Petrologic Relations Research Articles

Fluoritites Produced by Crystallization of Carbonate–Fluoride Magma

Abstract Fluorite-dominated rocks are occasionally found in association with carbonatites, but their geologic and petrologic relations are rarely reported. The Dunkeldyk area of the Pamir mountains in south-eastern Tajikistan contains dikes of distinctive rocks composed of calcite, fluorite, celestine-barite, sulfides, apatite, with minor quartz, biotite, and REE fluorcarbonates. The dikes have sharp contacts with the host (meta-)sedimentary rocks and layering with ribbons, ranging from fluorite-bearing calcite carbonatites to fluoritites (rocks with >50% fluorite). The fluoritites are characterized by high Ca, F, Ba, Sr, REE, and S coupled with anomalously low O. The geologic relations and textures suggest a magmatic origin of the dikes from melts close to calcite–fluorite eutectic that experienced nucleation-controlled differentiation during the crystallization of dikes and the formation of fluoritite cumulates in larger intrusions. The Dunkeldyk dikes demonstrate that sizable geological bodies of fluorite-dominated rocks could form from carbonate–fluoride melts originating from the differentiation of alkaline silicate magmas.

Using zircon U–Pb ages and trace element chemistry to constrain the timing of metamorphic events, pegmatite dike emplacement, and shearing in the southern Parry Sound domain, Grenville Province, Canada

The integration of zircon U–Pb ages and trace element chemistry with structural and petrologic relations from a range of sample types provides important temporal constraints on the tectono-metamorphic evolution of the southern Parry Sound domain (PSD), Ontario, Canada, and the processes attending development of the underlying Twelve Mile Bay shear zone (TMBSZ). Intact granulites preserve ca. 1145 Ma ages, slightly younger than those farther to the north in the interior PSD, but similar to that of the underlying granulite-to-amphibolite facies Parry Sound shear zone. Weaker zircon HREE enrichment in sheared and retrogressed samples with partially resorbed garnet porphyroblasts (containing abundant zircon inclusions) suggests that the 1128–1143 Ma ages common to sheared rocks in the southern PSD record an earlier phase of metamorphism and deformation, and not the timing of shear zone development. Deformed pegmatite dikes bounded by sheared and retrogressed wall rocks from across the southern interior PSD consistently record ca. 1100 Ma ages, indicating synchronous pegmatite emplacement across the transect and constraining the bounding amphibolite-facies shear zones to ≤1100 Ma. TMBSZ samples retain evidence for (1) the older ( ca. 1145 Ma) high-grade metamorphic event, (2) pegmatite emplacement and shear deformation at ca. 1100 Ma, and (3) a later ( ca. 1070 Ma) shearing event. Combined with published structural and petrologic data, these ages indicate that pegmatite emplacement and shearing in the TMBSZ was synchronous with that in the southern interior PSD; further confirming published models for TMBSZ development. Additionally, the data suggest that the Twelve Mile Bay assemblage (within the TMBSZ) experienced high-grade metamorphism synchronously with the interior PSD, and may therefore be correlative with the basal PSD to the north.

Aluminous sapphirine granulites from the Eastern Ghats Belt (India): Phase relations and relevance to counterclockwise P-T history

In the Eastern Ghats Belt, 7-9-3 sapphirine in biotite-free Crd + Crn ± Sil ± Spl restitic domains occurs interlayered with Opx + Crd + K-feldspar assemblages extensively replaced by biotite. Natural settings and experimental phase relations indicate the restitic assemblages were derived by low pressure ( 800 °C) anatexis of pelitic protoliths. In the restitic domain, sapphirine formed by the reaction Crn + Crd + Spl = Spr. Volumetrically minor amounts of sillimanite associated with sapphirine were produced by the ancillary reaction Crn + Crd = Spr + Sil. The decomposition of sapphirine + cordierite to En + Sil symplectites is attributed to the reaction Spr + Crd = En + Sil; occasional spinel in the symplectite may have formed by the reaction Spr = Spl + Sil + En. A quantitative activity-corrected M(F)AS grid constructed for Grt and Qtz-absent Mg, Al-rich metapelites suggests that sapphirine stabilization was induced by loading at ∼4.5 kbar, 900 °C. The decomposition of sapphirine occurred during cooling down to 700 °C at 8 Kbar in a thickened crust. The record of low-P prograde heating that led to high T (> 800 °C, P max attained broadly synchronous with P max at ∼9 kbar. The phase petrological relations in the sapphirine granulites strongly indicate that near-peak T was attained prior to the attainment of P max value. The initial high thermal anomaly is reconciled with asthenospheric heating below a thinned crust, and does not correspond with the thermal condition in the upper zone of a crust of normal thickness experiencing asthenospheric perturbation. Since loading induces temperature increase due to shallowing of geotherms, the cooling of the thickened crust can only be explained by conductive heat loss to “cooler” thrust slices above the hot crust, and/or asthenospheric depression. The post-T max cooling in the CCW orogen suggests the continuance of thermal relaxation in the thickened crust in its attempt to attain isostatic stability with its environs.

