Decompressed eclogites in the Sveconorwegian (–Grenvillian) orogen of SW Sweden: petrology and tectonic implications

Relict eclogites and associated high-pressure rocks are present in the Eastern Segment of the SW Swedish gneiss region (the tectonic counterpart of the Parautochthonous Belt of the Canadian Grenville). These rocks give evidence of Sveconorwegian eclogite facies metamorphism and subsequent pervasive reworking and deformation at granulite and amphibolite facies conditions. The best-preserved eclogite relics suggest a clockwise P–T –t history, beginning in the amphibolite facies, progressing through the eclogite facies, decompressing and partially reequilibrating through the high- and medium-pressure granulite facies, before cooling through the amphibolite facies. Textures demonstrate the former coexistence of the plagioclase-free assemblages garnet+clinopyroxene+quartz+rutile+ilmenite, garnet+clinopyroxene+ kyanite+rutile, and garnet+kyanite+quartz+rutile. The former existence of omphacite is evidenced by up to 45 vol.% plagioclase expelled as small grains within large clinopyroxene. Matrix plagioclase is secondary and occurs expelled from clinopyroxene or in fine-grained, granulite facies reaction domains formed during resorption of garnet and kyanite. Garnet shows preserved prograde growth zoning with rimward increasing pyrope content, decreasing spessartine content and decreasing Fe/(Fe+Mg) ratio, but is partly resorbed and reequilibrated at the rims. P–T estimates from microdomains with clinopyroxene+plagioclase+quartz+garnet indicate pressures of 9.5–12 kbar and temperatures of 705–795 °C for a stage of the granulite facies decompression. The preservation of the prograde zoning suggests that the rocks did not reside at these high temperatures for more than a few million years, and chemical disequilibrium and ‘frozen’ reaction textures indicate heterogeneous reaction progress and overstepping of reactions during the decompression through the granulite facies. Together these features suggest a rapid tectonic exhumation. The eclogite relics occur within a high-grade deformation zone with WNW–ESE stretching and associated oblique normal-sense, top-to-the-east (sensu lato) displacement, suggesting that extension was a main cause for the decompression and exhumation. Probable tectonic scenarios for this deformation are Sveconorwegian late-orogenic gravitational collapse or overall WNW–ESE extension.     

Eclogites with a short-lived granulite facies overprint in the Moldanubian Zone,Czech Republic: petrology,geochemistry and diffusion modelling of garnet zoning

The petrology and geochemistry of a newly discovered suite of high-pressure garnet + clinopyroxene-bearing rocks from the Monotonous Series of the Moldanubian Zone of the Bohemian Massif, southwest Czech Republic have been investigated. Three types [common eclogites, quartz ± kyanite ± (clino)zoisite eclogites and garnet-hornblende-clinopyroxenites] are distinguished by petrography and geochemistry. All underwent a significant degree of partial breakdown under granulite and amphibolite facies conditions during exhumation. Important features include the growth of orthopyroxene in breakdown domains after garnet and omphacite and anorthite + spinel ± corundum ± exceedingly peraluminous sapphirine replacing kyanite. Garnet zoning and inclusion patterns support a prograde evolution from low pressures for at least some of the samples. The post-eclogite stage granulite facies overprint indicates that high temperatures prevailed during exhumation, but preservation of zoning in some garnets and the results of diffusion modelling suggest that this overprint took place over a very short time-scale. The geochemical and petrological results allow characteristic differences to be recognized between these eclogites and metabasites found in other tectonic units of South Bohemia and consequently the assigning of all high-pressure rocks to a single, now disrupted, tectonic unit is a gross simplification that seriously misrepresents the tectono-metamorphic history of the region.     

Formation of eclogite, and reaction during exhumation to mid-crustal levels, Snowbird tectonic zone, western Canadian Shield

A re‐evaluation of the P–T history of eclogite within the East Athabasca granulite terrane of the Snowbird tectonic zone, northern Saskatchewan, Canada was undertaken. Using calculated pseudosections in combination with new garnet–clinopyroxene and zircon and rutile trace element thermometry, peak metamorphic conditions are constrained to ～16 kbar and 750 °C, followed by near‐isothermal decompression to ～10 kbar. Associated with the eclogite are two types of occurrences of sapphirine‐bearing rocks preserving a rich variety of reaction textures that allow examination of the retrograde history below 10 kbar. The first occurs as a 1–2 m zone adjacent to the eclogite body with a peak assemblage of garnet–kyanite–quartz interpreted to have formed during the eclogite facies metamorphism. Rims of orthopyroxene and plagioclase developed around garnet, and sapphirine–plagioclase and spinel–plagioclase symplectites developed around kyanite. The second variety of sapphirine‐bearing rocks occurs in kyanite veins within the eclogite. The veins involve orthopyroxene, garnet and plagioclase layers spatially organized around a central kyanite layer that are interpreted to have formed following the eclogite facies metamorphism. The layering has itself been modified, with, in particular, kyanite being replaced by sapphirine–plagioclase, spinel–plagioclase and corundum–plagioclase symplectites, as well as the kyanite being replaced by sillimanite. Petrological modelling in the CFMAS system examining chemical potential gradients between kyanite and surrounding quartz indicates that these vein textures probably formed during further essentially isothermal decompression, ultimately reaching ～7 kbar and 750 °C. These results indicate that the final reaction in these rocks occurred at mid‐crustal levels at upper amphibolite facies conditions. Previous geochronological and thermochronological constraints bracket the time interval of decompression to <5–10 Myr, indicating that ～25 km of exhumation took place during this interval. This corresponds to minimum unroofing rates of ～2–5 mm year^?1 following eclogite facies metamorphism, after which the rocks resided at mid‐crustal levels for 80–100 Myr.     

