Showing papers on "Granulite published in 2013"

The robustness of the Zr-in-rutile and Ti-in-zircon thermometers during high-temperature metamorphism (Ivrea-Verbano Zone, northern Italy)

Abstract: This study investigates the behaviour of the Zr-in-rutile and Ti-in-zircon thermometers in granulite facies metapelites from the Ivrea-Verbano Zone lower crustal section. U–Pb ages of zircon constrain the timing of regional amphibolite–granulite facies metamorphism to 316 ± 3 Ma and record zircon recrystallisation and resetting of U–Pb ages at 276 ± 4 Ma and 258 ± 3 Ma. Zr-in-rutile thermometry records peak contact metamorphic temperatures related to intrusion of mafic magmatic rocks and gives peak temperatures between 900–930 °C and 1,000–1,020 °C that are consistent with the geological settings of the samples. Ti-in-zircon temperatures of 700–800 °C and 810–870 °C record growth or re-equilibration of zircon after cooling from peak temperatures. Ti-in-quartz thermometry for one sample records both peak and retrograde temperatures. Some rutiles in all samples record resetting of Zr-in-rutile temperatures at ~750–800 °C. Electron microprobe profiles across individual rutiles demonstrate that Zr expulsion occurred by recrystallisation rather than by diffusive exchange. Exsolution of small needles of baddelyite or zircon from rutile is an important method of Zr redistribution, but results in no net Zr loss from the grain. The demonstration that Zr-in-rutile thermometry can robustly record peak temperatures that are not recorded by any other thermometer emphasises the relevance of this technique to investigating the evolution of high-grade metamorphic terranes, such as those that characterise the lower crust.

Origin and ascent history of unusually crystal-rich alkaline basaltic magmas from the western Pannonian Basin

Abstract: The last eruptions of the monogenetic Bakony-Balaton Highland Volcanic Field (western Pannonian Basin, Hungary) produced unusually crystal- and xenolith-rich alkaline basalts which are unique among the alkaline basalts of the Carpathian–Pannonian Region. Similar alkaline basalts are only rarely known in other volcanic fields of the world. These special basaltic magmas fed the eruptions of two closely located volcanic centres: the Bondoro-hegy and the Fuzes-to scoria cone. Their uncommon enrichment in diverse crystals produced unique rock textures and modified original magma compositions (13.1–14.2 wt.% MgO, 459–657 ppm Cr, and 455–564 ppm Ni contents). Detailed mineral-scale textural and chemical analyses revealed that the Bondoro-hegy and Fuzes-to alkaline basaltic magmas have a complex ascent history, and that most of their minerals (∼30 vol.% of the rocks) represent foreign crystals derived from different levels of the underlying lithosphere. The most abundant xenocrysts, olivine, orthopyroxene, clinopyroxene, and spinel, were incorporated from different regions and rock types of the subcontinental lithospheric mantle. Megacrysts of clinopyroxene and spinel could have originated from pegmatitic veins/sills which probably represent magmas crystallized near the crust–mantle boundary. Green clinopyroxene xenocrysts could have been derived from lower crustal mafic granulites. Minerals that crystallized in situ from the alkaline basaltic melts (olivine with Cr-spinel inclusions, clinopyroxene, plagioclase, and Fe–Ti oxides) are only represented by microphenocrysts and overgrowths on the foreign crystals. The vast amount of peridotitic (most common) and mafic granulitic materials indicates a highly effective interaction between the ascending magmas and wall rocks at lithospheric mantle and lower crustal levels. However, fragments from the middle and upper crust are absent from the studied basalts, suggesting a change in the style (and possibly rate) of magma ascent in the crust. These xenocryst- and xenolith-rich basalts yield divers tools for estimating magma ascent rate that is important for hazard forecasting in monogenetic volcanic fields. According to the estimated ascent rates, the Bondoro-hegy and Fuzes-to alkaline basaltic magmas could have reached the surface within hours to few days, similarly to the estimates for other eruptive centres in the Pannonian Basin which were fed by “normal” (crystal and xenoliths poor) alkaline basalts.

Osumilite–melt interactions in ultrahigh temperature granulites: phase equilibria modelling and implications for the P–T–t evolution of the Eastern Ghats Province, India

