An evaluation of orogenic kinematic evolution utilizing crystalline and magnetic anisotropy in granitoids

The Silurian-Devonian Galway Granite Complex (GGC ~425-380Ma) is defined here as a suite of granitoid plutons that comprise the Main Galway Granite Batholith and the Earlier Plutons. The Main Batholith is a composite of the Carna Pluton in the west and the Kilkieran Pluton in the east and extends from Galway City ~130km to the west. The Earlier Plutons are spatially, temporally and structurally distinct, situated northwest of the Main Batholith and include the Roundstone, Omey, Inis and Letterfrack Plutons. The majority of isotopic and structural data currently available pertain to the Kilkieran Pluton, several tectonic models have already been devised for this part of the complex. These relate emplacement of the Kilkieran Pluton to extension across a large east-west Caledonian lineament, i.e. the Skird Rocks Fault, during late Caledonian transtension. No chronological data have been published that directly and accurately date the emplacement of the Carna Pluton or any of the Earlier Plutons. There is also a lack of data pertaining to the internal structure of these intrusions. Accordingly, no previous study has established the mechanisms of emplacement for the Earlier Plutons and only limited work is available for the Carna Pluton. As a consequense of this, constituents of the GGC have not previously been placed in a context relative to each other or to regional scale Silurio-Devonian kinematics. The current work focuses on the Omey, Roundstone and Carna Plutons. Here, results of detailed field and Anisotropy of Magnetic Susceptibiliy (AMS) fabric studies are presented. This work is complemented by geological mapping that focuses on fault dynamics and contact relationships. Interpretation of AMS data is aided by rock magnetic experiment data and petrographic microstructural evaluations of representative samples. A new geological map of the the Omey Pluton demonstrates that this intrusion has a defined roof and base which are gently inclined parallel to the fold hinge of the Connemara Antiform. AMS and petrographic data show the intrusion is cross cut by NNW-SSE shear zones that extend into the country rock. These pre-date and were active during magma emplacement. It is proposed that the Omey pluton was emplaced as a discordant phacolith. Pre-existing subvertical D5 faults in the host rock were reactived during emplacement, due to regional sinistral transpression, and served as centralised ascent conduits. A central portion of the Roundstone Pluton was mapped in detail for the first time. Two facies are identified, G1 forms the majority of the pluton and coeval G2 sheets cross cut G1 at the core of the pluton. NNW-SSE D5 faults mapped in the country rock extend across the pluton. These share a geometrical relationship with the distribution of submagmatic strain in the pluton and parallel the majoity of mapped subvertical G2 dykes. These data indicate that magma ascent was controlled by NNW-SSE conduits that are inherently related to those identifed in the Omey Pluton. It is proposed that the Roundstone Pluton is a punched laccolith, the symmetry and structure of which was controlled by pre-exising host rock structures and regional sinistral transpressive stress which presided during emplacement. Field relationships show the long axis of the Carna Pluton lies parallel to mulitple NNW-SSE shear zones. These are represented on a regional scale by the Clifden-Mace Fault which cross cuts the core of this intrusion. AMS and petrographic data show concentric emplacement fabrics were tectonically overprinted as magma cooled from the magmatic state due to this faulting. It is proposed that the Clifden-Mace Fault system was active during ascent and emplacement of the magma and that pluton inflation only terminated as this controlling structure went into compression due to the onset of regional transtension. U-Pb zircon laser ablation inductively coupled mass spectrometry (LA-ICP-MS) data has been compiled from four sample sites. New geochronological data from the Roundstone Pluton (RD1 = ± 3.2Ma) represent the oldest age determination obtained from any member of the GGC and demonstrates that this pluton predates the Carna Pluton by ~10Ma and probably intruded synchronously with the Omey Pluton (~422.5 ± 1.7Ma). Chronological data from the Carna Pluton (CN2 = 412.9 ± 2.5Ma; CN3 = 409.8 ± 7.2Ma; CN4 = 409.6 ± 3.6Ma) represent the first precise magma crystallisation age for this intrusion. This work shows this pluton is 10Ma older than the Kilkieran Pluton and that the supply of magma into the Carna Pluton had terminated by ~409Ma. Chronological, magnetic and field data have been utilised to evaluate the kinematic evolution of the Caledonides of western Ireland throughout the construction of the GGC. It is proposed that the GGC was constructed during four distinct episodes. The style of emplacement and the conduits used for magma transport to the site of emplacement was dependent on the orientation of local structures relative to the regional ambiant stress field. This philosophy is used to critically evaluate and progress existing hypotheses on the transition from regional transpression to regional transtension at the end of the Caledonian Orogeny.

