HT/HP Metamorphism and Crustal Anatexis

The Jamari complex (gabbro-diorite-tonalite/enderbite-granodiorite/charnoenderbite) represents a Paleoproterozoic (ca. 1.76e1.74 Ga) magmatic arc emplaced along the southwestern Amazonian Craton in central-eastern Rondônia, Brazil. Comprising metaplutonic and metasedimentary rocks, the Jamari complex constitutes the polydeformed Paleoproterozoic basement of Rondônia state. Together with regional data from the Rondônia-Juruena Province, our findings suggest that the southwestern border of the Amazonian Craton was the site of magmatic arc evolution from at least ca. 1.78 to 1.63 Ga. Elemental geochemical data show that intermediate/acid orthogneisses are metaluminous to mildly peraluminous, and have a medium to high-K, calc-alkaline signature, suggesting that they were formed in an immature Andean-type magmatic arc. Gabbros have signatures similar to tholeiites of active continental margin and diorites show characteristics compatible with rocks of tholeiite/calc-alkaline active continental margin volcanic arc. The chemical data of the Jamari complex felsic plutonic rocks show general trends of increasing contents of incompatible elements (K 2 O, Rb, Nb, Th, La, Ba and Sr) and decreasing contents of compatible elements (Ni, V, Sc, MgO, Fe 2 O 3 , Al 2 O 3 , CaO and TiO 2 ) with increasing SiO 2 . Although these variations are consistent with closed system fractional crystallization processes, the wide variation of Rb/ Zr, La/Sm, K/Rb, Nb/Y, Th/Y and Th/Yb in the felsic rocks may indicate random crustal contamination during the evolution of these rocks. Normalized trace element patterns show enrichment in LILEs (Rb, Ba, K, Th and Ce) relative to HFSEs (Nb, Zr, P and Ti) and are very similar to calc-alkaline subduction-related rocks from orogenic belts. The Jamari complex represents the western extension of similar metaplutonic rocks (Juruena Complex, Mato Grosso), occurring along some 500 km of the Paleoproterozoic Madeirinha orogen (1.78e1.63 Ga). During this event the rocks were metamorphosed under upper amphibolite to granulite facies conditions that only mildly disturbed their igneous characteristics. Zircon UePb crystallization ages (ID-TIMS, SHRIMP and LA-ICP-MS) set the acretional phase of the magmatic arc in Rondônia between 1.76 and 1.74 Ga. Metamorphic mineral paragenesis and textural features in these rocks, combined with geochronologic data, indicate that metamorphic conditions in the study area reached the granulite facies (T ¼ ca. 750e800 C, P ¼ 7 to 8 kbar) in a tectonothermal collisional event that occurred between 1.67 and 1.63 Ga. The Jamari complex in this region was subsequently reworked during the Rondonian-San Ignácio Orogeny (1.50e1.30 Ga), a tectonic episode characterized by crytical metamorphic mineral assemblages and anatexis, suggesting upper-amphibolite-facies metamorphism. The youngest tectonic event recognized in the Jamari complex is associated with tectonic reactivation, deformation, thermal overprint, and magmatism related to the Sunsás Orogeny (1.30e0.95 Ga). Its effects are represented by extensive development of shear zones (Ji-Paraná system), mylonitic belts, rifts and sedimentary deposits, and post-collisional A-type intrusions. Nd isotopic data of the high-grade orthogneisses show a wide range of ε Nd values (À1.5 to þ5.6) and a wide spectrum of T DM model-* Corresponding author. Geological Survey of Brazil (CPRM), SGAN 603,

The Menderes Massif is a major polymetamorphic complex in Western Turkey. The late Neoproterozoic basement consists of partially migmatized paragneisses and metapelites in association with orthogneiss intrusions. Pelitic granulite, paragneiss and orthopyroxene-bearing orthogneiss (charnockite) of the basement series form the main granulite-facies lithologies. Charnockitic metagranodiorite and metatonalite are magnesian in composition and show calc-alkalic to alkali-calcic affinities. Nd and Sr isotope systematics indicate homogeneous crustal contamination. The zircons in charnockites contain featureless overgrowth and rim textures representing metamorphic growth on magmatic cores and inherited grains. Charnockites yield crytallization age of ~590 Ma for protoliths and they record granulite-facies overprint at ~ 580 Ma. These data indicate that the Menderes Massif records late Neoproterozoic magmatic and granulite-facies metamorphic events. Furthermore, the basement rocks have been overprinted by Eocene Barrovian-type Alpine metamor-phism at ~42 Ma. The geochronological data and inferred latest Neoproterozoic–early Cambrian palaeogeographic setting for the Menderes Massif to the north of present-day Arabia indicate that the granulite-facies metamorphism in the Menderes Massif can be attributed to the Kuunga Orogen (600–500 Ma) causing the final amalgamation processes for northern part of the Gondwana.

