Showing papers on "Metamorphism published in 1997"

The Lu–Hf dating of garnets and the ages of the Alpine high-pressure metamorphism

Abstract: It remains controversial whether burial and exhumation in mountain belts represent episodic or continuous processes1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20. Regional patterns of crystallization and closure ages of high-pressure rocks may help to discriminate one mode from the other but, unfortunately, metamorphic geochronology suffers from several limitations. Consequently, no consensus exists on the timing of high-pressure metamorphic events, even for the Alps—which have been the subject of two centuries of field work. Here we report lutetium–hafnium (Lu–Hf) mineral ages on eclogites from the Alps as obtained by plasma-source mass spectrometry. We find that the Lu/Hf ratio of garnet is particularly high, which helps to provide precise ages. Eclogites from three adjacent units of the western Alps give (from bottom to top) diachronous Lu–Hf garnet ages of 32.8 ± 1.2, 49.1 ± 1.2 and 69.2 ± 2.7Myr. These results indicate that the Alpine high-pressure metamorphism did not occur as a single episode some 80–120Myr ago6,7,10,18, but rather that burial and exhumation represent continuous and relatively recent processes.

Zirconium abundance in granulite-facies minerals, with implications for zircon geochronology in high-grade rocks

Abstract: Zircon growth in high-grade metamorphic rocks may be triggered by net transfer reactions involving the breakdown of a phase bearing zirconium (Zr). We have measured the concentration of Zr in the major minerals in granulite-facies rocks of differing bulk composition. Both garnet and hornblende contain tens of parts per million Zr, and no other major phase contains significant Zr. Simple calculations show that reaction of either garnet or hornblende to form non–Zr-bearing phases will release sufficient Zr to account for at least some new zircon growth. U-Pb ages from new zircon, grown as a result of either hornblende or garnet breakdown, are not expected to record the time of peak metamorphism, but rather will record the time of particular metamorphic reactions, allowing direct correlation of zircon ages with petrologically derived pressure-temperature-time paths. This approach offers the potential for more rigorous interpretation of the metamorphic significance of zircon ages than has previously been possible.

Do U-Pb zircon ages from granulites reflect peak metamorphic conditions?

Abstract: Granulite facies metamorphic events are constrained commonly through application of U-Pb zircon geochronometry. Zircon growth related to high-grade metamorphism is interpreted as reflecting the age of peak pressure-temperature ( P-T ) conditions. However, these ages obtained from granulites need to be interpreted with considerable care. Under conditions of high-grade metamorphism, it is important that the possible presence of melt is considered. Our modeling of partial melting and its impact on zircon stability implies that zircon crystallization in hot, isothermally uplifted granulites could postdate the pressure peak of the P-T path. In a case study of felsic granulites from the Bohemian massif of Variscan central Europe, it appears likely that most zircons in the rocks would have grown after they were exhumed to medium pressure levels. Thus, zircon growth related to high-grade metamorphism should not be automatically assumed as reflecting the age of peak P-T conditions.

New evidence for coesite in eclogite and gneisses: Defining an ultrahigh-pressure province in the Western Gneiss region of Norway

Abstract: Widespread evidence of ultrahigh-pressure metamorphism is reported from the Nordfjord and Stadtlandet area of the Western Gneiss region in Norway, outlining a distinct ultrahigh-pressure province. Coesite has been found as inclusions in garnet, omphacite, or kyanite in eclogite from 5 new localities, and quartz pseudomorphs after coesite occur in eclogite minerals at an additional 12 localities. Quartz pseudomorphs after coesite, within garnet and clinozoisite, are reported for the first time from associated gneisses, which is evidence for in situ ultrahigh-pressure metamorphism, at least at a local scale. The boundary to an ultrahigh-pressure province is defined by a metamorphic break from lower pressure ( 2.8 GPa) within a 10-km-wide mixed zone in the Nordfjord area. This zone may mark the disrupted tectonic boundary to a small ultrahigh-pressure unit and casts doubt on the assumption that the Western Gneiss region behaved as a coherent terrane during burial and exhumation.

