Petrogenesis And Geodynamic Implications Of Early Cretaceous Granitic Rocks In The Tengchong Block,SW China

The various granitic rocks would be generated by the subducting and closure processes of Meso-Tethyan ocean.Therefore,tracking their petrpgenesis will help us to clarify the geochemical characteristics by the evolutional processes of spatial-temporal relationship,nature of deep crustal materials and their geodynamic setting.The northern magmatic belt,as the products of the subducting and closure of Bangong-Nujiang Meso-Tethyan ocean,which was distributed from the western part in the central and northern Lhasa Block of Tibet to southeastern part in the Tengchong Block of its southeastern extension.However,the subducting and closure processes had caused the completedly different geological and magmatic responce:there are abundant Early-Cretaceous basalts-andesites-dacites-rhylites and mafic dykes-granitoids in the central and northern Lhasa Block,which were generated by the subducting,detach,roll-back and slab break-off of Bangong-Nujiang Meso-Tethyan oceanic lithosphere;In contrast to,the abundantly Early-Cretaceous intermediate to felsic granitoids and minor dacites and rhylites were distributed in the Tengchong Block.Therefore,some crucial geological problems would be proposed:why are there different geological and magmatic response between the western and southeastern part?Whether both of them were triggered by the similarly subducting,detach,roll-back and slab break-off of Bangong-Nujiang Meso-Tethyan oceanic lithosphere or not?If it is,what was the petrogenesis of these granitoids and how have they experienced in the geodynamic processes such as the subducting,detach,roll-back and break-off?Discussing these perplexing problems would improve us to understand the petrogenesis of granitoids that were generated at the evolutional processes of subducting and closure of Bangong-Nujiang Meso-Tethyan ocean,clarify the different evolutional characteristics of spatial-temporal and geochemistry in the western and southeastern part,and build the comprehensive geodynamic processes by the subducting and closure of Bangong-Nujiang Meso-Tethysn ocean.Hence,we present the LA-ICP-MS zircon U-Pb age,in-situ Lu-Hf isotoptic,whole-rock major and trace element,whole-rock Sr-Nd isotopic,and mineral chemical data of the Dulongjiang,Pianma,Donghe,Menglian and Xiaotang-Mangdong pluton in the Tengchong Block,SW China,aim to:(1)figure out the evolutional discipline of spatial-temporal distributions and geochemical characteristics of the Early-Cretaceous granitic magmatism in the Tengchong Block,and(2)try to build the petrogenic and geodynamic evolutionary model of the Early-Cretaceous magmatism by the subducting and closure of Bangong-NujiangMeso-Tethyan ocean.Then several results would be proposed:1.The Early-Cretaceous magmatism in the Tengchong Block was mainly composed by the intermediate to felsic granitoids.To the west,which were seperated by the Gudong-Tengchong fault,and to the east,which were separated by the Nujiang-Lushui-Ruili fault.Their zircon U-Pb age was mainly developed from ca.130Ma to ca.111Ma,and three stages should be focused on:(1)ca.130-122Ma,the rocks were mainly developed in the Donghe area of the northeastern Tengchong and Dulongjiang area of the northern Gaoligong,these granodiorites-monzogranites-syenogranites are weakly peraluminous and high-K calc-alkaline;(2)ca.122-116Ma,the rocks were developed in the Xiaotang-Mangdong and Menglian area in the eastern Tengchong Block,these quartz monzodiorite-granodiorites are Na-rich calc-alkaline;(3)ca.114-111Ma,the magmatism in this stage was mainly developed in the Lushui-Pianma area of the south-middlle part of Gaoligong.From east to west,the Na-rich calc-alkaline quartz diorites were gradually changed to high-K calc alkaline granodiorites and monzogranites.The LA-ICP-MS zircon U-Pb geochronology was focused on the various granitic plutons in the eastern Tengchong Block.The Dulongjiang granodiorites and their enclaves in the northern part of Gaologong belt were 124Ma and 133-130Ma;The Donghe monzogranites in the northeastern Tengchong Block were 124-120Ma,and the syenogranites in the western part were 130Ma;The Xiaotang-Mangdong quartz