Petrogenesis And Continental Geodynamic Implications Of Late Cretaceous To Early Eocene Granitic Rocks In Tengchong Block

The Tengchong Block is active continental margin arc,related to the eastward subduction of Neo-Tethys and collision of Indian-Asian continents,and has massive Late Cretaceous to Early Eocene granitic rocks.These granitic rocks could provide important information for research on the oceanic subduction and continental collision.Compared with the abundant achievements of northward subduction of Neo-Tethys and collision of Indian-Asian continents,the research on eastward process is relatively less.The northward and eastward subduction of Neo-Tethys and collision of Indian-Asian continents produce the Mesozoic to Cenozoic granitic rocks in magmatic arc area,which have different geochemical signatures.The Hf isotope of most of granitic rocks in Gangdese belt is depleted,but enriched in Tengchong Block.In addition,there is a suit of volcanic rocks in Lhasa Block,Linzizong volcanic rocks that resulted from the northward subduction of Neo-Tethys and collision of Indian-Asian continents.However,it is lack of coeval volcanic rocks in Tengchong Block.Thus,what is the dynamics mechanism that caused the different magmatism during different directions of subduction and collision?This study selects the Late Cretaceous to Early Eocene granitic rocks as research subjects to discuss the petrogenesis of the granitic rocks during the process of oceanic subduction and continental collision,then attempts to analyze the process of eastward subduction of Neo-Tethys and Indian-Asian collision from igneous rocks,and reveals the thermal structures of subducted oceanic crust and mantle.This dissertation selects the representative granitic bodies in Tengchong Block,including Guyong,Husa,Bangwan,Xima-Tongbiguan plutons.The systemic field investigation,petrography,zircon U-Pb age,geochemistry,mineral chemistry,Sr-Nd-Pb and zircon Lu-Hf isotopic analyses have conducted for these granitic rocks.Combined with the previous researches,this dissertation argues the geochronological framework and spatial-temporal evolution characteristics of granitic rocks,the relationship between the formation of granitic rocks and the Neo-Tethyan subduction and Indian-Asian collision.This study yields the opinions as following:1.The Late Cretaceous to Early Eocene granitic magmatism in Tengchong was concentrated in 76Ma-50Ma,and could be divided into two stages:the high-K calc-alkaline granite-granodiorite generated at 76Ma-64Ma,calc-alkaline to high-K calc-alkaline granite-granodiorite-quartz diorite formed at 55Ma-50Ma.The detailed researches on zircon U-Pb age in representative granitic plutons suggest that the western Guyong granites have formation ages of 65±1Ma to 64±1Ma,and the eastern Guyong granites have crystallization ages of 76Ma-68Ma referring to literatures,and the lithology in two parts of Guyong pluton is homogeneous;the Husa granodiorites have crystallization age of 64±1Ma.The Bangwan pluton is a complex massif,and the biotite granites have formation ages of 51 51±1Ma to 49±1Ma,the granodiorites have formation age of 50±1Ma,The Xima-Tongbiguan granodiorites-quartz diorites have crystallization ages of 54±1Ma to 51±1Ma.The abundant mafic microgranular enclaves are enclosed in the Bangwan granodiorites and Xima-Tongbiguan coarse-grain granodiorites-quartz diorites and crystallization ages of 50±1Ma and 52±1Ma to 51±1Ma,respectively.The products of mafic magmatism in Xima-Tongbiguan area have metagabbros and fine-grain quartz diorites,and the crystallization ages of 51±1Ma and 53±1Ma,respectively.According to the characteristics of geochronology and petrology for the Late Cretaceous to Early Eocene granitic rocks in Tengchong Block,the granitic magmatism was concentrated in 76Ma-50Ma,and these granitic rocks could be divided two stage:(1)Late Cretaceous to Early Paleocene(76Ma-64Ma),the stage of forming high-K calc-alkaline granitic rocks including Guyong granites and Husa granodiorites;(2)Early Eocene(55Ma-50Ma),the stage of forming Nabang calc-alkaine granodiorites,Xima-Tongbiguan high-K calc-alkaline coarse-grain granodiorites-quartz diorites,and Bangwan high-K calc-alkaline granodiorites and biotite granites.The data for Nabang granodiorites refer to the literatures.The abundant mafic microgranular enclaves are enclosed in Early Eocene granitic rocks,and coeval with the Early Eocene mafic rocks,which imply the closely relationship between granitic and mafic magmatism.2.The geochemical and isotopic characteristics