Geochemistry of the Longsheng Ophiolite from the southern margin of Yangtze Craton, SE China

Abstract: Geochemical and Sm-Nd isotopic data are presented for basaltic volcanic rocks from the Longsheng Ophiolite, which were crystallized at 977 ± 10 Ma and tectonically emplaced into the Danzhou Group (Banxi equivalent) along the southern margin of the Yangtze Block, SE China during the Neoproterozoic. The volcanic rocks have undergone low-grade alteration and metamorphism, displaying high Na2O contents (>5%) and a large variation of mobile elements, but a relatively small range of immobile elements. With enrichment of Th and LREEs, relatively low high-field-strength elements and other REEs, and the Ta-Nb-Ti negative anomaly, the volcanic rocks are geochemically similar to basalts forming in subduction-related environments, typical of the “supra-subduction zone” ophiolites. Nd isotopes of the volcanic rocks preclude a MORB-like depleted mantle source for the Longsheng Ophiolite, though their Sm-Nd isotopic systems have been rehomogenized during the late Neoproterozoic alteration. The Longsheng volcanic rocks, therefore, are interpreted to have been generated either by extension and thinning of a continental arc, or at a backarc side of continental arc during the early stage of the backarc spreading.

Back arc extension and denudation of Mediterranean eclogites

Abstract: Eclogite-facies rocks exposed in Mediterranean back arcs are delimited from above by low-angle normal faults and detachments. Nevertheless, our work demonstrates that these extensional structures associated with back arc extension played only a limited role in removing the overburden from above the eclogites. Extension in Mediterranean back arcs began in the late Oligocene or early Miocene, but the pressure - temperature - time (P-T-t) paths of eclogite-facies rocks exposed in these areas indicate that a major part of the overburden, several tens of kilometers, has been removed from above these rocks prior to the Oligo-Miocene. We show that the time period bracketed between the peak of eclogite metamorphism (Eocene in the central Aegean, probably Upper Cretaceous in Corsica and the Betics) and the onset of back arc extension in the Oligo-Miocene was characterized by thrust faulting. In the central Aegean, Corsica, and the Betics, eclogite-bearing units were partly unroofed and then overthrusted lower-pressure units. We emphasize that, with one exception (Tinos island, Greece), the entire inventory of extensional contacts operated subsequently to the overthrusting of the eclogites above the lower-grade sequences. Thus Mediterranean back arc extension lags behind a major part of the denudation process, and is superposed on orogenic wedges that contain eclogite-facies rocks at relatively shallow structural levels. We emphasize that the mode of occurrence of eclogites in Mediterranean back arc regions involves a continuum of in-situ crustal accretion below the eclogites, widespread P-T paths that show cooling or isothermal decompression, and lower-grade rocks at the bottom of the structural pile. Thus instead of reflecting whole - crust back arc extension, the tectonic style associated with the denudation of Mediterranean eclogites better fits an active accretionary-wedge setting. This is similar to the mode of occurrence of eclogite-facies rocks in mountain belts, such as the western Alps, where decompression was synorogenic and back arc extension played no role.

Eclogitic metasediments from the tso morari area (ladakh, himalaya) : evidence for continental subduction during india-asia convergence

Abstract: Metasediments in the Tso Morari area (Ladakh, Himalaya) provide new insights into the Higher Himalayan metamorphism in the northwestern part of the Himalayan belt. Whole-rock analyses and petrologic observations show that the metasediments correspond to Fe-rich metapelites, Mg-rich metapelites, intermediate metapelites and metagreywackes of the Indian continental margin. Jadeite + chloritoid + paragonite + garnet in the Fe-rich metapelites indicate pressures of 20 ±2 kbar at temperatures of 550 ±50 °C according to major element partitioning thermobarometry, stability fields of minerals and Thermocalc P-T estimates. These results are consistent with P-T estimates on other metasediments and with the occurrence of eclogites. Subsequent retrogression at the eclogite-blueschist facies transition (from 18 to 13 kbar and 540 ±50 °C) was followed by an increase in temperature to 630 ±30 °C at amphibolite facies conditions. The metamorphic evolution is related to subduction of the Indian continental margin beneath the southern Asian margin at the onset of the Indian-Eurasian collision.

