Biotite Dehydration Melting Research Articles

Sulphate incorporation in monazite lattice and dating the cycle of sulphur in metamorphic belts

Microgeochemical data and transmission electron microscope (TEM) imaging of S-rich monazite crystals demonstrate that S has been incorporated in the lattice of monazite as a clino-anhydrite component via the following exchange Ca2+ + S6+ = REE3+ + P5+, and that it is now partly exsolved in nanoclusters (5–10 nm) of CaSO4. The sample, an osumilite-bearing ultra-high-temperature granulite from Rogaland, Norway, is characterized by complexly patchy zoned monazite crystals. Three chemical domains are distinguished as (1) a sulphate-rich core (0.45–0.72 wt% SO2, Th incorporated as cheralite component), (2) secondary sulphate-bearing domains (SO2 >0.05 wt%, partly clouded with solid inclusions), and (3) late S-free, Y-rich domains (0.8–2.5 wt% Y2O3, Th accommodated as the huttonite component). These three domains yield distinct isotopic U–Pb ages of 1034 ± 6, 1005 ± 7, and 935 ± 7 Ma, respectively. Uranium–Th–Pb EPMA dating independently confirms these ages. This study illustrates that it is possible to discriminate different generations of monazite based on their S contents. From the petrological context, we propose that sulphate-rich monazite reflects high-temperature Fe–sulphide breakdown under oxidizing conditions, coeval with biotite dehydration melting. Monazite may therefore reveal the presence of S in anatectic melts from high-grade terrains at a specific point in time and date S mobilization from a reduced to an oxidized state. This property can be used to investigate the mineralization potential of a given geological event within a larger orogenic framework.

The occurrence of fluor-wagnerite in UHT granulites and its implications towards understanding fluid regimes in the evolution of deep crust: a case study from the Eastern Ghats Belt, India

We report the occurrence of a rare phosphate mineral, fluor-wagnerite (Mg1.91–1.94Fe0.06–0.07Ca<0.01) (P0.99–1.00O4)(OH0.02–0.17F0.98–0.83) from the Eastern Ghats Belt of India, an orogenic belt evolved during Meso- to Neoproterozoic time. The host rock, i.e. high- to ultrahigh temperature (UHT) granulites (~1000 °C, 8–9 kbar) of the studied area was retrogressed after emplacement to mid-crustal level (800–850 °C, 6–6.5 kbar) as deduced from their pressure-temperature histories. Based on mineral chemical data and micro-Raman analyses, we document an unusual high Mg-F-rich chemistry of the F-wagnerite, which occur both in peak metamorphic porphyroblastic assemblages as well as in the retrograde matrix assemblage. Therefore, in absence of other common phosphates like apatite, fluor-wagnerite can act as an indicator for the presence of F-bearing fluids for rocks with high X Mg and/or fO2. The occurrence of F-rich minerals as monitors for fluid compositions has important implications for the onset of biotite dehydration melting and hence melt production in the deep crust. We propose that fluor-wagnerite can occur as an accessory mineral associated with F-rich fluids in lower-mid crustal rocks, and F in coexisting minerals should be taken into consideration when reconciling the petrogenetic grid of biotite-dehydration melting.

Anatectic record and P–T path evolution of metapelites from the Wulashan Complex, Khondalite Belt, North China Craton

The Wulashan Complex in the Khondalite Belt, the North China Craton contains high-grade garnet-cordierite-sillimanite metapelitic rocks with anatectic record. The rocks preserve representative metamorphic assemblages of peak, post-peak near-isothermal decompression, and decompressional cooling stages. Combined with petrographic observation and P–T pseudosections of residual bulk compositions, a clockwise P–T path was inferred involving near-isothermal decompression and decompressional cooling that followed the peak metamorphism. Furthermore, based on phase equilibria modeling for the melt-reintegrated composition, a whole, clockwise P–T path with a possible pre-peak segment was inferred. In the pre-peak field of garnet+biotite+K-feldspar+plagioclase+sillimanite+quartz+magnetite+liquid, garnet was first introduced by consuming biotite, quartz, and plagioclase via the biotite dehydration melting reaction of Bt+Sil+Qz±Pl→Grt+Kfs+Melt. The peak metamorphic assemblage was interpreted to be garnet+biotite+K-feldspar+plagioclase+sillimanite+quartz+ilmenite+magnetite+liquid at 830–860°C and 9.5–11kbar. The following post-peak near-isothermal decompressional assemblage is garnet+biotite+cordierite+K-feldspar+plagioclase+quartz+magnetite+ilmenite+liquid at 840–880°C and 6.0–7.5kbar, resulting in matrix cordierite isolated from garnet due to the biotite decompressional dehydration melting reaction of Bt+Sil+Qz±Pl→Crd+Kfs±Ilm+Melt. Furthermore, subsequent decompressional cooling processes resulted in symplectites of cordierite and biotite+plagioclase mantling garnet due to the breakdown reactions: Grt+Sil+Melt→Crd+Bt+Fe-oxide, and Grt+Melt→Bt+Qz±Pl. Melt that remained in the host rocks formed thin films of plagioclase or K-feldspar distributed along plagioclase, quartz, and sillimanite grain boundaries. The results indicate that the Khondalite Belt experienced continent–continent subduction and collision followed by exhumation and cooling between the Yinshan and Ordos Blocks in the Western Block of the North China Craton, and the rocks of the Wulashan Complex were subjected to granulite-facies metamorphism accompanied by partial melting events.

