Rock Geochemistry And Chronology Of The Longling Pingda Granite In Western Yunnan

Longling area is located in the central and western part of Tengchong-Baoshan block,in Western Yunnan,where magmatic activity is intense,intrusive rocks and volcanic rocks are exposed in a large area,forming a large-scale complex rock body,which spatially forms a north-south magmatic belt roughly parallel to the Bangong-Nujiang junction zone,and It is one of the most important metallogenic prospects for non-ferrous and precious metals in China.In addition to Ordovician granites,Mesozoic-Cenozoic magmatic rocks are also developed in Pingda area.However,their geochemical characteristics,genetic relationship with early granites and tectonic evolution significance are still lacking in-depth study.For this reason,this dissertation chooses granite in Pingda area as the research object,through field geological survey and sample collection,using petrofacies,principal and trace geochemical tracing of rocks,zircon U-Pb dating and Hf isotope tracing analysis,summarizes and identifies its petrology and geochemical characteristics,and defines its diagenetic age.The main achievements of this study are as follows:(1)Pingda granite body occurs in Pinghe composite rock body with an exposed area of about 50 km².The rock type is mainly biotite monzonitic granite.The mineral composition and structure of the rock are relatively uniform,and the whole rock is gray-white.The lithological characteristics are medium-fine grained granite structure.The main ferromagnesia minerals are biotite,a small amount of muscovite,feldspar and quartz are mostly other-shaped granular,and some samples are greatly affected by weathering.(2)The major element characteristics of petrochemical composition of Pingda granite show that Pingda granite belongs to high-K calc-alkaline granite with higher K₂O/Na₂O value and larger Al saturation index(A/CNK).The trace element characteristics show obvious negative Eu anomaly(delta Eu=0.30-0.60),enrichment of light rare earth elements[(La/Yb)_N=4.4-43.9],enrichment of large ion lithophile elements and Pb,and loss of high field strength elements.The variation range of zircon epsilon Hf(t)values in Pingda rock mass samples ranges from-12.61--3.37,and the corresponding Hf isotope model ages are concentrated at 1.8 to 2.5 Ga.Pingda granite is S-type granite,mainly derived from remelting of ancient crustal materials.Pingda granite source area is dominated by poor clay sandstone.(3)Zircon U-Pb dating of two granite samples shows that the diagenetic ages of Pingda granite are 83.44 Ma and 83.04 Ma,respectively,indicating the Late Cretaceous emplacement events.(4)In Late Cretaceous,Tengchong massif entered the intracontinental post-collision orogenic environment,accompanied by a large number of collision-post collision granite emplacement,while Baoshan massif,due to the compression from Tengchong massif,entered the continental-continental collision stage with intense folding orogeny and regional uplift,and the lower crust partially melted into synmagma.Pingda-Tong collision granite was formed by the upwarping location of the fault passage.