Emplacement mechanism of the main granite pluton of the Lavras do Sul intrusive complex, South Brazil, determined by magnetic anisotropies

Magnetic fabric and rock magnetism studies were performed on apparently isotropic granite facies from the main intrusion of the Lavras do Sul Intrusive Complex pluton (LSIC, Rio Grande do Sul, South Brazil). This intrusion is roughly circular (similar to 12 x 13.5 km), composed of alkali-calcic and alkaline granitoids, with the latter occupying the margin of the pluton. Magnetic fabrics were determined by applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). The two fabrics are coaxial. The parallelism between AMS and AARM tensors excludes the presence of a single domain (SD) effect on the AMS fabric of the granites. Several rock-magnetism experiments performed in one specimen from each sampled site show that for all sites the magnetic susceptibility is dominantly carried by ferromagnetic minerals, while mainly magnetite carries the magnetic fabrics. Lineations and foliations in the granite facies were successful determined by applying magnetic methods. Magnetic lineations are gently plunging and roughly parallel to the boundaries of the pluton facies, except at the few sites in the central facies which have a radial orientation pattern. In contrast, the magnetic foliations tend to follow the contacts between the different granite facies. They are gently outerward-dipping inside the pluton, and become either steeply southwesterly dipping or vertical towards its margin. The lack of solid-state and subsolidus deformations at outcrop scale and in thin sections precludes deformation after full crystallization of the pluton. This evidence allows us to interpret the observed magnetic fabrics as primary in origin (magmatic) acquired when the rocks were solidified as a result of processes reflecting magma flow. The foliation pattern displays a dome-shaped form for the main LSIC-pluton. However, the alkaline granites which outcrop in the southern part of the studied area have an inward-dipping foliation, and the steeply plunging magnetic lineation suggests that this area could be part of a feeder zone. The magma ascent probably occurred due to ring-diking. (C) 2008 Elsevier B.V. All rights reserved.

Low delta18O zircon grains in the Neoarchean Rum Jungle Complex, northern Australia: An indicator of emergent continental crust

The timing of widespread continental emergence is generally considered to have had a dramatic effect on the hydrological cycle, atmospheric conditions, and climate. New secondary ion mass spectrometry (SIMS) oxygen and laser-ablation–multicollector–inductively coupled plasma–mass spectrometry (LA-MC-ICP-MS) Lu-Hf isotopic results from dated zircon grains in the granitic Neoarchean Rum Jungle Complex provide a minimum time constraint on the emergence of continental crust above sea level for the North Australian craton. A 2535 ± 7 Ma monzogranite is characterized by magmatic zircon with slightly elevated δ18O (6.0‰–7.5‰ relative to Vienna standard mean ocean water [VSMOW]), consistent with some contribution to the magma from reworked supracrustal material. A supracrustal contribution to magma genesis is supported by the presence of metasedimentary rock enclaves, a large population of inherited zircon grains, and subchondritic zircon Hf (εHf = −6.6 to −4.1). A separate, distinct crustal source to the same magma is indicated by inherited zircon grains that are dominated by low δ18O values (2.5‰–4.8‰, n = 9 of 15) across a range of ages (3536–2598 Ma; εHf = −18.2 to +0.4). The low δ18O grains may be the product of one of two processes: (1) grain-scale diffusion of oxygen in zircon by exchange with a low δ18O magma or (2) several episodes of magmatic reworking of a Mesoarchean or older low δ18O source. Both scenarios require shallow crustal magmatism in emergent crust, to allow interaction with rocks altered by hydrothermal meteoric water in order to generate the low δ18O zircon. In the first scenario, assimilation of these altered rocks during Neoarchean magmatism generated low δ18O magma with which residual detrital zircons were able to exchange oxygen, while preserving their U-Pb systematics. In the second scenario, wholesale melting of the altered rocks occurred in several distinct events through the Mesoarchean, generating low δ18O magma from which zircon crystallized. Ultimately, in either scenario, the low δ18O zircons were entrained as inherited grains in a Neoarchean granite. The data suggest operation of a modern hydrological cycle by the Neoarchean and add to evidence for the increased emergence of continents by this time

