Showing papers on "Metamorphism published in 1999"

Internal morphology, habit and U-Th-Pb microanalysis of amphibolite-to-granulite facies zircons: geochronology of the Ivrea Zone (Southern Alps)

Abstract: Several types of growth morphologies and alteration mechanisms of zircon crystals in the high-grade metamorphic Ivrea Zone (IZ) are distinguished and attributed to magmatic, metamorphic and fluid-related events. Anatexis of pelitic metasediments in the IZ produced prograde zircon overgrowths on detrital cores in the restites and new crystallization of magmatic zircons in the associated leucosomes. The primary morphology and Th-U chemistry of the zircon overgrowth in the restites show a systematic variation apparently corresponding to the metamorphic grade: prismatic (prism-blocked) low-Th/U types in the upper amphibolite facies, stubby (fir-tree zoned) medium-Th/U types in the transitional facies and isometric (roundly zoned) high-Th/U types in the granulite facies. The primary crystallization ages of prograde zircons in the restites and magmatic zircons in the leucosomes cannot be resolved from each other, indicating that anatexis in large parts of the IZ was a single and short lived event at 299 ± 5 Ma (95% c. l.). Identical U/Pb ages of magmatic zircons from a metagabbro (293 ± 6 Ma) and a metaperidotite (300 ± 6 Ma) from the Mafic Formation confirm the genetic context of magmatic underplating and granulite facies anatexis in the IZ. The U-Pb age of 299 ± 5 Ma from prograde zircon overgrowths in the metasediments also shows that high-grade metamorphic (anatectic) conditions in the IZ did not start earlier than 20 Ma after the Variscan amphibolite facies metamorphism in the adjacent Strona–Ceneri Zone (SCZ). This makes it clear that the SCZ cannot represent the middle to upper crustal continuation of the IZ. Most parts of zircon crystals that have grown during the granulite facies metamorphism became affected by alteration and Pb-loss. Two types of alteration and Pb-loss mechanisms can be distinguished by cathodoluminescence imaging: zoning-controlled alteration (ZCA) and surface-controlled alteration (SCA). The ZCA is attributed to thermal and/or decompression pulses during extensional unroofing in the Permian, at or earlier than 249 ± 7 Ma. The SCA is attributed to the ingression of fluids at 210 ± 12 Ma, related to hydrothermal activity during the breakup of the Pangaea supercontinent in the Upper Triassic/Lower Jurassic.

Seismic Consequences of Warm Versus Cool Subduction Metamorphism: Examples from Southwest and Northeast Japan

Abstract: Warm and cool subduction zones exhibit differences in seismicity, seismic structure, and arc magmatism, which reflect differences in metamorphic reactions occurring in subducting oceanic crust. In southwest Japan, arc volcanism is sparse and intraslab earthquakes extend to 65 kilometers depth; in northeast Japan, arc volcanism is more common and intraslab earthquakes reach 200 kilometers depth. Thermal-petrologic models predict that oceanic crust subducting beneath southwest Japan is 300° to 500°C warmer than beneath northeast Japan, resulting in shallower eclogite transformation and slab dehydration reactions, and possible slab melting.

Growth, annealing and recrystallization of zircon and preservation of monazite in high-grade metamorphism: conventional and in-situ U-Pb isotope, cathodoluminescence and microchemical evidence

Abstract: Zircon and monazite from granulite- to amphibolite-facies rocks of the Vosges mountains (central Variscan Belt, eastern France) were dated by ion-microprobe and conventional U-Pb techniques. Different granulites of igneous (so-called leptynites) and sedimentary origin (kinzigites) and their leucosomes were dated at 334.9 ± 3.6, 335.4 ± 3.6 and 336.7 ± 3.5 Ma (conventional age 335.4 ± 0.6 Ma). Subsequent growth stages of zircon were distinguished by secondary electron (SEM) and cathodoluminescence (CL) imaging: (1) subsolidus growth producing round anhedral morphologies and sector zoning; (2) appearance of an intergranular fluid or melt phase at incipient dehydration melting that first resulted in resorption of pre-existing zircons, followed by growth of acicular zircons or overgrowths on round zircons consisting of planar growth zoning; (3) advanced melting producing euhedral prismatic zircons with oscillatory zoning overgrowing the sector zones. Two further lithologies, the Kaysersberg granite and the Trois-Epis units, were both formerly considered as migmatites. The intrusion of the Kaysersberg granite was dated at 325.8 ± 4.8 Ma. The Trois-Epis unit was found to be the product of volume recrystallization of a former granulite, which occurred under amphibolite-facies conditions 327.9 ± 4.4 Ma ago. The amphibolite-facies overprint of the Trois-Epis zircons led to the complete rejuvenation of most of the zircon domains by annealing and replacement/recrystallization processes. Annealing is assumed to occur in strained lattice domains, which are possibly disturbed by high trace element contents and/or large differences in decay damage between adjacent growth zones. Investigation of cathodoluminescence structures reveals that the replacement occurs along curved chemical reaction fronts that proceed from the surface towards the interior of the zircon. The monazite U-Pb system still records the age of high-grade metamorphism at around 335 Ma. The chemical reagent responsible for the rejuvenation of zircon obviously left the monazite unaffected.

