A rare earth element study of complex zircons from early Archaean Amitsoq gneisses, Godthåbsfjord, south-west Greenland

We present high spatial resolution ion-microprobe rare earth element (REE) data for discrete growth phases of complex polyphase zircons from early Archaean Amitsoq gneisses, outer Godthabsfjord, SW Greenland. In Matsuda diagrams, the two major growth phases, >3.8 Ga cores and ca. 3.65 Ga rims, have steep positive slopes from La to Lu, prominent positive Ce anomalies and negative Eu anomalies that are consistent with growth in a melt. Exceptions to this are non-cathodolurnmescent zircon developed between the cores and rims, sometimes truncating zoning in the cores, and late Archaean prismatic tip overgrowths, both of which exhibit flatter light REE (LREE) patterns and have small or no Eu anomaly, which we interpret as the result of metamorphism and/or small-degree, isolated partial melting. Our data support previous interpretations that the ca. 3.65 Ga zircon phase was generated in a melt, with the >3.8 Ga phase representing either original protolith zircons in a large degree partial melt or inherited zircons in an introduced magma. Regardless which of these two interpretations is correct for these, and similar, rocks in the outer GodthAbsfjord, the 3.65 Ga event will have profoundly affected isotopic systems and obscured beyond recognition any earlier igneous features such as cross-cutting relationships, which may only be assigned a minimum 3.65 Ga age. (C) 2003 Elsevier Science B.V. All rights reserved.

Integrated Pb- and S-isotope investigation of sulphide minerals from the early Archaean of southwest Greenland

We present high-spatial resolution secondary ion mass spectrometry (SIMS) measurements of Pb and S isotopes in sulphides from early Archaean samples at two localities in southwest Greenland. Secondary pyrite from a 3.71 Ga sample of magnetite-quartz banded iron formation in the Isua Greenstone Belt, which has previously yielded unradiogenic Pb consistent with its ancient origin, contains sulphur with a mass independently fractionated (MIF) isotope signature (Delta(33)S =+3.3 parts per thousand). This reflects the secondary mineralization of remobilized sedimentary S carrying a component modified by photochemical reactions in the early Archaean atmosphere. It further represents one of the most extreme positive excursions so far known from the early Archaean rock record. Sulphides from a quartz-pyroxene rock and an ultramafic boudin from the island of Akilia, in the Godth (a) over circle bsfjord, have heterogeneous and generally radiogenic Pb isotopic compositions that we interpret to represent partial re-equilibration of Pb between the sulphides and whole rocks during tectonothermal events at 3.6, 2.7 and 1.6 Ga. Both these samples have Delta(33)S=0 (within analytical error) and therefore show no evidence for MIF sulphur. These data are consistent with previous interpretations that the rock cannot be proven to have a sedimentary origin. Our study illustrates that SIMS S-isotope measurements in ancient rocks can be used to elucidate early atmospheric parameters because of the ability to obtain combined S and Pb-isotope data, but caution must be applied when using such data to infer protolith. When information from geological context, petrography and chronology (i.e. by Pb isotopes) is combined and fully evaluated, Delta(33)S signatures from sulphides and their geological significance can be interpreted with a higher degree of confidence. (c) 2005 Elsevier B.V All rights reserved.

Uranium and thorium contents of coexisting gneisses, granites and pegmatites

U, Th and K contents of gneisses, granites and pegmatites of the Precambrian shield complex of S. E. Mysore have been determined by gamma ray spectrometry. Th/U ratios in most gneisses and granites are found to have values in the range 5–15, being higher than the accepted value of about 3.5 for crustal material.

Evolution of high-pressure mafic granulites and pelitic gneisses from NE Madagascar: Tectonic implications