Garnet-cordierite-sillimanite bearing rocks from the contact aureole of the Precambrian Loon Lake pluton in Chandos Township, southeastern Ontario were analyzed for the major and rare-earth elements. In comparison with the associated Apsley biotite gneisses, they are rich in A1, Mg, and Fe, low in Si, Na, and K and their REE distribution patterns show a depletion of light REE with a negative Eu anomaly. These rocks are probably residuum left after partial melting of biotite gneiss. Leucogranite associated with the GCS rocks may represent the extracted anateetic material. It is suggested that some of the garnet cordierite-sillimanite gneisses which frequently occur in high-grade regionally metamorphosed areas of the Grenville Province of the Canadian Shield may also be of a similar residual origin as proposed by Lal and Nioorhouse (1969).

The metapelitic schists of Jandagh or simply Jandagh metapelites can be divided into four groups
based on mineral assemblages: (1) quartz–muscovite schists, (2) quartz–muscovite–biotite schists, (3) gar
net–muscovite–chlorite schists, and (4) garnet–muscovite–staurolite schists. The Jandagh garnet–musco
vite–chlorite schists show the first appearance of garnets. These garnets contain 58–76% almandine, 1–18%
spessartine, and 8–20% grossular. Microprobe analysing across the garnets demonstrates an increase in Mg#
from core to rim. This is a feature of the prograde metamorphism of metapelites. Wellpreserved garnet
growth zoning is a sign that metapelites were rapidly cooled and later metamorphic phases had no effect here.
The appearance of staurolite in garnetmuscovitechlorite schists signifies a beginning of the amphibolite fa
cies. The absence of zoning in staurolite suggests that its formation and growth during prograde metamor
phism occurred at a widely spaced isograde. Thermobarometric investigations show that the Jandagh
metapelites were formed within a temperature range of 400–670°C and pressures of 2.0–6.5 kbar. These re
sults are in agreement with the mineral paragenetic evidence and show the development of greenschist and
amphibolite facies in the area studied.

1] Many migmatites represent crystallized partially molten crust and therefore record the mechanisms and pathways of orogenic crustal flow. Field and microstructural methods may be insufficient to characterize the planar and linear elements of rock fabric in migmatites due to obscured flow fabrics or protracted deformation. In the Naxos dome (Greece), we test the anisotropy of magnetic susceptibility (AMS) as a tool for recovering mineral fabric symmetry and the kinematic axes of flow in migmatites. Measurements of 155 migmatite samples yield dominantly low values (<300 × 10 −6 [SI]) of bulk magnetic susceptibility (K m ) consistent with biotite being the dominant carrier of the AMS. Higher values of K m , thermomagnetic, hysteresis, and microstructural data, however, suggest a ferromagnetic contribution from magnetite in a subset of samples (N = 15). Using electron backscatter diffraction (EBSD) analysis, we establish the correspondence of the biotite subfabric with the AMS and structural fabric of the Naxos migmatites. EBSD data from biotite suggests that magnetic lineation in these dominantly paramagnetic migmatites arises from a zone axis orientation of biotite crystals organized about the direction of viscoplastic flow. Over a range of spatial scales, migmatitic foliation and magnetic foliation are well correlated. The magnetic lineation recovered by AMS displays a coherent organization despite the heterogeneous structure and composition of the Naxos migmatites. These data suggest that the apparent complexity of migmatites masks a simpler flow regime controlled by bulk viscoplastic flow. Furthermore, our study demonstrates the utility of the AMS method for studying the dynamics of partially molten orogenic crust.

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 identifi cation 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 Mnpoor 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 identifi ed in the Wet Mountains will greatly facilitate the recognition of other such exhumed sections.

In hot orogens, gneiss domes are a response to upper crustal stretching and lower crustal flow. Two-dimensional thermal-mechanical modeling shows that localization of extension in the upper crust triggers, in the deep crust, oppositely verging horizontal flows that converge beneath the extended region. Upon viscous collision, both flowing regions rotate upward to form two upright domes of foliation (double domes) separated by a steep median high-strain zone. In such systems, horizontal shortening in the infrastructure develops in an overall extensional setting. Dome material follows a complex depth-dependent strain history, from shearing in the deep crustal channel, to contraction upon viscous collision in the median high-strain zone, to extension upon advection into the shallow crust. This depth-dependent strain history is likely a general feature of dome evolution, and is arguably well preserved in double domes such as the Montagne Noire (France) and Naxos (Greece) gneiss domes.

