Ordovician Magmatic Arc Stratum of the Middle Qilian Block

Abstract

Abstract:

Seduction-collision of the Caledonian tectonic belt in the Qilian unit formed in the Ordovician period. By geologic mapping, analysis of environment of sedimentation and study of rock geochemistry, it is considered that the rock formation of Late Ordovician epoch in Yeniutai, Subei County is the important part of magmatic arc of the northern margin in the western part of the middle Qilian block. Companied with the intermediate-acid intrusive rocks of Late Ordovician volcanic arcs, this stratum is mainly composed of slightly metamorphosed basic and intermediate volcanic rocks and conglomerate, sandstone, slate and a few fossil-bearing limestones from bottom of peaceful and deep water reducing environment of marine facies to the upper of alongshore underwater river-estuarine environment with alluvial-fluvial sedimentary sandwiched in the upper part. Material source is the magmatic arc intrusive rock, volcano-sedimentary rock and antiquity crystalline basement in the neighbor area, which reflects a progradational sequence in ascending order with lower water and shrinkage of the basin. The transformations of volcanic rock in ascending order are from basic to middle volcanic rock,from tholeiite of island arc sodium alkaline basalts to potassium alkaline basalts, from extravasations facies to burst facies, generally reflecting the characteristic of mature magmatic arc volcanic rock assemblage which is related to extrusion of the tectonic environment. The assemblage of magmatic arc stratum,intrusive rock and tectonic-ophiolitic mélange belt suggests characteristic of the continental margin proximal of Late Ordovician oceanic crust in west section of northern Qilian Mountains southdipping.

Key words: the north margin of middle Qilian block, Yeniutai, Late Ordovician period, magmatic arc, sedimentary-volcano stratum, seduction-collision

CLC Number:

WANG Yong-He JIAO Yang-Quan LI Jian-Xing LEI Gui-Xia. Ordovician Magmatic Arc Stratum of the Middle Qilian Block[J]. Geoscience, 2008, 22(5): 724-732.

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