Showing papers on "Granulite published in 1998"

Thermal evolution of Archean basement rocks from the eastern part of the North China craton and its bearing on tectonic setting

Abstract: The basement rocks in the eastern zone of the North China craton are composed predominantly of pretectonic tonalitic-trondhjemitic-granodioritic gneisses and syntectonic granitoids, with rafts of supracrustal rocks consisting of ultramafic to felsic volcanic and sedimentary rocks, metamorphosed over a range of conditions from greenschist to granulite facies. Most mafic granulites, amphibolites, and some pelitic gneisses and schists preserve the prograde, peak, and post-peak textures. The prograde metamorphic stage is indicated by mineral inclusions within minerals of the peak stage, represented by the assemblages of hornblende + plagioclase + quartz ± biotite in mafic granulites, chlorite + actinolite + epidote + plagioclase + quartz in amphibolites, and biotite + plagioclase + quartz in pelitic gneisses. The peak stage is shown by assemblages of orthopyroxene + clinopyroxene + garnet + plagioclase + quartz in the mafic granulites, hornblende + plagioclase + quartz + garnet in garnetiferous amphibolites, ...

Continuation of the Mozambique Belt Into East Antarctica: Grenville‐Age Metamorphism and Polyphase Pan‐African High‐Grade Events in Central Dronning Maud Land

Abstract: The about 500 km long coastal stretch of central Dronning Maud Land (DML), East Antarctica, is critical for understanding both Gondwana and Rodinia assembly. In common Gondwana reconstructions central DML lies at the potential southern extension of the Mozambique Belt. We report the first extensive geochronological study of magmatic and metamorphic rocks from the area. These new U‐Pb SHRIMP zircon and Sm‐Nd‐data of rocks sampled during the German international GeoMaud 1995/96 expedition indicate that the oldest rocks in central DML are Mesoproterozoic in age. The crystallization ages of metavolcanic rocks were determined at c. 1130 Ma. Syn‐tectonic granite sheets and plutons give ages of c. 1080 Ma, contemporaneous with metamorphic zircon growth at granulite facies conditions. An anorthosite intrusion and a charnockite are dated at c. 600 Ma. Subsequent metamorphism is recorded for at least two different episodes at c. 570–550 Ma and between 530 to 515 Ma. The latter metamorphic event reached granulite fa...

U-Pb monazite ages in amphibolite- to granulite-facies orthogneiss reflect hydrous mineral breakdown reactions: Sveconorwegian Province of SW Norway

Abstract: In the Rogaland–Vest Agder terrain of the Sveconorwegian Province of SW Norway, two main Sveconorwegian metamorphic phases are reported: a phase of regional metamorphism linked to orogenic thickening (M1) and a phase of low-pressure thermal metamorphism associated with the intrusion of the 931 ± 2 Ma anorthosite-charnockite Rogaland igneous complex (M2). Phase M1 reached granulite facies to the west of the terrane and M2 culminated locally at 800–850 °C with the formation of dry osumilite-bearing mineral associations. Monazite and titanite U-Pb geochronology was conducted on 17 amphibolite- to granulite-facies orthogneiss samples, mainly from a suite of 1050 +2/−8 Ma calc-alkaline augen gneisses, the Feda suite. In these rocks, prograde negatively discordant monazite crystallized during breakdown of allanite and titanite in upper amphibolite facies at 1012–1006 Ma. In the Feda suite and other charnockitic gneisses, concordant to slightly discordant monazite at 1024–997 Ma probably reflects breakdown of biotite during granulite-facies M1 metamorphism. A spread of monazite ages down to 970 Ma in biotite ± hornblende samples possibly corresponds to the waning stage of this first event. In the Feda suite, a well defined monazite growth episode at 930–925 Ma in the amphibolite-facies domain corresponds to major clinopyroxene formation at the expense of hornblende during M2. Growth or resetting of monazite was extremely limited during this phase in the granulite-facies domain, up to the direct vicinity of the anorthosite complex. The M2 event was shortly followed by cooling through ca. 610 °C as indicated by tightly grouped U-Pb ages of accessory titanite and titanite relict inclusions at 918 ± 2 Ma over the entire region. A last generation of U-poor monazite formed during regional cooling below 610 °C, in hornblende-rich samples at 912–904 Ma. This study suggests: (1) that monazite formed during the prograde path of high-grade metamorphism may be preserved; (2) that monazite ages reflect primary or secondary growth of monazite linked to metamorphic reactions involving redistribution of REEs and Th, and/or fluid mobilisation; (3) that the U-Pb system in monazite is not affected by thermal events up to 800–850 °C, provided that conditions were dry during metamorphism.

