trench sediments, respectively, that were juxtaposed in the early Cretaceous.
Late Triassic to late Jurassic Murihiku terrane volcaniclastic sediments are folded into a regional syncline and have been diagenetically altered. There is a general relationship between zeolite occurrence, clay mineralogy, vitrinite reflectance and stratigraphic position. Youngest Jurassic sediments contain heulandite, analcime and stilbite, whereas late Triassic to mid-Jurassic sediments have laumontite and heulandite (in detail the zeolite distribution is complicated). Tuffaceous horizons on the eastern limb of the syncline are calcitized rather than zeolitized. Post-diagenetic fractures associated with uplift are laumontite-filled. The inferred geothermal gradient is c. 15° C km⁻¹.
The Waipapa terrane is an accretionary complex dominated by imbricated terrigenous sediments of Triassic and Jurassic age with enclosed Permian to Jurassic pelagic sediments and basalts. Late Jurassic sediments are massive volaniclastic sandstones. The sediments are non-foliated, and metamorphic minerals in the massive sandstones have crystallized in specific domains. The observed metamorphic succession of prehnite-pumpellyite and pumpellyite-actinolite facies assemblages was overprinted in the imbricated rocks during a thermal event that was late in the deformation sequence and broadly coincident with hydraulic fracturing and veining.
The metamorphic successions in the two terranes and their relationships to structural features are in excellent accord with accretionary complex models.     

Multiple intrusions and low-pressure metamorphism in the central Old Woman Mountains, south-eastern California: constraints from thermal modelling

The Old Woman Mountains in south-eastern California are a Late Cretaceous low-pressure metamorphic terrane where multiple magmatic intrusions generated broad regions of elevated metamorphic temperatures. In the Scanlon Gulch area, two sheet-like, Late Cretaceous granitoid plutons are in contact with the Scanlon shear zone, a 1-km-thick sheet of isoclinally folded and transposed metamorphic rocks. The metaluminous Old Woman granodiorite underlies the shear zone and the peraluminous Sweetwater Wash granite overlies it. Both plutons record emplacement ages of ∼74 Ma. Thermobarometry and phase relations in the shear zone suggest that peak metamorphism was at 650 ± 50† C and 4.3 ± 0.5 kbar. Late Cretaceous metamorphic temperatures were less elsewhere in the Old Woman Mountains, away from the intrusions.
One-dimensional thermal models are used to investigate how differences in the time between the emplacement of plutons would affect the thermal evolution of the central Old Woman Mountains. The prediction of a thermal history inferred from petrological and thermochronological data requires the rapid emplacement of the two plutons around the shear zone; simulations with delays of more than 1 Myr in the emplacement of the second pluton failed to predict peak metamorphic temperatures. Calculations which consider only the emplacement of a single pluton yield metamorphic temperatures that are too low. The time separating the intrusions is by far the most sensitive parameter in the calculations; assumptions concerning the treatment of the initial geothermal gradient and the latent heat of crystallization have relatively small effects on the predicted thermal histories. Our results suggest that for certain geometries, relatively short-lived magmatic events involving rapid emplacement of multiple intrusions can produce low-pressure metamorphism.     

东南沿海早白垩世火山岩的极低级变质作用研究

中国东南沿海广泛分布的白垩纪火山岩中,发育一套具特殊岩相学特征的火山岩地层,以浙江高坞组和福建南园组为代表,其主体为熔结凝灰岩,以深灰色厚层致密块状、含大量长石和石英晶屑、大面积出露但假流动构造模糊为突出特征,而明显区别于其它白垩纪地层组的熔结凝灰岩,表明其具有独特的成岩过程。伊利石结晶度研究表明,高坞组和南园组火山岩特殊的岩相学特征与其成岩后受到极低级变质作用,从而发生一定程度重结晶有关,这一过程使其岩石结构构造趋于均一化。通过对上下火山岩系火山岩的伊利石结晶度、b0值及多型特征对比分析表明,东南沿海这一大范围的火山岩极低级变质作用,可能是早白垩世晚期"闽浙运动"后岩石圈发生强烈伸展,导致区域大规模热隆作用的反映,并与上下火山岩系之间区域性不整合面的形成、长乐-南澳构造带强烈的变质变形事件及东南沿海大规模金铜矿、非金属等成矿作用密切相关,具有重要的研究意义。     

Microcrystals of Th-rich monazite (La) with a negative Ce anomaly in metadiorite and their role for documenting Cretaceous metamorphism in the Slavonian Mountains (Croatia)