Tectonothermal evolution of the Mayuan Assemblage in the Cathaysia Block; implications for Neoproterozoic collision-related assembly of the South China Craton

Abstract: The Mayuan assemblage in the Cathaysia Block, Southeast China, consists of felsic paragneiss, pelitic schist, greenschist, amphibolite, marble, calcsilicate, and quartzite that underwent three episodes of deformation (D1-D3) and four episodes of metamorphism (M1-M4) in the early Neoproterozoic. The M1 assemblage consists of mineral inclusions defining an early foliation (S1) within porphyroblasts, represented by chlorite muscovite biotite plagioclase quartz in pelitic schist and actinolite chlorite epidote albite quartz enclosed in amphibolite. M2 coincides with the development of the regional schistosity (S2) and represents the formation of the porphyroblasts and growth of matrix minerals, resulting in development of prograde metamorphic zones (chlorite-biotite, garnet, staurolite, and kyanite zones). M3 is simultaneous with the third phase of deformation (D3) and produced sillimanite-bearing mineral assemblages in pelitic schist and hornblende-bearing assemblages in amphibo- lite. The last metamorphic episode M4 gave rise to the retrogressive assemblage chlorite muscovite in pelitic rock and actinolite chlorite epidote in amphibolite. The sequence of mineral assemblages and history of metamorphic reactions built from the petrogenetic grid of pelites suggest a near-isothermal decompression clockwise P-T path for the Mayuan pelitic schists. Using the TWEEQU software program, the garnet-biotite thermometer and garnet- muscovite-biotite-plagioclase barometer yield P-T conditions for M1 of 5.5 to 6.0 kb and 450° to 500°C and conditions for the garnet, staurolite, and kyanite zones of M2 of 6.0 to 7.0 kb and 550° to 600°C, 6.0 to 7.5 kb and 600°C and 11.0 to 11.5 kb and 600°C. The P-T conditions of M3 were estimated at 570° to 625°C and 4.0 to 4.5 kb using the muscovite-biotite thermometer and hornblende-plagioclase geothermo- barometer. The garnet-chlorite thermometer yields temperatures of 300° to 400°C for M4, but the pressures of M4 cannot be quantitatively estimated because of the lack of a suitable geobarometer. These P-T estimates also define an near- isothermal decompression clockwise P-T path, which is involved in initial crustal thickening followed by rapid exhumation and final cooling, and is related to amalgamation of the Cathaysia and Yangtze Blocks to form the South China craton.

Evaporites: Their Evolution and Economics

Abstract: Interpreting evaporite textureBrine evolution and mineralogyEvaporite basins and their stratigraphic evolutionIndicators and effects of dissolution: "the evaporite that was"Salt tectonicsMeta-evaporites Evaporites as a mineral resourceEvaporite-metal associations: lower temperature and diageneticEvaporite-metal associations: brines, magma and metamorphism

Geological implications of a permeability-depth curve for the continental crust

Abstract: The decrease in permeability ( k ) of the continental crust with depth ( z ), as constrained by geothermal data and calculated fluid flux during metamorphism, is given by log k = −14 − 3.2 log z, where k is in meters squared and z is in kilometers. At moderate to great crustal depths (>∼5 km), this curve is defined mainly by data from prograde metamorphic systems, and is thus applicable to orogenic belts where the crust is being thickened and/or heated; lower permeabilities may occur in stable cratonic regions. This k-z relation implies that typical metamorphic fluid flux values of ∼10−11 m/s are consistent with fluid pressures significantly above hydrostatic values. The k-z curve also predicts that metamorphic CO2 flux from large orogens may be sufficient to cause significant climatic effects, if retrograde carbonation reactions are minimal, and suggests a significant capacity for diffuse degassing of Earth (1015–1016 g/yr) in tectonically active regions.

