Migmatite Dome Research Articles

Time–temperature–fluid evolution of migmatite dome crystallization: Coupled U–Pb age, Ti thermometry, and O isotopic ion microprobe depth profiling of zircon and monazite

Fluid infiltration of high-grade terrains is commonly difficult to track; however, the use of secondary ionization mass spectrometry (SIMS) depth profiling analysis of zircon in high-grade metamorphic rocks allows acquisition of micron-scale U–Pb geochronology, Ti in zircon thermometry, and oxygen isotope measurements that can be used to decipher temporal and physicochemical records of orogens. This investigation focuses on a migmatite dome in a Cordilleran metamorphic core complex that records a long history of partial melting, deformation, and fluid flow during crustal thickening and/or rapid orogenic collapse. SIMS depth-profiling analyses of the outermost rims of zircon and monazite from samples representing a range of crustal levels elucidate the nature of the deep crustal environment during the final stages of extension and orogenic collapse from 58 to 50 Ma, following a major episode of granitoid emplacement and partial melt crystallization. The results from ultrathin (0.25 to 5.00 µm) outer rims of zircon demonstrate that fluid infiltration driven by amphibolite facies metamorphic devolatilization and/or igneous crystallization persisted at deep crustal levels until just before the time that the rocks were rapidly exhumed to upper crustal levels. Collectively, the results indicate that metamorphic fluids continued to infiltrate the gneiss dome from ca. 58 to 50 Ma. The overlap in ages from the zircon rims and monazite with argon cooling ages suggests that the dome cooled rapidly from > 650 °C to 300 °C from 51–47 Ma. The results further demonstrate that the evolution of high-grade terrains, including the fluid history, can be revealed in unprecedented detail through the combination of depth profiling analyses of U–Pb age, Ti thermometry, and O isotopes.

A case study of lateral spreading: the Precambrian Svecofennian Orogen

Abstract We have studied the crustal structures of the Palaeoproterozoic Svecofennian ( c. 1.9 Ga) Orogeny with the help of large scale seismic reflection surveys (FIRE 1–3), preliminary structural field work and geological and geophysical databases. The central part of the orogen is occupied by the Central Finland Granitoid Complex, which comprises two suites of granitoid rocks and associated mafic and volcanic rocks. The complex and the surrounding supracrustal belts are cut and deformed by numerous shear zones and faults; here divided into six groups. The most prominent reflections are usually shear zones or faults on outcrops. The granitoid complex is interpreted as a deep, lower-level section of an old core complex, where the younger granitoid intrusions form the basins and older granitoid intrusions and associated volcanic rocks form the horsts. The upper–middle crust detachment zone is exposed at the northeastern edge of the complex and middle crust is exposed in the migmatitic domes at northern and western margins. The seismic reflection sections display a frozen image of orogenic thickening and lateral spreading. The decoupling of the upper, middle and lower crust during spreading resulted in the formation of layered superstructure–infrastructure of the crust.

Paleocene-Eocene migmatite crystallization, extension, and exhumation in the hinterland of the northern Cordillera: Okanogan dome, Washington, USA

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 Ar39 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/

5: Late Variscan mineralizing systems related to orogenic processes: The French Massif Central

