Stable isotope constraints on the Al2SiO5'triple-point' rocks from the Proterozoic Priest pluton contact aureole, New Mexico, USA

The Priest pluton contact aureole in the Manzano Mountains, central New Mexico preserves evidence for upper amphibolite contact metamorphism and localized retrograde hydrothermal alteration associated with intrusion of the 1.42 Ga Priest pluton. Quartz–garnet and quartz–sillimanite oxygen isotope fractionations in pelitic schist document an increase in the temperatures of metamorphism from 540 °C, at a distance of 1 km from the pluton, to 690 °C at the contact with the pluton. Comparison of calculated temperature estimates with one‐dimensional thermal modelling suggests that background temperatures between 300 and 350 °C existed at the time of intrusion of the Priest pluton. Fibrolite is found within 300 m of the Priest pluton in pelitic and aluminous schist metamorphosed at temperatures >580 °C. Coexisting fibrolite and garnet in pelitic schist are in oxygen isotope equilibrium, suggesting these minerals were stable reaction products during peak metamorphism. The fibrolite‐in isograd is coincident with the staurolite‐out isograd in pelitic schist, and K‐feldspar is not observed with the first occurrence of fibrolite. This suggests that the breakdown of staurolite and not the second sillimanite reaction controls fibrolite growth in staurolite‐bearing pelitic schist. Muscovite‐rich aluminous schist locally preserves the Al₂SiO₅ polymorph triple‐point assemblage – kyanite, andalusite and fibrolite. Andalusite and fibrolite, but not kyanite, are in isotopic equilibrium in the aluminous schist. Co‐nucleation of fibrolite and andalusite at 580 °C in the presence of muscovite and absence of K‐feldspar suggests that univariant growth of andalusite and fibrolite occurred. Kyanite growth occurred during an earlier regional metamorphic event at a temperature nearly 80 °C lower than andalusite and fibrolite growth. Quartz–muscovite fractionations in hydrothermally altered pelitic schist and quartzite are small or negative, suggesting that late isotopic exchange between externally derived fluids and muscovite, but not quartz, occurred after peak contact metamorphism and that hydrothermal alteration in pelitic schist and quartzite occurred below the closure temperature of oxygen self diffusion in quartz (<500 °C).     

沂水杂岩中变泥砂质岩石的岩石化学特征及年代

含夕线石十字石榴二云斜长片麻岩是沂水杂岩中首次发现的一种少见的变泥砂质岩石,包裹于沂水生心官庄岩浆杂岩体中,为残留的变质表壳岩透镜体,它经历了两期变质作用的改造.早期高角闪岩相变质与区域麻粒岩相变质有关,峰期矿物共生组合主要为:石榴子石(中心域)+黑云母±白云母+斜长石+石英,M1峰期变质温压条件为:T=660±10℃,P=5.7±0.3kb;晚期角闪岩相变质矿物共生组合为:十字石+石榴子石(边部域)+黑云母±白云母+斜长石±夕线石+石英,以形成大量自形-半自形十字石和具有明显的成分环带的石榴子石为特征,晚期石榴子石的形成由核部→边部经历了一降温降压过程,石榴子石核部:T=650±10℃,P=7.7±0.5Kb,石榴子石边部:T=578±10℃,P=4.7±0.1kb;晚期变质作用早期(石榴子石成核)阶段与埋深导致的部分熔融有关,晚期石榴子石生长阶段与岩浆热事件有关.锆石SHRIMP U-Pb定年结果表明:碎屑锆石不一致线上交点年龄为2695±32Ma,代表变泥砂质岩石源区岩浆岩的结晶年龄,变泥砂质岩石的早期变质变质作用年龄小于此值;晚期变质作用年龄为2537±5Ma.     

