Late Mesozoic Magmatic Records For Transition Of Tectonic Regimes Of Dabie Orogenic Belt

The relationship between large-scale magmatism in early Cretaceous and crust-crust collision in late Triassic and the time and mechanism of regional structural stress field changing from compressional to extensional are major issues in studying of Dabie Orogen. Based on a studying of magmatic sequence in early Cretaceous in Shangcheng area, it is shown that the large-scale magmatism was the result of lithosphere delamination, large-scale lithosphere delamination made the regional structural stress field changed from compressional to extensional and made the ground surface began to subside, the time of these changes is ~ 130Ma.Shangcheng Pluton and Jingangtai volcanic rocks distribute over the northern margin of Dabie Mountain, in the middle part of the North Huaiyang belt. New Observations indicate that Jingangtai volcanic rocks uncomformally overlie the Shangcheng Pluton. Zircon SHRIMP U-Pb age of Shangcheng Pluton and Jingangtai volcanic rocks is 130.2 Ma and 129 Ma, they are produced in Nearly the same time, both in early Cretaceous.Shangcheng Pluton meanly consists of granite and montzogranite; all rocks can be classified into KCG-type granite. All rocks have high content of Al2O3, Sr, high Sr/Y and low content of Y, Yb, light and heavy REE show obvious fractionation,(La/Yb)N=23.0~72.5, HREE are comparatively depleted, and there is no Eu anomaly (δEu=0.90~1.05). They have the same geochemical characteristics with Adakites, which indicates that the magma came from partial melting of an over-thickened (>50 km) eclogite-facies lower crustal root beneath the orogen.Jingangtai volcanic rocks are a suit of mainly effusive phase rocks and some sub-volcanic phase rocks. They belong to calc-alkalic series, on the SiO2-K2O diagram all samples fall in the fields of high-K calc-alkalic series and shoshonite series. The volcanic rocks show typical calc-alkalic differentiation trend. The REE distribution patterns show obvious LREE fractionation, (La/Yb)N=17.0~20.2, flat HREE and weak to intermediate negative Eu anomalies (δEu=0.63~0.93). These characters indicate the magma came form partial melting of normal lower crustal.The geochemical characters of Shangcheng Pluton and Jingangtai volcanic rocks change from adakitic rocks to normal calc-alkalic roaks. These changes show the depressing of magma souces region. During 130.2~129Ma, the crustal thickness in Shangcheng region quickly changed from over-thickened to normal, which indicates the lithosphere delamination had occurred.Emplacement mechanism and depth of magma emplacement show the Shangcheng Pluton had the characters of forceful emplacement and had undergone syn-emplacement erosion. During 130~129Ma, Shangcheng Pluton was quickly erosoned from several kilometers beneath to the ground surface. From magma produced to emplacement of Shangcheng Pluton, the region had experienced intensively compressional uplift. The regional compressional force was just right the same force which made the crustal thicked. The well-preserved volcanic apparatuses and thick weathering crust indicate that there is no obviously erosion in Shangcheng region after eruption of Jingangtai volcanoes, the regional lithosphere had been extensional for a long time. Between 130~129Ma, the regional structural stress field changed from compressional to extensional.Large-scale magmatism in Dabie Orogen in early Cretaceous is the products of lithosphere delamination. Large-scale lithosphere delamination made the regional structural stress field changed from compressional to extensional. Instead of large-scale uplift and erosion, the ground surface began to subside.