西南天山变质俯冲杂岩带的内部结构——来自木扎尔特剖面的启示

西南天山高压-超高压变质带是世界上少有的经历深俯冲的增生杂岩带,是古天山洋向北俯冲的结果。针对该俯冲杂岩带内部结构的研究目前仍存在争论。本文以木扎尔特地区一条长约4 km的南北向剖面为例,对西南天山高压-超高压变质带的野外特征、矿物学和变质演化研究进行了综述。目前的研究表明,木扎尔特地区存在超高压和高压两类硬柱石榴辉岩,但绝大部分都经历了强烈的退变质和变形改造,被蓝片岩相或绿片岩相矿物组合取代。这些变基性岩在空间上构成北部和南部两个榴辉岩带,二者为构造接触。木扎尔特超高压硬柱石榴辉岩与其围岩经历相似的峰期压力,构成西南天山超高压带的西端。与东侧阿克牙孜地区超高压榴辉岩相比,它们在变形特征、岩石组合和变质演化方面表现出一定的独特性,很可能说明深俯冲板片在折返过程中沿构造带走向存在差异变质-变形演化。这些基础研究对全面认识冷俯冲增生杂岩带的变质演化及其俯冲和折返的地球动力学机制具有重要意义。     

Metamorphism of ultrahigh‐pressure eclogites from the Kebuerte Valley,South Tianshan,NW China: phase equilibria and P–T path

Eclogites from the Kebuerte Valley, Chinese South Tianshan, consist of garnet, omphacite, phengite, paragonite, glaucophane, hornblendic amphibole, epidote, quartz and accessory rutile, titanite, apatite and carbonate minerals with occasional presence of coesite or quartz pseudomorphs after coesite. The eclogites are grouped into two: type I contains porphyroblastic garnet, epidote, paragonite and glaucophane in a matrix dominated by omphacite where the proportion of omphacite and garnet is >50 vol.%; and type II contains porphyroblastic epidote in a matrix consisting mainly of fine‐grained garnet, omphacite and glaucophane where the proportion of omphacite and garnet is <50 vol.%. Garnet in both types of eclogites mostly exhibits core–rim zoning with increasing grossular (X_gr) and pyrope (X_py) contents, but a few porphyroblastic garnet grains in type I eclogite shows core–mantle zoning with increasing X_py and a slight decrease in X_gr, and mantle–rim zoning with increases in both X_gr and X_py. Garnet rims in type I eclogite have higher X_py than in type II. Petrographic observations and phase equilibria modelling with pseudosections calculated using thermocalc in the NCKMnFMASHO system for three representative samples suggest that the eclogites have experienced four stages of metamorphism: stage I is the pre‐peak temperature prograde heating to the pressure peak (P_max) which was recognized by the garnet core–mantle zoning with increasing X_py and decreasing X_gr. The P–T conditions at P_max constrained from garnet mantle or core compositions with minimum X_gr content are 29–30 kbar at 526–540 °C for type I and 28.2 kbar at 518 °C for type II, suggesting an apparent thermal gradient of ～5.5 °C km^?1. Stage II is the post‐P_max decompression and heating to the temperature peak (T_max), which was modelled from the garnet zoning with increasing X_gr and X_py contents. The P–T conditions at T_max, defined using the garnet rim compositions with maximum X_py content and the Si content in phengite, are 24–27 kbar at 590 °C for type I and 22 kbar at 540 °C for type II. Stage III is the post‐T_max isothermal decompression characterized by the decomposition of lawsonite, which may have resulted in the release of a large amount of fluid bound in the rocks, leading to the formation of epidote, paragonite and glaucophane porphyroblasts. Stage IV is the late retrograde evolution characterized by the overprint of hornblendic amphibole in eclogite and the occurrence of epidote–amphibole facies mineral assemblages in the margins or in the strongly foliated domains of eclogite blocks due to fluid infiltration. The P–T estimates obtained from conventional garnet–clinopyroxene–phengite thermobarometry for the Tianshan eclogites are roughly consistent with the P–T conditions of stage II at T_max, but with large uncertainties in temperature. On the basis of these metamorphic stages or P–T paths, we reinterpreted that the recently reported zircon U–Pb ages for eclogite may date the T_max stage or the later decompression stage, and the widely distributed (rutile‐bearing) quartz veins in the eclogite terrane may have originated from the lawsonite decomposition during the decompression stage rather than from the transition from blueschist to eclogite as previously proposed.     

