Petrogenesis Of The Mesozoic Granites In The Zhuguangshan Area In The Nanling Region And Their Implications For The Uranium Mineralization

Granitoids are often closely associated with economic mineralization and evolution of the continental crust.Mesozoic granitic rocks widely occur in the South China Block(SCB),and formed in three episodes,i.e.,the Triassic(also referred to as the "Indosinian"),Jurassic("Early Yanshanian")and Cretaceous("Late Yanshanian").Investigations of these granitoids can provide important information on the tectonic evolution and geodynamics mechanism of the SCB during the Mesozoic.The Nanling Region in the interior of the SCB is famous for its large scale Mesozoic granitic rocks and mineralization.Its W-Sn and Nb-Ta reserves rank No.1 in the world.Meanwhile,granite-related U deposits in China also mainly distributed in the Nanling Region.Therefore,studies on the Mesozoic granites in the Nanling Region could improve our understanding on the Mesozoic tectonic evolution and metallogeny of hydrothermal uranium deposits associated with granites in the SCB.The Mesozoic granites and granite-related U deposits were well developed in the Zhuguangshan area in the middle of the Nanling Region.This study focus on some Mesozoic granites in the Zhuguangshan area,including the Pingtian monzogranite,the Miantuwo biotite granite,the Lingxi granodiorite,and some granites in the Zhuguangshan Complex(the Ledong two-mica granite,the Zhaidi two-mica granite,the Baiyun biotite granite,the Longhuashan biotite granite,the Jiangnan biotite granite,the Changjiang biotite granite,the Sanjiangkou two-mica granite and the Jiufeng biotite granite).This dissertation presents comprehensive observation and analytical data of petrology,in situ zircon U-Pb geochoronological,mineralogical,whole rock elemental and Sr-Nd isotopic and in situ zircon Lu-Hf isotopic compositions on these granites,with the aims to constrain the petrogenesis and magma sources of them,and thus can provide new insights in understanding the Mesozoic tectonic evolution process of the SCB and the sources of granite-related uranium mineralization.This study revealed that both the emplacement ages of the Miantuwo and Pingtian plutons are 233 Ma.They show metaluminous to slightly peraluminous A-type granite affinity.Results by using zircon saturation thermometer and apatite saturation thermometer indicate that the magmatic temperatures were 800?980 0C for both granites.Both Miantuwo and Pingtian granites show relatively high initial 87Sr/88Sr ratios,low ?Nd(t)values and low to moderate zircon ?Hf(t)values.Based on these data,it is suggested that these two A-type granites were derived from the partial melting of existing mafic to intermediate rocks in the lower crust in response to the underplating and/or intraplating of mantle-derived magma.The Zhaidi,Ledong,Jiangnan,Baiyun and Longhuashan plutons are dated as between 234 Ma and 231 Ma.The Jiangnan granite displays metaluminous to peraluminous affinity and high CaO/Na2O ratios(0.15?0.89),indicating that it might be generated by partial melting of a quartzo-feldspathic protolith.The Zhaidi granite shows strongly peraluminous S-type granite affinity and low CaO/Na2O ratios(0.11?0.22),indicating that it was derived from partial melting of a pelitic protolith.The Ledong,Baiyun and Longhuashan granites show a transition affinity between A-type granite and S-type granite with features:wide ranges of A/CNK ratios,high field strength elements contents and Ga/Al ratios,relatively wider ranges of the ?Nd(t)values compared with those of the Zhaidi granites,whereas the zircon ?Nd(t)values of the Ledong,Baiyun and Longhuashan granites are relatively stable.We suggest that the protolith of the Ledong,Baiyun and Longhuashan granites was a mixture of pelitic rocks and igneous rocks.The differences in geochemical composition of A-and S-type granites in the study area were originated from protolith compositions and different conditions for partial melting.Combining our results and previous studies on other Triassic granitoids in the SCB,a tectono-magmatic evolution model of the interior of SCB was proposed here.During the Early-Middle Triassic(before 234 Ma),the interior of the SCB was dominated with a compressional tectonic environment in response to the collisions between the SCB and ambient blocks to its south and north.Only a small amount of S-type granites were formed within continental interior of the SCB during this period.A tectonic transition from the compressional environment to a post-collision extensional environment began at approximately 233 Ma as indicated by the occurrence of A-type granites in the interior of the Cathaysia Block.In response to this extension,a large amount of granites occurred mainly in the Cathaysia Block during 233?224 Ma.The tectonic transition in the Yangtze Block may have occurred at approximately 225 Ma in the Yangtze Block,which isindicated by the temporal and spatial distribution of granitic activities:most granites occurring in the Jiangnan Orogeny during 223?214 Ma.After 214 Ma,the granitic activities in the SCB gradually weakened,marking the end of the tectonic cycle from syn-collisional compression to post-collisional extension.The emplacement of the Jiufeng,Sanjiangkou and Changjiang plutons were during 168?157 Ma.The Jiufeng granite displays I-type granite affinity.Its magma sources are mainly intermediate to felsic igneous rocks and may contain minor mantle materials.The Changjiang granite belongs to S-type granite and may have been derived from partial melting of a quartzo-feldspathic protolith with a little pelitic proportion.The Sanjiangkou granite shows S-type granite affinity and may have been generated by partial melting of a pelitic protolith.And the Lingxi granodiorite was formed at approximately 100 Ma,showing geochemical features of adakitic rocks.It is suggested that the most likely generation mechanism of the Lingxi pluton is partial melting of a thickened Proterozoic lower continental crust at a pressure>12 kbar(or crust thickness>40 km).We propose that the crust was thickened to over 40 km by a compressive event occurring during the late Early Cretaceous which was consistent with an unconformity found between strata of late Jurrassic or early Early Cretaceous and strata of early Late Cretaceous.After this event,the Cathaysia Block experienced a lithospheric extension and thinning probably driven by the high-angel paleo-Pacific subduction.With the extenation of lithosphere,the lower crust was heated to partial melting by upwelling asthenospheric materials,resulting in generation of the Lingxi adakitic granodiorite and other coeval granitoids in the Cathaysia Block.Combining our study of the U-bearing granites in the Zhuguangshan Complex and previous studies of other typical U-bearing granites in the Nanling Region and Europe,it is proposed that the U-bearing granites share the following features:(1)Most of them belong to two-mica granites or muscovite-bearing biotite granite.(2)They were derived from partial melting of ancient crustal materials without actual material addition from mantle-derived magmas.(3)The melt of U-bearing granites mainly experienced the process of magmatic-hydrothermal interaction.A certain degree of the magmatic-hydrothermal interaction could promote the differentiation of the U and Th.The U-bearing granites usually have high relative content of uranium in the Th-U-Zr/5 diagram.Based on these conclusions,we have evaluated the uranium mineralization potential of the Triassic A-type granites in the SCB.The results show that the Qingxi and Xitian granites may have high uranium mineralization potential,whereas the Miantuwo and Pingtian A-type granites in this study display low mineralization potential.