Porphyroblastic manganaxinite metapelagites with incipient garnet in prehnite–pumpellyite facies, near Meyers Pass, Torlesse Terrane, New Zealand

Moderately manganiferous siliceous pelagites near Meyers Pass, Torlesse Terrane, South Canterbury, New Zealand, have been metamorphosed in the prehnite–pumpellyite facies. A conodont colour index measurement suggests T _max in the range 190–300 °C. Porphyroblastic manganaxinite, manganoan pumpellyite, manganoan chlorite and trace spessartine-rich garnet and sphalerite have formed in an extremely fine-grained quartz–albite–berthierine–phengite–titanite groundmass. Porphyroblastic manganaxinite semischists and schists are distinctive rocks in prehnite–pumpellyite to lower-grade greenschist and blueschist facies of New Zealand and Japan. Mn in the manganoan pumpellyites substitutes for Ca in W sites. Total Fe/(Fe+Mg) ratios in chlorite are dependent on oxidation state, being ≤0.22 in red hematitic hemipelagites, and ≥0.61 in low-f O₂ grey metapelagites. In the low-f O₂ metapelagites, manganoan berthierine with little or no chlorite is inferred in the groundmass and iron-rich chlorite occurs as porphyroblasts and veinlets, whereas in the red rocks, Mg-rich chlorite occurs both in groundmasses and veinlets. Variably high Si in the manganoan chlorites correlates with evidence for contaminant phases. The Mn content of chlorite contributing to garnet growth is dependent on metamorphic grade; incipient spessartine indicates a saturation value of 6–8% MnO in chlorite in low-f O₂ rocks at Meyers Pass. Lower MnO contents are recorded for otherwise analogous rocks with increasing metamorphic grade, but at a given grade coexisting chlorite and garnet are richer in Mn where f O₂ is high. Manganaxinite and manganoan pumpellyite also contributed to reactions forming grossular–spessartine solid solutions. Formation of garnet in siliceous pelagites is dependent on both Mn and Ca content. The spessartine component increases with grade into the greenschist facies. Partial recrystallization of berthierine to chlorite and the growth of porphyroblastic patches of other minerals was facilitated by brittle fracture and access of fluids to an otherwise impermeable matrix; to this extent the very low-grade metamorphism was episodic.     

Raman spectrum of carbonaceous material: a possible metamorphic grade indicator for low-grade metamorphic rocks

Raman spectral analyses of carbonaceous material (CM) extracted from pelitic samples along two sections traversing the metamorphic belt of Taiwan were carried out in the present study. The results show similar spectral variations of CM with metamorphic grade as those documented in the literature. However, continuous sampling from zeolite facies through prehnite–pumpellyite facies to greenschist facies metamorphic rocks in the present study does reveal some interesting features on the Raman spectra of CM that were not noted before. Both the Raman D (disordered-)/O (ordered-) peak area (i.e. integrated intensity) ratio and the D/O peak width (i.e. full width at half maximum, FWHM) ratio of the CM decrease with progressive metamorphism, but the most prominent change in the D/O peak area ratio occurs in samples of lower greenschist facies metamorphic grade, while the most significant decrease in the D/O peak width ratio occurs in samples near the boundary of prehnite–pumpellyite facies and greenschist facies. This phenomenon is interpreted as a result of the decoupling of the changing rates of in-plane crystallite size and degree of defects of CM with progressive metamorphism. It is postulated that the Raman spectrum of CM can serve as a metamorphic grade indicator to distinguish samples of prehnite–pumpellyite facies metamorphic grade from those of greenschist facies metamorphic grade.     

A Petrographic and Mineralogical Study of Volcanic Rocks from the Mayaxueshan Area, North Qilian Fold Belt, NW China

The Ordovician volcanic rocks in the Mayaxueshan area have been pervasively altered or metamorphosed and contain abundant secondary minerals such as albite, chlorite, epidote, prehnite, pumpellyite, actinolite, titanite, quartz, and/or calcite. They were denoted as spilites or spilitic rocks in terms of their petrographic features and mineral assemblages. The metamorphic grades of the volcanic rocks are equivalent to that of the intercalated metaclastic rocks. This indicates that both the spilitic volcanic rocks and metaclastic rocks in the Mayaxueshan area have formed as a result of Caledonian regional metamorphism. We suggest that the previously denoted spilitic rocks or altered volcanic rocks should be re-denoted as metabasalts or metabasaltic rocks. The metamorphic grade of the volcanic rocks increases with their age: prehnite-pumpellyite facies for the upper part of the Middle Ordovician volcanic rocks, prehnite-pumpeilyite to lower greenschist facies for the lower part of the Middle Ordovician vol     