Metastable staurolite–cordierite assemblage of the Bossòst dome: Late Variscan decompression and polyphase metamorphism in the Axial Zone of the central Pyrenees

A kilometre-scale shear zone is recognized in the Cambro–Ordovician schist of the Bossòst dome, a Variscan metamorphic and structural dome in the Axial Zone of the central Pyrenees. Non-coaxial deformation is recorded by rotated garnet and staurolite porphyroblasts following regional metamorphism M₁, while coaxial conditions prevailed during later contact metamorphic M₂ growth of andalusite and cordierite. Mineral compositions and bulk rock analyses show that garnet–staurolite–andalusite–cordierite assemblages are significantly enriched in Mg and Mn over the garnet–staurolite assemblage, which lacks sufficient Mg for cordierite to form. The garnet–staurolite assemblage preserves conditions during M₁, estimated by AFM diagrams and P–T pseudosections to be 5.5 kbar and 580 °C, respectively. Pseudosections also indicate that staurolite is not a stable phase in cordierite–andalusite assemblages of M₂, suggesting polyphase metamorphism and decompression along a clockwise P–T path for the staurolite–cordierite–andalusite assemblages. This concurs with proposed extensional tectonics along the regional shear zone. To cite this article: J.E. Mezger et al., C. R. Geoscience 336 (2004).     

Separate or shared metamorphic histories of eclogites and surrounding rocks? An example from the Bohemian Massif

Eclogite boudins occur within an orthogneiss sheet enclosed in a Barrovian metapelite‐dominated volcano‐sedimentary sequence within the Velké Vrbno unit, NE Bohemian Massif. A metamorphic and lithological break defines the base of the eclogite‐bearing orthogneiss nappe, with a structurally lower sequence without eclogite exposed in a tectonic window. The typical assemblage of the structurally upper metapelites is garnet–staurolite–kyanite–biotite–plagioclase–muscovite–quartz–ilmenite ± rutile ± silli‐manite and prograde‐zoned garnet includes chloritoid–chlorite–paragonite–margarite, staurolite–chlorite–paragonite–margarite and kyanite–chlorite–rutile. In pseudosection modelling in the system Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O (NCKFMASH) using THERMOCALC, the prograde path crosses the discontinuous reaction chloritoid + margarite = chlorite + garnet + staurolite + paragonite (with muscovite + quartz + H₂O) at 9.5 kbar and 570 °C and the metamorphic peak is reached at 11 kbar and 640 °C. Decompression through about 7 kbar is indicated by sillimanite and biotite growing at the expense of garnet. In the tectonic window, the structurally lower metapelites (garnet–staurolite–biotite–muscovite–quartz ± plagioclase ± sillimanite ± kyanite) and amphibolites (garnet–amphibole–plagioclase ± epidote) indicate a metamorphic peak of 10 kbar at 620 °C and 11 kbar and 610–660 °C, respectively, that is consistent with the other metapelites. The eclogites are composed of garnet, omphacite relicts (jadeite = 33%) within plagioclase–clinopyroxene symplectites, epidote and late amphibole–plagioclase domains. Garnet commonly includes rutile–quartz–epidote ± clinopyroxene (jadeite = 43%) ± magnetite ± amphibole and its growth zoning is compatible in the pseudosection with burial under H₂O‐undersaturated conditions to 18 kbar and 680 °C. Plagioclase + amphibole replaces garnet within foliated boudin margins and results in the assemblage epidote–amphibole–plagioclase indicating that decompression occurred under decreasing temperature into garnet‐free epidote–amphibolite facies conditions. The prograde path of eclogites and metapelites up to the metamorphic peak cannot be shared, being along different geothermal gradients, of about 11 and 17 °C km^?1, respectively, to metamorphic pressure peaks that are 6–7 kbar apart. The eclogite–orthogneiss sheet docked with metapelites at about 11 kbar and 650 °C, and from this depth the exhumation of the pile is shared.     

Regional vs contact metamorphism of garnet metapelites in the vicinity of Late Variscan granites (Central Armorican Domain, Brittany, France)