The French Massif Central constitutes an exceptional study area due to the diversity of its metallic deposits, its internal position in the Variscan belt, and the abundance of available geological, geophysical and metallogenic data obtained within the GeoFrance 3D programme. The deposits, formed towards the end of the orogenic evolution, represent the economic products of two distinct mineralizing systems, a Au ± Sb hydrothermal system and a W ± Sn and rare-metals magmatic–hydrothermal system, which were simultaneously active during a short time span between ca. 310 and 300 Ma. Two types of gold deposit can be distinguished on the basis of their depth of emplacement: “deep-seated” gold deposits developed under lithostatic to hydrostatic pressure during rapid exhumation, and “shallow” gold deposits emplaced under hydrostatic pressure with no significant uplift. Deposits of W ± Sn and rare-metals were emplaced in the upper crust during final crystallization of specialized magmas after their rapid ascent, perhaps enhanced by simultaneous regional uplift. The gold-bearing systems are associated with a complex network of re-activated crustal-scale faults initially active during the period between 335 and 315 Ma. Normal motion along the faults, coeval with 335 to 315 Ma granite–migmatite domes, played a major role in the 3D distribution of the hydrothermal plumbing system. Gold and related metals were carried within huge hydrothermal cells, which reached ca . 100 km by 10 km in area, and 30 km in depth. In contrast, granites rich in magmatophile elements (W, Sn, rare-metals) generated smaller hydrothermal cells (10 km by 10 km in area, and < 6 km deep). Extraction of metals, by both deep-seated fluids and specialized magmas, occurred during granulitization of the lower crust at 300 ± 15 Ma. In the French Massif Central, the genesis of the two late Carboniferous mineralizing systems coincided with the end of syn-collisional extension and ended just before post-collisional extension.

Flow of partially molten crust and origin of detachments during collapse of the Cordilleran Orogen

Abstract In metamorphic core complexes two types of detachments develop, coupled by flow of partially molten crust: a channel detachment and a rolling-hinge detachment. The channel detachment, on the hinterland side of the orogen, represents the long-lived interface that separates the partially molten crust flowing in a channel from the rigid upper crustal lid. On the foreland side of the core complex, a rolling-hinge detachment develops. This detachment dips toward the foreland, probably affects the whole crust, and its geometry is governed by strain localization at the critical interface between cold foreland and hot hinterland. Activation of the rolling-hinge detachment drives rapid decompression and melting, leading to the diapiric rise of migmatite domes in the footwall of the detachment. A kinematic hinge (switch in sense of shear) separates the two types of detachments. Structural, metamorphic and geo/thermochronological studies in the Shuswap core complex (North American Cordillera), combined with an anisotropy of magnetic susceptibility study of leucogranites concentrated in the detachments, suggest that this orogen collapsed rapidly through the development of channel and rolling-hinge detachments in the early Eocene. The kinematic hinge is currently located approximately 40 km west of the footwall in which it originated, corresponding to a mean exhumation rate of &gt;5 km Ma −1 , which explains the near-isothermal decompression recorded within the migmatite dome.

Ross Sea mylonites and the timing of intracontinental extension within the West Antarctic rift system

There are few direct constraints on the timing and style of faulting in the Ross Sea sector of the West Antarctic rift system, although Cretaceous plate reconstructions indicate that Ross Sea extension between East and West Antarctica occurred prior to breakup of the Gondwana margin ca. 80 Ma. Mylonitic gneisses dredged from the eastern Ross Sea indicate shear-zone deformation considerably earlier, at 98-95 Ma. Strain analysis of fab- rics indicates 85%-100% extension. Overprinting brittle structures record translation of shear-zone gneisses into the upper crust. Samples yield sensitive high-resolution ion- microprobe U-Pb zircon ages of 102-97 Ma, correlated to Byrd Coast Granite onshore, and concordant 40 Ar/ 39 Ar biotite and K-feldspar ages of 98-95 Ma, indicating that granites were mylonitized soon after emplacement and cooled rapidly. Apatite fission-track data corroborate this rapid cooling event, and reveal a second rapid cooling event ca. 80 Ma. Evidence for contemporaneous deformation and a similar thermal evolution at Deep Sea Drilling Project Site 270 on the Ross Sea central high and for a migmatite dome on land attests to the regional extent of intracontinental extension. Extension occurred at a time of complex microplate interactions along the Cretaceous active Gondwana margin, sug- gesting that distributed deformation in the overriding Antarctic plate may be related to plate boundary dynamics.

