Anatectic Rocks Research Articles

Petrogenesis of monzonites and leucogranites in the Ama Drime Massif: Implications for sources and differentiation of Himalayan leucogranites

Determining the sources and the evolution of the Himalayan leucogranites is crucial to deciphering the teotono-magmatic evolution history of the Himalayan Orogen. Various magmatic rocks, including the monzonites, two-mica leucogranites, and tourmaline leucogranites, are exposed as dykes in the Paleoproterozoic orthogneisses of the Ama Drime Massif (ADM), providing new insights into the Himalayan magmatism. This contribution presents systematic petrological, geochronological, and geochemical studies of these magmatic rocks from the ADM to constrain their emplacement timing, protolith, and petrogenesis. Geochronological results reveal two episodes of magmatism in the ADM, represented by the 13.5 Ma monzonites and the 10 Ma two-mica and tourmaline leucogranites. Despite having different ages, the two episodes of magmatism in the ADM both originated from the melting of the Paleoproterozoic orthogneisses. However, various magmatic rocks underwent different differentiation processes during emplacement. Based on mineral assemblages and geochemical features, the monzonites were formed by the accumulation of plagioclase, biotite, zircon, and monazite. For the leucogranites, the systematic variations in mineral assemblages, whole-rock geochemistry, and mineralogical chemistry demonstrate that the two-mica leucogranites gradually evolved into the tourmaline leucogranites by the separation of plagioclase, biotite, zircon, and monazite. Combining this contribution and previous studies, the Himalayan leucogranites might evolve from the peraluminous melts produced by the melting of Himalayan anatectic rocks according to fractional crystallization, with the formation of complementary cumulate rocks. The involvement of orthogneisses in the source suggests that the Himalayan leucogranites can not be simply classified into S-type granites.

Deformation characteristics of the high-grade metamorphic and anatectic rocks in the Daqingshan Paleoproterozoic orogenic belt, Inner Mongolia: A case study from the Shijiaqu-Xuehaigou area

The high-grade granulite-phase metamorphic and anatectic rocks are widespread in the Daqingshan area, which is an archetypical of the Early Paleoproterozoic Orogenic Belt (POB) in the North China Carton (NCC), providing an ideal object to delineate the rheology of partially molten lower crust. The multiple approaches of SEM-EBSD fabric analysis, electron probe microanalyses (EPMA) and LA-ICP-MS zircon U-Pb geochronology have been carried out on the metamorphic and anatectic rocks from the Shijiaqu-Xuehaigou area in the Daqingshan. The results of zircon U-Pb dating show that the anatexis process in the POB took place more or less prior to ca. 1920 Ma, subsequently, the main plastic deformation event occurred at ca. 1915–1870 Ma. The macro- and microscopic structures suggest that the Xuehaigou and Shijiaqu areas show the characteristics of the diatexite and metatexite zones, respectively. In the diatexite zone, appearance of abundant melts tends to form schollen and schlieren structures in the high-grade metamorphic and anatectic rocks. The deformation mechanism of plagioclase is mainly represented by diffusion creep and particle flow. In contrast, in the metatexite zone, the deformation pattern is represented by stromatic structure with light-colored bands paralleling to the main gneissosity, and the deformation mechanism of plagioclase is characterized by dislocation creep. Besides, the deformation mechanism of amphibole in the lower crustal rocks from both zones is mostly dominated by dislocation creep. Significantly, the presence of melts in the high-grade metamorphic and anatectic rocks probably weaken their strength, and may have aided to form the sub-horizontal channel, contributing significantly to the subsequent exhumation of the lower crustal rocks in the orogenic belt.

Early Paleogene Magmatism in the Pinaleño Mountains, Arizona: Evidence for Crustal Melting of Diverse Basement Assemblages during the Laramide Orogeny

Abstract Granitic rocks, interpreted to be related to crustal melting, were emplaced into regions of thickened crust in southern Arizona during the Laramide orogeny (80–40 Ma). Laramide-age anatectic rocks are exposed as plutons, sills, and dike networks that are commonly found in the exhumed footwalls of metamorphic core complexes. This study investigates newly discovered exposures of granodioritic–leucogranitic rocks from three intrusive phases in the footwall of the Pinaleño–Jackson Mountain metamorphic core complex of southeastern Arizona, called the Relleno suite. Zircon U–Pb geochronology indicates that the suite was emplaced from 58 to 52 Ma. Zircon Lu/Hf isotope geochemistry, whole-rock Sr and Nd isotope geochemistry, and mineral O isotope geochemistry were used to investigate the source of these rocks and evaluate whether they are related to crustal anatexis. Average zircon εHf(t) values of the suite range from −4.7 to −7.9, whole-rock εNd(i) and 87Sr/86Sr(i) values range from −9.4 to −11.8 and 0.7064 to 0.7094 respectively, and quartz δ18OVSMOW values range from 6.8 to 9.4 ‰. Isotopic and geochemical data of these rocks are consistent with derivation from and assimilation of intermediate–mafic (meta)igneous rocks, at deep crustal levels, and are supported by thermodynamic melt models of Proterozoic igneous rocks equivalent to those exposed in the Pinaleño Mountains. In comparison with other Laramide-age anatectic granites in SE Arizona, those exposed in the Pinaleño Mountains are temporally similar but present compositional and isotopic differences that reflect melting and assimilation of different lithologies, producing distinct mineralogical and isotopic characteristics. The results suggest that crustal melting during this interval was not limited to metasedimentary protoliths and may have affected large portions of the deep crust. The early Paleogene Relleno suite in the Pinaleño Mountains strengthens the relationship between crustal melting and regions of thickened crust associated with the Sevier and Laramide orogenies.