Naturally occurring graphite cones

Carbon, boron nitride, and other materials that form nanotubes are also able to form conical shapes. Even though the potential applications of cone arrays as electron emitters and other devices are very promising, understanding of their structure and formation mechanisms is still very limited compared to nanotubes and other carbon structures. Moreover, the cones have only been synthesized in a mixture with other shapes, but never as continuous arrays. It appears, however, that we can learn from nature how to produce large carbon cone arrays. We here report the first-known natural occurrence of large arrays of conical graphite crystals. These occur on the surfaces of millimeter-sized polycrystalline spheroidal aggregates of graphite. Cone heights range from less then a micron to 40 μm, which is larger than any other carbon cones reported in the literature. They are also observed to dominate sample surfaces. The surface topography of the cones and petrologic relations of the samples suggest that the cones formed from a metamorphic fluid. Unlike most laboratory produced cones, the natural cones have a wide distribution of apex angles, which supports a disclination model for cone-helix structures.

Non-magmatic fracture-controlled hydrothermal systems in the Idaho Batholith: South Fork Payette geothermal system

Non-magmatic, fracture-controlled hydrothermal systems located within and near the Idaho batholith have been examined and evaluated for potential application as natural analogues to high-level nuclear waste repositories. Detailed geochemical, petrologic, and structural studies of hot springs along the South Fork of the Payette River (SFPR) in central Idaho, USA, have been combined to assess the nature of a hydrothermal aquifer system along a major brittle shear zone. Geochemical modeling of thermal spring water chemistry with SOLVEQ [Reed, M.H., 1992. SOLVEQ: Users Manual. Unpublished Report, Dept. Of Geological Sciences, University Of Oregon, Eugene, OR.] indicates reservoir temperatures of 85°C to 160°C, and an alteration assemblage consisting of several clays, chlorite, serpentinites, quartz, albite, microcline, and micas. Geothermometry calculations, coupled with independent estimates of geothermal gradients based on heat flow and regional geology, allow estimation of reservoir depths ranging from 2.4 to 6.7 km. Comparison of trace element, rare earth element, and SOLVEQ alteration assemblages supports the hypothesis that waters equilibrated with granitic rocks at depth and ascended to the surface in a short time span. Stable isotope data (δD=−138.4‰ to −148.4‰ and δ 18O=−17.5‰ to −19.4‰) indicate that thermal waters of meteoric origin have interacted with country rocks at a very high water/rock (W/R) ratio. Systematic variations in δD values and the amount of δ 18O shift for thermal waters suggest that recharge probably occurs at much higher elevations than discharge. Fracture orientations, petrologic relations, and geochemical differences between adjacent springs delineate separate hydrothermal systems along the major structure defining the course of the SFPR. Hydrothermal systems along the SFPR exist primarily as small circulation cells which are probably influenced by minor changes in geothermal gradients associated with different rock types. These subsystems are dependent on periodic seismic or microseismic activity to maintain flowpaths. A working model of deep, fracture-controlled hydrothermal convection systems based on the SFPR Geothermal System may be used to evaluate the long-term behavior of artificial hydrothermal systems associated with high-level nuclear waste repositories.

