Showing papers in "International Geology Review in 1999"
TL;DR: This article presented the results of mapping selected cross-sections across the margins of the Chinese Tien Shan, an intracontinental mountain belt that formed in response to the India-Eurasia collision.
Abstract: We present the results of mapping selected cross-sections across the margins of the Chinese Tien Shan, an intracontinental mountain belt that formed in response to the India-Eurasia collision. This belt contains significant lateral variation in topography, structure, and stratigraphy at all scales, and our estimated rates of shortening also reveal a distribution of shortening that varies laterally. At the largest scale, it consists of two major high mountain ranges in the west that merge eastward into a complex, single high mountain belt with several distinct ranges, then separates farther eastward into several low mountain ranges in the south and a single narrow high mountain range in the north. Active fold-and-thrust belts along parts of the north and south flanks of the Tien Shan involve only Mesozoic and Cenozoic sedimentary cover, which varies in both stratigraphy and structure from east to west. The southern fold-and-thrust belt decreases in width and complexity from west to east and ends before rea...
216 citations
TL;DR: The Global Geoscience Transect SVEKA (GGT/SVEKA) as mentioned in this paper is a 160 km wide, 840 km long strip in the central part of the Fennoscandian shield.
Abstract: The Global Geoscience Transect SVEKA (GGT/SVEKA); a 160 km wide, 840 km long strip in the central part of the Fennoscandian shield, covers the western part of the Archean Karelian Province in the northeast, crosses the boundary zone between the Karelian P
205 citations
TL;DR: In this article, the isotopic systematics of the ultrahigh pressure (UHP) metamorphic rocks in the Dabie-Sulu belt, east China, appear to be rather different from what were expected.
Abstract: Because of a complicated metamorphic history, the isotopic systematics of the ultrahigh-pressure (UHP) metamorphic rocks in the Dabie-Sulu belt, east China, appear to be rather different from what were expected. Depending on the degree of retrograde metamorphism and on the retentivity of isotopes, the radiogenic isotopic systematics in the UHP metamorphic rocks yielded a wide range of radiometric ages. Some of these ages are geologically meaningful, but others may not be. In some fine-grained UHP metamorphic rocks, Sm/Nd isotopic systematics appear to be in equilibrium among the UHP phases, showing the best estimate for the age of peak metamorphism at 226 ± 3 Ma. On the other hand, retrograde overprinting often makes the interpretation of isotopic data more difficult. It is common to find that the Sm/Nd and Rb/Sr isotopic systematics among the UHP phases and retrograde phases are not in equilibrium. Regression of isotopic data involving both UHP and retrograde minerals in isotopic correlation diagrams oft...
166 citations
TL;DR: In the Qinling-Dabie-Sulu belt of eastern China, the Kokchetav Complex of northern Kazakhstan, the Maksyutov Complex of the southern Urals, the Dora-Maira massif of the Western Alps, and the Western Gneiss Region of Norway mark profound intracontinental collisional sutures as discussed by the authors.
Abstract: In Eurasia, the Qinling-Dabie-Sulu belt of eastern China, the Kokchetav Complex of northern Kazakhstan, the Maksyutov Complex of the southern Urals, the Dora-Maira massif of the Western Alps, and the Western Gneiss Region of Norway mark profound intracontinental collisional sutures. Adjacent regions exhibit scant evidence of contemporaneous calc-alkaline volcanism/plutonism. Each ultrahigh-pressure (UHP) metamorphic complex contains mineralogic and textural relics of coesite ± diamond as well as other very high P, moderate-T phases such as K-rich clinopyroxene, Mg-rich garnet, ellenbergerite, lawsonite, Al-rutile, glaucophane, high-Si phengite, and the phase assemblages coesite + dolomite, magnesite + diopside, and talc + kyanite, diopside, jadeite, or phengite. In each of these well-studied Eurasian complexes, maximum pressures approached or exceeded 2.8 GPa. Deep-seated recrystallization of old, cool continental crust took place during Phanerozoic time. Subduction zones constitute the only known plate-t...