The age of orogenesis in the Nambucca Slate Belt: A K‐Ar study of low‐grade regional metamorphic rocks

K‐Ar ages of biotite and hornblende from undeformed granodiorite plutons and of slaty and phyllitic rocks, ranging from prehnite‐pumpellyite metagreywacke to greenschist fades, have been determined in an attempt to define the age of orogenesis in the eastern part of the Nambucca Slate Belt. The plutons have K‐Ar ages of 226–227 m.y. (biotite) and 228–231 m.y. (hornblende) that provide a younger age limit for deformation. The lower grade metamorphic rocks yield a range of ages including some comparable with the depositional age of the rocks as indicated by fossils. Rocks of pumpellyite‐actinolite and greenschist facies give a more coherent group of ages which suggest orogenesis at about 250–255 m.y. Specimens of these latter rocks that have been affected by a later structural episode than that during which slaty cleavage formed, yield slightly older ages, which may result from the inclusion of minor amounts of environmental excess ⁴⁰Ar.

Support for the 250–255 m.y. age comes from previously determined radiometric ages from the western part of the Slate Belt, although the presence of granitic bodies perhaps as old as 289 m.y., some closely associated with high‐grade regional metamorphic rocks, may indicate the presence of additional earlier orogenic movements in this region.     

酸性岩的变质相

在沸石相变质条件下,花岗岩里浊沸石交代了斜长石和石英,在酸性火山岩里产生明矾石、埃洛石或高岭石。经受绿纤石-葡萄石相变质的花岗岩,其中黑云母变为钙铝榴石、帘石、绿纤石和葡萄石集合体,同时斜长石发生绢云母化。绿片岩相内酸性岩的浅色矿物有石英、微斜长石、钠长石和绿帘石,暗色矿物有绿泥石和黑云母。在角闪岩相变质的酸性岩中,开始出现中、基性斜长石,其中暗色矿物黑云母的镁铁比值要大于角闪石的镁铁比值。经受麻粒岩相变质后,紫苏花岗岩的矿物组成没有变化,但有铀、钍和钾的迁出。     

Occurrence of pumpellyite in hydrothermally altered basalts from the Vema fracture zone (mid-Atlantic ridge)

Metabasalts with abundant pumpellyite have been dredged in the Vema fracture zone, Atlantic ocean, and contain prehnite+pumpellyite±epidote+chlorite+white mica. The prehnite — pumpellyite association in these rocks differs from the prehnite-epidote association for most of the prehnite — pumpellyite facies metabasalts from the ocean crust described previously. The occurrence of pumpellyite is discussed in terms of temperature conditions, and oxygen fugacity and the pumpellyiterich metabasalts are believed to be recrystallized by hydrothermal circulation of seawater at about 250° C under a very low pressure (<1 kb).The bulk composition of the rocks demonstrates a strong chemical modification during hydrothermal metamorphism, similar to what is observed under greenschist facies conditions, except for potassium which can be uptaken from seawater by the rocks.     

Metamorphic evolution of the central Southern Cross Province,Yilgarn Craton,Western Australia

Two contrasting styles of metamorphism are preserved in the central Southern Cross Province. An early, low‐grade and low‐strain event prevailed in the central parts of the Marda greenstone belt and was broadly synchronous with the first major folding event (D₁) in the region. Mineral assemblages similar to those encountered in sea‐floor alteration are indicative of mostly prehnite‐pumpellyite facies conditions, but locally actinolite‐bearing assemblages suggest conditions up to mid‐greenschist facies. Geothermobarometry indicates that peak metamorphic conditions were of the order of 250–300°C at pressures below 180 MPa in the prehnite‐pumpellyite facies, but may have been as high as 400°C at 220 MPa in the greenschist facies. A later, higher grade, high‐strain metamorphic event was largely confined to the margins of the greenstone belts. Mineral assemblages and geothermobarometry suggest conditions from upper greenschist facies at P–T conditions of about 500°C and 220 MPa to upper amphibolite facies at 670°C and 400 MPa. Critical mineral reactions in metapelitic rocks suggest clockwise P–T paths. Metamorphism was diachronous across the metamorphic domains. Peak metamorphic conditions were reached relatively early in the low‐grade terrains, but outlasted most of the deformation in the higher grade terrains. Early metamorphism is interpreted to be a low‐strain, ocean‐floor‐style alteration event in a basin with high heat flow. In contrast, differential uplift of the granitoids and greenstones, with conductive heat input from the granitoids into the greenstones, is the preferred explanation for the distribution and timing of the high‐strain metamorphism in this region.     