Abstract: The exposed residual crust in the Eastern Ghats Province records ultrahigh temperature (UHT) metamorphic conditions involving extensive crustal anatexis and melt loss. However, there is disagreement about the tectonic evolution of this late Mesoproterozoic–early Neoproterozoic orogen due to conflicting petrological, structural and geochronological interpretations. One of the petrological disputes in residual high Mg–Al granulites concerns the origin of fine-grained mineral intergrowths comprising cordierite + K-feldspar ± quartz ± biotite ± sillimanite ± plagioclase. These intergrowths wrap around porphyroblast phases and are interpreted to have formed by the breakdown of primary osumilite in the presence of melt trapped in the equilibration volume by the melt percolation threshold. The pressure (P)–temperature (T) evolution of four samples from three localities across the central Eastern Ghats Province is constrained using phase equilibria modelling in the chemical system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO). Results of the modelling are integrated with published geochronological results for these samples to show that the central Eastern Ghats Province followed a common P–T–t history. This history is characterized by peak UHT metamorphic conditions of 945–955 °C and 7.8–8.2 kbar followed by a slight increase in pressure and close-to-isobaric cooling to the conditions of the elevated solidus at 940–900 °C and 8.5–8.3 kbar. In common with other localities from the Eastern Ghats Province, the early development of cordierite before osumilite and the peak to immediate post-peak retrograde reaction between osumilite and melt to produce the intergrowth features requires that the prograde evolution was one of contemporaneous increasing pressure with increasing temperature. This counter-clockwise (CCW) evolution is evaluated for one sample using inverse phase equilibria modelling along a schematic P–T path of 150 °C kbar−1 starting from the low P–T end of the prograde P–T path as constrained by the phase equilibria modelling. The inverse modelling is executed by step-wise down temperature reintegration of sufficient melt into the residual bulk chemical composition at the P–T point of the 1 mol.% melt isopleth at each step, representing the melt remaining on grain boundaries after each prograde drainage event, to reach the melt connectivity transition (MCT) of 7 mol.%. The procedure is repeated until a plausible protolith composition is recovered. The result demonstrates that clastic sedimentary rocks that followed a CCW P–T evolution could have produced the observed mineral assemblages and microstructures preserved in the central Eastern Ghats Province. This study also highlights the role of melt during UHT metamorphism, particularly its importance to both chemical and physical processes along the prograde and retrograde segments of the P–T path. These processes include: (i) an increase in diffusive length scales during the late prograde to peak evolution, creating equilibration volumes larger than a standard thin section; (ii) the development of retrograde mineral assemblages, which is facilitated if some melt is retained post-peak; (iii) the presence of melt as a weakening mechanism and the advection of heat by melt, allowing the crust to thicken; and (iv) the effect of melt loss, which makes the deep crust both denser and stronger, and reduces heat production at depth, limiting crustal thickening and facilitating the transition to close-to-isobaric cooling.

The South Tibetan detachment system facilitates ultra rapid cooling of granulite-facies rocks in Sikkim Himalaya

Abstract: [1] The eastern Himalaya is characterized by a region of granulites and local granulitized eclogites that have been exhumed via isothermal decompression from lower crustal depths during the India-Asia collision. Spatially, most of these regions are proximal to the South Tibetan detachment system, an orogen-parallel normal-sense detachment system that operated during the Miocene, suggesting that it played a role in their exhumation. Here we use geo- and thermochronological methods to study the deformation and cooling history of footwall rocks of the South Tibetan detachment system in northern Sikkim, India. These data demonstrate that the South Tibetan detachment system was active in Sikkim between 23.6 and ~13 Ma, and that footwall rocks cooled rapidly from ~700 to ~120 °C between ~15-13 Ma. While active, the South Tibetan detachment system exhumed rocks from mid-crustal depths, but an additional heat source such as strain heating, advected melt and/or crustal thinning is required to explain the observed isothermal decompression. Cessation of movement on the South Tibetan detachment system produced rapid cooling of the footwall as isotherms relaxed. A regional comparison of temperature-time data for the eastern South Tibetan detachment system indicates a lack of synchronicity between the Sa'er-Sikkim-Yadong section and the NW Bhutan section. To accommodate this requires either strike-slip tear faulting or local out-of-sequence thrusting in the younger segment of the orogen.

