The Spatial-temporal Distribution,Sequence And Petrogenesis Of Lincang Granites In Western Yunnan

The Sanjiang Tethys tectonic belt in western Yunnan,located in the southeastern margin of the Tibet plateau,preserves numerous magmatic records of Proto-and Paleotethys orogeny,which is an important window to study the tectono-magmatic activities and associated dynamic mechanisms during the oceanic and continental conversion.The Lincang granitic batholith,the largest complex batholith in this area,has attracted many researchers to study its petrology,geochronology and geochemistry,which has improved our understanding of its petrogenesis and tectonic setting.However,the previous studies mainly focused on the Triassic monzongranites,lacking the systematic and detailed disintegration of intrusive rock units.Besides,precise determination for crystallization age of each intrusive rock unit,and discussion on the genetic relationship between various granitic rocks and their enclaves are also lack.These factors restrict the further understanding of tectono-magmatic evolution,dynamic mechanism and crust evlution in the Sanjiang area.Thus,this paper mainly researches on the granites and their enclaves,through the detailed studies on field geology,petrology,geochronology,petrological geochemistry and their isotope characteristics,to construct the spatial-temporal frame and petrological evolution sequence,to discuss its petrogenesis and implications on the Tethyan tectonic evolution and crustal evolution.Combing with field geological investigation and published data,the Lincang batholith can be divided into eastern and western sections by Kangtai-Douge fault,with assemblages of granodiorite,monzonite and syenite in the western section,but granodiorite and monzonite in the eastern section.Studies on field geology,geochronology and petrography indicate that the Lincang batholith can be divided into several petrological sequences,including the Ordovician syenogranite sequence,the Permian granodiorite sequence and the Triassic monzongranite.The enclaves can be divided into two types,i.e,metamorphic enclaves and magmatic enclaves,and the former one comprise of granulite,schist and metamorphic volcanic rock,while the latter one comprise of intermediate-mafic enclaves and felsic enclaves.Zircon U-Pb geochronology shows that syenogranites were formed at ca.474-465Ma,the granodiorites were formed in Permian,while monzongranites were emplaced in Late Triassic.The different types of monzongranites were formed from 230Ma to 220Ma,from early to late episodes the crystallization sequence is fine-grained(230.38±0.82Ma),porphyritic fine-grained(229.4±1.6Ma),porphyritic medium grained(228.72±0.80Ma),coarse grained(225.3±1.2Ma)and porphyritic coarse grained monzongranite(220.0±1.7Ma).The magmatic enclaves(237±0.72Ma)were formed a little earlier than the host monzongranite,while the metamorphic enclaves show a wide range of zircon ages(3.2Ga?480Ma)with the youngest peak at 490 Ma.Geochronology studies show that the Lincang granite batholith were comprised by Prototethy-related Ordovician granites and Paleotethy-related Permian-Triassic granites.Whole-rock geochemistry study shows that the Ordovician syenogranites have high contents of Si and Alk,but low contents of Mg O with K2O/Na2O of 1.13-2.0,ASI of 1.46-2.46 and DOI of 80.37-96.08.They are enriched in LILE,such as Rb,Th,K,U,and are depleted in HFSE,such as Ba,Nb,Sr,P,Ti.The samples show strong light REE enrichment with a negative Eu anomalies(LREE/HREE=3.92?18.30).Zr saturation temperature yields an average value of 769.18?.The mineral geochemistry and zircon Lu-Hf isotopes(?Hf(t)=-5.47?-2.73,t_MD2=1.48?1.63Ga)indicate that the Ordovician syenogranites were sourced from Mesoproterozoic crust.The Permian granodiorites have high values of A/CNK(>1.1)identical to peraluminous series,and they have similar REE and trace elements compositions to the Ordovician syenogranites,but the Permian granodiorites have higher Zr saturation temperature(790.10?).The Sr-Nd isotopes of feldspar and apatite show that they were derived from ancient continental crust(⁸⁷Sr/⁸⁶Sr=0.7349-0.7369,¹⁴³Nd/¹⁴⁴Nd=0.5120-0.5122).The Triassic monzongranites have high contents of Si,compared with the eastern section,the western section is enriched in Na and Mg,but depleted in K.All the Triassic monzongranites are enriched in LILE but depleted in HFSE,and show medium differentiation of REE with negative Eu anomalies.Ziron Lu-Hf isotopes and the Sr-Nd isotope of feldspar and apatite indicate that the Triassic monzongranites are mainly sourced from Late-Paleoproterozoic to Early-Mesoproterozoic crust(?Hf(t)=-15.57?-8.91,t_DM2=1.62?1.99Ga,⁸⁷Sr/⁸⁶Sr=0.7259?0.7340,¹⁴³Nd/¹⁴⁴Nd=0.5119?0.5128).Petrographic,geochemical and geochronological studies show that the metamorphic enclaves most likely to be the basement remnants of Lincang Block,which may represent the magma source of Lincang granites.By contrast,the magmatic enclaves show similar zircon Lu-Hf and feldspar Sr isotopes to the host monzongranite.Combining with their petrology and geochemistry features,we propose the magmatic enclaves could not represent the underplated basaltic magma,but represent the crystal mush that derived from identical source to the host granites.The crystal mush(237Ma)were re-activated by the later magmatism(230-220Ma)with some residues remained to form the magmatic enclaves.