Tectonothermal Evolution of the Middle Crust in the Trans-Hudson Orogen, Baffin Island, Canada: Evidence from Petrology and Monazite Geochronology of Sillimanite-bearing Migmatites

Abstract Hall Peninsula, on SE Baffin Island, Canada, exposes a lithologically diverse mid-crustal section of the Paleoproterozoic Trans-Hudson Orogen, which features Paleoproterozoic supracrustal strata overlying Archean crystalline basement. Recent mapping, combined with petrological studies and U–Pb monazite dating, provides new insights into the tectonothermal history of the internal zone of the orogen, as well as mid-crustal orogenic processes in general. Peak metamorphic grade is constrained to progressively increase westward from mid-amphibolite- to granulite-facies conditions and three regional deformation events are recognized: east-directed thin-skinned crustal shortening (D 1 ); east-vergent, thick-skinned thrusting and folding (D 2 ); late-orogenic, north–south-directed, thick-skinned folding (D 3 ). Equilibrium phase diagram modeling constrained by garnet compositional transects in pelite indicates peak mid-amphibolite-facies conditions on the eastern peninsula of ∼720–740°C and 6·25–7·25 kbar, followed by cooling and late muscovite growth. In accordance with field evidence for extensive biotite dehydration melting, peak metamorphic conditions of granulite-facies pelite on the western peninsula are estimated at ∼810–890°C and ∼6·1–7·35 kbar. In situ U–Pb monazite geochronology (laser ablation inductively coupled plasma mass spectrometry) constrains the timing of peak metamorphism and concurrent D 1 deformation throughout the peninsula to between c. 1850 and 1825 Ma, coincident with the accretion of crustal blocks and arc terranes during the amalgamation of the orogenic upper plate. A younger population of c. 1800 Ma monazite implies fluid-assisted dissolution–reprecipitation of older monazite, possibly related to the terminal collision of the amalgamated upper plate Churchill domain with the lower plate Superior craton. Combined with published observations, the integrated geochronological and petrological data suggest that the crustal section exposed on Hall Peninsula remained at elevated temperatures (&amp;gt;550°C) for ∼100 Myr after the thermal peak. Despite significant crustal thickening and sustained high temperatures, which are comparable with those of large, hot orogens, the crustal section records no evidence of mid-crustal ductile (channel) flow or rapid exhumation during orogenic collapse.

Timing of anatexis and melt crystallization in the Socorro–Guaxupé Nappe, SE Brazil: Insights from trace element composition of zircon, monazite and garnet coupled to U[sbnd]Pb geochronology

The timing of partial melting and melt crystallization in granulite facies rocks of the Socorro–Guaxupé Nappe (SGN), Brazil is constrained using a combination of imaging techniques, LA-ICP-MS and EPMA dating, trace element geochemistry and thermobarometry. (Orthopyroxene)-garnet-bearing migmatite that records extensive biotite dehydration melting shows evidence for a clockwise P–T–t path. UHT peak conditions were attained at 1030±110°C, 11.7±1.4kbar, with post-peak cooling to 865±38°C, 8.9±0.8kbar. Cryogenian igneous inheritance of ca. 720–640Ma is identified in oscillatory zoned zircon cores (n=167) with steep HREE patterns. Resorbed, Y-rich monazite cores preserve a prograde growth stage at 631±4Ma prior to the partial melting event, providing an upper age limit for the granulite facies metamorphism in the SGN. REE-rich, Th-depleted monazite related to apatite records the initial stages of decompression at 628±4Ma. Multiple monazite growth episodes record melt crystallization events at 624±3Ma, 612±5Ma and 608±6Ma. Stubby, equant “soccer ball” zircon provide evidence for melt crystallization at 613±2Ma and 607±4Ma. The excess scatter in zircon and monazite age populations between 629±4 and 601±3Ma is interpreted as discontinuous and episodic growth within this age range, characterizing a prolonged metamorphic event in the SGN lasting ca. 30m.y. The development of Y+HREE-rich monazite rims at ca. 600Ma documents retrograde garnet breakdown, extensive biotite growth and the final stages of melt crystallization. Th-rich, Y+HREE-poor monazite rims at ca. 590Ma record monazite recrystallization.