Geochemical Features and Tectonic Setting of Greenstones from Kunimiyama, Northern Chichibu Belt, Central Shikoku, Japan

Abstract. We report whole‐rock chemical data for the greenstones from the Kunimiyama area in the Northern Chichibu Belt and their implications on the tectonic setting of these rocks. The Kunimiyama greenstones are associated with stratiform fer‐romanganese deposits or bedded cherts in the northern part of the study area, but are closely associated with a thick limestone block or bedded cherts in the southern part. The constituent minerals of greenstones are albitized plagioclase, clinopy‐roxene, chlorite, calcite, epidote, pumpellyite, prehnite, quartz, celadonite, sericite and opaque minerals such as iron oxyhy‐droxide and hematite. These mineral assemblages, epidote + pumpellyite + chlorite and chlorite + pumpellyite + prehnite, suggest that the metamorphic grade of greenstones from the Kunimiyama area is prehnite‐pumpellyite facies. The whole‐rock chemical compositions of greenstones associated with ferromanganese deposits are generally similar to those of normal mid‐ocean ridge basalt (N‐MORB). In contrast, the chemical compositions of the greenstones associated with the limestone block are comparable to those of ocean island alkaline basalt. Greenstones associated with bedded cherts are of enriched MORB and ocean island basalt, as well as N‐MORB origins, suggesting they probably formed as a result of plume‐related MOR volcanism in the Panthalassa Ocean in Early Permian and by tectonic mixing of ocean island basalts with oceanic ridge crustal fragments during accretion/subduction processes. These geological and geochemical lines of evidence suggest that the Kunimiyama greenstones are allochthonous blocks of accreted oceanic crust and seamounts. The ferromanganese deposits are frequently accompanied by reddish greenstones. Compared to common greenish greenstones, the reddish greenstones are characterized by high MnO and rare earth element contents and distinct negative Ce anomalies, implying a slight contribution of hydro thermal component forming the ferromanganese deposits.     

Reactions leading to the formation and breakdown of stilpnomelane in the Otago Schist, New Zealand

Semi‐pelitic rocks ranging in grade from the prehnite–pumpellyite to the greenschist facies from south‐eastern Otago, New Zealand, have been investigated in order to evaluate the reactions leading to formation and breakdown of stilpnomelane. Detrital grains of mica and chlorite along with fine‐grained authigenic illite and chlorite occur in lower‐grade rocks with compactional fabric parallel to bedding. At higher grades, detrital grains have undergone dissolution, and metamorphic phyllosilicates have crystallized with preferred orientation (sub)parallel to bedding, leading to slaty cleavage. Stilpnomelane is found in metapelites of the pumpellyite–actinolite facies and the chlorite zone of the greenschist facies, but only rarely in the biotite zone of the greenschist facies. Illite or phengite is ubiquitous, whereas chlorite occurs only rarely with stilpnomelane upgrade of the pumpellyite‐out isograd. Chemical and textural relationships suggest that stilpnomelane formed from chlorite, phengite, quartz, K‐feldspar and iron oxides. Stilpnomelane was produced by grain‐boundary replacement of chlorite and by precipitation from solution, overprinting earlier textures. Some relict 14 Å chlorite layers are observed by TEM to be in the process of transforming to 12 Å stilpnomelane layers. The AEM analyses show that Fe is strongly partitioned over Mg into stilpnomelane relative to chlorite (K_D≈2.5) and into chlorite relative to phengite (K_D≈1.9). Modified A′FM diagrams, projected from the measured phengite composition rather than from ideal KAl₃Si₃O₁₀(OH)₂, are used to elucidate reactions among chlorite, stilpnomelane, phengite and biotite. In addition to pressure, temperature and bulk rock composition, the stilpnomelane‐in isograd is controlled by variations in K, Fe³⁺/Fe²⁺, O/OH and H₂O contents, and the locus of the isograd is expected to vary in rocks of different oxidation states and permeabilities. Biotite, quartz and less phengitic muscovite form from stilpnomelane, chlorite and phengite in the biotite zone. Projection of bulk rock compositions from phengite, NaAlO₂, SiO₂ and H₂O reveals that they lie close to the polyhedra defined by the A′FM minerals and albite. Other extended A′FM diagrams, such as one projected from phengite, NaAlO₂, CaAl₂O₄, SiO₂ and H₂O, may prove useful in the evaluation of other low‐grade assemblages.     

