Cretaceous metamorphism in the Austroalpine Matsch Unit (Eastern Alps): The interrelation between deformation and chemical equilibration processes

In the polymetamorphic Austroalpine Matsch Unit (European Eastern Alps) Cretaceous upper greenschist facies metamorphism overprinted Variscan and Permian magmatic and metamorphic assemblages. Mineral compositional and (micro-)structural data of metapelites and metapegmatites document different mechanisms of interrelated deformation and (re-)equilibration during Cretaceous overprinting: i) Microfractures in relic garnet represented pathways for material transport, and thus established material exchange between intragranular domains and the matrix. Major element equilibration by fast diffusion along microfractures contrasts with limited volume diffusion in adjacent host garnet. ii) Syn-tectonic breakdown of staurolite initially to paragonite, then chloritoid allows correlating reaction progress with the formation of different fracture sets. iii) Syn-tectonic mineral growth with shape-preferred orientation in foliation domains contrasts with radial growth in microlithons and strain shadows of the mylonitic foliation. iv) Syn-tectonic unmixing of pre-existing oligoclase (an_14–16) produced fine-grained aggregates of two supposedly coexisting plagioclase-phases (an_3–6 and an_20–25) in strain shadows of the oligoclase-clasts. v) Pre-existing deformation-induced heterogeneities in the spatial distribution of phases and their preferred orientation influence the kinetics of phase equilibration. Understanding the mechanisms of the mutual interrelation between deformation and phase equilibration is a prerequisite for deducing PT-constraints from strained metamorphic rocks. New garnet—whole rock Sm-Nd data from metapegmatites indicate their emplacement at 263–280 Ma and provide an important age constraint on the interrelated deformation and re-equilibration processes.     

内蒙古锡林浩特地区中元古代锡林浩特岩群的厘定及其意义

通过大比例尺填图和剖面研究, 将内蒙古锡林浩特地区的"锡林郭勒杂岩"解体分为3大部分: 一套表壳岩、晚元古代基性-超基性侵入岩和早古生代酸性侵入岩.在此基础上根据变质岩的岩性组合和变形变质特点, 可将其中的表壳岩化分为4个岩性段: 黑云(石榴石)斜长片麻岩, 间夹多层条纹状斜长角闪岩, 在其顶部多见薄层含磁铁石英岩; 中粗粒黑云斜长片麻岩和细粒长英质片麻岩; 混合岩化条带状黑云(角闪)斜长片麻岩, 含石榴石黑云二长片麻岩夹条纹状斜长角闪岩组合; 夕线石黑云斜长片麻岩, 条纹状黑云斜长片麻岩, 间夹含石榴石黑云母石英片岩.通过对比区域内宝音图群、艾勒格庙组、白乃庙群等古老地块的岩石组合, 发现在原岩建造、变质级别、沉积环境等特征上均有区别, 应将这套表壳岩单独厘定为锡林浩特岩群, 它们组成了锡林郭勒微陆块前寒武变质基底岩系, 其地质特征的研究对锡林郭勒微陆块基底的形成和演化及华北板块与西伯利亚板块的构造关系演化有重要意义.      

Polymetamorphic evolution of pelitic schists and evidence for Permian low-pressure metamorphism in the Vepor Unit, West Carpathians

