Basalt And Rhyolite Association In Eastern Bogda: The Structural Dynamic Significance

Bimodal volcanic rocks are a volcanic suite lack of intermediate rocks,and the composition discontinuity is called Daly gap.Bimodal volcanic suite of the Xiaohongliu Valley is composed of basalt and rhyolite,and SiO₂ content exhibits a gap between 47.16%and 76%.The basalts belong to tholeiitic series with high Na₂O and lowK₂O (K₂O/Na₂O=0.04-0.2),and TiO₂ content(1.66%-1.71%)is similar to MORB.The rocks are slightly enriched in LREE(LREE/HREE=1.16-1.21,(La/Yb)_N=1.79-1.91)with weakly Eu positive anomalies(δEu= 1.07-1.12,average is 1.09),and the fractionation of HREE is not obvious((Gd/Yb)_N=1.05-1.16).The trace elements are characterized by enriched HFSE and depleted LILE.The rhyolites belong to calc-alkaline series with high Si,K₂O+Na₂O contents(SiO₂=76%-79.76%,K₂O+Na₂O=6.08%-7.58%)and low TiO₂ content(TiO₂=0.168%-0.228%,average is 0.22%).The trace elements are characterized by high∑REE,enriched LREE,obviously Eu negative anomalies(δEu=0.17-0.2,average is 0.18)and the fractionation of HREE is not obvious((Gd/Yb)_N=1.59-2.21).The PM-normalized spider diagram shows the content of trace elements is high with enrichment in strongly incompatible elements and intensive depletion in Sr,P,Ti.Sm-Nd model ages of the basalts are from 645±18 to 699±16Ma, and(¹⁴³Nd/¹⁴⁴Nd)₀ andεNd vary within 0.512603-0.512618 and 5.63-5.89.The Rb-Sr isochron age of the rhyolites is 296±2Ma,(⁸⁷Sr/⁸⁶Sr)₀ is from 0.70432 to 0.70522.The isotopic characteristics indicate that the basalts are derived from a depleted mantle,and the rhyolites are originated from the partial melting heated by contemporaneous intrusion of basaltic magma.Under the extension setting,the bimodal volcanic rocks are formed by the eruption of basic and acid magmas.The discovery of bimodal volcanic rocks in Xiaohongliu Valley shows that the Bogda rift,initiated in middle early-Carboniferous and accompanying with subduction of the Mailakeli-Takezhale oceanic basin and expansion of the south continental margin of Junggar plate,developed to the climax in late- Carboniferous.The crust thickness (16-19km)and extension speed(2cm/a-5 cm/a)suggest the fast development of Bogda rift.The closure and collision of Bogda rift happened about 307.1±1.3Ma and creat the regional uplift,the transitional period from collision and compression to stretching and extension is 298.4±0.76Ma,and from then on,the Bogda orogen entered the post-orogenic stage.The finding of Taersu ore indicates that the minerogenic conditions are mature,and copper ore related to bimodal volcanic rocks and copper and gold deposits related to quartz reef in the region could be a new ore-prospecting strategy.