Hf isotopic ratios in zircon reveal processes of anatexis and pluton construction

Heterogenous isotopic ratios in magmatic rocks have been variously interpreted to be a result of magma mixing or inheritance from the source rock. Isotopic heterogeneities in zircons in the source may be homogenised in the melt by magma flow and chemical diffusion. This implies that the degree of homogenisation of the inherited Hf isotopic signal can be indicative of the degree of magma homogenisation, which depends on the nature and duration of magma flow before crystallisation. In order to understand the systematics of Hf isotope transfer from zircons in the source to those crystallised from the melt, we measured Hf isotopic ratios of magmatic and inherited detrital zircons in migmatites from turbidites of the Puncoviscana Formation in NW Argentina, as well as a granite domain within the sequence. Detrital zircons show ∼30 Eps units of variation in Hf isotope values. Anatectic zircons also show a large range of isotopic values but it is narrower than, and lies within, the range of the detrital zircons. Using analysis of variance (ANOVA), we found that the Hf isotope ratios of zircon rims are partly controlled by their cores and partly by the mean value of the magma. This demonstrates the effect of a boundary layer around dissolving detrital zircon grains that contains a high concentration of the core's isotopic signature (the “core effect”). Anatectic systems can be divided into four end-members on the basis of the extent of the core effect, the degree of magma homogenisation, and nature of the magma prior to zircon crystallisation. Recognising these end-members allows the nature of processes in anatectic rocks that homogenise and modify Hf isotopic ratios to be constrained.

The role of Ediacaran synkinematic anatectic rocks and the late-orogenic charnockitic rocks in the development of the hot Araçuaí belt

The orogenic evolution of the hot Araçuaí belt is characterized by continuous magmatic activity, with magmas of different compositions recording a long-lived (∼630 to ∼530 Ma) tectono-thermal evolution in response to convergence between the Congo and São Francisco continents during the West Gondwana amalgamation. Geochemical data from the Carlos Chagas domain (CCD) and the Nova Venécia Complex (NVC), in the hinterland of this belt, show that these rocks contain high amounts of heat producing elements – HPEs – (Th, U, and K) and have a dominant peraluminous signature, suggesting that they result from partial melting of continental crust. The CCD is intruded by post-collisional charnockites that have a dominant shoshonitic signature, interpreted as representing magmas from an enriched mantle reservoir with a variable amount of crustal contamination. Detailed morphological investigations at the macro- and micro-scales show that the CCD contains remnants of residuum material from metamorphic reactions associated with textures that attest to melt crystallization. This reinforces the interpretation that the CCD is locally derived, i.e., it represents an in-source subhorizontal rheologically weak layer of migmatites and parauthoctonous granites that formed during the orogenic thickening. The CCD likely triggered the formation of an orogenic plateau and a geological setting in which high temperature conditions could be sustained for tens of millions of years. In such a setting, continuous heat supply from radiogenic decay might have been the main heat source for the compositionally diverse magmatism during most of the orogenic evolution of the Araçuaí belt.

Muscovite Dehydration Melting in Silica-Undersaturated Systems: A Case Study from Corundum-Bearing Anatectic Rocks in the Dabie Orogen