Ultrahigh-temperature metamorphism from an unusual corundum plus orthopyroxene intergrowth bearing Al-Mg granulite from the Southern Marginal Zone, Limpopo Complex, South Africa

The Southern Marginal Zone of the Limpopo Complex is composed of granite-greenstone cratonic rocks reworked by a Neoarchean high-grade tectono-metamorphic event. Petrographic and mineral chemical characterization of an Al-Mg granulite from this zone is presented here. The granulite has a gneissic fabric with distinct Al-rich and Si-rich layers, with the former preserving the unusual lamellar (random and regular subparallel) intergrowths of corundum and symplectic intergrowth of spinel with orthopyroxene. The Al-rich layer preserves mineral assemblages such as rutile with orthopyroxene + sillimanite +/- A quartz, Al-rich orthopyroxene (similar to 11 wt%), spinel + quartz, and corundum in possible equilibrium with quartz, while the Si-rich layer preserves antiperthites and orthopyroxene + sillimanite +/- A quartz, all considered diagnostic of ultrahigh-temperature metamorphism. Application of Al-in-opx thermometry, ternary feldspar thermometry and construction of suitable pressure-temperature phase diagrams, compositional and model proportion isopleth results indicate P-T conditions as high as similar to 1,050-1,100 A degrees C, and similar to 10-12 kbars for the Al-Mg granulite. Our report of ultrahigh-temperature conditions is significant considering that the very high temperature was reached during decompression of an otherwise high-pressure granulite complex (clockwise P-T path), whereas most other ultrahigh-temperature granulites are linked to magma underplating at the base of the crust (counterclockwise P-T path).

Geological evolution of two crustal scale shear zones. Part I. The Rand thrust complex, northwestern Mojave Desert, California. Part II. The Magdalena metamorphic core complex, north central Sonora, Mexico

The geology and structure of two crustal scale shear zones were studied to understand the partitioning of strain within intracontinental orogenic belts. Movement histories and regional tectonic implications are deduced from observational data. The two widely separated study areas bear the imprint of intense Late Mesozoic through Middle Cenozoic tectonic activity. A regional transition from Late Cretaceous-Early Tertiary plutonism, metamorphism, and shortening strain to Middle Tertiary extension and magmatism is preserved in each area, with contrasting environments and mechanisms. Compressional phases of this tectonic history are better displayed in the Rand Mountains, whereas younger extensional structures dominate rock fabrics in the Magdalena area.

In the northwestern Mojave desert, the Rand Thrust Complex reveals a stack of four distinctive tectonic plates offset along the Garlock Fault. The lowermost plate, Rand Schist, is composed of greenschist facies metagraywacke, metachert, and metabasalt. Rand Schist is structurally overlain by Johannesburg Gneiss (= garnet-amphibolite grade orthogneisses, marbles and quartzites), which in turn is overlain by a Late Cretaceous hornblende-biotite granodiorite. Biotite granite forms the fourth and highest plate. Initial assembly of the tectonic stack involved a Late Cretaceous? south or southwest vergent overthrusting event in which Johannesburg Gneiss was imbricated and attenuated between Rand Schist and hornblende-biotite granodiorite. Thrusting postdated metamorphism and deformation of the lower two plates in separate environments. A post-kinematic stock, the Late Cretaceous Randsburg Granodiorite, intrudes deep levels of the complex and contains xenoliths of both Rand Schist and mylonitized Johannesburg? gneiss. Minimum shortening implied by the map patterns is 20 kilometers.

Some low angle faults of the Rand Thrust Complex formed or were reactivated between Late Cretaceous and Early Miocene time. South-southwest directed mylonites derived from Johannesburg Gneiss are commonly overprinted by less penetrative north-northeast vergent structures. Available kinematic information at shallower structural levels indicates that late disturbance(s) culminated in northward transport of the uppermost plate. Persistence of brittle fabrics along certain structural horizons suggests a possible association of late movement(s) with regionally known detachment faults. The four plates were juxtaposed and significant intraplate movements had ceased prior to Early Miocene emplacement of rhyolite porphyry dikes.

In the Magdalena region of north central Sonora, components of a pre-Middle Cretaceous stratigraphy are used as strain markers in tracking the evolution of a long lived orogenic belt. Important elements of the tectonic history include: (1) Compression during the Late Cretaceous and Early Tertiary, accompanied by plutonism, metamorphism, and ductile strain at depth, and thrust driven? syntectonic sedimentation at the surface. (2) Middle Tertiary transition to crustal extension, initially recorded by intrusion of leucogranites, inflation of the previously shortened middle and upper crustal section, and surface volcanism. (3) Gravity induced development of a normal sense ductile shear zone at mid crustal levels, with eventual detachment and southwestward displacement of the upper crustal stratigraphy by Early Miocene time.

