Gabbros from southern Gangdese:implication for mass exchange between mantle and crust.

Abstract

Gabbro as predominant rock of an ultramafic to marie rocks belt is occurring in southern Gangdese,Tibet,paralleling with Yarlung Zangbo Suture.Field investigation has found the gradually transitional relationship between the gabbro and granitoid with abundant marie macro-granular enclaves (MME).Petrological,geochemical and isotopic studies have proved that the gabbro is directly similar to primary basaltic magma with SiO_2 of 49%～55% and Mg~# of 49～66.The REE pattern is flat and slightly declining with no Eu anomaly,showing less magma differentiation taking place.Ratio of ~(87)Sr/~(86) Sr varies in range of 0.7036～07051,while ~(143)Nd/~(144)Nd ratio is 0.5128～0.5131 withe_(Nd) of 2.34～6.87.The ratio of ~(206)Pb/~(204)Pb,~(207)Pb/~(204)Pb,~(208)Pb/~(204) Pb are averagely 17.9095,15.4986 and 38.0176 respectively.All the characters indicated that the gabbro is likely resulted from partial melting in prevalent mantle reservoir (PREMA) and some magma mixing during intruding up.SHRIMP zircon U-Pb dating for gabbro samples gave 52.5～40.0Ma of crystallized age,which is quite similar with the gabbro dating of 47.0～48.9 Ma in Quxu.It probably represents that the gabbro intrusions are formed in 53～40Ma.The paper therefore inferred that partial melting was induced in mantle wedge due to decompression and heating following slab-breakoff during the continental collision in Eocene.Then the basaltic magma is underplating below and then intruding up into the low crust.It is the underplated mafic magma that resulted in the partial melting in the ernst and then magma mixing following gabbro magma invading up into the crust along Gangdese belt,forming the Gangdese giant magmatic belt.