Phase equilibrium modelling and monazite microprobe dating were used to characterize the polymetamorphic evolution of metapelites from the northern part of the Vepor Unit, West Carpathians. Three generations of garnet and associated metamorphic assemblages found in these rocks correspond to three distinct metamorphic events related to the Variscan orogeny, a Permian phase of crustal extension and the Alpine orogeny. Variscan staurolite‐bearing and Alpine chloritoid‐bearing assemblages record medium‐temperature and medium‐pressure regional metamorphisms reaching 540–570 °C/5–7.5 kbar and 530–550 °C/5–6.5 kbar respectively. The Permian metamorphic assemblage involves garnet, andalusite, sillimanite, biotite, muscovite, plagioclase and corundum and locally forms silica‐undersaturated andalusite‐biotite‐spinel coronas around older staurolite. The transition from andalusite to sillimanite indicates a prograde low‐pressure and medium‐temperature metamorphism characterized by temperature increase from 500 to 650 °C at ～3 kbar. As accessory monazite is abundant in the rocks, an attempt was made to derive its age of formation by means of electron microprobe‐based Th‐U‐Pb chemical dating. Despite the polymetamorphic nature of the metapelites, the monazite yielded uniform Permian ages. Microstructures confirm that monazite was formed in relation to the low‐pressure and medium‐temperature paragenesis, and the weighted average ages obtained for two different samples are 278 ± 5 and 275 ± 12 Ma respectively. The virtual lack of Variscan and Alpine monazite populations points to interesting aspects concerning the growth systematics of monazite in metamorphic rocks.     

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.     

Upper‐amphibolite facies partial melting of paragneisses from the Epupa Complex,NW Namibia,and relations to Mesoproterozoic anorthosite magmatism

The evolution of the mineral assemblages and P–T conditions during partial melting of upper‐amphibolite facies paragneisses in the Orue Unit, Epupa Complex, NW Namibia, is modelled with calculated P–T–X phase diagrams in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O system. The close concordance of predictions from the phase diagrams to petrographic observations and thermobarometric results documents that quantitative phase diagrams are suitable to explain the phase relationships in migmatitic upper‐amphibolite facies low‐ and medium‐pressure metapelites, which occur in many high‐grade metamorphic terranes worldwide. Different mineral assemblages in the migmatitic metapelites of the Orue Unit reflect regional discrepancies in the metamorphic grade: in a Northern Zone, early biotite–sillimanite–quartz assemblages were replaced via melt‐producing reactions by cordierite‐bearing assemblages. In a Southern Zone, they were replaced via melt‐producing reactions by garnet‐bearing assemblages while cordierite is restricted to rare metapelitic granofelses, which preserve Grt–Sil–Crd–Bt peak assemblages. Peak‐metamorphic conditions of 700–750 °C at 5.5–6.7 kbar in the Southern Zone and of ～750 °C at 4.5 kbar in the Northern Zone are estimated by integrating thermobarometric calculations with data from calculated mineral composition isopleths. Retrograde back‐reactions between restite and crystallizing melt are recorded by the replacement of garnet by biotite–sillimanite and/or biotite–muscovite intergrowths. Upper‐amphibolite facies metamorphism and partial melting (c. 1340–1320 Ma) in the rocks of the Southern Zone of the Orue Unit, which underwent probably near‐isobaric heating–cooling paths, are attributed to contact metamorphism induced by the coeval (c. 1385–1319 Ma) emplacement of the Kunene Intrusive Complex, a huge massif‐type anorthosite body. The lower‐pressure metapelites of the Northern Zone are interpreted to record contact metamorphism at an upper crustal level.     

P–T evolution of the Precambrian Metamorphic Complex,NW Iran: a study of metapelitic rocks

The Mahneshan Metamorphic Complex (MMC) is one of the Precambrian terrains exposed in the northwest of Iran. The MMC underwent two main phases of deformation (D₁ and D₂) and at least two metamorphic events (M₁ and M₂). Critical metamorphic mineral assemblages in the metapelitic rocks testify to regional metamorphism under amphibolite‐facies conditions. The dominant metamorphic mineral assemblage in metapelitic rocks (M₁) is muscovite, biotite I, Garnet I, staurolite, Andalusite I and sillimanite. Peak metamorphism took place at 600–620°C and ∼7 kbar, corresponding to a depth of ca. 24 km. This was followed by decompression during exhumation of the crustal rocks up to the surface. The decrease of temperature and pressure during exhumation produced retrograde metamorphic assemblages (M₂). Secondary phases such as garnet II biotite II, Andalusite II constrain the temperature and pressure of M₂ retrograde metamorphism to 520–560°C and 2.5–3.5 kbar, respectively. The geothermal gradient obtained for the peak of metamorphism is 33°C km⁻¹, which indicates that peak metamorphism was of Barrovian type and occurred under medium‐pressure conditions. The MMC followed a ‘clockwise’ P–T path during metamorphism, consistent with thermal relaxation following tectonic thickening. The bulk chemistry of the MMC metapelites shows that their protoliths were deposited at an active continental margin. Together with the presence of palaeo‐suture zones and ophiolitic rocks around the high‐grade metamorphic rocks of the MMC, these features suggest that the Iranian Precambrian basement formed by an island‐arc type cratonization. Copyright © 2010 John Wiley & Sons, Ltd.     

