The Genesis Of Early Cretaceous Granites And Molybdenite Deposits In And Around Xinxian County,Henan Province

In this study,we carried out a comprehensive research including zircon U-Pb dating,petrogeo-chemistry,Sr-Nd-Pb-Hf isotopes,molybdenite Re-Os dating,ore S-Pb isotopes,H-0 isotopes and fluid inclusions on the early Cretaceous granite and correlative Mo deposits in and around the Xinxian County in order to investigate their age,petrogensis,rock source and geochemical dynamic background.The Xinxian granitic batholith yields three zircon238U-206Pb ages of 125.5±1.5 Ma(sampleX10-10),123.6±2.2Ma(sample J11-06)and 122.7±1.3Ma(sample X11-14).In sample J11-06 and sample X11-14,three-generation old inherited zircons of Neoproterozoic(-750 Ma),Paleoproterozoic(?2320 Ma)and Archean(-2796 Ma)were found.The Xiadain batholith and parasitical mafic microgranular enclaves(MMEs)yield zircon 238U-206Pb ages of 125.7410.96Ma and 125.3±1.4Ma,respectively.The Lingshan batholith yields zircon 238U-20bPb ages of 127.4±2.1 Ma and also contains several Neoproterozoic(-750 Ma)inherited zircons.Both the major and trace elements of the three batholiths show a property of continental crust.They are characterized by high Si,K,but low Ca,Fe and Mg and depleted Cr,Co and Ni etc.compatible element.These three batholiths are enriched in light REE and large ion lithophile elements(Rb,K,Th,U)and Pb,but are depleted in middle and heavy REE,high field strength elements(Nb,Ta,and Ti).It is noteworthy that the Lingshan batholith display more obvious differentiation between light REE and heavy REE and weaker Eu negative anomaly than the Xinxian and Xiadian batholiths,with(La/Yb)N ratios of 19.33^63.20(average 37.22),8.67-42.00(average 20.35)and 8.25-26.94(average 17.75),and with ?Eu values of 0.61-0.92(except sample JD11-06=1.01,average 0.82),0.21-0.85(average 0.56)and 0.42-0.83(average 0.56),respectively.In addition,Lingshan batholith has higher content of Sr and Ba than Xinxian and Xiadian batholiths,without Sr and Ba negative anomaly.The Sr-Nd isotopic compositions of three batholiths are distinct.The Xinxian and Lingshan batholiths show enriched(87Sr/86Sr)i ratios[(87Sr/86Sr)i Xinxia=0.7070?0.7090;(87Sr/86Sr),Lingshan= 0,7068?0.7086]and relatively low ?Nd(t)values[?ENd(t)Xinxia,?-20.83?-17.46;?Nd(t)Linshan =-21.18?-14.14].While the Xiadian host ganite batholith shows less enriched(87Sr/86Sr)i ratios and higher ?Nd(t)values[?Nd(t)xia dian bost ghaniht =-14.27?-11.62,(87Sr/86Sr)i xia tian host granite=0.7089?0.7092].The MMEs from Xiadian batholith have different(87Sr/86Sr)i ratios but similar ?Nd(t)values with host granite[(87Sr/86Sr)i xiadian MMEs=0.7065-0.7076,?Nd(t)xiadian MMEs=-13.71?-12.01]suggesting different sources between host granite and MMEs.The Xinxian batholith shows complicated Hf isotopic compositions.The early Cretaceous magma zircons display enriched?Hf(t)and old two-stage model age TDM2[weighted mean ?Hf(t)=-22.08±0.69 and weighted mean TDM2=2554±30 Ma for sample X11-14;weighted mean ?Hf(t)=-23.0210.68 and weighted mean TDM2=2612±42 Ma for sample J11-06,t=125 Ma].The Neoproterozoic zircons display relatively enriched?Hf(t)and young two-stage model age TDM2[weighted mean ?Hf(t)=-7.78±0.8 and weighted mean TDm2=2054±65 Ma for sample X11-14;weighted mean ?Hf(t)=-6.4013.9 and weighted mean TDM2=21121190 Ma for sample J11-06,t=Neoproterozoic ages].Paleoproterozoic and Archean inherited zircon show depleted ?Hf(t)and very old two-stage model age TDM2[?Hf(t)=-14.73,TDM2=3755 Ma for the Paleoproterozoic zircon,and ?Hf(t)=-0.52,TDM2=3288 Ma for Archean zircon].The early Cretaceous magma zircons of Ling shan batholith display enriched ?Hf(t)and old two-stage model age TDM2[?Hf(t)=-30.5?