The occurrence of high-pressure mafic-ultramafic bodies within major shear zones is one of the indicators of paleo-subduction. In mafic granulites of the Andriamena complex (north-eastern Madagascar) we document unusual textures including garnet-clinopyroxene-quartz coronas that formed after the breakdown of orthopyroxene-plagioclase-ilmenite. Textural evidence and isochemical phase diagram calculations in the Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2 system indicate a pressure-temperature (P-T) evolution from an isothermal (780 degrees C) pressure up to c. 24 kbar to decompression and cooling. Such a P-T trajectory is typically attained in a subduction zone setting where a gabbroic/ultramafic complex is subducted and later exhumed to the present crustal level during oceanic closure and final continental collision. The present results suggest that the presence of such deeply subducted rocks of the Andriamena complex is related to formation of the Betsimisaraka suture. LA-ICPMS U-Pb zircon dating of pelitic gneisses from the Betsimisaraka suture yields low Th/U ratios and protolith ages ranging from 2535 to 2625 Ma. A granitic gneiss from the Alaotra complex yields a zircon crystallization age of ca. 818 Ma and Th/U ratios vary from 1.08 to 2.09. K-Ar dating of muscovite and biotite from biotite-kyanite-sillimanite gneiss and garnet-biotite gneiss yields age of 486 +/- 9 Ma and 459 +/- 9 Ma respectively. We have estimated regional crustal thicknesses in NE Madagascar using a flexural inversion technique, which indicates the presence of an anomalously thick crust (c. 43 km) beneath the Antananarivo block. This result is consistent with the present concept that subduction beneath the Antananarivo block resulted in a more competent and thicker crust. The textural data, thermodynamic model, and geophysical evidence together provide a new insight to the subduction history, crustal thickening and evolution of the high-pressure Andriamena complex and its link to the terminal formation of the Betsimisaraka suture in north-eastern Madagascar. (C) 2015 Elsevier B.V. All rights reserved.

Metamorphic evolution and P-T path of gneisses from Goiás Magmatic Arc in southwestern Goiás State

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

The Eclogitic Gneisses of the Cabo Ortegal Complex: provenance and tectonothermal evolution (zircon U-Pb/ Lu-Hf methods)

El cinturón Varisco se formó cuando varios continentes interactuaron para acabar formando el supercontinente de Pangea. La tesis que aquí se presenta se enmarca en este contexto general de interacción continental y desarrollo orogénico. El principal objetivo es explorar la formación, las fuentes y el desarrollo de un terreno metamórfico de alto grado involucrado en la orogénesis Varisca. En esta tesis se hace especial hincapié en los Gneises Eclogíticos (Gneises Bandeados), los cuales están incluidos en el miembro de alta presión y alta temperatura (HP–HT) del Alóctono Superior del Complejo de Cabo Ortegal (NO de Iberia). También se aborda el estudio de otros miembros del Alóctono Superior, como la banda principal de eclogitas y los Gneises de Cariño, con el fin de tener más herramientas para abordar la evolución tectonotermal general. Para cumplir con los objetivos de esta tesis se han aplicado varios métodos. El primer grupo de métodos tenían por objetivo reconocer y describir el objeto de estudio, desde un punto de vista macroscópico (cartografía y representación detallada de secciones geológicas) hasta un punto de vista microscópico (observación y descripción de láminas delgadas). Una vez terminado el reconocimiento de las diferentes litologías y de la revisión bibliográfica pertinente, se llevaron a cabo determinaciones isotópicas detalladas. Se aplicaron métodos isotópicos U–Pb, Lu–Hf y Sm–Nd en las formaciones estudiadas para conocer las áreas fuente de los materiales detríticos, las edades de cristalización, las fuentes ígneas primigenias y la edad y las condiciones del metamorfismo de alto grado desarrollado en las rocas ígneas estudiadas...

Metamorphic investigation of mid-P aluminous gneisses from the central Grenville Province

This is a metamorphic study of mid-P anatectic aluminous gneisses from the Manicouagan and lac du Milieu areas of the central Grenville Province. The rocks are derived from hydrothermally altered felsic protoliths and were metamorphosed at granulite facies conditions during the Grenvillian orogeny. The samples come from three locations separated by several tens of kilometers and exhibit a wide range of textures and bulk compositions. However, they all have the same peak mineral assemblage: garnet + biotite + quartz + K-feldspar +/- plagioclase +/- sillimanite with retrograde cordierite in some, and show evidence of partial melting and melt loss. In terms of mineralogy and bulk composition, the samples were divided into two groups, sillimanite-rich and sillimanite-poor, with a high and low Alumina index in the AFM space, respectively. Phase equilibria modeling in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O– TiO₂–O (NCKFMASTHO) system using Thermocalc constrained the P–T field of the peak mineral assemblage at 800–900ºC and 6–11kbar, with melt solidification in the range of 800–865ºC and 6–8kbar. The presence of sillimanite inclusions in garnet, and of only scarce, retrograde cordierite, is consistent with moderate dP/dT gradient ‘hairpin’ P– T paths, which were similar between the three locations. This study also investigated the role of Fe3+ on phase stability in mid-P aluminous systems. Fe³⁺ is problematic because although it is incorporated in the NaCKFMASTHO system, it is rarely measured in modeled minerals and rocks and its value is generally assumed. Biotite may contain significant amounts of Fe³⁺, and these were analysed by Mössbauer spectroscopy in selected samples, where they were found to be low (0-4%). In addition, the effect of increasing the bulk Fe³⁺ in the mid-P portion of phase diagrams was modeled. This increase added new minor phases and changed the phase proportions, as well as shifted phase boundaries to a small degree, but P–T paths remained largely unaffected. Finally, the two methods commonly used in phase equilibria modeling to account for melt loss were compared. In some cases there were major differences in the topologies between the ‘melt reintegration’ and ‘adding water’ methods, but the former method is the most consistent with the rock data, and should be the method of choice.