Recrystallization of quartzofeldspathic rocks in a paleozoic thrust belt, Grand Lake, Newfoundland: Implications for garnet-biotite thermometry

Migmatitic granitic gneiss and associated garnetiferous granite at Grand Lake are tectonically interleaved with high-pressure ( 9 kbar) metapelites that contain Barrovian (Ky-St-Grt-Bt-Ms-Rt) mineral assemblages. The migmatites contain metabasites that are compositionally similar to the latest Proterozoic Long Range dykes, suggesting that the gneisses correlate to Grenvillian inliers in western Newfoundland.

In the metapelites, the Barrovian porphyroblastic assemblage overprints microfolds and is itself overprinted by greenschist-facies assemblages in thrust-related high-strain zones. Evidence that the quartzofeldspathic rocks also experienced Barrovian metamorphism is provided by low Ti/>Al amphiboles in metabasites in the gneiss, and, despite the migmatitic character of these rocks, the presence of growth-zoned garnets with bell-shaped compositional profiles (e.g., rimward increase in X_Prp and decrease in X_Sps) similar to those exhibited by garnet in the metapelite. The range of garnet-biotite paleotemperatures for the quartzofeldspathic rocks is only marginally higher (T_max=530–660°C) than that determined for the metapelites (T_max=500–615°C). There are no systematic differences in Grt-Bt temperatures between (proto) mylonitic rocks and nearby precursors.

These results indicate that the quartzofeldspathic rocks and metapelites share a common, post-migmatization (of the gneisses) metamorphic history; they may well have originally been linked by a basement/cover relationship. Mineralogical evidence for the earlier, higher grade metamorphic history of the migmatites has virtually been eradicated even outside the high-strain zones. In this area, there is no clear relation between qualitatively-estimated superimposed strain and the degree of resetting of the Grt-Bt geothermometer.     

Metapegmatites in the western Bohemian massif: ages of crystallisation and metamorphic overprint, as constrained by U–Pb zircon, monazite, garnet, columbite and Rb–Sr muscovite data

Granitic metapegmatites of two crystalline units of the western Bohemian massif, the Zone of Erbendorf-Vohenstrauss in Germany (ZEV), and the Zone of Teplá-Doma?lice in the Czech Republic (ZTD) have been dated by means of U–Pb and Rb–Sr methods. Ages interpreted to reflect emplacement and crystallisation of the pegmatites were found consistently to be approximately 480?Ma, as constrained by U–Pb analyses of primary magmatic zircon, monazite, garnet and columbite, and by Rb–Sr analyses of large pegmatitic muscovites. Later Devonian amphibolite-facies metamorphism caused ductile shearing of the pegmatites, leading to partial recrystallisation of pegmatitic material. A metamorphic, fine-grained generation of muscovite yielded consistent Rb–Sr ages of 371–376?Ma for both the ZEV and the ZTD, interpreted as dating the end of deformational activity. The Rb–Sr system of the large pegmatitic muscovites turned out to remain closed up to metamorphic temperatures of >600?°C. Deformation at elevated temperatures is identified as the dominant mechanism for opening of the Rb–Sr system of primary muscovites: apparent ages grade towards Devonian ages as the muscovites become more deformed and fragmented. The data derived from the metapegmatites point to a similar or common tectonometamorphic evolution for the ZEV and the ZTD since the Ordovician. Furthermore, the magmatic formation of granitic pegmatites implies an upper crustal position of the intruded rocks in Lower Ordovician times, clearly documenting the existence of two distinct metamorphic cycles. In contrast to this, in the neighbouring part of the Moldanubian, only unmetamorphosed granitic pegmatites with intrusion ages around 320?Ma are observed.     