Tectonic evolution of the southern Laurentian Grenville orogenic belt

Abstract: The Grenville orogenic belt along the southern margin of Laurentia records more than 300 m.y. of orogenic activity culminating in arc-continent and continent-continent collision ca. 1150–1120 Ma. Exposures in Texas provide a unique profile across the Grenville orogen from the orogen core to the cratonal margin. In the Llano uplift of central Texas, ca. 1360–1232 Ma upper amphibolite–lower granulite facies, polydeformed supracrustal and plutonic rocks represent the core of the collisional orogen. This exposure contains a suture between a 1326–1275 Ma exotic island-arc terrane and probable Laurentian crust and records A-type subduction. In west Texas, 1380–1327 Ma amphibolite to greenschist facies, polydeformed supracrustal rocks are thrust over ca. 1250 Ma carbonate and volcanic rocks along the cratonal margin. The carbonate and volcanic rocks form a narrow thrust belt with post–1123 Ma synorogenic sedimentary rocks, which grade into undeformed sedimentary rocks northward on the Laurentian craton. The Texas basement reveals a consistent but evolving tectonic setting for the southern margin of Laurentia during Mesoproterozoic time. This paper summarizes recent advances in our knowledge of the Texas basement and proposes plate models to explain the tectonic evolution of this margin during Mesoproterozoic time. The orogenic history is strikingly similar to that of the Canadian Grenville orogen and requires a colliding continent off the southern Laurentian margin during the assembly of Rodinia.

Geochronology and geochemistry of a Mesozoic magmatic arc system, Fiordland, New Zealand

Abstract: The Median Tectonic Zone in Eastern Fiordland, SW New Zealand, comprises a tectonically disrupted belt of Mesozoic magmatic arc rocks related to subduction along the palaeo-Pacific margin of Gondwana. New ion microprobe (SHRIMP) U–Pb zircon ages confirm that the bulk of the plutonic rocks in eastern Fiordland range from Mid-Jurassic to Early Cretaceous (168–137 Ma) in age. Carboniferous age granitoids occur in SW Fiordland, along the western side of, and within the zone. Triassic plutonic rocks appear to be restricted to the eastern side of the zone. The Mid-Jurassic–Early Cretaceous igneous rocks (collectively referred to as the Darran Suite) are cut by several plutons of Na-rich granitoid (Separation Point Suite) that give ages of c. 124 Ma, slightly older than equivalent rocks in the NW part of the South Island. Early Cretaceous granulite facies orthogneisses (126–119 Ma) in western Fiordland (Western Fiordland Orthogneiss) are considered to be the lower crustal equivalent of the Separation Point plutons. The majority of the Darran Suite rocks are I-type, hornblende-bearing calc-alkaline igneous rocks, most likely derived from melting in the mantle wedge above a subducting slab of oceanic lithosphere. In contrast, the Separation Point-type plutons are Na-rich, alkali-calcic granitoids with high concentrations of Sr (typically >500 ppm and up to 1000 ppm) and low concentrations of Y (≤5 ppm) and heavy REE ( 87 Sr/ 86 Sr initial ratios of c. 0.7038, and a Nd values of c. +3 at 120 Ma. Their geochemistry is consistent with melting of a mafic protolith of garnet amphibolite mineralogy. Mafic Darran Suite rocks have the appropriate chemical and isotopic compositions to generate the Western Fiordland Orthogneiss and the higher level Separation Point type plutons. We suggest that the sudden appearance of large volumes of Na-rich magma during the Early Cretaceous was triggered tectonically, perhaps by thrusting of the Median Tectonic Zone arc beneath western New Zealand. Melting of basal arc underplate at depths of >40 km would then have generated Na-rich granitoids, leaving residues of garnet + clinopyroxene + amphibole.