Continental subduction and exhumation of UHP rocks. Structural and geochronological insights from the Dabieshan (East China)

In the Dabieshan, the available models for exhumation of ultrahigh-pressure (UHP) rocks are poorly constrained by structural data. A comprehensive structural and kinematic map and a general cross-section of the Dabieshan including its foreland fold belt and the Northern Dabieshan Domain (Foziling and Luzenguang groups) are presented here. South Dabieshan consists from bottom to top of stacked allochtons: (1) an amphibolite facies gneissic unit, devoid of UHP rocks, interpreted here as the relative autochton; (2) an UHP allochton; (3) a HP rock unit (Susong group) mostly retrogressed into greenschist facies micaschists; (4) a weakly metamorphosed Proterozoic slate and sandstone unit; and (5) an unmetamorphosed Cambrian to Early Triassic sedimentary sequence unconformably covered by Jurassic sandstone. All these units exhibit a polyphase ductile deformation characterized by (i) a NW–SE lineation with a top-to-the-NW shearing, and (ii) a southward refolding of early ductile fabrics. The Central Dabieshan is a 100-km scale migmatitic dome. Newly discovered eclogite xenoliths in a Cretaceous granitoid dated at 102 Ma by the U–Pb method on titanite demonstrate that migmatization post-dates HP–UHP metamorphism. Ductile faults formed in the subsolidus state coeval to migmatization allow us to characterize the structural pattern of doming. Along the dome margins, migmatite is gneissified under post-solidus conditions and mylonitic–ultramylonitic fabrics commonly develop. The north and west boundaries of the Central Dabieshan metamorphics, i.e. the Xiaotian–Mozitan and Macheng faults, are ductile normal faults formed before Late Jurassic–Early Cretaceous. A Cretaceous reworking is recorded by synkinematic plutons. North of the Xiaotian–Mozitan fault, the North Dabieshan Domain consists of metasediments and orthogneiss (Foziling and Luzenguang groups) metamorphosed under greenschist to amphibolite facies which never experienced UHP metamorphism. A rare N–S-trending lineation with top-to-the-south shearing is dated at 260 Ma by the 40Ar/ 39Ar method on muscovite. This early structure related to compressional tectonics is reworked by top-to-the-north extensional shear bands. The main deformation of the Dabieshan consists of a NW–SE-stretching lineation which wraps around the migmatitic dome but exhibits a consistently top-to-the-NW sense of shear. The Central Dabieshan is interpreted as an extensional migmatitic dome bounded by an arched, top-to-the-NW, detachment fault. This structure may account for a part of the UHP rock exhumation. However, the abundance of amphibolite restites in the Central Dabieshan migmatites and the scarcity of eclogites (found only in a few places) argue for an early stage of exhumation and retrogression of UHP rocks before migmatization. This event is coeval to the N–S extensional structures described in the North Dabieshan Domain. Recent radiometric dates suggest that early exhumation and subsequent migmatization occurred in Triassic–Liassic times. The main foliation is deformed by north-verging recumbent folds coeval to the south-verging folds of the South Dabieshan Domain. An intense Cretaceous magmatism accounts for thermal resetting of most of the 40Ar/ 39Ar dates. A lithosphere-scale exhumation model, involving continental subduction, synconvergence extension with inversion of southward thrusts into NW-ward normal faults and crustal melting is presented.

The Velay dome (French Massif Central): melt generation and granite emplacement during orogenic evolution

This paper is a synthesis of available data on the Velay dome that include both small- and large-scale lithologic and structural mapping, strain analysis, isotope geochemistry, geochronology and pressure–temperature estimates. The Velay dome, one of the largest granite–migmatite domes of the Variscan Belt, formed during orogenic collapse at around 300 Ma. Its study allows an assessment of the thermal and geodynamic context leading to voluminous crustal anatexis of the Variscan orogenic crust. A first melting stage developed in connection with south-verging thrust zones during the Early Carboniferous, leading to a crustal thickening estimated at 20 km minimum. The involvement of fertile lithologies and the intrusion of plutons of deep origin contributed to the development of water-saturated melts. The volume of biotite granite extracted from melt during this period was limited. The second phase of melting, corresponded to generalized melting of gneiss achieved by biotite-dehydration melting reactions and accompanied by the generation of cordierite-bearing granites. At this stage, crustal-scale detachment faults were active and partially obliterated the earlier structures. The new structures were progressively tilted to the vertical at the margin of the Velay dome due to the southward and lateral ballooning of the granitic dome. The reconstructed P, T path indicate that the large volume of melt produced was a consequence of a significant increase in temperature at the onset of biotite dehydration melting. At the base of the crust, this melting event is coeval with granulite facies metamorphism associated to underplating of mantle-derived magmas as suggested by the geochemical signature of Late Paleozoic lower crustal xenoliths sampled by Cenozoic volcanoes and with the isotopic signature of the late granitic intrusions. Accordingly, it is proposed that asthenospheric upwelling was responsible for the temperature increase favoring melting of hydrous minerals.