Metamorphic evolution of relict lawsonite‐bearing eclogites from the (U) HP metamorphic belt in the Chinese southwestern Tianshan

In the Chinese southwestern Tianshan (U)HP belt, former lawsonite presence has been predicted for many (U)HP metamorphic eclogites, but only a very few lawsonite grains have been found so far. We discovered armoured lawsonite relicts included in quartz, which, on its part, is enclosed in porphyroblastic garnet in an epidote eclogite H711‐14 and a paragonite eclogite H711‐29. H711‐14 is mainly composed of garnet, omphacite, epidote and titanite, with minor quartz, paragonite and secondary barroisite and glaucophane. Coarse‐grained titanite occasionally occurs in millimetre‐wide veins in equilibrium with epidote and omphacite, and relict rutile is only preserved as inclusions in matrix titanite and garnet. H711‐29 shows the mineral assemblage of garnet, omphacite, glaucophane, paragonite, quartz, dolomite, rutile and minor epidote. Dolomite and rutile are commonly rimed by secondary calcite and titanite respectively. Porphyroblastic garnet in both eclogites is compositionally zoned and exhibits an inclusion‐rich core overgrown by an inclusion‐poor rim. Phase equilibria modelling predicts that garnet cores formed at the P‐peak (490–505 °C and 23–25.5 kbar) and coexisted with the lawsonite eclogite facies assemblage of omphacite + glaucophane + lawsonite + quartz. Garnet rims (550–570 °C and ~20 kbar) grew subsequently during a post‐peak epidote eclogite facies metamorphism and coexisted with omphacite + quartz ± glaucophane ± epidote ± paragonite. The results confirm the former presence of a cold subduction zone environment in the Chinese southwestern Tianshan. The P–T evolution of the eclogites is characterized by a clockwise P–T path with a heating stage during early exhumation (thermal relaxation). The preservation of lawsonite in these eclogites is attributed to isolation from the matrix by quartz and rigid garnet, which should be considered as a new type of lawsonite preservation in eclogites. The complete rutile–titanite transition in H711‐14 took place in the epidote eclogite facies stage in the presence of an extremely CO₂‐poor fluid with X(CO₂) [CO₂/(CO₂ + H₂O) in the fluid] <<0.008. In contrast, the incomplete rutile–titanite transition in H711‐29 may have occurred after the epidote eclogite facies stage and the presence of dolomite reflects a higher X(CO₂) (>0.01) in the coexisting fluid at the epidote eclogite facies stage.     

Petrology of coesite-bearing eclogite from Habutengsu Valley, western Tianshan, NW China and its tectonometamorphic implication

Coesite inclusions in garnet have been found in eclogite boudins enclosed in coesite‐bearing garnet micaschist in the Habutengsu Valley, Chinese western Tianshan, which are distinguished from their retrograde quartz by means of optical characteristics, CL imaging and Raman spectrum. The coesite‐bearing eclogite is mainly composed of porphyroblastic garnet, omphacite, paragonite, glaucophane and barroisite, minor amounts of rutile and dotted (or banded) graphite. In addition to coesite and quartz, the zoned porphyroblastic garnet contains inclusions of omphacite, Na‐Ca amphibole, calcite, albite, chlorite, rutile, ilmenite and graphite. Multi‐phase inclusions (e.g. Czo + Pg ± Qtz, Grt II + Qtz and Chl + Pg) can be interpreted as breakdown products of former lawsonite and possibly chloritoid. Coesite occurs scattered within a compositionally homogenous but narrow domain of garnet (outer core), indicative of equilibrium at the UHP stage. The estimate by garnet‐clinopyroxene thermometry yields peak temperatures of 420–520 °C at 2.7 GPa. Phase equilibrium calculations further constrain the P–T conditions for the UHP mineral assemblage Grt + Omp + Lws + Gln + Coe to 2.4–2.7 GPa and 470–510 °C. Modelled modal abundances of major minerals along a 5 °C km^?1 geothermal gradient suggests two critical dehydration processes at ～430 and ～510 °C respectively. Computed garnet composition patterns are in good agreement with measured core‐rim profiles. The petrological study of coesite‐bearing eclogite in this paper provides insight into the metamorphic evolution in a cold subduction zone. Together with other reported localities of UHP rocks from the entire orogen of Chinese western Tianshan, it is concluded that the regional extent of UHP‐LT metamorphism in Chinese western Tianshan is extensive and considerably larger than previously thought, although intensive retrogression has erased UHP‐LT assemblages at most localities.     

The Habutengsu metapelites and metagreywackes in western Tianshan,China: metamorphic evolution and tectonic implications

The HP‐UHP metamorphic belt of western Tianshan in northwestern China is a rarely preserved oceanic UHP terrane which consists predominantly of meta‐siliciclastic rocks, occasionally accompanied by lens‐shaped metabasites. The metapelites and metagreywackes from the Habutengsu Valley and adjacent area within this belt contain quartz, albite, garnet, white mica, chlorite and rutile/titanite, with or without minor amounts of barroisite, glaucophane, clinozoisite, allanite, graphite, carbonate and tourmaline. Included in coarse‐grained garnet, pseudomorphs of clinozoisite + paragonite after lawsonite are common, seldom also together with inclusions of chloritoid, jadeite and glaucophane. In the northern Habutengsu area, garnet is compositionally characterized by similar cores with consistently low‐Ca content. Similar garnet armouring coesite has been reported in UHP schists from the same area. Deduced P–T conditions during formation of these Ca‐poor garnet cores are 25–31 kbar and 430–510 °C, which are consistent with the computed stability of the observed assemblage Grt + Gln + Lws ± Jd ± Cld in the coesite stability field. Thus, the occurrences of the UHP metapelites and metagreywackes define an internally coherent UHP unit in the north of the Habutengsu area, the spatial extension of which is much larger than previously known. Compared with the northern ones, the southern metapelites and metagreywackes in the Habutengsu area consist of similar minerals and have similar bulk rock compositions but significantly different garnet chemistry, indicating an abrupt variation in P–T conditions during garnet growth. The derived conditions initiating the garnet growth for the southern rocks in a similar range (18–21 kbar and 450–500 °C) and thus constrain a coherent HP unit in the south of the Habutengsu area. The juxtaposition of two exhumed slices of contrasting metamorphic grades probably indicates the change of subduction dynamics of the palaeo‐Tianshan oceanic crust, the subduction polarity (from south to north) of which accounts for the spatial relationship between these two units.     

