Characteristics And Origin Of The Acid Intrusive Rocks In The Bangong Ophiolitic Melange Zone In Ritu

Bangong-Nujiang suture zone is an important plate boundary, which traverses the Tibetan Plateau with an extension of about 2, 000km from east to west. Ban-gong-Nujiang suture zone is mainly made up of ophiolitic melange which is distrib-uted in large scale, the width of which from north to south is generally dozens of ki-lometers. It is not only a key issue to understand the formation of the Tibetan Plateau and the evolution of Tethys, but also an important mineral concentration enrichment zone.On the base of having investigated the western part (in Ritu county) of Ban-gong-Nujiang ophiolitic melange belt and its ultramafites being rich in nickel in detail, I choose small acid rock masses in ophiolitic melange zone as the main object of study in this thesis. After detailed investigation in the wild and systemic observation of the representative acid rock masses, I have obtained some achievements as follows by isotope dating in zircon and geochemical analysis of total rock main elements, mi-cro elements and rare elements in acid rock masses.(1) The small acid rock masses in ophiolitic melange zone occur in small stock or small dyke. Their major axis direction is from east to west, in accord with the direc-tion of regional structural lineaments. The rock type is Granodiorite-porphyrite, which was formed by injected acid magma after the formation of ophiolitic melange zone.(2) The results of two Granodiorite-porphyrite samples by dating LA-MC-ICP-MS isotope of U-Pb in zircon are 96.10±0.53 Ma and 96.0±1.1 Ma, both of which represent products of the magmatic intrusion belonging to the Late Cretaceous of Mesozoic.(3) Granodiorite-porphyrites of Late Cretaceous are calc-alkaline series, belonging to sodium type or the general type rocks types. The rocks are rich in LREE and large ion lithophile elements (LILE), without anomalous Eu. The depletion of Nb and Ta means that they have the geochemistry characteristics of the micro element of typical arc magmatite.(4) Granodiorite-porphyrites have high LREE/HREE raitos (13.4～18.9) and have the geochemistry characteristics of adakite. It can be inferred that the magma oragi-nated from partial melting of eclogite in mantle wedge (subduction zone).(5) The Tethyan Ocean of this area once went through the Triassic-Early Jurassic spreading, Middle-Late Jurassic northward and southward bidirectional subduction, Late Triassic-Early Cretaceous residual ocean (sea) basin and Early-Late Cretaceous continent-arc (continent) collision.