PALEOZOIC PRECOLLISION AND POSTCOLLISION CALC-ALKALINE MAGMATISM IN THE QINLING OROGENIC BELT, CENTRAL CHINA, AS DOCUMENTED BY ZIRCON AGES ON GRANITOID ROCKS

Based on large-scale reconnaissance mapping, we identified two calc-al kaline plutonic assemblages from the northern Qinling orogenic belt. c entral China. The older assemblage of intrusions, closely associated a nd deformed coevally with their host volcanic are sequences, seems to represent the fractionation product of basaltic are magma. It therefor e predates the collision of the North China Block with the Central Qin ling island-arc system that developed in a SW Pacific-type oceanic dom ain south of the North China Block. Single-zircon Pb-207/Pb-206 evapor ation dating yielded early to middle Ordovician ages for this assembla ge, with a relatively small range from 487.2 +/- 1.1 to 470.2 +/- 1.3 Ma. Intrusions of the younger assemblage are largely undeformed and tr uncate structures shown in rocks of the older assemblage. They are int erpreted as post-collisional calc-alkaline granitoids. Single zircon d ating provided an age of 401.8 +/- 0.8 Ma for the younger assemblage, consistent with earlier work that defines an age range from c. 420 to 395 Ma. Our data favour a tectonic model involving formation and amalg amation of island are and microcontinent terranes between ca. 490 and 470 Ma ago to create the Central Qinling Zone which subsequently colli ded with the North China Block prior to c. 400 Ma ago. A late Precambr ian age of 762.0 +/- 0.7 Ma for a granitoid gneiss at the northern mar gin of the Yangtze Block supports a Gondwana affinity for this large c ontinental block.