Petrology And Geochemistry Features Of The Volcanic Rock And Ophiolite In Mianxian-Lueyang Suture Zone, Qinling Orogenic Belt

The Mianxian— Lueyang ophiolite belt forms a EW—trending zone more than 160km long.It extends along Bashan arcuate fault zone to Wuliba area, Xixiang county in the east and to Kangxian county, Gansu province in the west. It is the suture zone between Qinling and Yangtze plates. It is a tectonic melange, consisting of tectonic blocks of ultrabasic rocks, gabbros (accumulative gabbros), diabasic dike swarms, mid—ocean ridge basalts and island arc volcanic rocks. The peridotites are mainly harzburgite and dunite. Their REE distribution patterns can be summarized as two types:LREE depletion and with remarkable Eu enrichment; slight LREE enrichment and with Eu deficiency. Both gabbros and diabasic dike swarms exhibit a LREE enrichment, showing geochemical features of comagmatic igneous rocks. The mid—ocean ridge basalts in this region show a strong iron enrichment trend but do not show a silicon enrichment trend. That indicates the magma after which originated from the upper mantle by equilibrium partial melting withstanded fractional crystallization of olivine and pyroxene and did not exercise fractional crystallization of magnetite and / or ilmenite, reflecting low fo₂of the magma system.They show a LREE depletion and their Ti / V, Th / Ta, Th / Yb, Ta / Yb indicate that their primary magma originated from depletive mantle of asthenosphere by high degree equilibrium partial melting, must be considered as component of the ophiolite.It implys a MORB— type ophiolite association and an ancient ocean basin between Qinling and Yangtze plates during middle Palaeozoic--early Mesozoic era. The island arc volcanic rocks can be divided into tholeiitic and calc-alkaline associations.They evolve along two different trends but be linked together at the root. That indicates that they derived from a same primary magma system and belong to comagmatic igneous rocks, reflecting distinctive features of an inmature to a transitional stage island arc type. Their primary magma system originated from the upper mantle wedge above the slab by low degree partial melting. In short, on the basis of studies of tectonic, geochemistry, ophiolite and volcanic rock petrotectonic assemblages, it is evident that Mianxian—Lueyang suture zone represents an ancient ocean basin which was closed during the Indosinian period and eventually formed a complicated ophiolite melange with components of remnants of oceanic—ridge ophiolites, island—arc volcanic rocks and island—arc ophiolites.