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Mafic Dykes (mafic + dyke)
Selected AbstractsDiscovery of a Miocene Mafic Dyke from the Western Hills of Beijing and its Geological ImplicationsACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 3 2009Zhicheng ZHANG Abstract: The present study is the first report of a Miocene mafic dyke from the Dahuichang, in the Western Hills of Beijing. The dyke cuts the fossil-dated Changxindian Formation of Eocene sequences and yields K-Ar ages of 14,15 Ma. The dyke is fine-grained diabase and has 49.84%,50.81% SiO2 and 3.56,3.97% Na2O+K2O, high TiO2 (1.65%,1.93%) and MgO (7.36%,9.85%), and low K2O (<1.22%) contents, with Na2O>K2O and slightly varied magnesium numbers (Mg#=55.54,62.74). In trace elements geochemistry, the dyke is very similar to the Miocene basalts from Jining and Hanuoba. The enrichment of light rare earth elements ([La/Yb]N=5.03,6.12) and large ion lithophile elements (LILEs), no negative Eu anomalies, relatively high Cr (265,326 ppm) and Ni (155,262 ppm), and almost constant V concentrations (194,213 ppm) reveal that the composition close to the primary basaltic magma from an enriched-mantle source, with little crustal contamination and fractional crystallization. The basaltic magma was possibly derived from the upwelling asthenosphere mantle beneath eastern China during the Miocene lithospheric thinning. [source] Carbonate melting and peperite formation at the intrusive contact between large mafic dykes and clastic sediments of the upper Palaeozoic Saint-Jules Formation, New-Carlisle, QuebecGEOLOGICAL JOURNAL, Issue 1 2006P. Jutras Abstract The base of an upper Palaeozoic graben-fill in eastern Canada was affected by mafic dyke intrusions shortly after deposition, resulting in the formation of peperite. Complex magma,sediment interactions occurred as the melts mingled with the wet and poorly consolidated clastic material of this sedimentary basin, which is separated from underlying rocks by the Acadian unconformity (Middle Devonian). As a result of these interactions, the mafic rocks are strongly oxidized, albitized and autobrecciated near and above the unconformity, where blocky juvenile clasts of mafic glass and porphyritic basalt have mingled with molten or fluidized sediments of the upper Palaeozoic Saint-Jules Formation, forming a peperite zone several metres thick. In contrast to most peperite occurrences, the New-Carlisle peperites are associated with the tip of dykes rather than with the sides of sills or dykes. We argue that more heat can be concentrated above a dyke than above a sill, as the former provides a more efficient and focused pathway for heated waters to invade the poorly consolidated host sediments. Superheated groundwaters that issued from the sides of the dykes appear to have promoted melting of carbonate components in calcareous sedimentary rock clasts of the Saint-Jules Formation, locally generating carbonate melts that contributed to the mingling of juvenile and sedimentary clasts in the peperite. Copyright © 2005 John Wiley & Sons, Ltd. [source] Neoproterozoic Mafic Dykes and Basalts in the Southern Margin of Tarim, Northwest China: Age, Geochemistry and Geodynamic ImplicationsACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 3 2010Chuanlin ZHANG Abstract: Neoproterozoic rifting-related mafic igneous rocks are widely distributed both in the northern and southern margins of the Tarim Block, NW China. Here we report the geochronology and systematic whole-rock geochemistry of the Neoproterozoic mafic dykes and basalts along the southern margin of Tarim. Our zircon U-Pb age, in combination with stratigraphic constraint on their emplacement ages, indicates that the mafic dykes were crystallized at ca. 802 Ma, and the basalt, possibly coeval with the ca. 740 Ma volcanic rocks in Quruqtagh in the northern margin of Tarim. Elemental and Nd isotope geochemistry of the mafic dykes and basalts suggest that their primitive magma was derived from asthenospheric mantle (OIB-like) and lithospheric mantle respectively, with variable assimilation of crustal materials. Integrating the data supplied in the present study and that reported previously in the northern margin of Tarim, we recognize two types of mantle sources of the Neoproterozoic mafic igneous rocks in Tarim, namely the matasomatized subcontinental lithospheric mantle (SCLM) in the northern margin and the long-term enriched lithospheric mantle and asthenospheric mantle in the southern margin. A comprehensive synthesis of the Neoproterozoic igneous rocks throughout the Tarim Block led to the recognition of two major episodes of Neoproterozoic igneous activities at ca. 820,800 Ma and ca. 780,740 Ma, respectively. These two episodes of igneous activities were concurrent with those in many other Rodinian continents and were most likely related to mantle plume activities during the break-up of the Rodinia. [source] What Happened in the Trans-North China Orogen in the Period 2560-1850 Ma?ACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 6 2006Guochun ZHAO Abstract: The Trans-North China Orogen (TNCO) was a Paleoproterozic continent-continent collisional belt along which the Eastern and Western Blocks amalgamated to form a coherent North China Craton (NCC). Recent geological, structural, geochemical and isotopic data show that the orogen was a continental margin or Japan-type arc along the western margin of the Eastern Block, which was separated from the Western Block by an old ocean, with eastward-directed subduction of the oceanic lithosphere beneath the western margin of the Eastern Block. At 2550-2520 Ma, the deep subduction caused partial melting of the medium-lower crust, producing copious granitoid magma that was intruded into the upper levels of the crust to form granitoid plutons in the low- to medium-grade granite-greenstone terranes. At 2530-2520 Ma, subduction of the oceanic lithosphere caused partial melting of the mantle wedge, which led to underplating of mafic magma in the lower crust and widespread mafic and minor felsic volcanism in the arc, forming part of the greenstone assemblages. Extension driven by widespread mafic to felsic volcanism led to the development of back-arc and/or intra-arc basins in the orogen. At 2520-2475 Ma, the subduction caused further partial melting of the lower crust to form large amounts of tonalitic-trondhjemitic-granodioritic (TTG) magmatism. At this time following further extension of back-arc basins, episodic granitoid magmatism occurred, resulting in the emplacement of 2360 Ma, ,2250 Ma 2110,21760 Ma and ,2050 Ma granites in the orogen. Contemporary volcano-sedimentary rocks developed in the back-arc or intra-arc basins. At 2150-1920 Ma, the orogen underwent several extensional events, possibly due to subduction of an oceanic ridge, leading to emplacement of mafic dykes that were subsequently metamorphosed to amphibolites and medium- to high-pressure mafic granulites. At 1880-1820 Ma, the ocean between the Eastern and Western Blocks was completely consumed by subduction, and the closing of the ocean led to the continent-arc-continent collision, which caused large-scale thrusting and isoclinal folds and transported some of the rocks into the lower crustal levels or upper mantle to form granulites or eclogites. Peak metamorphism was followed by exhumation/uplift, resulting in widespread development of asymmetric folds and symplectic textures in the rocks. [source] | ||||||||||