Tectono‐metamorphic evolution of the Karakorum Metamorphic complex (Dassu–Askole area, NE Pakistan): exhumation of mid‐crustal HT–MPgneisses in a convergent context

AbstractThe South Karakorum margin, east of the Himalayan syntaxis, consist of an E–W elongated zone of young (10–3 Ma) high‐grade metamorphic rocks (M2) and related migmatitic domes. This late tectono‐metamorphic event post‐dates the Palaeogene (55–37 Ma) phase of thickening of the belt featured by NW–SE structures and associated M1 amphibolite facies metamorphism (0.7 GPa, 700 °C). This M2 metamorphism is characterised by low‐pressure, high‐temperature conditions coeval with migmatite formation in response to a thermal increase ofc. 150 °C compared to M1, culminating at a temperature ofc. 770 °C and a pressure of 0.5–0.6 GPa. Rapid exhumation of migmatitic domes, at a rate of 5 mm yr−1, was accommodated by vertical extrusion, in the core of E–W crustal‐scale folds. These crustal‐scale folds formed in response to N–S syn‐collisional shortening and were enhanced by thermal weakening of the migmatised continental crust.M2 metamorphism is spatially and temporarily associated with granitoids showing a mantle affinity, firmly suggesting that this could be the advective heat source for the granite and syenite generation and the subsequent migmatisation of the mid‐crustal level. Such relationships between a mantle‐related magmatism and a high‐temperature metamorphism in a convergent shortening context are suggestive of the breakoff of the subducted Indian slab since 20 Ma.

Where is the North China–South China block boundary in eastern China?

Research Article| February 01, 2001 Where is the North China–South China block boundary in eastern China? Michel Faure; Michel Faure 1Institut des Sciences de la Terre d'Orléans, Bâtiment Géosciences, Université d'Orléans, 45067 Orléans Cedex 2, France Search for other works by this author on: GSW Google Scholar Wei Lin; Wei Lin 1Institut des Sciences de la Terre d'Orléans, Bâtiment Géosciences, Université d'Orléans, 45067 Orléans Cedex 2, France Search for other works by this author on: GSW Google Scholar Nicole Le Breton Nicole Le Breton 1Institut des Sciences de la Terre d'Orléans, Bâtiment Géosciences, Université d'Orléans, 45067 Orléans Cedex 2, France Search for other works by this author on: GSW Google Scholar Geology (2001) 29 (2): 119–122. https://doi.org/10.1130/0091-7613(2001)029<0119:WITNCS>2.0.CO;2 Article history received: 20 Jun 2000 rev-recd: 13 Oct 2000 accepted: 26 Oct 2000 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Michel Faure, Wei Lin, Nicole Le Breton; Where is the North China–South China block boundary in eastern China?. Geology 2001;; 29 (2): 119–122. doi: https://doi.org/10.1130/0091-7613(2001)029<0119:WITNCS>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Qinling-Dabieshan belt results from the collision of the North China and South China blocks. The eastern extension of the belt (the Sulu area) consists of several stacked units: from top to bottom, these are (1) a weakly metamorphosed slate-sandstone unit of Neoproterozoic-Paleozoic age, (2) a gneiss-quartzite unit metamorphosed under high-pressure (P) conditions, (3) a marble-amphibolite unit, (4) an ultrahigh-P unit that was very deeply subducted and then exhumed, and (5) a migmatitic dome resulting from the melting of ultrahigh-P rocks. All these units underwent the same deformation, which was characterized by a top-to-the-northwest extensional ductile shearing, during their exhumation. Granulite facies restites in the migmatite are not significantly different from granulitized eclogites derived from the ultrahigh-P rocks. The migmatite exhibits the same structural and petrologic evolution as that determined for the other units. The resemblance of petrologic and structural features and the lack of ocean-basin rock shows that the boundary between the North China block and South China block must be placed north of the Sulu area. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Tectonics of the Dabieshan (eastern China) and possible exhumation mechanism of ultra high‐pressure rocks

