Geochemical characteristics and protoliths of the marble rock group of the khondalite series in the Daqing Mountains, Inner Mongolia

Abstract：The marble rock group is the top one of three rock groups of the khondalite series in the Daqing Mountains area, Inner Mongolia. It, as a suite of Mg-rich marble, is characterized by development of very thick and extensive dolomitic marble, with small amount of clastic rocks at the bottom. Besides dolomite，the mineral composition also commonly includes one or several Mg-rich silicate minerals such as serpentinized olivine, diopside and phlogopite. The rocks are rich in CaO, MgO and LOI, reflecting the geochemical characteristics of carbonate rocks. The CaO＋MgO content is more than 44.96％ and the CaO/MgO ratio is relatively low, being 1.37-10.94; so the protolith should be dolostone that has a very low content of fragments and mainly formed by chemical deposition. The sedimentary environment of the protoliths graded from the paralic deltaic facies at the initial stage through the littoral-neritic faces to the enclosed sea basin facies which was separated by barrier islands under dry climatic conditions. The tectonic environment also passed from the metastable to stable one. This suggests that the protoliths of the suite of metamorphosed sedimentary strata may have formed on the passive continental margins, but were more likely to be similar to intracratonic aulacogen or rift deposits such as the Mesoproterozoic Changchengian System and Jixianian System.