新疆西南天山木扎尔特一带低压麻粒岩相变质作用P-T轨迹及其地质意义

本文首次报道在新疆西南天山木扎尔特一带发现了二辉石麻粒岩和麻粒岩相变质的堇青石榴矽线石片麻岩。二辉石麻粒岩的矿物组合为单斜辉石-斜方辉石-黑云母-角闪石-斜长石-石英。堇青石榴矽线石片麻岩矿物组合为堇青石-矽线石-石榴石-黑云母-斜长石-石英。岩石学和矿物学特征表明它们是典型的低压麻粒岩相变质岩石,其变质作用经历了两期演化：a．峰期麻粒岩相变质,T=681～705℃,P=5．4～5．8kbar;b．峰后角闪岩相退变阶段,T：571～637℃．P=4．7～5．3kbar。其变质作用P-T轨迹具有逆时针近等压降温(IBC)的特点,代表该地区可能为塔里木板块向伊犁．中天山板块俯冲过程中,在陆壳一侧所产生的陆源岩浆弧区域,由于受到下部岩浆热源的影响,在拉伸环境下出现低压麻粒岩相变质。通过分析低压麻粒岩相岩石与其南部高压一超高压变质带的大地构造位置和年代关系,我们认为该地区的低压麻粒岩相变质岩石可能与其南部的西天山高压一超高压变质带组成了双变质带。     

Subduction and exhumation mechanisms of ultra‐high and high‐pressure oceanic and continental crust at Makbal (Tianshan,Kazakhstan and Kyrgyzstan)

The Makbal Complex in the northern Tianshan of Kazakhstan and Kyrgyzstan consists of metasedimentary rocks, which host high‐P (HP) mafic blocks and ultra‐HP Grt‐Cld‐Tlc schists (UHP as indicated by coesite relicts in garnet). Whole rock major and trace element signatures of the Grt‐Cld‐Tlc schist suggest a metasomatized protolith from either hydrothermally altered oceanic crust in a back‐arc basin or arc‐related volcaniclastics. Peak metamorphic conditions of the Grt‐Cld‐Tlc schist reached ~580 °C and 2.85 GPa corresponding to a maximum burial depth of ~95 km. A Sm‐Nd garnet age of 475 ± 4 Ma is interpreted as an average growth age of garnet during prograde‐to‐peak metamorphism; the low initial εΝd value of ?11 indicates a protolith with an ancient crustal component. The petrological evidence for deep subduction of oceanic crust poses questions with respect to an effective exhumation mechanism. Field relationships and the metamorphic evolution of other HP mafic oceanic rocks embedded in continentally derived metasedimentary rocks at the central Makbal Complex suggest that fragments of oceanic crust and clastic sedimentary rocks were exhumed from different depths in a subduction channel during ongoing subduction and are now exposed as a tectonic mélange. Furthermore, channel flow cannot only explain a tectonic mélange consisting of various rock types with different subduction histories as present at the central Makbal Complex, but also the presence of a structural ‘dome’ with UHP rocks in the core (central Makbal) surrounded by lower pressure nappes (including mafic dykes in continental crust) and voluminous metasedimentary rocks, mainly derived from the accretionary wedge.     

新疆西天山超高压变质榴辉岩

在新疆西天山3类榴辉岩中均发现了超高压变质矿物：在第Ⅰ类与蓝片岩互层的榴辉岩中发现了柯石英假像；在第Ⅱ类枕状榴辉岩的绿辉石中发现了石英出溶叶片；在第Ⅲ类产于大理岩中呈透镜状的方解石/白云石榴辉岩中发现了变质成因的菱镁矿。详细的岩石矿物学研究表明，第Ⅰ类榴辉岩3个阶段的变质演化温压条件为：UHP前变质阶段（356-433℃，0.8-1.0GPa）、峰期UHP榴辉岩相阶段（496-598℃,2.572-2.666GPa)和退变绿帘蓝片岩相阶段（500-530℃，1.0-1.2GPa），并建立了在NCMASCH（Na2O-CaO-MgO-Al2O3-SiO2-CO2-H2O）体系中出现蓝闪石、石榴石、绿辉石、菱镁矿、白云石、黝帘石、柯石英、CO2和H2O的相平衡关系。经变质反应分析和相平衡理论计算西天山含菱镁矿的蓝闪石榴辉岩的变质峰期温压条件为：2.7-2.8GPa和525-607℃，XCO2低于0.006。超高压变质矿物的发展表明西天山榴辉岩经历了超高压变质作用，所谓的南天山造山带可能是世界上规模最大的由洋壳俯冲形成的超高压变质带。     