南秦岭佛坪片麻岩穹隆变质作用及与岩浆作用的关系

南秦岭佛坪片麻岩穹隆是国内片麻岩穹隆的典型代表之一,记录了碰撞造山到后造山伸展等多阶段构造演化信息,对揭示秦岭造山带早中生代构造演化具有重要的意义.岩石组合、岩相学、矿物化学和传统温压计研究表明,佛坪片麻岩穹隆具有明显的变质分带特征,以穹隆核部为中心,向南北两侧的盖层变质温度逐渐降低,压力并没有出现随地温梯度的相应变化.不同类型岩石的锆石LA-ICP-MS U-Pb定年和变泥质岩的独居石SHRIMP U-Pb定年给出了一致的变质时代,为206~196 Ma.结合南秦岭早中生代岩浆作用研究,认为佛坪片麻岩穹隆递增变质作用与三叠纪末期的岩浆底辟有关,片麻岩穹隆形成于后碰撞伸展环境.      

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

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.     

Polymetamorphism in garnet micaschists of the Saualpe Eclogite Unit (Eastern Alps,Austria), resolved by automated SEM methods and EMP–Th–U–Pb monazite dating

Polymetamorphic garnet micaschists from the Austroalpine Saualpe Eclogite Unit (Kärnten, Austria, Eastern Alps) display complex microstructural and mineral–chemical relationships. Automated scanning electron microscopy routines with energy dispersive X‐ray (EDX) spectral mapping were applied for monazite detection and garnet mineral–chemical characterization. When the Fe, Mg, Mn and Ca element wt% compositions are used as generic labels for garnet EDX spectra, complex zonations and porphyroblast generations can be resolved in complete thin sections for selective electron‐microprobe analyses. Two garnet porphyroblast generations and diverse monazite age populations have been revealed in low‐Ca and high‐Al‐metapelites. Garnet 1 has decreasing Mn, constant Ca and significantly increasing Mg from cores to rims. Geothermobarometry of garnet 1 assemblages signals a crystallization along a M1 prograde metamorphism at ~650 °C/6–8 kbar. Sporadic monazite 1 crystallization started at c. 320 Ma. Subsequent pervasive 300–250 Ma high‐Y and high‐Gd monazite 1 formation during decompression coincided with the intrusion of Permian and Early Triassic pegmatites. Monazite 1 crystallized along the margin of garnet 1. Coronas of apatite and allanite around the large 320–250 Ma monazite signal a retrogressive stage. These microstructures suggest a Carboniferous‐to‐Early‐Permian age for the prograde M1 event with garnet 1. Such a M1 event at an intermediate‐P/T gradient has not yet been described from the Saualpe, and preceded a Permo‐Triassic low‐P stage. The M2 event with garnet 2 postdates the corona formation around Permian monazite. Garnet 2 displays first increasing X_Ca at decreasing X_Mg, then increasing X_Ca and X_Mg, and finally decreasing X_Ca with increasing X_Mg, always at high Ca and Mg, and low Mn. This records a P–T evolution which passed through eclogite facies conditions and reached maximum temperatures at ~750 °C/14 kbar during decompression‐heating. A monazite 2 population (94–86 Ma) with lower Y and Gd contents crystallized at decreasing pressure during the Cretaceous (Eo‐Alpine) metamorphism M2 at a high‐P/T gradient. The Saualpe Eclogite Unit underwent two distinct clockwise metamorphic cycles at different P–T conditions, related to continental collisions under different thermal regimes. This led to a characteristic distribution pattern of monazite ages in this unit which is different from other Austroalpine basement areas.     