Microcrystals (1–15 μm) of unusual monazite (La) with 41–47 mol% cheralite [ThCa(PO₄)₂] component and a strong negative Ce anomaly are described from a metadiorite from the SW Slavonian Mountains, Psunj, Croatia. The dioritic host rock still shows a relictic igneous fabric on macroscopic scale. However, metamorphic reaction textures can be recognized in thin section. These include partial recrystallization of igneous plagioclase to albite coupled with the formation of epidote. Furthermore, partial replacement of igneous hornblende by a fine-grained orthoamphibole-chamosite-epidote paragenesis can be observed and replacement of ilmenite by titanite. The compositions of the metamorphic minerals indicate upper greenschist facies conditions (460–500 °C according to two-feldspar geothermometry) under a high oxygen fugacity. Microstructures show that the monazite crystals belong to the metamorphic paragenesis and formed at the expense of magmatic allanite. Their negative Ce anomalies reflect the oxidizing conditions of metamorphism. Application of the xenotime in monazite solvus geothermometer provides unrealistically high temperatures of ~500–660 °C which disagree with the greenschist facies metamorphic paragenesis. We interpret that the presence of cheralite has a profound effect on the nature of the monazite-xenotime solvus curve and hence the existing calibrations of this geothermometer may be generally unsuitable for cheralite-rich monazite. An important geological result is that the Th-U-total Pb ages of the monazite grains are uniformingly Upper Cretaceous. Our data thus suggest that the imprint of the Alpine orogeny on the Slavonian Mountains was stronger than presumed until now.     

Rb-Sr and U-Pb isotope studies on migmatites from the Schwarzwald (Germany): constraints on isotopic resetting during Variscan high-temperature metamorphism

Mineral and isotope studies were undertaken on migmatites from the Schwarzwald, Moldanubian zone of the Variscan belt. The aims of the study were to date the migmatite formation and to determine the processes involved in migmatization in order to evaluate their influence on isotopic resetting. Textural evidence and the comparison of mineral compositions from leucosomes and mesosomes of two centimetre-scale migmatite profiles, respectively, suggest that migmatitic textures and mineral assemblages were formed by metamorphic segregation (deformation-enhanced mass transport) rather than by partial melting (anatexis). The results of Rb-Sr thin-slab dating on these profiles indicate that Sr isotopes were not completely reset during migmatization. No true isochron ages, but ages of approximate isotopic homogenization were obtained on the thin slabs by calculating ⁸⁷Sr/⁸⁶Sr ratios back to various stages in their evolution. The coincidence of these Rb-Sr data with U-Pb ages of monazites from migmatites and non-migmatitic gneisses shows that gneisses and migmatites were formed during the same high-temperature event in the Carboniferous (330-335 Ma). The observation that high-temperature metamorphism failed to equilibrate Sr isotopes on the centimetre-scale imposes limitations on the use of conventional whole-rock isochron techniques in dating migmatites.     

Cordierite-bearing schists and gneisses from Timor, eastern Indonesia: P-T conditions of metamorphism and tectonic implications

In the Boi Massif of Western Timor the Mutis Complex, which is equivalent to the Lolotoi Complex of East Timor, is composed of two lithostratigraphical components: various basement schists and gneisses; and the dismembered remnants of an ophiolite. Cordierite-bearing pelitic schists and gneisses carry an early mineral assemblage of biotite + garnet + plagioclase + Al-silicate, but contain no prograde muscovite; sillimanite occurs in a textural mode which suggests that it replaced and pseudomorphed kyanite at an early stage and some specimens of pelitic schist contain tiny kyanite relics in plagioclase. Textural relations between, and mineral chemistries of, ferro-magnesian phases in these pelitic chists and gneisses suggest that two discontinuous reactions and additional continuous compositional changes have been overstepped, possibly with concomitant anatexis, as a result of decrease in P_load during high temperature metamorphism. The simplified reactions are: garnet and/or biotite + sillimanite + quartz + cordierite + hercynite + ilmenite + excess components. P-T conditions during the development of the early mineral assemblage in the pelitic gneisses are estimated to have been P + 10 kbar and T > 750°C, based upon the plagioclase-garnet-Al-silicate-quartz geobarometer and the garnet-biotite geothermometer. P-T conditions during the subsequent development of cordierite-bearing mineral assemblages in the pelitic gneisses are estimated to have been P + 5 kbar and T + 700°C with X_H2O < 0.5, based upon the Fe content of cordierite occurring in the assemblage quartz + plagioclase + sillimanite + biotite + garnet + cordierite coexisting with melt. Final equilibration between some of the phases suggests that conditions dropped to P > 2.3 kbar and T > 600°C. A similar exhumation P-T path is suggested for the pelitic schists with early metamorphic conditions of P > 6.2 kbar and T > 745°C and subsequent development of cordierite under conditions in the range P = 3-4 kbar and T = 600-700°C. The tectonic implications of these P-T estimates are discussed and it is concluded that the P-T path followed by these rocks was caused by decompression during rifting and synmetamorphic ophiolite emplacement resulting from processes during the initiation and development of a convergent plate junction located in Southeast Asia during late Jurassic to Cretaceous time.     

