Based on thin section and cathodoluminescence imaging

this paper discussed the types of diagenetic facies and the characteristics of lithofacies of Ordovician carbonate rocks in Ordos Basin.In combination with carbon and oxygen stable isotopic characteristics

we also studied the geochemical characteristics of each diagenetic facies.During the early Paleozoic

the Ordovician carbonate platform was in a near-surface diagenetic tract and can be divided into four diagenetic environments and facies including normal seawater

evaporative seawater

early meteoric freshwater and hot seawater according to the diagenetic medium.The δ

18

O and δ

13

C of diagenetic facies of normal seawater

hot seawater and evaporative seawater are generally high

but tend to decrease along with the increasing bioclastic content.In contrast

the δ

18

O and δ

13

C of the early freshwater diagenetic facies

are generally low.During the late Paleozoic

the top of the Ordovician was in the epidiagenetic tract.According to the rock types of the current residual strata

the epidiagenetic tract can be divided into limestone weathering diagenetic facies in the Fengfeng Formation and dolomite weathering diagenetic facies in the Majiagou Formation.Their δ

18

O and δ

13

C values are significantly lower than that of the early freshwater diagenetic facies.At the end of the Late Paleozoic

the Ordovician Formation was in the moderate-deep burial diagenetic tract at a depth of 3 000-4 000 m

which can be divided into a closed system diagenetic facies and an open system diagenetic facies.The δ

18

O and δ

13

C values of the closed system diagenetic facies are similar with their original rocks.In contrast

the δ

18

O and δ

13

C values of the open system diagenetic facies are much lower than their original rocks because of the isotope fractionation effect in the hot environment.These results indicate that the petrographic characteristics and the δ

18

O and δ

13

C va-lues of carbonate rocks are important indicators for identification of the diagenetic environment and classification of diagenetic facies of carbonate rocks.