Showing papers on "Granulite published in 1982"

The transformation of amphibolite facies gneiss to charnockite in southern Karnataka and northern Tamil Nadu, India

Abstract: Amphibolite facies metamorphic grade gives way southward to the granulite grade in southern Karnataka, as acid gneisses develop charnockite patches and streaks and basic enclaves develop pyroxenes. Petrologic investigations in the transitional zone south of Mysore have established the following points: 1) The transition is prograde. Amphibole-bearing gneisses intimately associated with charnockite at Kabbal and several similar localities are not retrogressive after charnockite, as proved by patchy obliteration of their foliation by transgressive, very coarse-grained charnockite, high fluorine content of biotite and amphibole in gneisses, and high large-ion lithophile element contents in gneisses and charnockites. These features are in contrast to very low fluorine in retrogressive amphiboles and biotites, very low large-ion lithophile element contents, and zonal bleaching of charnockite, in clearly retrogressive areas, as at Bhavani Sagar, Tamil Nadu. 2) Metamorphic temperatures in the transitional areas were 700°–800° C, pressures were 5–7 kbar, and H2O pressures were 0.1–0.3 times total pressures, based on thermodynamic calculations using mineral analyses. Dense CO2-rich fluid inclusions in the Kabbal rocks confirm the low H2O pressures at the first appearance of orthopyroxene. Farther to the south, in the Nilgiri Hills and adjacent granulite massif areas, peak metamorphic temperatures were 800°–900° C, pressures were 7–9 kbar, and water pressures were very low, so that primary biotites and amphiboles (those with high F contents) are rare. 3) The incipient granulite-grade metamorphism of the transitional areas was introduced by a wave of anatexis and K-metasomatism. This process was arrested by drying out under heavy CO2 influx. Charnockites so formed are hybrids of anatectic granite and metabasite, of metabasite and immediately adjacent gneiss, or are virtually isochemical with pre-existing gneiss despite gross recrystallization to granulite mineralogy. These features show that partial melting and metasomatism are attendant, rather than causative, in charnockite development. Copious CO2 from a deep-crustal or mantle source pushed ahead of it a wave of more aqueous solutions which promoted anatexis. Granulite metamorphism of both neosome and paleosome followed. The process is very similar to that deduced for the Madras granulites by Weaver (1980). The massif charnockites, for the most part extremely depleted in lithophile minor elements, show many evidences of having gone through the same process.

Garnet–orthopyroxene barometry for granulites and peridotites

Abstract: An experimentally calibrated geobarometer based on the solubility of alumina in orthopyroxene coexisting with garnet is presented. This geobarometer is based on experiments in the FeO–MgO–Al2O3–SiO2 and CaO–FeO–MgO–Al2O3–SiO2 systems performed at high temperatures and pressures (800–1,200 °C and 5–20 kbar). Applying the geobarometer to a variety of natural garnet–orthopyroxene assemblages from granulite facies terrains and other occurrences, yields reliable and consistent pressure estimates for these samples.

Geochronological studies of the Bohemian massif, Czechoslovakia, and their significance in the evolution of Central Europe