Corundum-bearing anatectic aluminous rocks are exposed in the deeply subducted North Dabie complex zone (NDZ), of Central China. The rocks consist of corundum, biotite, K-feldspar and plagioclase, and show clear macro- and micro-structural evidence of anatexis by dehydration melting of muscovite in the absence of quartz. Mineral textures and chemical data integrated with phase equilibria modeling, indicate that coarse-grained corundum in leucosome domains is a peritectic phase, reflecting dehydration melting of muscovite through the reaction: Muscovite = Corundum + K-feldspar + Melt. Aggregates of fine-grained, oriented, corundum grains intergrown with alkali feldspar in the mesosome domains are, instead, formed by the dehydration melting of muscovite with aluminosilicate, through the reaction: Muscovite + Al-silicate = Corundum + K-feldspar + Melt. P-T pseudosections modeling in the Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2 system constrains peak pressure-temperature (P-T) conditions at 900–950 °C, 9–14 kbar. The formation of peritectic corundum in the studied rocks is a robust petrographic evidence of white mica decompression melting that has occurred during the near-isothermal exhumation of the NDZ. Combined with P-T estimates for the other metamorphic rocks in the area, these new results further confirm that the NDZ experienced a long-lived high-T evolution with a near-isothermal decompression path from mantle depths to lower-crustal levels. Furthermore, our new data suggest that white mica decompression melting during exhumation of the NDZ was a long-lasting process occurring on a depth interval of more than 30 km.

The Origin of Garnets in Anatectic Rocks from the Eastern Himalayan Syntaxis, Southeastern Tibet: Constraints from Major and Trace Element Zoning and Phase Equilibrium Relationships

AbstractAmphibolite- and granulite-facies metamorphic rocks are common in the eastern Himalayan syntaxis of southeastern Tibet. These rocks are composed mainly of gneiss, amphibolite and schist that underwent various degrees of migmatization to produce leucogranites, pegmatites and felsic veins. Zircon U–Pb dating of biotite gneiss, leucocratic vein and vein granite from the syntaxis yields consistent ages of ∼49 Ma, indicating crustal anatexis during continental collision between India and Asia. Garnets in these rocks are categorized into peritecitc and anatectic varieties based on their mode of occurrence, mineral inclusions and major- and trace-element zoning. The peritectic garnets mainly occur in the biotite gneiss (mesosome layer) and leucocratic veins. They are anhedral and contain abundant mineral inclusions such as high-Ti biotites and quartz, and show almost homogeneous major-element compositions (except Ca) and decreasing HREE contents from core to rim, indicating growth during the P- and T-increasing anatexis. Peak anatectic conditions at 760–800°C and 9–10·5 kbar are well constrained by phase equilibrium calculations, mineral assemblages, and garnet isopleths. In contrast, anatectic garnets only occur in the vein granite. They are round or subhedral, contain quartz inclusions, and exhibit increasing spessartine and trace-element contents from core to rim. The garnet–biotite geothermometry and the garnet–biotite–plagioclase–quartz geobarometry suggest that the anatectic garnets crystallized at ∼620–650°C and 4–5 kbar. Some garnet grains show two-stage zoning in major and trace elements, with the core similar to the peritectic garnet but the rim similar to the anatectic garnet. Mineralogy, whole-rock major- and trace-element compositions and zircon O isotopes indicate that the two types of leucosomes were produced by hydration (water-present) melting and dehydration (water-absent) melting, respectively. The leucocratic veins contain peritectic garnet but no K-feldspar, have lower whole-rock K2O contents and Rb/Sr ratios, higher whole-rock CaO contents and Sr/Ba ratios, and show homogeneous δ18O values that are lower than those of relict zircons, indicating that such veins were produced by the hydration melting. In contrast, the vein granite contains peritectic garnet and K-feldspar, has higher whole-rock K2O contents and Rb/Sr ratios, lower whole-rock CaO contents and Sr/Ba ratios, and shows comparable δ18O values with those of relict zircons, suggesting that this granite were generated by the dehydration melting. Accordingly, both hydration and dehydration melting mechanisms have occurred in the eastern Himalayan syntaxis.

The monazite record of pluton assembly: Mapping manaslu using petrochronology

The Manaslu pluton in the Nepal Himalaya presents an ideal opportunity to examine the history of pluton assembly as recorded by the accessory mineral monazite. A combined dataset of 1407 individual U-Th/Pb and trace element and 795 Sm/Nd spot analyses from 29 specimens enables a detailed comparison with both existing whole rock isotope data and structurally lower metamorphic and anatectic rocks. A systematic decrease in monazite age and Gd/Yb along with a concomitant increase in εNdi, Y, and heavy rare earth elements with decreasing structural level indicates emplacement and crystallization of the pluton occurred from the top down in two broad episodes, one at ∼22 Ma and the other at ∼19 Ma. Monazite in structurally lower specimens contain a greater range in inherited age and Nd isotopic components than structurally higher specimens indicating less efficient monazite dissolution and melt homogenization in these rocks. We infer the difference in age and isotopic variation to result from a change in melt source and/or melt forming reaction. Variations in monazite 143Nd/144Nd (0.5116 – 0.5122) and Sm/Nd (0.1 – 0.2) within the sample suite reflect the isotopic heterogeneity of the source region and differences in modal abundance of apatite, respectively. This study illustrates the potential for combining isotopic and trace element information retained in monazite to extract information on the source region and assembly of plutonic rocks; insight that is otherwise lost in bulk rock isotopic measurements. Combined in-situ monazite U-Th/Pb and Sm/Nd, therefore, presents a means to better understand the history of minimum temperature melts in collisional orogenic systems.