The Dabieshan is divided into three tectonic domains. The Southern Dabieshan is a stack of allochtons, from top to bottom: (i) unmetamorphosed Cambrian–Early Triassic series unconformably covered by Jurassic sandstone; (ii) weakly metamorphosed Proterozoic slate; (iii) HP rocks mostly retrogressed into greenschist facies; (iv) UHP allochton; (v) UHP‐free gneisses. These units exhibit a NW–SE lineation and top‐to‐the‐NW shearing reworked by S‐verging folds. The Central Dabieshan is a migmatitic dome superimposed on UHP metamorphism and bounded by a detachement fault responsible for the exhumation of the UHP unit during its retrogression into amphibolite facies. In the Northern Dabieshan, early foliation and N–S trending compressional lineation are deformed by N‐verging folds coeval to the syn‐exhumation ductile structures of the Central Dabieshan. A geodynamic model involving coeval thrusting and normal faulting is discussed.

Structure and kinematics of Upper Cenozoic extensional detachment on Naxos and Paros (Cyclades Islands, Greece)

This paper presents a study of the ductile and brittle deformation on Naxos and Paros islands (Cyclades, Greece). Previous maps and studies of the two islands have shown that a major low‐angle fault zone separates surface rocks above the contact from an initially deep‐seated unit below, showing a metamorphic evolution from high to low pressures. Structural analysis, as well as available stratigraphical, metamorphic, and geochronological data taken together demonstrate that this fault zone is a major normal‐sense detachment zone dipping to the north. Rapid denudation of footwall rocks subsequent to high temperature metamorphism, at an estimated rate of 1.8–9.5 mm/yr, attests for tectonic unroofing during regional‐scale top‐to‐the north ductile shearing. The change from ductile to brittle behavior of the footwall rocks together with a progressive localization of high strain intensity deformations just below the hangingwall is explained by the progressive cooling of the uprisen footwall of the detachment. Mio‐Pliocene clastic sediments in the hangingwall represent the infilling of half grabens opened in between major normal faults that are synthetic to the underlying ductile shear zone. These sediments are as old as (Aquitanian‐25 Ma), or younger than the earliest recognized evidence of ductile extension in the footwall. This provides a minimum age for the onset of extension in the Cyclades, which appears significantly older than maximum ages reported up to now (13–5 Ma). Structural data strongly suggest that the detachment fault was initially rather low dipping (≈35°). An evolutionary model is proposed, in which migmatite domes in the footwall correspond to the uprise of the lower ductile crust between two separating upper crustal blocks, during a process of asymmetric boudinage of the crust. This detachment model applies to a previously thickened continental lithosphere, which then suffers thermal relaxation and weakening, allowing extensional deformation to reach a climax during and subsequent to high temperature metamorphism. In the Cyclades, crustal‐scale extension started after Early Cenozoic thrusting, while the crust was still thick, or less likely, before late underthrusting below the present surface.