Abstract: U–Pb zircon and Rb–Sr whole-rock analyses from various gneisses and plutonie rocks of the Moldanubian and Moravo-Silesian zones and the stable foreland of the Hercynian (Variscan) orogenic belt indicate that most of the crust in Central Europe was first formed during the Cadomian orogeny which straddles the Precambrian–Cambrian boundary. Zircons, however, have a memory of older ages which correspond with those of events known in Fennoscandia. The new radiometrie data are consistent with the stratigraphie record in that they do not provide any evidence for a major early Palaeozoic tectonothermal event between the Cadomian and Hercynian orogenies.Granulites from two localities in the Moldanubian zone yield U–Pb zircon ages of 345 ± 5 Ma; discordant zircon data points indicate that the granulite facies metamorphism was not of long duration. Tectonic units containing these high grade rocks were emplaced amongst amphibolite facies rocks during an event of widespread shearing which has been dated at 341 ± 4 Ma on the basis of a lower U–Pb zircon intercept age from one of the sheared gneisses and 338 ± 3 Ma U–Pb ages from monazites. Rb–Sr muscovite ages of 331 ± 5 Ma from pegmatites axial planar to asymmetrical folds date the last stage of SE-directed simple shear. A Rb–Sr whole-rock isochron of 331 ± 4 Ma from a principal magmatic type of the Central Bohemian pluton confirms the field evidence that the large NE-trending plutons of the Moldanubian zone were emplaced during a late stage of the deformation. The strong disturbance of the U–Pb zircon isotopic system in the sheared gneisses suggests U loss while a high U/Th ratio in monazite from one of these tectonised rocks suggests the simultaneous passage of hydrothermal fluids. Thus a crustal source is indicated for the uranium deposits of the Moldanubian zone.Critical to any plate tectonic model for the development of the Middle European Hercynides was the existence of an ocean in Early Devonian times which separated a North European continent from a South European continent(s). The northward movement of the South European continent over a shallowly-dipping subduction zone and subsequent continental collision can explain the high T–low P metamorphism and the imbricated tectonic style of the Moldanubian zone and adjacent Moravo-Silesian zone along the southeastern Hercynian foreland. The temporal separation of granulites and granites implies distinct conditions of formation and it has been suggested that the plutonism, following on from the imbrication of the Cadomian crust, was initiated by the subduction of wet oceanic sediments.

A partisan review of Proterozoic anorthosites

Abstract: Most anorthosites of the massif type crystallized in the episode 1.7-1.2 Gyr, with a pronounced peak of the age distribution near 1.4 Gyr. They were emplaced anorogenically at depths as shallow as 7 km, where the ambient temperature of country rocks was probably less than 250'C. Depths of emplacement may have been as great as 25 km or more in rare cases; the greater depths of equilibration estimated from granulite facies metamorphism may be incorrectly interpreted as emplacement depths, but in any case they are demonstrably not required or characteristic of anorthosite emplacement. Penetrative deformation and metamorphism of anorthosites are post-emplacement accidents of the local geologic history, and are not directly caused by the presence of anorthosites. Granitic rocks (mangerite-charnockite suite) associated with anorthosite are in general later or contemporaneous products of crustal anatexis, with chemical and isotopic signatures distinct from the anorthosites and their residua. Such granitic rocks should not, therefore, be summed with the anorthositic rocks to obtain bulk compositions. The magmas that produced most anorthosites were dry, as shown by high-temperature mineralogy and anhydrous mineral assemblages in contact aureoles. Residua from their crystallization are ferrodiorites to ferrosyenites typical of closed-system fractionation. These residua were locally and frequently ejected into contemporaneously molten granite, where they formed pillows and cooled rapidly. The overall chemistry of anorthosites and residua is broadly tholeiitic and consistent with derivation from the mantle. Olivine-bearing magmas locally ranged from leucotroctolite (anorthosite) to later but coexisting picrite or melatroctolite in the same pluton, confirming a wide spectrum of magma types. A signal feature of troctolitic and noritic magmas is their low augite content, implying high content of spinel component. Large anorthosite complexes such as Nain and Harp Lake consist of many plutons representing repeated injections of separate magma batches with varying chemistry. The abundant true anorthosites, richer in plagioclase than magmas cosaturated with a mafic phase, must represent plagioclase enrichment by either mechanical or chemical processes or both. The role of kinetics in nucleation and solidification of such rocks may be centrally important. It is proposed that hyperfeldspathic (plagioclase-supersaturated) liquids were generated by quasi-isothermal extraction of mafic minerals from tholeiitic magma enroute to and at the site of emplacement, and that such a kinetic process was uniquely permitted in an environment of aborted continental rifting. Anorthositic rocks may have much to say about the episodic versus continuous geochemical evolution of the earth's mantle.