Late Variscan granites intruded Brioverian (Upper Proterozoic) and Lower Paleozoic pelitic sequences to the north of the South Armorican shear zone. In the vicinity of the granites, Brioverian garnet micaschists contain pre/syn-S₂ assemblages with garnet + staurolite and post-S₂ assemblages with staurolite ± andalusite. Andalusite appeared pre/syn- and post-S₂ in garnet-free micaschists. The garnets in the Brioverian micaschists are zoned with increasing Mg and decreasing Mn and Ca from core to inner rim. Only poor garnet zonations occur in Paleozoic hornfelses of enclaves in the Rostrenen granite. The results of a microstructurally controlled application of garnet–biotite geothermometers and garnet–plagioclase geobarometers are similar to P–T trends obtained by the Gibbs method of garnet zonation modelling in the system NCFMnMASH. The P–T paths of a pre/syn-S₂ regional metamorphism are clockwise between 500–550°C/8 kbar and 700°C/5 kbar, followed by cooling decompression. They contrast with isobaric contact metamorphism between 500 and 700°C at 2.5–3 kbar in Paleozoic hornfelses. This points to a two-stage Variscan metamorphism with a pre-granitic pressure-dominated event in the Brioverian micaschists, followed by Late Variscan contact metamorphism, and suggests the existence of a pre-granitic tectonic boundary between the micaschists and overlying low-grade sequences.     

拉萨地块林芝杂岩体石榴角闪岩的地球化学特征和年代学研究

本文对位于青藏高原拉萨地体东南缘林芝杂岩中的两类石榴角闪岩进行了详细的地球化学和锆石U-Pb年代学研究。这两类石榴角闪岩分别为硅不饱和的含十字石石榴角闪岩和硅饱和的含石英石榴角闪岩。含十字石石榴角闪岩矿物组合为十字石、石榴子石、角闪石、钠云母、绿泥石、斜长石。石英石榴角闪岩矿物组合为石榴子石、角闪石、石英、斜长石、黑云母。岩石学及变质相平衡研究表明两类石榴角闪岩均经历了高压角闪岩相变质作用。含十字石石榴角闪岩和石英石榴角闪岩具有MORB的地球化学特征,锆石U-Pb年代学分析获得了800~200 Ma的206Pb/238U年龄范围,出现了~430 Ma、~268 Ma和~216 Ma年龄峰值。~430 Ma年龄可能和拉萨地体岩浆活动有关,~268 Ma变质年龄和~216 Ma变质年龄和拉萨地块经历的高压变质作用有关。其中~268 Ma年龄和拉萨地块内部松多高压带榴辉岩的峰期变质年龄一致,而~216 Ma年龄和榴辉岩的围岩含石榴子石片岩年龄一致。对比该区域的年代学研究成果,这表明林芝杂岩体不仅经历了中新生代的变质和岩浆再造活动,还经历了古特提斯洋闭合,南北拉萨地块发生碰撞的晚二叠世的高压变质作用和三叠纪的中压变质作用。     

川西丹巴地区变质岩的Rb—Sr年代学研究

丹巴地区自中生代以来经历了多期的变形、变质和岩浆作用。根据变质程度差异划分出6个变质带：矽线石带、蓝晶石带、十字石带、石榴子石带、黒云母带和绢云母－绿泥石带。蓝晶石带十字石片岩和石榴石带斜长角闪岩与变玄武岩的矿物Rb－Sr等时线年龄分别为149.0±7.1Ma，160.0±13.0Ma和150.2±2.4Ma，它们代表该区最主要的一次区域动热变质事件年龄。本文还结合已有的年代学资料，讨论了丹巴地区变质作用演化史     

Polymetamorphism, zircon growth and retention of early assemblages through the dynamic evolution of a continental arc in Fiordland, New Zealand

