Geochemical Characteristics And Petrogenesis Of Paleoproterozoic Pyroxenite,Syenite And Carbonatite In Fengzhen Area,North China Craton

As one of the oldest cratons in the world,the North China Craton provides an ideal lab to study the evolution of early Earth and Precambrian geology.Carbonatites are exposed in some areas of the North China Craton,and the genesis of carbonatites is usually closely related to the Earth’s deep carbon cycle and mineralization of rare elements.Carbonatites usually outcrops together with alkaline rocks and mafic rocks in the fields.And their genetic relation is still in debate.This PhD dissertation presents a geochemical study on a set of pyroxenite,syenite and carbonatite dykes in Fengzhen area,where the Trans-North China Orogen joined with the Khondalite Belt.The results are used to reveal the genetic mechanism of this complex.On the basis of petrogenesis study,apatites from different rock types are analyzed for U-Pb ages,trace element compositions and Sr-Nd isotope ratios.The results reveal the detailed magmatic process of carbonatite magma in continental crust.In order to determine the genetic relationship of the pyroxenite,syenite and carbonatite,that is,whether the pyroxenite is mantle xenolith captured by carbonatite,independent magmatic rock,early crystallized cumulate from carbonate magma,or the products of the reaction between carbonate magma and the crust;and whether the syenite is differentiated from carbonate magma or mixed with crustal materials,various geochemical methods were employed in this study.The pyroxenites show low SiO2(40-49 wt%),high CaO(>20 wt%),and are enriched in light rare earth elements,strongly depleted in high field strength elements(Nb,Ta,Zr,Hf),consistent with the characteristics of carbonatite dykes.The zircon U-Pb ages of pyroxenites and syenites are 1789 ± 13 Ma and 1793&14 Ma,respectively,which are consistent with the metamorphic age of wall-rock granites(1839 ± 19 Ma).The whole-rockεNd(t)and εHf(t)values of the pyroxenites are-4.4 to-5.1 and-3.8 to-6.9,respectively.Similarly,the seynites have εNd(t)and εHf(t)values of-3.7 to-4.7 and-4.1 to-5.3,respectively.Both are similar to those of the carbonatite dykes but significantly lower than those of wall-rock granites.The δ18O values of clinopyroxene fall in the range of+6.6‰ to+11.3‰,which are much higher than the mantle value.Calcite from the carbonatite yield δ18O andδ13C value of 9.3 ‰-15.0 ‰ and-8.0‰--3.2 ‰,respectively.These lines of geochemical evidence indicate that the pyroxenites are not mantle xenolith or derived directly from mafic magma,but closely related to the carbonatite dykes and the wall-rock granites.Since the Ba/La and Ba/Mn ratios of the syenites are much higher than those of the carbonatite,an immiscibility origin is excluded.Mineral chemistry of clinopyroxene,olivine,calcite and phlogopite from the carbonatite,pyroxenite and syenite all suggest they are formed by contamination of felsic rocks in different degrees.Modeling shows that the Nd isotope compositions and trace element compositions of the pyroxenites and syenites fall on the mixing trends between the carbonatite dykes and wall-rock granites.These results demonstrate that the pyroxenites and syenites in the Fengzhen area were formed when carbonatite magma intruded and reacted with felsic crustal rocks.This provides a canonical case for the formation of pyroxenite and syenite by carbonatite magma reaction with crustal rocks.To decipher how apatite can record the reaction process between carbonatite magma and crustal rocks,apatites from the Fengzhen carbonatite,pyroxenite,and kfs-bearing pyroxenite were analyzed for U-Pb ages,major and trace elements,and Sr-Nd isotope compositions.The U-Pb ages of apatite in three pyroxenites are 1777±18 Ma,1847 ± 15 Ma and 1806 ±14 Ma.And the U-Pb age of apatite from a Kfs-bearing pyroxene is 1782 ± 18 Ma.These results,similar to the zircon U-Pb dating results,confirm that these rocks were formed in the Paleoproterozoic.The apatites in carbonatite can be divided into two groups,the first group with higher Sr content,total REE content,Sr/Y ratio and(La/Yb)N ratio than the second group,indicating that the first group of apatite was crystallized earlier than the second group.However,the total REE content(1652-14242 ppm)and Sr content(1213-7329 ppm)of both types of apatite are lower than those in the pyroxenite(∑REE:12064-28455 ppm;Sr:6001-15686 ppm).And they have slightly negative Eu anomalies(Eu/Eu*of about 0.8).This suggests that the apatite in the carbonatite crystallized during or after the reaction with the crust.The total REE and Sr contents of apatite enclosed in diopside from pyroxenite are lower than those of intergrain apatites,indicating that the latter crystallized after some degrees of crust reaction or clinopyroxene crystallization.The apatite in the kfs-bearing pyroxenite(17FZ10)can be divided into two groups.The first group are larger and occur as intergrain crystals wheras the second group are smaller and enclosed in potassic feldspar.The first group are significantly enriched in LREEs,Sr,and REE contents,with(La/Yb)N ratios higher than the second group.The first group of apatite formed after the reaction of carbonate magma with felsic crust.The second group is inherited from the wall rock granite,and the composition is similar to apatite in felsic granite.The Sr content of apatite from kfs-bearing pyroxenite(17FZ26)is very low(231-256 ppm),and shows a significant negative Eu anomaly.These apatites record the separation and crystallization of clinopyroxene and feldspar.The SO3 content of apatites in the pyroxenite is higher than those in the kfs-bearing pyroxenite,which is consistent with the fact that pyroxenite requires higher contribution of carbonate magma with high oxygen fugacity than Kfs-bearing pyroxenite.The in-situ Sr-Nd isotopes of apatite are generally similar to those of the whole rock.In particular,the apatite and whole-rock(87Sr/86Sr)i of the kfs-bearing pyroxenite is close to that of wall-rock granite,which records higher proportion of crustal reaction.Based on the(87Sr/86Sr)i of the carbonatite,the deposition time of the sedimentary carbonate involved in the mantle source of carbonatite is estimated to be about 2.4 Ga,indirectly constraining the age of subducted oceanic crust.