Dehydration melting and the granulite transition in metapelites from southern Namaqualand,S. Africa

In a prograde amphibolite-granulite transition zone in the Namaqualand Metamorphic Complex, metapelites show an interbanding of the amphibolite facies association biotite+sillimanite+quartz with the granulite facies association garnet+cordierite+K-feldspar. Relict graded bedding shows that compositional banding is of sedimentary origin. The garnet-cordierite-K-feldspar gneisses contain quartzofeldspathic segregations surrounding garnets, and have more Fe-rich bulk compositions than the biotite-sillimanite schists.The contrasting asemblages could have formed at the same pressure and temperature provided that a(H₂O) was systematically lower in the garnet-cordierite-K-feldspar layers. The a(H₂O) reduction resulted from the production of silicate melt by a vapour-absent continuous Fe-Mg reaction such as biotite+sillimanite+quartz=garnet+K-feldspar+liquid which affects Fe-rich compositions before vapour-absent melting occurs in more Mg-rich rocks. The segregations represent the solid and liquid products of the reaction.Such processes imply local control of a(H₂O), and indicate that this granulite transition did not result from a regional influx of metasomatising fluids.     

Retrograde Schists of the Amphibolite Facies at broken hill,New South Wales

Aluminous, mafic, felsic, calcareous, and sulphide‐rich rocks have been involved in localized deformation and retrograde metamorphism at Broken Hill, western New South Wales, where retrograde schist‐zones intersect high‐grade, regional metamorphic rocks of the lower granulite facies (or the amphibolite‐granulite facies transition). Although technically retrograde, the schists contain mineral assemblages indicative of the lower amphibolite facies. The schist‐zones were formed by local folding, apparently as part of the third stage of deformation in the Broken Hill area.     

Correlation between the Wonominta and the Broken Hill Blocks in western New South Wales: Evidence from Nd isotope studies of metasediments

Nd isotope studies of the oldest metasedimentary rocks from the Wonominta Block, western New South Wales reveal that these samples have a model age (T_DM) of 1780–2010 Ma, slightly younger than that of low‐grade Willyama Supergroup metasediments (1920–2160 Ma), and significantly younger than those ages previously reported from high‐grade rocks of the Broken Hill Block (2200–2300 Ma). These differences have important implications for tectonic reconstruction in this region and support a model of transitional tectonics from the Broken Hill to Wonominta Blocks, as suggested by earlier geochemical studies of mafic rocks. Those studies revealed that the mafic rocks from the basal sequence of the Wonominta Block may have formed in a back‐arc basin, developed from a propagating rifting, an environment contiguous to that in which Willyama Supergroup was deposited. These results also carry significant implications for tectonic reconstruction of eastern Australia.     

Stratigraphic subdivision and correlation of the upper part of the Willyama Complex,New South Wales

Within the complexly deformed and metamorphosed middle Precambrian Willyama Complex in western New South Wales, two distinct sedimentary sequences are recognized at the top of the succession. The fine‐grained carbonaceous Bijerkerno Beds conformably overlie a very thick psammopelitic unit termed the Wookookaroo Beds. These two sequences record different depositional histories, though they probably have similar source areas. The base of the Wookookaroo Beds is terminated against a significant tectono‐stratigraphic boundary, which may represent a major stratigraphic break within the Willyama Complex. Beneath this boundary, felsic and mafic gneisses and high‐grade metasediments are correlated with mine‐sequence rocks such as are developed within the vicinity of the main Broken Hill lode. Correlations between the stratigraphy, as established at Bijerkerno, and other areas from the Willyama Complex, help provide a fuller understanding of this Precambrian terrain.     