-21.67,weighed mean=-26.7±1.4;TDM2=2535?3078 Ma,weighed mean=2845±83Ma].While the Neoproterozoic inherited zircons show depleted ?Hf(t)and young two-stage model age TDM2[?Hf(t)=-30.5?-21.67,weighed mean=-26.7±1.4,TDM2=253 5-3078 Ma,weighed mean= 2845±83Ma].The host granite and parasitical MMEs of Xiadian batholith show similar Hf isotopic compositions with enriched ?Hf(t)values and old two-stage model age TDM2[?Hf(t)host granite=-20.24?-14.07,weighted mean=-15.98±0.49;TDm2 host gran,w,=2062-2446 Ma,weighted mean=2190136Ma;?Hf(t)MMEs=-23.40?-14.28,weighted mean=-16.89±0.9;TDM2 MMES =2077-2642 Ma,weighted mean = 2235±55Ma].All above geochemical evidences prove that the early Cretaceous granites in western Dabie area have complex petrogeniss and source.Lingshan batholith with adakite property should form under a thicken crust condition(>50km).While the Xinxian and Xiadian bathoiths were formed under a non-thicken crust condition(<35km).Hf isotopic trace of Xinxian and Lingshan batholiths indicate that the early Cretaceous granites in western Dabie area were derived form a mixed source mainly including the Neoproterozoic rocks and some ancient Paleoproterozoic and Archean slab.In Xinxian County area,the Bananzhai,Dayinjian,Damaoling and Xiawa small stocks yield zircon U-Pb dating of 123.2±1.1Ma(MSWD = 0.52,2?,N =16),124.9±1.3Ma(MSWD =0.51,2a,N=12),123.811.8Ma(MSWD =2.7,2a,N=15)and 124.4±2.1Ma(MSWD= 4.2,2?a,N=15),respectively,indicating they are contemporary and belong to same magmatism.All these four stocks have similar major and trace element compositions.They are enriched in light REE and large ion lithophile elements(Rb,K,Th,U)and Pb,but are depleted in middle and heavy REE,high field strength elements(Nb,Ta,and Ti)and Sr and Ba.They also have similare Sr-Nd isotopic compositions with relatively high(8'Sr/86Sr);ratios,enriched eNd(t)values and Paleoproterozoic two-stage model TDM2 values[(87Sr/86Sr)i=0.706518?0.707116,average=0.706861,?Nd(t)=-14.22?-14.92,average=-14.55,TDM2=2.20-2.27Ga,average=2.23 Ga for Baoanzhai;(87Sr/86Sr)i=0.706379-0.707173,average=0.706931,?Nd(t)--15.62--15.17,average=-15.41,TDM2=2.15-2.18Ga,average=2.17 Ga for Dayinjian stock;(87Sr/86Sr)i=0.707434--0.707926,average=0.707649 for Damaoling stock;(87Sr/86Sr)i=0.710016-0.710443,average=0.710155,?Nd(t)=?-17.9482?-15.4738,average=-16.9072,TDM2=2.18-2.38Ga,average=2.29 Ga for Xiawa stock].They also have consistent Pb isotopic compostions.[(206Pb/204Pb)i=16.423?16.699(207Pb/204Pb)i?15.285?15.345,(208Pb/204Pb)i=37.334?37.633 for Baoanzhai stock;(206Pb/204Pb),=16.793?16.979,(207Pb/204Pb)i=15.379?15.487,(208Pb/204Pb)i?37.559?37.976 for Dayinjian stock;(206Pb/204Pb)i=16.234?16.470,(207Pb/204Pb)i=15.274?15.312,(208Pb/204Pb)i=37.184?37.328 for Damaoling stock;(206Pb/204Pb)i=15.4536?16.6879,(207Pb/204Pb)i=15.2272?15.3559,(208Pb/204Pb)i=37.3254?37.8044 for Xiawa stock.Similarly,they have alike Lu-Hf isotopic compositions[?Hf(t)=-21.67?-19.19,average=-20.39;TDM2-2.38?2.53Ga,average=2.45 Ga for Bananzhai stock;eHf(t)=-24.8?-17.8,average=-20.9;TDM2=2.31?2.75 Ga,average=2.50 Ga for Dayinjian stock;?Hf(t)=-23.5?