Tectonic implications from sapphirine-bearing lithologies, south-west Grenville Province, Canada

Sapphirine has been found in two types of magnesian, metabasic lenses from tectonite zones within the Central Gneiss Belt of the south-west Grenville Province, Canada. The first type (association I) comes from a lenticular mafic lens within highly tectonized anorthosite, the second type (association II) comes from meta-eclogitic pods with foliated amphibolite rims. In each case the sapphirine-bearing assemblages record a wealth of reaction textures. The primary mineralogy in association II is represented by high alumina clinopyroxene, garnet and kyanite ± plagioclase and records pressures of around 14-16 kbar; in association I the primary mineralogy is represented by plagioclase, two pyroxenes and possibly olivine but here the equilibrium pressure is unknown.
The host gneisses equilibrated at approximately 8 to 10 kbar and 700-750°C by continuous cation exchange reactions during and after the culmination of the Grenvillian orogeny at 1.16-1.0 Ga. It is unlikely that the higher pressures recorded in the meta-eclogitic pods represent an earlier high-pressure metamorphism as the pods are restricted to shear zones. A tectonic mode of emplacement into a crust undergoing granulite facies metamorphism is more likely. Sapphirine formed by discontinuous decompression reactions; in association II this involved a reaction between garnet and kyanite and resulted in the formation of magnesian granulite facies assemblages. At the same time primary clinopyroxene became much less aluminous by evolving plagioclase. Pressures and temperatures from coexisting phases, that are believed to have equilibrated at the same time as sapphirine formation, are estimated as 11 to 12 kbar and 750°C. These probably represent the peak conditions for granulite facies metamorphism in the south-west Grenville Province.     

Petrology of ultrahigh-pressure rocks from the southern Su-Lu region, eastern China

Abstract In the Su-Lu ultrahigh- P terrane, eastern China, many coesite-bearing eclogite pods and layers within biotite gneiss occur together with interlayered metasediments now represented by garnet-quartz-jadeite rock and kyanite quartzite. In addition to garnet + omphacite + rutile + coesite, other peak-stage minerals in some eclogites include kyanite, phengite, epidote, zoisite, talc, nyböite and high-Al titanite. The garnet-quartz-jadeite rock and kyanite quartzite contain jadeite + quartz + garnet + rutile ± zoisite ± apatite and quartz + kyanite + garnet + epidote + phengite + rutile ± omphacite assemblages, respectively. Coesite and quartz pseudomorphs after coesite occur as inclusions in garnet, omphacite, jadeite, kyanite and epidote from both eclogites and metasediments. Study of major elements indicates that the protolith of the garnet-quartz jadeite rock and the kyanite quartzite was supracrustal sediments. Most eclogites have basaltic composition; some have experienced variable 'crustal'contamination or metasomatism, and others may have had a basaltic tuff or pyroclastic rock protolith.
The Su-Lu ultrahigh- P rocks have been subjected to multi-stage recrystallization and exhibit a clockwise P-T path. Inclusion assemblages within garnet record a pre-eclogite epidote amphibolite facies metamorphic event. Ultrahigh- P peak metamorphism took place at 700–890° C and P >28 kbar at c . 210–230 Ma. The symplectitic assemblage plagioclase + hornblende ± epidote ± biotite + titanite implies amphibolite facies retrogressive metamorphism during exhumation at c . 180–200 Ma. Metasedimentary and metamafic lithologies have similar P-T paths. Several lines of evidence indicate that the supracrustal rocks were subducted to mantle depths and experienced in-situ ultrahigh- P metamorphism during the Triassic collision between the Sino-Korean and Yangtze cratons.     