柴西缘花土沟超高温变质地体中的镁铝麻粒岩岩石成因及其与熔体行为的关系

镁铝麻粒岩泛指一类全岩化学成分富镁、铝的麻粒岩相变质岩,是研究超高温变质作用的峰期变质条件和变质演化历史的重要对象,但目前对它的原岩属性和岩石成因的认识仍十分有限。本文以柴达木地块西缘的花土沟超高温变质地体为例,在野外调查基础上,对镁铝麻粒岩和泥质片麻岩进行了岩相学和全岩地球化学分析,发现镁铝麻粒岩与含浅色体的泥质片麻岩的SiO₂、TiO₂、P₂O₅ 含量相似,^TFe₂O₃、Al₂O₃、MnO、CaO、Na₂O含量虽有差异但变化范围存在交集。此外,两类岩石具有相似的微量和稀土元素配分曲线,结合两者的矿物组合也存在相似性,提出花土沟镁铝麻粒岩的原岩可能是与泥质片麻岩类似的泥质沉积岩。从低角闪岩相变泥质岩到含浅色体的泥质片麻岩,再到镁铝麻粒岩,其全岩化学成分向着贫铝、钙、钾、钠,富铁、镁的趋势变化。其中,高X_Mg值（0.51~0.69）是镁铝麻粒岩与其他泥质片麻岩（X_Mg=0.34~0.43）的最大差别。通过对变泥质岩的相平衡模拟和理论计算,发现部分熔融和熔体丢失能解释大部分的变化趋势,但基本不影响全岩X_Mg值;只有在进变质升温过程中丢失含石榴子石的熔体才能造成变泥质岩的镁铝麻粒岩化。此外,富石榴子石的泥质残留体相比附近的含浅色体泥质片麻岩,贫硅、钠、钾,富集铝、铁、镁、锰、钙,重稀土元素含量显著高于后者,上述地球化学特征对应石榴子石熔体的加入而后长英质熔体的丢失,支持野外观察到的熔体携带石榴子石迁移的现象。最后,对镁铝麻粒岩只呈透镜体产出的原因做出了推测,即熔体很难带着石榴子石完成长距离迁移,只有被长英质正片麻岩包围的泥质沉积物,其进变质加热阶段形成的熔体才能携带石榴子石完全迁出原岩,促成变泥质岩透镜体的镁铝麻粒岩化,目前仍需更多的相关研究来验证这一推测。在世界其他高温-超高温变质岩区,石榴子石熔体的迁出和泥质岩的镁铝麻粒岩化可能也不同程度有所保留和记录。     

Pan-African decompressional P-T path recorded by granulites from central Dronning Maud Land, Antarctica

A high-grade metamorphic complex is exposed in Filchnerfjella (6–8°E), central Dronning Maud Land. The metamorphic evolution of the complex has been recovered through a study of textural relationships, conventional geothermobarometry and pseudosection modelling. Relicts of an early, high-P assemblage are preserved within low-strain mafic pods. Subsequent granulite facies metamorphism resulted in formation of orthopyroxene in rocks of mafic, intermediate to felsic compositions, whereas spinel + quartz were part of the peak assemblage in pelitic gneisses. Peak conditions were attained at temperatures between 850–885 °C and 0.55–0.70 GPa. Reaction textures, including the replacement of amphibole and garnet by symplectites of orthopyroxene + plagioclase and partial replacement of garnet + sillimanite + spinel bearing assemblages by cordierite, indicate that the granulite facies metamorphism was accompanied and followed by decompression. The observed assemblages define a clock-wise P-T path including near-isothermal decompression. During decompression, localized melting led to formation of post-kinematic cordierite-melt assemblages, whereas mafic rocks contain melt patches with euhedral orthopyroxene. The granulite facies metamorphism, decompression and partial crustal melting occurred during the Cambrian Pan-African tectonothermal event.     

Constraints on the early metamorphic evolution of Broken Hill, Australia, from in situ U-Pb dating and REE geochemistry of monazite

The Broken Hill Pb-Zn deposit, New South Wales Australia, is hosted in granulite facies gneisses of the Southern Curnamona Province (SCP) that have long been known to record a polydeformational and polymetamorphic history. The details of this potentially prolonged tectonothermal history have remained poorly understood because of a historical emphasis on conventional (i.e. grain mount) U-Pb zircon geochronology to reveal details of the sedimentary, magmatic and metamorphic history of the rock that crops out in the vicinity of the city of Broken Hill. An alternative approach to unravelling the metamorphic history of the granulite facies gneisses in and around Broken Hill is to date accessory minerals, such as monazite, that participate in sub-solidus metamorphic reactions. We have taken advantage of the high spatial resolution and high sensitivity afforded by SHRIMP monazite geochronology to reconstruct the early history of the metamorphic rocks at Broken Hill. In contrast to previous studies, in situ analysis of monazite grains preserved in their original textural context in polished thin sections is used. Guided by electron microprobe X-ray maps, SHRIMP U-Pb dates for three distinct monazite compositional domains record pulses of monazite growth at c. 1657 Ma, c. 1630 Ma and c. 1602 Ma. It is demonstrated that these ages correspond to monazite growth during lower amphibolite facies, upper amphibolite facies and granulite facies metamorphism, respectively. It is speculated that this progressive heating of the SCP crust may have been driven by inversion of the upper crust during the Olarian Orogeny that was pre-heated by magmatic underplating at c. 1657 Ma.     