Deformation‐driven,regional‐scale metasomatism in the Hamersley Basin,Western Australia

Mafic volcanic rocks of the Fortescue Group form the lowermost stratigraphic unit of the 100,000 km² Hamersley Basin on the southern margin of the Archean Pilbara Craton, Western Australia. A regional burial metamorphic gradient extends across the basin from prehnite–pumpellyite facies in the north to greenschist facies in the south. Phase equilibria modelling of mafic rocks with the computer program thermocalc , in subsets of the system Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–Fe₂O₃, successfully reproduces observed metamorphic mineral assemblages, giving conditions of ~210 °C, 2 kbar in the north and 335 °C, 3.2 kbar in the south. Superimposed on this metamorphic gradient, regional‐scale metasomatism in the Fortescue Group progressively produces a suite of prehnite‐bearing and pumpellyite–quartz/epidote–quartz‐dominated assemblages. Further modelling of variably metasomatized samples consistently estimates conditions of 260–280 °C, 2.5–3 kbar across the basin. All modelled samples were likely metasomatized at approximately the same structural level, following regional deformation during the Ophthalmian orogeny. Folding during the Ophthalmian orogeny produced topographic and/or tectonic driving forces for regional‐scale fluid flow, pushing metasomatic fluid northwards across the Hamersley Basin. These new phase equilibria calculations support previous interpretations linking the Ophthalmian orogeny, fluid flow and upgrading of Hamersley iron ore deposits. We propose an extension of this fluid flow to the Fortescue Group, the metasomatism of which may have contributed a source of Fe to the Hamersley iron ore deposits.     

青藏高原碰撞造山背景造山型金矿床:构造背景、地质及地球化学特征

青藏高原碰撞造山背景下形成了雅鲁藏布江缝合带及哀牢山造山带两条造山型金矿带。雅鲁藏布江缝合带包含马攸木、念扎、邦布及折木朗金矿等;该矿带形成于拉萨地块及特提斯喜马拉雅地层序列地壳初始缩短加厚的背景(59~44Ma),与林子宗火山岩和高压变质岩同期形成。控矿构造主要以EW向展布。金以自然金形式赋存在石英硫化物脉及石英脉两侧以绿片岩相变质为主的千枚岩及板岩中。哀牢山造山带包含镇沅、金厂、大坪及长安金矿等,主要形成于35~26Ma,成矿背景为区域发生大规模走滑剪切,矿区内分布有成矿前期的煌斑岩及富碱斑岩。控矿构造主要以NW-SE向展布,围岩变质级低于雅鲁藏布江缝合带。C-S-H-O-Pb同位素变化较大,整体雅鲁藏布江缝合带及哀牢山造山带造山型金矿成矿流体主要来源于深部地幔流体、围岩地层的变质流体及岩浆流体,成矿围岩的差异性也会导致同位素的变化性。     

Petrochemical characterization of mafic rocks from the Ligurian ophiolites,southern Apennines

New mineralogical and chemical data for ophiolitic rocks from the southwesternmost Liguride Units are presented in order to constrain their ocean-floor origin and subsequent emplacement in an accretionary wedge. Their complete petrochemical evolution is particularly well preserved in the southern Apennine metabasites. Metadolerites show amphibolite and greenschist facies mineral assemblages of ocean-floor metamorphism. Metabasalts display greenschist facies ocean-floor metamorphism and spilitic alteration. Veins cutting the mafic rocks show mineral assemblage of the prehnite–pumpellyite metamorphic facies. HP/LT orogenic metamorphism, reflecting underplating of the ophiolitic suite at the base of the Liguride accretionary wedge during subduction of the western Tethys oceanic lithosphere produced a mineral assemblage typical of the lawsonite–glaucophane facies. Bulk-rock chemistry suggests that the mafic protoliths had a MORB-type affinity, and were affected by ocean-floor rodingitic and/or spilitic alteration. Hydrothermal alteration-induced LREE mobility and LREE enrichment may be correlated with the ocean-floor metamorphism.     