An overview and tectonic synthesis of the pre-Tertiary very-high-pressure metamorphic and associated rocks of Java, Sulawesi and Kalimantan, Indonesia

Abstract: High-pressure metamorphic rocks are widely distributed in Cretaceous accretionary complexes throughout Java, Sulawesi (formerly Celebes) and southeast Kalimantan (Indonesian Borneo). Many of these rocks occur as imbricate slices of carbonate, quartzose and pelitic schists of shallow marine or continental margin parentage, interthrust with subordinate basic schists and serpentinite. They are predominantly of low-to-intermediate metamorphic grade (300 60 km), however, are sporadically exposed, usually as tectonic blocks, throughout the Cretaceous accretionary complexes. They include eclogite, garnet–glaucophane rock (P = 18–24 kbar, T = 580–620 °C), and jadeite–garnet–quartz (?coesite) rock (?P > 27 kbar, T = 720–760 °C) in Bantimala, southwest Sulawesi; eclogite and garnet granulite in west central Sulawesi; eclogite and jadeite-glaucophane-quartz rock (P ∼ 22 kbar, T ∼ 530 °C) in Luk Ulo, Central Java; and Mg–chloritoid-bearing whiteschists (P ∼ ?18 kbar) in the Meratus Mountains, southeast Kalimantan. Garnet lherzolites from depths of > 60 km are also associated with schists in east central Sulawesi (P = 22–28 kbar, T = 1000–1100 °C), west central Sulawesi (P = 16–20 kbar, T = 1050–1100 °C); and garnet pyroxenite (P ∼ 20 kbar, T ∼ 850 °C) occurs as blocks with pyrope–kyanite amphibolite, eclogite and blueschist, within Miocene conglomerate in Sabah, northeast Borneo. Many of the metamorphic rocks were probably recrystallized in a north-dipping subduction zone at the margin of the Sundaland craton in the Early Cretaceous. Exhumation may have been facilitated by the collision of a Gondwanan continental fragment with the Sundaland margin at ca 120–115 Ma.

The age and origin of a thick mafic–ultramafic keel from beneath the Sierra Nevada batholith

