Petrogenesis And Tectonic Significance Of The Late Mesozoic I-and A-type Composite Granite Plutons In Coastal Fujian And Zhejiang Provinces

I-and A-type granitic complexes,as a special rock association,are generally related with deep thermodynamic processes including asthenosphere upwelling or underplating of mantle-derived magmas,and can provide abundant information about crust-mantle interaction and tectonic evolution process.Therefore,such lithological associations have now become an ideal object in probing the composition and evolution mechanism of materials in the interior of the earth and in revealing the tectono-magmatic relations.Late Mesozoic A-type granites within coastal Fujian and Zhejiang Provinces,SE China,generally coexist with I-type granites,constituting typical I-and A-type composite granitic complex.These A-type granites can be further divided into aluminous A-type(AA-type)and peralkaline A-type(PA-type)subgroups,thus the complexes within SE China Coast can be correspondingly divided into I-and PA-type,and I-and AA-type composite granitic plutons.Previous studies have examined the late Mesozoic granites within SE China Coast at different levels,however the petrogenetic processes and tectonic setting involved in the generation of the I-and A-type complexes within the coastal granite belt still remain controversial.In this paper,we present comprehensive LA-ICP-MS zircon U-Pb geochronological,petrological,mineralogical,whole-rock elemental and Sm-Nd isotopic and LA-MC-ICP-MS zircon Lu-Hf isotopic data for the typical Zhoushan I-and PA-type complex in northeastern Zhejiang and the Zhangpu I-and AA-type complex in southeastern Fujian,with the aims to precisely constrain the emplacement ages of these I-and A-type granites,identify the nature of their magma sources and magma process,then reveal the role of mantle-derived magma during the generation of granitic magmas and crust-mantle interaction processes,and finally elucidate the differences in the genesis between PA-and AA-type granites and the genetic relationship of I-and A-type granites,thus can shed new insights in understanding the late Mesozoic tectono-magmatic evolution of SE China Coast.The Zhoushan composite granitic complex in the northeast coast of Zhejiang Province is composed of the northern alkali-feldspar granites and the southern alkali granites.The former were emplaced at 98～96 Ma,usually have low 104Ga/Al ratios with the presence of calcic amphibole and magnesium biotite and thus can genetically be classified as I-type granites.In contrast,the latter formed slightly later at 88～86 Ma,and are typical peralkaline A-type granites,as indicated by the presence of arfvedsonite and aegirine.Both them thus make up a typical composite I-and PA-type granitic complex.The Zhangpu composite granite pluton occurs in the southeast coast of Fujian Province,and is a big batholith formed by multi-stage magmatism.It consists mainly of granodiorite,biotite granite,and alkali-feldspar granite.The Baishishan,Jingangshan and Wushan alkali-feldspar granites were emplaced at 92-86 Ma.They generally have well-developed micrographic texture and miarolitic cavities,and high 104Ga/Al ratios with the presence of spessartine and Mn-rich muscovite,and thus can genetically be divided as aluminous A-type granites.The other plutons were emplaced significantly earlier at 119～91 Ma,and generally have low AKI values and 104Ga/Al ratios,indicative of calc-alkaline I-type granites.Therefore,the Zhangpu complex is a typical composite I-and AA-type granitic complex.Both the Zhoushan and the Zhangpu I-and A-type complexes are highly siliceous,K-rich,depleted in Ba,Sr,P,and Eu,and enriched in large ion lithophile elements(Rb,Th,U,Pb)and light rare earth elements(LREE),but there are also geochemical differences between the I-type and coexisting A-type granites.The A-type granites within Zhoushan complex are peralkaline and F-rich,and have slightly higher HREE and HFSE concentrations,and more distinct negative Ba,Sr,and Eu anomalies than the coexisting calc-alkaline,metaluminous to mildly peraluminous I-type granites.All of the granites within Zhangpu complex are metaluminous to mildly peraluminous and high-K calc-alkaline,but the AA-type granites generally have higher AKI values and HFSE concentrations,and more distinct negative Ba,Sr,and Eu anomalies than the coexisting I-type granites.However,compared with the Zhoushan PA-type granites,these AA-type granites still have relatively lower HREE and HFSE concentrations,and less distinct negative Ba,Sr,and Eu anomalies.These indicate that A-type granites within the complexes experienced more intensive fractional crystallization than the coexisting I-type granites,and the Zhangpu AA-type granites suffered relatively lower degree of magma fractionation compared with the Zhoushan PA-type granites.Based on a synthesis of the geochronological and geochemical data,it is suggested that diversity of the magma sources makes an important contri’ ation to the generations of the two types of composite granite plutons.The I-and P type granites within the Zhoushan complex were derived from K-rich basaltic.s and residual granulitic source,respectively.The I-type granites within th Zhangpu complex originated from the same magma source as the Zhoushan I-tyre granites,however,the coexisting AA-type granites derived from felsic infracrustal source.All the granites in this study show variable zircon Hf isotopic compositions,and both their εNd(t)andεHf(t)values all plot above the isotope evolution domain for the regional crustal basement,indicating the involvement of mantle-derived magma during their generation.Of note,the Zhoushan PA-type granites appear to have more radiogenic Nd and Hf isotope compositions than the coexisting I-type granites,while the Zhangpu AA-type granites show similar Nd-Hf isotopic compositions to those of the coexisting I-type granites,implying there are significant differences in the mixing ratio of crustal-and mantle-derived magmas during the formation of PA-and AA-type granites.It suggests that origins of the Zhoushan I-and PA-type granites are mainly controlled by the variations in the mixing ratio between crustal-and mantle-derived magmas,the nature of source rocks and the degree of magma evolution,and that of the Zhangpu I-and AA-type granites are mainly controlled by the nature of their source rocks as well as the degree of magma evolution.Overall,generations of the both types of I-and A-type complexes were the synthetical results of multiple factors,such as components of magma source,mixing ratio of crustal-and mantle-derived magmas,degrees of magma differentiation,intensity of tectonic extension,and fault system.Gradually intensifying regional extension may be a major dynamic mechanism responsible for the emplacement of I-type granites and subsequently A-type granites within the composite granitic complexes.And,spatial distribution of the PA-and AA-type granites in coastal Fujian and Zhejiang Provinces are closely related with the dissecting depth of faults,i.e.,greater cutting depth of fault would cause more intensive underplating of mantle-derived magmas,being conducive to the generation of PA-type granites,whereas to the formation of AA-type granites.