The petrogenesis of the Early Permian Variscan granites of the Cornubian Batholith: Lower plate post-collisional peraluminous magmatism in the Rhenohercynian Zone of SW England

The Early Permian Cornubian Batholith was generated during an extensional regime following Variscan convergence within the Rhenohercynian Zone of SW England. Its component granites can be classified, using mineralogical, textural and geochemical criteria, into five main types, all of which are peraluminous (A/CNK>1.1): G1 (two-mica), G2 (muscovite), G3 (biotite), G4 (tourmaline) and G5 (topaz). G1 granites formed through up to 20% muscovite and minor biotite dehydration melting of a metagreywacke source at moderate temperatures and pressures (731–806°C, >5kbar). Younger G3 granites formed through higher temperature, lower pressure (768–847°C, <4kbar) biotite-dominated melting of a similar source. Partial melting was strongly influenced by the progressive lower–mid crustal emplacement of mafic igneous rocks during post-Variscan extension and a minor (<5%–10%) mantle-derived component in the granites is possible. Two distinct fractionation series, G1–G2 and G3–G4, are defined using whole-rock geochemical and mineral chemical data. Variations in the major elements, Ba, Sr and Rb indicate that G1 and G3 granites underwent 15%–30% fractionation of an assemblage dominated by plagioclase, alkali feldspar and biotite to form more evolved G2 and G4 granites, respectively. Decreasing whole-rock abundances of Zr, Th and REE support the fractionation of zircon, monazite, apatite and allanite. Subsolidus alteration in G2 and G4 granites is indicated by non-primary muscovite and tourmaline and modification of major and trace element trends for G3–G4 granites, particularly for P2O5 and Rb. Topaz (G5) granites show low Zr, REE and extreme enrichment in Rb (up to 1530ppm) and Nb (79ppm) that cannot be related in a straightforward manner to continued differentiation of the G1–G2 or G3–G4 series. Instead, they are considered to represent partial melting, mediated by granulite facies fluids, of a biotite-rich restite following extraction of G1 and/or G3 magmas; they do not exhibit the typical geochemical characteristics of intraplate A-type granites.

Petrogenesis and tectonic implications of the Yadong leucogranites, southern Himalaya

The leucogranites in the Higher Himalayan Sequence (HHS) provide a probe to elucidate the crustal melting of continental collisional orogen. An integrated geochemical and geochronological study of the Yadong leucogranites, southern Himalaya, shows that these rocks have relatively high SiO2 contents of 69.77 to 75.32wt.% and alumina saturation index (A/CNK) of 1.09–1.40, typical of peraluminous granites. They show moderately fractionated REE patterns with negative Eu anomalies, and are characterized by enriched LILE (Rb and Cs) and depleted HFSE (Zr, Hf, Nb and Ta). LA–ICP–MS U–Pb zircon dating of ten samples yields crystallization ages ranging from 21.0 to 11.7Ma. The zircons have variable εHf(t) values of −26.3 to −3.5 and corresponding Hf two-stage model ages of 2.77–1.33Ga. The present study reveals that the muscovite–biotite leucogranites (2ML) have higher TiO2, MgO, CaO, Sr, Ba and Zr contents, lower Rb/Sr ratios than the tourmaline–muscovite leucogranites (TML). Zircon and monazite saturation thermometry results show that the melt temperatures (681–784°C) of the 2ML are 20–80°C higher than those (663–705°C) of the TML. Combining with previous results, we propose that the TML were derived from the muscovite-dehydration melting, whereas the 2ML dominantly resulted from the biotite-dehydration melting during the prograde metamorphism of the pelitic and felsic granulites of the HHS. Therefore, the Himalayan leucogranites were probably formed during the subduction of the Indian crust following the India and Asia collision.

Using nanogranitoids and phase equilibria modeling to unravel anatexis in the crustal footwall of the Ronda peridotites (Betic Cordillera, S Spain)

Anatexis in the crustal footwall of Ronda peridotites (Betic Cordillera, S Spain) is apparently related to the hot emplacement of this mantle slab over metasedimentary rocks. In this study, we combine the analysis of melt inclusions (MI) and phase equilibria calculations on quartzo-feldspathic mylonites (former migmatites) occurring at the contact with the mantle rocks, in the region of Sierra Alpujata (Ojén unit). The goal is to better characterize anatexis in these rocks and to provide new constraints on the geodynamic evolution of the crustal footwall. Such data are important for understanding the mechanisms of crustal emplacement of the mantle slice. The quartzo-feldspathic mylonites are characterized by the mineral assemblage Qtz+Pl+Kfs+Sil+Grt+Ilm+Bt±Ap±Gr. Clusters of MI are observed both at the core and toward the rim of peritectic garnet. In each cluster, MI range from totally glassy to nanogranitoids, consisting of Qtz+Kfs+Bt+Ms+Pl aggregates. The trapped melt is leucogranitic and peraluminous with variable Na2O/K2O values and low H2O contents (≈2–4wt%). Phase equilibria modeling in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2–TiO2–O2–C (MnNCaKFMASHTOC) system with graphite-saturated fluid constrains the P–T conditions of melting at ≈6kbar, ≈820°C. MI data support the fluid-absent character of melting. The investigated MI represent the primary anatectic melts produced during prograde anatexis of the host rocks via biotite dehydration melting. Field, compositional, and textural observations indicate that mylonitic migmatites represent strongly deformed former diatexites. The comparison between the new data and some recently published information on migmatites located further from the contact with the peridotites and toward the bottom of the crustal footwall, raises some important issues which question the previously proposed geodynamic models for this region. Among them, (i) the crustal footwall at Sierra Alpujata constitutes an inverted metamorphic sequence, not only in terms of temperature but also in pressure, and (ii) the Ojén unit does not seem to represent a coherent high-P portion of a continental subduction system.