The Marguerite Amphibolite and associated rocks in northern Fiordland, New Zealand, contain evidence for retention of Carboniferous metamorphic assemblages through Cretaceous collision of an arc, emplacement of large volumes of mafic magma, high‐P metamorphism and then extensional exhumation. The amphibolite occurs as five dismembered aluminous meta‐gabbroic xenoliths up to 2 km wide that are enclosed within meta‐leucotonalite of the Lake Hankinson Complex. A first metamorphic event (M1) is manifest in the amphibolite as a pervasively lineated pargasite–anorthite–kyanite or corundum ± rutile assemblage, and as diffusion‐zoned garnet in pelitic schist xenoliths within the amphibolite. Thin zones of metasomatically Al‐enriched leucotonalite directly at the margins of each amphibolite xenolith indicate element redistribution during M1 and equilibration at 6.6 ± 0.8 kbar and 618 ± 25 °C. A second phase of recrystallization (M2) formed patchy and static margarite ± kyanite–staurolite–chlorite–plagioclase–epidote assemblages in the amphibolite, pseudomorphs of coronas in gabbronorite, and thin high‐grossular garnet rims in the pelitic schists. Conditions of M2, 8.8 ± 0.6 kbar and 643 ± 27 °C, are recorded from the rims of garnet in the pelitic schists. Cathodoluminescence imaging and simultaneous acquisition of U‐Th‐Pb isotopes and trace elements by depth‐profiling zircon grains from one pelitic schist reveals four stages of growth, two of which are metamorphic. The first metamorphic stage, dated as 340.2 ± 2.2 Ma, is correlated with M1 on the basis that the unusual zircon trace element compositions indicate growth from a metasomatic fluid derived from the surrounding amphibolite during penetrative deformation. A second phase of zircon overgrowth coupled with crosscutting relationships date M2 to between 119 and 117 Ma. The Early Carboniferous event has not previously been recognized in northern Fiordland, whereas the latter event, which has been identified in Early Cretaceous batholiths, their xenoliths, and rocks directly at batholith margins, is here shown to have also affected the country rock. However, the effects of M2 are fragmentary due to limited element mobility, lack of deformation, distance from a heat source and short residence time in the lower crust during peak P and T. It is possible that many parts of the Fiordland continental arc achieved high‐P conditions in the Early Cretaceous but retain earlier metamorphic or igneous assemblages.     

Early Namaqua low-pressure metamorphism: deformation and porphyroblast growth in the Zoovoorby staurolite schist, South Africa

This paper describes the deformation and metamorphism recorded in the Zoovoorby staurolite schist, a sliver of pelitic supracrustal material in the 1.3–1.0 Ga eastern Namaqua Province, South Africa. The supracrustal Biesjepoort Group, of which the schist is a part, has undergone at least four phases of deformation (D₁–D₄). D₁ and D₂ are preserved in the pelitic schists; staurolite and garnet grew during D₁, with staurolite growth persisting to the very earliest D₂ crenulation. Andalusite, found in more Mg-rich schists, grew during D₂, overprinting both S₁ schistosity and S₀ banding. S₂ has been rotated both with respect to S₁ (preserved as parallel orientated inclusion trails in garnet and staurolite) and with respect to its original orientation (preserved as open D₂ crenulations in staurolite). Staurolite is dissolved against S₂ in zones of progressive shear. The pseudomorphing of staurolite and andalusite by cordierite, and the preservation of relic grains of both minerals in a wide range of garnet–cordierite pelites throughout the eastern Namaqua Province infers that what is preserved fortuitously in the Zoovoorby locality is representative of the early metamorphic history of a much larger terrane. The high thermal gradients needed to attain estimated conditions of 540–550° C and 1.6–2.4 kbar require substantial heat input. Large amounts of foliated (syn-D₂) granite amongst the supracrustal succession are inferred to be the result of delamination of a thickened crust at a destructive plate margin, generating an elevated thermal gradient during D₁–D₂ times.     

Grenvillian-aged orogenesis in the Palaeoproterozoic Gascoyne Complex, Western Australia: 1030–950 Ma reworking of the Proterozoic Capricorn Orogen

In situ SHRIMP U–Pb geochronology of monazite and xenotime in pelitic schists from the central Gascoyne Complex, Western Australia, shows that greenschist to amphibolite facies metamorphism occurred between c. 1030 and c. 990 Ma. Monazite from an undeformed rare‐element pegmatite from the same belt gives a ²⁰⁷Pb/²⁰⁶Pb age of c. 950 Ma, suggesting that peak metamorphism and deformation was followed by pegmatite intrusion and coeval granite magmatism. Metamorphism in the central Gascoyne Complex was previously interpreted as Barrovian, largely based on the identification of kyanite in peak metamorphic assemblages, and has been attributed to intense crustal shortening and substantial tectonic thickening during Palaeoproterozoic continent–continent collision. However, the stable Al₂SiO₅ polymorph has been identified in this study as andalusite rather than kyanite, and the prograde assemblages of staurolite–garnet–andalusite–biotite–muscovite–quartz indicate temperatures of 500–550 °C and pressures of 3–4 kbar. These data show that the Palaeoproterozoic Gascoyne Complex underwent an episode of Grenvillian‐aged intracontinental reworking concentrated in a NW–SE striking corridor, during the Edmundian Orogeny. Until now, the Edmundian Orogeny was thought to have involved only reactivation of structures in the Gascoyne Complex, along with deformation and very low‐ to low‐grade metamorphism of Mesoproterozoic cover rocks some time between 1070 and 755 Ma. However, we suggest that it involved regional amphibolite facies metamorphism and deformation, granite magmatism and pegmatite intrusion between c. 1030 and c. 950 Ma. Therefore, the Capricorn Orogen experienced a major phase of tectonic reworking c. 600 Myr later than previously recognized. Our results emphasize the importance of in situ geochronology integrated with petrological studies in order to link the metamorphic history of a terrane with causally related tectonic events.     