The Okanogan gneiss dome, Washington, is located in the hinterland of the North American Cordillera and is part of a chain of metamorphic core complexes containing gneiss and migmatite domes exhumed during Eocene extension of thickened crust. U-Pb sensitive high-resolution ion micro-probe (SHRIMP) analyses of zircon, monazite, and titanite, and 40Ar-39Ar analyses of biotite from migmatites exposed in the footwall of the Okanogan detachment, coupled with a detailed structural analysis, document the timing and duration of migmatite crystallization and indicate coeval crystallization, extensional deformation, and exhumation of the dome. Okanogan migmatites are folded and deformed, and preserve successive generations of leucosomes generated by synkinematic anatexis.

Analyses of migmatite samples from a high-melt fraction subdome near Stowe Mountain suggest that the Okanogan dome records a history of migmatite crystallization spanning at least 12 m.y., as indicated by 206Pb/238U ages ranging from ca. 61 to 49 Ma for new zircon growth and rim overgrowths attributed to migmatite crystallization. Zircons from a granodiorite in a domain of diatexite near Stowe Mountain preserve rims that have a mean 206Pb/238U age of 51.1 ± 1.0 Ma for the youngest population attributed to migmatite crystallization. Zircon from folded and discordant granitic leucosome in the diatexite domain yields a calculated 206Pb/238U age of 53.5 ± 0.5 Ma for migmatite crystallization. Zircon from discordant leucosome of the metatexite domain has a mean 206Pb/238U age of 59.8 ± 0.5 Ma, with ages as young as ca. 53 Ma attributed to final crystallization of the leucosome. Core domains of zircon samples have 206Pb/238U ages that range from ca. 85 to 70 Ma and are interpreted to be related to an earlier phase of the orogeny. Monazite from two samples gives 206Pb/238U crystallization ages of 52.9 ± 0.6 Ma for the granodiorite diatexite and 52.0 ± 0.6 Ma for nearby boudinaged and foliated layers of biotite granodiorite. One sample of folded granitic leucosome in metatexite contains titanite with a mean 206Pb/238U age of 47.1 ± 0.5 Ma. The ca. 47 Ma age for titanite is similar to biotite 40Ar-39Ar ages of 48.0 ± 0.1 Ma, 47.9 ± 0.2 Ma, and 47.1 ± 0.2 Ma for samples collected from the upper detachment surface downward over 1.5 km of structural thickness into the migmatite domain.

Crystallization of the Okanogan migmatites was therefore coeval in part with upper crustal extension and ductile flow of the mid-crust. Leucosome crystallization largely ceased by ca. 49 Ma, followed by rapid cooling of footwall rocks through ~325 °C by ca. 47 Ma. These data are similar to crystallization ages in migmatites from other domes in the northern Cordillera hinterland, suggesting that crustal anatexis was widespread over much of the mid-crust during Paleocene to Eocene time, coeval with extension and exhumation of orogenic middle crust.

Gneiss domes are commonly cored by quartzofeldspathic rocks that provide little information about the pressure–temperature–fluid history of the domes. Three northern Cordilleran migmatite domes (Thor-Odin and Valhalla/Passmore, British Columbia, Canada; Okanogan, Washington, USA), however, contain Mg–Al-rich orthoamphibole-cordierite gneiss as layers and lenses that record metamorphic conditions and pressure–temperature (P–T) path information not preserved in the host migmatite. These Mg–Al-rich rocks are therefore a valuable archive of metamorphic conditions during dome evolution, although refractory rocks such as these commonly contain reaction textures that may complicate the calculation of metamorphic conditions. In the Okanogan dome, Mg–Al-rich layers are part of the Tunk Creek unit, which occurs at the periphery of an underlying migmatite domain. Bulk compositional layers (mm- to m-scale) consist of gedrite-dominated, hornblende-dominated and biotite-bearing layers that contain variable amounts of gedrite, hornblende, anorthite, cordierite, spinel, sapphirine, corundum, kyanite, biotite and/or staurolite. The presence of different compositional layers (some with reaction textures, some without) allows systematic analysis of metamorphic history by a combined petrographic and phase equilibrium analysis. Gedrite-dominated layers containing relict kyanite preserve evidence of the highest-P conditions; symplectitic and coronal reaction textures around kyanite indicate decompression at high temperature. Gedrite-dominated layers lacking these reaction textures contain layers of sapphirine and spinel in apparent textural equilibrium and record a later high-T–low-P part of the path. Phase equilibria (pseudosection) analysis for layers that lack reaction textures indicates metamorphic conditions of 720–750 °C at a range of pressures (>8 to <4 kbar) following decompression. Elevated crustal temperatures and concordant structural fabrics in the Tunk Creek unit and underlying migmatite domain suggest that the calculated P–T conditions recorded in Tunk Creek rocks were coeval with anatexis, extension, and dome formation in Palaeocene–Eocene time. In contrast to orthoamphibole-cordierite gneiss in the other Cordilleran domes, the Tunk Creek unit occurs as a discontinuous km-scale layer rather than as smaller (m-scale) pods, is more calcic, and lacks garnet. In addition, kyanite did not transform to sillimanite, and spinel commonly occurs as a blocky matrix phase in addition to vermicules in symplectite. These differences, along with the compositional layering, allow an analysis of bulk composition v. tectonic (P–T path) controls on mineral assemblages and textures. Pseudosection modelling of different layers in the Tunk Creek unit provides a basis for understanding the metamorphic history of these texturally complex, refractory rocks and their host gneiss domes, and other such rocks in similar tectonic settings.

