Petrogenesis And Tectonic Setting Of The Ca. 1.75 Ga Mafic Dyke Swarms In The Mt. Zhongtiao And Mt. Songshan Areas

Mafic dykes are believed to have derived from tholeiitic magmas under extension setting. Abundant mafic dykes with similar occurrences can distribute linearly to form mafic dyke swatms in a very short time. Mafic dyke swarms emplaced in different levels of the crust depending on the continental solidification and the orientation of ancient stress. They are thus important indicators of the extension of lithospheric mantle (or crust), and important for the reconstruction of the ancient plates.Mafic dyke swarms in the North China Craton (NCC) formed in several period, of which the～1.75 Ga ones are the largest in scale and are one of the most important Precambrian tectono-thermal events in NCC. Therefore, they provide important constrains on the tectonic evolution of the NCC and the lithospheric mantle under NCC in this period. In this study, the mafic dyke swarms in the Mt. Zhongtiao and Mt. Songshan areas are chosen because they were not well documented preciously. And we can compare to the mafic dyke swarms in the adjacent area, in order to realize the macroscopical characteristic of the mafic dyke swarms in NCC. The petrogenesis and tectonic setting of the mafic dyke swarms are discussed on the basis of their petrology, geochemistry, and U-Pb isotope geochronology.A SHRIMP zircon U-Pb age of 1776±35 Ma from Mt. Songshan area is obtained in this study, interpreted to the crystallization age of the dyke swarms.The mafic dyke swarms is dominant of NNW- and EW-oriented, with dipping angles >75°. The rocks composed of the mafic dykes are mainly diorite, with minor gabbroic diorite and gabbro. Primary minerals are plagioclase and pyroxene, and minor hornblende, biotite, magnetite, ilmenite, quartz and apatite.Two geochemical series are identified in these dykes: highly fractionated series and slightly fractionated series. The rocks of the slightly fractional series have relative low FeOt, TiO₂, and K₂O content, and high MgO content to the highly fractional ones. They show distinctive depletion of high field strength elements (HFSE, such as Nb, Ta, Zr, Hf) and large ion lithophile elements (LILE, such as Rb, Ba, La), similar to OIB. The rocks have eNd(t) values from -6.2 to -8.1, the ²⁰⁶Pb/²⁰⁴Pb, ratios from 16.05 to 17.64, indicating that the slightly fractionated series may have derived from the PREMA (Prevalent mantle) source possibly affected by the highly fractionated series.The highly fractionated series have relative high FeOt, TiO₂, K₂O content, and low MgO content compared with the slightly fractionated series. They also have distinctive depletion of HFSE and enrichment of LILE similar to those for the slightly fractionated series, but the concentrations of elements are higher in the highly fractionated seres than those for the slightly fractionated series. TheεNd(t), and ²⁰⁶Pb/²⁰⁴Pb_t values of the rocks vary from -6.2 to -8.1 and 15.57 to 16.54, respectively,indicating that the highly fractionated series were derived from enrich mantle source possibly modified by the ancient subducted crust.The highly fractionated series can be further identified to high-Fe (FeOt>12.8%) and low-Fe group (FeOt<12.8%) by the FeOt content. The high-Fe group has higher FeOt, TiO₂ and MgO content than the low-Fe group. The rocks of the high Fe group show an iron-enriched and silica-decreased trend (Fenner trend), whereas the rocks of the low-Fe group a silica-enriched and iron- decreased trend (Bowen trend), which are possibly related to the different fO₂ in magma. In summary, the active interval of the dykes is about 1800～1750 Ma. The similarity of geochemistry, geometry, and related magmatic products between the mafic dykes and the Xiong'er Group volcanic rocks indicates that the mafic dykes are possibly the counterpart of Xiong'er Group volcanic rocks. They both may have formed by continental rifting, related to plume magmatism.