108 citations
TL;DR: In this paper, robust geochronological methods for age determinations of the southernmost segment of the Neoproterozoic terranes of Brazil, namely the Dom Feliciano Belt, provide important clues for unraveling the complex evolution of the Brasiliano/Pan-African orogeny in this southwestern portion of the Gondwana supercontinent.
Abstract: The introduction of robust geochronological methods for age determinations of the southernmost segment of the Neoproterozoic terranes of Brazil, namely the Dom Feliciano Belt, provides important clues for unraveling the complex evolution of the Brasiliano/Pan-African orogeny in this southwestern portion of the Gondwana supercontinent Except for associated small schist belts and post-orogenic foreland basins, the belt is represented in this region of southeastern South America by the Pelotas Batholith Precise SHRIMP U/Pb zircon geochronological techniques based on the study of 95 individual spots on 74 zircon crystals (three samples) and on Nd-isotopic determinations (three samples) are used to assess the late Neoproterozoic history of the belt, especially the orthogneisses interleaved with the batholithic plutons Three petrotectonic associations were selected for detailed isotopic investigations—the Pinheiro Machado syncollisional monzogranites, the widespread Piratini gneiss tonalitic xenoliths, and t
104 citations
TL;DR: In this article, the authors reviewed the occurrences and petrographic characteristics of ultra-high pressure eclogites in exhumed ultrahigh pressure metamorphic terrenes within certain major Phanerozoic continental plate.
Abstract: Ultrahigh-pressure eclogites are metabasic rocks that have equilibrated at pressures within the coesite P-T stability field. This paper reviews their occurrences and petrographic characteristics in exhumed ultrahigh-pressure metamorphic terrenes within certain major Phanerozoic continental plate—collision belts. Unequivocal identification of ultrahigh-pressure eclogites depends on the presence of relict coesite or of polycrystalline quartz pseudomorphs after earlier coesite. However, preservation of such distinctive petrographic evidence is at best sporadic, leading to serious problems over whether or not particular eclogite samples have experienced “peak” pressures consistent with coesite stability. Accordingly, we also review here other petrographic, mineral-assemblage, and mineral-chemistry features that may aid in the identification of ultrahigh-pressure eclogites. In addition, we discuss the quantitative evaluation of P-T conditions for formation based on various reaction equilibria, including the do...
99 citations
TL;DR: Exsolution microstructures have been identified in ultramafic and eclogitic minerals from several orogenic belts, including the Western Gneiss Region, Norway; Alpe... as discussed by the authors.
Abstract: Exsolution microstructures have recently been identified in ultrahigh-pressure (UHP) ultramafic and eclogitic minerals from several orogenic belts, including the Western Gneiss Region, Norway; Alpe...
89 citations
TL;DR: Garnet peridotites and pyroxenites have been reported from 11 of the 15 or so high-pressure/ultrahigh pressure (HP/UHP) terranes in Eurasia as discussed by the authors.
Abstract: Garnet peridotites and pyroxenites have been reported from 11 of the 15 or so high-pressure/ultrahigh-pressure (HP/UHP) terranes in Eurasia. Most of these ultramafic rocks are Mg-Cr types, derived from depleted upper mantle, but some are more Fe-rich and originated by crystallization in ultramafic-mafic igneous complexes. The peridotites are polymetamorphic, with HP/UHP garnet-bearing assemblages being followed by a succession of retrograde assemblages related to exhumation and cooling; some also contain evidence for a pre-HP/UHP stage, such as spinel inclusions in garnet or the presence of Ti-clinohumite. Equilibration conditions have been calculated from all available analyses of garnetiferous assemblages, by application of the olivine-garnet Fe-Mg exchange thermometer and the Alin-orthopyroxene barometer, resulting in two distinct P-T regimes for garnet peridotites—one at high P/T in the coesite and diamond fields, and another at low P/T in the vicinity of the spinel-to-garnet transition. Garnet perido...
87 citations
TL;DR: In this paper, it was shown that pyrope megablasts contain late, crack-related granulite-facies mineral assemblages, which reflect very lo...
