A Quantitative Study Of The Indosinian Magmatic Flare-up In The Zhongdian Arc,Northwest Yunnan Province:Implication For The Formation Of The Porphyry Copper Deposits

Situated in the southern part of Yidun arc in the southeastern Tibetan Plateau,the Zhongdian arc hosts widespread Late Triassic porphyry copper deposits,and records a Indosinian magmatic activity with steady low-volume magmatic lulls punctuated by short-lived high-volume flare-ups,which is likely attributable to the westward slab subduction of Garze-Litang oceanic crust.Previous studies mainly focus on the genesis of individual deposit or rock body,e.g.geochronology,isotope geochemistry,petrology,however,rarely studies focus on the temporal-spatial relationship between magmatism and the subduction system around the Zhongdian arc from a quantitative perspective.Frequency distribution calculated from igneous zircon U-Pb ages is commonly utilized to intuitively interpret the duration of magmatic events.Recently,a new concept of fractal density and a local singularity analysis?LSA?method have been applied to analyze the geometric property of the zircon U-Pb age peak anomalies,which are linked to deeply rooted avalanches associated with short spurts of convection during the formation of supercontinents and continental crust growth.In this paper,the igneous zircon U-Pb dating data collected from the Zhongdian arc is analyzed by LSA method from the perspective of fractal density to interpret the causational relationship between age peak and subduction process of Garze-Litang oceanic crust.The results show that the age density around the age peak of 215 Ma can be well fitted and described by power law functions,and the strong singularity?6?>0.8)indicates the occurrence of extreme geological events.Meanwhile,systematic changes in hafnium isotopic composition of igneous zircons with time reflect the upwelling of asthenospheric mantle materials,characterized by a rapid increase in_Hf??values which reach the highest around the peak 215 Ma.Moreover,the log-log plot which shows the cumulative number-age distribution reveals another transitional age of 235 Ma.By comparison of the magmatic intensity between western and eastern belts of the Zhongdian arc,it is indicated that the eastward magmatic migration might begin at ca.235 Ma.In this paper,the Sr/Y and?La/Yb?_N values of the intermediate-acid magmatic rocks are also used to quantify the crustal thickness of the Zhongdian arc during Late Triassic.The crustal thickness shows an overall increasing trend with time,and might reach more than 60 km.The northern and southern parts of the Yidun arc have significant differences in the nature of the overlying blocks.The presence of the thickened lower crust under the Zhongdian arc is probably one of the favorable conditions for the formation of porphyry copper deposits in the area.Finally,based on the above research,a new comprehensive model was established to show the relationship among the Indosinian magmatic flare-up,the formation of porphyry copper deposits,and the entire subduction process in the Zhongdian arc.It is proposed that the Indosinian magmatic flare-up in the Zhongdian arc may be caused by superimposition of slab subduction,steepening of slab dip,and slab breakoffs.The research in this paper provides a reference direction for depicting the variation of magmatic activity from a quantitative perspective.