Late Archaean to early Proterozoic granitoid magmatism and high-grade metamorphism in the central Limpopo belt, South Africa

Abstract: The Central Zone of the Limpopo belt in southern Africa has previously been interpreted as a segment of Archaean crust which experienced its main deformation and metamorphism around 2.7 Ga ago. We report new single zircon U/Pb and Pb/Pb ages for granitoid gneisses, supracrustal rocks and anatectically derived granitic melt patches in the area around Messina, South Africa. The Sand River Gneiss is a composite suite of tonalitic to trondhjemitic rocks with protolith ages between 2.6 and 3.2 Ga. The Singelele gneiss, a heterogeneous granodioritic to quartz monzonitic rock, has protolith ages between 2.55 and 2.58 Ga. Since both the Sand River and Singelele gneisses experienced polyphase high-strain ductile deformation this must have occurred later than 2.55 Ga ago. Granulite-facies pelitic gneisses of the Beit Bridge Complex contain abundant spherical, multifacetted zircons which reflect new zircon growth near or at the peak of metamorphism. These zircons provide ages with a mean at 2026.5±6.3 Ma which is interpreted as reflecting a high P – T event (>10 kbar, 825±25°C). Granitic melt patches in the metapelites as well as in the Sand River Gneiss and anatectic granites are probably related to rapid near-isothermal decompression to below 3–5 kbar and 600–750°C). These rocks contain new magmatic zircons which yielded a mean age of 2005.6±4.4 Ma and probably reflect a crustal melting event resulting from rehydration of the granulitic assemblage. Our zircon data support previous suggestions for only one single granulite-facies event in the Central Zone, and we suggest that this event occurred c. 2027 Ma ago. Since most of the deformation seen in the gneisses of the Messina area must have occurred later than 2.55 Ga ago, it is likely that the ‘Limpopo Orogeny’, at least in the Central Zone, is not an Archaean event but took place in the early Proterozoic.

Proterozoic Events in the Eastern Ghats Granulite Belt, India: Evidence From Rb‐Sr, Sm‐Nd Systematics, and Shrimp Dating

Abstract: Metamorphic and protolith ages of five rock types (mafic granulite, orthopyroxene granulite, leptynite, sillimanite granite, and metapelite) from Rayagada, in the north‐central part of the Eastern Ghats Granulite Belt (EGGB), India, were determined from Rb‐Sr and Sm‐Nd whole rock and mineral isochrons in combination with SHRIMP U‐Pb zircon data. Most of the whole rock isochron ages in both Sm‐Nd and Rb‐Sr systems point to either ∼1450 or ∼1000, Ma, and the mineral isochron ages are ∼1000, ∼800, and ∼550 Ma. SHRIMP U‐Pb zircon ages of ∼940 Ma were obtained from metapelite, which are in close agreement with the Sm‐Nd and Rb‐Sr isochron ages. From all these data, four age clusters (∼1450, ∼1000, ∼800, and ∼550 Ma) have been noted. The 1450 Ma ages are interpreted to represent igneous protolith formation of mafic granulite and leptynite. The 1000 Ma age cluster is regarded as the intrusion ages of sillimanite granite, and charnockite, and associated granulite facies metamorphism. Two other age clusters (800 a...

Metamorphic field gradients in the Central Alps

Abstract: Metamorphic field gradients were determined across the entire amphibolite grade Central Alps (c. 50×100 km). P–T were calculated from 116 samples acquired from our own field work, from samples provided to us by others, and from rocks with mineral compositions described in the literature. Only fluid-conserved equilibria were used to determine P–T . The use of an internally consistent thermodynamic database and mineral solid solution models makes the results robust and reduces relative errors. The results are presented in contour maps. Temperature increases from 500 to 550 °C along the limit of amphibolite grade metamorphism in the north and west, to c. 675 °C toward the south at the Insubric line near the town of Bellinzona. Maximum recorded pressures of c. 7 kbar are in a central region c. 20 km north of the Insubric line, and decrease both to the north (5.5 kbar) and south (4.5 kbar). The P–T results indicate that there is a relatively large area that reached conditions in the sillimanite stability field but developed neither sillimanite nor fibrolite; this is interpreted as a result of kinetic constraints on nucleation and growth because of the small amounts of thermal overstep (<40 °C) of the kyanite-sillimanite phase boundary. Comparison of P–T conditions with carbonate isograds in the region indicate that fluids present during metamorphism were not dominated by a homogeneous external source. Examination of the two-dimensional distribution of pressure and temperature in the context of thermal and tectonic models indicates that two thermal pulses affected the Central Alps during the Tertiary. In the second, heat affected only the southern parts of the area and overprinted the previously established P–T gradients.