The complexities of zircon crystllazition and overprinting during metamorphism and anatexis: An example from the late Archean TTG terrane of western Shandong Province, China

There are different viewpoints on metamorphic and anatectic zircons recording ages of 2.45–2.48Ga or even younger in some areas of the North China Craton where both late Neoarchean and late Paleoproterozoic tectono-thermal events are well developed. These are: 1) partial resetting of the U-Pb isotopic system in the late Neoarchean zircons, 2) metamorphism lasting from the late Neoarchean to the earliest Paleoproterozoic, and 3) earliest Paleoproterozoic metamorphism as separate different event. Western Shandong Province is an area where the late Neoarchean tectono-thermal event is widely developed but the late Paleoproterozoic event has not been identified. This provides an opportunity to understand the geological processes around the Archean-Proterozoic boundary. Based on a field study, we carried out SHRIMP U-Pb zircon dating on seventeen samples of ∼2.5Ga old metamorphic and anatectic rocks, including tonalite, trondhjemite, granodiorite, gabbro, quartz diorite, granite and paragneiss with primary emplacement or depositional ages of 2.52–2.68Ga. Anatectic zircons show some textural and compositional features: a) Homogenous or blurred oscillatory zoning, b) high U contents and low in Th/U ratios (commonly <0.3), c) rare idiomorphic morphologies, and d) commonly containing inherited (xenocrystic) cores. We conclude that the strong late Neoarchean event is widespread in western Shandong and is limited between ∼2.50 and 2.54Ga. In contrast, apparent 2.45–2.48Ga metamorphic zircon ages in some other areas of the North China Craton may be the result of overprinting and partial recrystallization of Neoarchean metamorphic zircons during the late Paleoproterozoic event. The Archaean rocks of western Shandong can be divided into three belts, namely Belts A, B and C from the northeast to the southwest. The difference between Belts A and B in ∼2.5Ga metamorphic and anatectic intensity may indicate that the former was elevated to a higher crustal level compared with the latter at the end of the Neoarchean.

Himalayan leucogranites and migmatites: nature, timing and duration of anatexis

AbstractWidespread anatexis was a regional response to the evolution of the Himalayan‐Tibetan Orogen that occurred some 30 Ma after collision between Asia and India. This paper reviews the nature, timing, duration and conditions of anatexis and leucogranite formation in the Greater Himalayan Sequence (GHS), and compares them to contemporaneous granites in the Karakoram mountains. Himalayan leucogranites and associated migmatites generally share a number of features along the length of the mountain front, such as similar timing and duration of magmatism, common source rocks and clockwise P–T paths. Despite commonalities, most papers emphasize deviations from this general pattern, indicating a fine‐tuned local response to the dominant evolution. There are significant differences in P–T– conditions during anatexis, and timing in relation to regional decompression. Further to that, some regions underwent a second event recording melting at low pressures. Zircon and monazite ages of anatectic rocks range between c. 25 and 15 Ma, suggesting prolonged crustal melting. Typically, a single sample may have ages covering most of this 10 Ma period, suggesting recycling of accessory phases from metamorphic rocks and early‐formed magmas. Recent studies linking monazite and zircon ages with their composition, have determined the timing of prograde melting and retrograde melt crystallization, thus constraining the duration of the anatectic cycle. In some areas, this cycle becomes younger down section, towards the leading front of the Himalayas, whereas the opposite is true in other areas. The relationship between granites and movement on the South Tibetan Detachment (STD) reveals that fault motion took place at different times and over different durations requiring complex internal strain distribution along the Himalayas. The nature and fate of magmas in the GHS contrast with those in the Karakoram mountains. GHS leucogranites have a strong crustal isotopic signature and migration is controlled by low‐angle foliation, leading to diffuse injection complexes concentrated below the STD. In contrast, the steep attitude of the Karakoram shear zone focused magma transfer, feeding the large Karakoram‐Baltoro batholith. Anatexis in the Karakoram involved a Cretaceous calcalkaline batholith that provided leucogranites with more juvenile isotopic signatures. The impact of melting on the evolution of the Himalayas has been widely debated. Melting has been used to explain subsequent decompression, or conversely, decompression has been used to explain melting. Weakening due to melting has also been used to support channel flow models for extrusion of the GHS, or alternatively, to suggest it triggered a change in its critical taper. In view of the variable nature of anatexis and of motion on the STD, it is likely that anatexis had only a second‐order effect in modulating strain distribution, with little effect on the general history of deformation. Thus, despite all kinds of local differences, strain distribution over time was such that it maintained the well‐defined arc that characterizes this orogen. This was likely the result of a self‐organized forward motion of the arc, controlled by the imposed convergence history and energy conservation, balancing accumulation of potential energy and dissipation, independent of the presence or absence of melt.

