The formation of gabbro and diabase in the Xigaze ophiolite: Implications for a solw-spreading ridge

The Xigaze ophiolite is located in the central segment of the Yarlung Zangbo Suture Zone,and its genesis and tectonic setting remain controversial. The ophiolite has a serpentinized peridotites in the lower part,and the ultramafic cumulates are absent in the mantle-crust transition zone. Small amounts of gabbro dikes with isotropic or rhythmic textures intruded into the mantle peridotites or diabase sills. The diabase sills generally intruded along the interface of the mantle peridotites or intruded into the mantle peridotites as diabase dikes. Based on studies of field relationships and geochemical features,the authors hold that,the Xigaze gabbro dikes,instead of being formed by in-situ crystallization of magma chamber in the oceanic crust,might have resulted from the intrusion of crystals-enriched magmas from magma pockets which were distributed in crust-mantle boundary and evolved in variable degrees into silicate-rich minerals. The diabase sills might have been formed from the intrusion of mafic magmas along the structurally weak boundary. The detachment faults probably resulted in the uplift and exhumation of the upper mantle which triggered the upwelling and partial melting of asthenospheric mantle. The gabbro dikes and diabase sills in the Xigaze ophiolite might have originated from small amounts of supply and discrete intrusion of magmas in a slow-spreading ridge.