Resarch On Geochemistry Characteristics Of The Middle Devonian Magmatic Rocks In Ashele Copper District

Altay orogenic belt is an important part of Central Asian Orogenic Belt(Sengoret al,1993;Xiao et al,2004),Which is a metallogenic belt that rich in copper,gold,zinc-lead, nickel and rare metallic (Li, Be, Nb, Ta). In this paper, petrographic,chronological, geochemical analyses have been carried out on the Middle Devonianmagmatic rocks in the Ashele Basin at the South Margin of the Altay Orogenic belt.Some conclusions have been drawn:(1) LA-ICP-MS analyses of zircons from Dacite rock of the Ashele Fm. in theAshele Basin yield weighted mean206Pb/238U ages of379.4±0.78Ma(MSWD=7.0)and387.0±4.2Ma(MSWD=6.3), and garanitic plutons are375.5±1.0Ma (MSWD=1.3),387.1±1.7Ma(MSWD=1.11) and378.5±0.9Ma(MSWD=1.9), respectively. TheMiddle Devonian existed magmatic activities in the Ashele Basin, which is consistedwith the characteristics of the South Margin of Altay, and the deposit hosted in theAshele Formation (eg. Ashele copper deposit) were formed after375Ma.(2) The garanitic plutons in Ashele Basin are high in the concentrations of SiO2(54.30%to77.55%), total alkali (Na2O+K2O)(3.02%to6.59%) and Al2O3(12.06%to18.68%),and low in the concentrations of P2O5(<0.14%) and MgO+FeO (<5%). In addition, they areenriched in sodium (Na2O/K2O=2.97to14) with high A/CNK values (1.09to3.18), which aredefined as low-K calalkaline peraluminious granites. They display noticeable negative anomaly ofSr,Nb,Ta,Ti and δEu (0.46to1.04), obvious enrichment of LREE and Rb,Ba,Th characteristics.(3) Conbined with the tectonic evolution of the southern margin of the Altay, we suggest thatthe Middle Devonian magmatic rocks in the Ashele Basin at the South Margin of theAltai Orogenic belt probably have been generated in island arc environment. The AsheleBasin is down-faulted Basin formed in compression tectonic environment ofDevonian period.