Late Sveconorwegian (Grenville) high-pressure granulite facies metamorphism in southwest Sweden

Mafic granulite, garnet amphibolite and charnockite occur in the southwest Swedish part of the Baltic Shield. This part is generally considered to be the continuation of the Grenville collisional belt in Canada. The area with granulite facies rocks, the Southwest Swedish Granulite Region (SGR), is considerably larger than previously thought. The SGR is bounded to the east and west by two major tectonic zones. The first quantitative age data and P–T determinations for the high-grade metamorphism in the SGR are presented.
Conventional geothermobarometry was applied to mafic granulites from five localities. The estimated P–T conditions for the peak of metamorphism range from 705°C and 8.1 kbar at Hallandsås in the south, to 770°C and 10.5 kbar at Ullared in the north (medium- to high- P granulite facies conditions). Sm–Nd geochronology on minerals from the mafic granulites at Hallandsås and Ullared give late Sveconorwegian (Grenville) ages of 907 ± 12 and 916 ± 11 Ma for the high-grade metamorphism, which is considerably younger than previously thought.
Our results stress the hitherto underestimated importance of the late Sveconorwegian high-grade metamorphism in the southwestern part of the Baltic Shield.     

Petrogenesis of kyanite‐ and corundum‐bearing mafic granulite in a meta‐ophiolite,SE Turkey

Although ophiolitic rocks are abundant in Anatolia (Turkey), only in rare cases have they experienced high‐grade metamorphism. Even more uncommon, in Anatolia and elsewhere are high‐grade meta‐ophiolites that retain an oceanic lithosphere stratigraphy from upper crustal mafic volcanic rocks through lower crustal gabbro to mantle peridotite. The Berit meta‐ophiolite of SE Turkey exhibits both features: from structurally higher to lower levels, it consists of garnet amphibolite (metabasalt), granulite facies metagabbro (as lenses in amphibolite inferred to be retrogressed granulite) and metaperidotite (locally with metapyroxenite layers). Whole‐rock major and trace‐element data indicate a tholeiitic protolith that formed in a suprasubduction setting. This paper presents new results for the metamorphic P–T conditions and path of oceanic lower crustal rocks in the Berit meta‐ophiolite, and an evaluation of the tectonic processes that may drive granulite facies metamorphism of ophiolite gabbro. In the Do?an?ehir (Malatya, Turkey) region, granulite facies gabbroic rocks contain garnet (Grt)+clinopyroxene (Cpx)+plagioclase (Pl)+corundum (Crn)±orthopyroxene (Opx)±kyanite (Ky)±sapphirine (Spr)±rutile. Some exhibit symplectites consisting of Crn+Cpx, Ky+Cpx and/or coronas of garnet (outer shell) around a polygonal aggregate of clinopyroxene that in some cases surrounds a polygonal aggregate of orthopyroxene. Coronitic and non‐coronitic textures occur in proximity in mm‐ to cm‐scale layers; corona structures typically occur in plagioclase‐rich layers. Their formation is therefore related primarily to protolith type (troctolite v. gabbro) rather than P–T path. Phase diagrams calculated for a kyanite‐rich granulite, a plagioclase‐rich non‐coronitic granulite, and a plagioclase‐rich coronitic granulite (taking into account changes in effective bulk composition during texture development) predict peak conditions of ~800°C, 1.1–1.5 GPa; these conditions do not require invoking an unusually high geothermal gradient. In the coronitic metagabbro, reaction textures formed along the prograde path: Crn–Cpx symplectites grew at the expense of garnet, sapphirine and plagioclase. Peak conditions were followed by isobaric cooling of ~150°C. Hornblende–plagioclase thermometry results for host amphibolite (Hbl+Pl±Crn±Grt±relict Cpx) indicate retrograde conditions of 620–675°C and 0.5–0.8 GPa accompanied by infiltration of H₂O‐rich fluid. This anticlockwise P–T path differs from an isothermal decompression path previously proposed for these rocks based on the presence of symplectite. Metamorphism of the ophiolitic rocks was driven by closing of the southern Neotethys Ocean, as oceanic lithosphere was obducted (most SE Anatolian ophiolites) or underthrust (Berit meta‐ophiolite). This was followed by subduction of a continental margin, driving cooling of the Berit granulite after the thermal peak at depths of ~40 km.     

An Early Palaeozoic HP/HT granulite–garnet peridotite association in the south Altyn Tagh, NW China: P–T history and U-Pb geochronology