High-pressure granulites of the Podolsko complex, Bohemian Massif: An example of crustal rocks that were subducted to mantle depths and survived a pervasive mid-crustal high-temperature overprint

The Podolsko complex, Bohemian Massif, is a mid-crustal migmatite–granite dome exposed along a tectonic boundary separating the upper crust from the deeply eroded interior of the Variscan orogen, referred to as the Moldanubian Zone. This study examines metamorphic history of mafic and felsic granulites that occur in this complex as minor lenses or layers hosted in pervasively anatectic rocks. The mafic granulite contains garnet with preserved high-Ca cores, which based on pseudosection modelling indicates pressure conditions near the coesite stability field at temperatures of ca. 550–600°C. The relicts of an earlier eclogite-facies stage have been overprinted by a later granulite-facies assemblage consisting of ternary feldspar, orthopyroxene, and spinel in the mafic granulite and sillimanite and spinel in the felsic granulite. Composition of younger garnet (in rims and as smaller grains) in both granulites suggests that a near isothermal decompression of these rocks was followed by heating that reached temperature of ca. 900°C at pressure of ca. 0.5GPa. It is thus concluded that the granulites underwent at least two temporally separate tectonometamorphic events: they were first subducted to mantle depths and exhumed rapidly at relatively low temperatures and then near isobarically heated at mid-crustal levels. The preservation of earlier eclogite-facies garnet in the mafic granulite indicates that the latter event was short-lived and was followed by near isobaric cooling. The geologically brief granulite-facies metamorphism was previously explained as a result of slab break-off and mantle upwelling after the main phase of microplate convergence in the Bohemian Massif. To put the Podolsko complex into a broader tectonic context, we synthesize the available petrologic and structural data from the correlative (U)HP assemblages of the Moldanubian Zone to suggest that they typically do not preserve structural record of the subduction stage, only rarely preserve an early exhumation phase to the middle crust, and most, if not all, have been extensively overprinted by late-stage processes at shallower crustal levels. This synthesis also reveals that kinematics and thus tectonic processes driving the final exhumation of the (U)HP rocks were diverse and may vary from one location to another. These local settings include exhumation in the footwall of major extensional detachments, due to folding and subsequent lateral extrusion, and along crustal-scale strike-slip shear zones. The extensive late-stage overprint masking the earlier (pre-340Ma) subduction history is greatly exemplified in the Podolsko complex where the granulites are only small relics in pervasively anatectic rocks melted at mid-crustal conditions and then rapidly exhumed along extensional detachments to upper-crustal levels.

Thermal conditions during deformation of partially molten crust from TitaniQ geothermometry: rheological implications for the anatectic domain of the Araçuaí belt, eastern Brazil

Abstract. During the Neoproterozoic orogeny, the middle crust of the Araçuaí belt underwent widespread partial melting. At the regional scale, this anatectic domain is characterized by a progressive rotation of the flow direction from south to north, suggesting a 3-D deformation of the anatectic middle crust. To better determine whether melt volumes present in the anatectic middle crust of the Araçuaí orogen were large enough to allow a combination of gravity-driven and convergence-driven deformation, we used the titanium-in-quartz (TitaniQ) geothermometer to estimate the crystallization temperatures of quartz grains in the anatectic rocks. When possible, we compared these estimates with thermobarometric estimates from traditional exchange geothermobarometers applied to neighboring migmatitic kinzigites. TitaniQ temperatures range from 750 to 900 °C, suggesting that quartz starts crystallizing at minimum temperatures of ≥ 800 °C. These results, combined with the bulk-rock chemical composition of diatexites, allows the estimation of a minimum of ~ 30% melt and a corresponding viscosity of ~ 109–1010 Pa s. Such a minimum melt content and low viscosity are in agreement with interconnected melt networks observed in the field. Considering that these characteristics are homogeneous over a wide area, this supports the finding that the strength of the middle crust was severely weakened by extensive partial melting, making it prone to gravity-driven flow and lateral extrusion.