Eclogites and Blueschists of the Pam Peninsula, NE New Caledonia: a Reappraisal

Abstract: High-P rocks of the Pam Peninsula, NE New Caledonia, are divided into three zones: (1) an uppermost ferroglaucophane-lawsonite zone of Cretaceous to Eocene metasediments and metavolcanics of the Diahot terrane that experienced peak conditions involving P=7-9 kbar and T=400±58°C; (2) albite-epidote-omphacite zone Diahot terrane rocks that experienced blueschist facies conditions of P=12·6±1·2 kbar and T=570±36°C; (3) lowermost metabasic eclogites of uncertain age that form the Pouebo terrane, which experienced high-P conditions of P=23·9±3·0 kbar and T∼600°C. Eclogite occurs as metre- to kilometre-scale pods in coarse grained hydrous mineral-rich 'glaucophanite' formed during hydration and decompression of the Pouebo terrane. Metamorphism and deformation were consequent to 44-51 Ma Eocene convergence, when sedimentary and ophiolitic nappes were thrust over the eclogites in a SW direction; white mica ages constrain metamorphism to have ended by 37±1 Ma. Large steps in metamorphic grade are coincident with SW-dipping and NE-dipping faults that separate the three zones and were formed during two stages: (1) comparatively slow uplift and hydration of the Pouebo terrane before it was juxtaposed with the albite-epidote-omphacite zone at P∼14 kbar; (2) comparatively rapid uplift of both the Pouebo terrane and the albite-epidote-omphacite zone to form a domal core of eclogite flanked by significantly lower-grade rocks to the SW and NE.

Geochronology of the northern Idaho batholith and the Bitterroot metamorphic core complex: Magmatism preceding and contemporaneous with extension

Abstract: Granitic plutonism and extension are broadly contemporaneous in many metamorphic core complexes. However, the relationship between magmatism and extension is rarely unambiguous. The northern Idaho batholith (Idaho-Bitterroot batholith), Montana and Idaho, composes the footwall for most of the Bitterroot metamorphic core complex and thus is an ideal area for assessing the relationships between magmatism and extension. We analyzed zircon from six samples of granitic rock from the Idaho-Bitterroot batholith using the SHRIMP (II) ion microprobe. Three samples of mylonitic granite from the Bear Creek pluton, Lost Horse Canyon, give a weighted mean 206 Pb/ 238 U age of 54.3 ± 0.7 Ma. A protomylonitic granite from the central part of the Bitterroot core complex (also Bear Creek pluton) gives a similar 206 Pb/ 238 U age of 54.6 ± 0.8 Ma. Mylonitic megacrystic granite from Sweathouse Canyon yields an age of 63.6 ± 0.6 Ma. A granite sample from the Lochsa Canyon, in the central Idaho-Bitterroot batholith, gives an age of 56.7 ± 1.0 Ma. Inherited zircon from the granitoids ranges in age from 800 to 1820 Ma, but the majority of grains have formation ages of 1750–1800 Ma. This suggests that Paleoproterozoic crust dominates the source region of the Idaho-Bitterroot batholith. Hornblende 40 Ar- 39 Ar age spectra for mafic dikes intruded during late-stage crystallization of main-phase granite in the central Idaho-Bitterroot batholith suggest crystallization of the main-phase plutons in this area at ca. 57 Ma. New and previously published 40 Ar- 39 Ar and K-Ar apparent ages of biotite and muscovite from the Lochsa River area and the western and central Bitterroot core complex are 50 to 47 Ma. Younger mica ages (46–43 Ma) are restricted to the vicinity of the Bitterroot mylonite zone. These results indicate that the cessation of main-phase magmatism within the Bitterroot metamorphic core complex migrated east with time, and that most of the plutons in the core complex were intruded during the Paleocene and early Eocene. When the regional tectonic setting changed from compression to extension at ca. 50 Ma, the late stages of mid-crustal, peraluminous plutonism appear to have been localized within the Bitterroot core complex. The presence of the youngest mid-crustal plutons in this area may have focused extensional deformation leading to the thick mylonite zone, as a consequence of rheological contrasts with cooler areas to the east and west. A progression of K-Ar and 40 Ar- 39 Ar cooling ages from west to east within the core complex part of the batholith is consistent with top-to-the-east shear indicators in the mylonite zone. Thermochronology indicates that the western part of the Bitterroot metamorphic core complex was below ≈350°C at the same time as the last stage of granite emplacement and metamorphism in the east. Therefore, the transition from mylonitization to brittle deformation to inactivity of the shear zone was progressive from west to east across the core complex from ca. 50 to 44 Ma. These features offer an explanation for the previously enigmatic occurrence of amphibolite facies ductile deformation in the eastern part of the core complex coincident with emplacement of epizonal, alkali-feldspar granite plutons along the western side of the complex.