Prehnite-Pumpellyite to Greenschist Fades Transition in the Karmutsen Metabasites, Vancouver Island, B.C.

Mineral paragenescs in the prehnite-pumpellyite to greenschistfades transition of the Karmutsen metabasites are markedly differentbetween amygdule and matrix, indicating that the size of equilibriumdomain is very small. Characteristic amygdule assemblages (+chlorite + quartz) vary from: (1) prehnite + pumpeUyite + epidote,prehnite + pumpellyite + calcite, and pumpellyite + epidote+ calcite for the prehnite-pumpellyite facies; through (2) calcite+ epidote + prehnite or pumpellyite for the transition zone;to (3) actinolite + epidote + calrite for the greenschist facies.Actinolite first appears in the matrix of the transition zone.Na-rich wairakites containing rare analcime inclusions coexistwith epidote or Al-rich pumpellyite in one prehnite-pumpellyitefacies sample. Phase relations and compositions of these wairakite-bearingassemblages further suggest that pumpellyite may have a compositionalgap between 0.10 and 0.15 X_Fe?. Although the facies boundaries are gradational due to the multi-varianceof the assemblages, several transition equilibria are establishedin the amygdule assemblages. At low X_co2, pumpellyite disappearsprior to prehnite by a discontinuous-type reaction, pumpellyite+ quartz + CO₂ = prehnite + epidote + calcite + chlorite + H₂O,whereas prehnite disappears by a continuous-type reaction, prehnite+ CO₂ = calcite + epidote + quartz-l-H₂O. On the other hand,at higher X_CO2 a prehnite-out reaction, prehnite + chlorite+ H₂O + CO₂ = calcite + pumpellyite + quartz, precedes a pumpellyiteoutreaction, pumpellyite + CO₂ = calcite + epidote + chlorite +quartz + H₂O. The first appearance of the greenschist faciesassemblages is defined at both low and high XCOj by a reaction,calcite + chlorite + quartz = epidote + actinolite+ H₂O + CO₂.Thus, these transition equilibria are highly dependent on bothX_Fe³⁺ + of Ca-Al silicates and X_H20 of the fluid phase. Phaseequilibria together with the compositional data of Ca-Al silicatesindicate that the prehnite-pumpellyite to greenschist faciestransition for the Karmutsen metabasites occurred at approximately1.7 kb and 300?C, and at very low X_co2, probably far less than0.1.     

Zones of Progressive Regional Burial Metamorphism in Part of the Tasman Geosyncline, Eastern Australia

A succession of metamorphic zones can be recognized along theMolong Geanticline, an ancient rise within the Tasman Geosynclineof Eastern Australia. In order of increasing grade, the zonesin the volcanogenic rocks are: (1) an albite-quartz zone, containingneither prehnite, pumpellyite, actinolite, nor biotite; (2)a prehnite zone; (3) a prehnite-pumpellyite zone; (4) an actinolitezone; and (5) a biotite zone. Zeolite associations are not developed.Zones 4 and 5 lie within the greenschist facies. The rocks alongthis geanticline are generally only slightly deformed, lackingcleavage. The grade of this metamorphism within the Molong Geanticlineincreases partly in accordance with increasing stratigraphicdepth and partly increases eastwards towards the adjacent HillEnd Trough. Within the Trough, rocks show metamorphic assemblagesakin to those of Zones 4 and 5 as defined within the MolongGeanticline, but deformation is more extreme and a regionalcleavage is characteristic. In the region about the Hill EndTrough and the adjacent geanticlines there is an apparent variationfrom essentially static burial metamorphism of the geanticlinesthrough transitional types to tectonic dynamothermal metamorphismof the trough. Both metamorphisms are envisaged as having arisenduring the same general period of metamorphism, but deformationof the geanticlines was much less than deformation in the interveningHill End Trough.     