The Realm of Ultrahigh-Pressure Metamorphism

Abstract: The discovery of diamond and coesite in crustal rocks is compelling evidence that continental material has experienced pressures that can only be achieved at mantle depths. At least 20 terranes of unequivocal continental crust containing diamond or coesite are now recognized around the globe; their study constitutes a new field in petrology called ultrahigh-pressure metamorphism. The idea that continents do not subduct has given way to the notion that Earth has been sufficiently cool since the Cryogenian (~850 Ma) to allow density changes to drive continental crust into the mantle during collision. Some of this crust is exhumed to the surface, some pools at the Moho, and the rest sinks into the mantle. In this issue, microscopic observations, phase-equilibrium modeling, geochronology, and geodynamic modeling track the journey of crustal rocks to the mantle and back to Earth's surface. * Buoyancy : the upward force exerted on the crust by the mantle; the term also refers to the difference in density between the mantle and the oceanic or continental crust because density determines the amount of displacement (floating or sinking) of the crust with respect to the mantle. Coesite : a high-pressure polymorph of SiO2. Coesite is more dense than quartz and is stable at depths greater than 90 km ( P > 2.6 GPa) at 600 °C. Coesite is the main index mineral of UHP metamorphism, but it can also form by shock from a meteorite impact. Delamination : a process whereby rocks at the base of the crust become dense, perhaps by overthickening or gravitational instability, and sink into the mantle. These so-called mantle drips have been imaged seismically and produced in geodynamic models. Diamond : a dense, high-pressure polymorph of carbon, in which each carbon atom is bonded to four other carbon atoms in a rigid tetrahedron, resulting in its cubic symmetry. Diamond is also an index mineral of UHP metamorphism. Eclogite : a metamorphosed mafic rock that contains garnet and omphacite, but not plagioclase. Other common minerals in eclogite are kyanite, quartz/coesite, phengite, epidote-group minerals, magnetite, rutile, and zircon. Exhumation : the movement of a buried rock towards Earth's surface. Exhumation is distinct from uplift, which is simply the increase in elevation of the surface. Garnet peridotite : an ultramafic metamorphic rock composed of garnet + olivine ± pyroxene, sometimes called orogenic peridotite because such rocks are found in collisional orogens. Garnet peridotites are commonly derived from the sublithospheric mantle, but they can also represent minor ultramafic cumulates associated with crustal mafic intrusions. High-pressure (HP) metamorphism : metamorphism at pressures above the calcite to aragonite transition. Na pyroxenes are also stable in this field. HP rocks occupy the blueschist, eclogite, and HP granulite facies. Metamorphic facies : the collection of mineral assemblages from rocks of all bulk compositions that crystallize at the same pressure and temperature conditions. The greenschist, blueschist, amphibolite, granulite, and eclogite facies boundaries are established based on the mineral assemblages in metamorphosed mafic rocks. Omphacite : a dense, Na-rich clinopyroxene, with a minimum of 20% jadeite component. Jadeite is the pure Na pyroxene (NaAlSi2O6) end-member. Pseudomorph : the result of the replacement of one mineral by another, where the new mineral assumes the form of the original. Relamination : a process whereby continental rocks that have reached mantle depths—or their melt products—are returned to the base of the crust. Stishovite : the second high-pressure polymorph of SiO2, formed by the conversion of coesite at mantle depths in excess of 200 km (~7 GPa) at 600 °C or 300 km (~10 GPa) at 1400 °C. Natural stishovite has only been observed in meteorite impact structures. Terrane : a crustal block with a distinct stratigraphy, structure, and geologic history. Exotic terranes are fault-bounded blocks that have a foreign origin compared to the surrounding rocks. Most UHP terranes are of known provenance and belong to a craton, although exotic UHP terranes that originated as continental arcs are also known. A terrain can be thought of as any physical landmass, and the term sometimes refers to topography, as in “rough terrain.” Ultrahigh-pressure (UHP) metamorphism : metamorphism of crustal rocks that occurs at pressures in the coesite stability field. UHP metamorphism is identified by the presence of coesite or diamond, or by the equivalent pressure–temperature conditions calculated from thermodynamic models of mineral compositions.

Reinterpretation of metamorphic age of the Hengshan Complex, North China Craton

Abstract: Hengshan granulite facies terrane consists of tonalite-trondhjemite-granodiorite (TTG) gneisses and minor high-pressure mafic granulite blocks. LA-ICP-MS zircon dating for two TTG gneiss samples indicates metamorphic ages of 1916.7±9.9 Ma and 1850–1930 Ma, and magmatic protolith age of ca. 2.5 Ga, with similar HREE-rich patterns. Phase equilibria modelling in ZrO2-bearing system reveals that zircon abundance varies inversely with melt abundance in suprasolidus metamorphism, zircon growth occurs with melt crystallization during cooling, and thus, the newly grown zircon can only record the age of retrograde metamorphism. Consequently, we suggest that the Hengshan granulite facies terrane experienced slowly uplifting and cooling during 1.85–1.93 Ga in an extensional setting under middle-crust, while the collision orogeny corresponding to peak stage of high-pressure granulite may have happened much earlier.

Extreme extension across Seram and Ambon, eastern Indonesia: evidence for Banda slab rollback