56A. Petrological Relations between Tholeiitic and Calc-Alkalic Series at Zao Volcano, Northeast Japan.(Abstracts of Papers Presented at the Spring Meeting of the Society)

56A. Petrological Relations between Tholeiitic and Calc-Alkalic Series at Zao Volcano, Northeast Japan.(Abstracts of Papers Presented at the Spring Meeting of the Society)

Deep Environment of Volcanogenic Epithermal Mineralization: Proposed research drilling at Creede, Colorado

Proposed research drilling in the Creede mining district will constrain relations between epithermal veins and the deeper thermal, fluid flow, and volcanic structural environment. Major targets include a deep inferred heat source in the form of a concealed intrusion and possibly associated stacked porphyry‐type mineralization; thermal, chemical, and hydrologic evolution of the fossil hydrothermal system in comparison with active geothermal environments; and temporal, structural, and petrologic relations between the mineralization and the hosting silicic caldera cluster.

Geochemical and petrological relations in some deccan basalts, Western Maharashtra, India

Abstract Petrographic and geochemical data are given for some basaltic rocks from the Koynaghat, Ambaghat and Panvel sections of the western Deccan volcanic province. This study confirms geochemical features established earlier for the Deccan basalts but brings out minor additional characters. Mineralogical and major-element compositions of the basaltic flows from the Koyna and Panvel sections indicate tholeiitic affinity; the Ambaghat flows exhibit a slight affinity towards alkali basalt. Rare earth element (REE) distribution patterns and trace-element abundances suggest minor fractionation of olivine and plagioclase during the evolution of the flows. The general similarity of chemical and mineralogical features over a wide area and the lack of conspicuous inter-element relationships suggest that the flows reflect the combined effects of partial melting, minor mineral fractionation and selective crustal contamination.

The geology of the polymetamorphic complex of Ios, Cyclades, Greece and its significance for the Cycladic Massif

The geological structure of the island of Ios, Cyclades, Greece, is a dome consisting of an augengneiss core and a mantle of garnet-mica schists that together form the Basement which is overthrust by a marble-schist Series. This Series is mainly made up of meta-volcanics and metamorphosed sediments, presumably Mesozoic in age. It is a tectonically composed pile of marbles alternating with glaucophane schists, actinolite schists and chlorite schists. Petrological relations and isotope dating indicate the polymetamorphic character of Ios. Two Alpine metamorphic phases, M1 and M2, and relicts of a high grade metamorphic or magmatic phase (Mo), that only affected the Basement, are demonstrated. Radiometric ages obtained for the Basement confirm the interpretation that the Mo phase is Pre-Alpine. The M1 and M2 were dated at 43 Ma and 25 Ma respectively. The P-T conditions of metamorphism are estimated as 9–11 kb and 350 to 400° C for the M1 phase and 5–7 kb and 380–420° C for the M2 phase. The metamorphic history of the Cyclades is discussed and it is suggested that Pre-Alpine Basement occurs also on Sikinos and Naxos.

Contrasted petrological relations between tholeiitic and calc-alkalic series from Funagata volcano,northeast Japan.

Funagata volcano, on the volcanic front of northeast Japan arc, is situated at about 35km northwest of Sendai city, Miyagi prefecture. It is made up of three major volcanic bodies, consisting predominantly of lavas with subordinate pyroclastic rocks, of which about 95 vol. % is calc-alkalic andesite and about 5 vol. % is tholeiitic basalt and andesite. Chemical and mineralogical data are presented to show the contrasted petrological relations between tholeiitic and talc-alkalic series from Funagata volcano. The different compositional trends between both series in the 1/K2O-oxides diagram suggest that the calc-alkalic magma was not produced by continuous fractional crystallization of the tholefitic magma. Analysed mineral phases in the tholeiitic rocks display systematic compositional variations, obeying the process of simple fractional crystallization, On the other hand, mineralogical evidences of some calc-alkalic rocks, such as disequilibrium phenocryst assemblages and reverse zoning of phenocrysts, indicate that magma mixing may have played a key role in their formation. Calculation of magmatic temperatures by pyroxene geothermometer indicates that the tholeiitic magmas crystallized under about 1180-1000°C, whereas the calc-alkalic magmas did under almost constant 950°C. The liquidus temperatures of plagioclase at 1 atm were estimated to be nearly the same range (1070-1210°C) between tholeiitic and calc-alkalic series. The difference between the calculated 1 atm liquidus temperature and pyroxene equilibration temperature was supposed to be ascribed to the difference of H2O contents in magmas at the time of phenocrysts crystallization. Evaluated H20 contents on the basis of experimental data of Sekine et al. (1979) range from 0.5-1.6 wt. % in the tholeiitic magma and 2.0-3.8 wt. % in the calc-alkalic magma. The extremely low H2O contents of the tholeiitic magma imply that the primary magma of the tholeiitic series is unlikely to have originated by hydrous partial melting of mantle peridotite.