Ocean floor and subduction record in the Zermatt‐Saas rodingites,Valtournanche, Western Alps

Multiscale structural analysis and petrological modelling were used to establish the pressure‐peak mineral assemblages and pressure–temperature (P–T) conditions recorded in the rodingites of the upper Valtournanche portion of the oceanic Zermatt‐Saas Zone (ZSZ; Western Alps, northwestern Italy) during Alpine subduction. Rodingites occur in the form of deformed dykes and boudins within the hosting serpentinites. A field structural analysis showed that rodingites and serpentinites record four ductile deformation stages (D1–D4) during the Alpine cycle, with the first three stages associated with new foliations. The most pervasive fabric is S2 that is marked by mineral assemblages in serpentinite indicating pressure‐peak conditions, involving mostly serpentine, clinopyroxene, olivine, Ti‐clinohumite and chlorite. Three rodingite types can be defined: epidote‐bearing, garnet–chlorite–clinopyroxene‐bearing and vesuvianite‐bearing rodingite. In these, the pressure‐peak assemblages coeval with S2 development involve: (i) epidote_II + clinopyroxene_II + Mg‐chlorite_II + garnet_II ± rutile ± tremolite_I in the epidote‐bearing rodingite; (ii) Mg‐chlorite_II + garnet_II clinopyroxene_II ± vesuvianite_II ± ilmenite in the garnet–chlorite–clinopyroxene‐bearing rodingite; (iii) vesuvianite_II + Mg‐chlorite_II + clinopyroxene_II + garnet_II ± rutile ± epidote in vesuvianite‐bearing rodingite. Despite the pervasive structural reworking of the rodingites during Alpine subduction, the mineral relicts of the pre‐Alpine ocean floor history have been preserved and consist of clinopyroxene porphyroclasts (probable igneous relicts from gabbro dykes) and Cr‐rich garnet and vesuvianite (relicts of ocean floor metasomatism). Petrological modelling using thermocalc in the NCFMASHTO system was used to constrain the P–T conditions of the S2 mineral assemblages. The inferred values of 2.3–2.8 GPa and 580–660 °C are consistent with those obtained for syn‐S2 assemblages in the surrounding serpentinites. Multiscale structural analysis indicates that some ocean floor minerals remained stable under eclogite facies conditions suggesting that minerals such as vesuvianite, which is generally regarded as a low‐P phase, could also be stable in favourable chemical systems under high‐P/ultra‐high‐pressure (HP/UHP) conditions. Finally, the reconstructed P–T–d–t path indicates that the P/T ratio characterizing the D2 stage is consistent with cold subduction as estimated in this part of the Alps. The estimated pressure‐peak values are higher than those previously reported in this part of ZSZ, suggesting that the UHP units are larger and/or more abundant than those previously suggested.     

Cold subduction of the Neotethys: the metamorphic record from finely banded lawsonite and epidote blueschists and associated metabasalts of the Nagaland Ophiolite Complex,India

In this study, we have deduced the thermal history of the subducting Neotethys from its eastern margin, using a suite of partially hydrated metabasalts from a segment of the Nagaland Ophiolite Complex (NOC), India. Located along the eastern extension of the Indus‐Tsangpo suture zone (ITSZ), the N–S‐trending NOC lies between the Indian and Burmese plates. The metabasalts, encased within a serpentinitic mélange, preserve a tectonically disturbed metamorphic sequence, which from west to east is greenschist (GS), pumpellyite–diopside (PD) and blueschist (BS) facies. Metabasalts in all the three metamorphic facies record prograde metamorphic overprints directly on primary igneous textures and igneous augite. In the BS facies unit, the metabasalts interbedded with marble show centimetre‐ to metre‐scale interlayering of lawsonite blueschist (LBS) and epidote blueschist (EBS). Prograde HP/LT metamorphism stabilized lawsonite + omphacite (X_Jd = 0.50–0.56 to 0.26–0.37) + jadeite (X_Jd = 0.67–0.79) + augite + ferroglaucophane + high‐Si phengite (Si = 3.6–3.65 atoms per formula unit, a.p.f.u.) + chlorite + titanite + quartz in LBS and lawsonite + glaucophane/ferroglaucophane ± epidote ± omphacite (X_Jd = 0.34) + chlorite + phengite (Si = 3.5 a.p.f.u.) + titanite + quartz in EBS at the metamorphic peak. Retrograde alteration, which was pervasive in the EBS, produced a sequence of mineral assemblages from omphacite and lawsonite‐absent, epidote + glaucophane/ferroglaucophane + chlorite + phengite + titanite + quartz through albite + chlorite + glaucophane to lawsonite + albite + high‐Si phengite (Si = 3.6–3.7 a.p.f.u.) + glaucophane + epidote + quartz. In the PD facies metabasalts, the peak mineral assemblage, pumpellyite + chlorite + titanite + phengitic white mica (Si = 3.4–3.5 a.p.f.u.) + diopside appeared in the basaltic groundmass from reacting titaniferous augite and low‐Si phengite, with prehnite additionally producing pumpellyite in early vein domains. In the GS facies metabasalts, incomplete hydration of augite produced albite + epidote + actinolite + chlorite + titanite + phengite + augite mineral assemblage. Based on calculated T–M(H₂O), T–M(O₂) (where M represents oxide mol.%) and P–T pseudosections, peak P–T conditions of LBS are estimated at ~11.5 kbar and ~340 °C, EBS at ~10 kbar, 325 °C and PD facies at ~6 kbar, 335 °C. Reconstructed metamorphic reaction pathways integrated with the results of P–T pseudosection modelling define a near‐complete, hairpin, clockwise P–T loop for the BS and a prograde P–T path with a steep dP/dT for the PD facies rocks. Apparent low thermal gradient of 8 °C km^?1 corresponding to a maximum burial depth of 40 km and the hairpin P–T trajectory together suggest a cold and mature stage of an intra‐oceanic subduction zone setting for the Nagaland blueschists. The metamorphic constraints established above when combined with petrological findings from the ophiolitic massifs along the whole ITSZ suggest that intra‐oceanic subduction systems within the Neotethys between India and the Lhasa terrane/the Karakoram microcontinent were also active towards east between Indian and Burmese plates.     