Abstract: New results acquired in the coesite-bearing terrane of the Dora-Maira massif, Italian Western Alps, indicate that the UHP unit extends over less than 45 km2 and is bounded by two lower-grade continental units. The contact to the footwall unit is a late, lowangle, top-to-the-SW extensional one, which probably cuts across the original thrust contact. The protolith of the pyrope “whiteschist” of the UHP unit is shown to be the metasomatic transformation product of granitic rock. Nearend member magnesiochloritoid and magnesiostaurolite occur in pyrope megablasts. Ordering in mag-nesiostaurolite leads to cross-hatched twinning and superstructures with either a doubled a or doubled c parameter. The Si3.5 phengite-3T from the same whiteschist contains nano- to microscale quartz platelets and talc layers parallel to (001) of the host mica, interpreted as in situ decompression products of the mica. Some of the pyrope megablasts contain late, crack-related granulite-facies mineral assemblages, which reflect very lo...
72 citations
TL;DR: Petrological and isotopic evidence suggests that the protolith of the Sulu ultrahigh pressure (UHP) terrane was Precambrian continental crust consisting of granite, granodiorite, gabbro, marble, and basic dikes, with local granulite-facies assemblages as mentioned in this paper.
Abstract: Petrological and isotopic evidence suggests that the protolith of the Sulu ultrahigh-pressure (UHP) terrane was Precambrian continental crust consisting of granite, granodiorite, gabbro, marble, and basic dikes, with local granulite-facies assemblages. Around 220 Ma this unit of continental rocks was buried to depths up of ∼120 km within the mantle. Structures formed during exhumation suggest highly mobile behavior of acidic rocks, even under conditions of very low water activity. Petrological studies show that the Sulu terrane underwent isothermal decompression, which implies relatively rapid exhumation, and suggests that the role of melting during exhumation may have been underestimated. The later stages of exhumation are associated with NW-SE-directed tectonic transport and the formation of at least one major normal detachment.
65 citations
TL;DR: In this article, the authors compared the isotope characteristics of UHP and ultramafic rocks from the Alpine, Hercynian, and Pan-African belts of northern Africa with those from the Dabie orogen of central China.
Abstract: Sm-Nd isotope tracer techniques are powerful tools in identification of the protolith nature of UHP and HP rocks and can be used to constrain modeling of tectonic processes of continental collision. UHP rocks may have diverse origins, and not all of them carry the same significance for subduction of continental blocks. In this paper, Sm-Nd isotopic data are compiled for UHP and HP rocks, mostly represented by eclogites and garnet peridotites, from the Alpine, Hercynian (Variscan), and Caledonian belts of western Europe; the Pan-African belts of northern Africa; and the Ross belt of Antarctica. These data then are compared with the isotopic characteristics of the UHP rocks from the Dabie orogen of central China. Except for the coesite-bearing quartzitic metasediments of Dora-Maira (Western Alps), which are clearly of continental origin, all HP and UHP rocks (eclogites and ultramafic rocks) from the Alpine, Hercynian, and Pan-African belts have oceanic affinities with the characteristic positive eNd(T) valu...
TL;DR: Recently, regional structural studies and detailed mapping at a scale of 1:10,000 in several key areas in the Dabie massif have revealed that the present regional tectonic pattern of ultrahigh-pressure and high-pressure metamorphic belts was formed mainly by extensional processes that postdate the Triassic collision between the Sino-Korean and Yangtze cratons as discussed by the authors.
Abstract: Recently, regional structural studies and detailed mapping at a scale of 1:10,000 in several key areas in the Dabie massif have revealed that the present regional tectonic pattern of ultrahigh-pressure and high-pressure metamorphic belts was formed mainly by extensional processes that postdate the Triassic collision between the Sino-Korean and Yangtze cratons. The tectonic pattern is characterized by an ancient metamorphic core complex, in which the most prominent feature is the development of multilayered extensional detachment zones on a crustal scale. This pattern controls the present spatial distribution of the ultrahigh-pressure metamorphic (UHPM) and high-pressure metamorphic (HPM) rocks, and obviously records large-scale subhorizontal extensional flow under amphibolite-facies conditions when the UHPM and HPM rocks were returned to lower-middle crustal levels. Based mainly on the crustal-scale tectonic scenario and combined with previous petrological and geochronological studies, the Dabie Mountains...