High‐pressure kyanite‐bearing felsic granulites in the Bashiwake area of the south Altyn Tagh (SAT) subduction–collision complex enclose mafic granulites and garnet peridotite‐hosted sapphirine‐bearing metabasites. The predominant felsic granulites are garnet + quartz + ternary feldspar (now perthite) rocks containing kyanite, plagioclase, biotite, rutile, spinel, corundum, and minor zircon and apatite. The quartz‐bearing mafic granulites contain a peak pressure assemblage of garnet + clinopyroxene + ternary feldspar (now mesoperthite) + quartz + rutile. The sapphirine‐bearing metabasites occur as mafic layers in garnet peridotite. Petrographical data suggest a peak assemblage of garnet + clinopyroxene + kyanite + rutile. Early kyanite is inferred from a symplectite of sapphirine + corundum + plagioclase ± spinel, interpreted to have formed during decompression. Garnet peridotite contains an assemblage of garnet + olivine + orthopyroxene + clinopyroxene. Thermobarometry indicates that all rock types experienced peak P–T conditions of 18.5–27.3 kbar and 870–1050 °C. A medium–high pressure granulite facies overprint (780–820 °C, 9.5–12 kbar) is defined by the formation of secondary clinopyroxene ± orthopyroxene + plagioclase at the expense of garnet and early clinopyroxene in the mafic granulites, as well as by growth of spinel and plagioclase at the expense of garnet and kyanite in the felsic granulite. SHRIMP II zircon U‐Pb geochronology yields ages of 493 ± 7 Ma (mean of 11) from the felsic granulite, 497 ± 11 Ma (mean of 11) from sapphirine‐bearing metabasite and 501 ± 16 Ma (mean of 10) from garnet peridotite. Rounded zircon morphology, cathodoluminescence (CL) sector zoning, and inclusions of peak metamorphic minerals indicate these ages reflect HP/HT metamorphism. Similar ages determined for eclogites from the western segment of the SAT suggest that the same continental subduction/collision event may be responsible for HP metamorphism in both areas.     

柴北缘都兰高压麻粒岩的锆石U-Pb定年及其地质意义

在柴北缘高压-超高压变质带的东端都兰地区,高压麻粒岩以透镜体的形式存在于石榴白云母片岩、花岗质片麻岩以及斜长角闪岩中。高压麻粒岩的主体为基性麻粒岩,并含少量中酸性麻粒岩。基性麻粒岩主要由石榴子石、单斜辉石、斜长石和石英等组成,而中酸性麻粒岩峰期矿物组合为:石榴子石+斜长石+钾长石+蓝晶石+石英±单斜辉石。根据显微构造和反应结构特征,主要识别出3期变质作用:①峰期高压麻粒岩相阶段(M1);②退变质高角闪岩相阶段(M2);③绿片岩相/低角闪岩相阶段(M3)。选取典型的中酸性麻粒岩样品进行了锆石LA-ICP-MSU-Pb原位定年分析,获得加权平均年龄为446.9±6.5Ma,且CL图像显示锆石内部发育石榴子石、单斜辉石、斜长石等矿物包体,反映锆石可能形成在峰期高压麻粒岩相变质条件下。岩石学和年代学结果显示都兰高压麻粒岩和邻近的榴辉岩同时形成于同一俯冲带的不同热构造环境,高压麻粒岩并非榴辉岩热松弛作用形成的,两者具有各自独立的变质演化历史。     

造山带中成对出现的高压麻粒岩与榴辉岩及其地球动力学意义

在一些典型碰撞造山带中,高压麻粒岩与榴辉岩在空间和时间上密切相关,它们之间的关系对揭示碰撞造山带的造山过程和造山机制具有重要意义.本文以中国西部的南阿尔金、柴北缘及中部的北秦岭造山带为例,详细陈述了这3个地区榴辉岩和相关的高压麻粒岩的野外关系、变质演化和形成时代,目的是要建立大陆碰撞造山带中榴辉岩和相关高压麻粒岩形成的地球动力学背景模式.南阿尔金榴辉岩呈近东西向分布在江尕勒萨依,玉石矿沟一带,与含夕线石副片麻岩、花岗质片麻岩和少量大理岩构成榴辉岩一片麻岩单元,榴辉岩中含有柯石英假象,其峰期变质条件为P=2.8～3.0GPa,T=730～850℃,并在抬升过程中经历了角闪岩-麻粒岩相的叠加;大量年代学研究显示其峰期变质时代为485～500Ma.南阿尔金高压麻粒岩分布在巴什瓦克地区,包括高压基性麻粒岩和高压长英质麻粒岩,它们与超基性岩构成了一个大约5km宽的构造岩石单元,与周围角闪岩相的片麻岩为韧性剪切带接触.长英质麻粒岩和基性麻粒岩的峰期组合均具有蓝晶石和三元长石(已变成条纹长石),形成的温压条件为T=930～1020℃,P=1.8～2.5GPa,并在退变质过程中经历了中压麻粒岩相变质作用叠加.锆石SHRIMP测定显示巴什瓦克高压麻粒岩的峰期变质时代为493～497Ma.都兰地区的榴辉岩分布柴北缘HP-UHP变质带的东端,在榴辉岩和围岩副片麻岩中均发现有柯石英保存,形成的峰期温压条件为T=670～730℃和P=2.7～3.25GPa,退变质阶段经过了角闪岩相的叠加;榴辉岩相变质时代为420～450Mao都兰地区的高压麻粒岩分布在阿尔茨托山西部,高压麻粒岩包括基性麻粒岩长英质麻粒岩,基性麻粒岩的峰期矿物组合为Grt+Cpx+Pl±Ky±Zo+Rt±Qtz,长英质麻粒岩的峰期矿物组合为：Grt+Kf+Ky+Pl+Qtz.峰期变质条件为T=800～925℃,P=1.4～1.85GPa,退变质阶段经历了角闪岩-绿片岩的改造,高压麻粒岩的变质时代为420～450Ma.北秦岭榴辉岩分布在官坡-双槐树一带,榴辉岩的峰期变质组合为Grt+Omp±Phe+Qtz+Rt,所计算的峰期温压条件为T=680～770℃和P=2.25～2.65GPa,年代学数据显示榴辉岩的变质时代为500Ma左右.北秦岭高压麻粒岩分布在含榴辉岩单元的南侧松树沟一带,包括高压基性麻粒岩和高压长英质麻粒岩,与超基性岩在空间上密切伴生,高压麻粒岩的峰期温压条件为T=850～925℃,P=1.45～1.80GPa,锆石U-Pb年代学研究显示其峰期变质时代为485～507Ma.以上三个实例显示,出现在同一造山带、在空间上伴生的高压麻粒岩和榴辉岩有各自不同的变质演化历史,但榴辉岩中的榴辉岩相变质时代和相邻的高压麻粒岩中的高压麻粒岩相变质作用时代相同或相近,这种成对出现的榴辉岩和高压麻粒岩代表了它们同时形成在造山带中不同的构造环境中,即榴辉岩的形成于大陆俯冲带中,而高压麻粒岩可能形成在俯冲带之上增厚的大陆地壳根部.     

