Petrogenesis and geodynamic implication of Middle Ordovician quartz monzoporphyry at central Lajishan,Qilian Mountains
Granitoids are widespread over the Earth's crust.As products of magmatism from crustal materials,their geo-chemical compositions,features of rock association and spatial and temporal distribution collectively recorded properties of their magmatic sources,magmatic evolutional processes,as well as geodynamic mechanism for their formation.Based on petrography,zircon U-Pb geochronology,and whole-rock geochemistry and Sr-Nd isotopes on Keqianyakou quartz monzo-porphyry at central Lajishan,Qilian Orogen Belt,together with other available researches on coeval magmatism at this re-gion,the authors are able to make discussions on petrogenesis of the quartz monzoporphyry and deep geodynamic setting for their generation.The result turns out that the age of quartz monzonite porphyry is(466±5)Ma.it has relatively high SiO2(63.57％-64.04％)and Na2O(7.08％-7.36％)and low CaO(1.19％-2.52％)and K2O(1.53％-1.96％)content.In trace elemental spider diagram,they are characterized by enrichment of Ba,U and Pb and depletion of Zr,Hf and Sr.Additionally,they have slightly negative Eu anomaly(Eu*=0.80-0.91)and obviously fractionated rare earth elemental distribution((La/Yb)N=13.00-15.72).They have uniform Sr and Nd isotopic compositions with(87Sr/86Sr)i=0.705 4-0.706 2 and eNd(t)=0.87-1.03.Synthetic study manifests they are derived from partial melt-ing of thickened basaltic crustal materials.The quartz monzoporphyry,together with other coeval magmatic rocks in this region,shows a decreasing trend in magmatic crystallization age towards the suture,which indicates their generation at a tectonic setting of rollback of the subducted oceanic slab.The decreasing(La/Yb)N value with ages for the 470-460 Ma granitoids in Lajishan tectonic belt implies persisting extension of lithosphere during that period.
granitoidspetrogenesisslab rollbackQilian Orogenic Belt
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