Origin,age, and significance of deep‐seated granulite‐facies migmatites in the Barrow zones of Scotland,Cairn Leuchan,Glen Muick area

Petrological modelling of granulite‐facies mafic and semipelitic migmatites from Cairn Leuchan, northeast Scotland, has provided new constraints on the pressure (P) and temperature (T) conditions of high‐grade metamorphism in the type‐locality Barrow zones. Phase diagrams constructed in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O₂ system have constrained the P?T conditions of peak metamorphism in the Glen Muick region of the upper sillimanite zone (Sill+Kfs) to have been at least ～840°C at ～9 kbar (high‐P granulite facies). These conditions are ～120°C and ～3 kbar higher than those recorded by lower sillimanite zone (Sill+Ms) units located only a few kilometres away to the southeast at Glen Girnock, indicating the presence of a significant thermal and barometric high exposed within the Scottish Dalradian, and supporting previous suppositions of a potential tectonic break between the two regions. U–Pb zircon geochronology performed on these mafic migmatites produced ages of c. 540–470 Ma from grains with both igneous and metamorphic morphological characteristics. Their basaltic protoliths likely formed during a period of volcanism dated at c. 570 Ma, associated with passive‐margin extension prior to the onset of Iapetus Ocean closure, and high‐grade metamorphism and partial melting is interpreted to have taken place at c. 470 Ma, synchronous with sillimanite‐grade metamorphism recorded elsewhere in the Dalradian. These high‐grade Cairn Leuchan lithologies are interpreted as representing a fragment of Grampian Terrane lower crust that was exhumed via displacement along a steeply dipping tectonic discontinuity related to the Portsoy–Duchray Hill Lineament, and are not pre‐Caledonian Mesoproterozoic basement, as suggested by some previous studies. Veins within some mafic migmatites in the Cairn Leuchan area, composed almost entirely (>80%) of garnet, with minor quartz, plagioclase, amphibole, and clinopyroxene, are interconnected with leucosomes and are interpreted to represent former garnet‐bearing melt segregations that have been locally drained of almost all melt. Thus, mafic components of the lower crust, currently underlying relatively lower grade metasediments exposed to the southeast, may represent a potential source rock for widely documented, post‐orogenic felsic plutons, sills, and dykes that occur throughout the Grampian Terrane.     

The origin of the albite-rich rocks enclosing the cobaltian pyrite deposit at Thackaringa,N.S.W., Australia

A stratabound disseminated and massive cobaltian pyrite deposit at Thackaringa, 30 km SW. of Broken Hill occurs in banded albite-quartz-biotite rocks which are conformable with regional structure and stratigraphy. The albite rocks are associated with pelitic and psammitic metasediments, amphibolite and minor quartzgahnite rocks. The deposit has undergone granulite facies metamorphism and there is no evidence that pyrite has reacted to form pyrrhotite. It is suggested that the albite rocks were an analcimized tuffaceous rock in which disseminated pyrite formed by a volcanic exhalative process. A massive cobaltian pyrite body is enclosed by a metamorphosed hydrothermal alteration zone characterised by an increase in quartz, magnesian fluorobiotite and Rb.     

Mineralogy and pressure–temperature–time path of Cretaceous granulite gneisses, south-eastern San Gabriel Mountains, southern California