Sm–Nd dating and cooling history of Scourian granulites, Sutherland

Abstract: The Scourian gneisses of the Lewisian complex of north-west Scotland have been the subject of many geochronological investigations. Sm–Nd whole-rock measurements suggest that the protoliths of the Lewisian complex differentiated 2.92±0.05 Gyr ago from an approximately chrondritic mantle1. Rb–Sr and Pb–Pb whole-rock ages between ∼2.8 and 2.6 Gyr reflect subsequent depletion in Rb and U (refs 2, 3) which has generally been associated with granulite facies metamorphism. Zircons which are thought to have crystallized during this metamorphism have also given U–Pb ages of 2.66 Gyr (ref. 4). This granulite facies metamorphism, involving pressures in excess of 10 kbar and peak temperatures estimated between 820±50 °C (ref. 5) and 1,250 °C (refs 6,7), affects a terrain including supracrustal rocks7 and thus implies a major tectonic event. We report here the first geochronological data measured on mineral assemblages that have also provided estimates of the conditions of metamorphism. Such measurements are possible because the major minerals of granulite facies assemblages show an unusually favourable spread in Sm/Nd ratios; garnet, especially, is strongly enriched in Sm (ref. 8). Data for three garnetiferous basic gneiss assemblages suggest that closure of the Sm–Nd system occurred significantly later than the peak of the metamorphism.

Sm–Nd, Rb–Sr and U–Th–Pb systematics of granulite facies rocks from Fyfe Hills, Enderby Land, Antarctica

Abstract: Sm–Nd, Rb–Sr and U–Th–Pb systematics of granulite facies rocks from Fyfe Hills, Enderby Land, Antarctica

Geobarometry, Geothermometry, and Late Archean Geotherms from the Granulite Facies Terrain of South India

Abstract: Geobarometry and geothermometry of garnet-bearing chamockitic and metapelitic assemblages from the high-grade terrain from South India indicate three fields of equilibration. High pressure granulites ($P = 8.3 \pm 1.0 kb, T = 760 \pm 40^{\circ}C$) are restricted to the charnockites and mafic granulites of the north slopes of the Nilgiris. These place a minimum constraint of ~30 km on the maximum thickness of late Archean crust. Medium pressure granulites ($P = 6.4 \pm 1.0 kb, T = 735 \pm 40^{\circ}C$) characterize the charnockites of the central Nilgiris and the Madras granulites to the east. Low pressure granulites ($P = 5.0 \pm 1.0 kb, T = 700 \pm 20^{\circ}C$) characterize the charnockites and metapelites south of the Bhavani shear zone, although migmatites from the Kodaikanal massif may reflect somewhat higher temperatures. All three granulite fields lie on a metamorphic geotherm (piezothermic array) which is strongly convex to the temperature axis and characteristic of convective heat transfer. The P...

Osumilite in the sapphirine-quartz terrane of Enderby Land, Antarctica; implications for osumilite petrogenesis in the granulite facies

Abstract: Osumilite is an important rock-forming mineral at l0 exposures in the Late Archean Napier complex of coastal Enderby Land (50-52'E), Antarctica. Mineral assemblages of low compositional variance (all with quartz-rplagioclase) are osumilite-garnet-sillimanite -r spinel, osumilite-sapphirine-sillimanite, osumilite-garnet-orthopyroxene-sillimanite, osumilite-orthopyroxene-biotite, and in a rock nearly free of iron (atomic Mg/(Mg+Fe) = 0.89) from Reference Peak (67'15'S, 50"29'E), osumilite-enstatite (EnerFse)-sillimanite. The Antarctic osumilite departs from the theoretical composition (K, Na) (Mg,Fe,Mn)zAlr (SiroAlto3o in that (Mg+Fe+Mn) ranges from2.20to2.39; A1,4.45 to 4.67; and Si, 10.05 to 10.30. These variations approximately obey the substitution (Mg+Mn+Fe) + Si : 2Al. Atomic Me/(Mg+Mn+Fe) ratio (X14j of Antarctic osumilite ranges from 0.79 to 0.97, and is greater than X1a, of associated cordierite and biotite. Antarctic osumilite-bearing have moderate to high Mg/Fe ratios (atomic Mg/(Mg+Fe) = 0.43 to 0.89) and are peraluminous. A petrogenetic grid in the system K2O-FeO-MgO-AlzOrSiO2 for quartz (present in all assemblages), osumilite, K-feldspar, garnet, sapphirine, cordierite, orthopyroxene, and sillimanite can be constructed given the constraint that the univariant reaction osumilite = cordierite * enstatite + K-feldspar + quartz in the iron-free subsystem intersects the stable portion of the univariant reaction in the MgO-Al2O3-SiO2 system: cordierite : enstatite + sillimanite -t qtartz. Pressure-temperature slopes of reactions involving osumilite but not cordierite are positive; slopes of reactions involving both osumilite and cordierite are negative, or in one case, parallel to the pressure axis. Osumilite is predicted to be stable in rocks of appropriate bulk composition at temperatures above 750'C and at total (lithostatic) pressures less than 8 or 9 kbar. M. Olesch and F. Seifert's recent experimental work and Olesch's calculations indicate that osumilite would be stable under hydrous conditions only when P11r6 = Prot.l < I kbar; at higher total pressures, H2O partial pressure must be much less than total pressure. Minimum temperatures for the appearance of osumilite-garnet and osumilite-sillimanite assemblages are in general higher than those needed for osumilitecordierite-orthopyroxene, and pressures of 5 or 6 kbar may be needed to stabilize osumilite-garnet or osumilite-sillimanite at geologically accessible temperatures. As the physical conditions proposed here for osumilite stability overlap the estimated conditions in many granulite-facies terranes, osumilite can be expected to occur at more localities than have been reported to date.

