Study On Ophiolite And Chromitite In Western Yarlung Zangbo, Tibet

In Tibet, the Yarlung Zangbo ophiolite belt reaches up to 1500 km, and can be divided into three parts: the east (Qushui - Motuo), the middle (Angren - Renbu) and the west (westward of Saga). The west area is further divided into Dajiweng - Saga ophiolite zone in the north and Daba - Xiugugabu ophiolite zone in the south. The previous research work was focus on the east and middle areas. Only few works has been done in the west area because of the bad working conditions. In this paper, the Daba - Xiugugabu ophiolite zone and its associated chromitites are studied in more detail for the first time, and made considerable progress as follows:1. The boundaries of major ophiolite massifs and their internal lithologic units are more accurately determined. All massifs are composed of peridotite, lacking of the upper magmatic units in the typical ophiolite suite. The massifs are large consisting of distinguished lithologic units ("facies"). Among them dunite "fades" and dunite-harzburgite "facies" are of majority. The boundaries between the "facies" are transitive. Lithologic types of the massifs are mainly harzburgite and dunite, with subordinated lherzolite (orthopyroxene: 5~7%). The textures and structures are similar to those of metamorphic rocks, and micro- plastic deformation of minerals is ubiquitous. There are a few mafic-ultramafic dikes intruded into the mantle peridotite. They are dissimilar to the sheeted dyke swarm in ophiolite profile.2. Comparing to chondrite, rare earth element (REE) abundances of mantle peridotites in this area are depleted to highly depleted. Their chondrite-normalized patterns are divided into four types: â… , LREE depleted pattern, â…¡, LREE enriched pattern, â…¢, MREE depleted pattern, or "U" -shaped pattern, â…£, Ce positive anomaly pattern. These patterns are more complex in comparison to those from the east and middle parts of the Yarlung Zangbo and Alpine peridotite. Ce positive anomaly pattern is unique in this area, having same special petrogenic implications. Incompatible trace elements are depleted in comparison to the primary mantle. Their mantle-normalized patterns are obviously different from those of MORB, and vary from one to another massif. PGE abundances and distribution patterns are in accordance with primitive mantle, showing little fractionation. The distribution patterns of REE and trace elements of mafic-ultramafic dikes are in accordance with those of N-MORB. Pb, Sr, Nd isotopic compositions of dikes has a close affinity to the Indian Ocean MORB, but it is obviously different from Eastern Mediterranean ophiolites, such as Vourinos, Semail and Troodos.3. The Daba-Xiugugabu ophiolite might represent ancient ocean crust relics of India continental northern a marginal ocean basin in the north of India plate. The ancient ocean crust has geochemical signature that is similar to mid-ocean ridge setting, as well as to the island arc transformation setting. So the tectonic setting belongs neither to SSZ (supra-subduction zone), nor to mid-ocean ridge, but to thepassive continental marginal ocean basin. The formation and evolution period are approximately coeval with the Neo-Tethys. The formation age of Dab-Xiugugabu ophiolite is mid-Jurassic to early cretaceous (123-173 Ma).4. Major chromite orebodies in this area could be formed by ore magma crystallization. Metallic elements released from primitive pyrolite and concentrated into melts by partial melting of pyrolite. Subsequently Cr-enriched silicate melt broke down into two conjugated part due to liquid immiscibility, one of which is so-called ore magma. Finally, ore magma crystallized to become massive chromite orebodies.5. Based on the geotectonic setting of ophiolite, massif scale, lithologic types, petrochemistry and geochemistry, and minerallization characteristics, and comparing to the chromitite-bearing ophiolites within and outside China, the author considers Daba-Xiugugabu ophiolite has a good potential for metallogenic prospect. Four exploration prospecting areas are suggested: (1) Northwest in Laangcuo massif, especially its dunite facies and surrounding harzburgite;(2) North in Dongpo massif, especially the dunite - harzburgite facies;and (3) Dunite facies in southeastern Dongpo massif;(4) Dunite- harzburgite facies in Dangqiong massif. Among them Laangcuo massif is the most potential target for prospecting breakthrough.