Composition, sources, and mechanism of continental crust growth in the Lake zone of the Central Asian Caledonides: I. Geological and geochronological data |
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| Authors: | V V Yarmolyuk V P Kovach V I Kovalenko E B Salnikova A M Kozlovskii A B Kotov S Z Yakovleva and A M Fedoseenko |
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| Abstract: | Data on the composition, inner structure, and age of volcanic and siliceous-terrigenous complexes and granitoids occurring
in association with them in the Caledonian Lake zone in Central Asia are discussed in the context of major relations and trends
in the growth of the Caledonian continental crust in the Central Asian Foldbelt (CAFB). The folded structures of the Lake
zone host basalt, basalt-andesite, and andesite complexes of volcanic rocks that were formed in distinct geodynamic environments.
The volcanic rocks of the basalt complex are noted for high concentrations of TiO2 and alkalis, occur in association with fine-grained siliceous siltstone and siliceous-carbonate rocks, are thus close to
oceanic-island complexes, and were likely formed in relation to a mantle hotspot activity far away from erosion regions supplying
terrigenous material. The rocks of the basalt-andesite and andesite complexes have lower TiO2 concentrations and moderate concentrations of alkalis and contain rock-forming amphibole. These rocks are accompanied by
rudaceous terrigenous sediments, which suggests their origin in island-arc environments, including arcs with a significantly
dissected topography. These complexes are accompanied by siliceous-terrigenous sedimentary sequences whose inner structure
is close to those of sediments in accretionary wedges. The folded Caledonides of the Lake zone passed through the following
evolutionary phases. The island arcs started to develop at 570 Ma, their evolution was associated with the emplacement of
layered gabbroids and tonalitetrondhjemite massifs, and continued until the onset of accretion at 515–480 Ma. The accretion
was accompanied by the emplacement of large massifs of the tonalite-granodiorite-plagiogranite series. The postaccretionary
evolutionary phase at 470–440 Ma of the Caledonides was marked by intrusive subalkaline and alkaline magmatism. The Caledonides
are characterized by within-plate magmatic activity throughout their whole evolutionary history, a fact explained by the accretion
of Vendian-Cambrian oceanic structures (island arcs, oceanic islands, and back-arc basins) above a mantle hotspot. Indicators
of within-plate magmatic activity are subalkaline high-Ti basalts, alkaline-ultrabasic complexes with carbonatites and massifs
of subalkaline and alkaline gabbroids, nepheline syenites, alkaline granites, subalkaline granites, and granosyenites. The
mantle hotspot likely continued to affect the character of the lithospheric magmatism even after the Caledonian folded terrane
was formed. |
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