Petrology of thermally metamorphosed pelitic rocks in the Champawat Area,Kumaun Himalaya

In the Champawat area, Kumaun Himalaya, greenschist facies regionally metamorphosed rocksviz chlorite-phyllite and schist have been subjected to thermal metamorphism due to emplacement of batholithic granite/granodiorite body. As a consequence, biotite, garnet, andalusite, fibrolite, sillimanite and perthite minerals have formed in the contact rocks. The conspicuous absence of cordierite and staurolite reported from such aureole rocks is due to higher FeO/MgO ratio of the bulk rock composition in the former while the absence of staurolite is due to low Al₂O₃/FeO+MgO ratio in the schists. AFM diagram demonstrates that in muscovite-bearing schist, the bulk composition of chlorite- and cordierite-bearing rocks are restricted to low FeO/MgO side and thus the restricted occurrence of former and the absence of latter in the contact rocks of the area. This is further evident from the common occurrence of almandine-rich garnet in the rocks.     

赣中变质岩带的组成及构造变质变形特征

赣中变质岩带不是简单的一套震旦纪地层 ,而是由结晶基底中元古界中深变质岩系 (斜长角闪岩的Sm Nd全岩等时线年龄为 1113± 4 9Ma)和褶皱基底变质较浅的震旦系组成。两者之间以具热流体参与的混合岩化、韧性剪切带和递进变质三位一体组成的动热变质带接触。结晶基底经历了4期构造变质变形的叠加改造 ,每期构造变形都在变质岩石构造单元内留下各种变形形迹 ,变质作用表现为时间上的递进和空间上的叠加演化系列 ,是一套以众多的不平衡结构和多相共生混存的矿物组合 ,热变质带为一套动热变质塑性变形带 ,空间上依次形成绢云母—绿泥石带、黑云母带、石榴石带、十字石带以及夕线石带     

Polymetamorphic history of a relict Permian hornfels from the central Gran Paradiso Massif (Western Alps,Italy): a microstructural and thermodynamic modelling study

A petrological and thermobarometric study of the Lago Teleccio hornfelses was undertaken to reconstruct the polymetamorphic evolution and constrain the P–T conditions of Permian contact metamorphism. The Lago Teleccio metasedimentary rocks record a Variscan regional metamorphism characterized by amphibolite facies mineral assemblages including quartz, plagioclase, K‐feldspar (Kfs 1), biotite, garnet (Grt 1) and staurolite; this was followed by a late‐Variscan mylonitization event. Metamorphism of the Variscan metamorphic rocks at the contact with a Permian granitic intrusion produced static recrystallization and/or new growth of quartz, garnet (Grt 2), plagioclase, K‐feldspar (Kfs 2), cordierite, green spinel, biotite and prismatic sillimanite (Contact 1). This thermal event, which occurred at a peak pressure of 0.23–0.35 GPa, temperature of 670–700 °C and a_H2O of 0.75–1, was followed either during post‐contact metamorphism cooling or, more likely, during the early‐Alpine metamorphism by the breakdown of cordierite into an anhydrous kyanite + orthopyroxene + quartz assemblage. The poorly developed early‐Alpine eclogite facies metamorphism (Alpine 1) was characterized by relatively anhydrous mineral associations and low strain, which locally produced coronitic and pseudomorphous microstructures in metasedimentary rocks, with scanty formation of jadeite, zoisite and a new high‐pressure garnet (Grt 3). Greenschist facies retrogression (Alpine 2) was characterized by the local development of a chlorite‐ and muscovite‐bearing mineral association, suggestive of aqueous fluid incursion. In the hornfelses, the limited extent of metamorphic overprinting is suggested by the fine grain size of the Alpine mineral associations, which formed at the expense of the Permian contact metamorphic associations, and was favoured by the anhydrous mineralogy of the hornfelses.     