Elucidation of the metamorphic core complex evolution just described was facilitated by fortuitous preservation of a unique assemblage of rocks and structures. The "type" stratigraphy utilized for regional correlation and strain analysis includes a Jurassic volcanic arc assemblage overlain by an Upper Jurassic-Lower Cretaceous quartz pebble conglomerate, in turn overlain by marine strata with fossiliferous Aptian-Albian limestones. The Jurassic strata, comprised of (a) rhyolite porphyries interstratified with quartz arenites, (b) rhyolite cobble conglomerate, and (c) intrusive granite porphyries, are known to rest on Precambrian basement north and east of the study area. The quartz pebble conglomerate is correlated with the Glance Conglomerate of southeastern Arizona and northeastern Sonora. The marine sequence represents part of an isolated arm? of the Bisbee Basin.

Crosscutting structural relationships between the pre-Middle Cretaceous supracrustal section, younger plutons, and deformational fabrics allow the tectonic sequence to be determined. Earliest phases of a Late Cretaceous-Early Tertiary orogeny are marked by emplacement of the 78 ± 3 Ma Guacomea Granodiorite (U/Pb zircon, Anderson et al., 1980) as a sill into deep levels of the layered Jurassic series. Subsequent regional metamorphism and ductile strain is recorded by a penetrative schistosity and lineation, and east-west trending folds. These fabrics are intruded by post-kinematic Early Tertiary? two mica granites. At shallower crustal levels, the orogeny is represented by north directed thrust faulting, formation of a large intermontane basin, and development of a pronounced unconformity. A second important phase of ductile strain followed Middle Tertiary? emplacement of leucogranites as sills and northwest trending dikes into intermediate levels of the deformed section (surficial volcanism was also active during this transitional period to regional extension). Gravitational instabilities resulting from crustal swelling via intrusion and thermal expansion led to development of a ductile shear zone within the stratigraphic horizon occupied by a laterally extensive leucogranite sill. With continued extension, upper crustal brittle normal faults (detachment faults) enhanced the uplift and tectonic denudation of this mylonite zone, ultimately resulting in southwestward displacement of the upper crustal stratigraphy.

Strains associated with the two ductile deformation events have been successfully partitioned through a multifaceted analysis. R_f/Ø measurements on various markers from the "type" stratigraphy allow a gradient representing cumulative strain since Middle Cretaceous time to be determined. From this gradient, noncoaxial strains accrued since emplacement of the leucogranites may be removed. Irrotational components of the postleucogranite strain are measured from quartz grain shapes in deformed granites; rotational components (shear strains) are determined from S-C fabrics and from restoration of rotated dike and vein networks. Structural observations and strain data are compatable with a deformation path of: (1) coaxial strain (pure shear?), followed by (2) injection of leucogranites as dikes (perpendicular to the minimum principle stress) and sills (parallel to the minimum principle stress), then (3) southwest directed simple shear. Modeling the late strain gradient as a simple shear zone permits a minimum displacement of 10 kilometers on the Magdalena mylonite zone/detachment fault system. Removal of the Middle Tertiary noncoaxial strains yields a residual (or pre-existing) strain gradient representative of the Late Cretaceous-Early Tertiary deformation. Several partially destrained cross sections, restored to the time of leucogranite emplacement, illustrate the idea that the upper plate of the core complex bas been detached from a region of significant topographic relief. 50% to 100% bulk extension across a 50 kilometer wide corridor is demonstrated.

Late Cenozoic tectonics of the Magdalena region are dominated by Basin and Range style faulting. Northeast and north-northwest trending high angle normal faults have interacted to extend the crust in an east-west direction. Net extension for this period is minor (10% to 15%) in comparison to the Middle Tertiary detachment related extensional episode.

