The Geochemical Characteristics And Geological Significance Of Yushigou And Dachadaban Volcanic Rocks In The North Qilian Orogen

Ophiolite sequences in the North Qilian orogen distribute in two belts: the southern belt and northern belt.Ophiolite suites have been well-documented in Yushigou Ophiolite suite and Dachadaban Ophiolite suite.Ophiolite,as the residual phase of oceanic crust,is a series of rock assemblage.It is vital significance for recognizing the formation and subduction of old oceanic lithosphere.The volcanic rocks in North Qilian mountains formed in Early Paleozic,which mostly suffered different degrees of alteration.This paper,has carried on the detailed analysis for the basalts in Yushigou,basalts and basaltic andesites in Dachadaban by means of geochronology and geochemistry.Zircon U-Pb dating indicates that the gabbros in Yushigou and dachadaban ophiolite suites formed at 522±5 Ma and 503±10 Ma,respectively.In the primitive mantle-normalized spider diagrams of incompatible elements and the chondrite-normalized distribution diagrams of rare earth elements,basalts in Yushigou ophiolite sequence display similar MORB-type,which maybe form in mid-oceanic ridges spreading environment.Basalts in Dachadaban show characteristics of MORB-type,which may be formed in back-arc spreading environment.They are similar with Izu back-arc basin in present-day western pacific.Basaltic andesites in Dachadaban have characteristics of boninites.The formation of basaltic andesites maybe associate with subduction of North Qilian oceanic lithosphere,which are possible to be produced from early arc magmatism to development of back-arc basin.During 550 to 446 Ma,the North Qilian orogenic belt recorded three stages of tectonic evolution.Stage I(~550 to 517 Ma): The North Qilian ocean kept oceanic ridge spreading,and formed the southern ophiolitic belt including Yushigou ophiolite before seafloor subduction began.Stage II(~520 to 487 Ma): Seafloor began to subduct.Though partial melting of mantle wedge,than resulting in formation of Zoulangnanshan arc.With increasing the degree of partial melting of the mantle wedge,boninites were produced in Dachadaban area.Stage III(~490 to 446 Ma): The North Qilian ocean was extending in back-arc basin,forming the northern ophiolitic belt including Dachadaban ophiolite.