Application Of Micro Beam Analysis And Fission Track Thermochronology For The Tectonics And Mineralization

Microbeam analysis technology has become the basic and advanced technique for the study of earth and planets,which can be used in analyzing the mineral distribution and chemical composition on the micro scale.The fission track chronology has unique advantages in the investigation of denudation histories and long-term landscape evolution of various geological settings,basin analysis,thermal history evolution,and preservation change of deposit.The conjunction of the two methods provides new insights for geological researchWe obtained 9 apatite fission track ages and 10 zircon fission track ages.Thermal history modelling of zircon and apatite fission track of Eastern Kunlun Orogenic Belt(EKOB)revealed five major episodes of cooling stage of?200–170 Ma,?170–100 Ma,?100–50 Ma,?50–18 Ma,and?18–0 Ma.Stage one represents the collision between the Qiangtang and Kunlun tectonic blocks and the closure of the Songpan-Ganzi-Bayan Har Triassic turbidite basin.In stage two,the northward subduction of the Neo-Tethys oceanic plate accentuated sediment transport to the actively closing ocean basin between the Lhasa and Qiangtang terranes;the far-field effect of the collision caused the cooling of the EKOB.Stage three is characterised by tectonic quiescence,which ended at?50 Ma due to the collision of India and Asia.In stage four,the establishment of a strike-slip fault system and fold-thrust belt in the EKOB resulted the exhumation.Stage five saw rapid cooling in response to the most recent compressional event associated with fault development in the EKOB.The full scan which including thin section,TIMA and BSE can be a useful tool to analyze the petrographic characteristics of minerals.We revealed 24 typical digital petrographic pictures and summarized the distribution characteristics of 13 typical minerals in the Bayan Obo deposit.The situ U-Pb of zircon in the section from quartz conglomerate in Wu dyke reveal two intercept age group of 2520±65 Ma,1847±24 Ma and 1973±16 Ma,representing regional sources of basal and cover material,respectively.The situ U-Pb concordant ages of monazite are 621±28 Ma,754±68 Ma,and 689±22 Ma,which indicates that the coarse-grained monazite is not formed by recrystallization of fine-grained monazite in dolomite.The situ U-Pb concordant age of coarse-grained dolomite is 340±34 Ma,which is the result of the reset of its closed system that due to subduction of the Palaeo-Asian oceanic plate during the Caledonian.The mineralization of the Bayan Obo REE is a continuous process,ranging from1393±142 Ma to 429±24 Ma.The Bayan Obo deposit mainly underwent four thermal events of?1.3 Ga,?1.0 Ga,?0.8 Ga,and?0.4 Ga,which is also indicative of four phases of mineralization time.The age difference between different samples and points is the result of the nuclear decay or/and the reset of the closure temperatures.A qualitative retention series of the Bayan Obo deposit can be suggested as follows:Cl=Na>K>(Cs,Rb,Pb,Cu,Zr)>U=Th>Ti>Y=Ba>F=Mg>REE=Sr>Mn=Fe,representing the decreasing tendency of the elements to partition into a volatile phase.This sequence is contributed to the transport and deposition of REEs,and finally formed the alkali-rich country rock in the Bayan Obo deposit.The fenitization caused the Sr content and ⁸⁷Sr/⁸⁶Sr ratio,which is also the origin of the coarse monazite.In hydrothermal solutions of the Bayan Obo deposit,the main REE transporting ligands are chloride and sulfate,whereas fluoride likely plays an important role as depositional ligands.And the Iron exists and migrates in the form of Fe-Cl complex which leads to it deposited in the Na-K rich mineral that formed due to the fenitizaiton.