of Late Cretaceous to Early Paleocene granites and granodiorites reveal that the Guyong granites were derived from partial melting of pelite-rich rocks and the Husa granodiorites were derived from partial melting of mixing sources of ancient and juvenile crustal rocks.The Early Eocene granitic magmatism represents the anatexis in the different crustal levels.The biotite granites were derived from partial melting of ancient crustal rocks,and the granodiorites-quartz diorites were products of mixing of mantle-derived mafic and crust-derived granitic magma.The granodiorites-quartz diorites have abundant mafic microgranular enclaves,representing the coeval mafic magma,and implying the Early Eocene partial melting events of mantle wedge.The Late Cretaceous-Early Paleocene granitic rocks could be subdivided into relatively high silicic Guyong granites and low silicic Husa granodiorites.The Guyong granites have SiO2 contents of 70.64%-76.45%,K2O contents of 4.14%-5.08%,A/CNK ratios of 1.01-1.12,belonging to peraluminous and high-K calc-alkaline series,and have variable CaO/Na2O ratios of 0.1-0.7,high initial Sr ratios of 0.706511-0.711753,?Nd(t)values of-9.2 to-11.6,suggesting that the granites were derived from partial melting of pelite-rich ancient crustal rocks.The Husa granodiorites have relatively low SiO2 contents of 64.50%-66.99%,K2O contents of 3.10%-3.78%,high Na2O contents of 3.44%-3.85%,A/CNK ratios of 0.96-0.97,belonging to metaluminous high-K calc-alkaline series,and have high initial Sr ratio of 0.716496,?Nd(t)value of-16.5,but variable Hf isotopic composition,?Hf(t)values of 3.4 to-18.1,indicating that the granodiorites were derived from partial melting of mixing of ancient and juvenile crustal rocks.In addition,there are a little high-silicic granitic rocks that were derived from low-degree partial melting of juvenile crustal rocks in Tengchong Block.Thus,the sources for the Late Cretaceous to Early Paleocene granitic rocks are mainly comprised of ancient crustal and a little juvenile rocks.According to Sr/Y ratios of Husa granodiorites,the crustal thickness of Tengchong Block is thin during Late Cretaceous to Early Paleocene.The Early Eocene granitic rocks in Tengchong Block are variable,and have SiO2 contents of 54.08%-75.41%,forming deep crustal hot zone in the lower crust,as the magmatic products at different levels in crust,represented by Bangwan and Xima-Tongbiguan plutons.Bangwan pluton is a complex massif,and consists of biotite granites and granodiorites.The biotite granites have high SiO2 contents of 74.33%-75.41%,K2O contents of 5.57%-6.23%,A/CNK ratios of 1.00-1.09,belonging to peraluminous and high-K calc-alkaline series,high CaO/Na2O and Al2O3/TiO2 ratios,low Mg#,high initial Sr ratios of 0.710526-0.713002,?Nd(t)values of-7.4 to-9.4,EHf(t)values of-0.7 to-9.7,suggesting that they were derived from partial melting of greywackes in ancient crust.The granodiorites in Bangwan have SiO2 contents of 67.81%-69.57%,K2O contents of 4.48%-5.95%,A/CNK ratios of 0.97-1.03,belonging to metaluminous to peraluminous and high-K calc-alkaline series.The Bangwan and Late Cretaceous-Early Paleocene Husa granodiorites have identical occurrence locations and lithology,but different geochemical signatures.Bangwan granodiorites have more enriched K,Rb,Th,U and REE,and abundant mafic microgranular enclaves that is mixing of mafic and granitic magma,implying the Early Eocene mafic magmatism in the region.The comparison results show that the enriched compositions inject into the parent granitic magma during formation of the Bangwan granodiorites.The Bangwan granodiorites have high initial Sr ratios of 0.712751-0.713073,?Nd(t)values of-9.0 to-11.6,?Hf(t)values of-1.0 to-10.8,indicating that the granitic magma was generated from partial melting of ancient crust rocks.The geochemistry of mafic microgranular enclaves and high-K calc-alkaline to shoshonitic mafic rocks in this region are similar,enriched in K,Th,U and LREE,high initial Sr ratios and enriched Nd-Hf isotopic compositions,which suggest that the mafic magma in the Bangwan area was derived from mantle wedges that was metasomatized by melts/fluids released after breaking of phlogopite and allanite in subducted sediments during increasing temperature.Thus,the Bangwan granodiorites were generated by mixing of crust-derived granitic magma and mantle-derived mafic magma.The granitic rocks in Xima-Tongbiguan have variable compositions,SiO2 contents of 54.08%-70.09%,and the lithology mainly consists of granodiorites and quartz diorites,that