Titanite U–Pb isotopic and major and trace element compositions of one mafic garnet granulite from a rare suite of lower crustal xenoliths (e.g., eclogite, garnet pyroxenite, and mafic garnet granulite) hosted in Early Cretaceous dioritic porphyries in the Xu–Huai area along the southeastern margin of the North China Craton (NCC) were analyzed by laser ablation ICP-MS. Titanite occurs as granular grains or coronary rims on rutile. The coronary titanite is clearly a secondary product of rutile decomposition. The granular titanite exhibits zonation in U–Pb age and chemical composition. Petrographic and geochemical evidence suggests that the zonation was formed by thermal diffusion and later fluid-assisted recrystallization. Occurrences of granular titanite between garnet grains point to a pressure of > 10 kbar, while inclusions of rutile inside granular titanite rims imply that the pressure might have reached 15 kbar. Granular titanite cores give U–Pb ages of 237–241 Ma and Zr-temperatures of 794–831 °C at 10 kbar and 850–892 °C at 15 kbar, indicating high-pressure granulite-facies metamorphism. Together with previous P–T estimates of coeval eclogite-facies xenoliths, a geotherm of above 60 mW m− 2 is implied. The geotherm plots below the temperature field of amphibole dehydration melting, consistent with presence of abundant amphibole. This geotherm is similar to that of the Kohistan arc, which has preserved a 12-km-thick dense lower crust, but significantly cooler than the geotherm of the Talkeetna arc, where most of the dense lower crust has been foundered. Our results provide new evidence for Triassic thickened dense lower crust along the southeastern margin of the NCC. By comparison with the Kohistan and Talkeetna arc crusts, we suggest that this dense lower crust was not hot enough to be foundered in the Triassic. Foundering must have occurred in the Jurassic–Cretaceous in order to explain the present-day seismic velocity structure characterized by a sharp Moho, overall slow velocities in the lower crust, and a thin crustal thickness in the Xu–Huai area and other parts of the eastern NCC. We suggest that the Jurassic–Cretaceous foundering was related to the Pacific subduction. On one hand, the Jurassic subduction may have further thickened the southeastern margin of the NCC prior to Cretaceous extension, leading to greater instability of the lower crust. On the other hand, the subduction-related magma provided heat and water that weakened the lower crust, resulting in the final foundering. The large contrast in mineralogy between the Xu–Huai eclogite-facies xenoliths and nearby Nüshan garnet-free granulite xenoliths entrained by Quaternary basalts indicates > 20 km removal of the lower crust along the southeastern margin of the NCC.

The trace and Rare Earth Element (REE) characteristics of igneous rocks provide significant evidence of the origin and evolution of magmas and are useful to investigate the tectono-magmatic origin of Precambrian terranes. A simple assumption is that magmas that are generated by similar geological processes possess the same geochemical attributes. These attributes facilitate a comparison through tectonic discriminatory diagrams (e.g. Floyd and Winchester, 1975; Pearce and Cann, 1973; Pearce et al., 1984; Winchester et al., 1995). Although the interpretation of geochemical data for magmatic processes and the interaction of magmas with crustal materials are sometimes complex (e.g. Castro et al., 1991; Chappell, 1996; Frost and Mahood, 1987; Neves and Vauchez, 1995), geochemical data in combination with other geological evidence have proved essential for the interpretation of the petrogenesis and tectonic setting of crustal igneous provinces.