High-grade metamorphic processes which influence Archaean gold deposits, with particular reference to Big Bell, Australia

Abstract Mineralogical and geochemical evidence indicates that partial melting and desulphidation have occurred in the Big Bell gold deposit. Decarbonation may also have occurred, to account for the lack of a carbonate alteration halo; this is compatible with the present data, but difficult to test. The Big Bell deposit consists of auriferous sulphide-bearing (‘lode’;) schists with muscovite and K-feldspar, and surrounding biotite schists, all derived by intense premetamorphic alteration of rocks of mafic composition. Assemblages which include cordierite-sillimanite-K-feldspar-garnet-biotite-quartz suggest peak metamorphic conditions of 4–5 kbar, and 650–700° C, based on phase relations, geobaro-meters and garnet-biotite Fe-Mg exchange partitioning. Partial melting occurred at peak metamorphism, particularly in the altered mafic rocks in and around the deposit, and its occurrence may have been essential to the preservation of the deposit. Melting greatly limited the importance of devolatilization reactions, resulting in negligible aqueous fluids and no means of removing appreciable gold. Minor gold loss may have accompanied desulphidation. A diversity of complex metamorphic assemblages occurs around the mine, compared to the assemblages developed regionally; variable bulk rock composition influences this contrast, but there is no evidence of higher metamorphic grades at the mine, nor that this might have been the prime control on the different assemblages in this narrow belt. It is suggested that the Big Bell and Hemlo deposits are the higher metamorphic grade equivalents of the more abundant greenschist facies gold deposits within Archaean greenstone belts. This interpretation is favoured by the host rock setting and geochemical characteristics of Big Bell. Alternative models that suggest that this class of deposit is a new type must account for the absence of high-grade equivalents of the greenschist facies deposits and also the lack of low-grade equivalents of the Big Bell/Hemlo type. Archaean gold deposits in high-grade metamorphic terrains have undergone a series of processes that are not recorded in the more typical gold deposits of the greenschist facies.     

Mineral assemblages and phase equilibria of metabasites from the prehnite–pumpellyite to amphibolite facies,with the Flin Flon Greenstone Belt (Manitoba) as a type example

An exceptionally well-exposed part of the Flin Flon Greenstone Belt (Manitoba/Saskatchewan) is used to characterize the mineral assemblage evolution associated with prehnite–pumpellyite through amphibolite facies metamorphism of basalts. Data from these rocks are combined with a large literature data set to assess the ability of current thermodynamic models to reproduce natural patterns, evaluate the use of metabasic rocks at these grades to estimate pressure–temperature (P–T) conditions of metamorphism, and to comment on the metamorphic devolatilization that occurs. At Flin Flon, five major isograds (actinolite-in, prehnite- and pumpellyite-out, hornblende-in, oligoclase-in, and actinolite-out) collectively represent passage from prehnite–pumpellyite to lower amphibolite facies conditions. The evolution in mineral assemblages occurs in two narrow (~1,000 m) zones: the prehnite–pumpellyite to greenschist facies (PP-GS) transition and greenschist to amphibolite facies (GS-AM) transition. Across the GS-AM transition, significant increases in the hornblende and oligoclase proportions occur at the expense of actinolite, albite, chlorite, and titanite, whereas there is little change in the proportions of epidote. The majority of this mineral transformation occurs above the oligoclase-in isograd within the hornblende–actinolite–oligoclase zone. Comparison with thermodynamic modelling results suggests data set 5 (DS5) of Holland and Powell (1998, Journal of Metamorphic Geology, 16 (3):309–343) and associated activity–composition (a–x) models is generally successful in reproducing natural observations, whereas data set 6 (DS6) (Holland & Powell, 2011, Journal of Metamorphic Geology, 29 (3):333–383) and associated a–x models fail to reproduce the observed mineral isograds and compositions. When the data from Flin Flon are combined with data from the literature, two main pressure-sensitive facies series for metabasites are revealed, based on prograde passage below or above a hornblende–albite bathograd at ~3.3 kbar: a low-pressure ‘actinolite–oligoclase type’ facies series, characterized by the appearance of oligoclase before hornblende, and a moderate- to high-pressure ‘hornblende–albite type’ facies series, characterized by the appearance of hornblende before oligoclase. Concerning the PP-GS transition, the mineral assemblage evolution in Flin Flon suggests it occurs over a small zone (<1,000 m), in which assemblages containing true transitional assemblages (prehnite and/or pumpellyite coexisting with actinolite) are rare. This contrasts with thermodynamic modelling, using either DS5 or DS6, which predicts a wide PP-GS transition involving the progressive appearance of epidote and actinolite and disappearance of pumpellyite and prehnite. Patterns of mineral assemblages and thermodynamic modelling suggest a useful bathograd (‘CHEPPAQ bathograd’), separating prehnite–pumpellyite-bearing assemblages at low pressures and pumpellyite–actinolite-bearing assemblages at higher pressures, occurs at ~2.3 to 2.6 kbar. Observations from the Flin Flon sequence suggests devolatilization across the GS-AM transition (average: ~1.8 wt% H₂O) occurs over a very narrow interval within the actinolite–hornblende–oligoclase zone, associated with the loss of >75% of the total chlorite. By contrast, modelling of the GS-AM transition zone predicts more progressive dehydration of ~2 wt% H₂O over a >50°C interval. Observations from the field suggest devolatilization across the PP-GS transition occurs over a very narrow interval given the rarity of transitional assemblages. Modelling suggests fluid release of 1.0–1.4 wt% resulting from prehnite breakdown over a ~10°C interval. This fluid may not be entirely lost from the rock package due to involvement in the hydration of igneous mineralogy across the PP-GS transition as observed in the Flin Flon sequence.     

