Petrogenesis Of The Middle-late Triassic Intermediate-acid Intrusive Rocks In The Yidun Terrane And Its Geological Significances

Sanjiang region is located in the eastern Tethys tectonic domain.Its complex of geological history,strong tectonism,magmatism and mineralization has been paid broad attention to by the foreign and native researchers.The Yidun terrane,located in the eastern part of the Tibetan Plateau,is an important component of the Sanjiang Tethyes tectonic domain,where not only develops the Mesozoic tectonic-magmatic activity,but also is an important poly-metallic metallogenic area.In this dissertation,we conduct a comprehensive study on the middle to late Trassic dioritic rocks and granitoids(and enclave),including zircon U-Pb dating,mineralogy,whole-rock elemental and Sr-Nd and zircon Hf isotope analyses.The main conclusions are summarized below.(1)The Xuejiping diorite porphyry from southern part of the Yidun terrane,including the ore-bearing dioritic porphyries(mainly located near the fault zone)and ore-barren dioritic porphyries(away from the fault zone),were emplaced at?216 Ma.Both the ore-bearing and ore-barren dioritic porphyries show an adakitic affinity,such as medium SiO2(56.06wt%?66.51 wt.%),MgO(2.51 wt%?4.69 wt%),Sr/Y(18.2?97.6)and low Cr(9.71 ppm?38.4 ppm),Ni(7.97 ppm?40.4 ppm),Y(12.1 ppm?24.8 ppm),with weak negative Eu anomalies(?Eu=0.81?1.11).Combining the elemental and isotopic signatures indicate that the Xuejiping dioritic porphyries were derived from the remelting of the thickened lower crust corresponding to the garnet-amphibolite source.(2)The Dongcuo and Shenmu granite rocks dated at?216 Ma,are characterized by high Si,A1,K,and low Fe,Mg,most of which are high potassium calc-alkaline feature and peraluminous,showing a similar nature to I-type granite.And they show high(87Sr/86Sr)i =0.706573-0.729698),negative ?Nd(t)values(-8.82 to-6.13),it notably reflect that the these granites mainly originated from partial fusion of the Mesoproterozoic crustal rocks.The Dongcuo granite rocks are the product of the crystallization of Shenmu granites magma.(3)U-Pb zircon dating for the enclaves from Shenmu pluton yielded a same crystallization age with Shenmu host granites,at ca.216 Ma.Geochemically,the enclaves show low SiO2(57.78 wt.%?58.45 wt.%),high MgO(ave=3.79 wt.%),Mg#(ave=50.82),Fe2O3(ave=7.27 wt.%).The Shenmu enclaves show(87Sr/86Sr)i=0.707443,?Nd(t)=-6.78.and ?Hf(t)=-17.8?1.98,along with their igneous microtextures,the presence of acicular apatites and hyperplasia of plagioclase edge of the enclaves,imply that the enclaves are homologous magma mixing.(4)There was a strongest magmatism in the Triassic period(?230 Ma to 216 Ma)of the Yidun terrane and the Songpan-Ganzi block on both sides of the Ganzi-Litang suture zone,based on the rock assemblage characteristics and regional depositional characteristics,the Mid-Late Triassic magmatism in the region is formed in the post-collisional extentional setting associated with the partial litho spheric delamination.The study of this paper further constrains the closure time of the Ganzi-Litang ocean at least in the early Middle Triassic.(5)Based on the characteristics of the coeval adakitic magmatic rocks and mineralization on both sides of the Ganzi-Litang suture suture,we believe that the source region of the dioritic porphyries related to the copper polymetallic deposit from the southern Yidun terrane may not have the characteristics of high oxygen fugacity and rich fluid,the high oxygen fugacity and rich fluid conditions required for mineralization may be related to the lithosphere mantle that is metasomatized by the ancient Tethys subduction on the west side of the suture.