40Ar-39Ar and Rb-Sr geochronology of high-pressure metamorphism and exhumation history of the Tavsanli Zone, NW Turkey

Abstract: Geochronological investigations in high- and ultra-high-pressure metamorphic rocks are problematic since firstly the low temperatures lead to fine grain size and disequilibrium assemblages, and secondly the problem of “excess argon” affects 40Ar-39Ar systematics, the most commonly used isotopic system. The Tavsanli Zone is a belt of high-pressure low-temperature (HP-LT) rocks spanning NW Turkey and is one such region where previous geochronological studies have produced a range of estimates for the age of HP-LT metamorphism, raising the question of whether they are geologically significant. This study presents new data from the Tavsanli Zone; 40Ar-39Ar ages are in the range 60 Ma to 175 Ma, whilst Rb-Sr ages are restricted to 79.7 Ma to 82.8 Ma, confirming the presence of excess argon. Detailed ultra-violet laser ablation microprobe (UVLAMP) studies have revealed younger 40Ar-39Ar ages in the cores of coarser white micas, which in conjunction with 40Ar-39Ar ages from the finest grained lithologies and the Rb-Sr white mica crystallisation ages, constrain the post-HP-LT metamorphism exhumation rates of these rocks. Petrological and regional constraints suggest that syn-subduction exhumation and cooling took place initially by synchronous subduction and exhumation by underplating. This is followed by a phase of syn-continent-continent collision at a rate of approximately 1.5 mma−1 and exhumation to the surface via thrusting. The 40Ar-39Ar hornblende data from a granodiorite intruding the HP-LT rocks constrain the later parts of exhumation path. This study highlights the importance of a multi-system geochronological approach when attempting to determine the history of HP-LT rocks.

The tectonic evolution of the Kohistan‐Karakoram collision belt along the Karakoram Highway transect, north Pakistan

Abstract: The Kohistan arc terrane comprises an intra-oceanic island arc of Cretaceous age separating the Indian plate to the south from the Karakoram (Asian) plate to the north within the Indus suture zone of north Pakistan. The intra-oceanic arc volcanics (Chalt, Dras Group) were built on a foundation of dominantly mid-ocean ridge basalt (MORB)-related amphibolites of the Kamila Group. The subarc magma chamber is represented by multiple intrusions of a huge gabbro-norite complex (Chilas complex), which includes some ultramafic assemblages of residual mantle harzburgite and dunite, layered cumulates, and hornblendites cut by late stage dikes of hornblende + plagioclase pegmatites. The Chilas complex norites intrude the Gilgit metasediments of lower amphibolite and greenschist facies in northern Kohistan, which also form xenolithic roof pendants within the top of the Chilas complex. Along the southern margin of Kohistan, Jijal and Sapat complex ultramafics (dunites, harzburgites and websterites) form remnant suprasubduction zone ophiolitic mantle rocks along the hanging wall of the Main Mantle Thrust, the Cretaceous obduction plane along which Kohistan was emplaced onto Indian plate rocks. Garnet granulites of the Jijal complex, formed at 12–14 kbars, represent original magmatic lower crustal rocks subducted to depths of at least 45 km and metamorphosed during high-pressure and high-temperature subduction of earlier arc-related rocks. Obduction of the Sapat ophiolite and Kohistan arc occurred between ∼75 and 55 Ma. The closure of the Shyok suture zone separating Kohistan from the Karakoram plate must have occurred prior to 75 Ma, the age of the Jutal basic dikes which crosscut the closure-related fabrics, mainly late north directed backthrusting in the lower Hunza valley. Andean-type granitoid (gabbrodiorite-granodiorite-granite) emplacement along the Kohistan-Ladakh batholith ended at the time of India-Asia collision, ∼ 60–50 Myr ago. Postcollisional crustal thickening along the Karakoram led to multiple episodes of metamorphism from latest Cretaceous and throughout the Tertiary. Sillimanite grade metamorphism in Hunza was actually pre-India-Asia collision and may have resulted from the earlier Kohistan collision. Localized and sporadic crustal melting episodes across northern Kohistan (Indus confluence and Parri granite sheets) and the southern Karakoram (Hunza dikes and Sumayar and Mango Gusar leucogranites) occurred from 51 to 9 Ma and culminated in the huge Baltoro monzogranite-leucogranite intrusion 25–21 Myr ago. A vast network of leucogranitic and pegmatitite dikes containing gem quality aquamarine + muscovite ± tourmaline ± garnet ± biotite quartz are younger than 5 Ma and form the final phase of intrusion in the Haramosh area and parts of the southern Karakoram area.

