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Deep Crust (deep + crust)
Selected AbstractsContinental basalts in the accretionary complexes of the South-west Japan Arc: Constraints from geochemical and Sr and Nd isotopic data of metadiabaseISLAND ARC, Issue 1 2000Hiroo Kagami Abstract The Ryoke Belt is one of the important terranes in the South-west Japan Arc (SJA). It consists mainly of late Cretaceous granitoid rocks, meta-sedimentary rocks (Jurassic accretionary complexes) and mafic rocks (gabbros, metadiabases; late Permian,early Jurassic). Initial ,Sr (+ 25, + 59) and ,Nd (, 2.1,,5.9) values of the metadiabases cannot be explained by crustal contamination but reflect the values of the source material. These values coincide with those of island arc basalt (IAB), active continental margin basalt (ACMB) and continental flood basalt (CFB). Spiderdiagrams and trace element chemistries of the metadiabases have CFB-signature, rather than those of either IAB or ACMB. The Sr,Nd isotope data, trace element and rare earth element chemistries of the metadiabases indicate that they result from partial melting of continental-type lithospheric mantle. Mafic granulite xenoliths in middle Miocene volcanic rocks distributed throughout the Ryoke Belt were probably derived from relatively deep crust. Their geochemical and Sr,Nd isotopic characteristics are similar to the metadiabases. This suggests that rocks, equivalent geochemically to the metadiabases, must be widely distributed at relatively deep crustal levels beneath a part of the Ryoke Belt. The geochemical and isotopic features of the metadiabases and mafic granulites from the Ryoke Belt are quite different from those of mafic rocks from other terranes in the SJA. These results imply that the Ryoke mafic rocks (metadiabase, mafic granulite) were not transported from other terranes by crustal movement but formed in situ. Sr,Nd isotopic features of late Cretaceous granitoid rocks occurring in the western part of the Japanese Islands are coincident with those of the Ryoke mafic rocks. Such an isotopic relation between these two rocks suggests that a continental-type lithosphere is widely represented beneath the western part of the Japanese Islands. [source] Granulite facies thermal aureoles and metastable amphibolite facies assemblages adjacent to the Western Fiordland Orthogneiss in southwest Fiordland, New ZealandJOURNAL OF METAMORPHIC GEOLOGY, Issue 5 2009A. H. ALLIBONE Abstract In southwest New Zealand, a suite of felsic diorite intrusions known as the Western Fiordland Orthogneiss (WFO) were emplaced into the mid to deep crust and partially recrystallized to high- P (12 kbar) granulite facies assemblages. This study focuses on the southern most pluton within the WFO suite (Malaspina Pluton) between Doubtful and Dusky sounds. New mapping shows intrusive contacts between the Malaspina Pluton and adjacent Palaeozoic metasedimentary country rocks with a thermal aureole ,200,1000 m wide adjacent to the Malaspina Pluton in the surrounding rocks. Thermobarometry on assemblages in the aureole indicates that the Malaspina Pluton intruded the adjacent amphibolite facies rocks while they were at depths of 10,14 kbar. Similar P,T conditions are recorded in high- P granulite facies assemblages developed locally throughout the Malaspina Pluton. Palaeozoic rocks more than ,200,1000 m from the Malaspina Pluton retain medium -P mid-amphibolite facies assemblages, despite having been subjected to pressures of 10,14 kbar for > 5 Myr. These observations contradict previous interpretations of the WFO Malaspina Pluton as the lower plate of a metamorphic core complex, everywhere separated from the metasedimentary rocks by a regional-scale extensional shear zone (Doubtful Sound Shear Zone). Slow reaction kinetics, lack of available H2O, lack of widespread penetrative deformation, and cooling of the Malaspina Pluton thermal anomaly within c. 3,4 Myr likely prevented recrystallization of mid amphibolite facies assemblages outside the thermal aureole. If not for the evidence within the thermal aureole, there would be little to suggest that gneissic rocks which underlie several 100 km2 of southwest New Zealand had experienced metamorphic pressures of 10,14 kbar. Similar high- P metamorphic events may therefore be more common than presently recognized. [source] Microstructural tectonometamorphic processes and the development of gneissic layering: a mechanism for metamorphic segregationJOURNAL OF METAMORPHIC GEOLOGY, Issue 1 2000Williams The Mary granite, in the East Athabasca mylonite triangle, northern Saskatchewan, provides an example and a model for the development of non-migmatitic gneissic texture. Gneissic compositional layering developed through the simultaneous evolution of three microdomains corresponding to original plagioclase, orthopyroxene and matrix in the igneous rocks. Plagioclase phenocrysts were progressively deformed and recrystallized, first into core and mantle structures, and ultimately into plagioclase-rich layers or ribbons. Garnet preferentially developed in the outer portions of recrystallized mantles, and, with further deformation, produced garnet-rich sub-layers within the plagioclase-rich gneissic domains. Orthopyroxene was replaced by clinopyroxene and garnet (and hornblende if sufficient water was present), which were, in turn, drawn into layers with new garnet growth along the boundaries. The igneous matrix evolved through a number of transient fabric stages involving S-C fabrics, S-C-C, fabrics, and ultramylonitic domains. In addition, quartz veins were emplaced and subsequently deformed into quartz-rich gneissic layers. Moderate to highly strained samples display extreme mineralogical (compositional) segregation, yet most domains can be directly related to the original igneous precursors. The Mary granite was emplaced at approximately 900 °C and 1.0 GPa and was metamorphosed at approximately 750 °C and 1.0 GPa. The igneous rocks crystallized in the medium-pressure granulite field (Opx,Pl) but were metamorphosed on cooling into the high-pressure (Grt,Cpx,Pl) granulite field. The compositional segregation resulted from a dynamic, mutually reinforcing interaction between deformation, metamorphic and igneous processes in the deep crust. The production of gneissic texture by processes such as these may be the inevitable result of isobaric cooling of igneous rocks within a tectonically active deep crust. [source] Relationship between Crustal 3D Density Structure and the Earthquakes in the Longmenshan Range and Adjacent AreasACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 4 2009Jisheng ZHANG Abstract: This paper presents the 3D density structure of crust in the Longmenshan range and adjacent areas, with constraints from seismic and density data. The density structure of crust shows that the immense boundary plane of density distribution in relation to the Longmeshan fault belt is extended downward to ,80 km deep. This density boundary plane dips towards the northwest and crosses the Moho. With the proximity to the Longmenshan fault belt, it has a larger magnitude of undulation in the upper and middle crust levels. Density changes abruptly across Longmeshan fault belt. Seismic data show that most of the earthquakes in the Longmenshan area after the 2008 Ms8.0 Wenchuan Earthquake occurred within the upper to middle crust. These earthquakes are clearly distributed in the uplifted region of the basement. A few of them occurs in the transitional zone between the uplifted and subsided areas. But most of the earthquakes distributes in transitional zone from subsided to uplifted areas in the upper and middle crust where relatively large density changes occurr The 3D density structure of crust in the Longmenshan and adjacent areas can thus help us to understand the pattern of overthrusting from the standpoint of deep crust and where the earthquakes occurred. [source] | ||||||||||