Intraplate Magmatism And Its Tectonic Implications

It is well known that the North Margin of North China Plate is modified by the orogenesis of the Paleo-Asian Ocean tectonic regime and its east part was overprinted by the Pacific tectonic regime in the late Mesozoic and Cenozoic. This raises its important significances for the reconstruction of the Mesozoic tectonic strength field in the East China. However, the Early Mesozoic geological history at this area is not well understood till now. Recently, the discoveries of large and super-large poly-metallic deposits and the new interpretations of some important geological evidences suggest that the tectonism- magmatism- mineralization was remarkable at the North Margin of North China Block in the Early Mesozoic. So, we took the Guandi mafic intrusion as object, and synthetically studied it from Geology, Petrology, Mineralogy, Geochemistry and chronology poly-overviews, for the petrologic constrain of continental dynamics in this area.Guandi intrusions have small-scale, Au-Ag enrichment and apt mafic characteristics. Most of the rocks are gabbros- diabases, and traversed by granodiorite veins. We reported whole rocks Rb-Sr isotopic geochronological data of Guandi intrusions is 234Ma, which is powerful evidence to the existence of tectonic-magmatism- mineralization activities in Early Mesozoic. Geochemistry characteristics analysis show that Guandi rocks have high Al (>17%), low Ti(<1.3%), high K (K₂O =0.76-2.32%, average is 1.53%) characteristics, which belong to the post-collision calc-alkaline basalt series. The intrusion show right-oriented linear REE patterns with LREE enrichment, depleted HFSE and strongly negative Nb, Ta anomalies as well as moderately enriched Sr isotopes and obviously varied Nd isotopic compositions. Radioactive isotope ⁸⁷Sr/⁸⁶Sr initialized average value is 0.7048, and ε Nd value is -5.3, which show magma come of enriched mantle, and assimilated by the lower melt compositions from the eclogite face mafic rocks. The analysis of sections of plagioclase mineral, we can see the probability of two kind of magmas mixing. Thermodynamics calculation show clinopyroxene formed under the temperature of 1120¹150°C and 1170-1200°C, which implied magma evolved obviously into two stages. The deeply studies show the GD intrusions was mixed by magmas from mantle and from lower crust, without the effect of subduction system.We brought forward the model of "partial delamination" , according to the GD above characteristics and tectonic thermo-system conditions of magma genesis. Delamination induced asthenosphere decompressional melt to basalt magma, delaminated Lower crust partial melted to form the feldspar melt. The two kind of magmas happen mixing at their ascending to up layers. At the same time, Middle and Up crust come into being local spreading tectonics, creating favorable channels for ascending magmas. So, mixed magma ascended by the tectonic crannies and formed small intrusion like GD rocks and more diorite and mafic-intermediate dyke swarms of Early Mesozoic.Sum up, the north margin of North China have not modified and overlapped by the Pacific tectonic regime, intense tectonic-magmatism were the results of delamination of thickened lithosphere at the terminal Paleozoic. Small scale of magmatism had important instruction of poly-metallization to the west of ChiFeng City.