Paleofluids and recent fluids in the upper continental crust: results from the German Continental Deep Drilling Program (KTB)

Abstract: The two German Continental Deep Drilling Program (KTB) boreholes provided samples for studies of fluid inclusions (paleofluids), “free fluids” of the crystalline basement, and their fingerprints on the chemical and isotopic composition of minerals and rocks to 9101 m depth, which allowed reconstruction of the evolution of fluids, their migration pathways, and their sources. Aqueous fluids were largely lost during the Devonian amphibolite facies metamorphism. Thereafter, radiogenic, nucleogenic, and fissiogenic gases, together with NH4-fixed nitrogen, were released from host rocks and partly enclosed in secondary inclusions. During the Hercynian uplift, Na-Cl fluids (formation water) infiltrated and dissolved noble gases and N2 largely originating from the host rocks. In the course of the Cretaceous denudation, high-salinity Ca-Na-Cl brines, possibly derived from Permo-Carboniferous sediments but altered by fluid/rock interaction, migrated into their present position. This fluid introduced low-maturity hydrocarbons released together with nitrogen from early metamorphic organic-rich sediments. The 4000-m fluid from the KTB pilot hole pumping test, which was analyzed chemically and isotopically, seems to be a mixture of an ascending basement brine and a descending paleometeoric water, from which the late alteration minerals calcite and laumontite precipitated in fractures. The calcite is neither chemically nor isotopically in equilibrium with the recovered “free fluid.” Hydraulic tests indicate a communicating system of fractures between the boreholes with a distinct matrix and fracture porosity.

Glaucophane-bearing eclogites in the Tso Morari dome (eastern Ladakh, NW Himalaya)

Abstract: Lenses of glaucophane-bearing eclogites have been discovered within the Cambro-Ordovician metagranitic basement and in the Permian metasedimentary cover of the Tso Morari dome (eastern Ladakh, India). Petrological and thermobarometrical evidence shows that these rocks underwent a high-pressure event (P> 16 kbar, probably near 20±3 kbar) associated with relatively low-temperature conditions (T = 580±60 °C). The beginning of decompression was isothermal down to l 1 ± 2 kbar. Subsequently, temperature increased to 610±70°C under amphibolite-facies conditions. This record of high-pressure and relatively low-temperature metamorphism in the Himalayan belt implies that an early subduction of the whole NW part of the Indian continental margin to a minimum depth of 70+10 km occurred during the India-Asia convergence. Two different units can be distinguished in the Higher Himalaya of Ladakh-Zanskar, recording two successive and distinct orogenic events: (i) the North Himalayan Massif related to the continental subduction and (ii) the High Himalayan Crystalline slab related to the intracontinental subduction.