Abstract: We present evidence for a thick (∼100 km) sequence of cogenetic rocks which make up the root of the Sierra Nevada batholith of California. The Sierran magmatism produced tonalitic and granodioritic magmas which reside in the Sierra Nevada upper- to mid-crust, as well as deep eclogite facies crust/upper mantle mafic–ultramafic cumulates. Samples of the mafic–ultramafic sequence are preserved as xenoliths in Miocene volcanic rocks which erupted through the central part of the batholith. We have performed Rb-Sr and Sm-Nd mineral geochronologic analyses on seven fresh, cumulate textured, olivine-free mafic–ultramafic xenoliths with large grainsize, one garnet peridotite, and one high pressure metasedimentary rock. The garnet peridotite, which equilibrated at ∼130 km beneath the batholith, yields a Miocene (10 Ma) Nd age, indicating that in this sample, the Nd isotopes were maintained in equilibrium up to the time of entrainment. All other samples equilibrated between ∼35 and 100 km beneath the batholith and yield Sm-Nd mineral ages between 80 and 120 Ma, broadly coincident with the previously established period of most voluminous batholithic magmatism in the Sierra Nevada. The Rb-Sr ages are generally consistent with the Sm-Nd ages, but are more scattered. The 87Sr/86Sr and 143Nd/144Nd intercepts of the igneous-textured xenoliths are similar to the ratios published for rocks outcroping in the central Sierra Nevada. We interpret the mafic/ultramafic xenoliths to be magmatically related to the upper- and mid-crustal granitoids as cumulates and/or restites. This more complete view of the vertical dimension in a batholith indicates that there is a large mass of mafic–ultramafic rocks at depth which complement the granitic batholiths, as predicted by mass balance calculations and experimental studies. The Sierran magmatism was a large scale process responsible for segregating a column of ∼30 km thick granitoids from at least ∼70 km of mainly olivine free mafic–ultramafic residues/cumulates. These rocks have resided under the batholith as granulite and eclogite facies rocks for at least 70 million years. The presence of this thick mafic–ultramafic keel also calls into question the existence of a “flat” (i.e., shallowly subducted) slab at Central California latitudes during Late Cretaceous–Early Cenozoic, in contrast to the southernmost Sierra Nevada and Mojave regions.

Time of formation and peak of Variscan HP-HT metamorphism of quartz-feldspar rocks in the central Erzgebirge, Saxony, Germany

Abstract: The Variscan Erzgebirge represents an antiform with a core of gneisses and mica schists, surrounded by a phyllitic mantle. The Gneiss-Eclogite Unit (GEU), in the central part, is a composite tectonometamorphic assemblage characterized by a HP-HT imprint and comprises migmatitic para- and orthogneisses, HT mylonites, HP granulites, eclogites and garnet peridotites. It is tectonically sandwiched between two major units with distinctly lower PT histories. The GEU experienced a characteristic “kinked” retrograde PT path after HP-HT equilibration with: (1) strong near-isothermal decompression at high temperatures; (2) extensive re-equilibration at medium pressures, followed (3) by rapid cooling during continued uplift. We dated zircons (Pb-Pb evaporation) from granitoid orthogneisses and metapelites of the GEU. The orthogneisses contain euhedral, long-prismatic zircons of igneous origin that provided protolith ages between 470 and 524 Ma. Metapelites retain well-preserved granulite-facies mineral assemblages and contain spherical, multifaceted metamorphic zircons that grew near the peak of HP/HT metamorphism. Inclusions of prograde HP phengite (∼15 kbar) and rutile are included in one such zircon. Metamorphic zircons of three samples from different localities yielded identical 207Pb/206Pb ages of 340.5 ± 0.7 Ma, 341.2 ± 0.5 Ma and 341.6 ± 0.5 Ma respectively. Consideration of these zircon ages with published 39Ar/40Ar white mica ages suggests fast cooling and uplift rates in excess of 50 °C/Ma and 4 km/Ma. This is typical for large-scale extensional tectonic unroofing of the ultra-deep part of a fossil, thickened Variscan continental crust (>60 km) during continuing continental collision and orogenic collapse.

Composition of fluids in the lower crust inferred from metamorphic salt in lower crustal rocks

Abstract: Knowledge of the rheological properties of the lower crust and the metamorphic processes that operate there is important for our understanding of orogenic processes and granite genesis. The rheological properties critically depend on whether fluids are present in the lower crust1,2 and, if present, on their composition3,4,5,6. Fluid-inclusion7,8,9 and phase-equilibria4,10 studies of lower crustal granulites have shown that fluids with low water activities (due to the presence of dissolved components such as CH4, N2, CO, CO2 and chlorides)11 are present at least episodically in the lower crust. Here we report the occurrence of a solid salt solution (NaCl–KCl) found together with chlorine-rich amphibole and biotite in lower crustal granulites. A desiccation mechanism explains how salt and chlorine-rich minerals formed from an originally water-rich fluid through a short-lived series of hydration reactions in the granulites, during which chlorine was progressively enriched in the fluid. Consequently, it would appear that fluid was present in the lower crust in only small amounts and was not stable over geologically long periods of time, leading to the conclusion that the lower crust is devoid of a free fluid phase during most of its history.