Earliest Paleoproterozoic high-grade metamorphism and orogenesis in the Gawler Craton, South Australia: The southern cousin in the Rae family?

Neoarchean–earliest Paleoproterozoic rocks of the Gawler Craton preserved within the Mulgathing Complex, South Australia, record metamorphism and deformation during an earliest Paleoproterozoic tectonothermal event known as the Sleafordian Orogeny. We present metamorphic data from pelitic gneisses of the Mulgathing Complex from three locations (Challenger, Mt Christie and Blackfellow Hill) in order to constrain the pressure–temperature–time (P–T–t) evolution of this orogeny. In situ U-Pb dating via laser ablation-inductively coupled plasma mass spectrometry, shows that monazites within garnet porphyroblasts have ages ca. 2470 Ma, providing an estimate of the timing peak P–T conditions. Monazite crystallisation from melt occurred at ca. 2460 Ma in the presence of peritectic garnet either late on the prograde path during biotite-dehydration melting, or immediately following peak-T. Lobate and patchy rims in leucosome-hosted monazite suggest a dissolution-reprecipitation process occurred during cooling across the elevated dry solidus at ca. 2440 Ma. Monazite younger than ca. 2430–2420 Ma that is located at grain boundaries between matrix minerals likely reflects monazite crystallisation during cooling across the dry solidus and/or subsolidus mineral breakdown during the waning stages of the thermal event. Phase diagram modelling of pelitic gneisses suggests peak metamorphic conditions at the three localities overlap at ∼7kbar and 850°C with apparent thermal gradients in the order of 125–135°C/kbar (∼35–45°C/km). Metamorphism involved a tight anticlockwise P–T–t path based on mineral assemblage paragenesis and indicates a slow, shallow decompressive-cooling path, sub-parallel to the apparent thermal gradient, in the order of few degrees per Myr. A protracted magmatic history involving bimodal magmatism associated with a thinned and underplated extensional basin in the Mulgathing Complex was likely a thermal precursor to the high thermal gradient metamorphism and convergent (transpressive) deformation. A significant period of post 2.4 Ga quiescence in the Gawler Craton subsequent to basin inversion suggests that this orogenesis may have occurred interior to a plate margin. The similarity in timing and style between the Sleafordian Orogeny in the Gawler Craton and the 2.5–2.28 Ga Arrowsmith Orogeny of the Rae Craton supports the notion that these terranes, together with the broader Gawler-Adélie Craton of the Mawson Continent, the North Australia Craton, elements of the North China, Sino-Korean and Dharwar cratons formed part of a family of Neoarchean terranes, described as the Rae family of cratons, with a distinctive tectono-metamorphic record as compared with other Archean cratons. We suggest the Mawson Continent is the southern member of this family, dominated as it is by terranes now present in the northern hemisphere.

The Blaník Gneiss in the southern Bohemian Massif (Czech Republic): a rare rock composition among the early palaeozoic granites of Variscan Central Europe

Metamorphosed and deformed tourmaline-bearing leucogranites with a Cambro-Ordovician formation age are widespread in the Monotonous Group of the Variscan southern Bohemian Massif, Czech Republic. The rocks, known locally as Blanik gneiss, are strongly peraluminous and classify as phosphorus-rich low-T, S-type granite. The magma formed from a metapelitic source, most likely through muscovite dehydration melting. With respect to its low-T origin and the abundance of tourmaline, the Blanik gneiss is exotic within the spectrum of Early Palaeozoic granites of the Variscan fold belt of Central Europe. Coeval granitic gneisses in the neighbouring Gfohl unit of the Bohemian Massif can be classified as higher T S-type granites and were probably generated through biotite dehydration melting. The geochemical differences between the Early Palaeozoic granitic magmatism in the Gfohl unit and the Monotonous Group support models claiming that these two geological units belonged to independent peri-Gondwana terranes before the Variscan collision. It is suggested here, that the Gfohl unit and the Monotonous Group represent zones of higher and lower heat flow within the Early Palaeozoic northern Gondwana margin, respectively. The geochemical data presented in this study could be helpful for terrane correlations and palaeogeographic reconstructions.