Ultra high temperature metamorphism of mafic granulites from South Altyn Orogen,West China: A result from the rapid exhumation of deeply subducted continental crust

The South Altyn orogen in West China contains ultra high pressure (UHP) terranes formed by ultra‐deep (>150–300 km) subduction of continental crust. Mafic granulites which together with ultramafic interlayers occur as blocks in massive felsic granulites in the Bashiwake UHP terrane, are mainly composed of garnet, clinopyroxene, plagioclase, amphibole, rutile/ilmenite, and quartz with or without kyanite and sapphirine. The kyanite/sapphirine‐bearing granulites are interpreted to have experienced decompression‐dominated evolution from eclogite facies conditions with peak pressures of 4–7 GPa to high pressure (HP)–ultra high temperature (UHT) granulite facies conditions and further to low pressure (LP)–UHT facies conditions based on petrographic observations, phase equilibria modelling, and thermobarometry. The HP–UHT granulite facies conditions are constrained to be 2.3–1.6 GPa/1,000–1,070°C based on the observed mineral assemblages of garnet+clinopyroxene+rutile+plagioclase+amphibole±quartz and measured mineral compositions including the core–rim increasing anorthite in plagioclase (X_An = 0.52–0.58), core–rim decreasing jadeite in clinopyroxene (X_Jd = 0.20–0.15), and TiO₂ in amphibole (Ti^M2/2 = 0.14–0.18). The LP–UHT granulite facies conditions are identified from the symplectites of sapphirine+plagioclase+spinel, formed by the metastable reaction between garnet and kyanite at <0.6–0.7 GPa/940–1,030°C based on the calculated stability of the symplectite assemblages and sapphirine–spinel thermometer results. The common granulites without kyanite/sapphirine are identified to record a similar decompression evolution, including eclogite, HP–UHT granulite, and LP–UHT granulite facies conditions, and a subsequent isobaric cooling stage. The decompression under HP–UHT granulite facies is estimated to be from 2.3 to 1.3 GPa at ~1,040°C on the basis of textural records, anorthite content in plagioclase (X_An = 0.25–0.32), and grossular content in garnet (X_Grs = 0.22–0.19). The further decompression to LP–UHT facies is defined to be >0.2–0.3 GPa based on the calculated stability for hematite‐bearing ilmenite. The isobaric cooling evolution is inferred mainly from the amphibole (Ti^M2/2 = 0.14–0.08) growth due to the crystallization of residual melts, consistent with a temperature decrease from >1,000°C to ~800°C at ~0.4 GPa. Zircon U–Pb dating for the two types of mafic granulite yields similar protolith and metamorphic ages of c. 900 Ma and c. 500 Ma respectively. However, the metamorphic age is interpreted to represent the HP–UHT granulite stage for the kyanite/sapphirine‐bearing granulites, but the isobaric cooling stage for the common granulites on the basis of phase equilibria modelling results. The two types of mafic granulite should share the same metamorphic evolution, but show contrasting features in petrography, details of metamorphic reactions in each stage, thermobarometric results, and also the meaning of zircon ages as a result of their different bulk‐rock compositions. Moreover, the UHT metamorphism in UHP terranes is revealed to represent the lower pressure overprinting over early UHP assemblages during the rapid exhumation of ultra‐deep subducted continental slabs, in contrast to the cause of traditional UHT metamorphism by voluminous heat addition from the mantle.     