monzodiorites-granodiorites in the eastern Tengchong Block were 123-120Ma,and their enclaves were 121Ma,the Menglian granodiorites and their enclaves in the eastern Tengchong Block were 118-116Ma and 118Ma.Together with data in the literatures,the weighted mean age of Xiaotang-Mangdong pluton was ca.122Ma and the Menglian pluton was ca.117Ma,the gradually young trend was developed from west to east;The Pianma quartz diorites-granodiorites-monzogranites in the middle-southern Gaoligong belt were 114-111Ma.Therefore,we may suggest that the Early-Cretaceous magmatism in the Tengchong Block show more concentrated spatial and temporal than those in the central and northern Lhasa Block,which were also triggered by muti-stage magmatic response.Meanwhile,the abundant symbiotic mafic enclaves in the granitic pluton indicate that the probably mantle-derived mafic magmas play a significant role in the genesis.2.The Early-Cretaceous granitic magmatism in the Tengchong Block was weakly peraluminnous,calc-alkaline to high-K calc-alkaline,evolved whole-rock Sr-Nd-Pb isotopic compositions and variable zircon Hf isotopic components.Their amphiboles are mainly magnesian-hornblende,a few was ferro-tschermakite,and their plagioclases were mainly labradorite-andesine but a few was bytownite.Dulongjiang granodiorites were derived from recycled subducting sediments-derived melts by the mantle-like materials in the mantle wedge;Donghe granites were typically high fractionated ?-type granites,which derived from partial melting of lower crustal materials;The Xiaotang-Mangdong and Menglian quartz monzodiorites-granodiorites were derived from melting of various basaltic lower crustal components,triggered by the underplate mantle-deirved magmas;The Pianma quartz diorites-granodiorites-monzogranites were derived from various deep crustal materials by mantle-like magmas.The abundant mafic enclaves and variable zircon Hf components indicate that the mantle-crust interaction and magma mixing play the crucial role in the origin of the primitive magmas.The Dulongjiang granodiorites are high-K,calc-alkaline and slightly peraluminous(A/CNK = 0.96-1.11).The high Th/La and La/Sm ratios,Th and Nb content,low Nb/Ta,Ba/Th,U/Th and Zr/Nb ratios,similar to rocks formed by sediment-derived melts.The uniformly high initial 87Sr/86Sr(0.7103-0.7126),low sNd(t)values(-8.3 to-11.2)and negative and variable zircon ?Hf(t)values(-11.7 to-1.0)imply its continental crustal sources.The enclaves are basaltic to andesitic and have high MgO(3.82-5.48 wt.%),variations of zircon Hf isotopic compositions and contain magnesian-hornblende and labradorite-bytownite,both indicate a possible mantle-like origin.In consideration of regional geology,we infer that the rocks were derived from melting of recycled subducted oceanic sedimentary materials in the mantle wedge;The Donghe granites display features typical of highly fractionated ?-type granites:high SiO2 contents(>71 wt.%),high K contents(K2O = 3,88-5,66 wt.%),calc-alkaline character,slight peraluminosity(A/CNK = 1.02-1.16),and a highly differentiated index ranging from 84 to 96.In addition,as SiO2 contents increase,the rare earth element(REE)abundances,especially heavy REE abundances,and REE pattern slopes change gradually,but the negative Eu anomalies increase sharply,while the degree of enrichment in Rb,Th,U,and Pb and depletion in Ba,Nb,Sr,P,and Ti are enhanced;The Menglian granodiorites and their enclaves show calc-alkaline and sodium-rich nature,enrichment in large-ion lithophile elements(LILEs),and variable depletion in zircon Hf isotopic compositions.Euhedral amphiboles in both granodiorites and associated enclaves are magnesian-hornblende with high Mg and Ca and contain euhedral plagioclase inclusions of labradorite to andesine(An36-57)composition,which was most likely derived through the mixing of partial melts derived from juvenile basaltic lower crust and a minor evolved component of ancient crustal sources;The Xiaotang-Mangdong quartz monzodiorite-granodiorites are also sodium-rich and show calc-alkaline trend with negative zircon Hf isotopic compositions(-5.6 to +0.6).The MMEs in the host intrusions are monzogabbro with variable and depleted zircon Hf isotopic compositions.The amphiboles in the both host