| Monzonitic rocks(including quartz monzonites or quartz monzodiorites)usually show geochemical characteristics intermediate between those of the calc-alkaline and alkaline rock series,and are generally grouped into high potassic calc-alkaline series.The volume of monzonitic rocks worldwide is relatively small,and they mainly occur as individual intrusive stocks,or coexist with granites forming composite complexes.These rocks are generally occcurred in an extensional tectonic setting ranging from post-collision to within-plate,and are spatially and temporally associated with deep thermodynamic processes including asthenosphere upwelling or injection of mafic magmas,thus can provide plentiful information about crust-mantle interaction process and magma evolution in these settings.Therefore,the monzonitic rocks has now become an important target in probing the composition,evolution and dynamic mechanism of materials in the interior of the earth and the interaction relationships between different spheres.Late Mesozoic monzonitic intrusions are well developed in Zhejiang Province.They are distributed roughly along a EW-trending belt to the south of the Jiangshan-Shaoxing fault in the Cathaysia Block.Typical examples of these intrusions include the Muchen pluton in Longyou County,the Matou pluton in Suichang County,the Dalai pluton in Wuyi County,the Huangtanyang and Maoliling plutons in Linhai County,and the Kanggu pluton in Sanmen County,etc.In this paper,we present comprehensive LA-ICP-MS zircon U-Pb geochronological,petrological,geochemical,Sr-Nd isotopic and LA-MC-ICP-MS zircon Lu-Hf isotopic data for these monzonitic intrusions(including monzonites and their coexisting rocks),aiming to precisely constrain their emplacement ages,and to comprehensively reveal their geological and geochemical characteristics,genetic types,petrogenesis,genetic relationships with their coexisting rocks,and geodynamic implications.This study has also unraveled the nature of mantle sources and mechanism of crust-mantle interaction associated with these magmatism,thus can shed new insights in understanding the Late Mesozoic tectono-magmatic evolution of SE China.The Late Mesozoic monzonitic intrusions in Zhejiang province are composed mainly of quartz monzonites and quartz monzodiorites.Mafic microgranular enclaves(MMEs)are abundant in most of these plutons.The MMEs consist mainly of monzodiorites,and show different shapes,mainly in plastic shape.Zircon U-Pb ages show that these intrusions were emplaced at 112?100 Ma,with the detailed crystallisation ages of 112 Ma for quartz monzonites and their MMEs in Muchen pluton,of 101?100 Ma for quartz monzonites from Matou pluton and quartz monzodiorites from Dalai pluton,of 108 Ma for quartz monzonites from Huangtanyang pluton,of 106?105 Ma for quartz monzodiorites and fine-grained granites from Maoliling pluton,and of 110?105 Ma for quartz monzodiorites and 110 Ma for MMEs from Kanggu pluton.Geochemically,the Late Mesozoic monzonitic intrusions in Zhejiang Province show intermediate to acidic,subalkaline to calc-alkaline,metaluminous to weakly peraluminous,and K-rich in composition,with SiO2 from 57.2%to 68.6%.These monzonitic intrusions can be divided into two groups,i.e.,the eastern and the western parts of Zhejiang Province,bounded by the Zhenghe-Dapu fault.Monzonites from the western part show higher and variable alkali,potassium,aluminum concentrations,whereas those from the eastern part are relatively homogeneous.Meanwhile,MMEs from the western part also show higher alkali and potassium and lower silica than those from the eastern part.In addition,all the Late Mesozoic monzonitic rocks are enriched in large ion lithophile(e.g.,Rb,Th,U and Pb)and light rare earth elements,depleted in high-field strength elements(e.g.,Nb,Ta,P and Ti).Monzonitic rocks from the western part have higher concentrations of heavy rare earth elements with variable degrees of Eu,Ba,and Sr negative anomalies,whereas those from the eastern part show homogeneous in rare earth element contents and with unconspicuous Eu,Ba,Sr anomalies.Most monzonitic rocks show relatively high ?Nd(t)values with relatively young two-stage Nd model ages.However,monzonitic rocks from western part have lower initial 87Sr/86Sr ratios(ISr)and more depleted Nd isotopic compositions than those from the eastern part(ISr=0.7058?0.7083,?Nd(t)=-5.60?-2.29 for those from western part,and Isr=0.7079?0.7090,?Nd(t)=-8.06?-6.84 for those from eastern part,respectively).MMEs from western part(?Nd(t)=-2.45?+0.72)also exhibit more depleted Nd isotopic compositions than those from eastern part(?Nd(t)=-7.16).These monzonitic rocks contain some zircons with unusually low ?Hf(t)values and relatively old two-stage Nd model ages(?Hf(t)=-12.9,TDM2=1.95 Ga for those from western part,and ?Hf(t)=-13.3,TDM2=1.97 Ga for those from eastern part).In general,monzonitic rocks from western part show more depleted Hf isotopic composition than those from eastern part with EHf(t)values up to+1.3 and to-4.4,respectively.Hf isotopic compositions for their MMEs share similar characteristics,with EHf(t)values up to+5.8 in the western part and up to-3.8 in the eastern part,respectively.The integrated geochronological,petrological,geochemical,and Sr-Nd-Hf isotopic data indicate that MMEs from these monzonitic rocks were generated by various degrees of mixing between mantle-derived mafic magmas and crustally derived silicic magmas.The quartz monzodiorites and fine-grained granites from Maoliling pluton were likely share a common magma source,and were generated by different degrees of differentiation during magma ascent.Investigations also show that melts from both enriched lithospheric mantle and depleted asthenospheric mantle have contributed to the generation of the primitive basaltic magmas associated with the genesis of these monzonitic plutons,and more depleted mantle melts have involved in the generation of the monzonites from the western part than those form the eastern part.Most monzonitic intrusions in Zhejiang province were generated by mixing of mantle-derived,mafic magmas and felsic magmas produced by partial melting of Palaeoproterozoic crustal materials,however,the Dalai monzodiorites were probably generated by olivine-and pyroxene-dominated fractional crystallisation from basaltic magmas,which were in turn produced by mixing between melts from depleted asthenosphere and subduction-enriched mantle.All the studied monzonitic rocks were generated in a post-collisional extensional tectonic setting,and their generation period(112?100 Ma)might correspond to the period with most intensive crust-mantle interaction in the coastal area of Zhejiang and Fujian Provinces.The Late Mesozoic monzonitic rocks in Zhejiang Province can be generated solely by fractionation of basaltic magmas,but most of them were produced by mixing between mantle-derived and crustally-derived magmas.The formation of these monzonitic intrusions was mainly triggered by rollback subduction of the palaeo-Pacific plate and subsequently induced underplating of mantle-derived mafic magmas. |
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