Incipient eclogite facies metamorphism in the Moldanubian granulites revealed by mineral inclusions in garnet

We investigated several mineral phases and their replacement products which occur as inclusions in garnets from felsic and mafic granulites of the Gföhl Unit in the Moldanubian Zone. The most important mineral inclusions, Ti-rich muscovite and omphacite, were used for the reconstruction of the metamorphic history of granulites. Some inclusions were transformed during high-temperature granulite facies metamorphism, partial melting and decompression to other phases, and so the original mineral can only be deduced from the inclusion morphology and reaction products. These inclusions have columnar shapes and consist of K-feldspar + kaolinite, albite + Fe-oxide, plagioclase + Fe-oxide, or albite + K-feldspar, respectively. The pseudomorphs with albite/plagioclase occur in a Ca-rich garnet that shows prograde zoning. Pressure–temperature (PT) evolution, derived from mineral assemblages in granulite and based on the inclusions, suggests a prograde metamorphism from amphibolite through eclogite to granulite facies conditions with subsequent amphibolite facies overprint during exhumation. The estimated PT trajectory for the studied granulites, which also host lenses or boudins of eclogites and garnet peridotites, allows reconstruction of the complete clockwise metamorphic path that is consistent with subduction geotherm prior to the tectonic amalgamation within the continental collisional root.     

Ultrahigh Pressure Metamorphism and Tectonic Evolution of Southwestern Tianshan Orogenic Belt,China: A Comprehensive Review

Recently, a huge ultrahigh‐pressure (UHP) metamorphic belt of oceanic‐type has been recognized in southwestern (SW) Tianshan, China. Petrological studies show that the UHP metamorphic rocks of SW Tianshan orogenic belt include mafic eclogites and blueschists, felsic garnet phengite schists, marbles and serpentinites. The well‐preserved coesite inclusions were commonly found in eclogites, garnet phengite schists and marbles. Ti‐clinohumite and Ti‐chondrodite have been identified in UHP metamorphic serpentinites. Based on the PT pseudosection calculation and combined U‐Pb zircon dating, the P‐T‐t path has been outlined as four stages: cold subduction to UHP conditions before ～320 Ma whose peak ultrahigh pressure is about 30 kbar at 500oC, heating decompression from the Pmax to the Tmax stage before 305 Ma whose peak temperature is about 600oC at 22kbar, then the early cold exhumation from amphibolite eclogite facies to epidote‐amphibolite facies metamorphism characterized by ITD PT path before 220 Ma and the last tectonic exhumation from epidote amphibolite facies to greenschist facies metamorphism. Combining with the syn‐subduction arc‐like 333‐326 Ma granitic rocks and 280‐260 Ma S‐type granites in the coeval low‐pressure and high‐temperature (LP‐HT) metamorphic belt, the tectonic evolution of Tianshan UHP metamorphic belt during late Cambrian to early Triassic has been proposed.     

Exhumation of the lower crust during crustal shortening: an Alice Springs (380 Ma) age for a prograde amphibolite facies shear zone in the Strangways Metamorphic Complex (central Australia)

Foliated garnet-bearing amphibolites occur within the West Bore Shear Zone, cutting through granulite facies gneisses of the Strangways Metamorphic Complex. In the amphibolites, large euhedral garnet (up to 3 cm) occurs within fine-grained recrystallized leucocratic diffusion haloes of plagioclase–quartz. The garnet and their haloes include a well-developed vertical foliation, also present in the matrix. This foliation is the same as that cutting through the unconformably overlying Neoproterozoic Heavitree Quartzite. The textures indicate syn- to late kinematic growth of the amphibolite facies mineral assemblages.
All mineral assemblages record an arrested prograde reaction history. Noteworthy is the growth of garnet at the expense of hornblende and plagioclase, and the breakdown of staurolite–hornblende to give plagioclase–gedrite. These dehydration reactions indicate increasing P – T  conditions during metamorphism, and suggest heating towards the end of a period of intense deformation. Temperature estimates for the garnet–amphibolite and related staurolite–hornblende assemblages from the shear zone are about 600 °C. Pressure is estimated at about 5 kbar.
An Sm–Nd isochron gives an age of 381±7 Ma for the peak metamorphism and associated deformation. This age determination confirms that amphibolite facies conditions prevailed during shear zone development within the Strangways Metamorphic Complex during the Alice Springs Orogeny. These temperature conditions are significantly higher than those expected at this depth assuming a normal geothermal gradient. The Alice Springs Orogeny was associated with significant crustal thickening, allowing exhumation of the granulite facies, Palaeoproterozoic, lower crust. Along-strike variations of the tectonic style suggest a larger amount of crustal shortening in the eastern part of the Alice Springs Orogeny.     