SHRIMP monazite and zircon geochronology of high‐grade metamorphism in New Zealand

Abstract: Ion microprobe dating of zircon and monazite from high-grade gneisses has been used to (1) determine the timing of metamorphism in the Western Province of New Zealand, and (2) constrain the age of the protoliths from which the metamorphic rocks were derived. The Western Province comprises Westland, where mainly upper crustal rocks are exposed, and Fiordland, where middle to lower crustal levels crop out. In Westland, the oldest recognisable metamorphic event occurred at 360-370 Ma, penecontemporaneously with intrusion of the mid-Palaeozoic Karamea Batholith (c. 375 Ma). Metamorphism took place under low-pressure/high-temperature conditions, resulting in upper-amphibolite sillimanite-grade metamorphism of Lower Palaeozoic pelites (Greenland Group). Orthogneisses of younger (Cretaceous) age formed during emplacement of the Rahu Suite granite intrusives (c. 110 Ma) and were derived from protoliths including Cretaceous Separation Point suite and Devonian Karamea suite granites. In Fiordland, high-grade paragneisses with Greenland Group zircon age patterns were metamorphosed (M1) to sillimanite grade at 360 Ma. Concomitant with crustal thickening and further granite emplacement, M1 mineral assemblages were overprinted by higher-pressure kyanite-grade metamorphism (M2) at 330 Ma. It remains unclear whether the M2 event in Fiordland was primarily due to tectonic burial, as suggested by regional recumbent isoclinal folding, or whether it was due to magmatic loading, in keeping with the significant volumes of granite magma intruded at higher structural levels in the formerly contiguous Westland region. Metamorphism in Fiordland accompanied and outlasted emplacement of the Western Fiordland Orthogneiss (WFO) at 110-125 Ma. The WFO equilibrated under granulite facies conditions, whereas cover rocks underwent more limited recrystallization except for high-strain shear zones where conditions of lower to middle amphibolite facies were met. The juxtaposition of Palaeozoic kyanite-grade rocks against Cretaceous WFO granulites resulted from late Mesozoic extensional deformation and development of metamorphic core complexes in the Western Province.

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

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

Crustal Age Domains and the Evolution of the Continental Crust in the Mozambique Belt of Tanzania: Combined Sm–Nd, Rb–Sr, and Pb–Pb Isotopic Evidence

Abstract: African reworking. Eclogite-facies metapelites of the Early Prot- Mozambique Belt; Sr‐Nd‐Pb isotopes erozoic Usagaran Belt likewise exhibit Archean Nd model ages, but higher Pb isotopic ratios are consistent with last recrystallization of feldspar at 2 Ga. Granulites with Nd model ages from 1 to 1·5 Ga only occur in NE Tanzania; because of their restricted range INTRODUCTION in Pb isotopic composition they are interpreted as juvenile additions The study of ancient high-grade gneiss belts provides during late Proterozoic time. Granulites of the W Uluguru Mts important insights into the dynamics of deep-seated orohave Nd model ages between 2·1 and 2·6 Ga, and highly variable genic processes that often cannot be observed in modern feldspar Pb isotope composition indicating possible derivation from active orogenic belts because most of these expose only cratonic and/or Usagaran material, reworked and mixed with a the upper brittle parts of the continental crust. In such old small proportion of younger Proterozoic material during the Pan- and eroded belts, Pb and Nd isotopes supply particularly African orogeny. This could indicate the suture zone between a valuable information on crustal genesis, evolution and western Archean‐Proterozoic continental mass and juvenile arc- terrane amalgamation and can be used to distinguish terranes docking on from the east during subduction of the Mo- between old reworked and juvenile crust. zambique Ocean. The combined isotope data provide strong evidence This study combines Nd and Sr isotope systematics of that parts of the East African crust grew by lateral accretion of whole rocks and Pb isotopes from leached feldspars Early and Mid Proterozoic segments onto an Archean nucleus. to investigate the assembly and crustal history of the However, the Neoproterozoic (Pan-African) orogeny not only led to Proterozoic, polymetamorphic Mozambique Belt. The diVerent chemical properties of the elements determine addition of new crust in the NE of Tanzania, but also reworked