Mid-Cretaceous granulite gneisses crop out in a narrow belt in the Cucamonga region of the south-eastern foothills of the San Gabriel Mountains, southern California. Interlayered mafic granulites and pelitic, carbonate, calc-silicate and quartzofeldspathic metasediments record hornblende granulite subfacies metamorphism at approximately 8 kbar and 700–800°C. Regional deformation and formation of banded gneisses ceased by c. 108 Ma. although mafic-intermediate magmatism and high-grade metamorphism continued locally as late as c. 88 Ma. Garnet zoning in metapelitic gneisses suggests that peak metamorphism was followed locally by a period of near-isobaric cooling, but this interpretation requires diachronous cooling of the granulite belt which cannot be demonstrated without detailed thermo-chronological data. It is more likely that the entire terrane remained at granulite facies P–T conditions until 88 Ma, followed by rapid uplift associated with juxtaposition against adjacent middle and upper crustal arc terranes. Uplift occurred between c. 88 and 78 Ma at rates of approximately 1–2 km Ma^-1. The geotectonic evolution of the Cucamonga granulites is similar to mid-Cretaceous high- P granulites in the Sierra Nevada and Salinian block of central California. Late Cretaceous uplift common to these granulites may provide an important tectonic link between dismembered Mesozoic batholithic terranes in the California Cordillera.     

Metamorphic evolution of exhumed middle to lower crustal rocks in the Mojave Extensional Belt, southern California, USA

The Waterman Metamorphic Complex of the central Mojave Desert was exposed as a consequence of early Miocene detachment-dominated extension. However, it has evidence consistent with a more extensive geological history that involves collision of a crustal fragment(s), tectonic thickening by overthrusting and two periods of extension. The metamorphic complex contains granitoid intrusives and felsic mylonitic gneisses as well as polymetamorphic rocks that include marble, calc-silicate, quartzite. mafic granulite, pyribolite, amphibolite, migmatite and biotite schist. The latter group of rocks was affected by an initial series of high-grade metamorphic events (M1 and M2) and a localized lower grade overprint (M3). The initial metamorphism (M1) can be separated into two stages along its high-grade P–T path: M1a, a granulite facies metamorphism at 800–850° C and 7.5–9 kbar and Mlb, an upper amphibolite facies overprint at 750–800° C and 10–12 kbar. M1a developed mineral assemblages and textures consistent with granulite facies conditions at a reduced activity of H₂O and is associated with intense ductile deformation (D1) and minor local partial melting. M1b overprinted the granulite assemblages with a series of hydrous phases under conditions of increasing pressure and H₂O activity and is accompanied by little or no deformation. M2 developed at lower pressures and temperatures (650–750° C, 4.5–5.5 kbar) and is distinguished by a second local overprint of hydrous phases that reflects an input of aqueous fluids probably associated with the intrusion of a series of granitic dykes and veins. Effects of M3 are confined to the Mitchel detachment zone, an anastomosing early Miocene detachment fault, and are characterized by local ductile/brittle deformation (D2) of the pre-existing high-grade rocks and granitoid intrusives and by the production of mylonites and mylonitic gneisses under greenschist facies conditions (300–350° C, 3–5 kbar). The initial overprint (M1a) represents metamorphism, devolatilization and minor partial melting of supracrustal rocks under granulite facies conditions as a consequence of tectonic and, possibly, magmatic thickening. The increasing pressure transition of M1a to M1b reflects a period of continued compressional tectonism, thrusting and influx of H₂O, in part, locally related to crystallization of partial melts. The near isothermal decompression between M1b and M2 probably represents a pre-112-Ma extensional episode that may have been the result of a decompressional readjustment of a thickened crust. Following the initial extensional event, the metamorphic complex remained at depths of 10–17 km for at least 90 Ma until it was uplifted following Miocene extension. M3 develops locally in response to this second extensional period resulting from the early Miocene detachment faulting.     

Role of H2O in the formation of garnet coronas during near‐isobaric cooling of mafic granulites: the Tasiusarsuaq terrane,southern West Greenland

The Mesoarchaean Tasiusarsuaq terrane of southern West Greenland consists of Tonalite–trondhjemite–granodiorite gneisses and, locally, polymetamorphic mafic and ultramafic rocks. The terrane experienced medium‐pressure granulite facies conditions during M_1A in the Neoarchean, resulting in the development of two‐pyroxene melanosome assemblages in mafic granulites containing garnet‐bearing leucosome. Reworking of these rocks during retrogression introduced garnet to the melanosome in the form of overgrowths, coronas and grain necklaces that separate the mafic minerals from plagioclase. NCFMASHTO pseudosection modelling constrains the peak metamorphism during M_1A to ～850 °C and 7.5 kbar at fluid‐saturated conditions. Following M_1A, the rocks retained their M_1A H₂O content and became fluid‐undersaturated as they underwent near‐isobaric cooling to ～700 °C and 6.5–7 kbar, prior to reworking during M_1B. These low H₂O contents allowed for the formation of garnet overgrowths and coronas during M_1B. The stability of garnet is greatly increased to lower pressure and temperature in fluid‐absent, fluid‐undersaturated mafic rocks, indicating that fluid and melt loss during initial granulite facies metamorphism is essential for the introduction of garnet, and the formation of garnet coronas, during retrogression. The occurrence of garnet coronas is consistent with, but not unique to, near‐isobaric cooling paths.     