intrusions and the enclaves show Al-rich ferro-tschermakite composition.We infer that the quartz monzodiorite-granodiorites were derived from magmas generated by the melting of ancient basaltic rocks in the lower arc crust induced by the underplating of mantle-derived mafic magmas;There are the distinct sources regions for the Pianma quartz diorite and granodiorite-monzogranite association.The Pianma quartz diorites are sodic,calc-alkaline and have high Mg#(52-54)values.They also have elevated initial 87Sr/86Sr(0.7070 to 0.7092)and low ?Nd(t)(-5.2 to-7.6),with variable zircon ?Hf(t)values(+5.7 to-9.0).Those data suggest that the quartz diorites were derived from partial melting of ancient basaltic lower crust by a mantle-derived magma in source region.The granodiorite-monzogranite association has high-K calc-alkaline,weakly peraluminous characters.They show lower Nb/Ta(5.57 to 13,8),CaO/Na2O(0.62 to 1.21),higher Al2O3/TiO2(24.4 to 44.4)ratios,more evolved whole-rock Sr-Nd and zircon Hf isotopic signatures,all of which suggest derivation from mixed basaltic and metasedimentary source rocks in a deep crustal zone.In summary,the Early-Cretaceous granitoids show various petrological and geochemical features,which could be caused by various magmatic processes,thereby,we can use them to clarify the geodynamic setting in the deep crust.3.In contrast to the Early-Cretaceous magmatism in the central and northern Lhasa Block,there are the absence of basaltic-anseditic volcanics in the Tengchong Block.The Early-Cretaceous granitic rocks show more evolved geochemical features than those in the central and northern Lhasa Block.From ca.122Ma,the more Na-rich calc-alkaline rocks were ocuured with depleted zircon Hf components,and their enclaves are mantle-derived.These features indicate that the more mantle-derived materials have the significant role in the nature,from ca.122Ma,although the crust-derived materials were also predominantly.The age of Early-Cretaceous magmatism in the Tengchong Block was range from ca.l 30Ma to 111Ma,which mainly contained diorites,quartz diorites,granodiorites,monzogranites and syenogranites,mafic enclaves were included in them.Their mineral components mainly contained plagioclase + K-felspar + quartz + biotite ± hornblende ±muscovite.The rocks have intermediate to felsic SiO2 content range from 58.8 wt.%to 77.9 wt.%,and their enclaves were range from 47.7 wt.%to 59.8 wt.%.The A/CNK of rocks was range from 0.91 to 1.44,most of them are weakly peraluminous.The rocks have various Mg#range from 22 to 65,and most are>45.But since 122Ma,the more high Mg#and Na-rich rocks are abundantly occurred in.The rocks have relative moderate Nb/Ta ratios between the mantle and crust,but their Sr/Y ratios are very low(most of them<20),which also show variable zircon Hf components range from-18.9 to +12.1,but evolved whole-rock 87Sr/86Sr(0.7067-0.7124)and ?Nd(t)(-7.1--10.1).4.There are three stage granitic magmatism range from ca.130Ma to 110Ma in the Tengchong Block,which were generated at the setting of the detach,roll-back and break-off of subducting Bangong-Nujiang Meso-Tethyan oceanic lithosphere.The first stage was ca.130-122Ma,the Dulongjiang high-K calc-alklaine granodiorites were derived from melting of recycled subducted sediments by the subducting Bangong-Nujiang Meso-Tethyan oceanic lithosphere,and the Donghe high fractionated I-type granites were also identified in the northeastern Tengchong Block,which indicate the significantly evolutional processes of crystallization-fractionation following the partial melting of crustal materials;The second stage was ca.122-116Ma,the two distinctive Na-rich granitoids and their enclaves were triggered by the roll-back of subducting Bangong-Nujiang Meso-Tethyan oceanic lithosphere,both of them were greatly affected by the underplate mantle-derived magmas.Meanwhile,there are enhanced mantle-derived contributions since 122Ma from west to east;The third stage was ca.114-111Ma,the quartz diorite,granodiorite and monzogranites were products of slab break-off of subducting Bangong-Nujiang Meso-Tethyan oceanic lithosphere,and the geochemical variation between quartz diorite and granodiorite-monzogranite could be caused by their distinctive source in the deep crust.