Geology, geochemistry, and U–Pb geochronology of the Archean (2.74 Ga) Serra do Rabo granite stocks, Carajás Metallogenetic Province, northern Brazil

The Carajás region, located in the southeastern part of the Amazon Craton, has been considered one of the most important mineral provinces in the world. The Serra do Rabo Granite (SRG) crops out near the eastern termination of the Carajás fault as two granite stocks, elongated approximately in an E–W direction, concordant with the regional structures. Leucomicrocline granite, hornblende–microcline granite, biotite–hornblende–microcline granite, hornblende syenogranite, and subordinate aplite are identified. The granites are grayish pink and coarse to medium grained and have mainly hypidiomorphic granular texture. Granophyric textures are common. The accessory minerals are ilmenite, apatite, zircon, allanite, and rare pyroxene.The SRG rocks are either massive or foliated, with a slightly anastomosed continuous S₁ foliation (E–W/subvertical) outlined by the preferred orientation of quartz, feldspars, and mafic minerals. Locally, decimeter- to meter-wide mylonite/ultramylonite bands (S_1m) occur along the E–W foliation. The S₁ foliation was developed under higher temperatures than those of the S_1m mylonite foliation. The SRG structural evolution was controlled by progressive deformation under decreasing temperature, indicative of syntectonic emplacement. The SRG also has relatively high SiO₂, K₂O, and Na₂O contents; high FeO*/(FeO*+MgO) ratios; high Zr, Ba, Nb, and Ga; and very high rare-earth element contents. The chemical signature is moderately alkaline and metaluminous, comparable to that of the A-type, A2, and ALK-3 granites. The origin of the SRG magmas may be related to the partial melting of crustal sources, such as previously metamorphosed calc-alkaline granites.The SRG crosscuts supracrustal rocks, promoting low-pressure/high-temperature metamorphism. The interaction between regional compressive stresses and the ballooning effect of the granite stocks promoted slight aureole flattening and rheological changes in the supracrustal rocks. The U–Pb zircon age of 2743±1.6 Ma is interpreted as the age of zircon crystallization, granite stock emplacement, and regional horizontal shortening. Other 2.7 Ga syntectonic alkaline granites (e.g. Estrela, Plaquê, Planalto) have been reported in the region.     

Zircon sensitive high mass-resolution ion microprobe U–Pb and fission-track ages for gabbros and sheeted dykes of the Taitao ophiolite, Southern Chile, and their tectonic implications

Abstract   The Late Miocene–Pliocene Taitao ophiolite is composed of a complete sequence of classic oceanic lithosphere and is exposed approximately 50 km southeast of the Chile triple junction, where the Chile Ridge subducts beneath the South American Plate. Gabbros and ultramafic rocks are folded into a complex pattern, but only evidence for block rotation has been reported in the overriding sheeted dyke complex. In the present study, sensitive high mass-resolution ion microprobe U–Pb and fission-track dating methods were applied to zircon crystals separated from gabbros and sheeted dykes. Two sets of radiometric ages of gabbros range between 5.9 ± 0.4 and 5.6 ± 0.1 Ma. These ages coincide within their error ranges and imply rapid intrusion and cooling of gabbros. The U–Pb age of a dacite dyke intruded into the sheeted dyke complex was determined to be 5.2 ± 0.2 Ma. These data indicate that the magmas of the Taitao ophiolite were formed during the 6 Ma Chile Ridge collision event and emplaced in a shorter period than previously thought. A short segment of the Chile Mid-oceanic Ridge must have been emplaced during the 6 Ma event.     

东秦岭松树沟超基性岩体“热”侵位时代新知

松树沟超镁铁质岩体位于秦岭商-丹断裂以北,是该地区出露最大的超基性岩体,并以侵入方式与秦岭杂岩接触.该岩体主要由细粒纯橄榄岩、中粗粒纯橄榄岩和斜辉橄榄岩组成.利用LA-ICPMS锆石U-Pb法对采自该岩体北侧接触变质带的榴闪岩进行锆石定年,获得206pb/238U变质年龄为518±19 Ma.以上工作初步证实该岩体大致在0.52 Ga左右的构造热事件中侵位于秦岭杂岩中.     

Emplacement History of Pasupugallu Gabbro Pluton, Eastern Ghats Belt, India: a Structural Study

Structural mapping of the Pasupugallu pluton, an elliptical intrusive gabbro-anorthosite body, emplaced into the western contact zone between the Eastern Ghats Mobile Belt and the Archaean East Dharwar Craton, along the east coast of India, reveals concentric, helicoidal and inward dipping magmatic and/or tectonic foliations. We identify a <1 km-wide structural aureole characterized by pronounced deflection of regional structures into margin parallel direction, mylonitic foliations with S-C fabrics, sigmoidal clasts, moderately plunging stretching lineations, non-cylindrical intrafolial folds, and stretched elliptical mafic enclaves in the aureole rocks. Our results suggest that the pluton emplacement is syn-tectonic with respect to the regional ductile deformation associated with the terrane boundary shear zone at the western margin of the Eastern Ghats. We present a tectonic model for the emplacement of the pluton invoking shear-related ductile deformation, rotation and a minor component of lateral expansion of magma. The intrusive activity (1450-800 Ma) along the western margin of the Eastern Ghats can be correlated with the significant event of recurring mafic, alkaline and granitic magmatism throughout the global Grenvillian orogens associated with the continent-continent collision tectonics possibly related to the amalgamation and the breakup of the supercontinent Rodinia.     