Ultrahigh temperature granulite metamorphism (1050 °C, 12 kbar) and decompression in garnet (Mg70)–orthopyroxene–sillimanite gneisses from the Rauer Group, East Antarctica

Abstract: Highly magnesian and aluminous migmatitic gneisses from Mather Peninsula in the Rauer Group, Eastern Antarctica, preserve ultrahigh temperature (UHT) metamorphic assemblages that include orthopyroxene+sillimanite±quartz, garnet+sillimanite±quartz and garnet+orthopyroxene±sillimanite. Garnet that ranges up to XMg of 71.5 coexists with aluminous orthopyroxene that shows zoning from cores with 7.5–8.5 wt% Al2O3 to rims with up to 10.6 wt% Al2O3 adjacent to garnet. Peak P–T conditions of 1050 °C and 12 kbar are retrieved from Fe–Mg–Al thermobarometry involving garnet and orthopyroxene, in very good agreement with independent constraints from petrogenetic grids in FeO–MgO–Al2O3–SiO2 and related chemical systems. Sapphirine, orthopyroxene and cordierite form extensive symplectites and coronas on the early phases. The specific reaction textures and assemblages involving these secondary phases correlate with initial garnet XMg , with apparent higher-pressure reaction products occurring on the more magnesian garnet, and are interpreted to result from an initial phase of ultrahigh temperature near-isothermal decompression (UHT-ITD) from 12 to 8 kbar at temperatures in excess of 950 °C. Later textures that involved biotite formation and then partial breakdown, along with garnet relics, to symplectites of orthopyroxene+cordierite or cordierite+spinel may reflect hydration through back-reaction with crystallizing melts on cooling below 900–850 °C, followed by ITD from 7 to 8 kbar to c. 5 kbar at temperatures of 750–850 °C. The tectonic significance of this P–T history is ambiguous as the Rauer Group records the effects of Archean tectonothermal events as well as high-grade events at 1000 and 530 Ma. Late-stage biotite formation and subsequent ITD can be correlated with the P–T history preserved in the Proterozoic components of the Rauer Group and hence with either 1000 or 530 Ma collisional orogenesis. However, whether the preceding UHT-ITD history reflects a temporally unrelated event (e.g. Archean) or is simply an early stage of either the late-Proterozoic or Pan-African tectonism, as recently deduced for similar UHT rocks from other areas of the East Antarctica, remains uncertain.

High‐pressure granulite facies metamorphism in the Pan‐African belt of eastern Tanzania: P–T–t evidence against granulite formation by continent collision