The Fosdick Mountains form an E-W trending migmatite dome in the northern Ford Ranges of Marie Byrd Land, Antarctica. Pervasively folded migmatites derived from lower Paleozoic greywacke and middle Paleozoic plutonic rocks constitute the dome. New field research documents a transition from melt-present to solid-state deformation across the south flank of the dome, and a mylonitic shear zone mapped for 30 km between Mt. Iphigene and Mt Richardson. Kinematic shear sense is dextral normal oblique, with top-to-the-SW and -WSW transport. A U-Pb age of 107 Ma, from a leucosome-filled extensional shear band, provides a melt present deformation age, and a U-Pb age of 96 Ma, from a crosscutting granitic dike, gives a lower age limit for deformation. The shear zone, here named the South Fosdick detachment zone, forms the south flank of the migmatite dome and was in part responsible for the exhumation of mid-crustal rocks.

The composite intrusions of Drumadoon and An Cumhann crop out on the SE coast of the Isle of Arran, Scotland and form part of the larger British and Irish Palaeogene Igneous Province, a subset of the North Atlantic Igneous Province. The intrusions (shallow-level dykes and sills) comprise a central quartz–feldspar-phyric rhyolite flanked by xenocryst-bearing basaltic andesite, with an intermediate zone of dark quartz–feldspar-phyric dacite. New geochemical data provide information on the evolution of the component magmas and their relationships with each other, as well as their interaction with the crust through which they travelled. During shallow-crustal emplacement, the end-member magmas mixed. Isotopic evidence shows that both magmas were contaminated by the crust prior to mixing; the basaltic andesite magma preserves some evidence of contamination within the lower crust, whereas the rhyolite mainly records upper-crustal contamination. The Highland Boundary Fault divides Arran into two distinct terranes, the Neoproterozoic to Early Palaeozoic Grampian Terrane to the north and the Palaeozoic Midland Valley Terrane to the south. The Drumadoon Complex lies within the Midland Valley Terrane but its isotopic signatures indicate almost exclusive involvement of Grampian Terrane crust. Therefore, although the magmas originated at depth on the northern side of the Highland Boundary Fault, they have crossed this boundary during their evolution, probably just prior to emplacement.

The origin and life cycle of ocean islands have been debated since the early days of Geology. In the case of the Canary archipelago, its proximity to the Atlas orogen led to initial fracture-controlled models for island genesis, while later workers cited a Miocene-Quaternary east-west age-progression to support an underlying mantle-plume. The recent discovery of submarine Cretaceous volcanic rocks near the westernmost island of El Hierro now questions this systematic age-progression within the archipelago. If a mantle-plume is indeed responsible for the Canaries, the onshore volcanic age-progression should be complemented by progressively younger pre-island sedimentary strata towards the west, however, direct age constraints for the westernmost pre-island sediments are lacking. Here we report on new age data obtained from calcareous nannofossils in sedimentary xenoliths erupted during the 2011 El Hierro events, which date the sub-island sedimentary rocks to between late Cretaceous and Pliocene in age. This age-range includes substantially younger pre-volcanic sedimentary rocks than the Jurassic to
Kirsten Zaczek, Valentin R. Troll, Mario Cachao, Jorge Ferreira, Frances M. Deegan, Juan Carlos Carracedo, Vicente Soler, Fiona C. Meade & Steffi Burchardt

Miocene strata known from the older eastern islands and now reinstate the mantle-plume hypothesis as the most plausible explanation for Canary volcanism. The recently discovered Cretaceous submarine volcanic rocks in the region are, in turn, part of an older, fracture-related tectonic episode.

Migmatite domes are common in metamorphic core complexes. Dome migmatites deform in the partially molten or magmatic state and commonly record complex form surfaces, folds, and fabrics while units mantling the dome display a simpler geometry, typically formed by transposition during crustal extension. We use field observations and magnetic fabrics in the Naxos dome (Greece) to quantify the complex flow of anatectic crust beneath an extensional detachment system. The internal structure of the Naxos dome is characterized by second-order domes (subdomes), pinched synforms, and curved lineation trajectories, which suggest that buoyancy-driven flow participated in dome evolution. Subdomes broadly occur within two compartments that are separated by a steep, N-S oriented, high-strain zone. This pattern has been recognized in domes formed by polydiapirism and in models of isostasy-dominated flow. The preferred model involves a combination of buoyancy- and isostasy-driven processes: the Naxos dome may have been generated by regional N-S extension that triggered convergent flow of partially molten crust at depth and the upwelling of anatectic migmatites within the dome. This pattern is complicated by gravitational instabilities and/or overturning of the high melt fraction crust leading to the growth of subdomes. As the migmatites within the Naxos dome reached a higher structural level, they were affected by regional top-to-the-NNE kinematics of the detachment system. Dome formation therefore occurred by a combination of coeval and coupled processes: upper crustal extension, deep crust contraction during convergent flow of anatectic crust, diapirism and/or density-driven crustal convection forming subdomes, and north directed detachment kinematics.