Tourmaline KAr ages compared to other radiometric dating systems in Alpine anatectic leucosomes and metamorphic rocks (Cyclades and southern Spain)

KAr ages of tourmalines from Alpine anatectic leucocratic rocks in the migmatite dome on Naxos, Greek Cyclades, and along ultramafic intrusions in southern Spain are older than the corresponding mineral ages. They are interpreted as cooling ages closely approaching the age of magmatism, suggesting a closure temperature exceeding that of hornblende. RbSr whole rock analyses from metagranites and gneisses of the Palaeozoic suite of the Mulhacen Complex of the Betic Cordilleras, southern Spain, yield errorchrons of between 275 and 191 Ma. Several KAr tourmaline ages of these rock units are higher than the corresponding whole-rock ages, pointing to the incorporation of excess radiogenic Ar. Of the young tourmalines, three of them give ages of 80–85 Ma, indicating possible examples of incomplete degassing and/or possible incorporation of excess radiogenic Ar in the mineral during the Alpine metamorphism. The tourmaline ages are compared with the ages of the coexisting micas. These have a range of KAr ages consistently higher than the corresponding young Alpine RbSr ages of 15.6–12.5 Ma, pointing to a complex behavior for radiogenic Ar during the Alpine metamorphism.

Structural and metamorphic evolution of the Gabon Orogenic Belt: Collision tectonics in the lower proterozoic?

Abstract The Archaean granite-gneiss terrain of central Gabon comprises medium- to high-grade gneisses metamorphosed at 3.15 Ga, forming the Monts Cristal and underlying the Mitzic region, intruded by and enclosed as enclaves in granitoids 3.0–2.6 Ga old that form the extensive Chaillu and North Gabon blocks. These rocks are unconformably overlain in the east of Gabon by Early Proterozoic Francevillian sedimentary rocks, between 2.3 and 2.0 Ga old, forming the Franceville and Booue Basins. The Francevillian is overthrust on its west side by the Ogooue Metamorphics, themselves considered to be of Early Proterozoic age. These rocks crop out in a broad synclinorium consisting essentially of paragneisses, with orthogneisses appearing locally at the base. The overall tectonic setting is discussed in terms of the evolution of the various domains from west to east. 1. (1) On the west flank of the synclinorium is a migmatitic-granulitic dome, the Abamie Dome, in which structures diverge away from the core. Metamorphism is of high-temperature, intermediate- to low-pressure type. 2. (2) On the east flank of the Ogooue synclinorium the medium-grade Ogooue Metamorphics (sillimanite-biotite, kyanite-staurolite, staurolite-muscovite-biotite) have been thrust eastwards on to the low-grade (kyanite-sericite) Francevillian. 3. (3) The Francevillian is parautochthonous, being widely detached from its Archaean basement owing to the effects of the same eastward-directed tangential tectonics. 4. (4) Locally, slices of Archaean basement have been tectonically incorporated. The structural cross-section illustrates a history of diapiric uprise passing eastward into tangential tectonics that is typical of classical collision belts. The suture marking the collision is interpreted to be located in the Abamie migmatite dome, where the nappes are rooted. The Gabon Orogenic Belt is correlated with the collision belts of about the same age on the eastern margin of the Congo craton and with the Trans-Amazonian Orogenic Belt of the Guyana Shield.

The pre-Cretaceous deep-seated tectonics of the Abukuma massif and its place in the structural framework of Japan

The structures and microstructures of the Takanuki and Hitachi areas in the Abukuma massif, Northeast Japan are described. In the Takanuki area, the basic Gosaisho series thrusts the pelitic Takanuki ones in a HP metamorphic context. The nappe structure is afterwards refolded by a migmatitic dome: the Samegawa dome, in a HT metamorphic context. Microtectonic analysis shows that the nappe was transported from south to north along the stretching lineation. Geometric features suggest that the Samegawa dome was emplaced by diapirism. The role of the thrust surface as an instable interface promoting the doming is emphasized. The Hitachi metamorphic rocks composed of basic schist, limestone and sandstone shist thrust the pelitic rocks of the western Hitachi gneisses. As for the Takanuki area, the thrusting occurred in ductile synmetamorphic conditions with a north or northeastward displacement. Owing to lithologic, petrologic, structural similitudes, the nappe of the Hitachi metamorphic rocks and that of the Gosaisho series are unified into a unique nappe with a northward motion. The emplacement occurred between late Permian and late Cretaceous likely in late Jurassic. The allochthonous units of the Abukuma massif are correlated with the Green Schist nappe described in Southwest Japan, since they are surrounded by the same zones, namely the Tanba zone and the Kurosegawa-Kitakami one. Moreover both in Southwest and Northeast Japan, the emplacement of the Green Schist nappes is due to a shear deformation inducing rotational structures along the stretching lineation indicating the same sense of transport, that is eastward in Southwest Japan and northward in Northeast Japan, owing to the late bending of the Japanese Islands. The late Jurassic nappe structure is obliquely overprinted by a HT metamorphism, Ryoke in Southwest Japan, Abukuma in Northeast Japan, and afterwards cut by late faults as the Median Tectonic Line or the Tanakura fault, giving rise to the present complexity.

