Comparision Of Geochronology And Origin Of The Late Mesozoic Granitoids From Major Metamorphic Core Complexes In The North China Craton And Their Significance

Extensive crustal extension occurs in northeast Asian continent especially in the North China Craton since Late Mesozoic. Metamorphic core complexes (MCCs) constitute a typical extensional phenomenon in this province. Almost all of the MCCs were invaded by different period granitoid in their footwall, which is characteristics of the MCCs. Currently, the geochronological frame, rock assemblages; origin and evolution of these granitoids, particularly their relationship with MCCs are unclear. Study on these questions will allow us to understand well the formation time and the mechanism of the MCCs.Based on previous studies, this paper tentatively studied the deformation characteristics, Zircon U-Pb chronology, and geochemistry of the Late Mesozoic granitoids in the Yunmengshan, Louzidian, Yiwulushan and Liaonan MCCs.Following achievments are have been made.1. Zircon U-Pb dating by LA-MC-ICPMS yielded ages of 126.6±1.8Ma of the Fengjiayu pluton, 122.5±1.8Ma of the Xibailianyu pluton, and 127.4±1.2Ma of the Baicha pluton in the Yunmengshan MCC area.Those plutones cut discordantly across the shear zone foliation and is not deformed themself. Their ages may provide upper age limit on the Yunmengshan MCC. Ziron U-Pb dating by SHRIMP yielded ages of 124.8±2.4Ma of the Anjiayingzi pluton and 155.9±1.6Ma of the Maanshan pluton in the Lozidian MCC area, further define that the plutons emplaced in Late Mesozoic. Zircon U-Pb dating by LA-MC-ICPMS yield ages form 204Ma to 232Ma of the other plutons in Louzidian MCC, suggesting that these plutons were Early Mesozoic plutons, not Late Mesozoic plutons confirmed as before. SHRIMP U-Pb dating of the magmatic zircon from the mylonitized granite vein which intruded in the Jinzhou detachment fault yields an age of 129±2Ma, constraint the formation time of the Liaonan MCC.2. Base on the previous date and our research, three stages of the Late Mesozoic granitoids in these MCCs are tentatively identified 175-141Ma,135-120Ma and about 113Ma, with two peaks of 165-150Ma and 130-120Ma.The Late Mesozoic granitoids in the MCCs have similar characteristics in rock assemblages, mainly include moyite and adamellite, showing metaluminous or weakly peraluminous, high-K calc-alkaline, high Sr, enrich in LREE, LILE with negative in HFSE,Ta,Nb,Ti and no or littleδEu.The early granitoids have higher SiO2, K2O+ Na2O, A/CNK or ASI, andΣREE than late granitoids. The early granitoids show ferruginous and S-I type feature while the late magnesian and I-A feature.The Nb/La, Nd/Th, Ti/Y, Ti/Zr, Nb/Ta and Zr/Hf value which can show source region is between the mantle-derived and crust-derived and vary greatly. In combination with the previous studies, it is suggested that the granitoids were derived from mantle and the crust.3. Preliminary analysis of the deformation feature and comparision to the age from the strong deformation to the no deformation granitoids limit the formation time of the MCCs, the Yunmengshan MCC formed during 144-123Ma, the Louzidian MCC formed during 150-124Ma, the Yiwulushan MCC formed during 163-123Ma, the Liaonan MCC formed during 129-113Ma.4. This study confirms previous suggestions. High Sr, low Y granite magmatism developed all over region from 165-120Ma. During 165-150Ma, some granitoids with characteristic of S-I type may be related to partril melting of the lower crust caused by the thicking crust resulted from nappe extrusion.During 150-120Ma, the deep crust might start strong thinning and granitic magma were generated in the deep crust. At that time, as the heterogeneity of crustal thickness, A-type granites occured in some part regions of the North China craton. At about 125Ma, with the uplifting and cooling of the MCCs, a large number of A-type granites appeared.