Metamorphic petrology and zircon geochronology of high-grade rocks from the central Mozambique Belt of Tanzania: crustal recycling of Archean and Palaeoproterozoic material during the Pan-African orogeny

New data on the metamorphic petrology and zircon geochronology of high‐grade rocks in the central Mozambique Belt (MB) of Tanzania show that this part of the orogen consists of Archean and Palaeoproterozoic material that was structurally reworked during the Pan‐African event. The metamorphic rocks are characterized by a clockwise P–T path, followed by strong decompression, and the time of peak granulite facies metamorphism is similar to other granulite terranes in Tanzania. The predominant rock types are mafic to intermediate granulites, migmatites, granitoid orthogneisses and kyanite/sillimanite‐bearing metapelites. The meta‐granitoid rocks are of calc‐alkaline composition, range in age from late Archean to Neoproterozoic, and their protoliths were probably derived from magmatic arcs during collisional processes. Mafic to intermediate granulites consist of the mineral assemblage garnet–clinopyroxene–plagioclase–quartz–biotite–amphibole ± K‐feldspar ± orthopyroxene ± oxides. Metapelites are composed of garnet‐biotite‐plagioclase ± K‐feldspar ± kyanite/sillimanite ± oxides. Estimated values for peak granulite facies metamorphism are 12–13 kbar and 750–800 °C. Pressures of 5–8 kbar and temperatures of 550–700 °C characterize subsequent retrogression to amphibolite facies conditions. Evidence for a clockwise P–T path is provided by late growth of sillimanite after kyanite in metapelites. Zircon ages indicate that most of the central part of the MB in Tanzania consists of reworked ancient crust as shown by Archean (c. 2970–2500 Ma) and Palaeoproterozoic (c. 2124–1837 Ma) protolith ages. Metamorphic zircon from metapelites and granitoid orthogneisses yielded ages of c. 640 Ma which are considered to date peak regional granulite facies metamorphism during the Pan‐African orogenic event. However, the available zircon ages for the entire MB in East Africa and Madagascar also document that peak metamorphic conditions were reached at different times in different places. Large parts of the MB in central Tanzania consist of Archean and Palaeoproterozoic material that was reworked during the Pan‐African event and that may have been part of the Tanzania Craton and Usagaran domain farther to the west.     

超高压变质岩的放射性同位素体系及年代学方法

要深刻理解同位素在超高压变质及退变质过程中的地球化学行为对获得超高压变质岩准确并有明确意义的年龄值是非常重要的。对Sm—Nd,Rb—Sr同位素体系,只有变质矿物同位素体系达到平衡才能给出精确有意义的等时线年龄。研究表明,与副变质岩互层的细粒榴辉岩的高压变质矿物之间,或者强退变质岩石的退变质矿物之间,其Nd,Sr同位素可以达到平衡;然而高压变质矿物与退变质矿物之间Nd,Sr同位素不平衡。由于全岩样品总是含有数量不等的退变质矿物,因此石榴石＋全岩Sm—Nd法或多硅白云母＋全岩Rb—Sr法将有可能给出无地质意义的年龄。通常低温榴辉岩的高压变质矿物之间存在Nd同位素不平衡。超高压变质岩多硅白云母所含过剩Ar主要源于榴辉岩原岩中角闪石在变质分解时释放出来的放射成因Ar。因此,不舍榴辉岩的花岗片麻岩多硅白云母基本不舍过剩Ar。对变质锆石成因的准确判断是正确理解锆石U-Pb年龄意义的关键。本文对不同成因锆石的判别标志及年龄意义做了总结,并指出将阴极发光图形,锆石痕量元素组成及矿物包裹体鉴定相结合是进行锆石成因鉴定的有效方法。高压变质或退变质增生锆石组成单一,是理想变质定年对象。然而变质重结晶锆石域常是重结晶锆石和继承晶质锆石的混合区,因而给出混合年龄。只有完全变质重结晶锆石才能给出准确变质时代。     