大别超高压变质过程中流体作用的微量元素效应及其地球化学意义

对大别地区角闪岩相－麻粒岩相和榴辉岩相岩石进行较系统的地球化学研究,探讨了超高压俯冲变质及折返过程中元素的再分配行为,并就其对变质过程中的部分熔融作用和流体交代作用的指示进行了分析。研究表明,大别榴辉岩原岩在其俯冲过程中发生了部分熔融作用,并在其早期脱水和后期退变质作用过程中与变质流体相之间产生了交换。脱水作用使榴辉岩Ｎｂ／Ｔａ比值升高,但后期的退变质作用使榴辉岩与其寄主围岩间的Ｎｂ／Ｔａ比值相对     

Triassic to Early Jurassic (c. 200 Ma) UHP metamorphism in the Central Rhodopes: evidence from U–Pb–Th dating of monazite in diamond‐bearing gneiss from Chepelare (Bulgaria)

Evidence for ultrahigh‐pressure metamorphism (UHPM) in the Rhodope metamorphic complex comes from occurrence of diamond in pelitic gneisses, variably overprinted by granulite facies metamorphism, known from several areas of the Rhodopes. However, tectonic setting and timing of UHPM are not interpreted unanimously. Linking age to a metamorphic stage is a prerequisite for reconstruction of these processes. Here, we use monazite in diamond‐bearing gneiss from Chepelare (Bulgaria) to date the diamond‐forming UHPM event in the Central Rhodopes. The diamond‐bearing gneiss comes from a strongly deformed, lithologically heterogeneous zone (Chepelare Mélange) sandwiched between two migmatized orthogneiss units, known as Arda‐I and Arda‐II. Diamond, identified by Raman micro‐spectroscopy, shows the characteristic band mostly centred between 1332 and 1330 cm^?1. The microdiamond occurs as single grains or polyphase diamond + carbonate inclusions, rarely with CO₂. Thermodynamic modelling shows that garnet was stable at UHP conditions of 3.5–4.6 GPa and 700–800 °C, in the stability field of diamond, and was re‐equilibrated at granulite facies/partial melting conditions of 0.8–1.2 GPa and 750–800 °C. The texture of monazite shows older central parts and extensive younger domains which formed due to metasomatic replacement in solid residue and/or overgrowth in melt domains. The monazite core compositions, with distinctly lower Y, Th and U contents, suggest its formation in equilibrium with garnet. The U–Th–Pb dating of monazite using electron microprobe analysis yielded a c. 200 Ma age for the older cores with low Th, Y, U and high La/Nd ratio, and a c. 160 Ma age for the dominant younger monazite enriched in Th, Y, U and HREE. The older age of c. 200 Ma is interpreted as the timing of UHPM, whereas the younger age of c. 160 Ma as granulite facies/partial melting overprint. Our results suggest that UHPM occurred in Late Triassic to Early Jurassic time, in the framework of collision and subduction of continental crust after the closure of Paleotethys.     

内蒙古林西—乌兰浩特地区二叠系极低级变质作用与生烃潜力

极低级变质作用对年轻造山带构造演化和油气勘探有重要意义,近年来成为地质学家探讨的热点领域。文章以中国东北地区南缘石炭—二叠系为研究对象,探讨了其极低级变质作用的特征。通过对兴蒙造山带东段林西—乌兰浩特地区二叠纪地层的伊利石结晶度、b₀值和多型等参数进行测试分析发现,伊利石结晶度Kübler指数在0.175~0.47之间,大多数集中于0.25~0.35,平均值为0.30;样品b₀值主要在9.0155~9.0316 ?之间,平均值为9.0249 ?;伊利石多型主要为2M₁。以上结果表明研究区二叠系林西组和哲斯组主要经历了葡萄石-绿纤石相的极低级变质作用,变质温度200~370 ℃,压力为中压,同时表明其具有较高的生烃潜力。结合二叠系生烃期研究和区内广泛发育的白垩纪侵入体推测,二叠系极低级变质作用可能发生在早白垩世,且与岩体侵入导致的地温梯度升高有关。