Complex weathering in drylands: implications of 'stress' history for rock debris breakdown and sediment release

Weathering studies have often sought to explain features in terms of a prevailing set of environmental conditions. However, it is clear that in most present-day hot desert regions, the surface rock debris has been exposed to a range of weathering environments and processes. These different weathering conditions can arise in two ways, either from the effects of long-term climate change acting on debris that remains relatively static within the landscape or through the spatial relocation of debris from high to low altitude. Consequently, each fragment of rock may contain a unique weathering-related legacy of damage and alteration — a legacy that may greatly influence its response to present-day weathering activity. Experiments are described in which blocks of limestone, sandstone, granite and basalt are given ‘stress histories’ by subjecting them to varying numbers of heating and freezing cycles as a form of pre-treatment. These imposed stress histories act as proxies for a weathering history. Some blocks were used in a laboratory salt weathering simulation study while others underwent a 2 year field exposure trial at high, mid and low altitude sites in Death Valley, California. Weight loss and ultrasonic pulse velocity measurements suggest that blocks with stress histories deteriorate more rapidly than unstressed samples of the same rock type exposed to the same environmental conditions. Laboratory data also indicate that the result of imposing a known ‘weathering history’ on samples by pre-stressing them is an increase in the amount of fine sediment released during salt weathering over a given period of time in comparison to unstressed samples.

Granite Unveiling

As part of the celebration of the 200th anniversary of Sir Isaac Brock's birth on October 6, 1969, a piece of granite from Isaac Brock's childhood home in Guernsey was unveiled along with a plaque commemorating the ties between the General, the University, and Guernsey. The granite had been donated by Sir William Arnold, Bailiff of Guernsey, two years prior and had been in the possession of the university since that time before it was unveiled. The granite block was integrated into a wall in the Thistle Complex. It has since been relocated and is now part of a wall in the Walker Complex. Pictured here from left to right are: Sir William Arnold, Mrs. Arnold, Dr. Gibson and Governor General Michener.

Major- and trace-element composition of REE-rich turkestanite from peralkaline granites of the Morro Redondo Complex, Graciosa Province, south Brazil

Turkestanite, a rare Th- and REE-bearing cyclosilicate in the ekanite-steacyite group was found in evolved peralkaline granites from the Morro Redondo Complex, south Brazil. It occurs with quartz, alkali feldspar and an unnamed Y-bearing silicate. Electron microprobe analysis indicates relatively homogeneous compositions with maximum ThO(2), Na(2)O and K(2)O contents of 22.4%, 2.93% and 3.15 wt.%, respectively, and significant REE(2)O(3) abundances (5.21 to 11.04 wt.%). The REE patterns show enrichment of LREE over HREE, a strong negative Eu anomaly and positive Ce anomaly, the latter in the most transformed crystals. Laser ablation inductively coupled plasma mass spectrometry trace element patterns display considerable depletions in Nb, Zr, Hf, Ti and Li relative to whole-rock sample compositions. Observed compositional variations suggest the influence of coupled substitution mechanisms involving steacyite, a Na-dominant analogue of turkestanite, iraqite, a REE-bearing end-member in the ekanite-steacyite group, ekanite and some theoretical end-members. Turkestanite crystals were interpreted as having precipitated during post-magmatic stages in the presence of residual HFSE-rich fluids carrying Ca, the circulation of which was enhanced by deformational events.

Microgranitic Enclaves as Products of Self-mixing Events: a Study of Open-system Processes in the Maua Granite, Sao Paulo, Brazil, Based on in situ Isotopic and Trace Elements in Plagioclase

Felsic microgranular enclaves with structures indicating that they interacted in a plastic state with their chemically similar host granite are abundant in the Maua Pluton, SE Brazil. Larger plagioclase xenocrysts are in textural disequilibrium with the enclave groundmass and show complex zoning patterns with partially resorbed An-rich cores (locally with patchy textures) surrounded by more sodic rims. In situ laser ablation-(multi-collector) inductively coupled plasma mass spectrometry trace element and Sr isotopic analyses performed on the plagioclase xenocrysts indicate open-system crystallization; however, no evidence of derivation from more primitive basic melts is observed. The An-rich cores have more radiogenic initial Sr isotopic ratios that decrease towards the outermost part of the rims, which are in isotopic equilibrium with the matrix plagioclase. These profiles may have been produced by either (1) diffusional re-equilibration after rim crystallization from the enclave-forming magma, as indicated by relatively short calculated residence times, or (2) episodic contamination with a decrease of the contaminant ratio proportional to the extent to which the country rocks were isolated by the crystallization front. Profiles of trace elements with high diffusion coefficients would require unrealistically long residence times, and can be modeled in terms of fractional crystallization. A combination of trace element and Sr isotope data suggests that the felsic microgranular enclaves from the Maua Pluton are the products of interaction between end-member magmas that had similar compositions, thus recording `self-mixing` events.