Oxygen isotope evidence for shallow emplacement of Adirondack anorthosite

Abstract: Oxygen isotopic analysis of wollastonites from the Willsboro Mine, Adirondack Mountains, New York reveals a 400-ft wide zone of 18O depletion at anorthosite contacts. Values of δ18O vary more sharply with distance and are lower (to −1.3) than any yet reported for a granulite fades terrain. Exchange with circulating hot meteoric water best explains these results and implies that the anorthosite was emplaced at relatively shallow depths, <10 km, in marked contrast to the depth of granulite fades metamorphism (23 km). These 18O depletions offer the first strong evidence for shallow emplacement of anorthosite within the Grenville Province and suggest that regional metamorphism was a later and tectonically distinct event.

Crustal xenolith magnetic properties and long wavelength anomaly source requirements

Abstract: Granulite xenoliths, probable components of the lower continental crust, are a primary source of information about the magnetization of the lower crust. Magnetization values for lower crustal xenoliths from three tectonic settings (converging plate margin, rift valley, and continental intraplate region) demonstrate that metabasic rocks in the granulite facies have magnetization values consistent with magnetizations inferred for modeled sources of long wavelength anomalies. Measured Curie points for granulite xenoliths are near 560-570°C except for those from rift zones and other regions where anhydrous, probably reducing lower crustal conditions exist in a steep geothermal gradient. In samples from the reducing environments Curie points <300°C are measured. The lower crust could be the most magnetic crustal layer. Satellite magnetic anomalies may serve to delineate magnetization provinces which may be related to the tectonic and chemical evolution of continents.

Proterozoic age and cumulate origin for granulite xenoliths, Lesotho

Abstract: Rare earth element (REE), 143Nd/144Nd and 87Sr/86Sr analyses on a range of granulite xenoliths from kimberlite pipes in Lesotho demonstrate that useful whole rock ages can be obtained from such lower crustal material. The characteristic positive Eu-anomalies, relatively low REE abundances and large variation in Sr contents suggest that many of the xenoliths are crystal cumulates rather than residua after partial melting. They yield an Sm–Nd whole rock age of 1,400 ± 100 Myr and are unrelated to the younger magmatic rocks of the Karoo.

Mineralogy of mafic and Fe.Ti oxide-rich differentiates of the Marcy anorthosite massif, Adirondacks, New york

Abstract: Rocks enriched in mafic silicates, Fe-Ti oxides and apatite form a minor but ubiquitous facies of nearly all Proterozoic massif-type anorthosite complexes. In the Marcy massif of the Adirondack highlands, New York, the mafic rocks have two modes of occurrence: l) as conformable segregations interpreted as cumulate layers in the border zones, and 2) as dikes throughout the massif, interpreted as having crystallized from residual liquids extracted at varying stages during differentiation. Primary mineral compositions in these rocks are commonly preserved, despite the effects of subsolidus recrystallization in the high pressure granulite facies. In the mafic rock suite, primary compositions of mafic silicates reveal an iron enrichment trend which is broadly similar to that of basic layered intrusions, but suggestive of somewhat higher crystallization temperatures. Textural relationships suggest the following crystallization sequence: plagioclase, pigeonite + augite, hemo-ilmenite * magnetite, apatite. Fe-rich olivine replaced pigeonite in the lateststage residual liquids. The mineralogy and field relationships of these mafic rocks are consistent with their origin as diferentiates from the same melts which produced the anorthosites. This conclusion agrees with recent geochemical data that suggest an independent origi-n for the spatially associated mangerite-charnockite suite.