Metasedimentary melting in the formation of charnockite: Petrological and zircon U-Pb-Hf-O isotope evidence from the Darongshan S-type granitic complex in southern China

Charnockites are Opx-bearing igneous rocks commonly found in high-grade metamorphic terranes. Despite being volumetrically minor, they show a wide range in both bulk geochemistry and intensive parameters. They form a characteristic component of the AMCG (anorthosite–mangerite–charnockite–granite) suite, but their association with typical S-type granites is less well-known. The Darongshan S-type granitic complex (DSGC) in Guangxi Province, southern China, contains granites varying in mafic silicate mineral assemblages from Bt+Crd (Darongshan suite) to Opx+Grt+Bt+Crd (Jiuzhou suite) and Opx+Crd±Bt (Taima suite), corresponding to a geochemical transition from magnesian calc-alkalic to ferroan calc-alkalic. However, its genesis, even the accurate age of intrusion, remains highly contentious despite intensive research. In order to understand the genesis of charnockite and its genetic relationship with S-type granite; here, we first determined zircon U-Pb ages of each suite using a SIMS on the basis of a detailed petrological study. Zircon U-Pb ages show that all suites of the complex were emplaced contemporaneously at ca. 249Ma. Monazite apparent U-Pb ages are indistinguishable from zircon U-Pb ages within analytical error. Further in situ zircon Hf-O isotope analyses reveal that the granitic complex was dominantly derived from reduced melting metasedimentary rocks (δ18Ozircon=ca. 11‰; εHf(t)zircon=ca. −10; Δlog FMQ≤0; Mn in apatite oxybarometer) with rare material input from the mantle. The variation in δ18O (7.8‰–12.9‰) is more likely a result of hybridization, whereas that in εHf(t) (−31.9 to −1.8) is a result of both hybridization and disequilibrium melting. The variation in mineralogy and geochemistry may be interpreted as a result of entrainment of peritectic garnets from biotite-dehydration melting. Nevertheless, heat input from mantle through basaltic intrusion/underplating is considered to play a major role in high-temperature (>850°C) melting at mid-crustal levels (i.e. the cordierite stable field) for generation of the granitic complex. We interpret that the granites were intruded in a back-arc setting and basaltic magmatism was directly associated with slab roll-back and tearing during the latest Permian and early Triassic times.

High-temperature metamorphism and crustal melting: Working with melt inclusions

The application of melt inclusion (MI) studies to migmatitic and granulitic terranes is a recent, small-scale approach for a better understanding of melting in the continental crust. In order to show the role of anatectic MI in providing a wealth of microstructural and compositional information on high-temperature metamorphism and crustal anatexis, we review a series of studies on the crustal footwall of the Ronda peridotites (Betic Cordillera, S Spain), which consists of an inverted metamorphic sequence with granulite-facies rocks showing extensive melting on top and amphibolites-facies rocks at the bottom. We studied the microstructures and geochemistry of small (2-10 µm) primary MI hosted in peritectic garnet of metatexites at the bottom of the migmatitic sequence and of diatexites close to the contact with the mantle rocks. The occurrence of MI is a proof that the investigated rocks were partially melted at some time in their history, despite other microstructures indicating the former presence of melt in mylonite were erased by deformation. MI show a variable degree of crystallization ranging from totally glassy to fully crystallized (nanogranites), consisting of Qtz+Pl+Kfs+Bt+Ms aggregates (often modal Kfs > Pl in mylonites). Piston cylinder remelting experiments led to the complete rehomogenization of nanogranites in metatexites at the conditions inferred for anatexis. Compositions of investigated MI are all leucogranitic and peraluminous and differ from those of coexisting leucosomes and from melts calculated by phase equilibria modeling. Systematic compositional variations have been observed between MI in metatexites and mylonites: the former commonly show higher H 2 O, CaO, Na 2 O/K 2 O and lower FeO. The compositions of MI in metatexites and mylonites are interpreted to record the composition of the anatectic melts produced from a peraluminous greywacke i) on, and immediately after crossing, the fluid-saturated solidus of this metasedimentary rock, and ii) during anatexis via biotite dehydration melting at increasing temperature, respectively. While partial melting at the bottom of the migmatitic sequence likely started in the presence of an aqueous fluid phase, MI data support the fluid-absent character of the melting event in mylonites. Anatectic MI should therefore be considered as a new and important opportunity to understand the partial melting processes.

Formation of the Jurassic South China Large Granitic Province: Insights from the genesis of the Jiufeng pluton