Himalayan Metamorphism and Its Tectonic Implications

The Himalayan range exposes a spectacular assemblage of metamorphic rocks from the mid- and deep crust that have fostered numerous models of how the crust responds to continental collisions. Recent petrogenetically based petrologic and geochronologic studies elucidate processes with unprecedented resolution and critically test models that range from continuum processes to one-time events. The pronounced metamorphic inversion across the Main Central Thrust reflects continuum thrusting between ca. 15 and 20 Ma, whereas exposure of ultrahigh-pressure rocks in northwestern massifs and syntaxis granulites reflects singular early (≥45 Ma) and late (≤10 Ma) exhumation events. Multiple mechanisms including wedge collapse and flow of melt-weakened midcrust are debated to explain pressure-temperature trajectories, patterns of thinning, and thermal overprinting. A geochronologic revolution is under way in which spatially resolved compositions and ages of accessory minerals are combined in a petrogenetically valid context to recover specific temperature-time points and paths. Combined chemical and chronologic analysis of monazite is now well established and titanite is particularly promising, but recent zircon data raise questions about anatectic rocks and their use for investigating tectonism.

Petrogenesis of migmatites and leucogranites from Sierra de Molinos, Salta, Northwest Argentina: A petrologic and geochemical study

In Sierra de Molinos, Eastern Cordillera, NW Argentina the relationship between different components of an anatectic system developed in high temperature and low pressure conditions (<800°C and 4–6kbar) can be observed. This allows the direct insight on the origin of migmatites and granites by anatexis of psammo–pelitic metasedimentary rocks. Four principal rock types have been recognized in Sierra de Molinos 1) metasedimentary rocks, assigned to the Puncoviscana Formation, 2) metatexite migmatites, 3) diatexite migmatites and 4) anatectic granitoids including: leucogranites, trondhjemites and pegmatite/aplitic dykes. The metamorphic grade increases from phyllites and schists at sub-greenschist and greenschist facies in the west, to upper amphibolite facies migmatites in the east. The mineral assemblages indicate a progressive increase in temperature without a significant change in pressure. The ages obtained for granitoids and migmatites are identical within analytical errors and indicate that the metamorphic peak and anatectic granite generation occurred at ca. 470Ma.We use petrological and geochemical variations to investigate the formation of granite magma from migmatites that derived from metasedimentary protoliths. This provides evidence of the processes that occur during the evolution (melting, segregation and crystallization stages) of partially molten terranes, for which particular components: protolith, residuum and melt fractions are identified. Mass balance calculations are used to estimate the degree of partial melting and the degree of melt extraction in migmatites. Petrological and compositional data suggest that migmatites from this region are the result of open system processes and that fluid-absent melting played a major role in the formation of migmatites and leucogranites. The geochemical evidence suggests that the compositional variation within the anatectic rocks from Molinos can be understood as a result of 1) variable degrees of melt–residuum separation and 2) fractional crystallization of the anatectic melt. Compared to the protolith, most of the migmatites have melt-depleted compositions (up to 0.25 of melt extraction). In the same fashion most of the leucosomes represent cumulate products of fractional crystallization. This means that they have lost some liquid of evolved composition and represent evidence of the melt extraction pathways and linkage to the magma ascent conduits.

40Ar/39Ar geochronological constraints on the formation of the Dayingezhuang gold deposit: New implications for timing and duration of hydrothermal activity in the Jiaodong gold province, China

China's largest gold resource is located in the highly endowed northwestern part of the Jiaodong gold province. Most gold deposits in this area are associated with the NE- to NNE-trending shear zones on the margins of the 130–126Ma Guojialing granite. These deposits collectively formed at ca. 120±5Ma during rapid uplift of the granite. The Dayingezhuang deposit is a large (>120t Au) orogenic gold deposit in the same area, but located along the eastern margin of the Late Jurassic Linglong Metamorphic Core Complex. New 40Ar/39Ar geochronology on hydrothermal sericite and muscovite from the Dayingezhuang deposit indicate the gold event is related to evolution of the core complex at 130±4Ma and is the earliest important gold event that is well-documented in the province. The Dayingezhuang deposit occurs along the Linglong detachment fault, which defines the eastern edge of the ca. 160–150Ma Linglong granite–granodiorite massif. The anatectic rocks of the massif were rapidly uplifted, at rates of at least 1km/m.y. from depths of 25–30km, to form the metamorphic core complex. The detachment fault, with Precambrian metamorphic basement rocks in the hangingwall and the Linglong granitoids and migmatites in the footwall, is characterized by early mylonitization and a local brittle overprinting in the footwall. Gold is associated with quartz–sericite–pyrite–K-feldspar altered footwall cataclasites at the southernmost area of the brittle deformation along the detachment fault. Our results indicate that there were two successive, yet distinct gold-forming tectonic episodes in northwestern Jiaodong. One event first reactivated the detachment fault along the edge of the Linglong massif between 134 and 126Ma, and then a second reactivated the shears along the margins of the Guojialing granite. Both events may relate to a component of northwest compression after a middle Early Cretaceous shift from regional NW–SE extension to a NE–SW extensional regime.