TL;DR: Omphacite with 5.02 wt% Na2O, inter-grown with orthopyroxene with Mg# 83.7, represents the first example of a diamondiferous websterite paragenesis including Na-clinopyroxenes.
Abstract: Eclogitic (E-type) and related parageneses of natural diamonds are represented by suites of diamond inclusions and xenoliths of diamondiferous eclogites. Major-element data are presented for 32 coexisting minerals forming 19 bimineralic and trimineralic inclusions from diamonds, including omphacite-orthopyroxene (1 sample), garnet-omphacite (5 samples), garnet-coesite (5 samples), omphacite-coesite (2 samples), garnet-picroilmenite (2 samples), garnet-kyanite (1 sample), omphacite-phlogopite (2 samples), and garnel-omphacite-phlogopite (1 sample). Major-element variations of coexisting minerals are typical of corresponding eclogites. Omphacite with 5.02 wt% Na2O, inter-grown with orthopyroxene with Mg# 83.7, represents the first example of a diamondiferous websterite paragenesis including Na-clinopyroxene. This indicates a broader range in mineral compositions of E-type-related websteritepyroxenite-associated diamonds than known previously. This unique websterite-pyroxenitic mineral assemblage represents ...
TL;DR: The Taquarembo Plateau plutono-volcanic association (TPPVA) as mentioned in this paper represents a portion of the last episode of post-Brasiliano/Pan-African collisional magmatism in southern Brazil.
Abstract: The Taquarembo Plateau plutono-volcanic association (TPPVA), a magmatic association related to a silica-saturated alkaline series, represents a portion of the last episode of post-Brasiliano/Pan-African collisional magmatism in southern Brazil. It was preceded by a postcollisional high-K calc-alkaline and shoshonitic magmatism, which is more pronounced near the collisional belt. The TPPVA is a sequence of volcanic, volcaniclastic, and volcanogenic sedimentary deposits with hypabyssal associated rocks, lying on Paleoproterozoic granulites. Isotope data suggest that this alkaline postcollisional magmatism occurred over the period from 580 to 537 Ma. Two sequences of magmatic liquids, which evolved by mineral fractionation from low-Ti-P and high-Ti-P basaltic magmas, were identified. The former includes mildly alkaline silica-saturated basalts, metaluminous monzodioritic intrusions, and rhyolitic lavas, whereas the second includes hawaiites, mugearites, syenitic intrusions, and peralkaline to intermediate la...
TL;DR: In this article, high δ13C values up to 11%PDB occur in Paleoproterozoic dolostones from the Aravalli Supergroup, western India.
Abstract: High δ13C values up to 11%PDB occur in Paleoproterozoic dolostones from the Aravalli Supergroup, western India. Correlation of high δ13C with high δ18O up to 23%SMOW in the studied carbonates suggests that pre-metamorphic δ13C values were above 10%PDB. The data are consistent with worldwide positive excursions in the δ13C of marine bicarbonate. The positive excursion was contemporaneous with sedimentation.
TL;DR: The distribution and characterization of UHP rocks within the Western Gneiss Region (WGR) of the Norwegian Caledonides is reviewed in this article, which indicates an overall regional trend of increased T and P to the northwest.