The magma source and thermal regime for the formation of Large Granitic Provinces (LGPs) have been the subjects of much debate. As mineralogical, geochemical and isotopic variations of plutonic rocks preserve records of crustal thermal evolution, the Jiufeng pluton was comprehensively studied to gain insights into material and heat sources and tectonic process in the formation of the South China LGP. SIMS zircon U-Pb analyses of a biotite granite (158.9±1.3Ma), three two-mica granites (stage I: 159.7±1.2Ma, stage II: 158.6±1.2Ma and 156.8±1.2Ma) and a muscovite granodiorite (159.0±4.4Ma) of the Jiufeng pluton suggest that they are coeval and were emplaced in a short period of no more than 5.3m.y. Limited variations in whole-rock Nd isotope (εNd(t)=−12.6 to −9.5) and zircon Hf-O isotopes (εHf(t)=−12.0 to −6.3, δ18O=6.8‰−9.4‰) suggest that the Jiufeng pluton was mainly derived from melting of a common metasedimentary source, possibly with a minor basaltic contribution. We consider the geochemical variations of the Jiufeng pluton are primarily a result of incremental assembly of magma batches produced from rapid step-like transition from fluid-saturated to fluid-absent melting of the source. The muscovite granodiorite, with high Na2O (>3.80wt.%; K2O/Na2O=~1), is interpreted to have been produced by fluid-saturated melting at low temperature (~650°C). High P2O5 (0.09–0.17wt.%), zircon saturation temperature (TZr=769–816°C) and La/Yb ratios (8.4–55.8) of the stage II two-mica granite support its formation from high-temperature (>800°C) biotite-dehydration melting, whereas lower P2O5 (<0.02wt.%), TZr (685–742°C) and La/Yb (<3) of the stage I two-mica granite suggest its generation at lower temperature, likely by muscovite-dehydration melting. We propose that extensive emplacement of basaltic melts in the lower crust most likely drove the rapid increase of mid-crustal (~20km) temperature (~50°C/m.y.) and widespread crustal melting for the formation of the Jurassic South China LGP. Therefore, formation of the LGP signifies prominent crustal growth as well as crustal reworking in an intraplate setting and was likely a response to flat-slab delamination and foundering.

Long-lived high-temperature granulite-facies metamorphism in the Eastern Himalayan orogen, south Tibet

Abstract The Namche Barwa Complex exposed in the Eastern Himalayan Syntaxis, south Tibet, underwent high-pressure (HP) and high-temperature (HT) granulite-facies metamorphism and associated anatexis. The HP pelitic granulites contain garnet, kyanite, sillimanite, cordierite, biotite, quartz, plagioclase, K-feldspar, spinel, ilmenite and graphite. These minerals show composite reaction texture and varying chemical compositions and form four successive mineral assemblages. Phase equilibrium modeling constrains the P–T conditions of 10–12 kbar and 550–700 °C for the prograde stage, 13–16 kbar and 840–880 °C for the peak-metamorphic stage, and 5–6 kbar and 830–870 °C for the late retrograde stage, indicating that the HP granulites recorded a clockwise P–T path involving the early heating burial and anatexis through dehydration melting of both muscovite and biotite, and the late isothermal decompression and gradual melt crystallization under HT granulite-facies conditions. The zircon U–Pb dating reveals that the HT granulite-facies metamorphism probably initiated at ca. 40 Ma, and lasted to ca. 8 Ma. Therefore, the present study provides robust evidence for a long-lived HT metamorphism and associated anatexis in the deeply buried Indian continent and important constraints on the leucogranite generation and tectonic evolution of the Himalayan orogen.

Timing, duration and inversion of prograde Barrovian metamorphism constrained by high resolution Lu–Hf garnet dating: A case study from the Sikkim Himalaya, NE India

We investigated Lu–Hf isotopic systematics in garnets from gradually higher grade metamorphic rocks from the first appearance of garnet at c. 500 °C to biotite dehydration melting at c. 800 °C in the Sikkim Himalaya, India. Exceptionally precise Lu–Hf ages obtained for the Barrovian metasedimentary sequence in the Lesser Himalaya (LH) correspond to the time of early garnet formation on a prograde path and show remarkable correlation with increasing metamorphic grade and decreasing structural depth. The youngest age is reported for the garnet zone (10.6±0.2 Ma) and then the ages become progressively older in the staurolite (12.8±0.3 Ma), kyanite (13.7±0.2 Ma) and sillimanite (14.6±0.1 Ma) zones. The oldest age of 16.8±0.1 Ma was recorded at the top of the sequence in the zone marking the onset of muscovite dehydration melting, directly below the Main Central Thrust (MCT). These ages provide a tight constraint on the timing and duration of the Barrovian sequence formation which lasted about 6 Ma. The age pattern is clearly inverted with respect to structural depth but shows “normal” correlation with the metamorphic grade, i.e. earlier garnet growth in higher grade rocks.The timing of the peak metamorphic conditions estimated for the garnet and kyanite zones suggests that thermal climax occurred in all zones within a relatively short time span about 10–13 Ma ago. The metamorphic inversion in the Sikkim Himalaya must therefore have occurred after the metamorphic peak was attained in all grades.Lu–Hf garnet ages in the overthrust Higher Himalaya (HH) continue to be older but do not show a clear progression. There is about 6 Ma jump to 22.6±0.1 Ma within the MCT zone and even older ages of 26–28 Ma were obtained for migmatites from the lower part of the HH. The Lu–Hf system in garnets from the HH records the time of melting at or near the thermal peak rather than dates initial garnet growth as in the LH.The age pattern obtained in this study provides precise time constraints on the continental collision models which must take into account early and prolonged anatexis in the HH, progressive delay in initial garnet growth in different Barrow zones in the LH, nearly contemporaneous metamorphic peak in all grades and post peak inversion of the coherent LH block during exhumation.