Tectonothermal history of the western Lachlan Fold Belt, Australia: insights from white mica studies

Detailed b lattice parameter and illite crystallinity (IC) studies of K-white micas in slates from the Stawell and Ballarat-Bendigo Zones (SZ, BBZ) in the western Lachlan Fold Belt of Victoria, Australia, reveal a metamorphic pattern characterized by regional metamorphism associated with crustal thickening and younger contact metamorphism accompanied by deformation. The IC data indicate that rocks regionally metamorphosed prior to the intrusion of the Early and Late Devonian granitoids, vary in grade from epizonal (greenschist facies) to diagenetic (zeolite facies) and that most are of epizonal to anchizonal (prehnite–pumpellyite facies) grade. In the BBZ, a decrease in grade from west to east occurs. Across fault zones, IC values show little change, indicating that limited vertical displacement has occurred. This is in accord with the thin skinned deformation model proposed for the western Lachlan Fold Belt. The b lattice parameters (x=9.022 Å; n=137; σ_n=0.009) indicate baric conditions intermediate between those of New Hampshire (P=Al₂SiO₅ triple point) and Otago (intermediate P ). Thus, a moderately low geothermal gradient existed 450–430 Ma ago, when these rocks were deformed. K_D Fe/Mg (actinolite)/Fe/Mg (chlorite) values (0.52–0.70) obtained from coexisting actinolite and chlorite in metabasites from fault zones support the moderately high-P (c. 4 kbar) metamorphism suggested by the b cell parameter values. The metamorphic conditions indicated by these data are contrary to the low-P/high-T conditions proposed by previous authors, who inferred an intimate association between deformation, granitoid intrusion and gold mineralization. The b lattice parameter of white micas in slates adjacent to Early Devonian (c. 400 Ma) granitoids with schist bearing aureoles in the north-eastern part of the BBZ (x=9.002 Å; n=27; σ_n=0.007), indicate pressures in the order of c. 2.5 kbar which are in accord with those obtained from andalusite–cordierite and zoisite–garnet bearing assemblages observed in the higher grade metapelitic and calcareous rocks. This contrasts with the higher pressure (c. 4 kbar) existing during regional metamorphism and implies that c. 6.5–8 km of metasedimentary rocks in the BBZ were removed before the emplacement of the Early Devonian granitoids. Metamorphic assemblages in hornfelses associated with Late Devonian granitoids indicate a further 5–6 km of metasediment were removed in the next 40 Ma prior to their emplacement. This study shows the value of white mica studies in elucidating the tectonothermal history of a low-grade metamorphic terrane dominated by metapelitic rocks.     