The effects of layering and local stresses in composite volcanoes on dyke emplacement and volcanic hazards

Since most volcanic eruptions are fed by dykes, any assessment of volcanic hazards in an area must include an evaluation of the probability of injected dykes either reaching the surface or becoming arrested. Composite volcanoes are normally composed of alternating stiff (high Young's modulus) and soft (low Young's modulus) layers. Numerical models indicate that during unrest periods with magma-chamber inflation, the local stresses in composite volcanoes commonly prevent dyke-fed eruptions: while the stresses in the stiff layers may favour dyke propagation and seismogenic faulting, the local stresses in the soft layer remain seismically quiet and favour dyke arrest. Geodetic and field studies also indicate that most dykes never reach the surface, and that only a small fraction of the magma volume injected from a chamber erupts at the surface. I propose that for a dyke-fed eruption to occur, all the layers along the potential pathway of the dyke must have local stresses that favour magma-driven extension-fracture propagation. Thus, the stress field along the pathway must be homogenised. To cite this article: A. Gudmundsson, C. R. Geoscience 337 (2005).     

Emplacement kinematics of nepheline syenites from the Terrane Boundary Shear Zone of the Eastern Ghats Mobile Belt,west of Khariar,NW Orissa: Evidence from meso- and microstructures

Nepheline syenite plutons emplaced within the Terrane Boundary Shear Zone of the Eastern Ghats Mobile Belt west of Khariar in northwestern Orissa are marked by a well-developed magmatic fabric including magmatic foliation, mineral lineations, folds and S-C fabrics. The minerals in the plutons, namely microcline, orthoclase, albite, nepheline, hornblende, biotite and aegirine show, by and large, well-developed crystal faces and lack undulose extinction and dynamic recrystallization, suggesting a magmatic origin. The magmatic fabric of the plutons is concordant with a solid-state strain fabric of the surrounding mylonites that developed due to noncoaxial strain along the Terrane Boundary Shear Zone during thrusting of the Eastern Ghats Mobile Belt over the Bastar Craton. However, a small fraction of the minerals, more commonly from the periphery of the plutons, is overprinted by a solid state strain fabric similar to that of the host rock. This fabric is manifested by discrete shear fractures, along which the feldspars are deformed into ribbons, have undergone dynamic recrystallization and show undulose extinction and myrmekitic growth. The shear fractures and the magmatic foliations are mutually parallel to the C-fabric of the host mylonites. Coexistence of concordant solid state strain fabric and magmatic fabric has been interpreted as a transitional feature from magmatic state to subsolidus deformation of the plutons, while the nepheline syenite magma was solidifying from a crystal-melt mush state under a noncoaxial strain. This suggests the emplacement of the plutons synkinematic to thrusting along the Terrane Boundary Shear Zone. The isotopic data by earlier workers suggest emplacement of nepheline syenite at 1500 +3/−4Ma, lending support for thrusting of the mobile belt over the craton around that time.     

Possible lava tube system in a hummocky lava flow at Daund,western Deccan Volcanic Province,India

A hummocky flow characterised by the presence of toes, lobes, tumuli and possible lava tube system is exposed near Daund, western Deccan Volcanic Province, India. The lava tube system is exposed as several exhumed outcrops and is composed of complex branching and discontinuous segments. The roof of the lava tube has collapsed but original lava tube walls and fragments of the tube roof are seen at numerous places along the tube. At some places the tube walls exhibit a single layer of lava lining, whereas, at other places it shows an additional layer characterised by smooth surface and polygonal cracks. The presence of a branching and meandering lava tube system in the Daund flow, which represents the terminal parts of Thakurwadi Formation, shows that the hummocky flow developed at a low local volumetric flow rate. This tube system developed in the thinner parts of the flow sequence; and tumuli developed in areas where the tube clogged temporarily in the sluggish flow.     

望云山复式花岗岩体磁性组构特征及其动力学意义

为了分析望云山复式花岗岩体侵位时的动力学状态，对该岩体进行了岩石磁组构参数测定，磁化率的变化特征反映了望云山得式岩体侵位过程中构造环境的变化和单元岩石的暗色矿物组成及其性程度的高低；磁面理的产状与岩体形态和岩石单元分布相吻合，清晰地反映了不同构造单元岩浆侵位方式；磁线理指示岩体的侵位中心，线理表现形式不一，标示侵位方式与受构造应力的不同；晚期岩浆对早期岩浆中的磁线理有一定的改造作用，磁线理方向基本上反映了岩浆原始流动状态；从磁化率椭球参数可以看出岩浆侵位的应变状态；岩浆应力场分布体现了岩浆侵位方式及其所处构造环境，岩体岩石磁组构参数佐证了区域构造,围岩构造及岩体内部其他构造所反映的岩体侵位机制。     

Experiments simulating surface deformation induced by pluton emplacement

We present centrifuge experiments to study the surface deformation induced by shallow pluton emplacement in a rheologically stratified crust. Sand simulates the topmost brittle crust; plastilina and denser silicone represent more and less competent crustal portions, respectively; lighter silicone simulates a buoyant intrusion. In the models, density differences affect the rate of intrusion but not their evolution or shape, whereas viscosity and strength stratifications control both the shape and rate of the intrusions. With a higher viscosity contrast (10^2–4) between the intrusion and the embedding media, the rise of the lighter silicone induces a laccolith-like intrusion, responsible for doming and thinning of the overburden; an apical depression may form, inducing silicone extrusion. Conversely, with a lower (10¹) viscosity contrast, the overburden and the intrusion exhibit a lens-shaped form, with a broad central depression bordered by an upward flexure towards the periphery. A sag in the floor of the intrusion is commonly observed; no silicone extrusion occurred. The intrusion is a hybrid between a laccolith and a lopolith. The comparison with nature (1) confirms roof uplift as an important means of accommodating space during pluton emplacement and (2) suggests that, where roof uplift plays a major role, pluton emplacement can induce a well-correlated sequence of events at surface: doming, the development of a depression and extrusion.