Dating the shock wave and thermal imprint of the giant Vredefort impact, South Africa

Abstract: U-Pb geochronology of single grains of zircon and monazite has been used to date an episode of intense postimpact metamorphism in the core of the deeply eroded Vredefort impact structure of South Africa. Results from two basement units exposed in the uplifted central region indicate that the impact and a later pyroxene hornfels metamorphic event were penecontemporaneous at 2020 ± 3 Ma. Discovery of a synimpact to postimpact dike of norite that intruded at 2019 ± 2 Ma is the first recognition of mafic igneous activity related to impact. The dike is either derived from a Sudbury-type impact melt layer (since eroded) or is the product of decompression melting of Kaapvaal mantle in response to the ablation of >15 km of crust at the center of the crater. The combined heating effects of the shock wave and impact-triggered magmas are thought to have created the 300 km 2 thermal imprint of the asteroid collision with Kaapvaal craton, and account for the nearly coeval timing relationship between core metamorphism and shock revealed by this study.

Metamorphic field gradients in the Central Alps

Abstract: Metamorphic field gradients were determined across the entire amphibolite grade Central Alps (c 50◊100 km) P-T were calculated from 116 samples acquired from our own field work, from samples provided to us by others, and from rocks with mineral compositions described in the literature Only fluid-conserved equilibria were used to determine P-T The use of an internally consistent thermodynamic database and mineral solid solution models makes the results robust and reduces relative errors The results are presented in contour maps Temperature increases from 500 to 550°C along the limit of amphibolite grade metamorphism in the north and west, to c 675°C toward the south at the Insubric line near the town of Bellinzona Maximum recorded pressures of c 7 kbar are in a central region c 20 km north of the Insubric line, and decrease both to the north (55 kbar) and south (45 kbar) The P-T results indicate that there is a relatively large area that reached conditions in the sillimanite stability field but developed neither sillimanite nor fibrolite; this is interpreted as a result of kinetic constraints on nucleation and growth because of the small amounts of thermal overstep (<40°C) of the kyanite- sillimanite phase boundary Comparison of P-T conditions with carbonate isograds in the region indicate that fluids present during metamorphism were not dominated by a homogeneous external source Examination of the two-dimensional distribution of pressure and temperature in the context of thermal and tectonic models indicates that two thermal pulses aected the Central Alps during the Tertiary In the second, heat aected only the southern parts of the area and overprinted the previously established P-T gradients

A revised Archaean chronology for the Napier Complex, Enderby Land, from SHRIMP ion-microprobe studies

Abstract: The long and complex Archaean evolution of the Napier Complex of Enderby Land, characterized by high-grade metamorphism and several strong deformations, is reassessed in the light of new SHRIMPU–Pb zircon dating results bearing on the ages of protoliths and possible regional extents of distinct Archaean tectonothermal events. Initial felsic igneous activity occurred over a significant time interval c. 3800 Ma ago. An age of 2980±9 Ma for the emplacement of charnockite at Proclamation Island might date the oldest tectonothermal event to be recognized in the Napier Complex. An ensuing, very-high grade, previously imprecisely dated tectonothermal event occurred at 2837±15 Ma. U–Pb zircon ages ranging from 2456+8/−5 Ma to 2481±4 Ma date a subsequent, protracted high-grade tectonothermal event. Whereas the ~2840 Ma event is of regional importance in the Amundsen Bay-Casey Bay area, it is possible that the ~2980 Ma event was of only moderate grade, minor importance, or even absent, in that part of the Complex. If so, the apparent trend to very-high temperature metamorphism in the Tula and Scott mountains compared with the Napier Mountains may reflect two distinct metamorphic events rather than a simple baric and thermal gradient. The oldest crustal component in the Napier Complex appears to have been of igneous derivation. Zircon populations in paragneisses at Mount Sones are similar to those in the nearby orthogneisses, which therefore may have been basement. Another paragneiss, in the Casey Bay area, yields no zircons older than 2840 Ma, probably indicating that pre-3000 Ma crust, which is now located nearby, was not exposed at the time of sedimentation there. The isotopic data are quite complex, particularly in rocks that experienced postcrystallization metamorphic temperatures of 1000°C or more. It is postulated that this complexity, which was largely the product of migration of radiogenic Pb within the zircon grains in ancient times, and produced local excesses of this element with respect to its parent U, was caused by volume diffusion at these abnormally high regional crustal temperatures.

