The Early Paleozoic Granitic Magmatism And Its Implication Of The Kontum Massif,Central Vietnam

Early Paleozoic granitoids are widely distributed in the Kontum massif,whereas their petrogenesis and geodynamic settings are still matter of dispute.In this study,granitic plutons including Ben Giang diorite,Dien Binh granitoids(diorite and granite),Chu Lai and Dai Loc granite were analyzed on petrography,zircon U-Pb geochronology,whole-rock elemental and zircon-Hf isotopic compositions to above issues.Combined with available data,we address the spatial and temporal distribution of early Paleozoic granitic magmatism and their petrogenesis and reveal the corresponding geodynamics process responsible for the early Paleozoic crustal and tectonic evolution of the Kontum massif.Main conclusions are as follows:(1)The early Paleozoic granitoids in the central Kontum massif reveal I-and S-type characteristics and probably the products of northward subduction and closure of Tamky-Phuocson ocean as well as ensuing syn-collision processes.Of which,the Ben Giang diorite,Dien Binh diorite and granite were I-type granite generated in the slab subduction stage at 485-453 Ma,whereas Chu Lai and Dai Loc(S-type granite)were latter products of syn-collision from 431 to 422 Ma.(2)Early Ordovician(485-473 Ma)Ben Giang diorites show low Si O₂(54.44-56.85 wt.%)contents and relatively high Mg^#(47-53)and reveal metaluminous,calc-alkalic characteristics.They also display enrichment in LREE over HREE((La/Yb)_N=10.4-12.3)and weakly negative Eu anomalies(?Eu=0.83-0.97)as well as positive anomaly of LILE(e.g.,Ba,Pb,Sr)and negative anomaly of HSFE(e.g.,Nb,Ta and Ti).Combined with their positive zircon?_Hf(t)values(+6.2-+10),melting of metasomatized mantle wedge induced by ocean slap subduction was proposed.(3)Late Ordivician(457-453 Ma)Dien Binh horblende-bearing diorites and granites have large range of Si O₂(61.15-70.55 wt.%)and show slightly peraluminous characteristics as well as relatively low Mg^#(35-43)and Cr(1.35-15.7 ppm),Ni(3.12-11.8 ppm)contents.They are enriched in LREEs over HREEs((La/Yb)_N=7.7-20.1)and characterized by negatives Eu anomalies(Eu/Eu*=0.46-0.87).Taking their enrichment of Ba,Pb,Sr and remarkable depletions in Nb,Ta and Ti and negative zircon?_Hf(t)values(-8.7 to-4.2)into consideration,reworking of an acient continental mafic crust during subduction was ascribed to their genesis.(4)The Silurian Chu Lai and Dai Loc muscovite-bearing granites(431-422 Ma)are characterized by relatively high Si O₂ contents(72.21-77.06 wt.%)and total alkalis(K₂O+Na₂O=7.74-9.39 wt.%).They show strong peraluminous with high A/CNK ratios of 1.12-1.21 which is an indicator of S-type granite affinity.They are also enriched in LREEs over HREEs((La/Yb)_N=1.1-8.6)and characterized by obvious negative Eu anomalies(Eu/Eu*=0.17-0.3).They have zircon?_Hf(t)values of-6.0 to+0.6and old two stage Hf model ages of 1.5 to 1.9 Ga,indicating the reworking of ancient crust which perhaps dominated by sedimentary rocks.(5)In combination with the widespread occurrences of coeval high-pressure metamorphism,we consider an advancing accretionary orogenesis was responsible for the generation of the early Paleozoic arc magmatism in the Kontum massif.Such magmatic arc system was dominated the early Paleozoic evolution of the northern Gondwana margin during the subduction of the Proto-Tethys.