The trace and Rare Earth Element (REE) characteristics of igneous rocks provide significant evidence of the origin and evolution of magmas and are useful to investigate the tectono-magmatic origin of Precambrian terranes. A simple assumption is that magmas that are generated by similar geological processes possess the same geochemical attributes. These attributes facilitate a comparison through tectonic discriminatory diagrams (e.g. Floyd and Winchester, 1975; Pearce and Cann, 1973; Pearce et al., 1984; Winchester et al., 1995). Although the interpretation of geochemical data for magmatic processes and the interaction of magmas with crustal materials are sometimes complex (e.g. Castro et al., 1991; Chappell, 1996; Frost and Mahood, 1987; Neves and Vauchez, 1995), geochemical data in combination with other geological evidence have proved essential for the interpretation of the petrogenesis and tectonic setting of crustal igneous provinces.

The Okanogan gneiss dome, Washington, is located in the hinterland of the North American Cordillera and is part of a chain of metamorphic core complexes containing gneiss and migmatite domes exhumed during Eocene extension of thickened crust. U-Pb sensitive high-resolution ion microprobe (SHRIMP) analyses of zircon, monazite, and titanite, and 40 Ar-39 Ar analyses of biotite from migmatites exposed in the footwall of the Okanogan detachment, coupled with a detailed structural analysis, document the timing and duration of migmatite crystallization and indicate coeval crystallization, extensional deformation, and exhumation of the dome. Okanogan migmatites are folded and deformed, and preserve successive generations of leucosomes generated by synkinematic anatexis. Analyses of migmatite samples from a highmelt fraction subdome near Stowe Mountain suggest that the Okanogan dome records a history of migmatite crystallization spanning at least 12 m.y., as indicated by 206 Pb/ 238 U ages ranging from ca. 61 to 49 Ma for new zircon growth and rim overgrowths attributed to migmatite crystallization. Zircons from a granodiorite in a domain of diatexite near Stowe Mountain preserve rims that have a mean 206 Pb/ 238 U age of 51.1 ± 1.0 Ma for the youngest population attributed to migmatite crystallization. Zircon from folded and discordant granitic leucosome in the diatexite domain yields a calculated 206 Pb/ 238 U age of 53.5 ± 0.5 Ma for migmatite crystallization. Zircon from discordant leucosome of the metatexite domain has a mean 206 Pb/ 238 U age of 59.8 ± 0.5 Ma, with ages as young as ca. 53 Ma attributed to fi nal crystallization of the leucosome. Core domains of zircon samples have 206 Pb/ 238 U ages that range from ca. 85 to 70 Ma and are interpreted to be related to an earlier phase of the orogeny. Monazite from two samples gives 206 Pb/ 238 U crystallization ages of 52.9 ± 0.6 Ma for the granodiorite diatexite and 52.0 ± 0.6 Ma for nearby boudinaged and foliated layers of biotite granodiorite. One sample of folded granitic leucosome in metatexite contains titanite with a mean 206 Pb/ 238 U age of 47.1 ± 0.5 Ma. The ca. 47 Ma age for titanite is similar to biotite 40 Ar-39 Ar ages of 48.0 ± 0.1 Ma, 47.9 ± 0.2 Ma, and 47.1 ± 0.2 Ma for samples collected from the upper detachment surface downward over 1.5 km of structural thickness into the migmatite domain. Crystallization of the Okanogan migmatites was therefore coeval in part with upper crustal extension and ductile fl ow of the mid-crust. Leucosome crystallization largely ceased by ca. 49 Ma, followed by rapid cooling of footwall rocks through ~325 ºC by ca. 47 Ma. These data are similar to crystallization ages in migmatites from other domes in the northern Cordillera hinterland, suggesting that crustal anatexis was widespread over much of the mid-crust during Paleocene to Eocene time, coeval with extension and exhumation of orogenic middle crust.

The Okanogan gneiss dome, Washington, is located in the hinterland of the North American Cordillera and is part of a chain of metamorphic core complexes containing gneiss and migmatite domes exhumed during Eocene extension of thickened crust. U-Pb sensitive high-resolution ion micro-probe (SHRIMP) analyses of zircon, monazite, and titanite, and 40Ar-39Ar analyses of biotite from migmatites exposed in the footwall of the Okanogan detachment, coupled with a detailed structural analysis, document the timing and duration of migmatite crystallization and indicate coeval crystallization, extensional deformation, and exhumation of the dome. Okanogan migmatites are folded and deformed, and preserve successive generations of leucosomes generated by synkinematic anatexis. Analyses of migmatite samples from a high-melt fraction subdome near Stowe Mountain suggest that the Okanogan dome records a history of migmatite crystallization spanning at least 12 m.y., as indicated by 206Pb/238U ages ran...