Structural,petrological and chemical analysis of syn‐kinematic migmatites: insights from the Western Gneiss Region,Norway

Evidence of melting is presented from the Western Gneiss Region (WGR) in the core of the Caledonian orogen, Western Norway and the dynamic significance of melting for the evolution of orogens is evaluated. Multiphase inclusions in garnet that comprise plagioclase, potassic feldspar and biotite are interpreted to be formed from melt trapped during garnet growth in the eclogite facies. The multiphase inclusions are associated with rocks that preserve macroscopic evidence of melting, such as segregations in mafic rocks, leucosomes and pegmatites hosted in mafic rocks and in gneisses. Based on field studies, these lithologies are found in three structural positions: (i) as zoned segregations found in high‐P (ultra)mafic bodies; (ii) as leucosomes along amphibolite facies foliation and in a variety of discordant structures in gneiss; and (iii) as undeformed pegmatites cutting the main Caledonian structures. Segregations post‐date the eclogite facies foliation and pre‐date the amphibolite facies deformation, whereas leucosomes are contemporaneous with the amphibolite facies deformation, and undeformed pegmatites are post‐kinematic and were formed at the end of the deformation history. The geochemistry of the segregations, leucosomes and pegmatites in the WGR defines two trends, which correlate with the mafic or felsic nature of the host rocks. The first trend with Ca‐poor compositions represents leucosome and pegmatite hosted in felsic gneiss, whereas the second group with K‐poor compositions corresponds to segregation hosted in (ultra)mafic rocks. These trends suggest partial melting of two separate sources: the felsic gneisses and also the included mafic eclogites. The REE patterns of the samples allow distinction between melt compositions, fractionated liquids and cumulates. Melting began at high pressure and affected most lithologies in the WGR before or during their retrogression in the amphibolite facies. During this stage, the presence of melt may have acted as a weakening mechanism that enabled decoupling of the exhuming crust around the peak pressure conditions triggering exhumation of the upward‐buoyant crust. Partial melting of both felsic and mafic sources at temperatures below 800 °C implies the presence of an H₂O‐rich fluid phase at great depth to facilitate H₂O‐present partial melting.     

The Ivrea—Verbano and Strona-Ceneri Zones, Northern Italy: A cross-section of the continental crust—New evidence from seismic velocities of rock samples

Seismic velocities under confining pressures to 10 kbar have been measured for rocks of the Ivrea—Verbano and Strona—Ceneri Zones of northern Italy, a metamorphic complex thought to represent a cross-section of the continental crust and crust—mantle boundary. Laboratory-determined compressional wave velocities for schists and gneisses of the amphibolite facies found in the upper levels of the section (having an average density of 2.74 g/cm³) average 6.45 km/sec at pressures between 6 and 10 kbar. These increase with depth to values greater than 7.1 km/sec for amphibolites and rocks of the amphibolite—granulite facies transition and to 7.5 km/sec. (average density 3.06 g/cm³) in intermediate and mafic granulite facies rocks near the base of the section. Compressional wave velocities then abruptly increase to 8.5 km/sec in ultramafic complexes near the Insubric Line. Regional geophysical surveys show that P_g is 6.0 km/sec (density of 2.7 g/cm³), P* is 7.2–7.4 km/sec (density of 3.1 g/cm³) and P_n is 8.1 km/sec, values which are in agreement with the laboratory data when effects of temperature are taken into consideration. Estimated thicknesses of exposed rock units are in reasonable agreement with thicknesses determined for crustal layers in seismic refraction experiments. The agreement between the regional crustal structure and the laboratory-determined values of velocity and density provides strong evidence for the hypothesis that the rocks of this metamorphic complex represent a cross-section of the continental crust of the Po Basin.Using the Ivrea—Verbano and Strona—Ceneri sequence as a model of the continental crust, the crust of northern Italy is shown to consist of a thick series of metamorphic rocks with greenschist facies rocks occupying the uppermost levels. These grade downward into amphibolite facies gneisses and schists with occasional granitic intrusives. The Conrad discontinuity is marked by a change from silicic and intermediate amphibolite facies gneisses to intermediate and mafic granulite facies rocks in which hydrous minerals diminish in abundance and thus represents a distinct transition in terms of both composition and metamorphic grade. The lower crust is dominated by a heterogeneous series of mafic and metapelitic rocks in the granulite facies. Importantly, metasedimentary rocks of intermediate silica content found in the complex can have compressional wave velocities equivalent to velocities in mafic rocks suggesting that the lower continental crust everywhere is not necessarily mafic in composition. Ultramafic complexes near the Insubric Line may represent the upper mantle of the continent and their setting suggests that the continental crust-upper mantle boundary is sharp and is not isochemical.     

