PRELIMINARY STUDY OF THE METAMORPHOSED YANLINGGUAN FORMATION OF TAISHAN GROUP OF XINTAI, SHANDONG, CHINA

Resumo

The Yanlingguan Formation of lhe Arehean Taishan Group of Xintai, Shandong province. China, mostly metamorphosed to amphibolite fades of the low-to moderate-pressure series and with a metarnorphic age of about 2,500 Ma, was originally a dominantly basic volcano-sedimentary series 1,000-1,400 m thick, containing subordinate basic intrusions and ultrabasic rocks, with the lowermost part influenced by migmatization. It is sub-divided into two sub-formations, each constituting a "major" eruption-sedimentational cycle and containing two to four minor ones, showing an up-ward succession of lower tuff or lava (plagioclase-amphibolite), main lava and upper tuffaceous sandy rocks (hornblende-leptitic rocks] often with intercalations of semipelitic types (Moine-type biotite-granulite). The basic lava usually corresponds to the high-ferromagnesian and especially high-ferreous type of the tholeiitic basalt. Judging from petrochemical data, the eruption could have taken place for part of the volcano-sedimentational period in a transitional belt between the island arc area and deep marine basin. All other geological characteristics, however, reveal that the rocks were accumulated mostly in water masses under conditions of crustal unstability. ln fact, part of them were formed in subaqueous and subaerial and even, lagoon environments.