Abstract: To constrain the tectonic history of the Pan-African belt in Tanzania, we have studied the P–T evolution of granulites from northern and eastern Tanzania representative for a large part of the southern Pan-African belt of East Africa (e.g. Pare, Usambara, Ukaguru and Uluguru Mountains). Thermobarometry (conventional and multireaction equilibria) on enderbites and metapelites gives 9.5–11 kbar and 810±40 °C during peak metamorphism at 650–620 Ma. This is consistent with the occurrence of both sillimanite and kyanite in metapelites and of the high-P granulite facies assemblage garnet–clinopyroxene–quartz in mafic rocks. Peak metamorphic conditions are surprisingly similar over a very large area with N-S and E-W extents of about 700 and 200 km respectively. The prograde metamorphic evolution in the entire area started in the kyanite field but evolved mainly within the sillimanite stability field. The retrograde P–T evolution is characterized by late-stage kyanite in metapelites and garnet–clinopyroxene coronas around orthopyroxene in meta-igneous rocks. This is in agreement with thermobarometric results and isotopic dating, indicating a period of nearly isobaric and slow cooling prior to tectonic uplift. The anticlockwise P–T path could have resulted from magmatic underplating and loading of the lower continental crust which caused heating and thickening of the crust. Substantial postmetamorphic crustal thickening of yet unknown age (presumably after 550 Ma) led subsequently to the exhumation of high-P granulites over a large area. The results are consistent with formation of the Pan-African granulites at an active continental margin where tonalitic intrusions caused crustal growth and heating 70–100 Ma prior to continental collision. The P–T–t path contradicts recent geodynamic models which proposed tectonic crustal thickening due to continental collision between East and West Gondwana as the cause of granulite formation in the southern part of the Pan-African belt.

SHRIMP monazite and zircon geochronology of high-grade metamorphism in New Zealand

Abstract: Ion microprobe dating of zircon and monazite from high-grade gneisses has been used to (1) determine the timing of metamorphism in the Western Province of New Zealand, and (2) constrain the age of the protoliths from which the metamorphic rocks were derived. The Western Province comprises Westland, where mainly upper crustal rocks are exposed, and Fiordland, where middle to lower crustal levels crop out. In Westland, the oldest recognisable metamorphic event occurred at 360-370 Ma, penecontemporane- ously with intrusion of the mid-Palaeozoic Karamea Batholith (c. 375 Ma). Metamorphism took place under low-pressure/high-temperature conditions, resulting in upper-amphibolite sillimanite-grade meta- morphism of Lower Palaeozoic pelites (Greenland Group). Orthogneisses of younger (Cretaceous) age formed during emplacement of the Rahu Suite granite intrusives (c. 110 Ma) and were derived from protoliths including Cretaceous Separation Point suite and Devonian Karamea suite granites. In Fiordland, high-grade paragneisses with Greenland Group zircon age patterns were metamorphosed (M1) to sillimanite grade at 360 Ma. Concomitant with crustal thickening and further granite emplace- ment, M1 mineral assemblages were overprinted by higher-pressure kyanite-grade metamorphism (M2) at 330 Ma. It remains unclear whether the M2 event in Fiordland was primarily due to tectonic burial, as suggested by regional recumbent isoclinal folding, or whether it was due to magmatic loading, in keeping with the significant volumes of granite magma intruded at higher structural levels in the formerly contiguous Westland region. Metamorphism in Fiordland accompanied and outlasted emplacement of the Western Fiordland Orthogneiss (WFO) at 110-125 Ma. The WFO equilibrated under granulite facies conditions, whereas cover rocks underwent more limited recrystallization except for high-strain shear zones where conditions of lower to middle amphibolite facies were met. The juxtaposition of Palaeozoic kyanite-grade rocks against Cretaceous WFO granulites resulted from late Mesozoic extensional defor- mation and development of metamorphic core complexes in the Western Province.

Influence of Fluid and Deformation on Metamorphism of the Deep Crust and Consequences for the Geodynamics of Collision Zones

Abstract: A survey of natural transitions from igneous rocks, amphibolite, and granulite, to eclogite demonstrates that the transitions may take place over cm-scale distances parallel to fluid fronts. Rocks ranging in composition from basaltic to granitic show incomplete reactions over the whole range of high pressure to ultrahigh-pressure conditions (500°C and 1.2 GPa to 800°C and >3.0 GPa), indicating overstepping of reaction boundaries of 1 GPa. When fluid becomes available metastable crust may react forcefully and release earthquakes as indicated by occurrence of eclogite-facies pseudotachylite.