甘肃敦煌东巴兔山地区敦煌杂岩的变质 变形特征及时代限定

敦煌地块位于塔里木克拉通的东南缘,是塔里木克拉通前寒武纪岩石出露的重要地区之一。敦煌地块包括北阿尔金 地区和敦煌地区,区域内出露的最古老岩石分别被称为米兰杂岩和敦煌杂岩,文章研究重点是敦煌地区出露的敦煌杂岩。 敦煌杂岩由TTG质片麻岩和变质表壳岩组成,代表性岩石组合包括英云闪长质片麻岩、奥长花岗质片麻岩、花岗闪长质片 麻岩、石榴石斜长角闪岩、石榴石黑云母片岩以及大理岩。文章对东巴兔山干沟地区的花岗闪长质片麻岩样品进行了 LA-ICP-MS锆石U-Pb测年,对石榴石黑云母片岩样品进行了矿物电子探针分析,同时对敦煌杂岩开展了构造变形研究,目 的是揭示敦煌杂岩的形成时代、构造变形特征以及变质温压条件。花岗闪长质片麻岩样品的结晶年龄为2057±75 Ma,并且 记录了~1.88 Ga和~400 Ma两期变质作用,分别与古元古代和古生代的造山事件相关。构造变形研究表明敦煌杂岩记录了两 期变形作用,分别为区域上的右形韧性剪切作用（S1） 和南北向褶皱作用（S2）,且右形韧性剪切作用（S1） 和~400 Ma的变 质作用是配套同期的。此外,对石榴石黑云母片岩样品的分析表明,~400 Ma这期变质作用变质峰期的矿物组合主要为石 榴石+黑云母+斜长石+石英,且变质温压条件为667~690 ℃/0.88~0.89 GPa,代表了高角闪岩相的变质作用。     

Quantitative determination of metamorphic reaction history: mass balance relations between groundmass and mineral inclusion assemblages in metamorphic rocks

Qualitative and quantitative information about metamorphic reaction history and PT paths may be obtained from mineral inclusions in garnet by comparing the mineralogy, distribution, and compositions of paragenetically-related inclusions with minerals in the groundmass assemblage. Using the algebraic technique of singular value decomposition (SVD), we document mass balance relations between inclusion and groundmass assemblages in metapelitic rocks from two metamorphic terranes that experienced different peak metamorphic conditions, and whose transition from inclusion to groundmass assemblage records different PT path segments relative to peak conditions. We calculate mass balances relating an inclusion assemblage consisting in part of armored relics of chloritoid to groundmass mineral assemblages in a kyanite-staurolite mica schist from the Solitude Range, British Columbia, and an inclusion assemblage of kyanite, staurolite, and rutile to groundmass minerals in a sillimanite-cordierite gneiss from the Skagit Gneiss, North Cascade Range, Washington. Mass balances for each rock are consistent with reaction histories inferred from petrographic observations. In the Solitude Range schist, the results of mass balance calculations are consistent with the growth of staurolite and garnet at the expense of chloritoid during prograde metamorphism and suggest that chlorite, although not preserved as an inclusion, was involved in initial staurolite growth. In the Skagit sillimanite gneiss, mass balance relations exist between the inclusion suite, which formed during high pressure metamorphism, and the associated groundmass assemblage, which equilibrated at high temperature but much lower pressure. Mass balance does not exist between the groundmass of the Skagit sillimanite gneiss and the groundmass of a nearby kyanite-staurolite schist that has been proposed as a possible lower-grade equivalent of the sillimanite-bearing rocks. These results indicate that, although compositional modification and selective preservation of minerals must be taken into account, mineral inclusion suites may nevertheless preserve enough compositional information to allow reconstruction of complete or nearly complete pre-existing assemblages. This information may not be retrievable from any other source if no lower-grade equivalents of the rocks of interest are exposed.     

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 ± Kfeldspar + biotite + muscovite + kyanite ± sillimanite), 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°C at 7.3–7.4 kbar for the M1 stage, 661–800°C at 9.4–11.6 kbar for the M2 stage, and 579–713°C 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.     