Mineral chemistry and thermobarometry of plutonic, metamorphic and anatectic rocks from the Tueyserkan area (Hamedan, Iran)

The study area is a part of the NW to SE striking Sanandaj–Sirjan metamorphic belt in western Iran. The Alvand Pluton, consisting of rocks that range in composition from gabbro to granite, is the major magmatic rock complex of this area. Gabbroic rocks include olivine gabbro, gabbro norite, norite and gabbro. Rocks around the Alvand Pluton were subjected to different P–T conditions due to polymetamorphism. Common metamorphic rocks are meta-pelites, but some meta-psammites, meta-basites and meta-carbonates also occur. Slates, phyllites, schists, migmatites and hornfelses are major rock units of meta-pelites in the metamorphic sequence. Based on mineral chemistry, the highest temperature of crystallisation (1300°C) was determined for the olivine gabbros, and the lowest temperature (950°C) was calculated for the hornblende-bearing gabbros. Clinopyroxene–plagioclase barometry suggests that pressures near 5 to 6 kbars prevailed during the crystallisation of the various mafic rocks. P–T estimates yield maximum temperatures of 700–750°C at 5–6 kbars for the high-grade metamorphic rocks from the metamorphic aureole around the pluton. These results indicate that the heat released from the Alvand Pluton (T = 950–1300°C), which intruded the metamorphic rocks at middle and upper crustal levels, was sufficient to cause partial melting leading to formation of the metatexites, diatexites and restite-rich S-type granites. During this process, part of the deep-seated gabbro-dioritic rocks were transported to higher crustal levels by viscous, enclave- and crystal-rich granitic magmas of the partial melting zone.

The cordierite‐bearing anatectic rocks of the higher Himalayan crystallines (eastern Nepal): low‐pressure anatexis, melt productivity, melt loss and the preservation of cordierite

AbstractCordierite‐bearing anatectic rocks inform our understanding of low‐pressure anatectic processes in the continental crust. This article focuses on cordierite‐bearing lithologies occurring at the upper structural levels of the Higher Himalayan Crystallines (eastern Nepal Himalaya). Three cordierite‐bearing gneisses from different geological transects (from Mt Everest to Kangchenjunga) have been studied, in which cordierite is spectacularly well preserved. The three samples differ in terms of bulk composition likely reflecting different sedimentary protoliths, although they all consist of quartz, alkali feldspar, plagioclase, biotite, cordierite and sillimanite in different modal percentages. Analysis of the microstructures related to melt production and/or melt consumption allows the distinction to be made between peritectic and cotectic cordierite. The melt productivity of different prograde assemblages (from two‐mica metapelite/metagreywacke to biotite‐metapelite) has been investigated at low‐pressure conditions, evaluating the effects of muscovite v. biotite dehydration melting on both mineral assemblages and microstructures. The results of the thermodynamic modelling suggest that the mode and type of the micaceous minerals in the prograde assemblage is a very important parameter controlling the melt productivity at low‐pressure conditions, the two‐mica protoliths being significantly more fertile at any given temperature than biotite gneisses over the same temperature interval. Furthermore, the cordierite preservation is promoted by melt crystallization at a dry solidus and by exhumation along P‐T paths with a peculiar dP/dT slope of about 15–18 bar °C−1. Overall, our results provide a key for the interpretation of cordierite petrogenesis in migmatites from any low‐P regional anatectic terrane. The cordierite‐bearing migmatites may well represent the source rocks for the Miocene andalusite‐bearing leucogranites occurring at the upper structural levels of the Himalayan belt, and low‐P isobaric heating rather than decompression melting may be the triggering process of this peculiar peraluminous magmatism.

MnNCKFMASH Phase Relations in Cordierite–Orthopyroxene Migmatitic Gneisses, Southern India: Implications for Low-pressure Crustal Melting under Granulite-facies