A petrogenetic grid for eclogite and related facies under high‐pressure metamorphism

Abstract: The petrogenetic grid between the eclogite and other high-pressure/temperature (P/T) metamorphic facies in a basaltic system is constructed by considering barroisite as one of the important phases in high-P/T metamorphism and by using previous petrological data combined with Schreinemakers' analysis and slope calculation. In the constructed petrogenetic grid, the eclogite facies is bounded by the blueschist and epidote–amphibolite facies with negative-slope reactions at lower temperatures (450–550 °C) and by the epidote–amphibolite, amphibolite and granulite facies with positive-slope reactions at higher temperatures (> 550–600 °C). The eclogite facies does not contact the greenschist facies, and the lowest P condition for the eclogite facies exists at the boundary between the eclogite and epidote–amphibolite facies. The temperature range of the epidote–amphibolite facies increases with increasing pressure until 8–11 kbar and then decreases up to 13–15 kbar. Compared to boundaries of other facies, boundaries of the eclogite facies may have wider P–T ranges. The boundary between the blueschist and eclogite facies occurs over a large temperature range from 450 to 620 ± 30 °C, and the transitions between the eclogite and amphibolite or high-pressure granulite facies occur over a pressure range in excess of 6–10 kbar.

Halogen-bearing minerals in syenites and high-grade marbles of Dronning Maud Land, Antarctica: monitors of fluid compositional changes during late-magmatic fluid-rock interaction processes

Abstract: Meta-sedimentary rocks including marbles and calcsilicates in Central Dronning Maud Land (CDML) in East Antarctica experienced a Pan-African granulite facies metamorphism with peak metamorphic conditions around 830 ± 20 °C at 6.8 ± 0.5 kbar which was accompanied by the post-kinematic intrusion of huge amounts of syenitic (charnockitic) magmas at 4.5 ± 0.7 kbar. The marbles and calcsilicates may represent meta-evaporites as indicated by the occurrence of metamorphic gypsum/anhydrite and Cl-rich scapolite that formed in the presence of saline fluids with X NaCl in the range 0.15–0.27. The marbles and calcsilicates bear biotite, tremolite and/or hornblende and humite group minerals (clinohumite, chondrodite and humite) which are inferred to have crystallized at about 650 °C and 4.5 kbar. The syenitic intrusives contain late-magmatic biotite and amphibole (formed between 750 and 800 °C) as well as relictic magmatic fayalite, orthopyroxene and clinopyroxene. Two syenite and two calcsilicate samples contain fluorite. Corona textures in the marbles and calcsilicates suggest very low fluid-rock ratios during the formation of the retrograde (650 °C) assemblages. Biotite in all but two syenite samples crystallized at log(f H 2 O/f HF) ratios of 2.9 ± 0.4, while in the calcsilicates, both biotite and humite group minerals indicate generally higher log(f H 2 O/f HF) values of up to 5.2. A few samples, though, overlap with the syenite values. Log(f H 2 O/f HCl) derived from biotite covers the range 0.5–2.6 in all rock types. Within a single sample, the calculated values for both parameters vary typically by 0.1 to 0.8 log units. Water and halogen acid fugacities calculated from biotite-olivine/orthopyroxene-feldspar-quartz equilibria and the above fugacity ratios are 1510–2790 bars for H2O, 1.3–5.3 bars for HF and 7–600 bars for HCl. The results are interpreted to reflect the reaction of relatively homogeneous magmatic fluids [in terms of log(f H 2 O /f HF)] derived from the late-magmatic stages of the syenites with both earlier crystallized, still hotter parts of the syenites and with adjacent country rocks during down-temperature fluid flow. Fluorine is successively removed from the fluid and incorporated into F-bearing minerals (close to the syenite into metamorphic fluorite). In the course of this process log(f H 2 O /f HF) increases significantly. Chlorine preferably partitions into the fluid and hence log(f H 2 O /f HCl) does not change markedly during fluid-rock interaction.