Showing papers on "Granulite published in 2009"

Metamorphic evolution of major tectonic units in the basement of the North China Craton: Key issues and discussion

Abstract: The basement of the North China Craton consists of three micro-continental blocks (Eastern Block, Yinshan Block and Ordos Block) and three Paleoproterozoic mobile belts ( Khondalite Belt, Trans-North China Orogen and Jiao-Liao-Ji Belt) These tectonic units are distinctly different in metamorphic time and P-T evolution Metamorphism of the Late Archean basement rocks in the Eastern and Yinshan Blocks occurred at ～2 5Ga, characterized by anticlockwise P-T paths involving isobaric cooling, suggesting the origin of the metamorphism related to the underplating of numerous mantle-derived magmas The major metamorphic event of the Khondalite Belt occurred at ～195Ga, and its metamorphic evolution is characterized by clockwise P-T paths involving near-isothermal decompression, which is interpreted as a result of collision between the Yinshan and Ordos Block to form the Western Block The metamorphism of the Trans-North China Orogen occurred at ～185Ga, and is also characterized by clockwise P-T paths involving near-isothermal decompression, reflecting the final amalgamation of the united Western Block and the Eastern Block to form the coherent basement of the North China Craton The metamorphism of the Paleoproterozoic Jiao-Liao-Ji Belt is featured by paired metamorphic zones of which the northwestern zone consists of the North Liaohe, Lading and Fenzishan groups, metamorphism of which is characterized by medium-pressure-type clockwise P-T paths, whereas the southeastern zone consists of the South Liaohe, Ji'an and Jingshan groups, metamorphism of which is characterized by low-pressure-type anticlockwise P-T paths Compared with the traditional 1 : 4,000,000 metamorphic map of China published in 1986, the newly compiled metamorphic evolution map of the North China Craton can be better used to reflect metamorphic histories of different tectonic units in the craton Although magmatic arc, continental rifting and mantle plume models can all be applied to interpretations of the metamorphism of the Eastern Block with IBC-type anticlockwise P-T paths, a mantle plume model is favored because it can reasonably explain the presence of an 800km wide igneous province in which all rocks were nearly simultaneously emplaced at 260～250Ga, numerous komatiites or komatiitic rocks, biomodal volcanic rocks, domiform structures, etc In the North China Craton, the exposures of typical high-pressure basic granulites with garnet-clinopyroxen-plagioclase-quartz assemblage and high-pressure pelitic granulites with kyanite-Kf-feldspar assemblage are only restricted to the northern segment of the Trans-North China Orogen and the southern segment of the Jiao-Liao-Ji Belt, and their formation was related to subduction and collision operative in these Paleoproterozoic mobile belts The local ultrahigh temperature (UHT) metamorphism with anticlockwise P-T paths in the Khondalite Belt, represented by sapphirine granulites in the Daqingshan and Jining Complexes, most likely resulted from underplating of mantle-derived magmas during the post-collisional extension at ～192Ga, as indicated by the sapphirine granulites which are always in direct contact with ～192Ga basic dykes

Anatomy of Zircons from an Ultrahot Orogen: The Amalgamation of the North China Craton within the Supercontinent Columbia

Abstract: We report SHRIMP U‐Pb, rare earth element, Hf isotope, and laser Raman spectroscopic data on zircons from ultrahigh‐temperature (UHT) granulites from the Jining Complex of the Khondalite Belt in the Western Block of the North China Craton (NCC). These UHT rocks form part of an ultrahot orogen that formed along the collisional margin of the NCC associated with the tectonics of assembly of the Paleoproterozoic supercontinent Columbia. Despite the core‐rim textures displayed by some of the UHT zircons, their age values sharply converge within error, yielding weighted mean 207Pb/206Pb ages of ca. 1.92 Ga, indicating growth by recrystallization under extreme thermal conditions. In general, the zircons show moderate heavy rare earth element enrichment with sharp positive Ce and negative Eu anomalies. The Hf isotope data from the UHT zircons also display a fairly uniform character, with the majority of them characterized by positive eHf values, without any indication of mixing between reworked crust and...