An amagmatic origin of carlin-type gold deposits

Abstract: Carlin-type deposits are major sources of gold, yet their origins are enigmatic. Suggested genetic models make connections to magmatism, regional metamorphism, or regional extension. Depositional mechanisms are uncertain as well. We propose on the basis of geologic, physical, and chemical reasoning, a genetic model in which meteoric fluids were circulated by heat released during crustal extension. These fluids interacted at depth with the sedimentary rock pile and scavenged gold. Upon upwelling, these fluids interacted with various lithologies and/or other fluids and produced the characteristic alteration and metal suites of these deposits. To test the viability of this amagmatic model, we have investigated certain physical and chemical constraints implicit to the model. Heat balance calculations indicate that ample surface waters could be heated to appropriate temperatures and circulated during rapid crustal extension. Mass balance calculations indicate that solution transport efficiencies of <10 percent can account for several times the amount of known alteration and mineralization. In contrast, magma-driven or metamorphic fluids appear to be less adequate sources. A two-step, source to deposit approach was used to simulate fluid-rock interaction and mass transport implied in the amagmatic model. Fluids with a fixed Sigma Cl were first equilibrated with various potential source rocks (arkose, graywacke, and pelite, each with various f O2 and f S2 buffer assemblages) at 300 degrees C and 1 kbar to simulate reaction at depth, perhaps near the brittle-ductile transition. These fluids were then reequilibrated under conditions indicated for deposit formation (225 degrees -150 degrees C, 0.5 kbars) to investigate processes including cooling, wall-rock reaction, and mixing which could be important to ore deposition. Gold transport was favored for low-chlorinity, intermediate oxidation state (below pyrrhotite-pyrite-magnetite), slightly acid fluids from sulfide-bearing sources (i.e., equilibration with arkosic rocks). These conditions are not conducive to base metal transport and explain the rarity of base metals in Carlin-type systems. When reacted under deposit level conditions, these calculated ore fluids leach carbonate and deposit quartz. The volume ratio of carbonate dissolved to quartz precipitated is approximately 0.2 to 2.5, and increases when mixing with a second fluid is considered. Arkose-equilibrated source fluids can precipitate gold by combinations of wall-rock reaction with a decarbonated host, mixing with various sulfide-poor fluids, and reaction with iron-rich host rocks. Cooling alone is ineffective over this temperature interval. Although most deposits are hosted by decarbonated marls, geochemical calculations suggest that other depositional environments are feasible, consistent with observation. Although here applied to the amagmatic hypothesis, these chemical calculations are not mechanism specific and might also be appropriate to metamorphic or magma-driven hydrothermal systems. These results demonstrate that Carlin-type deposits can result from the coincidence of an effective driving force (rapid, major crustal extension), a source of dilute fluids, and chemically appropriate sets of both source and host rocks. The aqueous chemistry involved is unexceptional and shares many features with other types of moderate- to low-temperature gold-bearing hydrothermal systems--it is the geologic setting, and perhaps the fluid driving force, that is unusual. An amagmatic model is consistent with both deposit-scale and regional-scale observations; it rationalizes Carlin-type systems in the framework of southwestern North American metallogeny and it implies multiple possible mechanisms for gold mineralization. Detachment-type gold systems may be the high-oxidation state, high-salinity analogues of Carlin-type systems, formed by major extension, but from contrasting lithologic and fluid sources.

L'Evolution polycyclique de la chaine hercynienne

Abstract: The Mid-European Palaeozoic Belt is often considered as the result of a continuous continental convergence from late Silurian to Carboniferous. However, some stratigraphic, magmatic and structural data support a discontinuous evolution model. In late Silurian, the Eo-Variscan stage corresponds to a continental subduction responsible for a high pressure metamorphism. During their exhumation, the high pressure rocks are partly melted. In North Massif central, the Eo-Variscan rocks are already outcropping in Middle Devonian. Since that time, evidence for a calc-alkaline magmatic arc is found in the Massif central. Vosges and Massif armoricain. This arc is due to the southward subduction of the Rheic Ocean. Tensional events such as the Brevenne rift, ligne des klippes and Bolazec areas in Massif central. Vosges and Massif armoricain respectively occur in the upper plate. These lines of evidence show that the Medio-Variscan period is heterogeneous since extensional tectonics are followed by compressional ones belonging to the Hercynian Belt proper. Therefore, the Variscan belt results of two orogenic cycles.

