| According to the research on field geological and petrologic characteristics ofgranite from the Gaudeanmus area, the main types of granite in the Gaudeanmus areaare basement-hosted gneissic granite, porphyritic granite, biotite granite and alaskite.The alaskite have been divided into six types, those are A, B, C, D, E and F typealaskite, based wholly on observable field characteristics of colour, grain size, texture,radiometric measurements by scintillometer and macro-scale mineralogy. Cross-cutting and structural relationships have been used to create a chronological sequencefrom type A which is the oldest to type F which is the youngest. The mineralcomposition for alaskite shows a larger range, and neither alaskite in the traditionalspeaking, nor typical granitic pegmatite.Using zircon LA-ICP-MS dating methods, we have been established petrogeneticchronology frame for granite in the area. The results show as follow. The age ofporphyritic granite formed at552.5±2.2Ma, biotite granite formed at540.2±3.9Ma,A type alaskite fromed at547.4±3.6Ma, B type alaskite formed at537.8±4.3Ma, Ctype alaskite formed at525.4±2.6Ma and D type alaskite formed at497±5.5Ma.Major, trace element and Sr-Nd-Pb isotopes studies show that granite from theGaudeanmus area is derived from melting of crust, and all belong to S-type granites.Abbabis biotite granite gneiss is mainly derived from remelting of rock from poorclay source. The porphyritic granite mainly is derived from remelting of poor clayclastic rock from ancient basement. The alaskite is mainly derived from remelting ofancient basement, however, its trace elements and Sr-Nd isotopic compositionindicate that the complexity of the source or the component of source area existheterogeneity in some extent. The alaskite is not the product of the same source bybatch melting or fractional crystallization, but they have their own separate magmachamber.The porphyritic granite and A type alaskite were formed in the main collisionstage, due to partial melting of ancient basement by crust rustal shortening andthickening, which caused by collsion of the Kalahari craton and Congo craton. Thebiotite granite is formed in the subsequent "soft collision" orogenic stage, and crustalthickening effect is not obvious in syn-collisional stage. There are a small amount ofbasaltic magma formed in the stage, which is caused by lithospheric delamination andasthenosphere upwelling, and then mantle-derived magmas and crustal-derived magma mixed, and ascended along deep faults and assimilated wall-rock andemplacement condensation. The deep-seated crustal rock generated amount of water-bearing fluids by dehydration with collapse of orogenic belt, sink of upwellingasthenosphere, direction change of micoroplate movement the change of micro-platedirection within orogenic tectonic activation, tectonic activate in orogenic belt andcrustal extrusion in crosswise. The fluids ascend by thrust shear zone, and madeextensive melt of the heterogeneous Pre-Damara basement rock, thus theemplacement of B, C, D, E and F type alaskite accomplished.The research on geochronological shows that diagenesis of alaskite and uraniummineralization occurred at the same time in the Gaudeanmus area. The source materialcomposition for alaskite and tectonic setting share controlled the mineralization ofalaskite. |
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