Geochronology of granulitized eclogite from the Ama Drime Massif: Implications for the tectonic evolution of the South Tibetan Himalaya

Abstract: [1] The Ama Drime Massif (ADM) is an elongate north-south trending antiformal feature that extends ∼70 km north across the crest of the South Tibetan Himalaya and offsets the position of the South Tibetan Detachment system. A detailed U(-Th)-Pb geochronologic study of granulitized mafic eclogites and associated rocks from the footwall of the ADM yields important insights into the middle to late Miocene tectonic evolution of the Himalayan orogen. The mafic igneous precursor to the granulitized eclogites is 986.6 ± 1.8 Ma and was intruded into the paleoproterozoic (1799 ± 9 Ma) Ama Drime orthogneiss, the latter being similar in age to rocks previously assigned to the Lesser Himalayan Series in the Himalayan foreland. The original eclogite-facies mineral assemblage in the mafic rocks has been strongly overprinted by granulite facies metamorphism at 750°C and 0.7–0.8 GPa. In the host Ama Drime orthogneiss, the granulite event is correlated with synkinematic sillimanite-grade metamorphism and muscovite dehydration melting. Monazite and xenotime ages indicate that the granulite metamorphism and associated anatexis occurred at <13.2 ± 1.4 Ma. High-grade metamorphism was followed by postkinematic leucogranite dyke emplacement at 11.6 ± 0.4 Ma. This integrated data set indicates that high-temperature metamorphism, decompression, and exhumation of the ADM postdates mid-Miocene south directed midcrustal extrusion and is kinematically linked to orogen-parallel extension.

Thermochronology of the Chernorud granulite zone, Ol’khon Region, Western Baikal area

Abstract: Structural-petrologic and isotopic-geochronologic data on magmatic, metamorphic, and metasomatic rocks from the Chernorud zone were used to reproduce the multistage history of their exhumation to upper crustal levels. The process is subdivided into four discrete stages, which corresponded to metamorphism to the granulite facies (500–490 Ma), metamorphism to the amphibolite facies (470–460 Ma), metamorphism to at least the epidote-amphibolite facies (440–430 Ma), and postmetamorphic events (410–400 Ma). The earliest two stages likely corresponded to the tectonic stacking of the backarc basin in response to the collision of the Siberian continent with the Eravninskaya island arc or the Barguzin microcontinent, a process that ended with the extensive generation of synmetamorphic granites. During the third and fourth stages, the granulites of the Chernorud nappe were successively exposed during intense tectonic motions along large deformation zones (Primorskii fault, collision lineament, and Orso Complex).

Unraveling Sedimentary Provenance and Tectonothermal History of High‐Temperature Metapelites, Using Zircon and Monazite Chemistry: A Case Study from the Eastern Ghats Belt, India

Abstract: The geochemical behavior of detrital zircon and monazite during granulite facies anatexis in metapelites from the Eastern Ghats Belt (EGB), India, is explored using U‐Pb geochronology, Hf isotopes, and trace elements. In a metapelite from the Ongole Domain, detrital zircon reequilibrated by coupled dissolution‐reprecipitation and diffusion reaction during ultrahigh‐temperature metamorphism at 1.63 Ga. The event completely reset the U‐Pb systems, but Hf isotopes and trace elements were only partially reequilibrated. Overgrowths on the altered cores date migmatization at 1.61 Ga. Monazite yields metamorphic ages similar to those of zircon. In metapelites from the Eastern Ghats Province (EGP), detrital zircon grains give 2.44–1.40‐Ga ages and metamorphic ones 1.2–0.5‐Ga ages. Metamorphic components include detrital grains reequilibrated by coupled dissolution‐reprecipitation in the presence of anatectic melt and newly crystallized overgrowths and grains. In reequilibrated domains, the U‐Pb system wa...

Upper Temperature Limits of Orogenic Gold Deposit Formation: Constraints from the Granulite-Hosted Griffin’s Find Deposit, Yilgarn Craton