Abstract Cordierite-orthopyroxene migmatitic gneisses exposed in Achankovil unit of the Kerala Khondalite Belt, southern India show evidences of melting, melt extraction and in-situ crystallization of melt under granulite-facies conditions. The sequential mineral assemblages garnet + biotite + orthopyroxene + plagioclase + quartz (± melt) in the mesosomes and garnet + biotite + orthopyroxene + cordierite + plagioclase + K-feldspar + quartz + melt in the melanosomes makes the Achankovil cordierite-orthopyroxene migmatitic gneisses a good example of anatectic rocks, where substantial melt fractions remained in-situ during decompression and cooling. Therefore, the rocks provide an opportunity to investigate deep crustal processes and record of rheological (thermal and mechanical) reequilibration prevailed during the final stages of orogeny. The significance of cordierite formation and its possible relationship with melt formation are investigated applying theoretical calculations in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (MnNCKFMASH) system. Results of numerical modelling of the mineral assemblages in pressure–temperature–composition (P–T–X) pseudosections using Perple_X infer that the sequence of reactions involving formation of cordierite-orthopyroxene-melt assemblage is consistent with an isothermal decompression (with a pressure drop of &amp;gt;1.5 kbars) at high temperatures (&amp;gt;800 °C), forming leucosomes. Biotite dehydration melting reactions, occurring above 4.5 kbars constrain prograde arm of the P–T trajectory and is interpreted as a product of crustal thickening, which was followed by rapid decompression. The final stage of exhumation is characterized by rehydration of cordierites in the melanosome by melt-solid interactions at exceptionally low-pressure (~3.2 kbars) conditions. The high-temperature isothermal decompression inferred from the mineral reactions and P–T–X pseudosections constitute a clockwise P–T path for the exhumation of the lower crust. This clockwise P–T path is consistent with the common tectonic model accepted for the genesis of granulite-facies migmatites during crustal thickening and later unroofing, accompanied with arc-continent collision. Our conclusions indicate low–P metamorphism and anatexis can be traced to convergent setting, where melt buoyancy considerably decreases density of the lithosphere and modifies rheology leading to rapid exhumation of the lower crust. Therefore, the crustal evolution in the Kerala Khondalite Belt is correlated with two stage processes: (i) thickening of the crust in relation to a continental-arc setting, followed by (ii) exhumation along a high-temperature stable geotherm with sufficient pressure release associated with syn- to post-convergence transpression and transtension.

Geochemical and isotopic (Sr, Nd and O) constraints on sources for Variscan granites in the Western Carpathians - implications for crustal structure and tectonics

* �Correspondingauthor A Sr, Nd, and O isotopic study of Variscan granitoid rocks from the Western Carpathians reveals the dominance of hete- rogeneous crustal sources for the most of the granitic rocks. Their neodymium crustal index (NCI) is 0.4 to 1.0 (mainly 0.6-0.8). Initial ( 87 Sr/ 86 Sr) 350 of 0.7053 to 0.7078 and e Nd (350) of -0.6 to -6.9 preclude a simple mantle and/or crustal origin for most of granitoids and suggest more complex sources, such as vertically zoned lower crust consisting of old metaigne- ous, amphibolitic and metasedimentary rocks. Apparent crustal residence ages, indicated by two-stage depleted-mantle Nd model ages ranging from 1.6 to 1.1 Ga, are comparable with other segments of the European Variscan belt. The whole rock δ 18 O (SMOW) values of the granites are heterogeneous and range from 8.4 ‰ in tonalites to 11.3 ‰ in leucogranites, reflecting source compositions ranging from mafic to silicic. Petrographically, these granites are representative of common crustal anatectic rocks with magmatic muscovite; however, their isotopic signature reflects petrogenesis related to subduction pro - cesses at active continental margins. Recent metamorphic, sedimentary and/or structural studies of the Variscan basement of the Western Carpathians suggest a continental collisional rather than a volcanic arc setting. The Western Carpathian granitic rocks were most likely generated by partial melting of mainly recycled Proterozoic crustal material during subduction- collisional processes of the Variscan orogeny, with possible input of mafic magmas from the mantle. These mafic magmas may have served as a heat source for melting of the lower crust.

Remnants of Early Carboniferous I-type granodiorite plutons in the Bavarian Forest and their bearing on the tectonic interpretation of the south-western sector of the Bohemian Massif (Bavarian Zone)

3 �AcademyofSciences,�InstituteofRockStructureandMechanics,�VHolesovickach�41,�182�09�Prague�8,�CzechRepublic * �Correspondingauthor The Bavarian Zone of the south-western Bohemian Massif is a late Variscan high heat-flow region, characterized by numerous granite intrusions and large amounts of LP-HT anatectic rocks, the latter comprising meta- and diatexites derived from sedimentary and igneous protoliths. Imaging and SIMS U-Pb dating of zircon from a distinctive type of hornblende-bearing diatexite, with a presumed igneous protolith, was done to gather information on both metamorphism and pre-anatectic crustal evolution. Zircon crystals have two phases of growth zoning, with oscillatory-zoned grains (visible through cathodoluminescence, CL, imaging) modified and surrounded by relatively uniform, high luminosity zircon. Analyses on oscillatory-zoned zircon yield an age of 344 ± 2 Ma (2σ), interpreted as the formation age of the igneous protolith. More luminescent overgrowths, interpreted as having formed during anatexis, were dated at 323 ± 3 Ma. Based on these dates, and on geochemical similarities, the protolith can be perfectly correlated with the high-K calc-alkaline Blatna suite of the Central Bohemian Plutonic Complex, c. 70 km to the north. This correlation strengthens tectonic models interpreting the Bavarian Zone as a reheated but integral constituent of the Variscan collisional crust of the Bohemian Massif, as opposed to an independent basement terrane assembled by late-Variscan orogenesis.