Metamorphic history of eclogites and country rock gneisses in the Aktyuz area,Northern Tien‐Shan,Kyrgyzstan: a record from initiation of subduction through to oceanic closure by continent–continent collision

Eclogites and related high‐P metamorphic rocks occur in the Zaili Range of the Northern Kyrgyz Tien‐Shan (Tianshan) Mountains, which are located in the south‐western segment of the Central Asian Orogenic Belt. Eclogites are preserved in the cores of garnet amphibolites and amphibolites that occur in the Aktyuz area as boudins and layers (up to 2000 m in length) within country rock gneisses. The textures and mineral chemistry of the Aktyuz eclogites, garnet amphibolites and country rock gneisses record three distinct metamorphic events (M1–M3). In the eclogites, the first MP–HT metamorphic event (M1) of amphibolite/epidote‐amphibolite facies conditions (560–650 °C, 4–10 kbar) is established from relict mineral assemblages of polyphase inclusions in the cores and mantles of garnet, i.e. Mg‐taramite + Fe‐staurolite + paragonite ± oligoclase (An_<16) ± hematite. The eclogites also record the second HP‐LT metamorphism (M2) with a prograde stage passing through epidote‐blueschist facies conditions (330–570 °C, 8–16 kbar) to peak metamorphism in the eclogite facies (550–660 °C, 21–23 kbar) and subsequent retrograde metamorphism to epidote‐amphibolite facies conditions (545–565 °C and 10–11 kbar) that defines a clockwise P–T path. thermocalc (average P–T mode) calculations and other geothermobarometers have been applied for the estimation of P–T conditions. M3 is inferred from the garnet amphibolites and country rock gneisses. Garnet amphibolites that underwent this pervasive HP–HT metamorphism after the eclogite facies equilibrium have a peak metamorphic assemblage of garnet and pargasite. The prograde and peak metamorphic conditions of the garnet amphibolites are estimated to be 600–640 °C; 11–12 kbar and 675–735 °C and 14–15 kbar, respectively. Inclusion phases in porphyroblastic plagioclase in the country rock gneisses suggest a prograde stage of the epidote‐amphibolite facies (477 °C and 10 kbar). The peak mineral assemblage of the country rock gneisses of garnet, plagioclase (An_11–16), phengite, biotite, quartz and rutile indicate 635–745 °C and 13–15 kbar. The P–T conditions estimated for the prograde, peak and retrograde stages in garnet amphibolite and country rock are similar, implying that the third metamorphic event in the garnet amphibolites was correlated with the metamorphism in the country rock gneisses. The eclogites also show evidence of the third metamorphic event with development of the prograde mineral assemblage pargasite, oligoclase and biotite after the retrograde epidote‐amphibolite facies metamorphism. The three metamorphic events occurred in distinct tectonic settings: (i) metamorphism along the hot hangingwall at the inception of subduction, (ii) subsequent subduction zone metamorphism of the oceanic plate and exhumation, and (iii) continent–continent collision and exhumation of the entire metamorphic sequences. These tectonic processes document the initial stage of closure of a palaeo‐ocean subduction to its completion by continent–continent collision.     

High-pressure amphibolite facies dynamic metamorphism and the Mesozoic tectonic evolution of an ancient continental margin, east-central Alaska