Sapphirine, Kornerupine, and Sillimanite + Orthopyroxene in the Charnockitic Region of South India

Abstract: Sapphirine, kornerupine, and the associations sillimanite-orthopyroxene and sillimanite-gedrite are reported from 23 localities (or areas of closely spaced localities) in Fermor's (1936) charnockitic region and in transitional zones to Fermor's non-charnockitic region in south India (Andhra Pradesh, Tamil Nadu, and southern Karnataka). The rocks containing these minerals and mineral associations are believed to have a pelitic sedimentary precursor. They are in general closely associated with khondalite (garnet-sillimanite gneiss), quartzite, and calc-silicate rocks, and less commonly with metabasites of presumed metavolcanic origin. However, the sapphirine occurrences in the Sittampundi anorthosite complex, where sapphirine and sillimanite-gedrite are found in reaction zones between anorthosite and chromite layers, are an exception. Assemblages of sapphirine with garnet and sillimanite are not rare, but sapphirine in direct contact with quartz, which has not previously been reported from south India, was found at Paderu, Vishakhapatnam District, A. P. Kornerupine occurs with sillimanite at three localities but with orthopyroxene only at Ganguvarpatti, a new locality for kornerupine. Temperatures and pressures calculated from pyroxenes, garnets, and feldspars in associated rocks suggest that sapphirine, kornerupine, and sillimanite-orthopyroxene crystallized at 800-850°C and 6-10 kbar in the charnockitic region and sapphirine at 780°C in a transition zone between the granulite and amphibolite-facies near Sargur. Water partial pressure during the metamorphism was significantly less than total pressure. Regional differences in pressure are reflected in the distribution of mineral assemblages: sillimanite-orthopyroxene is found in Madras and Andhra Pradesh, but is conspicuously absent in the Palni Hills, where spinel-cordierite is an important assemblage. Sapphirine and sillimanite-orthopyroxene (and sillimanite-gedrite) are restricted to rocks relatively rich in Al 2 O 3 with high Mg/Fe ratios relative to khondalite. Kornerupine appears to be confined to rocks containing boron. These special rock compositions (except Sittampundi) are believed to reflect unusual compositions of the precursor pelitic sediments rather than changes in chemistry during metamorphism.

Fluid evolution and graphite genesis in the deep continental crust

Abstract: Metamorphism in deep continental crust is often accompanied by carbon-rich, H2O-poor fluids1–5, and is characterized by the development of granulite facies mineral assemblages3–5. Graphite, a minor phase in many high-grade metamorphic rocks6–17, has also been recognized in recent xenoliths of deep crustal material18, demonstrating its presence in modern deep continental crust. Although graphite is most common in biotite schists, it has also been recorded in pyroxene gneisses, two feldspar-quartz pegmatites, marbles and quartzo-feldspathic gneisses11,15–19. The presence of supracrustal sequences in granulite facies regions6–19 suggests that the latter developed through progressive metamorphism and dehydration of shallow level crustal material. Much of this material must have experienced a metamorphic recrystallization in the amphibolite facies, accompanied by the development of hornblende. Although graphite in the high-grade rock may develop from oxidation of organic material, this may not be the most common means for graphite genesis. In much of West Greenland14 and in southern Norway12 the low-grade, hornblende-bearing material does not contain significant quantities of organic compounds, although graphite is found in higher-grade metamorphic equivalents. This strongly suggests that introducing CO2 into deep crustal material leads to fluid evolution and graphite formation. I outline here the evolution of fluid composition in the C–O–H system which is in equilibrium with common crustal mineral assemblages. Thermodynamic data for CO2–H2O mixtures20 and mineral components21 are used to model the chemical changes in fluid composition which accompany introduction of CO2 into various types of rock. The results demonstrate that graphite genesis is a direct consequence of deep crustal metamorphism in the presence of a CO2-rich fluid phase. In addition, the calculations demonstrate that the deep continental crust will not be a reservoir for large volumes of methane-rich gas.