Constraining the prograde and retrograde P-T-t path of Eocene HP rocks by SHRIMP dating of different zircon domains: inferred rates of heating, burial, cooling and exhumation for central Rhodope, northern Greece

Abstract: Ion microprobe (SHRIMP) dating was carried out on different zircon domains from metamorphic rocks of the HP-HT terrane of central Rhodope, northern Greece, to constrain the timing of prograde and retrograde stages within a single tectono-metamorphic cycle. A well determined P-T-t(relative) path for the metamorphic rocks of this terrane was used as a petrological basis for the geochronological investigations. Ion microprobe work was assisted by cathodoluminescence (CL) images of the zircon crystals. The geochronological results revealed that Hercynian continental crust was subducted during the Eocene. Several stages of the Eocene tectono-metamorphic cycle – including both the prograde and retrograde parts of the P-T path above ca 300 °C, 0.3 GPa – were dated using zircons from the following rock types: (1) A deformed quartz vein probably formed at ca 300 °C, 0.3 GPa. Zircons in this vein precipitated from a hydrothermal fluid; they yielded an age of 45.3 ± 0.9 Ma which corresponds to the time of a low-T prograde stage of metamorphism. (2) In kyanite eclogites, zircons were entirely reset during eclogite-facies metamorphism. Resetting was very probably enhanced by the presence of fluids derived by H2O liberating reactions close to the P-peak. They yielded an age of 42.2 ± 0.9 Ma. (3) Orthogneisses surrounding the kyanite eclogites contained zircons with magmatic oscillatory zoned cores, which yielded Hercynian ages of 294 ± 8 Ma (age of granitic protolith formation), whereas CL-bright, metamorphic rims yielded, like the eclogite zircons, ages of 42.0 ± 1.1 Ma. Therefore, both the eclogites and orthogneisses are interpreted to have approached maximum depth at around 42 Ma. (4) In a leucosome of a migmatized orthogneiss, oscillatory zoned zircons yielded an age of 40.0 ± 1.0 Ma. At this time the rocks reached maximum temperatures during early decompression. (5) A late pegmatite crosscutting the schistosity of amphibolites contained oscillatory zoned zircons that yielded a crystallization age of 36.1 ± 1.2 Ma. Thus, the whole tectono-metamorphic cycle above ca 300 °C, 0.3 GPa lasted from 45.3 ± 0.9 Ma to 36.1 ± 1.2 Ma, that is 9.2 Ma with an extreme error value of 2.1 Ma. Based on combined SHRIMP and petrological data, the average rates of heating and burial during subduction (above ca 300 °C, 0.3 GPa) are >94 °C/Ma and >15 mm/a, respectively. Rates of cooling and exhumation (also above 300 °C, 0.3 GPa) are calculated to be >128° C/Ma and >7.7 mm/a. The Eocene age of metamorphism in central Rhodope implies that the terrane of, at least, central Rhodope and the Cyclades very probably was part of the same continental crust.

Timing of prograde metamorphism in the Zanskar Himalaya

Abstract: The timing of the prograde metamorphism of continental crust buried during the collision of India and Eurasia is of importance to our understanding of the mechanics of mountain building and crustal metamorphism. Our knowledge of the Himalaya indicates that continental collision (at 55–50 Ma) culminated in crustal melting and leucogranitic magmatism at 24–18 Ma. However, the timing of events in the intervening 30 m.y. is poorly understood. Here we present Sm-Nd isotope analyses of garnets from the Zanskar Himalaya that also preserve information on crustal and orogenic evolution during that period. The prograde P-T-t paths indicate that heating and burial (from 6 to 10 kbar) continued until at least 25–30 Ma and imply peak temperatures (∼700 °C) close to those necessary to cause the crustal melting in the Himalaya at ca. 20 Ma, without a requirement for additional heat sources such as shear heating on major crustal shear zones. The persistence of high temperatures as late as 20 Ma in the Himalaya may be related to postcollisional thrusting south of the suture zone and within the Indian plate at 40–35 Ma.