Abstract: . The island of Seram, which lies in the northern part of the 180°-curved Banda Arc, has previously been interpreted as a fold-and-thrust belt formed during arc-continent collision, which incorporates ophiolites intruded by granites thought to have been produced by anatexis within a metamorphic sole. However, new geological mapping and a re-examination of the field relations cause us to question this model. We instead propose that there is evidence for recent and rapid N–S extension that has caused the high-temperature exhumation of lherzolites beneath low-angle lithospheric detachment faults that induced high-temperature metamorphism and melting in overlying crustal rocks. These "Kobipoto Complex" migmatites include highly residual Al–Mg-rich garnet + cordierite + sillimanite + spinel + corundum granulites (exposed in the Kobipoto Mountains) which contain coexisting spinel + quartz, indicating that peak metamorphic temperatures likely approached 900 °C. Associated with these residual granulites are voluminous Mio-Pliocene granitic diatexites, or "cordierite granites", which crop out on Ambon, western Seram, and in the Kobipoto Mountains and incorporate abundant schlieren of spinel- and sillimanite-bearing residuum. Quaternary "ambonites" (cordierite + garnet dacites) emplaced on Ambon were also evidently sourced from the Kobipoto Complex migmatites as demonstrated by granulite-inherited xenoliths. Exhumation of the hot peridotites and granulite-facies Kobipoto Complex migmatites to shallower structural levels caused greenschist- to lower-amphibolite facies metapelites and amphibolites of the Tehoru Formation to be overprinted by sillimanite-grade metamorphism, migmatisation, and limited localised anatexis to form the Taunusa Complex. The extreme extension required to have driven Kobipoto Complex exhumation evidently occurred throughout Seram and along much of the northern Banda Arc. The lherzolites must have been juxtaposed against the crust at typical lithospheric mantle temperatures in order to account for such high-temperature metamorphism and therefore could not have been part of a cooled ophiolite. In central Seram, lenses of peridotites are incorporated with a major left-lateral strike-slip shear zone (the "Kawa Shear Zone"), demonstrating that strike-slip motions likely initiated shortly after the mantle had been partly exhumed by detachment faulting and that the main strike-slip faults may themselves be reactivated and steepened low-angle detachments. The geodynamic driver for mantle exhumation along the detachment faults and strike-slip faulting in central Seram is very likely the same; we interpret the extreme extension to be the result of eastward slab rollback into the Banda Embayment as outlined by the latest plate reconstructions for Banda Arc evolution.

The Itsaq Gneiss Complex of Greenland: Episodic 3900 to 3660 Ma juvenile crust formation and recycling in the 3660 to 3600 Ma Isukasian orogeny

Abstract: From the 3000 km2 Eoarchean Itsaq Gneiss Complex (IGC) of Greenland, zircon U-Pb dating of numerous meta-granitoid and orthogneiss samples is integrated with geologic observations, whole rock geochemistry and a strategic subset of zircon Hf and whole rock Nd isotopic measurements. This shows that there are multiple episodes of TTG suite formation from ∼3890 to 3660 Ma, characterized by zircon initial eHf≈0 and whole rock initial eNd of > +2. These rocks mostly have geochemical signatures of partial melting of eclogitized mafic sources, with a subset of high magnesian, low silica rocks indicating fusion by fluid fluxing of upper mantle sources. The TTG suites are accompanied by slightly older gabbros, basalts and andesites, which have geochemical signatures pointing to magmas originating from fluid fluxing of upper mantle sources. The data show the formation of juvenile crust domains in several discrete events from ∼3900 to 3660 Ma, probably at convergent plate boundaries in an environment analogous, but not identical to, modern island arcs. In the Isua area, a northern ∼3700 Ma terrane formed distal from a predominantly ∼3800 Ma terrane. These terranes were juxtaposed between 3680 and 3660 Ma—respectively the age of the youngest rocks unique to the northern terrane and the lithologically distinctive ultramafic-granitic Inaluk dykes common to both terranes. This shows the assembly of different domains of juvenile rocks to form a more expansive domain of “continental” crust. A rare occurrence of high-pressure granulite is dated at ∼3660 Ma, demonstrating that assembly involved tectonic crustal thickening. This continental crust was then reworked in the 3660 to 3600 Ma Isukasian orogeny. In the northern part of the Isua area, 3660 to 3600 Ma granites were emplaced into ∼3700 Ma tonalites. The earliest granites are nebulous, and sigmoidal schlieric inclusions within them demonstrate ductile extension. Younger granite sheets were emplaced into extensional ductile-brittle fractures. These granite-tonalite relationships are overprinted by widespread development of late Eoarchean (pre-3500 Ma Ameralik dyke) brittle-ductile extensional cataclastic textures, together demonstrating that extension was polybaric. The southern part of the Isua area largely escaped 3660 to 3600 Ma high temperature processes and has sparse granite sheets commonly focused into coeval shear zones. In the rest of the complex, deeper crustal levels during the Isukasian orogeny are widely preserved. These experienced upper amphibolite to granulite facies moderateto low-pressure syn-kinematic metamorphism, forming complex migmatites rich in granitic-trondhjemitic neosome. The migmatites were intruded by composite ferrogabbro and granite bodies, in which syn-magmatic extensional features are locally preserved. Thus 3660 to 3600 Ma crustal recycling involved elevated crustal thermal gradients in an extensional regime. Crustal melts formed in the Isukasian orogeny have zircon initial eHf