Abstract: The crustal continuum model has been the dominant model for formation of orogenic gold deposits for more than 25 years. This model is based partly on the timing of mineralization at the Griffin’s Find gold deposit, located in the southwestern Yilgarn Block in Western Australia, which was previously interpreted to have formed via influx of gold-bearing hydrothermal fluid under granulite facies conditions at 700° to 750°C and 500 to 700 MPa. In this study, new petrogenetic constraints are placed on the timing of mineralization at Griffin’s Find. Peak metamorphism is here redefined to have involved conditions of 820° to 870°C and at least 550 MPa. This metamorphism caused significant partial melting of silicate assemblages within and surrounding the deposit, initially through dehydration melting and then decompression melting under fluid-absent conditions. Typical orogenic hydrothermal fluids (which have X(CO2) from 0.04 to 0.30) cannot have been added to the rock under these conditions. This statement is supported by the quartz-calcite assemblages preserved within the deposit, as these would have been completely destroyed if typical orogenic fluids had been added at temperatures beyond ~750°C. Infiltration of a high X(CO2) or X(CH4) fluid during peak metamorphism is precluded by the ubiquitous presence of biotite and cordierite throughout the deposit. Gold sulfide textures are consistent with solid-state prograde and retrograde metamorphic reactions in some samples, and with development of a gold-rich polymetallic melt in others. The presence of gold and sulfides in textural equilibrium (well-rounded and subspherical morphologies) with peak metamorphic minerals, particularly cordierite, indicates that extensive fluid influx cannot have occurred after peak metamorphism, as these silicates would have been completely retrogressed to hydrous phases. Thus mineralization at Griffin’s Find must have been introduced prior to granulite facies metamorphism. Textures in mineralized microcline-rich gneiss imply original mineralization temperatures within the greenschist facies, similar to the conditions of formation for other orogenic gold deposits. The results of this study have thus removed the high-temperature end of the crustal continuum model. Given the difficulty of transmitting hydrothermal fluids through rocks beyond 600° to 650°C without causing partial melting (migmatization), we suggest that gold deposits cannot form at temperatures beyond these. For the same reasons, mineralizing fluids cannot be sourced from high-grade metamorphic rocks. Thus, we find that gold deposits cannot form over a metamorphic continuum. Orogenic gold deposits should thus be thought of as a mesothermal phenomenon.

The Eastern Ghats Belt … A polycyclic granulite terrain

Abstract: The Eastern Ghats Belt is a polycyclic granulite terrain along the east coast of India whose western boundary is marked by a shear zone along which the granulites are thrusted over the cratonic units of the Indian shield, and its northern margin is marked by the presence of a number of fault-bounded blocks. Recent work has convincingly brought out that there are domains within the belt having different evolutionary histories. The segment south of the Godavari Rift went through a high grade thermo-tectonic event at ∼1.6-1.7 Ga. North of the Godavari Rift in a narrow zone along the western boundary the last high-grade metamorphic event is of late Archaean age. A series of alkaline plutons along the western boundary zone testifies to a rifting episode at ∼1.3-1.5 Ga. In the major part of the EGB the metamorphism is broadly of Grenvillian age, with two major thermo-tectonic pulses at ∼1.1-1.2 Ga and ∼0.95-1.0 Ga. But high grade conditions persisted for a long period and younger thermal events of ∼0.65 Ga to ∼0.80 Ga are locally recorded. There are differences in the tectonometamorphic histories of different domains, but the tectonic significance of these differences remains uncertain. Pan-African (0.50-0.55) thermal overprints are common and become conspicuous along the western boundary zone. The thrusting of the Eastern Ghats granulites in a hot state over the cratons to the west is of Pan-African age. In the Rodinia assembly (∼0.9 Ga) the Eastern Ghats and the Rayner-Napier Complexes of Antarctica were contiguous, but the pre-Rodinia configuration of these terrains remains unclear. At ∼0.8 Ga during the Rodinia break up Greater India rifted apart from East Antarctica, and only later it docked with Australia-East Antarctica at 530-550 Ma. The continuation of the East Antarctic Pan-African orogenic belts into the Eastern Ghats is yet to be ascertained.

Origin of a Mesozoic granite with A-type characteristics from the North China craton: highly fractionated from I-type magmas?