Abstract Ductilely deformed amphibolite facies tectonites comprise two adjacent terranes in east-central Alaska. These terranes differ in protoliths, structural level and cooling ages. A structurally complex zone of gently north-dipping tectonites separates the two terranes. The northern, structurally higher Taylor Mountain terrane includes garnet amphibolite, biotite ± hornblende gneiss, marble, quartzite, metachert, pelitic schist and cross-cutting granitoids of intermediate composition (including the Late Triassic to Early Jurassic Taylor Mountain batholith). Lithological associations and isotopic data from the granitoids indicate an oceanic or marginal basin origin for the Taylor Mountain terrane. ⁴⁰Ar/³⁹Ar metamorphic cooling ages from the Taylor Mountain terrane are latest Triassic to earliest Middle Jurassic. The southern, structurally lower Lake George subterrane of the Yukon-Tanana terrane is made up of quartz-biotite schist and gneiss, augen gneiss, pelitic schist, garnet amphibolite and quartzite; we interpret it to comprise a continental margin and granitoid belt built on North American crust. Metamorphic cooling ages from the Lake George subterrane are almost entirely Early Cretaceous. Geothermobarometric analysis of garnet rims and adjacent phases in garnet amphibolite and pelitic schist from the Taylor Mountain terrane and Lake George subterrane indicate peak metamorphic conditions of 7.5-12 kbar at 555-715° C in the northern part of the Taylor Mountain terrane, in which NNE-vergent shear fabrics are preserved; 6.5-10.8 kbar at 520-670° C within the contact zone between the two terranes, in which NW-vergent shear fabrics predominate; and 6.8-11.8 kbar at 570-700° C in the Lake George subterrane of the Yukon-Tanana terrane, in which NW-vergent shear is recorded in the northern part of the study area and SE-vergent shear in the southern part. Where the two shear-sense directions occur together in the northern Lake George subterrane and, locally, in the contact zone, fabrics that record NW-vergent shear are more penetrative and preceded fabrics that record SE-vergent shear. We interpret the pressure, temperature, kinematic and age data to indicate that the metamorphism of the Taylor Mountain terrane and Lake George subterrane took place during different phases of a latest Palaeozoic through early Mesozoic shortening episode resulting from closure of an ocean basin now represented by klippen of the Seventymile-Slide Mountain terrane. High- to intermediate-pressure metamorphism of the Taylor Mountain terrane took place within a SW-dipping (present-day coordinates) subduction system. High- to intermediate-pressure metamorphism of the Lake George subterrane and the structural contact zone occurred during NW-directed overthrusting of the Taylor Mountain, Seventymile-Slide Mountain and Nisutlin terranes, and imbrication of the continental margin in Jurassic time. The difference in metamorphic cooling ages between the Taylor Mountain terrane and adjacent parts of the Lake George subterrane is best explained by Early Cretaceous unroofing of the Lake George subterrane caused by crustal extension, recorded in its younger top-to-the-SE fabric.     

Discovery of Early Tonian Calc-alkaline and Shoshonitic Metamafic Rocks from the North Purulia Shear Zone, Chhotanagpur Gneissic Complex, Eastern India: Implications of Proterozoic Sub-continental Lithospheric Mantle

Reports of shoshonitic rocks in Precambrian terrains are relatively rare. Pl-Grt amphibolites and Hbl-Bt mafic granulites occurring in the migmatitic gneisses of the Chhotanagpur Gneissic Complex(CGC) show calc-alkaline and shoshonitic characteristics. Relict porphyritic, sub-ophitic and poikilitic textures are noted in these rocks. Their parent magma was emplaced during the waning phase of the regional metamorphism. Geochemically, these metamafics are similar to the Group Ⅲ potassic and ultrapo...     

Prograde garnet growth along complex <Emphasis Type="Italic">P–T–t</Emphasis> paths: results from numerical experiments on polyphase garnet from the Wölz Complex (Austroalpine basement)

Garnet in metapelites from the Wölz Complex of the Austroalpine crystalline basement east of the Tauern Window characteristically consists of two growth phases, which preserve a comprehensive record of the geothermal history during polymetamorphism. From numerical modelling of garnet formation, detailed information on the pressure–temperature–time (P–T–t) evolution during prograde metamorphism is obtained. In that respect, the combined influences of chemical fractionation associated with garnet growth, modification of the original growth zoning through intragranular diffusion and the nucleation history on the chemical zoning of garnet as P and T change during growth are considered. The concentric chemical zoning observed in garnet and the homogenous rock matrix, which is devoid of chemical segregation, render the simulation of garnet growth through successive equilibrium states reliable. Whereas the first growth phase of garnet was formed at isobaric conditions of ～3.8 kbar at low heating/cooling rates, the second growth phase grew along a Barrovian P–T path marked with a thermal peak of ～625°C at ～10 kbar and a maximum in P of ～10.4 kbar at ～610°C. For the heating rate during the growth of the second phase of garnet, average rates faster than 50°C Ma^?1 are obtained. From geochronological investigations the first growth phase of garnet from the Wölz Complex pertains to the Permian metamorphic event. The second growth phase grew in the course of Eo-Alpine metamorphism during the Cretaceous.     