Abstract: The distribution and characterization of UHP rocks within the Western Gneiss Region (WGR) of the Norwegian Caledonides is reviewed. While recent studies have documented a significantly increased number of eclogite localities preserving mineralogical evidence for Scandian-aged UHP metamorphism, much uncertainty remains over the regional extent of any UHP province because of the widespread overprinting by retrograde amphibolite-facies assemblages (especially in the dominant gneisses) during exhumation of the terrain. Based on current observations, the UHP metamorphic province may be limited to a northwest region of only∼4000 km2, although an enigmatic mixed zone of HP (quartz-stable) and UHP (coesite-stable) eclogites extends a minimum of 5 km farther south and east in the Outer Nordfjord area. Quantitative P-T evaluation of key mineral reaction equilibria for eclogites sampled across the WGR indicates an overall regional trend of increased T and P to the northwest. This is consistent with Baltic plate rock...
TL;DR: The lead-isotopic database of over 200 analyses from nearly 90 samples of non-mare basalt, lunar highland rocks (>3.9 Ga) delineate at least three isotopically distinct signatures that in some combination can be interpreted to characterize the systematics of the entire database as discussed by the authors.
Abstract: The present lead (Pb)-isotopic database of over 200 analyses from nearly 90 samples of non-mare basalt, lunar highland rocks (>3.9 Ga) delineate at least three isotopically distinct signatures that in some combination can be interpreted to characterize the systematics of the entire database. Two are fairly new sets of lunar data and are typical of Pb data from other solar-system objects, describing nearly linear arrays slightly above the “geochron” values, with 207Pb/206Pb values <0.9. In aggregate, those data allow a relatively new interpretation of the Pb-isotopic evolution of the Moon, helping to identify and characterize a planetary evolutionary stage not preserved by Earth rocks. From the Pb-isotopic compositions, a source 238U/204Pb (μ) value can be calculated; differences in μ values from one Pb signature to another can have important implications on the age and evolution of Moon-forming events. In particular, the U/Pb ratio is one method of measuring the fractionation between refractory and volati...
TL;DR: In this paper, a comparison of measured shear and compressional wave velocities in the middle crust of the Hayward fault with laboratory measurements suggests that the middle layer is a diabase (oceanic) crust.
Abstract: Compilation of seismic transects across the central and northern California Coast Ranges provides evidence for the widespread tectonic emplacement beneath the margin of a slab of partially subducted oceanic lithosphere. The oceanic crust of this lithosphere can be traced landward from the former convergent margin (fossil trench), beneath the Coast Ranges, to at least as far east as the Coast Range/Great Valley boundary. Comparison of measured shear and compressional wave velocities in the middle crust beneath the Hayward fault with laboratory measurements suggests that the middle crust is a diabase (oceanic crust). Both of these observations are consistent with recent models of the high heat flow and age progression of Neogene volcanism along the Coast Ranges based on tectonic emplacement (stalling) of young, hot oceanic lithosphere beneath the margin, but appear to contradict the major predictions of the slab-gap or asthenospheric-window model. Finally, the Neogene volcanism and major strike-slip faults ...
TL;DR: In this article, two processes are suggested to explain how UHP rocks are exhumed from mantle depths: removal of the overburden either by erosion or by extension, whereas the uplifting of...
Abstract: Two processes are suggested to explain how UHP rocks are exhumed from mantle depths. One is removal of the overburden either by erosion or by extension, whereas the other involves the uplifting of ...
TL;DR: In this paper, the authors evaluate metasomatism as a process with respect to the fluid composition involved in mantle xenoliths, by compositional variations within and between minerals, and by the introduction of secondary minerals.
Abstract: In mantle xenoliths, metasomatism is recorded by compositional variations within and between minerals, and by the introduction of secondary minerals. However, metasomatism has not been quantitatively evaluated as a process with respect to the fluid composition involved. Diamondiferous eclogites from the Udachnaya kimberlite provide a unique suite of samples that allow a semiquantitative estimation of metasomatic fluid composition. The basis of our analysis involves comparison of reconstructed whole-rock compositions with measured whole-rock analyses. Primary minerals in these samples are relatively homogeneous, and permit the use of modal analyses and mineral chemistry for reconstruction of “pristine” whole-rock compositions. The metasomatic overprint, which is similiar in all samples studied, has produced depletions in SiO2, Na2O, and FeO and enrichments in TiO2, K2O, MgO, and LREE. Secondary minerals from the samples are interpreted as the direct result of metasomatism (i.e., typical metasomatic mineral...