Widespread 18O depletion in some precambrian granulites of Australia

In the Strangways Range a broad tract of lower Proterozoic mafic and silicic granulites with δ ¹⁸O = 0.1 to 7.3% is depleted in ¹⁸O on average by 2–47% compared with high-grade gneisses and granitoids of Canada and other shields. The Fraser Range mafic granulites (δ¹⁸O ~ 7.2%) are enriched with respect to unaltered sea-floor basalts (~5.7%).In some rocks depletion in ¹⁸O could be related to dehydration during granulite-facies metamorphism and removal of the resultant products of partial melting. In other rocks pre-granulite reaction between heated seawater and hot basic intrusives seems to be a plausible mechanism of depletion in ¹⁸O. A direct correlation between depletion in ¹⁸O and the abundance of brown granulite hornblende suggests that ¹⁸O-depleted water was present in certain mafic rocks before the onset of granulite metamorphism, whereas in others brown hornblende was introduced during a phase of the granulite facies metamorphism itself.     

The deformation and recrystallization of biotite in the Woodroffe Thrust mylonite zone

The deformation and recrystallization microstructures in biotite from the Woodroffe Thrust mylonites are described and interpreted. The degree of strain causing recrystallization and the nucleation mechanisms differ across the mylonite zone. These differences are associated with the contrast in water content between the granulite and amphibolite facies felsic gneisses on either side of the zone. p]In moderately mylonitized granulite facies felsic gneisses (0.1–0.6% H₂O) subgrains form in intensely deformed host biotite and recrystallization mechanisms involve subgrain rotation both on host grain boundaries and associated with kink band bulge. In the amphibolite facies felsic gneisses (0.9–1.2% H₂O) the biotite recrystallizes by a mechanism involving localized internal kinking of the host and subsequent migration of high angle boundaries generated on the kink limbs. This combined with rotation due to the concurrent deformation generates high angle grain boundaries around the entire original kink limb and thus a new grain.     

The deformation and recrystallization of quartz in a mylonite zone, Central Australia

Hydrolytic weakening in quartz has been extensively demonstrated by experimental deformation of single crystals and aggregates. This paper describes the deformation and recrystallization microstructures and preferred orientations of quartz in a mylonite zone separating granulite facies (0.2% H₂O) from amphibolite facies (1.0% H₂O) acid gneisses. The transition from slightly deformed country rock on both sides to the ultimate product of mylonitization (a phyllite) is described and the following major differences are noted:

1. (1) The strain prior to recrystallization is higher on the granulite side.

2. (2) Misorientations across deformation-band boundaries are much higher on the granulite side.

3. (3) Subgrains and new grains are considerably smaller at the same stage of recrystallization on the granulite side.

4. (4) Preferred orientation of [0001] developes more rapidly with respect to strain on the amphibolite side.

5. (5) There is a closer orientation relationship between host and recrystallized (new) grains on the granulite side.

The microstructures and preferred-orientation development on both sides are related to concurrent ductile deformation, dynamic recovery and recrystallization processes. The differences between the two sides is attributed to the difference in bulk H₂O content of the rocks and a resultant difference in strain rate. The suggested effect of water on recovery processes is favoured over its possible role in slip processes.     