Low‐grade metamorphism of mafic lavas,Upper Permian Broughton Formation,Sydney Basin

Coexisting primary minerals and hydrous alteration minerals in basalt lavas of the Upper Permian Broughton Formation of the Sydney Basin are indicative of the involvement of a hydrothermal fluid phase during low‐grade metamorphism. Variation and zonation of alteration phases in vesicles and vugs indicate that the alteration minerals developed in response to several episodes of precipitation, with early CO₂‐rich fluids producing assemblages rich in calcite and chlorite‐smectite while later CO₂‐poor fluids precipitated Ca‐zeolites, prehnite and pumpellyite. Vesicular parts of flows typically show much higher contents of alteration minerals than more massive parts of the same flow, but no systematic increase in either the style or intensity of alteration with increasing depth in the lava pile is evident. The presence of Ca‐zeolites, prehnite, pumpellyite and rare epidote suggests uppermost zeolite facies to lowermost prehnite‐pumpellyite facies metamorphism. Stability relationships of the metamorphic phases based on experimental and theoretical studies, used in conjunction with measured parameters for modern geothermal systems, indicate a peak metamorphic temperature of ～200–230°C while the extant stratigraphy indicates that the maximum depth of burial was ～ 1200 m. Alteration developed in response to circulation of hot, aqueous fluids generated by thermal convection cells associated with the Permian lavas and/or a large buried intrusion.     

Large-scale gold mineralization in eastern China induced by an Early Cretaceous clockwise change in Pacific plate motions

Orogenic gold (Au) deposits are the most important type, accounting for more than half of the world's proven Au reserves. They are mainly controlled by three key factors: (1) abundant andesitic rocks (SiO₂ of 55–60 wt.%) at depth, which have systematically higher Au contents than other rock types; (2) a pervasive transition from greenschist facies to amphibolite facies metamorphism within a short period, which releases S^2?-rich fluids that may scavenge Au from host rocks; and (3) deformation and fracturing under a compressive/transpressive tectonic regime. Orogenic belts at convergent margins are the best places for such mineralization because convergent margins are rich in andesites; the transition from greenschist to amphibolite facies recrystallization commonly occurs as a result of collision, compression, and thickening at convergent margins, forming large amounts of Au-rich fluids within a short period of time; and strong deformation and fracturing during orogenic processes provide channels for fluid transportation. Moreover, the overlying plate is injected and enriched by auriferous fluids released during amphibolite facies metamorphism of the subducting plate. The Pacific plate changed course by ～80° (from SW to NW) at approximately 125–122 Ma, reflecting an altered thermal structure and the elevation of the South Pacific plate attending the appearance of the plume head that formed the Ontong Java large igneous province. Consequently, the tectonic regime changed from extension to compressive/transpressive in eastern China, causing deformation, thickening, and metamorphism of the overriding plate, especially along weak crustal belts (e.g. overlying plates of palaeosutures), which resulted in world-class mineralization of orogenic Au deposits. During this process, pyrite changed to pyrrhotite during the transition from greenschist to amphibolite facies, releasing sulphur. Sulphur mobilized and scavenged Au and other chalcophile elements into metamorphic ore-forming fluids. A series of NE-trending compressive faults were formed at ?120 Ma as a result of continuous compression of the subducting Pacific plate, releasing these ore-forming fluids. Auriferous carbonate-rich quartz veins and/or metasomatized Au-bearing wall rocks were formed due to the decompression of the ascending ore-forming fluids. Orogenic belts along the margins of the North China craton and the Jiangnan block were the most favourable regions for mineralization. Compared with the former, the latter has much smaller proven Au reserves. However, more exploration is needed along the margins of the Jiangnan block. Promising targets include accessory faults and kink points of large, NE-trending Cretaceous faults that transect greenschist facies metamorphic rocks of the Niuwu and Jingtan Groups, etc.     

贵州水银洞超大型卡林型金矿同位素地球化学特征

贵州水银洞金矿是黔西南地区一个重要的超大型卡林型金矿,也是国内外少见的一个"高品位、大吨位"卡林型金矿。为了探讨矿床的物质来源问题,在前人研究资料的基础上,对矿床各个中段不同矿体的硫化物、碳酸盐矿物进行了系统采样,进行了S、C、O同位素的测试分析。同时对前人所作的放射性同位素Pb研究成果进行了整理分析。研究表明,水银洞金矿的成矿物质来源具有多源性。结合区域成矿地质背景,认为区域上卡林型金矿的物质来源具有壳幔结合的特点。