An experimental study of effect of pre‐existing fabric on deformation of foliated mylonite at high temperature and pressure

We performed deformation experiments on a foliated mylonite under high temperature and pressure conditions in this study. To investigate the effect of pre‐existing fabric on the rheology of rocks, our samples were drilled from natural mylonite with the cylinder axis parallel to the foliation (PAR) and perpendicular to the foliation (PER). We performed 25 tests on seven PAR samples and 21 tests on seven PER samples at temperatures ranging from 600 to 890 °C, confining pressures ranging from 800 to 1400 MPa, and steady‐state strain rates of 1 × 10−4, 1 × 10−5 and 2.5 × 10−6 s−1. In the temperatures of 600–700 °C, the deformation is accommodated by semi‐brittle flow, with the average stress exponent being 6–7 assuming power law flow; in the temperature range of 800–890 °C, deformation is mainly by plastic flow, with an average stress exponent of n = 3 and activation energies of Q = 354 ± 52 kJ/mol (PER and PAR samples). The experimental results show that the strengths of PER samples are higher than those of PAR samples. Deformation microstructures have been studied by optical and electron microscopy. The original foliation of PER samples is destroyed by deformation and replaced by a new foliation, but the deformation of PAR samples followed the original foliation. Electron backscatter diffraction (EBSD) measurements show a strong lattice preferred orientation (LPO) of the quartz c axis fabrics of the starting samples and deformed PER and PAR samples. However, the c axis fabric of quartz in experimentally deformed PER and PAR samples varied with temperature and strain rate is different from that seen in the starting mylonite sample. The initial quartz c axis fabric of the starting mylonite sample has been transformed into a new fabric during experimental deformation. Dehydration melting of biotite and hornblende occurred in both PER and PAR samples at temperatures of 800–890 °C. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Petrogenesis of the Ramba leucogranite in the Tethyan Himalaya and constraints on the channel flow model

Abstract The Himalayan leucogranites are critical for understanding the geodynamic process of Himalayan orogenesis. In this article, we present petrological, geochronological and geochemical results of the Ramba leucogranites that crop out in the center of the Ramba dome within the Tethyan Himalaya. U–(Th)–Pb dating of zircon, monazite and xenotime revealed three episodes of leucogranitic magmatism in the Ramba dome at ca. 44 Ma, ca. 28 Ma and ca. 8 Ma, respectively. The ca. 44 Ma and ca. 28 Ma magmatisms are both represented by strongly deformed porphyritic two-mica granite gneiss dykes that intruded into the margin of the dome. These granite gneisses have high CaO and Sr contents, relatively high Na/K, Sr/Y and low Rb/Sr ratios, and much lower initial 87Sr/86Sr ratios but higher eNd(t) and eHf(t) values than the ancient Indian crustal material. These results indicate that the granite gneisses were not derived from metasedimentary rocks of the High Himalayan Sequence but most likely were derived from partial melting of mafic rocks at a thickened crustal condition. The ca. 8 Ma leucogranites consist of a two-mica granite pluton in the core of the dome and garnet-bearing granite dykes in the margin of the dome. Major and trace element compositions indicate that these Miocene leucogranites are typical S-type granites and were most likely derived from muscovite and/or biotite dehydration melting of metapelites. The relatively lower TiO2, TFeO, MgO and CaO contents, higher Rb concentration and Rb/Sr ratio, and more strongly negative Eu anomaly of the garnet-bearing granite indicate that it was more evolved than the coeval two-mica granite. The non-CHARAC (CHarge-And-Radius-Controlled) trace element behavior and the tetrad effect of the REE distribution of the garnet-bearing granite further suggest that it is a highly fractionated granite. Besides, our study detected some Asian-like materials in the Ramba leucogranites, such as Mesozoic-aged inherited zircons and the relatively depleted Sr–Nd–Hf isotopic compositions of some leucogranite samples. These features of leucogranites in the Tethyan Himalaya have been previously considered as direct geological evidence for the channel flow model. However, detailed geochronological and geochemical studies of the country rocks suggest that these Asian-like materials are most likely sourced from the country rock rather than the Asian lithosphere, thus they may not be used as solid evidence for the channel flow model.

High to ultrahigh temperature contact metamorphism and dry partial melting of the Tasiuyak paragneiss, Northern Labrador