Sapphirine-kornerupine rocks from the Reynolds Range,central Australia: constraints on the uplift history of a Proterozoic low pressure terrain

Sapphirine-kornerupine-bearing rocks from the Reynolds Range, Northern Territory, Australia preserve spectacular metamorphic reaction textures that provide valuable insights into the regional metamorphic uplift history. The rocks occur in pods that are several meters in diameter within high-temperature, low-pressure (750 to 800°C and 4 to 5 kbar) granulite facies exposures of the early Proterozoic Lander Rock beds, a laterally extensive sequence of folded pelitic and quartzose metasediments. The pods are not associated with large volumes of partial melts and are likely to have formed by metasomatism near the peak of M₂ metamorphism. The rocks in the pods consist of high-temperature Mg- and Al-rich minerals such as boron-free korneurpine, and are coarse-grained (0.5 to >15 cm), non-foliated, and locally nearly monomineralic. The growth of the coarse minerals in the pods largely post-dated the high-grade regional metamorphic D₂ fabric and completely reconstructed the precursor rocks. The retrograde metamorphic reaction textures show that the early retrogression from the M₂ granulite facies conditions was characterized not by isobaric cooling as previously proposed, but by nearly isthermal decompression. These data imply that the Reynolds Range did not follow a simple anticlockwise P-T-t path. Because rocks such as these preserve information from a only restricted portion of the metamorphic history and can preserve evidence of decompression reactions more clearly than many more ordinary lithologies, they can be especially important for discerning metamorphic P-T-t paths.This paper is a contribution to IGCP Project 304, Lower Crustal Processes.     

Clay mineral,geochemical and Sr–Nd isotopic fingerprinting of sediments in the Murray–Darling fluvial system,southeast Australia

The Willyama Supergroup of the Broken Hill region in southern Australia consists of supracrustal sedimentary and magmatic rocks, formed between 1810 and 1600 Ma. A statistical analysis of nearly 2000 SHRIMP U–Pb zircon spot ages, compiled from published and unpublished sources, provides evidence for three distinct tectonostratigraphic successions and four magmatic events during this interval. Succession 1 includes Redan Geophysical Zone gneisses and the lower part of the Thackaringa Group (Cues Formation). These rocks were deposited after 1810 Ma and host granite sills of the first magmatic event (1710–1700 Ma). Succession 2 includes the upper Thackaringa Group (Himalaya Formation), the Broken Hill Group and the Sundown Group and was deposited between 1710 and 1660 Ma. These rocks all contain detrital zircons from the first magmatic event (1710–1700 Ma) and in some cases from the second magmatic event (1690–1680 Ma). The second magmatic event (1690–1680 Ma) was bimodal, resulted from crustal extension, and was coeval with deposition of the Broken Hill Group and deepening of the basin. With this event a mafic sill swarm focused in the Broken Hill Domain. Mafic sills lack any trace of inheritance, unlike the granitoids that commonly contain inherited zircons typical of the supracrustal sediments. Succession 3, the Paragon Group and equivalents were deposited after 1660 Ma, but before a regional metamorphic event at 1600 Ma. Metamorphism was closely followed by inversion of the succession into a fold‐and‐thrust belt, accompanied by a fourth late to post‐orogenic magmatic event (ca 1580 Ma) characterised by granite intrusion and regional acid volcanism (the local equivalents of the Gawler Range Volcanics in South Australia).     

Rare-earth element geochemistry of Madras granulites

Archaean granulites from the type charnockite locality of Pallavaram, Madras City, Tamil Nadu, southern India consist of a bimodal suite of basic and silicic orthogneisses, associated with metasedimentary gneisses (khondalites). Charnockite is the dominant rock type. Basic granulites display a tholeiitic trend of strong iron enrichment accompanied by an increase in the concentration of the rare earth elements (REE), and the development of appreciable negative europium anomalies. These trends are considered to reflect low pressure fractional crystallisation of pyroxene-plagioclase assemblages under conditions of lowf _{O 2}. Ultramafic granulites may represent the cumulus material of such a process. The silicic granulites (charnockites) are generally K₂O rich and in marked contrast to the low K₂O (tonalitic) silicic gneisses which dominate most granulite facies terrains. Their REE abundances, however, embrace the complete range of patterns observed in both K-rich and K-poor Archaean gneisses. The presence of a large number of pre-granulite facies potassic pegmatites in the area suggests metasomatism of an originally less potassic suite of rocks. Trace element considerations lead to a model whereby metasomatism and partial fusion of silicic gneisses in the terrain preceded the granulite facies metamorphic event. This sequence of events is best related to fluctuations in the composition of metamorphic fluids in the lower crust.