Tectonics of the Dabieshan (eastern China) and possible exhumation mechanism of ultra high‐pressure rocks

Abstract: The Dabieshan is divided into three tectonic domains. The Southern Dabieshan is a stack of allochtons, from top to bottom: (i) unmetamorphosed Cambrian–Early Triassic series unconformably covered by Jurassic sandstone; (ii) weakly metamorphosed Proterozoic slate; (iii) HP rocks mostly retrogressed into greenschist facies; (iv) UHP allochton; (v) UHP-free gneisses. These units exhibit a NW–SE lineation and top-to-the-NW shearing reworked by S-verging folds. The Central Dabieshan is a migmatitic dome superimposed on UHP metamorphism and bounded by a detachement fault responsible for the exhumation of the UHP unit during its retrogression into amphibolite facies. In the Northern Dabieshan, early foliation and N–S trending compressional lineation are deformed by N-verging folds coeval to the syn-exhumation ductile structures of the Central Dabieshan. A geodynamic model involving coeval thrusting and normal faulting is discussed.

Metamorphism, Melting, and Extension: Age Constraints from the High Himalayan Slab of Southeast Zanskar and Northwest Lahaul

Abstract: We present evidence for two distinct stages of Tertiary metamorphism (M1 and M2) in the High Himalayan Slab of southeast Zanskar and northwest Lahaul, as well as evidence for an older, pre‐Himalayan metamorphism (pre‐M1). The M1 was a regional Barrovian‐type event related to crustal shortening and thickening of the Indian plate margin, while M2 was associated with crustal melting and the emplacement of the Gumburanjon leucogranite into the Zanskar Shear Zone at the top of the slab. U‐Pb dating of metamorphic and magmatic accessory phases constrains the timing of M1 between 30 and 37 Ma and the crystallization and emplacement age of the Gumburanjon leucogranite at 21–22 Ma. Inherited accessory phases in metamorphic and magmatic samples suggest that the protoliths of the slab are at least Lower Paleozoic in age and that they experienced a major pre‐M1 thermal perturbation at ca. 450–500 Ma. Whether this was associated with a regional Barrovian‐type metamorphism or whether it was a thermal event rel...

Sm/Nd, Rb/Sr, and40Ar/39Ar Isotopic Systematics of the Ultrahigh-Pressure Metamorphic Rocks in the Dabie-Sulu Belt, Central China: A Retrospective View

Abstract: Because of a complicated metamorphic history, the isotopic systematics of the ultrahigh-pressure (UHP) metamorphic rocks in the Dabie-Sulu belt, east China, appear to be rather different from what were expected. Depending on the degree of retrograde metamorphism and on the retentivity of isotopes, the radiogenic isotopic systematics in the UHP metamorphic rocks yielded a wide range of radiometric ages. Some of these ages are geologically meaningful, but others may not be. In some fine-grained UHP metamorphic rocks, Sm/Nd isotopic systematics appear to be in equilibrium among the UHP phases, showing the best estimate for the age of peak metamorphism at 226 ± 3 Ma. On the other hand, retrograde overprinting often makes the interpretation of isotopic data more difficult. It is common to find that the Sm/Nd and Rb/Sr isotopic systematics among the UHP phases and retrograde phases are not in equilibrium. Regression of isotopic data involving both UHP and retrograde minerals in isotopic correlation diagrams oft...

Chronology of deformation within the turbidite‐dominated, Lachlan orogen: Implications for the tectonic evolution of eastern Australia and Gondwana