Trace element behaviour during migmatization. Evidence for a complex melt-residuum-fluid interaction in the St. Malo migmatitic dome (France)

The St. Malo migmatitic dome represents an interesting example wherein migmatites arise from the anatexis of the surrounding gneisses. Petrographical and chemical data suggest that leucosome compositions are compatible with partial melting of the quartzo-feldsphathic fraction of the parent gneiss. The contribution of the incongruent melting of biotite to the melt does not exceed 5% of the parent rock. Petrogenetic modelling based on experimental data and assuming non modal batch melting show that the K, Rb, Ca, Sr, U and Th chemical patterns of these migmatites result in fact from the interaction of several mechanisms, namely: equilibrium partial melting, mixing between melts and refractory minerals (biotite and accessories), melt removal and late hydrothermal alteration. Zr, Y and Th which are mostly hosted in accessory minerals are significantly withheld from the melts and remain stored in melanosomes (metatexites) except when leucosomes are affected by mixing (diatexites). U is frequently enriched in the leucosomes as well as in some melanosomes suggesting external supply.

Dating the events of metamorphism and granitic magmatism in the Alpine orogen of Naxos (Cyclades, Greece)

An isotopic dating investigation (66 K-Ar and 34 Rb-Sr analyses) provided the geochronological framework for the Alpine events of metamorphism and granitic magmatism on Naxos. The oldest phase of high-pressure/medium-temperature metamorphism, M1, was dated by Rb-Sr and K-Ar analyses of paragonites, phengitic muscovites and muscovites at 45±5 Ma (Middle Eocene). Most of the record of the M1 phase has been eraded by a second phase of medium-pressure/high-temperature metamorphism, M2, which induced a metamorphic zonation with anatectic melting in the highest-grade part, the migmatite dome. Most K-Ar dates of M2 hornblendes, muscovites, biotites and tourmalines range from about 21 Ma in the lower-grade part (biotite-chloritoid zone) to about 11 Ma in the migmatite dome. From the pattern of K-Ar mineral dates it is concluded that the M2 phase took place 25±5 Ma ago (Late Oligocene/Early Miocene) and was followed by a prolonged cooling history until about 11 Ma ago (Late Miocene), when the ambient temperature in the migmatite dome had decreased to below 400−360 °C. A Rb-Sr whole-rock isochron analysis of a granodioritic mass dated the intrusion (and the associated M3 phase of contact-metamorphism) at 11.1±0.7 Ma (Late Miocene), with an initial 87Sr/86Sr ratio of 0.7112 ±0.0001. A local phase of low-grade retrograde metamorphism, M4, probably related to Late Alpine overthrusting, was dated at about 10 Ma (Late Miocene).

Carbon, hydrogen, and oxygen isotope studies of the regional metamorphic complex at Naxos, Greece