AbstractContact aureoles of the anorthositic to granitic plutons of the Mesoproterozoic Nain Plutonic Suite (NPS), Labrador, are particularly well developed in the Palaeoproterozoic granulite facies, metasedimentary, Tasiuyak gneiss. Granulite facies regional metamorphism (MR),c. 1860 Ma, led to biotite dehydration melting of the paragneiss and melt migration, leaving behind biotite‐poor, garnet–sillimanite‐bearing quartzofeldspathic rocks. Subsequently, Tasiuyak gneiss within ac. 1320 Ma contact aureole of theNPSwas statically subjected to lower pressure, but higher temperature conditions (MC), leading to a second partial melting event, and the generation of complex mineral assemblages and microstructures, which were controlled to a large extent by the textures of theMRassemblage. This control is clearly seen in scanning electron microscopic images of thin sections and is further supported by phase equilibria modelling. Samples collected within the contact aureole near Anaktalik Brook, west of Nain, Labrador, mainly consist of spinel–cordierite and orthopyroxene–cordierite (or plagioclase) pseudomorphs afterMRsillimanite and garnet, respectively, within a quartzofeldspathic matrix. In addition, some samples contain fine‐grained intergrowths of K‐feldspar–quartz–cordierite–orthopyroxene inferred to be pseudomorphs after osumulite. Microstructural evidence of the former melt includes (i) coarse‐grained K‐feldspar–quartz–cordierite–orthopyroxene domains that locally cut the rock fabric and are inferred to represent neosome; (ii) very fine‐ to medium‐grained cordierite–quartz intergrowths interpreted to have formed by a reaction involving dissolution of biotite and feldspar in melt; and (iii) fine‐scale interstitial pools or micro‐cracks filled by feldspar interpreted to have crystallized from melt. Ultrahigh temperature (UHT) conditions during contact metamorphism are supported by (i) solidus temperatures &gt;900 °C estimated for all samples, coupled with extensive textural evidence for contact‐related partial melting; (ii) the inferred (former) presence of osumilite; and (iii) titanium‐in‐quartz thermometry indicating temperatures within error of 900 °C. TheUHTenvironment in which these unusual textures and minerals were developed was likely a consequence of the superposition of more than one contact metamorphic event upon the already relatively anhydrous Tasiuyak gneiss.

Relationship between syndeformational partial melting and crustal-scale magmatism and tectonism across the Wet Mountains, central Colorado

In the Wet Mountains of central Colorado, we document evidence for increasing metamorphic grade and associated higher amounts of partial melting along a transect from northwest to southeast. Field observations of structural orientation and style, qualitative assessment of strain intensity, analysis of metamorphic mineral assemblages, and macroscopic identification of leucosomes and migmatites are complemented by the use of melt microstructures to carefully document the presence and locations of former partial melt and to identify melt-producing reactions. In the northwest Wet Mountains, migmatitic foliation is moderately well developed, and partial melting occurred via granite wet melting and muscovite-dehydration melting, with rare melt pseudomorphs remaining. At Dawson Mountain in the central part of the range, inferred former melt channels are preserved along grain and subgrain boundaries, deformation appears more intense, and anatexis occurred through biotite-dehydration melting. Farthest to the south, the highest intensity of strain is inferred, with very closely spaced foliations, abundant dynamic recrystallization, and local mylonitization occurring in rocks of granitic composition, and partial melting occurring via granite wet melting. Metapelitic rocks experienced biotite-dehydration melting and contain garnet with Mn-rich rims and Mn-poor cores mantled by plagioclase, decussate biotite, and quartz, textures indicating back-reaction between melt and garnet. These textures indicate there was abundant melt within these highest-grade rocks and extensive melt-rock interaction. Throughout the Wet Mountains, deformation is concentrated in areas where melt-producing reactions occurred, and melt appears to be localized along deformation-related features, suggesting a correlation between partial melting and deformation. The northern Wet Mountains contain few plutons, whereas the central and southern portions of the Wet Mountains contain more pervasive dikes and sills and may contain more former melt as a result of both higher metamorphic grade and widespread thermal insulation. The Wet Mountains represent an exhumed section of formerly molten middle crust located at the transition between upper and lower crust and provide insight into processes ongoing at depth in modern orogenic belts. The microstructures indicative of former partial melt, textures associated with melt-rock interaction, and melting reactions we have identified in the Wet Mountains will greatly facilitate the recognition of other such exhumed sections.

Petrogenesis of synorogenic high-temperature leucogranites (Damara orogen, Namibia): Constraints from U–Pb monazite ages and Nd, Sr and Pb isotopes

Two suites of leucogranites were emplaced at 508±5.9Ma in the Okombahe District of the Damara belt (Namibia) synchronous with the peak of regional high-temperature metamorphism. The Sr (87Sr/86Srinit: 0.707 to 0.711), Nd (εNdinit: −4.5 to −6.6), and Pb isotopic (206Pb/204Pb: 18.51–19.13; 207Pb/204Pb: 15.63–15.69; 208Pb/204Pb: 38.08–38.66) compositions indicate that these peraluminous S-type granites were derived from mid- to lower-crustal rocks, which are slightly different to the metapelitic rocks into which they intruded. Since the leucogranites are unfractionated and show no evidence for assimilation or contamination, they constrain the temperature and pressure conditions of their formation. Calculated Zr and LREE saturation temperatures of ca. 850°C indicate high-temperature crustal melts. High Rb/Sr and low Sr/Ba ratios are consistent with biotite dehydration melting of pelitic source rocks. Qz–Ab–Or systematics reveal that melting and segregation for the least fractionated samples occurred at ca. 7kbar corresponding to a mid-crustal level of ca. 26km. However, there is no evidence for a mantle component that could have served as a local heat source for crustal melting. Therefore, the hot felsic magmas that formed close to the time of peak metamorphism are the result of long-lasting high temperature regional metamorphic conditions and intra-crustal collision.