Abstract: Ar-Ar data from fabric-forming white mica in slates, syntectonic quartz veins and granitic mylonites constrain the timing of metamorphism, deformation, and exhumation in the Lachlan orogen, Australia These data also help define the tectonic evolution of the Tasmanides during Paleozoic time The Lachlan orogen formed by the progressive accretion of a thick turbidite fan sequence and volcanic terrains to Gondwana during the closing of a small marginal ocean or back arc basin This tectonic setting was similar to the present western and southwestern Pacific region Accretion of the Lachlan orogen to Gondwana occurred by closing of the basin system by subduction-accretion processes and some translation The process is typified in the western Lachlan orogen by a major eastward migrating deformation involving chevron folding and faulting over an eastward propagating decollement/melange zone and is recorded by Ar-Ar mica ages ranging from ∼455 Ma in the western part to ∼390 Ma in the eastern part In the central Lachlan orogen, deformation migrated southwestward from ∼440–430 to 405 Ma away from the high-temperature Wagga-Omeo metamorphic complex, where deformation/metamorphism occurred between >440 and 400 Ma In the north, ∼400 Ma mica ages record deformation and inversion of structures in the Cobar basin In the eastern Lachlan orogen, Ar-Ar mica dates range from 450 to 340 Ma Ages of 455–445 Ma are yielded by the Narooma accretionary complex, 405–390 Ma ages are found along the major thrust faults bounding high-grade metamorphic complexes, and 360–340 Ma cooling ages are found in the inverted extensional basins (eg, Hill End) and related structural zones The Ar-Ar results also document periods of reactivation on early-formed structures during later deformation elsewhere in the orogen

The History of Granulite-Facies Metamorphism and Crustal Growth from Single Zircon U-Pb Geochronology: Namaqualand, South Africa

Abstract: The Namaqualand Metamorphic Complex is a well-exposed, INTRODUCTION Mesoproterozoic, low-pressure, amphibolite–granulite-facies terrane flanking the Archaean Kaapvaal Craton of southern Africa. Previous isotopic dating in the region suggests an ~150 my period of prograde granulite-facies metamorphism and episodic granite emplacement The mid-crustal granulite-facies problem in the mid-crust. In contrast, thermal modelling suggests that Granulite-facies terranes are rocks of the Earth’s lower suband superjacent magmatic accretion should not have exceeded and middle crust that equilibrated at high pressures (P ) 30 my in duration. This enigma is resolved by precise U–Pb zircon and temperatures (T ). Their petrology and geoSHRIMP dating of the major orthogneissic units of the region. chronology commonly preserve both prograde and retroThese data point to Kibaran crustal growth at 1220–1170 Ma, grade characteristics. Because these terranes reflect a which occurred on the margins of a Palaeoproterozoic (2000–1800 number of different crustal and tectonic processes, their Ma) continental nucleus. A later, distinct, orogenic episode, here origin is important in understanding the nature of contermed the Namaquan (time equivalent of the Grenvillian), involved tinental growth and crustal evolution. crustal thickening and magmatism at 1060–1030 Ma and was Granulite-facies rocks typically reflect P–T conditions responsible for, and coeval with, the peak of metamorphism. Lowof 6–9 kbar and 750–850°C and comprise anhydrous P granulite-facies metamorphism resulted from advective heating mineral assemblages that point to conditions of reduced and crustal thickening by magmatic accretion over a 30 my interval. water activity (Harley, 1989). In the lower crust some granulite-facies rocks are thought to be residues of partial melting that has moved melt and volatiles to higher crustal levels. Alternatively, granulite-facies rocks may form where mutually soluble CO2–H2O-rich fluids stream upwards through the crust, causing local reduction in volatile content and accompanying mineral phase changes along the fluid flow paths (Harley, 1989). In the

Ultra-high Temperature Metamorphism of Metapelitic Granulites from Kondapalle, Eastern Ghats Belt: Implications for the Indo-Antarctic Correlation