Metamorphic evolution and geochemistry of kyanite eclogites in central Rhodope,northern Greece

In central Rhodope of northern Greece, kyanite eclogites were discovered in the area of Thermes. They are strongly overprinted and exhibit a multi-stage development of minerals and mineral assemblages formed during successive stages of the exhumation. The initial high-pressure assemblage was garnet+omphacite+kyanite+zoisite+phengite+rutile+quartz. Corundum, Fe-Mg-spinel, sapphirine and högbomite occur as products of a first, high-temperature overprint, still at high pressures, whereas various symplectites [corundum-plagioclase (pl), spinel-pl, sapphirine-pl, clinopyroxene-pl, biotite-pl, amphibole-pl] grew during subsequent stages of the exhumation. Diablastic amphibole+plagioclase formed as end-products of the amphibolitization. According to geochemical data, the protoliths of the kyanite eclogites were basalts to basaltic andesites with “volcanic arc” affinities. For the high-pressure stage of metamorphism, minimum PT conditions were around 19 kbar, 700°C, while for the initial stages of the overprint, high-pressure granulite-facies conditions prevailed (T>800°C, at P>15 kbar). The PT conditions of the amphibolite facies were 8–11 kbar, 580–690°C. The kyanite eclogites of Thermes record the highest temperatures of metamorphism within the whole of Rhodope.     

P-T-t Evolution of Ultrahigh-Temperature Granulites from the Saxon Granulite Massif, Germany. Part I: Petrology

The granulites of the Saxon Granulite Massif equilibrated athigh pressure and ultrahigh temperature and were exhumed inlarge part under near-isothermal decompression. This raisesthe question of whether P–T–t data on the peak metamorphismmay still be retrieved with confidence. Felsic and mafic granuliteswith geochronologically useful major and accessory phases haveprovided a basis to relate P–T estimates with isotopicages presented in a companion paper. The assemblage garnet +clinopyroxene in mafic granulite records peak temperatures of1010–1060°C, consistent with minimum estimates ofaround 967°C and 22·3 kbar obtained from the assemblagegarnet + kyanite + ternary feldspar + quartz in felsic granulite.Multiple partial overprint of these assemblages reflects a clockwiseP–T evolution. Garnet and kyanite in the felsic granulitewere successively overgrown by plagioclase, spinel + plagioclase,sapphirine + plagioclase, and biotite + plagioclase. Most ofthis overprinting occurred within the stability field of sillimanite.Garnet + clinopyroxene in the mafic granulite were replacedby clinopyroxene + amphibole + plagioclase + magnetite. Thehigh P–T conditions and the absence of thermal relaxationfeatures in these granulites require a short-lived metamorphismwith rapid exhumation. The ages of peak metamorphism (342 Ma)and shallow-level granitoid intrusions (333 Ma) constrain thetime span for the exhumation of the Saxon granulites to     

Eclogite facies relics and a multistage breakdown in metabasites of the KTB pilot hole,NE Bavaria: implications for the Variscan tectonometamorphic evolution of the NW Bohemian Massif

Complex reaction textures in coronitic metagabbros and retrograded eclogites of the KTB pilot and an adjacent drilling provide evidence for a multistage metamorphic history in the Variscan basement of the NW Bohemian Massif. The eclogites show complete metamorphic recrystallization leaving no textural or mineral relics of their igneous precursors. In contrast, textural relics of the igneous protolith are still preserved in the metagabbros where the metamorphic overprint under high pressure conditions achieved only partial replacement of the initial assemblage plagioclase + augite + amphibole (+olivine or orthopyroxene?) + ilmenite to form the eclogite facies assemblage garnet + omphacite + kyanite + zoisite + quartz+rutile. The garnets in the metagabbros occur in the typical ‘necklace’ fashion at the borders between the original plagioclase and mafic phase domains. In the same rocks, omphacite formed by a topotactic reaction mechanism replacing igneous augite as well as in smaller grains at the margins of the texturally igneous clinopyroxene where it occurs without fixed orientation with respect to the relict phase. Both eclogites and metagabbros show a partial breakdown under high pressure granulite (transitional to high pressure amphibolite) facies conditions during which omphacite broke down to vermicular symplectites of diopside + plagioclase. A later pervasive medium pressure metamorphism under amphibolite facies conditions led to the development of assemblages dominated by hornblende + plagioclase+titanite: phases prevailing in the overwhelming majority of the surrounding metabasites. Subsequent vein-associated retrogression produced minerals typical of the greenschist to zeolite facies. All metamorphic stages may be represented in a single thin section but although the overall reaction sequence is apparent, the obvious disequilibrium in the rocks makes the use of conventional geothermobarometry difficult. However, calculations made by assuming an approach to domainal equilibrium show that both the eclogite facies and early breakdown occurred above 10 kb. As the metamorphic unit hosting these particular metabasites is generally characterized by pressures below 10 kb these results have important implications for understanding the tectonometamorphic evolution of the region. The relationship between the studied rocks and other units in the NW Bohemian Massif exhibiting a multistage metamorphic evolution is discussed and possible tectonic models evaluated.     