西阿尔卑斯Zermatt-Saas洋壳深俯冲超高压变质带与我国新疆西南天山超高压变质带的比较

根据对西阿尔卑斯Zernatt-Saas带的野外地质考察及有关资料的总结,详细地介绍了西阿尔卑斯Zermatt-Saas洋壳深俯冲超高压变质带的地质概括、岩石组成和变质作用演化过程,并着重与我国新疆西南天山洋壳深俯冲超高压变质带进行了比较,探讨了这两个洋壳深俯冲超高压变质带在变质作用和地质演化方面的异同。在此基础上,总结了这两个目前世界上已知最典型的洋壳深俯冲超高压变质带的基本地质特征是都具有保存完好的洋壳岩石组合特征和较低的地热梯度。     

西大别浒湾变质带洋壳和陆壳耦合俯冲的锆石U-Pb年龄记录及其动力学意义

西大别浒湾高压变质带是研究秦岭-大别-苏鲁造山带演化的关键区域.本文对该变质带熊店和学河两地的两个榴辉岩样品进行了LA-(MC)-ICPMS锆石U-Pb定年、微量元素分析及Hf同位素测定.熊店榴辉岩岩浆锆石得到的年龄为406±14Ma,具有高的εHf(t)值(εHf(t)=11.3±1.3),年轻的亏损地幔模式年龄(tDM=578±52Ma),其来源可能为亏损地幔,进而说明它们的原岩可能为古特提斯洋壳物质;学河榴辉岩岩浆锆石的年龄为703±8Ma,具有略低的εHf(t)值(εHf(t)=4.11±0.94),较老的亏损地幔模式年龄(tDM=1105±37Ma),其原岩可能为扬子克拉通新元古代裂谷岩浆作用产生的新生陆壳物质.这些结果表明浒湾地区存在原岩形成于新元古代和志留纪两个时期的榴辉岩.熊店榴辉岩中变质锆石的微量元素特征与岩浆锆石类似,可能为完全重结晶成因锆石,其206Ph/238U加权平均年龄为316±1Ma,代表了洋壳榴辉岩榴辉岩相峰期变质的最早时间.学河榴辉岩变质锆石以低Th/U、Nb/Ta比值为特征,其REE组成模式为不明显的Eu负异常,HREE呈平坦型.这些特征反映了这些锆石形成时出现了较大数量的石榴子石与金红石,而缺乏长石.根据锆石Ti温度计计算学河榴辉岩变质锆石形成的温度范围是704～741℃,与榴辉岩相变质温度一致.它们对应的206Pb/238U加权平均年龄为312±3Ma,可作为浒湾变质带榴辉岩相峰期变质年龄,这一结果表明浒湾变质带陆壳成因榴辉岩存在石炭纪榴辉岩相变质事件.空间上相近的古生代洋壳和新元古代陆壳具有相同的榴辉岩相变质年龄表明,浒湾变质带的洋壳和陆壳榴辉岩可能存在石炭纪的耦合俯冲作用.     

P–T path of high-pressure/low-temperature rocks and tectonic implications in the western Tianshan Mountains, NW China

The Chinese western Tianshan high-pressure/low-temperature (HP–LT) metamorphic belt, which extends for about 200 km along the South Central Tianshan suture zone, is composed of mainly metabasic blueschists, eclogites and greenschist facies rocks. The metabasic blueschists occur as small discrete blocks, lenses, bands, laminae or thick beds in meta-sedimentary greenschist facies country rocks. Eclogites are intercalated within blueschist layers as lenses, laminae, thick beds or large massive blocks (up to 2 km² in plan view). Metabasic blueschists consist of mainly garnet, sodic amphibole, phengite, paragonite, clinozoisite, epidote, chlorite, albite, accessory titanite and ilmenite. Eclogites are predominantly composed of garnet, omphacite, sodic–calcic amphibole, clinozoisite, phengite, paragonite, quartz with accessory minerals such as rutile, titanite, ilmenite, calcite and apatite. Garnet in eclogite has a composition of 53–79 mol% almandine, 8.5–30 mol% grossular, 5–24 mol% pyrope and 0.6–13 mol% spessartine. Garnet in blueschists shows similar composition. Sodic amphiboles include glaucophane, ferro-glaucophane and crossite, whereas the sodic–calcic amphiboles mainly comprise barroisite and winchite. The jadeite content of omphacite varies from 35–54 mol%. Peak eclogite facies temperatures are estimated as 480–580 °C for a pressure range of 14–21 kbar. The conditions of pre-peak, epidote–blueschist facies metamorphism are estimated to be 350–450 °C and 8–12 kbar. All rock types have experienced a clockwise P–T path through pre-peak lawsonite/epidote-blueschist to eclogite facies conditions. The retrograde part of the P–T path is represented by the transition of epidote-blueschist to greenschist facies conditions. The P–T path indicates that the high-pressure rocks formed in a B-type subduction zone along the northern margin of the Palaeozoic South Tianshan ocean between the Tarim and Yili-central Tianshan plates.     