TL;DR: In this article, the authors summarized tectonic settings and mineral parageneses of several recognized ultra-high pressure (UHP) terranes other than the well-studied Kokchetav Massif of northern Kazakhstan, the Western Gneiss Region of Norway, the Dora-Maira massif of the Western Alps, and the Dabie-Sulu terrane of east-central China.
Abstract: This paper summaries tectonic settings and mineral parageneses of several recognized ultrahigh-pressure (UHP) terranes other than the well-studied Kokchetav Massif of northern Kazakhstan, the Western Gneiss Region of Norway, the Dora-Maira massif of the Western Alps, and the Dabie-Sulu terrane of east-central China. Diamond-bearing terranes include the Beni Bousera/Ronda peridotite massif, the Erzgebirge Crystalline Complex, mantle peridotite from a Tibetan ophiolite, and possibly the Maksyutov Complex. Coesite-bearing UHP terranes are the Zermatt-Saas area, Western Alps; the Mali eclogites from West Africa; the Makabal complex of western Tien Shan; the Bohemian massif; and the newly reported Central Indonesia terrane and Himalayan eclogites from the upper Kaghan Valley of Pakistan. Except for the diamond-bearing Tibetan ophiolite, most UHP terranes share similar petrotectonic assemblages and lie within major continental collision belts in Eurasia and Africa. Using new approaches (including new geobaromet...
TL;DR: In this paper, the authors report on a recent finding of granulitic and gabbroic xenoliths in the Rincon de Parangueo and La Cintura maars, within the Valle de Santiago volcanic field.
Abstract: We report on a recent finding of granulitic and gabbroic xenoliths in the Rincon de Parangueo and La Cintura maars, within the Valle de Santiago volcanic field. The field is formed by several maars, cinder cones, and shield volcanoes and is located at the northern sector of the extensive Michoacan-Guanajuato volcanic field, characterized by abundant Neogene cinder cones and medium-sized shield volcanoes. The maars are distributed along a belt elongated NW-SE, apparently related to a regional fault system that has been documented in the west-central sector of the Trans-Mexican volcanic belt (TMVB). Two types of xenoliths have been distinguished—i.e., hypersthene-diopside granulites and gabbroic xenoliths—which are present in the Rincon de Parangueo and La Cintura maars, respectively. The silica content in the granulites has a narrow range between 46.9 and 50.9 wt%, plagioclase crystals are fractured and display reaction borders, and crystals appear to have grown slowly and show no compositional zoning. Sil...
TL;DR: The main felsic lithologies experiencing ultra-high pressure metamorphism (UHPM) are described, with a focus on rock association, nature, and composition of the peak minerals, as well as nature of the protolith.
Abstract: The main felsic lithologies experiencing ultrahigh-pressure metamorphism (UHPM) are described, with a focus on rock association, nature, and composition of the peak minerals, as well as nature of the protolith. Felsic rocks usually preserve poor evidence of UHPM, being pervasively deformed and retrogressed during exhumation. Locally, however, unstrained lithologies are preserved that show UHPM minerals and even microstructures inherited from the protolith. Among the number of localities where UHPM has been reported, the following have been considered in detail— the southern Dora-Maira massif and Lago di Cignana in the Western Alps, the Dabie Shan-Sulu region in northeastern China, the Kokchetav massif in northern Kazakhstan, and the Western Gneiss Region of the Scandinavian Caledonides. At Lago di Cignana, metasediments of oceanic affinity are carbonate-bearing garnet-phengite-quartz schist and oxidized manganiferous quartz schists, all preserving rare coesite relics. In the Dora-Maira massif, the Brossas...
TL;DR: The Coast Range Ophiolite serpentine may have been exposed by uplift and deep erosion combined with imbricate thrusting and westward thinning of the Great Valley sediments.