Tectonic setting and U/Pb zircon dating of the plutonic Socorro Complex in the Transpressive Rio Paraíba do Sul Shear Belt, SE Brazil

The Precambrian Rio Paraíba do Sul Shear Belt comprises a 200-km-wide anastomosing network of NE-SW trending ductile shear zones extending over 1000 km of the southeastern coast of Brazil. Granulitic, gneissic-migmatitic, and granitoid terrains as well as low- to medium-grade metavolcanosedimentary sequences are included within it. These rocks were affected by strong contractional, tangential tectonics, due to west-northwestward oblique convergence of continental blocks. Subsequent transpressional tectonics accomodated large dextral, orogen-parallel movements and shortening. The plutonic Socorro Complex is one of many deformed granites with a foliation subparallel to that of the shear belt and exposes crosscutting relationships between its tectonic, magmatic, and metamorphic structures. These relationships point to a continuous magmatic evolution related to regional thrusts and strike slip, ductile shear zones. The tectonic and magmatic structural features of the Serra do Lopo Granite provide a model of emplacement by sheeting along shear zones during coeval strike-slip and cross shortening of country rocks. Geochronological data indicate that the main igneous activity of Socorro Complex spanned at least 55 million years, from the late stage of the northwestward ductile thrusting (650 Ma), through right-lateral strike slip (595 Ma) deformation. The country rocks yield discordant age data, which reflect a strong imprint of the Transamazonian tectono-metamorphic event (1.9 to 2.0 Ma). We propose a model for the origin of calcalkaline granites of the Ribeira Belt by partial melting of the lower crust with small contributions of the lithospheric mantle during transpressional thickening of plate margins, which were bounded by deep shear zones. The transpressional regime also seems to have focused granite migration from deeper into higher crustal levels along these shear zones.

Geochemistry and Pb-207/Pb-206 zircon ages of granitoids from the southern portion of the Tamboril-Santa Quiteria granitic-migmatitic complex, Ceara Central Domain, Borborema Province (NE Brazil)

The Tamboril-Santa Quiteria Complex is an important Neoproterozoic granitic-migmatitic unit from the Ceara Central Domain that developed from ca. 650 to 610 Ma. In general the granitoids range in composition from diorite to granite with predominance (up to 85%) of granitic to monzogranitic composition with biotite as the main mafic AFM phase. Geochemical and Pb-207/Pb-206 evaporation zircon geochronology studies were applied in a group of these abundant monzogranitic rocks from the region of Novo Oriente in the southern portion of the Ceara Central Domain. In this area the granitoids are weakly peraluminous biotite granitoids and deformed biotite granitoids of high-K calc-alkaline and ferroan composition, which we interpreted as primary magmas (segregated diatexites) derived from the partial melting of crustal material. The close temporal relation of this magmatism with local eclogitic and regional high temperature metamorphism in Ceara Central Domain point out to an orogenic setting, arguably emplaced during the collisional stage. Subordinate coeval juvenile mantle incursions are also present. This crustally derived magmatism is the primary product of the continental thickening that resulted from the collision between the rocks represented by the Amazonian-West African craton (Sao Luiz cratonic fragment) to the northwest and the Paleoproterozoic-Archean basement of the Borborema Province to the southeast along the Transbrasiliano tectonic corridor. (C) 2011 Elsevier Ltd. All rights reserved.

Complex, 3D strain patterns in a synkinematic tonalite batholith from the Aracuai Neoproterozoic orogen (Eastern Brazil): Evidence from combined magnetic and isotopic chronology studies