Petrology and metamorphism of glaucophane eclogites in Changning-Menglian suture zone,Bangbing area,southeast Tibetan Plateau: An evidence for Paleo-Tethyan subduction

High/ultrahigh-pressure (HP/UHP) metamorphic complexes, such as eclogite and blueschist, are generally regarded as significant signature of paleo-subduction zones and paleo-suture zones. Glaucophane eclogites have been recently identified within the Lancang Group characterized by accretionary mélange in the Changning-Menglian suture zone, at Bangbing in the Shuangjiang area of southeastern Tibetan Plateau. The authors report the result of petrological, mineralogical and metamorphism investigations of these rocks, and discuss their tectonic implications. The eclogites are located within the Suyi blueschist belt and occur as tectonic lenses in coarse-grained garnet muscovite schists. The major mineral assemblage of the eclogites includes garnet, omphacite, glaucophane, phengite, clinozoisite and rutile. Eclogitic garnet contains numerous inclusions, such as omphacite, glaucophane, rutile, and quartz with radial cracks around. Glaucophane and clinozoisite in the matrix have apparent optical and compositional zonation. Four stages of metamorphic evolution can be determined: The prograde blueschist facies (M₁), the peak eclogite facies (M₂), the decompression blueschist facies (M₃) and retrograde greenschist facies (M₄). Using the Grt-Omp-Phn geothermobarometer, a peak eclogite facies metamorphic P-T condition of 3000–3270 MPa and 617–658°C was determined, which is typical of low-temperature ultrahigh-pressure metamorphism. The comparison of the geological characteristics of the Bangbing glaucophane eclogites and the Mengku lawsonite-bearing retrograde eclogites indicates that two suites of eclogites may have formed from significantly different depths or localities to create the tectonic mélange in a subduction channel during subduction of the Triassic Changning-Menglian Ocean. The discovery of the Bangbing glaucophane eclogites may represent a new oceanic HP/UHP metamorphic belt in the Changning-Menglian suture zone.©2021 China Geology Editorial Office.     

Oriented growth of garnet by topotactic reactions and epitaxy in high‐pressure,mafic garnet granulite formed by dehydration melting of metastable hornblende‐gabbronorite (Jijal Complex,Kohistan Complex,north Pakistan)

Garnet growth in high‐pressure, mafic garnet granulites formed by dehydration melting of hornblende‐gabbronorite protoliths in the Jijal complex (Kohistan palaeo‐island arc complex, north Pakistan) was investigated through a microstructural EBSD‐SEM and HRTEM study. Composite samples preserve a sharp transition in which the low‐pressure precursor is replaced by garnet through a millimetre‐sized reaction front. A magmatic foliation in the gabbronorite is defined by mafic‐rich layering, with an associated magmatic lineation defined by the shape‐preferred orientation (SPO) of mafic clusters composed of orthopyroxene (Opx), clinopyroxene (Cpx), amphibole (Amp) and oxides. The shape of the reaction front is convoluted and oblique to the magmatic layering. Opx, Amp and, to a lesser extent, Cpx show a strong lattice‐preferred orientation (LPO) characterized by an alignment of [001] axes parallel to the magmatic lineation in the precursor hornblende‐gabbronorite. Product garnet (Grt) also displays a strong LPO. Two of the four 〈111〉 axes are within the magmatic foliation plane and the density maximum is subparallel to the precursor magmatic lineation. The crystallographic relationship 〈111〉_Grt // [001]_Opx,Cpx,Amp deduced from the LPO was confirmed by TEM observations. The sharp and discontinuous modal and compositional variations observed at the reaction front attest to the kinetic inhibition of prograde solid‐state reactions predicted by equilibrium‐phase diagrams. The P–T field for the equilibration of Jijal garnet granulites shows that the reaction affinities are 5–10 kJ mol.^?1 for the Grt‐in reaction and 0–5 kJ mol.^?1 for the Opx‐out reaction. Petrographic and textural observations indicate that garnet first nucleated on amphibole at the rims of mafic clusters; this topotactic replacement resulted in a strong LPO of garnet. Once the amphibole was consumed in the reaction, the parallelism of [001] axes of the mafic‐phase reactants favoured the growth of garnet crystals with similar orientations over a pyroxene substrate. These aggregates eventually sintered into single‐crystal garnet. In the absence of deformation, the orientation of mafic precursor phases conditioned the nucleation site and the crystallographic orientation of garnet because of topotaxial transformation reactions and homoepitaxial growth of garnet during the formation of high‐pressure, mafic garnet‐granulite after low‐pressure mafic protoliths.     