-15.1,average=-19.3;TDm2=2.13-2.64 Ga,average=2.39 Ga for Damaoling stock;?Hf(t)= =28.99?-14.19,average=-19.95;TDM2=2.07?2.99Ga,average=2.43Ga for Damaoling stock].Based on the results of the whole-rock geochemistry,geochronology and isotopic compositions,we conclude that the Bananzhai,Dayinjian,Damaoling and Xiawa small stocks were formed in the same magmatism.They originated from the partial melting of the subducted lower crust of Yangtze Block under a non-thickened lower crust condition(<35 km).The Baoanzhai and Dayinjian Mo deposits yield two Re-Os isochron ages of 122.5 ± 2.7 Ma and 124.54±0.66 Ma,respectively.The Baoanzhai and Dayinjian Mo deposits display similar S isotopic compositions with granite with ?34Sv-CDT ranging from 3.2‰to 5.1‰(average=3.87‰)and ?34Sv-CDT ranging from 3.90‰ to 10.70‰(average=7.44‰).Ores from the Baoanzhai and Dayinjian Mo deposits have similar initial Pb isotopic compositions with associated granites[(206Pb/204Pb)i=16.590?17.674(average=17.027),(207Pb)/(204Pb)=15.300?15.476(average =15.37),(208Pb)/(204Pb)= 37.419-37.911,(average=37.664)for Baoanzhai Mo deposit;(206Pb)/(204Pb)=7.041?17.319,averagee=17.139;(207Pb)/(204Pb)=15.397?15.416(average=15.404),(208Pb)/(204Pb)=37.732?38.236(average=37.910)for Dayinjian Mo deposite,respectively].Molybdenites from Baoanzhai and Dayinjia Mo deposits have low Re content(5.69 ppm?10.97 ppm for Bananzhai;8.28 ppm-54.75 ppm for Dayinjian).All above S-Pb-Re isotopes suggest a crustal and magmatic source for the metallogenic minerals in the western Mo deposits.Three types of primary fluid inclusions in the quartz veins formed in mainmineralization stage from Baoanzhai Mo deposit were identified,including:(1)aqueous-rich two-phase inclusions(L-type),(2)vapor-rich two-phase inclusions(V-type),and(3)daughter mineral-bearing three-phase inclusion(S-type).The analysis results indicate that the main homogenization temperatures intervals present at 220??240?,while the salinities range in two intervals of 2%?4%(NaCleq)and 46%-48(NaCleq),respectively.The densities of the trapped fluids vary from 0.58 to 1.05 g/cm3.Raman spectra of the inclusions indicate that water vapor is the dominant component with minor amount of CO2 in the gasous phase of the inclusions.The typical shift in oxygen isotope compositions suggest a mixing event between magmatic and meteoric water.In summary,the deposit is an intermediate-high temperature epithermal deposit,and the fluid mixing plays crucially significant roles in the mineralization.The Dabie-Sulu Orogen was formed by a continent-continent subduction/collision between the Yangtze Block and the North China Block during the Triassic at around 240 Ma.The Yangtze Block was subducted underneath the North China Block and possibly stayed on the top of the upper mantle for dozens to hundreds of million years.Until the Early Cretaceous period around 130 Ma,the collisional orogenic belt was transformed from compressional to extensional conditions,with the thickened orogenic belt becoming stretched and subsequently collapsed.The subducted Yangtze Block was heated by the upwelling of mantle materials and began to melt.Later on the derived melts intruded to the surface forming numerous post-collisional magmatic rocks in an extensional tectonic setting.The large-scale magmatism and mineralization events in the west Dabie area are concluded to formed in this context.