Abstract: We report geochronological, geochemical and isotopic data for the Mesozoic Shangshuiquan granite from the northern margin of the North China craton. The granite is highly fractionated, with SiO2 > 74%. Occurrence of annitic biotite, high contents of alkalis (K2O + Na2O), Rb, Y, Nb and heavy rare earth elements, high FeOt/MgO, low contents of CaO, Al2O3, Ba, and Sr, and large negative Eu anomalies, makes it indistinguishable from typical A-type granites. A mantle-derived origin for the rocks of the granite is not favored because their high initial 87Sr/86Sr (≥0.706) and low eNd (t) (<−15) are completely different from either those of the lithospheric or asthenospheric mantle. In fact, their Sr–Nd isotopes fall within the range of Sr–Nd isotopic compositions of the Archean granulite terrains and are comparable to those of Mesozoic crustal-derived I-type granitoids in the region. Therefore, the Shangshuiquan granite is considered to be dominantly derived from partial melting of the ancient lower crust. Its parental magmas prove to be similar to I-type magmas and to have undergone extensive fractionation during its ascent. This is supported by the fact that some of the nearby Hannuoba feldspar-rich granulite xenoliths can be indeed regarded as the early cumulates in terms of their mineralogy, chemistry, Sr–Nd isotopes and zircon U–Pb ages and Hf isotopes. It is furthermore argued that some of highly fractionated granites worldwide, especially those with A-type characteristics and lacking close relationship with unfractionated rocks, may in fact be fractionated I-type granites. This suggestion can explain their close temporal and spatial associations as well as similar Sr–Nd isotopes with I-type granites. Our study also sheds new light on the petrogenesis of deep crustal xenoliths.

Probing the basement of southern Tibet: evidence from crustal xenoliths entrained in a Miocene ultrapotassic dyke

Abstract: A variety of felsic and mafic granulites and ultramafic rocks occur as xenoliths within a 12.7 Ma ultrapotassic dyke intruding Xigaze flysch immediately to the north of the Yarlung–Tsangpo suture zone in southern Tibet. Garnet–clinopyroxene–plagioclase–quartz thermobarometry on mafic granulite xenoliths gives temperatures of 1130–1330 °C and pressures between 22 and 26 kbar indicating equilibration in the high-pressure and ultrahigh-temperature granulite field and defining a geotherm of c . 16 °C km −1 . Ultramafic xenoliths consist mainly of hornblende and biotite, probably of restitic crustal rather than mantle origin, and attained peak metamorphic conditions of 920–1130 °C and 17–24 kbar, whereas felsic granulites equilibrated at 870–900 °C at an inferred pressure of 17 kbar. In situ U–(Th)–Pb laser ablation inductively coupled plasma mass spectrometry dating of zircons shows that protoliths may include Proterozoic basement rocks, Late Cretaceous calc-alkaline tonalites of the Gangdese batholith root and/or remnants of a Neo-Tethyan oceanic arc. Certain zircons from a felsic granulite and an ultramafic xenolith have mean 206 Pb/ 238 U ages of 16.8 ± 0.9 Ma and 15.6 ± 0.6 Ma, respectively, and monazites from a micaceous xenolith yielded a mean 208 Pb/ 232 Th age of 14.4 ± 0.4 Ma. These results show that the southern Tibet basement reached a thickness of c . 80 km by 17–14 Ma at the latest and has remained unchanged until the present day. Supplementary material: Mineral analyses used for pressure–temperature estimations and results of 40 Ar/ 30 Ar laser single-grain fusion experiments are available at http://www.geolsoc.org.uk/SUP18325.

Eclogite–high-pressure granulite metamorphism records early collision in West Gondwana: new data from the Southern Brasília Belt, Brazil

Abstract: Nappes in the southern sector of the Southern Brasilia Belt record suturing of the Paranapanema Block and Socorro–Guaxupe Arc with a subducted passive margin on the western side of the Sao Francisco Craton. We report secondary ion mass spectrometry U–Pb zircon ages that for the first time constrain the age of: (1) retrograded eclogite from a block along the tectonic contact beneath the uppermost nappe in a stack of passive margin-derived nappes; (2) high-pressure granulite-facies metamorphism in the uppermost passive margin-derived nappe; (3) high-pressure granulite-facies metamorphism in the overlying arc-derived nappe. Rare zircons from a retrograded eclogite yield a 206 Pb/ 238 U age of 678 ± 29 Ma, which we interpret as most likely to date close-to-peak- P metamorphism and to provide a minimum age for detachment of the overlying passive margin-derived nappe from the subducting plate. Zircon associated with ilmenite in samples from two structural levels in the passive margin-derived high-pressure granulite nappe yields 206 Pb/ 238 U ages of 648 ± 12 and 647 ± 11 Ma, and Ti-in-zircon crystallization temperatures from c . 860 °C down to c . 785 °C, but skewed toward the lower part of the range. These data indicate zircon formation during cooling from around peak T to the solidus, consistent with the high- T retrograde P – T path deduced from microstructures linked to phase assemblage fields in isochemical phase diagrams. Rb–Sr multi-mineral–whole-rock isochrons from two samples from close to the bottom of this nappe date formation of a retrograde sillimanite-bearing penetrative fabric to c . 590 Ma at temperatures of c . 750 °C (based on Ti-in-quartz thermometry). Rare zircons from leucosome in high-pressure granulite from the overlying arc-derived nappe yield a 206 Pb/ 238 U age of 622 ± 28 Ma and Ti-in-zircon crystallization temperatures from c . 970 °C down to c . 820 °C, which we interpret to record formation of zircon during cooling from peak high-pressure granulite-facies conditions. These ages indicate that the first stage of craton amalgamation in West Gondwana may have occurred earlier than previously inferred. Supplementary material: Geochemical and geochronological data are available at http://www.geolsoc.org.uk/SUP18377.