This work combines structural and geochronological data to improve our understanding of the mechanical behaviour of continental crust involving large amount of magma or partially melted material in an abnormally hot collisional belt. We performed a magnetic and geochronological (U/Pb) study on a huge tonalitic batholith from the Neoproterozoic Aracual belt of East Brazil to determine the strain distribution through space and time. Anisotropy of magnetic susceptibility, combined with rock magnetism investigations, supports that the magnetic fabric is a good proxy of the structural fabric. Field measurements together with the magnetic fabrics highlight the presence in the batholith of four domains characterized by contrasted magmatic flow patterns. The western part is characterized by a gently dipping, orogen-parallel (similar to NS) magmatic foliation that bears down-dip lineations, in agreement with westward thrusting onto the Sao Francisco craton. Eastward, the magmatic foliation progressively turns sub-vertical with a lineation that flips from sub-horizontal to sub-vertical over short distances. This latter domain involves an elongated corridor in which the magmatic foliation is sub-horizontal and bears an orogen-parallel lineation. Finally the fourth, narrow domain displays sub-horizontal lineations on a sub-vertical magmatic foliation oblique (similar to N150 degrees E) to the trend of the belt. U/Pb dating of zircons from the various domains revealed homogeneity in age for all samples. This, together with the lack of solid-state deformation suggests that: 1) the whole batholith emplaced during a magmatic event at similar to 580 Ma, 2) the deformation occurred before complete solidification. and 3) the various fabrics are roughly contemporaneous. The complex structural pattern mapped in the studied tonalitic batholith suggests a 3D deformation of a slowly cooling, large magmatic body and its country rock. We suggest that the development of the observed 3D flow field was promoted by the low viscosity of the middle crust that turned gravitational force as an active tectonic force combining with the East-West convergence between the Sao Francisco and Congo cratons. (C) 2012 Elsevier Ltd. All rights reserved.

Magnetic fabric of Araguainha complex impact structure (Central Brazil): Implications for deformation mechanisms and central uplift formation

The weakening mechanisms involved in the collapse of complex impact craters are controversial. The Araguainha impact crater, in Brazil, exposes a complex structure of 40 km in diameter, and is an excellent object to address this issue. Its core is dominated by granite. In addition to microstructural observations, magnetic studies reveal its internal fabric acquired during the collapse phase. All granite samples exhibit impact-related planar deformation features (PDFs) and planar fractures (PFs), which were overprinted by cataclasis. Cataclastic deformation has evolved from incipient brittle fracturing to the development of discrete shear bands in the center of the structure. Fracture planes are systematically decorated by tiny grains (<10 mu m) of magnetite and hematite, and the orientation of magnetic lineation and magnetic foliation obtained by the anisotropies of magnetic susceptibility (AMS) and anhysteretic remanence (AAR) are perfectly coaxial in all studied sites. Therefore, we could track the orientation of deformation features which are decorated by iron oxides using the AMS and AAR. The magnetic fabrics show a regular pattern at the borders of the central peak, with orientations consistent with the fabric of sediments at the crater's inner collar and complex in the center of the structure. Both the cataclastic flow revealed from microstructural observations and the structural pattern of the magnetic anisotropy match the predictions from numerical models of complex impact structures. The widespread occurrence of cataclasis in the central peak, and its orientations revealed by magnetic studies indicate that acoustic fluidization likely operates at all scales, including the mineral scales. The cataclastic flow made possible by acoustic fluidization results in an apparent plastic deformation at the macroscopic scale in the core. (C) 2012 Elsevier B.V. All rights reserved.

Micro-structural and compositional variations of hydrothermal epidote-group minerals from a peralkaline granite, Corupá Pluton, Graciosa Province, South Brazil, and their petrological implications

Epidote-group minerals, together with albite, quartz, fluorite, Al-poor and Fe-rich phyllosilicates, zircon, and minor oxides and sulphides, are typical hydrothermal phases in peralkaline alkali-feldspar granites from the Corupá Pluton, Graciosa Province, South Brazil. The epidote-group minerals occur as single crystals and as aggregates filling in rock interstices and miarolitic cavities. They display complex recurrent zoning patterns with an internal zone of ferriallanite-(Ce), followed by allanite-(Ce), then epidote-ferriepidote, and an external zone with allanite-(Ce), with sharp limits, as shown in BSE and X-ray images. REE patterns show decreasing fractionation degrees of LREE over HREE from ferriallanite to epidote. The most external allanite is enriched in MREE. LA-ICP-MS data indicate that ferriallanite is enriched (>10-fold) in Ti, Sr and Ga, and depleted in Mg, Rb, Th and Zr relative to the host granite. Allanite has lower Ga and Mn and higher Zr, Nb and U contents as compared to ferriallanite, while epidote is enriched in Sr, U and depleted in Pb, Zr, Hf, Ti and Ga. The formation of these minerals is related to the variable concentrations of HFSE, Ca, Al, Fe and F in fluids remaining from magmatic crystallization, in an oxidizing environment, close to the HM buffer. L-MREE were in part released by the alteration of chevkinite, their main primary repository in the host rocks.