Geochronological and petrological constraints on Palaeoproterozoic granulite facies metamorphism in southeastern margin of the North China Craton

In the southeastern margin of the North China Craton, high-pressure (HP) granulite facies meta-basic rocks exposed as bands or lenses in the Precambrian metamorphic basement (e.g. Bengbu) and as xenoliths in Mesozoic intrusions (e.g. Jiagou) are characterized by the assemblage garnet + clinopyroxene + plagioclase + quartz + rutile ± Ti-rich hornblende. Cathodoluminescence imaging and mineral inclusions reveal that most zircon from the three dated samples displays distinct core-mantle-rim structures. The cores show typical igneous zircon characteristics and give ages of 2.5–2.4 Ga, thus dating the protolith of the metabasites. The mantles formed at granulite facies conditions as evidenced by inclusions of the HP granulite mineral assemblage garnet + clinopyroxene + rutile + plagioclase + quartz ± hornblende and Ti-rich biotite and yield ages of 1839 ± 31, 1811 ± 19 and 1800 ± 15 Ma. An inclusion-free rim yields an age of 176 ± 2 Ma with the lower Th/U ratio of 0.02. The geochronological and preliminary petrological data of this study suggest that the lower crust beneath the southeastern margin of the North China Craton formed at 2.5–2.4 Ga and underwent HP granulite facies metamorphism at c. 1.8 Ga. This HT-HP metamorphic event may be ascribed to large-scale crustal heating and thickening related to mantle-derived magma underplating at the base of the lower crust, as evidenced by widespread extension, rifting and related mafic magma emplacement in the North China Craton during this period. The age of 176 ± 2 Ma most likely records the late amphibolite facies retrogression occurring during exhumation.     

Two-stage decompression in orthopyroxene–sillimanite granulites from Forefinger Point, Enderby Land, Antarctica: implications for the evolution of the Archaean Napier Complex

Magnesian metapelites of probable Archaean age from Forefinger Point, SW Enderby Land, East Antarctica, contain very-high-temperature granulite facies mineral assemblages, which include orthopyroxene (8–9.5 wt% Al₂O₃)–sillimanite ± garnet ± quartz ± K-feldspar, that formed at 10 ± 1.5 kbar and 950 ± 50°C. These assemblages are overprinted by symplectite and corona reaction textures involving sapphirine, orthopyroxene (6–7 wt% Al₂O₃), cordierite and sometimes spinel at the expense of porphyroblastic garnet or earlier orthopyroxene–sillimanite. These textures mainly pre-date the development of coarse biotite at the expense of initial mesoperthite, and the subsequent formation of orthopyroxene (4–6 wt% Al₂O₃)–cordierite–plagioclase rinds on late biotite.
The early reaction textures indicate a period of near-isothermal decompression at temperatures above 900°C. Decompression from 10 ± 1.5 kbar to 7–8 kbar was succeeded by biotite formation at significantly lower temperatures (800–850°C) and further decompression to 4.5 ± 1 kbar at 700–800°C.
The later parts of this P–T evolution can be ascribed to the overprinting and reworking of the Forefinger Point granulites by the Late-Proterozoic ( c . 1000 Ma) Rayner Complex metamorphism, but the age and timing of the early high-temperature decompression is not known. It is speculated that this initial decompression is of Archaean age and therefore records thinning of the crust of the Napier Complex following crustal thickening by tectonic or magmatic mechanisms and preceding the generally wellpreserved post-deformational near-isobaric cooling history of this terrain.     

P–T–t paths and tectonic history of an early Precambrian granulite facies terrane, Jining district, south-east Inner Mongolia, China

Abstract The widespread khondalite series of south-east Inner Mongolia consists largely of biotite–sillimanite–garnet gneiss and quartzo-feldspathic gneiss with some marble and mafic granulite layers. It has experienced two metamorphic events at c. 2500 and 1900–2000 Ma.
A pre-peak stage of the first metamorphism at T = 600–700°C and P > 6–7 kbar is recognized by the relict amphibolite facies assemblage Ky–Grt–Bt–Pl–Qtz and 'protected'inclusions of biotite, hornblende, sodic plagioclase and quartz in garnet or orthopyroxene. The peak stage, with T = c. 800 ± 50°C and P 8–10 kbar, is characterized by the widespread granulite facies assemblages Sil–Grt–Bt–Kfs–Pl–Qtz in gneiss and Opx–Cpx–Pl ± Hbl ± Grt in granulite. The P–T–t path suggests that the supracrustal sequence was buried in the lower crust by tectonic thickening during D1–D2.
The beginning of the second metamorphism is characterized by further temperature rise to 700°C or more at lower pressure. This stage is manifested by the appearance of cordierite after garnet, fibrolite (Sil2) after biotite in gneiss and transformation of Hbl1 into Opx2 and Cpx2 in granulite. Coronas of symplectitic Opx2 + Pl2 surrounding Grt1 and Cpx1 in mafic granulite are interpreted as products of near-isothermal decompression. The P–T–t path may be related tectonically to waning extension of the crust by the end of the early Proterozoic.