The Geochronology,geochemistry,petrogenesis of Southern Yare Pluton and Its Constrains for the Metallogenic Conditions of Miocene Adakite-like Porphyry in Lhasa Terrane

Author:Yu Feng

Supervisor:Hou Zengqian


Degree Year:2015





Porphyry Cu polymetallic deposit as an important type of mineral deposit provides huge amounts of mineral resources for the world.Gangdese porphyry copper belt located in Lhasa Terrane has received excessive attention as the birthplace of the Continental collision metallogenic theory.Miocene porphyry system is found to be the large ore deposit host and has evident characteristics of adakite.However,research work has been focused on the east part of the Lhasa Terrane,but rarely on the west of 87E°.This dissertation chose the Southern Yare pluton on the west side as the target area which is a typical barren porphyry body,integrated the geochemical analyses of Zircon in-situ U-Pb isotopic dating,Hf-O isotopes,major elements and trace elements of whole rocks,and Sr-Nd-Pb isotope,and deduced the origin and evolution processes of the magma.Based on these findings,this dissertation derived the constraints of metallogenic conditions for Miocene adakite-like porphyry deposits by comparison.Southern Yare pluton is composed of magma from two ages,i.e.,the Oligocene(50Ma)magmatism and Miocene(16Ma).The Oligocene magmatism is mainly composed of biotite adamellite which contains dioritic enclaves,monzonite and gabbro-diorite dike.Intrusive rocks are the I-type,alkaline-subalkaline series.Oligocene rocks have the right-leaning REE pattern,rare of HFSE such as Nb,Ta and Ti and enriched in LILE such as Rb,Th,and U.Zircon in-situ Hf isotope data shows the depleted feature and the O isotopic character is much the same as Mantle zircon.Sr-Nd-Pb isotope data shows the feature of magma mixing.Oligocene basic magma is believed to originate from the partial melting of enriched mantle and the intrusive magma from the lower crust which was modified by the mantle wedge.Miocene magmatism is consisted of granite porphyry,orthophyre,dioritic porphyrite and some felsic dikes.The porphyry system in this study has the characteristics of Adakite,such as high Sr/Y ratio,low Y,lower Mg and high Al concentrations.Miocene rocks have the consistent models of REE pattern and spider map,showing no Eu abnormal and mid-REE abnormal,which indicates that the magma did not experience strong crystal fractionation.From zircon in-situ Hf isotope data,Miocene porphyry shows obvious enrichment signature.Zircon O isotope and Sr-Nd-Pb isotope data also indicate that there was ancient crustal contribution to the Miocene magma.So it is assumed that the Adakite-like rocks originated from partial melting of the thickened juvenile mafic lower crust and then underwent strong contamination from the ancient crust during upwelling.By the investigation on this set of barren porphyry,overall Southern Yare pluton’s Miocene porphyry system shows enriched isotope characteristic compared to ore bearing porphyry,and lack of mantle source contribution.It suggests that the contribution from mantle to the magma origin is the necessary condition for the metallogenesis of porphyry system.However,the relative high O2 fugacity is not the necessary and sufficient condition for mineralization.