Many migmatites represent crystallized partially molten crust and therefore record the mechanisms and pathways of orogenic crustal flow. Field and microstructural methods may be insufficient to characterize the planar and linear elements of rock fabric in migmatites due to obscured flow fabrics or protracted deformation. In the Naxos dome (Greece), we test the anisotropy of magnetic susceptibility (AMS) as a tool for recovering mineral fabric symmetry and the kinematic axes of flow in migmatites. Measurements of 155 migmatite samples yield dominantly low values (<300 × 10 −6 [SI]) of bulk magnetic susceptibility (K m) consistent with biotite being the dominant carrier of the AMS. Higher values of K m , thermomagnetic, hysteresis, and microstructural data, however, suggest a ferromagnetic contribution from magnetite in a subset of samples (N = 15). Using electron backscatter diffraction (EBSD) analysis, we establish the correspondence of the biotite subfabric with the AMS and structural fabric of the Naxos migmatites. EBSD data from biotite suggests that magnetic lineation in these dominantly paramagnetic migmatites arises from a zone axis orientation of biotite crystals organized about the direction of viscoplastic flow. Over a range of spatial scales, migmatitic foliation and magnetic foliation are well correlated. The magnetic lineation recovered by AMS displays a coherent organization despite the heterogeneous structure and composition of the Naxos migmatites. These data suggest that the apparent complexity of migmatites masks a simpler flow regime controlled by bulk viscoplastic flow. Furthermore, our study demonstrates the utility of the AMS method for studying the dynamics of partially molten orogenic crust.

Keywords: partial melting carbonatites nanogranites garnet melt inclusions nanocarbonatites
Carbonatites are peculiar magmatic rocks with mantle-related genesis, commonly interpreted as the products of melting of CO 2-bearing peridotites, or resulting from the chemical evolution of mantle-derived magmas, either through extreme differentiation or secondary immiscibility. Here we report the first finding of anatectic carbonatites of crustal origin, preserved as calcite-rich polycrystalline inclusions in garnet from low-to-medium pressure migmatites of the Oberpfalz area, SW Bohemian Massif (Central Europe). These inclusions originally trapped a melt of calciocarbonatitic composition with a characteristic enrichment in Ba, Sr and LREE. This interpretation is supported by the results of a detailed microstructural and microchemical investigation, as well as re-melting experiments using a piston cylinder apparatus. Carbonatitic inclusions coexist in the same cluster with crystallized silicate melt inclusions (nanogranites) and COH fluid inclusions, suggesting conditions of primary immiscibility between two melts and a fluid during anatexis. The production of both carbonatitic and granitic melts during the same anatectic event requires a suitable heterogeneous protolith. This may be represented by a sedimentary sequence containing marble lenses of limited extension, similar to the one still visible in the adjacent central Moldanubian Zone. The presence of CO 2-rich fluid inclusions suggests furthermore that high CO 2 activity during anatexis may be required to stabilize a carbonate-rich melt in a silica-dominated system. This natural occurrence displays a remarkable similarity with experiments on carbonate–silicate melt immiscibility, where CO 2 saturation is a condition commonly imposed. In conclusion, this study shows how the investigation of partial melting through melt inclusion studies may unveil unexpected processes whose evidence, while preserved in stiff minerals such as garnet, is completely obliterated in the rest of the rock due to metamorphic re-equilibration. Our results thus provide invaluable new insights into the processes which shape the geochemical evolution of our planet, such as the redistribution of carbon and strategic metals during orogenesis.