Some Remarks on Melting and Extreme Metamorphism of Crustal Rocks

Abstract: Typically melting occurs during decompression in ultra-high-pressure terranes, along the evolution from peak pressure to peak temperature in medium-temperature eclogite-high-pressure granulite terranes and by simple prograde heating in granulite facies and ultra-high-temperature (UHT) metamorphic terranes. The source of heat must be due to one or more processes among thickening and radiogenic heating, viscous dissipation, and heat from asthenospheric mantle. Melt-bearing rocks become porous at a few vol% melt initiating an advective flow regime. As the melt volume approaches and exceeds the melt connectivity transition (∼7 vol% melt), melt may be lost from the system in the first of several melt-loss events. In migmatites and residual granulites, a variety of microstructures indicates the former presence of melt at the grain scale and leucosome networks at outcrop scale record melt extraction pathways. This evidence supports a model of focused melt flow by dilatant shear failure at low melt volume as the crust weakens with increasing melt production. Melt ascent is initiated as ductile fractures but continues in dykes that propagate as brittle fractures. Crustal rocks undergo melting via a sequence of reactions beginning with minimal melt production at the wet solidus (generally 50) vol% melt, depending on the fertility of the protolith composition and the intensive variables). At temperatures above the stability of the hydrate, assuming significant melt loss by this point, low-volume melt production continues by consumption of feldspar(s) and quartz at UHT conditions (generally < 10 vol% melt at peak UHTM conditions). Significant melt loss is a contributory factor to achieving UHTs because dehydration of the system limits the progress of heat-consuming melting reactions among the residual phases in the source.

Geochronological and petrological constraints on Palaeoproterozoic granulite facies metamorphism in southeastern margin of the North China Craton

Abstract: In the southeastern margin of the North China Craton, high-pressure (HP) granulite facies meta-basic rocks exposed as bands or lenses in the Precambrian metamorphic basement (e.g. Bengbu) and as xenoliths in Mesozoic intrusions (e.g. Jiagou) are characterized by the assemblage garnet + clinopyroxene + plagioclase + quartz + rutile ± Ti-rich hornblende. Cathodoluminescence imaging and mineral inclusions reveal that most zircon from the three dated samples displays distinct core-mantle-rim structures. The cores show typical igneous zircon characteristics and give ages of 2.5–2.4 Ga, thus dating the protolith of the metabasites. The mantles formed at granulite facies conditions as evidenced by inclusions of the HP granulite mineral assemblage garnet + clinopyroxene + rutile + plagioclase + quartz ± hornblende and Ti-rich biotite and yield ages of 1839 ± 31, 1811 ± 19 and 1800 ± 15 Ma. An inclusion-free rim yields an age of 176 ± 2 Ma with the lower Th/U ratio of 0.02. The geochronological and preliminary petrological data of this study suggest that the lower crust beneath the southeastern margin of the North China Craton formed at 2.5–2.4 Ga and underwent HP granulite facies metamorphism at c. 1.8 Ga. This HT-HP metamorphic event may be ascribed to large-scale crustal heating and thickening related to mantle-derived magma underplating at the base of the lower crust, as evidenced by widespread extension, rifting and related mafic magma emplacement in the North China Craton during this period. The age of 176 ± 2 Ma most likely records the late amphibolite facies retrogression occurring during exhumation.