Deformation history inferred from magnetic fabric in the southwestern Okcheon metamorphic belt, Korea

Magnetic fabric and rock-magnetic studies have been carried out for the non-fossiliferous, low- to medium-grade metasedimentary rocks in the southwestern part of the Okcheon metamorphic belt (OMB). Two major metamorphic events in the study area were previously recognized: regional metamorphism (M₁) in the late Carboniferous to early Permian and contact metamorphism (M₂) due to the intrusion of granite in the middle Jurassic. The metamorphic grade of the study area increases from the biotite zone in southeast through the garnet zone to the staurolite zone towards the northwest. Magnetic fabrics of the study area are generally well defined and can be characterized according to the metamorphic zones. Magnetic foliation is the dominant magnetic fabric in the biotite zone, while magnetic lineation prevails in both garnet and staurolite zones. We interpret the metamorphism-related deformation history of the study area based on magnetic fabrics, magnetic mineralogy and previously reported metamorphic evolution as follows. Penetrative NW-dipping cleavage, represented by magnetic foliation, was formed in the study area by prevailing NW–SE shortening event during the M₁ regional metamorphism in the late Carboniferous–early Permian. This shortening event is interpreted to be associated with the collisional event between the North and South China blocks. Cleavages dipping steeply to the southeast in the staurolite zone are attributed to the pressure exerted from the intrusion of Jurassic granite in the northwestern area.     

Tectonic evolution of Proterozoic rocks in the Cimarron Mountains, northern New Mexico, USA

Abstract Portions of three Proterozoic tectonostratigraphic sequences are exposed in the Cimarron Mountains of New Mexico. The Cimarron River tectonic unit has affinities to a convergent margin plutonic/volcanic complex. Igneous hornblende from a quartz diorite stock records an emplacement pressure of 2–2.6 kbar. Rocks within this unit were subsequently deformed during a greenschist facies regional metamorphism at 4–5 kbar and 330 ± 50° C. The Tolby Meadow tectonic unit consists of quartzite and schist. Mineral assemblages are indicative of regional metamorphism at pressures near 4 kbar and temperatures of 520 ± 20° C. A low-angle ductile shear zone separates this succession from gneisses of the structurally underlying Eagle Nest tectonic unit. Gneissic granite yields hornblende pressures of 6–8 kbar. Pelitic gneiss records regional metamorphic conditions of 6–7 kbar and 705 ± 15° C, overprinted by retrogression at 4 kbar and 530 ± 10° C. Comparison of metamorphic and retrograde conditions indicates a P–T path dominated by decompression and cooling. The low-angle ductile shear zone represents an extensional structure which was active during metamorphism. This extension juxtaposed the Tolby Meadow and Eagle Nest units at 4 kbar and 520° C. Both units were later overprinted by folding and low-grade metamorphism, and then were emplaced against the Cimarron River tectonic unit by right-slip movement along the steeply dipping Fowler Pass shear zone. An argon isotope-correlation age obtained from igneous hornblende dates plutonism in the Cimarron River unit at 1678 Ma. Muscovite associated with the greenschist facies metamorphic overprint yields a ⁴⁰ Ar/³⁹ Ar plateau age of 1350 Ma. By contrast, rocks within the Tolby Meadow and Eagle Nest units yield significantly younger argon cooling ages. Hornblende isotope-correlation ages of 1394–1398 Ma are interpreted to date cooling during middle Proterozoic extension. Muscovite plateau ages of 1267–1257 Ma appear to date cooling from the low-grade metamorphic overprint. The latest ductile movement along the Fowler Pass shear zone post-dated these cooling ages. Argon released from muscovites of the Eagle Nest/Tolby Meadow composite unit, at low experimental temperatures, yields apparent ages of c. 1100 Ma. Similar ages are not obtained north-east of the Fowler Pass shear zone, suggesting movement more recently than 1100 Ma.     

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.