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

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

Distinct zircon U–Pb and O–Hf–Nd–Sr isotopic behaviour during fluid flow in UHP metamorphic rocks: evidence from metamorphic veins and their host eclogite in the Sulu Orogen,China

Fluid plays a key role in metamorphism and magmatism in subduction zones. Veins in high‐pressure (HP) to ultrahigh‐pressure (UHP) rocks are the products of fluid‐rock interaction, and can thus provide important constraints on fluid processes in subduction zones. This contribution is an integrated study of zircon U–Pb and O–Hf, as well as whole‐rock Nd–Sr isotopic compositions for a quartz vein, a complex vein, and their host eclogite in the Sulu UHP terrane to decipher the timing and source of fluid flow under HP‐UHP metamorphic conditions. The inherited magmatic zircon cores from the host eclogite constrain the protolith age at c. 750 Ma. Their variable ε_Hf(t) values from ?1.11 to 2.54 and low δ¹⁸O values of 0.32–3.40‰ reflect a protolith that formed in a rift setting due to the breakup of the supercontinent Rodinia. The hydrothermal zircon from the quartz and the complex veins shows euhedral shapes, relatively flat HREE pattern, slight or no negative Eu anomaly, low ¹⁷⁶Lu/¹⁷⁷Hf ratios, and low formation temperatures of 660–690 °C, indicating they precipitated from fluids under HP eclogite facies conditions. This zircon yielded similar U–Pb ages of 217 ± 2 and 213 ± 3 Ma within analytical uncertainty, recording the timing of fluid flow during the exhumation of the UHP rock. It is inferred that the fluids might be of internal origin based on the homogeneity of δ¹⁸O values of the hydrothermal zircon from the quartz (?2.41 ± 0.13‰) and complex veins (?2.35 ± 0.12‰), and the metamorphic grown zircon of the host eclogite (?2.23 ± 0.16‰). The similar ε_Nd(t) values of the whole rocks also support such a point. Zircon O and whole‐rock Nd isotopic compositions are therefore useful to identify the source of fluid, for they are major and trace components in minerals involved in metamorphic reactions during HP‐UHP conditions. On the other hand, the hydrothermal zircon from the veins and the metamorphic zircon from the host eclogite exhibit variable ε_Hf(t) values. Model calculation suggests that the Hf was derived from the breakdown of major rock‐forming minerals and recycling of the inherited magmatic zircon. The variable whole‐rock initial ⁸⁷Sr/⁸⁶Sr ratios might be caused by subsequent retrograde metamorphism after the formation of the veins.     

Petrology,geochemistry and P–T–t path of lawsonite‐bearing retrograded eclogites in the Changning–Menglian orogenic belt,southeast Tibetan Plateau

The Changning–Menglian orogenic belt (CMOB) in the southeastern Tibetan Plateau, is considered as the main suture zone marking the closure of the Palaeo‐Tethys Ocean between the Indochina and Sibumasu blocks. Here, we investigate the recently discovered retrograded eclogites from this suture zone in terms of their petrological, geochemical and geochronological features, with the aim of constraining the metamorphic evolution and protolith signature. Two types of metabasites are identified: retrograded eclogites and mafic schists. The igneous precursors of the retrograded eclogites exhibit rare earth element distribution patterns and trace element abundance similar to those of ocean island basalts, and are inferred to have been derived from a basaltic seamount in an intra‐oceanic tectonic setting. In contrast, the mafic schists show geochemical affinity to arc‐related volcanics with the enrichment of Rb, Th and U, and depletion of Nb, Ta, Zr, Hf and Ti, and their protoliths possibly formed at an active continental margin tectonic setting. Retrograded eclogites are characterized by peak metamorphic mineral assemblages of garnet, omphacite, white mica, lawsonite and rutile, and underwent five‐stage metamorphic evolution, including pre‐peak prograde stage (M₁) at 18–19 kbar and 400–420°C, peak lawsonite‐eclogite facies (M₂) at 24–26 kbar and 520–530°C, post‐peak epidote–eclogite facies decompression stage (M₃) at 13–18 kbar and 530–560°C, subsequent amphibolite facies retrogressive stage (M₄) at 8–10 kbar and 530–600°C, and late greenschist facies cooling stage (M₅) at 5–8 kbar and 480–490°C. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) U–Pb spot analyses of zircon show two distinct age groups. The magmatic zircon from both the retrograded eclogite and mafic schist yielded protolith ages of 451 ± 3 Ma, which is consistent with the ages of Early Palaeozoic ophiolitic complexes and ocean island sequences in the CMOB reported in previous studies. In contrast, metamorphic zircon from the retrograded eclogite samples yielded consistent Triassic metamorphic ages of 246 ± 2 and 245 ± 2 Ma, which can be interpreted as the timing of closure of the Palaeo‐Tethys Ocean. The compatible peak metamorphic mineral assemblages, P–T–t paths and metamorphic ages, as well as the similar protolith signatures for the eclogites in the CMOB and Longmu Co–Shuanghu suture (LCSS) suggest that the two belts formed part of a cold oceanic subduction system in the Triassic. The main suture zone of the Palaeo‐Tethyan domain extends at least 1,500 km in length from the CMOB to the LCSS in the Tibetan Plateau. The identification of lawsonite‐bearing retrograded eclogites in the CMOB provides important insights into the tectonic framework and complex geological evolution of the Palaeo‐Tethys.     