The Ivrea zone as a model for the distribution of magnetization in the continental crust

Abstract: Units are identified within the Ivrea zone of northern Italy exhibiting magnetic susceptibilities greater than 00005 cgs, saturation magnetization values above 0009 emu/cu cm, and Curie points as high as 570-580 C Amphibolites from the granulite-amphibolite facies transition, and the mafic-ultramafic granulite facies lithologies exhibit high values of initial susceptibility and saturation remanence, are laterally continuous, and may be considered as a deep crustal source for long-wavelength anomalies in low-geothermal gradient areas Evidence is presented which suggests that such mafic-ultramafic bodies as those exposed in the Toce valley were synmetamorphic additions to the base of the crust

K, Rb, Sr, Ba, U and Th geochemistry of the Lapland Granulites (Fennoscandia). LILE fractionation controlling factors

Abstract: The LILE geochemical patterns of the three main lithological units (graywacke-shale metasedimentary sequence, tholeiitic metaigneous rocks and migmatitic rocks) of the Lapland Granulite belt are described. K, Ba, Sr and Th concentrations in metasediments are nearly similar to average continental crust, whereas Rb and U are unevenly impoverished. In particular graphitic metashales and calcsilicate rocks are not significantly depleted in uranium. Tholeiitic metaigneous rocks comprises metavolcanics which present K/Rb ratios similar to metasediments, and metaplutonics with LILE abundances close to those of the low-K-tholeiites. Migmatites show wide range in LILE content. Metatexites and diatexites have higher K, Rb, Th and U concentrations and similar K/Rb ratios with respect to equivalent unmobilized rocks. Potassic pegmatoids are strongly enriched K, Rb, Ba and Th but moderately in Sr and U. Plagioclasic pegmatoids and ferromagnesian restites are rich in Sr and poor in other LIL elements.

The Formation of Garnet in Olivine-Bearing Metagabbros from the Adirondacks

Abstract: A regional study of olivine-bearing metagabbros in the Adirondacks has permitted testing of the P(pressure)-T(temperature)-X(composition) dependence of garnet-forming reactions as well as providing additional regional metamorphic pressure data. Six phases, olivine, orthopyroxene, clinopyroxene, garnet, plagioclase and spinel, which can be related by the reactions: orthopyroxene+clinopyroxene+spinel +anorthite=garnet, and forsterite+anorthite=garnet occur together both in coronal and in equant textures indicative of equilibrium. Compositions of the respective minerals are typically Fo25–72, En44–75, En30–44Fs9–23Wo47–49, Pp13–42Alm39–63Gr16–20, An29–49 and Sp16–58. When they occur in the same rock, equant and coronal garnets are homogeneous and compositionally identical suggesting that chemical equilibrium may have been attained despite coronal textures. Extrapolating reactions in the simple CMAS system to granulite temperatures and making thermodynamic corrections for solid solutions gives equilibration pressures (using the thermometry of Bohlen et al. 1980b) ranging from about 6.5 kb in the Lowlands and southern Adirondacks to 7.0–8.0 kb in the Highlands for the assemblage olivine-plagioclase-garnet. These results are consistent with inferred peak metamorphic conditions in the Adirondacks (Valley and Bohlen 1979; Bohlen and Boettcher 1981). Thus the isobaric retrograde path suggested by Whitney and McLelland (1973) and Whitney (1978) for the formation of coronal garnet in olivine metagabbros may not be required. Application of the same equilibria gives >8.7 kb for South Harris, Scotland and 0.9 kb for the Nain Complex. Disagreement of the latter value with orthopyroxeneolivine-quartz barometry (Bohlen and Boettcher 1981) suggests that the use of iron-rich rocks (olivines ≧Fa50) results in errors in calculated pressures.

Polymetamorphic relations in iron ores from the Iron Quadrangle, Brazil: The correlation of oxygen isotope variations with deformation history

Abstract: This study deals with the oxygen isotope composition of hematite-rich ore bodies in the Iron Quadrangle, Brazil. The area studied can be divided into two different regions: a western (W) zone of greenschist facies assemblages and an eastern (E) zone of amphibolite facies with transitions into granulite facies assemblages.