Abstract: The Santa Clara Formation was drastically affected by emplacement of serpentine diapirs in very recent times. Serpentine in flat thrust sheets formerly belonged to the lower unit of the Coast Range Ophiolite, which underlay the Jurassic—Cretaceous clastic sediments of the Great Valley Sequence. The regionally extensive Coast Range fault had emplaced all the aforementioned rocks, including the flat thrust sheets of serpentine, over the Mesozoic Franciscan Complex. This complex is exposed in many places in the area and presumably is basement for the entire region. Serpentine bodies within the Franciscan Complex were derived from peridotite wedges unrelated to the Coast Range Ophiolite. Exposure of the Coast Range Ophiolite serpentine may have been assisted by uplift and deep erosion combined with imbricate thrusting and westward thinning of the Great Valley sediments, broadly exposing the basal Coast Range Ophiolite serpentine. Plio-Pleistocene, non-marine Santa Clara Formation was deposited on Coast Range ...
TL;DR: In this paper, the stability relations of kyanite, piezotite, OH-topaz, and diaspore are reviewed in the system SiO2, the reactions bounding the coesite stability field as well as the rheology of coesitites and the kinetics of the Coesite-quartz transformation are discussed.
Abstract: Recent experimental studies have addressed mineral phases that can be expected to occur in metasedimentary rocks at ultrahigh-pressure conditions. These studies are based mainly on the P-T location of reaction curves using reversals and the determination of compressibility data as well as the compositional change of solid solution phases with P and T in fixed mineral assemblages. In the system SiO2, the reactions bounding the coesite stability field as well as the rheology of coesitites and the kinetics of the coesite-quartz transformation are discussed. In the system Al2O3-SiO2-H2O, the stability relations of kyanite, piezotite, OH-topaz, and diaspore are reviewed. In MgO-Al2O3-SiO2-H2O, investigations on the enstatite polymorphs, talc, the 10 A phase, MgMgAl-pumpellyite, and pyrope, which can coexist with coesite, are reported. Experimental studies on Mg-chloritoid, Mg-staurolite, and chlorite also are reviewed. In FeO-Al2O3-SiO2-H2O, chloritoid, staurolite, and ferrosilite are addressed. In K2O-Al2O3-S...
TL;DR: In this paper, the in situ determination of critical curves in pressure-temperature diagrams, along which the hydrous melts and silicate-bearing vapors coexisting at low pressures merge compositionally, and form a single-phase supercritical fluid stable at higher pressures.
Abstract: The outstanding development of recent years in the experimental petrology of granite systems with water is the in situ determination of critical curves in pressure-temperature diagrams, along which the hydrous melts and silicate-bearing vapors coexisting at low pressures merge compositionally, and form a single-phase supercritical fluid stable at higher pressures. Their locations, extending from pressures of ∼15 kbar near 1000°C to ∼25 kbar near 700°C, suggest that previous determinations of excess-water granite melting at higher pressures were influenced by quench problems. Under the conditions of ultrahigh-pressure metamorphism, therefore, granites in the presence of water do not melt, but are more or less gradually dissolved in the supercritical fluid with rising temperature, i.e., without any sharp “melting temperature.” The anhydrous compositions of the supercritical fluids change from strongly incongruent, mainly potassium silicatic at low temperatures via ultrapotassic, leucititic compositions to c...
TL;DR: In this paper, two anorthosite massifs in the Eastern Ghats share similar structural constitutions, internal differentiation histories, and overall thermal-tectonic patterns of evolution.
Abstract: Two anorthosite massifs in the Eastern Ghats share similar structural constitutions, internal differentiation histories, and overall thermal-tectonic patterns of evolution. The (1) circular to near-circular structural patterns both inside and close-to-border outside of the plutons; (2) the merging of these structures with the straight to gently flowing, essentially unidirectional structural trend of the granulite basement within short distances from the border of the plutons; (3) the increase of strain intensity near the border of the plutons; (4) the small but recognizable differences in the dip of the anorthosite flow layers; and (5) the foliation of the granulites, all are strong indications that these anorthosites were emplaced as syntectonic diapirs. In both massifs, anorthosite is by far the dominant lithology, noritic varieties being subordinate and generally formed as narrow dike-like bodies and pods, pockets, and irregular patches near the border. The striking features of the whole-rock and miner...