are associated with coeval abundant different types of mafic microgranular enclaves,enriched biotite-rind MMEs and absence of rind MMEs.Xima-Tongbiguan granodiorites-quartz diorites have K2O contents of 2.24%-4.45%,A/CNK ratios of 0.92-1.01,belonging to metaluminous to peraluminous and high-K calc-alkaline series;high initial Sr ratios of 0.708502-0.709175,?Nd(t)values of-5.2 to-6.5,?Hf(t)values of 1.9 to-7.5,and the feldspars occur reverse growth zone.Combined with geochemical and mineral chemical results,the granodiorites-quartz diorites in Xima-Tongbiguan pluton were produced by mixing of mafic and granitic magma.The source rocks for granitic rocks are mainly comprised of ancient crustal rocks,juvenile crustal rocks and coeval mafic magmatism.The isotopic compositions show that the metagabbros have ?Hf(t)values of 1.8 to 10.9,and ?Nd(t)values of-3.4,indicating the decoupling between Nd and Hf isotopes,which resulted from the interaction and mixing between granitic and mafic magma.The mafic magma was derived from depleted mantle according to their zircon Lu-Hf isotopic compositions.The mafic microgranular enclaves enclosed in granitic rocks are the mixing products of evolved mafic magma and granitic magma,and different types of enclaves show different stages of interaction between mafic and granitic magma.After mafic magma moving into granitic magma and then rapid solidification,the minerals in enclaves could be interaction with host or residual granitic melts,such as amphibole changing into biotite.During this process,the enclaves could be broken and Ca+Fe+Mg move in the melts,leading to these compositions increase in the granitic magma.3.Early Eocene granodiorites-quartz diorites with SiO2 contents of 60%-70%have the increasing of enriched compositions from west to east,and the variations of granitic and mafic rocks are synchronousAs the active continental margin arc related to the eastward Neo-Tethyan subduction and Indian-Asian continental collision,the Tengchong Block has mature crust.The Eocene granitic rocks(including quartz diorites and granodiorites)with SiO2 contents of 60%-70%show increasing enriched compositions from west to east,such as K,Th,U and LREE,and the increasing initial Sr ratios,decreasing Nd-Hf isotopic ratios,but decreasing Pb isotopic ratios.This trend between gradually increasing enriched compositions with Sr-Nd-Hf isotope and Pb isotopic compositions are opposite.The compositional variation of granitic rocks is synchronous to the mafic rocks,such as the granitic rocks change from Na-rich to K-rich and the mafic rocks change from low-K tholeiitic to high-K calc-alkaline-shoshonitic series.Synthesized the petrogenesis of Bangwan and Xima-Tongbiguan granitic rocks,the composition variation of intermediate to acid igneous rocks in active continental margin arc is caused by mixing of different mantle-derived mafic magma and crust-derived granitic magma,and this variation depends upon the composition variation of mafic magma.4.The activity of granitic magmatism from Late Cretaceous to Early Eocene in the Tengchong Block is gradually westward migration,and the thermal structure of crust-mantle vary from cool crust-cool mantle in Late Cretaceous-Early Paleocene to cool crust-warm mantle in Early Eocene,caused by the initial collision between Indian and Asian continents.The Late Cretaceous-Early Paleocene magmatism occurs at the east of Yingjiang County,and the Early Eocene magmatism occurs at Yingjiang County and the west area.The activity of granitic magma from Late Cretaceous-Early Paleocene to Early Eocene in the Tengchong Block is gradually westward migration.The Late Cretaceous to Early Eocene igneous rocks is mainly granitic rocks,and lack of coeval mafic rocks in region,but the Early Eocene igneous rocks is characterized by coexisted granitic and mafic rocks and granitic rocks have abundant enclaves and depleted Hf isotopic compositions,which indicate that the mantle materials involved in the formation of granitic rocks.The changing of rock assemblage with time reveals the thermal structure of subducted oceanic crust and mantle wedge at the Tengchong Block,as active continental margin arc related to eastward Neo-Tethys subduction and Indian-Asian continental collision,from cool crust-cool mantle in Late Cretaceous-Early Paleocene to cool crust-warm mantle in Early Eocene.The changing of thermal structure resulted from the initial collision between Indian and Asian continents,and it is further confirmed that the eastward initial Indian-Asian collision at 55Ma.