Fluorine end-member micas and amphiboles

Abstract: The near end-member minerals fluorphlogopite (xF = 0.96) and fluortremolite (xF : 0'82) have been found in Grenville marbles near Balmat, New York. These micas and amphiboles, like other fluorine-rich minerals reported in the literature, are extremely low in iron. The substitution of F for OH is partly responsible for stabilizing these minerals in the granulite facies marbles of the Adirondacks. Fluorine-rich amphiboles and micas are more common than generally recognized. A literature review shows that many amphiboles and micas have more than fifty percent of the interlayer site occupied by fluorine. This degree of solid solution qualifies these phases as independent minerals, but they are not currently recognized by the I.M.A. We propose that fluorbiotite, fluorphlogopite, fluoractinolite, fluorarfvedsonite, fluoredenite, fluorhastingsite, fluorpargasite, fluorrichterite, fluorriebeckite, fluortremolite, fluoredenitic hornblende, fluorhastingsitic hornblende, fluorpargasitic hornblende, fluortremolitic hornblende, fluorferro-edenite, and fluormagnesio-arfvedsonite be formally applied as mineral names for these phases. Identification of the mineralogically and petrologically important solid solution of fluorine for hydroxyl is currently obscured by use of names that imply hydroxyl end-members.

Seismic crustal structure of the Proterozoic North Australian Craton between Tennant Creek and Mount Isa

Abstract: Seismic recording along a 600-km traverse across the southern part of the North Australian craton indicates that lateral velocity variations exist in the middle/lower crust. Taken together with other data from the Craton, there is evidence for midcrustal velocities increasing from east to west and from south to north. These trends could result from a greater proportion of high-velocity granulites being emplaced at higher levels in the crust. Between Tennant Creek and Mount Isa the velocity structures near the surface are compatible with an assemblage of younger and older metamorphic rocks intruded by granites. Near-surface (0–5 km) P wave velocities on the Tennant Creek Inlier increase from 5.47 km/s at the surface to about 6.2 km/s at 6-km depth and correspond with low-grade metamorphics of the Warramunga Group interspersed with weathered granites overlying amphibolite facies rocks of older metamorphic domains. In the Mount Isa Geosyncline, velocities of 6.03–6.15 km/s correspond to the Leichhart Metamorphics interspersed with granites. In the middle crust, P wave velocities of 6.85 km/s occur at a depth of 26 km near Tennant Creek, whereas such velocities are not evident until depths of about 37 km near Mount Isa. There is therefore a trend for midcrust velocities to increase from east to west. The lower crust is characterized by velocities of 7.3–7.5 km/s, and the upper mantle velocities of 8.16–8.20 km/s are reached at depths of 51–54 km. Exposed granulite facies rocks in central Australia are low in heat-producing elements and are compatible with the low surface heat flow generally measured in Precambrian Australia. These data taken together with laboratory velocity measurements on worldwide samples reported elsewhere indicate that middle to lower crustal rocks could be an assemblage of granulite facies rocks varying from pyroxene granulite in the middle crust, through hornblende granulite, to garnet granulite in the lower crust.

Petrologic variations in quaternary volcanic rocks, British Columbia, and the nature of the underlying upper mantle

Abstract: Volcanic activity has produced Late Tertiary and Quaternary cinder cones and flows between the Snake River Plain, U.S.A. and the Yukon Territory, Canada. The rock types include basanites, alkali olivine basalts, high-iron basalts, hawaiites, ankaramites, nephelinites, and olivine tholeiites. The alkali olivine basalts, basanites and hawaiites sampled are chemically similar to rocks from the mid-Atlantic islands. Associated with the volcanic rocks are xenoliths of ultramafic rocks, gabbros, granites and granulites. Seismic data indicate that the Moho throughout the region dips eastward at a very shallow angle. The low velocity zone has been located beneath southern British Columbia and displays a topographic high trending northwest-southeast. The nephelinite was erupted from near the crest of this high with less undersaturated lavas erupted from along its flanks. The suite of ultramafic xenoliths spans a greater variety of rock types than can be generated by maximum amounts of partial melting of a uniform source material to produce the lavas in the region. Calculated residual olivine compositions in equilibrium with the magmas at low velocity zone depths and liquidii temperatures are more iron-rich than the typical lherzolite xenolith olivine. This suggests that the residua from the partial melting episodes which produced the volcanic rocks are different from the upper mantle lid above the low velocity zone as represented by the ultramafic xenoliths.