TL;DR: In this article, high-resolution aeromagnetic data delineate offset and/or truncated magnetic rock bodies of the Franciscan Complex that define the location and structure of, and total offset across, the San Andreas fault in the San Francisco Bay region.
Abstract: Recently acquired high-resolution aeromagnetic data delineate offset and/or truncated magnetic rock bodies of the Franciscan Complex that define the location and structure of, and total offset across, the San Andreas fault in the San Francisco Bay region. Two distinctive magnetic anomalies caused by ultramafic rocks and metabasalts east of, and truncated at, the San Andreas fault have clear counterparts west of the fault that indicate a total right-lateral offset of only 22 km on the Peninsula segment, the active strand that ruptured in 1906. The location of the Peninsula segment is well defined magnetically on the northern peninsula where it goes offshore, and can be traced along strike an additional ~6 km to the northwest. Just offshore from Lake Merced, the inferred fault trace steps right (northeast) 3 km onto a nearly parallel strand that can be traced magnetically northwest more than 20 km as the linear northeast edge of a magnetic block bounded by the San Andreas fault, the Pilarcitos fault, and th...
TL;DR: Paragenetic, textural, and chemical characteristics of micas from 10 rare-metal granitic stocks and the associated greisens were examined in order to identify the metallogenetic processes of the host granitoids.
Abstract: Paragenetic, textural, and chemical characteristics of micas from 10 rare-metal granitic stocks and the associated greisens were examined in order to identify the metallogenetic processes of the host granitoids. The investigated granitoids and type occurrences can be categorized as: (1) metaluminous, Nb + Zr + Y-enriched alkali granite (e.g., Hawashia, Ineigi, and a stock northwest of Um Naggat); (2) peraluminous, Ta > Nb + Sn ± W + Be-enriched Li-albite granites (e.g., Nuweibi, Igla, and Abu Dabbab); and (3) metasomatized, Nb » Ta + Sn + Zr + Y + U ± Be ± W-enriched apogranites (e.g., Um Ara, Abu Rusheid, Mueilha, and Homr Akarem). Mica of the alkali granite is of the annite-siderophyllite series, and is characterized by an average FeO∗ of 28.14, low MgO of 0.05, a mean Fe∗/(Fe∗ + Mg)atom. value of 0.996, TiO2 of 0.69, enhanced Al2O3 of 14.91, MnO of 0.58, Li2O of 0.26, and moderate to low F of 0.86. These characteristics are representative of the relatively highly evolved nature of the annite-siderophyl...
TL;DR: The geochemistry of Laramide granitic rocks from central Sonora was studied to relate possible compositional variations to the assimilation of different crustal sources as discussed by the authors, and the results showed that the Laramide plutons intruded a thick section of Upper Proterozoic and Paleozoic miogeoclinal strata, whereas south of it, the intrusives were emplaced in Lower Ordovician to Permian eugeoclinal rocks accreted to the continental margin.
Abstract: The geochemistry of Laramide granitic rocks from central Sonora was studied to relate possible compositional variations to the assimilation of different crustal sources. Most of the studied rocks are granodiorites collected near the southern boundary of the Paleozoic North American continent. North of this boundary, the Laramide plutons intruded a thick section of Upper Proterozoic and Paleozoic miogeoclinal strata, whereas south of it, the intrusives were emplaced in Lower Ordovician to Permian eugeoclinal rocks accreted to the continental margin between Late Permian and Middle Triassic times. Whole-rock Na2O, TiO2, and P2O5 contents are slightly higher in plutons intruded in cratonic and miogeoclinal rocks, whereas MgO and CaO seem to be higher in plutons emplaced in eugeoclinal rocks. The samples located in the north are characterized by steeper chondrite-normal-ized REE slopes and generally well developed negative europium anomalies, whereas the group of granitoids in the south have flatter REE slopes...