Micas in experimentally shocked gneiss

Biotites and muscovites from a gneiss have been experimentally shocked between 18 and 70 GPa using powder-propellant guns at NASA Johnson Space Center and at the California Institute of Technology. This study shows that shock in biotite and muscovite can produce homogeneous and devolatilized glasses within microseconds. Shock-deformed micas display fracturing, kinking, and complex extinction patterns over the entire pressure range investigated. However, these deformation features are not a sensitive pressure indicator. Localized melting of micas begins at 33 GPa and goes to completion at 70 GPa. Melted biotite and muscovite are optically opaque, but show extensive microvesiculation and flow when observed with the SEM. Electron diffraction confirms that biotite and muscovite have transformed to a glass. The distribution of vesicles in shock-vitrified mica shows escape of volatiles within the short duration of the shock experiment. Experimentally shocked biotite and muscovite undergo congruent melting. Compositions of the glasses are similar to the unshocked micas except for volatiles (H2O loss and K loss). These unusual glasses derived from mica may be quenched by rapid cooling conditions during the shock experiment. Based on these results, the extremely low H2O content of tektites may be reconciled with a terrestrial origin by impact. Release of volatiles in shock-melted micas affects the melting behavior of coexisting dry silicates during the short duration of the shock experiment. Transportation and escape of volatiles released from shock-melted micas may provide plausible mechanisms for the origin of protoatmospheres on terrestrial planets, hydrothermal activity on phyllosilicate-rich meteorite parent bodies, and fluid entrapment in meteorites.

Geologic map of the Valley Mountain 15’ quadrangle, San Bernardino and Riverside Counties, California: U.S. Geological Survey Geologic Quadrangle Map GQ–1767, scale 1:62,500

The Valley Mountain 15’ quadrangle straddles the Pinto Mountain Fault, which bounds the eastern Transverse Ranges in the south against the Mojave Desert province in the north. The Pinto Mountains, part of the eastern Transverse Ranges in the south part of the quadrangle expose a series of Paleoproterozoic gneisses and granite and the Proterozoic quartzite of Pinto Mountain. Early Triassic quartz monzonite intruded the gneisses and was ductiley deformed prior to voluminous Jurassic intrusion of diorite, granodiorite, quartz monzonite, and granite plutons. The Jurassic rocks include part of the Bullion Mountains Intrusive Suite, which crops out prominently at Valley Mountain and in the Bullion Mountains, as well as in the Pinto Mountains. Jurassic plutons in the southwest part of the quadrangle are deeply denuded from midcrustal emplacement levels in contrast to supracrustal Jurassic limestone and volcanic rocks exposed in the northeast. Dikes inferred to be part of the Jurassic Independence Dike Swarm intrude the Jurassic plutons and Proterozoic rocks. Late Cretaceous intrusion of the Cadiz Valley Batholith in the northeast caused contact metamorphism of adjacent Jurassic plutonic rocks...

K-Ar ages of biotite and muscovite from Pangong metamorphic complex, Shyok suture zone, India: Implications for the youngest post-collision metamorphic event in Ladakh Himalaya

The garnet-kyanite-staurolite and garnet-biotite-staurolite gneisses were collected from a locality within Lukung area that belongs to the Pangong metamorphic complex in Shyok valley, Ladakh Himalaya. The kyanite-free samples have garnet and staurolite in equilibrium, where garnets show euhedral texture and have flat compositional profile. On the other hand, the kyanite-bearing sample shows equilibrium assemblage of garnet-kyanite-staurolite along with muscovite and biotite. In this case, garnet has an inclusion rich core with a distinct grain boundary, which was later overgrown by inclusion free euhedral garnet. Garnet cores are rich in Mn and Ca, while the rims are poor in Mn and rich in Fe and Mg, suggesting two distinct generations of growth. However, the compositional profiles and textural signature of garnets suggests the same stage of P -T evolution for the formation of the inclusion free euhedral garnets in the kyanite-free gneisses and the inclusion free euhedral garnet rims in the kyanite-bearing gneiss. Muscovites from the four samples have consistent K-Ar ages, suggesting the cooling age (∼ 10 Ma) of the gneisses. These ages make a constraint on the timing of the youngest post-collision metamorphic event that may be closely related to an activation of the Karakoram fault in Pangong metamorphic complex.