CRUSTAL EVOLUTION OF SOUTHEASTERN CHINA - ND AND SR ISOTOPIC EVIDENCE

We present a synthesis of crustal evolution in SE China based on exten sive Nd and Sr isotopic data compiled from the literature for intrusiv e granitoids, volcanic, sedimentary and metamorphic rocks from three m ajor tectonic units of SE China: Dabie, Yangtze and Cathaysia. Overall , igneous rocks of Phanerozoic ages possess initial epsilon(Nd)(T) val ues from -21 to +5 and depleted-mantle model ages (T-DM) from 2.3 to 0 .5 Ga. Sedimentary and metamorphic rocks have a T-DM range of 3.3 to 1 .1 Ga. The model age data indicate that the most important crustal for mation took place in the Proterozoic, possibly with a very minor propo rtion produced in the late Archaean. This finding clearly suggests tha t the crustal evolution in SE China, including the Yangtze craton and Cathaysia, is distinctly different from the Archaean-dominated North C hina Block. The Dabie terrane, due to its lack of clear indication of Archaean crustal signal, might belong more logically to the Yangtze Bl ock. The large database shows an oceanward younging of T-DM for Phaner ozoic igneous rocks in SE China, with the oldest ages found in Cathays ia Interior and the youngest in coastal Fujian and Taiwan. epsilon(Nd) (T) values show parallel systematic variations. Two NE-striking low-T- DM zones characterized by high-REE granites in the middle of SE China are identified, but their tectonic significance is not yet clear. With in SE China, timing of the Yangtze-Cathaysia collision (Proterozoic vs . Mesozoic) has been controversial for many years. While the Mesozoic thrust model may be supported by a recent Ar-40/Ar-39 age study that r eveals Triassic to Early Jurassic ages for mylonitic rocks from the sa me fracture zone, a Proterozoic collision model seems to be favoured b ecause (1) abundant Neoproterozoic ages have been obtained recently fo r a variety of rocks occurring along the Jiangshan-Shaoxing fracture z one and for ophiolite suites of the Banxi Group, and (2) many lines of evidence have been found to argue against the Banxi Group as a melang e complex and a long displaced thrust sheet. This issue is not yet res olved by geochronological means, but the present synthesis of Nd isoto pic data and model ages and recent palaeomagnetic studies of Lower to Middle Triassic rocks appear to refute the hypothesis of Mesozoic coll ision. (C) 1998 Elsevier Science B.V. All rights reserved.