Geological Characteristics And Significance Of Early Paleozoic Alkali Volcanic In South-Qinling, Langao-Pingli, Shan'xi Province

There was a NW alkali-ultrabasic-acidic complex in Langao-Pingli in south Qinling structure belt. Many geologists had studied the petrology, geochemistry and geochronology in Bijiashan of Langao-Pingli belt, some of them have founded ultramafic xenoliths in the belt. But in rock naming, occurrence adscription still remain difference, and study less in west Langao town and Yerenzhai of Pingli town now, needing further research.This paper takes the author in west of Langao town-east of Miaogou of Ziyang town area's igneous which controlled by Hongchunba fracture and magmatite of Yerenzhai-Pingyang in Pingli town as studying objects, use methods of field investigations and studying indoor, handling up to date theory and advanced testing means, described igneous characteristic through the geological occurrence, petrology, geochemistry and geochronology with a great deal of information and analytic data, discussed its magmatic evolvement process and geotectonic background, draw main conclusion as follows:1) Alkali-ultrabasic and medium-acidic rocks were dominating in investigation area. Among them, microstructure modulus pyroxenite and trachyle are most widely distributed, with geographical features: from west to east, the lithology changed from dominating microstructure (glass) pyroxenite, included albite basalt and cryptoexplosive breccia to dominating trachyle, included microstructure modulus pyroxenite and subvolcanic. There were intrusive and eruptible geological occurrence, and the latter was main. The microstructure (glass) modulus pyroxenite and trachyle was intruded by subvolcanic, then intruded by carbonatite dike.2) Clinopyroxene and albite were dominating mineral in microstructure (glass) pyroxenite, included olivie, no orthopyroxene, plagioglase, feldspathide or alkaline melanocratic mineral which was caused by enrichment in CaO and depleted in Al₂O₃;. From SiO₂ content, showing ultrabasic features, and it was metasomatismed by carbonatete obviously.3) Most pyroxenes have girdle in clinopyroxene of different generation in microstructure (glass) pyroxenite. The EPMA testing result showing that pyroxene and albite of different occurrence have different oxide content. 4) Rock has bimodal characteristic. Ultrabasic rock was enrichment in TiO₂, Na₂O, CaO, TFeO, depleted in Al₂O₃; Medium-acidic rock was enrichment in SiO₂,TiO₂, depleted in CaO, FeO, Na₂O>K₂O. In REE and trace element patterns , both of them was enrichment in LREE, and K, Nb, Ta, Hf, Zr, depleted in Rb, Sr, Y. and Eu anomaly. They were the comagmatic product.5) Magma source is possibly phlogopite-garnet-clinopyroxence-bearing mantle which was many times metasomatismed by carbonatete. The magma had experienced certain crystallization differentiation and slight crust contamination. Tectonic setting was continental rift.