Petrological relations in some Icelandic basaltic hyaloclastites

Petrological relations in some Icelandic basaltic hyaloclastites

Regional Geology of Yukon-Tanana Upland, Alaska: ABSTRACT

The basic geologic framework of the Yukon-Tanana Upland, Alaska, a mountainous region of about 30,000 sq mi between the Yukon and Tanana Rivers, was delineated primarily by L. M. Prindle and J. B. Mertie, Jr., in the early part of this century. The subsequent recognition of large-scale offset along the Tintina fault, which bounds the eastern upland on the north, has required a reconsideration of the regional stratigraphic and structural relations. The northwestern part of the upland is dominantly underlain by a sedimentary sequence consisting of rocks which range in age from Cambrian to Mississippian. Cretaceous and Tertiary sedimentary rocks unconformably overlie the older sequence. The Cambrian is apparently underlain by a thick section of grits, quartzites, phyllites, and quartz-mica schists. Pre-Silurian volcanic rocks, mafic and ultra-mafic rocks of probable Devonian age, and Permo-Triassic diabase and volcanic rocks are also present. These sedimentary End_Page 2480------------------------------ and igneous rocks are cut by granitic plutons of Cretaceous and Tertiary age. The central and eastern parts of the upland are underlain by a metamorphic complex with rocks that range from lower greenschist to amphibolite facies. Fossils date the parent sediments of some green-schist facies rocks as Paleozoic. Radiometric dates from several localities in the metamorphic complex indicate that Precambrian, Ordovician, and Jurassic-Cretaceous thermal events are recorded in the metamorphic history. Mesozoic granodiorite and quartz monzonite batholiths and smaller granitic plutons of Mesozoic and Tertiary ages intrude the crystalline schists. Locally, unmetamorphosed Cretaceous and/or Tertiary sedimentary rocks are in unconformable or fault contact with the older rocks. Tertiary volcanic rocks which range in composition from rhyolite to basalt overlie the older rocks in small but significant parts of the eastern upland. Ultramafic intrusions, mostly small and serpentinized, also are present. Work has progressed to the point where the sedimentary rocks in the upland can be correlated reasonably well with those in other parts of Alaska, but interregional correlation of the metamorphic terranes must await additional clarification of structural and petrologic relations. End_of_Article - Last_Page 2481------------

Precambrian Age Determinations in the Western San Gabriel Mountains, California

Isotopic age determinations (U-Th-Pb) on zircons from pegmatite and granophyre dikes in the western San Gabriel Mountains have yielded concordant ages of about 1,200 million years. From structural and petrological relations, this is both a minimum and a probable age for the San Gabriel Mountains anorthosite complex. It is also a minimum age for the Mendenhall gneiss, a retrograded charnockitic granulite which has been intruded by the anorthosite complex. Several zircon fractions from the gneiss yield consistent but moderately discordant apparent ages as follows: $Pb^{206}/U^{238} = 1,295 m.y., Pb^{207}/U^{235} = 1,355 m.y., Pb^{207}/ Pb^{206} = 1,440 m.y.$ The $Pb^{207}/Pb^{206}$ age is considered minimal for the zircon and, considering genetic uncertainties, a possible minimum age for the parental rock of the gneiss. These age determinations provide the first compelling evidence of Precambrian rocks on the west coast of North America. Their intimate involvement in the great fault systems of southern Ca...

Phosphatization at Malpelo Island, Colombia

A petrographic study is presented of three specimens from Malpelo, a volcanic island in the eastern equatorial Pacific Ocean. Two of the specimens show appreciable alteration by phosphatic solutions from guano: an amygdaloid in which the feldspars have been completely destroyed and replaced and a phosphate rock composed essentially of phosphosiderite and strengite, a dimorphous pair with the composition FePO 4 ·2H 2 O. The third is a comparatively unaltered augite andesite having a rather low silica content and possibly containing anemousite. The petrologic relations among aluminum-iron and calcium phosphates are discussed briefly.