Petrogenetic Characteristics And Structural Implications Of The Early Neoarchean Intrusions In Western Shandong Province

The growth pattern of the earth's early continental crust and the mechanism of crust-mantle dynamics have been the focus of international Precambrian research.Although it has been generally agreed that the Archean is the main period of continental crust growth,the specific crust growth mode and crust-mantle dynamics mechanism in the Archean period are still very controversial.Some researchers proposed mantle plume or vertical tectonic mechanism related to subduction,while others proposed lateral accretion model related to subduction or combined action mechanism of mantle plume-island arc.In this paper,the meso basic intrusive bodies in the early Neoarchean period in the west of Shandong Province are taken as the research objects.Through the study of field geological characteristics,microscopic slice identification,whole rock principal and trace analysis,zircon U-Pb chronology and Lu-Hf isotope characteristics,more evidence is provided for the structural evolution characteristics of the early Neoarchean period in the west of Shandong Province.Predecessors' research results revealed that in the period of > 2.70 Ga,WSP area formation the variable komatiite in tholeiite primarily supracrustal rocks,overall in the Qixingtai–Sujiagou,such ultrabasic rock series and the global scope of craton inner mantle column structure formed under the background of tholeiite,is formed by partial melting mantle plume in different depth.During the period of 2.69-2.65 Ga,variable gabbro and variable diorite with island-arc origin were formed,which were mainly distributed in the Taishan area.The samples of this group had a slightly REE right-leaning distribution pattern,and the slight negative anomaly of Nb?Ta and Ti indicated that they were the products of plate subduction.At the same time,due to the high value of crustal heat flow in the Archean,it may be a series of small scale,low angle thermal subduction.In addition,calc-alkaline island-arc basaltic rocks with ?2.6ga have been found in Mengjiatun area in this paper,which have obvious right-leaning REE distribution pattern and negative Nb?Ta and Ti anomalies,indicating that they are not caused by mantle plume but by island arc.Previous studies have found 2618±36Ma of high-mg tholeiite basalts in the north of Dawangzhuang,southeast of Mengyin and east of Yiyuan.Such samples have distinct REE distribution patterns and negative Nb?Ta and Ti anomalies,higher MgO values and lower TiO2 values,and the TTG rocks produced in the same period also have features of island-arc origin.Crustal heat flow value at the time of the ~ 2.60 Ga gradually tend to be rigid,with the decrease of the subduction Angle and depth increases gradually,subduction slab and mantle wedge gradually began to interact,subduction slab continuous dehydration by partial melting,the molten part of the continuous enrichment of large ion lithophile elements and light rare earth elements,eventually forming calcium alkaline arc basaltic rocks.To sum up,the rock assemblage types,geochemical characteristics and petrogenesis of the intermediate-base intrusions indicate that these intrusions were formed in a combined mantle plumes and plate tectonics.Combined with the rock assemblage characteristics of the komatiite-tholeiite basalt-enriched basalt,? 2.75 Ga is the oceanic basin outburst period,2.70-2.65 Ga is the arc-mantle joint period,and 2.65-2.60 is the subduction stage.