At Naxos, Greece, a migmatite dome is surrounded by schists and marbles of decreasing metamorphic grade. Sillimanite, kyanite, biotite, chlorite, and glaucophane zones are recognized at successively greater distances from the migmatite dome. Quartz-muscovite and quartz-biotite oxygen isotope and mineralogie temperatures range from 350 to 700°C. The metamorphic complex can be divided into multiple schist-rich (including migmatites) and marblerich zones. The δ 18O values of silicate minerals in migmatite and schist units and quartz segregations in the schist-rich zones decrease with increase in metamorphic grades. The calculated δ 18O H 2O values of the metamorphic fluids in the schist-rich zones decrease from about 15‰ in the lower grades to an average of about 8.5‰ in the migmatite. The δD values of OH-minerals (muscovite, biotite, chlorite, and glaucophane) in the schist-rich zones also decrease with increase in grade. The calculated δD H 2O values for the metamorphic fluid decrease from −5‰ in the glaucophane zone to an average of about −70‰ in the migmatite. The δD values of water in fluid inclusions in quartz segregations in the higher grade rocks are consistent with this trend. Theδ 18O values of silicate minerals and quartz segregations in marble-rich zones are usually very large and were controlled by exchange with the adjacent marbles. The δD values of the OH minerals in some marble-rich zones may reflect the value of water contained in the rocks prior to metamorphism. Detailed data on 20 marble units show systematic variations of δ 18O values which depend upon metamorphic grade. Below the 540°C isograd very steep δ 18O gradients at the margins and large δ 18O values in the interior of the marbles indicate that oxygen isotope exchange with the adjacent schist units was usually limited to the margins of the marbles with more exchange occurring in the stratigraphic bottom than in the top margins. Above the 540°C isograd lower δ 18O values occur in the interior of the marble units reflecting a greater degree of recrystallization and the occurrence of Ca-Mg-silicates. Almost all the δ 13C values of the marbles are in the range of unaltered marine limestones. Nevertheless, the δ 13C values of most marble units show a general correlation with δ 18O values. The CO 2 H 2O mole ratio of fluid inclusions in quartz segregations range from 0.01 to 2. Theδ 13C values of the CO 2 range from −8.0 to 3.6‰ and indicate that at some localities CO 2 in the metamorphic fluid was not in carbon isotopic equilibrium with the marbles.

Magnetic Anomalies of Precambrian Shields and some Problems of their Geological Interpretation

The predominance of extended zones of linear anomalies as well as of curved, ring-shaped, and other complicated anomalies, is characteristic of the magnetic field over ancient shields and platforms. The zones of linear anomalies reflect large tectonic features of the ancient geosynclines—deep faults and the associated structural facies zones. Curved anomaly zones characterize regions of deep-seated folding which are peculiar to ancient shields and platforms.Three large groups of deep structures may be distinguished in terms of the magnetic effects they produce. The first group includes domes (arches) composed essentially of nonmagnetic granitoids surrounded by stable magnetically active metamorphic zones. Structures of this group are most extensively developed in the inner zones of ancient geosynclines.A characteristic feature of the second group is magnetic anomalies over migmatite domes, whose minima extend over bordering synclines composed of nonmagnetic rocks. The trend of individual magnetic horizons is concordant with the direction of banding in the migmatites.The third group includes deep structures whose synclinal and anticlinal forms are composed of rocks showing an increased magnetization. Mapping of such structures in the magnetic field presents considerable difficulties.The autocorrelation functions of anomalous magnetic fields over ancient shields indicate the intricate nature of the fields. A zero-correlation radius of a high-frequency component ranges from 5 to 10 km; that of a long-wave component amounts to about 40 km.Two layers corresponding to the sources of anomalies are identified in the crustal cross section. The depths to lower boundaries of the bodies responsible for the local magnetic anomalies have shown that most of the sources are confined to a surface layer which is not more than 10 km thick. The source of regional magnetic anomalies is attributable to magnetic inhomogeneities of the earth's crust, lying at depths from 10 to 30 km.

De quelques particularites de la chaine alpine d'Hidaka au Japon

Abstract The Hidaka alpine range is characterized by metamorphism in the andalusite-sillimanite-cordierite facies and is located at the southern end of the SSE-NNW-trending tectonic arc of Hokkaido. The axial zone of the Hidaka range is composed of migmatites which formed at the expense of metamorphic rocks in the surrounding series of Mesozoic age. Gabbros, some of which contain peridotite, outcrop in a longitudinal zone paralleling the structural trend. The concomitant relationship between these two series is suggested by the occurrence of gabbros in or on the borders of the migmatite domes and by the causal relationship between the basic intrusions and the migmatizations.