Geochronology And Tectonic Significance Of Basic Rocks In Chifeng Area

Chifeng area across two tectonic units-the North China plate and the Xingmeng orogenic belts. On the basis of detailed field surveys, this paper selects a series of rocks as the object on petrological, geochemical and geochronological studies.Toudaoyingzi hornblende gabbro located on North China plate on southern area of Chifeng, the rock invaded into Crystalline limestone of Cambrian Jinshan group, LA-ICP-MS U-Pb zircon analyses reveal that the emplacement age of hornblende gabbro is 368.5 ± 3.1Ma. A series of basic rock dikes on Xingmeng orogenic belts Shows strong deformationand Chaoyangou gabbro dike’s LA-ICP-MS U-Pb zircon age is 386.3 ± 3.7Ma.Those basic rocks have similar geochemical features, they are tholeiitic basalts, characterized by low Si and K, high Mg and Na, enrichment in large ion lithospheric element(LILE, Sr, Ba) and LREE, depletion in high field strength elements(HFSE, Nb、Ta、Zr、Hf) and HREE, but the fractionation between HREE and LREE is not strong.The magma is derived from enriched mantel,and contaminated by crustal substance.Those basic rocks on Chifeng area have extensional tectonic background and display bimodal volcanic rocks characteristics all together with the Devonian igneous rocks along north margin of North China block. This extension lasted whole Devonian.