Abstract: A suite of quartz- and corundum-bearing metapelitic granulites, intruded by layered gabbronorite-pyroxenite-anorthosite at Kondapalle, Eastern Ghats Belt, preserves a multitude of reaction textures involving oxide and silicate minerals that attest to several prograde and retrograde reactions. In the quartz-bearing associations, the reactions are: (a) biotite + sillimanite + quartz →garnet + liquid; (b) garnet + sillimanite →spinel (+ magnetite) + quartz; (c) Fe2TiO4 + O2 →ferrian ilmenite + magnetite; (d) reversal of reaction (b); (e) Fe2O3-rich ilmenite + plagioclase + quartz →Fe2O3-poor ilmenite + garnet + O2. Reactions in the corundum-bearing associations are: (f) spinel + biotite + sillimanite →garnet + liquid; (g) biotite + sillimanite →garnet + Ti-rich spinel + corundum + liquid; (h) biotite + sillimanite →garnet + corundum + liquid; (i) Fe2TiO4 + FeAl2O4 + O2 →ferrian ilmenite + Fe3O4 + Al2O3 (in ilmenite); (j) garnet + corundum →spinel + sillimanite. To examine the paragenetic evolution of the metapelitic granulites, a petrogenetic grid for the KFMASH system at high temperatures and pressures, involving quartz and corundum, was constructed. The sequence of inferred reactions documents an anticlockwise heating-cooling path. Reintegrated compositions of spinel (with >10 mol % Fe2TiO4) and feldspars indicate ultra-high temperature (UHT) of metamorphism (>1000°C), comparable with the liquidus temperature of the enclosing magmatic rocks. Crystallization pressures inferred for the magmatic rocks and the pressure constraints imposed by the petrogenetic grid on the metapelite assemblages indicate that the emplacement of the igneous suite and the accompanying UHT metamorphism occurred in the lower crust (>8 kbar). Reported U-Pb cooling ages of monazite and allanite from a late pegmatite suggest the UHT metamorphism to be older than 1600 Ma. The deduced P-T history and the absence of Grenvillian high-grade metamorphism in the study area provide important constraints on the configuration of East Gondwana, in particular on the continuation of the Napier-Rayner terrane boundary into the Eastern Ghats Belt.

Application of the Restite Model to the Deddick Granodiorite and its Enclaves -a Reinterpretation of the Observations and Data of Maas et al. (1997)

Abstract: Mafic S-type granites of the Lachlan Fold Belt always contain a INTRODUCTION distinctive suite of enclaves, dominated by schists and gneisses of high Maas et al. (1997) have proposed complex models of metamorphic grade, generally with lesser numbers of microgranular crustal melting and magma mixing to account for the enclaves. Maas et al. (1997, Journal of Petrology 38, chemical and isotopic features of several kinds of enclaves 815–841) recently described enclaves from the S-type Deddick in the S-type Deddick Granodiorite of the Lachlan Fold Granodiorite. They concluded that the metasedimentary enclaves Belt (LFB), exposed at McKillops Bridge. That grancould be either source lithologies or accidental xenoliths, and that odiorite is a member of the Bullenbalong Suite (area the most common microgranular enclaves formed as globules of ~3100 km; White & Chappell, 1988), itself part of the hybrid magma that mingled with the host magma. All features of very extensive and slightly more diverse Bullenbalong the metasedimentary enclaves are consistent with an origin as lithic Supersuite (total area ~8750 km plus ~4000 km of restite fragments from the source, and their ubiquity in the mafic Svolcanic rocks); it occurs near the southern extremity of type granites and their rarity in both felsic varieties and in I-type those granites. These are the rocks that have been used granites conform with such an origin. The argument that these as primary examples of restite and of the restite model enclaves are not in chemical equilibrium with the host granite in S-type granites (Hine et al., 1976; White & Chappell, because they do not complement its composition is not valid, as they 1977; Chappell et al., 1987; Chappell, 1996b), and the represent less fertile parts of the source that did not melt to the point enclosed enclaves have been regarded by us as lithic of disaggregation. Unaltered cores of microgranular enclaves in the restite (White & Chappell, 1988; Chen et al., 1989; S-type granites have compositions that do not match those of any Chappell & White, 1991; White et al., 1991; Wyborn known igneous rock and an origin by metamorphism of a calcareous et al., 1991). sediment is probable. Partial melting during that metamorphism Maas et al. (1997) mostly reject the presence of restite accounts for the pseudo-igneous textures seen in these enclaves. enclaves in the Deddick Granodiorite. However, the

Occurrence, age, and implications of the Yagan–Onch Hayrhan metamorphic core complex, southern Mongolia

Abstract: Mylonitic rocks associated with the south-dipping detachment fault of the Yagan–Onch Hayrhan metamorphic core complex in southernmost Mongolia indicate subhorizontal south-southeast–directed extension in the Early Cretaceous; synkinematic biotites give 40 Ar/ 39 Ar ages of 129 to 126 Ma. The Yagan–Onch Hayrhan core complex demonstrates that late Mesozoic localized high-strain extension, recently recognized in other parts of eastern Asia, also occurred in Mongolia. The presence of Mesozoic metamorphism at Onch Hayrhan, previously presumed to be Precambrian, brings into question the existence of the South Gobi microcontinent.