Metamorphism of the Basement of the Qilian Fold Belt in the Minhe-Ledu Area, Qinghai Province, NW China

The basement of the central Qilian fold belt exposed along the Minhe-Ledu highway consists of psammitic schists, metabasitic rocks, and crystalline limestone. Migmatitic rocks occur sporadically among psammitic schist and metabasitic rocks. The mineral assemblage of psammitic schist is muscovite + biotite + feldspar + quartz ± tourmaline ± titanite ± sillimanite and that of metabasitic rocks is amphibole + plagioclase + biotite ± apatite ± magnetite ± pyroxene ± garnet ± quartz. The migmatitic rock consists of leucosome and restite of various volume proportions; the former consists of muscovite + alkaline feldspar + quartz ± garnet ± plagioclase while the latter is either fragments of psammitic schist or those of metabasitic rock. The crystalline limestone consists of calcite that has been partly replaced by olivine. The olivine was subsequently altered to serpentine. Weak deformations as indicated by cleavages and fractures were imposed prominently on the psammitic schists, occasionally on me     

孟中榴辉岩形成和演化和P-T轨迹

本文研究了江苏东海县孟中榴辉岩富铝单斜辉石中石榴子石出溶作用,利用温压计对榴辉岩中各阶段矿物组合进行了温压估算,由此建立了孟中榴辉岩形成和演化的P-T轨迹。结果表明,该榴辉岩经历了地幔条件下由于降温和可能的升压作用,富铝单斜辉石中出溶石榴子石并形成榴辉岩;榴辉岩构造侵位到高角闪岩相片麻岩基底中;退化变质这3个阶段的演化过程。      

胶东海阳所堆积辉长岩的变质反应结构、石榴石形成及P—T演化

海阳所堆积辉长岩由橄长岩、橄榄辉长岩和辉长岩组成。在橄长岩和橄榄辉长岩中发育有典型的变质反应结构：主要为橄榄石与斜长石之间形成由斜方辉石、尖晶石、角闪石和石榴石等矿物组成的多期次次变边，并有三个不同世代变质矿物，早期Cpx＋Opx＋Spl，中期Amp，晚期Grt；期次是钛铁矿与斜长石之间形成石榴石次变边，相对比较简单，只有一个世代变质矿物，为Grt＋Amp＋Rut或Grt＋Rut岩中石榴石是通过斜长石与角闪石或斜长石与钛铁矿之间的变质反应形成的，虽为峰值变质作用的产物，但变质反应的期次及类型不同导致了所形成石榴石的温度和压力有所不同。堆积辉长岩形成演化的温压计算表明，堆积辉长岩在经过近等压降温的岩浆作用之后的变质作用早期，仍为近等压降温，而晚期则表现为近等温升压。这一特殊的P－T演化可能反映了堆积辉长冷侵位与深俯冲特征。     

Constraining the conditions of Barrovian metamorphism in Sikkim,India: P–T–t paths of garnet crystallization in the Lesser Himalayan Belt

Garnet crystallization in metapelites from the Barrovian garnet and staurolite zones of the Lesser Himalayan Belt in Sikkim is modelled utilizing Gibbs free energy minimization, multi‐component diffusion theory and a simple nucleation and growth algorithm. The predicted mineral assemblages and garnet‐growth zoning match observations remarkably well for relatively tight, clockwise metamorphic P–T paths that are characterized by prograde gradients of ～30 °C kbar^?1 for garnet‐zone rocks and ～20 °C kbar^?1 for rocks from the staurolite zone. Estimates for peak metamorphic temperature increase up‐structure toward the Main Central Thrust. According to our calculations, garnet stopped growing at peak pressures, and protracted heating after peak pressure was absent or insignificant. Almost identical P–T paths for the samples studied and the metamorphic continuity of the Lesser Himalayan Belt support thermo‐mechanical models that favour tectonic inversion of a coherent package of Barrovian metamorphic rocks. Time‐scales associated with the metamorphism were too short for chemical diffusion to substantially modify garnet‐growth zoning in rocks from the garnet and staurolite zones. In general, the pressure of initial garnet growth decreases, and the temperature required for initial garnet growth was reached earlier, for rocks buried closer toward the MCT. Deviations from this overall trend can be explained by variations in bulk‐rock chemistry.