1] Many migmatites represent crystallized partially molten crust and therefore record the mechanisms and pathways of orogenic crustal flow. Field and microstructural methods may be insufficient to characterize the planar and linear elements of rock fabric in migmatites due to obscured flow fabrics or protracted deformation. In the Naxos dome (Greece), we test the anisotropy of magnetic susceptibility (AMS) as a tool for recovering mineral fabric symmetry and the kinematic axes of flow in migmatites. Measurements of 155 migmatite samples yield dominantly low values (<300 × 10 −6 [SI]) of bulk magnetic susceptibility (K m ) consistent with biotite being the dominant carrier of the AMS. Higher values of K m , thermomagnetic, hysteresis, and microstructural data, however, suggest a ferromagnetic contribution from magnetite in a subset of samples (N = 15). Using electron backscatter diffraction (EBSD) analysis, we establish the correspondence of the biotite subfabric with the AMS and structural fabric of the Naxos migmatites. EBSD data from biotite suggests that magnetic lineation in these dominantly paramagnetic migmatites arises from a zone axis orientation of biotite crystals organized about the direction of viscoplastic flow. Over a range of spatial scales, migmatitic foliation and magnetic foliation are well correlated. The magnetic lineation recovered by AMS displays a coherent organization despite the heterogeneous structure and composition of the Naxos migmatites. These data suggest that the apparent complexity of migmatites masks a simpler flow regime controlled by bulk viscoplastic flow. Furthermore, our study demonstrates the utility of the AMS method for studying the dynamics of partially molten orogenic crust.

Rory McFadden 1 , Christine Smith Siddoway 2 , Christian Teyssier 1 , CM Fanning 3 , and Seth C. Kruckenberg 1 . (1) Department of Geology and Geophysics, University of Minnesota, 310Pillsbury Drive SE, Minneapolis, MN 55455, (2) Department of Geology, The Colorado ...

Differentiation of the Earth into its major spherescrust, mantle and corehas proceeded dominantly through magmatic processes involving melting and melt separation. Models that describe these differentiation processes are guided by elemental abundances in the different reservoirs. Elements are fractionated between coexisting phases during partial melting, and geochemical models are generally based on the fundamental assumption that trace-element equilibrium is established between the partial melts and the restitic minerals. The element pair niobium and tantalum is key to the distinction of different melting regimes involved in crustal differentiation, but equilibrium partition models have largely failed to reproduce the Nb/Ta patterns observed in nature, posing a long-standing geochemical conundrum. Here we demonstrate that kinetic fractionation of Nb and Ta by diffusion may have produced the low Nb/Ta observed in the continental crust. On the basis of the diffusivities of Nb and Ta in rutile (TiO 2 ) determined experimentally in this study, we conclude that equilibrium cannot be expected for the natural range of grain sizes, temperatures and time scales involved in partial melting of crustal rocks. Instead, the observed fractionation of the geochemical twins, Nb and Ta, in the silicate Earth most likely proceeds by partialas opposed to completeequilibration of rutile and melt. Hence, the assumption of bulk equilibrium during partial melting for the processes of crustal differentiation may not be justified, as is demonstrated here for Nb/Ta. The concept presented here is based on kinetic fractionation melting and explains the observed low Nb/Ta ratio of the continental crust.

The Beni Bousera peridotite massif (Internal Rif , Morocco) is formed in a major part of Spinel-bearing lherzolite rimed by a layer 100m thick of garnet-bearing peridotite which is in direct contact with HP-HT granulite metamorphic rocks (16Kbar, 860°C). According to detailed recent study, this shearing contact between these two formations shows the presence of metamorphic ultramafic intercalations underneath the deformed granulites. Foliation is well distinct and allocate in both rocks with a succession of metamorphic micro-zones with very diverse mineral assemblages. Its originality comes from the spatial arrangement of 3 centimetric zones separating garnet-spinel bearing peridoties from garnet-kyanite bearing granulites: - Phlogopite, orthopyroxene, spinel zone. - Corundum, sapphirine, kornerupine zone. - Sillimanite, spinel zone. Geochemistry of the different phases shows peraluminous (corundum, kornerupine sapphirine, spinel) and magnesian (phlogopite, enstatite) assemblages, the P-T conditions estimated using thermocalc are 9 to 10Kbars and 700 to 900°C in the saphirine - sillimanite - corundum stability domain. These thermobarometric conditions reveal that these rocks -forming the top of the peridotite massif- have incurred a metamorphic evolution at high temperatures, which is related to an isothermal decompression after or during the setting up of these rocks at the base of the crust (60 km thick). However, the kornerupine could have formed sequentially under nearly constant P-T conditions (sillimanite stability field) during the infiltration of fluid in the rocks. These hydrothermal processes are also involved and drive the peridotites to a metasomatic contamination at the contact with fluid-rich crustal rocks that leads to crystallization of phlogopite and amphiboles. This contact is possibly a shear zone crossing the limit crust- mantle. The data show that in the Rif mountains the granulites and formerly diamond bearing ultrabasic mantle rocks are uplifted in the same geodynamic environment and give very strong constraints on the Alpine metamorphic evolution of the whole Alboran domain. Key words: Phlogopite, Kornerupine, Corundum, Sapphirine, Metasomatism, Peridotites, Rif