Fluid pathways and high‐P metasomatism in a subducted continental slice (Mt. Emilius klippe,W. Alps)

The Mt. Emilius klippe (Western Alps, Italy) corresponds to a segment of the stretched Adriatic continental margin metamorphosed at granulite facies during Permian. This slice was subducted during the early Cenozoic Alpine subduction with the underlying eclogite facies remnants of the Tethyan seafloor (Zermatt‐Saas zone). Near the base of the Mt. Emilius massif, there is a shear zone with eclogite facies hydrofracture systems associated with deformation‐induced re‐equilibration of granulites during high‐P metamorphism. In the basal part of the massif, a pluri‐hectometre domain of sheared mafic boudins is hosted in the granulitic paragneiss. In these mafic boudins, there are garnetites, garnet veins and clinopyroxenites, as well as clinozoisite and calcite veins. These features record multiple events of fracture opening, brecciation, boudinage and parallelization of structures coevally with fluid–rock interaction, metasomatism and volume change. This integrated petrological, micro‐textural and geochemical investigation illustrates the multiplicity and the chemical variability of fluid sources during prograde to peak metamorphic evolution in the lawsonite–eclogite‐facies field (at ~2.15–2.4 GPa, 500–550 °C) during subduction of the Mt. Emilius slice. The calcite veins crosscutting the garnetites have relatively low δ¹⁸O_VSMOW values (～+6.5‰) near those for marble layers (and nearby calcsilicates) embedded within the metasomatized granulites (+8 to +10‰). It is proposed that infiltration of externally‐derived H₂O‐rich fluids derived from the plate interface flushed the marbles, promoting decarbonation followed by short‐distance transport and re‐precipitation along garnetite fractures. This study highlights the importance of inherited structural heterogeneities (such as mafic bodies or sills) in localizing deformation, draining fluids from the downgoing plate and creating long‐lasting mechanical instabilities during subduction zone deformation.     

Constraining peak P–T conditions in UHP eclogites: calculated phase equilibria in kyanite‐ and phengite‐bearing eclogite of the Tromsø Nappe,Norway

Kyanite‐ and phengite‐bearing eclogites have better potential to constrain the peak metamorphic P–T conditions from phase equilibria between garnet + omphacite + kyanite + phengite + quartz/coesite than common, mostly bimineralic (garnet + omphacite) eclogites, as exemplified by this study. Textural relationships, conventional geothermobarometry and thermodynamic modelling have been used to constrain the metamorphic evolution of the Tromsdalstind eclogite from the Tromsø Nappe, one of the biggest exposures of eclogite in the Scandinavian Caledonides. The phase relationships demonstrate that the rock progressively dehydrated, resulting in breakdown of amphibole and zoisite at increasing pressure. The peak‐pressure mineral assemblage was garnet + omphacite + kyanite + phengite + coesite, inferred from polycrystalline quartz included in radially fractured omphacite. This omphacite, with up to 37 mol.% of jadeite and 3% of the Ca‐Eskola component, contains oriented rods of silica composition. Garnet shows higher grossular (X_Grs = 0.25–0.29), but lower pyrope‐content (X_Prp = 0. 37–0.39) in the core than the rim, while phengite contains up to 3.5 Si pfu. The compositional isopleths for garnet core, phengite and omphacite constrain the P–T conditions to 3.2–3.5 GPa and 720–800 °C, in good agreement with the results obtained from conventional geothermobarometry (3.2–3.5 GPa & 730–780 °C). Peak‐pressure assemblage is variably overprinted by symplectites of diopside + plagioclase after omphacite, biotite and plagioclase after phengite, and sapphirine + spinel + corundum + plagioclase after kyanite. Exhumation from ultrahigh‐pressure (UHP) conditions to 1.3–1.5 GPa at 740–770 °C is constrained by the garnet rim (X_Ca^Grt = 0.18–0.21) and symplectite clinopyroxene (X_Na^Cpx = 0.13–0.21), and to 0.5–0.7 GPa at 700–800 °C by sapphirine (X_Mg = 0.86–0.87) and spinel (X_Mg = 0.60–0.62) compositional isopleths. UHP metamorphism in the Tromsø Nappe is more widespread than previously known. Available data suggest that UHP eclogites were uplifted to lower crustal levels rapidly, within a short time interval (452–449 Ma) prior to the Scandian collision between Laurentia and Baltica. The Tromsø Nappe as the highest tectonic unit of the North Norwegian Caledonides is considered to be of Laurentian origin and UHP metamorphism could have resulted from subduction along the Laurentian continental margin. An alternative is that the Tromsø Nappe belonged to a continental margin of Baltica, which had already been subducted before the terminal Scandian collision, and was emplaced as an out‐of‐sequence thrust during the Scandian lateral transport of nappes.