U-Pb isotopic evidence for the accretion of different crustal blocks to form the Lewisian Complex of northwest Scotland

Abstract: Single zircon and titanite U-Pb SHRIMP data presented for tonalite-trondhjemite-granodiorite (TTG) suite gneisses and an ultramafic rock from the northern and central regions of the Lewisian Complex of northwest Scotland, show that protolith ages of tonalitic gneisses in the northern region (2800–2840 Ma) are significantly younger than those in the central region (2960–3030 Ma). Further evidence of a major (2490–2480 Ma) metamorphic event in the central region is documented by a metamorphic zircon associated with a granulite facies ultramafic body. A dioritic gneiss from the northern region has also been dated at c. 2680 Ma. The northern region therefore does not comprise reworked central region rocks and consequently the old models for the evolution of the Lewisian which were based upon this concept need replacing. It is instead proposed that two distinct crustal blocks, now the northern and central regions, were tectonically juxtaposed along a boundary corresponding to the Laxford Front. Juxtaposition would appear to have occurred in Proterozoic times, as it must have postdated the 2490–2480 Ma (?Inverian) metamorphism recorded only in the central region, and the emplacement of granite sheets restricted to the northern side of the boundary. The first recorded event common to both regions is resetting of titanite ages associated with c. 1750 Ma Laxfordian amphibolite facies metamorphism. Zircon inheritance in rocks of both regions is scarce. Within one zircon from the northern region a c. 3550 Ma core was found. This represents the oldest known material from the region.

Mesozoic tectonics and metamorphism in the Pequop Mountains and Wood Hills region, northeast Nevada: Implications for the architecture and evolution of the Sevier orogen: Discussion and reply

Abstract: The Pequop Mountains–Wood Hills–East Humboldt Range region, northeast Nevada, exposes a nearly continuous cross section of Precambrian to Mesozoic strata representing middle to upper crustal levels of the Mesozoic hinterland of the Sevier orogen. These rocks preserve the transition from unmetamorphosed Mesozoic upper crust to partially melted middle crust. Integration of new structural, metamorphic, and U-Pb thermochronologic data from the Wood Hills and Pequop Mountains, coupled with a regional tectonic reconstruction, reveals substantial Cretaceous metamorphism, contraction, and extension in the Sevier hinterland in northeast Nevada. We report two phases of contraction not previously recognized that are accommodated by top-to-the-southeast thrust faults, the Windermere and Independence thrusts. Contraction was succeeded by two phases of extension along west-rooted normal faults, the Late Cretaceous Pequop fault and Tertiary Mary9s River fault system. The earliest phase of thrust faulting resulted in as much as 30 km of crustal thickening and an estimated minimum of 69 km of shortening along an inferred fault called the Windermere thrust. The timing of this thrusting event is bracketed between Late Jurassic (ca. 153 Ma) and Late Cretaceous (84 Ma). Relaxation of crustal isotherms following and perhaps during thrusting resulted in Barrovian-style metamorphism of footwall rocks, and partial melting of metapelite at deep levels. Peak metamorphism was attained ca. 84 Ma, and by this time hinterland crustal thickening had reached a maximum. During 84–75 Ma another minor pulse of shortening and thickening along the Independence thrust was followed by partial exhumation of the metamorphic rocks and as much as 10 km of crustal thinning along the Pequop fault. Thus the interval from 84 to 75 Ma in northeast Nevada marks a fundamental, and apparently permanent, change from horizontal contraction to extension in the upper to middle crust in the hinterland. Final exhumation of the metamorphic rocks was accomplished by the Tertiary Mary9s River fault system. Our data indicate that much of the metamorphism and some of the contraction in the Sevier hinterland in northeast Nevada, which was previously thought to be largely Late Jurassic, is actually Cretaceous in age. Furthermore, the data indicate that widespread metamorphism of the middle crust is a byproduct of tectonic burial, and that hinterland and foreland thrust faulting were coeval, suggesting that thrust faults in the Sevier orogen do not form a simple foreland younging sequence.